Highly Active and Chemoselective Reduction of Halogenated

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Highly Active and Chemoselective Reduction of Halogenated Nitroarenes Catalysed by Ordered Mesoporous Carbon Supported Platinum Nanoparticles Yao Sheng, Xueguang Wang, Zhikang Xing, Xiubin Chen, Xiujing Zou, and Xionggang Lu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00948 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 20, 2019

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ACS Sustainable Chemistry & Engineering

Highly Active and Chemoselective Reduction of Halogenated Nitroarenes Catalysed by Ordered Mesoporous

Carbon

Supported

Platinum

Nanoparticles Yao Sheng, Xueguang Wang,* Zhikang Xing, Xiubin Chen, Xiujing Zou, and Xionggang Lu* State Key Laboratory of Advanced Special Steel, School of Materials Science and Engineering， Shanghai University, 99 Shangda Road, BaoShan District, Shanghai 20444, China. E-mails:

[email protected]

(X.

G.

Wang),

[email protected]

(X.

G.

Lu),

[email protected] (Y. Sheng), [email protected] (X. J. Zou), [email protected] (Z. K. Xing), [email protected] (X. B. Chen).

1 Environment ACS Paragon Plus

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ABSTRACT: Highly-dispersed Pt nanoparticles (~2.2 nm) on ordered mesoporous carbon (Pt/CMK-3−HQ) were first prepared through a two-step impregnation route with aqueous solutions of 8-hydroxyquinoline (8-HQ) and H2PtCl6, respectively. The Pt/CMK-3−HQ quantitatively converted various halogenated nitroarenes to the corresponding haloanilines using hydrazine

hydrate

with

unprecedented

activities

(e.g.,

turnover

frequency

for

o-

chloronitrobenzene was 30.2 s–1) and exhibited high stability with 20 cycles without decrease in catalytic efficiency. The high activity and chemoselectivity of Pt/CMK-3−HQ were attributed to the cooperation effect between Pt and N species, promoting cleavage of hydrazine to generate more Pt-H– and N-H+ species for reduction of nitro groups, and weakening the interaction between halogen groups and Pt atoms for activation of C-halogen bonds.

KEYWORDS:

Pt

nanoparticles,

Heterogeneous

catalysis,

Chemoselective reduction, Ordered mesoporous carbon.


Halogenated

nitroarenes,

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INTRODUCTION Direct reduction of nitro aromatics into the corresponding anilines is a fundamental reaction in organic chemistry.1,2 As key chemical intermediates for the production of agricultural chemicals, pharmaceuticals and dyestuffs,3-5 halogenated anilines are traditionally obtained by the reduction of the corresponding halogenated nitroarenes using stoichiometric amounts of reducing reagents like Fe/Zn and sulfides. However, these reduction processes produce large amounts of metal sludge and hazardous wastewater containing acids and sulfur, causing serious environmental pollution.6,7 Heterogeneous catalytic reductions over solid metal catalysts have attracted much interest as a sustainable and efficient protocol because the catalysts can be easily separated and recovered from the reaction mixtures for reuse.8,9 The chemoselective reduction of nitroarenes has been reported over transition metal catalysts using various hydrogen donors such as gas hydrogen, formic acid, hydrazine hydrate, sodium borohydride, hydrosilanes, and so on. 10-14 Among them, H2 and hydrazine are recognized as the most ideal green reducing reagents, which generate only water and/or N2 as by-products.15-19 Supported noble metal catalysts like Pt, Pd, Ru and Rh have been widely investigated for the selective reduction of various substituted nitroarenes to the corresponding anilines using H2 or hydrazine due to their high activity up to now.20-25 However, for halogenated nitroarenes, these catalysts frequently generate some unwanted dehalogenated by-products which seriously lower the product quality, limiting their large-scale industrial applications.26-30 Hydrodehalogenation reaction has been demonstrated to be sensitive to the metal particle size and the support nature over supported metal catalysts.31-37 Apart from the geometric factors, the electronic properties of metal sites also greatly affect the dehalogenation reaction.38,39 Generally, the electron-deficient metal species do not favor the dehalogenation in the hydrogenation of



