Peiyu Zhao ,Guojie Zhang *,Huangyu Yan ,Yuqiong Zhao

1 Heze Branch,Qilu University of Technology (Shandong Academy of Sciences),Biological Engineering Technology Innovation Center of Shandong Province,Heze 274000,China

2 Key Laboratory of Coal Science and Technology,Ministry of Education and Shanxi Province,Taiyuan University of Technology,Taiyuan 030024,China

Keywords:Solid adsorbents Amine functionalized CO2 capture Adsorption Review

ABSTRACT Global warming and associated global climate change have led to serious efforts towards reducing CO2 emissions through the CO2 capture from the major emission sources.CO2 capture using the amine functionalized adsorbents is regard as a direct and effective way to reducing CO2 emissions due to their large CO2 adsorption amount,excellent CO2 adsorption selectivity and lower energy requirements for adsorbent regeneration.Moreover,large number of achievements on the amine functionalized solid adsorbent have been recorded for the enhanced CO2 capture in the past few years.In view of this,we review and analyze the recent advances in amine functionalized solid adsorbents prepared with different supporting materials including mesoporous silica,zeolite,porous carbon materials,metal organic frameworks(MOF)and other composite porous materials.In addition,amine functionalized solid adsorbents derived from waste resources are also reviewed because of the large number demand for cost-effective carbon dioxide adsorbents and the processing needs of waste resources.Considering the importance of the stability of the adsorbent in practical applications,advanced research in the capture cycle stability has also been summarized and analyzed.Finally,we summarize the review and offer the recommendations for the development of amine-based solid adsorbents for carbon dioxide capture.

1.Introduction

The anthropogenic excessive CO2emissions over the past few decades have been identified as the main cause of the greenhouse effect and related climate change [1,2].Moreover,CO2emissions due to burning of fossil fuels and their derivatives is regard as the most important source of CO2[3].Unfortunately,the consumption of fossil fuels will continue to increase for industrial production in the next few decades.It is without a doubt that the excessive anthropogenic carbon dioxide emissions will continue to have the major impact on global warming and its environmental impact.Therefore,effective capture and separation of CO2has become the most importance to effectively cope with the global warming [2,4].Simultaneously,with the reduction of nonrenewable fossil fuels,CO2also can be used as effective carbon resource.As shown in Fig.1,CO2can be transformed into many valuable chemicals[5].Therefore,the use of captured carbon dioxide as the carbon resource to produce value-added fine chemicals is highly desirable for addressing energy needs in the future[5,6].Among different emission sources,coal-fired power plant is the most important source of anthropogenic CO2emission throughout the world [7],as shown in Fig.2.Therefore,CO2capture and sequestration from coal-fired power is a direct solution to reduce CO2emissions.At present,there are three main capture methods available for carbon dioxide capture from the coal-fired power plants,including pre-combustion capture,postcombustion capture and oxyfuel combustion [6].Compared with the pre-combustion capture and oxyfuel combustion,the postcombustion CO2capture technology is regarded as the most feasible method due to its flexible operation and minimal requirements for retrofitting existing power plants [6,7].It is only necessary to carry out a simple retrofitting based on the existing power plant and install a carbon dioxide capture unit.The flow chart of postcombustion CO2capture process is shown in Fig.3 [8,9].The flue gas will be treated before entering the CO2capture unit to reduce the content of impurity gases.

Fig.1.Chemical utilization of CO2 [5].

Fig.2.Breakdown of the dominant CO2 emission sources [7].

CO2capture from flue gas by absorption with liquid amine is the commercially available post-combustion CO2capture method[9,10].However,this capture technology still has many inherent shortcomings.For instance,the amine component is easily degraded during the heating process,the trapping equipment is easily corroded,and the regeneration of the liquid amine absorbent requires a lot of energy consumption,etc[11,12].To overcome such drawbacks for the amine chemical absorption technology,large number of solid porous materials were proposed for CO2adsorption [13].These adsorbents have relatively lower of energy consumption and fast adsorption kinetics.And CO2adsorption amount is relatively stable after multiple CO2adsorption cycles.However,most solid porous adsorbents have the small adsorption amount at high temperature and low pressure due to the physical adsorption characteristics [8,14].Meanwhile,these adsorbents usually suffer from the poor selectivity and poor water resistance[8].

Thus,in order to compensate for mentioned above problems,incorporating amine species into the solid porous materials to form the amine functionalized adsorbents for carbon dioxide capture has been developed and widely studied [15,16].This composite adsorbent not only has the large specific surface area and pore volume but also has a high affinity for carbon dioxide molecules.Simultaneously,the adsorbent also has very high selectivity for carbon dioxide due to the introduction of amine species.These promising porous materials include mesoporous silicon [17–19],activated carbon[20–22],metal–organic frameworks[23–25],zeolite [26,27] and so on.Compared with the liquid absorbents,this amine-containing solid adsorbent has a lower regenerative energy consumption and there is no corrosion to the capture equipment.What’s more,these solid amine adsorbents also show the good selectivity and higher tolerance to water[28,29].Therefore,amine loaded solid adsorbent is now regarded as a prospective carbon dioxide capture technology.

Fig.3.Simplified block diagram of post-combustion CO2 capture [8,9].

A lot of work has been made to improve the CO2adsorption performance of amine loaded adsorbent since the appearance of solid amine adsorbents,which is mainly concentrated on the following aspects:developing new support materials with excellent pore structures;developing new preparation methods;optimizing and improving amine species [30].The rapid research progress has yielded positive results.For examples,in order to improve the adsorption performance,various porous materials with excellent pore structure have been developed and optimized for the amine functionalization,such as hierarchical pore silica monolith[17,19,31],carbon monolith with ultrahigh pore volume [32] and 3D printed amine modified adsorbent [33,34].Simultaneously,many new preparation methods have been developed and applied.For instance,the freeze-drying method was applied for the development of amine modified adsorbents[35].And the functionalized PEI with 1,2-epoxybutane was developed to enhance the stability and sulfur resistance of solid amine adsorbent [36,37].

Although,several review papers have already been documented about solid adsorbent for CO2capture,which typically involve the physical or chemical adsorption of CO2using various porous materials as adsorbents [12,13].However,review papers focusing on the development of amine functionalized adsorbent for enhanced carbon dioxide capture are sparse,despite significant progress in the field of amine functional solid adsorbents have been recorded over the past few years.Furthermore,it is still problem to prepare the amine functionalized solid adsorbent with high adsorption capacity,fast kinetic,higher tolerance to impurities and minimal in operational cost,especially for the cyclic stability of the adsorbent [30,38].The adsorbent must have good stability and without loss of CO2capture performance after hundreds or even thousands of CO2adsorption cycles during practical applications.However,the number of cycles of adsorbent testing is not satisfactory in most of the reported paper,with 10 cycles being the most common.

Thus,we focus on emphasizing the CO2capture property of solid amine CO2adsorbents and reviewing recent research progress.Through comparing the property of amine functionalized CO2solid adsorbents and emphasizing some inadequacies,the purpose of this review is to help readers understand the current state and limitations of this field,finally to promote the development of the amine functionalized solid adsorbents.In this work,the preparation method of the solid amine adsorbent is first introduced.The CO2adsorbents are then organized in detail according to the different porous material and classified as amine functionalized silicabased adsorbents,amine functionalized carbon-based adsorbents,amine functionalized zeolite-based sorbents,amine functionalized MOF-based adsorbents,amine functionalized other composite material adsorbents.Moreover,amine functionalized solid adsorbents derived from waste resources are also reviewed because of the large number demand of cost-effective CO2adsorbents and the processing needs of waste resources.Then,considering the importance of the stability of the adsorbent in practical applications,advanced research in the CO2adsorption/desorption cycle stability has been summarized and analyzed in detail.Finally,we summarize the review and offer the recommendations for the development of amine-base CO2solid adsorbents.

2.Synthesis of Amine Functionalized Solid Adsorbents

Amine-based solid adsorbents are considered a promising alternative to liquid ammonia absorption processes due to their low regeneration energy consumption and higher CO2adsorption capacity [15].With the emphasis on CO2emission control and the development of porous material synthesis technology,more and more researchers have prepared CO2amine-based solid adsorption by loading amine species materials onto the porous supports.According to the combination of the amine species material and the support material,the preparation method of the amine-based solid CO2adsorption adsorbent can be classified into three types,namely,impregnation method,grafting method and direct synthesis method [11,39].

2.1.Impregnation method

Fig.4.Schematic diagram of preparation of amine-based solid adsorbent by impregnation [42].

The impregnation method is to load organic amine species into pores and on the surface of the mesoporous support material by wet impregnation to obtain the amine functionalized CO2adsorbent [40,41].The impregnation method can introduce large amount of amine species,and thus a higher amine-based loading can be obtained.The adsorbent obtained by impregnation method has been classified into Class 1 [11].The preparation process is shown in Fig.4 [42].To prepare the adsorbent with high performance,there are many types of amine that can be selected for the preparation of amine-base solid adsorbents,as listed in Table 1[43].Among them,the PEI and TEPA are the most widely used amine species due to its high stability and nitrogen content.The amount of amine loaded into the porous material is determined by the total pore volume of the porous material and the amine density.The theoretical loading amount of the organic amine can be obtained from the pore volume of the support and the amine density.If the amount of loaded amine exceeds the theoretical loading of the support,the amine species will be attached to the outer surface of the support,which can be clearly observed from the state of the prepared adsorbent.A suitably prepared solid amine adsorbent is typically in the form of a flowing powder.If the amine species is overloaded,the amine species will agglomerate on the surface of the support,which adhered fine porous material particles to form larger agglomerated powders.And even the slurry mixture will appear.At this point,the available amine active sites provided by the adsorbent are instead reduced.Thus,adsorbents with the appropriate amine loading can provide a large amount of available amine active sites and higher amine utilization for CO2capture.Therefore,the adsorption performance of the adsorbent is mainly determined by the loading amount and distribution of the amine group.That is to say,the pore structure properties of the carrier have a crucial effect on the capture performance of the amine modified adsorbent[44].The effect of the support structure property on the performance of the adsorbent would be described in detail below.The impregnation method has many advantages.For instance,the preparation process of the adsorbent is relatively simple,and the loading of the amine is high.The impregnated samples often show high CO2adsorption amount.However,the transfer of carbon dioxide within the adsorbent is limited and the amine utilization is lower because the supported amine occupies a large part of the internal pores of the support material [45].Simultaneously,in the preparation of amine-impregnated solid amine adsorbent,the volatile organic solvent such as ethanol,methanol is generally used.And generally,the organic solvent is removed by the heat drying method to prepare an adsorbent.This method consumes energy and is prone to amine loss and collapse of the pore structure during heating [35].To overcome the above disadvantages,freeze-drying technology is recently used to prepare the adsorbents by Wanget al.[35].Water is used as the solvent for organic amines during the preparation of the adsorbent by freeze-dried.The resultant mixtures of support and amine species were exposed to liquid nitrogen.The frozen water is then removed by sublimation.The freeze-drying method used to prepare the adsorbent not only can reduce the energy consumption,but also form new voids in the loaded organic amine layer due to the removal of water during the freeze-drying process,which would facilitate the gas diffusion and the exposure of the active site of the organic amine.The impregnation method has high applicability to the carrier material.Therefore,many carriers can be used as carriers for the preparation of adsorbents by impregnation method,such as zeolite molecular sieves [46–48],porous carbon materials[49,50],mesoporous molecular sieves [51,52] and MOFs material[25,53].In the following content we will detail the application condition of each support.

