Synthesis of Mesoporous Î³-Alumina Supported CoâBased Catalysts

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Synthesis of Mesoporous #-Alumina Supported Co-Based Catalysts and Their Catalytic Performance for Chemoselective Reduction of Nitroarenes Haigen Huang, Mingwu Tan, Xueguang Wang, Man Zhang, Shuqiang Guo, Xiujing Zou, and Xionggang Lu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14513 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Applied Materials & Interfaces

Synthesis of Mesoporous γ-Alumina Supported Co−Based

Catalysts

and

Their

Catalytic

Performance for Chemoselective Reduction of Nitroarenes Haigen Huang,† Mingwu Tan,‡ Xueguang Wang,*, † Man Zhang,† Shuoqiang Guo,† Xiujing Zou,† and Xionggang Lu*, † †

State Key Laboratory of Advanced Special Steel, School of Materials Science and Engineering,

Shanghai University, Shanghai 200072, China ‡

Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University,

Xiamen 361005, China KEYWORDS: mesoporous alumina, cobalt, nitroarene, aromatic amine, reduction

ABSTRACT: Mesoporous γ-alumina (γ-MA)-supported cobalt oxides (Co3O4) with large surface areas and narrow pore size distributions were first prepared through one-pot hydrolysis of metal nitrates. The obtained Co3O4/γ-MA materials were impregnated with a water–ethanol solution of 1,10-phenanthroline, followed by the treatment of 700 oC in N2 atmosphere, generating Co−NC/γ-MA catalysts containing N-doped graphitic carbon (NC). The Co−NC/γ-MA catalysts

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maintained mesoporous structure of γ-MA, and Co3O4 was reduced to metallic Co nanoparticles highly-dispersed in the γ-MA frameworks. Metallic Co species had a strong interaction with NC in the matrices, avoiding the surface oxidation of Co particles. The Co−NC/γ-MA catalysts exhibited superior catalytic activity and quantitatively reduced a variety of functionalized nitroarenes to the corresponding arylamines with hydrazine hydrate in ethanol at near room temperature, affording the yields of > 99%. The recycling test of 2-chloronitrobenzene as a model reaction showed no detectable change in catalyst performance after the tenth cycle reactions.

1. INTRODUCTION The selective reduction of nitro compounds is one of the most fundamental chemical reactions for the production of amines, which serve as important intermediates and key precursors in the manufacture of numerous agrochemicals, pharmaceuticals, polymers, and fine chemicals.1−4 Traditional non-catalytic processes are carried out for the reduction of nitro groups using stoichiometric reducing agents such as Fe, Zn, Sn, and metal sulfides, causing serious problems in product separation, corrosion hazard of the reactor, large amount of waste acids or bases, and unwanted byproducts such as hydroxylamine.5−7 Therefore, considerable efforts have been focused on establishing efficient and selective catalytic reduction of nitro compounds to replace non-catalytic processes. Heterogeneous catalytic reaction is in general preferred because of the ease of separation and recycling compared with the homogeneous one. Supported precious metal-based nanocatalysts have extensively applied for the chemoselective reduction of functional nitroarenes to arylamines. However, most of them do not satisfy the dual requirements of activity and selectivity. Pt-group


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(Pt, Pd, Rh，Ru，etc.) catalysts afford high intrinsic activity for the hydrogenation of nitro aromatics, but poor chemoselectivity for the nitro group reduction when other reducible functional groups such as halogen, alkene, nitrile, carbonyl, ester, amide, and hydroxyl groups, are present in the same molecule,8−17. and other noble metal (Au, Ag, etc.) catalysts show high selectivity, but are always not active even under the harsh conditions.18−22 In addition, the high cost and limited availability of these precious metals also restrict their widespread applications for numerous industrial processes. Non-noble transition metal catalysts (Fe, Co, Ni, etc.) have been shown to be effective for the selective hydrogenation of nitro compounds as catalysts or supports.23−28 Especially, cobaltbased catalyst supported on alumina is an important material system in the field of heterogeneous catalysis for the hydrotreating, hydrogenation and combustion process.29,30 From the fundamental viewpoint, it is known that the catalytic properties of metal catalysts have close correlations with the natures of metal species and support, concentration of metal species, interaction between metal species and support, the characteristics and distribution of surface metal species, and textural properties; while these influence factors depend primarily upon preparation routes and selection of heat treatment conditions, etc. However, because they are easily oxidized in air, metallic Co nanoparticles were seldom investigated for the liquid-phase organic reactions in a sequence batch reactor.31−33 It is still a great challenge to develop inexpensive and recyclable non-precious metallic Co nanomaterials for batch liquid-phase reactions. Carbon-supported cobalt and iron oxide catalysts, which were modified by nitrogen-doped graphene layers from the pyrolysis of metal acetate complexes with nitrogen ligands, have recently been demonstrated to be effective for the chemoselective hydrogenation of nitroarenes.34−38 However, the catalytic reductions on these metal catalysts frequently require


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strict reaction conditions (≥ 100 oC) and the aid of additives (acid, base and metal salts) so as to achieve high catalytic activity and satisfactory conversion likely due to low ability of metal oxides for generating and stabilizing the hydride and protons.39 Therefore, our objective is to prepare highly stable supported metallic Co nanocatalyst that enables the selective nitroarene reduction in a liquid phase medium under mild reaction conditions. Nitroarenes have been catalytically reduced using various hydrogen sources including sodium borohydride,40 pinacol,41 formic acid,42 hydrazine,43,44 and molecular hydrogen.45 Among them, hydrazine hydrate (N2H4·H2O) is the most attractive hydrogen donor for the reduction of nitro groups, as it not only is abundant, inexpensive and relatively safe, but also produces harmless nitrogen gas and water as the only by-products, avoiding the production of large amounts of waste in the resulting mixture.46 Conventional supported metallic Co catalysts are frequently prepared by the impregnation method. However, the one-pot method is often advantageous over the impregnation route because it minimizes processing steps and provides a more uniform dispersion of metal species without closing the framework pores. Recently, we have first synthesized γ-alumina (γ-MA) with a high specific surface area and uniform mesoporous framework via one-pot hydrolysis and condensation of metal nitrates.47 Herein, we extended this procedure to prepare γ-MA-supported cobalt oxides (Co3O4), and successfully combined them with 1,10-phenanthroline. The obtained phenanthroline/Co3O4/γ-MA composites were treated in a N2 flow to produce γ-MA-supported metallic Co nanocatalysts containing N-doped graphitic carbon (NC). Their catalytic behaviors were intensively investigated for selective reduction of nitroarenes with hydrazine hydrate, and also were compared to the counterpart prepared by the traditional impregnation method and to noble metal Pd, Pt, Au) catalysts supported on γ-MA. The Co−NC/γ-MA catalysts exhibited


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superior catalytic activity, and quantitatively reduced various functional nitroarenes to the arylamines in ethanol without any additive at near room temperature, affording the yields of > 99%.

2. EXPERIMENTAL 2.1. Chemicals and Catalyst Preparation. All reagents of AR grade were purchased from Sinopharm Chemical Reagent Co., Ltd. Co−NC/γ-MA composites were synthesized by a simple two-step approach. Co3O4/γ-MA materials were first synthesized by one-step hydrolysis and condensation of metal nitrates, then was impregnated with a water-ethanol solution of 1,10-phenanthroline, and finally the obtained solid was calcined at 700 oC in a flow of N2 (Scheme 1). Pure γ-MA was prepared according to the procedure reported in our previous study.47 For the xCo3O4/γ-MA materials with different weight percentage contents of cobalt (x = 5, 10, 15, 20, and 25), the preparation procedure was the same as that of pure γ-MA except use of Co(NO3)2·6H2O. Typically, 0.1 mol of Al(NO3)3·9H2O and required amounts of Co(NO3)2·6H2O were added into 50 mL of deionized water under magnetic stirring for ~0.5 h at 70 oC. Then, 1 mol L−1 (NH4)2CO3 aqueous solution was pumped (1.0 mL min−1) into the above mixed aqueous solution at a stirring rate of 450 rpm using a syringe pump until a transparent pink gel was suddenly generated, where the pumping time was ~2.5 h. The as-prepared gel was statically aged at 30 oC for 48 h, and then was laid on a culture dish at 100 oC for 24 h. Then, the as-prepared NH4NO3/Co−Al hybrid was further treated at 200 oC and 450 oC for 10 h, respectively, to generate mesoporous xCo3O4/γ-MA. The color of the xCo3O4/γ-MA turned from dark green to black with the increase in the Co content.


