Three-shell Cu@Co@Ni Nanoparticles Stabilized with A Metal

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Three-shell Cu@Co@Ni Nanoparticles Stabilized with A Metal-Organic Framework for Enhanced Tandem Catalysis Jia-Lu Sun, Yu-Zhen Chen, Bang-Di Ge, Jin-Hua Li, and Guo-Ming Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18584 • Publication Date (Web): 17 Dec 2018 Downloaded from http://pubs.acs.org on December 18, 2018

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ACS Applied Materials & Interfaces

Three-shell Cu@Co@Ni Nanoparticles Stabilized with A Metal-Organic Framework for Enhanced Tandem Catalysis Jia-Lu Sun, † Yu-Zhen Chen,*,† Bang-Di Ge,† Jin-Hua Li*,† and Guo-Ming Wang† †College

of Chemistry and Chemical Engineering, Qingdao University, Qingdao,

Shandong 266071, P. R. China Supporting Information Placeholder KEYWORDS: metal-organic framework, ammonia borane, three-layered core-shell structure, cascade reaction.

ABSTRACT: Transition metal catalysts, particularly featuring triple-layered core-shell structure are very promising for practical application, while reports on their synthesis and catalytic application for a cascade reaction were very rare. In this work, tiny Cu@Co@Ni core-shell nanoparticles (~3.3 nm) containing Cu core, Co middle shell, and Ni outer shell stabilized by MOF were successfully synthesized to give quadruple-layered Cu@Co@Ni/MOF at moderate conditions. The catalyst exhibited superior catalytic performance toward the in situ hydrogenation of nitroarenes using the H2 generated from the hydrolysis of ammonia borane (NH3BH3) under mild conditions. Interestingly, the Cu@Co@Ni/MOF also performed excellent catalytic activity toward CO oxidation reaction, which outperforms those of noble metal 1 ACS Paragon Plus Environment

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catalysts. To our knowledge, this is the first report on transition metal nanoparticles with three-layered core-shell structure stabilized by MOF as a cooperative catalyst for cascade reaction and CO oxidation.

1. INTRODUCTION Aromatic amine compounds, representing all-important feedstocks in industrial field, have been widely applied in the production of dyes, polymers, pharmaceuticals, agrochemicals and flame retardant.1-4 Typically, p-chloroaniline, as an important fine chemical plays a significant role in synthetic dyes and pharmaceutical industries.1,3 The general preparation of aromatic/aliphatic amines is the reduction of their corresponding nitro derivatives with hydrogen molecule as reducing agent in the presence of noble metal-based catalysts (Pt, Pd, etc.).1,5,6 Given that H2 gas is hard to dissolve in most solvents, the hydrogenation of nitro derivatives generally conducts relatively slow. In this regard, ammonia borane (NH3BH3, AB) is believed to be an ideal candidate as hydrogen source owing to its affluent hydrogen (19.6 wt %), non-pollution, high stability, and excellent solubility in water solvents.7-12 Compared with the moderate reducing agent of alcohol, which usually needs high temperature, NH3BH3 can guarantee the good reducibility for metal ions at mild conditions.13,14 Moreover, the hydrogen in situ evolved from hydrolysis of NH3BH3 is highly dispersed in reaction solution and thus could increase the contact area with substrate, and then greatly promoting the reduction of nitro group based on catalysts. However, the progress on multifunctional catalysts that can simultaneously realize the cascade 2 ACS Paragon Plus Environment

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catalysis for dehydrogenation of NH3BH3 and reduction of nitro compounds remains a largely unexplored field.15-17 Although very few cases about noble metal-based catalysts have been reported, the high cost and limited resource greatly restricted their large-scale applications.5 Transition metal catalysts, especially multicomponent metal compounds, have attracted considerable interest in heterogeneous catalysis because of their low-cost and unique electronic structure.18-24 Trimetallic catalysts, particularly featuring triple-layered core-shell structure, may be a prominent candidate to substitute for noble metals because of the more active sites explored on their surface. Hitherto, the synthesis of above nanoparticles (NPs) usually requires multiple steps of long duration and most contains noble metals and large size (e.g., [email protected]@Ni0.48, > 10 nm).24-26 Generally, metal NPs with smaller sizes have higher catalytic activities. Therefore, it would be more desirable and challenging to develop triple-layered transitional metal core-shell NPs with tiny sizes (< 5 nm). In this aspect, choosing suitable porous material as support is crucial for the synthesis of ultrafine NPs. Metal-organic

