2014-Deactivation of ZSM-5 zeolite during catalytic steam cracking of n-hexane

Deactivation of ZSM-5zeolite during catalytic steam cracking of n -hexaneAritomo Yamaguchi a ,b ,⁎,Dingfeng Jin a ,Takuji Ikeda a ,Koichi Sato a ,Norihito Hiyoshi a ,Takaaki Hanaoka a ,Fujio Mizukami a ,Masayuki Shirai a ,ca Research Center for Compact Chemical System,National Institute of Advanced Industrial Science and Technology (AIST),4-2-1Nigatake,Miyagino,Sendai 983-8551,Japanb JST PRESTO,4-2-1Nigatake,Miyagino,Sendai 983-8551,JapancDepartment of Chemistry and Bioengineering,Faculty of Engineering,Iwate University,Ueda 4-3-5,Morioka,Iwate 020-8551,Japana b s t r a c ta r t i c l e i n f o Article history:Received 8April 2014Received in revised form 15May 2014Accepted 18May 2014Available online 7June 2014Keywords:ZSM-5n -Hexane steam cracking Dealumination Coke deposition PropyleneH-ZSM-5catalyst was initially active for the steam cracking of n -hexane to produce propylene and ethylene;however,the H-ZSM-5catalyst deactivated.Understanding the deactivation reason of the ZSM-5catalyst during the catalytic steam cracking is essential to develop the durable catalyst.It was revealed that the dealumination of ZSM-5occurred under the steam flow at the reaction temperature,from the characterization results of H-ZSM-5and pre-steamed H-ZSM-5by SEM,XRD,ICP,N 2adsorption,27Al MAS NMR,and NH 3-TPD.The coke deposition also occurred during the steam cracking of n -hexane as suggested by TG-DTA result of the used H-ZSM-5catalyst.The used H-ZSM-5catalysts were regenerated by calcination for a removal of coke deposition and the regenerat-ed catalysts were used for the n -hexane steam cracking again.The initial conversion over the regenerated H-ZSM-5was partially recovered,indicating that the H-ZSM-5catalyst was deactivated reversibly by the coke deposition and irreversibly by the dealumination during the n -hexane steam cracking.©2014Elsevier B.V.All rights reserved.1.IntroductionCatalytic cracking plays a signi ficant role in petroleum re finery pro-cesses.Fluid catalytic cracking (FCC)is widely operated in a worldwide commercial system,which can convert heavy naphtha into valuable products such as lique fied high octane gasoline and petroleum gases (LPG)[1–3].The catalysts for FCC process typically consist of Y-zeolite,active alumina,an inert binder,and an inert matrix.The catalysts are facing two problems to be overcome:dealumination of Y-zeolite and coke deposition [4].The combustion of the deposited coke is carried out to regenerate the catalysts in the regenerator vessel and it provides the heat to the FCC reactor,where the general FCC process operates without external heating.Thus,the feedstock is required to be convert-ed to some extent into coke to keep the heat balance.The dealumination of Y-zeolite can be suppressed by a hydrothermal treatment (ultra-stable Y,USY)[5,6]and a loading of rare earth [7,8].Light ole fins such as propylene and ethylene are key raw materials to produce plastics,pharmaceuticals and other chemicals,and their world-wide demands and consumptions are increasing [9,10].Recently,the propylene production from the FCC process is enhanced by ZSM-5additives [11,12]and accounts for almost 30%of worldwide propylene supply [13].On the other hand,huge energy is currently consumed in petrochemical industry to produce the light ole fins from naphtha by thermal steam cracking without catalysts at more than 1073K [14]and worldwide propylene is produced from steam cracking (ca.70%).Development of a catalysis technology to produce the light ole fins from naphtha is required for the decrease of reaction temperature (873–973K),leading to not only energy saving but also less carbon di-oxide emission.Several types of zeolites have been investigated as cat-alysts for the cracking [15–18].Among these zeolites,ZSM-5catalysts are effective for the naphtha cracking to provide high yields of the light ole fins [19–30];however,the deactivation of