Partial Oxidation of Methane to Synthesis Gas over Hexaaluminates LaMAl11O19-δ catalysts

Available online at www.sciencedirect.comScienceDirectP|Joumal of NarualGa CheristyJournal of Natural Gas Chemistry 16(2007)60- 63SCIENCE PRESSArticlePartial Oxidation of Methane to Synthesis Gas overHexaaluminates LaMAl11O19- -8 catalysts .Zhanlin Xu*，Lina Zhao，Fang Pang，Liang Wang，Chunyan NiuCollege of Chemistry, Jilin Normal University, Siping 136000, Jilin, China[ Manuscript received August 14, 2006; revised November 27, 2006 ]Abstract: A series of M-substituted hexaaluminates LaMAl1O19-δ (M=Fe, Co, Ni, Mn, and Cu)were prepared and characterized by XRD, XPS, TPR and TGA techniques, respectively. They exhibiteddiferent reducibility and catalytic activity for partial oxidation of methane (POM) to synthesis gas. Amongthe LaMAl1O19. -δ samples, LaNiAl11O1g- δ showed the best catalytic activity for the topic reaction andselectivity for synthesis gas at 780 °C for 2 h. The conversion of CH4 was over 99.2%, and the productselectivity for both CO and H2 was above 90.3%.Key words: partial oxidation; CH4; synthesis gas; hexaaluminates; LaMAluO1g- -6; Fe; Co; Ni; Mn; Cu1. Introductioninvestigated mainly focused on the transition metal(e.g. Ni, Co, and Fe) [5-7] and the noble metal (e.g.The process of partial oxidation of methaneRu, Rh, Pd, Pt, and Ir) supported catalysts [1,2,8,9].(POM) to synthesis gas has received worldwide at-Ni-based catalysts received much more attention fortention in recent years [1- .3], because this process hastheir high activity and low cost. However, a reactionseveral advantages over the steam reformation or theon Ni-based catalysts starts only at higher tempera-dry reformation: (1) POM reaction is a mild exother-ture [10], and the formation of carbon deposition ismic reaction, and therefore the industrial processmuch faster than that on the noble metals [11].based upon POM is energy saving. (2) The molarTherefore, it is very important to improveratio of H2 to CO in the resultant synthesis gas isthe state of supported transition metal catalystsclose to 2, when POM reaction is carried out in thein order to overcome these disadvantages.Instoichiometric ratio. This kind of synthesis gas isthe present work, the authors describe a simplean ideal feedstock for downstream processes, such aspreparation method of M-substituted hexaaluminatesmethanol synthesis and the Fischer- Tropsch reaction,LaMAl1O19-5 (M=Fe, Co, Ni, Mn, and Cu), inetc. (3) POM can be carried out under the conditionwhich M ions, as active components, were inlayed inof very high gas hourly space velocity (GHSV), whichthe hexaaluminate lattices to substitute part of therequires less investment and less production scale toAl ions. The reducibility and catalytic activity ofachieve the same or larger capacity for the process.LaMAlO19- -8 were investigated.POM reaction to synthesis gas can be carried outwithout the employment of a catalyst, but it occurs2. Experimentalat very high temperature, usually when heated above中国煤化工1127 °C [4]. The employment of catalyst can facilitate2.1the light-off of POM and promote it to reach ther-MHCNMHGmodynamic equilibrium. The catalysts which wereA series of hexaaluminates， LaMAl1O19- -δ* To whom correspondence should be addressed. Te: 0434- 3291890; E mail: xzl@jnu.edu.cn.This work was supported by the Education Department of Jilin Province and Science and