Experimental evidence for two different aluminum-bonded silanol species, Si(OSi) (OA1)2OH and Si(OSi)2(OA1)OH, in a mildly dealuminated faujasite is presented. A sample of a low unit cell size (Si/AI=4.1) templated Y zeolite was prepared for this study to minimize interferences from framework structures highlycoordinated to aluminum, such as Si(OSi)(OA1)3 and Si(Oml)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/27A1 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 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.