An Investigation on Gas Transport Properties of Cross-Linked Poly

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Environmental and Carbon Dioxide Issues

An Investigation on Gas Transport Properties of XLPEGDA and XLPEGDA/TiO2 Membranes with Focus on CO2 Separation Ali Ghadimi, Somayeh Norouzbahari, Vahid Vatanpour, and Fereidoon Mohammadi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00545 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 27, 2018

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An Investigation on Gas Transport Properties of XLPEGDA and XLPEGDA/TiO2 Membranes with Focus on CO2 Separation Ali Ghadimia,*, Somayeh Norouzbaharib, Vahid Vatanpourb and Fereidoon Mohammadia a

Faculty of Petrochemicals, Iran Polymer and Petrochemical Institute, 14965-115 Tehran, Iran Tel: +98 21 48662489; Fax: +98 21 44787021; E-mail address: [email protected] b

Faculty of Chemistry, Kharazmi University, 15719-14911 Tehran, Iran

Abstract Poly(ethylene oxide) (PEO)-based membranes are known as outstanding candidates for carbon dioxide (CO2) separation as the major greenhouse gas responsible for global warming. In this paper, gas transport properties (solubility, permeability and diffusivity) of neat and nanocomposite cross-linked poly(ethylene glycol diacrylate) (XLPEGDA) membranes were investigated for CO2 as well as CH4, C2H4, C2H6, C3H8, H2 and N2 gases. XLPEGDA as a low molecular weight PEO, has not been much studied compared to other PEO-based membranes such as poly(ether-block-amide) (PEBA) for CO2 capture. To make the conducted research more practical, the operating conditions were selected near to industrial operational conditions, i.e. at the temperature range of 35-75 ˚C and pressures up to 16 bar. All membranes were synthesized by UV photopolymerization. To prepare nanocomposite membranes, inorganic titanium dioxide (TiO2) nanoparticles were incorporated within the polymeric matrix prior to its cross-linking. Structural properties of the prepared membranes were characterized by scanning electron microscopy energy dispersive X-ray (SEM-EDX), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and density analysis. DSC and FTIR

1 ACS Paragon Plus Environment

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results confirmed completeness of the crosslinking reaction. SEM images showed homogeneous structure of the membranes and rather uniform dispersion of the TiO2 nanoparticles. It was found that incorporation of the TiO2 nanoparticles, more specifically at 3 wt% loading, results in enhancement of CO2 permeability and solubility by 39% and 18.5%, respectively. Furthermore, CO2 selectivity values over the investigated light gases including H2, CH4 and N2 increased by 16.2, 15.6 and 26.6%, respectively. Keywords: Gas transport properties; XLPEGDA; CO2 separation; TiO2 nanoparticles

Introduction Membrane gas separation has found widespread application owing to advancements in fabrication of membranes offering both desirable permeability and selectivity in recent decades13

. Separation of condensable components from light gas streams such as CO2 from N2, H2 and

CH4, using reverse-selective polymeric membranes is of great value for many practical purposes 4

. For instance, post-combustion CO2 capture from flue gas of the fossil-fuel-fired power plants

to mitigate aversive environmental effects of the greenhouse gases. The other noticeable application is for natural gas sweetening, where CO2 along with the other acid gas H2S are removed to prevent numerous operational problems such as corrosion of the pipelines, poisoning of the catalysts and reducing the heating value of natural gas 5-7. Poly(ethylene oxide) (PEO)-based membranes are principally known as prominent candidates for CO2 separation 8. Due to the presence of polar ether oxygen units/linkages in their backbone, exhibiting high affinity to CO2 molecules via dipole-quadrupole interactions, more CO2 sorption, permeation and subsequently CO2/light gases selectivity values are achieved 2 ACS Paragon Plus Environment

8, 9

. Nevertheless,

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despite the advantages of PEO particularly for CO2 capture, it suffers some drawbacks. Polar polymers like PEO, due to their high chain packing, show considerable tendency for chain crystallization compared to non-polar ones10 . High crystallinity of PEO which results in low gas solubility and permeability is also challenging for preparation of defect-free films as the selective/skin layer in composite membranes11, 12. In fact, PEO crystals restrict the permeation of gas penetrants by increasing penetration path tortuosity. For instance, CO2 permeability of fully amorphous PEO is about 12 times more than that of the semi-crystalline one13. Different approaches have been employed to suppress PEO crystals formation as follows: (1) applying low molecular weight PEO; (2) fabrication of block copolymers with short length PEO in their soft segments and (3) PEO cross-linking 14. Among PEO-based membranes, poly(ether-block-amide) (PEBA) ones have gained extensive attention in the field of gas separation15-20. However, the low molecular weight PEO or poly(ethylene glycol) (PEG)-based membranes such as cross-linked poly(ethylene glycol diacrylate) (XLPEGDA) with high ethylene oxide (~ 82 wt%) and extremely low crystal content (~ 0%), have not been much studied. Lin and Freeman8,

21

studied cross-linked

amorphous PEO membranes using PEGDA. Their solubility and permeation experiments were performed in the temperature ranges of -20-35 ˚C and -20-45 ˚C, respectively. However, most of the industrial processes involving CO2 separation, e.g. from natural gas, synthesis gas and flue gas streams, operate at temperatures above 35 °C. Car et al.22 studied performance of Pebax® 1657 blends with PEG as thin film composite membranes for CO2 separation from gas mixtures containing CH4, N2 and H2. They results demonstrated improved permeabilities (more than two times compared to neat Pebax®) and selectivities by increasing PEG content up to 50%. The improvements were assigned to presence of more ether oxygen units resulted from PEG addition


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and their affinity for CO2 molecules. Moreover, recent publications have revealed that composite membranes prepared with low molecular weight cross-linked PEO, provide high CO2 permeance and selectivity values, as well 12, 23. Therefore, in view of XLPEGDA intrinsic capability in CO2 separation, it seems to be a substantial need to investigate gas transport properties of this low molecular weight PEO-based membrane, more specifically at conditions near to industrial operational conditions. For instance, removal of CO2 from products of the methane reforming process after low temperature shift unit (LTS) in ammonia production companies is performed at 65 ˚C. In addition, natural gas sweetening is usually carried out at about 35-45 ˚C over a high pressure range depending on the gas field pressure even up to 70 bar. Flue gas of the fossil-fuel-fired power plants, i.e. main source of the CO2 emission to atmosphere is also of high temperature but near atmospheric pressure24, 25. On the other hand, dispersion of impermeable inorganic nanoparticles in the polymeric matrix of membranes, both glassy and rubbery ones, is principally conducted to promote separation performance and overcome the trade-off between permeability and selectivity26. This improvement is mainly attributed to combination of both organic and inorganic membranes favorable characteristics, including high permeation, thermal and chemical stability and mechanical strength. For instance, Patel et al.

