C Catalyst Generation via

Research Article pubs.acs.org/journal/ascecg

Hydroxyl Group-Regulated Active Nano-Pd/C Catalyst Generation via in Situ Reduction of Pd(NH3)xCly/C for N‑Formylation of Amines with CO2/H2 Yujing Zhang,†,‡ Hongli Wang,† Hangkong Yuan,† and Feng Shi*,† †

State Key Laboratory for Oxo Synthesis and Selective Oxidation, Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, No.18, Tianshui Middle Road, Lanzhou 730000, China ‡ University of Chinese Academy of Sciences, No. 19A, Yuquanlu, Beijing 100049, China S Supporting Information *

ABSTRACT: The reductive N-formylation of amines using CO2 and hydrogen is a promising means of incorporating CO2 into value-added chemicals. To date, there has been a lack of heterogeneous catalyst systems that are sufficiently active and selective for N-formylation of primary amines with CO2 and H2. For the first time, we report that a highly active palladium nanoparticle supported on an hydroxyl group-functionalized carbon material has been designed for the N-formylation of aliphatic primary amines with CO2 and H2. XPS, XRD, FTRaman, and TEM characterizations revealed the adsorbing of Pd(NH3)xCly onto the carbon support during catalyst preparation, followed by in situ reduction to generate active nano-Pd particles. The catalytic activity of the Pd/C catalysts can be tuned efficiently by the hydroxyl group, which can modulate the hydrophilic/hydrophobic properties of the carbon surface, and promote the adsorption of CO2 and the amines near the Pd sites. The results described here may promote the design of an active catalyst for CO2 recycling and N-formyl amine synthesis. KEYWORDS: N-formylation, Amines, Pd/C, Catalyst

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INTRODUCTION The utilization of carbon dioxide (CO2) as a feedstock for fine chemical synthesis has attracted considerable attention due to the fact that CO2 is a cheap, renewable, abundant, and green C1 resource.1−10 In the last decades, great efforts have been devoted to explore efficient catalyst systems for CO 2 conversion. Among them, the reduction of CO2 in the presence of amines is a promising pathway for the formation of Nmethylamines11−24 or N-formamides, which are value-added fine chemicals. In particular, formamide derivatives, such as N,N-dimethylformamide (DMF), are important solvents and key intermediates in the synthesis of adhesives, pesticides, and drugs and formulation of polymers.25,26 More recent works have focused on development of new catalytic methodologies for the conversion of CO2 to formamides using hydrosilanes27−39 and borane15,40 as the reductants. However, in comparison with hydrosilanes and boranes, hydrogen is the cleanest, atom economical, and economic reductant. Furthermore, products are prone to be isolated from the reaction mixtures when hydrogen was applied. Since the initial report by Adkins and co-workers on RANEY Ni-catalyzed reductive formylation of amines with CO2 and H2,41 a series of homogeneous or heterogeneous transition-metal catalysts such as ruthenium,42−53 palladium,54,55 platinum,56 copper,57−59 iridium,60 iron,61 and others62 have been developed. © 2017 American Chemical Society

Although extensively studied, N-formylation of primary amines with CO2 and H2 remains a formidable challenge in this field. The pioneering work by Adkins et al. first reported the formylation of N-amylamine with CO2 and H2, albeit in only 45% yield.41 In 2003, Jessop et al. discovered attractive formylation of aniline with CO2 and H2 in the presence of RuCl2(PMe3)4 and a stoichiometric amount of DBU.49 In 2005, a successful formylation of 3-methoxypropylamine with H2 and supercritical CO2 was achieved in the presence of ruthenium catalysts.51 In 2015, Ding and co-workers reported an elegant homogeneous ruthenium-catalyzed N-formylation of aliphatic primary amines with CO 2 and H 2 . 53 In contrast to homogeneous catalysts, heterogeneous catalysts are easily reused and recycled. However, there have been no examples of heterogeneous catalysts46,51,55,57−60 for N-formylation of primary amines with CO2 and H2. Therefore, the discovery of a heterogeneous catalyst for N-formylation of primary amines with CO2 and H2 is highly desired. It is well known that adsorption and activation of CO2 are usually the key steps in the transformation of CO2. We envisaged that surface functionalization of the support of the Received: February 6, 2017 Revised: May 23, 2017 Published: June 1, 2017 5758

