Water-stable ZIF-300/Ultrason®mixed-matrix membranes for selective CO2 capture from humid post combustion flue gas

Muhammad Sarfraz *,M.Ba-Shammakh

1 Department of Polymer and Process Engineering,University of Engineering and Technology,Lahore 54890,Pakistan

2 Department of Chemical Engineering,King Fahd University of Petroleum and Minerals,Dhahran 31261,Saudi Arabia

1.Introduction

Global warming issues can be resolved to a great extent by controlling CO2emissions by its separation from post combustion flue gases,thus protecting the world environment[1].CO2capture or sequestration from a carbon-enriched gas mixture is one of the most cost effectively feasible strategies to control carbon releases[2].Amongst other typical carbon capture operations,gas separation process using polymer-based mixed-matrix membranes has gained significant importance due to its low energy requirement,high efficiency,easiness of scale up,simple design,uncomplicated function,economical operating costs and capital and environmental kindliness[3].

The key parameters of a superior quality gas separation membrane include improved permselectivity and separation factor,good mechanical strength,improved chemical and thermal stability,and good operational stability[4].Due to its inherent structural constraints(chain mobility and inter-chain spacing),a glassy polymer membrane being highly permeable is generally less selective[5];it requires considerable improvements for practical applications.A number of glassy polymers(such as poly(vinyl acetate),polydimethylsiloxane,polyimide,poly-(1,4-phenylene ether-ether-sulfone),polysulfone,Ultem®,Matrimid®)and inorganic nanofillers(like zeolites,carbon nanotubes[6],structured mesoporous and nonporous silica[7],microporous metal organic frameworks(MOFs)[8],and carbon molecular sieves[9])have been used to fabricate hybrid membranes providing improved separation performance in comparison to their bare-polymer counterparts.The major advantages of developing MMMs by doping thermoplastic polymer matrix with microporous nanocrystals of MOF materials comprise the capability of coupling the easy processability and casting,improved chemical and mechanical performance of polymers with the superior gas separation effectiveness,variable pore sizes,adjustable surface functionalities,and large surface areas of microporous nanomaterials[10].

Owing to their high CO2permeability and CO2/N2permselectivity values,hybrid membranes fabricated from commercially available polysulfone and its different grades have acquired considerable research attention for carbon capture.Selective chemical functionalization or formulation of a composite membrane by integrating microporous nanofillers into base polymer matrix are valuable strategies to further augment gas permeability through a flexible polymer.Incorporation of HKUST-1 contents into polysulfone to obtain HKUST-1/PSF hybrid membranes leads to improve both CO2permeability and CO2/N2selectivity[11].Fabrication of composite membranes by doping Udel®(a commercial grade of polysulfone)with varying contents of mesoporous silica spheres helped to improve CO2separation performance[12].

MOFs microporous nanocrystals are significant materials to formulate proficient mixed membranes for gas separation owing to their adjustable nano-sized dimensions,high surface areas,superior wetting features,improved chemical and thermal stability,and unique surface functionality[10].Since the hydrothermally stable crystals of ZIF-300 selectively detain CO2gas from dry and wet CO2/N2gas mixtures[13],their insertion into glassy polymers is anticipated to enhance separation efficiency of hybrid membranes under both dry and wet environments.Hydrophobic organic linkers combined with zinc salt result into microporous isoreticular-structured ZIF-300 crystals rendering controlled pore size of few nanometers.Contrary to several MOF crystalline materials[14-16],ZIF-300 crystals do not disintegrate at elevated temperatures and effectively detain CO2from CO2/N2/H2O mixture subjected to genuine post combustion conditions.The volumetric composition of an ordinary flue gas stream consists of 75%N2,15%CO2,6%H2O and 4%detectable gases[15].

