Zeolites synthesis conditions

In the search for wide-pore zeolite synthesis conditions, therefore, efforts should be concentrated on the field where zeolites are produced with a large amount of isomorphous substitution. 

Typical zeolite synthesis conditions are strongly alkaline, enabling silica and alumina to be dissolved at concentrations that permit a sufficient supply of species to nuclei and growing crystals. At these high pHs the alumina exists as tetrahedral aluminate Al(OH)4 species, whereas silica exists as a variety of oligomeric species in solution (Section 5.3.3). The hydroxide ion is also acting as a mineraliser under these conditions, i.e. it increases solubility and catalyses the subsequent formation of Si-O-Si bonds. Fluoride ions can also perform this function, even at neutral pH, and this is described further in Section 5.4.3. 

The organic templates could remain intact under zeolite synthesis conditions (high pH) and be cleavable at conditions that would not destroy the assembled zeolite. After the synthesis... 

A1 3Q MAS NMR spectroscopy and DFT/MM calculations were employed to study the A1 siting in eleven differently synthesized samples of the ZSM-5 zeolite. The results of our study reveal that the occupation of the distinguishable framework T sites by A1 and the concentration of A1 in these T sites are neither random nor controlled by a simple rule. They both depend on the conditions of the zeolite synthesis. We further found that at least 10 out of the 24 distinguishable framework T sites are occupied by A1 in our ZSM-5 samples. 

We have demonstrated that a combined experimental (27A1 3Q MAS NMR) and theoretical (QM-Pot employing the bare framework model) approach represents a powerful tool for the determination of the local geometry of framework A104 tetrahedra, the prediction of27A1 isotropic chemical shifts in hydrated silicon rich zeolites, and the identification of A1 siting in the framework of silicon-rich zeolites. Experimental evidence is provided for the occupation of at least 10 out of 24 distinguishable framework T sites by A1 atoms in silicon-rich ZSM-5. The conclusion is reached that the A1 distribution over the framework T sites is neither random nor controlled by a simple rule, but depends on the conditions of the zeolite synthesis. 

Zeolite synthesis around a metal complex was introduced by Balkus [40]. The metal complex is added to the zeolite synthesis mixture and is incorporated into the zeolite structure during the zeolite synthesis. Of course, this procedure is only applicable when the metal complex is soluble in the synthesis mixture and can withstand the hydrothermal synthesis conditions. Another requirement is that the zeolite structure-directing agent added must be removable by a milder... 

Table 10.1 Summary of common zeolite structures used in membrane applications along with important properties and typical synthesis conditions. 

Table 1. Synthesis conditions for ferrisilicate analogs of zeolites.

A lot of work [4,68,69] has been done using A1-NMR and Si-NMR spectra on the isostructural synthetic zeolite fl to determine Si/Al distribution in the two tetrahedral sites. These data clearly indicate that Si and Al distribution in zeolite (I is not random in nature, with an A1tz/A1ti ratio in the range 0.9-1.6, depending on the different Si/Al ratios and different synthesis conditions, and with a preferential location in the 6-memberedring, as in natural mazzite. 

Besides the conventional zeolites, several novel zeolite analogues such as the ALPOs (aluminophosphates), MeALPOs (divalent-metal (Me) substituted aluminophos-phates), SAPOs (silicon substituted aluminophosphates) and so on have been synthesized (Davis Lobo, 1992). Wilson et al. (1982) first reported the synthesis of microporous ALPOs. ALPO synthesis differs from zeolite synthesis in that it involves acidic or mildly basic conditions and no alkali metal ions. Some members in the ALPO... 

The hydrothermal method has been employed in recent years to synthesize a variety of solids that include aluminium phosphates (ALPOs) and other microporous transition-metal phosphates and transition-metal polychalcogenides (Davis Lobo, 1992 Haushalter Mundi, 1992 Liao Kanatzidis, 1990, 1992). Unlike zeolites, synthesis of ALPOs requires acidic or mildly basic conditions and no alkali metal cations. A typical synthetic mixture for making ALPO consists of alumina, H3PO4, water and an organic material such as a quaternary ammonium salt or an amine. The hydrothermal reaction occurs around 373-573 K. The use of fluoride ions, instead of hydroxide ions as mineralizer, allows synthesis of novel microporous materials under acidic conditions (Estermann et al, 1991 Ferey et ai, 1994). 

The section on crystallization comprises zeolite synthesis, kinetics and mechanism of formation, stability relationships, recrystallization processes as well as the genesis of natural zeolites. Recent advances in this field have been surveyed, and some new perspectives have been outlined in the review by E. M. Flanigen. Most of the studies in this field are still empirical because of the complexity of the systems involved. Considerable progress has been made, however, towards a better understanding of the processes and mechanisms governing zeolite crystallization. It is not unreasonable to expect that conditions for synthesizing new zeolite structure types can eventually be predicted. 

