Oxidative catalytic cracking of n-butane to lower alkenes

Applied Catalysis A:General206(2001)237–244Oxidative catalytic cracking of n-butane to lower alkenesover layered BiOCl catalystNorihito Kijima a,∗,Koichi Matano b,Masao Saito b,Tomo Oikawa b,Tomohiro Konishi b,Hiroyuki Yasuda a,Toshio Sato a,Yuji Yoshimura aa National Institute of Materials and Chemical Research,1-1Higashi,Tsukuba,Ibaraki305-8565,Japanb Japan Chemical Industry Association,3-2-4Kasumigaseki,Chiyoda-ku,Tokyo100-0013,JapanReceived16February2000;received in revised form27April2000;accepted1May2000AbstractCatalytic performance of BiOCl catalyst has been investigated for the oxidative cracking of n-butane to lower alkenes. Results are compared with those of isostructural LaOCl and SmOCl catalysts.On the BiOCl catalyst,oxidative dehydrogena-tion and combustion were major reactions at∼500◦C;above∼600◦C oxidative cracking reaction to produce ethylene and propylene began to be significant.At the temperature as low as625◦C,52%yield of overall C2–C4alkenes was achieved,and then decreased above this temperature.The conversions of n-butane and oxygen were maintained constants within6h periods at the reaction temperatures of550and600◦C.However,both conversions decreased slightly with time-on-stream at650◦C. BiOCl catalyst shows rather high alkenes’selectivity and suppresses deep oxidation reactions for the oxidative cracking of n-butane,as compared to the LaOCl and SmOCl catalysts.The overall yield of C2–C4alkenes for the BiOCl catalyst is higher than the values for the LaOCl and SmOCl catalysts over the temperature range of500–600◦C.©2001Elsevier Science B.V. All rights reserved.Keywords:Oxidative cracking;n-Butane;Alkenes;BiOCl catalyst;LaOCl catalyst;SmOCl catalyst1.IntroductionPresently,light alkenes are produced by the ther-mal cracking of naphtha as well as ethane and ethane–propane mixtures.This process is strongly endothermic and hence energy-intensive,for it has to be carried out at high reaction temperatures around 800–900◦C.Furthermore,other unwanted reactions leading to large quantities of coke also occur at such high reaction temperatures.From the viewpoint of the exhaustion of fossil fuels and the destruction of global environment,we must establish simple and ∗Corresponding author.Fax:+81-298-61-4532.E-mail address:nkijima@nimc.go.jp(N.Kijima).energy-saving processes of converting naphtha to use-ful basic chemicals,such as ethylene and propylene. Oxidative catalytic cracking of higher alkanes to lower alkenes would be an informative reaction to investigate the feasibility of catalytic converting of naphtha to lower alkenes with co-feeding oxygen as oxidants.Previously,we have reported oxidative cracking of n-butane over rare earth oxides[1].La2O3, Nd2O3,Sm2O3,Eu2O3,and Gd2O3catalysts showed a rather high yield of C2and C3alkenes(20–28%) even though the reaction temperature was as low as 600◦C.On the contrary,CeO2,Pr6O11,and Tb4O7 catalysts exhibited poor alkene selectivity,where the major product was carbon dioxide.The catalytic per-formances of these rare earth oxides in the oxidative0926-860X/01/$–see front matter©2001Elsevier Science B.V.All rights reserved. PII:S0926-860X(00)00598-6238N.Kijima et al./Applied Catalysis A:General206(2001)237–244Fig.1.Schematic drawing of the crystal structure of A OCl(A=Bi, La,Sm)of a tetragonal PbFCl-type structure with space group of P4/nmm.cracking of n-butane were similar to those observed in oxidative coupling of methane(OCM)reaction[2–4]. These detailed experimental results will be published elsewhere[5].It has been known that oxychlorides show good catalytic activities and selectivities in the OCM re-action[6–10].The isostructural BiOCl,LaOCl,and SmOCl have a tetragonal PbFCl-type structure with space group of P4/nmm(No.129).This structure is known to have a layered structure