1 research outputs found
Diffusion Properties of Hexane in Pseudomorphic MCM-41 Mesoporous Silicas Explored by Pulsed Field Gradient NMR
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<sub>2</sub> adsorption,
SEM, TEM, etc.) in order to give a better picture of diverse porous
materials