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

Abstract

International audiencePulsed 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 surfaceto-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

Probing Mass Transfer in Mesoporous Faujasite‐Type Zeolite Nanosheet Assemblies

“…The potential of this type of porous materials has been impressively demonstrated in ref. [13,25,26] Herein, we present the first results of a systematic PFG NMR diffusion study in assemblies of mesoporous FAU-type zeolite nanosheets. PFG NMR diffusion measurements of guest molecules in this material are complicated, however, by their relatively short nuclear magnetic relaxation time in the medium-pore-size MFI zeolites.…”

Diffusion of alcohols and aromatics in a mesoporous MCM-41 material

“…It was confirmed that for monomodal porosities the diffusivities increases when the pore size increases: pore diameters of 3.8 nm,6.0 nm, and 18 nm lead to diffusion coefficients of 4.1 Â10 À10 m 2 s À1 , 5.9 Â 10 À10 m 2 s À1 , and 50 Â 10 À10 m 2 s À1 , respectively. The greater values of diffusivities obtained in the work of Adem et al [55] was probably due to the smaller molecular size of hexane compared to the molecular size of probe molecules used in the present work. With the increase in the molecular size, less and less probe molecules can enter the micropores which yields a more mesopore controlled process and a lower activation energy.…”

“…In addition to that, the diffusion in small mesopores plays an important role. Adem et al [55] studied diffusion properties of hexane in pseudomorphic MCM-41 mesoporous silicas by Pulsed Field Gradient (PFG) NMR. The study of silicas with different particle sizes and different pore sizes and their MCM-41 pseudomorphs as a function of time allowed to demonstrate the possibilities of this technique.…”

The Mechanism of Pseudomorphic Transformation of Spherical Silica Gel into MCM-41 Studied by PFG NMR Diffusometry

“…Different degrees of transformation were obtained not, as reported in most of the earlier work, e.g., in [24], by interrupting the synthesis after different synthesis durations, but by using different volumes of the CTAOH solution per unit mass of the initial silica gel. Thus, the transformation stopped when the CTAOH supplied for dissolution of the initial silica gel and “re-precipitation” of the MCM-41 product was consumed.…”