Microporous Ceramic Tubule Based and Dendrimer-Facilitated

Article pubs.acs.org/IECR

Microporous Ceramic Tubule Based and Dendrimer-Facilitated Immobilized Ionic Liquid Membrane for CO2 Separation Xingming Jie, John Chau, Gordana Obuskovic, and Kamalesh K. Sirkar* Otto H. York Department of Chemical, Biological and Pharmaceutical Engineering, Center for Membrane Technologies, New Jersey Institute of Technology, Newark, New Jersey 07102, United States ABSTRACT: A solvent evaporation method has been used to prepare a supported ionic liquid (IL) membrane in a microporous ceramic tubule-based module; its separation performance for CO2−N2 and CO2−He mixtures has been reported here over a pressure range of 50−250 psig (344.7−1724 kPag) and temperatures up to 100 °C. Solutions of IL-ethanol having different IL contents were used to prepare 1-butyl-3-methylimidazolium dicyanamide ([bmim][DCA]) membranes having different thicknesses. Membranes prepared from 25 wt % IL in ethanol solution showed the best gas permeation and separation performance. Ionic liquid membranes prepared from 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([emim][Tf2N]) under the same conditions displayed a slightly higher gas permeance, but its separation capability was somewhat lower. Tests at higher temperatures using the [bmim][DCA] membrane generated obviously higher gas permeances, but the ideal separation factors of CO2 over N2 and He dropped sharply. These tests were carried out with a dry feed gas. Adding polyamidoamine (PAMAM) dendrimer generation 0 to IL did not show any improvement when the feed gas was dry. After the feed gas was humidified, both the permeance and the separation factor of CO2 over N2 and He were improved greatly. This is due to lower viscosity caused by dissolved water in the IL membrane and additional reactions between CO2 and tertiary amine units of dendrimer in the presence of water. A set of gas mixture-based test results confirmed the facilitating effects of dendrimer to enhance CO2 permeation with humidified feed gas.

1. INTRODUCTION Governments around the world are making efforts to reduce carbon dioxide emission by separating CO2 from released gas mixtures for later sequestration;1,2 this has been necessitated by large amounts of CO2 emission caused by anthropogenic activities which has raised earth’s surface temperature and altered our living environment in dangerous and harmful ways.3 Currently gas absorption in a solvent is still the most widely used industrial method for CO2 removal. However, this process needs high capital investment; further absorbent regeneration usually requires high temperature resulting in high energy consumption. Compared with solvent absorption, separating CO2 by membrane permeation shows obvious advantages e.g., simple process, easy to scale up, no phase change (low energy cost), and environmentally friendly. There are a variety of membrane-based techniques for CO2 capture.4 Among various membrane techniques being investigated, facilitated transportbased liquid membrane especially supported IL membrane has recently attracted a lot of attention. Since ILs have high thermal stability and extremely low vapor pressure, supported IL membrane will be stable. Based on reported research results, usually ILs have much higher CO2 solubility than gases such as N2 and H2. Shiflett and Yokozeki5 and Yokozeki et al.6 have demonstrated that solubility selectivity of CO2 over H2 can be as much as 30−300 at room temperature for the IL of 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]). Yuan et al.7,8 carried out solubility selectivity tests for CO2 over N2 with different ILs and found that 1,1,3,3-tetramethylguanidium lactate (TMGL) had a CO 2 −N 2 solubility selectivity of around 59.6. Meindersma et al.9 reviewed the application of task-specific ionic liquids for intensified separation and suggested that 1© XXXX American Chemical Society

butyl-3-methylimidazolium dicyanamide ([bmim][DCA]) could be a good choice for CO2 separation. All these solubility test results revealed that carbon dioxide separation from other gases using supported IL membrane could be possible and applicable. There are quite a few studies to prepare supported IL membrane using different porous materials including porous polymeric membranes as support for gas separation, especially in the area of CO2 separation;10−16 some of them have involved higher temperatures and pressures. Using porous hydrophilic polytetrafluoroethylene (PTFE) membrane as support, Hanioka et al.17 prepared a novel supported liquid membrane (SLM) based on a task-specific ionic liquid to achieve selective and facilitated CO2 transport. Using ILs and Nafion composite membranes, Yoo et al.18 developed SLMs for CO2 separation with good CO2−CH4 separation factors. Vangeli et al.19 prepared chemically stabilized IL membranes with nanoporous ceramic supports and tested the separation factors of CO2 over CO at high temperatures using the prepared SLM. Close et al.20 prepared nanoporous alumina-supported IL membranes and investigated the effect of the support on separation performance of the CO2−N2 system. There have been also several reports about the influence of water on SLM performance. Zhao et al.21 investigated the effect of water in the IL on the separation performance of supported ionic liquid membrane for Special Issue: Doraiswami Ramkrishna Festschrift Received: March 31, 2015 Revised: June 4, 2015 Accepted: July 2, 2015

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DOI: 10.1021/acs.iecr.5b01214 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Physical Properties of ILs [bmim][DCA] and [emim][Tf2N]24,25,28 IL

formula

density at 25 °C (g/mL)

viscosity at 20 °C (cP)

surface tension at 25 °C (mN/m)

thermal decomposition point (°C)

purity (%)

[bmim][DCA] [emim][Tf2N]

C10H15N5 C8H11F6N3O4S2

1.06 1.52

39.14 35.55

48.5 37.5

300a 455

>98 >97

a Reference 24 recommends maximum operating temperature range 393−433 K. Reference 25 identifies 240 °C as the initiation temperature for decomposition.

Figure 1. Molecular structures of two tested ILs.

