Zeolites by confined space synthesis – characterization of the acid sites in nanosized ZSM-5 by ammonia desorption and 27Al/ 29Si-MAS NMR spectroscopy

Abstract

ZSM-5 zeolites prepared by confined space synthesis, i.e., by crystallization inside a mesoporous matrix, are characterized by X-ray powder diffraction (XRPD), ammonia temperature-programmed desorption (TPD), 27Al and 29Si-MAS NMR spectroscopy, and transmission electron microscopy (TEM). It is shown by XRPD that confined space synthesis can be used for the preparation of pure and highly crystalline ZSM-5 with a uniform crystal size distribution governed by the pore size of the inert matrix. Ammonia desorption demonstrates that the small zeolite crystals have the same number of acid sites as large crystals when the Si/Al ratio is similar. The number of acid sites have been determined by ammonia TPD and converted into framework Si/Al ratios. These ratios are higher than the bulk Si/Al ratios but agree with those obtained from 27Al/ 29Si-MAS NMR. Furthermore, the 27Al MAS NMR spectra show that the synthesis conditions are important in order to prevent the formation of non-framework Al and that the best result is obtained using NaAlO 2 as the aluminum source. By proper control of the synthesis conditions, it is possible to produce a ZSM-5 zeolite with a framework Si/Al=50 and without non-framework aluminum. The relative numbers of Si*(OSi) 4, HOSi*(OSi) 3, and AlOSi*(OSi) 3 units have been determined from 29Si-MAS NMR in combination with the Si/Al ratios from 27Al-MAS NMR. These results indicate a higher number of HOSi*(OSi) 3 sites for ZSM-5 zeolites of the nm size when compared to those of the μm size. Transmission electron micrographs provide independent support for the formation of small zeolite crystals.