STUDIES OF MULTICOMPONENT COMPLEX OXIDE CATALYSTS FOR THE OXIDATIVE COUPLING OF METHANE Ⅳ CATALYSTS LiLa_(1-x)Mn_xO(2-λ)(X=0-1)

A series of catalysts LiLa_(1-x)Mn_xO_(2-λ)(x=0—1)were examined for the oxidative couplingof methane.A high C_2 yield of 22％ was obtained.The structures and physico-chemical properties ofsamples have been studied by means of XRD,IR,XPS and SEM.It was found that some complex ox-ides could be formed as X value changed,among them the K_2NiF_4-type oxide La_2Mn_(1-y)Li_yO_(4-λ)was theactive phase for methane coupling,while the Li_2MnO_3 and LiMn_2O_4 were the active phases for completeoxidation of re.ethane.The increase of catalytic activity for methane coupling was attributed to theincrease of the amount of active site,namely,oxygen vacancy in oxide La_2Mn_(1-y)Li_yO_(4-λ)     

甲烷氧化偶联Ti-La-Li系混合氧化物催化剂

研究了Ti-La-Li三元氧化物的组成、结构及其对甲烷氧化偶联反应的催化性能；用XRD、IR、XPS和SEM等方法对催化剂进行表征，结果表明：在LiTi_xLa_(1-x)O_2系列催化剂中，随x值的不同，可生成LaTi_(1－y)Li_yO_(3-λ)、Li_2TiO_3、La_(0.66)TiO_(2.993)、La_2O_3和Li_(1.33)Ti_(1.66)O_4几种物相，其中，钙钛矿到三元复合氧化物LaTi_(1-y)Li_yO_(3-λ)是甲烷氧化偶联反应的主要活性相，活性位Li~＋-O~－-Ti~(3＋)的形成是活性提高的主要原因．Li_2TiO_3和La_(0.66)TiO_(2.993)是深度氧化活性相，而Li_(1.33)Ti_(1.66)O_4既无偶联活性，也无深度氧化活性．     

STUDIES OF MULTICOMPONENT COMPLEX OXIDE CATALYSTS FOR THE OXIDATIVE COUPLING OF METHANE Ⅲ CATALYSTS LiLa_(1-X)Ni_XO_(2-λ)(X=0-1)

The catalysts LiLa_(1-x)Ni_xO_(2-λ),(x=0-1)were examined systematically for the oxidativecoupling of methane,and the active phase La_2Ni_(1-y)Li_yO_(1-λ)was identified by means of XRD,IR,SEMand so on.A high C_2 yield of 21.5％ with a selectivity of 67.2％ at 1023 K was obtained.The activitytowards C_2 formation arised with the increasing of the amount of active sites,oxygen vacancies Vo,which were formed in the complex oxides La_2Ni_(1-y)Li_yO_(1-λ).     

A STUDY OF METHANE OXIDATIVE COUPLING OVER La-Th-O MIXED OXIDE CATALYSTS

The compositions and structures of La-Th-O mixed oxides as well astheir catalytic activity for methane oxidative coupling have been studied bymeans of XRD,XPS and IR.The results indicated that the solid solution ofLa_xTh_(1-x)O_(2-&)with the fluorite structure was formed in the range of 0≤x≤0.6and it was the main active phase for oxidative coupling of methane(OCM).     

STUDIES OF MULTICOMPONENT COMPLEX OXIDE CATALYSTS FOR OXIDATIVE COUPLING OF METHANE VII.EFFECT OF CALCINATION TEMPERATURE ON Ti-La-Li PEROVSKITE-TYPE OXIDE CATALYSTS

The correlations of the calcination temperature.structure and catalvtic activity of the LiLa_（0.5）Ti_（0.5）O_(2-(?)) catalvsts with main phase and major active phase of perovskite-type trmary complex oxide LaTi_(l-y)Li_yO_(3-λ）m the Oxidative coupling of methane(OCM)have been studied The surface and bulk structures of the catalysts were characterized by means of XRD,XPS. IR.BFT and so on. The results clearly indicated that the effect of calcination temperature on the activity for the oxidative coupling of methaneis twofold On one hand.high calcination temperature is favoragble for Lisubstitution for Ti~(3 )into the lattice of LaTiO_3 and the production of moreoxygen vacancies at which active oxygen species are formed However,excessivelv high calcmation temperature makes Li~ substitution for Ti~(3 )less due to a httle change of structure or phases of the catalvst On the otherhand,the conversion of CH_4 drops because of the decrease of surface area,when the calcination temperature is raised.     

