The effect of protic ionic liquids incorporation on CO2 separation performance of Pebax-based membranes

Haiyan Jiang,Lu Bai,Bingbing Yang,Shaojuan Zeng,Haifeng Dong,3,Xiangping Zhang,3,*

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

2 School of Future Technology,University of Chinese Academy of Sciences,Beijing 100049,China

3 Advanced Energy Science and Technology Guangdong Laboratory,Huizhou 516003,China

Keywords:CO2 Protic ionic liquids Membranes Pebax

ABSTRACT The separation of carbon dioxide (CO2) is of great importance for environment protection and gas resource purification.The ionic liquids (ILs)-based gas separation membrane provides a new chance for efficient CO2 separation,while high permeability and selectivity of membranes is a great challenge.In this study,the influence of two protic ILs with different anion ([TMGH][Im] and [TMGH][PhO]) on the CO2 separation performance of the prepared ILs/Pebax blended membranes were systematically investigated at different temperature.The results showed the CO2 permeability exhibits the rising trend for ILs/Pebax blended membranes with the increment of IL content.Especially,the[TMGH][Im]with low viscosity and high CO2 absorption capacity leads to the blended membranes showing better CO2 permeability and ideal CO2 selectivity than that of membranes with [TMGH][PhO] at high IL content.Besides,with operating temperature increasing,the gas permeability of 20% (mass) [TMGH][Im]/Pebax blended membrane increases due to the decreasing viscosity of IL and the rising chain mobility of polymer.Inversely,the gas selectivity shows decreasing trend because CO2 absorption capacity obviously decreased at higher temperature.

1.Introduction

Excessive emission of CO2cause greenhouse effect,such as glaciers melting,sea level rising and so on,which affect the normal production and bring the irreversible consequences [1].Carbon capture,utilization and storage (CCUS) has been recognized the most effective measures to relieve this effect and take full advantage of carbon resource.Especially,the CO2separation and capture is an important part and many technologies have been developed,such as cryogenic distillation,absorption,adsorption and membrane separation [2–4].Among above mentioned methods,the membrane separation has attracted a lot of attentions because of lower energy consumption,environmental friendliness as well as easy scale up [5,6].The development of membrane materials is crucial for application.At present,the conventional membrane materials are polymers including poly(ether-b-amide) (Pebax),polyimides (PI),polysulfone (PSF) and so on,in which Pebax has become hot due to their availability,processability and excellent mechanical properties [7–9].Importantly,the polar ether oxygen group in the Pebax has affinity to CO2,which is beneficial for CO2/non-polar gas separation [10–12].However,the neat Pebax membranes like other polymeric membranes also suffer from the trade-off effect between permeability and selectivity [13].Therefore,modifying polymers to improve the gas separation performance is significant.

Ionic liquids(ILs)have been viewed as a new kind of green solvent due to their non-volatility,structural designability and high gas affinity[14–16].Due to these unique advantages,ILs were usually introduced into polymeric membranes to tune the microstructures,modify the gas affinity of polymer matrix,hence improving the gas separation performance[17–19].ILs instead of organic solvent in supported liquid membranes avoid the problems of easy volatilization of active components,which greatly improve the stability and gas separation performance of supported liquid membranes [20].The morphological structure changed from isolated ionic domains to the continuous phase in blended membranes after embedding ILs in polymer matrix[21].Besides,gas transport channels could be constructed in matrix mixed membranes by taking full advantage of high gas affinity and confinement effect of ILs[22,23].Furthermore,ILs also acted as mobile carrier to facilitate gas transport through membranes [24,25].Although many different ILs-based membranes have been developed,blending ILs with polymer is a simple and efficient method to modify the polymeric membranes and improve the gas separation performance.Li et al.[26] prepared the conventional imidazolium-based ILs blended membranes and [Bmim][NTf2]/Pebax 1657 exhibited higher CO2permeability due to the affinity between anion of IL and CO2molecule.Mahdavi et al.[27] fabricated IL gel membranes by blending[Bmim][PF6] with Pebax 1074.The CO2permeability increased from 58.6 to 148.3 barrer (1 barrer=10-10cm3(STP).cm.cm-2.s-1-.cmHg) and CO2/CH4selectivity decreased slightly with the increment of IL content.The rising pressure caused higher permeability for both CO2and CH4.Fam et al.[28] studied the effect of feed pressure and temperature on [Emim][BF4]/Pebax 1657 gel membrane.The results suggested that CO2flux was enhanced significantly with increasing pressure.Interestingly,the CO2flux of gel membrane with high IL content declined with rising temperature,which was different from neat Pebax membrane and gel membrane with low content.Dai et al.[29] studied the triethylene tetramine (TETA)-based task specific ILs/Pebax 2533 blended thin film for CO2separation at different conditions,and the results suggested that CO2permeability and CO2/N2selectivity obviously improved under wet condition and low pressure of feed gas due to the facilitated transport mechanism.It is worth noting that the investigation of ILs especially functionalized ILs and the operating conditions affecting membrane separation performance is significant.