Majorchallenges regarding carbon capture from postcombustion flue gas can be addressed by using ZIF-300 material due to its improved CO2adsorption capacity,high CO2/N2selectivity and low regeneration enthalpy subjected to moistcircumstances[13].Expecting the improvement in gas separation performance(both under dry and wet conditions)by inserting them into glassy polymers,ZIF-300 nanocrystals have been incorporated into glassy Ultrason®(a commercial grade of polysulfone)matrix to fabricate ZIF-300/US mixed-matrix membranes for effective carbon capture.The central focus of the present work is to investigate the dependence of CO2permeability and CO2/N2permselectivity on ZIF-300 contents,the effect of moisture contents on CO2separation efficiency of ZIF-300/US MMMs,and the prediction and comparison of experimentally-acquired gas permeation data with two-and threephase permeation models.

2.Experimental and Data Validation

2.1.Materials

Synthesizing ingredients of ZIF-300i.e.zinc nitrate hexahydrate,2-methylimidazole and 5(6)-bromobenzimidazole were purchased from Merck Chemical Company.Methanol,chloroform,andN,N-dimethylformamide(DMF)solvents were obtained from Aldrich Chemical Company.Commercial-grade Ultrason®S 6010,having density 1.25 g·cm-3and LS average molecular weight～35000,was obtained from Sigma Aldrich.The glassy polymer and all reagentgrade chemicals were utilized in the obtained form without any processing.He,N2and CO2gases used for gas sorption and permeation experiments were highly pure.

2.2.Formulation of ZIF-300 nanocrystals

ZIF-300 nanocrystals were preparedviacold synthesis process by dissolving zinc nitrate hexahydrate(67.8 mg),2-methylimidazole(23.4 mg)and 5(6)-bromobenzimidazole(56.4 mg)in a 10-ml mixture of distilled water(0.5 ml)and DMF(9.5 ml)in a 20-ml vial and sonicating for 10 min.The prepared solution was then gently stirred on a hot plate for 70 h by maintaining the temperature at 50°C to obtain brown slurry of ZIF-300 nanocrystals.Centrifugation was employed to separate suspended nanocrystals from mother liquor.The resulting nanocrystals were purified by daily washing them with 7-ml fresh DMF for five times followed by solventexchange with anhydrous methanol thrice a day at room temperature for another three days.Spent methanol was decanted followed by washing the nanocrystals thrice with chloroform.

2.3.Preparation of ZIF-300/US MMMs

Prior to fabricate composite membranes of varying compositions all the constituents(US and ZIF-300)were degassed at 100°C under vacuum for 20 h to get rid of adsorbed moisture and/or gases.Neat Ultrason®membrane was fabricated by dispersing 1-g polymer pellets in 7-ml DMF solvent,followed by vigorously stirring at room temperature for20 h untila thick viscous solution was obtained.The hybrid membranes doped with varying amounts of ZIF-300 nanocrystals were fabricated by incorporating 1-g base polymer in 5-ml DMF followed by robust stirring at room temperature for 20 h to get a thick solution.Specified amount of ZIF-300 nanocrystals was redistributed in1 ml DMF solvent,sonicated for 10 min,and uniformly dispersed solution was obtained.Both the viscous solutions were combined collectively into a beaker and stirred for 12 h until a homogenized thick solution was acquired.After adjusting their gate height,the viscous solutions were distributed on clean glass plates and knife cast to get thin membranes.The entrapped solvent with in the membranes was allowed to slowly evaporate atroom temperature for 20 h.The membranes were peeled off from the glass plates and dried at 80 °C(12 h),100 °C(20 h)and 160°C(20 h)to make them bone-dry.

2.4.Membranes characterization methods

The characterization techniques used to measure various properties of ZIF-300 nanocrystals and bare polysulfone and composite membranes include X-ray diffraction,scanning electron microscopy,thermal gravimetric analysis and gas sorption analysis.The nanofiller's fractional volume(ΦD)incorporated into the hybrid membranes can be determined by the following equation:

heremand ρ respectively symbolize mass and density of continuous polymer phase C(Ultrason®)and dispersed filler's phase D(ZIF-300 nanocrystals).

