A Stable Polyoxometalate-Based Metal–Organic Framework as Highly

2 days ago - Synopsis. A copper-containing 3D POMOF, HENU-1, which is based on Dawson-type phosphotungstates and in situ-generated ligands, has ...
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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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A Stable Polyoxometalate-Based Metal−Organic Framework as Highly Efficient Heterogeneous Catalyst for Oxidation of Alcohols Dandan Li, Qiaofei Xu, Yingguang Li, Yueting Qiu, Pengtao Ma, Jingyang Niu,* and Jingping Wang* Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, College of Chemistry and Chemical Engineering, Henan University, Kaifeng, 475004, Henan, P. R. China

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ABSTRACT: A novel copper-containing 3D polyoxometalate-based metal− organic framework (POMOF), H[Cu5ΙCuΙΙ(pzc)2(pz)4.5{P2W18O62}]·6H2O (HENU-1, HENU = Henan University; Hpzc = pyrazine-2-carboxylic acid, pz = pyrazine), was successfully isolated by a one-step hydrothermal method. In this compound, the {P2W18} polyanion acts as a seven-connected linker bridging adjacent 2D double-layer networks, as well as a template to induce the formation of the desired 3D framework. Particularly, the pz ligands are generated from pzc ligands in situ during the reaction process. HENU-1 exhibits not only good stability in air but also tolerance to acidic and basic media. It was first employed as a highly efficient heterogeneous catalyst for the oxidation of 1-phenylethanol into acetophenone, which shows 97% yield using tert-butyl hydroperoxide as oxidant with a turnover frequency of up to 9690·h−1, and was reused for at least five cycles without significant catalytic activity loss. No POM leaching or framework decomposition was observed in our study.

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solving the above problems.24−29 Specifically, the accessible ordered porous structure of POMOFs can facilitate substrate diffusion at the molecule level to realize the homogeneity of the heterogeneous catalysis.30−32 More importantly, the spatial distances between the POM anions can be significantly increased via the immobilization of functional POMs into MOFs, thereby improving the dispersity of anion clusters and exposing more active sites.33,34 Inspired by that, catalytically active POM precursors were screened first, to be assembled with low-cost transition metals and organic ligands under hydrothermal conditions, so as to directly obtain the POMOF material employed as an efficient and stable heterogeneous catalyst for the oxidation of alcohols. Preliminary studies indicated that Dawson-type phosphotungstates exhibit better catalytic activities on the selective oxidation of 1-phenylethanol into acetophenone among several potential active POM species (Table S1). Considering from the synthetic perspective, however, the immobilization of functional Dawson anions into MOFs is not simple owning to their larger steric hindrance, compared with Keggin clusters.35−44 The mixedligand systems consisting of two linkers have proven an impactful approach in directing the construction of an open framework to encapsulate Dawson anions directly.35,36,40 Especially, the bifunctional pyrazine carboxylic acid ligand is a good candidate because of its capacity for pyrolysis in situ to give a secondary ligand under certain hydrothermal conditions.

INTRODUCTION The oxidation of alcohols into carbonyl compounds is a frequently important research topic in the laboratory, for aldehyde/ketone products take up the key position in the chemical industry including the fine chemical and pharmaceutical industry.1,2 Traditionally, various stoichiometric amounts of oxidizing agents (e.g., MnO2, CrO3, SeO2, KMnO4, K2Cr2O7, etc.) have been widely used in industry3 but under severe conditions such as high pressure or temperatures and/or with strong mineral acids.4 Also, a mass of environmentally harmful toxic inorganic salts is usually generated from these chemical transformations.5 Hence, it is still a challenge to design and construct an effective, sustainable, and eco-friendly oxidative catalytic platform. Polyoxometalates (POMs), as a subset of metal oxides, have been employed rather extensively as catalysts for various organic reactions over the past few decades, because of their unique redox properties and tunable acidity.6−12 Recently, many POM-based catalyst systems have been developed for the oxidation of inactive alcohols;13−20 however, among them, only lots of noble metals-containing (Ru, Pd, Au) catalysts exhibit high efficiency on the alcohol oxidation reaction,17,21−23 which is not suitable for the mass preparation of cheaper fine chemicals. Also, the inherent shortcomings of poor stability and low exposure of active sites impeded the further application of POM-based catalysts. In this regard, POM-based metal−organic frameworks (POMOFs), which maximize the superiority of POMs and MOFs, have already proven to be an effective strategy for © XXXX American Chemical Society

