Designed synthesis of porous carbons for the separation of light hydrocarbons

Shuang Xu,Ru-Shuai Liu,Meng-Yao Zhang,An-Hui Lu

State Key Laboratory of Fine Chemicals,Liaoning Key Laboratory for Catalytic Conversion of Carbon Resources,School of Chemical Engineering,Dalian University of Technology,Dalian 116024,China

Keywords:Porous carbon Adsorption Separation Hydrocarbons

ABSTRACT The separation of light hydrocarbon mixtures(C1-C3)generated from petrochemical industry is vital and challenging process for obtaining valuable pure chemical feedstocks.In comparison to the energy intensive conventional separation technologies (cryogenic distillation,absorption and hydrogenation),the adsorptive separation is considered as a low energy cost and high efficiency process.Porous carbons have been demonstrated as excellent adsorbents for the separation of light hydrocarbons,owing to their designable structure and tailorable properties.This review summarizes the recent advances of using porous carbons as adsorbents for the separation of light hydrocarbons,including methane/nitrogen,methane/alkane,methane/carbon dioxide,ethylene/ethane and propylene/propane.We discuss the separation mechanisms and highlight the material features including pore structure,surface chemistry and target molecular properties that determine the separation performance.Furthermore,the challenges and development direction associated with carbonaceous adsorbents for light hydrocarbon separation are discussed,meanwhile the guidelines for the design of porous carbons are proposed.

1.Introduction

The utilization of clean energy resources instead of coal and oil has long been one of the sustainable development goals [1-4].Light hydrocarbon mixtures (C1-C3),mainly derived from natural gas,shale gas,biogas,oil field gas and refinery gasetc.,are important clean energy resources,in which natural gas,mainly consisting of CH4,is expected to become the worldwide energy supply before 2030 [5].However,the gas resources cannot be directly used due to the various impurities,such as CO2,N2,H2O and other trace gases,as listed in Table 1[6-17].The presence of these impurities would not only reduce the conversion rate and energy content,but also cause corrosion events during the storage and transportation process [18].Moreover,in these gas resources,C2H4and C3H6are important chemical feedstocks to manufacture their downstream products of polyethylene and polypropylene.Therefore,to achieve the sustainable utilization of these gas resources,we need to establish highly efficient separation of light hydrocarbons.

Generally,the suggested separation process is placed downstream the exhaust gas cleaning section of the plant,including pretreatment,sorption-separation and purification units.Taking the separation of refinery gas as an example(Fig.1),(1)in the pretreatment unit,the water vapor,acidic gases(H2S,CO2,SO2)and C4+are firstly removed through the condensation,deacidification and dehydration technology,then the gases undergo compression and are cooled down to 303-313 K using the heat exchanger in order to further separate the gases [19].(2) In the sorption-separation unit,the cryogenic distillation,absorption,membrane separation and adsorptive separation are the common methods.The cryogenic distillation relies on the difference of boiling point of the target molecules,which is suitable for gas flow of～107m3·d-1(Fig.1)with the product gas purity of ＞95% [20].This process should be conducted at low temperature (183 K) and high pressure(～2 MPa),suffering from high energy consumption and costintensive.The absorption processes,especially oil absorption for light hydrocarbons separation,rely on the difference of gas solubility in absorbent at 193-273 K [21,22].The flow rate can reach about 108m3·d-1[21].Nevertheless,the purity of gas product usually in the range of 50%-85%due to the injection of additional solvents during desorption process,resulting in the difficulty of the production of polymer grade olefin[23,24].The membrane separation has been applied to the removal of CO2or N2from natural gas,and the plant can purify feed gases of～106m3·d-1[25,26],however,the separation of light hydrocarbons through this technology is mostly in the laboratory development stage [27,28] While the adsorptive separation is an appropriate candidate technology with economically acceptable flow rate of 106m3·d-1[29,30].(3)In the purification unit,the flexible integration of different separation processes can achieve energy saving and obtain the gas products with high purity.For example,an adsorptive technology can be coupled with cryogenic distillation units,in which the product gas contains 98%CH4and 96%N2can be recovered simultaneously[31].Consideration of the property of feed gas,product specification,available market and economy consumption,we should flexibly choose and design the specific separation process for practical applications.According to the above analysis,the continuous adsorption-desorption cycles can be realized based on physical adsorption through the multiple fixed-beds operating in adsorptive separation process.In this case,the adsorbed component can be recovered at ambient temperature and low pressure (0.1-0.01 kPa),thus,adsorptive separation attracts increasingly attention compared to other technologies due to its low energy consumption,flexible operation and running continuous with mild regeneration condition.Therefore,we mainly discuss the adsorptive separation technology for separating light hydrocarbons.

The preparation and design of efficient adsorbents such as zeolites,silica materials,metal organic frameworks (MOFs),covalent organic frameworks (COFs) and porous carbon materials,etc.[6,32,33]is very imperative to the separation of the light hydrocarbons by means of the adsorptive separation process.Considering the moisture sensitivity and regeneration of adsorbents in industry process,the utilization of porous carbon materials for selective separation has garnered increasing attention.Many typical commercial carbons have shown a good gas adsorption and separation performance,for instance,the Takeda CMS 4A displays a kinetic separation performance towards C3H6/C3H8mixture[34],likewise,the Takeda CMS 3 K has the separation ability for the mixture of CH4/N2due to the difference in the diffusion kinetics [35],however,their equilibrium selectivity is unsatisfied.Despite enormous endeavors have been spent to enhance the selectivity of light hydrocarbon separationviamodifying surface chemistry and pore structure of porous carbons,it is still a challenge for hydrocarbon separation especially the target molecule pairs with close physical property,including CH4/N2,C2H4/C2H6and C3H6/C3H8,because the inert surfaces of carbonaceous materials provide similar interactions between the adsorbates and adsorbents.

In this review,we give a detailed overview of recently documented use of porous carbons as adsorbents for the adsorption and separation of light hydrocarbons.We aim to understand the mechanism of adsorption and separation,and analyze the correlations among pore structure,surface chemistry and separation performance.We would like to provide deep insights to researchers for the rational design of carbonaceous adsorbents.

2.Separation of Light Hydrocarbons

Fig.1.Simplified process flow diagram for the recovery of refinery gas.Note:depending on the available markets,feed gas properties and product specifications,the units designed are different.Note:the gas flow mentioned in this part was calculated at 293 K and 101.325 kPa.

Fig.2.Proposed adsorptive separation mechanisms.

For overall process of gas adsorption on the adsorbent,the adsorbate transports from the exterior surface of a particle to macropore surface and micropore channels,and diffuses from micropore channels to micropore surface or internal crystalline structure,in which the intraparticle diffusion is the determining step [36].Typically,the mechanism of separation includes the molecular sieving,kinetic and thermodynamic equilibrium effects(Fig.2).(1)molecular sieving effect:namely the steric mechanism,is mainly based on the match-degree between the size/shape of adsorbates and the pore structure of adsorbents;(2)kinetic effect:in this case,the separation performance is driven by the different diffusion rates of adsorbates;(3) thermodynamic equilibrium effect:separation is determined by the interactions of adsorbents with the adsorbed molecules,including dispersion,electrostatic and induction interactions [37].These interactions depend on the nature of the adsorbed light hydrocarbon molecules (e.g.,dipole moment,quadrupole moment and polarizability,Table 1) and the properties of the pore wall and surface of adsorbents(e.g.,polarity and non-polarity),thus,this mechanism is relatively intricate.In general,the surface properties of the adsorbents such as polarity corresponds to its affinity with polar substances.For instance,the difference of polarizability and quadrupole moment makes the major contributions for separating the non-polarity and nonpolarity molecule mixtures (C2H4/C2H6,CH4/N2,etc.),while for the separation of polarity and non-polarity molecule pairs (C3H8/CH4,C2H6/C3H8,etc.) and the polarity-polarity molecules (C3H6/C3H8,etc.),the one with high dipole moment is preferentially bound by adsorbent.For other complex gas mixtures,the mechanism should be analyzed according to the actual situations.