2.2.Grafting method

The grafting process is carried out by reacting the aminosilane with the surface functional group of the support material to support the amine group to the surface of the support material[39].Because the grafting method relies on the chemical bond between the amine group and the carrier,the adsorbent prepared by the grafting method has better chemical stability.Furthermore,the support pore structure changes little after the introduction of aminosilane.Therefore,the graft prepared adsorbent has the higher carbon dioxide diffusion rate compared with impregnation prepared adsorbent.The grafting amine adsorbents have been classified into Class 2[11].The commonly used aminosilanes are also listed in Table 1.

Table 1Amine species commonly used in impregnation and grafting method [43]

Since the grafting process relies on a chemical reaction,it is required that the surface of the carrier has the rich hydroxyl functional group[39].Therefore,the grafting method is generally applicable to a carrier material such as a porous silicon material rich in hydroxyl groups (including zeolite,mesoporous silica molecular sieve,etc.) [48,54,55] and MOFs materials [56].The amount of amine species introduced on to adsorbent during the preparation of grafted adsorbent is closely related to the amount of hydroxyl functional groups on the surface of the support.However,due to the limited functional groups in the support for chemical reactions,the number of amine groups capable of grafting is limited.Therefore,the amount of carbon dioxide adsorbed by the material is relatively low.But,the adsorbent prepared by the method has high amine efficiency and cycle stability [45].

2.3.Direct synthesis

The direct synthesis method is to polymerize the amine polymer monomer in situ in the porous material.In the synthesis of the support material,an amino functional group is introduced into the material to prepare an amine-based solid adsorbent material.The sorbents prepared by one pot synthesis have been classified into Class 3 [11].At present,the direct synthesis method of the carrier material generally uses silicon-based molecular sieves [57,58] and MOFs [23,25].The direct synthesis method also relies on chemical bonding.Compared with the impregnation and grafting methods,the synthesis process is only one step,which reduces the operation process,and the amino group distribution in the obtained amine-based solid adsorption material is more uniform.Simultaneously,the amine volatilization problem would be alleviated by covalently attaching amines into the adsorbent [58].In addition to the above three methods of preparing the adsorbent,in order to enhance the property of CO2adsorbents of amine modified solid adsorbent,a new synthesis routes have been proposed to prepare CO2adsorbents,which consists of two consecutive functionalization steps:grafiting and impregnation [59–61].This method combines the advantages ofimpregnation and grafting method.The impregnated organic amine can be better dispersed in the pores of the material due to the steric hindrance effect of grafted aminosilane.Thus,more organic amine active sites are exposed for carbon dioxide capture.Compared with the single impregnation method,the adsorbent prepared by this method has improved gas diffusion rate and increased amine efficiency.Compared with the single grafting method,the adsorbent prepared by this method has a higher carbon dioxide adsorption amount.

3.Amine-functionalized Solid Adsorbents

3.1.Amine-functionalized porous silica adsorbents

3.1.1.Amine impregnated silica adsorbents

Amine impregnated silica adsorbents are considered as the most practical adsorbents in all types of amine-containing solid adsorbents[15].Large amount of porous silica was widely studied as support materials since the appearance of solid amine adsorbents because of their excellent pore structure and abundant surface hydroxyl groups [15,16].Therefore,various types of porous silicas,such as MCM-41 [55,62–64],SBA-15 [15,51,52],have been applied for amine impregnation.

The PEI impregnated MCM-41 adsorbent was prepared by Sohail Ahmedet al.[62].The capture capacity was 2.26 mmol·g-1with 50% (mass) TEPA at 100 °C.It is known that the MCM-41 materials have a single and smaller pore size.The smaller mesopores are easily blocked by introduced amine species during the preparation of the adsorbent,resulting in a decrease in the exposed organic amine capture sites[17]and increase the resistance of CO2transfer in the pores.In response to the above problems,much work has been made to investigate the compromise between the amine density and pore blockage,using 2D and 3D porous material[43,65].The results suggest that when the introduced amine occupies too much of pore volume in the porous material,the decrease of pore space would lead to the more difficult transfer of CO2molecules to the amine active sites.And the amount of introduced amine has a balance effect on the CO2capture property of amine-containing solid adsorbents.Impregnated amine can offer more active sites in one way.At the same time,too much amine will block up the channels of the adsorbents in another opposite way.This situation will lead to a decrease of the CO2capture capacity due to the decrease of effective and available active site for the CO2capture.Although the solid amine CO2adsorbents have many significant advantages and promising prospects for CO2capture in the large-scale,how to load large number of amine while keeping the better dispersion of active sites in the adsorbent is still a challenging task in the development process of amine-containing solid adsorbents.

Recently,much work has been made to solve the above problems via optimizing the porous structure of material.Many new support materials with excellent pore structure,such as silica monolith with hierarchical pore structure (HPS) [17,19,31],mesoporous multilamellar silica vesicles (MMSV) [66] and MSU-J with connective channel and large pore volume[18],have been applied to accommodate more amine species to obtain larger carbon dioxide capture amount with high amine efficiency and rapid capture dynamics.Zhanget al.[65]studied the effect of supports structure on the CO2capture property of amine functionalized solid adsorbent.The results indicated that the residual pores in the amine functionalized adsorbent play a vital role in carbon dioxide capture amount.The adsorbent with 3D pore structure displays larger carbon dioxide capture amount and amine efficiency than those with 2D pore material due to the better dispersion and lower diffusion resistance.

To further illustrate the influence of 3D support on carbon dioxide adsorptive performance,Kishoret al.[67] studied the role of structural parameters of porous silica support on the carbon dioxide capture property of amine modified solid adsorbents.The results indicated that larger pore size and 3D pore structure can promote the CO2transfer to the amine sites,and large pore volume can promote the dispersion of loaded amine in the support.Thus,the large pore size and pore volume could obviously enhance the capture performance of CO2.Therefore,many 3D silicon-based materials with excellent pore structure performance have been synthesized as support materials for amine impregnated adsorbents.Jiaoet al.[18].developed amine impregnated porous silica MSU-J,which has 3-D connective worm-hole framework structured and larger pore volume of 1.93 cm3·g-1.Compared to the TEPA impregnated SBA-15,TEPA modified MSU-J displays faster carbon dioxide adsorption rate due to the better dispersion of amines.The carbon dioxide capture amount of MSU-J is 3.17 mmol·g-1,which is almost 2.5 times than the capture amount of SBA-15 with 20% (mass) TEPA loading.It is known from the above report that the presence of worm-hole framework structured promotes efficient dispersion of amines species in the support.Therefore,Wanget al.[66] prepared the mesoporous multilamellar silica vesicle(MMSV)and applied them as the effective support material for the preparation of PEI modified adsorbent.This MMSV support has the large pore volume for amine loading and the multilamellar structure can promote the better dispersion of PEI in the cell of the support.More importantly,the mesopores on the multilamellar structure layer can improve the fast diffusion of CO2.Thus,the fast diffusion and the better dispersion of PEI would effectively improve CO2capture property.Among various PEI modified MMSV adsorbent,when the 60% (mass) PEI was loaded in the MMSV,the adsorbent had optimum carbon dioxide capture amount of 4.73 mmol·g-1at 90 °C.

Based on the research mentioned above,some important basic characteristics can be summarized for excellent amine modified adsorbents.The large pore size can impede the accumulation of supported amines in the support,large pore volume can improve the amine loading for CO2capture and large mesopores can promote the transfer of carbon dioxide molecules within the adsorbent [17].Currently,Conventional amine modified adsorbent always faces the compromise between improved amine loading and increased carbon dioxide transfer resistance.Therefore,in order to surmount the shortcomings of single pore support and make the best of the superiority of each pore,one feasible method to the problem mentioned above was proposed by utilizing porous material with hierarchical pore structure as the carrier for amine loading.This silica support with multimodal pore network would provide the advantage of each channel combined in the single pore support.

A kind of novel porous silica monoliths with trimodal pore structure has been prepared and modified with PEI by Chenet al.[17].In order to illustrate and compare the effect of the pore structure of the support on the adsorption performance of carbon dioxide,the commercially silicone gel having a single pore size is also supported by PEI.The CO2capture amount of PEI modified trimodal silica monoliths increased with the loaded PEI content increased until 70% (mass).The PEI impregnated trimodal silica monoliths displays the optimal carbon dioxide capture amount of 3.91 mmol·g-1,while the PEI impregnated silica gel displays the optimal carbon dioxide capture amount of only 1.77 mmol·g-1with 40%(mass) PEI loading.These works suggest that the support with hierarchical pore networks can overcome the limitations of single pore support and make the best of the superiority of different pores sizes.Therefore,the hierarchical pore networks could disperse the amine into the various levels pore to prevent the accumulation of loaded amine and promote the carbon dioxide transfer in the adsorbent [66].These works demonstrate that the rational design of support material could effectively relieve the compromise between loaded amine contain and gas diffusion barrier.

The improvement CO2capture amount of amine impregnated adsorbents is ascribed to the increase of accessible adsorption sites in the adsorbents.The support having an excellent pore structure can promote the dispersion of the loaded amine,thereby exposing more carbon dioxide adsorption sites in the amine modified adsorbent.Thus,the porous material with large pore volumes such as mesoporous multilamellar silica vesicle and hierarchical pore silica monolith exhibit higher CO2capture amount than MCM-41 and SBA-15.But,further enhancement of the CO2adsorption amount of the adsorbents has become difficult problem due to the limitations of the supported amine content.Thus,in addition to the optimization of the support pore structure,Wanget al.[68]developed a novel technique to maintain the high capture amount and rapid adsorption kinetics in the adsorbentsviapromote the diffusion of carbon dioxide molecules within the supported amine layer.The surfactant is co-impregnated with PEI into the adsorbent during the preparation of the adsorbent,such as Pluronic F127,CTAB and Span 80.These co-impregnated surfactants can make extra carbon dioxide diffusion channel that would promote carbon dioxide molecule transfer in the loaded amine layers would expose more capture active sites.Thus,the CO2capture capacity and amine efficiency were dramatically enhanced due to the more exposed active sites in amine and surfactant co-impregnated adsorbent.Moreover,the amine and surfactant co-impregnated adsorbent displayed better capture dynamics and cycle stability property.Based on the above research,it can be found that compared with the strategy of optimizing support pore structure,this simple and feasible method of introduction surfactant additives also can significantly improve CO2capture property of solid amine adsorbent.The introduction surfactant additives can create extra diffusion pathways for the CO2in the loaded organic amine layer.As a result,the diffusion resistance of CO2can be reduced on the one hand,and more organic amine sites can be exposed in the adsorbent on the other hand,thereby promoting CO2capture and capture kinetics.

