Conclusions

The overall aim of this thesis was to synthesize crystalline microporous materials through several new synthetic routes which are different from the conventional hydrothermal route or trial-and-error method. In Chapter 4, 5 and 6, top-down treatment methods, ionic liquid assisted routes and ionothermal synthesis were applied for synthesizing zeolites and zeolite analogues, respectively. In Chapter 7, a MOF material was synthesized targeting the hemilabile property by carefully selecting the ligands.

Two Ge-zeolites, IWW and ITH, were selected for the top-down treatments.1 The experiments showed that the chemical compositions and the Ge distributions in the parental zeolites had crucial effects on the structure of the products. A suitable chemical composition of the pillar units was a key parameter for the successful disassembly process. The IWW zeolite with a D4R composition of [6Ge, 2Si] was fully pulled apart into layer structures but the IWW zeolite with a D4R composition of [4Ge, 4Si] was only disassembled to a connected structure after acidic treatment. For ITH zeolites, the experiments indicated that if the framework contained some pure silica or high silica

D4Rs, the zeolites were hydrolytically stable to acidic treatment or partially hydrolysed.

ITH-zeolites with only [4Si, 4Ge] D4Rs in the framework can be fully separated. Meanwhile, the Ge distribution also affected the structures of the products from the top- down treatment. For IWW zeolites, the Ge occupied the D4R sites exclusively. The layer structure kept intact after hydrolysis. Attempts at reassembling the hydrolysis products to the 3D zeolitic structures led to the recovery of the IWW structure. However, for the ITH zeolites, the Ge atoms locate in both the layer and the D4R sites. The ITH zeolites with low Si/Ge ratio can be successfully separated but the layer structure was unstable to harsh chemical treatment conditions, which obstructed the following reassembly treatment.

In Chapter 7, a copper MOF material was synthesized using 2-sulfoterephthalic acid (STP) and 4, 4’-bipyridine (Bpy) as the ligands. The STP contains one sulfonate and two carboxylate groups. The variable temperature single crystal structure analysis revealed that the MOF material obtained was flexible and can reversibly transform to a

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new structure upon dehydration. The weakly bonded sulfonate group changed its coordination mode upon dehydration, leading to the structural transformation. Meanwhile, the carboxylate groups were strong enough to retain crystallinity and the framework. The successful synthesis of such a hemilabile MOF material proved that the strategy of targeted synthesis of the flexible MOFs with ligands containing both weaker and stronger coordinating groups could be generalized to other compounds.

In these two chapters, both the Ge-zeolites and the sulfonate-containing MOF show structural flexibility and thus are used to induce targeted structural transformations. Both of the designable compounds have a common feature, which is that they contain weaker bonds in their structures: the Ge-zeolites contain Ge-O bonds that are less hydrolytically stable than a Si-O bond; for the sulfonate-containing MOF, the bonding strengths of the sulfonate-metal bonds are weaker than the carboxylate-metal bonds. Transformations are implemented by exploiting these weaker bonds in the structures. It is noted that to achieve a designable transformation, the weaknesses should be in the right place. The differences in bonding strength make it possible to exploit the weakness while keeping the rest of the structure intact. The work in these two chapters inspire a new way of tailoring crystalline microporous materials.

In Chapter 5, an ionic liquid assisted strategy for the synthesis of zeolites was described. A TON zeolite was synthesized using [Emim]Br as the SDA. The synthetic conditions were investigated and the products were characterised. The results indicated that the presence of the ionic liquid was important for the crystallization of the zeolite and played the role of SDA. The ILA strategy exhibits many advantages such as the readily recyclable SDA; the high yield of the zeolites and the minor production of waste water. This ILA strategy is thus considered as a new “green chemistry” synthetic route for zeolites.

In Chapter 6, a new ionic liquid, 1-(2-hydroxyl-ethyl)-3-methylimidazolium chloride ([HOEmim]Cl), was synthesized and used as an SDA and solvent for the ionothermal synthesis of aluminophosphates. The product was a triclinic deformed CHA type framework. Moreover, large single crystals were obtained at higher temperature, which may be because of the increase of the mass transfer of the crystal growth. The structural analysis indicated the [HOEmim] cations were decomposed to 1, 3-dimethyl imidazolium cations as the SDA for CHA framework.

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In Chapter 5 and 6, utilizing ionic liquids for the syntheses of microporous materials was investigated. To conclude, ionic liquids are suitable SDAs for the synthesis of both zeolites and zeolite analogues. The synthetic flexibility of ionic liquids indicates the possibility of synthesizing zeolites and zeolite analogues with different framework structures. For instance, in the synthesis of aluminophosphates, the ionic liquids can simultaneously play the role of the solvent and this is known as ionothermal synthesis strategy, however, for the synthesis of zeolites using common ionic liquids, a certain content of water is still required. Although a true ionothermal synthesis of zeolites was not achieved in Chapter 5, utilizing ionic liquids to assist the synthesis of zeolites still has some advantages, especially for environmental benefit.