The oxidative coupling of Methane could take place at 723 K over the La2O3 catalyst prepared by the precipitation method, which was about 100 K lower than the startup temperature over the commercial La2O3 catalyst. Under the conditions of 723 K, CH4/O-2 molar ratio of 3, and GHSV = 7 500 ml/(g . h), the conversion of methane and selectivity for C-2 hydrocarbons reached 26.6% and 40.8%, respectively. The characterization results showed that there was only hexagonal La2O3 over the two catalyst samples, but the catalyst prepared by precipitation had a larger specific surface area and a distinct desorption peak of adsorbed O-2 at 598 K

Wan Huilin

Wang Lihua

Weng Weizheng

Yi Xiaodong

万惠霖

伊晓东

翁维正

CHINESE JOURNAL OF CATALYSIS (CHINESE VERSION)

Xiamen University Institutional Repository

Article ID : 025329837 (2006) 0820653203 Communication : 653～655Received Date : 2006205220. 　First Author : WAN G Lihua , female , born in 1973 , PhD student .Corresponding Author : WEN G Weizheng. Tel : (0592) 2186569 ; E2mail : wzweng @xmu. edu. cn.WAN Huilin. Tel : (0592) 2186569 ; E2mail : hlwan @xmu. edu. cn.Foundation Item : Supported by the Ministry of Science and Technology of China (2005CB221408) , the National Natural Science Foundationof China (20433030 , 20021002 , and 20423002) , and the Key Scientific Project of Fujian Province , China (2005HZ0123)Low Temperature Oxidative Coupling of Methane over a La2O3 CatalystWANG Lihua , YI Xiaodong , WENG Weizheng , WAN Huilin( Depart ment of Chemist ry , S tate Key L aboratory of Physical Chemist ry of Solid S urf aces , Institute ofPhysical Chemist ry , Xiamen U niversity , Xiamen 361005 , Fujian , China)Abstract : The oxidative coupling of methane could take place at 723 K over the La2O3 catalyst prepared by the precipitation method ,which was about 100 K lower than the startup temperature over the commercial La2O3 catalyst . Under the conditions of 723 K ,CH4/ O2 molar ratio of 3 , and GHSV = 7 500 ml/ (g·h) , the conversion of methane and selectivity for C2 hydrocarbons reached2616 % and 4018 % , respectively. The characterization results showed that there was only hexagonal La2O3 over the two catalystsamples , but the catalyst prepared by precipitation had a larger specific surface area and a distinct desorption peak of adsorbed O2 at598 K.Key words : lanthanum oxide ; precipitation method ; low temperature ; oxidative coupling of methaneCLC number : O643 　　　Document code : A　　Since Otsuka et al [1 ] reported that the rare earthoxides ( REOs) , with the exception of Ce , Pr , andTb oxides , showed very high C2 selectivity for theoxidative coupling of methane (OCM) under the con2dition of extremely high CH4/ O2 ratios , REOs havebeen extensively studied as OCM catalysts[2 - 8 ] . Inprevious research , the REO catalysts for OCM havebeen generally studied at high temperatures ( ≥873K) . Little attention has been paid to a low tempera2ture ( < 773 K) OCM reaction , except for some au2thors who have tried using the reaction heat as an en2ergy source[9 ] . Recently , Miao et al [10 ] used a Sr2TiO3 catalyst with a larger specific surface area for theOCM reaction , which showed better low temperatureactivity than that of a conventional Sr TiO3 catalyst .Therefore , our focus is on preparing a catalyst for theOCM reaction at low temperatures. In this study , weprepared a La2O3 catalyst using a precipitation methodand investigated the catalytic performance for the lowtemperature OCM reaction. The specific surface areaand the capacity for adsorbed oxygen of the catalystswere investigated , and the correlation between thecapacity for adsorbed oxygen and the catalytic activityat low temperatures was interpreted.　　