Article pubs.acs.org/IECR
Transition-Metal-Exchanged Nanocrystalline ZSM‑5 and MetalOxide-Incorporated SBA-15 Catalyzed Reduction of Nitroaromatics Balwinder Kaur,† Mahesh Tumma,† and Rajendra Srivastava*,† Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar-140001, Punjab, India ABSTRACT: Nanocrystalline ZSM-5 was synthesized by a one-step synthetic route using the conventional ZSM-5 synthesis composition but in the presence of propyltriethoxysilane as an additive. Transition metal ion-exchanged nanocrystalline ZSM-5 catalysts were obtained by the ion exchange of nanocrystalline ZSM-5. The one-step direct synthetic route was used to prepare metal-oxide-incorporated SBA-15 catalysts under milder acidic conditions than are conventionally employed for the synthesis of Si-SBA-15. The catalysts were investigated in the reduction of nitroaromatics in the presence of sodium borohydride as a reducing agent. Among the catalysts investigated, copper ion-exchanged nanocrystalline ZSM-5 exhibited the highest activity. Although the copper ion-exchanged nanocrystalline ZSM-5 is recyclable, but for efficient recycling (with no loss in activity), it is better for the catalyst to be ion-exchanged once before being used in the next cycle.
1. INTRODUCTION Zeolites are widely used in water purification (because of their ion-exchange capability), gas separation (because of their regular porous nature), and catalysis (because of their tunable acidity, channels, and micropores).1−4 Zeolites also play important roles in the petroleum refining and fine chemical industries because of their acidic properties. However, the role of zeolites in the fine chemical industries is limited because of the microporous characteristics of zeolites, which provide diffusion constraints for reactant and product molecules.5,6 Zeolites having intra- or intercrystalline mesoporosity have attracted significant attention because of their improved diffusion, catalytic activity, selectivity, and retardation against deactivation.7−14 Several attempts have been made to prepare nanosized zeolites from colloidal precursors under mild hydrothermal conditions, including mesopore generation using soft and hard templates or dealumination of zeolites.7−18 A variety of silane-containing soft templates and multiquaternary ammonium salts have been investigated in the synthesis of mesoporous zeolites.11−16 Carbon nanoparticles, carbon nanotubes, mesoporous carbons, and polymer beads have all been used as hard template for the synthesis of mesoporous zeolites,9,17−19 and functionalized polymers have been used as soft templates in the synthesis of mesoporous ZSM-5.20,21 Facile diffusion of reactant and product molecules through the mesopores present in mesoporous zeolites is responsible for their high catalytic and electrocatalytic activities compared to those of conventional microporous zeolites.22−25 The reduction of aromatic nitro compounds to the corresponding amines is an important organic functionalgroup transformation, because the amino group serves as a site for further derivatization.26 Amines are important intermediates for the production of many pharmaceuticals, photographic materials, agrochemicals, polymers, dyes, and rubber materials.27 Aromatic amines are generally prepared from the corresponding aromatic nitro compounds by catalytic reduction or a noncatalytic process.28 The conventional noncatalytic routes based on metal/acid pairs involve several operational limitations such as difficulty in product isolation, low reduction © 2013 American Chemical Society
efficiency, and waste production (serious environmental concerns). Therefore, researchers have shifted their attention to catalytic reduction because of the better product quality and lower generation of pollution. Precious metal-based catalytic reductions of nitro compounds have been widely investigated, but such reaction systems are expensive and usually sensitive to both air and moisture.29−32 To overcome these limitations, heterogeneous catalysts such as metal/metal salt mixtures supported on activated carbon,33,34 iron oxide-based catalysts,35,36 clays,37 and iron nanoparticles38 have been investigated for the nitro reduction. Our research is focused on the synthesis and applications of zeolites,12−16 and we are exploring the catalytic activity of nanocrystalline zeolites.25 In this study, transition-metalexchanged nanocrystalline ZSM-5 materials (hereafter denoted as M-Nano-ZSM-5) were prepared and investigated in the catalytic reduction of nitroaromatics. For comparative purposes, metal oxide incorporated SBA-15 materials (hereafter denoted as M-SBA-15) were also prepared and investigated in the catalytic reduction of nitroaromatics. To the best of our knowledge, the catalytic reduction of nitroaromatics using a nanocrystalline-ZSM-5 based route is reported here for the first time.
