Ti active sites

Although the complete mechanism for each of the previously described reactions is not known, substantial details have been worked out. First, it is clear that Ti is incorporated into the framework of the silicalite structure. Too much Ti (more than about 2.5%) in the preparation steps forms nonframework TiOz crystallites, which decompose H202. Second, the rate enhancement due to methanol suggests a tight association at the Ti active site as shown in Fig. 6.8.37,38 This is supported by the fact that methanol oxidizes much more slowly than other alcohols.47 This tight coordination of methanol is proposed to increase the electrophilicity of the Ti-coordinated H202 and facilitate oxygen transfer to the alkene.31... 

Ti = active site generated from TiMe3Cp and B(C6F5)3... 

As a further example of a hydroxyl-assisted epoxidation, geraniol and nerol bearing two isolated C=C double bonds were regioselectively epoxidized with TS-1 at the 2-position (near the OH group), as reported by Kumar et al. (795). On the basis of these results, Kumar et al. (195) proposed that the transition state of the epoxidation of allylic alcohols involves coordination of the alcoholic functional group to the Ti active site and that the double bond interacts with one of the peroxidic oxygen atoms, not with the titanium site (Scheme 9). 

Results of oxidation of unsaturated alcohols are shown in Table 3. Both 2-penten-1 -ol and 3-methyl-2-buten-1 -ol exhibited higher reactivity than cyclohexene. A decrease around 20-50% in catalytic activity of organically functionalized samples has been observed. This is probably due to the inhibition of access of the rather hydrophilic substrates to the Ti-active sites surrounded by the organic groups of increased hydrophobicity. It is noteworthy that the epoxidation was favorable for the organically functionalized samples whereas the alcohol oxidation was retarded. 

In summary, the organically functionalized Ti-substituted MCM-41 materials have been successfully synthesized by one-step synthesis method with a varied Ti-incorporation of the Si/Ti ratio from 50 to 600. The hydrothermal treatment resulted in the increase of Ti-incorporation. The epoxidation selectivity was improved by organic functionalization than alcohol oxidation probably due to the increased hydrophobicity nearby the Ti-active sites. 

The catalytic properties of Del-Ti-MWW have been compared with those of other titanosilicates in the epoxidation of cyclic alkenes (Table 4.4). The TON decreased sharply for TS-1, Ti-beta and 3D Ti-MWW with increasing molecular size of cyclic alkenes. Ti-MCM-41 with mesopores, however, showed higher TONs for cyclooctene and cyclododecene. This implies that the reaction space is extremely important for the reactions of bulky molecules. The delamination of Ti-MWW increased the TON greatly for not only cyclopentene but also bulkier cycloalkenes. Especially, the catalytic activity of Del-Ti-MWW was about 6 x higher than that of Ti-MWW for cyclooctene and cyclododecene. Del-Ti-MWW even turned out to be superior to Ti-MCM-41 in the epoxidation of bulky substrates. This should be due to the high accessibility of Ti active sites in Del-Ti-MWW. Thus the delamination was able to change Ti-MWW into an effective catalyst applicable to reactions of bulky substrates. 

It can be seen that, under similar conditions, the reactivity of the dialkyl sulfides is directly linked to their molecular size Et2 S > Pr2 S > Bu2 S, and saturated sulfides are more reactive than allyl or aryl sulfides Pr2 S > Me S Allyl > Allyb S > Me S Ph > Ph2 S. These results can be explained, first, if we take into account the relative easiness of thioethers accessibility to the Ti active sites of the catalytic species located in the zeolite framework. The diffusion of the bulkier molecules, such as Ph2S is very difficult even inside the large pores of Ti-beta zeolite. Secondly, the reactivity of thioethers is in agreement with the nucleophilicity of the sulfur atom, so that alkyl sulfides are more easily oxidized than allyl or aryl sulfides by H2O2 (an electrophilic oxidant) in agreement with reported results [1-9]. It must be pointed out, that in the case of allyl methyl sulfide and di-allylsulfide, the epoxidation of the allyl system is not observed under our experimental conditions. 

In that Table it can be seen that TS-1 is much more active than Ti-Beta for the oxidation of n-hexane, the activity per Ti site being about 20 times higher in the former. However, this difference in activity is strongly reduced in the case of cyclohexane, provided this molecule has serious problems to diffuse inside the pores of TS-1, and hence, to reach the Ti active sites. It has also to be noticed that Ti-Beta shows a higher selectivity to cyclohexanone than TS-1 at a similar conversion level. 

Li P, Xin Y, Li Q, Wang Z, Zhang Z, Zheng L (2012) Ce-Ti Amorphous Oxides for Selective Catalytic Reduction of NO with NH3 Confirmation of Ce-O-Ti Active Sites. Environ Sci Tech 46 (17) 9600-9605... 

