Sublimation-Induced Sulfur Vacancies in MoS2 Catalyst for One-Pot

Jul 19, 2019 - Sublimation-Induced Sulfur Vacancies in MoS2 Catalyst for One-Pot Synthesis of Secondary Amines .... The distributions of pore size and...
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Research Article Cite This: ACS Catal. 2019, 9, 7967−7975

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Sublimation-Induced Sulfur Vacancies in MoS2 Catalyst for One-Pot Synthesis of Secondary Amines Yunrui Zhang,† Yongjun Gao,*,† Siyu Yao,*,‡ Siwei Li,‡ Hiroyuki Asakura,§,∥ Kentaro Teramura,§,∥ Haijun Wang,† and Ding Ma*,‡

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†

Key Laboratory of Analytical Science and Technology of Hebei Province, College of Chemistry and Environmental Science, Hebei University, 071002 Baoding, P. R. China ‡ Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering and College of Engineering, and BIC-ESAT, Peking University, Beijing 100871, P. R. China § Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ∥ Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan S Supporting Information *

ABSTRACT: MoS2 catalysts with abundant S and Mo defects have been developed for the one-pot reductive amination of nitro compounds with aldehydes to synthesize secondary amines. The combination of multiple structural characterizations demonstrates that the density of S vacancies can be tuned by changing the thermal sublimation temperature. The experimental results and DFT calculations demonstrate that S vacancies on the surface of MoS2 are the active sites for the hydrogenation of the intermediate imines to the final products secondary amines.

KEYWORDS: reductive amination, sulfur vacancies, one-pot strategy, cascade reaction, molybdenum sulfide

1. INTRODUCTION

In the previous studies, a number of precious metal-based catalysts have been used in the one-pot synthesis of secondary amines from nitro compounds and aldehydes. For instance, Gum-acacia-stabilized palladium nanoparticles have been reported to efficiently catalyze this reaction under mild conditions.8 Other Pd-based catalysts, such as Pd−Ag alloy and AuPd@Fe3O4, were also active for the conversion of nitroarenes.9,10 Considering the reduction of the price of catalyst, some nonprecious cobalt-based7,11,12 and iron-based13 catalysts have been reported recently to be able to catalyze the reductive amination reactions with organic compounds (formic acid for example) and molecular hydrogen as the reducing reagents. However, due to the poor activity of the 3d transition metals, the optimal reaction conditions of these systems are generally higher than 150 °C. Constructing an inexpensive catalyst with good low-temperature activity and selectivity is still a big challenge for the cascade reductive amination reactions.

As the important building blocks for pharmaceutical, agricultural, and other fine chemicals, functionalized secondary amines are among the most important reactants and intermediates in today’s synthetic chemistry.1−3 In general, the functionalized secondary amines can be synthesized via Nalkylation of primary amines with corresponding alcohols or the reductive amination of nitro compounds with aldehydes in the presence of hydrogen.4−6 On the basis of the criterions of green and sustainable chemistry, it is highly desirable to develop a one-pot catalytic system for the reductive amination of nitro compounds to fabricate secondary amines to avoid the tedious separation/purification steps and decrease the release of waste.7 According to the reaction mechanism, the reductive amination of nitro compounds with aldehydes can be seen as a cascade reaction of a serial of elementary steps including the reduction of the nitro group to primary amine, the reversible formation of imines from the coupling of aldehydes with amine, and the reduction of imines into secondary amines. Therefore, the hydrogenation rate of the catalysts is one of the key factors to enhance the efficiency of the cascade reaction. © XXXX American Chemical Society

