Hydroisomerization of n-hexadecane over a Pd–Ni2P/SAPO-31 bifunctional catalyst: synergistic effects of bimetallic active sites

A series of bifunctional catalysts are prepared by loading Pd, Ni2P and bimetallic Pd–Ni2P on di-n-butylamine(DBA)-templated SAPO-31 molecular sieves through either impregnation or impregnation combined with temperature-programmed reduction methods. All bifunctional catalysts are characterized using XRD, N2 adsorption, 27Al and 31P MAS NMR, SEM, TEM, IR spectroscopy of adsorbed pyridine, XPS, H2 chemisorption, ICP, and TG–DTG measurements. The catalytic performance over all prepared bifunctional catalysts is compared for n-hexadecane hydroisomerization. The obtained results demonstrate that the 0.05Pd–4Ni2P/S31 catalyst produces a higher iso-C16 yield of 72.7% with n-C16 conversion of 83.1% compared with the other two catalysts. This result is attributed to two reasons: i) the bimetallic Pd–Ni2P component possesses stronger (de)hydrogenation functionality than Ni2P does, and ii) the bimetallic bifunctional catalyst has a better balance between metal and acid functionality than the monometallic catalysts do. Moreover, the 0.05Pd–4Ni2P/S31 catalyst also shows the best catalytic stability among all catalysts, since the n-C16 conversion and iso-C16 selectivity are still above 80% and 90%, respectively, even after 100 h of long-term testing. Therefore, the present study has provided a novel idea for the design of bimetallic bifunctional catalysts for long-chain n-alkane hydroisomerization.