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As for the preparation of the NC/γ-MA (x = 0) and xCo−NC/γ-MA (x = 5, 10, 15, 20, and 25) catalysts, in a typical procedure, 1.8 g of the γ-MA or xCo3O4/γ-MA was added into 40 mL of a mixed solution of water and ethanol (v/v = 1:1) containing 0.6 g of 1,10-phenanthroline monohydrate (C₁₂H₈N₂H₂O), and was sonicated for 30 min, and then the solvent was evaporated at 40 oC under stirring. The obtained phenanthroline/xCo3O4/γ-MA mixture was dried at 100 oC overnight in air in an oven, and finally treated at 700 oC in a tubular furnace in a 50 mL min−1 flow of high purity N2 for 2 h with a heating ramp of 5 oC min−1. For comparison, 20Co−NC/γ-MA-imp was also developed by a two-step impregnation approach. Namely, the γ-MA was first impregnated with an aqueous solution of Co(NO3)2·6H2O and calcined at 450 oC for 10 h in air in a muffle furnace; then, the obtained 20Co3O4/γ-MA-imp was further impregnated with a water–ethanol solution of 1,10-phenanthroline monohydrate, followed by the treatment of 700 oC in a tubular furnace in a 50 mL min−1 flow of high purity N2 for 2 h. 2.0%Pd/γ-MA, 2.0%Pt/γ-MA and 2.0%Au/γ-MA catalysts containing 2.0 wt% of the noble metal were obtained by the impregnation of γ-MA support with aqueous solutions of PdCl2, H2PtCl6, and HAuCl4, respectively.


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Scheme 1. Preparation procedure of the Co−NC/γ-MA catalysts. 2.2. Catalyst Characterization. N2 sorptions were carried out using a Micromeritics ASAP 2020 Sorptometer at liquid nitrogen temperature (−196 oC). Before the analysis, the samples were degassed at 300 °C for 10 h. The specific surface areas (SBET) were calculated using the Brunauer–Emmett–Teller (BET) method. Pore size distributions were obtained using the desorption branches and the Barrett–Joyner–Halenda (BJH) method. The BJH pore sizes (Dp) were read from the maximum of the pore size distribution curves, and the average pore sizes (Da) were estimated by BJH method. The pore volumes were taken at the P/P0 = 0.990 single point. Transmission electron microscopy (TEM) images were obtained with an accelerating voltage of 200 kV with a JEOL JEM-2010F field emission microscope operating at 200 kV and high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) was employed


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to obtain chemical mappings of the elements using the energy-dispersive X-ray (EDX) spectroscopy detector. X-ray diffraction (XRD) patterns were obtained on a Rigaku D/MAX2200 apparatus employing Cu Kα radiation (λ = 0.1542 nm) at a voltage of 40 kV and a current of 40 mA in the angular range of 10°to 80°with the scan rate of 2 o/min. The Co crystallite sizes were calculated using the full-widths at half maximum (FWHM) of the Co (200) peaks by the Scherrer equation. Temperature programmed reduction (TPR) was performed on a homemade fixed-bed reactor. Prior to the test, 100 mg sample was put in a quartz reactor and was treated in an Ar atmosphere (30 mL min−1) at 300 °C for 0.5 h, and then cooled to 50 oC. After this pretreatment, H2-TPR was performed with a gas mixture of 5 vol% H2 in Ar at 30 mL min−1. The temperature was raised to 1000 oC at a heating rate of 10 °C min−1. The amount of H2 uptake was measured with a thermal conductivity detector (TCD). Surface electronic states were analyzed by X-ray photoelectron spectroscopy (XPS) on an ESCALAB 250Xi spectrometer equipped with monochromatized Al Kα radiation (hν = 1486.6 eV) with a basis pressure of ca. 1 × 10–9 Torr. All binding energies were calibrated using the binding energy of C 1s peak at 284.6 eV as a reference. Raman spectra were carried out at ambient temperature with a 785-nm HPNIR excitation laser on a Renishaw Raman spectrometer with an Olympus microscope and a chargecoupled device (CCD) detector. The laser power was 15 mW and a total of 20 acquisitions were taken for each spectrum. The factual amounts of Co elements in the catalysts were determined on a Perkin Elmer emission spectrometer by inductively coupled plasma atomic emission spectrometry (ICP-AES). 100 mg of sample dried at 200 °C for 24 h was dissolved into 15 mL of aqua regia by heating under vigorous stirring for 30 min. After cooling, a proper amount of deionized water was added in, then filtered and washed with water. The filtrate was adjusted to 100 mL accurately. Finally, 10 mL of the above solution was further diluted 10 times with water


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for analysis. The analysis of C and N elements was carried out on a Perkin Elmer 2400 CHN elemental analyzer. 2.3. Catalytic Reaction. The selective reduction of nitro aromatics was conducted in a 25 mL Telfon-lined stainless steel autoclave with an interior diameter of about 40 mm with magnetic stirring in the temperature range of 20−80 oC. After the completion of the reaction, the catalyst was rapidly separated from the mixture by filtration. n-Decane (100 μL) was introduced into the solution as standard and the solution was dried with anhydrous Na2SO4. The products were analyzed by gas chromatography-mass spectrometry (GC-MS, Shimadzu GCMS-QP2010 Plus) and GC-FID (Varian CP-3800) with a capillary column (column VF-1 ms, 15 m, 0.25 mm, 0.25 μm) and a flame ionization detector (FID). Each catalytic reaction was repeated more than three times to confirm the reproducibility of the result. The conversion of nitro compound i, noted Conv. (i) (Eqs. (1)), is defined as the percentage of the molar amount of the nitro compound i consumed to the molar amount of nitro compound i added in the reactor before the reaction: Conv. (i) = (N(i, before) − N(i, after))/N(i, before)  100%

(1)

where N(i, before) and N(i, after)) are the molar number of nitro compound i before and after the reaction in the reactor, respectively. The selectivity of the amine product j generated by the reduction of nitro compound i with hydrazine, noted as Sel. (j) (Eqs. (2)), is defined as the percentage of the j molar amount to the nitro compound i consumed after the reaction in the reactor: Sel. (j) = Nj/(N(i, before) − N(i, after))  100%

(2)

where N(j) is the molar number of the amine compound j in the reactor after the reaction.


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3. RESULTS AND DISCUSSION 3.1. Textural and Physicochemical Properties of Co3O4/γ-MA. N2 sorption measurement was first used to investigate the textural properties of the materials. Figure 1 illustrates N2 sorption isotherms and BJH pore size distributions of the xCo3O4/γ-MA (x = 0, 5, 10, 15, 20, and 25) materials. All the xCo3O4/γ-MA samples exhibited characteristic type IV isotherms with clear hysteresis loops, which was an indication of mesoporous materials. BJH pore size analysis indicated that the xCo3O4/γ-MA materials possessed narrow pore size distributions in the range of 2−5 nm. Nevertheless, the pore size distribution peaks were generally weakened with increasing the Co content. In terms of the 20Co3O4/γ-MA-imp, there were similar N2 sorption isotherm and BJH pore size distribution to the γ-MA, indicating that mesoporous structure of γMA was still retained during the preparation process.

180 Volume adsorbed (cm g , STP)

a

b 0.3

x=0 x=5 x = 10 x = 15 x = 20 x = 25 20Co3O4/-MA-imp

1

dV/dD (cm g nm )

1

150

1

120

0.2

3

3

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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90

x=0 x=5 x = 10 x = 15 x = 20 x = 25 20Co3O4/-MA-imp

60 30 0.0

0.2

0.4 0.6 0.8 Relative pressure (P/P0)

1.0

0.1

0.0 2

3

4

5 6 7 Pore size (nm)

8

9

10

Figure 1. (a) N2 adsorption–desorption isotherms and (b) BJH pore size distributions of the xCo3O4/γ-MA and 20Co3O4/γ-MA-imp samples.