Received: December 24, 2018

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DOI: 10.1021/acs.inorgchem.8b03589 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

The Procedure of the Catalysis for the Oxidation of Alcohols. The alcohols (1 mmol), tert-butyl hydroperoxide (TBHP) (3 mmol) catalyst (0.2 mol %), and CH3CN (1 mL) were added into a tube at room temperature. The mixture was further stirred under the prescribed conditions on the parallel reactor. All of target products were identified by GC−MS, and the yields were monitored by GC with methylbenzene as internal standard. The catalyst was recovered by filtration when the reaction mixture was cooled to room temperature at the end of each cycle, and then washed thoroughly (at least three times) with acetonitrile, and dried in an oven at 120 °C overnight to be reused for the next round. X-ray Crystallography. A suitable crystal of HENU-1 was selected and placed on a Bruker Apex-II CCD diffractometer. The diffraction intensity data were collected at a temperature of 296.15 K using a graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) as the radiation source. Using Olex2,48 the structure was solved with the ShelXT structure solution program using Intrinsic Phasing and refined with the ShelXL refinement package49 using Least Squares minimization on F2. In the last refinement cycles, all the heavy atoms were refined anisotropically and the remaining non-hydrogen atoms were isotropically refined. The lattice water molecules were located by using a Fourier map and further determined by TGA results. The hydrogen atoms of the organic ligands were placed in calculated positions, while those H atoms from water molecules were added into the formula directly. A summary of the crystallographic data and structural determination of HENU-1 is listed in Table 1, and

Besides, taking into consideration that the copper sites are generally regarded as one of the active centers in most oxidation reactions,13,45,46 we chose copper acetate, {P2W18} anions, and bifunctional ligand, pyrazine-2-carboxylic acid (Hpzc), as the reactants to construct a novel POMOF catalyst for the oxidation of alcohols. In this work, we reported a copper-containing 3D POMOF based on Dawson-type phosphotungstates and in situgenerated ligands, namely, H[Cu5ΙCuΙΙ(pzc)2(pz)4.5{P2W18O62}]·6H2O (HENU-1; Hpzc = pyrazine-2-carboxylic acid, pz = pyrazine), where the pzc ligands are partly pyrolyzed to generate the pz secondary ligands (Scheme 1), and both of Scheme 1. Schematic Diagram of Ligands Decomposition in Situ

them participate in the construction of the framework. Particularly, HENU-1 exhibits not only good stability in air but also tolerance to acidic and basic media. Furthermore, it was employed as an efficient heterogeneous catalyst for the oxidation of 1-phenylethanol into acetophenone, which has shown 97% yield with a high turnover frequency (TOF) of 9690 h−1 under certain conditions, and reused for at least five runs without obvious decrease in catalytic performance.

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Table 1. Crystal Data and Structure Refinement for HENU1 compound empirical formula formula weight T/K crystal system space group a/Å b/Å c/Å α/deg β/deg γ/deg V/Å3 Z ρcalc/g cm−3 μ/mm−1 crystal size/mm3 2θ range/deg index ranges reflns collected independent reflns data/restraints/parameters GOF on F2 R1, wR2 [I ≥ 2σ(I)] R1, wR2 [all data]

