Co-doping of Li/MgO, a well-known catalyst for the oxidative coupling of
methane, was investigated. It is demonstrated that Gd3+ and Fe3+ can be used
as spin sensors in these solids to investigate the structure via EPR
spectroscopy. These aliovalent ions occupy Mg2+ sites in the lattice; the
expected coupling with charge-compensating neighboring Li+ was detected. A
strong increase of the activity was observed. However, all samples suffered
from deactivation. The solubility of Gd3+ in MgO turned out to be inhibited.
No such restriction was observed for Fe3+

Arndt, S.

Dinse, K. P.

Görke, O.

Otremba, T.

Schlingmann, T.

Schomäcker, R.

Schubert, H.

Simon, U.

Institutional Repository of the Freie Universität Berlin

Li/MgO with Spin Sensors as Catalyst for the Oxidative Coupling of MethaneU. Simona,∗, S. Arndtb,∗, T. Otrembab, T. Schlingmanna, O. Go¨rkea, K.-P. Dinsec, R. Schoma¨ckerb, H. Schuberta,∗∗aTechnische Universita¨t Berlin, Department of Material Science, Englische Straße 20, 10587 Berlin, GermanybTechnische Universita¨t Berlin, Department of Chemistry, Straße des 17. Juni 124, 10623 Berlin, GermanycFreie Universita¨t Berlin, Arnimallee 14, 14195 Berlin, GermanyAbstractCo-doping of Li/MgO, a well-known catalyst for the oxidative coupling of methane, was investigated. It is demon-strated that Gd3+ and Fe3+ can be used as spin sensors in these solids to investigate the structure via EPR spectroscopy.These aliovalent ions occupy Mg2+ sites in the lattice; the expected coupling with charge-compensating neighboringLi+ was detected. A strong increase of the activity was observed. However, all samples suffered from deactivation.The solubility of Gd3+ in MgO turned out to be inhibited. No such restriction was observed for Fe3+.Keywords: Oxidative Coupling of Methane, Li/MgO, Transition Metal Oxides, Rare Earth Oxide, CationSubstitution, Spin Sensor, EPR1. IntroductionThe oxidative coupling of methane (OCM) (Equa-tion 1) is an attractive direct route for the conversionof methane into value added compounds. Li/MgO wasconsidered to be a potential candidate for a practical ap-plication, despite the doubts on the stability.CH4 + O2 → C2H6 or C2H4 + H2O (1)A very detailed investigation has shown, that it suf-fers from an intrinsic instability [1–3], prohibiting anypractical application. Furthermore, Li evaporates fromthe catalyst and only a content of approximately 0.01 -0.03 wt% Li in MgO is stable [1, 2, 4].To improve the catalytic performance of Li/MgO, itwas doped with numerous additional metal oxides [5–24]. In many cases an improved stability and/or higherselectivity was found, however, deactivation of thesecatalysts was often retarded, but still observed.Moreover, doping did not contribute to an under-standing of the catalytic system. In an excellent reviewarticle, Gellings and Bouwmeester commented that un-fortunately many of the publications, dealing with co-doped Li/MgO, have not interpreted the results in terms∗These authors contributed equally; the order was determined byPaper, Scissors, Stone.∗∗Corresponding Author. Fax: 0049-30-314-28534, Tel: 0049-30-314-24833Email address: schubert@ms.tu-berlin.de (H. Schubert)of the charge compensating defects and the influence ofthe dopants thereon [25].The present study shows how the co-dopants Fe andGd can be used as spin sensors for the EPR spectroscopyto investigate the structure of such materials. Fe and Gdhave been chosen as co-dopants, not because they couldresult in a better catalytic performance then comparedto catalysts reported in the literature, but because be-cause these high spin ions can serve as sensitive probesfor the local structure. However, the anticipated stabi-lization of the Li-content by these charge compensatingions (Gd3+, Fe3+) might lead to an improved stability ofthe catalytic