which is con-structed by the combination of the chloride ion layer and the metal oxygen layer as shown in Fig.1[11]. In an earlier report[6],Williams and coworkers have compared the catalytic performances of A OCl(A=Bi, La,Sm)catalysts in the OCM reaction and reported that the BiOCl catalyst shows high catalytic activity, high selectivity to C2-hydrocarbons,and exception-ally large C2H4/C2H6ratio.Khan and Ruckenstein [9]have reported the catalytic performance of im-proved BiOCl/Li2CO3/MgO catalyst.The Li2CO3is believed to have a stabilizing effect on the catalyst. They have indicated that not only Cl but also Bi en-hances C2selectivities on the basis of compositional analyses by X-ray photoelectron spectroscopy(XPS). Recently,Ueda and coworkers[7,8,12–15]have re-ported the catalytic performances of several layered bismuth oxychloride catalysts in the OCM reac-tion and oxidative dehydrogenation of ethane(ODE)reaction.They have proposed that the surface chlorine on the bismuth oxychlorides is a key element in the C–H activation of alkanes,since the activity strongly depends on the type of layered structures and the content of chlorine sheets[8].We expect that the BiOCl catalyst has an oxidative cracking activity as well as a high oxidative dehy-drogenation activity in the oxidation of n-butane.In this paper,we report the catalytic performance of the BiOCl catalyst in the oxidative cracking of n-butane to lower alkenes.We also compare the catalytic per-formances of the BiOCl,LaOCl,and SmOCl cata-lysts in this reaction.A brief report has been given for the catalytic performance of the BiOCl catalyst in the oxidative cracking of n-butane[16].2.Experimental2.1.Catalyst preparation and characterization BiOCl catalyst was a commercial product(Wako Pure Chemical Ind.Ltd.).LaOCl catalyst was pre-pared from La2O3(99.9%)and LaCl3·7H2O(99.9%).The La2O3was highly unstable in air due to the for-mation of a surface coating of La(OH)3.Hence,the weight of the La2O3was measured as soon as pos-sible after calcination at1000◦C for3h.The dried La2O3was mixed with LaCl3·7H2O in an agate mor-tar.After mixing,the powder was calcined in air at 800◦C for6h.SmOCl catalyst was prepared by heat treatment of SmCl3·6H2O(99.5%)in static air at 600◦C for6h.Powder X-ray diffraction patterns of each fresh catalyst confirmed the single-phase spec-imens.All the diffraction peaks of these fresh cata-lysts were indexed in terms of tetragonal symmetry with space group of P4/nmm.The lattice constants were a=3.89and c=7.38Åfor BiOCl,a=4.12and c=6.86Åfor LaOCl,and a=3.97and c=6.70Åfor SmOCl,respectively.The BET surface areas of each fresh catalyst were6.3m2/g for BiOCl,2.5m2/g for LaOCl,and6.8m2/g for SmOCl,respectively.The powder X-ray diffraction patterns of the cat-alysts were measured by a conventional diffracto-meter using diffracted beam monochromatized Cu K␣radiation.The BET surface areas of the catalysts were deter-mined by the single point BET method by measuringN.Kijima et al./Applied Catalysis A:General206(2001)237–244239the adsorption of nitrogen(30mol%balance He)at liquid nitrogen temperature.2.2.Catalytic testThe catalytic tests were carried out at atmospheric pressure in a conventionalfixed-bed continuous-flow reactor.The reactor was a quartz tube(10mm i.d.) whose temperature was controlled by an electric fur-nace.The reaction temperature was measured by a chromel-alumel thermocouple located in the cata-lyst bed.The high-purity gas feed,which contained n-C4H10(99.9%),O2(99.9%),and N2(99.9%),was used in the experiments.Theflow rate of these gases was controlled by a thermal massflow controller. The products of the reaction were separated with the columns of PEG-1500,MS-5A,SP-1700,Shimalite Q,Porapak Q,Porapak N,and Porapak QS,and an-alyzed by an on-line gas chromatograph(Shimadzu GC-8A)equipped with a thermal conductivity