Ni-Al2O3-Mn 合成气甲烷化

Journal of Natural Gas Chemistry21(2012)170–177Ni/Al2O3catalysts for syngas methanation:Effect of Mn promoterAnmin Zhao,Weiyong Ying∗,Haitao Zhang,Hongfang Ma,Dingye Fang State Key Laboratory of Chemical Engineering,Engineering Research Center of Large Scale Reactor Engineering and Technology,Ministry of Education,East China University of Science and Technology,Shanghai200237,China[Manuscript received July7,2011;revised August22,2011]AbstractNi/Al2O3catalysts with different amounts of manganese ranging from1to3wt%as promoter were prepared by co-impregnation method.The catalysts were characterized by N2physisorption,XRD,TPR,SEM and TEM.Their catalytic activity towards syngas methanation reaction was also investigated using a fixed-bed integral reactor.It was demonstrated that the addition of manganese to Ni/Al2O3catalysts can increase the catalyst surface area and average pore volume,but decrease NiO crystallite size,leading to higher activity and stability.The effects of reaction temperature,pressure and weight hourly space velocity(WHSV)on carbon oxides conversion and CH4formation rate were also studied.High carbon oxides conversion,CH4selectivity and formation rate were achieved at the reaction temperature range of280−300◦C.Key wordsmanganese promotion;nickel catalysts;syngas methanation1.IntroductionMethanation reaction between carbon oxides and hydro-gen over nickel catalysts to produce methane was firstly re-ported by Sabatier and Senderens in1902[1].Since then, it has been widely used in many processes,including pu-rification of gas in ammonia synthesis and hydrogen produc-tion[2,3].The interest to the reaction as a promising route to produce synthetic natural gas(SNG)has remarkably grown during the last few years.Recently,a great deal of works has been directed towards SNG production by gasification of the coal or biomass to synthesis gas and subsequent methanation of the synthesis gas to SNG[4−8].This route can improve the security of energy supply and reduce the emission of green house gas[9].Extensive studies have been conducted on several metal-based catalytic systems using Ni[10−12,14],Ru[15−17],Co [18]and Fe[19]on various oxide supports(Al2O3[11,20], SiO2[21,22],ZrO2[23,24]and TiO2[25]).Ni-based cata-lysts were the most extensively studied because of its high activity and selectivity for methane formation as well as its low cost[26,27].Meanwhile,Nattaporn Lohitharn et al.[28]reported that introduction of Mn as promotor onto a precipitated Fe catalyst for Fischer-Tropsch synthesis sig-nificantly improves its catalytic activity.However,up to date scarce study has been conducted to investigate the pro-motion effect of Mn on Ni/Al2O3catalysts for carbon ox-ides methanation.Our preliminary experiments have revealed that Ni/Al2O3catalyst with24wt%nickel content exhibits good activity for carbon oxides methanation.In order to study the impact of different Mn loadings on the activity and stability of carbon oxides methanation over the Ni-based catalyst,a series of Ni/Al2O3catalysts with Ni loading of 24wt%were modified by different amounts of Mn ranging from1to3wt%.The catalysts were prepared through co-impregnation method and characterized using N2physisorp-tion,XRD,H2-TPR,SEM and TEM.Effects of reaction tem-perature,pressure and weight hourly space velocity(WHSV) on its activity were investigated aiming to design a potential effective catalyst for SNG.The stability of the catalysts under high reaction temperature was also examined up to1200min on stream.2.Experimental2.1.Preparation of catalyst24Ni/Al2O3catalysts with different Mn loadings were de-noted as X Mn-NA(X=0,1,2,and3),where X represents the Mn loading(wt%),N stands for Ni and A denotes Al2O3. Ni(NO3)2·6H2O,50wt%Mn(NO3)2solution and Al2O3were∗Corresponding author.Tel/Fax:+86-21-6425-2192;E-mail:wying@This work was supported by the National Science and Technology Supporting