Hydroisomerization of n-decane over the Pd/ZSM-22 bifunctional catalysts: The effects of dynamic and static crystallization to the zeolite

摘要

Two series of pure nanosized ZSM-22 zeolites (Z-22 (xD) and Z-22 (xS)) with different SiO2/Al2O3 molar ratios are synthesized with 1,6-diaminohexane (DAH) template by the hydrothermal method under dynamic and static crystallization conditions, respectively. The effects of crystallization conditions on the characteristic properties of ZSM-22 zeolites are investigated by XRD, SEM, HR-TEM, N-2 adsorption, Si-29 and Al-27 MAS NMR, NH3-TPD, Py-IR and H-2 chemisorption measurements. The results show that Z-22 (xS) samples reveal smaller crystal size, larger mesoporosity, weaker Bronsted acid strength and lower Bronsted acid density compared with Z-22 (xS) samples. The Pd/Z-22 (xD) and Pd/Z-22 (xS) bifunctional catalysts are prepared by loading 0.5 wt% Pd on the corresponding zeolite samples, respectively, and the catalytic performance over all bifunctional catalysts is tested in the hydroisomerization of n-decane. The catalytic test results indicate that the Pd/Z-22 (xD) catalysts show higher selectivity for both mono-branched and multi-branched iso-decanes than those of Pd/Z-22 (xS) catalysts, because of the smaller crystal size and lower Bronsted acidity. Therefore, the dynamic hydrothermal crystallization is an effective way to synthesize ZSM-22 zeolites with improved catalytic behaviors for the n-alkane hydroisomerization.