Boron Sites in Borosilicate Zeolites at Various Stages of Hydration Studied by Solid State NMR Spectroscopy

Abstract

The local structures of framework boron atoms in borosilicate zeolites B-β, 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. ^(11)B 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 ^(11)B MAS NMR at two different fields (11.7 and 19.6 T), ^(11)B MQMAS, ^(11)B CPMAS, and ^(11)B 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 °C). 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 ^1H NMR are correlated with B[3] and B[3]-I sites, respectively. The presence of a new hydroxyl proton at 3.8 ppm in ^1H NMR that showed selective correlation with B[3]-I in HETCOR NMR was also identified.