Water in Interaction with Acid Sites in H-ZSM-5 Zeolite Does Not Form Hydroxonium Ions. A Comparison between Neutron Scattering Results and ab Initio Calculations

Abstract

Inelastic neutron scattering (INS) has been used to study the adsorption of water, at different concentrations, in H-ZSM-5. INS is the only vibrational technique where the intensities can be calculated with reasonable accuracy from atomic displacements. This feature is used here to simulate the INS spectra of the two possible structures resulting from water interaction with the Brönsted acid sites of the zeolite: hydrogen-bonded water or hydroxonium ion. The atomic displacements for the two structures are derived from recent ab initio MP2 calculations (Krossner, M.; Sauer, J. J. Phys. Chem. 1996, 100, 6199-6211). The comparison between experimental and calculated INS spectra confirms that the first water molecule is attached to the acid site via two hydrogen bonds, in agreement with the conclusion made by Krossner and Sauer. Hydroxonium ions are not found in H-ZSM-5; however, this protonated species might be present in zeolites with a different structure.

Density Functional Theory Study of Proton Mobility in Zeolites: Proton Migration and Hydrogen Exchange in ZSM-5

“…The adsorption energy of water to the zeolite, defined as the difference in energy between gas-phase and adsorbed water, E Ads , is -16.9 kcal/mol for the structure shown in Figure 4a and -16.0 kcal/mol for the structure shown in Figure 4c. As noted in Table 2, these values lie within the range reported in previous theoretical studies, [11][12][13][14][15][16] -13.5 to -22.0 kcal/mol. The range of different values reported is a consequence of the quantum chemical method and zeolite model used in each of the studies.…”

Application of Inelastic Neutron Scattering to the Methanol-to-Gasoline Reaction Over a ZSM-5 Catalyst

“…4a) shows three bands at *800, 1080 and 1200 cm -1 . This is closely similar to the spectrum of dehydrated HZSM-5 reported in this region by Jobic et al [31]. These authors assigned a band at 1080 cm -1 to the in-plane deformation of bridging OH groups (the Brønsted acid sites).…”

Intracrystalline Diffusion in Zeolites Studied by Neutron Scattering Techniques

“…The Debye-Waller factor decreases the intensity of the fundamentals as Q increases, Eq. (8), and the intensity is redistributed into multiphonon terms [19]. The simulated INS spectrum, Figure 4d, is in reasonable agreement with the spectrum measured for bulk para-xylene, Figure 4c.…”

“…The technique is called inelastic neutron scattering (INS). The scattering function can be calculated in several ways: -from ab initio quantum calculations [8]; -from molecular dynamic simulations [9]; -from the Wilson's GF matrix method [10].…”