Recent progress of green sorbents-based technologies for low concentration CO2 capture

Yuanyue Zhao ,Yihui Dong ,Yandong Guo *,Feng Huo, *,Fang Yan ,Hongyan He

1 College of Mathematics Science,Bohai University,Jinzhou 121013,China

2 Beijing Key Laboratory of Ionic Liquids Clean Process,CAS Key Laboratory of Green Process and Engineering,State Key Laboratory of Multiphase Complex Systems,Institute of Process Engineering,Chinese Academy of Sciences,Beijing 100190,China

Keywords:Low concentration CO2 capture Ionic liquids Manned closed spaces

ABSTRACT The increased concentration of CO2 due to continuous breathing and no discharge of human beings in the manned closed space,like spacecraft and submarines,can be a threat to health and safety.Effective removal of low concentration CO2 from the manned closed space is essential to meet the requirements of long-term space or deep-sea exploration,which is an international frontier and trend.Ionic liquids(ILs),as a widespread and green solvent,already showed its excellent performance on CO2 capture and absorption,indicating its potential application in low concentration CO2 capture.In this review,we first summarized the current methods and strategies for direct capture from low concentration CO2 in both the atmosphere and manned closed spaces.Then,the multi-scale simulation methods of CO2 capture by ionic liquids are described in detail,including screening ionic liquids by COSMO-RS methods,capture mechanism by density functional theory and molecular dynamics simulation,and absorption process by computational fluid dynamics simulation.Lastly,some typical IL-based green technologies for low concentration CO2 capture,such as functionalized ILs,co-solvent systems with ILs,and supported materials based on ILs,are introduced,and analyzed the subtle possibility in manned closed spaces.Finally,we look forward to the technology and development of low concentration CO2 capture,which can meet the needs of human survival in closed space and proposed that supported materials with ionic liquids have great advantages and infinite possibilities in the vital area.

1.Introduction

With the gradual deepening of human use of mineral fuels such as coal,oil,and natural gas,CO2emissions have caused a series of environmental problems and climate change.Development and implementation of novel and affordable technological solutions to reduce greenhouse gas (i.e.CO2) emissions are a vital step toward fossil fuel-based sustainable energy.The increased 2%concentration of CO2[1]beyond the scope of human body in typical manned closed spaces,such as naval submarines,spacecraft,space stations,and refuges,could seriously threaten the lives and health of cabin crew [2,3].Capture technologies from low concentrations CO2have attracted more and more attention in closed spaces and the emissions of CO2in the atmosphere.The removal of CO2from these closed systems to support the self-sustainment and longterm operation of the human body is of great significance for the development of long-term space programs or deep-sea survey technology [4].

Many efforts have been devoted to achieving highly efficient CO2capture.At present,the traditional methods of CO2capture for large-scale point sources with high concentrations CO2are mature.However,the work on direct capture methods of CO2with low concentration,including direct capture from the air or closed spaces,is still ongoing.Furthermore,the core for the CO2capture is the development of the green adsorbent and capture process.In general,the synthesis of most available CO2adsorbents,such as polymer composite materials [5],are not environmentally friendly,which is known as a serious violation of the principles of green chemistry (GCP) [6].It is necessary to develop green and environmentally friendly absorbents and develop economical process for CO2capture.

Among a series of new-developed absorbents,Ionic liquids(ILs)have emerged as potential candidates for CO2capture owing to their unique properties such as unique molecular structures,negligible vapor pressure,special functional groups,high thermal stability,and designability [7–19].Many theoretical and experimental research works already focused on the studies and application of ILs on CO2capture;however,few works have been done on the application of ILs in manned closed space and direct capture in the atmosphere so far.In summary,we summarized the current methods and technologies on low concentration CO2capture,including the traditional methods,new materials,and ILs-based methods in this review.We notice that there have been many excellent reviews in CO2capture.Thus,this review is not intended to be extensive but mainly discusses the recent advances of the manned closed space and brief summarization on low concentration CO2capture IL-based technologies,especially the potential application of IL-based technologies on low concentration CO2capture in manned closed space.

Fig.1.CO2 capture by amine-functionalized silica materials.Reprinted with permission from Ref.[26].Copyright 2015 American Chemical Society.

2.Current Technologies on Low Concentration CO2 Capture

In the atmosphere,for large-scale deployment of the carbon capture and storage (CCS) process,only ‘‘direct air capture”(DAC)could reduce the global atmospheric CO2concentration,but control technology was challenging.And in manned closed space,due to the difficulty of resupplying supplies,it was necessary to consider regeneration materials to alleviate these challenges.It was worth noting that,in both areas,we focus on identifying and clearly illustrate the key technologies that applied widely in the recent decade.

