Abstract
2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibers (TOCNs), which have a high-density of exposed carboxylic acid groups on their crystalline surfaces, effectively act as acid catalysts in acetal hydrolysis. Carboxy-free cellulose nanofibers, polymeric carboxylic acids, and homogeneous acetic acid do not show significant catalytic activity under the same reaction conditions. Mercerized TOCNs differing from the original TOCNs in a crystalline structure were also ineffective, which suggests that the unique nanoarchitectural features of TOCNs, such as regularly aligned carboxylic acid groups, large specific surface areas, and structural rigidity, must be major factors in the acceleration of acetal hydrolysis. Kinetic analysis suggested that substrates and/or acid catalyst species were concentrated on the TOCN crystalline surfaces, which significantly enhanced the catalytic activity.
Highlights
Cellulose nanofibers (CNFs) are promising natural nanomaterials for achieving a sustainable society because of their abundance, renewability, and attractive physicochemical properties for advanced materials applications[1,2,3,4,5,6]
Neither an HCl solution of pH 4.0 nor the supernatant of pH 4.0 obtained by complete removal of TOCNs from the mixture was effective in the hydrolysis reaction, clearly suggesting the unique features of TOCNs as a heterogeneous acid catalyst
Physically pulverized CNFs with no carboxylic acid groups showed low catalytic efficiency. These results presumably indicate that the dense interfacial carboxy groups on the TOCN crystalline surfaces play a crucial role in accelerating the hydrolysis reaction
Summary
Cellulose nanofibers (CNFs) are promising natural nanomaterials for achieving a sustainable society because of their abundance, renewability, and attractive physicochemical properties for advanced materials applications[1,2,3,4,5,6]. CNFs are attractive supporting materials for heterogeneous catalysts, especially for metal nanoparticles, because of their designable functions, large surface areas, and high chemical resistances. The use of highly dispersed TOCNs on which bare metal nanoparticles as catalysts in a number of reactions were supported has been reported[31,32,33,34,35]. A number of catalytic reactions have been performed with metal nanoparticles immobilized on CNFs, few attempts have been made to use CNFs themselves as catalysts, despite their designable functionalities. Chitosan and surface-deacetylated chitin nanofibers, which are amino analogs of CNFs, are useful platforms for functional moieties Aerogels of these nanofibers successfully catalyze Knoevenagel condensation and aldol reactions[39,40]. The kinetic profiles of the reaction and the effect of the carboxylate density of the TOCNs on the catalytic performance are discussed in terms of enhancement of the catalytic efficiency in the interfacial hydrolysis reactions of acetals
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