Partial Oxidation of Methanol Over Highly Dispersed Vanadia Supported on Silica SBA-15
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Catalysis Letters Vol. 105, Nos. 1–2, November 2005 ( 2005) DOI: 10.1007/s10562-005-7997-x
Partial oxidation of methanol over highly dispersed vanadia supported on silica SBA-15 C. Hessa,d,*, Ian J. Drakec, James D. Hoefelmeyera, T. Don Tilleya,b, and Alex T. Bella,c a
Chemical and Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, 94720, USA b Department of Chemistry, University of California, Berkeley, CA, 94720, USA c Department of Chemical Engineering, University of California, Berkeley, CA, 94720, USA d Present address: Department of Inorganic Chemistry, Fritz Haber Institute of the Max Planck Society, Faradayweg 4–6, 14195, Berlin, Germany
Received 12 May 2005; accepted 6 August 2005
The partial oxidation of methanol to formaldehyde (FA) was studied over highly dispersed vanadia supported on mesoporous silica SBA-15 (VOx/SBA-15). VOx/SBA-15 catalysts were prepared by a novel grafting/ion-exchange method and characterized using UV–VIS- and Raman spectroscopy. The resulting surface vanadium oxide species (0–2.3 V/nm2), grafted on the inner pores of the SBA-15 silica matrix, consist of tetrahedrally coordinated monomeric and polymeric vanadia. The VOx/SBA-15 catalysts are active and highly selective for the production of FA between 300 and 400 C. Comparison of the reactivity results with literature data reveals that a better catalytic performance can be obtained over vanadia supported on mesoporous silica in comparison with conventional silica samples with the same vanadium loading. Raman characterization of the catalyst after reaction at high conversion indicates that dispersed vanadia partly agglomerates into vanadia crystallites during methanol oxidation. KEY WORDS: supported vanadium oxide; vanadia; SBA-15; methanol oxidation; spectroscopy; Raman.
1. Introduction Mesoporous SBA-15 silica is a promising new support material for catalysts. First, its large internal surface area (>800 m2/g) and OH concentration allow for the dispersion of a large number of catalytically active sites. Secondly, it possesses uniform hexagonal channels, which can be tuned from 5 to 30 nm, and a very narrow pore-size distribution [1]. Therefore, SBA15 allows for rigorous control of the surface geometry at the mesoscale, which is an important ingredient for the design of catalysts [2]. Its large pores permit access to bulky reagents. Furthermore, the thick framework walls (3.1–6.4 nm) of SBA-15 provide high hydrothermal stability that exceeds those of other mesoporous materials e.g. thinner-walled MCM-41 [3]. Although of great potential use for catalytic applications, there have been only few reports on the preparation and/or reactivity behavior of SBA-15 supported catalysts. Only recently, SBA-15 supported vanadia was used for the photo-induced oxidation of methane to formaldehyde (FA) [4] and oxidative dehydrogenation (ODH) of propane [5]. Supported vanadia catalysts possess unique properties for a number of reactions such as the oxidation of sulfur dioxide to sulfur trioxide [6], oxidation of
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