Preparation of Butadiene-Bridged Polymethylsiloxane/Ethylcellulose/1-Carboxymethyl-3-methylimidazolium Chloride Ternary Composite Membranes for Gas Separation.

Excessive CO2 emissions have resulted in global warming and are a serious threat to the life of people, various strategies have been implemented to cut carbon emissions, and one of them is the use of a gas separation membrane to capture CO2 effectively. In this experiment, the butadiene-bridged polymethylsiloxane (BBPMS)/ethyl cellulose (EC)/ionic liquid (IL) ternary composite membranes were prepared by EC as a substrate, BBPMS, and IL as additives in tetrahydrofuran under high-speed stirring and coated on the membrane. The membrane structure was characterized by a Fourier transform infrared spectrometer and scanning electron microscope, and the membrane properties were tested by a membrane tensile strength tester, thermal weight loss analyzer, and gas permeability meter. The results show that the surface of the ternary composite membrane is dense and flat with a uniform distribution, and the membrane formation, heat resistance, and mechanical properties are good. The permeability coefficient of the ternary composite membrane for CO2 reached 1806.03 Barrer, which is 20.00 times higher than that of the EC/IL hybrid matrix membrane. The permeability coefficient of O2 reached 321.01 Barrer, which is 19.21 times higher than that of the EC/IL membrane. When the doping amount of BBPMS is 70-80%, the O2/N2 gas permeation separation of the BBPMS/EC/IL ternary composite membrane is close to the Robertson 2008 curve. It is always known that in the gas separation process the membrane material is the most crucial factor. The success of this experiment points to a new direction for the preparation of new membrane materials.

Introduction

As a major greenhouse gas, the increasing amount of CO2 in the atmosphere and oceans is not only contributing to global warming and climate change but is also having a significant impact on the growth of some plants and microorganisms.[1] On the other hand, as an abundant and readily available non-toxic carbon resource, CO2 has been used as an important feedstock for the production of chemicals, fuels, and polymers.[2] Therefore, effective, simple, and cost-effective separation and capture of carbon dioxide is important for reducing the greenhouse effect and utilizing carbon resources.[3−5]

Carbon dioxide separation and capture technologies mainly include temperature swing adsorption technology,[6] porous solid adsorption,[7] and membrane separation.[8−10] Among them, the membrane separation methods are widely used for their economic, efficient, and convenient advantages.[11,12] The separation of gases, such as CO2/CH4, CO2/N2, and O2/N2 by membrane separation methods, has a history of being used for several decades.[13] Various polymeric materials with excellent formability and tunable properties, such as ethyl cellulose (EC), silicone rubber, polyolefins, polyimides, and polysulfone, have been widely used as membrane materials for gas separation.[14−16] EC is produced using natural cellulose and has good membrane making properties, high tensile strength, and carbon dioxide as well as oxygen selectivity; therefore, it occupies an important position in the field of carbon dioxide and oxygen separation.[17] However, the carbon dioxide permeability coefficient of the pure membrane of ethylcellulose is not high and to improve the gas separation performance, modification or co-blending of ethylcellulose is a good method, by adding various carbon-based hybrid matrix materials such as zeolites,[18] silica,[19] carbon nanotubes,[20] titanium dioxide,[21] and ionic liquids (IL).[22] Although these methods can increase the permeability or/and selectivity of CO2, their increment is limited due to poor compatibility and a non-uniform distribution. Therefore, it is still important to develop high-performance polymeric gas separation membrane materials with high gas separation performance, good stability, mechanical properties, durability, easy preparation process, and low cost.

In recent years, research on the application of polysiloxanes in the direction of gas separation membranes has become more and more extensive,[23] but little research has been reported on the use of diene-based compounds bridged with polymethylhydrosiloxanes (BBPMS). Diene-bridged polymethylsiloxane has become an important research direction for gas separation membrane materials due to its many unique features, such as simple reaction conditions, high gas permeability, low cost input, and high structural variability.[24,25]

EC as an abundantly available, renewable, and low cost non-ionic cellulose ether was used in a lot of fields, such as food, filtration, microencapsulation, and medicine.[26] Remarkably, EC with a large number of ethyl groups and the unsubstituted hydroxyl groups resulted in it possessing easy film formation[27] and good gas separation performance.[28,29]