CO2−N2; they found a small addition of water in [bmim][BF4] improved the membrane performance. Kasahara et al.22 have carried out similar research and concluded that strong water holding ability of tetrabutylphosphonium proline resulted in large absorption of CO2 and thus increased its permeability. The ionic liquids investigated in this study as a supported liquid membrane were [bmim][DCA] and [emim][Tf2N]. It is known from the literature23 that with an increase in the length of the alkyl side chain of the cation, CO2 solubility increases, whereas the CO2 diffusion coefficient decreases. Both properties CO2 solubility and CO2 diffusivity are important in the membrane separation process of interest. The maximum operating temperature recommended for [bmim][DCA] is between 393 and 433 K24 from another reference the value for the start of its decomposition is 240 °C.25 The highest temperature employed in this study, namely 100 °C, is clearly below the maximum operating temperature range. In the present study, supported IL membranes were prepared using the solvent evaporation method applied to a microporous ceramic tubule-based module. The IL membrane thickness was controlled by the IL concentration in the starting solution. Individual permeances of CO2, N2, and He were measured over a pressure range of 50−250 psig (344.7−1724 kPag) and temperatures up to 100 °C. High temperature test performance of prepared IL membranes has been examined at high pressures. High temperature tests have also been carried out to study the effect of adding PAMAM dendrimer of generation 0 to the IL membrane and improving its performance with and without water in the feed gas. At the end, a set of tests with gas mixtures was carried out to confirm the facilitating effect of dendrimer in IL. Our earlier studies of such a mixture of the IL [bmim][DCA] and PAMAM dendrimer generation 0 as an absorbent in a pressure swing membrane absorption process at higher temperatures around 75−100 °C and pressures around 150−250 psig (1034−1724 kPag) are detailed in refs 26 and 27.

shown in Figure 1. Ethanol of purity higher than 99.95% was purchased from Sigma-Aldrich (St. Louis, MO) and was used as received. Polyamidoamine (PAMAM) dendrimer (generation 0) was purchased from Dendritech (Midland, MI 48642). Its molecular structure is shown in Figure 2. It was received as a

Figure 2. Structure of PAMAM dendrimer of generation 0.

dendrimer-methanol solution in which dendrimer concentration was 62.35 wt %. To get pure dendrimer, the solution was subjected to vacuum for several days under relatively high temperature (about 60 °C) to remove the methanol. 2.2. Ceramic Membrane Module Washing and Drying. The ceramic membrane module was obtained from Media and Process Technology (Pittsburgh, PA). As shown in Figure 3, the module contains a single ceramic tubule in a stainless steel housing. The surfaces of the ceramic tube and the pores as supplied were completely hydrophobized by a nonafluorohexylsilane coating. Table 2 provides detailed dimensional information on the ceramic membrane module. It is to be noted that the pore size of the top surface is 5 nm, whereas the bulk has a pore size of ∼0.2 μm. As a result, the breakthrough pressure for this membrane module for the ionic liquid [bmim][DCA] in the 5 nm pores is quite high.26,29 Before preparing a supported IL membrane, the module should be washed with ethanol and then dried to make sure that the micropores of the ceramic membrane are open. At first ethanol was introduced into the shell side which was then connected to a N2 cylinder. Under a N2 pressure of 300 psig (2068 kPag), ethanol was pushed to pass through the micropores and remove any residual high boiling point solvent such as IL remaining in the micropores from earlier studies. Then ethanol on the shell side was removed, and N2 at 300 psig (2068 kPag) pressure was introduced to the shell side to remove any ethanol left in the micropores. At the end, the

2. MATERIAL AND METHODS 2.1. Materials. Ionic liquids, 1-butyl-3-methylimidazolium dicyanamide ([bmim][DCA]) and 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([emim][Tf2N]), were purchased from EMD Chemicals (Philadelphia, PA) and used as received. Their detailed physical properties as identified in ref 28 are listed in Table 1; the purities provided by the manufacturer are also listed. Their molecular structures are B


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Industrial & Engineering Chemistry Research

Figure 3. Ceramic tubule membrane module.

Table 2. Dimensional Characteristics of Ceramic Tubule Membrane Module module

ODa (cm)

IDb (cm)

Lc (cm)

pore size (μm)

pore size of the top surface (nm)

VVFd

tubule no.

surface areae (cm2)

ceramic

0.57

0.35

44.0

∼0.2

5

∼0.4

1

78.75

a

OD: outer diameter of tubule. bID: inner diameter of tubule. cL: effective tubule length. dVVF: void volume fraction. eCalculation was based on outer diameter of tubule; membrane porosity, 0.4.

ceramic membrane module was put inside a PV-222 oven (Espec Corp, Hudsonville, MI) which was kept at 100 °C for 8 h to evaporate any residual ethanol. This process is always repeated before preparing a different IL membrane to make sure that the porous ceramic tubule is in its original state. 2.3. Preparation of Ceramic Tubule Based Supported IL Membrane. Figure 4 illustrates the steps followed to prepare a microporous ceramic tubule based supported IL membrane. First a certain amount of IL/ethanol solution was put into the tube side of the dried ceramic membrane module; then using a certain N2 pressure, the solution was pushed into the micropores. Next the excess solution in the tube side was removed, and the membrane module was put into an oven at 100 °C for a certain time; flow of a carrier gas such as N2 in the tube side and vacuum applied on the shell side helped to remove ethanol in the IL solution forming a supported IL membrane. As shown in Figure 4(f), the IL membrane existed in the micropores close to the outer surface because of the vacuum applied on the shell side. The membrane thickness will be influenced by the IL concentration in the starting ethanol solution. 2.4. Gas Permeation Test of Ceramic Tubule Based IL Membrane. Figure 5 shows the gas permeation test system used. The ceramic membrane module was put into an oven to control the exact test temperature desired. When the feed gas was changed, before making a permeance measurement, the gas in the module tube side was completely replaced to eliminate any possible influence. There was an extra water supply system especially for humidified gas test. Feed gas introduced from cylinder entered the humidifier located in the oven to be fully humidified before entering the membrane shell side. Water was injected by a syringe pump at a certain flow rate. A RH transducer was connected between the humidifier and the membrane module to measure the relative humidity of the feed gas. For the humidified feed gas permeation test, there will be a water vapor absorber at the permeate side to remove any permeated water vapor. The permeation flux was measured with a soap bubble flowmeter; the gas permeance, (P/l), (gas permeation flux per unit driving pressure difference) is calculated by eq 1

P /l =

pp Vp

Vm RT At(pr − pp )

(1)

Here pp is the pressure of the permeate side (cm Hg), pr is the pressure of the retentate side (cm Hg), Vp is the volume of permeant gas during the test time (cm3), Vm is the molar volume of gas under standard conditions (cm3(STP)/mol), t is the test time (s), A is the effective membrane area (cm2), l is the membrane thickness (cm), R is the gas constant (J/mol-K), and T is the test temperature (K). The unit of permeance is GPU (1GPU = 10−6 cm3 (STP)/ (cm2-s-cm Hg)). The ideal separation factor of gases i and j, α (i/j), is defined by eq 2: α(i /j) = (P /l)i /(P /l)j