2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals used in the study were of AR grade and were used as received without further purification. Tetraethylorthosilicate (TEOS), propyltriethoxysilane (PrTES), tetrapropylammonium hydroxide (TPAOH), poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (EO20PO70EO20, MW = 5800; P123), sodium borohydride (NaBH4), and nitroaromatics were purchased from Aldrich. CuCl2·2H2O, CuSO4·5H2O, MnCl2·4H2O, Received: Revised: Accepted: Published: 11479
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NiCl2·6H2O, CoCl2·6H2O, FeCl2·4H2O, FeSO4·7H2O, SnCl4· 5H2O, and Na2WO4·2H2O were obtained from Loba Chemie Pvt. Ltd. 2.2. Catalyst Preparation. Nano-ZSM-5 was synthesized according to the reported procedure.25 In a typical synthesis, 1.2 g of sodium aluminate (53 wt % Al2O3, 43 wt % Na2O, Riedel-de Haën) was dissolved in 25 mL of distilled water (solution A). In a separate vessel, 2.06 g of PrTES was mixed with 25 mL of TPAOH (1 M aqueous solution) (solution B). Solutions A and B were mixed, and the resultant solution was stirred for 15 min under ambient conditions, until it became clear. Then, 18.7 g of TEOS was added, and stirring was continued for 6 h. The molar composition of the gel mixture was 90 TEOS/10 PrTES/2.5 Al2O3/3.3 Na2O/25 TPAOH/ 2500 H2O. This mixture was transferred to a Teflon-lined autoclave and hydrothermally treated at 443 K for 3 days under static conditions. The final product was filtered, washed with distilled water, and dried at 373 K. The material was calcined at 823 K for 4 h under flowing air. ZSM-5 was synthesized at 443 K using the same synthesis composition as used for NanoZSM-5 but without the PrTES additive. Samples of NanoZSM-5 (1 g) were cation-exchanged into M-Nano-ZSM-5 (where M = Cu, Ni, Co, Mn, Fe) by repeating the ion exchange twice with 1 M aqueous solution of the metal source (50 mL) at 343 K for 4 h. Metal oxide incorporated SBA-15 materials (Si/M = 20) were synthesized according to the reported procedure using the molar gel composition TEOS/0.05 metal precursor/0.016 P123/0.46 HCl/127 H2O.39 In a typical synthesis, 1.78 g of P123 was added to 24 mL of water. After 4 h of stirring, a clear solution was obtained. A dilute aqueous HCl solution (0.92 g of HCl + 20 mL of water) was added, and the solution was stirred for another 2 h. Then, 4 g of TEOS and the required amount of metal source were added, and the mixture was stirred for 24 h at 313 K. The reaction mixture was then transferred into an autoclave and aged under static conditions for 48 h at 373 K to obtain the product. The resultant solid was filtered, washed, dried at 373 K, and calcined at 823 K for 6 h. M-SBA-15 (where M = Cu, Ni, Co, Mn, Fe, Sn, W) synthesized with an input Si/ M ratio of 20 are denoted as M-SBA-15(20). 2.3. Characterization. X-ray diffraction (XRD) patterns were recorded both in the low-angle range (0−5°) and in the wide-angle range (5−80°) on a PANalytical X’PERT PRO diffractometer using Cu Kα radiation (λ = 0.1542 nm, 40 kV, 20 mA) and a proportional counter detector. Nitrogen adsorption measurements at 77 K were performed on an Autosorb-IQ Quantachrome Instruments volumetric adsorption analyzer. Samples were outgassed at 573 K for 2 h in the degas port of the adsorption apparatus. The specific surface area was determined by the Brunauer−Emmett−Teller (BET) method using the data points for relative pressures (P/P0) in the range of about 0.05−0.3. The pore diameter was estimated using the Barret−Joyner−Halenda (BJH) model. Scanning electron microscopy (SEM) measurements were carried out on a JEOL JSM-6610LV instrument to investigate the morphology of the materials. During SEM investigations, energy-dispersive X-ray spectroscopy (EDS) was utilized to map various elements present in the sample. The chemical composition of the samples was also estimated by atomic absorption spectrophotometry (model AAS 5EA, Analytik Jena, Jena, Germany). 2.4. Catalytic Reactions. In a typical procedure, a nitroaromatic compound (1.0 mmol) was dissolved in 10 mL of solvent (1:1 ethanol/water mixture). NaBH4 (5.0 mmol) and
catalyst (50 mg) were added, and the reaction mixture was stirred at 313 K. The progress of the reaction was monitored through thin layer chromatography (TLC) and gas chromatography (GC). After completion of the reaction, the reaction mixture was extracted by ethyl acetate and dried over anhydrous Na2SO4 to afford the product. Catalytic reactions performed using various catalysts were also analyzed by GC (Yonglin 6100 instrument; BP-5 column, 30 m × 0.25 mm × 0.25 m). The products were identified by authentic samples obtained from Aldrich. In some cases, products were also purified using column chromatography.
3. RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. Nano-ZSM-5 was obtained by the addition of PrTES to the synthesis composition of ZSM-5. PrTES contains only three hydrolyzable moieties with one hydrophobic propyl group that is unfavorable for the formation of extended tetrahedral SiO2 linkages. Consequently, the zeolite growth is significantly retarded at the organic and inorganic interfaces, resulting in the formation of nanocrystalline zeolites. Mesopores are formed in Nano-ZSM-5 because of the crystal packing of these nanosized zeolite particles. Under the present weakly acidic synthesis conditions of MSBA-15, TEOS and MCl2 are hydrolyzed and protonated in acidic medium, forming ethanol, SiOH2+, HCl, and M− OH2+. The positively charged silica species are attracted electrostatically to the anionic portion of the surfactant ion pair (S+X−) forming electrical triple layer, where halide ions coordinate through Coulombic interactions to the protonated silica groups. Therefore, under these conditions, partially condensed silica species are able to form MOSi bonds in M-SBA-15 materials. Among the catalysts investigated in this study, Cu-NanoZSM-5 (among the M-Nano-ZSM-5 materials) and Cu-SBA15(20) [among the M-SBA-15(20) materials] exhibited higher activities in the reduction of nitroaromatics. Therefore, detailed characterizations of Cu-Nano-ZSM-5 and Cu-SBA-15(20) are presented in this article. Cu-ZSM-5 and Cu-Nano-ZSM-5 exhibited MFI framework structures with high phase purity, as confirmed by XRD (Figure 1). The XRD patterns of other MNano-ZSM-5 materials were found to be similar to that of CuNano-ZSM-5, confirming the high phase purity even after metal ion exchange (Figure 2). The small-angle powder XRD pattern of Cu-SBA-15(20) exhibited three well-resolved diffraction peaks in the 2θ range of 0.7−2° that correspond to the diffraction of the (100), (110), and (200) planes (Figure 1a). These peaks are characteristic of the hexagonally ordered structure of SBA-15.39 The powder XRD pattern of Cu-SBA15(20) at wide angles (5−80°) showed a broad diffraction peak at 2θ = 23° (Figure 1), which indicates the amorphous nature of SBA-15. This observation confirms that CuO was incorporated into the mesopore wall of the SBA-15 materials and that no CuO was present on the external surface of CuSBA-15(20). Similar results were also observed for other metaloxide incorporated SBA-15 materials prepared in this study (data not shown). The N2 adsorption isotherm of Cu-Nano-ZSM-5 showed a type-IV isotherm similar to that of the mesoporous materials (Figure 3). The major difference in the isotherm of Cu-NanoZSM-5 with respect to that of Cu-ZSM-5 is a distinct increase in N2 adsorption in the range of 0.4 < P/P0 < 0.95, which is interpreted as capillary condensation in intercrystalline mesopore void spaces. The mesopores show a broad pore 11480
