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A molybdate-incorporated cooperative catalyst: High efficiency in the assisted tandem catalytic synthesis of cyclic carbonates from CO2 and olefins
Molecular Catalysis 461 (2018) 10–18
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A molybdate-incorporated cooperative catalyst: High efficiency in the assisted tandem catalytic synthesis of cyclic carbonates from CO2 and olefins ⁎
Zhuolin Shia, Guiqin Niua, Qiuxia Hana,b, , Xiaoyun Shia, Mingxue Lia,
T
⁎
a
Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan, 475004, China b State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, Liaoning, 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Assisted tandem catalysis Bifunctional catalyst Molybdate Metal-organic frameworks Cyclic carbonates
Currently, substantial interest is focused on developing assisted tandem catalysis to optimize the reaction conditions for each step in the process. By combining a molybdate oxidation catalyst and alkaline μ3-OH tricopper (II) cores into a metal-organic framework (MOF), a bifunctional catalyst, Cu3(μ3-OH)2(4,4′-BPY)(MoO4)2 (CuMoBPY, BPY = 4,4′-bipyridine), was achieved and successfully explored for the assisted tandem catalytic conversion of olefins into value-added cyclic carbonates. CuMo-BPY displayed a three-dimensional (3D) extended network of two-dimensional (2D) inorganic CuMo layers interconnected with 4,4′-bipyridine groups. Two potential catalytic sites, Mo(VI) oxide (an oxidant for the epoxidation of olefins) and the alkaline μ3-OH of tricopper (II) cores (for the capture and activation of CO2), exhibit an orderly distribution and spatial matching in the framework, which can provide further compatibility with the Mo=Ot-activated epoxidation intermediate to drive tandem catalysis in a single workup stage.
1. Introduction Tandem catalysis is an ideal catalysis process that involves two or more mechanistically distinct reactions promoted by a single catalyst. Currently, this approach has aroused much interest within the synthetic community in an effort to enhance the efficiency of chemical synthesis [1–3]. Tandem catalysis includes two general subcategories: autotandem catalysis and assisted tandem catalysis [4–7]. Autotandem catalysis can proceed without the addition of other reactants or alteration of the reaction conditions [8–10]. Assisted tandem catalysis, by contrast, can be a useful approach for positioning two or more incompatible ingredients within one catalyst through high precision and control, thus promoting the generation of the desired transition state. In particular, additional reagents may be required for a specific catalytic cycle to perform this function [11–13]. Cyclic carbonates are a class of important chemical intermediates that are widely applied in urea and polymer synthesis, as gasoline additives, as electrolytes in lithium battery devices and for other applications [14–17]. A tandem catalytic procedure that processes an olefin through two discrete catalytic steps is the most desirable method for the environmentally benign conversion of CO2 to cyclic carbonates [18–20]. Recently, several efficient catalytic systems for the tandem
transformation of cyclic carbonates from olefins have been developed. In 2009, Sun’s group reported the direct synthesis of styrene cyclic carbonates from styrene and CO2 with a composite system of Au/Fe (OH)3-ZnBr2/[Bu4N]Br under 4 MPa CO2 at 80 °C [21]. In 2011, Hu’s group explored a method to directly synthesize cyclic carbonates from olefins and CO2 by a MoO2(acac)2-quaternary ammonium salt system [22]. In 2015, Wang’s group presented the direct synthesis of cyclic carbonates with Mn-salen catalysis [23], and Gao’s group investigated the one-pot synthesis of propylene carbonate from propylene and CO2 with a system of H3PW12O40-TBABr under 3 MPa CO2 at 140 °C [24]. In 2016, Kholdeeva’s group described the direct synthesis of cyclic carbonates with a Ti-containing catalyst under 0.8 MPa CO2 at 50–70 °C [25]. Although the explorations above have performed excellently in the preparation of cyclic carbonate, most of these approaches include composite systems and require multiple components and high temperatures and pressures. Therefore, there is still a strong imperative to search for new types of heterogeneous catalysts that can unite the active ingredients of multicomponent systems and various catalytic sites into one single framework. Furthermore, the achievement of compatibility between the reaction intermediates and synergy among multiple catalytic cycles continues to be demanding and challenging.
