Influence of the Ni/P ratio and metal loading on the performance of NixPy/SBA-15 catalysts for the hydrodeoxygenation of methyl oleate

• Nickel phosphide forms mainly spherical nanoparticles inside the SBA-15 channels.
• Depending of the Ni/P ratio phases with different stoichiometry are obtained.
• The highest activity for methyl oleate hydrotreating corresponds to the Ni2P phase.
• The catalyst based on Ni2P present high selectivity to HDO products.
• Phosphide particles with smaller size favor the production of n-octadecane.

A series of high-performance catalysts based on nickel phosphide particles supported on SBA-15 have been synthesized by temperature-programed reduction of a nickel phosphate, previously impregnated in the mesostructured silica support. In these samples the loading of the active phase varied between 10% and 20% in a Ni basis, while nominal Ni/P ratios ranged from 0.5 to 3. The formation of different active phases, Ni2P, Ni12P5 or Ni3P, depending on the P/Ni proportion was verified by X-ray diffraction (XRD). The study of these catalysts by transmission electron microscopy (TEM) and Energy-Dispersive X-ray spectroscopy (EDX) revealed that the majority of the phosphide nanoparticles present a relatively uniform diameter evenly distributed within the SBA-15 channels. All the nickel phosphide catalysts, as well as a reference Ni/SBA-15 catalyst, were tested for the hydrodeoxygenation (HDO) of methyl oleate (C17H33–COO–CH3) in a high pressure continuous flow reactor. This compound was used as a convenient surrogate of triglyceride molecules present in vegetable oils, and following catalytic hydrotreating yields mainly n-alkanes at 30 bars and temperature in the 250–340 °C range. In all cases lower contact time and reaction temperature decrease the conversion but favor the formation of C18 alkanes, which is the preferred product in terms of carbon atom economy. In addition, this study reveals that activity and selectivity are affected remarkably by the Ni/P ratio. Maximum conversion is achieved for the catalysts prepared with an initial Ni/P ratio of 1, while selectivity to C18 is larger for the catalyst with an initial Ni/P ratio of 2. These catalysts contain predominantly the Ni2P phase, which seem to be the most active for this process, although the presence of Ni12P5 in the case of the catalysts with Ni/P = 2 appears to be also positive. On the basis of the product distribution it can be concluded that both hydrodeoxygenation and decarboxylation/decarbonylation reactions occur simultaneously over the nickel phosphide catalysts, whereas decarboxylation and cracking are the prevailing processes over the Ni/SBA-15 catalyst. Owing to these high yields of long-chain paraffins, SBA-15-supported nickel phosphide catalysts can be considered as a very promising catalyst for the transformation of vegetable oils into green diesel.

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