Preparation and High Performance of Cellulose Acetate Films by Grafting with Imidazole Ionic Liquid.

Cellulose acetate (CA) grafted with imidazole ionic liquids (CA-ILs) was synthesized by reacting CA with imidazole ionic liquids ([HO2CMmim]Cl, [HO2CEtmim]Cl, and [HO2CMmim]Br) by using tetrahydrofuran (THF) as the solvent and pyridine as the catalyst. The CA and CA-IL films were fabricated by using the casting solution method. The CA-IL films exhibited good film forming ability and mechanical properties. The successful grafting of CA with imidazole ionic liquids was confirmed by Fourier transform infrared (FTIR), 1H NMR, scanning electron microscopy (SEM), and elemental analysis, and the grafting degrees were 2.24, 2.45, and 3.30%, respectively. The CO2 permeation properties of the CA-IL films were 65.5, 105.6, and 88.3 Barrer, increased up to 2.0, 3.2, and 2.7 times, respectively, as compared to pure CA (32.6 Barrer). The CO2/CH4 selectivities of the CA-IL films were 15.6, 12.6, and 19.2, increased up to 1.7, 1.4, and 2.1 times, respectively, as compared to pure CA (9.26). Therefore, it can be concluded that the imidazole ionic liquids are immensely useful for improving the gas separation performance of CA films.

Introduction

pan class="Chemical">pan>n class="Gene">Gaspan> film sepapan>ration tn class="Chemical">papan>n class="Chemical">echnology is an important research part in the field of n class="Gene">gas separation, and compared with the traditional pan>n class="Gene">gas separation technology (such as amine absorption, pressure swing adsorption, cryogenic separation, etc.), it has a great application prospect with low-energy consumption, without phase change, high efficiency, and environmental friendliness in oxygen or nitrogen concentration, oil refining, chemical production, carbon dioxide capture, and other fields.[1−3] As the core of gas film separation technology, the film materials with high permselectivity are the purpose of researchers.[4] Polymeric films have some applicable advantages, such as good film forming ability, low cost of raw materials, environmental resistance, and simple operation.[1,2] In this case, a variety of materials have been used for gas separation films in the past few decades. Among these films, polymeric films have been applied in industrial gas separations for decades,[5−7] the main ones being polyimide (PI),[8] poly(vinylidene fluoride) (PVDF),[9] poly(dimethyl siloxane) (PDMS),[10] polysulfone (PSF),[11] polyetherimide (PEI),[12] and cellulose derivatives.[13,14] In addition to the advantages of polymeric films, these films have an inevitable limitation in the field of gas separation applications, which is called the tradeoff of permeability and selectivity.[15,16] Nonetheless, polymeric films are still good candidates for gas separation, but more points of view are necessary to enhance the gas permselectivity above the tradeoff.[17]

pan class="Chemical">pan>n class="Chemical">Cellulosepan>-based materials have bpapan>n class="Chemical">ecn>ome a necessary part of the gas separation because they have a moderate chain structure, high mechanical strength, and good flexibility.[18−22] All kinds of cellulose derivatives, such as ethyl cellulose (EC), cellulose acetate (CA), trifluoroacetylated cellulose, and silyl cellulose, have been prepared to be used for the gas separation film materials.[23−25] Of those film materials, CA was found to be the most suitable and most widely developed membrane material for gas separation and enrichment.[23] As such, there is always a tradeoff between gas permeance and selectivity. Surprisingly, gas separation performance of film materials could be realized by chemical or physical modification to change their microstructure, such as ionic liquids (ILs) as grafting agents and inorganic materials as additives, or by blending with other polymers in the CA polymeric matrix.[24−31]

