Ag Nanoparticle-Bearing Poly(vinylidene fluoride) Nanofiber Mats as

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Ag Nanoparticle-Bearing Poly(vinylidene fluoride) Nanofiber Mats as Janus Filters for Catalysis and Separation Lei Miao, Guojun Liu, and Jian Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20759 • Publication Date (Web): 28 Jan 2019 Downloaded from http://pubs.acs.org on January 29, 2019

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ACS Applied Materials & Interfaces

Ag Nanoparticle-Bearing Poly(vinylidene fluoride) Nanofiber Mats as Janus Filters for Catalysis and Separation Lei Miaoa,b, Guojun Liub,*, and Jiandong Wangb a Foshan b

University, 18 Jiangwan 1st Road, Foshan, Guangdong, P. R. China, 528000

Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6

Correspondence author: Guojun Liu (Tel: 613-533-6996 E-mail: [email protected]) Abstract.

Hydrophilic Ag nanoparticles were pattern-deposited onto one side of an

electrospun poly(vinylidene fluoride) (PVDF) nanofiber mat, yielding a Janus filter.

The

filter was used to separate a receiver cell from a reactor cell that contained an aqueous 4nitrophenol/NaBH4 solution.

Upon contact with this solution, the Ag nanoparticles catalyzed

the reduction of 4-nitrophenol to 4-aminophenol.

After the reaction reached completion,

ethyl acetate was added into the reactor to extract the product.

During this process, the ethyl

acetate containing 4-aminophenol also selectively permeated regions of the hydrophobic yet oleophilic PVDF mat that were not covered by Ag nanoparticles. after the evaporation of ethyl acetate.

The product was obtained

This paper demonstrates the first use of a Janus

membrane in a catalytic separatory reactor that catalyzes a chemical reaction and then facilitates the eventual separation of the formed product via filtration.

Keywords:

Janus Membranes, Janus Filters, Nanoparticles, Catalysis, Separatory Reactor,

and Separation.

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I.

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Introduction

Hydrophobic filters that reject water but allow the selective permeation of water-insoluble organic solvents (oils) are highly useful for various applications.

For example, large barrels

bearing openings covered by such filters can be used to collect crude oil from the surface of a lake or an ocean after an oil spill.1-3

Such a filter can also be used in the design of a separatory

reactor that automatically separates hydrophobic products formed from a reaction mixture in water.4-5 A Janus filter, possessing two sides that have different wettability or serve different functions, offers numerous advantages over superhydrophobic filters in many applications.6-7 For example, a Janus filter possessing both a hydrophobic and a hydrophilic side, respectively, induces a unidirectional force that draws water droplets from the hydrophobic side, where the water droplets bead up and possess a higher internal Laplace pressure, to the hydrophilic side where water spreads.8

As a consequence of this force, such a filter collected water from

condensing fog more efficiently than traditional homogeneous hydrophobic or hydrophilic filters.9

Such a force has also been leveraged to induce oil migration across a Janus filter from

the oleophobic to the oleophilic side, facilitating the more efficient or cleaner separation of oil from water than was achievable with a conventional homogeneous superhydrophobic filter.10 Such a high efficiency was also observed during the separation of oil from oil-in-water emulsions using Janus filters that possessed both a hydrophilic de-emulsifying side and a superhydrophobic side.11-13

Aside from the normal inner Laplace force from the hydrophilic

to the hydrophobic side for these filters, the additional de-emulsifying function facilitated the coalescence of emulsified oil droplets and enabled oil separation at filter pore diameters that were orders of magnitude larger than the emulsion droplets prior to their coalescence. Another example includes the stacking of two filters with one filter capable of catalyzing the photo-degradation of water-soluble dyes and the other allowing the selective permeation of oil

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and the subsequent use of the resultant dual-layer Janus filter to not only remove oily contaminants from water but also photo-decompose water-soluble dyes.14

An additional

example invoked the use of a Janus filter with the hydrophilic side bearing an enzyme in contact with an aqueous reaction mixture and the water-impermeable superhydrophobic side exposed to CO2 or air.15-17

Such a design greatly promoted gas diffusion to the enzyme and increased

the rate of enzyme-catalyzed reactions that required CO2 or oxygen as a substrate. Despite these prior examples, we are not aware of reports on the use of Janus filters in the design of a catalytic separatory reactor that catalyzes a chemical reaction and then facilitates the eventual separation of the formed product via filtration.