the ZSM-5catalysts is an issue of the greatest concern in commercial ole fin production from naphtha.The deactivation of ZSM-5catalysts is also mainly caused by dealumination of ZSM-5and coke deposition [4].In the case of cata-lytic cracking without steam flow,the deactivation of ZSM-5catalysts is mainly caused by the coke deposition [25–28].The deactivated catalyst due to the coke deposition can be regenerated by calcination on a labo-ratory scale;however,the coke deposition causes pressure drop and collapse of catalyst pellets on a commercial scale as well as the catalyst deactivation.Thus,the steam is used as an inexpensive diluent of naph-tha during the catalytic cracking for the decrease of the coke deposition [31];however,the catalysts are also deactivated by the dealumination of ZSM-5in the case of catalytic steam cracking [32–34].The deactiva-tion behavior of ZSM-5catalyst during the catalytic steam cracking of naphtha is essential to develop the catalyst life;however,contributionsFuel Processing Technology 126(2014)343–349⁎Corresponding author at:Research Center for Compact Chemical System,National Institute of Advanced Industrial Science and Technology (AIST),4-2-1Nigatake,Miyagino,Sendai 983-8551,Japan.Tel.:+81222373010;fax:+81222375226.E-mail address:a.yamaguchi@aist.go.jp (A.Yamaguchi)./10.1016/j.fuproc.2014.05.0130378-3820/©2014Elsevier B.V.All rightsreserved.Contents lists available at ScienceDirectFuel Processing Technologyj o u r n a l h o m e p a g e :w w w.e l s e v i e r.c o m/l o c a t e /f u p r o cof the dealumination and coke deposition to the catalyst deactivation have not been investigated in detail.In this article,we investigated the deactivation of H-ZSM-5for the steam cracking of n -hexane,which was a model compound of the naphtha.To understand effects of the dealumination,we prepared the H-ZSM-5catalysts pretreated in the steam flow,which was expected to be model catalysts deactivated by only dealumination without n -hexane flow under the reaction condition.We characterized H-ZSM-5and pre-steamed H-ZSM-5in detail using SEM,XRD,ICP,N 2adsorption,27Al MAS NMR,and NH 3-TPD to elucidate the deactivation reason by clarifying the structural changes of H-ZSM-5under the steam flow.We also investigated the regeneration of deactivated catalyst by calcina-tion to understand the contributions of the dealumination and coke de-position to the catalyst deactivation.We found that H-ZSM-5during the steam cracking of n -hexane was deactivated by a main reason of the dealumination and partially by the coke deposition.H-ZSM-5catalysts with high resistance against the dealumination of ZSM-5under steam flow are required for the improvement of the dura-bility for the steam cracking of n -hexane,which has been investigated by chemical modi fication such as phosphorus [24,33,35–42]and lantha-num loading [22,43–46].The phosphorus modi fication could especially enhance the resistance against the dealumination of ZSM-5and severalmodel structures of P-ZSM-5have been proposed [33,47–49].In all pro-posed structures,the phosphorus modi fication produced new species including aluminum and phosphorus atoms.We previously found that both amount of phosphorus loading and calcination temperature in flu-enced the catalyst durability [42]and that 80%of initial activity was maintained over the phosphorus-modi fied ZSM-5catalyst calcined at 1073K for the steam cracking for 30h.2.Experimental 2.1.CatalystH-ZSM-5powder (SiO 2/Al 2O 3=60)used in this work was pur-chased from JGC Catalysts and Chemicals Ltd.A part of H-ZSM-5zeolite was pretreated in flowing steam (69kPa)diluted with N 2(32kPa)at 923K for 5h or 10h,which was denoted as H-ZSM-5-st5or H-ZSM-5-st10,respectively.2.2.Catalytic activity testCatalytic steam cracking of n -hexane was carried out in a packed-bed reactor.The catalysts (0.2g,pellet diameter 0.5–1.0mm)were placed within a quartz tube (10mm diameter)with a K-type thermo-couple enclosed in a quartz sheath in contact