(2)

The ideal separation factor has been determined most often by using pure component permeance values of individual gases. For gas mixture permeation tests, the feed gas mixture was introduced into the shell side of the ceramic tubule. Feed gas was released at the other port with a much higher flow rate than that on the permeate side to maintain an almost uniform composition in the shell side. Carrier gases either He (for CO2−He mixture) or N2 (for CO2−N2 mixture) at a controlled flow rate was passed through the tubule tube side to bring any permeated gas out to a Quantek Model 906 CO2 Analyzer (Grafton, MA) to determine its CO2 concentration. Based on the feed gas mixture CO2 concentration, the flow rates of the carrier gas and the permeated gas, one can calculate the CO2 concentration in the permeated gas. As in the pure gas permeation tests, for humidified feed gas permeation tests, there was a water vapor absorber at the permeate side to remove any possible permeated water vapor.

3. RESULTS AND DISCUSSION 3.1. Gas Permeation Performance of the Original Ceramic Tubule Membrane Module. Before preparing a supported IL membrane with the microporous ceramic tubule membrane module, its original gas permeation performance was tested with pure gases such as CO2, N2, and He since this could partly reveal its microstructure. Dry feed gas was used to make the test. C


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Figure 4. Various steps in the process of preparing a microporous ceramic tubule based supported IL membrane.

smaller molecular size, He permeated the fastest. When the test pressure increased from 10 psig to 40 psig (68.95 to 275.8 kPag), its permeance increased from 466 GPU to 499 GPU. It should be pointed out here that the pure gas permeation test was always repeated before preparing a different IL membrane in order to find out if there is any change of gas permeation behavior. Details of such permeation performances will not be reported here since at the most there was a maximum of 15% permeance difference which was considered acceptable. 3.2. Influence of IL Concentration on Performance of the Prepared Membrane. During the process of forming an IL membrane in the micropores of the ceramic tubule by solvent evaporation, IL concentration in the solvent is very important since it will directly determine the final IL membrane thickness. In order to find an appropriate IL concentration for the corresponding membrane formation, IL membranes were prepared from 15 wt %, 25 wt %, and 35 w% [bmim][DCA] in ethanol, and their gas permeation and separation performances

Results presented in Figure 6 show that at room temperature the ceramic tubule membrane possessed stable and high values of gas permeance in the pressure range tested. For all three gases tested, the values of the permeance were several hundred GPU. Among them N2 displayed the lowest gas permeance of around 220 GPU since it has the largest molecular size. When the test gas pressure increased from 10 psig to 40 psig (68.95 to 275.8 kPag), N2 permeance increased from 207 GPU to 224 GPU indicating a reasonable behavior. Next came carbon dioxide. As shown in Figure 6, when the test gas pressure increased from 10 psig to 40 psig (68.95 to 275.8 kPag), CO2 permeance increased from 407 GPU to 426 GPU. Usually CO2 is considered as a condensable polar gas; it is believed that it has a kind of strong affinity with the inner surface of the micropores that will facilitate its permeation through the pores in the ceramic tubule. Thus, even though it has a molecular size close to that of N2, its permeance was still much larger. However, it did not exceed the permeance of a much smaller gas, helium. Compared with CO2 and N2, because of its much D


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Figure 5. Gas permeation performance test system.

Figure 6. Gas permeation performance of ceramic membrane module at room temperature.

former had a value around 19.4−22.8, and the latter showed a value around 6.6−9.1. It is clear that in the prepared IL membrane, CO2 permeated much faster than helium which has a much smaller molecular size. Usually for glassy polymeric gas separation membranes of cellulose acetate, polyimide, etc., a gas having a smaller molecular size permeates faster since these membranes are characterized as “diffusivity-selective” membranes. Here in the IL membrane, even though He has a higher diffusion coefficient than CO2 by a factor of 1.2 (at 100 °C) to 2 (at 25 °C), the solubility of CO2 is substantially higher than that of He (2.9 at 100 °C and 33.2 at 25 °C).29 Thus, carbon dioxide permeates much faster than helium which has a much smaller molecular size since the permeability coefficient P is a product of the diffusion coefficient and the solubility coefficient. The present

were tested. The detailed results are shown respectively in Figures 7, 8, and 9. 3.2.1. IL Membrane Prepared from 15 wt % IL in Ethanol Solution. The IL membrane prepared from 15 wt % IL ([bmim][DCA]) in ethanol solution presented a high gas permeation performance (Figure 7) at room temperature. When the test pressure was increased from 50 psig to 250 psig (344.7 kPag to 1724 kPag), CO2 permeance increased from 11.0 to 15.3 GPU, an approximate increase of 50%. Compared with that for CO2, pressure increase did not show an obvious influence on the gas permeances of He and N2. Nitrogen permeance increased from 0.57 to 0.72 GPU; He permeance was quite stable at ∼1.63−1.68 GPU. In terms of separation performance, the ideal separation factors of CO2 over N2 and over He were relatively stable at various test gas pressures. The E


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Figure 7. Influence of test pressure on IL membrane gas permeation (a) and separation (b) performance at room temperature (prepared from solution of 15.0 wt % [bmim][DCA] in ethanol).

