Synthetic Chemistry and Multifunctionality of an Amorphous Ni-MOF

Article Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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Synthetic Chemistry and Multifunctionality of an Amorphous NiMOF-74 Shell on a Ni/SiO2 Hollow Catalyst for Efficient Tandem Reactions Bowen Li and Hua Chun Zeng* Department of Chemical and Biomolecular Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, 119260 Singapore Downloaded via IDAHO STATE UNIV on July 22, 2019 at 07:43:05 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Core−shell particles, a unique class of functional materials, have received increasing research interest for the past 2 decades owing to their exceptional performance in many technological fields. For catalysis, coating a core catalyst with a shell could effectively enhance core stability and catalytic activity, provide reactant/product selectivity, add stimuli-responsive smart features, and so forth. Despite the rapid advancement made for core−shell materials, it is rare to see such shells displaying more than one or two functional roles in a single reaction system. Herein, we have developed an amorphous Ni-MOF-74 coating process for a hollow sphere made of silica-supported Ni nanoparticles. Multifunctional catalysts prepared by integration of metal−organic frameworks (MOFs) and metals were mainly limited to noble metals, whereas our catalyst successfully integrates highly dispersed transition-metal nanoparticles with MOFs. Our MOF shell possesses four distinct functionalities for tandem imination: to prevent detachment and segregation of catalytic Ni nanoparticles, to act as an acid catalyst, to avoid over-hydrogenation of the desired product via molecular sieving, and to suppress the undesired byproduct via promoting competitive reaction with a sizesieveable product. As a result, this MOF shell enables Ni/SiO2 to serve as a potential alternative to noble metal catalysts in this tandem reaction, and chemical transformation of the reactant substrate to a targeted product can be achieved more effectively.

1. INTRODUCTION Core−shell systems have attracted continuous research interest in the synthesis of functional materials as they provide a feasible way to integrate materials with different length scales, compositions, and morphologies into the same construct.1−5 Using the desired core and shell materials, numerous core− shell structures have been constructed to demonstrate exemplary performances in the fields of biology, catalysis, gas sensing, energy storage, and so forth.6−13 In heterogeneous catalysis, more specifically, covering active cores with shell materials could provide enhanced catalytic stability, selectivity to desired reaction products, stimuli-responsive reactivity, additional synergistic activity, and so forth.14−22 Although some extensively studied shell materials (e.g., silica and polymer) have shown their various functions in heterogeneous catalysis, it is important to point out that the coating shell of a core−shell catalyst typically possesses single functionality in a specific reaction environment.23−25 In view of the complex reaction systems we are facing, creating new core−shell structures endowed with multiple functionalities within a single shell is highly desirable because they could greatly simplify conventional stepwise reactions into one-pot tandem reactions with much lower synthesis and separation costs. © XXXX American Chemical Society

In search of potential shell materials with multiple functionalities, metal−organic frameworks (MOFs) have attracted our attention because of their flexibility of tunable pore channels and tailorable compositions for shell-engineering. Both intrinsic (e.g., acidic/basic sites of MOFs serve as active catalysts) and extrinsic (e.g., engineered porous channels act as diffusional barriers) features have been well explored when utilizing MOF-derived integrated catalysts toward complicated reactions.26−30 Despite the fact that utilization of both selectivity and reactivity of MOFs in the same tandem reaction system has yet to be demonstrated, we believe that MOFs are possible candidates to construct multifunctional coating shells.31 Application of MOFs alone in tandem reactions has been proven possible, especially for MOFs with acidic and basic sites at the same time.28,32 On a more common ground, incorporation of catalytically active metal (or alloy) nanoparticles (NPs) with MOFs has received tremendous research interest in the synthesis of multifunctional new catalysts.33−35 To the best of our knowledge, however, incorporation of Received: May 26, 2019 Revised: June 24, 2019 Published: June 25, 2019 A

DOI: 10.1021/acs.chemmater.9b02070 Chem. Mater. XXXX, XXX, XXX−XXX

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Chemistry of Materials

Figure 1. (a) Stepwise synthesis of Ni/SiO2@amNi-MOF-74 hollow spheres, (b) amNi-MOF-74 formation via oxidation−etching−coordination process using a Ni NP solid precursor, and (c) illustration for multifunctionality of amNi-MOF-74 in tandem imination of nitrobenzene with benzaldehyde.

reaction system: to protect catalytically active Ni NPs, to act as a solid acid catalyst, to prevent over-hydrogenation of the desired product via size-screening, and to promote competitive reaction with a size-sieveable product. For the first time, we have demonstrated the significance of combing multifunctional MOF shells with low-cost earth-abundant metal catalysts to carry out the desired tandem reaction.