The catalyst was prepared by precipitationmethod. After dissolving lanthanum nitrate ( ChinaMedicine ( Group) Shanghai Chemical Reagent Cor2poration , AR , 015 g/ ml ) and N H4 HCO3 ( ChinaMedicine ( Group) Shanghai Chemical Reagent Cor2poration , AR , 015 g/ ml) , respectively , in a smallamount of distilled water , the N H4 HCO3 solution wasdropped into the lanthanum nitrate solution slowly( n (N H4 HCO3)∶n (lanthanum nitrate) = 313) , withsome N H3·H2O also dropped into the mixed solutionto make p H = 9. Then the mixture was stirred atroom temperature. After stirring for 6 h , the precipi2tate was filtered , washed , dried at 383 K , and thencalcined in static air for 6 h at 1 073 K (La2O3 (A) ) .A commercial La2O3 catalyst ( China Medicine( Group ) Shanghai Chemical Reagent Corporation ,99199 %) was directly calcined at 1 073 K withoutany pretreatment (La2O3 (B) ) . All the catalysts usedfor the OCM reaction were pelletized , crushed , andsieved to 40- 80 mesh.　　X2ray diff raction (XRD) analysis was carried outto determine the catalyst st ructure using a PanalyticalX’pert Pro MPD X2ray diff ractometer with Cu Kα(λ= 01154 06 nm) radiation. A continuous scan modewas used to collect 2θ data from 20°to 70°with a01016°step increment . The voltage and current were40 kV and 30 mA , respectively. The specific surfacearea of the catalysts was measured by N2 adsorption(BET method) on a Tristar 3000 apparatus. Tem2perature2programmed desorption of adsorbed O2 (O22第 27 卷 第 8 期 催 　化 　学 　报 2006 年 8 月Vol. 27 No. 8 Chinese Journal of Catalysis August 2006TPD) was performed in a quartz reactor (i. d. 5 mm)from 298 to 1 073 K at a linear heating rate of 10 K/min in a flow of helium (991999 % purity , Linde Gas( Xiamen) Ltd , 20 ml/ min) . Before collecting theTPD data , the catalyst (400 mg) packed in the reac2tor was pretreated i n sit u with air (Linde Gas ( Xia2men) Ltd , 20 ml/ min) at 1 073 K for 30 min andthen was cooled down from 1 073 to 298 K in air. At298 K , the catalyst was purged by helium (20 ml/min) for 90 min in order to remove the physically ad2sorbed O2 . The products desorbed during TPD weredetected online by a Balzers OmniStar 200 quadrupolemass spectrometer.　　The catalytic activity of the La2O3 catalysts issummarized in Table 1. The OCM reaction can takeplace at 723 K. The conversion of CH4 and C2 selec2tivity over the La2O3 catalyst prepared by precipita2tion (La2O3 (A) ) were 2616 % and 4018 % , respec2tively , under the conditions of n (CH4) ∶n (O2) = 3and GHSV = 7 500 ml/ ( g·h) . But under the sameconditions , there was no C2 hydrocarbon product until823 K over the commercial La2O3 catalyst ( La2O3(B) ) . These results indicated that the startup tem2perature over La2O3 (A) was about 100 K lower thanthat of La2O3 (B) . The methane conversion and selec2tivity for C2 over La2O3 (B) were less than those ofLa2O3 (A) catalyst in the temperature range of 823 -873 K. The higher activity at a low temperature overLa2O3 (A) catalyst can be att ributed to its larger spe2cific surface area (12 m2/ g) , which favors the disper2sion of active centers.Table 1 　Catalytic performance of the La2O3 catalystsCatalystA BETm2/ gT/ KConversionof CH4 ( %)Selectivity ( %)CO CO2 C2H4 C2H6 C2Yield ofC2 ( %)La2O3 (A) 12. 