Isomorphous Substitution of Si

The contribution of boron-dependent acidity to the activity of the catalysts has been questioned. Chu et al. concluded, after examining [BJ-ZSM-5 for acid-catalyzed reactions, that the activity was mainly, if not entirely, due to trace amounts of framework aluminum. It was even demonstrated that the B-containing catalyst was less active in the methanol conversion than the B-free catalyst. According to Coudurier et al., who used microcalorimetry and infrared spectroscopy, the presence of boron in the pentasil framework gives rise to a very weak acidity only. Sayed et al., ° synthesizing ZSM-5 catalysts containing both aluminum and boron, also concluded that the boron-associated sites are to be considered 

ZSM-5 catalysts containing zirconium and/or hafnium and titanium are also more selective in the conversion of methanol to olefins. The C2 C4 olefin s selectivity was increased from 56% on [Al]-ZSM-5 to 65% and 64% on [Zr] [Al]-ZSM-5 and [Ti] [Al]-ZSM-5, respectively. 

More recently, these metallosilicates were studied in order to optimize the selectivity toward light olefins.At complete conversion of methanol, the selectivity toward olefins on the various metallosilicates varied as follows  

Romannikov et al. studied the catalytic properties of beryllium-silicates with a zeolite-type structure. Methanol conversion yielded primarily olefins, while on the isostructural aluminum-silicate catalyst paraffins and aromatics were obtained. The methanol conversion was considerably higher on the [AlJ-ZSM-5 than on the [Be]-ZSM-5, except in one example (Table 12). 

The higher selectivity toward light olefins observed with Be-silicate could be explained by the higher reaction temperature. 

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An extremely versatile catalyst for a variety of synthetically useful oxidations with aqueous hydrogen peroxide is obtained by isomorphous substitution of Si by Ti in molecular sieve materials such as silicalite (the all-silica analogue of zeolite ZSM-5) and zeolite beta. Titanium(IV) silicalite (TS-1), developed by Enichem (Notari, 1988), was the progenitor of this class of materials, which have become known as redox molecular sieves (Arends et al., 1997). 

TS-l and titanium silicalite-2 (TS-2) are microporous solid materials made of Si02 and Ti02 that have silicalite structures (TS-1 has the ZSM-5 structure and TS-2, the ZSM-11 structure) modified by isomorphous substitution of Si(IV) with Ti(IV). TS-1 and TS-2, the former being most studied, show similar properties in catalysis of H202 oxidations. 

If isomorphic substitution of Si(IV) by AI(III) occurs in the tetrahedral sheet, the resulting negative charge can distribute itself over the three oxygen atoms of the tetrahedron (in which the Si has been substituted) the charge is localized and relatively strong inner-sphere surface complexes (Fig. 3.10a) can be formed. 

A new microporous solid material has been obtained made of Ti02 and Si02 (TS-1) which has a silicalite-1 structure modified by isomorphous substitution of Si(IV) with Ti(IV), Its synthesis takes place in the presence of tetraalkylammonium bases under carefully controlled conditions,... 

A unique titanium(IV)-silica catalyst prepared by impregnating silica with TiCLt or organotitanium compounds exhibits excellent properties with selectivities comparable to the best homogeneous molybdenum catalysts.285 The new zeolite-like catalyst titanium silicalite (TS-1) featuring isomorphous substitution of Si(IV) with Ti(IV) is a very efficient heterogeneous catalyst for selective oxidations with H2C>2.184,185 It exhibits remarkable activities and selectivities in epoxidation of simple olefins.188,304-306 Propylene, for instance, was epoxidized304 with 97% selectivity at 90% conversion at 40°C. Shape-selective epoxidation of 1- and 2-hexenes was observed with this system that failed to catalyze the transformation of cyclohexene.306 Surface peroxotitanate 13 is suggested to be the active spe-... 

Figure 1 shows the FT-1R spectra of samples dispersed in KBr. All the spectra display a strong band at 960 cm 1. This band has been assigned to Si-O-Ti bonds [14] or to Si-OH groups [15, 16]. It is usually taken as the evidence for isomorphous substitution of Si by Ti, but it cannot be used to determine quantitatively the content of titanium into the framework of mesoporous materials [17]. In addition, the broad pattern between 3700 and 3000 cm 1, originated from hydrogen-bonded surface OH groups as well as from adsorbed H20 [18], decreases dramatically in the silylated samples. 

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. 

In a quantum-chemical MNDO-PM3 level study of the hydration of the Mg2+ cation located in a ditrigonal cavity of the basal surface of clays [99], the most favorable area of Mg2+ cation location was predicted to be in the vicinity of the AIO4 tetrahedron formed by the isomorphic substitution of Si for Al in the silica-oxygen sheet. The authors have showed the important role of the hydrogen bond formation between the water molecules and the oxygen atoms of the silica-oxygen sheet in the Mg-ion hydration. This was confirmed in several MC simulation studies [65, 66]. 

