2-Ethyl-9,10-anthraquinone hydrogenation over Pd/polymers - Effect of polymers-Pd(II) chlorocomplexes interactions

Two polymers, namely poly(4-vinylpyridine) (PVP) and polyaniline (PANI), were used as the supports for palladium catalysts acting in 2-ethyl-9,10-anthraquinone (EAQ) hydrogenation, a key step in the industrial production of H2O2. The nature of PVP and PANI interactions with various chlorocomplexes of Pd(II) coexisting in PdCl2-HO-HCl solutions was studied using IR (mid-IR, far-IR), UV-Vis and XPS spectroscopies. It was found that the type of interactions involving nitrogen atoms of the polymers depended mainly on the acidity of PdCl2 solution. Protonation of polymers (via acid-base reactions) as well as coordination of Pd2+ ions by nitrogen atoms of the polymers took place in highly acidic PdCl2 solution (2 M HCl) containing predominantly anionic [PdCl4](2-), [PdCl3(H2O)](-) complexes (series A of experiments). In the weakly acidic PdCl2 solutions (0.66 x 10(-3) M HCl) (series B of experiments) containing predominantly electrically neutral [PdCl,(H2O)(2)] complexes, hydrolysis of the complex proceeded as the main process resulting in precipitation of palladium oxide on PVP. In the case of PANI in solution B, the redox mechanism was involved resulting in the reduction of some Pd2+ to Pd-0 accompanied by partial oxidation of the polymer chain. As a consequence of various mechanisms of polymers reactions with Pd2+ ions, surface morphology of the final catalysts, characterized by XRD and SEM methods, was different. It was found that dispersion of palladium in Pd/PVP and Pd/PANI catalysts (1-10 wt.% Pd) influenced the course of EAQ hydrogenation. The presence of large palladium particles promoted reactions leading to the formation of the so-termed "degradation products" not capable of hydrogen peroxide formation. Pd/PVP catalysts (series B) exhibited higher activity. Selectivity of EAQ hydrogenation in their presence was better than that seen for Pd/PANI catalysts. (C) 2001 Elsevier Science B.V. All rights reserved.