Ethanolamine Purification by Nanofiltration through PIM-1 and Carbon

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Ethanolamine Purification by Nanofiltration Through PIM-1 and Carbon Membranes: A Molecular Simulation Study Krishna M. Gupta, Qi Shi, Lev Sarkisov, and Jianwen Jiang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07043 • Publication Date (Web): 23 Aug 2017 Downloaded from http://pubs.acs.org on September 2, 2017

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The Journal of Physical Chemistry

Ethanolamine Purification by Nanofiltration through PIM-1 and Carbon Membranes: A Molecular Simulation Study Krishna M. Gupta1, Qi Shi,1,2 Lev Sarkisov,3 Jianwen Jiang1* 1

Department of Chemical and Biomolecular Engineering, National University of Singapore, 117576, Singapore Department of Applied Physics, Nanjing University of Science and Technology, Nanjing 210094, China 3 School of Engineering, University of Edinburgh, Edinburgh, EH9 3JL, United Kingdom 2

ABSTRACT: Economical purification of high-value chemicals after biomass conversion is highly desirable. Herein, we report a molecular simulation study to investigate ethanolamine (ETA) purification from its mixture with water and ammonia by nanofiltration. Two different classes of membranes are considered including a polymer of intrinsic microporosity (PIM-1) and two activated carbons namely curved corannulene (CRNL) and functionalized CRNL with –OH (CRNL-(OH)2). Water flux and permeability through the membranes are found to decrease in the order of CRNL-(OH)2 > PIM-1 > CRNL. As attributed to its hydrophilic nature, CRNL-(OH)2 exhibits the highest flux and permeability. Ammonia is also observed to permeate through the three membranes. Nevertheless, 100% ETA retention is achieved by CRNL-(OH)2, indicating the suitability of CRNL-(OH)2 for optimal ETA purification. Due to strong affinity, water near CRNL-(OH)2 exhibits substantially longer residence time than near PIM-1 and CRNL. Furthermore, the lifetime of hydrogen bonds for water in the membranes follows the reverse trend of water flux. The simulation study provides microscopic insights into the dynamic and structural properties of water, ETA as well as ammonia in the three membranes, and suggests that CNRL-(OH)2 might be an interesting candidate for ETA purification.

*Email: [email protected]

1

ACS Paragon Plus Environment


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1. Introduction Biomass is an abundant resource of renewable feedstock on the Earth and there has been increasing interest to convert biomass into fuels and valuable chemicals.1 As the major component of biomass, cellulose has a global quantity of 700,000 billion tons. However, only 0.1 billion tons of cellulose is being used to produce paper, textiles, pharmaceuticals, etc.2 Currently, cellulose is primarily converted into C, H, and O-based fuels and chemicals such as bioethanol, sugar alcohols/polyols, furan and organic acids.3-5 On the other hand, N-containing chemicals are often more valuable and widely used in pharmaceuticals, CO2 fixation, textiles and many more.6,7 For example, ethanolamine (ETA) is extensively used for scrubbing acidic gases and as an important feedstock in the production of detergents, emulsifiers, chemical intermediates, etc. The world demand of ETA exceeds 2 million tons per year.8 According to the price list of the Independent Chemical Information Service, ethylene glycol (EG) is about $1200/ton. Intriguingly, by replacing one O with N in EG, ETA costs $2000/ton. Therefore, the conversion of cellulosic biomass to ETA has received considerable attention.9 After the conversion, however, the solution usually contains ETA, ammonia (used as N source during conversion) and a large amount of water. Traditional technique to purify ETA using thermal distillation is energy intensive and may decompose ETA. In this context, technically feasible and economically viable ETA purification is highly desirable. Since

introduced

in

1984,

nanofiltration

(NF)

has

emerged

as

a

promising

separation/purification technology under relatively low operating pressure.10 NF was first employed for water treatment and it is now commonly applied in the purification of chemicals, food and bioproducts.11,12 In NF, solvent permeates through a membrane and solute is retained on the basis of molecular size sieving and/or the Donnan exclusion. The membrane used plays a

