Synergetic effect between the metal and acid sites of Pd/SAPO-41 bifunctional catalysts in n-hexadecane hydroisomerization

Abstract

Silicoaluminophosphate molecular sieves (SAPO-41) with different SiO₂/Al₂O₃ ratios were synthesized using di-n-butylamine (DBA) as template. All of the samples were characterized by XRD, SEM, TEM, N₂ adsorption, ²⁹Si MAS NMR and IR spectroscopy of adsorbed pyridine. Based on the SAPO-41 molecular sieve, a series of nPd/S41(x) bifunctional catalysts with different SiO₂/Al₂O₃ molar ratios (x = 0.2–0.7) and Pd loadings (n = 0.1–1.0 wt%) were prepared. Their catalytic performance was investigated in n-hexadecane hydroisomerization using a fixed-bed flow reactor system. The influence of the acidity and ratio between the accessible Pd sites and Brønsted acid sites $\left( {{{{{C}_{{Pd}}}} \mathord{\left/ {\vphantom {{{{C}_{{Pd}}}} {{{C}_{{{H}^{+}}}}}}} \right. \kern-\nulldelimiterspace} {{{C}_{{{H}^{+}}}}}}} \right)$ on the catalytic performance of the prepared samples was studied by adjusting the composition of the initial gel in the synthesis of SAPO-41 molecular sieves and the Pd loadings during preparation process. The results indicated that the 0.8Pd/S41(0.6) catalyst exhibited excellent catalytic performance due to a suitable acidity, high Pd dispersion and appropriate ${{{{C}_{{Pd}}}} \mathord{\left/ {\vphantom {{{{C}_{{Pd}}}} {{{C}_{{{H}^{+}}}}}}} \right. \kern-\nulldelimiterspace} {{{C}_{{{H}^{+}}}}}}$ ratio (0.58), which was beneficial for the synergetic effect between the (de)hydrogenating and acid functions of the bifunctional catalyst. The highest iso-hexadecane yield of 89% was obtained at an n-hexadecane conversion of 94% over the 0.8Pd/S41(0.6) catalyst.