Interaction of Hydrocarbons and Water With ZSM5

The coadsorption of water and n-hexane on HZSM5 and NaZSM5 was studied by means of i.r. spectroscopy and thermogravimetry. Under the experimental conditions used, both molecules interact selectively with H⁺ and Na⁺, if adsorbed as single components. Adsorbed on HZSM5, water forms hydroxoniums ions, n-hexane is hydrogen bonded to the proton of the SiOHAl group. Both molecules form electron pair donor-acceptor complexes with the Na⁺ cations of the NaZSM5. Between 20 and 30 mol% of the water molecules, which were present on ZSM5 after equilibration with 10⁻³ mbar water, were displaced by n-hexane adsorption. With HZSM5, the coadsorption of n-hexane at equilibration pressures higher than 10⁻³ mbar supressed the formation of the hydroxonium structure. The i.r. spectra suggest that water is displaced from and n-hexane preferentially adsorbed on the SiOHAl groups. With NaZSM5, no appreciable change of the i.r. spectra as compared to those of the pure components was observed. The water molecules seem to remain adsorbed on the Na⁺ cations. Despite the different structures of the coadsorbates, the concentration of exchangable protons or metal cations seems to be the most important factor determining the concentration of adsorbed water.