with the catalyst bed.Water and n -hexane were introduced using syringe pumps into the re-actant stream by vaporizing into a stainless line kept at 423K.All trans-fer lines from the injection point to the gas chromatograph were kept above 423K to avoid condensation.Dinitrogen gas as an internal stan-dard for gas chromatography analysis was metered by an electronic flow controller.The reaction conditions were the following:reaction temperature,923K;WHSV (hexane),11h −1;W/F,8.0g-cat h/mol-hexane;P hexane ,14kPa;P H2O ,69kPa;and P N2,18kPa.The concentra-tions of reactant and products were measured by on-line gas chroma-tography using a Molecular Sieve 13X column (for dinitrogen,carbon monoxide,and methane)and a Porapak Type Q column (for carbon dioxide,methane,ethane,and ethylene)with a thermal conductivity detector and using a SP1700column (for methane,propane,propylene,n -butane,i -butane,n -pentane,and n -hexane)and a Gaskuropack54column (for methane,n -heptane,benzene,toluene,and xylene)with a flame ionization detector.Conversion of n -hexane and product yield based on carbon were de fined as given below.Conversion %ðÞ¼1−mol of unreacted n ‐hexane in productsmol of reactant n ‐hexaneÂ100ð1ÞYield based on carbon C %ðÞ¼mol of carbon atom in productmol of carbon atom in reactant n ‐hexaneÂ100ð2Þ2.3.CharacterizationScanning electron microscopy (SEM)images were measured using a JEOL JSM-5600LV operated at 20keV.Sample was coated with gold using a vacuum deposition equipment.X-ray diffraction (XRD)patterns of the catalysts were recorded using a Rigaku SmartLab with Cu K αradiation (λ=0.15406nm)under 30mA current and 40kV voltages in the 2θrange of 5–80°with a 2θstep size of 0.02.Chemical composition analysis of silicon and aluminum was car-ried out using an inductively coupled plasma (ICP)atomic emission spectrometry (SPS4000,SII NanoTechnology Inc.)after dissolution of the H-ZSM catalysts using sulfuric acid,nitric acid,and hydro fluoricacid.Fig.1.Conversion of n -hexane during steam cracking at 923K.The catalysts:□H-ZSM-5;●H-ZSM-5-st5;▲H-ZSM-5-st10.Dotted line indicates n -hexane conversion during thermal cracking without H-ZSM-5catalysts.Fig.2.Product distribution during the steam cracking of n -hexane at 923K over the (a)H-ZSM-5,(b)H-ZSM-5-st5,and (c)H-ZSM-5-st10,and (d)without H-ZSM-5catalysts.BTX indicates the total amounts of benzene,toluene,o -xylene,m -xylene,and p -xylene,and C2–C4indicates paraf fin hydrocarbon with from two to four carbon atoms.344 A.Yamaguchi et al./Fuel Processing Technology 126(2014)343–349Nitrogen adsorption and desorption measurements at 77K were carried out on a Autosorb-1(Quantachrome Instruments Co.,USA)for samples degassed at 473K for 2h.The speci fic surface area values of the catalysts were determined by the Brunauer –Emmett –Teller (BET)method.27Al solid-state magic angle spinning (MAS)nuclear magnetic reso-nance (NMR)experiments were performed on a Bruker AVANCE 400WB spectrometer (Bruker BioSpin,Japan)operated at 104.261MHz with a 12kHz spinning frequency using a 4mm MAS probe.A single pulse sequence was applied with a pulse length of 2.1μs.The 27Al chem-ical shifts were referenced to AlCl 31M aqueous solution.Temperature-programmed desorption of ammonia (NH 3-TPD)was carried out on a TPD-1-AT (Bel Japan,Inc.).The sample (ca.50mg)was loaded in a quartz tube and pretreated at 773K in flowing helium for 2h.After cooling in flowing helium to 373K,the sample was saturat-ed in 5%ammonia diluted with helium (0.33cm 3s −1)for 30min,after which the flowing gas was switched to helium with a flow rate of 0.5cm 3s −1for 1h.Finally,the sample was heated at a constant rate of 10K min −1to 953K.The ammonia signal was analyzed with an on-line quadrupole mass spectrometer.Thermogravimetric and differential thermal analysis (TG-DTA)were carried out on a TG-DTA2100SA thermal analyzer (Bruker AXS,Japan)at a heating rate of 10K min −1in flowing dry air.3.Results and discussion3.1.Catalytic steam cracking