Well-dispersed rhenium nanoparticles on three-dimensional carbon nanostructures: Efficient catalysts for the reduction of aromatic nitro compounds

Journal of Colloid and Interface Science 506 (2017) 271–282

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Well-dispersed rhenium nanoparticles on three-dimensional carbon nanostructures: Efficient catalysts for the reduction of aromatic nitro compounds Pitchaimani Veerakumar a,b,⇑, Pounraj Thanasekaran c, King-Chuen Lin a,b,⇑, Shang-Bin Liu a a b c

Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan Department of Chemistry, National Taiwan University, Taipei 10617, Taiwan Institute of Chemistry, Academia Sinica, Taipei 11529, Taiwan

g r a p h i c a l a b s t r a c t Ordered mesoporous carbon supported ReNPs (Re/OMC) exhibited superior performances compared to their unsupported colloidal metal nanoparticles.

a r t i c l e

i n f o

Article history: Received 4 May 2017 Revised 22 June 2017 Accepted 17 July 2017 Available online 18 July 2017 Keywords: Catalytic reduction Rhenium nanoparticles Sodium borohydride Rate constant Toxic pollutants

a b s t r a c t Rhenium nanoparticles (ReNPs) supported on ordered mesoporous carbon (OMC) as a catalyst (Re/OMC) through a solvent-evaporation induced self-assembly (ELSA) method were prepared. The synthesized heterogonous catalyst was fully characterized using X-ray diffraction, field emission transmission electron microscopy, N2 sorption, metal dispersion, thermogravimetric analysis, Raman, Fourier-transform infrared, and X-ray photon spectroscopies. In addition, the catalyst was applied to reduce the aromatic nitro compounds (ANCs) for the first time in aqueous media and the reactions were monitored by following the intensity changes in the UV–vis absorption spectra with respect to time. This method provides the advantages of obtaining a high rate constant (k), green reaction conditions, simple methodology, easy separation and easy workup procedures. Moreover, the catalyst can be easily recovered by centrifugation, recycled several times and reused without any loss of activity. The higher activity of this catalyst was attributed to higher dispersion and smaller particle size of ReNPs as observed from FE-TEM and XRD results. Ó 2017 Published by Elsevier Inc.

⇑ Corresponding authors at: Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei 10617, Taiwan. E-mail addresses: [email protected] (P. Veerakumar), [email protected] (K.-C. Lin). http://dx.doi.org/10.1016/j.jcis.2017.07.065 0021-9797/Ó 2017 Published by Elsevier Inc.

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1. Introduction Over the past decade, transition metal nanoparticles (MNPs) supported catalysts have received wide attention as heterogeneous catalysts due to their environmental benignity, low cost, structural integrity and high catalytic activities in various reactions [1–4]. By taking advantages of extraordinary properties of these nanomaterials including large specific surface area, high electrical conductivity and superior strength, the assembled 3D porous structures offer excellent opportunities for building high performance supercapacitors and electrochemical sensors [5]. Nevertheless, the catalytic activity of rhenium (Re) based nanomaterials used in the catalyst is limited compared to other metal catalysts [6–10]. Extensive effort has thus been devoted herein to synthesizing the carbon supported rhenium nanoparticles (ReNPs) and studied their catalytic activities. Several new techniques including pulsed-laser decomposition [11], electrodeposition [12], gamma irradiation [13], chemical vapour deposition (CVD) [14], wet chemical reduction [15,16], solvothermal [17], colloidal and microemulsion and other methods [18–20] were adopted to synthesize ReNPs materials. Apart from rhenium nanoparticles, other catalytic active MNPs, such as RuNPs [21], OsNPs [22,23], IrNPs [24], AuNPs [25] and, AgNPs [26] were also used as catalysts for the reduction of aromatic nitro compounds (ANCs). Rhenium metals are the most promising catalytic materials that could be used as versatile catalysts. Apart from bare metals, ReNPs may serve as catalysts while they usually supported on polymer [27], DNA scaffold [28], carbon [29–31], and c-Al2O3 [32], etc. Ordered mesoporous carbon materials (OMCs) as a support for transition metal catalysts [33,34] offer several advantages including easy modification, elimination of nanoparticle agglomeration, relatively low cost, high mechanical strength, chemical stability, and a pore structure along with an attractive surface chemistry. In addition, these nanocatalysts can make the products easily removable and the catalysts recyclable [35]. Over the last several years, numerous review articles have specifically addressed the toxicity and mutagenicity of ANCs. For example, the remediation of ANCs is of interest because it is a mutagen, anaemia and skin irritation [36]. Moreover, the reduced intermediates and products (such as amino-, and hydroxylaminoANCs) often have greater toxicity and/or aqueous solubility than their parent compounds. Its ingestion can cause various health