highly dispersed metal NPs within MOFs has been greatly limited with only noble metals. Although there are many advancements exploring the potential of replacing noble metals with low-cost transition metals in catalysis applications, integration of such low-cost transition-metal NPs with MOFs has been rarely explored.36 Herein, we report the synthesis of a multifunctional amorphous Ni-MOF-74 (amNi-MOF-74) coating shell onto a hollow sphere made of silica-supported nickel NPs (Ni/SiO2). By utilizing preinstalled Ni NPs as a solid precursor, this catalyst successfully integrates highly dispersed Ni NPs (lowcost transition metal) with amorphous Ni-MOF-74 (Figure 1a,b). In addition to the multifunctional shell, Ni/SiO2 hollow spheres are used instead of conventional solid core or rattle structure. This provides controlled spatial distribution of Ni NPs at the interface between the SiO2 support and amNi-MOF74 shell. Compared with typical impregnation approaches where metal NPs are distributed throughout the porous MOF support, this core−shell structure with uniform amNi-MOF-74 shell thickness provides short and identical diffusion length for reactants to NPs, allowing better control over reaction extent. We have then explored the application potential of this integrated catalyst as an alternative in replacing noble metal catalysts in complex reaction systems. The above-prepared catalyst was tested in a tandem system (imination of nitrobenzene with benzaldehyde) in which noble metals such as Pt and Pd were predominantly used as catalysts.30,37,38 Apart from many side reactions, the desired tandem process requires hydrogenation of nitrobenzene to aniline and subsequent condensation of aniline with benzaldehyde to produce an imine. In many cases, this imine product is then hydrogenated again to produce an amine as the final product. In particular, noble metals are prevailingly used for this reaction as they could catalyze the hydrogenation of nitrobenzene to aniline while remain inactive for the hydrogenation of benzaldehyde to benzyl alcohol. At our experimental conditions, however, Ni NPs could not be used as a catalyst for this tandem reaction on its own, as it will catalyze the hydrogenation of both nitrobenzene and benzaldehyde at the same time. Condensation reaction between aniline and benzaldehyde is thus impossible using such a low-cost transition metal. Nevertheless, integration of Ni NPs with amNi-MOF-74 shell can prevent such undesired hydrogenation, making Ni as a possible replacement of noble metals in this tandem reaction. In fact, our amNi-MOF-74 shell has displayed four distinct functionalities for this tandem

2. EXPERIMENT SECTION 2.1. Chemicals and Materials. The following chemicals were used as received without further purifications: ammonium hydroxide solution (NH4OH, 25 wt % NH3 in water, Merck), ethanol (C2H5OH, ≥99.8%, VWR), tetraethyl orthosilicate (TEOS, Si(OC2H5)4, ≥99.0%, Sigma-Aldrich), nickel(II) sulfate hexahydrate (NiSO4·6H2O, ≥99.0%, Sigma-Aldrich), sodium acetate anhydrous (NaOOCCH3, ≥99.0%, Fluka), 2,5dihydroxyterephthalic acid (H4DOBDC, C8H6O6, >98.0%, TCI), dimethylformamide (DMF, C3H7NO, ≥99.8%, Merck), palladium(II) chloride (PdCl2, 99%, Sigma-Aldrich), polyvinylpyrrolidone (PVP, (C6H9NO)n, average molecular weight 40 000, Sigma-Aldrich), hydrochloric acid (HCl, 37% in water, Merck), dodecane (CH3(CH2)10CH3, >99.0%, Alfa Aesar), nitrobenzene (C6H5NO2, ≥99.0%, Sigma-Aldrich), and benzaldehyde (C6H5CHO, ≥99.0%, Sigma-Aldrich). 2.2. Synthesis of SiO2 Spheres. In a typical synthesis, 5 mL of ammonia solution was mixed with 46 mL of ethanol and stirred for 5 min. TEOS (2.5 mL) was then added to the solution under constant magnetic stirring. The final solution was stirred for 4 h, followed by centrifugation to collect the product. The produced white precipitate was then washed twice with ethanol and dried in an electric oven at 80 °C for 1 h. 2.3. Synthesis of Nickel Silicate Hollow Spheres. Typically, 25 mg of as-synthesized SiO2 powder was dispersed in 30.5 mL of deionized water and subjected to ultrasonication for 30 min. Then, 1 mmol of sodium acetate was dissolved in 2.0 mL of deionized water and transferred to the SiO2 suspension together with 2.5 mL of 0.02 M nickel sulfate solution. The resultant mixture was sonicated for 15 min before transferred to a Teflon-lined reactor and sealed inside a stainless steel autoclave. Hydrothermal treatment was performed at 160 °C for 10 h using an electric oven. The greenish product (namely, NiSiO, or in the chemical formula of Ni3Si2O5(OH)4) was separated from the solution by centrifugation and washed twice with ethanol. The sample was then dried at 80 °C for 1 h. B


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Chemistry of Materials 2.4. Hydrogen Reduction of Nickel Silicate Hollow Spheres. In a typical synthesis, 20 mg of the above nickel silicate powder was calcined in a tube furnace under the H2 flow of 50 mL/min at 650 °C for 6 h with a ramp rate of 3 °C/ min. A black powder (Ni/SiO2) was produced after the H2 reduction. 2.5. Synthesis of amNi-MOF-74 Coating Shell. The Ni/ SiO2 sample (20 mg) produced from the hydrogen reduction was dispersed in 4 mL of DMF and sonicated for 15 min. This solid−liquid suspension was then mixed with 28 mL of 0.25 M H4DOBDC in DMF solution. This final mixture was kept at 110 °C using an oil bath for 12 h to achieve an amNi-MOF-74 coating shell with desired thickness. 2.6. Synthesis of NiPd/NiSiO Hollow Spheres. The nickel silicate (NiSiO) hollow spheres prepared according to the previous description were subjected to H2 reduction for 4 h at 500 °C. This reduced sample was dispersed in ethanol with 1 mg/mL concentration. PVP was dissolved in water to prepare 22.5 mg/mL of solution. Pd in water solution (56 mM) was prepared by dissolving PdCl2 in 0.3 M HCl solution. The above PVP solution (100 μL) was mixed with 7 mL of deionized water and 25 μL of the Pd solution. After stirring for 5 min, 1 mL of the above reduced NiSiO hollow spheres in ethanol was added and stirred for another 30 min. The final product was collected via centrifugation and washed twice with ethanol. 2.7. Synthesis of NiPd/NiSiO@amNi-MOF-74. The synthetic procedure was similar to the synthesis of amNiMOF-74 coating shell, except that 20 mg of Ni/SiO2 was replaced by 20 mg of NiPd/NiSiO. 2.8. Tandem Imination of Nitrobenzene with Benzaldehyde. High-pressure hydrogenation reactions were carried out using a 100 mL Parr reactor. In a typical procedure, 10 mg of the catalyst was dispersed in 10 mL of ethanol. Subsequently, 50 μL of dodecane, 22 μL of nitrobenzene, and 35 μL of benzaldehyde were introduced. This liquid mixture was then transferred to a glass liner and sealed within a stainless steel autoclave. This autoclave was pressurized to 430 psi gauge with H2 gas and purged three times at this pressure. The temperature was then increased to 80 °C while the pressure inside the autoclave reached 490 psi. During the catalytic reaction, the solution was stirred using a magnetic stirrer at 500 rpm. After reaction, the autoclave was allowed to cool for 10 min before releasing the H2 gas. The reaction solution was centrifuged to recover the catalyst and dried in a vacuum oven. The liquid solution was filtered through a membrane and analyzed by gas chromatography (GC). 2.9. Material Characterization. The size and morphology of as-synthesized samples were investigated by transmission electron microscopy (TEM, JEM-2010, FETEM-2100F, accelerating voltage: 200 kV). The crystallographic structure was determined by a powder X-ray diffractometer (Bruker D8 ADVANCE) equipped with a Cu Kα radiation source. The composition and elemental distribution were analyzed by inductively coupled plasma−optical emission spectroscopy (ICP−OES, PerkinElmer Optima 7300 DV), energy-dispersive X-ray (EDX) spectroscopy (Oxford Instruments), and X-ray photoelectron spectroscopy (XPS) with binding energy referenced to the C 1s peak at 284.8 eV (XPS, AXIS-HSi, Kratos Analytical). More structural information was also gathered using Fourier transform infrared (FTIR) spectroscopy (Bio-Rad FTS-3500ARX) and thermogravimetric analysis (TGA-2050, TA Instruments). Analysis of the products after