IL membranes are therefore called “solubility-selective” as reported earlier.30,31 3.2.2. IL Membrane Prepared from 25 wt % IL in Ethanol Solution. Figure 8 shows the results of an IL membrane prepared from a higher concentration of the same IL in an ethanol solution (25 wt %) at room temperature. Higher IL concentration means more IL which will lead to a thicker liquid membrane reducing the gas permeance. When the test pressure was increased from 50 to 250 psig (344.7 kPag to 1724 kPag), CO2 permeance increased from 1.54 to 2.72 GPU which is much smaller than that of the membrane prepared from 15 wt % IL-ethanol solution. Nitrogen and helium also showed increased permeances with the feed gas pressure. A sharp increase in permeance was observed at a feed gas pressure of 250 psig (1724 kPag), especially for N2. Permeation performance will determine the separation performance. So when the

feed gas pressure was increased from 50 psig to 250 psig (344.7 kPag to 1724 kPag), the separation factor of CO2 over N2 correspondingly showed a sharp decrease from 66 to 20.1. It is believed that 250 psig (1724 kPag) gas pressure may be too high and may have generated some defects leading to a higher N2 permeance: much lower separation factor will be inevitable. At 200 psig (1379 kPag) feed gas pressure, the IL membrane from 25 wt % solution still possessed an ideal separation factor of CO2 over N2 as high as 38.2 which is quite encouraging. Though helium also showed an obvious permeance increase at 250 psig (1724 kPag), the separation factor of CO2 over He was relatively stable with a value ∼15.1 to 20.3 for various gas pressures tested. 3.2.3. IL Membrane Prepared from 35 wt % IL in Ethanol Solution. The IL membranes prepared from 35 wt % ILethanol solution showed the lowest gas permeances among all F


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three IL membranes tested at room temperature (Figure 9). Its CO2 permeances increased from 1.36 to 1.74 GPU when the feed gas pressure was increased from 50 to 250 psig (344.7 kPag to 1724 kPag). Both N2 and He showed also corresponding increase in permeance. Similar to the performances of the two earlier membranes, the ideal separation factor of CO2 over He showed little change under various test pressures. The value remained around 22.5−25.5; this was the highest among three IL membranes tested. An increase in the test pressure reduced the ideal separation factor of CO2 over N2. The value dropped from 67.2 to 41.8 when the test pressure was increased from 50 to 250 psig (344.7 kPag to 1724 kPag). It still showed an ideal separation factor of 41.8 for CO2

over N2 at 250 psig (1724 kPag); this value is much higher than 20.1 of 25 wt % solution-based IL membrane. This means that a thicker IL membrane prepared from a 35 wt % IL solution in ethanol had lower gas permeance but yielded a better gas separation performance at a higher pressure. In separations based on gas permeation through polymeric membranes, there is usually a trade-off between the gas permeance and the separation performance. Higher permeances usually lead to a lower separation factor; satisfactory separation performance could be achieved often by sacrificing the permeance. A similar behavior is observed here; however, here it is most likely due to generation of defects in thinner liquid membranes resulting from lower IL concentration in the G


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performances were tested. As can be seen from Figure 10, [emim][Tf2N] membrane showed relatively steady permeances in the test pressure range of 50 to 100 psig (344.7 to 689.5 kPag); when the test pressure was increased further to 125 psig (861.8 kPag) and 150 psig (1034 kPag), gas permeances for all three gases tested increased (especially for He and N2). In the low test pressure range (50 to 100 psig (344.7 to 689.5 kPag)), CO2 permeance was around 1.98 GPU which is higher than the corresponding value of ∼1.60 GPU for [bmim][DCA] membrane (Figure 7a). This is most likely due to the higher reported solubility of CO2 in [emim][Tf2N] compared to that in [bmim]DCA].32 When the test pressure was higher than 100 psig (689.5 kPag), gas permeances increased sharply; this is likely to be due to possible membrane defects at higher pressures. Compared with [bmim][DCA], the surface tension of [emim][Tf2N] is lower; further it may not have the same

alcohol solution. Therefore, the mechanisms for two apparently similar observances are very different. The IL membranes prepared from 15 wt % IL-ethanol solution had much higher gas permeances, while its gas separation factors were unacceptable. The IL membranes prepared from 35 wt % solution displayed the best separation performance, while its gas permeances were the lowest. So based on the test results listed in Figures 7, 8, and 9, 25 wt % solution should be the best choice to prepare an IL membrane since it displayed relatively satisfactory gas permeances and separation factors (except at 250 psig (1724 kPag)) at the same time. 3.3. Comparison between IL Membranes Prepared from [bmim][DCA] and [emim][Tf2N]. Based on the test results described in section 3.2.2, a different supported IL membrane was prepared using 25 wt % solution of [emim][Tf2N] in ethanol, and its gas permeation and separation H


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Figure 10. Influence of test pressure on IL membrane gas permeation (a) and separation (b) performance at room temperature (prepared from solution of 25.0 wt % [emim][Tf2N] in ethanol).

higher pressures. When the feed gas pressure was 150 psig (1034 kPag), the separation factor of CO2 over N2 dropped to 7.26, and that for CO2 over He decreased to 3.95; both were much smaller compared with the corresponding values at low feed pressures. Compared to the supported IL membrane based on [bmim][DCA], even though the [emim][Tf2N] membrane showed relatively larger gas permeances of CO2, it did not show any better separation performance over [bmim][DCA] membranes even at lower feed pressures. Thus, our continued study mainly focused on the supported [bmim][DCA] membrane. 3.4. Influence of Test Temperature on the IL Membrane Performance. Since in some cases (e.g., shifted syngas) selective CO2 separation has to be implemented at higher temperatures and higher pressures, it is important to investigate the effect of the test temperature on gas permeation

level of affinity with the surface of the microstructure of the inner ceramic micropores. Therefore, certain tiny membrane defects may be generated at high gas pressures. Another possible explanation could be that after many times of high pressure displacement of the IL, ethanol washing and high temperature drying, the microporous structure of the ceramic tubule may be somewhat altered in certain locations causing defects when running at higher pressure in experiments carried out near the end of the series of experiments. Gas permeances will determine the corresponding ideal separation factor directly. So as shown in Figure 10b, the separation factor of CO2 over N2 remained in the range of 29.0 to 27.6 for the lower test pressure range, and the separation factor of CO2 over He was stable around 12.0. When the test pressure was increased to over 100 psig (689.5 kPag), both separation factors dropped sharply since compared with CO2, N2, and He showed much a larger increase in permeance at I


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Figure 11. Influence of the test temperature on IL membrane gas permeation (a) and separation (b) performance (prepared from solution of 25.0 wt % [bmim][DCA] in ethanol) at 150 psig (1034 kPag).