frameworks

(MOFs),27-34

characterized

large

surface

area,

exceptionally high porosity and fine-tunable structure, undoubtedly are ideal hosts to stabilize metals for prolonged catalysis.35-52 However, the reduction of some transition metal ions (e.g., Co2+, Ni2+) with low electrode potential usually requires multi extreme conditions, such as NaBH4 or H2 reduction under high temperature.18-24 Actually, these harsh conditions often cause the collapse of porous MOF framework and the aggregation of metal NPs. Luckily, Cu ion with high reduction potentials 3 ACS Paragon Plus Environment


could be readily reduced by a peaceable reducing agent, NH3BH3. The generating copper hydride (Cu-H) and succedent metal hydride (M-H) with strong reducibility can continuously reduce other M2+ salts during the reaction process.25

Cu2+ Co2+ Ni2+

DSA

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R

R

NO2

Cu@Co@Ni NPs

NH2

Yield > 99% in situ H2 H2

Figure

1.

Schematic

illustration

showing

the

in

situ

preparation

of

Cu@Co@Ni/MIL-101 for cascade catalysis involving dehydrogenation of NH3BH3 and subsequent hydrogenation of nitroarenes. Remembering the above points, a mixed Cu, Co and Ni precursor was rationally introduced into a hydrophilic MOF, through a double solvent approach (DSA).42 The metal ions were sequentially in situ reduced with NH3BH3 to afford Cu@Co@Ni NPs with average size of ~3.3 nm mainly located inside MIL-101 pores. The obtained Cu@Co@Ni/MOF exhibits superior catalytic performance in cascade reaction of NH3BH3 dehydrogenation and nitroarene hydrogenation than most precious metal catalysts (Figure 1). To our knowledge, this is the first example of MOF stabilized triple-layered transitional metal core-shell NPs catalyst for the tandem catalysis. 4 ACS Paragon Plus Environment

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2. EXPERIMENTAL SECTION 2.1 Materials and Instrumentation. Etanol (>99%), cobalt nitrate hexahydrate (Co(NO3)2∙6H2O, 99%), nickel nitrate hexahydrate (Ni(NO3)2∙6H2O, 99%), copper nitrate hexahydrate (CuCl2∙2H2O, 99%), zinc(II) nitrate tetrahydrate (Zn(NO3)2·4H2O, 98%), methanol (>99%), terephthalic acid (>99.0%), 2-methylimidazole (>98.0%), polyvinylpyrrolidone (PVP, MW = 55000) active carbon (AC) and zeolite (Na2O·Al2O3·xSiO2·yH2O) were from Sinopharm Chemical Reagent Co., Ltd. X-ray photoelectron spectroscopy (XPS) measurements were performed with an ESCALAB 250 high-performance electron spectrometer using monochromatized AlKa (hν = 1486.7 eV) as the excitation source. Power X-ray diffraction (PXRD) were measured on a Rigaku Saturn 70 diffractometer at 113 K with Mo-Kα radiation (λ = 0.71073 A). Nitrogen sorption measurement was conducted using a Micromeritics ASAP 2020 system at 77 K. The transmission electron microscopy (TEM) was acquired on JEOL-2010 with an electron acceleration energy of 200 kV. The reaction progress were monitored by a gas chromatography (GC, Shimadzu 2010 Plus). The contents of Cu, Co and Ni species in the samples were analyzed by an Optima 7300 DV inductively coupled plasma atomic emission spectrometer (ICP-AES). Solid-state 1H NMR spectra were recorded on a Agilent DD2 (400 MHz). 500 mg of Cu2+/Co2+/Ni2+@MIL-101 and MIL-101 were placed into a mixture solvent of water/methanol (v:v = 1:1, 50 mL) involving NH3BH3 (60 mg) or NaBH4 (0.6 M), respectively. After reaction for 5 min, the obtained Cu@Co@Ni/MIL-101 and MIL-101 were seperated from solution by 5 ACS Paragon Plus Environment


