Abstract
A wide series of copolymer materials with various contents of 4-vinyl-diisopropyl-phtalate ester (10–90 mol%), divinylbenzene (1–11 mol%) and styrene, as monomers, were obtained by radical copolymerization. In the last steps of the synthesis, diisopropyl ester functionalities were converted into the form of N-hydroxyphthalimide (NHPI) rings. The obtained materials with the NHPI groups immobilized in the copolymer structure were studied by various physicochemical techniques, including FT-IR, UV-Vis-DR, XPS, elemental analysis, and tested as catalysts in aerobic oxidation of p-methoxytoluene in the presence of Co(II) acetate co-catalyst. Conversion of the aromatic substrate was correlated with the NHPI content and cross-linking degree. The best catalytic performance (conversions higher than 23%) was achieved for the copolymer catalysts containing 60% and 30% of 4-vinyl-diisopropyl-phtalate ester. At too high concentrations of NHPI and DVB, some of the NHPI groups were transformed into inactive (C=O)-N=O species or not available due to embedding inside the copolymer structure. The mechanism of the process involving both NHPI centers, forming phthalimide N-oxyl (PINO) radicals, and Co(II) cations was discussed. Stability of the developed catalysts was also tested. The opening of imide rings took place during the catalytic process, resulting in the formation of carboxyl groups and the release of hydroxylamine molecules. The deactivated catalyst could be easily regenerated by repeating two last steps of closing imide ring.
Highlights
The processes of oxidation of organic compounds, consisting of an introduction of covalently bonded oxygen atom(s) into a molecule, are widespread both in nature and many fields of human activity
Various amounts of VDPE monomer were incorporated with styrene and DVB into the copolymer structure using the developed copolymerization strategy
A wide series of the synthesized materials allowed the analysis of the influence of the copolymer composition on its catalytic activity in the aerobic PMT oxidation
Summary
The processes of oxidation of organic compounds, consisting of an introduction of covalently bonded oxygen atom(s) into a molecule, are widespread both in nature (e.g., biochemical reactions, biomass combustion) and many fields of human activity (e.g., industrial chemical synthesis, combustion of fossil fuels). The by-product of this reaction is N2 O (recognized as a greenhouse gas), which is formed in amount of about 300 kg per 1 ton of the desired product An advantage of this process is its relatively high selectivity (~83%), in view of low substrate conversion of 3–5%. In the presence of NHPI (20 mol%), cobalt(II) and manganese(II) salts, acetic acid, at 100 ◦ C and 0.1 MPa O2 , terephthalic acid was achieved at the yield of 99%. In the oxidation of 1-phenylethanol with the use of VO(acac) as a cocatalyst, acetophenone was obtained as the only product with the yield of ~40% Another approach to the synthesis of NHPI-based catalysts for oxidizing alcohols to corresponding ketones was to obtain a derivative that could be attached to ionic liquids, as described by Koguchi et al [25].
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