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Title: Alkene epoxidation catalysed by molybdenum (VI) supported on polymers
Author: Mbeleck, Rene
ISNI:       0000 0004 2680 2079
Awarding Body: University of Strathclyde
Current Institution: University of Strathclyde
Date of Award: 2009
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In 1963, Bruce Merrifield introduced the solid-phase peptide synthesis technique using an insoluble macromolecular protecting group for synthesising peptides. Since then organic chemistry involving solid supports has been developed as an efficient technique for the synthesis of a large number of organic compounds, and therefore a significant interest has been raised in catalysis. This thesis deals with the synthesis of new supported Mo catalysts active, selective and stable for the epoxidation of olefins on an industrial scale using a continuous reactive distillation rig and also in multi-step fine chemical syntheses. Here I report on the immobilisation of molybdenum (VI) complexes supported on polymer beads or resins and an inorganic MCM-41 support for the catalytic epoxidation of alkenes with alkyl hydroperoxides as the oxidants. Specifically Mo0₂(acac)₂ has been successfully supported on a commercially sourced polybenzimidazole (PBI) resin using a simple ligand exchange reaction. In addition a number of chloromethylated polystyrene resins has been synthesised by suspension copolymerisation of appropriate mixture of vinyl benzyl chloride (VBC) and divinylbenzene (DVB), in the presence of porogenic solvents when required. The VBC residues have been reacted with several different donors ligands 2-aminomethylpyridine (AMP), 2-aminophenol (2-AMPH), glycine (GLYC), iminodiacetic acid (lMDA) ethylenediaminetetraacetic acid (EDTA) and phenylalanine (Phe) in order to create binding sites for the metal on each polymer matrix. However the inorganic MCM-41 support was first modified with aminopropyl trimethylsilane, then the thiophene-2-carboxyaldehyde ligand was attached in order to generate binding sites for the metal on this support. Ligand exchange with Mo0₂(acac)₂ again allows Mo (VI) species to be supported on these supports. Each resin supported Mo has been activated by treatment with t-butylhydroperoxide (TBHP) and then investigated as a heterogeneous catalyst in the epoxidation of cyclohexene and styrene using TBHP. An ¹H NMR spectroscopic analytical procedure has been used to monitor the progress of each epoxidation. In order to check the stability of each of these supported Mo catalysts prior to using them in a reactive distillation column, extensive study of recycling of each support Mo catalyst was undertaken by monitoring the use of a sample of each in up to 5-10 successive batch epoxidations. At the same time the supernatant solution from each epoxidation has been isolated, reduced to a dry residue, and the latter employed as a potential catalyst in an epoxidation reaction. The latter procedure is a very powerful methodology for the detection of low levels of catalyst (Mo) leaching in the heterogeneous reactions. PBI.Mo, polystyrene-supported Mo and inorganic supported Mo catalysts have proved to be very active, maintaining this activity over 5 to 10 cycles. However, the residues from supernatant solutions have also proved to be catalytically active demonstrating the occurrence of Mo leaching from PBI.Mo and polystyrene-supported Mo catalysts. In the case of the PBI supported Mo catalyst (PBI.Mo), the level of leaching fell substantially over 10 cycles until the rate of epoxidation was close to that of a control epoxidation reaction carried out in the absence of any Mo species. Leaching from one of the polystyrene-supported Ps.AMP.Mo (25) catalysts was more pronounced and remained significant even after several cycles. However, a second sample of polystyrene-supported Ps.AMP.Mo (26) with a higher ligand: metal ratio showed a much lower tendency to leach. This result has now been confirmed with a third sample of this type of polymer catalyst (Ps.AMP.Mo (27). Other polystyrene resin supported Mo catalysts employing the 2-AMPH, GLYC, IMDA and EDTA ligands have also been tested. These are all very active cyclohexene epoxidation catalysts and extensive recycling and leaching experiments have also been carried out in order to assess again if these ligands offer improved stability towards Mo leaching. A similar study has also been carried out with the inorganic MCM-4I.SB.Mo (32) catalyst. Several of these supported Mo catalysts have also been assessed on a small scale e.g. for fine chemical syntheses, and they also show relatively good conversion of olefins to their corresponding epoxides using different solvents or no solvent. PBI.Mo has also been tested in a reaction distillation column. It shows very high reproductive conversion and good selectivity in the case of cyclohexene epoxidation with conversion of TBHP up to 97% after 1 h.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral