Solid-state NMR studies of silanol groups in mildly and highly dealuminated faujasites

Experimental evidence for two different aluminum-bonded silanol species, Si(OSi)(OAl)₂OH and Si(OSi)₂(OAl)OH, in a mildly dealuminated faujasite is presented. A sample of a low unit cell size (Si/Al = 4.1) templated Y zeolite was prepared for this study to minimize interferences 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 ¹H/²⁹Si cross-polarization/magic-angle-spinning NMR experiments, while bonding of silicon to aluminum was observed by ²⁹Si/²⁷Al dipolar-dephasing-difference NMR experiments. The newly identified sites are of interest because these aluminum-bonded silanol sites may dehydrate to form strong Brønsted 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 mildly dealuminated samples of both ZSM-5 and faujasite in previous studies. As an extension of these studies, the structure of a highly dealuminated faujasite by solid-state ²⁹Si NMR methods was also examined. Our purpose is to resolve conflicting chemical shift assignments in the literature. Based on the results of a variable-contact-time ¹H/²⁹Si cross-polarization experiment, new signals with chemical shifts of -102 and -104 ppm are assigned to silicon atoms that are directly connected through bridging oxygen atoms to silicon atoms in silanol groups.