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Magnetic polyaniline-chitosan nanocomposite decorated with palladium nanoparticles for enhanced catalytic reduction of 4-nitrophenol
Molecular Catalysis 439 (2017) 72–80
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Magnetic polyaniline-chitosan nanocomposite decorated with palladium nanoparticles for enhanced catalytic reduction of 4-nitrophenol Mohamad M. Ayad a,b,∗ , Wael A. Amer a , Mohammed G. Kotp a a b
Chemistry Department, Faculty of Science, Tanta University, Tanta 31527, Egypt Institute of Basic and Applied Sciences, Egypt-Japan University of Science and Technology, New Borg El-Arab City, Alexandria 21934, Egypt
a r t i c l e
i n f o
Article history: Received 7 April 2017 Received in revised form 19 June 2017 Accepted 20 June 2017 Keywords: Nanocomposites Polyaniline Magnetite Palladium Heterogenous catalysis
a b s t r a c t In this article, palladium (Pd) nanoparticles were deposited on a smart surface composed of a synthetic polymer, a natural polymer and a magnetic inorganic material, polyaniline (PANI), chitosan (CS) and magnetite (Fe3 O4 ), respectively to form Pd@PANI-CS-Fe3 O4 nanocomposite. The parent magnetic nanocomposite was prepared via in situ polymerization of aniline in the presence of CS. Exploitation of the polymerization side product, ferrous chloride (FeCl2 ), produced Fe3 O4 that imparted the nanocomposite with magnetic character. Pd nanoparticles were then stabilized @ the nanocomposite via reduction of Pd ions. X-ray electron diffraction (XRD), Fourier transform infrared (FTIR), transmission electron microscope (TEM), selected area electron diffraction (SAED) as well as energy dispersive X-ray (EDX) were employed to characterize Pd@PANI-CS-Fe3 O4 nanocomposite. In addition, vibrating sample magnetometer (VSM) was used to investigate the magnetic property of Pd@PANI-CS-Fe3 O4 nanocomposite. The reduction of the toxic 4-nitrophenol (4-NP) to a safer form, 4-amino phenol (4-AP), was employed to examine the catalytic efficacy of Pd@PANI-CS-Fe3 O4 nanocomposite as a heterogenous nanocatalyst. Pd@PANI-CS-Fe3 O4 represented a high kinetic rate up to 0.222 min−1 . © 2017 Elsevier B.V. All rights reserved.
1. Introduction The treatment of toxic dyes and nitroaromatic compounds in wastewater has attracted great global attention nowadays. The difficulty of removing such compounds, through traditional degradation systems, arises from their biological and chemical stability [1–3]. Despite their use in different fields such as pharmaceuticals, textiles dyes, explosives, pigments, plastics, fungicidal agents and industrial solvents, these highly hazardous nitroaromatic compounds have no replacements in the human life [4,5]. As an example of these compounds, 4-nitrophenol (4-NP) is a mononitrophenol that is carcinogenic for living organisms, may accumulate in the food chain [6,7] and damages liver, kidney, central nervous system of human and animal’s blood causing various fatal diseases [8]. 4-NP was classified as a priority pollutant, by the
∗ Corresponding author at: Chemistry Department, Faculty of Science, Tanta University, Tanta 31527, Egypt and Institute of Basic and Applied Sciences, Egypt-Japan University of Science and Technology, New Borg El-Arab City, Alexandria 21934, Egypt. E-mail addresses: [email protected], [email protected] (M.M. Ayad). http://dx.doi.org/10.1016/j.mcat.2017.06.023 2468-8231/© 2017 Elsevier B.V. All rights reserved.
Environmental Protection Agency (EPA), as a result of its environmental stability, nonbiodegradablility and its easy penetration to underground water through soil and hence, EPA limited its upper concentration to be at maximum probability only 10 parts per billion in natural water [9–12]. The reduction was proposed as an effective route toward getting rid of 4-NP [13,14] producing 4aminophenol (4-AP) as the reduction product. 4-AP is not only safer than the original compound [15] but it can be used in different fields such as analgesic and antipyretic drugs, photographic developers, dyes manufacturing, and corrosion inhibitors [16–19]. Although reduction is a suitable solution, it usually requires high temperatures and hydrogen pressure [20] and hence, researchers looked for mild conditions for this reduction reaction via decreasing the kinetic barrier in the electrons movement between the electron donor (reducing agent) like sodium boron hydride (NaBH4 ) and the electron acceptor represented in the nitrocompound [19,21]. It is worth mentioning that reducing 4-NP, using hydrides in the normal conditions without existence of metals, has no advance within time passing [21–23]. Noble metal nanoparticles (NPs) such as Ag, Au, Pt, Ir, Pd and Cu-based catalysts are often used for lowering the kinetic barrier for organic synthesis reactions, redox reactions and decomposition reactions of pollutants [24–29]. Due to the amazing
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physiological characteristics of noble metals NPs, they are different from their corresponding bulk counterparts which cannot be used in the same applications. Despite the growing use of noble metal NPs in catalysis reactions, they have a great problem in their agglomeration which requires a suitable supporting substrate to overcome this drawback [28,29]. Naked Pd NPs act as one of the most effective noble metals in catalyzing many reactions such as hydrogenation reactions for unsaturated olefins [32,34–37] and carbon-carbon coupling reactions (Suzuki–Miyaura and Heck reactions) [30–34]. Like other noble metals, Pd is rarely abundant in nature which renders it highly expensive for industrial applications. An easy route to enhance the mass activity of the particles is to decrease their particle size as possible to enlarge the surface area by creating more active sites in little weight [21]. Pd NPs were applied in many fields such as membranes [38], probes [39], fuel cells [40], oxygen reduction [41], lithium ion batteries [42], and heterogenous oxidation of alcohols [43]. Different types of polymers were employed as supporting surfaces for anchoring Pd NPs [11,14,28,43]. Chitosan (CS), as a renewable natural biopolymer, is featured by its biocompatibility, environmental friendliness, high mechanical strength, good film forming ability and low cost and is considered as the second widespread biopolymer in nature [44–47]. CS is composed of -(1,4)-linked-2-deoxy-2-amino-d-glucopyranose units. These characteristics expands the applications of CS in our life including food packing, water treatment, separation membranes, drug carrier, tissue scaffolds, and other biomedical fields [46,48]. In spite of the great characteristics of CS, it has a poor electrical conductivity and poor stability which made CS away from motivation response [49] and as a result, chemists blend it with other polymers to enhance its properties [50,51]. Conjugated conducting polymers, such as polyaniline (PANI), polypyrrole (PPY), polyacetylene and polythiophene were used since 1970 in electronics, biosensors, actuators and photo thermal therapy [52–55]. Among these polymers, PANI received considerable attention due to its controllable conductivity, easy synthesis and good environmental stability [56,57]. Based on the above promising properties, combining CS and PANI is expected to produce good properties. Bagheri et al. [58] elucidated that the combination of a conducting polymer with CS could increase the sorption sites which in turn lead to exploit the resulting composite as a good sorbent for naproxen isolation. Furthermore, Dhayal and Khan [59] enhanced the surface morphology of PANI using CS by synthesizing PANI-CS nanocomposite that was employed as an immunosensor for ochratoxin-A. The great progress in hydrogel materials composed of conducting polymers with CS with their fantastic features (such as fast responsive and high adsorption rate toward pollutants from aqueous solutions) added much applications to that composite definitely [60,61]. Magnetic NPs are one of the most promising research avenues not only for their fundamental scientific research but also for technological applications such as medicine [62], magnetic resonance imaging [63], magnetic inks [64] and other industrial fields [65,66]. Furthermore, the inclusion of magnetic NPs into other materials helps in their separation from the medium by applying an external magnetic field [30,67]. Although homogenous catalysts provide readily-accessible active sites by dissolving in reaction media, they have limited usage due to the recycling problem of the catalysts and the problems in contaminating the products with byproducts. Therefore, durable and reusable heterogeneous catalysts are favorable from a sustainable standpoint. Moreover, several metal particles or NPs were anchored on various substrates to work as industrial heterogeneous catalysts for different practical applications [68]. Shokouhimehr et al. [35,68,69] represented the highest importance of magnetic
