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Development of novel hydrophilic ionic liquid membranes for the recovery of biobutanol through pervaporation
Journal of Environmental Management 251 (2019) 109618
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Journal of Environmental Management journal homepage: http://www.elsevier.com/locate/jenvman
Research article
Development of novel hydrophilic ionic liquid membranes for the recovery of biobutanol through pervaporation Zabia Sajjad a, Mazhar Amjad Gilani b, Abdul-Sattar Nizami c, Muhammad Roil Bilad d, Asim Laeeq Khan a, * a
Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus, Pakistan Department of Chemistry, COMSATS University Islamabad, Lahore Campus, Pakistan Center of Excellence in Environmental Studies (CEES), King Abdulaziz University, Jeddah, Saudi Arabia d Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 8, Perak, Malaysia b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Ionic liquids membranes Pervaporation Biobutanol separation Cleaner production Climate change
This paper aims to develop novel hydrophilic ionic liquid membranes using pervaporation for the recovery of biobutanol. Multiple polyvinyl alcohol (PVA) membranes based on three commercial ionic liquids with different loading were prepared for various experimental trials. The ionic liquids selected for the study include tributyl (tetradecyl) phosphonium chloride ([TBTDP][Cl]), tetrabutyl phosphonium bromide ([TBP][Br]) and tributyl methyl phosphonium methylsulphate ([TBMP][MS]). The synthesized membranes were characterized and tested in a custom-built pervaporation set-up. All ionic liquid membranes showed better results with total flux of 1.58 kg/m2h, 1.43 kg/m2h, 1.38 kg/m2h at 30% loading of [TBP][Br], [TBMP][MS] and [TBTDP][Cl] respec tively. The comparison of ionic liquid membranes revealed that by incorporating [TBMP]MS to PVA matrix resulted in a maximum separation factor of 147 at 30 wt% loading combined with a relatively higher total flux of 1.43 kg/m2h. Density functional theory (DFT) calculations were also carried out to evaluate the experimental observations along with theoretical studies. The improved permeation properties make these phosphonium based ionic liquid a promising additive in PVA matrix for butanol-water separation under varying temperature conditions.
1. Introduction There has been significant interest in the production of biofuels through fermentation of sustainable biomass resources due to the increasing concerns of fossil fuels shortage and climate change (Petra novic et al., 2017; Tan et al., 2014). Bioethanol and Biobutanol are the most popular energy sources among biofuels (Ko et al., 2017). Owing to the properties of high octane number, energy content (Sindhu et al., 2019), low vapor pressure and excellent combustion characteristics in the engine as compared to ethanol, butanol is considered to be the most promising fuel among many other alternative fuels (Li et al., 2016; Szwaja and Naber, 2010). Noteworthy research has been carried out in different aspects of acetone-butanol-ethanol (ABE) fermentation process in recent years owing to the availability of a wide range of fermentative substrates (Li et al., 2013). Butanol is commercially synthesized using petrochemical route whose cost is linked with the price of crude oil. However, biobutanol is produced by ABE fermentation of renewable
biomass for sustainable development. The ABE products are formed in a ratio of 3:6:1 (Algayyim et al., 2018; Brito and Martins, 2017). During the production of butanol in fermentation broth, inhibition of microbial growth is the most challenging part (Liu et al., 2013b). That is why butanol is harmful to microorganism, Clostridium acetobutylicum, used in the process (Zak et al., 2015). In traditional fermentation broth, the solvent concentration is not exceeding 20 g/L whereas butanol concentration is less than 14 g/L (Tan et al., 2014). Certain species of Clostridium have been investigated to make this process economically viable. Separation coupled with fermentation is suggested to increase butanol tolerance of microorganisms. These hybrid processes of fermentation such as adsorption (Levario et al., 2012; Silva et al., 2017), liquid-liquid extraction (Cascon et al., 2011; Sas et al., 2018), gas stripping (Ezeji et al., 2003; Rincon et al., 2019), or pervaporation (Liu et al., 2011) improve butanol extraction from the fermentation broth and separation efficiency. Among these separation methods, pervapo ration is a promising method to recover biobutanol because of low
* Corresponding author. E-mail address: [email protected] (A.L. Khan). https://doi.org/10.1016/j.jenvman.2019.109618 Received 27 April 2019; Received in revised form 10 September 2019; Accepted 21 September 2019 Available online 26 September 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.
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Journal of Environmental Management 251 (2019) 109618
energy cost as it requires latent heat to penetrate through the membrane, easy operation and does not contaminate the targeted product like liquid-liquid extraction (Chapman et al., 2008; Shao and Huang, 2007). The efficiency of pervaporation mainly depends on (1) the choice of membrane material for a particular separation application, (2) an interaction between components of feed and polymer, (3) the membrane stability and mechanical strength, and (4) high selectivity and perme ation flux (Sairam et al., 2006). The fundamental processes governing the transport through pervaporation membranes include (1) sorption of molecules (gas/liquid) in membrane surface, (2) diffusion of species sorbed across the membrane body, (3) and desorption of sorbed species at the permeate interface. These three steps correspond to the solution-diffusion theory (Jyothi et al., 2019; Ong et al., 2016). The polymeric and mixed matrix membranes filled with inorganic filler have been widely used in pervaporation applications (Jia and Wu, 2016). Polyvinyl alcohol (PVA) was the first material that was used in perva poration and is now extensively used for the dehydration of alcohols because of its unique properties of hydrophilicity and film formation (Thorat et al., 2017). The PVA based membranes are highly hydrophilic due to the pres ence of hydroxyl groups. Their hydrophilicity is controllable, and their chemical resistant properties make them efficient for pervaporation dehydration (Qiao et al., 2005). These membranes also show high chemical stability and better mechanical strength (Liu et al., 2013a). However, a limitation of the PVA based membranes is the low selectivity that makes them less commercially viable. Hence, several modifications have been recommended to increase the performance of PVA based pervaporation. These modifications involve the synthesis of composite and mixed matrix membranes using highly selective support and incorporation of various nanoparticles in the PVA matrix (Dong et al., 2006; Peters et al., 2006). In the last decade, ionic liquid membranes have gained significant attention because of their advanced properties (Matsumoto et al., 2011). Ionic liquids have high thermal stability, less viscosity, low vapor pressure, high electrical conductance, low volatility and high ability to dissolve a range of compounds (Petra and Katalin, 2011). Their prop erties can be further tuned because of the cationic and anionic combi nations (Heitmann et al., 2012; Matsumoto et al., 2011; Petra and Katalin, 2011). They have been used for the recovery of numerous sol vents from the aqueous solution (Cascon and Choudhari, 2013; Mai et al., 2013; Neves et al., 2011). Rdzanek et al. (2015) studied perva poration of quaternary ABE mixture through hydrophobic supported ionic liquid membranes. Two ionic liquids namely, trihexyl(tetradecyl) phosphonium tetracyanoborate and 1-hexyl-3-methylimidazolium tet racyanoborate were incorporated in polyether block amide matrix, and pervaporation experimentation was carried out at 37 � C with 3 wt% butanol concentration feed solution. The results showed that by using ionic liquid membranes, permeate flux and selectivity were influenced, and butanol concentration was raised by 10 times in permeate as compared to feed. In another study (Thorat et al., 2017), studied PVA-IL hydrophilic composite membranes for IPA-water mixture separation. Four ionic liquids namely, 1-n-butyl-3-methylimidazolium chloride (BMIMCl), 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF4), 1-hexyl-3-methylimidazolium chloride (HMIMCl) and 1-octyl-3-methylimidazolium chloride (OMIMCl) were examined in the study. They reported that longer the length of alkyl chain of ionic liquid, higher was the selectivity of the membrane towards the water. This fact is supported by an argument that alkyl chain length increases the van der Waals and coulombic interactions that ultimately increase the selectivity of the membrane. Researchers have laid the foundation of ionic liquid membranes for pervaporation, but still, there is a long way to discover the nature of ionic liquid that best fits with the membrane material for a specific feed mixture separation, which was the focus of this study. This study, for the first time, aims to develop and investigate the novel hydrophilic membranes based on PVA and three different ionic
liquids were selected in order to efficiently separate butanol-water mixture while keeping the stability issues of the membranes in consid eration. The selected ionic liquids include tributyl (tetradecyl) phos phonium chloride ([TBTDP][Cl]), tetrabutyl phosphonium bromide ([TBP][Br]) and tributyl methyl phosphonium methylsulphate ([TBMP] [MS]). These ionic liquids have different degrees of hydrophilicity, are commercially available and novel for the desired applications. The findings of this study would allow researchers to compare the perfor mance and behavior of ionic liquids when they are incorporated in PVA films. 2. Materials and methods 2.1. Materials and fabrication of membranes PVA was purchased from Sigma-Aldrich (USA). The distilled water (pH 6.5 � 0.2) as a solvent was prepared in the departmental laboratory. Other materials include hydrophilic ionic liquids, tributyl (tetradecyl) phosphonium chloride ([TBTDP][Cl]), tetrabutyl phosphonium bromide ([TBP][Br]) and tributyl methyl phosphonium methylsulphate ([TBMP] [MS]). All the ionic liquids were purchased from Io-Li-Tech (Germany). The chemical structures of all these ionic liquids are shown in Table 1. For membrane fabrication 10 wt% PVA solution in distilled water was prepared. This solution was stirred for 3 h. The stirring was carried out on a digitally controlled magnetic hot plate stirrer (MS7-H550-S) fol lowed by heating at 60 � C. After getting a homogenous solution, ionic liquid in different loadings with respect to polymer matrix was added in the solution and stirring was continued for another 4 h. After casting the solution in Teflon Petri dish, they were allowed to evaporate at room temperature for 24 h. In order to remove residual solvent, these mem branes were thermally treated at 100 � C for 1 h and peeled off. The composition of all prepared membranes is shown in Table 2. 2.2. Pervaporation set-up Pervaporation experiments were carried out in custom-built crossflow pervaporation cell. The detailed working of the set-up is reported by (Khan et al., 2018), and schematics are presented in Fig. S1. A 10% aqueous solution of butanol was used as feed and permeation studies were carried out at 25, 35, 45, 55 and 65 � C. The effective area of the membrane was 16 cm2. In order to achieve a steady state, the membrane was allowed to equilibrate for 10 h with feed solution before starting the experiment. The feed was pumped under atmospheric pressure at a constant feed rate of 1 L/min. The retentate was recycled back to the feed tank. The vacuum was maintained at the permeate side with the help of a vacuum pump. The condensed permeate was analyzed in a gas chromatograph equipped with a thermal conductivity detector (7890A, Agilent, USA). The flux (J) was calculated by dividing the mass of permeate (Q) by the product of the interval time (t) and area of mem brane (A), as shown in Equation (1): J¼
Q At
(1)
The separation factor (α) was calculated by Equation (2):
αi=j ¼
yi=yj xi=xj
(2)
where x and y are mole fractions of components in permeate and feed respectively and subscript i and j represent water and butanol, respectively. Pervaporation is an activated process, so the behavior of flux was analyzed at various temperatures and the following Arrhenius Equation was used to explain this behavior of flux dependence on temperature.
