ADSORPTION OF METHANOL ON ZSM-5 ZEOLITES

The interaction of methanol with a NaZSM-5 and a HZSM-5 zeolite was in vestigated by means of temperature-programmed desorption (TPD) and ads orption measurements. For both zeolites the desorption curves are stru ctured, indicating different adsorption sites. In the case of HZSM-5 t he desorption at higher temperatures is accompanied with a partial con version of methanol. It was investigated how the course of the reactio n depends on the initial adsorbed amount of methanol. Using a regulari zation method, desorption energy distribution functions have been calc ulated. The energy range of these distributions correlates well with t he heats of adsorption in the literature. The desorption energy distri butions between 50 and 65 kJ mol(-1), which can be attributed to a non specific interaction, reflect the formation of stronger hydrogen bonds in the methanol clusters at higher loading on HZSM-5 than on NaZSM-5. The shape of the desorption energy distribution of NaZSM-5 in the ran ge 65-100 kJ mol(-1), specific for the interaction of methanol with Na + cations, shows two clearly distinguished maxima, indicating two kind s of Na+ cations. The obtained results for the HZSM-5 zeolite reflect the energetic heterogeneity of the bridging Si-OH-Al groups. Both, the shape and wideness of adsorption energy distributions extracted from measured adsorption isotherms agree well with the determined desorptio n energy distributions.