Physicochemical and catalytic properties of palladium supported on poly(o-methoxyaniline)

Palladium was introduced into a conjugated polymer poly(o-methoxyaniline) (POM) by reacting the powdered polymer with aqueous solution of PdCl2 of low acidity (PdCl2: 2.3 × 10−3 mol/dm3, HCl: 0.66 × 10−3 mol/dm3). Various Pd2+ complexes with Cl−, H2O, OH− ligands coexisted in this solution but predominated [PdCl2(H2O)2] ones. Several techniques like X-ray powder diffraction, scanning electron microscopy, X-ray photoelectron and Raman spectroscopy, extended X-ray absorption fine structure have been used to characterize the poly(o-methoxyaniline)-Pd systems. In particular, the state of Pd species in the Pd/POM of various content of palladium (2-8 wt.% Pd) and chemical changes in the polymer matrix induced by insertion of palladium were studied. The protonation and redox reactions involved on palladium incorporation resulted in palladium ions and Pd metal in the final samples. Metallic Pd produced due to spontaneous reduction of palladium ions by the polymer formed large crystalline particles 200-1000 nm in size. The Pd2+ species in the form of anionic complexes like [PdCl4]2− acted as the counterions at low content of palladium (2-4 wt.% Pd). At high palladium content (8 wt.% Pd), several atoms like Cl, N and/or O were identified by extended X-ray absorption fine structure (EXAFS) technique in the nearest environment of Pd atoms. The structural groups of POM (like N groups and/or OCH3) as well as H2O, OH− molecules are, therefore, considered as probable species in the coordination sphere of palladium. The catalytic properties were studied for the as-prepared Pd/POM and the samples additionally reduced with aqueous solution of NaH2PO2. They were used in the hydrogenation of CC bonds in maleic acid (MAC) and CO groups in 2-ethylanthraquinone (eAQ) at 60 °C and atmospheric pressure of hydrogen using xylene-octanol-2 or water medium. The correlation between Pd/POM activities and the content of Pd metal was found. Activation of the as-prepared Pd/POM with NaH2PO2 improved their catalytic properties. Much higher and much stable activities were then obtained in both MAC and eAQ hydrogenation reactions.