CO2 adsorption on carbon molecular sieves 2012 164 280-7

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CO 2adsorption on carbon molecular sievesA.Wahby,J.Silvestre-Albero,A.Sepúlveda-Escribano ⇑,F.Rodríguez-ReinosoLaboratorio de Materiales Avanzados,Departamento de Química Inorgánica –Instituto Universitario de Materiales de Alicante (IUMA),Universidad de Alicante,Apartado 99,E-03080Alicante,Spaina r t i c l e i n f o Article history:Available online 19July 2012Keywords:Carbon molecular sieves CO 2adsorptionImmersion calorimetrya b s t r a c tThe effect of the textural properties of a series of commercial carbon molecular sieves (CMS),prepared from different polymeric precursors,on their ability for CO 2adsorption at different temperatures has been studied.The adsorbents have been characterized by N 2and CO 2adsorption at 77and 273K,respectively,together with measurements of immersion calorimetry into liquids of different molecular dimensions.The studied CMSs cover a wide range of porosity,from purely microporous carbons to samples containing wide micropores as well as a certain proportion of mesoporosity.Studies of CO 2adsorption,at atmospheric pressure (1bar)and three different temperatures (273,298and 323K),have shown that a high CO 2adsorption capacity requires the presence of a well-developed microporosity,as well as a high volume of narrow micropores.On the other hand,narrow micropores seem to be the key factor leading to a max-imum capacity of CO 2adsorption,even at temperatures close to that of anthropogenic emissions of CO 2.Ó2012Elsevier Inc.All rights reserved.1.IntroductionEnergy consumption,together with the progressive growth of world’s population,has soared during the 20th century,led by a great revolution entitled ‘‘transport’’.Moreover,the invention of vehicles,energy-powered trains and aircrafts has created a new world that has become progressively dependent on the use of fossil fuels such as gasoline,diesel and jet-fuel.However,the environ-ment is already paying ‘‘the tax’’of this spectacular socio-economic progress.In fact,serious environmental problems due to emissions of many pollutants from the combustion of solid,liquid and gas fuels in various mobile/stationary energy systems,together with the harmful emissions generated by industrial plants,have been reported as major global problems involving not only pollutants such as NOx,SOx and soot,but also greenhouse gases like carbon dioxide and methane.There are more concerns about global cli-mate change [1–3]and,therefore,an increased interest in reducing emissions of greenhouse gases,particularly CO 2[4–7].The reuse of the latter is nowadays a challenging task due to the continuous and signiﬁcant rise in atmospheric CO 2concentrations,the increased consumption of carbon-based energy worldwide,the exhaustion of available sources of carbon and the low efﬁciency of the current energy systems.Nevertheless,the reuse of CO 2undergoes several limitations,such as:(i)cost of storage,separation,puriﬁcation and transport;and (ii)energy requirement for chemical conversion (source and cost of reagents).There are several methods to sepa-rate CO 2from ﬂow gases for large-scale applications [8].Among them,the most widely used are cryogenic distillation,membrane puriﬁcation,liquid absorption and adsorption by means of porous solids.Cryogenic distillation is not considered to be practical for CO 2separation because of the involved energy consumption.Membrane-based separation of CO 2has been widely investigated [9].Traditionally,absorption using basic solvents (mainly amine solutions)has been widely used [10,11],although this technology suffers important drawbacks.In this sense,adsorption processes using porous sorbents can be an excellent alternative to the other mentioned processes for CO 2removal/recovery by physical adsorp-tion.Zeolites,activated carbons,carbon molecular sieves,meso-porous silicas and,more recently,metal–organic framework materials (MOF)have been proposed as promising candidates for CO 2adsorption [9,12–16].Carbon molecular sieves (CMS)are carbonaceous materials with a narrow pore size distribution,endowed with a selective adsorp-tion capacity of certain components of a mixture.They can dis-criminate molecules on the basis of size,shape or on a difference in adsorption equilibrium or,even,in adsorption rate.Thus,micro-porous CMS require the presence of a speciﬁc porous network con-taining pore mouths of molecular dimensions,together with a relatively high micropore volume.These features will confer them with a high adsorption capacity and selectivity into a given appli-cation.Hence,a proper molecular sieve must exhibit high adsorp-tion capacity and fast adsorption kinetics of certain components of a gas mixture which leads to high selectivity.CMS are commonly prepared from a variety of carbonaceous materials such as cellulosic precursors [17,18],coals [19],carbon ﬁbers [20,21],res-ins [22,23],etc.In general,there are two main methods to manu-facture microporous CMS;the ﬁrst one is based on controlled1387-1811/$-see front matter Ó2012Elsevier Inc.All rights reserved./10.1016/j.micromeso.2012.06.034Corresponding author.Tel.:+34965903974;fax:+34965903454.E-mail address:asepul@ua.es (A.Sepúlveda-Escribano).pyrolysis of a carbon precursor and the other one is based on the modiﬁcation of the existing porous structure by means of carbon vapor deposition technique(CVD)[24,25].The last one is particu-larly considered to be an appropriate technique,and it has received considerable attention in the last few years.The CVD process leads to activated carbons with a tailored porosity if the reduction of the pore mouth is controlled.In practice,commercial microporous CMS are mainly produced by controlled deposition of pyrolytic car-bon at the pore mouth.Theﬁnal porous structure is deﬁned by the nature of the precursor and the pyrolysis conditions applied.It is worth mentioning that CMS,compared to conventional zeolites, have some advantages such as higher hydrophobicity,higher resis-tance to both alkaline and acid media,thermal stability under inert atmosphere at higher temperatures,and higher selectivity towards planer molecules.Within the wide range of industrial applications of CMS,typical examples are:separation/puriﬁcation of binary gas mixtures,for instance,separation of linear and branched hydrocar-bons,removal of CO2from gas/air steams,separation of N2and O2 from air,and so on.[17,18,20,26–31].In this sense,the aim of the present work is to analyze the por-ous structure of commercial carbon molecular sieves recently developed by Supelco ing a combination of N2and CO2 adsorption at77and273K,respectively,together with immersion calorimetry measurements into liquids of different molecular dimensions.Additionally,the adsorption capacity of CO2at atmo-spheric pressure and different temperatures(273,298and323K) will be analyzed.The effect of temperature on CO2adsorption capacity will be studied and correlated with the porous structure of the different CMS.The presence of a well-deﬁned porous struc-ture(pore mouth opening)will allow determining the optimum size required for CO2adsorption.This correlation will allow per-forming a relative simulation of the real conditions of pressure and temperature of the typical CO2released from industrial chimneys.2.ExperimentalCarbon molecular sieves were prepared from polymeric precur-sors.The selected CMS cover different pore size distributions,and all of them are commercially available from Supelco.Preparation conditions and particle size distribution,for some CMS,were de-tailed elsewhere[32].Several techniques have been used in order to analyze the porous structure of these CMS.Adsorption isotherms of N2at77K and CO2 at273K were carried out using a fully automated manometric equipment,designed and constructed by the Advanced Materials group(LMA),now commercialized as N2Gsorb-6[33].Before the adsorption experiments,samples were degassed at423K during 4h under vacuum(10À7bar).The‘‘apparent’’surface area was obtained applying the BET method in the relative pressure range p/p0=0.001–0.1.The total micropore volume(V0)was deduced from the N2adsorption data using the Dubinin–Radushkevich(DR)equa-tion,whereas the mesoporous volume(V meso)was obtained as the difference between the total pore volume(V t),corresponding to the amount adsorbed at p/p0%0.95,and V0.The pore volume corre-sponding to the narrow microporosity(V n)was obtained by apply-ing the D-R equation to the CO2adsorption data at273K[34].Immersion calorimetry measurements into liquids of different molecular dimensions(dichloromethane,0.33nm;benzene, 0.37nm;cyclohexane,0.48nm;2,2-dimethylbutane,0.56nm and a-pinene,0.7nm)were carried out in a Setaram Tian-Calvet C80D calorimeter at303K.A complete description of the experi-mental setup can be found elsewhere[35].Brieﬂy,prior to the experiment the samples were outgassed at423K for4h in a glass tube connected to vacuum equipment.After completing the heat treatment,the bulb containing the sample was sealed in vacuum and then introduced into the calorimetric cell containing the immersion liquid.Once thermal equilibrium was reached,the tip of the glass bulb was broken and the wetting liquid was allowed to contact the sample.The heat evolution resulting from the inter-action between the liquid and the clean surface was registered as a function of time.The integration of the signal,after making the appropriate corrections(those arising from the breaking of the tip(exothermic)and from the evaporation of the immersion liquid toﬁll the void volume of the bulb with the vapor at the corre-sponding vapor pressure(endothermic)),provides the total immer-sion enthalpy(ÀD H imm).Both mentioned corrections were previously calibrated using empty glass bulbs with different vol-umes.Experimental errors related to the measurement of immer-sion enthalpies are below3–4%.The total area accessible to the wetting liquid was estimated from the immersion enthalpy(J/g) of the CMS by using a nonporous graphitized carbon black(V3G; S BET:62m2/g)as a reference.The CO2adsorption isotherms on the different CMS were per-formed on the manometric equipment previously described,at atmospheric pressure and temperatures of273,298and323K. Before the adsorption experiment,samples were degassed at 423K for4h under vacuum(10À7bar).3.Results and discussion3.1.N2and CO2adsorption isothermsFig.1shows the N2adsorption–desorption isotherms for the dif-ferent CMS.Additionally,Table1collects the‘‘apparent’’BET surface area(S BET),the total