Ye Yuan,Yurui Pan,Menglong Sheng,Guangyu Xing,Ming 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,Haihe Laboratory of Sustainable Chemical Transformations,Tianjin University,Tianjin 300350,China

3 College of Chemical Engineering and Safety,Binzhou University,Engineering Research Center for Industrial Wastewater Treatment and Reuse of Shandong Province,Binzhou 256600,China

Keywords:CO2/N2 separation Polyvinylamine Polymeric membranes Facilitated transport membranes

ABSTRACT Membrane technology features inspiring excellence from numerous separation technologies for CO2 capture from post-combustion gas.Polyvinylamine (PVAm)-based facilitated transport membranes show significantly high separation performance,which has been proven promising for industrial scale-up.However,commercialized PVAm with low molecular weight and excessive crystallinity is not available to prepare high-performance membranes.Herein,the synthesis process of PVAm was optimized by regulating polymerization and acidic hydrolytic conditions.The membranes based on PVAm with a molecular weight of 154 kDa and crystallinity of 11.37% display high CO2 permeance of 726 GPU and CO2/N2 selectivity of 55 at a feed gas pressure of 0.50 MPa.Furthermore,we established a PVAm synthesis reactor with an annual PVAm solution(1%(mass)) capacity of over 7000 kg and realized the scaled-up manufacture of both PVAm and composite membranes.

1.Introduction

Carbon dioxide capture from post-combustion gas has become increasingly essential to the reduction of greenhouse gas emissions to mitigate global warming issues[1].Compared with various separation technologies,membrane separation technology has attracted wide attention due to the advantages of low fixed investment,obvious operating flexibility,small footprint,and environmental friendliness [2-4].Based on the selective reversible reaction of the functional groups (named carriers) to the target component (CO2),the facilitated transport membranes can simultaneously enhance the differences in the solution and diffusion of the target component with other components,so they can break through the Robeson upper bound of ordinary polymer dense membranes [5,6].

To date,polymeric materials of facilitated transport membranes for CO2separation,including polyethyleneimine (PEI) [7],polyvinylamine (PVAm) [8,9],and polyallylamine (PAAM) [10,11],exhibit excellent separation performance,among which PVAm is a kind of linear polymer(Fig.1)with ample primary amine groups serving as fixed functional carriers to facilitate the delivery of CO2[12].The mechanism of reversible interaction between the primary amine groups and CO2is as follows:

Fig.1.Molecular structure of PVAm.

Owing to its water-soluble properties,PVAm does not need to be formulated into solutionsviavolatile or toxic organic solvents,thereby guaranteeing environmental friendliness during its largescale preparation.In addition,the simple synthetic route,good toughness,excellent pressure resistance,and ease of scale-up are all conducive for PVAm to be applied in preparing membranes[13-19].Hoet al.[20-23]developed a series of PVAm based membranes,among which the PVAm/PG membranes prepared by PVAm with a molecular weight of 12.7 MDa and piperazine glycinate(PG)as mobile carriers achieved CO2permeance of 839 GPU and CO2/N2selectivity of 161 at 57°C.Hagget al.[14,24]developed the PVAm/PVA (polyvinyl alcohol) blend membranes and demonstrated that PVA provided mechanical strength by entangling with PVAm chains to create a supporting network that could efficiently retain water within the polymer matrix,thus enhancing CO2separation performance.Wanget al.[8,25-31] conducted long-term and indepth studies on facilitated transport membranes based on PVAm modifiedviacrosslinking,blending,or copolymerizing with smallmolecule amines to comprehensively regulate the multilevel structure and strengthen multiple selective permeation mechanisms.For example,Qiaoet al.[8] crosslinked PVAm with piperazine to prepare PVAm-PIP/PSf composite membranes with CO2permeance of 6500 GPU and CO2/N2(20/80,%(vol)) selectivity of 277 at 0.11 MPa.Liet al.[31]copolymerized monomers containing amine groups,carbonate groups,and quaternary ammonium groups to achieve a synergistic effect between multiple functional groups.The prepared membranes showed CO2permeance of 1842 GPU and CO2/N2selectivity of 160 at a feed pressure of 0.11 MPa.

