The heterogeneity of the hydroxyl groups in chabazite

Abstract

Two different clusters that have the topology of chabazite but different shapes have been used as a model for the Brønsted sites in chabazite. One of the clusters consists of eight tetrahedral atoms ͑8T͒ arranged in a ring and the other represents an intersection of two 8T rings. The adsorption of water and methanol on the two stable proton positions in chabazite has been studied using the B3LYP functional. The coordination of water and methanol with respect to the zeolite fragments were found to be similar, but with methanol situated closer to the acid site than water. The anharmonic zeolite OH stretch frequencies were found to be in the range of 2170-2500 cm Ϫ1 and 1457-2074 cm Ϫ1in the presence of water and methanol, respectively. As a measure of the acidity of the bridging hydroxyl groups in chabazite the shift of the zeolite OH stretch frequency upon adsorption has been used. We have found that the proton attached to the oxygen atom O 1 to be more acidic than the proton attached to the oxygen atom O 3 . Also, in the closed ring clusters the zeolite hydroxyl groups are more acidic than in the open clusters. This is not due to a steric effect as the orientation of the adsorbates with respect to the zeolite site is very similar for both clusters. The anharmonicities of the zeolite O-H bond account for about 40% in the redshift upon the adsorption of water or methanol.

Surface Structure and Acidity Properties of Mesoporous Silica SBA-15 Modified with Aluminum and Titanium: First-Principles Calculations

“…26 More accurate prediction of vibrational frequencies have been reported by inclusion of anharmonic contributions. 33,55 However, harmonic calculations still provide reasonable estimates for Brønsted acid properties at the surface. 17,25 The Bader charge method was used to quantify the electronic charge associated with specific atomic species.…”

Ammonia IRMS-TPD measurements and DFT calculation on acidic hydroxyl groups in CHA-type zeolites

“…1. The nature of the four structurally-different acid sites in CHA-type zeolites has been studied by quantum chemical calculations; 3,[14][15][16]27,[30][31][32][33][34] the number of atoms in the unit cell of CHA is very small (36 atoms), which enables a rapid and easy calculation. However, experimental measurements of the solid acidity of CHA-type zeolites have not been carried out sufficiently; therefore no comparison between experimental and theoretical values has been performed.…”

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

“…We have studied adsorption of water on both proton sites, 35 and we have found that a more stable complex is formed with the proton position O 3 than O 1 . In this paper all calculations for the bare zeolite clusters and complex with water have been done for the proton attached to O 3 .…”