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centrifugation and the solvent inside MOF pores was subsequently evaporated in hydrogen atmosphere for 5 min at room temperature. Then, the samples were immediately measured with solid-state 1H NMR. 2.2 Preparation of catalysts. 2.2.1 Preparation of MIL-101: Typically, a mixture of Cr(NO3)3∙9H2O (2 g), aqueous HF (0.14 mL) and terephthalic acid (0.83 g) in the presence of de-ionized water (24 mL) was placed into an autoclave and reacted at 200 °C for 8 h.47 The obtained green powder of MIL-101 was first purified in water for 24 h followed by in ethanol for twice at reflux temperature, and was subsequent purified by NH4F solution. The MIL-101 was finally dried for 24 h at 60 °C under vacuum prior to the further use. 2.2.2 Preparation of ZIF-8: Typically, a mixture of 2-methylimidazole (3.70 g) with 80 mL methanol was added to the another 80 mL of methanol involving 1.68 g of Zn(NO3)2∙6H2O under stirring for 24 h.54 The white product was separated from solution and washed with methanol, and finally dried for 12 h in vacuo at 200 oC. 2.2.3

Preparation

of

Cu@Co@Ni/MIL-101,

Cu/MIL-101,

Cu@Co/MIL-101,

Cu@Co@Ni/PVP,

Cu@Ni/MIL-101,

Cu@Co@Ni/AC

and

Cu@Co@Ni/zeolite: A certain amount of Cu2+@MIL-101, Cu2+/Co2+@MIL-101, Cu2+/Ni2+@MIL-101, and Cu2+/Co2+/Ni2+@MIL-101 were successfully synthesized according to the reported procedures by double solvent approach (DSA).56 These obtained precursors or Cu2+/Co2+/Ni2+/PVP or Cu2+/Co2+/Ni2+/AC or Cu2+/Co2+/Ni2+/ 6 ACS Paragon Plus Environment

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zeolite were placed into the water solution (20 mL) involving NH3BH3 (15 mg). During the reduce process, the Cu ions would be first reduced to form Cu NPs as cores and the intermediate Cu-H species would promote the subsequent reduction of Co2+ prior to Ni2+ for the sedimentation of Co middle shell NPs. Similarly, the generated Co-H species would be act as strong reducing agent for the reduction of Ni2+ to give Ni outer shell NPs. Finally, the Cu@Co@Ni/MIL-101 catalyst would be obtained during the hydrolysis of ammonia borane. 2.2.4

Preparation

of

Co/MIL-101,

Ni/MIL-101,

CuCoNi/MIL-101

and

CuCoNi/ZIF-8 reduced by NaBH4 for activity comparison: The aqueous solution Co(NO3)2∙6H2O (0.094 mol/L), Ni(NO3)2∙6H2O (0.094 mol/L), and/or CuCl2∙2H2O (0.01748 mol/L) with desired volume of solution and 20 mL of pure water were added into a flask with 50 mg of MIL-101 or ZIF-8. The fresh NaBH4 (0.6 M, 5 mL) solution was charged into above flask with consequent stirring to afford Co/MIL-101, Ni/MIL-101, CuCoNi/MIL-101 or CuCoNi/ZIF-8. 2.3 Catalytic Activity Evaluation. 2.3.1 Catalytic performance evaluation for Cu@Co@Ni/MIL-101 catalyst toward the dehydrogenation reaction of ammonia borane only: Typically, metal precursors such as Cu2+/Co2+/Ni2+@MIL-101 with moderate concentration and 20 mL of pure water were placed in a round-bottomed flask (25 mL). The reduction initiated when 15 mg of NH3BH3 was put into above flask. For the catalytic durability experiments, the same amount of NH3BH3 (15 mg) was added to induce the catalytic reaction. For comparison, catalyst Cu/MIL-101, Cu@Co/MIL-101 (Cu/Co molar ratio 7 ACS Paragon Plus Environment


