Micro-imaging of transient guest profiles in nanochannels.

Zeolites of type ferrierite are exploited as a host system for monitoring the evolution of guest concentration (methanol) in nanoporous host materials upon adsorption. Additional transport resistances at the crystal surface have been removed so that uptake is exclusively controlled by the diffusion resistance of the pore space. Since the crystal shape deviates from a simple parallelepiped, the primary imaging data do not immediately reflect true local concentrations. A simple algorithm is developed which overcomes this complication. The determined transient concentration profiles ideally comply with the requirements for the application of the Boltzmann-Matano integration method for determining diffusivities. The resulting diffusivities (along the direction of the "10-ring channels") are found to exceed those along the 8-ring channels by three orders of magnitude.