Page 272 - Lignocellulosic Biomass to Liquid Biofuels

228 Lignocellulosic Biomass to Liquid Biofuels

increase hydrogenation reaction of CO. The addition of low-quantity
ruthenium could increase Co reduction and enhance hydrogen turnover
frequency (TOF). Furthermore, inclusion of platinum and palladium
improves Co reduction, while it reduces the TOF [91]. In general, the
addition of platinum as promoter remarkably enhances the FT synthesis
process and methane production but reduces the C 51 production [92].
Also, rhenium is frequently used as promoter for Co catalysts, which
enhances the diffusion of Co supported on TiO 2 by avoiding the cluster
of CoO x [53]. Storsæter et al. [93] reported that rhenium doping on CO
catalysts could promote selectivity of C 51 hydrocarbons and increase the
production rate.
However, reaction rate of Co catalysts could be increased by the addi-
tion of certain metal oxides during FT synthesis [25,47,53]. The addition
of ZrO 2 metal oxide as promoter increased the C 51 selectivity and con-
sumption of CO during FT synthesis with Co/SiO 2 catalysts [95].
Jongsomjit et al. [96] observed that the addition of ZrO 2 on a 20 wt.%
Co/Al 2 O 3 saturated γ-Al 2 O 3 and enhanced the reducibility of cobalt by
avoiding the origination of aluminate on catalysts surface. They also
reported that a higher amount of zirconium increased the olefin:paraffin
ratio in C 2 C 17 and reduced the hydrogen production ability of cobalt
catalysts, which was promoted by ZrO 2 [87]. It was also reported in litera-
ture that the addition of manganese could enhance the reaction rate of FT
synthesis with higher CO conversion [97]. The paraffin-to-olefin ratio in
hydrocarbons could be significantly improved by supplement with a large
amount of manganese in Mn Co/TiO 2 . The inclusion of MnO x
decreased the CH 4 selectivity and increased the C 51 selectivity, which
could be attained by the selection of the optimum preparation method
[36,97,98].
Furthermore, the rare earth oxides could be useful as promoters for
cobalt-based catalysts during FT synthesis. The supplementation of La 2 O 3
to 20 wt.% Co/SiO 2 increased the Co time yield and decreased the CH 4
and C 2 C 4 hydrocarbons selectivity. The increasing ratio of La/Co ratio
enhanced the olefin to paraffin ratio in C 2 C 5 hydrocarbons [88,89]. The
presence of La 2 O 3 could not modify the inherent properties of the cobalt
species but probably increased the concentration of active sites. The
La 2 O 3 -promoted Co/Al 2 O 3 catalysts improved the selectivity toward
hydrocarbons, while the formation of olefins was increased with the
increase in La content. However, a very high amount of La 2 O 3 loading
could increase the CH 4 selectivity by decreasing the C 51 selectivity.