Ionic liquid is a kind of liquid ionic compound consisting of organic cations and organic (inorganic) anions with the melting n class="Chemical">point at room temperature or near room temperature. It was developed as a new green reaction medium in various industrial fields.[32] Bpan class="Chemical">pan class="Chemical">ecpan>ause of the good solubility and selpapan>n class="Chemical">ectivity of ionic n>n class="Chemical">liquids for CO2, ionic liquids are mainly used for the separation of CO2 in the gas separation film technology; especially, the ionic liquids containing functional imidazole groups are more remarkably effective.[33,34] Authors such as Nikolaeva et al. reported a new poly(diallyldimethyl ammonium)-bis-(trifluoromethylsulfonyl)imide (P(CA)(Tf2N)) membrane for CO2 separation; the ideal CO2/N2 adsorption selectivity of (P(CA)(Tf2N)) was constantly up to 10 bar. The mixed gas permeation tests showed that the P[CA][Tf2N]-based film with a 5 μm-thick selection layer had a 2-fold higher CO2 flow than conventional CA.[35] These results present that the successfully modified CA with IL did not only improve the permeance but also enhance the process stability in a wide range of pressures and concentrations of CO2/CH4 and CO2/N2 gas mixtures.

In this n class="Chemical">pan>per, we continue to work on predicting gas separation in the n class="Chemical">EC blended ionic n>n class="Chemical">liquids and EC grafted with ionic liquids.[36] Based on the CA as a good gas separation film with good flexibility and mechanical strength and imidazole ionic liquids as a good additive with good solubility and selectivity for CO2, the CA grafted with imidazole ionic liquids films was fabricated. The molecular structure of graft CA, morphology, mechanical properties, and CO2 separation performance of membranes were studied subsequently.

Experimental Section

Materials

pan class="Chemical">pan class="Chemical">Cellulose acetatepan> (CA) and ionic papan>n class="Chemical">liquids were purchased from Chembee (Shanghai), and Greenchem ILs (Lanzhou) were used as reactants. Cellulose acetate (the acetyl content was 32.0 wt %; the hydroxyl content was 8.7 wt %.) was different from the purity and type of cellulose acetate used in the literature, resulting in a certain deviation from the literature. Tetrahydrofuran, pyridine, and methanol were purchased by Kaitong Chemical Reagent Co. Ltd. (Tianjin) and were employed after distillation.

Synthesis of CA-ILs

pan class="Chemical">pan class="Chemical">Cellulose acetatepan> grafted with papan>n class="Chemical">imidazole ionic liquids (CA-IL1, CA-IL2, and CA-IL3) was synthesized by reacting cellulose acetate with imidazole ionic liquid (IL1: [HO2CMmim]Cl, IL2: [HO2CEtmim]Cl, and IL3: [HO2CMmim]Br), according to Scheme .

Scheme 1

Synthesis of CA Grafted with Imidazole Ionic Liquids (CA-IL1, CA-IL2, and CA-IL3)

pan class="Chemical">pan>n class="Chemical">Cellulose acetatepan> (2.00 g, 0.04 mmol) was placed into a flask under the protpapan>n class="Chemical">ecn>tion of N2. Then, pyridine (2.0 mL, 24.8 mmol) and THF (50 mL) were added into the flask at room temperature. Subsequently, imidazole ionic liquid (IL1: [HO2CMmim]Cl) (3.66 g, 20.7 mmol) was injected dropwise into the above solution, and it was stirred at 70 °C for 48 h. After the reaction solution was cooled to room temperature, it was poured into a large amount of methanol (1000 mL) for a week. The formed precipitate was concentrated by centrifugal separation and filtered, and the obtained solid was dried in vacuo at 50 °C to give a white solid (CA-IL1) (yield: 78.1%). The CA-IL2 and CA-IL3 were obtained by a similar method with yields of 79.3 and 73.3%, respectively. The yield was calculated using the following equation (eq )where MIL is the added mass of ionic liquid, MCA is the added quantity of CA, and M is the mass of the obtained graft product.

