(678e) Nanoscale Characterization of Heteroatom Distributions in Boro- and Alumino-Silicate Frameworks

The local environments and distributions of heteroatoms in boro- or alumino-silicate materials are known to have strong influences on their macroscopic adsorption and reaction properties. This is hypothesized to be due to the variations of Brønsted acidity caused by the molecularly distinct surroundings of boron or aluminum atoms incorporated into different framework sites. Experimental characterization of such heteroatom distributions at the molecular-level (sub-nm) has been challenging, due to the complicated local order and disorder of heteroatom-containing silicate frameworks to which diffraction and other conventional techniques are insensitive. Solid-state nuclear magnetic resonance (NMR) spectroscopy overcomes many of these challanges by being sensitive to the local environments and spatial and/or bonding interactions of heteroatoms. In particular, two-dimensional (2D) NMR methods allow interactions of specific ¹¹B-O-²⁹Si or ²⁷Al-O-²⁹Si framework species to be identified and resolved by detecting their through-space (dipole-dipole) or through-bond scalar (J) interactions. Here, we analyze the local structures and distributions of boron and aluminum atoms in semi-crystalline boro- and alumino-silicates by directly determining the molecular site connectivities of ¹¹B or ²⁷Al heteroatoms and adjacent ²⁹Si atoms in the framework. The 2D NMR results, together with density-functional-theory (DFT) calculations, indicate that boron heteroatoms can be preferentially incoporated into specific ²⁹Si sites in layered and zeolitic boro-silicate frameworks, whereas aluminum heteroatoms appear to be broadly distributed in otherwise identical layered alumino-silicates directed using the same structue-directing agents as layered borosilicates. The results provide new insights on local heteroatom environments and distributions in boro- and alumino-silicates that are expected to aid understanding of their adsorption and reaction properties.