DOI: 10.1021/acssuschemeng.7b00363 ACS Sustainable Chem. Eng. 2017, 5, 5758−5765

Research Article

ACS Sustainable Chemistry & Engineering

remove the base, and a carbon material was obtained. A series of carbon materials were prepared using this method by varying the type of base. The carbon materials that were not treated with a base are denoted C-none. C-KOH, C-NaOH, C-K2CO3, C-Na2CO3, and CK3PO4 were prepared using various bases (that is, KOH, NaOH, K2CO3, Na2CO3, K3PO4, respectively) with a fixed ratio of wet RF gel to base (that is, 1:1). The oxidation process was carried out by adding 40 mL of nitric acid (14.5 M, 65% HNO3) to 4.0 g of C-KOH placed in a roundbottomed flask. The mixture was heated at 25, 50, 80, or 110 °C on a hot plate with constant stirring for 1 h, then washed with distilled water to neutral and decanted, and the solid sample was dried at 80 °C in air. The carbon materials that were not treated with nitric acid are denoted C-0. C-1, C-2, C-3, and C-4 were prepared at different temperatures (that is, 25, 50, 80, or 110 °C). Typical Procedure for Carbon-Supported Nano-Pd Catalyst Preparation. Here, 0.25 g of C-3 and 0.08 mL of H2PdCl4 (aqueous solution, 29.93 mg/mL) were added into a 10 mL aqueous solution of ammonia hydroxide (1 mol/L). After further stirring for 24 h at room temperature, the water was removed under vacuum. It was dried at 80 °C in air for 6 h and then reduced under hydrogen flow at 200 °C for 2 h. The resulting catalyst samples were denoted as Pd/C-3. Following, the catalysts ware characterized by BET surface-area analysis (BET), transmission electron microscopy (TEM), X-ray power diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), and the reaction products were characterized by nuclear magnetic resonance spectrum (NMR). XRD measurements were conducted by using an STADIP automated transmission diffractometer (STOE) equipped with an incident beam curved germanium monochromator with Cu Kα1 radiation and current of 40 Kv and 150 mA, respectively. The XRD patterns were scanned in the 2θ range of 10−80°. XPS were obtained using a VG ES-CALAB 210 instrument equipped with a dual Mg/Al anode X-ray source, a hemispherical capacitor analyzer, and a 5 keVAr+ iron gun. The electron binding energy was referenced to the C 1s peak at 284.8 eV. The background pressure in the chamber was less than 10−7 Pa. The peaks were fitted by Gaussian−Lorentzian curves after a Shirley background subtraction. For quantitative analysis, the peak area was divided by the elementspecific Scofield factor and the transmission function of the analyzer. Nitrogen adsorption−desorption isotherms were measured at 77 K by using a Quanta chrome autosorb iQ2 instrument. The pore size distribution was calculated from the desorption isotherm by using the Barrett, Joyner, and Halenda (BJH) method. Prior to measurements, the samples were degassed at 200 °C for 10 h, at a rate of 10 °C/min. TEM was carried out by using a Tecnai G2 F30 S-Twin transmission electron microscope operating at 300 kV. Single-particle EDX analysis was performed by using a Tecnai G2 F30 S-Twin field emission TEM in STEM mode. For TEM investigations, the catalysts were dispersed in ethanol by ultrasonication and deposited on carbon-coated copper grids. NMR spectra were measured by using a Bruker ARX 400 or ARX 100 spectrometer at 400 MHz (1H) and 100 MHz (13C). All spectras were recorded in CDCl3, and chemical shifts (d) are reported in ppm relative to tetramethylsilane referenced to the residual solvent peaks. Catalytic Performance Test. The catalytic activity was tested by N-formylation of piperidine as the model reaction. Here, 1.0 mmol of amine, 50 mg of catalyst, and 2 mL of methanol were added into a 100 mL autoclave equipped with a magnetic stirrer. The autoclave was sealed and exchanged with CO2 three times and reacted at 105 °C (oven temperature 130 °C) under 1 MPa CO2 and 3 MPa H2 for 24 h. Then, it was cooled to room temperature, and 10 mL ethanol was added. The reaction mixture was analyzed by GC-MS (Agilent 7890B/ 5977A). The GC yield was determined by GC-FID (Agilent 7890A) using biphenyl as the internal standard, and the isolated yields were obtained by flash column chromatography.

