Greener and Other Approaches To Synthesize Fe and Pd

Chapter 3

Downloaded by MIT on April 18, 2013 | http://pubs.acs.org Publication Date (Web): April 17, 2013 | doi: 10.1021/bk-2013-1124.ch003

Greener and Other Approaches To Synthesize Fe and Pd Nanoparticles in Functionalized Membranes and Hydrogel V. Smuleac, L. Xiao, and D. Bhattacharyya* Department of Chemical and Materials Engineering, University of Kentucky, Lexington, Kentucky 40506 *E-mail: [email protected]

Nano-scale materials have applications in diverse fields, such as catalysis, electronics, and medical science. In the pollution remediation field Fe and Fe/Pd nanoparticles (NPs) are being used to detoxify chlorinated organics. The use of unsupported NPs often leads to aggregation and loss to environment. Our approach is based on direct synthesis of iron-based NPs in membranes and temperature responsive hydrogel supports by using “greener” approach. In our current work, we investigated a simple and fast method of Fe and Fe/Pd NPs synthesis in polyacrylic acid functionalized polyvinylidene fluoride (PAA/PVDF) membranes, using “green” reducing agents (green tea extract and epicatechin), as an alternative to the commonly used reducing agent, sodium borohydride. In addition, we have synthesized reactive Fe/Pd NPs in hydrogel, P(NIPAAm-AA (poly- N-isopropylacrylamide-acrylic acid). Both membrane and hydrogel immobilized NPs have been applied to successful dechlorination of TCE and various PCBs. For example, the surface area normalized reaction rates for TCE ranged between 0.008 to 0.04 L/m2h with Fe/Pd systems. Membrane polymer support and hydrogel had insignificant loss of NPs to environment.

© 2013 American Chemical Society In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Introduction In recent years, nanoscale materials and nanoparticles (NPs) in particular, received a great attention due to their unique physico-chemical and thermodynamic properties, different from those of bulk materials (1). As a consequence, these materials find applications in diverse fields, such as catalysis, electronics or medical science (2–5). NP synthesis with the desired properties is one of the most exciting and challenging aspects of modern nanotechnology. Among various synthesis procedures, green chemistry techniques show a great potential by using economical, non-toxic and biodegradable materials, thus being an attractive alternative for conventional methods. A variety of materials originating from bio-renewable natural sources, such as plant surfactants (6), waste biomass (7), vitamins (8–11), aminoacids (12), pomace (wine waste) (13), polysaccharides (14), glucose (15), polyvinylpyrrolidone (16), oleic acid (17), as well as extracts of coffee (18), green tea (19–23), black tea (24) and plant leaves and seeds (25–29) have emerged as replacements for well established chemicals. These materials are nontoxic, biodegradable, and act as both dispersive and capping agents, thus minimizing the NPs oxidation and agglomeration (30). Various metallic NPs such as Au, Ru, Pd, Pt, Ag and Fe were synthesized using these novel techniques. Our research group has been involved in synthesis of supported Fe and bimetallic Fe/Pd NPs in membranes and hydrogels, and their application toward remediation of toxic chloro-organics. Membrane’s open structure and high internal surface area ensure a high NP loading and easy accessibility of the pollutant to the active site; in addition these can be operated in convective mode (permeation through the membrane). NPs incorporation in membranes can also be conducted in diffusive (soaking) mode that involved long (10-12 h) processing time. In our current work, we investigated a simple and fast method of Fe and Fe/Pd NPs synthesis in polyacrylic acid functionalized polyvinylidene fluoride (PAA/PVDF) membranes, using “green” reducing agents (green tea extract and epicatechin), as an alternative to the commonly used reducing agent, sodium borohydride. All synthesis steps have been conducted in convective mode (mounting the membrane in a filtration cell) thus significantly reducing the synthesis time (from 12 h to less than 2 h) and promoting the NP formation inside membrane pores, rather than pore mout and external surface. In addition, we have synthesized reactive Fe/Pd NPs in hydrogel, P(NIPAAm-AA) (polyN-isopropylacrylamide-acrylic acid). The membranes containing the Fe and bimetallic Fe/Pd NPs have been characterized by permeability studies and FTIR, SEM, TEM, EDX and selected area electron diffraction (SAED) pattern techniques. The applications of these immobilized NP systems are towards remediation of toxic chloro-organics (such as trichloroethylene and/or polychlorinated biphenyls) from water.

