Self-assembly of rare-earth Anderson polyoxometalates on the surface of imide polymeric hollow fiber membranes potentially for organic pollutant degradation

Accepted Manuscript Self-assembly of rare-earth Anderson polyoxometalates on the surface of imide polymeric hollow fiber membranes potentially for organic pollutant degradation Lei Yao, Lizhi Zhang, Yuan Zhang, Rong Wang, Sunee Wongchitphimon, ZhiLi Dong PII: DOI: Reference:

S1383-5866(15)00332-9 http://dx.doi.org/10.1016/j.seppur.2015.05.045 SEPPUR 12439

To appear in:

Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

1 February 2015 17 May 2015 18 May 2015

Please cite this article as: L. Yao, L. Zhang, Y. Zhang, R. Wang, S. Wongchitphimon, Z. Dong, Self-assembly of rare-earth Anderson polyoxometalates on the surface of imide polymeric hollow fiber membranes potentially for organic pollutant degradation, Separation and Purification Technology (2015), doi: http://dx.doi.org/10.1016/ j.seppur.2015.05.045

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Self-assembly of rare-earth Anderson polyoxometalates on the surface of imide polymeric hollow fiber membranes potentially for organic pollutant degradation Lei Yaoa,c, Lizhi Zhanga,b, Yuan Zhanga,b, Rong Wanga,b,*, Sunee Wongchitphimona,b, ZhiLi Dongc,*

a

Singapore Membrane Technology Centre, Nanyang Environment and Water

Research Institute, Nanyang Technological University, 637141 Singapore b

School of Civil and Environmental Engineering, Nanyang Technological University,

639798 Singapore c

School of Materials Science and Engineering, Nanyang Technological University,

639798 Singapore

*Corresponding authors: Rong Wang & Zhili Dong School of Civil and Environmental Engineering, Nanyang Technological University, 639798 Singapore. Tel.: +65 6790 5327; fax: +65 6791 0676. E-mail addresses: [email protected] (R. Wang), [email protected] (Z. Dong)

1

Abstract In this study, the self-assembly of rare-earth Anderson polyoxometalates (POMs) on the surfaces of imide polymeric hollow fiber membranes was designed for fabrication of novel POMs-functionalized interfacial composite membranes. The rare-earth Anderson POM ([Gd(H2O)7Cr(OH)6Mo6O18]n) nanoparticles with controllable size and distribution were successfully constructed. Experimental results revealed that the self-assembly was a surface-induced growth process. The silanol groups on the membrane surfaces generated by the (3-aminopropyl)trimethoxysilane pretreatment were essential for the self-assembly process because of the potential hydrogen bonding (–OH···POMs) and coordination bonding (–OH–Gd) interactions. This work provided a simple but practical method not only for the fabrication of novel POMs-functionalized

membranes,

but

also

for

the

synthesis

of

POMs

nanoarchitectures. In addition, the potential application of the as-prepared POMs-functionalized hollow fiber membranes in degradation of organic pollutant has also been explored. The idea of interfacial membrane contactor has been utilized for the catalytic wet air oxidation of phenol under mild conditions. A three-phase (gas/catalyst/liquid) interface was successfully built up, which enhanced the catalytic efficiency. It is anticipated that these novel POMs-functionalized membranes can promisingly be used as catalytic membrane contactors for wastewater treatment under mild conditions. Keywords: 2

polyoxometalates; hollow fiber membrane; self-assembly; nanoparticle; catalytic membrane contactor.

3

1. Introduction Polyoxometalates (POMs) are unique metal-oxygen cluster macroanions with a broad compositional range and an enormous structural diversity [1-3]. Despite of their endless structural variety, they can commonly be divided into three broad subsets: isopolyanions, heteropolyanions, and molybdenum blue or molybdenum brown reduced POMs [4]. Isopolyanions are composed of a metal oxide framework without the internal heteroatom/heteroanion, which are mostly represented by the Lindqvist structure [5] and decavanadates [6]. Heteropolyanions centring on a heteroatom that forms a central polyhedronare by far the most explored subset of POMs with emphasis on the archetypal Keggin type [XM12O40]n–, Wells-Dawson type ([X2M18O62]n−), Anderson-Evans type ([XM6O24]n−) and Dexter-Silverton type ([XM12O42]n−) (X: the heteroatom; M: the addenda atom). The class of Molybdenum blue and molybdenum brown reduced POM clusters, which is one of the most recent and exciting developments in POMs chemistry, relates to giant cluster species. Because of the versatile nature in terms of structure, size, and tunable properties such as redox potential, acidity, charge distribution and solubility in various media, which can be manipulated by choosing proper constituent elements and counter cations, POMs are receiving considerable attention in extensive fields such as catalysis, materials, energy, medicine and biology [7-10]. Among their diversified properties, their function as catalysts is the most popular research field for either oxidation reactions or acid-dependent processes. POMs, as catalysts, are generally resistant to 4

