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The Structure of Molybdenum-Heteropoly-Acids Under Conditions of Gas Phase Selective Oxidation Catalysis: a Multi-Method in Situ Study

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The Structure of Molybdenum-Heteropoly-Acids Under Conditions of Gas Phase Selective Oxidation Catalysis: a Multi-Method in Situ Study Applied Catalysis A: General, 256 (2003) 1-2, 291-317 The Structure of Molybdenum-Heteropoly-Acids under Conditions of Gas Phase Selective Oxidation Catalysis: A Multi-Method in situ Study. F.C. Jentoft, S. Klokishner, J. Kröhnert, J. Melsheimer, T. Ressler, O. Timpe, J. Wienold, R. Schlögl* Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG, Faradayweg 4-6, 14195 Berlin, Germany * Corresponding author: e-mail [email protected], phone +49 30 8413 4400, fax +49 30 8413 4401 Recieved 11 February 2003; accepted 25 March 2003 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-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 pre catalyst and the pre- cursor oxide mixture is the final deactivated state of the catalyst. Keywords: diffuse reflectance, polyoxometallate, deactivated, in situ, thermal analysis, EXAFS, XRD, UV-vis-NIR spectroscopy, semi-empirical theory, FTIR spectroscopy, CO adsorption Introduction stream at 300 K and/or when temperature rises. The widths Vanado-molybdo-phosphates of the type of of the distribution of desorption temperatures and their start- H4PVMo11O40*yH2O (HPA) and H4-xCsx-PVMo11O40*yH2O ing points depend on sample composition (structure) and on experimental conditions (kinetics). After the removal of (CsxA, x=1-4) have been extensively studied as active cata- lysts for the selective oxidation of several alkanes, aldehydes crystal and constitutional water further oxygen evolution and acids [1,2,3,4,5,6]. These compounds contain networks takes place and the systems undergo internal redox reaction of MO6 (M=V, Mo) octahedra, which resemble discrete [10]. This complex reactivity that is partly reversible with fragments of metal oxide structures [7]. The Mo and V ions temperature calls for an in-depth structural study in order to are distributed randomly in the mixed HPA and CsxA com- identify the true nature of the catalytically active material. pounds. Structural studies revealed that the vanadium may Only then the often-quoted chemical diversity of HPA sys- be in the primary structure in the as-synthesized state, but is tems can fully be exploited for catalytic applications. definitively located in the secondary structure as a vanadyl Central to redox catalysis is the knowledge of the electronic group after the HPA was used as catalyst [8,9]. structure of the active phase. This can be studied in situ HPA crystallize with a large number of water molecules that using optical spectroscopy. An essential advance in the stud- are present in two distinctly different forms: crystal and ies of optical spectra of catalysts of different stages of their structural water. Water is lost under the action of a gas transformation was achieved by applying the in situ diffuse Preprint of the Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG (for personal use only) (www.fhi-berlin.mpg.de/ac) The Structure of Molybdenum-Heteropoly-Acids under Conditions of Gas Phase Selective Oxidation Catalysis: A Multi-Method in situ Study. F.C. Jentoft et al., Applied Catalysis A: 2 General, 256 (2003) 1-2, 291-317 accepted March 2003 reflectance UV/Vis/near-IR spectroscopy (DRS) [10,11], Redox transformations require in addition to the availability which proved to be a suitable technique for probing both d-d of electrons (that are absent in stoichiometric parent HPA) and charge transfer transitions at reaction temperature and certain acid-basic properties of the bi-functional catalysts. A under realistic gas compositions [12]. A number of groups frequently cited property of HPA is their strong acidity that have reported exsitu optical spectra [13,14,15,16,17]. It is is related to the presence of structural water. As this water known from these studies that the DRS method detects even may be absent under reaction conditions it is adequate to small changes in spectral features connected with water loss probe the acid-base properties of activated samples with a or chemical reduction. This method was applied to investi- suitable method. The acidity of solid heteropoly compounds gate the reduction-reoxidation of HPA and Cs2A by metha- [38] has been investigated through interaction with bases nol and ethanol and to correlate structural changes with combined with different methods of analysis: (i) Hammett catalytic data [18]. indicators + visual inspection [39], (ii) ammonia + tempera- The method exhibits limitations similar to those of other ture programmed