Project Details
Projekt
Transport in Porous Media - Simulations of Diffusion in Heterogeneous, Rough Nanoporous Materials
Applicant
Professor Dr. Marc-Olivier Coppens
Subject Area
Chemical and Thermal Process Engineering
Term
from 2003 to 2010
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 5406677
This project aims at investigating the effect of geometric heterogeneity, and in particular surface roughness, on diffusion in nanoporous materials, in particular for applications to chemical reaction engineering. Despite the importance of this problem to chemical reaction engineering, many key problems remain unsolved. The seeming lack of structure of amorphous, rough nanoporous materials has led to phenomenological treatments, lumping the geometric heterogeneity or "disorder" in a correction factor, the so-called tortuosity factor. Diffusion in these materials therefore remains badly understood, even qualitatively. This project aims at further investigating earlier findings in my group that Knudsen self- and transport diffusion are affected in a different way by roughness, by using more extended dynamic Monte-Carlo simulations, analytical treatment, and molecular dynamics simulations. In zeolites, self- and transport diffusion depend in a different way on molecular occupancy: molecules may not be able to pass each other, and the motion of a molecule is influenced by that of other molecules, leading to correlations in its trajectory, and significant deviations from mean-field, averaged, predictions. It is proposed here to look beyond zeolites, as geometric heterogeneity may lead to unforeseen phenomena in rough mesoporous materials, especially in the Knudsen regime, where molecular motion is governed by molecule-wall collisions (as opposed to molecule-molecule collisions). Our first investigations indicate that surface heterogeneity in these materials, which include common silicas and aluminas, may have an important impact on diffusion, to the extent that correlations induced by the surface affect self-diffusion in an important way, but do not necessarily affect transport diffusion. The former is the one that matters for catalysis, yet the latter is the one that governs separations. Moreover, experimental tools like PFG-NMR and sorption do not measure the same diffusivity, as is well appreciated in zeolite science; in mesoporous materials, however, it is commonly assumed that self- and transport diffusion are the same. Our Monte-Carlo and molecular dynamics simulations should help investigating the effect on diffusion of roughness or heterogeneity in general, beyond what is possible with only experimental means, and allow us to study the effect on catalysis and separations.
DFG Programme
Priority Programmes
International Connection
Netherlands