XRD characterization of the hybrid membranes was accomplished to con firm whether the nanofillers crystalline structure is not damaged even after their addition into the polymer matrix.The membrane small specimens were put on silicon substrate placed in a sample holder.Powder-XRD radiation diagrams of all the prepared membranes were recorded by Bruker D8 X-ray DiffractometerviaCu Kαradiation(λ=0.15406 nm)functioned at 45-mA current,40-kV voltage and step size increase of 0.02°in 2θ.

Hitachi S-4300SE/N SEM instrument was employed to scan the membrane surface to get relevant micrographs aiming to check their morphological configuration and matrix- filler interfacial aspects.Prior to testing,the membrane samples were prepared by cryofracturing them in liquid nitrogen followed by coating their external surfaces with a delicate golden film to circumvent electrons charging.The SEM device was run at 20 kV to take fine quality images.

Thermal gravimetric analysis was employed to determine thermal constancy and other associated attributes of membrane materialsviaTGA/SDTA 851(Mettler Toledo)system operated in air.Testing specimen was heated from room temperature to 700°C by maintaining the heating rate at 10 °C·min-1.Differential scanning calorimetry(DSC)was carried outviaNetzsch DSC 200F3 Calorimeter to assess glass transition temperature(Tg)of the membranes.The equipment was operated in nitrogen atmosphere being flown at a rate of 50 ml·min-1and heated from 40 to 200 °C while maintain a heating rate of 5 °C·min-1.

Quantachrome Autosorb iQ Gas Sorption Analyzer was used to get N2and CO2adsorption isotherms at different temperatures of 77 and 298 K.After cutting them into minute bits,the membrane specimens were degassed,subjected to vacuum(＜0.1 Pa),at100°C for 5 h.The isotherms obtained at 77 K were analyzed to obtain different microporous properties(such as N2and CO2gas uptakes,microporous volume,specific Langmuir and BET surface areasetc.)of the membrane materials.The sorption measurements evaluated at a temperature of 298 K in the pressure range of 0.1 to 1×105Pa were employed to assess physisorption data of N2and CO2gases.

2.5.Gas permeation measurements

Single gas permeation cell operated in constant-volume(variable pressure)mode at a temperature of 298 K[17-18]was utilized to evaluate membrane transport properties(permeability)and separation performance(ideal selectivity)for N2and CO2gases.Average thickness of the membranes,in the range of 70-120 μm,was measured by a digital micrometer and the membrane sample was placed in the permeation cell to perform permeation test.The permeate-side line of the cell was evacuated by keeping the upstream valve closed and downstream valve connecting to a vacuum pump.Once the vacuum was generated on the permeate side,the valve between vacuum pump and permeateside line was shutdown and feed-side valve opened to uphold a constant pressure(e.g.,104Pa)on feed-side for a definite time period(i.e.,2 h)to register the permeation measurements.The upstream pressure was next intermittently raised followed by taking the reading after 1 h of stabilization for each step.To ensure their perfection and certainty,at least three replicas were fabricated and assessed equivalent to every membrane specimen.

Gas permeability of the membrane can be assessed using the following relation:

wherePi,A,V,R,T,l,ΔPiand ΔPi/dtdenote gas permeability(Barrer,1 Barrer=10-9mol·m-2· s-1·Pa-1),membrane effective area(cm2),downstream chamber volume(cm3),universal gas constant(6236.56 cm3·cmHg·mol-1·K-1,1 cmHg=13.3322 Pa),absolute temperature(K),membrane thickness(cm),pressure difference across the membrane(psi,1 psi=6894.76 Pa),and gas permeation rate(psi·s-1)of componentirespectively.

Diffusion coefficient(D)of the membrane was determinedviadiffusivity(D)vs.time(θ)lag relationship as suggested by Paul and Kemp[19]:

hereVpandVddenote volume fractions of polymer and filler phases respectively;yandKrepresent adsorption parameters to be calculated from Langmuir adsorption isotherm.