halogenated aromatics.40,27 Recently, ordered mesoporous carbon-supported metal catalysts have attracted increasing interest because of their large surface area, large pore volume, narrow mesoporous size distribution and excellent chemical stability; and their catalytic performance can be improved by modulating the surface properties on the support, metal particle size, and chemical states, which have strong dependence on preparation method, thermal treatment and element doping.41,15 Herein, we have demonstrated for the first time that Pt nanoparticles supported on ordered mesoporous carbon CMK-3 modified with 8-hydroxyquinoline (8-HQ) (Pt/CMK-3−HQ) can be used as a highly efficient heterogeneous catalyst for the chemoselective reduction of halogenated nitroarenes to the haloanilines with hydrazine hydrate with unprecedented activities. The Pt/CMK-3−HQ is applicable to a variety of halogen-substituted nitroarenes, and the haloaniline selectivity of almost 100% is achieved at the complete conversion. The Pt/CMK-3−HQ catalyst is easily recovered from the reaction mixture and reused without any decrease in its efficiency. EXPERIMENTAL Materials and Preparation Pluronic P123 (EO20PO70EO20, Mav = 5800) was purchased from Aldrich. Other some chemicals of reagent grade including all nitro compounds and solvents, hexahydrate (H2PtCl6·6H2O), N2H4·H2O aqueous solution (80 wt%), 8-hydroxyquinoline, and so on, and commercial activated carbon (AC), γ-alumina (γ-Al2O3), silica (SiO2) and 5%Pt/C catalyst were purchased from Sinopharm Chemical Reagent Co., Ltd. Deionized water was used to prepare all solutions. All chemicals and solvents were used as received without further purification. Before use, all supports were treated in air or N2 at 550 oC for 6 h to remove water and impurities.


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Ordered mesoporous silica SBA-15 was obtained with the synthesis temperature of 100 oC and the calcination in air at 550 oC for 6 h according to the document.42 The ordered mesoporous carbon CMK-3 was synthesized by replication using SBA-15 as hard template and sucrose as carbon source, which was carbonized in a tube furnace in a flow of high-purity N2 at 800 oC and washed with 5 wt% hydrofluoric acid to remove the silica according to the document.43 The Nmodified ordered mesoporous carbon CMK-3−HQ was prepared by the impregnation method. Briefly, 0.6 g of CMK-3 powder was added into 10 mL of ethanol solution containing 0.2 g of 8hydroxyquinoline under stirring at room temperature for 1 h. Then the mixture solution was evaporated under stirring at 40 oC, and dried at 100 oC overnight. Finally, the solid was treated in a tube furnace in a flow of high-purity N2 at 800 oC for 6 h. 2%Pt/CMK-3−HQ catalyst with a nominal loading of 2 wt% Pt was prepared by an ultrasound-assisted impregnation strategy. Typically, 0.6 g of CMK-3−HQ powder was dispersed in 45 mL of deionized water under ultrasound conditions, and then 1.23 mL of H2PtCl6·6H2O aqueous solution with a Pt concentration of 0.05 mol L–1 was added into the solution for 10 min. After this, the mixture was evaporated under stirring at 40 oC. Finally, the solid was directly treated at 200 oC for 3 h in a 30 vol% H2/N2 flow. For comparison, 2%Pt/CMK-3, 2%Pt/SiO2, 2%Pt/γ-Al2O3 and 2%Pt/AC catalysts were also prepared by the same method. Catalyst Characterization The actual amounts of Pt elements in the catalysts were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES) conducted on a Perkin Elmer emission spectrometer. The analysis of C and N elements was carried out on a PerkinElmer 2400 CHN elemental analyzer. The N2 adsorption and desorption isotherms were recorded on a 5 Environment ACS Paragon Plus


