Rafat Ahmad

Reaction Pathways in n-Pentane Conversion Catalyzed by Tungstated Zirconia: Effects of Platinum in the Catalyst and Hydrogen in the Feed

Abstract The catalytic isomerization of n-pentane catalyzed by tungsted zirconia (WZ) was investigated to elucidate the effects of H2 in the feed and platinum in the catalyst. In the reaction catalyzed by WZ with or without platinum, when no H2 was present, a complex reaction network was observed, associated with organic deposits on the catalyst, yielding mainly cracked products. The alkane is inferred to be activated in a redox step forming W5+ on the catalyst and unsaturated intermediates that react to form polyalkenylic surface species, which were detected by in situ UV–visible spectroscopy. Promotion of WZ with platinum dramatically improved its catalytic activity and the isomerization selectivity in n-pentane conversion. The improvement was only marginal in the absence of H2, but it became substantial in the presence of H2, with the conversion increasing from 3 to 55% for the platinum-promoted catalyst, which underwent only little deactivation. The selectivity for isopentane was about 95% at 523 K. The results indicate that the complex reaction network operative with the WZ catalyst is suppressed on the platinum-containing catalysts in the presence of H2. A fast and selective monomolecular isomerization reaction takes over in this case. The adsorbed unsaturated C5 intermediates are rapidly desorbed via hydrogenation on the reduced tungstate surface. This rapid desorption minimizes the formation of higher-molecular-weight organic species such as polyalkenyls that are necessary for the complex reaction path observed with the unpromoted catalyst. The observed side products are interpreted not as cracking products accompanying the acid-catalyzed isomerization reaction but instead as hydrogenolysis products formed directly on the platinum particles.

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.

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.

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.

Improved experimental setup for in situ UV-vis-NIR spectroscopy under catalytic conditionsPresented at the International Congress on Operando Spectroscopy, Lunteren, The Netherlands, March 2?6, 2003.

A commercial UV-vis-NIR spectrometer (Lambda 9, PerkinElmer) equipped with an integrating sphere was supplemented with a construction to measure in situ diffuse reflectance spectra from room temperature to 723 K in the range from 250 to 2500 nm. The new construction was compared to a previously developed setup, which features a highly reflecting ceramic spacer that bridges the 12 mm between the integrating sphere and microreactor. In the new construction, the catalyst bed is in the isothermal zone of a tubular furnace and the 120 mm space between the optical quartz window of the microreactor and the integrating sphere are bridged with a specially formed light conductor made of quartz. Improvements include a stable position of the powder bed, a smaller bed volume (ca. 0.27 cm3), a reduction of thermal radiation, and a higher signal level and thus better signal-to-noise-ratio. The performance of the setup is demonstrated by two examples, (i) spectra recorded at 723 K during the activation of a zirconia sample, and (ii) spectra recorded during n-pentane isomerization over sulfated zirconia at room temperature with simultaneous gas phase analysis.