Evaluation of transition metal phosphides supported on ordered mesoporous materials as catalysts for phenol hydrodeoxygenation

A series of transition metal phosphides (Ni2P, Co2P and MoP) have been synthesized by temperature programmed reduction of the corresponding metal phosphate precursors loaded over mesostructured Al-SBA-15, mesoporous gamma-Al2O3 (m-Al2O3) and ordered mesoporous carbon (CMK-3). Both the dispersion and metal phosphide phases attained are strongly influenced by the support features, such as their acidic and textural properties. XRD, TEM and H-2 chemisorption results revealed that the MoP phase was probably formed but it underwent a fast re-oxidation in air. On the other hand, metal phosphide formation was hindered over m-Al2O3 as only metallic Ni and Co were detected. All the materials prepared have been evaluated as catalysts in hydrodeoxygenation (HDO) using phenol as a bio-oil model compound. The highest phenol conversions were attained with the catalysts based on the acidic supports (Al-SBA-15 and m-Al2O3). Nevertheless, Co2P/Al-SBA-15, Ni2P/m-Al2O3 and Co2P/m-Al2O3 yielded cyclohexanol as the main product denoting very low HDO efficiency. In contrast, Ni2P/Al-SBA-15 showed remarkable catalytic properties, being the only catalyst that provided almost full phenol conversion and extremely high HDO efficiency, with cyclohexane selectivity higher than 90%. This may be due to a synergetic effect between the high electron deficiency, generated by the Ni alpha+ (0 < alpha < 1) species owing to an electron transfer from Ni to P and the different acidic sites present in the catalyst.