Bert F. Sels

Recent Advances in the Catalytic Conversion of Cellulose

Concerns about the depletion of fossil fuel reserves, the impact of anthropogenic CO2 emissions, and increasing energy demands have encouraged the exploration of new catalytic procedures for converting cellulosic biomass into valuable platform chemicals and renewable fuel components. The development of these sustainable catalytic transformations could potentially provide a long-term solution to the industrial dependence on fossil carbon, requiring in 2025 production of up to 30% of raw materials for the chemical industry from renewable resources. With an abundance of approximately 720 billion tonnes, that is, 40% of the annual net yield of photosynthesis, cellulose is the world’s largest organic raw material resource. Whereas nature renews 40 billion tonnes every year, no more than 200 million tonnes of this nonedible biomass are processed, mainly as a raw material for paper and packaging industry. The blueprints of the “new” cellulose chemistry are based on some key elements, namely controlled depolymerization of the biopolymer and catalytic cascade reactions (e.g. , hydrogenation, hydrogenolysis, oxidation), which, when put together, yield a pool of molecules that can be used for the synthesis of industrial intermediates and fine chemicals. One of the methods for chemical degradation of cellulose is the acid-catalyzed hydrolytic cleavage into its glucose monomers, which are, for example, of high interest for further fermentation into bioethanol. 6] An excellent review on cellulose hydrolysis as an entry point into biorefinery schemes has recently been published by Rinaldi and Sch th. Also as an introduction, we recommend more general reviews on the challenges and issues involved in the catalytic processing of biomass. In this Minireview, we focus on the impressive scope of recent catalytic advances in the conversion of cellulose over solid acid and multifunctional catalysts, the direct conversion into furan-based or valeric biofuels, liquid alkenes, alkyl glycosides, and cellulose dissolution and processing in ionic liquids. Particular emphasis will be on concepts known from heterogeneous and multistep catalysis. Before the different catalytic strategies are discussed, structural aspects as well as specific chemical and physical properties of cellulose will be briefly addressed, as this knowledge is a prerequisite for the rational design of new catalytic transformations.

Layered double hydroxides exchanged with tungstate as biomimetic catalysts for mild oxidative bromination

The manufacture of a range of bulk and fine chemicals, including flame retardants, disinfectants and antibacterial and antiviral drugs, involves bromination. Conventional bromination methods typically use elemental bromine, a pollutant and a safety and health hazard. Attempts to develop alternative and more benign strategies have been inspired by haloperoxidase enzymes, which achieve selective halogenation at room temperature and nearly neutral pH by oxidizing inorganic halides with hydrogen peroxide,. The enzyme vanadium bromoperoxidase has attracted particular interest, in this regard, and several homogeneous inorganic catalysts mimicking its activity are available, although they are limited by the requirement for strongly acidic reaction media. A heterogenous mimic operating at neutral pH has also been reported, but shows only modest catalytic activity. Here we describe a tungstate-exchanged layered double hydroxide that catalyses oxidative bromination and bromide-assisted epoxidation reactions in a selective manner. We find that the catalyst is over 100 times more active than its homogeneous analogue. The low cost and heterogeneous character of this system, together with its ability to operate efficiently under mild conditions using bromides rather than elemental bromine, raise the prospect of being able to develop a clean and efficient industrial route to brominated chemicals and drugs and epoxide intermediates.

Hydrotalcite-like anionic clays in catalytic organic reactions

Hydrotalcite-like anionic clays (HTs; or layered double hydroxides, LDHs) can be used as precursors to mixed oxides, but their catalytic potential reaches much further. This review demonstrates that HTs can be successfully applied in a broad spectrum of organic reactions, with advantages such as improved activity, selectivity, metal dispersion, less waste production, and an improved recuperation of immobilized catalysts. HTs can be used in the as-synthesized form or after different pretreatments. The HT as such is a solid base or, depending on the elemental composition of its octahedral layers, may have redox properties. Specific metals can be incorporated either as a cation in the octahedral layer or as an anion via exchange. The anion-exchange strategy is especially successful in the context of heterogenizing homogeneous catalysts. A particular advantage for base catalysis is that the number and strength of the basic sites can be tuned precisely to a specific reaction. HTs are excellent materials to design bifunctional redox-base catalysts or to control the acid–base properties around a heterogenized metal complex. Potential applications of HTs range from the production of large-scale basic chemicals to the synthesis of small-scale specialty chemicals.

