Jan Geboers

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.

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.

Catalytic production of levulinic acid from cellulose and other biomass-derived carbohydrates with sulfonated hyperbranched poly(arylene oxindole)s

Innovative catalyst design holds the key to fundamental advances in the transformation of cellulose to chemicals and transportation fuels, both of which are vital to meet the challenge of increasing energy costs and the finite nature of fossil fuel reserves. Here we report on the functionalization, characterization and successful application of sulfonated hyperbranched poly(arylene oxindole)s for the direct catalytic conversion of cellulose to levulinic acid. The use of water-soluble hyperbranched polymers in combination with ultrafiltration is conceptually novel and opens new horizons in the aqueous-phase processing of cellulose substrates with various degrees of crystallinity. Compared to most conventional types of acid catalysts, these highly acidic polymers demonstrate superior catalytic performance in terms of both activity and selectivity. Additionally, this molecular approach can be successfully transferred to the acid-catalyzed degradation of other abundant biomass resources, including starch, inulin and xylan.

Tuning the acid/metal balance of carbon nanofiber-supported nickel catalysts for hydrolytic hydrogenation of cellulose.

Carbon nanofibers (CNFs) are a class of graphitic support materials with considerable potential for catalytic conversion of biomass. Earlier, we demonstrated the hydrolytic hydrogenation of cellulose over reshaped nickel particles attached at the tip of CNFs. The aim of this follow-up study was to find a relationship between the acid/metal balance of the Ni/CNFs and their performance in the catalytic conversion of cellulose. After oxidation and incipient wetness impregnation with Ni, the Ni/CNFs were characterized by various analytical methods. To prepare a selective Ni/CNF catalyst, the influences of the nature of oxidation agent, Ni activation, and Ni loading were investigated. Under the applied reaction conditions, the best result, that is, 76 % yield in hexitols with 69 % sorbitol selectivity at 93 % conversion of cellulose, was obtained on a 7.5 wt % Ni/CNF catalyst prepared by chemical vapor deposition of CH(4) on a Ni/γ-Al(2)O(3) catalyst, followed by oxidation in HNO(3) (twice for 1 h at 383 K), incipient wetness impregnation, and reduction at 773 K under H(2). This preparation method leads to a properly balanced Ni/CNF catalyst in terms of Ni dispersion and hydrogenation capacity on the one hand, and the number of acidic surface-oxygen groups responsible for the acid-catalyzed hydrolysis on the other.

Direct catalytic conversion of cellulose to liquid straight-chain alkanes

High yields of liquid straight-chain alkanes were obtained directly from cellulosic feedstock in a one-pot biphasic catalytic system. The catalytic reaction proceeds at elevated temperatures under hydrogen pressure in the presence of tungstosilicic acid, dissolved in the aqueous phase, and modified Ru/C, suspended in the organic phase. Tungstosilicic acid is primarily responsible for cellulose hydrolysis and dehydration steps, while the modified Ru/C selectively hydrogenates intermediates en route to the liquid alkanes. Under optimal conditions, microcrystalline cellulose is converted to 82% n-decane-soluble products, mainly n-hexane, within a few hours, with a minimum formation of gaseous and char products. The dominant route to the liquid alkanes proceeds via 5-hydroxymethylfurfural (HMF), whereas the more common pathway via sorbitol appears to be less efficient. High liquid alkane yields were possible through (i) selective conversion of cellulose to glucose and further to HMF by gradually heating the reactor, (ii) a proper hydrothermal modification of commercial Ru/C to tune its chemoselectivity to furan hydrogenation rather than glucose hydrogenation, and (iii) the use of a biphasic reaction system with optimal partitioning of the intermediates and catalytic reactions. The catalytic system is capable of converting subsequent batches of fresh cellulose, enabling accumulation of the liquid alkanes in the organic phase during subsequent runs. Its robustness is illustrated in the conversion of the raw (soft)wood sawdust.

Conversion of (Ligno)Cellulose Feeds to Isosorbide with Heteropoly Acids and Ru on Carbon

The catalytic valorization of cellulose is currently subject of intense research. Isosorbide is among the most interesting products that can be formed from cellulose as it is a potential platform molecule and can be used for the synthesis of a wide range of pharmaceuticals, chemicals, and polymers. A promising direct route from cellulose to isosorbide is presented in this work. The strategy relies on a one-pot bifunctional catalytic concept, combining heteropoly acids, viz. H(4)SiW(12)O(40), and redox catalysts, viz. commercial Ru on carbon, under H(2) pressure. Starting from pure microcrystalline cellulose, a rapid conversion was observed, resulting in over 50% isosorbide yield. The robustness of the developed system is evidenced by the conversion of a range of impure cellulose pulps obtained by organosolv fractionation, with isosorbide yields up to 63%. Results were compared with other (ligno)cellulose feedstocks, highlighting the importance of fractionation and purification to increase reactivity and convertibility of the cellulose feedstock.

Thiol-promoted catalytic synthesis of diphenolic acid with sulfonated hyperbranched poly(arylene oxindole)s

Acid-catalyzed condensation of levulinic acid and phenol into high yields of diphenolic acid (>50%) is possible with a combination of sulfonated hyperbranched polymers and thiol promotors, either added as a physical mixture or bound to the polymer by ion-pairing.

Design of Ru–Zeolites for Hydrogen‐Free Production of Conjugated Linoleic Acids

While conjugated vegetable oils are currently used as additives in the drying agents of oils and paints, they are also attractive molecules for making bio-plastics. Moreover, conjugated oils will soon be accepted as nutritional additives for functional food products. While current manufacture of conjugated vegetable oils or conjugated linoleic acids (CLAs) uses a homogeneous base as isomerisation catalyst, a heterogeneous alternative is not available today. This contribution presents the direct production of CLAs over Ru supported on different zeolites, varying in topology (ZSM-5, BETA, Y), Si/Al ratio and countercation (H(+), Na(+), Cs(+)). Ru/Cs-USY, with a Si/Al ratio of 40, was identified as the most active and selective catalyst for isomerisation of methyl linoleate (cis-9,cis-12 (C18:2)) to CLA at 165 °C. Interestingly, no hydrogen pre-treatment of the catalyst or addition of hydrogen donors is required to achieve industrially relevant isomerisation productivities, namely, 0.7 g of CLA per litre of solvent per minute. Moreover, the biologically most active CLA isomers, namely, cis-9,trans-11, trans-10,cis-12 and trans-9,trans-11, were the main products, especially at low catalyst concentrations. Ex situ physicochemical characterisation with CO chemisorption, extended X-ray absorption fine structure measurements, transmission electron microscopy analysis, and temperature-programmed oxidation reveals the presence of highly dispersed RuO(2) species in Ru/Cs-USY(40).

Bridging the gap between cellulose chemistry and heterogeneous catalysis

Although cellulosic biomass offers a promising alternative as an abundant renewable resource in the production of biofuels and platform chemicals, so far only a few studies have reported its aqueous-phase conversion into glucose or sugar alcohols using solid chemocatalysts. The principal reason is that these polymeric biomolecules with semi-crystalline structure cannot penetrate the pores of conventional heterogeneous chemocatalysts. New advances in the conversion of cellulose thus require the design of efficient multifunctional catalytic systems with sterically accessible acid and metal sites.