Carbides of early transition metals have attracted attention as catalysts for various purposes, including hydrotreating, isomerization, hydrogenation, hydrogenolysis and, most recently, hydrodeoxygenation. In view of these possible applications, it would be desirable to tune the catalytic behavior of carbides. One way to systematically vary properties is to partially substitute anions or cations in a given carbide. This approach has been used in the literature, but the number of well-defined families of high-surface-area carbides has remained small because of the challenging synthesis. Our goal is to devise routes to single-phase mixed metal carbides and establish structure-activity relationships. We report on the successful synthesis, characterization and catalytic performance of two series of single-phase mixed metal carbides. Molybdenum is combined with more oxophilic metals, viz. tungsten or niobium, to retain surface oxygen and add an acid function to the hydrogenation function. Toluene serves as a surrogate reactant for heavy oil compounds, as it can undergo all relevant reactions including hydrogenation, ring contraction and opening, and hydrogenolysis. Carbides were made through carburization of oxidic precursors. Two routes to homogeneous mixed metal precursors were explored: hydrothermal synthesis and, for compositions not accessible by this method, flash-freezing followed by freeze-drying. Thermal analysis of the carburization process showed that mixed precursors are more easily reduced than monometallic precursors, and that molybdenum and tungsten are predominantly reduced by H2 whereas niobium is reduced by CH4. X-ray diffractograms of the Mo-W carbides revealed exclusively M2C(hP3) structure (M=metal). Mo-Nb carbides rich in molybdenum crystallized in Mo2C(hP3) structure, whereas niobium-rich carbides crystallized in MC(cF8) structure. Surface areas of the passivated mixed metal carbides were between 14 and 22 m2/g for the Mo-W series and between 24 and 102 m2/g for the Mo-Nb series. Several trends in catalytic behavior with composition and structure emerged. For both series of mixed metal carbides, CO sorption capacity and turnover frequency for toluene-to-methylcyclohexane conversion at 250 °C increased with increasing molybdenum content. At 400 °C, ring contraction was observed in addition to ring hydrogenation and hydrogenolysis; and the selectivity towards ring contraction was highest for niobium-containing samples with MC(cF8) structure.

Single phase mixed metal carbides: Synthesis and catalytic properties

254th National Meeting (2017)

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