The effects of process variables on the activity of a Co/HZSM-5 zeolit
e bifunctional catalyst for the conversion of syngas to hydrocarbons w
ere studied. The catalyst was prepared by incipient-wetness impregnati
on from aqueous solutions of cobalt and thorium nitrate, the latter us
ed as promoter. The reactions were carried out in a fixed-bed integral
reactor. A simple two-level factorial design was not sufficient to de
scribe the dependence of C-6+ yield and C-6+ selectivity on the proces
s variables. A composite factorial design led to two response surfaces
, describing the dependence of yield and selectivity on temperature (2
40-320 degrees C), space velocity (0.5-1.3 h(-1)), CO/H-2 feed ratio (
0.5-1.9 molar) and pressure (0.1-4.3 MPa), with a deviation error of s
imilar to 20%. The catalyst showed a better selectivity towards higher
hydrocarbons than a physical mixture of ZSM-5 zeolite and a conventio
nal Fischer-Tropsch catalyst and only slow deactivation, affecting mai
nly the C-6+ selectivity. This deactivation is probably due to coke de
position on the active surface sites, that is, cobalt particles and ac
id sites of the zeolite.