PublicationsTake a look at our recent peer-reviewed publications or book chapters!
Reductive catalytic fractionation: state of the art of the lignin-first biorefinery 20-01-2019
Reductive catalytic fractionation (RCF) of lignocellulose is an emerging biorefinery scheme that combines biomass fractionation with lignin depolymerisation. Central to this scheme is the integration of heterogeneous catalysis, which overcomes the tendency of lignin to repolymerise. Ultimately, this leads to a low-Mw lignin oil comprising a handful of lignin-derived monophenolics in close-to-theoretical yield, as well as a carbohydrate pulp. Both product streams are considered to be valuable resources for the bio-based chemical industry. This Opinion article sheds light on recently achieved milestones and consequent research opportunities. More specifically, mechanistic studies have established a general understanding of the elementary RCF steps, which include (i) lignin extraction, (ii) solvolytic and catalytic depolymerisation and (iii) stabilisation. This insight forms the foundation for recently developed flow-through RCF. Compared to traditional batch, flow-through RCF has the advantage of (i) separating the solvolytic steps from the catalytic steps and (ii) being a semi-continuous process; both of which are beneficial for research purposes and for industrial operation. Although RCF has originally been developed for ‘virgin’ biomass, researchers have just begun to explore alternative feedstocks. Low-value biomass sources such as agricultural residues, waste wood and bark, are cheap and abundant but are also often more complex. On the other side of the feedstock spectrum are high-value bio-engineered crops, specifically tailored for biorefinery purposes. Advantageous for RCF are feedstocks designed to (i) increase the total monomer yield, (ii) extract lignin more easily, and/or (iii) yield unconventional, high-value products (e.g. alkylated catechols derived from C-lignin). Taking a look at the bigger picture, this Opinion article highlights the multidisciplinary nature of RCF. Collaborative efforts involving chemists, reactor engineers, bioengineers and biologists working closer together are, therefore, strongly encouraged.
From rational design of a new bimetallic MOF family with tunable linkers to OER catalysts 02-01-2019
Innovative bimetallic MOFs offer more possibilities to further tailor the properties of MOFs, which have attracted great attention for wide applications. However, it is still a great challenge to rationally design bimetallic MOFs due to the lack of a tunable and reasonable hybrid structure architecture. Herein, a new series of bimetallic metal–organic frameworks (MOFs) with tunable pillar linkers were prepared by a one-step synthesis method. These bimetallic MOFs retain the same crystal structure when the mole fraction (based on metal) of the two metals changes from 0 to 1 and both metal ions occupy random nodal positions. The incorporation of a second metal cation has a large influence on the intrinsic properties (e.g. thermal stabilities and band gaps) of the MOFs. Furthermore, these bimetallic MOFs were used as self-sacrificial templates to prepare bimetal oxide catalysts for the oxygen evolution reaction (OER). After pyrolysis, a porous and hierarchical honeycomb-like structure with carbon network covered (bi)metal oxides is formed. Among all the bimetallic MOF-derived catalysts, CoNi1@C showed the best performance for the OER with the lowest Tafel slopes (55.6 mV dec−1) and overpotentials (335 mV on a glassy carbon electrode and 276 mV on Ni foam) at a current density of 10 mA cm−2, which is higher than those of state-of-the-art Co–Ni mixed oxide catalysts derived from MOFs for the OER. Our results indicate that the incorporation of a second metal ion is a promising strategy to tailor the properties of MOFs. More importantly, this new bimetallic MOF family with tunable linkers is expected to serve as a flexible assembly platform to offer broad possibilities for practical applications of MOFs.
Mechanism of selective benzene hydroxylation catalyzed by iron-containing zeolites 27-11-2018
A direct, catalytic conversion of benzene to phenol would have wide-reaching economic impacts. Fe zeolites exhibit a remarkable combination of high activity and selectivity in this conversion, leading to their past implementation at the pilot plant level. There were, however, issues related to catalyst deactivation for this process. Mechanistic insight could resolve these issues, and also provide a blueprint for achieving high performance in selective oxidation catalysis. Recently, we demonstrated that the active site of selective hydrocarbon oxidation in Fe zeolites, named α-O, is an unusually reactive Fe(IV)=O species. Here, we apply advanced spectroscopic techniques to determine that the reaction of this Fe(IV)=O intermediate with benzene in fact regenerates the reduced Fe(II) active site, enabling catalytic turnover. At the same time, a small fraction of Fe(III)-phenolate poisoned active sites form, defining a mechanism for catalyst deactivation. Density-functional theory calculations provide further insight into the experimentally defined mechanism. The extreme reactivity of α-O significantly tunes down (eliminates) the rate-limiting barrier for aromatic hydroxylation, leading to a diffusion-limited reaction coordinate. This favors hydroxylation of the rapidly diffusing benzene substrate over the slowly diffusing (but more reactive) oxygenated product, thereby enhancing selectivity. This defines a mechanism to simultaneously attain high activity (conversion) and selectivity, enabling the efficient oxidative upgrading of inert hydrocarbon substrates.
Sulfonated mesoporous carbon and silica-carbon nanocomposites for biomass conversion. 15-11-2018
Sulfonated mesoporous carbon or silica-carbon nanocomposite materials possess a large amount of accessible SO3H acid groups, which may have versatile applications as solid acid catalysts in biomass conversion. The mesopores can facilitate the transportation of the large biomass substrates and the targeted products. The hydrophobicity of carbon ensures the hydrothermal stability of the materials, which is essential since biomass conversion usually occurs in polar circumstances (e.g., in water), and it can facilitate the adsorption of reactants and the desorption of formed H2O during the conversion simultaneously. The other weak acid groups of carbon, like phenolic OH and COOH groups may help the adsorption of reactants or even exert a synergistic catalytic function. With the co-existence of silica phase, the mesopores can be maintained under harsh conditions, e.g., during the sulfonation synthesis step in concentrated H2SO4 at a high temperature. Furthermore, the hybrid silica-carbon surface can provide specific polarity from the synergy of both kinds of components and offer potentiality for multi-functionalization. Herein, the synthesis and fabrication of such sulfonated mesoporous carbon and silica-carbon nanocomposite wherein CeSO3H is confined in mesoporous channels is reviewed. Their state-of-the-art use in catalytic biomass-related conversion such as fatty acids esterification, carbohydrates conversion and furan-derivative condensation, are discussed in detail. The stability issues of the sulfonated carbon or silica-carbon nanocomposites for the specific catalytic reactions are specifically addressed. Finally, a general conclusion is drawn from the above and a future outlook for this class of upcoming materials in terms of synthetic challenges and catalytic application is presented.
Direct upstream integration of biogasoline production into current light straight run naphtha petrorefinery processes. 24-09-2018
There is an urgent need to address environmental problems caused by our transportation systems, which include the reduction of associated CO2 emissions. In the short term, renewable drop-in fuels are ideal, as they allow a direct integration into the existing infrastructure. However, preferably they would perform better than current alternatives (for example, bioethanol) and be synthesized in a more efficient way. Here we demonstrate the production of biogasoline with a direct upstream integration into processes in existing petrorefinery facilities that targets the 10% bio-based carbon in accordance with the current European Union directives (for 2020) for biofuels. To achieve this goal, we show the valorization of (hemi)cellulose pulp into light naphtha using a two-phase (H2O:organic) catalytic slurry process. A C5–C6alkane stream, enriched with bio-derived carbon and compatible with further downstream petrorefinery operations for (bio)gasoline production, is automatically obtained by utilizing fossil light straight run naphtha as the organic phase. The ease of integration pleads for a joint petro/bio effort to gradually produce bio-enriched gasolines, wherein the chemical compounds of the bio-derived fraction are indistinguishable from those in current high-quality gasoline compositions.
Functionalised heterogeneous catalysts for sustainable biomass valorisation. 18-09-2018
Efficient transformation of biomass to value-added chemicals and high-energy density fuels is pivotal for a more sustainable economy and carbon-neutral society. In this framework, developing potential cascade chemical processes using functionalised heterogeneous catalysts is essential because of their versatile roles towards viable biomass valorisation. Advances in materials science and catalysis have provided several innovative strategies for the design of new appealing catalytic materials with well-defined structures and special characteristics. Promising catalytic materials that have paved the way for exciting scientific breakthroughs in biomass upgrading are carbon materials, metal–organic frameworks, solid phase ionic liquids, and magnetic iron oxides. These fascinating catalysts offer unique possibilities to accommodate adequate amounts of acid–base and redox functional species, hence enabling various biomass conversion reactions in a one-pot way. This review therefore aims to provide a comprehensive account of the most significant advances in the development of functionalised heterogeneous catalysts for efficient biomass upgrading. In addition, this review highlights important progress ensued in tailoring the immobilisation of desirable functional groups on particular sites of the above-listed materials, while critically discussing the role of consequent properties on cascade reactions as well as on other vital processes within the bio-refinery. Current challenges and future opportunities towards a rational design of novel functionalised heterogeneous catalysts for sustainable biomass valorisation are also emphasized
Spectroscopic Identification of the α-Fe/α-O Active Site in Fe-CHA Zeolite for the Low-Temperature Activation of the Methane C-H bond. 31-08-2018
The formation of single-site α-Fe in the CHA zeolite topology is demonstrated. The site is shown to be active in oxygen atom abstraction from N2O to form a highly reactive α-O, capable of methane activation at room temperature to form methanol. The methanol product can subsequently be desorbed by online steaming at 200 °C. For the intermediate steps of the reaction cycle, the evolution of the Fe active site is monitored by UV–vis–NIR and Mössbauer spectroscopy. A B3LYP-DFT model of the α-Fe site in CHA is constructed, and the ligand field transitions are calculated by CASPT2. The model is experimentally substantiated by the preferential formation of α-Fe over other Fe species, the requirement of paired framework aluminum and a MeOH/Fe ratio indicating a mononuclear active site. The simple CHA topology is shown to mitigate the heterogeneity of iron speciation found on other Fe-zeolites, with Fe2O3 being the only identifiable phase other than α-Fe formed in Fe-CHA. The α-Fe site is formed in the d6r composite building unit, which occurs frequently across synthetic and natural zeolites. Finally, through a comparison between α-Fe in Fe-CHA and Fe-*BEA, the topology’s 6MR geometry is found to influence the structure, the ligand field, and consequently the spectroscopy of the α-Fe site in a predictable manner. Variations in zeolite topology can thus be used to rationally tune the active site properties
Catalytic Strategies Towards Lignin-Derived Chemicals. 27-08-2018
Lignin valorization represents a crucial, yet underexploited component in current lignocellulosic biorefineries. An alluring opportunity is the selective depolymerization of lignin towards chemicals. Although challenged by lignin’s recalcitrant nature, several successful (catalytic) strategies have emerged. This review provides an overview of different approaches to cope with detrimental lignin structural alterations at an early stage of the biorefinery process, thus enabling effective routes towards lignin-derived chemicals. A first general strategy is to isolate lignin with a better preserved native-like structure and therefore an increased amenability towards depolymerization in a subsequent step. Both mild process conditions as well as active stabilization methods will be discussed. An alternative is the simultaneous depolymerization-stabilization of native lignin towards stable lignin monomers. This approach requires a fast and efficient stabilization of reactive lignin intermediates in order to minimize lignin repolymerization and maximize the envisioned production of chemicals. Finally, the obtained lignin-derived compounds can serve as a platform towards a broad range of bio-based products. Their implementation will improve the sustainability of the chemical industry, but equally important will generate opportunities towards product innovations based on unique biobased chemical structures.
Catalytic lignocellulose biorefining in n-butanol/water: a one-pot approach toward phenolics, polyols, and cellulose. 01-08-2018
Lignocellulose constitutes an alluring renewable feedstock for the production of bio-based chemicals. In this contribution, we propose a chemocatalytic biorefinery concept that aims to convert lignocellulosic biomass (Eucalyptus sawdust) into (i) lignin-derived (mono)phenolics, (ii) hemicellulose-derived polyols, and (iii) a cellulose pulp. This is achieved by processing biomass in an equivolumetric mixture of n-butanol and water at elevated temperature (200 °C), in the presence of Ru/C and pressurised hydrogen (30 bar). During this one-pot Reductive Catalytic Fractionation (RCF) process, the hot liquor enables the extraction and solvolytic depolymerisation of both lignin and hemicellulose, while the catalyst and reductive environment are essential to hydrogenate reactive intermediates (coniferyl/sinapyl alcohol and sugars) toward stable target products (phenolics and polyols, respectively). After the catalytic reaction, the solid carbohydrate pulp (mainly cellulose) is easily retrieved upon filtration. Phase separation of n-butanol and water occurs upon cooling the liquor (<125 °C), which offers a facile and effective strategy to isolate lignin-derived phenolics (n-butanol phase) from polyols (aqueous phase). The three resulting product streams provide a versatile platform for down-stream conversion, en route to bio-based chemicals. A proof-of-concept experiment using a 2 L batch reactor demonstrates the scalability potential. Furthermore, this contribution highlights that the conversion of each biopolymer is influenced in a different way by reaction parameters like catalyst, hydrogen pressure, temperature, and acidity (HCl). The key challenge is to find suitable conditions that allow (close-to-)optimal valorisation of all constituents.
Propylphenol to Phenol and Propylene over Acidic Zeolites: Role of Shape Selectivity and Presence of Steam. 30-07-2018
This contribution studies the steam-assisted dealkylation of 4-n-propylphenol (4-n-PP), one of the major products derived from lignin, into phenol and propylene over several micro- and mesoporous acidic aluminosilicates in gas phase. A series of acidic zeolites with different topology (e.g., FER, TON, MFI, BEA, and FAU) are studied, of which ZSM-5 outperforms the others. The catalytic results, including zeolite topology and water stability effects, are rationalized in terms of thermodynamics and kinetics. A reaction mechanism is proposed by (i) analyzing products distribution under varying temperature and contact time conditions, (ii) investigating the dealkylation of different regio- and geometric isomers of propylphenol, and (iii) studying the reverse alkylation of phenol and propylene. The mechanism accords to the classic carbenium chemistry including isomerization, disproportionation, transalkylation, and dealkylation, as the most important reactions. The exceptional selectivity of ZSM-5 is attributed to a pore confinement, avoiding disproportionation/transalkylation as a result of a transition state shape selectivity. The presence of water maintains a surprisingly stable catalysis, especially for ZSM-5 with low acid density. The working hypothesis of this stabilization is that water precludes diphenyl ether(s) formation in the pores by reducing the lifetime of the phenolics at the active site due to the high heat of adsorption of water on H-ZSM-5, besides counteracting the equilibrium of the phenolics condensation reaction. The water effect is unique for the combination of (alkyl)phenols and ZSM-5.
