Acetic acid hydrodeoxygenation on molybdenum carbide catalysts

摘要

As-synthesized molybdenum carbide shows 98% selectivity (deoxygenated products) and stable chemical conversion for 30 h time-on-stream after an initial deactivation period of approximate to 9 h for vapor phase hydrodeoxygenation of acetic acid at a low temperature (403 K) and atmospheric pressure. Space-time variation studies show acetaldehyde as a primary unstable product, ethanol as a secondary unstable product, and ethyl acetate and ethylene as secondary stable products suggesting a sequential reaction pathway for acetic acid deoxygenation on Mo2C. The concurrent half and zero-order dependence of acetic acid HDO rates on H-2 and acetic acid pressure, respectively, suggests that catalytic sites for H-2 activation are distinct from those required for the activation of acetic acid consistent with prior reports for deoxygenation of aromatic ethers and alcohols. Catalyst surface evolution by oxygen (O:Mo-bulk approximate to 0.3) and carbon (C:Mo-bulk approximate to 0.1) deposition from the reactant oxygenate was noted using temperature programmed surface reaction (TPSR) with hydrogen post reaction. Higher O*/Mo deposition with acetic acid in reference to H2O, CO2, and aromatic ethers at similar oxygenate pressures suggests that the identity of the oxygenate determines its proficiency for heteroatom deposition on fresh carbidic materials. Catalytic site densities were estimated via in situ titration using 2,2-dimethylpropanoic acid (DMPA) as a reagent to calculate a turnover frequency (TOF) of (9 +/- 1) x 10(-4) mol s(-1) mol(DMPA)(-1). The X-ray diffraction patterns and X-ray photoelectron spectra of passivated Mo2C catalyst samples before and after acetic acid reaction indicate the presence of a carbide/oxycarbide phase, even though the bulk structure of orthorhombic -Mo2C is retained.