Synthesis and characterization of the titanium doped nanostructural V2O5

Using scanning and analytical transmission electron microscopies (TEM), the morphology and structure of nanostructurally assembled V2O5 doped with Ti has been studied. It was found that the bulk structure of the oxide particles crystallized in rod-like shape is of the V2O5 type whereas Ti atoms are located mainly on the thin surface layer of the rods. Such surface coating is nonuniform and contains up to 3 at.% of titanium. Modification of the oxide sample with titanium atoms seems to stabilize the V2O5 structure against electron beam irradiation

Oxidative dehydrogenation of 1-butene to 1,3-butadiene over a multicomponent bismuth molybdate catalyst: influence of c3–c4 hydrocarbons

The influence of light hydrocarbons, such as n-butane, isobutane, propylene, cis- and trans-2-butenes, and isobutene on the oxidative dehydrogenation of 1-butene to 1,3-butadiene over BiMoKNiCoFePOx/SiO2 catalyst has been studied using a gas flow reactor. The inhibition effect of the listed hydrocarbons on the target reaction increased in the order of n-butane ~ isobutane < propylene < 2-butenes < isobutene. In addition, in contrast to 1-butene, isobutene has shown significant contribution to coke formation. It was suggested, that the coke formation and therefore the rate of the catalyst regeneration exercise a significant influence on the efficiency of 1-butene transformation into 1,3-butadiene in the concurrent presence of other hydrocarbons

Oxidative dehydrogenation of 1-butene to 1,3-butadiene over a multicomponent bismuth molybdate catalyst: influence of c3–c4 hydrocarbons

The influence of light hydrocarbons, such as n-butane, isobutane, propylene, cis- and trans-2-butenes, and isobutene on the oxidative dehydrogenation of 1-butene to 1,3-butadiene over BiMoKNiCoFePOx/SiO2 catalyst has been studied using a gas flow reactor. The inhibition effect of the listed hydrocarbons on the target reaction increased in the order of n-butane ~ isobutane < propylene < 2-butenes < isobutene. In addition, in contrast to 1-butene, isobutene has shown significant contribution to coke formation. It was suggested, that the coke formation and therefore the rate of the catalyst regeneration exercise a significant influence on the efficiency of 1-butene transformation into 1,3-butadiene in the concurrent presence of other hydrocarbons

Preparation of a single phase (MoVW)5O14-mixed oxide catalyst

Motivation Mixed oxide catalysts containing molybdenum, vanadium and tungsten are widely used in industry for partial oxidation reactions [1, 2, 3]. Previous work revealed (MoVW)5O14 to be the active phase of the catalyst. The partial oxidation from acrolein to acrylic acid is performed on such a system [4, 5]. This catalytic system is characterised by high long term stability, excellent turn-over rates and high selectivity. Different preparation steps are necessary to form a single phase, crystalline, ternary oxide (MoVW)5O14 as a model catalyst. In a previous paper [6] it was suggested that a precursor of this oxide is already formed in solution. Therefore this poster is dedicated to propose a structure of the dissolved species and a reaction mechanism leading to the formation of this structure in solution. Experimental For the synthesis of this oxide, solutions of ammonium heptamolybdate, ammonium metatungstate, and vanadyl oxalate were spray-dried followed by different thermal treatments. The structures of the materials formed in solution were studied using UV/Vis Raman and ESR spectroscopy. Results It is suggested from this data that a molecular structure is already formed in solution which seems to be closely related to that of the final crystalline Mo5O14-type oxide. Raman spectroscopy shows bands at 964, 943, 912, 821, 792, 709 and 682 cm-1. The bands at 943 and 792 cm-1 could be assigned to AHM. Bands at 964, 879, 821, 709, und 682 cm-1 do not belong to AHM and point to a polymeric species This result could be corroborated by UV/Vis and ESR spectroscopy. Moreover ESR shows that the state of oxidation of molybdenum and tungsten is +6. The state of oxidation of vanadium is +4. Vanadium exists as vanadyl type. The spray-dried sample shows bands at 943, 872, and 818 cm-1. A higher degree of polymerisation in the dried sample could be responsible for the shift of the band at 872 cm-1 compared to the spectrum in solution 879 cm-1. Literature [1] Hibst, H.,Unverricht, S. (BASF), DE 19815281 A 1. [2] Tanimoto, M., Himeji-shi, H., Mihara, I., Aboshi-ku, H., H., Kawajiri, T., Himeji-shi, H., (Nippon Shokubai), EP 0 711 745 B1. [3] Tenten, A., Hibst, H., Martin, F-G., Marosi, L., Kohl, V., (BASF), DE 4405514 A1. [4] Mestl, G., Linsmeier, C., Gottschall, R., Dieterle, M., Find, J., Herein, D., Jäger, J., Uchida, Y., Schlögl, R., J. Mol. Catal. A 162 (2000) 455-484. [5] Dieterle, M., Mestl, G., Jäger, J., Hibst, H., Schlögl, R., J. Mol. Catal. A 174 (2001) 169-185. [6] Knobl, S., Zenkovets, G. A., Kryukova, G. N., Ovsitser, O., Dieterle, M., Mestl, G. , Schlögl, R., J. Catal, submitted