of n-hexane over H-ZSM-5and pre-steamed H-ZSM-5Fig.1shows the n -hexane conversion during the catalytic steam cracking at 923K over H-ZSM-5and pre-steamed H-ZSM-5,which were pre-treated in steam flow at the same water pressure and tem-perature as the steam cracking reaction.The conversion of n -hexane over the H-ZSM-5catalyst decreased from 100to 50%at 4h and be-came 34%at 16h,indicating that the H-ZSM-5catalyst showed a high initial activity but deactivated quickly during the catalytic steam cracking.On the other hand,the initial conversions of n -hexane over the H-ZSM-5-st5and H-ZSM-5-st10catalysts at 1h were 62%and 48%,respectively,indicating that the initial conversion de-creased with increasing pre-treatment time of the steaming.This re-sult implied that the pre-treatment in the steam flow reduced the activity of H-ZSM-5and that the deactivation of the H-ZSM-5catalyst during the n -hexane steam cracking was partially caused by the steam flow,which was used as a diluent and carrier of n -hexane.The reason of deactivation by the steam flow will be discussed in the Section 3.2from the characterization results of H-ZSM-5,H-ZSM-5-st5,and H-ZSM-5-st10.Fig.2shows product yields during the catalytic steam cracking of n -hexane at 923K over H-ZSM-5and pre-steamed H-ZSM-5.The prod-uct yields from n -hexane steam cracking over H-ZSM-5at 1h were pro-pylene 34%,ethylene 18%,methane 5.3%,paraf fin hydrocarbon of C2–C418%,butene 8.8%,and BTX (benzene,toluene,and xylene)3.9%,respec-tively,and these yields decreased at 16h to propylene 13%,ethylene 8.9%,methane 3.4%,C2–C44.1%,butene 4.4%,and BTX 0.7%,respective-ly.The product yields from n -hexane steam cracking over H-ZSM-5-st5(H-ZSM-5-st10)at 1h were propylene 26(19)%,ethylene 13(11)%,methane 4.9(3.7)%,paraf fin hydrocarbon of C2–C411(10)%,butene 6.4(5.3)%,and BTX 1.3(0.6)%,respectively (Fig.2(b)and (c)),as sum-marized in Table 2.The n -hexane conversions over H-ZSM-5-st5and H-ZSM-5-st10at 1h were 62%and 48%,respectively,which was almost similar to the conversions over the H-ZSM-5catalyst at 2h (59%)and 6h (49%).The product distributions over the H-ZSM-5catalyst at 2h (6h)were propylene 23(18)%,ethylene 11(11)%,methane 3.6(3.5)%,paraf fin hydrocarbon of C2–C413(11)%,butene 6.6(5.9)%,and BTX 1.4(0.5)%(Table 2),respectively,which were almost the same as those over H-ZSM-5-st5and H-ZSM-5-st10at 1h in the case of similar conversion,as shown in Table 2.The ratio of propylene to eth-ylene yield was about 1.9in the case of high conversion of n -hexane (60–80%)regardless of the catalysts.The ratio decreased with decreas-ing conversion of n -hexane and became about 1.5at the n -hexane con-version of ca.30%.The product yields without catalysts were propylene 11%,ethylene 7.6%,methane 2.6%,C2–C44.8%,butene 3.9%,and BTX 0.1%,respectively (Fig.2(d))which were almost the same as those over H-ZSM-5,H-ZSM-5-st5,and H-ZSM-5-st10at 16h,indicating that these catalysts completely deactivated at 16h and did not work as catalysts for n -hexane cracking.3.2.Characterization of H-ZSM-5and pre-steamed H-ZSM-5The particle sizes of H-ZSM-5were 0.1–0.3μm,as shown in SEM image (Fig.S1).The particle sizes were not changed by the steam treat-ment for 5and 10h (H-ZSM-5-st5and H-ZSM-5-st10).Fig.3shows XRD patterns of the H-ZSM-5,H-ZSM-5-st5,and H-ZSM-5-st10catalysts.The XRD pattern,which was assigned to MFI structure,was not changedTable 1Chemical composition of silicon and aluminum,speci fic surface area,and acid amount of H-ZSM-5,H-ZSM-5-st5,and H-ZSM-5-st10.CatalystChemical composition a /wt.