hydrogenation reaction was performed by GC (Agilent 7890A with a flame ionization detector).

3. RESULTS AND DISCUSSION 3.1. Material Synthesis and Characterization. Synthesis of Ni/SiO2 hollow spheres with a uniform coating shell of amorphous Ni-MOF-74 (namely, Ni/SiO2@amNi-MOF-74) was carried out using Ni NPs as a solid precursor (Figure 1a). Unlike most MOFs synthesized from liquid solution, the current synthesis of amNi-MOF-74 started with the Ni metal in order to have a better morphology control.39,40 Briefly, nickel silicate (Ni3Si2O5(OH)4, or denoted as NiSiO) hollow spheres were synthesized using a sacrificial hard template of silica spheres as described in our previous work.41 Further reductive transformation with hydrogen gas produced silica-supported Ni NPs while preserving the hollow morphology (eq 1). Use of Ni/SiO2 hollow spheres as a template/precursor has brought us several advantages. First, this chemical reduction approach is assumed to provide a better metal−support interaction. This might help to retain Ni NPs in their original position during the MOF synthesis. Second, this Ni/SiO2 hollow structure can support Ni NPs with much less SiO2 usage. The solid core of SiO2 that is not participating in any reaction has been hollowed to give better material utilization. Moreover, such a hollow catalyst has low density and is more mixable and floatable in liquid phase. As this catalyst is designed for complex organic synthesis, which is mostly carried out in liquid phase, it is easier to obtain homogeneous dispersion of hollow catalyst in solution media. Compared to similar structures where Ni NPs are deposited within the porous silica support, our Ni/SiO2 hollow structure ensures that all Ni NPs are positioned on the external surface of the SiO2 support, which is crucial for the subsequent formation of Ni-MOF-74. Because Ni NPs are evenly distributed on the surface, the releasing Ni2+ ions could immediately interact with the organic linkers present in the solution to form the desired amorphous Ni-MOF-74. If the nickel NPs are positioned within a porous silica matrix, it would exert a diffusional barrier for both Ni2+ ions and organic linkers to overcome. This would then prevent the formation of Ni-MOF-74. In addition, this Ni/SiO2 @a mNi-MOF-74 structure ensures immediate interaction of reactants with Ni NPs after diffusing through the amorphous MOF shell. Thus, the additional diffusion barrier of porous silica can be avoided. Ni3Si 2O5(OH)4 + 3H 2 → 3Ni + 2SiO2 + 5H 2O

(1)

As can be seen from the TEM images of Figure 2, the Ni NPs formed from the above reduction are highly dispersed with a uniform size distribution, which is further confirmed by EDX mapping and line scan (Figures S1 and S2). These homogeneously distributed Ni NPs (sized at round 4 nm) are ideal as a nickel source because we can expect a uniform coverage of MOFs on top of the Ni/SiO2 hollow sphere. Indeed, an amNi-MOF-74 shell was obtained when the hollow Ni/SiO2 spheres were suspended in DMF and mixed with 2,5dihydroxyterephthalic acid ((HO)2C6H2-1,4-(CO2H)2, or C8H6O6, or H4DOBDC) solution for 12 h at 110 °C (Figure 3); all of the Ni/SiO2 hollow spheres were encapsulated by the amNi-MOF-74 coating shell. On the other hand, the use of spherical Ni NPs with no alloying has ruled out slow Ni diffusion via Kirkendall effect. All Ni2+ ions are released by etching of surface NiO, leading to a faster release rate and faster nucleation process. As a result, amorphous Ni-MOF-74 is produced. The rapid MOF-74 nucleation process is also C