The Synthesis and Structure of a Single Phase, Nanocrystalline MoVW Mixed Oxide Catalyst of the Mo5O14-Type

The different preparation steps are characterized for the single-phase, crystalline, ternary oxide (MoVW)5O14, which is important for catalytic, mild selective oxidation reactions. For the synthesis of this oxide, solutions of ammonium heptamolybdate, ammonium metatungstate, and vanadyl oxalate were spray-dried followed by different thermal treatments. The structures of the materials formed at each preparation step, starting from the precursor to the final product, were studied using scanning and transmission electron microscopy, X-ray powder diffraction, thermal analysis, and Raman spectroscopy. Raman spectroscopy was also applied to shed some light into the aqueous chemistry of the mixed precursor solutions. Raman data indicate that a molecular structure which seems to be closely related to that of the final crystalline Mo5O14-type oxide is already formed in solution. X-ray diffraction revealed that the thermal treatment steps strongly affect the degree of crystallinity of the ternary Mo5O14 oxide. Transmission electron microscopy with energy-dispersive microanalysis confirmed the presence of V and W in the molybdenum oxide particles and gave evidence for the (010) plane as the most developed face of the crystals of this phase. Details of the structural transformation of this system at the different preparation and calcination steps are discussed in relation to their performance in the selective partial oxidation of acrolein to acryli

