Pervaporation composite membranes for ethyl acetate production

Chemical Engineering and Processing 87 (2015) 81–87

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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Pervaporation composite membranes for ethyl acetate production Anastasia Penkova a, *, Galina Polotskaya a,b , Alexander Toikka a a b

St. Petersburg State University, Department of Chemical Thermodynamics & Kinetics, Universitetsky pr. 26, Peterhof, St. Petersburg 198504, Russia Institute of Macromolecular Compounds, Russian Academy of Sciences, Bolshoy pr. 31, St. Petersburg 199004, Russia

A R T I C L E I N F O

A B S T R A C T

Article history: Received 11 July 2014 Received in revised form 22 October 2014 Accepted 22 November 2014 Available online 25 November 2014

This paper continues research on the physicochemical features of nanocomposites of poly(2,6-dimethyl1,4-phenylene oxide) (PPO) and fullerene C60 with the purpose of using them as a selective layer of composite membranes in pervaporation coupling esterification of acetic acid with ethanol to produce ethyl acetate. The C60–PPO/MFFC composite membranes containing up to 3 wt%C60 were prepared in this work. SEM was employed in visualizing the internal morphology of the membranes. The nature of the interaction between PPO and C60 molecules in composite was studied by NMR. TG analysis reveals the fact of increasing thermal destruction point of the membrane due to certain structural changes of the PPO matrix upon incorporation of the C60 molecules. Transport properties of composite membranes with a selective layer containing up to 3 wt%C60 were studied in the pervaporation of the quaternary mixture: ethanol, acetic acid, ethyl acetate, and water in order to identify the availability of their use in the hybrid process “esterification + pervaporation”. The strong permeability enhancement due to the introduction of C60 particles in the PPO network was observed whereas the selectivity was almost steady. The best complex of transport properties was obtained from the 2%C60–PPO/MFFC membrane. ã 2014 Elsevier B.V. All rights reserved.

Keywords: Pervaporation Hybrid process Polymer membranes PPO Fullerene

1. Introduction The increase in industrial applications of pervaporation using membrane separation technology is due to the numerous advantages of this method. In some cases to enhance performance of chemical synthesis or technological production, the pervaporation can be involved in engineering design of hybrid processes [1–3]. One of the perspective hybrid processes is combining esterification and pervaporation for the purpose of esters production. Esterification of carboxylic acids and alcohols is a typical example of an equilibrium-limited reaction that produces ester and water as products. Nearing reaction equilibrium, the conversion is generally low and increases cost by using an excess of reactant for the shifting from equilibrium [4]. Accordingly, removing reaction products in the course of the reaction inhibits chemical equilibrium. Therefore, removing the ester or water by pervaporation is a useful technology for ester synthesis reaction. The reaction studied for the application of pervaporation is an esterification of acetic acid with ethanol to produce ethyl acetate and water:

* Corresponding author. Tel.: +7 812 428 48 05. E-mail address: [email protected] (A. Penkova). http://dx.doi.org/10.1016/j.cep.2014.11.015 0255-2701/ ã 2014 Elsevier B.V. All rights reserved.

CH3 COOH þ C2 H5 OH

!

CH3 COOC2 H5 þ H2 O

In our previous work we studied a similar case involving the synthesis of methyl acetate [5]. The fundamentals of esterification facilitated by pervaporation are presented in the paper [6]. To isolate one of the reaction products, either ester or water, different types of polymer membranes can be used [5]. In the majority of known cases, hydrophilic membranes have been applied, which promote the release of water from the reaction vessel (tank) [7–15]. The removal of the ester, not water, is economically viable because the ester elimination not only diverts the reacting system from equilibrium, but also promotes the separation of an important industrial solvent from the reaction mixture. For ester separation, membranes based on hydrophobic materials such as polydimethylsiloxane are often used [16–18]. Use of PDMS membranes is not always efficient because these membranes require the procedure of PDMS cross-linking and show low selectivity. In our previous works [19–22] dense membranes based on poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) modified by fullerene C60 (up to 2 wt%) were developed and studied in the pervaporation of mixtures containing species that are reagents and products of ethyl acetate synthesis. The conditions of the pervaporation