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of 0.33/0.67), Cu@Ni/MIL-101 (Cu/Ni molar ratio of 0.33/0.67), Ni/MIL-101, Co/MIL-101, Cu@Co@Ni/PVP, Cu@Co@Ni/AC or CuCoNi/MIL-101 (Cu/Co/Ni molar ratio of 0.33/0.33/0.33) with the same total metal percentage of 0.5 wt%, was investigated for the catalysis and all other conditions were kept unchanged. 2.3.2 Catalytic performance evaluation for Cu@Co@Ni/MIL-101 catalyst toward the cascade reaction: In general, a mixture of metal precursor Cu2+/Co2+/Ni2+@MIL-101 (Cu/Co/Ni molar ratio of 0.33/0.33/0.33, total metal percentage of 0.5 wt%) and nitroarene was highly dispersed in a mixture solvent of water and methanol (v:v = 1:1, 20 mL) placed in round-bottomed flask (25 mL). The reaction started when NH3BH3 (15 mg) was added into the flask. Catalytic conversion of nitroarene hydrogenation reaction was tracking and identified by gas chromatography. For the catalytic recycling experiments, the same amount of NH3BH3 (15 mg) and nitroarene (0.1 mmol) were added into the reaction system to induce the reaction under magnic stirring. 2.3.3 Catalytic performance evaluation for Cu@Co@Ni/MIL-101 catalyst toward the CO oxidation reaction: The 25 mg of Cu@Co@Ni/MIL-101 catalyst was placed in a quartz tube reactor. The mixed reaction gas (1% CO and 99% dry air) was fed at a flow rate of 30 mL/min. The catalysis system was heated to the desired reaction temperatures at a heating rate of 2 °C/min and then kept for 30 min to reach a steady state. Meanwhile, the composition of the eﬄuent gas was on-line monitored

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with GC-14C gas chromatograph. The CO conversion was calculated from the change of CO concentrations in the inlet and outlet gases. 3. RESULTS AND DISCUSSION The

mesoporous

MOF,

Cr(III)-MIL-101

with

molecular

formula

of

Cr3F(H2O)2O[(O2C)C6H4(CO2)]3·nH2O (n ≈ 25), was selected as a host material to confine metal NPs, owing to its high surface area, two giant cavities of 2.9 and 3.4 nm, and excellent stability in alcohol/water.53 A certain volume of aqueous solution containing Cu, Co and Ni ions was dropwise into the suspended MIL-101 dispersed in the hexane. The doohickey in the generation of triple-layered structure mainly depend on their different reduction potentials (ϕθCu(II)/Cu(0) = +0.34 eV vs. SHE; ϕθCo(II)/Co(0) = -0.282 eV vs. SHE; ϕθNi(II)/Ni(0) = -0.257 eV vs. SHE) and the weak reducing agent. The Cu2+ was firstly reduced in presence of NH3BH3 and the generated Cu core was active for promoting the catalytic hydrolysis of AB. Co2+ would be subsequently reduced to Co NPs prior to Ni2+ by Cu-H species, might forming the intermediate Co-H species, which further promotes the reduction of Ni2+ to give final Cu@Co@Ni NPs loaded into MIL-101. The similar reduction mechanisms have been proposed by previous works for various synthesis of core-shell NPs with NH3BH3 as reducing agent, such as Cu@Co,19 Ag@Co@Ni24 and Au@Co@Fe26 NPs. It’s challenging to detect these M-H species because of their transient property. For proving the presence of M-H species, we tried to investigate the states of H in Cu@Co@Ni/MIL-101 by solid-state

H

NMR

measurements.