However,commercial PVAm usually serves as flocculant,and its inherently undesirable properties restrict its large-scale application for membranes [32].For example,PVAm with low molecular weights reveals regular chain segments and high crystallinity,resulting in the diminution of the free volume of the prepared membranes and attenuating the cohesion with the hydrophobic gutter layer,thus degrading the permselectivity [33,34].In contrast,PVAm with immoderate molecular weights is difficult to dissolve and hydrolyze,which reduces its production efficiency [35].In addition,membranes prepared by PVAm with high crystallinity possess fewer effective carriers and exhibit high mass transfer resistance,thereby reducing both the solubility and diffusivity of CO2[10,36].

Therefore,in this work,by synergistically optimizing the synthesis conditions,the molecular weight and crystallinity of PVAm were precisely controlled to obtain composite membranes with excellent CO2separation performance.First,PVAm with different molecular weights was synthesized by adjusting the monomer concentration,initiator concentration and polymerization temperature.Second,the crystallinity of PVAm was adjusted by optimizing the hydrolytic degree involving the catalyst concentration,hydrolytic temperature,and hydrolytic duration.Then,the highperformance composite membranes were manufactured by casting the prepared PVAm-PVA solution on modified polysulfone (mPSf)membranes with polydimethylsiloxane(PDMS)as the gutter layer.Finally,the physicochemical properties of PVAm and the separation performance for the CO2/N2mixture of the composite membranes were further investigated.

2.Experimental

2.1.Materials

Ethyl orthosilicate(TEOS),dibutyltin dilaurate(DBD),and polyvinyl alcohol (PVA 1799) were purchased from Aladdin Reagent Co.,Ltd..Polydimethylsiloxane (PDMS) was supplied by Shin-Etsu Chemical Co.,Ltd.N-Vinylformide (NVF,98%) and 2,2′-azobis(2-m ethylpropionamidine) dihydrochloride (AIBA,98%) were supplied by Sigma Aldrich Corporation.NVF was pretreated by reduced pressure distillation and stored at -10 °C.AIBA was dispersed in ethanol and then recrystallized and stored at 0 °C.All other reagents were not further pretreated.Hydrochloric acid (HCl) and sodium hydroxide (NaOH) were supplied by Tianjin Fengchuan Chemical Reagent Co.,Ltd.Ethyl alcohol and n-heptane were purchased from Tianjin Yuanli Chemical Co.,Ltd.Quaternary ammonium anion exchange resin was purchased from Tianjin Xingnan Yuneng Polymer Technology Co.,Ltd.and activated by using 5 %(-mass) HCl and 5 %(mass) NaOH aqueous solutions before utilization.Deionized water was produced by a laboratory water purification system.Polysulfone (PSf) ultrafiltration membranes with a molecular weight cut-off of 45000 Da were provided by Jozzon Membrane Technology Co.Ltd.

2.2.Synthesis of PVAm

Different from the current commercial method of alkali hydrolysis,PVAm used for CO2separation was synthesized by acidic hydrolysis to obtain an optimal hydrolytic degree [29].As shown in Fig.2,a certain amount of NVF and AIBA was dissolved in deionized water.Then,free radical polymerization was carried out in a flask equipped with a stirrer at a certain temperature for 8 h under a persistent nitrogen atmosphere.After polymerization,a certain concentration of HCl was added to the flask,and acidic hydrolysis was carried out at a certain temperature for different reaction durations.Then,the solution was poured into excessive ethanol to precipitate white deposits,which were then dried in a vacuum oven at 40 °C for 48 h.Finally,the obtained PVAm-HCl polymer was dissolved in deionized water and treated with excessive activated basic anion exchange resin,and PVAm aqueous solution was obtained by suction filtration.