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heterogeneous catalysts not only in their easy separation from solutions under magnetic field but also their reusability as well as their chemical stability and thus the authors used the prepared magnetic nanocatalysts for different reduction reactions. Murugadoss et al. [70] chose pristine CS as a supporting surface for anchoring Ag NPs that were employed for the reduction of 4-NP. In addition, Ag NPs@PANI were fabricated and used as a catalyst for 4-NP reduction [71]. In a few words, PANI with its fantastic properties such as thermal, electrical and catalytic properties when combined with a biocompatible and a biodegradable polymer (CS) in a magnetic matrix supported with Pd NPs will take us not only toward a novel catalysis system but also for different future applications. Therefore, this work represents the synthesis, characterization and catalytic application of a smart surface that combine the mechanical and conducting properties in addition to the magnetic character as a supporting substrate for anchoring Pd NPs. Here in, PANI-CS matrix was prepared through a simple method. What’s more, some authors used multiple steps to prepare magnetic core [72–74] but here the synthesis byproduct was exploited to add a magnetic feature to the prepared nanocomposite. Moreover, the amino and hydroxyl groups in CS and PANI skeletons played a prominent role in chelating Pd ions to enhance their reduction to form Pd metals on the surface of the nanocomposite. Various characterizations tools such as Fourier transform infrared (FTIR), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM) and vibrating sample magnetometer (VSM) were employed to ensure as well as investigate the formation and the characteristics of the nanocomposite. The reduction reaction of 4-NP to 4-AP was studied to elucidate the catalytic activity of the heterogenous Pd@PANI-CSFe3 O4 nanocatalyst. 2. Experimental 2.1. Chemicals Aniline (Adwic, Egypt) was purified via double distillation using zinc dust [75]. Glacial acetic acid, methanol (Adwic, Egypt), CS (Acros, USA, molecular weight: 100,000–300,000) as well as FeCl3 (98%, SISCO, India) were used without further purification. NaOH pellets (lobachemie, India), sodium boron hydride (NaBH4 , Johnson Matthey, UK), palladium chloride (PdCl2 , Kishide Chemicals, Osaka Yakuken, Japan) and 4-NP (Sigma Aldrich) were used as received. 2.2. Preparation of PANI Previous contexts were followed to polymerize aniline using FeCl3 as an oxidizing agent [76]. Briefly, 0.05 M of pure aniline was prepared in 50 mL of 0.1 M HCl and then 50 mL of 0.154 M FeCl3 solution was added quietly with constant stirring to the aniline solution. The reaction was kept under mechanical stirring for 3 h. Finally, the product was filtered and washed with distilled water and methanol several times and then dried at 50 ◦ C. 2.3. Preparation of PANI-CS-Fe3 O4 nanocomposite The current protocol depends on the procedures described by Ayad et al. [77]. Briefly, 1 g of CS was added to 100 mL of 2% acetic acid with stirring till complete solubility. 0.5 mL of aniline was added to CS solution under mechanical stirring for 1 h. Under cold conditions (in an ice bath), 2.51 g of FeCl3 was added to the previous mixture and the flask was kept under mechanical stirring overnight. 25 mL of 0.5 M NaOH solution was added to the matrix and the resulting precipitate was harvested and washed with dis-
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Scheme 1. Formation mechanism of Pd@PANI-CS-Fe3 O4 [The locations of Fe3 O4 and Pd are arbitrary]. Reaction conditions: 100 mL distilled water, CS = 1 g, Aniline = 8.4 mmoles, FeCl3 = 15 mmoles. [PdCl2 ] = 9 × 10−4 M, Mechanical stirring.
tilled water and methanol several times. Finally, the product was dried at 50 ◦ C for 18 h. 2.4. Synthesis of Pd@PANI-CS-Fe3 O4 nanocomposite 0.3 g of the fabricated PANI-CS-Fe3 O4 nanocomposite was added to 60 mL of 10 mg PdCl2 dissolved into (50:50) water-ethanol mix-
ture. The solution was left under ultrasonic dispersion for 15 min then the mixture was left in an oven at 60 ◦ C for 1/2 h. Afterward, the precipitate was collected and washed with excess distilled water and methanol up to 5 times. Finally, the product was left to dry overnight.
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2.5. Catalytic activity of Pd@PANI-CS-Fe3 O4 nanocomposite toward the reduction of 4-NP A typical catalytic reduction reaction of 4-NP was carried out in a well stoppered cuvette. 2.5 mL of 7 mM 4-NP was added to the cuvette followed by the addition of 0.5 mL of NaBH4 (5 mg/mL). Finally, 1 mg of Pd@PANI-CS-Fe3 O4 nanocomposite was added and the reaction progress was followed via UV–vis spectrophotometer. 3. Characterization XRD patterns were measured by GNR APD-2000 PRO diffractometer with Cu Ka radiation (40 KV, 30 mA) at a step scan mode. FTIR spectrums were recorded using a Bruker, Tensor 27 FTIR spectrophotometer with a frequency range from 4000 cm−1 to 400 cm−1 . Size and morphology of the nanocomposite were observed by TEM (JEM-2100F) at 200KV. The nanocomposite elements, their corresponding amount percentages as well as atomic percentages were determined by using EDX. Induced coupled plasma-atom emission spectrometer (ICP-AES, Agilent 720 ICPOES, USA) was used to measure the percentage of the elements. VSM on physical property measurement system was used to record the magnetic feature. UVD-2960 (Labomed-Inc) spectrometer was exploited to record UV–vis absorption spectra. 4. Results and discussion PANI-CS-Fe3 O4 was fabricated through one-step simple route. In situ chemical polymerization of aniline was processed via FeCl3 in the presence of CS. During the polymerization reaction, ferrous chloride (FeCl2 ) was produced as a result of the reduction of FeCl3 . In the alkaline media, FeCl2 reacted with excess FeCl3 to give the opposite hydroxides Fe(OH)2 and Fe(OH)3 and consequently Fe3 O4 NPs were formed due to the mixing of stoichiometric ratios 1:2 of Fe+2 and Fe+3 ions at pH ranged between (8–12) according to Eq. (1) [78]. The synthesized PANI-CS-Fe3 O4 nanocomposite inherited the magnetic character from the produced Fe3 O4 . Fe2+ + 2Fe3+ + 8OH− → Fe3 O4 + 4H2 O
(1)
For anchoring Pd NPS step, there are two intermediate stages. The first stage is chelation of Pd cations on deprotonated amines and hydroxyls groups of CS and PANI followed by reduction of the adsorbed cations to Pd NPs employing ethanol as a reducing agent leading to the formation of Pd@PANI-CS-Fe3 O4 nanocomposite [30]. The reduction of Pd cations with a primary alcohol depends on its oxidation to the corresponding aldehyde and the reduction of Pd cations to the opposite metal. Bendicho et al. [79] proved that ethanol is the best reducing alcohol for Pd ions to precipitate Pd NPs. The reaction is expected to proceed according to Eq. (2). The whole reactions are summarized in Scheme 1. CH3 CH2 OH + PdC12 → CH3 CHO + Pd + 2H+ + 2Cl−
(2)
FTIR spectroscopy was employed to confirm the structure and the formation of Pd@PANI-CS-Fe3 O4 nanocomposite via comparing it with the pristine components. Fig. 1A exhibits the FTIR spectrum of PANI and the main peaks at 1571 cm−1 , 1472 cm−1 , 1301 cm−1 , 1121 cm−1 and 803 cm−1 are related to nitrogen quinone (Q) structure, benzene ring (B) structure vibration, C N stretching vibration, C H in-plane vibration and out-of-plane bending vibrations of C H, respectively [80]. The FTIR spectrum of the pristine CS ascribed by Yavuz et al. [51] coincides with the measured CS spectrum (Fig. 1B) and it shows a peak at 3436 cm−1 attributed to stretching NH2 as well as two peaks at 2880 cm−1 and 1654 cm−1 related to C H stretching and NH2 bending, respectively. After the formation of PANI-CS-Fe3 O4 nanocomposite, the main CS peak
Fig. 1. FTIR spectra of PANI (A), CS (B), PANI-CS-Fe3 O4 nanocomposite (C), and Pd@PANI-CS-Fe3 O4 nanocomposite (D).