2.1.Low concentration CO2 capture method in the atmosphere

Due to the enormous amount and low concentration CO2in the atmosphere,direct capture of CO2in the atmosphere is a long-term problem and always meets difficulty.In fact,low-concentration CO2capture in the atmosphere we mentioned is referred to the removal of CO2from large point sources.Air quality,an important parameter which is closely related to human health and environmental livability,has always been considered and detected [22].Currently,CO2removal has become a popular research,but research on large point sources CO2absorption is extremely difficult and generally lacks applicability [23,24].In 2016,a review was given by Sanz-Pérez et al.on the studies of non-regenerative chemical adsorbents,including alkaline solvents,supported amine/ammonium materials,and metal organic framework(MOF)materials,in the‘‘direct air CO2capture”field[25].In recent years,regenerative methods based on organic materials have been developed,mainly including hydroxide materials and solid amines loaded-organic materials on CO2capture.For example,the supported amine materials are uniquely suited to extract CO2from Ultra dilute gas mixtures in air.As unique low temperature chemical adsorbents,supported amine materials can operate CO2spontaneously from ambient air,while being regenerated under mild conditions using heat or the combination of heat and vacuum.Stephanie et al.reported the design of amine-functionalized silica materials in CO2separations[26],as shown in Fig.1.Furthermore,many new materials,such as hyperbranched amino silica materials,and amine-oxide hybrid compositions,also have been extensively studied for CO2extraction from simulated ambient air(415 mg·L–1of CO2).Goeppert et al.[27]reported a new CO2adsorbent,which was obtained based on the reaction of pentaethylenehexamine (PEHA) or tetraethylenepentamine (TEPA) with propylene oxide(PO).They are prepared in‘‘one-pot”by impregnation on silica support in water.These polyamine/epoxide adsorbents splayed a much-improved stability compared with those unmodified amine counterparts,especially under oxidative conditions.Meanwhile,these materials can capture CO2more efficiently and could be regenerated in a humid environment.

Meanwhile,the conversion after CO2capture is also of great significance.For example,Prakash and Olah put forward the production of methanol through chemical recycling of captured CO2[28].The extremely low vapor pressure,nontoxic nature,easy renderability,and high reactivity of NaOH/KOH toward CO2adsorption make them ideal for scrubbing CO2even from low concentration sources in air and converting it to value-added products.Subsequently,their team demonstrated the first example of an alkali hydroxide-based system for CO2capture and conversion to methanol [29].In the integrated system,CO2was efficiently captured by an ethylene glycol solution and subsequently hydrogenated to CH3OH using Ru-PNP catalysts.Qaroush et al.[30]reported a simple aqueous guanidine adsorbent using guanidinium hydrogen bonding that captures CO2from the atmosphere.The results showed that separating CO2from air by this way required low energy and chemical input,and offers the prospect for low-cost CO2capture technologies.In summary,the cost [31],adsorption capacity,kinetics [32],multi-cycle stability,and selectivity [33]should be considered comprehensively when selecting a suitable adsorbent for DAC.In addition,the binding force between CO2and the adsorption site should be strong enough due to that the CO2in the atmosphere is extremely rare(about 400 mg·L–1,which is about 1/350 of the concentration of CO2in the flue gas).Therefore,ILs is a suitable adsorbent for DAC due to its excellent chemical adsorption performance and good chemical tunability.

Gin et al.[34]reported that ILs could capture CO2in either a transformation configuration due to their high thermal stability and low volatility.Recently,Fennell et al.[35]proved that these properties provide an opportunity to regenerate the solvent at a very wide range of temperatures and pressures.Compared with using traditional aqueous liquid capture media,ILs providing an excellent opportunity for process optimization.Most of the CO2solubility work in ILs had been carried out at high pressures and with pure CO2gas streams [36–39],but it was not applicable for carbon capture and storage(CCS).Up to Welton et al.[40]provided a systematic design method for IL adsorbents,adjusting the function of ILs,especially the function group of cation of ILs.Then,Cui et al.[41]discussed on the topic of active-site functionalized ILs in CCS applications at large scale.They proposed that tunning the basicity of functionalized ILs.If cost considerations could be balanced successfully,this idea would show great potential on atmosphere CO2capture.

2.2.Low concentration CO2 capture method in manned closed spaces

Unlike the situation in atmosphere,the CO2capture in enclosed space such as the manned closed spaces is quite unique and more difficult.In order to maintain the health of the staff in the space station,the control of CO2concentration is crucial.Meanwhile,a CO2collection system is needed to ensure the CO2concentration remains at the prescribed level in the capsule.Currently,available CO2adsorption methods in manned closed spaces included solid amine adsorption,electrochemical adsorption,metal hydroxide adsorption,molecular sieve adsorption,and membrane separation,as shown in Fig.2.In the short-term manned space flight,metal hydroxide adsorption method is mainly used.In the medium and long-term manned space flight,molecular sieve adsorption method is a mainstream approach,which has passed the application test of the sky laboratory and has considerable development potential.In the space flight of longer time and longer distances(such as larger space stations,lunar bases,and even future Mars bases),due to the difficulty of resupplying supplies and the need for reliability and efficiency technologies are also more stringent,the current methods are mostly non-regenerative and no longer applicable.It is necessary to consider regeneration materials to promote the development of controllable and sustainable regeneration technology.

Nowadays,the controllable technologies of CO2concentration in manned closed spaces can be mainly divided into three types:non-regenerative,physicochemical regenerative,and controlled ecological regeneration types.

