Propene epoxidation processes

T. A. Nijhuis, Towards a new propene epoxidation process, PhD Thesis, Delft University of Technology, 1997. 

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. 

It has recently been found that NEt3 is a gas-phase promoter for propene epoxidation by supported gold catalysts [245]. In more recent studies, Hughes et al. reported that catalytic amounts of peroxides could initiate the oxidation of alkenes with 02, without the need for sacrificial H2 [243]. The process worked for a range of substrates (cyclohexene, ds-stilbene, styrene and so on) and even in the absence of solvent hence, we may refer to this as green technology. 

The epoxidation of propene is analogous to that of ethylene catalyzed by silver. However, the selectivity is much lower. Due to the pronounced oxidation sensitivity of the allyl CH3-group, excessive combustion occurs as a side reaction. The heterogeneous process has no practical significance, therefore, as it has to compete with a highly selective liquid phase epoxidation process. 

Oxidation is extremely important both from a scientific and a practical point of view. Without oxidation life would not exist. In the chemical industry, too, oxidation is probably the most important process. A major example is the combustion of fossil fuels. This process is usually uncatalyzed, but sophisticated catalytic processes do exist. Examples in the inorganic industry are the oxidation of sulphur dioxide and ammonia in the manufacture of sulphuric acid and nitric acid, respectively. In the petrochemical industry many catalytic synthesis processes are carried out, for example the production of ethylene and propene epoxide, phthalic acid anhydride. An example which has recently also become important is the catalytic combustion of hydrocarbons in flue gases. Table 5.2 gives a list of examples of oxidation catalysis in industry [93]. 

With an overall capacity of lOOOOta" , it is a modest process when compared to recent applications of TS-1 in ammoximation and propene epoxidation. The introduction of digital photography, which no longer needs hydroquinone for the development of silver emulsions, risks the continuity of the process, unless the selectivity to catechol is greatly improved or new uses are developed for hydroquinone. 

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. 

Since the early 1980s, EniChem has been a pioneer in the development of the process, holding a portfolio of patents [10c,dj. The integration of HP synthesis by means of alkylanthraquinone/alkylanthrahydroquinone (RAQ/RAHQ) cycle technology, with PO production, by means of propene epoxidation with H P, is possible because of the peculiar properties of the TS-1 catalyst (Scheme 6.5). TS-1 can selectively epoxidize propene using diluted HP [12]. A water- methanol mixture is the solvent for the epoxidation the alcohol is necessary to obtain a sufficient reaction rate. Therefore, a cost-saving feature in this process is the fact that the crude H P produced can be used directly in the epoxidation of propene. Moreover, integration of the two processes is also allowed by the easily accomplished separation of propene and PO from the water-methanol mixture. Methanol, after separation and purification, can be recycled to the epoxidation step. 

Scheme 6.5 RAQ/RAHQ process for the generation of HP, and integration with propene epoxidation.

Scheme 6.6 Reactions involved in the EniChem process for propene epoxidation with in situ generation of HP.

Figure 6.4 Simplified flow-sheet ofthe integrated insitu HPPO (a) and H PPO (b) processes for propene epoxidation with generation of HP, developed by EniChem.

Table 6.3 Catalytic performance in propene epoxidation with H P in the Degussa/Uhde process [20].

PO-only Routes Several Approaches for Sustainable Alternatives 349 Table 6.5 Performance of catalysts in the Lyondell DOPO process for propene epoxidation with O2. 

A major challenge that remains is the hydrogen efficiency of the catalyst. Since the propene epoxidation is performed in the presence of hydrogen, it is desirable that the hydrogen is used only in the epoxidation reaction and is not converted directly into water. At this time none of the catalysts have a sufficiently high hydrogen efficiency to be able to run a process profitable and, therefore, this remains one of the key challenges to be solved for this catalyst system for propene epoxidation. 

Although chiral catalysts continue to dominate the literature in this arena, there are a number of novel achiral alternatives. Examples of the latter are a manganese porphyrin/tetrabutylammonium periodate system, useful for neutral homogeneous conditions [94TL945], as well as a polybenzimidazole-supported molybdenum(VI) catalyst suitable for industrial application in the Halcon process for propene epoxidation [94CC55]. 

Chapter 14 by Jun Kawahara and Masatake Haruta reviews AuNP-catalyzed propene epoxidation by dioxygen and dihydrogen. Propylene oxide, which is processed to polyurethane polyols and propylene ycols, is one of the most important chemical feedstocks whose efficient, free of side produd and selective synthesis is hi ly challenging, the... 

I 14 Au NP-catalysed Propene Epoxidation by Dioxygen and Dihydrogen a) Cumene recycling process by Sumitomo Chemical Co. Ltd. 

We have made a rough comparison between gas-phase propene epoxidation with O2 and H2 and the current industrial processes for PO production. In order to... 

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. 

In this paper the application of an improved Temporal Analysis of Products (TAP) reactor system to the propene epoxidation is described. The sensitivity of the Multitrack (MULTIple Time Resolved Analysis of Catal5rtic Kinetics) set-up is an important advantage compared to conventional TAP reactors. Now the analysis of reactions with low conversions (and, therefore, small amounts of products) has become possible. Furthermore, without the necessity of signal averaging, transient processes can be monitored in real-time. 

The availability of an industrial process for producing TS-1 allowed the development of several selective oxidations, all using hydrogen peroxide as a versatile and environmentally friendly oxidizing agent which gives water as the only co-product. The epoxidation of propene (Hyprox process), the phenol hydroxylation and the ammoximation of cyclohexanone are now H202-based industrial processes benzene hydroxylation to phenol is currently under advanced evaluation. 

The one general exception to the rule that ethers don t typically undergo Sn2 reactions occurs with epoxides, the three-membered cyclic ethers that we saw in Section 7.8. Epoxides, because of the angle strain in the three-membered ring, are much more reactive than other ethers. They react with aqueous acid to give 1,2-diols, as we saw in Section 7.8, and they react readily with many other nucleophiles as well. Propene oxide, for instance, reacts with HC1 to give l-chloro-2-propanol by Snj2 backside attack on the less hindered primary carbon atom. We ll look at the process in more detail in Section 18.6. 

The first variant works with isobutane as the hydroperoxide precursor, which is oxidized to TBHP by molecular oxygen. During the epoxidation of propene, TBHP is transformed to ferf-butanol, which is converted to methyl ferf-butyl ether. The second procedure employs ethylbenzene, which is oxidized by molecular oxygen to phenyl ethyl hydroperoxide, which transfers an oxygen to propene and so is reduced to phenylethanol. This by-product of the process is converted to styrene, a versatile bulk chemical. 

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