Propene direct epoxidation

T. A. Nijhuis, B. J. Huizinga, M. Makkee, and J. A. Moulijn, Direct epoxidation of propene using gold dispersed on TS-1 and other titanium-containing supports, Ind. Eng. Chem. Res. 38, 884—891... 

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... 

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]. 

Haruta s research work on the catalysis of gold has initiated a new possibility for the direct epoxidation of propene using hydrogen and oxygen [7,169] ... 

T. A. Nijhuis, T. Visser, B. M. Weckhuysen, Mechanistic study into the direct epoxidation of propene over gold/titania catalysts, /. Phys. Chem. B 109 (2005) 19309. 

Gold nanoparticle catalysts have a good potential for a future process for the direct epoxidation of propene. In a single reactor, propene can be selectively epoxidized using a hydrogen-oxygen mixture. Catalysts containing titanium... 

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. 

A direct correlation between the concentration of the titanium oxo species and epoxidation activity was proposed by Lin and Frei (133). Loading TS-1/H202 with propene after evacuation, they observed by FTIR difference spectroscopy the loss of the bands characterizing propene (at 1646 cm-1) and TiOOH (at 837 and 3400 cm-1). Figure 48 is the infrared difference spectrum recorded immediately after loading the propene on TS-1/H202 Fig. 49 includes the spectra recorded 80 and 320 min later. 

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. 

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>. 

Degussa, in turn, has recently announced the commercialization of a HPPO process, jointly developed with the engineering company Uhde. Parallel to this, Degussa with Headwaters is also working on the direct synthesis of H2O2, for which a demonstration plant was completed in 2006 [151]. According to the news release, hydrogen peroxide will be obtained in the new process as a dilute methanol solution to be used directly in the epoxidation of propene. 

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. 

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