High Catalytic Performance of a CeO2-Supported Ni Catalyst for

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High Catalytic Performance of CeO2-Supported Ni Catalyst for Hydrogenation of Nitroarenes Fabricated via Coordination-Assisted Strategy Wei She, TianQinJi Qi, Mengxing Cui, Peng-Fei Yan, Ng Seik Weng, Weizuo Li, and Guang-Ming Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 Apr 2018 Downloaded from http://pubs.acs.org on April 11, 2018

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ACS Applied Materials & Interfaces

High Catalytic Performance of CeO2-Supported Ni Catalyst for Hydrogenation of Nitroarenes Fabricated via Coordination-Assisted Strategy ‡

Wei She,† Tianqinji Qi,§ Mengxing Cui,§ Pengfei Yan,† Seik Weng Ng, Weizuo Li,*† and Guangming Li*† †

Key Laboratory of Functional Inorganic Material Chemistry (MOE); School of Chemistry and

Materials Science, Heilongjiang University, Harbin, 150080, Heilongjiang, China. §

State Key Laboratory of Fine Chemicals, Department of Catalysis Chemistry and Engineering,

School of Chemical Engineering, Dalian University of Technology, Dalian 116024, Liaoning, China. ‡

Institute of Postgraduate Studies and Research, University of Malaya, 50603, Kuala Lumpur,

Malaysia.

KEYWORDS: two-dimensional lanthanide complexes, uncoordinated groups, oxygen vacancies, strong metal-support interaction, hydrogenation of nitrobenzene

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ABSTRACT A family of two-dimensional salen type lanthanide complexes were synthesized through facile solution-diffusion (SD) method. The two-dimensional lanthanide complexes were characterized by single-crystal X-ray diffraction (SCXRD) and X-ray photoelectron spectroscopy (XPS) analytical techniques. The SCXRD and XPS analyses reveal that the obtained two-dimensional structures are rich in uncoordinated imine (-CH=N-) groups located on the skeleton of salen type organic ligand, which remains strong coordinated ability with metal ions. On the basis of this unique-feature, a highly strong metal-support interaction (SMSI) and dispersed CeO2 supported Ni catalyst (Ni/CeO2-CAS) was firstly synthesized via a coordination-assisted synthetic (CAS) method, which exhibits a much better catalytic activity in the hydrogenation of nitrobenzene than the traditional Ni/CeO2-IWI catalyst prepared by incipient wetness impregnation. The origin of improved catalytic activity of Ni/CeO2-CAS as well as the role of Ni@Ce-H2salen is revealed by using the diverse characterizations. Based on the comparative characterization results, the superior catalytic performance of Ni/CeO2-CAS to Ni/CeO2-IWI could have resulted from the smaller and highly dispersed Ni nanoparticulates, the intensified Ni-CeO2 interaction, the enhanced NiO reducibility, as well as the higher concentration of oxygen vacancy, favoring the H2 dissociation and adsorption of nitrobenzene reactant. The Ni/CeO2-CAS catalyst also exhibits high catalytic performance for reduction of diverse nitroarenes to their corresponding functionalized arylamines. We anticipated that this coordination-assisted strategy may provide a new way for preparing other highly oxide-supported catalysts with potential applications in various catalytic reactions. 1. INTRODUCTION

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Arylamines, widely used as ubiquitous intermediates for the production of various fine chemicals, and pharmaceuticals,1-10 are generally prepared by chemoselective reduction of nitroarenes. Among the numerous metal catalysts, transition-metal (Fe, Co, Ni, etc) catalysts have attracted increasing interest in hydrogenation reactions in comparison to precious catalysts because of their low cost and availability in industrial applications.11-20 Among them, the supported metal catalysts, especially for Ni-based catalysts, exhibit outstanding catalytic performance in hydrogenation of nitroarenes ascribed to their metal-support interfacial structure.21-28 Generally, incipient wetness impregnation (IWI) has been a common method to prepare the supported Ni catalysts. However, the non-selective Ni particles on the surface of supports through this process are inevitable. These supported Ni catalysts are prone to irreversible suffer from severe sintering problems, via either particles migration-coalescence or Ostwald ripening processes. Hence, the formation of Ni nanoparticles (NPs) with a low dispersity and weak metal-support interaction, thereby hinder their long term use.29,30 It is demonstrated that the highly dispersed with SMSI supported Ni-based catalysts play essential roles on catalytic activity for hydrogenation reaction, which are favourable for adsorption of the nitroarenes and dissociation of H2 molecules, as well as acceleration of electron transfer between metal and support.31 Therefore, numerous impregnated strategies were proposed to improve catalytic performance of supported Ni catalysts including as atomic layer deposition (ALD),32,33 microwave-assisted synthesis,34 and so on.35,36 However, further improvements of Ni dispersion and metal-support interaction for supported Ni catalysts are highly desirable. Recently, another more attractive strategy has been developed that employing poly-dimensional coordination polymer (including lanthanide, alkaline earth,

