Production of high-grade diesel from palmitic acid over activated carbon-supported nickel phosphide catalysts

• Pure Ni2P and Ni12P5 crystallites formed on the AC by controlling Ni/P ratios.
• Ni12P5 presence profitably affected the dispersion of Ni2P.
• High dispersion of Ni12P5 and Ni2P favors the oil yield and C15 selectivity.
• The synergy of Ni12P5 and Ni2P facilitated the formation of branched alkanes.
• High oil yield of 56.0% with a high heating value of 46.5 MJ kg−1 was obtained.

A series of activated carbon (AC)-supported nickel phosphide catalysts were prepared; characterized using XRD, XPS, TEM, and NH3-TPD techniques; and evaluated for the deoxygenation of palmitic acid. The formation of Ni2P and/or Ni12P5 on the surface of AC could be controlled by controlling the Ni/P molar ratios. With low Ni/P molar ratios from 0.5 to 0.8, only crystalline Ni2P formed. Both Ni2P and Ni12P5 formed with Ni/P ratios of 1.0 and 1.5, whereas only Ni12P5 formed with a Ni/P ratio of 2.0. As the Ni/P ratio further increased (Ni/P ≥ 3.0), crystalline Ni formed in addition to Ni12P5. The deoxygenation activities of the NixP/AC catalysts were strongly dependent on the types and dispersion of the nickel phosphide. The oil yield and C15 selectivity on the catalysts followed the sequence Ni1.5P/AC > Ni2.0P/AC > Ni1.0P/AC ≈ Ni3.0P/AC > Ni4.0P/AC > Ni0.5P/AC ≈ Ni0.8P/AC, Ni1.0P/AC > Ni1.5P/AC ≈ Ni0.8P/AC > Ni0.5P/AC ≈ Ni2.0P/AC > Ni3.0P/AC > Ni4.0P/AC, respectively. The high activity was attributed to the coexistence and high dispersion of Ni2P and Ni12P5, which were favorable for branched alkanes formation, C15 selectivity improvement and oil yield increase. Due to the high-grade diesels (HV = 46.5 MJ kg−1) obtained, NixP/AC can be considered to be a very promising catalyst for transforming fatty acids into high-grade diesel.

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