2


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crucial role in the performance of NF and the key characteristics of an ideal NF membrane are high solvent permeation and solute retention, as well as high chemical and mechanical stability. In this study, the objective is to explore ETA purification from ETA/ammonia/water mixture by NF through two classes of membranes. The first is a polymer of intrinsic microporosity (PIM). Due to the presence of spiro-centers in backbone, PIMs have a contorted structure with intrinsic microporosity potentially useful for separation.13 To date, most studies for PIMs have been largely focused on gas separation. Nevertheless, few experimental studies have been reported to use PIMs for liquid separation. For example, PIM-1, in addition to poly(4-methyl-1pentene) (PMP) and poly[1-(trimethylsilyl)-1-propyne] (PTMSP), was examined to quantify the effects of polymer and solute nature during NF.14 By coupling with NF, PIM-1 was used for solvent swing adsorption and found to possess either solvent- or solute-selective transport depending on the nature of solvent.15 The second class is activated carbon membranes, which are versatile materials with specific characteristics such as commercially available, relatively inexpensive, highly stable under a broad range of conditions.16 Herein, we evaluate and compare the NF performance of PIM-1 and carbon membranes using molecular dynamics (MD) simulation. With tremendous growth of computational power, molecular simulation has increasingly become a robust tool in materials science and engineering. Microscopic insights from simulation can provide the underlying physics and subsequently guide the rational design of materials. In Section, 2, the molecular models of PIM-1 and carbon membranes, as well as ETA, water and ammonia, are briefly described and followed by simulation methods. In Section 3, water and ammonia flows, as well as ETA retention through the membranes are presented; furthermore, the dynamics and structure of water in the membranes are examined on the basis of

3



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survival time and hydrogen bonding. Finally, the concluding remarks are summarized in Section 4. 2. Models and Methods PIM-1 can be synthesized by a polycondensation reaction from 5,5’,6,6’-tetrahydroxy3,3,3’,3’-tetramethyl-1,1’-spirobisindane (Figure 1a).13 The PIM-1 chain was constructed using ten monomers, terminated with hydrogen atoms and energetically optimized. Then, fourteen polymer chains were used to generate a membrane using the Amorphous Cell module in Materials Studio.17 Finally, the system was equilibrated by a 7-step compression and relaxation scheme. As listed in Table S1, the 7-step scheme consisted of (1) energy minimization at 0 K for 10 ps to eliminate possible close contacts between atoms. (2) 300 ps isothermal and isobaric (NPT) MD simulation at 300 K and 3000 bar. (3) 100 ps isothermal and isochoric (NVT) MD simulation at 800 K. (4) 100 ps NVT MD simulation at 300 K. (5) 300 ps NPT MD simulation at 300 K and 1000 bar. (6) repeat step (3-5) 29 times. We found that sufficient number of cycles was necessary during this compression/relaxation scheme. (7) 10000 ps NPT MD simulation at 300 K and 1 bar to reach the final structure. The density of the PIM-1 membrane was predicted to be 0.975 g/cm3, close to experimental data and previous simulated values.18,19 A polymer membrane often swell in a solvent, which has a significant effect on membrane property and performance. Consequently, the swelling of the PIM-1 membrane in water was simulated at 300 K and 1 bar (Figure S1) and the swollen membrane as shown in Figure 1b was used for the NF of ETA/water/ammonia mixture. (b) Swollen PIM-1 (a) monomer