Received: April 8, 2019 Revised: July 15, 2019 Published: July 19, 2019 7967

DOI: 10.1021/acscatal.9b01429 ACS Catal. 2019, 9, 7967−7975

Research Article

ACS Catalysis

ESCALAB 250XI XPS spectrometer. The Fourier transform infrared spectroscopy (FTIR) analysis was carried out on a Bruker ALPHA FTIR spectrometer. Raman spectra were collected on a laser confocal micro-Raman spectroscope (LabRAM HR800, Horiba Jobin Yvon). The samples were placed on a glass slide and measured directly with the laser wavelength 532 nm. The nitrogen adsorption−desorption isotherms are measured on a surface area and pore-size analyzer (V-sorb-2800, Gold APP Instruments, Beijing). The samples were first pretreated at 120 °C for 4 h in vacuum. The BET surface area is achieved according to the adsorption data in the relative pressure (P/P0) range 0.05−0.35. The distributions of pore size and pore volume are based on Barrett−Joyner−Halenda (BJH) analysis of adsorption isotherm data. Mo K-edge XAS measurements were conducted at the public beamline, BL01B1, SPring-8 (Hyogo, Japan). The incident X-rays were monochromatized with a Si(311) double crystal monochromator. Higher harmonic X-rays radiation was removed by setting the glancing angle of the Rh-coated X-ray mirrors to 1.5 mrad. The incident (I0) and transmitted (It) Xray fluxes were measured using ion chambers filled with N2 (50%)/Ar (50%) and Ar (75%)/Kr (25%), respectively. The photon energy was calibrated at the inflection point of the Mo K-edge X-ray absorption near-edge structure (XANES) spectrum of Mo metal foil to 20 000 eV. Data were reduced using Athena and Artemis software version 0.9.25 included in the Demeter package. 2.4. Catalytic Reaction. In general, nitroarenes (0.5 mmol), aldehydes (0.7 mmol), catalyst (10 mg), and solvent (3 mL) were added into an autoclave (stainless steel 316, 20 mL) sequentially. The reactor was purged with hydrogen twice and then charged with hydrogen pressure to 2 MPa. The autoclave was heated in an oil bath at 120 °C for 5 h. After reaction, the reaction mixture was cooled to room temperature, and dodecane (50 μL) was added into the system as the internal standard. Then, the mixture was diluted with ethyl acetate (20 mL) and filtered into a sample vial for analysis on a gas chromatography instrument equipped with an HP-5 column FID detector (Agilent GC 7820). 2.5. Density Functional Theory (DFT) Calculations. The DFT calculations are performed using the Vienna ab initio simulation package (VASP). The projector-augmented wave method was adopted to describe the interaction between the ionic cores and electrons. The approach taken by VASP is based on the local density approximation (LDA). The calculation precision is normal, and a 1 × 1 × 1 Monkhorst−Pack K-point sampling was used for all calculations. The value of the plane wave cutoff is 400 eV. The van der Waals interactions corrections were not taken into consideration. A supercell 7 × 7 MoS2 plane involving the consecutive S vacancies enlarged from 0 to 6 is used as model catalysts. The lattice type is 3D triclinic, where a = b = 25.328 Å and c = 18.410 Å. The adsorption energy for hydrogen or imine on the MoS2 plane is calculated on the basis of the following equation, Ea = E(CAT+sub) − ECAT − Esub. In the formula, E(CAT+sub) is the total energy of MoS2 catalyst adsorbing substrates, such as hydrogen or imine; ECAT is the total energy of MoS2 catalyst; and Esub is the total energy of free substrate.

Molybdenum sulfide is one of the most representative layered compounds with a planar Mo atom layer sandwiched between two sulfur ion layers. Due to the lack of accessible Mo sites in the bulk MoS2 structure, the molybdenum sulfide is generally unreactive. Only at the structural defects could the MoS2 be able to activate small molecules, such as H2.14−16 Hence, manufacturing a high density of defects in MoS2 is the key measure to improve the catalytic activity of MoS2 in hydrogenation reactions. Although MoS2 has been applied to catalyze the hydrogenation of nitro compounds with hydrazine hydrate or hydrogen as the reducer,17−20 no research concerning the one-pot reductive amination of nitro compounds with aldehydes has been reported. Herein, we report that by utilizing the thermal sublimation method at different temperatures it is possible to tune the density of S vacancies on the MoS2 catalyst synthesized via the hydrothermal method. Under the optimal conditions, the thermal-treated MoS2 catalysts exhibit over 85% yield to target secondary amine products at only 120 °C in the one-pot reductive amination of nitrobenzene with benzaldehyde. The correlation of the catalytic performances with structural characterizations and DFT calculations demonstrates that when the density of sulfur vacancies in the MoS2 catalysts is higher, the higher catalytic activity to hydrogenate the intermediate imine to final secondary amine can be achieved. The MoS2 catalyst has also exhibited excellent substrate adaptability in the synthesis of secondary amines.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Ammonium molybdate (AR, 99%), carbon disulfide (GC, 99%), ethyl acetate (AR, 99.5%), and ethanol (AR, 99.8%) are produced by Tianjin Kermel Chemical Reagent Co., Ltd. Nitrobenzene (AR, 99%) is produced by Tianjin Fuchen Chemical Reagent Factory. Benzaldehyde (AR, 99%) was purchased from Tianjin Guangfu Fine Chemical Research Institute. Other nitro-compounds and dodecane (AR, 99%) were purchased from Aladdin Industrial Corporation. All chemicals were used without further treatment. 2.2. Synthesis of the CAT. The hydrothermal process for synthesizing MoS2 is based on a previous report.21 Typically, (NH4)6Mo7O24·4H2O (0.9 g) was dissolved in deionized water (20 mL) in a small beaker (50 mL) and transferred into a 100 mL autoclave (stainless steel). Then, carbon disulfide (10 mL) was introduced into the solution. The autoclave was sealed and heated at 400 °C for 4 h in a muffle furnace. The resulting product was filtered, washed several times with deionized water, and finally freeze-dried for 12 h. The product was named as CAT. CAT (0.5 g) was then further calcined at 450 °C for 1 h in nitrogen atmosphere (40 mL/min) in a tube furnace, and the sample was named as CAT-450. If CAT-450 is exposed to air for a few days, the catalytic activity will decrease dramatically. The deactivated CAT-450 is named as CAT-450D. Sodium molybdate was used instead of ammonium molybdate to synthesize the references. The corresponding references were named as CAT-Na and CAT-Na-450. The commercial MoS2 is named as MoS2-com. 2.3. Catalyst Characterizations. Powder X-ray diffraction (XRD) patterns were recorded on the Bruker D8 ADVANCE diffractometer with Cu Kα (λ = 1.5418 Å) radiation at 40 kV and 20 mA. The morphology characterizations of CAT and CAT-450 were researched on a transmission electron microscope (TEM, Tecnai G2 20). The X-ray photoelectron spectroscopy (XPS) data were collected on a Thermo 7968

