Diffusion of methanol in NaX crystals: Comparison of i.r., ZLC, and PFG-n.m.r. measurements

Abstract

A new method of following sorption/desorption rates in zeolite crystals by monitoring the transient response of the surface temperature by i.r. has been developed. Unlike conventional sorption rate measurements, this method is not limited to isothermal or quasi-isothermal conditions, and the rapid response of the i.r. detector means that the method can be applied to study relatively fast systems. The shape of the response curves can 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 has been confirmed by a detailed study of sorption/desorption rates for methanol in NaX crystals. At room temperature, the results obtained by the new method are consistent with the data obtained, for the same system, by both zero-length column (ZLC) and pulsed field gradient (PFG) n.m.r. self-diffusion measurements. The PFG n.m.r. 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 (11 kJ/mol). Sorption rates in successive experiments, after “regeneration” at 300°C, show a regular decline accompanied by an increasing surface resistance to mass transfer. This was attributed to a slow buildup of “coke” at the crystal surface.