In order to further broaden the research of EC as gas separation membrane materials, this paper is based on the groups’ previous adequate research on the blended membrane of 1-carboxymethyl-3-methylimidazolium chloride salt (IL) and EC.[26] A series of butadiene-bridged BBPMS/EC/1-carboxymethyl-3-methylimidazolium chloride (IL) composite membranes with butadiene-bridged BBPMS contents of 40, 50, 60, 70, 80, and 90% were prepared by incorporating polysiloxane. The structural, mechanical, and thermal properties were characterized and tested, and a series of hybrid matrix membranes were focused on O2/N2 and CO2/N2 permeation performance and separation selection performance. It is always known that in the gas separation process, the membrane material is the key factor.[30] In this paper, we propose and prepare an effective and simple EC-based gas separation membrane with preferential permeability to carbon dioxide. Successful preparation of new membrane materials opens up new paths for the development of separation membranes.

Experimental Section

Materials

EC was purchased from Chembee (Shanghai, China). The EC (M70) was 40–100 MPa·s, 95% pure, and contained 5% toluene/isopropanol = 80:20. PMHS and Karstedt’s catalyst were purchased from Kejunchi Technology Co. (Shenzhen, China). 1-Carboxymethyl-3-methylimidazole chloride salt (IL) was purchased from Aulico New Material Technology Co. (Qingdao, China). Butadiene was purchased from Inokai Technology Co. (Beijing, China). Tetrahydrofuran (THF) was purchased from Comio Chemical Reagent Co. (Tianjin, China), the THF is analytically pure, 98% purity.

Preparation of Butadiene-Bridged BBPMS

According to Scheme , a three-necked flask was evacuated for 10 min and flushed with nitrogen. Then, 5 mL of butadiene (approximately 15% of hexane), 0.1 mL of the Karstedt’s catalyst, and 10 mL of THF were added to a three-necked flask. The mixture was stirred at 85 °C for 1 h. Subsequently, 1 mL of PMHS was added and stirred for another 4 h to obtain a colorless and clear solution.

Scheme 1

Synthesis of Butadiene-Bridged BBPMS

Preparation of BBPMS/EC/IL Ternary Blended Membrane

EC (1.80 g, 7.58 mmol) and THF (20 mL) were placed in a flask and stirred at high speed. After the EC was completely dissolved, a well-mixed methanol solution containing 1-carboxymethyl-3-methylimidazolium chloride salt (IL) (8%, 156.52 mg) was added, and then the reaction was continued for 2 h to obtain EC (IL: 8 wt %) homogeneous solution. The reaction was carried out at room temperature.

The BBPMS synthesized in Materials section was added to an EC (IL: 8 wt %) homogeneous solution and continued to react for 1 h at room temperature to obtain a homogeneous cast membrane solution; the cast membrane solution was filtered through a 200 mesh filter and then the membrane was spread on a clean glass plate and dried naturally at room temperature for 12 h. After 12 h of vacuum drying at room temperature, the membrane was removed from the glass plate to obtain a BBPMS/EC/IL ternary composite membrane. The composite membrane composition of this study is shown in Table .

Table 1

Composition of Composite Membranes

no.	sample	BBPMS addition amount (mL)	BBPMS/loadinga (wt %)	composite membranes/matrixb (wt %)
1	IL/EC	0.00	0.00	100.00
2	BBPMS	30.00	100.00	0.00
3	MMM(40%)	1.30	40.00	60.00
4	MMM(50%)	1.90	50.00	50.00
5	MMM(60%)	2.80	60.00	40.00
6	MMM(70%)	4.40	70.00	30.00
7	MMM(80%)	7.50	80.00	20.00
8	MMM(90%)	16.90	90.00	10.00

Mass fraction of BBPMS in the composite membrane.

Mass fraction of EC (IL: 8 wt %) in the composite membrane.