DOI: 10.1021/acscatal.9b01429 ACS Catal. 2019, 9, 7967−7975

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Table 1. Catalytic Performance of MoS2 Samples in the One-Pot Reductive Amination of Nitrobenzene with Benzaldehydea

selectivity (%) entry

catalyst

solvent

T/°C

t/h

conversion (%)

a

b

c

d

e

1 2 3 4 5 6b 7 8 9 10 11 12 13 14 15c

CAT CAT-450 CAT-450 CAT-Na CAT-Na-450 CAT-450 CAT-450 CAT-450 CAT-450 CAT-450 CAT-450 CAT-450-D MoS2-com

ethanol ethanol ethanol ethanol ethanol ethanol ethanol ethanol tetrahydrofuran acetonitrile H2O ethanol ethanol ethanol ethanol

120 120 120 120 120 120 100 110 120 120 120 120 120 120 120

4 4 5 5 5 5 5 5 5 5 5 5 5 5 19

100 100 100 0.4 67.9 100 100 100 98.8 100 62.1 6.3 0 0 100

12.4 73.2 85.2 3.8 5.9 52.1 36.9 74.4 32.8 13.1 2.3 5.5 0 0 83.7

17.8 6.8 0.7 16.7 44.5 43.4 21.3 3.9 37.4 35.1 74.9 14.1 0 0 1.2

62.9 11.4 6.0 70.2 44.0 1.3 38.2 12.1 29.8 51.4 22.8 35.0 0 0 0

6.8 7.0 0 5.1 3.0 2.8 2.5 7.4 0 0.4 0 41.2 0 0 6.6

0 1.6 8.1 4.1 2.6 0.8 1.1 2.1 0 0 0 4.1 0 0 8.4

CAT

a

Reaction conditions: nitrobenzene 0.5 mmol, benzaldehyde 0.7 mmol, catalyst 10 mg, solvent 3 mL, H2 2 MPa, conversion and selectivity were determined by GC. bNitrobenzene 0.5 mmol, benzaldehyde 0.5 mmol; the other conditions are the same as those in footnote a. cNitrobenzene 5 mmol, benzaldehyde 7 mmol, catalyst 80 mg; the other conditions are the same as those in footnote a.

Figure 1. (a) Recycling experiment of CAT-450. (b) Products distribution along with reaction time.

3. RESULTS AND DISCUSSION 3.1. Catalytic Activity. The catalytic hydrogenation of nitrobenzene to aniline over hydrothermal synthesized CAT catalyst has been evaluated in the presence of 2 MPa H2 (Table S1). At 120 °C, 38% yield of aniline can be achieved in 1 h. Extending the reaction time to 4 h, the yield of aniline reached 98%. In comparison, the commercial MoS2 shows no activity for the hydrogenation reaction. When an appropriate amount of benzaldehyde is introduced into the reaction system, it is normal that the commercial MoS2 is unable to catalyze the cascade secondary amine formation reaction. However, although nitrobenzene could be totally hydrogenated within 4 h over CAT, the aniline was still the main product, and the selectivities to N-benzyl aniline (desirable

product) and N-benzylideneaniline (intermediate) were only 12.4% and 17.8% (Table 1, entry 1), respectively. In addition, a small amount of 2-phenylquinoline and its hydrogenated product 2-phenyl-1,2,3,4-tetrahydroquinoline were detected as side products, which is due to the reaction of ethanol solvent with the reaction substrates and further hydrogenation.22 To produce more defects in the MoS2, we tried to anneal the CAT catalyst at above 450 °C (the sublimation temperature of MoS2) and used the treated catalysts for the reaction. Under the same reaction conditions, CAT-450 presented 100% conversion of nitrobenzene and 73.2% selectivity to N-benzyl aniline (Table 1, entry 2). Further extending reaction times to 5 h, the selectivity to N-benzyl aniline reached 85.2% (Table 1, entry 3), suggesting that the thermal treatment has a positive 7969

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Table 2. Catalytic Performance of CAT-450 in the One-Pot Reductive Amination of Nitroarenes with Aldehydes

