Propene oxide, direct epoxidation

The direct conversion of propene to its epoxide, in near quantitative yields, with aqueous H202 will be environmentally more benign. One of the unique features of TS-1 as a solid oxidation catalyst is its ability to utilize aqueous H202 as the oxidant for such conversions. This ability of TS-1 derives from the fact that silicalite-1 is hydrophobic, in contrast to the hydrophilic amorphous Ti-Si02. Consequently, hydrophobic reactants, such as alkenes, are preferentially adsorbed by TS-1, thus precluding the strong inhibition by H20 observed with amorphous Ti-Si02. 

The most convenient route to lluorinatcd epoxides is the direct epoxidation of alkenes. Since first reports of a general method using alkaline hydrogen peroxide at low temperature,- several alternative synthetic approache.s have been developed. Molecular oxygen under free-radical conditions has been used to oxidize alkenes such as tetrafluoroethene or hexafluoro-propene however, internal epoxides are formed most conveniently using hypochlorites. The products, e. g. oxirane 14 from alkene 13, are usually obtained in high... 

The same differential behavior can be observed with amine nucleophiles. For example, calcium triflate promotes the aminolysis of propene oxide 84 with benzylamine to give 1-(A -benzyl)amino-2-propanol 85, the result of attack at the less substituted site <03T2435>, and which is also seen in the solventless reaction of epoxides with heterocyclic amines under the catalysis of ytterbium(III) triflate <03SC2989>. Conversely, zinc chloride directs the attack of aniline on styrene oxide 34 at the more substituted carbon center <03TL6026>. A ruthenium catalyst in the presence of tin chloride also results in an SNl-type substitution behavior with aniline derivatives (e.g., 88), but further provides for subsequent cyclization of the intermediate amino alcohol, thus representing an interesting synthesis of 2-substituted indoles (e.g., 89) <03TL2975>. 

Finally, it is noted that the direct epoxidation of propene with molecular oxygen is potentially more economically attractive than all of the coproduct processes currently in operation. It is indeed a holy grail in oxidation chemistry. Notwithstanding the extensive research on this topic in the last three decades, an industrially viable method for the direct epoxidation of propene has not been forthcoming. Sumitomo has recently announced [67] that they will commercialize a coproduct free route to PO but this probably involves an alkyl hydroperoxide e.g. cumene) oxidant with recycling of the alcohol coproduct [68]. 

Key Words Direct propylene epoxidation. Propylene oxide, Gold, Titanium, Propene, Au/Ti catalysts. Catalysis by gold. Titanium silicalite, TS-1, Gold/TS-1, Hydrogen peroxide, Kinetics, Design of experiments, Deposition-precipitation, Ammonium nitrate, Selective oxidation, Alkene epoxidation, Density functional theory, DFT calculations, QM/MM calculations. 2008 Elsevier B.v. 

Both catalysts supported on silica produce a small amoimt of propene oxide at the highest temperatures however, the selectivities remain low, proving the necessity of titania on the support to have an effective epoxidation catalyst. Furthermore, these catalysts by far have the lowest hydrogen efficiency because of the direct oxidation of hydrogen into water, making the economics of these catalysts very unattractive. 

Highly dispersed titanium oxide species on silica prepared by the sol-gel method catalyse the selective epoxidation of propene by molecular oxygen.59 This is potentially very significant as the new commercial route to propene oxide is based on the reaction of propene with hydrogen peroxide catalysed by a mixed Ti-Si oxide the direct reaction with oxygen has clear advantages. 

The direct vapor phase epoxidation of propene using dioxygen (O2) and dihydrogen (H2) can be achieved using supported gold catalysts and is expected to contribute to future industrial processes for propene oxide production. At present, the following conclusions can be drawn ... 

An interesting extension of this concept is to use a CO2/H2/O2/H2O mixture containing a catalyst for H2O2 fomiation in the CO2 phase directly for the epoxidation of alkenes [44], e.g., in a direct synthesis of propene oxide (Scheme 2) [45]. It has been shown that H2O2 will react with aqueous bicarbonate to form percarbonate... 