Preparation of Gas Separation Films

The corresponding homogeneous memn class="Chemical">pan class="Chemical">pan class="Chemical">brpn>an>anes of CA and CA-ILs were prepan class="Chemical">pan>red through the coating solution method (concentration of 5 wt % in pan class="Chemical">THF) on clean glass plates and evaporating solvents at room temperature. Finally, the obtained films (thickness: 120.7–134.6 μm) were dried under vacuum at 25 °C for 12 h. The thickness of the films was measured by a thickness gauge (the thickness gauge was purchased from Shanghai Liuling Instrument Factory, the model was a CH-1-B hand-type millimeter thickness gauge, the graduation value was 0.001 mm, the measurement range was 0–1 mm, and the error was about ≤0.007 mm).

Characterization of the CA and CA-IL Films

A Fourier transform infrared spn class="Chemical">pan class="Chemical">pan class="Chemical">ecpn>an>trometer (Sppan class="Chemical">pan>n class="Chemical">ectrum Two, PE compan>ny, Waltham, Massachusetts, USA) was used to characterize the molecular structures of CA and CA-ILs. The 1H NMR (400 MHz) spectra were recorded on a JEOL LEOLEX-400 spectrometer with 16 scans in DMSO-d6 (LEOLEX-400, JOEL Japan). The content of the nitrogen element in the grafted CA was measured using an organic elemental analyzer (PE2400 SERIES II CHNS/O, PerkinElmer, Waltham, Massachusetts, USA). The microstructures of the pure CA and CA-IL films were studied by scanning electron microscopy (SEM) (JSM-6490, JOEL Japan). Mechanical properties of the pure CA and CA-IL films were tested by a film tensile testing machine (XLW(PC)-500 N, Sumspring, Jinan, China) at 25 °C.

The Grafting Degrees of CA-IL Films

The organic elemental analyzer was used to mark the content of the pan class="Chemical">pan>n class="Chemical">nitrogenpan> element in the grafted CA, and then, the mass fraction of the ionic liquid in CA was calculated using the following formula (eq )[36]where d is the grafting degree of ILs in CA. N% is the mass fraction of N, which was determined by using an elemental analyzer. The content of the papan>n class="Chemical">nitrogenn> element is expressed in wt %. MIL and MN are the molpan class="Chemical">ecular weights of the ionic liquid and N element, resppan class="Chemical">ectively.

Gas Separation

The test of mixed pan class="Chemical">pan class="Gene">gaspan> permeation was performed using a differential pressure papan>n class="Gene">gas transmission instrument (GTR-11MH type, GTR TEC Corporation, Kyoto, Japan; the test area was 0.785 cm2. The instrument test temperature was 34 °C, the test pressure was maintained at 49 KPa, the content of the mixed gas was the same, and the test pressure was 0.1 MPa. The carrier gas was H2, and the pressure was 0.5 MPa). The permeability coefficients of the mixed gases were measured by the gas chromatographic method using the differential pressure gas transmission instrument (GTR-11MH type), and the gas permeability coefficient P was calculated using the following relation (eq )where q is the transmission volume (mL), K is the auxiliary positive coefficient (the fixed value is 2); it is the setting point instrument by factory, l is the film thickness (cm), p is the permeability pressure (cmHg), t is the measurement time (s), and a is the area of the gas permeation film (the fixed value is 0.785 cm2).

In this experiment, the n class="Chemical">pan class="Chemical">pan class="Gene">gaspn>an> sepan class="Chemical">pan>ration factor was calculated using the following relation (eq )where pan class="Chemical">PA and PB correspond to single pan class="Gene">gases A and B, resppan class="Chemical">ectively, and they can be calculated from eq .

Results and Discussion

Characterization of CA and CA-ILs

The final desired CAs grafted with ionic pan class="Chemical">pan>n class="Chemical">liquidspan> (Scheme ; CA-papan>n class="Gene">IL1n>, CA-IL2, and CA-IL3) were synthesized through an esterification reaction from the carboxyl groups in imidazole ionic liquid reaction with hydroxyl groups in CA under the protection of N2 at 70 °C for 48 h in THF. The mechanism of pyridine-catalyzed esterification can be seen in the previous research.