Such a filter could have a

hydrophobic side, for example, which would allow the selective permeation of hydrophobic products or those dissolved in a hydrophobic solvent, while the other side would facilitate catalysis.

In this paper, we report the fabrication of a Janus filter consisting of an electrospun

hydrophobic poly(vinylidene fluoride) (PVDF) nanofiber mat bearing pattern-deposited Ag nanoparticles on one side.

We also discuss the use of the filter as a catalytic separatory reactor.

Such a reactor was constructed by separating a receiver cell from the reactor cell with our filter. Upon contact with an aqueous 4-nitrophenol (4-NP)/NaBH4 solution, the Ag nanoparticles catalyzed the reduction of 4-NP to 4-aminophenol (4-AP).

After the reaction reached

completion, ethyl acetate was added into the reactor to extract 4-AP.

While the ethyl acetate

could not permeate the water-impregnated Ag nanoparticle layer, it selectively permeated the Ag-free regions that consisted solely of PVDF.

Thus, the Janus filter served dual roles,

facilitating both catalysis and the separation of the extracted product by filtration. The reduction of 4-NP to 4-AP was chosen as the model reaction because this reaction has been frequently used in the past to assess the catalytic function of silver nanoparticles.18 Further, 4-AP is used as a photographic developer and a corrosion inhibitor and is the final precursor to the drug acetaminophen.

Additionally, 4-NP is a pollutant of environmental

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concern. To prepare the targeted Janus filter, we first fabricated a PVDF nanofiber mat via electrospinning (Scheme 1A).19-20

PVDF was selected for this role due to its chemical

inertness, low surface tension of 30.3 mN/m, and insolubility in a wide range of organic solvents.21-22

To produce Ag nanoparticles, we electro-sprayed a SnCl2 solution through a

mask onto a fiber mat (Scheme 1B).

After mask removal and solvent evaporation (Scheme

1C), the SnCl2-bearing mat was soaked in a Ag+ solution to reduce the Ag+, yielding Ag nanoparticles near the deposited SnCl2 (Scheme 1D)23 and leaving the uncoated regions free for the permeation of organic solvents.

The Ag nanoparticles were securely attached to the

mat by heating the Janus filter (hereon called Ag-mat) at 150 °C, which was close to PVDF’s melting temperature of 160 °C.24

Scheme 1. Pathway toward a nanofiber mat bearing pattern-deposited Ag nanoparticles: (A) PVDF mat production via electrospinning; (B) SnCl2 spraying onto the PVDF fiber mat under a star-shaped paper mask; (C) mask removal to yield a mat bearing a SnCl2 pattern; (D) soaking in a Ag+ solution to yield a mat patterned by Ag nanoparticles; and (E) thermal treatment to stabilize the Ag nanoparticle pattern. We note that there have been many reports on electrospun nanofiber mats supporting noble metal nanoparticles.25-29

However, most of these prepared composite mats were

subsequently immersed into reaction mixtures and used as supported catalysts to facilitate reactions and catalyst recovery.

In one case, a reaction mixture was passed through such a

mat to effect catalysis without separating this mixture from its products.30

Thus, the use of a

mat bearing noble metal nanoparticles in a catalytic separatory reactor design is unprecedented.

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II. Materials.

Experimental Section

PVDF pellets (Mw = 275 kDa and Mn = 107 kDa), tin(II) chloride (reagent

grade, 98%), silver nitrate (ACS reagent), 4-NP ( ≥ 99.5 %), 4-AP (≥ 98%), acetone (ACS reagent, 99.5%) and hexane (mixture of isomers, ≥ 98.5% by HPLC) were purchased from Sigma-Aldrich.

Ethyl acetate (reagent grade), ethanol (anhydrous), and N,N-

dimethylformamide (DMF, reagent grade) were purchased from Caledon Laboratories Ltd. (Ontario, Canada).