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experiment and the C60–PPO composite preparation were identical to the applied conditions in this paper. These membranes are selective to the ester and exhibit a good separation factor but low flux in the separation of binary and multicomponent mixtures containing ethyl acetate. For example, in the works [20–22], using the dense PPO membrane in the pervaporation of a fourcomponent mixture with equimolar ratio of reagents—16.7 mol% of acetic acid, 16.7 mol% of ethanol, 33.3 mol% ethyl acetate and 33.3 mol% water (18.9 wt% acetic acid, 14.6 wt% ethanol, 55.2 wt% ethyl acetate and 11.3 wt% water)—led to the permeate containing 81.6 wt% ethyl acetate. However, the flux was only 0.059 kg/m2, which is insufficient for a hybrid process on an industrial scale. Thus, establishing high selective properties in dense membranes based on PPO make them promising for use in ethyl acetate production. However, low permeability is a significant disadvantage. Flux is largely determined by the thickness of a membrane, which is about 60 mm in the case of dense PPO membranes. Improving the fluxes is possible by decreasing membrane thickness: it may lead to the increase of fluxes but simultaneously causes a deterioration of the mechanical properties. One of the ways to preserve the mechanical characteristics may be the development of composite membranes. In this paper we continue research on the structure and physicochemical properties of the C60–PPO nanocomposites (up to 3 wt%) for use in membrane technology, e.g., as a selective layer of composite membranes. The transport properties of composite membranes with different concentration of C60 were investigated in pervaporation of a four-component mixture—ethanol, acetic acid, ethyl acetate, and water—to identify their prospective use in the hybrid process “esterification + pervaporation”. In studying pervaporation of quaternary mixtures, the results of experiments on the separation of binary mixtures water–ethanol and water– ethyl acetate were taken into account [19,21]. 2. Experimental 2.1. Materials PPO with a molecular weight of 172,000 and intrinsic viscosity 1.58 dl/g (Brno, Czech Republic) and fullerene C60 of 99.9% purity (NeoTechProduct, Research & Production Company, Russia) were used for the work. Ethyl acetate, ethanol, acetic acid, toluene, and chloroform were purchased from Vecton (Russia). The MFFC microfiltration fluoroplastic composite hydrophobic membrane (Vladipor, Russia) was composed of a porous fluoroplast F42L layer on polypropylene support; the pore size was 0.05 mm, with a total porosity of 80%. 2.1.1. Preparation of composites The C60–PPO composites were prepared by mixing solutions of PPO in chloroform (2 wt% PPO) and fullerene C60 in toluene (0.14 wt % C60) in amounts that provided the required content of fullerene in the composite (up to 3 wt% C60). The resulting solution was allowed to stand for 3–4 days for interactions to take place between the polymer and fullerene C60 molecules. Next, the composite solution was sonicated for 40 min and filtered to remove dust contaminants. 2.1.2. Membrane preparation Dense membranes based on PPO and composites C60–PPO with the thickness 60 mm were obtained by casting a 8 wt% polymer solution on a cellophane surface. The solvent was removed by evaporation at 40  C; the membrane was separated from the substrate and dried in a vacuum oven at 40  C up to the constant weight.

