Runlin Han*,Yongli Xie,Xufeng Ma

School of Petroleum and Chemical Engineering,Dalian University of Technology,Panjin 124221,China

Keywords:MXene Crosslinking Polyimide membrane Mixed Matrix Membrane(MMM)Solvent resistance

ABSTRACT MXene isa novel 2Dlamellar material w ith excellent hydrophilicity and permselectivity.MXene wasintroduced in the P84 polymer matrix and the matrix was crosslinked w ith triethylenetetramine(TETA)to improve the permselectivity and solvent resistance of the polyimide membrane.The membrane was characterized w ith SEM,AFM and ATR-FTIR,and effects of MXene content on the membrane morphology and separation performance are investigated.The membrane prepared w ith 18%P84 and 1%MXene show s high rejection(100%)to gentian violet(408)and high fl ux(268 L·m-2·h-1)at 0.1 MPa and ambient temperature.MXene endows the membrane with much water channel and denser functional layer w hich improves the membrane performance obviously.The membrane shows excellent solvent resistance to dimethylformamide(DMF),acetone and methanol after crosslinking with TETA during the 18 days of immersion.

1.Introduction

Nano fi ltration(NF)membraneswith nominal molecular weight cutoff from 200 to 1000 are extensively used in(w aste)w ater treatment thanks to their excellent performance,low cost and relative energy ef ficiency[1].The most prevalent nano fi ltration membranes are thin fi lm composite(TFC)membranes w ith a thin dense top layer and a porous support on non-woven substructure for mechanical strength[2].The dense top layer is typically made of polyamide(PA)via interfacial polymerization[3].However,this PAlayer ishighly reactive tow ardsoxidizing agents such as chlorine and its derivatives w hich are often used as w ater disinfectants and bactericides[4].The classical uncrosslinked nano fi ltration(NF)membranes prepared through non-solvent induced phase inversion are unstable in organic solvents,again demonstrating their poor chemical resistance.In contrast,polyimide(PI)exhibits high thermal and chemical stability,and good mechanical properties.PImembranes w ith elevated chlorine resistance are prepared by interfacial polymerization of m-phenylene diamine(MPD)and 1,2,4,5-benzene tetracarbonyl chloride (BTC)w ith subsequent thermal imidization[5].PIis also widely used for membranes in the fi eld of solvent resistant nano fi ltration(SRNF),since it can easily be crosslinked w ith polyamines and polyalcohols[6-11].These crosslinking agents can react on both ends with polyimide chains,resulting in breakage of imide bonds and formation of intermolecular amide bonds[12,13].

Recently,a new 2Dmaterials named ‘MXene’has attracted signi ficant research interest because it exhibits hydrophilic surfaces,good structural and chemical stability[14-16].MXene is a type of layered 2D material,w hich is produced by Al etched from MAX phase material(w here M represents an early transition metal,A corresponds to IIIA or IV A group elements,and X is Cor N)pow ders in HF solutions.MXene is studied in preparation of composite membrane for its excellent hydrophilicity,permselectivity and 2D layer structure w hich is a potential membrane material[17-19].

The combined mixed matrix membrane(MMM)w ith comprehensive performance is attractive and easy to scale up[20,21].Polyimide(PI)(P84)is often used to prepare organic solvent nano fi ltration(OSN)membranes with crosslinking agent and well-studied in the literature because of its membrane forming ability and permselectivity[20,22].In this w ork,triethylenetetramine(TETA)is used to improve the solvent resistance of P84 membrane after the nascent membrane is formed,w hile MXene is used to prepare P84/MXene MMM w ith comprehensive performance.Effects of MXene content on membrane performance are investigated in detail and membranes are characterized w ith SEM,AFM,FTIRand solvent resistance measurements.