at 1654 cm−1 (Fig. 1B) was shifted to 1636 cm−1 (Fig. 1C) and a similar behavior was reported previously by Yavuz et al. [51]. The appearance of new peaks at 595 cm−1 and 449 cm−1 (Fig. 1C) prove the presence of magnetite and they are assigned to Fe-O vibration [73,75]. A small shift in wavenumber appeared after anchoring Pd NPs in which the peaks related to quinone ring and benzene ring vibration shifted from 1570 cm−1 and 1470 cm−1 to 1579 cm−1 and 1480 cm−1 , respectively [30,70,81]. The crystallinity and the phases of the samples were investigated by using XRD. As observed in Fig. 2A, peaks at 2 of 30.2◦ , 35.74◦ , 43.12◦ , 53.51◦ , 57.19◦ and 62.78◦ can be indexed as [hkl] to [220], [311], [400], [422], [511] and [440] planes of face centered cubic (FCC) of Fe3 O4 present in PANI-CS-Fe3 O4 nanocomposite [80,82]. The appearance of two diffraction peaks at 2 equivalent to 40.1◦ and 46.8◦ (Fig. 2B) could be indexed to [111] and [200] lattice plane of FCC Pd crystals, respectively [83–86]. The obtained diffractions coincide with the FTIR results and prove the formation of magnetite and Pd NPs. The average size of Pd NPs was found to be 3 nm by using the lattice [111] plane at 2 = 40.1◦ and applying Deybe-scherer equation Eq. (3) [14]. =
K ¯  cos
(3)
where, is the mean particle size, K is Dimensionless shape factor, ¯ is The X-ray wavelength, ˇ is The line broadening at half the maximum intensity (FWHM), is Bragg angle. TEM was used for focusing insight into the structure of samples and their morphologies. Fig. 3A shows the TEM image of the parent PANI-CS-Fe3 O4 nanocomposite. After decoration with Pd NPs, the TEM image of PANI-CS-Fe3 O4 nanocomposite shows the presence of Fe3 O4 and Pd NPs with size ranged between 8 and 10 nm. What’s more, Fe3 O4 and Pd NPs are well-distributed onto the PANICS matrix without serious aggregations (Fig. 3B). High resolution
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Fig. 2. XRD patterns of the prepared PANI-CS-Fe3 O4 nanocomposite (A) and Pd@PANI-CS-Fe3 O4 (B).
Fig. 3. TEM images of PANI-CS-Fe3 O4 nanocomposite (A), Pd@PANI-CS-Fe3 O4 nanocomposite (B), HRTEM of Pd@PANI-CS-Fe3 O4 nanocomposite (C), and SAED of single Pd nanoparticle (D).
Fig. 4. EDX of Pd@PANI-CS-Fe3 O4 (A), elemental mapping of Pd@PANI-CS-Fe3 O4 nanocomposite; image used for mapping (B), iron distribution (C), and Pd distribution (D).
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Fig. 5. Magnetization curve of Pd@PANI-CS-Fe3 O4 nanocomposite.
TEM (HRTEM) image shown in Fig. 3C proved the presence of Fe3 O4 NPs in Pd@PANI-CS-Fe3 O4 nanocomposite. The lattice fringes of the nanocomposite represent interplanner distance around 0.16 nm between the strips that corresponds to the lattice plane [220] of Fe3 O4 as shown in Fig. 3C. Selected area electron diffraction (SAED) as observed (Fig. 3D) showed bright rings which elucidate the crystalline structure that is consistent with the obtained XRD results [30]. EDX pattern reveals the elemental composition and proved the inclusion of C, O, Fe and Pd in Pd@PANI-CS-Fe3 O4 nanocomposite as shown in the MAP directory (Fig. 4A). Pd signals pointed that the PdCl2 was successfully reduced to Pd NPs. Masses normality% of Fe and Pd were calculated from EDX to be 65.82 and 19.7, respectively. Furthermore, the atomic percentages of Fe and Pd were equivalent to 47.12% and 7.40%, respectively. The distribution of iron and Pd in the Pd@PANI-CS-Fe3 O4 nanocomposite was demonstrated via elemental mapping technique as shown in Fig. 4 (B-D), which proves that Fe3 O4 and Pd NPs were formed over PANI-CS matrix. Moreover, the prepared nanocatalyst was analyzed by ICP and the percentage of Fe and Pd elements was found to be 57% and 3.5%, respectively. The magnetic property of the synthesized matrix was studied using VSM. As shown in Fig. 5, the hysteresis loop of the fabricated Pd@PANI-CS-Fe3 O4 nanocomposite showed non-coercive force property or remanence values at room temperature and indicated the ferromagnetic property of Pd@PANI-CS-Fe3 O4 nanocomposite with saturation magnetization (Ms) about 0.3 emu/g. Pd NPs supported onto different substrates such as CS and PANI were exploited as an effective catalyst for different reactions. Vincent and Guibal synthesized Pd@CS hollow fibers and tested their catalytic activity toward nitrophenols degradation using sodium formate as a reducing agent and the authors reported that the degradation efficiency has the maximum rate at high temperatures [87]. In addition, Cs@Pd hybrid was used as a catalyst for nitrophenol degradation employing sodium formate in addition to hydrogen gas regimes as reducing agents [88]. Patel et al. [89] synthesized Pd NPs supported on PANI as an effective heterogenous catalyst for dehalogenation reactions. To investigate the catalytic properties of Pd@PANI-CS-Fe3 O4 nanocomposite, the reduction reaction of 4-NP to 4-AP in presence of NaBH4 was chosen as a reaction model. This reaction has become a benchmark reaction to assess the catalytic activity of metal NPs embedded in various substrates [22,23]. It is worth noting that metal NPs were used to accelerate electron transfer from donor (BH4 − ) to acceptor (4NP) [14] and thus the reaction was expected to proceed via the formation of two reactive species Pd-BH3 − and Pd-H as described in Eq. (4). The Langmuir–Hinshelwood mechanism demonstrated the contri-
Fig. 6. UV–vis absorption spectra for the reduction of 4-NP to 4-AP using (A) 1 mg and (B) 2 mg of Pd@PANI-CS-Fe3 O4 nanocatalyst. Reaction conditions: [4NP] = 7 mM, [NaBH4 ] = 0.2 mM, T = 298 K.
