Catalytic Strategies for Converting Lignocellulosic Carbohydrates to Fuels and Chemicals

The goal of biomass pretreatment is to efficiently recover and process carbohydrate polymers (and their respective monomeric sugars) comprising lignocellulosic biomass. Here, the use of these carbohydrates is considered as a sustainable feedstock for industrial carbon. Carbohydrates are markedly distinct from the fossil resources predominately used in the modern production of both fuels and chemicals. The implications of these differences are outlined, particularly in terms of upgrading strategies associated with each feedstock. Chemically, sugars are extensively functionalized molecules because each carbon in the molecule is bound to an oxygen atom. Compared to petroleum-sourced alkane feedstocks, this functionality imparts a generally low energy density and high reactivity; they therefore require processing strategies that differ from current petroleum-based approaches. If they are to yield presently available consumer products, carbohydrates must undergo extensive chemical transformations enabling fuel and chemical production. To achieve the desired transformations, there are multiple options available. Although a general overview is provided, the focus of this chapter is catalytic methods to achieve selective transformations of biomass-derived intermediates. Catalysis allows tailored control of chemical selectivity in upgrading strategies, which is imperative for achieving high-yield production of intermediates from over-functionalized feedstocks (sugars). Upgrading strategies typically proceed through multiple intermediate stages. In this regard, heterogeneous catalysis is particularly attractive because it enables process intensification through condensed strategies that couple multiple transformations in single reactors with straightforward catalyst recovery. Finally, solid catalysts are characteristically robust and may be applied over a broad range of reaction conditions. As such, they can expedite biorefining strategies by simultaneously allowing selective and rapid feedstock conversion at high temperatures and feed concentrations. A variety of catalytic pathways are considered by which the functionality of carbohydrates and derived intermediates may be reduced (e.g., hydrogenolysis to improve energy density for producing fuel additives) or increased (e.g., oxidation to facilitate production of specialty chemicals). This chapter assembles a general roadmap of the multiple options available for value-added processing of carbohydrates, offering readers both a historical view of advances in the field and a look forward to ways in which catalysis will continue to shape the advance of biorefining to a commercial scale.