2.2.1.Non-regenerative adsorption technology

In short-term manned spaceflight,the CO2adsorption methods mainly include the metal hydroxide-based adsorption and solid amine-based adsorption,which were non-regenerative but reliability and could provide higher working performance.For the metal hydroxide-based adsorption,previous studies showed that the primary metal oxyhydroxides are tended to bond with CO2.In contrast,the metal hydroxide with larger surface area and open metal sites tended to have a high affinity for CO2molecules through physical adsorption[42].Many international certifications(American Mercury Project,Gemini Project,Apollo,artificial satellite,space laboratory,and Russian Ascend,Soyuz,Salute,etc.) all used the metal hydroxide adsorption method,due to the advantages such as large adsorption capacity,small size,lightweight,stable adsorption performance,safety,and high reliability.However,the disadvantage is that regeneration of the metal hydroxide is difficult after adsorbing CO2,and it needs to be replaced regularly,thus reducing the closedness of the system.

The solid amine-based adsorption method,which is regarded as another widely-used non-regenerative technology,especially in nautical missions,has attracted more and more attention due to its low energy consumption [43].In recent years,many works focused on the studies of the physical properties (structure,pore size,etc.) of solid amine materials on CO2capture [44,45].Some solid amine materials modified by Tetraethylenepentamine(TEPA)could be used for the removal of CO2in closed spaces effectively through the impregnation method [46–48].Fu et al.prepared the solid amine adsorbent by a suspension polymerization of divinylbenzene (DVB) with acrylonitrile,followed by aminating with TEPA.The adsorbent shows a maximum adsorption capacity of CO2(82.8 mg·g–1) [49].Tan et al.found that the PSF-TEPA membrane-based solid amine showed excellent CO2adsorption performance and stable regeneration property,and the adsorption performance of CO2increased with the increment of temperature and the CO2volume fraction,where the maximum adsorption amount could reach 102.13 mg·g–1[50].

2.2.2.Regenerative technology

In the mid/long-term manned space flight,the non-regenerative CO2consumption technologies are facing the problem of increasing quality and fast resupply difficulties,which have promoted the development of regenerative CO2capture technologies.In the current adsorption method,the molecular sieve adsorption showed considerable development potentials and has been regarded as a widely-used method in mid/long-term manned space flight.Meng et al.prepared two kinds of molecular sieve materials to investigate the CO2performance and satisfy with special requirements of manned space flight [51].The results also showed that these molecular sieve materials are proved to have much better adsorption capacity on low concentration CO2(≤1.0%in volume)than the other adsorbents.At present,the four-bed molecular sieves (4-BMS) are the most mature adsorption materials in the molecular sieve adsorption method.These 4-BMS were used to remove CO2in a carbon dioxide removal assembly (CDRA) in a manned spacecraft,which included the CO2adsorption and reduction possess,and a CO2collection system was required to ensure that the CO2concentration remained at the prescribed level.As shown in Fig.3,the wet air entered the desiccant bed I from the cabin firstly,and then was loaded with silica gel and zeolite-13X materials in order to remove the water.After that,the dry air entered into the CO2adsorption bed I,where the zeolite-5A molecular sieve was used to adsorb the CO2selectively [53].

Fig.2.CO2 removal process and standard adsorption methods in manned closed spaces.

Fig.3.Schematic diagram for 4-BMS adsorption/desorption processes.Reprinted with permission from Ref.[52].Copyright 2016 American Society of Civil Engineers.

In the space flight of longer time and distances (such as larger space stations,lunar bases,and even future Mars bases),the requirements become harsher mainly due to the improved difficulty of resupplying supplies.Therefore,to meet the requirements of sustainable development,green solvents that can reduce the energy consumption and environmental impact caused by chemical production have emerged.Bio-regenerative materials and ILs,which were regarded as ‘‘representatives”green solvents,were widely used in the CO2capture process,and showed outstanding adsorption performance.These regenerative materials could react with the adsorbed CO2and then produced the oxygen and water,which could reduce the energy consumption and environmental impact caused by chemical production.Several bio-regenerative materials have been reported for CO2capture.For example,Heydarinasab et al.[54]synthesized the mesoporous chitosan-SiO2nanoparticles (NPs),which exhibited great performance for CO2capture and showed a maximum CO2adsorption capacity of 193.16 mg·g–1.Besides,Yu et al.[55]prepared a superhydrophobic PVDF/Si-R hollow fiber membrane contactor as bio-solvents to capture low concentration CO2from flue gas.The results indicated that this hollow fiber membrane contactor showed better CO2absorption performance than the conventional solvent of monoethanolamide (MEA),based on the examination of the CO2absorption flux,efficiency,absorption capacity,bio-solvent flow rate,and feed gas flow rate,respectively.

Meanwhile,ILs as one type of effective media on the CO2capture,due to the outstanding characteristics of ILs,especially high solubility in solutions of ILs and aqueous or organic solvents with significant aqueous content.[41]Since the 12 GCPs were set by Anastas and Warner [56],green chemistry has become a field that attracted much attention and is committed to preventing environmental problems.So far,most researchers have devoted themselves to the study of the physical properties of ILs,but there is still a lack of exploration on the renewability of IL,which was prevent waste,atom economy,benign solvents,and auxiliaries,use of regenerative feedstocks etc.Recent years,the toxicity,biodegradability,and sustainability of ILs was improved with the continuous synthesized technologies development.[57,58]Among them,compared with other biomolecules,amino acids (AAs)have significant advantages of being cheap and abundant in resources,and could act as cations and/or anions.Up to now,many scholars have done a lot of research on the performance of amino acid ionic liquids(AAILs)of different cations(including imidazole cations of different chain lengths,choline,amines,phosphonium) are used for CO2capture and have shown excellent results [12,59,60].Green solvents have shown good application potential in low concentration CO2capture,therefore,we have reason to believe that these bioregenerative materials and ILs can be fully utilized on the low concentration CO2capture of manned closed spaces in future practical applications.