Ethyl acetate was distilled prior to use.

Exception of ethyl acetate, all

of the chemicals were used as received without further purification. PVDF Nanofiber Mats. To prepare a PVDF nanofiber mat, 0.90 g of PVDF pellets and 4.1 g of an acetone and DMF mixture at v/v = 3/7 were stirred at 120 °C to dissolve the polymer. The resultant yellow polymer solution was placed into a desiccator during subsequent cooling to room temperature to minimize moisture uptake.

The PVDF solution (4.0 mL) was then

transferred to a syringe with a 22-gauge stainless steel needle and this syringe was mounted into a pump.

To achieve electrospinning, a voltage of 12.0 kV was applied between the needle

and a smooth aluminum foil with dimensions of 9.0 cm × 9.0 cm that were placed 15.0 cm apart.

The PVDF solution was then pumped from the syringe at a rate of 0.25 mL/h.

The

thickness of the electrospun nanofiber mat was governed by the duration of the electrospinning time.

After 16 h of electrospinning, the fabricated mat had a thickness of ~ 250 μm.

Patterned Deposition of SnCl2.

Immediately after the electrospinning of a PVDF mat,

four hollow five-pointed stars that had been cut from paper were placed on the four quadrants of a nanofiber mat as masks. 1.0 mm, respectively.

The height and sideline width of such a star were 23.0 mm and

To the masked mat was then electrospun 0.50 g 2.6 × 10-3 mol of SnCl2

that was dissolved in 10 mL of ethanol.

During this deposition, the distance and voltage drop

existing between the needle and the Al foil were 15.0 cm and 15.0 kV, respectively. Ag Nanoparticle Formation.

The SnCl2-bearing nanofiber mat was cut into four 5



sections with dimensions of 4.5 cm × 4.5 cm.

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Ignoring loss due to the landing of the electro-

sprayed SnCl2 solution outside of the area covered by the nanofiber mat, we calculated a SnCl2 amount of 0.125 g or 6.6 × 10-4 mol on each of the mat sections.

Each mat piece was then

soaked for 24 h without stirring in 225 mg or 1.32 × 10-3 mol of AgNO3 dissolved in 30 mL of water.

After a mat section was removed, it was carefully rinsed with de-ionized water at least

thrice to remove residual Ag+, Sn2+, NO3-, or Cl-.

Lastly, the sections were placed in an oven

at 150 °C for 20 min to sinter the formed Ag nanoparticles and to strengthen their adhesion to the mat. SEM Analysis.

Scanning electron microscopy (SEM) images were obtained using

mostly secondary electrons on a FEI Quanta 250 instrument that was operated in the environmental mode at an accelerating voltage of 10 kV.

Energy dispersive X-ray

spectroscopy was performed with an EDAX Element detector. Areal Ag Loading Density.

To determine the areal loading density of Ag, a piece of

Ag-coated PVDF nanofiber mat sized at 2.0 cm × 2.0 cm bearing no uncoated regions was immersed into 20 mL of DMF at 120 °C.

After the PVDF mat had completely dissolved, the

DMF solution was carefully discarded, leaving behind the black Ag nanoparticle powder. The powder was washed with DMF twice and then with acetone twice.

The cleaned Ag

nanoparticle powder was dried at 120 °C for 30 min before its mass was determined gravimetrically and used to calculate the mass of Ag that had been loaded per unit area of PVDF mat. 4-NP Reduction and Separation.

The H-shaped cell shown in Figure 1 was used to

demonstrate the catalytic and separation functions of the Ag-mats.

The two half-cells of the

device were connected via two perforated rubber disks and were held in place with a clamp. Sandwiched between the perforated disks was a PVDF mat bearing one hollow star pattern that was surrounded by Ag nanoparticles.

The side of the mat bearing this Ag nanoparticle pattern

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was kept in contact with the reaction mixture and the effective contact area of the mat with this reaction mixture was 3.8 cm2.