EXPERIMENTAL SECTION

Materials and Methods. All chemical reagents were purchased from the commercial supplier and used without further purification, except for the precursor K6P2W18O62·15H2O was prepared following ref 47 and further identified by IR spectrum. The IR spectrum was obtained on a Bruker VERTEX 70 IR spectrometer through KBr pellet in the range of 400−4000 cm−1 region. Elemental analyses (C, H, and N) were performed on an Elementar VarioElcube CHNS analyzer. ICP-OES analyses (Cu, P, and W) were performed with a Perkin-Elemer Optima 2100DV spectrometer. The thermal gravimetric analyses (TGA) were measured in a flowing N2 atmosphere on a NETZSCH STA 449 F5 Jupiter thermal analyzer from 35 to 970 °C with a heating rate of 10 °C min−1. The powder X-ray powder diffraction (PXRD) patterns were carried out on a Bruker D8 Advance instrument with Cu Kα radiation (λ = 1.5418 Å). X-ray photoelectron spectra (XPS) were recorded with an Axis Ultra X-ray photoelectron spectrometer. GC analyses were performed on a Bruker 450-GC with a flame ionization detector equipped with a 30 m column (GsBP-5, 0.25 mm internal diameter and 0.25 μm film thickness) with nitrogen as carrier gas. Synthesis of Compound H[Cu5ICuII(pzc)2(pz)4.5{P2W18O62}]· 6H2O (HENU-1). A mixture of K6P2W18O62·15H2O (0.906 g, 0.20 mmol) and Cu(OAc)2·2H2O (0.256 g, 1.28 mmol) was dissolved in 5 mL of distilled water under stirring. Then, pzc (0.164 g, 1.32 mmol) was added into the resulting solution and stirred for another 30 min. After that, the pH value was adjusted to 1.98 by adding 4 M HCl dropwise and the suspension was sealed in a 25 mL Teflon-lined autoclave and further heated at 160 °C for 4 days under autogenous pressure. After slow cooling to room temperature at a rate of 10 °C h−1, black block crystals of HENU-1 were isolated and further collected by washing with distilled water, and then air-dried to give a yield of 40% (based on W). Elemental analysis (%) calcd for C28H24Cu6N13O67P2W18 (Mr = 5367.08): Cu, 7.10; P, 1.15; W, 61.66; C, 6.27; H, 0.451; N, 3.39. Found: Cu, 7.02; P, 1.05; W, 61.72; C, 6.39; H, 0.404; N,3.48. IR (KBr, cm−1): 3462 (w), 1635 (s), 1603 (s), 1425 (w), 1356 (w),1161 (w), 1088(s), 961 (w), 912 (w), 794 (s).

HENU-1 C28H24Cu6N13O67P2W18 5367.08 296.15 triclinic P1̅ 13.136(4) 17.177(5) 20.598(7) 89.288(6) 87.601(6) 70.447(6) 4376(2) 2 4.073 25.114 0.22 × 0.17 × 0.14 3.198−50.198 −14 ≤ h ≤ 15, −19 ≤ k ≤ 20, −20 ≤ l ≤ 24 22717 15409 15409/2/665 1.003 R1 = 0.0895, wR2 = 0.2099 R1 = 0.1597, wR2 = 0.2605

the selected bond lengths and angles are given in Table S2. CCDC 1869003 contains the supplementary data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre.

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RESULTS AND DISCUSSION Structural Description. Single-crystal structural analyses showed that HENU-1 crystallizes in the triclinic P1̅ space group. In this compound, the Dawson-type POMs, {P2W18}, as guests, have been introduced into a 2D Cu/pz/pzc doubleB

