α-alumina as catalyst support in Co Fischer-Tropsch synthesis and the effect of added water; encompassing transient effects

Abstract

Three catalyst supports containing from 84 to 100% α-alumina prepared by heat treatment of γalumina have been impregnated with cobalt and rhenium. The catalysts were tested for Fischer-Tropsch synthesis (FTS) under dry and enhanced water vapor pressure conditions. Both activity and selectivity to higher hydrocarbons respond positive to water added or generated in situ by the reaction. A linear trend between formation of methane and C5+ products were found, but displaced to higher C5+ values compared to catalysts on γ-alumina. Calculation of chain propagation probabilities (αn) for α-alumina supported catalysts discloses that the first step, characterized by α1, increases the most under higher water partial pressure. Moreover, α1 is significantly higher for αcompared to all γ-alumina supports irrespective of pore sizes of the latter. These results are ascribed to suppression of hydrogen coverage on the cobalt surface, linked to more regular cobalt crystallites, accompanied by enhanced water assisted generation of CHx polymerization monomers. Surprisingly, the following α2 probability is comparably low for α-alumina supports, although it increases significantly with water concentration. Linear correlations are found between each pair of parameters α1, α2, α4 and SC5+; giving support to a mechanistic model where all products are interlinked, including methane. Transients observed when water was added or removed from the system are ascribed to pore diffusion. Selectivities in these periods follow closely the general selectivity trends found for different process conditions.

Effect of Co-Feeding Inorganic and Organic Molecules in the Fe and Co Catalyzed Fischer–Tropsch Synthesis: A Review

“…This sintering process is prominently facilitated by high water partial pressure, such as >4 bar [47]. Claeys and van Steen [8] reported a noticeable increase in the formation rates of the product and significant changes in the selectivity to the FT product, in particular, lower methane selectivity and enhanced chain growth ensuing from the effect of water. The flow rate of the main reactants (CO and H 2 ) is another FT operating process parameter that affects the performance of the FT processes.…”

“…For instance, there have been controversies about the co-feeding of water to the FT reactor. Several researchers have investigated the addition of water to elucidate the water effect on both activity and selectivity [1][2][3][4] on the catalyst deactivation [5][6][7], the kinetics and mechanism [8][9][10], and product distribution [3]. Due to the different views on the effect of water co-feeding in syngas during FTS, some authors have focused their studies on shedding light on the subject of positive effects of water co-feeding on the FT rate, which was observed for some, but not all, Co-based catalysts [9].…”

Significance of C3 Olefin to Paraffin Ratio in Cobalt Fischer–Tropsch Synthesis

“…From Equation (1), a stoichiometric amount of water is produced for each carbon in the product. In a recent series of papers on cobalt Fischer-Tropsch synthesis [1][2][3], we have demonstrated that According to the above mechanisms, the primary product of FTS is olefins, and paraffins are mostly produced by secondary hydrogenation. Indeed, this was rationalized in terms of secondary hydrogenation of olefins to paraffins that increases with residence time, chain length, and catalyst particle size [7][8][9].…”

Terahertz pulsed imaging as a new method for investigating the liquid transport kinetics of α-alumina powder compacts

“…However, the TPI method is currently providing one-dimensional (1D) information and further work can be conducted to further develop the technique and overcome such limitation. In industry, ↵-alumina is used as a catalyst support for various processes, such as steam reforming (Yeboah et al, 1995) and Fischer-Tropsch catalysis (Rytter et al, 2019). It is important to study the liquid transport kinetics of alumina pellets prepared over a range of di↵erent compaction and firing conditions to explore whether a combination of sintering temperature and compaction pressure can produce a strong but still su ciently porous pellet.…”