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enough, the distinction among these three is not obvious. However, carbon is only present in the amorphous MOF shell and it can be isolated from Si/Ni/O (Figure S5). When the obtained sample, Ni/SiO2@amNi-MOF-74 (12 h), was examined with powder X-ray diffraction (PXRD), only peaks corresponding to SiO2 and Ni NPs were observed (Figure 4a).41−44 Because of the amorphous nature of the shell, diffractions corresponding to the crystallographic phase of NiMOF-74 were absent from our PXRD pattern. As such, however, we arrive at the conclusion that the amNi-MOF-74 shell is indeed an amorphous MOF based on the following four considerations: (i) coating shell was formed in a solution containing only nickel, silica, MOF-74 linker (H4DOBDC), and solvent (DMF). It is highly unlikely for nickel to react with silica to form back nickel silicate which requires the presence of both water and base under hydrothermal conditions.16 Furthermore, diffractions representing nickel silicate were not detected after the shell coating. Formation of Ni-MOF-74, on the other hand, has been reported to be viable under similar conditions;30 (ii) our FTIR spectra displayed two new absorption peaks emerged after the shell formation [Figure 4b, Ni/SiO2@amNi-MOF-74 (12 h)]. Similar IR peaks were also found for H4DOBDC linkers coordinated with nickel ions in nanosized MOF-74.45 Comparing our catalysts with the crystalline Ni-MOF-74 sample, we could see that most peaks before 1200 cm−1 are not distinguishable for our sample because of the presence of SiO2 peaks. There are two major peaks at 1405 and 1557 cm−1 for crystalline Ni-MOF-74 because of O−H bending in carboxylic acid and C−H bending of ring structure. These two peaks are also observed for our samples with a clear shift to lower wavenumber regions. It is probably caused by difference in bond strengths due to neighboring environment because our samples are amorphous and such disordered bonds are generally weaker as compared to the crystalline ones. Nevertheless, the presence of such absorption peaks makes the composition of the shell more likely to be amNi-MOF-74; (iii) XPS measurement revealed the state of surface Ni atoms. The presence of binding energy peak (of Ni 2p3/2) at 856.1 eV is characteristic for divalent Ni2+ ions in Ni-MOF-74 (Figure 4c).46 Such an observation further confirms the formation of coordinative bonds between Ni2+ and the carboxylate anion; and (iv) TGA curves of Ni/SiO2@ amNi-MOF-74 were measured by heating the as-prepared sample under an air flow (20 mL/min and 10 °C/min, Figure 4d). Both continued weight loss before 200 °C and rapid weight loss after 250 °C were indeed characteristics of MOF74 as a result of organic linker decomposition.47 The concaveup shape observed after 350 °C was due to simultaneous oxidation reaction of both pre-existing Ni NPs and newborn Ni NPs produced from the decomposition of amNi-MOF-74 in this TGA measurement. On the basis of TEM images for samples taken after 4, 12, and 16 h of reaction, continuous shell formation is evident with its shell thickness increasing over time (Figures 5 and S6−S9). Although a barely visible amNi-MOF-74 shell is found after reacting for 4 h (Figure 5a,b), a much thicker shell is observed for a 16 h sample (Figure 5c,d). This time-dependent growth process is further confirmed with XPS measurements (Figure 6). After hydrogen reduction, surface Ni was presented in the form of metallic Ni as measured from XPS (Figure 6a).48 When this H2 reduced sample was mixed with the H4DOBDC linker for 4 h, XPS revealed the formation of amNi-MOF-74 from the binding energy of Ni. A significant peak

Figure 2. Representative (a−d) TEM images of Ni/SiO2 hollow spheres at different magnifications produced from the H2 reduction of a nickel silicate hollow precursor.

Figure 3. Representative (a) SEM and (b−d) TEM images of the Ni/ SiO2@amNi-MOF-74 structure.

confirmed by the fact that Ni leaching to the bulk solution is negligible, indicating that all leaching Ni2+ ions have been converted to MOF before reaching the bulk solution. The morphological aspects of Ni/SiO2@amNi-MOF-74 were investigated with scanning electron microscopy (SEM) and TEM. It is clearly visible that the attained product is monodisperse, comprising a Ni/SiO2 hollow core and a welldefined amNi-MOF-74 shell with a uniform thickness (Figure 3b−d). The mean diameter of the overall sphere is measured to be 260 nm while the shell thickness is about 15 nm. The hollow interior of this product has been illustrated directly from certain broken spheres revealed in the SEM image (Figure 3a). EDX elemental mappings and line scans also confirmed the hollow morphology and homogeneous distribution of Ni, O, and Si across the structure (Figures S3 and S4). There is no clear distinction between Si and Ni/O as the hollow core itself (Ni/SiO2) contains all these three elements. When the formed amorphous Ni-MOF-74 shell is not thick D


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Figure 4. (a) PXRD measurements of NiSiO, Ni/SiO2, and Ni/SiO2@amNi-MOF-74 samples. (b) FTIR spectrum of Ni/SiO2, Ni/SiO2@amNiMOF-74 with different MOF shell thicknesses, and crystalline Ni-MOF-74. (c) XPS spectra of Ni 2p3/2 for the Ni/SiO2@amNi-MOF-74 (12 h) sample. (d) TGA curves for Ni/SiO2@amNi-MOF-74 samples with different MOF shell thicknesses (Table S1).

eventually the total absence of metallic Ni0 signal (Figures 4c and 6c). It is important to note that this absence of Ni0 signal do not imply full consumption of Ni NPs. Instead, it suggests that the formed Ni-MOF-74 layer is thick enough to prevent XPS from detecting Ni NPs underneath. On the basis of PXRD measurements, Ni NPs still exist after the MOF formation process (Figure 4a). A similar time-dependent change has been observed for FTIR spectra (Figure 4b). The absorption signals of amNi-MOF-74 were not detected for the 4 h sample, probably because of the limited amount of the carboxylate linker present. The weight percentage of amNi-MOF-74 after each reaction time was evaluated based on the TGA measurements of the decomposed carboxylate linker (Figures 4d and S10 and Table S1). A drastic weight loss was found when the reaction time increased from 4 to 12 and 16 h, indicating an increase in the weight percentage of amNi-MOF74. The decomposition temperature had shown a slight increase when the reaction time was extended. A similar trend has been observed for crystalline Ni-MOF-74 materials where a smaller crystal size results in a lower linker decomposition temperature.45 Total Ni wt % and the Ni distribution between Ni NPs and amNi-MOF-74 are calculated using the TGA technique, assuming no Ni leaching to the bulk solution. These calculated results agree well with the total Ni wt % measured from EDX mappings, and hence, we could conclude that the Ni leaching during MOF synthesis is negligible (Table S1). This negligible Ni leaching could be used as evidence to prove rapid Ni-MOF-74 nucleation, which in turn explains why the resulted MOF is amorphous in our experiment.