oxidative and thermal degradation. And in proper pH ranges, they are also resistant to hydrolytic degradation. Their properties which are centrally important in catalysis such as reduction potential, solubility, acidity and polarity can be varied adequately due to their sizable, modifiable and large class [11]. By virtue of their high stability, low cost, high efficiency and environmentally friendly nature, POMs are recognized as potent catalysts in fine chemical synthesis as well as environmental applications [12, 13]. Membrane technology is an emerging technology that has been utilized to separate substances over the past decades. With the inspiration of the multi-functional characteristics of natural membranes, various functional compounds or groups have been incorporated into/onto the synthetic membranes, imparting additional unique properties to the membrane and extending their applications [14]. In recent years, POMs-functionalized membranes have attracted great interest. Ma et al. [15] fabricated POM-poly(ionic liquid)s modified polypropylene membranes for photocatalytic degradation of dyes. Popa et al. [16] reported polyoxomolybdate-based poly(vinyl alcohol-co-ethylene) membranes, which can be used as selective membranes for chemical protection. Horan et al. [17] prepared a highly proton-conductive membrane based on divinylsilyl-11-silicotungstic acid, butyl acrylate and hexanedioldiacrylate for fuel-cell applications. However, to the best of our knowledge, the exploration of such POMs-functionalized membranes is still in its infancy and very limited reports are available in this area [16-22]. 5

To survey those limited reports, it was found that the common method to fabricate POMs-functionalized membranes is to blend POMs directly with polymers or blend zeolite or active carbon supported POMs with polymers [16, 17, 19, 21, 23]. In such a mixed-matrix composite membrane, however, the functional compounds were enclosed by the polymeric matrix, to some extent restricting their functional activities in contrast with those interfacial composite membranes which have been the main trend in development of functionalized membranes [14]. Consequently, it would be more practical to fabricate an ultra-thin POMs layer over a polymeric membrane support. However, owing to their hydrophilic nature and high crystalline energy, the assembly of POM clusters onto the membrane surfaces had remained a challenge. Among the very limited work, the assembly of POMs on the membrane surfaces was mainly realized by the plasma technology [18, 20] or complicated grafting method under oxygen-free condition with toxic organic solvent (toluene) [15]. Plasma radiation allowed the generation of desired functional groups (e.g. –NH2) on membrane surfaces, which can be used as anchor groups for the immobilization of POMs by protonation and Coulomb interaction [18, 20]. The grafting method comprised UV-initiated grafting polymerization of a synthesized ionic liquid monomer for binding POMs by ion exchange [15]. Both of them have drawbacks involving the relatively high capital cost, harsh reaction conditions and scale-up difficulty.

6

In the current study, POMs-functionalized interfacial composite membranes were constructed by the self-assembly of the Anderson-type POMs anion ([CrMo6H6O24]3−) and the lanthanide countercation (Gd3+) on the surface of imide polymeric hollow fiber membranes. Herein, hollow fiber membranes were utilized as substrate instead of those commonly reported flat sheet membranes [15, 20] by virtue of the advantages such as large membrane surface area to volume ratio, high membrane packing density, scalability from R&D to production, etc. The Anderson-type POM ([XM6O24]n−) consists of a metal-oxygen octahedron at the center surrounded by six MO6 octahedra. In contrast with classical Keggin or Wells-Dawson type POMs with tetrahedrally coordinated hetero-atoms in the center to form polyhedral geometries, the Anderson-type polyoxoanion exhibits attractive planar structure. Each addenda atom (Mo or W) has two terminal O atoms, which makes them possess high reactivity and capacity to coordinate with countercations [24-26]. The rare-earth metal ion (Gd3+) was selected as an unique countercation in consideration of its high coordination number. This work aims at offering a simple but practical method for fabrication of novel POMs-functionalized hollow fiber membranes. In our attempt to explore applications of the POMs-functionalized membranes, catalytic wet air oxidation (CWAO) of phenol in wastewater was studied. For those reported POMs-functionalized catalytic membranes [15, 18-20], catalytic degradation of organic pollutants in waste water was totally explored in the form of photocatalytic reactors. As long as the artificial light is used, their full-scale 7

applications will be rather limited [27]. In contrast, CWAO is a promising way to treat organic pollutants that are too toxic to biologically degrade and too dilute to incinerate. This process typically involves the oxidation of organic compounds in a stirred tank reactor where the oxygen in air acts as a green oxidant while catalysts are utilized to reduce the operating temperature and pressure. In the current study, the basic idea is to use the POMs-functionalized hollow fiber membranes as an interfacial catalytic membrane contactor (CMC). CMCs in which catalytic reactions take place at the membrane interface have attracted increasing attention for their potential to conduct CWAO in wastewater treatment under mild conditions [28-30]. Compared with conventional CWAO reactor systems such as slurry stirrers or trickle beds, CMCs can show superior catalytic properties, usually 3–5 times, due to the establishment of well-defined gas-catalyst-liquid contact avoiding diffusional limitations [31, 32]. Furthermore, through immobilizing catalysts onto the membrane, ease of recovery and separation of catalysts can be attained. According to survey of literatures, the limited investigation of CMCs mainly focused on the tubular ceramic membranes loaded with noble metals such as Pt, Pd and Ru [28, 29, 31, 33-35]. Although their catalytic performances were satisfying, the high cost was still the main constraint. Taking these into account, low-cost POMs-functionalized polymeric hollow fiber membranes in the current study were evaluated on the CWAO performance as CMC in degrading aqueous phenol under mild conditions. 8