detec-tor(TCD)for inorganic gases and aflame ionization detector(FID)for hydrocarbons.N2in the feed was used as an internal standard for calculations of con-version and selectivity.The conversion and selectivity were calculated based on the reacted n-butane.The carbon balances were nearly100%.Powder catalysts were pressed binder-free,crushed, and sieved to the particle size of75–150␮m.The catalysts were diluted with inert quartz particles (150–710␮m particle size)and mounted in the mid-dle of the reactor.To minimize the extent gas phase reactions,the volume in the reactor except for catalyst was packed with the quartz particles.The catalytic zone was heated up to500◦C in aflow of O2and kept at500◦C for1h before catalytic test.Then,the reactant mixture(n-C4H10/O2/N2)was introduced into the reactor.A blank test,using only the quartz particles,was run at the same reaction condition. 3.Results and discussion3.1.Oxidative cracking of n-butane over BiOCl catalyst3.1.1.Effect of reaction temperatureFig.2shows the effects of the reaction temperature on the conversions and selectivities to products forthe Fig.2.Effects of reaction temperature on(a)the conversions of n-C4H10(240N.Kijima et al./Applied Catalysis A:General206(2001)237–244 always the major products and a small amount ofmethane was observed.Any oxygen and chlorine in-serted products were not detected within our resolutionbelow625◦C and extremely small amounts of uniden-tified products were observed at650◦C.Oxidative dehydrogenation and combustion weremajor reactions at∼500◦C;above∼600◦C oxida-tive cracking reaction began to be significant.Asthe temperature increased,the selectivity to ethyl-ene increased and the selectivities to propylene andbutenes decreased.Selectivity to both carbon monox-ide and carbon dioxide increased with increase inthe temperature.The CO/CO2ratio increased slightlyat high temperatures,suggesting that n-butane con-verts into carbon monoxide in a gas-phase reactionat the high temperatures in the presence of molec-ular oxygen.The yield of overall C2–C4alkenes(ethylene+propylene+butenes+butadiene)reached amaximum for a value of52%at the temperature aslow as625◦C,and then decreased above this tem-perature.At the temperature of650◦C,34%yield ofethylene plus propylene was achieved.3.1.2.Durability of the catalytic performanceFig.3shows the time-course of conversions ofn-butane and oxygen at550,600and650◦C overtheFig.3.Time course of conversions of n-butane(᭹)and oxygen (᭺)at650◦C;n-butane(᭿)and oxygen(ᮀ)at600◦C;and n-butane(᭜)and oxygen(᭛)at550◦C in the oxidative cracking of n-butane over the BiOCl catalyst(3.0g).Feed composition is n-C4H10/O2/N2=2/8/40and totalflow rate is50cm3/min.Fig.4.Powder X-ray diffraction patterns of BiOCl catalyst.The diffraction patterns are measured(a)before the reaction and after the reactions at(b)550◦C for6h,(c)600◦C for6h,and(d) 650◦C for6h,respectively.Peaks with solid circles are those of Bi24Cl10O31.BiOCl catalyst.As shown in Fig.3,when the reaction temperatures were550and600◦C,the conversions of n-butane and oxygen were maintained constants within6h periods.The alkenes’selectivities did not change under the above conditions.On the other hand,both the conversions as well as the selectivities to alkenes decreased slightly with time-on-stream at the reaction temperature of650◦C.Fig.4presents the powder X-ray diffraction pat-terns of the BiOCl catalyst before the reaction and after the reactions at550◦C for6h,600◦C for6h, and650◦C for6h,respectively.As seen in Fig.4(b), BiOCl catalyst is stable for long periods under the reaction at550◦C.On the other hand,small amounts of new Bi24O10Cl31phases appeared after the reac-tions at600and650◦C,although the main structures were still present.The lattice constants a and c of the BiOCl catalysts did not change after the reactions. BiOCl catalyst