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Figure 2 presents the TEM images of the γ-MA (x = 0) and typical xCo3O4/γ-MA materials with different Co contents (x = 10, and 20). It could be found that the xCo3O4/γ-MA samples showed a homogenous wormhole-like mesoporous structure with a pore size of ca. 2–5 nm, similar to the γ-MA. This result demonstrated that after the addition of Co, mesoporous γ-Al2O3 frameworks were maintained and pore walls were fabricated through cross-linking of small γAl2O3 and/or Al−Co composite oxide nanoparticles together.47 For the 20Co3O4/γ-MA-imp (Figure 2d), there were aggregated particles observed in the TEM images as excepted. Combined with the results of N2 sorptions, one concluded that the homogeneous wormhole-like mesopores were formed with open mouths of similar sizes to pore channels, by which the internal active sites were easily accessible to reactants in the xCo3O4/γ-MA materials.


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Figure 2. TEM images of the representative xCo3O4/γ-MA materials. (a) γ-MA, (b) 10Co3O4/γMA, (c) 20Co3O4/γ-MA, and (d) 20Co3O4/γ-MA-imp.

Table 1. Textural Properties of the Prepared Materials sample

SBET (m²g−1)

Micro. S.A. VP (cm³g−1) DP (nm) (m2 g−1)

Da (nm)

γ-MA

276

0

0.27

3.2

3.1

5Co3O4/γ-MA

241

0

0.23

3.4

3.1

10Co3O4/γ-MA

225

0

0.22

3.4

3.3

15Co3O4/γ-MA

204

0

0.21

3.7

3.3

20Co3O4/γ-MA

203

0

0.20

3.7

3.3

25Co3O4/γ-MA

168

0

0.19

3.7

3.7

20Co3O4/γ-MA-imp

165

6

0.18

3.7

3.7

The basic textural properties of the xCo3O4/γ-MA materials calcined in air at 450 oC are summarized in Table 1. Compared with the γ-MA, the xCo3O4/γ-MA (x = 5, 10, 15, 20, and 25) materials showed a clear decline in BET surface area, pore volume, but the pore sizes of the


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samples exhibited a slight increase. With raising the Co content, the surface areas and pore volumes of the xCo3O4/γ-MA gradually decreased, nevertheless, their pore sizes had no noticeable change. For instance, the 5Co3O4/γ-MA sample showed a specific surface area (SBET) of 241 m2 g−1, a pore volume (Vp) of 0.23 cm3 g−1 and a pore size (Dp) of 3.4 nm, whereas those of the 25Co3O4/γ-MA were 168 m2 g−1, 0.19 cm3 g−1 and 3.7 nm, respectively. This result implicated that the number of mesopores decreased with increasing Co content in the xCo3O4/γMA. Table 1 also demonstrated that there were hardly microporous surface areas (micro. S.A.) in the xCo3O4/γ-MA materials. These results could be due to several factors, such as the change in mass density of the catalysts, the coverage of γ-Al2O3 surface by Co oxides, and the interaction between Al2O3 and Co oxides.48,49 Compared with the 20Co3O4/γ-MA, the 20Co3O4/γ-MA-imp exhibited smaller specific surface area and pore volume, but similar pore size probably due to plugging of parts of pore mouths in the γ-MA support and aggregates of Co oxides particles. Figure 3 presents the XRD patterns of the xCo3O4/γ-MA (x = 0, 5, 10, 15, 20, and 25) samples calcined in air at 450 oC. All the samples presented three strong diffraction peaks, which were assigned to the (311), (400) and (440) reflections for spinel phase, respectively. This result indicated that aluminum oxide species for the xCo3O4/γ-MA were converted into γ-Al2O3 or reacted with Co species to form cubic Co−Al spinel phases. It is known that the diffraction peak positions of γ-Al2O3 (PDF 10–0425), Co3O4 (PDF 43–1003), and Co−Al spinels such as CoAl2O4 (PDF 44–160) are so similar that their diffraction peaks are difficult to be separated from each other due to the superimposition. However, in the case of the Co3O4 and stoichiometric CoAl2O4 spinels, the (311) diffraction peak is the strongest and the relative intensity of the (311) and (440) reflections is in the range of 2.6−3.1, whereas for γ-Al2O3, the (440) peak is the strongest and the intensity ratio of the (311) to (440) reflections is 0.80. In


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addition, because the ionic radii of Co3+ and Co2+ are 0.063 nm and 0.074 nm, respectively, which are larger than 0.057 nm of Al3+, the Co3O4 and CoAl2O4 spinels have larger lattice parameters than γ-Al2O3. When either Co2+ ions were diffused into the lattice of γ-Al2O3 to generate Co2+−Al3+ spinel (CoδAl(8/3−2/3δ)O4, 0 < δ ≤ 1) or Al3+ ions were exchanged with Co2+ ions into the tetrahedral sites of Co3O4 spinel to produce a mixed Co3+−Al3+ oxide with a tentative overall formula of

or Co3A1O6 stoichiometry as a result of solid-state

reactions, where Co3+ ions occupied all the octahedral spinel sites as in Co3O4 (Note: calculations based on ligand field theory indicates a relative octahedral site preference energy of Co3+ with respect to A13+),50 the diffraction peaks of γ-Al2O3 should move toward smaller 2θ values for the xCo3O4/γ-MA materials. Therefore, the formation of CoδAl(8/3-2/3δ)O4 and Co3A1O6 crystallites could be distinguished with the change in the relative intensities and in the shifts of the diffraction peaks for γ-Al2O3. It could be observed in Figure 3 that compared with those for the γ-MA, the diffraction peaks for the xCo3O4/γ-MA materials gradually shifted toward lower 2θ angles, moreover, both the peak intensities and the intensity ratios of the (311) and (440) peaks exhibited clear increases with raising the Co content in the gel precursors. For example, the 2θ value of the (440) reflection and the intensity ratio of the (311) to (440) peaks for the 5Co3O4/γ-MA was 66.6o and 1.02, while those for the 25Co3O4/γ-MA became 65.6o and 2.31, respectively. These results suggested that CoδAl(8/3−2/3δ)O4 (0 < δ < 1) and/or Co3A1O6 spinels should have been formed, probably due to homogeneous mixing and strong interaction between alumina and Co ions by the one-pot gelation procedure. Since the Co3O4, Co3A1O6, and CoAl2O4 spinels possessed almost identical diffraction peaks including both the peak positions and the relative peak intensities, they could not be differentiated from each other in the XRD patterns.50,51 In Figure 3, there was no diffraction peak associated with CoO in the XRD patterns.


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Moreover, CoO is not a stable phase in air at temperatures of 350−700 oC.50 Thus, it could be judged that the Co species existed in the form of either Co3O4 or Co−Al composites in the xCo3O4/γ-MA matrices. Compared with the 20Co3O4/γ-MA, the 20Co3O4/γ-MA-imp in Figure 3g exhibited stronger diffraction peaks and smaller widths of the diffraction peaks for the Co3O4 and/or Co−Al spinels, indicating the presence of larger Co3O4 and/or Co−Al crystallites as expected, which was consistent with the TEM result in Figure 2. -Al2O3



g

Intensity (a.u)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




* Co3O4, Co3A1O6 or CoAl2O4      

f e d c b a

20

30

40

2 /

o

50

60

70

Figure 3. XRD patterns of the xCo3O4/γ-MA materials with (a) x = 0, (b) x = 5, (c) x = 10, (d) x = 15, (e) x = 20, (f) x = 25, and (g) 20Co3O4/γ-MA-imp. In order to make clear the phase distribution of various Co species, the interaction of Co species with support and the oxidation state of Co species on the surface, the xCo3O4/γ-MA (x = 0, 5, 10, 15, 20, and 25) materials were further characterized by TPR and XPS techniques. Figure