Titania-Silica Catalysts For Lactide Production From Renewable Alkyl Lactates: Structure-Activity Relations 20-07-2018
Different Ti-Si catalysts, viz. TiO2 supported on amorphous SiO2 or Si-MCM-41, TiO2-SiO2 xerogels and Ti-zeolites (TS-1 and Ti-beta) were compared in terms of activity and selectivity for the direct conversion of methyl lactate to lactide in the gas phase. Except for Ti-beta, all catalysts exhibit a high lactide selectivity of 88-92% at conversions below 50 %. From DR UV-VIS spectroscopy, it is evidenced that the catalytic activity of tetrahedral TiO4 sites is higher than of polymerized TiO5 or octahedral TiO6 counterparts, irrespective of the catalyst structure, an analysis supported by ToF-SIMS measurements. A kinetic analysis shows that the catalytic activity is proportional to the number of vacant sites on the catalyst surface. Thus, the activity increase observed for tetrahedral TiO4 sites may be attributed to an increased number of vacant sites (e.g. two for TiO4, zero for TiO6). Lactide productivity thus highly benefits from an increased dispersion of Ti-sites on the catalyst surface, and could be increased by a factor of 2.5 (up to 10 gLD gcat-1 h-1) when TiO2 is dispersed on a Si-MCM-41 support, with higher surface areas than amorphous SiO2 gels.
Kinetics of Homogeneous and Heterogeneous Reactions in the Reductive Aminolysis of Glucose with Dimethylamine. 05-07-2018
The reductive aminolysis of glucose with dimethylamine (DMA) as aminating agent has been investigated experimentally as well as via kinetic model construction. A fed-batch reactor configuration was used at following conditions: temperatures ranging between 383 K–398 K, total pressures from 6.0 MPa to 7.5 MPa, an overall catalyst to glucose ratio of 9–31 gcat mol−1, an overall DMA to glucose ratio of 12–24 mol mol−1 and an overall hydrogen to glucose ratio of 5–10 mol mol−1. Such a reactor configuration, combined with a controlled feeding rate of the sugar, allowed to significantly avoid degradation reactions. The main desired products, i.e., dimethylaminoethanol (DMAE) and tetramethylethylenediamine (TMEDA), were obtained after amination followed by retro-aldol cleavage with maximum selectivity, amounting up to 15% and 60% respectively. Retro-aldol cleavage after amination proceeds at lower temperatures, evidenced by an activation energy of 60 kJ mol−1, than without amination, where activation energies amounting to 140 kJ mol−1 have been reported. A higher catalyst to glucose ratio leads to more parallel side products such as N,N-dimethylglucamine and 4‐dimethylamino‐1,2,3‐butanetriol. The effects of temperature and catalyst to glucose ratio were much more pronounced than that of the total pressure or the ratio of the aminating agent to glucose. The developed kinetic model is based on the most prominent homogeneous bulk phase and heterogeneously catalyzed reactions and accounts quantitatively for degradation reactions. A statistically and physically significant model which could satisfactorily reproduce the experimental observations, was thus obtained.
Second-Sphere Effects on Methane Hydroxylation in Cu-Zeolites 28-06-2018
Two [Cu2O]2+ cores have been identified as the active sites of low temperature methane hydroxylation in the zeolite Cu-MOR. These cores have similar geometric and electronic structures, yet different reactivity with CH4: one reacts with a much lower activation enthalpy. In the present study, we couple experimental reactivity and spectroscopy studies to DFT calculations to arrive at structural models of the Cu-MOR active sites. We find that the more reactive core is located in a constricted region of the zeolite lattice. This leads to close van der Waals contact between the substrate and the zeolite lattice in the vicinity of the active site. The resulting enthalpy of substrate adsorption drives the subsequent H atom abstraction step—a manifestation of the “nest” effect seen in hydrocarbon cracking on acid zeolites. This defines a mechanism to tune the reactivity of metal active sites in microporous materials
One-step synthesis of stereo-pure l, l lactide from l-lactic acid 01-06-2018
Poly (lactic acid) (PLA) is extensively used as an eco-friendly compound for many applications. The synthesis of stereo-pure lactide, specifically l,l-lactide which is the most desired isomer for the synthesis of high-quality PLA is very important. In this work, various materials including MOFs and simple bases have been applied as catalysts for the Lactide synthesis. Herein we report the one-step synthesis of l,l-lactide with high selectivity and yield (99%) in the absence of racemization by applying a cost-effective Cs2CO3 catalyst. The novel described procedure is even expected to be very efficient for industrial applications
Branching-first: synthesizing C-C skeletal branched bio-based chemicals from sugars 25-04-2018
A novel strategy to bio-based chemicals with a branched carbon skeleton is introduced. Hereto, small sugars, such as 1,3-dihydroxyacetone, are coupled catalytically to obtain branched C6 sugars, such as dendroketose, in high yield at mild conditions. By bringing this branching step up front, at the level of the sugar feedstock (branching-first), new opportunities for the synthesis of useful chemicals arise. Here, we show that the branched sugar can be efficiently valorized into (i) new branched polyols and (ii) short branched alkanes. The first route preserves most of the original sugar functionality by hydrogenation with Ru/C, and renders access to branched polyols with three primary alcohol groups. These molecules are potentially interesting as plasticizers, crosslinkers or detergent precursors. The second valorization route demonstrates a facile hydrodeoxygenation of the branched sugars towards their corresponding branched alkanes (e.g. 2-methylpentane). The highest alkanes yields (> 65 mol% C) are obtained with a Rh/C redox metal catalyst in a biphasic catalytic system, following a HDO mechanism. In the short term, commercial integration of these mono-branched alkanes, in contrast to branched polyols, is expected to be straightforward, because of their drop-in character and well-known valuable octane booster role when present in gasoline. Accordingly, the branching-first concept is also demonstrated with other small sugars (e.g. tetroses) enabling the production of branched C6-C8 sugars, and thus also branched C5-C8 alkanes after HDO.
Supported Molecular Catalysts 24-04-2018
This Editorial presents the Special Issue on Supported Molecular Catalysts as a forum to discuss recent advances in the immobilization and heterogenization of molecular catalysts. The scope ranges from organic synthesis and highly selective reactions through to photocatalysis and electrocatalysis, including advances in enzyme immobilization methodology. The outcome is a wide spectrum of papers encompassing all the old formal domains of catalysis research of both primary and secondary research.
Silica–Carbon Nanocomposite Acid Catalyst with Large Mesopore Interconnectivity by Vapor-Phase Assisted Hydrothermal Treatment 23-04-2018
Mesostructured silica–carbon nanocomposites with large mesopore interconnectivity are created from sucrose as sustainable carbon source using a mild vapor-phase assisted hydrothermal treatment procedure. The resultant mesostructured silica–carbon nanocomposite can be readily sulfonated to provide a strong acid catalyst with high sulfonic acid density, or the carbon phases of the nanocomposite can be removed by calcination to produce a silica material with ultrahigh porosity (Vpore = 1.25 to 1.34 cm3 g–1). A superior catalytic activity is demonstrated for the solvent-less condensation of 2-methylfuran with furfural; both product yield and conversion rate surpass that of reference catalysts such as their counterparts from dry pyrolysis and the commercial strong acid resins. The enhanced catalytic activity is attributed to the higher SO3H acid density (0.64 to 1.08 mmol g–1), the larger and better communicating mesopores (Vmeso = 0.38 to 0.82 cm3 g–1) and the abundant presence of surface oxygen-containing functional groups on the vapor-phase assisted hydrothermally treated samples. The origin of the well-developed large interconnecting mesopores is investigated and discussed. The mild hydrothermal treatment causes local etching of the original mesopores in the precursor material, creating unexpected interconnectivity between the pores, while the original micropores are basically eliminated during the treatment. Therefore, the here specified hydrothermal treatment provides a promising method to conventional pyrolysis for the efficient and eco-friendly synthesis of highly mesoporous silica–carbon nanocomposites and modification of their physicochemical properties.
Shape selectivity vapor-phase conversion of lignin-derived 4-ethylphenol to phenol and ethylene over acidic aluminosilicates: Impact of acid properties and pore constraint 08-04-2018
Selective dealkylation of alkylphenols, the opposite reaction of the more commonly studied phenol alkylation, may represent an important reaction in the production of base chemicals like phenol and olefins from fossilized and raw lignocellulosic matter. This study reports the first thermodynamics and kinetics studies of the vapor-phase conversion of ethylphenol (EP) over acidic γ-Al2O3, amorphous (ASA) and crystalline aluminosilicates (like ferrierite, ZSM-22, ZSM-5, beta, and USY) in the absence of hydrogen and noble metals, as a way to produce phenol and ethylene. The reaction was studied deliberately in presence of steam to get stable time on stream catalysis. The thermodynamic analysis shows an endothermal EP conversion to phenol and ethylene, favoured at high reaction temperature, while isomerisation, disproportionation and transalkylation are thermodynamically preferred at low temperature. The kinetic study examines the role of the catalytically active sites; it reveals the importance of site constraint on the activity, selectivity and stability, and shows the complex temperature dependency of the dealkylation. Both Brønsted and Lewis acid sites are active, but multifactor dependency (such as acid strength and site accessibility) complicates the establishment of simple quantitative relationships with the acid type. EP does not enter the micropores of ferrierite and ZSM-22, as suggested by adsorption experiments. Kinetics without significant diffusion limitations were obtained with ZSM-5, beta and USY. Thus, in absence of intracrystalline diffusion limitation (as verified by calculations using reported effective diffusivities, and substantiated by a comparably high apparent activation energy for all zeolites), the increased reaction turnover rate with increasing pore size from medium to large pore zeolites is largely explained by a change in reaction pathway (from monolecular to bimolecular) to convert EP to phenol and ethylene. A pathway proceeding through fast thermodynamically favourable bimolecular reactions occurs in the spatially non-constrained pores and crystal surface, whereas monomolecular reactions take place in the micropores of ZSM-5. Despite the lower rate, the selectivity over ZSM-5 strongly benefits from active site confinement, being responsible to achieve quantitative formation of phenol and ethylene from ethylphenol. The excellent performance of ZSM-5 thus accords with its shape selective property that avoids undesired side reactions such as the sterically demanding bimolecular reactions like disproportionation, transalkylation and C-C cracking, and severe cokes formation.
Catalytic Reductive Aminolysis of Reducing Sugars: Elucidation of Reaction Mechanism 06-04-2018
A catalytic reductive aminolysis of reducing monosaccharides into short ethylene diamines (or C2diamines) was recently communicated by our group (Pelckmans et al. Angew. Chem. Int. Ed.2017, 56, 14540–14544). Here, a general mechanism for this novel reaction is proposed based on the results of a combined experimental and theoretical study. The mechanism involves hemiaminal formation and subsequent dehydration to produce a zwitterionic iminium intermediate, which undergoes fast C–C cleavage as a result of intramolecular deprotonation, followed by hydrogenation of the formed unsaturated amine intermediate. The role of the amine in facilitating the C–C scission is explained in detail and supported by DFT calculations. Different catalysts, carbohydrate substrates, and reaction conditions were tested to validate the proposed reaction mechanism. Reductive aminolysis of sugars is preferably carried out in the presence of a passivated silica(-alumina) supported Ni catalyst and an alkyl amine using 75–85 bar H2 at 125–130 °C. The water content in the reaction mixture should be kept below 33 wt % to favor dehydration equilibria in the mechanism, while the amine-to-glucose molar ratio should be kept high, preferably larger than 6, to favor the amination equilibria. The reaction rate experiences a strong solvent dependency. For instance, the presence of MeOH enhances the rate of C2 diamine formation, as compared to the use of tetrahydrofuran (THF). DFT calculations show that presence of MeOH beneficially affects both the kinetics of the nucleophilic amine attack and the C–C bond scission. These selective rate enhancements result in a 2- to 3-fold increase of the C2 diamine yield. Among a series of aminating agents, reductive aminolysis with N-methylethanolamine (MEOA) shows a 92 C% yield to the corresponding C2 diamine (BHEDMEDA). The high yield is explained by the formation of a heterocyclic oxazolidine intermediate. Since its formation occurs H2 free, a two-step one-pot production protocol, decoupling C–C scission and hydrogenation, is proposed to achieve highest C2 diamine yield.
Structural characterization of a non-heme iron active site in zeolites that hydroxylates methane 02-04-2018
Iron-containing zeolites exhibit unprecedented reactivity in the low-temperature hydroxylation of methane to form methanol. Reactivity occurs at a mononuclear ferrous active site, α-Fe(II), that is activated by N2O to form the reactive intermediate α-O. This has been defined as an Fe(IV)=O species. Using nuclear resonance vibrational spectroscopy coupled to X-ray absorption spectroscopy, we probe the bonding interaction between the iron center, its zeolite lattice-derived ligands, and the reactive oxygen. α-O is found to contain an unusually strong Fe(IV)=O bond resulting from a constrained coordination geometry enforced by the zeolite lattice. Density functional theory calculations clarify how the experimentally determined geometric structure of the active site leads to an electronic structure that is highly activated to perform H-atom abstraction.
Recrystallization on Alkaline Treated Zeolites in the Presence of Pore-Directing Agents 14-03-2018
In previous works aiming at understanding the mesoporous network after alkaline treatment in the presence of organic additives, conventional bulk characterization techniques led to the conclusion that the dissolved zeolite does not undergo any kind of recrystallization [Verboekend, D., Cryst. Growth. Des. 2013, 13, 5025−5035]. Here for the first time, we demonstrate using the data obtained from 1H and 129Xe NMR spectroscopy that such recrystallization does occur, which leads to the formation of a very thin coating of the mesopore walls. This demonstration is done on a beta (BEA) zeolite treated in the presence of TPA+ in an alkaline solution. The formation of a small amount of nanosized crystals or embryonic phases of silicalite-1 (MFI) zeolite is evidenced, as well as their homogeneous dispersion on the mesoporous surface of the beta zeolite. We think that these results may explain why a homogeneous mesopore size distribution is obtained, when organic pore-directing agents are used in the zeolite hierarchization process performed in an alkaline medium
Straightforward sustainability assessment of sugar-derived molecules from first-generation biomass 16-02-2018
Today, there is a general consensus in academic literature that second generation biomassis the only valuable carbohydrate resource for non-food applications. In the meantime, fermentation of sugars towards bio-ethanol, mainly from first generation biomass, is becoming widespread at an industrial level. This paper tries to investigate if there can be a valuable role for edible resources (here: sugars) in the chemical industry without affecting the food security. Moreover, a connection is proposed between a broad range of multiple technologies and sustainable resources, with main attention to the native C skeleton and functionality of biomass. Going deeper in sustainability, this paper selected four criteria, taking into account the entire valorisation route, to apply to the most promising carbohydrate-derived molecules. By doing this, the most promising chemicals and working points from a sustainable point of view are highlighted.