The solubility coefficient(S)of the membrane was calculatedviafollowing equation:

Ideal selectivity(αij)of gasioverjwas computed from the eq.given below:

Here(Si/Sj)and(Di/Dj)denote respectively the solubility-and diffusion-based selectivity terms.

2.6.Predicting permeability of ZIF-300/US MMMs

Gas permeation through a membrane can be estimatedviadifferent theoretical models[20]based on ideal(two-phase system)and nonideal(three-phase system)morphologies of composite membranes.A two-phase permeation model based on perfect morphology of MMMs can be characterized by a continuous polymer matrix phase and a homogeneously distributed nanoparticles phase possessing a faultless,defect free and non-deformable polymer- filler interface.A non-ideal morphology representing imperfections,defects and faults at polymer- filler interface leads to a three-phase system which takes into consideration the polymer- filler interface along with matrix and filler phases.

Gas permeability of a composite membrane relative to its bare polymer counterpart(Pr)can be predictedviafollowing expressions based on two-phase permeation models.

The three-phase permeation model used to anticipate the relative permeability of a gas through a composite membrane can be expressed as follows:

3.Results and Discussion

The end results of different characterization techniquesi.e.,XRD,SEM,TGA,gas sorption and gas permeation are briefly described here.

3.1.Powder X-ray diffraction

It is a valuable technique to examine the nanofillers impact on polymer chains arrangementin MMMs and to substantiate the continuation of ordered structure of crystalline materials after incorporating them into polymers.In order to ascertain actuality of crystalline of CNTs and ZIF-300 nanocrystals in MMMs,XRD quantifications were recorded in the 2θ range of2°-50°(see Fig.1).Intensity peaks arising at2θlocations of 5.3°,6.5°,11.3°,17.2°and 22.9°in XRD schemes of composite membranes con firm the dispersion of crystalline structure of isoreticular ZIF-300 nanocrystals[13]within Ultrason®matrix.The preservation of crystalline structure of these nanocrystals even after their incorporation into the polymer matrix is well supported by XRD patterns of MMMs.Also the peak heights at particular 2θ levels correspond to respective nanofillers loadings in MMMs.In addition,the centralization of a specific broad peak at 2θ position of 17.2°is associated with base polymer membrane.Also the minor reallocation of the wide peak of un filled Ultrason® from a 2θ position of 17.2°(d-spacing=0.521 nm)to 17.3°(d-spacing=0.517 nm),indicating the reduction in polymer inter-chain distance,can be ascribed to intense filler-polymer interactions.Decrease in inter-chain dimensional attitude endorses CO2/N2permselectivity enhancement due to the fact of size exclusion.

3.2.Morphology of ZIF-300/US MMMs

The characteristic features of polymer matrix and incorporated nanofillers mainly dictate the distinctive interior microstructural features and morphology of fabricated MMMs.In order to explore their morphological features,nanofillers distribution within polymer substance and polymer- filler interfacial adhesion,cross sectional photomicrographs of composite membranes were probedviaSEM.The internal surfaces of all MMMs doped with varying amounts of nanofillers depicted continuous appearances,virtually independent of interfacial spaces.Cross sectional outlook of these composite membranes demonstrated faultless interrelated morphology,effective adhesion at polymer- filler interface and uniform dispersal of nanofillers in polymer matrix.

Fig.1.XRD patterns of pure ZIF-300,pure Ultrason®,and ZIF-300/US MMMs containing 10%,20%,30%and 40%by mass of ZIF-300.

Fig.2.Scanning electron micrographs of pure Ultrason®(A),ZIF-300/US MMMs containing 10%(B),20%(C),30%(D),40%(E)by mass of ZIF-300,and pure ZIF-300(F).