Measurements

A Fourier transform infrared (FTIR) spectrometer (Spectrum Two, PE company, Waltham, Massachusetts, USA) was used to characterize the molecular structures of the functional groups in mixed matrix membranes. The surface structure of gas separation membranes were studied by scanning electron microscopy (SEM, JSM-6490, JOEL, Japan). The thickness of the membranes was measured by a thickness gauge (CH-1-B, Liuling Instrument Factory, Shanghai, China). The graduation value was 0.001 mm, the measurement range was 0–1 mm, and the error was about ≤0.007 mm. Mechanical properties were analyzed with a membrane tensile testing machine (XLW(PC)–500N, Sumspring, Jinan, China) at 25 °C. A thermogravimetric analyzer (Q5000IRS type, American TA Co., Ltd., USA) was used to test the thermal performance of the membranes. During the test, the membranes were heated to 550 °C. The pure gas permeation property tests were performed using a fixed-volume pressure increase instrument time-lag apparatus (VAC-V2 type, Labthink instrument Co., Ltd, Jinan, China) at 34 °C. The membranes were first placed in a stainless-steel cell, and the disk was sealed with rubber O-rings to avoid leakage. Then, both the upstream and downstream sides of the system were degassed at ambient temperature to ensure that the system was not disturbed. Next, pure gases (CO2, O2, and N2) were fed into the membrane individually, and the permeability of each pure gas was measured at a feed pressure of 2 bar. The permeability coefficients of the mixed gas were measured by a gas chromatographic method using a differential pressure gas transmission instrument (GTR-11MH type) at 34 °C.[31]

Results and Discussion

Infrared Analysis of Membranes

The peaks and absorption bands of components in the obtained ternary composite membranes were observed and assigned by FTIR spectrometry. Judging from Figure , the two weak spectral bands of 2954 and 2840 cm–1 belong to the C–H bond stretching vibration of methyl and methylene groups in EC, respectively. The Si–CH3 absorption peak of BBPMS appears at 1261 cm–1. However, the Si–H characteristic absorption peak of BBPMS does not appear at 2165 cm–1, probably because the double bond in 1-carboxymethyl-3-methylimidazolium chloride (IL) reacts with the Si–H bond, making the Si–H bond completely reacted. The absorption peak at 1568 cm–1 originates from the vibration of the imidazole ring skeleton. Therefore, it can be proved that the presence of IL, EC, and BBPMS in the mixed matrix membrane, and the gradual decrease of the methyl/methylene peak in the IR spectrum with the gradual increase of the doping amount of BBMPS, proves that the amount of EC gradually decreases, which is also consistent with the doping ratio of the composite membrane.

Figure 1

FTIR spectrum of the ternary composite membrane [(a): MMM(90%), (b): MMM(80%), (c): MMM(70%), (d): MMM(60%), (e): MMM(50%), and (f): MMM(40%)].

FTIR was used to verify the change of the double bond of the EC, IL, BBPMS, and ternary composite membrane (because the IR images of different scales of ternary composite membranes are similar, they are referred to by the same curve). As shown in Figure , the C=C characteristic absorption peak of IL does appear at 1680 cm–1 and the Si–H characteristic absorption peak of BBPMS does appear at 2165 cm–1. Both the C=C double bond and Si–H are missing in the final ternary composite membranes, which proves the successful reaction of IL with BBPMS.

Figure 2

FTIR spectrum of the EC, IL, BBPMS, and ternary composite membrane.

SEM Analysis of Membranes

Figure gives the optical pictures of the EC/IL membrane and the BBPMS/EC/IL ternary composite membranes. Compared to the colorless and transparent EC/IL membrane, the ternary composite membrane become less and less transparent as BBPMS increases. However, the ternary composite membranes also show the same excellent flexibility with repeated curling without deformation or breakage as the EC/IL membrane.

Figure 3

Optical photos [(A) transparency of the membrane varies with the amount of BBMPS doping and (B) flexibility of the membrane] of the EC/IL membrane (g) and BBPMS/EC/IL mixed matrix membranes [(a): MMM(90%), (b): MMM(80%), (c): MMM(70%), (d): MMM(60%), (e): MMM(50%), and (f): MMM(40%)].