Technology office of Siping Municipality..Journal of Natural Gas Chemistry Vol. 16 No. 1 200761(M=Fe, Co, Ni, Mn, and Cu), were prepared as fol-reaction, the catalyst was reduced at 900 °C in a flowlows: La(NO3)3-.6H2O, transition metal nitrates, andof 10%H2-90%Ar mixture gas for 30 min. The exitAl(NO3)3.9H2O were dissolved in dstilled water withgas was analyzed by a gas chromatograph (Shimadzua molar ratio of 1:1:11, respectively, then the aqueousGC-8A) equipped with a TCD, using a Porapak-Qsolution was slowly added to a polyethylene glycol-and a 5 A molecular sieve column. The catalytic ac-isopropyl alcohol solution under magnetic stirring.tivities were investigated in a temperature range fromThe mixture was evaporated to dryness at 80。C,400C to 800 °C.and stored in an oven to remove polyethylene gly-col and also to decompose the nitrates. After being3. Results and discussionground into fine powder, the sample was calcined at400 °C for 2 h, fllweld by calcination at a tempera-3.1 XRD studiesture higher than 1200 °C for 3 h.Figure 1 shows the XRD patterns of M2.2 Catalyst characterizationsubstituted hexaaluminates LaMA11O19- 8 (M=Fe,Co, Ni, Mn and Cu) before reaction.The crystalline structures of hexaaluminatesLaMAl1O19- 8 (M=Fe, Co, Ni, Mn, and Cu) weredetermined by X- ray powder diffraction (XRD) (Shi-madzu XD-3A diffractometer) using Ni filter and CuKa radiation, at 30 kV and 20 mA. Diffraction peaksrecorded for the 20 value range of 15°- 80° have been. mn JMuuu Mhenused to identify' the structure of the samples. Thebinding energy of samples was measured by X-rayphotoelectron spectroscopy (XPS) (V G ESCA MarkII) using Al Ka radiation. The measurements wereoperated at a pass energy of 50 eV and a step size ofMhmmMmon0.05'eV. The reducibility of samples was characterized306C80by a temperature programmed reduction (TPR) tech-201(° )nique using 0.2 g of catalyst embedded in a fixed-bedFigure 1. XRD patterns of M-substituted hexaalumi-quartz tube with an inner diameter of 8 mm. Beforenates LaMAlO19-8reaction, the samples were heated at a temperature of(1) LaCuAl1019- -8, (2) LaMnAlO1g -8, (3) LaCoAl1019. _8,300 °C for 30 min and then cooled to room tempera-(4) LaFeAlO19- -8, (5) LaNiAl11O19-8 .ture in Ar flow; subsequently, the reactor was heatedfrom room temperature to 1100 °C at a linear heatingIt is found that the series of samples exhibit al-rate of 20 °C /min in 10%H2- 90%Ar mixture gas flowmost the same crystalline structure, namely, the sameat a rate of 30 ml/ min. The effluent gases were passeddiffraction peaks (20). The characteristic diffractionthrough a dryer flll with 5 A molecular sieves, andpeaks of all samples are at 31.9， 33.80 and 35.99,then were analyzed using a gas chromatograph (Shi-respectively.' The crystalline structure of the hexaa-madzu GC-8A) equipped with a thermal conductivityluminates LaMAl11O19- -δ obtained hereby is consis-detector (TCD). The amount of carbon deposited ontent with that of the samples synthesized by the hy-the catalysts was determined by a thermogravimetricdrolysis of metal alkoxides method and the aerogel-analyzer (Perkin-Elmer TGA7).derived approach as reported in literature [12]. Fur-thermore, our experimental results confirmed that2.3 Catalytic activity teststhe structures of these hexaaluminates were extremelystable. After reaction at high temperature and afterThe reaction of POM was carried out under