Plan(No.2006BAE02B02). Copyright©2012,Dalian Institute of Chemical Physics,Chinese Academy of Sciences.All rights reserved. doi:10.1016/S1003-9953(11)60350-2Journal of Natural Gas Chemistry V ol.21No.22012171supplied by Sinopharm Chemical Reagent Co.,Ltd.Both de-sirable amounts of Ni(NO3)2·6H2O and50wt%Mn(NO3)2 solution were mixed or dissolved in50mL deionized water under vigorous stirring and kept at60◦C for1h to obtain a mixed solution.Catalysts were prepared by impregnation of Al2O3with the mixed solution.After impregnation at60◦C for12h,the samples were dried at120◦C for12h and cal-cined in static air at550◦C for6h.2.2.Characterization of catalystsNitrogen adsorption-desorption isotherms were obtained with a Micrometrics ASAP2020apparatus.Prior to N2ad-sorption,the samples were degassed under vacuum at200◦C for4h.Specific surface areas were measured by the multi-point Brunauer-Emmet-Teller(BET)method.Total pore vol-ume and sizes were evaluated using the standard Barrett-Joyner-Halenda(BJH)treatment.Powder X-ray diffraction(XRD)patterns were recorded on a Rigaku D/Max2550using Cu Kαradiation at40kV and100mA.XRD patterns were recorded over a2θrange of 10o−80o and a step size of0.02o.Hydrogen temperature-programmed reduction(H2-TPR) measurements were carried out on Micrometrics AutoChem II2920.Prior to the H2-TPR measurements,200mg sample placed in a quartz U-tube reactor was pretreated in Ar stream at500◦C for0.5h and then cooled to ambient temperature. H2-TPR was then conducted with a gas mixture of10vol% H2in Ar at a flow rate of50mL·min−1and temperature was raised to800◦C with a heating rate of10◦C·min−1.Hydro-gen consumption was monitored by a thermal conductivity de-tector(TCD).Scanning electron microscopy(SEM)and transmission electron microscope(TEM)were employed to the morpho-logic characterization.SEM measurements were performed on a JSM-6360LV electron microscope.TEM investigations were carried out on a JEM-1400operating at100kV.In or-der to obtain suitable samples,the catalyst powders were dis-persed in ethanol by ultrasonic and then a drop of the solution was placed on a carbon-coated copper grid.2.3.Syngas methanationSyngas(carbon oxides and hydrogen)methanation reac-tions were carried out in a12mm(inner diameter)fixed-bed reactor.The length of the uniform temperature zone was over 140mm.Typically500mg catalyst was used,and the total gas flow rate was100mL·min−1,corresponding to a weight hourly space velocity(WHSV)of12000mL·g−1·h−1.Cat-alyst diluted with a certain amount of inert Al2O3was set-tled in the uniform temperature zone,where the axial temper-ature was measured with a thermocouple.The catalyst was heated from ambient temperature to450◦C with a heating rate of1◦C·min−1and then reduced in situ by pure hydro-gen for2h.After this pretreatment,the catalyst was cooled to250◦C under nitrogen flow and the reaction started as gas flow was switched to the syngas(6.6vol%CO,1.8vol%CO2, 67.8vol%H2,and25.9vol%N2).Flow rates of all gases were monitored by calibrated mass flow controllers(Brooks 5850E).Gas composition of the inlet and outlet was ana-lyzed by an on-line gas chromatograph(Agilent7890A se-ries)equipped with one FID(flame ionization detector)and two TCD(thermal conductivity detector).Hydrocarbons from C1to C6were analyzed by a FID after separation with a HP-PLOT“S”Al2O3column.N2,CO and CO2were analyzed by a TCD after separation with two Propack Q and one5A molecular sieve columns.H2was analyzed using a TCD after separation with one Propack Q and one5A molecular sieve column.To calculate carbon oxides conversion,formation rate and selectivity of methane,the effluent gas was analyzed after steady-state operation under each condition.The selec-tivity of a particular product was calculated based on carbon efficiency using the formula n i C i/Σn i C i,where n i and C i are the carbon number and molar concentration of the i th product, respectively.The conversions of