Pulsed field gradient NMR studies of diffusion in MCM-41 mesoporous solids

Pulsed field gradient NMR is able to monitor the displacement of guest molecules in porous materials without interfering with the microstructure and microdynamics of the system under study. Using this technique, the propagation pattern of benzene, n-hexadecane. propylenecarbonate. ethylbenzene and diethyl ether in ordered mesoporous materials of type MCM-41 was studied. The evaluation of the self-diffusion of benzene allows a quick and easy assessment of the transport resistance in the nanoporous material acting over diffusion length scales of 1-10 mum. The fast diffusivities observed with benzene. ethylbenzene and diethyl ether in MCM-41 suggest that the transport processes for these sorbate molecules are governed by gas or vapour phase diffusion, which is reduced by interactions with the silica walls. Measurements over shorter diffusion lengths ( less than or equal to 1 mum), which were realised using n-hexadecane. showed that the self-diffusion in the hexagonal channels of MCM-41 is anisotropic. The finite value of the self-diffusion component perpendicular to the channel axis indicates the existence of permeable defects like ''windows in the silica matrix. (C) 2001 Elsevier Science B.V. All rights reserved.