problems including eye, liver or kidney damages, aplastic anaemia, cyanosis, respiratory organs, as well as damage to neurological system [37]. Some ANCs are high-energy explosives. For example, 2,4,6-trinitrotoluene (TNT), 2,4,6-trinitrophenol (TNP) or picric acid are the primary constituents that were adopted in many unexploded landmines worldwide [38]. Therefore, it is necessary to remove such highly toxic pollutants. As the reduced products of ANCs, aromatic amino compounds (AACs) have attracted enormous attention in the chemical industry. They can also be used to synthesize drugs, pesticides, and explosives. In addition, these are key intermediates in many important fine chemicals, agrochemicals, pharmaceuticals, and polymer building blocks [39]. In the course of our studies on the development of heterogeneous catalysts, we report the potentiality of the Re/OMC catalyst that has been tested for the first time as a catalyst for the reduction of ANCs at room temperature, as illustrated in Scheme 1. In addition to the good catalytic properties, the synthesized catalyst could be easily separated from the reaction mixture and reused six times without loss of activity, indicative of its potential application in chemical industries. 2. Results and discussion 2.1. Structural properties A highly mesoporous 3D structure of OMC was prepared via pyrolysis of phloroglucinol-formaldehyde resol by evaporation induced self-assembly (EISA) strategy (see Supporting Information, hereafter denoted as SI). Decomposition of dirhenium decacarbonyl (Re2(CO)10, 217 mg, 0.33 mmol) used as a metal precursor may produce small ReNPs, which are embedded on the interior pore walls of the 3D carbon matrix (Scheme 1). Unlike previous methods adopted to synthesize on mesoporous carbon [8] and MWCNTs [29], the rhenium precursor Re2(CO)10 decomposes to the ReNPs upon microwave irradiation as shown in Eq. (1).

Re2 ðCOÞ10 ! 2Re þ 10CO "

Structural characterization based on X-ray diffraction pattern reveals that OMC showed two strong peaks appeared at 2h = 24.6° and 44.6°, which can be attributed to the amorphous graphitic carbon (0 0 2) and (1 0 0), respectively (Fig. 1a) [40]. The related XRD pattern of the Re/OMC catalyst showed the peaks at 2h = 37.7°, 40.6°, 43.0°, 56.6°, 68.4°, 75.3°, and 82.5°, confirming the formation of hexagonal rhenium. These results indicate that Re/OMC catalyst possessed a very small size 2–5 nm of Re particle with a hexagonal closely-packed (hcp) structure of the bulk Re, which is in good agreement with previously published reports [11,40]. This fact verifies that ReNPs have been successfully loaded on to the three dimensional OMC material with rather good dispersion of the metallic phase. No other peaks have been observed; it is indicated that the rhenium dispersions have a highly textured, nanocrystalline structure with preferential growth orientation [20]. The crystallite size of Re in Re/OMC sample was determined according to the Scherrer formula (2):

d¼

Scheme 1. Illustrated synthesis of Re/OMC nanocomposite from phloroglucinolformaldehyde resin and its morphology and catalytic application.

ð1Þ

0:9k b cos h

ð2Þ

where k is the wavelength, b is the full width at half maximum (FWHM) of the Bragg’s peak corrected using the corresponding peak in sample, and h is the Bragg’s angle. Besides, pristine OMC and Re/OMC samples were also characterized by Raman spectroscopy (Fig. 1b). Both samples show the D and G bands at 1351 and 1598 cm1, respectively, which are in agreement with those reported [41]. Moreover, we observed an

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Fig. 1. (a) XRD patterns, (b) Raman spectra, (c) N2 sorption isotherms and their pore size distributions (inset), and (d) TGA and DTG curves of OMC and Re/OMC catalyst.

additional peak at 2742 cm1 for the 2D and 2902 cm1 for the D + G band, implying that the ReNPs are successfully imbedded inside the OMC matrix. Given the N2 adsorption/desorption isotherm measurements, both OMC and Re/OMC samples exhibited type-IV isotherm (Fig. 1c) with type-H1 hysteresis loop, indicating the existence of mesoporosity [42]. This result indicated notable decreases in the surface area and pore volume of the Re/OMC sample upon loading of ReNPs onto the 3D OMC matrix. Note that a substantial fraction of small mesopores in OMC sample was blocked by ReNPs as the pore diameter of OMC decreased. The textural properties of as-prepared catalyst are summarized in Table 1. Thermogravimetric analysis (TGA Fig. 1d) of as prepared samples showed the initial weight loss of water around ca.11.6 (OMC) and 13.1 wt% (Re/OMC) at about 110 °C. Then, the nanocomposite became stable until it reached to 510 °C, after which a weight loss ca. 75.7 wt% (Re/OMC) at 640 °C and 88.4 wt % (OMC) at ca. 680 °C was observed. These weight losses are

probably caused by the decomposition of the OMC framework and the presence of rhenium oxides [33]. The morphology of these samples was observed by fieldemission transmission electron microscopy (FE-TEM) analysis, which showed that the highly well-ordered 3D hexagonal carbon frameworks (Fig. 2a–c) were fully loaded with well-dispersed ReNPs with an average size range of 5 ± 0.2 nm (Fig. 2d–h). This result was also confirmed by H2 chemisorption study. A single ReNP with a d-spacing of 0.377 nm corresponding to the (1 0 1) plane was also shown in Fig. 2h. In the electron diffraction (SAED) pattern, the diffraction spots of Re observed from Re/OMC catalyst (Fig. 2i) revealed a mean particle size of 5 ± 0.2 nm. This result is consistent with the view of FE-TEM, showing that the highly dispersed ReNPs are successfully fixed into the interior part of the OMC walls. Energy dispersive X-ray spectroscopy (EDX) analysis of the Re/OMC catalyst verified the existence of rhenium, carbon, oxygen and copper elements (See Fig. S1, SI). The peaks of elemental Cu at about 1 and 8 keV corresponding to the copper grid were excluded during elemental analysis. The weight