4


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Figure 1. (a) PIM-1 monomer (b) PIM-1 membrane after swelling in water. Color code: C, grey; O, red; N, blue; H, white. The activated carbons were mimicked by the random packing of curved corannulene (CRNL) and functionalized CRNL with –OH (CRNL-(OH)2) (Figure 2a).20-22 These models were previously proposed to examine adsorption in high-performance carbon adsorbents (Maxsorb). Here, we envision that they can also represent amorphous carbon membranes. A CRNL membrane was generated by packing 120 CRNLs with a target density of 0.9 g/cm3. We found that if its density was too small, ETA retention was poor; on the other hand, water permeation was low if its density was too large. As a consequence, the density was chosen to be 0.9 g/cm3. Among tenth configurations generated, the one with the lowest energy was selected. By replacing two opposite H atoms in CRNL with –OH groups and structure optimization, CRNL(OH)2 membrane was constructed. It is intriguing to point out that the CRNL-(OH)2 membrane can be considered, to a certain extent, as a simple model for graphene oxide. As shown in Figure 2b, the two carbon membranes had structural similarity, allowing us to unambiguously assess the effect of hydrophilicity. Figure S2 further shows the interconnected and disconnected voids in the two carbon membranes, which were estimated by Zeo++23 with a probe of 2.68 Å in diameter, corresponding to the kinetic diameter of water. Similar patterns of interconnected and disconnected voids are observed in both carbon membranes. To gauge the structural similarity, the void size distributions (VSDs) were estimated. Both membranes possess similar peaks in the range of 4 – 14 Å.

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Figure 2. (a) Corannulene (CRNL) and corannulene functionalized with –OH (CRNL-(OH)2) (b) CRNL and CRNL-(OH)2 membranes (on the xy plane). Color code: C, grey; O, red; H, white. To simulate a NF process for ETA purification, the initial simulation system is illustrated in Figure 3. The left chamber (feed) was filled with ETA/water/ammonia mixture (5 wt% ETA, 10 wt% NH3 and 85 wt% water), whereas the right chamber (permeate) contained pure water. The membrane (PIM-1, CRNL-(OH)2 or CRNL) was positioned in the middle. The pressures on the left and right graphene plates were 51 and 1 bar, respectively, corresponding to a pressure gradient of 50 bar as the driving force for NF. The nonbonded interactions in the system were mimicked by Lennard-Jones (LJ) and Coulombic potentials

U nonbonded

 σ 12  σ 6  qi q j ij ij = ∑ 4ε ij   −    + ∑   4πε 0 rij  rij   rij   

(1)

where εij and σij are the well depth and collision diameter, rij is the distance between atoms i and j, qi is the atomic charge of atom i, and ε0 = 8.8542 × 10-12 C2N-1m-2 is the permittivity of vacuum. The bonded interactions (for PIM-1) were described by 2 1 U stretching = ∑ kr ( rij − rij0 ) 2

(2)

2 1 U bending = ∑ kθ (θijk − θijk0 ) 2

(3)

6


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5

0 U dihedral = ∑ Cn [cos(φijkl − φijkl )]n

(4)

n=0

where k r , kθ and Cn are the force constants; rij , θijk and φijkl are bond lengths, angles and 0 dihedrals, respectively; rij0 , θ ijk0 and θ ijkl are the equilibrium values. All the potential parameters

of PIM-1 were adopted from the OPLS-AA force field,24 and water from the three point potential (TIP3P) model.25 For ETA and NH3, the LJ and bonded potential were based on the OPLS-AA force field, while their atomic charges were estimated from density functional theory implemented in Gaussian 09 package.26 Both ETA and NH3 were optimized using B3LYP functional at 6-311+G(d,p) basis set and the electrostatic potentials were calculated at 6311++G(d,p) basis set. Thereafter, the atomic charges were determined by the restricted electrostatic potential method, as listed in Tables S2 and S3. For carbon membranes, the atomic charges and LJ potential parameters were from previous studies20-22 and the atoms were assumed to be rigid during simulation. The carbon atoms in graphene plates were described with the parameters as used for carbon nanotubes.27

1 bar

51 bar

graphene

feed solution

membrane

water

1 bar

graphene

Figure 3. A simulation system to mimic NF for the separation of ETA through a membrane. Color code: Green, ETA; red, NH3; yellow, water.

