Page 270 - Lignocellulosic Biomass to Liquid Biofuels

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comparatively lower because liquid phase does not occur under reaction
conditions. The small catalyst particles (B100 μm diameter) are exciting
in the reactor and carbon deposition on the catalyst surface does not inter-
rupt operation process. The second pathway of Fe catalyst supports the
highest catalyst activity where major hydrocarbon is in the liquid phase
under low reaction temperature (B230°C). The low-temperature reac-
tion supports paraffin production and could be further processed for
obtaining pure products. Thus iron catalysts are very flexible and could
produce a wide variety of petroleum products, such as olefin and paraffins
[6,54,55].

7.3.2 Role of Co catalyst

FT synthesis using the Co catalysts shows high activity up to 60% 70%
conversion in a single process, along with significantly high stability and
selectivity of linear hydrocarbons, and it has insignificant activity on CO
conversion. FT synthesis over Co catalysts increases the CH 4 formation at
higher temperatures, which limits its application at lower temperatures
respect to the Fe catalysts [54,79 81]. The C 2 C 4 and CH 4 selectivity
are higher over Co catalysts at 250°C or above temperatures, while the
CH 4 selectivity is less than 15% at 200°C and below. However, pressure
plays a crucial role in FT synthesis for products selectivity [6]. The deacti-
vation and regeneration of Co catalysts is challenging, which involves
sequential treatment of hydrogen and steam or solvent wash [82,83], and
the deactivation is a major challenge due to high costs of Co-based cata-
lysts [84]. Oxidation of Co forms H 2 O and CO 2 and leads to carbon
deposition on the catalysts surface, which is the major source of deactiva-
tion of the catalyst though long-term effect on Co-based catalysts [85].
The metal-supported Co-synthesized catalysts could improve the activity
and the extensive life of the catalysts, together with the products selectiv-
ity. However, the performance and structure of Co catalysts depend on
the type of catalytic support material. The key function of the catalyst
support is to diffuse Co and produce stable Co metal particles in the cata-
lysts after reduction [25,86].
Several studies have demonstrated that the addition of noble metals
and transition metal oxides as distinctive promoters, such as Pt, Ru, and
Re, can increase the reduction of Co and known as active stage of FT
process. Also, the properties of different promoters are summarized in
Table 7.3. Tsubaki et al. [91] found that the addition of Co/SiO 2 could