乙炔在ZSM-5分子筛上的吸附机制及吸附态性质

Mechanism of Acetylene Adsorption on ZSM-5 Zeolites and Characteristics of the Surface Species

在有微量水存在下研究了乙炔在HZSM-5和NaZSM-5上的吸附和脱附.在80℃下,NaZSM-5上乙炔的饱和吸附量约为HZSM-5上的5倍,但在200℃以上时吸附的乙炔只有少量可存留于NaZSM-5表面.这与乙炔在HZSM-5上的较强吸附显著不同.依据FT-IR对乙炔吸附态的测定,研究了乙炔在ZSM-5分子筛上的吸附机制及吸附态性质.乙炔与HZSM-5中B酸中心上的水作用形成乙烯醇,后者在升温时变为碳正离子结合在分子筛的Al-O--Si上构成强吸附物种.在NaZSM-5上,乙炔只通过与Na+位上的水作用形成乙烯醇,后者与沸石孔道中的阳离子以静电结合构成弱吸附物种.与HZSM-5的情况不同,NaZSM-5上的吸附物种在升温过程中被活化前即分解成乙炔脱附.

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 Br0nsted acid sites to form vinyl alcohol. The latter is converted to carbonium ions bound to Al-O^--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.