catalysts produced nearly 100% conversion with product selectivity
reaching 90%. The catalysts were stable for 10 h with negligible
coke deposition. Therefore, although catalyst regeneration strategies
are available for restoring the activity of coked catalyst,
preventing the deactivation in situ will be more economical for the
industry. The enhanced activity and stability properties demonstrated
by the metals and phosphorous modifiers present an
alternative for improving the prospects of the overall process.
However, further studies are still essential to fully establish the
most appropriate modifier loading suitable for commercial
applications and the applicable reaction conditions.
Recent HDO and HDC studies with other transition metals
A summary of some recent studies on the behaviors of the
transition metals-based catalysts during the hydroprocessing of
some vegetable oils under variable conditions has also been
presented in Table 2 [76–83]. In all the cases, the reactions proceed
via the HDO/HDC mechanism, with the side reactions being mainly
of methanation and water-gas types. The main reaction products
were n- and i-alkanes with carbon chains in the range of C7 to C18
(i.e. potentials for biojet fuels). Among these, the higher alkanes
(i.e. C16 to C18) could be successfully cracked to lighter liquid
alkanes with 15 or less-carbon numbers over hydrocracking
catalysts like the acidic zeolites prior to hydroisomerization. It
could be established that, catalytic activity was dependent on
several factors that include the nature of the catalyst, operation
conditions, nature of the support, metal loading and in some
instances the incorporation of a second or third metal. According to
Srifa and co-workers [76], increasing the hydrogen/oil ratio from
1000 to 2000 led to the formation of mainly liquid alkanes (i.e.
selectivity increased from 0 to 100%). At the lower ratio of 1000,
only solid alkanes were produced (28%). These compounds can
however be converted into the C5 to C15 alkanes by hydrocracking
with zeolite catalysts. NiMo catalysts supported with Al2O3 have
generally demonstrated good activity toward alkanes’ production,
with limited degree of side reactions than supported CoMo
catalysts. Modification of Al2O3 with B2O3 may have a negative
effect on the alkanes yield. Similarly, SiO2 being less acidic was
more favorable to production of alkanes than H-Y for the Ni2P
catalyst under constant conditions. According to Zarchin et al. [77],
with 25 wt.% Ni2P/SiO2, the soybean oil yields 84% of desired
alkanes compared to 78% obtained with 25 wt.% Ni2P/H-Y catalyst.
Promotion of Ni with Mo or W produces a comparable activity
under constant conditions. Modification of NiMo/Al2O3 system
with 1–25 wt.% of Ce showed slight modification to the hydrotreating
properties of jatropha oil. At both low and high loadings
(i.e. 1–25 wt.%), the selectivity to alkanes was in the range of 89–
91% [82]. Lowering the reaction temperature from 320 to 280 8C
had a positive effect on the production of alkanes from palm oil
using 7 wt.%Ni/SAPO-11 catalyst. It could be seen that, the
selectivity increased from 44 to 79% when the temperature was
reduced from 320 to 280 8C [83]. A similar trend could be observed
by increasing the loading of Ni particles. When the loading was
increased from 2 to 7 and 9 wt.%, the selectivity to alkanes
increased from 60 to 67 and 68%, respectively [79].
Catalyst deactivation during HDO/HDC of vegetable oils
Catalyst deactivation involved the decay of catalytic performance
(i.e. activity) or the associated reduction in selectivity to
desired reaction products over time. The mode of the deactivation
process is usually dependent on the reaction being carried out.
Catalyst replacement usually posed a great challenge to refineries
due to the costs involved. Therefore, understanding the processes
of catalyst deactivation and the appropriate strategies for
mitigating the catalyst deactivation would be very beneficial for