WATER IN INTERACTION WITH ACID SITES IN H-ZSM-5 ZEOLITE DOES NOT FORMHYDROXONIUM IONS - A COMPARISON BETWEEN NEUTRON-SCATTERING RESULTS AND AB-INITIO CALCULATIONS

Inelastic neutron scattering (INS) has been used to study the adsorpti on of water, at different concentrations, in H-ZSM-5. INS is the only vibrational technique where the intensities can be calculated with rea sonable accuracy from atomic displacements. This feature is used here to simulate the INS spectra of the two possible structures resulting f rom water interaction with the Bronsted acid sites of the zeolite: hyd rogen-bonded water or hydroxonium ion. The atomic displacements for th e two structures are derived from recent ab initio MP2 calculations (K rossner, M.; Sauer, J. J. Phys. Chem. 1996, 100, 6199-6211). The compa rison between experimental and calculated INS spectra confirms that th e first water molecule is attached to the acid site via two hydrogen b onds, in agreement with the conclusion made by Krossner and Sauer. Hyd roxonium ions are not found in H-ZSM-5; however, this protonated speci es might be present in zeolites with a different structure.