Ionic liquid-modified MCM-41-polymer mixed matrix membrane for butanol pervaporation

Because of the preferential butanol selectivity of some ionic liquids (ILs), an increasing amount of research has appeared regarding their application in butanol separation. In this research, two ionic liquids, namely, 1-ethyl-3-vinylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([EVIM][Tf2N], IL1) and N-octyl-pyridinium bis[(trifluoromethyl)sulfonyl]imide ([OMPY][Tf2N], IL2), were applied to modify a mesoporous molecular sieve MCM-41. The IL-modified MCM-41 samples were characterized by XPS, BET, XRD, SEM and TEM. The ionic liquid-modified MCM-41 was incorporated into the polymer PEBA to prepare mixed matrix membranes to study the influences of the filling of IL-modified MCM-41 and operating conditions on the performance of the mixed matrix membrane for butanol pervaporation. The results indicated that the pervaporation performance of the PEBA membrane was enhanced by the incorporation of IL-modified MCM-41. When the temperature of the feeding liquid was 35°C and the mass fraction of butanol was 2.5 wt%, the 5% MCM-41-IL2-PEBA membrane showed a permeation flux of 421.7 g m−2 h−1 and a separation factor of 25.4. The permeation flux and the separation factor of the membrane increased as the temperature of the feeding liquid increased. The results of the long-period experiment suggested that the 5% MCM-41-IL2-PEBA membrane exhibited high stability within 100 h of operation.

Because of the preferential butanol selectivity of some ionic liquids (ILs), an increasing amount of research has appeared regarding their application in butanol separation. In this research, two ionic liquids, namely, 1-ethyl- 3

-vinylimidazolium bis[(trifluoromethyl)sulfonyl]imide ([EVIM][Tf 2 N], IL1) and N-octyl-pyridinium bis[(trifluoromethyl)sulfonyl]imide ([OMPY]
[Tf 2 N], IL2), were applied to modify a mesoporous molecular sieve MCM-41. The IL-modified MCM-41 samples were characterized by XPS, BET, XRD, SEM and TEM. The ionic liquid-modified MCM-41 was incorporated into the polymer PEBA to prepare mixed matrix membranes to study the influences of the filling of IL-modified MCM-41 and operating conditions on the performance of the mixed matrix membrane for butanol pervaporation. The results indicated that the pervaporation performance of the PEBA membrane was enhanced by the incorporation of IL-modified MCM-41. When the temperature of the feeding liquid was 35°C and the mass fraction of butanol was 2.5 wt%, the 5% MCM-41-IL2-PEBA membrane showed a permeation flux of 421.7 g m −2 h −1 and a separation factor of 25.4. The permeation flux and the separation factor of the membrane increased as the temperature of the feeding liquid increased. The results of the long-period experiment suggested that the 5% MCM-41-IL2-PEBA membrane exhibited high stability within 100 h of operation. the pervaporation performance. When the mass ratio of PVDF-co-HFP to [P 6,6,6,14 ][dca] was 1:2.5, the butanol permeation flux and separation factor of the ionic liquid gel-based membrane were 226 g m −2 h −1 and 68, respectively.
Although IL offers significant advantages over traditional organic solvents, there are some disadvantages, such as high viscosity and recycling difficulty. In addition, the use of IL is also faced with industrial problems, such as high costs, resulting from the large consumption of expensive materials. Therefore, it is useful to immobilize IL on some supports [28,29]. Connecting IL to a carrier such as a molecular sieve is a common immobilizing method. After immobilizing the ionic liquid, the molecular sieves were functionalized or modified, sharing the advantages of a molecular sieve and the ionic liquid simultaneously.
The mesoporous molecular sieve MCM-41 has a high specific surface and pore volume, and ionic liquids containing the anion [Tf 2 N] − have good solubility for butanol. To improve the permeation flux and separation factor of the pervaporation membrane for butanol aqueous solutions simultaneously, in this study, two ionic liquids with the hydrophobic anion [Tf 2 N] − were selected to couple with a MCM-41 mesoporous sieve to prepare IL-modified MCM-41. Then, the IL-modified MCM-41 was incorporated into the PEBA membrane to prepare a series of mixed matrix membranes. The IL-modified MCM-41 was characterized with XPS, BET, XRD, SEM and TEM. The morphology, thermal stability and swelling behaviour of the mixed matrix membranes were analysed. Afterwards, pervaporation experiments were conducted to evaluate the membrane performance and stability of the mixed matrix membranes.

