Green design of poly(m-phenylene isophthalamide) (PMIA) based thin

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Green design of poly(m-phenylene isophthalamide) (PMIA) based thin-film composite membranes for organic solvent nanofiltration (OSN) and concentrating lecithin in hexane Dan Hua, Susilo Japip, Kai Yu Wang, and Tai-Shung Chung ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02021 • Publication Date (Web): 10 Jun 2018 Downloaded from http://pubs.acs.org on June 10, 2018

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ACS Sustainable Chemistry & Engineering

Green design of poly(m-phenylene isophthalamide) (PMIA) based thin-film composite membranes for organic solvent nanofiltration (OSN) and concentrating lecithin in hexane

Dan Hua†‡, Susilo Japip†, Kai Yu Wang†, Tai-Shung Chung∗†

†

Department of Chemical & Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, Singapore 117585

‡

College of Chemical Engineering, Hua Qiao University, No.668 Jimei Avenue, Xiamen, Fujian, China 361021. *

Corresponding author. Tel.: (+65) 6516 6645; fax: (+65) 6779 1936. E-mail address: [email protected] (T. S. Chung).

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ABSTRACT: A green method to fabricate poly(m-phenylene isophthalamide) (PMIA) based thin-film composite membranes has been developed for organic solvent nanofiltration (OSN). For the first time, the porous PMIA membrane substrates are cast from dope solutions prepared by an environmentally benign ionic liquid and the thin-film selective layer is synthesized on top of the glutaraldehyde (GA) modified PMIA substrate (mPMIA) via imine condensation between hyper-branched polyethyleneimine (HPEI) using water as the reaction media. Experimental results show that both HPEI and GA concentrations play important roles in controlling the thickness and free volume properties of the selective layer, and thus influence the OSN separation performance significantly. The newly designed composite membranes have an ethanol permeance varying from 3.2 to 21.4 LMH/bar while the corresponding MWCO value changing from 470 to 730 Da when both HPEI and GA concentrations reduce from 1 to 0.5 wt%. In summary, the newly fabricated membranes not only possess satisfactory permeances for solvents like methanol, ethanol, acetonitrile, and hexane but also have good separation performance for concentrating lecithin in hexane, which is in great demand by the food industry. Therefore, this work may offer a green and sustainable method to design environmentally benign composite membranes for OSN.

KEYWORDS: Poly(m-phenylene isophthalamide) (PMIA), thin-film composite membrane, hyperbranched polyethyleneimine (HPEI), glutaraldehyde (GA), organic solvent nanofiltration.

INTRODUCTION Organic solvent nanofiltration (OSN), also known as solvent-resistant nanofiltration


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(SRNF), is an emerging separation technology is an emerging separation technology and in high demand by pharmaceutical, catalyst, food and petrochemical industries for solvent recovery, solvent exchange, and solute concentration or purification.1-9 Similar to aqueous nanofiltration (NF), OSN membranes possess pore sizes in the range of about 0.5-2.0 nm with the molecular weight cut-off (MWCO) between 200 and 1000 Da. However, due to their applications in harsh solvents, OSN membranes must be solvent-resistant and chemically stable. Otherwise, they would be directly dissolved in organic solvents or gradually lose separation performance due to solventinduced swelling problems. To date, commercially available OSN membranes are very limited, most of which are made from cross-linked polyimides, silicone rubber and poly(ether ether ketone).2,

10-13

Therefore, novel OSN membranes made from

alternative polymeric and inorganic materials as well as fabrication approaches have received global attention with aims to achieve superior-performance and lower the fabrication costs.14-23 Although inorganic membranes can mitigate the compaction and swelling problems, they are brittle and expensive caused by complicated and highenergy fabrication. Therefore, polymeric membranes have been widely studied due to their low cost, flexibility, easy fabrication and scale-up.

Among various studied polymeric materials for OSN, the aromatic polyamide family has received great interest because of its high thermal, chemical and thermo-oxidative stabilities, as well as excellent mechanical properties. Moreover, it can be further functionalized using aldehydes.24 However, most of polyamide-related OSN membranes are fabricated by synthesizing a thin polyamide layer on via interfacial polymerization between diamine and acyl chloride on top of cross-linked polymeric substrates.25-27 In contrast, there are few studies using polyamides as the porous



substrates for the fabrication of composite OSN membranes possibly because of the difficulties to dissolve polyamides in common solvents.

Poly(m-phenyleneisophthalamide) (PMIA) is a commercially available aromatic polyamide which has shown promising membrane applications in the areas of desalination and nanofiltration for the removal of dyes and organic compounds.8, 28-33 However, in all reported studies, PMIA was dissolved by dimethylacetamide (DMAc) with LiCl as the additive at elevated temperatures ranging from 80 to 120 oC. Since DMAC is an environmentally unfriendly solvent and harmful to pregnant women,34 it inspires us to seek greener solvents to process PMIA so that one can minimize the chemical toxicity and volatility of DMAc at high temperatures.