3.ILs-based Technologies on Low Concentration CO2 Capture

As mentioned above,due to the difficulty of replenishment and low efficiency,traditional solvent absorption technologies are limited in the field of CO2capture.At the same time,most used technologies are non-regenerative.Therefore,these technologies are not optimal for sustainable development.As a green solvent,ILs have low toxicity and good biocompatibility,the CO2capture technology based on ILs could be considered to promote the development of controllable and sustainable regeneration technologies.Since Brennecke et al.[7]first reported that ILs could capture CO2,the more scientific research on ILs as CO2absorbent has been carried out all over the world due to the advantages of excellent CO2absorption capacity,as well as the designable structure and performance.Therefore,we mainly review the potential of ILsbased technologies on the low concentration CO2capture from two parts:theoretical guidance on and the advance of practical application.

3.1.Theoretical guidance on CO2 capture by ILs

Molecular simulation and modeling are well suited to investigate the ‘‘structure–property”relationships of materials at the molecular level and to establish designable principles on the ILs on CO2adsorption.Conductors-like Screening Model (COSMO) is used to select the types of ILs and predict the behavior of the ILs in bulk on CO2capture.Density Functional Theory(DFT)is used to calculate the interaction site and energy between the ILs and CO2.Molecular dynamics (MD) simulations is regarded as an effective method for investigating the absorption mechanism [61–65]and molecular microstructures of ILs on CO2capture[66,67].The computational fluid dynamics (CFD) method is used to describe the transport and diffusion phenomenon in ILs such as the bubble behavior and mass transfer properties on CO2capture[68].Hence,theoretical calculations methods are useful methods for providing design guidance of adsorbents and development of the CO2capture process.

3.1.1.COSMO model for adsorbents screen

The COSMO for Real Solvents (COSMO-RS) proposed by Klamt and co-workers is a predictive model that can be used for initial screening of ILs for various applications [69–71].Wang et al.[72]proposed a screening method by COSMO-RS for the molecular design of ILs on CO2capture and predicted the Henry’s law constants of CO2in 408 different kinds of ILs.It was found that the ILs with the anion of tris(pentafluoroethyl)trifluoro phosphate([FEP]) screened by COSMO showed an improved capability on CO2capture.Zhang et al.[73]used COSMO-RS to screen suitable ILs and then used process simulation to evaluate these screened ILs on decarbonization.The toxicity and viscosity of ILs are taken into the consideration,the [Bmim][NTf2]screened from 90 different kinds of ILs by COSMO showed the best CO2solubility and selectivity (see Fig.4).

Yang et al.[74]briefly reviewed the latest progress of conductor-like screening model-segmented activity coefficient(COSMO-SAC)in predicting the liquid–liquid equilibrium of IL systems.They evaluated the prediction accuracy of different chemical combinations.Liu et al.[75]indicated the hydrogen bond donor,hydrogen-selective bond acceptor,and solvent surface tension of ILs by the combination of group contribution (GC) and GCCOSMO.Santiago et al.[76]selected the most appropriate ILs before conducting experimental tests by using COSMO-RS and analyzed the potential industrial applications of ILs in CO2capture.Palomar et al.[77]optimized 50 different kinds of ILs through the COSMO continuous medium solvation method and calculated the Henry’s law constant of CO2in ILs by COSMO-RS method in order to complete the kinetic screening of these optimized ILs.Benguerba et al.[78]calculated the theoretical solubility of CO2into different choline chloride and phosphonium based deep eutectic solvents(DESs)by using COSMO-RS method,which showed a good agreement with the predicted values and the experimental results.In summary,COSMO model can be used to describe the intermolecular interactions in ILs system as pairwise interactions of surface segments.This description is much more efficient than the description via electrostatic and vdW interactions.Thus,a highly efficient calculation of the chemical potential of any solute in a pure or mixed solvent is enabled,which provides access to most of the relevant thermodynamic parameters of ILs systems.Nevertheless,the prediction results of COSMO are not satisfactory for many systems.Although the relative error of the predicted system can be roughly judged,the total deviation of various compounds cannot replace the deviation of a single system in practical applications.A large amount of experimental data is still need to optimize parameters for model modification.

3.1.2.DFT calculations

In order to better understand the CO2absorption mechanism,DFT calculations showed as an efficient method to elucidate the key role of anions and cations of ILs in the CO2capture process and investigate the interactions between ILs and CO2.Brennecke et al.[15]studied the mechanism of CO2adsorption by AAILs using DFT calculations.The predicted reaction stoichiometry enthalpies were in line with calorimetric measurement experiments.Seyedhosseini et al.[79]found that the electric charge could transfer from the nitrogen atom of the－NH2in AAILs to the carbon atom of CO2through the natural bond orbital by using DFT calculations.Prakash and Nathan[80]calculated the interaction energy and natural bond orbital in [Lys]-CO2and [Lys]--H2O-CO2complexes through DFT calculations,and found that the complexes showed the significant stability due to multiple-site cooperative interactions of CO2with the COO－ group and N1site.Li et al.[81]extended the proton transfer mechanism by using DFT calculations to combine imidazole and ammonium ILs and found that the carbene molecules formed after proton transfer could react strongly with CO2.Fu et al.[82]demonstrated the absorption mechanism through DFT calculations and confirmed that the anions of ILs could react with CO2to form carbamate.Moreover,the charge analysis reasonably proved the diversity of absorption capacity of CO2in various ILs.Mehrdad et al.[83]studied the interactions between the amino acid anion-based ILs and CO2by DFT calculations.They found that the ILs with anions [Gly]showed higher CO2absorption capacity than other two anions such as [Ala]and[Val].

