Page 269 - Lignocellulosic Biomass to Liquid Biofuels

Fischer Tropsch synthesis of syngas to liquid hydrocarbons 225

with dried sample at 400°C for 8 h, which was pressed into pellets and
crushed and sieved to obtain 300 600 μm particles for FT synthesis in a
fixed-bed reactor. Luo and Davis [73] prepared Fe catalysts using an aque-
ous solution of ferric nitrate obtained from Fe (NO 3 ) 3 9H 2 O and tetra-
ethyl or thosilicate and mixed to attain a preferred Fe:Si ratio. The
mixture of iron nitrate and tetraethyl orthosilicate was added to a continu-
ously stirred tank reactor (CSTR) precipitation vessel together with a
stream of 30% ammonium hydroxide to achieve pH of 9.0. The mixture
from the CSTR was filtered using a vacuum drum filter and washed twice
with deionized water, and then it was dried for overnight at 110°C. Also,
the Fe:Si catalyst powder was impregnated in an appropriate amount of
aqueous K 2 CO 3 and Cu(NO 3 ) 2 solution to produce the desired atomic
ratio of Fe:Si:K:Cu (100:4.6:1.44:2.0). The final catalyst was dried for
overnight at 110°C and calcined at 350°C in an air flow for 4 h [73].
However, the choice and level of promoters are important for catalyst
preparation with low selectivity to CH 4 and high selectivity to heavy
hydrocarbon production with preferred olefin and oxygenate content in
the products. Also, higher level of promoter could have adverse effect on
catalyst activity; an optimum level is required [74 76]. Several promoters,
such as copper, manganese, silica, and potassium, are added for increased
activity and improved stability of iron catalyst. Copper improves the
reduction of iron oxide phase from hematite (Fe 2 O 3 ) to magnetite
(Fe 3 O 4 ) then further to iron metal or iron carbide [55]. Silica is a struc-
tural promoter used to stabilize the surface area and also has the chemical
effect on Fe catalyst properties [46,62,77]. The initial activity of FT syn-
thesis is reduced with increased manganese due to weak carburization.
Also, it is able to limit the reoxidation of Fe carbides to Fe 3 O 4 and aug-
ment additional carburization of the catalyst, which increase light olefin
selectivity. Potassium is reflected to promote CO adsorption and reduce
H 2 adsorption, which could enhance chain growth [75,78]. It increases
both the FT activity and the olefin yield via inhibition of secondary
hydrogenation reactions and decreases the CH 4 selectivity. Potassium
could also enhance the catalytic activity for FT synthesis and the rate of
WGS reactions [6,7,25,62 64,71].
The iron catalysts are used for two pathways selectivity toward the
desired products. One pathway supports the production of lower molecu-
lar weight olefinic hydrocarbons during FT synthesis, and it is known as
Sasol Synthol process. High reaction temperature (B340°C) promotes
Sasol Synthol process, and the average molecular weight of products is