Preparation of the IL-modified MCM-41
An appropriate amount of MCM-41 was weighed and added to the nitrate solution. The solution was stirred at room temperature for 24 h, followed by washing with distilled water and ethanol. The washed specimen was then dried at 120°C for 12 h (to activate the molecular sieve) [30]. A quantity of 2 g activated MCM-41 was weighed and placed in a 250 ml three-necked flask. Next, 100 ml of toluene (treated by refluxing with metal sodium followed by distillation), 5 ml of 3-chloropropyltrimethoxysilane and 2 ml of triethylamine (as the catalyst) were added to the flask, which was then mixed by magnetic stirring at 80°C for 24 h. After the reaction, the mixture was cooled to room temperature and was vacuum filtered. Then, the product was washed using toluene, methanol, a water/methanol mixture (1 : 1), distilled water and methanol to obtain a white solid powder. The powder was dried in a vacuum at 80°C for 8 h. Subsequently, the silanized molecular sieve, MCM-41-Si, was produced.
The prepared MCM-41-Si was placed into a 250-ml three-necked flask, with 100 ml acetonitrile added as a solvent. A quantity of 0.005 mol ionic liquid (IL) was also added to the flask. The mixture received magnetic stirring at 80°C for 24 h. When the reaction was completed, the mixture was cooled to room temperature and was vacuum filtered. The product was washed using ethyl acetate, 0.1 mol l −1 hydrochloric acid, distilled water and methanol, sequentially. The resulting white solid was dried in an oven at 80°C for 12 h. The modified molecular sieves were labelled MCM- 41

Preparation of the mixed matrix membranes
The IL-modified MCM-41, as described in §2.2, was added to butanol. The solution received ultrasonic dispersion for 30 min. PEBA granules were added to the butanol, which was magnetically stirred in a water bath at a constant temperature of 70°C for 1 h. Subsequently, the ultrasonic dispersed molecular sieve was added to the PEBA solution. The solution was stirred in a water bath at 70°C for another 2 h until the membrane casting solution was evenly mixed. It was then left standing for defoaming. When the membrane casting solution was cooled to 40°C, it was poured onto a horizontal glass plate. The membrane was scraped using a glass slicker and kept at room temperature for a period of time. When most of the solvent volatilized, the membrane was moved into an oven and dried at 60°C. The membrane was not peeled off until the solvent had been completely volatilized. The membranes modified with IL-modified MCM-41 mass fractions of 2 wt%, 5 wt% and 10 wt% were denoted as 2% MCM-41-IL-PEBA, 5% MCM-41-IL-PEBA and 10% MCM-41-IL-PEBA, respectively.  The prepared membranes were cut into samples of the same size, which were then dried thoroughly in the drying oven at 60°C. Subsequently, the samples were weighed and immersed into the feeding liquid with a certain concentration at a constant temperature. The membranes were picked up after a set time interval. The remaining liquid on the membrane surfaces was wiped quickly using Kimwipes (Kimberly Clark). Subsequently, the membranes were weighed and placed in the feeding liquid again. This operation was repeated until there was no significant change in the mass of the membrane samples, which suggested that the swelling had reached equilibrium. The swelling degree (DS%) can be calculated by equation (2.1) as follows:

Characterization
where W d and W s are the mass (g) of the dry membranes and the mass of samples at swelling equilibrium, respectively.

