1. Introduction
Bioethanol is one of the attractive renewable fuels that are produced from biomass [
1,
2,
3]. Compared to gasoline, ethanol has higher octane ratings. Hence, bioethanol engines show better thermal efficiency. Top-quality grade ethanol is utilized as an antiseptic and solvent in the medicinal field [
4,
5]. The azeotropic composition of aqueous bioethanol is challenging to purify with conventional distillation as it is economically not viable, and it makes use of carcinogenic chemicals [
6,
7]. Instead, the pervaporation, which is a membrane-based process, is preferred because of its efficiency and economic benefits. In pervaporation, membrane plays a significant role in separating the aqueous–organic mixture because of their ability to separate the azeotropic mixture efficiently [
8,
9].
In pervaporation, the separation efficiency depends on the membrane’s selective affinity towards one of the components of the mixture. The vacuum is generated on the permeate side, whereas the feed side will be at the atmospheric pressure; this leads to the generation of the pressure difference between the two sides, which acts as a driving force for the separation process.
Research in the pervaporation area is mainly directed towards designing an efficient membrane material. The materials must encompass properties such as film-forming ability, high selectivity, higher permeation flux, good mechanical strength, and good shelf life. Research in this direction has shown that hydrophilic materials are always suitable for the dehydration of alcohols, and many such membrane materials have been studied for dehydration of alcohols [
10,
11,
12,
13].
Among all the hydrophilic materials, poly(vinyl alcohol) (PVA) is considered to be promising because of its cost-effectiveness, hydrophilicity, and excellent membrane-forming properties [
14,
15,
16,
17,
18]. However, the PVA membrane suffers from excessive swelling, which leads to poor pervaporation efficiency; to overcome this, many modification techniques such as grafting, crosslinking, blending, as well as the addition of nanomaterials are employed to strengthen the membrane and solve the problem of excessive swelling [
19,
20].
In recent advances, Hanshuo et al. fabricated PVA/SO
42−-anodic Al
2O
3 membranes by dip coating method. The dual functional flat composite membranes had shown enhanced PV performance [
21]. Chaudhari et al. prepared poly(vinyl alcohol)–tetraethyleorthosilicate membrane and modified it through layer by layer deposition of poly(vinyl amine)/silicotungstic acid polyelectrolyte. In terms of the membrane’s layer by layer modification, the authors achieved a higher flux [
22]. In order to enhance the PV properties of PVA, Thorat et al. prepared PVA/ionic liquid mixed matrix membranes by solvent evaporation method using four ionic liquids: 1-n-butyl-3-methylimidazolium chloride (BMIMCl), 1-hexyl-3-methylimidazolium chloride (HMIMCl), 1-octyl-3-methylimidazolium chloride (OMIMCl), and 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF4). The membrane performance was based on the number of alkyl groups. It was observed that the higher alkyl groups of cation were responsible for high selectivity towards the water, whereas lower alkyl groups were responsible for high flux [
23].
Nanomaterials possess incredible properties such as high thermal stability, high surface area, good optical activity, and conductive properties, which can be utilized to enhance the membrane performance by their incorporation into the membrane matrix [
24,
25,
26]. Amongst all the nanomaterials, graphene is a very interesting nanomaterial as it possesses a two-dimensional layered structure (honeycomb-like structure), which makes it thermally and mechanically highly stable as well as providing it a high surface area. The layered arrangement and small interlayer space make the graphene highly selective and an excellent additive. Moreover, its adhesive nature with silane groups makes it more effective and stable in the membranes [
27]. Liange et al. fabricated graphene oxide/polyacrylonitrile composite membranes for pervaporation applications and revealed that graphene oxide-based films show preferential water transport [
28]. Dharupaneedi et al. prepared chitosan nanocomposite membranes with embedded functionalized graphene sheets and subjected them to isopropanol and ethanol dehydration via pervaporation. In the study, they also revealed that functionalized graphene sheets enhanced the water permeation and decreased the alcohol permeation through the membrane [
29]. These studies created curiosity in us to understand the effect of graphene more systematically.
In this research work, we attempted to study the effect of graphene on the PV efficiency by loading the graphene into the benzaldehyde 2 sulphonic sodium salt acid (B2SA)-grafted PVA–tetraethylorthosilicate (TEOS) hybrid membranes. The morphological and chemical properties of the developed membranes were analyzed by means of FTIR, wide-angle X-ray diffraction (WAXD), thermogravimetric analysis (TGA), SEM, mechanical analysis, and contact angle analysis. Then, the effect of graphene loading on the PV separation of azeotropic aqueous bioethanol was broadly studied.
