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

• Hydrodeoxygenation of palmitic acid on supported and bulk Ni2P or MoP.
• Using different preparation routes allows tuning the particle size of phosphides.
• Ni2P is intrinsically more active and better for decarbonylation than MoP.
• The preparation route determines the exposed surface of active phosphide.
• Alumina influences the HDO network and phosphide functionality.

The use of a series of bulk and supported Ni2P 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 Ni2P), and the use of low reduction temperatures (applied to Ni2P/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) < Ni2P/Al2O3-LT (low temperature synthesis) < Ni2P/Al2O3-TPR < MoP, whereas the rates normalized per metal site (TOF) followed the trend: MoP/Al2O3-TPR < MoP < Ni2P-CA < Ni2P/Al2O3-TPR < Ni2P/Al2O3-LT. Thus, the Ni2P 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 Ni2P phase more selective towards CC bond cleavage than bulk Ni2P or MoP/Al2O3-TPR.

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