Fabrication of polyimide and functionalized multi-walled carbon nanotubes mixed matrix membranes by in-situ polymerization for CO2 separation

Fabrication of polyimide and functionalized multi-walled carbon nanotubes mixed matrix membranes by in-situ polymerization for CO2 separation

Accepted Manuscript Fabrication of polyimide and functionalized multi-walled carbon nanotubes mixed matrix membranes by in-situ polymerization for CO2...

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Accepted Manuscript Fabrication of polyimide and functionalized multi-walled carbon nanotubes mixed matrix membranes by in-situ polymerization for CO2 separation Haixiang Sun, Tao Wang, Yanyan Xu, Wen Gao, Peng Li, Q. Jason Niu PII: DOI: Reference:

S1383-5866(16)30977-7 http://dx.doi.org/10.1016/j.seppur.2017.01.015 SEPPUR 13477

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

8 July 2016 6 January 2017 6 January 2017

Please cite this article as: H. Sun, T. Wang, Y. Xu, W. Gao, P. Li, Q. Jason Niu, Fabrication of polyimide and functionalized multi-walled carbon nanotubes mixed matrix membranes by in-situ polymerization for CO2 separation, Separation and Purification Technology (2017), doi: http://dx.doi.org/10.1016/j.seppur.2017.01.015

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Fabrication of polyimide and functionalized multi-walled carbon nanotubes mixed matrix membranes by in-situ polymerization for CO2 separation Haixiang Suna,b*, Tao Wangb, Yanyan Xub, Wen Gaob, Peng Lia, Q. Jason Niua* a

State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East

China), Qingdao 266580, P.R. China b

College of Science, China University of Petroleum (East China), Qingdao 266580,

P.R. China

*Corresponding authors. Tel.: +86 532 86981135; fax: +86 532 86981135 E-mail address: [email protected] (Dr. Sun) [email protected] (Dr. Niu)

1

Abstract Mixed matrix membranes (MMMs) consisting of acid-treated functionalized multi-walled carbon nanotubes (MWCNTs) incorporated into polyimide (PI) polymer were fabricated for gas separation. The functional groups and dispersion properties of the MWCNTs were characterized by Fourier transform infrared spectroscopy (FTIR), transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The results showed that the functionalized MWCNTs were cut into short ropes and acid oxidation introduced oxygen containing functional groups such as hydroxyl (OH) and carboxylic acid (COOH) on the surface of MWCNTs. Based on the assumption of strong interfacial interactions between the functionalized MWCNTs and PI matrix, the MWCNTs were first incorporated into poly(amic acid) (PAA) to form a highly dispersed solution, then the PI/MWCNTs nanocomposites were formed after in-situ chemical imidization. Scanning electron microscopy (SEM) indicated that the functionalized MWCNTs were dispersed homogenously in the PI matrix. The gas separation performance showed that the CO2 permeability coefficient increased 292% at 3 wt.% MWCNTs incorporated into the MMMs, and there was a 145% increase in the selectivity of CO2/N2 and a 144% increase in the selectivity of CO2/CH4. This result demonstrates that the carboxylic and hydroxyl groups have a strong interaction with CO2, which increases the solubility coefficient of the polar gases. The in-situ polymerization approach of fabricated MMMs improves the interfacial interaction between the dispersed nanomaterials and the polymer matrix, which can be utilized in the practical gas separation technology. 2

Keywords: Multi-walled carbon nanotubes; Mixed matrix membrane; Gas separation; Membrane structure 1. Introduction Carbon dioxide (CO2) is one of the primary greenhouse gases that contribute to global warming and climate change

[1]

. Energy-efficient and scalable carbon capture

thus stands as one of the greatest current challenges

[2]

. Compared with traditional

separation technologies such as cryogenic techniques, absorption and pressure swing adsorption technology [3], membrane separation has evolved as a green and affordable alternative

[4]

, owing to its intrinsic advantages such as high efficiency, easy

intensification, simple operation and low capital and operating costs

[5]

. However,

membranes designed for gas separations have been known to have a near-universal trade-off between permeability and selectivity as shown in upper bound curves developed by Robeson

[6]

. Therefore, mixed matrix membranes (MMMs) have been

studied as an alternative approach to solve the trade-off phenomena of polymeric membranes in gas separation as well as water treatment. MMMs are defined as inorganic fillers, such as zeolites[7], metal organic frameworks (MOFs)[8,9], carbon molecular sieves[10], silica[5,11], carbon nanotubes (CNTs)[12-14] and graphenes[15], disperse at a nanometer level in polymer matrix[16,17], which combine the following benefits of both phases: the superior gas transport properties and thermal resistance of molecular sieves with the desirable mechanical properties, low price, and good processability of polymers[18,19]. The choice of a suitable polymer is fundamentally important for the separation properties of prepared membranes. To date, many types of polymers, such as polysulfone (PS), polyvinyl acetate (PVAc), phenolic resins and polyimide (PI) have been used extensively. Among these kinds of polymers, PI represents an impressive 3

potential in terms of its industrial viability due to its high intrinsic permeation selectivity as well as excellent thermal and chemical stability. However, gas permeation properties of the existing PI materials need continuous exploitation to better meet the requirements for practical applications

[20]

. One of the facile and

promising approaches to enhance gas separation performance of polymer membranes is to introduce inorganic materials during the preparation of MMMs. For example, Rahman et al. prepared nanocomposite membranes by incorporation of poly(ethylene glycol) functionalized polyoctahedral oligomeric silsesquioxanes (PEG-POSS) into PEBEX. The results indicated that CO2 permeability increased evidently and the selectivity was not affected with the nanocomposite membranes compared to the pristine PEBEX [21]. Compared with MOFs and ZIFs, CNTs seem to be more advantageous for their extraordinary smooth walls and large diameter pores

[13, 22]

. With unique structural and

transport properties, such as excellent mechanical, thermal, and electrical properties along with low density, CNTs have attracted much interest since they were discovered in 1991[23,24]. The gas separation performance of the polymer materials are enhanced significantly even at low concentrations of CNTs. Cong

[14]

investigated the

combination of single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) in brominated poly(2,6-diphenyl-1,4-phenylene oxide) (BPPOdp) membranes. The resulting MMMs showed improvements in CO2 permeability and tensile modulus at low CNTs concentration. Li

[13]

explored the incorporation of CNTs and graphene

oxide (GO) into a Matrimids matrix. The results showed an improvement in gas separation performance attributed to a synergistic effect of these two types of fillers. Overall, the CNTs rendered high permeability due to the extraordinary smooth walls of CNTs, while GO increased the selectivity. 4

However, there is a major disadvantage for the dispersion of pristine CNTs as fillers, and functionalization of the surface of CNTs is an effective solution to solve this problem. Ismail

[25]

demonstrated that the purification of MWCNTs with acid

mixtures and surface functionalization with 3-aminopropyltriethoxysilane (APTES) gave a dramatic impact on the MMMs gas permeation properties. Sanip

[26]

showed

that the functionalization treatment of MWCNTs with beta-cylcodextrin (beta-CD) improved the solubility and homogeneous dispersion of CNTs in the MMMs. Their study showed that the addition of CNTs to polymeric membranes improved the separation properties of the membranes to a certain extent. Ge

[27]

reported that

carboxyl modified MWCNTs were prepared via H2SO4/HNO3 treatment. They demonstrated that the carboxyl groups on the MWCNTs had a stronger interaction with CO2, which increased the solubility of the polar gases and limited the solubility of non-polar gases, thus benefiting the CO2/N2 gas selectivity. In addition, the gas permeation fluxes of the derived nanocomposite membranes increased by about 67% without sacrificing selectivity at 5 wt.% MWCNTs introduced. For gas separation, the interfacial interaction between nanoparticles and polymer matrix, the properties of separated gases and the preparation method of membranes are very important factors to the final separation performance. The main problems encountered in the fabrication of CNTs-reinforced composites are agglomeration of CNTs and weak interfacial interactions between the CNTs and the matrix

[28]

. In this

study, in order to achieve better dispersion of MWCNTs, MWCNTs were functionalized via acid-treatment. Moreover, it is known that in-situ polymerization plays an important role for making polymer/CNT nanocomposites

[29]

. Therefore, in

this paper, we have attempted to prepare PI/MWCNTs nanocomposites using in-situ polymerization approach (i.e., solution-casting followed by subsequent imidization) to 5

get strong interfacial interactions between the MWCNTs and PI. To our best knowledge, this is the first time that fabricates the functionalized MWCNTs mixed matrix membranes by in-situ polymerization method for gas separation. The chemical structure and morphology of the MWCNTs were confirmed by Fourier transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The aggregate morphology of MWCNTs in PI matrix was studied with X-ray diffraction (XRD) and scanning electron microscopy (SEM). The permeability coefficient of pure gases and gas separation selectivity in PI membrane and PI/MWCNTs MMMs were determined by constant volume–variable pressure approach. The corresponding separation mechanism of nanoparticles in MMMs on gas separation was also discussed.

2. Experimental 2.1. Materials The main monomers required for PI formation, 3,3’,4,4’-benzophenonetetracarboxylic dianhydride (BTDA) and 3,3’-dimethyl-4,4’-diaminodiphenylmethane (DMMDA), were purchased from Sigma-Aldrich. N-methyl-2-pyrrolidone (NMP) (used after dehydration) was supplied by Sinopharm Chemical Reagent Factory (China). Acetic anhydride and pyridine were purchased from Shanghai Reagent Factory (China) and used directly without further treatment. Sulfuric acid (98 vol%) and nitric acid (70 vol%), were purchased from Xilong Chemical Co., Ltd.(China). High-purity MWCNTs (Purity: >95wt.%; average diameter: 20–30 nm; length: 10–30μm) were bought from Chengdu Organic Chemicals Co., Ltd. Chinese

6

Academy of Sciences. Pure CO2, N2 and CH4 were obtained from Tianyuan Gas Company, Tsingdao. 2.2. Preparation of acid-treated MWCNTs In order to improve the MWCNTs dispersion in the polymer matrix, 0.8g of the pristine MWCNTs were sonicated in 100 mL of a mixture of concentrated H2SO4 /HNO3 in a volume ratio of 3:1 for 6 h

[30]

. The resulting product was washed with

deionized water by physical sedimentation three times. After that the wet powder was filtered with a 0.45μm polytetrafluoroethylene (PTFE) membrane filter, and washed by deionized water until the filtrate reached a constant pH of 6. The product was then dried at 75°C in a vacuum oven for 24h. 2.3. Synthesis of the polyimide A typical procedure for polymerization was used as follows[31] (Fig. 1). DMMDA and BTDA were dried at 120°C and 260°C under vacuum for 12h and 18h, respectively. Then the mixture of the molar equivalent DMMDA and BTDA was added to NMP to make a slurry with 10% (w/v) solid content. The solution was equilibrated in an ice bath in a nitrogen atmosphere for 12 h to form a viscous polyamide acid (PAA) solution. Finally, an equimolar mixture of acetic anhydride and pyridine was added to the solution and incubated for 6 h at room temperature to prepare the PI solution. The solution was subsequently precipitated in deionized water under vigorous stirring, washed several times in deionized water, and dried in a vacuum oven at 50°C for 6 h to produce solid PI powder.

7

2.4. Preparation of PI/MWCNTs MMMs The preparation of PI/MWCNTs nanocomposites was as follows: Acid-treated MWCNTs were dispersed in NMP solvent by sonication for 2 h. Then the MWCNTs dispersion was added to PAA solutions prepared above to form a series of PAA/MWCNTs suspensions with MWCNTs concentrations of 0, 0.127, 0.336, 1.0, 2.0, 3.0 and 4.0 wt.% to PAA solid contents. The chemical imidization of the PAA/MWCNTs solutions were carried out as the last step described in 2.3 and the products of PI/MWCNT nanocomposites were obtained in-situ polymerization approach (i.e., solution-casting followed by subsequent imidization). Pure PI membrane and MMMs incorporated with pristine and functionalized carbon nanotubes were fabricated by solution-casting method. PI or PI/MWCNTs nanocomposite was dissolved in NMP to get a l7.8 wt.% polyimide solution and stirred for 24 h. The obtained solutions were cast onto glass plates and then the MMMs were dried in a dry air-flowing oven at 45°C for 12h at first, and then dried at 100°C in a vacuum oven for 24h. All MMMs thicknesses were approximately 15−20μm. 2.5. Characterization of the materials Fourier transform infrared spectroscopy (FTIR) of pristine MWCNTs, acid-treated MWCNTs, PI membrane and PI/MWCNTs MMMs were measured using a BRUKER TENSORII FT-IR spectrometer in the wavenumber range of 4000−600 cm−1. The morphology of pristine and acid-treated MWCNTs were analyzed by a transmission electron microscope (TEM, JEM 2100F, JEOL, Japan). Surface 8

chemistry of pristine and acid-treated functionalized MWCNTs was monitored with X-ray photoelectron spectroscopy (XPS, Thermo escalab 250Xi, Thermo Electron, USA). A monochromatic Al Ka X-ray was used at 1486.6 eV. Cross sections of the PI/MWCNTs MMMs were characterized by scanning electron microscopy (SEM, S-4800, Hitachi, Japan) after being coated with a conductive layer of sputtered gold. The X-ray diffraction (XRD) spectra of PI and PI/MWCNTs were obtained with PA Nalytical X’Pert PRO Materials Research Diffractometers (Netherlands) using a voltage of 40 kV and current of 40 mA. The X-rays (λ=1.5418Å) were generated by a Cu Kα source. Each pattern was collected in the 2θ from 5° to 40° in the repetition mode (three times) with a total duration of approximately 0.4h at selected times of hydration. The d-spacing of the PI and PI/MWCNTs MMMs, which was the distance between diffracting plane, was calculated by substituting the scattering angle, 2θ, of the perk into the Bragg equation d

 2 sin 

(1)

where λ is the wavelength of the Cu Kα ray (nm) and 2θ is the diffraction angle (o). 2.6. Procedure for gas permeability measurements The pure gas (CO2, N2 and CH4) permeability measurements were carried out by a Pressure Permeability Tester (Labthink Instruments Co., Ltd.). The experiments were performed at the temperature of 15°C. The circular membrane with effective area of 4.95 cm2 was mounted in a permeation cell prior to degassing the whole apparatus. The feed and the permeate cell were evacuated using a vacuum pump for more than 4 h to maintain degree of vacuum less than 20 Pa, and then the feed gas 9

was introduced into the membrane cell at 0.1 MPa. When the pressure change of the permeate side remained constant over time, the test was completed. Gas permeability coefficient (P) can be calculated as follows[32]: P

273 VL dp 76 ATp0 dt

(2)

Where P is the permeability coefficient of a membrane to a gas and its unit is in Barrer (1 Barrer = 110-10cm3cm cm-2s-1cmHg-1), V is the volume of the down-stream chamber (cm3), L and A are the thickness (cm) and the effective area of the membrane (cm2), respectively, T is the experimental temperature (K), and the pressure of the feed gas in the up-stream chamber is given by p0 in cmHg. The ideal selectivity αA/B is defined as follows:

 A/ B 

PA PB

(3)

where PA and PB represent the permeability coefficient of pure gas CO2 and N2, or CO2 and CH4 (Barrer), respectively.

3. Results and discussion 3.1. Characterization of multi-walled carbon nanotubes 3.1.1. TEM As revealed by the TEM images shown in Fig. 2, the average diameter of pristine and acid-treated functionalized MWCNTs is in the range of 20–30 nm. The length of the pristine MWCNTs is about tens of microns and they are randomly and loosely entangled together without any particle-like impurities. However, the acid-treated 10

MWCNTs are suspended and manipulated as individual macromolecules without entanglement. The length of acid-treated MWCNTs is only a few hundreds of nanometers, which is much shorter than that of the pristine ones. The reason is that the reaction is easy to start at the defect sites such as the –CH2 and –CH groups and heptatomic rings

[33]

. Moreover, the pristine MWCNTs are generally closed-ended

(signed out by circles in Fig. 2a) and the acid-treated MWCNTs are generally open-ended (signed out by circles in Fig. 2b) [34]. In addition, both the pristine and acid-treated MWCNTs graphitic walls are smooth, indicating that the tubular structure of the MWCNTs do not undergo any destructive process during the course of functionalization. 3.1.2. FTIR The FTIR spectra of pristine and acid-treated MWCNTs are depicted in Fig. 3. The perk at the wavenumber 1500cm-1 in Fig. 3(a) corresponds to the characteristic carbon skeleton, and the peaks at 2914 cm-1 and 2846 cm-1 are representative of the C-H stretching of the alkyl chain[12]. The hydroxyl groups (–OH) and the C=C double bond were observed at 3434cm-1 and 1630 cm-1, respectively. For the acid-treated MWCNTs in Fig. 3(b), the peaks at 3435cm-1 and 1051 cm-1 attribute to –OH on the surface of the MWCNTs[28]. The increase in the relative intensities of these two perks indicates that there are more –OH on the MWCNT surface after the acid treatment. In addition, two new perks appear at 1714cm-1 and 1164 cm-1 in correspondence to the C=O and C-O stretching vibrations of the carboxylic groups (–COOH), respectively[24]. All these observations confirm that acid oxidation has introduced 11

–COOH and–OH on the surface of MWCNTs. 3.1.3. XPS XPS is a useful tool for analyzing the functional groups on the membrane surface. All functional groups could be semiquantitative analysis by detecting binding energies change as well as the local chemical state. Fig. 4 presents C1s XPS spectra of the pristine and acid-treated MWCNTs. Both spectra indicate four kinds of C atoms in different functional groups: namely C-C (284.88 eV), C-O (285.80 eV), C=O (287.18 eV) and C(O)O (289.50 eV). The peak at 291.18 eV corresponds to a shakeup feature of the aromatic structure present in the MWCNTs

[24]

. It can be seen from Fig. 4(b)

that after the acid treatment, the peaks at 285.80, 287.18 and 289.50 eV of the modified MWCNTs are attributed to the C-O, C=O and C(O)O groups, respectively, which are much higher than those of the pristine MWCNTs. The peaks at 533.08 and 285.08 eV in the XPS survey spectra of pristine and acid-treated MWCNTs shown in Fig. 5 correspond to O1s and C1s, respectively. Clearly the intensity of the O1s increases after the acid treatment. As summarized in Table 1, the oxygen atomic percentage in the sample increases from the pristine MWCNTs of 2.03% to the functionalized MWCNTs of 6.05%, indicating that the acid treatment process effectively generates more functional groups on the MWCNTs surface. This observation is in line with the result of FTIR spectra as seen earlier. 3.2. Characterization of the MMMs 3.2.1. SEM In the mixed matrix membranes, the morphology of the dispersed phase strongly 12

influences the gas transport properties. Cross-sectional morphology of the membranes characterized by SEM is displayed in Fig. 6. A crater-like and dense structure is observed in both pure PI membrane and PI/MWCNTs mixed matrix membranes

[35]

,

which indicate that the addition of MWCNTs has little influence on the basic structure of PI matrix. The bright dots and lines are the ends of the MWCNTs. When 2 wt.% pristine MWCNTs were incorporated into the PI matrix, the resulting MMM shows a non-uniform dispersion agglomeration and cluster forming of nanotubes (Fig. 6b). However, the cross section image of MMM with 2 wt.% acid-treated MWCNTs shows a well dispersion of MWCNTs in the polymer matrix (Fig. 6c), and the MMM presents more MWCNTs dots than that of the membrane with 2 wt.% pristine MWCNTs. The reason is attributed to shear force by the HNO3/H2SO4 acid oxidation, which induces the dispersion of acid-treated MWCNTs in the PI matrix more homogeneously than that of the pristine ones. When the content of the acid-treated MWCNTs in the matrix increases to 3 wt.% (Fig. 6d), there are more MWCNTs dots in the field of view, and the MWCNTs still show good dispersibility in the matrix. However, the MWCNTs agglomerations and clusters are clearly observed in Fig. 6(e) with 4 wt.% acid-treated MWCNTs incorporated into the PI matrix, indicating that the MWCNTs are poorly dispersed at higher loadings

[27]

. The SEM image in Fig. 6(f) shows that some nanotubes protrude

out of the dense top layer near the surface of the synthesized MWCNTs mixed matrix membrane. It suggests that there are affinity and strong interfacial interactions between the PI matrix and carbon nanotubes, which are important parameters for the 13

preparation of highly selective MMMs [28]. 3.2.2. FTIR Fig. 7 displays the FTIR spectra of the pure PI membrane and PI/MWCNTs MMMs. As for the pure PI membrane in Fig. 7(a), the characteristic absorption peaks for the imide functional groups at wavenumber of 1780, 1720 and 1380 cm-1 are attributed to the asymmetric stretch of C=O, the symmetric stretch of C=O and the stretching vibration of C–N, respectively. The peak at 725 cm-1 corresponds to the deformation of the imide ring. These four peaks are the characteristic peaks of the PI group

[36,37]

. For the MMMs in Fig. 7(b), the peak at 1720 cm-1 divides into two

absorption peaks at 1677cm-1 and 1727cm-1. This may be attributed to the interfacial interactions between the PI and MWCNTs. The result has been confirmed by the SEM images. 3.2.3. XRD To investigate the influence of MWCNTs on the arrangement of polymer chains, the crystalline structure of MMMs is analyzed by XRD. Fig. 8 represents the XRD spectra of pure PI membrane and MMMs with different acid-treated MWCNTs loadings, and Fig. 9 represents the XRD spectra of MMMs with different kinds of MWCNTs. Pure PI membrane in Fig. 8(a) reveals two characteristic peaks at 2θ=16.40° and 22.40°, which indicate the semi-crystalline structure of the sample

[38]

.

The most prominent XRD peak is used to estimate the d-spacing of the polymer

[39]

.

The pure PI polymer shows the inter-chain spacing of 0.54nm, which is in line with the one reported for PI (0.58nm) [35]. It is shown from Fig. 8 that the incorporation of 14

MWCNTs decreases the intensity of the two characteristic peaks. The reason is that the interaction between PI and MWCNTs may disrupt the primary crystalline pattern and chain packing

[2]

. It also can be seen that the peak of pure PI at 2θ=16.40° shifts

to lower values as the loading of acid-treated MWCNTs increase from 1 to 3 wt.%. The d-spacing of the polymer increases slightly with the addition of MWCNTs. These shifts indicate an increase in intersegmental spacing of the PI polymer, which is anticipated to create more free volume after the addition of MWCNTs. Based on the interaction of the MWCNTs and the polymer segmental chains, the increase in free volume of the polymer matrix as a result of the disruption of the polymer chains packing also demonstrated an ascending trend of the permeability

[14]

. However, with

4 wt.% acid-treated MWCNTs incorporated, the d-spacing of the polymer decreases slightly due to the poor dispersion of MWCNTs at higher loadings. For the synthesized MMMs containing acid-treated MWCNTs as shown in Fig. 9(c), the intensity of both peaks is weaker than that of the MMMs with pristine MWCNTs (Fig. 9b) and pure PI membrane (Fig. 9a), and the peak at 2θ=16.40° shifts to a lower value. This indicates that functionalized MWCNTs show better dispersion than that of the pristine MWCNTs in the polymer matrix. 3.3. Gas separation performances The effect of functionalized MWCNTs in MMMs was investigated by synthesizing a series of membranes with different content of untreated and acid-treated MWCNTs loadings of 1-4 wt.%, and the permeability coefficient of CO2, N2 and CH4, the permeation selectivity of CO2/N2 and CO2/CH4 were obtained. The 15

gas transport data are summarized in Table 2. The gas transport properties as a function of MWCNTs loading are depicted in Fig. 10. Fig. 10 as well as Table 2 illustrates that the MMMs with acid-treated MWCNTs represents better gas separation performance than those of the MMMs containing pristine MWCNTs. The CO2 permeability increases with the increase of MWCNTs loading in the polymer matrix obviously. The CO2 permeability coefficient increases 292% from 2.31 Barrer with the pure PI membrane to 9.06 Barrer with the MMMs containing 3 wt.% acid-treated MWCNTs loading. When the MWCNTs content increases to 4 wt.%, the CO2 permeability coefficient shows a decreasing trend, although still higher than the permeation of pure PI membrane. As clearly represented in Fig. 10, the MWCNTs content has no obvious influence on the permeability coefficient of N2 and CH4, and the permeability coefficient of CH4 is a little higher than that of N2. This reason may be attributed to the critical temperatures of gases decreasing in the following order: CO2 (304.2 K) > CH4 (190.7 K) > N2 (126.1 K). The higher condensability has the higher solubility of the gas in the polymer matrix [16]

, which leads to a higher permeability coefficient. From the above SEM images, it

can be seen that MWCNTs are well dispersed in the polymer matrix at lower MWCNTs loadings. This indicates that acid treatment forces the nanotubes to disintegrate into small fragments. In this case the nanocomposite membrane shows higher gas permeability based on the well-dispersed MWCNTs serving as channels to transport gas molecules more effectively. Moreover, XRD spectra reveal that the strong interaction between polymer-chain segments and carbon nanotubes disrupts the 16

polymer-chain packing and thus enhances the gas permeability coefficient due to more free volume of the polymer

[40]

. As a result, the well-dispersed acid-treated

MWCNTs could insert into the polymers chains more effectively and the gas permeability coefficient increases correspondingly. However, at higher MWCNTs loading ratio, such as the loading above 4 wt.%, there are agglomerations and clusters of nanotubes formed in the polymer matrix, which leads to a slight deterioration of the gas permeability coefficient. Fig. 11 illustrates the separation selectivity of CO2/N2 (αCO2/N2) and CO2/CH4 (αCO2/CH4) in MMMs with pristine and different content of acid-treated MWCNTs. The selectivities for CO2/N2 and CO2/CH4 both follow the same trend as CO2 permeability coefficient. The highest αCO2/N2 and αCO2/CH4 are found to be 37.74and 24.49, respectively. The polar carboxylic groups and/or hydroxyl groups on the MWCNTs play a significant role on the gas selectivity. It has been reported in the literature that the carboxylic group and hydroxyl group on the surface of the MWCNTs have stronger interactions with polar gases [1], such as CO2 than with non-polar gases, such as N2 and CH4. As a consequence, the MMMs reveals higher selectivity than the pure PI membrane, especially the MMMs incorporated with acid-treated MWCNTs. The separation selectivity of CO2/N2 is similar to the result of Khan’s research

[41]

,

however, the permeability coefficient of CO2 and N2 is much lower than that of Khan’s. The reason is that the polymer of intrinsic microporosity (PIM) contains the contorted spirobisindane unit, which is attributed to the outstanding gas permeability. From the both results, it can be concluded that the groups on the MWCNTs have 17

strong interaction with CO2 which increases the solubility of polar gas and limit the solubility of nonpolar gas, which is advantageous for CO2/N2 selectivity increase. Based on the gas separation results of PI/MWCNTs MMMs, a reasonable explanation of the gas separation mechanism could be proposed as in Fig. 12. As for the acid-treated MWCNTs, the short-cut MWCNTs disperse in the MMMs in different directions. During the course of the gases passing through the membranes, the vertical MWCNTs could act as the channels for the gases. Therefore in the PI/MWCNTs MMMs, the increased gas permeability coefficient might be attributed to the ‘highway’ composed by MWCNTs. On the other hand, the polar functional groups on the surface of carbon nanotubes make a great contribution to the gas selectivity. The presence of functional groups, such as the hydroxyl groups and carboxyl groups strongly interact with CO2. In that case, the solubility coefficient of polar gas enhances and correspondingly the increases of gas permeability. In order to evaluate the effect of in-situ polymerization on the gas separation performance, the permeability coefficient of CO2, N2 and ideal selectivity of CO2/N2(αCO2/N2) in MMMs with different membrane fabricating methods are illustrated in Fig. 13 and Fig. 14. It can be seen that the MMMs fabricated with in-situ polymerization method have higher gas separation performance than that of the MMMs fabricated with the solution mixing method. In particular, there is a 48.1% increase in the CO2 permeability coefficient from 6.12 Barrer of the MMMs using solution mixing method to 9.06 Barrer of the MMMs using in-situ polymerization method and a 29.5% increase in the selectivity of CO2/N2 (from 29.14 to 37.74) at 18

3wt.% acid-treated MWCNTs loading. This improvement is attributed to the homogeneous dispersion of MWCNTs in the PI matrix as well as the strong interfacial interaction between the PI matrix and acid-treated MWCNTs by in-situ polymerization method [29].