performance.2. Experimental Part2.1. PreparationIn a catalyst the content of the different metal oxidesis calculated in atom percent (at%) based on the totalcation content, shown in Equation 2.Cation A [at%] =Cation A [mol]∑all Cations [mol]× 100% (2)Aqueous solutions of Mg(NO3)2 were prepared bydissolving Mg(NO3)2 × 6H2O (p.A., Merck) in distilledH2O. The solution of Fe(NO3)3 × 9H2O (p.A., Riedel deHaen) and Gd(NO3)3 × 6H2O (99,9 %, Sigma Aldrich)was prepared the same way. The nitrate solutions werePreprint submitted to Catalysis Communications August 24, 2011concurrently added drop wise to stirred ammonia solu-tion while keeping the pH value above 11. The gelati-nous precipitates were rinsed with distilled H2O andmixed with aqueous LiOH solution (LiOH×H2O p.A.,Riedel de Haen) with appropriate Li concentrations ina tubular mixer. The prepared combination of oxideswere:1. MgO2. Gd/MgO3. Gd-Li/MgO4. Fe-Li/MgOFinally, the solution was quick-frozen using liquidN2. Afterwards, it was freeze-dried over at least 72 h us-ing a freeze-dryer (Gamma 2-20 (Christ)). Finally, thesamples were calcined at 900 °C for 1 h in MgO cru-cibles. A scheme of the preparation procedure is shownin Figure 1.Figure 1: Delineation of the applied preparation procedure.Some samples were too fluffy after calcination; there-fore, they were pressed at 100 bar for 10 minutes andsubsequently crushed. All samples were sieved and onlythe fraction ≤ 200 µm was used for testing to exclude in-ternal mass transfer effects.2.2. CharacterizationThe Li-content of the different samples was quanti-fied via atomic absorption spectroscopy, using a AASNovAA 400 G device from Analytik Jena via flame.The Fe and Gd-content was determined via induc-tively coupled plasma atomic emission spectroscopy,using a Horiba Scientific ICP Model Ultima 2.The specific surface area was determined by a Mi-cromeritics Gemini III 2375 Surface Area Analyzer, us-ing N2 adsorption at -196 °C. Before measuring, thesamples were degassed at 300 °C and 0.15 mbar at leastfor 30 minutes. The surface areas were calculated by themethod of Brunauer, Emmett and Teller.Powder X-Ray diffractograms were obtained (CuKα1radiation, with a wavelength 0.154 nm) using a BrukerAXS D8 ADVANCE X-ray diffractometer.EPR experiments were performed with a BRUKERE680 spectrometer. The spectrometer was operated inconventional continuous wave (cw) as well as in pulsedmode. Data were taken from ”as prepared” compounds.2.3. CatalysisA 6-fold parallel reactor (Integrated Lab SolutionsBerlin and Premex Reactor AG) was used for the de-termination of the catalytic activity, with packed-bed,linear, tubular reactors made of quartz glass. For eachcatalytic run, 50 mg catalyst were diluted with approx-imately 1.5 ml quartz sand (Merck). Below and abovethe catalyst bed, a small amount of pure quartz sand wasput to ensure proper heat transfer. The applied reactionconditions were: 750 °C reactor temperature, 60 ml/mingas flow and a feed gas composition of CH4:O2:N2 =4:1:4. The analysis was performed with a gas chro-matograph Agilent 7890 A. The conversion and selec-tivity is calculated with a mass balance based on the in-let and outlet concentration of reactants and products,see Equations 3 and 4, taking into account the differentnumbers of C-atoms in the different molecules. The car-bon balance was always well above 95 %. The forma-tion of CO was not observed in any of the experiments,CO2 was the only observed total oxidation product andand no visible formation of deposited carbon was found.X =∑(Reaction Products)∑(Reaction Products) + unconverted Reactant(3)SA =Product A∑(Reaction Products)(4)3. Results and DiscussionIn Table 1 an overview over the target and the actualloading of Li and the Gd or Fe-content, the BET surfacearea and the present phases, detected via XRD is given.The intended