(5ci) 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.