Selective hydrogenolysis of C-O bonds in small oxygenates is a critical step for the conversion of biomass-pyrolysis oils into fuels and high-value chemicals, such as α,ω-diols from furanic and pyranic species. Metal phosphides (MPs) are promising catalysts for C-O bond rupture as they are inexpensive, chemically and thermally stable, and selectively cleave C-X bonds (where X=S, N, or O) over C-C bonds. Selectivities for sterically-hindered C-O (3C-O) bonds are significantly higher than for unhindered C-O (2C-O) bonds on NixP (x = 2-2.4) and much greater than 3C-O bond selectivities on Ni. The underlying reasons for these differences, however, are not fully understood. Here, we use kinetic analysis of reaction networks and modulation excitation in situ infrared spectroscopy (MES) to identify reactive intermediates and active sites for C-O rupture in 2-methyltetrahydrofuran (2-MTHF) on NixP and Ni clusters. Rate measurements and inhibition studies show adsorption and dehydrogenation steps of 2-MTHF are quasi-equilibrated and precede kinetically relevant C-O bond rupture within partially dehydrogenated intermediates bound at acidic active sites on NixP. Ratios of 3C-O bond rupture rates to those for 2C-O bonds increase with [H2]1/2, which suggests H-content of the reactive intermediates for 3C-O and 2C-O rupture differ by one H-atom. Sites that bind the reactive intermediates for both 3C-O and 2C-O bond rupture resemble those that bind CO*, NH3*, and H* on all NixP catalysts, which implies that C-O bond rupture selectivity depends primarily on differences between the coordination of reactive intermediates to the same active site and not on ensemble size. MES experiments reveal spectral features of reactive intermediates by suppressing those of inactive surface species. Pure-component spectra (extracted by principal component analysis) indicate two distinct reactive surface species exist on NixP, which is consistent with kinetically and theoretically proposed structures of 3C-O and 2C-O reactive intermediates. The maximum coverage of the 3C-O intermediate coincides with the greatest [H2], which combined with kinetic data, strongly suggests this pure-component spectra portrays the 3C-O reactive intermediate. Kinetic and spectroscopic observations demonstrate that [H2] and temperature control selectivity, in part, by manipulating the coverage of reactive intermediates for each C-O bond rupture pathway over a single acidic active site on NixP and Ni clusters.

Mechanistic and spectroscopic evidence for active site identity and reactive intermediate structures during C-O bond rupture in small oxygenates

253rd National Meeting (2017)

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