and the loading of Li, Fe and Gd of theprepared catalysts agree very well. The ratio TM/Li isin both cases near 1 and the BET surface area does notvary too strongly.XRD patterns of the Gd-Li/MgO sample indicatethe formation of Gd2O3 besides the main phase MgO,shown in Figure 2. Thus, a low solubility of Gd3+ in2Table 1: Li, Gd and Fe-content of the different catalysts, the ratio dopant to Li, the specific surface area, determined before reaction, the samplecolor and the detected phases.Target-Content Measured-Content Atom Ratio BET Color XRD PhasesCatalyst Li [at%] Dop. [at%] Li [at%] Dop. [at%] [Dop./Li] [m2/g]MgO - - - - - 34.4 white MgOGd/MgO - 0.7 - 0.7 - 37.0 white MgO, Gd2O3Fe-Li/MgO 0.5 0.5 0.5 0.5 1.0 29.4 light yellow MgOGd-Li/MgO 0.7 0.7 0.7 0.8 1.1 16.7 white MgO, Gd2O3MgO can be assumed, which could be caused by thelarger ionic radius of Gd3+ of 94 pm (coordination num-ber 6) compared to the radius of Mg2+ of 72 pm (coor-dination number 6). No such restrictions were observedfor Fe3+ (ionic radius: 55 pm for coordination number6). The Fe-Li/MgO sample showed solely the XRDpeaks of the MgO phase indicating an entire incorpora-tion of the Li and Fe ions in MgO. The peak positions ofthe MgO pattern were marginally affected. An overviewof the detected phases is shown in Table 1.Figure 2: XRD patterns of MgO, Fe-Li/MgO and Gd-Li/MgO (bottomto top).EPR spectra of Gd-Li/MgO samples exhibit the char-acteristic powder pattern of Gd3+ in its S = 7/2 spin state.Replacing Mg2+ substitutionally, the local site symme-try significantly restricts the number of spin Hamiltonparameters as was described by Abraham et al. for asingle crystal of MgO [27]. For co-doping, local Gd3+ -Li+ clusters are anticipated. Such an arrangement wouldbreak the local site symmetry, introducing for instanceotherwise forbidden second rank tensor elements. Forthe Gd-Li/MgO sample, a small but significant devia-tion from the single crystal parameter set [27] is found,as expected being caused by co-doping with Li. Com-paring with Gd/MgO powder samples, an almost iden-tical EPR pattern is observed (see Figure 3). Appar-ently, the preparation and sintering process used inducesFigure 3: EPR spectra of Gd/MgO (blue trace) and Gd-Li/MgO (redtrace) taken at 80 K. The narrow spikes can be attributed to traceamounts of Mn2+.Figure 4: EPR spectrum of Fe-Li/MgO taken at 80 K. The dotted lineis a least squares fit resulting in B40 = 5 MHz and B44 = 30 MHz, quiteclose to the values of 5.16 MHz and 25.6 MHz, determined by Bolduet al. from single crystals data [26].3distortions from perfect cubic symmetry, which in turnalso lead to the appearance of the observed additionalspin Hamilton parameters. Using pulsed EPR insteadof conventional cw detection, a true EPR absorptionspectrum can be detected instead of a field derivativepattern. Using this detection method, additional broadsignals covering a much larger field range from 220 to480 mT could be recorded exclusively for the co-dopedsample, indicative of a significant distortion of the localsymmetry. We ascribe this pattern as resulting from theformation of next neighbor correlated pairs. A detailedanalysis will be presented in a forthcoming publication.Cw EPR signals observed for the Fe-Li/MgO powdersamples can be fitted by using spin Hamilton parametersvery similar to the parameter set reported by Boldu etal. for Fe-doped single crystals of MgO [26], as shownin Figure 4. This indicates that the topology of the Fesite is not significantly disturbed by adjunct Li ions. Be-cause of very fast spin relaxation, no pulsed EPR spectracould be detected. For this reason no effect of co-dopingcould be detected by EPR for this compound.The CH4 conversion, the selectivity for C2 and CO2(CO formation was not observed) for all 3 materials isshown in Figure 5. The