Ionic liquids (ILs) can be considered as green solvents due to their negligible volatility in addition to their good thermal and chemical stabilities. Zhao et al. prepared of PMIA fibers by synthesizing 6 kinds of ILs for dissolving PMIA.35, 36 1-nbutyl-3-methylimidazolium chloride ([BMIM]Cl) was the best solvent among them in terms of solubility, dissolution time, and solution stability. They also found that the best dissolution method was to heat the PMIA/[BMIM]Cl mixture at a temperature between 70 and 110oC in a vacuum oven to avoid moisture sorption. In addition, all their PMIA solutions prepared from ILs showed higher stability than that prepared from DMAc/LiCl. To make the dissolution method simpler, we aim to utilize an alternative IL, 1-ethyl-3-methylimidazolium acetate ([EMIM]OAc), which has been reported to be a good green solvent to dissolve polybenzimidazole37, 38 and cellulose.39

In addition to using a greener solvent to prepare porous PMIA substrates, the second


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objective of this research aims at a greener method to fabricate the top selective layer for OSN applications. Currently, interfacial polymerization, polymer coating, and layer-by-layer (LBL) are the leading methods to fabricate OSN composite membranes.9, 26, 40 Nonetheless, the interfacial polymerization requires a hazardous solvent, e.g. hexane, as the organic phase and the polymer coating involves the evaporation of organic solvents. In addition, the membranes made from polymer coating and LBL may have weak long-term stability because the interactions between the selective layer and the substrate as well as among layers are commonly based on physical bonding. To overcome these concerns, we aim to fabricate the selective layer based on covalent reaction and use a much more environmentally friendly solvent, i.e., water, as the reaction medium. Previous works on pervaporation composite membranes for solvent dehydration showed that the selective layer formed by means of the imine condensation between HPEI and aldehyde possessed good and stable separation performance.41,

42

Moreover, P84 and SPPSU nanofiltration membranes

crosslinked by HPEI and then GA showed enhanced solute rejections.43,

44

This

inspires us to synthesize this type of selective layer with a controllable thickness and free-volume properties on top of porous PMIA substrates using water as the green media to form OSN composite membranes.

Therefore, the overall target of this work is to design (HPEI-GA)/GA modified PMIA (referred to as mPMIA) composite membranes via 3 steps: (1) fabricating porous PMIA substrates using IL [EMIM]OAc as a green solvent; (2) modifying PMIA by GA to get mPMIA, which has enhanced interactions with the selective layer; (3) synthesizing the selective layer on top of mPMIA by means of imine condensation between HPEI and GA in water media. The effects of GA modification duration and



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reagent concentrations in the 3rd step on membrane morphology, free volume properties and OSN performance of various solvents would be systematically studied.

EXPERIMENTAL SECTION Materials Commercially available fibrous poly(m-phenylene isophthalamide) (PMIA) and an ionic liquid, 1-ethyl-3-methylimidazolium acetate ([EMIM]OAc, ≥95.0%, HPLC) were purchased from Sigma Aldrich to prepare dope solutions for membrane substrates.

Both

glutaraldehyde

(GA,

Alfa

Aesar)

and

hyperbranched

polyethyleneimine (HPEI, Mw = 750,000, Sigma Aldrich) with a concentration of 50 wt% in aqueous solutions were acquired to prepare solutions for the fabrication of PMIA based composite membranes. Polyethylene glycols with molecular weights (Mw) of 35,000 and 100,000 g/mol (i.e., PEG35K and PEG100K, respectively) from Merck and deionized (DI) water were used to prepare solutions for filtration tests of PMIA substrates. Moreover, solvents such as ethanol, isopropanol, acetone, acetonitrile, tetrahydrofuran (THF), dimethylformamide (DMF), N-Methyl-2pyrrolidone (NMP) and hexane of analytical reagent grade were utilized as received. Meanwhile, various dyes including methylene blue (MB, 320 g/mol), rhodamine 6G (R6G, 479 g/mol), eosin Y (EY, 648 g/mol), brilliant Blue-R (BBR, 826 g/mol), and rose bengal (RB, 1018 g/mol) purchased from Sigma-Aldrich were employed in OSN tests. Their molecular structures are illustrated in Figure S1 in the Supporting Information. Besides, a valuable food additive L-α-lecithin (lecithin, Sigma-Aldrich) was used to prepare lecithin/hexane feed solutions for concentration experiments.