To analyze the physicochemical properties of PVAm and the corresponding membranes,PVAm synthesized under different polymerization conditions was named as PVAm-P-a-b-c,where a represented the monomer (NVF) concentration (20%-60 %(mass)),b representedthe initiator(AIBA)concentration(0.0713-0.57g·L-1),and c represented the polymerization temperature (55-70 °C).In addition,the subsequent PVAm with different hydrolytic reaction conditions was fixed as PVAm-H-a-b-c,where a represented the catalyst (HCl) concentration (4%-16%(mass)),b represented the hydrolytic temperature(50-85°C),and c represented the hydrolytic duration(2-6 h).

2.3.Preparation of membranes

The preparation procedure of the modified polysulfone ultrafiltration membrane (mPSf) was similar to our previous work [37].PDMS solution of 0.50 %(mass) was obtained by mixing 0.50 g PDMS,0.40 g TEOS and 0.40 g DBD with 98.70 gn-heptane.Then,mPSf membranes were prepared by coating PDMS solution on the PSf ultrafiltration membranes with a wet coating thickness of(70±5)μm and then dried at 30°C and 40%RH in an artificial climate chamber (Climacell 222 R,Germany).Next,the casting solution was obtained by adding 1.0 g PVA solution of 0.05 %(mass) to 10.0 g PVAm solution of 0.10 %(mass).Then,the PVAm-PVA/mPSf membranes were fabricated by coating PVAm-PVA aqueous solution onto the mPSf substrate with a 150 μm wet coating thickness.Finally,the membranes were dried at 30°C and 40%RH in the artificial climate chamber.

2.4.Characterization

Fig.2.Flow diagram of the two-step synthesis of PVAm.

The functional groups of the synthesized PVAm were characterized by attenuated total reflection flourier transformed infrared spectroscopy(ATR-FTIR,FTS-6000,Bio-Rad,USA).The crystallinity of PVAm was investigated by X-ray diffraction (XRD,D2 Discover diffractometer,Bruker Corporation,Germany) with Cu Kα radiation between 3° and 50° at 1(°)·min-1and then analyzed by MDI Jade 6.0.An organic element analyzer (Elementar Vario EL cube,Germany) was used to determine the content of C,H and N elements in PVAm to calculate the hydrolytic degree.The glass transition temperature (Tg) of PVAm was measured by thermal gravimetric and differential scanning calorimetry (TG-DSC)(STA449F3,Germany) with the heating rate of 10 °C·min-1.The molecular weight of PVAm was characterized by gel permeation chromatography (GPC) (PL-GPC120,Polymer Laboratories,Canada).The viscosity of the PVAm aqueous solution (1%(mass))was measured by a rotating viscometer (NDJ-5S,Shanghai Genggeng,China).The morphology of the membrane was screened by scanning electron microscopy (SEM,Nova NanoSEM 430,FEI,USA).

2.5.Gas permeation experiments

The gas separation performance of the composite membranes was tested on a laboratory-made gas permeance analysis platform,as presented in Fig.3.The feed CO2/N2mixed gas (15/85 by volume) was humidified to be saturated and then entered the membrane cell at certain pressures.Helium was used as the sweeping gas to drive the permeate gas into the gas chromatograph(7890B,Agilent,USA)for composition analysis with a specific flow rate measured by an electronic flowmeter[38].The gas permeance and the CO2/N2selectivity can be calculated as:

whereRirepresents the permeance of the gas (CO2or N2) with the gas permeance unit (GPU,1 GPU=10-6cm3(STP)·cm-2·s-1-·cmHg-1) as the unit in this paper;Qiis the volume flow rate of the gas composition (CO2or N2) under standard conditions with units of cm3(STP)·s-1;ΔPiis the partial pressure difference of the gas component on the feed gas side and the permeate side,whose unit is cmHg (1 cmHg=1.33 kPa);Ais the effective membrane area in cm2;α is the CO2/N2selectivity of the composite membranes.

3.Results and Discussion

3.1.Regulation of PVAm molecular weight

3.1.1.Effect of monomer concentration

The ATR-FTIR spectra(Fig.S1,in Supplementary Material)illustrate that PVAm synthesized under different polymerization conditions exhibits similar chemical structures with amino groups,methylene groups,and amide bonds,indicating the successful synthesis of PVAm.