⁎ Corresponding authors at: Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan, 475004, China. E-mail addresses: [email protected] (Q. Han), [email protected] (M. Li).
https://doi.org/10.1016/j.mcat.2018.10.003 Received 27 July 2018; Received in revised form 27 September 2018; Accepted 1 October 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.
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(hv = 1486.6 eV) achromatic X-ray source. The vacuum inside the analysis chamber was maintained at 1 × 10−9 Pa during analysis. Thermogravimetric (TG) analysis was conducted on a Mettler-Toledo TGA/SDTA 851e instrument, with a heating rate of 10 °C min–1, heated from 25 to 800 °C under nitrogen. The morphology and structure of sample catalysts were characterized using a field-emission scanning electron microscope (SEM) (ZEISS, MERLIN Compact) at an accelerating voltage of 15.0 keV. The N2 absorption-desorption isotherms for determining the Brunner − Emmet − Teller (BET) surface area and Barrett − Joyner−Halenda (BJH) pore width distribution were obtained with an ASAP 2460 Micromeritics instrument, using N2 as the adsorbent at 77 K. 1H nuclear magnetic resonance (NMR) spectra were recorded on a Varian INOVA-400 MHz type (1H, 400 MHz; 13C, 400 MHz) spectrometer. The chemical shifts are reported in ppm relative to CDCl3 (d = 7.26) for 1H NMR.
Homogeneous molybdate species have demonstrated high conversion and selectivity for the synthesis of epoxides [26–29]. However, their applicability is limited by poor catalyst separation and reusability. The design of active, environmentally benign and recyclable heterogeneous molybdate-incorporated catalysts is expected to have a major impact on applications of oxidation catalysts. In 2016, Hupp’s group designed a Mo(VI) oxide-deposited MOF for cyclohexene epoxidation that exhibited excellent catalytic activity, with a yield eventually reaching 93% [30]. Nevertheless, the structure and net content were only simulated and computationally determined. The lack of direct single-crystal X-ray diffraction analyses will limit the speculation and verification of the catalytic mechanism. Furthermore, the method to modify molybdate active sites on MOF by solvothermal deposition is more complex than a one-pot procedure. Herein, by incorporating simple MoO42−, copper (II) ion, and 4,4′bipyridine (BPY) bridging links in a mixed solution of H2O and CH3CN under hydrothermal conditions, we report the design and synthesis of the MOF-based material CuMo-BPY. Although the structure of CuMoBPY was reported by Nocera’s group in 2005, only the magnetic properties were described in detail [31]. In this work, we synthesized CuMo-BPY from different precursors and conducted primary research on the catalysis of the material. We envisioned that the incorporation of a Mo(VI) oxide as an oxidation catalyst and the fixing and activation of μ3-OH tricopper (II) cores for CO2 molecules into an MOF would be a powerful approach to devise a tandem catalytic process for the chemical transformation of olefins to cyclic carbonate (Scheme 1). In addition, CuMo-BPY possesses intrinsic crystalline properties, which could provide precise knowledge about the nature and distribution of catalytically active sites as well as the potential interactions between the catalytic sites and the adsorbed substrates.
