Functional Cross-Linked Electrospun Polyvinyl Alcohol Membranes and Their Potential Applications

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range of applications.[1–3] It is a suitable polymer for the novice electrospinning to produce nanofibers as it is inex-pensive, nontoxic, and water soluble. However, due to its water solubility, it needs to be stabilized, for example, by cross-linking to enable its practical application in water-based environments to be realized. There are several studies demonstrating cross-linking of electrospun PVA membranes and their water stabilization.[4–9] Three major cross-linking methods have been described in the litera-ture but each of these methods has its own problems in scaling up and cross-linking efficiency.

(i) Immersion of electrospun membranes into an organic solvent

(ii) Exposure of electrospun membranes to reagent vapor (iii) Addition of a cross-linker/catalyst to electrospinning

solutions

The first approach involving the use of an organic solvent is the simplest approach. Solvents such as

Polyvinyl alcohol (PVA) is a water soluble polymer that requires further treatment to be

sta-bilized before it can be used in aqueous environments. Electrospun PVA is cross-linked by

incorporating cross-linking agents directly into the electrospinning solution followed by

post-electrospinning thermal treatments to attain stability in aqueous environments.

Previ-ously published works on post-treatments include glutaraldehyde vapor exposure or soaking

in organic solvents such as ethanol. However, these treatments incur lots of difficulties and

hazards especially in scale production. In this study, with a view of imminent scale-up

pro-duction required, fabricating electrospun cross-linked PVA is investigated without using

cat-alysts, toxic vapor exposure, or solvent treatment. To produce cross-linked electrospun PVA

membranes, citric acid, maleic acid, and polyacrylic acid are,

respectively, added to PVA solution prior to electrospinning.

Two potential applications are examined; the first is to use

the membranes as produced for metal uptake in aqueous

sys-tems. The second application is for ammonia adsorption after

decorating the membranes with a metal organic framework,

copper benzene-1,3,5-tricarboxylate (HKUST-1).

Functional Cross-Linked Electrospun Polyvinyl

Alcohol Membranes and Their Potential

Applications

Yen Bach Truong,* Jonghyun Choi, James Mardel, Yuan Gao, Sabrina Maisch,

Mustafa Musameh, Ilias Louis Kyratzis

Dr. Y. B. Truong, Dr. J. Mardel, Dr. Y. Gao, S. Maisch, Dr. M. Musameh, Dr. I. L. Kyratzis

CSIRO Manufacturing

Private Bag 10, Clayton South, Victoria 3168, Australia E-mail: yen.truong@csiro.au

Dr. J. Choi

The New Zealand Institute for Plant and Food Research Ltd. Private Bag 3230, Waikato Mail Centre

Hamilton 3240, New Zealand

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/ mame.201700024.

1. Introduction

Polyvinyl alcohol (PVA) is a water soluble polymer that has been extensively studied in the electrospinning literature. PVA electrospun fibrous membranes with well-controlled uniform nanosized fiber diameter have been tested for a

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ethanol,[7,10] acetone,[9] and methanol[8] have been used. This process is extremely slow requiring 3–48 h. This would pose an efficiency problem for a scale-up produc-tion with the required volume of solvent and amount of time. This approach, however, is an effective method to increase the crystallinity of PVA due to the removal of residual water resulting in the intermolecular polymer hydrogen bonding.[8]

The second approach involving the exposure to a cross-linking agent in vapor form is also a common way of cross-linking electrospun PVA. The vapors that have been used include glutaraldehyde,[9] glyoxal,[11] and ammonia.[12] Glutaraldehyde vapor cross-linking has been used at different conditions, concentration, and time in a number of studies to stabilize electrospun PVA. Destaye et al.[4] used 2.5 m glutaraldehyde for 48 h at room temperature while Yang et al.[13] used 50% glutaraldehyde for 2 h at 25 °C. Glutaraldehyde vapor has also been used by Pattama et al.[6] at 37 °C for 8 h and by Lin et al.[1] for 3 d at room temperature. The vapor exposure method for cross-linking electrospun PVA membranes generally requires a closed vessel environment and long periods of time making it impractical for scale-up.

