Layered-silicate or zeolite-based heterogeneous catalysts exhibit high activities and selectivities in a variety of important processes in petroleum refining or fine chemical manufacture. They function by promoting chemical reactions at surface acid sites near heteroatoms, such as aluminum or boron. The resulting macroscopic catalytic reaction properties are believed to depend strongly on several factors, including framework topologies, heteroatom contents, and heteroatom distributions, the molecular-level (<nm) understanding of which are not well understood. This is due in part to the limited experimental means of probing the complicated local order and disorder of heteroatom-containing silicate frameworks by using conventional scattering and spectroscopy techniques. By comparison, solid-state nuclear magnetic resonance (NMR) spectroscopy is sensitive to local electronic and nuclear environments as well as the bonding interactions of heteroatoms in silicate frameworks. In particular, two-dimensional (2D) NMR methods are able to identify and selectively resolve signals from 11B-O-29Si or 27Al-O-29Si framework moieties, based on their through-bond (scalar, J) interactions, which allow boron and aluminum distributions in silicates to be determined. Here, we analyze the local structures and distributions of boron atoms in the borosilicate zeolite B-SSZ-70 prepared under different conditions. The 2D NMR results indicate that boron heteroatoms are incorporated selectively into specific Si sites in B-SSZ-70, which furthermore depend on the type of organic structure-directing agents used. These results provide detailed insights into local boron environments and their distributions in borosilicate zeolites, which are expected to enable synthesis protocols, compositions, and structures of zeolites and other heteroatom-containing siliceous catalysts to be optimized to enhance their catalytic properties.

Measuring boron heteroatom distributions in borosilicate zeolite catalysts

247th National Meeting (2014)

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