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

Abstract

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 γ-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 H₂ 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^α+ (0 < α < 1) species owing to an electron transfer from Ni to P and the different acidic sites present in the catalyst.