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Fabrication of PMIA membrane substrates Porous PMIA substrates were fabricated via a non-solvent induced phase inversion process. Firstly, a homogeneous polymer dope solution containing 20 wt% PMIA was prepared by dissolving PMIA in [EMIM]OAc at 85°C for at least 48 h. The solution was then degassed in an oven at 80°C overnight. Before casting, both the glass plate and casting knife were preheated at 60°C. Then the polymer dope was poured onto the glass plate and cast slowly by the casting knife with a gap of 150 µm. The glass plate with the nascent polymer film was immediately immersed in a water coagulation bath at an ambient temperature to complete the phase inversion. The fabricated membrane substrates were stored in fresh daily-changed DI water for at least 3 days to remove the residual solvent. Afterward, the membrane substrates underwent a 30-minute solvent exchange using isopropanol twice and then stored in isopropanol.

The fabrication of HPEI-GA/mPMIA composite membranes Firstly, the porous PMIA substrates were modified in a 1 wt% GA aqueous solution at 70°C for a certain period (i.e., 0.5, 6, 24 h) before synthesizing the selective layer. The modified PMIA membranes were named as mPMIA. Secondly, the mPMIA membranes were immersed in an HPEI aqueous solution followed by a GA aqueous solution with the same concentration at 70°C for 0.5 h. Consequently, the selective layer was formed due to the reaction between HPEI and GA. The subsequent composite membranes were named as (HPEI-GA-x%)/mPMIA, where x represents the reagent (i.e., HPEI and GA) concentrations in the 2nd step, x= 0.1, 0.5, 0.75 and 1 wt%.

Characterizations

of

PMIA substrates and

HPEI-GA/PMIA


composite


membranes A field-emission scanning electron microscope (FESEM, JEOL JSM-6700LV) was used to observe the morphology of PMIA membranes. Both Fourier transform infrared spectroscopy under the attenuated total reflectance mode (FTIR-ATR, Bruker) and X-ray photoelectron spectroscopy (XPS, Kratos AXIS His spectrometer) were employed to analyze the chemical functionalities. The binding energy of C1s at 284.5 eV was taken as a reference. In addition, one type of slow beam positron annihilation spectroscopy, Doppler broadening energy spectroscopy (DBES) was used to study the variations of free volume and depth profile of the (HPEI-GA)/mPMIA composite membranes. The DBES spectra were characterized by R parameter as a function of positron implantation energy ranging from 0.1 keV to 15 keV, where R parameter is defined as the ratio of the total counts from ortho-positronium 3γ (o-3γ) annihilation to the total counts from the 511 keV peak region (due to 2γ annihilation).45

Solvent immersion tests In order to evaluate the solvent stability of the PMIA substrates and composite membranes, they were immersed in solvents such as methanol, ethanol, acetone, acetonitrile, THF, NMP, DMF, and hexane for 1 month. The weight loss in each solvent was obtained from the weight difference between the dried membranes before and after immersion.

Water-based filtration and OSN experiments Both the water-based filtration and OSN experiments were conducted using dead-


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ended permeation cells under a certain pressure of N2. The pure water/solvent flux (J, L m-2 h-1, abbreviated as LMH) and permeance (P, L m-2 h-1 bar-1, abbreviated as LMH/bar) were calculated using Eq. 1 and 2, respectively. =

(1)

= ∆

(2)

where Q is the volumetric ﬂow rate of the permeating solvent (L/h), A is the effective membrane area for filtration (m2), and ∆p is the transmembrane pressure (bar). Solute rejection, RT, was obtained from Eq. 3.

% = 1 −

× 100 (3)

To estimate the pore size of PMIA substrates, filtration tests using PEG35K and PEG100K aqueous solutions with each feed concentration of 200 ppm were conducted. The feed and permeate concentrations were analyzed by a total organic carbon analyzer (Analytik Jena, Germany). While in the OSN tests, five dyes with different Mw ranging from 320 to 1018 g/mol were used as model solutes. The feed solutions were prepared by separately dissolving each dye in ethanol with a concentration of 50 ppm. At least three permeate samples were collected to ensure the reproducibility of the results and at least two identical membranes were tested for each

filtration

condition.

As

for

the

lecithin

concentrating

experiments,

lecithin/hexane solutions with concentrations of 2000 ppm and 10 wt% were continuously concentrated, where the permeance, rejection and the concentrations at both feed and permeate sides were monitored during the entire testing period.



To investigate the separation stability of the (HPEI-GA)/mPMIA composite membranes for a longer time, the (HPEI-GA-1%)/mPMIA composite membrane was chosen to run pure ethanol for 210 h first, followed by another 210-h test to separate an R6G/ethanol mixture with a concentration of 50 ppm at 5 bar. The feed concentration was adjusted and kept constant by using the recycled permeate and additional fresh ethanol during the long-duration filtration tests.

