Cotter, T.

Schlögl, R.

Trunschke, A.

English

MPG.PuRe

Temperature programmed reduction-carburisationDehydrogenation of propane over a bimetalliccarbide catalystThomas Cotter, Annette Trunschke and Robert SchlöglFritz-Haber-Insitut der Max-Planck-Gesellschaft, BerlinIntroductionEarly transition metal carbides are interesting candidates for use in a number of catalytic reactions. They have been shown to exhibit similar reactivities to those of noble metals under arange of catalytic conditions.1 Much of the subsequent research has focused on hydrocarbon transformation reactions, but they also demonstrate potential in oxidation chemistry. In such non-oxidic systems there exists the potential to probe the role of surface and subsurface oxygen in catalysis (cf. Mars von Krevelen mechanism). In order to study these materials, molybdenum carbide was chosen as a basis material from which to generate polymetallic doped-carbides. It is anticipated that such doping will confer improved catalytic properties to the final material.Objectives• Develop a flexible synthetic route to molybdenum carbide with high surface area• Explore synthesis of bimetallic Mo/V carbide in order to tune reactivity• Test and compare carbide materials for reactivity in the dehydrogenation (DH) and oxydehydrogenation (ODH) of propane• Study the influence of feed conditions on the carbide activity and stabilityN2H2Custom rotary furnaceWith N2 shieldingN2 purge flowCH4O2meterO2WasteACFHIDH versus ODH over Mo2C Catalytic setup• The carbide is formed from theprecursor oxide in a temperature programmed reaction under an atmosphere of H2/CH4 (4:1)• To achieve high surface area material(50 – 100 m2/g) a high space velocity is used in combination with full rotationalagitation of the reacting material• Quantities of material (>10g) may be produced using this method and additionally treated in O2 - free conditionsCharacterization of Mo2C and Mo(V)Cx• Electron microscopy indicates the V-containingmaterial has a fibrous, high aspect ratio morphology• N2 sorption measurements show exceptional surface area (100 – 110m2/g) of the V-modified carbide• SEM and porosimetry calculations indicate a more highly ordered mesostructure for V modification• X-ray spot diffraction shows ordered crystallinity overa large spot, indicative of a topotactic transformationSynthesis of h-MoO3 precursorsSynthesis of the complex molybdate is carried out by a controlled precipitation in acidic media followed by an aging period (2h)2θ22 24 26 28 30 32 34 36 38 40 4202004002θ- Mo4O11 (orthorhombic)h-MoO3 h-(Mo,V)O3Ammonium heptamolybdate (AHM)[+ Ammonium metavanadate(AMV)]HNO3 (aq)  @ 75°CpH = 1.0meterSafety valve0.0 0.2 0.4 0.6 0.8 1.004080120V [cc/g]p/p01µm 1µmcf. Mo2CMo(V)CxHe Heating oven Analytics(Agilent® T/C readerReductive pretreatment and surface properties of Mo(V)Cx67C3H6 YieldConditions• Temperature = 773K• DH: 10%C3H81µmMo(V)CxIn situ powder X-ray diffraction Influence of precursor on mesostructure and morphologyh-MoO3→ (Mo4O11)→ MoO2 → Mo2C h-Mo(V)O3→ Mo(V)O2 → Mo(V)Cx101m2/g80m2/gNH4+122 24 26 28 30 32 34 36 38 40 420200400300-399°C400-499°C500-599°C600-699°C700°C750°C (hold)50°CTEM study of Mo(V)Cx with spot diffraction showing regular crystallinity (inset)SEM micrographs of Mo(V)Cx (above) showing the fibrous morphology of nano-sized inverse poresContrasting SEM micrograph (right) of Mo2C shown alongside an N2 BET isotherm (left) illustrating the carbide porosity and surface areaPost catalysis characterisation• Carbide unstable to oxidation by ODH feed leading to severe deactivation• Dehydrogenation deactivates via deposition of C - no observed phase change• Dehydrogenation