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and transition) serve as ideal sacrificial templates for fabricating various supported metal catalysts.37-39 Among them, 2D lanthanide coordination polymers have received extensive interests due to the fascinating properties that arise from the anisotropic molecular structure.40 However, rational design and direct-synthesis of novel 2D lanthanide coordination polymer containing uncoordinated groups (-NH2, -COOH, NO3-, etc.) located at the organic chelating linkers without postsynthetic-modifications still remains challenge. These challenges may stem from limited stability/solubility of 2D lanthanide coordination polymer, and the extra-uncoordinated groups located at the organic chelating linkers. Fortunately, the multidentate salen type organic ligands could provide more potentials for the designing of unique polymer framework due to their freedom of conformations.41 Therefore, a series of 2D lanthanide coordination polymer containing uncoordinated imine (-CH=N-) groups were isolated by reaction of a flexible N,N’-bis(5-methylsalicylaldehyde)cyclohexane-1,2-diamine

(H2salen)

with

Ln(NO3)3·6H2O. This unique 2D structure could anchor Ni ions and prevent their aggregations with a subsequent calcination process. Further, a recyclable and highly dispersed Ni/CeO2-CAS catalyst has been firstly fabricated for hydrogenation of nitroarenes to corresponding functionalized arylamines via coordination-assisted strategy. Various characterizations techniques were performed to explore the enhanced catalytic activity of the Ni/CeO2-CAS in comparison to the traditional Ni/CeO2-IWI catalyst. Our findings may provide a new method for design and synthesis more active Ni-based catalysts for diverse applications. 2. EXPERIMENTAL SECTION

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2.1. Chemicals and Reagents. Ln2O3 (CeO2, Pr2O3, Nd2O3, Ho2O3, Tb2O3, Er2O3 and Yb2O3), 1,2-diaminocyclohexane and 5-methylsalicylaldehyde were purchased from Aladdin Chemicals Co., Ltd. 2.2. Synthesis of Complexes. The H2salen ligand was synthesized based on our previously reported.42,43 The H2salen lanthanide complexes were synthesized by using solution diffusion (SD) method. Typically, the ligand H2salen (0.30 mmol, 105.0 mg) was dissolved in 2 mL dichloromethane and methanol with stirring at 20 oC. Then, 0.15 mmol Ln(NO3)3 ⋅ 6H2O was slowly added to above solution drop by drop. The suitable crystals for X-ray analysis were obtained within two days. 2.3. Preparation of Ni/CeO2-CAS and Ni/CeO2-IWI Catalysts. The Ni/CeO2-CAS catalyst was prepared by using coordination-assisted strategy. The complex Ce-H2salen (200.0 mg) was added to 50 mL methanol and stirred for 1 h. Then, the Ni(NO3)2 ⋅ 6H2O (18.3 mg) was slowly added to above solution. The reaction products were dried at 105 oC for 12 h. The obtained yellow solid was calcinated at 650 oC for 5 h in muffle furnace, and the NiO/CeO2-CAS was reduced with H2/N2 (20/80, vol/vol) at 650 °C for 4 h, which is denoted as Ni/CeO2-CAS. The referenced Ni/CeO2-IWI catalyst was synthesized by IWI method. For CeO2-IWI support, the ceric ammonium nitrate (0.36 mmol, 200 mg) was dissolved in 40 mL deioned water under magnetic stiring for 2 h, the resulting solution was hydrothermal treatment at 105 oC for 20 h. The resulting products were separated by filtration, and dried at 105 oC for 12 h. The as-synthesized sample was subsequently calcinated at 650 oC for 5 h in tube furnace. The Ni/CeO2-IWI catalyst with 10 wt% loading was prepared by IWI method. The as-synthesized NiO/CeO2-IWI sample was reduced at 650 °C for 4 h under

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the H2/N2 (20/80, vol/vol) to obtain the Ni/CeO2-IWI catalyst, which is labled as Ni/CeO2-IWI. 2.4. Characterization. Powder X-ray diffraction (Rigaku D/Max-3B X-ray diffractometer). Thermal analysis was conducted on a Perkin-Elmer STA 6000 (heating rate of 10 oC min-1 in air). High resolution TEM and high angle annular dark-field scanning TEM (HAADF-STEM) images were obtained on an FEI Tecnai F30 instrument coupled with an element energy-dispersive X-ray spectrometer (EDS). SEM images were obtained on a JEOL JSM-5600LV SEM/EDX instrument coupled with an element energy-dispersive X-ray spectrometer (EDS). The Ni content was measured by using the Optima 2000DV ICP-AES technique. Single-crystal X-ray diffraction (SCXRD, Rigaku R-AXISRAPID). The obtained data was processed by using OLEX2 program.42,43 Temperature programmed reduction (H2-TPR, FINESORB 3010). Catalyst (∼ 100 mg) was placed in a U-shape quartz tube, then purged under pure Ar (30 cm3 min-1) flow at 300 oC for 0.5 h, and then cooled down to 20 oC. After that, it was reduced with H2/Ar (10/90, vol/vol) (30 ml min-1) up to 750 °C (ramp rate of 10 °C min-1). Raman spectroscopy (Thermo Scientific DXR Raman microscope), X-ray photoelectron spectroscopy (XPS, ESCALAB 250). Temperature programmed desorption of H2 (H2-TPD, FINESORB 3010). Samples were first reduced at 650 o

