Article pubs.acs.org/IECR
Magnetic Pt Catalyst for Selective Hydrogenation of Halonitrobenzenes Weichen Du, Shuixin Xia, Renfeng Nie, and Zhaoyin Hou* Key Lab of Applied Chemistry of Zhejiang Province, Department of Chemistry, Zhejiang University, Hangzhou, 310028, China S Supporting Information *
ABSTRACT: Surfactant-free, H2-reduced Pt NPs were successfully fabricated on the surface of Fe3O4. Characterizations disclosed that Pt NPs (3.1 nm on average) dispersed evenly on Fe3O4. This catalyst is extremely active and selective for hydrogenation of chloronitrobenzenes, convenient, and suitable for cyclic utilization. The study showed that the activity of Pt depended mainly on its particle size and that the Fe3O4 support is most favorable for this reaction.
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INTRODUCTION Nanoparticles and nanocrystals that are well dispersed in a reaction mixture always exhibit prominent activity because of their large surface area, high utilization of surface atoms, and easy accessibility to reactants. These nanoparticles bring new challenges and insights into traditional catalysis, and many achievements have been reported in recent years.1,2 However, catalysts in nanoparticles suffer from serious problems with thermal stability due to the Ostwald ripening effect3 and must be protected by a surfactant. The recovery and reuse of these nanoparticles are becoming an obstacle from the perspective of green chemistry and industrial application. Many efforts have been reported to solve these problems. It was reported that the aggregation of silver NPs could be effectively suppressed under the protection of starch,4 and the morphology of Pt NPs could also be controlled by an optimized ratio between sodium polyacrylate and Pt 2+.5 However, protective surfactants surrounding NPs can hinder the free access of substrates to the active sites. Another popular approach is fixing the NPs to a support to improve thermal stability and ease of recovery.6 Nonetheless, traditional impregnation methods require calcination and reduction at high temperature, and these steps can cause aggregation of the metal NPs. More recently, a combined in situ reduction and deposition of metal NPs on a support using sodium borohydride, 7 ethylene glycol, 8 poly(vinylpyrrolidone),9 and formaldehyde10 as reductant and/or protectant has attracted the attention of several groups. It was confirmed that the distribution of metal particles could be controlled under the protection of surfactants. SBA-15,9 graphene,11 α-Fe2O3,12 and Fe3O413 have been reported as efficient supports for Pt NPs. Among these reported supports, magnetic Fe3O4 is superior for its ease of separation. With more than 4 000 000 tons produced per year, aromatic chloramines (CANs) play an important role in the production of dyes, herbicides, pesticides, medicines, and functional polymers.14 CANs are produced via the hydrogenation of the corresponding chloronitrobenzenes (CNB) over Raney Ni,15 Ni/TiO2,16 Ru/SnO2,17 and Ag/SiO2.18 It has been reported that in the hydrogenation of m- and p-CNB, the yields of mand p-CAN were 81.4% and 84.2% over Raney Ni along with 18.3% and 13.3% of the hydrodechlorination products.15 For © 2014 American Chemical Society
the hydrogenation of o-CNB over Ni/TiO2, Ru/SnO2, and Ag/ SiO2, the product selectivity to o-CAN was higher than 99%, and their corresponding specific activities were 48.8, 154.8, and 57.1 molo‑CAN/(molmetal·h),16−18 respectively. However, hydrodechlorination occurred easily in the hydrogenation of m-CNB, and toxic inhibitors were added to suppress this side reaction.19,20 These additives sometimes decrease the hydrogenation rate of nitroarenes and extra steps need to be adopted for the separation.14,16 In this work, surfactant-free, H2-reduced Pt NPs were successfully fabricated on the surface of Fe3O4 and applied in the selective hydrogenation of CANs. The performance of the Pt/Fe3O4 was compared with that of MgO-, Al2O3-, SiO2supported Pt NPs, and Pt/Fe3O4 prepared with different reductants (Pt/Fe3O4−HH: reduced with hydrazine hydrate; Pt/Fe3O4−SB: reduced with sodium borohydride).