27

investigated the effects of methacrylate

functionalized fumed silica (FS) incorporation, up to 10 wt% on mechanical and transport properties of XLPEGDA membranes. Their prepared nanocomposite membranes offered improved mechanical properties; however, transport properties and selectivity remained rather invariant. Comer et al.

28

prepared PEGDA/MgO nanocomposite membranes with high loading

contents of the inorganic filler from 0 to 30 wt% and studied glass transition temperature (Tg) 4 ACS Paragon Plus Environment

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and gas transport properties. It was found that incorporation of MgO nanoparticles has increased the rubbery modulus and Tg compared to the neat membranes. However, a systematic decrease in CO2 permeability values with increasing the MgO nanoparticles content, accompanied by a small reduction in CO2/light gas selectivity were observed even at low loadings of the nanoparticles. In the present work, UV photopolymerization technique was applied to synthesize crosslinked amorphous PEO membranes using PEGDA oligomers. The transport properties of a number of gas penetrants in the prepared membranes were thoroughly investigated over temperatures ranging from 35 to75 ˚C (305-348) K, and a pressures up to 16 bar which are both near to industrial conditions. The solubility and diffusivity coefficients along with sorption and diffusion activation energies for H2 and N2 in XLPEGDA were obtained in this work that are not reported elsewhere to the best of our knowledge. TiO2 nanoparticles at loading contents up to 3 wt% were incorporated into PEGDA polymeric matrix, and the effects of their incorporation on gas transport properties were studied, as well.

2. Background The permeability coefficient of a gas penetrant i through a non-porous polymeric membrane , is expressed based on the solution-diffusion theory as follows 29:

=

(1)

where and stand for the solubility and diffusion coefficients of the gas penetrant i in the

polymeric matrix, respectively. The ratio of pure gas permeability coefficients of two gases i and j yields the ideal selectivity of a membrane for gas i over gas j ⁄ , characterizing its ability to separate them30:

⁄ =

=

(2)


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where ⁄ and ⁄ designate the “solubility selectivity” and “diffusivity selectivity”, respectively. The temperature dependency of permeability and solubility coefficients are given by van’t Hoff–Arrhenius type equations31:

= exp = exp

−

(3)

−Δ

= exp

(4)

−

(5)

where , and denote pre-exponential factors, and , and designate the permeation activation energy, sorption enthalpy and diffusion activation energy, respectively. R is the universal gas constant and T shows the absolute temperature. Moreover, is also defined as follows 32:

=

(6)

where is the concentration of gas penetrant i dissolved in the polymeric matrix at upstream surface, and

represents its fugacity in contact with the polymer to consider departure from

ideal gas state at high pressures. The Flory-Huggins (FH) interaction parameter ! is calculated using the following equation as a representative of polymer-penetrant interactions 33:

"#

$%&

= "#' + 1 − ' + !1 − ' *

where

$%&

(7)

and ' denote the fugacity at saturation state and volume fraction of the sorbed

gas penetrant, respectively. ' is expressed as follows 34:

' = +1 +

22414 01 / .

(8)


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where . indicates the penetrant partial molar volume. The infinite dilution solubility

coefficient of a gas penetrant in a polymeric matrix 2 , is determined as follows 35 :

2 = "45 67 →

=

22414 9:; −1 − !2 $%& .

(9)

where !2 stands for the FH interaction parameter at infinite dilution. Additionally, ! is

generally expressed as a function of T and ' as given in Eq. (10) 36:

! = ! +

!1 + !* 1 − '

(10)

where ! , !1 and !* denote the adjustable parameters. The concentration-averaged FH interaction parameter (!̅ ) is calculated according to Eq. (11) 35:

!̅ =

1

'=%>

?

A7BCD

! ' @'

(11)

where '=%> is the maximum gas penetrant volume fraction within the polymeric matrix.

3. Experimental section 3.1. Materials PEGDA (# = 14, where n denotes the average number of the ethylene oxide units according to the supplier’s reported molecular weight) was purchased from Shin-Nakamura Chemical Co. China. 1-hydroxyl-cyclohexyl phenyl ketone (HCPK) as photo-initiator and TiO2 nanoparticles were obtained from Sigma-Aldrich Co., Germany. Carbon dioxide (CO2), methane (CH4), ethylene (C2H4), ethane (C2H6), propane (C3H8), hydrogen (H2) and nitrogen (N2) gases, all with the purity of 99.9 mol% were supplied by Farafan Gas Tehran Co., Iran. The chemicals were used as received. The main characteristics of the chemicals applied are outlined in Table 1. Table 1. The main characteristics of the applied PEGDA, HCPK and TiO2 nanoparticles.


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PEGDA

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TiO2 nanoparticles

HCPK

Property

Value

Molecular weight (g mol-1)

708

Property

Value

Property

Value

204.26

Particle size (nm)

22

Molecular weight (g mol-1) Surface area Specific gravity at 25˚C

1.121

Melting point (˚C)

50

70 (m2 g-1)

Viscosity (mPa s) at 25˚C

106

Melting point (˚C)

12-17

Tg (˚C)

-23

Density (g cm-3) Flash point (˚C)

164

3.9

TiO2 content > 99% (wt%)

3.2. Experimental setups Sorption/solubility measurements were carried out based on the pressure decay approach using a dual volume method

37, 38

. Pure gas permeation measurements were conducted applying

an apparatus using the constant volume/variable pressure method. The details of the applied experimental set ups have been described elsewhere

39, 40

and are not given here for the sake of

conciseness. 3.3. Membrane preparation At the first step, 0.1 wt% HCPK based on PEGDA weight was added to pure PEGDA polymer as photo-initiator. It was then magnetically stirred (model: Heidolph) at ambient temperature to completely dissolve HCPK and obtain a homogenous solution for at least 30 min. The prepared solution was then degassed using an ultrasonic bath (model: Sonorex Digitec) for about 15 min to eliminate bubbles. In the next step, the degassed solution was sandwiched between two clean and dry glass plates separated by spacers to control the polymer film thickness. It was then exposed to 312 nm ultraviolet (UV) light in a UV chamber for about 5


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min. Eventually, after cross-linking, the prepared solid films with thickness of about 250 µm were separated from the glass plates. To prepare nanocomposite membranes, TiO2 nanoparticles at the loading of interest based on the weight of the PEGDA polymeric matrix, were dispersed in PEGDA prior to adding the initiator (HCPK). The solution was then sonicated for 20 min to ensure that the TiO2 nanoparticles were homogeneously dispersed. Thereafter, the membrane preparation procedure was followed exactly similar to that of the neat ones. 3.4. Membrane characterization The prepared XLPEGDA and XLPEGDA/TiO2 nanocomposite membranes, following the procedure described in previous section, were characterized by different analyses including scanning electron microscopy energy dispersive X-ray (SEM-EDX), Fourier transform infrared (FTIR) spectroscopy, differential scanning calorimetry (DSC) and density analysis. The applied instruments for SEM-EDX, FTIR, and DSC were VEGA-TESCAN, BRUKER (VERTEX 80) spectrometer and METTLER TOLEDO DSC 822e. The buoyancy method was employed for determination of density values.