0 673 1. 2 0 　 100 　 0 　 0 　 0 　 0 　723 26. 6 16. 7 42. 5 21. 6 19. 2 40. 8 10. 8773 28. 8 13. 8 43. 0 23. 9 19. 3 43. 2 12. 4823 29. 3 11. 3 43. 3 25. 7 19. 7 45. 4 13. 3873 29. 4 9. 9 45. 9 25. 5 18. 7 44. 2 13. 0La2O3 (B) 0. 9 723 0 0 0 0 0 0 0773 2. 3 0 100 0 0 0 0823 27. 5 19. 9 37. 1 21. 3 21. 7 43. 0 11. 8873 28. 4 18. 2 38. 3 22. 4 21. 1 43. 5 12. 4Reaction conditions : CH4/ O2 molar ratio = 3 , GHSV = 7500 ml/ (g·h) , catalyst = 400 mg.The data was obtained after 30 min on stream.Fig 1 　XRD patterns of different La2O3 catalyst samples(1) La2O3 (A) , (2) La2O3 (B)　　The specific surface area of these two catalystsamples was different , but the XRD patterns asshown in Fig 1 were approximately the same. Theposition of the diff raction peaks fitted well with thatof hexagonal La2O3 ( 2θ = 261094°, 291111°,291947°, 391489°, 461035°, 521132°, 531890°,551397°, 551896°, 601459°, 621261°, and671307°) . There was no evidence of a new phase.Fig 2 　O22TPD prof iles of different La2O3 catalyst samples(1) La2O3 (A) , (2) La2O3 (B)　　Fig 2 shows the results of TPD of adsorbed oxy2gen from the two La2O3 catalyst samples. The O22456 催 　化 　学 　报 第 27 卷TPD from the La2O3 catalyst prepared by precipita2tion (La2O3 (A) ) showed a distinct peak at 598 K ,but there was no signal observed on the commercialLa2O3 catalyst (La2O3 (B) , 019 m2/ g) . These resultsindicated that La2O3 (A) with a larger specific surfacearea (12. 0 m2/ g) had more defects on the catalystsurface[11 ] that adsorb and activate O2 . Thus , theLa2O3 (A) catalyst showed excellent activity at lowtemperature.　　In summary , a La2O3 catalyst prepared by pre2cipitation method had a larger specific surface areaand showed excellent activity at a low temperature.The startup temperature was about 100 K lower thanthat of a commercial La2O3 catalyst . The result wasinterpreted as due to the difference in the dispersionof the active centers and the capacity for adsorbedoxygen.References1 　Otsuka K , Jinno K , Morikawa A. Chem L ett , 1985 , (4) :4992 　Long R Q , Wan H L . A ppl Catal A , 1997 , 159 (122) : 453 　Lin Ch H , Campbell K D , Wang J X , Lunsford J H. JPhys Chem , 1986 , 90 (4) : 5344 　Campbell K D , Zhang H , Lunsford J H. J Phys Chem ,1988 , 92 (3) : 7505 　Lacombe S , Zanthoff H , Mirodatos C. J Catal , 1995 , 155(1) : 1066 　Kus S , Otremba M , Taniewski M. Fuel , 2003 , 82 (11) :13317 　Lacombe S , Geantet C , Mirodatos C. J Catal , 1994 , 151(2) : 4398 　Maitra A M , Campbell I , Tyler R J . A ppl Catal A ,1992 , 85 (1) : 279 　Wang J X , Chou L J , Zhang B , Song H L , Yang J , ZhaoJ , Li Sh B. Catal Com m un , 2006 , 7 (2) : 5910 　Miao J W , Song G H , Fan Y N , Zhou J . Chin J Catal ,2005 , 26 (7) : 53311 　Long R Q , Wan H L . J Chem Soc , Faraday T rans ,1998 , 94 (8) : 1129氧化镧催化剂上的低温甲烷氧化偶联反应王丽华 , 　伊晓东 , 　翁维正 , 　万惠霖(厦门大学化学系 , 固体表面物理化学国家重点实验室 , 物理化学研究所 , 福建厦门 361005)摘要 : 采用沉淀法制备了氧化镧催化剂 ,并考察其催化低温甲烷氧化偶联反应的性能. 催化剂性能测试结果表明 ,在 723 K ,CH4/ O2 摩尔比 = 3 和 GHSV = 7 500 ml/ (g·h) 的条件下 ,甲烷的转化率和 C2 的选择性分别达 2616 %和 4018 % , 比商品化氧化镧催化剂的启动温度低 100 K. 催化剂的表征结果表明 ,沉淀法制备的氧化镧催化剂与商品化的氧化镧催化剂都为六方晶相的氧化镧 ,但前者具有较大的比表面积 ,并且在 598 K处出现一个明显的 O2 脱附峰.关键词 : 氧化镧 ; 沉淀法 ; 低温 ; 甲烷氧化偶联( Ed YHM)556第 8 期 王丽华 等 : 氧化镧催化剂上的低温甲烷氧化偶联反应

Low temperature oxidative coupling of methane over a La(2)O(3)catalyst

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Low temperature oxidative coupling of methane over a La(2)O(3)catalyst

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