Figure 5. Representative TEM images of Ni/SiO2@amNi-MOF-74 structure after reacting with H4DOBDC for (a,b) 4 h and (c,d) 16 h.

corresponding to metallic Ni0 was also observed, indicating an extremely thin amNi-MOF-74 shell formation (Figure 6b). As XPS technique can probe a sampling depth of up to 10 nm, a signal of imbedded Ni NPs could be detected when the coating shell is less than 10 nm in thickness. A possible signal from uncovered Ni NPs has been excluded (see our model tandem reaction later). Further extending the reaction time would lead to the formation of a thicker shell of amNi-MOF-74 and E


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Figure 6. Ni 2p XPS spectra for (a) Ni/SiO2 hollow spheres, (b) Ni/SiO2@amNi-MOF-74 (4 h) hollow spheres, and (c) Ni/SiO2@amNi-MOF-74 (16 h) hollow spheres.

2Ni + O2 → 2NiO

(2)

C8H6O6 → 4H+ + C8H 2O6 4 −

(3)

+

2+

NiO + 2H → Ni

+ H 2O

0 Ni(s) + PdCl4 2 −(aq) → Ni(aq)2 + + Pd(s)0 + 4Cl(aq)−

(6)

Prior to the formation of amNi-MOF-74, interestingly, the Ni NPs produced from H2 reduction could facilitate the deposition of Pd NPs in the presence of PVP to control the particle size (Figures 7a,b and S17−S23). Galvanic replacement reaction between solid Ni NPs and dissolved PdCl42− precursor serves as a driving force for Pd reduction and the formation of NiPd NPs. As the standard reduction potential of the PdCl42−/Pd pair [0.59 V vs standard hydrogen electrode (SHE)] is higher than that of the Ni2+/Ni pair (−0.25 V vs SHE),50 spontaneous galvanic replacement reaction could take place in which Ni0 has been oxidized to Ni2+ and leached into solution while Pd2+ has been reduced to metallic state and deposited/alloyed onto Ni NPs (eq 6). The possible formation of bimetallic NiPd NPs is affirmed from PXRD measurement and EDX elemental mappings (Figures 7e and S16). The peak observed in between Pd(111) and Ni(111) is evident for the formation of bimetallic NiPd alloy NPs. Furthermore, the XPS measurement of Pd 3d has also confirmed the existence of Pd0 metallic state (Figure 7f). Partially reduced Ni/NiSiO hollow spheres prepared at lower reduction temperature and duration were used for this reaction because the small board peak at around 2θ = 20° in the PXRD plot indicates the coexistence of SiO2 and unreduced NiSiO. Fully reduced Ni/SiO2 was not used here as it would give rise to larger NiPd alloy NPs which are not preferred as catalysts (Figure S22). After the alloy formation, the same amNi-MOF-74 synthesis could be carried out later to produce a similar shell structure but with more complex metal compositions (Figures 7c,d and S23). Success in incorporation of noble metals into nickel could effectively broaden the application of our catalyst because formation of bimetallic alloy NPs has been able to provide catalytically synergistic effects in many cases. In particular, noble metals are well known for their excellent catalytic activity for numerous reactions. As such, the ability to incorporate noble metals into our catalyst provides possibilities to tailor this catalyst to suit other complex reaction systems. Although such effects are not observed in the current work, it might be still beneficial when this catalyst is applied to other reaction systems. 3.2. Tandem Imination of Nitrobenzene with Benzaldehyde by Ni/SiO2@amNi-MOF-74. Adoption of MOFs in catalysis has been explored extensively in recent years, as this class of materials can provide unique catalytic activity and product selectivity.18,51−56 We believe that both catalytically active sites and tunable pore structures of MOFs could be utilized at the same time to accomplish complex

(4)

2Ni2 + + C8H 2O6 4 − → Ni 2C8H 2O6 (i. e. , Ni 2DOBDC) (5)

Depending on the above time-resolved measurements, the formation of an amNi-MOF-74 shell was proposed to follow an oxidation−etching−coupling process using Ni NPs as a solid precursor (Figure 1b). As the formation process of the shell was carried out using a round-bottom glass flask with a condenser mounted on top, ambient air was able to dissolve in the DMF solvent causing the surface oxidation of Ni NPs (eq 2). On the other hand, because H4DOBDC is acidic in nature (eq 3), nickel oxide was etched to release Ni2+ ions, which were in turn used as a metal source and coupled with deprotonated organic linkers (eqs 4 and 5). By conducting control experiments, it is proposed that the oxidation is the rate-limiting step in this process. The hydrogen-reduced Ni/ SiO2 sample was collected and placed in a glass vial for 2 days to allow the oxidation of nickel to take place. This aged sample was subjected to the same reaction condition for 12 h, producing a thicker shell as compared to samples without aging (Figure S11). Moreover, this MOF formation process could be affected by solvent composition. When a small amount of water was added as a cosolvent to DMF, amNi-MOF-74 shells with a thickness ca. 50 nm was formed after 12 h (Figure S12). From the line scans of this thick shell sample, it is well demonstrated that the peaks of Si are smaller in width as compared to the peaks of C/Ni/O (Figure S13). This serves as better evidence to conclude that Si is absent in the shell. At this shell thickness, no crystalline Ni-MOF-74 peaks are detected from our PXRD measurement (Figure S14). On the other hand, N2-sorption isotherm of this thick-shelled sample shows a typical type IV isotherm, indicating the presence of porosity despite the MOF being amorphous. The pore size distribution plot has a clear peak at 1.1 nm, agreeing well with the reported pore dimension of Ni-MOF-74 (Figure S15).49 Because this thick-shelled sample is also amorphous, we believe that it is reasonable to extend this result to other thinner amNi-MOF-74 shells. Constant stirring and hence homogeneity of the reaction mixture are other essential keys for the shell uniformity. When the reaction was carried out using autoclaves without stirring under solvothermal condition, the amNi-MOF74 shell was synthesized together with a number of uncovered Ni/SiO2 spheres (Figure S16). F