C in pure H2 flow (30 mL min-1) for 1 h, and then the reactor was cooled to 50 oC in Ar, and

purged by Ar for 5 h at 50 oC to eliminate the physically adsorbed H2. The desorption experiment of H2 was conducted by heating to 800 oC at a ramp rate of 10 °C min−1. The Ni dispersions of the Ni/CeO2-CAS and Ni/CeO2-IWI were conducted on FINESORB 3010 automated equipment. The sample was reduced with H2 (20/80, vol/vol) at 650 °C for 1.5 h, purged at 400 °C with pure He for 1 h. After that, the system was cooled to 35 °C and CO

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chemisorption experiment was performed. The Ni dispersion was calculated from the Eq. 1, where (DM, metal dispersion), (SF, stoichiometry factor), (VS, volume of CO chemisorbed), (MW, metal weight), and (M, molecular weight of Ni).29 DM (%)=100 × (

Vs × SF )× M MW × 22414

(1)

2.5. Catalytic Reactions and Product Analysis. 2.4 mmol Nitrobenzen and 30 mg catalyst were added into 40 ml ethanol, and then transferred to autoclave (100 mL). Before experiments, the reactor was purged three times with N2 to replace air in the autoclave. After that, the reactor was purged three times with pure H2 to replace the N2, respectively. The reaction mixture was isolated by filtration and analyzed by using GC-MS and GC. 3. RESULTS AND DISCUSSION 3.1. Characterization of Lanthanide Complexes and Ni/CeO2-CAS Catalyst. The identity of structure towards Ln-H2salen was confirmed by SCXRD (Figure 1). As a representative example, the structure of Tb-H2salen was solved in the cubic space group with a lattice parameter of a = 16.3738 Å (Table S1), in which H2salen organic linkers were expanded by binuclear secondary building units (SBUs) consisting of a central O2- bonded to three Tb3+ ions (Figure 1a). An important point to notice in this SBUs, each Tb3+ ion is coordinated to phenolic hydroxy group from the H2salen ligand and nitrate radical, while the uncoordinated -CH=N- groups located at the H2salen ligands are left (Figure 1b). Furthermore, the H2salen ligands are connected by the binuclear SBUs to form the sheet-type structure (Figure 1c), which stacks together to give the 2D framework (Figure 1d, 1e). In addition, the XRD patterns of obtained Ln-H2salen complexes (Figure S3) are consistent with the simulated patterns, confirming that the crystalline samples are homogenous. According to previous reports,44,45 the reducible supports, especially for CeO2 (reversible Ce3+/Ce4+ redox), may forebode strong interaction between the surface of CeO2 and 7



nitro groups. Therefore, the Ce-H2salen lanthanide complex is selected as precursor of support for hydrogenation of nitrobenzene reaction.

Figure 1. (a) Structure unit of Tb-H2salen; (b) Skeleton of structure unit of Tb-H2salen; (c) Skeleton of 2D structure for Tb-H2salen; (d and e) Topology of 2D structure for Tb-H2salen. The selected crystal data: a (Å) = 16.3738, b (Å) = 16.3738, c (Å) = 56.467, V (Å) = 13111, ρ (g /cm3) = 1.525, R1, [I>2σ(I)] = 0.0492, wR2, [I>2σ(I)]=0.1436, GOF on F2 is 1.058. Scheme 1 demonstrates the synthetic procedure of Ni/CeO2-CAS catalyst. In stage I, the Ni ions on Ni@Ce-H2salen was metalated with Ce-H2salen ligand under stirred at 20 o

C. Before catalytic hydrogenation of nitroarene, the Ni@Ce-H2salen precursor was

calcinated in air and then reduced with dilute H2 at 700 oC to afford Ni/CeO2-CAS catalyst (stage II). The high yielding synthesis of Ni@Ce-H2salen under mild reaction conditions is expected since Ce-H2salen possesses abundantly free imine (-CH=N-) groups, in which Ni2+ ions can be absorbed within 2D layers by the strong coordination interaction between d-orbitals of Ni atom and lone pair electron of nitrogen located at the skeleton of the ligands.45 The FT-IR UV-Vis and XPS analyses were further conducted