Pervaporation experiment
In this study, the equipment for the PV experiment was provided by Tianjin University Beiyang Chemical Equipment Co., Ltd. When the feeding liquid was heated to a preset temperature, it was pumped from the liquid feeding tank using a circulating pump. The liquid flowed into the membrane chamber through the flow meter. In the membrane chamber, the feeding liquid was separated using PV. The permeated components, also known as the penetrants, were collected using a liquid nitrogen trap, whereas the remaining liquid was returned to the feeding tank via liquid circulation. The experimental temperature error was controlled to within ±0.5°C. The feeding liquid flow was measured using a rotor flow meter. The vacuum in the downstream side of the membrane chamber was greater than 0.1 MPa; the effective area of the PV membrane in the membrane chamber was 3.6 × 10 −3 m 2 . A gas chromatograph (GC, 7900, TECHCOMP (HOLDINGS) LIMITED) was used to determine the compositions of the feeding liquid and the penetrants. The chromatographic column was an HP-FFAP (50 m × 0.2 mm × 0.3 µm) capillary column. A hydrogen flame ionization detector was used as the detector. Nitrogen was the carrier gas. The column temperature was 120°C; the temperatures of the sample injector and the detector were 160°C. The separating performance of the PV membrane was primarily evaluated using the separation factor α, the permeation flux J (g m −2 h −1 ) and the pervaporation separation index (PSI) as follows: where C i and C j are the mass fractions of butanol and water, respectively, in the penetrant; and W, A, t and J represent the mass of penetrant (g), the effective area of the membrane (m 2 ), the effective operating time of the PV experiment (h), and the permeation flux (g m −2 h −1 ), respectively. PSI is a usually used index to compare the comprehensive separation performance of different membranes. The contents of the IL-specific elements S and F in MCM-41-IL2 were 1.1% and 2.0%. Based on the XPS results, it was assumed that the ionic liquids could be effectively coupled with MCM-41 through hydrogen bonds and the nucleophilic reaction between anions of the ionic liquids and silanized MCM-41.

Pore structure analysis
The N 2 adsorption-desorption isotherms of MCM-41 and ionic liquid-modified MCM-41 are shown in figure 2. The structural parameters of an IL-modified mesoporous molecular sieve are shown in table 2. Figure 2 and table 2 suggest that the pore diameter, the specific surface area and the pore volume decreased after IL modification. This suggests that some pores were occupied after the IL and MCM-41 were coupled. Because IL2 has a longer side chain (octyl) in the cation than IL1 (ethyl + vinyl), the pore volume of MCM-41-IL2 decreased more than that of MCM-41-IL1. However, the pore diameters of the modified MCM-41 still remained between 2 and 5 nm, which suggests that the mesoporous structure was not damaged.

Characterization of the membranes 3.2.1. Scanning electron microscope
The SEM images of the surfaces and cross sections of the PEBA and the modified PEBA membranes are shown in figure 5. It can be seen from figure 5a,b that the pristine PEBA membrane was flat at its surface with a uniform cross section. Figures 5c,e are the SEM images of the surfaces of modified PEBA membranes with different types of IL. The surface of 5% MCM-41-IL2-PEBA was smoother than that of 5% MCM-41-IL1-PEBA. As shown in figure 5d,f, the molecular sieves were tightly wrapped by PEBA and evenly distributed on the membrane surface or cross section; the membranes were dense without any defects.

Swelling experiment
The variation of the degree of swelling for the modified PEBA membrane in the butanol solution with a mass fraction of 2.5 wt% over time is shown in figure 6. For the first 2 h, the degrees of swelling for both  figure 7a,b. The data show that the degrees of swelling for the modified membranes increased in both butanol solution and pure water as the filling amounts of the two modified MCM-41 samples increased. With the increase of filling amount, water molecules tended to form hydrogen bonding with the Si-OH on the molecular sieve [34]. Therefore, the degree of swelling for the modified membrane in pure water increased with the filling amount. Comparing the degrees of swelling for the same mixed matrix membrane in pure water and in a butanol solution suggests that the degrees of swelling in the butanol solution were higher than those in pure water. The reason is that butanol is preferably adsorbed by PEBA [33]. In addition, organophilic ionic liquid enhanced the sorption of butanol on mixed matrix membranes.

Thermogravimetric analysis
The thermogravimetric (TG) analyses of PEBA, 5% MCM-41-PEBA, 5% MCM-41-IL1-PEBA, and 5% MCM-41-IL2-PEBA are shown in figure 8, which indicate that the PEBA membrane was highly thermostable because it started decomposing at approximately 210°C. The thermal decomposition temperature of the membrane with IL-modified MCM-41 was near that of the PEBA membrane. This suggested that the addition of IL-modified MCM-41 made little difference to the thermostability of the PEBA membrane. Because 210°C is much greater than the PV process temperature, the modified membrane prepared in this study could meet the pervaporation requirements.