4. Conclusions In this work, the MMMs based on polyimide and MWCNTs nanomaterials were prepared using in-situ polymerization approach. The MWCNTs were functionalized via acid-treatment method. FTIR and XPS spectra confirmed that acid oxidation had introduced carboxylic and hydroxyl groups on the surface of MWCNTs, and TEM images showed that the acid-treated MWCNTs were cut into short ropes. Therefore, a homogeneous dispersion of MWCNTs throughout PI matrix and strong interfacial adhesion between the MWCNTs and the polymer matrix was obtained. The XRD patterns obtained for the semi-crystalline of PI/MWCNTs nanocomposite membranes revealed that the intensities of the characteristic peaks decrease with an increase in the MWCNT concentrations; this indicated that the interaction of MWCNTs and PI may disrupt the primary crystalline lattice. Moreover, the characteristic peak at 2θ 16.40° shifted to the low-angle region suggesting that a high MWCNT loading increased the polymer intersegmental spacing. The MMMs incorporated with acid-treated MWCNTs represented a better gas separation performance than those containing of pristine MWCNTs, and the gas permeability coefficient increased with the increase of MWCNTs loading. Specifically, there was a 292% increase in the CO2 permeability 19

coefficient and an equally obvious increase in the selectivity of CO2/N2 and CO2/CH4 at 3wt.% acid-treated MWCNTs loading due to the fine dispersion of MWCNTs and the presence of oxygen containing functional groups such as hydroxyl (OH) and carboxylic acid (COOH). However, at higher loading ratio, there were agglomerations and clusters of nanotubes formed in the polymer matrix, which led to a slight deterioration of the gas permeance. As a result, in-situ polymerization approach of the fabricated MMMs increases the interfacial interaction between the polymer matrix and the dispersed nanomaterials, which significantly improved the gas permeability without sacrificing the separation selectivity of MMMs.

Acknowledgements The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No. 21406268), the Shandong Provincial Natural Science Foundation (No. ZR2014BM005) and the Fundamental Research Funds for the Central Universities (No. 14CX05034A).

References [1] T. Dasgupta, S.N. Punnathanam, K.G. Ayappa, Effect of functional groups on separating carbon dioxide from CO2/N2 gas mixtures using edge functionalized graphene nanoribbons, Chem. Eng. Sci. 121 (2015) 279–291. [2] S.F. Wang, Z.Z. Tian, J.Y. Feng, et al., Enhanced CO 2 separation properties by incorporating poly(ethylene glycol)-containing polymeric submicrospheres into polyimide membrane, J. 20

Membr. Sci. 473 (2015) 310–317. [3] A.C. Lua, Y. Shen, Preparation and characterization of polyimide–silica composite membranes and their derived carbon–silica composite membranes for gas separation, Chem. Eng. J. 220 (2013) 441–451. [4] V. Abetz, T. Brinkmann, M. Dijkstra, et al., Developments in membrane research: from material via process design to industrial application, Adv. Eng. Mater. 8 (2006) 328–358. [5] B. Zornoza, S. Irusta, C. Téllez, J. Coronas, Mesoporous silica sphere-polysulfone mixed matrix membranes for gas separation, Langmuir 25 (2009) 5903–5909. [6] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 61 (1991) 165–185. [7] E. Barankova, N. Pradeep, K.V. Peinemann, Zeolite-Imidazolate Framework (ZIF-8) Membrane Synthesis on A Mixed-Matrix Substrate, Chem. Commun. 49 (2013) 9419−9421. [8] S. Basu, A. Cano-Odena, Ivo F.J. Vankelecom, MOF-containing mixed-matrix membranes for CO2/CH4 and CO2/N2 binary gas mixture separations, Sep. Purif. Technol. 81 (2011) 31–40. [9] M. Arjmandi, M. Pakizeh, Mixed matrix for

improved polyetherimide

membranes incorporated with cubic-MOF-5

gas separation membranes: Theory and experiment,

J. Ind. Eng. Chem. 20 (2014) 3857–3868. [10] D.Q. Vu, W.J. Koros, S.J. Miller, Mixed matrix membranes using carbon molecular sieves–I. Preparation and experimental results, J. Membr. Sci. 211 (2003) 311–334. [11] B. Zornoza, C. Téllez, J. Coronas, Mixed matrix membranes comprising glassy polymers and dispersed mesoporous silica spheres for gas separation, J. Membr. Sci.368 (2011) 100–109. [12] Y.N. Zhao, B.T. Jung, L. Ansaloni, W.S. Winston Ho, Multiwalled carbon nanotube mixed 21

matrix membranes containing amines for high pressure CO 2/H2 separation, J. Membr. Sci. 459 (2014) 233–243. [13] X.Q. Li, L. Ma, H.Y. Zhang, et al., Synergistic effect of combining carbon nanotubes and graphene oxide in mixed matrix membranes for efficient CO2 separation, J. Membr. Sci.479 (2015) 1–10. [14] H.L. Cong, J.M. Zhang, M. Radosz, Y.Q. Shen, Carbon nanotube composite membranes of brominated poly(2,6-diphenyl-1,4-phenylene oxide) for gas separation, J. Membr. Sci. 294 (2007) 178–185. [15] J. Shen, G.P. Liu, K. Huang, et al., Membranes with fast and selective gas-transport channels of laminar graphene oxide for efficient CO2 capture, Angew. Commun. 54 (2015) 578 –582. [16] X.Q. Li, Y.D. Cheng, H.Y. Zhang, et al., Efficient CO2 capture by functionalized graphene oxide nanosheets as fillers to fabricate multi-permselective mixed matrix membranes, Appl. Mater. Interfaces 7 (2015) 5528−5537. [17] Y. Li, G. He, S. Wang, et al., Recent advances in the fabrication of advanced composite membranes, J. Mater. Chem. A 35 (2013) 10058−10077. [18] A.F. Ismail, P.S. Goh, S.M. Sanip, M. Aziz, Transport and separation properties of carbon nanotube-mixed matrix membrane, Sep. Purif. Technol. 70 (2009) 12–26. [19] T.S. Chung, L.Y. Jiang, Y. Li, S. Kulprathipanja, Mixed matrix membranes (MMMs) comprising organic polymers with dispersed inorganic fillers for gas separation, Prog. Polym. Sci. 32 (2007) 483-507. [20] T.C. Merkel, H. Lin, X. Wei, R. Baker, Power plant post-combustion carbon dioxide capture: an opportunity for membranes, J. Membr. Sci. 359 (2010) 126–139. 22