benefit selective CO2 permeation the least. Therefore, the separation factor, as can be seen from Figure 11b, encountered a sharp drop when the test temperature increased. Over the temperature range, the CO2−N2 separation factor dropped from 41.5 to 10.3; that for CO2 over He dropped from 18.7 to only 4.9. This means that pure IL membrane formed in micropores of the ceramic tubule may not be suitable for CO2 separation at high temperatures. Another set of tests focusing on the effect of pressure on gas permeation and separation performance with the IL membrane prepared from 25 wt % solution was carried out at 100 °C. Detailed results are shown in Figure 12. When the test temperature was kept at 100 °C, even a low test pressure of 50 psig did not bring about higher ideal gas separation factors. Unlike that at room temperature where lower test pressures show much better separation performance, high temperature reduces the IL viscosity (increasing gas diffusivity) and at the same time reduces the solubility of CO2 in IL.29,33 In other words, high temperature will reduce the solubility selectivity of CO2 over the other two gases greatly, and this should be the

and separation performance of the prepared IL membranes. Figure 11 illustrates the test results with an IL membrane prepared from 25 wt % IL containing ethanol solution in the temperature range of 25−100 °C and a pressure of 150 psig (1034 kPag). It is clear that the test temperature has considerable influence. Permeances for all three gases tested increased with the test temperature. Higher temperature decreases the viscosity of IL; thus gas diffusion in IL will be faster. This should be the main reason for permeance increase with temperature (Note: He solubility in the IL increases somewhat with temperature in this range.33). In the tested temperature range, He permeance increased from 0.106 GPU to 0.599 GPU about 5.6 times increase; nitrogen permeances increased from 0.048 GPU to 0.283 GPU, almost 6-fold larger. Compared with them, CO2 showed the smallest permeance increase from 1.973 GPU to 2.923 GPU, only about a 50% increase. This behavior is due to the fact that higher temperatures will decrease the CO2 solubility in the IL considerably.33 At room temperature CO2 can permeate through the IL membrane much faster mainly due to its much higher solubility in IL; thus temperature increase will J


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Figure 12. Influence of test pressure on IL membrane gas permeation (a) and separation (b) performance (prepared from solution of 25.0 wt % [bmim][DCA] in ethanol) at 100 °C.

3.5. Influence of Adding PAMAM Dendrimer to IL on the Prepared IL Membrane Performance. 3.5.1. Optimal Dendrimer Concentration Determination. Pure PAMAM dendrimer generation 0 liquid provides a high concentration of primary and tertiary amine functional groups such as −NH2, ≡N. The primary amine group −NH2 is known to react strongly with CO2 to produce a carbamate ion and a protonated base (this reaction does not need water). For the tertiary amine groups to be active, water is required. There have been several reports of using such a dendrimer to facilitate CO2 solubility and permeation.33−35 To investigate if addition of this dendrimer to the IL can improve the prepared IL membrane performance especially at higher temperatures, the first step

main reason for the low separation factors at various tested pressures. As mentioned earlier, high temperature CO2-containing feed gases are encountered often; a high separation factor may not be expected from a pure nontask-specific IL membrane supported in microporous ceramic tubules. Thus, it is important to deliberate on how to improve IL membrane performance at higher temperatures. Here a PAMAM dendrimer gen 0 could play an important role to enhance IL membrane performance at higher temperatures since it can directly react with CO2 generating a much larger solubility of CO2; this is likely to be beneficial for high temperature tests. K


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Figure 13. Influence of dendrimer concentration on gas permeation (a) and separation (b) performance of prepared dendrimer-containing IL membranes at 100 psig (689 kPag) and room temperature.

taken involved finding an appropriate dendrimer concentration in the IL for membrane preparation. Three dendrimer-IL mixtures having dendrimer concentrations of 10, 25, and 50 wt % were dissolved in ethanol with fixed 25 wt % IL concentration and then used to prepare dendrimer-containing IL membranes. As shown in Figure 13a, adding dendrimer to the IL did not show any improvement to the membrane permeation performance at room temperature and 100 psig (689.5 kPag) with dry feed gas. Compared with pure IL membrane, adding dendrimer greatly reduced its gas permeance. For example, CO 2 permeance of pure IL membrane was 1.54 GPU, while the value for dendrimer containing IL membrane dropped sharply to the range of 0.019−0.037 GPU. This could be attributed to the large viscosity increase caused by adding dendrimer to the IL. It is much more difficult for a gas to permeate through a viscous liquid; that was why CO2 permeance dropped so much. Although the dendrimer was expected to facilitate the permeation of CO2, it appears that this advantage had been nullified by the deleterious influence of viscosity. As shown in

Figure 13b, adding dendrimer to the IL also reduced the ideal separation factors of CO2 over N2 and He at room temperature in the presence of a dry feed gas. Further comparison among different dendrimer-containing IL membranes showed that 25 wt % dendrimer concentration is likely to be a better choice. At first, it showed the highest carbon dioxide permeance. Its permeance was higher than 10 wt % dendrimer containing IL membrane which could be explained by its higher CO2 solubility due to higher dendrimer content. On the other hand, even though 50 wt % dendrimer containing IL membrane had a much higher dendrimer content, it was too viscous to utilize this advantage; thus its CO2 permeance was also lower than that of the 25 wt % dendrimer-containing IL membrane. Second in the case of ideal separation factors, 25 wt % dendrimer containing IL membrane also showed much higher values than 10 wt % while being only slightly lower than that of 50 wt %. Therefore, the next tests focused on 25 wt % dendrimer-containing IL membrane. 3.5.2. High Temperature Test Performance of DendrimerContaining IL Membrane. The objective behind adding L


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Figure 14. Influence of the test temperature on gas permeation (a) and separation (b) performance of 25.0 wt % dendrimer-containing IL membrane at 100 psig (689 kPag).

helium showed an increase from 0.007 GPU to 0.314 GPU. In terms of ideal separation factors, as shown in Figure 14b, as the temperature increased from 25 to 100 °C, the separation factors of CO2 over N2 dropped obviously from 26.2 to 10.6 and that for CO2 over He dropped from 5.2 to 3.0. Compared with the results in Figure 11, one does not notice any obvious performance improvement by adding dendrimer to IL as was expected. Continued tests were carried out at different pressures at a fixed 100 °C test temperature using the prepared 25 wt % dendrimer-containing IL membrane. As shown in Figure 15a, when the test pressure increased, gas permeances also showed an increase, especially at pressures higher than 100 psig (689.5 kPag). It is believed that the much more permeance increase

dendrimer to the IL and forming a dendrimer-containing IL membrane was to improve the membrane performance at higher temperatures. To test if dendrimer addition will facilitate CO2 permeation at high temperature or not, tests similar to those in section 3.4 were carried out; the feed gas was dry. As shown in Figure 14a, for a 25 wt % dendrimer-containing IL membrane and 100 psig (689.5 kPag) feed gas pressure, permeances of all tested gases increased when the test temperature increased. As explained earlier, temperature increase will reduce the viscosity of dendrimer-containing IL; thus it is easy to understand the basis for a corresponding increase in gas permeances. Over the test temperature range, permeance of CO2 increased from 0.038 GPU to 0.952 GPU, that for N2 had an increase from 0.001 GPU to 0.090 GPU; M