The third approach involves incorporating polycarbox-ylic acids under acidic conditions in the electrospinning solution followed by a postelectrospinning heat treat-ment to form cross-linking via an esterification reaction. Ding et al.[14] added 8 wt% of glyoxal with phosphoric acid as a catalyst prior to electrospinning, then heat treated for 5 min at 120 °C followed by catalyst removal using soxhlet extraction with tetrahydrofuran. Qin and Wang[2] mixed maleic acid with PVA in a ratio of 1:10 wt:wt at pH 5 (adjusted with sulfuric acid) to obtain cross-linked elec-trospun PVA membranes. On the other hand, Yang et al.[11] added maleic anhydride and adjusted pH to 2–3 with sulfuric acid. They determined the optimum molar ratio of PVA:maleic anhydride to be 20:1 and calculated the degree of cross-linking by membrane mass loss in water. Cay and Miraftab[15] produced cross-linked electrospun PVA membrane by adding 5% 1,2,3,4-butanetetracar-boxylic acid with sodium hypophosphite mono hydrate (2:1 ratio) as a catalyst followed by heat treatment of the membrane at 180 °C for 2 min. Gao et al.[5] added 15% of N,N′-trimethylene-bis[2-(vinylsulfony) acetamide] as a cross-linking agent in 3% acetic acid and heat cured the sample at 150 °C for 10 min and achieved a material that was stable in hot water for 2 h.

This study outlines methods of stabilizing PVA suit-able for scale up using cross-linking agents. The reactions were performed without the use of a catalyst to prevent degradation due to high acidity or changes in viscosity caused by untimely cross-linking prior to electrospinning. Three cross-linking agents including two small molecules (maleic acid and citric acid) and a polymer (polyacrylic

acid) were investigated. The stabilized electrospun PVA membranes were then evaluated for use in two potential applications including water remediation for metal ion uptake and for ammonia adsorption after growing model metal organic frameworks (MOFs) onto the electrospun fibers.

2. Experimental Section

2.1. Materials

Polyvinyl alcohol (MW 100 000 g mol−1, Chem-Supply), maleic acid

(Sigma-Aldrich), citric acid anhydrous (LR grade, Chem-Supply), 25% polyacrylic acid in water (MW 345 000 g mol−1, Polysciences,

Inc.), copper acetate (CuAce, Acros Organics), 1,3,5-benzenetri-carboxylic acid (BTC) (Acros Organics), ethanol (AR grade Merck), zinc chloride (ZnCl2, Sigma-Aldrich), copper (II) chloride (CuCl2,

Sigma-Aldrich) and ammonia (NH3, Sigma-Aldrich), super

acti-vated carbon (Product No. 0530HT, average particle size <100 nm, SkySpring Nanomaterials, Inc.) were used.

2.2. Methods

2.2.1. Electrospinning and Stabilization of PVA

PVA was dissolved in deionized water and stirred on a hot plate stirrer at 60 °C for 3 h. Maleic acid, citric acid, and 25% polyacrylic acid (PAA) solution were respectively added to the PVA solutions and mixed at room temperature prior to electrospinning. Three different formulations (citric acid/PVA, maleic acid/PVA, and PAA/PVA) dissolved in water were electrospun by using the fol-lowing experimental setup. The setup consisted of a spinneret (23 G), a 5 mL syringe, a syringe pump (model NE-1000 New Era Pump Systems, Inc., Farmingdale, NY, USA), a rotating drum collector, and a high voltage supply (Spellman high voltage DC power supply, USA). An applied voltage of 20 kV was used with a 15 cm spinneret-to-collector distance. Aluminum foil was wrapped around a drum collector rotating at 30 mm s−1. After

electrospinning the membranes were thermally cured at 130 °C for 30 min in an oven. The stability of the cross-linked mem-branes was tested by immersing a small piece of the membrane into a bath of deionized water for 24 h at room temperature. The membranes were dried before observing their integrity by scan-ning electron microscopy (SEM).