2. Experimental 2.1. Chemicals and Materials Polyetherimide (PEI) (Ultem1000 resin) was purchased from General Electric Plastics. Poly(amide-imide) (PAI) (Torlon® 4000T–MV) was purchased from Solvey Advanced Polymers. They are pre-dried and used as membrane materials for the hollow fiber substrate fabrication. (3-aminopropyl)trimethoxysilane (APTMS, CAS#13822-56-5) used as a coupling agent and GdCl3·6H2O (CAS#13450-84-5) used as a countercation agent to POMs were purchased from Sigma-Aldrich. Isopropanol (IPA, CAS#67-63-0) and N-methyl-2-pyrrolidone (NMP, >99.5%, CAS#872-50-4) as solvents were purchased from Merck. Na3[CrMo6H6O24]·8H2O was prepared according to the literature [36]. 2.2. Membrane preparation Two kinds of imide polymeric membranes were produced in our lab. The PEI and PAI hollow fiber membranes were fabricated by a dry-jet wet spinning technique and used as substrates for the self-assembly of POMs. The homogenous dope solution for spinning was prepared by dissolving the pre-dried polymer (PEI or PAI) and the additive in NMP solvent under stirring at 60 °C for 3 days. Then the dope solution was cooled down to room temperature, degassed under vacuum and finally extruded with bore fluid through a spinneret into the tap water with a certain air gap. The detailed spinning conditions are listed in Table 1. The spinning condition for PAI membrane was the same as that reported in our previous study [37]. 9

2.3. Membrane modification APTMS solution was prepared with a mixture of IPA and distilled water at 1:1 (wt./wt.). Before the self-assembly process, the PEI membrane was pretreated by immersing into 2 wt.% APTMS solution at 60 °C or 4 wt.% APTMS solution at 80 °C for 2 h, respectively. The PAI membrane was pretreated by immersing into 2 wt.% APTMS solution at 60 °C for 1 h. Then the membranes were washed by water and dried in air. In a typical self-assembly process, the APTMS-treated PEI or PAI membrane was immersed into an aqueous solution of GdCl3 ·6H2O (0.3 wt.%) at 60 °C for 20 min. After that, an aqueous solution of Na3[CrMo6H6O24]·8H2O (5 wt.%) was added (GdCl3·6H2O:Na3[CrMo6H6O24]·8H2O, 0.24:1 (wt./wt.)). The mixed system was slowly stirred for 90 min (for PEI membrane) or 20 min (for PAI membrane). Finally, the membranes were washed by ethanol/water mixed solution (1:1, vol./vol.), and dried in air for further characterization. 2.4. Membrane characterization Molecular weight cut-off (MWCO) was performed to characterize the pore size of the original membranes. Each module consisting of six hollow fibers with an effective length of 20 cm was tested in a lab-scaled cross-flow filtration set up. A 2000 ppm dextran solution (Mw 6–500 kDa) was circulated under a constant pressure of 14.7 psi (1 bar). The feed and permeate solution after 1 h stabilization were taken to determine the dextran molecular weight distribution through the gel permeation 10

chromatography (GPC) on a Polymer Laboratories-GPC 50 plus system. The MWCO of the membranes was defined as the molecular weight at 90% rejection. The porosity of the fabricated membrane was calculated based on the density measurements of the membranes with the equation shown below:

where

is the density of the membrane and

is the density of PEI

or PAI polymer.

is calculated from the weight and volume of the as-spun

membranes and

is 1.27 g/cm3 for PEI and 1.45 g/cm3 for PAI. Tensile

strength test of the original hollow fiber membranes were carried out using Zwick 0.5kN universal testing machine to evaluate the mechanical stability. Gas permeation tests were carried out before running the catalytic performance tests. The air which was controlled by a mass flow controller was introduced at the lumen side of the hollow fiber module and the permeate was collected at the shell side. The permeate side was maintained at atmospheric pressure and measured by a digital bubble flow-meter (Bios Defender 510). Electron microscopy studies were carried out with a field emission scanning electronic microscope (FESEM, JEOL JSM-7600F) equipped with an energy dispersive X-ray (EDX) spectrometer and a transmission electron detector (TED). FESEM in second electron imaging (SEI) mode was conducted at 5 kV while FESEM in TED mode was conducted at 30 kV. For preparing the TED observation sample, 11

the membrane was first embedded in EpofixTM (Electron Microscopy Sciences) and cured completely. Then the polymerized block with membrane cross-section was carefully trimmed and sectioned with a glass knife using an ultramicrotome (Leica EMFCS) and left floating on water held in a boat. The glass knife was freshly prepared by a knife maker (Leica EMKMR2) every time before trimming and the thickness of the sections was controlled at 100 nm. The sections were then retrieved from the water surface and mounted onto the TEM copper grids and dried in air before observation. The measurement of crystallinity was carried out by using the Shimadzu 6000 X-ray diffractometer at 40 kV and 40 mA with a 2θ step size of 0.02° and a scan rate of 2°/min. ATR-FTIR spectra were collected at room temperature over a scanning range of 650–4000 cm−1 with a resolution of 4.0 cm−1, using an IR Presitige-21 FT-IR (Shimadzu, Japan). Dynamic contact angle measurement of the membranes was conducted with a tensiometer (DCAT11 Data physics, Germany). 2.5. Catalytic performance test The experimental set-up utilized to test catalytic performance of the modified hollow fiber membranes was reported in our previous work [38]. The proposed experimental mode is shown in Fig. 1. The hollow fiber membranes were mounted into a membrane module, using a tight seal separating the liquid feed on the shell side from the gas phase on the lumen side. The gas/liquid interface was then located on the membrane/catalyst

