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.

Efficient synthesis of renewable fuels remains a challenging and important line of research. We report a strategy by which aqueous solutions of gamma-valerolactone (GVL), produced from biomass-derived carbohydrates, can be converted to liquid alkenes in the molecular weight range appropriate for transportation fuels by an integrated catalytic system that does not require an external source of hydrogen. The GVL feed undergoes decarboxylation at elevated pressures (e.g., 36 bar) over a silica/alumina catalyst to produce a gas stream composed of equimolar amounts of butene and carbon dioxide. This stream is fed directly to an oligomerization reactor containing an acid catalyst (e.g., H ZSM-5, Amberlyst-70), which couples butene monomers to form condensable alkenes with molecular weights that can be targeted for gasoline and/or jet fuel applications. The effluent gaseous stream of CO2 at elevated pressure can potentially be captured and then treated or sequestered to mitigate greenhouse gas emissions from the process.

Reaction pathways and kinetics governing the Rucatalyzed hydrogenation of levulinic acid (LA) in the aqueous phase to form γ-valerolactone (GVL) were considered in a packed bed reactor. GVL can be produced by two distinct hydrogenation pathways; however, over Ru/C at temperatures below 423 K, it forms exclusively via intramolecular esterification of 4-hydroxypentanoic acid (HPA). Over Ru/C, reasonable hydrogenation rates of LA to HPA were observed at near-ambient temperatures (e.g., 0.08 s −1 at 323 K), but GVL selectivities are poor (<5%) under these conditions. Apparent barriers for LA hydrogenation and HPA esterification are 48 and 70 kJ mol −1 , respectively, and GVL selectivity improves at higher temperatures alongside increasing mass transfer limitations in 45−90 μm catalyst particles. Reactivity and selectivity trends in LA hydrogenation below 343 K are well-described by an empirical kinetic model capturing sequential hydrogenation and esterification. Coupling stacked beds of Ru/ C and Amberlyst-15 delivers high GVL yields (∼80%) at near ambient temperatures (323 K) and practical residence times.

Experimental and technoeconomic analysis of a catalytic process for the conversion of whole biomass into jet fuel.

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γ-Valerolactone (GVL) has been identified as a promising, sustainable platform molecule that can be produced from lignocellulosic biomass. The chemical flexibility of GVL has allowed the development of a variety of processes to prepare renewable fuels and chemicals. In the present work involving a combination of computational and experimental studies, we explore the factors governing the ring-opening of GVL to produce pentenoic acid isomers, as well as their subsequent decarboxylation over acid catalysts or hydrogenation over metal catalysts. The ring-opening of GVL has shown to be a reversible reaction, while both the decarboxylation and hydrogenation reactions are irreversible and kinetically controlled under the conditions studied (temperatures from about 500 to 650 K). The most significant contributor to lactone reactivity toward ring-opening is the size of the ring, with γ- lactones being more stable and less readily opened than δ- and ε-analogues. We have observed that the presence of either a C═C double bond or a lactone (which opens to form a C═C double bond) is necessary for appreciable rates of decarboxylation to occur. Olefinic acids exhibit higher rates of decarboxylation than the corresponding lactones, suggesting that the decarboxylation of alkene acids provides a lower energy pathway to olefin production than the direct decarboxylation of lactones. We observe lower rates of decarboxylation as the chain length of alkene acids increases; however, acrylic acid (3-carbon atoms) does not undergo decarboxylation at the conditions tested. These observations suggest that particular double bond configurations yield the highest rates of decarboxylation. Specifically, we suggest that the formation of a secondary carbenium ion in the β position leads to high reactivity for decarboxylation. Such an intermediate can be formed from 2- or 3-alkene acids which have at least four carbon atoms.

Alkylphenol solvents allow a more effective production of biofuels from corn stover by enabling selective extraction and hydrogenation of levulinic acid to γ‐valerolactone, and by increasing the final concentration of GVL through successive extraction/hydrogenation steps. The versatility of alkylphenol solvents may lead to their use in other biomass conversion processes utilizing mineral acids for biomass deconstruction.

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