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

Abstract

Brønsted acidity of H–chabazite (CHA) zeolites (Si : Al₂ = 4.2) was investigated by means of ammonia infrared-mass spectrometry/temperature-programmed desorption (IRMS-TPD) methods and density functional calculations. Four IR bands were observed at 3644, 3616, 3575 and 3538 cm⁻¹, and they were ascribable to the acidic OH groups on four nonequivalent oxygen sites in the CHA structure. The absorption band at 3538 cm⁻¹ was attributed to the O(4)H in the 6-membered ring (MR), and ammonia adsorption energy (ΔU) of this OH group was the lowest among the 4 kinds of OH groups. The other 3 bands were assigned to the acidic OH groups in 8MR. It was observed that the ΔU in 8 and 6MR were 131 (±3) and 101 kJ mol⁻¹, respectively. On the other hand, the density functional theory (DFT) calculations within periodic boundary conditions yielded the adsorption energies on these OH groups in 8 and 6MR to be ca. 130 and 110 kJ mol⁻¹, respectively, in good agreement with the experimentally-observed values.