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Table 1 Ionic Liquids used in the study. Full name
Abbreviation
Structure
Tributyl(tetradecyl)phosphonium chloride
[TBTDP][Cl]
435.158
Tetrabutyl phosphonium bromide
[TBP][Br]
339.33
Tributylmethylphosphonium methylsulphate
[TBMP][MS]
328.45
analyzer DSA-30 (KRUSS, Germany). In order to confirm the presence of ionic liquids and their interaction with PVA matrix, membranes were characterized by Fourier Transform Infrared (FTIR) spectra that was recorded on FTIR-Spectrometer (Thermo-Nicolet 6700 P, USA). Mem branes were analyzed for thermal analysis using Thermogravimetric Analysis (TGA - SDT Q600 TA Instruments) having a ramp rate of 10 � C/ min under nitrogen atmosphere. The morphology of membranes was observed with the help of Scanning Electron Microscope (SEM) of TESCAN Vega LMU having variable pressure. The membranes were fractured for cross-section analyses by immersing in liquid nitrogen and were sputter-coated with gold to increase their conductivity.
Table 2 Abbreviation and composition of membranes. Membrane
Composition
PVA PVA-S-10
Pure PVA þ0% IL 90% PVA solution sulphate (IL) 80% PVA solution sulphate (IL) 70% PVA solution sulphate (IL) 90% PVA solution (IL) 80% PVA solution (IL) 70% PVA solution (IL) 90% PVA solution 80% PVA solution 70% PVA solution
PVA-S-20 PVA-S-30 PVA-Cl-10 PVA-Cl-20 PVA-Cl-30 PVA-Br-10 PVA-Br-20 PVA-Br-30
J ¼ Jo exp ð
þ10% Tributyl methyl phosphonium methyl þ 20% Tributyl methyl phosphonium methyl þ 30% Tributyl methyl phosphonium methyl þ 10% Tributyl tetradecyl phosphonium chloride þ 10% Tributyl tetradecyl phosphonium chloride
2.4. Computational techniques
þ 30% Tributyl tetradecyl phosphonium chloride
All simulations were performed on the Gaussian 16 software. The geometries of all structures were optimized using hybrid B3LYP method (Lee et al., 1988) with the 6–31 þ G (d, p) basis set without any geometrical constraints. The geometries of the optimized structures were plotted with Avogadro 1.2.0 (Hanwell et al., 2012). The frequency calculations were performed at the same level of theory to confirm the optimized geometries as true minima with no imaginary frequency (Pople et al., 1981). The complexation energies (Ec) have been calcu lated from the following Equation (4): � (5) Ec ¼ EIL solvent ðEIL þ Esolvent
þ 10% Tetrabutyl phosphonium bromide (IL) þ 20% Tetrabutyl phosphonium bromide (IL) þ 30% Tetrabutyl phosphonium bromide (IL)
EJ Þ RT
(3)
where J is the flux, Jo is the pre-exponential factor, and Ej is the acti vation energy of flux. 2.3. Membrane uptake and characterization
where EIL-solvent, EIL, and Esolvent are the total energies for the ionic liquid-solvent complex, ionic liquid, and solvent, respectively.
The sorption of solvent in the membranes was calculated using the gravimetric method. 1 � 1 cm piece of membranes of known weight was immersed in 90 wt% butanol solution at the room temperature. These films were then allowed to equilibrate for 24 h. After the removal of membranes from the solution, they were wiped with tissue paper to remove the excessive liquid and weighed. The following Equation (3) was used to calculate liquid sorption ‘S’ (g of liquid/100 g of dry poly mer) in the membrane: S¼
ms
m0 m0
:100
MW (g/mol)
3. Results and discussion 3.1. Membrane characterization Fig. 1 presents the water contact angle of the membranes. Pure PVA membrane showed an angle of 51� . The obtained value is almost consistent with the literature data. It can be noted that water contact angle for all the membranes was lower than 90� , which proves the hy drophilic character of these membranes (Dmitrenko et al., 2018; Kujawski et al., 2017). On increasing the content of ionic liquid in membranes, the water contact angle was decreased for all three ionic liquids, implying that hydrophilicity of membranes was increased as ionic liquid loading was enhanced. This further helped to build up the
(4)
where ms and m0 are the weight of swollen and dried membrane respectively. The hydrophilicity of membranes was determined by contact angle measurement using a sessile drop method with the help of contact angle 3
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results of surface and cross-section of membranes are shown in Figs. 2 and 3, respectively. It can be seen that the surface of pure PVA mem brane was smooth with some random pores that were present only on the surface in agreement with the reported literature (Kittur et al., 2018; Sairam et al., 2006). Fig. 2 (b) showed that with the incorporation of [TBMP][MS] in PVA matrix, membranes became more porous. These pores were uniformly distributed throughout the surface of membranes. Because of the presence of these pores, the diffusion of feed solution was expected to be higher through the surface. However, the cross section of these membranes (Fig. 3b) was dense without the presence of any pores and voids. It was expected that the porous surface behaved like a skin layer that increased the contact with feed solution and the rest of the membrane would impart the selective separation characteristics. Clearly, there were no agglomerates in ionic liquid membranes that can be ascribed to good adhesion and suitable compatibility between the PVA and ionic liquid, as both are hydrophilic (Raeisi et al., 2019). Images in Fig. 2 (c) depicted that the addition of [TBTDP][Cl] in PVA matrix resulted in the formation of a membrane that was rough. The roughness was increased with the increased loading of the ionic liquid. The same behavior was observed earlier in a reported study (Zhang et al., 2019). Image of PVA-Cl-30 (Fig. 3) showed that the top layer of the membrane was highly rough, and roughness continued throughout the cross-section. This entire change in the surface structure and cross-section of these membranes as compared to the neat PVA mem brane was attributed to the nature of [TBTDP][Cl] ionic liquid, or due to the presence of long tetradecyl chain present in ionic liquid. Fig. 2 (d) showed that the membrane with minimum loading of [TBP][Br] had a porous surface. However, on higher loadings, ionic liquid was highly embedded in the polymer matrix as the pores were disappeared and the surface became rough. Moreover, a closer look at a cross-section of [TBP][Br] membranes showed that all these membranes were completely smooth and non-porous. The difference in morphology of membranes comprising different ionic liquids was likely due to the difference in anion, hydrophilicity and alkyl chain length of each ionic liquid and their interactions with the polymer matrix.