2.1.Separation of C1

Methane (CH4),as one of the most important C1energy resources,is the dominant component of coal bed methane,natural gas,landfill gas,refinery gas and biogas with different concentration proportions depending on the source of gas(Table 1).Separation of CH4from these gas sources plays a significant role in energy conservation and emission reduction[38,39].The main reasons can be summarized into three aspects:(1) natural gas has long been considered as an attractive clean fuel to alleviate the reliance on oil because the low ration of C/H of CH4can emit reduced CO2concentration after combustion [40];(2) for the utilization of natural gas,the presence of impurities including N2,CO2and hydrocarbons can reduce the calorific values and conversion rate,and give rise to the pipeline corrosion and unsafe events [5];(3) as one of the greenhouse gases,the greenhouse effect of CH4is 20 times higher than that of CO2,leading to the climate change.Therefore,to reduce energy and environmental issues and increase the commercial values,the separation and purification of CH4reaching a certain purity is necessary.

2.1.1.CH4/N2 separation

Coal mine methane (CMM) usually consists of 20%-50% (vol)CH4and 30%-70% (vol) N2as well as 20%(vol) other gases (CO2,O2,H2O,etc.),because a large amount of air need to be injected into the coal seams to enhance the extraction of coalbed methane(CBM)during the mining process[41-43]In this case,the low concentration of CH4(＜30% (vol)) cannot be used as the fuels and chemicals,consequently,every year about 20-29 billion cubic meters CBM are directly released into the atmosphere,amounting to 200 million tons of standard coal [44,45].Separation and recycling of CH4from CBM can save 65%-70% (vol) resources and alleviate energy shortage [46,47].Based on the current studies,removal of N2from CH4viaadsorptive technologies can be achieved by using molecular sieving,equilibrium driven processes and kinetic separation.However,because of the tiny difference in molecular size between CH4and N2(0.38 nmvs.0.36 nm),the separation by molecular sieving is nearly impossible.Thus,the main separation mechanisms rely on the equilibrium and kinetics effects.

Fig.3.(a) CO2,CH4 and N2 adsorption at 298 K of OTSS-derived carbons.(b) IAST selectivities of CO2/N2 (15:85),CO2/CH4 (10:90) and nitrogen content of OTSS-derived carbons[54].(c)Interaction energy between CH4/N2 molecules and the surface carboxylic/hydroxyl/aldehyde groups of the samples.(d)Absolute difference of the interaction energies (ΔIE) between CH4 and N2 on functional groups [60].

In the case of equilibrium mechanism,since both CH4and N2are nonpolar molecules without dipole moments,the higher polarizability of CH4determines the greater interactions of gas with the surface of porous carbons than that of N2(CH4:26.0 × 10-25cm3vs.N2:17.6×10-25cm3).In this regard,one most straightforward way is to enhance the affinity of CH4with adsorbents by depositing chemicals(such as Br2or ICl)[48]and introducing heteroatoms or plasma treatment in CH4/N2atmosphere to control the surface chemistry of porous carbons [49].Nitrogen element doping is one of the most pervasive methods to improve CH4/N2separation performance of porous carbons due to the improvement of surface polarity by integrating electronegative N species (pyridinic-N(N-6),pyrrolic-/pyridonic-N(N-5)and graphitic-N(N-Q))into carbon frameworks [50],thus providing a CH4-philic environment to trap CH4molecules [51,52].Fenget al.[53] investigated the influence of N-species for CH4adsorption and showed the pyridine N facilitates the enhancement of positive adsorptive sites.Zhanget al.[54] reported the N-rich porous carbons (OTSSs) with high surface areaviaNaNH2activation of oil-tea seed shells at 623 K.It exhibited a good CH4/N2selectivity of 5.6 and CH4capacity of 0.93 mmol·g-1at 298 K and 100 kPa.They found the proportion of pyrrole/pyridine species had no obvious relationship regards to the separation performance of CH4/N2mixture,whereas,the content of pyrrole N specie had a positive correlation for the adsorption performance of CO2compared to CH4due to the higher polarizability of CO2than that of CH4(29.1 × 10-25cm3vs.CH4:26.0×10-25cm3)(Fig.3(a),(b)).The similar phenomenon was also observed by Fan and co-workers [55].Liet al.[56] prepared Ndoped wool derived porous carbon(N-WAPC)with high N content of 14.48%(mass) through the urea modification followed by KOH activation.The CH4adsorption capacity was 1.01 mmol·g-1with CH4/N2selectivity of 7.62 at 298 K and 100 kPa.Other researchers also reported the N doping porous carbons prepared by thermal treatment of N-containing precursors.For instance,the porous covalent triazine-based framework (CICTF) was regarded as raw material to fabricate porous carbon with high N content of 12%(atom) [57],which exhibited a CH4/N2separation selectivity of 8.6.Moreover,the activated carbons modified by basic groups(such N-H)were fabricated by immersing them into the different concentration (2-8 mol·L-1) ammonia solution followed by washing and drying [58],and showed 38.4% higher separation coefficient of CH4/N2(5.33) than that of the original activated carbons(3.85) [59].

The effects of O-atom doping in porous carbons for the CH4separation performance were also investigated.Tanget al.[60]reported the rice-based binderless granular carbon adsorbents(PRC-850) with rich O-groups (hydroxyl/carboxyl/aldehyde groups),showing CH4/N2selectivity of 5.7 and CH4uptake of 1.12 mmol·g-1.They calculated the interaction energies (IEs) of CH4and N2molecules with surface O-functional groups by theoretical calculation,and found that the IE between CH4molecules and O-groups was higher than N2molecules(Fig.3(c)).They also indicated that the carboxyl group was the dominant surface group to improve CH4/N2selectivity through calculating the absolute difference of interaction energy(ΔIE)between CH4and N2on functional groups(Fig.3(d)).Meanwhile,the co-effect of N and O heteroatoms on gas adsorption and separation of ultramicroporous carbon (PS-2-450) was also investigated by Liu and co-workers [61].The results indicated that the improvement of CH4adsorption capacity at low pressure (＜15 kPa) was mainly attributed to the introduction of the polar groups.

Recently,Wanget al.[62] also studied the influence of surface polarity of porous carbons for CH4/N2separation performance.They prepared a series of heteroatom-doped porous carbons(PZS-derived carbons) by pyrolysis of the polymeric precursor of poly(cyclophosphazene-co-4,4′-sulfonyldiphenol (PZS) at high temperature,and the N,O,P,S were distributed evenly within the carbon skeleton.They explored the surface polarity through monitoring the hydrophilicity of PZS-derived carbons.The results indicated that the water contact angles gradually decreased from 23°to 9°with the increasement of carbonization temperature from 1073 to 1223 K,which indicated the enhancement of surface polarity.The IAST selectivity of CH4/N2of PZS-derived carbons ranged from 3.73 to 6.04,which was proportional to surface polarity,and the PZS-900 showed the highest CH4uptake of 1.88 mmol·g-1at 298 K and 100 kPa.