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adsorption at P/P0 > 0.6 is characteristic of capillary condensation within mesopores. The empty cylindrical mesopores with a hysteresis at higher relative pressures (P/P0 = 0.6) confirmed that the CuO particles did not plug the pores of SBA-15. These interpretations indicate that, under the present mild acidic conditions, SiOCuOSi linkages are formed in the corona region. Surface area, pore volume, and pore diameter were obtained from the sorption data (Table 1). The surface area and pore volume of M-Nano-ZSM-5 were found to be similar to those of Nano-ZSM-5 (Table 1). SEM images indicate that the spheroid-shaped Cu-NanoZSM-5 particles were composed of very small nanocrystals. Based on SEM and N2 adsorption, one can conclude that the Cu-Nano-ZSM-5 sample contained intercrystalline mesopores, which were formed by the packing of nanosize zeolite particles of 10−20-nm size (Figure 4a). Many wheat-like domains aggregated into a rope-like microstructure can be seen for CuSBA-15(20) (Figure 4a). EDS mapping confirmed the presence of finely dispersed copper ions in the Nano-ZSM-5 matrix (Figure 4b). The metal contents in the M-Nano-ZSM-5 and MSBA-15(20) samples were obtained by atomic absorption spectrophotometry (Table 1). 3.2. Catalytic Activity. 4-Nitroaniline was chosen as a model substrate, and Cu-Nano-ZSM-5 was chosen as a catalyst to determine the optimum reaction conditions. The reduction of 4-nitroaniline by NaBH4 in the absence of zeolite catalyst was extremely slow, and the reaction was not initiated even after 10 h at 313 K. Thus, a combination of zeolite catalyst and NaBH4 was investigated. The effect of solvent on this reaction was also investigated (Table 2). It was found that organic solvents such as tetahydrofuran (THF), toluene, dichloromethane, and acetonitrile failed to initiate the reaction, whereas the reduction proceeded efficiently in water or ethanol. Interestingly, the reaction did not proceed well in methanol or isopropanol. The differences in the catalytic activities obtained in different solvent media are explained later in this article. It was found that, among the solvents investigated,
Figure 1. XRD patterns of Cu-ZSM-5, Cu-Nano-ZSM-5, and Cu-SBA15(20) materials. Inset: (a) Low-angle powder X-ray diffraction pattern of Cu-SBA-15(20).
size distribution in the range of 2−10 nm. Cu-SBA-15(20) exhibited a type-IV isotherm with an H1 hysteresis loop, according to the IUPAC classification (Figure 3), which is characteristic of mesoporous materials with one-dimensional cylindrical channels. A sharp increase in the volume of N2
Figure 2. XRD patterns of Ni-Nano-ZSM-5, Co-Nano-ZSM-5, Mn-Nano-ZSM-5, Fe-Nano-ZSM-5, and recovered Cu-Nano-ZSM-5 materials. 11481
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Figure 3. (a) N2 adsorption/desorption isotherms and (b) pore size distributions of Cu-ZSM-5, Cu-Nano-ZSM-5, and Cu-SBA-15(20) materials.