Fig. 1. Effect of ionic liquid loading on degree of swelling of membranes based on (a) [TBTDP][Cl] (b) [TBP][Br] (c) [TBMP][MS] Ionic liquids.
hypothesis of achieving better solubility and crosslinking behavior for all three ionic liquids in the PVA matrix. The membranes comprising of [TBTDP][Cl] showed the highest value of contact angle that was decreased from 47� to 36� when ionic liquid was increased from 10% to 30%, proving them to be the least hydrophilic in nature. The membranes filled with [TBP][Br] ionic liquid showed the most hydrophilic behavior as their water contact angle showed the most significant drop. The hy drophilicity of [TBMP][MS] membranes was in between the membranes based on other two ionic liquids. This change in membrane behavior was due to the physical properties of different ionic liquids leading to a difference in their interaction towards the water. In order to confirm the presence of functional groups in membranes, FTIR spectra was obtained (Fig. S2). In all membranes, a characteristic strong band appeared at 3500 cm 1 corresponding to O–H bond stretch. The addition of ionic liquids in the PVA matrix caused no significant difference as absorbance of O–H band remained the same in all mem branes. A band appeared between 1500 and 2000 cm 1 represents C–C stretching (Thorat et al., 2017). In Fig. S2 (a) teethed shaped peaks were appeared between 1200 and 1300 cm 1, corresponding to the asym metric stretching vibrations of SO3 group present in ionic liquid ([TBMP][MS]) (Boroglu et al., 2011). Moreover, in Fig. S2 (b) and (c) C–H stretching vibrations appeared in between 2900 and 3000 cm 1. All TGA curves showed a first degradation step with a little weight loss at a temperature range of 25–143 � C which is due to the removal of physically bonded water molecules (Kasai et al., 2018). Then a decom position range was observed in between 250 and 400 � C which may correspond to the release of water molecule from hydroxyl group of PVA molecular chains (Cheng et al., 2018). The curves showed a weight loss of about 2% for PVA-S-30 membrane, 5% for PVA-Cl-30 and 8% for neat PVA film from 50 � C to 300 � C (Fig. S3). This weight loss might have occurred due to the removal of residual solvent molecules. PVA-S-30 and pure PVA membrane appeared to be the most stable till 600 � C with gradual weight loss. All other membranes except PVA-Br-20 were completely decomposed in a temperature range of 400 � C–500 � C. Whereas membranes with 20% concentration of [TBP]Br showed the most different behavior. It lost 28% of its weight suddenly even before 100 � C and completely decomposed at 300 � C. This showed that higher loading of [TBP]Br made the membrane highly unstable. However, all the membranes were stable enough to run within the operating range of their actual application in the pervaporation process. The morphology of all membranes was studied using SEM and the
3.2. Degree of swelling The degree of swelling reflects the affinity between permeating molecules and the material of the membrane. It gives the information regarding permeation which is directly related to the performance of membranes (Badi et al., 2019). Fig. 4 showed the degree of swelling of membranes immersed in 10% aqueous solution of butanol. The swelling degree of pure PVA membrane was 38.88% while all other ionic liquid membranes showed swelling higher than this. It was observed that swelling of all membranes increased at higher the ionic liquid contents. This is because of the increase in hydrophilicity that enhanced the in finity towards water (Kwon et al., 2018). The highest degree of swelling was observed in [TBP][Br] membranes. This behavior of Br based IL membranes matched well with the contact angle of these membranes as shown in Fig. 1. The swelling induced in membranes, synthesized with [TBTDP][Cl] ionic liquid, was minimum due to the limited interaction of these films with water molecules. This is in correspondence to the results of water contact angle that showed the highest contact angle values for these membranes. Moreover, membranes comprised of [TBMP][MS] showed that their degree of swelling lied in between the membranes made of other two ionic liquids. 3.3. Pervaporation performance of membranes The experimental results of permeation flux for 10 wt% butanolwater feed mixture at 35 � C are presented in Fig. 5. The permeation flux of all ionic liquid membranes was higher than the neat PVA mem brane. In all ionic liquid membranes, an increase in flux was noticed with an increase in ionic liquid loading. Similar behavior was observed by (Thorat et al., 2017). The trend was higher in [TBP][Br] membranes 4
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Journal of Environmental Management 251 (2019) 109618
Fig. 2. SEM images of the surface (a) pure PVA membrane (b) membranes prepared with [TBMP][MS] (c) membranes prepared with [TBTDP][Cl] (d) membranes prepared with [TBP][Br].
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Fig. 3. SEM images of cross-section (a) pure PVA membrane (b) membranes prepared with [TBMP][MS] (c) membranes prepared with [TBTDP][Cl] (d) membranes prepared with [TBP][Br].
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The separation factor of water through all membranes was also observed. Fig. 6 showed that the separation factor of [TBMP][MS] and [TBTDP][Cl] membranes was increased with ionic liquid loading. Whereas in [TBP][Br] membranes, the separation factor was gradually dropped when the amount of ionic liquid was increased from 10% to 30%. Incorporation of [TBMP][MS] showed the highest value of the separation factor that increased with the concentration of ionic liquid in the matrix. These results correspond to the contact angle measurements that showed enhanced hydrophilicity of membranes upon addition of [TBMP][MS]. The increased hydrophilicity facilitates the diffusion of water molecules while acting as a barrier for butanol transport, thereby leading to higher separation factor. Moreover, the separation factor of [TBTDP][Cl] membranes was lower as compared to [TBMP][MS] membranes when ionic liquid content was increased beyond 10 wt%. [TBTDP][Cl] based membranes showed higher water contact angle than [TBMP][MS], resulting in lower separation factor. The possible reason for the sharp decline in [TBP][Br] membranes was the plasticization effect induced by super-hydrophilic [TBP][Br] ionic liquid. This plasti cization effect was visible in Fig. 4, where uptake of these membranes was the highest. Due to this plasticization effect, the passage of both water and butanol molecules became extremely high through the membrane matrix that ultimately resulted in a reduction of separation factor. However, it should be noted that despite of slightly higher values of sorption of [TBMP][MS] and [TBTDP][Cl], butanol transport through these membranes remained limited and separation factor of these membranes showed an increasing trend. Theoretical investigations were carried out to probe the reasoning of selective uptake of water molecules by the membranes as compared to butanol molecules. For this purpose, [TBMP][MS] was selected having the best separation efficiency, among the ionic liquids used in this study. Complexation energies of [TBMP][MS] with water and butanol were calculated at density functional theory (DFT) level. For comparison purpose, only a single molecule of adsorbate (water or butanol) and [TBMP][MS] was considered. The optimized geometries of the com plexes are shown in Fig. S4. Both molecules interact with the anion of the ionic liquid through hydrogen bonding. The interaction distances (hydrogen bonds) of water and butanol with the ionic liquid were 1.79 Å and 1.81 Å, respectively. The shorter bond length between water and ionic liquid exhibited stronger interaction. As a result, more complex ation energy was released (50.2 kJ/mol). On the other hand, the complexation energy between butanol and the ionic liquid was esti mated to be 46.0 kJ/mol. The greater complexation energy in case of water and liquid ionic complex was responsible for selective uptake of water.
Fig. 4. Effect of ionic liquid loading on degree of swelling of membranes based on (a) [TBTDP][Cl] (b) [TBP][Br] (c) [TBMP][MS] Ionic liquids.
Fig. 5. Effect of ionic liquid loading on total flux.
that confirm that these membranes are highly permeable. This fact can be explained by higher hydrophilicity of [TBP][Br] membranes that was observed through their minimum water contact angles and the highest swelling degree of these membranes. The hydrophilicity induced by incorporation of [TBP][Br] makes polymer chains more flexible leading to the high free volume available for the passage of permeating mole cules (Penkova et al., 2018b). Moreover, [TBTDP][Cl] membranes showed lower total flux at all points of ionic liquid loading. The same result of permeation was expected for these membranes because their swelling degree and hydrophilicity was minimum of all the other membranes. This showed that the incorporation of [TBTDP] makes polymeric chains of PVA flexible up to a certain extent. On the other hand, the total flux of [TBMP][MS] membrane lies in between the other two ionic liquid membranes at all points of ionic liquid loading. Therefore, these membranes were not hydrophilic like [TBP][Br] membranes and their flux was not as low as [TBTDP][Cl] membranes. Their swelling results (Fig. 4c) and water contact angle (Fig. 1c) revealed the same fact.
Fig. 6. Effect of ionic liquid loading on the separation factor. 7
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The effect of feed temperature on total flux was also investigated. Fig. S5 showed that the total flux of all membranes increased as the temperature was increased from 25 � C to 65 � C. This was due to the reason that increasing the temperature, chain flexibility was increased which further enhanced the passage of permeating molecules through the membrane matrix. Moreover, all the membranes with 30% con centration of ionic liquid, showed a higher value of total flux due to their higher hydrophilicity than membranes with lower ionic liquid contents. The activation energy for all the membranes was calculated and shown in Table S1. The ionic liquid membranes had lower activation energy than neat PVA membrane. Similar behavior was reported in an earlier study (Penkova et al., 2018a). Therefore, these results supported the argument that additional pathways are provided for the molecules to permeate at a higher rate by incorporating ionic liquids and hence the fluxes are high.
