Characterization of Silica-Supported Vanadium(V) Complexes Derived from Molecular Precursors and Their Ligand Exchange Reactions Gordon L. Rice and Susannah L. Scott* Department of Chemistry, University of Ottawa, Ottawa, Ontario, Canada K1N 6N5 Received July 10, 1996. In Final Form: November 13, 1996 X The reaction of OdVX3, where X is Cl or O i Pr, with the hydroxyl-terminated silica surface gives the well-defined surface complexes tSiOVOX2. These complexes have been characterized by 51 V magic angle spinning and 13 C cross polarization magic angle spinning NMR spectroscopy and infrared spectroscopy. The surface complexes undergo clean ligand replacement reactions with alcohols, similar to the reactions of analogous molecular vanadium complexes and relevant to the understanding of mechanisms in catalysis. Introduction Vanadium-containing solid catalysts are widely used for the partial oxidation of hydrocarbons, 1-3 oxidation of SO 2 , and selective catalytic reduction (SCR) of NO x . 4 Mixed oxides generally show higher catalytic activity and more selectivity than bulk, crystalline V 2 O 5 . 5 The mixed oxide catalysts are believed to consist of a two-dimensional vanadium oxide overlayer stabilized by interaction with a high surface area oxide such as silica or titania. The mechanisms of the reactions catalyzed by these materials continue to be targets of active research. On silica, oxidation of organic substrates has been proposed to occur at sites containing one or more terminal vana- dium-oxygen bonds. 6 There is also evidence that the nature of the interaction between vanadium and the oxide support exerts a strong influence on reactivity. The atomic level structure of these materials has been studied by vibrational spectroscopy, 5,7 51 V solid-state NMR, 8 extended X-ray absorption fine structure/X-ray absorption near edge structure (EXAFS/XANES), 9 and by comparison to mo- lecular model compounds. 10,11 A variety of surface struc- tures on silica have been proposed, with various degrees of condensation ranging from isolated mononuclear va- nadium pseudotetrahedra to two-dimensional rafts of V 2 O 5 . 6,12 A significant advance in controlling the molecular-level architecture of these materials is their preparation from volatile molecular precursors. Using this strategy, it should be possible to synthesize surface species whose composition is very uniform within a given sample. The homogeneous nature of the vanadium environment makes it amenable to molecular spectroscopies, as well as to kinetic studies of reactions at the gas-solid interface. In order to prepare such a material, the reaction conditions must be reproducible and carefully controlled, especially for the presence of water vapor. In particular, the reaction temperature must not exceed the thermal stability of the molecular precursors or the surface complexes. In this paper, we report the preparation and charac- terization of well-defined silica-supported vanadium(V) coordination complexes which are analogous to known molecular complexes. We also demonstrate the reactivity of these surface complexes in ligand exchange reactions with alcohols and tert-butyl hydroperoxide. Experimental Section Preparation and Characterization of Silica-Supported Vanadium Complexes. Surface complexes were prepared from two different vanadium precursors. OdV(O i Pr)3 (Alfa-Aesar) was separated from its 2-propanol impurity by a trap-to-trap distillation using liquid N 2 and CO2/acetone cold baths. A similar procedure was used to remove HCl from OdVCl3 (Aldrich). Both vanadium reagents were stored in grease-free glass bulbs equipped with high-vacuum stopcocks and were transferred into reaction vessels using standard breakseal and high vacuum techniques. Pyrogenic silica (Degussa Aerosil-200, 200 m 2 /g) was used in all experiments. A standard pretreatment procedure was followed in order to ensure reproducibility. For transmission infrared experiments, silica was either pressed at 125 kg/cm 2 into a self-supporting disk of diameter 1.6 cm (2-4 mg of silica/ cm 2 ) or spread in a thin film onto a 25 mm diameter ZnSe disk (0.1-0.5 mg of silica/cm 2 ). In all other experiments, silica was compacted by pressing into pellets (20-30 mg/cm 2 ) which were then finely ground in a mortar. In all reactions, silica-500 was prepared by calcination in 200 Torr O 2 (Air Products, ultrapure carrier grade) at 500 °C for 2 h, followed by dehydroxylation for 2 h at 500 °C in dynamic vacuum (10 -4 Torr). To prepare silica-200 and silica-25, the calcination step was omitted. The silica was simply heated to the appropriate temperature under dynamic vacuum for 2 h. Infrared experiments were performed in high-vacuum in situ IR cells (volume ca. 200 mL) equipped with KCl windows. Transmission spectra were recorded on a dry-air purged Mattson Research Series FTIR equipped with a DTGS detector. For both background and sample spectra, 32 scans were recorded at a resolution of 2 cm -1 . The HCl and 2-propanol liberated by reactions of OdVCl3 and OdV(O i Pr)3, respectively, with silica were analyzed by quantitative IR spectroscopy in the in situ cells. At the end of each experiment, chemisorbed vanadium was extracted from the silica with 1 M H2SO4 to give solutions containing ca. 0.1 mg of V/mL. These solutions were treated with 30% aqueous H2O2 (0.03 mL/mL sample solution) to form the strongly absorbing red-brown peroxovanadium complex. 13 * Author to whom correspondence should be addressed. 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Chem. 1994, 4, 141-188. 1545 Langmuir 1997, 13, 1545-1551 S0743-7463(96)00679-8 CCC: $14.00 © 1997 American Chemical Society