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Investigation on the catalytic activity of aminosilane stabilized gold nanocatalysts towards the reduction of nitroaromatics
Colloids and Surfaces A 528 (2017) 48–56
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Investigation on the catalytic activity of aminosilane stabilized gold nanocatalysts towards the reduction of nitroaromatics P. Viswanathan, T.S. Bhuvaneswari, R. Ramaraj
MARK
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School of Chemistry, Centre for Photoelectrochemistry, Madurai Kamaraj University, Madurai, 625 021, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Aminosilane Gold nanoparticles Catalysis Nitroaromatics reduction
In situ preparation of gold nanoparticles (AuNPs) in the presence of silicate matrix, N1-(3-trimethoxysilylpropyl) diethylenetriamine (TPDT) and cetyltrimethylammonium bromide (CTAB), using strong reducing agents sodium borohydride (NaBH4) and hydrazine (N2H4) was established and their catalytic abilities were examined by choosing industrially important model reactions, in order to find the best reducing agent for the preparation of catalytically more active AuNPs. The prepared AuNPs were characterized using UV–vis absorption spectroscopy, XRD, HRTEM, EDX and SAED analyses. The AuNPs produced by NaBH4 showed relatively good mono-dispersion and small size than the AuNPs formed by N2H4. The catalytic activity of AuNPs produced by NaBH4 was found to be better than the AuNPs produced by N2H4 towards the reduction of various nitroaromatics. The turn over frequency (TOF) values obtained for the catalytic reduction of nitrobenzene (NB), 4-nitrophenol (4-NP) and 4nitroaniline (4-NA) using the TPDT-Au-CTAB-NaBH4 catalyst are 0.312, 0.714 and 1.00 s−1, respectively. Moreover, the rate constant (k) value obtained for the reduction of 4-NP is very high when compared to the reported results.
1. Introduction In recent years, extensive investigations on the metal nanoparticles (MNPs) have been carried out by researchers to understand their physical, chemical and catalytic properties, not only to gain scientific knowledge but also to find technological applications [1–4]. When the size of the metal approaches to nanometre regime, i.e. MNPs show significant changes in their electrical, optical, and catalytic properties. While comparing the MNPs to their respective bulk materials, a noticeable change in the reduction potential is observed for MNPs of different sizes as the Fermi potential of nanoparticles (NPs) becomes more negative and this interesting property makes them good candidates as catalysts for various electron transfer processes [5,6]. However, the stability of MNPs is the major problem associated with nanocatalysts due to their high unfavourable surface energies. Hence, for the effective use of MNPs as catalyst, they must be stable and protected from agglomeration. For the stabilization of MNPs, a variety of stabilizing agents are available such as amines, phosphines, thiols, micelles, dendrimers, polymers, and biomolecules [7–10]. In this sense, silicate matrix encapsulated MNPs seems to be advantageous because of their stability, reusability, safer operations, easy scale up, and good catalytic properties [11]. In recent years, our group has reported the catalytic behaviour of various mono- and bi-metallic NPs stabilized by
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Corresponding author. E-mail address: [email protected] (R. Ramaraj).
http://dx.doi.org/10.1016/j.colsurfa.2017.05.040 Received 3 March 2017; Received in revised form 25 April 2017; Accepted 20 May 2017 Available online 25 May 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.
amine functionalized silicates [12–16]. Nitroaromatics are the class of organic compounds, which are widely known as toxic materials released from chemical industry, diesel and gasoline engines and are widely distributed in the environment and threatening the human population [17]. However, the reduced forms of nitroaromatics i.e. aromatic amines are important starting materials for the preparation of dyes, pharmaceuticals, agricultural products, surfactants and polymers [18]. Moreover, aromatic amines are less toxic when compared to nitroaromatics. Therefore, it is important to develop a simple and efficient method for the catalytic conversion of nitroaromatics into aromatic amines. Conventional method of preparation of aromatic amines is the reduction of nitro compounds using catalytic hydrogenation and a variety of other reduction conditions [19,20]. Among the several reducing agents, metals in the presence of acid are widely employed for the reduction of nitroaromatics. But, this method is environmentally hazardous and untidiness [21]. Hence, in the recent years, NaBH4 in water has been used as hydride source and reducing agent. But, the reduction of nitro compounds only with NaBH4, in the absence of catalyst is an extremely slow process. Hence, catalyst is necessary to accomplish the reduction of nitroaromatic compounds in the presence of NaBH4. A variety of MNPs such as Ag [11,18], Au [22], AuAg [23], PdAg [24], Pt/C [25], and NiPt [26] have been used as catalyst to drive the
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reduction of nitrocompounds. Among these MNPs, gold is the widely studied catalyst due to its intriguing catalytic properties [27,28]. Moreover, the properties of MNPs, which are prepared in the solution phase can be tuned by varying its composition and preparation method, especially the reducing agent employed for the reduction of metal salts [29]. Hence, in the present work, we report a facile preparation of AuNPs in the presence of amine functionalized silicate matrix and a surfactant using two different reducing agents, NaBH4 and N2H4. So prepared AuNPs were successfully applied for the catalytic conversion of various nitroaromatics and their catalytic activities were compared. 2. Experimental section 2.1. Materials and methods Chloroauric acid (HAuCl4), cetyltrimethylammonium bromide (CTAB) and N1-(3-trimethoxysilylpropyl)diethylenetriamine (TPDT) were received from Sigma-Aldrich. All other chemicals are analytical grade and were received from Merck. All glassware was thoroughly cleaned with aqua regia (1:3 HNO3/HCl v/v) (caution: Aqua regia is a powerful oxidizing agent and it should be handled with extreme care) and rinsed extensively with distilled water before use. UV–vis absorption spectra were recorded using Agilent Technologies 8453 spectrophotometer. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses were conducted on a TECNAI-T20 operated at 200 kV. The specimen for the HRTEM analysis was prepared by dropping the colloidal solution onto a carbon coated copper grid and dried at room temperature. X-ray diffraction (XRD) pattern was recorded using PAN X’pert Pro instrument. 2.2. Preparation of AuNPs Fig. 1. UV–vis absorption spectra obtained for TPDT-Au-CTAB (a), CTAB-Au (b), TPDTAu (c), A) NaBH4 reduced and B) N2H4 reduced AuNPs.