To further observe the distribution of BBPMS in the ternary composite membranes by SEM analysis. Figure gives the surface SEM images of the EC/IL membrane (g), (h) pure BBPMS, and BBPMS/EC/IL ternary composite membranes [(a): MMM(90%), (b): MMM(80%), (c): MMM(70%), (d): MMM(60%), (e): MMM(50%), and (f): MMM(40%)]. According to Figures and S1, the surface of EC/IL membrane is dense, flat and smooth. Compared with the EC/IL membrane, the surface changes of the BBPMS/EC/IL ternary composite membranes became more and more obvious with the increase of BBPMS content.However, the surface of the membranes were still uniformly distributed without agglomeration, indicating that homogeneous ternary composite membranes of EC-doped BBPMS and IL were successfully prepared.

Figure 4

Surface SEM images of the EC/IL membrane (g) and BBPMS/EC/IL mixed matrix membranes [(a): MMM(90%), (b): MMM(80%), (c): MMM(70%), (d): MMM(60%), (e): MMM(50%), (f): MMM(40%), and (h): BBPMS].

Mechanical Property Analysis

As the gas separation membranes, they should have enough tensile strength and elongation at break during use. Figure shows the test data on the tensile strength and elongation at break of the EC/IL membrane and BBPMS/EC/IL ternary composite membranes with membrane thicknesses between 117 and 136 μm. From Figure , it can be seen that the tensile strength and elongation at break of the composite membranes increased when the doping amount of BBPMS was 40 and 50% compared with the undoped membranes. The elongation at break and tensile strength of the composite membrane reached the maximum when the doping amount of BBPMS was 50%. When the doping amount of BBPMS exceeded 50%, the elongation at break and tensile strength started to decrease. In addition, with the increase of BBPMS doping, the elongation at break of the ternary composite membranes eventually became smaller than that of the EC/IL membrane. However, the tensile strength was always higher than that of the EC/IL membrane. This may be due to the increase of BBPMS doping, the silicon hydrogen bond in the polysiloxane breaks to form a hydrogen bond, and then the broken hydrogen bond combines with the −OH in the EC. The BBPMS/EC/IL hybrid membranes formed a kind of structure with interpenetrating networks.[31]

Figure 5

Mechanical properties of the EC/IL membrane and BBPMS/EC/IL membrane.

Thermal Property Analysis

The thermogravimetric (TG) test plots of the EC/IL membrane and BBPMS/EC/IL ternary composite membrane are given in Figure . From Figure , it can be seen that the temperature at 5% weight loss is in the range of 260–280 °C. With the increase of BBPMS doping, the weight loss temperature gradually increases, which is due to the fact that BBPMS is a cross-linked polymer whose thermal stability is better than that of EC. The residual amount of the mixed matrix membrane at 600 °C increased with the increase of BBPMS doping. The presence of BBPMS in a prepared hybrid matrix membrane was also demonstrated by the TG test.

Figure 6

TG analysis of the EC/IL membrane and BBPMS/EC/IL membrane.

Gas Separation Properties

The prepared BBPMS/EC/IL ternary composite membranes were tested for the single component gas permeability of CO2, N2, and O2, and the test data are shown in Table . According to the results in Table , the BBPMS complex membrane shows good gas transmission performance. CO2 permeability of the BBPMS complex membrane is up to 1417.01 Barrer, O2 permeability of the BBPMS complex membrane is up to 248.63 Barrer, and N2 permeability of the BBPMS complex membrane is up to 97.12 Barrer. In contrast, its separation performance is not that good. The PCO/PN permselectivity values of the BBPMS membrane is 14.59 and the PO/PN permselectivity values of the BBPMS complex membrane is 2.56. Meanwhile, it can be seen that the gas permeability of the membranes gradually increased with the increase of the BBPMS doping amount, and the gas separability first increased and then gradually decreased. When the doping of BBPMS reaches 90%, the CO2 permeation of the MMM(90%) membrane was 1806.03 Barrer, which is 20.04 times higher than that of the IL/EC membrane, which is due to the high gas permeability of the introduced BBPMS structure itself.[32] Combining the solubility and diffusion coefficients of CO2, O2, and N2 gases listed in Table , it can be found that the CO2 permeation was dominated by dissolution, and its solubility has a large increase with the addition of BBPMS, which also proved that the addition of BBPMS has shown great improvement in the CO2 permeation. Generally, in the prepared BBPMS/EC/IL mixed matrix membranes compared to the prepared EC/IL membranes, the separation factor of CO2/N2 increases significantly and the O2/N2 separation coefficients are not very different. (The calculation formula has been placed in the Supporting Information).

