Microstructure and catalytic performances of chitosan intercalated montmorillonite supported palladium (0) and copper (II) catalysts for Sonogashira reactions

Abstract

In this study, an efficient heterogeneous catalytic material including Pd⁰ nanoparticles and Cu²⁺ cations supported on montmorillonite/chitosan (MMT/CS) composite was prepared by solution intercalation and complexion methods. The valence states of Pd (both Pd(0) and Pd(II) coexisting) and Cu (mainly Cu(II)) of the Pd⁰/Cu²⁺@MMT/CS catalyst were confirmed by the X-ray photoelectron spectroscopy (XPS) characterization. The d₀₀₁ spacing was enlarged from 1.25nm (MMT) to 1.94nm (Pd⁰/Cu²⁺@MMT/CS). Pd⁰/Cu²⁺@MMT/CS catalyst had obviously bigger specific surface area (S_BET) and total pore volume (V_p) than pure MMT. High resolution transmission electron microscopy (HR-TEM) observation of the Pd⁰/Cu²⁺@MMT/CS catalyst showed that separated Pd⁰ nanoparticles sized below 3nm dispersed well both in the interlayer space and surface of MMT layers. The positron annihilation lifetime spectroscopy (PALS) was very sensitive to the microstructure changes caused by the formation of nano particles Pd⁰ after reduction of Pd²⁺/Cu²⁺@MMT/CS to Pd⁰/Cu²⁺@MMT/CS. The prepared Pd⁰/Cu²⁺@MMT/CS catalysts are highly active for the Sonogashira reactions of aromatic halides and alkynes in H₂O/ether solution, and can be recycled 6 times. The leaching of Cu species is much quicker than Pd species during recycling, which should be the main reason for the decrease in efficiency of the recycled Pd⁰/Cu²⁺@MMT/CS catalysts.