The molecular weights of PVAm were measured by GPC.As shown in Table 1,the molecular weights of the synthesized PVAm continue to increase with increasing monomer concentration.This can be explained by the universal equations of the polymerization rate and the kinetic chain length of the polymer [39]:

where,Ris the rate of polymerization.ν is the kinetic chain length of the polymer.kd,kp,andktare rate constants for the initiator decomposition,propagation,and termination.fis the initiator efficiency factor.[I]is the concentration of initiator,and[M]is the concentration of monomer.

The polymerization rate increases linearly with the monomer concentration and with the square root of the initiator concentration.In addition,the kinetic chain length of the polymer is also proportional to the concentration of the monomer and inversely proportional to the square root of the initiator concentration[40].Therefore,the higher the monomer concentration,the faster the rate of polymerization and the larger the average polymer chain length.However,the ‘‘gel effect” and ‘‘a(chǎn)uto-acceleration”phenomena become more pronounced with increasing monomer concentration [41],so the concentration of the monomer and initiator should be limited to a certain value.

From Table 1,with increasing monomer concentration,the crystallinity of PVAm displays a dropping trend,which is due to the reduced mobility of the polymer segments and the declining regularity of molecular chains.Simultaneously,the decrease in crystallinity of PVAm means more effective functional carriers,which is conducive to CO2transport in the membrane[33].In addition,with a higher monomer concentration,the viscosity of the PVAm aqueous solution(1%(mass))increases correspondingly.Viscosity is a manifestation of the voluminous character of randomly coiled long-chain molecules.The higher the molecular weight within a given series of linear polymer homologues,the greater the increase in viscosity produced by a given weight concentration of polymer [39].And the coating solution with higher viscosity penetrates less into the substrate,making the membrane more permeable [20].

Fig.3.Schematic of the laboratory-made platform for membrane performance test.

Table 1 Physicochemical properties of PVAm under different polymerization conditions.

Fig.4 shows that with increasing monomer concentration,the permselectivity of the prepared membranes improves to a certain extent.For example,under feed gas pressure of 0.50 MPa,the CO2permeance of the PVAm-P-50-0.57-70-PVA/mPSf membranes reaches 398 GPU,and the corresponding CO2/N2selectivity is 52,enhancing by 65% and 63%,respectively,compared to that of the PVAm-P-20-0.57-70-PVA/mPSf membranes.Since PVAm-P-50-0.57-70 with low crystallinity possesses a loose accumulation of polymer segments,it increases the fractional free volume of the prepared membranes,thereby increasing the CO2diffusion coefficient.In addition,low crystallinity of PVAm means that more effective functional carriers are located in the non-crystalline regions where the selective and reversible reaction between CO2and carriers occurs,thus improving the efficiency of functional groups[29,33,35].

In addition,as shown in Fig.4,the CO2permeance and CO2/N2selectivity of PVAm-PVA/mPSf membranes render a downward trend simultaneously as the feed gas pressure increases from 0.15 MPa to 0.50 MPa,which manifests the typically facilitated transport mechanism [42].This can be attributed to the following reasons.It is generally believed that the CO2permeance of the facilitated transport membranes consists of that dedicated by the solution-diffusion mechanism and the facilitated transport mechanism [43].With the increasing feed pressure,the contribution of the CO2flux derived from facilitated transport declines due to the gradually saturated functional amine groups,thus leading to the attenuated CO2permeance.However,the permeance of N2that penetrates through the membranes by the solution-diffusion mechanism practically remains unchanged.Therefore,the corresponding CO2/N2selectivity of PVAm-PVA/mPSf membranes decreases as the increasing feed gas pressure.

3.1.2.Effect of initiator concentration

Fig.4.CO2/N2 mixture separation performance of PVAm-PVA/mPSf membranes(PVAm-P-a-b-c refers to the membranes that were prepared by PVAm-P-a-b-c-PVA coated on PDMS gutter layer).(a) CO2 permeance,(b) CO2/N2 selectivity.Feed gas: CO2/N2 (15/85 by volume) mixture.Test temperature: 25 °C.

