Zeolites synthesis framework

Assembly of zeolite seeds, protozeolite clusters, etc, into mesoporous frameworks -zeolite synthesis mixture is crystallized for a certain period of lime to generate zeolite fragments and then treated with surfactant to assemble the mesoporous structure [58]... 

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. 

Framework infrared has also been used to look for the formation of a zeolite during synthesis. Since many of the secondary building units can be detected in the infrared spectrum, it is possible to see zeolite formation at very early stages in the synthesis. In fact, zeolite formation can be detected in the infrared before crystallinity is observed by X-ray diffraction. Most of the reported work has been done by sampling the zeolite synthesis at various stages, isolating the soUds and measuring infrared spectra of the dried samples. 

In zeolite synthesis, large cations such as tetramethylammonium (NMe4" ) and tetrapropylammonium (N(C3H7)4" ) can be used as a template around which the aluminosilicate framework crystallizes with large cavities to accommodate the ion. On subsequent heating the cation is pyrolysed, but the structure retains the cavities. Such structures formed around a single molecule template, with pore sizes between 200 and 2000 pm, are known as microporous. 

The synthesis of the open-framework zeolites improved the number of accessible active sites for catalysis dramatically. It is estimated that ZSM-5 has a turnover of more than 300 molecules per active site per minute during the cracking process, and that other... 

In this review, we focus on cluster models of Br0nsted acid sites, bridging hydroxyl groups that result from the incorporation of trivalent aluminum atoms into the siliceous framework during synthesis. These sites are by no means the only active sites within zeolites, but they are among the best characterized. 

Control of pH is critical in the determination of the Si/Al ratio. As the pH increases, the ability of the silicate to condense decreases because of a decrease in the amount of Si-CCspecies relative to Si-OH. The anionic form is necessary in order for the initial nucleophilic attack to take place. In contrast, the condensation rate of Al(OH) 4 remains constant and so aluminium-rich zeolites crystallise preferentially at high pH and vice versa. Zeolite synthesis also depends on a wide range of experimental parameters, including concentrations and degree of supersaturation, the source of the framework materials, solvent... 

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],... 

Elements such as B, Ga, P and Ge can substitute for Si and A1 in zeolitic frameworks. In naturally-occurring borosilicates B is usually present in trigonal coordination, but four-coordinated (tetrahedral) B is found in some minerals and in synthetic boro- and boroaluminosilicates. Boron can be incorporated into zeolitic frameworks during synthesis, provided that the concentration of aluminium species, favoured by the solid, is very low. (B,Si)-zeolites cannot be prepared from synthesis mixtures which are rich in aluminium. Protonic forms of borosilicate zeolites are less acidic than their aluminosilicate counterparts (1-4). but are active in catalyzing a variety of organic reactions, such as cracking, isomerization of xylene, dealkylation of arylbenzenes, alkylation and disproportionation of toluene and the conversion of methanol to hydrocarbons (5-11). It is now clear that the catalytic activity of borosilicates is actually due to traces of aluminium in the framework (6). However, controlled substitution of boron allows fine tuning of channel apertures and is useful for shape-selective sorption and catalysis. 

There are many curious features of zeolite synthesis. Thus, different framework structures are formed by the same amine, and the same framework structure is also formed by the use of different amines [11]. Such observations have prompted investigations into the role of the amines in the formation of these structures. The role of the organic amine in the synthesis of zeolites can be classified into three types templating , structure-directing and space-filling. Here,... 

Fig. 3.3 Relationship between unit cell size and framework al content 3.2.2.2 MFI and BEA zeolite synthesis...

Site III). Each pseudo unit cell contains 27 water molecules, and 20 of these form a pentagonal dodecahedron that fines the a-cage (Figure 17). This water structure is found in some water clathrates and in LTA surrounds the central Na ion with its attendant three water molecules. It is cation water assemblages like this that are hkely to be the precursors for the formation of zeolite frameworks in synthesis gels (see Section. 4.2.3). 

These calculations yield, subject to some simplifying assumptions, relative T-site alumimun substitution energies computed (1) for the thermodynamic equilibrium state, (2) at zero K and (3) for models devoid of non-firamework species. Framework zeolites, metastable structures, are produced under luetic control and if, as indicated by the most recent calculations, the relative T-site substitution energies for the (Cerent sites are not grossly disparate, the actual distributions in reed materitds will be determined by the particular conditions of synthesis. As the molecular-level mechanisms of zeolite sjmthesis remain obscure, we especially need some experimental indicator of which sites are actually adopted by aluminum in real MFI-framework materials. 

There are about a hundred zeolite framework types. Some of these are found in nature, and are classified as minerals. However, most zeolite framework types are not found in nature but are synthetic, with the first having been reported in 1956. More are reported every year due to active research efforts in zeolite synthesis. 

N. Rajic, N.Z. Logar, and V. Kaucic, A Novel Open Framework Zincophosphate Synthesis and Characterization. Zeolites, 1995, 15, 672-678. 

This method provides an alternative to the removal of the organic SDA occluded in the framework of porous materials, which can overcome some drawbacks of high-tempera-ture calcination such as the release of NO resulting from the combustion of organic SDAs (most SDAs are quaternary ammonium cations or amines). Recycling of the SDAs can significantly decrease the overall zeolite-synthesis cost. 

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