at-the TPR experiment, respectively, the peak positionsmospheric pressure in a tubular fixed-bed quartz re-(20) a中国煤化Iiples were still theactor with an inner diameter of 8 mm. The reactionsameaction (Figure 1)，gas consisted of CH4 and O2 with a molar ratio of 2:1indicaMYHC N M H Gucture of bexaalu-and a flow rate of 30 ml/min. A total of 0.2 g of cat-minates LaMAl1O19- -8 remains unchanged after re-alyst was fixed by two layers of quartz wool. Beforeaction, and only a part of Mo+ ions in the hexaalumi-62Zhanlin Xu et al./ Journal of Natural Gas Chemistry Vol. 16 No.1 2007nate lattices can be reduced to M0 at a high tempera-face elements La, Al, and 0 are almost not affectedture. These results show that M-substituted hexaalu-by the substituted-M, that is, their valence state isminates LaMAl1O19-8 (M=Fe, Co, Ni, Mn, and Cu)unchanged basically. Thus, based upon the bindingcan be used as catalysts for the reaction process ofenergy data, it can be concluded that their oxidationpartial oxidation of CH4 to synthesis gas.states are La3+, Al3+, and 02-, respectively. Basedupon the binding energy data of XPS, it can also be3.2 XPS resultsassured that the oxidation states of the substituted-M in hexaaluminate lattices are Ni2+, Fe3+, Co2+,Table 1 gives the binding energy of surface el-Mn2+ and Cu2+ , respectively. This result shows thatements of hexaaluminates LaMAl1O19- - 5 measuredthe substituted-M is inlayed in hexaaluminate latticesusing XPS. It is found that the binding energies of sur-with a stable valence state.Table 1. Binding energy of surface elements of hexaaluminates LaMAlO19-8Binding energy(eV)LaMAl1019-8 .MLa 3ds/2M 2p1/2Al 2pO 1sLaNiAl1O19- 8Ni835.6856.274.1LaFeAluO1g- δFe835.5711.873.9530.9LaCoAl11O19- -δCo835.474.2531.2LaMnA11019-8 .Mn836.0642.774.5531.5LaCuAl11019- 8Cu3.3 TPR characterizationin hexaaluminate lattices can be partly reduced toM0 at high temperature, and the reduction state M0As shown in Figure 2，the TPR profiles ofwas highly distributed on the surface of hexaalumi-LaMAl11019-8 (M=Fe, Co, Ni, Mn, and Cu) in-nate samples, and acts on the ions in hexaaluminatedicate that different M- substituted hexaaluminateslattices. M° is the active center for partial oxidationLaMAl1O19- 8 exhibit the reduction profiles withof methane to synthesis gas. It is suggested that this isdifferent shapes. LaCuAl11O19- -8 has two discon-the main reason for which M- substituted hexaalumi-nected reduced peaks, and the XRD and XPS analysisnate LaMAl11O19- 8 show different catalytic activityproved that the Cu ion inlayed in hexaaluminate lat-due to the differences in the substituted-M (M= =Fe,tices has only Cu2+, therefore, the reduced processCo, Ni, Mn, and Cu).of Cu2+ is from Cu2+ to Cu+ at the lower temper-atures between 650 °C and 750 °C, and from Cu+to Cu° at the higher temperatures between 750 °C to820 °C. LaFeAl11O19- 8 has similar reduced profiles;5)_the reduction process of Fe3+ is from Fe3+ to Fe2+at the lower temperature between 500 °C to 730 °C,4)and from Fe2+ to Fe0 at the higher temperature be-3)tween 750 °C to 900 °C. Both LaNiAl1O19- δ andLaCoAl1O19- δ have rather high reduction tempera-2)tures, and they begin to be reduced at about 750 °C,1)and exhibit reduction profiles with similar shapes.The reduction peak of LaMnAl11O19- 8 is wider than500700900that of other M substituted hexaaluminates, and itsTemperature(C)reduction temperature is between 