Propene epoxide

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. 

C[bicarbonate] and NMR were used to demonstrate that the first product in the metabolism of propene epoxide is acetoacetate, which is then reduced to (3-hydroxybutyrate (Allen and Ensign 1996). 

The various spectroscopic techniques had revealed that Ti4+ ions in TS-1, Ti-beta and, Ti-MCM-41 are 4-coordinate in the dehydrated state. Tetrapodal Ti(OSi)4 and tripodal Ti(OH)(OSi)3 are the main Ti species. Upon exposure to H20, NH3, H202, or TBHP, they increase their coordination number to 5 or 6. On samples in which the Ti4+ has been grafted onto the silica (referred to as Ti f MCM-41), a dipodal Ti species (Ti(OH)2(OSi)2) may also be present. As a result of interaction with the oxidant ROOH (R = H, alkyl), the formation of 7)1- and p2-peroxo (Ti-O-O-), hydroperoxo (Ti-OOH), and superoxo (Ti02 ) species has been observed experimentally (Section III). A linear correlation between the concentration of the p2-hydroperoxo species and the catalytic activity for propene epoxidation has also been noted from vibration spectroscopy (133). 

Research Focus Epoxidation of propylene using palladium/titanium zeolite-1 as catalyst Originality This propene epoxidation method occurs in a neutral medium and is... 

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. 

Propene epoxidation. Stoukides and Vayenas have studied the epoxi-dation of propene over silver catalysts.71 73 A Langmuir-Hishelwood type model was used to explain the results of work performed between 290 and 400°C.71 As with the work on ethylene oxidation, two types of oxygen were proposed to be involved, molecular and atomic oxygen responsible for partial and total oxidation respectively. 

Figure 9.11 The DEMiS reactor for propene epoxidation with HP vapors (courtesy of Uhde).

Table 14.2 Selectivities and yields (%) for propene epoxidation using gold on titanosilicate catalysts.70...

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]. 

Fig. 8. Mossbauer spectra recorded at T = 4.2 K of gold catalysts on various supports used for propene epoxidation. The average size of the particles is given in the legend. Figure according to Goossens et al. (165).

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. 

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. 

On the other hand, propene epoxidation over Ag/a-alumina is notably inefficient, yielding epoxide selectivities in the order of only a few percent. As a... 

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. 

Table 6.2 Comparison of catalytic performance with EBHP versus different heterogeneous Ti-based catalytic systems in propene epoxidation.

As an example. Table 6.2 compares the performance of Ti-MCM-41 [9] with a mesoporous silica-grafted Ti and a microporous Ti silicalite (TS-1) for propene epoxidation with EBHP. With TS-1, the pore diameter is 5.5 A, which imposes restrictions and limits the size of the molecules to be oxidized the diffusion of the oxidant is inhibited, and hence the conversion attained is less than with the mesoporous materials. The difference between the mesoporous catalyst and TS-1 is even greater when the epoxidation of propene is carried out with CHP. The affinity for propene is increased by conferring hydrophobic properties to the surface, by means of silylation. 

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.

The rate of catalyst deactivation is a function of the TS-1 crystal size [12a, 13a] with larger crystals, the slow diffusion of the epoxide solvolysis products (especially with more hindered products) makes the blockage of pores more likely. In propene epoxidation, polyethers are mainly responsible for this phenomenon, when the catalyst is used in consecutive reaction cycles. The activity of the catalyst can be restored by washing it with a solvent or by calcining it at temperatures higher than 500 °C. 

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].

It has been proposed that the methanol molecule reacts with the oxygen molecule rather than with Pd, to form a peroxy intermediate HOCH2OOH, which regenerates the peroxoheteropoly-compound species that is active in propene epoxidation [29i]. 

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. 

Duma and Honicke were the first to report the successful use of N2O in propene epoxidation. A PO yield of 5% was obtained over silica-supported iron oxide catalysts promoted with Na ions [43bj. The pore shape and diameter of the support as well as iron oxide dispersion are crucial parameters in the reaction [43b,cj. Doping vdth alkali metal may also considerably affect the Fe dispersion, and favor epoxidation over allylic oxidation [43fj. Further modification by boron can also significantly enhance the catalytic performance of the K-doped FeO /SBA-lS catalyst [43gj. 

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