432
• Carbide Mechanism: This mechanism initiates FT reaction by the adsorption of
carbon monoxide gas on the surface of the active metal catalyst that results in the
formation a metal carbide on the catalyst surface. This chemisorbed carbon monoxide, a C–O bond bridged between metal carbide nodes on the catalyst surface,
dissociates into C and O species. In the next step, hydrogen molecules (H–H)
dissociate to form H atoms which are used in the hydrogenation of the carbide
groups to produce the first C 1 intermediate methyl species (–CH 2 –). The methyl
species can react with hydrogen atoms to form methane gas or act as alkyl chain
extenders (monomeric units) to form the final product. The formation of this
intermediate removes oxygen in the form of water and has been assumed to be
the suitable mechanism for oxygen removal since the FT synthesis was first
demonstrated.
• CO-Insertion Mechanism: In this mechanism, a second carbon monoxide molecule inserts between the metal and initially chemisorbed carbon monoxide molecule, i.e., metal-alkyl bond. This step is the key process in forming C–C
intermediate which then undergoes hydrogenation and polymerization to form
an alkyl chain. This mechanism mostly explains FT synthesis carried out by iron
and ruthenium-based catalysts.
• Hydroxy-Carbene Mechanism: As the name implies, this mechanism proceeds
via a hydroxy carbene (CHOH) intermediate formed during the chain growth
process in the reaction. The first chemisorbed CO on the metal will undergo a
hydrogenation reaction to form the hydroxy carbene intermediate. Condensation
of two intermediates removes oxygen as water in the coupling reaction to form a
C–C bond. The surface oxidation of this intermediate propagates chain growth in
the reaction. The chain growth termination of this reaction pathway consequently
produces mostly oxygenated hydrocarbons such as aldehydes, ketones, ethers,
and alcohols.
1.2.1 Adsorption Mechanisms in FT Reaction Kinetics
It is worth noting that different mechanisms have been proposed to predict the possible routes by which the FT reaction could proceed. The desired product selectivity
and optimal reactor design require an in-depth understanding of reaction kinetics.
However, an understanding of the kinetics of this complex reaction system is fairly
difficult, and not much kinetic studies for FT synthesis in microreactors have been
reported. The kinetic performance evaluation of the catalyst requires rate expression
models that account for the consumption of reactants considering product distribution in the reaction system. The Anderson-Schulz-Flory (ASF) model, which is
based on the statistical distribution of the FT products, as governed by the chain
growth probability, is typically used to predict the product distribution of the FT
reaction. These kinetic models are usually developed by collecting extensive experimentation data by varying conditions like space velocity, temperature, pressure,
reactant flowrates, type of catalyst, syngas molar ratio with eliminating heat and
mass transfer losses. All the kinetic models based on the chain propagation and
N. Mohammad et al.
• Carbide Mechanism: This mechanism initiates FT reaction by the adsorption of
carbon monoxide gas on the surface of the active metal catalyst that results in the
formation a metal carbide on the catalyst surface. This chemisorbed carbon monoxide, a C–O bond bridged between metal carbide nodes on the catalyst surface,
dissociates into C and O species. In the next step, hydrogen molecules (H–H)
dissociate to form H atoms which are used in the hydrogenation of the carbide
groups to produce the first C 1 intermediate methyl species (–CH 2 –). The methyl
species can react with hydrogen atoms to form methane gas or act as alkyl chain
extenders (monomeric units) to form the final product. The formation of this
intermediate removes oxygen in the form of water and has been assumed to be
the suitable mechanism for oxygen removal since the FT synthesis was first
demonstrated.
• CO-Insertion Mechanism: In this mechanism, a second carbon monoxide molecule inserts between the metal and initially chemisorbed carbon monoxide molecule, i.e., metal-alkyl bond. This step is the key process in forming C–C
intermediate which then undergoes hydrogenation and polymerization to form
an alkyl chain. This mechanism mostly explains FT synthesis carried out by iron
and ruthenium-based catalysts.
• Hydroxy-Carbene Mechanism: As the name implies, this mechanism proceeds
via a hydroxy carbene (CHOH) intermediate formed during the chain growth
process in the reaction. The first chemisorbed CO on the metal will undergo a
hydrogenation reaction to form the hydroxy carbene intermediate. Condensation
of two intermediates removes oxygen as water in the coupling reaction to form a
C–C bond. The surface oxidation of this intermediate propagates chain growth in
the reaction. The chain growth termination of this reaction pathway consequently
produces mostly oxygenated hydrocarbons such as aldehydes, ketones, ethers,
and alcohols.
1.2.1 Adsorption Mechanisms in FT Reaction Kinetics
It is worth noting that different mechanisms have been proposed to predict the possible routes by which the FT reaction could proceed. The desired product selectivity
and optimal reactor design require an in-depth understanding of reaction kinetics.
However, an understanding of the kinetics of this complex reaction system is fairly
difficult, and not much kinetic studies for FT synthesis in microreactors have been
reported. The kinetic performance evaluation of the catalyst requires rate expression
models that account for the consumption of reactants considering product distribution in the reaction system. The Anderson-Schulz-Flory (ASF) model, which is
based on the statistical distribution of the FT products, as governed by the chain
growth probability, is typically used to predict the product distribution of the FT
reaction. These kinetic models are usually developed by collecting extensive experimentation data by varying conditions like space velocity, temperature, pressure,
reactant flowrates, type of catalyst, syngas molar ratio with eliminating heat and
mass transfer losses. All the kinetic models based on the chain propagation and
N. Mohammad et al.