zone,

leading

to 12

the

favorable

three-phase

contact

(gas/solid/liquid). Aqueous phenol solution (2 mM, 200 mL) was chosen as the model feed to test the catalytic performance of the membranes. The system was firstly stabilized for 20 min. Then samples were collected at 20-min intervals for 100 min at room temperature while the air flow rate was kept at 50 mL/min under 0.5 bar with a mass flow controller and the feed solution flow rate was maintained at 177 mL/min. The effect of gas flow pressure on the catalytic performance of membranes was also carried out under the pressure of 0.5, 1.0, 1.5 and 2.0 bar respectively. The decomposition

of

phenol

was

monitored

using

high-performance

liquid

chromatography (HPLC, Perkin Elmer Series 200) analysis, which was carried out using an Inertsil ODS-3 column and a Series 200 UV/Vis detector at 269 nm with methanol and water (60/40, vol./vol.) as the mobile phase at a flow rate of 0.5 mL/min.

3. Results and Discussion 3.1. Membrane characterization Properties of as-spun PEI and PAI hollow fiber membrane substrates in terms of dimension, MWCO, porosity and mechanical strength are listed in Table 2. Both as-spun PEI and PAI substrates fall into the category of ultrafiltration membranes. SEM observations on the surface of PEI hollow fiber membranes before and after the modification are shown in Fig. 2. It can be observed in Fig. 2a that POMs can hardly be attached onto the PEI membrane without the pretreatment of APTMS. 13

After reacting with 2 wt.% APTMS solution at 60 °C for 2 h, spherical particles with a diameter range from 50 to 300 nm were observed on the membrane surface (Fig. 2b). The morphologies of the particles could be further tailored when the membrane was pretreated with 4 wt.% APTMS solution at 80 °C for 2 h. As shown in Fig. 2c, a layer of nanoparticles with a diameter from 30 to 100 nm were embedded on the surface of the membrane. It indicates that the extent of reaction between PEI and APTMS plays a vital role in the assembly of nanoparticles. Smaller nanoparticles with narrower diameter distribution and more coverage on the membrane surface can be achieved if the PEI membrane reacts with APTMS more sufficiently. As is shown in Fig. 2d, the pretreatment with APTMS did not change the surface morphology of the PEI membrane obviously. Homogenously distributed small pores around 14 nm without defects can be observed, which indicates a good stability of PEI membrane under the chemical treatment of APTMS solution. In order to get a clear view of the assembly layer on the membrane surface, FESEM observation of the membrane cross-section in TED mode was carried out. As is shown in Fig. 3a, a layer of nanoparticles, closed to each other, with a distinct contrast to the membrane substrate can be observed with a thickness around 70 nm, which suggests that the overall distribution of these nanoparticles is in the form of single-layer on the membrane surface. The inorganic assembled layer appears darker than the organic membrane matrix and Epofix resin because the contrast in the FESEM-TED mode is dependent on the atomic number of the object elements. The EDX spectrum of the membrane 14

surface (Fig. 3c) further confirmed the presence of Cr, Mo and Gd, suggesting the successful self-assembly of a rare-earth POM on the surface of PEI membranes. In addition, tiny dark dots with sizes smaller than 25 nm can be observed in the membrane matrix. Since the amount is limited, it cannot be confirmed by EDX whether they are POMs. Thus, FESEM-TED of the original PEI membrane was performed. It can be seen in Fig. 3b that the membrane substrate is homogenous and the boundary between the Epofix resin and the membrane is not obvious. The weak contrast could be explained as similar atomic number and thus inexistence of inorganic components. This result indicates that during the self-assembly process, trace rare-earth Anderson POMs also formed in the membrane matrix with amorphous morphologies instead of nanoparticles. In contrast with PEI membranes, nanoparticles with a diameter around 50 nm and higher coverage on the PAI membrane surface were attained (see Fig. 4a and 4b), which should be attributed to the fact that PAI is more reactive to APTMS than PEI membranes. Fig. 4c shows the cross-section view of the skin layer of the PAI hollow fiber membranes. The self-assembled rare-earth Anderson POMs layer can be observed on the top of the membrane skin layer with a thickness of around 0.5 m, which suggests that the overall distribution of the rare-earth POMs nanoparticles is in the form of multi-layer on the PAI membrane surface. Without the APTMS pretreatment, the rare-earth POMs can hardly be incorporated on the PAI membranes

