(765f) 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.