heterogeneous catalyst might promote the adsorption and activation of CO2. Carbon materials, including amorphous carbon, ordered mesoporous carbon, graphite/graphene (oxide), and carbon nanotubes, are widely applied in many catalytic transformations in modern organic chemistry.63−65 Noteworthy, carbon material is easy to be functionalized, which provides the possibility as a potential candidate for the preparation of functional support to promote CO2 adsorption. For example, the functionalized carbon material with a hydroxyl group, which can adsorb CO2 especially when a suitable amount of base was added, can be smoothly realized via oxidation.66−69 On the basis of our previous work,55 palladium is an active catalyst for the N-formylaton of amines with CO2 and H2. Therefore, we envisioned that hydroxyl groupfunctionalized carbon-supported nano-Pd might establish an efficient approach to realize N-formylation of primary amines with CO2 and H2. Herein, a series of carbon-supported nanoPd catalysts were prepared with a precipitation−deposition method, and the results showed that the carbon supports with higher hydroxyl group density provide better catalytic performance in the reductive N-formylation of amines with CO2 and H2 (Scheme 1). Scheme 1. N-Formylation of Amines with CO2/H2 Catalyzed by Supported Nano-Palladium Catalyst

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EXPERIMENTAL SECTION

All solvents and chemicals were obtained commercially and were used as received. Typical Procedure for Carbon Support Preparation. Carbon materials were prepared via sol−gel polymerization of resorcinol and formaldehyde with Na2CO3 as a catalyst (Figure 1). First, a wet RF gel was prepared by polymerization of resorcinol and formaldehyde at 80 °C using a hydrothermal method. Then, the wet RF gel was mixed with KOH or another base and heated at 800 °C under a nitrogen flow. Next, the carbonized sample was washed with deionized water to

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RESULTS AN DISCUSSION Catalyst Screening and Reaction Conditions Optimization. In order to find a suitable carbon material to check the

Figure 1. Illustration of Pd/C catalyst preparation. 5759


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ACS Sustainable Chemistry & Engineering Table 1. Catalyst Screeninga

entry

sample

yield (%)b

1 2 3 4 5 6

Pd/C-none Pd/C-KOH Pd/C-NaOH Pd/C−K2CO3 Pd/C−Na2CO3 Pd/C−K3PO4

27 51 34 30 36 27

Figure 2. Contents of surface oxygenated groups by Boehm titration.

a

Reaction conditions: 1.0 mmol piperidine, 50 mg Pd/C (1 wt % Pd, 0.47 mol % Pd to 1a), 2 mL methanol, 105 °C, 0.5 mmol KOH, 1/3 MPa CO2/H2, 5 h. bYield determined by gas chromatography using biphenyl as internal standard.

Table 2. Catalyst Screening

Table 4. Physicochemical Properties of Catalysts

a

different catalysts

SBET (m2/g)

Vpa (cm3/g)

Dpb (nm)

Pd/C-0 Pd/C-1 Pd/C-2 Pd/C-3 Pd/C-4

744 875 878 702 202

0.44 0.46 0.48 0.40 0.20

1.18 1.04 1.09 1.14 1.94

a

Pore volume measured at the single point of P/P0 = 0.99. bBJH desorption average pore diamete. entry