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(Figure 2). The peak located at 1640 cm-1 is stretching vibration of -CH=N- group.46 The intensity and half-peak width changes of -CH=N- peak suggests that there is existence of the strong interaction between Ni ions and free -CH=N- groups.46 A similar trend is observed by the UV-Vis analysis, the intensity changes and position shifts of the peaks further indicate that the nickel nitrate interacts strongly with free -CH=N- groups, which is also confirmed by the significant difference in a colour change of reaction solution from yellow orange to light brown once methanol solutions of nickel nitrate and Ce-H2salen are then mixed under continuous stirring a few seconds (Figure 2b inset). The TG-DSC analysis was performed on the complexes Ce-H2salen and Ni@Ce-H2salen, and the TG-DSC curves are presented in Figure S4. From Figure S4a, there is no weight loss peak for Ce-H2salen below 300 oC, suggesting that the coordinated solvents are not present in the Ce-H2salen framework. In the TGA curve of the Ni@Ce-H2salen (Figure S4b), the first weight loss is about 12% (below 300 oC) which is attributed to the weight loss of the coordinated solvent molecules with Ni2+ ions. The second weight losses peaks for two complexes occur at a temperature range of 300-400 oC, which corresponds to the decomposition of the H2salen ligand and nitrate radical.

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Scheme 1. Schematic illustration of preparing Ni/CeO2-CAS catalyst. To gain further insight into the interaction between Ni2+ ions and Ce-H2salen ligand, XPS investigation was also performed. From Figure 2c, the visible peaks towards Ni 2p for complex Ni@Ce-H2salen can be observed, suggesting that the successful introduction of Ni precursor in Ni@Ce-H2salen extracted from the reaction mixture. Figure 2d shows the N 1s spectra of both complexes Ni@Ce-H2salen and Ce-H2salen. The peaks at ∼398.0 and ∼405.0 eV are attributed to imine nitrogen located at the skeleton of ligands and NO3species, respectively.47 For Ni@Ce-H2salen, significant differences are observed that appearance of Nx-Ni peak (∼400.0 eV) along with lower peak intensity towards imine nitrogen in comparison with Ce-H2salen, directly confirming that the strong coordination interaction between imine groups and Ni2+ ions, which is in good agreement with above FT-IR and UV-Vis analyses. Noticeably, the strong coordinated ability with Ni2+ ions and free -CH=N- groups located at the skeleton of the ligands can facilitate to immobilize the Ni single atoms within 2D layers and strengthen the metal-support interaction, and prevent their aggregation during calcination process at high temperature. 10


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Figure 2. FT-IR (a), Uv-Vis (b) and XPS spectra of complexes Ni@Ce-H2salen and Ce-H2salen. (c) Ni 2p region; (d) N 1s region. (Inset in Figure 2b is photo of the aqueous solutions of Ce-H2salen and Ni@Ce-H2salen.) The SEM/EDX investigation was carried out on Ni/CeO2-CAS and Ni/CeO2-IWI catalysts, as shown in Figure 3. For each SEM image of catalyst, the corresponding EDX mappings of Ni, Ce and O coupled with EDX are exhibited. From the mapping analysis, it can be observed that more uniform distribution of the Ni species on Ni/CeO2-CAS surface in comparison with those of on Ni/CeO2-IWI, indicating that the coordination-assisted method facilitates to enhance the dispersion of Ni species. EDX spectra of Ni/CeO2-CAS catalyst demonstrates more progressive decrease in the Ni signal than that of Ni/CeO2-IWI, further suggesting that Ni is uniformly distributed in the CeO2 lattice. The enhanced Ni dispersity of Ni/CeO2-CAS catalyst compared with traditional Ni/CeO2-IWI

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was further confirmed by the following HAADF-STEM-EDS, CO chemisorption and XPS analyses.

Figure 3. Ni/CeO2-CAS (A) and Ni/CeO2-IWI (B) catalysts: (a-c) Ni, Ce, and O mapping (d) SEM micrograph, and (e) EDX spectrum.

The TEM analysis was also conducted on the Ni/CeO2-CAS and Ni/CeO2-IWI catalysts (Figure 4). As shown in Figure 4a, the Ni/CeO2-CAS catalyst exhibits relatively regular nanosheets without any obvious agglomeration. For Ni/CeO2-IWI catalyst (Figure 4c), severe agglomerates are presented in some of regions. In some agglomerates (Figure 4c), the high-resolution image illustrates the fringe spacing of the (111) plane was measured at 2.03 Å (Figure 4a, inset), suggesting that the coordination-assisted

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impregnated method could enhance the dispersion of Ni species in comparison with the Ni/CeO2-IWI catalyst. The estimated mean particle sizes of both catalysts were 12.3 and 23.4 nm, respectively, which suggests that using of the coordination-assisted impregnated method can efficiently increase the dispersion of Ni and diminution the size of Ni particles on the CeO2-CAS support.

Figure 4. The TEM images of Ni/CeO2-CAS (a) and Ni/CeO2-IWI (c) (The insets are the particle size distribution histograms and Ni crystal lattice); The HADDF-STEM images of Ni/CeO2-CAS (b) and Ni/CeO2-IWI (d) catalysts as well as the corresponding EDS mapping of O, Ni and Ce elements.

