Shape selectivity of zeolite for hydroisomerization of long-chain alkanes

Abstract

Shape-selective catalysis is an essential topic for fundamental research and industrial application of n-alkane hydroisomerization. Zeolites with a 1D channel are extensively studied, while there are few quantitative reports on the ideal channel size and the accurate size effect on product isomer molecule structures. In this work, a series of 1D zeolites ZSM-22 (4.6 × 5.7 Å), ZSM-12 (5.6 × 6.0 Å) and MOR (6.5 × 7.0 Å) with a variable pore size are prepared, characterized and tested for n-dodecane hydroisomerization. Their shape selectivity effect on the detailed product distribution and the reaction pathway are fully discussed combining the results of molecule size calculations and molecular dynamics simulations. The reaction activity is determined by exposed Brønsted acid sites to reaction molecules, and is highly correlated with the zeolitic channel size. And the matching degree between the reaction molecules and the channel size is discovered to be the key for product distribution. A 0.3 Å pore size difference in zeolite channels is discovered to be critical for isomer selectivity, corresponding to a 0.3 Å size difference between a mono-branched isomer and di-branched ones with branches located on different carbon atoms. As a result, the ZSM-12 based catalyst exhibits a maximum selectivity to multi-branched isomers of 55% at a high conversion of 90%, while the ZSM-22 catalyst gives a multi-branched isomer selectivity barely reaching 17%. Furthermore, the large pore of MOR offers less steric constraints on isomer products but causes serious cracking. Thus, an optimal channel dimension range is discovered between 5.6 × 6.0 Å and 6.5 × 7.0 Å for the maximum multi-branched isomers at high conversion, which highlights a direction for the development of efficient isomerization catalysts.