In a typical experiment, 5 mL of 5 mM CTAB aqueous solution was mixed with 25 μL of 1 M TPDT solution and stirred for 15 min. To this solution, 50 μL of 0.1 M HAuCl4 solution was added and stirred for 5 min. A 0.3 mL of 0.05 M ice-cold NaBH4 was slowly added to the above solution and stirring was continued for another 3 h. Colour of the solution changed from light yellow to wine red immediately after the addition of NaBH4. This confirms the formation of AuNPs and the NPs represented as TPDT-Au-CTAB-NaBH4. The prepared AuNPs were stable for more than a month. CTAB-Au-NaBH4, TPDT-Au-NaBH4, were prepared either in the presence of only CTAB or TPDT by following the same procedure. Hydrazine reduced AuNPs (TPDT-AuCTAB-N2H4, CTAB-Au-N2H4, TPDT-Au-N2H4) were prepared using 0.3 mL of 0.05 M N2H4 by following the same procedure as mentioned above.
of MNPs are highly dependent on the reducing agents used to reduce the metal salts. Hence, it is meaningful to compare the catalytic activities of AuNPs produced by these two reducing agents. The absorption spectroscopy is a fundamental tool to characterize the AuNPs and which can be effectively used to corroborate the particle size and shape and oxidation state of AuNPs derived from TEM and XPS studies [30]. The surface plasmon resonance (SPR) bands of MNPs are strongly dependent on their size, shape, composition and their local environment [10,31]. Fig. 1 shows the SPR band observed for NaBH4 and N2H4 reduced AuNPs. AuNPs prepared in this work by using NaBH4 and N2H4 showed a strong absorption band around 520 nm, which confirms the formation of gold nanostructures in the presence of TPDT and CTAB. TPDT-Au-CTAB-NaBH4, CTAB-Au-NaBH4 and TPDT-AuNaBH4 showed the absorption bands (λmax) at 518, 527 and 523 nm, respectively. Similarly, TPDT-Au-CTAB-N2H4, CTAB-Au-N2H4 and TPDT-Au-N2H4 showed the absorption bands at 521, 523 and 534 nm, respectively. These sharp SPR bands of AuNPs may be due to the formation of mono-dispersed AuNPs. However, when the absorption spectrum of TPDT-Au-CTAB-N2H4 was compared with other AuNPs prepared here, TPDT-Au-CTAB-N2H4 showed a broad absorption band around 750 nm along with the main SPR band of Au at 521 nm. This longitudinal band is attributed to the formation anisotropic gold nanostructures with rod-like nature. The rod-like nature of TPDT-AuCTAB-N2H4 gold nanostructure was further confirmed through HRTEM studies (Fig. 2D & E). From HRTEM studies of TPDT-Au-CTAB-N2H4, it is observed that some bend rod-like structures were formed in the presence of CTAB and TPDT in addition to the spherical AuNPs. Hence, the absorption studies clearly reveal the formation of AuNPs in the presence of TPDT and CTAB.
2.3. Catalytic reduction of nitroaromatics Catalytic reduction of nitroaromatics was carried out as follows: 0.1 mL of 2 mM of nitroaromatic compound solution was mixed with 1.150 mL of water followed by 0.75 mL of 0.056 M NaBH4. To this mixture 10 μL of AuNPs was added and the reaction progress was monitored using UV–vis absorption spectroscopy. 3. Results and discussion 3.1. Absorption spectral studies Preparation of AuNPs can be achieved through variety of methods, among them, chemical reduction of metal salts is a more common method, due to its efficiency, low cost and ease of synthesis. Though many reducing agents are available for the preparation of MNPs, NaBH4 and N2H4 are the widely used reducing agents. Moreover, the properties 49
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Fig. 2. HRTEM images obtained for TPDT-Au-CTAB-NaBH4 (A & B) and TPDT-Au-CTAB-N2H4 (D & E) and their corresponding SAED patterns C & F, respectively.
NaBH4 (Fig. 2C) and N2H4 (Fig. 2F) reduced AuNPs correspond to (111) crystal planes of gold. Therefore, HRTEM studies clearly reveal that spherical AuNPs with a reasonably good mono-dispersity was achieved when NaBH4 employed as reducing agent, whereas, in the case of N2H4, anisotropic AuNPs were formed in the presence of TPDT and CTAB. Fig. 3 shows the powder XRD pattern obtained for TPDT-Au-CTABNaBH4 and TPDT-Au-CTAB-N2H4. XRD profile showed a single intense peak centred at the 2θ value of 37.8°, which corresponds to the (1 1 1) lattice plane of the face-centred cubic structure of Au crystals, whereas the peaks belonging to other lattice planes are quite weak/not observed. Mandal et al. observed similar type of XRD pattern for spongy gold nanocrystals [32]. The observed 2θ (37.8) value was found to be in good agreement with the standard data base value. Hence, the observed single intense diffraction peak indicates that the Au atoms are unidirectionally arranged in the AuNPs.
3.2. HRTEM and XRD studies The size and shape profile of Au nanostructures formed in the presence of CTAB and TPDT was analyzed by recording HRTEM images. Fig. 2 shows the HRTEM images obtained for both NaBH4 and N2H4 reduced AuNPs. A well-dispersed spherical AuNPs with an average particle size of 6 nm were formed in the presence of CTAB and TPDT, when NaBH4 was used as reducing agent. Whereas, the mixture of spherical and bend-rod like AuNPs were formed when N2H4 was employed as reducing agent. This bend-rod like Au nanostructures are responsible for hump observed around 750 nm in absorption spectrum (Fig. 1B(a)). The d spacing values obtained from SEAD pattern of both
3.3. Catalytic studies Catalytic reduction of nitroaromatics in the presence of NaBH4 was chosen to investigate the catalytic activity of the prepared AuNPs. The absorption changes involved during the reduction reaction of 4-NP over metal and semiconductor nanocomposites catalyst provides a simple way to monitor the reaction kinetics using the spectroscopic method [11]. In neutral or acidic condition, 4-nitrophenol exhibits a strong absorption band at 317 nm. Upon the addition of NaBH4 to 4nitrophenol, the increased alkalinity of the solution leads to the formation of 4-nitrophenolate ions, which shows a new absorption band at 400 nm [22,33]. Similarly, aqueous solutions of 4-NA and NB shows the strong absorption band at 378 [34] and 265 nm [35], respectively, in the presence of NaBH4. In the absence of AuNPs, reduction of nitroaromatics is extremely slow, but in the presence of AuNPs catalyst, time dependent absorption spectra showed a progressive decrease in the absorption peak at 280, 378 and 400 for NB, 4-NA and 4-NP, respectively, with the appearance of new peaks at 230, 305 and 300 nm, corresponding to aniline, p-phenylenediamine and paminophenol, respectively (Fig. 4). These observations suggest that
Fig. 3. XRD patterns of TPDT-Au-CTAB-NaBH4 (a) and TPDT-Au-CTAB-N2H4 (b) NPs.