Promising bulk production of a potentially benign bisphenol A replacement from a hardwood lignin platform† 01-02-2018
A full lignin-to-chemicals valorisation chain – from hardwood over bissyringols to aromatic polyesters (APEs) – is established for renewable 4-n-propylsyringol (PS), the main product from catalytic hydrogenolysis of (native) hardwood lignin. To do so, reagent-grade PS was produced from birch wood via reductive catalytic fractionation (RCF) and isolated in 34 wt% yield on lignin basis. Additional early-stage theoretical calculations, based on both relative volatility (α) and distillation resistance (Ω) as well as Aspen Plus® simulations, predict that the isolation of PS by means of distillation is economically feasible at industrial scales ($85–95 per ton of propylphenolics at 200–400 kt a−1 scale). Subsequent stoichiometric acid-catalysed condensation with formaldehyde unveils a remarkably high 92 wt% selectivity towards the dimer 3,3′-methylenebis(4-n-propylsyringol) (m,m′-BSF-4P), which is isolated in >99% purity by facile single-step crystallisation. The striking dimer selectivity is ascribed to the synergetic interplay between the activating methoxy groups and the oligomerisation-inhibiting propyl chain. Next, an in vitro human oestrogen receptor α (hERα) assay was performed to ensure safe(r) chemical design. The bissyringyl scaffold displays reduced potency (∼19–45-times lower affinity than bisphenol A) and lower efficacy (∼36–45% of BPA's maximum activity). Lastly, to assess the functionality of the safe(r) bissyringol scaffold, it was converted into an APE. The APE displays a Mw = 43.0 kDa, Mn = 24.4 kDa, Tg = 157 °C and Td,5% = 345 °C. In short, (i) the feasibility and scalability of the feedstock, (ii) the simplified process conditions, (iii) the reduced in vitro oestrogenicity, and (iv) the functionality towards polymerisation, make this bissyringol a renewable and potentially benign bisphenol replacement, capable for production at bulk scale.
Catalytic gas-phase production of lactide from renewable alkyl lactates 22-01-2018
A new route to lactide, key building block of the bioplastic polylactic acid, is proposed via a continuous catalytic gas-phase transesterification of renewable alkyl lactates in a scalable fixed-bed setup. Supported TiO2/SiO2 catalysts are highly selective to lactide, with only minimal lactide racemization. The solvent-free process allows for easy product separation and recycling of unconverted alkyl lactates and recyclable lactyl intermediates. The catalytic activity of TiO2/SiO2 catalysts was strongly correlated to their optical properties by DR UV-VIS spectroscopy. Catalysts with high band gap energy of the supported TiO2 phase, indicative of a high surface spreading of isolated Ti centers, show the highest turnover frequency per Ti site.
Chemicals from lignin: an interplay of lignocellulose fractionation, depolymerisation, and upgrading 10-01-2018
In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.
Synthetic & Catalytic Potential of Amorphous Mesoporous Aluminosilicates Prepared by Post‐Synthetic Aluminations of Silica in Aqueous Media 10-01-2018
Amorphous aluminosilicates catalysts have been used industrially on a large scale for almost a century. However, the influence of the pH on the alumination of silica in aqueous solutions has remained largely unclear. Herein, room temperature aluminations of different mesoporous amorphous silicas (fumed silica, dried silica gel, SBA-15, MCM-41, and COK-12) with aqueous solutions of various pH (3-13) are explored. The aqueous solutions are prepared using different aluminum sources (Al(NO3)3 or NaAlO2) and alkaline additives (NaOH or NH4OH). The decoupling of pH and Al source using alkaline additives results in a vast experimental potential to prepare unique aluminosilicates, where an important role is played by the pH development during the treatment. The bulk and surface composition, acidity, aluminum coordination, morphology, hydrothermal stability, and porosity of the obtained materials are characterized. Optimal samples possess large surface areas and superior acidities (up to 50% higher) and outstanding stabilities compared to aluminosilicates prepared via state of the art methods. The obtained materials are evaluated in a series of acid-catalyzed model reactions. The potential of the obtained materials is emphasized by the similar or superior acidity and catalytic performance compared to several benchmark industrial silica-alumina-based catalysts.
Perspective on Lignin Oxidation: Advances, Challenges, and Future Directions 08-01-2018
Lignin valorization has gained increasing attention over the past decade. Being the world’s largest source of renewable aromatics, its valorization could pave the way towards more profitable and more sustainable lignocellulose biorefineries. Many lignin valorization strategies focus on the disassembly of lignin into aromatic monomers, which can serve as platform molecules for the chemical industry. Within this framework, the oxidative conversion of lignin is of great interest because it enables the formation of highly functionalized, valuable compounds. This work provides a brief overview and critical discussion of lignin oxidation research. In the first part, oxidative conversion of lignin models and isolated lignin streams is reviewed. The second part highlights a number of challenges with respect to the substrate, catalyst, and operating conditions, and proposes some future directions regarding the oxidative conversion of lignin.
Advancing the Use of Sustainability Metrics in ACS Sustainable Chemistry & Engineering 02-01-2018
Iron and Copper Active Sites in Zeolites and Their Correlation to Metalloenzymes 19-12-2017
Metal-exchanged zeolites are a class of heterogeneous catalysts that perform important functions ranging from selective hydrocarbon oxidation to remediation of NOx pollutants. Among these, copper and iron zeolites are remarkably reactive, hydroxylating methane and benzene selectively at low temperature to form methanol and phenol, respectively. In these systems, reactivity occurs at well-defined molecular transition metal active sites, and in this review we discuss recent advances in the spectroscopic characterization of these active sites and their reactive intermediates. Site-selective spectroscopy continues to play a key role, making it possible to focus on active sites that exist within a distribution of inactive spectator metal centers. The definition of the geometric and electronic structures of metallozeolites has advanced to the level of bioinorganic chemistry, enabling direct comparison of metallozeolite active sites to functionally analogous Fe and Cu sites in biology. We identify significant parallels and differences in the strategies used by each to achieve high reactivity, highlighting potentially interesting mechanisms to tune the performance of synthetic catalysts
Fed-batch production of green coconut hydrolysates for high-gravity second-generation bioethanol fermentation with cellulosic yeast 01-11-2017
The residual biomass obtained from the production of Cocos nucifera L. (coconut) is a potential source of feedstock for bioethanol production. Even though coconut hydrolysates for ethanol production have previously been obtained, high-solid loads to obtain high sugar and ethanol levels remain a challenge. We investigated the use of a fed-batch regime in the production of sugar-rich hydrolysates from the green coconut fruit and its mesocarp. Fermentation of the hydrolysates obtained from green coconut or its mesocarp, containing 8.4 and 9.7% (w/v) sugar, resulted in 3.8 and 4.3% (v/v) ethanol, respectively. However, green coconut hydrolysate showed a prolonged fermentation lag phase. The inhibitor profile suggested that fatty acids and acetic acid were the main fermentation inhibitors. Therefore, a fed-batch regime with mild alkaline pretreatment followed by saccharification, is presented as a strategy for fermentation of such challenging biomass hydrolysates, even though further improvement of yeast inhibitor tolerance is also needed.
Low‐Temperature Reductive Aminolysis of Carbohydrates to Diamines and Aminoalcohols by Heterogeneous Catalysis 13-10-2017
Short amines, such as ethanolamines and ethylenediamines, are important compounds in today's bulk and fine chemicals industry. Unfortunately, current industrial manufacture of these chemicals relies on fossil resources and requires rigorous safety measures when handling explosive or toxic intermediates. Inspired by the elegant working mechanism of aldolase enzymes, a novel heterogeneously catalyzed process—reductive aminolysis—was developed for the efficient production of short amines from carbohydrates at low temperature. High-value bio-based amines containing a bio-derived C2 carbon backbone were synthesized in one step with yields up to 87 C%, in the absence of a solvent and at a temperature below 405 K. A wide variety of available primary and secondary alkyl- and alkanolamines can be reacted with the carbohydrate to form the corresponding C2-diamine. The presented reductive aminolysis is therefore a promising strategy for sustainable synthesis of short, acyclic, bio-based amines.
Bio-based amines through sustainable heterogeneous catalysis 18-09-2017
The production of amines from biomass is a growing field of interest. Particularly the amination of bio-based alcohols receives a lot of attention. In this review, we discuss recent progress in the development of efficient heterogeneous catalysts. The substrate scope for the production of bio-based amines is not limited to (hemi)cellulosic alcohols. Other platform chemicals that originate from different biomass fractions, such as lignin, oils, chitin and protein, are also suitable feedstock for the production of amines. This comprehensive review first provides an overview of the available bio-based feedstock candidates. The following section is devoted to the sustainable reaction routes that are available to carry out the desired amination reactions. Next, state-of-the-art technologies are summarized for each substrate class, focussing on heterogeneous catalysis. Special attention is dedicated to the sustainability of the discussed reaction routes. Finally, a critical discussion is provided, together with current challenges and future perspectives regarding the industrial production of bio-based amine chemicals.
Heterogeneous catalysis for bio-based polyester monomers from cellulosic biomass: advances, challenges and prospects 07-09-2017
It is a 21st century challenge to develop a more sustainable chemical industry where fossil-based resources are, where possible, preferentially replaced by renewable alternatives. Bio-based polymers, in particular those derived from cellulose or other carbohydrates, are often considered benign alternatives for petrochemical plastics. The majority of bioplastic precursors are currently derived from fermentation or biotechnology. Chemocatalytic routes to both similar and new polymer building blocks are emerging in an effort to mitigate challenges related to carbohydrate fermentation, such as waste generation and costly product purification. This review critically surveys recent developments in applying heterogeneous catalysis for the production of bio-based polyester monomers from cellulose or cellulose-derived carbohydrates. Highlighted target molecules include various α-hydroxy acids or esters (e.g. lactic and glycolic acid, lactide and methyl vinyl glycolate), furandicarboxylic acid, ethylene glycol and isosorbide. The production of lactic acid from glycerol will exceptionally be included as well, as an oversupply of glycerol might contribute to non-negligible amounts of lactic acid in the future. Where possible, remaining challenges and future prospects are highlighted.
Identification of α-Fe in High-Silica Zeolites on the Basis of ab Initio Electronic Structure Calculations 24-08-2017
α-Fe is the precursor of the reactive FeIV═O core responsible for methane oxidation in Fe-containing zeolites. To get more insight into the nature and stability of α-Fe in different zeolites, the binding of Fe(II) at six-membered-ring cation exchange sites (6MR) in ZSM-5, zeolite beta, and ferrierite was investigated using DFT and multireference ab initio methods (CASSCF/CASPT2). CASPT2 ligand field (LF) excitation energies of all sites were compared with the experimental DR-UV–vis spectra reported by Snyder et al. From this comparison it is concluded that the 16000 cm–1 band of α-Fe, observed in all three zeolites, can uniquely be assigned to a high-spin square-planar (SP) Fe(II) located at a 6MR with an Al–Si–Si–Al sequence, where the Al atoms are positioned opposite in the ring and as close to each other as possible. The stability of such conformations is also confirmed by the binding energies obtained from DFT. The bands at 10000 cm–1 in the experimental spectra, assigned to spectator Fe(II), are attributed to six-coordinated trigonal-prismatic Fe(II) species, as calculated for the γ-site in ZSM-5. The entatic effect of the zeolite lattice on the stability of the SP sites was investigated by making use of the unconstrained Fe(II) model complex FeL2 (with L = [Al(OH)4]−). The SP conformer is approximately 2 kcal/mol more stable than the tetrahedral form, indicating that the SP coordination environment of α-Fe is not imposed by the zeolite lattice but rather electronically preferred by Fe(II) in the environment of four O ligands. A significant contribution to the stability of the SP conformer is provided by mixing of the doubly occupied 3dz2 orbital with the higher lying 4s
Lewis acid catalysis on single site Sn centers incorporated into silica hosts 15-07-2017
Tetrahedral Sn built into microporous silica frameworks such as zeolites and structured mesoporous silica can be used as heterogeneous Lewis acid catalysts. These materials have recently attracted much attention, as they show remarkable activity and selectivity in a wide range of reactions. A prominent example is the conversion of carbohydrates into platform and commodity chemicals such as lactic acid or alkyl lactates, where the activity and selectivity of Sn-based materials remains unsurpassed compared to Sn-free catalysts. Some of the materials show water-tolerant behavior and can therefore also be used in aqueous systems. In this work, a literature overview regarding synthesis of Sn-containing silica materials is given, as well as a synopsis of the characterization tools which can be used to unravel the structure of the catalytic active site. The application of such Sn-containing materials for diverse catalytic reactions is reviewed, with special emphasis on the effects of the catalyst characteristics on the catalytic activity and stability.
Barriers and Chemistry in a Bottle: Mechanisms in Today’s Oxygen Barriers for Tomorrow’s Materials 28-06-2017
The stability of many organic compounds is challenged by oxidation reactions with molecular oxygen from the air in accordance with thermodynamics. Whereas glass or metal containers may protect such products, these packaging types also offer severe disadvantages over plastics. Large-scale packaging, especially for food and beverage industries, has shifted towards polymeric materials with passive and active oxygen barrier technologies over the last decades. Even though patent literature is flooded with innovative barrier systems, the mechanisms behind them are rarely reported. In a world where packaging requirements regarding recyclability and safety are continuously getting stricter, accompanied by the appearance of emerging applications for plastic oxygen barriers (such as organic semi-conductors), research towards new materials seems inevitable. To this cause, proper in-depth knowledge of the existing solutions is a prerequisite. This review therefore attempts to go deep into the problems at hand and explain the chemistry behind the existing solution strategies and finally discusses perspectives suggesting new applications such as organic light-emitting diodes (OLEDs) and solar cells.
Lignin-first biomass fractionation: the advent of active stabilisation strategies 22-06-2017
During the past decade, a growing scientific community is eagerly seeking for effective lignin valorisation approaches. Thought-out utilisation of the world's most abundant resource of bio-aromatics could substantially augment the profitability of future lignocellulosic biorefineries. From a multitude of complementary valorisation opportunities (e.g., composite materials, dispersants, carbon fibres), harnessing lignin as renewable feedstock for chemicals forms an alluring challenge. However, a root cause that hampers its full exploitation, is an historically grown and deeply ingrained (mis)conception, stating that lignin is merely considered as a subordinate opportunity to derive some extra added-value, without being of primary concern. Unfortunately, this mind-set doesn't reckon with the fact that lignin is prone to irreversible degradation, leading to recalcitrant condensed structures which are difficult to disassemble into a handful of chemicals. In response, new biorefinery schemes are being developed, wherein the valorisation of lignin is regarded as one of the primary targets. At the heart of these alternative biorefineries are fractionation strategies that aim to prevent structural lignin degradation, hereby enabling an efficient and selective lignin-to-aromatic conversion. Of particular interest are fractionation methods that implement active stabilisation mechanisms that prohibit the problem of lignin condensation, without compromising the structural integrity of the carbohydrates. This new and emerging biorefinery paradigm is termed lignin-first, and includes two distinct strategies to actively prevent structural degradation during biomass fractionation, namely (i) tandem depolymerisation–stabilisation of native lignin, and (ii) active preservation of β-O-4 bonds.