42 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Materials and Methods Chemicals


Potassium persulfate was purchased from EM Science. Deionized ultra-filtered water (DIUF) was purchased from Fisher Scientific. Acrylic acid (AA), potassium tetrachloropalladate (II), sodium borohydride, epicatechin, N-isopropylacrylamide (NIPAAm), poly (ethylene glycol) 600 dimethacrylate (PEG600DMA), 4-(4-dimethylaminophenylazo) aniline (DMPA), ethanol (>99.5%) were purchased from Sigma-Aldrich and ethylene glycol (EG) from Mallinckrodt. Hydrophilized PVDF microfiltration membranes, with a thickness of 125 μm and nominal pore size of 650 nm were obtained from Millipore Corporation. Characterization of Nanoparticles (NPs) Surface and cross-section of membrane/hydrogel and nanoparticles were examined by Hitachi S-4300 Scanning Electron Microscope (SEM). The samples were coated with gold for imaging purposes. A JEOL 2010F high-resolution Transmission Electron Microscopy (TEM) equipped with energy dispersive X-ray spectrometer (EDX) were used to observe the NPs morphology and analyze the elemental composition. A drop of nanoparticle solution was placed on a standard TEM copper (Cu) grid and then dried the samples in vacuum oven. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) (Varian 7000e) was used to determine the presence of functional groups in membrane hydrogel. The samples were placed on the diamond crystal and the spectrum was obtained between 500 and 4000 cm-1 for 32 scans at a resolution of 8 cm-1. The UV-Visible spectra were recorded in a Varian Cary Bio300 UV-visible absorption spectroscopy.

Results and Discussion Aqueous Phase in Situ Polymerization of Acrylic Acid in PVDF Membranes Our goal is to synthesize NPs inside a membrane domain and this approach involves metal cation exchange followed by reduction to metallic NPs. Therefore, the first step was to attach ion exchange groups on the membrane. Although various membrane materials can be used, PVDF was chosen for its high chemical and thermal stabilities with melting point in the range between 162-172 °C (31). PVDF membranes were functionalized with poly(acrylic acid) by in situ polymerization of acrylic acid. A schematic was shown in Figure 1, the PVDF membrane (hydrophylized membranes were used, for proper wetting) was dipped in the polymerization solution for 2 min, sandwiched between two Teflon plates and placed in an oven at 90 °C for 4 hours. The polymerization was greatly influenced by several parameters, such as monomer and cross linker concentration, pH, initiator amount etc (32). The polymerization solution contained 30 wt% acrylic (cross-linker, added in a 1:10 molar ratio of EG to acrylic acid), and 1 wt% potassium persulfate (initiator). Raising the temperature is necessary 43 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


for the formation of ester bonds between the ethylene glycol (crosslinker) and carboxylate on the formed polyacrylate. Ethylene glycol is a bidentate molecule (binds to two –COOH groups), and was used to prevent PAA leaching from the membrane. In order to maintain free –COOH groups for ion exchange, the amount of cross-linking agent had to be kept low. Under our experimental conditions at least 80% of the –COOH groups were free, this was established by quantifying the entrapment capacity of Ca2+ with –COOH groups (33, 34). Nitrogen gas was continuously supplied to remove oxygen which acted as an inhibitor for the polymerization reaction.

Figure 1. In situ polymerization of acrylic acid in PVDF membranes using green chemistry (no organic solvent used). 44 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.

Fe and Bimetallic Fe/Pd NPs Synthesis in PAA-Functionalized PVDF Membranes


The flowchart for NPs synthesis in PAA/PVDF membranes as well as the properties of the base - PVDF membrane is shown in Figure 2. NPs synthesis in the PAA/PVDF membranes involved a two-step procedure and involves cation exchange (on -COO- groups from PAA) followed by their reduction. Prior to Fe2+ ion exchange, PAA-functionalized PVDF membranes were immersed in NaCl (5 to 10 % wt) solution at pH 10 (adjusted with 0.1M NaOH) for at least 3 h to convert the -COOH to -COONa form. In the next step, the membrane was washed with DIUF until the pH of the washing solution became neutral.

Figure 2. Convective flow apparatus for nanoparticle synthesis in the PAA/PVDF membranes, synthesis procedure, base PVDF membrane characteristics and photos of the membranes containing Fe NPs, reduced by A) epicatechin and B) tea extract. The membrane was mounted in a filtration apparatus (also shown in Figure 2), and a solution of FeCl2 solution at a pH of 5.5 (adjusted with 0.1M NaOH) was convectively permeated (flux of 12.6 x 10-4 cm3/cm2s) through the membrane. The concentration of Fe2+ can be varied, depending on the amount of Fe desired to be immobilized in the membrane. Typically the feed solution volume and concentration were 200 mL and 180 mg/L Fe2+, respectively. Next a solution of green tea extract (50 ml, 20 g/L) or a solution of epicatechin (50 mL, 1 g/L), at pH 5 was permeated through the membrane, in order to reduce the Fe2+ to Fe NPs. The green tea extract was prepared by immersion of 1 tea bag (containing 1 g green tea, 100% purity) in boiling water for 15-20 minutes; the bag was removed and the liquid was filtered using a 450 nm syringe membrane (polytetrafluoroethylene, PTFE). 45 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