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EXPERIMENTAL SECTION Catalyst Preparation. Ferroferric oxide microspheres were synthesized according to the literature procedure.21 FeCl3· 6H2O (>99%, 13.5 g, 50 mmol), NaAc·3H2O (>99%, 36.0 g), and poly(ethylene glycol) (PEG 20000, 10.0 g) were dissolved together in 400 mL of ethylene glycol (99.5%) and stirred vigorously for 30 min. The mixture was transferred to and sealed in a stainless autoclave (500 mL) with a Teflon inner layer. The autoclave was heated to 200 °C and kept at this temperature for 8 h under stirring. After reaction, it was cooled to room temperature. The resulting products were separated by an external magnetic field, washed with ethanol, and dried at 60 °C overnight. Ferroferric oxide powder (1.0 g) and 5 mL of an aqueous solution of H2PtCl6 (containing 0.05 g of Pt) were added to 15 mL of N,N-dimethylformamide (DMF, 99.5%). The mixture was stirred vigorously, sealed in a stainless autoclave (100 mL) with a glass inner layer, and then reduced by H2 (1.0 MPa) at 40 °C for 9 h under stirring. The resulting products were Received: Revised: Accepted: Published: 4589
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separated by an external magnetic field, washed with ethanol, and dried at 60 °C in vacuum overnight. The loading amount of platinum in freshly prepared catalyst detected by inductively coupled plasma atomic emission spectroscopy (ICP-AES) is 4.93%. At the same time, MgO-, Al2O3-, and SiO2-supported Pt catalysts (with a 5.0% loading amount of Pt) were also synthesized by a method similar to that used for Pt/Fe3O4, and these catalysts were used in the reaction without further treatment. Free Pt NPs were prepared by reduction of H2PtCl6 in a mixed solution DMF and PVP under H2 atmosphere (1.0 MPa).22 On the surface of the Fe3O4 support, the reduction of H2PtCl6 was also performed with hydrazine hydrate and sodium borohydride as reducing agents (see experimental section in Supporting Information), and the prepared catalysts are denoted as Pt/Fe3O4−HH and Pt/Fe3O4−SB. Characterization. X-ray diffraction (XRD) patterns were collected on a Rigaku diffractometer (D/MAX 2550/PC, 18 kW) using Cu Kα radiation (100 kV, 40 mA, λ = 0.1542 nm). Scanning electron microscopy (SEM) images were taken on a scanning electron microscope (Leo Series VP 1430, Germany). Transmission electron microscopy (TEM) images were detected using an accelerating voltage of 200 kV (TEM, JEOL-2020F). Samples were dispersed on a Cu grid after supersonic-wave shaking for 5 min. The magnetic properties of the samples were investigated by a superconducting quantum interference device magnetometer (SQUID, MPMS-XL-5). Xray photoelectron spectroscopy (XPS) measurements were performed on a Kratos Axis Ultra DLD system with a base pressure of 10−9 Torr. Hydrogenation of Chloronitrobenzenes. Hydrogenation of chloronitrobenzenes was carried out in a 100 mL stainless steel autoclave with a glass inner layer and equipped with a magnetic stirrer. Catalyst was dispersed in 30 mL of ethanol, followed by the addition of a certain amount of substrate. The autoclave was sealed, purged with H2, pressurized to 1.0 MPa, and subsequently stirred with a magnetic stirrer at a rate of 1000 rpm at the desired temperature. Finally, the mixture was separated by an external magnetic field. All reactants were analyzed by GC (HP 5890), and products were confirmed by GC-MS (Agilent 68905973N).