Composite membranes were prepared by casting 2 wt% PPO (or C60–PPO) solutions in chloroform on the surface of an MFFC support consisting of a fluoroplast F42L layer and polypropylene base. To create a selective layer with 4–6 mm thickness, 0.03 ml of polymer solution per 1 cm2 of the membrane was split. Then, the composite membrane was dried in a vacuum oven at 40  C up to constant weight. 2.2. Characterization 2.2.1. Nuclear magnetic resonance (NMR) NMR spectra were recorded at the Center of Magnetic Resonance of the St. Petersburg State University using a BrukerAvance III spectrometer operating at a magnetic field strength of 9.39 T, corresponding to Larmor frequency of 100.6 MHz for 13C nuclei. Experiments were carried out using Bruker wide-bore 3.2 mm MAS probe. Chemical shifts are referenced relative to the low field signal of adamantane at 38.48 ppm. Two types of experiments were applied to both PPO and C60–PPO samples: (i) direct polarization of 13C with high power 1 H decoupling at a MAS rate of 5 kHz; and (ii) cross-polarization from 1H with spinning sideband suppression (CP TOSS) using contact pulse durations of 2 ms at a MAS rate of 7 kHz. 2.2.2. Thermal gravimetric (TG) analysis Thermal degradation measurements of the membrane samples were performed at the Resource Center of Thermal Analysis and Calorimetry of St. Petersburg State University using TG 209 F3 Iris Thermo-microbalance (Netzsch) at a heating rate of 10  C min and airflow of 50 ml/min. 2.2.3. Scanning electron microscopy Scanning electron microscopy (SEM) micrographs were obtained with a microscope Zeiss Merlin SEM. Membranes were submerged in liquid nitrogen for 5 min and fractured perpendicularly to the surface. The prepared specimens were uncoated and the fracture surfaces and sides membranes were observed under the SEM microscope using secondary electrons at 1 kV. 2.2.4. Swelling study Two types of dry non-porous membranes from PPO and from fluoroplast F42L were immersed in liquid at 20  C and atmospheric pressure for 7 days. At definite intervals, the swollen membranes were taken out from the weighing bottles, carefully wiped with filter papers to remove residue liquid on the membrane surface, and then quickly weighed. The experiment was carried out until the swollen membranes obtained a constant weight that indicated a state of sorption equilibrium. Then, the membranes were placed in a vacuum box at 40  C for 7 days to control the weight of the dry membranes. The liquids under the study were water, ethanol, acetic acid, and ethyl acetate. The degree of the swelling was calculated by the following equation: Sw ¼

Ms  Md  100 Md

(1)

where Ms is the weight of a swollen membrane in equilibrium state and Md is the weight of a dry membrane. 2.2.5. Pervaporation test Pervaporation properties were studied using a laboratory cell with an effective membrane area of 14.8 cm2 at 20  C with stirring. A downstream pressure of <101 mmHg was maintained. The permeate was collected in a liquid nitrogen trap, weighed, and then analyzed by gas chromatography. A gas chromatograph “Chromatec Crystal 5000.2” (Russia) with thermal conductivity detector was used.

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The fluxes, J (kg/m2 h), were determined as an amount of liquid transported through the unit of the membrane area per hour. Pervaporation separation index (PSI) was calculated by the following equation: ! C pEA 1 (2) PSI ¼ J  C fEA where CpEA is the concentration of ethyl acetate in the permeate, CfEA is the concentration of ethyl acetate in the feed. Pervaporation experiments were carried out at least three times for each of membrane type to study the reproducibility and calculate experimental errors. 3. Results and discussion 3.1. Dense film characterization The possibility of donor-acceptor binding between PPO and C60 molecules in composites was shown by IR spectroscopy, luminescence, etc., in work [23]. In this work, the character of interaction between PPO and C60 molecules was investigated by nuclear magnetic resonance. The 13C NMR spectrum of neat Buckminster fullerene C60 (Fig. 1a) contains only a sharp single line with a chemical shift of 142.68 ppm [24]. After incorporation into the PPO matrix, its signal shifts ca. 1.2 ppm to the high-field and broadens (Fig. 1c). This change indicates an interaction between fullerene molecules and the polymer matrix, which slows down fast isotropical reorientation of the former. However, other conclusions follow from the spectrum of the C60–PPO sample obtained under crosspolarization (CP) conditions (Fig. 1e) in which 13C magnetization is developed due to energy transfer from 1H nuclei via a heteronuclear magnetic dipolar interaction and thus is sensitive to internuclear distances and the mobility of molecules or functional groups involved [25]. It is evident that the C60 signal is small in comparison with the signal in the direct 13C observation experiment (Fig. 1c). This observation might indicate an insufficient interaction of the C60 molecules with the PCO matrix: they are either rotating quickly or are relatively far from the vicinity of protonated polymer fragments. Also, a comparison of the crosspolarization spectra of the samples C60–PPO and PPO (Fig. 1d and e)