micropore volume(V0),the total pore volume (V t),and the mesopore volume(V meso),obtained from the N2adsorp-tion data at77K,together with the volume corresponding to the narrow micropores(V n),obtained from the CO2adsorption iso-therms at273K[34].As it can be observed in Fig.1,the N2adsorp-tion isotherms are quite different among the different samples, showing that these CMS samples exhibit completely different tex-tural properties depending on both the polymer precursor and the synthesis conditions applied.There are:(i)pure microporous CMS (carbons C-1012,C-1018,C-1021,C-G60and C-SIII)exhibiting a type I isotherm,characteristic of typical microporous carbons;(ii) a pure mesoporous sample(carbon C-1016),with the amount ad-sorbed sharply increasing above a relative pressure of0.8,due to the presence of large sized mesopores and(iii)three carbons (C-569,C-1000and C-1003)with molecular sieving properties at micropore and mesopore scale,the later producing a sharp increase in the amount adsorbed at high relative pressures(above p/p0%0.8).A rigorous inspection of the different adsorption isotherms al-lows a more detailed description of the porous structure of these carbons.In this sense,although sample C-1012presents a wide plateau at high relative pressure range,characteristic of strictly microporous adsorbents,the wide knee of the N2isotherm at low relative pressures(below p/p0%0.2)indicates the presence of a wide micropore size distribution.Thisﬁnding is also conﬁrmed by comparing the large difference between the volume of narrow micropores(V n),deduced from the CO2adsorption data,and the volume of total micropores(V0),deduced from the N2adsorption data.It is noteworthy to mention that in the absence of kinetic restrictions the comparison of these two values(V0and V n)pro-vides a good evaluation toward the micropore size distribution. In fact,these two values are very similar for carbons exhibiting a narrow and homogeneous microporosity,while they become dif-ferent(V0>V n)with the widening of microporosity[34].Samples C-1018,C-1021,C-G60and C-SIII also exhibit type I isotherms.However,the porous structure of samples C-1018andA.Wahby et al./Microporous and Mesoporous Materials164(2012)280–287281C-1021is more complex.Both samples show a small type H2hys-teresis loop in the relative pressure (p/p 0)range from 0.4to 0.8,that reﬂects an additional contribution of a certain proportion of mesopores.The comparison of the adsorption data from these two samples highlights the difference between the volume of nar-row micropores and the total microporosity,despite the clear sim-ilarity observed in the development of porosity.In fact,(V 0ÀV n )is greater in the case of C-1018compared to C-1012,implying a larger portion of wide micropores in the former carbon.Sample C-G60exhibits a ﬂat isotherm proﬁle at high relative pressures,as in the case of carbon C-1012.Although the knee in the adsorp-tion isotherm is narrower at relative pressures below 0.2,carbon C-G60exhibits a wide pore size distribution with wider micropores (V 0>V n ).Sample C-SIII also shows a ﬂat proﬁle at high relative pressures.However,the narrower knee at low relative pressures (p/p 0<0.1)denotes the presence of a narrow microporosity,with a pore size distribution even narrower and more homogenous,since the values of V 0and V n are rather similar.As described above,samples C-1000and C-1003show,in addi-tion to the high N 2adsorption capacity at low relative pressures,a high adsorption at relative pressures above 0.8.A similar situation is observed for carbons C-569and C-1016,despite the absence of a high N 2adsorption at p/p 0<0.1.This behavior can be attributed,in the case of sample C-1016,to the nature of the material,since it is a non-microporous graphitized polymeric carbon material.In the case of sample C-569,the presence of a highly narrow and homog-enous microporosity can clearly be observed.In all four cases,the high N 2adsorption capacity is accompanied by a type H3–H1hysteresis loop on carbons C-1000,C-1003and C-1016,and by a type H2hysteresis loop on sample C-569.In principle,the high adsorption capacity together with the hysteresis loop reﬂects the presence of mesopore volume ranging from V meso =0.2to 0.6cm 3/g (Table 1).The ‘‘apparent’’surface area of the synthesized CMS ranges from 77m 2/g,in the case of nonporous sample C-1016,up to 2000m 2/g on sample C-1012.In summary,textural characterization results obtained from gas adsorption measurements show that the carbon molecular sieves prepared from polymeric precursors cover a wide range of pore sizes,depending on both the nature of the polymer precursor and the synthesis conditions used.CMS studied range from purely microporous materials to CMS that combine a well-developed microporosity together with a secondary porosity in the large micropores -small mesopores range.It is clear that some of them should not be structurally considered as proper molecular sieves.3.2.Immersion calorimetry measurementsThe heat of immersion of certain porous solids into liquids can be used to evaluate the porous structure,as well as the surface chemistry of the material [35–37].In the absence of speciﬁc inter-actions at the solid–liquid interface,the heat of immersion can be considered as an indirect measurement of the surface area accessi-ble to a certain molecule.Consequently,the selection of the appro-priate liquid in terms of molecular dimensions can be used to evaluate the surface area accessible to each molecule,that is,the experimental micropore size distribution.Fig.2shows the enthalpyTable 1Textural characteristics of the different carbon molecular sieves deduced from the N 2and CO 2adsorption isotherms at 77and 273K,respectively.aSample S BET (m 2/g)V 0(cm 3/g)V meso (cm 3/g)V t (cm 3/g)V n (cm 3/g)V 0ÀV n C-1016770.060.500.560.010.05C-5695480.220.200.420.200.02C-10216950.280.070.350.240.04C-10187570.300.060.360.230.07C-SIII 9920.400.000.400.330.07C-100010100.400.440.840.300.10C-G6011470.450.000.450.360.09C-100312600.480.59 1.080.340.14C-101220000.730.110.840.460.27aS BET :‘‘Apparent’’surface area calculated using the BET method;V 0:Micropore volume calculated by applying the DR equation to the N 2adsorption data at 77K;V t :Total pore volume obtained from the amount of N 2adsorbed at p/p0%0.95;V meso :Mesopore volume obtained by subtracting V t from V 0;V n :Volume of narrow micropores calculated by applying the DR equation to the CO 2adsorption data.282 A.Wahby et al./Microporous and Mesoporous Materials 164(2012)280–287of immersion(J/g)for the different CMSs into various liquids,cov-ering a wide range of kinetic diameters,from0.33nm(dichloro-methane)up to0.7nm(a-pinene).Carbon C-1012exhibits the highest enthalpy of immersion for all liquids studied,in agreement with the large surface area available on this sample.Nevertheless, the presence of similar values of immersion enthalpy indepen-dently of the size of the probe molecule clearly reﬂects the absence of important molecular sieving effects for the liquids used,up to 0.70nm,i.e.a wide micropore size distribution.For carbons C-1003,C-1000and C-G60there is a continuous but relatively small decrease in the enthalpy of immersion when increasing the molec-ular size of the probe molecule.The relatively high enthalpy of immersion observed for a large molecule such as a-pinene on these samples reﬂects the absence of signiﬁcant restrictions for mole-cules below0.7nm.By contrast,samples C-SIII,C-1018,C-1021 and C-569present a large heat of immersion into DCM(0.33nm) and benzene(0.37nm)that sharply decreases,thus indicating the inaccessibility of the inner porosity to molecules of a similar dimen-sion to cyclohexane(0.48nm)and2,2-DMB(0.56nm).N2and CO2 adsorption measurements described above showed that carbon molecular sieves C-569and C-SIII exhibit a narrow pore size distri-bution,since the volume of narrow micropores,deduced from the CO2adsorption data,was quite similar to the total volume of micropores,deduced from the N2adsorption[34].This observation is clearly conﬁrmed by calorimetric measurements.Carbons C-1018and C-1021have a slightly wider micropore size distribu-tion as deduced from the slightly larger difference between V n and V0,and the higher enthalpy of immersion for cyclohexane. These results suggest that carbons C-569,C-SIII,C-1021and C-1018are microporous carbon molecular sieves with a narrow pore mouth(pore diameter60.48nm for theﬁrst two and 60.56nm for the other two).Finally,sample C-1016exhibits a low enthalpy of immersion($7J/g)independently of the immer-sion liquid.This behavior is explained based on the textural charac-terization results which showed a low‘‘apparent’’surface area,in close agreement with the available surface areas obtained from immersion calorimetry(Table2).3.3.CO2adsorption at atmospheric pressureThe CO2adsorption,separation and/or storage on carbon materials by means of physical/chemical adsorption or combina-tion of both processes have created great expectations because of the promising results reported in the literature[38–41].As mentioned above,the nature of the precursor and the preparation conditions,more speciﬁcally,post-synthesis treatments as well as the pore structure(micropore volume,total surface area,pore size/shape,etc.)are key parameters deﬁning the total adsorption capacity.In order to evaluate the potential performance of CMS on this issue,CO2adsorption isotherms at different temperatures (273,298and323K)were carried out at atmospheric pressure (<1bar).The proﬁle of the CO2adsorption isotherms at273K is rather similar for all CMSs(Fig.3a),except in the case of sample C-1016 for which the adsorption capacity is practically nil over the whole pressure range.This observation anticipates that CO2adsorption requires the presence of micropores,since carbon C-1016does not adsorb CO2even at low temperatures.Taking a closer look at the other CMS,there is a clear increase in the amount of CO2ad-sorbed with the development of porosity,up to a maximum on sample C-1012,with a total amount adsorbed of232mg/g,i.e.23.3wt.