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Another alternative is to hydrogenate ketones to produce alcohols that can be
dehydrated to produce nonene which can be coupled by acid-catalyzed oligomerization
[32,57]. Smaller ketones can be converted to alkenes, which also undergo oligomerization to produce the final mixture of C 6 –C 27 alkenes that can be hydrogenated over
Pt/Nb 2 O 5 to produce liquid alkenes to be used as jet fuels or diesel blenders [29–42].
Bond et al. [58] reported that GVL can undergo ring opening to produce pentenoic
acid and isomers which subsequently undergo decarboxylation to produce equimolar
mixture of butenes and carbon dioxide. Both reactions occur on solid acid catalyst SiO 2 /
Al 2 O 3 . The butene monomers products can be coupled by oligomerization over an acid
catalyst to form C 8
+ alkenes that can be converted to jet fuels upon hydrogenation. More
details of this reaction chemistry are described by Alonso et al. [32].
7.3.3 FurFuryl And hydroxymeThyl FurFuryl
Furfuryl is produced from the hemicellulose pentose fractions of biomass. xylose
is the predominant pentose and hemicellulosic arabinose is found to a lesser extent
in most of the feedstock. Furfuryl can be sold as a solvent or converted to furfuryl
alcohol, which in turn can be converted to THF and LA as shown by Hayes et al. [8].
Furfuryl alcohol is a monomer of furan resins that are mainly used as foundry binders. It is produced by hydrogenation of furfuryl. THF is produced by decarbonylation
of furfuryl to furan followed by catalytic hydrogenation [40]. Furfuryl alcohol, when
boiled in ethyl methyl ketone in the presence of HCl, gives rise to 90%–93% yield
of LA [17].
HMF and furfuryl are also precursors of liquid hydrocarbon fuels and are an
option for the production of linear alkanes in the molecular weight range appropriate for diesel and jet fuels. Since furans can be produced from both cellulose
and hemicellulose, they utilize the larger fraction of available lignocellulosic feedstock. Furfuryl and HMF can be produced with good selectivity (90%) from xylose
and fructose in biphasic reactors; the yields for glucose are lower. The addition of
dimethyl sulfoxide (DMSO) improves the selectivity of HMF from fructose. In the
presence of water, HMF is readily hydrated to LA and formic acid. Furfuryl can
be extracted from water using solvents such as THF, butanol, and methyl isobutyl
ketone (MIBK), and by adding salts to the aqueous phase [8,32].
Dumesic et al. [29–42] have shown different strategies to upgrade HMF to liquid fuels. HMF can be converted to DMF over Cu–Ru/C catalyst by hydrogenolysis. DMF can be used as a blender in transportation fuels. Higher hydrocarbons
are produced by aldol condensation with ketones. Single condensation of HMF
produces C 9 intermediates that can react with HMF again to produce C 15 intermediates [32]. The condensation products are hydrogenated and dehydrated over a
bifunctional catalyst with metal and acid sites to produce linear C 9 or C 15 alkanes
that can be easily separated from water [32]. Aldol condensation can be coupled
with hydrogenation steps using a bifunctional catalyst such as Pd/MgO–ZrO 2
leading to high yields of condensation products at 326–353 K [59]. The selective
hydrogenation of HMF and furfuryl can also be converted to C 12 and C 10 alkanes
through a series of reaction steps involving self-condensation and hydrogenation/
dehydration, respectively [32].