2.2. Procedure for assisted tandem catalysis from olefins to cyclic carbonate in the CuMo-BPY/TBHP/TBABr system The assisted tandem catalytic reaction was carried out heterogeneously with 10 mmol olefin, 0.01 mmol catalyst, and 20 mmol tbutylhydroperoxide (TBHP, 70% in decane), without extra solvent, under stirring in a 25 mL stainless steel autoclave equipped with a magnetic stirrer. The mixture was then placed in an oil bath and heated to the desired temperature for a predetermined time. After completion of the reaction, 0.1 mmol tetrabutylammonium bromide (TBABr) was added to the reactor without separating the above mixture, CO2 was charged into the autoclave, and the pressure (0.5 MPa) was kept constant during the reaction. The autoclave was placed into an oil bath and heated to the desired temperature. After the desired time, the excess gases were vented. The remaining mixture was degassed and fractionally distilled under reduced pressure. The organic extracts were dried over Na2SO4, evaporated to dryness and purified by column chromatography on silica gel with eluent solvents (dichloromethane/petroleum ether = 4/1) to obtain pure cyclic carbonate. The yields were directly determined by 1H NMR analysis of the reaction solution.
2. Experimental 2.1. Materials and methods All solvents and reagents used in this study were reagent grade and used without further purification. All chemicals were of reagent-grade quality, obtained from commercial sources and used without further purification. The elemental analysis (EA) of C, H, and N was performed on a Vario EL III elemental analyzer. Inductive coupled plasma (ICP) emission spectrometry was performed on a JarrelAshJ-A1100 spectrometer. The infrared (IR) spectra were recorded from a solid sample pelletized with KBr on a JASCO FT/IR-430. The powder X-ray diffractometry (PXRD) spectra were obtained on a Rigaku D/Max-2400. The X-ray photoelectron spectroscopy (XPS) analyses were performed on an ESCALAB 250 XI (Thermo Fisher, USA) spectrometer with an Al Ka
3. Results and discussion 3.1. Structural description An asymmetric unit of CuMo-BPY consists of a BPY ligand, one tetrahedral {MoO4} group, and two crystallographically distinct Cu(II) atoms (Fig. S1). As shown in Fig. 1a, both crystallographically independent Cu(II) ions adopt distorted octahedral geometries. Cu(1) is coordinated to one nitrogen atom from a BPY ligand, two μ3-OH oxygen atoms from {MoO4}, one μ2-OH oxygen atom from {MoO4} and two cis
Scheme 1. Synthetic procedure of CuMo-BPY, showing the pattern for tandem catalytic synthesis of cyclic carbonates from CO2 and olefins. Color code: Cu (sky blue), Mo (green), N (blue), O (red), C (gray); hydrogen atoms are omitted for clarity. 11
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Fig. 1. (a) The coordination environment of the two crystallographically independent atoms Cu(1) and Cu(2). (b) The connection scheme of Cu(II) and Mo(VI) in the inorganic layer along the c-axis. (c, d) The 3D open network of CuMo-BPY with two particular pores. The hydrogen atoms are omitted for clarity. all hydrogen atoms are omitted for clarity.
Fig. 2. (a) The TG curve of CuMo-BPY. (b) The IR spectra of fresh CuMo-BPY catalyst (top line) and the catalyst CuMo-BPY (bottom line) impregnated with TBHP. (c) TPD−CO2 curve of the CuMo-BPY catalyst. (d) The IR spectra of fresh CuMo-BPY (top line) catalyst and the catalyst CuMo-BPY (bottom line) impregnated in an atmosphere of 0.5 MPa CO2. 12
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Fig. 3. SEM images of CuMo-BPY display the morphology and structure of CuMo-BPY nanorods with high resolution.
chemical and thermal stability and meets most of the prerequisites as an ideal platform for heterogeneous catalysis. The results show excellent consistency with the X-ray structure.