Fig.4.Based on COSMO-RS model for screening suitable ILs.Reprinted with permission from Ref.[73].Copyright 2016 American Chemical Society.

In addition,Wang et al.[84]used the DFT calculation to verify the low cost and excellent performance of a series of aminebased ILs on CO2capture.They found that several functionalized ILs with pyridine-containing anion showed excellent CO2capacity through multi-site cooperation interactions.Tao et al.[85]introduced a new strategy for multi-molar absorption of CO2by activating a carboxylate group in ILs.They employed the DFT calculations to investigate the interaction of different anions with CO2.The results showed that the reduction of the negative inductive effect of amino groups in ILs could stimulate the carboxylate group to attract CO2efficiently,resulting in a significant increase in CO2adsorption capacity.Wang et al.[86]also proposed a novel preorganization and cooperation strategy for highly efficient and reversible capture of low-concentration CO2using acylamido-based ILs.The cooperative interactions between CO2and multiple active sites in the preorganized anion could result in superior CO2capacity and excellent reversibility.DFT calculation can be used to describe the configuration accurately,energy analysis,orbital composition,charge and bond analysis of finite-scale systems,and wireless periodic repetitive systems.However,quantum chemical calculation only can be applied to vacuum or rarefied gas molecules and the intermolecular interaction in the case is negligible.While the liquid phase was associated with many industrial chemical processes or multi-phase systems,quantum chemical calculation method is no longer applicable.

3.1.3.MD simulation

MD simulation was used to calculate the physical properties and understand the microscopic mechanism of the CO2adsorption process and studied the behavior of ILs[87,88].MD simulation can be used to simulate the movement of a molecular system based on Newtonian mechanics,and calculate the configuration integral of the system,as well as the thermodynamics and other macroscopic properties.However,MD simulation has the limitation on the time scale.If the simulation increases 1 ns,the calculation amount will increase N times.So,the system cannot be too large.The potential function and numerical algorithm have a greater impact on the accuracy of the simulation.In order to improve the accuracy of the potential function,ab initio MD calculation based on local density functional theory,quantum chemical analysis parameter fitting and Monte Carlo method are used.Combination is expected to become the best method for studying potential functions.With the continuous development of computer performance,the application range of ab initio MD calculations that get rid of empirical potential functions will continue to expand,and the accuracy of calculations will continue to increase.Theodorou et al.[89]understood the dynamic properties and the solubility and diffusivity of CO2by performing MD simulations using fully atomic systems(see Fig.5).Wang et al.[90]used the Grand Canonical Monte Carlo(GCMC)method to study the effect of trace water vapor on the capture of low-concentration CO2in 5A zeolite particles.The calculation results showed that when the water vapor concentration is less than 0.1 mg·L–1,the CO2adsorption capacity increased by 0.7–53.4%,while the water vapor concentration is greater than 0.3 ppm,the CO2adsorption capacity decreased.It was known that the mixture solubility and diffusivity of binary gas-IL mixtures is critical for the design of ILs,where the MD simulation is an effective method to calculate the solubility and self-diffusion coefficients.Maginn et al.[91]used atomistic MC and MD simulation to obtain solubility,diffusion selectivity of gas in ILs in order to test the ideal permeation selectivity hypothesis.Wang et al.[92]identified the sparse hydrogen bond interaction network constructed by CO2and metal-based ILs (MBILs) through MD simulation.Furthermore,the dynamical properties including residence time and self-diffusion coefficient were studied,and the results showed that the CO2absorption capacity of MBILs increases in the order Cl-→[ZnCl4]2-→[CuCl4]2-→[CrCl4]-→[FeCl4]-.

Shaikh et al.compared dry and wet conditions to identify factors contributing to CO2 solubility and selectivity at room temperature through MD simulation[64].The MD results were useful for mimicking the mechanism for CO2absorption on AAILs and its effect on physical properties such as the fractional free volume,diffusion coefficient,and hydrogen bonding.Venkatnathan et al.[93]investigated the molecular mechanism of high pressure CO2absorption in [P4444][Lys]by using MD simulations and achieved a maximum of 1.08 molar absorption of CO2.Aparicio et al.[94]studied the surface properties of [Emim][Gly]in contact with a vacuum,CO2,SO2and flue gas phases,respectively.RDF and number density results showed [Gly-]-water interaction was remarkably larger than any other ion–gas interaction,leading to fast and efficient adsorption of water molecules hindering and delaying CO2capture at the interface.

Fig.5.Structure and local dynamics in the presence of CO2 .Reprinted with permission from Ref.[89].Copyright 2020 American Chemical Society.1?=0.1 nm.

Fig.6.CO2 mass fraction field of pure [bmim][BF4 ]during bubble coalescence.Reprinted with permission from Ref.[97].Copyright 2015 Elsevier Ltd.