14). Compared with other catalytic systems, such as cobalt catalyst and Pd-based catalysts, this MoS2-based catalytic system is low-cost and efficient (Table S2). More importantly, when the scale of the reaction was enlarged to 10 times under the optimized conditions, that is, 5 mmol of nitrobenzene and 7 mmol of benzaldehyde as substrates, 83.7% yield of N-benzyl aniline can be achieved with 80 mg of CAT450 under 2 MPa H2 at 120 °C in 19 h (Table 1, entry 15). This result points out that the catalytic system has the potential to be applied in practical scenarios. To understand the reaction mechanism, the products distribution at different reaction times was monitored (Figure 1b). It was found that the conversion of nitrobenzene increased steadily along with the reaction time, while the selectivity of the target product N-benzylaniline started increasing rapidly after 2 h of reaction time. The selectivity of other N containing molecules showed a trend of first an increase and then a decrease with the increasing reaction time, which is a typical characteristic of the reaction intermediate. On the basis of the reaction mechanism, the reaction between primary amine with aldehyde is a reversible reaction, while the hydrogenation of the imine and the decomposition of imine are competitive reactions. The kinetic behavior of the reactants and intermediates also indicates that the formation of imine and NC bond hydrogenation is slower than the nitro group hydrogenation. The imine hydrogenation to secondary amine is the rate-limiting step of the cascade reaction. To confirm this conclusion, control experiments starting with intermediate imine have been performed. With CAT-450 as the catalyst, 0.5 mmol of N-benzylideneaniline can be smoothly converted into N-benzyl aniline under 2 MPa H2 at 120 °C only in 0.5 h. However, CAT cannot catalyze this reaction under the same reaction conditions, and no N-benzyl aniline was detected.

influence on the yield of secondary amine product. To further confirm the effectiveness of thermal sublimation, we used sodium molybdate as the precursor and obtained another kind of CAT-Na using the same hydrothermal synthesis procedure. The CAT-Na and CAT-Na-450 catalysts exhibited a similar trend in the reaction (Table 1, entries 4 and 5), although with relatively weaker activity. These results demonstrate that partial sublimation is an efficient method to improve the catalytic activity of MoS2. If the cascade reaction starts with equal molar nitrobenzene and benzaldehyde, the selectivity to N-benzyl aniline was only 52.1%, and a large amount of aniline was left because a part of benzaldehyde reacted with ethanol to generate byproducts, such as benzaldehyde diethyl acetal (Table 1, entry 6). Therefore, excess benzaldehyde or nitrobenzene is essential to ensure the maximum yield of the target product. We further optimized the thermal sublimation condition (Table S1) with the hydrogenation of nitrobenzene as a probe reaction. The CAT catalysts treated at 400−550 °C exhibit an almost identical promotion effect for the hydrogenation of nitrobenzene. However, when the treatment was done over 600 °C, the catalysts lost the activity dramatically. The optimization of the reaction temperature and solvent (Table 1, entries 7−11) points out that a lower reaction temperature would decrease yield of target products, and ethanol is the best solvent for the cascade reaction. The CAT450 catalyst can be recycled easily, and only a little compromise of catalytic performance is observed after the fourth run (Figure 1a). However, if CAT-450 was exposed to air for a few days, the catalyst (denoted as CAT-450-D) would deactivate and lose the hydrogenation ability (Table 1, entry 12). As control experiments, commercial MoS2 donated as MoS2-com has no catalytic activity for this reaction, which is the same as with the blank reaction (Table 1, entries 13 and 7970

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Figure 2. (a) HRTEM and TEM (inset) images of CAT, edge defects in circles and surface defects in rectangles. (b) HRTEM image of CAT-450, edge defects in circles and surface defects in rectangles. (c) SEM image of CAT. (d) SEM image of CAT-450. (e) XRD patterns of MoS2-com, CAT, CAT-450, CAT-450-D, and CAT-800. (f) Raman spectra of MoS2-com, CAT, CAT-450, CAT-450-D, and CAT-800. (g) XANES spectra of MoS2-com, CAT, and CAT-450; inset, magnification of curves in the circle. (h) Fourier transform of EXAFS spectra of MoS2-com, CAT, and CAT450.

Only a small quantity of decomposed products such as aniline, benzaldehyde, and benzaldehyde diethyl acetal can be detected. Therefore, enhancing the hydrogenation rate of the imine hydrogenation will significantly promote the yield of the desired product. We further expand the reaction substrates to a serial of functional groups modified nitroarenes and benzaldehyde derivatives (Table 2). Both the electron-donating and electron-withdrawing substituents are compatible with the process, resulting in a better yield (72.0−93%) of the secondary amines. It is regular that the nitroarenes containing an electron-donating group have a higher yield than those containing an electron-withdrawing group. 3.2. Characterizations. Detailed structural characterizations have been performed to elucidate the role of

thermal-treated MoS2 in the one-pot cascade reductive amination reactions, especially in the hydrogenation of the imine intermediate. From the high-resolution TEM (HRTEM) images (Figure 2a,b), it could be observed that the CAT and CAT-450 catalysts are layered structures with generally 3−8 layers stacked together. The edge defects (in the circle) and surface defects (in the rectangle) are frequently observed compared to commercial MoS2 (Figure S1c). The TEM image of CAT shows that CAT is composed of MoS2 sheets (Figure 2a, inset). SEM images of CAT and CAT-450 also present flowerlike MoS2 sheets (Figure 2c,d). Compared with the commercial ones (Figure S1), this relatively thin layered structure makes the CAT and CAT-450 expose a more active surface. The X-ray powder diffraction (XRD) profiles of the CAT and CAT-450 catalysts (Figure 2e) suggested that both 7971