However,the additives mentioned above are all the nonreactive additives.They hardly react with carbon dioxide molecule because of the nonreactive nature in the process of carbon dioxide adsorption.And the CO2capture amount are even reduced when the content of the additive in the adsorbent is too high.Thus,Liuet al.[69]proposed the use of ionic liquids 1-ethyl-3-methylimidazolium acetate with chemical reaction characteristics to carbon dioxide molecule as the additive for the preparation of solid CO2adsorbents.PEI and 1-ethyl-3-methylimidazolium acetate were coimpregnated onto the SBA-15.The results indicated that after the addition of 1-ethyl-3-methylimidazolium acetate,the carbon dioxide capture amount and amine efficiency of PEI modified SBA-15 adsorbent were increased by 86%and 270%,respectively.This kind of additive not only can reduce the CO2diffusion barrier in the inner amine layer,but also exhibit chemical reactivity to CO2.Mesostructured cellular silica foams were prepared and impregnated with TEPA and imidazoles to enhance the carbon dioxide capture amount by Quyenet al.[70].When the mesostructured cellular silica foams were loaded with 30% (mass) 4-methylimidazole and 40% (mass) TEPA,the adsorbent displayed the optimal carbon dioxide capture amount of 4.88 mmol·g-1at 40 °C.The result indicated that the CO2capture amount of TEPA modified adsorbent relys on the spread of carbon dioxide molecule and protons to the carbon dioxide capture active sites.The added imidazole can act as the amine active site by accepting protons,thereby increasing the amount of adsorption.

The addition of surfactants and ionic liquids could break the bulk amine films,allowing more capture sites to be exposed and more diffusion channels to be formed in the interior of the loaded amine layers.Thus,the amine and additives co-impregnated adsorbents represent marked improvement CO2adsorption amount,amine efficient as well as the adsorption dynamics.Compared with the strategy of optimizing the support pore structure,the introduction of additives in the preparation process of adsorbent is easier and more feasible in terms of practical operation.Thus,it offers a simple and general strategy to designing amine functionalized adsorbents solid with high capture property.

3.1.2.Amine grafted silica adsorbents

Graft process of the sorbents is carried out by reacting an aminosilane with the surface silanol functional group of the support material to support the amine group to the surface of the support material.Thus,the amount of silanol groups in the pore surface of silica support has significant influence on the amount of grafted amine[39,71].So,similar to the amine impregnated silica adsorbent,the performances of the silica carrier also play a significant role in the carbon dioxide property of the amine grafted adsorbent.

The abundant silanol groups on the pore surface of the siliconbased carrier will contribute to the improve the amount of grafted amine,thereby increasing the amount of CO2adsorbed.However,in general,compared with amine impregnated adsorbents,conventional chemical grafting method has significantly lower CO2adsorption capacity due mainly to the lower amine loading [39].Simultaneously,the compromise between grafted amine amount and CO2capture amount in the capture process of amine grafted adsorbent is also observed because of the large molecular size of grafting agents or limited pore size of carrier[72].To achieve high amine contents in the amine grafted silica adsorbents without the compromise between grafted amine amount and CO2capture amount,many studies have been conducted on the relationship between carrier pores,grafting agents and CO2adsorption properties.Horiet al.[73]studied the capture performance of porous silica adsorbents modified with different grafting agents.The effect of the aminosilane grafting agent length and the pore size of the carrier on the CO2 capture performance was studied.

CO2capture amount was enhanced by increasing the grafted aminosilane amount.While,carbon dioxide capture amount of modified silicas with small pores were obviously reduced with high aminosilane grafted amount due to the pore blockage occurred at high aminosilane density.For aminosilanes with polymer length,high amine efficiency can be obtained for the grafted porous silicas with large channel size.The influence of the structure property on the carbon dioxide adsorption property of triamine-grafted SBA-15 with different pore size was studied by Lashakiet al.[72].The results indicated SBA-15 with large pore size displyed the maximum enhancement of surface amine density due to the decreased of the space hindrance in the SBA-15 support.The tethered triamine species in the large pore size were highly accessible,leading to the high CO2capture amount and amine efficiencies.The triamine-grafted supports with large pore presented the highest surface density of amine groups,highest CO2capture amount of 1.88 mmol·g-1and fastest capture dynamics.Therefore,it is very important to carefully select the molecular weight of aminosilane and the pore size of carrier in the process of developing amine grafted adsorbent with high capture property.

Although the amine grafted adsorbent has better thermal stability,the amine grafted adsorbent has lower CO2capture capacity due to the limitation of grafted aminosilane amount in the adsorbent.Amine impregnated adsorbents can introduce a large number of amino groups to achieve higher carbon dioxide capture capacity,but the cycle stability of these adsorbents is poor[39].To combine the advantages of two methods for preparing adsorbents,a new method for functionalization of adsorbents,called ‘‘double functionalization”,has been developed by Sanzet al.[59].The carrier is first grafted with the aminosilane and then impregnated with amine species.This new functionalization method for preparing adsorbents can offer higher amine content and improved CO2capture capacity and adsorbent cyclic stability.The pore-expanded SBA-15 is grafted with APTMS or TMPTA and then impregnated with TEPA or PEI for the preparation of the double functionalization adsorbent.When 50%(mass) TEPA was loaded on the APTMS grafted SBA-15,this adsorbent has the CO2capture amount of 5.31 mmol·g-1at 75 °C.Compared with the single impregnation or grafting adsorbent,more amine active sites can interact with CO2in the double functionalization adsorbent due to better dispersion of the impregnated amine.The APTS and TEPA cofunctionalized MCM-41 adsorbent was prepared by Wanget al.[60] using the double functionalization method.Among the prepared adsorbent,when the 40% (mass) TEPA was impregnated on the MCM-41 with 30%(mass)APTS,the adsorbent showed highest carbon dioxide adsorption capacity of 3.50 mmol·g.Linet al.[61]also reported TEPA or PEI impregnated multi-level pore adsorbent for carbon dioxide adsorption based on the TMPTA grafted ZSM-5/KIT-6.When the 60% (mass) TEPA was loaded on the TMPTA grafted ZSM-5/KIT-6,the composite adsorbent displayed the optimal carbon dioxide capture amount of 6.28 mmol·g-1at 75 °C.Compared with the single TEPA modified ZSM-5/KIT-6,CO2capture amount,amine efficiency and capture kinetics have been significantly improved.

Fig.5.Schematic illustration of the strategy and motivation of synthesis of 3D MPS-supported amine adsorbent [76].

In addition to the impregnation or grafting adsorbents,amine functionalized silica adsorbent prepared by direct synthesis also has better capture performance.Klinthonget al.[74,75] also prepared PEI functionalized silica powders (PEI-MSP) adsorbent via one-pot synthesis method.The PEI is directly introduced into the raw material for preparing the silicon powder (aqueous solution of tetraethyl orthosilicate) during the silica powders preparation.The optimized PEI-MSP adsorbent displays a high CO2adsorption capacity of 3.34 mmol·g-1under 15% CO2in N2at 95 °C.Simultaneously,compared with the PEI impregnated SBA-15 adsorbent,this synthesis method not only saves 96% of preparation time but also saves 50% of chemical reagents,thereby decreasing the cost of the obtained adsorbents.A novel efficient carbon dioxide adsorbent was developed by Liuet al.[76]via in situpolymerization of Ncarboxyanhydride of L-alanine from amine modified 3D interconnected silica adsorbents.The synthesis process is shown in Fig.5.Compared to previously reported ads orbents,these adsorbents have the advantages of rapid capture dynamics,large carbon dioxide capture amount,and better stability because of the covalently attachment between the silica carrier and the introduced amine.This amine modified 3D adsorbent shows the CO2adsorption amount of 3.86 mmol·g-1at 50°C.However,in view of the preparation cost of the adsorbent,this preparation method has higher cost due to the use of more expensive template,so new synthesis methods with less expensive template should be developed.The adsorbent synthesized by the one-step sol–gel method using aminosilane has better stability than the adsorbent directly synthesized by using an organic amine and a silicon source.Fanet al.[58] developed amine functionalized silica aerogel using one-step sol–gel method.The formed silica aerogel adsorbent has abundant micropore,mesopore and macropore.Moreover,the pore structure property and the amount of introduced amine could be adjusted by changing the synthetic solution.The CO2capture amount is as high as 6.45 mmol·g-1at 1 bar with ambient air.Furthermore,the CO2adsorption amount remained unchanged after 50 adsorption cycles.Amine functionalized silica adsorbents and the CO2adsorption property are summarized in Table 2.

Table 2Summary of amine functionalized silica-based adsorbents and their performance in CO2 capture

Table 3Summary of amine functionalized carbon-based adsorbents and their performance in CO2 capture

Amine functionalized silica-based adsorbents show a great prospect for post-combustion carbon dioxide capture due to the large carbon dioxide capture amount.Based on these studies,it can be found that the silica support material can be applied to various adsorbent preparation methods because of their excellent structure and surface performance [15].Thus,various types of silica material were developed as the carriers for CO2capture.At the same time,many new preparation methods have also been developed to improve CO2capture performance.However,the compromise between capture amount and capture dynamics can be observed regardless of the classes of the supported adsorbents,even in class 2 and class 3 adsorbents.Therefore,these results indicated the pore structure of the support,rather than the amine modified method,is the most important factor to control the adsorption performance and the compromise mentioned above.Overall,the structural properties of the support in many ways determines how effective increasing the amine loading will help to improve CO2adsorption properties.Thus,the optimization and design of the support and preparation process is still critical to prepare high performance adsorbent.