As shown in Table 1,as the initiator concentration decreases from 0.57 g·L-1to 0.0713 g·L-1,the molecular weight of PVAm increases from 46 kDa to 121 kDa,which is because a lower initiator concentration slows down the polymerization process and postpones the termination of polymer chain propagation,resulting in chain lengthening [39,44].However,when the concentration of AIBA is less than 0.0713 g·L-1,it is difficult to generate sufficient free radicals to initiate polymerization since the radicals decomposed by AIBA are surrounded by monomer molecules and solvent molecules to form stable molecules and cause the cage effect,thereby greatly reducing the efficiency of the initiator[40].Simultaneously,the crystallinity of PVAm decreases with decreasing initiator concentration owing to the disturbing regularity of molecular accumulation.Moreover,with decreasing AIBA concentration and,the viscosity of the PVAm aqueous solution(1%(mass))gradually increases,reaching a peak at 154.0 mPa·s.

Fig.5 illustrates that as the AIBA concentration decreases,the CO2permeance and CO2/N2selectivity of the prepared membranes are both enhanced.For example,under a feed gas pressure of 0.50 MPa,the CO2permeance of the PVAm-P-50-0.0713-70-PVA/mPSf membrane achieves 524 GPU,an increase of 31.7%compared to PVAm-P-50-0.57-70-PVA/mPSf,and the corresponding CO2/N2selectivity increases by 17.11%,which is due to the following reasons.(1) The increasing molecular weight of PVAm reduces the regularity of PVAm molecular chains by diminishing the crystallinity and increasing the free volume of the membrane,thus obtaining better gas separation performance.(2) PVAm with high molecular weight and low crystallinity possesses more free primary amine groups,which strengthens the facilitated transport effect of the membrane.(3) The moderate molecular weight of PVAm leads to the appropriate interaction between the carrier groups,resulting in moderate viscosity of the coating solution and better spreadability on the substrate.The better cohesion of the PVAm solution reduces interfacial defects,improving the membrane performance.

3.1.3.Effect of polymerization temperature

Fig.5.Effect of initiator concentration on CO2 permeance and CO2/N2 selectivity of PVAm-PVA/mPSf membranes (PVAm-P-a-b-c refers to the membranes that were prepared by PVAm-P-a-b-c-PVA coated on PDMS gutter layer).Feed gas: CO2/N2(15/85 by volume) mixture.Feed gas pressure: 0.50 MPa.Test temperature: 25 °C.

The effect of polymerization temperature on the average molecular weight,crystallinity and viscosity of PVAm is also represented in Table 1.With temperature reducing from 70 °C to 55 °C,the molecular weight of PVAm first increases to a maximum of 154 kDa at 60°C and then plummets to 44 kDa.Since the propagation reaction usually requires only activation energy of approximately 5 kcal·mol-1(1 cal=4.184 J),the rate does not vary rapidly with temperature.In contrast,the transfer reaction requires higher activation energies than the chain-growth reaction.This means that the average molecular weight will be more affected by the transfer reaction at higher temperatures.When allowances are made for chain transfer,the molecular weight passes through a maximum as the temperature is decreased.Therefore,when the temperature is above the maximum,owing to an enhanced transfer reaction,which is not conducive to chain growth,the product molecular weight is lower with increases in the temperature [20,39,40].Simultaneously,the crystallinity of PVAm decreases with decreasing polymerization temperature,and a significant reduction in the crystallinity occurs at 55 °C because of the insufficient polymerization reaction caused by the intense cage effect.In addition,the gel appearance and increase in viscosity with the duration extended to 72 h were not observed during the synthesis procedure of PVAm-P-50-0.0713-55,indicating the difficulty of producing polymers with high molecular weights when the temperature was lower than 55 °C.As the polymerization temperature decreases,the viscosity of the PVAm aqueous solution(1%(mass))first increases and then decreases,which is consistent with the changing trend of molecular weight.The lower molecular weight of PVAm attenuates the entanglement of chain segments,allowing the molecules to flow with less resistance,thus decreasing the viscosity of the obtained solution.Therefore,PVAm-P-50-0.57-60 exhibits the largest molecular weight of 154 kDa and the largest aqueous solution (1%(mass)) viscosity of 227.5 mPa·s among all types of synthesized PVAm.In addition,Fig.6 shows that the effect of temperature on the separation performance of the PVAm-PVA/mPSf membranes displays the same changing trend as that of the molecular weight of PVAm.The membranes prepared by PVAm polymerized at 60°C exhibits CO2permeance of 654 GPU and CO2/N2selectivity of 65 at a feed gas pressure of 0.50 MPa.