Recently,Deng et al.[95]used MD simulation and combined with MC and machine learning (ML) to study the adsorption and diffusion characteristics of low-concentration CO2with 6013 different experimental CoRE-MOFs.Among these MOFs,14 MOFs were screened out successfully with the best performance and found that the key to capture low concentration CO2is the diffusion performance of CO2in the MOF.Meanwhile,the MOF with pore limiting diameter close to the CO2dynamic diameter showed higher CO2diffusion and separation selectivity.This indicated that the synthesis of new MOFs possessed the potential on low concentration CO2capture from the manned closed places.

3.1.4.CFD simulation

In recent years,many researchers used CFD methods to study bubble behavior and mass transfer in IL-CO2systems,which could overcome the difficulty of experimentally measuring flow patterns and concentration distributions.At present,most works of IL systems using CFD simulation use empirical and straightforward relationships to describe the physical and chemical properties of these complex systems.Huang et al.[96]proposed CFD combined of the ion fragment contribution method to predict the density,viscosity,and surface tension through the microstructure of ILs,which indicated that the new ILs could be designed more accurately for the CO2capture process.Meanwhile,CFD method provides another effective method to study the transport properties,which overcomes the inherent difficulties of determining the flow field.However,the interfacial mass transfer of single bubble in ILs was never discussed.Especially,the effect of viscosity reduction of ILs after absorbing CO2has not been considered in the CFD method,limiting the understanding of bubble behavior and CO2mass transfer in IL systems.Bao et al.[97]used a CFD method to study the ILs in CO2capture system with two improvements.One improvement was to introduce the resistance equation to the IL system into the hydrodynamic model,and the other was considering the influence of the CO2concentration in the liquid phase on the viscosity of ILs(see Fig.6).Based on this method,bubble behavior and mass transfer characteristics of ILs on CO2capture can be accurately described.Abdeltawab et al.[98]investigated three ILs inside a 3D flat bubble column and explained the flow pattern,liquid velocity magnitude,CO2holdup and bubble size distribution using CFD model.

3.2.ILs-based technologies advance of practical application on CO2 capture

Moreover,among a series of ILs,the AAILs possess higher CO2absorption rates than those of most other solvents such as ethanolamine derivatives,pure amino acids,and amino acid salts,etc.[85].However,the viscosity of pure ILs increased with the CO2adsorption increased,which could reduce the mass transfer properties of ILs.Han et al.[99]found that adding the polyethylene glycol(PEG)as co-solvent in the ILs to form the mixture could reduce the viscosity and capture CO2effectively.Furthermore,another effective way to reduce the viscosity of ILs and improve the CO2mass transfer rates was to support the ILs on other solid materials,which is formed like the support ILs membranes to enlarge the formation of gas–liquid interface [100–103].

3.2.1.Amino acid ionic liquid absorption

AAILs are potential green substitutes of aqueous amine solutions for CO2capture.The typical chemical structures of these AAILs were shown in Fig.7.We summarized different types of AAILs on CO2capture,and compared the adsorption capacity of these AAILs on low concentration (10 vol%) CO2capture and full concentration CO2capture,as shown in Table 1.According to the molar ratio and adsorption capacity,the functionalized AAILs,[P4442][Suc]and [P4442]2[IDA],possessed better performance on CO2capture in these two different comparison systems.However,the maximum absorption capacity of CO2in a low concentration system is still limited to the less active sites in these functionalized AAILs and the rapid increase of the viscosity of these ILs during CO2capture [41,111].In order to address these issues,many works have focused on the improvement of ILs on CO2capture through the decrease of the viscosity and increase the active sites of these ILs through functionalization.

Since viscosity is one of the most important characteristics which could affect the CO2adsorption capacity,development new functionalized AAILs with low viscosity is critical [13].After the first finding by Jiang et al.[60]in 2008 that the tetraalkylammoniums-based AAILs,which was representative ammonium based AAILs,showed low viscosity and high thermal stability,more and more ammonium based AAILs were developed.Ji et al.[112]designed several tetraethylammonium amino acid ILs and found that these ILs showed lower viscosities than other literature reported.The ILs could be regenerated after CO2adsorption.In addition,the CO2was found to be adsorbed easily and directly in the air by the choline which comes from the nature.Based on these,the AAILs functionalized by choline cation-can be regarded as a cation of environmentally friendly degradable ILs and are widely used in CO2capture[113–115].For example,Bhattacharyya et al.synthesized a novel series of choline based AAILs by ether,which could effectively reduce the viscosity of ILs and lead to a high CO2capture capacity [116].

Fig.7.Chemical structures of the AAILs.

Table 1 The comparison of CO2 absorption by AAILs in literature

Meanwhile,phosphonium cation is a relatively cheap and easily available thermally stable cation,which could affect the physical properties of AAILs.Goodrich et al.synthesized the phosphonium cation-based AAILs,[P66614][Gly],[P66614][Ala],[P66614][Sar],and[P66614][Ile]and investigated the CO2capture performance from post combustion flue gas [117].Moghadam et al.[118]used the functionalized AAILs,[P4444][Pro]-based tough gel membrane and investigated the separation properties of CO2from CO2/N2mixtures with very low CO2concentration (500–1000 mg·L–1).The effects of different aliphatic chains on cation of these phosphonium based AAILs on structural properties and dynamical properties were also investigated by Shaikh et al.[119].The results showed that the long-chain cations could increase the energy barrier which was attributed to the hydrophobicity and bulkiness of the cations,leading to the better CO2absorption capacity.

