Physicochemical and Catalytic Properties of HZSM-5 Zeolites Dealuminated by the Treatment with Steam

We studied the physicochemical properties (by 27Al MAS NMR, XPS, and IR spectroscopy) and catalytic activity (in n-heptane cracking) of a series of HZSM-5 zeolites treated at 770 K with water vapor at various partial pressures (0, 7, 13, 40, and 93 kPa). The treatment with steam removed Al from framework tetrahedral positions (as evidenced by 27Al MAS NMR studies) and yielded a decrease in the amount of acidic Si−OH−Al groups. Quantitative IR studies of pyridine sorption showed that the amount of Si−OH−Al in parent and in dealuminated zeolites was very close to the amount of tetrahedral Al which remained in the framework. The Al atoms removed from the tetrahedral positions migrated (as given by XPS) to the surface. Only a small amount of these removed Al created Lewis acid sites. IR studies suggested that the vacancies created by removal of Al were filled with Si atoms migrating from other places in the zeolite. From IR studies of the desorption of ammonia it was concluded that mild dealumination (with 7 kPa of H2O) increased the acidic strength of Si−OH−Al groups. The experiments of chlorobenzene sorption suggested that this increase was in the first order due to removal (by steam treatment) of the less acidic hydroxyls. The more severe dealumination decreased the acidic strength of Si−OH−Al groups. The results of catalytic tests of n-heptane cracking agreed well with the IR results concerning acidity. The mild dealumination resulted in an increase in the catalytic activity which can be related to the increase in the acidic strength of Brönsted sites which compensate the decrease in the number of sites. The further decrease in the cracking activity (for more severely dealuminated zeolites) may be explained by the decrease in both concentration and acidic strength of Brönsted sites. The cracking activity of our HZSM-5 zeolites was higher when hydrogen was used (instead of nitrogen) as the carrier gas. This indicates that hydrogen transfer plays an important role in n-heptane cracking.