SOLID STATE CHEMISTRY OF La_(1-x)Li_(x)MnO_3 AND RELATED COMPOUND

A series of perovskite type oxides La_(1-x)A_(x)MnO_3(x=0.1 for A=Li,Na,K;x=0.1～0.5 for A=Li)have been prepared by impregnation.Experimental results showed that the substitution of La~(3 ) by Li~ inLaMnO_(3 ?) greatly increased the selectivity to ethane and ethylene for theoxidative coupling of methane.Temperature-programmed desorption of oxygenproved the presence of oxygen vacancies in the oxide lattice.The higher Mn~(4 )/Mn_t ratio in oxide made the formation of oxygen vacancies easier on the oxidesurface.The general formula of the oxides is La_(1-x)Li_(x)Mn＇V＇_(y)O_(3-y),V=vacancy.     

OXIDATIVE COUPLING OF METHANE OVER SrO-La_2O_3/CaO CATALYSTS Ⅳ. EFFECT OF HOT SPOT TEMPERATURE AND CO_2 IN FEED

The phenomenon of hot spot temperature and the effect of CO_2 in thefeed on the catalytic performance were studied in the oxidative coupling ofmethane over the La_2O_3/CaO(LC)and SrO-La_2O_3/CaO(SLC)catalysts.There was an abrupt change in the activity of the catalyst when the reactiontemperature increased from 873 to 923 K if no inert gas was used in the feed.This is mainly caused by the total oxidation of methane,releasing the combus-tion heat and resulting in a local over-heated spot in the catalyst bed.Then thereaction of oxidative coupling of methane was initiated.It is found that the ad-dition of CO_2 in the feed caused a severe effect on the catalytic performances ofthe SLC catalysts,especially if the SrO content was high.But no effect couldbe detected for the LC catalyst.The deactivation of the SLC catalyst is mainlyattributed to the formation of SrCO_3,which is inactive or much less active thanthe surface SrO species and is difficult to dissociate at the reaction condition.     

STUDY ON OXYGEN ADSORPTION AND CATALYTIC PROPERTIES OF TiO_2-BASED CATALYSTS

The active oxygen species and active sites for MOC(methane ox-idative coupling)over TiO_2-based catalysts are studied by ESR method.Oxy-gen chemisorption on TiO_2,La/TiO_2 and Li/TiO_2 was measured by dynamicESR technique from 295 K to 105 K.The process of oxygen adsorption onTiO_2 is discussed.The spectra show that O_2~- ions exist largely on TiO_2 but lit-tle on Li/TiO_2 due to the decrease of surface density of electrons caused bythe substitution of Li~+ for lattice Ti~(4+) in Li/TiO_2.O~- was not detected on allsamples.Both O_2~- and O~- are not likely to be the active species in MOC reac-tion.Two triplet peaks in ESR spectra of Li/TiO_2 are attributed to Ti~(3+)(Ⅱ)and Ti~(3+)(Ⅲ)of low coordination numbers,which are active in the presenceof oxygen.Sites of low coordinated Ti~(3+) or Ti~(4+) are suggested to be responsi-ble for the formation of active oxygen species.     

STUDY ON OXID TIVE COUPLING OF METHANE Ⅱ.CATALY TIC PERFORMANCE AND CHARACTERIZATION OF TiO_2-BASED CATALYSTS

The catalytic performance of Mn/TiO_2,La-Mn/TiO_2,Li-La-Mn/TiO_2 etc for the oxidetive coupling of methane(OCM)was investignted.Thecatalysta were cheracterised with X-ray diffraction(XRD)and X-rayphotoelectron spectroscopy(XPS).The results reveal that catalyst Li-La-Mn/TiO_2 exhibits high activity and C_2 selectivity;Ti in this catalyst exists asTi~( 4) state;Li can promote the formation of lanthanum tituate via theinteraction between La and TiO_2;the formed La_2Ti_2O_7 and La_4Ti_9O_(24) aredistributed in the inner surface layer and Mn exists in outer surface layer in lowvalence states.The high activity and C_2 selectivity of catalyst Li-La-Mn/TiO_2are intimately related to the valence states of Mn,Li,La and theirdistribution on the catulyst surface layer.     