Acknowledgment
3.4. Practical implications of this study
Algayyim, S.J.M., Wandel, A.P., Yusaf, T., Hamawand, I., 2018. Production and application of ABE as a biofuel. Renew. Sustain. Energy Rev. 82, 1195–1214. Badi, S.M., Sajjan, A.M., Banapurmath, N.R., Kariduraganavar, M.Y., Shettar, A.S., 2019. Preparation and characterization of B2SA grafted hybrid poly (vinyl alcohol) membranes for pervaporation separation of water-isopropanol mixtures. Chemical Data Collections 22, 100245–100260. Boroglu, M., Cavus, S., Boz, I., Ata, A., 2011. Synthesis and characterization of poly (vinyl alcohol) proton exchange membranes modified with 4, 4-diaminodipheny lether-2, 2-disulfonic acid. Express Polym. Lett. 5, 470–478. Brito, M., Martins, F., 2017. Life cycle assessment of butanol production. Fuel 208, 476–482. Cascon, H.R., Choudhari, S.K., 2013. 1-Butanol pervaporation performance and intrinsic stability of phosphonium and ammonium ionic liquid-based supported liquid membranes. J. Membr. Sci. 429, 214–224. Cascon, H.R., Choudhari, S.K., Nisola, G.M., Vivas, E.L., Lee, D.-J., Chung, W.-J., 2011. Partitioning of butanol and other fermentation broth components in phosphonium and ammonium-based ionic liquids and their toxicity to solventogenic clostridia. Separ. Purif. Technol. 78, 164–174. Chapman, P.D., Oliveira, T., Livingston, A.G., Li, K., 2008. Membranes for the dehydration of solvents by pervaporation. J. Membr. Sci. 318, 5–37. Cheng, P.-I., Hong, P.-D., Lee, K.-R., Lai, J.-Y., Tsai, Y.-L., 2018. High permselectivity of networked PVA/GA/CS-Agþ-membrane for dehydration of Isopropanol. J. Membr. Sci. 564, 926–934. Dmitrenko, M., Penkova, A., Kuzminova, A., Missyul, A., Ermakov, S., Roizard, D., 2018. Development and characterization of new pervaporation PVA membranes for the dehydration using bulk and surface modifications. Polymers 10, 571–590. Dong, Y., Zhang, L., Shen, J., Song, M., Chen, H., 2006. Preparation of poly (vinyl alcohol)-sodium alginate hollow-fiber composite membranes and pervaporation dehydration characterization of aqueous alcohol mixtures. Desalination 193, 202–210. Ezeji, T., Qureshi, N., Blaschek, H., 2003. Production of acetone, butanol and ethanol by Clostridium beijerinckii BA101 and in situ recovery by gas stripping. World J. Microbiol. Biotechnol. 19, 595–603. Hanwell, M.D., Curtis, D.E., Lonie, D.C., Vandermeersch, T., Zurek, E., Hutchison, G.R., 2012. Avogadro: an advanced semantic chemical editor, visualization, and analysis platform. J. Cheminf. 4, 17–49. Heitmann, S., Krings, J., Kreis, P., Lennert, A., Pitner, W., G� orak, A., Schulte, M., 2012. Recovery of n-butanol using ionic liquid-based pervaporation membranes. Separ. Purif. Technol. 97, 108–114. Jia, Z., Wu, G., 2016. Metal-organic frameworks based mixed matrix membranes for pervaporation. Microporous Mesoporous Mater. 235, 151–159. Jyothi, M., Reddy, K.R., Soontarapa, K., Naveen, S., Raghu, A.V., Kulkarni, R.V., Suhas, D., Shetti, N.P., Nadagouda, M.N., Aminabhavi, T.M., 2019. Membranes for dehydration of alcohols via pervaporation. J. Environ. Manag. 242, 415–429. Kasai, D., Chougale, R., Masti, S., Chalannavar, R., Malabadi, R.B., Gani, R., 2018. Influence of Syzygium cumini leaves extract on morphological, thermal, mechanical, and antimicrobial properties of PVA and PVA/chitosan blend films. J. Appl. Polym. Sci. 135, 46188–46204. Khan, A., Ali, M., Ilyas, A., Naik, P., Vankelecom, I.F., Gilani, M.A., Bilad, M.R., Sajjad, Z., Khan, A.L., 2018. ZIF-67 filled PDMS mixed matrix membranes for recovery of ethanol via pervaporation. Separ. Purif. Technol. 206, 50–58. Kittur, A.A., Madhu, G.M., Rashmi, S.H., Rajanna, S.S., Yaranal, N.A., 2018. Polymeric nanocomposites for dehydration of isopropyl alcohol–water mixtures by pervaporation. Chem. Chem. Technol. 12, 310–317. Ko, C.-H., Yu, F.-C., Chang, F.-C., Yang, B.-Y., Chen, W.-H., Hwang, W.-S., Tu, T.-C., 2017. Bioethanol production from recovered napier grass with heavy metals. J. Environ. Manag. 203, 1005–1010. Kujawski, J.K., Kujawski, W.M., Sondej, H., Jarzynka, K., Kujawska, A., Bryjak, M., Rynkowska, E., Knozowska, K., Kujawa, J., 2017. Dewatering of 2, 2, 3, 3-tetra fluoropropan-1-ol by hydrophilic pervaporation with poly (vinyl alcohol) based Pervap™ membranes. Separ. Purif. Technol. 174, 520–528. Kwon, Y., Chaudhari, S., Kim, C., Son, D., Park, J., Moon, M., Shon, M., Park, Y., Nam, S., 2018. Ag-exchanged NaY zeolite introduced polyvinyl alcohol/polyacrylic acid mixed matrix membrane for pervaporation separation of water/isopropanol mixture. RSC Adv. 8, 20669–20678.
A. L. Khan would like to thank the Higher Education Commission (HEC), Pakistan for their grant under NRPU Project # 3514. The authors would like to thank Dr. Farasat Iqbal for facilitation and guidance in performing SEM analysis. The authors would also like to thank IT section of COMSATS University Islamabad, Lahore Campus for providing computing facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvman.2019.109618. References
Today, the global energy economy is dominated by fossil fuels. The renewable energy sources account for 9.8% of the total world’s energy supply. In the automotive fuel market, the share of biofuels is expected to grow in the coming decades. Among biofuels, bioalcohols are being used most widely. Since butanol has four carbon structure, so it has more oxygen contents and thus leads to reduce soot emissions. Increased attention of researchers towards bioalcohols make pervaporation potentially viable for biobutanol separation from the fermentation broth. Various membrane materials have been screened for their suit ability for this process. Use of ionic liquids in a polymer matrix is a breakthrough in the field of pervaporation. A wide range of attractive properties of ionic liquids makes such membranes competitive for this application. Although the separation factor offered by these membranes is not very high, but it happened on account of high total flux, as there is always a trade-off between the two. Moreover, in real systems, a membrane with excellent performance relies not only on high separation factor and permeation flux but also on long term thermal stability (Liu et al., 2015). The reported ionic liquid membranes can perform at high temperature. PVA membranes were among the first commercialized membranes for pervaporation and are still available commercially from DeltaMem AG (Switzerland) as part of their PERVAP™ product line of flat sheet membranes. Similarly, AzeoSep™-2002 membranes from PetroSep Membrane Research (Canada) are said to be made of PVA on a poly(acrylonitrile) (PAN) support (Vane, 2019). The longevity of PVA in the industry reflects the functional stability and performance of this polymer. Use of ionic liquids that further improve the performance of PVA in terms of separation factor and permeation flux, is expected to raise the viability of these membranes. 4. Conclusions Hydrophilic ionic liquid-based PVA membranes were developed, characterized and their performance was compared for the separation of the butanol-water mixture. Hydrophilicity of membranes was in the order of [TBP][Br] > [TBMP][MS] > [TBTDP][Cl] and so was the total flux. The separation factor of [TBMP][MS] and [THTDP][Cl] mem branes was increased while that of [TBP][Br] membranes was decreased on increasing ionic liquid loading. This was because of the plasticization effect induced by super-hydrophilic nature of [TBP][Br] ionic liquid. Overall [TBMP][MS] membranes had the highest separation factor and relatively high total flux among all other membranes. The higher interaction energy of [TBMP][MS] with water compared to butanol justified the trend in selective separation. The study findings could open a new outlook for future developments of ionic liquids based membranes for pervaporation.
