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

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 masstransfer 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 k J/tool). 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. Butterworth-Heinemann provides, in principle, an alternative way of measuring the sorption rate and, hence, the time constant for intracrystalline diffusion. This approach offers two major advantages: It does not require a near isothermal system and the response time of an infrared detector is short, so the method should be applicable to the study of fairly rapid sorption processes. We report here a new experimental technique, developed at the CNRS laboratory, that depends on monitoring, by an infrared detector, the surface temperature of a sample of zeolite Crystals following a small change in sorbate pressure. This technique must be distinguished from the method recently introduced by Karge and Niessen 3 in which FTi.r. measurements are used to monitor the adsorbed phase concentration and diffusivities are then determined from the (isothermal) transient sorption curve. To confirm the validity of the new method, the same system (methanol-NaX zeolite) has also been studied by the pulsed field gradient (PFG) n.m.r, method and by the zero-length column (ZLC) method. This particular