Figure 1. Photographs showing a catalytic separatory reactor in action: A) 0 and B) 90 min after the addition of an aqueous 4-NP (25.0 μg/mL) and NaBH4 solution (1.5 mg/mL) into the left half-cell; C) 0 and D) 30 min after the addition of 10-mL ethyl acetate into the left half-cell. To perform the catalytic reduction of 4-NP, 20.0 mL of an aqueous solution containing 4NP at concentrations of 25.0, 37.5, or 50.0 μg/mL and NaBH4 at a concentration of 1.50 mg/mL was added into the left half-cell (the reaction cell) of the catalytical separatory reactor.

To

ensure data reproducibility among the different runs, we tried our best to ensure that the reaction cell was placed at the same position relative to the stirring hot plate, that the stirring was performed using the same speed of 1000 rpm, and that the same magnetic stirring bar (with a diameter and length of 0.50 and 3.0 cm, respectively) during each run.

At 15 min time

intervals, aliquots of 0.50 mL were retrieved from the reactor and added into 5.0 mL of deionized water.

The absorbances of the diluted samples at 400 nm were recorded using a

UV-vis spectrometer (Cary 300 Bio, Varian) to monitor the progress of the reaction. To demonstrate 4-AP extraction by ethyl acetate, a 20.0 mL aqueous solution containing 4-NP at 50.0 μg/mL and NaBH4 at 1.50 mg/mL was prepared in the reaction cell.

After

stirring overnight at 1000 rpm to fully reduce 4-NP to 4-AP, 10 mL of ethyl acetate was poured into the cell.

After ~ 5 mL of ethyl acetate containing the extracted 4-AP was collected in the

receiving cell, it was transferred into a graduated cylinder to allow volume determination and

7



then into a spectrophotometric cell for absorbance analysis at 310 nm.

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Meanwhile, another

5.0 mL of ethyl acetate was added into the reaction cell to replenish the ethyl acetate.

This

process of ethyl acetate removal, absorbance analysis, and addition was repeated until more than 90% of the product was extracted from the reaction cell. Solvent Flux Measurements. To determine the flux of chloroform, ethyl ether, or ethyl acetate across the Ag-mat, 20.0 mL of deionized water was added into the left half-cell under stirring at 1000 rpm prior to the addition of 5.0 mL of one of the solvents.

Some time was

required for an added solvent to wet the PVDF fabric before permeation occurred and this delay time varied from solvent to solvent.

To make it more consistent for each solvent, the PVDF

mat was wet with 50 μL of hexadecane that was dispensed onto the PVDF side of the mat using a syringe with a bent needle.

This wetting triggered the immediate permeation of the

extracting solvent and timing thus started immediately after the application of the hexadecane. To minimize solvent evaporation, the two half-cells were covered by parafilm.

The volume

of the solvent that had permeated to the other side was measured 5 min later and was used to calculate the mean solvent flux using the known liquid-contacting membrane area of 3.8 cm2 (permeation area was only ~ 22% of this area) and permeation time of 5 min.

The

reproducibility of the flux data was evaluated by repeating the experiment 7 times. Contact Angle Measurements. Static water contact angles (CAs) were measured using a TBU 90E optical contact angle goniometer (Dataphysics, Germany).

A liquid droplet

volume of 5.0 µL was used for these measurements and a picture was taken of a droplet ~ 5 s after its dispensing from a syringe.

The CA was automatically determined by the instrument

using OCA 15Pro software (Dataphysics, Germany) by fitting the droplet cross-section to a circular shape.

The reported values represented the averages from at least 10 parallel

measurements.

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III. Nanofiber Mats.

Results and Discussion

PVDF nanofiber mats were fabricated by electrospinning a 18.0%

(m/m) PVDF sample that had been initially dispersed in DMF/acetone (v/v = 7/3).

While

PVDF was soluble in polar solvents such as DMF, DMSO, and N,N-dimethylacetamide, the non-solvent acetone was added into DMF mainly to expedite solvent evaporation during electrospinning, which ensured the rapid vitrification of the viscous electrospun threads.31 Figure 2 shows top-down and cross-sectional SEM images of a PVDF nanofiber mat. The average diameter of the PVDF nanofibers as estimated from Figure 2A was 247 ± 71 nm. Meanwhile, the mat thickness as estimated from the drawn line in Figure 2B was 229 ± 12 µm.