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4 presents the TPR profiles of the xCo3O4/γ-MA samples. The Co contents strongly affected the reducibility of the Co species and the interaction between the Co species and alumina in the matrices. All the xCo3O4/γ-MA (x = 5, 10, 15, 20, and 25) samples showed two major broadened H2 consumption profiles in the ranges of 500–770 oC and 770–1000 oC, respectively. The former could be assigned to the two-step reduction of highly-dispersed Co3O4 (or surface Co3+ ions) to CoO and then to Co0 on the surface with a strong interaction with γ-alumina support or the reduction of Co3+−Al3+ oxide crystallites to metallic Co; and the latter was associated with the reduction of Co2+ ions from CoδAl(8/3-2/3δ)O4 spinel.50−53 Both of them were strengthened with the increase in the Co content. This results demonstrated that the increase in the Co content enhanced the amounts of both surface Co3+ ions and/or Co3+−Al3+ oxide crystallites on the support surface and CoδAl(8/3-2/3δ)O4 spinel in the xCo3O4/γ-MA materials, which were consistent with the XRD patterns in Figure 3. Regardless of Co contents, there were hardly H2 consumptions observed in the low-temperature region of less than 500 oC due to free or bulk-like Co3O4 crystallites.52 These results demonstrated that the diffraction peaks for spinel phases of the xCo3O4/γ-MA materials in the XRD patterns of Figure 3 were dominantly due to the Co−Al spinels instead of bulk-like Co3O4 crystallites. It was observed that when the Co content was increased to 20 wt% in the gels, the H2 consumption in the temperature region of 500–770 oC could be clearly deconvoluted into two curves peaked at ~570 oC and ~675 oC, respectively. In combination of the analyses of TPR profiles on alumina-supported Co oxides in the previous literature.50−54 Such a large difference of these two reduction temperatures (> 100 oC) could not be caused by the two-step reduction of Co3+ ions (Co3+ → Co2+ → Co0), but seemed to arise from the individual reduction of surface Co3+ ions and Co3+−Al3+ oxide crystallites, respectively.50 Generally, the starting reduction temperatures for all types of the Co species shifted to lower


16

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reduction temperatures, and the ratios of low-temperature H2 consumptions to high-temperature ones clearly increased with raising the Co content in the gels. These results suggested that the interaction between the Co species and the support, and the relative amounts of CoδAl(8/3-2/3δ)O4 spinel abated with increasing the Co content as expected. In the case of the 20Co3O4/γ-MA-imp, there were three H2 consumption regions observed in the TPR profile (Figure 4g). It was clear that compared with those of the 20Co3O4/γ-MA sample, the total peak areas of H2 consumptions associated with Co3O4 crystallites, surface Co3+ ions and Co3+−Al3+ oxide crystallites exhibited an obvious increase, while the one due to the CoδAl(8/3-2/3δ)O4 spinel had a significant decline. It should be noted that TPR peak locations of supported Co oxide species showed a high complexity due to the interaction between Co species and support, which have a close relation with the nature of support, preparation method, particle size, Co content, calcination conditions, and reduction gas composition, therefore, several interpretations for the Co oxide species supported on alumina in published literature have still been ambiguous and even sometimes contradictory up to now.50−54


17


570 378 H2 consumption (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 54

885

675

g

770 500

f e d c b a

983

200 300 400 500 600 700 800 900 1000 o Temperature ( C) Figure 4. TPR profiles of the xCo3O4/γ-MA samples with (a) x = 0, (b) x = 5, (c) x = 10, (d) x = 15, (e) x = 20, (f) x = 25, and (g) 20Co3O4/γ-MA-imp. Figure 5 presents the XPS spectra of the Co 2p region of the xCo3O4/γ-MA (x = 5, 10, 15, 20, and 25) and 20Co3O4/γ-MA-imp materials. All the xCo3O4/γ-MA samples exhibited a broadened Co 2p3/2 primary spectrum, which could be deconvoluted into two symmetrical components peaked within a binding energy (BE) range of 780.2–780.8 eV and a BE range of 781.8–782.4 eV, respectively. It was known that the BE value for Co 2p3/2 was positioned at 279.6–780.5 eV for pure Co3O4, and the BE value for Co 2p3/2 was located at 781.7–782.0 eV for stoichiometric CoAl2O4 with a reference of the BE value of C 1s at 284.8 eV.55−58 Therefore, the former was assigned to highly-dispersed Co3O4 with a strong interaction with the support and/or Co3+−Al3+ oxide crystallites on the support surface; and the latter was definitely attributed to the CoδAl(8/32/3δ)O4

(0 < δ < 1) spinel in the xCo3O4/γ-MA materials. It was found that the Co 2p3/2 BEs for


18

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both of the Co species gradually moved to smaller values with increasing the Co content in the gels. For instance, the Co 2p3/2 BE values for these two types of Co species on the surface of the 5Co3O4/γ-MA were 780.8 eV and 782.4 eV, respectively, while those for the 25Co3O4/γ-MA became 780.2 eV and 781.8 eV, respectively. This result revealed that the interactions between the Co species and alumina were weakened with increasing the Co content. The spectrum intensities of both highly-dispersed Co3O4 and/or Co3+−Al3+ oxide and CoδAl(8/3-2/3δ)O4 spinel were strengthened with the Co content in the gels, indicating the increase in their amounts on the surface. However, it was clearly seen that the peak area ratio of the former to the latter also increased with raising the Co content. This result implied that the relative amounts of highlydispersed Co3O4 and/or Co3+−Al3+ oxide were augmented in the xCo3O4/γ-MA materials. This point was also supported by the attenuation of the relative intensities of the shakeup satellite peaks at approximately 5 eV above their main peaks.56−58 Since the xCo3O4/γ-MA materials were proved to have similar porous structures and pore size distributions as illustrated in Figure 1 and Table 1, the relative intensities of XPS spectra for Co 2p could generally reflect the changes in the composition of different Co species on the support surface. Thus, it could be concluded that the amounts of Co species on the support surface had similar variation trend to those in the xCo3O4/γ-MA materials revealed in the XRD patterns in Figure 3 and the TPR profiles in Figure 4. As for the 20Co3O4/γ-MA-imp, the spectrum peaks for Co 2p3/2 further shifted to lower BE values due to a weaker interaction with the support, and the total amount of Co species on the surface was obviously increased relative to that of the 20Co3O4/γ-MA.


19


780.8 782.4

(a) 20Co3O4/-MA-imp

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 54

x = 25 x = 20 x = 15 x = 10 x=5 775

780

785 790 795 Binding energy (eV)

800

805

810

Figure 5. Co 2p XPS spectra of the xCo3O4/γ-Al2O3 and 20Co3O4/γ-MA-imp materials. 3.2. Characterization of xCo−NC/γ-MA. The above-prepared xCo3O4/γ-MA (x = 0, 5, 10, 15, 20, and 25) materials were impregnated with a water-ethanol solution of 1,10-phenanthroline to form the phenanthroline/xCo3O4/γ-MA composites, followed by the treatment of 700 oC under N2 atmosphere, where the xCo−NC/γ-MA catalysts generated were proven to exhibit the optimum catalytic performance for the selective reduction of nitroarenes to the arylamines with hydrazine hydrate in the preliminary control experiments. The final contents of Co, C and N in the xCo−NC/γ-MA catalysts were determined by ICP-AES and CHN elemental analyzer and the results are summarized in Table 2. The actual Co contents increased from 3.8 to 19.1 wt% with raising the initial ones from 5 to 25 wt% used for the xCo3O4/γ-MA materials. However, the actual amounts of C and N elements deposited gradually declined with increasing the Co content. This could be explained by two aspects. On one hand, the decrease in the number


20

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of mesopores in the xCo3O4/γ-MA with the Co content resulted in more 1,10-phenanthroline molecules adsorbed outside the pores, which were directly desorbed without condensation; and on the other hand, the presence of Co could promote the decomposition and gasification of phenanthroline in the pores. Note that in all the xCo−NC/γ-MA catalysts, the C/N molar ratios were in general increased with the Co content, and also higher than 6 of the stoichiometric value of 1,10-phenanthroline molecule as the C and N source. This result suggested that the N atoms should be gasified more easily than the C atoms in the pyrolysis of phenanthroline, and the addition of Co could further improve the relative gasification rate of N to C under the present treating conditions. In the 20Co−NC/γ-MA-imp, the amounts of C and N deposited were apparently much lower than those in the 20Co−NC/γ-MA. This phenomenon might be caused mainly by larger amounts of phenanthroline adsorbed outside the pores due to the smaller specific surface area and pore volume as shown in Table 1. Figure 6 displays the typical STEM image and individual Co, C, and N elemental mapping images for the representative xCo−NC/γ-MA materials. The EDX spectra indicated that the 20Co−NC/γ-MA catalyst consisted mainly of Co, C, N, O and Al elements. Table 2. Actual Compositions of Co, C and N Elements of the Prepared Catalysts sample

Co (wt%)

C (wt%)

N (wt%)

C/N (mol/mol)

NC/γ-MA

‒

4.69

0.73

7.50

5Co−NC/γ-MA

3.8

4.76

0.69

8.05

10Co−NC/γ-MA

7.8

4.62

0.58

9.29

15Co−NC/γ-MA

11.2

4.50

0.54

9.72

20Co−NC/γ-MA

15.4

4.31

0.46

10.9

25Co−NC/γ-MA

19.1

3.56

0.41

10.13

20Co−NC/γ-MA-imp

17.5

1.81

0.17

12.42


21


Al

Counts (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 54

O C Co

Co

N 1

2

3 4 5 Binding energy (KeV)

6

7

Figure 6. Representative STEM image and elemental mapping images of Co, C and N for the 20Co−NC/γ-MA.