Figure 1. XRD patterns of Fe3O4 (a) and Pt/Fe3O4 (b). The inset is the magnified pattern from 30° to 50°.
agrees with the XRD analysis. Pt/Fe3O4 nanospheres reveal morphology properties similar to those of the support (Figure S1b). Pt nanoparticles cannot be identified on the surface of the Fe3O4 nanospheres in the SEM image. High resolution TEM images at the edge of a single Pt/Fe3O4 nanosphere (Figure 2b) shows the oriented and ordered lattice fringes of Fe3O4. A dspacing value of 0.486 nm (dFe3O4) corresponds to that of the Fe3O4 (111) planes.23 Besides, Pt nanoparticles were dispersed evenly on the surface of the Fe3O4 nanospheres and remained small in size (Figure 2b and 2c), exposing mainly the (111) planes with a d-spacing value of 0.225 nm (Figure 2c). The inserted particle distribution diagram in Figure 2c indicates that most Pt nanoparticle diameters had a size range of 2−4 nm and an average Pt particle size of 3.1 nm. Figure 3 shows the XPS spectra of Pt 4f and Fe 2p in Pt/ Fe3O4. Both spectra were calibrated on the basis of C 1s (284.6 eV).24 The deconvolution spectrum of the Pt 4f region from the spin orbital splitting of the 4f7/2 and 4f5/2 states reveals two pairs of doublets which are associated with different chemical environments. The pair with the stronger intensity is attributed to metallic Pt at 71.2 eV (Pt 4f7/2) and 74.6 eV (Pt 4f5/2).11,25 This value is slightly higher than that of bulk Pt metal (71.0 eV, 4f7/2), which reveals the strong interaction between Pt and Fe3O4 and/or the local environmental effects of the support.26 Another pair with higher binding energies at 72.2 eV(Pt 4f7/2) and 75.6 eV (Pt 4f5/2) corresponds to Ptδ+ existing in the form of Pt−O−Fe groups anchored on the Fe3O4 nanosphere surface.27,28 Metallic platinum occupies 85.86% of the total Pt, and 14.14% exists in an oxidation state, which were calculated from the intensity of the metallic Pt and Ptδ+ peaks. Two broad peaks at 711.6 and 725.2 eV corresponding to Fe 2p3/2 and Fe 2p1/2, which are the characteristic symbols of Fe3O4, were detected in the Fe 2p spectrum (Figure 3b), and they were in accordance with the values provided in the literature.29,30 A peak around 720 eV, which is regarded as the charge transfer satellite of Fe 2p3/2, was not detected, indicating the formation of a mixture of Fe(II) and Fe(III) oxides such as Fe3O4.29 The elemental compositions of Pt/Fe3O4 analyzed by XPS and ICPAES are summarized in Table 1. The surface composition of Pt (5.4%, relative atomic percentage measured by XPS) is higher than its bulk composition (0.8%, relative atomic percentage disclosed in ICP-AES), which reveals the surface enrichment
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RESULTS AND DISCUSSION Characterization of Materials. The XRD spectra of Fe3O4 and Pt/Fe3O4 are shown in Figure 1. Sharp, strong peaks indicate that both prepared materials are of high crystallinity. The crystallite size of pure Fe3O4, possessing a face-centered cubic structure, is 20.0 nm, as calculated by Debye−Scherrer’s formula upon the distinct (311) diffraction peak. Compared with the XRD pattern of Fe3O4, few changes can be observed from the Bragg peaks of ferroferric oxide after Pt was loaded, which indicates that H2 reduction has scarcely any effect on the morphology of Fe3O4. The tiny, broad peak at 40.1° (the inset image in Figure 1), corresponding to the Pt (111) plane, proves the existence of Pt particles and their good dispersion on Fe3O4. Figure S1 (Supporting Information) shows the SEM images of prepared Fe3O4 and Pt/Fe3O4. Pure Fe3O4 particles exhibit a spherical appearance, with a mean diameter of 250 nm in a narrow size distribution (Figure S1a). As we can roughly see in the SEM image, these Fe3O4 particles are aggregates of much smaller nanoparticles with sizes of dozens of nanometers, which 4590
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Figure 2. TEM images of Pt/Fe3O4. Overview of Pt/Fe3O4 nanospheres (a) and enlarged images of Pt/Fe3O4 nanospheres (b, c): dFe3O4 = 0.486 nm (b) and Pt NP distribution on Fe3O4: dPt = 0.225 nm (c).
Figure 3. XPS spectra of Pt 4f (a) and Fe 2p (b) in Pt/Fe3O4.