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Journal of Environmental Management 251 (2019) 109618 Pople, J., Schlegel, H., Krishnan, R., DeFrees, D., Binkley, J., Frisch, M., Whiteside, R., Hout, R., Hehre, W., 1981. Molecular orbital studies of vibrational frequencies. Int. J. Quantum Chem. 20, 269–278. Qiao, X., Chung, T.-S., Guo, W.F., Matsuura, T., Teoh, M.M., 2005. Dehydration of isopropanol and its comparison with dehydration of butanol isomers from thermodynamic and molecular aspects. J. Membr. Sci. 252, 37–49. Raeisi, Z., Moheb, A., Sadeghi, M., Abdolmaleki, A., Alibouri, M., 2019. Titanate nanotubes–incorporated poly (vinyl alcohol) mixed matrix membranes for pervaporation separation of water-isopropanol mixtures. Chem. Eng. Res. Des. 145, 99–111. Rdzanek, P., Heitmann, S., Gorak, A., Kaminski, W., 2015. Application of supported ionic liquid membranes (SILMs) for biobutanol pervaporation. Separ. Purif. Technol. 155, 83–88. Rincon, C.A., De Guardia, A., Couvert, A., Le Roux, S., Soutrel, I., Daumoin, M., Benoist, J.C., 2019. Chemical and odor characterization of gas emissions released during composting of solid wastes and digestates. J. Environ. Manag. 233, 39–53. Sairam, M., Patil, M.B., Veerapur, R.S., Patil, S.A., Aminabhavi, T.M., 2006. Novel dense poly (vinyl alcohol)–TiO2 mixed matrix membranes for pervaporation separation of water–isopropanol mixtures at 30� C. J. Membr. Sci. 281, 95–102. Sas, O.G., Dominguez, I., Gonzalez, B., Dominguez, A., 2018. Liquid-liquid extraction of phenolic compounds from water using ionic liquids: literature review and new experimental data using [C2mim] FSI. J. Environ. Manag. 228, 475–482. Shao, P., Huang, R., 2007. Polymeric membrane pervaporation. J. Membr. Sci. 287, 162–179. Silva, T.L., Morales-Torres, S., Castro-Silva, S., Figueiredo, J.L., Silva, A.M., 2017. An overview on exploration and environmental impact of unconventional gas sources and treatment options for produced water. J. Environ. Manag. 200, 511–529. Sindhu, R., Gnansounou, E., Rebello, S., Binod, P., Varjani, S., Thakur, I.S., Nair, R.B., Pandey, A., 2019. Conversion of food and kitchen waste to value-added products. J. Environ. Manag. 241, 619–630. Szwaja, S., Naber, J.D., 2010. Combustion of n-butanol in a spark-ignition IC engine. Fuel 89, 1573–1582. Tan, H., Wu, Y., Zhou, Y., Liu, Z., Li, T., 2014. Pervaporative recovery of n-butanol from aqueous solutions with MCM-41 filled PEBA mixed matrix membrane. J. Membr. Sci. 453, 302–311. Thorat, G.B., Gupta, S., Murthy, Z., 2017. Synthesis, characterization and application of PVA/ionic liquid mixed matrix membranes for pervaporation dehydration of isopropanol. Chin. J. Chem. Eng. 25, 1402–1411. Vane, L.M., 2019. Membrane materials for the removal of water from industrial solvents by pervaporation and vapor permeation. J. Chem. Technol. Biotechnol. 94, 343–365. Zak, M., Klepic, M., Stastna, L.C., Sedlakova, Z., Vychodilova, H., Hovorka, S., Friess, K., Randova, A., Brozova, L., Jansen, J.C., 2015. Selective removal of butanol from aqueous solution by pervaporation with a PIM-1 membrane and membrane aging. Sep. Purif. Technol. 151, 108–114. Zhang, L., Li, Y., Liu, Q., Li, W., Xing, W., 2019. Fabrication of ionic liquidsfunctionalized PVA catalytic composite membranes to enhance esterification by pervaporation. J. Membr. Sci. 584, 268–281.
Lee, C., Yang, W., Parr, R.G., 1988. Development of the Colle-Salvetti correlation-energy formula into a functional of the electron density. Phys. Rev. B 37, 785–789. Levario, T.J., Dai, M., Yuan, W., Vogt, B.D., Nielsen, D.R., 2012. Rapid adsorption of alcohol biofuels by high surface area mesoporous carbons. Microporous Mesoporous Mater. 148, 107–114. Li, S., Qin, F., Qin, P., Karim, M.N., Tan, T., 2013. Preparation of PDMS membrane using water as solvent for pervaporation separation of butanol–water mixture. Green Chem. 15, 2180–2190. Li, Y., Shen, J., Guan, K., Liu, G., Zhou, H., Jin, W., 2016. PEBA/ceramic hollow fiber composite membrane for high-efficiency recovery of bio-butanol via pervaporation. J. Membr. Sci. 510, 338–347. Liu, D., Liu, G., Meng, L., Dong, Z., Huang, K., Jin, W., 2015. Hollow fiber modules with ceramic-supported PDMS composite membranes for pervaporation recovery of biobutanol. Separ. Purif. Technol. 146, 24–32. Liu, G., Wei, W., Jin, W., 2013. Pervaporation membranes for biobutanol production. ACS Sustain. Chem. Eng. 2, 546–560. Liu, G., Wei, W., Wu, H., Dong, X., Jiang, M., Jin, W., 2011. Pervaporation performance of PDMS/ceramic composite membrane in acetone butanol ethanol (ABE) fermentation–PV coupled process. J. Membr. Sci. 373, 121–129. Liu, S., Liu, G., Zhao, X., Jin, W., 2013. Hydrophobic-ZIF-71 filled PEBA mixed matrix membranes for recovery of biobutanol via pervaporation. J. Membr. Sci. 446, 181–188. Mai, N.L., Kim, S.H., Ha, S.H., Shin, H.S., Koo, Y.-M., 2013. Selective recovery of acetonebutanol-ethanol from aqueous mixture by pervaporation using immobilized ionic liquid polydimethylsiloxane membrane. Korean J. Chem. Eng. 30, 1804–1809. Matsumoto, M., Murakami, Y., Kondo, K., 2011. Separation of 1-butanol by pervaporation using polymer inclusion membranes containing ionic liquids. Solv. Extr. Res. Dev. (Jpn.) 18, 75–83. Neves, C.M., Granjo, J.F., Freire, M.G., Robertson, A., Oliveira, N.M., Coutinho, J.A., 2011. Separation of ethanol–water mixtures by liquid–liquid extraction using phosphonium-based ionic liquids. Green Chem. 13, 1517–1526. Ong, Y.K., Shi, G.M., Le, N.L., Tang, Y.P., Zuo, J., Nunes, S.P., Chung, T.-S., 2016. Recent membrane development for pervaporation processes. Prog. Polym. Sci. 57, 1–31. Penkova, A.V., Dmitrenko, M.E., Ermakov, S.S., Toikka, A.M., Roizard, D., 2018. Novel green PVA-fullerenol mixed matrix supported membranes for separating water-THF mixtures by pervaporation. Environ. Sci. Pollut. Control Ser. 25, 20354–20362. Penkova, A.V., Dmitrenko, M.E., Savon, N.A., Missyul, A.B., Mazur, A.S., Kuzminova, A. I., Zolotarev, A.A., Mikhailovskii, V., Lahderanta, E., Markelov, D.A., 2018. Novel mixed-matrix membranes based on polyvinyl alcohol modified by carboxyfullerene for pervaporation dehydration. Separ. Purif. Technol. 204, 1–12. Peters, T., Poeth, C., Benes, N., Buijs, H., Vercauteren, F., Keurentjes, J., 2006. Ceramicsupported thin PVA pervaporation membranes combining high flux and high selectivity; contradicting the flux-selectivity paradigm. J. Membr. Sci. 276, 42–50. Petra, C., Katalin, B.B., 2011. Application of ionic liquids in membrane separation processes. In: Kokorin, A. (Ed.), Ionic Liquids: Applications and Perspectives, first ed. IntechOpen, Hungary, pp. 561–586. Petranovic, Z., Besenic, T., Vujanovic, M., Duic, N., 2017. Modelling pollutant emissions in diesel engines, influence of biofuel on pollutant formation. J. Environ. Manag. 203, 1038–1046.
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Journal of Environmental Management journal homepage: http://www.elsevier.com/locate/jenvman
Research article
Development of novel hydrophilic ionic liquid membranes for the recovery of biobutanol through pervaporation Zabia Sajjad a, Mazhar Amjad Gilani b, Abdul-Sattar Nizami c, Muhammad Roil Bilad d, Asim Laeeq Khan a, * a
Department of Chemical Engineering, COMSATS University Islamabad, Lahore Campus, Pakistan Department of Chemistry, COMSATS University Islamabad, Lahore Campus, Pakistan Center of Excellence in Environmental Studies (CEES), King Abdulaziz University, Jeddah, Saudi Arabia d Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, 32610 8, Perak, Malaysia b c
A R T I C L E I N F O
A B S T R A C T
Keywords: Ionic liquids membranes Pervaporation Biobutanol separation Cleaner production Climate change
This paper aims to develop novel hydrophilic ionic liquid membranes using pervaporation for the recovery of biobutanol. Multiple polyvinyl alcohol (PVA) membranes based on three commercial ionic liquids with different loading were prepared for various experimental trials. The ionic liquids selected for the study include tributyl (tetradecyl) phosphonium chloride ([TBTDP][Cl]), tetrabutyl phosphonium bromide ([TBP][Br]) and tributyl methyl phosphonium methylsulphate ([TBMP][MS]). The synthesized membranes were characterized and tested in a custom-built pervaporation set-up. All ionic liquid membranes showed better results with total flux of 1.58 kg/m2h, 1.43 kg/m2h, 1.38 kg/m2h at 30% loading of [TBP][Br], [TBMP][MS] and [TBTDP][Cl] respec tively. The comparison of ionic liquid membranes revealed that by incorporating [TBMP]MS to PVA matrix resulted in a maximum separation factor of 147 at 30 wt% loading combined with a relatively higher total flux of 1.43 kg/m2h. Density functional theory (DFT) calculations were also carried out to evaluate the experimental observations along with theoretical studies. The improved permeation properties make these phosphonium based ionic liquid a promising additive in PVA matrix for butanol-water separation under varying temperature conditions.