than the SBA-15-based catalysts. Among the M-SBA-15 catalysts, only Cu-SBA-15(20) was found to be active (Table 3). However, the activity of Cu-SBA-15(20) was much lower than the activity of Cu-ZSM-5 and Cu-Nano-ZSM-5 (Table 3). Among the M-Nano-ZSM-5 catalysts, Cu-Nano-ZSM-5 exhibited the highest activity. The highest catalytic activity of CuNano-ZSM-5 is due to the intrinsic catalytic activity provided by copper ions for the nitro reduction when compared to other transition-metal ions investigated in this study. (Details of the mechanism of nitro reduction and the differences in the catalytic activities of various M-Nano-ZSM-5 materials are provided later in this article.) However, the activity of CuNano-ZSM-5 was found to be much higher than the activity of Cu-ZSM-5 (Table 3). The high dispersion of copper ions on the large-surface-area Nano-ZSM-5 matrix and intercrystalline mesopores (for facile diffusion of reactant and product molecules) is responsible for the high activity of Cu-NanoZSM-5. The activity of Cu-Nano-ZSM-5 is also dependent on the number of exchanges (amount of Cu loading during ion exchange) (Table 3). The scope of this reaction was extended to other nitroaromatics such as nitrobenzene and substituted nitrobenzene. A wide range of substituted nitroaromatics were reduced by this method to produce the corresponding aromatic amines. The reduction of nitrobenzene required longer than the reduction of substituted nitrobenzene (Table 4). The reduction of p-chloronitrobenzene proceeded selectively, without any dehalogenation, at a much higher rate (Table 4). In general, the catalytic reduction was very clean and gave aromatic amines in high yield. No intermediate products, such as hydroxylamine or hydrazine, were detected after the reaction. Whereas the products were obtained just by extraction with ethyl acetate, column chromatography was performed to obtain the products for spectroscopic investigations. In the reduction of nitroaromatics by the combination of NaBH4 with Cu-Nano-ZSM-5 in ethanol/water medium, the evolution of hydrogen gas was identified. Very fine black
Table 1. Textural Characteristics of Several Catalysts Investigated in This Study
catalyst Cu-ZSM-5 Nano-ZSM-5 Cu-Nano-ZSM-5 Ni-Nano-ZSM-5 Co-Nano-ZSM-5 Fe-Nano-ZSM-5 Mn-Nano-ZSM-5 Cu-SBA-15(20) Cu-Nano-ZSM-5 (recycled)
Si/Al (output)
total surface area (m2/g)
external surface area (m2/g)
total pore volume (cm3/g)
transitionmetal content (mg/g of catalyst)
22.5 23.6 24.3 24.7 24.5 24.8 23.8 − −
273 492 476 483 475 480 470 823 470
52 246 234 240 237 243 230 629 233
0.198 0.412 0.400 0.407 0.400 0.405 0.396 1.268 0.397
16.6 − 16.9 16.2 16.0 15.5 15.9 34.8 15.5
ethanol/water (1:1) was found to be the best reaction medium for the reduction of nitroaromatics, when the dissolution of the reactants and the catalytic activities are taken into account (Table 2). The effects of the amount of catalyst and substrate/ NaBH4 ratio were also investigated to determine the optimum reaction conditions (Figure 5). It can be noted that, for a constant substrate/NaBH4 ratio of 1:2, only 80% yield of the product was observed even after 12 h of the reaction. The reaction took place even at 298 K; however, it required a significantly longer reaction time. For example, it took only 2 h at 313 K for the complete conversion of p-nitroaniline to pphenylinediamine, whereas 7 h was required for the complete conversion of p-nitroaniline at 298 K. Having analyzed all of the reaction conditions, the following conditions were selected for a detailed investigation: nitroaromatics, 1 mmol; catalyst, 50 mg; NaBH4, 5 mmol; solvent, 10 mL, ethanol/water = 1:1; reaction temperature, 313 K. All catalysts prepared in this study were evaluated under the optimized reaction conditions for the reduction of p-nitroaniline. The zeolite-based catalysts were found to be more active 11482
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Figure 4. (a) SEM images of Cu-Nano-ZSM-5 and Cu-SBA-15(20) and (b) EDS elemental maps of representative elements in Cu-Nano-ZSM-5.
Table 2. Influence of the Solvent in the Reduction of pNitroaniline over Cu-Nano-ZSM-5a
3R−NO2 + 12H 2 → 3R−NH 2 + 6H 2O
sample no.