4. Results and discussion 4.1. Gas sorption in XLPEGDA membranes The obtained sorption isotherms of gas penetrants through prepared neat XLPEGDA membranes at the temperatures of 308, 318, 328, 338 and 348 K (35-75) °C are presented in Figure 1. The reported data by Lin and Freeman8 at 308 K (35°C) are shown in this figure, as well. As can be seen, the data points obtained in this study match the reported ones in the literature. Moreover, due to the low sorption of N2 and H2 light gases into XLPEGDA caused by their low condensability, their sorption isotherms were plotted using only two data points. 9 ACS Paragon Plus Environment

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Figure 1. The sorption isotherms of gas penetrants over temperature range of 308-348 K through XLPEGDA membrane; red markers display reported values at 308 K in the literature 8.

A relatively simple method was employed to ensure the consistency of N2 and H2 sorption data obtained in this study via comparison of the solubility ratio of these two gases in XLPEGDA with the other rubbery polymers and liquids. The chemical structure of liquid solvents affects their gas sorption properties and these systematic effects are considered as a guideline for designing solubility selective polymers. Indeed, solubility selectivity of polymers is rather similar to that of liquid solvents with same chemical structures41. One of the rational justifications for the applied approach is that the Flory-Huggins model at the infinite dilution state is in accordance with the obtained predictions from the regular-solution theory for solubility parameters42, 43.


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The solubility ratio of N2 and H2 gases in many liquids has a limited range of 1.2 to 2.2 44. For instance, the reported values for this ratio is 1.7 and 1.4 in alcohols and carbon disulfide, respectively whereas for hydrocarbon liquids, it lies between 1.9 to 2.2 at 25˚C and 1 atm

44

.

Thus, the obtained value in the present study which is equal to 1.41, could be considered rational. Moreover, the value of this ratio for PDMS is 1.82, and for different grades of Pebax® consisting of 80PTMEO/PA12 (Pebax® 2533), 53PTMEO/PA12 (Pebax® 4033), 55PEO/PA12 (Pebax® 1074) and 57PEO/PA6 (Pebax® 1657), its values are 1.48, 1.25, 1.71 and 1.77, respectively 34, 45. PA12, PA6 and PTMEO represent nylon 12, nylon 6 and poly(tetramethylene oxide), respectively. 4.1.1. Solubility coefficients of the gas penetrants in XLPEGDA Figure 2 parts (a) and (b) illustrates the values of infinite dilution solubility 2 and solubility selectivity of the investigated gas penetrants through prepared XLPEGDA membranes, respectively.


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Figure 2. Plot of (a) 2 and (b) infinite dilution solubility selectivity of the gas penetrants in prepared XLPEGDA membranes over temperature range of 308-348 K.

As depicted in Figure 2 (a), CO2 and H2 have the highest and lowest 2 values, respectively. This behavior can be attributed to the considerable favorable interactions between CO2 molecules and ether oxygen units in the polymeric matrix of XLPEGDA and conversely the extremely low condensability of H2 molecules 14, 40, 46. Indeed, the solubility of a gas penetrant in a given polymer, is mainly affected by its condensability which is typically characterized by its normal boiling point temperature E , critical temperature F and/or the Lennard-Jones

potential well depth G ⁄H, likewise interactions between the solute and polymer. This

interaction is typically determined by the FH interaction parameter 47. Regarding to our previous study48, enthalpy of condensation of the gas penetrants can be related to their normal boiling point temperatures via our proposed correlation, as well. If the gas penetrants do not exhibit any specific interactions with the polymeric matrix, their solubility is mainly increased by increasing their condensability. If this assumption holds for all the investigated gas penetrants in this work, their solubility would be expected to obey the following trend:


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C3H8> C2H6 > CO2> C2H4> CH4> N2> H2 In fact, this trend is obeyed in PDMS where CO2 does not exhibit any particular interactions with the polymeric matrix

35

. Nonetheless, when there exists outstanding interactions between

the polymeric matrix and gas penetrant, the order of the solubility values would depend on both polymer-penetrant interactions as well as penetrant condensability. In this research, it was found that the solubility values through XLPEGDA obey the following trend as also depicted in Figure 2 (a): CO2> C3H8> C2H4> C2H6> CH4> N2> H2 CO2 molecules have the highest solubility among other gases considered in this study, even compared to C3H8 with higher critical temperature. In fact, higher solubility of CO2 can be justified with its particular interaction with the polymeric matrix of XLPEGDA, i.e. the presence of the polar ether oxygen units. Indeed, optimum materials for achieving the highest CO2 solubility and also solubility selectivity of CO2 over light gases should have a solubility parameter of about 21.8 MP0.5. Moreover, Among polar units in polymeric backbones, polar ether oxygen units have the highest CO2 solubility compared to those of other polar units such as nitriles, acetates, carbonyls and amides14. Therefore, high CO2 solubility values observed in this work is mainly related to presence of the ether oxygen units having a solubility parameter near the optimum value”. C2H4 is also an exception that with a lower condensability compared to C2H6, exhibits a higher solubility coefficient. This unexpected behavior can be justified by interaction between double bond of the C2H4 molecule and polar ether oxygen units in the XLPEGDA polymer49. Furthermore, for condensable gas penetrants i.e. CO2, C3H8, C2H4 and C2H6, solubility coefficients descend as temperature ascends. This reduction is due to the lower condensability at 14 ACS Paragon Plus Environment

