Adsorption of methanol on ZSM-5 zeolites

The interaction of methanol with a NaZSM-5 and a HZSM-5 zeolite was investigated by means of temperature-programmed desorption (TPD) and adsorption measurements. For both zeolites the desorption curves are structured, indicating different adsorption sites. In the case of HZSM-5 the desorption at higher temperatures is accompanied with a partial conversion of methanol. It was investigated how the course of the reaction depends on the initial adsorbed amount of methanol. Using a regularization method, desorption energy distribution functions have been calculated. The energy range of these distributions correlates well with the heats of adsorption in the literature. The desorption energy distributions between 50 and 65 kJ mol(-1), which can be attributed to a nonspecific 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 range 65-100 kJ mol(-1), specific for the interaction of methanol with Na+ cations, shows two clearly distinguished maxima, indicating two kinds 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 desorption energy distributions.