Andreas Jentys

Publications of Andreas Jentys

• Co and Mn polysiloxanes as unique initiator-catalyst-systems for the selective liquid phase oxidation of o-xylene.

  Authors: Tobias Förster, Stephan A Schunk, Andreas Jentys, Johannes A Lercher

  Chemical communications (Cambridge, England). 01/2011; 47(11):3254-6.

  Co and Mn polysiloxanes are unique catalyst/initiator systems for the liquid phase oxidation of o-xylene showing higher activity than Co naphthenates, which is related to the weak interaction of theCo and Mn polysiloxanes are unique catalyst/initiator systems for the liquid phase oxidation of o-xylene showing higher activity than Co naphthenates, which is related to the weak interaction of the polar products with the hydrophobic surface as well as the absence of hydroxyl groups and surrounding oxygenates limiting radical quenching.

• Enhancement of Sorption Processes in the Zeolite H-ZSM5 by Postsynthetic Surface Modification.

  Authors: Stephen J Reitmeier, Oliver C Gobin, Andreas Jentys, Johannes A Lercher

  Angewandte Chemie (International ed. in English). 01/2009;

• SOx storage materials under Lean-Rich cycling conditions--Part II: influence of Pt, H2O, and cycling time.

  Authors: Hendrik Dathe, Peter Haider, Andreas Jentys, Johannes A Lercher

  The journal of physical chemistry. B. 01/2007; 110(51):26024-32.

  The role of Pt and the influence of the reaction conditions during lean-rich cycling experiments were studied on a second generation SOx trapping material. The combination of the Generalized 2-DThe role of Pt and the influence of the reaction conditions during lean-rich cycling experiments were studied on a second generation SOx trapping material. The combination of the Generalized 2-D Correlation Analysis, 2-D Sample-Sample Correlation Analysis, and Factor Analysis using the MCR-ALS technique was applied to identify the reactive species. Transient surface sulfate species were formed under oxidative reaction conditions (lean mode) and decomposed under reducing reaction conditions (rich operation mode). The reduction of this species was identified to be the main contribution to the SO2 release observed under dynamic flow conditions. Pt facilitates the formation of sulfates but also catalyzes the reduction of the transient surface sulfate species leading to a higher amount of SO2 released under rich conditions. In the presence of water, this effect was diminished, which was found to be mainly a result of the suppressed formation of surface sulfate species caused by the faster transport of SO2 into the bulk phase of the SOx trapping component (BaCO3). Increasing the time under reducing conditions in the cycles leads to an enhanced reduction of the surface during rich conditions. The presence of water did not influence the bulk type species. It is proposed that for effective SO2 storage materials, strong SOx adsorption sites on the surface, the presence of water, and a short time under reducing conditions are essential.

• SO(x) storage materials under lean-rich cycling conditions. Part I: Identification of transient species.

  Authors: Hendrik Dathe, Peter Haider, Andreas Jentys, Johannes A Lercher

  The journal of physical chemistry. B. 07/2006; 110(22):10729-37.

  The SO(x) uptake of second generation sulfur trapping materials was studied by in situ IR spectroscopy under lean-rich cycling conditions. The combination of advanced chemometric methods includingThe SO(x) uptake of second generation sulfur trapping materials was studied by in situ IR spectroscopy under lean-rich cycling conditions. The combination of advanced chemometric methods including generalized 2D correlation analysis, 2D sample-sample correlation analysis, and multivariate curve resolution with alternating least squares allowed the detection of the species involved in the storage process. The formation of the bulk sulfate species was always accompanied by the consumption of carbonates. The reduction of a transient surface sulfate species was identified as the key parameter in the storage process under dynamic conditions. Three distinct reaction regimes were differentiated on the industrial materials under SO(x) trapping conditions being imperceptible from conventional spectra.

• On the trapping of SOx on CaO-Al2O3-based novel high capacity sorbents.

  Authors: Hendrik Dathe, Andreas Jentys, Peter Haider, Ellen Schreier, Rolf Fricke, Johannes A Lercher

  Physical chemistry chemical physics : PCCP. 05/2006; 8(13):1601-13.