DOI: 10.1021/acs.inorgchem.8b03589 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry layered net and further bridged the adjacent networks to give a 3D POMOF. Specifically, the asymmetry unit of HENU-1 is composed of one [P2W18O62]6− polyanion, five Cu+, one Cu2+, two deprotonated pzc ligands, four and a half pz ligands, and six lattice water molecules (Figure S1). For the purpose of charge balance, one proton was added directly to give the final chemical formula of H[Cu5ΙCuΙΙ(pzc)2(pz)4.5{P2W18O62}]· 6H2O. The {P2W18} anions show a typical Dawson-type polyanions structure which offers more profuse coordination sites to capture transition metals, compared with Keggin-type POMs. Bond valence sum (BVS)50 calculations indicated that the P center and W atoms are all conventionally in +5 and +6 oxidation states, while copper atoms are in mixed-valence states where Cu1, Cu2, Cu3, Cu5, Cu6 are all +1 valences apart from that the Cu4 is +2 oxidation state, exceptionally (Table S3). The presence of mixed-valence copper is further evidenced by XPS (Figure S7a). In HENU-1, there are six crystallographically independent copper atoms (Cu1, Cu2, Cu3, Cu4, Cu5, and Cu6), which exhibit two kinds of coordination modes (Table S3). The bond lengths of Cu−O (1.85−2.83 Å) and Cu−N (1.84−2.02 Å) are within a reasonable range. The O/N−Cu−N/O bond angles vary from 81.4(12)° to 177.8(11)° (Table S2). It is worth noting that the secondary ligands pz are generated from pzc ligands in suit through pyrolysis under certain condition. Each linear pz ligand bridges two Cu atoms (Cu1, Cu2, Cu3, Cu5, and Cu6) in a μ2-coordination fashion (Figure S2a). Two deprotonated pzc ligands in a central symmetric position are bridged by Cu4 atom and further attached to two Cu atoms (Cu3, Cu1; Cu5, Cu6), respectively (Figure S2b). In this coordination mode, two linear pz ligands and one tetra-connected pzc ligand alternately interconnect two adjacent copper atoms to form a [Cu8(pz)4(pzc)2] macrocycle (A) which possesses a 14.45 × 12.15 Å window (Figure S3a). Furthermore, the macrocycle A fused with another two neighboring ones by edge-sharing to give a 1D loop containing chain (Figure 1a). Beyond that, a similar alternating mode is employed for the rest of the organic ligands, three pz ligands and one pzc ligand, to link adjacent copper atoms. As a result, a novel [Cu10(pz)6(pzc)2] macrocycle (B) is generated. And the window size of macrocycle B is about 18.67 × 12.92 Å (Figure S3b). Differently, each macrocycle B is linked with another six identical ones to give a 2D layer (Figure 1b). Such two parallel 2D layers are bridged by the 1D loop containing chains mentioned above in a vertical direction via N4-Cu4-N5 and O63-Cu4-O65 linkages. Thus, a 3-connected Cu/pz/pzc double-layered net with the new {6.72}{62.7}{7.82}2{72.8} topology is formed (Figure 1c). It should be pointed out that, from the a-axis direction, the 2D bilayer structure possesses a super large Z-shaped void with a size range of 17.18 × 15.13 Å which is precisely accessible for the encapsulation of Dawsontype POMs (12.3 × 10.5 Å). The {P2W18} anion, as a sevenconnected inorganic template, is incorporated into the host double-layered network (Figure 1d). Two adjacent polyanions are linked by two O2−Cu3−O4 bonds to form a dimer, which fits the large void exactly. In addition, the introduction of POMs joints two identical double-layered networks to give a resulting 3D complicated POMOF structure through the Cu-O linker, which stabilizes the entire framework to some extent (Figure 1e). From the topology view, if the {P2W18} polyanions are considered as 7-connected nodes, Cu1, Cu2, Cu3, Cu5, and Cu6 are considered as 4-connected nodes, and

Figure 1. (a) 1D loop containing chain in 2D host double-layered framework; (b) 2D layer based on edge-sharing large rings; (c) 2D host double-layered framework; (d) coordination mode of {P2W18} cluster; and (e) 3D framework of HENU-1.

pzc ligands are considered as 3-connected nodes, the whole structure of HENU-1 can be simplified as a 3D (3,4,7)connected framework with a new topology {3.4.5.62.7}{3.6.73.8}{3.62.73}{3.63.72}{32.4.5.67.74.83.93}{62.7}2{65.8} (Figure 2). The solvent-accessible void in HENU-1 is 13.1% calculated by using the PLATON program.51

Figure 2. (a) The topology network of the 2D host double-layered net and (b) 3D framework of HENU-1.

