(10f) Understanding Local Environments of Aluminum Heteroatoms in Bulk and Mesostructured Zeolite Catalysts

The chemical reaction activities and/or selectivities of porous catalysts depend strongly on the local compositions and structures near active site moieties, as well as the mass transport of reactant and product molecules to and from such sites. In zeolite catalysts, the solid acidity depends on the locations, local structural environments, and distributions of heteroatoms (e.g., Al, B) in the silicate framework. Recently, a variety of approaches have been pursued for synthesizing zeolitic materials with a combination of meso- and nanoscale porosities, which have resulted in improved transport and catalytic performance, especially for large molecules. However, the specific locations of heteroatoms and their correlations with macroscopic Brønsted acidity are challenging to understand and control, in part because of subtle differences in local bonding geometries and distributions of nearest-neighbor environments near heteroatom sites. Despite the lack of long-range atomic ordering of heteroatoms in zeolite frameworks, their local structural features can be understood by combining spectroscopic, scattering, and macroscopic property analyses that provide complementary insights across multiple length and time scales. In particular, solid-state 2D ²⁷Al{²⁹Si} nuclear magnetic resonance (NMR) measurements are sensitive to through-bond scalar (J) couplings, as manifested by correlated signals from covalently bonded framework moieties. Using these methods, the connectivities of ²⁷Al heteroatoms with nearby ²⁹Si framework sites are established in conventional bulk and surfactant-directed mesostructured zeolites with BEA and MFI topologies. These analyses reveal that Al heteroatoms are incorporated into distinct framework sites that manifest differences in their respective Brønsted acidities, as measured by the ³¹P isotropic chemical shifts of adsorbed phosphine oxide probe molecules. Such molecular-level insights about the local compositions and structures near the catalytically active sites are expected to aid the development of new zeolite catalysts with improved macroscopic reaction, adsorption, and transport properties for diverse engineering applications.