[(Fig._1)TD$IG]

reveals some noticeable differences that are possibly due to structural changes in the matrix upon incorporation of the fullerene C60 molecules. The effect of fullerene C60 additives in the PPO matrix on the thermal stability was determined by a TG analysis of the samples. Fig. 2 demonstrates that thermal destruction of PPO films begins at 370  C, while thermal destruction of the C60–PPO composite occurs at 420  C and rises dramatically, approximately in one point. This result can be explained as due to structural changes of the matrix upon incorporation of the C60 molecules. It is known that fullerene C60 molecules are capable of strong intermolecular interaction because of many conjugate links in the structure of C60 [26]. When C60 content exceeds 0.15 mol%, opportunities for interaction between C60 and C60 molecules in the PPO–C60 system may increase, which leads to physical linking of polymer chains [27]. SEM was applied in visualizing the internal morphology of membranes. SEM micrographs of fracture surfaces perpendicular to the membrane surfaces, so called cross-sections, are shown in Fig. 3. A pure PPO membrane (Fig. 3a) exhibited comparatively uniform morphology. The structure of the 2%C60–PPO membranes changed (Fig. 3b), which also proves that the formation of nanocomposites occurs through the interaction of the fullerene C60 with polymer chains. 3.2. Transport properties The transport properties of membranes were studied in pervaporation of mixtures of reagents and products of ethyl acetate synthesis reaction (i.e., ethanol, acetic acid, water, and ethyl acetate). Table 1 lists the physical properties of substances under study, which are called penetrants in pervaporation. Data on the solubility parameter (d) of substances can be used to predict the solubility of a polymer in these liquids [28]. According to the solubility theory [29], the less the difference in solubility parameters of polymer and penetrant |Dd|, the better the solubility of the penetrant in the polymer. The solubility parameter of PPO is equal to 18.2 (MJ/m3)1/2 [28]. In the case of ethyl acetate |Dd| is equal to 0.1 and minimal. So, ethyl acetate solubility should be preferential for PPO compared to the other components of the mixture. To use the PPO selective properties with respect to the ethyl acetate in the real process, we developed composite membrane that ensures high performance of pervaporation. The composite membrane consisted of a thin selective PPO top layer (4 mm)

[(Fig._2)TD$IG]

Fig. 1. 13C MAS and CP/MAS NMR spectra of nanocomposite 2%C60–PPO, (c) and (e), respectively; 13C MAS and CP/MAS NMR spectrum of neat PPO, (b) and (d), respectively; and (a) 13C MAS NMR spectrum of neat fullerene C60.

83

Fig. 2. TG curves of PPO and 2%C60–PPO dense films.

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[(Fig._3)TD$IG]

Fig. 3. SEM micrographs of cross-section of (a) PPO and (b) 2%C60–PPO dense films.

Table 1 Properties of substances. Substance

Mol. weight

Density, g/cm3

Mol. volume, cm3/mol

Viscosity, mPa s

Solubility parameter, d,(MJ/m3)1/2

Ethanol Acetic acid Ethyl acetate Water

46.07 60.05 88.10 18.02

0.789 1.049 0.901 0.998

40.40 119.10 97.78 18.07

1.20 1.15 0.44 0.89

26.0 25.7 18.6 48.1

deposited on a MFFC microporous support, which provides mechanical strength and does not resist to penetrant transport. The composite membrane was prepared with consideration to the peculiarities of selective layer formation established in the work [30,31]. Fulfilling the conditions necessary for casting PPO solution 1  [h]c  8, where [h] is an intrinsic viscosity (dl/g) and c the polymer solution concentration (g/dl), assured the formation of defectless membranes [31]. In our case, the value [h] c = 1.58  2 = 3.16 was used. Fig. 4 illustrates a cross-section of PPO/MFFC composite membrane. The SEM micrograph shows the uniform structure of the dense PPO top layer and that a part of the porous MFFC support exhibited a spongy structure. Fig. 4 also demonstrates the excellent adhesion of the selective top layer to the porous support. According to SEM data, the thickness of the top layer is equal to 4 mm. The transport properties of the PPO/MFFC composite membrane were studied in pervaporation of quaternary mixtures of various compositions. In the real hybrid process the reaction mixtures with different concentrations of species can be obtained

[(Fig._4)TD$IG]

Fig. 4. SEM micrographs of cross-section of the PPO/MFFC composite membrane.