Table 1 Physical properties of the as-prepared OMC and Re/OMC materials. Sample

OMC Re/OMC a b c d

Surface areaa (m2 g1)

Pore volumea (cm3 g1)

STotal

SMicrob

SMesob

VTotal

VMicrob

1468 1386

344.1 275.3

1123.9 1110.7

0.1499 0.1103

1.0207 0.8883

SBET surface area from BET method, and total pore volume (VTot) calculated at P/P0 = 0.99. Microporous surface areas (SMicro) and pore volumes (VMicro) obtained from t-plot analysis. Average pore size (Dp) derived by the BJH method using the adsorption branch of the isotherm. Metal dispersion measured by H2 chemisorption at 323 K.

Dpc (nm)

Dmd (%)

ID/IG

6.19 4.95

– 0.22

0.99 0.99

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Fig. 2. FE-TEM images of (a–c) bare OMC, (d–g) Re/OMC nanocatalysts, (h) single Re NP, and (i) their SAED pattern.

% of Re, C, and O was confirmed in the range of 32.94, 62.76, 4.30, respectively, as expected with a previously published report [11]. The surface properties of as-prepared materials were further examined by X-ray photoelectron spectroscopy (XPS), as shown in Fig. 3a. The intense Re 4f peaks from the Re loading sample were displayed in Fig. 3b. In general, rhenium exists in five positive oxidation states, Re0, 3+ Re , Re4+, Re6+ and Re7+[43]. The XPS survey spectrum of Re/OMC showed that it exhibits two well-resolved spectral lines at 40.7 and 42.9 eV, which are assigned to the Re 4f7/2 and Re 4f5/2 spin–orbit components, respectively. These results imply that the metallic Re0 was thermally reduced from the Re2(CO)10 under inert conditions. The high resolution C1 s spectrum (Fig. 3c) can be deconvoluted into three peaks located at 284.7, 285.8, and 288.7 eV, which are ascribed to CAC, CAO, and CAC@O, respectively. The oxygen O 1s spin-orbits yield the characteristic peaks with binding energy (BE) at 531.2 (CAO), 532.8 (C@O) and 533.2 eV (CAOC/CAOH), as shown in Fig. 3d. Moreover, the Re 4f7/2 BE (40.7 eV) corresponding to the metallic rhenium in the Re/OMC sample is similar to that determined previously for rhenium nanopowder (40.5 eV) [44]. 2.2. H2-TPR analysis The TPR profiles of as-prepared bare OMC and Re/OMC catalysts are shown in Fig. 4. It is an ideal tool for examining the reducibility, and the interaction between the metal species and support, which provides the information regarding the active state of the solid cat-

alysts [8,45]. Mitra et al. reported that consumption of H2 over the bare carbon catalyst showed a broad maximum peak at the range of 500–800 °C, accompanied by methane emission and the hydrogenation of surface functional groups on the carbon support [46]. A similar phenomenon was observed for OMC, whereas the reduction profile of Re/OMC consisted of two peaks. Obviously, at a low temperature, the peaks at approximately 320 °C was caused by the reduction of small rhenium oxide species, while gasification/methane (CH4) formation on the carbon support generated a broader peak centred at 600 °C [47]. 2.3. FT-IR analysis The FT-IR spectra of Re2(CO)10, polymeric resin containing Re2(CO)10, and OMC supported rhenium catalysts (Re/OMC) are shown in (Fig. S2, SI) before and after carbonization. The pristine Re2(CO)10 exhibited C@O horizontal and C@O vertical stretching vibrations at 2071, and 1978 cm1, and 2011 cm1, respectively. The ReAC stretching vibration band in Re precursor of Re2(CO)10 was observed at 590 cm1 [48]. The intensities of these characteristic peaks mostly disappeared, when carbonized at high temperature, indicating that the Re2(CO)10 was completely decomposed into metallic Re. This result is consistent with the previous reports [49], and also indicates the strong interaction as well as active involvement during incorporation of ReNPs onto the 3D carbon matrix.

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Fig. 3. XPS survey spectra of (a) OMC and Re/OMC, (b) expanded XPS profiles of Re 4f, (c) C 1s, and (d) O 1s regions of the Re/OMC catalyst.

Fig. 4. H2-TPR profiles of (a) pristine OMC, and (b) Re/OMC catalysts.

