Synthesis and Catalytic Performance of Hierarchical HZSM-5 Zeolites By Recrystallization

Synthesis and Catalytic Performance of Hierarchical HZSM-5 Zeolites by Recrystallization ^*

Zhenheng Diao, Guozhu Liu, Guozhu Li, Jijun Zou, Li Wang^**

Key Laboratory of Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China.

 

Abstract

Over the last years numerous efforts have been put forth to the design of hierarchical zeolites. Desilication performed by the treatment with NaOH has recently received significant attention, while a major problem often encountered is the loss of crystallinity and acid sites, which causes the decrease in the catalytic performance. Tetraalkylammonium hydroxides (TAAOH) were also used as alkali source because tetraalkylammonium cations could act as pore-directing agents under alkaline conditions and more uniform intracrystal mesopores could be obtained than in sodium hydroxide solution. In this work, considering the differences of model effects and pore-directing effects among TMAOH, TEAOH, TPAOH and TBAOH, we prepared four kinds of hierarchical zeolites (denoted as HZ5-TMAR, HZ5-TEAR, HZ5-TPAR and HZ5-TBAR) by desilication with TAAOH and recrystallization with CTA⁺ and TAA⁺ as templates, and investigated the effect of hierarchical structure on catalytic performances.

The results of XRD indicate that HZ5-TMAR, HZ5-TEAR and HZ5-TPAR are ZSM-5/MCM-41 composites. While HZ5-TBAR is pure ZSM-5 phase and no MCM-41 phase could be detected. Compared to others, HZ5-TEAR exhibits higher intense of the characteristic diffraction peaks of MCM-41 phase, which indicates that more ordered MCM-41 structure was formed during recrystallization. TEM reveals that HZ5-TMAR, HZ5-TEAR and HZ5-TPAR are all core-shell structured zeolites with ZSM-5 single crystal as the core. The shell of HZ5-TEAR and HZ5-TMAR are composed of pure MCM-41 phase with thickness of 56 nm and 30 nm respectively, and the shell of HZ5-TPAR is the mixture of MCM-41 and ZSM-5. Pyridine sorption experiments demonstrate that there is no significant difference among the number of Lewis acid sites of all zeolites, and the number of total or strong Brønsted acid sites decreased by the following order: HZSM-5> HZ5-TPAR> HZ5-TMAR> HZ5-TEAR> HZ5-TBAR. While the number of accessible Brønsted acid sites, determined by 2,6-di-tert-butylpyridine sorption, decrease by the following order: HZ5-TEAR> HZ5-TMAR> HZ5-TPAR> HZ5-TBAR> HZSM-5, which is in accord with their external specific surface areas. It seems that thicker and more ordered MCM-41 shell of HZ5-TEAR could increase external specific surface area and improve the accessibility of Brønsted acid sites.

In order to investigate effect of hierarchical structure on the catalytic activity, the catalytic cracking of n-dodecane over hierarchical zeolites and parent HZSM-5 were investigated in the form of coatings on the inside of a tubular reactor at 550 ^oC and 4 MPa. All hierarchically structured zeolites showed improved catalytic activity and stability compared to the parent HZSM-5 zeolite and the conversion of n-dodecane decreased by the following order: HZ5-TEAR > HZ5-TMAR >HZ5-TPAR > HZ5-TBAR. Virtually, catalytic activity was directly related to the number of accessible Brønsted acid sites. We proposed that the number of accessible Brønsted acid sites is responsible for catalytic activity, and the hierarchical structure of HZ5-TEAR is superior for catalytic cracking of n-dodecane because the thicker and more ordered MCM-41shell could provide more accessible Brønsted acid sites than others. Moreover, the shell could act as a pre-cracking location and provide superior diffusion passage for reactants and products.

---

*Financial support from the National Natural Science Fund of China (U1232134) is gratefully acknowledged.

^**Corresponding authors: (Tel: 022-27892340, Email: wlytj @tju.edu.cn)