CRITICAL REVIEW www.rsc.org/greenchem | Green Chemistry Catalytic conversion of biomass to biofuels David Martin Alonso, Jesse Q. Bond and James A. Dumesic* Received 25th March 2010, Accepted 14th July 2010 DOI: 10.1039/c004654j Biomass has received considerable attention as a sustainable feedstock that can replace diminishing fossil fuels for the production of energy, especially for the transportation sector. The overall strategy in the production of hydrocarbon fuels from biomass is (i) to reduce the substantial oxygen content of the parent feedstock to improve energy density and (ii) to create C–C bonds between biomass-derived intermediates to increase the molecular weight of the final hydrocarbon product. We begin this review with a brief overview of first-generation biofuels, specifically bioethanol and biodiesel. We consider the implications of utilizing starchy and triglyceride feedstocks from traditional food crops, and we provide an overview of second-generation technologies to process the major constituents of more abundant lignocellulosic biomass, such as thermochemical routes (gasification, pyrolysis, liquefaction) which directly process whole lignocellulose to upgradeable platforms (e.g., synthesis gas and bio-oil). The primary focus of this review is an overview of catalytic strategies to produce biofuels from aqueous solutions of carbohydrates, which are isolated through biomass pretreatment and hydrolysis. Although hydrolysis-based platforms are associated with higher upstream costs arising from pretreatment and hydrolysis, the aqueous solutions of biomass-derived compounds can be processed selectively to yield hydrocarbons with targeted molecular weights and structures. For example, sugars can be used as reforming feedstocks for the production of renewable hydrogen, or they can be dehydrated to yield furfurals or levulinic acid. For each of the platforms discussed, we have suggested relevant strategies for the formation of C–C bonds, such as aldol condensation of ketones and oligomerization of alkenes, to enable the production of gasoline, jet, and Diesel fuel range hydrocarbons. Finally, we address the importance of hydrogen in biorefining and discuss strategies for managing its consumption to ensure independence from fossil fuels. 1. Introduction An important current focus of research in chemistry, engineer- ing, agriculture, and environmental policy is the development of clean technologies that utilize a sustainably produced feedstock to the largest extent possible. 1 This research is especially impor- tant in the transportation fuel sector which is strongly dependent on petroleum, a non-renewable fossil source of carbon. However as the worldwide supply of petroleum diminishes, it is becoming increasingly expensive and, accordingly, less attractive as a carbon source. Furthermore, the combustion of fossil fuels or their derivatives for the production of heat and power is associated with a net increase in greenhouse gas levels worldwide. 2–4 In contrast to the present situation, where the entirety of demand is met by a single source (i.e., petroleum), a more flexible system drawing from multiple energy sources should be an attractive long term solution. Vehicles powered by electricity, solar energy, hydrogen fuel cells, and biofuels are all being actively researched to reduce our dependence on petroleum as a source of energy. Nevertheless, these new technologies require time to be economically and technically viable. The situation is further exacerbated by the lack of Department of Chemical and Biological Engineering University of Wisconsin Madison, WI 53706 an infrastructure to support cutting-edge technologies like hydrogen fuel cells, and change will thus come slowly to a market currently governed by preferences and habits that are based on widespread availability of liquid hydrocarbon fuels. In this respect, liquid biofuels derived from renewable plant mass, are unique in their similarity to the currently preferred fuel sources. As such, their implementation does not require extensive changes to the transportation infrastructure and the internal combustion engine. Thus, the use of biomass as a renewable source of carbon for the production of transportation fuels is a promising alternative that is realizable on short time scales. For example, bioethanol and biodiesel are currently used commercially as blending agents for petroleum-derived gasoline and Diesel fuels. Presently in the petrochemical industry, crude oil is fraction- ated and refined to produce various grades of liquid transporta- tion fuel, and hydrocarbon feedstocks are functionalized to produce intermediates and speciality chemicals. The analogous concept of biorefining would be similar in scope, with the key difference being that biomass—rather than petroleum— would be utilized as a renewable source of carbon 5,6 that can be transformed into fuels and valuable chemicals within a single facility. Furthermore, in the production of heat and power, the utilization of biomass derivatives mitigates the release of greenhouse gas emissions through cycles of regrowth and combustion, 2,7 as illustrated in Fig. 1. This journal is © The Royal Society of Chemistry 2010 Green Chem., 2010, 12, 1493–1513 | 1493 Downloaded by University of Washington on 15 November 2010 Published on 03 September 2010 on http://pubs.rsc.org | doi:10.1039/C004654J View Online