2.Experimental

2.1.Materials and instruments

The average particle size of the Ti3AlC2used in the experiment w as about 3μm(98%,Beijing Fusiman).It w asetched w ith HF(49 wt%,Aladdin)to get layer MXene.First,1g Ti3AlC2is dispersed in 10 ml HFw ith strong stirring for 10 h then fi ltrated w ith de-ionized w ater.The MXene is dried at 60°Cfor 24 h before utilization.The P84/MXene mixed matrix membraneswere prepared with phase inversion method.P84(HP Polymer Inc.,Mw:25000)w as used to prepare the casting solution w ith MXene as the inorganic additive.TETA w as introduced to crosslink the nano fi ltration membrane to get solvent resistant nano fi ltration membrane.N,N-dimethylacetamide(DMAc),N,N-dimethylformamide(DMF),acetone,methanol and other chemicals used in the experiments w ere all of analytical purity grade,and w ere used w ithout further puri fi cation.A fl at sheet dead-end fi ltration setup w as used to evaluate the performance of the nano fi ltration membrane w ith a magnetic stirrer below the membrane to alleviate the concentration polarization as show n in Fig.1.The dye concentration was analyzed w ith UV-Vis Spectrometer(721,Shanghai Youke).

Fig.1.Flow process chart of test(1.Nitrogen gascylinder;2.stop valve;3.reducing valve;4.pressure maintaining valve;5.membrane evaluation apparatus;6.feed inlet;7.emptying hole;8.discharge hole).

2.2.Membrane preparation

Eighteen percent(w/w)P84 solutions in solvent DMAc w ere used as the membrane solution.In order to prepare P84/MXene mixed matrix membrane,inorganic additive MXene w ith different contents is added in the casting solution w ith strong stirring.The membranes w ith the inorganic additive w ere prepared by the same method except a certain amount of MXene added in the casting solution.The casting solution is stirred and degassed before use.The membranes w ere prepared on a glass plate w ith phase inversion method and the evaporation time of 5 s in the air.The coagulation bath w as fi lled with water at the temperature of about 20°C.The thickness of prepared membrane is about 150-200μm.The generated membranes w ere dipped in the crosslinking reagent solution w ith 1 w t%TETA at 80°Cfor 10 min.Then the excessive solution in the membrane w as w ashed by de-ionized w ater and stored in de-ionized w ater before use.

2.3.Membrane characterization

SEM(FEI Nova NanoSEM 450)is used to characterize the morphology of the mixed matrix membrane with the gold sputtering.The membrane w as also tested w ith AFM(Dimension ICON,Bruker)in the 2 μm × 2 μm area.FTIR-ATR spectrum obtained by Nicolet-20DXB w as used to characterize the membrane w ith or w ithout MXene additive.The performance of the membranes is mainly described by product fl ux(F)and dye rejection(R).The separation performance is characterized w ith the dead-end membrane set-up w ith a stirring apparatus.The membranes were pressed under 0.2 MPa for 30 min in order to obtain stabilized membrane performance.The concentrations of Gentian Violet solution were fi xed 0.1 g·L-1.The membrane performances including dye solution fl ux and rejection t w ere measured under the pressure of 0.1 MPa at 20°C.The permeation fl ux(F)was calculated as follows:

w here V is the total volume of the solution permeated during fi ltration process;A is the valid membrane area and the effective membrane area is 41cm2;and t is the operation time.Rejection,R,is calculated using the follow ing equation:

w here Cpand Cfare the concentration of the permeate solution and the feed solution,respectively.All the experimentson fl ux and rejection are repeated for three times.

Membranesprepared w ith different ingredientsare listed in Table 1.

Table 1 Substance constitution of the nano fi ltration membrane

3.Results and Discussion

3.1.Morphology of P84/MXene membraneswith different MXene contents

The layered MXene material used in thiswork ischaracterized with SEM(Fig.2).The MXene etched w ith HFhas obvious layered structure w hich is potential channel of mass transfer and the channel size is about 200 nm as shown in the inset of Fig.2.After the P84 is well-dissolved in DMAc,a certain amount of MXene pow dersisadded into casting solution followed by 30-min sonication and strong stirring to obtain uniform dispersion.

Fig.2.The layered MXene(the magni fi cation of inset is 200000×).

Fig.3.Effect of MXene content on the morphology of membranes(left:the top surface morphology and the magni fi cation timesof inset is100000×.Right:the cross-section morphology and the magni fi cation times of inset is 10000×).

