Zhang, Shuyao, Huang, Yuling, Lin, Ting, Xue, Meiling, Liu, Shirong, Deng, Hanhui, and Li, Yiqun
Dip catalysts, in which the catalytic species are immobilized on solid planar supports, have drawn increasing attention due to their ease of fabrication, excellent catalytic performance, convenient separation and reusability, and environmental friendliness. A special feature is that they can turn the reaction on/off instantly by simple insertion/removal from the reaction medium. Embedding metal catalysts in biodegradable films that act as dip catalysts in organic reactions is relatively new, and therefore the facilitation of their use is necessary. Biodegradable carboxymethylcellulose (CMC) and polyvinyl alcohol (PVA) have film-forming properties due to their inter/intramolecular hydrogen bonds between numerous carboxylic and hydroxy groups. Citric acid (CA), a biodegradable tricarboxylic acid, is widely used as a crosslinker due to its esterification of hydroxy groups of polymers. To obtain biodegradable dip catalysts, we propose the incorporation of CA into a CMC–PVA composite film to enhance mechanical strength through cross-linking CMC and PVA, and to provide sufficient carboxylic and hydroxy groups for the coordination of Cu(II) and Pd(II) ions, thereby yielding Cu@CMC–CA–PVA and Pd@CMC–CA–PVA dip catalysts. Both dip catalysts were extensively characterized using ICP-AES, XPS, FT-IR, XRD, TGA, FE-SEM, TEM, and EDS analysis. The XPS analysis confirmed Cu(I) in the fresh Cu@CMC–CA–PVA catalyst, while the used sample exhibited a mixture of Cu(I) and Cu(II), likely due to partial oxidation of Cu(I) during the reaction under air conditions. Similarly, Pd(II) ions immobilized on the fresh Pd@CMC–CA–PVA were reduced to Pd(0) nanoparticles (NPs) by hydroxy groups of CMC and PVA with a high average diameter distribution of 6.53 nm. The catalytic activity of both dip catalysts was firstly evaluated separately for the aldehyde-alkyne-amine (A3) coupling reactions and Suzuki–Miyaura cross-couplings. The results demonstrated that both Cu@CMC–CA–PVA and Pd@CMC–CA–PVA catalysts have good catalytic activity to give excellent yields. In addition, these dip catalysts were easily retrieved with tweezers from the reaction system to turn off reaction and re-introduced in successive batches to turn on the reaction for repeated use. After six recycles, the catalysts still maintain good performance without noticeable loss in catalytic activity. These facilely prepared, conveniently deployed, highly active, good recyclable, and eco-friendly dip catalysts are expected to hold promise as practical tools in organic synthesis. A composite film comprising carboxymethylcellulose (CMC), polyvinyl alcohol (PVA), and citric acid (CA) enables the fabrication of Cu@CMC–CA–PVA and Pd@CMC–CA–PVA dip catalysts, featuring sufficient carboxylic and hydroxy groups for coordination with Cu(II) and Pd(II) ions, respectively. Both catalysts exhibit strong activity in A3 coupling reactions and Suzuki–Miyaura cross couplings. [ABSTRACT FROM AUTHOR]