Hydrocarbons obtained through FTS from a mixture of CO and H₂, also known as synthesis gas, are one of the promising sources of hydrocarbon feedstocks for further use in chemical and petrochemical industries [1,2,3,4]^[1][2][3][4]. The composition of hydrocarbon mixtures depends on both the catalyst properties and the process parameters (Figure 1). The products of FTS usually follow the molecular weight distribution (MWD) [1,2^{[1][2][3][4][5][6]},3,4,5,6], which is very wide and nonselective for any products being determined by the competition between the processes of growth and hydrocarbon chain termination. For example, high molecular weight n-paraffins are selectively formed in the presence of the Co-based catalysts on alumina, silica or titania supports. Such a mixture of hydrocarbons should be further processed at high temperature and pressure in excess of hydrogen for future use.

An alternative process is FTS aimed at the direct production of liquid hydrocarbons (synthetic oil) from CO and H₂ [6,7,8,9]^[6][7][8][9]. The idea of combining metal sites active in FTS with acid sites of zeolite active in the cracking and isomerization of hydrocarbons was proposed in the middle of 1970s and continues to be of keen interest to the scientific community [6,7,8,9,10,11,12,13,14,15,16]^{[6][7][8][9][10][11][12][13][14][15][16]}. Such catalysts are routinely referred to as bifunctional or hybrid. The composition of the hydrocarbons obtained in their presence depends on the relative rates of reactions on acid and metal sites. The latter depend on concentrations of metal and zeolite sites, their location relative to each other and properties of the support pore system. Therefore, one has to consider the role of mass transfer while developing new bifunctional FTS catalysts ^[17]. In order to provide intensive mass transfer on the surface of the pellet, it is necessary to reduce diffusion limitations or average molecular weight of the formed hydrocarbons. In the first case, an extended system of macro- and meso-pores is necessary; in the second case, a source of atomic hydrogen or an additional function of the catalyst, —for example, provided by sites with Bronsted acidity—is necessary.

In order to obtain hydrocarbons with a narrower MWD, it is necessary to study both the mutual influence of the components of bifunctional catalysts and the location of metal/acid sites. The composition of FTS products can be controlled by changing the number or/and strength of acid sites that are determined by Si/Al ratio ^[13].

2. The Role of Cobalt in the Formation of Fischer–Tropsch Synthesis Products

Thermodynamic calculations show that hydrocarbons of any molecular weight, type and structure except acetylene can be formed from CO and H₂ in the presence of Co-containing catalysts [1,2,3,4,18]^{[1][2][3][4][18]}. However, the FTS contains a lot of consecutive and parallel reactions and thermodynamic calculations based on the assumptions about concurrent equilibrium, so it only allows to evaluate the probability of products formation approximately. In addition, the rates of each reaction depend on the process parameters: for example, with an increase in temperature, the probability of forming unsaturated hydrocarbons and aldehydes increases, and the probability of forming saturated hydrocarbons decreases. With an increase in pressure, the content of heavy hydrocarbons increases. At low-temperature FTS in the presence of Co and Fe catalysts, the fraction of branched products up to C₁₄ does not vary significantly with molecular mass and hydrogen saturation ^[19]. An increase in the hydrogen content in the synthesis gas favors the formation of saturated linear hydrocarbons. In the case of CO content, an increase favors the formation of olefins and aldehydes, while the degree of carbonization of the catalyst increases. As a result, the actual composition of FTS products differs significantly from thermodynamic calculations. The main FTS reactions correspond to the polymerization mechanism where the single fragment is CH_x monomer formed by interaction of CO and H₂ [1,2,3,4,18,20]^{[1][2][3][4][18][20]}: Side reactions in the case of low-temperature FTS in the presence of Co catalysts are as follows: formation of methane by direct CO hydrogenation; and water gas shift reaction. In the case of high-temperature FTS, the Boudouard reaction is a predominant side reaction. In addition, CO and H₂ can interact to form oxygen-containing compounds such as alcohols [1^{[1][2][4][20]},2,4,20], although the probability of their formation is higher in the presence of Fe than Co. It is necessary to take into account the activity of metal sites in the secondary transformation of FTS-generated hydrocarbons such as hydrogenation, re-adsorption of α-olefins followed by their subsequent inclusion in the chain growth, hydrogenolysis and isomerization [9,21,22,23,24]^{[9][21][22][23][24]}. Secondary transformations of hydrocarbons depend on the carbon chain length and can have an effect on the composition of the resulting products. Kuipers E.W. et al. ^[21] used flat model catalysts—a poly-crystalline cobalt foil and Co particles deposited on SiO₂—since it seemed easier for interpretation of experimental results than in the case of porous system modeling. Hydrogenation of olefins was the main secondary reaction on cobalt foil and depended on the chain length. As a result, it caused an exponential increase in the paraffin/olefin ratio with carbon number but did not lead to deviation from Anderson–Schulz–Flory (ASF) distribution. In the case of model catalyst Co/SiO₂, α-olefins were mainly re-included into the chain growth process, causing an increase in the growth probability. To a lesser extent, hydrogenolysis proceeded, and as a result of which, the number of carbons atoms in long hydrocarbons was reduced due to the consecutive methane elimination. The total effect of this process was expressed in sigmoid product distributions with a high selectivity to middle distillates. Jam S. et al. ^[22] carried out a two-stage FTS and observed an increase in C₇₊ n-paraffins content. According to the authors, it was a result of re-adsorption of C₂–C₆ α-olefins with their subsequent inclusion in the growing chain. At the first stage of a fixed-bed reactor, an iron catalyst active in the synthesis of light olefins was used. The experiments were performed at a temperature of 320 °C with a ratio of H₂/CO = 2. At the second stage, a ruthenium-promoted cobalt catalyst was used, and the experiments were performed at a temperature of 220 °C with a ratio of H₂/CO = 2. A comparative analysis of the results obtained for each catalyst separately and during two-stage FTS allowed to establish that the complete disappearance of C₂–C₆ α-olefins is explained by an increase in C₇₊ n-paraffins content and not from their direct hydrogenation to the corresponding n-paraffins. It was shown in [23,24]^[23][24] that in hydrocarbons obtained by FTS in the presence of both Co and Fe catalysts, methyl-branched hydrocarbons were identified, but ethyl-branched and dimethyl-branched hydrocarbons were not observed. The total amount of branched alkanes is about 5% in the presence of Co catalyst, while in the presence of Fe, it is about 25%. In order to explain the results, the authors proposed the alkylidene mechanism of isomerization. According to this mechanism, branched hydrocarbons are produced by re-adsorption of olefins ^[19]. The alkylidene mechanism predicts that (1) there will be «kink» corresponding to C₂ on the MWD since the ethylidene is more active than alkylidene in combination with the monomer MCH; (2) the branched hydrocarbons are monomethyl-branched with little amount of ethyl or multimethyl-branched hydrocarbons; (3) the formation of branched hydrocarbons will not obey ASF kinetics or the growth probability that branched hydrocarbons will be larger than linear chain hydrocarbons. The experiments with the deuterium have shown that the formation of 2-alkenes is different from the formation of 1-alkenes. Therefore, the contribution of secondary transformations of hydrocarbons in the total composition of FTS products in the presence of Co catalysts is a promising direction to create catalytic systems for the selective production of narrow hydrocarbons’ fractions—for instance, direct formation diesel fraction from CO and H₂ without additional stages of hydrotreatment requiring high energy costs. Zeolites are the most used catalysts for their components which are active in the transformations of hydrocarbons.