Hybrid membranes doped with varying loadings of nanofiller exhibit uninterrupted phases for almost all MMMS(Fig.2)due to homogeneous distribution of tiny nanocrystals of ZIF-300.The phase continuity at filler matrix boundary leads to voids-free interface in all MMMs.Nanofiller distribution in MMMs filled with low loadings(10 wt%)of ZIF-300 nanocrystals was sparse(Fig.2B).Hybrid membranes doped with moderate contents(20 wt%-30 wt%)of ZIF-300 nanocrystals manifested consistent dispersion of nanofiller in polymer matrix(see Fig.2C,D).High loadings(40 wt%)of ZIF-300 nanoparticles depict signs of slightly rough surface and marginal aggregation(Fig.2E).Specific spatial arrangement of chabazite-type ZIF-300 nanocrystals within polymer matrix lead to smooth,voids-free and crack-less morphology corresponding to their defined loadings.

3.3.Thermal gravimetric analysis

The effect of incorporating ZIF-300 nanocrystals into neat Ultrason®matrix,determined in terms of thermal stability,phase transitions,physical and chemical performanceetc.,by carrying out thermal gravimetric analysis(TGA)coupled with derivative thermal gravimetric(DTG)analysis[collectively called(TGA-DTG)analyses]of bare Ultrason®and composite membranes in the temperature range of 40-700°C.

Thermal phase transitions of fabricated membranes were depicted by TGA decomposition profiles(see Fig.3A)in terms of specimen mass loss taking place in two steps.The two-stage mass loss takes place due to desolvation and pyrolysis processes in the temperature ranges of 100-200 °C and 510-640 °C respectively.The former occurs on account of liberation of volatile solvent molecules(DMF,water,CCl4etc.)from the membrane pores.The latter owes to the degradation of Ultrason®structure into its constituting entities(C,benzene,SO2,toluene,phenol,xylene,styreneetc.)accompanied by the degeneration of organic ligands of imidazole frameworks(5(6)-bromobenzimidazole and 2-methylimidazole).Minimal mass contents of the nanofillers added to hybrid membranes were demonstrated by the ash leftover in the pan on completion of the experiment.

Td5%andTd10%values,defined as the temperatures atwhich a material sample loses its 5%and 10%mass respectively,facilitates to assess its thermal constancy[21].These values are controlled by the loadings of constituting nanofillers of composite membranes and normally fall in the temperature range of 165-175 °C and 210-450 °C respectively.Differential mass loss(DTG)curves and glass transition temperature(Tg)of the polymer specimen impart essential information on extent of stiffness of polymer chains in composite membranes.Improvement in these values(especiallyTg)by increasing nanofillers loadings indicate enhanced MMMs stiffness owing to restricted polymer chains movement which is a direct consequence of polymer chain-to-nanofiller interactions.The 1st and 2nd DTG peaks in Fig.3B render necessary data about pyrolysis rates of the specimen.Values of various important thermal properties of composite membranes are outlined in Table 1.

Table 1 Characteristic temperatures of membrane materials acquired from TGA-DTG data

As compared to unloaded Ultrason®membrane,thermal decomposition of MMMs taking place at elevated temperatures might be ascribed to enhanced filler-polymer interactions and improved thermal constancy of ZIF-300 nanocrystals.As maintained by 2nd DTG peaks,thermal stability of hybrid membranes was ameliorated by raising loadings of nanofiller and was found maximum for 40 wt%doping of ZIF-300 nanocrystals.The MMMs can be used free of danger in carbon capture applications because the highest temperature noticed in various combustion and gas separation processes falls in the range of 30-350°C[22].

Fig.3.TGA-DTG curves:TGA(A)and DTG(B)of pure ZIF-300,pure Ultrason®,and ZIF-300/US MMMs containing 10%,20%,30%and 40%by mass of ZIF-300.

