Research On Hydroxylation Of Benzene To Phenol In Supported V Species-O₂-reducing Agent Systems

Phenol is an important raw material for organic chemical industry, the traditional synthesis method generally exists the disadvantage of multistep process, low atomic utilization rate, pollution of the environment. The direct catalytic hydroxylation of benzene to phenol, needs only one step to preparation of product. This process accords with the view point of Green Chemistry. However, the bond energy of C-H bond on benzene ring is very high, benzene is difficult to be activated, and directly introducing hydroxyl into benzene ring with great challenge. The purpose of this study is to optimize the reaction conditions, and find effective reducing agent in the hydroxylation of benzene to phenol with molecular oxygen as an oxidant, to improve the conversion of benzene and selectivity of phenol.The effect of reaction temperature on hydroxylation of benzene to phenol by molecular oxygen as an oxidant over V/SiO2, prepared by a sol-gel process has been investigated with different reducing agents. The results show that the reaction temperature has an important impact on reduction power of the different reducing agents. Benzoquinone is also an effective reducing agent in this catalyst system, benzoquinone and hydroquinone may be transformed into each other. The temperature also influence on amount of leaching of V species into solution, the experiments show that the reaction is mainly heterogeneous catalysis. With each of reducing agent, H2O2was formed and reaction scheme is similar to a Fenton type reaction. The effect of vanadium loadings, amount of reducing reagent on the reaction has been investigated. Results show that under the optimal reaction(0.1g V/SiO2(5wt%),1ml benzene,3mmol reducing agent,7ml acetic acid,3ml H2O,1MPa O2, and10h.), we obtained143%yield of phenol with hydroquinone as reducing agent.The V-M/SiO2-Al2O3(M=Cu、Fe、Cr、Co)were synthesized by impregnation process. Second metals (Cu, Fe, Cr, Co) were incorporated into V/SiO2-Al2O3catalyst to enhance the catalytic activity for the hydroxylation of benzene with molecular oxygen as the oxidant. The results show that the copper has the synergic effect on the vanadium species. With hydroquinone as a reducing agent, under optimal reaction conditions, the yield of phenol was increased from6.6%over the Cu-free catalyst to12.8%over V-Cu/SiO2-Al2O3. Among the reducing agents investigated, hydroquinone is the most effective reducing agents at higher reaction temperature on the synergistic catalytic influence. The catalysts were characterized by means of IR, H2-TPR and TEM. The characterization results show that the addition of Cu can made the catalyst active component better dispersed on the surface of the support.