Compositional variation in FeXMo2−XP catalysts alters their Lewis acidities, leading to modulated catalytic performance in the hydrodeoxygenation of phenol.

We investigated the influence of P incorporation into a Ni catalyst on ethane dehydrogenation (EDH). Density functional theory calculations on model Ni(111) and Ni2P­(001) surfaces reveal that surface P generally decreases adsorption energies of fragments relevant to EDH at surface Ni sites but that P itself participates in binding some of these intermediates. These nonlinear influences of P cause CH3CH2–H activation to occur with similar facility on metal and phosphide surfaces, while CH2CH–H activation, an indicator of coking tendency, has much greater barriers on the phosphide. We prepared Ni and Ni–P catalysts on an SBA-15 support to test these predictions. A Ni–P catalyst with a 2:1 ratio (Ni2P­(2)/SBA-15), corresponding to the Ni2P phase, showed >80% ethylene selectivity during EDH at 873 K, compared to <1% ethylene selectivity on Ni/SBA-15, and maintained this selectivity up to 4 h time-on-stream. Diffuse reflectance infrared Fourier transform spectroscopy observations following ethylene exposure and heating under an inert flow indicate the appearance of carbon deposits on Ni/SBA-15 compared to ethylene desorption from Ni2P­(2)/SBA-15, consistent with predicted adsorption energy trends. Thermogravimetric analysis of spent EDH catalysts indicates significantly less carbon deposition on Ni2P­(2)/SBA-15 relative to Ni/SBA-15. The results highlight the potential of metal phosphides as selective and robust alkane dehydrogenation catalysts.

Bimetallic phosphides are promising materials for biomass valorization, yet many metal combinations are understudied as catalysts and require further analysis to realize their superior properties. Herein, we provide the synthesis, characterization, and catalytic performance of a variety of period 4 and 5 solid solutions of molybdenum-based bimetallic phosphides (MMoP, M = Fe, Co, Ni, Ru). From the results, the charge sharing between the metals and phosphorus control the relative oxidation of Mo and reduction of P in the lattice, which were both indirectly observed in binding energy shifts in X-ray photoelectron spectroscopy (XPS) and absorption energy shifts in X-ray absorption near-edge spectroscopy (XANES). For MMoP (M = Fe, Co, Ni), the more oxidized the Mo in the bimetallic phosphide, the higher the selectivity to benzene from phenol via direct deoxygenation at 400 °C and 750 psig. This phenomenon was observed in the bimetallic materials synthesized across period 4, where aromatic selectivity and degree of Mo oxidation both decreased in the following order FeMoP ≫ CoMoP > NiMoP. Alternatively, in the case of MMoP (M = Fe, Ru), the P in RuMoP is more oxidized compared to that in FeMoP, and the selectivity toward the hydrogenation pathway increased due to the interaction between the aromatic rings and the P species on the surface. For RuMoP and NiMoP, cyclohexanol was selectively produced from phenol with >99% selectivity when the reaction temperature was lowered to 125 °C at 750 psig, whereas FeMoP and CoMoP were not active under these conditions. Last, complete deoxygenation of phenol to benzene, cyclohexane, and cyclohexene was accomplished using mixtures of RuMoP and FeMoP in flow and batch experiments. These results highlight the versatility and wide applicability of transition metal phosphides for biomass conversions.

Different binding modes of the reactant on different catalysts determine the hydrodeoxygenation selectivity.