Jiajia Ren ,Zheng Li ,2,Yifeng Chen ,2,Zhuhong Yang ,*,Xiaohua Lu

1 State Key Laboratory of Material-Oriented Chemical Engineering,Nanjing Tech University,Nanjing 210009,China

2 Energy Engineering,Division of Energy Science,Lulea University of Technology,Lulea 97187,Sweden

Keywords:CO2 adsorption Amino acid ionic liquid Supported ionic liquid sorbent Adsorption kinetics

A B S T R A C T Supported ionic liquid(IL)sorbents for CO2 capture were prepared by impregnating tetramethylammonium glycinate([N1111][Gly])into four types of porous materials in this study.The CO2 adsorption behavior was investigated in a thermogravimetric analyzer(TGA).Among them,poly(methyl methacrylate)(PMMA)-[N1111][Gly]exhibits the best CO2 adsorption properties in terms of adsorption capacity and rate.The CO2 adsorption capacity reaches up to 2.14 mmol·g-1 sorbent at 35°C.The fast CO2 adsorption rate of PMMA-[N1111][Gly]allows 60 min of adsorption equilibrium time at 35°C and much shorter time of 4 min is achieved at 75°C.Further,Avrami's fractional-order kinetic model was used and fitted well with the experiment data,which shows good consistency between experimental results and theoretical model.In addition,PMMA-[N1111][Gly]remained excellent durability in the continuous adsorption–desorption cycling test.Therefore,this stable PMMA-[N1111][Gly]sorbent has great potential to be used for fast CO2 adsorption from flue-gas.

1.Introduction

The CO2concentration in atmosphere has risen sharply due largely to the rapid consumption of fossil fuels in past decades,which is generally considered the main reason for climate change and global warming[1,2].It is extremely urgent to capture CO2from large emission sources,such as power plants.Nowadays,the most widely used technology for CO2capture is aqueous amine scrubbing,which has the advantages of fast absorption rate,excellent absorption capacity and low cost[3].Despite significant progress in technology optimization,it still suffers from some inherent drawbacks including solvent volatilization,equipment corrosion,and extensive energy consumption for sorbent regeneration[3,4].Therefore,the environment-benign sorbent and technology for efficient and economical CO2capture have already aroused a worldwide attention[5,6].

Ionic liquids(ILs)are considered as promising alternatives for CO2capture because of their distinctive properties such as extremely low vapor pressure,good thermal stability and high CO2solubility[7–9].In particular,some functional ILs containing amino groups(–NH2),derived from inexpensive amino acids,are attractive for CO2separation[10,11].Nevertheless,the high viscosity and the high cost of ILs impede their industrialization[12,13].An effective strategy to circumvent this issue is to immobilize ILs into porous solid materials,which could significantly enhance the rate of mass transfer and minimize the use of ILs[14,15].In recent years,such strategy has been widely used to synthesize supported IL sorbents by an impregnation–evaporation method to obtain good CO2sorption performance.For example,Wang et al.impregnated 1-ethyl-3-methylimidazolium lysine([EMIM][Lys])and other different AA(i.e.,Gly,Ala,Arg)-based[EMIM][AA]s into porous poly(methyl methacrylate)(PMMA)microsphere support for CO2capture,all the sorbents show a better adsorption performance in pure CO2atmosphere at 40°C compared to the pure ILs due to the good distribution of ILs in PMMA[16,17].I.H.Arellano and co-worker studied the adsorption performance of some other sorbents,which were prepared by impregnating 1-ethyl-3-methylimidazolium tri[bis(trifluoromethylsulfonyl)imide]zincate(EZT3)into SiO2,nano-SiO2and SBA-15[18,19].The EZT3-modified sorbents achieved a good adsorption capacity of 4.7 mmol·g-1sorbent in pure CO2at 40°C.This is the highest CO2adsorption capacity reported so far on supported IL sorbent,but the main disadvantage is the very long equilibration time.In our previous work,titanium oxide(P25)was used as support to immobilize 1-aminopropyl-3-methyIimidazolium bromide([APMIm]Br)for CO2capture[14].The synergistic effect between support and[APMIm]Br was identified as the main reason for the enhanced efficiency of CO2capture.The author ascribed this behavior to the difference of film thickness of IL,which was caused by the interaction between P25 surface and ILs.Until now,numerous porous materials such as silica gel(SG)[20],MCM-41[21],and activated carbons[22]have also been proposed for IL immobilization,it is still a great challenge to evaluate the effectiveness of supports since the IL loading,category and operating condition are all different in reported works.To acquire a fundamental understanding of the CO2adsorption behavior on supported IL sorbents,a systematic investigation is therefore required.