RESULTS AND DISCUSSION Characterizations of PMIA, mPMIA and (HPEI-GA)/mPMIA composite membranes Surface Morphology The morphologies of PMIA substrates cast from the PMIA/IL (20/80 wt%) polymer solution are shown in Figure S2 in the Supporting Information. The PMIA substrate possesses a dense top surface, a sponge-like cross-section with only a few finger-like marcovoids and a dense structure near the top surface. Even though the pores are difficult to be observed from the surface, the pore size of the PMIA substrate is within an ultrafiltration range because it has a pure water permeance of 168.6 LMH/bar and rejections against PEG35K and PEG100K of 61.5% and 91.4%, respectively.

Figure 1 compares the surface and enlarged cross-section morphologies of the PMIA substrate, mPMIA substrate, and (HPEI-GA)/mPMIA composite membrane. The morphologies of PMIA and mPMIA are very similar, indicating that the GA modification does not affect the membrane structure. However, the surface of (HPEIGA)/mPMIA composite membrane shows a fibrous and rougher surface with a denser cross-section, suggesting the successful formation of a selective layer. Moreover,


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Figure 2 shows the photos and FESEM images of (HPEI-GA)/mPMIA composite membranes with different reagent concentrations. When both HPEI and GA concentrations increase, the membrane surface turns from light yellow to dark orange. Besides, the fibrous structure of the surface becomes more obvious and the crosssection looks denser. However, it is still difficult to accurately determine the selective layer thickness, even though the layer could be clearly observed from an oblique view (see Figure S3). Therefore, PAS is employed to characterize the evolution of pores across the membrane thickness in the later section.

Figure 1. FESEM images of surface and cross-section morphologies of the PMIA substrate, mPMIA, and the (HPEI-GA)/mPMIA composite membranes.



Figure 2. Photos and FESEM morphologies of the (HPEI-GA)/mPMIA composite membranes fabricated with different HPEI and GA concentrations of (a) 0.1%, (b) 0.5%, (c) 0.75% and (d) 1%. The mPMIA is the PMIA substrate modified with GA for 6 h.

Chemical analyses Prior to chemical analyses, the reaction mechanism during each fabrication step of (HPEI-GA)/mPMIA composite membranes is studied. Figure 3 illustrates possible reaction mechanisms. Modification of PMIA by GA not only introduces –CHO but also creates –OH functional groups on the main chain of PMIA and transforms its secondary amine groups into tertiary amine groups. Afterward, the reaction in the second step forms the HPEI-GA layer on the mPMIA substrate and generates C=N groups because of the imine condensation between –NH2 and –CHO groups, leading to more hydrocarbon chains due to the introduction of HPEI.


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Figure 3. The proposed reaction mechanism during each fabrication step of (HPEIGA)/mPMIA composite membranes

To prove this hypothesis, the pristine PMIA membrane and GA modified one are firstly analyzed by ATR-FTIR. The results in Figure 4a show that both the spectra possess amide groups owing to the PMIA substrate, which occur at around 1650 cm-1 and 1535 cm-1, 3300 cm-1 representing the C=O stretching, N-H bending, and N-H stretching, respectively.42,

46-48

By using the invariable aromatic C=C peak (1605

cm−1) as the reference, one can estimate the chemical variations of membranes before and after the GA modification.49 Although the mPMIA membrane and the pristine PMIA have quite similar spectra due to the limited degree of reaction, the former has relatively lower intensity of N-H stretching (3300 cm-1) than the latter (i.e., decreases from 0.338 to 0.269). This confirms the existence of the GA modification. In addition, the (HPEI-GA)-1%/PMIA membrane shows greatly enhanced relative intensity of CH stretching at 2945 and 2853 cm−1 due to the introduction of HPEI during the reaction. More interestingly, the C-H intensity increases with an increase in reagent concentration, as shown in Figure S4. However, the peak representing the C=N



groups cannot be observed since they overlap with C=O groups at 1650 cm−1.

Figure 4. (a) ATR-FTIR spectra and (b) N1s XPS spectra of (i) PMIA, (ii) mPMIA and (iii) (HPEI-GA)-1%/mPMIA membranes.

Since XPS is more sensitive to the surface chemistry, it is used to further investigate the aforementioned membranes. Figure 4b compares their high-resolution N1s XPS spectra. The pristine PMIA membrane only possesses a single peak at the binding energy of 399.8 eV, which can be ascribed to the amide groups (–NH-C=O) of PMIA. After the GA modification, the amide groups convert to >N-C=O groups. More interestingly, Figure S5 and Table S1 show a decrease in –NH-C=O intensity but an increase in >N-C=O intensity with a longer GA modification duration, indicating the GA modification is successful. Moreover, the appearance of imine groups (C=N) at 398.7 eV of the (HPEI-GA)-1%/mPMIA membrane in Figure 4b proves the reaction between HPEI and GA. While the peaks at 399.6 eV and 401.4 eV represent neutral


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amine groups (e.g. –NH2, -NH- and –N

Green design of poly(m-phenylene isophthalamide) (PMIA) based thin

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