THE ROLE OF LATTICE AND GASEOUS OXYGEN ON THE OXIDATIVE COUPLING OF METHANE STUDIED BY A PULSE REACTION TECHNIQUE

A comparative study of the role of lattice and gaseous oxygen in theoxidative coupling of methane over Na~ /Cao and Ca_xSr_(1-x)TiO_3 perovskiteoxide catalysts was performed by using a pulse reaction technique with CH_4,O_2 and mixtures of CH_4and O_2.It was found that there is an oxygen species onthe Surface of Cao which is active only for the total oxidation of methane at thereaction temperature.These oxygen species.once reacted with methane andconsumed,can be regenerated by the adsorption of oxygen molecules from thegas phase or by the migration of the lattice oxysen from the bulk onto thesurface.In contrast,no such oxygen species and no reaction can be detected bymethane pulsed over Ca_xSr_(1-x)TiO_3perovskite oxide catalysts.Na~ can heavilysuppress the non-selective oxygen species on the surface,and When 5mol%Na~ was incorporated onto the surface of Cao,the sample showed no reactivityto methane.When oxygen exists in the gas phase,or the pulse containsmethane and oxygen,the reaction over(Cao is mai     

OXIDATIVE COUPLING OF METHANE OVER SrO-La_2O_3/CaO CATALYSTS Ⅲ.THE REACTION MECHANISM

In the previous kinetics study,we found that there are differencesin the reaction behavior and the pathway of CO_x formation between the LC andSLC Catalysts.In this paper,several mechanisms have been tested.It is con-firmed that the oxidative coupling of methane over the LC follows bangmuir-Hinshelwood mechanism and an adspecies of oxyen in its molecular form(O_2) is the active species.While the reaction over the SLC catalyst follows amodified Rideal-Redox mechanism and an adspecies of oxygen in its atomicform (O~-) is the active species.     

Li_2SO_4-Mn_xO_y/TiO_2 CATALYST FOR ETHYLENE SYNTHESIS BY CATALYTIC OXIDATION OF METHANE

The synthesis of ethylene by catalytic oxidation of methane over Li-2SO_4-Mn_xO_y/TiO_2 wascarried out in a conventional fixed bed flow reactor.A 246-h life test of the catalyst under the condi-tions of normal pressure,1073K,2g of catalyst,and flow rate(ml/min)CH_4:O_2:N_2=15:6:15showed that the C_2 yield and C_2 selectivity were 30.8％(C_2H_4 19.8％)and 82.5％ respectively.XRDpatterns of this multicomponent catalyst revealed that the support TiO_2 was mostly converted intofutile,MnTiO_3 was the main active component,LiMnO_2,Li_2TiO_3 and TiOSO_4 were the active andoxidation-controlling components.     

OXIDATIVE COUPLING OF METHANE AT LOWER TEMPERATURE

A simple review is given to the recent work of the oxidative couplingof methane at lower temperature. One emphasis is on the different systems oflow-temperature catalysts under conventional CH_4/O_2 cofeed conditions,andanother is on the investigations of low-temperature oxidative coupling ofmethane in the presence of steam in the feed. Other approaches,e.g.,oxidative coupling of methane at elevated pressure and moderate temperature,preparing ethylene by oxidative coupling reaction of methane on laser-activatedsolid surface, are also concerned.     

OXIDATIVE COUPLING OF METHANE OVER Na_2O-La_2O_3/BaCO_3 CATALYSTS

The oxidative coupling of methane over various alkali metal oxidespromoted LaO_2O_3/BaCO_3 catalysts and the effects of Na_2O content on the performance of Na_2O-La_2O_3/Ba_2CO_3 catalyst have been studied.It was found thatNa_2O promoted La_2CO_3/BaCO_3 catalyst showed advantages of high CH_4 conver-sion,C_2 selectivity and C_2H_4/C_2H_6 ratio,Na_2O might affect the properties ofcatalyst through elctronic and geometric effuts.The highest C_2yield(19.0一20.6%)over Na_2-9wt%La_2O_3/BaCO_3 catalysts was obtained in the Na_2Ocontent range of 1.0—3.0%.Effects of reaction conditions on OCM over3wt% Na_2O-9wt%La_2CO_3/BaCO_3 catalyst were also investigated.The catalystswere characterized by BET,TPD,and XRD.TPD studies on 3wt% Na_2O-9wt%La_2O_3/BaCO_3 catalyst demonstrated that CO_2,CH_4.O_2could bestrongly on the catalyst.This might be related to the activation of CH_4 and theformation and reseneration of active oxygen soecies.     

