MetalâOrganic Frameworks Supported on Nanofiber for Desalination

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Applications of Polymer, Composite, and Coating Materials

Metal-organic frameworks supported on nanofiber for desalination by direct contact membrane distillation Fan Yang, Johnson Efome, Dipak Rana, Takeshi Matsuura, and Christopher Lan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01371 • Publication Date (Web): 08 Mar 2018 Downloaded from http://pubs.acs.org on March 8, 2018

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ACS Applied Materials & Interfaces

Metal-organic frameworks supported on nanofiber for desalination by direct contact membrane distillation Fan Yang, Johnson E. Efome*, Dipak Rana, Takeshi Matsuura, Christopher Lan Industrial Membrane Research Institute, Department of Chemical and Biochemical Engineering, University of Ottawa. 161 Louis Pasteur Ottawa ON Canada, K1N 6N5

Abstract Among other applications, metal-organic frameworks (MOFs) are slowly gaining grounds as fillers for desalination composite membranes. In this study, superhydrophobic poly (vinylidene fluoride) (PVDF) nanofibrous membranes were fabricated with MOF (Iron-BTC) loading of up to 5 wt. % via electro-spinning on a nonwoven substrate. To improve attachment of nanofibers unto the substrate, a substrate pre-treatment method called ‘solvent basing’ was employed. Iron content in the nanofiber, measured by dispersive X-ray spectroscopy (EDS), increased proportionally with the increase of MOF concentration in the spinning dope, indicating a uniform distribution of MOF in the nanofiber. The water contact angle increased up to 138.06±2.18° upon incorporation of 5 wt. % MOF and a liquid entry pressure of 82.73 kPa could be maintained, making the membrane useful for DCMD experiments. The membrane was stable for the entire operating period of 5 h, exhibiting 2.87 kg/m2h of water vapor flux and 99.99% NaCl (35 g/L) rejection when the feed and permeate temperature were 48℃ and 16℃, respectively. Immobilization of MOF on nanofibers with the enhanced attachment was proven by inductively coupled mass spectrometry (ICP-MS) analysis, by which no Fe2+ could be found in the permeate to the detection limit of ppt.

Keywords: metal organic framework; nanofibers; desalination; direct contact membrane distillation; electrospinning

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1. Introduction Water is already a scarce commodity in most countries especially in the developing world because of the growing population, industrialization, irrational pollution and food production playing their individual and collective roles to the shortage. Although usage of existing groundwater helps in alleviating the situation of drinkable water scarcity, the economic tide is slowly turning in favor of seawater desalination. Currently, nanotechnology tools like electrospinning have become a key player since it can fabricate high porous, hydrophobic/hydrophilic membranes employed to improve the performance, surface properties and anti-fouling of a long-term desalination membrane system like membrane distillation 1. Membrane distillation (MD) is an emerging thermally-driven separation technology that is drawing the attention of researchers for several merits such as, smaller distillation equipment, lower operating temperature, very high salt rejection and lower hydraulic pressures required compared to conventional membrane separation processes like reverse osmosis (RO), ultra-filtration and nano-filtration 2. However, membrane distillation suffers from several limitations: low permeate flux of MD compared with others, heat loss due to conduction and deterioration of permeate water quality induced by pore wetting or fouling 3. Several configurations of MD have been developed including; direct contact membrane distillation (DMCD), vacuum membrane distillation (VMD), air gap membrane distillation (AGMD) and sweeping gas membrane distillation (SGMD). From above configurations, DCMD is the most appropriate for desalination due to the facile operability and simple equipment 4. In DCMD, partial vapor pressure difference between hot feed and chilled permeate is the main driving force. As a hot aqueous feed stream comes into contact with one side of the membrane surface while cold permeate stream with the other side, evaporation 2


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occurs at the hot side of the membrane and the vapor is driven through membrane pores by vapor pressure difference between the two sides, and condenses at the permeate side 5. Aqueous feed solution should be prohibited to enter membrane pores because of the hydrophobic surface of membranes. Hence, membrane materials are often limited to nonreactive and thermally stable polymers of high hydrophobicity, typically polyvinylidene (PVDF), polystyrene (PS) and polytetrafluoroethylene (PTFE) 6. Moreover, the pore size should be large enough to allow high flux, while it should be small enough to prevent the pore wetting. In order to achieve this goal, membranes are fabricated by phase inversion, thermallyinduced phase separation and electrospinning 7. Among these, nanofiber membranes that are prepared via electrospinning exhibits unique characteristics such as high porosity, large specific surface area and adjustable fiber thickness 8. Recent studies have proven that incorporation of nano-additives enhances MD performance due to increased pore sizes, enhanced surface roughness and mechanical stability 9. Metal-organic frameworks (MOFs) are those that have been used as fillers in mixed matrix membranes10,11. MOFs are coordination networks that consist of metal ions coordinated to organic ligands adsorption and separation

