Influence of bimodal pore size distribution of Ru/Co/ZrO 2–Al 2O 3 during Fischer–Tropsch synthesis in fixed-bed and slurry reactor

Abstract

Cobalt-based catalysts with 20 wt.% Co and 0.5 wt.% Ru supported on the coprecipitated ZrO 2–Al 2O 3 mixed oxide were prepared by three different methods, such as slurry precipitation (SLP), physical-mixing of precipitates (PMP) and conventional impregnation (IMP) to investigate their catalytic performance during Fischer–Tropsch (FT) synthesis. Pore size distribution suggested that the catalysts prepared by SLP and PMP possessed a bimodal pore size distribution data in contrast to the IMP catalyst possessing a unimodal pore size distribution. Information obtained using transmission electron microscopy (TEM) revealed the presence of homogeneously dispersed cobalt clusters in the case of SLP catalyst, whereas heterogeneously dispersed cobalt clusters were observed on IMP and PMP catalysts. Although Ru/Co/ZrO 2–Al 2O 3 catalyst prepared by IMP showed low CO conversion, it showed high C 8+ selectivity due to the existence of large cobalt clusters with high reducibility. In case of Ru/Co/ZrO 2–Al 2O 3 catalyst prepared by SLP method, the studies on cobalt cluster size reveals that the highest activity was obtained at 5 h of aging time. The activity of the catalysts performed in slurry reactor is much higher on SLP catalyst showing a bimodal pore size distribution due to the facile mass-transfer of heavy hydrocarbons. The functionality of the catalysts during FT synthesis was interpreted in terms of their structural aspects and reducibility of cobalt species obtained with different preparation methods and the type of reactor.