Fig.4.(a)The separation coefficient based on GCMC simulation.(b)Correlation of separation coefficient with pore volume for the range 0.7-1.3 nm reduced to unit surface area [71].

Besides the modification of the surface chemical property,the regulation of pore structures including pore volume and pore size is another effective route to improve the adsorption and separation performance[63].Many researchers have found that the gas selectivity is mainly affected by micropores,while mesopores and macropores almost have no effects on the selectivity of CH4/N2mixture [64-67].The effective pore size for CH4enrichment has been studied from 0.5 to 1.3 nm.The pores with a width smaller than 0.31 nm could inhibit the penetration of both CH4and N2[68],while the pores with the width range from 0.5 to 0.8 nm would exhibit optimal enrichment performance for CH4[15,69,70].For example,Liuet al.[71] investigated the effect of pore structure of porous carbon on the separation coefficient for CH4/N2mixture.They reported that the separation coefficient varied basically with pore size,and reached a highest value in the pore of width 0.75 nm,while the effect of pore size could be neglected when the pore size is larger than 1.3 nm (Fig.4(a)).Afterwards,they also explored the influence of pore volume on separation coefficient,and found that pore volume of the specified pore size(0.7-1.3 nm) per unit surface area correlated linearly with the separation coefficient of CH4/N2with a correlation coefficient of 0.94(Fig.4(b)).Therefore,the design of adsorbent with increased specific pore volumes with the pore sizes ranging 0.7-1.3 nm facilitates the adsorption and separation of CH4.The similar result also has been observed by Chen and co-workers [72].

Donget al.[73] reported the granular oil-tea-shell derived carbon adsorbent (GOC-2) with the pore size of approximately 0.5-0.6 nm,which exhibited a high CH4uptake of 1.82 mmol·g-1at 298 K and 100 kPa among carbon granules.Their group [74] also prepared cactus derived porous carbons (CGUCs) with narrow ultramicropore size(＜0.7 nm)through anin situK+ionic activation method,in which the KOH was used as the activator,and the carboxyl and hydroxy groups on cactus and glucose could immobilize K+ions and subsequently the ultramicropores could be created followed by removal of K+ions during the carbonization process(Fig.5).The obtained CGUCs showed applicable gas mixture separation performance of CO2/N2,CO2/CH4and good CH4/N2selectivity range of 5.0-6.7.After that,starch-based ultramicroporous carbons (SC-6) were prepared by the similar activation procedure,which showed CH4uptake as high as 1.86 mmol·g-1with the CH4/N2IAST selectivity of 5.7 at 298 K and 100 kPa [75].Recently,Liuet al.[76] reported ultramicroporous carbon with pore size distribution of 0.4-0.7 nm through thein situactivation mechanism of K+ion.Differently,they used the KOH alkalized KMnO4solution as a facile activating agent to pretreat carbon precursor.After activation,NH3was used to remove the alkali metal.They found that KMnO4can provide an oxidizing condition and increase the content of the carboxyl group of precursors,thus enhancing the ions exchange between K+and H+in carboxyl group.In this case,more alkaline metal can be loaded on precursors,thus obtaining porous carbon with abundant ultramicropores after activation.The obtained porous carbon (AC-KP) showed a high surface area of 1549 m2·g-1calculated by N2adsorption isotherm and ultramicropore volume of 0.50 cm3·g-1calculated by CO2adsorption at 273 K.Notably,the AC-KP exhibited high CH4capacity of 3.38 mmol·g-1at 273 K and 1.87 mmol·g-1at 298 K.

Fig.5.(a) The schematic illustration of the synthesis process of CGUCs.(b,c) NLDFT pore size distribution of CGUCs [74].

Fig.6.(a)Idealized hypothetical ultramicropore size distribution for CMS membranes pyrolyzed at three final pyrolysis temperatures[81].(b)Adsorption isotherms of CH4,N2 and O2 on three CMSs at 298 K.(c) Adsorption uptake curves of CH4,N2,and O2 CMSs at 298 K [16].

Fig.7.(a)SEM images of microporous carbon spheres(MCS)and(b)nanosheets(MCN).(c)First-order linear plot of the CO2 adsorption for MCN and MCS[86].(d)AFM image of PCNPs and corresponding cross-section analysis for the dashed orange line in the image.(e) Micropore size distribution and micropore volume determined by CO2 adsorption at 273 K.(f) Comparison of kinetic adsorption of CH4 for PCNPs [87].

Notably,for equilibrium separation of CH4/N2,the improvement of surface polarity can not only boost the CH4adsorption capacity,but also results in improved N2adsorption capacity due to the close polarizability between N2and CH4molecule,thus limiting the enhancement of CH4/N2selectivity[77].One effective strategy for this separation issue relies on the kinetic separation by precise controlling pore structure,further realizing the difference of gas diffusion rate in the adsorbent [78].Carbon molecular sieves(CMS) have been widely studied for CH4and N2separation due to the abundant ultramicropores and micropore structures [79].In these cases,the match degree between the pore structures and gas molecules size or shape is key[80].Ninget al.[81]studied the influence of ultramicropore size for N2/CH4permselectivity and proposed the idealized hypothetical ultramicropore size distribution (Fig.6(a)).It showed that the ultramicropore distribution shifted to the lower size with increased pyrolysis temperature from 773 K to 1073 K,thus forming the tightly packed CMS structure.The permeabilities and diffusion coefficients increased with temperature,while the sorption coefficients decreased.Moreover,the chemical vapor deposition is a common strategy to tailor the pore structure.For instance,carbon molecular sieve (CMS-1023 K) was prepared by using benzene as depositing agent on activated carbon [82].The results demonstrated that the CH4adsorption capacity increased with the increase of deposition temperature and reached a maximum adsorption capacity of 1.41 mmol·g-1and high kinetic selectivity of 35.3 at deposition temperature of 1023 K,then the selectivity decreased as the temperature exceeded 1023 K.This result indicated that the increased deposition temperature was beneficial to the formation of microporous channels.And they found the carbon deposition was effective in producing constrictions at the pore entrances at 1023 K.However,the higher temperature than 1023 K led to the pore shrinkage and collapse,further resulting in the low adsorption performance.

For the practical upgrading coal mine methane,besides the main components of N2and CH4,the 10%-16% (vol) of O2is also the non-negligible component,consequently,the separation for ternary mixtures of CH4/N2/O2has been investigated.Yanget al.[16] studied the breakthrough curves of simulated coal bed methane with the feed gas composition of 30% (vol) CH4/14.7%(vol) O2/55.3% (vol) N2.The results showed higher adsorption strength between CH4and adsorbents than that of N2and O2due to the higher polarizability of CH4(2.6 × 10-3nm3) compared to N2(1.4 × 10-3nm3) and O2(1.2 × 10-3nm3) (Fig.6(b)).And O2had longer breakthrough times compared to CH4and N2,that was ascribed to its greater adsorption kinetics (the diffusion time constantD/r2:O21.46 × 10-2s-1;CH44.77 × 10-6s-1;N26.97 × 10-4s-1).The kinetic selectivity for N2/CH4was calculated to be 4.46 at 298 K (Fig.6(c)).

