Evolution in structure and function of transition metal carbides during the catalytic activation of C-O, C-H, and C-C bonds

Abstract

Transition metals such as Mo or W can form interstitial carbides and related interstitial formulations by occlusion of atoms such as C*, H*, O* and N*. Diverse combinations of bulk and surface stoichiometries catalyze a multitude of reactions ranging from entirely metallic hydrogenolysis and (de)hydrogenation to entirely acidic dehydration. Transition metal carbides exhibit extreme oxophilicity, and oxygen-modification results in persistent suppression of metallic characteristics and genesis of acidic properties. The carbidic penchant for cleaving C-O bonds makes these formulations promising catalysts for hydrodeoxygenation or COx activation chemistries. Bulk structure and catalytic function modification resulting from varied synthesis and reaction conditions necessitates in situ studies to quantify the effects of catalyst evolution. Isopropanol (IPA) and acetone reactions with and without O2 co-feed at 354 - 430 K were used to quantify the effects of surface oxidation on acidic and metallic characteristics of MoCx and WCx catalysts including a-Mo2C, b-Mo2C, WC, and W2C. Exposure of a freshly synthesized carbide to O* (O2, IPA or acetone) initiated suppression of metallic and basic functionality (de/hydrogenation and carbonyl condensation, respectively). Dehydration rates per gram could be reversibly tuned by over an order of magnitude using an O2 co-feed over MoCx and WCx catalysts by reversibly creating Brønsted acid sites on the carbide surface without altering bulk carbidic structure as evinced by X-ray diffraction. Oxidation decreased surface areas by an order of magnitude from ~68 to 9 m2 g-1. MoCx and WCx catalysts exhibited IPA-saturated surfaces with zero-order dependence upon IPA pressure from 0.05 – 5 kPa, and intrinsic activation energies over all MoCx and WCx catalysts under all reaction conditions ranged from 89 – 116 kJ mol-1. Brønsted acid site densities were assessed using in situ 2,6-di-tert-butylpyridine (DTBP) titrations; DTBP-normalized turnover frequency (TOF) values for MoCx and WCx catalysts were all within the range of 0.07 – 0.31 propylene s-1 (DTBP site)-1 while dehydration rates per gram varied by nearly two orders of magnitude. IPA dehydration proceeded via an E2 elimination mechanism limited by β-hydrogen scission as evinced by a kinetic isotope effect (KIE) of ~1.8. Carbonyls (acetone and propanal) are deoxygenated over H2-activated Mo2C via sequential C=O hydrogenation to equilibrium and subsequent rate-determining IPA dehydration over Brønsted acid sites at 369 K that is kinetically independent of H2 pressure from 10 – 82 kPa. Mo2C catalyzes propane dehydrogenation and hydrogenolysis with >95% selectivity to dehydrogenation in absence of H2, >95% selectivity to hydrogenolysis to CH4 with H2 co-feed, or >80% selectivity to CO via reforming with H2/CO2 co- feed at 823 K. The fraction of oxidized (O*) and carbidic (*) catalytically active surface sites can be assessed from measured effluent PCO2/PCO ratios due to an established reverse water gas shift equilibrium under steady state reaction conditions. Dehydrogenation rates can be quantitatively described using a two-site reverse Horiuti-Polyani mechanism over three distinct site pairings: (i) O*-O*, (ii) O*-*, and (iii) *-*, as evidenced by distinct dehydrogenation rate constants and activation energies in absence of co-feed, in presence of CO2 co-feed, and in presence of combined H2/CO2 co-feed. O*-* site pairs exhibited the highest dehydrogenation rate constant per gram of catalyst (kO*-* = 0.078 μmol s-1 gcat-1 kPaC3H8-1 > kO*-O* ~ k*-*). Propane dehydrogenation can be used as a probe to rigorously model transient evolution of the extent of surface oxidation and to extract kinetic dehydrogenation rate constants with near quantitative agreement with rate parameters obtained from steady state modeling (kO*-* = 0.065 μmol s-1 gcat-1 kPaC3H8-1).