Hierarchically nanoporous zeolites with three-dimensional mesostructure

Abstract

The work presented in this thesis addresses the synthesis, and characterization of hierarchically mesoporous-microporous zeolites with three-dimensional mesostructure.

Nanoporous materials are of scientific and technological importance owing to their ability to absorb and interact with other ions and molecules on their large surface area. Nanoporous materials can be classified into three types depending on their pore size: 1) microporous [$\Box$ < 2 nm], 2) mesoporous [2 < $\Box$ < 50 nm], and 3) macroporous [$\Box$ > 50 nm] materials. Zeolites are a subset of microporous materials, and are composed of crystalline aluminosilicate frameworks. Roughly, 200 unique zeolite frameworks have been identified so far. Depending on the framework type of zeolites, the micropore diameters, shapes, and connectivities are specified. Some zeolites are excellent solid acid catalysts, cation-exchangers and support materials for other nanoparticles. In addition, zeolites exhibit high chemical, thermal, and mechanical stability. Owing to these remarkable features, zeolites have diverse applications and account for more than 40% of all solid catalysts used currently in the chemical industry. Nevertheless, applications of zeolites are limited to only small molecules that can diffuse through the microporous framework. To extend zeolite applications beyond the microporous range, tremendous efforts have been direct toward the synthesis of crystalline zeolites with mesopores. Mobil Company, in 1992, reported on the synthesis of ordered mesoporous silica MCM-41 possessing a uniform mesopore diameter, where cetyltrimethyl ammonium bromide (CTAB) was used as a mesopore-generating agent. In this synthesis, the mesopores were generated by a supramolecular assembly (i.e., micelle) of the CTAB organic surfactants. This synthetic route has since been further exploited in the synthesis of mesoporous silicas or aluminosilicates with various structures such as lamellar and cubic structures. The mesopores were also tunable with uniform size in a range of 2 to 20 nm by changing the surfactant molecular size or by the addition of a swelling agent. MCM-41 has attracted a great deal of attention as an alternative zeolite material. However, contrary to initial expectations, the MCM-41 mesoporous molecular sieves transpired to be insufficient in terms of acidity and hydrothermal stability for catalytic applications. This was due to the noncrystalline (i.e., amorphous) nature of the mesopore walls. Thus, ordered mesoporous materials built with crystalline alumonisilicates have been long sought over the twenty years. A di-quaternary ammonium surfactant was recently tested as a structure-directing agent for such ordered mesoporous crystalline materials, but the synthesis yielded 2-dimensional MFI zeolite nanosheets. The objective of this thesis is to synthesize crystalline mesoporous aluminosilicates in which the mesopore walls are zeolite-like microporous aluminosilicate frameworks. The mesoporous aluminosilicates built with zeolitic frameworks can extend the application scope of zeolites into the mesoporous range. In order to obtain the crystalline mesoporous aluminosilicates, we have rationally designed zeolite structure-directing surfactants that can simultaneously generate mesopores and microporous crystalline frameworks (i.e., dual-porogenic surfactants). The mesostructures were generated by aggregation of a number of surfactant molecules while the zeolitic crystalline frameworks were directed by quaternary ammonium groups of the surfactants. One notable member of this dual-porogenic surfactant family had a molecular formula of $[C_{18}H_{37}-N{+}(CH_3)_2-C_6H_{12}-N_{+}(CH_3)_2-C_6H_12-N^{+}(CH_3)_2-C_{18}H_{37}][Br^{-}]_3$. By using this surfactant, a hexagonally ordered mesoporous molecular sieve built with a 1.7 nm thick crystalline microporous framework was synthesized. The mesostructure was changed to a three-dimensionally interconnected disordered mesostructure by changing the structure of the dual-porogenic surfactants. The wall thickness was precisely controlled according to the molecular length of head groups. The mesopore diameters were also tailorable according to the surfactant tail length or by the addition of hydrophobic swelling agents. The resultant materials were characterized with X-ray diffraction, Ar adsorption isotherm, electron microscopy, and nuclear magnetic resonance. Among the series of hierarchically nanoporous zeolites, the physicochemical properties of hexagonally ordered hierarchical zeolites were rigorously investigated. The results showed that hexagonally ordered hierarchical zeolites exhibited high thermal/hydrothermal stability comparable to that of conventional zeolites. The acidic strength was somewhat weaker than that of the conventional zeolites. Nevertheless, the acidity was much stronger than that of amorphous mesoporous aluminosilicates such as MCM-41. Furthermore, we disclosed a simple rule of thumb of how to rationally design dual-porogenic surfactants that can generate hierarchically mesoporous-microporous zeolites. The rule can be simply described as follows: a conventional zeolite-structure-directing agent can be covalently joined to a hydrophobic tail through $-(Me)_2N^{+}-$ to obtain a dual-porogenic surfactant. Based on the surfactant design rule, surfactants equipped with enantiomeric pore-generating agents were synthesized to obtain enantioselective, crystalline microporous solids. This approach, however, has seen only minor success thus far. In addition to the synthesis of chiral zeolites, the present synthetic strategy using dual-porogenic surfactants would be further extended to other inorganic materials, such as aluminophosphates, chalcogenides.