It is clearly desirable to be able to alter the Si/Al ratio of the framework, particularly in the case of zeolites X, Y and ZSM-5. This can be conveniently achieved by "secondary synthesis", i.e. by isomorphous substitution of Si or A1 on the tetrahedral sites after the completion of the original zeolite crystallization (1-4). We shall demonstrate that ... 

It is clear that in the case of MFI, the zeolite pore entrances should not be considered as rigid apertures. Instead, zeolite framework topologies can show flexibility. While the O-Si-0 angle in the tetrahedral unit is rigid (109 + 1 °), the Si-O-Si angle between the units can vary between 145 and 180°. Based on isomorphous substitution of Si by other T-atoms in the framework [18], framework defects [19], cation positions, changes in the water content [16], external forces on the crystalline material [20] and upon adsorption of guest molecules [21] phase transitions can occur that have a dramatic influence in particular cases on the framework atom positions. 

New materials consisting of amorphous silica with regular pore structure, therefore called mesoporous molecular sieves, have recently been described [7]. Isomorphous substitution of Si by Ti has been attempted by performing the synthesis in the presence of titanium compounds. Ti-MCM have been tested for oxidation of hydrocarbons in liquid phase, using HjOj or hydroperoxides as oxidants [8-10]. 

Kaolinite is a 1 1 (T-O) phyllosilicate. The fundamental unit of its structure is an extended sheet of two constituents a silica-type layer of composition (Si4O10)4- and a gibbsite-type layer of composition (0H)6A14(0H)204 (see schematic representation in Fig. 10). Ideally, kaolinite crystals are not permanently charged. However, due to isomorphic substitution of Si by Al at the siloxane surface, kaolinite platelets carry a small, permanently negative charge (Van Olphen, 1977). Lim et al. (1980) and Talibudeen (1984) postulate that the permanent charge of kaolins is caused by contamination with small amounts of 2 1 phyllosilicates rather than a consequence of isomorphic substitution. 

The isomorphous substitution of Si" " by Ti and Sn" ", V"" in the silicalite-2 (ZSM-11, MEL) framework has been the subject of some recent reports [1-4]. The possibility of substituting silicon by zirconium in the framework of ZSM-5 (MFI) by hydrothermal analysis has been reported but not really substantiated by experimental evidence [5-9]. We have recently attempted the isomorphous substitution of Si" by Zr ions in the MFI and MEL framework by hydrothermal synthesis by using ZrCU as a source [10,11]. In these reports, we have shown that about 0.6 - 1.0 Zr atom per unit cell could be incorporated in the framework positions and that those well dispersed Zr ions are capable of catalyzing hydroxylation of phenol with aqueous H2O2. This report describes the hydrothermal synthesis of Al- free zirconium silicate molecular sieves with MEL structure using two different sources viz., zirconium (IV) tetrachloride and zirconium (IV) acetylacetonate and their physico-chemical characterization and a comparison of the samples. 

In the recent years isomorphous substitution of Si by foreign elements (3) as B(4-6), Fe(5), Ga(5), Ti(7), etc has been largely studied and has been shown to result in different catalytic properties. New materials as AlP04-n family has also opened new fields of interests and prospectives with the possibility of isomorphous substitution by many elements (8,9), as Si, Co, Mn and many others. Carbon and sulfide-type molecular sieves have also been synthesized. 

Zeolites contain voids of molecular dimensions which permit molecular sieving. This feature is ideal for selective separation of compounds. Pores of molecular dimension with specific shapes will also enable size- and shape-selective processes in catalytic reactions. In addition, acid-base or redox properties needed for catalytic conversions are introduced in zeolites by isomorphous substitution of Si atoms in the crystalline framework by other elements such as Al, Ti, Ga, etc. 

Similar effects as found upon incorporation of Al into the faujasite framework (vide supra) were observed when heteroatoms isomorphously substituted Si framework cations of siHcalite-1 (i.e., Al-free MFI structure, [Si]MFI) or siliceous sodalite ([Si]SOD) In fact, the band shifts may be not very dramatic, since usually only small degrees of isomorphous substitution of Si by introduction of Al or, similarly, B, Ti, V, Ga, Fe and V are considered. 

The isomorphic substitution of some Al + for Si" " " in the tetrahedral sheet and some Mg + for Al + in the octahedral sheet (or to some local partial charges on Si-O-Si surfaces) was suggested to occur in the minerals of the kaolin group [63], which can lead to the generation of the surface cations and the presence of local charge defects [64]. The basal siloxane surfaces of kaolinite are believed by many [65-68] to carry a constant structural charge due to the isomorphous substitution of Si" + by Al + [69]. Natural kaolinites do contain very small amount of Mg [70] and Mg-rich kaolinite was synthesized [71]. 

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