2. Materials and Methods
2.1. Materials
Graphene was procured from United Nanotech Innovations Pvt. Ltd., Banglore, India. Tetraethylorthosilicate (TEOS) was procured from E. Merck Ltd., Mumbai, India. Benzaldehyde 2 sulphonic sodium salt acid (B2SA) and poly(vinyl alcohol) were purchased from spectrum reagents and chemicals Pvt. Ltd., Cochin, India. Chemicals purchased were used in the study without further purification as they were reagent-grade chemicals.
2.2. Membrane Preparation
The PVA solution (4 mass%) was made by dissolving the PVA in double-distilled water and kept for constant stirring for half a day. The solution was then filtered to remove the undissolved particles. To the above solution, we mixed in a known quantity of tetraethylorthosilicate (6 g), and the mixture was stirred for 1 day at 60 °C. The resultant homogenous solution was then casted on a clean and smooth glass plate by means of a casting knife in pollution-free atmosphere. The membrane was allowed to dry properly at room temperature. A fully dried membrane was then peeled off and named as M. B2SA-grafted hybrid PVA membrane was prepared by mixing a known quantity of B2SA (7.5 mass%) to the above-prepared solution and stirred for 1 day at 60 °C. The remaining procedure followed was similar to the procedure followed for the hybrid PVA membrane preparation and subsequently the prepared membrane was named as M-B2SA [
19]. With the intention of studying the effect of graphene on pervaporation performance of sulfonated PVA–TEOS hybrid membranes, known amounts (0.5%, 1%, 1.5%, and 2%) of different mass% of graphene were added with respect to PVA and stirred for 4 h, being kept in a sonicator for 30 min to obtain a uniform suspension. Further, the suspension was spread on a glass plate and subjected to drying for 3 days. Then, the membranes were carefully peeled off and were named as M-0.5 G, M-1 G, M-1.5 G, and M-2 G, respectively. Upon further enhancement in the content of graphene, the membrane peeling became difficult because of agglomerated graphene. Therefore, we stopped the membrane development at 2 mass% graphene. The thickness of the developed membranes was assessed at various positions using a thickness gauge (Peacock dial thickness gauge). The uniform thickness 50 ± 2 μm was obtained for the fabricated membranes. The membrane preparation scheme for the graphene loaded sulfonated PVA–TEOS hybrid membrane is illustrated in
Figure 1.
2.3. Fourier Transform Infrared (FTIR) Spectroscopy
Spectrum two FTIR with Diamond ATR (PerkinElmer Singapore Pte. Ltd., 28, Ayer Rajah Crescent, no. 08-01, Singapore 139959) was used to study the interactions between the chemicals used in the developed membranes. FTIR analysis was recorded in the range of 500 to 4000 cm
−1. KBr method was employed for the analysis [
30].
2.4. Wide-Angle X-ray Diffraction (WAXD)
Philips Analytical X-ray Diffractometer (Rigaku SmartLab SE, Tokoyo, Japan) was used to analyze the solid-state morphology of the developed membranes. Membrane samples were scanned in the range 5° to 50° for the angle 2θ at the rate of 8°/min.
2.5. Differential Scanning Calorimetry (DSC)
Differential scanning calorimeter (DSC Q20, TA Instruments, Waters LLC., New Castle, DE, USA) was used to study the crystallinity of fabricated membranes. Weight of samples ranging from 9 to 10 mg were subjected to heating from atmospheric temperature to 700 °C under nitrogen atmosphere at a heating rate of 10 °C per minute.
2.6. Thermogravimetric Analysis (TGA)
A thermogravimetric analyzer named SDT Q600 (TA Instruments, Waters LLC., New Castle, DE, USA) was used to study the thermal analysis of the fabricated membranes under the nitrogen atmosphere. The heating rate was maintained at 10 °C/min.
2.7. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray (EDX) Analysis
The surface morphology and elemental composition of the graphene was analyzed using a scanning electron microscope (SEM) and energy dispersive X-ray (EDX) analysis named JEOL-JSM-IT500, (Tokyo, Japan). Further, membranes were examined using a scanning electron microscope (SEM), and all the membranes were dried properly prior to the analysis and were sufficiently covered with a sputtered gold layer of 400 Å.