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Figure 15. Influence of pressure on gas permeation (a) and separation (b) performance of 25.0 wt % dendrimer-containing IL membrane at 100 °C.

described next will focus on a humidified feed gas using the prepared 25 wt % dendrimer-containing IL membrane. As shown in Figure 5, keeping the oven temperature at 100 °C, water was injected to the humidifier at a flow rate of 1.0 mL/min; according to the relative humidity transducer, feed gas leaving the humidifier had a relative humidity of ∼95.2 to 98.8%. Detailed results are presented in Figure 16. It is clear that adding water to the feed gas improved gas permeances considerably, especially for CO2. When the feed gas pressure was 100 psig (689.5 kPag), dry feed gas could generate a CO2 permeance of 0.647 GPU, while with a humidified feed gas, CO2 permeance increased to 6.683 GPU which was more than ten times that of the dry feed gas value. In terms of the separation factor, when dry feed gas was introduced, separation factors of CO2 over N2 were around 9.51 which was also much

was caused by IL membrane defects since similar phenomena was found with formerly prepared [emim][Tf2N] membrane as was mentioned in section 3.3. Figure 15b showed the change in ideal separation factors for different feed pressures at 100 °C. Actually for the separation factors of CO2 over N2, a sharp decrease could be seen when the test pressure was higher than 100 psig (689.5 kPag). Compared with it the separation factors of CO2 over He were relatively stable around 2.5, a quite low value. 3.5.3. High Temperature Test Performance of DendrimerContaining IL Membrane with Humidified Feed Gas. Test results in section 3.5.2 clearly showed that adding dendrimer to IL and forming a dendrimer-containing IL membrane did not bring any obvious performance improvement as expected when tested at a high temperature with dry feed gas. The results N


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Figure 16. Influence of pressure on gas permeation (a) and separation (b) performance of 25.0 wt % dendrimer-containing IL membrane at 100 °C (humidified feed gas).

could be explained as follows. First, a large amount of water in feed gas will lead to its absorption in the dendrimer containing IL reducing the viscosity of the membrane liquid; a better gas permeation performance was achieved. Second, although the tertiary amine group ≡N of dendrimer is not supposed to react directly with CO2 like a primary or a secondary amine because they lack the proton needed in the deprotonation step, experimental results indicate that tertiary amines show considerable reactivity with CO2 in the presence of water. This means adding water to the dendrimer-IL solution will definitely increase the extent of reaction between dendrimer and CO2, thus higher permeation-based facilitation of CO2 could be generated.36

lower than 33.3 with humidified feed gas. This meant adding water to feed gas facilitated CO2 permeation in dendrimercontaining IL membrane much more than that of N2. When the feed gas pressure exceeded 100 psig (689.5 kPag), the ideal separation factor of CO2 over N2 decreased sharply due most likely to the membrane defects described earlier. The ideal separation factor of CO2 over He was not influenced by the feed gas pressure and was around 4.49 to 5.63 which was also much higher than 2.5 from dry feed gas. Results in Figure 16 are encouraging since adding water to the feed gas enhanced CO2 permeation through dendrimercontaining IL membrane greatly; further both gas permeance and separation performance improved at the same time. This O


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Industrial & Engineering Chemistry Research 3.6. Gas Mixture Permeation Test with Supported IL Membranes. In this section, the results of a set of tests with gas mixtures are considered. A carrier gas flow-rate of 60 mL/ min was allowed to flow through the tube side. Detailed results are presented in Figure 17. As can be seen from Figure 17a,

influence of temperature was investigated using the pure IL membrane with a dry 40% CO2/60% He mixture representing shifted syngas composition where He is acting as a surrogate for H2 at a feed pressure of 100 psig (689.5 kPag). As was seen earlier with the test results of pure gases, when the temperature increased, permeances of gas mixture species also increased; while the separation factor dropped leading to a lower CO2 concentration in the permeated gas. At room temperature, the permeance of the gas mixture was 0.45 GPU, and CO2 concentration in the permeate side was as high as 88.5%; based on it a separation factor of about 11.5 for CO2 over He was calculated. When the test temperature increased to 100 °C, the permeance increased to 1.22 GPU, while CO2 concentration in the permeate side dropped sharply to 42.1%; and the calculated separation factor of CO2 over helium was only 1.09. Figure 17b gives the results from another dry gas mixture, 14% CO2/86% N2. Similar influence of the test temperature could be seen. At room temperature, permeance for this gas mixture was 0.22 GPU, CO2 concentration in the permeate side reached 82.5%, and the separation factor of CO2 over nitrogen was about 28.9. When the test temperature increased to 100 °C, permeance increased to 0.31 GPU, while CO2 concentration in the permeate side dropped to 34.2%; correspondingly the separation factor of CO2 over N2 was only 3.2. To investigate if adding dendrimer in IL will facilitate CO2 permeation when tested with humidified feed gas mixtures as in pure gas tests, 25% dendrimer in IL membrane was tested at 100 °C with a 100 psig humidified feed gas (having a relative humidity∼ 98.8%). The results are presented in Figure 17c. For a 40% CO2/60% He mixture, the permeance for the gas mixture was 2.86 GPU; the CO2 concentration in the permeate side was 66.5% leading to a separation factor of CO2 over He about 3.0; with a 14% CO2/86% N2 mixture, permeance for the gas mixture was 0.75 GPU, and CO2 concentration in the permeate side was as high as 75.6% leading to a separation factor of about 19.0 for CO2 over N2. If one compares the results in Figure 17c with those of Figures 17a and 17b, it is clearly seen that adding dendrimer to IL and forming a mixed supported liquid membrane greatly improved its CO 2 permeation and separation performance at high temperatures in the presence of humidity in the feed gas, even though the performance was not as good as pure gas test results under the same conditions. Much higher permeances and separation capabilities compared with pure IL membrane could be achieved in the presence of water in feed gas at the same time. 3.7. Analysis of Prepared IL Membranes Based on “Solution-Diffusion” Mechanism. As described earlier, three IL membranes were prepared from IL-ethanol solutions with different IL concentrations. Different IL concentrations mean different membrane thicknesses formed in the micropores of the ceramic tubule. According to the “solution-diffusion” mechanism for gases permeating through membrane, gas solubility is not influenced by IL membrane thickness, while it definitely needs more time to diffuse through a thicker IL membrane. Thus, it is necessary to make an analysis based on gas permeation performance of prepared IL membranes. Carbon dioxide permeation data tested at 100 psig (689.5 kPag) and room temperature from four different IL membranes were selected to make the analysis; detailed results are presented in Table 3. In order to simplify the analysis, micropores of the ceramic tubule were assumed to be uniformly cylindrical. According to the IL-ethanol solution concentration and density, we can calculate the IL membrane thickness in

