A recoverable dendritic polyamidoamine immobilized TEMPO for efficient catalytic oxidation of cellulose

Polyamidoamine (PAMAM) dendrimers of G1.0 and 2.0 were synthesized by the repeated Michael addition and ester aminolysis of ethylenediamine and methyl acrylate. Through the reductive amination reaction of primary amines in PAMAM and carbonyl groups in 4-oxo-2,2,6,6-tetramethylpiperidine-1-oxyl (4-oxo-TEMPO), the water-soluble PAMAM immobilized TEMPO (PAMAM-TEMPO) was successfully prepared. The obtained PAMAM-TEMPO was characterized by Fourier transform infrared spectroscopy (FT-IR) and ultraviolet-visible spectrophotometer (UV-vis). PAMAM-TEMPO was used as catalyst instead of free TEMPO for selective catalytic oxidation of primary hydroxyl groups in cellulose with water as reaction medium. The results showed that the catalytic performance of G1.0 PAMAM-TEMPO with 28.8% TEMPO loading was equivalent to free TEMPO. After salting out the supernatant of oxidation mixture, PAMAM-TEMPO was recovered by extraction with N,N-dimethylformamide and reused for further oxidation cycles. No significant reduction in catalytic performance was found after 4 oxidation cycles. The recovery of PAMAM-TEMPO after each cycle was about 90%. By sonication of oxidized cellulose obtained with G1.0 PAMAM-TEMPO as catalyst, the individualized cellulose nanofibers with approximately 10 nm in diameter were successfully prepared. This is the first report on the use of immobilized TEMPO catalysts comparable to the performance of free TEMPO to oxidize cellulose in water.