Stability Tests. The phase purity was first certified by PXRD measurement. As can be seen from Figure 3, the experimental value is in good agreement with the simulated value, which proves that the collected samples are pure. In addition, HENU-1 exhibits good thermal stability before 320 °C. According to the TG curve, HENU-1 shows the first weight loss step of 2.1% within 200 °C, corresponding to the removal of six lattice water molecules (Figure S8a). At this moment, the framework of HENU-1 still remains intact, which was further confirmed by PXRD after heating the sample in an oven at different temperatures for 12 h (Figure S8b). Starting from 320 °C, the skeleton starts to collapse. The observed second weight loss step occurred in the range of 320−740 °C of 11.4%, which is ascribed to the decomposition of organic molecules, and the third weight loss step between 740 and 950 °C corresponds to the partial collapse of {P2W18} clusters. Additionally, the framework is air-stable with good crystallinities maintained after several months. Anything else, the solution stability of HENU-1 was further investigated. It is steadily undissolved in water and common organic solvents, C

DOI: 10.1021/acs.inorgchem.8b03589 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 3. PXRD patterns of HENU-1 after 24 h immersion in (a) acidic and (b) basic aqueous solutions with the pH range of 1−12.

Compared to HENU-1, only 24% yield of acetophenone could be obtained in the presence of the POM precursor (K6P2W18O62·15H2O). Notably, Cu(OAc)2 (Table 2, entry 3) behaved more active than other raw materials including the separate {P2W18} precursor and dual organic ligands (Table 2, entries 2, 4, and 5), which implied that copper atoms played a vital role in this transformation. In addition, the catalytic activity of the P2W18/Cu mixture is better than that of any single constituent, indicating that there would be the synergic effects between POM units and copper atoms in HENU-1 (Table 2, entry 6). The P2W18/Cu/pz/pzc mixture has no superior catalysis than P2W18/Cu, suggesting the ligands pz, pzc do not have an obvious contribution to the system (Table 2, entry 7). Furthermore, compared with the P2W18/Cu/pz/ pzc mixture, the higher catalytic efficiency of HENU-1 demonstrates that the formation of the framework, which possesses large solvent accessible voids, may have a positive effect on the process. Optimization of Catalytic Oxidation of 1-Phenylethanol with HENU-1. In order to find the optimal reaction conditions for the oxidation of 1-phenylethanol with HENU-1, the influence of various reaction parameters was studied in detail. Initially, the dosage of catalyst came into the first research object. As shown in Table 3, with the increasing amount of catalyst (from 0.05 to 0.2 mol %), the acetophenone yields rose from 94% to 97% (Table 3, entries 1−3), correspondingly. It is obvious that reaction activity was positively correlated with catalyst dosage in a certain range. Similarly, a higher dosage of the oxidants is also beneficial to the conversion of this reaction (Table 3, entries 3−5). Keeping the other parameters constant, when the temperature goes up from 50 to 60 °C, the yield increases somewhat (about 4%). As the temperature continues to rise, however, the production rate is fixed at 97% without improvement (Table 3, entries 3, 6, and 7). Therefore, the optimal reaction temperature can be locked at 60 °C. In addition, the influence of solvent on catalytic activity is also an important factor to be considered. For the template reaction of the oxidation of 1-phenylethanol to acetophenone, it is observed that there is already a yield of 69% in the solvent-free system (Table 3, entry 12). When some organic solvents, such as toluene, octane, and methanol, were, respectively, introduced into the catalyst system, the yields were significantly reduced to 56%, 59%, 46% (Table 3, entries 8−10). In contrast, when acetonitrile and dichloromethane

