the industry. Several catalysts deactivation routes are known.
These include fouling (i.e. the deposition of materials like coke that
cover the catalyst active sites), poisoning by the interaction of
catalyst with poisons such as gases, water, metals, etc. in the
reaction stream or produced during the reaction, catalyst
decomposition by heat (i.e. thermal degradation) and agglomeration
of the active catalyst particles by sintering process (i.e.
crystallite growth).
Catalyst deactivation during HDO/HDC of vegetable oils depend
on the nature of the catalyst and the compositions of the vegetable
oil feedstock. Noble metal catalysts are associated with low
thermal stability and could therefore be sintered at elevated
temperatures. At reaction temperatures reaching 500 8C, active Pd
or Pt particles could be sintered by agglomeration (i.e. crystallite
growth) [37]. This reduces accessibility be reactants due to the
reduction in the active catalyst surface area with a consequent
reduction in the yields of hydrocarbon products (particularly the
C5 to C15 alkanes required for jet fuels). The hydrodeoxygenation
(HDO), in particular, produces water molecules that can destroy
active noble metal catalyst sites by poisoning due to high
sensitivity to poisons of these catalysts [38]. Catalyst containing
sulfur such as CoMoS are more susceptible to deactivation by
impurities like fatty acids and lipid-based impurities or water in
the vegetable oil feeds [59]. These impurities strongly interact with
active CoMoS catalyst sites and caused poisoning with a great
negative consequence of lowering catalytic activity. Therefore,
reaction with refined vegetable oils would be a very good
alternative. Another problem with sulfur-containing catalysts is
the removal of the sulfur species at high temperature via
hydrogenation (i.e. interaction of catalyst with hydrogen employed
for the reaction). The H2S produced under this condition can also
serve as a catalyst poison. The poisoning normally occurs through
the strong interaction of these impurities. This trigger active sites
blockage, destroy geometric catalyst structure and consequently
cause activity loss. The reaction selectivity is usually shifted