Integrating lignin valorization and bio-ethanol production: on the role of Ni-Al 2 O 3 catalyst pellets during lignin-first fractionation 13-06-2017
Reductive catalytic fractionation (RCF) of lignocellulosic biomass is a promising lignin-first biorefinery strategy that yields nearly theoretical amounts of phenolic monomers by performing solvolytic delignification and lignin depolymerization in presence of a reducing catalyst, here Ni-Al2O3. This contribution attempts to elucidate the precise role of the catalyst, with respect to lignin solubilization, depolymerization and stabilization. The presented experiments unambiguously show that the solvent, under the applied conditions (methanol at 523 K), is largely responsible for both the initial release of lignin fragments from the lignocellulose matrix and their further depolymerization to shorter phenolics. The catalyst is merely responsible for the hydrogenation of reactive unsaturated side-chains in the solubilized lignin intermediates, leading to the formation of stable phenolic monomers and short oligomers. This catalytic reduction essentially prevents undesirable repolymerization reactions towards a condensed (high MW) lignin product. Since a solid–solid interaction between catalyst and wood is not required for the stabilization of soluble lignin products, the use of catalyst pellets (confined in a reactor basket) as a means to facilitate catalyst recuperation and clean pulp production, is justified. After optimizing the process with regard to mass transfer limitations, above 90% delignification of birch wood is achieved, producing a lignin oil that contains over 40% phenolic monomers, of which 70% consists of 4-n-propanolguaiacol and -syringol. In addition, multiple catalyst recycling experiments are successfully performed. Catalyst fouling is appointed as a primary cause of deactivation, though catalytic activity can be fully restored by thermal H2-treatment. Simple filtration of the reaction mixture finally affords a catalyst-free and delignified pulp, containing most of the initial cellulose and hemicellulose (93% glucose and 83% xylose retention). This pulp is converted into bio-ethanol, through simultaneous saccharification (accelerase trio enzyme mix) and fermentation (GSE16-T18-HAA1* yeast). A first and unprecedented trial led to a 73% bio-ethanol yield.
Acidic mesostructured silica-carbon nanocomposite catalysts for biofuels and chemicals synthesis from sugars in alcoholic solutions 05-06-2017
Sulfonated mesostructured silica-carbon nanocomposites with varying carbon content, acidic site density and porosity, obtained via the one-pot evaporation induced self-assembly (EISA) synthesis, were used here to convert sugars into useful chemicals and biofuel components in alcoholic solvent. The nanocomposites show a remarkable catalytic performance in ethanol, yielding up to 80%, predominantly ethyl levulinate, 5-ethoxymethylfurfural and 2-(diethoxymethyl)-5-(ethoxymethyl)furan. Fructose is the sugar substrate of choice, but the sulfonated composites are also able to convert di- and polymeric forms of fructose. Due to a lack of a glucose-to-fructose isomerization ability, the composites are unable to form the above products from the glucose resources (glucose and cellulose), ethyl glucoside being the dominant product from these feedstocks. The composite has a peculiar hierarchical pore architecture, which is stable on shelf in ambient for at least six years. While the mesoporosity facilitates entrance and fast transport (even of soluble poly-carbohydrates like inulin and cellulose polymers), the presence of microporosity is beneficial to attain fast sugar catalysis. Since the microporosity is associated with voids in the carbon phase (preferably pyrolyzed at 400 °C), composites with high carbon contents are preferred. Due to the fast transport, reactions with fructose in ethanol run in the chemical regime in the applied thermal conditions. Kinetic inspection of the reaction further clarifies the complex network of consequent and parallel reactions, demonstrating that HMF is the main precursor of humins, while the formation of EL directly from HMF should also be considered. While this observation corroborates the protective role of alcohols like ethanol, this work also concludes based on a series of reactions in different alcohols and water, that the presence of water plays a crucial role in the HMF-to-humins formation. While alcohols are known to stabilize HMF, the unprotected HMF is fairly stable in non-aqueous reaction circumstances like in tert-butanol solvent.
Sustainable bisphenols from renewable softwood lignin feedstock for polycarbonates and cyanate ester resins 27-04-2017
The selective reductive catalytic depolymerisation of softwood lignin (e.g. pine, spruce) yields predominantly 4-n-propylguaiacol (4PG; 15–20 wt% on lignin basis), an interesting platform chemical for bio-based chemistry. This contribution specifically shows promising technical, sustainable and environmental advantages of such a bio-phenol for various polymer applications. The bisphenolic polymer precursor, 5,5′-methylenebis(4-n-propylguaiacol) (m,m′-BGF-4P), was therefore first synthesized by acid-catalysed condensation, and its synthesis and isolation are compared with shorter chain analogs, viz. 4-methyl- and 4-ethylguaiacol. A thorough GC-GPC/SEC analysis of the crude condensation mixture was developed to assess the purity of the isolated dimers. Isolation is done by a single-step crystallization, yielding 57 wt% of m,m′-BGF-4P in >99% purity. This pure m,m′-BGF-4P bisphenol displays a notably reduced potency to activate human estrogen receptor alpha (hERα; EC50 at 10−5 M) in comparison with commercial bisphenols, and is therefore useful for future polymer applications. As a proof of concept, polycarbonates and cyanate ester resins were prepared from m,m′-BGF-4P and compared to other bisphenols. The polycarbonate had Mn = 5182 g mol−1, Tg = 99 °C, Tm = 213 °C, Td,5% = 360 °C, and displayed improved processability in common solvents, as opposed to the methylated and ethylated bisguaiacols. A fully cured resin disk exhibited a Tg = 193 °C, Td,5% = 389 °C and a water uptake of only 1.18% after being immersed in 85 °C water for four days. These results underscore the potential of the intrinsic functionality of lignin-derived building blocks to transcend the scope of renewability.
Zeolites as sustainable catalysts for the selective synthesis of renewable bisphenols from lignin‐derived monomers 24-04-2017
Alternative biobased bisphenols from lignocellulosic biomass are not only favorable to reduce the environmental impact of current petroleum-derived plastics, but they can be simultaneously beneficial for health issues related to bisphenol A (BPA). Additionally, conventional BPA synthesis entails a large excess of unrecoverable homogeneous acid catalyst (e.g., HCl) or unrecyclable thermolabile sulfonated resins. In this report, zeolites are proposed as recoverable and thermally stable solid acids for the Brønsted-acid-catalyzed condensation between 4-methylguaiacol and formaldehyde to selectively produce renewable bisphenols. It is found that the Brønsted-acid-site density plays a pivotal role for catalyst performance. In particular, the cheap and environmentally friendly FAU 40 exhibits outstanding activity (turnover frequency of 496 h−1) and selectivity (>95 %), outperforming even the best benchmark catalyst. Additionally, the zeolite can be easily recycled without activity loss after regeneration by coke burn-off. The catalytic zeolite system also seems very promising for other lignin-derived alkylphenols, alkylguaiacols, and alkylsyringols.
Unconventional Pretreatment of Lignocellulose with Low‐Temperature Plasma 10-01-2017
Lignocellulose represents a potential supply of sustainable feedstock for the production of biofuels and chemicals. There is, however, an important cost and efficiency challenge associated with the conversion of such lignocellulosics. Because its structure is complex and not prone to undergo chemical reactions very easily, chemical and mechanical pretreatments are usually necessary to be able to refine them into the compositional building blocks (carbohydrates and lignin) from which value-added platform molecules, such as glucose, ethylene glycol, 5-hydroxymethylfurfural, and levulinic acid, and biofuels, such as bioderived naphtha, kerosene, and diesel fractions, will be produced. Conventional (wet) methods are usually polluting, aggressive, and highly energy consuming, so any alternative activation procedure of the lignocellulose is highly recommended and anticipated in recent and future biomass research. Lignocellulosic plasma activation has emerged as an interesting (dry) treatment technique. In the long run, in particular, in times of fairly accessible renewable electricity, plasma may be considered as an alternative to conventional pretreatment methods, but current knowledge is too little and examples too few to guarantee that statement. This review therefore highlights recent knowledge, advancements, and shortcomings in the field of plasma treatment of cellulose and lignocellulose with regard to the (structural and chemical) effects and impact on the future of pretreatment methods.
Scalable Synthesis of Acidic Mesostructured Silica–Carbon Nanocomposite Catalysts by Rotary Evaporation 09-01-2017
A practical and scalable synthesis for the mass production of well-ordered mesoporous silica–carbon composites by using a fast rotary-evaporation-induced self-assembly method in the absence of any additional support is presented.
Four Years of ACS Sustainable Chemistry & Engineering: Reflections and New Developments 03-01-2017
ACS Sustainable Chemistry & Engineering (ACS SCE) was launched in January 2013, with a scope that included green chemistry, green engineering, and the grand challenges for sustainability in the chemical enterprise. As Editors, we have been honored by the response of the research community to the journal. Submissions and published content have grown by an order of magnitude since our launch, and as our fourth year came to a close, we were approaching 800 published articles per year. These publications are being widely cited, and the journal’s impact factor, which currently stands at 5.27, is on an upward trajectory. Given the number and high quality of submissions that ACS SCE is attracting, we chose this editorial venue at the start of our fifth year to comment on the scope and future directions of ACS SCE, providing clarifications based on our collective editorial experience.
Identifying Sn Site Heterogeneities Prevalent Among Sn‐Beta Zeolites 09-11-2016
In recent years, various protocols on preparing Lewis acidic Sn-β zeolite hydrothermally and postsynthetically have been reported. However, very little is known about the effects of different synthesis protocols on the Sn(IV) speciation in the final material. Even the effects of individual synthesis parameters within a certain preparation method have not been studied systematically. Here, we demonstrate that hydrothermally synthesized Sn-β zeolites prepared via very similar recipes show significantly different 119Sn-NMR spectra, suggesting different Sn site speciation. Among postsynthetically prepared Sn-β zeolites, less variation in the resulting 119Sn-NMR spectra have been observed, indicating a more reproducible synthesis procedure compared to hydrothermal synthesis in fluoride media. This work highlights the importance of 119Sn-NMR measurements to elucidate the precise local geometry of the Sn heteroatoms in Sn-β, and the need to quantify the number of reactive Sn sites on each sample that participate in a given catalytic reaction, in order to accurately compare materials prepared by different routes.
Synthesis of aluminum-containing hierarchical mesoporous materials with columnar mesopore ordering by evaporation induced self-assembly 01-11-2016
The incorporation of aluminum into the silica columns of hierarchical mesoporous materials (HMMs) was studied. The HMMs were synthesized by a combination of hard and soft templating methods, forming mesoporous SBA-15-type silica columns inside the pores of anodic aluminum oxide membranes via evaporation induced self-assembly (EISA). By adding Al-isopropoxide to the EISA-mixture a full tetrahedral incorporation of Al and thus the creation of acid sites was achieved, which was proved by nuclear magnetic resonance spectroscopy. Electron microscopy showed that the use of Al-isopropoxide as an Al source for the HMMs led to a change in the mesopore ordering of silica material from circular hexagonal (donut-like) to columnar hexagonal and a 37% increase in specific surface (BET surface). These results were confirmed by a combination of nitrogen physisorption and small-angle X-ray scattering experiments and can be attributed to a swelling of the P123 micelles with isopropanol. The columnar mesopore ordering of silica is advantageous towards the pore accessibility and therefore preferential for many possible applications including catalysis and adsorption on the acid tetrahedral Al-sites.
Fast catalytic conversion of recalcitrant cellulose into alkyl levulinates and levulinic acid in the presence of soluble and recoverable sulfonated hyperbranched poly(arylene oxindole)s 27-10-2016
Sulfonated hyperbranched polymers were recently reported to efficiently mimic cellulase activity, producing large quantities of glucose from cellulose. The polymer structure allows tuning of the acid properties in terms of active site confinement and acid strength, while being sufficiently flexible to strongly interact with a solid carbohydrate. Whereas previous research focussed on catalysis in water, herein the sulfonated hyperbranched poly(arylene oxindole)s (SHPAOs) were used in alcoholic media, converting cellulose into alkyl glucosides and alkyl levulinates. Interestingly high reaction rates were noticed in the alcoholic solvent, ethanol being the solvent of choice. Unlike most previous reports, low reaction temperature, high cellulose concentrations and no external pressure were employed. A chlorinated SHPAO, denoted as 5-Cl-SHPAO, due to its high acid strength, exhibits the best catalytic efficiency, yielding 79% ethyl glucoside (EG) in 1 h and 60% ethyl levulinate (EL) in 6 h, the latter value being considerably higher than those of the reference sulfuric acid (29%) and 2-naphthalenesulfonic acid (42.5%) under similar reaction conditions. Worth mentioning is a combined ethyl glucosides and ethyl levulinate (levulinic acid) yield of >90% from microcrystalline cellulose at complete conversion. The cellulose reaction runs in a chemical regime in the temperature range of 150 to 190 °C, 160 °C being the most optimal with regard to the reaction speed and product yields. Time profiles and analysis of the product distributions reveal fast formation of alkylglucosides, while their conversion is the slowest step in the cascade to alkyl levulinate. Besides being very fast, reaction rates in an alcoholic solvent appear less affected by the properties of the cellulose. Therefore, even large particles of highly crystalline cellulose are easily converted to high alkyl levulinate yields. Obtaining a high levulinic acid (LA) yield directly from cellulose appears difficult, also in the presence of a hyperbranched polymer. Therefore, a two-stage catalytic strategy that uses the facile formation of alkyl levulinate from cellulose in alcohol in the presence of catalytic amounts of 5-Cl-SHPAO is proposed. After alcoholic evaporation of the alkyl levulinate product solution, aliquots of water are added to hydrolyse the product into LA. As this reaction in the presence of the remaining soluble catalyst is complete, a 60% LA yield from microcrystalline cellulose is demonstrated. Catalyst recovery is demonstrated through nanofiltration. Due to the soluble character of the hyperbranched catalyst in the alcoholic solvent, it is easily separated from the solid humins, and recovered from the solution over a commercial low molecular weight cut-off filter. The recovered catalyst showed comparable catalytic activity (per catalyst weight) and product selectivity
Enhanced Acidity and Accessibility in Al-MCM-41 through Aluminum Activation 11-10-2016
Incorporating aluminum is the most widely applied and industrially relevant method to functionalize amorphous silica. However, established protocols yield predominately poorly distributed and inaccessible Al species, and as a result only ∼10–15% of the present aluminum gives rise to the acid sites, hampering the overall catalytic potential. Herein, the influence of alkaline activations with aqueous NaOH and NH4OH on the porosity, acidity, and catalytic properties of Al-MCM-41 is studied. By performing room temperature activations in 0.01–0.1 M NaOH or 0.5 M NH4OH, the Ostwald ripening of silica in alkaline media is exploited, which results in high mass retention yields (100–74%) and a controlled transformation of the 3.6 nm mesopores of the parent material to a broad pore range from 3 to ∼12 nm. Electron microscopy indicates the presence of additional interconnected intraparticle porosity, whereas no significant change in the shape and size of the original particles is observed. Elemental analysis reveals that the optimal alkaline activation with 0.05 M NaOH leads to a decrease in the Si/Al ratio at the surface, despite an increase in the bulk Si/Al ratio. 27Al magic angle spinning nuclear magnetic resonance spectroscopy demonstrates a large conversion of octahedral Al into tetrahedral Al, doubling the purely tetrahedral fraction from 30 to 60%. Pyridine-probed Fourier transformed infrared spectroscopy shows a doubling of the Brønsted and Lewis acidity after activation. The compositional and spectroscopic results are ratified by monitoring the relative accessibility of the acid sites, i.e., effective acidity (mol acid sites per mol Al). The alkaline activation enhances the effective acidity by increasing access to the Al sites trapped inside the pore wall and by reincorporation of the octahedral Al as accessible tetrahedral sites. As a result, an unprecedented effective acidity is obtained after the Al incorporation, which is substantiated using a novel accessibility concept. The catalytic potential of the activation protocol is demonstrated by quadrupling the catalytic activity for the acid-catalyzed alkylation of toluene with benzyl alcohol, an over-50% activity gain, a slightly enhanced selectivity, and a strongly reduced coking in the acid-catalyzed coupling of furfural with sylvan
Synergetic effects of alcohol/water mixing on the catalytic reductive fractionation of poplar wood 03-10-2016
One of the foremost challenges in lignocellulose conversion encompasses the integration of effective lignin valorization in current carbohydrate-oriented biorefinery schemes. Catalytic reductive fractionation (CRF) of lignocellulose offers a technology to simultaneously produce lignin-derived platform chemicals and a carbohydrate-enriched pulp via the combined action of lignin solvolysis and metal-catalyzed hydrogenolysis. Herein, the solvent (composition) plays a crucial role. In this contribution, we study the influence of alcohol/water mixtures by processing poplar sawdust in varying MeOH/water and EtOH/water blends. The results show particular effects that strongly depend on the applied water concentration. Low water concentrations enhance the removal of lignin from the biomass, while the majority of the carbohydrates are left untouched (scenario A). Contrarily, high water concentrations favor the solubilization of both hemicellulose and lignin, resulting in a more pure cellulosic residue (scenario B). For both scenarios, an evaluation was made to determine the most optimal solvent composition, based on two earlier introduced empirical efficiency descriptors (denoted LFDE and LFFE). According to these measures, 30 (A) and 70 vol % water (B) showed to be the optimal balance for both MeOH/water and EtOH/water mixtures. This successful implementation of alcohol/water mixtures allows operation under milder processing conditions in comparison to pure alcohol solvents, which is advantageous from an industrial point of view.