Micromeritics ASAP 2020 sorptometer at −196 °C, and the samples were degassed for 10 h at 200 °C before analysis. The specific surface areas (SBET) were calculated via the Brunauer– Emmett–Teller (BET) method. The single-point pore volume (VP) was calculated from the adsorption isotherm at a relative pressure of 0.990. The pore diameter distributions were calculated using the adsorption branches of the N2 adsorption−desorption isotherms by the Barrett–Joyner–Halenda (BJH) method. For the ordered mesoporous materials, the BJH pore sizes (Dp) was obtained from the maximum of the pore size distribution curves, and for the 2%Pt/SiO2, 2%Pt/γ-Al2O3, 2%Pt/AC and 5%Pt/C catalysts, mean pore diameters were obtained by the Barrett–Joyner–Halenda (BJH) method. TEM and HAADF-STEM micrographs were taken with a JEOL JEM-2010F field emission microscope operating at 200 kV. The sample was prepared by drying an ethanolic dispersion of the well-ground catalyst powder on a holeycarbon-coated copper grid. XPS spectra were obtained on an ESCALAB 250Xi spectrometer equipped with monochromatized Al Kα radiation (hν = 1486.6 eV) operated at ca. 1 × 10–9 Torr. Before the XPS measurement, the supported Pt catalysts were ex-situ treated at 200 oC for 3 h in a 30 vol% H2/N2 flow. The spectra were calibrated using the binding energy of C 1s peak at 284.6 eV. Catalytic Reactions and Product Analyses The selective reduction of nitro aromatics with hydrazine hydrate as the hydrogen donor was conducted under ambient atmosphere in a 100 mL two-neck round-bottom flask with a condenser. In a typical experiment, 10 mmol of o-chloronitrobenzene, appropriate amounts of catalysts, and 20 mL of ethanol were ultrasonically dispersed in the flask at room temperature. After this, the flask was transferred into a water bath at the set temperature of 80 oC with an accuracy of ± 0.1 oC and magnetically stirred for ~30 min, and then 40 mmol of hydrazine 6 Environment ACS Paragon Plus

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hydrate was rapidly added into the reaction solution under stirring at a stirring rate of 900 rpm. The liquid solution was separated from the reaction mixture at appropriate reaction intervals with a filtering syringe for analysis. The selective hydrogenation of nitro aromatics with H2 as the hydrogen donor was carried out in a 100 mL stainless steel high pressure reactor with magnetic stirring. In a typical reaction, 10 mmol of o-chloronitrobenzene, appropriate amounts of catalysts, and 20 mL of ethanol were first introduced into the reactor. Prior to the reaction, the reactor was flushed three times with 0.5 MPa H2 and depressurized to ambient pressure. Then, the reaction mixture was heated to 80 oC, and reacted at 2.0 MPa H2 pressure and at a stirring rate of 900 rpm. After the reaction, the reactor was rapidly cooled to room temperature and the remaining H2 was discharged slowly. The reaction mixture was separated by filtration for analysis. Each reaction was repeated at least three times. The products were analyzed by gas chromatography-mass spectrometry (GC-MS) (Shimadzu GCMS-QP 2010 Plus) and GC (Varian CP-3800) with a capillary column (column CP-SIL 5 CB 50 m length, 0.32 mm internal diameter, 5 μm film thickness) and a flame ionization detector. After finishing the reaction, the catalyst was filtered and washed with ethanol, then was refluxed in 100 mL fresh ethanol for 1 h, followed by filtration and washing again with ethanol. Finally, after the solvent was removed, the whole organic products were obtained to calculate the mass balance. It was found that the resulting products accounted for > 98% on the basis of the mole number of the starting reactants. The recyclability of the 2%Pt/CMK-3–HQ with hydrazine hydrate as the hydrogen donor was studied using o-chloronitrobenzene as the model compound in a 2000 mL flask. In the first cycle, the reaction conditions of 200 mmol o-chloronitrobenzene, 100 mg catalysts, 400 mL ethanol,