2.2.2. Characterization of PVA Membranes

The SEM analysis was carried out by using a Philips XL30 field emission scanning electron microscope to observe fiber mor-phology and the presence of copper benzene-1,3,5-tricarboxylate (HKUST-1) crystals. The membrane samples were mounted onto an aluminum stub using conductive carbon tape and coated with iridium via sputter coating (50 mA for 45 s). The SEM images were taken at an acceleration voltage of 2 kV. Infrared spectra were carried out using a Nicolet 6700 Fourier transform infrared spectroscopy (FTIR) (Thermo Scientific) with the setting of absorbance mode and 32 scans, to monitor cross-linking of maleic acid, citric acid, and PAA with PVA, respectively.

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2.2.3. Uptake of Zinc and Copper by Stabilized Membranes

Zinc chloride and copper chloride were used as model metal spe-cies to test the metal ion uptake capacity of the membranes. Individual 10 ppm standard solutions of Zn and Cu, and a third solution of a mixture of both Zn and Cu were prepared in deion-ized water for assessing uptake capacity. Circular discs (25 mm) of the electrospun membranes were punched out after curing and weighed before placing into 10 mL of each of the three solu-tions and placed onto orbital shaker and gently shaken for 16 h. The solution concentration of the different metal species was measured by inductively coupled plasma using a Varian 730-ES Axial before and after immersion. The uptake capacity for each metal species and the mixture of zinc and copper were then calculated.

2.2.4. Growing HKUST-1 onto the Stabilized Membranes The HKUST-1 was grown onto the membrane by immersing the stabilized PVA membranes into a bath of saturated copper ace-tate (CuAce) for 5 min and then rinsing in ethanol for 1 min, and immersing into 1 × 10−3 m BTC for 5 min followed by rinsing in

ethanol for 1 min as shown in Figure 1. This cycle was repeated five times.

2.2.5. Ammonia Adsorption Jar Test

A modified 4.5 L polyethylene jar with a wide top was used as the test jar. Two holes were drilled into the lid, one in the middle and the second hole was located halfway between the lid center and the edge of the lid. The Dräger tube is inserted through the middle hole and sealed in with the silicone sealant. The wire was passed through the second hole into the jar with a stainless steel mesh attached to the wire end hanging ≈10 cm from the top of the jar. The electrospun sample (≈20 mg) was secured to the stainless steel mesh. A sample schematic is shown in Figure 2. The sample was weighed accurately before and after ammonia absorption. The sample and a known amount of ammonia (using 3 mL of 0.08 wt% NH3) were added to the jar and sealed using parafilm.

The system was allowed to equilibrate for 2 h with gentle shaking in the first hour prior to measuring residual NH3 content using a

Dräger tube. The residual ammonia in the jar was sampled after 2 h during a Dräger tube and pump system. Residual NH3 can be

read directly from the tube by a color change on the Dräger tube. The result is calculated based on residual NH3 per gram of sample.

Figure 2 illustrates the steps of the ammonia adsorption test.

3. Results and Discussion

3.1. Cross-Linking of Electrospun PVA Membranes

The chemical structures of the three additives used in this study are shown in Figure 3. They all have carboxylic Figure 1. Schematic description of the four-step process for the

growth of HKUST-1 onto stabilized PVA membrane surfaces.

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acid groups which can be used to react with the hydroxyl groups on the PVA to form ester bonds.

The fabrication of electrospun cross-linked PVA mem-branes by the addition of polycarboxylic acid cross-linkers has been achieved by several researchers.[2,3,5,11,15] Although to the best of our knowledge, there has been no report on the use of citric acid as a cross-linking agent of electrospun PVA membranes. Xiao et al.[3] was able to make water stable PAA–PVA composite by electrospin-ning a 10% polymer solution of a 1:1 ratio of PAA–PVA and curing at 130 °C for 45 min. The cured 1:1 PAA–PVA membrane was used to the extract copper ions.