To the best of our knowledge,a systematic study on the CO2adsorption in supported IL materials is not available yet and the best supporting material candidate used for IL loading is still unknown.Therefore,a further investigation of CO2adsorption behavior on ILimmobilized sorbents was conducted in this work.As the supports of IL-impregnated sorbents,the materials should possess clear superiority,such as high porosity,low cost,high mechanical and chemical stability[14,16,20,21].In addition,the materials with the different microstructures will be helpful for the selection of a better support.So,SG,MCM-41,P25 and PMMA were selected and the best one was further research.It is generalized that the higher CO2adsorption amount will be obtained with more active sites on the materials.[N1111][Gly]containing an amine group was chosen due to its small molar mass[23,24],which means that more ILs can be impregnated into the support with a fixed pore volume.Also,the IL is cost effective because it can be synthesized by a simple method and inexpensive precursors[25,26].In this study,the effects of support,IL loading and operation conditions on CO2adsorption performance were investigated.Avrami's fractional-order kinetic model was used to simulate the dynamic adsorption curve and the applicability of the model was also investigated.

2.Experimental

2.1.Material

[N1111][Gly]was supplied by Lanzhou Institute of Chemical Physics,Chinese Academy of Sciences,China.PMMA beads(Diaion HP-2MG)were purchased from Sigma-Aldrich Chemical.MCM-41 was purchased from Nanjing XFNANO Materials Tech Co.,China.SG was purchased from Qingdao Haiyang Chemical Co.,China.P25 was purchased from Degussa.Ethanol(analytical reagent)was purchased from Lingfeng Chemical Reagent Co.,China.N2and CO2(99.99 vol%)was obtained from Nanjing Tianhong gas factory.SG was activated in a muffle furnace at 400°C for 4 h.The supports were dried and degassed in a vacuum oven at 80°C for 24 h before use.All pristine material looks creamy white and in a powder form except for PMMA microsphere.

2.2.Immobilization of[N1111][Gly]

Immobilization of[N1111][Gly]onto porous supports was carried out with the aid of ethanol;for details,refer to the previously reported impregnation–vaporization method[14,27].Specifically,the desired amount of[N1111][Gly]was firstly dissolved in ethanol.Then,solid particles were added to the above solution with the desired weight percentage and further stirred for 2 h.The formed turbid liquid was placed in a rotary evaporator(RE-52AA,Rongsheng,China)under vacuum at 60°C to remove ethanol.At last,the sorbents were dried at 80°C for 12 h in a vacuum oven.The prepared sorbents were denoted as S-x,where S represents the type of support and x represents the mass percentage of ILs in the sorbent.

It's worth reminding that the resulting sorbents were white fine particles at an appropriate feed ratio.However,at a higher feed ratio(about ＞60 wt%),the resulting sorbents were clay-like material and could not be used any further.

2.3.Characterization

The[N1111][Gly]loading in the prepared sorbents can be calculated from the feed ratio in the physical impregnation–vaporization process and the error is allowed[28].

The microstructural property of different supports(PMMA,SG,MCM-41,P25)before and after IL loading was characterized by N2adsorption at-196°C with a Micromeritics Tristar II 3020 Analyzer(Micromeritics,USA).Prior to the testing,the samples were degassed at 80°C for 8 h.The specific surface area was determined by the Brunauer–Emmett–Teller(BET)method.The total pore volume(Vtotal)was obtained as the volume of liquid nitrogen adsorbed at a relative pressure of 0.98.The pore size distribution of sorbents was derived from the desorption branches of isotherms based on the Barrett–Joyner–Halenda(BJH)theory.

