that were at least five times those obtained with 0.2 wt.%Pt/
H-ZSM-5. Therefore, structural effects are critical during the
hydroisomerization reaction [130].
Conclusions
Hydrodeoxygenation (HDO) and hydrodecarboxylation (HDC)
are the first steps for converting the vegetable oils into mostly
n-alkanes suitable for upgrade to biojet fuels. Noble metals were
the earlier catalysts to be tested and later replaced with other
transition metals-based catalysts, due to many reasons including
catalyst deactivation by poisoning, production of cracking species
and process costs. Reaction mechanism over most of the catalyst
systems proceeds by serial hydrogenation of unsaturated glycerides
followed by simultaneous HDO and HDC reactions. A number
of factors have been identified to affect the catalytic activity and
selectivity to alkane compounds. The nature of the vegetable oil
feedstock is a very critical factor. Oils with low degree of
unsaturation (e.g. olive oil) are more favorable. Catalyst supports
with moderate acidity like Al2O3 yield excellent upgrading
properties at moderate reaction temperatures closer to 300 8C.
Modification of active metal catalyst sites by incorporation of a
second or a third metal reduces the chance of catalyst deactivation,
retard cracking and favor alkanes yield by synergism. These
properties are superior of NiMo-based catalysts even without
sulfidation and at low hydrogen pressures. However, Co containing
catalysts like MoCo, MoCoS could be deactivated by poisoning,
fouling or loss of sulfur species. With the NiMo-based catalysts,
complete conversion of vegetable oils with excellent selectivity to
alkanes and suppressed side reactions can be achieved, especially
when the nano-structured materials are appropriately employed.
However, devoted effort is still necessary to clearly establish the
actual reaction mechanism and the main active sites of each catalyst
system under consideration. This can be a way forward to achieving
optimal performance. Although some catalyst regeneration methods
that were at least five times those obtained with 0.2 wt.%Pt/H-ZSM-5. Therefore, structural effects are critical during thehydroisomerization reaction [130].ConclusionsHydrodeoxygenation (HDO) and hydrodecarboxylation (HDC)are the first steps for converting the vegetable oils into mostlyn-alkanes suitable for upgrade to biojet fuels. Noble metals werethe earlier catalysts to be tested and later replaced with othertransition metals-based catalysts, due to many reasons includingcatalyst deactivation by poisoning, production of cracking speciesand process costs. Reaction mechanism over most of the catalystsystems proceeds by serial hydrogenation of unsaturated glyceridesfollowed by simultaneous HDO and HDC reactions. A numberof factors have been identified to affect the catalytic activity andselectivity to alkane compounds. The nature of the vegetable oilfeedstock is a very critical factor. Oils with low degree ofunsaturation (e.g. olive oil) are more favorable. Catalyst supportswith moderate acidity like Al2O3 yield excellent upgradingproperties at moderate reaction temperatures closer to 300 8C.Modification of active metal catalyst sites by incorporation of asecond or a third metal reduces the chance of catalyst deactivation,retard cracking and favor alkanes yield by synergism. Theseproperties are superior of NiMo-based catalysts even withoutsulfidation and at low hydrogen pressures. However, Co containingcatalysts like MoCo, MoCoS could be deactivated by poisoning,
fouling or loss of sulfur species. With the NiMo-based catalysts,
complete conversion of vegetable oils with excellent selectivity to
alkanes and suppressed side reactions can be achieved, especially
when the nano-structured materials are appropriately employed.
However, devoted effort is still necessary to clearly establish the
actual reaction mechanism and the main active sites of each catalyst
system under consideration. This can be a way forward to achieving
optimal performance. Although some catalyst regeneration methods
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