Study of the Effect of Surface Modification and Sulfur Impurities in Syngas on the Fischer-Tropsch Performance of Cobalt Catalysts.

摘要

The effect of surface modification of the silica support on the Fischer-Tropsch (FT) activity and the selectivity of cobalt catalyst were studied. The catalysts were characterized by N2- physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and hydrogen chemisorption. FT was carried out in a fixed-bed reactor at 230 °C, 20 bar and 9000 sccm/h/gcat, using syngas with H2 /CO = 2.0. Surface modification with chelating agents (CAs), i.e., nitrilotriacetic acid (NTA) and ethylenediaminetetracetic acid (EDTA), results in a smaller crystallite size of cobalt oxides after the calcination. In addition, the CA-modified catalysts contain well-dispersed cobalt oxides and higher number of catalytic sites on the surface. The CO conversions and the hydrocarbon yields, i.e., gC/Kgcat/h, are notably higher for the CA-modified catalysts at different operating temperatures. The structure-activity relationships based on the findings confirm the advantage of CA modification of the support during catalyst synthesis. In the next step, catalysts were subjected to poisoning by 10 and 50 ppm sulfur in the syngas. At both the concentrations, the sulfur is noted to adsorb irreversibly on the surface and to cause permanent loss in the activity. In the presence of sulfur, the performance of both CA-modified catalysts is better compared to that of the base catalyst. The sulfur poison affects the hydrogenation and the chain-propagation ability of the catalysts, and shifts the product selectivity towards short-chain hydrocarbons with a higher percentages of olefins. Highly dispersed CA-modified catalysts have a longer life in the presence of sulfur poison.