15

(Fig. 4d), which further indicates that the APTMS plays a vital role in the assembly process. The XRD pattern (Fig. 5a) indicates the self-assembled nanoparticles are crystalline. Moreover, it was found that the XRD patterns were highly consistent with the theoretically calculated results (Fig. 5b) from the reported single-crystal data of [LnIII(H2O)7Cr(OH)6Mo6O18]n·4nH2O [39]. This suggests that the crystalline structure of

the

self-assembled

nanoparticles

is

isomorphous

to

that

of

[LnIII(H2O)7Cr(OH)6Mo6O18]n·4nH2O. Consequently, the crystalline structure of the nanoparticles

should

be

crystallized

as

[Gd(H2O)7Cr(OH)6Mo6O18]n·4nH2O

(GdCrMo6) that consists of 1D zigzag chains built from alternate polyanions [Cr(OH)6Mo6O18]3− and hydrated lanthanide cations [Gd(H2O)7]3+. From this point of view, the self-assembled nanoparticles should be regarded as the nanocrystals that are regularly assembled by the 1D zigzag chains of [Gd(H2O)7Cr(OH)6Mo6O18]n through inter-chain hydrogen-bonding interactions. As is known, one of recent progresses on POMs has been made in the synthesis of POMs nanoarchitectures (nanoparticles, nanowires or nanotubes) [40-43]. However, the development of POMs-based nanoarchitectures is rather slow because POMs clusters usually tend to aggregate to form bulk crystalline solids that are hard to process due to strong hydrogen-bonding interactions [9]. Herein, it is very interesting that a simple self-assembly between the Anderson-type POMs anion ([CrMo6H6O24]3−) and the lanthanide cation (Gd3+) led to the formation of POMs nanoparticles on the surface of membranes. 16

It is noticed that the pretreatment of both the PEI and PAI membranes by APTMS to generate silanol surfaces is essential for the self-assembly process. Without the generation of silanol groups, rare-earth Anderson POMs can hardly be incorporated on the membranes only through Van der Waals force. The result suggests that the self-assembly of the POMs nanoparticles on the surface of membranes is a surface-induced growth process, in which the silanol surface is in favor of the growth of the rare-earth Anderson POMs. In the pretreatment, the incorporation of APTMS onto the surface of membranes was through an imide ring opening reaction accompanied with hydrolysis of silane groups to generate silanol surface. This process can be described as in Scheme 1b. In the subsequent self-assembly of the rare-earth Anderson POMs, silanol surface initially interacted with the lanthanide ions (Gd3+) through coordination bond (–OH–Gd). After the addition of POMs, coordination bonding (–Gd–POMs) and abundant hydrogen bonding (–OH···POMs) interactions occurred in the mixed system, promoting the formation and growth of nanoparticles. It was interesting to observe that the morphology of the self-assembled POMs nanoparticles on the PEI was different from that on the PAI substrate. In contrast to a monolayer of nanoparticles on PEI membranes, a multilayer of nanoparticles with a thickness of approximately 0.5 μm was attained on PAI membranes even under the pretreatment with lower concentration of APTMS solution and much shorter reaction time. It should be due to that the PAI is more chemically reactive than the PEI in the imide ring opening 17

process so that more silanol groups which induced a better coverage and stacking of the self-assembled nanoparticles were generated. Therefore, the self-assembly of nanoparticles can be tailored through adjusting the APTMS pretreatment conditions as well as the selection of different imide polymeric membrane materials. The proposed self-assembly mechanism of rare-earth Anderson POMs on the membranes was further confirmed by FTIR analysis. Both the original PEI and PAI membranes exhibited the typical imide peaks at 1778 (symmetric C=O stretching), 1719 (asymmetric C=O stretching) and 1379 cm−1 (C–N–C stretching) (marked by dashed squareness in Fig. 6). After the incorporation of POMs onto the membranes, those imide peaks were diminished due to the pretreatment of APTMS which opened the imide rings and transformed them into the amide groups [37, 44, 45]. On the other hand, the POMs-functionalized membranes present the typical vibration bands of the Anderson-type POMs anion [CrMo6H6O24]3− at 1598, 1103, 945 and 891 cm−1 in Fig. 6a and 1610, 1110, 923 and 893 cm−1 in Fig. 6b [39, 46, 47]. A broader band around 3550–3230 cm−1 region (O–H stretching) was also observed and should be attributed to the water of crystallization in the rare-earth Anderson polyoxometalate [Gd(H2O)7Cr(OH)6Mo6O18]n·4nH2O.

The

intensity

of

imide

peaks

of

the

POMs-functionalized PAI membrane was much weaker than those of the POMs-functionalized PEI membrane, while the intensity of [CrMo 6H6O24]3− characteristic vibration bands for the POMs-functionalized PAI membrane was stronger than those for POMs-functionalized PEI membrane. This result agrees with 18

the observation that the PAI membrane was more reactive and easier to form a multilayer of nanoparticles. 3.2. CWAO performance of POMs-functionalized membrane contactors Before