In this work,two protic ILs/Pebax blended membranes including tetramethylgunidinium phenol[TMGH][PhO]and tetramethylgunidinium imidazole [TMGH][Im] were developed.These two protic ILs were selected due to their low viscosity and different CO2absorption capacity,further affecting the CO2separation performance of membranes.Then the effect of protic IL anion,IL content and the operating temperature on CO2separation performance of the blended membranes was systematacially investigated.The structures,morphologies,and thermal properties of the prepared protic ILs/Pebax blended membranes were studied by various characterizations.

2.Experimental

2.1.Materials

Pebax 2533 containing 80% (mass) of polyether (PE) and 20%(mass) polyamide (PA) was purchased from Arkema Inc.It is referred to as ‘‘Pebax”in the following section.The protic ILs[TMGH][PhO] and [TMGH][Im] were provided from Shanghai Chengjie Chemical Co.,Ltd.The structures of ILs and Pebax were shown in Fig.1.The ethanol was purchased from Macklin Co.,Ltd.The above chemicals were used without further purification.The high purity gases(>99.99%),containing N2,CH4and CO2,were offered by Beijing Beiwen Gas Factory.

Fig.1.The structure of [TMGH][PhO] (a) and [TMGH][Im] (b).

2.2.Membranes fabrication

A certain amount of Pebax pellets were dissolved in ethanol at 80°C under reflux for 6 h to obtain 8 wt%membrane solution.After that,a certain amount of ILs were added into the above solution.Then mixed solution was stirred at least 2 h under 40 °C and cast onto a glass plate at the ambient conditions for one day.Finally,all the membranes were dried for 24 h at 60 °C under vacuum.The ILs/Pebax blended membranes are named X% (mass) [TMGH][Im]/Pebax and X%(mass)[TMGH][PhO]/Pebax,where X represents the content of the ILs based on the mass of the Pebax in this study.The membrane thicknesses were measured with a digital thickness gauge (SD-201,Yuanhengtong Instrument Factory).The thickness of prepared membranes is about 100–110 μm.

2.3.Membranes characterizations

The Fourier transform infrared spectrum (FTIR) of all samples were collected using a Thermo Nicolet 380 spectrometer in the range of 400–4000 cm-1in attenuated total reflectance mode.Thermo-gravimetric analysis(TGA)of samples were obtained from room temperature to 600 °C with the rate of 10 °C.min-1at N2atmosphere.The X-ray diffraction (XRD) of membranes were recorded using a Smartlab (9 kW) at a rate of 15°C.min-1from 5 to 60° 2θvalues.The differential scanning calorimetry (DSC) of samples was performed at heat rate of 10 °C.min-1from -90 °C to 200 °C under N2atmosphere.The morphologies of membranes were examined using scanning electron microscopy at 5 kV(SU8020 SEM).Prior to scanning cross-sectional SEM images,the samples should be fractured in liquid nitrogen and coated with gold.The contact angle test between ILs and neat Pebax membrane was performed on the KRUSS DSA100 in a static drop mode.The water content of ILs were measured by Mettler Toledo Coulometric KF titrator C20.And water content of [TMGH][Im] and [TMGH][PhO] is 0.0875% and 0.0771%,respectively.

2.4.Gas separation measurement

The permeability of CO2,N2and CH4were tested in a gas permeameter (VAC-V2,Labthink).The gas permeability of the membranes was calculated as follows:

where P (barrer) is the permeability,dp/dt (Pa.s-1) represents the pressure increasing rate at stable state,V (cm3) means the downstream volume,A (cm2) stands for the effective membrane area,T(K)represents the operating temperature,L(cm)refers to the thickness of a membrane,T0and P0is standard temperature and pressure(273.15 K,101.325 kPa) and (p1-p2) is transmembrane pressure difference (cm Hg).

The ideal selectivity αA/Bcan be obtained according to Eq.(2):

The diffusion coefficient(D)can be calculated from Eq.(3),the θ denotes lag time.