Each system was initially subject to energy minimization using the steepest descent method, then velocities were generated according to the Maxwell−Boltzmann distribution, finally MD simulation was conducted at 300 K for 150 ns. The temperature was controlled by the velocity7



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rescaled Berendsen thermostat with a relaxation time of 0.1 ps. In all the three dimensions, periodic boundary conditions were applied. The equations of motion were integrated by the leapfrog algorithm with a time step of 2 fs. A cutoff of 14 Å was used to calculate the LJ interactions and the particle-mesh Ewald method was used to evaluate the Coulombic interactions with grid spacing of 1.2 Å. The PIM-1 membrane was allowed to fluctuate; to avoid its drift, however, the PIM-1 atoms in small four regions were fixed as illustrated in Figure S3. Three sets of independent simulations with different initial seeds were performed and the properties were averaged. All the MD simulations were performed using the GROMACS v5.0.6 package.28

3. Results and discussion 3.1. Water flow and ETA retention Generally, the performance of a membrane is assessed by solvent permeability and solute rejection. Therefore, water flow and ETA retention through the PIM-1, CRNL-(OH)2 and CRNL membranes are analyzed. Upon initializing simulation, both water and ammonia flow from the left chamber (feed) and some enter into the right chamber (permeate). As time lapses, more water and ammonia molecules are accumulated in the permeate chamber, implying the occurrence of NF. Figure 4 shows the net water flows Nw through the three membranes, each averaged over three independent simulations. For each membrane, the Nw increases almost linearly after a time lag tlag, which indicates a steady flow. The occurrence of tlag is due to the fact that the membrane is initially dry, thus water needs to fill in the membrane before entering the permeate chamber. The tlag is approximately 12 ns for PIM-1 and CRNL-(OH)2, whereas 35 ns for CRNL. Despite a similar structure between CRNL-(OH)2 and CRNL, the tlag for CRNL(OH)2 is substantially shorter. This is because CRNL-(OH)2 contains hydrophilic –OH groups,

8


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which have strong affinity for water and facilitates water flow. As illustrated in Figure 5, water density at the interface of CRNL-(OH)2 is higher than that of CRNL. In other words, more water molecules are located near CRNL-(OH)2 before entry; consequently, CRNL-(OH)2 is filled by water within a shorter tlag. 160

120

(a)

(b)

90

Nw (t)

120

Nw (t)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

60

40

30

0

0 0

30

60

90

120

CRNL-(OH)₂ CRNL

0

150

30

60

90

120

150

t (ns)

t (ns)

Figure 4. Net water flows through (a) PIM-1 and (b) CRNL-(OH)2 and CRNL.

CRNL-(OH)2

CRNL

increase

Figure 5. Density contours of water at the interfaces of CRNL-(OH)2 and CRNL.

Based on Figure 4, water flux Jw can be calculated by

/ ∙ ∙

9


(5)


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where N0 is the Avogadro constant (6.022 × 10²³), Mw is the molecular weight of water (18.015 g/mol), A is the membrane cross-section area and t is time duration. The t is from 20 to 150 ns for PIM-1 and CRNL-(OH)2, and from 50 to 150 ns for CRNL. Water flux, as well as membrane thickness and area are listed in Table 1. The flux decreases as CRNL-(OH)2 > PIM-1 > CRNL. Through the hydrophilic CRNL-(OH)2, the flux is about 50 % higher than the hydrophobic counterpart CRNL. This is consistent with the observation in the reverse osmosis (RO) of water through polymer membranes29,30 and 2D functional covalent-organic framework membranes.31 However, it is in remarkable contrast to the RO through Zeolitic Imidazolate Framework (ZIF) membranes, where hydrophobic ZIFs were found to enhance water flux.32-34 The reason is that well-defined channels exist in ZIFs and they dominate water flow; water experiences weaker interaction in hydrophobic channels and hence flows faster. Instead, the channels in amorphous PIM-1, CRNL-(OH)2 and CRNL are irregularly distributed and water flow is primarily governed by the entry of water into the membrane. As already seen in Figure 5, water has as larger density at the interface of CRNL-(OH)2 than CRNL; thus water can more easily enter into CRNL-(OH)2. Table 1. Membrane thickness and area, water flux and permeability. Membrane PIM-1 CRNL(OH)2 CRNL

