Influence of Nanoscale Intimacy and Zeolite Micropore Size on the Performance of Bifunctional Catalysts for n-Heptane Hydroisomerization

In this study, Pt nanoparticles on zeolite/gamma-Al2O3 composites (50/50 wt) were located either in the zeolite or on the gamma-Al2O3 binder, hereby varying the average distance (intimacy) between zeolite acid sites and metal sites from closest to nanoscale. The catalytic performance of these catalysts was compared to physical mixtures of zeolite and Pt/gamma-Al2O3 powders, which provide a microscale distance between sites. Several beneficial effects on catalytic activity and selectivity for n-heptane hydroisomerization were observed when Pt nanoparticles are located on the gamma-Al2O3 binder in nanoscale proximity with zeolite acid sites, as opposed to Pt nanoparticles located inside zeolite crystals. On ZSM-5-based catalysts, mostly monobranched isomers were produced, and the isomer selectivity of these catalysts was almost unaffected with an intimacy ranging from closest to microscale, which can be attributed to the high diffusional barriers of branched isomers within ZSM-5 micropores. For composite catalysts based on large-pore zeolites (zeolite Beta and zeolite Y), the activity and selectivity benefitted from the nanoscale intimacy with Pt, compared to both the closest and microscale intimacies. Intracrystalline gradients of heptenes as reaction intermediates are likely contributors to differences in activity and selectivity. This paper aims to provide insights into the influence of the metal-acid intimacy in bifunctional catalysts based on zeolites with different framework topologies.