Boron sites in borosilicate zeolites at various stages of hydration studied by solid state NMR spectroscopy

The local structures of framework boron atoms in borosilicate zeolites B-beta, B-SSZ-33 and B-SSZ-42 have been studied in the course of hydration/dehydration by employing solid-state NMR methods. In particular, characterization of trigonal boron sites has been studied in great detail. B-11 MAS NMR spectra showed that boron trigonally coordinated to the framework (B(OSi)(3), denoted as B[3]) can be readily transformed to a defective trigonal boron site (B(OSi)(2)(OH), denoted as B[3]-I) as a result of hydration. The presence of B[3]-I sites was proven by utilizing a number of different NMR methods including B-11 MAS NMR at two different fields (11.7 and 19.6 T), B-11 MQMAS, B-11 CPMAS, and B-11 2D HETCOR experiments. The B[3]-I species can be converted into B[3] upon dehydroxylation, but its presence can also be sustained even after very high-temperature treatment (at least up to 500 degreesC). The formation of deboronated species, B(OH)(3), in distorted form was detected even under a mild hydration treatment. HETCOR NMR revealed that hydroxyl protons with chemical shifts at 2.4 and 3.3 ppm in H-1 NMR are correlated with B[3] and B[3]-I sites, respectively. The presence of a new hydroxyl proton at 3.8 ppm in H-1 NMR that showed selective correlation with B[3]-I in HETCOR NMR was also identified.