Bentrup, U.

Brückner, A.

Eckelt, R.

Jacobi, B.

Kubias, B.

Schlögl, R.

MPG.PuRe

Mesoporous VOx-MCM materials - promising catalysts for selective 2 and 4 electron oxidationsB. Kubias1), B. Jacobi1), R. Eckelt2), A. Brückner2), U. Bentrup2), R. Schlögl1)1) Department of Inorganic Chemistry, Fritz Haber Institute  of  the Max Planck SocietyFaradayweg 4-6, D 14195 Berlin, Germany2) Institute for Applied Chemistry Berlin-AdlershofRichard-Willstätter-Straße 12, D 12489 Berlin, GermanyAbstractCommonly, selective gas phase oxidations of hydrocarbons to oxygenates are performed over catalysts with low specific surfaces. Against this background, it was the aim of this study to explore the catalytic potential of surface-rich mesoporous VOx/SiO2 materials in the selective oxidations of butadiene (1,3-b) to furan (F) and/or crotonaldehyde (CA). These one-step oxidations of butadiene still remain unsolved and two of the challenging tasks of the selective oxidation of n-C4 hydrocarbonsbecause of the tendency of both target products to react to maleic anhydride.Vanadosilicate mesoporous MCM molecular sieves are promising catalysts due to their ability to disperse vanadium oxides to isolated vanadium oxide species in order to limit their oxidation activity. The samples were characterised by chemical as well as spectroscopic methods to derive structure-reactivity relationships. They exhibited remarkable sum F and CA selectivities up to 55 % at 50 % 1,3-b conversion and proved to be more selective than catalysts obtained by impregnation. Independent of the type of preparation, V tends to be incorporated into the SiO2 lattice only to a certain extent due to a tendency of segregation. The results show that VOx/SiO2 catalysts, having a high dispersion of VOx, are suitable catalysts for the desired 2 and 4 electron oxidations in contrast to catalysts with active sites consisting of larger ensembles of transition metal ions, which catalyse multi electron oxidations to more highly oxidised products.AACApplied CAtalysisResults of characterisation:51V solid state NMRResults of characterisation:Increasing V content:formation of BrØnsted sites (BS)additionally strong Lewis sites (LS) at 10V/MCM-41Adsorption of pyridineGround spectrum after reaction:identification of ads. cyclic anhydride + oxygenatesdeposites are displaced by pyridine at RTBSLSLSAnhydride BSLSEx situ-EPR at 77 K250 –250 –750 – 1250ppm– 800–4000ppmVoctVtetrVtetrVoctV / %101,80,6VOx-coordinationin dependence on vanadium content1) Samples 0.6, 1.8, and 3.0 wt% V2O5: vanadosilicates; sample 5 wt%: impregna-ted; conditions  see left  2) T = 300oC 3)  not determined  4) MA = maleic anhydridebefore calcinationafter calcinationHydration/dehydration of  3.0 wt% sample (left), hydrated samples (right): - H2O addition (before calcination): a part of tetrahedrally coordinated VVbecomes octahedrally coordinated: surface V - Dehydration (by calcination): after calcination –both “wall“ and surface VVtetrahedrally coordinated- little V content: V only in walls200 300 400 500B0/ mT200 300 400 500B0/ mTbefore catalysis after catalysis1.8 %3 %5 %10 %+ O2 + H2OO+ ½ O2 O- Growing content of VOx: increasingly bigger shares of cluster-shaped VO2+sites,  decrease of shares of isolated sites- Increase of the amount of VO2+ sites after catalysis- no cluster formation observed at low VOx content: stable VOx site dispersionResults of catalytic testsOxidation of butadiene on VOx/MCM-48:Dependence of the conversion rate of 1,3-b on residence time overdifferent VOx/MCM-48 catalysts (mcat= 0.3 g, T = 325 oC)-Mass specific rates of 1,3-b consumption increase up to 3 wt% V2O5loading: dispersed and accessible active sites available up to this levelDependence of formation of F and CA on VOx contentand method of preparation1)VOxcontentwt%SMAXF%XC4H6MAXF%SMAXCA%XC4H6MAXCA%SMAXMA%XC4H6MAXMA%0.62) 25 30 n.d.3) n.d.3) 01.8 40 40 15 20 0.5 703.0 24 52 12 20 4.8 705.0 23 25 n.d.3) n.d.3) <5 25015304560750.0 0.5 1.0 1.5 2.0tmod [gs/mlN]X 