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elevated temperatures. In contrast, increasing temperature does not have a significant influence on the solubility coefficient of light gas penetrants, such as H2, N2 and CH4 which are already at their supercritical states. As a consequence, it is expected that the solubility selectivity values of CO2 against the other investigated gases decrease at higher temperatures. For instance, by increasing temperature from 308 to 348 K, the infinite dilution solubility selectivity values for CO2/N2, CO2/H2, CO2/CH4, CO2/C2H4 and CO2/C2H6 descend from 25.1, 40.9, 11.2, 2.3 and 3.1 to 20.3, 12.9, 6.7, 1.8 and 2.2, respectively. In Table 2 the values of infinite dilution solubility of considered gas penetrants in XLPEGDA, PDMS, two grades of poly(ether block amide) (PEBA) including Pebax® 1657 and Pebax® 1074, along with semi-crystalline poly(ethylene oxide) (SC-PEO) and amorphous PEO (Am-PEO) at the temperature of 308 K are given. As can be seen, the obtained results in this study are in accordance with the literature data for XLPEGDA 8. Table 2. Values of the infinite dilution solubility (cm3 (STP) cm-3 bar-1) of the gas penetrants in XLPEGDA, PDMS, Pebax® 1657, Pebax® 1074, SC-PEO and Am-PEO at 308 K. XLPEGDA Lin and Gas

This

®

Pebax PDMS 35

Freeman penetrant

®

Pebax

1657

50

1074

SC-PEO 49

Am-PEO 49

50

work 8

CO2

1.35

1.4

1.27

0.881

0.914

0.37

1.3

H2

0.034

__

0.049

0.0057

0.0085

__

__

N2

0.048

__

0.089

0.010

0.0144

__

__

CH4

0.12

0.13

0.415

0.0564

0.067

0.078

0.28

C2H4

0.58

0.64

__

__

__

0.17

0.62


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C2H6

0.44

0.52

2.172

0.305

0.422

0.12

0.41

C3H8

0.77

1.2

4.936

0.854

1.2

0.34

1.3

As shown in Table 2, for the all gas penetrants studied, the solubility values within PDMS are higher than those in XLPEGDA except for CO2. Higher solubility of CO2 in XLPEGDA is due to its favorable interactions with ether oxygen units within the XLPEGDA polymeric matrix as described earlier. On the other hand, high solubility of the gas penetrants through PDMS can be referred to the concept of polarity and cohesive energy density (CED). As the polymer polarity decreases, CED is also decreased which leads to lessening the energy required to open a hole of molecular size in the polymeric matrix as a sorption site . Hence, the solubility of gas penetrants would be increased through nonpolar rubbery polymers such as PDMS compared to the polar ones like PEO based polymers. XLPEGDA contains 82 wt% PEO 8, so the solubility values of gas penetrants are expected to be almost similar to those of Am-PEO. It should be pointed out that crystallization of ether oxygen segments restricts solubility through a semi-crystalline polymeric matrix; thereby, solubility values through SC-PEO are much lower than the other considered polymers, as given in Table 2. Pebax® 1657 and 1074 contain 60 and 45 wt% PEO in their chemical structure, respectively. Hence, it is expected the solubility values in these two copolymers stand between Am-PEO (or XLPEGDA) and SC-PEO. Additionally, higher solubility values of the penetrants in Pebax® 1074 compared to Pebax® 1657, can be relegated to the lower Tg of PA12 (hard block of Pebax® 1074) versus PA6 (hard block of Pebax® 1657). The reported Tg values for pure PA12 and PA6 are 36 °C and 51°C, respectively 51. As stated before, the FH interaction parameter represents the polymer-penetrant interaction in a way that lower values of this parameter shows higher favorable interactions and vice a versa 35.


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In Figure 3 values of ! obtained for the gas penetrants of this research, have been plotted against the sorbed penetrant volume fractions at 308 K.

Figure 3. The obtained FH interaction parameter of the gas penetrants in XLPEGDA at 308 K.

As evidenced by this figure, ! values for the investigated gas penetrants in this work obey the following trend: CO2< N2< C2H4< CH4< C2H6< C3H8< H2 This trend shows that CO2 has the most favorable interaction compared to the other gas penetrants with XLPEGDA due to its polarizability and high level of interaction with the polar ether oxygen units in XLPEGDA. In contrast, H2 with the highest value of FH parameter has the lowest level of solubility. Additionally, lower FH parameter of C2H4 compared to CH4, C2H6 and C3H8 can be explained with the interaction of double bonds in the olefins and polar ether oxygen units in XLPEGDA. Interestingly, it was observed that FH parameter of H2 in XLPEGDA and PDMS are almost equal, while FH parameter of N2 in XLPEGDA is considerably higher than that in PDMS. This observation might seem logical due to the fact that XLPEGDA is of higher polarity and CED compared to PDMS. 17 ACS Paragon Plus Environment

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In Table 3 the concentration-averaged FH interaction parameter of the gas penetrants (χJ ) at 308 K for PEGDA and PDMS calculated by Eq. (11) along with the literature data 8, 35 are given. As can be seen, the calculated values in this work and the reported ones by Lin and Freeman are in good accordance. Table 3. Comparison of χJ values for XLPEGDA and PDMS. XLPEGDA PDMS 35

Gas penetrant This work

CO2

H2

N2

CH4

C2H4

C2H6

!̅ = 0.96

!̅ = 2.74

!̅ = 1.43

!̅ = 1.91

!̅ = 1.61

!̅ = 2.25

Lin and Freeman

! !1

!* ! !1

!* ! !1

!* ! !1

!* ! !1

!* ! !1

8

3.92 372.78

!̅ = 0.97

!̅ = 0.585

-

!̅ = 2.563

-

!̅ = 0.678

!̅ = 1.37

!̅ = 0.139

!̅ = 1.49

-

!̅ = 2.10

-

-1.82 -2.03 751.26 2.57 -0.00 1080.03 -2.03 -5.88 1081.09 4.42 -0.20 444.06 0.457 1.38 767.06


8

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Energy & Fuels

!* !̅ = 2.35

C3H8

! !1

!*

-1.58 -0.50 !̅ = 2.19

927.18

!̅ = 0.247

-0.10

Figure 4 depicts the obtained values of 2 of the gas penetrants in this study from 308 to 348 K along with the data of Lin and Freeman

8

in the temperature range of 253-308 K for the

selected gas penetrants versus reciprocal of the absolute temperature. The obtained data of Lin and Freeman

8

were extrapolated as demonstrated in this figure via dotted lines using the

reported values for ∆$ and 2 by these researchers. This figure might be used as a reference for the whole temperature range of 253-308 K.

Figure 4. Infinite dilution solubility coefficients of the gas penetrants over temperature range of 308348 K of this work and 253-308 K in the literature 8.

4.1.2. The relation between solubility coefficient and heat of sorption 19 ACS Paragon Plus Environment

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Page 20 of 44

In Figure 5, values of against −∆$ for the gas penetrants have been depicted via filled black circles, whereas the reported values by Lin and Freeman

8

are shown by unfilled red

triangles.

