Bulk and γ-Al₂O₃-supported Ni₂P and MoP for hydrodeoxygenation of palmitic acid

The use of a series of bulk and supported Ni₂ and MoP materials in the hydrodeoxygenation of palmitic acid, shows that their catalytic performance can be tuned by the presence of Al₂O₃ as a support. Al₂O₃ promotes acid-catalyzed pathways, and influences the phosphide functionality. A series of strategies can be followed to successfully decrease the phosphide particle size, i.e., the use of citric acid (applied to bulk Ni₂), and the use of low reduction temperatures (applied to Ni₂/Al₂O₃) during the preparation steps. The effects of synthesis parameters and the support on the properties of the phosphides were determined by, e.g., X-ray diffraction, transmission electron microscopy, BET analysis, CO adsorption and NH3-TPD. Small particle size of phosphides does not necessarily lead to a large exposed surface of metal phosphide due to residual carbon or to agglomeration of phosphide particles. The specific activities (per gram of material) follow the trend MoP/Al₂O₃-TPR (high temperature synthesis) <Ni₂P-CA (citric acid in the synthesis) <Ni₂/Al₂O₃-LT (low temperature synthesis) <Ni₂/Al₂O₃-TPR < MoP, whereas the rates normalized per metal site (TOF) followed the trend: MoP/Al₂O₃-TPR <MoP<Ni₂-CA <Ni₂/Al₂O₃-TPR <Ni₂/Al₂O₃-LT. Thus, the Ni₂ phase is intrinsically more active than MoP, although the overall activity is determined by the interplay between intrinsic activity and exposed active surface. The conversion of palmitic acid was achieved in a trickle bed flow reactor at varying temperature and residence times. The model reaction follows three different pathway: Hydrodeoxygenation (HDO): C₁₅H₃₁COOH→C₁₅H₃₁CHO→C₁₆H₃₃OH→C₁₆H₃₄; decarboxylation/decarbonylation (DCO): C₁₅H₃₁COOH→[C₁₅H₃₁CHO]→C₁₅H₃₂; and esterification: C₁₅H₃₁COOH+ C₁₆H₃₃OH→C₁₅H₃₁COOC₁₆H₃₃. The presence of Al₂O₃ increases the esterification rates due to relative high acidity, and makes the supported Ni₂ phase more selective towards CC bond cleavage than bulk Ni₂ or MoP/Al₂O₃-TPR.