  Calcium-aluminum mixed oxide based materials doped with Na and Mn were explored as sulfur trapping materials. The materials showed a three times higher total storage capacity and a higher time onCalcium-aluminum mixed oxide based materials doped with Na and Mn were explored as sulfur trapping materials. The materials showed a three times higher total storage capacity and a higher time on stream with complete SO2 removal compared to a second generation SOx trapping material which was mesoporous with calcium mainly present in oxidic form. Combining in situ XANES at the S K-edge and IR spectroscopy the key properties of the storage materials and the affiliated storage processes were identified. CaO-Al2O3 acts as the primary support and storage component, while Na+ cations adjust the base strength and enhances the storage capacity. Manganese cations provide the appropriate oxidation capacity in absence and presence of up to 10% water. The transport into the bulk phase, which is markedly influenced by a layer of sorbed water, is the rate-limiting step in presence of Mn cations. In the absence of manganese cations the oxidation step appears controlling the rate. The overall reaction network, identified by in situ IR spectroscopy and the 2D Correlation Analysis, is similar on all materials.

• Spectroscopic characterization of cobalt-containing mesoporous materials.

  Authors: Torbjørn Vrålstad, Wilhelm R Glomm, Magnus Rønning, Hendrik Dathe, Andreas Jentys, Johannes A Lercher, Gisle Oye, Michael Stöcker, Johan Sjöblom

  The journal of physical chemistry. B. 04/2006; 110(11):5386-94.

  Cobalt-containing mesoporous materials that have been prepared using different procedures have been comparatively characterized by transmission electron microscopy/energy-dispersive X-rayCobalt-containing mesoporous materials that have been prepared using different procedures have been comparatively characterized by transmission electron microscopy/energy-dispersive X-ray spectroscopy (TEM/EDS), extended X-ray absorption fine structure spectroscopy (EXAFS), X-ray absorption near edge spectroscopy (XANES), and ultraviolet-visible (UV-vis), near-infrared (NIR), and mid-infrared (mid-IR) spectroscopies, and the results provide new insights into the local environment and properties of cobalt in this type of material. TEM/EDS analyses have shown that tetraethyl orthosilicate (TEOS) may be less appropriate as a silicon source during the syntheses of cobalt-containing mesoporous materials, because the distribution of cobalt throughout the framework may become uneven. EXAFS has been determined to be the most suitable method for direct verification of framework incorporation, by identifying silicon as the backscatterer in the second shell. Such a direct verification may not be obtained using UV-vis spectroscopy. From EXAFS analyses, it is also possible to distinguish between surface-bound and framework-incorporated cobalt. There is a good agreement between the results obtained from XANES and UV-vis regarding the coordination symmetry of cobalt in the samples. The presence of cobalt in the silica framework has been determined to create Lewis acid sites, and these acid sites are suggested to be located at tetrahedral cobalt sites at the surface.

• In situ S K-edge X-ray absorption spectroscopy for understanding and developing SOx storage catalysts.

  Authors: Hendrik Dathe, Andreas Jentys, Johannes A Lercher

  The journal of physical chemistry. B. 12/2005; 109(46):21842-6.

  In situ S K-edge XANES experiments were carried out on second-generation SO(x)() trapping materials under oxidizing and reducing conditions. The experiments clearly show that the strong release ofIn situ S K-edge XANES experiments were carried out on second-generation SO(x)() trapping materials under oxidizing and reducing conditions. The experiments clearly show that the strong release of SO(2) under rich conditions at plug flow conditions is caused by the facilitated reduction of sulfite species on Pt. In the absence of Pt the sulfite species were stable under reducing conditions, while maintaining a similar total SO(2) uptake capacity. Thus, SO(x)() trapping materials without a noble metal are a clearly better option. The enhancing effect on the SO(x)() storage process of water present in the gas mixture is attributed to the formation of a higher sulfate fraction in the samples. The application of the in situ S K-edge XANES technique clearly reveals new information and insights on the behavior of the sulfur in the trapping process compared to that from the ex situ measurements and is therefore essential for designing new SO(x)() trapping materials.

