Large pore molecular sieves

Chapter 3 Synthesis of large pore alumosilicates

The generation of mesopores in the range of 15–20 nm by alkaline treatment of a unidimensional high silica content zeolite, namely ZSM-12 was achieved. The factors that affect the generation of mesopores were thoroughly investigated. For the ZSM-12 with Si/Al ratio of 58, additional mesopores with volume in the range 0.05–0.61 cm³ g⁻¹ were generated without destroying the microporous framework of ZSM-12 by varying the NaOH solution concentration, operation temperature and time. The concentration of NaOH solution was found to be the most dominant factor affecting the desilication process. Meanwhile, parent ZSM-12s with different Si/Al ratios in the range of 31–500 were alkali treated in order to investigate the influence of Si/Al ratios on the desilication of ZSM-12. It was found that the content of tetrahedrally coordinated aluminum in the zeolite frame work also controls the formation of mesopores by facilitating desilication process in alkaline medium. For the ZSM-12 with relatively low Si/Al ratios ranging from 31 to 58, relatively high NaOH concentration was favorable for the generation of mesopores while preserving the microporous structure and acidic property of ZSM-12. However, for the samples with higher Si/Al ratios, lower NaOH concentrations favored the creation of mesopores. In terms of uniform mesopore generation, ZSM-12s with Si/Al ratios in the range of 31–58 were more appropriate. Desilicating the as synthesized ZSM-12 samples (prior to calcination) is much more difficult than desilicating calcined ZSM-12 samples of comparable Si/Al ratios. The diffusivity of long chain hydrocarbons through the framework of alkaline treated ZSM-12 samples improved compared to the untreated ones. The ease to control the mesoporous network developed in ZSM-12 by alkaline treatment offers a new and economic path for the improvement of diffusion in the zeolite framework.

Synthesis of ZSM-12 using tetraethylammonium bromide (TEABr) as the template was investigated. Among the various parameters that affect the crystallization of ZSM-12, aluminum content of the gel, OH⁻/SiO₂ and TEA/SiO₂ ratios were the important determinants. Systematic variations of these parameters revealed that the TEABr-assisted synthesis had many similarities to the synthesis using TEAOH template reported earlier, with the exception that the OH⁻/SiO₂ ratios had to be maintained at lower values. Furthermore, the OH⁻/SiO₂ ratios favorable for ZSM-12 formation lie in a very narrow range. The source of alkalinity also affected the rate of crystallization and the composition of the product. The crystallization was found to be faster and better incorporation of aluminum in the zeolite framework was obtained when NaOH was used to provide alkalinity rather than KOH. Successful synthesis of highly crystalline ZSM-12 samples with Si/Al ratio around 30 was achieved using a minimal amount of relatively inexpensive TEABr (TEA/SiO₂=0.125).

Aluminum-27 NMR spectroscopy unambiguously revealed that all aluminum atoms are incorporated in the zeolite framework in tetrahedral coordination. The NMR line-widths of aluminum signals of the calcined samples were significantly larger than those with template incorporated samples. Spin-lattice relaxation times, conventional and rotating frame, as well as magic angle spinning (MAS) cross-polarization data with variable contact time support that there is a significant proton reservoir in the aluminum framework. The NMR data indicate that many distorted tetrahedral sites are formed upon removal of the template and some of these sites contain Al–OH moieties.

An attempt was made to synthesize MAZ-type zeolites with enhanced Si/Al ratio by adding organic molecules to conventional aluminosilicate hydrogels for MAZ zeolites containing sodium and tetramethylammonium cations. Molecular models of candidate templates were built and optimized inside the 12-membered ring (12-MR) pore of the MAZ structure. The hexamethonium cation, 1,6-diaminohexane and 1,6-hexanediol show the most favorable calculated Lennard-Jones potential. The crystallization in presence of these organic molecules led to the formation of MAZ-type zeolite. The samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive analysis of X-rays (EDAX), chemical analysis, nitrogen adsorption, thermogravimetric analysis/differential thermal analysis (TGA/DTA), ²⁹Si and ²⁷Al magic-angle spinning (MAS) nuclear magnetic resonance spectroscopy (NMR) and ¹³C cross-polarization (CP)/MAS NMR. The hexamethonium cation and 1,6-diaminohexane are incorporated into the MAZ crystals, whereas 1,6-hexanediol is not. Incorporation of 1,6-diaminohexane caused a higher incorporation of silicon in the framework, especially in the T1 sites delineating the 12-MR channels. The upper limit of silicon incorporation was investigated by systematically reducing the aluminum content of the gel. The highest Si/Al ratio achieved by this approach was 5.3. A molecular crystallization mechanism for mazzite is proposed, explaining this upper limit as well as the crystal morphology changes depending on the aluminum content of the synthesis gel.

Photocatalytic water oxidation to O₂ takes place at the solid/water phase boundary of an appropriately prepared thin AgCl layer in the presence of a small excess of Ag⁺. This water oxidation reaction shows self-sensitization: as the reaction proceeds, the sensitivity is extended from the near-UV—visible towards the red range. This means that new photo active colour centres are formed upon near UV illumination composed of reduced silver species. The quantum yield per redox equivalent for O₂ evolution upon illumination with near UV light (340–390 nm) is ∼0.8 and it is the same upon illumination with blue light (420–480 nm). In the green range it is ∼0.5. We discuss parameters controlling these reactions. Some of our observations parallel those made by Henry Fox Talbot 160 years ago when he made his first “photogenic drawings” on silver chloride containing paper.

Zeolite microcrystals are investigated as hosts for supramolecular organization of clusters, complexes and molecules. They offer possibilities to design precise and reversible functionalities which have the potential to become useful in a solar energy conversion system because in favourable cases very stable materials have been obtained. The possibility to arrange zeolite microcrystals of good quality and narrow size distribution as dense monograin layers on different types of substrates allows the discovery of specific properties. In the present context, three functionalities are of special importance: intrazeolite ion transport, intrazeolite charge transport and intrazeolite excitation energy transport. All of them have been clearly demonstrated experimentally although there are still some controversies going on. The zeolite acts as a host in each case mentioned. It is not actively involved in the corresponding processes, but provides the necessary geometrical and chemical environment.

Favourable conditions for finding an efficient antenna device are a high concentration of monomeric dye molecules with a large Förster energy transfer radius, high luminescence quantum yield and ideal geometrical arrangement of the chromophores, as well as an optimal size of the device. Very interesting supramolecular devices can be realized by enclosing dyes inside a microporous material such that the volume of the cages and channels is able to uptake monomers only, but not aggregates.

The preparation of metallosilicates with MTW structure with pores consisting of 12-membered rings and pore openings of 5.5×5.9in the presence of the redox metals Cr, Fe, Mn and V is described. The incorporation of Cr(III), Fe(III) and V(IV) in the lattice was confirmed by physico-chemical characterization. In the Mn-silicate, the major part of the Mn cations was present as extraframework manganese oxide. After the calcination, oxidation to Cr(V) and Cr(VI), V(V) and Mn(VII) occurred while for the Fe-silicate the formation of extraframework iron oxide was observed. The metallosilicates were tested as catalysts for cyclohexane oxidation with aqueous H₂O₂ or tert-butylhydroperoxide. Cr, V and Mn MTW showed good activities, which increased in the absence of water, indicating that the metallosilicates are hydrophilic.