A FTIR and TPD examination of the distributive properties of acid sites on ZSM-5 zeolite with pyridine as a probe molecule

The amount, type, origin and strength distribution of acid sites on ZSM-5 zeolite were analyzed with pyridine as a probe molecule, and with the aid of FTIR and interrupted-temperature programmed desorption techniques. Desorption activation energy (DAE) was used as a measure of the acid strength. The acid sites were divided into four sets each with a mean strength of DAE 70, 90, 120 and 150 kJ mol−1. Brönsted (B) sites were found dominant in the set with DAE 150 kJ mol−1, while Lewis (L) sites prevail for the DAE 120 kJ mol−1 set. For the other two sets both B and L sites are important. The B sites with DAE 70 kJ mol−1 were attributed to terminal silanol groups, and the B sites with DAE 90 kJ mol−1 were due to bridged hydroxyl and extraframework AlOH groups.