%(Table3).The amount of CO2adsorbed at273K for these CMSs(except for sample C-1016)ranges from153mg/g(C-569)to 232mg/g(C-1012).Table2Total surface area available for dichloromethane(0.33nm),benzene(0.37nm),cyclohexane(0.48nm),2,2-dimethyl-butane(0.56nm)and a-pinene(0.7nm)obtained from immersion calorimetry measurements at303K for the different CMS.BET surface area is included for the sake of comparison.Sample S BET(m2/g)S DCM(m2/g)S BZ(m2/g)S cyclohexane(m2/g)S2,2DMB(m2/g)S a-pinene(m2/g)C-1016776471705649C-5695487266661058576C-10216958367282279554C-101875791987448212965C-SIII99211501143762824C-10001010118010921108907844 C-G60114715271264114581159C-100312601265129912871030884 C-1012200016761678163414571584A.Wahby et al./Microporous and Mesoporous Materials164(2012)280–287283The proﬁle of the CO2adsorption at298K is rather similar among the different CMS,at least at low relative pressures,the dif-ferentiation among samples being clearer close to atmospheric pressure(Fig.3b).In addition,the results presented in Table3 show that the total CO2adsorption capacity ranges from100mg/ g on sample C-569up to a maximum of164mg/g,in the case of the C-G60carbon.Finally,the CO2adsorption isotherms at323K maintain the observed similarity concerning the adsorption pro-ﬁles for all CMS at273and298K(Fig.3c).To better understand the role of the porous structure in the CO2 adsorption behavior of CMS at the three studied temperatures,the total amount adsorbed at atmospheric pressure was correlated with the different textural parameters obtained from the N2 adsorption measurements at77K(BET surface area,the total284 A.Wahby et al./Microporous and Mesoporous Materials164(2012)280–287volume of pores,V t )and CO 2at 273K (i.e.the total volume of nar-row micropores,V n ).Fig.4a shows that the amount of CO 2ad-sorbed is practically independent of the total volume of pores.However,Fig.4b shows that there is a somewhat better relation-ship between the CO 2adsorption and the BET surface area.Since no correlation was found with the total pore volume one has to as-sume that there should be a speciﬁc pore size which is the key fac-tor deﬁning the total adsorption capacity on the studied materials.In this sense,Fig.4c shows the correlation between the adsorbed amount of CO 2and the total volume of narrow micropores,ob-tained by applying the DR equation to the CO 2adsorption data at 273K.In this case,a better correlation can be clearly observed,thus conﬁrming the importance of a speciﬁc porosity for CO 2adsorption/capture [42,43].On the basis of the textural character-ization results described above,it is clear that CO 2adsorption at atmospheric pressure and different temperatures requires the presence of a high volume of narrow micropores together with a well-developed ‘‘apparent’’surface area.In this sense,carbon C-1012exhibits the highest adsorption capacity (232mg/g)at 273K among all CMS studied.This sample,with a wider pore size distribution (V 0ÀV n =0.27),exhibits the highest BET surface area (2000m 2/g)and the maximum micropore volume (0.73cm 3/g)as well as the maximum volume of narrow micropores (0.46cm 3/g)already suggested as primarily responsible for the observed up-take.Despite these excellent properties,sample C-1012exhibits a slightly lower CO 2uptake than expected from the extrapolation of the other samples (Fig.4c).This ﬁnding must be related to the wide micropore size distribution of carbon C-1012(Fig.2);this means that although the volume of narrow micropores is high in this carbon,not all narrow micropores (the widest ones)are effec-tive for an optimum adsorption capacity;a comparison of samples C-1012and C-SIII reﬂects a similar adsorption capacity in spite of the larger textural parameters for the former sample.The presence of a narrow micropore size distribution,i.e.narrow micropores below 0.48nm (see Fig.2)clearly conﬁrms the necessity of a well-developed speciﬁc porosity below 0.5nm.In these narrow micropores,the overlapping potential allows a better packing of the CO 2molecules giving rise to a larger adsorption capacity.Carbons C-G60,C-SIII and C-1003exhibit quite similar CO 2adsorp-tion capacities (around 220mg/g)at 273K,since these samples possess a very similar volume of narrow micropores (around 0.35cm 3/g).In summary,the total amount of CO 2adsorbed at 273K increases with increasing volume of narrow micropores,thus conﬁrming the observed correlations.Excluding sample C-1016,the calculated CO 2densities for these CMSs (Table 4),considering only the narrow micropore volume,range from 0.77g/cm 3for samples with a narrow pore size distri-bution (e.g.C-569),to an average value of 0.50g/cm 3in the case of samples with a wider pore size distribution (e.g.C-1012).The high-est density of CO 2adsorbed is presented by sample C-569,with theTable 3Total amount of CO 2adsorbed at 1bar and different temperatures (273,298and 323K)for the different carbon molecular sieves.Amount of CO 2adsorbed (mg/g)Sample/Temperature 273K 298K 323K C-101223213279.3C-102116410772.6C-100321812583.8C-56915310068.3C-SIII 22214595.2C-10167.5 4.08 3.37C-G60224164109C-100017712378.1C-101815711780.2Table 4Average CO 2density on narrow micropores for the different carbon molecular sieves at 273K and 1bar.Sample CO 2density (g/cm 3)C-10120.5043C-10210.6833C-10030.6412C-5690.7650C-SIII 0.6727C-1016-C-G600.6222C-10000.5900C-10180.6826A.Wahby et al./Microporous and Mesoporous Materials 164(2012)280–287285narrowest micropore size distribution(V0ÀV n=0.02).However, this sample exhibits the lowest CO2adsorption capacity for all studied CMS because of low V n and S BET values.This observation must be attributed to the presence of a certain mesoporosity,not contributing to the adsorption of CO2,together with the presence of a poor-developed narrow microporosity.Consequently,in order to reach a high amount of adsorbed CO2(mg/g),the presence of a high volume of narrow micropores,together with a high adsorp-tion density(g/cm3),and a narrow pore size distribution seem mandatory requirements.To conﬁrm thisﬁnding,the isosteric heat of CO2adsorption(Q)was calculated for the different CMSs as a function of the surface speciﬁc coverage n x and at the different temperatures,by applying the Clausius–Clapeyron equation[44].Fig.5shows that the isosteric heat of adsorption remains con-stant or slightly increases with coverage on most carbons,the in-crease being larger for samples C-1018,C-569and C-1021. Carbons C-569and C-SIII exhibit the maximum values of Q,which practically remain constant on the latter($25.4kJ/mol).Excluding sample C-1016(for which the isosteric heat of adsorption de-creases from9to7.7kJ/mol when the surface coverage increases), the increase in isosteric heat with increasing surface coverage can be explained by the presence of intermolecular interactions be-tween the adsorbed CO2species.By contrast,sample C-1016, which is non-microporous,exhibits a very limited CO2adsorption capacity that implies a lower value of Q.The decrease in isosteric heat when increasing surface coverage suggests the energetic het-erogeneity of the adosorption sites(the mesoporous size distribu-tion).Table5shows the values of isosteric heat of adsorption at zero coverage for all studied carbonsðq0stÞ;excluding the heat gen-erated by interactions between CO2adsorbed molecules.The value of q0stranges from10kJ/mol on the non-microporous carbon (C-1016)to25kJ/mol in the case of microporous CMSs with a nar-row pore size distribution(e.g.C-569and C-SIII).The high value of the isosteric heat of adsorption at zero coverage,indicates the pres-ence of strong interactions between CO2adsorbed molecules and the walls of the narrow micropore.In general,CMSs with a high volume of narrow micropores,together with a narrow pore size distribution,exhibit a high adsorption capacity at the three studied temperatures,as well as a high density of CO2adsorbed in the nar-row micropores(e.g.C-SIII).In summary,the analysis of a series of commercial carbon molecular sieves indicates that a narrow micropore size distribu-tion,a pore entrance below0.48nm and,indirectly,a maximum density of CO2adsorbed,are the necessary conditions for an opti-mal adsorption capacity[40].At the three studied temperatures, samples with a high volume of narrow micropores exhibit higher amounts of CO2adsorbed.These narrow pores are postulated to be the key factor of the CO2adsorption process.On the other hand, the densities calculated for the different CMS of the adsorbed CO2, at atmospheric pressure,strengthen the need for a narrow pore size distribution.This is conﬁrmed by the isosteric heat of adsorp-tion calculated,for all samples at zero coverage.4.ConclusionsA series of commercial carbon molecular sieves(CMS)prepared from different polymeric precursors have been characterized by means of adsorption of N2at77K and CO2at273K,together with measurements of immersion calorimetry into liquids of different molecular dimensions.Experimental results show that these CMSs cover a wide range of porosity,from purely microporous carbons to samples containing wide micropores together with a certain proportion of mesoporosity.CO2adsorption studies on these well-characterized CMS,at atmospheric pressure(1bar)and three different temperatures(273,298and323K)conﬁrm that a high CO2adsorption capacity requires the presence of a well-developed microporosity,as well as a high volume of narrow micropores. Additionally,the narrow micropores seem to be the key factor, leading to a maximum capacity of CO2adsorption,even when per-forming the adsorption at a temperature similar to that of anthro-pogenic emissions of CO2.AcknowledgementsAuthors acknowledgeﬁnancial support from Generalitat Valen-ciana(PROMETEO/2009/002-FEDER).References[1]S.A.Levin,Ecology73(1992)1943.[2]J.M.Melillo,A.D.Mcguire,D.W.Kicklighter,B.Moore,C.J.Vorosmarty,A.L.Schloss,Nature363(1993)234.[3]D.S.Schimel,Glob.Change Biol.1(1995)77.[4]J.Paul,C.-M.Pradier(Eds.),Carbon Dioxide Chemistry:Environmental Issues,Royal Society of Chemistry,Cambridge,UK,1994.p.405.[5]M.M.Halmann,M.Steinberg,Greenhouse Gas Carbon Dioxide Mitigation:Science and Technology,Lewis Publishers,Boca Raton,FL,1999.p.568. [6]M.M.Maroto-Valer, C.S.Song,Y.Soong,Environmental Challenges andGreenhouse Gas Control for Fossil Fuel Utilization in the21st Century, Kluwer Academic/Plenum Publishers,New York,2002.p.447.[7]DOE/NETL,Carbon Sequestration.Technology Roadmap and Program Plan Department of Energy,National Energy Technology Laboratory,May 2005.[8]D.Aaron,C.Tsouris,Sep.Purif.Rev.40(2005)321.[9]S.Choi,J.H.Drese,C.W.Jones,Chem.Sus.Chem.2(2009)796.[10]P.V.Danckwerts,Chem.Eng.Sci.34(1979)443.[11]P.V.Danckwerts,M.M.Sharma,Trans.Inst.Chem.Eng.44(1966)244.[12]K.T.Chue,J.N.Kim,Y.J.Yoo,S.H.Cho,R.T.Yang,Ind.Eng.Chem.Res.34(1995)591.Table5Isosteric heat of adsorption at zero loading(kJ/mol)for thedifferent CMS.Sample q0st(kJ/mol)C-101218.792C-102115.215C-100317.552C-56924.285C-SIII25.176C-10169.9421C-G6023.036C-100022.324C-101821.415286 A.Wahby et al./Microporous and Mesoporous Materials164(2012)280–287。