Figure 5. Plot of versus −∆$ ; filled circles: this study ( * = 0.94); unfilled triangles: Lin and

Freeman 8 * = 0.92; filled and unfilled black rectangles: upper and lower bounds, respectively 52.

A practical point that can be deduced from this figure is that a linear relationship can be established between the logarithm of and −∆$ . The following correlations for and ∆$ in rubbers (elastomers) have been already developed by D.W. van krevelen 52:

"TU = −5.5 − 0.005G ⁄H V 0.8

(12)

100W ∆ = 1 − 0.01G ⁄H V 0.5

(13)

"# = X − Y∆

(14)

As a result, might be linearly correlated to −∆$ as also shown in Figure 5:


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Energy & Fuels

where a and b are fitting parameters. For all the considered gas penetrants in this study, except CO2 regarding to Eqs. (12) and (13), the parameter a can vary from -1.036 to -3.569 and parameter b is a constant value equal to 0.138

52, 53

. The upper and lower bounds for

against ∆$ , based on Eqs. (12) and (13), using V0.8 and V0.5, respectively, have been also shown in Figure 5 by filled and unfilled black rectangles. Moreover, the acceptable areas for H2 and CH4 have been specified by yellow and pink rectangles in this figure. As depicted, the experimental points for these two penetrants obtained in this work, lie in these expected zones. The obtained values for the parameters a and b appeared in Eq. (14), were found to be -3.224 and 0.131, respectively via linear fitting * = 0.94 of the experimental data. In Table 4, the enthalpies of sorption (∆ ), mixing (∆= ) and condensation (∆F ) for the

investigated gas penetrants are tabulated. Generally, ∆= of the solutes within the polymers obeys the Hildebrand equation as follows 54:

∆= = Z** .* [* − [1 * = !1

(15)

where Z* is the volume fraction of the polymer and with assuming dilution of the solutes, it

can be considered equal to unity. The values of !1 have been already given in Table 3. Lin and

freeman8 obtained ∆ values from the sorption experiments, calculated ∆F at 273 K and then estimated ∆= by subtracting ∆F from ∆ (∆= = ∆ − ∆F ) 8. In this study, ∆ values

were obtained from the sorption experiments, ∆= values were calculated via the Hildebrand

equation, i.e. Eq. (14) and ∆F values were then attained by subtracting ∆= from ∆ (∆F =

∆ − ∆= ). Moreover, to assure the accuracy of ∆ = values obtained from Eq. (14), we also calculated ∆F values via our developed correlation in our previous contribution48 using normal

boiling point temperatures.


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Page 22 of 44

Table 4. The enthalpies of sorption (∆ ), mixing (∆= ) and condensation (∆F ) of the gas penetrants in XLPEGDA.

∆\] (kJ mol-1)

∆\_

∆\^(kJ mol-1)

(kJ mol-1)

This

This

Gas

Lin and Lin and

penetrant

Lin and

This work

∆\_ 48 (kJ mol-1)

Freeman Freeman

8

work

Freeman

8

work 8

CO2

-16.29±1.3

-17±1

-3.1

-6.8±1

-13.2±1.3

-10.2

-14.71

H2

-0.27±0.12

--

6.2

--

-6.5±0.12

--

-2.66

N2

-0.86±0.25

--

8.9

--

-9.5±0.25

--

-5.15

CH4

-3.60±0.89

-1±2

9.0

--

--

-10.89

-5.4

-17.95

-9.1

-17.21

-16.5

-19.54

12.6±0.89 -

-7.5±1.4

C2H4

3.7

-2.1±1.4

10.03±1.47

13.7±1.47 -

C2H6

-8.62±1.10

-6.4±1.4

6.4

2.7±1.4 15.0±1.10

-

-9.4±1.4

C3H8

7.7

7.1±1.4

12.92±2.03

20.6±2.03

* calculated at triple point temperature of CO2

As can be seen from Table 4, the obtained values for ∆F in this work are rather in

accordance with the developed correlation for ∆F in our previous work48. Therefore, it can be concluded that the estimated values for ∆= by the Hildebrand equation are reliable.

4.2. Gas permeation and diffusion in XLPEGDA membranes


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Energy & Fuels

As demonstrated earlier in Figure 2, by considering the values of solubility selectivity of various gas penetrants in XLPEGDA, it can be deduced that the prepared membranes are promising for CO2 separation from light gases, i.e. H2, N2 and CH4. As a matter of fact, separation of CO2 from these gases is of high value for CO2 removal from synthesis gas, flue gas and natural gas streams as stated earlier. Therefore, the permeability measurements were carried out for CO2, H2, N2 and CH4 gases over a temperature and pressure ranges of 308 to 348 K and 2 to 16 bar, respectively. Figure 6 shows CO2 permeability in Barrer (1 Barrer = 10-10 cm3 (STP) cm cm-2 s-1 cmHg-1) versus transmembrane pressure difference ∆ in bar, along with the values of ideal selectivity ⁄ of CO2 over H2, CH4 and N2 gases.


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Page 24 of 44

Page 25 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

Figure 6. Plots of CO2 permeability and ideal selectivity over light gases versus pressure in XLPEGDA membranes over the temperature range of 308-348 K.

As can be seen, at all temperatures investigated, the CO2 permeability ascends with increase of ∆ with more intensity at lower temperatures. This observation can be justified by plasticization effect of CO2 on the polymeric matrix of XLPEGDA. In fact, when considerable amount of a diluent gas like CO2 is absorbed by a rubbery polymer, the polymer Tg decreases which brings about more polymer chain flexibility as well as more fractional free volume (FFV) 55

. However, this behavior is suppressed at higher temperatures due to lower gas solubility at

these temperatures. Generally, the effect of pressure/fugacity on permeability of a gas penetrant is considered via the following equation 56:

= 9:; 5∆

(16)

where ` is the infinite dilution permeability, 5 is an adjustable coefficient at constant

temperature and ∆ is the penetrant fugacity difference between upstream and downstream of the membrane. This equation is often simplified as follows 32:

= 40 1 + 5 *

(17)

Table 5 presents the obtained values of ` and 5 for the considered gas penetrants over the

entire selected temperature and pressure ranges of this work. Table 5. Obtained values of ` (Barrer) and 5 (bar-1) for the selected gas penetrants in this work. Temperature (K)