catalytic performance of pureMgO is low, however, it does not deactivate signifi-cantly.Co-doping with charge compensating Fe leads to asignificantly increased CH4 conversion, however, to-tal oxidation prevails. Using Gd instead for chargecompensation, an initial activity comparable with Fe-Li/MgO at a significantly higher C2 selectivity, how-ever the XRD results indicate a formation of Gd2O3 be-side Gd-MgO solid solution. This could be responsiblefor the higher activity, as Gd2O3 exhibits a considerableOCM activity [28]. However, this catalysts also suffersfrom deactivation. A steady state has not been reachedwithin 16 h.4. Summary and ConclusionCo-doping of Li/MgO catalysts with charge compen-sating ions was performed with a constant atomic ra-tio of Li to dopant of 1:1. These aliovalent ions turnedout to occupy Mg2+ sites in the lattice. The solubilityof Gd3+ in MgO turned out to be limited. No such re-strictions were observed for Fe3+. The anticipated Li-stabilization by co-doping Li/MgO with Fe3+ and Gd3+apparently seems to take place. An indication of the ex-pected coupling with charge compensating neighboringLi+ was detected via EPR for the Gd-Li/MgO sample.Therefore, the application potential of dopants as EPRspin sensors for solids has been demonstrated.Figure 5: The CH4 conversion (top) and the C2 selectivity (middle)and the selectivity for CO2 (CO formation was not observed) forMgO, Fe-Li/MgO and Gd-Li/MgO.4The co-doping results in active catalytic materials,however, deactivation is not suppressed. Doping withFe leads to a significantly increased CH4 conversion,however, total oxidation is dominating. Gd-Li/MgOshowed an improved performance with respect to con-version and selectivity, however, the activity might bedue to the formation of Gd2O3 on the MgO, as detectedvia XRD.AcknowledgmentThis work is part of the Cluster of Excellence “Uni-fying Concepts in Catalysis” coordinated by the Tech-nische Universita¨t Berlin. Financial support by theDeutsche Forschungsgemeinschaft (DFG) within theframework of the German Initiative for Excellence isgratefully acknowledged. We thank Prof. Thomas forthe permission to use his XRD diffractometer.References[1] S. Arndt. Stability of Lithium doped Magnesium Oxide and ZincOxide Catalysts for the Conversion of Natural Gas. PhD thesis,Technical University Berlin, 2010.[2] S. Arndt, U. Simon, S. Heitz, A. Berthold, B. Beck, O. Go¨rke,J.D. Epping, T. Otremba, Y. Aksu, E. Irran, G. Laugel,M. Driess, H. Schubert, and R. Schoma¨cker. Li-doped MgOFrom Different Preparative Routes For The Oxidative CouplingOf Methane. submitted to Top. Catal.[3] S. Arndt, G. Laugel, S. Levchenko, R. Horn, M. Baerns,M. Scheﬄer, R. Schlo¨gl, and R. Schoma¨cker. Li/MgO - TheDrosophila Catalyst for Methane Oxidative Coupling. A Criti-cal Assessment. submitted to Catal. Rev. Sci. Eng.[4] A.G. Anderson and T. Norby. Liquid Phases in Li:Mgo asstudied by thermoanalytical Methods, Electron Microscopy, andElectrical Conductivity Measurements. Catal. Today, 6:575–586, 1990.[5] F.P. Larkins and M.R. Nordin. The Effects of Transition MetalOxides on the Methane Oxidative Coupling Activity of dopedMgO Catalysts I. Zinc and Manganese. J. Catal., 130:147–160,1991.[6] S.J. Korf, J.A. Roos, L.J. Veltman, J.G. van Ommen, and J.R.H.Ross. Effect of Additives on Lithium Doped Magnesium OxideCatalysts used in the Oxidative Coupling of Methane. Appl.Catal, 56:119–135, 1989.[7] R. Mariscal, J. Soria, M.A. Pena, and J.L.G. Fierro. Featuresof Li-Mn-MgO Catalysts and Their Relevance in the OxidativeCoupling of Methane. J. Catal., 147:535–543, 1994.[8] V.R. Choudhary, A.M. Rajput, D.B. Akolekar, and V.A. Se-leznev. Oxidative Conversion of Methane to C2-Hydrocarbonsover Lithium, Manganese, Cadmium and Zinc promoted MgOCatalysts I. Catalyst Performance in Presence of Free Oxygen.Appl. Catal, 62:171–187, 1990.