合成氨净化工段工艺流程

合成氨净化工段工艺流程英文回答：The purification process in the ammonia synthesis section is crucial for ensuring the production of high-quality ammonia. This process involves the removal of impurities, such as carbon dioxide, water, and hydrogen sulfide, from the synthesized ammonia gas.The first step in the purification process is the removal of carbon dioxide. This is typically done using an absorption column, where the ammonia gas is brought into contact with a solvent, such as monoethanolamine (MEA). The carbon dioxide in the gas reacts with the MEA to form a soluble compound, which can then be separated from the ammonia gas.After the removal of carbon dioxide, the next step is the removal of water. Water is a common impurity in the ammonia gas and can have detrimental effects on downstreamprocesses. To remove water, the ammonia gas is passed through a series of adsorption beds containing a desiccant, such as molecular sieves. The desiccant selectively adsorbs the water molecules, allowing the dry ammonia gas to pass through.The final step in the purification process is the removal of hydrogen sulfide. Hydrogen sulfide is a toxic and corrosive impurity that needs to be removed to meet product specifications. This is typically done using a chemical reaction with a scavenger, such as iron oxide or zinc oxide. The scavenger reacts with hydrogen sulfide to form a solid compound, which can then be separated from the ammonia gas.Overall, the purification process in the ammonia synthesis section involves the removal of carbon dioxide, water, and hydrogen sulfide from the synthesized ammonia gas. This ensures the production of high-quality ammonia that can be used in various industrial applications.中文回答：合成氨净化工段的工艺流程对于确保高质量氨的生产至关重要。