The molpan class="Chemical">pan>n class="Chemical">ecpan>ular structures of the CA and CA-ILs were characterized by FTIR. As shown in Figure (CA, CA-papan>n class="Gene">IL1n>, CA-IL2, and CA-IL3), the absorption peak at 1760 cm–1 was credited to the C=O bond stretching vibration of the ester group by esterification reaction from the hydroxyl groups in CA reaction with the carboxyl groups in imidazole ionic liquid; the two peaks around 1620 and 1640 cm–1 were ascribed to the C=N and C=C bond stretching vibrations of imidazole groups in ionic liquid, respectively. The two weak peaks around 2960 and 2860 cm–1 can be ascribed to the C–H bond stretching vibration of methyl and methylene in CA, respectively. Similarly, the peaks at 3080, 3020, and 1400 cm–1 were attributed to the C–H bond stretching vibration in the imidazole-based double bond in ionic liquid. In addition, the observed peaks at 1470, 1250, and 1348 cm–1 were attributed to the C–N bond stretching vibrations of the imidazole group. For the analysis based on IR spectra, the obtained products were tentatively concluded that they are the grafted products.

Figure 1

FTIR spectra of CA, CA-IL1, CA-IL2, and CA-IL3.

FTIR sppan class="Chemical">pan class="Chemical">ecpan>tra of CA, CA-papan>n class="Gene">IL1, CA-class="Chemical">n>n class="Gene">IL2, and CA-IL3.

n class="Chemical">1Hn> NMR spectra were used to elucidate the molecular structures and evaluate the distribution of graft groups of the as-prepared CA-ILs. The typical spectra of 1H NMR of pure CA and the prepared CA-ILs are shown in Figure . In the case of pure CA, the broad peaks from 3.9 to 4.9 ppm are attributed to the proton resonance on the AGU units. The signal at 2.1 ppm was ascribed to protons of OH. The strong signal at 1.8 ppm was assigned to methyl protons of acetyl. Compared with the pure CA, in the case of the CA-ILs, in addition to having the peaks of CA, the CA-ILs also have the peaks of corresponding ionic liquids. Most obviously, there are no signals from the carboxyl hydrogen of ionic liquids. At the same time, the peak areas have decreased at 2.1 ppm from the protons of OH in the CA-ILs, and this is due to the esterification reaction from the carboxyl groups in imidazole ionic liquid reacted with hydroxyl groups in CA. Based on the observed peak assignments and the corresponding integrals, the chemical structure factors of the CA-ILs, there is a reason to believe that the CA grafted with ionic liquids was prepared successfully.

Figure 2

1H NMR spectra of CA, CA-IL1, CA-IL2, and CA-IL3.

1H NMR sppan class="Chemical">ectra of CA, CA-pn>an class="Gene">IL1, CA-IL2, and CA-IL3.

The micropan class="Chemical">pan>n class="Chemical">ecpan>onomics of films were observed by SEM. Figures and show the surface and cross-spapan>n class="Chemical">ecn>tional photographs, respectively, of (Figures a and a) the pure CA film and (Figures b–d and b–d) CA-IL films. From the SEM image, (Figures b–d and b–d) CA-IL films are very similar to the CA film (Figures a and a), and all of them clearly demonstrate a smooth, non-porous, and dense morphology. In addition, the cross-sectional images have also given similar phenomena. This indicated that CA-ILs are grafting products but not blended products. In combination with the IR analysis, we can validate that the CA grafted with ionic liquids was synthesized successfully.

Figure 3

Surface SEM photographs of CA (a), CA-IL1 (b), CA-IL2 (c), and CA-IL3 (d).

Figure 4

Cross-sectional SEM images of CA (a), CA-IL1 (b), CA-IL2 (c), and CA-IL3 (d).

Surface SEM photographs of CA (a), CA-pan class="Chemical">pan class="Gene">IL1pan> (b), CA-papan>n class="Gene">IL2 (c), and CA-n>n class="Gene">IL3 (d).