Thickness (nm)

Area (nm2)

Flux (kg/m2⋅h)

Permeability [10-7 kg⋅m/(m2⋅h⋅bar)]

7.0

16.0

5977.6 ± 10.4

8.37 ± 0.01

6.3

9.1

6888.5 ± 11.6

8.68 ± 0.01

6.0

9.9

4544.5 ± 15.6

5.49 ± 0.02

Water permeability can be calculated by Pw = J w l / ∆p , where l is the membrane thickness, ∆p is pressure difference between the feed and permeate sides. As listed in Table 1, PIM-1 and

10


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CRNL-(OH)2 possess close permeability, which is higher than CRNL. Additionally, ammonia in the feed solution has to be separated from ETA. Figure S4 shows ammonia flows through PIM-1, CRNL-(OH)2 and CRNL membranes. Compared with water (Figure 4), ammonia exhibits a lower flux because ammonia has a greater molecular size compared with water.35 The performance of ETA purification is further quantified by ETA retention. Figure 6 shows the density distributions of ETA along the z-axis in the three membrane systems at 0 and 150 ns. Initially, ETA resides in the feed chamber. At the final stage, most ETA still remains in the feed chamber, whereas a few crosses the membrane interface and enters into the membrane. Specifically, ETA is able to pass through PIM-1 into the permeate side (indicated by red circle in Figure 6a). While ETA enters into the central region of CRNL-(OH)2, it remains there and cannot pass through (Figure 6b) within 150 ns. On the other hand, ETA only resides at the interface of CRNL (Figure 6c), but cannot enter into the interior due to hydrophobicity. This interesting observation for ETA entering into the interior of CRNL-(OH)2 but not pass through is attributed to two factors. First, although there exist voids with size larger than ETA (~ 5.2 Å) (see Figure S2c), these voids in the membrane are surrounded by small voids and not interconnected. Second, the permeation of ETA is restrained by the strong interaction between ETA and hydrophilic CRNL-(OH)2. To elucidate, the radial distribution functions g(r) between the atoms of ETA and membrane were calculated

gij ( r ) =

N ij ( r, r + ∆r )V 4π r 2 ∆r N i N j

(6)

where r is the distance between atoms i and j, Nij (r, r + ∆r ) is the number of atom j around i within a shell from r to r + ∆r, V is the system volume, Ni and Nj are the numbers of atoms i and j, respectively. As shown in Figure S5, a sharp peak is seen at 1.9 Å for the H7 atom of

11



ETA around the O atom of CRNL-(OH)2. This implies a strong interaction between ETA and CRNL-(OH)2, and thus ETA preferentially resides in CRNL-(OH)2 membrane. As a comparison, there is a relatively lower peak at a much longer distance 5.1 Å for ETA around the N atom of PIM-1, suggesting a weaker interaction between ETA and PIM-1. Overall, with excellent ETA retention and high water permeability, CRNL-(OH)2 could be a promising candidate for the separation of ETA from ETA/water/ammonia mixture.

12

12

0 ns 150 ns

(a)

10

ρN (nm-3)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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feed

8

membrane

12

10

(c)

0 ns 150 ns

(b)

8

permeate

6

6

4

4

4

2

2

2

5

10

15

20

25

30

permeate

6

0

0 0

membrane

membrane

permeate

0

feed

8

feed

0 ns 150 ns

10

0

5

z (nm)

10

15

20

25

30

z (nm)

0

5

10

15

20

25

30

z (nm)

Figure 6. Density profiles of ETA along the z-axis in (a) PIM-1, (b) CRNL-(OH)2, and (c) CRNL at t = 0 and 150 ns. Each membrane is between the two dashed lines.

