| Selective catalytic oxidation of hydroxyl compounds is a common method for preparing carbonyl compounds.The nitroxide radical catalyst has the advantages of mild oxidation conditions and high selectivity,so it is widely used in the selective catalytic oxidation of various alcohol compounds.N-hydroxyphthalimide(NHPI)has attracted much attentions because of its low price.Due to its poor solubility in water and low molecular weight,NHPI is difficult to recover,limiting its application.In this work,polyethylene glycol(PEG)and polyethylene glycol monomethyl ether(m PEG)were used as carriers to prepare macromolecular NHPI catalysts,and their catalytic oxidation performances for benzyl alcohol and cellulose were studied.Firstly,the chlorinated trimellitic anhydride was bonded to the PEG/m PEG through an esterification reaction.Hydroxylamine hydrochloride was used to hydroxyl aminate the acid anhydride bond to obtain the water-soluble macromolecular catalyst called PEG-NHPI/m PEG-NHPI.The prepared water-soluble macromolecular catalyst was characterized by infrared spectroscopy,nuclear magnetic resonance spectroscopy,gel permeation chromatography and ultraviolet-visible spectrophotometer.Furthermore,the effects of carrier molecular weight and reactant ratio on the loading of NHPI were studied.The results showed that the smaller the molecular weight of the carrier,the higher the loading of NHPI.When the ratio of chlorinated trimellitic anhydride to hydroxyl was 1:1,the loading efficiency was higher.Secondly,the catalytic oxidation conditions of NHPI for selective oxidation of benzyl alcohol was optimized.In the two-phase system of dichloromethane-water(the dosage of benzyl alcohol was 11.5 mmol),effects of the reaction time,the amount of NHPI and the amount of Na Cl O on the yield of benzaldehyde were explored,and the optimal reaction conditions were determined: The reaction time was 45 min,the amount of NHPI was 0.14 mmol,and he amount of Na Cl O was 4.7 mmol.Under these conditions,the yield of benzaldehyde was 55%.The water-soluble macromolecular NHPI catalyst for the catalytic oxidation reaction of benzyl alcohol was studied.The results showed that the product yield for oxidation of benzyl alcohol was up to 77%,much higher than that of NHPI.Finally,for the NHPI-catalyzed oxidation of cellulose,effects of reaction time,oxidant dropping method,oxidant addition amount and reaction medium on the catalytic performance were explored,and the optimized catalytic reaction process was obtained: In a pure water system,10 mmol of sodium hypochlorite solution was added dropwise under acidic conditions and reacted for 10 h.The carboxyl group content of oxidized cellulose obtained under this condition was 0.99 mmol/g,which was equivalent to 68% of traditional TEMPO oxidized cellulose and the degree of polymerization was 114,which was 1.1 time higher than that of TEMPO oxidized cellulose.The water-soluble macromolecular NHPI catalyst for the catalytic oxidation reaction of cellulose was studied.When the water-soluble macromolecular NHPI catalyst was used in selective catalytic oxidation of primary hydroxyl groups in the cellulose,the catalytic performance was equivalent to 95% of NHPI level.The degree of polymerization of resulting oxidized cellulose was 126,10% higer thant that of NHPI oxidized cellulose.After the supernatant was recycled for five times,the catalytic performance did not change significantly.The structure of the macromolecular catalyst recovered by the extraction method was not changed significantly.The recovered catalyst still showed a good catalytic effect on the oxidation of cellulose.These results fully demonstrate that,loading NHPI onto PEG/m PEG,can achieve the high catalytic and recyclable properties. |
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