The density of hydroxyl groups(ρ–OH)on the surface of SG and P25 were characterized by means of TGA analysis as described in the previous works[18,27,29].Briefly,about 10 mg of samples(SG,P25,MCM-41)was placed in a sample cell filled with flowing N2at a flow rate of 40 ml·min-1.The samples were heated from 35°C to 120°C with a heating rate of 10°C·min-1and maintained at 120°C for 10 min(Step 1).Then,the P25 sample was heated to 500°C with a heating rate of 20°C·min-1.The SG or MCM-41 sample was heated to 800°C with the same heating rate(Step 2).The density of acidic hydroxyl groups on the surface of sample was calculated as follows:

Here,SBETis the specific surface area,MH2Ois the molecular weight of water,NAis the Avogadro constant,and WT1and WT2are the weights of samples at 120°C and 500°C(or 800°C),respectively.α is a calibration factor(0.625).

The samples of PMMA and sorbents with different IL loading were characterized by X-ray diffraction(XRD)(D8 Advance,Bruker,Germany)in the range of 2θ=5°–80°.And the morphologies were observed under scanning electron microscopy(SEM)(JSM-7800F).TGA(TG209-F3,±0.0001 mg,Netzsch)was used to characterize the thermal stability of the samples.TGA analysis was performed in a pure N2atmosphere at a flow rate of 40 ml·min-1.

2.4.Adsorption measurements

CO2capture test is performed with a thermogravimetric analyzer(TGA)(TG209-F3,±0.0001 mg,Netzsch)[14,27].CO2/N2mixture with CO2concentration of 10 vol%was used for adsorption study.The gas flow rate was controlled at 100 ml·min-1.In a general way,about 20 mg of the[N1111][Gly]-supported sorbent was placed in a sample cell and heated to 100°C in a N2atmosphere at a flow of 100 ml·min-1for about 40 min to remove the adsorbate and moisture.The temperature was then adjusted to 35°C(or other desired adsorption temperature).Afterwards,the CO2/N2mixture gas was introduced until no evident weight gain was observed.The desorption process was implemented by introducing pure N2into the sample cell with a flow rate of 100 ml·min-1at 100°C.The weight change of the sorbent was recorded continuously.CO2adsorption capacity in mmol·g-1sorbent was calculated from the mass change of the samples during the adsorption process.In the sorbents'regenerability test,this process was conducted for 10 cycles to check the stability of the sorbents.

3.Results and Discussion

3.1.Effect of the support

Commercially available porous material(SG,PMMA,P25,MCM-41)with different pore size and porosity were selected to immobilized[N1111][Gly].The porous support has a small quantity of physical CO2adsorption,however,[N1111][Gly]reacts with CO2with stoichiometry of 2:1 to form carbamate[30].And porous support's contribution to CO2adsorption reduces after IL loading because most of the pores are occupied by ILs.Therefore,it is expected that ILs play a more significant role in CO2adsorption,while the role of support itself cannot be ignored as well based on the CO2adsorption amounts listed in Fig.1.The adsorbed amounts of CO2on the supported-[N1111][Gly]sorbents roughly follow the decreasing order of PMMA ＞SG ≈P25 ＞MCM-41,when the IL loading is no more than 40 wt%.When the IL loading reaches 40 wt%,the resulting sorbents based on PMMA and SG exhibit larger CO2capacity.

Fig.1.CO2 adsorption capacity of supported-[N1111][Gly]sorbents at 35°C from 10 vol%CO2 balance with N2.

As listed in Table 1,MCM-41,with the large surface area and pore volume among the various supports,shows the lowest CO2adsorption capacity.This unusual phenomenon may be derived from the different dispersion states of ILs formed on the surface of different supports.Due to the interaction of the surface tension between the host and guest,the ILs tend to fill the small pores of support[31,32].Therefore,small pore leads to a serious pore filling or occlusion during IL impregnation and results in a decreased approachability of the active adsorption sites to CO2molecules.Given all that,it is suggested that the support with small pore size is not suitable for IL immobilization.