Methods for preparation and characterisation of heterogeneous catalysts

Whilst much research effort was spent on optimisation of catalyst performance, less attention has been drawn to problems concerning catalyst preparation. It is commonly known that catalyst research is facing two problems: the Materials gap and the Pressure gap. The “Materials gap” describes the discrepancy between commercial catalyst material that is often too complex to be successfully characterised and (single crystal) model catalysts that are often not able to achieve good product rates. The “Pressure gap” addresses the problem that surface investigations are commonly performed under UHV conditions whereas commercial processes are carried out at ambient or high pressure. As a consequence information of reaction mechanisms or the “real structure” under reaction conditions is very limited. Although catalysis experiments on single crystals led to new information about catalyst behaviour, it is now commonly believed that the main catalytic processes happen on centres with a high- but often unidentified- number of defects. A major task for catalyst preparation is therefore to produce highly defective metastable material. New syntheses have to be developed that fulfil many more requirements such as to ensure high reproducibility and products easy to characterise. Whilst the former can be achieved by monitoring each reaction step in-situ, the latter is taken care of by preparing thin films on a substrate. These new preparation methods will be demonstrated on the example of MoVW supported catalysts, which are used in industry for the synthesis of acrylic acid [1-5]. Despite this industrial importance, there is still a lack of information concerning structure formation during synthesis and the atomic arrangements with respect to different preparation routes and element ratios. Earlier work [6-9] showed a significant increase in selectivity for partial oxidation products in the presence of a Mo5O14 type structure. This structure, which was first identified by Kihlborg et.al. [10], is built up by pentagonal bipyramids and octahedrally coordinated metal centres [Figure 1]. It is metastable until crystallisation and oxidative decomposition into binary oxide phases occurs under high oxygen partial pressure (air and above). The element ratio is (Mo0.68V0.23W0.09)5O14. At the same time binary molybdenum based oxides doped with different elements such as Nb, W and Ta have been synthesised and their structure was identified as that of the Mo5O14-type [11, 12]. These phases were found to be stable at a wide temperature range. For the synthesis of this oxide, solutions of ammonium heptamolybdate, ammonium metatungstate, and vanadyl oxalate were spray-dried and subsequently calcined in air and helium. The Mo5O14 structure is an idealised endpoint that is formed under reduced oxygen partial pressure during the organisation process of a mixture of oligo anions, which are generated in solution. It is therefore necessary to characterise not only the structure itself but also the full preparation process with all intermediates. It seems plausible that different thermal treatments of the precursor solutions affect a) the composition of the usually mixed phase catalysts and b) the crystallite sizes of the different constituting phases. Thus, the understanding of the aqueous precursor chemistry is required to control the preparation of such mixed oxide catalysts. Furthermore, subsequent drying and activation procedures from the liquid precursor to the active and selective catalyst are of paramount importance for the development of the optimal catalytic performance. A preparation that is based on understanding of the system would allow precise control of the phase composition of the mixed oxide catalyst, the crystallite size, the crystallinity, and the morphology of the active phase. A developed synthesis routine thus could lead to defined crystallite sizes or even nano-crystalline (MoVW)5O14 mixed oxide catalysts. Moreover, it offers a versatile path to control its elementary composition. Effects of crystallite size / morphology and elemental composition could be studied separately on the catalytic performance. To this end, some steps of the developed aqueous preparation procedure are characterised by in situ micro Raman spectroscopy. The important, subsequent drying process as well as further activation and formation procedures are investigated by in situ Raman spectroscopy, HREM and XRD. Comparison with Raman spectra of well defined, single-crystalline reference oxides [13] can be used to assign the obtained spectra during these catalyst preparation routes to certain oxides, such as MoO2, Mo4O11, Mo8O23, MoO3, or Mo5O14. A different approach is currently carried out to synthesize the MoVW oxide by a Sol gel method. The Sol-gel chemistry is widely used to synthesize metal oxides by inorganic polymerisation of molecular precursors in organic media (alcohols, hydrocarbons). The low synthesis temperatures often lead to the formation of oxides with amorphous or metastable phases, which are not observed using other synthesis routes. The sol-gel synthesis of molybdenum oxides has received little attention, especially in comparison with transition metal oxides such as TiO2, V2O5 and WO3. The overall aim of this work is the rational preparation of molybdenum-based oxides via sol-gel synthesis of alkoxide precursors. The work concentrates on the mechanisms of solid formation from solution by in-situ measurements (Raman and UV-vis) in order to find new synthesis methods for high surface molybdenum oxides

Preparation and characterisation of single phase (MoVW)5O14-type catalyst material

MoVW based materials are highly effective catalysts for partial oxidation reactions such as conversion of acrolein to acrylic acid. They offer a high selectivity, high yields and a good long term stability. Preceding work has identified the catalytically active phase of the MoVW catalyst and characterised it by Raman spectroscopy. The current work has been carried out to synthesise and characterise this active (MoVW)5O14 type structure