2.4. Catalytic studies The catalytic behavior of these samples was studied in the reduction reaction among the following compounds: nitrobenzene (NB), 4-nitrophenol (4-NP), 2-nitroaniline (2-NA), 4-nitroaniline (4-NA), 2,4-di-nitrophenol (2,4-DNP) and 2,4,6-tri-nitrophenol (2,4,6-TNP)) in aqueous medium using sodium borohydride (NaBH4) as a hydrogen generator. Currently, general conversion of a nitro group into an amino group is mostly achieved by treatment with various reducing reagents, such as NaBH4 [33,50,51], H2 and CO gas [52], N2H4 [53], ammonia-borane (NH3BH3) [54], 1,1,3,3-tetramethyl disiloxane (TMDS) [55], and triethyl silicon hydride (Et3SiH) [56], under acidic/basic/neutral conditions. Among the various reducing agents, NaBH4 is the most commonly used as a mild, selective reducing agent, which is adopted

to be the best economical option for the reduction reactions under normal pressure and room temperature in the presence of transition metal catalysts, although preparation is an intricate procedure (high temperature and pressure were always required) in the case of silicon-based reagents. In some studies, a relatively high H2 pressure was required, but might lead to explosion if careless during experiments. It is well known that the final products (AACs) are very useful and important industrial intermediate, which are applicable in many fields including analgesic and antipyretic drugs, photographic developer, corrosion inhibitor, anticorrosionlubricant, and manufacture of various dyes and polymers, etc., [57]. To optimize the reaction conditions, the reduction of 4-NP was first carried out as a model reaction in the presence of different source of catalysts under various conditions with the aim of optimizing the yield, and the results are summarized in Table 2. Recently, various functional groups containing bare carbon materials are employed as metal free catalyst in catalytic reduction reactions [58]. In order for optimization, the catalytic performances during reduction of 4-NP over OMC without ReNPs involved were also investigated. The bare OMC showed less catalytic activity that resulted in trace yield over a reaction time of 20 s (Table 2, entry 1). By comparison, the same reaction was carried out using the rhenium precursor Re2(CO)10 as the catalyst, but no reaction was observed. Generally, metal carbonyls are soluble in nonpolar and polar organic solvents but insoluble in water [59], and hence, they do not cause the reduction reaction in aqueous medium (Table 2, entry 2). The commercially available Re2O3 catalyst gave a low yield of reduction product, (typically ca. 68%) as shown in (Table 2, entry 3). In contrast, the current Re/OMC catalyst shows a much better reactivity, and gives an excellent product yield as shown in (Table 2, entry 4–9). While increasing the amount of Re/OMC catalyst from 0.01 to 0.05 mg, a little effect on the reduction efficiency was found (Table 2, entries 4–7). Upon increasing the

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Table 2 Optimization of the reaction conditions for the reduction of 4-NP.a

a b c

Entry

Catalyst

Amount (mg)

Time (s)

k (s1)

Conversion (%)b

1 2 3 4 5 6 7 8 9

OMC Re2(CO)10 ReO3 Re/OMC Re/OMC Re/OMC Re/OMC Re/OMC Re/OMC

0.02 0.02 0.02 0.01 0.02 0.03 0.05 0.02 0.05

20 20 20 20 20 20 20 30 30

– – 0.1148 0.1375 0.1498 0.1500 0.1502 0.1499 0.1508

Trace NRc 68 95 99 99 99 99 99

Reaction conditions: 4-NP (1.8 mL, 0.1 mM), NaBH4 (0.5 mL, 0.1 M) and water (3 mL). GC conversion on the basis of 4-nitrophenol. No reaction.

reaction time (30 s) under the same condition, there are no appreciable changes in the product yields as compared to 20 s (Table 2, entries 7 and 8). The catalytic reduction of NB could be apparently considered as a pseudo-first order reaction with respect to the concentration of NaBH4. Thus, the kinetic equation of this catalytic reaction is readily solved and the reaction rate constant (k) was obtained from the slope of the plots [10]. As the concentration of NaBH4 is higher than that of 4-NP, the reduction rate is found to be independent of NaBH4 concentration, and the reaction could be considered as a pseudo-first order with respect to the concentration of 4-NP. It is worth noting that our protocol catalytic conditions are significantly milder (0.1 M of NaBH4 and room temperature) than those used in other studies, in which similar reduction reactions were examined; however, either large excess of NaBH4 (100-fold excess) [60] or hast conditions (>100 °C) are required [61]. Since ReNPs are more sensitive to oxygen, the ReNPs are covered with a thin layer of ReO3. Although partial oxidation of the ReNPs on the surface of ReO3 particle may have a strong influence on the catalytic efficiency to some extent, the catalyst can be reused for six cycles still with a high activity. 2.5. Reduction of nitrobenzene Recently, considerable interest has been focused on MNPs supported materials as heterogeneous catalysts for the reduction of NB system and for evaluation of the catalytic activity [51]. The development of an efficient and reliable technique for the catalytic reduction of aromatic nitro compounds is a demanding task. In this system, the nitro compounds act as an electronic relay system, in which the electron transfer takes place from donor BH 4 to acceptor nitro (ANO2) groups. We explored the catalytic activity of our synthesized Re/OMC catalyst using NaBH4 as a reducing agent. Consequently, the progress of the reduction reaction can be readily followed by means of UVvis absorption spectrophotometry (Fig. 5a). Upon the reduction of NB using Re/OMC catalyst in the presence of NaBH4, the absorption spectra of NB showed that the main peak at kmax = 275 nm disappeared gradually while the peak at kmax = 232 nm of aniline (AN) was slowly increased (indicated by arrows) within a period 25 s, indicating a successive reduction of NB to AN [62]. As shown in Fig. 5a, two isobestic points at kmax = 216 and 241 nm are clearly visible. Obviously, the appearance of isobestic point at 241 nm indicates the formation of aniline peak without any side reactions [63]. A good linear correlation (R2 > 0.9995) of ln (At/A0) vs reduction time was observed, confirming the pseudo first-order kinetics. The rate constant (k) is calculated as 0.1482 s1, and the corresponding kinetic plot is given in Fig. 5a (inset). The reduction reaction did not proceed in the absence of catalyst as well as in the absence of reductant. We selected a few important studies from the literature [51,62–64]

