Manipulation of hydroisomerization performance on Pt/ZSM-23 by introducing Al₂O₃

ZSM-23 zeolite was successfully synthesized in a dual-template system, and ZSM-23-Al₂O₃ composites with different ratios were also prepared. The hydroisomerization performance of Pt/ZSM-23 catalyst was manipulated by introducing Al₂O₃, and the influence of Al₂O₃ on physicochemical properties was investigated by XRD, SEM, TEM, N₂ physical adsorption-desorption and NH₃-TPD characterizations. The results showed that Al₂O₃ improved the dispersion of Pt, significantly reduced the acid sites concentration of the catalyst, and regulated the metal-acid balance in quantitative. The suitable metal-acid balance concentration could improve the selectivity of isomers and suppress the cracking reactions. Meanwhile, Al₂O₃ dispersed the ZSM-23 grains, which improved the dispersion and increased the number of exposed pores in ZSM-23. Thus the diffusion efficiency of reactants and intermediates could be promoted and the isomer products selectivity could be improved. All composite catalysts showed high selectivity of isomer products, among which, Pt/(9Z-1Al) had the highest yield of isomer products due to its suitable metal-acid concentration balance, reached 60% at 340 ℃, which was a significant improvement compared with Pt/ZSM-23 (42%). When the reaction temperature was lower than 310 ℃, the pore mouth mechanism dominated in Pt/ZSM-23, while the key-lock mechanism was significantly strengthened at higher reaction temperature. After the introduction of Al₂O₃, more adjacent pores in ZSM-23 were exposed and the key-lock mechanism became the domination, which led to a large number of 7/8Me-C₁₅ isomers.