However,there still existed some disadvantages of these choline based AAILs such as their strong water absorption and easily decomposition when heated,which could limit their applications on CO2capture.Furthermore,a mixture of two ILs could effectively improve these disadvantages.For example,Gouveia et al.[120]found that adding the [C2mim][C(CN)3]into AAILs could decrease the viscosity of the IL after CO2adsorption,while adding the[Emim][C(CN)3]could further improve the flexibility and finetuning ability of AAILs.Chen et al.[121]found that mixing AAIL with[Hmim][Ac]hybrids could slow the increase of their viscosity after CO2absorption and showed high CO2adsorption capacity compared with the pure AAILs,indicating that this hybrid material is considered to be an effective CO2absorbent.

On the other hand,increasing the active sites of the ILs is another effective way to improve CO2adsorption performance.For example,Chen et al.[85]reported a new strategy for multimolar absorption of CO2through activating the carboxylate group in AAILs efficiently.The multi-molar of amino-functionalized AAILs on CO2capture could reach 2 mol CO2·(mol IL)–1after 24–48 h due to the existence of two amino groups and was equivalent to the increase of the adsorption sites [122].Huang et al.[86]designed a novel strategy for superior capture capacity of lowconcentration CO2.The constrained succinimide anion could improve the CO2capture with low concentration through multiple adsorption sites.They also found that the cooperative interactions between CO2and multiple active sites are the driving force for the superior CO2adsorption capacity and excellent reversibility.

3.2.2.ILs co-solvent on CO2capture

Although the performance of pure ILs on CO2capture is widely recognized,the viscosity of ILs is high for its hard to use.Since the finding that adding co-solvent into pure ILs,such as water,polymer,alcohol,deep eutectic solvent,could decrease the viscosity of the ILs [123].The co-solvent-based method could be another effective way to explore the ILs co-solvent on CO2capture.Among a series of co-solvents,the poly (ethylene glycol) (PEG) material,which is a typical low-cost and less viscous co-solvent,has been used as ILs co-solvent in CO2capture.Li et al.[99]first demonstrated that the IL/PEG200 mixture could effectively capture CO2 and the molar fraction of CO2to IL can slightly surpass 0.5 and the chemical absorption theoretical maximum value of CO2.Xie et al.[114]reported that adding PEG200 into [Choline][Pro]showed a better performance on CO2adsorption than the amino functional ILs.Meanwhile,Scovazzo et al.[124]reported that the ideal gas selectivity of the[Choline][Pro]/PEG200 mixture was better than the imidazole-based ILs due to its low cost,good biodegradability,and non-toxicity,even though the CO2permeation fluxes were the same [125,126].Muhammad et al.[127]showed that the CO2absorption capacity in [P4444][Gly]/PEG400 mixture could reach 1.23 mol CO2/mol absorbent due to the good CO2solubility.Li et al.[128]found that adding the PEG400 in the ILs could create a better gas–liquid interface and promote the mass transfer and intensifies of CO2adsorption.Pandey et al.[129]found that the CO2absorption capabilities in ILs with PEG as co-solvents increased with the increase of the molecular weights of PEG.

3.2.3.ILs supported materials on CO2capture

As we mentioned above,adding co-solvent is an efficient way to adjust the viscosity of the ILs,but the mass transfer rates on CO2adsorption is still limited.According to the principle of nonequilibrium thermodynamics,increasing the driving force of the CO2transfer process and reducing the resistance is an effective way to overcome the high viscosity of ILs and increase the rate of the CO2transfer process.Zhang et al.[14,130]proved that the ILs supported on porous SiO2showed fast and reversible CO2absorption with the comparison of the bulk ILs.The capacity rate of CO2was significantly increased due to the supported large surface area of SiO2.Li et al.[76]synthesized [apaeP444][AA]-ILs and immobilized them into porous SiO2.The results showed that these ILs-supported SiO2 had excellent sorption capacity and selectivity of CO2and could be used repeatedly.Wang et al.[131]found that the [Emim][Gly]supported on the surface of PMMA could greatly improve the CO2 absorption performance.Xie et al.[132,133]reported that the mixture of [Bmim][Ac]and [APMim][ Br]supported on the TiO2surface could enhance the CO2adsorption and transfer coefficient [134].Wang et al.[135]found that the[Emim][Lys]supported on the porous microsphere PMMA exhibited good long-term stability and the highest CO2capacity compared to other counterparts.Recently,Chun et al.[136]reported that the mixture of [N4444][2-Op]and [P4444][2-Op]supported on the mesoporous SiO2with 13.5 nm pore diameter could achieve 90% saturated adsorption capacity of 3.72 mol CO2/mol IL,which was 12.76 times higher than that of the pure [P4444][2-Op].