800 mmol N2H4·H2O, 80 °C and 18 min were used, where the o-chloronitrobenzene conversion was ca. 82%. The catalyst after each run was recovered by a simple filtration and washing with ethanol, followed by drying at 100 oC. Considering the catalyst loss in the treatment process, the amount of catalyst was changed for each of the following cycles, but the 2%Pt/CMK-3–HQ/oCNB/ethanol/hydrazine ratio and reaction time were always kept same as those for the first cycle. RESULTS AND DISCUSSION The actual contents of N and Pt elements in the obtained catalysts were determined by ICPAES and a CHN elemental analyzer, and the results are summarized in Table 1. The 2%Pt/CMK3 and 2%Pt/CMK-3−HQ exhibited typical type IV isotherms with H1-type hysteresis loops and narrow pore size distributions, similar to pure CMK-3 (Supporting Information Figure S1), indicating that these two catalysts still maintained the mesoporous structure of the CMK-3 after treated with 8-HQ and loaded with Pt. Nevertheless, compared with the CMK-3, their specific surface areas, pore volumes and pore sizes showed a certain decline due to the incorporation of Pt, N and C from 8-HQ (Table 1). The mesostructured ordering of the 2%Pt/CMK-3 and 2%Pt/CMK-3−HQ were further confirmed by the low-angle X-ray diffraction (XRD) patterns with a strong (100) peak at 2θ = ~1.05o (Supporting Information Figure S2a).


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Table 1. N, Pt contents and physical properties of the catalysts SBET

Vp

(m2 g−1)

(cm3 g−1)

–

1182

1.29

3.8a

2.30

–

783

0.71

3.5a

2%Pt/CMK-3

–

1.96

987

1.01

3.4a

4

2%Pt/CMK-3–HQ

2.28

1.78

720

0.67

3.3a

5

2%Pt/SiO2

–

1.87

563

0.65

4.8b

6

2%Pt/γ-Al2O3

–

1.81

210

0.43

6.7b

7

2%Pt/AC

–

1.88

1480

0.95

2.6b

8

5%Pt/C

–

4.89

1434

1.30

3.5b

9

Spent 2%Pt/CMK-3–HQ

2.27

1.73

712

0.66

3.3a

Entry

Catalyst

N (wt%)

Pt (wt%)

1

CMK-3

–

2

CMK-3–HQ

3

D (nm)

a

Obtained by the maximum of the pore size distribution curve. bAverage pore size by BJH method The wide-angle XRD patterns of both 2%Pt/CMK-3 and 2%Pt/CMK-3−HQ presented two broadened diffraction peaks at 2θ = ~23o and ~43o, respectively (Supporting Information Figure S2b), indicating graphitic carbon.44,45 The 2%Pt/CMK-3 showed a diffraction peak of metal Pt at 2θ = 39.8o, whereas for the 2%Pt/CMK-3−HQ, no Pt peak was found. This result revealed that the doped N benefited the Pt dispersion on the surface, likely owing to the complexing and stabilizing effect.46 TEM and HAADF-STEM images in Figure 1 indicated that the Pt particles were homogeneously dispersed in the mesoporous framework of both 2%Pt/CMK-3 and 2%Pt/CMK-3−HQ; however, the Pt particle sizes (~2.2 nm) for the 2%Pt/CMK-3−HQ were much smaller than those (~5.6 nm) for the 2%Pt/CMK-3, in agreement with the XRD results. Besides, the elemental mapping images of the 2%Pt/CMK-3−HQ were shown in Figure 2, which also demonstrated that both N and Pt species were homogeneously dispersed in the matrix. 9 Environment ACS Paragon Plus


The incorporation of N atoms in the sp2 graphene framework was identified by the N 1s XPS spectra in Figure 1d, in which two strong peaks at 398.5 eV, 400.9 eV were assigned to pyridinic-type N and graphite-type N, respectively, and the weak peak at 402.9 eV was attributed to pyridinic N-oxide.47,48 It seemed that there were very few pyrrolic-type N atoms in the 2%Pt/CMK-3−HQ. The Pt 4f XPS spectra for the 2%Pt/CMK-3 in Figure 1e showed two relatively symmetric binding energy (BE) curves peaked at 71.8 eV and 75.2 eV, which could be assigned to Pt0,49,50 since it was reduced at 200 oC with H2 before the XPS analysis. However, this BE value was remarkably higher than the BE of bulk Pt (4f7/2 = 71.0 eV).50,51 This result demonstrated that there was a strong interaction between Pt particles and CMK-3. When the CMK-3 was treated with HQ, the Pt 4f BEs shifted to higher values at 72.6 eV and 76.0 eV, respectively, and the curve was slightly broadened. This result could be explained by the interaction between the surface Pt atoms and the pyridinic N to form Ptδ+–Nδ– bond.52