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Fig. 4. Time-dependent absorption spectral changes obtained for the borohydride reduction of NB (A, D), 4-NP (B, E) and 4-NA (C, F) using TPDT-Au-CTAB-NaBH4(A-C) and TPDT-AuCTAB-N2H4 (D-E) as catalyst.
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Fig. 5. Plots of ln(A) versus time for the reduction of NB (A), 4-NP (B) and 4-NA (C) using TPDT-Au-CTAB- NaBH4 (a) and TPDT-Au-CTAB-N2H4 (b) as catalyst.
4-NA and 4-NP when compared to TPDT-Au-CTAB-NaBH4 catalyst. Table 1 shows the comparison of rate constants obtained for the catalytic reduction of nitroaromatics using various AuNPs catalysts prepared in this work. From the Table 1, it is clear that the TPDT-AuCTAB-NaBH4 shows higher k and knor values for the reduction of NB, 4NP and 4-NA when compared to TPDT-Au-CTAB-N2H4 and all the other catalysts prepared here. Overall, NaBH4 reduced AuNPs having better catalytic activity than N2H4 reduced AuNPs, except TPDT-Au-N2H4 in comparison with TPDT-Au-NaBH4. Hence, it was found out that the NaBH4 reduced AuNPs show better catalytic activity than N2H4 reduced AuNPs. The better catalytic activity of TPDT-Au-CTAB-NaBH4 was ascribed to the following fact that the AuNPs formed in the presence of CTAB and TPDT was very small and uniform when the NaBH4 was employed as reducing agent, whereas N2H2 produces the anisotropic and bigger AuNPs in the identical environment. This variation in size causes the better catalytic activity of AuNPs. In addition to smaller size, NaBH4 reduction of Au ions might have created a more number of lowcoordinated Au atoms (active sites) over the AuNPs surface. Table 2 compares the catalytic performance of present catalyst with already reported literatures for the reduction of 4-NP and confirms the better catalytic performance of present catalytic system over others. Owing to the best catalytic activity of TPDT-Au-CTAB-NaBH4 catalyst, it was also applied for the catalytic reduction of 2-chloro 4nitrophenol and 2,6-dichloro 4-nitrophenol into their corresponding amino compounds (Fig. 9). This shows the effective catalytic property of TPDT-Au-CTAB-NaBH4 catalyst towards the reduction of nitro groups even in the presence chloro substituent on the benzene ring. The electron withdrawing effect of chloro substituent present in the benzene ring lowers the electron density on the nitro group and facilitates the electron accepting nature of nitro group during the reduction when compared to 4-NP. Consequently, the reduction of nitro groups in 2chloro 4-nitrophenol and 2, 6-dichloro 4-nitrophenol becomes faster than the reduction of 4-NP in the similar environment.
prepared AuNPs catalyst can successfully catalyze the reduction of nitroaromatics. In the catalytic reduction of nitrobenzene, the characteristic absorption band of NB observed at 265 nm at zero time was red-shifted to 280 nm during the course of reduction reaction. This redshift is attributed to the formation of azobenzene intermediate during the course of the reduction of NB over gold catalyst in the presence of sodium borohydride, as reported by Grassi, et al. [36]. However, at the end of the reaction, the starting material and the formed intermediate were converted into product, aniline, which was confirmed by the characteristics absorption band of aniline at 230 nm. In the catalytic experiments, the concentration of NaBH4 is excess when compared to the concentration of nitroaromatics and the rate of the reaction is independent of NaBH4 concentration. Hence, the catalytic reduction reaction follows the pseudo-first order kinetics and the rate constant (k) values were calculated from the slope of ln(A) versus time plot. Moreover, the normalized rate constant (knor) was also calculated, which is associated with the amount of catalyst used, i.e. knor = kapp/m, where m is mass of the catalyst used. Here, rate constant values were normalized using the gold concentration used in the preparation of the catalyst. Fig. 4 shows the catalytic reduction of NB, 4-NP and 4-NA in the presence of TPDT-Au-CTAB-NaBH4 and TPDT-Au-CTAB-N2H4 catalysts. Fig. 5 shows the plots of ln(A) versus time obtained for the catalytic reduction of NB (Fig. 5A), 4-NP (Fig. 5B) and 4-NA (Fig. 5C) using TPDT-Au-CTAB-NaBH4 and TPDT-Au-CTAB-N2H4 as catalyst. The rate constant values obtained for the catalytic reduction of NB, 4-NP and 4NA on TPDT-Au-CTAB-NaBH4 catalyst are 3.16 × 10−2, 1.10 × 10−1 and 1.47 × 10−1 s−1, respectively, whereas the rate constant values obtained for the catalytic reduction of NB, 4-NP and 4-NA on TPDT-AuCTAB-N2H4 catalyst are 1.69 × 10−2, 5.10 × 10−2 and 5.90 × 10−2, respectively. Hence, Fig. 4 and Fig. 5 clearly reveal that the catalytic reduction of nitroaromatics was faster in the presence of TPDT-AuCTAB-NaBH4 catalyst when compared to the TPDT-Au-CTAB-N2H4 catalyst. The turn over frequency values obtained for the catalytic reduction of NB, 4-NP and 4-NA on TPDT-Au-CTAB-NaBH4 catalyst are 0.312, 0.714 and 1.00 s‐1, respectively. The isosbestic points obtained during the reduction of 4-NA (Fig. 4C and 4F) suggest the formation of single product, p-phenylenediamine from 4-NA. Figs. 6 and 7 shows the catalytic reduction of nitroaromatics using CTAB-Au and TPDT-Au NPs catalysts, respectively and Fig. 8 compiles the corresponding kinetic plots obtained for them. The catalytic reduction of NB to aniline over TPDT-Au-CTAB-NaBH4 catalyst was found to complete within the time period of 64 s, whereas TPDT-AuCTAB-N2H4 needs 120 s and all other AuNPs catalyst prepared here took more than 64 s for the same. A complete reduction of 4-NP and 4NA at TPDT-Au-CTAB-NaBH4 catalyst was accomplished with 24 and 20 s, respectively, whereas, TPDT-Au-CTAB-N2H4 takes 70 and 44 s for the reduction of 4-NP and 4-NA, respectively. Moreover, all other catalyst prepared here, taking more time for the catalytic reduction of
4. Conclusion In summary, a simple method for the preparation of AuNPs dispersed in silicate matrix was established using NaBH4 and N2H4 as reducing agents and the reduction of nitroaromatics was chosen as the model reaction to study the catalytic behaviour of AuNPs. NaBH4 reduced AuNPs shows strong and single SPR band at 518 nm while the N2H4 reduced AuNPs shows the two SPR bands due to anisotropic nature. NaBH4 reduced AuNPs having better catalytic activity than the N2H4 reduced AuNPs. A faster conversion of nitroaromatics into aromatic amine was observed at NaBH4 reduced AuNPs and rate constants obtained for these reactions were higher than the already reported values. Hence, these results conclude that the morphology, optical and catalytic properties of AuNPs are highly dependent on the reducing agent used for the reduction of metal salts. 52
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Fig. 6. Time-dependent absorption spectral changes obtained for the borohydride reduction of NB (A, D), 4-NP (B, E) and 4-NA (C, F) using CTAB-Au-NaBH4 (A-C) and CTAB-Au-N2H4 (DE) as catalyst.