shows fairly poor stability in our reac-tion above600◦C,which is consistent with the results of the OCM reaction around700◦C[6–8].The Bi24O10Cl31is known to have a layered struc-ture and to show less activity and less selectivity toN.Kijima et al./Applied Catalysis A:General 206(2001)237–244241C 2hydrocarbons than the BiOCl catalyst in the OCM reaction [7,8].According to the literature [8],the cat-alytic performance of BiOCl catalyst decreases with formation of Bi 24O 10Cl 31phase in the OCM reaction at 720◦C.The catalytic activity and selectivity in the oxidation of n -butane can be affected by the formation of Bi 24O 10Cl 31phase.At the reaction temperature of 650◦C,the catalytic activity decreases slightly with time-on-stream and the Bi 24O 10Cl 31formation was observed after the reaction.On the other hand,BiOCl catalyst exhibited a stable activity at 550◦C and any extra phases were not detected by XRD measurement.Although the Bi 24O 10Cl 31phase was formed during the reaction at 600◦C,BiOCl catalyst also showed a stable activity.This used catalyst provided similar cat-alytic performances to the fresh one at 550◦C.A vari-ation of catalytic performance would not be observed in our experiment since the amount of the Bi 24O 10Cl 31formation was extremely small.3.1.3.Oxygen partial pressure dependencyFig.5shows the partial pressure effect of oxygen on the converted rate of n -butane and the yield of overall C 2–C 4alkenes in the oxidative crackingofFig.5.Partial pressure effects of oxygen on the converted rate of n -butane (᭹)and the yield of overall C 2–C 4alkenes (᭺)in the oxidative cracking of n -butane at 600◦C over the BiOCl catalyst (3.0g).Total flow rate is 50cm 3/min and partial pressure of n -butane is 4.1kPa (const.).All data are collected within 1h reaction.The solid curvature shows a fitted Langmuir’s curve.n -butane at 600◦C over the BiOCl catalyst.We con-cluded that the catalytic effect would only be observed with co-feeding oxygen since the n -butane conversion at P O 2=0kPa was less than 5%,which was compa-rable with the value of the thermal pyrolysis without catalyst.As seen in Fig.5,the oxygen partial pressure has a positive effect for the converted rate of n -butane,which indicates that the presence of gas phase oxygen plays an important role in the reaction.The converted rate of n -butane increased abruptly with increasing the partial pressure of oxygen and then gradually reached to a constant value at higher oxygen partial pressures.When the n -butane conversion increased,the selectiv-ity to alkenes decreased and the selectivity to CO x in-creased (not given in the figure).The yield of overall C 2–C 4alkenes increased with increasing the partial pressure of oxygen and above P O 2=15kPa,reached about 38%,as shown in Fig.5.An attempt has been made to fit the P O 2ver-sus P O 2/V C 4dependence to the Langmuir’s formula without dissociative adsorption:V C 4=abP O 2(1+aP O 2)(1)where V C 4is the converted rate of n -butane,P O 2is the partial pressure of oxygen,and a and b are constants.The experimental result can be reproduced well by Langmuir’s formula without dissociative adsorption.The solid curvature in Fig.5shows a fit with the Lang-muir curvature.Wealso attempted to make the fit of the P O 2versus P O 2/V C 4depended on Langmuir’s formula with dissociative adsorption.However,the result could not be reproduced by the formula with dissociative adsorption.This result suggests that the rate-determining step is the reaction of n -butane with O 2molecular species adsorbed on the BiOCl cata-lyst.Similar partial pressure dependencies have been observed in oxidative cracking of n -butane over rare earth oxides [17].3.1.4.Reaction pathsFig.6presents the variation of the selectivities with the conversion of n -butane at 600◦C over the BiOCl catalyst.As shown in Fig.6,the selectivity