Catalytic Strategies for the Conversion of Alternative Feedstocks to Fuels and Chemicals Via Selective C-C Bond Formation and Oxygen Removal

Current economic, environmental, and geopolitical issues may detrimentally affect the future of sustainable production of liquid transportation fuels from petroleum.  One possible approach to help ameliorate these effects is to synthesize gasoline, diesel, and jet fuel-range hydrocarbon compounds from alternative feedstocks such as coal, natural gas, and biomass.  Catalytic processing of these feedstocks requires high selectivity to the desired hydrocarbons in a limited number of steps to be cost-competitive with petroleum refining, and herein, we present catalytic systems for the conversion of functional molecules and H₂/CO streams which can be derived from natural gas, coal, and/or biomass into hydrocarbons which comprise current gasoline, diesel, and jet fuel.

One approach consists of a two-step process involving initial removal of most of the oxygen from biomass-derived carbohydrates on Pt-Re/C catalysts at modest temperatures (~520 K) and pressures (5-20 bar).  In this step, sorbitol and glucose are converted to a mixture of hydrophobic molecules consisting of monofunctional hydrocarbon intermediates such as alcohols, ketones, carboxylic acids, and heterocyclic compounds with 4-6 carbon atoms.  The H₂ necessary for this initial oxygen removal is produced in situ via reforming of a portion of the carbohydrate to H₂/CO_x gas streams.  These monofunctional hydrocarbons retain the necessary functionality such that they may be used directly in the chemical industry as chemical intermediates or solvents.  Alternatively, these molecules may be upgraded to a variety of hydrocarbons with the appropriate molecular weight, energy content, and combustion properties for liquid fuel applications via selective C-C bond formation combined with removal of the remaining oxygen.  Strategies for this subsequent C-C bond formation include aromatization/alkylation on acid catalysts to produce gasoline range hydrocarbons, aldol-condensation of ketones and secondary alcohols on a bi-functional metal-base catalyst to produce diesel and jet fuels, and ketonization of organic acids to ketones on a mixed metal oxide followed by aldol-condensation to produce diesel and jet fuels. 

An additional approach for the conversion of alternative feedstocks to fuels is processing of light oxygenates derived from biomass, coal, and natural.  These light oxygenates can be derived from biomass conversion processes similar to step one of the previous approach or from initial conversion of biomass/coal/natural gas to H₂/CO followed by synthesis to light oxygenates.  The upgrading of light oxygenates occurs via co-homologation reactions with alkane byproducts from petroleum refining on solid acid catalysts (such as zeolites) at low temperatures (~473 K) and pressures (total pressure of 1 bar).  Specifically, low value light alkanes, such as isobutane, can be upgraded to gasoline range alkanes via co-homologation with dimethyl ether in the presence of adamantane.  Selective C-C bond formation occurs between alkene species derived from the alkane co-feed and methylating species derived from dimethyl ether.  High selectivity to 2,2,3-trimethylbutane (research octane number = 112) is achieved as a result of a chain growth pathway involving preferential methylation of growing chains leading to structures that cannot readily change chain length by cracking or isomerization.  Adamantane acts a dehydrogenation/hydrogenation co-catalyst that facilitates the conversion of the alkane co-feed to alkenes which serve as primary chain growth centers.

The strategies presented here represent advancements toward the production of fuels and chemicals from biomass, coal, and natural gas by achieving high selectivity to various types of hydrocarbon fuels in a limited number of flow reactors.