3.2.Amine-functionalized carbon-based adsorbents

Porous carbon materials are available in many forms.Common porous carbon materials include active carbons,graphene,biochar and carbon nanotubes,etc.[11].These porous carbon materials show great application prospects in the fields of adsorption,water treatment and fuel cells due to their large specific surface area and pore volume,high mechanical stability,good electrical conductivity and surface chemical inertness [8].These porous carbon materials are also widely applied in the CO2adsorption separation,but the CO2adsorption amount and selectivity are poor at low pressures and high temperatures due to these physical adsorption characteristics [14].Therefore,in order to increase the capture amount and selectivity of carbon dioxide,the porous carbon material must be surface modification through increasing the affinity of carbon materials for carbon dioxide molecules [8].Activated carbon has the advantages of mature preparation technology,large specific surface and abundant microporous structure.Moreover,the pore structure of the activated carbon could be adjusted by changing the preparation conditions [8,32,49].In addition,the introduction groups on the surface of porous carbon also could be adjusted by different modification conditions [93,94].A large amount of porous carbon material has been developed for carbon dioxide adsorption.The incorporation of various basic groups and the regulation of pore size on the porous carbon was widely stud-ied for improving the CO2affinity to increase the CO2capture amount.

Activated semicoke is used to prepare solid amine adsorbents as support by Wanget al.[22].The semicoke was first activated by N2and was then modified by TEPA.Compared with raw semicoke,N2-activated semicoke exhibited more abundant pore structure for the TEPA loading.The results indicated that the CO2capture amount of N2-activated semicoke was up to 3.24 mmol·g-1with 10%(mass) TEPA loading at 60 °C.The biochar was activated by Chatterjeeet al.[93] through ultrasonic irradiation and amination for carbon dioxide capture.The ultrasonic irradiation biochar was activated with theN-(3-dimethylaminopropyl-N′-ethylcarbodiimide hydrochloride and hydroxybenzotriazole.The TEPA was then impregnated into the biochar.The results indicated that CO2capture amount was significantly improved for ultrasound-treated amine-activated biochar.The maximum CO2capture amount was 2.79 mmol·g-1at 70°C.Compared to the traditional carbon activation at high temperatures,this ultrasonic irradiation process is carried out at room temperature,which is economically feasible strategy for the biochar preparation.

It can be seen from the above literature that the capture performance of amine modified porous carbon adsorbent mainly depends on the structure of carbon materials and the loading of organic amine modifiers.Therefore,porous carbon materials with adjustable pore structure has always been the focus of adsorption materials research.To better load organic amine modifier to improve CO2capture performance,mesoporous carbon and hierarchically porous carbon material have been developed successively for CO2capture.Wanget al.[20] developed novel solid amine adsorbent by impregnating the PEI and polymer-based surfactant on the mesoporous carbon(MC)for the CO2capture at low temperature.The mesoporous carbon has the pore volume of 3.1 cm3·g-1and interconnected 3D framework.These excellent pore structures can promote dispersion and facilitate CO2rapid diffusion in the mesoporous carbon.Thus,these adsorbents display the higher CO2adsorption capacity of 4.67 mmol·g-1at 30 °C for pure CO2.The excellent pore structure and the introduction of surfactant effectively improved the CO2capture amount and amine utilization efficiency.

Fig.6.Photograph of the MCSs(a),SEM images of the MCSs(b,c)and MCS-50(d,e)with different magnification [95].

The mesoporous carbon spheres(MCSs)was developed by Wanget al.[95] through the suspension polymerization assisted sol–gel method using silica sol as hard template.The photograph and SEM images of the MCSs was shown in Fig.6.The MCSs has the abundant mesoporous structure,millimeter size and ultra-large pore volumes.The MCSs was then modified with PEI for CO2capture.The optimized adsorbent exhibits high CO2adsorption capacity of 3.71 mmol·g-1at 75°C with 20%(mass)PEG and 50%(mass)PEI loading for 15%CO2.Gadipelliet al.[32]developed TEPA modified carbon-monolith platform for efficient CO2capture.The carbon monolith has the excellent hierarchical pore structure and large pore volume of 5.35 cm3·g-1,which can effective promote dispersion of loaded TEPA.The highly dispersion of loaded TEPA in the carbon-monolith platform exhibits much improved CO2capture properties.When the carbon-monolith was loaded with 80%(mass)TEPA,the CO2adsorption capacity can up to 5.6 mmol·g-1at 75°C with 15%CO2.At the same time,the results show that the excellent hierarchical pore structure carbon-monolith platform in the can improve the cycle stability of the adsorbent to some extent.

The surface area,pore size and the pore volume are important indicators to determine whether porous carbon materials are beneficial to organic amine loading.The larger pore size and pore volume of carbon support can facilitate the loading and dispersion of the organic amine.In particular,the hierarchically pore carbon materials with a larger pore volume are more advantageous for the carbon dioxide adsorption amount of amine modified solid adsorbent.At present,the preparation of porous carbon materials by template method is still a widely used method by researchers.The removal of the template requires the use of strong acid or alkali,and the recovery of the template is difficult,which causes economic and environmental waste.The development of new templating agents has great significance for the synthesis and application of porous carbon.

Carbon nanotubes were also widely applied for the gas adsorption because of their special physical and chemical performance,such as high mechanical strength,superior electrical properties,and high adsorption capacity [96–98].These special structures facilitate the loading of organic amines.By choosing the appropriate pore size and optimum conditions,CNT could be considered as a promising material for CO2capture.The TRI functionalized MWCNT containing hydroxyl groups was prepared by Molyanyanet al.for the CO2capture[99].The CO2capture capacity of TRI functionalized MWCNT for pure carbon dioxide was 2.5 times than those for raw MWCNT.The linear polyethylenimine (LPEI) and polythylenimine (BPEI) was supported on CNT for reversible CO2adsorption by Zhouet al.[100].Compared with the CO2capture amount of LPEI modified CNT,the BPEI (2.43 mmol·g-1) modified CNT has larger capture amount.Simultaneously,LPEI modified CNT adsorbent shows better cyclic stability than BPEI modified CNT with steam.So,LPEI modified CNT adsorbent is more conducive to CO2capture.And the CNT-LPEI required less energy for the regeneration,which promises lower energy consumption.The phenylenediamine grafted MWCNTs was prepared by Huet al.for CO2captureviaan activation-grafting process [50].The amine modified process was shown in Fig.7.The loading amounts of PDA were changedviaadjusting the activation time and the activator amount.The results indicated that the pore size of MWCNTs increased from 9.68 to 23.80 nm when the phenylenediamine is introduced.The CO2capture amount of aminated MWCNTs was significantly improved.The phenylenediamine modified MWCNTs has the CO2capture amount of 0.59 mmol·g-1,while the capture amount of the pristine MWCNTs was only 0.17 mmol·g-1.The increase in carbon dioxide adsorption capacity is mainly due to the large number of amine adsorption sites provided by the grafted phenylenediamine.Kelleret al.[101] reported the PEI functionalized hollow fiber CO2adsorption material.The hollow fiber microtubes material is consist of the (MWCNT) with abundant porous structure and lager surface area.The tubular geometry of microtubes can better promote the dispersion of amines and gas diffusion.The CO2capture amount increases with increasing PEI fractions.When the microtubes was loaded with 20% (mass) PEI,the microtubes has the maximum CO2capturer amount of 2.11 mmol·g-1.Furthermore,these hollow fibers have the better thermal stability and the large CO2capture amount.Thus,the tubular MWCNTs may provide a support system to immobilize the introduced amine species in the effective nanospace.

Graphene is a two-dimensional honeycomb lattice structure carbon material with unique performance,such as optical properties,mechanical strength,electrical conductivity and thermal conductivity,etc.So,the application research of graphene has become a research hotspot in the field of nanotechnology in recent years[102].Among them,a lot of reach has been carried out to explore their application for CO2capture because of the large specific surface area and the low production cost [102–104].To make graphene better for carbon dioxide capture,the reaches on graphite materials for CO2capture are mainly focused on surface functionalisation and synthesized hybrid materials.

Fig.7.Pathway of the amino functionalization process.(a) Carboxylation of MWCNTs.(b) Activation of carboxylated MWCNTs.(c) Grafting PDA onto the carboxylated MWCNTs [50].

As the important derivative of graphene,large amount of oxygen-containing reactive groups on the GO surface are beneficial for the chemical functional modification of GO.Shinet al.[104]developed PEI functionalized graphite oxide (GO) adsorbent for CO2capture.The results indicate that the CO2adsorption capacity increased as the increase of PEI content.The optimal CO2adsorption capacity was 0.75 mmol·g-1with the 60% (mass) PEI loading.Although the performances of pore structure of PEI modified GO are obviously reduced,the CO2capture amount of PEI modified GO was obviously increased.Gadipelliet al.[105] developed a kind of graphene networksviathermal-shock exfoliation of grapheneoxide(exfGO).This graphene networks have obviously hierarchical meso-microporous.Simultaneously,it has ultrahigh pore volumes,up to 6 cm3·g-1.The hierarchical hyperporous graphene networks was then loaded with TEPA for the CO2capture.It is exciting that the amine loading amount in the exfGO samples up to 6 g·g-1.Moreover,the hierarchical meso-/microporous structure can effectively promote the dispersion of loaded amine.Therefore,the CO2capture amount is up to 7.0 mmol·g-1at 75 °C with the flue gas conditions.

The EDA functionalized graphene oxid aerogel was developed by Prunaet al.viaone-step hydrothermal technique [106].The effects of oxidation conditions and raw graphite on the graphene oxide aerogel were investigated.The results indicate that the distribution of oxygen species and the raw graphite in modified aerogels lead to the different N doped structures.The graphene oxide aerogel originated from expanded graphite had the optimal capture amount of 1.18 mmol·g-1at 25 °C.Simultaneously,Heet al.[107] proposed a novel route for the develop of PEI functionalized CO2GO adsorbents.GO aqueous solution is first dripped into the PEI solution to form PEI-GO gel particles,and then the PEI-GO gel particles are freeze-drying.The PEI functionalized GO gel particles adsorbents exhibited better CO2adsorption amount of 2.91 mmol·g-1at 75 °C.Bhowmiket al.[108] developed mesoporous g-Al2O3nanorod/reduced graphene oxide (g-Al2O3NR/RGO).This mesoporous composite was functionalized with PEI for CO2capture.It has the larger surface area and thermal conductivity.The PEI modified composite showed an excellent CO2capture amount of 1.14 mmol·g-1with simulated flue gas at 75 °C.More important,the overheating of the amine functionalized adsorbent in the CO2capture process can be restricted to some extent due to the high thermal conductivity of mesoporous g-Al2O3nanorod/reduced graphene oxide support.Thus,the thermal degradation of loaded PEI species also can be relieved.