The solubility coefficient (S) can be obtained from Eq.(4):

3.Results and Discussion

3.1.Structure and morphology analysis

The ILs and membrane materials were characterized by FTIR spectroscopy to confirm their chemical structure and analyze the interactions between ILs and Pebax.The FTIR spectra of the ILs,neat Pebax membrane and ILs/blended membranes are displayed in Fig.2.For neat membrane,the characteristic peaks at around 1104.3,1638.6,1734.8,2850.5 and 3303.1 cm-1are respectively attributed to the C-O,H-N-C-O,C-O,N-CH2and N-H stretching vibrations [30,31].For pure [TMGH][Im],the characteristic peaks at 2936.8,2607.2,1592.3 and 1064.3 cm-1are assigned to CH3,N-CH3,C-N and C-N stretching vibrations of cation[TMGH]+,respectively.The 1505.93 and 1411.07 cm-1peaks are related to stretching vibrations in imidazole ring of anion [Im]-[32].For pure [TMGH][PhO],the characteristic peaks of CH3,N-CH3,C-N and C-N also could be observed at 2943.9,2589.4,1602 and 1065.3 cm-1of [TMGH]+.And the stretching vibrations of benzene ring at 1471.6 and 1410.1 cm-1could be found in the spectra of [TMGH][PhO] [33].It is worth noting that there is no chemical change in all blended membranes comparing with pure ILs and neat membrane,implying that protic ILs are blended physically with Pebax matrix.Besides,the minor blue shift of N—H stretching vibration has been noticed in ILs/Pebax blended membranes comparing with the neat Pebax membrane.For example,the peak of N—H stretching vibrations shifts from 3303.1 to 3306.5 cm-1for 20% (mass) [TMGH][Im]/Pebax blended membrane,indicating that there is hydrogen bond interaction between ILs and Pebax.

Fig.2.FTIR spectra of ILs,neat Pebax membrane and [TMGH][PhO]/Pebax blended membranes (a),[TMGH][Im]/Pebax blended membranes (b).

Fig.3.SEM images of pure Pebax membrane(a),(f),10%(mass)[TMGH][Im]/Pebax(b),(g),20%(mass)[TMGH][Im]/Pebax(c),(h),10%(mass)[TMGH][PhO]/Pebax(d),(i)and 20% (mass) [TMGH][PhO]/Pebax blended membranes (e),(j).

Fig.4.XRD patterns of [TMGH][PhO]/Pebax blended membranes (a) and [TMGH][Im]/Pebax blended membranes (b).

Fig.5.TGA thermograms of neat membrane,pure ILs and ILs/Pebax blended membranes.

The neat Pebax membrane and the ILs/Pebax blended membranes were characterized by SEM and XRD to study the influence of ILs on the morphology and microstructure of membranes.As shown in Fig.3,ILs/Pebax blended membranes remain similar morphology with neat membrane from the surface and crosssection observed by SEM,implying that the anion and content of ILs have no obvious effect on the morphology of membranes.The XRD patterns of neat Pebax membrane and IL/Pebax blended membranes were shown in Fig.4.The neat Pebax membrane has a broad peak located at 2θ=19.8°,which was identified as the interchain spacing of crystalline PA regions.And the diffraction peaks at 2θ=11.2° and 22.5° with lower intensity were related to amorphous PE segments of Pebax structure [34].All blended membranes contain same peaks with neat membrane which implies that the addition of ILs does not influence the microstructure of membrane materials.But it is noted that the intensity of diffraction peaks at 2θ=11.2° and 22.5° decrease by adding ILs probably due to the reduction of crystallinity of the PE segment,which is beneficial for gas molecule transport.

3.2.Thermal properties analysis

The TGA were conducted to study the thermal stability of membrane materials.The results were shown in Fig.5.And the corresponding temperature at 5% mass loss was defined as decomposition temperature (Td).The neat Pebax has a singlestep decomposition behavior with a Tdof 367.7 °C,while Tdof[TMGH][Im] and [TMGH][PhO] is 82.1 and 108.8 °C,respectively.And ILs/Pebax blended membranes also show similar one-step decomposition behavior but the Tddecreases.The reason is that the addition of IL breaks the orderly arrangement of the molecular chains in the polymer,leading to a decrease in the crystallinity of the membrane.

Fig.6.DSC thermograms of [TMGH][PhO]/Pebax blended membranes (a) and [TMGH][Im]/Pebax blended membranes (b).

