Covalent organic frameworks-incorporated thin film composite membranes prepared by interfacial polymerization for efficient CO2 separation

Haoqing Xu,Wenyan Feng,Menglong Sheng,Ye Yuan,Bo Wang,Jixiao Wang,Zhi Wang,*

1 Chemical Engineering Research Center,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300350,China

2 Tianjin Key Laboratory of Membrane Science and Desalination Technology,State Key Laboratory of Chemical Engineering,Collaborative Innovation Center of Chemical Science and Engineering,Tianjin University,Tianjin 300350,China

3 Life and Health Research Institute,School of Chemistry and Chemical Engineering,Tianjin University of Technology,Tianjin 300384,China

Keywords:Covalent organic frameworks CO2/N2 separation In situ interfacial polymerization Compatibility Covalent bonds

ABSTRACT Thin film composite (TFC) membranes with nanofillers additives for CO2 separation show promising applications in energy and environment-related fields.However,the poor compatibility between nanofillers and polymers in TFC membranes is the main problem.In this work,covalent organic frameworks(COFs,TpPa-1)with rich-NH-groups were incorporated into polyamide(PA)segment via in situ interfacial polymerization to prepare defect-free TFC membranes for CO2/N2 separation.The formed covalent bonds between TpPa-1 and PA strengthen the interaction between nanofillers and polymers,thereby enhancing compatibility.Besides,the incorporated COFs disturb the rigid structure of the PA layer,and provide fast CO2 transfer channels.The incorporated COFs also increase the content of effective carriers,which enhances the CO2 facilitated transport.Consequently,in CO2/N2 mixed gas separation test,the optimal TFC (TpPa0.025-PIP-TMC/mPSf) membrane exhibits high CO2 permeance of 854 GPU and high CO2/N2 selectivity of 148 at 0.15 MPa,CO2 permeance of 456 GPU (gas permeation unit) and CO2/N2 selectivity of 92 at 0.5 MPa.In addition,the TpPa0.025-PIP-TMC/mPSf membrane also achieves high permselectivty in CO2/CH4 mixed gas separation test.Finally,the optimal TFC membrane showes good stability in the simulated flue gas test,revealing the application potential for CO2 capture from flue gas.

1.Introduction

Excessive carbon dioxide (CO2) emissions have caused global climate change and frequent natural disasters,resulting in severe damage to the Earth’s ecosystem [1].Simultaneously,CO2separation plays an essential role in energy gas purification,such as CO2removal from flue gas (mainly N2and CO2) [2] and natural gas (mainly CH4and CO2) [3].Compared to other CO2separation technologies,membrane separation technology exhibits great application potential due to the advantages of low energy consumption,environmental friendliness,small area requirements,and flexible operating conditions [4,5].

Membrane performance,including permeance and selectivity,significantly affects energy consumption in the membrane separation process.Taking CO2capture from flue gas as an example,under the target of capturing 90% CO2with 95% purity,the membrane performance needs to achieve approximately 500 GPU (gas permeation unit)with CO2/N2selectivity of 70 at 0.5 MPa to make the membrane separation process economically competitive with the chemical absorption process [6,7].Considering the fact that the permeability of most membrane materials ranges from 100 to 10,000 Barrer (1 Barrer=10-10cm3(STP).cm.cm-2.s-1.cmHg-1)[8],it seems hard to achieve the above competitive membrane performance with the membrane thickness of tens to hundreds of micrometers.On the contrary,defect-free thin membranes with a thickness under 1 μm exhibit great potential in practical application [6].