such as ethanol, acetonitrile, toluene, methanol, dichloromethane, and octane. Remarkably, this POMOF also exhibits excellent tolerance in acidic and basic aqueous solutions with the pH range of 1−12, which can be further demonstrated by PXRD after 24 h immersion (Figure 3). It is rare for MOFs to remain stable in both acidic and basic aqueous solutions, especially for POMOFs, which are always disassembled in the alkaline environment for the inherent acidity of POMs. In a word, good chemical stability is essential for the further study of heterogeneous catalytic properties. Catalytic Tests. The successful encapsulation of functional Dawson anions and good chemical stability offer the HENU-1 promising potential for further catalytic studies. Concretely, the catalytic activity of as-synthesized HENU-1 was evaluated in the oxidation of 1-phenylethanol into acetophenone using TBHP as the oxidant under mild conditions. And the results of parallel tests have demonstrated that HENU-1 possessed better catalytic activity for the oxidation of 1-phenylethanol compared with the catalytically active precursors. As shown in Table 2, running the experiment without any catalyst at 60 °C for 0.5 h resulted in the formation of the desired product acetophenone only with a yield of 11% (Table 2, entry 1). However, great promotion in yields (11% to 84%) was observed distinctly when HENU-1 employed as heterogeneous catalysts were added (Table 2, entry 8), which indicates its superior catalytic activity in the oxidation of 1-phenylethanol. Table 2. Catalytic Oxidation of 1-Phenylethanol with Different Catalystsa entry

cat.

reaction system

yieldb/%

1 2 3 4 5 6 7 8

P2W18 Cu(OAc)2 pz pzc P2W18/Cu P2W18/Cu/pz/pzc HENU-1

homogeneous homogeneous homogeneous homogeneous homogeneous homogeneous heterogeneous

11 24 58 16 14 74 72 84

a

Reaction conditions: 1-phenylethanol (1 mmol), TBHP (3 mmol), catalyst (0.2 mol %), CH3CN (1 mL), T = 60 °C, t = 0.5 h. bGC yields for target product acetophenone were based on methylbenzene as internal standard. All of the products were identified by GC−MS spectra and GC spectra. D

DOI: 10.1021/acs.inorgchem.8b03589 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

acetophenone is not significantly reduced after at least five cycles, which fully indicates the stability and recyclability of the catalyst. Meanwhile, PXRD patterns and IR spectrum proved that the framework of catalyst HENU-1 remains intact after catalysis (Figure S9). Furthermore, the ICP results also showed that there was no W element detected in the recycled filtrate, further demonstrating the stability of catalyst HENU-1 during five contiguous runs. Scope of Substrate. To evaluate the applicability of HENU-1, various alcohols were used as substrates under the optimal reaction conditions, including electron-donating and electron-withdrawing groups substituted 1-phenylethanol derivatives, aliphatic alcohols, and cyclopentanol, which was carried out in the presence of catalyst HENU-1. The obtained results are listed in Table 4. Both electron-withdrawing and electron-donating 1-phenylethanol derivatives can be effectively oxidized to the corresponding products using HENU-1 as catalyst. For instance, a high yield (96%) can be simultaneously observed in the transformation of methyl/ methoxyl or bromo substituted 1-phenylethanol into acetophenone derivatives (Table 4, entries 1−3). However, the substrate with large steric hindrance could not achieve a good yield of related product under the same conditions (85%; Table 4, entry 7). In terms of aliphatic alcohols substrates, the low yields of corresponding products were obtained due to the inherent inert reactivity of substrates (Table 4, entries 8−10). Kinetic Study. To clarify the kinetic state of the catalytic oxidation of 1-phenylethanol, a series of catalytic experiments were conducted at different temperatures and monitored at intervals. As shown in Figure S10b, the yield and C/C0(C0 − C) are plotted against the reaction time at optimal 60 °C, respectively, in which C0 and C represent the corresponding initial and transformed concentration. The linear fitting of the data proves that the catalytic process conforms to second-order kinetics (R2 = 0.9992), and the value of the reaction rate constant k was calculated to be 0.14332 mol L−1 min−1 according to eq 1. The same linear fittings were performed for the catalytic reaction data at other temperatures (50 °C, 70 °C). The results showed that the catalytic experiments were all consistent with the kinetic characteristics of the second-order reaction within the temperature range tested (Figure S10a,c), and the corresponding rate constants k were 0.10846 mol L−1 min−1, and 0.18595 mol L−1 min−1, respectively (Figure S10e). As shown in Figure S10d, the yield of product displayed a gradual upward trend at different temperatures within the 60