1. Introduction There has been significant interest in the production of biofuels through fermentation of sustainable biomass resources due to the increasing concerns of fossil fuels shortage and climate change (Petra novic et al., 2017; Tan et al., 2014). Bioethanol and Biobutanol are the most popular energy sources among biofuels (Ko et al., 2017). Owing to the properties of high octane number, energy content (Sindhu et al., 2019), low vapor pressure and excellent combustion characteristics in the engine as compared to ethanol, butanol is considered to be the most promising fuel among many other alternative fuels (Li et al., 2016; Szwaja and Naber, 2010). Noteworthy research has been carried out in different aspects of acetone-butanol-ethanol (ABE) fermentation process in recent years owing to the availability of a wide range of fermentative substrates (Li et al., 2013). Butanol is commercially synthesized using petrochemical route whose cost is linked with the price of crude oil. However, biobutanol is produced by ABE fermentation of renewable
biomass for sustainable development. The ABE products are formed in a ratio of 3:6:1 (Algayyim et al., 2018; Brito and Martins, 2017). During the production of butanol in fermentation broth, inhibition of microbial growth is the most challenging part (Liu et al., 2013b). That is why butanol is harmful to microorganism, Clostridium acetobutylicum, used in the process (Zak et al., 2015). In traditional fermentation broth, the solvent concentration is not exceeding 20 g/L whereas butanol concentration is less than 14 g/L (Tan et al., 2014). Certain species of Clostridium have been investigated to make this process economically viable. Separation coupled with fermentation is suggested to increase butanol tolerance of microorganisms. These hybrid processes of fermentation such as adsorption (Levario et al., 2012; Silva et al., 2017), liquid-liquid extraction (Cascon et al., 2011; Sas et al., 2018), gas stripping (Ezeji et al., 2003; Rincon et al., 2019), or pervaporation (Liu et al., 2011) improve butanol extraction from the fermentation broth and separation efficiency. Among these separation methods, pervapo ration is a promising method to recover biobutanol because of low
* Corresponding author. E-mail address: [email protected] (A.L. Khan). https://doi.org/10.1016/j.jenvman.2019.109618 Received 27 April 2019; Received in revised form 10 September 2019; Accepted 21 September 2019 Available online 26 September 2019 0301-4797/© 2019 Elsevier Ltd. All rights reserved.
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energy cost as it requires latent heat to penetrate through the membrane, easy operation and does not contaminate the targeted product like liquid-liquid extraction (Chapman et al., 2008; Shao and Huang, 2007). The efficiency of pervaporation mainly depends on (1) the choice of membrane material for a particular separation application, (2) an interaction between components of feed and polymer, (3) the membrane stability and mechanical strength, and (4) high selectivity and perme ation flux (Sairam et al., 2006). The fundamental processes governing the transport through pervaporation membranes include (1) sorption of molecules (gas/liquid) in membrane surface, (2) diffusion of species sorbed across the membrane body, (3) and desorption of sorbed species at the permeate interface. These three steps correspond to the solution-diffusion theory (Jyothi et al., 2019; Ong et al., 2016). The polymeric and mixed matrix membranes filled with inorganic filler have been widely used in pervaporation applications (Jia and Wu, 2016). Polyvinyl alcohol (PVA) was the first material that was used in perva poration and is now extensively used for the dehydration of alcohols because of its unique properties of hydrophilicity and film formation (Thorat et al., 2017). The PVA based membranes are highly hydrophilic due to the pres ence of hydroxyl groups. Their hydrophilicity is controllable, and their chemical resistant properties make them efficient for pervaporation dehydration (Qiao et al., 2005). These membranes also show high chemical stability and better mechanical strength (Liu et al., 2013a). However, a limitation of the PVA based membranes is the low selectivity that makes them less commercially viable. Hence, several modifications have been recommended to increase the performance of PVA based pervaporation. These modifications involve the synthesis of composite and mixed matrix membranes using highly selective support and incorporation of various nanoparticles in the PVA matrix (Dong et al., 2006; Peters et al., 2006). In the last decade, ionic liquid membranes have gained significant attention because of their advanced properties (Matsumoto et al., 2011). Ionic liquids have high thermal stability, less viscosity, low vapor pressure, high electrical conductance, low volatility and high ability to dissolve a range of compounds (Petra and Katalin, 2011). Their prop erties can be further tuned because of the cationic and anionic combi nations (Heitmann et al., 2012; Matsumoto et al., 2011; Petra and Katalin, 2011). They have been used for the recovery of numerous sol vents from the aqueous solution (Cascon and Choudhari, 2013; Mai et al., 2013; Neves et al., 2011). Rdzanek et al. (2015) studied perva poration of quaternary ABE mixture through hydrophobic supported ionic liquid membranes. Two ionic liquids namely, trihexyl(tetradecyl) phosphonium tetracyanoborate and 1-hexyl-3-methylimidazolium tet racyanoborate were incorporated in polyether block amide matrix, and pervaporation experimentation was carried out at 37 � C with 3 wt% butanol concentration feed solution. The results showed that by using ionic liquid membranes, permeate flux and selectivity were influenced, and butanol concentration was raised by 10 times in permeate as compared to feed. In another study (Thorat et al., 2017), studied PVA-IL hydrophilic composite membranes for IPA-water mixture separation. Four ionic liquids namely, 1-n-butyl-3-methylimidazolium chloride (BMIMCl), 1-hexyl-3-methylimidazolium tetrafluoroborate (HMIMBF4), 1-hexyl-3-methylimidazolium chloride (HMIMCl) and 1-octyl-3-methylimidazolium chloride (OMIMCl) were examined in the study. They reported that longer the length of alkyl chain of ionic liquid, higher was the selectivity of the membrane towards the water. This fact is supported by an argument that alkyl chain length increases the van der Waals and coulombic interactions that ultimately increase the selectivity of the membrane. Researchers have laid the foundation of ionic liquid membranes for pervaporation, but still, there is a long way to discover the nature of ionic liquid that best fits with the membrane material for a specific feed mixture separation, which was the focus of this study. This study, for the first time, aims to develop and investigate the novel hydrophilic membranes based on PVA and three different ionic
liquids were selected in order to efficiently separate butanol-water mixture while keeping the stability issues of the membranes in consid eration. The selected ionic liquids include tributyl (tetradecyl) phos phonium chloride ([TBTDP][Cl]), tetrabutyl phosphonium bromide ([TBP][Br]) and tributyl methyl phosphonium methylsulphate ([TBMP] [MS]). These ionic liquids have different degrees of hydrophilicity, are commercially available and novel for the desired applications. The findings of this study would allow researchers to compare the perfor mance and behavior of ionic liquids when they are incorporated in PVA films. 2. Materials and methods 2.1. Materials and fabrication of membranes PVA was purchased from Sigma-Aldrich (USA). The distilled water (pH 6.5 � 0.2) as a solvent was prepared in the departmental laboratory. Other materials include hydrophilic ionic liquids, tributyl (tetradecyl) phosphonium chloride ([TBTDP][Cl]), tetrabutyl phosphonium bromide ([TBP][Br]) and tributyl methyl phosphonium methylsulphate ([TBMP] [MS]). All the ionic liquids were purchased from Io-Li-Tech (Germany). The chemical structures of all these ionic liquids are shown in Table 1. For membrane fabrication 10 wt% PVA solution in distilled water was prepared. This solution was stirred for 3 h. The stirring was carried out on a digitally controlled magnetic hot plate stirrer (MS7-H550-S) fol lowed by heating at 60 � C. After getting a homogenous solution, ionic liquid in different loadings with respect to polymer matrix was added in the solution and stirring was continued for another 4 h. After casting the solution in Teflon Petri dish, they were allowed to evaporate at room temperature for 24 h. In order to remove residual solvent, these mem branes were thermally treated at 100 � C for 1 h and peeled off. The composition of all prepared membranes is shown in Table 2. 2.2. Pervaporation set-up Pervaporation experiments were carried out in custom-built crossflow pervaporation cell. The detailed working of the set-up is reported by (Khan et al., 2018), and schematics are presented in Fig. S1. A 10% aqueous solution of butanol was used as feed and permeation studies were carried out at 25, 35, 45, 55 and 65 � C. The effective area of the membrane was 16 cm2. In order to achieve a steady state, the membrane was allowed to equilibrate for 10 h with feed solution before starting the experiment. The feed was pumped under atmospheric pressure at a constant feed rate of 1 L/min. The retentate was recycled back to the feed tank. The vacuum was maintained at the permeate side with the help of a vacuum pump. The condensed permeate was analyzed in a gas chromatograph equipped with a thermal conductivity detector (7890A, Agilent, USA). The flux (J) was calculated by dividing the mass of permeate (Q) by the product of the interval time (t) and area of mem brane (A), as shown in Equation (1): J¼
Q At
(1)
The separation factor (α) was calculated by Equation (2):
αi=j ¼
yi=yj xi=xj
(2)
where x and y are mole fractions of components in permeate and feed respectively and subscript i and j represent water and butanol, respectively. Pervaporation is an activated process, so the behavior of flux was analyzed at various temperatures and the following Arrhenius Equation was used to explain this behavior of flux dependence on temperature.
