Diffusion-Selective Tandem Catalysis for Alkane Hydroisomerization

The molecular diffusion behaviors in catalytic systems can influence product selectivity. Such a phenomenon, analogous to shape selectivity in zeolite catalysis, can be referred to as diffusion-selectivity. However, diffusion-selectivity is often neglected, and its connotation has scarcely been revealed. Herein, a tandem diffusion system of n-alkene and i-alkene intermediates in catalytic hydroisomerization was precisely modulated for the demonstration of intermediate diffusion-selective effects. In a series of Pt/zeolite composites, the diffusion distance of n-alkene intermediates from Pt sites towards zeolitic acid sites was prolonged by depositing Pt nanoparticles at the external surface of zeolite, leading to a reduced surface permeability by 20%, which facilitated the dispersion of n-alkene intermediates and mitigated side-reactions. On the contrary, the diffusion length of i-alkene intermediates inside zeolites decreased with reduced channel length, resulting in an enhanced intracrystal diffusion by two orders of magnitude, which avoided the long-time residence in zeolite and thus the secondary cracking of i-alkene intermediates. In this case, a high isomer yield of 62% can be obtained, outperforming the counterparts and those reported in the literature. The proposed diffusion-selective catalysis can be generalized to reactant and product diffusion-selectivity.