22

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The XRD patterns of the xCo3O4/γ-MA materials treated with 1,10-phenanthroline under N2 atmosphere at 700 oC are illustrated in Figure 7. It could be seen that the obtained xCo−NC/γMA (x = 5, 10, 15, 20, and 25) catalysts generated three new diffraction peaks at 44.2°, 51.5°and 75.8o corresponding to cubic metallic Co (111), (200) and (220) reflections (PDF 15–0806), respectively, and meanwhile, the peak intensities of the Co−Al spinels were obviously weakened compared with those of the xCo3O4/γ-MA materials illustrated in Figure 3. It was known that the CoδAl(8/3-2/3δ)O4 spinel could not be reduced below 800 oC even under H2 atmosphere. This result demonstrated that the Co3+−Al3+ spinel crystallites together with well-dispersed Co3O4 species on the surface were responsible for the formation of metallic Co during the treatment procedure. With raising the Co content, the peak intensity of metallic Co increased and the peak width decreased, which were indicative of the increase in the Co crystallite sizes. When the Co content was increased to 25 wt%, the average size of the Co crystallites was 15.6 nm. On the other hand, it was found that the diffraction peaks associated with the Co−Al spinel phases also shifted to higher 2θ values after the treatment due to the reduction of the Co3+−Al3+ oxide species by carefully comparing the XRD patterns of the xCo3O4/γ-MA materials and the xCo−NC/γ-MA catalysts in Figure 3 and Figure 7, respectively. For the 20Co−NC/γ-MA-imp, the relative intensity of the diffraction peaks corresponding to the Co3O4 and Co−Al spinel phase also showed a drastic decrease, but the diffraction peaks of metallic Co formed were more strengthened than those in the 20Co−NC/γ-MA. Correspondingly, the mean Co crystallize size became 19.4 nm. For all the catalysts, the diffraction peaks associated with carbon species were not observed likely due to the lower carbon content and the presence of carbon in the form of very thin surface layers or amorphous structure, which was XRD invisible. Therefore, the carbon structure in the xCo−NC/γ-MA catalysts was analyzed by Raman spectra (not shown). All the


23


xCo−NC/γ-MA (x = 0, 5, 10, 15, 20, and 25) and 20Co−NC/γ-MA–imp catalysts exhibited a defect (D) band at ~1350 cm−1, and a graphite (G) band at ~1580 cm−1, which corresponded to the disordered graphitic carbon and the graphitization degree, respectively.34,35 -Al2O3



*

g Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 54



*

*





* CoAl2O4  Co *  

*

 

f e d c b a 20

30

40

50 o 2/

60

70

80

Figure 7. XRD patterns of the xCo−NC/γ-MA catalysts with (a) x = 0, (b) x = 5, (c) x = 10, (d) x = 15, (e) x = 20, (f) x = 25, and (g) 20Co−NC/γ-MA-imp. Figure 8 presents the representative TEM images of the xCo−NC/γ-MA and 20Co−NC/γ-MAimp catalysts. All the catalysts possessed homogenous wormhole-like mesoporous structures, closely resembling those before the treatment. Due to the perturbation of deposited graphitic carbon on the catalyst surface and poor contrast between alumina and metal particles, it is difficult to define the boundaries of the Co particles. However, it could still be observed that the darker Co particles were highly dispersed in the mesoporous γ-MA frameworks for the xCo−NC/γ-MA catalysts, and the sizes of Co particles were located mainly in the range of 10−20


24

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nm when the Co content was raised to 25 wt%, in agreement with the XRD results in Figure 7. In the TEM image of 20Co−NC/γ-MA-imp, there was a wide particle size distribution of Co particles including metal Co crystallites and aggregates of crystallites.

Figure 8. TEM images of the representative xCo−NC/γ-MA catalysts. (a) NC/γ-MA, (b) 5Co−NC/γ-MA, (c) 10Co−NC/γ-MA, (d) 15Co−NC/γ-MA, (e) 20Co−NC/γ-MA, (f) 25Co−NC/γ-MA, and (g) 20Co−NC/γ-MA-imp.


25


Page 26 of 54

The textural properties of the xCo3O4/γ-MA (x = 0, 5, 10, 15, 20, and 25) materials after the treatment of 1,10-phenanthroline at 700 oC were determined by the N2 sorption measurement. Figure 9 illustrates the N2 adsorption and desorption isotherms and BJH pore size distributions of the obtained xCo−NC/γ-MA catalysts. All the xCo−NC/γ-MA catalysts exhibited the similar shape N2 adsorption and desorption isotherms and narrow pore size distributions to the counterparts before the treatment displayed in Figure 1. This result demonstrated that the mesoporous structures of the xCo3O4/γ-MA materials were still retained in the xCo−NC/γ-MA catalysts after the phenanthroline treatment, as illustrated in the TEM images of Figure 8. Table 3 lists the textural parameters of the xCo−NC/γ-MA catalysts. The xCo−NC/γ-MA catalysts generally exhibited similar specific surface areas and pore sizes, but smaller pore volumes, compared with the counterparts before the treatment. This result demonstrated that phenanthroline was pyrolyzed into N-doped graphitic carbon inside the pores of the xCo−NC/γMA catalysts. Certain amounts of microporous surface areas in the xCo−NC/γ-MA catalysts in Table 3 might result from the deposited carbon which was porous. On the other hand, the higher treatment temperature, the reduction of the Co oxide species on the surfaces, structural shrinkage as well as blockage of pores of the alumina in the treatment process might also affect the textural properties of the catalysts. The 20Co−NC/γ-MA-imp in Figure 9 also showed very similar N2 sorption isotherm and pore size distribution curve to the 20Co3O4/γ-MA-imp before the treatment with phenanthroline.


26

Page 27 of 54

140

a

b

120 1

dV/dD (cm g nm )

0.15

1

100 80

3

3

x=0 x=5 x = 10 x = 15 x = 20 x = 25 20Co-NC/-MA-imp

0.20

-1

Volume adsorbed (cm g , STP)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


x=0 x=5 x = 10 x = 15 x = 20 x = 25 20Co-NC/-MA-imp

60 40 20

0.10

0.05

0.00

0.0

0.2

0.4 0.6 0.8 Relative pressure (P/P0)

1.0

2

3

4

5 6 7 Pore size (nm)

8

9

10

Figure 9. (a) Nitrogen adsorption-desorption isotherms and (b) BJH pore size distributions of the xCo−NC/γ-MA and 20Co−NC/γ-MA-imp catalysts. Table 3. Textural Properties of the Catalysts after Phenanthroline Treatment sample

SBET (m²g−1) Micropore S.A. (m2 g−1)

VP (cm³g−1) DP (nm)

Da (nm)

NC/γ-MA

262

19

0.21

3.5

3.3

5Co−NC/γ-MA

245

18

0.21

3.5

3.8

10Co−NC/γ-MA

218

32

0.17

3.5

3.7

15Co−NC/γ-MA

186

7

0.16

3.6

3.6

20Co−NC/γ-MA

169

7

0.15

3.6

3.5

25Co−NC/γ-MA

159

5

0.15

3.5

3.7

20Co−NC/γ-MA-imp

155

0

0.16

3.3

3.9

XPS spectra were employed to investigate the elemental composition, chemical state and the interaction of elements on the catalyst surface, and the results are illustrated in Figure 10. The XPS survey spectra in Figure 10a revealed that the xCo−NC/γ-MA (x = 5, 10, 15, 20, and 25) and 20Co−NC/γ-MA-imp catalysts consisted of Al, C, N, O and Co elements on the surface and


