Surface diffusion in porous catalysts

Abstract

This paper presents the application of pulsed field gradient (PFG) nuclear magnetic resonance (NMR) to observe surface diffusion of 1-octene in porous 1 wt% Pd/θ-Al₂O₃ catalyst trilobes. We demonstrate for the first time the ability to identify diffusion on the pore surfaces unambiguously at ambient conditions in saturated porous media; this technique is applicable to microporous and mesoporous materials in general. At very short observation times, two distinct diffusion regimes are present. These are associated with the bulk pore and pore surface diffusion of 1-octene; using the model proposed by Kärger for two site exchange we determined the diffusion coefficients of these regimes to be 1.3 × 10⁻⁹ and 1.7 × 10⁻¹¹ m² s⁻¹, respectively, and the mean residence time of a molecule on the pore surface to be 150 ms. Treatment of the catalyst trilobes with a silane surface coating is seen to influence the surface such that a surface diffusion coefficient is no longer observed, supporting the interpretation that the molecular dynamics of surface diffusing species are influenced strongly by their interaction with hydroxyl groups on the alumina surface. This technique will enable further study and improved understanding of molecular transport in porous catalysts used in liquid-phase, heterogeneous catalytic processes.