The amine functionalized carbons material adsorbent and the CO2capture property is summarized in Table 3.Carbonaceous porous support (including active carbons,carbon nanotubes and graphite oxide) has the advantages of abundant pore structure,high specific surface area,good chemical stability and wide source.However,the current preparation technology of carbons-based support material still presence some shortcoming.For example,the preparation process of active carbon requires the higher energy consumption because of the heating procedure with higher temperature.Therefore,it is very necessary to develop the preparation method of carbon-based support material with low energy consumption.Compared with conventional approach via furnace,the microwave heating can save heating time and reduces energy consumption,because microwave heating can uniformly heat the sample from the inside to the outside.In the amine impregnated process of porous carbon,since the absence of large mesoporous structure in the microporous porous carbon,the pores of the porous carbon carrier are easily clogged as the amount of amine impregnation increases.This situation results in a lower adsorption amount of the amine-modified porous carbon.Therefore,it is necessary to enrich the mesoporous structure of porous carbon material for the increase of the loaded amine content and the improvement dispersion of the loaded amine species.Accordingly,apart from the lower preparation costs,in order to make these carbon support even better compared with other support material,the tune and design of the pore structure in the carbon materials should be taken to obtain the superior CO2capture performance.In particular,the development of hierarchical porous carbon with large pore volume and the connected pore network should be taken seriously

3.3.Amine-functionalized zeolite-based adsorbents

Zeolite molecular sieve is an aluminosilicate crystal containing large number of microporous structures.According to its siliconaluminum ratio and crystal structure,it can be classified into X type,Y type,A type,MFI type and the like [11,114].The CO2adsorption by zeolite molecular sieve is mainly physical adsorption,relying on the effect of ion dipole and synergistic interaction of carbonate [12].The carbon dioxide adsorption property of zeolite is significantly affected by the adsorption temperature and environmental pressure.In addition,zeolite molecular sieves can only be used for CO2capture in anhydrous environments due to the zeolite have the strong adsorption of polar water molecules[114].Therefore,much work has been carried out to improve the CO2adsorption property of zeolite.Among them,the CO2capture amount and selectivity of zeolite molecular sieves are greatly improved after functionalization of organic amines.Simultaneously,amine-modified zeolite have higher carbon dioxide capture amount in the hydrous environments [28,29].

Since the zeolite molecular sieve contains the large amount of microporous structure,the amine loading is limited when the zeolite material was used as support,which in turn leads to a small adsorption amount of the amine-modified zeolite adsorbent.To increase the loading of organic amines,many amines modified mesopores zeolite were developed.Chenet al.prepared the 13X zeolite with abundant mesoporous structure (Meso-13X).In the process of synthesis,the dimethyloctadecyl[3-(trimethoxysilyl)propyl] ammonium was applied as the mesopore agent [115].The meso-13X was then modified with PEI to form the amine-containing solid adsorbent for carbon dioxide capture.The PEI functionalized meso-13X adsorbent displays larger carbon dioxide adsorption amount than the PEI functionalized 13X zeolite under the same PEI loading because of the presence of the mesoporous structure.The loaded amine can be better dispersed in the large mesoporous pore.The mesoporous pore volume in the support also can promote the diffusion of carbon dioxide.Compared with the zeolite 13X,PEI functionalized Meso-13X adsorbent exhibits much larger carbon dioxide adsorption selectivity and higher carbon dioxide adsorption amount of 1.82 mmol·g-1at 100 °C.Compared with traditional zeolite 13X materials,ZSM-5 materials with larger mesoporous structure can be loaded more amine,and the existence of mesoporous structure is conducive to the dispersion of PEI in the channels.Wanget al.[116]also synthesized mesoporous ZSM-5 zeolite as the support material and then impregnated with PEI for carbon dioxide capture.The carbon dioxide capture amount of ZSM-5-PEI adsorbent at different temperatures was studied.The ZSM-5 with 30% (mass) PEI loading exhibits the highest carbon dioxide capture amount of 1.96 mmol·g-1at 120 °C.And this adsorbent has relatively high desorption capacity level (97.85%).The binder-containing zeolite 4A was modified with the different aliphatic amines for CO2capture by Pandaet al.[117].The result shows that the CO2capture properties of the amine functionalized binder-containing zeolite 4A are primarily influenced by the nature structural properties of the amine such as carbon chain length and linear/branched chain structural.Among then,the binder-containing zeolite 4A with iso-butylamine loading presented the highest carbon dioxide capture amount of 2.56 mmol·g-1at 25 °C and 1 bar.Simultaneously,this adsorbent has high CO2/N2selectivity values (335) due to the introduction of the aliphatic amines with strong affinity for carbon dioxide.

To increase the CO2capture amount of amine-modified zeolite adsorbent,the zeolite material with the large pore size and pore volume has been further developed to increase the amine loading and facilitate the distribution of the active sites.Kalantarifardet al.[118] synthesized ZSM-5 with large pore volume.To obtain the large CO2capture amount,the ZSM-5 was then functionalized with EDA.The influence of additives such as granite,bentonite and starch on the structural properties of ZSM-5 were studied during the preparation process.The results indicated that the presence of starch can promote the form of the high surfaces area and large mesopores in the ZSM-5.The presence of large pore volume and mesopores in the ZSM-5 increase the loaded amine content and promote the dispersion of amine.The highest CO2capture capacity obtained of synthesized ZSM-5 was 6.13 mmol·g-1at 75 °C with 30% CO2.

As described in the silicon-based sorbent,the composite porous support material with abundant hierarchical pore structure,which can provide better structure conditions for amine loading and gas transport.The development of porous supports with hierarchical pore structure has attracted attention in recent years.5A zeolite coated with mesoporous silicon shell was developed Liuet al.[27]to increase the gas diffusion and reduce the influence of water on zeolite adsorption CO2.This zeolite with mesoporous silicon shell was named as 5A@MSA and the structure is shown in Fig.8.Then this support was modified with PEI to improve the CO2adsorption performance.After the PEI loading,the presence of mesoporous shell can prevent the diffusion of water to the zeolite interior to achieve a high carbon dioxide adsorption amount.When the 30% (mass) PEI was loaded on the support,the optimal carbon dioxide adsorption amount of 5.05 mmol·g-1was obtained at 25°C with 70%relative humidity.This result show that the optimization and design of 5A zeolite can achieve high capture amount by combining the high CO2affinity of PEI and the better water resistance.Amine modified zeolite adsorbents and their CO2adsorption property are summarized in Table 4.

Fig.8.Schematic illustration of CO2 and H2O molecules diffusion in the 5A@MSA core–shell hybrids [27].

Table 4Summary of amine functionalized zeolite-based and MOF-based adsorbents and their performance in CO2 capture

It is known from the above documents that the carbon dioxide adsorption performance of the amine modified zeolite adsorbent can be remarkably improved by optimizing the zeolite structure of the support.Among them,the synthesis of hierarchical pore zeolite and the reaming of zeolite are the development direction of the amine modified zeolite carbon dioxide adsorbents.The excellent hierarchical pore structure and large pore volume in the zeolite can effectively promote the dispersion of the supported amine and increase the carbon dioxide adsorption amount.In addition,the effect of water vapor on the adsorption of the zeolite can be effectively reduced by designing the internal structure of the zeolite and combining the performance advantages of the amine.When the zeolite is used to adsorb CO2from the mixture containing water,the presence of water will reduce the adsorption of carbon dioxide and can cause framework collapse of zeolite adsorbs.Thus,the optimization and development of zeolite support will be conducive to develop the efficient CO2capture technologies for the practical application under complex adsorption conditions.

3.4.Amine-functionalized MOF-based adsorbents

Metal organic framework(MOFs)is promising novel adsorbents for CO2capture.This material has the characteristics of developed porosity,large surface area,tunable pore and easy adjustment of surface functional groups [23].Therefore,the MOFs has been extensively studied in the field of CO2capture and separation.In general,the material has high carbon dioxide capture amount under high pressure due to the high surface and mesoporous volume.However,it has low carbon dioxide adsorption capacity compared with other porous adsorbent at low pressure [128].In order to increase adsorption amount of MOFs to carbon dioxide from the flue gas,the researchers introduced the amine-based groups into the surface of materials and successfully prepared various aminebased solid adsorbents based on MOFs.Since the porous structure,active site and preparation process of MOFs materials are suitable for the introduction of amine-based materials,the impregnation method,grafting method and direct synthesis method are all suitable for the synthesis of amine-based solid adsorbent materials supported by MOFs.

Fig.9.Monolith-supported amine-containing MOF for CO2 capture [121].

Fig.10.TEPA-functionalization(in blue)of Mg-MOF-74 and its effect on CO2(in red) adsorption depending on degree of saturation (s-TEPA being saturated particle)[129].

Xianet al.[53] developed the PEI functionalized ZIF-8 CO2adsorbent.The results indicated that CO2capture amount of PEI modified ZIF-8 was up to 1.61 mmol·g-1,which is much higher than the capture amount of the ZIF-8.More importantly,it was found that the CO2capture amount of PEI functionalized ZIF-8 was enhanced 1.99 mmol·g-1in the hydrous environment,which has increases of 23.6% compared with the capture amount in the anhydrous environment.The PEI-functionalized MOF film,mmen-M2(dobpdc)(M=Mg and Mn),supported on the structured monolith contactor is explored by Darunteet al.[121],as shown in Fig.9.PEI functionalized MOF film was synthesized on the monolith surface through the rapid drying of MgO on the contactor and hydrothermal treatment.The CO2capture amount of the contactor loaded mmen-Mg2(dobpdc) film is 2.37 mmol·g-1using 10% (vol)CO2.Simultaneously,it shows the better cyclic stability property.Suet al.[129] prepared the Mg-MOF-74 adsorbent with TEPA.The CO2capture performance of the functionalized MOF was studied.The CO2capture amount of the TEPA modified Mg-MOF-74 is up to 6.1 mmol·g-1at 25 °C and 1 bar,which is superior to that of the raw MOF because of the introduction of the TEPA species with strong adsorption sites.The results indicated the introduced amine content in the Mg-MOF-74 has strongly influence on the CO2capture property.The schematic of the effect of functionalization degree of Mg-MOF-74 adsorbent is shown in Fig.10.Before the amine functionalized,the water and carbon dioxide can diffuse into the interior of the Mg-MOF-74,and the ingress of water can destabilize the structure of Mg-MOF-74.When the TEPA is loaded onto the surface of the Mg-MOF-74,the formed TEPA layer can prevent the moisture from entering the interior of the Mg-MOF-74,thereby protecting the superior structural properties.TEPA-MOF makes the adsorbent have better water resistance and maintains high carbon dioxide capture amount.However,if too much TEPA is loaded onto the surface of the Mg-MOF-74,the formed thick amine layer would prevent carbon dioxide from entering the interior of the support and thereby reduce the amount of carbon dioxide adsorbed.This work emphasizes the importance of the understanding the partitioning of the introduced amine content within the support,which would contribute to the design and optimization of the amine modified MOF to achieve more efficient carbon dioxide adsorption performance.