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Fig. 7. Time-dependent absorption spectral changes obtained for the borohydride reduction of NB (A, D), 4-NP (B, E) and 4-NA (C, F) using TPDT-Au-NaBH4 (A-C) and TPDT-Au-N2H4 (DE) as catalyst.
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Fig. 8. Plots of ln(A) versus time for the reduction of NB (A), 4-NP (B) and 4-NA (C) using TPDT-Au-CTAB-NaBH4 (a) and TPDT-Au-CTAB-N2H4 (b) as catalyst.
Table 1 Summary of the rate constants obtained for the catalytic reduction nitroaromatics using AuNPs catalyst prepared in the present work. Catalyst
TPDT-Au-CTAB-NaBH4 CTAB-Au-NaBH4 TPDT-Au-NaBH4 TPDT-Au-CTAB-N2H4 CTAB-Au-N2H4 TPDT-Au-N2H4
kapp and knor for the reduction of NB
kapp and knor for the reduction of 4-NP
kapp and knor for the reduction of 4-NA
kapp (s−1)
knor (mg−1 s−1)
kapp (s−1)
knor (mg−1 s−1)
kapp (s−1)
16.03 5.32 11.31 8.57 0.48 14.97
1.10 × 10−1 4.80 × 10−2 4.20 × 10−2 5.10 × 10−2 0.70 × 10−2 3.30 × 10 −2
55.83 24.36 21.31 25.88 3.55 16.75
1.47 × 10 4.83 × 10 2.49 × 10 5.90 × 10 0.76 × 10 2.33 × 10
3.16 × 10 1.05 × 10 2.23 × 10 1.69 × 10 9.63 × 10 2.95 × 10
−2 −2 −2 −2 −4 −2
knor (mg−1 s−1) −1 −2 −2 −2 −2 −2
74.61 24.51 12.63 29.94 3.85 11.82
Table 2 Comparison of the catalytic performances of present AuNPs catalyst with already reported catalysts towards the reduction of 4-NP. Catalyst material
Catalyst (mol%)
NaBH4 equivalent
kapp (s−1)
References
ZnNPs Se-Fe3O4-CoNPs Pt–Au pNDs/RGOs CNT/piHP PdNPs Fe@Au–ATPGO Microgels-PdNPs 2D graphene oxide/ SiO2–Au CTAB-Au NPs Au@DHBC NPs PGMA@PAH@Au NPs Methyl-imidazoliumbased ionic polymer-AuNPs PAMAM-AuNPs PPI-AuNPs rCD-AuNPs CeO2-AuNPs Au nanocages Au–Ag (0.7:0.3)/silica Au@PVP Cyclodextrin-AuNPs Polyaniline-AuNPs Fe3O4-PdNPs Ppy/TiO2-PdNPs SBA-15-PdNPs CuNPs PEDOT-PPS-Pd NPs TPDT-Au-CTAB-NaBH4
10 – – 4 – 2.1 –
50 40 – 80 – 100 –
– 6.54 × 10−3 3.8 × 10−3 5 × 10−3 1.4 × 10−3 1.5 × 10−3 17 × 10−3
[37] [38] [39] [40] [41] [42] [43]
1.25 0.5 3.2 20
500 300 400 88
6.1 × 10−3 9.5 × 10−−3 – 3.3 × 10−2
[44] [45] [46] [47]
0.10 0.10 5.2 × 10−5 – 4.5 × 10−6 0.8 1.56 17.6 17.6 10 2.6 100 666.7 77 5
100 100 75 1333 300 750 147 44 44 139 11 1000 167 Excess 210
7.91 × 10−3 1.47 × 10−2 9.75 × 10−4 1.28 × 10−2 9.3 × 10−2 5.6 × 10−2 1.02 × 10−2 4.65 × 10−3 11.7 × 10−3 3.3 × 10−2 1.22 × 10−2 1.2 × 10−2 1.6 × 10−3 6.58 × 10−2 1.10 × 10−1
[48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] Present catalyst
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Fig. 9. Time-dependent absorption spectral changes obtained for the borohydride reduction of 2-chloro 4-nitrophenol (A), 2, 6-dichloro 4-nitrophenol (B) using TPDT-Au-CTAB-NaBH4 as catalyst. [27] [28] [29] [30]
Acknowledgments RR acknowledges the financial support received from the CSIREmeritus Scientist scheme (No. 21(1006)/15/EMR-II), New Delhi, India. PV is the recipient of Senior Research Fellowship under UGCBSR scheme, India.
