Selective Catalysis for Renewable Feedstocks and Chemicals

Cheap fossil oil resources are becoming depleted and crude oil prices are rising. In this context, alternatives to fossil fuel-derived carbon are examined in an effort to improve the security of carbon resources through the development of novel technologies for the production of chemicals, fuels, and materials from renewable feedstocks such as biomass. The general concept unifying the conversion processes for raw biomass is that of the biorefinery, which integrates biofuels with a selection of pivot points towards value-added chemical end products via so-called “platform chemicals”. While the concept of biorefining is not new, now more than ever there is the motivation to investigate its true potential for the production of carbon-based products. A variety of renewable chemicals have been proposed by many research groups, many of them being categorized as drop-ins, while others are novel chemicals with the potential to displace petrochemicals across several markets. To be competitive with petrochemicals, carbohydrate-derived products should have advantageous chemical properties that can be profitably exploited, and/or their production should offer cost-effective benefits. The production of drop-ins will likely proceed in short term since the markets are familiar, while the commercial introduction of novel chemicals takes longer and demands more technological and marketing effort.

Rather than describing elaborate catalytic routes and giving exhaustive lists of reactions, a large part of this review is devoted to creating a guideline for the selection of the most promising (platform) chemicals derived via chemical-catalytic reaction routes from lignocellulosic biomass. The major rationale behind our recommendations is a maximum conservation of functionality, alongside a high atom economy. Nature provides us with complex molecules like cellulose and hemicellulose, and it should be possible to transform them into chemical products while maintaining aspects of their original structure, rather than taking them completely apart only to put them back together again in a different order, or turning them into metabolites and CO₂. Thus, rather than merely pursuing energy content as in the case of biofuels, the chemist sees atom efficiency, functional versatility, and reactivity as the key criteria for the successful valorization of biomass into chemicals.

To guide the choice of renewable chemicals and their production, this review adopts the original van Krevelen plots and develops alternative diagrams by introducing a functionality parameter F and a functionality index F:C (rather than O:C). This index is more powerful than the O index to describe the importance of functional groups. Such plots are ideal to assess the effect of several reaction types on the overall functionality in biomass conversion. The atom economy is an additional arbitrator in the evaluation of the reaction types. The assessment is illustrated in detail for the case of carbohydrate resources, and about 25 chemicals, including drop-ins as well as novel chemicals, are selected.

Most of these chemicals would be difficult to synthesize from petrochemicals feeds, and this highlights the unique potential of carbohydrates as feedstocks, but, importantly, the products should have a strong applied dimension in existing or rising markets. Ultimately, the production scales of those markets must be harmonized to the biomass availability and its collection and storage logistics.

The synthesis and chemistry of 5-(hydroxymethyl)furfural (HMF), 5-(chloromethyl)furfural (CMF), and levulinic acid (LA), three carbohydrate-derived platform molecules produced by the chemical-catalytic processing of lignocellulosic biomass, is reviewed. Starting from the historical derivation of these molecules and progressing through modern approaches to their production from biomass feedstocks, this review will then survey their principal derivative chemistries, with particular attention to aspects of commercial relevance, and discuss the relative merits of each molecule in the future of biorefining.

This review discusses topical chemical routes and their catalysis for the conversion of cellulose, hexoses, and smaller carbohydrates to lactic acid and other useful α-hydroxy acids. Lactic acid is a top chemical opportunity from carbohydrate biomass as it not only features tremendous potential as a chemical platform molecule; it is also a common building block for commercially employed green solvents and near-commodity bio-plastics. Its current scale fermentative synthesis is sufficient, but it could be considered a bottleneck for a million ton scale breakthrough. Alternative chemical routes are therefore investigated using multifunctional, often heterogeneous, catalysis. Rather than summarizing yields and conditions, this review attempts to guide the reader through the complex reaction networks encountered when synthetic lactates from carbohydrate biomass are targeted. Detailed inspection of the cascade of reactions emphasizes the need for a selective retro-aldol activity in the catalyst. Recently unveiled catalytic routes towards other promising α-hydroxy acids such as glycolic acid, and vinyl and furyl glycolic acids are highlighted as well.

Hydrogenolysis of C–O bonds is becoming more and more important for the production of biomass-derived chemicals. Since substrates originated from biomass usually have high oxygen content and various kinds of C–O bonds, selective hydrogenolysis is required. Rhenium or molybdenum oxide modified rhodium and iridium metal catalysts (Rh-ReO _x , Rh-MoO _x , and Ir-ReO _x ) have been reported to be effective for selective hydrogenolysis. This review introduces the catalytic performance and reaction kinetics of Rh-ReO _x , Rh-MoO _x , and Ir-ReO _x in the hydrogenolysis of various substrates, where selectivity is especially characteristic. Based the model structure of the catalysts and the reaction mechanism, the role of the oxide components is to make the interaction between the OH groups in the substrates and the catalyst surface, and the role of metal components is to dissociate hydrogen molecule heterolytically to give hydride and proton.

The development of sustainable chemical processes for the conversion of highly oxygenated biomass feedstocks to chemical products requires efficient and selective processes for partial oxygen removal and refunctionalization. Here we review the development of the deoxydehydration (DODH) reaction, which converts vicinal diols (glycols) to olefins. Uncatalyzed deoxygenative eliminations were first established. The catalyzed DODH reactions have largely employed oxo-rhenium catalysts and a variety of reductants, including PR₃, dihydrogen, sulfite, and alcohols. A variety of glycol and biomass-derived polyol substrates undergo the DODH reaction in moderate to good efficiency, regioselectively, and stereoselectively. Observations regarding selectivity, mechanistic probes, and computational studies support the general operation of a catalytic process involving three basic stages: glycol condensation to an M-glycolate, reduction of the oxo-metal, glycol condensation to produce a metal-glycolate, and alkene extrusion from the reduced metal-glycolate. Recent practical developments include the discovery of non-precious V- and Mo-oxo DODH catalysis.

The development of sustainable routes to fine chemicals, liquid fuels, and polymeric materials from natural resources has attracted significant attention from academia, industry, the general public, and governments owing to dwindling fossil resources, surging energy demand, global warming concerns, and other environmental problems. Cellulosic material, such as grasses, trees, corn stover, or wheat straw, is the most abundant nonfood renewable biomass resources on earth. Such annually renewable material can potentially meet our future needs with a low carbon footprint if it can be efficiently converted into fuels, value added chemicals, or polymeric materials. This chapter focuses on various renewable monomers derived directly from cellulose or cellulose platforms and corresponding sustainable polymers or copolymers produced therefrom. Recent advances related to the polymerization processes and the properties of novel biomass-derived polymers are also reviewed and discussed.

Lignin comprises 15–25% of terrestrial biomass and is the second most abundant source of renewable carbon after cellulose. However, its structural heterogeneity frustrates efforts for its selective conversion into biobased chemicals. Catalyst design for lignin transformation offers an opportunity to improve selectivity, and, hence, improve lignin’s utility as a raw material in chemical production. Catalytic deconstruction and conversion of lignin has been examined using a variety of thermochemical treatments, analogous to those used in the petrochemical industry. However, the complex nature of these products limits their utility. More recently, greater focus has been given to an understanding of lignin’s molecular level structure, and designing catalysts that can be targeted to key individual structural units within the biopolymer. This review gives a sense of the field by providing a representative description of recent developments in some of the primary technologies employed for lignin conversion and approaches that promise to improve selectivity.