Figure 2.

A) Top-down and B) cross-sectional SEM images of a PVDF nanofiber mat.

Patterned Deposition of SnCl2. mat via electro-spraying.

SnCl2 was deposited onto one side of a PVDF nanofiber

Ethanol was utilized as the spraying solvent, which selectively

dissolved SnCl2 but not PVDF.

We discovered that the resultant SnCl2 salt after ethanol

evaporation adhered well to the PVDF mat only if the SnCl2 solution was electro-sprayed immediately after the fabrication of a PVDF mat before the mat was completely dry.

We

suspect that some SnCl2 had diffused into the swollen fibers and eventually underwent

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crystallization within these fibers to form partially-embedded particles that offered stronger adhesion to the PVDF fibers.

To selectively deposit SnCl2 in only certain regions on one face

of a PVDF mat, five-pointed hollow stars cut from paper were used to mask the mat prior to the electro-spraying procedure. Figure 3A shows a SEM image of one region of a SnCl2-bearing mat.

The white triangle

represented one corner of the hollow center on which SnCl2 had been deposited.

The darker

region around this corner corresponded to the SnCl2-free domain which was originally protected by the border of the paper star.

This result was clearer in Figure 3B, which showed

a distribution map for the elements Sn and F that was obtained via energy dispersive spectroscopy (EDS).

These images confirmed the deposition of SnCl2 into the hollow center

of the star-shaped mask.

Figure 3. SEM and EDS mapping images a PVDF mat after the patterned deposition of SnCl2: A) an SEM image, B) an EDS map, C) and D) SEM images of SnCl2-covered regions, and E) an SEM image of a SnCl2-free region. Figures 3C-3E show enlarged SEM images of two SnCl2-coated regions and a SnCl2-free 10


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region.

In the region shown in Figure 3C, SnCl2 seemed to have formed a conformal coating

surrounding the PVDF fibers.

These composite fibers had an average diameter of 343 ± 73

nm, which was substantially larger than that of 247 ± 71 nm for the pristine PVDF nanofibers shown in Figure 2A or in Figure 3E depicting a SnCl2-free region.

In the regions where more

SnCl2 was deposited (Figure 3D), SnCl2 formed a defective layer, blocking most of the interfiber pores.

The SnCl2 spraying was non-uniform probably due to the intrinsic non-

uniformity of the spray.

Alternatively, the seeping of the sprayed SnCl2 solution into the

matrix in selected regions could also be responsible for the observed differences in different regions. PVDF Mats Bearing Pattern-Deposited Ag Nanoparticles.

SnCl2 is normally used to

reduce Ag+ to yield seed Ag nanoparticles on surfaces and the seed particles are then used to catalyze the further growth of the original seed particles via the reduction of Ag+ with another reducing agent.23, 32

The second reducing agent can be an aldehyde, for example, and is

typically so chosen that its oxidized product is readily removed from the system after the reaction.

We chose to use only SnCl2 to reduce Ag+ for convenience.

Indeed, such a

protocol would yield Ag nanoparticles that co-existed with the side products that were probably SnO2 or Sn(OH)4.33-35

However, this non-ideality did not interfere with the demonstration of

the performance of a separatory reactor, the primary goal of this research, because Ag nanoparticles thus produced were effective in catalyzing the reduction of 4-NP.36 Figures 4A and 4B respectively show photographs of the Ag-bearing and Ag-free sides of a Ag-mat.

The dark regions on the Ag-bearing side (Figure 4A) corresponded to those

covered by the reduced Ag nanoparticles, which were formed in regions that were originally laden with SnCl2.

Silver occurred only in regions where SnCl2 was deposited despite the

solubility of SnCl2 in water (178.0 g in 100.0 g of water at 10 °C),37 probably because the dissolution of SnCl2 was a slower process than Ag+ diffusion and was the rate-determining step 11



for Ag nanoparticle formation.