Table 1 Physical and surface chemistry properties of supports

In general,the supports with low specific surface area and small total pore volume are unfavorable for CO2capture,since ILs can be easily agglomerated in such porous supports[22,33].SG acquires larger surface area and pore volume than that of P25,better dispersion of ILs on the surface could be expected and superior CO2capture performance.However,it is apparent that the supported-[N1111][Gly]sorbents(SG,P25)have commensurable CO2adsorbed amounts,when the IL loading ranges from 10 to 30 wt%(Fig.1).This can probably be attributed to the fact that the density of the hydroxyl groups(–OH)on the surface of P25 was much higher than that of SG(Table 1).It was found that the presence of these–OH groups on the support surface would form a hydrogen bond readily with the amino group of[N1111][Gly].Such interaction between surface–OH group and immobilized[N1111][Gly]could lead to a good dispersion and increase the chance of CO2–[N1111][Gly]interaction[34].Another reason is the relatively larger pore size.Compared with SG,relatively large pores in P25 are favorable for the distribution of ILs and reduce the aggregation caused by pore filling or clogging.When the IL loading reaches to 40 wt%,the P25 support could not afford to load more ILs and consequently reduce the CO2adsorption capacity.In addition,unlike the support of P25,the lower surface hydroxyl density(ρ–OH=0.61 nm-2)in MCM-41 could not reduce the IL aggregation phenomenon caused by the small pore size.

Compared to other supports,PMMA acquires a hierarchical structure and larger specific surface area,which shows the best adsorption capacity for[N1111][Gly]with loading from 10%to 50 wt%.Theoretically,for the technical field of CO2separation,the preferable supported ionic liquid sorbent morphology is the one with the internal pore and surface completely filled with the ILs[35].In such a way,the sorbent could take full advantage of the CO2absorption capacity of ILs.The amount of CO2adsorbed by support IL sorbents is attributed to two factors[12].One is the mass transfer rate,and the other is CO2absorption capacity of the IL supported on the porous supports.Obviously,compared to other porous material,PMMA with higher porosity and relative larger pore could immobilize more[N1111][Gly]in the case of good distribution.Therefore,PMMA impregnated with more ILs could adsorb more CO2.Moreover,the fixed bed or fluidized bed usually used for the adsorption process requires sufficient particle size and anti-wear capability of the sorbent.PMMA support acquires appropriate size(300–710 μm,Table 1)and sufficient robustness,while other supports are in powder form and need a further fabrication,which may influence the CO2capture[36].Based on these facts,PMMA was selected as the support for further systematic investigation.

3.2.Effect of the[N1111][Gly]loading

The effects of[N1111][Gly]loading(0–60 wt%,sorbent basis)on CO2adsorption characteristics at 35°C in 10 vol%CO2/N2mixture gas and the microstructural property of as-fabricated[N1111][Gly]-impregnated PMMA supported sorbents were also studied.

Fig.2 introduces the XRD patterns of PMMA and sorbents with different IL loading.A broad peak was obtained from these samples at about 2θ=15°and 29°,which indicated that the PMMA and the resulting sorbents are a semi-crystalline polymer[37].After impregnation with ILs,the IL-impregnated sorbents remained the character of PMMA but with small shift,indicating that the loading of ILs did not affect the structure of PMMA,and the interaction between ILs and PMMA caused the small shifting of the broad peaks.With the increase of IL loading,the broad peak became weaker and broader indicating the increase of amorphous phase in the IL-impregnated sorbents.

Fig.2.XRD patterns of PMMA and sorbents with different IL loading.

Fig.3.(a)Dynamic CO2 capacity for PMMA-x with different[N1111][Gly]loading at 35°C from 10 vol%CO2 balance with N2.(b)N2 adsorption/desorption isotherms and pore size distributions of different[N1111][Gly]loading sorbents.

