Hydrogenation of 2-ethyl-9,10-anthraquinone on Pd-polyaniline(SiO2) composite catalyst: The effect of humidity Original Research Article

The role of humidity in the hydrogenation of 2-ethyl-9,10-anthraquinone (eAQ) was studied using palladium catalysts supported on silica (SiO2) coated with polyaniline (PANI). Polymer was deposited on the support, SiO2, during “in situ” polymerization of aniline. Two series of catalysts (0.5 wt.% Pd) differing in ordering of PANI macromolecules in the polymer film were used. The role of humidity was studied in the desired eAQ to eAQH2 (2-ethyl-9,10-anthrahydroquinone) reduction (quinone–hydroquinone stage), and in the consumption of primary and desirable product, eAQH2. The influence of humidity, especially in the course of hydroquinone form transformation, varied depending on the morphology of polymer film. At the high level of humidity a trend in the reactivity change of catalysts with thin and denser polymer film is similar to that for typical 0.5% Pd/SiO2 catalyst. On the other hand, reactivity of catalysts with “brush-like” ordering of PANI macromolecules does not change even at high level of humidity in the system. On these catalysts, the increase in humidity level has no influence on directing the consumption of eAQH2, and on the course of hydrogenolytic reactions producing 2-ethylanthrone (eAN). The hydroquinone form eAQH2 was exclusively transformed by phenyl ring saturation to give H4eAQH2 (active quinone, 2-ethyl-5,6,7,8-tetrahydro-9,10-anthrahydroquinone) with strong suppression of hydrogenolytic reactions yielding degradation products. This advantageous reactivity observed for all the PANI film-containing catalysts is related to the modification of environment surrounding of the Pd centres by the presence of hydrophobic polymer. Its role may be considered as the influence of polymer on the accessibility of reactants to the Pd centres and the geometry of reactant molecules adsorption.