[31] [32] [33] [34] [35] [36]
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Investigation on the catalytic activity of aminosilane stabilized gold nanocatalysts towards the reduction of nitroaromatics P. Viswanathan, T.S. Bhuvaneswari, R. Ramaraj
MARK
⁎
School of Chemistry, Centre for Photoelectrochemistry, Madurai Kamaraj University, Madurai, 625 021, India
A R T I C L E I N F O
A B S T R A C T
Keywords: Aminosilane Gold nanoparticles Catalysis Nitroaromatics reduction
In situ preparation of gold nanoparticles (AuNPs) in the presence of silicate matrix, N1-(3-trimethoxysilylpropyl) diethylenetriamine (TPDT) and cetyltrimethylammonium bromide (CTAB), using strong reducing agents sodium borohydride (NaBH4) and hydrazine (N2H4) was established and their catalytic abilities were examined by choosing industrially important model reactions, in order to find the best reducing agent for the preparation of catalytically more active AuNPs. The prepared AuNPs were characterized using UV–vis absorption spectroscopy, XRD, HRTEM, EDX and SAED analyses. The AuNPs produced by NaBH4 showed relatively good mono-dispersion and small size than the AuNPs formed by N2H4. The catalytic activity of AuNPs produced by NaBH4 was found to be better than the AuNPs produced by N2H4 towards the reduction of various nitroaromatics. The turn over frequency (TOF) values obtained for the catalytic reduction of nitrobenzene (NB), 4-nitrophenol (4-NP) and 4nitroaniline (4-NA) using the TPDT-Au-CTAB-NaBH4 catalyst are 0.312, 0.714 and 1.00 s−1, respectively. Moreover, the rate constant (k) value obtained for the reduction of 4-NP is very high when compared to the reported results.
1. Introduction In recent years, extensive investigations on the metal nanoparticles (MNPs) have been carried out by researchers to understand their physical, chemical and catalytic properties, not only to gain scientific knowledge but also to find technological applications [1–4]. When the size of the metal approaches to nanometre regime, i.e. MNPs show significant changes in their electrical, optical, and catalytic properties. While comparing the MNPs to their respective bulk materials, a noticeable change in the reduction potential is observed for MNPs of different sizes as the Fermi potential of nanoparticles (NPs) becomes more negative and this interesting property makes them good candidates as catalysts for various electron transfer processes [5,6]. However, the stability of MNPs is the major problem associated with nanocatalysts due to their high unfavourable surface energies. Hence, for the effective use of MNPs as catalyst, they must be stable and protected from agglomeration. For the stabilization of MNPs, a variety of stabilizing agents are available such as amines, phosphines, thiols, micelles, dendrimers, polymers, and biomolecules [7–10]. In this sense, silicate matrix encapsulated MNPs seems to be advantageous because of their stability, reusability, safer operations, easy scale up, and good catalytic properties [11]. In recent years, our group has reported the catalytic behaviour of various mono- and bi-metallic NPs stabilized by
⁎
Corresponding author. E-mail address: [email protected] (R. Ramaraj).
http://dx.doi.org/10.1016/j.colsurfa.2017.05.040 Received 3 March 2017; Received in revised form 25 April 2017; Accepted 20 May 2017 Available online 25 May 2017 0927-7757/ © 2017 Elsevier B.V. All rights reserved.
amine functionalized silicates [12–16]. Nitroaromatics are the class of organic compounds, which are widely known as toxic materials released from chemical industry, diesel and gasoline engines and are widely distributed in the environment and threatening the human population [17]. However, the reduced forms of nitroaromatics i.e. aromatic amines are important starting materials for the preparation of dyes, pharmaceuticals, agricultural products, surfactants and polymers [18]. Moreover, aromatic amines are less toxic when compared to nitroaromatics. Therefore, it is important to develop a simple and efficient method for the catalytic conversion of nitroaromatics into aromatic amines. Conventional method of preparation of aromatic amines is the reduction of nitro compounds using catalytic hydrogenation and a variety of other reduction conditions [19,20]. Among the several reducing agents, metals in the presence of acid are widely employed for the reduction of nitroaromatics. But, this method is environmentally hazardous and untidiness [21]. Hence, in the recent years, NaBH4 in water has been used as hydride source and reducing agent. But, the reduction of nitro compounds only with NaBH4, in the absence of catalyst is an extremely slow process. Hence, catalyst is necessary to accomplish the reduction of nitroaromatic compounds in the presence of NaBH4. A variety of MNPs such as Ag [11,18], Au [22], AuAg [23], PdAg [24], Pt/C [25], and NiPt [26] have been used as catalyst to drive the
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reduction of nitrocompounds. Among these MNPs, gold is the widely studied catalyst due to its intriguing catalytic properties [27,28]. Moreover, the properties of MNPs, which are prepared in the solution phase can be tuned by varying its composition and preparation method, especially the reducing agent employed for the reduction of metal salts [29]. Hence, in the present work, we report a facile preparation of AuNPs in the presence of amine functionalized silicate matrix and a surfactant using two different reducing agents, NaBH4 and N2H4. So prepared AuNPs were successfully applied for the catalytic conversion of various nitroaromatics and their catalytic activities were compared. 2. Experimental section 2.1. Materials and methods Chloroauric acid (HAuCl4), cetyltrimethylammonium bromide (CTAB) and N1-(3-trimethoxysilylpropyl)diethylenetriamine (TPDT) were received from Sigma-Aldrich. All other chemicals are analytical grade and were received from Merck. All glassware was thoroughly cleaned with aqua regia (1:3 HNO3/HCl v/v) (caution: Aqua regia is a powerful oxidizing agent and it should be handled with extreme care) and rinsed extensively with distilled water before use. UV–vis absorption spectra were recorded using Agilent Technologies 8453 spectrophotometer. High resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analyses were conducted on a TECNAI-T20 operated at 200 kV. The specimen for the HRTEM analysis was prepared by dropping the colloidal solution onto a carbon coated copper grid and dried at room temperature. X-ray diffraction (XRD) pattern was recorded using PAN X’pert Pro instrument. 2.2. Preparation of AuNPs Fig. 1. UV–vis absorption spectra obtained for TPDT-Au-CTAB (a), CTAB-Au (b), TPDTAu (c), A) NaBH4 reduced and B) N2H4 reduced AuNPs.
