Nanofibrous γ-Al_2O_3 as support for Co-based Fischer-Tropsch catalysts: Pondering the relevance of diffusional and dispersion effects on catalytic performance

摘要

Hierarchical macro-mesoporous nanofibrous γ-alumina (Al_2O_3_nf) synthesized by a surfactant-templated route has been applied for the first time as support for preparing RuCo catalysts at two different loading levels (20 wt% Co-0.5 wt% Ru and 30 wt% Co-1.0 wt% Ru). Equivalent catalysts involving four commercial aluminas (Sasol, mean pore size ranging from 6.0 to 32.4 nm) have also been prepared for comparison purposes. The materials have been exhaustively characterized (XRD, elemental analysis, ICP-OES, N_2 physisorption, Hg intrusion porosimetry, TEM-HRTEM, FE-SEM, H_2 chemisorption, and H_2-TPR) and evaluated for Fischer-Tropsch synthesis (FTS) in a fixed-bed reactor under realistic conditions. Nanofibrous alumina displayed simultaneously the highest specific surface area (321 m~2/g) and the largest macroporosity. The introduction of the original nanofibrous morphology allowed us to conclude that Co dispersion is mainly driven by support surface area rather than by pore size. By taking advantage of these properties, RuCo/Al_2O_3_nf catalysts bearing both the highest metal dispersion and macroporosity have been used in this work to ponder the relevance of dispersion and intrapellet diffusion on FTS catalyst performance, which is of paramount importance for designing improved Co-based FTS catalysts. While initial (TOS → 0) FTS rates were related to cobalt dispersion through the recently reported particle size-TOF dependence, pseudo-steady-state FTS activity and selectivity were dictated by both dispersion and support porosity, evidencing the kinetic relevance of CO and α-olefin intrapellet diffusion through the liquid phase filling the catalyst pores under working conditions. At high metal loadings, where the relative population of intrinsically less active and C_(5+) selective Co~0 nanoparticles is decreased, RuCo/Al_2O_3_nf catalyst displayed the highest cobalt-time-yield and diesel productivity.