Pressure Swing Adsorption

COMMENTARIESPressure Swing AdsorptionIntroductionPressure swing adsorption(PSA)is a very versatile technology for separation and purification of gas mix-tures.Some of the key industrial applications include (a)gas drying,(b)solvent vapor recovery,(c)fraction-ation of air,(d)production of hydrogen from steam-methane reformer(SMR)and petroleum refinery off-gases,(e)separation of carbon dioxide and methane from landfill gas,(f)carbon monoxide-hydrogen sepa-ration,(g)normal isoparaffin separation,and(h)alcohol dehydration.There are several hundred thousand PSA units operating around the world servicing these and other applications.In fact,PSA has become the state-of-the-art separation technology for application areas a-d listed above,and the sizes of these units range from very small(∼300SCFD units for the production of90% O2from air for personal medical use)to very large(∼100 MMSCFD units for the production of99.999+%hydro-gen from an SMR).Many of these processes are de-scribed in the published books and review papers on the subject.1-13The growth in the research and development of PSA technology has been phenomenal since the first U.S. patent on the subject,authored by C.W.Skarstrom,was granted in1960.14A recent survey showed that∼600 U.S.patents on PSA were issued in the application areas a,c,and d alone during1980-2000,while the number of published papers with PSA as the keyword exceeded800during the period of1970-2000.15The concept of PSA for gas separation is relatively simple.Certain components of a gas mixture are selectively adsorbed on a microporous-mesoporous solid adsorbent at a relatively high pressure by contacting the gas with the solid in a packed column of the adsorbent in order to produce a gas stream enriched in the less strongly adsorbed components of the feed gas. The adsorbed components are then desorbed from the solid by lowering their superincumbent gas-phase par-tial pressures inside the column so that the adsorbent can be reused.The desorbed gases are enriched in the more strongly adsorbed components of the feed gas.No external heat is generally used for desorption.Many different nomenclatures are used to describe these concepts.A PSA process carries out the adsorption step at a superambient pressure,and the desorption is achieved at a near-ambient pressure level.A vacuum swing adsorption(VSA)process undergoes the adsorp-tion step at a near-ambient pressure level,and the desorption is achieved under vacuum.A pressure-vacuum swing adsorption(PVSA)process utilizes the benefits of both concepts.Although simple in concept, a practical PSA/VSA process can be fairly complex because it involves a multicolumn design where the adsorbers operate under a cyclic steady state using a series of sequential nonisothermal,nonisobaric,and non-steady-state steps.These include adsorption,de-sorption,and a multitude of complementary steps which are designed to control the product gas purity and recovery and to optimize the overall separation perfor-mance.9The research trends are to(a)produce purer products at higher recovery,(b)lower adsorbent inven-tory and energy of separation,and(c)increase the scale of application at a lower overall cost.Unique PSA cycles are also designed to simulta-neously produce two pure products from a multicompo-nent feed gas(e.g.,O2and N2from air,CO2and H2from SMR off-gas,and CO2and CH4from landfill gas)as well as to produce a product gas containing a component which is not initially present in the feed gas(ammonia synthesis gas from SMR off-gas).These examples dem-onstrate the wide flexibility of PSA process designs.10,16,17 The key reasons for such growth in this technology are as follows:18(a)An extra degree of thermodynamic freedom for describing the adsorption process introduces immense flexibility in PSA process design as compared with other conventional separation tools such as distillation,ex-traction,or absorption.(b)Numerous microporous-mesoporous families of adsorbents(new or modified)like activated carbons, zeolites,aluminas,silica gels,and polymeric sorbents exhibiting different adsorptive properties for separation of gas mixtures(equilibria,kinetics,and heats)are available.(c)The optimum marriage between a material and a process in designing the PSA separation scheme pro-motes innovations.(d)Many PSA process paths can be designed for the same separation objectives.A good example of items a-d listed above can be found in the area of air fractionation by adsorption. Approximately390U.S.patents were issued on the subject during the last20years.15The processes are designed to(a)separately produce∼23-95%O2from air,(b)separately produce98-99.99+%N2from air,and (c)simultaneously produce∼90+%O2and99+%N2 from air.Many different zeolites having different ther-modynamic selectivities and capacities for adsorption of N2over O2are employed,and the processes are tailor-made to fit the zeolite properties in order to produce the specific product demands for cases a and c.11Many different carbon molecular sieves having different ki-netic selectivities and capacities for adsorption of O2 over N2are employed for case b with appropriate process designs for controlling the N2product purity and recovery.19,20These processes use many different de-signs of PSA,VSA,or PVSA cycles and operating conditions in order to achieve the final goals.A key success in this area has been the lowering of the specific power(<10KWD/ton)of O2production(∼90+%)from air below that of the conventional cryogenic distillation route by the use of LiX zeolite as the N2selective1389Ind.Eng.Chem.Res.2002,41,1389-139210.1021/ie0109758CCC:$22.00©2002American Chemical SocietyPublished on Web02/09/2002adsorbent,21,22while retaining the traditional advantage of lower capital cost for a PSA/VSA process.This opens up the door for increasing the sizes of O2-PSA processes beyond today’s standards.Parallel developments can be cited in the other application areas.9,10,12,13It is expected that this trend will continue in the future.New PSA cycles using old or new adsorbents(preferably tailor-made for each other)will continue to be developed in the existing and new application areas.Good combina-tions of PSA processes and adsorbents are needed for bulk separation of N2and CH4at high pressures, separation of dilute O2from Ar,and bulk separation of propane-propylene mixtures.Emerging ConceptsSeveral very interesting PSA process developments are emerging.They include(a)rapid PSA cycles,(b) novel PSA adsorber designs,(c)sorption-enhanced reac-tion processes(SERP),and(d)high-temperature PSA (HTPSA)cycles.(a)The rapid PSA processes are designed to increase the productivity of the processes by an order of magni-tude by using total process cycle times of seconds instead of minutes as in the case of the conventional PSA cycles.This is achieved by simply running a conventional cycle faster using novel hardware(e.g., rotary valves)23or by changing both the adsorber and the process cycle designs.11However,separation ef-ficiency and performance may be compromised(lower product purity or recovery)because of time limitations in incorporating the complementary cyclic steps and limitations caused by adsorption kinetics.24(b)Faster PSA cycles are often limited by the hydro-dynamic constraints(gas maldistribution,adsorbent fluidization,column pressure drop,etc.),which limit gas flow rates through packed beds.Radial bed adsorbers where the adsorbent is placed between two concentric cylinders and gas flows radially through the packed section may alleviate many of these problems.These adsorbers,however,are more expensive.Many such designs have been proposed and developed which allow faster cycles,higher gas throughputs at lower pressure drops,and complete absence of fluidization.25,26An interesting possibility will be the development of a PSA cycle using a rotary bed adsorber which has been successfully employed for practicing thermal swing adsorption concepts.27(c)The SERP is a hybrid concept where an equilib-rium-controlled reaction is carried out in the presence of an adsorbent which selectively removes one of the undesired reaction products from the reaction zone,thus increasing the yield and the rate of formation of another desired product by Le Chatelier’s principle.The adsor-bent is then regenerated periodically using the prin-ciples of PSA.The concept has also been called pressure swing reactor.28Very recently,a novel process called SERP-H2was demonstrated in a pilot-scale unit for production of CO-and CO2-free(<40ppm)H2by SMR using a CO2selective chemisorbent and a SMR catalyst inside the reactor.29,30The primary impurity in the90+ mol%H2product(dry basis)was unreacted CH4.A high conversion of CH4to H2at a much lower reaction temperature than the conventional SMR process could be achieved.The concept may be useful for direct production of CO-free H2in a fuel cell application. (d)A HTPSA(∼200°C)process was developed for sequestering and recovering CO2from a hot,wet waste gas.31This process also used a chemisorbent for CO2 (∼10-20mol%in feed),and it recovered∼80%of feed CO2as an essentially pure CO2product(dry basis).The concept may be very attractive for controlling green-house effects by removing CO2from a hot and wet flue gas without cooling the gas and removing the water. PSA Process DesignDespite such growth in the practical applications of this technology,the design and optimization of a PSA system still largely remains an experimental effort.A priori design of a practical PSA system that can guarantee the commercial specifications without the use of supporting data from a bench-or pilot-scale process rig may not yet be feasible for the following two reasons. First,most of the practical PSA processes are fairly complex,involving a number of sequential but interact-ing unsteady-state cycle steps.It may be possible to formulate a rigorous mathematical framework(model) to describe such processes,but it is usually expensive and time-consuming to solve such models with the accuracy and reliability needed for industrial design.It requires repeated numerical solutions of a set of coupled nonlinear partial differential equations in the time and space domain with different initial and boundary condi-tions defining the steps of the process for any given cycle until a cyclic steady-state solution is achieved.The computation time for such calculations is often prohibi-tive,and model simplification becomes imperative.10 Second,fundamental understanding of the multicom-ponent gas-solid interactions(thermodynamic and kinetic)that govern the performance of a PSA adsorber is very limited because of their complexity.An accurate description of these interactions must be available before the mathematical process model can be solved.10 It is essential that these interactions are known under all conditions of temperature,pressure,and composition prevailing in a PSA adsorber during all steps of the process.Because these conditions may vary widely in a practical PSA process,the measurement and correlation of such a massive volume of data are often impractical. One needs to predict these interactions from a minimum source of experimental data for the system of interest. Although much progress has been made in this area, the state of the art is often not adequate for satisfying the general design needs.A common practice is to develop a simplified and specific model for the PSA process of interest and use simplistic descriptions(models or empirical)of the gas-solid interactions for the relevant system,to evaluate approximately the effects of the design variables on the performance of the PSA process and finally obtain a crude optimum design.The effort is always closely tied to experimental verification and fine-tuning by measur-ing actual process data from a pilot plant.Such models have often proved to be very useful for process optimiza-tion and screening new ideas.10The partial differential equations to be solved are the coupled mass,momentum,and heat(both gas and adsorbed phases)conservation equations describing the state of the adsorber for each step of the PSA process. The key input data for solving these equations are(i) the multicomponent adsorption equilibria,(ii)the multicomponent adsorptive/desorptive mass-transfer characteristics,and(iii)the multicomponent isosteric heats of adsorption for the system of interest.1390Ind.Eng.Chem.Res.,Vol.41,No.6,2002There lies the crux of the problem.The design engineer must be able to predict these input data from a limited source of experimental data because the range of conditions(pressure,temperature,and gas composi-tion)encountered within the adsorber of a practical PSA process can be immense.Although numerous efforts have been made in the past30years to develop techniques to predict multi-component adsorption characteristics(thermodynamic and kinetic)from pure gas adsorption characteristics of the components,these models have met with only limited success.10,32The state of the art does not allow a priori selection of these methods for the system of interest without extensive experimental testing of the method.This obviously defeats the purpose.Currently, an extensive experimental database is needed(equilib-ria and kinetic)for the design of each separation system of interest.The quality of simulation of a PSA process perfor-mance using a process model largely depends on the detailedness of the model and the accuracy of the input data.33The results can be extremely sensitive to small errors(say(10-15%)in the input data for many process designs because these models often act as amplifiers of errors.On the other hand,many industrial PSA processes require very stringent product specifica-tions.For example,a PSA process for production of H2 from SMR off-gas must have a product H2purity of 99.999+mol%containing less than10ppm of CO x impurities.Furthermore,a difference of(2%in the estimation of the product H2recovery from the feed gas can make or break the economics of a process design for a medium-sized unit(∼50MMSCFD H2).These critical industrial demands are not often appreciated, and a large volume of published literature on PSA models fails to seriously evaluate the quality of the model and the input data by comparing the model calculations with actual experimental process data only qualitatively.Nature of the ProblemThe problem at hand is by no means simple.The adsorbents of practical use,such as activated carbons, aluminas,silica gels,and zeolites,can be very hetero-geneous microporous solids.The first three types of adsorbent contain intricate networks of interconnected micro-and mesopores of various shapes and sizes,which give rise to a nonuniform distribution of gas-solid and gas-gas interaction fields within the adsorbent mass. Heterogeneity in these adsorbents is also caused by differences in the chemical nature of the surfaces at different parts of the adsorbent mass.The pore struc-tures of the zeolites may be ideally well-defined and uniform,but energetic heterogeneity for adsorption is introduced by lattice defects,the presence of hydrated and nonhydrated ion species of one or more kinds at different locations within the framework,the presence of trace moisture within the pore structure,nonuniform hydrolysis of the zeolite framework during regeneration, the existence of distributed Si/Al ordering of the frame-work,etc.34Another problem is introduced by the dissimilar nature of the adsorbate molecules in a gas mixture (sizes,shapes,polarities,etc.).This can severely com-plicate their interactions with the heterogeneous porous solid surfaces and with each other inside the pores of the adsorbent.It is apparent that realistic and accurate estimation of single-component or multicomponent gas-solid or gas-gas interactions within such distributed energy fields inside the adsorbent pores(which often remains unknown)will be difficult,if not impossible.Even the current experimental methods to characterize the pore structures of real heterogeneous microporous solids are very crude and full of uncertainty.The challenge,therefore,is to develop methods for quantitative characterization of adsorbent heterogeneity and to accurately predict multicomponent gas adsorp-tion characteristics(equilibria,kinetics,and heats) using a limited data source.The models,however,must also take into account dissimilar adsorbate properties and lateral interactions between the adsorbates at higher coverages.The models must also be analytical in order to be practically useful.This area should be a subject of serious research by adsorption scientists for many years to come because the problem is complex and success will not be easy.Unfortunately,the topic is being ignored in the recent years.There is also a desperate need to generate and compile a multicomponent gas adsorption database.A large volume of pure gas and some binary gas adsorp-tion equilibrium data are currently available in the literature,35but multicomponent gas equilibrium data are rare.The situation is identical for gas adsorption kinetics data.36The data for heats of adsorption are only emerging.37-39These data will be needed to test the reliability of the theoretical models.In conclusion,it can be said that the development of practical PSA processes for gas separation has been extremely successful despite the fact that an“in-depth”understanding of the complex physicochemical phenom-enon governing this technology may be lacking.Much more basic research is needed before a priori design of PSA processes can be made with acceptable accuracy and confidence.A judicious balance between experi-mental process development and theoretical designs using process models will continue to be the state of the art in the near future.Literature Cited(1)Ruthven,D.M.Principles of Adsorption and Adsorption Processes;John Wiley:New York,1984.(2)Yang,R.T.Gas Separation by Adsorption Processes;But-terworth:London,1987.(3)Suzuki,M.Adsorption Engineering;Kodansha:Tokyo, 1990.(4)Ruthven,D.M.;Farooq,S.;Knaebel,K.S.Pressure Swing Adsorption;VCH Publishers:New York,1994.(5)Crittenden,B.;Thomas,W.J.Adsorption Technology and Design;Butterworth-Heinemann:Oxford,U.K.,1998.(6)Keller,G.E.;Anderson,R.A.;Yon,C.M.Adsorption.In Handbook of Separation Process Technology;Rousseau,R.W.,Ed.; John Wiley:New York,1987;Chapter12.(7)Humphrey,J.L.;Keller,G. E.Adsorption.Separation Process Technology;McGraw-Hill:New York,1997;Chapter4.(8)Sircar,S.Adsorption.In The Engineering Handbook;Dorf, R.C.,Ed.;CRC Press:Boca Raton,FL,1996;Chapter59.(9)Sircar,S.Pressure Swing Adsorption Technology.In Ad-sorption Science and Technology;Rodrigues,A.E.,et al.,Eds.; Kluwer Academic Publishers:Dordrecht,The Netherlands,1989; pp285-321.(10)Sircar,S.Pressure Swing Adsorption:Research Needs in Industry.Proceedings of Third International Conference on Fun-damentals of Adsorption,Sonthofen,Germany,1989;Mersmann, A.B.,Scholl,S.E.,Eds.;AIChE:New York,1989;pp815-843.Ind.Eng.Chem.Res.,Vol.41,No.6,20021391(11)Sircar,S.;Rao,M.B.;Golden,T.C.Fractionation of Air by Zeolites.In Studies in Surface Science and Catalysis;Dab-rowski,A.,Ed.;Elsevier Science:New York,1998;Vol.120A,pp 395-423.(12)Sircar,S.;Rao,M.B.;Golden,T.C.Drying Gases and Liquids by Activated Alumina.In Adsorption on New and Modified Inorganic Solvents;Dabrowski,A.,Tertykh,V.A.,Eds.;Studies in Surface Science and Catalysis;Elsevier Science:New York, 1996;Vol.99,pp629-646.(13)Sircar,S.;Golden,T.C.Sep.Sci.Technol.2000,35,667.(14)Skarstrom,C.W.U.S.Patent2,944,627,1960.(15)Sircar,S.Adsorption2000,6,359.(16)Sircar,S.Sep.Sci.Technol.1990,25,1087.(17)Fuderer,A.U.S.Patent4,375,363,1983.(18)Sircar,S.Adsorpt.Sci.Technol.2001,19,347.(19)Knoblauch,K.;Heimbach,H.;Harder, B.U.S.Patent 4,548,799,1985.(20)Lemcoff,N.C.;Gmelin,R.C.U.S.Patent5,176,722,1993.(21)Hirano,S.Dynamic Adsorption Properties of Li Ion-Exchanged Zeolite Adsorbents.In Fundamentals of Adsorption; Kaneko,K.,Ed.;2001;Vol.7,in press.(22)Kirner,J.F.U.S.Patent5,268,023,1993.(23)Keefer,B.G.;Doman,D.G.WIPO International Publica-tion No.W097/39821,1997.(24)Sircar,S.;Hanley,B.F.Adsorption1995,1,313.(25)Poteau,M.;Eteve,S.U.S.Patent5,232,479,1993.(26)Smolarek,J.;Leavitt,F.W.;Nowobilski,J.J.;Bergsten,E.;Fassbaugh,J.H.U.S.Patent5,759,242,1998.(27)Hirose,T.;Kuma,T.Honeycomb Rotor Continuous Ad-sorber for Solvent Recovery and Dehumidification.2nd Korea-Japan Symposium on Separation Technology,1990.(28)Vaporciyan,G.G.;Kadlec,R.H.AIChE J.1987,33,1334.(29)Sircar,S.;Hufton,J.R.;Nataraj,S.U.S.Patent6,103,143, 2000.(30)Waldron,W.E.;Hufton,J.R.;Sircar,S.AIChE J.2001, 47,1477.(31)Sircar,S.;Golden,C.M.A.U.S.Patent6,322,612,2001.(32)Sircar,S.AIChE J.1995,41,1135.(33)Hartzog,D.G.;Sircar,S.Adsorption1995,1,133.(34)Sircar,S.Adsorption Technology s A Versatile Separation Tool.In Separation Technology s The Next Ten Years;Garside,J., Ed.;Institute of Chemical Engineers:Rugby,Warwickshire,U.K., 1994;pp47-72.(35)Valenzuela,D.P.;Myers,A.L.Adsorption Equilibrium Data Handbook;Prentice-Hall:Englewood Cliffs,NJ,1989.(36)Ka¨rger,J.;Ruthven,D.M.Diffusion in Zeolites and Other Porous Solids;John Wiley:New York,1992.(37)Dunne,J.;Rao,M.B.;Sircar,S.;Gorte,R.J.;Myers,A.L. Langmuir1997,13,4333.(38)Cao,D.V.;Sircar,S.Ind.Eng.Chem.Res.2001,40,156.(39)Cao,D.V.;Sircar,S.Adsorption2001,7,73.Shivaji SircarAir Products and Chemicals,Inc.,Allentown,Pennsylvania18195IE01097581392Ind.Eng.Chem.Res.,Vol.41,No.6,2002。