[21] M.M. Rahman, V. Filiz, S. Shishatskiy, et al., PEBAX® with PEG functionalized POSS as nanocomposite membranes for CO2 separation, J. Membr. Sci.437 (2013) 286-297. [22] D.S. Sholl, J.K. Johnson, Making high-flux membranes with carbon nanotubes, Science 312 (2006) 1003–1004. [23] S. Iijima, Helical microtubules of graphitic carbon, Nature 354 (1991) 56-58. [24] P.C. Ma, J.K. Kim, B.Z. Tang, Functionalization of carbon nanotubes using a silane coupling agent, Carbon 44 (2006) 3232–3238. [25] A.F. Ismail, N.H. Rahim, A. Mustafa, et al., Gas separation performance of polyethersulfone/multi-walled carbon nanotubes mixed matrix membranes, Sep. Purif. Technol. 80 (2011) 20–31. [26] S.M. Sanip, A.F. Ismail, P.S. Goh, et al., Gas separation properties of functionalized carbon nanotubes mixed matrix membranes, Sep. Purif. Technol. 78 (2011) 208–213. [27] L. Ge, Z.H. Zhu, V. Rudolph, Enhanced gas permeability by fabricating functionalized multi-walled carbon nanotubes and polyethersulfone nanocomposite membrane, Sep. Purif. Technol. 78 (2011) 76–82. [28] J.H. Lee, K.Y. Rhee, S.J. Park, Silane modification of carbon nanotubes and its effects on the material properties of carbon/CNT/epoxy three-phase composites, Compos. Part A: Appl. Sci. Manufac. 42 (2011) 478–483. [29] T.X. Liu, Y.J. Tong, W.D. Zhang, Preparation and characterization of carbon nanotube/polyetherimide nanocomposite films, Compos. Sci. Technol. 67 (2007) 406–412. [30] T. Saito, K. Matsushige, K. Tanaka, Chemical treatment and modification of multi-walled carbon nanotubes, Physica B. 323 (2002) 280–283. 23

[31] B.B. Yuan, M. Cao, H.X. Sun, et al., Preparation of a Polyimide Nanofiltration Membrane for Lubricant Solvent Recovery, J. Appl. Polym. Sci. 131 (2014) 40338-40346. [32] H.X. Sun, C. Ma, B.B. Yuan, et al., Cardo polyimides/TiO2 mixed matrix membranes: Synthesis, characterization, and gas separation property improvement, Sep. Purif. Technol. 122 (2014) 367–375. [33] J. Zhang, H.L. Zou, Q. Qing, et al., Effect of Chemical Oxidation on the Structure of Single-Walled Carbon Nanotubes, J. Phys. Chem. B. 107 (2003) 3712-3718. [34] M.A. Aroon, A.F. Ismail, M.M. Montazer-Rahmati, et al., Effect of chitosan as a functionalization

agent

on

the

performance

and

separation

properties

of

polyimide/multi-walled carbon nanotubes mixed matrix flat sheet membranes, J. Membr. Sci. 364 (2010) 309–317. [35] F.Y. Li, Y. Li, T.S. Chung, et al., Facilitated transport by hybrid POSS®-Matrimid®-Zn2+ nanocomposite membranes for the separation of natural gas, J. Membr. Sci. 356 (2010) 14–21. [36] P.S. Tin, T.S. Chung, Y. Liu, et al., Effects of cross-linking modification on gas separation performance of Matrimid membranes, J. Membr. Sci. 225 (2003) 77–90. [37] Z. Ahmad, F. Al Sagheer, A. Al Arbash, et al., Synthesis and characterization of chemically cross-linked polyimide–siloxane hybrid films, J. Non-Cry. Sol. 355 (2009) 507–517. [38] J. Ahmad, M.B. Hägg, Development of matrimid/zeolite 4A mixed matrix membranes using low boiling point solvent, Sep. Purif. Technol. 115 (2013) 190–197. [39] F. Aziz, A.F. Ismail, Preparation and characterization of cross-linked Matrimid® membranes using para-phenylenediamine for O2/N2 separation, Sep. Purif. Technol. 73 (2010) 421–428. 24

[40] T.C. Merkel, B.D. Freeman, R.J. Spontok, et al., Ultrapermeable, reverse-selective nanocomposite membrane, Science 296 (2002) 519.

[41] M. M. Khan, V. Filiz, G. Bengtson, et al., Functionalized carbon nanotubes mixed matrix membranes of polymers of intrinsic microporosity for gas separation, Nanoscale Res. Lett. 7 (2012) 504.

25

Figure Captions Fig. 1. Procedure for the preparation of PI and PI/MWCNTs Fig. 2. TEM image of the pristine (a) and functionalized (b) MWCNTs Fig. 3. FTIR spectra of the pristine (a) and functionalized (b) MWCNTs Fig. 4. C1s XPS spectra of the pristine (a) and functionalized (b) MWCNTs Fig. 5. XPS survey scans of the pristine (a) and functionalized (b) MWCNTs Fig. 6. SEM images of the cross-section of MMMs incorporated with different MWCNTs loading. 0 wt. % (a), 2 wt. % pristine MWCNTs (b), 2 wt. % acid-treated MWCNTs (c), 3 wt. % acid-treated MWCNTs (d) and 4 wt. % acid-treated MWCNTs (e), and the acid-treated nanotubes protruding from the membrane matrix (f) Fig. 7. FTIR spectra of the pure PI membrane (a) and MMMs incorporated with acidtreated MWCNTs (b) Fig. 8. XRD patterns of the pure PI membrane (a), and the MMMs incorporated with different acid-treated MWCNTs loading. 1% (b), 2% (c), 3% (d) and 4 wt% (e) Fig. 9. XRD patterns of the pure PI membrane (a), the MMMs incorporated with pristine MWCNTs (b), and the MMMs incorporated with acid-treated MWCNTs(c) Fig. 10. Effects of MWCNTs loading level on the gas permeability at 288 K and a feed pressure of 0.1 MPa: CO2 permeability of PI/pristine MWCNTs 26

