Pd/Chitosan Nanoparticle Catalysts Prepared by Solid Mortar Grinding for Hydrogenation of Nitroarenes

Herein, we report a rational and highly sustainable strategy for the gram-scale synthesis of solid chitosan polymer-supported palladium nanoparticles (Pd/Chit) without the use of toxic solvents or expensive purification processes. Pd nanoparticles (NPs) are deposited onto a chitosan polymer using both ascorbic acid and NaOH. The former served as a mild reducing agent in the conversion of PdCl2 to Pd NPs, and the latter was crucial in anchoring the resulting Pd NPs to the chitosan support during solid grinding. The combination of ligand adsorption kinetics measurements and Langmuir adsorption isotherm studies of Pd/Chit reveals that the deposited Pd NPs have a higher available surface area and a large number of accessible active sites, which is in sharp contrast with other Pd NP catalysts reported using solution methods. As a result, in the presence of NaBH4 as a hydrogen source, the Pd/Chit demonstrated enhanced catalytic activity for the reduction of a range of nitroarenes into corresponding amines. Furthermore, a kinetics investigation of the reduction of nitroarenes under optimal reaction conditions showed that Pd/Chit has substantially lower activation energy than other reported Pd NP catalysts. Because of the novel interface structure at the Pd–chitosan site as well as the greater number of accessible active sites at the Pd NP sites, the Pd/Chit possesses excellent catalytic activity and selectivity. The development of more renewable biopolymer-supported metal catalysts for heterogeneous catalysis may therefore be encouraged by the present scalable solid-state method for the sustainable synthesis of Pd/Chit catalysts.