Table 3. Optimization of the Different Reaction Conditions for the Catalytic Oxidation of 1-Phenylethanol with HENU1a

entry

cat./mol %

TBHP/equiv

T/°C

solvent

yieldb/%

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

0.05 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

3 3 3 2 1 3 3 3 3 3 3 3

60 60 60 60 60 50 70 60 60 60 60 60

CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN toluene octane MeOH CH2Cl2

94 95 97 91 78 93 97 56 59 46 71 69

a

Reaction conditions: substrate (1 mmol), t = 3 h, solvent (1 mL or none). bGC yields for target product acetophenone were based on methylbenzene as internal standard. All of the products were identified by GC−MS spectra and GC spectra.

were used as solvents, severally, the yields were visibly improved (Table 3, entries 3, 11). Especially, the catalyst in acetonitrile exhibits the best performance for the process where an excellent yield of 97% has been obtained (Table 3, entry 3). In particular, compared with the reported POMbased catalysts, the catalyst HENU-1 has lower requirements on reaction conditions but possesses superior catalytic performance for the oxidation of 1-phenylethanol (Table S4). Recyclability and Stability Tests. For the purpose of verifying the heterogeneous system of the catalyst HENU-1, a hot filtration test was performed. The reaction mixture was filtered to remove HENU-1 after 5 min reaction, while the filtrate continued to react for another 25 min. And the 1phenylethanol conversion finally reached 55.7%, demonstrating that the oxidation reaction does not proceed after the removal of the catalyst (Figure 4a). In addition, the cyclability of catalyst HENU-1 for the oxidation of 1-phenylethanol was studied under the optimal conditions. The catalyst can be easily separated and reused in the next round of experiments. As presented in Figure 4b, the yield of target product

Figure 4. (a) Hot filtration test for the HENU-1 catalyst. (b) Recyclability of the HENU-1 catalyst. E

DOI: 10.1021/acs.inorgchem.8b03589 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 4. Catalytic Oxidation of Different Substratesa

a Reaction conditions: substrate (1 mmol), TBHP (3 mmol), catalyst (0.2 mol %), CH3CN (1 mL), T = 60 °C, t = 3 h; bGC yields for target product were based on methylbenzene as internal standard. All of the products were identified by GC−MS spectra and GC spectra.

Scheme 2. Mechanism for Oxidation of 1-Phenylethanol Catalyzed by HENU-1

32% at 70 °C; that is, the TOF is as high as 9690 h−1, which showed impressive catalytic performance among the POMbased catalysts used for the oxidation of alcohols (Table S4). Encouraged by the outstanding catalytic performance of HENU-1, enlarged-scale test of the 1-phenylethanol substrate (from 1 to 10 mmol, 0.2 mol % catalyst, 5 mL of CH3CN, 60 °C) was further conducted, and turnover number (TON) reaches 4000 after 3 h with an acetophenone yield of 80%.

min detection time range when HENU-1 was used in the system. In addition, increasing the reaction temperature from 50 to 70 °C leads to a promotion in the yield from 87% to 92%, indicating that the activity of this catalytic reaction was significantly affected by the temperature. On this basis, activation energy (Ea) of the oxidation of 1-phenylethanol catalyzed by HENU-1 under optimal conditions was calculated according to the Arrhenius law [eq 2] for 24.829 kJ mol−1 (Figure S10f). Besides, the activation enthalpy was determined at ΔH⧧ = 22.060 kJ mol−1 [eq 3], the activation entropy at ΔS⧧ = −228.624 J mol−1 K−1 [eq 4], and the Gibbs energy at ΔG⧧ = 97.049 kJ mol−1 [eq 5]. The negative entropy indicates that there is a correlation process for the rate-limiting step.52 In particular, the conversion of the substrate in 1 min reached F