in presence of steam leads to partialoxidation Mo2C → MoO2• Surface area and phase composition can be partly stabilised for steam DH** 20 40 60 80050100150200250Y Axis TitleX  Axis Title BComparison of Mo2C and Mo(V)Cx• V-doped carbide shows higher activity under similar conditions• U-tube reactor; ID = 4 mm• Bed length = 25mm (±0.5°C)• Length/ID = 5• GHSV: 500 < x < 120000Conclusions• High surface area Mo2C was produced from vanadium-doped h-MoO3 with varying V content• A novel mesostructured bimetallic carbide was prepared with exceptional morphologyReactivity explored for the activation of propane:• Oxycarbide more active phase for dehydrogenation• Metallic carbide surface exhibits increased stability to oxidation• Mo2C bulk phase can be preserved in a balance of steam oxidation and carburisation-reductionN2O2C3H8Helium (GC dilution)High-flow MFCsLow-flow MFCsWaste gas6890 GC)P 2400°CT-controller150°C• Mo/V catalysts are pre-reduced in H2 at different temperatures to explore the influence of surface composition on product distribution• Freshly reduced carbides show similar initial reactivity to hydrogenolysis/dehydrogenation• Oxycarbide species is more highly selective to dehydrogenation• More highly reduced material exhibits greater stability to oxidation• Most active species lies between molybdenum carbide and molybdenum dioxide oxidation state0123450:00 1:00 2:00 3:00 4:00 5:00 6:00DHODHSteam-DH%YT (h:mm)• ODH: 10%C3H8 + 1.5%O2• Steam DH: 10%C3H8 + 10%H2O Total flow – 40 mln/min, Inert = N2Mo2C (290mg) diluted 1:1 (w/w) with SiC– pretreatment in H2 @773KSieve fraction 315µ < x < 400µ GHSV = 7600h-100:00 01:00 02:00 03:00 04:00 05:0002468101214405060708090100 XC3H8 (Mo2C) SC3H6 (Mo2C) XC3H8 (MoVCx) SC3H6 (MoVCx)Conversion/Selectivity (%)Absolute time (hh:mm)Mo2C Mo(V)CxX(C3H8) 87% 88%S(C3H6) 2.3% 3.6%After 4 hours on stream @773K10%C3H8 /12%H2O /78%N2Flow – 40 mln/min; GHSV = 7600h-1Mo2C (290mg) diluted 1:1 (w/w) with SiC-pretreatment in H2 @773K400 401 402 403 404 405 4060123456cm°CThermostat(steam saturator)20 40 60 80020040060080010001200Y Axis TitleX Axis Title20 4 0 6 0 8 002 0 0Y Axis TitleX  A x is  T i t le BPost DH*20 40 60 802θPost ODH*(MoO3)Post steam-DH*MoO2Mo2C20 40 60 80Steam DH @ 823K (16h)**2θ*feed composition as aboveTemp(K)blank 773 823 873BET**  (m2/g)57 50 54 -** Over Mo(V)Cx with feed:C3H8/H2O/N2 = 18/12/7001020304050607080901000:00 2:00 4:00 6:00 8:00CO2COCH4C2H2C2H6C3H6• 773K reaction temperature• 450mg Mo(V)Cx• Feed:– C3H8/H2O/N2 =  16:13:71– Total flow 20ml; – GHSV = 3800 h-1T (h:mm)%STOS product selectivities - 773K H2 pretreatment024681012141618200:00 2:00 4:00 6:00 8:00Conversion C3H8Yield C3H601020304050607080901000:00 3:00 6:00 9:00 12:00CO2COCH4C2H2C2H6C3H6024681012141618200:00 3:00 6:00 9:00 12:00Conversion C3H8Yield C3H6• 973K reaction temperature• 450mg Mo(V)Cx• Feed:– C3H8/H2O/N2 =  18:14:68– Total flow 20ml; – GHSV = 3800 h-1TOS product selectivities - 973K H2 pretreatment%ST (h:mm)ACKNOWLEDGEMENTSInternational Max Planck Research School for Complex Surfaces in Material ScienceAC Reactivity Group, Dr. Raoul Naumann, Dr. Olaf Timpe, Edith Kitzelmann, Gisela Lorenz, Daniel BrenneckeREFERENCESR.B. Levy, and M. Boudart, Science 181 (1973) 547-549.O. Mougin, J.-L. Dubois, F. Mathieu, and A. Rousset, Journal of Solid State Chemistry 152 (2000) 353-360.T. Cotter, F. Girgsdies, W. Zhang, A. Trunschke, R. Schlögl; in preparation

Dehydrogenation of propane over a bimetallic carbide catalyst

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Dehydrogenation of propane over a bimetallic carbide catalyst

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MPG.PuRe