Nitroxide polymer mediated oxidation of cellulose : preparation and applications

Abstract

2, 2, 6, 6-Tetramethylpiperidine-N-oxyl radical (TEMPO) is a stable nitroxyl radical, as well as a highly efficient selective oxidation catalyst. It can efficiently catalyze the selective oxidation of C6 primary hydroxyl groups of cellulose to carboxyl groups in water for the production of oxidized cellulose, which is widely used in a variety of high-tech fields. However, the TEMPO-mediated oxidation system has two major disadvantages: difficulty in separating the catalyst from the aqueous reaction mixture; serious depolymerizaiton of cellulose. In this thesis, TEMPO was loaded onto a polymer carrier with a defined molecular structure to enable TEMPO recycling and reduce the degradation of cellulose. Firstly, an acrylamide-vinylamine copolymer PVAm-supported TEMPO catalyst (PVAm-T) was designed and developed for selective catalytic oxidation of cellulose. PVAm was prepared by using the Hoffmann degradation of polyacrylamide (PAM) method. The amine groups in the copolymer reacted with the carbonyl groups in 4-oxo-TEMPO to form the catalyst PVAm-T. It was used as a catalyst, instead of free TEMPO, for selective catalytic oxidation of the C6 primary hydroxyl groups of cellulose in water. The study found that the catalyst has good catalytic performance and a low degree of degradation of cellulose. The carboxyl content of oxidized cellulose was equivalent to 76% of the level of free TEMPO. Furthermore, the catalyst PVAm-T was easy to recycle by dialysis and the recycling performance was excellent. Interestingly, it was found that PVAm-T could effectively reduce the degradation of oxidized cellulose. Secondly, G1.0 polyamidoamine (PAMAM) dendrimers was synthesized by repeated Michael addition method and ester aminolysis of ethylenediamine and methyl acrylate. Through reductive amination reaction of the primary amines in PAMAM and the carbonyl groups in 4-oxo-TEMPO, the water-soluble PAMAM immobilized TEMPO (G1.0 PAMAM-TEMPO) was successfully prepared. A polyethylene glycol monomemethyl ether (mPEG) modified PAMAM-TEMPO catalyst, called mPEG-G1.0 PAMAM-Tx, was prepared by means of reaction of the aldehyde group in mPEG-CHO and the amino group in PAMAM-TEMPO. mPEG-G1.0 PAMAM-Tx was used as a catalyst instead of free TEMPO for the selective catalytic oxidation of primary hydroxyl groups in cellulose to carboxyl groups with water as the reaction medium. The results showed that the cellulose catalytic performance of mPEG-G1.0 PAMAM-T30 was equivalent to 84% of the level of free TEMPO and that the depolymerization of cellulose was also greatly reduced. Interestingly, the positive charge and suitable size of mPEG-G1.0 PAMAM-Tx effectively reduced the formation of C6 aldehydes and C2/C3 ketones. After extracting the supernatant of the oxidation mixture with dichloromethane, mPEG-G1.0 PAMAM-Tx was recovered and re-used for further oxidation cycles. No significant reduction in catalytic performance was found after four oxidation cycles. Following this process, the applications of TEMPO oxidized cellulose (TOC) were studied. TOC has abundant carboxyl groups and so adsorbs heavy metal ions extremely well. However, TOC has a small particle size and abundant carboxyl groups and is easily dispersed in water, making separation difficult. To address this issue, PAMAM-modified magnetic nanoparticle supported TEMPO catalysts, called Gn PAMAM-Tx-MNP, were prepared and used to prepare the TOC. Gn PAMAM-Tx-MNP and TOC were crosslinked by glutaraldehyde to give a magnetic oxidized cellulose/nano-Fe3O4 composite (TOC-Gn PAMAM-Tx-MNP). TOC-Gn PAMAM-Tx-MNP was applied as an adsorption material to adsorb Pb2+ in water. The adsorption capacity of TOC-Gn PAMAM-Tx-MNP was up to 109 mg/g, i.e. equivalent to that of TOC with the same carboxy content. Furthermore, it was easily recovered by magnetic separation and the cycling performance was good. 4-nitrophenol (4-NP) is a highly toxic compound. Ag nanoparticles prepared with PAMAM as a template (PAMAM-Ag) can achieve the effect of uniform dispersion of Ag nanoparticles and can be used as a catalyst to rapidly transform 4-NP into 4-aminophenol (4-AP). However, re-use is difficult. To address this issue, PAMAM-Ag was covalently attached to TOC by an aldehyde-amine reaction between TOC-containing aldehyde groups and PAMAM-Ag containing amino groups. The TOC immobilized PAMAM-Ag (TOC-PAMAM-Ag) was used to catalyze the reaction of reducing 4-NP to 4-AP. It was found that the imine bonds contained in TOC-PAMAM-Ag were transformed into stable amine bonds in the catalytic reduction process, making TOC-PAMAM-Ag stable. The effect of the catalyst loading amount and carboxyl content was investigated and it was found that increasing the carboxyl content is beneficial in improving catalytic activity. The cyclic catalytic performance was stable, and the catalytic activity remained at a good level after several cycles.