Cobalt supported on morphologically tailored SBA-15 mesostructures: The impact of pore length on metal dispersion and catalytic activity in the Fischer–Tropsch synthesis

A series of wide-pore (11 nm) SBA-15 mesostructures displaying markedly different morphologies and pore lengths (0.3–5.7 μm) have been applied as support for 1 wt%Ru–20 wt%Co catalysts. Additionally, a reference catalyst supported on conventional narrow (7 nm)-pore SBA-15 displaying high pore length (6.9 μm) has also been synthesized. The catalytic supports and metal-loaded catalysts have been characterized by ICP-OES, N2-physisorption, SEM, TEM, XRD, H2-chemisorption, H2-TPR and TPO-MS, and evaluated for the Fischer–Tropsch synthesis (FTS) under realistic conditions (T = 493 K, P = 2.0 MPa, XCO = 55%). During calcination of catalyst precursors, the deleterious effect of the endo-generated nitrate-derived gaseous oxidants (NO2, N2O) on metal dispersion is diminished in short-pore catalysts due to a shorter pore-residence time, as suggested by TPO-MS. Besides, during H2 reduction, not only the d(Co3O4) but also the porous structure affects the H2-TPR profile for the catalysts, suggesting a higher pore-residence time for the formed water leading to a significant metal sintering for long-pore catalysts. Pore length is shown to be more influent than pore diameter (PD = 7–11 nm) on the attained metal dispersion. Due to its effect on metal dispersion, at constant PD = 11 nm, shortening the SBA-15 pores to ≤1.2 μm leads to 50–67% higher initial FTS Co-time-yields than for catalysts with conventional morphology (pore length >5 μm). As a result of these higher initial rates, due to a d(Co0) around the optimal 10 nm, and a lower rate loss during the transitory state (TOS = 0–6 h), short-pore SBA-15 catalysts display very high steady cobalt-time-yields of >250 × 10−3 mol CO/gCo h. Besides, enhanced selectivity toward the desired C5+ hydrocarbons is obtained with short-pore RuCo/SBA-15 catalysts likely due to a lower diffusional barrier for CO to the active sites.

1%Ru–20%Co catalysts supported on wide-pore SBA-15 mesostructures having shortened pore lengths (≤1.2 μm) show enhanced metal dispersion and superior Fischer–Tropsch activity and C5+ selectivity than the counterpart catalysts prepared on SBA-15 silicas displaying conventional morphologies with curved and long (>5 μm) pores.Figure optionsDownload as PowerPoint slide