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 (SBET) and total pore volume (Vp) 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.