Heterogeneous conjugation of vegetable oil with alkaline treated highly dispersed Ru/USY catalysts 25-09-2016
Heterogeneous metal catalysts enable the direct conjugation of linoleic acid tails in vegetable oil to their conjugated linoleic acid (CLA) isomers. CLA-enriched oils are useful as renewable feedstock for the chemical industry and as nutraceutical. Up to now, a solvent-free process for conjugated oils without significant formation of undesired hydrogenation products was not existing. This work shows the design of Ru/USY catalysts able to directly conjugate highly unsaturated vegetable oils such as safflower oil in absence of solvent and hydrogen. Key is fast molecular transport of the bulky reagent and reactive product triglycerides in the zeolite crystal. A two-step zeolite post-synthetic treatment (with NH4OH and acetate salt) was applied to create the necessary mesoporosity. More open zeolite structures allow for a faster conjugation reaction, while securing a fast removal of the reactive conjugated triglycerides, otherwise rapidly deactivating through fouling and pore blockage by polymers. The best Ru/USY catalyst in this contribution is capable of producing exceptionally high yields of conjugated oils, containing up to almost 30 wt% conjugated fatty acid tails in safflower oil, at an initial production rate of 328 gCLA mL−1 h−1 per gram metal catalyst.
Selective Conversion of Lignin-Derivable 4-Alkylguaiacols to 4-Alkylcyclohexanols over Noble and Non-Noble-Metal Catalysts 06-09-2016
Recent lignin-first catalytic lignocellulosic biorefineries produce large quantities of two potential platform chemicals, 4-n-propylguaiacol (PG) and 4-n-propylsyringol. Because conversion into 4-n-propylcyclohexanol (PCol), a precursor for novel polymer building blocks, presents a promising valorization route, reductive demethoxylation of PG was examined here in the liquid-phase over three commercial hydrogenation catalysts, viz. 5 wt % Ru/C, 5 wt % Pd/C and 65 wt % Ni/SiO2–Al2O3, at elevated temperatures ranging from 200 to 300 °C under hydrogen atmosphere. Kinetic profiles suggest two parallel conversion pathways: Pathway I involves PG hydrogenation to 4-n-propyl-2-methoxycyclohexanol (PMCol), followed by its demethoxylation to PCol, whereas Pathway II constitutes PG hydrodemethoxylation to 4-n-propylphenol (PPh), followed by its hydrogenation into PCol. The slowest step in the catalytic formation of PCol is the reductive methoxy removal from PMCol. Moreover, under the applied reaction conditions, PCol may react further into hydrocarbons. The following criteria are therefore essential to reach a high PCol yield: (i) catalytic pathway II is preferred as this route does not involve stable intermediates; (ii) reactivity of PMCol should be higher than that of PCol, and (iii) the overall carbon balance should be high. Both the catalyst type and the reaction conditions have a substantial impact on the PCol yield. Only the commercial Ni catalyst meets the three criteria, provided the reaction is performed at 250 °C in hexadecane. Additional advantages of this solvent choice are a high boiling point (low operational pressure in closed reactor systems), high solubility of PG and derived products, high thermal, reductive stability, and easy derivability from fatty biomass feedstock. This Ni catalyst also showed an excellent stability in recycling runs and is capable of converting highly concentrated (up to 20 wt %) PG in hexadecane. Ru and Pd on carbon showed a low PCol yield, as they are not conform the three criteria. Low hydrogen pressure favors Pathway II, resulting in a very high PCol yield of 85% at 10 bar. Catalytic conversion of guaiacol, 4-methyl- and 4-ethylguaiacol in comparable circumstances showed similarly high yields of the corresponding cyclohexanols.
Opportunities of Immobilized Homogeneous Metathesis Complexes as Prominent Heterogeneous Catalysts 25-08-2016
A plethora of strategies for immobilizing well-defined metathesis catalysts on solid supports have been developed over the past decades. Whereas most reviews on classical heterogeneous metathesis catalyst designs mainly discuss covalent binding approaches, we here extended this topic with non-covalent surface science methods, a research area that gained interest recently. Within this context, the main focus was set on the immobilization of ruthenium alkylidenes or so-called Grubbs complexes, as these catalysts are known for their high activity, stability and commercial viability. Besides giving an overview of different immobilization strategies, the wide diversity of supported Grubbs catalysts also allows for discussing implications of attaching Grubbs catalysts that go beyond the traditional benefits of heterogeneous catalysis, such as easy catalyst separation. More specifically, this Review will summarize several unique opportunities of immobilized homogeneous catalysts in liquid-phase metathesis transformations in terms of impact on activity, stability, and product selectivity, as to highlight the economic potential, but without ignoring possible shortcomings and limitations.
Internal architecture of coffin‐shaped ZSM‐5 zeolite crystals with hourglass contrast unravelled by focused ion beam‐assisted transmission electron microscopy 19-08-2016
Optical microscopy, focused ion beam and transmission electron microscopy are combined to study the internal architecture in a coffin-shaped ZSM-5 crystal showing an hourglass contrast in optical microscopy. Based on parallel lamellas from different positions in the crystal, the orientation relationships between the intergrowth components of the crystal are studied and the internal architecture and growth mechanism are illustrated. The crystal is found to contain two pyramid-like components aside from a central component. Both pyramid-like components are rotated by 90° along the common c-axis and with respect to the central component while the interfaces between the components show local zig-zag feature, the latter indicating variations in relative growth velocity of the two components. The pyramid-like intergrowth components are larger and come closer to one another in the middle of the crystal than at the edges, but they do not connect. A model of multisite nucleation and growth of 90° intergrowth components is proposed.
The active site of low-temperature methane hydroxylation in iron-containing zeolites 18-08-2016
An efficient catalytic process for converting methane into methanol could have far-reaching economic implications. Iron-containing zeolites (microporous aluminosilicate minerals) are noteworthy in this regard, having an outstanding ability to hydroxylate methane rapidly at room temperature to form methanol1,2,3. Reactivity occurs at an extra-lattice active site called α-Fe(II), which is activated by nitrous oxide to form the reactive intermediate α-O4,5; however, despite nearly three decades of research5, the nature of the active site and the factors determining its exceptional reactivity are unclear. The main difficulty is that the reactive species—α-Fe(II) and α-O—are challenging to probe spectroscopically: data from bulk techniques such as X-ray absorption spectroscopy and magnetic susceptibility are complicated by contributions from inactive ‘spectator’ iron. Here we show that a site-selective spectroscopic method regularly used in bioinorganic chemistry can overcome this problem. Magnetic circular dichroism reveals α-Fe(II) to be a mononuclear, high-spin, square planar Fe(II) site, while the reactive intermediate, α-O, is a mononuclear, high-spin Fe(IV)=O species, whose exceptional reactivity derives from a constrained coordination geometry enforced by the zeolite lattice. These findings illustrate the value of our approach to exploring active sites in heterogeneous systems. The results also suggest that using matrix constraints to activate metal sites for function—producing what is known in the context of metalloenzymes as an ‘entatic’ state6—might be a useful way to tune the activity of heterogeneous catalysts.
Compositional and structural feedstock requirements of a liquid phase cellulose-to-naphtha process in a carbon- and hydrogen-neutral biorefinery context 27-07-2016
Processing raw (ligno)cellulosic feedstock into renewable light naphtha alkanes could lead to a gradual replacement of fossil feedstock for the production of chemicals, materials and fuels. The production of drop-in alkanes is a preferable short term strategy because of its practical implementation and integration in existing infrastructure and processes. A handful of promising cellulose-to-alkane biorefinery initiatives were recently reported, both processing in gas and liquid phase. This contribution presents a detailed study of the two-liquid phase hydrodeoxygenation of cellulose to n-hexane under relatively mild circumstances, proceeding through the recently communicated HMF route, in presence of a soluble acid and Ru/C metal catalyst. Two main points were addressed here: (i) the importance (or not) of the lignocellulose pretreatment and purification to the alkane yield, and (ii) the renewability of the consumed hydrogen in the process. A systematic study of the effect of cellulose purity, crystallinity, degree of polymerization and particle size (surface area) on the light naphtha yield was performed to tackle the first part. As fibrous cellulose with large particles was the most favourable feedstock with regard to alkane yield and as the presence of hemicellulose and lignin impurities had no effect on the cellulose-to-naphtha conversion, costly mechanical and purification steps are redundant to the process, in contrast to their notable importance in other cellulose valorisation processes (e.g. to glucose, sorbitol, isosorbide and acids). The second point regarding sustainable hydrogen supply is discussed in detail by calculating hydrogen and carbon mass and energy balances of the chemical conversions, assuming selected scenarios among others to recuperate the hydrogen by steam-reforming of waste streams (like gaseous C<6 hydrocarbons and aqueous polyol fractions) and (partial) aromatization of the C6 fraction into benzene. The study shows potential to integrate the liquid phase cellulose-to-naptha (LPCtoN) technology into a self-sufficient biorefinery, in which the chemical processes may run without consumption of external (non-renewable) hydrogen, carbon and energy, except for solar light.
ACS Sustainable Chemistry & Engineering's Impact Factor Rises 05-07-2016
Depolymerization of 1,4-polybutadiene by metathesis: high yield of large macrocyclic oligo(butadiene)s by ligand selectivity control 20-06-2016
Herein, we demonstrate a practical high yield preparation of large macrocyclic oligo(butadiene)s, preferably the C16 to C44 fraction, from commercial 1,4-polybutadiene by exploring intramolecular backbiting using a series of commercially available Ru catalysts. Product contamination with linear fragments is restricted by using high molecular weight 1,4-polybutadiene with a low content of 1,2-constructs (vinyl groups). The distribution of the cyclic compounds is largely dependent on the nature of the ligand structure of the Ru catalyst. Kinetic inspection of the reaction reveals a two-step mechanism involving (i) backbiting of the linear polymer with initial formation of large macrocycles followed by (ii) tandem ring-opening ring-closing metathesis predominantly leading to thermodynamically favorable t,t,t-cyclododecatriene (CDT). In particular, second-generation Ru catalysts with N-heterocyclic carbene (NHC) ligands favor undesired CDT formation. First-generation catalysts, presumably due to their high barriers for formation of the intermediate metallacyclobutane, selectively form the C16 to C44 macrocyclic oligo(butadiene) fraction. For example, reaction of (HMW, 98% cis)-polybutadiene with a first-generation Ru catalyst almost yields 90% C16–C44 cyclic oligo(butadiene)s.
Reductive splitting of hemicellulose with stable ruthenium-loaded USY zeolites 06-06-2016
Reductive catalytic splitting to sugar alcohols is a promising technology to valorize (hemi)cellulosic feedstock. This contribution focuses on the conversion of arabinoxylan (AX), a common hemicellulose polymer, to pentitols like xylitol and arabitol in the presence of ruthenium-loaded H-USY zeolites. Both acid and metal sites on the catalyst play a crucial role in the bifunctional catalytic mechanism. Overall, the reaction mechanism involves hydrolysis of AX into shorter (less reactive) xylan oligomer intermediates (XOs), which are in turn hydrolysed into sugar monomers. The first step occurs fast in hot liquid water, but the second step which is rate limiting, requires acid catalysis. Literature has reported successful XO hydrolysis with soluble acids. However, USY zeolites, being non-corrosive instead of the former, are able to hydrolyse XOs more efficiently, likely due to their strong mesopore adsorption capacity. Once formed, the monomeric sugars should be hydrogenated on the metal sites as fast as possible, as otherwise undesired competitive acid catalysed side-reactions will occur. While another catalyst like Ru on carbon can also be used in the one-pot approach close proximity of the two sites, e.g. in the pores of the USY zeolite, is beneficial for the pentitol selectivity, as long as they are well harmonised. After searching for the ideal dual site balance, exceptionally high pentitol yields up to 90 mol% were achieved after only 5 h of reaction. Comparison with earlier reported cellulose reactions shows a narrowing of the ideal acid-to-metal range, besides a shift to lower ratios. Initial regeneration studies show a stable Ru/USY catalytic system able to perform multiple reaction runs with retention of activity and selectivity.
Introducing the Inaugural ACS Sustainable Chemistry & Engineering Lectureship Awards 06-06-2016
Synthesis of Novel Renewable Polyesters and Polyamides with Olefin Metathesis 31-05-2016
Unsaturated and hydroxyl-functionalized C6-dicarboxylic acids were successfully synthesized via olefin metathesis from methyl vinyl glycolate (MVG), a renewable α-hydroxy C4-ester product from Lewis-acid carbohydrate conversion. Addition of a second-generation Hoveyda–Grubbs catalyst to neat MVG leads to a near quantitative yield of dimethyl-2,5-dihydroxy-3-hexenedioate (DMDHHD). Additional hydrolysis and hydrogenation steps form interesting polymer building blocks like 2,5-dihydroxy-3-hexenedioic acid (DHHDA) and 2,5-dihydroxyadipic acid (DHAA). Their use in polyester and polyamide synthesis is demonstrated after determination of their physical and spectroscopic characteristics. Copolymerization of DHHDA with l-lactic acid for instance produces a cross-linked poly(l-lactic acid-co-DHHDA) polyester. Proof of cross-links is ascertained by NMR and FTIR. Substantial impact on the melting, thermal, and polar properties of PLA are observed already at low amounts of DHHDA (0.1 mol %) in accord with the presence of cross-links in the polymer. Biobased polyamides were also synthesized by equimolar reaction of DHHDA with hexamethylenediamine, producing a renewable polyamide analogue of the petroleum-based nylon-6,6. Interestingly, the as-synthesized polyamide (α-bishydroxylated unsaturated polyamide, HUPA) possesses similar thermal stability as nylon-6,6 but shows different chemical properties as a result of the double bond and α-hydroxy functionality.
