ABSTRACT
Catalysts with two different cobalt loadings (12 and 25%) supported on wide-pore and narrow-pore alumina (Degussa C-aluminoxide and Condea Vista Catalox SBA150 alumina, respectively) were prepared to study the effect of co-fed water during Fischer-Tropsch synthesis in a continuously stirred tank reactor (CSTR) operated at 220°C, 2.03 MPa, and H2/CO = 2:1. The amount of co-fed water was varied between 5 and 30 vol%, while total space velocity of the feed and the partial pressure of H2 and CO in the feed were kept constant during all experiments. The average pore diameters of the wide-pore and narrow-pore supports were 22.9 and 9.5 nm, respectively. In the 12% Co/Degussa C-alumina catalyst, the major fraction of cobalt clusters (size 12.2 nm) are believed to be located within the pore, while in the 25% Co/Degussa C-alumina (cluster diameter, 19.5 nm), 12% Co/SBA150 alumina (cluster diameter, 8.5 nm), and 25% Co/SBA150 alumina (cluster diameter, 9.2 nm), the cobalt clusters are thought to be outside the pore structure. A negative effect of co-fed water on catalytic activity was observed for all four catalysts. However, in the case of 12% Co/Degussa C-alumina catalyst, this negative effect was severe and, in particular, irreversible when 25 vol% water was added, while for the other three catalysts this effect was either marginal (25% Co/Degussa C-alumina) or significant (12 and 25% Co/SBA150 alumina), but always reversible even when up to 30% water was added. The CO2 selectivity of all four catalysts was found to increase with the addition of water, suggesting the participation of the water-gas shift reaction in parallel with Fischer-Tropsch synthesis. The calculated C5+ selectivity of catalysts with 25% cobalt loading (for both wide-pore and narrow-pore supports) increased with co-fed water, while the CH4 selectivity decreased. The differences in catalytic performance can be explained in terms of relative sizes of cobalt clusters, differences in pore volumes and pore networks of supports, location of cobalt clusters within pore networks (i.e., within the pore or outside the pore), and relatively higher residence time of water vapor within the pore, compared to the outside of the pores.