Cross-span class="Chemical">pan class="Chemical">ecpan>tional SEM images of CA (a), CA-papan>n class="Gene">IL1 (b), CA-pan class="Gene">IL2 (c), and CA-pan class="Gene">IL3 (d).

In this experiment, the final n class="Chemical">products were washed with large pan class="Chemical">pan class="Chemical">methanolpan> at room temperature for 1 week; its purpose is to wash the raw ionic liquid to the n class="Chemical">papan>n class="Chemical">methanol solution. The yields and grafting degrees of the final products are summarized in Table (CA-n>n class="Gene">IL1, CA-IL2, and CA-IL3), and the yields were about 73.3–79.3%, respectively. The grafting degrees of CA-ILs were about 2.34–3.30%, which further indicated that the grafted CA was successfully synthesized.

Table 1

Yields, Grafting Degrees, and the N Element Contents of CA-ILs

sample	yield (%)	N element content (%)	grafting degree (%)
CA-IL1	78.1	0.37	2.34
CA-IL2	79.3	0.36	2.45
CA-IL3	73.3	0.42	3.30

Mechanical Properties of Films

Since the obtained films will be used in the pan class="Chemical">pan>n class="Gene">gaspan> sepapan>ration, the mn class="Chemical">papan>n class="Chemical">echanical properties were tested. As shown in Table , given the test data of the CA and CA-IL films, although the tensile strength and elongation at n class="Chemical">break of the CA-IL films have dn>n class="Chemical">ecreased slightly from being pure, the elasticity moduli of the CA-IL films (Table ; CA-IL1, CA-IL2, and CA-IL3) were 174, 163, and 167 MPa, increased up to 1.7, 1.6, and 1.6 times, respectively, as compared with pure CA film (104 MPa), and the results indicated that the CA-ILs are grafted films. In the phenomenon shown in Table , given the different mechanical properties, the most possible reason is that the molecular structure of CA was changed due to the fact that the CA is grafted with ionic liquids; the molecules are covalently bonded and form strong intermolecular interactions. This eventually led to an increase in the elasticity modulus of the grafted film.

Table 2

Mechanical Properties of CA and CA-IL Filmsa

sample	thickness (μm)	elongation (%)	elastic modulus (MPa)	tensile strength (MPa)
CA	125.5	37.8	104	52.7
CA-IL1	134.6	25.1	174	43.8
CA-IL2	128.5	26.4	163	42.9
CA-IL3	120.7	28.4	167	47.5

Tested at 5.00 mm/min speed. The standard spline had a length of 50 mm and a width of 10 mm.

Gas Separation Properties of Films

The uniform and transn class="Chemical">pan>rent casting solutions of pure CA and the grafted CA were achieved by stirring in THF. The casting solutions were subsequently coated on 10 cm diameter clean glass plates, the solvent was evaporated for 12 h at 25 °C, and the membranes were detached from the plates and dried in vacuo at 25 °C for 24 h to give the homogeneous membranes with similar thicknesses.

To obtain the grafted CA mempan class="Chemical">pan>n class="Chemical">brpan>ane performance in industrial applications, the mixed papan>n class="Gene">gasn> permeances and the selectivities of the obtained grafted membranes are shown in Table . As compared to the CO2/CH4 separation factors of the pure CA membrane (9.26), the CO2/CH4 separation factors of the CA-IL films (Table ; CA-IL1, CA-IL2, and CA-IL3) were up to 15.6, 12.6, and 19.2, respectively. By investigating this reason, we ascribed that the imidazolium group of ionic liquid can interact with carbon dioxide, which raises the adsorption of CO2 gas molecules and continues to undergo the penetration CO2 in the films, ultimately resulting in higher CO2 permeability than pure CA film. Another CH4 gas molecule has a non-polar molecule with a regular tetrahedral structure, but the three ionic liquids added were polar substances. According to the similarity-intermiscibility theory, the solubility of CH4 in the film was poor, which leads to a poor permeation of methane. Therefore, the CA-IL membranes show high CO2/CH4 selectivity.[37]