desorption (TPD) [40,41], IR spectroscopy optical methods. The optical bands arising from intra- and [42], calorimetry [43,44], and (iii) pyridine + TPD, IR spec- interatomic transitions exhibit significant widths and thus troscopy [45,46]. limited resolution. Due to strong coupling of outer valence The use of Hammett indicators is difficult because many electronic states with the vibrational states of the solid the heteropoly compounds are strongly colored. Not only the bands are additionally broadened. A further loss in resolu- surface of heteropoly acids interacts with vapors; many po- tion caused by data acquisition above room temperature has lar molecules enter the bulk, usually in a number related to to be accepted as consequence of the dynamic nature of the the amount of protons per formula unit [47] (formation of activated systems [11,19]. The multiple redox states of acti- clathrates?). The reaction with the strongly basic molecules vated HPA give rise to band systems strongly overlapping ammonia and pyridine leads to the stoichiometric formation and thus requiring data analysis based on theoretical predic- of ammonium and pyridinium salts. This reaction is not tions in order to derive meaningful electronic structural in- easily reversible by evacuation at moderate temperatures. formation. Ammonium salts only start to decompose at higher tempera- The parent structures of HPA that constitute highly active ture, e.g. (NH4)3PMo12O40 releases most of its NH3 at 700 K materials for selective oxidation reactions (e.g. methacrolein [41]. Upon heating pyridinium salts of H3+xPMo12-xVxO40 in to methacrylic acid [20]) contain as common motif the Keg- TPD experiments H2O, CO, CO2, and N2 were detected gin anion. Incompletely salified CsxA (e.g. CsxH3- besides pyridine, indicating that a fraction of the pyridine x[PMo12O40] (2 ≤x<3), Pn-3m (No. 244), a = 11.85 Å [21]), was oxidized. The strong bases ammonia and pyridine can, for instance, is applied on the industrial scale. It has been thus, not be considered as probe molecules for the heter- proposed that the active phase of the HPA under reaction opoly compounds within the definition of acidity as consti- conditions corresponds to the intact and undistorted Keggin tuting an equilibrium reaction between a conjugated acid- structure and, hence, that the catalytic reactivity of the mate- base pair. The heat measured during interaction with the rial could be understood based on the initial structure of the "probe" is actually the heat for the salt formation. It is thus HPA [22,23]. Conversely, the stability of the Keggin anions possible to estimate the difference in the enthalpies of for- during thermal treatment and under catalytic conditions, the mation between the acid and the salt. The often discussed homogeneity of the partially salified HPA, and the correla- difference in stability can be quantified; as an example -1 tion of the structure of the HPA and its catalytic activity are (NH4)3PMo12O40 is about 270 kJ mol more stable than the under debate [24,25,26,27]. Mixed phases of partially sali- corresponding acid. fied HPA have been proposed forming a core-shell system The salt formation interferes with the detection of further of the Cs3A-salt and the free acid under catalytic conditions acid sites, because all additionally offered probe molecules [25]. Furthermore, it has been suggested that the molybde- interact with the ammonium or pyridinium salt and not with num cations [28] as well as the vanadium cations [29,30] the original compound. Ammonia and pyridine are thus migrate from the Keggin anion to cationic positions in the good to titrate the acidic protons, but all further interpreta- structure [31,32] and that the heteropoly acids become par- tions are questionable. Bielański et al. [43] only found Brøn- tially reduced under reaction conditions [33,34]. sted and no Lewis sites on a series of H3+xPMo12-xVxO40 This migration of cations from the Keggin anion has been samples using ammonia as a probe, equally Serwicka et al. speculated to be responsible for the catalytic activity of the [47] detected ad(b)sorbed pyridine “predominantly” in its heteropoly oxomolybdates with Keggin structure protonated form after adsorption on H3+xPMo12-xVxO40. [25,29,30,33,35,36,37]. The result of the migration is a cu- Another probe tested was carbon monoxide, which was ad- bic molybdenum autosalified heteropolyacid (Pn-3m, a = sorbed at 100 K on H3PMo12O40 and its cesium salts [48]. -1 11.853 Å) derived from triclinic H3[PMo12O40]*13H2O un- The spectra showed three different bands at 2165 cm , as- der reduction conditions with propene and hydrogen at tem- signed to adsorption on OH groups, 2154 cm-1, assigned to peratures above 600 K.
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