DOI: 10.1021/acscatal.9b01429 ACS Catal. 2019, 9, 7967−7975

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ACS Catalysis Table 3. Structural Parameters Obtained from the Mo K-Edge EXAFS Spectra of MoS2 Catalysts sample MoS2-com CAT AT-450

shell Mo−S Mo−Mo Mo−S Mo−Mo Mo−S Mo−Mo

bond length (Å) 2.41 3.17 2.41 3.17 2.41 3.17

coordination number 2

± ± ± ±

6 (S0 = 0.96) 6 (S02 = 0.95) 5.5 ± 0.4 4.7 ± 0.9 5.1 ± 0.4 4.3 ± 0.6

0.01 0.01 0.01 0.01

σ2

E0 shift (eV)

0.003 0.003 0.003 0.004 0.003 0.004

3.1 3.1 2.9 2.9 2.8 2.8

Figure 3. (a) Nitrogen adsorption-desorption isotherms of MoS2-com, CAT, CAT-450, and CAT-450-D. (b) Pore-size distribution of MoS2-com, CAT, CAT-450, and CAT-450-D based on BJH analysis of the adsorption isotherms.

demonstrating that there are both sulfur and molybdenum vacancies in a single MoS2 layers. In the thermal-treated CAT450 the CNMo−S and CNMo−Mo value further decreases to only 5.1 and 4.3, confirming the further increase in the density of S and Mo vacancies after the sublimation. The analysis of the sublimation substances (Figure S2) also confirms the loss of S and Mo atoms after the thermal treatment. These structural characterization results suggest that it is the variation of the S and Mo vacancies in the MoS2 single layer rather than the crystal structures of the MoS2 that is related with the hydrogenation activities. Raman spectroscopy is a powerful nondestructive manner to study crystal structure, layer thickness, and defects of MoS2. As shown in Figure 2h, two Raman-active peaks at about 380 and 407 cm−1 can be observed, representing the in-plane E12g and out-of-plane A1g vibration of hexagonal MoS2, respectively.26 The relatively larger E12g peak width and weaker intensity of CAT compared with CAT-450 and CAT-450-D suggest that the crystal structure of CAT is not perfect and can be improved by the annealing manner in a nitrogen atmosphere at 450 °C.27 The lower intensity of in-plane E12g peaks also reveals that the three samples are composed of thin MoS2 sheets with a basal-edge-rich property and defects.28 Nitrogen adsorption−desorption experiments illustrate that CAT possesses a larger surface area than commercial MoS2 (217.3 m2/g versus 4.4 m2/g) (see Figure 3). Moreover, CAT has a kind of mesoporous structure with 2.5 nm of pore size and 0.93 cm3/g of pore volume. CAT-450 exhibits a larger BET surface area, wider pore diameter, and higher total pore volume than CAT, suggesting that thermal anneal procedure create more pores or enlarge the pore size. Compared with

of the catalysts maintain the crystal structures of the 2HMoS2.23 Compared with the commercial MoS2, the slight shift of the diffraction peak of the (002) facet in the CAT and CAT450 samples suggests the increasing of d-spacing between adjacent MoS2 layers in the synthesized MoS2. Meanwhile, it is clear that the intensity ratios of (100) planes (I(100)) and (002) planes (I(002)) of the CAT and CAT-450 samples are much larger than that of the commercial molybdenum disulfide, indicating that the average number of the stacked MoS2 layers in the hydrothermal synthesized MoS2 is significantly reduced.24 The correlation of the XRD pattern with the reaction performance indicates that the edge rather than the plane of the MoS2 layers is active for the hydrogenation reaction. However, the detailed crystal structures of the CAT and CAT-450 catalysts are almost identical. To understand the reason for the significantly different reactivity of these two catalysts, the XAFS method which is more sensitive to the local environment was applied to investigate the thermal sublimation-induced structural changes. As shown in Figure 2f, compared with MoS2-com, the X-ray absorption of CAT at about 20 010 eV is lower, which maybe results from the defects or sulfur vacancies in CAT. After CAT was annealed at 450 °C in a nitrogen atmosphere, the absorption further decreased. The Fourier transform of EXAFS spectra of the three samples shows two peaks at 1.9 and 2.8 Å, respectively (Figure 2g), which correspond to the Mo−S and Mo−Mo (in the same layer) coordination shells, respectively.25 Detailed fitting results of the Mo K-edge EXAFS spectra (Table 3) revealed clearly that the coordination numbers of Mo−S (CNMo−S = 5.5) and Mo−Mo (CNMo−S = 4.7) shells are both smaller than the theoretical values, 7972