Table 3

CO2 Permeabilities and CO2/CH4 Separation Factors of the CA Film and CA-IL Films

sample	PCO2 (Barrer)a	PCH4 (Barrer)	PCO2/PCH4
CA	32.6	3.52	9.26
CA-IL1	65.5	4.21	15.6
CA-IL2	105.6	8.38	12.6
CA-IL3	88.3	4.60	19.2

1 Barrer = 10–10 cm3(STP)·cm·cm–2·s–1·cmHg–1.

1 Barrer = 10–10 cm3(pan class="Chemical">pan class="Gene">STpan class="Chemical">Ppan>)·cm·cm–2·s–1·cmHg–1.

In this study, the CA-IL films not only present high n class="Chemical">pan class="Chemical">pan class="Chemical">CO2pn>an>/pan class="Chemical">pan>n class="Chemical">CH4 sepan>ration factors (12.6–19.2) but also present good CO2 permeabilities (65.5–105.6 Barrer), and they are higher than that of pure CA membrane (32.6 Barrer). For the three kinds of CA-IL membranes, the CA-IL2 membrane exhibited the highest CO2 permeability coefficient (105.6 Barrer) with the lowest CO2/CH4 separation factor (12.6), and this is due to the CA-IL2 having a long carboxyethyl spacer in the ionic liquid, which can improve the gas permeability coefficient. The CA-IL3 membrane exhibited the highest CO2/CH4 separation factor (19.2) with an intermediate CO2 permeability coefficient (88.3 Barrer) (Table ; CA-IL3), which was past the 1991 Robeson’s upper bound (Figure ).[38,39] The separation performances of the CA-IL1 and CA-IL2 membranes were lower than that of the 1991 prior upper bound, but they were higher than that of pure CA film (Figure ). In conclusion, the gas permeation properties of the pure CA and CA-IL films give the result CA-IL3 > CA-IL2 > CA-IL1 > CA. In general, the halogen atoms have a highly electronegative and electron-absorbing induction effect, F > Cl > Br > I.[40] In this experiment, the electron donating capacity of anions is Br– > Cl–; the free electrons of bromide in the CA-IL3 membrane can interact with CO2. At the same time, the imidazole group can make them more stable. As a result, the CA-IL3 film having bromide has shown the best gas separation performance.

Figure 5

CO2/CH4 selectivities and CO2 permeabilities of the CA film and CA-IL films in relation to Robeson’s upper bound (1991 and 2008).

pan class="Chemical">pan class="Chemical">CO2pan>/papan>n class="Chemical">CH4 selpan class="Chemical">ectivities and pan class="Chemical">CO2 permeabilities of the CA film and CA-IL films in relation to Robeson’s upper bound (1991 and 2008).

Conclusions

In summary, three kinds of CA grafted with pan class="Chemical">pan>n class="Chemical">imidazole ionic liquidspan> (CA-papan>n class="Gene">IL1n>, CA-IL2, and CA-IL3) were synthesized by cellulose acetate (CA) reaction with imidazole ionic liquids ([HO2CMmim]Cl, [HO2CEtmim]Cl, and [HO2CMmim]Br) via an esterification reaction, and the yields of CA-ILs were about 73.3–79.3%. The grafted CA-IL structure was characterized by FTIR and 1H NMR. The grafting degrees of CA-ILs were 2.34–3.30% by elemental analysis. The CA-ILs exhibited good film forming ability, and their homogeneous phase compact nonporous membranes showed excellent mechanical properties. The carbon dioxide permeance (PCO2) and CO2/CH4 separation factors of the CA-IL membranes were almost 2 times as compared to the pure CA film. Among the three grafted CA films, the CA-IL3 film was the best, and the permeability (PCO2) was up to 88.3 Barrer with the highest permselectivity (PCO2/PCH4 = 19.2) due to the fact that the bromide ion in CA-IL3 can interact well with CO2, which promotes CO2 permeability. Taking the results together, the imidazole ionic liquids are useful grafting agents to raise the gas separation performances of the economical CA membranes.