3.2. Water Dynamics and Structure in the Membranes To provide microscopic insight into water dynamics and structure in the membranes, survival time and hydrogen bonding of water are examined. The survival time correlation functions (STCFs) were calculated from36 N (t ) STCF ( t ) = = N 0 ( t0 )

∑

N 0 ( t0 ) i =1

δ i ( t0 , t 0 + t )

N 0 (t0 )

(7)

where N(t) is the total number of water molecules that stay near a given atom over a time span t, N0 is the initial number of water molecules at t = t0, and δi(t) is a binary function equal to unity if the ith water molecule remains near the given atom from t0 to t0 + t without escaping over this interval and zero otherwise. The multiple time-origin method was used to estimate the ensemble 12


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averaged STCFs in order to improve statistical accuracy. Figure 7a plots the STCFs of water near the N and C atoms of PIM-1. A remarkably fast decay is observed for water near the C atom. Due to the steric hindrance of two pentagonal rings, water is not able to access the C atom thus resulting in fast decay; nevertheless, water stays substantially longer near the N atom. This is attributed to easy accessibility of the N atom to water and their strong interaction. Although not shown, the g(r) between water and the N atom is similar to Figure S5 between ETA and the N atom. Figure 7 b shows the STCFs of water near the O and H atoms of CRNL-(OH)2 and CRNL. Clearly, the STCF decays more slowly for water near CRNL-(OH)2 than near CRNL. The reason is that water interacts with hydrophilic CRNL-(OH)2 more strongly than with hydrophobic CRNL. On this basis, water flux through CRNL-(OH)2 should be intuitively lower. Nevertheless, water density (or solubility in term of solution-diffusion mechanism) in CRNL(OH)2 is higher, and overall, the flux through CRNL-(OH)2 is higher. 1

1 (a)

(b)

0.8

O of CRNL-(OH)₂ H of CRNL

0.8

0.6

STCF(t)

STCF(t)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


N C

0.4 0.2

0.6 0.4

O

H

0.2

0

0 0

100

200

300

400

500

600

0

200

400

600

800

1000

t (ps)

t (ps)

Figure 7. Survival time correlation functions of water around (a) the N and C atoms of PIM-1 (b) the O and H atoms of CRNL-(OH)2 and CRNL, respectively.

Table 2. Parameters to fit the survival time correlation functions. atom

A

B

βs

βl

τs (ps)

τl (ps)

N

0.723

0.279

PIM-1 0.762

0.638

27.361

91.993

13



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C O H

0.500

0.500 0.397 0.500 CRNL-(OH)2 and CRNL 0.867 0.137 0.707 0.498 0.886 0.117 0.750 0.471

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0.012

1.000

33.786 26.822

535.707 251.130

To further describe the decay of STCFs, a stretched function with two exponential terms37 was used to fit the STCF data βs

Ae−(t /τ s ) + B e

−( t /τ l )βl

(8)

where τs and τl are the short- and long-time decay constants, βs and βl are stretching parameters that signify the non-exponential trend of STCFs. Table 2 lists the values of τs, τl, βs, and βl fitted to the data in Figure 7. The τs quantifies the fast dynamics of water as attributed to the frequent vibration and liberation of water. Similar behavior was observed for water near protein36,37 and cellulose.38 The residence time (decay constant) of water near the C atom of PIM-1 is negligibly small, due to the inaccessibility of the C atom. In contrast, the residence time is very large near the N atom because of strong interaction between water and the N atom. In the same spirit, the residence time near CRNL-(OH)2 is larger than near CRNL, indicting a longer residence time near the hydrophilic membrane. Water structure in the membranes is characterized by hydrogen bonding. Specifically, two geometrical criteria were implemented to define a hydrogen bond: (1) the distance between a donor and an acceptor ≤ 0.35 nm (2) the angle of hydrogen-donor-acceptor ≤ 30°