Besides supporting the IL on the mesoporous or polymer materials,the supported ILs-membranes (SILMs) are now widely applied for CO2capture due to their unique properties [33,137–139].For example,Aroua et al.[33]showed that the [Emim][NTf2]supported membrane on the CO2absorption is almost doubled with an average selectivity factor of CO2/O2around 5 times compared to blank contactor system.Currently,there are many alternative support materials for SILMs that would impact the mechanical stability of the membrane.Graphene is one such material with great promise for use in the design of an IL-based membrane.Recently,Maurya et al.[140]proposed the threedimensional monolayer graphene with superior mechanical properties as a mechanically robust SILMs,which showed the excellent CO2/CH4separation performance.Peng et al.[141]constructed a highly CO2-philic separation membrane by nanoclosed the ILs into graphene oxide nanoslits,leading to the outstanding separation performance for CO2.Recently,Gurkan et al.[142]reported that the ILs capsules could be used to absorb CO2in the air (as shown in Fig.8),and these ILs capsules can regeneration.Their research group first synthesized a new type of ILs-based capsules via encapsulating [Emim][2-CNpyr]with polyurea (PU) and graphene oxide(GO) sheets,where the CO2selectivity and capture rate can be adjusted by selecting appropriate ILs.Compared with the most advanced materials,the synthesized capsules have good stability,recyclability,and excellent CO2absorption capacity.

In addition,metal–organic frameworks (MOFs) as a new support material have been widely-used in the field of CO2separation[143–147]due to their high surface area and special pore structure[148,149].Especially,MOFs modified by the amino group also display high CO2adsorption capacity [150].However,many MOFs were not performed well when capture CO2containing water due to the competition of water with CO2on the adsorption sites[151–153].Peter et al.[154]synthesized two waters table MOFs(Al-PyrMOF and Al-PMOF) containing the hydrophobic adsorbaphores,and found that the CO2capture performance was not affected by water.Cadiau et al.[155]reported that a hydrolytically stable fluorinated MOF,with a periodic array of open metal coordination sites and one-dimensional channel could remove water vapor from gas streams containing CO2,N2,CH4,and higher hydrocarbons typical of natural gas.

In summary,the novel adsorption materials with multifunctional synergy formed by the combination of support materials(PMMA,SiO2,TiO2,graphene,MOFs,etc.) with ILs is expected to be applicated in low concentration CO2capture,especially at manned closed spaces.

4.Conclusions and Outlook

Fig.8.Schematics of the CO2 breakthrough setup.Reprinted with permission from Ref.[142].Copyright 2020 American Chemical Society.

This review mainly focuses on the summarization of the research progress of green solvent in CO2capture,especially in low concentration CO2capture.It is mainly due to that the capture technology of CO2with high quantity in the atmosphere is already mature.Still,in the field of spacecraft and submarines within the manned closed spaces,the concentration of CO2is low but it will accumulate to a certain extent and endanger people’s life.Therefore,low concentration CO2capture is very important.First,we review the current low concentration CO2capture technologies in the atmosphere and manned closed space.However,these mature technologies are limited by the large size,mass,and poor reproducibility of absorbent materials for long-term space programs or deep-sea surveys.Therefore,it is urgent to develop greener,more efficient,and more sustainable capture technologies.Considering the excellent performance of IL,especially in the field of CO2capture,we first summarized the molecular simulation research on IL capture CO2.Furthermore,we analyzed the experimental technologies on pure ILs,IL with co-solvent of ILs and ILs loading on solid materials in the application of CO2capture.However,there is no relevant research on ILs as CO2adsorbents in manned closed spaces currently.We mainly consider its limitations from the following aspects:Firstly,the most significant disadvantages of ILs is that they are expensive.The hydroxide adsorbents (e.g.LiOH,KOH and Ca (OH)2) are still the most mainstream technologies used in naval submarines and space stations due to their excellent adsorption capacity,high capture efficiency,and low cost [20,21].And the synthesis process of ILs is complicated and the use of water or organic solvents is required.It not only increases production costs,but also causes the low purity of the target ILs.Secondly,there are many purification steps for ILs that is difficult and purity is very important for ILs.The IL purification treatment needs to preheat the ILs,requires high vacuum,uses complex equipment,and the effect is still not ideal.Therefore,the purity and preparation difficulties of ILs directly affect their physical and chemical properties and their application on low concentration CO2capture.In addition,Lu et al.[156–158]simulated a manned closed system and their work focused on evaluating the K2CO3/AC adsorbent at 20–60 °C,0.2%–1.0% CO2,0.5%–2.5% H2O partial pressure conditions.But the experimental measurement results of IL adsorbents for lower CO2partial pressure are not clear.Therefore,our follow-up work mainly considers to prove the low dependence of IL on CO2partial pressure.We will analyze the effect of CO2chemisorption on 12 types of tetrabutylphosphonium-based and trihexyltetradecylphosphonium-based AAILs ([P4444][X]and[P66614][X],X=[Gly],[Im],[Pro],[Suc],[Lys],[Asp]) by DFT calculations and MD simulations.Next,we will select several ILs and loaded them on the polymer interface to simulate the ILs adsorbent at partial pressure conditions of manned closed system to evaluate their high-efficiency absorption capacity and further filters to guide experiment design.We put forward on the IL loading on solid materials for CO2capture with low concentration.Furthermore,it is expected to obtain suitable experimental condition and partial pressure of CO2for low concentration capture through the molecular simulation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21878295,22078024),the Natural Science Foundation of Beijing (2192052),and the Project funded by Liaoning Provincial Department of Education (LQ2020001).