Flame-made MoO 3 /SiO 2 –Al 2 O 3 metathesis catalysts with highly dispersed and highly active molybdate species Damien P. Debecker a,⇑ , Bjoern Schimmoeller b,⇑ , Mariana Stoyanova c , Claude Poleunis d , Patrick Bertrand d , Uwe Rodemerck c , Eric M. Gaigneaux a a Institute of Condensed Matter and Nanoscience – Molecules, Solids and Reactivity (IMCN/MOST) 1 , Université catholique de Louvain, Croix du Sud 2/17, 1348 Louvain-La-Neuve, Belgium b Particle Technology Laboratory, Institute of Process Engineering, Department of Mechanical and Process Engineering, ETH Zurich, CH-8092 Zurich, Switzerland c Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany d Institute of Condensed Matter and Nanoscience – Bio- and Soft Matter (IMCN/BSMA), Université catholique de Louvain, Croix du Sud 1, 1348 Louvain-La-Neuve, Belgium article info Article history: Received 7 October 2010 Revised 3 November 2010 Accepted 5 November 2010 Available online 13 December 2010 Keywords: Flame spray pyrolysis Propylene metathesis Dispersion Supported molybdenum oxide Alkene valorisation Si–Al mixed oxide Carbene abstract MoO 3 /SiO 2 –Al 2 O 3 catalysts are prepared via flame spray pyrolysis and evaluated in the self-metathesis of propene to ethene and butene. Their specific surface area ranges between 100 and 170 m 2 g 1 depending on the MoO 3 loading (1–15 wt.%, corresponding to Mo surface density between 0.3 and 6.1 Mo atoms per nm 2 ). The catalysts were characterized by N 2 -physisorption, X-ray diffraction (XRD), Raman spectros- copy, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectroscopy (ToF-SIMS). The silica–alumina matrix condenses first in the flame and forms non-porous spherical particles of 5–20 nm, followed by the dispersion of Mo oxide at their surface. Depending on the MoO 3 loading, different MoO x species are stabilized: dispersed and amorphous molyb- dates (mono- and oligomeric) at low loadings (<5 wt.%, <1.5 Mo nm 2 ) and crystalline MoO 3 species at higher loadings. Raman spectroscopy suggests the presence of monomeric species for surface densities of 0.3, 0.5 and 0.8 Mo nm 2 . The formation of MoAOAMo bonds is, however, clearly established by ToF-SIMS from surface densities as low as 0.5 Mo nm 2 . At 1.5 Mo nm 2 , crystallites of b-MoO 3 (2– 3 nm) are detected and further increasing the loading induces the formation of bigger a- and b-MoO 3 crystals (around 20 nm). The speciation of Mo proves to have a marked impact on the metathesis activity of the catalysts. Catalysts with high Mo loading and exhibiting MoO 3 crystals are poorly active, whereas catalysts with low Mo loading (<5 wt.%) perform well in the reaction. The catalyst loaded with only 1 wt.% of MoO 3 (0.3 Mo nm 2 ) is the most active, reaching turn over frequencies seven times higher than refer- ence catalysts reported in the literature. Moreover, the specific metathesis activity is clearly inversely correlated to the degree of condensation of the molybdenum oxide phase (as evaluated by ToF-SIMS). The latter finding indicates that monomeric MoO x species are the main active centres in the olefin metathesis. Ó 2010 Elsevier Inc. All rights reserved. 1. Introduction Light olefin metathesis attracts particular interest for the petro- chemical industry as it enables the conversion of olefins as a func- tion of the market demand [1]. As a thermoneutral reaction it can be run at low energy and environmental cost. Catalyst recovery and ease of product separation make heterogeneous catalysis appropriate to industrial-scale production [2]. In this regard, supported molybdenum oxide systems have been widely investi- gated as active olefin metathesis catalysts [3]. A higher activity has been observed for SiO 2 –Al 2 O 3 supported MoO 3 catalyst com- pared to Mo oxide supported on pure SiO 2 or Al 2 O 3 supports [4– 6]. This higher performance has been explained by the higher acidic character of the silica–alumina surface [4]. Such silica–alumina supported MoO 3 catalysts have been syn- thesized via different chemistry-based methods in the liquid phase, such as impregnation [6–8], non-hydrolytic sol–gel method [9] and MoO 2 (acac) 2 anchoring [5,8] or via dry methods such as thermal spreading [10–12]. These conventional methods allow only limited control over physical properties of the materials and, except for the non-hydrolytic sol–gel process, are all multi-step methods, thus time demanding and requiring pre-made supports. Generally, impregnated catalysts with Mo surface densities <1.0 Mo atom 0021-9517/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2010.11.003 ⇑ Corresponding authors. Fax: +32 10473649 (D.P. Debecker), fax: +41 44 632 12 76 (B. Schimmoeller). E-mail addresses: damien.debecker@uclouvain.be (D.P. Debecker), schimmoeller @ptl.mavt.ethz.ch (B. Schimmoeller), eric.gaigneaux@uclouvain.be (E.M. Gaigneaux). 1 Note: IMCN and MOST are new research entities involving the group formerly known as ‘‘Unité de catalyse et chimie des matériaux divisés’’. Journal of Catalysis 277 (2011) 154–163 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat