Fischer–Tropsch Synthesis: Influence of CO Conversion on Selectivities, H2/CO Usage Ratios, and Catalyst Stability for a Ru Promoted Co/Al2O3 Catalyst Using a Slurry Phase Reactor

Abstract

The effect of CO conversion on hydrocarbon selectivities (i.e., CH 4 , C 5? , olefin and paraffin), H 2 /CO usage ratios, CO 2 selectivity, and catalyst stability over a wide range of CO conversion (12-94%) on 0.27%Ru-25%Co/Al 2 O 3 catalyst was studied under the conditions of 220°C, 1.5 MPa, H 2 /CO feed ratio of 2.1 and gas space velocities of 0.3-15 NL/g-cat/h in a 1-L continuously stirred tank reactor (CSTR). Catalyst samples were withdrawn from the CSTR at different CO conversion levels, and Co phases (Co, CoO) in the slurry samples were characterized by XANES, and in the case of the fresh catalysts, EXAFS as well. Ru was responsible for increasing the extent of Co reduction, thus boosting the active site density. At 1%Ru loading, EXAFS indicates that coordination of Ru at the atomic level was virtually solely with Co. It was found that the selectivities to CH 4 , C 5? , and CO 2 on the Co catalyst are functions of CO conversion. At high CO conversions, i.e. above 80%, CH 4 selectivity experienced a change in the trend, and began to increase, and CO 2 selectivity experienced a rapid increase. H 2 /CO usage ratio and olefin content were found to decrease with increasing CO conversion in the range of 12-94%. The observed results are consistent with water reoxidation of Co during FTS at high conversion. XANES spectroscopy of used catalyst samples displayed spectra consistent with the presence of more CoO at higher CO conversion levels.

Heterogeneous Catalyst Deactivation and Regeneration: A Review

“…The review includes a detailed consideration and analysis of the mechanisms and processes of sintering, oxidation, aluminate formation, and coking and carbide formation and under what operating conditions each is important. They summarize their and others' previous findings that oxidation primarily occurs on small (<2 nm) cobalt crystallites and at high partial pressures of water [362][363][364][365][366]. Further, they highlight the potentially complicated transformations between CoO and aluminates [362,364,367].…”

“…They summarize their and others' previous findings that oxidation primarily occurs on small (<2 nm) cobalt crystallites and at high partial pressures of water [362][363][364][365][366]. Further, they highlight the potentially complicated transformations between CoO and aluminates [362,364,367]. These complications highlight a complex mechanism that may be related to chemical-assisted sintering of Co FTS catalysts through a combination of the effect of CoO reduction during the initial activation of the catalysts and water exposure during operation.…”

Influence of Reduction Promoters on Stability of Cobalt/g-Alumina Fischer-Tropsch Synthesis Catalysts

“…The results from the previous section suggest that the former is dependent on Co crystallite size and P(H 2 O)/P(syngas) ratio, which is in turn influenced by conversion level. In a recent kinetic investigation, excursions of a Ru-promoted 25%Co/Al 2 O 3 catalyst (average cluster size of 5.0 nm) to high CO conversion levels (e.g., X CO > 75%) resulted in the oxidation of a fraction of cobalt clusters [21] (as demonstrated by changes in the lineshape of the XANES derivative spectra) and increases in CO 2 and CH 4 selectivities (Figure 9). The oxidized cobalt is active for WGS, and the additional H 2 produced therefrom tends to increase the C 1 product.…”

“…However, an atomically equivalent amount of Pt slightly (though not prohibitively) worsened the selectivities, and an attempt to use Pd to replace Pt resulted in a significantly poorer product distribution. In terms of differences in catalyst structure, the three commonly used promoters have, in a number of cases, been observed to be in atomic contact with Co (e.g., as an alloy), with no presence of promoter-promoter coordination at relatively low loadings (Re [18][19][20], Ru [21], Pt [22][23][24]). This is not always the case (e.g., Ru [9,16]), indicating that loading and preparation method are also factors that influence coordination environment.…”

Óxidos mixtos del tipo CoxOy/ MgO-Al2O3 y su promoción con rutenio como catalizadores para la síntesis Fischer-Tropsch

“…Esta hipótesis es coherente con los resultados de XRD, UV-vis y TPR. Algunos autores han reportado que en los sistemas de cobalto soportados sobre alúmina se observa una tendencia al incremento de CO 2 (y otros hidrocarburos) a medida que aumenta el porcentaje de conversión, (Ma, Jacobs, et al, 2011;Bukur, Pan, et al, 2012). El análisis elemental del catalizador Co x O y /Al 2 O 3 después de la reacción no mostró un contenido significativo de carbono, lo cual permite descartar la formación de residuos carbonosos y CO 2 a través de la reacción de Boudouard (CO → C + CO 2 ).…”