Abstract: Numbers of acid sites of a series of HZSM-5 zeolites with SiO₂/Al₂O₃from 31. 4 to 137. 1 were determined using IR and NH₃-TPD spectra. To calculate respectively the adsorption heats of Bronsted and Lewis acid sites, an equation, ln0/[(1 - 0)P]=Q/RT+C, was established with the help of Langmuir model, and the adsorption heats of Bronsted acid sites were calculated with this equation. It was found that there is a transition of the relationship between Al/u. c. and acidic concentration at Al/u. c.=3. 64. The calculated results show that the adsorption on Bronsted acid sites accords with Langmuir model and the adsorption heats of samples with different SiO₂/Al₂O₃are almost the same, but the strength of Lewis acid sites is heterogeneous and adsorption on Lewis acid sites does not accord with Langmuir model. The extinction coefficients of Bronsted and Lewis sites of HZSM-5 zeolites were also determined, ε_B=7. 52×10 cm·mol^-1and ε_L=1. 09×10 cm·mol^-1.

Key words: Adsorption heat, FTlR, HZSM-5 zeolite, SiO₂/Al₂O₃ratios, Acidic properties