As illustrated in Fig.3(a),the pristine PMMA and pure[N1111][Gly]show negligible CO2adsorption capacity of ～0.1 and ～0.2 mmol·g-1,respectively.When the[N1111][Gly]was immobilized into PMMA,the CO2adsorption capacity of these sorbents was dramatically increased due to the increased active adsorption sites.The adsorption curves in Fig.3(a)indicate that the CO2adsorption capacity was firstly increased with an increasing amount of[N1111][Gly]immobilized in the range of 10 wt%–50 wt%,and then decreased with a further increase of the IL loading to 60 wt%.The adsorption curve is similar to the variation of CO2adsorption with reaction time,when the IL loading is in the range of 10 wt%–50 wt%.Whereas,when IL loading increased to 60 wt%,the amount of ILs in the sorbent(PMMA-60)was overloaded and formed the abnormal state of a liquid encapsulated solid,just like pure IL state.Therefore,PMMA-60 required longer time to reach equilibrium adsorption than other sorbents because of the filling or occlusion of the pore channels.

The microstructural property of these sorbents,such as pore size,pore volume,and surface area,changed with increasing IL loading into PMMA.Such changes would lead to dramatically different CO2adsorption property. The isotherm adsorption–desorption curves were shown in Fig.3(b)and the microstructural characteristics were summarized in Table 2.Obviously,surface area and pore volume decreasedwith the increasing[N1111][Gly]loading,due to the occupied pore channels.However,the average pore size of these sorbents increased first and then decreased rapidly when the IL loading continuously increased.The pore-size distribution of different[N1111][Gly]loading sorbents could provide more information on the porous structure.As illustrated in Fig.3(b),PMMA mainly possesses mesopores as well as small portion of micropores and macropores.When IL was immobilized into the support,it could not uniformly cover all the holes of different sizes of the support.When IL loading was low(such as PMMA-10),the pore volume decreased in the micropore and mesopore range,especially at pore size below 4 nm.With further increasing the IL loading,the pores with size of ≤20 nm gradually disappeared.The pore size above 35 nm is available for PMMA-50,but almost disappears in the whole size range for PMMA-60.The above phenomenon indicated that ILs prefer to fill the small pores compared to large pores because the surface potential of small pores is higher than that of large pores[38].Because PMMA-50 exhibited the largest CO2adsorption capacity,the residual larger pores may be conducive to CO2diffusion into the sorbent during the entire adsorption process,exposing more adsorption active sites for CO2capture.

Table 2 Structural properties of PMMA-[N1111][Gly]sorbents with different ILs

Fig.4.SEM images of the PMMA(a),(e)and the as-prepared sorbents with different amounts of IL loading,PMMA-30(b),(f),PMMA-50(c),(g),PMMA-60(d),(h).Left:surface structures;right:cross-section structures.

The morphologies of the PMMA and the as-prepared sorbents with different amounts of[N1111][Gly]loading(30 wt%,50 wt%,60 wt%)were investigated by SEM,and the images were displayed in Fig.4.The surface morphologies and cross-section of PMMA were all featured with a hierarchical structure.With the increase of IL loading,the content of ILs on the inner and outer surfaces of the sorbent was increasing,and the pores were gradually filled or blocked.Close observation of the surface and cross-section of the sorbents(Fig.4,inset),the sorbent of PMMA-30 and PMMA-50 still had porous structures and the[N1111][Gly]was uniformly distributed inside and outside the microspheres.In contrast,the sorbent of PMMA-60 had a much denser IL layer both internal pore and surface.As with the BET results previously described,the pores of PMMA-60 almost completely disappeared(Fig.4(d)and(h)).These observations can draw such a conclusion that the remaining pore structure in the sorbent contributes to the diffusion of CO2,and further affects the adsorption capacity of sorbent.

In order to ensure as many adsorption active sites as possible,more ILs need to be loaded on the PMMA,but some pores should be available for CO2diffusion into the sorbent.Therefore,there is an optimized IL loading for the support,based on the trade-off between the residual porosity and the total available ILs.This is the reason that the sorbent of PMMA-50 has a larger CO2adsorption capacity than other ILimpregnated sorbents.