STUDIES OF MULTICOMPONENT COMPLEX OXIDE CATALYSTS FOR THE OXIDATIVE COUPLING OF METHANE Ⅱ CATALYSTS LiLn_(0.3)Ni(0.7)O_(2-λ)(Ln:Y,La,Ce,Pr,Nd,Sm,Eu)

A comparative study on the behaviour of a series of multicomponent oxides LiLn_(0.3)Ni_(0.7)O_(2-λ)(Ln:Y,La,Ce,Pr,Nd,Sm,Eu)in the oxidative coupling of methane has been made.All of the samplesbut Ln=Ce were found to give higher C_2 yield.The increasing in activity was attributed to the forma-tion of multicomponent complex oxides composed of Ni,Ln and Li ions and/or the synergism ofdifferent phases.     

STUDY OF THE CATALYTIC TECHNOLOGY IN THE OXIDATIVE COUPLING OF METHANE

The temperature of the catalyst bed in the oxidative coupling ofmethane would rise and be higher than the wall temperature when the amountof catalyst, the space velocity and the ratio of oxygen to methane wereincreased. Various aspects of the catalytic technology including the thickness ofthe catalyst bed, the mode of catalyst charge, the ratio of CH_4 to O_2 and thespece velocity were studied. An optimum temperature of the catalyst bed forhigher methane conversion and C_2 selectivity was investigated. It was foundthat the burning of methane in the gas phase to form CO occurred at certaintemperatures with some ratios of oxygen to methane. Additionally, the effectof adding water to the reaction feed gas was studied.     

THE STRUCTURE OF ACTIVE SITE AND THE ROLE OF BIVALENT CATION IN FERRITE-FERRIC OXIDE CATALYSTS FOR OXIDATIVE DEHYDROGENATION OF BUTENES

The relations between catalytic activities and compositions of the systems Mg_xZn_(1-x)Fe_2O_4, Co_xZn_(1-x)Fe_2O_4, Co_xMg_(1-x)Fe_2O_4, Cd_xZn_(1-x)Fe_2O_4 and Mg_xZn_(1-x)Fe_2O_4·0.17Fe_2O_3 have been studied by flow differential reactor, XRD, and derivative IR techniques. It has been found that the appropriate normal-inverse spinel combination substantially increases the activity for the title raction. The structure of the active site and the role of the bivalent cation in the title catalyst have been inferred.     

OXIDATIVE COUPLING OF METHANE OVER Sr-Pb-O AND Sr-Pb-O/γ-Al_2O_3 CATALYSTS

Two series of Sr-Pb-O and Sr-Pb-O/γ-Al_2O_3 catalysts of differentcomposition were investigated for the oxidative coupling of methane.Theeffects of reaction conditions on the catalytic behavior have been studied.Catalyst characterization was carried out by XRD,TPR,TPD and XPS tech-niques.It was found that a strong interaction took place between Sr-Pb-Oand γ-Al_2O_3 so that Sr_3Al_(32)O_(51)phase and a small amount of PbAl_2O_4 formedin the bulk and the surface was mainly enriched with Pb.A 30 wt% Sr-Pb-O/γ-Al_2O_3 catalyst gave a 30.4% methane conversion and a 50% C_2 selec-tivity at 750℃ and with a feed gas composition of CH_4:O_2=2.5:1.Theenrichment of Pb on the surface may be responsible for the higher catalyticactivity and selectivity.     

STUDY OF CATALYTIC BEHAVIOR OF PEROVSKITE-TYPE MIXED OXIDES La_(1-x)Sr_xFeO_(3-λ)(Ⅰ)——RELATIONS OF SOLID STRUCTURE AND DEFECT CHARACTER WITH CATALYTIC BEHAVIOR

According to X-ray structure analysis, it was found that there exist oxygen disordered defects in the crystal lattice of the samples without exception as x was changed from 0 to 1 in La_(1-x)Sr_xFeO_(3-x). With the doping of Sr~(2+), the crystal lattice was distorted extremely, even the crystal type varied. The results of TG showed a large amount of lattice oxygen released under high temperatures as x>0.5 in the samples. After comparison between the experiment results of the three systems La_(1-x)(Ca, Sr)_xMO_(3+x) (M=Mn, Fe and Co), it was concluded that the occurrence of solid defects is affected by the geometric factor and the catalytic activity is related to the concentration and the ordering extent of oxygen defects.     

OXIDATIVE COUPLING OF METHANE OVER La-Ba-O MIXED OXIDE CATALYSTS

The oxidative coupling of methane was examined over various rare earth-alkaline earthoxide complex catalysts in which the La-Ba-Na-O catalyst gave a higher C_2-yield more than 20% andshowed a considerable stability without a decrease during the period of a 500-h test.With a higherCH_4/O_2 ratio of 4:1 in mole,a C_2-selectivity of 65% and a C_2-yield of 19% were obtained.Thecatalyst combination effect was discussed.