39,40

. On

average, one water molecule forms approximately 2.3 hydrogen bonds in all the three membranes, which is lower than the value (3.5) in bulk water.25 This suggests that the entry of water from bulk phase into the membrane is energetically unfavorable; nevertheless, the energy

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loss of weakened hydrogen bonding is compensated by the interaction with the membrane. The relaxation of hydrogen bonds is examined by autocorrelation function40,41 c(t ) =

hij (t0 )hij (t0 + t ) hij (t0 ) hij (t0 )

(9)

where h(t ) = 1 if two water molecules are hydrogen bonded at time t and h(t ) = 0 otherwise. The ensemble average ⋅ ⋅ ⋅ is on all the pairs of hydrogen boned water molecules. Physically, c(t) measures the probability for two water molecules remaining hydrogen bonded at t0 as well as t + t0, In other words, c(t) quantifies how fast a hydrogen bond is relaxed. Figure 8 shows the c(t) in PIM-1, CRNL-(OH)2 and CRNL membranes, as well as in bulk phase. In the three membranes, the c(t) seem close to one other due to similar confinement for water (with ~ 2.3 hydrogen bonds); nevertheless, one can see the hierarchy is CRNL > PIM-1 > CRNL-(OH)2, which follows the decreasing trend of water flux. Obviously, at a higher water flux, fewer hydrogen bonds can remain in the membrane. Moreover, the c(t) in all the membranes are larger compared with bulk phase. Estimated from c (t = τ HB ) = e −1 ,42 the lifetimes τ HB are 4.3, 3.8 and 3.3 ps in CRNL, PIM-1 and CRNL-(OH)2, respectively. The lifetimes are longer than in bulk phase (1.4 ps), this is because the confinement and interaction of the membrane restrict water motion thus leading to a longer lifetime of hydrogen bonds.

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1 PIM-1 CRNL-(OH)₂ CRNL in bulk

0.8 0.6

c(t)

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0.4 0.2 0 0

5

10

15

20

t (ps)

Figure 8. Autocorrelation functions of hydrogen bonding in PIM-1, CRNL-(OH)2, CRNL and bulk phase, respectively.

4. Conclusions Molecular simulations have been performed to examine ETA purification from ETA/water/ammonia mixture through three NF membranes namely PIM-1, CRNL and CRNL(OH)2. Both water and ammonia exhibit good permeation. In the presence of hydrophilic –OH groups, CRNL-(OH)2 possesses the highest water flux and permeability among the three membranes. ETA is able to pass through PIM-1, but 100% retained by CRNL-(OH)2. Overall, CRNL-(OH)2 appears to be the best candidate among the three for ETA purification. From the survival time correlation functions, water is found to reside near the C atom of PIM-1 for very short time, but quite long near the N atom of PIM-1 due to preferential interaction between water and the N atom. Because of hydrophilic nature, the residence time of water near CRNL-(OH)2 is longer than near CRNL. For the hydrogen bonds of water, the lifetimes are 4.3, 3.8 and 3.3 ps in CRNL, PIM-1, and CRNL-(OH)2, respectively. This simulation study examines the potential of PIM-1, CRNL-(OH)2 and CRNL, particularly CRNL-(OH)2 for ETA purification. The

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microscopic and quantitative understanding from bottom-up is useful for the development of new NF membranes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Compression and relaxation scheme, swelling of PIM-1 membrane in water, interconnected and disconnected voids in CRNL-(OH)2 and CRNL, atomic types and charges in ETA and NH3, fixed regions of PIM-1 membrane, ammonia flows, radial distribution functions g(r) between ETA and membranes. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Phone: +65-65165083. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

We gratefully acknowledge the Ministry of Education of Singapore (R-279-000-462-112) and the National Research Foundation of Singapore (R-279-000-468-281) for financial support.

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Abstract Graphic

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Ethanolamine Purification by Nanofiltration through PIM-1 and Carbon

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