Influence of Ni crystallite size on deoxygenation of methyl laurate to hydrocarbons over Ni/SiO₂ catalyst

Three Ni/SiO₂ catalysts with different Ni crystallite sizes were prepared by the incipient wetness impregnation-drying-reduction and incipient wetness impregnation-drying-calcination-reduction methods. The catalysts were characterized by H₂-TPR, XRD, TEM, H₂ chemisorption, NH₃-TPD and TGA techniques. Their catalytic performances in the deoxygenation of methyl laurate to undecane (C₁₁) and dodecane (C₁₂) were evaluated in a fixed bed reactor. The effects of Ni crystallite size on the catalyst structure and performance were investigated. It was found that the impregnation-drying-reduction method gave smaller Ni crystallite size, and the high reduction temperature promoted the growth of Ni crystallite. With the increase of the Ni crystallite size, the turnover frequency of methyl laurate increased, while the total selectivity to C₁₁ and C₁₂ (s_C11+C₁₂), C₁₁/C₁₂ mol ratio and the selectivity to cracking products decreased. We suggest that the deoxygenation of methyl laurate on Ni/SiO₂ is structurally sensitive. The effects of weight hourly space velocity (WHSV) on performance of Ni/SiO₂ were also investigated. As WHSV increased, the methyl laruate conversion, s_C11+C₁₂, C₁₁/C₁₂ mol ratio and the selectivity to cracking products decreased. In addition, CO and CO₂ generated from the decarbonylation/decarboxylation pathway were converted to CH₄, indicating that Ni/SiO₂ had high activity for methanation. It was also found that the sintering of small Ni crystallites, the adsorption of organic compounds and carbon deposit led to catalyst deactivation.