550 °C and 850 °C.Figure 2. TPR profiles of M-substituted hexaalu-minates LaMAl11019-8From the TPR experimental results, it can be(1) LaMnAl1019- -8, (2) LaFeAl1O19-8, (3) LaCoAl019-8,seen that the series of M-substituted LaMAl019-8中国煤化工-gave different reduction temperatures and reductionprofiles with different shapes due to the difference of3.4YHCNMHGsubtituted-M (M=Fe, Co, Ni, Mn, and Cu). It isobserved that the substituted transition metal M6+The catalytic activity and selectivity of the re-Journal of Natural Gas Chemistry Vol. 16 No.1 200763duced hexaaluminates LaMAl1O19- -8 were examinedthe same reaction conditions. At the same time, thefor stoichiometric reaction of CH4 and O2 under at-Ni-subtituted hexaaluminate LaNiAl1O19-8 alsomospheric pressure, and the experimental results areshowed the best resistance to carbon, highest selec-given in Figure 3 and Figure 4, respectively.tivity for synthesis gas (CO and H2), and the amountof carbon deposition was only 1.37wt%, and the selec-100 Ftivity for synthesis gas (CO and H2) was above 90.3%80for the topic reaction at 780 °C for 2 h. The experi-60[mental results are shown in Figure 4.+ LaNiAl,Oas30 + LaCoAlnOIgs4. Conclusions25 τLaMnAIi,Owu+ LaCuAl,ORA series of M-substitutedhexaaluminatesLaMAl11O19- 5 (M=Fe, Co, Ni, Mn, and Cu) as15 Fnew catalysts for topic reaction have been preparedotthrough the decomposition of nitrates and oxides at4005060070000high temperature. The catalytic activity, reducibil-Reaction temperature (C)ity and selectivity for CO and H2 of the catalystsFigure 3. Effect of reaction temperatureonare strongly dependent on the substituted metalscatalytic activities of hexaaluminatesM in hexaaluminate lattices, and the reduced NLaMAl11019-。substituted hexaaluminate LaNiAl11019- 8 shows thebest catalytic activity for topic reaction and highest100 -selectivity for CO and H2.References30 上[1] Ashcroft A T, Cheetham A K, Foord J s, Green M(3y]L H, Grey C P, Murrell A J, Vernon P D F. Nature,0E1990, 344(6264): 3192] Torniainen P M, Chu X, Schmidt L D. J Catal, 1994,50 t146(1): 1[3] ZhuQ L, Zhao X T, Deng Y Q. J Natur Gas Chem,50 L2004, 13(4): 191400 450 500 550 600 650 700 750 800[4] Solbakken A. Stud Surf Sci Catal, 1991, 61: 447Temperature (C)5] Vermeiren W J M, Blomsma E, Jacobs P A. CatalFigure 4. Catalytic activity and selectivity of Ni-Today, 1992, 13(2-3): 427substituted LaNiAl11O19-6) Wang H Y, Ruckenstein E. J Catal, 2001, 199(2): 309(1) CH4 conversion, (2) seletivity for H2,(3) selectivity for CO[7]ZhuYR,LiZH,ZhouYH,WangHT.JNaturGasChem, 2005, 14(1): 1From Figure 3, it can be found that the different8}] Elmasides C, Kondarides D I, Neophytides s G,M-substituted hexaaluminates LaMAl1O19-8 ex-Verykios X E. J Catal, 2001, 198(2): 195[9] Hichman D A， Schmidt L D. Science, 1993,hibit different catalytic activities.Among the259(5093): 343LaMAl1O19- 8 samples, Ni- subtituted hexaalumi-[10] Choudhary V R, Prabhakar B, Rajput A M. J Catal,nate LaNiAl1O19- 8 showed the best catalytic activ-1995, 157(2): 752ity for the topic reaction, the conversion of CH4 was[11] Claridge J B, Green M L H, TsangS C, York A Pover 99.2% at 780 °C for 2 h, the catalytic activities ofE, Ashcroft A T, Battle P D. Catal Lett, 1993, 22(4):other M-substituted hexaaluminates LaMAl019-8.299were much lower than that of LaNiAl11O19- -8, and[1中国煤化工Cah, 1989 120(2):the conversion of CH4 was lower than 30% under:YHCNMHG