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The other side of the mat appeared essentially white (Figure

4B), suggesting that the electro-sprayed SnCl2 could not permeate the mat probably due to the rapid evaporation of ethanol during the electrospraying of SnCl2.

Thus, the mat was indeed

a Janus material as predicted.

Figure 4. Photographs of a Janus PVDF nanofiber-based mat bearing pattern-deposited Ag nanoparticles: viewed from the A) Ag-bearing side and B) the Ag-free side, respectively. The as-prepared Ag nanoparticles lacked adhesion and were easily scraped away with a spatula, for example.

However, these nanoparticles adhered much more strongly to this mat

after it had been heated in an oven at 150 °C for 20 min.

While the melting point of the PVDF

nanofibers was between 160 and 165 °C,38 the melting point of the Ag nanoparticles could be much lower than the temperature of 962 °C for bulk silver.39

Consequently, Ag nanoparticles

with diameters between 52 and 120 nm were observed to fuse together after they had been heated at 120 °C for 75 min.40 Thus, the improved adhesion between the Ag nanoparticles and the mat could be partially due to the sintering of the Ag nanoparticles and also due to the embedding of some Ag nanoparticles into the fibers. To determine the amount of Ag nanoparticles that had been deposited per unit area of PVDF mat, the PVDF fibers were dissolved in hot DMF. measured.

The residual solid mass was then

Dividing the determined solid mass by the area of the covered region thus yielded

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an areal loading density of 2.4 mg/cm2 for Ag and probably SnO2.

Figure 5. SEM images of a PVDF mat after Ag nanoparticle deposition: A) a corner of the central hollow section of the star, B) view of a Ag-covered region, and C) view of a Ag-free region. Also shown is D) an AFM image of a Ag-covered region. Figure 5A shows an SEM image of one region of a Ag-mat.

The contrast between one

of the lighter Ag-bearing corners within the hollow center of the star and the darker Ag-free border in this image was evident.

The differences between these two regions were further

seen in the enlarged images shown in Figures 5B and 5C.

While some of the Ag nanoparticles

in the Ag-laden region appeared to have sintered into plate-like structures (Figure 5B), the masked border was essentially free of Ag nanoparticles (Figure 5C). The AFM image in Figure 5D provided an even closer view of the Ag nanoparticles. Some of the Ag nanoparticles had indeed become fused together.

The distinct Ag

nanoparticles had a diameter of 66 ± 10 nm. Figures 6A and 6B display the F and Ag distribution maps that were determined by EDS

13



on the Ag-coated surface and along the cross-section of the Ag-mat.

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While the top-down

view again confirmed that the deposition of Ag nanoparticles followed the shape of the original mask, the cross-sectional image showed that the Ag nanoparticles were concentrated in a very thin layer and that the filter indeed possessed the predicted Janus structure.

Figure 6.

Ag distributions as mapped by EDS: A) on the surface of a patterned mat and B) across the thickness of the mat.

Wettability of the Ag-Mats. contact angles.

The two sides of our Ag-mat had very different water

On the PVDF and Ag nanoparticle-bearing sides, the values were 133 ± 1°

and 65 ± 3°, respectively (Figures 7A and 7B).

The high contact angle on the PVDF side

should not be surprising because the mat possessed surface roughness and a hydrophobic smooth PVDF film featured a water contact angle of 88°.41 In contrast, the water contact angle on the Ag particles was low because silver had a high surface tension of 1.2 × 103 mN/m,42 which facilitated water spreading.

Figure 7. Photograph of a water droplet on: A) the Ag-decorated side and B) the Ag-free side. Our further observations indicated that organic solvents such as chloroform, ethyl acetate, 14


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and diethyl ether readily wetted the Ag-free side either in the air or underwater, suggesting their ability to permeate across a PVDF nanofiber mat.

However, the organic solvents did not

permeate through a PVDF mat bearing a uniform rather than a patterned Ag nanoparticle layer that had been pre-wetted with water, likely due to the high affinity of the Ag particles for water. Consequently, we pattern-deposited Ag nanoparticles so that organic solvents could permeate through the regions that were free of Ag particles. Permeation Fluxes.