the

catalytic

degradation

test,

air

permeation

over

the

POMs-functionalized hollow fiber membranes was examined. Both the permeances of the POMs-functionalized PEI membrane (GdCrMo6-PEI) and PAI membrane (GdCrMo6-PAI) were more than 770 GPU (1 GPU = 1×10−6 cm3 (STP)/cm2 s cmHg). The results demonstrate that both of them possess a good permeance, which is vital in the catalytic process. The catalytic effect as a function of time is illustrated in Fig. 7. GdCrMo6-PEI exhibited a good phenol degradation performance at room temperature with 0.5 bar air overpressure, where the conversion of phenol oxidation for 100 min is c.a. 10% which is higher than that (c.a. 6.1% calculated on the basis of equivalent catalyst in mass) for the reported catalytic membrane contactor [29] using the noble metal Pt as catalyst on tubular ceramic membranes at room temperature with 4 bar air overpressure. The catalytic efficiency of GdCrMo6-PAI was not as high as GdCrMo6-PEI under the same operating conditions (T = 25 °C; air overpressure = 0.5 bar; effective interfacial area of membranes = 58.9 cm2) despite its more catalysts in amount (estimated as c.a. 9.7 mg by a rough calculation on the basis of the density and thickness of assembly layer) than GdCrMo6-PEI (c.a. 1.4 mg), which could be attributed to their different optimal working conditions by virtue of their different morphologies of POM assemblies as well as different membrane structure 19

characteristics. As a matter of fact, the air overpressure played an important role in the optimal working conditions. This will be further discussed in later sections. To better evaluate the catalytic performance of POMs-functionalized membranes, contrast experiments were carried out with phenol solutions (2 mM, 200mL) in the presence of 250 mg rare-earth Anderson POMs [Gd(H2O)7Cr(OH)6Mo6O18]n·4nH2O (GdCrMo6) suspension or Anderson POMs Na3[CrMo6H6O24]·8H2O (CrMo6) dissolved in the solution. Even though excessive amount of catalysts were utilized in the contrast experiments, an obvious enhancement of phenol degradation was still achieved by POMs-functionalized membranes, which implies that the unique liquid-catalyst-gas interface based on the membranes plays an important role in enhancing the catalytic activity. First-order kinetics with respect to phenol concentration was found for the catalytic degradation process with both GdCrMo6-PEI and GdCrMo6-PAI (Fig. 8). The rate of degradation of phenol can be described by the following equation:

where C is the phenol concentration (mM), t is the time (min), and k is rate constant (min−1). Best fitting for the experimental data of GdCrMo6-PEI led to k = 1.2 × 10−3 min−1 with the coefficient of determination R2 = 0.967. GdCrMo6-PAI exhibited better fit to the first-order kinetics model, where the rate constant was determined to be k = 6.26 × 10−4 min−1 with R2 = 0.995. 20

Phenol degradation as a function of air overpressure was further examined (Fig. 9) to evaluate their optimal operating parameters. Phenol degradation efficiency presented a negative relation with the air overpressure for GdCrMo6-PEI. As for GdCrMo6-PAI, with the increase of air overpressure from 0.5 to 1.0 bar, the effect of phenol degradation was remarkably enhanced. However, further increase of air overpressure from 1.0 to 2.0 bar led to reduced phenol degradation. Generally speaking, the increase of air overpressure applied would increase the solubility of O2 in the liquid phase that helps for the oxidation reaction rate. However, after the overpressure reached a threshold and further increased, the location of the gas-liquid interface might be no longer maintained in the catalytic layer of the membrane. This could explain why the phenol degradation efficiency first increased and then decreased for GdCrMo6-PAI. The above results indicate that the air overpressure, which may control the location of the gas–liquid interface, is crucial to define the optimal conditions in the process. According to Fig. 9, the optimal air overpressure for GdCrMo6-PAI should be around 1.0 bar, while the threshold of air overpressure for GdCrMo6-PEI should be not more than 0.5 bar. It can be seen that the approximately same phenol degradation efficiency was achieved by GdCrMo6-PEI at 0.5 bar and GdCrMo6-PAI at 1.0 bar, respectively. In order to further understand why the optimal air overpressures for GdCrMo6-PEI and GdCrMo6-PAI are different, the wettability of the membranes was investigated by dynamic contact angle measurements. The contact angles of original 21

PEI and PAI membranes were 78° and 76°, respectively. After the incorporation of POMs, their contact angles reduced dramatically to 39° and 28°, respectively. The remarkable decrease of contact angles should be attributed to the hydrophilic nature of the rare-earth Anderson POMs on the membrane surfaces. Compared with GdCrMo6-PEI, GdCrMo6-PAI was more hydrophilic, which was consistent with the observations that a multilayer of POMs nanoparticles were assembled on the PAI membranes, and thus higher air overpressure was preferred to hold the gas/catalyst/liquid three-phase interface. The above results indicate that the air overpressure, which plays an important role in the phenol degradation process through controlling the location of the gas-liquid interface, is closely related to the wettability of the membranes. In our previous work [38], a catalytic membrane contactor (CMC) was constructed

with

the

Keggin-type

POM,

decamolybdodivanadophosphate

([PV2Mo10O40]5−) as catalyst. Chemical deposition was utilized to immobilize the POMs onto the surface of PVDF hollow fiber membranes with a permanent charged polyelectrolyte. The polyelectrolyte as countercations for POMs helps for the capture of organic pollutant towards the catalyst. However, at the same time it may restrict the catalytic activity of the catalyst because the POMs are encapsulated by the polyelectrolyte which consists of organic constituent polymer chains. In contrast with the reported work, the higher efficiency of phenol degradation by the CMC constructed in the current study has been achieved under the same working 22

conditions. This should benefit from the POMs catalyst employing small inorganic constituent lanthanide ions (Gd3+) instead of polyelectrolytes as unique countercations which could not only minimize the negative effect of countercations on catalytic activity but also result in nano-architectural morphology. It is noted that the Anderson-type configuration of POMs also plays an important role in the formation of the nano-architectural morphology because of its special coordination pattern to lanthanide

ions.

