Propene epoxidation hydrogen oxidation

Similar to the basic surface studies discussed above, promoters often show markedly different behaviors depending on the alkene species used. Lambert and co-workers (68) reported a study of ethene and propene epoxidation with different promoters that showed no real correlation based on the promoter used. In the case of NOx species as promoters, there was no effect for the formation of propylene oxide, which is interesting considering the high activity of NO in formation of ethylene oxide. Also, addition of potassium ions into the NO promoter feed decreased both activity and selectivity for propylene oxide formation, again completely opposite to the behavior seen for EO. As in the other surface studies, the authors postulate a chemical effect from the presence of allylic hydrogens. 

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. 

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. 

The link between the deactivation and the epoxidation is made clear in Fig. 12.5, in which the hydrogen oxidation reaction (no propene) is observed to be stable for 25 h. Once propene is added to the feed thereafter, the catalyst starts deactivating rapidly. Removal of the propene in the feed stops the deactivation process and the catalytic activity gradually increases. The fact that even after 25 h, the activity is not back to its original level, indicates that the deactivating species are bonded strongly to the catalyst. 

FIGURE 12.5 Deactivation during epoxidation and hydrogen oxidation over 1 wt.% Au/Ti02-Twenty-five hours hydrogen oxidation only, followed by epoxidation for 15 h, followed by hydrogen oxidation only for 25 h. [0.3 g of catalyst, 50 Nml/min gas feedrate (6% H2,02, and from t = 25 to 40 h propene), total pressure 1.1 bar(a).]... 

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 manufacture and uses of oxiranes are reviewed in (B-80MI50500, B-80MI50501). The industrially most important oxiranes are oxirane itself (ethylene oxide), which is made by catalyzed air-oxidation of ethylene (cf. Section 5.05.4.2.2(f)), and methyloxirane (propylene oxide), which is made by /3-elimination of hydrogen chloride from propene-derived 1-chloro-2-propanol (cf. Section 5.05.4.2.1) and by epoxidation of propene with 1-phenylethyl hydroperoxide cf. Section 5.05.4.2.2(f)) (79MI50501). 

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... 

Commercially available 30% hydrogen peroxide solution can oxidize alkenes readily in the presence of a carbo-diimide promoter <1996SL649, 1998JOC2564, 1998JOC1730> (Equation 67). A method to epoxidize propene using aqueous hydrogen peroxide and a reaction controlled phase-transfer catalysts was developed <20040PD131>. 

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