The adsorption of methanol and water on H-ZSM-5

Abstract

The adsorption of water and methanol on H-ZSM-5 has been characterized by temperature-programmed desorption (TPD), thermogravimetric analysis, and transmission infrared spectroscopy. For water, adsorption of the first molecule at the cation site occurs with a heat of adsorption of 12.1 kcal/mol. The heat of adsorption drops rapidly when more than one water molecule per cation is adsorbed and all additional water can be removed by evacuation at room temperature. For methanol, TPD shows adsorption in two desorption states. The higher temperature state corresponds to one molecule adsorbed at each cation site, while the lower temperature state is more weakly adsorbed and may not be associated with the cation. No reaction products were observed during desorption, indicating that methanol was the only species present on the surface. In the coadsorption of water and methanol, methanol vapor was found to rapidly displace adsorbed water, while water could not displace adsorbed methanol. However, deuterium-exchange experiments indicate that water is not blocked from approaching the cation sites by adsorbed methanol. Possible implications of these results on reactions carried out in the presence of water are discussed.