Ethene can be very selectively epoxidized over supported silver catalysts. The last decades the mechanism of this epoxidation has been studied in great detail [1,2]. Epoxidation of propene using the same silver catalysts has not been successful. However, a direct gas-phase epoxidation process to produce propene oxide is highly desired. The mechanism of propene oxidation is currently being investigated in order to develop new catalysts. 

FIGURE 7.8 Computationally determined structure of a Au, cluster on an alumina support. This and related clusters are highly active for propene oxide formation via direct propene epoxidation. Reprinted with permission from Ref. [91]. John Wiley Sons. (See insert for... 

Because of their success in ethylene epoxidation, it is not surprising that specially modified Ag catalysts have received intense attention. Although promising developments have been reported, there is still no commercial process for the direct oxidation of propene into PO. Here is a real challenge for the scientific community. 

Isobutane oxidation is performed in the liquid phase at 130-160 °C and elevated pressures. Since this exceeds the critical temperature of isobutane (134 °C), products (TBA, t-butyl hydroperoxide (TBHP)) must be present to maintain a liquid phase. The epoxidation step is performed at 100-130 °C using 10-300 ppm of Mo. Since propene is a rather unreactive olefin, a high propene/TBHP molar ratio is used to suppress nonproductive decomposition of TBHP. The high propene concentration leads to very high operating pressures and high recycle costs. The PO and TBA products are purified by a combination of direct and extractive distillation. TBHP conversion and PO selectivity are in excess of 90 %. 

Up till now it has not been possible to carry out the analogous reaction with propene. Numerous researchers have attempted to develop a process for the direct oxidation of propene into propene epoxide (PO). Only indirect routes have, up to now, been applied in successful selective processes (see Section 5.5.4). Those indirect processes involve the use of hydrogen peroxide, organic peroxides and peracids, hypochlorides, etc. (see e.g. SMPO, Chapter 2). The reason that it is difficult to epoxidize propene is the facile formation of an allylic intermediate because the C-H groups in the methyl group become activated. 

For conversions lower than 5%, very high selectivity for PO based on propene can be obtained (e.g., higher than 90%) with the O2/H2 mixture (HOPO), whereas in the presence of O2 alone the selectivity is not higher than 50-60% even at very low propene conversion. In general, yields for the direct oxidation of propene are lower than 5%. As shown clearly in [43a], if all the results achieved in the gas-phase epoxidation of propene with various oxidants, that is, O2, O2 + H2, HP vapors or N2O, are compiled in a cumulative plot of PO selectivity versus propene conversion, a limit curve can easily be drawn up, which seems to indicate that the conditions needed to increase propene conversion are not compatible with good PO selectivity. Moreover, selectivity to PO with respect to hydrogen is still too low. 

The Dow Chemical Company also filed several patents [183, 184] on Au-based catalysts for hydro-oxidation (i.e., direct reaction with O2 in presence of Hj) of alkenes (propene and larger). One of them, focused on catalyst prepared starting with atomically precise Au clusters, claims stable catalyst activity over extended hfetime and improved hydrogen efficiency with selectivity of about 90% toward formation of epoxide [184]. 

In this way, the direct contact of O2 with the olefin is prevented and the radical process of addition across the double bond is avoided. Reactions (6.18) and (6.19) are slow and the selectivity towards the epoxide in reaction (6.18) strongly depends on the catalyst preparation, the nature of the metal, and the reaction temperature. Using propene, the formation of the epoxide is in concurrence with the formation of acetone and propionaldehyde. Moreover, depending on the preparation of the metal oxide, the same catalyst can push the reaction to the formation of acroleine or even to the total oxidation of propene to CO2 and water [117]. If, instead of the only olefin, a mixture of olefin and CO2 is admitted on the catalyst in its oxidized form, the carbonate is formed which can be recovered by condensation and the excess olefin recycled. 

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