Interfacial polymerization(IP)is one of the commonly reported methods to prepare thin film composite (TFC) membranes [9].Because of its self-inhibition property [10],the TFC membranes prepared by IP process own thin but crosslinked membrane structures [11],resulting in their high permeance and selectivity.Some researchers have reported improved membrane performance after incorporating functional groups in aqueous phase monomers[12–14].The functional groups with strong CO2affinity,such as amino groups,could reversibly react with CO2,and thereby serving as the carriers to promote the CO2transport across the membranes[15,16].However,with pressure increasing,a sharply decreased performance is also observed in such membranes due to the‘‘carrier saturation”phenomenon [12,13].Subsequent attempts have been made to improve the weak performance under higher pressure [17–19].One method is to enhance the CO2facilitated transport,which involves increasing the carrier content [14,17],the stoichiometric CO2loading of carriers [19],the reaction rate constant between CO2and carriers [19].These measures are conducive to relieve the carrier saturation phenomenon,thereby improving the membrane performance under high pressure.Another method is to introduce multi-permselective mechanisms into the membrane to optimize membrane structure and improve membrane performance [20].Our group recently constructed unimpeded CO2transport channels in a thin polyamide layer embedded with ZIF-8 nanoparticles via Swelling-controlled Nanofiller Positioning [21],which could be used to optimize the membrane structure for CO2separation conveniently.

With the development of advanced materials,nanofillers,such as porous organic cage (POC) [22,23],metal organic frameworks(MOFs) [24–27],and covalent organic frameworks (COFs) [28–31],have attracted increasing attention in the membrane separation technologies.Due to the inherent advantages of high porosity,large surface area,and adjustable pore size,nanofillers could serve as the additive of the TFC membranes to significantly improve the membrane performance.Numerous works of such TFC membranes have been reported in liquid separation [31–36].Hoek et al.[35]first reported the ultra-thin PA membranes containing NaA zeolite nanoparticles for reverse osmosis,which exhibited improved flux and selectivity than pure PA membranes.Livingston et al.[36]used several MOFs as additives to prepare TFC membranes for organic solvent nanofiltration (OSN).Wang et al.[34] confirmed that the improved membrane hydrophilicity due to the incorporated SNW-1 accounted for improved water flux.

TFC membranes with nanofillers additives also exhibit great potential in gas separation [37],but there are relatively few reported in the literature,mainly due to the following two reasons.On the one hand,the poor compatibility between nanofillers and polymers can often emerge with undesirable interfacial morphology,thus resulting in performance degradation [38–42].Many efforts have been taken to enhance interfacial compatibility,like constructing hydrogen bonds[17,41,43].On the other hand,once the nanofiller loadings increase to a certain value,the dramatic decrease in gas selectivity can be observed in previous research[40,41,44,45],indicating the formation of non-selective defects.The strategy of improving compatibility through hydrogen bonds and avoiding defects formation is unreliable under high pressure or high nanofiller loadings.By contrast,building covalent bonds,which are much stronger than hydrogen bonds and can improve compatibility between nanofillers and polymers,is a better strategy to incorporate nanofillers into TFC membranes [38,46,47].Wang et al.[38] used poly (ethylene glycol) diglycidyl ether(PEGDE) as a crosslinker and successfully connected UiO-66-NH2with poly (vinylamine) (PVAm) via covalent bonds.

As an emerging type of nanoporous materials,COFs are synthesized only by assembling organic building blocks connected by strong covalent bonds,and possess properties such as permanent porosity,relatively low density,and desirable thermal stability[34,48,49].The whole organic-organic covalent bond structure endows the COF with better affinity to organic polymers,showing a huge advantage over classical inorganic particles in the fabrication of composite membranes [50].Pores from COFs can provide pathways for gas molecules to pass through.Furthermore,the synthesized COFs have great potential to prepare highly stable membranes due to their high thermal and water stability.Therefore,the preparation of COFs-based TFC membranes is expected to be an effective method for gas separation.