In addition,the DSC analysis were performed to study phase transition behavior of membrane materials.As shown in Fig.6,the glass temperature (Tg) of neat Pebax is -75 °C.But the Tgof blended membranes increases,indicating the addition of IL limits the mobility of the polymer chains [35].Moreover,the neat Pebax membrane has two melting peaks at around 12 °C and 135 °C,which are assigned to crystalline parts of PE segment and crystalline PA segment,respectively [10].The melting peak at 12 °C is more obvious than melting peak at 135°C because of the higher PE segment content.The addition of ILs increases the melting point of PE because the strong affinity between soft segment and ILs.However,PA melting peak becomes smaller and the melting point slightly decreases,which indicates the decreasing content of polymer segments in the blended membranes [31].Similar results are also found in the [TMGH][PhO]/Pebax blended membranes as shown in Fig.6(b).The DSC results showed that the incorporation of ILs affects greatly thermal properties and crystallinity of blended membranes,which will influence the gas permeability of blended membranes.

3.3.Effect of ionic liquid on gas separation performance

The pure gas permeation experiments were performed at 23°C and 0.1 MPa to investigate the effect of structure and content of ILs on gas separation performance.The CO2permeability,CO2/N2selectivity and CO2/CH4selectivity of blended membranes were shown in Fig.7.The CO2permeability of neat Pebax membrane is 127 barrer,which is among the literature reported values[36,37].For [TMGH][Im]/Pebax blended membranes,the CO2permeabilty shows an increasing trend from 0 to 20% (mass) IL content.The first reason is that the CO2molecule can be interacted with nitrogen atom on the imidazole ring which promotes the transport of CO2.On the other hand,the [TMGH][Im] (6.44 mPa.s at 30 °C [38]) itself have the extreme low viscosity compare with most of conventional ILs (e.g.[Bmim][BF4],68.90 mPa.s at 30 °C[39]),which is beneficial for gas transport.And this low viscosity will not affect the transport of CO2in the membrane even if the content of ILs increases.Therefore,the blended membranes with 20% (mass) [TMGH][Im] possesses highest CO2permeability up to 216 barrer,which increase the 69% compared with the neat Pebax membrane.In terms of CO2/N2and CO2/CH4ideal selectivity,they both exhibit first decrease and then increase trend with the rising of [TMGH][Im] content.

However,CO2permeability and ideal selectivities of [TMGH][PhO]/Pebax blended membranes showed distinctive differences with that of [TMGH][Im]/Pebax membranes.One of the reasons is that the anion[Im]-and[PhO]-have the different basicity which affect the CO2transport behavior in the membrane.Generally speaking,the anion of ILs with lager dissociation constant (pKa)has higher CO2absorption capacity because it is more reactive with CO2.The pKaof [Im]-is 18.6 in the DMSO and corresponding the CO2absorption capacity is 0.64 mol CO2/mol IL.However,the pKaof [PhO]-decreased to 16.2,and the CO2absorption capacity decreased sharply to 0.05 mol CO2/mol IL [38].Therefore,the CO2permeability of [TMGH][PhO] blended membranes is lower than that of [TMGH][Im] membranes.Another reason is that the viscosity of [TMGH][PhO] (26.7 mPa.s at 30 °C [38]) is almost 4.4 times that of [TMGH][Im] at the same temperature,which have an adverse effect on CO2transport in the membrane,especially at the higher IL content.Besides,pure ILs have the different compatibility with neat membrane from contact angle testing results.The contact angle for [TMGH][Im] on pure membrane (49.7°) is lower than that for [TMGH][PhO] (79.9°),which means that the[TMGH][Im] possesses better compatibility with Pebax than[TMGH][PhO].Therefore,20% (mass) [TMGH][Im]/Pebax blended membrane have better CO2separation performance than that of[TMGH][PhO]/Pebax blended membranes.

Furthermore,CO2diffusion and solubility coefficients were obtained by the time lag method to further analyze the CO2transport behavior in the membranes.As shown in Fig.8,CO2diffusion coefficient shows rising trend with the increment of IL content in both ILs/Pebax blended membranes.The addition of ILs affects the intermolecular forces of Pebax chains based on the DSC and FTIR results and increases fractional free volume,thus CO2diffusion coefficient of ILs/Pebax membrane is enhanced.The CO2solubility coefficient is little changed in the[TMGH][Im]/Pebax blended membranes with the increasing IL content,while it exhibits reducing trend in the [TMGH][PhO]/Pebax membranes.These results indicated that the structure and content of ILs affects greatly CO2transport properties in the membranes.

Fig.7.CO2 permeability (a),CO2/N2 ideal selectivity (b) and CO2/CH4 ideal selectivity (c) of ILs/Pebax blended membranes.

