SOLID-STATE NMR-STUDIES OF SILANOL GROUPS IN MILDLY AND HIGHLY DEALUMINATED FAUJASITES

Experimental evidence for two different aluminum-bonded silanol specie s, Si(OSi)(OAl)(2)OH and Si(OSi)(2)(OAl)OH, in a mildly dealuminated f aujasite is presented. A sample of a low unit cell size (Si/Al=4.1) te mplated Y zeolite was prepared for this study to minimize interference s from framework structures highly-coordinated to aluminum, such as Si (OSi)(OAl)(3) and Si(OAl)(4). Bonding of the silicon to OH groups was observed by H-1/Si-29 cross-polarization/magic-angle-spinning NMR expe riments, while bonding of silicon to aluminum was observed by Si-29/Al -27 dipolar-dephasing-difference NMR experiments. The newly identified sites are of interest because these aluminum-bonded silanol sites may dehydrate to form strong Bronsted or Lewis acid sites and thus change the activity of the zeolite in FCC catalysts. Strong Lewis acid sites have been implicated as a source of enhanced cracking activity for mi ldly dealuminated samples of both ZSM-5 and faujasite in previous stud ies. As an extension of these studies, the structure of a highly dealu minated faujasite by solid-state Si-29 NMR methods was also examined. Our purpose is to resolve conflicting chemical shift assignments in th e literature. Based on the results of a variable-contact-time H-1/Si-2 9 cross-polarization experiment, new signals with chemical shifts of - 102 and -104 ppm are assigned to silicon atoms that are directly conne cted through bridging oxygen atoms to silicon atoms in silanol groups.