Although the CMSs are the effective adsorbents for gas separation,the increased mass-transport limitation is not negligible during separation process due to the long diffusion paths and narrow apertures of ultramicropores.In this regard,our group [83-85]have prepared and designed various two-dimensional(2D)porous carbons (nanoplates,nanosheets) with tunable thickness and controllable morphology by adjusting the chemical environment of benzoxazine polymerization.And the diffusion and adsorption kinetics in both gas phase and liquid phase were investigated based on two model carbon materials,i.e.microporous carbon nanosheets (MCN) and microporous carbon spheres (MCS) [86].The results exhibited the nanosheets has more rapid diffusion and adsorption kinetics than the nanospheres due to the shorter diffusion paths and larger exposed geometrical area of 2D structure(Fig.7(a)-(c)).

Recently,we reported [87] self-pillared ultramicroporous carbon nanoplates with narrow pore size distribution (0.48 nm)viamulticomponent sequential assembly method to further intensify the diffusion kinetics of CH4/N2separation (Fig.7(d),(e)).The pillared 2D carbons (PCNPs) showed fast diffusion time constant(1.18 × 10-3s-1),which was 2.3 times higher than their smooth counterparts (SCNPs,5.06 × 10-4s-1) (Fig.7(f)),and two orders of magnitude faster CH4diffusion kinetics than that of commercial CMSs (Takeda 3kt,1.19 × 10-5s-1) [16].The comparison of the adsorbent properties and performances is listed in Table 2.

2.1.2.C2-3Hx/CH4 separation

Natural gas is a potential alternative to conventional fossil fuels during transition to a decarbonized energy system,which includes various amounts of light hydrocarbons,such as 60%-70%(vol)CH4,7%-35% (vol) C2-3Hxand 5%-13% (vol) other gases (Table 1).Separation of C2-3Hxfrom CH4not only can upgrade the purity of natural gas to meet the pipeline quality (＞90% CH4) for its efficient utilization,but also can recycle the C2-3Hxas a supplementary resource for raw chemicals [88,89] Considerable efforts have been contributed to improve the adsorption and separation performance of C2-3Hx/CH4.In the earlier studies,the researchers have proposed and established the theory approaches [90] for estimating adsorption performance of alkanes on commercial activated carbon adsorbents,such as Calgon BPL activated carbon[91,92],Westvaco BAX-1100[93].In recent years,various homemade porous carbons have been explored.The asphalt-based activated carbons (A-ACs)were prepared and the adsorption capacities of C3H8and C2H6were 11.76 and 6.59 mmol·g-1at 298 K and 100 kPa,respectively,and the IAST-predicted selectivity for C3H8/CH4reached 88.8 [94].More recently,Keet al.[95] reported starch-based microporous carbon (SMCs) with high specific surface area of 1999 m2·g-1and a narrow pore size distribution within 0.5-2.0 nm.The SMCs showed ultrahigh C2H6/CH4selectivity of 27.1 under ambient conditions,and exhibited the highest adsorption capacity of C3H8to 8.39 mmol·g-1compared to other alkanes because of the strongest adsorption affinity of C3H8with the surface of the adsorbent(Fig.8(a),(b)).Moreover,the adsorption energy of alkanes on the different adsorption sites and aperture size of SMCs were calculated (Fig.8(c),(d)).The results demonstrated that the C3H8and C2H6could be preferentially adsorbed to surface oxygen containing groups than CH4,and the carbon wall of the micropore majorly contributed to the adsorption energy than oxygen groups,especially in the confined pore aperture of～0.65 nm.

Table 2Summary of the physicochemical properties and CH4/N2 adsorption separation performances of typical carbonaceous adsorbents

In the same period,Zhanget al.[96] fabricated algae-derived N-doped porous carbon (ANPCs) by grinding the mixture of algae,melamine and K2C2O4followed by carbonization process,which exhibited high C3H8adsorption capacity of 11.5 mmol·g-1and good dynamic separation performance (Fig.9).They also prepared N-doping activated hydrothermal carbon (NAHA) by using algae and glucose as precursors [97],and NANA showed C2H6/CH4selectivity of 32.6 with C2H6uptake of 3.7 mmol·g-1at 298 K and 100 kPa.N-doping can selectively enhance the adsorption heats of C2H6while had a negligible effect on that of CH4,and doped nitrogen groups can enhance the adsorption capacity[98].

Recently,researchers also discussed the influence of microporous structure and pore size distribution of porous carbons for light hydrocarbons separation.Xueet al.[99]examined the performance of S-graphite slit pore in selective separation of CH4over C2and C3by Grand Canonical Monte Carlo calculations(GCMC).They found the optimal pore size for CH4/C2H2and CH4/C2H4was 0.65 nm,for CH4/C2H6was 0.75 nm and for CH4/C3H6and CH4/C3H8was 1.0 nm.Maet al.[100] reported tobacco-based porous carbon (UCs) with high surface area of 3839 m2·g-1and abundant porosity with narrow mesopores (＜3 nm).The C3H8uptake displayed a good trend with cumulative volume of pore smaller than 3.8 nm and a good tendency between C2H6uptake and pore volume smaller than 3.5 nm.Afterwards,they further discussed the influence of micropore size on separation selectivity of C3H8/CH4and C2H6/CH4[101].They found that the adsorption density of gas molecules was relevant to micropore width.For example,theselectivity of C3H8/CH4mainly depended on the micropores of＜1.2 nm,and the pore width of 0.9 nm and 1.0 nm contributed to the highest C3H8/CH4selectivity due to the higher C3H8adsorption density than that of CH4.For C2H6,the C2H6/CH4selectivity primarily relied on micropore size of ＜1.0 nm,and the highest selectivity could be realized at pore size of 0.6 nm.

Fig.8.(a) Comparison of IAST-predicted C2H6/CH4 selectivity of SMCs and other porous materials.(b) Isotherms of CH4,C2H6,and C3H8 adsorption on SMCs at 298 K.(c)Calculated adsorption energy of alkanes on different adsorption sites of SMCs.(d) Calculated adsorption energy of alkanes on SMCs [95].

Our group [102] prepared unimodal ultramicroporous flat carbon nanoplates (FCPs) with more than 80% sp2carbonviathermoregulated phase-transition-assisted method (Fig.10(a),(b)).The thin 2D morphology favored the orientated growth of carbon crystallite during high temperature pyrolysis process,thus allowing the formation of single micropores size.The unimodal ultramicropores structure is responsible for the separation of light hydrocarbon molecules due to the enhanced host-guest van der Waals interactions originated from short-range attractive forces in uniform ultramicropores (Fig.10(c),(d)).More recently,Lidoped nanoporous carbons(NPCs)[103]were explored in the separation of C2and C3over C1using GCMC calculation and density functional theory (DFT),which revealed the Li+doping could enhance the affinity between NPCs and light hydrocarbon molecules,providing useful insights for promoting the development of novel adsorbents.For a better comparison,the structure parameters and the adsorption and separation performances of the adsorbents are shown in Table 3.