In a typical experiment, 5 mL of 5 mM CTAB aqueous solution was mixed with 25 μL of 1 M TPDT solution and stirred for 15 min. To this solution, 50 μL of 0.1 M HAuCl4 solution was added and stirred for 5 min. A 0.3 mL of 0.05 M ice-cold NaBH4 was slowly added to the above solution and stirring was continued for another 3 h. Colour of the solution changed from light yellow to wine red immediately after the addition of NaBH4. This confirms the formation of AuNPs and the NPs represented as TPDT-Au-CTAB-NaBH4. The prepared AuNPs were stable for more than a month. CTAB-Au-NaBH4, TPDT-Au-NaBH4, were prepared either in the presence of only CTAB or TPDT by following the same procedure. Hydrazine reduced AuNPs (TPDT-AuCTAB-N2H4, CTAB-Au-N2H4, TPDT-Au-N2H4) were prepared using 0.3 mL of 0.05 M N2H4 by following the same procedure as mentioned above.
of MNPs are highly dependent on the reducing agents used to reduce the metal salts. Hence, it is meaningful to compare the catalytic activities of AuNPs produced by these two reducing agents. The absorption spectroscopy is a fundamental tool to characterize the AuNPs and which can be effectively used to corroborate the particle size and shape and oxidation state of AuNPs derived from TEM and XPS studies [30]. The surface plasmon resonance (SPR) bands of MNPs are strongly dependent on their size, shape, composition and their local environment [10,31]. Fig. 1 shows the SPR band observed for NaBH4 and N2H4 reduced AuNPs. AuNPs prepared in this work by using NaBH4 and N2H4 showed a strong absorption band around 520 nm, which confirms the formation of gold nanostructures in the presence of TPDT and CTAB. TPDT-Au-CTAB-NaBH4, CTAB-Au-NaBH4 and TPDT-AuNaBH4 showed the absorption bands (λmax) at 518, 527 and 523 nm, respectively. Similarly, TPDT-Au-CTAB-N2H4, CTAB-Au-N2H4 and TPDT-Au-N2H4 showed the absorption bands at 521, 523 and 534 nm, respectively. These sharp SPR bands of AuNPs may be due to the formation of mono-dispersed AuNPs. However, when the absorption spectrum of TPDT-Au-CTAB-N2H4 was compared with other AuNPs prepared here, TPDT-Au-CTAB-N2H4 showed a broad absorption band around 750 nm along with the main SPR band of Au at 521 nm. This longitudinal band is attributed to the formation anisotropic gold nanostructures with rod-like nature. The rod-like nature of TPDT-AuCTAB-N2H4 gold nanostructure was further confirmed through HRTEM studies (Fig. 2D & E). From HRTEM studies of TPDT-Au-CTAB-N2H4, it is observed that some bend rod-like structures were formed in the presence of CTAB and TPDT in addition to the spherical AuNPs. Hence, the absorption studies clearly reveal the formation of AuNPs in the presence of TPDT and CTAB.
2.3. Catalytic reduction of nitroaromatics Catalytic reduction of nitroaromatics was carried out as follows: 0.1 mL of 2 mM of nitroaromatic compound solution was mixed with 1.150 mL of water followed by 0.75 mL of 0.056 M NaBH4. To this mixture 10 μL of AuNPs was added and the reaction progress was monitored using UV–vis absorption spectroscopy. 3. Results and discussion 3.1. Absorption spectral studies Preparation of AuNPs can be achieved through variety of methods, among them, chemical reduction of metal salts is a more common method, due to its efficiency, low cost and ease of synthesis. Though many reducing agents are available for the preparation of MNPs, NaBH4 and N2H4 are the widely used reducing agents. Moreover, the properties 49
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Fig. 2. HRTEM images obtained for TPDT-Au-CTAB-NaBH4 (A & B) and TPDT-Au-CTAB-N2H4 (D & E) and their corresponding SAED patterns C & F, respectively.
NaBH4 (Fig. 2C) and N2H4 (Fig. 2F) reduced AuNPs correspond to (111) crystal planes of gold. Therefore, HRTEM studies clearly reveal that spherical AuNPs with a reasonably good mono-dispersity was achieved when NaBH4 employed as reducing agent, whereas, in the case of N2H4, anisotropic AuNPs were formed in the presence of TPDT and CTAB. Fig. 3 shows the powder XRD pattern obtained for TPDT-Au-CTABNaBH4 and TPDT-Au-CTAB-N2H4. XRD profile showed a single intense peak centred at the 2θ value of 37.8°, which corresponds to the (1 1 1) lattice plane of the face-centred cubic structure of Au crystals, whereas the peaks belonging to other lattice planes are quite weak/not observed. Mandal et al. observed similar type of XRD pattern for spongy gold nanocrystals [32]. The observed 2θ (37.8) value was found to be in good agreement with the standard data base value. Hence, the observed single intense diffraction peak indicates that the Au atoms are unidirectionally arranged in the AuNPs.
3.2. HRTEM and XRD studies The size and shape profile of Au nanostructures formed in the presence of CTAB and TPDT was analyzed by recording HRTEM images. Fig. 2 shows the HRTEM images obtained for both NaBH4 and N2H4 reduced AuNPs. A well-dispersed spherical AuNPs with an average particle size of 6 nm were formed in the presence of CTAB and TPDT, when NaBH4 was used as reducing agent. Whereas, the mixture of spherical and bend-rod like AuNPs were formed when N2H4 was employed as reducing agent. This bend-rod like Au nanostructures are responsible for hump observed around 750 nm in absorption spectrum (Fig. 1B(a)). The d spacing values obtained from SEAD pattern of both
3.3. Catalytic studies Catalytic reduction of nitroaromatics in the presence of NaBH4 was chosen to investigate the catalytic activity of the prepared AuNPs. The absorption changes involved during the reduction reaction of 4-NP over metal and semiconductor nanocomposites catalyst provides a simple way to monitor the reaction kinetics using the spectroscopic method [11]. In neutral or acidic condition, 4-nitrophenol exhibits a strong absorption band at 317 nm. Upon the addition of NaBH4 to 4nitrophenol, the increased alkalinity of the solution leads to the formation of 4-nitrophenolate ions, which shows a new absorption band at 400 nm [22,33]. Similarly, aqueous solutions of 4-NA and NB shows the strong absorption band at 378 [34] and 265 nm [35], respectively, in the presence of NaBH4. In the absence of AuNPs, reduction of nitroaromatics is extremely slow, but in the presence of AuNPs catalyst, time dependent absorption spectra showed a progressive decrease in the absorption peak at 280, 378 and 400 for NB, 4-NA and 4-NP, respectively, with the appearance of new peaks at 230, 305 and 300 nm, corresponding to aniline, p-phenylenediamine and paminophenol, respectively (Fig. 4). These observations suggest that
Fig. 3. XRD patterns of TPDT-Au-CTAB-NaBH4 (a) and TPDT-Au-CTAB-N2H4 (b) NPs.