13,14

12

and have been used especially in

but more commonly in gas phase separation, owing to

their high specific surface area and porous structure. However, since most MOFs are hydrothermally unstable 15, only a few, especially those with zirconium and iron metal ion clusters, are stable enough in water to be used for water applications. For example, zirconium MOF UiO-66 was supported on alumina hollow fibers for desalination 16. The objective of this study is to develop a novel nanofiber membrane for DCMD by electrospinning MOF (Fe-BTC, Iron 1, 3, 5-Benzenetricarboxylate (F300)) containing PVDF spinning dope onto the nonwoven support material. 3



Since electrospinning is the membrane fabrication technique, the size of MOF is very important to ensure that the spinneret of the electrospinning can accommodate the particle if not blockage could occur and distort the fiber morphology and properties. Coupled with the commercial availability of F300, the viscosity of the dope solution obtained allows for nanofiber formation compared with other particles because of its small size. The main feature of this coating technique is to spray an optimized thin film layer of the solvent onto the support material prior to electrospinning, called solvent basing technique. After characterizing the membranes by various methods, the membranes are further subjected to DCMD test by using 35 g/L NaCl aqueous solution as feed. Furthermore, an attempt was made to predict the permeate flux by a mathematical model under various DCMD operating conditions.

Figure 1. Schematic of the DCMD experimental setup

4


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2. Experimental

2.1 Materials Poly (vinylidene fluoride) (PVDF) (Mw=410kDa; melting temperature, Tm=160.1℃) was supplied by Arkema Inc., Philadelphia, PA. Dimethyl acetamide (DMAc, >99%) and acetone used as solvent were purchased from Sigma Aldrich Inc., St. Louis, MO. The nonwoven support material TA3618 (thickness: 0.19mm) was obtained from Tianlue Advanced Textile Co. Ltd., (China). Metal organic framework Fe-BTC, Iron 1, 3, 5-Benzenetricarboxylate (F300) was purchased from Sigma Aldrich, with a BET surface area of 1300-1600 m2/g, and bulk density of 0.16-0.35 g/cm3. The MOF was used as purchased without any further treatment.

2.2 Membrane fabrication The electrospinning dope was prepared by dissolving 13 wt. % PVDF into DMAc/Acetone solvent in a weight ratio of 2:3, then MOF (1, 3, 5 wt. %) was added into the solution 17. The dope compositions are shown in Table 1.Composition of dope solution.

The suspension was stirred at 50 ℃ for 24 h, followed by overnight de-

gassing. Before electrospinning, DMAc solution was sprayed evenly over the support material and then 10 mL of the spinning dope was electrospun under the following spinning condition: flow rate, 0.15 mm/min; distance from the needle nozzle to the rotating aluminum foil collector, 150 mm; rotating speed, 140 rpm; the applied voltage, 15 kV. The electrospinning was conducted at room temperature (25℃) and 40% humidity. The fabricated nanofiber membranes were dried at room temperature for complete removal of residual solvent prior to further tests and characterizations. 5



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Table 1.Composition of dope solution

Membrane codea

PVDF (wt. %)

DMAc (wt. %)

Acetone (wt. %)

MOF (wt. %)b

PVDF

13

34.8

52.2

-

PV-1

13

34.8

52.2

1

PV-3

13

34.8

52.2

3

PV-5

13

34.8

52.2

5

a b

the numeric represents the concentration of MOF in the dope on the bases of dope solution. MOF/(PVDF+Acetone+DMAc+MOF) x100

2.3 Membrane characterization The liquid entry pressure of water ( ) was measured by a setup, the detailed description of which is in our previous work

17,18

. The dry membrane was cut into

small circular pieces (13.1 cm ), which were placed at the bottom of the liquid chamber. The setup was filled with De-ionized water (DI) and nitrogen gas was introduced to increase the pressure of water at a rate of 2 psig per 10 min. was recorded at the point where water droplets start flowing from the cell. The measurement was triplicated and the average was reported. The contact angle (CA) of the electrospun membrane sample was measured by the sessile drop method with a water droplet size of 1 µL using VCA Optima Surface Analysis System (AST Products Inc., Billerica, MA). Measurements were made at 20 different locations and the average was reported. The surface roughness (Ra) of the electrospun membrane was measured by atomic force microscopy (AFM) under room temperature. Tapping mode was applied at the surface of membrane samples by a NanoScope III AFM equipped with a 1533D scanner (Digital Instruments, Santa Barbara, CA). Random locations were taken to measure average Ra. Gravimetric method was applied to determine membrane porosity 6. Dry membranes 6


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were cut into small samples and then weighed. Subsequently, the membrane sample was immersed into butanol for 30 min and the weight of the wetted membrane, W2, was measured. The following equation was applied to calculate porosity:

=

Where W1 and W2 are the weight of wet and dry membrane, respectively. Ws and Wp are the weight of the support material and PVDF nanofibers, respectively, which can be obtained by weighing the support material before and after electrospinning.

