Preparation of oxidized celluloses in a NaBr/NaClO system using 2-azaadamantane N-oxyl (AZADO) derivatives in water at pH 10
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Catalytic oxidation using N-oxyl radicals in water at pH 10 was applied to wood cellulose to determine the influence of different N-oxyl radical chemical structures on the reaction kinetics and structures of oxidized celluloses. 2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO), 2-azaadamantane N-oxyl (AZADO), 1-methyl-AZADO (1-Me-AZADO), 9-azanoradamantane N-oxyl (Nor-AZADO), and 1,5-dimethyl-9-azanoradamantane N-oxyl (DMN-AZADO) were used as oxidation catalysts in a NaBr/NaClO system in water at room temperature and pH 10. The reaction time required for complete oxidation of wood cellulose by 5 mmol/g of NaClO using AZADO, 1-Me-AZADO, or Nor-AZADO was less than 20 min, while DMN-AZADO and TEMPO required 70 min and 120 min, respectively. The sodium carboxylate contents of the oxidized celluloses prepared using TEMPO and DMN-AZADO were 1.41 and 1.45 mmol/g, respectively, which were higher than those of oxidized celluloses prepared using other AZADO derivatives. The original cellulose I crystal structure, crystallinity, and crystal size of wood cellulose were mostly maintained in all oxidized celluloses, with oxidation selectively occurring at C6–OH groups on the crystalline cellulose microfibril surfaces. Oxidized celluloses prepared using DMN-AZADO and post-reduced with NaBH4 showed high carboxylate contents of ~ 1.41 mmol/g and the heist viscosity-average degrees of polymerization of ~ 800.
KeywordsTEMPO AZADO derivatives Oxidized cellulose Kinetics Carboxylate content Degree of polymerization
This research was supported by Core Research for Evolutional Science and Technology (CREST, Grant Number JPMJCR13B2) of the Japan Science and Technology Agency (JST). We thank Simon Partridge, Ph.D., from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.
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