During the reduction process (1-2 h length), the membrane (initially white) changed color to grey or black, depending on the amount of Fe immobilized (darker with the increase of the amount of Fe). Photos in Figure 2 shows two membranes containing Fe NPs, reduced with epicatechin (Figure 2A) (3 mg Fe) and tea extract (Figure 2B) (8 mg Fe). For comparison purposes, Fe NPs immobilized on the membrane were also prepared with sodium borohydride (50 mL, 10 g/L) as a reducing agent. Tea extract containing a number of polyphenols, has been well studied in nutrition science due to their numerous benefic effects and antioxidant properties in human body (35). The most common polyphenols are epicatechin and its ester forms with gallic acid. Polyphenols can directly complex with iron ions and then reduce to zero valent NPs. Cyclic voltammetry studies (36) showed that only dihydroxyl groups in ortho position form could complex with metal cations and one pair of ortho-dihydroxyl groups could exchange 2e- during the complexation with Fe2+. Therefore, in the case of epicatechin (with only 1 pair of ortho-dihydroxyl groups), the molar ratio of reductant to Fe2+ should be 1:1, for complete reduction. The reduction potential for epicatechin is 0.57 V, sufficient for the reduction of Fe2+ to Fe0 (-0.44 V). The standard reduction potentials for most polyphenols and flavonoids are in a range from 0.5 to 0.7 V (Table 1). However, it was reported that chelation of metals (such as Fe) to polyphenols could shift the redox potential toward either anodic or cathodic direction (making it more susceptible to either oxidation or reduction), as a function of the interacting polyphenol-metal species (37). Although the exact overall mechanism for metal NPs formation is not known, several studies (supported by FTIR data) showed that hydroxyl groups used to form the complex with the metal cation were converted to carbonyl during the reduction process (24, 25, 28). It was also shown that among a multitude of compounds present in the tea extract, flavonoids and polyphenols played the most significant role in metal cation complexation with subsequent metal NPs formation (24). In current study, the reduction of Fe was also confirmed by preparing NPs in homogeneous phase (mixing 10 mL FeCl2 solution (0.1-0.4 M Fe2+) with 10 mL tea extract) by UV spectra (shown in Figure 3). The blank tea extract has an absorption beginning at 500 nm, similar to FeCl2 solutions. The reaction between FeCl2 and tea extract was instantaneous and the color of the reaction mixture changed from yellow to black. After the reaction, the UV spectra had broad absorption at a higher wavelength, which increased as the concentration of FeCl2 changed from 0.01 to 0.04 M. In order to form Fe/Pd bimetallic NPs in the membrane, a solution containing a mixture of K2PdCl4 and tea extract was permeated through the membrane containing Fe NPs. In order to establish the appropriate reaction conditions to form Pd NPs, this reaction was firstly carried out in homogeneous phase (mixing 10 mL K2PdCl4 (0.03 M Pd) with10 mL tea extract) and analyzed by UV-Vis. It was shown previously (28) that Pd reduction with various plant extracts containing polyols proceeded slowly (12 h) when the reaction was carried out at 30 °C. 46 In Sustainable Nanotechnology and the Environment: Advances and Achievements; Shamim, N., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2013.


Table 1. Reduction Potential of Tea Polyphenols, Flavonoids, and Other Physiological Antioxidants. (adapted from reference (20, 38)) Antioxidant

Reduction potential (V)

(-)-Epicatechin

0.57

(-)-Epicatechin gallate

0.55

(-)-Epigallocatechin

0.43

Catechol

0.53

Theaflavin

0.51

Theaflavin digallate

0.54

Hesperidin

0.72

Rutin

0.60

Quercetin

0.33

3,5-Dihydroxy-anisol

0.84

Methyl gallate

0.56

Ascorbate

0.28

α-Tocopherol

0.48

Uric acid

0.59

Glutathion (Cysteine)

0.92

Here, the metal salt precursor (K2PdCl4) was reacted with tea extract both at room and elevated (80 °C) temperature (Figure 4); at room temperature (20°C), no color change or difference was observed in absorption spectra of the product (compared to the reactants) within 1 h reaction time. However, at elevated temperature (80 °C), in less than 30 min reaction time, the color of the reaction product turned to dark brown and the absorption spectra showed a broad band at a higher wavelength, indicating the formation of Pd NPs. This was further confirmed by TEM with SAED pattern, and it will be discussed later. Therefore, for bimetallic Fe/Pd NPs formation in the membrane, the mixture of K2PdCl4 and tea extract was permeated at 80 °C. In order to quantify the amounts of Fe and Pd, membranes containing NPs were digested by nitric acid solution (0.02L, 35%) to release the metals into the solution phase. The concentrations of Fe and Pd in the digested solutions were analyzed by a Varian SpectrAA 220 Fast Sequential atomic absorption spectrometer equipped with a Fisher Scientific hollow cathode lamp that was operated at a wavelength of 386.0 nm for Fe and 246.6 nm for Pd. The calibration plot was created using 4 different concentrations of Fe ranging from 25 to 200 mg/L with R2 = 0.9995 and average analytical error of 2%. In the case of Pd, the linear calibration range is between 0.2 and 28 mg/L Pd and the error of analysis was

Greener and Other Approaches To Synthesize Fe and Pd

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