The IRMOF-74-III was tethered with alkylamine functionalities to study the CO2adsorption property by Flaiget al.[130].In general,the ammonium carbamate was mainly generated on the amine-containing CO2adsorbents in the process of carbon dioxide adsorption.However,the results indicate the main product in the carbon dioxide adsorption process is carbamic acid for the modified IRMOF-74-III.The main reaction product can change to ammonium carbamate when the adsorbent is exposed to the H2O.The findings in this work affected the control of the CO2chemistry adsorption process in amine-modified MOF materials and emphasized the mediating role of H2O in amine-modified MOF adsorbents.Geet al.[25] prepared 2-aminoterephthalic acid modified MIL-53(Al)adsorbent through the microwave and ultrasound irradiation method.During the preparation of the adsorbent,the NH2-MIL-53(Al) could be generated quickly under the synergy of the microwave and ultrasound irradiation.Compared with traditional preparation methods,this synthesis method has the advantages of fast synthesis process and energy efficient.The prepared 2-aminoterephthalic acid modified MIL-53(Al) presented the better carbon dioxide capture amount of 1.41 mmol·g-1at 25 °C.Moreover,the prepared adsorbent has good carbon dioxide adsorption cycle stability performance.Amine modified MOFs CO2adsorbents and their CO2adsorption property are also summarized in Table 4.

Table 5Summary of amine functionalized other composite material adsorbents and their performance in CO2 capture

The MOFs material is the new type of adsorbent material,which has great application prospects for CO2capture.As described above,the MOFs has been extensively studied in the field of CO2capture and separation.The CO2adsorption amount of the amine-modified MOF adsorbent is remarkably improved under low pressure conditions as compared with the MOFs material alone.However,the adsorption amount of the amine modified MOFs adsorbent is lower than that of other amine modified adsorbents at normal pressure.Therefore,the CO2capture capacity of MOFs is needed to further increase.On the other hand,in addition to increasing the amount of carbon dioxide capture,the development of MOFs material with both capture and conversion functions should be carried out due to the abundant active metal site inside MOFs.In general,the raw materials required for the synthesis of MOF are mostly relatively expensive ligand materials [128].Thus,it is also necessary to develop the low-cost MOF adsorbent for CO2capture.

3.5.Amine-functionalized other support material adsorbents

The development of porous material synthesis technology has also expanded the research space of amine-based solid adsorption materials.In addition to zeolites,mesoporous silica molecular sieves,porous carbon and MOFs materials,some amine-based solid adsorbents based on other types of porous composite materials have been developed,such as TiO2nanotubes [131],mesoporous alumina [132],3D printed material [33],porous polymers [133],etc.

TiO2nanotube has caught much attention because of their mesopores,high surface area and excellent ion exchange.Songet al.[131]prepared the composite carbon dioxide adsorbents with high adsorption performancevialoading TEPA on the TiO2nanotubes (TiNT).When the 40% (mass) TEPA was loaded on the TiO2nanotubes,the TiNT sample presents the highest CO2capture amount of 4.37 mmol·g-1at 30°C.Guoet al.[134]synthesized the amine functionalized protoned titanate nanotube(PTNT) with different porous structure.The effects of amine types,pore structure and amine loading on carbon dioxide adsorption performance were investigated.The results indicated that pore volume of the material plays a crucial role in the carbon dioxide capture capacity of composite adsorbents.When the 60% (mass) TETA was loaded in the PTNT,the PTNT adsorbent shows the highest carbon dioxide capture amount of 4.33 mmol·g-1at 75 °C.Besides high capture amount of CO2,the TETA modified PTNTs adsorbent also exhibits better adsorption stability and reusability during 6 adsorption cycles.The mesoporous silica nanotubes (MSiNTs) was developed from halloysite nanotubes by Niuet al.[135] and then modified with PEI for CO2adsorbent.The halloysite nanotubes are calcined and treated with hydrochloric acid to form the mesoporous silica nanotubes(MSiNTs).Compared with the raw halloysite nanotubes,the surface area and pore volume of MSiNTs were significantly improved.The increased pore volume promotes PEI dispersion and increases CO2capture amount.when 50% PEI was loaded in the MSiNTs,the capture amount of the MSiNTs can reach 2.75 mmol·g-1with pure CO2at 85°C.This PEI modified adsorbent also shown the better adsorption stability during the 10 adsorption cycles.

Licciulliet al.[132] developed spherical alumina-silica carriersviadrip cast method.The structure performance of spherical alumina-silica was greatly affected by Si/Al ratio.Amount them,the prepared spherical alumina-silica with Si/Al ratio 5:1 exhibits the largest surface area,and pore volume.Simultaneously,the carrier also has the hierarchical mesoporous structure.When the carrier was modified with DEA by the wet impregnation,the optimal carbon dioxide adsorption amount of 0.91 mmol·g-1was obtained with 36% (mass) DEA loading.Zainabet al.[136] prepared a novel spider-web-like fibrous composite CO2adsorbent through electrospinning and impregnation method.Polyamide-6/carbon nanotube(PA/CNT) nano-fiber/nets composite membranes were applied as composite support for PEI impregnation,as shown in Fig.11.This composite membrane displays abundant connected pore structure.When the 75% (mass) PEI was loaded on the PA/CNT nano-fiber/nets support,the modified adsorbent has a CO2capture amount of 1.16 mmol·g-1at 25°C.And this composite adsorbent also exhibits excellent regenerability and stability during 12 adsorption cycles.Boron nitrides (BN) nanosheet was modified with PEI by Huanget al.to increase the CO2adsorption property [137].The result indicated that when the BN nanosheet was functionalized with 54.9% (mass) PEI,the carbon dioxide adsorption amount of BN nanosheets was obviously improved from 0.29 to 3.12 mmol·g-1at 75°C.Furthermore,the adsorbed CO2can be easy to be detached through N2at 75 °C.In addition,PEI modified BN adsorbent also show better cycle stability,the carbon dioxide capture amount just reduced 6.3% after 10 capture cycles.

Graphitic carbon nitride (g-C3N4) is easily obtained from inexpensive and readily available raw materials and has a wide range of applications.However,the nitrogen species inside the g-C3N4has a lower affinity for carbon dioxide molecules,resulting in a lower carbon dioxide adsorption amount if the g-C3N4is used alone to adsorb carbon dioxide.The g-C3N4was prepared using the urea as precursor and then functionalized with PEI by Penget al.[138].The result shows after the PEI impregnation in the g-C3N4,the carbon dioxide capture property is obviously increased.The optimal carbon dioxide capture amount of amine modified g-C3N4can reach 3.77 mmol·g-1at 100°C,which is much higher than the carbon dioxide captures by the raw g-C3N4materials.The carbon nanosheets (CNS) with large pore size and pore volume were developed by Huanget al.[139].Furthermore,this hierarchical carbon nanosheet could be prepared simplyviaone-step carbonization of glucose and dicyandiamide.The CNS was then impregnated with PEHA for CO2capture.Due to the hierarchical porous structure and large pore volume of CNS,PEHA can be better dispersed and the CO2capture amount can reach 5.0 mmol·g-1at 75 °C with 15% CO2.

Fig.11.Schematic illustration of the fabrication of PEI impregnated PA/CNT composite nanofibrous membranes [136].

Particle shaped CO2adsorbent has great practical application prospects for CO2capture,which can promote the contact of carbon dioxide and adsorbents and reduce the pressure drop in the capture unit,providing ideas for the development of adsorbents for large-scale applications.With the development of 3D printing technology,this technology has also been applied in the field of carbon dioxide capture.The threedimensional (3D)-printed 13X and 5A zeolite monoliths with porous structures was developed by Thakkaret al.for CO2capture [33].When the prepared 3Dprinted monoliths adsorbent contains 90% (mass) 5A zeolites,the CO2capture amount of the 3D-printed monoliths is 1.59 mmol·g-1at room temperature,which is comparable to the capture amount of powder zeolite adsorbents.Thakkaret al.[140] also developed the 3D-printed polymer-zeolite composite monoliths.The zeolite was first introduced into the polymer matrix to form the polymer/zeolite mixture.The polymer/zeolite is then 3D printed into the polymer-zeolite composite monolithic.When the 3D-printed polymer-zeolite composite monoliths contained 31%(mass)zeolite particles,the monoliths adsorbent exhibited carbon dioxideadsorption amount proportional to the zeolite loading.Moreover,the developed 3D printed monolith has excellent mechanical strength and is suitable for conventional particle packing systems.Thakkaret al.[34] further reported the formulation of 3D printed amino silica composite adsorbents monolithic using the presynthesized TEPA or PEI modified silica power adsorbents as raw material,as shown in Fig.12.The results show the aminemodified silica retains substantially physical and chemical performance in the 3D printed monolith.Moreover,the 3D printed amine-containing monolith shows similar CO2adsorption performance to the amine-modified silica powder adsorbent.This work examined the effect of raw material composition on the adsorption and structural properties of the 3D printed monolith adsorbent,which may facilitate the large-scale application of monolith CO2adsorbents.

Fig.12.The preparation process of 3D-printed aminosilica monoliths adsorbent and its adsorption effect for CO2 capture [34].

Porous polymers are also an important support material for amine modified CO2adsorbents.The polymer not only has high pore volume,but also can achieve pore regulation and functional modification by selecting different polymerization monomers and different chemical synthesis means.Therefore,polymer is promising support materials for amine loading.Liuet al.[133] developed the poly(divinylbenzene) (PDVB) with hierarchical pore structure via one-step polymerization of divinylbenzene (DVB).Moreover,this synthesis process does not require any templating agents and catalysts and the required raw material are readily available.The prepared PDVB has obvious hierarchical meso-macropores and large pore volume.The PEI modified PDVB adsorbent has the CO2capture amount of 2.93 mmol·g-1at 75°C and large CO2selectivity.And this PEI modified PDVB adsorbent also exhibits better adsorption cycle stability in the presence of water.Amine modified other composite material adsorbents and the CO2adsorption performance are listed in Table 5.

In view of the large-scale application of the adsorbent,the adsorbent should be prepared simply and rapid and the raw materials must be inexpensive and readily available.However,most of the composite carrier adsorbents reported exhibit limited adsorption performance and complex synthetic procedure.Therefore,there is still a need to develop the adsorbent support with a simple and rapid preparation method,readily available reagents and excellent structural properties such as hierarchical carbon nanosheet and hierarchically nanoporous poly(divinylbenzene)mentioned above.

