Titanium sites, framework

Unlike aluminum, titanium is tetravalent and can exhibit different oxidation states. Thus TS-1 is non-acidic if isomorphously substituted. TS-1 is relatively difficult to synthesize, which is probably one of the reasons the site structure was under debate for quite some time. TS-1 can only be synthesized with a maximum of 3 wt% Ti if more Ti is added extra-framework titanium is formed. Inihally, it was suggested from XAS that octahedral sites are formed in the silicalite framework [60]. However, later more and more groups suggested tetrahedral isomorphous substitution of the titanium sites in the MFl framework [61-63], It is now more or less generally accepted that the Ti is four-coordinate and has a Ti—O distance of 1.79-1.81 A. This is a significantly increased distance compared to the Si—O distance 1.605 A, which is constant for a vast number of oxides [4]. The increase in... 

The deeper oxidation of ethylbenzene over TS-2 can be explained with the slower diffusion of 1-phenylethanol and aeetophenone formed in the zeolite pores where they could undergo additional oxidation to aeetophenone or other products, respectively. Another possible reason could be some differences in the local geometry of the titanium sites due to the different framework structure of the two titanium silicalites. 

Titanium alkoxides are effective homogeneous catalysts for the epoxidation of substituted olefins. Their propensity for association to multinuclear species is also well-established (6). The active form of a homogeneous, enantioselective titanium-tartrate catalyst was demonstrated to be dinuclear in titanium (7). In contrast, heterogeneous catalysts consisting of titanium embedded in an aluminosilicate framework contain mostly isolated titanium sites (8), although the assertion that such sites are uniquely responsible for catalyst activity is based on... 

Scheme 9.2 An alternative scheme for the activation of H2O2 on framework titanium sites. ...

The presence of the framework titanium sites in the aluminophosphate molecular sieves (TAPO-5, -11, -31 and -36) were proved indirectly by the catalytic activity of these materials in the liquid-phase oxidation and epoxidation reactions by hydrogen peroxide. The incorporation of Ti(IV) centers in mesoporous hexagonal alnminophosphates was determined by catalytic activity in the oxidation of phenols at room temperature, where remarkable paraselectivity was achieved in TiHMA (188) and TAP (189). 

The quantification of the extra-framework titanium species in titanium silicalites of MFI structure, TS-1, was performed using either XANES at the Ti K-edge or XPS Ti (2p) photolines. In addition, two different framework sites, [Ti(OH)(OSi)3] and [Ti(OSi)4], were characterized in dehydrated samples using Diffuse Reflectance UV-visible, multiple scattering analysis of EXAFS, H and Si NMR spectroscopies. 

Epoxidation of alkeneic reactants is faster on titanium-grafted silicates (such as A, B and C) than on the coprecipitated titanosilicates (such as D and E). This difference was attributed to the fact that on extra-framework titanium-grafted silicates, the catalytically active sites are virtually all exposed and accessible, whereas on the coprecipitated material some of them may be buried within the silicate walls and, thus, cannot adsorb reactant molecules. 

Following the discovery of TS-1 [125], a titanium-substituted MFl, the use of zeolitic materials for oxidation increased significantly. The presence of the Ti atom in the framework of a zeolite structure provides a site-isolated Ti center, a situation not possible with other Ti-containing materials while also allowing shape-selective oxidations. The combination of the two effects gives highly active and selective oxidation reactions [126]. 

A number of heterogeneous systems have been developed for oxidation reactions using H2O2 as oxygen source . In 1981, Taramasso, Notari and collaborators at Enichem opened new perspectives in this field with the discovery of the Ti-silicalite (TS-1) ° , a new synthetic zeolite of the ZSM family. In the TS-1 zeolite, titanium atoms are located in vicariant positions in the place of Si atoms in the crystalline framework . The remarkable reactivity of TS-1 is likely ascribable to the site-isolation of tetrahedral Ti(IV) in a hydrophobic environment. TS-1 has proved to be an efficient catalyst for the epoxidation of unfunctionalized short-chain olefins, especially terminal ones (equation 28). In addition, polyunsaturated compounds are mainly converted into the mono epoxides (equation 29). 

Effect of sodium and aluminum on TS-1. The catalytic activities of aluminum and/or sodium containing TS-1 are depicted in Table IV. The data show that the addition of aluminum during the synthesis of TS-1 yields a material (TAS-1(D)) that has a lower conversion of n-octane oxidation and a smaller IR peak ratio. The existence of the acid sites due to the incorporation of aluminum into the framework of TS-1 may accelerate the decomposition of H2O2 to water and oxygen during the reaction. However, reducing the number of acid sites by exchanging with sodium ions only increases the conversion by 1% (Na/TAS-1(D)). Therefore, the addition of aluminum into the synthesis mixture most likely reduce the amount of titanium present in the sample. 

In some reports, the presence of the two forms of titanium has been described as arising from two types of sites identified as isolated titanium framework sites and Ti02 particles, and the reactions have been attributed to the catalytic activity of one or the other phase (Huybrechts et al., 1992). Since it is possible to obtain pure phase titanium silicates, it seems preferable to identify the Ti02 phase as an impurity. 

The effect of zeolite porosity on the reaction rate was also well demonstrated in liquid-phase oxidation over titanium-containing molecular sieves. Indeed, the remarkable activity in many oxidations with aqueous H2O2 of titanium silicalite (TS-1) discovered by Enichem is claimed to be due to isolation of Ti(IV) active sites in the hydrophobic micropores of silicalite.[42,47,68 69] The hydrophobicity of this molecular sieve allows for the simultaneous adsorption within the micropores of both the hydrophobic substrate and the hydrophilic oxidant. The positive role of hydrophobicity in these oxidations, first demonstrated with titanium microporous glasses,[70] has been confirmed later with a series of titanium silicalites differing by their titanium content or their synthesis procedure.[71] The hydrophobicity index determined by the competitive adsorption of water and n-octane was shown to decrease linearly with the titanium content of the molecular sieve, hence with the content in polar Si-O-Ti bridges in the framework for Si/Al > 40.[71] This index can be correlated with the activity of the TS-1 samples in phenol hydroxylation with aqueous H2C>2.[71] The specific activity of Ti sites of Ti/Al-MOR[72] and BEA[73] molecular sieves in arene hydroxylation and olefin epoxidation, respectively, was also found to increase significantly with the Si/Al ratio and hence with the hydrophobicity of the framework. 

Sufficient ammonium fluotitanate was added to replace all of the framework aluminum in the NH L with titanium. The LZ-229 product contained 7.2 wt.% Ti02. Perhaps because the sample was digested at reflux for only 30 minutes, only 25% of the aluminum was removed. However, 85% of the aluminum depleted sites were filled by Ti. A small amount of Si was also removed during the reaction. The change in defect structure was +0.046. 

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