Providing

the

Keggin-type

POMs

(e.g.

decamolybdodivanadophosphate ([PV2Mo10O40]5−)) were used in the current study, the similar self-assembly process would not proceed. A reasonable mechanism of the catalytic oxidation of organic pollutant by the POMs [Gd(H2O)7Cr(OH)6Mo6O18]n (GdCrMo6) in this work can be speculated as a radical reaction [48, 49]. To act as an initiator of the radical reaction, the GdCrMo6 was first reduced into GdCrMoVI5MoV by electrons that were available from organic pollutants in wastewater. These electrons were then transferred to oxygen molecules to generate various highly active oxygen species (Equation 3) for effective degradation of organic pollutants in wastewater. This mechanism is similar to that of other Anderson-type catalysts for aerobic oxidative process [50-52].

4. Conclusions Novel POMs-functionalized interfacial composite membranes were constructed by the self-assembly of the Anderson-type POMs anions ([CrMo6H6O24]3−) and the 23

rare-earth lanthanide cations (Gd3+) on the surface of imide polymeric hollow fiber membranes. The self-assembly process can be tailored by different modification parameters based on different imide polymeric membrane materials. The self-assembled POMs were crystalline in the form of nanoparticles with the size of around 50 nm. The silanol surface generated by the (3-aminopropyl)trimethoxysilane pretreatment was proved to play an important role to induce the self-assembly process. This study provides valuable information for the fabrication of novel POMs-functionalized interfacial composite hollow fiber membranes as well as the synthesis

of

POMs

nanoarchitectures.

Additionally,

for

the

reported

POMs-functionalized membranes, their potential application in catalytic degradation of organic pollutant has also been explored by using a promising catalytic membrane contactor set-up through CWAO process. The results revealed the importance of air overpressure to form the three-phase (gas/catalyst/liquid) interface during the catalytic process. This CWAO process for organic pollutant degradation under mild conditions demonstrates the potential of POMs-functionalized interfacial hollow fiber membranes as catalytic membrane contactors in wastewater treatment. In the end, considering the versatility of POMs in extensive fields [7-10], it is anticipated that the reported POMs-functionalized membranes in the current work can find other applications.

Acknowledgments 24

This research grant is supported by the Singapore National Research Foundation under its Environmental & Water Technologies Strategic Research Programme, which was administered by the Environment & Water Industry Programme Office (EWI) of the PUB (EWI RFP 0901-IRIS-04-06). We are also grateful to Singapore Economic Development Board for funding Singapore Membrane Technology Centre (SMTC). In addition, we would like to thank the financial support from the Singapore MOE Academic Research Fund (AcRF) Tier 1 RG 76/12 (M4011088.070). The electron microscopy was performed at the Facility for Analysis, Characterization, Testing and Simulation (FACTS) in NTU. Figures and Tables Fig. 1 Fig. 2

Fig. 3

Fig. 4

Fig. 5

The proposed experimental mode for catalytic degradation of organic pollutant in wastewater. SEM images of the surface of the rare-earth Anderson POMs modified PEI hollow fiber membranes: (a) without APTMS pretreatment, (b) pretreated with 2 wt.% APTMS solution at 60 °C for 2 h, and (c) pretreated with 4 wt.% APTMS solution at 80 °C for 2 h. (d) SEM image of the surface of PEI hollow fiber membrane pretreated with 4 wt.% APTMS solution at 80 °C for 2 h before the POMs assembly. FESEM-TED image of the cross-section view of (a) the rare-earth Anderson POMs assembled PEI hollow fiber membrane skin layer and (b) the original PEI hollow fiber membrane skin layer. (c) EDX spectrum of the rare-earth Anderson POM particles on the PEI hollow fiber membrane surface. SEM images of the rare-earth Anderson POMs modified PAI hollow fiber membranes with APTMS pretreatment: (a) membrane surface enlarged at 20,000×, (b) membrane surface enlarged at 50,000×, and (c) cross-section view of the membrane skin layer. (d) SEM image of the surface of the rare-earth Anderson POMs modified PAI hollow fiber membranes without APTMS pretreatment. XRD patterns of (a) the self-assembled nanoparticles and (b) calculated results from the single-crystal data of [LnIII(H2O)7Cr(OH)6Mo6O18]n·4nH2O [39] 25

