Investigation of the microporous structure and non-framework aluminum distribution in dealuminated nanosized HZSM-5 zeolite by 129Xe NMR spectroscopy

Abstract

Variations in the microporous structure and non-framework aluminum (Al NF) distribution of HZSM-5 zeolite with a crystal size less than 100 nm dealuminated by thermal and hydrothermal treatment have been investigated by xenon adsorption and 129Xe NMR spectroscopy. The secondary pores in the nanosized HZSM-5 zeolite are observed directly via 129Xe NMR spectroscopy. As evidenced by 129Xe NMR and XPS, for the calcined microsized HZSM-5, most of the Al NF may deposit on its external surface, a small amount of Al NF may be located in its channels. However, for the calcined nanosized HZSM-5, most of the Al NF may be located in the channels, while a small amount of Al NF may be located on the external surface. After hydrothermal treatment at lower temperature, the distribution of Al NF in dealuminated nanosized HZSM-5 is heterogeneous. It is not only present in the channels and on the external surface as in the case of microsized HZSM-5, but also near the pore entrances at the outer edge of the crystallites. However, at higher steaming temperatures, more Al NF in both the microsized and the nanosized HZSM-5 zeolites will migrate to their external surface.