Direct Hydroxylation of Benzene to Phenol with Molecular Oxygen over Phase Transfer Catalysts: Cyclodextrins Complexes w
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Direct Hydroxylation of Benzene to Phenol with Molecular Oxygen over Phase Transfer Catalysts: Cyclodextrins Complexes with Vanadium-Substituted Heteropoly Acids Hanqing Ge Æ Yan Leng Æ Changjiang Zhou Æ Jun Wang
Received: 16 January 2008 / Accepted: 1 March 2008 / Published online: 30 April 2008 Ó Springer Science+Business Media, LLC 2008
Abstract Cyclodextrins (CyDs) complexes with vanadium-substituted heteropoly acids (PMoVn-b-CyDs, n = 1, 2) were prepared by simple mixing and their structures were characterized by FT-IR. Among various catalysts, PMoV1-b-CyDs, an efficient phase transfer catalyst, exhibited the highest yield (13.1%) of phenol without observing the formation of catechol, hydroquinone and benzoquinone in direct hydroxylation of benzene to phenol in 80 vol% aqueous acetic acid with molecular oxygen and ascorbic acid used as the oxidant and the reducing reagent, respectively. The influences of the reaction temperature, the pressure of oxygen, the amount of ascorbic acid, the amount of catalyst, and the reaction time on the yield of phenol were investigated to obtain the optimal reaction conditions for phenol formation. Keywords Hydroxylation of benzene Phase transfer catalysis Phenol Vanadium-substituted heteropoly acid Cyclodextrins
1 Introduction Phenol is an important intermediate for the manufacture of petrochemicals, agrochemicals and plastics [1–3]. It is mainly produced by cumene process. However, this process consists of three steps, and has many disadvantages. For example, the yield of phenol remains only 5%, accompanied by great energy consumption especially in
H. Ge Y. Leng C. Zhou J. Wang (&) State Key Laboratory of Materials-Oriented Chemical Engineering, College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, China e-mail: [email protected]
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the distillation process for separation among the products and the reactants [4]. Hence, finding a one-step process for the production of phenol by direct hydroxylation of benzene attracts much attention in recent years [5–11]. The direct synthesis of phenol from benzene and nitrous oxide, a very toxic and global-warming related gas molecule, has been described [12], but this method is only cost-effective when nitrous oxide is available cheaply as a by-product [13]. Therefore, there is a need for the search of new processes to produce phenol without by-products, with a high selectivity and at the reaction conditions as mild as possible. Bahidsky et al. reported the gas phase hydroxylation of benzene to phenol catalyzed by the Cu-modified phosphate catalysts in the presence of oxygen and ammonia at 723 K and a very poor yield of phenol was obtained [14]. Lemke et al. investigated the direct hydroxylation of benzene to phenol by hydrogen peroxide on vanadium oxide catalysts supported on MCM-41, MCM-48, silica of Aerosil type and AMM [3]. Low to moderate yield of phenol and selectivity were achieved. As has been recently shown, active peroxide-type species can be obtained by an in situ re
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