solvent
product yield (%)
1 2 3 4 5 6 7 8 9
ethanol methanol isopropanol water ethanol/water (1:1) THF acetonitrile dichloromethane toluene
100 30 15 90 100 0 0 0 0
This observation and the proposed mechanism are consistent with literature reports, which also reported the formation of metal boride as an active species.40−44 Differences in the catalytic activities in various solvent media can be explained based on the following observations during the reaction: Neither hydrogen evolution nor black precipitates were observed when the reactions were performed in dichloromethane and toluene. However, hydrogen evolution was observed in the case of THF and acetonitrile, but no black precipitates were formed during the reduction of p-nitroaniline. Hydrogen evolution and black precipitates were both observed during the reduction of p-nitroaniline when ethanol, methanol, isopropanol, water, and ethanol/water were used as solvent media. Based on these observations, one can conclude that, for the reduction of nitroaromatics, evolution of hydrogen and formation of black precipitates are both required. In addition, the evolution of hydrogen was very fast when the reaction was performed in methanol, and the H2 evolution stopped within 20 min after the start of the reaction. In ethanol/water medium, the hydrogen evolution was very fast in the beginning of the reaction, but it continued at a low rate even after 2 h. This observation confirmed that all of the sodium borohydride was converted into H2 in less than 20 min and no hydrogen was available after 20 min for the reduction of the nitro compound.
a
Reaction conditions: p-nitroaniline (1 mmol; solvent (10 mL; catalyst (50 mg; NaBH4 (5 mmol; temperature (313 K; reaction time (2.0 h).
precipitates were observed on the wall of the reaction vessel. The reduction of nitroaromatics is probably due to the formation of the black boride precipitates, which catalyze the decomposition of NaBH4, strongly adsorb nitroaromatics, and activate them toward the reduction by NaBH4 (eqs 1−3). M‐Nano‐ZSM‐5 + NaBH4 → metal boride species
(1)
metal boride species
NaBH4 + 2H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NaBO2 + 4H 2
(3)
(2) 11483
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Figure 5. Effects of (a) the amount of catalyst and (b) the substrate-to-NaBH4 ratio on the complete reduction of p-nitroaniline using Cu-NanoZSM-5. Reaction conditions: (a) p-nitroaniline, 1 mmol; solvent, 10 mL, ethanol/water = 1:1, NaBH4, 5 mmol; temperature, 313 K; (b) pnitroaniline, 1 mmol; solvent, 10 mL, ethanol/water = 1:1; catalyst, 50 mg; temperature, 313 K.
Table 3. Reduction of p-Nitroaniline over Various Catalysts Investigated in This Studya sample no.
catalyst
time (h)
product yield (%)
TOFb (h−1)
1 2 3 4 5 6 7 8 9 10 11 12 13 14
Cu-ZSM-5 Cu-Nano-ZSM-5 Cu-Nano-ZSM-5c Cu-Nano-ZSM-5d Ni-Nano-ZSM-5 Co-Nano-ZSM-5 Fe-Nano-ZSM-5 Mn-Nano-ZSM-5 Cu-SBA-15(20) Ni-SBA-15(20) Co-SBA-15(20) Fe-SBA-15(20) Mn-SBA-15(20) W-SBA-15(20)
3.0 2.0 4.0 1.5 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0
95 100 96 100 65 25 16 7 12 0 0 0 0 0
24.2 37.6 22.4 13.2 23.4 8.9 5.5 2.5 2.2 − − − − −
Table 4. Catalytic Reduction of Nitroaromatics over CuNano-ZSM-5a.
a Reaction conditions: p-nitroaniline, 1 mmol; solvent, 10 mL, ethanol/ water = 1:1; catalyst, 50 mg; NaBH4, 5 mmol; temperature, 313 K. b Turnover frequency (TOF, h−1) is defined as the number of moles of product formed per mole of active species present in the catalyst per hour. cCu-ZSM-5 (one-time exchange, Cu content = 13.6 mg/g of catalyst). dCu-ZSM-5 (three-time exchange, Cu content = 63.8 mg/g of catalyst).
a
Reaction conditions: nitroaromatics, 1 mmol; solvent, 10 mL, ethanol/water = 1:1; catalyst, 50 mg; NaBH4, 5 mmol; temperature, 313 K. bTurnover frequency (TOF, h−1) is defined as the number of moles of product formed per mole of active species present in the catalyst per hour. cReaction conducted using 10 mmol of NaBH4.
