Shape-selective n-alkane hydroconversion at exterior zeolite surfaces

Abstract

A critical review of the adsorption and catalysis of n-and methylalkanes demonstrates that the interior surface of TON-and MTT-type zeolites dominates both adsorption and catalysis, and that the contribution from the exterior surface is negligible. For both n-and methylalkane isomers, the experimental Henry constants at the interior TON-type zeolite surface are more than an order of magnitude greater than those at the exterior surface. Molecular simulations on exclusively interior TON-type silica surface reproduce the adsorption isotherms of n-and methylalkane isomers remarkably well and suggest that even an isomer as bulky as 2,3-dimethylpentane could have access to the interior TON-type zeolite surface. Only the reference state used in solution thermodynamics affords an equitable comparison between internal and external surface thermodynamics. It indicates that methylalkanes adsorb in a structured fashion at the exterior TON-type zeolite surface when the interior surface is inaccessible. But the entropic penalty for this organized exterior surface "pore mouth" or "key-lock" adsorption is high, so that methylalkanes prefer adsorption at the interior surface when it is accessible. We speculate that CHA-and ERI-type sieves exhibit exterior surface catalysis in long n-alkane conversion, but the database remains too small to allow investigation of the full potential of shape selectivity in exterior zeolite surface catalysis.

Molecular Simulations of Zeolites: Adsorption, Diffusion, and Shape Selectivity

“…134 Typically these reference states have been left to the personal preference of the researchers, even though there are strong arguments in favor of adopting the same reference state as for nonelectrolyte dissolution. 135,136 The properties that can be compared with experiments include the adsorption isotherm, the isosteric heat of adsorption, and the Henry coefficient.…”

“…490 Apparently, these were considered inconclusive, for papers explaining product distributions by invoking exclusively exterior surface effects continue to be issued. 136,458,482,499 Maybe the exterior surface effects postulated for ≈0.4 nm ERI-, CHA-, and AFX-type zeolites 191,136 will stand up better to similar experimental scrutiny.…”

Modeling of Transport and Accessibility in Zeolites

“…Among them, they are widely used in the petrochemical industry for isomerization and cracking of hydrocarbons [181][182][183]. Zeolites become catalytically active by substitution of aluminum for silicon into the framework.…”

Understanding Transport in MFI‐Type Zeolites on a Molecular Basis

“…Because of the application of H-ZSM5 in large-scale petrochemical processes such as trans-alkylation and isomerization of aromatic molecules [8,39], shape selectivity has been the subject of various theoretical and experimental studies [20,21,[41][42][43]. Moreover, several novel and alternative reactions making use of shape selectivity [19,20,43] are currently explored, making the optimization and the development of novel generations of molecular sieves a demanding challenge for modern material research.…”

“…Moreover, several novel and alternative reactions making use of shape selectivity [19,20,43] are currently explored, making the optimization and the development of novel generations of molecular sieves a demanding challenge for modern material research. Especially, materials with predefined activities and selectivities would be desirable, and hierarchically structured, porous materials have been of special interest in this context [44].…”