In

the

spectra

of

MIL-101

and 9

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Cu@Co@Ni/MIL-101, sharp peaks at around 4.1 ppm were observed at the same position as for free H2 gas. A broad peak at lower field of ~6.5 ppm in the spectrum of Cu@Co@Ni/MIL-101 may be attributed to metal hydride (M-H) when hydrogen are concentrated on copper, cobalt or nickel surface (Figure S1).55 The evolution of solution color can be visibly monitored during the preparation progress for Cu@Co@Ni/MIL-101 (Figure S2). The powder X-ray diffraction (PXRD) patterns show that the crystallinity of MIL-101 structure is preserved very well after loading Cu@Co@Ni NPs and no identifiable peaks for metal NPs, implying the small metal NPs and the structure integrity of MOF framework and (Figure 2a). As shown in Figure 2b, the Brunauer-Emmett-Teller (BET) surface areas of as-synthesized MIL-101 and Cu@Co@Ni/MIL-101 are 3425 and 2148 m2/g, respectively, from the N2 sorption isotherms. The slight decrease in the N2 sorption amount indicated that most of Cu@Co@Ni NPs were deposited inside the cavities of MIL-101. Inductively coupled plasma atomic emission spectrometry (ICP-AES) has confirmed that the actual contents of Cu, Co and Ni and the Cu/Co/Ni ratios are close to the nominal values (Table S1). X-ray photoelectron spectroscopy (XPS) was applied to analyze the element and valence of metal NPs. The XPS result shows the clear 2p3/2 peaks emerging at 933.7, 778.5 and 853.5 eV can be assigned to zero-valence copper, cobalt and nickel in Cu@Co@Ni/MOF (5 wt%), respectively (Figure S3), indicating that almost all of the metal ions were successfully reduced by NH3BH3.


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Figure 2. (a) PXRD data of as-synthesized catalysts and simulated MIL-101. (b) N2 sorption

isotherms

of

as-synthesized

MIL-101,

Cu@Co@Ni/MIL-101

and

CuCoNi/MIL-101 at 77 K. (c) TEM image of Cu@Co@Ni/MIL-101 (inset: the size distribution for Cu@Co@Ni NPs) and (d) the corresponding HAADF-STEM images for several relatively larger metal nanoparticles. The transmission electron microscopy (TEM) image for Cu@Co@Ni/MIL-101 indicates the uniform dispersion of Cu@Co@Ni NPs and the average particle size is ~3.3 nm (Cu/Co/Ni molar ratio of 0.33/0.33/0.33 as representative catalyst) (Figure 2c). The direct reduction for the Cu2+/Co2+/Ni2+@MIL-101 by strong reducing agent of NaBH4 yielded CuCoNi alloy NPs with larger sizes of ca. ~7.4 nm due to a partly collapsed MIL-101 structure (Figure S4). It’s not hard to understand that most NPs with small sizes mainly escape into the MOF pores, while some metal NPs with larger 11 ACS Paragon Plus Environment


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sizes are located on the surface of MOF during the reduction. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) has been measured. As shown in Figure 2d, the core-shell nanostructure with a brightest core coated by a brighter inner shell and darker outer shell for each particle is faintly observable. To gain further core-shell structural feature of metal NPs, the Cu@Co@Ni NPs with larger sizes of ca. ~12 nm have been also synthesized by increasing the metal percentage to 5 wt% in Cu@Co@Ni/MIL-101. The triple-layered core–shell structure of Cu@Co@Ni NPs could be observed by TEM and the corresponding elemental mapping characterization results (Figure S5, Figure 3a). The EDS spectra of three line scan for the Cu@Co@Ni NP in Figure 3a unambiguously demonstrate the Cu core, Co inner shell and Ni outer shell (Figure 3b).


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Figure 3. (a) TEM image of Cu@Co@Ni/MIL-101 and the corresponding element mapping of Cu (blue), Co (red) and Ni (green) for Cu@Co@Ni NP. (b) STEM-EDS line-scan EDS spectra across a single Cu@Co@Ni NP. To evaluate the catalytic performance of triple-layered Cu@Co@Ni NPs, the reaction of NH3BH3 hydrolysis has been investigated. The reduction of metal ions was initiated along with the hydrolysis dehydrogenation by adding NH3BH3 into water solution containing Cu2+/Co2+/Ni2+@MIL-101. Figure 4a shows the H2 generation from aqueous NH3BH3 based on different catalysts with the same metal percentage (0.5 wt%) at environment temperature (20 oC). In comparison with monometallic Cu, Co and Ni, bimetallic Cu@Co and Cu@Ni, as well as CuCoNi alloy NPs, Cu@Co@Ni achieves the highest activity and releases ∼100% H2 in ca. 14 min. In order to better compare the catalytic performance of alloy and core-shell NPs, we select ZIF-8, a stable MOF under alkaline conditions, as support for CuCoNi NPs. To exclude the influence of size effect, the size of CuCoNi NPs in CuCoNi/ZIF-8 was adjusted to close to that of CuCoNi NPs in Cu@Co@Ni/MIL-101 with the same metal percentage (0.5 wt%) (Figure S6). The reaction of NH3BH3 hydrolysis over CuCoNi/ZIF-8 achieves complete conversion need at least 30 min, which is far slower than that of Cu@Co@Ni/MIL-101 (only need 14 min). This suggested that the increasing number of active sites provided by the triple-layered core-shell structure and the synergetic interaction among tri-metals may contribute significantly to the dehydrogenation of NH3BH3. In addition, the active tests over Cu@Co@Ni/MOF with various Cu/Co/Ni molar ratios indicate that the catalyst with molar ratio of 13 ACS Paragon Plus Environment


