The influence of citric acid on the synthesis and activity of high surface area MoP for the hydrodeoxygenation of 4-methylphenol

Unsupported, high surface area MoP was synthesized by adding citric acid (CA) to solutions of ammonium heptamolybdate and diammonium hydrogen phosphate followed by drying (397 K), calcination (773-973 K), and reduction in H-2 (923 K). A tetrameric Mo citrate precursor was formed after drying which decomposed into a monomer of the form, Mo(C4H5O3CN)O-2 center dot HPO4 following calcination. Reduction of this monomer produced CH4, CO, CO2, NH3, H2O, and MoP. The C content of the reduced MoP-CA catalysts decreased from 12.8 to 4.0 wt% as the calcination temperature increased from 773 to 973 K, whereas the surface area decreased from 136 m(2)/g to 53 m(2)/g and the MoP particle size increased from 5 to 9 nm, respectively. The residual C acted as a structural promoter of the calcined samples, limiting agglomeration of the MoP crystallites during reduction so that nanoparticles (5-9 nm) of MoP were produced. The HDO of 4-methylphenol over the MoP-CA catalysts was found to be structure insensitive with an initial turnover frequency (TOF) of 0.079 s(-1) at 623 K and 4.4 MPa H-2. The ratio of hydrogenation to direct deoxygenation of 4-methylphenol (TOFHYD:TOFDDO = 0.78:1) was independent of the MoP particle size and greater than that observed over MoS2. (C) 2012 Elsevier B.V. All rights reserved.