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Figure 4. (a) Full XPS spectra of CAT, CAT-450, and CAT-450-D. (b) S 2p XPS spectra of CAT, CAT-450, and CAT-450-D. (c) Mo 3d spectra of CAT, CAT-450, and CAT-450-D. (d) O 1s spectra of CAT, CAT-450, and CAT-450-D (O/Mo represents the ratio of atom number (At %) according to XPS analysis).

pot cascade reaction decreased significantly (Table S3, entry 6). In summary, the combination of multiple characterization methods demonstrates that the excellent catalytic performances of the CAT-450 catalyst are from the high density of surface defects in the MoS2 planes and the less occupancies of surface oxygen species at these defect sites. 3.3. DFT Calculations. To understand the function of the surface defects in the MoS2 on the reactivity of the one-pot cascade reductive amination of nitroarenes with aldehydes, DFT calculations were performed to evaluate the variation of surface S vacancies on the adsorption of the key reactants and intermediates of the reaction (Figure 5). On the basis of the calculation results, the adsorption energy of two H* decreases from −0.5 to −1.3 eV when the S vacancies were enlarged

CAT-450, the BET surface area and pore volume of CAT-450D decrease dramatically, resulting in the deactivation of CAT450-D. XPS spectra of CAT before and after thermal treatment are displayed in Figure 4. It is obviously observed that the content of oxygen in CAT-450 is extremely lower than that in CAT (Figure 4a,d; the O/Mo ratio 0.18 of CAT-450 versus 0.48 of CAT), suggesting that thermal treatment in the nitrogen atmosphere effectively removes the oxygen from the MoS2 catalyst surface. In the deactivated CAT-450-D catalysts, the O/Mo ratio is 0.40. The detailed XPS spectra at S 2p and Mo 3d regions reveal that the oxygen species could be either SOx or Mo(IV/VI)Ox.27,29,30 As these oxygen species mainly appear in the relatively low-activity catalysts such as CAT and CAT-450-D, it is inferred that O possibly occupies the defects of the MoS2 which tends to reduce the hydrogenation activity of the catalysts. The atomic ratios of S to Mo in CAT, CAT-450, and CAT-450-D are determined to be 1.81, 1.75, and 1.76, confirming that the treatment at 450 °C did create more sulfur vacancies. To further confirm the relationship between sulfur vacancies and catalytic performance of MoS2 catalysts, a series of MoS2 catalysts with different atomic ratios of S to Mo were prepared by processing presynthesized MoS2 in a nitrogen atmosphere at 450 °C for 30, 120, 180, and 420 s, respectively, (Table S3). It is obviously observed that the atomic ratio of S to Mo in CAT decreased from 2.00 to 1.76 (S/Mo of CAT-450 is 1.75) when the annealing time was extended. Correspondingly, the catalytic performance of these MoS2 catalysts became better along with the decrease of the atomic ratio of S to Mo (Table S3, entry 2−5). Furthermore, if the sulfur vacancies of CAT-450 were remedied by treating the CAT-450 in gaseous sulfur, the catalytic activity for the one-

Figure 5. Adsorption energies of hydrogen on MoS2 with different S vacancies (black) and adsorption energies of intermediate imine on defective MoS2 containing dissociated H* on consecutive S vacancies. 7973

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ACS Catalysis from 0 to 6 on a 7 × 7 MoS2 plane, indicating that the dissociated H* adsorbs more stably at the consecutive S vacancies (Figure 5, Figure S3). Meanwhile, the consecutive S vacancies also benefit the adsorption of the imine intermediate (Figure 5, Figure S4). This phenomenon could explain the positive effect of the S vacancies on the reaction. Meanwhile, the DFT calculation also reveals that the affinity of the nitrobenzene with the S vacancies is much larger than that of the benzaldehyde (Figure S5). As a result, the nitrobenzene reduction will be faster than benzaldehyde.

National Key R&D Program of China (2017YFB0602200). DFT calculations are supported by the High-Performance Computing Center of Hebei University. XAS measurements were performed at public XAS beamline, BL01B1, SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan) under the approval of JASRI (Proposal 2018A1429).