0.33/0.33/0.33 is the most active (Figure 4b). Interestingly, from the plots of the H2 generation versus reaction time during NH3BH3 dehydrogenation, an abduction time was observed before the reaction starting, which is mainly dependent on the content of Cu2+ under the identical reaction conditions. A lower concentration of Cu2+ leads to a prolonged induction period because of the slow produce of Cu-H reductive species due to the trace amounts of Cu precursor. In addition, the hydrolysis of AB over Cu@Co@Ni/MOF (Cu/Co/Ni molar ratio is 0.33/0.33/0.33) was investigated in the range of 20-40 oC (Figure S7). Evidently, the catalyst completed the reaction within 10 min at 25 oC and 5.5 min at 30 oC, corresponding to turnover frequency (TOF) values of 31 and 56 molH2·molcat.−1·min−1, respectively, which are well above those of transitional metal or noble metal-based catalysts,57-60 such as CuCo/MIL-101 (19.6 molH2·molcat.-1·min-1, 25 oC),58 Pd@Co (51 molH2·molcat.-1·min-1, 30 oC),59 and [email protected]@Ni0.48 (2481 mL·min-1·g-1, 25 oC)24 (Table S2). These results indicate the superior catalytic performance of Cu@Co@Ni/MOF.

a 35

b 35

Cu@Co@Ni/MOF Cu@Co/MOF Cu@Ni/MOF Cu/MOF CuCoNi/MOF

30 25

Hydrogen Evolution (mL)

Hydrogen Evolution (mL)

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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20 15 10 5

Ni/MIL-101 (NaBH4)

0

Co 0

10

20

30 T (min)

2+

or Ni 40

2+

or Co/MOF 50

60

30 25 20

Cu@Co@Ni/MOF

15

Cu:Co:Ni

10

0.8:0.1:0.1 0.33:0.33:0.33 0.1:0.45:0.45 0.06:0.47:0.47

5 0 0

5

10

15

20

25

30

35

40

45

T (min)

Figure 4. Plots of time versus volume of hydrogen produced from the NH3BH3 hydrolysis over (a) Cu@Co@Ni/MOF and CuCoNi/MOF (Cu/Co/Ni molar ratio of 0.33/0.33/0.33), Cu@Co/MOF (Cu/Co molar ratio of 0.33/0.67), Cu@Ni/MOF 14 ACS Paragon Plus Environment

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(Cu/Ni molar ratio of 0.33/0.67), Cu/MOF, Co/MOF (reduced with NaBH4), Ni/MOF (reduced with NaBH4) and Co2+/Ni2+@MOF. (b) Cu@Co@Ni/MOF with various Cu/Co/Ni

molar

ratios.

Reaction

conditions:

15

mg

NH3BH3,

50

mg

Cu2+/Co2+/Ni2+@MOF, 20 mL H2O, 20 oC. The total metal percentage is fixed at 0.5 wt% for all catalysts. Noteworthily, in addition to the representative Cu@Co@Ni/MOF with metal percentage of 0.5 wt%, the catalyst with other metal percentages of 1% and 2% have been also synthesized, and the reaction rate increased evidently with the increased total metal percentages (Figure S8). For comparison, PVP, zeolite and AC-protected Cu@Co@Ni NPs were prepared at the same conditions, which exhibited incomplete dehydrogenation of NH3BH3 (Figure S6) and the Cu@Co@Ni/zeolite performs poor recyclability (Figure S9a), possibly because of the lack of confinement effects for the Cu@Co@Ni NPs in zeolite (Figure S9b,c). The color of the solution gradually changed from colorless to brown to final black during the synthesis progress of Cu@Co@Ni/PVP, which may be due to the sequential reduction of Cu2+, Co2+ and Ni2+ by NH3BH3 (Figure S10). Encouraged by the extraordinary catalytic activity, recyclability and triple-layered structure of multifunctional Cu@Co@Ni/MOF catalyst in the hydrolysis of AB, the cascade reaction of NH3BH3 hydrolysis and subsequent nitroarene hydrogenation has been further investigated. The reduction of nitroarene was initiated along with the dehydrogenation process by adding NH3BH3 into a mixture solvent of H2O and methanol (v:v = 1:1) containing Cu2+/Co2+/Ni2+@MIL-101 and nitroarene. As 15 ACS Paragon Plus Environment