Spatially resolved observation of crystal-face-dependent catalysis by single turnover counting

Catalytic processes on surfaces have long been studied by probing model reactions on single-crystal metal surfaces under high vacuum conditions. Yet the vast majority of industrial heterogeneous catalysis occurs at ambient or elevated pressures using complex materials with crystal faces, edges and defects differing in their catalytic activity. Clearly, if new or improved catalysts are to be rationally designed, we require quantitative correlations between surface features and catalytic activity—ideally obtained under realistic reaction conditions. Transmission electron microscopy and scanning tunnelling microscopy have allowed in situ characterization of catalyst surfaces with atomic resolution, but are limited by the need for low-pressure conditions and conductive surfaces, respectively. Sum frequency generation spectroscopy can identify vibrations of adsorbed reactants and products in both gaseous and condensed phases, but so far lacks sensitivity down to the single molecule level. Here we adapt real-time monitoring of the chemical transformation of individual organic molecules by fluorescence microscopy to monitor reactions catalysed by crystals of a layered double hydroxide immersed in reagent solution. By using a wide field microscope, we are able to map the spatial distribution of catalytic activity over the entire crystal by counting single turnover events. We find that ester hydrolysis proceeds on the lateral {1010} crystal faces, while transesterification occurs on the entire outer crystal surface. Because the method operates at ambient temperature and pressure and in a condensed phase, it can be applied to the growing number of liquid-phase industrial organic transformations to localize catalytic activity on and in inorganic solids. An exciting opportunity is the use of probe molecules with different size and functionality, which should provide insight into shape-selective or structure-sensitive catalysis and thus help with the rational design of new or more productive heterogeneous catalysts.

Lactic acid as a platform chemical in the biobased economy: the role of chemocatalysis

Upcoming bio-refineries will be at the heart of the manufacture of future transportation fuels, chemicals and materials. A narrow number of platform molecules are envisioned to bridge natures abundant polysaccharide feedstock to the production of added-value chemicals and intermediate building blocks. Such platform molecules are well-chosen to lie at the base of a large product assortment, while their formation should be straightforward from the refined biomass, practical and energy efficient, without unnecessary loss of carbon atoms. Lactic acid has been identified as one such high potential platform. Despite its established fermentation route, sustainability issues – like gypsum waste and cost factors due to multi-step purification and separation requirements – will arise as soon as the necessary orders of magnitude larger volumes are needed. Innovative production routes to lactic acid and its esters are therefore under development, converting sugars and glycerol in the presence of chemocatalysts. Moreover, catalysis is one of the fundamental routes to convert lactic acid into a range of useful chemicals in a platform approach. This contribution attempts a critical overview of all advances in the field of homogeneous and heterogeneous catalysis and recognises a great potential of some of these chemocatalytic approaches to produce and transform lactic acid as well as some other promising α-hydroxy acids.

Interfacial synthesis of hollow metal–organic framework capsules demonstrating selective permeability

Metal–organic frameworks (MOFs) are a class of crystalline materials that consist of metal ions and organic ligands linked together by coordination bonds. Because of their porosity and the possibility of combining large surface areas with pore characteristics that can be tailored, these solids show great promise for a wide range of applications. Although most applications currently under investigation are based on powdered solids, developing synthetic methods to prepare defect-free MOF layers will also enable applications based on selective permeation. Here, we demonstrate how the intrinsically hybrid nature of MOFs enables the self-completing growth of thin MOF layers. Moreover, these layers can be shaped as hollow capsules that demonstrate selective permeability directly related to the micropore size of the MOF crystallites forming the capsule wall. Such capsules effectively entrap guest species, and, in the future, could be applied in the development of selective microreactors containing molecular catalysts. The intrinsically hybrid nature of metal–organic frameworks (MOFs) — microporous crystalline solids composed of metal ions and organic ligands — has been exploited to grow thin MOF films at the aqueous–organic interface of a biphasic reaction mixture. These materials exhibit selective permeability and can also be obtained as hollow capsules that have potential as microreactors.