to butenes (1-,2-cis -,and 2-trans -butene)decreases when the n -butane conversion increases,suggesting that consecutive reactions take place.The selectiv-ity to butadiene increases with the increase in the242N.Kijima et al./Applied Catalysis A:General206(2001)237–244Fig.6.Variation of the selectivities to C2H4(᭹);C3H6(᭡);C4H8 (᭢);C4H6(᭿);CH4(᭜);CO(ᮀ);CO2(᭺)with the conversion of n-butane at600◦C over the BiOCl catalyst.Feed composition is n-C4H10/O2/N2=2/8/40and totalflow rate is50cm3/min.All data are collected within1h reaction.n-butane conversion,reaching a maximum for a value of about15%and then decreases slightly at the high conversions.Thus,on the BiOCl catalyst,butadiene can be considered a secondary and unstable product. The selectivities to ethylene and propylene are nearly conversion-independent,whereas they change slightly at the high conversions.Ethylene and propylene as well as butenes would mainly be considered primary products in this reaction,since these selectivities do not approach zero at the lower conversions.The se-lectivities to methane are extremely small and the CH4/C3H6formation ratios do not have1,as seen in Fig.6.Therefore,the rate of methane formation would not be related that of propylene formation.The selectivities to carbon monoxide and carbon dioxide increased with the increase in n-butane conversion, suggesting that consecutive reactions occur.These tendencies of the variation of the selectivity were also observed even though the reaction temperature was as low as550◦C.The(C2H4+C3H6)/(C4H8+C4H6) ratio increased with increase in the temperature. The catalytic performances of the oxychloride cata-lysts as well as rare earth oxide catalysts in the oxida-tive cracking of n-butane are similar to those observed in the OCM and ODE reactions.Therefore,it can be considered that the oxidative cracking of n-butane would be initiated by hydrogen abstraction and butylradical formation.In particular at high reaction tem-peratures,the butyl radical would decompose to giveethylene and ethyl radical or propylene and methylradical due to␤-scission.At lower temperatures,thebutyl radical can be mainly dehydrogenated to pro-duce butene.The hydrogens abstracted from n-butaneare oxidized by molecular oxygen to give water.parison of catalytic performances of AOCl(A=Bi,La,Sm)catalystsThe results of the catalytic performance of the A OCl(A=Bi,La,Sm)catalysts are summarized in Table1.The catalytic performances of these catalysts at600◦Cdid not change within6h OCl and SmOClcatalysts showed high structural stabilities in this re-action on the basis of the X-ray analyses.As shownin Table1,BiOCl catalyst shows rather high alkenes’selectivity and suppresses deep oxidation reactionsfor the n-butane oxidation,as compared to the LaOCland SmOCl catalysts.This tendency is consistentwith the results of the OCM reaction over the A OCl(A=Bi,La,Sm)catalysts[6,7].The overall yield ofC2–C4alkenes for the BiOCl catalyst is higher thanthe values for the LaOCl and SmOCl catalysts overthe temperature range of500–600◦C under a chosenreaction condition(n-C4H10/O2/N2=2/8/40cm3/min) since the LaOCl and SmOCl catalysts show pooralkenes’selectivity and high CO x selectivity,whereasthey show rather high activity.While LaOCl and SmOCl catalysts show highstructural stabilities,BiOCl catalyst shows fairly poorstability,thus a desorption of a small amount of chlo-rine from the catalyst would occur at high reactiontemperatures.Khan and Ruckenstein[9]have re-ported that the desorption of chlorine from the BiOClcatalyst takes place at600–620◦C.The chlorine atomin gas phase is known to serve as a chain carrier in ahomogeneous reaction.Ahmed and Moffat[18]havesuggested that the chlorine in gas-phase may playsuch a role in the ODE reaction.On the other hand,it is reported that there is no correlation between therate of