467
2.3 Enol Mechanism
Storch et al. [32, 33] proposed the enol mechanism as an alternative path for product
formation wherein oxygen-containing species called enol were involved in the
mechanism. In the proposed mechanism, enol species form through hydrogenation
of adsorbed CO. Chain growth occurs through a condensation reaction between
oxygen-containing species (=CROH & = CHOH) with the release of a water molecule (Fig. 5). The occurrence of branched hydrocarbons is proposed to form with
the involvement of branched alcohol –CHROH surface species [5, 34]. In the final
step of chain growth, i.e., termination step, oxygenates (aldehydes and alcohols),
and α-olefins are formed through desorption. From the theory of the mechanism,
n- paraffin is not the primary product; it is a secondary product which forms by the
hydrogenation of primarily formed olefins. The primary formation of n-paraffin
would require an alternative reaction pathway. The mechanism gained strong support from Emmett et al. [35–39] through their results by detailed studies using
14
C-labeled alcohol in FT synthesis reaction. The
14
C-labeled alcohol was co-fed
with syngas, and the distribution of isotopically labeled product confirms that the
co-fed alcohol was able to initiate the chain growth.
2.4 CO Insertion Mechanism
The CO insertion mechanism was fully developed by Pichler, and Schulz [23], in
which adsorbed CO is proposed to be a monomer of the reaction. The chain initiates through the surface methyl group. The step of oxygen elimination from the
surface of oxygen-containing species is the crucial point at which it differentiates
the CO insertion mechanism from the alkyl mechanism. The chain propagates via
the CO insertion into the metal alkyl bond, yielding a surface acyl species (–C–
OR). Oxygen is eliminated from the acyl species in the form of water and enlarged
alkyl species (Fig. 6). The products, viz., n-paraffin and α-olefins paths are similar
to those proposed in the alkyl mechanism, i.e., hydrogenation or β-hydrogen elimination. Oxygen-containing species terminate directly to form alcohols and aldehydes. Hydrogen addition to an acyl species will lead to aldehyde formation. The
formation of ketones is proposed to occur through the addition of a surface alky
group to the acyl species. Many researchers reported the “CO insertion” mechanism as the particular pathway responsible for the formation of oxygenates
[40, 41].
Recent Advancements and Detailed Understanding of Kinetics for Synthesis Gas…
2.3 Enol Mechanism
Storch et al. [32, 33] proposed the enol mechanism as an alternative path for product
formation wherein oxygen-containing species called enol were involved in the
mechanism. In the proposed mechanism, enol species form through hydrogenation
of adsorbed CO. Chain growth occurs through a condensation reaction between
oxygen-containing species (=CROH & = CHOH) with the release of a water molecule (Fig. 5). The occurrence of branched hydrocarbons is proposed to form with
the involvement of branched alcohol –CHROH surface species [5, 34]. In the final
step of chain growth, i.e., termination step, oxygenates (aldehydes and alcohols),
and α-olefins are formed through desorption. From the theory of the mechanism,
n- paraffin is not the primary product; it is a secondary product which forms by the
hydrogenation of primarily formed olefins. The primary formation of n-paraffin
would require an alternative reaction pathway. The mechanism gained strong support from Emmett et al. [35–39] through their results by detailed studies using
14
C-labeled alcohol in FT synthesis reaction. The
14
C-labeled alcohol was co-fed
with syngas, and the distribution of isotopically labeled product confirms that the
co-fed alcohol was able to initiate the chain growth.
2.4 CO Insertion Mechanism
The CO insertion mechanism was fully developed by Pichler, and Schulz [23], in
which adsorbed CO is proposed to be a monomer of the reaction. The chain initiates through the surface methyl group. The step of oxygen elimination from the
surface of oxygen-containing species is the crucial point at which it differentiates
the CO insertion mechanism from the alkyl mechanism. The chain propagates via
the CO insertion into the metal alkyl bond, yielding a surface acyl species (–C–
OR). Oxygen is eliminated from the acyl species in the form of water and enlarged
alkyl species (Fig. 6). The products, viz., n-paraffin and α-olefins paths are similar
to those proposed in the alkyl mechanism, i.e., hydrogenation or β-hydrogen elimination. Oxygen-containing species terminate directly to form alcohols and aldehydes. Hydrogen addition to an acyl species will lead to aldehyde formation. The
formation of ketones is proposed to occur through the addition of a surface alky
group to the acyl species. Many researchers reported the “CO insertion” mechanism as the particular pathway responsible for the formation of oxygenates
[40, 41].
Recent Advancements and Detailed Understanding of Kinetics for Synthesis Gas…