In this work,we incorporated COFs into TFC membranes by in situ IP to form covalent bonds between COFs and polyamide(PA)in the TFC membranes,aiming to obtain TFC membranes with improved compatibility and enhanced performance.Herein,piperazine (PIP) was chosen as aqueous monomers,and trimesoylchloride (TMC) was chosen as organic monomers to prepare pure TFC membranes.TpPa-1,chemical-stable COF with rich secondary amine groups (-NH-),was chosen as the additive and dispersed in aqueous phase to prepare TFC membranes with nanofillers.The -NH-groups of TpPa-1 can react with the -COCl groups of TMC,thus forming the covalent bonds and improving the compatibility.The incorporated TpPa-1 is supposed to provide fast gas transfer channel and increase effective carriers in the membrane to enhance the CO2facilitated transport[18].In addition,the influence of the incorporation of TpPa-1,the loading of TpPa-1 and operation pressure on membrane separation performance were investigated.Finally,the optimal membrane was tested by the simulated flux gas to investigate the operational stability.

2.Experimental

2.1.Materials

1,3,5-Triformylphloroglucinol (Tp) (99% purity,J&K Chemical,China),1,4-diaminobenzene(Pa)(99%purity,J&K Chemical,China),ethanol (AR,Yuanli,China),and dimethylformamide (DMF) (AR,Yuanli,China) were purchased for the synthesis of TpPa-1.The polysulfone (PSf) ultrafiltration substrate with an average molecular weight cut-off of 45,000 Da was supplied by Jiuzhang Membrane Technology Co.Ltd.(China).Ditin butyl dilaurate(DBD) (95% purity,Aladdin,China),tetraethoxysilane (TEOS)(99.99% purity,Aladdin,China),n-heptane (AR,Yuanli,China),and Polydimethylsiloxane (PDMS) (Shin-Etsu Chemical,Japan)were obtained to modify the PSf substrate.Piperazine (PIP) (99%purity,Aladdin,China) and TpPa-1 as aqueous phase monomers,Na2CO3(AR;Aladdin,China)as acid absorbent,and trimesoylchloride(TMC)(99.5%;Aladdin,China)as organic phase monomer participated in the IP process.All the above chemicals were directly used without further purification.

2.2.Synthesis of TpPa-1

The TpPa-1 was synthesized by a facile procedure reported by Yang et al.[51].As shown in Fig.1,Tp (63 mg,0.30 mmol) and Pa(49 mg,0.45 mmol)were separately dissolved in 20 ml ethanol.After 3 min ultrasonication,the ethanol solutions were mixed in a 100 ml flask with 1 h continuous stirring at room temperature.The obtain TpPa-1 was collected by centrifugation and washed with DMF three times to remove the residual Tp and Pa.Finally,the purified TpPa-1 was reflux with ethanol for 1 h to exchange the DMF and dried in a vacuum oven at 313 K.

Fig.1.Synthesis route and chemical structure of TpPa-1.

2.3.Preparation of TFC membranes

2.3.1.Preparation of the mPSf substrate

The mPSf substrate,modified by crosslinked PDMS,can effectively prevent pore penetration of PA layer and maintain high CO2permeance [38,52].The crosslinked PDMS was obtained by dissolving PDMS (0.5 g),TEOS (1 g),DBD (1 g) in n-heptane(97.5 g) and stirring for 30 min.Then,the PDMS solution was coated on the wet PSf substrate with a distance of 70 μm between the substrate and the coating knife.Finally,the coated substrate was maintained in the artificial climate chamber at 303 K and 40% (RH) for 12 h.

