SINGLE-STEP THERMAL METHOD TO MEASURE INTRACRYSTALLINE MASS DIFFUSION IN ADSORBENTS

A single-step thermal method is used to measure intracrystalline mass diffusion. Sorption/desorption rates in zeolite samples (large crystals, monolayers of pellets, or even a single pellet) after a pressure or volume step are followed by monitoring the sample surface temperature by infrared detection. For a volume step, the pressure is also measured yielding the adsorbed mass. Sorption rates of water vapor in NaX are measured both on large 100-mu m crystals and pellets. This fast system corresponds to the limit of the pressure-step thermal method (using 100-mu m crystals). Sorption rates of methanol in the same large NaX crystals show good precision by the pressure-step method. The methanol results show that a surface barrier may occur after thermal regeneration of the sample in the presence of methanol traces. A major advantage of this method is that the shape of response curves can provide useful information on the nature of the mass-transfer resistance despite its limits. Sorption rates of methanol vapor on mordenite H (zeolon) pellets prove that the intracrystalline diffusivity may be extracted from pellet measurements for a slow diffusing species.