Detailed Kinetics of the Fischer–Tropsch Synthesis on Cobalt Catalysts Based on H-Assisted CO Activation

Abstract

A complete mechanistic kinetic model of the Fischer-Tropsch synthesis (FTS) over a Co/Al 2 O 3 stateof-the-art catalyst is developed here under conditions relevant to industrial operation. On the basis of the most recent findings on the reaction mechanism, here described according to the H-assisted CO dissociation theory and the alkyl chain growth mechanism, and on the basis of the latest indications available on the rate determining step involved in the CO activation process, rate expressions for all the steps leading to CO and H 2 consumption and n-paraffins, a-olefins and H 2 O formation are derived. Such expressions are functions of the molar fraction of CO, H 2 and olefins in the liquid phase surrounding the catalyst pellets, that in turn are related to the gas-phase pressure and composition, and of the surface coverage of the adsorbed species. Thermodynamic and kinetic parameters involved in the model are estimated by nonlinear multi-response regression on a complete set of FTS experimental data collected in a lab-scale tubular reactor at steady-state conditions (P = 8-25 bar, T = 210-235°C, H 2 /CO feed ratio = 1.8-2.7 mol mol -1 , GHSV = 2,000-7,000 cm 3 (STP) h -1 g cat -1). Both the experimental CO conversion and the hydrocarbon distribution (up to N = 50) in FTS are well predicted by the model with 13 adaptive parameters, without the need of introducing any empirical parameter. Analysis of the model offers insight in the rates of the elementary steps associated with the reaction mechanism and in the surface coverages of the adsorbed species. Such information explains the peculiar reactivity observed over cobalt-based Fischer-Tropsch catalysts, and can provide guidelines for the design of more active and selective catalytic materials.

Breaking the Fischer–Tropsch synthesis selectivity: direct conversion of syngas to gasoline over hierarchical Co/H-ZSM-5 catalysts

“…S6, ESI †). In line with other reports, 16 the methane level, higher than that of the ASF distribution, implies that a secondary reaction of CO hydrogenation takes place (in addition to FTS) for all the Co catalysts. As compared to Co/SiO 2 , the higher CH 4 molar fraction obtained over Co/zeolite catalysts (see inset in Fig.…”

A Single-Event MicroKinetic model for the cobalt catalyzed Fischer-Tropsch Synthesis

“…There is a general consensus that the reaction proceeds through a polymerization-like scheme where the monomeric building block is formed on the catalyst surface and is inserted in a growing chain [1]. This has led to the development of various microkinetic models for Co catalyzed FTS [15][16][17][18][19][20][21][22][23][24][25][26]. Klinke et al [16] developed a microkinetic model based on direct CO dissociation and chain growth by inserting CHx species.…”

“…The UBI-QEP [27][28][29] method was applied for the calculation of chemisorption enthalpies and activation energies [17]. The CO dissociation has been assessed by Visconti et al [18,19] in two consecutive papers and showed that CO dissociation is, most…”

“…probably, H assisted [19]. In both models, chain growth was considered to proceed via CH2 insertion in metal alkyl species.…”

Six-flow operations for catalyst development in Fischer-Tropsch synthesis: Bridging the gap between high-throughput experimentation and extensive product evaluation

“…23 Although this assumption is fairly relevant for many catalytic systems, several reports deal with non-ASF product distributions. 24 Dissimilar sites or (parallel) growth mechanisms in one catalyst, 25 altered α values in different carbon number regions, 26 and the so-called "bifunctional catalysts" that represent a "break" in ASF plot 27 are among such exceptions.…”