27


Page 28 of 54

subsurface layers. Table 4 lists the elemental composition of the surface of the xCo−NC/γ-MA. It could be seen that the Co contents on the catalyst surfaces gradually increased with raising the initial ones used for the gels, however, their values were always smaller than the counterparts determined by ICP. On the contrary, the contents of C and N on the surfaces were much higher than the corresponding ones of the xCo−NC/γ-MA obtained by the CHN elemental analysis, as listed in Table 2. These results implicated that the deposited C and N species were enriched mainly on the catalyst surfaces and might be covered on the Co species of the surfaces to some extent. In terms of the 20Co−NC/γ-MA-imp, the fact that the Co content on the surface was higher was likely due to the formation of larger crystallites of Co species on the pore mouths or outside the pores, which could not be covered by deposited carbon species. It was noteworthy that the C/N molar ratios were irregular on the surfaces. This might be caused by the inaccuracy of carbon measurement on the surface as a result of hydrocarbon adsorption during the sample evacuation inside the electron spectrometer.56 Figure 10b displays the XPS spectra of the Co 2p regions of the xCo−NC/γ-MA (x = 5, 10, 15, 20, and 25) and 20Co−NC/γ-MA-imp catalysts. It could be seen that all the samples exhibited a broad non-symmetric Co 2p3/2 primary peak, which could be decomposed into two components peaked at 781.3  0.3 eV and 782.3  0.2 eV, respectively. The XRD patterns in Figure 7 clearly demonstrated that the catalysts contained metallic Co crystallites and CoδAl(8/3-2/3δ)O4 spinel. Nevertheless, it was previously reported that the Co 2p3/2 BE was located at 777.7–778.1 eV for metallic Co (C 1s = 284.8 eV as reference).56−58 Thus, the former indicated unambiguously that the novel surface Co−Nχ species were formed with a strong interaction between the surface Co atoms of the Co nanoparticles and N-doped carbon, which accordingly resulted in a lower electron density at the Co atom;34,35,59 and the latter was ascribed to the unreduced CoδAl(8/3-


28

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2/3δ)O4

spinel in the xCo−NC/γ-MA catalysts, which showed almost the same BEs as the

counterparts in the xCo3O4/γ-MA catalysts. In addition, the nature of the support and the size and shape of metal nanoparticles might also affect the binding energy of the metal element to a certain extent.60,61 In the XPS spectra of the Co 2p regions, there were no peaks within a BE range of 780.1−781.0 eV associated with surface Co(II) species due to the surface oxidization of metallic Co in the xCo−NC/γ-MA catalysts,56−59 even after exposure to ambient air atmosphere for more than six months. This result demonstrated that metal Co atoms on the Co nanoparticles could not combine with oxygen in the ambient atmosphere during the storage process due to the presence of the NC, unlike single alumina support metal Co catalysts. The N 1s XPS spectra of the prepared catalysts in the Figure 10c could be fitted into three peaks positioned at the BEs of 398.5 eV, 399.5 eV and 401.0 eV, which were attributed to pyridine-type N, Co−Nx and/or pyrrolic-type N, and graphite-type N, respectively.34,59 As regards the C 1s XPS spectra displayed in Figure 10d, two kinds of carbon were observed for all the catalysts. The most intense line at 284.6 eV was attributed to C=C or C−C bonds from the pyrolysis of phenanthroline and hydrocarbon residues adsorbed on the surface; and the second weak feature at 288.6 eV was assigned to C−N=C bonds.34,59


29


O 1s

20CoNC/-MA-imp Co 2p

Al 2p C 1s N 1s

x = 25 x = 20 x = 15 x = 10

0

200

c

b

399.5 401.0

x = 25 x = 20 x = 15

x=5

x = 10

x=0

x=5 775 780 785 790 795 800 805 810 Binding energy (eV)

N 1s

d

20CoNC/-MA-imp

C 1s 284.6

Intensity (a.u.)

x = 20 x = 15 x = 10

398

20CoNC/-MA-imp 288.6

x = 25

396

Co 2p 20CoNC/-MA-imp

400 600 800 1000 1200 Binding energey (eV)

398.6

781.3 782.3

Intensity (a.u.)

Intensity (a.u.)

a

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 54

x = 25 x = 20 x = 15 x = 10

x=5

x=5

x=0

x=0

400 402 404 Binding energy (eV)

406

282

284

286 288 290 Binding energy(eV)

292

294

Figure 10. XPS spectra of the xCo−NC/γ-MA and 20Co−NC/γ-MA-imp catalysts. (a) XPS survey spectra, (b) Co 2p, (c) N 1s, and (d) C 1s.


30

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Table 4. Elemental Analysis on the Surfaces of the xCo-NC/γ-MA and 20Co-NC/γ-MA-imp Catalysts sample

Co (wt%)

C (wt%)

N (wt%)

C/N (mol/mol)

NC/γ-MA

‒

46.1

0.8

57.6

5Co−NC/γ-MA

3.5

19.1

1.9

11.7

10Co−NC/γ-MA

6.0

17.1

1.8

9.5

15Co−NC/γ-MA

7.8

18.0

1.8

10.0

20Co−NC/γ-MA

11.7

17.6

1.8

9.8

25Co−NC/γ-MA

14.1

17.9

1.6

11.2

20Co−NC/γ-MA-imp

24.8

11.3

1.3

8.7

3.3. Catalytic Reaction. In the preliminary study, 2-chloronitrobenzene was applied as a model compound to investigate the effect of Co content on the catalytic performance of the xCo−NC/γ-MA catalysts for the selective reduction of nitro aromatics. Table 5 summaries the catalytic activities and chemoselectivities of the xCo−NC/γ-MA and other some catalysts for comparison for the reduction of 2-chloronitrobenzene to 2-chlorobenzenamine with hydrazine hydrate in ethanol. The γ-MA, 20Co3O4/γ-MA and NC/γ-MA catalysts showed no activity (entries 2−4). Furthermore, the 20Co3O4/γ-MA−R reduced with H2 at 600 oC and protected with high-purity N2 exhibited a certain activity (~15%). However, when it was placed in ambient air for more than 24 h, the 20Co3O4/γ-MA−R (entry 5) was completely deactivated for the reduction of 2-chloronitrobenzene probably due to the surface oxidization of metallic Co particles. For the xCo−NC/γ-MA catalysts, the sufficiently high catalytic activities were achieved for the reduction of 2-chloronitrobenzene (entries 6−10). The conversions of 2-chloronitrobenzene increased with raising the Co content, and showed the maximum value of 72% at the Co content of 20 wt% in


31


Page 32 of 54

the initial gels. As the Co content was further increased, the catalytic activity started to decline, and the 2-chloronitrobenzene conversion decreased to 56% at the Co content of 25 wt%. These results were likely due to the increase in the sizes of Co crystallites leading to the decrease in the number of the surface active sites. It was noteworthy that the activities of the xCo−NC/γ-MA catalysts hardly had an observable change for this reaction after exposed to ambient atmosphere for more than six months. These results suggested that metal Co should be the active component of the xCo−NC/γ-MA for the reduction of 2-chloronitrobenzene; the presence of NC could inhibit the surface oxidation of Co particles; and moreover, the synergistic effect of metal Co and NC or the formation of surface Co−Nχ species significantly improved the catalytic performance of the xCo−NC/γ-MA for the reduction of 2-chloronitrobenzene. Compared with the 20Co−NC/γ-MA, the 20Co−NC/γ-MA-imp (entry 11) showed much lower conversion of 2chloronitrobenzene (42%) due to larger Co particles. In addition, the lower NC content in the 20Co−NC/γ-MA-imp also might have a certain effect on the catalytic activity. Some γ-MAsupported noble metal catalysts such as Pd, Pt and Au were listed in entries 12−14 of Table 5, which were widely investigated for the chemoselective reduction of nitro compounds. It was clear that their catalytic activities were significantly lower than those of the xCo−NC/γ-MA catalysts for the reduction of 2-chloronitrobenzene. In Table 5, it was found that all the tested catalysts showed a selectivity of > 99% for 2-chlorobenzenamine for the selective reduction of 2chloronitrobenzene without any dehalogenation. This might be explained by use of lower reaction temperature in the reaction process.


