Correlation between Brønsted Acid Strength and Local Structure in Zeolites

Abstract

As an index of acid strength, ammonia adsorption energies (E ads ) were calculated with density functional theory on cluster models of Brønsted acid sites belonging to FAU, BEA, MFI, FER, MWW, and MOR structures, which were selected because of the availability of experimental data and industrial importance.The calculated E ads were reasonably consistent with experimental results from the ammonia IRMS-TPD (infrared mass spectroscopy-temperature-programmed desorption) method. The calculated value was slightly (10-20 kJ mol -1 ) lower than the observed value, and its change with varying structure was approximately in agreement with the experiments. A thorough study was carried out to find the geometric parameters of the zeolite clusters (in the H and NH 4 forms) relevant to E ads and to discuss parameters controlling the acidic property. Hydrogen bonding interactions between ammonium cations and neighboring zeolitic oxygens were found to affect E ads observed in small cavities. When NH 4 + was stabilized in relatively open spaces (large cavities), acid strength was controlled by the local geometry of the Brønsted acid site, indicating a contribution of strain around Si(OH)Al to acid strength. In these cases, a shorter Al-O distance (a) gave a higher E ads . This is consistent with the explanation that Lewis acidic Al withdraws the electron charge of the SiOH contributing to Brønsted acid strength. A relationship was found between a and the distance (b) and planar angle (ω) between two triangles consisting of three oxygens each, which surrounded the Si(OH)Al unit, and finally, a relationship was found in which a smaller b and ω brought a higher E ads . The strain (compression) on atoms surrounding the Si(OH)Al unit is reflected in the extent of b and ω, and this contributes to vary Brønsted acid strength.

Ammonia IRMS-TPD Characterization of Brønsted Acid Sites in Medium-pore Zeolites with Different Framework Topologies

“…In such a case, the Al and H positions are selected based on preliminary energy calculations and/or spectroscopic researches, as we have shown in other types of zeolites [17][18][19]. However, the Al sitting in the TNU-9 zeolite has not been studied, and therefore, the selection for these calculations had to be carried out purely on assumptions.…”

“…The calculated and observed values are in reasonable agreement, supporting the validity of the IRMS-TPD technique. Moreover, it is pointed out that the microstructure around the acid site controls the acidic property, because the acid strength is thus varied by changing the structure of cluster models [19].…”

Measurements of Acidity of H-SSZ-35 by a Combined Method of IRMS-TPD Experiment and DFT Calculation

“…Figure 9 shows the dependence of E des on AlO distance. Again, the open symbols show the data points in MOR, MWW, MFI, FER, BEA and FAU [18]; from this figure, the acid site models in very small spaces which caused steric hindrance for NH 4 ? cation has been excluded.…”

“…Local geometry is believed to control the acid strength through bond angles and lengths around the AlOHSi unit. We have recently found that the AlO distance in the AlOHSi unit predominantly controls the charge of Brønsted OH group, presumably because Al center with Lewis acidity withdraws the electron of O in SiOH [18]. As a result, Brønsted acid strength (ammonia desorption energy) of the OH showed an inverse relation with the AlO distance.…”

“…Figure 8 shows a relationship between the Mulliken charge of OH and the AlO distance found on cluster and periodic models of acid sites at various positions in various structures. The open symbols were obtained by our previous study for MOR, MWW, MFI, FER, BEA and FAU [18]. The shorter AlO distance gave the more positive charge of OH.…”