Shaarani_2011_Chemical-Engineering-Journal

Chemical Engineering Journal 169 (2011) 180–185Contents lists available at ScienceDirectChemical EngineeringJournalj 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 /c ejAmmonia-modiﬁed activated carbon for the adsorption of 2,4-dichlorophenolF.W.Shaarani,B.H.Hameed ∗School of Chemical Engineering,University of Science Malaysia,Engineering Campus,14300Nibong Tebal,Penang,Malaysiaa r t i c l e i n f o Article history:Received 5November 2010Received in revised form 27February 2011Accepted 1March 2011Keywords:AmmoniaActivated carbonOil palm empty fruit bunch 2,4-Dichlorophenol Adsorption Isotherm Kineticsa b s t r a c tIn this study,an oil palm empty fruit bunch-based activated carbon was modiﬁed using ammonia aque-ous solution.The effect of the chemical modiﬁcation on the adsorption capacity of modiﬁed activated carbon (MAC)for 2,4-dichlorophenol (2,4-DCP)adsorption was investigated.The monolayer adsorption capacity of MAC was 285.71mg/g at 30◦C.Adsorption isotherm ﬁtting revealed that Langmuir model was applicable for the MAC.The adsorption kinetics of 2,4-DCP onto MAC was described by the pseudo-second-order kinetic model.The surface modiﬁcation of the activated carbon using ammonia was shown to be able to increase its adsorption capacity for 2,4-DCP.© 2011 Elsevier B.V. All rights reserved.1.IntroductionIn recent years,increasing concern over pollution of water courses arising from different anthropogenic activities has resulted in growing demand of activated carbons [1].They have proved to be effective for the removal of a wide variety of organic and inorganic pollutants dissolved in aqueous media,or from gaseous environ-ments [2].However,activated carbon is expensive which limits its large scale application [3].A potential method to reduce its cost is to produce activated carbon from low-cost materials such as agri-cultural by-products,which has attracted an increasing research interest in recent years [4].The excellent performances of activated carbons is well known to be closely related to the well developed porous structures,large surface area and a wide spectrum of surface functional groups.It is an established fact that the activated carbon surface can display acidic,basic and/or neutral characteristics depending on the pres-ence of surface functional groups [5].Thus,these days more studies have focused on how to modify as well as to characterize the sur-face functional groups of carbon materials in order to improve or enhance their afﬁnities towards certain contaminants [3,6,7].The heteroatoms on the surface of activated carbon took sig-niﬁcant role on its application.The heteroatoms of porous carbon surface mainly contained oxygen,nitrogen,hydrogen,halogen,etc.,which bonded to the edges of the carbon layers and gov-erned the surface chemistry of activated carbon [8].Among these∗Corresponding author.Tel.:+6045996422;fax:+6045941013.E-mail address:chbassim@m.my (B.H.Hameed).heteroatoms,the oxygen-containing functional groups as well as nitrogen-containing functional groups were the widely recognized and the most common species formed on the surface of carbons.The presence of nitrogen-containing functional groups was claimed to increase the basicity of activated carbon surface and thus increased the adsorption of acidic agents such as phenolic deriva-tives.Practically,nitrogen is introduced into structure of activated carbon according to several procedures including treatment with ammonia or urea [9,10].Adsorption of phenolic derivatives on activated carbons has been intensively investigated [11],but relatively less work is reported related to modiﬁed carbons [12].In this work,oil palm empty fruit bunch-based activated carbon was treated with ammo-nia solution to modify its surface functional groups.The adsorption performance of the modiﬁed activated carbon on 2,4-DCP was then evaluated.2.Materials and methods 2.1.2,4-Dichlorophenol2,4-Dichlorophenol (2,4-DCP)of analytical reagent grade (Merck)was used as an adsorbate in this study.2,4-DCP has a chem-ical formula of C 6H 3Cl 2OH,with a molecular weight of 163.0g/mol.2.2.Preparation of activated carbonOil palm empty fruit bunch (EFB)used for preparation of acti-vated carbon was obtained from United Oil Palm Industries Sdn Bhd,Malaysia.The procedure used to prepare the activated car-1385-8947/$–see front matter © 2011 Elsevier B.V. All rights reserved.doi:10.1016/j.cej.2011.03.002F.W.Shaarani,B.H.Hameed /Chemical Engineering Journal 169 (2011) 180–185181400.010001500200030004000.05.006.08.010.012.014.016.018.020.022.00%TMACPAC3399.931594.17976.973850.993737.022360.861542.431052.87828.41790.89607.06Wavenumber (cm -1)Fig.1.FTIR spectrum for PAC and MAC.bon was referred to our previous work [13].The precursor was washed,sun dried,and crushed to particle size of 1–2mm.The pre-treated precursor was then soaked with 20wt%of phosphoric acid solution with an impregnation ratio of 16:1(acid:precursor).Sub-sequently the precursor was ﬁltered from phosphoric acid solution and then was dehydrated in the oven at 105◦C for 24h.The impreg-nated precursor was then loaded in a stainless steel vertical tubular reactor placed in a tube furnace.It was then heated at a rate of 10◦C/min from room temperature to 450◦C under puriﬁed nitro-gen (99.995%)ﬂow of 150cm 3/min.Activation took place for 2h before it was cooled down to room temperature.The ﬁnal prod-uct was then washed with 0.1M hydrochloric acid and hot distilled water until the pH of the washing solution reached 6–7,then dried in the oven at 105◦C for 24h and ﬁnally kept in an airtight con-tainer for further use.This sample was known as pristine activated carbon (PAC).2.3.Modiﬁcation of activated carbonAn amount of 4g of the PAC was immersed in 150mL of 10wt %ammonia solution (analytical reagent grade)and left for 48h at room temperature.After this time the treated activated car-bons were separated from liquid phase by mean of ﬁltration and then dried at 105◦C for 24h.The chemically treated activated car-bons were stored in a desiccator to prevent moisture build up.The obtained sample was called modiﬁed activated carbon (MAC).2.4.FTIR of activated carbonSince MAC was modiﬁed via chemical treatment,changes in surface functional groups on MAC from PAC needed to be stud-ied.The surface functional groups of both the PAC and MAC were detected by Fourier Transform Infrared (FTIR)spectroscope (FTIR-2000,Perkin Elmer).The spectra were recorded from 4000to 400cm −1.2.5.Batch equilibrium studiesAdsorption was performed in a set of 250mL-Erlenmeyer ﬂasks where 200mL of 2,4-DCP solutions with various initial concentra-tions (25–250mg/L)were placed.Equal mass of activated carbon (0.2g)was added to each ﬂask and kept in an isothermal shaker (30,40or 50◦C)at 130rpm for 24h to reach equilibrium.The pH of the solution was kept original without any adjustment.Aque-ous samples were taken from the solution and the concentrationswere analyzed.All samples were ﬁltered prior to analysis in order to minimize interference of the carbon ﬁnes with the analysis.Each experiment was duplicated under identical conditions.The concen-trations of 2,4-DCP before and after adsorption were determined using a double beam UV–vis spectrophotometer (Shimadzu,Japan)at 280nm.The amount adsorbed at equilibrium,q e (mg/g),was calculated by:q e =(C 0−C e )V(1)where C 0and C e (mg/L)are the liquid-phase concentrations of 2,4-DCP at initial and equilibrium,respectively.V (L)is the volume of the solution and W (g)is the mass of dry adsorbent used.2.6.Effect of solution pHThe effect of solution pH on the 2,4-DCP removal was stud-ied at different initial pH (2–12).The pH was adjusted using 0.1M HCl and/or 0.1M NaOH and was measured using pH meter (Model Ecoscan,EUTECH Instruments,Singapore).The 2,4-DCP initial con-centration was ﬁxed at 100mg/L,with activated carbon dosage of 0.2g/100mL and temperature of 30◦C.3.Results and discussion3.1.Surface chemistry of activated carbonsFig.1shows FTIR spectra of the PAC and the MAC.Several broad peaks were detected on the spectra of both activated car-bons at bandwidths around 3850–3300which could be assigned to O–H stretching vibration of hydroxyl functional groups.On the other hand,the spectra of these samples show some differences.For example,the spectrum of the PAC exhibits one strong peak at 1594cm −1associated with C–O group whereas peak at 976cm −1was attributed to P–O–C carbons asymmetric stretching,interac-tion between aromatic ring vibration and P–C (aromatic)stretching and/or symmetrical stretching of PO 2and PO 3in phosphate–carbon complexes.Therefore,P-containing carbonaceous structures like acid phosphates and polyphosphates are formed in the samples carbonized in the presence of H 3PO 4[14].MAC displays presence of one strong peak at 2360cm −1which corresponds to the C C stretching vibrations in alkyne groups.In addition the spectrum of MAC exhibits the appearance of new bands related to N-containing species at 1542cm −1(cyclic amides [7])and 1052cm −1(C–N of the amine group [7])implying that the use of ammonia modiﬁcation182 F.W.Shaarani,B.H.Hameed/Chemical Engineering Journal169 (2011) 180–185Fig.2.Effect of solution temperature on2,4-DCP uptake on MAC at various initial concentrations.produced new nitrogen surface complexes.The results were com-parable with the work done by Przepiórski[15]which reported that more basic groups such as cyclic amides and amines groups were produced by ammonia-treated activated carbons.3.2.Effect of temperatureThe temperature has two major effects on the adsorption pro-cess.Increasing the temperature is known to increase the rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particles as a result of the reduced viscosity of the solution.In addition,changing the temperature alters the equilibrium capacity of the adsorbent for a particular adsorbate[16].The effect of temperature on adsorption of2,4-DCP on the MAC was investigated by varying the adsorption temperature at30,40 and50◦C.Fig.2shows the plot of adsorption capacity versus ini-tial2,4-DCP concentrations(25–250mg/L)at the three different temperatures.The adsorption of2,4-DCP on the MAC was found to gradually increase when the temperature was increased from 30to50◦C for all initial concentrations studied(25–250mg/L).In addition,an increase in temperature from30to50◦C increased the MAC monolayer adsorption capacities from285.71to312.50mg/g (Table1).This phenomenon indicates that the adsorption process is endothermic in nature.This may be due to an increasing number of molecules which may also acquire sufﬁcient energy to undergo an interaction with active sites at the surface.Furthermore,increas-ing temperature may produce a swelling effect within the internal structure of the activated carbons enabling large dyes to penetrate further as being reported byÖzdemir et al.