carbon oxides and methane formation rate were calculated by the formulas as follows: CO conversion(X CO,%):X CO=N in y CO,in−N out y CO,outN y×N y×m172Anmin Zhao et al./Journal of Natural Gas Chemistry V ol.21No.22012Moreover,among the three Mn promoted catalysts,1Mn-NA catalyst gives the highest surface area and further increase in the manganese loadings has a little contribution to the surface area,pore volume and average pore diameter of the catalysts.Table 1.Physical property of 0Mn-NA,1Mn-NA,2Mn-NA and 3Mn-NA Catalyst S/m 2·g −1V a /cm 3·g −1D b /nm D c /nmSupport60.70.23115.6−Figure 1.XRD patterns of different catalystsTPR technique allows us not only to characterize the metal-support interactions but also to elucidate the role of ad-ditives as promoters in the reduction and the influence of one or more phases on the reducibility of a specific compound in multi-component system [31].Figure 2shows the TPR re-sults of the samples,in which the overlapped peaks are de-convolved using Gaussian-type functions.The results reveal that the addition of manganese can change the reduction char-acteristics ofcatalysts.Figure 2.TPR profiles of different catalystsAccording to the peak temperatures in their TPR profiles and reported in literatures [31−36],the reducible NiO species could be classified into three types:α-type (surface amor-phous NiO or bulk NiO),β-type (weakly interacted with Al 2O 3or called Ni-rich phase [35])and γ-type (strongly inter-acted with Al 2O 3or called Al-rich phase [35]).Similar results can also be found in references [30,37,38].After a mathematical treatment of the TPR profile,three elementary peak temperatures that centered at 370−410◦C,480−583◦C and 540−660◦C,respectively,could be well de-convolved with R 2(higher than 0.99).The TPR profiles for the samples with different Mn loadings are compared and the corresponding peak temperatures and percent of every ele-mentary peak area are shown in Table 2.The quantitative re-sults of the TPR profiles (Table 2)show that the proportion of α-type in all reducible NiO precursors maintained ca.28%for all the catalysts studied in this work.The α-peak temperature shifts downwards with the addition of manganese,suggesting a weaker interaction between α-NiO and alumina support for the Mn-containing samples.Table 2.TPR data of 0Mn-NA,1Mn-NA,2Mn-NA and 3Mn-NAT (◦C)Fraction of total area (%)0Mn-NA410507542294724Journal of Natural Gas Chemistry V ol.21No.22012173of NiO represents Al-rich phase containing some Ni2+ions (also called Ni2+ions surrounded by Al3+ions as suggested by Scheffer et al.[36]).1Mn-NA catalyst possesses the high-est fraction ofγ-type NiO(32%)while3Mn-NA has the high-est fraction ofβ-type NiO(57%).Based on the literatures [31,33],the reduction ofα-andβ-type NiO would benefit the formation of large-sized Ni particles which are active at low temperatures.On the contrary,the reduction ofγ-type NiO will lead to the formation of small Ni particles which are not only active at low temperatures but also active and stable at high temperatures.Typical SEM and TEM images of the0Mn-NA and1Mn-NA catalysts are displayed in Figure3.The SEM micrographs of0Mn-NA and1Mn-NA catalysts are shown in Figure3.It is obvious that the addition of manganese can restrain agglomer-ation by preventing the nickel species migration on the granule surface.The TEM micrographs of0Mn-NA and1Mn-NA cat-alysts show that the NiO particles are dispersed in the cotton-likeγ-Al2O3networks.It is obvious that the NiO species in 1Mn-NA catalyst has betterdispersion. Figure3.Electron microscope micrographs of0Mn-NA and1Mn-NA3.2.Equilibrium conversion and stability testsMain reactions(Reactions(4)to(6))and carbon deposit reactions(Reactions(7)and(8))which occur in the methana-tion process are expressed as follows:CO+3H2←→CH4+H2OΔHθ=−206kJ/mol(4) CO2+4H2←→CH4+2H2OΔHθ=−165kJ/mol(5) CO+H2O←→CO2+H2ΔHθ=−41kJ/mol(6) 2CO←→CO2+CΔHθ=−173kJ/mol(7)CH4←→C↓+2H2ΔHθ=−41kJ/mol(8) As