Catalytic conversion of biomass to biofuels

TLDR

Biomass is a promising sustainable feedstock for energy, especially transportation, and its conversion to hydrocarbon fuels involves reducing oxygen content to improve energy density and forming C–C bonds to raise molecular weight, though hydrolysis‑based routes incur higher upstream costs yet allow selective production of hydrocarbons with targeted weights and structures. This review focuses on catalytic strategies for converting aqueous carbohydrate solutions derived from biomass pretreatment and hydrolysis into biofuels, and on managing hydrogen consumption to maintain fossil‑fuel independence. We review first‑generation biofuels (bioethanol, biodiesel), second‑generation technologies for lignocellulosic biomass (gasification, pyrolysis, liquefaction), and catalytic pathways such as sugar reforming to hydrogen, dehydration to furfurals or levulinic acid, and C–C bond formation via aldol condensation and alkene oligomerization to produce gasoline, jet, and diesel‑range hydrocarbons. The review concludes that controlling hydrogen usage is critical for achieving fossil‑fuel‑independent biorefining.

Abstract

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.