Direct hydroxylation of benzene

Direct hydroxylation of benzene to phenol could be achieved using zeolite catalysts containing rhodium, platinum, palladium, or irridium. The oxidizing agent is nitrous oxide, which is unavoidable a byproduct from the oxidation of KA oil (see KA oil, this chapter) to adipic acid using nitric acid as the oxidant. 

In addition to the epoxidation of olefins, zeolitic materials have been studied for other fine chemical transformations. Table 12.21 indexes the zeolites used for oxidative dehydrogenation of propane, direct hydroxylation of benzene to phenol and e-caprolactam synthesis. A recent review summarizes other reactions for which there is not enough space in the table [138, 139]. 

Oxidation to Phenols. Direct hydroxylation of benzene to phenol can be achieved in a free-radical process with H202 or 02 as oxidants.739-744 Metal ions [Fe(II), Cu(II), Ti(HI)] may be used to catalyze oxidation with H202. Of these reactions, the so-called Fenton-type oxidation is the most widely studied process.742 Oxidation in the presence of iron(II) sulfate was reported in early studies to yield phenol. Since phenol exhibits higher reactivity than benzene, varying amounts of isomeric dihydroxybenzenes were also formed. 

The direct hydroxylation of benzene and aromatics with a mixture of 02 and H2 have been performed by simultaneously mixing benzene, oxygen and hydrogen in the liquid phase using a very complicated system containing a multi-component catalyst, a solvent and some additives. Besides the possibility of an explosive gas reaction, these hydroxylations gave only very low yields, 0.0014—0.69% of phenol and aromatic alcohols. For example, Pd-containing titanium silicalite zeolites catalyzed... 

Reproducibly of results, however, was questioned, because the total oxidation reaction was underestimated [84]. Possibly, these discrepancies derive from the observation of structural changes of the Pd-based membrane during direct hydroxylation of benzene to phenol [85]. The surface state of Pd during the reaction could be divided into two major regions the oxidized region near the gas... 

Figure 8.10 Phenol synthesis by direct hydroxylation of benzene using a hydrogen-permeable membrane. (Adapted from [83]).

Bhaumik A., Mukherjee P. and Kumar R., Triphase catalysis over titanium-silicate molecular sieves under solvent-free conditions -1. Direct hydroxylation of benzene, J. Catal. (1998) 178 pp. 101-107. 

It is remarkable that this reaction and the direct hydroxylation of benzene cited above occur in a 3-phase system gas-liquid-solid. The oxidation of benzene constitutes an extraordinary case in that the liquid is a mixture of benzene and acetic acid and the gas phase contains both air and hydrogen] This suggests the formidable problems that chemical engineering had to solve to develop these processes. 

Direct Hydroxylation of Benzene to Phenol by Nitrous Oxide... 

TSl has also been used in commercial epoxidations of small alkenes. A major limitation with this catalyst is its small pore size, typical of many zeolite materials. This makes it unsuitable for larger substrates and products. Again like many zeolites, it is also less active than some homogeneous metal catalysts and this prevents it from being used in what would be a highly desirable example of a green chemistry process - the direct hydroxylation of benzene to phenol. At the time of writing, commercial routes to this continue to be based on atom-inefficient and wasteful processes such as decomposition of cumene hydroperoxide, or via sulfonation (Scheme 1.1-3). 

Figure 1.17 Reaction steps, reaction conditions, and typical results for the direct hydroxylation of benzene to phenol with modified Fe-ZSM-5 as the catalyst and NjO as the oxidizing agent according to the Solutia process. Adapted from Chemical Engineering, September 2004, p. i, with permission from Chemical Engineering.

GE has also patented a process for the direct hydroxylation of benzene to... 

A novel reactor design was proposed by Bortolotto and Dittmeyer " the direct hydroxylation of benzene was... 

Wang et al. tested an MR in which titanium silicalite-1 (TS-1) was incorporated as catalyst in a Pd-based membrane. The direct hydroxylation of benzene to phenol was performed with hydrogen and oxygen as coreactants in the MR, and the effect of reactor configuration was examined. 

The reaction results indicated that the direct hydroxylation of benzene required both hydrogen and oxygen coreactants. As already pointed out, the Pd membrane can catalyze the conversion of benzene to phenol as reported by Niwa et al., and the reaction was sensitive to the H2/O2 molar feed ratio, and its optimum was governed by membrane permeation rate, reactant composition, and reaction conditions. Nevertheless, benzene conversion and phenol yield remained low. 

Bortolotto L., DittmeyerR. 2010. Direct hydroxylation of benzene to phenol in a novel microstructured membrane reactor with distributed dosing of hydrogen and oxygen. Separation and Purification Technology 73 51-58. 

Al-Megren et al. (2013) apphed a semibatch membrane system and a continuous membrane reactor (flat-sheet membranes of polypropylene and polyethersuUbne) for the direct hydroxylation of benzene using H2O2 as an oxidant. With the help of water on the stripping side of the membrane unit, the produced phenol was recovered. They explored the effect of the hydrophUic and hydrophobic character of the membrane material on the phenol recovery at different flow rates of the feed and the stripping phases. The obtained results showed that the hydrophUic membrane has a better performance. What is more, the continuous removal of the phenol from the reaction side contributed to a reduction in byproduct (such as benzoquinone) formalion and a halt in biphenyl and tars formalion. The performance of a continuous system was better than the semibatch system, mainly because of the total amount of phenol recovered in the permeate. In particular, more than 25% of the produced phenol was recovered in the continuous membrane reactor whereas this value for the semibatch membrane system was less than 1%. 

Dittmeyer, R., Bortolotto, L. (2011). Modification of the catalytic properties of a Pd membrane catalyst for direct hydroxylation of benzene to phenol in a double-membrane reactor by sputtering of different catalyst systems. Applied Catalysis A General, 391, 311—318. 

Uriarte, A., Rodkin, M., Gross, M., et al. (1997). Direct Hydroxylation of Benzene to Phenol by Nitrous Oxide, Stud. Surf. Sci. Catal., 110 (Proceedings of the 3rd World Congress on Oxidation Catalysis), pp. 857-864. 

H. Ge, H., Leng, Y., Zhang, R, et al. (2008). Direct Hydroxylation of Benzene to Phenol with Molecular Oxygen over Pyridine-modified Vanadium-substituted Heteropoly Acids, Catal. Lett., 124 250-255. 

Laufer, W., Niederer, J. and Hoelderich, W. (2002). New Direct Hydroxylation of Benzene with Oxygen in the Presence of Hydrogen over Bifunctional Palladium/Platinum Catalysts, Adv. Synth. Catal., 344, pp. 1084—1089. 

Figure 4.18 Conceptual illustration of direct hydroxylation of benzene to phenol by using a Pd membrane reactor. Reproduced from [73]. With permission from Elsevier.

MWCNT Direct hydroxylation of benzene to phenol Defects Active oxygen generated by HjOj decomposition Low benzene conversion (6%) but very high phenol selectivity (99%) [128]... 

---