bridging oxygen atoms from tricopper (Fig. S2). O(1) and O(4) occupy the axial positions of the elongated octahedron; other atoms build the equatorial plane. Cu(2) is coordinated to four μ3-OH oxygen atoms from {MoO4} and two trans μ3-OH oxygen atoms from tricopper (II); two O (1) atoms occupy the axial positions of the elongated octahedron (Fig. S3). The relevant bond lengths and bond angles are listed in Table S1. The Cu(II) octahedra form complex chains of alternating corner- and edge-sharing Cu3(μ3-OH) triangles. The chains are then linked by three oxygen atoms of the molybdate anion to form Cu3(OH)2(MoO4)2 layers with {MoO4} groups distributed on two sides of the one-dimensional chains (Fig. 1b). The layers are bridged by BPY ligands to form a 3D framework with 1D channels along the b axis (Fig. 1c, S4). The most unique structural feature of CuMo-BPY is that two types of pores are arranged alternately in a square cross section with the approximate dimensions of 4.16 × 7.13 Å for pore A and 5.55 × 9.48 Å for pore B (Fig. 1d). Pore A is a pure inorganic pore with hydrophilicity conferred by the μ-OH copper-molybdenum cores, in which the alkaline μ-OH possesses the function of the capture and activation of CO2, as other studies in the literature have reported [32–34]. Pore B is alternately constituted by hydrophilic μ-OH copper-molybdenum cores and hydrophobic organic BPY ligands, as well as Mo6+ catalytic sites, which are exposed fully and point toward the inside of the pore, favoring contact with styrene and TBHP substrates. The orderly distribution and spatial matching in the framework of the oxidation catalytic sites of Mo (VI) and the alkaline μ-OH copper core can provide further compatibility with the Mo = Ot-activated epoxidation intermediate, driving the tandem catalytic process in a single workup stage.
3.2.2. IR spectra The IR spectra of CuMo-BPY are collected and shown in Fig. 2b (top line). The characteristic bands of ν(Mo = Ot) and ν(Mo–O–Mo) vibrations in CuMo-BPY are in the ranges of 927 and 772 cm–1. The resonances at 1615 and 1321 cm–1 are assigned to the ν(CeN) and ν(NeH) stretching vibrations of BPY, respectively. These characteristic vibration resonances confirm the existence of BPY ligands and {MoO4} in CuMo-BPY. The results of the IR spectra are quite consistent with those of X-ray diffraction. Additionally, the IR spectrum of the catalyst impregnated in TBHP revealed a new ν(OeO) band at 887 cm−1, which indicated that the active intermediate peroxymolybdate was formed during the epoxidation process (Fig. 2b). Additionally, the IR spectrum of the catalyst CuMo-BPY impregnated in an atmosphere of 0.5 MPa CO2 exhibits a weak peak in the vicinity of 1659 cm−1 (Fig. 2d), which indicates the fixation of CO2 molecules by the μ3-OH tricopper (II) core of CuMo-BPY and the formation of a tridentate linear adsorption state. The intense peak in the vicinity of 1098 cm−1 represents the fixation of CO2 molecules by the μ3-OH dicopper (II) core of CuMo-BPY and formation of a bidentate bridge state [32–34]. 3.2.3. Surface base properties of CuMo-BPY samples (CO2-TPD) CO2-TPD (temperature-programmed desorption of CO2) techniques have been widely used to probe the surface base properties of solid catalysts [35] and were therefore employed here to correlate the structure of the base sites with catalytic performance. A strong desorption peak for CO2 is observed at approximately 314.5 °C (Fig. 2c). Based on this result, the high temperature peak could be ascribed to strong basic sites generated by the existence of μ3-OH tricopper (II) cores in CuMo-BPY. Based on the results of CO2-TPD and IR analysis, we speculated that CO2 was adsorbed and activated by μ3-OH in the channels of CuMo-BPY.
3.2. Characterization of CuMo-BPY 3.2.1. Thermogravimetric analysis (TG) The TG curve of a pure sample of CuMo-BPY indicates one weightloss step between 270 and 446 °C, which is associated with the loss of BPY, with a total loss of 24.23% (calcd. 22.87%)(Fig. 2a). The entire structure collapsed completely beyond 446 °C. CuMo-BPY exhibits high 13
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Fig. 4. Corresponding EDS mappings with Cu (KA) and Mo (LA) of CuMo-BPY.