%SiO 2/Al 2O 3Speci fic surface area b /m 2g −1Acid amount c /mmol g −1SiliconAluminum H-ZSM-541.0 1.3160.33440.47H-ZSM-5-st542.7 1.3859.73050.043H-ZSM-5-st1042.91.3859.63200.029a Estimated by ICP analysis.b Estimated by N 2adsorption using BET method.cEstimated by NH 3-TPD analysis using the h-peak area.Table 2Product distribution and conversion of n -hexane during the steam cracking of n -hexane at 923K over the H-ZSM-5,H-ZSM-5-st5,and H-ZSM-5-st10catalysts.BTX indicates the total amounts of benzene,toluene,o -xylene,m -xylene,and p -xylene,and C2–C4indicates paraf fin hydrocarbon with from two to four carbon atoms.CatalystConversion of n -hexane/%Yield/%PropyleneEthylene Methane C2–C4Butene BTX H-ZSM-5-st5a 622613 4.911 6.4 1.3H-ZSM-5b592311 3.613 6.6 1.4H-ZSM-5-st10a 481911 3.710 5.30.6H-ZSM-5c4918113.5115.90.5Time on stream:a 1h,b 2h,and c 6h.345A.Yamaguchi et al./Fuel Processing Technology 126(2014)343–349signi ficantly by the treatment in the steam flow,indicating that the crystallinity of MFI structure remained after the steam treatment.The quantitative elemental analysis of silicon and aluminum showed that the SiO 2/Al 2O 3ratio was 60.3in H-ZSM-5(Table 1)and that it remained after the treatment in the steam flow (59.7and 59.6in H-ZSM-5-st5and H-ZSM-5-st10,respectively).The speci fic surface area was 344m 2g −1for H-ZSM-5(Table 1)and slightly decreased to 305and 320for H-ZSM-5-st5and H-ZSM-5-st10,respectively,but it seemed mostly un-changed by the steam pre-treatment.The 27Al MAS NMR spectra (Fig.4)drastically changed,in contrast with the results of XRD,elemental analysis,and surface area.A large peak around 55ppm in 27Al MAS NMR spectra was observed for the H-ZSM-5sample,which was attributed to tetrahedral aluminum in ZSM-5framework [50,51].The peak intensities at 55ppm in H-ZSM-5-st5and H-ZSM-5-st10became very small,indicating that dealumination from the tetrahedral sites of ZSM-5framework occurred during the treatment in steam flow at 923K.However,the peak around 0ppm,which was attributed to octahedral aluminum of extra-framework,could not be observed for H-ZSM-5-st5and H-ZSM-5-st10,indicating that aluminum atoms in the ZSM-5framework became distorted tetrahedral coordination or nonsymmetrical five-or six-coor-dination [51].NH 3-TPD pro files of H-ZSM-5,H-ZSM-5-st5,and H-ZSM-5-st10(Fig.5)showed that the NH 3desorption peak around 520–850K (h-peak),which was ascribed to strong acid sites [52,53],wasdecreased by the steam pre-treatment,consistent with dealumination from tetrahedral sites of ZSM-5framework.The strong acid amounts of H-ZSM-5,H-ZSM-5-st5,and H-ZSM-5-st10were estimated to be 0.47,0.043,and 0.029mmol g −1from the NH 3-TPD pro files (Table 1),indicating that the acid sites in H-ZSM-5were decreased less than one-tenth by dealumination with the steam flow.Thus,we concluded that the decrease of initial activity of the pre-steamed H-ZSM-5catalysts for n -hexane cracking (Fig.1)was caused by the dealumination of ZSM-5framework,i.e.,decrease of acid sites.The dealumination and decrease of strong acid amount in H-ZSM-5occurred greatly for 0–5h of pre-steaming and those proceeded slowly for 5–10h of pre-steaming as shown in Figs.4and 5,implying that dealumination occurred greatly at the initial stage of the steaming and then it proceeded slowly during the steaming.Thus,the H-ZSM-5catalyst showed initially very high ac-tivity (Fig.1);however,it deactivated at the initial stage of the reaction and then showed similar trend of deactivation.The initial conversions of n -hexane were not proportional to the amounts of strong acid for H-ZSM-5and pre-steamed H-ZSM-5catalysts.We used much amount of the catalysts to con firm their feasibility for commercial application;thus,the initial conversions were very high (e.g.100%).Under these conditions,the reaction kinetics could not be discussed because the conversion was integral value obtained at each vertical position in the catalyst bed;thus,only qualitative relationship between amount of acid sites and conversion wasmentioned.Fig.3.XRD patterns of (a)H-ZSM-5,(b)H-ZSM-5-st5,and (c)H-ZSM-5-st10.Fig.4.27Al MAS NMR spectra of (a)H-ZSM-5,(b)H-ZSM-5-st5,and (c)H-ZSM-5-st10.Fig.5.NH 3-TPD pro files of (a)H-ZSM-5,(b)H-ZSM-5-st5,and (c)H-ZSM-5-st10.Fig.6.TG-DTA of H-ZSM-5after the steam cracking of n -hexane at 923K for 16h.346 A.Yamaguchi et al./Fuel Processing Technology 126(2014)343–349Fig.6shows TG-DTA of the H-ZSM-5catalyst after the steam crack-ing of n -hexane at 923K for 16h.The exothermic peak around 880K and the weight loss were observed,which were attributed to the burn-ing of coke deposited on H-ZSM-5during the n -hexane steam cracking for 16h.The amount of coke deposition increased with time of stream and became 10.1wt.