chlorine loss from the catalyst and its activityin the ODE reactions over a Li+–MgO–Cl−catalyst[19]and a SrBi3O4Cl3catalyst[14],respectively.Inour experimental results of TPD analyses,the de-sorptions of the chlorine from the A OCl(A=Bi,La,N.Kijima et al./Applied Catalysis A:General206(2001)237–244243 Table1Catalytic performance of A OCl(A=Bi,La,Sm)catalysts in the oxidative cracking of n-butane aCatalyst Surfacearea(m2/g)Weight(g)Temperature(◦C)Conversion(%)Selectivity(%)n-C4H10O2C2H4C3H6C4H8C4H6CH4CO CO2BiOCl 6.3 3.0500 3.6 1.9 6.29.532.720.90.0 6.523.93.055015.5 6.616.811.333.315.80.4 5.516.93.060050.719.028.310.119.915.10.97.118.5 LaOCl 2.50.1500 6.4 5.9 2.94.010.30.00.031.950.60.155017.414.2 6.9 5.99.40.60.132.844.60.160038.323.813.78.3 6.50.70.530.439.8 SmOCl 6.80.01500 5.0 4.9 4.911.320.20.00.013.350.90.0155014.011.010.011.316.30.60.214.846.80.0160027.417.019.213.612.9 1.10.715.836.7 Blank b5000.4 4.80.07.473.00.00.00.09.6550 1.3 4.111.212.159.1 2.20.00.012.3600 3.9 3.915.012.941.3 4.3 1.19.215.0a Reaction conditions:totalflow rate=50cm3/min;feed composition:n-C4H10/O2/N2=2/8/40.Data collected within1h reaction.b Reactor packed only with the quartz particles was used.Sm)catalysts did not take place below550◦C at least. The powder X-ray diffraction patterns of these spent catalysts also indicated that these catalysts were sta-ble for long periods under the reaction at550◦C.As seen in Table1,it is clear that the BiOCl catalyst shows higher alkenes’selectivity than the LaOCl and SmOCl catalysts at the reaction temperature of550◦C, which seems to be intrinsic feature of the BiOCl cat-alyst.Therefore,it is probable that the difference of catalytic performances among A OCl(A=Bi,La,Sm) catalysts is ascribed to active centers on the surface of the catalysts rather than a homogeneous reaction due to gas-phase chlorine,although the contribution of the gas-phase reaction can not be ignored,in particular at high reaction temperatures.The BiOCl alone shows rather high alkenes’selec-tivity,although every A OCl(A=Bi,La,Sm)catalyst has surface chorine.More recently,the substitution ef-fects of Bi3−x La x ClO4system in the OCM reaction [8]and Bi3−x La x SrCl3O4system in the ODE reaction [13,15]have been reported.On both catalyst systems, the activities of alkanes and CO x formations increase with increase in La content.Thus,it can be considered that the lanthanum and samarium ions in the oxychlo-ride catalysts can form activated oxygen species for the deep oxidation of n-butane.However,the nature of active centers on the oxychloride catalysts is not well understood yet at the molecular level.Further in-vestigations are required to verify and clarify the na-ture of active centers as well as reaction mechanism in oxidation of alkanes on the oxychloride catalysts. 4.SummaryIn summary,the following aspects should be noted: 1.BiOCl catalyst has oxidative cracking activity toproduce ethylene and propylene as well as oxida-tive dehydrogenation activity,in particular at high reaction temperatures.2.The yield of overall C2–C4alkenes(ethylene+propylene+butenes+butadiene)reached a maxi-mum for a value of52%at625◦C,and then de-creased above this temperature.At the reaction temperature of650◦C,34%yield of ethylene plus propylene was achieved.3.The conversions of n-butane and oxygen weremaintained constants within6h periods at the reaction temperatures of550and600◦C.How-ever,both conversions decreased slightly with time-on-stream when the reaction temperature was 650◦C.4.BiOCl catalyst shows rather high alkenes’selectiv-ity and suppresses deep oxidation reactions for the oxidative cracking of n-butane,as compared to the LaOCl and SmOCl catalysts.244N.Kijima et al./Applied Catalysis A:General206(2001)237–244AcknowledgementsWe would like to thank Dr.T.Hayakawa,Dr.K. 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