To investigate the ef fi cacy of various contents of MXene in the P84 membrane,SEM analysis is utilized as show n in Fig.3.It is evident that the addition of MXene to the matrix of P84 membrane changes the smooth surface to rough surface in the low magni fi cation(×1000)SEM images because of high viscosity of casting solution and phase interface betw een the additive and polymer matrix at high MXene content.With the increase of MXene,no defect or obviousagglomeration is observed at high magni fi cation images in the inset of Fig.3.The membrane surfaces are all very smooth which demonstrates the good compatibility of these tw o materials.The asymmetric structure containing fi nger-like porous sub-layer is show n on the right-hand side of Fig.3.As can be seen,the top layers of the fabricated membranes are all dense w ith increase of MXene content.With the increase of MXene content,themembrane thicknessisalso decreased.However,themembrane fl ux is dominated by the dense layer of the membrane.So it is found that the membrane fl ux decreases at last even if the membrane thickness decline.When the MXene content increase further,the casting solution is very viscous w hich is similar as the characteristic of high polymer concentration.High viscosity and solid content of the casting solution w ill endow the membrane w ith denser functional layer and low er fl ux.Furthermore,a more thin membrane and a vast macro-pores are observed at the bottom of the prepared membranes.The change of membrane cross-section morphology may be induced by the faster exchange of water and the solvent in the phase inversion method because hydrophilic MXene has good af fi nity w ith water[23].

3.2.AFM images of the membrane with different MXene content

Fig.4 show s AFM images of membranes prepared w ith different MXene content w ith a scanning area of 2μm×2μm.In the small scanning area,pure P84 membrane hasa relatively rough surface w ith some peaks in the surface.With the increase of MXene content,the membrane surface show more small protuberances as shown in the Fig.4.Thistrend means that the MXene hasgood compatibility w ith thepolymer matrix.As show n in the inset of Fig.3,the membrane surface all dense and smooth at higher magni fi cation(×100000)for the low aggregation of MXene sheets.But at low magni fi cation(×1000),the membrane surface changes rougher because of high viscosity of the casting solution and rigid MXene blocks in the polymer matrix at high MXene content.

Fig.4.AFM images of membranes prepared w ith different MXene contents.

3.3.ATR-FTIRof the membranes prepared with different MXene contents

The ATR-FTIRspectra of P84/MXene mixed matrix membranes prepared w ith different MXene contents were characterized ranging from of 400-4000 cm-1.The chemical components of membranes prepared w ith different MXene content are illustrated in Fig.5.All the membranes w ere crosslinked with TETA and absorption bands include C═O stretching(at 1717,1779 cm-1),and C--N stretching(at 1362 cm-1)are observed in all spectrums.With the increase of MXenecontent,thecharacteristic peakschangeslittle,becausetheinorganic MXene does not have obvious absorption bands.

Fig.5.The ATR-FTIRspectra of the P84/MXene membrane(from top to the bottom are:membranes w ith 0,0.5%,1%,1.5%,2%MXene).

3.4.Effect of MXene content on the performance of P84/MXene membrane

P84/MXene mixed matrix membranes w ith different MXene content were prepared w ith phase inversion method and cross-linked w ith TETA to improve the comprehensive performance of the membrane.It can be found from Fig.6 that the rejections to gentian violet dye of membranes are all 100%demonstrating the dense functional layer and good compatibility of the membrane materials.With the increase of MXene content,the membrane fl ux increases obviously fi rst,then declines.The mixed matrix membrane show s great fl ux elevation because the layer MXene has excellent hydrophilicity and abundant w ater channel.It is demonstrated that regularly channeled lamellar MXene w ill greatly increase the membrane fl ux because layered MXenes are used as rigid building blocks and provide multiple w ater channel for the excellent permeation[24].

Fig.6.Effect of MXene content on the performance of P84/MXene membrane(0.1 MPa,0.1 g·L-1 gentian violet).

The membranes are also tested w ith Contact Angle Meter(WCA,SL200KB),and the data are the mean value of ten data show n in Table 2.With the increase of MXene content,the contact angle decreases w hich demonstrates improved hydrophilicity of the membrane surface with MXene incorporated in it,but with the further increase of MXene content,the membrane fl ux declines.The incorporation of inorganic additive will produce a lot of water channelsbecause of phase interface betw een additive and polymer matrix.The interlayer in the MXene particles also provides a great channel for w ater molecule which increases the membrane fl ux obviously.Although MXene particles are very hydrophilic,they are encapsulated in the polymer matrix which will not obviously affect the hydrophilicity of the membrane surface.How ever,w ith thefurther increase of MXenecontent,viscouscasting solution is formed and the denser functional layer is formed w hich w ill decrease the membrane fl ux.This is caused by the high viscosity of the casting solution w hich in turn delays the phase separation and improves the compactness of the functional layer[23].