To determine the permeation flux of an organic solvent under the

conditions normally used for product extraction, 5.0 mL of the organic solvent was added into 20.0 mL of water under stirring in the reaction cell and the volume of solvent collected in the receiving cell 5.0 min after the observation of the initial solvent permeation was then recorded. To estimate the effective permeation area of the Ag-mat, we note that the total contact area of the mat with the “extraction” mixture was 3.8 cm2 and only ~ 22% (Figure 4A) of this area was free of Ag nanoparticles and allowed permeation. 0.8 cm2.

Thus, the effective permeating area was ~

Using the effective permeation area and the permeated solvent volume, we

calculated the average permeation fluxes and the resultant data were shown in Figure 8.

Figure 8. Permeation fluxes across a Ag-mat measured for chloroform, diethyl ether, and ethyl acetate under simulated extraction conditions as a function of the number of test runs. The permeation fluxes were reported for each solvent from 7 separate trials. 15


The


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numbers fluctuated somewhat among different runs with the average values of 88 ± 1, 117 ± 2 and 157 ± 2 L/(m2·h) for ethyl acetate, diethyl ether, and chloroform, respectively. Reduction of 4-NP.

The reduction of 4-NP to 4-AP by NaBH4 is well-known.43-45 Past

studies have shown that a noble metal catalyst, such as Au or Ag nanoparticles,46 was required for this process because although the reduction of 4-NP by NaBH4 was spontaneous, this reaction was slow without a catalyst.47

While we fixed the NaBH4 concentration to 1.50

mg/mL, the investigated 4-NP concentrations were 25.0, 37.5 and 50.0 μg/mL, respectively. We confirmed the need for the Ag catalyst under our experimental conditions by showing that the reaction did not proceed within the relevant observation period if the Ag-mat was replaced with a Ag-free PVDF mat.

Figure 9. A) Comparison of UV-Vis absorption spectra of 4-NP and 4-AP in an aqueous NaBH4 solution at 1.50 mg/mL. Shown in B) is the evolution of UV-Vis absorption spectra of a 4-NP/NaBH4 mixture catalyzed by a Ag-mat. The initial 4-NP concentration for the experiment reported in B) was 3.4 μg/mL. The inset shows a photograph of samples collected from the reaction mixture at different times after they were diluted with water by 11.0-fold. UV-Vis spectrophotometric analysis was used to monitor 4-NP reduction.

Figure 9A

compares the absorption spectra of 4-NP and 4-AP in an aqueous NaBH4 solution.

NaBH4

was used to simulate the reaction media, which had a pH of ~10 due to the reaction of NaBH4 with water to generate hydrogen and sodium borate.

In the basic media, 4-NP exhibited a

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strong absorbance at 400 nm.

Conversely, the product 4-AP exhibited an absorbance signal

at 310 nm rather than 400 nm.

Thus, the disappearance of the 4-NP absorption peak at 400

nm was used to monitor the progress of the reaction (Figure 9B). Ag-Mat-Based Reactors.

We demonstrated the feasibility of a Ag-mat-based reactor

design by reducing 4-NP in the cell shown in Figure 1.

The kinetics of 4-NP reduction at

initial 4-NP concentrations of 25.0, 37.5 or 50.0 μg/mL and a NaBH4 concentration of 1.50 mg/mL were monitored via the decreases in the absorbance A of 4-NP at 400 nm. shows how -ln(A) of the three solutions changed with the reaction time.

Figure 10A

As time progressed,

-ln(A) increased or A decreased as predicted.

Figure 10. A) Increases in - ln(A) as a function of reaction time t for 4-NP solutions at different initial concentrations, where A denotes the absorbance at 400 nm. B) Variation in the measured k values as a function of the number of catalytic cycles performed using the same Ag-mat. The initial 4-NP concentration for these experiments was always 50.0 g/mL. The fact that the ln(A) data all fell onto straight lines suggested that this reaction followed pseudo first-order reaction kinetics.

