Propene direct oxidation

CH2 CH CH0. a colourless, volatile liquid, with characteristic odour. The vapour is poisonous, and intensely irritating to eyes and nose b.p. 53"C. It is prepared by the distillation of a mixture of glycerin, potassium sulphate and potassium hydrogen sulphate. It is manufactured by direct oxidation of propene or cross-condensation of ethanal with meth-anal. 

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. 

Figure 4. Acetone by direct oxidation of propene (a) reactor (oxygen), (b) separator, (c) reactor (propene), (d) flash column, (e) light ends distillation, (f) acetone purification.

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. 

New approaches under investigation include the direct oxidation of propene with molecular oxygen, eventually in the presence ofhydrogen ... 

Several companies are working on the direct oxidation of propene for instance Lyondell is operating a pilot plant in Newtown Square, PA, and intends to commercialize the technology by 2010. Shell Chemical is also working on a direct route to PO production, based on variations of the gold and silver catalysts it uses to make ethene oxide. 

In theory, it should be possible to obtain PO through direct oxidation of propene with oxygen, similarly to the industrial production of ethene oxide ... 

Plant design for the direct oxidation of propene would most likely be based on pure oxygen feed, rather than air, to gain yield advantage and lower capital costs. The minimal purge gas flow in an oxygen-based process makes it economically feasible to use a ballast gas (diluent) other than nitrogen. 

Many problems remain to be solved for these fascinating processes that directly oxidize propene with green oxidants in the gas phase (i) the rapid deactivation of the catalyst, due to the accumulation of heavy compounds, precursors for coke formation (ii) the need for gas-phase promoters, for example, NO, or chlorocarbons, which act to moderate activity and enhance PO selectivity even in the very first patents issued in this field, this was claimed to be a key feature for optimal performance [46] and (iii) the low space-time-yield achieved, due to the low conversion of propene and/ or the low residence time. 

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. 

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. 

Direct oxidation of propene to propene oxide (PO) with high selectivity and activity is yet to be achieved in heterogeneous catalysis, as was already indicated in the description on the preparation of the Au catalysts. Obviously, the most attractive reaction is the direct oxidation with oxygen, rather than using the previously discussed mixture of oxygen and hydrogen ... 

PROPENE The major use of propene is in the produc tion of polypropylene Two other propene derived organic chemicals acrylonitrile and propylene oxide are also starting materials for polymer synthesis Acrylonitrile is used to make acrylic fibers (see Table 6 5) and propylene oxide is one component in the preparation of polyurethane polymers Cumene itself has no direct uses but rather serves as the starting material in a process that yields two valuable indus trial chemicals acetone and phenol... 

Propene is an intermediate utilized in the chemical and pharmaceutical industries. The partial oxidation of propene on cuprous oxide (CU2O) yields acrolein as a thermodynamically imstable intermediate, and hence has to be performed under kinetically controlled conditions [37]. Thus in principle it is a good test reaction for micro reactors. The aim is to maximize acrolein selectivity while reducing the other by-products CO, CO2 and H2O. Propene may also react directly to give these products. The key to promoting the partial oxidation at the expense of the total oxidation is to use the CU2O phase and avoid having the CuO phase. 

In the direct ammoxidation of propane over Fe-zeolite catalysts the product mixture consisted of propene, acrylonitrile (AN), acetonitrile (AcN), and carbon oxides. Traces of methane, ethane, ethene and HCN were also detected with selectivity not exceeding 3%. The catalytic performances of the investigated catalysts are summarized in the Table 1. It must be noted that catalytic activity of MTW and silicalite matrix without iron (Fe concentration is lower than 50 ppm) was negligible. The propane conversion was below 1.5 % and no nitriles were detected. It is clearly seen from the Table 1 that the activity and selectivity of catalysts are influenced not only by the content of iron, but also by the zeolite framework structure. Typically, the Fe-MTW zeolites exhibit higher selectivity to propene (even at higher propane conversion than in the case of Fe-silicalite) and substantially lower selectivity to nitriles (both acrylonitrile and acetonitrile). The Fe-silicalite catalyst exhibits acrylonitrile selectivity 31.5 %, whereas the Fe-MTW catalysts with Fe concentration 1400 and 18900 ppm exhibit, at similar propane conversion, the AN selectivity 19.2 and 15.2 %, respectively. On the other hand, Fe-MTW zeolites exhibit higher AN/AcN ratio in comparison with Fe-silicalite catalyst (see Table 1). Fe-MTW-11500 catalyst reveals rather rare behavior. The concentration of Fe ions in the sample is comparable to Fe-sil-12900 catalyst, as well as... 

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. 

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