Producing jet fuel from biomass lignin: Potential pathways to alkyl-benzenes and cycloalkanes

Jet turbine fuels have not been completely replaced by biomass-derived fuels due, in part, to the lack of the aromatic and cycloalkane hydrocarbons in bio-jet fuels. Such molecules play a critical role in traditional jet fuels for combustion characteristics and material compatibility. To date, this problem has been addressed by blending bio-jet fuel with traditional petroleum-based oils. In order to produce a 100% biomass-derived jet fuel, a suitable bioresource is needed for synthesizing aromatic and cycloalkane fuel compounds. Lignin, the only biomass component rich in aromatic benzene ring structures, is currently underutilized for low-value heat or treated as process waste because of its high chemical resistance. In this review, a four-step pathway of pretreatment, depolymerization, hydrodeoxygenation (HDO), and alkylation to convert lignin into jet-fuel-range aromatic hydrocarbons and cycloalkanes is explored. While many studies about these processes can be found in the literature for biomass and bio-derived molecules, the purposes of those studies were often for something other than lignin valorization. For example, pretreatment processes focus on cellulose and hemicellulose as desired products while lignin is handled as a waste. Most HDO studies focus on the upper phase of bio-oil which contains primarily water and phenolic monomers, ignoring the bottom phase which contains the larger, main components from depolymerization. Many alkylation studies used single, pure components; by comparison, the feedstocks from HDO of lignin-derived bio-oil are mixtures of varying complexity. This review of each process can be used to guide pathway integration and optimization.