Figure 17. Test results from different supported IL membranes and gas mixtures (a) pure IL membrane, dry 40% helium/60% CO2, 100 psig (689 kPag); temperatures in oC; (b) pure IL membrane, dry 86% nitrogen/14% CO2, 100 psig (689 kPag); temperatures in oC; (c) 25% dendrimer in IL membrane, tested at 100 oC and 100 psig (689 kPag), humidified feed gas. P


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Industrial & Engineering Chemistry Research Table 3. Analysis of Gas Permeation Performance from Four Prepared IL Membranes IL-ethanol solution 15 25 35 25 a

wt wt wt wt

% % % %

[bmim][DCA] [bmim][DCA] [bmim][DCA] [emim][Tf2N]

density (g/mL)

permeance GPU

thickness (mm)

permeability (cal.) (barrer)a

permeability (ref 37) (barrer)

selectivity CO2/N2a

selectivity CO2/N2 (ref 37)

0.819 0.853 0.882 1.243

12.16 1.69 1.40 1.99

0.127 0.221 0.320 0.225

3860 933 1120 1119

1237 1237 1237 1702

21.0 50.0 55.0 27.6

56.7 56.7 56.7 23.1

These data were obtained at room temperature with a feed pressure of 100 psig (689.5 kPag).

permeances would have been obtained for equivalent selectivity. Further the same module was used numerous times through the demanding procedure of cleaning and reintroduction of the IL in the pores. This may have affected the pore surfaces causing reduction in selectivities at higher pressures in the later experiments.

micropores. Then based on the permeance, membrane thickness, and porosity, permeability of carbon dioxide in IL membrane could be obtained easily: permeability = gas permeance (GPU)/(porosity) × (membrane thickness (cm)) × 104

(3)

4. CONCLUSION

The unit of permeability calculated from eq 3 is barrer (1 barrer = 10−10 cm3 (STP)-cm/(cm2-s-cm Hg)). Since there have been some reports of carbon dioxide permeability in [bmim][DCA] and [emim][Tf2N] membranes, a comparison between the calculated permeability and the reported values has been presented. A search did not yield any carbon dioxide permeability data for [bmim][DCA] despite one reported value which seems quite different from the range it should be. Thus, the value reported for [emim][DCA] will be reported as permeability for [bmim][DCA] since there should not be a great difference between them. At first, from the first 3 rows of Table 3 it is clearly seen that with the increase of [bmim][DCA] concentration in ethanol solution the correspondingly prepared IL membrane had a larger thickness. Based on the permeance of carbon dioxide and membrane thickness from 15 wt % [bmim][DCA] solution, a carbon dioxide permeability of 3860 barrer was generated that was much higher than 1237 barrer reported in the literature. It is believed that this was because of possible membrane defects. Compared with it, calculated carbon dioxide permeability values of IL membranes from 25 and 35 wt % IL solutions were much closer to the reported value. Carbon dioxide permeability calculated from 25 wt % [emim][Tf2N] was 1119 barrer that was lower than 1702 barrer as reported. Currently the tortuosity data for the used ceramic tubule is missing, while of course it should be larger than 1.0; once this is considered, the calculated permeability data should be even closer to the reported values. In terms of CO2/N2 selectivity which is also listed in Table 3, it can be clearly seen that IL membrane generated from 15 wt % [bmim][DCA] solution exhibited CO2/N2 selectivity around 21.0 that is much lower than its intrinsic value of 56.7; a greatly improved CO2/N2 selectivity of 50.0 was seen when 25 wt % [bmim][DCA] solution was used to prepare the IL membrane. The highest CO2/N2 selectivity of 55.0 which is very close to the intrinsic value of 56.7 was achieved when [bmim][DCA] solution concentration was further increased to 35 w%. Compared with [bmim][DCA], IL membrane generated from 25 wt % [emim][Tf2N] solution exhibited CO2/N2 selectivity of 27.6 which is slightly higher than 23.1, its intrinsic value; this difference might be caused by a relatively lower test temperature in this work. The ILMs displayed high values of permeability. However, the ceramic membranes had quite a thick wall. If the wall thickness were much lower for ceramic or polymeric support membranes, then much thinner ILMs with much higher