The intercept on the adsorption axis, and also the value of c, diminishes as the amount of retained nonane increases (Table 4.7). The very high value of c (>10 ) for the starting material could in principle be explained by adsorption either in micropores or on active sites such as exposed Ti cations produced by dehydration but, as shown in earlier work, the latter kind of adsorption would result in isotherms of quite different shape, and can be ruled out. The negative intercept obtained with the 25°C-outgassed sample (Fig. 4.14 curve (D)) is a mathematical consequence of the reduced adsorption at low relative pressure which in expressed in the low c-value (c = 13). It is most probably accounted for by the presence of adsorbed nonane on the external surface which was not removed at 25°C but only at I50°C. (The Frenkel-Halsey-Hill exponent (p. 90) for the multilayer region of the 25°C-outgassed sample was only 1 -9 as compared with 2-61 for the standard rutile, and 2-38 for the 150°C-outgassed sample). 

In this article we critically review most of the literature concerning non-catalyzed, proton-catalyzed and metal-catalyzed polyesterifications. Kinetic data relate both to model esterifications and polyeste-rificatiom. Using our own results we analyze the experimental studies, kinetic results and mechanisms which have been reported until now. In the case of Ti(OBu)f catalyzed reactions we show that most results were obtained under experimental conditions which modify the nature of the catalyst. In fact, the true nature of active sites in the case of metal catalysts remains largely unknown. 

In our experiment, photocatalytic decomposition of ethylene was utilized to probe the surface defect. Photocatalytic properties of all titania samples are shown in table 2. From these results, conversions of ethylene at 5 min and 3 hr were apparently constant (not different in order) due to the equilibrium between the adsorption of gaseous (i.e. ethylene and/or O2) on the titania surface and the consumption of surface species. Moreover it can be concluded that photoactivity of titania increased with increasing of Ti site present in titania surface. It was found that surface area of titania did not control photoactivity of TiOa, but it was the surface defect in titania surface. Although, the lattice oxygen ions are active site of this photocatalytic reaction since it is the site for trapping holes [4], this work showed that the presence of oxygen vacancy site (Ti site) on surface titania can enhance activity of photocatdyst, too. It revealed that oxygen vacancy can increase the life time of separated electron-hole pairs. 

Steady-state molecular beam studies of the reaction of methylacetylene on reduced Ti02 (001) surfaces were undertaken to determine whether this reaction could be performed catalytically under UHV conditions. A representative experiment is presented in Figure 1. Prior to each experiment, the surface was sputtered and annealed to a temperature between 400 K and 550 K surfaces prepared in this manner have the highest fraction of Ti(+2) sites (ca. 30% of all surface cations) of any surface we have been able to create by initial sputtering [3]. Thus these are the surfaces most active for cyclotrimerization in TPD experiments [1]. Steady-state production of trimethylbenzene (as indicated by the m/e 105 signal detected by the mass spectrometer) was characterized by behavior typical of more traditional catalysts a jump in activity upon initial exposure of the crystal to the molecular beam, followed by a decay to a lower, constant level of activity over a longer time scale. Experiments of up to 6 hours in duration showed... 

A heterogeneous olefin epoxidation catalyst containing both V and Ti in the active site was prepared by sequential non-hydrolytic grafting. The silica was exposed first to VO(OiPr)3 vapor followed by Ti(0 Pr)4 vapor. Formation of propene is evidence for the creation of Ti-O-V linkages on the surface. Upon metathesis of the 2-propoxide ligands with BuOOH, the catalyst becomes active for the gas phase epoxidation of cyclohexene. The kinetics of epoxidation are biphasic, indicating the presence of two reactive sites whose activity differs by approximately one order of magnitude. 

We demonstrated that a series of Ti-FI catalysts 40 (Fig. 25) and 44-47 (Fig. 29) possessing a t-Bu, cyclohexyl, i-Pr, Me, and H ortho to the phenoxy-O (thus having various steric environments in close proximity to the active site) all initiate room temperature living ethylene polymerization, though, for the non-fluorinated congeners, the steric bulk of the substituent ortho to the phenoxy-O significantly influences product molecular weight (Table 6) [28, 33]. 

On the basis of crystallochemistry consideration and taking into account electron microscopy observations of the surface of crystals upon which some polymer was formed,99 Arlman and Cossee13 concluded that the active sites are located on crystal surfaces different from the basal (001) ones. In particular, these authors considered in detail active sites located on crystal surfaces parallel in the direction a — b of the unit cell defined as in Ref. 98. Figure 1.13 illustrates that, if we cut a TiCl3 layer parallel to the direction defined above, which corresponds to the line connecting two bridged Ti atoms, electroneutrality conditions impose that each Ti atom at the surface of the cut be bonded... 

Active sites located on crystal surfaces different from the basal (001) ones were also proposed by Allegra14 for which the Ti atom at the surface of the cut would be bonded to four Cl atoms only (bridged to further metal atoms) (Figure 1.14). In this case both octahedral sites of coordination for the monomer and growing chain (indicated by arrows in Figure 1.14) are equivalent because the surface atoms with relevant nonbonded interactions at the catalytic site are locally related by a twofold axis (dashed line in Figure 1.14). It is worth noting that this model site presents a local C2 symmetry as the isospecific metallocenes of the previous section. 

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