C /[C0(C0 − C)] = kt

(1)

ln k = −Ea /RT + ln A

(2)

ΔH ⧧ = Ea − RT

(3) DOI: 10.1021/acs.inorgchem.8b03589 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry k=

kBT ⊖ 1 − n ijj Δ⧧r Sm⊖(c ⊖) yzz ijj Δ⧧r Hm⊖(c ⊖) yzz zzexpjj− zz (c ) expjjj z j z h R RT k { k {

ΔG = ΔH ⧧ − T ΔS ⧧

Accession Codes

CCDC 1869003 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

(4) (5)

Mechanism Study. For the purpose of the determination of catalytic mechanism, a radical capture experiment was further carried out, and the result showed that the catalytic activity for the oxidation of 1-phenylethanol was significantly decreased after the addition of the radical scavenger butylated hydroxytoluene (BHT), demonstrating the nature of the radical reaction process (Table S5, entry 4). Whereafter, this catalytic system was proceeded under O2 and N2 atmospheres, respectively, but no obvious difference could be observed, which indicated that O2 molecules did not participate in the process (Table S5, entries 1−3). On the basis of the observations above, a possible reaction mechanism in Scheme 2 was proposed according to the literatures.4 Initially, TBHP (tBuOOH) reacted with Cu sites of POM-Cu(Ι) catalyst in a so-called Haber−Weiss catalytic mechanism, resulting in a transformation from tBuOOH into tert-butoxyl (tBuO·) radical and hydroxyl ion (OH−).53,54 Subsequently, accompanied by the leaving of a tBuOH, the generated tBuO· interacted with 1phenylethanol to form the radical (1), which would convert into the intermediate (2) with the effect of Cu(ΙΙ) sites in HENU-1. Afterward, the corresponding acetophenone was formed along with the departure of water molecules and the regeneration of active sites Cu(Ι), which was supported by the XPS data of Cu element in the HENU-1 after catalysis (Figure S7b). The formation of both tBuOH and H2O molecules has been detected during the transformation, which experimentally supported the proposed mechanism.

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Corresponding Authors

*E-mail: [email protected] (J.N.). *E-mail: [email protected] (J.W.). ORCID

Jingyang Niu: 0000-0001-6526-7767 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Natural Science Foundation of China (Grants 21571050, 21573056, 21771053, 21771054).

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REFERENCES

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CONCLUSION In summary, we reported a novel copper-containing 3D POMOF based on Dawson-type phosphotungstate and in situgenerated ligands, HENU-1, which was employed as a heterogeneous catalyst for oxidation of alcohols, exhibiting superior catalytic activity. The Cu-based POMOF displays a noninterpenetrating 3D framework with new topology, in which {P2W18} polyanions act as a seven-connected linker connecting adjacent 2D double-layer networks, as well as templates to induce the formation of the final framework. Notably, HENU-1 can efficiently catalyze the oxidation of 1phenylethanol, achieving a 97% yield of the target product acetophenone after 3 h with a high TOF as 9690 h−1. This work provides a new approach to the development of POMbased catalysts. In this regard, incorporating the active transition metal ions and POMs to assemble POMOFs through the bridging of organic ligands may become an effective method to construct a POM-based heterogeneous catalytic platform for various organic reactions. This work is ongoing in our group.

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AUTHOR INFORMATION

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03589. BVS, PXRD, IR, XPS, TG, and catalytic properties of HENU-1 and the structures of HENU-1 (PDF) G

DOI: 10.1021/acs.inorgchem.8b03589 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b03589 Inorg. Chem. XXXX, XXX, XXX−XXX