Therefore, only a 30% yield of the product was observed in methanol medium. In contrast, in the case of ethanol/water medium, H2 was available for a longer duration (throughout the reaction) for the reduction of nitroaromatics. Differences in the catalytic activities of various M-Nano-ZSM-5 materials investigated in this study can also be explained based on the H2 evolution and metal boride formation. In all of these cases, H2 evolution was noticed. However, a comparatively greater
amount of metal boride was formed in the case of Cu-NanoZSM-5, when compared to other M-Nano-ZSM-5 catalysts. After completion of the reaction, Cu-Nano-ZSM-5 was separated by simple filtration from the reaction mixture. Filtered Cu-Nano-ZSM-5 was washed with ethyl acetate 11484
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reaction (Figure 6b). No loss in catalytic activity was observed even after 10 recycling experiments, provided that the recovered catalyst was treated once with 1 M CuSO4 solution (regeneration step) and subsequently used in the reaction (Figure 6b). Cu-Nano-ZSM-5 was found to be stable even after its reuse, as confirmed by XRD and N2 adsorption studies. Note that no copper boride phase was observed in the XRD pattern of recovered Cu-Nano-ZSM-5 (after it had been washed with ethyl acetate and water) (Figure 2) Reductions of nitroaromatics using metal salts and metal complex catalysts have been reported.45−49 However, these catalysts are less active than the Cu-Nano-ZSM-5 catalyst reported in this study. The reported catalysts required longer reaction times (15−24 h) and higher reaction temperature (100 °C) for the reduction, when compared to our reaction conditions. Furthermore, the reported catalysts cannot be regenerated and recycled.
(three times) and water (three times). Recovered catalyst was dried in vacuum at 373 K for 8 h and reused in the reaction. A progressive decrease in the catalytic activity was observed during the recycling study (Figure 6a). The hot-filtration
4. CONCLUSIONS In summary, the reduction of nitroaromatics to aromatic amines using transition metal ion-exchanged Nano-ZSM-5 and metal oxide incorporated SBA-15 materials was investigated. Among the catalysts investigated, Cu-Nano-ZSM-5 exhibited the highest activity. The simple operation, use of an economical catalyst, mild reaction conditions, short reaction time, high yield of amines, reduction of a wide range of substituted nitroaromatics, and ease of regeneration and recyclability of the catalyst make this protocol an attractive route for the reduction of aromatic nitro compounds.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel.: +91-1881-242175. Fax: +91-1881-223395. Author Contributions †
B.K., M.T., and R.S. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Department of Science and Technology, New Delhi, India, is acknowledged for financial assistance (SB/S1/PC-91/2012). B.K. is grateful to CSIR, New Delhi, India, for a JRF fellowship. The authors also thank Dr. Kamal Kumar Choudhary, Department of Humanities and Social Sciences, IIT Ropar, for proofreading the manuscript and the Director, IIT Ropar, for constant encouragement.
Figure 6. (a) Recycling of Cu-Nano-ZSM-5 and (b) regeneration and recycling of Cu-Nano-ZSM-5 in the complete reduction of pnitroaniline. Reaction conditions: (a) p-nitroaniline, 1 mmol; solvent, 10 mL, ethanol/water = 1:1, NaBH4, 5 mmol; catalyst, 50 mg; temperature, 313 K; (b) p-nitroaniline, 1 mmol; solvent, 10 mL, ethanol/water = 1:1; NaBH4, 5 mmol; catalyst, 50 mg; temperature, 313 K; reaction time, 2 h.
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method was used to investigate the leaching behavior of catalytically active species during the reaction. Catalyst was removed from the reaction mixture after 30 min (40% product yield) and 60 min (70% product yield), and the reaction was continued in the absence of catalyst. It was found that the reaction proceeded further but at a very low rate, taking 30 and 24 h, respectively, to complete the reaction. Recycling and hotfiltration tests confirmed that a small fraction of catalytic active species was leached during the reaction in the form of metal boride. Because our aim was to develop an ecofriendly reusable catalyst, an attempt was made to regenerate the catalyst. Used catalyst was ion-exchanged once using 1 M CuSO4 solution, and the regenerated catalyst was used in the fresh reaction. The reaction was found to be completed within 2 h, confirming that the catalyst was regenerated and can be used for successive
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