for the purpose of comparison with our results. These findings are summarized together with our data (Table S1, SI). In particular, Guan et al. [64] have reported multifunctional core–shell catalysts (Au@C yolk–shell) for the catalytic reduction of NB, yielding k = 2.8  103 (12 min), which is significantly lower than our catalytic system.

2.6. Reduction of 4-nitrophenol The reduction of phenolic aromatic nitro compounds with noble metal catalysts in the presence of NaBH4 has been extensively investigated for the efficient production of aromatic amine compounds [65]. Inspired by excellent activity of Re/OMC for the catalytic reduction of NB, we have extended the catalytic activity measurement for another five model ANCs, namely 4-NP, 2-NA, 4-NA, 2,4-DNP and 2,4,6-TNP under identical experimental conditions. The catalytic activity of Re/OMC catalyst was tested for the reduction of 4-NP. In this case, the 4-NP solution exhibits a distinct absorption maximum at kmax = 380 nm; however, the peak shifted to kmax = 400 nm, immediately after the addition of NaBH4 solution as shown in Fig. 5b. The resulting colour change from light to bright yellow was due to the formation of 4-nitrophenolate ion in alkaline medium (Fig. S3, SI). The maximum absorption peak at kmax = 400 nm did not change over time, even after NaBH4 was added enough to generate small bubbles from NaBH4 upon reducing water to H2 gas according to the reaction (3):

NaBH4 þ 2H2 O ! NaBO2 þ 4H2 0

ð3Þ 0

The reduction of 4-NP (E = 0.76 V) by NaBH4 (E = 1.33 V) is thermodynamically feasible, but kinetically restricted in the absence of a catalyst in aqueous NaBH4 (Fig. S3, SI) [66]. The result confirms that the reduction reaction did not proceed in aqueous NaBH4 solution alone. Furthermore, the addition of catalyst to reaction mixture led to the decrease in the peak intensity of 4nitrophenolate ion at kmax = 400 nm with a concomitant appearance of a new peak at kmax = 300 nm (Fig. 5b), revealing that the new peak was ascribed to 4-aminophenol (4-AP), and the corresponding kinetic plot is given in Fig. 5b (inset). Notably, the spectra have well-defined isobestic points, indicative of the sole product formation without any by-product generation. The complete conversion of 4-NA can also be confirmed by the colour change of the solution from originally bright yellow to colourless (inset of Fig. S3, SI). Indeed, the 4-nitrophenolate molecules have an active AOH group (activating character), which remained unchanged, but the NO2 group was totally reduced to amine (4-AP). Given the rate constant estimate [33], the Re/OMC gives the largest value, ca. k = 0.1498 s1 as compared with other carbon matrix-supported AgNPs, which yields k = 5.33  103 s1 (see Table S2, SI). With

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277

Fig. 5. UV–vis absorption spectra for the successive reduction of (a) NB, (b) 4-NP, (c) 2-NA, (d) 4-NA, (e) 2,4-DNP and (f) 2,4,6-TNP using NaBH4 in the presence of Re/OMC at different time intervals and their corresponding insets show the plot of ln At/A0 vs time.

increasing the catalyst amount (from 0.01 mg to 0.05 mg), conversion of 4-NP increased from 95 to 99% as listed in Table 2. It is noteworthy that the synthesized Re/OMCs with an optimal k value of 0.1498 s1 for the catalytic reduction of 4-NP, indeed, show a better catalytic performances surpassing most of the metal nanoparticles, such as Ru0 nanochains (k = 2.8  101 min1) [21], Re nanoclusters (k = 0.06 min1) [28], and bimetallic Pt–Ni NPs (k = 11.6  102 min1) [50] catalysts. In addition, the reduction rate constant using Re/OMC catalyst is also much higher than previously reported carbon based Au@CMK-3-O catalyst (k = 0.132 min1), and Ag doped carbon spheres (k = 2.02  102 s1) [47,67]. These results indicate that Re/OMC is more active than the other mono/bimetallic nanomaterials. 2.7. Reduction of 2-nitroaniline and 4-nitroaniline Apart from the above reactions, the catalytic activity of Re/OMC was also tested for the reduction of 2-NA and 4-NA under the same conditions, and their results are displayed in Fig. 5c and d, respectively. In the presence of catalyst, the absorption peaks of 2-NA at kmax = 280 and 400 nm [68] gradually decreases but the peak at