• Sulfate formation on SOx trapping materials studied by Cu and S K-edge XAFS.

  Authors: Hendrik Dathe, Andreas Jentys, Johannes A Lercher

  Physical chemistry chemical physics : PCCP. 03/2005; 7(6):1283-92.

  The elementary steps during oxidative chemisorption of SO2 by a novel composite material consisting of highly disordered benzene tri-carboxylate metal organic framework materials with Cu as centralThe elementary steps during oxidative chemisorption of SO2 by a novel composite material consisting of highly disordered benzene tri-carboxylate metal organic framework materials with Cu as central cation and BaCl2 as a second component (Ba/Cu-BTC) and by a conventional BaCO3/Al2O3/Pt based material were investigated. EXAFS analysis on the Cu K-edge in Ba/Cu-BTC indicates the opening of the majority of the Cu-Cu pairs present in the parent Cu-BTC. Compared to Cu-BTC, the BaCl2 loaded material has hardly any micropores and has higher disorder, but it has better accessibility of the Cu2+ cations. This results from the partial destruction of the MOF structure by reaction between BaCl2 and the Cu cations. The SO2 uptake in oxidative atmosphere was higher for the Ba/Cu-BTC sample than for the BaCO3/Al2O3/Pt based material. XRD showed that on Ba/Cu-BTC the formation of BaSO4 and CuSO4 occurs in parallel to the destruction of the crystalline structure. With BaCO3/Al2O3/Pt the disappearance of carbonates was accompanied with the formation of Ba- and Al-sulfates. XANES at the S K-edge was used to determine the oxidation states of sulfur and to differentiate between the sulfate species formed. At low temperatures (473 K) BaSO4 was formed preferentially (53 mol% BaSO4, 47 mol% CuSO4), while at higher temperatures (and higher sulfate loading) CuSO4 was the most abundant species (42 mol% BaSO4, 58 mol% CuSO4). In contrast, on the BaCO3/Al2O3/Pt based material the relative concentration of the sulfate species (i.e., BaSO4 and Al2(SO4)3) as function of the temperature remained constant.

• Identifizierung von Oberflächenzentren an Pt-Mordenitkatalysatoren für die Hydroisomerisierung mittels Sondenmoleküladsorption

  Authors: Jutta Kröhnert, Sabine Wrabetz, Friederike C. Jentoft, Robert Schlögl, Stuart Thomson, Pierre-Alain Weiss, Wilhelm F. Maier, Christian Woltz, Andreas Jentys, Johannes A Lercher