Water contact angle measurement
Figure 9a-d shows the water CAs for the pristine PEBA membrane and the mixed matrix membranes. The water CAs for the pristine PEBA membrane was 77°. Since molecular sieve MCM-41 has abundant -OH, the water CA of the 5% MCM-41-PEBA membrane decreased to 60°, which meant that the hydrophilicity of 5% MCM-41-PEBA membrane was increased. The anion of IL1 and IL2 shows strong hydrophobicity because it has two trifluoromethyl (-CF3) groups. With the incorporation of ionic liquid-modified MCM-41, the water CAs of 5% MCM-41-IL1-PEBA membrane and 5% MCM-41-IL2-PEBA membrane increased to 85°and 88°, respectively, which meant that the membranes' hydrophobicity was enhanced. Due to IL2 has longer alkyl chain length in its cation, the hydrophobicity of 5% MCM-41-IL2-PEBA membrane was higher than that of 5% MCM-41-IL1-PEBA membrane. The influence of the MCM-41-IL1 contents on the PV performance is shown in figure 10a, where the temperature and mass fraction of the feeding liquid are 35°C and 2.5 wt%, respectively. Compared with the pristine PEBA membrane, the permeation flux and separation factor of the modified PEBA membrane were both enhanced when there was an appropriate amount of IL-modified MCM-41. On the one hand, the mesoporous structure of the molecular sieve facilitated butanol and water diffusion in the membrane, for which the permeation flux of the modified membrane was significantly improved. On the other hand, the IL-modified MCM-41 showed favourable butanol selectivity, which would promote butanol sorption. Therefore, the separation factor of the mixed matrix membrane was also enhanced. When the filling amount of MCM-41-IL1 was 5%, the modified PEBA membrane showed the best PV separation performance; its separation factor and permeation flux were 22. − -based ionic liquids with high hydrophobicity, therefore the separation factor of butanol/water could be enhanced when IL-modified MCM-41 was used to fill the membrane. IL2 performed better in selective butanol pervaporation because it has longer alkyl chain length in its cation. The pervaporation performance of these membranes was also consistent with the swelling experiment and water CA measurement results.

Effect of feed temperature
The influence of feed temperature on the separation factors and permeation fluxes of the pristine PEBA membrane, 5% MCM-41-IL1-PEBA and 5% MCM-41-IL2-PEBA is shown in figure 11a,b. The data show that the separation factors and permeation fluxes of both the pristine PEBA membrane and the mixed matrix membrane increased with feed temperature. One reason for this behaviour was that the small molecules in the feed moved faster and diffused in the membrane at a higher rate as the feed temperature increased. Another reason was that the segment movement of the PEBA matrix accelerated and the free volume of the membrane increased with temperature, which promoted the mass transfer process.

Stability of MCM-41-IL-PEBA membrane
According to a previous study by our research group, the separation factor of the IL-blended PEBA membrane decreased markedly with IL loss after it was used for 26 h [33]. However, the variations of the permeation flux and separation factor of 5% MCM-41-IL2-PEBA with time were within a small range ( figure 12). Within 100 h of operation, the permeation flux and the separation factor of 5% MCM-41-IL2-PEBA were 25.2 and 414.1 g m −2 h −1 on average, both of which were higher than those of the pristine PEBA membrane and showed favourable stability. This suggests that IL modification of MCM-41 is also an effective way to immobilize ionic liquid.

Conclusion
Hydrophobic ionic liquids 1-ethyl-3-vinylimidazolium bis[(trifluoromethyl)sulfonyl]imide (IL1) and N-octyl-pyridinium bis((trifluoromethyl)sulfonyl)imide (IL2) were used to modify the mesoporous molecular sieve MCM-41. XPS characterization results indicated that IL-specific elements S and F were observed in IL-modified molecular sieves. According to XRD and BET analyses, the porous structure of the molecular sieve remained unchanged after IL was introduced while the specific surface area, pore volume and pore diameter of the modified MCM-41 decreased slightly.
IL-modified MCM-41 was used as the filling agent to prepare MCM-41-IL-PEBA mixed matrix membranes with different filling contents. The incorporation of MCM-41-IL made little difference to membrane thermostability, i.e. the membrane was able to meet the requirements for PV separation. Butanol was preferentially adsorbed by the MCM-41-IL-PEBA mixed matrix membranes. Compared with the pristine PEBA membrane, the MCM-41-IL-PEBA mixed matrix membranes showed enhanced performance for butanol pervaporation when the filling content was appropriate. In all the membranes investigated in this research, MCM-41-IL2-PEBA with a filling content of 5% showed the