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(1) Magano, J.; Dunetz, J. R. Large-Scale Carbonyl Reductions in the Pharmaceutical Industry. Org. Process Res. Dev. 2012, 16 (6), 1156− 1184. (2) Seayad, A.; Ahmed, M.; Klein, H.; Jackstell, R.; Gross, T.; Beller, M. Internal Olefins to Linear Amines. Science 2002, 297 (5587), 1676−1678. (3) Abdel-Magid, A. F.; Mehrman, S. J. A Review on the Use of Sodium Triacetoxyborohydride in the Reductive Amination of Ketones and Aldehydes. Org. Process Res. Dev. 2006, 10 (5), 971− 1031. (4) Huang, M.; Li, Y.; Liu, J.; Lan, X.-B.; Liu, Y.; Zhao, C.; Ke, Z. A bifunctional strategy for N-heterocyclic carbene-stabilized iridium complex-catalyzed N-alkylation of amines with alcohols in aqueous media. Green Chem. 2019, 21 (2), 219−224. (5) Ş ahin, N.; Ö zdemir, N.; Gürbüz, N.; Ö zdemir, I.̇ Novel NAlkylbenzimidazole-Ruthenium (II) complexes: Synthesis and catalytic activity of N-alkylating reaction under solvent-free medium. Appl. Organomet. Chem. 2019, 33 (2), No. e4704. (6) Zhou, P.; Jiang, L.; Wang, F.; Deng, K.; Lv, K.; Zhang, Z. High performance of a cobalt−nitrogen complex for the reduction and reductive coupling of nitro compounds into amines and their derivatives. Sci. Adv. 2017, 3 (2), No. e1601945. (7) Zhou, P.; Yu, C.; Jiang, L.; Lv, K.; Zhang, Z. One-pot reductive amination of carbonyl compounds with nitro compounds with CO/ H2O as the hydrogen donor over non-noble cobalt catalyst. J. Catal. 2017, 352, 264−273. (8) Sreedhar, B.; Reddy, P. S.; Devi, D. K. Direct One-Pot Reductive Amination of Aldehydes with Nitroarenes in a Domino Fashion: Catalysis by Gum-Acacia-Stabilized Palladium Nanoparticles. J. Org. Chem. 2009, 74 (22), 8806−8809. (9) Chen, Y.-Z.; Zhou, Y.-X.; Wang, H.; Lu, J.; Uchida, T.; Xu, Q.; Yu, S.-H.; Jiang, H.-L. Multifunctional PdAg@MIL-101 for One-Pot Cascade Reactions: Combination of Host−Guest Cooperation and Bimetallic Synergy in Catalysis. ACS Catal. 2015, 5 (4), 2062−2069. (10) Cho, A.; Byun, S.; Kim, B. M. AuPd−Fe3O4 Nanoparticle Catalysts for Highly Selective, One-Pot Cascade Nitro-Reduction and Reductive Amination. Adv. Synth. Catal. 2018, 360 (6), 1253−1261. (11) Zhou, P.; Zhang, Z. One-pot Reductive Amination of carbonyl Compounds with Nitro Compounds by Transfer Hydrogenation over Co−Nx as catalyst. ChemSusChem 2017, 10 (9), 1892−1897. (12) Zhang, Q.; Li, S.-S.; Zhu, M.-M.; Liu, Y.-M.; He, H.-Y.; Cao, Y. Direct reductive amination of aldehydes with nitroarenes using biorenewable formic acid as a hydrogen source. Green Chem. 2016, 18 (8), 2507−2513. (13) Jagadeesh, R. V.; Stemmler, T.; Surkus, A.-E.; Junge, H.; Junge, K.; Beller, M. Hydrogenation using iron oxide−based nanocatalysts for the synthesis of amines. Nat. Protoc. 2015, 10, 548. (14) Han, S. W.; Cha, G.-B.; Park, Y.; Hong, S. C. Hydrogen physisorption based on the dissociative hydrogen chemisorption at the sulphur vacancy of MoS2 surface. Sci. Rep. 2017, 7 (1), 7152. (15) Polyakov, M.; Indris, S.; Schwamborn, S.; Mazheika, A.; Poisot, M.; Kienle, L.; Bensch, W.; Muhler, M.; Grünert, W. Mechanochemical activation of MoS2Surface properties and catalytic activities in hydrogenation and isomerization of alkenes and in H2/D2 exchange. J. Catal. 2008, 260 (2), 236−244. (16) Mao, J.; Wang, Y.; Zheng, Z.; Deng, D. The rise of twodimensional MoS2 for catalysis. Front. Phys.-BEIJING 2018, 13 (4), 138118.

4. CONCLUSIONS One-pot cascade nitro-reduction and reductive amination for synthesizing secondary amines is achieved by using defective MoS2 with sulfur vacancies. Thermal treatment of MoS2 in a nitrogen atmosphere is a key and effective strategy to improve the catalytic activity of MoS2 due to the sublimation of MoS2 and sulfur. Characterizations and control experiments demonstrate that sulfur vacancies can be created because the sublimation of sulfur is easier than MoS2. DFT calculations also confirm that a larger sulfur vacancies area on MoS2 can effectively adsorb and activate hydrogen, nitro compounds, and intermediate imines so that MoS2 with more sulfur vacancies promotes the one-pot cascade reaction to conduct smoothly. However, slow oxidation in air can deactivate the active MoS2 catalyst. It is unprecedented that one-pot reductive amination of nitroarenes with aldehydes for synthesizing secondary amines is achieved by using defective MoS2 as an effective catalyst.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.9b01429. Additional data and figures including structures, TEM images, HRTEM images, XRD patterns, and XPS spectra (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*Phone: (+86)-312-5079359. E-mail: [email protected]. *Phone: (+86)-10-62765948. E-mail: [email protected]. *Phone: (+86)-10-62758603. E-mail: [email protected]. ORCID