3.6.Solid amine adsorbents from waste resource

Currently,most solid adsorbent materials are facing high cost problems.Therefore,many researchers are working on the development of low cost solid adsorbent materials[150].Thus,developing low cost amine functionalized adsorbent with the excellent carbon dioxide capture amount is critical to the practical largescale application of adsorbents.Simultaneously,a lot of studies have focused on the re-development and utilization of waste resources to effectively solve the growing environmental problems caused by domestic or industrial waste [13].To highlight the importance of the combination of waste utilization and carbon dioxide capture,we introduce a separate part to report the amine modified CO2adsorbents derived from waste resources.

Fig.13.Schematic diagram of biomass waste promoted with polyamines as potential CO2 adsorbents [154].

Alhassanet al.[151]prepared the jatropha curcas activated carbon(JAC)through KOH activation using waste jatropha curcas shell as the raw material.The jatropha curcas activated carbon were then functionalized with TEA to prepare CO2adsorbent.The CO2capture amount of jatropha curcas activated carbon and TEA modified jatropha activated carbon were 1.5 and 1.77 mmol·g-1respectively.The results indicated that an inexpensive and effective porous carbon adsorbent could be easily obtained from the Jatropha curcas shell and functionalized with amine species to increase its carbon dioxide capture amount.Bagasse activated carbon was developed with ZnCl2-KOH activation by Weiet al.[152].The prepared bagasse activated carbon with excellent porous structure was then modified with TEPA.When the prepared activated carbon was loaded with 5% (mass) TEPA,the CO2capture amount of adsorbents can reach the 3.62 mmol·g-1at 60°C.Compared with adsorbent support that require a large amount of chemical raw materials to be synthesized and prepared,this porous carbon support derived from the by-product of sugar industry exhibits comparable CO2capture property.

A low-cost and readily available amine modified adsorbent was developed by Wanget al.using the wood ashes originated from the biomass materials combustion as the porous carrier [154].The preparation process is shown in the Fig.13.The influence of the type of wood ashes and the amine types on the CO2adsorption performance was examined in detail.Among them,the TEPA functionalized wood ashes originated from the dry stalks of rice and wheat have optimal CO2capture performance.When the wood ash was loaded with 45% (mass) TEPA,the adsorbent shown the highest carbon dioxide adsorption amount of 2.02 mmol·g-1at 60 °C with 5%CO2and 5%H2O.The desired adsorbent is also stable during 10 cyclic runs.The PEI modified porous silica nanoparticles(PSNs)was developed by Zenget al.[155]using rice husk as a silica source.This preparation method does not require any templating agent as compared to the conventional porous silica preparation method.Additionally,when the PSN was loaded with 55% (mass)PEI,the CO2capture amount of adsorbent can reach 3.61 mmol·g-1at 75°C with 15%CO2.The MCM-41 has been synthesized by Paneket al.[157] using the hydrothermal zeolite production by-product as the silicon source from pulverized coal fly ash (PFA).The PFA derived MCM-41 was then functionalized with PEI loading for carbon dioxide capture.The result indicated when the 60%(mass) PEI was impregnated in to this PFA derived MCM-41,the prepared adsorbent presented CO2capture amount of 2.95 mmol·g-1at 75°C with 15%(vol)CO2,which is larger than that of the commercial MCM-41.Since the preparation process of this ash-derived MCM-41 uses the waste silicate solution as the raw material and does not require preparatory work,it has the low production cost.The SiO2nanowire clusters was developed by Ouyanget al.fromfibrous sepiolite (Sep) [158] through the acid treatment.The obtained amorphous SiO2silica nanowires presences the large surface area of 320 m2·g-1.After the TEPA impregnation,the optimized CO2capture amount can reach 3.7 mmol·g-1at 75 °C when the 50% (mass) TEPA was supported,and the CO2capture amount kept 3.6 mmol·g-1after 10 adsorption cyclic tests.Liuet al.[159] directly used acid-modified sepiolite as the carrier to develop the low-cost CO2adsorbent by impregnating DETA.The results suggested that acid-activation and DETA supported amount can simultaneously enhance the carbon dioxide capture amount.The highest CO2capture amount of 1.65 mmol·g-1can be obtained when 0.8 g DETA was loaded into the acid-modified sepiolite.Simultaneously,the CO2capture amount can maintain 95.2% after 4 cycles testes.Moreover,this adsorbent could be easily regenerated at 75°C with N2.Duet al.[160]developed cheap mesoporous silica material using the coal gangue as the readily available raw materials through the hydrothermal reaction.The obtained porous silica has a mesoporous structure similar to MCM-41.And after DEA modification,the CO2capture amount can reach 1.9 mmol·g-1with 8% CO2at 25 °C.A summary of amine-functionalized solid adsorbents derived from waste resource and their CO2capture property is shown in Table 6.It can be found that the adsorbent support material derived from waste resources has a relatively simple preparation method and low cost.The carbon dioxide capture amount of these materials also could be improved through the surface amine groups modification.Therefore,with the requirement for large number of carbon dioxide adsorbent,the development of cost-effective adsorbents from waste resources will undoubtedly increase the competitiveness of such adsorbents in carbon dioxide capture.However,noted that amine-modified mesoporous material derived from bagasse,rice husk,wood ashes,coal gangue and sepiolite generally exhibits the slightly lower carbon dioxide capture amount compared with the amine-modified conventional mesoporous material.This is mainly due to the poor pore structure of the prepared support from waste materials such as the smaller surface area and pore volume.These poor pore structure would result in low amine supported amount and poor amine dispersion,and in turn limit the adsorption of CO2.Therefore,it is recommended to use waste to prepare high structural performance materials and then perform CO2adsorption.

Table 6Summary of amine-functionalized solid adsorbents derived from waste resource and their performance in CO2 capture

Table 7Summary of adsorbent regeneration and stability performance

Table 8Adsorption capacity and heat of regeneration performance of adsorbents [67]

4.Adsorbent Regeneration and Stability

In addition to the need for adsorbents with high CO2adsorption capacity,the stability of the CO2adsorption/desorption cycle of amine-containing solid adsorbents is another important factor that determines whether the adsorbents can really be applied in industrial applications [38].The adsorbent would be recycled hundreds or even thousands of times during actual industrial applications.But most papers only report less than a few dozen cycles,of which 10 cycles are the most common [15,30,38].To make the amine modified adsorbent closer to practical applications,therefore,this section summarizes the reported studies on the stability of the carbon dioxide adsorption/desorption cycle of modified adsorbent,in which the number of cycles involved in the adsorbent is as high as possible.

The reduced cycle stability of the amine modified CO2adsorbent is primarily due to the volatilization and degradation of the loaded amine and the incomplete desorption of CO2in the adsorption/desorption cycle [30,38].Simultaneously,acid impurities gas also can deactivate the amine-base adsorbent by the irreversible react with amine to form product which is not regenerable at desired conditions [154,165].The formation of such byproducts would decrease the CO2adsorption capacity and thus reduce the cyclic stability of the amine-base solid CO2adsorbent.The presence of oxygen also causes the decrease in the stability of the adsorbent because amine functional groups can be oxidized to other nitrogen-containing compounds in oxygen-containing environment [166].

To improve the chemical stability of the amine loaded adsorbents during the regeneration process,Liet al.[167] studied the effect of PEI type and PEI molecular weight on the stability of adsorbents in the CO2adsorption cycles process.The results indicated that both molecular weight and PEI type effected the cycle stability of PEI modified adsorbents.Moreover,compared with the molecular weight,the PEI type has the greater effect on cycle stability of adsorption.Furthermore,the cycle stability of adsorption raised with increase of branched PEI molecular weight.Compared with branched PEI,linear PEI is more difficult to volatilize and leach.The linear PEI loaded fumed silica was prepared and investigated for the CO2capture by Zhanget al.[168].The results show that although the linear capture dynamics of linear PEI loaded silica is similar to that of branched PEI,the desorption rate on linear PEI loaded silica is significantly faster,which is conducive to rapid capture cycle.Moreover,the CO2capture amount does not decrease significantly even in wet conditions after more than 100 adsorption cycles.

In addition to the use of large molecular weight amines,modification of amine molecules and synthesis of new polymeric adsorbents can also significantly improve the stability of the adsorbent[169,170].A kind of functionalized PEI with 1,2-epoxybutane(EB) was prepared by Choiet al.[170].After functionalization of PEI with 1,2-epoxybutane,the primary amine proportion gradually reduced,while the secondary amine and tertiary amine proportion gradually increased.Functionalized PEI was then supported onto silica for carbon dioxide capture.The functionalized PEI with 1,2-epoxybutane reduced the heat of adsorption and promoted the desorption of CO2in the adsorption cycle process.In addition,the functionalized PEI with 1,2-epoxybutane can obviously enhance the stability of CO2adsorption cycle because of the inhibition of urea formation and degradation of amine oxides.More recently,they also developed the 1,2-epoxybutane functionalized TEPA to overcome the poor thermal stability of TEPA [36].The TEPA was functionalized with the 1,2-epoxybutane/TEPA ratios of 2:1,3:1 and 4:1.As a result,the formed material has the O/N molar ratios of 0.42,0.64 and 0.82,respectively,as shown in Fig.14.Despite the carbon dioxide adsorption capacity gradually decreases with the increase of O/N molar ratios after 1,2-epoxybutane modification,the TGA results indicated that the more functionalized materials have higher thermal stability.As shown in Fig.15,after 10 adsorption and desorption cycle,the TEPA modified adsorbent lost much CO2capture amount,whereas the 1,2-epoxybutane modified material has no significant losses in CO2capture amount,confirming their higher thermal stability.

The oxygen contained in the flue gas would cause the oxidative degradation of the amine species and deactivation in the adsorbent during the actual application of the adsorbent,thus,Choiet al.prepared novel O2resistant amine-base solid adsorbent by combining two method[171].PEI is first functionalized with 1,2-epoxybutane to form the tethered 2-hydroxybutyl group as mentioned above.The chelating agent was preliminarily immobilized on the silica carrier to poison P.P.M.grade metal impurities (Fe and Cu).The combination of these two methods produced a significant synergistic effect.The obtained adsorbent exhibits a small loss of carbon dioxide capture amount (8.5%) and even after aging for 30 days in the flue gas containing O2at 110 110°C.Therefore,the preparation method may provide a feasible strategy for the development and practical commercial application of high oxidation stability solid amine adsorbent.More recently,these research group also provide a facile method to prevent the negative effects of SO2using a similar method [37].The PEI was first impregnated into silica support.The PEI supported at the external surface of the silica support was then alkylated with 1,2-epoxybutane.Therefore,amine supported at the external surface of the silica support was converted to tertiary amines.The synthesis process of the SO2-resistant solid amine adsorbent with the tertiary-amine-rich protection layer is shown in Fig.16 [37].The results indicated that the SO2adsorption process on the primary and secondary amines is irreversible,while the adsorption of SO2on tertiary amines is completely reversible.Thus,in the capture procedure of CO2in the flue gas containing SO2,SO2is reversibly captured by the exterior tertiary amine layer,and then desulfurized CO2is captured by the below PEI layer.This SO2-resistant adsorbent exhibited a small amount of loss of CO2capture amount(8.52%)even after 1000 CO2capture cycles in the presence of 50×10-6ppm SO2.It is reasonably expected that many different polyamines could be modified with the diverse functional epoxides to prepared amine-base CO2adsorbents with outstanding long-term cycle stability.