during reaction time. The compositions of feed and permeate in pervaporation of a quaternary system using a PPO/MFFC composite membrane are presented in Table 2. Data on performance as total flux are also listed in Table 2. The enrichment of the permeate by esterification products, especially ethyl acetate, was established for all compositions. The concentration of ethyl acetate in the permeates was much higher than in the feed. The amount of water in the permeate increased (except in Nos. 2, 5, and 7). The concentration of initial reagents changes in the opposite way: the ethanol amount in the permeates is less than in the feed (except No. 7), while acetic acid virtually does not pass through the membrane. Such low permeability of acetic acid in pervaporation of quaternary mixture was previously observed in the hybrid processes “pervaporation + reaction” of butyl acetate [32] and methyl acetate [5] synthesis. Table 2 lists data on the total flux, which depends on the composition of feed with definite regularity. The performance of the PPO/MFFC composite membrane increases with an increase of ethyl acetate content in the feed. An exception is feed No. 5, where the decrease in performance may have occurred because of a high water content in the feed. A noteworthy result is that use of the composite membrane leads to an increase in permeability and selectivity with respect to ethyl acetate. The reason may be in the specific interaction between components of the feed and the polymer of the support (fluoroplast F42L). To confirm this assumption, the sorption of films from fluoroplast F42L was determined. Equilibrium degrees of swelling (Sw) of PPO and fluoroplast F42L in pure liquids are presented in Table 3. Fluoroplast F42L is inert to water, and slightly sorb ethanol and acetic acid, but has very high sorption with respect to ethyl acetate (S = 103.4%). The data confirm that hydrophobic interactions between the support and ethyl acetate molecules may contribute to an improved separation factor of the composite membrane PPO/MFFC containing fluoroplast F42L. It significantly influences the pervaporative removal of ethyl acetate from the quaternary mixture. Previous works on dense film membranes [20–22] showed that the addition of fullerene in a PPO matrix leads to a positive change in the transport characteristics of membranes in pervaporation of the quaternary reacting mixture with a composition close to chemical equilibrium. These results are presented in Table 4. It is evident that

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Table 2 Data on pervaporation of quaternary mixtures using PPO/MFFC membrane. No

1 2 3 4 5 6 7

Feed, wt% 0.2% EA

Ethanol

Water

Acetic acid

EA

Ethanol

Water

80.1 73.7 58.2 54.9 55.2 38.1 17.9

7.5 11.8 31.6 4.5 14.6 21.1 8.6

2.6 10.1 2.6 0.3 11.3 1.1 35.2

9.8 4.4 7.6 40.3 18.9 39.7 38.3

92.6 92.0 87.9 93.9 85.9 78.1 67.4

4.2 3.5 8.5 2.9 5.6 14.6 20.5

3.2 4.5 3.6 3.2 8.5 7.3 12.1

Table 3 Swelling degree of PPO [21] and fluoroplast F42L. Solvents

Swelling degree, g/100 g polymer PPO

Fluoroplast F42L

Ethanol Acetic acid Water Ethyl acetate

13.0 0.5 23.6 0.5 0 23.4 0.4

2.3 0.2 7.5 0.4 0 103 2

Table 4 Pervaporation of the quaternary reacting mixture using PPO and 2%C60–PPO dense films [22]. Solvents

Feed wt% 0.1%

Ethanol Acetic acid Water Ethyl acetate

14.6 18.9 11.3 55.2

Permeate, wt% 0.1% PPO

2%C60–PPO

8.6 1.4 8.4 81.6

8.8 1.2 6.2 83.8

use of membranes modified by fullerene C60 leads to decreasing water content and increasing ethyl acetate content in permeate. The increase of selectivity occurs due to decreasing the free volume [22] as a result of interaction between PPO and C60. The rise of flux occurs due to formation of additional sorption centers on the membrane

[(Fig._5)TD$IG]