2.3.2.Preparation of TFC membranes

The TFC membranes with TpPa-1 (denoted as TpPax-PIP-TMC/mPSf membrane,where x refers to the mass ratio of TpPa and PIP) or without TpPa-1 (denoted as PIP-TMC/mPSf membrane)were prepared by IP process.The interfacial polymerization process of the TpPax-PIP-TMC is shown in Fig.2.It is possible that part of TpPa-1 is physisorbed in the PA network.The organic solution was prepared by dissolving TMC (0.1 g) in approximately 200 ml n-heptane.The aqueous solution was prepared by dissolving PIP(0.2 g) and Na2CO3(0.12 g) in approximately 400 ml deionized water.Then,a certain amount of TpPa-1 was added into the aqueous solution and sonicated for 1 h at room temperature.During the IP process,mPSf substrate was firstly immersed into 50 ml organic solution for 5 min.After removing the excess organic solution from the surface,the mPSf substrate was dried at 298 K until no visible solution remaining.Subsequently,the TMC-impregnated mPSf substrate was immersed into 100 ml aqueous solution for 3 min to form the thin selective layer.After that,the obtained TFC membranes were washed with deionized water three times to remove the byproducts and unreacted monomers.Finally,the washed TFC membranes were maintained in the artificial climate chamber at 303 K and 40% relative humidity for 12 h.

2.4.Characterization

2.4.1.Characterization of TpPa-1

The crystal structure of the prepared TpPa-1 was detected by the X-ray diffractometer (D/MAX-2500,Japan) in the 2θ range between 3° and 60°.The functional groups and chemical structure of the prepared TpPa-1 were confirmed by the FTS-6000 spectrometer (Bio-Rad,America).The morphologies of the prepared TpPa-1 were characterized by scanning electron microscopy (SEM,Nova NanoSEM 430,FEI,America)and transmission electron microscopy(TEM,JEOL JEM-2100F,Japan),respectively.The pore structural parameters of the prepared TpPa-1 were investigated by specific surface areas and pore size analyses (3H-2000PM2,Beishide,China).X-ray photoelectron spectroscopy (XPS,PHI-204 1600)and FTS-6000 spectrometer were used to confirm the reaction between TpPa-1 and TMC.

2.4.2.Characterization of the TFC membranes

The effects of TpPa-1 loadings on morphology and membrane thickness were investigated by the SEM.Meanwhile,the structures of TFC membranes were characterized by the FTS-6000 spectrometer.

2.5.Gas permeation experiments

The gas separation performance of the prepared TFC membranes was tested by the procedure reported in Ref.[14].Feed gas was first humidified to saturation by water vapor,and the whole test temperature was kept at 298 K.Pure He gas was chosen as the sweep gas for CO2/N2and CO2/CH4mixed gas test.After the feed gas was separated by the TFC membranes,the components of permeate gas were analyzed in the gas chromatograph (Agilent Technologies 7890B) with the flux of permeate gas detected by the soap-film flowmeter.In order to reduce the error,three membranes samples prepared under the same condition were tested,and the error bars in the figure represented the standard errors of the three samples.The relationship between permeance and permeability is as follow:

where P is permeability in Barrer (1 Barrer=10-10cm3.(STP)cm.cm-2s-1.cm.Hg-1),R is permeance in GPU (1 GPU=10-6cm3-.(STP).cm-2.s-1.cm.Hg-1),and l is the membrane thickness in μm.

Fig.2.The interfacial polymerization process of the TpPax-PIP-TMC.

3.Results and Discussion

3.1.Characterization of TpPa-1

The chemical structure of the as-synthesized TpPa-1 was investigated by FT-IR.As depicted in Fig.3,the carbonyl stretching bands(1636 cm-1)of Tp disappear,and the strong N-H stretching bands(3500～3300 cm-1)of Pa become weak,indicating the reaction occurrence.The new absorption bands at 1574 cm-1and 1252 cm-1correspond to C-C and C-N bonds,respectively,which are consistent with the previous report and confirm the successful formation of TpPa-1 [30].

