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

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 Xe-129 NMR spectroscopy. The secondary pores in the nanosized HZSM-5 zeolite are observed directly via Xe-129 NMR spectroscopy. As evidenced by Xe-129 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. (C) 2002 Elsevier Science Inc. All rights reserved