2.1.3.CO2/CH4 separation

In general,the biogas and landfill gas contain 35%-70% (vol)CH4and 15%-60%(vol)CO2[29,104],which are valuable raw materials in the chemical industry.Removal of CO2from biogas and landfill gas is the main step in the enrichment of CH4to achieve fuel grade quality,and to avoid corrosion of pipeline during transportation and utilization [105].It is commonly accepted that the surface interaction of adsorbent with CO2is stronger than that of CH4due to the higher quadrupole moment of CO2(CO2vs.CH4:4.3 × 1026esu·cm2vs.0,1 esu=3.336 × 10-10C).The equilibrium-based adsorbents have been widely studied [106-109].For example,the N-doped porous carbon with high specific surface area of 1984.7 m2·g-1and pore volume of 1.0 cm3·g-1were synthesized from shrimp-shells.The sample activated at 973 K(SA-1-700) exhibited CO2/CH4separation selectivity of 8 at 298 K and 100 kPa [110].The N-functional group can enhance the interaction forces between the adsorbents and CO2more than CH4[111],and facilitates the favorable selectivity at low pressure(15 kPa) [112].Liet al.[113] found the nitrogen doping can enhance the selectivity of CO2/CH4.They prepared microporous carbon with high nitrogen content (6.48%(mass)) (NAC-4) by using anthracite as the precursor combined with KOH activation and urea treatment,and it exhibited CO2/CH4selectivity of 5(Fig.11).

However,the nitrogen content of a porous carbon is usually determined by its nitrogen-containing precursor,and not easy to adjust[114].From this consideration,some researchers developed porous carbons with tunable nitrogen contentviaimpregnation of N-containing species.For instance,the activated carbon with different nitrogen content was prepared through impregnation of polyethyleneimine(PEI/AC).The mass percentages of PEI were ranged from 0.06% to 0.29% and the 0.26% PEI/AC showed optimal uptake of gases with the order of CO2?CH4＞O2＞N2[58].

Fig.9.(a) The schematic illustration of the synthesis process of ANPCs.(b) Light hydrocarbon adsorption-desorption isotherms.(c) The dynamic breakthrough curves for CH4/C2H4/C2H6/C3H6/C3H8/He (5/5/5/5/5/75) mixture gas at 298 K [96].

Furmaniaket al.[115] verified the synergetic effect of narrow nanopore and oxidized pore walls is the most important factor in improving CO2/CH4separation performance.Sulfur-doped microporous carbons(CKS-5)had also been prepared by using KOH as the activating agent and poly(sodium 4-styrenesulfonate) as carbon source.The acidic molecule (CO2) was beneficial to interact with the basic C-S group of adsorbents,while the interaction between CH4and S-containing functional group was weaker,thus resulting in the high selectivity of 5.86 for CO2/CH4[116].Sahaet al.[117]reported that the sulfur content could affect the selectivity of CO2/CH4,and the porous carbon with the highest heteroatom contents(S and P)showed the best selectivity for CO2/CH4although it had the lowest specific surface area and pore volume [118].

Besides the equilibrium-based adsorbents,kinetic-based adsorbents were also studied for the separation of CO2/CH4[119,120].The difference in diffusivity was used as the basis for kinetic separation.Kapooret al.[121] investigated the kinetic separation performance of CO2/CH4based on carbon molecular sieve and found that the stronger adsorptive CO2was also the faster diffusant,which was favorable for kinetic separation,especially when CH4was the desired product.The diffusion time constants of CO2and CH4were calculated to be 9.7×10-4s-1and 5×10-6s-1,respectively,and the diffusivity ratio for CO2/CH4was 180 at 298 K.In this case,CO2molecule could be adsorbed fast while CH4would take longer time to penetrate the pores.Rochaet al.[122]reported that the CO2could reach equilibrium after approximately 13 min,while CH4reached equilibrium after over 25 h (Fig.12(a)).And a roll-up of CO2breakthrough curve was caused by additional adsorption of CH4at longer time since the CH4adsorbed in the adsorbent was very strongly limited by diffusion(Fig.12(b)).Moreover,the adsorption-diffusion process for CO2/CH4separation was studied and discussed.For example,álvarez-Gutiérrezet al.[123]compared the breakthrough curves of homemade biomass-based(CS-H2O and CS-CO2) with commercial activated carbon (Calgon BPL) for CO2/CH4mixtures.The Calgon BPL displayed a more distended mass transfer zone than the CS-H2O and CS-CO2in the whole range of CO2(solid line) partial pressure,while only small differences were observed for CH4adsorption curves (dotted line)between the three carbons (Fig.12(c)-(e)).This revealed the slow kinetics for CO2adsorption on Calgon BPL than that of biomassbased activated carbon and the kinetics of CH4adsorption may not be relevant to the separation of CO2/CH4.

For evaluating the practical application of the adsorbents,the adsorption and separation property of CO2/CH4mixtures at high pressure is also important [124-126].Meatreet al.[127] reported the CH4storage capacity up to 5.5 mmol·g-1at 1000 kPa,and indicated the materials with pore size distributions around 0.6-0.9 nm are more suitable for adsorption of CH4.Also,Matrangaet al.[128]illustrated the pore of 0.8 nm is more effective to obtain the highest uptake of CH4adsorbedviathe simulation of CH4adsorption on an ideal porous graphite and the theoretical adsorbed amount around 7 mmol·g-1at 1000 kPa.Moreover,glucose-based carbon(Glc-Cs) with surface area of 3153 m2·g-1and pore volume of 2.06 cm3·g-1was prepared for estimating separation property of CO2/CH4,which showed CO2/CH4(0.1:0.9) selectivity of 2700 at 3000 kPa and the CH4storage capacity of 10 mmol·g-1[129].The excellent performance at high pressure is mainly attributed to the large pore volume and high surface area.Recently,a flexible nanoporous activated carbon cloth (C60-CC-PNP) was developed by introducing dispersed polypyrrole nanoparticles on surface of commercial viscose rayon cloth fiber followed by carbonization and activation,which exhibited a high reported values of CH4storage(7.5 mmol·g-1at 2000 kPa and 298 K)in porous carbon materials [130].The list of adsorbent properties and performances involved in this section is compared in Table 4.

Table 3A summary of structure parameters and adsorption performances of carbonaceous adsorbents for C2-3Hx/CH4

Fig.10.(a)SEM image of FCPs.(b)Pore size distribution of FCPs materials.(c)Gas adsorption isotherms at 298 K.(d)Breakthrough curves of x/CH4(10/90 v/v)at 298 K and 100 kPa [102].

2.2.Separation of C2 and C3 hydrocarbons

Apart from CH4,C2(C2H4,C2H6) and C3(C3H6,C3H8) are also vital energy resources and chemical feedstocks for petrochemicals[131].Generally,olefins(C2H4and C3H6)are produced through the thermal or catalytic steam cracking of paraffins (C2H6and C3H8),thus,crude olefins usually contain abundant paraffins (～50%(vol)),which should be separated to obtain valuable polymer grade olefins[132].Traditional olefin/paraffin separation mainly depends on energy-intensive cryogenic distillation under extreme conditions,which is accounting for about 0.3%of the global energy consumption [133-135].For example,the C2H4/C2H6separation is carried out at 248 K and 2300 kPa,and similarly,C3H6/C3H8separation is performed at 243 K and 3000 kPa in a distillation column containing over 100 trays[136].Adsorptive separation,as an alternative to distillation,has great potential to change the current energy-intensive separation process.Porous carbons due to their abundant porosity,high surface area and good moisture resistant property,have been demonstrated to be promising adsorbents for the adsorptive separation of olefins and paraffins mixtures.

Fig.11.(a)Schematic illustration of synthetic route and application.(b)The relationship between influence factors and CO2 uptake.(c)IAST predicted selectivity at 298 K for CO2/CH4 binary mixture assuming different proportions of 50/50 on the NACs [113].