refers to density of butanol, is the density of the support material and is the

density of PVDF nanofibers. The surface and the cross-section of the electrospun membranes and the MOF particles were observed by scanning electron microscopy (SEM), Tescan, Vega-II XMU with Oxford Inca Energy 250X EDS. The membranes were immersed into liquid nitrogen and fractured for investigating the cross-section. The SEM samples were gold sputtered to increase conductivity. Nanofiber diameter, fiber diameter distribution, thickness, mean pore size, mean pore size distribution and MOF morphology were determined from the SEM images by ImageJ software. Fifty locations were randomly selected at the surface to measure pore sizes and fiber diameters, and the average was reported. Twenty locations at the cross-sectional image were randomly selected to measure the nanofiber mat thickness and the average value was reported. The surface elemental composition of the nanofiber layer was determined by Energy Dispersive X-ray spectroscopy (EDS). Transmission electron microscope (TEM), FEI Tecnai G2 F20 TEM, was employed to characterize the MOF size and morphology. MOF samples were prepared by 7



sonicating in ethanol for 30 min and then 1 droplet of suspension was placed on TEM grids for analysis. ATR-FTIR analyses of the pristine MOF and the nano-fibrous MOF membranes were carried out using Agilent tech- Cary 630 (Agilent, Canada). Samples were pressed on a diamond prism and the infrared spectra were collected at 4 cm-1 resolution, 64 scans within a wavenumber range of 500-4500 cm-1 at room temperature. Temperature stability of the material was investigated by thermographic analysis (TGA) using Q5000 IR- TA Instruments under a ramp of 10 oC/min under air. Powered X-ray diffraction analysis of MOF crystals was carried out at room temperature on Rigaku Ultima IV powder diffractometer in Bragg-Brentano geometry, using Cu Kα radiation (λ = 1.5418 A). 2θ range of 2° to 32° was covered with 0.02° step width and 2°/min scan speed. The surface area of the purchased MOF was confirmed by BET using nitrogen at 77 K with a Micromeritics 3FLEX volumetric apparatus. Before the nitrogen adsorption measurements, the samples were degassed under a purge flow of nitrogen with 40 cm3/min at the temperature of 90℃ for 1 h. The data in the relative pressure (P/P0) range 0.05–0.2 were used to calculate the specific surface area with the BET equation.

2.4. DCMD experiments The DCMD experiments were conducted by the laboratory setup shown in Figure 1. Schematic of the DCMD experimental setup

for all fabricated membranes. The setup is

comprised of a hot feed and chilled permeate sides with each having a circulation pump. Thermometers and pressure gauges are spread all over the setup to measure temperature and pressure respectively. The membrane module has an inlet feed valve and the exit permeate. All pipes were well insulated to minimize heat loss. The membrane sample was placed in the DCMD cell with an effective area of 0.00385 m2 8


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and subjected to continuous DCMD test over a 5 h period. In this study, the feed side solution was 3.5 wt. % NaCl aqueous solution and DI water for permeate side with an initial conductivity < 2 µS/cm and the flow rate on both side was maintained at 1.5 L/min. DCMD experiments were performed at different feed temperatures (28, 38 and 48 ±1.5℃), while permeate temperature was kept at 16 ±1.3℃ respectively. During the test, the conductivity of feed and permeate was measured using a conductivity diameter (Oakton con 2700), and the permeate vapor flux was measured by a digital balance (Ohaus adventurer pro av8101) every 20 min.

2.4 Calculation of vapor flux The details of the model for the calculation of water vapor is given in the reference 19– 21

. A summary is given below;

2.4.1 Heat transfer Due to the heat transfer resistances at the boundary layer of the feed and permeate stream, the temperatures at the solution/membrane interface on the feed, Tmf, and the permeate side, Tmp, which govern the mass transport, are considerably different from the bulk temperatures of feed, Tf and permeate stream, Tp. It is known that, by solving a set of heat transfer equations, Tmf and Tmp can be calculated by "#

!

2#%

= = where

$% # & '% #% ( )* ∆,$ % '% (/ ) ! !$

!

$ # 3 ' # )* ∆,$%

!

' (/

) !

%$

(1)

(2)

km, δ, hf, hp, Jw and ∆Hv are the effective thermal conductivity of the membrane, membrane thickness, mass transfer coefficient on the feed side, mass transfer coefficient on the permeate side, flux of water vapor and latent heat of evaporation of 9



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water, respectively. Furthermore, km can be calculated by

5 = (1– )5 + 58

(3)

Where ε is the porosity of the membrane, ks and kv are the thermal conductivity of the solid and vapor phase in the membrane, respectively, and ε is the porosity of the membrane. The water vapor flux is given by

: = ; (

MetalâOrganic Frameworks Supported on Nanofiber for Desalination

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