Epoxide mechanism

The commonly held view of the uniqueness of Ag for ethylene epoxidation may soon change in view both of the propene epoxidation work of Haruta and coworkers on Au/Ti02 catalysts upon cofeeding H2 123 and also in view of the recent demonstration by Lambert and coworkers124 126 that Cu(lll) and Cu(110) surfaces are both extremely efficient in the epoxidation of styrene and butadiene to the corresponding epoxides. In fact Cu was found to be more selective than Ag under UHV conditions with selectivities approaching 100%.124-126 The epoxidation mechanism appears to be rather similar with that on Ag as both systems involve O-assisted alkene adsorption and it remains to be seen if appropriately promoted Cu124 126 can maintain its spectacular selectivity under process conditions. 

The exact epoxidation mechanism is still not quite clear. However, in all possible mechanisms, the interaction between Au and Ti02 is essential. According to a mechanism suggested by Hayashi et al [1], molecular oxygen adsorbed on Ti02 is activated, probably to a... 

The catalysts which have been tested for the direct epoxidation include (i) supported metal catalysts, (ii) supported metal oxide catalysts (iii) lithium nitrate salt, and (iv) metal complexes (1-5). Rh/Al203 has been identified to be one of the most active supported metal catalysts for epoxidation (2). Although epoxidation over supported metal catalysts provides a desirable and simple approach for PO synthesis, PO selectivity generally decreases with propylene conversion and yield is generally below 50%. Further improvement of supported metal catalysts for propylene epoxidation relies not only on catalyst screening but also fundamental understanding of the epoxidation mechanism. 

An important finding is that all peroxo compounds with d° configuration of the TM center exhibit essentially the same epoxidation mechanism [51, 61, 67-72] which is also valid for organic peroxo compounds such as dioxiranes and peracids [73-79], The calculations revealed that direct nucleophilic attack of the olefin at an electrophilic peroxo oxygen center (via a TS of spiro structure) is preferred because of significantly lower activation barriers compared to the multi-step insertion mechanism [51, 61-67]. A recent computational study of epoxidation by Mo peroxo complexes showed that the metallacycle intermediate of the insertion mechanism leads to an aldehyde instead of an epoxide product [62],... 

For the model olefin ethene, we again investigated various epoxidation mechanisms (Figure 7) [67]. As before for the group VI metals, insertion was found to exhibit significantly higher activation barriers. 

We point out that the mechanism sketched in path A of Scheme 11 is in agreement with the kinetic and spectroscopic data collected from several research groups. On the other hand, a series of contradictions was encountered in fitting the experimental data into the mechanism proposed in path B. Furthermore, several other papers have appeared in the last decade, based on both experimental results and theoretical calculations, supporting an epoxidation mechanism involving a direct oxygen atom transfer to olefins. For selected examples, see References 34, 145-155. 

However, in view of the more recent data on kinetics of the reaction with l,2-di(substituted phenyl)tetramethyldisilane (158) and the newly proposed epoxidation mechanism involving the 1,3-dipolar form of the peracid (130), the mechanism that follows also appears to be worth due consideration. 

Thus, kinetic equation (7.12) adequately describes experimental data and indicates higher probability of epoxidation mechanism shown in Figure 7.23. 

Keywords Atomic scale characterization surface structure epoxidation reaction 111 cleaved silver surface oxide STM simulations DFT slab calculations ab initio phase diagram free energy non-stoichiometry oxygen adatoms site specificity epoxidation mechanism catalytic reactivity oxametallacycle intermediate transition state catalytic cycle. 

The compehtion of one-electron pathways is sometimes detectable in the epoxidations catalyzed by transition metal catalysts [67]. However, in the epoxidahon of unhindered olefins on TS-1, the typical radical products are below the detection limits. Their presence could no longer be neglected when the rate of epoxidation is so slow as to become comparable to that of homolytic side reactions, for example with bulky olefins (see also Section 18.11). It is possible that, within these limits only, the epoxide is produced in part through the addition of a radical peroxy intermediate to the double bond [68, 69]. Even so, a homolytic pathway has again been proposed as a generally vahd epoxidation mechanism [7]. 

Scheme 18.7 Epoxidation mechanism of species a) from Figure 18.1.

Scheme 18.8 (a) Epoxidation mechanism of species (b) from Figure 18.1 (b) Epoxidation mechanism of species (c) from Figure 18.1. 

The number of metal zeolites and their application to the epoxidation of olefins rose in parallel from the late 1980s. TS-2, Ti,Al-P, Ti-P, Ti-MWW and, rarely, Ti-MOR are catalysts that have been studied in some detail [7-9, 35, 77-84]. TS-2 behaves, according to the few studies published, similarly to TS-1. The greater spaciousness of pores in Ti-Beta zeolites and of external cups in Ti-MWW allows the epoxidation, under mild conditions, of olefins unable to diffuse in TS-1 and TS-2, such as methylcyclohexenes, cyclododecene, norbornene, camphene and methyl oleate [80-83]. Steric constraints still prevail over electronic factors, however, as in medium pore Ti-zeolites, even in the epoxidation of linear olefins (Table 18.9). It is generally believed that active sites and epoxidation mechanisms are not significantly different from those of TS-1. 

Figure 9. Epoxidation mechanism proposed by Bartlett (53). The cis-olefin gives rise to a cis-epoxide.

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