Scheme 2. The reduction mechanism of 4-NP to 4-AP.
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Fig. 7. Time dependent absorbance change for reduction of 4-NP by NaBH4 in presence of Pd@PANI-CS-Fe3 O4 nanocomposite (A), plot of -lnAt /Ao versus time (B).
Fig. 8. Recovery of 1 mg of the catalyst.
bution of Pd NPs in the catalytic reaction as illustrated in Scheme 2 Eqs. (5)–(7). On adding NaBH4 to 4-NP solution, there wasn’t any change in the absorption intensity even after several days so the addition of a catalyst is required to increase the reaction rate and hence the progress of this reaction can be monitored [14,23]. Just adding Pd@PANI-CS-Fe3 O4 heterogenous nanocatalyst to 4-NP in the presence of NaBH4 , the reaction preceded successfully and the absorption of 4-NP at 400 nm reduced gradually with time in addition to the emergence of a new peak at 310 nm related to the reduction product 4-AP [90]. On using 1 mg and 2 mg of the nanocatalyst to investigate the effect of the amount of catalyst, the reaction ended in 14 min and 8 min (Fig. 6), respectively. Furthermore, the reaction could be visualized through the naked eye as the intense yellow color of 4-NP disappears due to the formation of 4-AP. Fig. 7A pointed clearly to the decrease of 4-NP concentration with time. As the concentration of 4-NP was much lower than that of the reducing agent, pseudo first order kinetic assumption was applied with regard to 4-NP alone [85]. The pseudo first order equation could be expressed as Eq. (9) in which, At refers to the absorbance at time t, Ao is the absorbance at zero minute and Kapp is the reaction apparent rate constant. dCt /dt = −Kapp Ct or−lnC t /Co = −Kapp tor−lnAt /Ao = −Kapp t
(9)
Table 1 Comparison the catalytic activity of various catalytic substrates for the reduction of 4-NP at room temperature. Catalyst
Dose (mg)
K (S−1 )
Reference
PPY-Pd nanocapsules Guar gum-s-PtNPs p(AMPS)-Cu PPy nanotubes-Ru PPY nanotubes-Pt p(AMPS)-Co Pd@PANI-CS-Fe3 O4
7 5 10 4 4 50 1
8 × 10−3 7 × 10−3 1.72 × 10−3 9 × 10−4 1 × 10−3 2 × 10−3 3.7 × 10−3
[85] [14] [91] [13] [13] [92] The present work
Linear relationship was obtained by applying pseudo first order kinetic assumption and plotting the relation between lnAt /Ao vs. time t as shown in Fig. 7b. The fabricated Pd@PANI-CS-Fe3 O4 heterogenous nanocatalyst showed a high rate of reaction up to 0.222 min−1 . The catalytic activities toward reduction of 4-NP for some metals NPs supported onto different substrates were summarized in (Table 1). These materials include Pt, Cu, Ru. Co as well as Pd NPs decorated onto various supporting substrates such as PPY nanotubes and PPY nanocapsules, guar gum in addition to p(2-acrylamido-2-methyle-1-propansulfonic acid) (APMS). After comparing the results carefully, it is clearly obvious that a high catalytic reaction rate was obtained using a very small amount
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dinitrophenol) and explosives (such as 2,4,6-trinitrotoluene and 2,4,6-trinitrophenol), etc. Most of these nitroaromatic compounds and nitrophenols are toxic and mutagenic for living organisms. Reduction is one of the routes to transform the toxic nitroaromatic compounds to a safer form, the aminoaromatic compounds and hence, there is an increasing demand of using novel nanocatalysts for this purpose. Therefore, in the future, it is important to check the catalytic behavior of Pd@PANI-CS-Fe3 O4 nanocatalyst toward the reduction of some of the aforementioned compounds. The effect of various substituents, in the nitroaromatic and nitroaliphatic moieties (electron withdrawing and electron repelling groups) in different positions, on the catalytic reduction behavior of the nanocatalyst will be investigated.
6. Conclusion
Scheme 3. The separation technique of the heterogenous nanocatalyst. Table 2 The efficacy of the recoveries runs. Run
1st
2nd
3rd
4th
5th
Efficacy
97.8%
96.55%
95.48%
94.16%
93.17%
Reaction conditions: 1 mg of Pd@PANI-CS-Fe3 O4 nanocomposite, [4-NP] = 7 mM, [NaBH4 ] = 0.2 mM, T = 298 K.
(1 mg) of Pd@PANI-CS-Fe3 O4 nanocatalyst as compared to the other catalytic systems with higher doses. Recovery of the catalyst is one of the critical parameters, which add a value to the catalyst so it is important to explore this factor in Pd@PANI-CS-Fe3 O4 nanocatalyst. To test the reusability of 1 mg from the magnetic nanocatalyst, it was separated after the first run via the magnetic separation process using an external magnet added to the wall of the reaction cuvette then the produced 4-AP was collected with a syringe bit by bit to preserve this small amount of catalyst as observed in Scheme 3. The recycled nanocatalyst was washed twice, by freshly distilled water, before the next reuse. The catalyst was recovered up to 4 runs with a small loss in its efficiency. The little time difference for every cycle, as shown in Fig. 8A, proves the fitness of this smart heterogeneous nanocatalyst. This time difference may be attributed to a small weight loss of the nanocatalyst during the separation process or a small leashing. Moreover, the efficacy percentage of the recovery times (Fig. 8B) was calculated via mathematical Eq. (10) reported by Zhang et al. [93]. In addition, the efficacy percentages for recoveries runs were epitomized in Table 2. ˛=
Co − Ct × 100% Co
(10)
where, Co is the initial concentration and Ct is the concentration at the termination stage. 4.1. Future perspectives Some important pharmacologically active compounds have a nitroaromatic group in their molecular structures (such as 4-R2-nitrophenol derivatives, R = H, OCH3 , CH3 , CF3 , or CN). In addition, most nitroaromatic chemical compounds are produced as a consequence of some industrial processes during the synthesis of plastics, polyurethanes, dyes, pesticides (such as diethyl-p-nitrophenyl monothiophosphate and 2-sec-butyl-4,6-
An easy route was followed to fabricate PANI-CS-Fe3 O4 magnetic nanocomposite accompanied by easy anchoring of Pd NPs on its surface. Different characterization techniques such as FTIR, XRD, VSM, EDX, TEM, ICP and SAED were used to ensure the fabrication and the characteristics of Pd@PANI-CS-Fe3 O4 before and after the decoration with Pd NPs. Pd@PANI-CS-Fe3 O4 heterogenous nanocatalyst showed a high efficacy toward the reduction of 4-NP to the opposite amino compound, 4-AP. Furthermore, the magnetic property of the synthesized nanocatalyst enhanced its recovery via magnetic separation from the reaction medium. Finally, Pd@PANI-CS-Fe3 O4 nanocomposite could be ascribed as a promising nanocatalyst for the reduction of 4-NP.