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expected, the nitroarenes were rapidly reduced to give target product of aniline within 5 min taking advantage of the hydrogen generated in situ from NH3BH3 hydrolysis (Table 1, entry 1). For practical application, we proceeded to investigate the catalytic scope of this reaction. Diverse substituted aromatic nitroarenes have been with electron-donating (-CH3, -NH2, -OH) and electron-withdrawing (-Cl, -Br) substituents were completely reduced with excellent yields (> 99%) within 5 min at environment temperature (20 oC) (entries 2-8). The m-nitroacetophenone was completely reduced in a slightly longer time (6 min, entry 9), while the aliphatic nitroarenes were rapidly reduced within 4 min at identical reaction conditions (entries 10,11). For comparison research, the catalytic conversion was as slow as ~16% after 5 h (entry 12) when the NH3BH3 was replaced by external hydrogen gas as reducing agent, while other conditions were kept invariant. The lower yield may be due to the weak molecular contact between substrate and hydrogen. In the absence of catalysts, no any products have been detected, which demonstrates NH3BH3 couldn’t directly reduce nitroarene (entry 13). The above catalysis results reflect that, when coupling with NH3BH3 hydrolysis,

Cu@Co@Ni/MOF

exhibited

superior

catalytic

activity

and

chemoselectivity for the reduction of different nitroarenes to the corresponding anilines.

Table 1. Cascade reactions of NH3BH3 dehydrogenation and nitroarene hydrogenation over Cu@Co@Ni/MOF with the Cu/Co/Ni molar ratio of 0.33/0.33/0.33.a 16 ACS Paragon Plus Environment

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R

Entry

Substrate

1 2 3

NO2

NH3BH3,Cu@Co@Ni/MOF R

Product NO2

H3C

NO2

H3C

NO2

NH2

NH2

H3C

H3C

NH2

Yield b

Time

>99 %

5 min

>99 %

5 min

>99 %

5 min

>99 %

5 min

4

NH2

NO2

H2N

5

Br

NO2

Br

NH2

>99 %

5 min

6

Cl

Cl

NH2

>99 %

5 min

7

HO

HO

NH2

>99 %

5 min

>99 %

5 min

>99 %

6 min

NH2

>99 %

4 min

NH2

>99 %

4 min

NH2

16 %

5h

-

-

NO2

NO2

8

NH2

Cl

Cl O

11

NH2

NO2

9

10

NH2

NO2

O

a

NH2

MeOH/H2O (v:v=1:1), 20 oC

NO2

NO2

12c

NO2

13d

NO2

-

Reaction

conditions: 0.1 mmol nitroarene, 15 mg NH3BH3, 50 mg (Cu/Co/Ni, 0.5 wt%), 10 mL MeOH, 10 mL H2O, 20 oC. b Catalytic yield was identified by gas chromatography. c NH3BH3 was replaced by hydrogen gas (1 bar). d Without any catalysts. Cu2+/Co2+/Ni2+@MOF



Furthermore, the stability and recyclability of Cu@Co@Ni/MOF has been investigated for practical application. Remarkably, it can be found that the catalytic performance and framework integrity of Cu@Co@Ni/MOF is well retained even after five consecutive usages for the cascade reaction without any dispose of the catalyst (Figure 5a, Figure S11). Strikingly, the sizes of Cu@Co@Ni NPs have well retained even after several runs by TEM observation, clearly suggesting its great recyclability, longevity and good confinement effect of MOF (Figure 5b). 100

a

b

80

Yield (%)