Since the first synthesis of TS-1 in 1983 [1], considerable efforts have been devoted to the synthesis of titanium-containing zeolites [2, 3]. Recently, Ti-beta, a large-pore molecular sieve, has been extensively studied [4, 5]. Owing to its unique large-pore channel system, Ti-beta seems to be more active than the medium-pore TS-1 catalyst for the oxidation of cyclic and branched alkenes with aqueous hydrogen peroxide. Under the usual synthesis conditions, however, Ti-beta crystallizes with some Al as a framework constituent [4], This leads to the presence of acid centers, which may have a detrimental effect on the activity or selectivity of this type of catalyst. Since 1992, the discovery of a new family of mesoporous molecular sieves has received much attention [6,7], Because of their mesoporous nature (20-100A), the Ti-MCM-41 zeolites may be useful as oxidation catalysts for larger molecules [8], In this... 

Since titanosilicates generally require specific synthesis conditions in comparison to silicalites and aluminosilicates, many efforts made to synthesize those of numerous zeolite structures have led to a very limited success. This has also been the case with the MWW zeolite. Although it is possible to hydrothermally synthesize MWW... 

Direct synthesis of metal-substituted zeolites has long been sought. However, since the post-synthesis modifications can be made under wide-ranging conditions (temperature, solvent, atmosphere, pH, etc.) far from those for the zeolite synthesis, the modifications of zeolites present us with powerful indirect methods for manipulating the properties of zeolites. Therefore, the fine-tuning of the properties of zeolites will continue to be achieved by developing various post-synthesis modification procedures as well as direct synthetic techniques. 

Rebrov et al. [158] synthesized ZSM-5 zeolite in micro channels. The main focus of this work was to assess the performance benefits of zeolitic coatings in micro channels compared with conventional zeolite-based pellets and powders. The coatings were performed in a sandwich of two plates of 1 cm length and width at a thickness of 2 mm. The plates carried seven channels each, which were 1 cm long and had a diameter of500 pm. The plates were positioned at a distance of280 pm from each other in the housing of the reactor. A zeolitic film of one crystal thickness was formed under the optimum synthesis conditions, which were determined as a water/silicon ratio of 130 and a template/aluminum ratio of 2 at a temperature of 130 °C after 35 h on a flat plate. The Si/Al ratio of the zeolite, which also lowers the crystal size when decreased, was optimized to a value of 28. 

The role of the template in the synthesis is not merely as a porogen on the contrary, it is also responsible for many key functions [5, 9, 10]. The template (typically cationic) balances the negative charge that characterizes zeolitic framework, due to the isomorphic substitution of Si(IV) by Al(III), prearranges the secondary building units (SBUs) toward the zeolitic framework, improves the gel synthesis conditions, especially the solubility of the silica precursors, and favors the thermodynamics of the reaction by stabilizing the porous zeolite framework. 

An alternative method used for entrapment of large complexes into zeolite crystals is known as the so-called zeolite synthesis method .[67 701 In this method transition metal complexes are added to the synthesis mixture from which a faujasite zeolite is obtained. Therefore, the complex should be stable and dissolved in the medium in the conditions of zeolite synthesis, i.e. at elevated pH (> 12) and temperature (around 100 °C). It is not entirely clear whether occluded complexes are positioned in faujasite supercages or in cracks or defects of the crystals. To assure occlusion of isolated MePc complexes rather than of their clusters, the occluded amounts must be limited, implying the use of very active complexes. Ru and CoPcF17 complexes have been reported to show good activity and resistance to leaching.[67 701... 

Hydrophobic zeolites, as well as the all-silica zeolites or zeolites with a very small aluminum content, possess high capacity for adsorbing organic compounds dissolved in water. Some recent studies demonstrated that hydrophobic, dealuminated zeolites adsorbed organic compounds from water as effectively as activated carbon [2,37,88,89,214], The hydrophobicity of zeolites is controlled basically by changing the Si/Al ratio in the framework by synthesis conditions and postsynthesis modification treatments [215],... 

With TPA the very fast nucleation prevents the formation of other zeolitic phases while the stability prevents transformations to other products. TEA, on the other hand, may be used for the synthesis of other Si-rich zeolites such as mordenite, ZSM-12, -20, —25, beta, and Nu-2 (20) as well since it evidently exerts no specific structure-directing influence during nucleation. Depending on the precise gel composition and synthesis conditions, it can thus easily be envisaged that other zeolites may form which are stabilized by TEA incorporation. 

It is important to know what species are present at the beginning of the reaction. Silicate and aluminate solutions have been well studied so that one can be reasonably sure what species are present in a given solution of known concentration and pH. Aluminate solutions have been shown to contain only one type of ion at high pH the tetrahedral AI(OH)4 ion (1). It is only when the pH drops towards neutral that other, polymeric ions appear which ultimately give way to AI(H20) 3+ in acid conditions. The tetrahedral aluminate ion is the important species for normal zeolite synthesis. 

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