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Magnetic polyaniline-chitosan nanocomposite decorated with palladium nanoparticles for enhanced catalytic reduction of 4-nitrophenol Mohamad M. Ayad a,b,∗ , Wael A. Amer a , Mohammed G. Kotp a a b
Chemistry Department, Faculty of Science, Tanta University, Tanta 31527, Egypt Institute of Basic and Applied Sciences, Egypt-Japan University of Science and Technology, New Borg El-Arab City, Alexandria 21934, Egypt
a r t i c l e
i n f o
Article history: Received 7 April 2017 Received in revised form 19 June 2017 Accepted 20 June 2017 Keywords: Nanocomposites Polyaniline Magnetite Palladium Heterogenous catalysis
a b s t r a c t In this article, palladium (Pd) nanoparticles were deposited on a smart surface composed of a synthetic polymer, a natural polymer and a magnetic inorganic material, polyaniline (PANI), chitosan (CS) and magnetite (Fe3 O4 ), respectively to form Pd@PANI-CS-Fe3 O4 nanocomposite. The parent magnetic nanocomposite was prepared via in situ polymerization of aniline in the presence of CS. Exploitation of the polymerization side product, ferrous chloride (FeCl2 ), produced Fe3 O4 that imparted the nanocomposite with magnetic character. Pd nanoparticles were then stabilized @ the nanocomposite via reduction of Pd ions. X-ray electron diffraction (XRD), Fourier transform infrared (FTIR), transmission electron microscope (TEM), selected area electron diffraction (SAED) as well as energy dispersive X-ray (EDX) were employed to characterize Pd@PANI-CS-Fe3 O4 nanocomposite. In addition, vibrating sample magnetometer (VSM) was used to investigate the magnetic property of Pd@PANI-CS-Fe3 O4 nanocomposite. The reduction of the toxic 4-nitrophenol (4-NP) to a safer form, 4-amino phenol (4-AP), was employed to examine the catalytic efficacy of Pd@PANI-CS-Fe3 O4 nanocomposite as a heterogenous nanocatalyst. Pd@PANI-CS-Fe3 O4 represented a high kinetic rate up to 0.222 min−1 . © 2017 Elsevier B.V. All rights reserved.
1. Introduction The treatment of toxic dyes and nitroaromatic compounds in wastewater has attracted great global attention nowadays. The difficulty of removing such compounds, through traditional degradation systems, arises from their biological and chemical stability [1–3]. Despite their use in different fields such as pharmaceuticals, textiles dyes, explosives, pigments, plastics, fungicidal agents and industrial solvents, these highly hazardous nitroaromatic compounds have no replacements in the human life [4,5]. As an example of these compounds, 4-nitrophenol (4-NP) is a mononitrophenol that is carcinogenic for living organisms, may accumulate in the food chain [6,7] and damages liver, kidney, central nervous system of human and animal’s blood causing various fatal diseases [8]. 4-NP was classified as a priority pollutant, by the
∗ Corresponding author at: Chemistry Department, Faculty of Science, Tanta University, Tanta 31527, Egypt and Institute of Basic and Applied Sciences, Egypt-Japan University of Science and Technology, New Borg El-Arab City, Alexandria 21934, Egypt. E-mail addresses: [email protected], [email protected] (M.M. Ayad). http://dx.doi.org/10.1016/j.mcat.2017.06.023 2468-8231/© 2017 Elsevier B.V. All rights reserved.
Environmental Protection Agency (EPA), as a result of its environmental stability, nonbiodegradablility and its easy penetration to underground water through soil and hence, EPA limited its upper concentration to be at maximum probability only 10 parts per billion in natural water [9–12]. The reduction was proposed as an effective route toward getting rid of 4-NP [13,14] producing 4aminophenol (4-AP) as the reduction product. 4-AP is not only safer than the original compound [15] but it can be used in different fields such as analgesic and antipyretic drugs, photographic developers, dyes manufacturing, and corrosion inhibitors [16–19]. Although reduction is a suitable solution, it usually requires high temperatures and hydrogen pressure [20] and hence, researchers looked for mild conditions for this reduction reaction via decreasing the kinetic barrier in the electrons movement between the electron donor (reducing agent) like sodium boron hydride (NaBH4 ) and the electron acceptor represented in the nitrocompound [19,21]. It is worth mentioning that reducing 4-NP, using hydrides in the normal conditions without existence of metals, has no advance within time passing [21–23]. Noble metal nanoparticles (NPs) such as Ag, Au, Pt, Ir, Pd and Cu-based catalysts are often used for lowering the kinetic barrier for organic synthesis reactions, redox reactions and decomposition reactions of pollutants [24–29]. Due to the amazing
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physiological characteristics of noble metals NPs, they are different from their corresponding bulk counterparts which cannot be used in the same applications. Despite the growing use of noble metal NPs in catalysis reactions, they have a great problem in their agglomeration which requires a suitable supporting substrate to overcome this drawback [28,29]. Naked Pd NPs act as one of the most effective noble metals in catalyzing many reactions such as hydrogenation reactions for unsaturated olefins [32,34–37] and carbon-carbon coupling reactions (Suzuki–Miyaura and Heck reactions) [30–34]. Like other noble metals, Pd is rarely abundant in nature which renders it highly expensive for industrial applications. An easy route to enhance the mass activity of the particles is to decrease their particle size as possible to enlarge the surface area by creating more active sites in little weight [21]. Pd NPs were applied in many fields such as membranes [38], probes [39], fuel cells [40], oxygen reduction [41], lithium ion batteries [42], and heterogenous oxidation of alcohols [43]. Different types of polymers were employed as supporting surfaces for anchoring Pd NPs [11,14,28,43]. Chitosan (CS), as a renewable natural biopolymer, is featured by its biocompatibility, environmental friendliness, high mechanical strength, good film forming ability and low cost and is considered as the second widespread biopolymer in nature [44–47]. CS is composed of -(1,4)-linked-2-deoxy-2-amino-d-glucopyranose units. These characteristics expands the applications of CS in our life including food packing, water treatment, separation membranes, drug carrier, tissue scaffolds, and other biomedical fields [46,48]. In spite of the great characteristics of CS, it has a poor electrical conductivity and poor stability which made CS away from motivation response [49] and as a result, chemists blend it with other polymers to enhance its properties [50,51]. Conjugated conducting polymers, such as polyaniline (PANI), polypyrrole (PPY), polyacetylene and polythiophene were used since 1970 in electronics, biosensors, actuators and photo thermal therapy [52–55]. Among these polymers, PANI received considerable attention due to its controllable conductivity, easy synthesis and good environmental stability [56,57]. Based on the above promising properties, combining CS and PANI is expected to produce good properties. Bagheri et al. [58] elucidated that the combination of a conducting polymer with CS could increase the sorption sites which in turn lead to exploit the resulting composite as a good sorbent for naproxen isolation. Furthermore, Dhayal and Khan [59] enhanced the surface morphology of PANI using CS by synthesizing PANI-CS nanocomposite that was employed as an immunosensor for ochratoxin-A. The great progress in hydrogel materials composed of conducting polymers with CS with their fantastic features (such as fast responsive and high adsorption rate toward pollutants from aqueous solutions) added much applications to that composite definitely [60,61]. Magnetic NPs are one of the most promising research avenues not only for their fundamental scientific research but also for technological applications such as medicine [62], magnetic resonance imaging [63], magnetic inks [64] and other industrial fields [65,66]. Furthermore, the inclusion of magnetic NPs into other materials helps in their separation from the medium by applying an external magnetic field [30,67]. Although homogenous catalysts provide readily-accessible active sites by dissolving in reaction media, they have limited usage due to the recycling problem of the catalysts and the problems in contaminating the products with byproducts. Therefore, durable and reusable heterogeneous catalysts are favorable from a sustainable standpoint. Moreover, several metal particles or NPs were anchored on various substrates to work as industrial heterogeneous catalysts for different practical applications [68]. Shokouhimehr et al. [35,68,69] represented the highest importance of magnetic