2.2.1.C2H4/C2H6 separation

The separation of C2H4/C2H6is a challenging topic due to their non-polar property and similar molecule size (Table 1),especially for carbonaceous materials,where their inert surface would result in the weak interactions between adsorbents and adsorbates.To improve C2H4/C2H6separation performance,one effective strategy is the introduction of cations(Cu+or Ag+)into carbon framework or surface,further enhancing the interaction between C2H4and porous carbon through π-complexation.For example,Gaoet al.[137] reported CuCl/ACvialoading CuCl2into commercial activated carbon followed by reduction process under N2atmosphere.The obtained CuCl/AC adsorbent with the copper loading of 8 mmol·g-1AC achieved a high C2H4adsorption capacity of 2.57 mmol·g-1and C2H4/C2H6adsorption selectivity of 69 at 303 K and 100 kPa.Jianget al.[138] synthesized mesoporous carbon CMK-3 by using mesoporous silica SBA-15 as the hard template and then incorporated CuCl into the carbon matrixviasolid-state grinding.The 0.8 mmol CuCl modified CMK-3 (8CuCl/CMK-3) endowed the nonselective CMK-3 with the C2H4/C2H6selectivity of 2.67.de Lucaet al.[139] investigated the role of Ag(I)in the selective adsorption of alkene over alkane by DFT simulation.The theoretical analysis showed thedorbitals of Ag overlapped with the π orbital of the carbon atoms within the graphene plane placed in front of the Ag functionality,thus rejection of the alkane.Then the authors calculated the adsorption energies of C2H4,C2H6,C3H6,and C3H8based on two-S-Ag(I)functionalized slit models of widths 1.54 and 0.53 nm,respectively,and found that the adsorption energies are relevant to the pore widths.For instance,the adsorption energies of C2H4and C3H6were 6.1 and 4.4 times higher than those of C2H6and C3H8depending on the pore size of 0.52 nm.When the calculated model was chosen as slit model of width 1.54 nm,the adsorption energies of C2H4and C3H6were 3.8 and 3.6 times higher than those of C2H6and C3H8,respectively.In addition,sulfurization of the carbon precursor favors the increase of cations content owing to the affinity of sulfur to cations.For example,Sahaet al.[140] prepared Ag(I)doped microporous carbonsviasulfurizing polymer derived carbon,in which the sodium thiosulfate as sulfurizing agent.The results indicated that the amount of Ag(I) functionalities was positively correlated with the content of sulfur functionalities on the surface of the porous carbon.The porous carbon with Ag(I)content of 2.5%(atom) showed C2H4/C2H6selectivity of 2.5 at 100 kPa.

Table 4Summary of structure properties and CH4/CO2 adsorption capacity and selectivity of carbonaceous adsorbents

Fig.12.(a)Diffusion of CH4 and CO2 at 298 K at low pressure(first adsorption equilibrium point)in CMS KP 407.(b)Simulated molar fraction of a‘‘long-term”breakthrough curve for CH4-CO2 mixture at 1000 kPa showing roll-up for methane and CO2[122].(c and d)CO2(solid lines)and CH4(dashed lines)breakthrough curves for CS-H2O(c),CSCO2 (d),and Calgon BPL (e) with 30%/70% (vol) CO2/CH4 (blue color),50%/50% (vol) CO2/CH4 (red color),and 65%/35% (vol) CO2/CH4 (green color) [123].

Although the selectivity of C2H4/C2H6has been improvedviaπcomplexation,the high adsorption heat is observed.It will result in the high energy consumption during the desorption of olefins[137,138].As an alternative,the introduction of heteroatoms (N and O)into the carbonaceous material can enhance the interaction between adsorbents and adsorbates.In this case,the C2H6can be preferentially adsorbed because the C2H6has a higher polarizability than that of C2H4,thus the stronger interaction of C2H6with the O/N atoms on the surface than that of C2H4[141,142].For example,Wanget al.[143] reported the polydopamine-based porous carbons (C-PDAs) with high N and O contents (N:3.86%(mass),O:19.56%(mass)).The C-PDAs exhibited higher adsorption capacity of C2H6than C2H4(7.93 mmol·g-1vs.6.61 mmol·g-1) at 298 K and 100 kPa.The DFT calculation showed that the binding energy of C2H6with the C-PDA substrate was higher than that of C2H4with the substrate (-24.72 kJ·mol-1vs.-20.44 kJ·mol-1),suggesting the higher adsorption enthalpy for C2H6over C2H4.This result further confirms the preferential adsorption of C2H6over C2H4.They also prepared the composite adsorbents by depositing polydopamine on the surface of asphalt-derived carbon (CPDA@AACs) [144],and the CPDA@A-ACs showed preferentially adsorbing C2H6over C2H4with a high C2H6uptake of 7.12 mmol·g-1at 298 K and 1000 kPa.The C2H6/C2H4adsorption selectivity predicted by IAST reached 3.0-20.6 below 100 kPa at room temperature.

A similar adsorption behavior of C2H6was also observed in the microporous glucosamine-based carbon adsorbents (MGAs) [145].The MGAs presented preferential adsorption of C2H6over C2H4with its C2H6adsorption capacity of 7.6 mmol·g-1at 298 K and 200 kPa and the IAST selectivity of C2H6/C2H4reached 6.76(Fig.13(a),(b)).The adsorption heat of C2H6(28.4 kJ·mol-1) was higher than that of C2H4(21.8 kJ·mol-1),implying that the interaction of C2H6with the surfaces of the adsorbent was stronger than that of C2H4(Fig.13(c)).Then the breakthrough curves of the binary C2H6/C2H4mixtures were investigated through a fixed bed with a total flow rate of 3 ml·min-1at 100 kPa,and showed good C2H6/C2H4separation performance (Fig.13(d)).

In addition,the effect of O element on C2H4/C2H6separation was also studied.Leeet al.[146] prepared microporous 3D graphene-like materialsviausing zeolites (Beta-zeolite,Faujasite(FAU)-and its hexagonal analog (EMT)-) as the templates,which showed preferential adsorption of C2H6and the obtained C2H4with purity of ＞99.9% (Fig.14(a)).Among the three zeolite-templated carbons (ZTCs),the beta-ZTC with the BET areas of 3200 m2·g-1and micropore volumes of 1.4 cm3·g-1exhibited superior C2H6selectivity of 1.7 at 100 kPa in C2H6/C2H4mixtures (Fig.14(b)-(e)).Unlike in the previously reported ethane-selective carbon materials,the C2H6adsorption properties were not significantly affected by the presence of oxygen groups.

Fig.13.(a)C2H6 and C2H4 adsorption isotherms of MGAs-750-3 at 298 K.(b)IAST-predicted selectivity for an equimolar C2H6/C2H4 mixture at 298 K.(c)Comparison of the isosteric heats of ethane and ethylene adsorption over MGA-750-3.(d)Experimental breakthrough curves of an equimolar C2H6/C2H4 mixtures through the fixed bed at 298 K and 100 kPa [145].

