Oxidation of benzene to phenol

The oxidation of benzene to phenol and 1,4-dihydroxybenzene (Figure 2.11a) (Hyman et al. 1985), both side chain and ring oxidation of ethyl benzene, and ring-hydroxylation of halogenated benzenes and nitrobenzene (Keener and Arp 1994). 

Tao Y, A Fishman, WE Bentley, TK Wood (2004) Oxidation of benzene to phenol, catechol, and 1,2,3-trihydroxybenzene by toluene 4-monooxygenase of Pseudomonas mendocina KR 1 and toluene... 

Nitrous oxide as an efficient oxygen donor was noticed when used in such a delicate reaction as the direct oxidation of benzene to phenol ... 

Hensen EJM, Zhu Q, van Santen RA. 2005. Selective oxidation of benzene to phenol with nitrous oxide over MFI zeolites. 2. On the effect of the iron and aluminum content and the preparation route. J Catal 233 136-146. 

Pd(II) Wacker-type catalysts were also studied.146 Selective oxidation of benzene to phenol by molecular oxygen in the presence of Pd and heteropolyacids have been published.147... 

Oxidation of benzene to phenol. This was attempted in the former U.S.S.R. and Japan on a pilot-plant scale. High yields were reported, but full-scale operation apparently was discontinued because of destruction of product by irradiation and the possibility of explosion in the reaction vessel. The latter danger can be controlled in the oxidation of halo-genated hydrocarbons such as trichloro- or tetrachloroethylenes, where a chain reaction leads to the formation of dichloro- or trichloro-acetic acid chlorides through the respective oxides. 

The ce-pyrrolidonate Pt(2.5 + )4 (19) was also found to catalyze the oxidation of benzene to phenol by hydrogen peroxide (121). By HPLC, ESR, and UV-Vis absorption spectroscopy, the main reaction pathway was confirmed to be Eq. (13). 

Another example of heterogeneous catalysis by oxo-ions is the one-step oxidation of benzene to phenol with nitrous oxide, N2O. Fe/MFI catalysts have, again been found to be very active. This catalysis was discovered by Iwamoto and has been extensively studied by the group of G. Panov in Novosibirsk. " Preparations of Fe/MFI which appear highly active for this reaction display poor activity for NOj reduction and those which are optimum for that process, are poor for benzene oxidation. This shows that different sites are used. Work by Jia et al. revealed that the active sites for benzene oxidation appear to be Fe-oxo-ions containing only one Fe ion. This does not necessarily mean that the sites are mononuclear. A recent work by Zhu et al. has rather suggested that the site consists of one Fe and one Al + ion, the latter ion having left the zeolite framework. ... 

J. Jia, K. S. Pillai, and W. M. H. Sachtler, One-step oxidation of benzene to phenol with nitrous... 

Q. Zhu, R. M. van Teeffelen, R. A van Santen, and E. J. M. Hensen, Effect of high-temperature treatment on Fe/ZSM-5 prepared by chemical vapor deposition of FeCls 11. Nitrous oxide decomposition, selective oxidation of benzene to phenol, selective reduction of nitrous oxide by MO-butane, J. Catal. 221, 575—583 (2004)... 

S. (2008) liquid phase oxidation of benzene to phenol by molecular oxygen over La catalysts supported on HZSM-5. Top. Catal, 47, 98-100. 

The incorporation of vanadium(V) into the framework positions of silicalite-2 has been reported by Hari Prasad Rao and Ramaswamy . With this heterogeneons oxidation catalyst the aromatic hydroxylation of benzene to phenol and to a mixtnre of hydroqninone and catechol conld be promoted. A heterogeneons ZrS-1 catalyst, which has been prepared by incorporation of zirconinm into a silicalite framework and which catalyzes the aromatic oxidation of benzene to phenol with hydrogen peroxide, is known as well in the literature. However, activity and selectivity were lower than observed with the analogous TS-1 catalyst. 

Fig. 7.190. Diagram of the reactor for partial oxidation of benzene to phenol during 02-H2 fuel cell reactions. (Reprinted from K. Otsuka, M. Kunieda, and H. Yamagata, J. Electrochem. Soc. 139 2382,1992, Fig. 1. Reproduced by permission of the Electrochemical Society, Inc.)...

