Celaya Sanfiz, A.

Girgsdies, F.

Hansen, T.

Jentoft, R.

Sakthivel, A.

Schlögl, R.

Trunschke, A.

Wild, U.

English

MPG.PuRe

North American Catalysis Society - 20thNorth American Meeting, June 17-22, 2007, Houston, USAPreprint of the Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG (for personal use only) (www.fhi-berlin.mpg.de/ac)0 4 8 12 16 200,0000,0020,0040,0060,008dVp/dDp(cm3 g-1nm-1)Pore diameter (nm)0,0 0,2 0,4 0,6 0,8 1,0020406080100120140160180Volume (cm3 g-1)Relative pressure p/p0Novel mesostructured Mo-V-Te-Nb mixed oxide catalysts for selective oxidation of propane to acrylic acid Almudena Celaya Sanfiz, Ayyamperumal Sakthivel, Annette Trunschke*, Thomas Hansen, Frank Girgsdies, Rolf E. Jentoft, Ute Wild, Robert SchlöglFritz Haber Institute of the Max Planck Society, Berlin 14195 (Germany)*trunschke@fhi-berlin.mpg.deIntroductionStructurally and chemically complex Mo-V-Te-Nb mixed oxides are particularly promising as catalysts for selective oxidation and ammoxidation of propane to acrylic acid and acrylonitrile, respectively [1]. The activated catalysts are crystalline materials essentially composed of two different orthorhombic phases, denominated as M1 and M2 [1], respectively, usually exhibiting specific surface areas less than 20 m2g-1. M1 corresponds to a complex three-dimensional structure assembled from MO6 and MO5 units (M=Mo,V,Nb) forming six-and seven-sided channels, stabilized by built-in tellurium. Although the importance of M1 for propane activation is indisputable, its precise mode of action remains a matter of ongoing discussion bearing models that consider specific metal positions on the ab plane of the M1 phase as active sites [2]. In different concepts, M1 is regarded as a reservoir of chemically and structurally dynamic surface species formed under reaction conditions at the M1 surface [3]. Hydrothermal synthesis enables the preparation of highly crystalline single-phase M1 catalysts without the need for further purification of the product by chemical treatments. To a certain extent, the specific surface area is adjustable by the thermal activation procedure. The resulting catalysts convert propane into acrylic acid with more than 80% selectivity whereas the activity increases with increasing specific surface area. Consequently, our efforts have been focused on nanostructuring MoVTeNbOx. PEO-PPO-PEO block copolymers have been successfully applied to prepare a thermally stable mesostructured MoVTeNbOx possessing x-ray amorphous bulk structure, which shows activity in the oxidation of propane to acrylic acid.Materials and MethodsIn the present work, gels with the metal stochiometry Mo1V0.25Te0.23Nb0.124 were prepared by combining aqueous solutions of(NH4)6Mo7O24· 4 H2O, VOSO4·x H2O, Te(OH)6, (NH4)2Nb2(C2O4)5, and optionally the triblock copolymer EO20PO70EO20. Black-purple colored products were obtained after hydrothermal synthesis at 403 K for 96 h. If present, the template was removed to various degrees by extraction as verified by thermal analysis. Finally, the materials were activated in argon at 873 K.Results and DiscussionHigh temperature treatment results in crystallization of pure M1, if the mixed oxide was prepared in absence of the template. In contrast, the block copolymer assisted synthesis leads to x-ray amorphous materials. The residual carbon content amounts to about 6 wt-% as shown by thermal analysis. Nitrogen adsorption-desorption measurements revealed a type IV isotherm with a sharp inflection at p/p0=0.7 and a broad hysteresis loop, characteristic of capillary condensation in the channels of a mesoporous material (Fig. 1). BJH analysis evidenced a broad pore size distribution in the range of 6-12 nm (Fig. 1). The porous nature of the material was confirmed by SEM showing pore openings ranging from 7 to 12 nm in diameter (Fig. 2). In agreement with the absence of small angle x-ray diffraction lines, TEM images confirm an irregular spatial arrangement of the pores. The surface areas of the novel materials amount 100 m2g-1, which is two orders of magnitude higher than that of the crystalline MoVTeNbOx reference (1 m2g-1). EDX assessed a similar chemical composition of amorphous and crystalline products with slightly increased vanadium content in the mesoporous material. The marked similarities in electronic structure, as shown in Figures 3-4 for, e.g., Mo and V, permits the new mesoporous material to serve as an model system to study the function of crystallinity on the reactivity of  Mo-V-Te-Nb mixed oxides. SignificanceThe synthesis of a stable mesoporous  Mo-V-Te-Nb mixed oxide is reported for the first time. Tuning of the novel materials may result in entirely new catalytic properties.Figure 1. N2 adsorption on template-free MoVTeNbOx activated at 873 K.Figure 2. SEM image of mesoporous MoVTeNbOx activated at 873 K.North American Catalysis Society - 20thNorth American Meeting, June 17-22, 2007, Houston, USAPreprint of the Department of Inorganic Chemistry, Fritz-Haber-Institute of the MPG (for personal use only) (www.fhi-berlin.mpg.de/ac)242 240 238 236 234 232 230 228 2260,00,40,81,23248 (crystalline MoVTeNbOx)3005 (mesoporous MoVTeNbOx)Normalized IntensityBinding Energy (eV)232.5232.6520 519 518 517 516 515 514 513-0,020,000,020,040,060,080,100,123248 (crystalline MoVTeNbOx)3005 (mesoporous MoVTeNbOx)Normalized IntensityBinding Energy (eV)Figure 3. Mo 3d XPS spectra. Figure 4. V 2p3/2 XPS spectra.References1. T. Ushikubo. K. Oshima, A. Kayou, M. Hatano, Stud. Surf. Sci. Catal. 112 (1997) 473.2. R.K. Grasselli, D.J. Buttrey, J.D. Burrington, A. Andersson, J. Holmberg, W. Ueda, J. Kubo, C.G. Lugmair, A.F. Volpe Jr, Top. Catal. 38 (2006) 7.3. J. Wagner et al., Top. Catal. 38 (2006) 51.

Novel mesostructured Mo-V-Te-Nb mixed oxide catalysts for selective oxidation of propane to acrylic acid

Novel mesostructured Mo-V-Te-Nb mixed oxide catalysts for selective oxidation of propane to acrylic acid

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