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 31

60 40 20 0

20 nm 1

2

3

Cycles

4

1

2

3 4 5 Diameter (nm)

6

5

Figure 5. (a) Catalytic recyclability of Cu@Co@Ni/MIL-101 toward the cascade reaction of the hydrolysis of NH3BH3 and hydrogenation of nitroarenes. Reaction conditions: 0.1 mmol nitrobenzene, 15 mg NH3BH3, 50 mg Cu2+/Co2+/Ni2+@MOF (Cu/Co/Ni, 0.5 wt%), 10 mL MeOH, 10 mL H2O, 20 oC. (b) TEM image for Cu@Co@Ni NPs in Cu@Co@Ni/MOF catalyst after five catalytic runs (inset: the corresponding size distribution for Cu@Co@Ni NPs). In order to demonstrate the excellent catalytic performance of Cu@Co@Ni/MOF catalyst, we investigated the CO oxidation reaction as the new application of this core-shell structure. Interestingly, the Cu@Co@Ni/MOF (0.5 wt%) exhibited superior 18 ACS Paragon Plus Environment

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catalytic activity toward CO oxidation and the temperatures for 100% conversion is approximately 180 oC (Figure S12), which is lower than 220 oC of noble metal catalyst, 2 wt% Au/ZIF-8,61 at similar reaction conditions. The above experimental results indicated that the Cu@Co@Ni/MOF catalyst exhibits widespread catalytic applications including NH3BH3 dehydrogenation, reduction and oxidation reactions. 4. CONCLUSION In summary, the pure transition metal catalysts with triple-layered core-shell structure stabilized by MOF have been design synthesized by rational incorporation of Cu2+/Co2+/Ni2+ ions inside the pores of MIL-101 via DSA followed by in situ reduction with NH3BH3. The resultant Cu@Co@Ni/MOF exhibits a superior catalytic performance for the tandem catalysis of the catalytic reduction of nitroarenes by the hydrogen from NH3BH3 hydrolysis and CO oxidation evaluation. The excellent activity for the nitroarene reduction can be attributed to the more exposed active sites provided by triple-layered core-shell structure and their synergetic interaction as well as the hydrogen, in situ generated from NH3BH3. Various nito compounds with different substituents can be completely reduced to the corresponding anlines within 5 min with >99% selectivity. Moreover, the five consecutive runs of the cascade reaction over Cu@Co@Ni/MOF catalyst unambiguously demonstrated its great recyclability and stability. Significantly, different with the previous reduction system of nitroarenes, current tandem pattern does not require stored hydrogen gas, which provides much efficiency and safety. We anticipate the obtained cost-effective



Page 20 of 31

catalyst for relative cascade reactions would open an new avenue for the improvement of transition metal catalysts in industrial field. ASSOCIATED CONTENT Supporting Information The following file is available free of charge via the Internet at http://pubs.acs.org. Experimental section; 1H NMR spectra, TEM images; XRD patterns; XPS spectra; ICP-AES analysis; CO oxidation; Table S1. AUTHOR INFORMATION Corresponding Author E-mail: [email protected] (G.-M. W.) *E-mail: [email protected] (J.-H. L.); [email protected] (Y.-Z. C.) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work is supported by the NSFC (21701093, 21601101, 21571111), the NSF of Shandong Province (ZR2017BB044), China Postdoctoral Science Foundation (2018T110664, 2017M622127) and Applied Basic Research Plan of Qingdao (17-1-1-31-jch).


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Table of Contents

In this work, tiny Cu@Co@Ni core-shell nanoparticles (~3.3 nm) containing copper core, cobalt middle shell, and nickel outer shell stabilized by MOF were successfully synthesized to give quadruple-layered Cu@Co@Ni/MOF at moderate conditions. The catalyst exhibited excellent catalytic performance toward the in situ hydrogenation of nitroarenes using the hydrogen generated from the hydrolysis of ammonia borane under mild conditions. Cu2+ Co2+ Ni2+ Cu@Co@Ni NPs

DSA

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


R NH2

R

in situ H2 H2

NO2

Yield > 99%

NH3BH3


Three-shell Cu@Co@Ni Nanoparticles Stabilized with A Metal

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