3.3.Effect of temperature

Since the optimal CO2adsorption performance was obtain in 50 wt%loading of[N1111][Gly],the temperature study within the range of 35 to 75°C was carried out on this sorbent.The adsorption results are summarized in Fig.5.

Fig.5.(a)CO2 adsorption of the PMMA-50 at different temperatures from 10 vol%CO2 balance with N2.(b)CO2 capacity vs temperature.

Apparently,CO2adsorption capacity decreased with increasing temperature.A plot of CO2capacity versus temperature(Fig.5(b))shows the linear decrease of CO2capacity as a function of temperature.The CO2equilibrium capacity reached to 2.14 mmol·g-1at 35°C and decreased to 1.58 mmol·g-1at 75°C,indicating the unfavorable adsorption at high temperature.This phenomenon is similar to other reported IL-supported sorbents[16,17],which can be explained from the weakened interaction between the active adsorption site and a sorbate(CO2)due to the exothermic nature of the reaction.As illustrated in Fig.5(a),the temperature-dependent sorption suggests a shorter equilibrium time at higher adsorption temperature.The equilibrium time of CO2adsorption took 60 min at 35°C,whereas,only 4 min was required at 75°C.This phenomenon indicates that high temperature is beneficial to improve CO2adsorption rate on PMMA-50 due to the accelerated diffusion of the adsorbed CO2molecules from the surface into the deeper active adsorption sites.According to our previous work[12],the CO2adsorption process in ILs was considered to comprise two steps:reaction and diffusion.In the reaction layer,CO2chemically or physically dissolves into ILs which was spread over the support.In the diffusion layer,the dissolved CO2diffuses from the surface of the reaction layer to the bulk phase of ILs.The equilibrium of CO2adsorption can be reached,when the chemical potential of CO2in the gas phase equals to that in the liquid phase.However,the decrease of CO2adsorption capacity on PMMA-50 with increasing temperature indicates that the dominating role of the exothermic reaction compared to diffusion.

Moreover,as illustrated in Fig.5,for PMMA-50,the highest CO2capacity was obtained at 35°C,however,higher adsorption capacity can be remained even at high temperature than other reported ILsupported sorbents,as shown in Table 3.This superiority makes the sorbent more widely used in the field of CO2capture.

Table 3 Comparative CO2 adsorption capacity of several supported hybrid sorbent materials

It is worth noting that 50%EZT3/SBA-15 sorbent shows the maximum adsorption capacity but low adsorption rate,which may impede their potential application.Currently,the benchmark value for commercialization is 2 mmol·g-1-sorbent[41,42],which could potentially reduce the cost of CO2sequestration.The capacity and rate performance of the sorbents developed in this work outperform the others and thus guarantee a promising future in practical application.

3.4.Adsorption kinetics performances and modeling

As mentioned above,for CO2capture,the sorbent should possess not only high equilibrium sorption capacity but also fast sorption rate.Therefore,adsorption kinetics is required to assess the performance of a sorbent.Recently,many kinetic models have been developed to simulate CO2adsorption.As reported by Wang et al.[43],Avrami's fractional-order kinetic model provided the best fitting for the adsorption behavior of CO2compared with other kinetic models.Avrami's fractional-order kinetic model considers both physical and chemical interactions between CO2and sorbents[43,44],which is especially important for supported IL sorbents.Therefore,the IL loading and operating temperature were chosen for dynamic study.The general form of the model is given as follows:

kA(1 min-1)is rate constants,qt(mmol-CO2·g-1-sorbent)and qe(mmol-CO2·g-1-sorbent)represent the adsorption capacity at a given time t and equilibrium time,and nAis the Avrami exponent.The experimental value and the relevant fits to the above kinetic model were shown in Fig.6.

Fig.6.CO2 adsorption kinetics on supported-[N1111][Gly]sorbents at different loading and fittings using Avrami's fractional-order kinetic model.

Obviously,Avrami's fractional-order kinetic model agreed well with the experimental results.The shapes of the fitted curves as predicted by the Avrami kinetic model were in good agreement with the experimental data.Moreover,as illustrated in Table 4,the squared correlation coefficient(R2)of regressions was above 0.9.It is therefore concluded that the CO2adsorption of PMMA-x is neither purely physisorption nor purely chemisorption.