X-Ray diffraction(XRD)experiments were performed to characterize the crystal structure of as-synthesized TpPa-1.The assynthesized TpPa-1 exhibits moderate crystallinity with 2θ peaks at 4.84°,8.80°and 27.24°(shown in Fig.4(a)),which are consistent with the literature and indicate the successful synthesis of TpPa-1[30].Compared to the work reported by Rahul Banerjee et al.[30]and Jin et al.[53],the peak at 4.84° is weaker,and this phenomenon can be caused by the synthesis condition .In this work,TpPa-1 was synthesized at room temperature,but in the literature TpPa-1 was synthesized at high temperature and high pressure.The weaker peak indicates the decreased crystallinity and can be conducive for TpPa-1 to disperse in the solution [54,55].The pore diameters of the as-synthesized TpPa-1 were analyzed at 77 K in N2atmosphere.As represented in Fig.4(b),the N2adsorptiondesorption isotherms exhibit a typical Type-ⅠⅠ behavior.The Brunauer-Emmett-Teller (BET) surface area of the as-synthesized TpPa-1 calculated by the BET method is about 171.27 m2.g-1.In addition,the pore size of the as-synthesized TpPa-1 calculated by the H-K method is mainly centered at 1.58 nm,as shown in Fig.4(c).At 1 bar (100 kPa),the CO2adsorption capacity of the as-synthesized TpPa-1 can reach 51.27 cm3.g-1at 273.15 K,as depicted in Fig.4(d),while the N2uptake can only reach 0.25 cm3-.g-1,indicating the priority adsorption for CO2of the assynthesized TpPa-1.As presented in Fig.S1 (in Supplementary Material),the characterization results of SEM and TEM show the as-synthesized TpPa-1 lamellar structures with an average particle size of 50 nm.Various characterization results of TpPa-1 indicate that the incorporation of as-synthesized TpPa-1 can provide gas transport channels in the membrane.

Fig.3.FTIR spectra of (a) Tp,(b) Pa and (c) TpPa-1.

3.2.The reaction between TpPa-1 and TMC

To confirm the reaction between TpPa-1 and TMC,a certain amount of TpPa-1 was added into the n-heptane solution of TMC(0.5 g.L-1).After stirring for 10 min,the reaction products were washed with n-heptane three times and then dried in the oven at 333 K.The purified reaction products were characterized by FTIR and XPS,respectively.Fig.5(a) shows the occurrence of the N-C-O stretching absorption band at 1627 cm-1,which indicates the nucleophilic reaction between TpPa-1 and TMC [34,47].As shown in Fig.5(b),the XPS spectra of N1s for the reaction products could be divided into peaks at the binding energy of 400.6 and 401.8 eV,corresponding to the N-H and N-C-O,respectively[34].The characterization results of both FTIR and XPS indicate the reaction between TpPa-1 and TMC,which implies the TpPa-1 could also react with TMC in the IP process to form amide bonds.

3.3.Structure of TFC membranes

As represented in Fig.6,the typical corrugated structures appear on the PIP-TMC/mPSf membrane surface,which results from the unstable primary film formed in the IP process[56,57].For TpPa0.0125-PIP-TMC/mPSf membranes,almost all TpPa-1 nanofillers are covered by the PA layer.As the nanofiller loadings increases,more nanofillers appear on the membrane surface.Especially when the nanofiller loadings increase to 5%,obvious aggregation of TpPa-1 fillers is observed in the TpPa0.05-PIP-TMC/mPSf membranes.The cross-section morphologies reveal the thicknesses of the membranes with different TpPa-1 loadings.The thickness (containing selective layer and PDMS layer) of PIP-TMC/mPSf,TpPa0.0125-PIP-TMC/mPSf,TpPa0.025-PIPTMC/mPSf,and TpPa0.05-PIP-TMC/mPSf films are about 142 nm,145 nm,151 nm,and 156 nm,respectively,indicating the TpPa-1 loadings have ignorable effects on the membrane thickness.As represented in Fig.S2,the thickness of PDMS layer is about 50 nm.

Fig.4.Characterization results of TpPa-1:(a) XRD patterns,(b) N2 adsorption (square symbols) and desorption isotherms (circle symbols),test temperature:77 K,(c) pore size distribution,test temperature:77 K,(d) CO2 and N2 single gas adsorption capacity,test temperature:273.15 K.