It was reported independently by three research groups that MFI-type zeolites selectively catalyze the reaction of N20 with benzene to give phenol C6H6 + N20 —> C6H5OH + N2 [93-96]. Fe/ZSM-5 shows remarkable performance in benzene hydroxylation to phenol with N20 as oxidant, which is the first example of a successful gas phase direct phenol synthesis from benzene [97]. No other catalysts show similar high performances to the Fe/ZSM-5 catalyst. At present, iron is the sole element capable of catalyzing the benzene-to-phenol reaction [98]. Direct oxidation of benzene to phenol by N20 has been commercialized in the so-called AlphOx process in Solutia Inc., US A, where N20 is obtained as a by-product in adipic acid production with nitric acid [97, 99, 100] a selectivity >95% to phenol is achieved at >40% conversion at around 4000 C. But the process is cost-effective only if N20 can be obtained cheaply as a by-product in adipic acid production. 

Considerable efforts have been made to understand ZSM-5-based catalysts for the selective oxidation of benzene to phenol by nitrous oxide. However, the nature of the active species remains unclear. The most important proposals for the active species are extraframework Fe species [101], Bronsted add sites [102] and Lewis and A1 sites [103, 104]. The activity is usually interpreted in terms of very small, possibly... 

H202 has been a popular oxidizing agent for decades in the textile, electronics and pulp and paper industries [125]. Applications of H202 in bulk chemistry have been reviewed in a few papers [126, 127]. In the H202 oxidation of benzene to phenol a selectivity of 90 + % has been reported with catalysts based on TS-1 [128] and/or Ti-MCM-41 [129] at reasonable conversion levels. However, H202 is too expensive compared to the frequently applied air and/or oxygen, and so there are no drivers for economically sustainable and economic application. 

The Fe ion is easily incorporated into zeolites. Ferrosilicates and ferrisilcates are often used as acid catalysts. Direct oxidation of benzene to phenol over Fe-MFI zeolites has been shown to be practical when N20 is used as an oxidant (AlphOx process) [112]. This process is suitable for the abatement ofhighly concentrated N20 (20-40%) gas discharged from adipic acid plants to meet the tightened regulation of emission levels of N20. 

Oxidation over zeolites The results of Iwamoto et al. on the oxidation of benzene to phenol [58] stimulated further efforts in the search for new and more efficient catalytic systems. As a result, in 1988, ZSM-5 zeolites were shown to be the best catalysts for this reaction [59-61]. Over zeolites, the reaction proceeded at much lower temperature and, which was even more important, with a very high selectivity, approaching 100%. Further studies involving many other groups [62-80] contributed much to the improvement of ZSM-5 catalysts. Some other type zeolites and FeP04 were shown to be also active [69, 70, 73, 80, 81], although their efficiency was inferior to that of ZSM-5. 

Table 7.6 Effect of oxidant on the oxidation of benzene to phenol over FeZSM-5 and Fe203a .

Reactions (7.14)-(7.16) represent the main mechanistic steps not only of the stoichiometric, but also of the steady state catalytic oxidation of benzene to phenol. The involvement of a-oxygen in the catalytic oxidation is most convincingly evidenced by a linear dependence of the reaction rate on the concentration of a-sites [134,135]. 

N20 emission at chemical plants and the methods of its abatement have been considered in several reviews [185-187]. The major emission is related to the preparation of nitric add and its use in oxidation processes, like those involved in the production of adipic acid, caprolactam, glyoxal, acrylonitrile, and so forth. Of them, the biggest emission is the offgases of adipic acid about 1 M MT N 20 per year with a concentration of 30-40%. Recovery and purification of N20 from these offgases for use in the oxidation of benzene to phenol are described by Uriarte [188], Some companies use these off-gases to obtain medical-grade nitrous oxide. 

The economic estimation based on the pilot unit results showed that ammonia contributes 70% to the prime cost of the N20. Using this result and the ammonia price 0.37 kg-1 [191], one can evaluate the N20 price to be 0.53 kg-1. Certainly, this cost far exceeds that of dioxygen. Therefore, for reactions producing inexpensive products, like the oxidation of methane to methanol, the application of N20 cannot be economically sound. However, this modest cost opens great N20 prospects for the preparation of more expensive chemical products. For instance, the theoretical expenditure for N20 in the oxidation of benzene to phenol is 17%, and in the oxidation of phenol to hydroquinone is 4%, of the cost of the target product. The commercial viability of such processes will depend primarily on their technological advantages rather than the cost of nitrous oxide. 

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