Titanium silicates mechanisms

Investigation of mechanisms of reactions catalyzed by titanium silicates has been limited to oxidation reactions with H202 as the oxidant, as described below. As was previously discussed, elements different from titanium and silicon in the catalyst materials change their properties. Catalytic activity of doubly substituted materials such as Ti-beta, H[Al,Ti]-MFI and -MEL, and H[Fe,Ti]-MFI and -MEL is considered separately because the acidic properties associated with the added element affect the composition of the reaction products. 

The radical mechanism has also been proposed as a general mechanism for oxidation of alkenes and aromatics, but several objections have been raised because of the absence of products typically associated with radical reactions. In classical radical reactions, alkenes should react also at the allylic position and give rise to allyl-substituted products, not exclusively epoxides methyl-substituted aromatics should react at the benzylic position. The products expected from such reactions are absent. Another argument was made against the radical mechanism based on the stereoselectivity of epoxidation. Radical intermediates are free to rotate around the C C bond, with the consequence that both cis- and /rani-epoxides are formed from a single alkene isomer, contrary to the evidence obtained with titanium silicates (Clerici et al., 1993). 

A crucial point in this mechanism is the formation of the peroxo radical, which requires the reduction of the Ti4+ to Ti3+—a process that is easier with highly dispersed Tiiv and should therefore be favored in titanium silicates in which the dispersion of Tilv is the highest ever observed. 

Radical reactions have been proposed to explain the mechanisms of many oxidations catalyzed by metals (Mimoun, 1987), but until now they were considered incompatible with the experimental evidence available for titanium silicate-catalyzed reactions. The recent results indicate that the rationalization of the observed facts with radical mechanisms is as plausible as that with other mechanisms. Indeed, for the oxidation of saturated hydrocarbons, the radical mechanisms are judged to account for the observations better than other mechanisms. Further investigations are needed to clarify these issues. 

The ammoximation of cyclohexanone to cyclohexanone oxime using TS-1 was invented in 1987 (221). The limited amount ofpubHshed work focuses on the mechanism (222) and the kinetics (223) of this reaction on titanium silicate with dilute hydrogen peroxide as oxidant. There is no agreement thus far regarding the reaction pathway. The following two hypotheses have been formulated (7,90,222b,224) ... 

While the mechanism for selective oxidation of an alkene using hydrogen peroxide, over these titanium silicates is not fully understood, it is believed that tetrahedral titanium sites atomically dispersed in a silica matrix are necessary. Furthermore, a titanium-oxo species has been proposed as a possible active species by several groups through EPR and UV-Vis spectroscopies, see Figure 4.27 ... 

The last few years have seen the extensive use of nanoparticles because of the small size of the filler and the corresponding increase in the surface area, allowing to achieve the required mechanical properties at low filler loadings. Nanometer-scale particles including spherical particles such as silica or titanium dioxide generated in-situ by the sol-gel process (4-8), layered silicates (9-12), carbon (13) or clay fibers(14,15), single-wall or multiwall carbon nanotubes (16,17) have been shown to significantly enhance the physical and mechanical properties of rubber matrices. 

Special methods of incorporation usually surface modification of filler is performed prior to filler incorporation into the polymer high concentration of calcium carbonate (40 wt%) provides a fihn which has properties similar to paper but surpasses paper in resistance to moisture chlorinated PE containing carboxyl groups was used as compatibilizer with calcium carbonate " masterbatches of titanium dioxide contain 40-50 wt% pigment " rubber modification to incorporate zirconium silicate without a loss of mechanical strength ... 

In the following sections, some work is presented in which the properties of nano- and microparticle filled composites were determined under variation of the filler contents. These materials were made on the basis of standard epoxy resins cured by amine hardeners. The nanofillers were aluminum oxide (AljOj, 13 nm), titanium dioxide (Ti02, 300 nm and 20 nm) and also calcium silicate (CaS103,5-10 /xm) microparticles. All these fillers are commercially available as powders. The composites were prepared by mechanical mixing using a Dissolver mixing device, as shown in Figure 3. 

Experimentally, oxolation is often much slower than olation. For example, condensation via oxolation of silicic acid Si(0H)4 around pH 3 is much slower than condensation via olation of aquo-hydroxo complexes of titanium and zirconium. It is possible (hat (he difference in mechanism would make olation intrinsically faster than oxolation. A comparison is difficult becau.se it would require the... 

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