Despite the thermodynamical equilibrium separation for C2H6/C2H4have been widely studied,the selectivity remains at an unsatisfactory level due to the similar interactions donated by the close molecular property of C2H6and C2H4.Carbon molecular sieve(CMS) membranes,a class of applicable porous carbonaceous materials with rigid ultramicropores,are generally fabricated through pyrolysis of polymer precursors,which enable efficient separation of similarly-sized gas pairs (such as C2H4and C2H6)[147,148].The selection of suitable polymer precursors and pyrolysis conditions are the key factors for the preparation of CMS membranes [149-151].Liuet al.[152] prepared a series of CMSs by pyrolyzing cation exchange resins based on sulfonated poly(styrene-co-divinylbenzene).By controlling the pyrolysis temperature,a series of CMS adsorbents with effective pore size in the range of 0.35-0.46 nm were prepared.The breakthrough performance of the CMS adsorbents was validated in a packed bed configuration using binary gas mixtures,exhibiting high separation factors of 43 for C3H6/C3H8and 10 for C2H4/C2H6.Similarly,the CMS dense films were fabricated from commercial polyimide Matrimid?for C2H4/C2H6separation[153].The optimized pyrolysis temperature of 948 K ensured both a high C2H4/C2H6selectivity of 12 and moderate C2H4permeability of about 15 Barrer.Salinaset al.[154] prepared PIM-1-derived CMS membranes (PIM:polymer of intrinsic microporosity) and obtained an C2H4/C2H6ideal selectivity of 13,but with a low C2H4permeability of 1.3 Barrer(1 Barrer=7.52×10-18m3·(STP)·m-2·m·s-1·Pa-1.CMS membranes derived from a polyimide with intrinsic microporosity(PIM-6FDAOH) were fabricated by the same group by pyrolysis at 1073 K,which had an ideal C2H4/C2H6selectivity of 17.5 [155].The high selectivity was mainly attributed to the reduced pore size and narrow pore size distribution from pore collapse at high pyrolysis temperature.In addition,the rigid slit-like ultramicropores limit the rotational freedom of the C2H6molecule,allowing C2H4to be preferentially transported in a small size and planar configuration.Therefore,the CMS membranes have higher C2H4/C2H6selectivity than that of other materials [156].Moreover,the graphene oxide(GO) membranes with dual transport mechanisms of molecular sieving and carrier-facilitated transport for C2H4/C2H6separation were explored [157].In detail,Ag+was introduced as facilitated transport carriers and the protic ionic liquids (3-propanolammonium nitrate and 1-ethylimidazolium nitrate)were impregnated into the galleries of the GO laminates to enhance the separation performance.Thus,the modified GO membranes exhibited super-high C2H4/C2H6selectivity of 215 with C2H4permeance of 72.5 gas permeation unit(GPU),surpassing most of the state-ofthe-art membrane towards C2H4/C2H6separation.For detailed comparison,the structure properties of adsorbents and separation performances mentioned above were listed in the Table 5.

Table 5Summary of selected carbonaceous adsorbents for C2H4/C2H6 separation

2.2.2.C3H6/C3H8 separation

C3H6,as the crucial chemical raw material for production of polypropylene,is produced mainly from steam cracking of naphtha and catalytic cracking of gas oils.In both cases,C3H8coexists with C3H6(50%-80% (vol)) as binary mixtures,thus it is necessary to remove the C3H8from mixtures to yield polymer grade C3H6(99.5%)[158].For the separation of olefins/alkanes,one of the popular methods is to design the porous adsorbents with ultramicropore structure.In previous study [159],the influence of the pore structure on the behavior of the C3H6and C3H8equilibrium and kinetic adsorption was discussed based on two types of commercial activated carbons,Westvaco BAX 1100 and Chemviron LAC 30-57 (named as BAX-AC and LAC-AC).The LAC-AC showed higher C3H6equilibrium adsorption capacity than that of BAX-AC(5.48 mol·g-1vs.4.9 mol·g-1) due to its high proportion of micropores in the pore volume,while the BAX-AC displayed faster gas diffusion rates (D/r2) for both C3H6and C3H8(C3H65.67 × 10-4s-1,C3H84.56 × 10-4s-1) than those of LAC-AC (C3H63.38×10-4s-1,C3H83.32×10-4s-1)at 303 K,because of the larger fraction of the mesopore volume of BAX-AC.After that,the influence of carbonization temperature on the preparation of CMS had also been studied [160].The results showed that the CMS adsorbents prepared at a carbonization temperature of 973 K were most suitable for separating C3H6/C3H8,and their selectivity of C3H6/C3H8reached 12.

Fig.14.(a) Schematic diagram of 3D graphene-like FAU-,EMT-,and beta-zeolite-templated carbons.(b) Adsorption isotherms of C2H6 (circle symbols) and C2H4 (triangle symbols)by adsorbents at 303 K.(c)IAST-predicted selectivity for an equimolar C2H6/C2H4 mixtures at 303 K.(d)C2H6/C2H4 separation performance on Beta-ZTC:C2H6/C2H4(1:1) mixtures breakthrough curves and (e) C2H6/C2H4 (1:9) mixtures breakthrough curves at 100 kPa and 283 K [146].

Further,the precursors of CMS have been investigated[161-165].One of the widespread precursors is polyvinylidene chloride copolymer (PVDC),for example,a series of carbon molecular sieve fibers (CMSF) [166] were prepared by a twostep pyrolysis of melt extruded PVDC fibers,in which the PVDC fibers were first pretreated in air at low temperature to dehydrochlorinate and crosslink,and then pyrolyzed in N2at high temperatures from 773 to 1673 K.The obtained CMSF adsorbents exhibited a C3H6/C3H8separation selectivity up to 29 and large C3H6working capacity of 54 mg·g-1,which is 86% higher than commercial MSC-4 K CMS.In order to systematically study the influence of precursor composition and pyrolysis conditions on the properties of PVDC derived carbon materials,Liuet al.[167] prepared a set of CMSs.The single-component adsorption isotherms of probe gases with kinetic diameters ranging from 0.33 to 0.62 nm was measured by a high throughput setup to screen these CMSs.Gas sorption results indicated that the effective pore sizes of CMSs are dependent on pyrolysis temperature and precursor crystallinity.The sample prepared at pyrolysis temperature of 1773 K had an effective pore size of 0.42 nm,and showed 8.5 times higher C3H6/C3H8separation factor,1.5 times C3H6capacity and 2.3 times working capacity than thatof commercial MSC-4 K,further,exhibited 16.7 times higher working capacity than that of zeolite 4A.