Table 2

Gas Permeability, Gas Solubility, and Diffusivity Coefficients of BBPMS Complex Membrane, EC/IL Membrane, and BBPMS/EC/IL Membrane Measured (Single-Component Gas Permeability)

		P(Bar)a			α		Sb			Dc
no.	samples	CO2	O2	N2	CO2/N2	O2/N2	CO2	O2	N2	CO2	O2	N2
1	MMM(90%)	1806.03	321.01	103.22	17.51	3.11	48.11	5.12	2.87	3.75	6.27	3.60
2	MMM(80%)	1034.09	208.03	55.32	18.72	3.76	26.52	3.99	1.24	3.90	5.21	4.46
3	MMM(70%)	585.99	124.82	30.73	19.14	4.07	14.23	2.50	0.97	4.13	4.99	3.16
4	MMM(60%)	405.98	76.05	20.23	20.09	3.76	10.20	2.21	0.69	3.98	3.44	2.94
5	MMM(50%)	350.06	46.53	13.10	26.73	3.55	8.52	1.45	0.57	4.11	3.21	2.31
6	MMM(40%)	212.09	29.42	10.44	20.39	2.83	6.25	1.32	0.39	3.39	2.23	2.69
7	IL/EC	90.10	16.71	3.83	23.52	4.36	3.12	2.01	0.11	2.89	0.83	3.39
8	BBPMS complex membrane	1417.01	248.63	97.12	14.59	2.56	30.44	4.56	2.14	4.66	5.45	4.54

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

Ln 10–3 cm3(STP) cm–3 cmHg–1.

Ln 10–6 cm2 s–1.

The gas permeation selectivity of the prepared ternary composite membrane was also compared with the Robeson curve.[30] CO2/N2 permeability of the BBPMS/EC/IL membranes were determined, and the results are shown in Figure a. When BBPMS were doped at 50, 70, 80, and 90%, the CO2/N2 capacity of the composite membrane exceeded the Robeson line of 1991. Especially, when the doping amount of BBPMS was 90%, the CO2/N2 capacity of the composite membrane has been closer to the Robeson line of 2008. The O2/N2 permeability of the BBPMS/EC/IL membranes were also determined, and the results are shown in Figure b. When BBPMS were doped at 70 and 80%, the O2/N2 capacity of the composite membrane exceeded the Robeson line of 1991. When the doping amount of BBPMS was 90%, the O2/N2 capacity of the composite membrane has been closer to the Robeson line of 1991. All the above data show that this ternary composite membrane idea is successful.

Figure 7

Plot of permselectivity vs permeability for the gas pairs (a) CO2/N2 and (b) O2/N2 (single component gas permeability).

The prepared BBPMS/EC/IL ternary composite membranes were tested for the mixed component gas permeability of CO2/N2 and O2/N2, and the test data are shown in Table . According to the results shown in Table , the BBPMS complex membrane shows good gas transmission performance. CO2 permeability of the BBPMS complex membrane is up to 1326.01 Barrer, O2 permeability of the BBPMS complex membrane is up to 224.62 Barrer, and the transmission amounts of N2 in different gas mixtures are 87.80 (CO2/N2) and 80.24 (O2/N2), respectively. In contrast, its separation performance is not so good. The PCO/PN permselectivity values of BBPMS membrane is 15.12, and the PO/PN permselectivity values of the BBPMS complex membrane is 2.80. Meanwhile, it can be seen that the gas permeability of the membranes gradually increased with the increase of the BBPMS doping amount, and the gas separability first increased and then gradually decreased. When the doping of BBPMS reached 90%, the CO2 permeation of the MMM(90%) membrane was 1577.02 Barrer, which is 20.12 times higher than that of the IL/EC membrane, which is due to the high gas permeability of the introduced BBPMS structure itself.[32] Combining the solubility and diffusion coefficients of CO2/N2 and O2/N2 gases in Table , it can be found that the CO2 permeation was dominated by dissolution, and its solubility has a large increase with the addition of BBPMS, which also proved that the addition of BBPMS shows great improvement in the CO2 permeation. Generally, in the prepared BBPMS/EC/IL mixed matrix membranes compared to the prepared EC/IL membranes, the separation factors of CO2/N2 and O2/N2 increase significantly. (The calculation formula has been placed in the Supporting Information).

