XE-129 NUCLEAR-MAGNETIC-RESONANCE STUDY ON A SOLID-STATE DEFECT IN HZSM-5 ZEOLITE

Abstract

HZSM-5 and NaZSM-5 zeolites of various Si/Al ratios ranging from 20 to 130 have been characterized by Xe-129 nuclear magnetic resonance (NMR) spectroscopy at 296 K, following evacuation at temperatures between 673 and 823 K. Increases in the aluminum content and evacuation temperature resulted in significant increases in the Xe-129 NMR line width and chemical shift of xenon adsorbed on HZSM-5. Similarly, the electron spin resonance (ESR) signal at g = 2.000 +/- 0.005 increased in intensity with increasing Al content or evacuation temperature. In the case of NaZSM-5, evacuation at high temperature did not affect the Xe-129 NMR spectrum, and the ESR signal disappeared. These results are attributed to paramagnetic solid-state defects in HZSM-5 arising from the hydroxyl groups that bridge the Al and Si atoms in the zeolite framework.