Topological loading of Cu(I) catalysts onto crystalline cellulose nanofibrils for the Huisgen click reaction

Topological loading of monovalent copper ions (Cu(I)) onto the crystal surfaces of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs), which have a large number of sodium (Na) carboxylate groups on their surfaces, was achieved through ion exchange between Na and Cu species. The Cu-loaded TOCNs were fabricated into aerogels by a freeze-dry treatment. The as-prepared Cu-TOCN aerogel demonstrated an excellent catalytic efficiency for the azide-alkyne Huisgen [3 + 2] cycloaddition reaction, indicating that the Cu(I) catalysts are highly dispersed and exposed on the entire surfaces of crystalline TOCNs, allowing effective contact with the reactants. The Cu-TOCN aerogel was recoverable from the reaction system and Cu(I) catalysts were extensively immobilized on TOCNs during the reaction. It is noteworthy that TOCNs can be easily obtained from native cellulose, which is the most abundant and renewable biomass. Thus, the Cu-TOCN aerogel is a high-performance and environment-friendly catalyst, and this novel technique is expected to comprise a promising approach toward green and sustainable development.