Total flux, kg/m2 h

Permeate, wt% 0.3%

1.1 0.2 0.76 0.08 0.53 0.07 0.39 0.01 0.37 0.01 0.14 0.01 0.093 0.003

surface; this fact can be confirmed by increasing the contact angle u of water (from uPPO = 91.2 to u2%C60–PPO = 94.4 ). Therefore, taking into account the positive influence of fullerene C60 additives in PPO and the high efficiency of membranes with a composite structure, further studies were carried out with C60–PPO/MFFC composite membranes. The morphology of the C60–PPO/MFFC composite membrane was similar to that presented in Fig. 4. Good adhesion between the selective layer and the support was also observed. The effect of the content of fullerene on the transport properties of composite membranes with various concentrations of C60 in the top layer was studied in pervaporation of a reacting mixture whose composition is close to chemical equilibrium (Table 4). The results are presented in Figs. 5 and 6. Fig. 5 shows the change of content of the quaternary feed mixture after pervaporation through C60–PPO/MFFC composite membranes containing 0.5, 2, and 3 wt% C60. The content of ethanol and acetic acid in the permeate is smaller than in the feed. This indicates that over the course of the synthesis, the initial reagents essentially remained in the reactor. The reaction products penetrated through the membranes by different modes. The water amount in the permeate decreased compared to the initial amount for composite membranes containing only 0.5 and 2 wt% C60, but in the case of 3 wt% C60, the water amount in the permeate increased. The main component of the permeate is ethyl acetate. Its amount essentially increases in the permeate in comparison with the feed, and a higher amount of ethyl acetate have been obtained through a 2%C60–PPO/MFFC membrane.

Fig. 5. Content of quaternary mixture in feed and in permeate in pervaporation using 0.5%C60–PPO/MFFC, 2%C60–PPO/MFFC, and 3%C60–PPO/MFFC membranes, 25  C.

86

[(Fig._6)TD$IG]

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related to the producing composite membranes with fullerenecontaining polymer selective layer intended for ester separation in hybrid process of alkyl acetate production. Proposed method consists in formation of the selective polymer top layer on the MFFC support that is microfiltration membrane from copolymer fluoroplast F42L and polypropylene base. Selective layer is the composite of fullerene C60 and PPO. Selective layer is formed by coating 2% solution of C60–PPO composite onto the surface of microporous support; and finally composite membrane is dried. Acknowledgments

Fig. 6. Dependence of total flux and pervaporation separation index (PSI) on fullerene E60 content in the top layer of E60–PPO/MFFC membranes for pervaporation of the quaternary mixture 18.9 wt% acetic acid, 14.6 wt% ethanol, 55.2 wt% ethyl acetate, and 11.3 wt% water, 25  C. The size of dots includes the range of experimental errors.

Fig. 6 shows the dependence of total flux and pervaporation separation index on the fullerene content in the top layer of the E60–PPO/MFFC membranes in pervaporation of the quaternary reacting mixture. The data show that an increase in the fullerene C60 content in the top layer increases the total flux. The composite membrane with 3%E60–PPO selective layer has the best permeability. The efficiency of the membranes was evaluated using the pervaporation separation index (PSI) determined by Eq. (2) as a product of total flux on the separation factor of ethyl acetate. The dependence of PSI on fullerene content in the top layer of the E60–PPO/MFFC membranes is also presented in Fig. 6. The pervaporation separation index increases with the rise of C60 content up to 2 wt% C60 and then decreases in the case of the 3% C60–PPO/MFFC membrane. Thus, it was established that the best transport properties can be obtained by using the 2%C60–PPO/MFFC membrane in pervaporation of the system with ethyl acetate synthesis. 4. Conclusions The suggested character of binding in C60–PPO compositions was verified by nuclear magnetic resonance study. 13C MAS and CP/ MAS NMR spectra of C60–PPO nanocomposites show a slight interaction between the C60 molecules and the PCO matrix. TG analysis reveals that increasing the temperature of the membrane results in thermal destruction due to certain structural changes of the matrix upon incorporation of the C60 molecules. E60–PPO/MFFC composite membranes with a thin selective layer containing up to 3 wt% C60 placed on the surface of an MFFC porous support were developed. The study of pervaporation of the quaternary reacting system ethyl acetate–ethanol–water–acetic acid shows that composite membranes possess high permeability; moreover, the E60–PPO/MFFC membranes exhibit higher selectivity with respect to ethyl acetate compared to the composite membrane unmodified by fullerene. The optimum content of fullerene in the selective layer is equal to 2 wt% C60. Pervaporation tests show that the 2%E60–PPO/MFFC composite membrane has a maximal pervaporation separation index and is promising for use in real hybrid processes (esterification + pervaporation). The RU patent fixes the preparation method of composite membranes with a fullerene-containing selective layer [33]. The invention is

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