Zeolites, framework silicates

M ions (among which Na+ from NaCl) interact with the aluminosilicate gel (and eventually direct its structure towards a zeolite framework by template effect), more readily than the Na+ which were originally associated with the silicate anions. 

When tripropylamine or tributylamine is used instead of the corresponding Alk N salt, ZSM-5/11 mixed phases (intergrowths ) are formed, suggesting that the Alk N species are less efficient in directing a specific structure. Unexpectedly, Bu N yields essentially a ZSM-5-rich phase while ZSM-11-rich phases are preferentially obtained with Pr N (XRD data). When an organic molecule acts as template towards (alumino)silicate species to form an ordered zeolitic framework, the latter is supposed to organize itself around the host organic species in such a way that a complete... 

Finally, some authigenic clay minerals are produced by the reaction of seawater with fresh volcanic glass. This commonly occurs near mid-ocean ridges and rises or where lava from coastal volcanoes flows into the sea. Clay minerals produced by this process are primarily smectites, such as montmorillonite, and a type of framework silicate called zeolites of which phillipsite and clinoptilite are the most common marine examples. Zeolites are characterized by three-dimensional frameworks with large cavities that... 

Recently, the preparation of metallosilicates with MFI structure, which are composed of silicone oxide and metal oxide substituted isomorphously to aluminium oxide, has been studied actively [1,2]. It is expected that acid sites of different strength from those of aluminosilicate are generated when some tri-valent elements other than aluminium are introduced into the framework of silicalite. The Bronsted acid sites of metallosilicates must be Si(0H)Me, so the facility of heterogeneous rupture of the OH bond should be due to the properties of the metal element. Therefore, the acidity of metallosilicate could be controlled by choosing the metal element. Moreover, the transition-metal elements introduced into the zeolite framework play specific catalytic roles. For example, Ti-silicate with MFI structure has the high activity and selectivity for the hydroxylation of phenol to produce catechol and hydroquinon [3],... 

If ZSM-5 type zeolite, which includes a considerable amount of aluminium in the zeolite framework, is treated with metal chloride vapor at elevated temperatures, the metallo-alumino-silicate will be prepared. When ZSM-5 (Si/A1=41) was treated with antimony trichloride vapor at 923 K for 2 h, two absorption... 

Examples of these framework silicates include the feldspars, zeolites and ultramarines (see Clays Molecularsieves). 

For pure Si-MCM-41. this band has been assigned to the Si-O stretching vibrations and the presence of this band in the pure siliceous is due to the great amount of silanol groups present. A characteristic absorption band at about 970 cm 1 has been observed in all the framework IR spectra of titanium-silicalites. It was also reported that the intensity of 970 cm 1 band increased as a function of titanium in the lattice[17] and this absorption band is attributed to an asymmetric stretching mode of tetrahetral Si-O-Ti linkages [18] in the zeolitic framework. The increase in intensity of this peak with the Ti content has been taken as a proof of incorporation of titanium into the framework. 

The band observed at 3745 cm"1 is similar in frequency to that of silanol groups in silica, and Angell and Schaffer (147) attributed it to a Si-OH-stretching vibration. No structural position was assigned with certainty, although it possibly arises from siliceous material occluded within the zeolite structure. It has also been ascribed to Si-OH groups terminating the zeolite framework. 

Conversely, in many other cases studied, the stabilization of hydrophobic clathrasils, zeosils and very high silica zeolitic frameworks is induced by neutral guest molecules that only fill the channels and cavities (11, 12). They are thought to form a solid solution on the growing crystals and thereby lower the chemical potential of the framework (13). The energy required to stabilize such a framework mostly derives from weak Van der Waals bonds between the guest molecule and the siliceous framework (12). In that respect, the temptation of ZSM-5 with alcohols (14) or ethers (15) would also have to be interpreted in terms of a pore filling model. 

Zeolites — are any of various natural or synthesized hydrous framework silicates, consisting of interlocked tetrahedrons of SiC>4 and AIO4, with a ratio (Si + Al)/0 equal to 0.5. Z. were firstly discovered and named in 1756, by the Swedish mineralogist Axel Fredrick Cronst-edt, in case of stilbite. Other natural z., as, e.g., analcime, heulandite, natrolite, and stilbite, are often formed near volcanic activity centers, have been formed from rocks by interaction with alkaline groundwaters. Synthetic z., produced since the 1930s, can be made by a slow crystallization process of a silica-alumina gel in the presence... 

As a result of the interfacial processes on rocks and soils, the structure and chemical bonds of the sorbed compounds can be changed. For this reason, different chemical reactions can be initiated in which the components of rocks or soils act as catalysts. The most important mineral catalysts are zeolites and clay minerals. Naturally, the different oxides also have catalytic effects, and nowadays some of them are being artificially produced for catalytic purposes such as framework silicates (zeolites), the most effective and selective catalysts in organic syntheses. The catalytic applications of zeolites are too wide to summarize in this book, so we deal with the catalytic effects of clay minerals. 

Zeolites, sometimes called porotectosilicates, are the most important framework silicates. A zeolite may be defined as an aluminosilicate with a framework structure... 

Catalyst life estimation tests were also conducted on four different catalysts used for aromatization of light hydrocarbons in this system, namely H-ZSM-5, Zn/H-ZSM-5, Ga/H-ZSM-5 and H-Ga-silicate. Based on the results of characterization of the spent catalysts, the primary cause of deterioration of catalyst activity is the elimination of active metal from the zeolite framework. 

Highly sihceous zeoHtes (with Si/Al ratios 1) are microporous framework alumino-sihcate materials. Discussion of the framework skeletal vibrahons of highly sihceous zeolites is similar to that reported above for silicas. The addition of aluminum in the framework causes shifts in the positions of the sole band. In particular, the asymmetric Si—O—Si stretching modes of framework silicates, usually observed as a complex very strong absorption in the region 1200-1000 cm" , tend to shift down a litde with A1 for Si subshtution. 

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