1,3-b[%]1.25 1.5 1.82.1 2.4 3.0ConclusionsEx situ- NMR as well as EPR and UV/Vis/DRS (ex- and in situ-) investigations:- VivOx and VvOx species: at small VOx concentrations mainly in wallsof pores; at higher V concentrations increasing shares of VOx onwalls of pores by segregationPyridine-DRIFTS:- Lewis and BrØnsted acidity, BrØnsted acidity at least partiallyassociated with the occurence of acidic VOH groupsMesoporous V-Si-O catalysts with highly dispersed VOx :-Preparation method decisive für catalytic performance: preparationof vanadosilicates leads to superior catalysts- are suitable for 2 and 4 electron oxidation of butadiene to CA and F , resp., (no „bulk structure“ of the active sites!)-provide electrophilic O species for selective oxidation- have suitable acid/base properties for the desorption of the targetproducts- Possible transport restrictions (high inner surface!) and high dispersion of VOx species do not result in a marked total oxidation of the formed products of the selective oxidation.Literature[1] B. Kubias, G.-U. Wolf, A. Wahab,U. Rodemerck, J. Radnik, H. Hibst, 4th World Congress on Oxidation Catalysis, September 16-21, 2001 Berlin, Germany, Conference proceedings 95[2] H. Berndt, A. Martin, A. Brückner, E. Schreier, D. Müller, H. Kosslick, G.-U. Wolf, and B. Lücke,  J. Catal. 191 (2000) 384[3] Y.-M. Liu, Y. Cao, S.-R. Yan, W.-L. Dai, K.-N. Fan, Catal. Letters 88 (2003)  61[4] Z. Luan, J. Xu, H. He., J. Klinowski, and L. Kevan, J. Phys. Chem. 100 (1996) 19595This work was supported by the Bundesministerium für Bildung und Forschung of  the FRG (Proj. No. C03C02780). The authors thank H. Berndt for providing impregnated VOx-SiO2samples and D. Müller for NMR investigations.Starting pointOxidation of butadiene (1,3-b)  to furan (F):to crotonaldehyde (CA):F: intermediate for synthesis of  tetrahydrofuran (Spandex, Lycra fibres)CA: Intermediate for production of sorbic acid (nutritionalsupplement)State of the art:F: VOx/MeO bulk catalysts:  insufficient S, Y (SF< 50%, YF < 30mol-%) and long term stability, V-Fe-Cs-Cl-O- catalystsdo not have any potential for further developments forprinciple reasons (instability of active sites) [1].CA: On VPO catalysts SCA<10 % (YCA< 6 %); reason: toohigh oxidation potential, inadaquate acid/base propertiesApproach: Mesoporous VOx/MCM materials are suitable catalysts forthe oxidation of CH4 to CH2O [2] and propan to propen [3]: --High dispersion of VOx on high inner surface, thereforelimitation of the oxidation ability at high catalytic activity.-Adjustment of  a suitable oxidation potential for theoxidation of C4H6   by variation of the VOx content of a  MCM material possible?Experimental and methods of characterisationCatalyst synthesis:-Conversion of TEOS with VO(COO)2 and NH3 in the presenceof CTAB at room temperature within 3h analogously to themethod described in [4], calcination of precorsors for 4h on air at 550 °C to vanadosilicate-Impregnation of MCM materials with a solution of VO(COO)2V silicates (impregnated materials see [2]):V content: 0.3-10 wt% V2O5 („0.3-10V/MCM“), Pore structure (analysisaccording to Barrett, Joyner, Halenda) :1-2 nm pore diameter; 0.5-0.7 ml/g volume of pores; 700-900 m2/g specific surface area; 1 µm diameter of primary particlesCharacterisation:- Bulk: NMR (Unity Plus-500, Varian)ex- and in situ-EPR (ELEXSYS 500-10/12)UV/Vis/DRS (Cary 400, Varian), XRD, LRS (results are not presented) -Surface: DRIFTS (IFS 66, Bruker)N2-Adsorption (Asap 2010, Micromeritics); Catalytic testing:- Fixed bed tube reactor (glass, i. d.: 11mm)  with minimal empty space; < 0.7g catalyst, grain diameter: 0.5- 0.8 mmTypical conditions:  T: 325 oC, GHSV: 2000  - 16000 h-11.14% C4H6, 3.5% O2, balance N2On line- GC (Shimadzu 17A): frequence of analyses: 3 h-1 ;                 NDIR photometry: CO2, CO; electrochemical measuring cell: O2

Mesoporous VO<sub>x</sub>-MCM materials - promising catalysts for selective 2 and 4 electron oxidations

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