An in-situ X-ray powder diffraction study of the adsorption of hydrofluorocarbons in zeolites

A time resolved in-situ synchrotron X-ray powder diffraction study of hydrofluorocarbon (HFC)-134 (CF2HCF2H) and -134a (CF3CFH2) adsorption on NaY has been performed, as a function of the temperature of the zeolite molecular sieve. Use of an image plate, allowed kinetic processes to be followed with a resolution of the order of a few seconds. The 111 reflection of the zeolite framework is very intense and particularly sensitive to adsorption of gases, whereas the weaker 220 reflection is fairly insensitive to gas sorption. The adsorption process could, therefore, be readily monitored by using a ratio of the integrated intensities of these two reflections. This ratio was found to be an accurate measure of gas loading level and loading levels could be estimated from this ratio by using a calibration curve established with samples loaded es-situ. Furthermore, good agreement was found between the experimentally measured value of this ratio, and that obtained by calculating the 111 and 220 structure factors from our earlier structural model for the HFC-134 bound to NaY (Grey, C. P.; Poshni, F. I.; Gualtieri, A. F.; Norby, P.; Hanson, J. C.; Corbin, D. R. J. Am. Chern. Sec. 1997, 119, 1981). A contraction of the unit cell parameter was observed that correlated with the HFC loading level. This contraction occurred abruptly at room temperature but was both smoother and slower at higher temperatures (100 degreesC). The kinetics of sorption depended strongly on temperature, lower temperatures reaching equilibrium more rapidly. This result was explained by an increase in the interparticle diffusion rate at higher temperatures, so that equilibrium is not reached until all the zeolite particles in the whole capillary tube are uniformly loaded with HFC molecules. At low temperatures, the HFC molecules are strongly bound to the crystallite particles so that the adsorption front fills the particles sequentially, moving steadily and rapidly through the sample.