3.4.Gas sorption analysis

N2and CO2adsorption isotherms obtained at various temperatures under relative pressure of～0-0.1 MPa were used to determine some important physical macroscopic properties(such as density,nanofillers fractional volumeetc.)and key microporous characteristics(such as total micropore volume,Brunauer-Emmett-Teller(SBET)and Langmuir(SLang)surface areas,N2and CO2uptakes,and CO2/N2adsorption selectivityetc.)of fabricated membranes.Membranes microporosity and surface area were measured by using N2adsorption isotherms obtained at 77 K.Loading capacities of N2and CO2gases in low pressure regime were assessed from N2and CO2adsorption isotherms at 298 K(Fig.4).As outlined in Table 2,these properties are subsequently improved by increasing contents of incorporated ZIF-300 nanocrystals.The improved adsorption properties accurately corroborated the genuineness of uniform nanofiller dispersion and better adhesion at polymer- filler interface,thus resulting in fine quality composite membranes.

The chabazite-type nanocrystals of ZIF-300 favorably adsorb quadrupolar CO2gas molecules owing to their typical structural and chemical properties.This preferential adsorption significantly improves CO2uptake for all MMMs in contrast to un filled Ultrason®membrane.The maximum CO2uptake of 0.7 mmol·g-1(equivalent to 12 cm3·g-1)at 298 K was noted for the MMM containing 40 wt%ZIF-300 nanocrystals;the CO2uptake can be further increased by lowering the operating temperature.Since the incorporated nanofillers have no specific chemical affinity for N2gas,its uptake for all the prepared membranes followed almost a linear relationship with applied pressure up to 0.1 MPa.The size difference between two gas molecules resulted in controlled N2adsorption as compared to CO2on account of reduced pore size of ZIF-filled MMMs.As compared to N2,CO2adsorption capacity of all the fabricated membranes was significantly high,particularly in low pressure regime.The CO2/N2ideal adsorption selectivities of MMMs appreciably improved with increasing loadings of nanofiller;this increase is more sensitive at low partial pressure.The gas sorption study strongly recommends the use of these prepared MMMs in packed bed columns as effi-cient material for CO2gas separation from post combustion flue gas where partial pressure of CO2gas is comparatively low.

Fig.4.CO2 and N2 adsorption isotherms for pure ZIF-300,pure Ultrason®,and ZIF-300/US MMMs containing 10%,20%,30%and 40%by mass of ZIF-300 at 298 K.

Table 2 Microporous properties of ZIF-300,Ultrason®,and ZIF-300/US MMMs

3.5.Gas permeation properties of MMMs

3.5.1.Dry and wet gas permeation

Permeability,ideal selectivity,and coefficients of diffusion and solubility of fabricated membranes were determinedviasingle gas(N2and CO2)permeation experiments at 298 K and an upstream pressure of 0.2 MPa subjected to both dry and wet conditions.Inclusion of ZIF-300 nanocrystals resulted in substantial enhancements in CO2permeability and CO2/N2ideal selectivity as shown in Figs.5 and 6 respectively.Permeation properties of CO2molecules and their effective separation potentiality from N2were not disturbed by the use of humidified gases in permeation experiments.

The chabazite-type structural topology of ZIF-300 nanocrystals coupled with their noticeable chemical affinity for CO2molecules over those of N2greatly enhances CO2permeability through the hybrid membrane.The optimum nanofillers composition rendering maximum throughput(measured in terms of CO2permeability)and gas purity(quantified in terms of CO2/N2permselectivity)was established to be 40 wt%ZIF-300 nanocrystals.CO2permeability and CO2/N2ideal selectivity of composite membrane associated with this specific loading was estimated to 27 Barrer and 26 respectively.

3.5.2.Diffusivity and solubility of MMMs

Coefficients of diffusivity(D)and solubility(S)computed for N2and CO2molecules penetrating through MMMs can help to understand gas permeation process through a membrane taking place by means of solution-diffusion mechanism.Figs.7 and 8 respectively represent important facts related to gas diffusivity and solubility for the fabricated membranes.The increasing contents of ZIF-300 in MMMs lead to drop the diffusivity coefficients of both N2and CO2molecules as shown in Fig.7.A significant drop in gas diffusivity coefficient took place due to pore size reduction of composite membranes caused by compact arrangement of ZIF-300 nanocrystals within the continuous polymer matrix.

