On the Way to a More Open Porous Network of a Co–Re/Al₂O₃ Catalyst for Fischer–Tropsch Synthesis: Pore Size and Particle Size Effects on Its Performance

Five different γ-alumina supports were used, one commercial and four prepared in a macro-mesoporosity range of 7–1000 nm by different preparation methods. A simple method was used to obtain a macro-mesoporous alumina support by modification of an initial commercial mesoporous alumina with a pore generating agent. Supports were used to prepare Fischer–Tropsch synthesis (FTS) catalysts. Supports and catalysts were characterized by several techniques and tested in a lab-scale fixed-bed reaction unit in the FTS at 493 K and 2 MPa, with small (<63 μm) and large (500–710 μm) catalyst particle size (PS). Catalytic results showed that with small catalyst PS, the behavior is similar among them. With large catalyst PS, C₅₊ selectivity considerably decreased and CH₄ selectivity increased for all catalysts due to diffusional restrictions. Nevertheless, the effect of diffusion limitations of reactants and products through catalyst pores were lower for catalysts with a higher mesoporosity (20–50 nm), and much lower for the catalyst obtained when the support was modified to add macroporosity between 100 and 1000 nm. These macropores improve the transport of reactants and heavy hydrocarbons produced between the gaseous phase and the active sites. As a consequence, a better performance of the catalyst is attained, reducing the non-desirable effects of a diffusion-limited regime.