Besides the PVDC,the cellulose,resin or other polymers including polyimide,polysulfone,polyethersulfone,poly(2,6-dimethyl-1,4-phenyleneoxide) and their composites can also be used as the precursors to fabricate the CMSs [168-170].Recently,Andradeet al.[171] reported a cellulose-based CMS (GLE800) with the micropore size distribution from 0.5 nm to 1.0 nm.The GLE800 was prepared from a single carbonization step at 1073 K and 120 min of soaking time under N2atmosphere.The adsorption capacities of C3H6and C3H8were 2.49 and 0.017 mmol·g-1at 298 K and 100 kPa,respectively.After that,they utilized phenolic resin as the precursor to prepare carbon adsorbent (MFFs) for C3H8/C3H6separation,which showed a high affinity for C3H8[172].The phenolic resin precursor was pre-treated with phosphoric acid,followed by carbonization at 1373 K and C3H6posttreatment,i.e.,the carbon adsorbents were stored in 200 kPa of C3H6for 1 to 12 days.SAXS analysis indicated rod-shaped pores for the optimal adsorbent sample with a bimodal size distribution with averages of 0.4 nm and 3.7 nm,and HRTEM images showed a network of earthworm micropores.The optimal adsorbent displayed an equilibrium selectivity of 2 at 298 K and 100 kPa.Liuet al.[173] reported a CMS,which was synthesized by pyrolysis from a gel-type strong acid cation exchanged resin (Dowex-X8),and it showed a C3H6/C3H8separation factor of 27 at 363 K and 600 kPa.The C3H6and C3H8diffusivities in the CMS were 1.0 × 10-9and 1.1 × 10-11cm2·s-1,respectively,at 363 K and 100 kPa.The high C3H6/C3H8diffusivity ratio of 90 is similar to that in zeolite 4A,while the C3H6diffusivity is more than 30 times higher than that in zeolite 4A.As an alternative,CMS membrane is an effective material for high efficiency hydrocarbons separation.Generally,the carbon membranes can be produced as hollow fiber,flat sheet,tubular,and supported or non-supported membranes.The precursor types and fabrication process are similar to that of CMS solid adsorbents.As early as 1987,Soffer and Koresh [174]have studied how high temperature treatments influence the development of pore structure of CMS membrane.They found that high temperature (up to 973 K) evacuation and mild temperature(up to 723 K) air oxidation treatments can lead to the opening of the pore structure,while further increasing the treatment temperature under vacuum or inert atmosphere will result in a gradual pore closure due to sintering.After that,researchers also devoted to study the effect of thickness and pore size of CMS membrane for gas separation [175-178].For example,Koros group investigated the effect of aging process to pore size of CMS membrane,and found that the pore size and degree of defective plate could be tunable in the temperature of 523-573 K under air for 20-45 min(they called this process super-hyperaging),thus achieving the controllable C3H6/C3H8separation performance [179].When the super-hyperaging process was carried out at 553 K for 30 min,the obtained CMS membrane showed C3H6permeance of 30.5 GPU with C3H6/C3H8selectivity of 12.8.The pore size obtained through the super-hyperaging is centered at 0.382-0.395 nm,which allowing the C3H6molecules to pass the membrane while restricting slightly larger C3H8molecules penetrant.Although CMS membranes have shown potential separation performance for olefins/paraffins,the fragility of CMS membranes still need to be overcome in practical application.

Very recently,the starch-based carbons materials (SC-M;M=Na,K,Rb)were prepared by the combination of hydrothermal carbonization andin situionic activation methods for separation of alkene/alkane(Fig.15(a),(b))[180].The SC-M had uniform ultramicropore and the pore structures could be tuned by modulating the alkali metal ion employed during activation(Fig.15(c),(d)).Among the obtained SC-M materials,SC-K showed C3H6and C4H6uptakes as high as 2.20 and 2.36 mmol·g-1,respectively,at 298 K and 100 kPa,and breakthrough experiments showed that the adsorption capacity ratio of C3H6/C3H8was 5.4 (Fig.15(e),(f)).

Ultramicropore is the key for the design of efficient adsorbents which can distinguish the two molecules having similar properties,and mesopore can provide the main transport channels for increasing gas adsorption and diffusion kinetics.Experimental and computational results indicated the gas adsorption and separation performance not only depends on the pore size,but is relevant to pore geometries in terms of pore shapes,tortuosity as well as connectivity [181,182].Thus,our group have explored the effect of pore geometry on gas separation property [183].We prepared a ultramicroporous carbon monolith with unique wiggling mesopores (MC-wiggle) by perturbing the well-organized micelle template with the binary molecular regulator of boric acid and ammonia.The MC-wiggle showed a tri-modal pore size distribution centered at 1.5,4.2 and 6.6 nm,and the ultramicropore size was determined centering at 0.5 nm.The kinetic studies indicated the diffusion rate on MC-wiggle for C3H6and C3H8was reduced compared with MC(non-wiggling counterpart)due to the wiggling transport behavior.For example,the equilibrium time for C3H6molecule on the MC and MC-wiggle reached 2 min and 6 min,respectively.The calculated kinetic selectivity reached 20 for MCwiggle,which is 6.1 times higher than that of MC.Such high kinetic selectivity is mainly because the diffusion difference between C3H6and C3H8was clearly enlarged on MC-wiggle.The C3H8molecule has higher molecular mass and larger dimensions than those of C3H6,thus,the C3H8molecules bounced back more readily upon collision with rugged regions,resulting in lower diffusivity of C3H8.The performances of carbon adsorbents discussed in this part are listed in Table 6.

Table 6Summary of selected carbonaceous adsorbents for C3H6/C3H8 separation

Fig.15.(a)The synthesis process of the starch-based carbon adsorbents(SC-M).(b)SEM images of SC-M.(c,d)The pore-size distribution of starch-based carbon adsorbents(SC-M) obtained by (c) CO2 adsorption at 273 K and (d) N2 adsorption at 77 K.(e) C3H6 and C3H8 adsorption isotherms on SC-K at 298 K within the pressure range of 0-100 kPa.(f) Breakthrough curves of SC-K at 298 K and 100 kPa with a constant flow rate of 1 ml·min-1 for C3H6/C3H8 mixture (50/50,volume ratio) [180].

3.Conclusions and Outlook

In this review,we have summarized the recent advances in utilizing the carbonaceous adsorbents for adsorptive separation of light hydrocarbons,and discussed the designed synthesis of porous carbons,applicability of separation mechanisms and structureperformance relationships.In a word,the separation mechanisms of light hydrocarbons based on carbonaceous adsorbents are predominantly concentrated on thermodynamic equilibrium separation and kinetics separation,while less for molecular sieving effect.The main reason is attributed to the difficulties in fine-tuning the pore aperture size in 0.02 to 0.1 nm scale increments in an ultramicroporous adsorbent with the suitable pore size ranging from 0.38 to 0.52 nm (the size scope of CH4to C3H8),especially for carbonaceous materials.This is the persistent challenge in the design of porous carbons.Although intensive success has been made,some issues in carbonaceous materials as the adsorbents for the separation of light hydrocarbons are still needed to be considered and solved:(1) Moisture sensibility should be considered.The introduction of doped atoms (N,O) can boost the interactions between adsorbates and adsorbents,however,the hydrophilia of surface is also enhanced due to the rich amounts of N and O elements,thus the weak moisture resistance.(2) The heat management and regeneration remain a concern for practical operation.The high light hydrocarbon adsorption capacity obtainedviaπcomplexation is concomitant with a high adsorption enthalpy,resulting in high energy consumption.(3)Gas adsorption and desorption kinetics need to be improved.The rapid adsorption and diffusion rate can boost the efficiency of separation process.Considering this viewpoint,the design of lamellar morphology and hierarchical pore structure is the straightforward approach.(4) Various carbon materials can be integrated for high purity products.In fact,it is difficult to separate multiple components mixtures by using single porous carbon,in this case,for different gas molecules,several porous carbons with diverse properties can be loaded in one fixed bed,thus to effectively separate complex mixtures.(5)The correlation between separation performance and pore geometry of porous carbon should be considered in the construction of carbonaceous adsorbent models.We believe that driven by the growing demand of light hydrocarbons separation,carbonaceous adsorbents will face a rapid and boosting development in the coming years.

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

The research was financially supported by the National Natural Science Foundation for Distinguished Young Scholars (21225312)and the Cheung Kong Scholars Program of China (T2015036),and the Fundamental Research Funds for the Central Universities(DUT20GJ215).