2.8. Mechanical Properties
The mechanical strength and elastic nature of the developed membranes were analyzed using the Universal Testing Machine (DAK system Inc., Maharastra, India). The analysis was performed 3 times, and the mean value was considered.
2.9. Swelling Measurement
Membrane sorption measurements were conducted at room temperature by considering the azeotropic mixture of water and bioethanol. Initially, the developed membranes were vacuum dried and weighed. Further, these membranes were then dipped in the azeotropic composition of bioethanol for 1 day in a closed bottle for equilibrium establishment. After this, the swollen membranes were carefully blotted and weighed. The percentage membrane sorption was calculated using the expression [
31,
32,
33]:
where
Wd and
Ws are the masses of the dry and swollen membranes, respectively.
2.10. Contact Angle Meter
To study the surface properties of the developed membranes at 30 °C, we measured the contact angle by the sessile drop method by means of a contact angle meter named Kyowa Interface measurement and analysis (Japan).
2.11. Pervaporation Experiments
A custom-made apparatus was used to carry out the pervaporation experiments. The schematic design of the pervaporation apparatus is illustrated in
Figure 2a.
The membrane with the surface area 15 cm
2 was sandwiched between permeate and the feed section. The feed section was equipped with a stirrer aided by a DC motor to maintain the uniform temperature and flow of the liquid mixture. The photographic image of the designed PV unit [
34] is illustrated in
Figure 2b. The membrane under the test was allowed to equilibrate with the feed mixture for about 1 h in the feed section at room temperature. Once the membrane attained the equilibrium in the PV apparatus, a vacuum of 31.325 kPa was induced. On the permeate section, vapors were condensed in the cold trap and collected in the form of liquid at the uniform interval of 1 h time periods. After that, the weight of the permeate was measured using a digital microbalance. KAFI smart Karl Fischer Titrator was used to analyze the composition of the permeate liquid in terms of percentage. The experiment was repeated 3 times, and the average results were considered. Then, the PV performance of the membranes was analyzed by measuring the separation selectivity (
αsep), total permeation flux (
J), and pervaporation separation index (
PSI) using the following expressions [
35,
36]:
where
A is the membrane area (m
2);
W is the mass of permeate (kg);
t is the permeation time (h);
PET and
Pw are the mass percent of ethanol and water in the permeate, respectively; and
FET and
Fw are the respective mass percent of ethanol and water in the feed.
The permeance (
Pi/l) was measured by using the following expression for all the membranes to obtain a clear idea about the fundamental properties of the membranes [
37,
38]:
In this expression, Ki and Di are the sorption and diffusion coefficient of the ith constituent, respectively; Pi is the permeability of the ith constituent, and are the vapor pressures of the ith constituent in permeate and feed, respectively; ji is the molar flux of ith component; and l is the membrane thickness.
4. Conclusions
In this research analysis, graphene-loaded, sulfonated PVA–TEOS hybrid membranes were developed by a solution casting method. These membranes were characterized by means of various characterizing techniques. In FTIR, the broad intensity peak appearing at 3318 cm−1 was enhanced systematically, which was due to the presence of O–H groups of water molecules present in the channel created due to the adhesion (Van der Waals force of attraction) between the graphene and the silane group of TEOS. From the WAXD patterns, it was revealed that as the graphene was incorporated in the B2SA-grafted PVA–TEOS hybrid membrane, the peak, which was at 2θ = 19.65°, was shifted to a higher 2θ angle of 20.32°, along with the drastic increment in the intensity of the peak. A higher 2θ angle also resulted in the lower d-spacing value of 4.36 Å, which resulted in enhanced selectivity of the membrane. TGA showed the higher thermal stability of the membrane after the incorporation of graphene into the matrix. DSC revealed that the graphene-incorporated membranes exhibited a higher area under the curve, which indicated that the membranes with graphene possessed a higher water-holding capacity. These results were also reflected in sorption studies and contact angle measurements, as these characterizations also revealed that the membrane M-2 G exhibited higher water-holding capacity. Enhancement in both total permeation flux and separation selectivity is not commonly observed because of the trade-off phenomenon. However, in this study, both the separation selectivity and the total permeation flux were enhanced simultaneously.