Acidic Properties of SSZ-33 and SSZ-35 Novel Zeolites: a Complex Infrared and MAS NMR Study

Abstract

Two novel zeolites SSZ-33 and SSZ-35 were investigated with respect to their acidic properties using different probe molecules to characterize the accessibility and acid strength of Lewis and Brønsted acid sites. Ammonia, pyridine, pivalonitrile, and acetonitrile-d3 were used as probe molecules, and the results were correlated with 27Al and 1H magic angle spinning (MAS) NMR. Both SSZ-33 and SSZ-35 zeolites were found to possess bridging Si−OH−Al groups of virtually uniform and high acid strength. For both SSZ-33 and SSZ-35, there is the typical presence of highly disturbed OH groups (IR band around 3500 cm-1), which amounts to almost half of the overall Brønsted acidity. It was found that almost all bridging Si−OH−Al groups in SSZ-33 are located in the 12-MR rings. Both acetonitrile-d3 and pyridine sorptions suggest the presence of two types of Lewis sites in SSZ-35, differing in acid strength and electron-acceptor properties, whereas in the SSZ-33 zeolite only one type is present. The relative strength of these sites is higher than that of the Brønsted type for SSZ-35 and is of comparable strength for SSZ-33. 1H and 27Al MAS NMR measurements during thermal treatment allowed the assignment of NMR peaks to different surface OH species and the establishment of their relation to IR bands. NMR spectroscopy enabled the quantitative analysis of both free and hydrogen-bonded OH groups separately, showing that for both zeolites the amount of disturbed sites is higher than the number of free OH groups.