[17].3.3.Effect of solution pHFig.3shows the effect of solution pH on the removal of2,4-DCP on the MAC.It was observed that the2,4-DCP removal was highly dependent on the pH of the solution which affected the sur-face charge of the activated carbon and the degree of ionization of the adsorbate.It was found that an increase in the solution pH led to a decrease in the2,4-DCP removal efﬁciency.The highest per-cent removal was achieved at pH2,with2,4-DCP uptake as high as97.24%whereas the lowest percent removal was recorded at pH12with only66.39%.2,4-DCP acts as a weak acid,the dissocia-tion of hydrogen ion from the phenol molecules strongly depended on the pH level of solution.More protons were available in acidic solution,thus increasing electrostatic attraction between molecu-lar species of2,4-DCP and the positively charged adsorptionsites,20406080100pH%RemovalFig.3.Effect of solution pH on2,4-DCP removal on MAC at30◦C(2,4-DCP initial concentration=100mg/L).thus causing an increase in the adsorption of2,4-DCP.As the pH of the solution increased,2,4-DCP dissociated,forming chlorophe-nolate anions while the surface functional groups on the activated carbons were either neutral or negatively charged.As a result of the electrostatic repulsion between the identical charges it then low-ered the adsorption capacity.This observation is similar with the trend reported by Hameed et al.[18]and Gao and Wang[19].3.4.Adsorption of2,4-DCPThe linear form of Langmuir isotherm[20]equation is given as: C eq e=1Q0C e+1Q0b(2) where C e is the equilibrium concentration of the adsorbate(mg/L), q e is the amount of adsorbate adsorbed per unit mass of adsor-bent(mg/g),Q0and b are Langmuir constants related to adsorption capacity and rate of adsorption,respectively.When C e/q e was plot-ted against C e,a straight line was obtained.Q0was calculated from the slope whereas b was found from the intercept(Fig.4).The linear form of Freundlich isotherm[21]is given by the fol-lowing equation:log q e=1n log C e+log K F(3) where C e(mg/L)is the equilibrium concentration of the adsorbate, q e(mg/g)is the amount of adsorbate adsorbed per unit mass of adsorbent,K F(mg/g(L/mg)1/n)and n are Freundlich constants with n giving an indication of how favorable the adsorption process.K F is the adsorption capacity of the adsorbent which can be deﬁned as the adsorption or distribution coefﬁcient and representsthe0.000.020.040.060.080.10252015105C e (mg/L)Ce/qe(g/L)ngmuir adsorption isotherm of2,4-DCP onto MAC at30◦C.F.W.Shaarani,B.H.Hameed /Chemical Engineering Journal 169 (2011) 180–185183Table 1Langmuir and Freundlich isotherm model constants and correlation coefﬁcients.Temperature (◦C)Isotherm Langmuir Freundlich Q 0(mg/g)K L (L/mg)R 2K F (mg/g(L/mg)1/n )1/n R 230285.710.2170.9949.2720.5850.9140303.030.2700.9958.8980.6250.9350312.504.0000.99319.5950.6200.93quantity of 2,4-DCP adsorbed onto activated carbon for a unit equi-librium concentration.The slope of 1/n ranging between 0and 1is a measure of adsorption intensity or surface heterogeneity,becom-ing more heterogeneous as its value gets closer to zero.A value for 1/n below one indicates a normal Langmuir isotherm while 1/n above one is indicative of cooperative adsorption.The plot of log q e versus log C e (Fig.5)gave a straight line with slope of 1/n whereas K F was calculated from the intercept value.The Langmuir and Freundlich adsorption constants evaluated from the isotherms with the correlation coefﬁcients for MAC are listed in Table 1.According to the data tabulated in Table 1,adsorp-tion of 2,4-DCP on MAC was found to follow the Langmuir isotherm model since the correlation coefﬁcient,R 2values obtained for Langmuir isotherm yielded the best ﬁt (close to unity)at all temper-atures,in comparison to Freundlich isotherm where the correlation coefﬁcient,R 2values obtained were only between 0.91and 0.93.Similar trend was observed for PAC where the Langmuir isotherm also gave the best ﬁt for the adsorption of 2,4-DCP at all range of temperatures studied with R 2values of 0.99[13].From Fre-undlich isotherm model,the values of 1/n attained were <1.This speciﬁed a normal Langmuir isotherm.The result indicated the homogeneous nature of the surfaces of the activated carbon,i.e.,each 2,4-DCP molecule/carbon adsorption had equal adsorption activation energy.The results also demonstrated the formation of monolayer coverage of 2,4-DCP molecule at the outer surfaces of MAC.The same trend was procured by several researchers [22–24]on the adsorption of 2,4-DCP on various adsorbents where the data gave a better ﬁt in Langmuir isotherm model in comparison to Freundlich isotherm model.The surface modiﬁcation on the oil palm empty fruit bunch-based activated carbon using ammonia solution was shown to be able to increase its adsorption capacity on 2,4-DCP,from 232.56[13]to 285.71mg/g (22.86%higher)at 30◦C.This enhancement was due to the electrostatic attraction between 2,4-DCP and the sur-face of MAC.2,4-DCP is predominantly present in anionic form.The chloride (–Cl)ion on the benzene ring,which increased the acidic character was responsible for forming anion on the oxygen atom of0.001.002.003.001.51.00.50.0-0.5Log C eL o g q eFig.5.Freundlich adsorption isotherm of 2,4-DCP onto MAC at 30◦C.the –OH group and also had a strong afﬁnity for activated carbon surfaces.The basic surface functional groups created by nitrogen-incorporation (via ammonia treatment)rendered the carbon more basic;and created a surface that was more positively charged.As a result of electrostatic attractions between the negative charge on 2,4-DCP compound and positive charge of modiﬁed carbon surface,hence the uptake of 2,4-DCP increased.This phenomenon is similar to previous work reported by Deng et al.[25]on removal of pen-tachlorophenol and 2,4-dichlorophenoxyacetic acid from aqueous solution by an aminated biosorbent.They reported that the surface charge on the aminated biomass played an important role in the sorption of anionic pollutants.Table 2lists the comparison of the maximum monolayer adsorption capacity of various types of chlorophenols on various adsorbents.The adsorption capacity of MAC for 2,4-DCP was rela-tively high and comparable to some previous works reported in the literature.3.5.Kinetic studiesA linear form of pseudo-ﬁrst-order model was described by Lagergren and Svenska [30]in the form:log(q e −q t )=log q e −k 12.303t (4)where q e and q t refer to the amount of 2,4-DCP adsorbed (mg/g)at equilibrium and at any time,t (h),respectively,and k 1is the equi-librium rate constant of pseudo-ﬁrst order sorption (1/h ).Values of k 1were obtained from the slopes of the linear plots of log(q e −q t )versus t ,as shown in Fig.6.The parameters of pseudo-ﬁrst-order kinetics model for MAC are summarized in Table 3.The pseudo-second-order equation [31]is expressed as:t q t =1k 2q 2e+ 1q et (5)where q e and q t are the adsorption capacities at equilibrium and at time t ,respectively (mg/g)and k 2is the rate constant of pseudo-second-order sorption (g/mg h).By plotting t /q t versus t (Fig.7),q e and k 2can be determined from slope and intercept.Table 3sum--112345t (h)l o g (q e - q t )Fig.6.Pseudo-ﬁrst-order kinetics for adsorption of 2,4-DCP on MAC at 30◦C.184 F.W.Shaarani,B.H.Hameed /Chemical Engineering Journal 169 (2011) 180–185Table 2Comparison of maximum monolayer adsorption capacity of various chlorophenols on various adsorbents.AdsorbentAdsorbateMaximum monolayer adsorption capacity (mg/g)References MAC 2,4-Dichlorophenol 285.71This work PAC2,4-Dichlorophenol 232.56[13]Maize cob carbon2,4-Dichlorophenol 17.94[22]Molecularly imprinted polymer (MIP)2,4-Dichlorophenol 183.80[23]Punica granatum (Pomegranate)peel 2,4-Dichlorophenol 75.80[24]Palm pith carbon2,4-Dichlorophenol 19.16[26]Oil palm empty fruit bunch 2,4-Dichlorophenol 27.25[27]Activated carbon ﬁber2,4-Dichlorophenol 372.00[28]Rattan sawdust activated carbon4-Chlorophenol188.68[29]Table 3Comparison of the pseudo-ﬁrst-order model and pseudo-second-order model for adsorption of 2,4-DCP on MAC at 30◦C.Initial 2,4-DCP concentration (mg/L)q e ,exp (mg/g)Pseudo-ﬁrst-order kinetic model Pseudo-second-order kinetic model q e ,cal (mg/g)k 1(1/h)R 2 q (%)q e ,cal (mg/g)k 2(g/mg h)R 2 q (%)2525.14 2.49 1.0840.9834.06025.19 1.3130.990.0775051.1417.17 1.3310.9825.11052.360.1660.990.896100101.9125.820.8150.9828.219100.000.1110.990.707150147.3253.730.7960.9924.012144.930.0480.990.613200195.07100.880.6950.9918.251188.680.0220.99 1.239250233.35119.210.4100.9918.488196.080.0240.996.0370.000.020.040.060.080.10t (h)t /q t (h r .g /m g )Fig.7.Pseudo-second-order kinetics for adsorption of 2,4-DCP on MAC at 30◦C.marizes the parameters of pseudo-second-order kinetics model for MAC.It can be seen from Table 3,that the correlation coefﬁcients,R 2obtained from the two kinetic models were greater than 0.9for all 2,4-DCP concentrations.The suitability of the model to describe the adsorption kinetics was further justiﬁed based on the normalized standard deviation value, q (%),which is deﬁned as:q (%)=[(q exp −q cal )/q exp ]2N −1×100(6)where N is the number of data points,q exp and q cal (mg/g)are the experimental and calculated adsorption capacity,respectively.The lower the value of q the better the model ﬁts.It is clearly shown in Table 3,that the pseudo-second-order kinetic model yielded the lower q values.This is in agreement with the R 2values obtained earlier and proves that the adsorption of 2,4-DCP onto the MAC could be best described by the pseudo-second-order kinetic model.The kinetic data obtained for the adsorption of 2,4-DCP using PAC [13]was also found to follow the pseudo-second-order kinetic model.These results proved that the chemical treatment on the activated carbon did not change its adsorption mechanism where the adsorption of 2,4-DCP on both the activated carbons was best described by the pseudo-second-order kinetic model.The results obtained were in accordance with the previous works carried out on adsorption of phenolic compounds on thebanana peel [32]and substituted phenols on activated carbon ﬁbers [33].4.ConclusionThe present investigation showed that the use of ammonia treat-ment to modify the oil palm empty fruit bunch activated carbon could enhance its adsorption capacity for 2,4-DCP.The equilibrium data for MAC were best represented by the Langmuir isotherm.The surface modiﬁcation of the activated carbon using ammonia was shown to be able to increase its adsorption capacity for 2,4-DCP from 232.56[13]to 285.71mg/g (22.86%higher).This was due to the basic surface functional groups created by nitrogen-incorporation (via ammonia treatment)which rendered the carbon more basic;and created a surface that was more positively charged and thus improved the activated carbon uptake.The adsorption kinetics was found to follow closely the pseudo second-order kinetic model.AcknowledgementThe authors acknowledge the research grant provided by the Universiti Sains Malaysia under the Research University (RU)Scheme (Project No.1001/PJKIMIA/814003).References[1]J.de Celis,N.E.Amadeo,A.L.Cukierman,In situ modiﬁcation of activated carbonsdeveloped from a native invasive wood on removal of trace toxic metals from 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化学吸附(co2-tpd)单位