  Einleitung Pt-promotierter Mordenit ist ein Katalysator für die Hydroisomerisierung von n Pentan. Wir haben Oberflächenzentren verschieden aktiver sowie Pt-freier Proben durch Adsorption von n-ButanEinleitung Pt-promotierter Mordenit ist ein Katalysator für die Hydroisomerisierung von n Pentan. Wir haben Oberflächenzentren verschieden aktiver sowie Pt-freier Proben durch Adsorption von n-Butan (Modellalkan) und CO charakterisiert und Daten aus IR-Spektroskopie, Mikrokalorimetrie und katalytischen Messungen korreliert. Experimente Pt-promotierter Mordenit wurde durch Imprägnierung von NH4-Mordenit (aus Na-Mordenit durch Ionenaustausch hergestellt) mit Hexachloroplatinat und Calcinierung bei 500 oder 800°C erhalten ("Pt-HM 500", "Pt-HM 800"). Eine Pt-freie Vergleichs-probe ("HM 500") wurde durch Calcinierung von NH4-Mordenit bei 500°C produziert. IR-Experimente wurden an freitragenden Presslingen in Transmission durchgeführt. Die Proben wurden entweder im Vakuum bei 250°C bzw. 450°C aktiviert oder bei 375°C im H2-Strom. Spektren wurden bei 298 K und stufenweiser Erhöhung des Drucks der Sonde (CO 4.7 bzw. n-Butan 2.5, Messer Griesheim) aufgenommen. Für die Adsorptionskalorimetrie wurde ein MS70 Calvet Kalorimeter (SETARAM) eingesetzt, das mit einem selbstgebauten Dosiersystem verbunden war. Die Proben wurden bei 450°C im Vakuum aktiviert, n-Butan wurde bei 40°C adsorbiert. Ergebnisse und Diskussion Aus Pulverröntgenbeugung und Elektronenmikroskopie ergab sich, dass die Proben Mordenit sowie eine grobdisperse Pt-Spezies enthalten. Im kombinatorischen (Kleinstmengen) und im parallelisierten (Grammmassstab) Testverfahren zeigte sich Pt-HM 500 deutlich aktiver für die n-Pentanisomerisierung als Pt-HM 800. Anhand von Thermoanalysedaten und IR-Spektren wurde ersichtlich, dass die verbrückten OH-Gruppen von Pt-HM 800 teilweise zerstört waren. Während bei Pt-HM 500 die OH-Schwingungen durch n-Butan Adsorption um ca. 100 cm-1 verschoben wurden (Abb. 1, links), war für Pt-HM 800 keine ausgeprägte Verschiebung zu erkennen. Entsprechend betrug die typische Adsorptionswärme von n-Butan an Pt-HM 500 54 kJ mol-1, an Pt-HM 800 hingegen nur 45 kJ mol-1. Adsorption von CO auf den Proben resultierte in Summe in mindestens acht verschiedenen Banden, die der Adsorption von CO an Lewis- (Al3+) und an Pt-Zentren zugeordnet werden konnten. Vorbehandlungstemperatur und -atmosphäre wirkten sich auf die OH-Gruppendichte und die Oxidationsstufe des Platins aus. Mit zunehmender Temperatur nahm die Anzahl der OH-Gruppen ab. Nach Aktivierung bei 450°C im Vakuum war offensichtlich noch oxidiertes Pt vorhanden (CO Banden oberhalb von 2100 cm-1). Die sehr hohen Anfangswärmen (Abb. 1) könnten auf eine exotherme Reaktion von n-Butan mit oxidiertem Pt hindeuten. Die zugehörigen Wärmesignale waren, wie es für eine Folgereaktion nach der Adsorption typisch ist, verbreitert. Nach Aktivierung in H2 wurde durch CO-Adsorption metallisches Pt detektiert, und die differentiellen Adsorptionswärmen entsprachen etwa denen des Pt-freien HM 500. Schlussfolgerungen Unterschiede in der Isomerisierungsaktivität von Pt-promotiertem Mordenit ließen sich auf die strukturelle Integrität und die Dichte von OH-Gruppen sowie auf die Eigenschaften der exponierten Pt-Oberfläche zurückführen. Abb. 1: n-Butan-Adsorption an Pt-Mordenit: IR-Spektren bei RT, Verschiebung der OH-Gruppen; Einfluß der Vorbehandlung auf die differentiellen Adsorptionswärmen bei 40°C. Das dieser Kurzzusammenfassung zugrundeliegende Vorhaben wurde mit Mitteln des Bundesministeriums für Bildung und Forschung unter dem Förderkennzeichen 03C0307 gefördert. Die Verantwortung für den Inhalt liegt bei den Autoren.

• Experimental and theoretical investigation of the sticking probability of aromatics on HZSM-5 and SiO2

  Authors: Stephan J. Reitmeier, Rino R. Mukti, Andreas Jentys, Johannes A. Lercher

  Studies in Surface Science and Catalysis.

  The elementary steps of sorption and transport of benzene, toluene and p-/o-xylene on MFI-type zeolites were studied by fast time-resolved rapid scan IR spectroscopy. The transport into the zeoliteThe elementary steps of sorption and transport of benzene, toluene and p-/o-xylene on MFI-type zeolites were studied by fast time-resolved rapid scan IR spectroscopy. The transport into the zeolite pore proceeds via a weakly bound physisorbed state on the external surface. Due to the weak interaction, very low sticking probabilities in the order of 10-7 for porous and non-porous materials were observed. The sticking coefficients increased in the series toluene, o-xylene, benzene, and p-xylene, attributed using to the statistical thermodynamic analysis, to the symmetry of the sorbate molecule, the sorption enthalpy of the sorbate which increases with molar mass and the space the sorbate molecule occupied on the surface.