Gas Parameter penetrant

CO2

abc

d × fcg

308

318

328

338

348

125.1

182.7

255.5

316.3

354.9

136

7.8

5.5

4.5

2.6


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

abc

H2

15.2

23.4

33.7

53.4

80.1

-5

-7

-0.7

-6

-1

5

9.1

14.4

25.9

34.2

0

-9

-12

-13

-6

abc

2.1

4.0

7.3

14.2

14.0

d × fcg

-5

-3

-7

5.4

23

d × fcg

CH4

N2

Page 26 of 44

abc

d × fcg

In Table 6 values of the gas penetrants permeability in XLPEGDA, PDMS and different grades of Pebax® including 1074, 1657, 2533 and 3533 are summarized at 308 K that might be used for comparison purposes. ®

Table 6. Permeability of the gas penetrants (Barrer) through XLPEGDA, PDMS, Pebax 1074, 1657, 2533 and 3533 at 308 K. XLPEGDA Gas

Lin and

Pebax

®

Pebax

®

Pebax

®

1074 45

1657 16

2533 16

3533 31

PDMS

This penetrant

Pebax®

Freeman work 8

CO2

125.1

111.8

3800

120

76.2

257

132

H2

15.2

13.2

890

12.2

7.8

47

20

N2

2.1

2.6

400

2.3

1.3

9.1

2

CH4

5.0

6.0

1200

__

4.1

31

__

The higher permeability values through PDMS compared to XLPEGDA is ascribed to higher solubility and diffusivity of the gas penetrants through this polymer. Higher solubility is owing to the lower required energy for mixing of the penetrants, while higher level of diffusivity comes from the higher chain mobility of PDMS compared to XLPEGDA. Moreover, permeability of CO2 through XLPEGDA is greater than those of Pebax® grades with PEO block in their soft


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Energy & Fuels

segment (i.e., Pebax® 1074 and Pebax® 1657) due to the higher content of ether oxygen units in XLPEGDA. Additionally, the permeability of light gases through these grades of Pebax® are lower than XLPEGDA. This reduction is a consequence of the presence of polar amide groups in the matrix of Pebax® that increases CED of the polymeric matrix. On the other hand, Pebax® grades with PTMEO as their soft segment (i.e., Pebax® 2533 and Pebax® 3533) have higher gas permeability because of their lower Tg and higher chain mobility in their polymeric matrix, as presented in Table 6. In the next step, it was initially attempted to determine the diffusion coefficients of the gas penetrants regarding to the concept of time-lag which is defined as intercept of steady-state region of the amount of accumulated pressure versus time axis in the constant volume method 57:

"* = 6h

(18)

where " and h designate the thickness of membrane and time-lag, respectively. Figure 7 displays the accumulated pressure of permeated CO2, H2, CH4 and N2 gases versus time at the temperature and pressure of 308 K and 2 bar, respectively.


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Figure 7. Accumulated pressure of the permeated gases as a function of time in a transient permeation experiment at 308 K and 2 bar.

As can be seen, the curves for H2, CH4 and N2 are slightly concave to the time axis. This behavior was not observed for CO2 which can be mainly attributed to higher permeability of CO2 compared to the other investigated gases. The observed behavior of H2, CH4 and N2 might have been due to the compaction of XLPEGDA at initial moments of gas contact with the membrane surface. Therefore, the diffusion coefficients of the gas penetrants were preferred to be calculated using the well-known solution-diffusion theory, i.e. = ⁄ . Figure 8 demonstrates the plot of infinite dilution permeability and diffusion coefficients versus 1⁄.


Page 28 of 44

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Energy & Fuels

Figure 8. Temperature dependency of the infinite dilution permeability and diffusion coefficients over 308-348 K, black lines and markers: this study; red lines and markers: Lin and Freeman21.

As can be seen, permeability and diffusion coefficients of the penetrants obeys van't Hoff– Arrhenius equation over the considered range of temperature as given by Eqs. (3) and (5), respectively. The reported values for ` and by Lin and Freeman

21

at temperatures ranging

from 253-318 K were extrapolated to predict values over the considered range of temperature

in this study. The predicted values are indicated by red dashed lines in Figure 8. A similar procedure was applied to predict diffusivity of CO2 and CH4 from 308 to 348 K using the reported values for ` and in the literature 21.

In Table 7 the values of , , and for CO2, N2, H2 and CH4 are given. As can be

seen, the activation energies of permeation and diffusion at lower temperatures are greater than the correspondent values at higher temperatures. Therefore, for XLPEGDA polymer, extrapolating the obtained results at lower temperature for predicting the gas transport properties at higher temperature would not provide accurate results.


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Page 30 of 44

Table 7. Activation energies and pre-exponential factors of permeation and diffusion through XLPEGDA. Temperature Parameter

CO2

N2

H2

CH4

23.6

45.4

40

43.7

0.0137

1.16

0.274

1.34

39

57

43

56

4.6

120

2.6

190

40.1

50

42.3

47.6

0.47

10.5

4.98

3.98

56

-

-

57

1.5

-

-

1.8

rang (K)

ij (kJ mol-1) (This work)

abc × fc0k (Barrer) (This work)

308-348

abc × fc0k (Barrer) 21

253-318

pbc × fc0k qdr s0f (This work)

308-348

ij lm dno0f 21

ip (kJ mol-1) (This work) ip lm dno0f 21

pbc × fc0g qdr s0f 21

253-318

4.3. Incorporation of TiO2 nanoparticles An investigation was carried out to study the effect of incorporation of inorganic TiO2 nanoparticles into the polymeric matrix of PEGDA on transport properties of the selected gas penetrants. For this purpose, adsorption isotherms of the gases, consisting of CO2, CH4, N2 and H2 on surface of the TiO2 nanoparticles were initially investigated as follows. 4.3.1. Gas adsorption onto TiO2 nanoparticles The obtained adsorption isotherms of investigated gases on the surface of TiO2 nanoparticles at 308 K are shown in Figure 9.


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Energy & Fuels

Figure 9. Adsorption isotherms of CO2, CH4, N2 and H2 onto TiO2 nanoparticles at 308 K.

As can be seen, adsorption capability of TiO2 nanoparticles for all the four investigated gases is considerably higher than that of XLPEGDA (c.f. Figure 1). The adsorption values of CO2, CH4, N2 and H2 onto the TiO2 nanoparticles are 11, 87, 197, and 124 times greater than the corresponding values for XLPEGDA, respectively. The obtained experimental data were correlated by applying the empirical Freundlich model 58 as follows:

t = u;1⁄v

(19)

where t and ; stand for the equilibrium concentration of gases on surface of the

nanoparticles and equilibrium pressure, respectively. u and # represent temperature dependent

fitting parameters. It is worth to note that, the Freundlich model does not constrain the amount of gas adsorbed with increasing pressure in contrast to the Langmuir isotherm. Table 8 presents the obtained Freundlich fitting parameters for the adsorption of selected gases onto the TiO2 nanoparticles. Table 8. Obtained Freundlich fitting parameters for adsorption of gases of interest onto TiO2 nanoparticles at 308 K. 31 ACS Paragon Plus Environment

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Page 32 of 44

Gas

K (cm3 (STP) cm-3 TiO2 bar-1/n)

n

CO2

14.97

1.54

CH4

10.12

1.62

N2

9.44

1.92

H2

4.64

1.16

4.3.2. Characterization of the prepared membranes The cross-sectional morphologies and dispersion of the TiO2 nanoparticles were studied by SEM-EDX analysis. The obtained images are shown in Figure 10.

