APPLICATIONS OF MESOSTRUCTURED CARBONACEOUS MATERIALS AS SUPPORTS FOR FISCHER-TROPSCH METAL CATALYST

Abstract

Mesoporous MCM-48 was synthesized and used as a template to synthesize mesoporous carbon (MC) materials. Polystyrene, the carbon source, together with sulfuric acid and toluene were added to the template (160 oC for 6 h) and this procedure generated a low surface area carbon supported/MCM-48 material. A repeat addition and carbonization step was needed to form the precursor carbon/MCM-48 material that was pyrolysed at 900 oC to generate graphitic mesoporous carbon materials. After removal of the silica template, mesoporous carbons were characterized by XRD, HR-TEM, Raman spectroscopy and surface area analysis. The effect of the amount of polystyrene as well as the role of the pyrolysis temperature on the final product was investigated. This synthesis methodology can readily be controlled to produce partially ordered graphitic mesoporous carbon supports with predictable pore width and surface area. In this study, the effect of three different supports (activated carbon (AC), mesoporous carbon (MC) and MCM-48) and of the potassium loading (2, 5 and 10 %) on the iron metal catalyst was investigated for the Fischer-Tropsch synthesis (FTS) activity. Characterization of the dried and calcined Fe catalysts by electron microscopy revealed that the iron was found on the surface of the supports and in most cases the Fe nano-particles aggregated with neighboring particles to form Fe nano-clusters. The three catalysts precursors supported on activated carbon with different amount of potassium: 15%Fe/2%K/AC, 15%Fe/5%K/AC and 15%Fe/10%K/AC, were evaluated for the FTS. It was observed that the three promoted and precipitated Fe/AC catalyst gave an initially high (~90 mol%) CO conversion that decreased with time on stream to ~30 mol% after ~200 h. On average, a CO2 yield of ~40% was obtained for all the three catalysts, indicating similar water gas shift reaction behavior. Also, three catalysts precursors supported on mesoporous carbon were prepared, 15%Fe/2%K/MC, 15%Fe/5%K/MC and 15%Fe/10%K/MC and evaluated for the FTS. The activity of the 2 and 5% K loaded catalysts was found to be similar (~40 mol% CO conversion). Whereas, the 10% loaded K catalysts showed slightly lower CO conversion when compared to the other two catalysts. The catalytic performance of 15%Fe/2%K/AC, 15%Fe/2%K/MC and 15%Fe/2%K/MCM-48 were compared. The type of the support had a marked effect on the product selectivities and product distributions of the catalysts. The carbon dioxide selectivity was shown to have been decreased more with 15%Fe/2%K/MCM-48.

The methane selectivity was low (below 6%) and stable, and the overall olefin fraction was found to be good for all the supported catalysts studied. The potassium promoter increased the hydrocarbon chain growth to C68 giving α-1 and α-2 both between 0.79 and 0.90 for all supported catalysts with an exception of MCM-48 supported Fe catalyst that selectively produced hydrocarbons up to C28.