kmax = 280 nm is found to be red-shifted to kmax = 290 nm with reaction time, which indicates that the reduction of 2-NA to ortho-phenylenediamine (o-PDA) appeared with time. The yellow coloured solution became colorless after the completion of the reaction (Fig. 5c). When 4-NA is reduced by NaBH4 alone, no appreciable change in the absorption spectrum was observed even after standing for 30 s. It indicates that the use of BH 4 ions alone is incapable of reducing 4-NA to p-PDA (see Fig. S4, SI). After the addition of a small amount (0.02 mg) of the Re/OMC catalyst, the absorption peak of 4-NP at kmax = 380 nm significantly decreases while the peak at kmax = 235 nm gradually increases as the reaction proceeds. Meanwhile a new peak appears at kmax = 308 nm, revealing that the reduction of 4-NA leads to form p-PDA (Fig. 5d) [69]. The time-dependent UV spectra for the successive reduction of 2-NA and 4-NA and their corresponding kinetic plots are given in Fig. 5c and d (Insets), respectively. The UV–vis spectra of 4-NA show the presence of an isobestic point at kmax = 263 nm, suggesting that the catalytic reduction of 4-NA gives the product p-PDA only. This reduction reaction proceeds via nitrophenolate ion intermediate in NPs or via hydroxylamine in 4-NA [70]. A linear relationship between ln(At/A0) and reaction time is obtained in

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Table 3 Catalytic performances for reduction of various ANCs using Re/OMC catalyst.a

a b

Entry

Reactant

Product

Time (s)

k (s1)

Conversion (%)b

1 2 3 4 5 6

Nitrobenzene 4-Nitrophenol 2-Nitroaniline 4-Nitroaniline 2,4-dinitrophenol 2,4,6-trinitrophenol

Aniline 4-aminophenol o-phenylenediamine p-phenylenediamine 2,4-diaminophenol 2,4,6-triaminophenol

25 20 25 30 20 20

0.1482 0.1498 0.1454 0.1472 0.1469 0.1442

98 100 99 99 98 97

Reaction conditions: 4-NP (1.8 mL, 0.1 mM), NaBH4 (0.5 mL, 0.1 M) and water (3 mL). GC conversion on the basis of ANCs.

the 2-NA and 4-NA reduction by Re/OMC catalyst (Fig. 5c and d). The plots for 2-NA and 4-NA well matched with the pseudo firstorder reaction, and their rate constant k were calculated to be 0.1454 and 0.1472 s1, respectively. In contrast, the rate constant of RuNPs [21] catalysed 2-NA reduction was calculated to be 3.4  102 min1, which is smaller than that of present catalyst. In another study, Cu/Co3O4 catalyst was reported to require a longer time (21 min) for the achievement of complete reduction of 4-NA (k = 17.1  102 min1) [69]. Likewise, the reduction rate constant of 4-NA with other monometallic or bimetallic NPs catalysts was compared. For instance, the rate constants k was evaluated to be 34.33 (10 min) for RuNPs [21], 3.87 (14 min) for OsNPs [23], 2.083 (86 min) for AuNPs [25], 7.59  102 (42 min) for AgNPs [26], 37.4 (8 min) for ReNPs [28], and 6.4  102 min1 (8 min) for bimetallic Pt-Ni nanoparticles [50] catalysts. The diversity of such noble metal catalysts showed a potentially great contribution in the reduction reactions. Despite, it is inconvenient to recover the noble metals from the reaction mixture and low kinetic stability, whereas, the use of OMC supports makes it easy to recover for recycling process. However, these noble metallic NPs showed a lower catalytic activity compared to our present catalyst owing to their lower surface area. Additionally, the high dispersion of noble metal catalysts on carbon supports with large surface areas, a benefit for catalytic applications [33]. Hence, we compiled the catalytic activity of Re/OMC with the other reported metal NPs based catalysts employed for the reduction of 2-NA and 4-NA (see Tables S3 and S4, SI). These results clearly suggest that the mesoporous Re catalyst possess a high catalytic activity compared to others reported. A wide variety of ANCs can be reduced to their corresponding AACs in near quantitative yields, and their results are summarized in Table 3.