Figure 10. SEM-EDX images of the prepared XLPEGDA and XLPEGDA/TiO2 nanocomposite membranes with different concentrations of TiO2 nanoparticles: (a) 0 wt%; (b) 0.75 wt%; (c) 1.5 wt%; (d) 3 wt%.

As can be seen, there is no conspicuous agglomeration of the incorporated TiO2 nanoparticles, more specifically at loading contents of 0, 0.75 and 1.5 wt%. However, at 3 wt% i.e. the highest loading content of the TiO2 nanoparticles investigated in this work, some agglomeration was observed. The FTIR and DSC spectrums are not presented here for brevity. The disappearance of 32 ACS Paragon Plus Environment

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Energy & Fuels

the acrylate double bonds in the FTIR spectrum, i.e. the peaks at 812, 1190 and 1410 cm-1, approved the cross-linking reaction has been occurred with a high conversion level close to 100% as also stated in the literature

13, 29, 59

. This high level of conversion comes from high

propagation rate constant of the cross-linking reaction as well as its low termination rate constant60. Indeed these phenomena makes PEGDA suitable for rather uniform dispersion of several kinds of materials such as nanoparticles. The measured values of Tg and density for the prepared membranes are given in Table 9. Table 9. Results of DSC and density analyses of the prepared XLPEGDA and XLPEGDA/TiO2 membranes. Membrane

0 wt%

0.75 wt%

1.5 wt%

2.25 wt%

3 wt%

Tg (℃)

-41.3

-41.6

-42.0

-43.1

-45.2

Density (g cm-3)

1.180

1.187

1.193

1.199

1.206

As can be deduced from Table 9, incorporation of the TiO2 nanoparticles has slightly decreased Tg of the prepared nanocomposite membranes compared to the neat one, leading to an increase in chain mobility of the polymeric matrix. This slight reduction in Tg values might be assigned to a few number of unreacted PEGDA monomers around the TiO2 nanoparticles resulting in an increase in chain mobility of the polymeric matrix. Moreover, the prepared nanocomposite membranes have higher density values which can be attributed to the higher density of TiO2 nanoparticles than that of PEGDA as also indicated in Table 1. 4.3.3. Gas sorption in PEGDA/TiO2 nanocomposite membranes


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Page 34 of 44

With assumption of no interaction between gas sorption into the polymeric matrix and its adsorption onto the TiO2 nanoparticles, gas concentration through nanocomposite membranes vF , can be estimated by applying a mixing rule as given in Eq. (20) 61:

vF = ∅y y + ∅t t + ∅z z

(20)

where ∅ and denote the volume fraction and gas penetrant concentration, respectively.

Subscripts nc, p, F and { stand for the nanocomposite membrane, polymer, filler (i.e., nanoparticles) and voids, respectively. ∅y , ∅t and ∅z are calculated as follows 62: 'y = 't =

|y ⁄}y |y ⁄}y = = |y }vF ⁄}y |t ⁄}t + |y ⁄}y + .z 1⁄}vF

(21)

|t ⁄}t |t ⁄}t = = |t }vF ⁄}t |t ⁄}t + |y ⁄}y + .z 1⁄}vF

(22)

'z = 1 − 'y − 't

(23)

where w and } represent the weight percent and density, respectively. It should be noted that

for all the prepared nanocomposite membranes, volume fraction values of the TiO2 nanoparticles were less than 1 vol.% (3 wt%). By neglecting 'z , i.e. with assumption of ideal mixing of the polymer and nanoparticles, Eq. (20) could be simplified as follows:

vF = ∅y y + ∅t t

(24)

Substitution of Eqs. (6) and (19) into Eq. (24), yields:

vF = ∅y + ∅t u;1⁄v

(25)

Figure 11 depicts the concentration of gas penetrants through prepared XLPEGDA/TiO2 nanocomposite membranes against the loading content of TiO2 nanoparticles at 308 K and 4 bar. The markers display experimental data, and the solid lines demonstrate fitted lines to these data. The dashed lines in this figure, show the predicted values by Eq. (25). The values of CO2


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Energy & Fuels

concentration are indicated using primary x and y axis in black and those of CH4, N2 and H2 are shown on secondary x and y axes in red.

Figure 11. Plot of gas concentration in XLPEGDA/TiO2 nanocomposite membranes at 308 K and 4 bar with the dashed lines obtained from Eq. (25).

The value of S in the first term of the right hand side of Eq. (25) was calculated by linear interpolation of the solubility coefficients of gases versus the equilibrium pressure of sorption through the XLPEGDA membranes. The second term was obtained by extrapolating the Freundlich model, i.e., Eq. (19). It was found that incorporation of TiO2 nanoparticles has enhanced the concentration of gas penetrants sorbed into the nanocomposite membranes. For instance, experiments revealed that by incorporating 3 wt% TiO2 nanoparticles, the concentrations of CO2, CH4, N2 and H2 increase about 18.5, 18.2, 10 and 8.8% compared to the neat XLPEGDA membrane, respectively. According to Eq. (25), it is expected that the concentration enhancement for CO2, CH4, N2 and H2 to be equal to 5, 47, 86 and 110%, respectively. However, for light gases, including CH4, N2 35 ACS Paragon Plus Environment

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Page 36 of 44

and H2, the measured concentrations are less than the predicted values as also shown in Figure 11. This discrepancy can be described by the fact that low solubility of light gases through the polymeric matrix restricts capability of nanoparticles for enchasing concentration of these penetrants through the filled membranes compared to the neat one. In contrast, the measured CO2 concentrations through the nanocomposite membranes with a loading content higher than about 2 wt%, exhibit a positive deviation from the predicted values by Eq. (25). As mentioned earlier, incorporation of TiO2 nanoparticles slightly descends the polymer Tg value and subsequently provides greater chain mobility for nanocomposite membranes in comparison to the neat polymeric matrix of XLPEGDA. This may descend the required energy for making a hole to mix CO2 molecule within the polymeric matrix. Therefore, it can be assumed that ∆= of the gas molecules is slightly reduced in presence of the TiO2 nanoparticles, resulting in higher concentrations of the sorbed CO2 in the nanocomposite membrane. 4.3.4. Gas permeation in XLPEGDA/TiO2 nanocomposite membranes Generally, presence of impermeable inorganic fillers, results in reduction of gas permeability. The Maxwell's model