32

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Table 5. Catalytic Performance of xCo−NC/γ-MA Catalysts for Selective Reduction of 2Chloronitrobenzene to 2-Chlorobenzenaminea

entry

catalyst

conv. (%)

sel. (%)

1

No catalyst

0

−

2

γ-MA

0

−

3

20Co3O4/γ-MA

0

−

4

NC/γ-MA

0

−

5

20Co3O4/γ-MA−R

0

−

6

5Co−NC/γ-MA

27

> 99

7

10Co−NC/γ-MA

31

> 99

8

15Co−NC/γ-MA

54

> 99

9

20Co−NC/γ-MA

72

> 99

10

25Co−NC/γ-MA

56

> 99

11

20Co−NC/γ-MA-imp

42

> 99

12

2.0%Pd/γ-MA

20

> 99

13

2.0%Pt/γ-MA

22

> 99

14

2.0%Au/γ-MA

16

> 99

a

Reaction conditions: 20 mg catalyst, 1 mmol 2-chloronitrobenzene, 6 mmol N2H4·H2O, 1 mL ethanol, 40 oC, 45 min.

Figure 11 illustrates the catalytic properties as a function of reaction time over the 20Co−NC/γ-MA catalyst for the selective reduction of 2-chloronitrobenzene to 2chlorobenzenamine. The conversion of 2-chloronitrobenzene increased with the reaction time.


33


When the reaction was conducted for 75 min, 2-chloronitrobenzene was completely converted to 2-chlorobenzenamine. It was known that the accumulation of hydroxylamines often took place in the reduction of nitroarenes because these intermediates were often less reactive as compared to the starting nitroarenes.42 However, during the entire reactions, there were no by-products including hydroxylamine, nitroso, azoxy and azo compounds found in the products except 2chlorobenzenamine.

100

80 Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 54

60

40

20

0

10

20

30 40 50 Reaction time (min)

60

70

80

Figure 11. Catalytic properties as a function of reaction time over the 20Co−NC/γ-MA catalyst. Reaction conditions: 20 mg catalyst, 1 mmol 2-chloronitrobenzene, 6 mmol N2H4·H2O, 1 mL ethanol, 40 oC. The reduction of 2-chloronitrobenzene was further used as a model reaction to optimize the reaction conditions. Some widely-accepted green reductive agents were first used for this reaction in various organic solvents over the xCo−NC/γ-MA catalysts. The reaction results demonstrated that only hydrazine hydrate exhibited sufficiently high reducibility, while the


34

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others like formic acid and molecular hydrogen, etc., could scarcely react with 2chloronitrobenzene in the tested range of 40−80 oC. Table 6 summarized the reaction results for the reduction of 2-chloronitrobenzene over the 20Co−NC/γ-MA catalyst under different reaction conditions (temperature, hydrazine, solvent and substrate concentration). It could be seen that the catalyst exhibited excellent catalytic activities and chemoselectivities to 2-chlorobenzenamine under each given conditions. The conversions of 2-chloronitrobenzene increased with the reaction temperature in the wide temperature range of 20−80 oC (entries 1−4). When the temperature was elevated to 80 oC, 2-chloronitrobenzene could be entirely and quantitatively converted to 2-chlorobenzenamine within 20 min. The content of hydrazine hydrate in ethanol solution had a significant effect on the catalytic performance of the 20Co−NC/γ-MA. The 2chloronitrobenzene conversion increased with enhancing hydrazine content and approached a stable value at the hydrazine/substrate molar ratio of 6 : 1 (entries 2, 5−8), but the selectivity to 2-chlorobenzenamine was always retained at > 99%. The reaction results listed in entries 2, 9−17 demonstrated the influence of several widely-used organic solvents such as methanol, ethanol, THF, toluene, ethyl acetate, ethyl ether and DMF, in the catalytic reduction of nitro compounds. All the tested solvents showed excellent catalytic activities and selectivity of > 99% to 2chlorobenzenamine. Among them, the mixed solution of 1 mL of ethanol and 1 mmol of 2chloronitrobenzene afforded the highest 2-chloronitrobenzene conversion (entries 2). It was found that all the tested reactions over the 20Co−NC/γ-MA catalyst could achieve full conversion of 2-chloronitrobenzene with a 2-chlorobenzenamine selectivity of > 99% by prolonging reaction time.


35


Page 36 of 54

Table 6. Effect of Reaction Conditions on the Catalytic Performance of 20Co−NC/γ-MA for Selective Reduction of 2-Chloronitrobenzene to 2-Chlorobenzenaminea

entry

temp. (oC)

N2H4 (equiv.) solvent (mL)

conv. (%)

sel. (%)

1

20

6

ethanol (1 mL)

42

> 99

2

40

6

ethanol (1 mL)

72

> 99

3

60

6

ethanol (1 mL)

93

> 99

4b

80

6

ethanol (1 mL)

> 99

> 99

5

40

0

ethanol (1 mL)

0

−

6

40

2

ethanol (1 mL)

35

> 99

7

40

4

ethanol (1 mL)

43

> 99

8

40

8

ethanol (1 mL)

73

> 99

9

40

6

methanol (1 mL)

63

> 99

10

40

6

THF (1 mL)

56

> 99

11

40

6

toluene (1 mL)

45

> 99

12

40

6

ethyl acetate (1 mL)

36

> 99

13

40

6

ethyl ether (1 mL)

55

> 99

14

40

6

DMF (1 mL)

15

> 99

15

40

6

ethanol (2 mL)

55

> 99

16

40

6

ethanol (4 mL)

51

> 99

17

40

6

ethanol (8 mL)

42

> 99

a

Reaction conditions unless otherwise noted: 20 chloronitrobenzene, 45 min. bReaction time: 20 min.

mg

catalyst,

1

mmol

2-


36

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In order to demonstrate the generality of the xCo−NC/γ-MA catalysts for the selective reduction of nitroarenes to the corresponding arylamines, a series of substituted nitroarenes were examined under the optimized reaction conditions (20 mg catalyst, 1 mmol substrate, 6 mmol N2H4·H2O, 1 mL ethanol, 40 oC), and the reaction results were summarized in Table 7. It could be seen that all the substituted nitroarenes could be smoothly and completely transformed at 40 o

C during a short period. The nitroarenes containing non-reducible groups in the benzene rings,

such as -CH3, CH3O-, F3C-, -CH3OH, -OH, -NH2, were completely converted into the corresponding amines in the yields of > 99% (entries 1−11). It has been reported that the dehalogenation was a serious issue in the selective reduction of halogen-substituted nitroarenes. Herein, various halogen (F, Cl, Br)-substituted nitroarenes could smoothly proceed up to the end to generate the corresponding aromatic amines without discernible dehalogenation (entries 12−22). The reaction results in entries 23−29 demonstrated that other some reducible groups like carboxy, nitrile, amide, sulphanilamide, and ester in the benzene rings could be tolerated in the reduction of nitro group. Note that 1,3-dinitrobenzene was fully transformed into 1,3diaminobenzene (entry 30). It was well-known that supported noble catalysts such as Pd, Pt, Rh, Ru, etc., exhibited poor chemoselectivity to the reduction of the nitro group when there were alkenyl groups present in the same molecule.14,16 Nevertheless, it was found that 4-nitrostyrene could be selectively transformed to 4-aminostyrene in the yield of > 99% over the 20Co−NC/γMA catalyst (entry 31). Interestingly, the 20Co−NC/γ-MA catalyst could also selectively reduce heterocyclic nitroarenes containing N element to the corresponding arylamines, affording the yields of > 99% (entries 32−35). The high chemoselectivity of the xCo−NC/γ-MA catalysts for the reduction of nitroarenes to aromatic amines is probably due to the higher reactivity of the nitro group than other reducible groups.66


37


Page 38 of 54

Table 7. Chemoselective Reduction of Various Nitroarenes over 20Co−NC/γ-MA Catalysta

entry

substrate

time (min)

conv (%)

sel (%)