Normally the gas solubility coefficient mainly depends on structural arrangement and chemical formulation of nanofillers incorporated into polymer matrix.In addition,the presence of a large number of basic functional groups and interaction sites on ZIF-300 nanocrystals also improves the permselectivity of CO2over N2.The increase in solubility coefficient of CO2molecules was noted to be very high as compared to those ofN2molecules for every MMM specimen.The gas solubility coefficient significantly increased by increasing ZIF-300 contents and was maximized for the hybrid membrane containing 40 wt%ZIF-300 nanocrystals as depicted in Fig.8.Owing to their specific chemical affinity for ZIF-300 nanocrystals,quadrupolar CO2molecules,ascompared to non-polar N2molecules,are preferentially adsorbed onto ZIF crystals.This large solubility difference greatly enhances solubility of CO2molecules as well as solubility-based CO2/N2selectivity.

Fig.5.Single gas CO2 and N2 permeability values of pure Ultrason® and ZIF-300/US MMMs with different ZIF loadings.(1 Barrer=10-9 mol·m-2·s-1·Pa-1).

Fig.6.CO2/N2 ideal selectivity values of pure Ultrason®and ZIF-300/US MMMs with different ZIF-300 loadings.

Fig.7.Pure gas coefficient of diffusion(D)for bare Ultrason®and ZIF-300/US MMMs having different ZIF-300 loadings.

Fig.8.Pure gas coefficient of solubility(S)for bare Ultrason®and ZIF-300/US MMMs having different ZIF-300 loadings.(1 atm=101325 Pa).

Diffusion coefficients values for N2and CO2gases were estimatedviatime-lag method[Eq.(3)].The enclosed termin Eq.(3)was corrected by making use of Langmuir parameters(Kandy)which in turn were calculated from adsorption isotherms for N2and CO2gases acquired at298 K.The values of diffusion coefficient obtained from Eq.(3)were used in Eq.(4)to calculate solubility coefficient values subjected to a pressure of1 bar and temperature 298 K.An increase in ZIF-300 contents in polymer matrix led to reduce CO2/N2selectivity(DCO2/DN2)based on diffusivity coefficient as described in Fig.6.Gas selectivity(SCO2/SN2)based on solubility coefficient was increased by the addition of ZIF-300 nanocrystals as displayed in Fig.6.The overall effect of gas permeationviasolution-diffusion mechanism resulted in slight increase in ideal selectivity ofCO2over N2[Fig.6].This improvement in permselectivity can be ascribed to the enlarged number of interaction sites and basic functional groups available on ZIF-300 nanocrystals.

The phenomenon of improved CO2/N2permselectivity can also be elucidated in terms of reduced polymer inter-chain spacing.As compared to CO2gas molecules,the adsorptive diffusion of N2molecules was restricted owing to pore size constriction taking place within the polymer matrix.Pore size reduction takes place because ZIF-300 nanocrystals occupy and narrow down the polymer inter-chain void spaces.Both the restricted N2diffusion and selective CO2solubility taking place within the hybrid membrane result in improved CO2/N2ideal permselectivity[Fig.6].The added nanofiller acted in such a way that the membrane structure provided precise openings for better CO2permeability and CO2/N2ideal selectivity.The comprehensive investigation established on enhanced performance of ZIF-300/US MMMs firmly corroborated the development of a real mixed matrix membrane for CO2capture from flue gas.3.5.3.Comparison with Robeson upper bounds

A chart plotted between CO2permeability and CO2/N2selectivity is used to compare the separation efficiency of prepared composite membranes with Robeson 1991-and 2008-upper bounds[23]as displayed in Fig.9.In contrast to unfilled Ultrason®membrane,permselectivity values of composite membranes were significantly increased by the insertion of ZIF-300 nanocrystals into Ultrason®matrix.The permselectivity value of hybrid membrane containing 10 wt%ZIF-300 lies close to Robeson upper bound line while those of MMMs comprising 20 wt%-40 wt%ZIF-300 contents lie on this line,signifying their superiority over most of the current membranes,an important achievement of this work.

