464
2.1 Alkyl Mechanism
The Alkyl mechanism is accepted by a large number of the research groups and has
been used for deriving rate expressions for FTS reaction. The reaction initiation
starts with the dissociative adsorption of CO where the generated surface oxygen is
rejected as either water on reacting with hydrogen or CO 2 on reacting with CO. The
surface carbon indulges in the formation of the CHX intermediates, viz., CH–,
CH 2 –, CH 3 –, in the consecutive steps. Here –CH 3 is the chain initiator, and –CH 2 – is
chain propagation monomer. Hydrocarbons form via the successive insertion of
these intermediates. In the next step, the growing chain terminates either via
β-hydrogen elimination, yielding α-olefins, or hydrogenation producing alkanes
[20, 24]. The hydrogen removal is a reversible reaction that enables the olefins to
re-adsorb in the growing chain and takes part in the secondary product formation
(Fig. 3).
The alkyl mechanism was a continuation of the carbide mechanism, and Brady
and Pettit [20] experimentally confirmed the presence of the intermediates of the
proposed mechanism. The authors tried to convert diazomethane (CH 2 N 2 ) both in
the presence and absence of hydrogen and found that ethene was the only product
in case of conversion without hydrogen and that the presence of hydrogen yielded a
spectrum of hydrocarbons similar to that of FTS reaction products. This, in turn,
confirms that –CH 2 – acts as a monomer in the polymerization process and is
involved in long-chain hydrocarbon formation. The results also reveal that the
hydrogenation step is important to form chain starter species –CH 3 . The absence of
long-chained hydrocarbons in the earlier case where hydrogen was not in the feed
indicates that the CH to –CH 2 – formation is an irreversible step; otherwise, the
decomposition of adsorbed CH 2 species would yield CH and H, and the adsorbed H
could be further used as a hydrogenating agent to facilitate the long change hydrocarbon. However, this was not evident in the product of the first case; hence, the
transformation of CH to –CH 2 – can be taken as an irreversible step.
The FT synthesis reaction also produces some amount of branched alkane and
oxygenates. In the alkyl mechanism, the steps related to the branched alkanes and
alcohols formation were not considered. Initially, it was proposed that the branched
hydrocarbons result from olefin such as propene reinsertion; however, later, it was
concluded that the amount of branched hydrocarbons from experimental results is
more than expected from olefin reinsertion [20]. So the modification in the alkyl
mechanism was incorporated by Schulz et al. [25] by the addition of some separate
pathways. The additional path includes the reaction between surface methyl species
(–CH 3 ) with alkylidene species (R-HC=), which was responsible for branched
alkane formation. The probable formation path of alkylidene species is via the reaction of the alkyl group with surface methylidyne (≡CH). For the oxygenate formation, Johnston and Joyner [26] proposed the involvement of surface hydroxyl
groups. They reported that the surface hydroxyl group reacts with an alkyl group to
form the alcohols. However, there was a lack of experimental evidence for the same,
i.e., the participation of surface hydroxyl groups in the formation of oxygenates.
Sonal et al.
2.1 Alkyl Mechanism
The Alkyl mechanism is accepted by a large number of the research groups and has
been used for deriving rate expressions for FTS reaction. The reaction initiation
starts with the dissociative adsorption of CO where the generated surface oxygen is
rejected as either water on reacting with hydrogen or CO 2 on reacting with CO. The
surface carbon indulges in the formation of the CHX intermediates, viz., CH–,
CH 2 –, CH 3 –, in the consecutive steps. Here –CH 3 is the chain initiator, and –CH 2 – is
chain propagation monomer. Hydrocarbons form via the successive insertion of
these intermediates. In the next step, the growing chain terminates either via
β-hydrogen elimination, yielding α-olefins, or hydrogenation producing alkanes
[20, 24]. The hydrogen removal is a reversible reaction that enables the olefins to
re-adsorb in the growing chain and takes part in the secondary product formation
(Fig. 3).
The alkyl mechanism was a continuation of the carbide mechanism, and Brady
and Pettit [20] experimentally confirmed the presence of the intermediates of the
proposed mechanism. The authors tried to convert diazomethane (CH 2 N 2 ) both in
the presence and absence of hydrogen and found that ethene was the only product
in case of conversion without hydrogen and that the presence of hydrogen yielded a
spectrum of hydrocarbons similar to that of FTS reaction products. This, in turn,
confirms that –CH 2 – acts as a monomer in the polymerization process and is
involved in long-chain hydrocarbon formation. The results also reveal that the
hydrogenation step is important to form chain starter species –CH 3 . The absence of
long-chained hydrocarbons in the earlier case where hydrogen was not in the feed
indicates that the CH to –CH 2 – formation is an irreversible step; otherwise, the
decomposition of adsorbed CH 2 species would yield CH and H, and the adsorbed H
could be further used as a hydrogenating agent to facilitate the long change hydrocarbon. However, this was not evident in the product of the first case; hence, the
transformation of CH to –CH 2 – can be taken as an irreversible step.
The FT synthesis reaction also produces some amount of branched alkane and
oxygenates. In the alkyl mechanism, the steps related to the branched alkanes and
alcohols formation were not considered. Initially, it was proposed that the branched
hydrocarbons result from olefin such as propene reinsertion; however, later, it was
concluded that the amount of branched hydrocarbons from experimental results is
more than expected from olefin reinsertion [20]. So the modification in the alkyl
mechanism was incorporated by Schulz et al. [25] by the addition of some separate
pathways. The additional path includes the reaction between surface methyl species
(–CH 3 ) with alkylidene species (R-HC=), which was responsible for branched
alkane formation. The probable formation path of alkylidene species is via the reaction of the alkyl group with surface methylidyne (≡CH). For the oxygenate formation, Johnston and Joyner [26] proposed the involvement of surface hydroxyl
groups. They reported that the surface hydroxyl group reacts with an alkyl group to
form the alcohols. However, there was a lack of experimental evidence for the same,
i.e., the participation of surface hydroxyl groups in the formation of oxygenates.
Sonal et al.