(625g) Correlating the Local Geometries to 11B NMR Chemical Shifts of Framework Boron within Borosilicate Molecular Sieves

Borosilicate molecular sieves are being explored as catalytic materials and/or as precursors to zeolites for catalytic applications. Since catalytic site-specific ordering can have significant effects on catalysis, elucidation of the position of these heteroatoms amongst crystallographically distinct T-sites will be necessary for understanding framework structure-catalytic activity/selectivity relationships. A local geometric parameter that distinguishes different T-sites, the bond angle between neighboring T-sites (e.g., Si-O-T angle), has been used for (alumino)silicates to locate Si and Al at specific T-sites based on its correlation with NMR chemical shifts. Recently, we provided the first correlation of B-O-T angle to ¹¹B NMR chemical shifts from minerals that have site-specific B and single-crystal structure solutions.

Here, we investigate the local geometry of framework B sites (B-O-T angle) within borosilicate molecular sieves, SSZ-53, SSZ-55, SSZ-59, and SSZ-82 by establishing a finite cluster model-based Density Functional Theory calculation approach. Further experimental NMR studies reveal that the isotropic ¹¹B chemical shifts linearly correlate with B-O-T angles. By combining the relevant results in borosilicate minerals, we present a correlation applicable to the entire class of three-dimensional, crystalline borosilicates.

Analysis on the bonding orbital hybridization suggests that the linearity of the correlation is attributed to the relationship between the local geometry and the electronic structure surrounding tetrahedral B. The B-O-T angles (136 ~ 144 deg.) are found to be narrower than Si-O-T angles (141 ~ 161 deg.) at the identical T-sites. The investigated ¹¹B chemical shifts cover most of the reported values for borosilicate molecular sieves (-1.4 ~ -4.5 ppm). Furthermore, the deconvolution of center-of-gravity resonances provides estimates of isotropic chemical shift and relative quantity of framework ¹¹B species based on their small quadrupolar coupling constant less than 0.17 MHz. Collectively, these results provide an effective tool for characterizing heteroatom sites within crystalline borosilicate molecular sieves.