Roles of phosphorous-modified Al2O3 for an enhanced stability of Co/Al2O3 for CO hydrogenation to hydrocarbons

Abstract

A phosphorous-modified γ-Al₂O₃ (P-Al₂O₃), where γ-Al₂O₃ support was prepared by sol-gel method with a high surface area of ∼350 m²/g, has been applied for a preparation of cobalt-supported Co/P-Al₂O₃ catalysts. The Co/P-Al₂O₃ catalysts having a different P/Al molar ratio were investigated to elucidate the roles of phosphorous species on the γ-Al₂O₃ to the catalytic stability and product distribution for CO hydrogenation to hydrocarbons. The γ-Al₂O₃ surface was partially transformed to aluminum phosphates after phosphorous modification, and the newly formed aluminum phosphate phases simultaneously altered the surface hydrophilicity and cobalt dispersion as well. The partial formation of tridymite aluminum phosphate (AlPO₄) phases on the P-Al₂O₃ support eventually enhanced the dispersion of the supported cobalt crystallites and suppressed the aggregation of cobalt nanoparticles by forming the strongly interacted cobalt crystallites on the P-Al₂O₃ surfaces. The phosphorous-modified Fischer-Tropsch synthesis (FTS) catalyst also significantly suppressed heavy hydrocarbon depositions due to an increased surface hydrophilicity originated from partially formed SiO₂-like tridymite AlPO₄ surfaces. A higher stability of the Co/P-Al₂O₃ catalyst at an optimal phosphorous content in the range of 0.5–1.0 mol% was attributed to homogeneously distributed cobalt crystallites and less deposition of heavy hydrocarbons by forming macro-emulsion droplets with the help of trace amount of alcohols formed during FTS reaction. This was confirmed by in-situ analysis of adsorbed intermediates with surface hydrophilicity and some surface characterizations such as crystallite size, reducibility, and electronic state of the supported cobalt nanoparticles.