MMMs(■), CO2 permeability of PI/acid-treated MWCNTs MMMs (▼), CH4 permeability of PI/pristine MWCNTs MMMs(☆),: CH4 permeability of PI/acid-treated MWCNTs MMMs(▽),: N2 permeability of PI/pristine MWCNTs MMMs(△), N2 permeability of acid-treated PI/acid-treated MWCNTs MMMs(□) Fig. 11. Effects of MWCNTs loading level on the gas separation selectivity at 288 K and a feed pressure of 0.1 MPa: CO2/N2 selectivity of PI/pristine MWCNTs MMMs (▽), CO2/N2 selectivity of PI/acid-treated MWCNTs MMMs(▼), CO2/CH4 selectivity of PI/pristine MWCNTs MMMs (□), CO2/CH4 selectivity of PI/acid-treated MWCNTs MMMs (●) Fig. 12. Gas transport mechanism of prepared MMMs Fig. 13. Effect of different membrane fabricating methods on the gas permeability of MMMs at 288 K and a feed pressure of 0.1 MPa: CO2 permeability of MMMs with in-situ polymerizing method (▼), CO2 permeability of MMMs with solution mixing method (●), N2 permeability of MMMs with in-situ polymerizing method (▲), and N2 permeability of MMMs with solution mixing method (■) Fig. 14. Effect of different membrane fabricating methods on CO 2/N2 selectivity of MMMs at 288 K and a feed pressure of 0.1 MPa: solution mixing method (■), and in-situ polymerizing method (●)

27

Fig. 1

28

Fig. 2

29

(a) 2914

1500

2846 1630

Transmittance(%)

(b)

1051 3434

1714

1164 1633

3435

4000

3500

3000

2500

2000

-1 Wavenumber(cm )

Fig. 3

30

1500

1000

C-C (284.88 ev)

Intensity a.u.

(a)

C-O (285.80 ev)

C=O (287.18 ev) CO(O) (289.50 ev) 291.18 ev

300

295

290

285

280

Binding Energy (ev)

C-C (284.78 ev)

Intensity a.u.

(b)

300

CO(O) (289.50 ev)

295

C=O (287.18 ev)

290

Binding Energy (ev)

Fig. 4

31

C-O (285.80 ev)

285

280

Intensity a.u.

C1s

(a)

O1s C1s

O1s

(b)

1200

900

600

Binding Energy( eV)

Fig. 5

32

300

0

Fig. 6

33

Transmittance (%)

(b)

(a) 1727

1780

2000

1677

1720

725

1380

1500

1000

500

-1

Wavenumber(cm )

Intensity

Fig. 7

e d c b a 10

20

30

2/degree

Fig. 8

34

40

-1

CO2 Permeability Coefficient (Barrer)

10

0 20

1 30

10

8 0.8

6 0.6

4 0.4

2

0.2

0

2

Fig. 10

35

3

MWCNTs content(wt%)

4

5

N2/CH 4 Permeability Coefficient (Barrer)

Intensity

c

b

a

2/degree 40

Fig. 9

1.0

40

Selectivity

30

20

10 -1

0

1

2

3

MWCNTs content (wt%)

Fig. 11

Fig. 12

36

4

5

Permeability Coefficient (Barrer)

10

8

6

4

2

0 -1

0

1

2

3

4

5

4

5

MWCNTs content(wt%)

Fig. 13

40

Selectivity(CO 2/N2)

35

30

25

20

15

-1

0

1

2

3

MWCNTs content (wt%)

Fig. 14

37

Table Captions Table 1 Elemental composition of the MWCNTs Table 2 Pure gas permeability coefficient and ideal selectivity of different membranes

Table 1. Elemental composition of the MWCNTs

Element (Atom%)

C

O

Pristine

97.97

2.03

Acid-treated

93.95

6.05

38

Table 2. Pure gas permeability coefficient and ideal selectivity of different membranes PCO2 /Barrer

PN2 /Barrer

PCH4 /Barrer

αCO2/N2

αCO2/CH4

——

2.31

0.15

0.23

15.40

10.04

PI+1 wt% pristine MWCNTs PI+2 wt% pristine MWCNTs PI+3 wt% pristine MWCNTs PI+4 wt% pristine MWCNTs PI+1 wt% acid-treated MWCNTs PI+2 wt% acid-treated MWCNTs

in situ polymerizing

3.32

0.18

0.27

18.44

12.30

in situ polymerizing

4.58

0.22

0.32

20.82

14.31

in situ polymerizing

5.44

0.24

0.35

22.67

15.54

in situ polymerizing

4.05

0.20

0.31

20.25

13.71

in situ polymerizing

4.79

0.18

0.30

26.61

15.97

in situ polymerizing

6.77

0.21

0.33

32.24

20.52

PI+3 wt% acid-treated MWCNTs

in situ polymerizing

9.06

0.24

0.37

37.74

24.49

PI+4 wt% acid-treated MWCNTs PI+1 wt% acid-treated MWCNTs PI+2 wt% acid-treated MWCNTs PI+3 wt% acid-treated MWCNTs PI+4 wt% acid-treated MWCNTs

in situ polymerizing

8.25

0.23

0.37

35.87

22.30

solution mixing

3.67

0.18

——

20.39

——

solution mixing

5.15

0.21

——

26.24

——

solution mixing

6.12

0.21

——

29.14

——

solution mixing

5.64

0.21

——

26.86

——

Membrane pure PI

Fabricate Method

39

Highlights 

Polyimide/functionalized MWCNTs MMMs were made via in-situ polymerization.



In-situ prepared composites demonstrated the better carbon nanotubes dispersion.



Functionalized MWCNTs improved the CO2 separation selectivity significantly.



The functionalized MWCNTs MMMs demonstrated increased thermal stability.

40