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Figure 8. Reaction pathways for tandem imination of nitrobenzene with benzaldehyde.

for nitrobenzene hydrogenation could likely hydrogenate imine as well.30,38,57 When noble metals such as Pt or Pd are used to catalyze this reaction, hydrogenation of benzaldehyde 3 to benzyl alcohol 6 is much slower as compared to hydrogenation of nitrobenzene 1 to aniline 2.38 This allows the subsequent condensation reaction between aniline and benzaldehyde to produce imine. Herein, Ni NPs have been explored as a possible alternative of noble metals for hydrogenation reactions. Using Ni NPs as an alternative catalyst, we found that both nitrobenzene and benzaldehyde were hydrogenated, and formation of imine/ amine was minimum (see entry 5, Table 1). However, incorporating Ni NPs with a multifunctional shell of amNiMOF-74 could solve this problem, and our integrated catalyst has achieved 91.2% selectivity toward valuable imine 4 product at 100% conversion of nitrobenzene 1 (see entry 6, Table 1). With only transition metal (Ni) as the hydrogenation catalyst, as shown above, the achieved selectivity in this work is comparable to state-of-the-art tandem imination (using noble metals as the hydrogenation catalyst).30,38 In particular, our amNi-MOF-74 shell has enabled Ni/SiO2 as a low-cost alternative to noble metal NPs by providing four distinct functionalities riding on top of the pristine catalytic ability of Ni NPs on the hydrogenation reaction (Figure 1c),61 which will be further addressed. First, amNi-MOF-74 shell coating is able to prevent both leaching and segregation of Ni NPs. On the basis of previous sample characterization, the size of Ni NPs is about 4 nm, which is much larger than the pore dimension of Ni-MOF-74 (1.1 nm).49 This indicates that growth by contact coalescence of Ni NPs could be effectively minimized by the amNi-MOF-74 shell coverage. Because of the presence of porous shell channels, the diffusion path for leached Ni to reach bulk solution or nearby NPs has been greatly extended, decreasing the possibility of both Ni leaching and growth of Ni NPs by Ostwald ripening. Second, the amNi-MOF-74 shell can provide acidic sites to catalyze condensation reaction between aniline 2 (produced from hydrogenation of nitrobenzene by Ni NPs) and benzaldehyde 3. To prove that, a Ni/SiO2 catalyst (10 mg) was subjected to the same reaction conditions without the presence of amNi-MOF-74 (entry 5, Table 1). Full conversion of both reactants has been achieved with aniline 2 and benzyl alcohol 6 being two primary products while a trace amount of amine 5 was also detected (yield is 7.3%) with no intermediate imine 4 found in the output. As calculated from Table S1, the Ni loading of Ni/SiO2 is much higher than that of Ni/SiO2@ amNi-MOF-74 samples. To exclude the possibility that higher Ni loading is causing the observed performance, we have conducted a comparison experiment with 40 mg of Ni/SiO2@ amNi-MOF-74 so that the total Ni loading is similar (entry 4,

Figure 7. Representative TEM images of (a, b) NiPd/NiSiO hollow spheres prepared by galvanic replacement of Pd precursor with partially reduced Ni/NiSiO and (c, d) NiPd/NiSiO@amNi-MOF-74 hollow spheres prepared from subsequent MOF synthesis; (e) PXRD pattern of NiPd/NiSiO with a broad peak in between Pd(111) and Ni(111) to suggest the possible formation of NiPd bimetallic NPs; and (f) Pd 3d XPS spectrum for NiPd/NiSiO.

chemical synthesis in a one-pot manner.31 Integrating multifunctional amNi-MOF-74 as the coating shell of Ni/ SiO2 hollow sphere enables Ni NPs to serve as a low-cost alternative to noble metal catalysts for tandem imination reaction. This reaction uses nitrobenzene (1, Figure 8) and benzaldehyde 3 as reactants. Benzylideneaniline (imine 4) is the desired product from this tandem reaction as it can serve as a valuable intermediate in many chemical productions.57−60 Because of the multistep nature of tandem reaction and complexity of reactants involved, many side reactions could occur (Figure 8). For instance, it is problematic to avoid overhydrogenation of imine 4 to amine 5 because the catalyst used G


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Chemistry of Materials Table 1. Tandem Imination of Nitrobenzene with Benzaldehyde b

entrya

imine 4 amine 5

1.5 1.5 1.5 1.5 1.5 14 14 14

31.9 31.0 21.8 85.8 100 100 100 N.A.

100 100 100 98.4 0 91.8 0 N.A.

0 0 0 1.56 100 8.2 100 N.A.

24.3 31.0 21.8 74.6 0 91.8 0 N.A.

0 0 0 1.20 7.3 8.2 100 N.A.

(16 h)

1.5

26.9

100

0

26.9

0

aniline 2 N.A. N.A. aniline 2 aniline 2, benzyl alcohol 6 N.A. aniline 2, benzyl alcohol 6 benzaldehyde diethyl acetal 7 N.A.