Fig.8.CO2 diffusion and solubility coefficients of [TMGH][PhO]/Pebax blended membranes (a) and [TMGH][Im]/Pebax blended membranes (b).

In addition,the 20% (mass) [TMGH][Im]/Pebax blended membrane is selected to study CO2separation performance at 0.015 MPa,which is a typical partial pressure of CO2in flue gases.The results showed that CO2permeability of 20% (mass) [TMGH][Im]/Pebax blended membrane is 197.3 barrer at 0.015 MPa and 23 °C,and ideal selectivity of CO2/N2is about 30.Compared with CO2separation performance at 1.0 bar of transmembrane pressure,the CO2permeability at 0.015 MPa is slightly decreased due to reduced driving force and declined CO2absorption capacity of ILs,while the CO2/N2ideal selectivity is improved.The gas separation performance of IL/Pebax blended membranes in this study were also compared with other reported ILs/Pebax membranes as shown in Table.1.The prepared membranes in this study contain less IL content and show better CO2separation performance.

Table 1 Comparison of separation performance of the reported Pebax-based membranes with this work

3.4.Effect of temperature on gas separation performance

The temperature has a significant effect on properties of each component in the membrane,such as ILs viscosity,CO2absorption capacity and chain mobility of polymers,thereby affecting the gas separation performance.Herein,the influence of temperature on gas separation performance was investigated from 10 °C to 50 °C at 0.1 MPa.The corresponding results were presented in Fig.9.For neat Pebax membrane and 20% (mass) [TMGH][Im]/Pebax blended membrane,all single-gas permeability gradually increased with the increase of operating temperature.The reason is that the mobility of polymers chains increases with the increase of temperature,which is beneficial to improve the free volume of polymer for gas transport.Moreover,the mobility of gas molecules was improved at higher temperature,which accelerate the gas through membrane.It is noted that CO2permeability of IL/Pebax blended membranes is enhanced with the increment of temperature due to the viscosity of ILs decreasing at higher temperature,which facilitates CO2diffusion.In general,CO2permeability of IL/Pebax blended membranes displays higher CO2permeability than neat Pebax membrane at different operating temperature.

Fig.9.CO2 permeability (a) and ideal selectivity (b) of neat Pebax membrane and 20% (mass) [TMGH][Im]/Pebax blended membranes at different temperatures.

What’s more,the increasing temperature generally reduces gas selectivity for most of polymeric membranes due to the wider pore distribution at higher temperature.This agrees with the results of neat Pebax membrane and 20%(mass)[TMGH][Im]/Pebax blended membrane as shown in Fig.9(b).But ideal selectivity of IL/Pebax blended membrane is still larger than that of the neat membrane at high temperature in that CO2permeability of IL/Pebax blended membrane increases faster than that of neat Pebax membrane.Besides,CO2absorption capacity remarkably decreases with the rising temperature.And the effect of temperature on CO2absorption capacity is greater than CH4and N2absorption capacity in IL due to interaction between CO2molecule and electronegative nitrogen of IL anion [38].This also results in decreased selectivity of 20%(mass)[TMGH][IM]/Pebax blended membrane at high temperature.It is suggested that membrane materials possess higher selectivity at lower temperature.And CO2/N2selectivity of 20%(mass) [TMGH][Im]/Pebax blended membrane at 10 °C reaches 31.1.

4.Conclusions

Two kinds of Pebax-based membranes with protic ILs [TMGH][Im] and [TMGH][PhO] were developed.The protic ILs were physically blended with Pebax,and the hydrogen bond interaction formed between them.Incorporating ILs into the Pebax matrix affected the phase transition behavior and the crystallinity of PE segments,which is beneficial for gas transport.The effect of anion and content of ILs on gas separation performance were investigated and the[TMGH][Im]/Pebax blended membranes exhibit better CO2permeability and ideal CO2selectivity because of the low viscosity and high CO2absorption capacity of[TMGH][Im].Besides,CO2permeability of [TMGH][Im]/Pebax blended membranes increases with the rising of IL content and operating temperature.The CO2ideal selectivity decreases firstly and then increases with the increment of IL content,but it is negatively correlated with high temperature.It is conceived that this work provides new insights into the development of functional ILs membranes for CO2separation.

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 Key R&D Program of China (2020YFA0710200),the Science Fund for Creative Research Groups of the National Natural Science Foundation of China(21921005),the Major Scientific and Technological Project of Shanxi Province of China (20201102005),the Youth Innovation Promotion Association of the Chinese Academy of Sciences(2020047) and the Innovation Academy for Green Manufacture,Chinese Academy of Sciences (IAGM2020C15).