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

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 p 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 (E-des) 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 E-des and wavenumber of stretching vibration of each portion were in agreement with the observed values. (C) 2013 Elsevier B.V. All rights reserved.