Friederike C. Jentoft

A new approach to well-defined, stable and site-isolated catalysts

Abstract A new concept to circumvent some of the problems that are hindering a rational metallic catalyst development is introduced. Investigation of conventional metal catalysts — which consist of supported metals, metal mixtures or alloys — is handicapped by the presence of a variety of active sites, their possible agglomeration, metal–support interactions as well as segregation of the components. In order to avoid most of the drawbacks, we employ well-defined, ordered and in-situ stable unsupported intermetallic compounds. Knowledge of the chemical bonding in the compounds and the defined neighbourhood of the active sites allows a rational approach to catalysts with excellent selectivity as well as long-term stability. The concept is demonstrated for the intermetallic compound PdGa, which is applied as catalyst for the selective hydrogenation of acetylene to ethylene.

Thermally and Chemically Induced Structural Transformations of Keggin-Type Heteropoly Acid Catalysts

Abstract Raman characterization revealed that the Keggin anion structure of H 4 PVMo 11 O 40 is inherently unstable upon heat treatment and loss of water. Vanadyl and molybdenyl species are expelled from the Keggin cage and defective Keggin structures are formed. These defective structures further disintegrate to presumably Mo 3 O 13 triads of the former Keggin. These Keggin fragments oligomerize at later stages to molybdenum oxygen clusters comparable to hepta- or octamolybdates. The final disintegration and structural reorganization product is MoO 3 . This disintegration and recondensation process seems to be strongly affected by the heating rate and hence the presence of water in the sample. Only partial expulsion of V occurred under moderate dehydration conditions. The absence of water during heat treatments stabilizes the intermediate defective structures. Raman spectroscopy proved that free polyacids are unstable under catalytic partial oxidation conditions. Therefore, it can be suggested that intact Keggin anions are not the active species within an operating partial oxidation catalyst. From this Raman spectroscopy study it may be inferred that the structurally reorganized intermediates are relevant for the catalytic action. The Raman investigations of the HPA decomposition additionally revealed a dependency of the decomposition process on the reactive atmosphere and the presence of Cs. The presence of Cs led to a partial stabilization of the structural disintegration products of PVMo 11 and to the formation of the thermodynamically stable, but catalytically inactive Cs 3 -salt. Cs also inhibited the condensation of MoO 3 -type oxides. O 2 present in the gas phase also led to stabilization of the structural reorganization intermediates. Importantly, the presence of water did not lead to a stabilization of the intact Keggin structure. In contrast, hydrolysis of the Keggin anions seemed to be enhanced compared to the water-free situation. This observation is of high importance because water is added to the feed in industrial partial oxidation reactions. Hence, under industrial conditions, HPA-derived catalysts are inherently unstable and cannot contain intact Keggin anions at their active surface. Catalytic partial oxidation conditions even led to a more pronounced structural reorganization and amorphous suboxides of the MoO 3− x type seemed to be formed. Hence, heteropolyacids have to be understood only as defined molecular precursor compound.

Solid-acid-catalyzed alkane cracking mechanisms: evidence from reactions of small probe molecules

This review is a summary of the mechanisms of catalytic cracking of small (C3-C6) alkanes. Most of the evidence has arisen from product distributions and kinetics of cracking of these alkanes, interpreted on the basis of solution carbocation chemistry and theoretical chemistry. Cracking of small alkanes catalyzed by solid acids such as the zeolite HZSM-5 proceeds by two mechanisms: (1) The unimolecular (protolytic cracking) mechanism, which proceeds via an alkanium ion formed by protonation of the alkane by the catalyst. This supposed transition state collapses to give either H2 and a carbenium ion or an alkane and a carbenium ion; the carbenium ions give up protons to the catalyst to form alkenes. The cracking products include methane and ethane as well as H2. (2) The classical (bimolecular) cracking mechanism, which involves carbenium ion chain carriers that react with the alkane reactant to abstract hydrides and generate carbenium ions that undergo β-scission. The products include alkanes and alkenes, but not methane, ethane, or H2. Because protolytic cracking gives alkene products, which are much stronger bases than alkanes, the alkenes become the predominant proton acceptors as conversions increase, and thus bimolecular cracking prevails at all but the lowest conversions. Protolytic cracking in the near absence of secondary reactions has been observed only for propane and n-butane at low conversions; secondary reactions appear to be generally significant for other alkanes. Although the product distributions are qualitatively understood, there are still inconsistencies in the literature of quantitative product distributions and kinetics, and more experimental work is needed with standard catalysts such as HZSM-5. Theoretical chemistry is leading to deeper understanding of the transition states, showing that cracking mechanisms involving bare carbocations are oversimplified. Rather, the catalyst surface must be included, and it has been simulated by clusters that are zeolite fragments. Surface alkoxides are more stable than surface carbenium ions, and cracking takes place by concerted bond breaking and formation. Theoretical activation energies for protolytic cracking of alkanes are close to experimental activation energies that have been corrected for the adsorption energy of the reactant, but it appears that more theoretical work (as well as better data) is required for satisfactory agreement of theory and experiment.