• Improving bifunctional zeolite catalysts for alkane hydroisomerization via gas phase sulfation

  Authors: Christian Woltz, Andreas Jentys, Johannes A. Lercher

  Journal of Catalysis.

  Pt-containing H-BEA zeolites were functionalized with sulfate anions via H2S chemisorption followed by oxidation. This treatment generates sulfate groups and Brønsted acidic sites, modifies thePt-containing H-BEA zeolites were functionalized with sulfate anions via H2S chemisorption followed by oxidation. This treatment generates sulfate groups and Brønsted acidic sites, modifies the supported metal particles, and leads to higher activity and selectivity for pentane hydroisomerization. The strength and the accessibility of the acid sites present in the parent material were not affected by this procedure. The concentration of Brønsted acidic sites and the catalytic activity for light alkane isomerization varied sympathetically. The parallel increase in isomerization selectivity indicates that either the residence time of the alkoxy intermediates decreases for the modified samples (suppressing undesired cracking reactions on the acid sites) or the selective decoration of the Pt particles with sulfur reduces hydrogenolysis on the metal particles.

• On the enhanced para-selectivity of HZSM-5 modified by antimony oxide

  Authors: Shourong Zheng, Andreas Jentys, Johannes A. Lercher

  Journal of Catalysis.

  HZSM-5 was modified with antimony oxide by solid-state reaction at 773 K to enhance diffusional constraints. Crystalline phases of antimony oxide were not detected at a Sb2O3 loading up to 10.2 wt%.HZSM-5 was modified with antimony oxide by solid-state reaction at 773 K to enhance diffusional constraints. Crystalline phases of antimony oxide were not detected at a Sb2O3 loading up to 10.2 wt%. Antimony oxide was found to strongly interact with the hydroxyl groups of the zeolite, i.e., silanol and bridging hydroxyl groups, and with extra-framework aluminum oxide. Only a small amount of Sb2O3 penetrated the pores of the zeolite, while the main fraction of Sb2O3 was deposited on the external surface of the zeolite crystals. With increasing antimony oxide loading a pronounced pore narrowing of the zeolite was observed, which suppressed the further penetration of antimony oxide into the pores of the zeolite. The dispersion of antimony oxide on the surface of the zeolite led to complete removal of unselective Brønsted acid sites and to a narrowing/blocking of the pore mouth openings of the zeolite. Both effects induced enhanced para-selectivity for toluene disproportionation.

• Hydroxyl groups in phosphorus-modified HZSM-5

  Authors: Andreas Jentys, Gerd Rumplmayr, Johannes A. Lercher

  Applied Catalysis.

  The effects of post-synthesis modification on the hydroxyl groups of HZSM-5 were investigated by IR spectroscopy and temperature-programmed desorption. Three model compounds [phosphoric acid,The effects of post-synthesis modification on the hydroxyl groups of HZSM-5 were investigated by IR spectroscopy and temperature-programmed desorption. Three model compounds [phosphoric acid, trimethylphosphine and trimethyl phosphite] were used for modification. Six different hydroxyl groups were observed with the samples investigated. They were assigned to silanol groups terminating the crystals (3745cm−1), SiOH in the interior (3725 cm−1), AlOH and POH groups (3683 and 3668 cm−1), bridged hydroxyls (3610 cm−1) and hydrogen-bonded internal SiOH groups (3500 cm−1). Phosphorus treatment was found to decrease the concentration of the strong Brønsted acids sites (bridging hydroxyl groups) by dealumination (H3PO4 and trimethyl phosphite treatment) or by chemisorption of the phosphorus compound (trimethylphosphine treatment). After H3PO4 and trimethyl phosphite treatment, the formal replacement of strong Brønsted acidic hydroxyls by H2PO4− groups occurs to some extent. The concentration of weak acid sites was found to increase after H3PO4 and trimethyl phosphite treatment and did not vary after trimethylphosphine treatment. Strong Brønsted acid sites are bridging hydroxyl groups, weak Brønsted acid sites, hydrogen-bonded SiOH groups at defect sites and, to some extent, POH groups.