(16 h)

1.5

28.4

100

0

28.4

0

N.A.

(16 h)

1.5

23.1

100

0

23.1

0

N.A.

(16 h)

1.5

16.5

100

0

16.5

0

N.A.

catalyst

9g

Ni/SiO2@amNi-MOF-74 run 1 Ni/SiO2@amNi-MOF-74 run 2 Ni/SiO2@amNi-MOF-74 run 3 Ni/SiO2@amNi-MOF-74 run 4

12g

imine 4 amine 5

nitrobenzene 1conversion (%)

Ni/SiO2@amNi-MOF-74 (4 h) Ni/SiO2@amNi-MOF-74 (12 h) Ni/SiO2@amNi-MOF-74 (16 h) Ni/SiO2@amNi-MOF-74 (12 h) Ni/SiO2 Ni/SiO2@amNi-MOF-74 (12 h) mSiO2@Ni/SiO2@mSiO2 Ni/SiO2@amNi-MOF-74 (12 h)

11g

yield (%)

time (h)

1 2 3 4e 5 6 7 8f

10g

selectivityd (%) c

side product

a General reaction conditions: 10 mg of the catalyst, 10 mL of ethanol, 50 μL of dodecane, 22 μL of nitrobenzene 1, and 35 μL of benzaldehyde 2 were mixed and reacted at 80 °C and 490 psi with constant stirring at 500 rpm using a magnetic stirrer. bTime of reaction starts when the temperature first reached 80 °C and stops when H2 gas was released after 10 min cooling. cConversion of nitrobenzene 1 was calculated based on GC measurements. dSelectivity of imine 4 was calculated as moles of imine/(moles of imine + amine) in the product mixture. e40 mg of the catalyst was used to ensure a similar Ni loading to 10 mg of Ni/SiO2. fReaction was carried out without nitrobenzene 1, using benzaldehyde 2 as the only reactant. g15 mg of the catalyst was used and recycled.

acidic sites were present due to unsaturated coordination between Ni2+ cations and carboxylate anions within the MOF structure.26,62 This finding is further demonstrated by Ni/SiO2 catalysts with different shell thicknesses of amNi-MOF-74 (achieved by using different reaction times for MOF formation: 4, 12, and 16 h). When the tandem reaction lasted for 90 min, all three samples have shown a significant amount of nitrobenzene 1 conversion with imine 4 as the only product detectable from the product mixture (entries 1−3, Table 1). Such a 100% selectivity confirms that a full coverage of amNiMOF-74 shell over the Ni/SiO2 hollow sphere has been achieved after 4 h of shell growth. However, the Ni/SiO2@ amNi-MOF-74 4 h catalyst has produced a significant amount of aniline 2 as the side product (entry 1, Table 1). This could be explained as insufficient acidic sites provided by amNiMOF-74 due to an extremely thin MOF shell coated in this sample. As reported in Table 1, the 16 h sample has given a lower conversion as compared to the 12 h sample, which could be explained by two factors. First, the amount of Ni remained as Ni NPs in the 16 h sample has reduced, compared to the 12 h sample (Table S1). This could reduce the hydrogenation rate of nitrobenzene as the reaction is catalyzed by Ni NPs. Second, the amNi-MOF-74 shell thickness has increased for the 16 h sample. This could lead to increased diffusional barrier, which will in turn slow down the conversion of nitrobenzene. Third, the amNi-MOF-74 shell could prevent over-hydrogenation from imine 4 to amine 5 by acting as a molecular sieve. To prove such a selectivity, the mesoporous silicaintercalated Ni/SiO2 prepared according to a previous method was used as a comparative catalyst (entry 7, Table 1).41 This catalyst also has certain acidic sites because of the presence of mesoporous silica coating shells.63 In this case, all intermediate imine 4 has been further hydrogenated to amine 5 based on our GC analysis of final products. Comparing the mesoporous silica shell with the amNi-MOF-74 shell, it is proposed that a narrower pore channel of amNi-MOF-74 is responsible for

Table 1). After reacting for 1.5 h, 85.8% nitrobenzene has been converted to aniline, imine, and amine. The yield for imine is as high as 74.6% with imine/amine selectivity being 98.4:1.56. The small amount of aniline suggests that the increase in the nitrobenzene hydrogenation rate due to increased Ni NP amount outweighs the increase in the condensation rate due to increasing MOF acidic sites. The drastically different amine/ imine yields achieved from the two catalysts with a similar amount of Ni NPs have confirmed two hypotheses: nitrobenzene 1 could easily diffuse into the MOF channel and get hydrogenated by Ni NPs to produce aniline 2 (Figure 9a), and condensation reaction between outward diffusing aniline 2 and inward diffusing benzaldehyde 3 should be largely catalyzed by the acidic sites of amNi-MOF-74 shell (Figure 9b). These

Figure 9. Schematic illustrations of multifunctionality of amNi-MOF74 shell: (a) allowing the diffusion in/out of nitrobenzene and aniline; (b) catalyzing condensation reaction between aniline and benzaldehyde; (c) preventing imine inward diffusion in specific orientation with CN bond exposed to Ni NPs for hydrogenation; and (d) catalyzing benzaldehyde diethyl acetal formation and preventing its further inward diffusion toward Ni NPs. Note that the function of amNi-MOF-74 shell on immobilization of Ni NPs has been illustrated in Figure 1c. H