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Table 1 Ionic Liquids used in the study. Full name
Abbreviation
Structure
Tributyl(tetradecyl)phosphonium chloride
[TBTDP][Cl]
435.158
Tetrabutyl phosphonium bromide
[TBP][Br]
339.33
Tributylmethylphosphonium methylsulphate
[TBMP][MS]
328.45
analyzer DSA-30 (KRUSS, Germany). In order to confirm the presence of ionic liquids and their interaction with PVA matrix, membranes were characterized by Fourier Transform Infrared (FTIR) spectra that was recorded on FTIR-Spectrometer (Thermo-Nicolet 6700 P, USA). Mem branes were analyzed for thermal analysis using Thermogravimetric Analysis (TGA - SDT Q600 TA Instruments) having a ramp rate of 10 � C/ min under nitrogen atmosphere. The morphology of membranes was observed with the help of Scanning Electron Microscope (SEM) of TESCAN Vega LMU having variable pressure. The membranes were fractured for cross-section analyses by immersing in liquid nitrogen and were sputter-coated with gold to increase their conductivity.
Table 2 Abbreviation and composition of membranes. Membrane
Composition
PVA PVA-S-10
Pure PVA þ0% IL 90% PVA solution sulphate (IL) 80% PVA solution sulphate (IL) 70% PVA solution sulphate (IL) 90% PVA solution (IL) 80% PVA solution (IL) 70% PVA solution (IL) 90% PVA solution 80% PVA solution 70% PVA solution
PVA-S-20 PVA-S-30 PVA-Cl-10 PVA-Cl-20 PVA-Cl-30 PVA-Br-10 PVA-Br-20 PVA-Br-30
J ¼ Jo exp ð
þ10% Tributyl methyl phosphonium methyl þ 20% Tributyl methyl phosphonium methyl þ 30% Tributyl methyl phosphonium methyl þ 10% Tributyl tetradecyl phosphonium chloride þ 10% Tributyl tetradecyl phosphonium chloride
2.4. Computational techniques
þ 30% Tributyl tetradecyl phosphonium chloride
All simulations were performed on the Gaussian 16 software. The geometries of all structures were optimized using hybrid B3LYP method (Lee et al., 1988) with the 6–31 þ G (d, p) basis set without any geometrical constraints. The geometries of the optimized structures were plotted with Avogadro 1.2.0 (Hanwell et al., 2012). The frequency calculations were performed at the same level of theory to confirm the optimized geometries as true minima with no imaginary frequency (Pople et al., 1981). The complexation energies (Ec) have been calcu lated from the following Equation (4): � (5) Ec ¼ EIL solvent ðEIL þ Esolvent
þ 10% Tetrabutyl phosphonium bromide (IL) þ 20% Tetrabutyl phosphonium bromide (IL) þ 30% Tetrabutyl phosphonium bromide (IL)
EJ Þ RT
(3)
where J is the flux, Jo is the pre-exponential factor, and Ej is the acti vation energy of flux. 2.3. Membrane uptake and characterization
where EIL-solvent, EIL, and Esolvent are the total energies for the ionic liquid-solvent complex, ionic liquid, and solvent, respectively.
The sorption of solvent in the membranes was calculated using the gravimetric method. 1 � 1 cm piece of membranes of known weight was immersed in 90 wt% butanol solution at the room temperature. These films were then allowed to equilibrate for 24 h. After the removal of membranes from the solution, they were wiped with tissue paper to remove the excessive liquid and weighed. The following Equation (3) was used to calculate liquid sorption ‘S’ (g of liquid/100 g of dry poly mer) in the membrane: S¼
ms
m0 m0
:100
MW (g/mol)
3. Results and discussion 3.1. Membrane characterization Fig. 1 presents the water contact angle of the membranes. Pure PVA membrane showed an angle of 51� . The obtained value is almost consistent with the literature data. It can be noted that water contact angle for all the membranes was lower than 90� , which proves the hy drophilic character of these membranes (Dmitrenko et al., 2018; Kujawski et al., 2017). On increasing the content of ionic liquid in membranes, the water contact angle was decreased for all three ionic liquids, implying that hydrophilicity of membranes was increased as ionic liquid loading was enhanced. This further helped to build up the
(4)
where ms and m0 are the weight of swollen and dried membrane respectively. The hydrophilicity of membranes was determined by contact angle measurement using a sessile drop method with the help of contact angle 3
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results of surface and cross-section of membranes are shown in Figs. 2 and 3, respectively. It can be seen that the surface of pure PVA mem brane was smooth with some random pores that were present only on the surface in agreement with the reported literature (Kittur et al., 2018; Sairam et al., 2006). Fig. 2 (b) showed that with the incorporation of [TBMP][MS] in PVA matrix, membranes became more porous. These pores were uniformly distributed throughout the surface of membranes. Because of the presence of these pores, the diffusion of feed solution was expected to be higher through the surface. However, the cross section of these membranes (Fig. 3b) was dense without the presence of any pores and voids. It was expected that the porous surface behaved like a skin layer that increased the contact with feed solution and the rest of the membrane would impart the selective separation characteristics. Clearly, there were no agglomerates in ionic liquid membranes that can be ascribed to good adhesion and suitable compatibility between the PVA and ionic liquid, as both are hydrophilic (Raeisi et al., 2019). Images in Fig. 2 (c) depicted that the addition of [TBTDP][Cl] in PVA matrix resulted in the formation of a membrane that was rough. The roughness was increased with the increased loading of the ionic liquid. The same behavior was observed earlier in a reported study (Zhang et al., 2019). Image of PVA-Cl-30 (Fig. 3) showed that the top layer of the membrane was highly rough, and roughness continued throughout the cross-section. This entire change in the surface structure and cross-section of these membranes as compared to the neat PVA mem brane was attributed to the nature of [TBTDP][Cl] ionic liquid, or due to the presence of long tetradecyl chain present in ionic liquid. Fig. 2 (d) showed that the membrane with minimum loading of [TBP][Br] had a porous surface. However, on higher loadings, ionic liquid was highly embedded in the polymer matrix as the pores were disappeared and the surface became rough. Moreover, a closer look at a cross-section of [TBP][Br] membranes showed that all these membranes were completely smooth and non-porous. The difference in morphology of membranes comprising different ionic liquids was likely due to the difference in anion, hydrophilicity and alkyl chain length of each ionic liquid and their interactions with the polymer matrix.
Fig. 1. Effect of ionic liquid loading on degree of swelling of membranes based on (a) [TBTDP][Cl] (b) [TBP][Br] (c) [TBMP][MS] Ionic liquids.
hypothesis of achieving better solubility and crosslinking behavior for all three ionic liquids in the PVA matrix. The membranes comprising of [TBTDP][Cl] showed the highest value of contact angle that was decreased from 47� to 36� when ionic liquid was increased from 10% to 30%, proving them to be the least hydrophilic in nature. The membranes filled with [TBP][Br] ionic liquid showed the most hydrophilic behavior as their water contact angle showed the most significant drop. The hy drophilicity of [TBMP][MS] membranes was in between the membranes based on other two ionic liquids. This change in membrane behavior was due to the physical properties of different ionic liquids leading to a difference in their interaction towards the water. In order to confirm the presence of functional groups in membranes, FTIR spectra was obtained (Fig. S2). In all membranes, a characteristic strong band appeared at 3500 cm 1 corresponding to O–H bond stretch. The addition of ionic liquids in the PVA matrix caused no significant difference as absorbance of O–H band remained the same in all mem branes. A band appeared between 1500 and 2000 cm 1 represents C–C stretching (Thorat et al., 2017). In Fig. S2 (a) teethed shaped peaks were appeared between 1200 and 1300 cm 1, corresponding to the asym metric stretching vibrations of SO3 group present in ionic liquid ([TBMP][MS]) (Boroglu et al., 2011). Moreover, in Fig. S2 (b) and (c) C–H stretching vibrations appeared in between 2900 and 3000 cm 1. All TGA curves showed a first degradation step with a little weight loss at a temperature range of 25–143 � C which is due to the removal of physically bonded water molecules (Kasai et al., 2018). Then a decom position range was observed in between 250 and 400 � C which may correspond to the release of water molecule from hydroxyl group of PVA molecular chains (Cheng et al., 2018). The curves showed a weight loss of about 2% for PVA-S-30 membrane, 5% for PVA-Cl-30 and 8% for neat PVA film from 50 � C to 300 � C (Fig. S3). This weight loss might have occurred due to the removal of residual solvent molecules. PVA-S-30 and pure PVA membrane appeared to be the most stable till 600 � C with gradual weight loss. All other membranes except PVA-Br-20 were completely decomposed in a temperature range of 400 � C–500 � C. Whereas membranes with 20% concentration of [TBP]Br showed the most different behavior. It lost 28% of its weight suddenly even before 100 � C and completely decomposed at 300 � C. This showed that higher loading of [TBP]Br made the membrane highly unstable. However, all the membranes were stable enough to run within the operating range of their actual application in the pervaporation process. The morphology of all membranes was studied using SEM and the
3.2. Degree of swelling The degree of swelling reflects the affinity between permeating molecules and the material of the membrane. It gives the information regarding permeation which is directly related to the performance of membranes (Badi et al., 2019). Fig. 4 showed the degree of swelling of membranes immersed in 10% aqueous solution of butanol. The swelling degree of pure PVA membrane was 38.88% while all other ionic liquid membranes showed swelling higher than this. It was observed that swelling of all membranes increased at higher the ionic liquid contents. This is because of the increase in hydrophilicity that enhanced the in finity towards water (Kwon et al., 2018). The highest degree of swelling was observed in [TBP][Br] membranes. This behavior of Br based IL membranes matched well with the contact angle of these membranes as shown in Fig. 1. The swelling induced in membranes, synthesized with [TBTDP][Cl] ionic liquid, was minimum due to the limited interaction of these films with water molecules. This is in correspondence to the results of water contact angle that showed the highest contact angle values for these membranes. Moreover, membranes comprised of [TBMP][MS] showed that their degree of swelling lied in between the membranes made of other two ionic liquids. 3.3. Pervaporation performance of membranes The experimental results of permeation flux for 10 wt% butanolwater feed mixture at 35 � C are presented in Fig. 5. The permeation flux of all ionic liquid membranes was higher than the neat PVA mem brane. In all ionic liquid membranes, an increase in flux was noticed with an increase in ionic liquid loading. Similar behavior was observed by (Thorat et al., 2017). The trend was higher in [TBP][Br] membranes 4
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Fig. 2. SEM images of the surface (a) pure PVA membrane (b) membranes prepared with [TBMP][MS] (c) membranes prepared with [TBTDP][Cl] (d) membranes prepared with [TBP][Br].