%based on the weight of H-ZSM-5at 16h (Fig.7),indicating that the deactivation of the H-ZSM-5catalyst during the n -hexane steam cracking would be also partially caused by the coke deposition.It is reported that deactivation of zeolites by coke deposition is caused by limitation or blockage of the reactant access to the active sites and that the coke formation rate and deactivation rate of H-ZSM-5are slower than those of H-Y [4,54,55].In the case of one-dimensional pore structure (Y-zeolite),the blockage of the reactant ac-cess occurs easily because the whole channel becomes inactive due to the pore plugging of both ends.On the other hand,ZSM-5zeolite has three-dimensional pore structure and the coke effect is not severe;however,ZSM-5zeolite also deactivates slowly because of the coke de-position.Urata et al.reported that coke was deposited on the externalsurface of H-ZSM-5crystallites and that the channel was blocked by the external coke [56].Konno et al.reported that diffusion limitation of n -hexane depended on crystal size of H-ZSM-5[57,58].The cracking over small H-ZSM-5with the crystal size of 90nm proceeded under reaction-controlling condition and the coke effect was small;however,that over large H-ZSM-5with the crystal size of 2.3μm proceeded under the transition condition of diffusion-controlling and reaction-controlling conditions and the coke deposition reduced the catalytic ac-tivity rapidly [57,58].The particle sizes of H-ZSM-5were 0.1–0.3μm from SEM image (Fig.S1)in this study.The deactivation of H-ZSM-5by the coke deposition was probably due to the diffusion limitation of n -hexane and could be regenerated by calcination.3.3.Regeneration of deactivated H-ZSM-5To investigate contributions of the dealumination and coke deposi-tion to the catalyst deactivation,the deactivated H-ZSM-5catalysts were regenerated by calcination under O 2(20%)/He(80%)flow at 893K for 5h for removal of the coke deposition and the regenerated cata-lysts were used for the n -hexane steam cracking again (Fig.8).The n -hexane conversion over H-ZSM-5at 5h was 53%(Fig.8(a))and the ini-tial conversion over the calcined H-ZSM-5was recovered partially to 66%and then decreased again with increasing time on stream,indicat-ing that the H-ZSM-5catalyst was deactivated reversibly by the coke de-position and irreversibly by the dealumination during the n -hexane steam cracking.The NH 3-TPD pro file of the calcined H-ZSM-5catalyst after the n -hexane steam cracking for 5h was measured to con firm the reason of deactivation (Fig.9).The NH 3-TPD pro file showed that amount of acid sites decreased drastically during the n -hexane steam cracking because of irreversible dealumination,resulting in the irrevers-ible deactivation of H-ZSM-5for the n -hexane steam cracking.If the H-ZSM-5-st5catalyst,which is pre-treated in the steam flow at the reaction temperature,is deactivated by dealumination to the same extent as the H-ZSM-5deactivation by dealumination during the n -hexane steam cracking,the initial conversion over the calcined H-ZSM-5catalyst used for 5h should be the same as that over the H-ZSM-5-st5catalyst.However,the initial conversion (66%)over the cal-cined H-ZSM-5catalyst used for 5h was a little higher than that (62%)over the H-ZSM-5-st5catalyst.The acid amount of the calcined H-ZSM-5catalyst was also larger than that of the H-ZSM-5-st5catalystFig.7.Amout of carbon deposition over the H-ZSM-5catalyst during steam cracking of n -hexane at 923K.Fig.8.Conversion of n -hexane during steam cracking at 923K over H-ZSM-5.The H-ZSM-5catalyst was treated under O 2(20%)/He(80%)flow at 893K for 5h after the n -hexane steam cracking ((a)5h and (b)10h)and used again for the cracking at 923K.Dotted line indicates n -hexane conversion during thermal cracking without H-ZSM-5catalysts.347A.Yamaguchi et al./Fuel Processing Technology 126(2014)343–349from the NH 3-TPD pro files (Fig.9).One possible reason for the differ-ence is that the coke deposition mainly occurred on the external surface of the H-ZSM-5catalyst during the n -hexane steam cracking [57,58]and that the pores of H-ZSM-5were covered with the coke deposition.The coke prevented not only n -hexane molecules but also steam from ap-proaching the pores of H-ZSM-5,leading to the result that the extent of dealumination during the n -hexane steam cracking was less than that during the steam treatment.The n -hexane conversion over H-ZSM-5at 10h was 38%(Fig.8(b))and then the deactivated catalyst was also regenerated by calcination at 893K.The initial conversion of the regenerated catalyst after the crack-ing 10h was 53%(Fig.8(b)),which was lower than that (66%)after the cracking for 5h (Fig.8(a)),indicating that the irreversible deactivation by dealumination proceeded during the n -hexane steam cracking from 5to 10h.The effect of coke deposition on the H-ZSM-5deactivation was in-vestigated by the n -hexane catalytic cracking without steam flow (Fig.10).The conversion of n -hexane decreased to 50%at 5h during the n -hexane steam cracking;on the other hand,the conversion of n -hexane decreased slightly to 96%at 5h during the n -hexane crackingwithout steam flow.The dealumination due to the steam flow did not proceed during the n -hexane cracking without steam flow;thus,the acid amount of H-ZSM-5remained much larger than that during the n -hexane steam cracking at 5h.The stability of H-ZSM-5for the n -hexane cracking without steam flow was better;however,the coke de-position was 4.9wt.%based on the weight of H-ZSM-5at 5h,which was much larger than that (1.5wt.%)during the n -hexane steam cracking,indicating that the amount of deposited coke was decreased by the steam flow.We observed that the amount of deposited coke on the phosphorus-modi fied ZSM-5catalyst,which showed the resistance against the dealumination of ZSM-5[42],was also decreased with an in-crease in steam pressure,implying that coke formation was suppressed by not low acidity of dealuminated ZSM-5but reaction between coke precursor and steam.The coke deposition causes pressure drop and col-lapse of catalyst pellets on a commercial scale;thus,it could not be used in commercial ole fin production from naphtha.We succeeded in consideration of dealumination and coke deposi-tion for the catalyst deactivation and revealed that H-ZSM-5was deactivated mainly by the dealumination in the case of n -hexane steam cracking at 923K.Thus,H-ZSM-5catalysts with high resistance against the dealumination of ZSM-5under steam flow are required for the improvement of the durability for the steam cracking of n -hexane.4.ConclusionsThe H-ZSM-5catalyst showed a high initial activity but deactivated quickly during the catalytic steam cracking.Both dealumination of ZSM-5and coke deposition occurred during the steam cracking of n -hexane,from the characterization results of H-ZSM-5and pre-steamed H-ZSM-5by SEM,XRD,ICP,N 2adsorption,27Al MAS NMR,NH 3-TPD,and TG-DTA.The used H-ZSM-5catalysts could be regenerat-ed partially by calcination for removal of the deposited coke.H-ZSM-5was deactivated mainly by the dealumination in the case of n -hexane steam cracking at 923K.Supplementary data to this article can be found online at /10.1016/j.fuproc.2014.05.013.AcknowledgmentsThis work was supported by the New Energy and Industrial Technology Development Organization (NEDO),Japan.References[1]J.E.Otterstedt,S.B.Gevert,S.G.Jäås,P.G.Menon,Fluid catalytic cracking of heavy(residual)oil fractions:a review,Applied Catalysis 22(1986)159–179.[2]J.Dwyer,D.J.Rawlence,Fluid catalytic cracking:chemistry,Catalysis 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