Yongjun Gao: 0000-0003-2059-3168 Hiroyuki Asakura: 0000-0001-6451-4738 Kentaro Teramura: 0000-0003-2916-4597 Haijun Wang: 0000-0001-8671-1601 Ding Ma: 0000-0002-3341-2998 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for the financial support of the following funders: National Natural Science Foundation of China (21773053, 21725301, 21821004, 91645115), Natural Science Foundation of Hebei Procince (B2017201084), Hebei provincial technology foundation for High-level talents (CL201601), the One Hundred Talent Project of Hebei Province (Grant E2016100015), Advanced Talents Incubation Program of Hebei University (801260201019), and the 7974

DOI: 10.1021/acscatal.9b01429 ACS Catal. 2019, 9, 7967−7975

Research Article

ACS Catalysis (17) Zhang, C.; Zhang, Z.; Wang, X.; Li, M.; Lu, J.; Si, R.; Wang, F. Transfer hydrogenation of nitroarenes to arylamines catalysed by an oxygen-implanted MoS2 catalyst. Appl. Catal., A 2016, 525, 85−93. (18) Li, Z.; Zhang, D.; Ma, J.; Wang, D.; Xie, C. Fabrication of MoS2 microflowers for hydrogenation of nitrobenzene. Mater. Lett. 2018, 213, 350−353. (19) Wang, J.; Zhang, Y.; Diao, J.; Zhang, J.; Liu, H.; Su, D. A MoS2 nanocatalyst with surface-enriched active sites for the heterogeneous transfer hydrogenation of nitroarenes. Chin. J. Catal. 2018, 39 (1), 79−87. (20) Zhang, C.; Wang, X.; Li, M.; Zhang, Z.; Wang, Y.; Si, R.; Wang, F. Chemoselective transfer hydrogenation to nitroarenes mediated by oxygen-implanted MoS2. Chin. J. Catal. 2016, 37 (9), 1569−1577. (21) Deng, J.; Li, H.; Xiao, J.; Tu, Y.; Deng, D.; Yang, H.; Tian, H.; Li, J.; Ren, P.; Bao, X. Triggering the electrocatalytic hydrogen evolution activity of the inert two-dimensional MoS2 surface via single-atom metal doping. Energy Environ. Sci. 2015, 8 (5), 1594− 1601. (22) Shahabi, D.; Tavakol, H. One-pot synthesis of quinoline derivatives using choline chloride/tin (II) chloride deep eutectic solvent as a green catalyst. J. Mol. Liq. 2016, 220, 324−328. (23) Hu, W.-H.; Han, G.-Q.; Dai, F.-N.; Liu, Y.-R.; Shang, X.; Dong, B.; Chai, Y.-M.; Liu, Y.-Q.; Liu, C.-G. Effect of pH on the growth of MoS2 (002) plane and electrocatalytic activity for HER. Int. J. Hydrogen Energy 2016, 41 (1), 294−299. (24) Yin, Z.; Zhao, J.; Wang, B.; Xu, Y.; Li, Z.; Ma, X. Insight for the effect of bridging S22- in molybdenum sulfide catalysts toward sulfurresistant methanation. Appl. Surf. Sci. 2019, 471, 670−677. (25) Zhang, H.; Lin, H.; Zheng, Y.; Hu, Y.; MacLennan, A. Understanding of the effect of synthesis temperature on the crystallization and activity of nano-MoS2 catalyst. Appl. Catal., B 2015, 165, 537−546. (26) Yan, Y.; Ge, X.; Liu, Z.; Wang, J.-Y.; Lee, J.-M.; Wang, X. Facile synthesis of low crystalline MoS2 nanosheet-coated CNTs for enhanced hydrogen evolution reaction. Nanoscale 2013, 5 (17), 7768−7771. (27) Yan, Y.; Xia, B.; Ge, X.; Liu, Z.; Wang, J.-Y.; Wang, X. Ultrathin MoS2 Nanoplates with Rich Active Sites as Highly Efficient Catalyst for Hydrogen Evolution. ACS Appl. Mater. Interfaces 2013, 5 (24), 12794−12798. (28) Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Synthesis of MoS2 and MoSe2 Films with Vertically Aligned Layers. Nano Lett. 2013, 13 (3), 1341−1347. (29) Tang, Y.-J.; Wang, Y.; Wang, X.-L.; Li, S.-L.; Huang, W.; Dong, L.-Z.; Liu, C.-H.; Li, Y.-F.; Lan, Y.-Q. Molybdenum Disulfide/ Nitrogen-Doped Reduced Graphene Oxide Nanocomposite with Enlarged Interlayer Spacing for Electrocatalytic Hydrogen Evolution. Adv. Energy Mater. 2016, 6 (12), 1600116. (30) Chu, X. S.; Li, D. O.; Green, A. A.; Wang, Q. H. Formation of MoO3 and WO3 nanoscrolls from MoS2 and WS2 with atmospheric air plasma. J. Mater. Chem. C 2017, 5 (43), 11301−11309.

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DOI: 10.1021/acscatal.9b01429 ACS Catal. 2019, 9, 7967−7975