[9] V.R. Choudhary, S.D. Sansare, A.M. Rajput, and D.B. Akolekar.Oxidative Conversion of Methane to C2-Hydrocarbons over Li,Mn, Cd and Zn promoted MgO Catalysts II: Performance offresh and reoxidised catalysts in the absence of free oxygen.Appl. Catal, 69:187–200, 1991.[10] T. Kanno and M. Kobayashi. The difference in the modifica-tion effects of Li on two MgO samples of higher and lower Mncontents. J. Mater. Sci. Lett., 16:126–127, 1997.[11] J.J. Zhang, X.G. Yang, Y.L. Bi, and K.J. Zhen. Role of LiIon in Li-Mn-MgO Catalyst for Oxidative Coupling of Methane.Catal. Today, 13:555–558, 1992.[12] F.P. Larkins and M.R. Nordin. Physico-Chemical and CatalyticProperties of Doped MgO Catalysts. Stud. Surf. Sci. Catal.,81:249–251, 1994.[13] D.J. McNamara, S.J. Korf, K. Seshan, J.G. van Ommen, andJ.R.H. Ross. The Effect of Nb2O5 and ZrO2 Additions on theBehavior of Li/MgO and Li/Na/MgO Catalysts for the OxidativeCoupling of Methane. Can. J. Chem. Eng., 69:883–890, 1991.[14] G.C. Hoogendam, K. Seshan, J.G. van Ommen, and J.R.H.Ross. Oxidative coupling of methane over doped Li/MgO cata-lysts. Catal. Today, 21:333–340, 1994.[15] S.J. Korf, J.A. Ross, J.A. Vreeman, J.W.H.C. Derksen, J.G. vanOmmen, and J.R.H. Ross. A Study of the Kinetics of the Ox-idative Coupling of Methane over a Li/Sn/MgO catalyst. Catal.Today, 6:417–426, 1990.[16] A.N.J. van Keulen, G.C. Hoogendam, K. Seshan, J.G. van Om-men, and J.R.H. Ross. The Role of Tin in Li/Sn/MgO Catalystsfor the Oxidative Coupling of Methane. J. Chem. Soc., Chem.Commun., 18:1546–1547, 1992.[17] R.H. Nibbelke, J. Scheerova´, M.H.J.M. de Croon, and G.B.Marin. The Oxidative Coupling of Methane over MgO-BasedCatalysts: A Steady-State Isotope Transient Kinetic Analysis.J. Catal., 106:106–119, 1995.[18] E.P.J. Mallens, J.H.B.J. Hoebink, and G.B. Marin. An Inves-tigation of the Oxygen Pathways in the Oxidative Coupling ofMethane over MgO-Based Catalysts. J. Catal., 160:222–234,1996.[19] K. Nagaoka, T. Karasuda, and K.I. Aika. The Effect of SnO2Addition to Li/MgO Catalysts for the Oxidative Coupling ofMethane. J. Catal., 181:160–164, 1999.[20] Y.L. Bi, K.J. Zheng, Y.T. Jiang, C.W. Teng, and X.G. Yang.Catalytic Oxidative Coupling of Methane over Alkali, AlkalineEarth and Rare Earth Metal Oxides. Appl. Catal, 39:185–190,1988.[21] S. Bartsch, J. Falkowski, and H. Hoffmann. Catalyst Develop-ment for Oxidative Methane Coupling. Catal. Today, 4:421–431, 1989.[22] S. Bartsch and H. Hoffmann. Investigations on a Ce/Li/MgO-Catalyst for the Oxidative Coupling of Methane. Catal. Today,6:527–534, 1990.[23] A. Santos, C. Finol, J. Coronas, M. Mene´ndez, and J. Santa-marı´a. Reactor Engineering Studies of Methane Oxidative Cou-pling on a Li/MgO Catalyst. Stud. Surf. Sci. Catal., 81:171–176,1994.[24] A. Santos, M. Mene´ndez, and J. Santamarı´a. Oxidative couplingof methane over Li/Sn/MgO catalysts. Use of a fluidized bedreactor at low gas velocities. Stud. Surf. Sci. Catal., 107:373–378, 1997.[25] P.J. Gellings and H.J.M. Bouwmeester. Solid state aspects ofoxidation catalysis. Catal. Today, 58:1–53, 2000.[26] J.L. Boldu´, E. Munoz, Y. Chen, and M.M. Abraham. EPR powerpattern analysis for cubic sites of Fe3+ in MgO. Journal ofChemical Physics, 80:574–575, 1984.[27] M.M. Abraham, L.A. Boatner, Y. Chen, J.L. Kolopus, and R.W.Reynolds. Cubic-Site EPR Spectra of Eu2+ and Gd3+ in MgOSingle Crystals. Physical Review B, 4:2853–2857, 1971.[28] O. Forlani and S. Rossini. Rare earths as catalysts for the ox-idative coupling of methane to ethylene. Mater. Chem. Phys.,31:155–158, 1992.5

Li/MgO with spin sensors as catalyst for the oxidative coupling of methane

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Li/MgO with spin sensors as catalyst for the oxidative coupling of methane

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