The regioselective dimerisation of phenol

[摘要] The polymerisation of 2,6-dimethylphenol for the production of carbon-carbon and carbon-oxygen coupled biphenol compounds is currently catalysed on an industrial scale using homogeneous phase copper/diamine complexes. These homogeneous systems are however unable to yield useful polymers from phenols with only one or no ortho-substituent. The stereoselective polymerisation of 2- and/or 6-unsubstituted phenols has been a challenge since the late 1950's. For the production of biphenol from phenol, both direct dehydrogenation and oxidative dehydrogenation can be envisaged. Catalytic systems for the selective carbon-carbon dimerisation of phenol are currently undeveloped. A commercial one- step regioselective synthesis of biphenol compounds from phenol would dramatically reduce production costs since phenol is a relatively cheap and readily available starting material. The aim of this research project was to develop a catalyst system(s) capable of selectively generatmg 4,4'-biphenol, 2,2'-biphenol and 4-phenoxyphenol from phenol, exploiting the compounds different molecular shapes and dimensions. Biphenols are used as lubricants, fuel additives, antioxidants, polymerisation intermediates and fungicides. This research project studied the one-step regioselective synthesis of biphenol compounds from phenol using the zeolite catalysts: Zeolite-Y and ZSM-5, the clays: Montmorillonite and Attapulgite and the mesoporous catalyst MCM-41. Zeolites are crystalline aluminosilicates whose structures contain molecular sized and shape selective cavities that can be used as molecular sieves and/or the location of active sites for heterogeneous catalysis. Shape selectivity was important for the dimerisation of phenol to biphenols. This investigation has demonstrated that the regioselective dimerisation of phenol can be realised using metal-ion exchanged zeolites and clays as catalysts. A new reaction mechanism has been proposed which demonstrates the need for both water and oxygen. The oxygen was required to oxidise Cu1+ back to Cu2+ and water used to convert BrOnsted acid sites back to Lewis base sites in order to complete the catalytic cycle. The results from this research project suggest the coupling of phenoxy radicals coordinated to the catalyst. The yield of phenolic dimers was controlled by the position of active sites and therefore adsorbed phenoxy radicals. Solvents used to dissolve the phenol had a considerable impact on the selectivity of phenol dimerisation. These solvent effects had not previously been investigated. Acetonitrile was found to be not only a solvent but also a catalyst modifier. The solvent used did not however affect the rate of reaction, rate-determining step or conversion of phenol. The selectivity of phenol dimerisation catalysed by Cu/Zeolite- Y or Cu/Attapulgite was due to the competitive adsorption between water, the solvent and phenol at copper exchanged sites.

[发布机构] University:University of Glasgow

[关键词] Chemical engineering