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Figure 1. TEM, HAADF-STEM images and particle size distributions of: (a1–c1) 2%Pt/CMK-3, (a2–c2) 2%Pt/CMK-3−HQ, (d) XPS N 1s spectra, and (e) Pt 4f XPS spectra.



Figure 2. HAADF-STEM image and elemental mapping images of N and Pt for the 2%Pt/CMK3–HQ.

We first compared the initial conversions of o-chloronitrobenzene (o-CNB) as a model compound over different Pt catalysts with N2H4·H2O and H2 as a reducing agent, respectively (Table 2). It was clearly revealed that the support natures strongly influenced the activities of Pt catalysts. The 2%Pt/CMK-3–HQ showed much higher catalytic activities than not only all the Pt catalysts on various other supports such as CMK-3, SiO2, γ-Al2O3 and activated carbon (AC) obtained by the same method, but also commercial 5%Pt/C catalyst using either N2H4·HO or H2. As a result, the turnover frequencies (TOFs, the number of converted substrate molecules per Pt


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atom per second on the basis of the total Pt atoms by ICP in Table 1) for the 2%Pt/CMK-3–HQ exhibited unprecedented high values of 30.2 s–1 for hydrazine and 20.3 s–1 for H2, respectively (Table 2), which, to the best of our knowledge, were also the highest TOFs ever reported for the reduction of functionalized nitroarenes over noble metal catalysts using either hydrazine or H2. For all Pt catalysts, the conversions of o-CNB increased with the reaction time and finally reached 100%; however, only the 2%Pt/CMK-3–HQ could catalyze o-CNB to o-chloroaniline (o-CA) with 100% selectivity with hydrazine, and other Pt catalysts always formed some dechlorinated aniline (Table 2), as reported in the previous document.53,54 Interestingly, the reduction rates of o-CNB and the o-CA selectivities with hydrazine were always higher than those with H2 either at 0.1 MPa (not shown) or even at 2.0 MPa (Table 2). This phenomenon implied that the o-CNB reduction was not performed through the decomposition hydrogenation route, although hydrazine was easy to be decomposed into N2 and H2.55 The 2%Pt/CMK-3–HQ was also tested for the reduction of various halogenated nitroarenes using N2H4·H2O and the results were listed in Table 3. Compared to nitrobenzene, all the halogenated nitroarenes exhibited much lower reaction rates; however, these substrates could smoothly be converted into the corresponding haloanilines almost without any by-product except that 2-iodo nitrobenzene generated ca. 20% deiodinated aniline (entry 11). Although the effect of the halogen atoms on benzene ring on the catalytic activity was rather complex, in general, the reduction activity for the nitrobenzene with halogen atom at the same position followed the following order：F > Cl > Br > I. Besides, the 2%Pt/CMK-3–HQ could also quantitatively transform halogenated N-heterocyclic nitroarenes to the corresponding halogenated Nheterocyclic arylamines with 100% conversion (entries 25, 26), but the reaction rates were relatively lower in comparison with the nonheterocyclic ones.



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Table 2. Selective reduction of o-CNB to o-CA over various catalystsa

Entry

Catalyst

1

CMK-3–HQ

2

2%Pt/CMK-3–HQ

3

2%Pt/CMK-3

4

2%Pt/SiO2

5

2%Pt/γ-Al2O3

6

2%Pt/AC

7

5%Pt/C

Reducing agent

Conv. (%)b

TOF (s–1)c

Sel. (%)d

N2H4·H2O

Highly Active and Chemoselective Reduction of Halogenated

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