Table 2 Contact angle of the MMMs

In order to investigate the solvent resistance of the MMMs,the membranes w ere immersed in the static acetone,DMFand methanol separately because these solvents w ith good solubilizing properties are often used in chemical industry.The membrane w as soaked in solvent then tested in dye/w ater solution and then soaked back again in solvent and tested every three days.It is found that the pure PImembrane w ithout crosslinking is very unstable in polar organic solvent.It w ill dissolve in the DMFand seriously sw ell in acetone and methanol in 10 min.So it is dif fi cult to test the permselectivity of the membrane after immersion in solvent.The control experiments show ed that P84/MXene membranes without crosslinking are also ruptured and dif fi cult to test after immersed in solvents like acetone,DMFand methanol for 1 day.As show in Fig.7,the rejection to gentian violet did not change during 18 days of immersion in acetone.At the same time,rejections of the membranes immersed in DMFand methanol also did not decline w ith time as show n in Figs.8 and 9.So TETA is an effective crosslinking agent that makes the P84/MXene MMMs thoroughly resistant to these common solvents.All of the membranes show obvious fl ux decline w hich may be ascribed to the fouling of membrane surface w hen dealing w ith dye solution.The membrane immersed in DMFcompared w ith membrane immersed in acetone and methanol shows greater fl ux decline,apparently because DMF is a good solvent of P84 and induces more serious sw elling of the polymer matrix,therefore small dye molecules will penetrate in the membrane pores and induce irreversible fouling.

Fig.7.Solvent resistance of the MMM w ith 1%MXene(0.1 g·L-1 gentian violet,0.1 MPa,acetone).

Fig.8.Solvent resistance of the MMM with 1%MXene(0.1 g·L-1 gentian violet,0.1 MPa,methanol).

Fig.9.Solvent resistance of the MMM with 1%MXene(0.1 g·L-1 gentian violet,0.1 MPa,methanol).

In order to evaluate the performance of the MMM w ith 1%MXene,different solutes are utilized in fi ltration solutions,respectively.From Table 3,the membrane show s highest rejection to gentian violet w ith molecular w eight of 408 Da but lower rejections to congo red(697)and methyl orange(327).It may be caused by that the congo red and methyl orange are all linear molecules w hich can easily pass though the membrane pores.The polyimide membranes crosslinked with amines are often positively charged because the remaining amine groups on the membrane surface show s positively charged character[13].So the P84/MXene membrane in this w ork should have the similar character and higher rejection to cations.The congo red and methyl orange are both negatively charged molecule which isanother reason for therelative low er rejection.In Economy'sw ork[13],the P84 membrane has a fl ux of 67 L·m-2·h-1at 1.38 MPa and high rejection to CaCl2(about 62%)and NaCl(about 30%)after crosslinking w ith polyethylenimine.In this w ork,our membrane fl ux is about 4 times higher at 0.1 MPa and the membrane hasa rejection of 100%to gentian violet(408).The membrane w ith loose structure has little rejection to salts,and high fl ux has a potential use in w aste w ater treatment w ith lower osmotic pressure and operation cost.

4.Conclusions

It w as found that the P84/MXene MMM has excellent performance with facile phase inversion method.The MXene with good hydrophilicity and w ater channel plays positive role on the membrane performance.At 0.1 MPa and ambient temperature,it's rejection to gentian violet is 100%w ith high fl ux about 268 L·m-2·h-1.The incorporation of inorganic additive w ill produce a lot of w ater channels because of the phase interface betw een additive and polymer matrix.The interlayer in the MXene particles also provides a great channel for w ater molecule w hich increases the membrane fl ux obviously.The denser functional layer w ith high rejection is formed because of high viscosity and solid content of the casting solution.After crosslinking w ith TETA at 80°Cfor 10 min,the membrane show s excellent solvent resistance.During static immersion in pure solvents such as DMF,acetone and methanol,the rejection to gentian violet does not decline,while the fl ux decreases w ith operation time.The membrane show s high rejection to some dyes but little rejections to salts w hich can be used in w aste w ater treatment w ith high salinity.

Table 3 Separation performance of the MMM w ith 1%MXene(0.1 MPa,dyes are tested w ith 0.1 g·L-1 solution while salts are tested with 1 g·L-1 solution)