2.8. Reduction of 2,4-dinitrophenol and 2,4,6-trinitrophenol The reduction of toxic nitro compounds into their amino analogues are one of the most important derivatives, which are receiving commercial attention in a wide range of areas such as photographic, pharmaceutical, and dye industries [71]. Various effective methods have been developed for the treatment of such toxic compounds [72,73]. However, their disposal into nearby water sources from industries cause serious environmental pollution and harm the ecosystem. In this work, we investigated the catalytic reduction of 2,4-DNP [69,70] and 2,4,6-TNP [74] using the same manner, and their corresponding linear plots of the products 2,4-diaminophenol (2,4-DAP) and 2,4,6-tri-aminophenol (2,4,6-TAP) were obtained by using Eq. (2), as shown in Fig. 5e and f, respectively. When 0.02 mg of the Re/OMC catalyst was added to the reaction mixture, the peak intensity of 2,4-DNP at kmax = 358 nm decreased with a concomitant increase in the peaks corresponding to 2,4-diaminophenol (2,4-DAP) at kmax = 298 nm. However, upon the addition of aqueous solution of NaBH4, the peak maxima immediately shifted towards 445 nm (due to generation of nitrophenolate ion). The peak at

kmax = 298 nm gradually increased which was attributed to the generation of DAP. The corresponding pseudo-first-order rate constant was calculated to be k = 0.1469 s1. As for the reduction of 2,4,6-TNP with the catalyst, the yellow colour of the reaction mixture started fading initially, and the absorption intensity shifted towards at kmax = 390 nm (nitrophenolate ion as intermediate) followed by the appearance of new peak at kmax = 305 nm, representing the formation of 2,4,6-tri-aminophenol (2,4,6-TAP) as shown in Fig. 5e. The reaction was completed within 20 s, giving a rate constant (k) ca. 0.1442 s1 (Fig. 5e), while comparing with MnFe2O4 (k = 0.01134 s1) [73], and ZnO nanorods (k = 0.141 min1) [74]. The reduction reaction became quite slow without using this catalyst (Figs. S5 and S6, SI). In summary, the reduction rates of the six nitro-aromatics performed using Re/OMC catalyst followed the order: 2,4,6-NP (k = 0.1442 s1) > 2-NA (k = 0.1454 s1) > 2,4 -DNP (k = 0.1469 s1) > 4-NA (k = 0.1472 s1) NB (k = 0.1482 s1) > 4-NP (k = 0.1498 s1). The order of reactivity of ANCs can be realized by the following two factors: (i) the formation of intermediates, and (ii) their reactivity. For the formation of nitrophenolate ion, the acidic strength of nitro phenols plays an important role during the reaction. Here, NaBH4 acts as a Lewis base. The order of acidic strength of the ANCs are: 2,4,6-TNP > 2,4-DNP > 4-NA > 2-NA > NB > 4-NP, and the stability of nitrophenolate ions follow the same order. This is due to the fact that the negative charge of oxygen in nitrophenolate ion is delocalized through the aromatic ring that is stabilized via resonance. In addition, inductive effect of nitro groups further results in the facile liberation of H+ ion. However, the fact that the reduction rate constant of 2,4-DNP exceeds other ANCs, which could be due to the electron donating ability of NH2 as well as decrease of the electronegativity of nitrogen in aniline moieties, as compared to oxygen of hydroxyl group in nitrophenol compounds, and thus, nitrophenolate ions. Also, 2,4-DNP and 2,4,6-TNP (picric acid) suffer from more steric hindrance compared with 2-NA and 4-NA upon orientation on the catalyst surface. Among these nitroaniline molecules, the lone pair of electron on the amino nitrogen atom is attracted toward the strong electron withdrawing NO2 groups, resulting in a resonating structure. Hence, the amino nitrogen becomes positively charged in 4-NA, by which an electrostatic interaction with our catalyst system occurs [75].

2.9. Simultaneous reduction of ANCs We, furthermore, investigated the simultaneous catalytic reactions of mixture of ANCs (4-NP, 4-NA, 2,4-DNP and 2,4,6-TNP), using the same reaction conditions as seen in Fig. 6. Upon the reduction of ANCs using this catalyst, the absorption peak of ANCs decreases at kmax = 400 nm within 25 s, indicating the complete reduction of ANCs, while the peak at kmax = 300 nm increases due to the formation of amine products with the isobestic points at kmax = 276 and 340 nm. Interestingly, the Re/OMC catalyst has ability to reduce many nitroarenes into their respective amines in aqueous medium simultaneously. The time evolution of UV/Vis

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279

Fig. 6. UV–visible spectra representing simultaneous reduction of mixture of all ANCs in the presence of Re/OMC catalyst (a) 0.02 mg, and (b) 0.05 mg under same experimental conditions.

absorption spectra for the simultaneous reduction of nitro compounds into their corresponding products (AACs) are shown in Fig. 6a and b. This may be due to simultaneous adsorption/ diffusion of all ANCs on the surface of Re/OMC catalyst. It was also observed that ANCs cannot be reduced by NaBH4 without Re/OMC catalyst involvement (see Fig. S7, SI). The rate constants, k, were obtained directly from the slopes of the straight lines, and the k values are found to be 0.1278 s1 (R2 = 0.994; Fig. 6c) and 0.1317 s1 (R2 = 0.9983 Fig. 6d) for the 0.02 and 0.05 mg of Re/OMC catalyst, respectively. The reduction rate constant (k) for each nitro compound was found to be higher than that for simultaneous reduction of ANCs. The sole product was formed without any by-product generation during the reduction process. This catalytic system is proved to be useful for reduction of various nitro compounds that were carried out individually as well as simultaneously. Our catalytic system may be an effective catalyst for the rapid reduction of these highly toxic ANCs into less toxic AACs, which are useful for other practical applications. 2.10. Mechanism of reduction reaction The as-prepared Re/OMC catalyst with enhanced catalytic activity can be attributed to the active surface area, and a large adsorption of reactants onto the highly porous 3D carbon matrix, as shown in Scheme 2. Initially, the active nitro groups on the surfaces of the catalyst were converted into nitrophenolate ion in the presence of NaBH4 followed by hydrogen transfer from BH 4 (which is adsorbed on an carbon matrix), leading to a rapid reaction between the reactive hydrogen (H+) and 4-(hydroxylamino) phenolate ion [63]. The rate of electron transfer at the catalyst surface can be influenced by two-steps: (i) adsorption of 4-nitrophenolate ion onto the catalyst surface, and (ii) interfacial electron transfer and desorption of 4-aminophenolate ion away from the surfaces. Finally, the AACs were formed, and the catalyst

