A new method of following sorption/desorption rates in zeolite crystals by monitoring the transient response of the surface temperature by infra-red was developed. Unlike conventional sorption rate measurements, this method was not limited to isothermal or quasi-isothermal conditions, and the rapid response of the infra-red detector meant that the method could be applied to study relatively fast systems. The shape of the response curves could provide useful information concerning the nature of the mass-transfer resistance, thus allowing the intrusion of a surface barrier to be detected. The validity of this technique was confirmed by a detailed study of sorption/desorption rates for methanol in NaX crystals. At room temperature, the results obtained by the new method were consistent with the data obtained, for the same system, by both zero-length column and pulsed field gradient (PFG) NMR self-diffusion measurements. The PFG NMR data show an unusual variation of self-diffusivity and activation energy with loading; the limiting activation energy, extrapolated to zero loading, agrees well with the ZLC value (11kJ/mol). Sorption rates in successive experiments, after so-called regeneration at 300C, exhibited a regular decline accompanied by an increasing surface resistance to mass transfer. This was attributed to a slow build-up of coke at the crystal surface.

Diffusion of Methanol in NaX Crystals: Comparison of IR, ZLC, and PFG-NMR Measurements. P.Grenier, F.Meunier, P.G.Gray: Zeolites, 1994, 14[4], 242–9