Fig.5.Products of TpPa-1 contacting with TMC:(a) FTIR spectra and (b) XPS spectra.

3.4.Gas separation performance

3.4.1.Effects of the incorporated COFs

Fig.6.SEM images of PIP-TMC/mPSf and TpPax-PIP-TMC/mPSf membranes:(a,e) PIP-TMC/mPSf membrane,(b,f) TpPa0.0125-PIP-TMC/mPSf membrane,(c,g) TpPa0.025-PIPTMC/mPSf membrane,(d,h) TpPa0.05-PIP-TMC/PSf membrane.

All prepared TFC membranes were tested by CO2/N2mixed gas(15/85 by volume) to investigate the effects of the incorporated COFs on membrane performance,and the results were presented in Figs.7 and 8.Compared to PIP-TMC/mPSf membranes,the permselectivity of TpPax-PIP-TMC/mPSf membranes exhibits a significant improvement under low pressure.At 0.15 MPa,the CO2permeance of TpPa0.025-PIP-TMC/mPSf membrane is 854 GPU,which is 2.16 times the CO2permeance of PIP-TMC/mPSf membranes (395 GPU).Meanwhile,CO2/N2selectivity TpPa0.025-PIPTMC/mPSf and PIP-TMC/mPSf membranes are 148 and 117,respectively.In addition,the CO2permeance of all TFC membranes decrease sharply with the increased pressure,exhibiting a typical facilitated transport behavior [38].The tertiary amine groups in both PIP and TpPa-1 can react reversibly with CO2like the Eq.(2),thus serving as carriers in the TFC membranes.When the pressure increased,the carriers are approached to be saturated,which results in the permeance decrease[18,41].Notably,the CO2permeance of TpPax-PIP-TMC/mPSf membranes decrease faster with the increasing pressure,which is consistent with the higher content of carriers in TpPax-PIP-TMC/mPSf membranes [18].

The incorporation of TpPa-1 can account for the improved membrane performance.First,the incorporated TpPa-1 can still provide efficient channels and increase the free volume for gas molecular transport.Consequently,the TpPax-PIP-TMC/mPSf membranes exhibit improved CO2and N2permeance.Second,the tertiary amine groups in the incorporated TpPa-1 increase the content of the carriers in the membranes,which enhances the CO2facilitated transport.Thus,the CO2can be preferentially absorbed into the channels (shown in Fig.4(d)),and the CO2/N2selectivity under low pressure improves.Finally,the incorporated TpPa-1 can form covalent bonds with polymers,which is conducive to optimizing the membrane structure and enhancing compatibility and plasticization resistance [38,47].

As presented in Fig.7(b) and Fig.S3,the N2permeance of PIPTMC/mPSf membranes tested by CO2/N2mixed gas increases at 1.5 MPa,while that tested by pure N2remains basically unchanged,indicating the occurrence of CO2-induced plasticization[18].However,such a phenomenon isn’t observed in TpPa-PIP-TMC/mPSf membranes,which proves that incorporating TpPa-1 can improve the compatibility and maintain high separation performance under high pressure.Consequently,the TpPa0.025-PIP-TMC/mPSf membrane achieves CO2permeance of 304 GPU and CO2/N2selectivity of 72.2 at 1.5 MPa.

Fig.7.CO2/N2 mixed gas separation performance of PIP-TMC/mPSf and TpPax-PIP-TMC/mPSf membranes:(a) CO2 permeance,(b) N2 permeance (15/85 by volume,filled symbols),test temperature:298 K.

Fig.8.CO2/N2 selectivity of PIP-TMC/mPSf and TpPax-PIP-TMC/mPSf membranes(15/85 by volume,filled symbols),test temperature:298 K.