Fig.9.Comparison of CO2/N2 separation performance of ZIF-300/USMMMs with other MOF containing MMMs obtained from literature data.The Robeson upper bounds 1991 and 2008 for polymer separation performance are also shown.

Fig.10.Comparison between experimentally determined CO2 permeability and those predicted by Maxwell,Bruggeman,Singh,and modified Felske models.

3.6.Comparison of experimental data with mathematical models

CO2permeability through the fabricated composite membranes was estimatedviatwo-and three-phase permeation models.As shown in Fig.10,the two-phase Maxwell and Singh models established good match with experimental data at low filler loadings,whereas the three-phase Bruggeman and modified Felske models more closely predicted the permeation performance at higher filler concentration.In contrast to three-phase models,relatively low experimental-vs.-theoretical discrepancy associated with two-phase models indicates approximately ideal morphology of the prepared hybrid membranes.

4.Conclusions

Polymer-based mixed-matrix membranes were developed by doping flexible Ultrason®polymer with ZIF-300 nanocrystals,having narrow particle size distribution,in varying loading levels.The prepared composite membranes were able to selectively and efficiently capture carbon dioxide from flue gas.The fabricated MMMs were found to be partially crystalline,thermally stable,microporous materials depicting consistent distribution of nanofiller and fine interfacial adhesion between polymer matrix and inorganic nanofiller as maintained by XRD,SEM,TGA and gas sorption experiments.As compared to un filled Ultrason®membrane,CO2permeability of composite membranes was enhanced by four times while the CO2/N2ideal selectivity stayed almost unchanged.Moreover the presence of moisture contents in permeating gases did not upset separation performance of composite membranes.Furthermore,the carbon capturing effectiveness of fabricated MMMs was found close to the Robeson upper bound.The values of CO2permeability and CO2/N2permselectivity of composite membranes are high enough to fulfill industrial applications subjected to elevated temperatures.

Nomenclature

Aeffective membrane area,cm2

Ddiffusion coefficient,cm2·s-1

Kadsorption parameter determined from Langmuir adsorption isotherm

Lmembrane thickness,cm

mmass of the specimen

Pgas permeability,Barrers(1 Barrer=10-9mol·m-2·s-1·Pa-1)

ΔP/dtgas permeation rate(psi·s-1,1 psi=6894.76 Pa)in terms of time rate of pressure

Δppressure difference across the membrane,psi(1 psi=6894.76 Pa)

Runiversal gas constant(=6236.56 cm3·cmHg·mol-1·K-1)(1 cmHg=13.3322 Pa)

Ssolubility coefficient,cm3(STP)·cm-3·cmHg-1(1 cmHg=13.3322 Pa)

Tabsolute temperature,K

Vcell downstream volume,cm3

yparameter to be determined from Langmuir adsorption isotherm

α membrane gas selectivity

β matrix rigidification or chain immobilization factor

γ ratio of interphase thickness to particle radius

δ ratio of outerradius of rigidifed interfacial matrix chain layer to radius of core particle

θ x-rays diffraction angle,(°)

λ permeability ratio

ρ density,g·cm-3

ψ function of packing volume fraction of filler particles

Ф fractional volume of fillers,%

Subscripts

C continuous phase

D dispersed phase

d5%5%specimen mass loss

d 10%10%specimen mass loss

eff effective

g glass transition

i interphase

igas‘i’

jgas‘j’

m polymer matrix

r relative

Acknowledgements

The authors are thankful to KACST—Technology Innovation Center on Carbon Capture and Sequestration(CCS),King Fahd University of Petroleum and Minerals,Dhahran,Kingdom of Saudi Arabia(KSA)for providing support for this work.

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