In summary,the regulation of polymerization conditions can effectively improve the molecular weight of PVAm.With increasing molecular weight,the crystallinity of the polymer decreases,and the separation performance of the prepared membranes is enhanced significantly.Furthermore,as another essential procedure to synthesize PVAm,the hydrolytic reaction also has a great effect on its effective carrier content and aggregation structures.Therefore,in the next sections,the crystallinity of PVAm was further optimized by adjusting the hydrolytic degree to obtain better permselectivity of the membranes.

3.2.Regulation of PVAm crystallinity

3.2.1.Effect of catalyst concentration

The ATR-FTIR spectra in Fig.S2 illustrate that PVAm synthesized with different hydrolytic degrees possesses similar chemical structures with primary amine groups and amide groups,which indicates that PVAm is successfully synthesized through an acidic hydrolytic reaction.

Table 2 shows that as the catalyst HCl concentration increases from 4%(mass)to 16%(mass),the hydrolytic degree of PVAm subsequently increases from 16.74% to 83.07%,while the corresponding crystallinity first decreases and then increases,reaching a minimum of 11.37% at the HCl concentration of 10 %(mass).This is because polymers with a high stereoregular structure,strong intermolecular force and low degree of crosslinking tend to exhibit high crystallinity[34,45].The synthesized PVAm with a low hydrolytic degree renders high crystallinity due to masses of regularly arranged formyl groups in chain segments.As the hydrolytic reaction continues,the strong electrostatic shielding effect from the generated primary amine groups partially disturbs the regularity of the polymeric segments,thereby decreasing the crystallinity of the synthesized PVAm [45].However,as the hydrolytic degree reaches over 55.40%,the repeating units -CH2CH(NH2)-in chain segments tend to be arranged regularly,which significantly improves the crystallinity.

Furthermore,with the increase in hydrolytic degree,the glass transition temperature (Tg) first decreases from 170.9 °C to 163.7 °C and then rapidly increases to 208.2 °C,which can be attributed to the following reasons.First,the crystalline regions of PVAm seriously hinder the segment movement in the noncrystalline region;thus,the changing trend ofTgwith the HCl concentration is similar to that of the crystallinity.Second,the strong intramolecular force of polar primary amine groups hydrolyzed from formyl groups confines the internal rotation of the σ bond,thereby attenuating the flexibility of the chain segments.Finally,the numerous intermolecular hydrogen bonds generated by the crosslinking amine groups also further hinder the movement of molecular segments,resulting in increasing polymeric glassiness.

Fig.6.Effect of polymerization temperature on CO2 permeance and CO2/N2 selectivity of PVAm-PVA/mPSf membranes(PVAm-P-a-b-c refers to the membranes that were prepared by PVAm-P-a-b-c-PVA coated on PDMS gutter layer).Feed gas:CO2/N2(15/85 by volume)mixture.Feed gas pressure:0.50 MPa.Test temperature:25 °C.

