Hydroxylation Of Benzene To Phenol Over Organic Molybdovanadophosphoric Heteropolyacid Salt Catalysts

Phenol is an important raw chemical. Currently, the industrialized process for the production of phenol from benzene based on multistep and indirect syntheses has several disadvantages, such as complicated synthetic routes, large consumption of raw materials, and serious environmental pollution. Therefore, the direct hydroxylation of benzene to phenol in a single-stage reaction has become one of the most challenging topics in the chemical industry, from an economical and environmental point of view. The catalytic hydroxylation of benzene with hydrogen peroxide (H2O2) as a clean oxidant has attracted some great attention. Vanadium-containing heteropolyacids (HPAs) have been used in the hydroxylation of benzene to phenol under various reaction conditions. However, the yield of phenol from the hydroxylation of benzene over the reported catalysts is still rather low.Based on the status of research and the existing problems of the reported catalysts, a preparation method of phase-transfer catalysts and a novel catalytic process were proposed for the hydroxylation of benzene to phenol with H2O2. In this MSc thesis, the direct hydroxylation of benzene with H2O2 as an objective reaction was systematically investigated in line with screening molybdovanadophosphoric heteropolyacid salt catalysts. Research was further conducted on the effects of the number of vanadium atoms and of organic amine cations in the heteropolyacid catalysts on the catalytic activity of benzene hydroxylation. Moreover, the reaction conditions were optimized. Finally, investigations were conducted on the intrinsic kinetics of benzene hydroxylation with H2O2 in detail. The main results are summarized as follows:1) The catalysts were prepared by attaching nitrogen-containing heterocyclic amines to the Keggin-structured molybdovanadophosphoric heteropolyacid H4PMo11VO40 and characterized by FT-IR, UV-vis and TG-DTA techniques. Among these catalysts, the pyridine anchored heteropolyacid ([(C5H5NH)+]4[PMo11VO4o]4-) shows the best catalytic properties, over which a phenol yield of 28.1%, much higher than that over the pure H4PMo11VO40 catalyst (21.2%), can be obtained under the optimized reaction conditions.2) A serial of organic molybdovanadophosphoric heteropolyacid salt catalysts with different organic carbon-chain lengths were prepared by a facile acid-base neutralization method and then a comparison was made for their catalytic activities in the hydroxylation of benzene. The results show that the triethylamine molybdovanadophosphoric heteropolyacid salt ({[(C2H5)3NH]+}4[PMo11VO40]4-) exhibits the highest activity for the hydroxylation of benzene with a selectivity of 88.2% for phenol and a phenol yield of 32.3% at the optimized reactions conditions: a mixture of acetic acid and acetonitrile (volume ratio 1:1) as solvent,0.2 g of catalyst,1 ml of benzene,3.5 ml of 30 wt.% H2O2 aqueous solution, a reaction temperature of 60℃, and a reaction time of 8 h. The promotion effects of the organic species present in the catalysts on the hydroxylation are probably related to the electronic interactions between organic species and heteropolyacid anion coupled with the pseudo-liquid-phase behavior of the heteropolyacid. Thus these organic amine anchored heteropolyacid catalysts can be referred to as "reaction-induced self-separation" catalysts, indicating that these catalysts could be recovered by a simple filtration at the end of the reaction.3) The results from the study of catalytic kinetics in the direct hydroxylation of benzene over{[(C2H5)3NH]+}4[PMo11VO40]4- show that the orders of reaction with respect to benzene and hydrogen peroxide are first order and the activation energy is 93.3 kJ·mol-1.4) A series of Bronsted acidic ionic-liquids were synthesized, and their structure and thermal stability were characterized by IR, and TG-DTG techniques. The results show that ionic liquids display a high thermal stability up to 573 K. In such ionic liquids as solvents, the direct hydroxylation of benzene to phenol with H2O2 over {[(C2H5)3NH]+}4[PMo11VO40]4- was investigated in order to provide the basis for process development in the direct production of phenol.