Water adsorption on zeolites: ab-initio interpretation of IR data

Abstract

Ab-initio 6-3 lG* OH frequency calculations show the water molecule complex with the bridging OH group of zeolites to be hydrogen-bonded. The experimental 3390-and 3695-cm-I frequencies of the complex are interpreted to belong to the water OH group interacting with the surface by two hydrogen bonds and to the other slightly perturbed water OH group, respectively. The broad -2900-, -2450-, and -1700-cm-I bands are explained on the basis of a theory of OH band profiles of hydrogen-bonded complexes to be the so-called

New route for light olefins production from chloromethane over HSAPO-34 molecular sieve

“…These three broad bands may come from the H-bonding complex and be attributed to the perturbed OH vibration, because they are quite similar to the triplet A, B, C bands of hydroxyl group as previously reported [21,22]. Usually, for the H-bonding complex, the A-B-C pattern with broad and intense absorbance at around 2800, 2400 and 1700 cm À1 , is caused by a Femi resonance between out-of-plane deformation vibration of bridged OH group and the perturbed hydrogen bound OH group due to O-HÁÁÁX-H formation.…”

(A,B,C) Triplet of Infrared OH Bands of Zeolitic H-Complexes

“…wavenumber/cm-' Figure 3. 500-4000 cm-l range of H-ZSM5 (1) and that induced by CD3CN adsorption at 295 K and 0.05 mbar (2). The inset illustrates the appearance of the "imaginary" positive and negative bands at 1860 and 1895 cm-l in the difference spectrum 3 as a result of the 1880 cm-' band subtraction of the unloaded zeolite (spectrum 1) from that negligibly shifted to higher frequency by the adsorption (spectrum 2).…”

Quantum chemical calculation of infrared spectra of acidic groups in chabazite in the presence of water

“…During the last decade, adsorption of water on zeolites has been extensively studied both experimentally [1][2][3][4][5][6][7][8] and theoretically [9][10][11][12][13][14][15][16][17][18][19][20] in order to understand the nature of the interaction of basic molecules with the Brøn-sted sites in the zeolites. The reaction can be studied by infrared spectroscopy because the frequencies of the vibrations of the zeolite hydroxyl group will change upon interaction with the water molecule.…”

“…Theoretical models have been made by representing the zeolite by clusters consisting of several tetrahedral atoms, [9][10][11][12][13][14][15][16][17] and a hydrogen-bonded water molecule was found to be a minimum on the potential energy surface and a protonated water molecule to be a transition state. The molecular dynamics of water in sodalite 18 and H-SAPO-34 ͑Refs.…”

“…The appearance of two or three bands has been explained with a Fermi resonance of the downward-shifted OH stretch with upwardshifted OH bending modes. Pelmenschikov et al 9,10 have proposed this model to hold also for the spectra of acidic zeolites interacting with basic molecules.…”