External surface Al-rich ZSM-48 zeolite with high efficient n-dodecane hydroisomerization activity

Abstract

ZSM-48 zeolites with exterior Al-rich (Z-48-Sx) are synthesized by the recrystallization method. The crystal structure of Z-48-Sx with the Al-rich outer layer is verified by analyzing the crystallization process of Z-48-Sx and the EDS of the ultrathin section. The diffusion properties of Z-48-S3 and Z-48-C are investigated by the zero-length column (ZLC) method, and the synthesized ZSM-48 zeolite with an Al-rich external layer effectively improves the diffusion ability. The effective diffusion constants of Z-48-S3 are 53 %–102 % higher than those of the conventional ZSM-48 (Z-48-C). Meanwhile, the values of turnover frequency (TOF) of Z-48-S3 are increased by 14 %–63 % at similar n-dodecane conversions compared with Z-48-C, which is closely related to the diffusion characteristics. In addition, the depth of hydroisomerization (Iiso) of Z-48-S3 is decreased by 65.3 %, and the isomerization selectivity is increased by 52.5 % at 340 °C compared with that of Z-48-C. Hence, the ZSM-48 molecular sieves synthesized by the recrystallization hydrothermal method are beneficial for controlling the depth of hydroisomerization of the catalyst and reducing the occurrence of the cracking reaction. Meanwhile, the isomer distribution does not change significantly compared with that of the conventional ZSM-48, and the 5-methylundecane is still the main isomerization product.