Catalytic Activity. At first, it was found that the hydrogenation of chloronitrobenzenes proceeded efficiently over Pt/Fe3O4 (see Table 2). Besides the desired m-CAN, mchloronitrosobenzene (m-CNSB) and 3,3′-dichloroazoxybenzene (3,3′-CAOB) were also detected during the reaction process over Pt/Fe3O4. According to the mechanism reported in published works,8,20,31 we think that m-CNB may be first reduced to m-CNSB and then m-CNSB further reduced to mN-chlorophenylhydroxylamine (m-CPHA) and m-CAN. 3,3′CAOB is the condensation product of m-CNSB and mCPHA.17,32 CAOB can also be reduced to CAN via a consecutive hydrogenation. The suggested pathways for hydrogenation of chloronitrobenzenes over Pt/Fe3O4 catalyst are presented in Scheme 1.
Table 1. Elemental Composition of Pt/Fe3O4 relative atomic percentage (%) element
surface composition (XPS)
bulk composition (ICP-AES)
O Fe Pt
54.0 40.6 5.4
56.7 42.5 0.8
from Pt. The magnetic properties analysis confirmed that both Fe3O4 and Pt/Fe3O4 exhibited superparamagnetic properties (see Figure S2 in Supporting Information). The magnetic saturation (Ms) value of Pt/Fe3O4 is lower than that of Fe3O4, which may be attributed to the partial replacement of Fe3O4 by Pt in per unit weight and the effect of Pt on the electronic structure of Fe3O4. 4591
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Table 2. Hydrogenation of m-CNB over Different Pt Catalystsa catalyst Pt/ Fe3O4− HH Pt/ Fe3O4− SB Pt/Fe3O4 Pt NPs Pt/SiO2 Pt/Al2O3 Pt/MgO
particle size of Pt (nm)
conversion (%)
selectivity for m-CAN (%)
yield (%)
activity (molCAN/ (molPt·h))b
5.6
7.9
69.1
5.5
1428.5
5.0
33.6
70.8
23.8
5558.1
3.1 4.9 4.6 6.0 7.1
100.0 12.2 10.1 19.2 9.5
96.6 52.5 54.5 60.9 54.7
96.6 6.4 5.5 11.7 5.2
45630.8 1466.6 1183.2 3278.4 1724.1
Among the tested free Pt NPs, Pt/MgO, Pt/Al2O3, Pt/SiO2, and Pt/Fe3O4 catalysts, Pt/Fe3O4 exhibited the best activity and the highest selectivity toward m-CAN (96.6%) under the same reaction conditions (see Table 2). These results showed that the activity of these catalysts tends to increase with the decreased particle size of Pt, which indicated that the main active sites for the reaction are on the surface of Pt particles. The activity per surface Pt atom in Pt/Fe3O4 that was measured at low conversion (32%, 5 min) reached 45630.8 molCAN/ (molPt·h). The lower activity of Pt/MgO, Pt/SiO2, and Pt/ Al2O3 might be attributed to larger size Pt particles (7.1 nm, 4.6 nm, 6.0 nm, calculated by Debye−Scherrer’s formula) formed on these supports. At the same time, the particle size of Pt in Pt/SiO2 (4.6 nm), free Pt NPs (4.9 nm), and Pt/Fe3O4−SB (5.0 nm) was simliar, but the activity of Pt/Fe3O4−SB was higher than that of Pt/SiO2 and free Pt NPs, and no hydrodechlorination product was detected over Pt/Fe3O4 catalyst during the reaction process. These results indicate that the Fe3O4 support is more favorable for this reaction because the nitro compound can also adsorb on Fe(II) sites besides Pt,33,34 which can be regarded as a supplementary means to concentrate nitro compounds on the surface of the catalyst. We also found that a strong reducing agent, such as hydrazine hydrate and sodium borohydride, is unfavorable for the dispersion of Pt on the surface of Fe3O4, and thus the yield of m-CAN decreased to 5.5% and 23.8% over Pt/Fe3O4−HH and Pt/Fe3O4−SB, respectively. These results further confirm that the main active sites are on the surface of Pt particles. According to the literature, the catalytic activity increases with the proportion of surface atoms, and a reduction in particle size is accompanied by an improvement in activity per unit catalyst.35−37 Pt/Fe3O4 also showed a superior activity in the hydrogenation of a series of CNBs (see Table 3). All the substrates were transformed into the corresponding CAN with high selectivity. For m-CNB, the conversion is still above 80% even when the amount of catalyst was reduced by half. With the hydrogenation process going on, the intermediates were finally transformed into CAN as well. The catalytic activity of Pt/ Fe3O4 decreased slightly, and the magnetic property of the catalyst remained after five cycles (see Figure S3 in Supporting Information), indicating its stability in the reaction system.
