No abstract

In situ X-ray absorption spectroscopy combined with on-line catalytic measurements using FT-IR spectroscopy unequivocally identified that metallic palladium is the more active phase in the aerobic oxidation of benzyl alcohol than palladium oxide. The aerobic oxidation of benzyl alcohol in cyclohexane at 50 degrees C was low over oxidized 0.5%Pd/Al2O3 and 5%Pd/Al2O3 catalysts. XANES and EXAFS showed that the catalysts in the as-received state were almost fully oxidized and no reduction of the palladium constituent was observed during time-on-stream. After in situ reduction by hydrogen-saturated cyclohexane, the catalysts were much more active (over 50 times) than before reduction. Both XANES and EXAFS uncovered that the palladium constituent was mainly in a reduced state under these conditions of high catalytic activity. This demonstrates that metallic palladium is the active phase for alcohol dehydrogenation.

Selective oxidation of benzyl alcohol to benzaldehyde with molecular oxygen over an alumina-supported palladium catalyst was performed with high rate at about 95% selectivity in supercritical carbon dioxide. The experiments in a continuous flow fixed-bed reactor showed that the pressure has a strong influence on the reaction rate. A marked increase of the rate (turnover frequency) from 900 h(-1) to 1800 h(-1) was observed when increasing the pressure from 140 to 150 bar. Video monitoring of the bulk fluid phase behavior and the simultaneous investigation by transmission and attenuated total reflection (ATR) infrared spectroscopy at two positions of the view cell showed that the sharp increase in activity is correlated to a transition from a biphasic to a monophasic reaction mixture. In the single phase region, both oxygen and benzyl alcohol are dissolved in the supercritical CO2 phase, which leads to a reduction of the mass transport resistances (both in the external fluid film and in the catalyst pores) and thus to the high reaction rate measured in the catalytic experiments. The phase transition could be effectively and easily monitored by transmission and ATR-IR spectroscopy despite the small concentration of the dense liquid like phase. Deposition of the Pd/Al2O3 catalyst on the ATR-crystal at the bottom of the view cell allowed to gain insight into the chemical changes and mass transfer processes occurring in the solid/liquid interface region during reaction. Analyzing the shift of the upsilon2 bending mode of CO2 gave information on the fluid composition in and outside the catalyst pores. Moreover, the catalytic reaction could be investigated in situ in this spectroscopic batch reactor cell by monitoring simultaneously the reaction progress, the phase behaviour and the catalytic interface.

No abstract