Quantitative analysis of acidic OH groups in zeolite by ammonia IRMS-TPD and DFT: Application to BEA

• The stretching frequency, number and strength of acidic OH in BEA were analyzed.
• The values calculated by DFT agreed with ammonia IRMS-TPD experiments.
• The interaction between H and surrounding oxygen atoms lowers the OH frequency.

The number and ammonia desorption enthalpy, as a measure of acid strength, of bridging OH groups divided into three portions based on the OH stretching frequency in zeolite β were measured by means of a method of ammonia infrared/mass spectroscopy (IRMS) temperature-programmed desorption (TPD). On the other hand, the optimized structures, wavenumbers and ammonia desorption energies (Edes) of all the geometrically possible acid sites located at crystallographically non-equivalent positions in a BEA structure were calculated with an aid of density functional theory (DFT). The calculated wavenumbers could be classified mainly into three portions. The interaction between H in the AlOHSi unit and oxygen atoms surrounding the unit is suggested to lower the OH frequency. The number, averaged Edes and wavenumber of stretching vibration of each portion were in agreement with the observed values.

Wavenumber distribution and acid strength of acidic OH groups in zeolite β were analyzed experimentally (ammonia IRMS-TPD) and theoretically (DFT), providing good agreement.Figure optionsDownload high-quality image (143 K)Download as PowerPoint slide