The Direct Hydroxylation Of Benzene Catalyzed By Pd-V Supported Catalysts In The Presence Of H₂ And O₂

So far, phenol that is a very important fine chemical intermediate in modern industry is commercially produced by the "Cumene Process" , which is characterized by multi-step procedure, serious pollution, and a large amount of acetone as by-product. Therefore, it is a new challenge that people are faced with to exploit cleaner, simpler and more environmentally friendly novel technique to replace the traditional one.Herein, a sealed and H₂-O₂ inner-circulated reaction apparatus designed by us is first employed to carry out the direct liquid hydroxylation of benzene in the presence of H₂ and O₂ based on the viewpoint that the reactants can be cheaply obtained and both the reaction procedures and conditions are simpler. By use of this apparatus, benzene hydroxylation can occur under the atmospheric pressure, it can not only avoid the potential explosion of the mixed gas under high pressure and numerous consumption of energy, but also largely reduce the losses of mixed gas and benzene, which is contrary to some reports describing that H₂ and O₂ was bubbled in benzene phase at atmospheric pressure and then was let out. Besides, in order to raise the formation rate of phenol and improve its selectivity, we have made a series of researches on the new exploitation of catalysts and catalytic reaction systems.In course of our study, a series of catalysts such as Pd/Hp and Pd/γ-Al₂O₃ were prepared through the process defined by impregnation, calcination and reduction. And they were found to have a small catalytic activity of hydroxylation of benzene in the presence of H₂ and O₂. In addition, the rate of formation of phenol increased with Pd loading using ACOH/H2O as reaction medium, while the converse result was obtainedin H3PO4/H2O medium. It may be interpreted that Pd site is not only able to catalyze the formation of H2O2 but also can catalyze its decomposition, and has low catalytic ability for the hydroxylation of benzene. The favorable effect of AcOH/H2O medium to phenol yield is likely that acetic acid favors the stability of H2O2 and thus enhances the utilization ofH2O2.In order to improve the activity of catalysts, we also have prepared Pd-V/HP and Pd-V/y-AkCb by co-impregnation method. The reaction results revealed that the activities of these Pd-V supported catalysts had been significantly improved in comparison with single Pd supported catalysts and increased with V/Pd value until 4 and 2 at which points Pd-V/Hp and Pd-V/y-Al2C>3 reached their optimum activities respectively, while Hp and Y-AI2O3 modified only by V had not any catalytic activity. y-AkOs as support was less beneficial to catalytic activity than HP, its reason might be that Pd-V components are dispersed very well on HP support than Y-AI2O3 one because Hp has by far higher internal area than Y-AI2O3. Furthermore, we also found out that the phenol selectivity could reach over 99% when using AcOWlhP as reaction medium but relatively low using H3PO4/H2O as reaction medium, which may be partly due to the degree of miscibility of benzene and reaction medium. The ACOH/H2O is more miscible with benzene than H3PO4/H2O, so ACOH/H2O as reaction medium can facilitate the hydroxylation of benzene, while in H3PO4/H2O the mass transfer is so more limited that benzene cannot contact H2O2 evenly and then the further oxidation of the formed phenol to dihydroxybenzenes occurs easily. Therefore, adding a small amount of sodium dodecyl sulfonate used as a surfactant toH2O/H3PO4 increased the phenol selectivity by improving the effect of mass transfer. The formation rate of phenol was very low only using glacial acetic acid as reaction medium, but it increased dramatically when adding a small amount of H2O to glacial acetic acid. Finally, the favorable effect of V modification and addition of H2O can be reasonably explained with a harmonious catalytic mechanism proposed by us.