化学吸附(co2-tpd)单位1. 简介化学吸附(co2-tpd)是一种用于评估材料对CO2吸附能力的重要分析方法。

该方法通过热脱附测量，评估材料的CO2吸附量和吸附能力。

在进行CO2捕获和储存研究的过程中，CO2-TPD单位的选择对于准确评估材料的性能非常重要。

2. CO2-TPD测试原理CO2-TPD测试通过加热样品，将已吸附的CO2从材料表面脱附出来，然后通过检测和记录脱附过程中释放的CO2量来评估材料的CO2吸附性能。

在CO2-TPD测试中，有两个重要的参数：脱附温度和脱附峰值。

3. 脱附峰值脱附峰值是指在CO2-TPD测试中，释放CO2的峰值信号。

脱附峰值的大小与材料对CO2的吸附能力有关，通常情况下，脱附峰值越大，表示材料对CO2的吸附能力越强。

在进行CO2-TPD测试时，测量和记录脱附峰值的大小非常重要。

4. 脱附温度脱附温度是指在CO2-TPD测试中，CO2开始从材料表面脱附的温度。

不同的材料对CO2的吸附能力和脱附温度有所不同，通过测量和记录脱附温度，可以评估材料对CO2的吸附特性。

在实际应用中，选择合适的脱附温度对于评估材料的CO2吸附能力至关重要。

5. CO2-TPD单位CO2-TPD单位通常可以通过比表面积和孔体积来表征，常用单位包括μmol/g、mmol/g和cm3/g等。

μmol/g是指每克材料对CO2的吸附量，mmol/g是指每克材料对CO2的吸附量，cm3/g是指每克材料的孔体积。

选择合适的CO2-TPD单位是评估材料CO2吸附性能的重要步骤，不同的CO2-TPD单位对于评估材料性能有不同的优劣。

6. CO2-TPD单位选择的考量因素在选择CO2-TPD单位时，需要考虑材料的实际应用和研究目的。

如果材料的CO2吸附量较小，通常选择μmol/g作为CO2-TPD单位更加合适；如果材料的CO2吸附量较大，通常选择mmol/g或cm3/g 作为CO2-TPD单位更加合适。

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CO2 adsorption on carbon molecular sieves 2012 164 280-7

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