28, 61, 63-66

is commonly employed to predict the permeability of gas

penetrants in heterogeneous materials as follows:

vF =

1 − 't 1 + 0.5't

(26)

where vF and stand for the gas permeability of filled/composite and unfilled/neat

polymers, respectively. 't is the volume fraction of the impermeable filler embedded into the polymeric matrix. Based on this model, presence of impermeable fillers reduces the permeability of gas penetrants mainly due to: (a) reduction in solubility by reducing the polymer content, indicated in the numerator of Eq. (26), and (b) reduction in diffusivity by increasing the path way 36 ACS Paragon Plus Environment

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Energy & Fuels

of diffusion, indicated in the denominator of Eq. (26). However, observations in this study did not obey the Maxwell’s model. In other words, presence of TiO2 nanoparticles, increased permeability of the gas penetrants through XLPEGDA/TiO2 nanocomposite membranes as shown in Figure 12.

Figure 12. The ratio of permeability in nanocomposite XLPEGDA/TiO2 membrane vF to that in the neat XLPEGDA .

This converse behavior has been also reported for some other polymers and nanoparticles which has been mainly relegated to increased fractional free volume (FFV) and diffusion coefficients of the gas penetrants 67-69. Additionally, as stated earlier, due to the slight decrease in Tg of the prepared nanocomposite membranes as shown in Table 9, the resulted more chain mobility/flexibility of the polymeric matrix, also brings about more permeation compared to the neat membrane. On the other hand, high reversible sorption of CO2 by TiO2 nanoparticles as depicted in Figure 9 has favorable influence on its permeability, as well.


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Page 38 of 44

In order to assist gaining a better insight into conducted experiments in the present study, the gas transport properties, consisting of permeability, solubility and diffusivity through the prepared neat and nanocomposite membranes have been presented in Table 10. Table 10. Gas transport properties of prepared XLPEGDA and XLPEGDA/TiO2 membranes at 308 K and 4 bar. TiO2 content

CO2

H2

CH4

(wt%)

Permeability (Barrer)

0

125.1

15.2

5

0.75

137.8

15.8

1.5

145.2

2.25 3

N2

CO2

H2

CH4

N2

CO2

H2

CH4

N2

Solubility (cm3 cm-3 bar-1)

Diffusivity (cm2 s-1)×107

2.1

1.37

0.034

0.11

0.050

6.85

33.53

3.41

3.15

5.1

2.1

1.41

0.034

0.11

0.050

7.33

34.85

3.48

3.15

16.5

5.2

2.2

1.43

0.032

0.12

0.052

7.62

38.67

3.25

3.17

159.4

17.3

5.5

2.3

1.51

0.035

0.13

0.053

7.92

37.07

3.17

3.25

173.5

18.1

6

2.3

1.63

0.037

0.13

0.055

7.98

36.69

3.46

3.14

As can be deduced from this table, by incorporation of the TiO2 nanoparticles, the permeability coefficients of CO2, N2, H2 and CH4 improve with the maximum values at 3 wt% (1 vol.%) loading of the TiO2 nanoparticles by 39, 10, 22 and 20%, respectively. As stated earlier, TiO2 nanoparticles owing to their considerable adsorption capacity, increase concentration of the sorbed penetrants in prepared nanocomposite membranes. Table 10 also shows that with incorporation of 3 wt% TiO2 nanoparticles, the solubility coefficients of CO2, N2, H2 and CH4 increase 18.5, 10.0, 8.8 and 18.2%, respectively. Moreover, CO2 selectivity values over H2, CH4 and N2 have been also enhanced by 16.2, 15.6 and 26.6%, respectively. As can be seen from Table 10, in contrast to the Maxwell's model, the diffusivity coefficients of gas penetrants through filled membranes are slightly higher than those of the neat ones. Indeed, for this lack of agreement, two reasons can be given as follows. Firstly, interparticle


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spacing between TiO2 nanoparticles (@ ) with the assumption of spherical particles, can be theoretically calculated based on the nanoparticle diameter (@y ) as given in Eq. (27) 70:

@ = @y ~

1⁄W − 1 6't

(27)

Based on this equation, for maximum loading of TiO2 nanoparticles investigated in this work, i.e. 3 wt%, @ value reaches 376 nm which compared to the kinetic diameter of gas penetrants cannot restrict their diffusion rate. Secondly, result of the DSC analysis has shown that the presence of nanoparticles slightly descends Tg of the prepared nanocomposite membranes compared to the neat one. Therefore, the higher diffusivity through the filled membranes can be attributed to their higher chain mobility. Figure 13 depicts separation performance of the prepared neat XLPEGDA and nanocomposite XLPEGDA/TiO2 membranes with reference to the Robeson’s upper bound lines for CO2/CH4, CO2/N2 and CO2/H2.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 13. CO2/CH4, CO2/N2 and CO2/H2 ideal selectivity values vs. CO2 permeability. The lines represent Robeson’s upper bounds 71.

The presented upper bounds in this figure are plotted using the Rowe et al.71 theoretical model. As can be seen, the prepared membranes are located in the right upper corner of the upper bound lines which indicates their proper performance for separation of CO2 from light gas streams. In addition, as shown in this figure, presence of the TiO2 nanoparticles has improved position of the nanocomposite membranes compared to the neat ones. 5. Conclusions The present work has mainly focused on gas transport properties of XLPEGDA and XLPEGDA/TiO2 nanocomposite membranes. Due to strong affinity of the polar ether linkages in this polymer for CO2, it is promising for CO2 separation purposes. The operational conditions were selected more practical for industrial separation applications, such as CO2 removal from natural gas, synthesis gas as well as flue gas streams compared to the available data in the literature. In addition, the solubility and diffusivity of H2 and N2 gases through XLPEGDA were obtained in this research for the first time. To improve gas separation performance of XLPEGDA, TiO2 nanoparticles were incorporated in the polymer matrix. Presence of the TiO2 nanoparticles up to 3 wt% loading led to an increase in solubility and permeability coefficients of CO2, as well as its selectivity over the investigated light gases. This enhancement could be mainly ascribed to high surface area and adsorption capacity of TiO2 nanoparticles. Indeed, low loading contents of the embedded nanoparticles could not restrict diffusion of the gas penetrants and adversely affecting their permeability. Acknowledgment


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An Investigation on Gas Transport Properties of Cross-Linked Poly

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