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Fig. 3. SEM images of cross-section (a) pure PVA membrane (b) membranes prepared with [TBMP][MS] (c) membranes prepared with [TBTDP][Cl] (d) membranes prepared with [TBP][Br].
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The separation factor of water through all membranes was also observed. Fig. 6 showed that the separation factor of [TBMP][MS] and [TBTDP][Cl] membranes was increased with ionic liquid loading. Whereas in [TBP][Br] membranes, the separation factor was gradually dropped when the amount of ionic liquid was increased from 10% to 30%. Incorporation of [TBMP][MS] showed the highest value of the separation factor that increased with the concentration of ionic liquid in the matrix. These results correspond to the contact angle measurements that showed enhanced hydrophilicity of membranes upon addition of [TBMP][MS]. The increased hydrophilicity facilitates the diffusion of water molecules while acting as a barrier for butanol transport, thereby leading to higher separation factor. Moreover, the separation factor of [TBTDP][Cl] membranes was lower as compared to [TBMP][MS] membranes when ionic liquid content was increased beyond 10 wt%. [TBTDP][Cl] based membranes showed higher water contact angle than [TBMP][MS], resulting in lower separation factor. The possible reason for the sharp decline in [TBP][Br] membranes was the plasticization effect induced by super-hydrophilic [TBP][Br] ionic liquid. This plasti cization effect was visible in Fig. 4, where uptake of these membranes was the highest. Due to this plasticization effect, the passage of both water and butanol molecules became extremely high through the membrane matrix that ultimately resulted in a reduction of separation factor. However, it should be noted that despite of slightly higher values of sorption of [TBMP][MS] and [TBTDP][Cl], butanol transport through these membranes remained limited and separation factor of these membranes showed an increasing trend. Theoretical investigations were carried out to probe the reasoning of selective uptake of water molecules by the membranes as compared to butanol molecules. For this purpose, [TBMP][MS] was selected having the best separation efficiency, among the ionic liquids used in this study. Complexation energies of [TBMP][MS] with water and butanol were calculated at density functional theory (DFT) level. For comparison purpose, only a single molecule of adsorbate (water or butanol) and [TBMP][MS] was considered. The optimized geometries of the com plexes are shown in Fig. S4. Both molecules interact with the anion of the ionic liquid through hydrogen bonding. The interaction distances (hydrogen bonds) of water and butanol with the ionic liquid were 1.79 Å and 1.81 Å, respectively. The shorter bond length between water and ionic liquid exhibited stronger interaction. As a result, more complex ation energy was released (50.2 kJ/mol). On the other hand, the complexation energy between butanol and the ionic liquid was esti mated to be 46.0 kJ/mol. The greater complexation energy in case of water and liquid ionic complex was responsible for selective uptake of water.
Fig. 4. Effect of ionic liquid loading on degree of swelling of membranes based on (a) [TBTDP][Cl] (b) [TBP][Br] (c) [TBMP][MS] Ionic liquids.
Fig. 5. Effect of ionic liquid loading on total flux.
that confirm that these membranes are highly permeable. This fact can be explained by higher hydrophilicity of [TBP][Br] membranes that was observed through their minimum water contact angles and the highest swelling degree of these membranes. The hydrophilicity induced by incorporation of [TBP][Br] makes polymer chains more flexible leading to the high free volume available for the passage of permeating mole cules (Penkova et al., 2018b). Moreover, [TBTDP][Cl] membranes showed lower total flux at all points of ionic liquid loading. The same result of permeation was expected for these membranes because their swelling degree and hydrophilicity was minimum of all the other membranes. This showed that the incorporation of [TBTDP] makes polymeric chains of PVA flexible up to a certain extent. On the other hand, the total flux of [TBMP][MS] membrane lies in between the other two ionic liquid membranes at all points of ionic liquid loading. Therefore, these membranes were not hydrophilic like [TBP][Br] membranes and their flux was not as low as [TBTDP][Cl] membranes. Their swelling results (Fig. 4c) and water contact angle (Fig. 1c) revealed the same fact.
Fig. 6. Effect of ionic liquid loading on the separation factor. 7
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The effect of feed temperature on total flux was also investigated. Fig. S5 showed that the total flux of all membranes increased as the temperature was increased from 25 � C to 65 � C. This was due to the reason that increasing the temperature, chain flexibility was increased which further enhanced the passage of permeating molecules through the membrane matrix. Moreover, all the membranes with 30% con centration of ionic liquid, showed a higher value of total flux due to their higher hydrophilicity than membranes with lower ionic liquid contents. The activation energy for all the membranes was calculated and shown in Table S1. The ionic liquid membranes had lower activation energy than neat PVA membrane. Similar behavior was reported in an earlier study (Penkova et al., 2018a). Therefore, these results supported the argument that additional pathways are provided for the molecules to permeate at a higher rate by incorporating ionic liquids and hence the fluxes are high.
Acknowledgment
3.4. Practical implications of this study
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A. L. Khan would like to thank the Higher Education Commission (HEC), Pakistan for their grant under NRPU Project # 3514. The authors would like to thank Dr. Farasat Iqbal for facilitation and guidance in performing SEM analysis. The authors would also like to thank IT section of COMSATS University Islamabad, Lahore Campus for providing computing facilities. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.jenvman.2019.109618. References
Today, the global energy economy is dominated by fossil fuels. The renewable energy sources account for 9.8% of the total world’s energy supply. In the automotive fuel market, the share of biofuels is expected to grow in the coming decades. Among biofuels, bioalcohols are being used most widely. Since butanol has four carbon structure, so it has more oxygen contents and thus leads to reduce soot emissions. Increased attention of researchers towards bioalcohols make pervaporation potentially viable for biobutanol separation from the fermentation broth. Various membrane materials have been screened for their suit ability for this process. Use of ionic liquids in a polymer matrix is a breakthrough in the field of pervaporation. A wide range of attractive properties of ionic liquids makes such membranes competitive for this application. Although the separation factor offered by these membranes is not very high, but it happened on account of high total flux, as there is always a trade-off between the two. Moreover, in real systems, a membrane with excellent performance relies not only on high separation factor and permeation flux but also on long term thermal stability (Liu et al., 2015). The reported ionic liquid membranes can perform at high temperature. PVA membranes were among the first commercialized membranes for pervaporation and are still available commercially from DeltaMem AG (Switzerland) as part of their PERVAP™ product line of flat sheet membranes. Similarly, AzeoSep™-2002 membranes from PetroSep Membrane Research (Canada) are said to be made of PVA on a poly(acrylonitrile) (PAN) support (Vane, 2019). The longevity of PVA in the industry reflects the functional stability and performance of this polymer. Use of ionic liquids that further improve the performance of PVA in terms of separation factor and permeation flux, is expected to raise the viability of these membranes. 4. Conclusions Hydrophilic ionic liquid-based PVA membranes were developed, characterized and their performance was compared for the separation of the butanol-water mixture. Hydrophilicity of membranes was in the order of [TBP][Br] > [TBMP][MS] > [TBTDP][Cl] and so was the total flux. The separation factor of [TBMP][MS] and [THTDP][Cl] mem branes was increased while that of [TBP][Br] membranes was decreased on increasing ionic liquid loading. This was because of the plasticization effect induced by super-hydrophilic nature of [TBP][Br] ionic liquid. Overall [TBMP][MS] membranes had the highest separation factor and relatively high total flux among all other membranes. The higher interaction energy of [TBMP][MS] with water compared to butanol justified the trend in selective separation. The study findings could open a new outlook for future developments of ionic liquids based membranes for pervaporation.
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