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.

Sulfated zirconia with ordered mesopores as an active catalyst for n-butane isomerization

Zirconia/surfactant composites were hydrothermally synthesized in aqueous sulfuric acid at 373 K using Zr(O-nPr)4 as oxide precursor and hexadecyl-trimethyl-ammonium bromide as template. Mesostructural features similar to those of MCM-41 were detected by X-ray diffractometry, with d=4.6 nm. A sample obtained from a starting mixture with Zr:S:CTAB = 2:2:1 was stable enough for removal of occluded organics. After calcination at 813 K, the d-value was 3.6 nm, the surface area 200 m2/g, and the mean pore diameter estimated by the BJH method 2.2 nm. Extended X-ray absorption fine structure analysis suggests Zr to be in a short-range structure (<4 Å) similar to that of Zr in monoclinic ZrO2. Scanning electron microscopy including energy dispersive X-ray analysis showed 1-5 μm sulfur-containing ZrO2 spheres. The material catalyzes the isomerization of n-butane to i-butane at 378 K with a steady activity in the order of magnitude of commercial sulfated ZrO2.

Structural and catalytic properties of sodium and cesium exchanged X and Y zeolites, and germanium-substituted X zeolite

Abstract The conversion of isopropanol in a fixed bed flow reactor was used as a test reaction for a number of faujasite-type zeolites which were modified in order to increase their basicity. Samples included a CsY zeolite with an intact faujasite structure and an exchange degree of nearly 100% prepared by solid-state ion exchange, a CsNaY obtained from CsY through exchange with aqueous NaCl solution, a CsNaX obtained from NaX and aqueous CsCl solution, and a Na(Ge)X, with Si replaced by Ge. At 623 K, an isopropanol partial pressure of 5 kPa in He, and a total feed flow of 90 ml/min, a catalyst mass of 50 mg, initial yields were as follows: NaY: 62% propene, CsNaY: 78% propene, NaCsX: 10% propene, 0.37% acetone, Na(Ge)X: 8% propene, 11% acetone. Conversion in the presence of CsY was 2 adsorption and infrared (IR) spectroscopy were used to probe basicity, and Na(Ge)X was the only sample to form monodentate carbonates upon CO 2 adsorption (bands at 1477 and 1428 cm −1 ). Further characterization with X-ray diffraction (XRD), transmission electron microscopy (TEM), thermal analysis, nitrogen sorption, and isopropanol sorption was necessary to properly interpret catalytic results by identifying samples which contained impurities, had blocked pore systems, or decomposed partially during activation or reaction.

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.

Deoxydehydration of Glycols Catalyzed by Carbon-Supported Perrhenate

Growing recognition that the Earth’s fossil-based, non-renewable resources are becoming depleted has sparked interest in the development of new chemical processes for the conversion of renewable biomass into chemicals and fuels. The high oxygen content of biomass feedstocks, such as carbohydrates and triglycerides, requires the development of selective oxygen-removal processes for the production of most chemicals and fuels. Traditionally, dehydration has been the primary research focus for the conversion of polyoxygenates. 2] Reductive processes have also been under investigation and the partial hydrodeoxygenation (hydrogenolysis) of sugars and polyols has been achieved with various selectivities and efficiencies by using both heterogeneous and homogeneous transitionmetal catalysts. Recently, a reaction that involves the reductive conversion of glycols into olefins, termed “deoxydehydration” (DODH), has received increasing attention (Scheme 1). The first catalytic

Gas phase contributions to the catalytic formation of HCN from CH4 and NH3 over Pt: An in situ study by molecular beam mass spectrometry with threshold ionization

Molecular beam mass spectrometry has been used for an in situ study of the Pt-catalyzed formation of hydrocyanic acid from methane and ammonia. The goal was to identify transient gas phase intermediates which would indicate homogeneous contributions to the reaction mechanism. A catalytic wall reactor operated at 1300 °C, 1013 mbar, and 74% HCN yield was connected via a molecular beam interface with a quadrupole mass spectrometer, which allowed the measurement of ionization- and appearance potentials by electron impact. Shape and width of the electron energy spread function were determined by analyzing the ionization efficiency curve of helium; the experimental uncertainty of the measured threshold values was found to be 0.6 eV. By use of the threshold ionization technique it could be shown that methylamine (CH3NH2) and methylenimine (CH2NH) are present in the gas phase under reaction conditions. The measured threshold potentials at m/z = 30 u (9.9 ± 0.6 eV) and m/z = 29 u (10.6 ± 0.6 eV) were unambiguously assigned to the appearance potential of CNH4+/CH3NH2 and the ionization potential of CNH3+/CH2NH, respectively. Both molecules dehydrogenate rapidly at reaction temperature to HCN so that they can be considered as true gas phase intermediates.