There is an industrial need to develop energy-efficient pathways for the production of building block olefins. One possible route is the oxidative dehydrogenation (ODH) of abundant light alkanes. Recently, the Hermans group demonstrated that boron-containing materials such as hexagonal BN (hBN) and boron nitride nanotubes (BNNT) are highly active and selective catalysts for the ODH of light alkanes.1-3 They hypothesized that oxygen functionalized edge sites at the surface of boron materials provide the catalytically active sites.1 In order to test this hypothesis we have applied high-resolution 11B solid-state NMR (SSNMR) spectroscopy to samples of hBN and BNNT before and after use as propane ODH catalysts.4 Suprisingly, the 11B SSNMR spectra of the spent catalysts held on stream for 24 hours show that both hBN and BNNT undergo substantial oxidation and hydrolysis to form a new phase denoted B2(OH)xO3-x/2 (with x ≤ 6). Given the long term stability (weeks) of these ODH catalysts, our findings suggests that the catalytically active sites could reside in the B2(OH)xO3-x/2 phase. Advanced 2D 1H-11B double resonance, 2D 11B MQMAS and surface enhanced dynamic nuclear polarization (DNP) SSNMR were applied to further probe the structure and ordering of B2(OH)xO3-x/2. These experiments suggest that there are significant morphological changes in the materials and that the surfaces of the spent catalysts are rich in hydroxide groups. We also describe how 1H and 11B SSNMR spectroscopy can be used to probe the boron coordination environment and detect clustering/dimerization of boron sites grafted on silica and immobilized in zeolites.

Characterization of ODH heterogeneous boron catalysts by high-resolution solid-state NMR spectroscopy

2019 Spring National Meeting

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