Silica as support and binder in bifunctional catalysts with ultralow Pt loadings for the hydroconversion of n-alkanes

Hydroconversion is a key step in the production of ultraclean fuels from renewable sources. This reaction is carried out using a bifunctional catalyst consisting of a base metal sulfide or a noble metal and a solid acid. Recently, we have shown that for Pt/Al₂O₃/ZSM-22 catalysts with low Pt loadings (≤0.01 wt%) it is advantageous – to both the activity as well as the isomer selectivity - to emplace the Pt on the zeolite crystallites instead of on the Al₂O₃ binder. When these low loadings of Pt were on the alumina binder, small clusters or even single atoms were present which were hard to reduce leading to inactivity of the catalysts. Herein, we explore the replacement of alumina by silica, and the performance of catalysts with ultralow Pt loadings on the conversion of longer-chain hydrocarbons. A series of Pt/SiO₂/ZSM-22 catalysts with varying Pt weight loadings (0.001, 0.005, 0.01, 0.05, 0.1 and 0.5 wt%) and location (on silica or on ZSM-22) was prepared and characterized using ICP, NH₃-TPD, HAADF-STEM and XAS. Their hydroconversion performance was evaluated using n-heptane and n-hexadecane as model feedstocks. As for the Pt/Al₂O₃/ZSM-22 catalysts systems, for Pt/SiO₂/ZSM-22 catalysts with low Pt loadings (≤0.01 wt% for n-heptane conversion) it was beneficial to have the Pt nanoparticles on the ZSM-22 crystals. Hydroconversion of n-hexadecane over Pt/SiO₂/ZSM-22 and Pt/Al₂O₃/ZSM-22 catalysts showed that for feedstocks with a higher molecular weight, higher Pt loadings (≥0.05 wt%) are required for sufficient catalytic performance. For the conversion of n-hexadecane it was beneficial to locate these higher amounts of Pt on the binder.