• The Energetic and Entropic Contributions Controlling the Orientation of Alkyl Substituted Aromatic Molecules in the Pores of MFI Zeolites

  Authors: Andreas Jentys, Rino R. Mukti, Johannes A. Lercher

  Studies in Surface Science and Catalysis.

  The energetic and entropic contributions controlling the orientation of benzene, toluene and pxylene on acidic (H/ZSM-5) and non acidic (Silicalite-1) MFI type materials were studied usingThe energetic and entropic contributions controlling the orientation of benzene, toluene and pxylene on acidic (H/ZSM-5) and non acidic (Silicalite-1) MFI type materials were studied using calorimetry to determine the integral uptake and IR spectroscopy for monitoring the individual coverage of the SiOH and SiOHA1 groups. The Langmuir model, used to describe the sorption isotherms, indicates the presence of sterically constrained sorption structures, which primarily differ in the decrease of entropy. Energetically the sorption is controlled by the interaction between the molecules and the pore walls, while the localized interaction with the bridging hydroxy groups is only a minor energetic contribution. However, if the molecules are located close to SiOHA1 groups perturbed hydroxy groups are formed, their frequency reflects the local constraint of the molecules at the sorption sites.

• Xylene isomerization with surface-modified HZSM-5 zeolite catalysts: An in situ IR study

  Authors: Shourong Zheng, Andreas Jentys, Johannes A. Lercher

  Journal of Catalysis.

  Xylene isomerization over parent and tetra-ethoxysilane-modified HZSM-5 was studied using time-resolved in situ IR spectroscopy to monitor the concentrations of reactants and products inside theXylene isomerization over parent and tetra-ethoxysilane-modified HZSM-5 was studied using time-resolved in situ IR spectroscopy to monitor the concentrations of reactants and products inside the pores. Surface silylation formed patches sealing pore mouths, enhancing the differences in diffusivity between p-xylene and o- or m-xylene via increased tortuosity of the transport pathway. The reaction rate of m-xylene isomerization was controlled by reactant diffusion; the selectivity, by restrictions in the transition state. For p- and o-xylene isomerization over the silylated zeolite, the greater local concentration of the reaction intermediate m-xylene compared with that of the reactant molecules in the pores indicated that the bulkiest isomer (i.e., m-xylene) was selectively retained in the pores. Its higher effective residence time in the pores allowed conversion to p- and o-xylene. Because p- and o-xylene were formed from m-xylene in a ratio of approximately 2, this led to enhanced p-xylene selectivity. Xylene isomerization catalyzed by Brønsted acid sites in the pore mouth of the zeolite was largely suppressed after silylation.

• Diffusion in Circularly Ordered Mesoporous Silica Fibers

  Authors: Hatem Alsyouri, Oliver C. Gobin, Andreas Jentys, Johannes A. Lercher

  JOURNAL OF PHYSICAL CHEMISTRY C. 115(17):8602-8612.

  Transport processes of n-hexane and mesitylene in circularly ordered mesoporous silica fibers have been investigated. Combining theoretical calculations based on the complex hierarchical structureTransport processes of n-hexane and mesitylene in circularly ordered mesoporous silica fibers have been investigated. Combining theoretical calculations based on the complex hierarchical structure with experimental results from frequency response experiments, three well-defined pathways for the transport pathways of molecules in the ordered mesoporous silica fibers are identified, that is, diffusion in axial, tangential, and radial directions, all occurring in parallel. The radial diffusion is the dominant process and is governed by intrawall transport. Transport in the axial direction overlaps with the radial diffusion and can be governed by either intrawall or mesopore transport. Cracks or defects at the surface enable diffusion in the tangential direction, which is the third kinetic process experimentally observed.