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Fig. 4. Time-dependent absorption spectral changes obtained for the borohydride reduction of NB (A, D), 4-NP (B, E) and 4-NA (C, F) using TPDT-Au-CTAB-NaBH4(A-C) and TPDT-AuCTAB-N2H4 (D-E) as catalyst.
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Fig. 5. Plots of ln(A) versus time for the reduction of NB (A), 4-NP (B) and 4-NA (C) using TPDT-Au-CTAB- NaBH4 (a) and TPDT-Au-CTAB-N2H4 (b) as catalyst.
4-NA and 4-NP when compared to TPDT-Au-CTAB-NaBH4 catalyst. Table 1 shows the comparison of rate constants obtained for the catalytic reduction of nitroaromatics using various AuNPs catalysts prepared in this work. From the Table 1, it is clear that the TPDT-AuCTAB-NaBH4 shows higher k and knor values for the reduction of NB, 4NP and 4-NA when compared to TPDT-Au-CTAB-N2H4 and all the other catalysts prepared here. Overall, NaBH4 reduced AuNPs having better catalytic activity than N2H4 reduced AuNPs, except TPDT-Au-N2H4 in comparison with TPDT-Au-NaBH4. Hence, it was found out that the NaBH4 reduced AuNPs show better catalytic activity than N2H4 reduced AuNPs. The better catalytic activity of TPDT-Au-CTAB-NaBH4 was ascribed to the following fact that the AuNPs formed in the presence of CTAB and TPDT was very small and uniform when the NaBH4 was employed as reducing agent, whereas N2H2 produces the anisotropic and bigger AuNPs in the identical environment. This variation in size causes the better catalytic activity of AuNPs. In addition to smaller size, NaBH4 reduction of Au ions might have created a more number of lowcoordinated Au atoms (active sites) over the AuNPs surface. Table 2 compares the catalytic performance of present catalyst with already reported literatures for the reduction of 4-NP and confirms the better catalytic performance of present catalytic system over others. Owing to the best catalytic activity of TPDT-Au-CTAB-NaBH4 catalyst, it was also applied for the catalytic reduction of 2-chloro 4nitrophenol and 2,6-dichloro 4-nitrophenol into their corresponding amino compounds (Fig. 9). This shows the effective catalytic property of TPDT-Au-CTAB-NaBH4 catalyst towards the reduction of nitro groups even in the presence chloro substituent on the benzene ring. The electron withdrawing effect of chloro substituent present in the benzene ring lowers the electron density on the nitro group and facilitates the electron accepting nature of nitro group during the reduction when compared to 4-NP. Consequently, the reduction of nitro groups in 2chloro 4-nitrophenol and 2, 6-dichloro 4-nitrophenol becomes faster than the reduction of 4-NP in the similar environment.
prepared AuNPs catalyst can successfully catalyze the reduction of nitroaromatics. In the catalytic reduction of nitrobenzene, the characteristic absorption band of NB observed at 265 nm at zero time was red-shifted to 280 nm during the course of reduction reaction. This redshift is attributed to the formation of azobenzene intermediate during the course of the reduction of NB over gold catalyst in the presence of sodium borohydride, as reported by Grassi, et al. [36]. However, at the end of the reaction, the starting material and the formed intermediate were converted into product, aniline, which was confirmed by the characteristics absorption band of aniline at 230 nm. In the catalytic experiments, the concentration of NaBH4 is excess when compared to the concentration of nitroaromatics and the rate of the reaction is independent of NaBH4 concentration. Hence, the catalytic reduction reaction follows the pseudo-first order kinetics and the rate constant (k) values were calculated from the slope of ln(A) versus time plot. Moreover, the normalized rate constant (knor) was also calculated, which is associated with the amount of catalyst used, i.e. knor = kapp/m, where m is mass of the catalyst used. Here, rate constant values were normalized using the gold concentration used in the preparation of the catalyst. Fig. 4 shows the catalytic reduction of NB, 4-NP and 4-NA in the presence of TPDT-Au-CTAB-NaBH4 and TPDT-Au-CTAB-N2H4 catalysts. Fig. 5 shows the plots of ln(A) versus time obtained for the catalytic reduction of NB (Fig. 5A), 4-NP (Fig. 5B) and 4-NA (Fig. 5C) using TPDT-Au-CTAB-NaBH4 and TPDT-Au-CTAB-N2H4 as catalyst. The rate constant values obtained for the catalytic reduction of NB, 4-NP and 4NA on TPDT-Au-CTAB-NaBH4 catalyst are 3.16 × 10−2, 1.10 × 10−1 and 1.47 × 10−1 s−1, respectively, whereas the rate constant values obtained for the catalytic reduction of NB, 4-NP and 4-NA on TPDT-AuCTAB-N2H4 catalyst are 1.69 × 10−2, 5.10 × 10−2 and 5.90 × 10−2, respectively. Hence, Fig. 4 and Fig. 5 clearly reveal that the catalytic reduction of nitroaromatics was faster in the presence of TPDT-AuCTAB-NaBH4 catalyst when compared to the TPDT-Au-CTAB-N2H4 catalyst. The turn over frequency values obtained for the catalytic reduction of NB, 4-NP and 4-NA on TPDT-Au-CTAB-NaBH4 catalyst are 0.312, 0.714 and 1.00 s‐1, respectively. The isosbestic points obtained during the reduction of 4-NA (Fig. 4C and 4F) suggest the formation of single product, p-phenylenediamine from 4-NA. Figs. 6 and 7 shows the catalytic reduction of nitroaromatics using CTAB-Au and TPDT-Au NPs catalysts, respectively and Fig. 8 compiles the corresponding kinetic plots obtained for them. The catalytic reduction of NB to aniline over TPDT-Au-CTAB-NaBH4 catalyst was found to complete within the time period of 64 s, whereas TPDT-AuCTAB-N2H4 needs 120 s and all other AuNPs catalyst prepared here took more than 64 s for the same. A complete reduction of 4-NP and 4NA at TPDT-Au-CTAB-NaBH4 catalyst was accomplished with 24 and 20 s, respectively, whereas, TPDT-Au-CTAB-N2H4 takes 70 and 44 s for the reduction of 4-NP and 4-NA, respectively. Moreover, all other catalyst prepared here, taking more time for the catalytic reduction of
4. Conclusion In summary, a simple method for the preparation of AuNPs dispersed in silicate matrix was established using NaBH4 and N2H4 as reducing agents and the reduction of nitroaromatics was chosen as the model reaction to study the catalytic behaviour of AuNPs. NaBH4 reduced AuNPs shows strong and single SPR band at 518 nm while the N2H4 reduced AuNPs shows the two SPR bands due to anisotropic nature. NaBH4 reduced AuNPs having better catalytic activity than the N2H4 reduced AuNPs. A faster conversion of nitroaromatics into aromatic amine was observed at NaBH4 reduced AuNPs and rate constants obtained for these reactions were higher than the already reported values. Hence, these results conclude that the morphology, optical and catalytic properties of AuNPs are highly dependent on the reducing agent used for the reduction of metal salts. 52
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Fig. 6. Time-dependent absorption spectral changes obtained for the borohydride reduction of NB (A, D), 4-NP (B, E) and 4-NA (C, F) using CTAB-Au-NaBH4 (A-C) and CTAB-Au-N2H4 (DE) as catalyst.