From the fitting of the data, we obtained the apparent

rate constants of (9.2 ± 0.3) ×, (7.8 ± 0.3) ×, and (5.5 ± 0.3) × 104 s-1, respectively, for reactions

17



Page 18 of 24

with the initial 4-NP concentrations of 25.0, 37.5, and 50.0 µg/mL. constant decreased as the initial 4-NP concentration increased.

Thus, the apparent rate

Following these curves, we

estimated that the times required to achieve a 99% reduction of the 4-NP concentrations were 83, 95 and 137 min, respectively, for the three cases. The reaction was highly reproducible.

Figure 10B shows the apparent first-order rate

constants k obtained from 7 repetitive runs at the initial 4-NP concentration of 50.0 µg/mL. The value fluctuated slightly around 5.5×10-4 s-1 among different runs. To explain the k variation trends, we note that 4-NP reduction invoked several elementary reaction steps including BH4- adsorption onto the surfaces of the Ag nanoparticles, its reaction with Ag to generate metal hydride,43 4-NP adsorption onto the surfaces of Ag nanoparticles, its reaction with the metal hydride to generate 4-AP, and the desorption of 4-AP from the metal surface.

Such a reaction could appear to be first order with respect to 2-NP because the

catalytic surface area was constant and the concentration of NaBH4 was much higher than that of 4-NP.

The fact that the apparent k value decreased as the initial 4-NP concentration

increased was likely because the catalytic surface area was constant and it would take a longer time to consume 4-NP as its initial concentration increased. We note that the reduction of 4-NP proceeded faster when catalyzed by a Janus fabric directly placed inside a stirring 4-NP solution.

This reduction in reaction rate catalyzed by

the mounted fabric should be due to the lower accessibility of Ag nanoparticles in the latter case by the reactant (Figure 1) and we anticipate that this rate can be increased with a better reactor design.

For example, the vent for a future reactor can be directly on the wall of a

cylindrical cell rather than at the end of a narrow cylindrical tube connected to the reactor. Product Separation by Ethyl Acetate Extraction and Filtration.

To demonstrate the

separation performance of our separatory reactor, we fully converted 4-NP to 4-AP and then added ethyl acetate in batches of 5.0 mL to extract the product 4-AP.

18


The amount of 4-AP

Page 19 of 24 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


extracted in this manner was determined via absorbance analysis at 310 nm.

In Figure 11, the

yield of separated 4-AP was plotted as a function of the permeated volume of ethyl acetate.

It

was possible to separate over 90% of the product.

Figure 11. Variation in the recovery yield of 4-AP as a function of the separated volume of ethyl acetate.

IV.

Conclusions

PVDF nanofiber mats were prepared via electrospinning.

Electrospraying a SnCl2

solution onto such a mat under a mask facilitated the patterned deposition of SnCl2.

The

deposited SnCl2 reduced Ag+ locally to yield Ag nanoparticles with a distribution pattern that complemented the original mask pattern used for SnCl2 deposition.

Both SnCl2 and Ag were

shown to concentrate on the sprayed side of the PVDF nanofiber mat.

Upon contact with an

aqueous solution of 4-nitrophenol and NaBH4, the Ag nanoparticles catalyzed the reduction of 4-nitrophenol to 4-aminophenol by NaBH4.

After 4-aminophenol formation, ethyl acetate

was added to the reacting cell that was separated from a collecting cell by the PVDF mat 19



bearing the deposited Ag nanoparticle patterns.

Page 20 of 24

Ethyl acetate together with the extracted

product permeated regions of the mat that were uncovered by the Ag nanoparticles.

Thus, an

improved version of such a Ag-mat and other related mats may be promising candidates for use in the design of catalytic separatory reactors in the near future.

Acknowledgement.

NSERC of Canada is thanked for sponsoring this research.

LM

thanks Foshan University for a visiting fellowship that sponsored his visit to Queen’s University and the support provided by the Guangdong Natural Science Foundation (2014A030310426, 2018A030313717).

GL thanks the Canada Research Chairs program for

a Tier I Canada Research Chair in Materials Science.

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Ag Nanoparticle-Bearing Poly(vinylidene fluoride) Nanofiber Mats as

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