Scheme 1

Fig. 6

Fig. 7

Fig. 8

Fig. 9

Table 1 Table 2

(a) Chemical structures of PEI (Ultem 1000 resin) and PAI (Torlon® 4000T–MV). (b) The reaction mechanism of PEI or PAI with APTMS. (c) Schematic diagram of the self-assembled nanoparticles of [Gd(H2O)7Cr(OH)6Mo6O18]n·4nH2O on the PAI membrane surface. (d) Ball-stick view of the structure of Anderson-type POM anion [CrMo6H6O24]3−. ATR-FTIR spectra for (a) the original and the POMs-functionalized PEI membranes and (b) the original and the POMs-functionalized PAI membranes. Catalytic degradation of phenol (2 mM, 200 mL) as a function of time with GdCrMo6-PEI, GdCrMo6-PAI (T = 25 °C; air overpressure = 0.5 bar; effective interfacial area = 58.9 cm2), GdCrMo6 and CrMo6 (T = 25 °C, 250 mg). First-order fitting for the catalytic degradation of phenol as a function of time with GdCrMo6-PEI and GdCrMo6-PAI (T = 25 °C; air overpressure = 0.5 bar; effective interfacial area = 58.9 cm2). Effect of air overpressure on phenol (2 mM, 200 mL) degradation with GdCrMo6-PEI and GdCrMo6-PAI (T = 25 °C; t = 20 min; effective interfacial area = 58.9 cm2). Dry-wet phase inversion spinning conditions. Properties of as-spun PEI and PAI hollow fiber membranes.

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30

Fig. 1. The proposed experimental mode for catalytic degradation of organic pollutant in wastewater.

Fig. 2. SEM images of the surface of the rare-earth Anderson POMs modified PEI hollow fiber membranes: (a) without APTMS pretreatment, (b) pretreated with 2 wt.% APTMS solution at 60 °C for 2 h, and (c) pretreated with 4 wt.% APTMS solution at 80 °C for 2 h. (d) SEM image of the surface of PEI hollow fiber membrane pretreated with 4 wt.% APTMS solution at 80 °C for 2 h before the POMs assembly.

Fig. 3. FESEM-TED image of the cross-section view of (a) the rare-earth Anderson POMs assembled PEI hollow fiber membrane skin layer and (b) the original PEI hollow fiber membrane skin layer. (c) EDX spectrum of the rareearth Anderson POM particles on the PEI hollow fiber membrane surface.

Fig. 4. SEM images of the rare-earth Anderson POMs modified PAI hollow fiber membranes with APTMS pretreatment: (a) membrane surface enlarged at 20,000×, (b) membrane surface enlarged at 50,000×, and (c) crosssection view of the membrane skin layer. (d) SEM image of the surface of the rare-earth Anderson POMs modified PAI hollow fiber membranes without APTMS pretreatment.

Fig. 5. XRD patterns of (a) the self-assembled nanoparticles and (b) calculated results from the single-crystal data of [LnIII(H2O)7Cr(OH)6Mo6O18]n·4nH2O [39].

Fig. 6. ATR-FTIR spectra for (a) the original and the POMs-functionalized PEI membranes and (b) the original and the POMs-functionalized PAI membranes.

Fig. 7. Catalytic degradation of phenol (2 mM, 200 mL) as a function of time with GdCrMo6-PEI, GdCrMo6-PAI (T = 25 °C; air overpressure = 0.5 bar; effective interfacial area = 58.9 cm2), GdCrMo6 and CrMo6 (T = 25 °C, 250 mg).

Fig. 8. First-order fitting for the catalytic degradation of phenol as a function of time with GdCrMo6-PEI and GdCrMo6-PAI (T = 25 °C; air overpressure = 0.5 bar; effective interfacial area = 58.9 cm2).

Fig. 9. Effect of air overpressure on phenol (2 mM, 200 mL) degradation with GdCrMo6-PEI and GdCrMo6-PAI (T = 25 °C; t = 20 min; effective interfacial area = 58.9 cm2).

Scheme 1 (a) Chemical structures of PEI (Ultem 1000 resin) and PAI (Torlon® 4000T–MV). (b) The reaction mechanism of PEI or PAI with APTMS. (c) Schematic diagram of the self-assembled nanoparticles of [Gd(H2O)7Cr(OH)6Mo6O18]n·4nH2O on the PAI membrane surface. (d) Ballstick view of the structure of Anderson-type POM anion [CrMo6H6O24]3−.

Table 1 Dry-wet phase inversion spinning conditions. Spinning Parameters

PEI

PAI

Dope composition (polymer/NMP/Additive)

20/79.4/0.6

12/85/3

Spinneret o.d./i.d. (mm)

1.5/0.7

1.5/0.7

Dope flow rate (g/min)

3.8

6.0

Bore fluid (H2O/NMP ) (wt.%)

80/20

100/0

Take-up speed

Free falling

Free falling

Bore fluid flow rate (mL/min)

3.0

7.0

Air gap distance (cm)

2

15

Spinning temperature (°C)

23

23

Table 2 Properties of as-spun PEI and PAI hollow fiber membranes. Properties

PEI

PAI

Fiber outer diameter (mm)

1.03

1.38

Fiber inner diameter (mm)

0.78

1.05

Fiber wall thickness (μm)

123

165

MWCO (kDa)

17

34

Porosity (%)

72

85

Tensile modulus (MPa)

108

86

Strain at break (%)

3.2

31.1

Highlights ► The POMs-functionalized membranes were constructed by a simple self-assembly method. ► The self-assembled POMs nanoparticles are crystalline with the size of around 50 nm. ► Catalytic degradation of phenol in water has been achieved under mild conditions. ► A gas-catalyst-liquid interface was built up to enhance the catalytic efficiency.

31