Investigation of organophilic montmorillonites supported palladium catalytic composites by combined positron annihilation lifetime updates spectroscopy and X-ray diffraction

A characterization of organophilic montmorillonite (OMMT) supports and organophilic montmorillonite supported palladium (Pd@OMMT) catalytic composites was carried out using positron annihilation lifetime spectroscopy (PALS) and X-ray diffraction (XRD). XRD method supplies the fundamental information on the interlayer spacing of OMMT. And PALS method supplies the complementary information on the free volume voids variations inside the interlayer space of OMMT, which is occupied with organic cations and Pd species. In PALS analyzing, the size of the channel-like free volume voids inside the interlayer space of OMMT are estimated by both cuboidal and cylindrical free volume voids models from o-Ps lifetime. The OMMT and Pd@OMMT composites show bigger interlayer spacing values (by XRD) but smaller free volume void size values (by PALS) than MMT. It should be due to the fact that the interlayer space becomes more crowded after organic modifier intercalation and Pd immobilization inside the interlayer space of MMT. The prepared Pd@OMMT catalytic composites show high activity and good recyclability in typical Heck coupling reactions. The correlations between the microstructure and catalytic performances of the organophilic montmorillonites supported palladium catalytic composites were discussed.