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subjected 15 mg of Ni/SiO2@amNi-MOF-74 (16 h) sample to repeated reaction runs under exactly the same high-temperature and high-pressure reaction conditions (entries 9−12, Table 1). On the basis of this experiment, good reactivity and selectivity have been achieved after four repeated runs (Figure 10). It is noted that the conversion of nitrobenzene dropped

imine 4/amine 5 selectivity. The molecular dimensions of imine 4 are estimated to be 1.1 nm × 0.5 nm × 0.4 nm. Mesoporous silica shells have pore channels larger than 3 nm in diameter, which is more than enough to accommodate the diffusion of imine 4 in any orientation toward Ni NPs in order to be hydrogenated. Our amNi-MOF-74 shell, on the other hand, has a much narrower pore dimension of only 1.1 nm.49 For molecule 4, diffusion within this narrow channel could only occur in one orientation, with its (0.5 nm × 0.4 nm) plane perpendicular to the channel, in which the CN bond could not come into contact with Ni NPs (Figure 9b). On the other hand, orientations exposing CN bond to Ni NPs, with its (1.1 nm × 0.5 nm) plane or its (1.1 nm × 0.4 nm) plane perpendicular to the channel, could hardly diffuse through the MOF channel because of the size-sieving effect (Figure 9c). Hence over-hydrogenation from imine 4 to amine 5 could be effectively inhibited or minimized (entries 1−4, Table 1). Finally, hydrogenation of benzaldehyde 3 has been prevented by promoting reversible acetalization reaction. As shown in the previous experiment of Ni/SiO2 without amNiMOF-74 shell, Ni NPs are able to hydrogenate benzaldehyde 3 to benzyl alcohol 6. However, such a hydrogenated product is not detected in the presence of our amNi-MOF-74 shell. It is known that the acidic sites of amNi-MOF-74 can catalyze the reversible acetalization reaction of benzaldehyde 3 with ethanol to benzaldehyde diethyl acetal 7, in which the molecular dimensions increase to 0.9 nm × 0.8 nm × 0.3 nm. This is confirmed by a controlled experiment performed by using only benzaldehyde as the reactant (entry 8, Table 1). We found that the majority of benzaldehyde 3 (81.6%) has been converted to benzaldehyde diethyl acetal 7 with no benzyl alcohol 6 produced in the presence of an amNi-MOF-74 shell. Because the size of benzaldehyde diethyl acetal 7 is comparable to the channel dimension of amNi-MOF-74, it will hardly diffuse further into the channel in any orientation (Figure 9d). Cooperative actions between acidic sites and narrow channel dimension have prevented the hydrogenation of benzaldehyde. It could be taken that benzaldehyde 3 in solution will be diffusing into the MOF channel and gets converted to benzaldehyde diethyl acetal 7 within the channel. This then prevents it from further diffusing into the shell channel and finally gets hydrogenated by Ni NPs. However, this will not cause permanent blockage of channel as the acetalization reaction herein is reversible. Benzaldehyde diethyl acetal 7 could easily revert to benzaldehyde 3 and undergoes condensation reaction with outward diffusing aniline 2. Thus, when both nitrobenzene and benzaldehyde are present, a more competitive condensation reaction between aniline 2 and benzaldehyde 3 continuously consumes benzaldehyde 3, lowering the concentration of benzaldehyde diethyl acetal 7 at the same time according to Le Chatelier’s principle. This can also explain why the byproduct from less competitive acetalization is negligible from the final product (entries 1− 4, Table 1). In addition to that, the result from the mSiO2@ Ni/SiO2@mSiO2 sample also supports this finding. Despite having acidic sites, the mesoporous channel of mSiO2 is not able to sieve out the diffusion of benzaldehyde diethyl acetal 7. As a result, benzyl alcohol is produced as the side product. MOF-based catalysts are often subjected to stability issues, and the amorphous structure tends to be less stable as compared to its crystalline counterpart. The preservation of multifunctionality of our catalyst under repeated reaction runs is then of our concerns. To investigate that aspect, we have

Figure 10. Conversion (red bar) and selectivity (purple bar) of tandem imination of nitrobenzene with benzaldehyde of each cycle using 15 mg of Ni/SiO2@amNi-MOF-74 (16 h).

slightly after four runs probably because of accumulated sample loss during each cycle. More importantly, however, the selectivity of imine over amine has been maintained at 100% throughout these runs, suggesting that the amorphous NiMOF-74 layer is reasonably stable to provide all its designed functionalities during these repeated runs.

4. CONCLUSIONS In summary, we have developed a new synthetic approach to produce a carboxylate-based uniform amNi-MOF-74 coating shell onto Ni/SiO2 hollow spheres by employing Ni NPs as a solid precursor. Integration of transition-metal NPs and MOFs has been achieved to produce a core−shell structured hollow catalyst. The developed Ni/SiO2@amNi-MOF-74 catalyst has been tested for tandem imination of nitrobenzene with benzaldehyde, while the multifunctional amNi-MOF-74 shell allows Ni NPs to function as a low-cost alternative for noble metals. In this tandem reaction system, the amNi-MOF-74 shell has displayed four distinct functional roles: to prevent leaching and segregation of active Ni NPs, to act as an acid catalyst, to overcome deep hydrogenation of the desired product via molecular sieving, and to suppress the side reaction product via promoting competitive reaction with a size-sieveable product. By exploiting the multifunctionality of coating shells, wellestablished industrial catalysts can work in an unprecedented manner in terms of process efficiency and cost reduction, as demonstrated by the high yield of the desired imine at full conversion using the above catalyst.

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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/acs.chemmater.9b02070. Detailed experimental procedures; TEM images of samples studied at different experimental conditions; XPS, XRD, TGA, and EDX patterns of respective I


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samples; and calculation of Ni distribution between Ni NPs and amNi-MOF-74 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Bowen Li: 0000-0001-7783-0131 Hua Chun Zeng: 0000-0002-0215-7760 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support provided by the Ministry of Education, Singapore, NUS, and GSK Singapore. This project is also partially funded by the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) program.

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Synthetic Chemistry and Multifunctionality of an Amorphous Ni-MOF

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