Epoxidation diffusional constraints

The advantage of this mesoporous TS-1 over samples prepared by the conventional route is illustrated in Fig. 34. The two samples behave similarly for the oxidation of linear reactant oct-1-ene. But a marked difference was observed for the oxidation of bulkier cyclohexene. Because of the absence of diffusional constraints, the catalytic epoxidation activity in the mesoporous TS-1 enhanced by almost an order of magnitude for the oxidation of the bulkier cyclohexene. 

The higher conversion in the presence of Ti-beta is probably a result of the higher temperature (343 v.v. 323 K). Diffusional constraints cannot account for the observed differences in selectivity. Ti-beta and TS-1 are distinctly more selective than the mesoporous material. Recalling that tetrapodal titanium sites are more predominant in the former two molecular sieves although tripodal titanium sites are the major surface species over the latter mesoporous material (Section II), we infer that the data indicate that high epoxidation selectivity is probably correlated with the presence of tetrapodal structures in these two molecular sieves. This correlation is discussed in Section VI. 

The rate also decreases with an increase in the chain length of the alkene molecule (hex-l-ene > oct-l-ene > dodec-l-ene). Although the latter phenomenon is attributed mainly to diffusion constraints for longer molecules in the MFI pores, the former (enhanced reactivity of terminal alkenes) is interesting, especially because the reactivity in epoxidations by organometallic complexes in solution is usually determined by the electron density at the double bond, which increases with alkyl substitution. On this basis, hex-3-ene and hex-2-ene would be expected to be more reactive than the terminal alkene hex-l-ene. The reverse sequence shown in Table XIV is a consequence of the steric hindrance in the neighborhood of the double bond, which hinders adsorption on the electrophilic oxo-titanium species on the surface. This observation highlights the fact that in reactions catalyzed by solids, adsorption constraints are superimposed on the inherent reactivity features of the chemical reaction as well as the diffusional constraints. 

The epoxidation rates of various alkenes relative to hex-l-ene on Ti-beta with H2O2 and TBHP are summarized in Table XV, In the absence of diffusional constraints, the branched alkenes are more reactive than the linear ones (see also Section V.C. 13),... 

A third difference concerns Ti-MWW only. The siting of Ti in different porous environments, that is in external pockets, in internal supercages and in sinusoidal 10-MR channels, leads to active species associated with different diffusional and steric constraints [79]. Thus, the epoxidation of bulky olefins can occur exclusively in external pockets, whereas the linear ones are not subject to site limitations. Ti-MWW is also an unusual catalyst in the epoxidation of stereoisomers. At odds with TS-1 and Ti-Beta zeolites, trons-olefins are epoxidized faster than their as analogues [85]. Though the mechanism is still unclear, a better fitting of the trans configuration to the tortuous nature of 10-MR channels could be an explanation. 

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