A [Cu2O]2+ core in Cu-ZSM-5, the active site in the oxidation of methane to methanol

Driven by the depletion of crude oil, the direct oxidation of methane to methanol has been of considerable interest. Promising low-temperature activity of an oxygen-activated zeolite, Cu-ZSM-5, has recently been reported in this selective oxidation and the active site in this reaction correlates with an absorption feature at 22,700 cm−1. In the present study, this absorption band is used to selectively resonance enhance Raman vibrations of this active site. 18O2 labeling experiments allow definitive assignment of the observed vibrations and exclude all previously characterized copper-oxygen species for the active site. In combination with DFT and normal coordinate analysis calculations, the oxygen activated Cu core is uniquely defined as a bent mono-(μ-oxo)dicupric site. Spectroscopically validated electronic structure calculations show polarization of the low-lying singly-occupied molecular orbital of the [Cu2O]2+ core, which is directed into the zeolite channel, upon approach of CH4. This induces significant oxyl character into the bridging O atom leading to a low transition state energy consistent with experiment and explains why the bent mono-(μ-oxo)dicupric core is highly activated for H atom abstraction from CH4. The oxygen intermediate of Cu-ZSM-5 is now the most well defined species active in the methane monooxygenase reaction.

Sulfonated silica/carbon nanocomposites as novel catalysts for hydrolysis of cellulose to glucose

Sulfonated silica/carbon nanocomposites were successfully developed as reusable, solid acid catalysts for the hydrolytic degradation of cellulose into high yields of glucose.

Reductive lignocellulose fractionation into soluble lignin-derived phenolic monomers and dimers and processable carbohydrate pulps

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.

Fast and Selective Sugar Conversion to Alkyl Lactate and Lactic Acid with Bifunctional Carbon–Silica Catalysts

A novel catalyst design for the conversion of mono- and disaccharides to lactic acid and its alkyl esters was developed. The design uses a mesoporous silica, here represented by MCM-41, which is filled with a polyaromatic to graphite-like carbon network. The particular structure of the carbon-silica composite allows the accommodation of a broad variety of catalytically active functions, useful to attain cascade reactions, in a readily tunable pore texture. The significance of a joint action of Lewis and weak Brønsted acid sites was studied here to realize fast and selective sugar conversion. Lewis acidity is provided by grafting the silica component with Sn(IV), while weak Brønsted acidity originates from oxygen-containing functional groups in the carbon part. The weak Brønsted acid content was varied by changing the amount of carbon loading, the pyrolysis temperature, and the post-treatment procedure. As both catalytic functions can be tuned independently, their individual role and optimal balance can be searched for. It was thus demonstrated for the first time that the presence of weak Brønsted acid sites is crucial in accelerating the rate-determining (dehydration) reaction, that is, the first step in the reaction network from triose to lactate. Composite catalysts with well-balanced Lewis/Brønsted acidity are able to convert the trioses, glyceraldehyde and dihydroxyacetone, quantitatively into ethyl lactate in ethanol with an order of magnitude higher reaction rate when compared to the Sn grafted MCM-41 reference catalyst. Interestingly, the ability to tailor the pore architecture further allows the synthesis of a variety of amphiphilic alkyl lactates from trioses and long chain alcohols in moderate to high yields. Finally, direct lactate formation from hexoses, glucose and fructose, and disaccharides composed thereof, sucrose, was also attempted. For instance, conversion of sucrose with the bifunctional composite catalyst yields 45% methyl lactate in methanol at slightly elevated reaction temperature. The hybrid catalyst proved to be recyclable in various successive runs when used in alcohol solvent.