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The high-angle annular dark field scanning transmission electron microscopy energy-dispersive spectroscopy (HAADF-STEM-EDS) mapping was also conducted to investigate the distribution of Ni, Ce, and O elements in the Ni/CeO2-CAS and Ni/CeO2-IWI catalysts. The results are shown in Figure 4 and Figure S5. The representative HAADF-STEM images coupled with corresponding elemental mappings (Figure 4) reveal that the smaller size and higher dispersion of Ni element on CeO2-CAS than those of CeO2-IWI. In addition, all elements including Ni, Ce, and O are shown in Figure S5. From the EDS spectra of Ni/CeO2-CAS catalyst (Figure S5a), a significant decrease in the intensity ratio values of Ni to Ce is observed in comparison with Ni/CeO2-IWI (Figure S5b), further suggesting that Ni is uniformly distributed in the CeO2 lattice, which is in good agreement with above SEM/EDX analysis. The structures of Ni/CeO2-CAS and Ni/CeO2-IWI catalysts were investigated by XRD analysis, and metallic Ni was also included for comparison. The XRD analysis (Figure 5) reveal that the diffraction peaks located at 44.5 and 51.8

o

in both catalysts

correspond to the characteristic of (111) and (200) reflection of metallic Ni, respectively.29,30 The CeO2 phase was identified by comparison with previous report.45 The XRD peaks of Ni(111) and Ni(200) for the Ni/CeO2-CAS catalyst are significantly weaker and broader than those of Ni/CeO2-IWI, which might have resulted from the lower Ni loading (Table 1), small and highly dispersed crystalline Ni nanoparticles on the surface of CeO2-CAS support, or Ni2+ ions are incorporated into the CeO2 lattice forming the Ce1-xNixO2 solid solution. The CO chemisorption experiment (Table 1) verifies that Ni/CeO2-CAS exhibit much higher Ni dispersity (11.92%) than that for Ni/CeO2-IWI (3.56%), suggesting that the coordination-assisted strategy could effectively improve the

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the Ni dispersion. The SEM/EDX, HAADF-STEM-EDS and CO chemisorption confirm that Ni/CeO2-CAS catalyst is of the higher Ni dispersion than that for Ni/CeO2-IWI, which favours the hydrogenation of nitroarenes reaction by way of the more exposed active Ni species. In addition, Table 1 lists the lattice parameters of ceria cubic phase in Ni/CeO2-CAS and Ni/CeO2-IWI catalysts. The profiles fitting of XRD data suggest that Ni/CeO2-CAS has a slightly smaller lattice parameter (5.410 Å) than that of Ni/CeO2-IWI (5.417 Å), which is ascribed to Ce3+ ions and oxygen vacancies in the catalyst. This observation further demonstrates that the uncoordinated imine groups located at the skeleton of CeO2 precursor can effectively anchor and stabilize the Ni nanoparticles.

Figure 5. XRD patterns of the Ni/CeO2-CAS and Ni/CeO2-IWI catalysts. (The metallic Ni was included for reference).

The reducibility of Ni/CeO2-CAS and Ni/CeO2-IWI catalysts, as well as interactions between Ni and CeO2 were investigated by H2-TPR analysis. The H2-TPR profiles are illustrated in Figure 6. The H2-TPR profiles for both as-synthesized catalysts exhibit quite different features. There are three reduction peaks for Ni/CeO2-CAS but only one peak for 15



Ni/CeO2-IWI catalyst. The α reduction peaks located at ∼380 oC for Ni/CeO2-CAS and Ni/CeO2-IWI catalysts can be assigned to the reduction of free NiO or the NiO specie with weakly interacting with CeO2 supports.45 The second reduction peaks located at 411 and 434 oC (β and γ, respectively) for Ni/CeO2-IWI catalyst can be assigned to the low reducible NiO with medium and strong interactions between Ni and CeO2 support,45 but no β and γ reduction peaks can be observed for Ni/CeO2-IWI catalyst. This change demonstrates that the SMSI is formed in the Ni/CeO2-CAS catalyst, which will result in a decrease in the Ni nanoparticles size, as identified by the TEM and XRD analyses. From the Table 1, the total H2 consumption corresponding to three (α, β and γ) reduction peaks (0.48 mmol g-1) and reduction degree (86.7%) on Ni/CeO2-CAS are obviously higher than those of Ni/CeO2-IWI catalyst (0.37 mmol g-1 and 68.5%, respectively), which are favorable for enhancing the catalytic activity of hydrogenation of nitrobenzene.

Figure 6. The H2-TPR profiles of Ni/CeO2-CAS and Ni/CeO2-IWI catalysts.

