Diffusion Properties of Hexane in Pseudomorphic MCM-41 Mesoporous Silicas Explored by Pulsed Field Gradient NMR

Abstract

Pulsed field gradient (PFG) NMR is a powerful tool to examine diffusion of adsorbates in porous systems. The use of mesoporous silicas with uniform particle sizes allowed us to demonstrate the possibilities of this technique. In particular, we confirmed that, in the Mitra mathematical approach of diffusion, the surface-to-volume ratio is related to the geometry of the whole particle and not of a single pore. Hexane diffusion measured by PFG-NMR was efficient to study innovative materials like pseudomorphic MCM-41 mesoporous silicas presenting different pore topologies. The thorough analysis of the diffusion data allows monitoring the extension of the restricted diffusion domain. This method gives quantitative information on diffusion processes in bimodal pore systems and permits to gain insight into the internal structure of the pseudomorphic materials at different synthesis times. For a simple pore geometry, it is observed that the diffusion coefficient increases with the pore size. However, when materials possess a bimodal pore system (as for the intermediate materials of the pseudomorphic transformation), the diffusion can either decrease or increase depending on the connectivity of the secondary large mesopores with the main mesoporous channels. By PFG-NMR it was possible to detect the rearrangement of the mesoporous network of MCM-41 with synthesis time and to confirm the time necessary for the ordered mesoporous channels of MCM-41 to run through the whole particle. This type of measurement can nicely complement usual characterization techniques (N 2 adsorption, SEM, TEM, etc.) in order to give a better picture of diverse porous materials.