Effect of promotion with ruthenium on the structure and catalytic performance of mesoporous silica (smaller and larger pore) supported cobalt Fischer–Tropsch catalysts

The effects of promotion with ruthenium on the structure of cobalt catalysts and their performance in Fischer–Tropsch synthesis were studied using MCM-41 and SBA-15 as catalytic supports. The catalysts were characterized by N2 physisorption, H2-temperature programmed reduction, in situ magnetic measurements, X-ray diffraction and X-ray photoelectron spectroscopy. It was found that monometallic cobalt catalysts supported by smaller pore mesoporous silicas (dp = 3–4 nm) had much lower activity in Fischer–Tropsch synthesis than their larger pore counterparts (dp = 5–6 nm). Promotion with ruthenium of smaller pore cobalt catalysts led to a considerable increase in Fischer–Tropsch reaction rate, while the effect of the promotion with ruthenium was less significant with the catalysts supported by larger pore silicas. Characterizations of smaller pore cobalt catalysts revealed strong impact of ruthenium promotion on the repartition of cobalt between reducible Co3O4 phase and barely reducible amorphous cobalt silicate in the calcined catalyst precursors. Smaller pore monometallic cobalt catalysts showed high fraction of barely reducible cobalt silicate. Promotion with ruthenium led to a significant increase in the fraction of reducible Co3O4 and in decrease in the amount of cobalt silicate. In both calcined monometallic and Ru-promoted cobalt catalysts supported by larger pore silicas, easy reducible Co3O4 was the dominant phase. Promotion with ruthenium of larger pore catalysts had smaller influence on cobalt dispersion, fraction of reducible cobalt phases and thus on catalytic performance.