(169h) Studying the Synthesis of Hierarchical Siliceous Zeolites By Post Synthetic Zeolite Surfactant-Templating Method

The microporous nature of the zeolites, composed of the narrow pore distribution of molecular dimensions and high internal surface area, result in shape selective properties for zeolite materials. These micropore structures (< 2 nm) however also impose slow diffusion rates within zeolites leading to transport limitation for the materials in the applications of catalysis and separation. This transport limitation can be tackled by introduction of mesopores (> 2 nm) in the zeolitic microporous structure and development of Hierarchical Molecular Sieves (HMSs).^1,2 A variety of strategies can be pursued to create mesoscale porosity, such as numerous templating techniques and post-synthetic methods. Post-synthetic modification of zeolites in the presence of surfactants under basic conditions is one of the easy, scalable, and low-cost approaches. The simplicity and efficiency of this “Surfactant Templating” strategy make it very attractive for industrial applications.³ However, the fundamentals of the formation of mesopores in zeolites using the surfactant-templating method have not been fully understood.⁴ The synthesis of hierarchical zeolites with micropore size in the range of 0.4 -1.2 nm (FAU, BEA, MFI, MOR) using surfactant-templating is very well studied. But when it comes to zeolite frameworks with relatively smaller micropore sizes, like CHA and LTA, the research area is unexplored. And these small pore molecular sieves have proved to be promising when it comes to separation of important industrial gases like CO₂, CH₄, propane, and propylene.^5,6 And hence the hierarchical small pore zeolites would play an even bigger role in the field of olefin/ paraffin and CO₂ gas separation.^7,8 So, it is important to study the fundamentals of surfactant templating, one of the most industrially scalable post-synthetic techniques to synthesize hierarchical molecular sieves and extend it to promising zeolitic frameworks.

According to one of the proposed mechanisms, local defects are created by cleavage of Si-O-Si which is followed by the uptake of cationic surfactant species at these defects.⁹ Since, defects are at the center of this proposed mechanism, we have explored surfactant templating of zeolitic frameworks with different defect densities and distributions. Along with those the effects of pore dimensions of zeolite and presence of aluminum in the microporous framework were also inspected to create the base knowledge so that this post-synthetic treatment be extended to zeolites with different physical and chemical features.

In this study, we carried out post-synthetic surfactant templating on a series of zeolites with different framework structure, aluminum content and defect densities. This templating treatment was performed at different concentrations of NaOH in presence of cetyltrimethylammonium ammonium bromide (CTAB). We demonstrated that the extent of mesopore incorporation in zeolites depends on the fraction of silicon atoms at the defect sites in the parent framework. This presents an opportunity to engineer a controllable post-synthetic method to incorporate hierarchy which is industrially scalable as well. It was also exhibited that the surfactant templating method is more effective for zeolites with large pore structure than small-pore zeolites. This might be attributed to different extents of CTAB uptakes. Heteroatoms in the zeolite frameworks also play a critical role in mesopore formation. For example, Al-containing zeolites displayed more resistance to the framework rearrangement necessary of mesopore formation.

References:

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