Towards biolubricant compatible vegetable oils by pore mouth hydrogenation with shape-selective Pt/ZSM-5 catalysts 31-05-2016
Pt/ZSM-5 catalysts with various crystal sizes were prepared via competitive ion-exchange, followed by a slow activation procedure. Even when using very large ZSM-5 crystals, highly dispersed Pt nano-clusters were contained within the zeolite crystal's voids, as ascertained by 2D pressure-jump IR spectroscopy of adsorbed CO and focussed ion-beam transmission electron microscopy. The shape-selective properties of the Pt/ZSM-5 catalysts were evaluated in the partial hydrogenation of soybean oil. Unique hydrogenation selectivities were observed, as the fatty acids located at the central position of the triacylglycerol (TAG) molecules were preferentially hydrogenated. The resulting oil has therefore high levels of intermediately melting TAGs, which are compatible with biolubricants due to their improved oxidative stability and still appropriate low-temperature fluidity. The TAG distribution in the partially hydrogenated soybean oil samples was independent from the zeolite crystal size, while the hydrogenation activity linearly increases with the crystal's external surface area. This trend was confirmed with a Pt loaded mesoporous ZSM-5 zeolite, obtained via a mild alkaline treatment. These observations imply and confirm a genuine pore mouth catalysis mechanism, in which only one fatty acid chain of the TAG is able to enter the micropores of ZSM-5, where the double bonds are hydrogenated by the crystal encapsulated Pt-clusters.
Lactide Synthesis and Chirality Control for Polylactic acid Production 10-05-2016
Polylactic acid (PLA) is a very promising biodegradable, renewable, and biocompatible polymer. Aside from its production, its application field is also increasing, with use not only in commodity applications but also as durables and in biomedicine. In the current PLA production scheme, the most expensive part is not the polymerization itself but obtaining the building blocks lactic acid (LA) and lactide, the actual cyclic monomer for polymerization. Although the synthesis of LA and the polymerization have been studied systematically, reports of lactide synthesis are scarce. Most lactide synthesis methods are described in patent literature, and current energy-intensive, aselective industrial processes are based on archaic scientific literature. This Review, therefore, highlights new methods with a technical comparison and description of the different approaches. Water-removal methodologies are compared, as this is a crucial factor in PLA production. Apart from the synthesis of lactide, this Review also emphasizes the use of chemically produced racemic lactic acid (esters) as a starting point in the PLA production scheme. Stereochemically tailored PLA can be produced according to such a strategy, giving access to various polymer properties.
Chapter 9 – Conversion of Biomass to Chemicals: The Catalytic Role of Zeolites 06-05-2016
Since the mid-20th century zeolites have been successfully applied in oil refining and petrochemistry, owing to the strong Brønsted acidity of their protonated form in a porous crystalline matrix. Yet, concerns about the excessive use of fossil fuels force researchers to develop processes for the production of fuels and chemicals from CO2-neutral feedstocks such as biomass, considered as the alternative and sustainable source of carbon for the production of future bio-derived chemicals. With their success in refinery and petrochemistry, there is increasing interest in the use of zeolites in biomass processing, and this has already resulted in the gradual entrance of zeolites in the conversion of biomass feedstocks. Many interesting biomass conversions have been demonstrated today using the unique acid and redox chemistry of zeolites. However, there are disadvantages inherent to the biomass conversion that need to be overcome before zeolite chemistry can play as important a role in the conversion of biomass as in the conversion of fossil feedstocks. These disadvantages include unstable products and complex conversion network schemes, the stability of zeolites in often polar (condensed) media and active site accessibility of large biomolecules. This chapter presents the major organic compounds in biomass feedstock and provides an overview of the numerous chemical reactions with these chemicals using zeolites in the bulk and fine chemistry. Developments and future challenges in the area are summarized.
Snβ-zeolite catalyzed oxido-reduction cascade chemistry with biomass-derived molecules 18-04-2016
High activity of post-synthetically synthesized Sn-beta, producing novel caprolactone polymer building blocks, is demonstrated in Meerwein–Ponndorf–Verley (MPV), Oppenauer (OPO), Baeyer–Villiger (BV) and cascade reactions thereof with biomass-derived molecules.
Tin triflate-catalyzed conversion of cellulose to valuable (α-hydroxy-) esters 15-02-2016
The direct conversion of cellulose with metal-triflate catalysts in methanol is investigated. SnII-triflate
remarkably catalyzes the formation of a mixture of useful -hydroxy esters such as methyl lactate, methyl
vinyl glycolate and methyl-4-methoxy-2-hydroxybutanoate, on top of methyl levulinate. Compared to
other metaltriflates or Sn salts,the catalytic features of SnII-triflate are distinct and linked to the interplay
between its Brønsted and Lewis acidic component. A total ester yield (carbon-based, mol%) in the 60%-
range could be obtained from cellulose after 2 h at 200 ◦C for cellulose loadings up to 20 g L−1 with 4.8 mM
of catalyst. The cascade reaction network, confirmed by feeding intermediates, highlights the importance
of a fast retro-aldol of the hexose intermediates – opposed to their dehydration – when -hydroxy esters
are targeted. By manipulating the triflate-to-Sn ratio, nearly 40% of -hydroxy esters can be produced in
a one-pot approach. Such mixtures could help fuel the demand for functional biodegradable polyesters.
Influence of Acidic (H3PO4) and Alkaline (NaOH) Additives on the Catalytic Reductive Fractionation of Lignocellulose 10-02-2016
Reductive catalytic fractionation of lignocellulose is a promising “lignin-first” biorefinery strategy wherein lignin is solvolytically extracted from the cell wall matrix and simultaneously disassembled, resulting in a stable lignin oil and a solid carbohydrate-rich residue. Herein, we report on the different influence of acidic (H3PO4) and alkaline (NaOH) additives on the Pd/C-catalyzed reductive processing of poplar wood in methanol (MeOH). It was found that the addition of small quantities of H3PO4 results in three rather than two product streams, since under acidic conditions both delignification and alcoholysis of hemicellulose are promoted, leaving behind a cellulose-rich pulp. The simultaneous acid-catalyzed fractionation of the carbohydrates into separate cellulose and hemicellulose streams provides opportunities for more efficient downstream conversion, as processing parameters can be tailored to the needs of both streams. Alkaline conditions, on the other hand, also enhance delignification, but additionally cause (i) the formation of lignin products other than those obtained under neutral and acidic conditions, (ii) a hampered degree of lignin depolymerization, and (iii) substantial loss of cellulose from the pulp. Further on, a modified process descriptor (LFFE: lignin first fractionation efficiency) was applied to evaluate the fractionation efficiency of lignocellulose in its three major constituents. According to this new efficiency measure, mildly acidic conditions performed best.
Advances in the Conversion of Short-Chain Carbohydrates: A Mechanistic Insight 31-01-2016
This chapter discusses recent insights in the conversion of short carbohydrates, viz., sugars containing four or less carbon atoms. Rather than summarizing product yields from such sugars and reported catalysts for the conversions, the focus lies on understanding the underlying mechanisms. These short carbohydrates can lead to a broad spectrum of products, ranging from platform chemicals such as lactic acid and ethylene glycol to high-value chemicals such as α-hydroxy-γ-butyrolactone and even fuels. Different synthesis strategies of these short carbohydrates include (1) a top-down approach from mono- or polysaccharides and (2) a selective bottom-up synthesis route from formaldehyde. Lewis acids play a major role in carbohydrate chemistry, and among these, Sn-based catalysts often show the highest activity. Whether dioses, trioses, or tetroses are used as substrate, Sn is able to convert them efficiently into α-hydroxy acids or esters, which are useful building blocks for renewable polyesters. Other reaction types such as isomerization, hydrogenation, and cross couplings are discussed briefly as well. Glycerol and glyoxal are no sugars, but their chemistry shows great resemblance to that of carbohydrates. Therefore, these compounds are also briefly accounted for in this chapter.
Water-soluble sulfonated hyperbranched poly(arylene oxindole) catalysts as functional biomimics of cellulases 06-01-2016
A new polymer acid catalyst, sulfonated hyperbranched poly(arylene oxindole), 5-OH–SHPAO, was prepared for selective cellulose hydrolysis. Its superior catalysis, showing high glucose selectivity at almost full cellulose conversion, is attributed to the presence of an hydroxyl group next to the sulfonic acid, therefore mimicking the separate acid–base pair in the cellulase active site.
Potential and challenges of zeolite chemistry in the catalytic conversion of biomass 21-12-2015
Increasing demand for sustainable chemicals and fuels has pushed academia and industry to search for alternative feedstocks replacing crude oil in traditional refineries. As a result, an immense academic attention has focused on the valorisation of biomass (components) and derived intermediates to generate valuable platform chemicals and fuels. Zeolite catalysis plays a distinct role in many of these biomass conversion routes. This contribution emphasizes the progress and potential in zeolite catalysed biomass conversions and relates these to concepts established in existing petrochemical processes. The application of zeolites, equipped with a variety of active sites, in Brønsted acid, Lewis acid, or multifunctional catalysed reactions is discussed and generalised to provide a comprehensive overview. In addition, the feedstock shift from crude oil to biomass involves new challenges in developing fields, like mesoporosity and pore interconnectivity of zeolites and stability of zeolites in liquid phase. Finally, the future challenges and perspectives of zeolites in the processing of biomass conversion are discussed.
Immobilized Grubbs catalysts on mesoporous silica materials: insight into support characteristics and their impact on catalytic activity and product selectivity 07-12-2015
Silica materials show a high ability to physisorb the 2nd generation Hoveyda–Grubbs catalyst (HG2) in organic solvents. The interaction with the complex, likely proceeding through hydrogen bonding, is particularly strong with surfaces rich in silanols, wherein geminal silanols show the highest affinity, and therefore mesoporous silicas are the supports of choice. As long as the silica material is sufficiently pure and free of cages, in which high HG2 concentrations can accumulate, the immobilization of HG2 occurs in a very stable manner. Despite the complex stability, exploration of HG2-loaded mesoporous silica supports in metathesis of cis-cyclooctene indicated significant diffusional and confinement effects, and therefore control of pore size, pore architecture and morphology in balance with the intrinsic catalytic activity is essential for catalyst design. As metathesis of cis-cyclooctene apparently proceeds through the initial formation of linear polymers, followed by backbiting forming cyclic oligomers, potential interference of mass transport and space restriction issues is not surprising. This study shows that the catalyst requirements are best met with the TUD-1 silica support (1.24 wt% HG2). Under such conditions, the heterogeneous catalyst performs as good as the homogeneous one, presenting a thermodynamic distribution of cyclic oligomers. The latter catalyst also showed high catalyst stability in a continuous fixed bed reactor, corresponding to a catalytic turnover number of 18000. The catalytic rates and catalyst stability are lower when operating in a diffusional regime, therefore long reaction times are required to reach the thermodynamic product distribution. Water removal from the catalyst is also important, not because of HG2 stability reasons, but of lower reaction rates which were measured for hydrated samples, likely due to inhibition of cis-cyclooctene uptake in the pores. Mild removal of physisorbed water before immobilization is therefore advised, for instance by thermal treatments, but care has to be taken to keep the silanol density high for firm HG2 immobilization and also to avoid formation of reactive siloxanes, which chemically react with and destroy HG2. Surprisingly, reactive siloxane formation conditions strongly depend on the silica type, with TUD-1 being fairly sensitive to their formation. Finally, the best HG2-loaded TUD-1 catalyst is used successfully in a broad set of other metathesis reactions.
The importance of pretreatment and feedstock purity in the reductive splitting of (ligno)cellulose by metal supported USY zeolite 23-11-2015
Reductive hydrolysis of cellulose to hexitols is a promising technology to valorize cellulose streams. Several catalytic systems have been reported to successfully process commercially available purified cellulose powders according to this technology. Ruthenium-loaded USY zeolites in the presence of minute amounts of HCl previously showed very high hexitol yields. This contribution first investigates into more detail the impact of several cellulose accessibility-related properties like cellulose crystallinity, particle size and degree of polymerization on the conversion rate and hexitol selectivity. Therefore, a series of commercial cellulose samples and several mechano- and chemotreated ones were processed with the Ru/H-USY–HCl catalytic system under standard hot liquid water conditions. The results reveal that the polymerization degree has a large impact on both the conversion rate and selectivity, but its impact fades for DPs lower than 200. From then on, the dominant parameters are the particle size and crystallinity. A second part addresses the influence of cellulose purity. Therefore, organosolv pulps of three lignocellulosic substrates (wheat straw, spruce and birch wood), optionally followed by a bleaching procedure, were processed under the same catalytic circumstances. Here factors like residual lignin content and acid buffer capacity appeared crucial, pointing to the necessity of a dedicated delignification and purification procedure step in order to form the most reactive cellulose feedstock for hexitol production. Complete removal of non-glucosic components is not required since processing of ethanol organosolv birch cellulose and bleached ethanol organosolv wheat straw cellulose, both containing about 6 wt% of lignin and minor contents of ashes and proteins, showed a similar hexitol yield, viz. 34–39%, to that derived from pure microcrystalline cellulose.
An Inner-/Outer-Sphere Stabilized Sn Active Site in β-Zeolite: Spectroscopic Evidence and Kinetic Consequences 10-11-2015
A highly active Sn site with Lewis acid properties is identified in post-synthetically synthesized Sn/DeAlβ catalyst, prepared by liquid-phase Sn grafting of a dealuminated β-zeolite. Though apparently similar Sn active-site structures have been reported for the post-synthetic and the conventional hydrothermal Snβ, detailed study of the electronic structure and redox behavior of Sn with EXAFS, XANES, DR UV–vis, and TPR clearly reveals dissimilarities in geometry and electronic properties. A model of the active Sn site is proposed using a contemporary interpretation of inner-/outer-sphere coordination, assuming inner-sphere coordination of SnIV with three framework SiO– and one outer-sphere coordination by a distant charge-balancing SiO–, resulting in a separated Lewis acid–base pair. Stabilization of this geometry by a nearby water molecule is proposed. In comparison with active Sn sites in a hydrothermally synthesized Snβ, those in the grafted dealuminated material are sterically less demanding for substrate approach, while the low inner-sphere coordination of Sn leads to a stronger Lewis acidity. Proximate silanols in the active-site pocket, identified by FTIR, 29Si MAS NMR, 1H–29Si CP MAS NMR, DR NIR, and TGA, may impact local reagent concentration and transition states stabilization by hydrogen bonding. The structural dissimilarity of the active Sn site leads to a different kinetic behavior. Kinetic experiments using two Lewis-acid-catalyzed reactions, Baeyer–Villiger and Meerwein–Ponndorf–Verley, show differences that are reaction-type dependent and have different entropic (like sterical demand and hydrogen bonding) and enthalpic contributions (Lewis acid strength). The active-site model, containing both inner- and outer-sphere ligands with the zeolite framework, may be considered as a general model for other grafted Lewis acid single sites.