As there was no current literature on the cross-linking of electrospun PVA by using citric acid, a systematic study was carried out on the amount of citric acid required to achieve adequate cross-linking. Citric acid was added to a 10% PVA solution in incremental amounts based on the weight of PVA and mixed at room temperature until the components completely dissolved. Table 1 summarizes the water stability tests of the cured PVA–citric acid branes as well as the PVA control. The PVA only mem-brane dissolved rapidly with no solid material remaining after 24 h. Increasing the amount of citric acid increased the stability of the cured PVA in aqueous solutions. PVA membrane with 5% citric acid swelled immediately and finally disintegrated after 24 h with only parts of the membrane remaining intact. An acceptable level of water stability was only achieved with the addition of 20% citric acid with the membrane staying intact after 48 h

incubation in water. Similar results were obtained with the 30% citric acid sample. SEM images provided more detailed confirmation of the stability as evidenced from its structure integrity (Figure 4).

The un-crosslinked and the 30% citric acid–PVA cross-linked membranes were analyzed by FTIR. The PVA peak at 1734 cm−1 (red) due to CO stretching was shifted to 1713 cm−1 (aqua) for the cured citric acid–PVA mem-brane due to ester bond formation as shown in Figure 5. A similar IR peak shift in relation to esterification has also been observed by Thomas et al.[16] who prepared porous citric acid–PVA scaffolds for use as biomaterials. They also observed a reduction in the OH band around the 3200 cm−1 and an intense CO stretch at 1715 cm−1 attrib-uted to ester bond was formation.

In a separate study, Sivakumar et al.[17] combined metal nitrates, PVA, and citric acid via a sol–gel method to grow cobalt ferrite magnetic particles. They proposed that the addition of citric acid resulted in two possible cross-linking pathways inter and intra molecular shown in Figure 6a.

Similarly, systematic studies were carried out for maleic acid–PVA and PAA–PVA, including water solubility testing and SEM imaging. For both malic acid and PAA, a minimum of 20% based on PVA mass was required to achieve stable cross-linked PVA membranes. The reaction between maleic acid and PVA has been reported by Qin and Wang[2] (reaction scheme shown in Figure 6). They used sulfuric acid as a catalyst and to adjust the pH to 5. They reported under these conditions only 10% maleic acid was required to achieve stable cross-linking. Unlike in this study, without the use of the sulfuric acid cata-lyst the quantity of maleic acid required for water stable membrane formation increased from 10% to 20%. IR anal-ysis also confirms the formation of cross-linking bond formation between maleic acid and PVA. Figure 7 shows the IR spectra of maleic acid (red), PVA (light blue), maleic acid–PVA membrane before curing (dark blue), and the cured maleic acid–PVA membrane (green). The maleic acid ester-carboxyl conjugate of cross-linked PVA shows a band at a lower wavenumber of 1713 cm−1 compared to Figure 3. Three additives used for cross-linking of PVA: a) citric acid, b) maleic acid, and c) polyacrylic acid.

Table 1. Stability of citric acid–PVA in water at room temperature.

Membrane type Weight loss after 24 h in water [%]

PVA only 100 (dissolves instantly)

PVA with 5% citric acid 73

PVA with 10% citric acid 34

PVA with 20% citric acid 3

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that of PVA at around 1733 cm−1 due to conjugation with CC of the maleic acid. Similar changes to peak wave-numbers have been observed by Qin and Wang.[2]

The chemical reaction between PVA and PAA is shown in Figure 8, indicating the ester cross-links between the two polymer chains. Unlike, the IR study performed by Xiao et al.[3] for reaction between PAA and PVA who found no difference between the CO peaks at 1710 cm−1 before and after curing, a slight shift of the same peak from 1708 cm−1 before curing to 1706 cm−1 after curing was observed in our study (Figure 8).

