Adsorption of water, methanol and acetonitrile in ZK-5 investigated by temperature programmed desorption, microcalorimetry and FTIR

The adsorption of water, methanol and acetonitrile in ZK-5 was investigated using temperature programmed desorption, microcalorimetry and FTIR spectroscopy. A multi-stage desorption in the TPD, steps in the isotherms and a stepwise decrease of the heat curves with increasing loading was found for H-ZK-5. The potassium form of ZK-5 shows a much less pronounced two step desorption behavior, pointing to the fact that K-ZK-5 is enthalpically less heterogeneous for polar molecules than the H-form. Contrary to the adsorption behavior of n-paraffins in ZK-5 the multi-stage desorption in the TPD profile and a step in the isotherm of those polar molecules can be related with the enthalpic heterogeneity of the adsorption sites in the cavities of H-ZK-5. For n-paraffins in ZK-5, however, it was found that discontinuities in the sorption behavior were determined by differences in sorption entropy. FTIR measurements with different coverage of methanol and acetonitrile on H-ZK-5 confirm the site heterogeneity of the acidic OH, but no pronounced preference between the HF or LF band could be detected.