Mf. Ciraolo et al., An in-situ X-ray powder diffraction study of the adsorption of hydrofluorocarbons in zeolites, J PHYS CH B, 105(13), 2001, pp. 2604-2611
A time resolved in-situ synchrotron X-ray powder diffraction study of hydro
fluorocarbon (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 zeol
ite framework is very intense and particularly sensitive to adsorption of g
ases, whereas the weaker 220 reflection is fairly insensitive to gas sorpti
on. 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 establishe
d with samples loaded es-situ. Furthermore, good agreement was found betwee
n the experimentally measured value of this ratio, and that obtained by cal
culating the 111 and 220 structure factors from our earlier structural mode
l 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, 19
81). A contraction of the unit cell parameter was observed that correlated
with the HFC loading level. This contraction occurred abruptly at room temp
erature but was both smoother and slower at higher temperatures (100 degree
sC). The kinetics of sorption depended strongly on temperature, lower tempe
ratures reaching equilibrium more rapidly. This result was explained by an
increase in the interparticle diffusion rate at higher temperatures, so tha
t equilibrium is not reached until all the zeolite particles in the whole c
apillary 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.