The separation performance of the prepared PVAm-PVA/mPSf membranes was tested by CO2/N2mixture(15/85 by volume)with saturated humidity at a feed gas pressure of 0.50 MPa.As shown in Fig.7,as the hydrolytic degree increases,both CO2permeance and CO2/N2selectivity first increase and then decrease,which is consistent with the changing trend of crystallinity.This is because the permselectivity of membranes is affected by both the carrier content and crystallinity of PVAm.In theory,the higher the hydrolytic degree,the higher the amino content in the polymer,and the higher the performance of the composite membranes.However,with the further increase in the hydrolytic degree (＞55.40%),the crystallinity of PVAm also increases,leading to the restriction of a portion of amino groups in the crystalline regions.On the other hand,if the composite membranes possess low crystallinity but insufficient carriers,the corresponding separation performance is also limited.Therefore,when the hydrolytic degree is lower than 55.40%,the regularly arranged ethyl formamide segments lead to high crystallinity.As the hydrolytic degree increases,the amino content continuously increases.PVAm with a disordered segment arrangement processes more effective facilitated carriers and lower crystallinity,thus enhancing the permselectivity of the prepared membranes.However,when the hydrolytic degree of PVAm reaches 55.40%,the content of polar amino groups further increases,whereas the increase in the number of single carriers in the fixed carrier membrane leads to enhanced crystallization properties.For the fixed carrier membrane,crystallization not only reduces the penetration area of CO2but also reduces the content of effective carriers.Therefore,the PVAm-PVA/mPSf membranes manufactured by PVAm with a hydrolytic degree of 55.40%(mass)show the highest CO2permeance of 726 GPU and CO2/N2selectivity of 55 at the feed gas pressure of 0.5 MPa.

3.2.2.Effect of hydrolytic temperature

As listed in Table 2,hydrolytic degree increases with increasing hydrolytic temper-ature,which is due to the endothermic hydrolytic reaction.However,polymeric materials are prone to aging due to the excessively high local temperature.In addition,as shown in Table 2 and Fig.8,it can be seen that as hydrolytic degree increases,the crystallinity of synthesized PVAm first decreases and then increases,reaching a minimum at a hydrolytic temperature of 70 °C,where the hydrolytic degree is 11.37%,and the CO2permeance and CO2/N2selectivity reached maxima of 726 GPU and 55,respectively.

Table 2 Crystallinity and glass transition temperature of PVAm with different hydrolytic degrees.

Fig.7.Effect of HCl concentration on CO2 permeance and CO2/N2 selectivity of PVAm-PVA/mPSf membranes (PVAm-H-a-b-c refers to the membranes that were prepared by PVAm-H-a-b-c-PVA coated on PDMS gutter layer).Feed gas: CO2/N2(15/85 by volume) mixture.Feed gas pressure: 0.50 MPa.Test temperature: 25 °C.

Fig.8.Effect of hydrolytic temperature on CO2 permeance and CO2/N2 selectivity of PVAm-PVA/mPSf membranes (PVAm-H-a-b-c refers to the membranes that were prepared by PVAm-H-a-b-c-PVA coated on PDMS gutter layer).Feed gas: CO2/N2(15/85 by volume) mixture.Feed gas pressure: 0.50 MPa.Test temperature: 25 °C.

3.2.3.Effect of hydrolytic duration

As listed in Table 2,with the extension of the hydrolytic reaction,the hydrolytic degree increases continually.As the hydrolytic reaction proceeds,the protonated primary amine groups with a strong electrostatic effect show repellent activity against H+in the solution,thus hindering the surrounding amide groups from being nucleophilically attacked by H+.Therefore,when the hydrolysis time exceeds 4 h,the hydrolysis reaction rate gradually slows down,ultimately decreasing the efficiency of the hydrolytic reaction.Table 2 and Fig.9 illustrate that as the hydrolytic reaction progresses,the crystallinity reaches a minimum at a hydrolytic duration of 4 h,where the CO2permeance also achieves a maximum of 726 GPU with a selectivity of 55 at the feed gas pressure of 0.5 MPa.Furthermore,as the hydrolytic degree increases,the glass transition temperature increases constantly from 166.4 °C to 192.6 °C.