the methanation reactions are highly exothermic and pressure-favorable,reactors are designed running at low tem-perature and relative high pressure with catalysts.For prac-tical operation of this process,it is essential to avoid both Reactions(7)and(8)since carbon formation and deposition would plug the catalyst bed and render it inoperative.Ther-modynamics and equilibrium calculations were performed at various temperatures ranging from250◦C to300◦C.The re-sults reveal that equilibrium conversion of both CO and CO2 are approached to100%under our experimental conditions. Therefore,the system is controlled by kinetics instead of equi-librium.Before the investigation to catalytic activity,stability of the samples was studied.Carbon oxides conversion and methane formation rate were employed to represent catalytic activity to accomplish the stability tests under this operat-ing conditions(450◦C,0.1MPa and600000mL·g−1·h−1;12.22vol%CO,4.67vol%CO2,75.33vol%H2and7.77 vol%N2).Results summarized in Figure4show that carbon oxides conversion decreases sharply with time on stream over 0Mn-NA.On the contrary,catalysts modified by Mn have bet-ter stability during the tests.1Mn-NA is the most stable cata-lyst among the four catalysts with CO conversion always over 95%and CH4selectivity over99%.This is mainly because catalyst1Mn-NA possesses the highest fraction ofγ-type NiO (32%)which is active and stable under high temperature.174Anmin Zhao et al./Journal of Natural Gas Chemistry V ol.21No.22012Figure4.Stability test results for the four catalysts3.3.Ef fects of temperature and Mn loadingThe catalytic activity of the four catalysts,with Mn loading from0to3wt%,was studied from250to320◦C.The reaction results obtained under the condition of 12000mL·g−1·h−1and0.1MPa are presented in Figure5, where conversions of carbon oxides and CH4formation rateare plotted as functions of reaction temperature.The conversions of both CO and CO2increase apparently with the increase of temperature.CO conversion achieves 100%above270◦C over catalyst1Mn-NA,2Mn-NA and 3Mn-NA.As for0Mn-NA,CO is also consumed completely above280◦C.On the other hand,CO2conversions over all the four catalysts show negative value when the reaction tem-perature is below270◦C.This is probably owing to water gas shift reaction,which may produce more CO2than that consumed in the methanation reaction[16,27].The CO2con-version on the catalyst promoted by Mn increases more obvi-ously compared with the conversion on0Mn-NA catalyst over 270◦C.The trend of CH4formation rate with temperature is similar to that ofCO.Figure5.Effect of temperature on the conversion of carbon oxides and CH4 formation rateIt should be noted that,in addition to methane(C1),hy-drogenation of carbon oxides under present reaction condi-tions also results in the production of higher hydrocarbons. Typical results obtained over2Mn-NA and3Mn-NA catalystsJournal of Natural Gas Chemistry V ol.21No.22012175are shown in Figure 6,where the selectivities towards hydro-genation products are plotted as functions of reaction tempera-tures.The main by-products formed below 290◦C are C 2–C 6hydrocarbons.Selectivities of methane (C 1)on 2Mn-NA and 3Mn-NA catalysts increase from ca.83%to ca.99%with temperature from 250to 290◦C and arrive nearly 100%above 300◦C.The results demonstrate that the addition of Mn has a little effect on product distribution while temperature plays an importantrole.Figure 6.Effect of temperature on product distributionThe promotion effect of Mn loading on CO conversion under 260◦C,0.1MPa and 12000mL ·g −1·h −1was plotted in Figure 5.The CO conversion increases rapidly from 52.8%to 75.8%with the Mn loading from 0to 1wt%,and the promo-tion effect would be inconspicuous when further increase the Mn loading from 1to 3wt%.An appropriate amount of Mn would improve the catalyst activity,increase the surface ba-sicity and enhance the carburization of