Table 3

Gas Permeability, Gas Solubility, and Diffusivity Coefficients of the BBPMS Complex Membrane, EC/IL Membrane and BBPMS/EC/IL Membrane Measured (Mixed Component Gas Permeability)

		P(Bar)a				α		Sb				Dc
		CO2/N2		O2/N2		CO2/N2	O2/N2	CO2/N2		O2/N2		CO2/N2		O2/N2
no.	samples	CO2	N2	O2	N2			CO2	N2	O2	N2	CO2	N2	O2	N2
1	MMM(90%)	1577.02	80.60	269.61	70.92	19.62	3.81	43.61	2.60	4.81	2.42	3.62	3.13	5.63	3.00
2	MMM(80%)	903.10	43.21	174.70	38.81	20.90	4.52	24.20	1.11	3.62	1.00	3.69	3.92	4.90	3.91
3	MMM(70%)	508.32	23.72	104.80	22.32	21.43	4.69	12.92	0.92	2.30	0.81	3.93	2.61	4.60	2.83
4	MMM(60%)	352.21	15.63	63.81	14.49	22.61	4.40	9.33	0.72	2.11	0.62	3.81	2.20	2.99	2.42
5	MMM(50%)	306.43	10.21	39.12	9.10	30.00	4.33	7.71	0.63	1.32	0.53	4.00	1.73	3.01	1.80
6	MMM(40%)	185.32	8.10	24.69	7.32	22.82	3.42	5.59	0.44	1.20	0.32	3.32	1.99	2.10	2.42
7	IL/EC	78.40	3.12	14.03	2.93	25.32	4.91	2.81	0.13	1.79	0.10	2.83	3.13	0.83	2.91
8	BBPMS complex membrane	1326.01	87.80	224.62	80.24	15.12	2.80	35.60	2.20	4.50	2.13	3.73	4.02	5.01	3.81

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

Ln 10–3 cm3(STP) cm–3 cmHg–1.

Ln 10–6 cm2 s–1.

The gas permeation selectivity of the prepared ternary composite membrane was also compared with the Robeson curve.[33] CO2/N2 permeability of the BBPMS/EC/IL membranes were determined, and the results are shown in Figure a. When BBPMS were doped at 50, 70, 80, and 90%, the CO2/N2 capacity of the composite membrane exceeded the Robeson line of 1991. Especially, when the doping amount of BBPMS was 90%, the CO2/N2 capacity of the composite membrane was closer to the Robeson line of 2008. The O2/N2 permeability of the BBPMS/EC/IL membranes were also determined, and the results are shown in Figure b. When BBPMS was doped at 70, 80, and 90%, the O2/N2 capacity of the composite membrane exceeded the Robeson line of 1991. When the doping amounts of BBPMS were 70 and 80%, the O2/N2 capacity of the composite membrane was closer to the Robeson line of 2008. All the above data show that this ternary composite membrane idea is successful.

Figure 8

Plot of permselectivity vs permeability for the gas pairs (a) CO2/N2 and (b) O2/N2 (mixed component gas permeability).

Conclusions

The ternary composite membranes with different ratios of BBPMS/EC/IL were prepared based on the previous study of EC/IL hybrid matrix membranes. The composite membranes have good mechanical properties and can maintain less than 5% thermal weight loss at 260 °C. The prepared composite membranes showed great improvement in the permeability of CO2 and O2. The CO2 permeation reaches 1806.03 Barrer at 90% doping of BBPMS, which is 20.04 times higher than that of the MMM(0%) membrane. O2 permeation reaches 321.01 Barrer, which is 19.21 times higher than that of the MMM(0%) membrane. Moreover, when the doping of BBPMS is 70 and 80%, the O2/N2 permeation performance of the membrane exceeded the Robertson 1991 curve and closed to the Robertson 2008 curve.

In summary, the BBPMS/EC/IL ternary composite membranes exhibited excellent gas separation performance, thermal stability, and mechanical property, which made it have more potential as a gas separation material.