sample

yield (%)b

1 2 3 4 5

Pd/C-0 Pd/C-1 Pd/C-2 Pd/C-3 Pd/C-4

13 11 17 19 16

influence of the hydroxyl group on the catalytic performance of the Pd/C catalyst, first, a series of carbon materials were prepared with sol−gel polymerization, followed by carbonization with or without the addition of base. By applying these carbon supports, a series of Pd/C catalysts were prepared with a precipitation−deposition method. The catalyst screening was performed with N-formylation of piperidine (1a) as the model reaction. The results suggested that 27% yield was obtained with Pd/C; none was used as catalyst (Table 1, entry 1). The treatment of the carbon supports can improve the catalytic performance of Pd/C remarkably, and applying Pd/C-KOH afforded 51% yield (entry 2). Lower yields to N-formyl piperidine were obtained if Pd/C-NaOH, Pd/C-Na2CO3, Pd/ C-K2CO3, and Pd/C-K3PO4 were used as catalysts (entries 3− 6). Therefore, C-KOH (C-0) was chosen as the standard carbon to prepare carbon materials with different hydroxyl group densities via oxidation with nitric acid at different temperatures, and C-1, C-2, C-3, and C-4 were produced via C0 oxidation at 25−110 °C. Then, these carbon materials were used as supports to prepare Pd/C catalysts, which were denoted as Pd/C-0, Pd/C1, Pd/C-2, Pd/C-3, and Pd/C-4. The catalytic performance of these catalysts was tested (Table 2). Clearly, the highest catalytic performance was observed with Pd/C-3, providing the N-formylation product in 19% yield at 105 °C for 2 h. (The oven temperature is 130 °C, while 105 °C is the actual temperature measured by thermocouple). Therefore, catalyst Pd/C-3 was used for the further optimization of the reaction conditions. Then, the influence of base on the catalytic reaction was evaluated. Clearly, the results can be improved slightly if KOH was added during the reactions (Table 3, entries 1−4). As similar results were obtained when 50% or 100% KOH was added, finally, we chose 50% KOH as the additive for further improvement. Then, the influence of solvents, i.e., MeOH, EtOH, 1,4-dioxane, CH2Cl2, and H2O, and the CO2/H2 ratios were tested, and the best result was obtained when 50% KOH

a

Reaction conditions: 1.0 mmol piperidine, 50 mg Pd/C (1 mol % Pd), 2 mL methanol, 105 °C, 0.5 mmol KOH, 1/3 MPa CO2/H2, 2 h. b Yield determined by gas chromatography using biphenyl as internal standard.

Table 3. Reaction Conditions Optimization for NFormylation of Piperidinea

entry

catalyst

solvent

KOH (mmol)

CO2/H2 (Mpa)

Yield of 1b (%)b

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

Pd/C-3 Pd/C-3 Pd/C-3 Pd/C-3 Pd/C-3 Pd/C-3 Pd/C-3 Pd/C-3 Pd/C-3 Pd/C-3 Pd/C-3 Pd/C-3 Pd/C-3

methanol methanol methanol methanol methanol methanol methanol methanol C2H5OH 1,4-dioxane CH2Cl2 H2O methanol

0 0.3 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

2:3 2:3 2:3 2:3 1:3 2:2 1:2 1:4 1:3 1:3 1:3 1:3 1:3

45 55 57 60 62 32 43 57 22 17 2 0 99(93d, 82e)

a

Reaction conditions: 1.0 mmol amine, 50 mg catalyst (1 wt % Pd, 0.47 mol % Pd to 1a), 105 °C, 5 h. bDetermined by GC-FID using biphenyl as the standard material. c24 h. dIsolated yield. eCatalyst was recovered and reused at the third run. 5760


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Figure 3. (a) XRD diffraction patterns of catalyst. (b) Raman pictures of catalyst.

Figure 4. XPS spectra of fresh catalyst (left) and used three times catalyst (right).

titration analysis was performed to determine the contents of surface oxygenated groups (Figure 2). The content of the surface number of phenol groups first dropped from 208 to 119 μmol/g, then increased from 119 to 808 μmol/g (Table S1). The same trend was observed in lactone groups (Table S1). However, the content of the surface number of carboxylic acid groups continuously increased from 0 to 2599 μmol/g (Table S1). With the increase in hydroxyl group density, the yield for N-formyl piperidine was improved to 19% yield when Pd/C-3 was used as the catalyst (Table 2, entry 4). It is similar to the trend of the surface number content of phenol and lactone groups given that the lactone groups are weak for the adsorption of CO2. Therefore, we supposed that phenol groups belonging to the support influenced the catalytic activity of the catalyst. However, the yield for N-formyl piperidine was decreased to 16% if the hydroxyl group density was further increased (Table 2, entry 5). It was well known that the treatment of carbon material at elevated temperature would cause collapse of its framework. We reasoned that decreasing the catalytic activity might result from collapse of the carbon material framework. So these carbon materials were characterized by BET surfacearea analysis (BET) (Table 4). The results of BET analysis confirmed our conjecture, and the BET surface areas dropped remarkably from 702 to 878 to 202 m2/g. This might be the main reason for the decrease in the catalytic activity when it was applied as the catalyst support. The N2 adsorption/desorption isotherm exhibited a steep increase (Figure S2), signifying the presence of microporosity with a pore size distribution at 0.2− 0.3 nm. The Barret−Joyner−Halenda (BJH) pore size distribution further revealed the mean size of mesopores to be 2−3 nm. Given that the size of an amine molecule is usually larger than the microporous size, the nano-Pd particles located inside mesopores or on the support surface might play a key role in N-formylation of the amines with CO2/H2.