Based on a single microporous ceramic tubule-based module, supported ionic liquid membranes were prepared by a solvent evaporation method. Prepared IL membranes were considered solubility-selective since on a comparative basis gas permeances were mainly determined by their solubilities in IL. At first a set of tests with [bmim][DCA]-based IL membranes prepared from ethanol solutions containing variable IL concentrations were carried out, and 25 wt % was determined to be the optimal concentration since prepared IL membrane presented satisfactory gas permeation and separation performance at the same time. Another IL membrane using similar optimized concentration of [emim][Tf2N] in ethanol solution was prepared. Test results showed that it had a slightly better permeation performance, while its gas separation capability was not comparable with that of the [bmim][DCA]-based membrane, especially at feed pressures higher than 100 psig. Further investigations showed that when the test temperature increased, permeances of [bmim][DCA] membrane increased, but its gas separation factors of CO2 over N2 and He dropped sharply since high temperature will reduce the solubility of CO2 in IL dramatically. Continued attempts of adding PAMAM dendrimer generation 0 to IL-ethanol solution to form a dendrimer containing IL membrane did not show any facilitating effect in the absence of moisture in the feed gas; the gas permeances decreased dramatically because of the much higher viscosity with dry feed gas. When tested at 100 °C in the presence of humidified feed gas, dendrimer-containing IL membrane showed much higher CO2 permeances and obviously improved separation factors over nitrogen and helium. Using a feed gas mixture test results confirmed the facilitating effects of dendrimer to enhance CO2 permeation with a humidified feed gas. These results reveal significant application potential for carbon dioxide separation at high temperature with prepared dendrimer-containing IL membrane in the presence of water. However, substrates with thinner walls are needed to improve permeances substantially. Removal of CO2 from lower temperature shifted syngas is an area where such a gas permeation-based separation technique may be potentially applicable. In such applications taking place at around 100 °C, higher temperature stability is important. Therefore, exploration of ILs having anions such as [TCM], [SCN] is of considerable interest. Q


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(18) Yoo, S.; Won, J.; Kang, S. W.; Kang, Y. S.; Nagase, S. CO2 separation membranes using ionic liquids in a Nafion matrix. J. Membr. Sci. 2010, 363, 72. (19) Vangeli, O. C.; Romanos, G. E.; Beltsios, K. G. Development and characterization of chemically stabilized ionic liquid membranesPart I: Nanoporous ceramic supports. J. Membr. Sci. 2010, 365, 366. (20) Close, J. J.; Farmer, K.; Moganty, S. S.; Baltus, R. E. CO2/N2 separations using nanoporous alumina-supported ionic liquid membranes: Effect of the support on separation performance. J. Membr. Sci. 2012, 390−391, 201. (21) Zhao, W.; He, G.; Zhang, L. Effect of water in ionic liquid on the separation performance of supported ionic liquid membrane for CO2/N2. J. Membr. Sci. 2010, 350, 279. (22) Kasahara, S.; Kamio, E.; Ishigami, T.; Matsuyama, H. Effect of water in ionic liquids on CO2 permeability in amino acid ionic liquidbased facilitated transport membranes. J. Membr. Sci. 2012, 415−416, 168. (23) González-Miquel, M.; Bedia, J.; Palomar, J.; Rodríguez, F. Solubility and diffusivity of CO2 in [hxmim][NTf2], [omim][NTf2], and [dcmim][NTf2] at T = (298.15, 308.15, and 323.15) K and pressures up to 20 bar. J. Chem. Eng. Data 2014, 59, 212. (24) Navarro, P.; Larriba, M.; García, J.; Rodríguez, F. Thermal properties of cyano-based ionic liquids. J. Chem. Eng. Data 2013, 58, 2187. (25) Fredlake, C. P.; Crosthwaite, J. M.; Hert, D. G.; Aki, S. N. V. K.; Brennecke, J. F. Thermophysical properties of imidazolium-based ionic liquids. J. Chem. Eng. Data 2004, 49, 954. (26) Jie, X. M.; Chau, J.; Obuskovic, G.; Sirkar, K. K. Preliminary studies of CO2 removal from pre-combustion syngas through pressure swing membrane absorption process with ionic liquid as absorbent. Ind. Eng. Chem. Res. 2013, 52, 8783. (27) Jie, X. M.; Chau, J.; Obuskovic, G.; Sirkar, K. K. Enhanced Pressure Swing Membrane Absorption Process for CO2 Removal from Shifted Syngas with Dendrimer-Ionic Liquid Mixtures as Absorbent. Ind. Eng. Chem. Res. 2014, 53, 3305. (28) Ionic liquid physical properties data were acquired from http:// en.solvionic.com/family/ionic-liquids (accessed July 1, 2015). (29) Chau, J.; Obuskovic, G.; Jie, X.; Sirkar, K. K. Pressure swing membrane absorption process for shifted syngas separation: Modeling vs. experiments for pure ionic liquid. J. Membr. Sci. 2014, 453, 61. (30) Yu, D.; Matteucci, S.; Stangland, E.; Calverley, E.; Wegener, H.; Anaya, D. Quantum chemistry calculation and experimental study of CO2/CH4 and functional group interactions for the design of solubility selective membrane materials. J. Membr. Sci. 2013, 441, 137. (31) Kelman, S.; Lin, H.; Sanders, E. S.; Freeman, B. D. CO2/C2H6 separation using solubility selective membrane. J. Membr. Sci. 2007, 305, 57. (32) Raeissi, S.; Peters, C. J. A potential ionic liquid for CO2separating gas membranes: selection and gas solubility studies. Green Chem. 2009, 11, 185. (33) Chau, J.; Obuskovic, G.; Jie, X.; Sirkar, K. K. Solubilities of CO2 and He in ionic liquid containing Poly (amido amine) (PAMAM) dendrimer Gen 0. Ind. Eng. Chem. Res. 2013, 52, 10484. (34) Kovvali, A. S.; Chen, H.; Sirkar, K. K. Dendrimer membranes: A CO2-selective molecular gate. J. Am. Chem. Soc. 2000, 122, 7594. (35) Kovvali, A. S.; Sirkar, K. K. Dendrimer liquid membranes: CO2separation from gas mixtures. Ind. Eng. Chem. Res. 2001, 40, 2502. (36) Ko, Y. G.; Shin, S. S.; Choi, U. S. Primary, secondary, and tertiary amines for CO2 capture: Designing for mesoporous CO2 adsorbents. J. Colloid Interface Sci. 2011, 361, 594. (37) Scovazzo, P. Determination of the upper limits, benchmarks, and critical properties for gas separations using stabilized room temperature ionic liquid membranes (SILMs) for the purpose of guiding future research. J. Membr. Sci. 2009, 343, 199.

AUTHOR INFORMATION

Corresponding Author

*Phone: 973-596-8447. Fax: 973-642-4854. E-mail: sirkar@njit. edu. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work supported by the Department of Energy, National Energy Technology Laboratory under Award Number DE-FE0001323. The senior author acknowledges many fruitful conversations with Prof. D. Ramakrishna both at IIT, Kanpur and here in the USA.

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Microporous Ceramic Tubule Based and Dendrimer-Facilitated

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