Synthesis of Hierarchical MOR-Type Zeolites with Improved Catalytic Properties.

Zeinab Mcheik,Tayssir Hamieh,Joumana Toufaily,T Jean Daou,Ludovic Pinard

doi:10.3390/molecules26154508

Abstract

Hierarchical MOR-type zeolites were synthesized in the presence of hexadecyltrimethylammonium bromide (CTAB) as a porogen agent. XRD proved that the concentration of CTAB in the synthesis medium plays an essential role in forming pure hierarchical MOR-type material. Above a CTAB concentration of 0.04 mol·L−1, amorphous materials are observed. These hierarchical mordenite possess a higher porous volume compared to its counterpart conventional micrometer crystals. Nitrogen sorption showed the presence of mesoporosity for all mordenite samples synthesized in the presence of CTAB. The creation of mesopores due to the presence of CTAB in the synthesis medium does not occur at the expense of zeolite micropores. In addition, mesoporous volume and BET surface seem to increase upon the increase of CTAB concentration in the synthesis medium. The Si/Al ratio of the zeolite framework can be increased from 5.5 to 9.1 by halving the aluminum content present in the synthesis gel. These synthesized hierarchical MOR-type zeolites possess an improved catalytic activity for n-hexane cracking compared to large zeolite crystals obtained in the absence of CTAB.

Highlights

Zeolites have a rigid framework structure with pores constrained in their sizes at the molecular level [1,2,3,4]
Hierarchical zeolites can be obtained with different approaches: (i) Post synthesis treatment using either a destructive approach, which consists of the dealumination or desilication of large zeolite crystals to create mesopores and macropores, or a constructive approach by assembling nanocrystals and creating hierarchical porosity [9,15] and (ii) one-shot synthesis using hard template strategy [16,17,18,19,20,21,22,23] or a soft templating
X-ray diffraction (XRD) proved that the concentration of CTAB in the synthesis medium plays an essential role in forming pure hierarchical MOR-type material

Summary

Introduction

Zeolites have a rigid framework structure with pores constrained in their sizes at the molecular level [1,2,3,4]. They exhibit attractive features that are superior to those of silica, such as their unique pores and their various framework compositions accessible by changing the Si/Al ratio or by substituting silicon atoms with atoms other than aluminum such as germanium or phosphorus. Several studies have shown that additional larger pores (usually mesopores) can overcome possible diffusion/transport limitations of the smaller micropores of the zeolite [9,13,14]. Hierarchical zeolites can be obtained with different approaches: (i) Post synthesis treatment using either a destructive (top-down) approach, which consists of the dealumination or desilication of large zeolite crystals to create mesopores and macropores, or a constructive (bottom-up) approach by assembling nanocrystals and creating hierarchical porosity [9,15] and (ii) one-shot synthesis using hard template strategy (use of carbon nanotubes or polymers or silica beads as reactors) [16,17,18,19,20,21,22,23] or a soft templating

Methods

Results

Conclusion