the catalyst.However,excessive addition of Mn retarded the catalyst carburization due to the enrichment of manganese on catalyst surface [39].We can draw the conclusion that 3Mn-NA is the most ac-tive catalyst among the four catalysts under present reaction conditions according to Figure 5.The order of carbon oxides conversion and CH 4formation rate on these catalysts in this section seems to be contradictory with the results obtained in the stability tests (1Mn-NA was the most stable catalyst).Thiscan be explained as follows:(1)It has been confirmed that Mn supported on Al 2O 3has hydrogenation activity in our ex-periment (see Figure 5).Thus,adding more Mn can result in the increase of carbon oxides conversion and CH 4forma-tion rate;(2)Catalysts using solo-manganese as active metal supported on aluminum oxide are not stable under high tem-perature (>400◦C),which is in line with the literature [19].Therefore,it can be concluded that 3Mn-NA is the most ac-tive catalyst at low temperature (250−320◦C)while 1Mn-NA is the most stable catalyst under high temperature (450◦C).3.4.Ef fect of space velocityWeight hourly space velocity has been designed in the range of 6000−60000mL ·g −1·h −1.Typical results ob-tained under 290◦C and 0.1MPa over catalyst 2Mn-NA and 3Mn-NA are depicted in Figure 7.CO conversion of 2Mn-NA and 3Mn-NA keeps above 96%while space velocity is below 30000mL ·g −1·h −1.Further increase of WHSV results in a drastic decrease of CO conversion to the extent of 58.2%and 59.9%at 60000mL ·g −1·h −1for 2Mn-NA and 3Mn-NA catalysts,respectively.CO 2conversion declines more dras-tically with the WHSV ,and even presents negative value at 60000mL ·g −1·h −1.This fact is mainly caused by watergasFigure 7.Effect of space velocity on carbon oxides conversion and CH 4formation rate176Anmin Zhao et al./Journal of Natural Gas Chemistry V ol.21No.22012shift reaction and it is also mentioned in Paraskevi’s work [40].As the curves of carbon oxides conversion are nearly linear from 30000to 45000mL ·g −1·h −1,meaning the entire catalyst bed works efficiently,one can judge that space veloc-ity chosen in that range seems to be optimal.Similar conclu-sion is also concluded by Ocampo’s research [41].3.5.Ef fect of pressureIn this section,catalytic performance of 2Mn-NA was studied by varying the pressure from 0.1to 2.0MPa.The test was carried out at 290◦C and 45000mL ·g −1·h −1.As is shown in Figure 8,the conversion of CO 2rises significantly from 9.5%to 85.3%with the increase of pressure from 0.1MPa to 2.0MPa.CO conversion increases from 80.0%to 99.3%with the increase of pressure from 0.1MPa to 1.0MPa,and then achieves 100%under higher pressure.Since metha-nation reaction is pressure-favorable,operated under high pressure can improve the utilization factor of reactors.There-fore,practical operating pressure depends largely on the syn-gas pressure from gasificationunit.Figure 8.Effect of pressure on carbon oxides conversion and CH 4formation rate4.ConclusionsNi/Al 2O 3catalysts with Mn loadings ranging from 0to 3wt%were prepared,and their properties were characterized.Several conclusions can be drawn from these investigations.First,adding Mn to Ni/Al 2O 3catalysts can increase the cata-lyst surface area,average pore volume and decrease NiO crys-tallite size slightly which is demonstrated through BET and XRD.Second,all catalysts promoted by Mn exhibit excellent catalytic activity and stability in carbon oxides methanation by comparison with Mn-free catalyst (0Mn-NA).Third,3Mn-NA catalyst is the most active at low temperature (250−300◦C)while 1Mn-NA catalyst is the most stable under high tem-perature (450◦C).Fourth,adding Mn to Ni/Al 2O 3cata-lysts can change CH 4formation rate and the product distri-bution slightly,but temperature plays a more important role.Fifth,operating at 280−450◦C,30000−45000mL ·g −1·h −1and around 2.0MPa would 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