An in situ approach to preparing Ni2P/SiO2 catalyst under mild conditions and its performance for the deoxygenation of methyl laurate to hydrocarbons

Abstract

Ni₂P/SiO₂ was in situ prepared from Ni/SiO₂via a phosphorization process using a dodecane solution containing triphenylphosphine (TPP) as the phosphorus source on a fixed-bed reactor. The influence of the phosphorization condition (nominal P/Ni molar ratio, temperature, WHSV of TPP and atmosphere) on the structure of the phosphorized samples was investigated. The sample structure was characterized by means of XRD, TEM, ICP-AES, TGA, N₂ sorption, and FT-IR and magnetic property. It was found that the phosphorization of metallic Ni to Ni₂P was promoted by increasing the phosphorization temperature and nominal P/Ni molar ratio and decreasing the WHSV of TPP. The phosphorization rate was much faster in the H₂ atmosphere than the N₂ one, ascribed to the formation of reactive H atoms on the Ni atoms that facilitated the cleavage of the P–C bond in PPT releasing more reactive PH₃/P. To prepare the well-crystallized Ni₂P/SiO₂ in the H₂ atmosphere, the minimum temperature (250 °C) and nominal P/Ni ratio (0.67) were necessary. Also, the Ni₂P crystallite size in Ni₂P/SiO₂ was determined by the Ni one in Ni/SiO₂, and no sintering took place during the phosphorization even at 400 °C. It is worth stating that there was a carbonaceous deposit formed on the in situ prepared catalysts, which was harmful for the catalyst activity for the deoxygenation of methyl laurate to hydrocarbons. The phosphorization condition greatly affected the performance of the resulting catalysts. On the whole, the Ni₂P/SiO₂ catalyst with good performance was prepared under a suitable phosphorization condition (i.e., 300 °C, nominal P/Ni ratio of 0.75, TPP WHSV of 0.5 h⁻¹, and H₂ atmosphere). Under the reaction conditions of 340 °C, 3.0 MPa, methyl laurate WHSV of 5 h⁻¹ and H₂/methyl laurate molar ratio of 25, it gave the conversion of methyl laurate and the total selectivity for C11 and C12 hydrocarbons higher than 98% and 96% during 100 h, respectively, exhibiting good stability. Finally, we propose a mechanism for the phosphorization of Ni/SiO₂.