3.4.2.Effects of the incorporated COFs loadings

As depicted in Fig.7,at 0.15 MPa,CO2and N2permeance of all TpPax-PIP-TMC/mPSf membranes increase with the increasing TpPa-1 loadings.As shown in Fig.8,the selectivity of CO2/N2increases significantly after adding TpPa-1.However,as the TpPa-1/PIP ratio increases,the CO2/N2selectivity show a downward trend.As the COF loading increases,the incorporated TpPa-1 in the TFC membranes can provide more channels for gas transport,which significantly improves the CO2permeance and N2permeance.Second,the increasing tertiary amine groups of the TpPa-1 further enhance the CO2facilitated transport.Consequently,the TpPa0.05-PIP-TMC/mPSf membranes achieve maximum CO2permeance of 1046 GPU at 0.15 MPa,while the TpPa0.0125-PIP-TMC/mPSf membranes achieve a maximum CO2/N2selectivity of 149 at 0.15 MPa.Merkel et al.[58] confirmed by computer simulation improving membrane permeance is more important than increasing selectivity to further reduce the cost of CO2capture from flue gas.Further reducing of the TpPa-1 loading may result in a slight increase in selectivity but a marked decrease in CO2permeance,which is not conducive to reducing carbon capture costs.

As the pressure increases,CO2permeance and the CO2/N2selectivity of TpPax-PIP-TMC/mPSf membranes decrease to different degrees.Notably,N2permeance of TpPa0.05-PIP-TMC/mPSf membranes in the mixed gas test and pure gas test increases at 0.5 MPa,indicating the formation of non-selective defects under relatively high pressure [38].The formation of non-selective defects can be attributed to the nanofillers agglomeration represented in Fig.6(d),which severely compromises the CO2/N2selectivity.

TFC membranes with higher TpPa-1 loadings exhibit a faster decreasing trend of membrane performance with increasing pressure between TpPa0.0125-PIP-TMC/mPSf and TpPa0.025-PIP-TMC/mPSf membrane,which can be attributed to two aspects.On the one hand,TpPa0.025-PIP-TMC/mPSf membrane contains more tertiary amine carriers,so as the pressure increases,the CO2permeance decreases faster.On the other hand,the oversized channels of TpPa-1 can also allow the transport of N2molecules,resulting in a much higher N2permeance in TpPa0.025-PIP-TMC/mPSf membrane than that in TpPa0.0125-PIP-TMC/mPSf membrane.Consequently,TpPa0.025-PIP-TMC/mPSf membrane exhibits a faster decreasing trend of CO2permeance and the CO2/N2selectivity with the increasing pressure.

3.4.3.Stability of the TpPa0.025-PIP-TMC/mPSf membrane

The impurity gases in the flue gas,such as SO2,NOx,O2,will affect the membrane performance in the process of CO2capture from flue gas [59,60].Hence,the CO2/N2mixed gas (15/85 by volume)and simulated flue gas(containing 14.5%CO2,6.5%O2,0.014%SO2,0.007% NOx,0.001% CO and balanced by N2,volume fraction)were used to test the separation performance and stability of the optimal TFC membrane (TpPa0.025-PIP-TMC/mPSf membrane).The test pressure and temperature were kept at 0.5 MPa and 298 K,respectively.As shown in Fig.9,the optimal membrane was first tested by the CO2/N2mixed gas for 12 h.The membrane performance maintains almost unchanged within the test period with CO2permeance of 456 GPU and CO2/N2selectivity of 92.Then feed gas was changed to simulated flue gas,and the optimal membrane was tested by simulated flue gas for 48 h.The CO2permeance decreases from 456 GPU to 401 GPU,exhibiting about 12.0%decrease,while the N2permeance maintains almost unchanged.This change can be attributed to the competitive adsorption between acid gas and CO2in the membrane.The acid gases like SO2and NOxcan also react with the tertiary amine groups of the PA layer and TpPa-1,thus occupying some carriers of the TFC membranes [61].By contrast,the transport of N2follows the solution-diffusion mechanism,and N2can not react with tertiary amine groups of the TFC membrane.Thereby,the competitive adsorption can not affect the transport of N2,and the N2permeance can maintain almost unchanged within the test period[62].Finally,when the feed gas was changed back to the CO2/N2mixed gas,the separation performance of the optimal membrane resumes the initial performance,indicating the simulated flue gas has no damage to the membrane structure.