Catalyst Design by NH4OH Treatment of USY Zeolite 30-10-2015
Hierarchical zeolites are a class of superior catalysts which couples the intrinsic zeolitic properties to enhanced accessibility and intracrystalline mass transport to and from the active sites. The design of hierarchical USY (Ultra-Stable Y) catalysts is achieved using a sustainable postsynthetic room temperature treatment with mildly alkaline NH4OH (0.02 m) solutions. Starting from a commercial dealuminated USY zeolite (Si/Al = 47), a hierarchical material is obtained by selective and tuneable creation of interconnected and accessible small mesopores (2–6 nm). In addition, the treatment immediately yields the NH4+ form without the need for additional ion exchange. After NH4OH modification, the crystal morphology is retained, whereas the microporosity and relative crystallinity are decreased. The gradual formation of dense amorphous phases throughout the crystal without significant framework atom leaching rationalizes the very high material yields (>90%). The superior catalytic performance of the developed hierarchical zeolites is demonstrated in the acid-catalyzed isomerization of α-pinene and the metal-catalyzed conjugation of safflower oil. Significant improvements in activity and selectivity are attained, as well as a lowered susceptibility to deactivation. The catalytic performance is intimately related to the introduced mesopores, hence enhanced mass transport capacity, and the retained intrinsic zeolitic properties.
Molecular design of sulfonated hyperbranched poly(arylene oxindole)s for efficient cellulose conversion to levulinic acid 29-10-2015
This contribution is about the design and synthesis of various sulfonated hyperbranched poly(arylene oxindole)s (SHPAOs) with different substituents via a convenient A2 + B3polycondensation and subsequent sulfonation as water-soluble and recyclable acid catalysts for the conversion of cellulose to levulinic acid (LA). Whereas their molecular weight (from 2.7 × 103 to 20.2 × 103), acid density (from 3.4 to 4.8 mmol H+ per g) as well as the polymer structure, viz. hyperbranched or linear analogues, only slightly affect the catalytic performance, the presence of electron-withdrawing substituents on the isatin polymer building block is key to their catalytic efficiency. Among all polymer catalyst designs studied, the use of 5-Cl-SHPAO provided the highest LA yield of almost 50%, directly obtained from ball-milled cellulose in aqueous medium at 165 °C, being twice the LA yield of that of unsubstituted SHPAOs. The presence of the 5-chloro-substituent substantially facilitates the hydrolysis of the glycoside bonds. The close vicinity of the oxindole functionality to the sulfonic acid group seems essential to realize such high hydrolysis rates, as chemical protection of the NH group or sulfonation at other positions lead to substantially lower LA yields. The presence of the 5-chlorine substituent also retards the glucose isomerisation rate, while slightly increasing the HMF conversion rate to LA. As a result, the catalytic reaction progresses in conditions of low concentrations of the most reactive intermediates, fructose and HMF, that otherwise could lead to considerable humin formation. Though hydrophobic interactions are usually invoked to explain such catalytic effects, this contribution suggests also a significant role of the steric proximity of the sulfonic acid group to the oxindole NH group, enabling a kinetic optimization of the reaction cascade through molecular design of the catalytically active acid site.
Thermally activated LTA(Li)–Ag zeolites with water-responsive photoluminescence properties 21-10-2015
Silver–zeolite composites are interesting materials with unique optical properties such as high external quantum efficiencies and large Stokes shifts. The selective formation of luminescent silver clusters within zeolite scaffolds can be achieved by varying silver guest and zeolite host conditions. Nevertheless, at present, the controlled synthesis of Ag–zeolite composites with responsive optical properties remains a challenge. In this report, silver–zeolite composites displaying a dynamical emission color change with respect to their water content were synthesized using LTA zeolites containing lithium cations as counter-balancing agents. An intense blue emission was encountered in partially hydrated LTA(Li)–Ag composites, at low silver loadings, whereas a green/yellow emission was observed in their fully hydrated state. The materials synthesized in this report possess high external quantum efficiencies, up to 62%, compared to their close analogues having Na, K, and Ca as counter-balancing ions. Due to the remarkable dynamical change in emission color depending on the hydration level of LTA(Li)–Ag composites, the use of these materials as luminescence-based humidity sensors is suggested.
Alkane production from biomass: chemo-, bio- and integrated catalytic approaches 08-09-2015
Linear, branched and cyclic alkanes are important intermediates and end products of the chemical industry and are nowadays mainly obtained from fossil resources. In search for alternatives, biomass feedstocks are often presented as a renewable carbon source for the production of fuels, chemicals and materials. However, providing a complete market for all these applications seems unrealistic due to both financial and logistic issues. Despite the very large scale of current alkane-based fuel applications, biomass definitely has the potential to offer a partial solution to the fuel business. For the smaller market of chemicals and materials, a transition to biomass as main carbon source is more realistic and even probably unavoidable in the long term. The appropriate use and further development of integrated chemo- and biotechnological (catalytic) process strategies will be crucial to successfully accomplish this petro-to-bio feedstock transition. Furthermore, a selection of the most promising technologies from the available chemo- and biocatalytic tool box is presented. New opportunities will certainly arise when multidisciplinary approaches are further explored in the future. In an attempt to select the most appropriate biomass sources for each specific alkane-based application, a diagram inspired by van Krevelen is applied, taking into account both the C-number and the relative functionality of the product molecules.
Influence of bio-based solvents on the catalytic reductive fractionation of birch wood 03-09-2015
Reductive catalytic fractionation constitutes a promising approach to separate lignocellulose into a solid carbohydrate pulp and a stable liquid lignin oil. The process is able to extract and convert most of the lignin into soluble mono-, di- and oligomers, while retaining most of the carbohydrates in the pulp. This contribution studies the impact of the solvent choice on both pulp retention and delignification efficiency. Several bio-derivable solvents with varying properties were therefore tested in the Pd/C-catalyzed reductive liquid processing of birch wood. Though a high solvent polarity favors delignification, a too polar solvent like water causes significant solubilization of carbohydrates. A new empirical descriptor, denoted as ‘lignin-first delignification efficiency’ (LFDE), is introduced as a measure of efficient wood processing into soluble lignin derivatives and solid sugar pulp. Of all tested solvents, methanol and ethylene glycol showed the highest LFDE values, and these values could be increased by increasing both reaction time and temperature. Moreover, substantial differences regarding the process characteristics and analyzed product fractions between these two different solvents were discussed extensively. Most striking is the impact of the solvent on the pulp macrostructure, with methanol yielding a pulp composed of aggregated fiber cells, whereas the ethylene glycol pulp comprises nicely separated fiber cells.
An Eco-friendly Soft Template Synthesis of Mesostructured Silica-Carbon Nanocomposites for Acid Catalysis 03-09-2015
The synthesis of ordered mesoporous silica-carbon composites was explored by employing TEOS and sucrose as the silica and carbon precursor respectively, and the triblock copolymer F127 as a structure-directing agent via an evaporation-induced self-assembly (EISA) process. It is demonstrated that the synthesis procedures allow for control of the textural properties and final composition of these silica-carbon nanocomposites via adjustment of the effective SiO2/C weight ratio. Characterization by SAXS, N2 physisorption, HRTEM, TGA, and 13C and 29Si solid-state MAS NMR show a 2D hexagonal mesostructure with uniform large pore size ranging from 5.2 to 7.6 nm, comprising of separate carbon phases in a continuous silica phase. Ordered mesoporous silica and non-ordered porous carbon can be obtained by combustion of the pyrolyzed nanocomposites in air or etching with HF solution, respectively. Sulfonic acid groups can be readily introduced to such kind of silica-carbon nanocomposites by a standard sulfonation procedure with concentrated sulfuric acid. Excellent acid-catalytic activities and selectivities for the dimerization of styrene to produce 1,3-diphenyl-1-butene and dimerization of α-methylstyrene to unsaturated dimers were demonstrated with the sulfonated materials.
Post-synthesis Snβ: An exploration of synthesis parameters and catalysis 29-08-2015
Snβ is probably one of the best water tolerating heterogeneous Lewis acids for liquid phase catalysis. Instead of applying the usual lengthy hydrothermal synthesis to prepare Snβ, this contribution uses a more hands-on two-step synthesis method, involving the grafting of Sn precursors in isopropanol under reflux conditions on a commercial β zeolite that was dealuminated in acid. Among several reference synthesis procedures, this Sn introduction method resulted in active Sn catalytic sites. Taking advantage of this practical method, several synthesis parameters were explored and their impact on the catalytic activity in four different Lewis acid catalyzed reactions is discussed. The adsorption isotherm of SnIV in isopropanol over a broad range of Sn salt concentrations at reflux temperature is presented and discussed in relation with FTIR spectroscopy, UV–vis absorption characteristics and the porosity of the materials. The study reveals a selective Sn uptake, up to 2 wt% Sn loading, into silanol nests of the dealuminated precursor, forming a diversity of mononuclear SnIV. Higher Sn loadings result in less active Sn (hydrous) extraframework oxide phases, which also cause partial blockage of the zeolite micropores. Depending on the reaction type under study, space time yield may increase with increasing Sn loading, but the activity per Sn is always lower. Therefore it is concluded that a preferred synthesis should form high contents of isolated Sn active sites, especially for sugar isomerization and intermolecular Meerwein–Ponndorf–Verley, while the other reaction types like Baeyer–Villiger is also sufficiently catalyzed by the small Sn oxide clusters, albeit less actively.
Synthesis, characterisation, and catalytic evaluation of hierarchical faujasite zeolites: milestones, challenges, and future directions 27-08-2015
Faujasite (X, Y, and USY) zeolites represent one of the most widely-applied and abundant catalysts and sorbents in the chemical industry. In the last 5 years substantial progress was made in the synthesis, characterisation, and catalytic exploitation of hierarchically-structured variants of these zeolites. Hererin, we provide an overview of these contributions, highlighting the main advancements regarding the evaluation of the nature and functionality of introduced secondary porosity. The novelty, efficiency, versatility, and sustainability of the reported bottom-up and (predominately) top-down strategies are discussed. The crucial role of the relative stability of faujasites in aqueous media is highlighted. The interplay between the physico-chemical properties of the hierarchical zeolites and their use in petrochemical and biomass-related catalytic processes is assessed
Confinement Effects in Lewis Acid-Catalyzed Sugar Conversion: Steering Toward Functional Polyester Building Blocks 24-08-2015
We report the use of solid Lewis acid catalysts for the conversion of tetrose sugars to four-carbon α-hydroxy acid esters (C4-AHA), which are useful as functional polyester building blocks. Sn-β was by far the most active and selective catalyst, yielding up to 80% methyl vinyl glycolate (MVG), methyl-4-methoxy-2-hydroxybutanoate (MMHB), and α-hydroxy-γ-butyrolactone (HBL) combined at 95% conversion. A very high turnover frequency (TOF) of 330 molC4-AHA molSn h–1 was attained using Sn-β, a more than 6-fold increase compared with homogeneous SnCl4·5H2O. It is shown that, using different Sn-based catalysts with various pore sizes, the product distribution is strongly dependent on the size of the catalyst pores. Catalysts containing mainly mesopores, such as Sn-MCM-41 or Sn-SBA-15, prefer the production of the more bulky MMHB, whereas microporous catalysts such as Sn-β or Sn-MFI favor the production of MVG. This effect can be further enhanced by increasing the reaction temperature. At 363 K, only 20% MVG is attained using Sn-β, but at 433 K, this increases to 50%. Using a kinetic analysis, it was found that, in microporous catalysts, steric hindrance near the Sn active site in the catalyst pores plays a dominant role in favoring the reaction pathway toward MVG. Moreover, the selectivity toward both products is kinetically controlled.
Alkylphenols to phenol and olefins by zeolite catalysis: a pathway to valorize raw and fossilized lignocellulose 17-08-2015
Selective conversion of alkylphenols to phenol and olefins is presented as a challenging key step in upgrading raw and fossilized lignocellulose. An exceptional and stable dealkylation performance is achieved by application of an acidic ZSM-5 zeolite, in which co-feeding of water is crucial to maintain catalytic activity. The role of water is attributed to competitive adsorption of water and phenol. The lignin-first pathway towards phenol yields a tenfold improvement of phenol compared to the state-of-the-art single-step lignocellullosic depolymerization techniques.
Polyimide mixed matrix membranes for CO2 separations using carbon–silica nanocomposite fillers 12-08-2015
Mixed matrix membranes (MMMs) have a potential to improve the separation performance of polymeric membranes while maintaining their advantages of easy processing and lower costs. In this work, series of MMMs were developed via solution casting by adding porous carbon–silica nanocomposite (CSM) fillers to a readily available Matrimid® membrane. CSMs were prepared by a hard template synthesis technique to get a tuneable porosity and surface chemistry which is controlled by the optimization of the filler porosity using carbon deposition, the pyrolysis conditions, and the maximization of polarity via oxygen functional groups. SEM images of the synthesized MMMs confirmed the good adhesion and dispersion of the fillers within the polymer matrix. The separation results demonstrate that the overall separation efficiency is increased by the addition of a carbon phase, providing an increased affinity for the CO2 gas molecules next to the creation of extra porosity and free volume. It was showed that significantly improved CO2 mixed gas selectivity and permeability for CO2:N2 and CO2:CH4 gas mixtures at 9 bar and 308 K was achieved. For gas mixtures with a 50:50 (CO2:N2) feed composition, a 2-fold and 6-fold increase of the mixed gas selectivity (up to 42.5) and permeability (up to 27 Barrer) compared to unfilled PI was achieved, respectively. The performance of the membranes was compared to the existing literature data.
Alkali Activation of AOD Stainless Steel Slag Under Steam Curing Conditions 03-08-2015
Crystalline argon oxygen decarburization slag, in powdery form, was investigated for its hydration potential by alkali activation and curing at 80°C. Na-silicate and K-silicate of the same modulus were used as activators. Isothermal calorimetry at 80°C indicated exothermic reactions in the slag pastes. When the slag mortars were cured under steam at 80°C appreciable gain in compressive strength was measured. This was attributed to C–S–H which was detected in TG, FTIR, and 29Si NMR analyses. Upon hydration at 90 d, the amount of crystalline phases decreased, whereas the XRD amorphous content in the slag increased. Electron microscopy showed the formation of different morphologies of reaction products depending on the alkaline activator employed. Presence of reaction rims around the crystalline phases with a major presence of Ca, Si, and O in the reacted matrix was observed in elemental maps.