Figure 5. (a) TEM images of fresh catalyst. (b) HADDF-STEM images and particle size distributions of fresh catalyst. (c) TEM images of used three times catalyst. (d) HADDF-STEM images and particle size distributions of used three times catalyst.

was added with a CO2/H2 ratio of 1:3 and methanol as solvent (entries 5−12). Finally, 99% GC yield to N-formyl piperidine was achieved by prolonging the reaction time to 24 h, and the isolated yield was 93% (Table 3, entry 13). Pd(NO3)2 was used instead of H2PdCl4 to prepare catalyst Pd/C-3, and the yield of N-formyl piperidine was 88% when it was applied in the piperidine N-formylation reaction, which indicated that the anion of the Pd precursor influences the catalytic performance of the final catalyst sample less. Also, this catalyst is easily recoverable by simple filtration, and it can be used directly without further treatment. To our delight, 82% yield was maintained when it was used at the third run; thus, this catalyst exhibits nice reusability. Catalyst Characterization. In order to prove that catalytic activity was influenced by hydroxyl group density, Boehm 5761


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ACS Sustainable Chemistry & Engineering Table 5. Results of N-Formylation of Secondary Aminesa

Table 6. Results of N-Formylation of Primary Aminesa

a

Reaction conditions: 1.0 mmol amine, 50 mg catalyst (1 wt % Pd, 0.47 mol % Pd to 1), 2 mL MeOH, 105 °C, 24 h.

Scheme 2. N-Formylation of Aniline with CO2/H2

Scheme 3. Control Experiment for Mechanism Study

In order to reveal the structure of the Pd/C catalysts, they were further characterized by XRD, XPS, FT-Raman, and TEM. First, the catalysts were characterized by XRD (Figure 3a and Figure S8), and there are no observable diffraction patterns of Pd species, which suggested that the Pd species were amorphous or highly dispersed. The Pd 3d5/2 peak at 335.9 eV is attributed to Pd0 (metallic palladium), while the Pd 3d5/2

a

Reaction conditions: 1.0 mmol amine, 50 mg catalyst (1 wt % Pd, 0.47 mol % Pd to 1), 2 mL MeOH, 105 °C, 24 h. b36 h. 5762