Fig.9.Separation performance stability of the TpPa0.025-PIP-TMC/mPSf membrane.Feed gas:CO2/N2 (15/85 by volume) mixed gas and simulated flue gas (14.5% (vol)CO2,6.5 l% (vol) O2,0.014% SO2,0.007% NOx,0.001% CO and balanced by N2).Feed gas pressure:0.5 MPa,test temperature:298 K.

3.4.4.CO2/CH4mixed gas separation performance of the TpPa0.025-PIPTMC/mPSf membrane

In order to investigate the application potential of purifying natural gas,the TpPa0.025-PIP-TMC/mPSf membrane also tested the CO2separation performance for CO2/CH4(10/90 by volume)mixed gas,as represented in Fig.S4.The CO2permselectivity for CO2/CH4system changes similar to that of CO2/N2system as feed gas pressure increases.However,under the equal value of feed gas pressure,CO2permeance for CO2/CH4system has a lower CO2permeability than that of CO2/N2system,which can be attributed to the stronger competition between CO2transport and CH4transport together with the smaller CO2content of 10%(vol).These factors can also result in a lower selectivity of CO2/CH4system than that of CO2/N2system.Hence,the lower CO2permeance and selectivity for CO2/CH4system are obtained.At 0.15 MPa,the TpPa0.025-PIP-TMC/mPSf membrane achieves CO2permeance of 632 GPU and CO2/CH4selectivity of 49.In addition,the TpPa0.025-PIP-TMC/mPSf membrane achieves N2permeance of 6 GPU in CO2/N2system and CH4permeance of 13 GPU in CO2/CH4system,indicating that the TpPa0.025-PIP-TMC/mPSf membrane has CH4/N2separation selectivity of about 2.

3.4.5.Performance comparison with other TFC membranes reported in the literature

Fig.10 and Table 1 list the CO2/N2separation performance of our work and other reported TFC membranes in the literature.The separation performance of the TpPa0.025-PIP-TMC/mPSf membrane successfully surpasses the 2008 Robeson CO2/N2upper bound limit and the 2019 Robeson CO2/N2upper bound limit,exhibiting the potential application for CO2capture from flue gas.

Table 1 Comparison of gas separation performance of related membranes (CO2/N2)

Fig.10.Separation performance of TpPa0.025-PIP-TMC/mPSf membrane and other various membranes from the reported literature with CO2/N2 mixed gas,presented on Robeson 2008 upper-bound line (1 Barrer=10-10 cm3 (STP).cm.cm-2.s-1.cmHg-1).

4.Conclusions

Herein,we successfully fabricated TFC membranes for efficient CO2separation through incorporating TpPa-1 into PA layer via in situ interfacial polymerization.The incorporated TpPa-1 nanoparticles can form covalent bonds with PA segment,which enhance compatibility and improve plasticization resistance.The TpPa-PIP-TMC/mPSf membrane,in the TpPa-1/PIP ratio of 0.025,exhibits high CO2permeance of 854 GPU (2.2 times of the PIPTMS/mPSf membrane) and high CO2/N2selectivity of 148 at 0.15 MPa and good stability in the simulated flue gas,revealing the application potential for CO2capture from flue gas.In addition,the fabricated membranes also demonstrate good separation performance for CO2/CH4mixed gas.Moreover,this design concept and simple method about incorporating covalent bonds into TFC membranes to enhance the stability of membrane structure show great potential in developing other high-performance membranes for gas separation.

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 research is supported by the National Key Research &Development Program of China (2017YFB0603400),the National Natural Science Foundation of China (21938007).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.02.014.