Die Genese von (MoVW)5O14 Precursoren in wässeriger Lösung

Motivation Molybdän-, Vanadium- und Wolframhaltige Suboxide finden vielfältigen Einsatz als Partialoxidationskatalysatoren. Beispielsweise wird die Partialoxidation von Acrolein zu Acrylsäure an einem solchen System durchgeführt. Kürzlich wurde (MoVW)5O14 phasenrein dargestellt. Verbindungen mit identischem Röntgenbeugungsmustern zeigten allerdings unterschiedliche katalytische Aktivität. Dies ist auf die unterschiedliche "Realstruktur" der Katalysatoren zurückzuführen [1]. Diese Arbeit soll den Reaktionsmechanismus in Lösung verdeutlichen, der zu einem Precursor führt, der dann beim Kalzinieren die Einphasigkeit des Systems gewährleistet. Ferner wird untersucht welche Parameter auf die Defekte im Festkörper Einfluss haben und damit die "Realstruktur" mitbestimmen. Experimentelles Eine detailierte Präparationsvorschrift findet sich in der Literatur [1]. Zur Synthese des Suboxides werden entsprechende Mengen von AHM, AMT und Vanadyloxalat gelöst, gemischt, sprühgetrocknet und anschließend kalziniert. Die Strukturen in Lösung werden mit UV/Vis, Raman und ESR Spektroskopie untersucht. Ergebnisse Eine Erniedrigung des pH-Wertes von pH=5,5 auf pH=3 führt zu einem starken Ansteigen der Absorption im Bereich von 30000 cm-1 bis 50000 cm-1. Diese signifikante Veränderung im Metall-Ligand Charge Transfer wird durch eine Protonierung der Sauerstoffatome verursacht. Durch die Zugabe von AMT wird dieser Effekt noch verstärkt. Zusätzlich zu diesem Effekt zeigt sich bei der portionsweisen Zugabe von Vanadyloxalat eine Bande zwischen 18000 cm-1 und 19000 cm-1. Die Blauverschiebung bei höheren Vanadiumkonzentrationen ist durch die fortschreitende Vernetzung bzw. Polymerisation zu erklären. Die Lage der Bande lässt auf einen Intervalence Charge Transfer Übergang schließen. Dieser Befund wird durch das ESR Experiment bestätigt. Je höher die Vanadyloxalatkonzentration, desto weniger isolierte Vanadyleinheiten liegen vor. Außerdem macht das ESR Spektrum deutlich, dass eine Vernetzung zu größeren oligomeren oder polymeren Einheiten stattfindet. Diese Spezies werden durch Vanadylbrücken miteinander verknüpft und können somit eine oligomere oder polymere Spezies in Lösung bilden (Schema 1). Ein Literaturvergleich zeigt, dass im vorliegenden pH-Bereich die Existenz von [HMo7O24]5- und [Mo8O26]4- sehr wahrscheinlich ist. Für [Mo36O112(H2O)16]8- spricht allerdings, dass dieses Ion eine pentagonale Bipyramide enthält, also genau die Struktureinheit die in Mo5O14 auftaucht

Propane Dehydrogenation over Cobalt Aluminates: Evaluation of Potential Catalytic Active Sites

Non-oxidative propane dehydrogenation (PDH) is becoming an increasingly important approach to propylene production, while cobalt-containing catalysts have recently demonstrated great potential for use in this reaction, providing efficiencies comparable to those of industrially employed Pt- and Cr-based catalytic systems. It is therefore essential to clarify the nature of their active sites, especially since contradictory opinions on this issue are expressed in the literature. In this study, efforts were made to determine the state of Co in cobalt aluminates (CoAl2O4-Al2O3) responsible for PDH under typical operating conditions (600 °C, 1 atm). It is shown that the catalyst with a low cobalt content (Co/Al = 0.1) ensured the highest selectivity to propylene, ca. 95%, while maintaining significant propylene conversion. The structural motifs such as cobalt oxide and metallic cobalt nanoparticles, in addition to tetrahedral Co2+ species in the CoAl2O4 spinel system, were evaluated as potential active-site ensembles based on the obtained catalytic performance data in combination with the XRD, H2-TPR, TEM and XPS characteristics of as-synthesized, spent and spent–regenerated catalysts. It is revealed that the most likely catalytic sites linked to PDH are the Co-oxide forms tightly covering alumina or embedded in the spinel structure. However, additional in situ tuning is certainly needed, probably through the formation of surface oxygen vacancies rather than through a deeper reduction in Co0 as previously thought