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Fig. 7. Time-dependent absorption spectral changes obtained for the borohydride reduction of NB (A, D), 4-NP (B, E) and 4-NA (C, F) using TPDT-Au-NaBH4 (A-C) and TPDT-Au-N2H4 (DE) as catalyst.
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Fig. 8. Plots of ln(A) versus time for the reduction of NB (A), 4-NP (B) and 4-NA (C) using TPDT-Au-CTAB-NaBH4 (a) and TPDT-Au-CTAB-N2H4 (b) as catalyst.
Table 1 Summary of the rate constants obtained for the catalytic reduction nitroaromatics using AuNPs catalyst prepared in the present work. Catalyst
TPDT-Au-CTAB-NaBH4 CTAB-Au-NaBH4 TPDT-Au-NaBH4 TPDT-Au-CTAB-N2H4 CTAB-Au-N2H4 TPDT-Au-N2H4
kapp and knor for the reduction of NB
kapp and knor for the reduction of 4-NP
kapp and knor for the reduction of 4-NA
kapp (s−1)
knor (mg−1 s−1)
kapp (s−1)
knor (mg−1 s−1)
kapp (s−1)
16.03 5.32 11.31 8.57 0.48 14.97
1.10 × 10−1 4.80 × 10−2 4.20 × 10−2 5.10 × 10−2 0.70 × 10−2 3.30 × 10 −2
55.83 24.36 21.31 25.88 3.55 16.75
1.47 × 10 4.83 × 10 2.49 × 10 5.90 × 10 0.76 × 10 2.33 × 10
3.16 × 10 1.05 × 10 2.23 × 10 1.69 × 10 9.63 × 10 2.95 × 10
−2 −2 −2 −2 −4 −2
knor (mg−1 s−1) −1 −2 −2 −2 −2 −2
74.61 24.51 12.63 29.94 3.85 11.82
Table 2 Comparison of the catalytic performances of present AuNPs catalyst with already reported catalysts towards the reduction of 4-NP. Catalyst material
Catalyst (mol%)
NaBH4 equivalent
kapp (s−1)
References
ZnNPs Se-Fe3O4-CoNPs Pt–Au pNDs/RGOs CNT/piHP PdNPs Fe@Au–ATPGO Microgels-PdNPs 2D graphene oxide/ SiO2–Au CTAB-Au NPs Au@DHBC NPs PGMA@PAH@Au NPs Methyl-imidazoliumbased ionic polymer-AuNPs PAMAM-AuNPs PPI-AuNPs rCD-AuNPs CeO2-AuNPs Au nanocages Au–Ag (0.7:0.3)/silica Au@PVP Cyclodextrin-AuNPs Polyaniline-AuNPs Fe3O4-PdNPs Ppy/TiO2-PdNPs SBA-15-PdNPs CuNPs PEDOT-PPS-Pd NPs TPDT-Au-CTAB-NaBH4
10 – – 4 – 2.1 –
50 40 – 80 – 100 –
– 6.54 × 10−3 3.8 × 10−3 5 × 10−3 1.4 × 10−3 1.5 × 10−3 17 × 10−3
[37] [38] [39] [40] [41] [42] [43]
1.25 0.5 3.2 20
500 300 400 88
6.1 × 10−3 9.5 × 10−−3 – 3.3 × 10−2
[44] [45] [46] [47]
0.10 0.10 5.2 × 10−5 – 4.5 × 10−6 0.8 1.56 17.6 17.6 10 2.6 100 666.7 77 5
100 100 75 1333 300 750 147 44 44 139 11 1000 167 Excess 210
7.91 × 10−3 1.47 × 10−2 9.75 × 10−4 1.28 × 10−2 9.3 × 10−2 5.6 × 10−2 1.02 × 10−2 4.65 × 10−3 11.7 × 10−3 3.3 × 10−2 1.22 × 10−2 1.2 × 10−2 1.6 × 10−3 6.58 × 10−2 1.10 × 10−1
[48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] Present catalyst
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Fig. 9. Time-dependent absorption spectral changes obtained for the borohydride reduction of 2-chloro 4-nitrophenol (A), 2, 6-dichloro 4-nitrophenol (B) using TPDT-Au-CTAB-NaBH4 as catalyst. [27] [28] [29] [30]
Acknowledgments RR acknowledges the financial support received from the CSIREmeritus Scientist scheme (No. 21(1006)/15/EMR-II), New Delhi, India. PV is the recipient of Senior Research Fellowship under UGCBSR scheme, India.
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