Table 4 Kinetic parameters of Avrami's fractional-order kinetic model for CO2 adsorption on supported-[N1111][Gly]sorbents at different loading

The corresponding kinetic constants of CO2adsorption in the supported-[N1111][Gly]sorbents were determined and listed in Table 4.The CO2adsorption capacity at equilibrium time(qe)increased with increasing[N1111][Gly]loading due to more active adsorption sites involved,and then decreased because of pore filling or blocking.While,the value of kadecreased continuously with increasing[N1111][Gly]loading,indicating that the mass transfer resistance of the adsorption process increases.All demonstrate that the appropriate amount of IL loading would provide more active adsorption sites,however,the pore filling or occlusion caused by excess IL loading would also impede the CO2diffusion into the deeper active adsorption sites.And the results are in good agreement with the discussion in Fig.3.

To study the effect of temperature on CO2capacity and adsorption rate,the experimental data of PMMA-50 performed at the temperature range from 35°C to 75°C were also fitted by Avrami's fractional-order kinetic model.It is not surprising to find that the CO2adsorption capacity at equilibrium time(qe)decreased with increasing temperature due to the exothermic nature of reaction.However,the value of kaincreased first and then decreased with increasing temperature.At a relatively low temperature region,the increase of temperature decreases the viscosity of the impregnated[N1111][Gly].As a result,this leads to less inhibition of CO2diffusion from surface to the bulk of ILs.But at a relatively high temperature region,the improved diffusion could not compensate for the accelerated desorption of CO2,therefore,the reduced CO2adsorption rate was observed(Fig.7 and Table 5).The same phenomenon has also been reported in other literature[38].

Fig.7.CO2 adsorption kinetics on PMMA-50 at different temperatures and fittings using Avrami's fractional-order kinetic model.

Table 5 Kinetic parameters of Avrami's fractional-order kinetic model for CO2 adsorption on PMMA-50 at different temperatures

3.5.Further evaluation for practical application

3.5.1.Operation time and selectivity

In practical industrial applications,CO2separation processes involve mixture gas that should be purified,therefore,both the capacity and selectivity are the critical factors for evaluating the separation performance of the sorbents[45].In order to evaluate the selectivity of the sorbent(PMMA-50),the adsorption capacity of pure CO2and pure N2was also investigated respectively.The CO2/N2selectivity of PMMA-50 was calculated based on the ratio of adsorption capacities for CO2versus N2at 35°C and ambient pressure[35,46].

As illustrated in Fig.8,the transients of CO2and N2adsorption show different adsorption reaction mechanisms.The rate of CO2chemisorption of this sorbent is faster,by 2 to 3 orders of magnitude,than that of physisorption of N2.The equilibrium adsorption capacity is excellent for pure CO2but negligible for pure N2,indicating excellent adsorption selectivity for CO2separation.As illustrated from the evolution of the CO2/N2selectivity with time,the selectivity drops sharply and reaches a balance afterwards.In the case of fully guaranteed CO2capacity,high CO2/N2selectivity of this sorbent can be obtained in a very short time.Therefore,this property can be fully exploited to regulate CO2separation operation time involving the IL-supported sorbents in an adsorbent chamber to avoid the excessive adsorption of N2.

Fig.8.Evolution of CO2/N2 selectivity during the transient stage of absorption for PMMA-50.

3.5.2.Effect of water vapor

Because the prepared sorbent is hydrophilic and the flue gas often contains a certain amount of water vapor,the influence of moisture on CO2capture performance of the sorbent is unavoidable.In the cyclic process of adsorption and desorption,the sorbents will be wetting and drying again and again.Therefore,it is important to investigate the effect of moisture on CO2adsorption performance.In order to examine the moisture effect on CO2capture,we used a bubbling method to pass a wet N2in a quartz tube with some sorbent at 35°C[47].After an hour,the water content was measured with an electronic balance,4.7 wt%water(based on the weight of wet sorbent)was adsorbed.Considering the CO2adsorption equilibrium time as well as the drying process in next desorption cycle,an hour of humidification is sufficient for PMMA-50 sorbent.Then,the wet sorbent was subjected to CO2adsorption experiment at 35°C.The results are shown in Fig.9.

