Probing Active Sites During Palladium-Catalyzed Alcohol Oxidation in “Supercritical” Carbon Dioxide

Abstract

Structural information has been gained during aerobic benzyl alcohol oxidation in ''supercritical'' carbon dioxide at 150 bar on alumina-supported palladium by X-ray absorption spectroscopy while monitoring simultaneously the performance of the catalyst. The reduction of the catalyst by benzyl alcohol could be monitored by the analysis of the near-edge region of the Pd K-edge. The palladium constituent was mainly in metallic state under operating conditions. Partial reoxidation was observed when only oxygen in ''supercritical'' carbon dioxide in the absence of alcohol was fed. The catalytic activity of the PdO x =Al 2 O 3 catalyst during benzyl alcohol oxidation was comparable to that in a conventional continuous fixed-bed reactor and depended on the oxygen concentration in the feed. The rate of alcohol conversion went through a maximum when the oxygen concentration was increased. At maximum rate, part of the palladium was in the oxidized state. Upon further increase of the oxygen concentration, the activity decreased because of the formation of surface palladium oxide. The reaction rate in ''supercritical'' carbon dioxide was strikingly higher than that observed for the corresponding liquid-phase oxidation.

Comparative in situ XAS investigations during aerobic oxidation of alcohols over ruthenium, platinum and palladium catalysts in supercritical CO2

“…95%) could be achieved [23]. The phase behaviour of the reaction mixture [25,27] and the oxidation state of the solid catalyst [28,29] were found to be crucial for the high catalytic activity. In particular, we showed that the reaction could be effectively investigated, understood and optimized only by applying the appropriate in situ spectroscopic techniques (combined transmission IR/ATR-IR and X-ray absorption spectroscopy).…”

“…Section 2 and Refs. [28,29]). During the experiments with the palladium catalyst the reaction rate measured in the in situ XAS flow cell was 1890 mol product mol Pd À1 h À1 (80 8C, 150 bar, 1.9 mol% alcohol, 1.9 mol% O 2 0.1 mol/min CO 2 ).…”

Aerobic Oxidations in Continuous Flow

“…In order to obtain operando structural analysis by X-ray absorption spectroscopy (XAS), the standard reactor design of the Baiker group (Scheme 10) was adapted for analytical applications (Fig. 2a) [88,89,96].…”

Oxidation of Alcohols with Molecular Oxygen

“…The results of some electrochemical [98,99] and EXAFS [100] studies have indicated a considerable oxygen coverage of the metal surface during alcohol oxidation. A feasible interpretation of this would be that oxygen interacts directly with the adsorbed alcohol (or alcoholate) in the rate-determining step, and this model is supported also by numerous kinetic studies [101,102].…”