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Table 1. Characterizations of the Ni/CeO2-CAS and Ni/CeO2-IWI Catalysts Ni Catalyst

Lattice

loadinga parameterb (%)

(Å)

Ni/CeO2-CAS

6.2

5.410

Ni/CeO2-IWI

7.7

5.417

a

Measured by ICP analysis;

b

DNi

c

(%)

H2 uptaked

Reduction

Ce3+/

(mmol g-1)

Degreed

(Ce3++Ce4+)e

(%)

(%)

86.7

10.9

0.42

68.5

7.4

0.31

α

β

γ

11.92 0.15 0.23 0.10 3.56 0.37

-

-

Calculated from XRD (Figure 4);

c

Ratio of AF2g/ADf

Determined by CO

Chemisorption experiments; dDetermined by H2-TPR analysis; eMeasured by Ce XPS analysis; f

Measured by Ce XPS spectra (Figure 6). Furthermore, H2-TPD (temperature-programmed desorption of H2) experiment was

employed to explore the hydrogen activation ability of Ni/CeO2-CAS and Ni/CeO2-IWI catalysts. The H2-TPD profiles of two catalysts (Figure S6) exhibit that the peaks appearing at low-temperature (< 400 oC) is weak physical-adsorption and the peak above 400 oC is strong chemisorbed of H2 on the surface of metallic Ni.48 In contrast to the Ni/CeO2-IWI catalyst, the TPD desorption peaks towards physical- and chemical-adsorption H2 on the Ni/CeO2-CAS shift to the higher temperature region suggest that the Ni/CeO2-CAS is of stronger adsorption ability of H2 than that for Ni/CeO2-IWI catalyst, which prompts the hydrogenation reaction and refrains from the formation of byproducts. Figure 7a exhibits the results of high-resolution peaks towards Ni 2p XPS spectra. The Ni 2p regions contain five easily discernible features for each catalyst: the main peak and its satellite cantered at ∼854 and ∼862 eV can be assigned to Ni 2p3/2, and the main peak and its satellite located at ∼872, ∼882 and ∼888 eV is attributed to Ni 2p1/2, respectively.30 It is found that the peaks of Ni 2p3/2 for calcinated Ni/CeO2-CAS are weaker than those of Ni/CeO2-IWI due to its lower loading, which is consistent with ICP analysis (Table 1). As compared to Ni/CeO2-IWI catalyst, the peaks corresponding to Ni

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2p1/2 spectra of Ni/CeO2-CAS shift to lower binding energy values, implying that generation of negatively charged Ni species in the oxygen-deficient Ni/CeO2-CAS catalyst. It suggests that there is existence of SMSI between Ni species and CeO2-CAS, which is also in good agreement with above H2-TPR analyses (Figure 6). Figure 7b exhibits the Ce 3d XPS spectra of two catalysts, and the quantitatively calculated results are also listed in Table 1. The Ce 3d XPS spectra are fitted with eight peaks.45 According to previous results, the peaks of α2 and β2 are ascribed to the 3d3/2 of the Ce3+ fractions, and the other six peaks are attributed to the 3d5/2 feature of the Ce4+ fractions.45 Generally, the Ce3+ fractions lead to formation of oxygen vacancies, and the concentrations of oxygen vacancies of total Ce are greatly associated with surface of Ce3+ fractions.8 As shown in the Table 1, the surface concentration of total Ce (Ce3+/Ce3++Ce4+) on Ni/CeO2-CAS is 10.9%, which is obviously higher than that for Ni/CeO2-IWI (7.4%), clearly suggesting that there is existence of more Ce3+ on CeO2-CAS support with oxygen vacancies in comparison with CeO2-IWI. This phenomenon is caused by the SMSI between the CeO2-CAS support and Ni species. The Ni/CeO2-CAS with more oxygen vacancies tends to adsorb the nitrobenzene and effectively enhance the catalytic activity.

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Figure 7. The Ni 2p (a) and Ce 3d (b) XPS spectra of calcinated Ni/CeO2-CAS and Ni/CeO2-IWI catalysts. The Raman analysis was also conducted to directly identify the oxygen vacancies and defects. Figure 8 exhibits the Raman spectra of two catalysts, and the quantitative calculated surface concentration of oxygen vacancies (AF2g/AD, where AF2g are integrated peaks areas located at ∼ 600 cm-1, and AD are integrated peaks areas located at ∼ 460 cm-1) are listed in Table 1. The broad band with high intensity around 460 cm-1 is assigned to 19



the F2g mode, and the weak bands at 226, 598, and 1158 cm-1 are attributed to second-order transverse acoustic (2TA) mode.8,45 The Raman spectra of Ni/CeO2-CAS catalyst exhibit a decrease in intensity of the F2g vibration band and red-shifted (from 462 to 458 cm-1) in comparison with Ni/CeO2-IWI catalyst, suggesting that oxygen vacancies and structural defects are formed. Moreover, from the Table 1, the AF2g/AD for Ni/CeO2-CAS catalyst (0.42) is higher than that of Ni/CeO2-IWI (0.31), further revealing the richness of surface defects in Ni/CeO2-CAS,8 which is consistent with XPS analysis results (Figure 7).