In summary,with increasing hydrolytic degree,the separation performance of the membranes first increases and then decreases in the wake of crystallinity.This is because the carrier content and crystallinity are two vital influencing factors on the performance of the membrane.With sufficient amino groups,the crystallinity of PVAm should be kept as low as possible to guarantee adequate participation of the amino groups for the CO2delivery in the membrane.Herein,when the hydrolytic degree of PVAm is regulated to 55.40% with an optimal crystallinity of 11.37%,the prepared composite membrane shows the highest CO2permselectivity.

3.3.Performance comparison of polymeric membranes for CO2 separation

Fig.10 shows the CO2permeance and CO2/N2selectivity of the prepared PVAm-PVA/mPSf membranes in this work,as well as other polymeric membranes for CO2separation reported in the literature.The PVAm-PVA/mPSf membranes display much higher CO2permeance and comparable gas selectivity of 1070 GPU and 87 respectively at 0.11 MPa,which lies in the region of optimum membrane properties for the separation of CO2from flue gas[52],exhibiting predictable potential in large-scale carbon capture from post-combustion gas.

3.4.Large-scale production of PVAm for CO2 separation

Fig.9.Effect of hydrolytic duration on CO2 permeance and CO2/N2 selectivity of PVAm-PVA/mPSf membranes (PVAm-H-a-b-c refers to the membranes that were prepared by PVAm-H-a-b-c-PVA coated on PDMS gutter layer).Feed gas: CO2/N2(15/85 by volume) mixture.Feed gas pressure: 0.50 MPa.Test temperature: 25 °C.

Fig.10.CO2/N2 selectivity and CO2 permeance comparison with polymeric membranes reported in the literature.The shaded area shows the region of optimum membrane properties for the separation of CO2 from flue gas [14,46-52].

After the key variables that affected the performance of PVAm for CO2separation were first identified and optimized,the transition from laboratory-scale synthesis to continuous batch production for membrane scale-up was further performed by our group.The synthesis reactors for PVAm manufacture are shown in Fig.S4,and the annual production capacity of PVAm solution(1%(mass))is over 7000 kg.As shown in Fig.S5,the FTIR spectra and XRD patterns data show that the PVAm prepared by both flasks in laboratory-scale and pilot scale reactor possesses the same molecular structure,thus demonstrating the successful amplified synthesis of PVAm.Furthermore,taking advantage of the synergistic effect of cationic polymer PVAm with anionic polymer sodium polyacrylate (PAAS),small molecular anionic surfactant sodium dodecyl sulfate (SDS) and polyvinyl alcohol (PVA),utilizing ‘‘coating and surface crosslinking” membrane fabrication technology which was designed and developed by our group,large-area defect-free multilayer composite membranes were continuously fabricated with a width of 1 m and showed high and stable CO2separation performance.At a feed gas pressure of 0.50 MPa,the prepared membrane displayed CO2permeance of 785 GPU and CO2/N2selectivity of 78 [53].

4.Conclusions

In summary,the effects of synthesis conditions on the molecular structure and physicochemical properties of PVAm were systematically ascertained,and the synthesis process of PVAm was optimized by regulating the molecular weight and crystallinity,thereby improving the CO2separation performance of the PVAm-PVA/mPSf composite membranes.The prepared membranes tested by mixing CO2/N2gas (15/85 by volume) at 0.50 MPa with saturated humidity exhibit the best CO2separation performance of CO2permeance at 726 GPU and the CO2/N2selectivity at 55,when the monomer concentration,initiator concentration,polymerization temperature,catalyst concentration,hydrolytic temperature,and hydrolytic duration are 50%(mass),0.0713 g·L-1,60°C,10%(-mass),70°C and 4 h,respectively.On this basis,1 m-width uniform defect-free composite membranes with high performance were successfully manufactured on a large scale,indicating the strong application potential of PVAm for CO2capture,which also hopefully provides a reference for membrane scaling up research.

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 was supported by the National Key Research and Development Program of China (2021YFB3801200),the National Natural Science Foundation of China (21938007) and the Natural Science Foundation of Hebei Province (E2020402036).All authors are also grateful to Rui Xu for his full support in this work.

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.06.005.