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ACS Sustainable Chemistry & Engineering peak at 337.5 eV is related to Pd2+ (palladium oxide or Pd(NH3)xCly). According to XPS data, Pd2+ was mainly formed on the surface of the fresh catalyst, and Pd0 was almost not formed (Figure 4 and Figure S6). The same results are afforded in the XPS spectra of Pd/C-3 which was submitted to CO2 + H2 before collecting the spectra (Figure S6). The possibility of surface oxidation by air was ruled out. We wonder whether Pd2+ can be active for the reductive formylation of the amines with CO2 and H2. So, the catalyst sample used three times was characterized by XPS, and formation of Pd0 (metallic palladium) on the carbon surface was observed, which indicated in situ formation of active Pd0 species during the reaction process. In addition, XPS spectra suggested that a small amount of K cation was retained into the catalyst, and the K/C atomic ratio had no obvious impact on the reactivity as below. Clearly, there is no relationship between the catalytic results and K cation inside the catalyst sample. In order to verify the real palladium species in catalyst Pd/C-3, it was further characterized by FT-Raman (Figure 3b and Figure S10), which revealed that Pd(NH3)xCly but not Pd2+ was mainly formed on the surface of the fresh catalyst.70 The HR-TEM images shown in Figure 5 and Figure S4 confirm the observations from the XPS spectra and FT-Raman spectra. The crystal lattices of fresh catalyst were Pd(NH3)xCly, and the crystal lattices of the catalyst sample used three times was Pd0 species. The TEM images showed that 63% Pd species in Pd/ C-3 was in the range of 0−1 nm, while it was mainly 1−4 nm in other catalysts. It should be noted that 86% of Pd species dropped in the range of 0−1 nm after Pd/C-3 was used three times. All these results clearly indicate that the fresh catalysts contain Pd(NH3)xCly immobilized on the surface of functional carbon-base materials, and the active nano-Pd was generated in situ during the reaction. In addition, CO2/NH3-TPD characterizations were performed (Figures S11 and 12). Unfortunately, less difference can be observed from the results of different samples. So the correlations between catalytic activity and whole acidic/basic groups on the catalyst surfaces cannot be built. Therefore, the −OH groups might be helpful to modulate the hydrophilic/ hydrophobic properties of the carbon surface, which can favor the adsorption of CO2 and the amines near the Pd sites because no important change of the density of acid and basic sites is observed as a function of the preparation method. CO2 adsorption tests with FT-IR suggested that other peaks of CO2 adsorption were not observed in addition to background peaks (Figure S13). With the optimized conditions in hand, the scope of this Pd/ C-3 catalyzed N-formylation of different secondary amines was further investigated, and the results were summarized in Table 5. By using substituted piperidines as substrates, the reaction proceeded successfully and furnished the desired products in 65−93% yields (2a−2d). In addition, other cyclic secondary amines, such as piperazine and morpholine, were also converted into the formamides in 68−89% yields under the optimized conditions (2e−2h). Moreover, pyrrolidine and 1,2,3,4-tetrahydroisoquinoline were converted into the corresponding formamides smoothly in 81% and 83% yields, respectively. The employment of other dialkyalmines, such as Me2NH, n-Pr2NH, and Oct2NH, led to good yields, too (2j− 2l). Inspired by the results in Table 5, N-formylation of primary amines was further investigated (Table 6). It should be mentioned that primary aliphatic amines with different alkyl

chains proceeded well, affording their corresponding products in moderate yields. The steric hindrance substrate 1u was also worked to provide 2u in 74% yield. It is noteworthy that benzylamine 1w and amphetamine 1x, bearing benzene ring on the substrates, can also be converted into desired products of 2w and 2x in 91% and 92% yields, respectively. When aniline was subjected to this reductive formylation protocol (Scheme 2), 49% conversion of aniline with 27% of Nformanilide and 6% N-cyclohexylformamide selectivities was obtained. These results indicated that in order to achieve the Nformylation of the aromatic amines, we should design a catalyst with nice catalytic performance in the reduction of CO2 while being resistant to hydrogenate the aromatic rings. To obtain insights into the mechanism of the N-formylation reaction, a control experiment was performed. Reaction of pepridine with CO2 was performed in the presence of 50 mg of Pd/C-3 in methanol at 105 °C for 2 h under 10 atm CO2 without the addition of H2. After the reaction mixtures were centrifuged and washed with methanol, the catalyst was further reacted in methanol at 105 °C for 5 h under 10 atm H2 and 20 atm N2, which afforded the desired formylation product in 0.9% yield with recovery of 1.4% pepridine. These results suggest that this N-formylation first reaction of amine with CO2 in the presence of Pd/C-3 and KOH form carbamate followed by hydrogenation to form the formylation product (Scheme 3).

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CONCLUSIONS In conclusion, an active catalyst can be obtained for the Nformylation of amines with CO2 and H2 by immobilizing nanoPd onto the hydroxyl group-functionalized carbon materials. The active nano-Pd/C structure was generated in situ from Pd(NH3)xCly/C during the reaction. In the presence of this catalyst, the N-formylation of a variety of amines, especially primary aliphatic amines, can be achieved under relatively mild conditions. The hydroxyl group on the carbon surface might be helpful to modulate hydrophilic/hydrophobic properties of the carbon surface and promote the adsorption of CO2 and the amines near the Pd sites. Further development of Nformylation of aromatic amines is underway in our laboratory.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b00363.

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Characterization results of catalysts and isolated products. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Feng Shi: 0000-0001-5665-4933 Author Contributions

F. Shi supervised the research work. Y. Zhang performed the reactions. Y. Zhang, H. Wang, H. Yuan, and F. Shi analyzed the data and wrote the manuscript. Notes

The authors declare no competing financial interest. 5763


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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (21633013).

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