Figure 8. The Raman spectra of calcinated Ni/CeO2-CAS and Ni/CeO2-IWI catalysts.

The reduction of nitrobenzene was employed as a model reaction to evaluate the activity and selectivity for Ni/CeO2-CAS and Ni/CeO2-IWI catalysts. As shown in Table 2, the contrast experiment with no catalyst shows that no reaction is processed. The temperature has a significant influence on the hydrogenation of nitrobenzene. At 150 oC, 20


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the conversion and yield to amine are 30.1 and 29.8%, respectively, under the reaction time of 1.5 h and reaction H2 pressure of 2.0 Mpa. The conversion and yield increase with increases in the temperature up to 210 oC (61.3 and 60.6%, respectively). According to previous report,32 the higher reaction temperature results in the occurrence of side reaction, thereby 210 oC was selected as the optimized reaction temperature. At 210 oC, the conversion and yield increase to 84.3 and 83.5% (entry 4), when the reaction time and H2 pressure are increased to 7.0 h and 2.0 Mpa, respectively. In contrast, Ni/CeO2-IWI was also tested under the reaction time of 7.0 h and reaction H2 pressure of 2.0 Mpa. According to Table 2 (entries 4 and 5), the Ni/CeO2-CAS catalyst exhibits much superior catalytic activity for reduction of nitrobenzene in comparison with the Ni/CeO2-IWI, indicating that the coordination-assisted strategy is effective in promoting the hydrogenation reaction. Table 2. The Catalytic Hydrogenation of Nitrobenzene to Amine with Ni/CeO2-CAS and Ni/CeO2-IWI Catalysts Temp Time PH2 Yield Entry Catalyst Con. (%) (oC) (h) (Mpa) (%) 1

no catalyst

150

5.0

1.5

0

0

2

Ni/CeO2-CAS

150

5.0

1.5

30.1

29.8

3

Ni/CeO2-CAS

210

5.0

1.5

61.3

60.6

4

Ni/CeO2-CAS

210

7.0

2.0

84.3

83.5

5

Ni/CeO2-IWI

210

7.0

2.0

64.2

63.9

Reaction conditions: 30 mg catalyst, 2.4 mmol nitrobenzene.

Correlation with the above characterization results, the enhanced Ni dispersion and CeO2-CAS with excess oxygen vacancies can effectively improve the catalytic performance for reduction of nitrobenzene. It is known that, nitro group has very strong electron-effect with supported Ni catalysts in the reduction of nitrobenzene. For 21



Ni/CeO2-CAS catalyst, the active metal Ni with higher dispersity and reducibility (identified by SEM/EDX, HAADF-STEM-EDS. XRD, CO chemisorption and H2-TPR analyses) can quickly dissociate H2 molecules, adsorb and diffuse nitrobenzene molecules in the hydrogenation reaction. The CeO2-CAS support with more oxygen vacancies (identified by Raman and XPS measurements), as strong Lewis-basic sites, prefer to adsorb nitrobenzene molecules on the surface of catalyst. Therefore, it is reasonable that Ni/CeO2-CAS catalyst can catalyze the hydrogenation of nitrobenzene with higher activity than traditional Ni/CeO2-IWI catalyst. Furthermore, the optimization for Ni/CeO2-CAS catalyst in hydrogenation reaction with various dosages of catalyst and nitrobenzene was carried out (Table S2). It reveals that 98.9% conversion and 98.7% yield was obtained (entry 3) when the dosages of catalyst decreases from 30 mg to 10 mg, and of nitrobenzene decreases from 2.4 mmol to 0.81 mmol, simultaneously. In heterogeneous catalytic reactions, the recyclability is also significantly important criterion to evaluate catalysts.32,37 Therefore, the recyclability test of the developed Ni/CeO2-CAS catalyst was conducted (Figure 9). From the Figure 9, the conversion and yield remain for seven cycles of reused catalyst without any loss in its catalytic activity and yield. This result further demonstrates that the developed Ni/CeO2-CAS catalyst prepared by coordination-assisted approach with its undegraded catalytic activity and yield endows it to be a potential catalyst for production of aniline via hydrogenation reaction.

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Figure 9. Catalytic activity and selectivity of developed Ni/CeO2-CAS catalyst for hydrogenation of nitrobenzene. Reaction conditions: Catalyst (30 mg), nitrobenzene (0.81 mmol), PH2= 2.0 MPa, T= 210 oC, t = 7 h. Inspired by the much superior catalytic performance of the Ni/CeO2-CAS, the reaction protocol was further extended to structurally diverse nitroarenes to further investigate the scope, selectivity, and yield of hydrogenation reaction catalyzed by the above developed Ni/CeO2-CAS catalyst (Table 3). As shown in Table 3, notably, the chloro-substituted nitrobenzenes are nearly fully reduced to corresponding chloro-anilines with excellent conversions and yields (entries 2-4). Furthermore, the high catalytic performance was obtained in the reduction of methyl-, hydroxyl-, formyl-, ester-, and carboxyl-substituted nitroarenes (entries 5-13), as well as multiamino nitroarenes (entries 14-16) without byproducts are detectable, further highlighting that the developed Ni/CeO2-CAS catalyst serves as a promising candidate for production aromatic amines via hydrogenation reaction from their corresponding nitroarenes.