3.2. Uptake of Copper and Zinc

The three types of cross-linked electrospun PVA mem-branes (20% citric acid–PVA, 20% maleic acid–PVA, and 20% PAA–PVA) were evaluated for their ability to take up copper and zinc from single metal CuCl2(aq) and ZnCl2(aq) solutions (10 ppm) and from the mixture of CuCl2(aq) and ZnCl2(aq) (10 ppm for each in the same solution).

Table 2 lists the capacities of the three different electro-spun membranes toward the different metal solutions. The membrane with the highest capacity membrane for both Figure 4. SEM images of citric acid–PVA membranes before and after water immersion.

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Figure 6. Cross-linking reactions: a) between citric acid and PVA, b) between maleic acid and PVA, and c) between PAA and PVA.

Figure 7. FTIR spectra of maleic acid (d), PVA membrane (b), maleic acid–PVA membrane before curing (c), and the cured maleic acid–PVA membrane (a).

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Cu and Zn is the PAA–PVA cross-linked membrane. Copper uptake was higher than zinc in the single metal solutions as well as the mixed metal solution. The citric acid–PVA membrane has preference for copper compared to zinc but the Cu uptake is low. The maleic acid–PVA membrane has low metal uptake relative to the PAA–PVA citric acid–PVA systems. The higher Cu uptake capacity

than Zn by PAA–PVA membrane agrees with the findings of Xiao et al.,[3] who found higher selectivity toward Cu compared to Ca ions. They attributed the uptake capacity of PAA–PVA mem-brane for copper is mainly attributed to the free carboxyl groups. The measured capacity of their membrane with 50% PAA was 0.142 mmol-Cu/g compared to our capacity of 0.110 mmol-Cu/g with 20% PAA which correlates approximately to amount of PAA in their membrane.

The capacity of the PAA–PVA membrane maybe due to the greater number of free carboxylic acids compared to the citric acid and maleic acid-based membranes as well as the close proximity of the alcohols groups from the PVA backbone. Further work is required to elucidate the reasons for the higher uptake of copper compared to zinc in the case of PAA–PVA and the higher uptake compared to the citric and maleic acid-based membranes. 3.3. Growth of HKUST-1

MOFs are crystalline nanoporous materials composed of metal clusters connected by multifunctional organic linkers. MOFs have sparked a large interest in the fields Figure 8. FTIR spectra of PAA (d), PVA (c), PAA–PVA membrane before curing (b), and the cured PAA–PVA membrane (a).

Table 2. Metal uptake capacity of cross-linked PVA membranes (mg metal per g membrane).

Membrane type/capacity 20% Citric

acid–PVA

20% Maleic acid–PVA

20% PAA–PVA

Cu2+ (mg g−1) from Cu only solution 1.04 1.49 7.11

Zn2+ (mg g−1) from Zn only solution 0 0 4.23

Cu2+ (mg g−1) from Cu + Zn solution 0.27 1.73 5.04

Zn2+ (mg g−1) from Cu + Zn solution 0 0 1.80

Figure 9. Photographs and SEM images of the a) cross-linked citric acid–PVA membrane and b) HKUST-1 growth on the citric acid–PVA membrane (20 µm scale bar for SEM images).

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of adsorption and catalysis where advantage is taken of their internal porosities and extremely large surface area.[18] The growth of HKUST-1 onto a range of smooth surfaces such as polymeric, gold, ceramic, and quartz have been studied.[19] Using a step-by-step process, Nan et al.[20] grew HKUST-1 onto porous α-aluminum discs, using 1:1 water:ethanol solvent system in a multistep process. In this work, HKUST-1 was grown on the surface of the three cross-linked electrospun membranes using ethanolic solu-tions. The color of the PVA–PAA membrane changed from off-white to light green after the first cycle and by the fifth cycle the membrane became completely blue-green as shown in Figure 9.

