Acetophenone hydrogenation on polymer-palladium catalysts.: The effect of polymer matrix

Catalytic properties of Pd supported on two polymers of similar basicity but different electrical properties, a pi-conjugated conducting-polypyrrole (PPY) and the electro-inactive poly(4-vinylpyridine) (PVP) have been studied in the hydrogenation of acetophenone (ACT) and compared with that of gamma-Al2O3 supported Pd. Experimental evidences provided by several techniques: X-ray photoelectron spectroscopy (XPS), scanning (SEM) and transmission electron (TEM) microscopy, X-ray diffraction (XRD) and temperature programmed desorption (TPD) of hydrogen show that both polymers PVP and PPY exhibited ability to stabilize finely dispersed palladium nanoparticles, better this ability is offered by electro-inactive PVP. Palladium nanoparticles within a narrow range of size 2 - 20 nm as well as very high surface concentration of Pd (22.2 at %) in agglomerates were established in the latter polymer supported catalysts. Distinctly lower surface concentration of Pd (1.8 at %) and crystalline Pd particles of dimension within a wide range, from 5 nm up to ca. 1500 nm appeared in the matrix of electroactive polymer - PPY. The hydrogenation of ACT to ethylbenzene (ETB) via 1-phenylethanol (ACP) ( as the intermediate) proceeded over all studied catalysts. The effects of solvents, Pd content, ACT concentration and the additives of ACP, ETB were also studied. The catalytic properties of Pd/PPY in terms of activity and selectivity significantly differ from those of Pd/PVP and Pd/Al2O3. Both latter catalysts offered high activity and selectivity in the C=O in ACT to C-OH reduction. Definitely lower activity and higher tendency towards the hydrogenolysis of C-OH in ACP reflected Pd/PPY catalysts. Such unprofitable properties of Pd/PPY can be attributed to relatively strong adsorption of all organic reactant ACT, ACP, ETB. A competition of the ACP and ETB with the ACT occurred only in the case of Pdcentres created in the electroactive polymer, whereas Pd sites dispersed in the electro-inactive PVP similarly as the ones in Al2O3 exhibited definitely more substrate - ACT specific character.