Scheme 2. Mechanism for the reduction of 4-NP using Re/OMC catalyst.

was recovered by centrifugation and reused for subsequent runs. It is noteworthy that the formation of 4-AP from 4-NP proceeds via the generation of 4-nitrophenolate ion and 4-(hydroxylamino) phenolate ion as the intermediates, which involve six-electron (6e), and six-proton (6H+) transfer, as shown in Scheme 2. 2.11. Reusability and stability To investigate the reusability, the Re/OMCs catalysts were separated from the catalytic reaction solution using an ultracentrifugation. In order to keep the amount of catalyst, we added

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Fig. 7. (a) Recycling test of the Re/OMC catalyst towards the 4-NP reduction, (b) XRD pattern, and (c, d) FE-TEM images of spent Re/OMC catalyst.

reactants to the resulting mixture for the next six cycles to investigate the reusability of the catalyst under the same experimental conditions. The conversion and activity of the catalyst were found to be slightly decreased for the same reaction time (20 s) even after running for 6 cycles. It was found that the rate constant (k) remained almost the same. Meanwhile, the conversion percentage for complete reaction gradually decreased with increasing the running cycles. For instance, the rate constant was ca. 0.1188 s1 during the 20 s time at sixth cycle (Fig. 7a). The slight decrease in the activity may be caused by the loss of some catalysts at the steps of centrifugation. Besides, the X-ray diffraction pattern of the spent Re catalyst remains the same (Fig. 7b). FE-TEM measurements also confirmed that the 3D micro/mesoporous structures of this catalyst are well maintained after six catalytic processes, further suggesting its excellent stability and long life (Fig. 7c and d). Recyclability is the main advantage of heterogeneous catalysis compared with homogeneous catalysis for practical applications. In our catalytic system, the solid catalyst was easily recovered from the reaction mixture by brief centrifugation or sedimentation process. It can be seen that the recyclable reduction of 4-NP in the presence of Re/OMCs catalyst exhibited similar catalytic activity as in the first three cycles of the reaction. The 4-NP conversion decreased from the fourth to sixth cycles, probably due to loss of catalyst during the recycling process. Thus, the Re/OMCs catalyst was recycled and reused for six successive reactions with a conversion of more than 90%, and the product yield was >90%. These facts confirmed that the catalyst is stable and active (Fig. 6b). Moreover, metal dispersion characterization data, before and after the catalytic reactions, are shown in the supporting information (Table S5, SI). The recycled catalyst so prepared from the hot filtration procedure was examined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). Only about 15 ppm loss

of Re species was obtained while comparing to that of fresh Re/ OMC catalyst (ca. 3.12 wt%; Fig. S1, SI), and a spent catalyst (ca. 2.95 wt%; Fig. S8, SI) obtained after six consecutive runs. These results clearly demonstrate that the Re/OMC catalyst is truly robust and heterogeneous in nature. 3. Conclusions In this work, we have reported a new and highly efficient methodology for the reduction of ANCs to AACs using Re/OMC catalyst in the presence of NaBH4 as a hydrogen resource in aqueous medium. We have demonstrated a solvent-evaporation induced self-assembly (ELSA) method to synthesize ReNPs catalysts supported on OMC materials, which possess an accessible ordered mesopore channels. This catalyst was characterized by FE-TEM, XRD and N2 adsorption/desorption isotherm measurements. FETEM studies showed the isolated ReNPs with an average diameter of 5 nm were formed and immobilized onto the OMC supports. The ReNPs/OMC catalysts were highly active for the reduction of ANCs to AACs and could be recycled for six consecutive runs without significant loss of catalytic activity. The catalytic activity was found to be higher while compared to some of the monometallic or bimetallic NPs catalysts reported [21–26,50]. The simplicity of the system, easy separation of catalyst, simple workup procedures and excellent yields make this method an attractive, environmentally acceptable synthetic tool for the reduction of various aromatic nitro compounds to their corresponding amines. All these features of OMC support with noble metal nanoparticle encourage researchers to continue to develop new heterogeneous catalysts. Therefore, our results may provide new ways to fabricate 3Dbased carbon materials, which can be extended to the fabrication of other sustainable applications such as energy and environmental, and electrochemical/biosensor.

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