Heterogeneously catalysed partial oxidation of acrolein to acrylic acid—structure, function and dynamics of the V–Mo–W mixed oxidesw Philip Kampe, a Lars Giebeler, b Dominik Samuelis, c Jan Kunert, a Alfons Drochner, a Frank Haaß, bc Andreas H. Adams, b Joerg Ott, a Silvia Endres, a Guido Schimanke, c Thorsten Buhrmester, b Manfred Martin,* c Hartmut Fuess* b and Herbert Vogel* a Received 4th January 2007, Accepted 2nd April 2007 First published as an Advance Article on the web 26th April 2007 DOI: 10.1039/b700098g The major objective of this research project was to reach a microscopic understanding of the structure, function and dynamics of V–Mo–(W) mixed oxides for the partial oxidation of acrolein to acrylic acid. Different model catalysts (from binary and ternary vanadium molybdenum oxides up to quaternary oxides with additional tungsten) were prepared via a solid state preparation route and hydrochemical preparation of precursors by spray-drying or crystallisation with subsequent calcination. The phase composition was investigated ex situ by XRD and HR-TEM. Solid state prepared samples are characterised by crystalline phases associated to suitable phase diagrams. Samples prepared from crystallised and spray-dried precursors show crystalline phases which are not part of the phase diagram. Amorphous or nanocrystalline structures are only found in tungsten doped samples. The kinetics of the partial oxidation as well as the catalysts’ structure have been studied in situ by XAS, XRD, temperature programmed reaction and reduction as well as by a transient isotopic tracing technique (SSITKA). The reduction and re-oxidation kinetics of the bulk phase have been evaluated by XAS. A direct influence not only of the catalysts’ composition but also of the preparation route is shown. Altogether correlations are drawn between structure, oxygen dynamics and the catalytic performance in terms of activity, selectivity and long-term stability. A model for the solid state behaviour under reaction conditions has been developed. Furthermore, isotope exchange experiments provided a closer image of the mechanism of the selective acrolein oxidation. Based on the in situ characterisation in combination with micro kinetic modelling a detailed reaction model which describes the oxygen exchange and the processes at the catalyst more precisely is discussed. Introduction This work was part of the priority programme of the German Research Foundation (DFG) ‘‘Bridging the gap between ideal and real systems in heterogeneous catalysis’’. Therein, the thermodynamics, kinetics and dynamics of several technically relevant catalytic systems were investigated. The philosophy of this approach was to answer the question: to what extent is it possible to extrapolate results from single crystal under UHV to the complex catalyst working under industrial conditions? We investigated the technically important selective oxidation of acrolein to acrylic acid. The aim was not to improve the industrial catalyst but to gain a better scientific understanding of the structure, function and dynamics of the catalyst based on V–Mo mixed oxides. For this, it was essential to find a model catalyst as simple as possible that still provides the key features (activity, selectivity, stability). To bridge the gap between ideal and real systems, different model catalysts (from the pure crystalline oxides to the amorphous V–Mo–W mixed oxides) were studied extensively using several probe molecules (from H 2 via CO to acrolein) in the whole pressure range (from UHV via inert gas to reaction gas under atmospheric pressure). History of acrylic acid synthesis Acrylic acid (AA) is, from a chemical point of view, the simplest unsaturated monocarboxylic acid. The most striking chemical property is its extraordinary propensity for polymer- isation, therefore handling during synthesis, storage, transport and work up requires great care and sophisticated knowledge. 1 AA is the intermediate for the production of:  Acrylic esters (53%z) (especially methyl-, ethyl-, n-butyl- and 2-ethylhexyl-)  Superabsorber polymers (31%)  Detergents (6%) a Darmstadt University of Technology, Faculty of Chemistry, Technical Chemistry, Petersenstr. 20, D-64287 Darmstadt, Germany. E-mail: vogel@ct.chemie.tu-darmstadt.de; Fax: +49 6151 163465; Tel: +49 6151 162165 b Darmstadt University of Technology, Institute for Materials Science Structure Research, Petersenstr. 23, D-64287 Darmstadt, Germany. E-mail: hfuess@tu-darmstadt.de; Fax: +49 6151 166023; Tel: +49 6151 164373 c RWTH Aachen University, Institute of Physical Chemistry, Landoltweg 2, D-52074 Aachen, Germany. E-mail: martin@pc.rwth- aachen.de; Fax: +49 241 80992128; Tel: +49 241 8094712 w Electronic supplementary information (ESI) available: In situ XANES spectra. See DOI: 10.1039/b700098g z These data refer to the US consumption in the year 2000. This journal is  c the Owner Societies 2007 Phys. Chem. Chem. Phys., 2007, 9, 3577–3589 | 3577 INVITED ARTICLE www.rsc.org/pccp | Physical Chemistry Chemical Physics