Fig.14.Functionalization of TEPA using different amounts of EB [36].

Fig.15.Cyclic stability of TEPA-containing materials with and without EB Functionalization [36].

Fig.16.Synthesis of an SO2-resistant amine-containing adsorbent with a tertiaryamine-rich protection layer [37].

Fig.17.P-dendrimers used to produce solid amine adsorbents with PEI [172].

By using a cross-linking reaction,the macromolecular polymer and the amine molecule are combined to form solid amine adsorbent,which also can improve the adsorption/desorption cycles.Samuelet al.[172] prepared the solid amine adsorbents for CO2capture through a cross-linking reaction between polyaldehyde phosphorous dendrimers and PEI.The crosslinked carbon dioxide adsorbents formed by different types of polyaldehyde Pdendrimers and PEI with different molecular weights were studied.The preparation process of crosslinked adsorbents is shown in Fig.17.The optimal property adsorbent,1-G0/600PEI,350 adsorption cycles testes were carried out for 700 hours.The adsorbent substantially maintains the capture amount of 2.98 mmol·g-1after the 350 adsorption cycles.In contrast,the carbon dioxide capture amount of the porous silica adsorbent with 37.5%(mass)PEI loading drops 16.6%after the 335 adsorption cycles due to the leach of the loaded PEI species.Parvaziniaet al.[173] prepared the ion exchange resins as amine-containing solid CO2adsorbents.Among them,VPOC1065 resin was modified with primary amines on the polymeric carrier of styrene crosslinked with divinylbenzene.VPOC1065 exhibits the adsorption amount of 1.75 mmol·g-1at 25 °C with 98% (vol) CO2.VPOC1065 displays excellent stability,the CO2capture amount just has 4.84%reduction after 275 adsorption/desorption cycles.

In addition to optimization of the amine structure and preparation process during the preparation of the adsorbent,the stability of adsorbent can also be improved by changing the test environment.The water vapor in the simulated flue gas can promote the increase of the capture amount by changing the mechanism of amine adsorption CO2.Simultaneously,the presence of water vapor also can improve the adsorption cycle stability of the adsorbent [165].Aliakbaret al.[174] prepared the PEI functionalized pore-expanded MCM-41.The effect of water vapor on the adsorption cycle performance was investigated.The results showed that the CO2adsorption capacity of PEI modified pore-expanded MCM-41 decreased by 14% after 120 adsorption cycles under drying conditions due to the evaporation and decomposition of PEI and the formation of urea during heating cycle.On the contrary,the adsorption capacity of adsorbent decreased by only 6% when the simulated gas containing water vapor (6% RH) was used.This result indicated that the presence of water vapor can reduce the loss of amine functional groups.Sayariet al.[175] also prepared the pore-expanded MCM-41 as the support for the surface graftingof 2-[2-(3-trimethoxysilylpropylamino) ethylamino] ethylamine.The influence of the water vapor on the CO2cycle stability of adsorbent was also investigated.When there is no water vapor,the adsorption capacity of TRI modified pore-expanded MCM-41 adsorbent drops by 15%after 750 cycles.In contrast,the adsorbent can maintain the stable adsorption capacity after 700 cycles of CO2adsorption in the presence of water vapor.The DRIFT spectrum result indicated the urea would form directly in the CO2adsorption/desorption process or form indirectly through the carbamate decomposition under dry conditions.In contrast,the formation of urea can be prevented when the water vapor is presence in the simulated flue gas.

Although the moisture can be used to improve adsorbent cycle stability in the process of CO2capture,studies have shown that when the silicon-based mesoporous support is exposed to water vapor for a long time,the pore structure of the support will collapse significantly,which will lead to a steady decrease of the adsorbent circulation [176].Therefore,there is need to develop the adsorbent support with long-term hydrothermal stability.The recent work on the regeneration and stability performance of adsorbent is summarized in Table 7.To further increase the stability and realize the practical application of adsorbent,it is still necessary to exploit the new synthetic methods of the adsorbent.For example,the development of adsorbent with both high stability and high sulfur resistance by optimizing the type of amine functional group.The regeneration strategy will also need to be carefully selected to avoid the irreversible reduction of CO2capture property due to urea formation during the adsorbent regeneration process.

Another important parameter of the adsorbents for carbon dioxide adsorption is the energy requirement in the form of adsorption and regeneration heat.Kishoret al.[67] calculated the adsorption heat and regeneration heat of PEHA-impregnated solid adsorbent by DSC tests.The regeneration heat (Q) is associated with the CO2capture amount of adsorbent as shown in equation:Q≈mCpΔT+ΔHr,wherem(kg) is the mass of adsorbent,Cp(-kJ·kg-1·K-1)is the heat capacity,ΔT/(K)is the temperature change in the regeneration process,and ΔHr(kJ·mol-1) is the heat of adsorption [67].The result is shown in Table 8.The result shows that the heat of adsorption of PEHA-impregnated adsorbents in the range of 53.4–72.7 kJ/mol CO2.And the heat of regeneration of adsorbents in the range of 58–78 kJ/mol CO2.Wanget al.[147]also calculated the adsorption heat of 2D/3D amine functionalised sorbents containing graphene silica aerogel and mesoporous silica.The results show the adsorption heat of adsorbent lies between 47 and 57 kJ/mol CO2for 2D/3D adsorbents loaded with TEPA from 30 to 80%(mass).However,the energy required in 30%MEA solution lies between 141 and 242 kJ/mol CO2,which is much higher than that with the solid amine adsorbent [184].

Thus,the carbon dioxide capture performance of amineimpregnated adsorbents are better than those of the conventional MEA process in terms of energy efficiency.It is expected that the regeneration heat can be greatly reduced by replacing the conventional MEA process with solid amine adsorbents adsorption process,because of the much lower heat capacity of solid adsorbents comparing to aqueous MEA.

5.Conclusion and Perspectives

Amine functionalized solid adsorbent has great potential in practical applications for post-combustion CO2capture due to these outstanding capture performances.And large number of achievements in the field of amine functionalized solid adsorbents have been recorded over the past few years.In this review,the recent development of solid amine CO2adsorbents has been thoroughly reviewed.All the solid amine adsorbents are organized in detail according to the different porous support and classified as amine functionalized porous silica-based adsorbents,amine functionalized carbon-based CO2adsorbents,amine functionalized zeolite-based CO2adsorbents,amine functionalized MOF-based CO2adsorbents,amine-functionalized other composite material adsorbents.For adsorbents using porous silicon,zeolite and porous carbon as supports,much of the work has focused on the optimization of the support structure and the development of new preparation methods.Many carrier materials with excellent pore structure have been developed for the improvement of CO2adsorption property,such as 3D hierarchical porous silicon,hierarchical porous zeolites,and carbon-monolith with ultrahigh hierarchical pore volume.For adsorbents using as MOF supports,the work has focused on the support surface modification.Although amine functionalized MOF adsorbents can be prepared by simple surface modification,the adsorption amount is lower than that of amine functionalized other porous material at normal pressure.With the requirement for large number of solid CO2adsorbents and the development of cost-effective adsorbents,another important part of this review paper is the introduction of solid amine CO2adsorbents from waste resources such as bagasse,oil sands coke,wood ashes,rice husk and pulverized coal fly.Finally,considering the importance of the amine functionalized adsorbent stability for the industrial applications,recent advances in the adsorbent cycle stability has been briefly reviewed.Fortunately,many efforts have been directed to improve their long-term cycling stability and the exciting achievements have been made,such as converting the primary/secondary amines into the tertiary aminesviaepoxide modification,chelating ppm-level metal impurities of PEI.

Compared with the liquid amine absorbents,the ideal CO2adsorbent should have the properties of high CO2capture capacity,considerable stability,fast adsorption kinetics,good resistance to impurities and low energy loss requirements.However,in order to achieve the above goals,there are still many unresolved problems in the development of solid amine adsorbents.For example,although the support material with outstanding structural property and high CO2capture capacity was developed,while the meticulous selection and combination of the multi-templates are often demanded,which virtually increases the complexity and cost of sample syntheses.Therefore,it is necessary to develop the highly efficient green synthesis method to simplify the synthesis step and reduce preparation cost such as the development of amine-containing silica aerogelviaone-step sol–gel method.Moreover,most reaches focus more on the properties of support than on the development of amine functionalized processes.When the amine is introduced to support,there is less investigation or discussion on the preferred occupancy scheme of amines for the different pore in the support.Effective introduction of amines species into different levels pore of the support may play a key role in determining important property of adsorbents such as amine accessibility and amine leaching propensity.Thus,it is very important to develop effective methods of amine introduction into practical support structures for the preparation of adsorbent with excellent capture performance.At present,most of the adsorbent studies use a mixture of CO2and N2as the simulated flue gas.However,many impurity gases such as SO2,NO2,O2,and water vapor in the real flue gas.Some of these impurity gases are advantageous for CO2adsorption such as water vapor.Some impurity gases can cause irreversible deactivation of the adsorbent such as SO2,NO2,and O2.Therefore,for the practical application of the adsorbent,the actual flue gas should be simulated as completely as possible in the evaluation of the adsorbent.In addition,most reported adsorbents are present in a powder state that results in greater heat and mass transfer resistance during the CO2adsorption process.Therefore,the monolith adsorbent with porous structure should attract more attention,which not only can improve the heat transfer and mass transfer of the adsorbent but also eliminate the molding process of the adsorbent in the later application.To realize the theoretical advantage of solid amines compared with aqueous solvents,adsorbent stability puzzler also must be solved owing to the industrial adsorbents would be exposed to thousands of CO2capture cycles.Simultaneously,the design,optimization and scale-up of capture procedure also need to get more attention due to the lack of the enough demonstration and industrial application experiences.

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 (21878200 and 21676174),International S&T Cooperation Program of Shanxi province(201703D421038),Shanxi Scholarship Council of China (2017-036),and Joint Fund of Shanxi Provincial Coal Seam Gas (2015012019).