Shape-selective zeolite catalysis for bioplastics production 03-07-2015
Biodegradable and renewable polymers, such as polylactic acid, are benign alternatives for petrochemical-based plastics. Current production of polylactic acid via its key building block lactide, the cyclic dimer of lactic acid, is inefficient in terms of energy, time, and feedstock use. We present a direct zeolite-based catalytic process, which converts lactic acid into lactide. The shape-selective properties of zeolites are essential to attain record lactide yields, outperforming those of the current multistep process by avoiding both racemization and side-product formation. The highly productive process is strengthened by facile recovery and practical reactivation of the catalyst, which remains structurally fit during at least six consecutive reactions, and by the ease of solvent and side-product recycling.
Influence of MgO precursors on mechanically activated forsterite synthesis 28-06-2015
Brucite-fumed silica and hydromagnesite-fumed silica mixtures were used to investigate the influence of MgO precursors on mechanically activated forsterite synthesis. The changes in morphology, chemical bond and phase composition of the ground and calcined mixtures were examined with scanning electron microscopy (SEM), Si 2p X-ray photoelectron spectroscopy (XPS) and 29Si magic angle spinning nuclear magnetic resonance (MAS-NMR), and X-ray diffraction (XRD), respectively. The XPS and MAS-NMR analyses show that high-energy milling generates more Mg–O–Si chemical bonds in the brucite-fumed silica mixture than in the hydromagnesite-fumed silica sample. This is because brucite has a higher concentration of Mg–OH bonds than hydromagnesite. However, single-phase forsterite forms at a higher temperature of 1000 1C in the milled brucite-fumed silica mixture than that of 800 1C in the ground hydromagnesite-fumed silica sample after the same grinding. The different forsterite completion temperature is probably due to the longer Mg2þ and Si4þ diffusion distance of over 500 nm in the former milled mixture than that of less than 300 nm in the latter ground sample.
Tuning the lignin oil OH-content with Ru and Pd catalysts during lignin hydrogenolysis on birch wood 11-06-2015
Liquid reductive processing of birch wood in the presence of commercial Ru/C or Pd/C catalysts yields about 50% of a select set of phenolic monomers and a variety of phenolic di- and oligomers, next to a solid carbohydrate pulp. Changing the catalyst from Ru/C to Pd/C drastically increases the OH-content of the lignin-derived products, in particular for the phenolic monomers.
Ruthenium indenylidene complexes bearing N-alkyl/N-mesityl-substituted N-heterocyclic carbene ligands 19-05-2015
We report on the synthesis and characterization of second generation ruthenium indenylidene catalysts bearing unsymmetrical N-heterocyclic carbene (NHC) ligands denoted as RuCl2(3-phenyl-1-indenylidene)(1-mesityl-3-R-4,5-dihydroimidazol-2-ylidene)(PCy3), in which R is methyl 8a, octyl 8b or cyclohexyl 8c. The characterization of 8a–c was performed by NMR spectroscopy, elemental analysis, IR, HRMS and single-crystal X-ray diffraction analysis. In addition, the catalytic activity of the obtained initiators was evaluated in various representative metathesis reactions. The results reveal that the complexes 8a–c, bearing an N-alkyl side on the NHC, show a faster catalytic initiation than the reference complex 2. Complex 8a, which performs the best among the investigated indenylidene complexes, exhibits slower initiation but better overall efficiency than its benzylidene analogue 1c, especially in a low catalyst loading.
Review of catalytic systems and thermodynamics for the Guerbet condensation reaction and challenges for biomass valorization 18-05-2015
The Guerbet condensation reaction is an alcohol coupling reaction that has been known for more than a century. Because of the increasing availability of bio-based alcohol feedstock, this reaction is of growing importance and interest in terms of value chains of renewable chemical and biofuel production. Due to the specific branching pattern of the alcohol products, the Guerbet reaction has many interesting applications. In comparison to their linear isomers, branched-chain Guerbet alcohols have extremely low melting points and excellent fluidity. This review provides thermodynamic insights and unravels the various mechanistic steps involved. A comprehensive overview of the homogeneous, heterogeneous and combined homogeneous and heterogeneous catalytic systems described in published reports and patents is also given. Technological considerations, challenges and perspectives for the Guerbet chemistry are discussed.
Spectroscopic Definition of the Copper Active Sites in Mordenite: Selective Methane Oxidation 26-04-2015
Two distinct [Cu–O–Cu]2+ sites with methane monooxygenase activity are identified in the zeolite Cu-MOR, emphasizing that this Cu–O–Cu active site geometry, having a ∠Cu–O–Cu ∼140°, is particularly formed and stabilized in zeolite topologies. Whereas in ZSM-5 a similar [Cu–O–Cu]2+active site is located in the intersection of the two 10 membered rings, Cu-MOR provides two distinct local structures, situated in the 8 membered ring windows of the side pockets. Despite their structural similarity, as ascertained by electronic absorption and resonance Raman spectroscopy, the two Cu–O–Cu active sites in Cu-MOR clearly show different kinetic behaviors in selective methane oxidation. This difference in reactivity is too large to be ascribed to subtle differences in the ground states of the Cu–O–Cu sites, indicating the zeolite lattice tunes their reactivity through second-sphere effects. The MOR lattice is therefore functionally analogous to the active site pocket of a metalloenzyme, demonstrating that both the active site and its framework environment contribute to and direct reactivity in transition metal ion-zeolites.
Reductive lignocellulose fractionation into soluble lignin-derived phenolic monomers and dimers and processable carbohydrate pulps 20-04-2015
A catalytic lignocellulose biorefinery process is presented, valorizing both polysaccharide and lignin components into a handful of chemicals. To that end, birch sawdust is efficiently delignified through simultaneous solvolysis and catalytic hydrogenolysis in the presence of a Ru on carbon catalyst (Ru/C) in methanol under a H2 atmosphere at elevated temperature, resulting in a carbohydrate pulp and a lignin oil. The lignin oil yields above 50% of phenolic monomers (mainly 4-n-propylguaiacol and 4-n-propylsyringol) and about 20% of a set of phenolic dimers, relative to the original lignin content, next to phenolic oligomers. The structural features of the lignin monomers, dimers and oligomers were identified by a combination of GC/MS, GPC and 2D HSQC NMR techniques, showing interesting functionalities for forthcoming polymer applications. The effect of several key parameters like temperature, reaction time, wood particle size, reactor loading, catalyst reusability and the influence of solvent and gas were examined in view of the phenolic product yield, the degree of delignification and the sugar retention as a first assessment of the techno-economic feasibility of this biorefinery process. The separated carbohydrate pulp contains up to 92% of the initial polysaccharides, with a nearly quantitative retention of cellulose. Pulp valorization was demonstrated by its chemocatalytic conversion to sugar polyols, showing the multiple use of Ru/C, initially applied in the hydrogenolysis process. Various lignocellulosic substrates, including genetically modified lines of Arabidopsis thaliana, were finally processed in the hydrogenolytic biorefinery, indicating lignocellulose rich in syringyl-type lignin, as found in hardwoods, as the ideal feedstock for the production of chemicals.
Selective Nickel-Catalyzed Conversion of Model and Lignin-Derived Phenolic Compounds to Cyclohexanone-Based Polymer Building Blocks 16-04-2015
Valorization of lignin is essential for the economics of future lignocellulosic biorefineries. Lignin is converted into novel polymer building blocks through four steps: catalytic hydroprocessing of softwood to form 4-alkylguaiacols, their conversion into 4-alkylcyclohexanols, followed by dehydrogenation to form cyclohexanones, and Baeyer–Villiger oxidation to give caprolactones. The formation of alkylated cyclohexanols is one of the most difficult steps in the series. A liquid-phase process in the presence of nickel on CeO2 or ZrO2 catalysts is demonstrated herein to give the highest cyclohexanol yields. The catalytic reaction with 4-alkylguaiacols follows two parallel pathways with comparable rates: 1) ring hydrogenation with the formation of the corresponding alkylated 2-methoxycyclohexanol, and 2) demethoxylation to form 4-alkylphenol. Although subsequent phenol to cyclohexanol conversion is fast, the rate is limited for the removal of the methoxy group from 2-methoxycyclohexanol. Overall, this last reaction is the rate-limiting step and requires a sufficient temperature (>250 °C) to overcome the energy barrier. Substrate reactivity (with respect to the type of alkyl chain) and details of the catalyst properties (nickel loading and nickel particle size) on the reaction rates are reported in detail for the Ni/CeO2catalyst. The best Ni/CeO2 catalyst reaches 4-alkylcyclohexanol yields over 80 %, is even able to convert real softwood-derived guaiacol mixtures and can be reused in subsequent experiments. A proof of principle of the projected cascade conversion of lignocellulose feedstock entirely into caprolactone is demonstrated by using Cu/ZrO2 for the dehydrogenation step to produce the resultant cyclohexanones (≈80 %) and tin-containing beta zeolite to form 4-alkyl-ε-caprolactones in high yields, according to a Baeyer–Villiger-type oxidation with H2O2.
Potential of Sustainable Hierarchical Zeolites in the Valorization of α-Pinene 03-03-2015
In the valorization of α-pinene, which is an important biomass intermediate derived from turpentine oil, hierarchical (mesoporous) zeolites represent a superior class of catalysts. Hierarchical USY, ZSM-5, and beta zeolites have been prepared, characterized, and catalytically evaluated, with the aim of combining the highest catalytic performance with the most sustainable synthetic protocol. These zeolites are prepared by alkaline treatment in aqueous solutions of NH4OH, NaOH, diethylamine, and NaOH complemented with tetrapropylammonium bromide. The hierarchical USY zeolite is the most attractive catalyst of the tested series, and is able to combine an overall organic-free synthesis with an up to sixfold activity enhancement and comparable selectivity over the conventional USY zeolite. This superior performance relates to a threefold greater activity than that of the commercial standard, namely, H2SO4/TiO2. Correlation of the obtained benefits to the amount of solid lost during the postsynthetic modifications highlights that the highest activity gains are obtained with minor leaching. Furthermore, a highly zeolitic character, as determined by bulk XRD, is beneficial, but not crucial, in the conversion of α-pinene. The alkaline treatments not only result in a higher overall activity, but also a more functional external surface area, attaining up to four times the pinene conversions per square nanometer. The efficiency of the hierarchical USY zeolite is concomitantly demonstrated in the conversion of limonene and turpentine oil, which emphasizes its industrial potential.
Cooperative Catalysis for Multistep Biomass Conversion with Sn/Al Beta Zeolite 26-12-2014
Lewis acid Snβ-type zeolites with varying amounts of Brønsted acid Al in the framework were synthesized using a simple two-step procedure comprising partial dealumination of β zeolite under action of acid, followed by grafting with SnCl4·5H2O in dry isopropanol. Characterization of the thus-prepared Al-containing Snβ (Sn/pDeAlβ) zeolites with ICP, (pyridine probed) FTIR, and 27Al MAS NMR demonstrates the presence of Brønsted acid framework AlIII. Tetrahedral Lewis acidic SnIV is present, as ascertained by a combination of techniques such as EPMA, 119Sn Möβbauer, XPS, (pyridine probed) FTIR, and UV–vis. A closed SnIV configuration was implied by comparing of 119Sn solid-state MAS NMR and deuterated acetonitrile probed FTIR spectra with literature. The catalytic activity of the Al-containing Snβ was tested for the conversion of 1,3-dihydroxyacetone (DHA) into ethyl lactate (ELA), proceeding via pyruvic aldehyde (PAL). Despite the difference in synthesis between the classic hydrothermal Snβ reference and Sn/pDeAlβ, the activity of Sn for the Lewis acid-catalyzed hydride shift of PAL to ELA is similar. Yet, the overall reaction rate of DHA into ELA is faster with Sn/pDeAlβ because Brønsted acidity of the remaining framework AlIIIfacilitates the rate-determining dehydration of DHA into PAL. Materials containing moderate amounts of Al (0.3 wt % Al) show the highest ELA productivities, leading to a record value of 2113 g ELA·kg catalyst–1·h–1 at 363 K. The cooperative effect of Lewis SnIV and Brønsted AlIII acid sites is verified by comparing catalytic data with physical mixtures of partially dealuminated β zeolite and Al-free Snβ.
Conceptual Frame Rationalizing the Self-Stabilization of H-USY Zeolites in Hot Liquid Water 09-12-2014
The wide range of liquid-phase reactions required for the catalytic conversion of biomass compounds into new bioplatform molecules defines a new set of challenges for the development of active, selective, and stable catalysts. The potential of bifunctional Ru/H-USY catalysts for conversions in hot liquid water (HLW) is assessed in terms of physicochemical stability and long-term catalytic performance of acid sites and noble metal functionality, as probed by hydrolytic hydrogenation of cellulose. It is shown that zeolite desilication is the main zeolite degradation mechanism in HLW. USY zeolite stability depends on two main parameters, viz., framework and extra-framework aluminum content. The former protects the zeolite lattice by counteracting hydrolysis of framework bonds, and the latter, when located at the external crystal surface, prevents solubilization of the zeolite framework which is the result of its low water-solubility. Hence, the hot liquid water stability of commercial H-USY zeolites, in contrast to their steam stability, increased with decreasing Si/Al ratio. As a result, mildly steamed USY zeolites containing a high amount of both Al species exhibit the highest resistance to HLW. During an initial period of transformations, Al-rich zeolites form additional protective extra-framework Al species at the outer surface, self-stabilizing the framework. A critical bulk Si/Al ratio of 3 was determined whereby USY zeolites with a lower Si/Al ratio will self-stabilize over time. Besides, due to the initial transformation period, the accessibility of the catalytic active sites is extensively enhanced resulting in a material that is more stable and drastically more accessible to large substrates than the original zeolite. When these findings are applied in the hydrolytic hydrogenation of cellulose, unprecedented nearly quantitative hexitol yields were obtained with a stable catalytic system.
Ternary Ag/MgO-SiO2 Catalysts for the Conversion of Ethanol into Butadiene 19-11-2014
Ternary Ag/Magnesia-silica catalysts were tested in the direct synthesis of 1,3-butadiene from ethanol. The influence of the silver content and the type of silica source on catalytic performance has been studied. Prepared catalysts were characterized by 29Si NMR, N2 sorption, small-angle X-ray scattering measurements, XRD, environmental scanning electron microscopy with energy dispersive X-ray analysis (ESEM/EDX), FTIR spectroscopy of adsorbed pyridine and CO2, temperature-programmed desorption of CO2 and UV/Vis diffuse reflectance spectroscopy. Based on these characterization results, the catalytic performance of the catalysts in the 1,3-butadiene formation process was interpreted and a tentative model explaining the role of the different catalytically active sites was elaborated. The balance of the active sites is crucial to obtain an active and selective catalyst to form 1,3-butadiene from ethanol. The optimal silver loading is 1–2 wt % on a MgO-silica support with a molar Mg/Si ratio of 2. The silver species and basic sites (Mg-O pairs and basic OH groups) are of prime importance in the 1,3-butadiene production, catalyzing mainly the ethanol dehydrogenation and the aldol condensation, respectively.