Structure Determination of Boron-Based Oxidative Dehydrogenation Heterogeneous Catalysts With Ultrahigh Field 35.2 T 11B Solid-State NMR Spectroscopy

Boron-based heterogeneous catalysts, such as hexagonal boron nitride (h-BN) as well as supported boron oxides, are highly selective catalysts for the oxidative dehydrogenation (ODH) of light alkanes to olefins. Previous catalytic measurements and molecular characterization of boron-based catalysts by B-11 solid-state NMR spectroscopy and other techniques suggest that oxidized/hydrolyzed boron clusters are the catalytically active sites for ODH. However, B-11 solid-state NMR spectroscopy often suffers from limited resolution because boron-11 is an I = 3/2 half-integer quadrupolar nucleus. Here, ultrahigh magnetic field (B-0 = 35.2 T) is used to enhance the resolution of B-11 solid-state NMR spectra and unambiguously determine the local structure and connectivity of boron species in h-BN nanotubes used as an ODH catalyst (spent h-BNNT), boron-substituted MCM-22 zeolite (B-MWW), and silica-supported boron oxide (B/SiO2) before and after use as an ODH catalyst. One-dimensional direct excitation B-11 NMR spectra recorded at B-0 = 35.2 T are near isotropic in nature, allowing for the easy identification of all boron species. Two-dimensional (2D) H-1-B-11 heteronuclear correlation NMR spectra aid in the identification of boron species with B-OH functionality. Most importantly, 2D B-11 dipolar double-quantum single-quantum homonuclear correlation NMR experiments were used to unambiguously probe boron-boron connectivity within all heterogeneous catalysts. These experiments are practically infeasible at lower, more conventional magnetic fields due to a lack of resolution and reduced NMR sensitivity. The detailed molecular structures determined for the amorphous oxidized/hydrolyzed boron layers on these heterogeneous catalysts will aid in the future development of next-generation ODH catalysts.