Jörg Melsheimer

Isomerization of n-Butane and of n-Pentane in the Presence of Sulfated Zirconia: Formation of Surface Deposits Investigated by In Situ UV-vis Diffuse Reflectance Spectroscopy

Catalytic performance and formation of carbonaceous deposits were studied simultaneously during alkane isomerization over sulfated zirconia in a fixed bed flow reactor with an optical window for in situ UV-vis diffuse reflectance spectroscopy. The reactions of n-butane (5 kPa) at 358 and 378 K and of n-pentane (0.25 kPa) at 298 and 308 K passed within 5 h or less through an induction period, a conversion maximum, and a period of deactivation; a steady activity of 41 and 47 µmol g -1 h -1 (isobutane formation) and ≈2.5 µmol g -1 h -1 (isopentane, both temperatures) remained. UV-vis spectra indicate the formation of unsaturated surface deposits; the band positions at 310 nm (n-butane reaction) and 330 nm (n- pentane) are within the range of monoenic allylic cations. More highly conjugated allylic cations (bands at 370 and 430 nm) became evident during n-butane reaction at 523 K. The chronology of events suggests that the surface deposits are (i) a result only of the bimolecular and not the monomolecular reaction mechanism, and (ii) are formed in a competitive reaction to the alkane products.

The structure of molybdenum-heteropoly acids under conditions of gas-phase selective oxidation catalysis: a multi-method in situ study

Abstract The present study focuses on the evidence about the existence of Keggin ions under various reactive conditions. The stability of the hydrated parent heteropoly acid (HPA) phases is probed in water, by thermal methods in the gas phase, by in situ X-ray diffraction and in situ EXAFS. An extensive analysis of the in situ optical spectra as UV-Vis-near-IR (NIR) in diffuse reflectance yields detailed information about the activated species that are clearly different from Keggin ions but are also clearly no fragments of binary oxides in crystalline or amorphous form. Infrared spectroscopy with CO as probe molecule is used to investigate active sites for their acidity. Besides OH groups evidence for electron-rich Lewis acid sites was found in activated HPA. All information fit into a picture of a metastable defective polyoxometallate anion that is oligomerised to prevent crystallisation of binary oxides as the true nature of the “active HPA” catalyst. The as-synthesized HPA crystal is thus a precatalyst and the precursor oxide mixture is the final deactivated state of the catalyst.

In situ UV‐VIS diffuse reflectance spectroscopy of reduction–reoxidation of heteropoly compounds by methanol and ethanol: a correlation between spectroscopic and catalytic data

The degree of reduction/oxidation of H4PVMo11O40 (HPA) and Cs2H2PVMo11O40 (CsPA) was studied during reduction and reoxidation by methanol, ethanol and mixtures with oxygen, respectively. The peak intensity of the intervalence charge transfer (IVCT) band, the apparent band gap energy (Eg*) and catalytic data were obtained by in situ UV‐VIS diffuse reflectance spectrosccopy (UV‐VIS‐DRS) and on‐line gas chromatography (GC), respectively. The peak intensity of the IVCT band and Eg* increase during the reduction of heteropoly compounds by the alcohols. The spectroscopic and catalytic data (conversion, selectivity) correlate in the transient state during the reoxidation process. It is shown that isolated Keggin anions act as precursors for the active states of the catalysts, which molecular structure cannot be deduced from UV‐VIS spectroscopy alone. UV‐VIS spectroscopy, however, can serve as a tool to determine the degree of reduction in future combined in situ UV‐VIS/Raman/XRD studied.

Optical bands of dodecanuclear compounds H4PVMo11O40·yH2O with Keggin structure. Semiclassical vibronic model

The problem of optical spectra of dodecanuclear clusters with the Keggin structure is considered. Charge-transfer and d–d-bands arising from intact and ill-defined species formed on different stages of disintegration of the heteropoly acid H4PVMo11O40·yH2O are investigated in detail. On the basis of the performed calculations the transformation of optical spectra of H4PVMo11O40·yH2O under increasing temperature and time on stream is explained.

Influence of dehydration effects on the optical spectra of H4PVMo11O40 in the visible and near infrared range: intra- and intercenter optical transitions in the V–Mo cluster

UV-Vis-NIR diffuse reflectance spectra of heteropoly acids of the type of H4PVMo11O40.xH2O were recorded with improved spectroscopic equipment for different times on helium stream at room temperature. A broad asymmetric absorption band with a maximum at 740-780 nm in the visible range and a shoulder at 900-1000 nm in the near infrared range is identified. With increasing time on stream the intensity of this band grows appreciably: simultaneously, the water peaks at 1430 and 1925 nm decrease. Focusing on a single mixed-valence V4+-Mo6+-cluster, a model is suggested for the description of the experimental results. The model takes into account electron transfer, low symmetry crystal fields and vibronic interaction. The absorption band shape calculation is performed within the semiclassical limits. The band shape is shown to be determined both by the optical d-d transitions in V4+ and Mo5- and the intervalence V4+-Mo6+ optical transition. Appreciable changes in the band intensity are attributed to the increase in the electron transfer parameter due to water loss. Quite good agreement with the experiment was found for a reasonable set of transfer and vibronic parameters.

In situ UV/Vis/near-IR diffuse reflection measurement of catalysts at temperatures up to 673 K

In situ UV/VIS/near-IR diffuse reflectance spectroscopy can be carried out on catalysts with a suitable microreactor at temperatures up to 673 K. The experimental difficulties in the application of such a reactor at higher temperatures could be solved as follows. Integrating sphere and reactor are connected by a ceramic part of high reflectivity and low thermal conductivity, i.e., the integrating sphere is enlarged. The signal/noise ratio increases by a factor of 2. The standard IR detector was replaced by a temperature-stabilized version. In addition watercooling jackets were mounted at the integrating sphere. The signal/noise ratio in the near-IR region was thus improved by a factor of 2–3 with a very good reproducibility of measurements. To overcome an alteration of catalyst spectra by thermal radiation, a series of reference spectra of quartz powder (SiO2) were recorded at several temperatures. These spectra are used to correct the catalyst spectra by dividing catalyst and SiO2 spectra.

UV/Vis/near-IR spectroscopic characteristics of H4−xCsxPVMo11O40(x=0, 2) catalyst under different temperatures and gas atmospheres

In order to understand the transformations of the Keggin-type H4−xCsxPV–Mo11O40 (x=0, 2) compounds with rising temperature over a long time in a stream of different gas atmospheres, in situ UV/Vis/near-IR spectroscopic studies were carried out. Diffuse reflectance spectra were recorded using an improved spectrometer and a suitable microreactor. Visible and near-IR peak intensities, peak positions and the band gap energies were determined from apparent absorption spectra. Propene, isopropanol, water and the oxidation products were analyzed by GC. The experimentally observed blue shift of the visible absorption band in the region of crystal water loss and the increase in the near-IR absorption were explained on the basis of quantum-mechanical calculations of the shapes and positions of the charge transfer and d–d bands arising from Mo5+–Mo6+ and V4+–Mo6+ pairs in intact and ill-defined fragments of the Keggin structure. It was concluded that with removal of crystal water during the action of He, He/H2O, propene, O2/propene and increasing temperature, reduced species with protons located at the bridging oxygens promote a blue shift of the visible band, while a large number of ill-defined species form the near-IR part of the spectra in the temperature range 326–600 K.