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heterogeneous catalysts not only in their easy separation from solutions under magnetic field but also their reusability as well as their chemical stability and thus the authors used the prepared magnetic nanocatalysts for different reduction reactions. Murugadoss et al. [70] chose pristine CS as a supporting surface for anchoring Ag NPs that were employed for the reduction of 4-NP. In addition, Ag NPs@PANI were fabricated and used as a catalyst for 4-NP reduction [71]. In a few words, PANI with its fantastic properties such as thermal, electrical and catalytic properties when combined with a biocompatible and a biodegradable polymer (CS) in a magnetic matrix supported with Pd NPs will take us not only toward a novel catalysis system but also for different future applications. Therefore, this work represents the synthesis, characterization and catalytic application of a smart surface that combine the mechanical and conducting properties in addition to the magnetic character as a supporting substrate for anchoring Pd NPs. Here in, PANI-CS matrix was prepared through a simple method. What’s more, some authors used multiple steps to prepare magnetic core [72–74] but here the synthesis byproduct was exploited to add a magnetic feature to the prepared nanocomposite. Moreover, the amino and hydroxyl groups in CS and PANI skeletons played a prominent role in chelating Pd ions to enhance their reduction to form Pd metals on the surface of the nanocomposite. Various characterizations tools such as Fourier transform infrared (FTIR), X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), transmission electron microscope (TEM) and vibrating sample magnetometer (VSM) were employed to ensure as well as investigate the formation and the characteristics of the nanocomposite. The reduction reaction of 4-NP to 4-AP was studied to elucidate the catalytic activity of the heterogenous Pd@PANI-CSFe3 O4 nanocatalyst. 2. Experimental 2.1. Chemicals Aniline (Adwic, Egypt) was purified via double distillation using zinc dust [75]. Glacial acetic acid, methanol (Adwic, Egypt), CS (Acros, USA, molecular weight: 100,000–300,000) as well as FeCl3 (98%, SISCO, India) were used without further purification. NaOH pellets (lobachemie, India), sodium boron hydride (NaBH4 , Johnson Matthey, UK), palladium chloride (PdCl2 , Kishide Chemicals, Osaka Yakuken, Japan) and 4-NP (Sigma Aldrich) were used as received. 2.2. Preparation of PANI Previous contexts were followed to polymerize aniline using FeCl3 as an oxidizing agent [76]. Briefly, 0.05 M of pure aniline was prepared in 50 mL of 0.1 M HCl and then 50 mL of 0.154 M FeCl3 solution was added quietly with constant stirring to the aniline solution. The reaction was kept under mechanical stirring for 3 h. Finally, the product was filtered and washed with distilled water and methanol several times and then dried at 50 ◦ C. 2.3. Preparation of PANI-CS-Fe3 O4 nanocomposite The current protocol depends on the procedures described by Ayad et al. [77]. Briefly, 1 g of CS was added to 100 mL of 2% acetic acid with stirring till complete solubility. 0.5 mL of aniline was added to CS solution under mechanical stirring for 1 h. Under cold conditions (in an ice bath), 2.51 g of FeCl3 was added to the previous mixture and the flask was kept under mechanical stirring overnight. 25 mL of 0.5 M NaOH solution was added to the matrix and the resulting precipitate was harvested and washed with dis-
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Scheme 1. Formation mechanism of Pd@PANI-CS-Fe3 O4 [The locations of Fe3 O4 and Pd are arbitrary]. Reaction conditions: 100 mL distilled water, CS = 1 g, Aniline = 8.4 mmoles, FeCl3 = 15 mmoles. [PdCl2 ] = 9 × 10−4 M, Mechanical stirring.
tilled water and methanol several times. Finally, the product was dried at 50 ◦ C for 18 h. 2.4. Synthesis of Pd@PANI-CS-Fe3 O4 nanocomposite 0.3 g of the fabricated PANI-CS-Fe3 O4 nanocomposite was added to 60 mL of 10 mg PdCl2 dissolved into (50:50) water-ethanol mix-
ture. The solution was left under ultrasonic dispersion for 15 min then the mixture was left in an oven at 60 ◦ C for 1/2 h. Afterward, the precipitate was collected and washed with excess distilled water and methanol up to 5 times. Finally, the product was left to dry overnight.
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2.5. Catalytic activity of Pd@PANI-CS-Fe3 O4 nanocomposite toward the reduction of 4-NP A typical catalytic reduction reaction of 4-NP was carried out in a well stoppered cuvette. 2.5 mL of 7 mM 4-NP was added to the cuvette followed by the addition of 0.5 mL of NaBH4 (5 mg/mL). Finally, 1 mg of Pd@PANI-CS-Fe3 O4 nanocomposite was added and the reaction progress was followed via UV–vis spectrophotometer. 3. Characterization XRD patterns were measured by GNR APD-2000 PRO diffractometer with Cu Ka radiation (40 KV, 30 mA) at a step scan mode. FTIR spectrums were recorded using a Bruker, Tensor 27 FTIR spectrophotometer with a frequency range from 4000 cm−1 to 400 cm−1 . Size and morphology of the nanocomposite were observed by TEM (JEM-2100F) at 200KV. The nanocomposite elements, their corresponding amount percentages as well as atomic percentages were determined by using EDX. Induced coupled plasma-atom emission spectrometer (ICP-AES, Agilent 720 ICPOES, USA) was used to measure the percentage of the elements. VSM on physical property measurement system was used to record the magnetic feature. UVD-2960 (Labomed-Inc) spectrometer was exploited to record UV–vis absorption spectra. 4. Results and discussion PANI-CS-Fe3 O4 was fabricated through one-step simple route. In situ chemical polymerization of aniline was processed via FeCl3 in the presence of CS. During the polymerization reaction, ferrous chloride (FeCl2 ) was produced as a result of the reduction of FeCl3 . In the alkaline media, FeCl2 reacted with excess FeCl3 to give the opposite hydroxides Fe(OH)2 and Fe(OH)3 and consequently Fe3 O4 NPs were formed due to the mixing of stoichiometric ratios 1:2 of Fe+2 and Fe+3 ions at pH ranged between (8–12) according to Eq. (1) [78]. The synthesized PANI-CS-Fe3 O4 nanocomposite inherited the magnetic character from the produced Fe3 O4 . Fe2+ + 2Fe3+ + 8OH− → Fe3 O4 + 4H2 O
(1)
For anchoring Pd NPS step, there are two intermediate stages. The first stage is chelation of Pd cations on deprotonated amines and hydroxyls groups of CS and PANI followed by reduction of the adsorbed cations to Pd NPs employing ethanol as a reducing agent leading to the formation of Pd@PANI-CS-Fe3 O4 nanocomposite [30]. The reduction of Pd cations with a primary alcohol depends on its oxidation to the corresponding aldehyde and the reduction of Pd cations to the opposite metal. Bendicho et al. [79] proved that ethanol is the best reducing alcohol for Pd ions to precipitate Pd NPs. The reaction is expected to proceed according to Eq. (2). The whole reactions are summarized in Scheme 1. CH3 CH2 OH + PdC12 → CH3 CHO + Pd + 2H+ + 2Cl−
(2)
FTIR spectroscopy was employed to confirm the structure and the formation of Pd@PANI-CS-Fe3 O4 nanocomposite via comparing it with the pristine components. Fig. 1A exhibits the FTIR spectrum of PANI and the main peaks at 1571 cm−1 , 1472 cm−1 , 1301 cm−1 , 1121 cm−1 and 803 cm−1 are related to nitrogen quinone (Q) structure, benzene ring (B) structure vibration, C N stretching vibration, C H in-plane vibration and out-of-plane bending vibrations of C H, respectively [80]. The FTIR spectrum of the pristine CS ascribed by Yavuz et al. [51] coincides with the measured CS spectrum (Fig. 1B) and it shows a peak at 3436 cm−1 attributed to stretching NH2 as well as two peaks at 2880 cm−1 and 1654 cm−1 related to C H stretching and NH2 bending, respectively. After the formation of PANI-CS-Fe3 O4 nanocomposite, the main CS peak
Fig. 1. FTIR spectra of PANI (A), CS (B), PANI-CS-Fe3 O4 nanocomposite (C), and Pd@PANI-CS-Fe3 O4 nanocomposite (D).
