Bertram Kimmerle

Aerobic Epoxidation of Olefins Catalyzed by the Cobalt‐Based Metal–Organic Framework STA‐12(Co)

A Co-based metal-organic framework (MOF) was investigated as a catalytic material in the aerobic epoxidation of olefins in DMF and exhibited, based on catalyst mass, a remarkably high catalytic activity compared with the Co-doped zeolite catalysts that are typically used in this reaction. The structure of STA-12(Co) is similar to that of STA-12(Ni), as shown by XRD Rietveld refinement and is stable up to 270 °C. For the epoxidation reaction, significantly different selectivities were obtained depending on the substrate. Although styrene was epoxidized with low selectivity due to oligomerization, (E)-stilbene was converted with high selectivities between 80 and 90 %. Leaching of Co was low and the reaction was found to proceed mainly heterogeneously. The catalyst was reusable with only a small loss of activity. The catalytic epoxidation of stilbene with the MOF featured an induction period, which was, interestingly, considerably reduced by styrene/stilbene co-epoxidation. This could be traced back to the formation of benzaldehyde promoting the reaction. Detailed parameter and catalytic studies, including in situ EPR and EXAFS spectroscopy, were performed to obtain an initial insight into the reaction mechanism.

Oscillatory behaviour of catalytic properties, structure and temperature during the catalytic partial oxidation of methane on Pd/Al2O3

Pd/Al(2)O(3) catalysts showed an oscillatory behaviour during the catalytic partial oxidation (CPO) of methane, which was investigated simultaneously by IR-thermography, X-ray absorption spectroscopy, and online mass-spectrometry to correlate the temperature, state of the catalyst and catalytic performance. The following stages were observed: (i) build-up of a temperature maximum in the first half of the catalyst bed, (ii) reduction of palladium in the end zone of the catalyst bed with a front moving toward the entrance zone, (iii) strong hot spot formation accompanied by reduction of palladium due to self-reduction leading to extinction of the process. The latter was the key driver for the oscillations and thus gave additional insight into the mechanism of partial methane oxidation.

Parallel structural screening of solid materials

X-Ray absorption spectroscopy using a microreactor array in combination with an X-ray camera is applied for fast parallel structural screening of a variety of differently prepared supported palladium and copper particles in the as-prepared state and after heat pre-treatments in different gas atmospheres.

Reduction and re-oxidation of Cu/Al2O3 catalysts investigated with quick-scanning XANES and EXAFS

In the present study the structure of copper catalysts on alumina support were investigated in situ and time resolved during reduction and re-oxidation at different temperatures with the quick-scanning EXAFS (QEXAFS) technique. Different impregnation times (2 min and 90 min) were chosen for the preparation which resulted in different copper species that show a strong variation in the reduction/re-oxidation behaviour. These dynamic changes as well as possible intermediate phases during the gas atmospheres changes were followed with up to 20 EXAFS spectra per second at the copper K-edge covering an energy range of 450 eV. The high time resolution provided new insights into the dynamics of the catalysts e.g. revealing Cu(I) as intermediate state during re-oxidation. Latest advances in the data acquisition hardware are leading to an improved data quality of spectra collected at the SuperXAS beamline. Thus, not only accurate analysis of the catalysts via XANES but also by EXAFS was possible. This is also due to the recent upgrade to monitor the Bragg angle directly with an encoder during the experiments.

Investigation of the ignition behaviour of the noble metal catalyzed catalytic partial oxidation of methane

Catalytic partial oxidation (CPO) of methane to hydrogen and carbon monoxide over Pt-Rh/Al2O3 and Pt/Al2O3 was studied in-situ with a new QEXAFS setup. The structural changes of the catalysts were investigated on the subsecond timescale during two reaction steps by recording both XANES and full EXAFS spectra: (1) heating and ignition in 6%CH4/3%O2/He, (2) periodic changes between the reaction gas mixture and H2 atmosphere. The results showed that the ignition occurred at lower temperatures for Pt-Rh/Al2O3 while it was completed in a significant shorter time interval for Pt/Al2O3. Some structural changes during the heating phase were detectable before the reaction ignited, especially for Pt/Al2O3, as reflected by the performed principle component analysis. However, a closer analysis of the FT-QEXAFS data did not evidence a defined intermediate. In addition, the composition of the gas atmosphere was altered between hydrogen and the reaction mixture, enabling modulation excitation spectroscopy. This technique was for the first time applied to QEXAFS data and resulted in significantly enhanced data quality.