As can be seen from Fig.9,after an hour of humidification,the CO2adsorption capacity of sorbent decreased significantly by about 17%,meanwhile,the adsorption rate was also slightly decreased.As far as we know,similar results were also observed in other sorbents,such as microporous zeolite 13X[48],MCM-41-TMGL[49].The reduced adsorption capacity can be attributed to the following two reasons:(1)the competitive adsorption of CO2and H2O in the pore channel and(2)the hydrogen bond interaction between[N1111][Gly]and water.In practical application,it is better to remove the moisture before CO2adsorption because of the negative influence of moisture on the CO2adsorption.

Fig.9.CO2 adsorption of PMMA-50 sorbent containing different amounts of water.

3.5.3.Cycle test for sorbent and thermal stability

To enable practical industrial application,the excellent cycling stability is required or even more important than high adsorption capacity.Therefore,ten consecutive cycles of adsorption/desorption test were conducted by using PMMA-50 with 10 vol%CO2balanced with N2at 35°C.The results are shown in Fig.10.After the PMMA-50 adsorbed CO2from mixture gas,the CO2saturated sorbent was regenerated by heating up to 100°C in N2flow.The cyclic data reveals that the CO2adsorption capacity of PMMA-50 is relatively stable indicating a good sorbent durability.After ten adsorption/desorption cycles,over 91%of the initial CO2adsorption capacity remained.And after ten cycles,the mass of sorbent was also reduced by about 5%.Consequently,the decrease of the CO2capacity could be attributed to the slight loss of ILs[20].

Fig.10.Recycling results for CO2 adsorption experiment of PMMA-50.

Furthermore,the thermal stability of PMMA-50 was also investigated.The samples of[N1111][Gly],PMMA and PMMA-50 were tested from 35°C to 800°C in N2atmosphere.As shown in Fig.11,the thermal stability of[N1111][Gly]was improved slightly when ILs were immobilized into the PMMA.And the TG curve of PMMA-50 drops slightly below 100°C,which was due to the removed moisture or solvent.In addition,as illustrated in the TG curve of PMMA-50,the sorbent showed two main mass losses during the whole process.The first one occurred between 160°C and 290°C,which was related to the degradation of[N1111][Gly][26].Another mass loss began at 300°C,which attributed to the decomposition of PMMA support[50].Therefore,it can be determined that the prepared sorbent existed stably at the adsorption and desorption temperature range.

Fig.11.TG curves of[N1111][Gly],PMMA and PMMA-50.

4.Conclusions

The immobilization of[N1111][Gly]into the porous materials is an effective method to preparing novel IL-supported sorbents with high CO2adsorption.The effects of supports,[N1111][Gly]loadings and temperatures on both CO2adsorption capacity and kinetics were studied in this work systematically.Besides the microstructural property,the CO2adsorption performances were also affected by the chemical properties of the support surface.Among all the prepared IL-supported sorbents,the PMMA impregnated with 50 wt%[N1111][Gly]exhibited the best CO2adsorption performance.According to the pore size distribution of PMMA-[N1111][Gly],the residual pores in the size of above 35 nm could result in fast CO2diffusion,which was closely related to CO2adsorption.The highest CO2adsorption capacity of 2.14 mmol·g-1sorbent was obtained in PMMA-50 at 35°C and ambient pressure.With the increase of temperature,the CO2adsorption capacity decreased,while the CO2adsorption rate is improved significantly.A short adsorption equilibrium time of 4 min was reached,when the operation temperature increases to 75°C.Considering the capacity and rate,the PMMA-50 should be the best candidate for CO2capture compared with other reported supported-IL sorbents.In addition,the CO2adsorption capacity of the resulting sorbent after ten cycles remains 91%of the original amount.Based on this work,it is evident that a strong potential was shown by PMMA-50 for CO2capture from flue-gas.