at 1654 cm−1 (Fig. 1B) was shifted to 1636 cm−1 (Fig. 1C) and a similar behavior was reported previously by Yavuz et al. [51]. The appearance of new peaks at 595 cm−1 and 449 cm−1 (Fig. 1C) prove the presence of magnetite and they are assigned to Fe-O vibration [73,75]. A small shift in wavenumber appeared after anchoring Pd NPs in which the peaks related to quinone ring and benzene ring vibration shifted from 1570 cm−1 and 1470 cm−1 to 1579 cm−1 and 1480 cm−1 , respectively [30,70,81]. The crystallinity and the phases of the samples were investigated by using XRD. As observed in Fig. 2A, peaks at 2 of 30.2◦ , 35.74◦ , 43.12◦ , 53.51◦ , 57.19◦ and 62.78◦ can be indexed as [hkl] to [220], [311], [400], [422], [511] and [440] planes of face centered cubic (FCC) of Fe3 O4 present in PANI-CS-Fe3 O4 nanocomposite [80,82]. The appearance of two diffraction peaks at 2 equivalent to 40.1◦ and 46.8◦ (Fig. 2B) could be indexed to [111] and [200] lattice plane of FCC Pd crystals, respectively [83–86]. The obtained diffractions coincide with the FTIR results and prove the formation of magnetite and Pd NPs. The average size of Pd NPs was found to be 3 nm by using the lattice [111] plane at 2 = 40.1◦ and applying Deybe-scherer equation Eq. (3) [14]. =
K ¯  cos
(3)
where, is the mean particle size, K is Dimensionless shape factor, ¯ is The X-ray wavelength, ˇ is The line broadening at half the maximum intensity (FWHM), is Bragg angle. TEM was used for focusing insight into the structure of samples and their morphologies. Fig. 3A shows the TEM image of the parent PANI-CS-Fe3 O4 nanocomposite. After decoration with Pd NPs, the TEM image of PANI-CS-Fe3 O4 nanocomposite shows the presence of Fe3 O4 and Pd NPs with size ranged between 8 and 10 nm. What’s more, Fe3 O4 and Pd NPs are well-distributed onto the PANICS matrix without serious aggregations (Fig. 3B). High resolution
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Fig. 2. XRD patterns of the prepared PANI-CS-Fe3 O4 nanocomposite (A) and Pd@PANI-CS-Fe3 O4 (B).
Fig. 3. TEM images of PANI-CS-Fe3 O4 nanocomposite (A), Pd@PANI-CS-Fe3 O4 nanocomposite (B), HRTEM of Pd@PANI-CS-Fe3 O4 nanocomposite (C), and SAED of single Pd nanoparticle (D).
Fig. 4. EDX of Pd@PANI-CS-Fe3 O4 (A), elemental mapping of Pd@PANI-CS-Fe3 O4 nanocomposite; image used for mapping (B), iron distribution (C), and Pd distribution (D).
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Fig. 5. Magnetization curve of Pd@PANI-CS-Fe3 O4 nanocomposite.
TEM (HRTEM) image shown in Fig. 3C proved the presence of Fe3 O4 NPs in Pd@PANI-CS-Fe3 O4 nanocomposite. The lattice fringes of the nanocomposite represent interplanner distance around 0.16 nm between the strips that corresponds to the lattice plane [220] of Fe3 O4 as shown in Fig. 3C. Selected area electron diffraction (SAED) as observed (Fig. 3D) showed bright rings which elucidate the crystalline structure that is consistent with the obtained XRD results [30]. EDX pattern reveals the elemental composition and proved the inclusion of C, O, Fe and Pd in Pd@PANI-CS-Fe3 O4 nanocomposite as shown in the MAP directory (Fig. 4A). Pd signals pointed that the PdCl2 was successfully reduced to Pd NPs. Masses normality% of Fe and Pd were calculated from EDX to be 65.82 and 19.7, respectively. Furthermore, the atomic percentages of Fe and Pd were equivalent to 47.12% and 7.40%, respectively. The distribution of iron and Pd in the Pd@PANI-CS-Fe3 O4 nanocomposite was demonstrated via elemental mapping technique as shown in Fig. 4 (B-D), which proves that Fe3 O4 and Pd NPs were formed over PANI-CS matrix. Moreover, the prepared nanocatalyst was analyzed by ICP and the percentage of Fe and Pd elements was found to be 57% and 3.5%, respectively. The magnetic property of the synthesized matrix was studied using VSM. As shown in Fig. 5, the hysteresis loop of the fabricated Pd@PANI-CS-Fe3 O4 nanocomposite showed non-coercive force property or remanence values at room temperature and indicated the ferromagnetic property of Pd@PANI-CS-Fe3 O4 nanocomposite with saturation magnetization (Ms) about 0.3 emu/g. Pd NPs supported onto different substrates such as CS and PANI were exploited as an effective catalyst for different reactions. Vincent and Guibal synthesized Pd@CS hollow fibers and tested their catalytic activity toward nitrophenols degradation using sodium formate as a reducing agent and the authors reported that the degradation efficiency has the maximum rate at high temperatures [87]. In addition, Cs@Pd hybrid was used as a catalyst for nitrophenol degradation employing sodium formate in addition to hydrogen gas regimes as reducing agents [88]. Patel et al. [89] synthesized Pd NPs supported on PANI as an effective heterogenous catalyst for dehalogenation reactions. To investigate the catalytic properties of Pd@PANI-CS-Fe3 O4 nanocomposite, the reduction reaction of 4-NP to 4-AP in presence of NaBH4 was chosen as a reaction model. This reaction has become a benchmark reaction to assess the catalytic activity of metal NPs embedded in various substrates [22,23]. It is worth noting that metal NPs were used to accelerate electron transfer from donor (BH4 − ) to acceptor (4NP) [14] and thus the reaction was expected to proceed via the formation of two reactive species Pd-BH3 − and Pd-H as described in Eq. (4). The Langmuir–Hinshelwood mechanism demonstrated the contri-
Fig. 6. UV–vis absorption spectra for the reduction of 4-NP to 4-AP using (A) 1 mg and (B) 2 mg of Pd@PANI-CS-Fe3 O4 nanocatalyst. Reaction conditions: [4NP] = 7 mM, [NaBH4 ] = 0.2 mM, T = 298 K.