The SEM images of three different membranes after five cycles of treatments are shown in Figure 10. It is clear from the SEM images that microparticles are present on all three PVA cross-linked electrospun membranes with the cross-linked citric acid–PVA membrane having the

greatest particle coverage compared to the cross-linked maleic acid–PVA or PAA–PVA membranes.

Infrared spectroscopy was used to confirm the growth of HKUST-1 on the citric acid–PVA membrane as shown in Figure 11. The most important peak on the spectra is at 729–730 cm−1 showing the presence of the benzene ring. Other significant peaks are at 761 and 1372–1448 cm−1. This agrees with IR analysis by a different study stating that the symmetric stretching of the carboxyl group in the HKUST-1 is detected between 1384 and 1405 cm−1.[21]

Results of residual ammonia per unit mass of adsorbant are listed in Table 3. The lower the residual ammonia values the greater the adsorbing capacity. Carbon nanopowder and HKUST-1 were included as a control for comparison purposes. HKUST-1 is much better at adsorbing ammonia than carbon nanopowder. The control membranes (without HKUST-1) were also Figure 10. HKUST-1 growth on a) citric acid–PVA, b) maleic acid–PVA, and c) PAA–PVA membranes.

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able to adsorb ammonia, both having values better than that of carbon nanopowder, although not quite as good as pure HKUST-1 MOF crystal. It should be noted that HKUST-1 adsorbs ammonia in the pores of its crystal lat-tice whereas for the electrospun membranes the mecha-nism is via H-bonding and/or ionic interactions to the carboxylic acid groups on the nanofibrous surface. In each case the HKUST-1 electrospun membrane adsorbed more ammonia than the electrospun membrane only system. This is because ammonia is capture via both mechanisms, adsorption into the HKUST-1 crystal lat-tice as well as interaction with the nanofiber surface. The addition of the MOF HKUST-1 to the nanofiber improved the adsorption of ammonia compared to only the electro-spun system. The lowest residual ammonia results were for HKUST-1 crystal and HKUST-1 decorated on the citric acid–PVA membrane which corresponded to a maximum removal of ≈87%.

4. Conclusions

Three different cross-linked PVA electrospun membranes namely citric acid–PVA, maleic acid–PVA, and PAA–PVA were successfully fabricated with good water stability. The electrospun membranes were produced by adding citric acid, maleic acid, and PAA to PVA solution prior to electrospinning followed by heat treatment to initiate the cross-linking process. The process is easy to scale up, does not need any catalyst which needs to be removed down-stream. Both citric acid–PVA and PAA–PVA cross-linked systems have been tested using a pilot-scale electro-spinner. A minimum of 20% cross-linking agent based on PVA weight is necessary to achieve good water. Their sta-bility in ethanol is evidenced through taking these mem-branes through layer-by-layer growth of HKUST-1 using ethanolic solutions of both precursors. The PAA–PVA membrane performed the best for the uptake of copper and/or zinc from aqueous systems while citric acid–PVA membranes provided the best surfaces for the growth of the MOF HKUST-1 as observed in SEM analysis (Figure 10).

Acknowledgements: This work was supported by CSIRO’s Manufacturing business unit.

Conflict of Interest: The authors declare no conflict of interest. Received: January 12, 2017; Revised: May 1, 2017; Published online: July 3, 2017; DOI: 10.1002/mame.201700024 Keywords: copper benzene-1,3,5-tricarboxylate; cross-linking; electrospinning; HKUST-1; metal organic frameworks; polyacrylic acid; polyvinyl alcohol

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Table 3. Ammonia adsorption results by different adsorbents.

Sample type Residual NH3 per 1 g of adsorbent [ppm] % Removal NH3

Negative control 9800 0

Carbon nanopowder 5330 45

HKUST-1 crystals 1184 87

PAA–PVA membrane 2631 73

HKUST-1 decorated PAA–PVA membrane 1490 85

Citric acid–PVA membrane 1962 80

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