a
Reaction condition: m-CNB (19.6 mmol), m-CNB:Pt = 5000:1, ethanol (30 mL), temperature (30 °C), time (1 h), H2 (1.0 MPa), 1000 rpm. bThe activity of surface Pt atoms in Pt/Fe3O4 was calculated at 32.0% conversion of m-CNB during the reaction (5 min). The activity of surface Pt atoms in other catalysts was calculated on the yield data at 1 h that is listed in column 5.
Scheme 1. Pathways of Chloronitrobenzene Hydrogenation over Pt/Fe3O4
Table 3. Hydrogenation of Different CNBs over Pt/Fe3O4a
a
Reaction condition: catalyst (15 mg), substrate (19.6 mmol), ethanol (30 mL), H2 (1.0 MPa), 1000 rpm. bCatalyst (7.5 mg). cCatalyst (7.5 mg), substrate (39.2 mmol). dMainly chloronitrosobenzene and azoxydichlorobenzene. 4592
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Table 4. Hydrogenation of Nitroarenes to Amines over Pt/Fe3O4a
Reaction conditions: substrate (19.6 mmol), catalyst (15 mg), ethanol (30 mL), H2 (1.0 MPa), temperature (20 °C), 1000 rpm. bTemperature (30 °C). cSubstrate (4.9 mmol), catalyst (3.8 mg), temperature (30 °C). dTemperature (40 °C).
a
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To fully understand the catalytic activity of Pt/Fe3O4, we applied this catalyst in the hydrogenation of a series of nitroarenes. As summarized in Table 4, these reactions proceeded efficiently regardless of the functional group substitution on the nitroarene. Nevertheless, the steric constraint, conjugative effect, and inductive effect of different substituents affect the catalyst, which reflects on the reaction condition and specific activity of the respective reaction. The hydrogenation of nitroaniline, for instance, required more critical conditions but exhibited activity relatively lower than that for methylnitrobenzene mainly because of the difference between NH2 and CH3 for the conjugative effect and inductive effect. It is noteworthy that hydrodehalogenation was still basically avoided in the hydrogenation of bromonitrobenzene. Thus, this catalytic system can greatly improve upon the situation where inhibitors are needed to suppress the hydrodechlorination in the industrial production of haloanilines.
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ASSOCIATED CONTENT
S Supporting Information *
Detailed synthesis of Pt/Fe3O4−HH and Pt/Fe3O4−SB, SEM images and magnetic properties of Fe3O4 and Pt/Fe3O4, additional catalytic data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Tel/Fax: +86-571-88273283. E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research work was supported by the National Natural Science Foundation of China (contracts 21273198, 21073159) and the Zhejiang Provincial Natural Science Foundation (grant no. LZ12B03001).
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CONCLUSION
In summary, a facile surfactant-free one-step approach to fabricate Pt NPs on Fe3O4 magnetic catalyst was reported in this work. Pt/Fe3O4 exhibited superior activity and extremely high selectivity for CAN in the hydrogenation of CNB compared to free Pt NPs and MgO-, Al2O3-, and SiO2supported Pt catalysts. The high efficiency could be attributed to the well-dispersed Pt NPs on the Fe3O4 surface and the strong interaction and electron transfer from Pt NPs to Fe3O4, and the synergistic reaction between Pt and Fe3O4 may also promote the catalytic activity. This catalyst is broad-spectrum for the hydrogenation of a series of nitroarenes and is convenient for recycled utilization.
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