Scheme 2. The reduction mechanism of 4-NP to 4-AP.
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Fig. 7. Time dependent absorbance change for reduction of 4-NP by NaBH4 in presence of Pd@PANI-CS-Fe3 O4 nanocomposite (A), plot of -lnAt /Ao versus time (B).
Fig. 8. Recovery of 1 mg of the catalyst.
bution of Pd NPs in the catalytic reaction as illustrated in Scheme 2 Eqs. (5)–(7). On adding NaBH4 to 4-NP solution, there wasn’t any change in the absorption intensity even after several days so the addition of a catalyst is required to increase the reaction rate and hence the progress of this reaction can be monitored [14,23]. Just adding Pd@PANI-CS-Fe3 O4 heterogenous nanocatalyst to 4-NP in the presence of NaBH4 , the reaction preceded successfully and the absorption of 4-NP at 400 nm reduced gradually with time in addition to the emergence of a new peak at 310 nm related to the reduction product 4-AP [90]. On using 1 mg and 2 mg of the nanocatalyst to investigate the effect of the amount of catalyst, the reaction ended in 14 min and 8 min (Fig. 6), respectively. Furthermore, the reaction could be visualized through the naked eye as the intense yellow color of 4-NP disappears due to the formation of 4-AP. Fig. 7A pointed clearly to the decrease of 4-NP concentration with time. As the concentration of 4-NP was much lower than that of the reducing agent, pseudo first order kinetic assumption was applied with regard to 4-NP alone [85]. The pseudo first order equation could be expressed as Eq. (9) in which, At refers to the absorbance at time t, Ao is the absorbance at zero minute and Kapp is the reaction apparent rate constant. dCt /dt = −Kapp Ct or−lnC t /Co = −Kapp tor−lnAt /Ao = −Kapp t
(9)
Table 1 Comparison the catalytic activity of various catalytic substrates for the reduction of 4-NP at room temperature. Catalyst
Dose (mg)
K (S−1 )
Reference
PPY-Pd nanocapsules Guar gum-s-PtNPs p(AMPS)-Cu PPy nanotubes-Ru PPY nanotubes-Pt p(AMPS)-Co Pd@PANI-CS-Fe3 O4
7 5 10 4 4 50 1
8 × 10−3 7 × 10−3 1.72 × 10−3 9 × 10−4 1 × 10−3 2 × 10−3 3.7 × 10−3
[85] [14] [91] [13] [13] [92] The present work
Linear relationship was obtained by applying pseudo first order kinetic assumption and plotting the relation between lnAt /Ao vs. time t as shown in Fig. 7b. The fabricated Pd@PANI-CS-Fe3 O4 heterogenous nanocatalyst showed a high rate of reaction up to 0.222 min−1 . The catalytic activities toward reduction of 4-NP for some metals NPs supported onto different substrates were summarized in (Table 1). These materials include Pt, Cu, Ru. Co as well as Pd NPs decorated onto various supporting substrates such as PPY nanotubes and PPY nanocapsules, guar gum in addition to p(2-acrylamido-2-methyle-1-propansulfonic acid) (APMS). After comparing the results carefully, it is clearly obvious that a high catalytic reaction rate was obtained using a very small amount
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dinitrophenol) and explosives (such as 2,4,6-trinitrotoluene and 2,4,6-trinitrophenol), etc. Most of these nitroaromatic compounds and nitrophenols are toxic and mutagenic for living organisms. Reduction is one of the routes to transform the toxic nitroaromatic compounds to a safer form, the aminoaromatic compounds and hence, there is an increasing demand of using novel nanocatalysts for this purpose. Therefore, in the future, it is important to check the catalytic behavior of Pd@PANI-CS-Fe3 O4 nanocatalyst toward the reduction of some of the aforementioned compounds. The effect of various substituents, in the nitroaromatic and nitroaliphatic moieties (electron withdrawing and electron repelling groups) in different positions, on the catalytic reduction behavior of the nanocatalyst will be investigated.
6. Conclusion
Scheme 3. The separation technique of the heterogenous nanocatalyst. Table 2 The efficacy of the recoveries runs. Run
1st
2nd
3rd
4th
5th
Efficacy
97.8%
96.55%
95.48%
94.16%
93.17%
Reaction conditions: 1 mg of Pd@PANI-CS-Fe3 O4 nanocomposite, [4-NP] = 7 mM, [NaBH4 ] = 0.2 mM, T = 298 K.
(1 mg) of Pd@PANI-CS-Fe3 O4 nanocatalyst as compared to the other catalytic systems with higher doses. Recovery of the catalyst is one of the critical parameters, which add a value to the catalyst so it is important to explore this factor in Pd@PANI-CS-Fe3 O4 nanocatalyst. To test the reusability of 1 mg from the magnetic nanocatalyst, it was separated after the first run via the magnetic separation process using an external magnet added to the wall of the reaction cuvette then the produced 4-AP was collected with a syringe bit by bit to preserve this small amount of catalyst as observed in Scheme 3. The recycled nanocatalyst was washed twice, by freshly distilled water, before the next reuse. The catalyst was recovered up to 4 runs with a small loss in its efficiency. The little time difference for every cycle, as shown in Fig. 8A, proves the fitness of this smart heterogeneous nanocatalyst. This time difference may be attributed to a small weight loss of the nanocatalyst during the separation process or a small leashing. Moreover, the efficacy percentage of the recovery times (Fig. 8B) was calculated via mathematical Eq. (10) reported by Zhang et al. [93]. In addition, the efficacy percentages for recoveries runs were epitomized in Table 2. ˛=
Co − Ct × 100% Co
(10)
where, Co is the initial concentration and Ct is the concentration at the termination stage. 4.1. Future perspectives Some important pharmacologically active compounds have a nitroaromatic group in their molecular structures (such as 4-R2-nitrophenol derivatives, R = H, OCH3 , CH3 , CF3 , or CN). In addition, most nitroaromatic chemical compounds are produced as a consequence of some industrial processes during the synthesis of plastics, polyurethanes, dyes, pesticides (such as diethyl-p-nitrophenyl monothiophosphate and 2-sec-butyl-4,6-
An easy route was followed to fabricate PANI-CS-Fe3 O4 magnetic nanocomposite accompanied by easy anchoring of Pd NPs on its surface. Different characterization techniques such as FTIR, XRD, VSM, EDX, TEM, ICP and SAED were used to ensure the fabrication and the characteristics of Pd@PANI-CS-Fe3 O4 before and after the decoration with Pd NPs. Pd@PANI-CS-Fe3 O4 heterogenous nanocatalyst showed a high efficacy toward the reduction of 4-NP to the opposite amino compound, 4-AP. Furthermore, the magnetic property of the synthesized nanocatalyst enhanced its recovery via magnetic separation from the reaction medium. Finally, Pd@PANI-CS-Fe3 O4 nanocomposite could be ascribed as a promising nanocatalyst for the reduction of 4-NP.
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