Abstract: Adsorption and desorption of acetylene on HZSM-5 and NaZSM-5 zeolites were investigated in the presence of trace water. The results showed that saturated adsorption of acetylene on NaZSM-5 was about five times as much as that on HZSM-5 at 80 ℃, but little acetylene could bear the temperature up to 200 ℃, which is in contrast to the strong adsorption of acetylene on HZSM-5. Based on the FT-IR results characterizing the adsorption states of acetylene on the zeolites as well as their evolution with increasing temperature, a mechanism of acetylene adsorption on ZSM-5 zeolites was proposed that acetylene reacts with the water adsorbed on Bronsted acid sites to form vinyl alcohol. The latter is converted to carbonium ions bound to Al[KG-45x]-[KG-20x]O-[KG-45x]-[KG-20x]Si, corresponding to the strongly adsorbed species on HZSM-5. On NaZSM-5, acetylene reacts with the water adsorbed on Na+ sites to form vinyl alcohol bound to the cations in zeolite through static attraction, corresponding to the weakly adsorbed species. Being different from the case of HZSM-5, the species on NaZSM-5 can not be further activated because they are decomposed to acetylene and desorbed with increasing temperature.

Key words: acetylene, ZSM-5 zeolite, adsorption, mechanism, adsorption state