23



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Table 3. Hydrogenation of Various Substituted Nitroarenes Catalyzed by Ni/CeO2-CAS Catalyst Entry

Substrate

Product

PH2 (MPa)

Time (h)

1

2.0

7.0

99.6/99.5

2

1.8

6.5

99.8/99.5

3

2.2

7.0

99.9/99.6

4

2.0

5

2.0

7.5

99.6/99.5

6

1.9

7.0

99.5/99.4

7

2.2

7.5

99.7/99.6

8

2.0

7.0

99.7/99.4

9

1.9

7.0

99.8/99.4

10

2.0

7.0

99.6/99.3

24


Sel./Yield (%)

99.7/99.5

7.0

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11

2.0

6.5

99.6/99.5

12

2.0

7.0

99.4/99.3

13

2.2

8.0

99.7/99.5

14

2.0

7.0

99.4/99.2

15

1.8

7.0

99.7/99.5

16

2.2

8.0

99.7/99.6

Reaction conditions: 30 mg catalyst, 0.81 mmol substituted nitroarenes, PH2 = 2.0 MPa, T = 210 o

C, t = 7 h The above characterization results demonstrate that the superior catalytic activity of

Ni/CeO2-CAS for reduction of nitrobenzene is of the cooperative effect between highly dispersed of Ni species and more oxygen vacancies defects. Thus, A plausible catalytic reaction process is proposed (Scheme 2). The CeO2-CAS support with rich surface of oxygen vacancies (identified by XPS and Raman analyses), which can serve as much more strong adsorbed sites for nitrobenzene in comparison with CeO2-IWI. Meanwhile, the small and highly dispersed Ni species on surface of CeO2-CAS support can accelerate dissociation of H2 into the active H species, which are favourable for quickly reducing the adsorbed nitrobenzene reactants.8 The aniline products desorb from CeO2-CAS support

25



upon the hydrogenation is completed. The high catalytic activity can be realized. This strategy must be valuable for other important hydrogenation reaction. Continued work is in progress.

Scheme 2. A plausible catalytic process for reduction of nitrobenzene catalyzed by Ni/CeO2-CAS catalyst. 4. CONCLUSIONS A family of novel two-dimensional lanthanide-based coordination polymer containing uncoordinated imine (-CH=N-) groups were synthesized by using the flexible H2salen organic ligand, and characterized by SCXRD analysis. Based on the potential strong coordination between uncoordinated imine (-CH=N-) groups and Ni ions, a novel CeO2-supported Ni catalyst (Ni/CeO2-CAS) enhanced on Ni disperisty, Ni-support interaction, NiO reducibility and concentration of oxygen vacancies was prepared via coordination-assisted approach. Strikingly, the developed Ni/CeO2-CAS catalyst exhibits superior catalytic activity of hydrogenation of nitrobenzene to traditional Ni/CeO2-IWI catalyst attributed to the cooperative effect between highly dispersed Ni species and more oxygen vacancies defects. Notably, for Ni/CeO2-CAS catalyst, the conversion and yield

26


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remain for seven cycles of reused catalyst without any loss in its catalytic activity and yield. Moreover, this highly efficient Ni/CeO2-CAS catalyst enables it to exhibit much superior catalytic activity and yield for the reduction of functionalized nitroarenes into their corresponding arylamines. Our finding may provide a new way for the design and synthesis of novel supported catalysts with excellent catalytic performance for other heterogeneous catalytic reactions.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications websiteat DOI: xxxx/acsami.xxxxxx. The synthesis for H2salen ligand and Ln-H2salen complexes; FT-IR, UV-Vis spectra and X-ray powder diffraction patterns of H2salen ligand and Ln-H2salen complexes; EDX spectra and H2-TPD profiles of Ni/CeO2-CAS and Ni/CeO2-IWI catalysts; Proposed pathways for the reduction of nitroarens on the Ni/CeO2-CAS and Ni/CeO2-IWI catalysts. For ESI and crystallographic data in CIF or other electronic format see DOI: xxxx/acsami.xxxxxx.

AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected], [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS

27



This work was financially supported by the National Natural Science Foundation of China (No. 21471051), Doctoral Fund of Ministry of Education of China (2017M621315), Heilongjiang Provincial Government Postdoctoral Science Foundation (LBH-Z17190), and State Key Laboratory of Fine Chemicals (KF1714).

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266x131mm (120 x 120 DPI)


High Catalytic Performance of a CeO2-Supported Ni Catalyst for

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