1

R=H

45

> 99

> 99

2

R = 4-Me

90

> 99

> 99

3

R = 2,4-Me

90

> 99

> 99

4

R = 4-OCH3

120

> 99

> 99

5

R = 4-CF3

45

> 99

> 99

6

R = 4-CH2OH

90

> 99

> 99

7

R = 2-OH

90

> 99

> 99

8

R = 4-OH

120

> 99

> 99

9

R = 2-CH3; 5-OH

90

> 99

> 99

10

R = 2-NH2

180

> 99

> 99

11

R= 3-NH2; 4-Me

90

> 99

> 99

12

R = 4-F

45

> 99

> 99

13

R = 2,4-F

45

> 99

> 99

14

R = 2,3,4-F

45

> 99

> 99

15

R = 4-Cl

75

> 99

> 99

16

R = 3-Cl

75

> 99

> 99

17

R=2-Me; 3-Cl

90

> 99

> 99

18

R = 2,4-Cl

75

> 99

> 99

19

R = 2-NH2; 5-Cl

180

> 99

> 99

20

R = 2-NH2; 4,5-Cl

180

> 99

> 99

21

R = 4-Br

90

> 99

> 99


38

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22

R = 2-Br; 5-F

90

> 99

> 99

23

R = 3-COOH

90

> 99

> 99

24

R = 3-Cl; 4-NH2; 5-CN

120

> 99

> 99

25

R = 4-CH2CN

90

> 99

> 99

26

R = 4-CONH2

90

> 99

> 99

27

R = 4-SO2NH2

90

> 99

> 99

28

R = 4-CO2CH2CH3

90

> 99

> 99

29

120

> 99

> 99

30

90

> 99

> 99b

31

75

> 99

> 99

32

90

> 99

> 99

33

90

> 99

> 99

34

120

> 99

> 99

35

180

> 99

> 99

a

Reaction conditions: 20 mg catalyst, 1 mmol substrate, 6 mmol N2H4·H2O, 1 mL


39


Page 40 of 54

ethanol, 40 oC. b1,3-Diaminobenzene.

The reduction of 2-chloronitrobenzene with hydrazine hydrate was used as the model reaction to investigate the stability of the 20Co−NC/γ-MA catalyst. The chemoselective reduction of 2chloronitrobenzene with ~72% conversion, at which the reaction was controlled by the chemical kinetics, was applied for the first cycle. The catalyst after every run was recovered by centrifugation, followed by washing with ethanol, and drying at 100 oC. The test results in Fig. 12 indicated that after the tenth cycling reaction, the 20Co−NC/γ-MA catalyst still retained the 2chloronitrobenzene conversion of ~72% and 2-chloronitrobenzene selectivity of > 99%. After the tenth run, the spent catalyst had no discernible variations in textural properties (SBET: 174 m2 g−1; Vp: 0.16 cm3 g−1; Dp: 3.6 nm), phase structure, Co particle size, and valence states of Co species. ICP analysis demonstrated that the contents of Co in the spent e 20Co−NC/γ-MA catalyst was 15.6 wt%, which was consistent well with that (15.4 wt%) of the fresh 20Co−NC/γ-MA catalysts shown in Table 2. Besides, the filtrate could not continue to react with hydrazine hydrate and Co element was not found within the detectable limitation by ICP-AES. This result revealed that there was no leaching of Co in the reaction procedure.


40

Page 41 of 54

Conversion

Selectivity

Conversion

and selectivity (%)

100 80 60 40 20 0 1

2

3

4 5 6 7 Cycle number

8

9

10

Figure 12. Reusable profiles of the 20Co−NC/γ-MA catalyst for the selective reduction of 2chloronitrobenzene to 2-chlorobenzenamine. Reaction conditions: 60 mg catalyst, 3 mmol 2chloronitrobenzene, 18 mmol N2H4·H2O, 45 min for the 1st cycle; the catalyst/2-

0.15

g 1, STP )

chloronitrobenzene ratio was identical to that for the first cycle in the following cycles 500 100

-Al2O

b

60 40 20 0.0

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

0.05

Internsity (counts)

(cm Volume adsorbed

0.10

400

80

*

*

3

b

* CoAl2O4



a

3

dV/dD (cm g1 nm1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60






 

Co

*

*



300





*

Spent

200 Fresh

100 0.00

2

3

4 5 6 7 Pore size (nm)

8

9

20

30

40

50

2 / o

60

70

80


41


14000

Intensity (kcps)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

d

Page 42 of 54

781.3 782.3

12000

10000

8000 775 780 785 790 795 800 805 810 Binding energy (eV)

Figure 13. (a) N2 sorption isotherms and pore size distribution, (b) XRD patterns, (c) TEM image, and (d) Co 2p XPS spectra, of the spent 20Co−NC/γ-MA catalyst for the selective reduction of 2-chloronitrobenzene. It is generally accepted that under reducing conditions, the reduction of nitroarenes into benzene amine goes through the two reaction pathways, i.e., direct and indirect. In the direct pathway, the nitro group is first reduced to the nitroso, and then to the hydroxylamine in two consecutive steps, and finally, the hydroxylamine is directly reduced to the amine; whereas in the case of the indirect pathway, the nitroso and hydroxylamine intermediates condense to produce azo intermediates, which were subsequently hydrogenated and cleaved to form benzene amine.62,63 However, in the selective reduction of 2-chloronitrobenzene as a model reaction over the xCo−NC/γ-MA catalyst, there were no nitroso, hydroxylamine and azo compounds detected in the products. In order to make clear the reduction pathway of nitroarenes in ethanol by hydrazine over the xCo−NC/γ-MA catalyst, other several separate experiments over the 20Co−NC/γ-MA

catalyst

were

designed.

It

was

noted

that

nitrosobenzene

and

phenylhydroxylamine could smoothly be reduced into benzene amine with hydrazine hydrate in


42

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ethanol at 40 oC, whereas the reduction of both azoxybenzene and azobenzene to benzene amine was very slow under the same conditions. These results demonstrated that the reduction of nitroarenes to aromatic amines proceeded over the xCo−NC/γ-MA catalysts dominantly via the direct pathway rather than the indirect pathway. It was demonstrated in Table 5 that metal Co was the active component of the xCo−NC/γ-MA catalyst for the reduction of 2-chloronitrobenzene, and the superior catalytic performance could be attributed to the cooperative effect of metal Co and NC or the formation of surface Co−Nχ species on the catalyst surfaces. Transition metal nitrides and carbides for hydrazine decomposition have studied extensively and exhibited excellent catalytic ability of hydrazine dissociation to H2 and N2.64,65 It can be assumed that hydrazine on surface Co−Nχ species of the xCo−NC/γ-MA catalyst had similar dissociation mechanism of hydrazine to on Co carbide and nitride in the reduction of nitro groups. Therefore, we proposed that hydrazine molecule might be adsorbed primarily on the Co atoms bonded with N or C atoms and then was dissociated into the active H* species, which were interacted with the adjacent nitro groups on the surface to form the nitroso intermediates. These nitroso compounds were fast reduced to hydroxylamine, and further to benzene amine.

4. CONCLUSIONS In summary, mesoporous γ-alumina (γ-MA) supported cobalt oxides (xCo3O4/γ-MA) with large surface areas and narrow pore size distributions were first prepared through one-pot partial hydrolysis of metal nitrates. The Co3O4/γ-MA materials were impregnated with a water–ethanol solution of 1,10-phenanthroline, followed by the treatment of 700 oC in N2 atmosphere, producing xCo−NC/γ-MA composites with N-doped graphitic carbon (NC). Metal Co


43


Page 44 of 54

nanoparticles, N and C elements were uniformly distributed in mesoporous γ-alumina framework and showed strong interactions among them. The xCo−NC/γ-MA composites were applied for a variety of functional nitroarenes to the corresponding arylamines with hydrazine, and showed high catalytic activity and selectivity, due to the synergistic effect of metal Co and NC or the formation of surface Co−Nχ species on the catalyst surfaces. The recycling tests and characterizations demonstrated that the Co−NC/γ-MA catalyst was highly stable and can be recycled for the chemoselective reduction of nitroarenes. The present preparation method can be extended to other transition metal catalysts such as Ni, Fe, Cu, etc., for other important reduction reactions. AUTHOR INFORMATION Corresponding Author *[email protected] (X. G. Wang); [email protected] (X. G. Lu). Fax: +86-21-56338244 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This research was supported by Open Project of State Key Laboratory of Advanced Special Steel of Shanghai University (SKLASS2015-Z052), National Basic Research Program of China (973 Program, No. 2014CB643403), the National Natural Science Foundation of China (No. 51574164) and Basic Major Research Program of Science and Technology Commission Foundation of Shanghai (No. 14JC1491400).


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

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Synthesis of Mesoporous γ-Alumina Supported Co−Based Catalysts and Their Catalytic Performance for Chemoselective Reduction of Nitroarenes

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Synthesis of Mesoporous Î³-Alumina Supported CoâBased Catalysts

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