The Synthesis of Green Palladium Catalysts Stabilized by Chitosan for Hydrogenation

Abstract

The proposed paper describes a simple and environmentally friendly method for the synthesis of three-component polymer–inorganic composites, which includes the modification of zinc oxide or montmorillonite (MMT) with chitosan (CS), followed by the immobilization of palladium on the resulting two-component composites. The structures and properties of the obtained composites were characterized by physicochemical methods (IRS, TEM, XPS, SEM, EDX, XRD, BET). Pd–CS species covered the surface of inorganic materials through two different mechanisms. The interaction of chitosan polyelectrolyte with zinc oxide led to the deprotonation of its amino groups and deposition on the surface of ZnO. The immobilization of Pd on CS/ZnO occurred by the hydrolysis of [PdCl4]2−, followed by forming PdO particles by interacting with amino groups of chitosan. In the case of CS/MMT, protonated amino groups of CS interacted with negative sites of MMT, forming a positively charged CS/MMT composite. Furthermore, [PdCl4]2− interacted with the –NH3+ sites of CS/MMT through electrostatic force. According to TEM studies of 1%Pd–CS/ZnO, the presence of Pd nanoclusters composed of smaller Pd nanoparticles of 3–4 nm in size were observed on different sites of CS/ZnO. For 1%Pd–CS/MMT, Pd nanoparticles with sizes of 2 nm were evenly distributed on the support surface. The prepared three-component CS–inorganic composites were tested through the hydrogenation of 2-propen-1-ol and acetylene compounds (phenylacetylene, 2-hexyn-1-ol) under mild conditions (T—40 °C, PH2—1 atm). It was shown that the efficiency of 1%Pd–CS/MMT is higher than that of 1%Pd–CS/ZnO, which can be explained by the formation of smaller Pd particles that are evenly distributed on the support surface. The mechanism of 2-hexyn-1-ol hydrogenation over an optimal 1%Pd–CS/MMT catalyst was proposed.