Different roles of CNTs in hierarchical HZSM-5 synthesis with hydrothermal and steam-assisted crystallization

Abstract

Hierarchical HZSM-5 zeolites with uniform mesopore distribution were synthesized by hydrothermal synthesis (HTS) and steam-assisted crystallization (SAC) methods using multi-walled carbon nanotubes (CNTs) as hard templates. Compared with the HTS method, the SAC method favored the utilization of CNTs because of lower phase separation degree. It was also found that the crystallization process of HZSM-5 was slowed down after the addition of CNTs in the HTS method, while was accelerated in the SAC method. Furthermore, after the addition of CNTs, the hierarchical samples prepared by the HTS method showed lower Si/Al ratios and higher acid amounts while slight changes in the acid properties were observed for those prepared by SAC method. The synthesized hierarchical HZSM-5 zeolites using the SAC method exhibit similar catalytic activities but better stabilities in the catalytic cracking of n-decane than the HTS samples as a result of the presence of more mesopores.