[36]. Two other peaks at 231.8 and 234.9 eV are attributed to Mo6+(3d5/2) and Mo6+(3d3/2), respectively [37]. All of these results further confirm the structural analyses. The porosity and specific surface area of the samples can be determined from N2 adsorption-desorption isotherms (Fig. 5d). The pore width of CuMo-BPY is generally ca. 13.88 nm. According to the N2 adsorption-desorption isotherms, the BET surface area and BJH pore volume of CuMo-BPY are approximately 635.76 m2 g−1 and 1.85 cm3 g−1, respectively.
3.2.4. Morphology characterization The morphology and structure of a CuMo-BPY sample were investigated with a field-emission scanning electron microscope (FESEM). As shown in Fig. 3, CuMo-BPY shows nanorod-like structure with widths in the range of 140–150 nm and lengths of up to 1.3–1.4 micrometers. As shown in the typical SEM images of some representative single rods, most of the nanorods are very straight and uniform with smooth surfaces, although some of the nanorods are slightly agglomerated. Additionally, energy-dispersive spectroscopy (EDS) presents the distribution of the individual elements in the CuMo-BPY sample (Fig. 4a–e). Elemental mapping collected from the same nanorod region identified the homogeneous distribution of Cu (red) and Mo (green). The different intensities of the green and red colors were assumed to be proportional to the metal composition in each sample. It can be concluded that both copper and molybdenum were homogeneously distributed in the framework of CuMo-BPY.
3.3. Assisted tandem catalysis in the CuMo-BPY/TBHP/TBABr system A two-step catalytic process for the epoxidation of olefins and the cycloaddition of CO2 with epoxide was studied separately in order to understand the compatibilities of these catalysts in terms of chemical stabilities in the process utilizing one-pot assisted tandem catalysis. To explore the CuMo-BPY/TBHP/TBABr assisted tandem-catalysis system, various reaction conditions were scanned; the reaction optimization results are presented in Table S3-8. Initially, we examined epoxidation using styrene and TBHP as the oxidant, along with CuMoBPY (0.1% mol ratio), in a heterogeneous reaction at 50 °C for 48 h. As shown in Table 1 (Entry 1a), the results exhibit excellent reaction
3.2.5. XPS spectra and BET analysis The XPS spectra of the Cu 2p and Mo 3d orbitals of the CuMo-BPY sample are displayed in Fig. 5. The peaks observed at 934.4 and 954.0 eV are assigned to Cu2+ (2p3/2) and Cu2+ (2p1/2), respectively 14
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Fig. 5. (a) Survey-scan XPS of the studied membranes and high-resolution XPS spectra of Mo 3d (b) and Cu 2p (c) of a CuMo-BPY sample. (d) N2 adsorptiondesorption isotherms of a CuMo-BPY sample. Table 1 Conversion in the two-step catalytic process: epoxidation of the olefins and cycloaddition of CO2 with epoxide by the CuMo-BPY/TBHP/TBABr system[a].
Yield (%)[c]
Entry
1a[b]
90.6
1b[b]
90.0
2a
86.5
2b
85.4
3a
91.6
3b
90.8
4a
88.2
4b
–
5a
89.5
5b
–
6a
20.5
7
Entry
Substrate
Product
Substrate
Product
Yield (%)[c]
– – 54.8
[a]Entries 1–6: Conditions: olefin: 10 mmol; CuMo-BPY: 0.01 mmol; TBHP (70% in decane): 20 mmol; TBABr: 0.1 mmol; CO2: 0.5 MPa; 50 °C, 96 h in total. [b] Twostep catalytic process for olefin epoxidation (1a) and the cycloaddition of CO2 with epoxide (1b). [c] The yield was determined by 1H NMR spectroscopy of the crude products.
biphenyl generated less than 20.5% conversion under the same reaction conditions (Table 1, Entry 6a). This fact revealed that substrate 6a is too large to be adsorbed into the channels. The results suggest that epoxidation indeed occurred in the channels of the MOF, not on the external surface. Subsequently, the coupling reaction was performed by adding the cocatalyst TBABr (0.1 mmol) and CO2 to the styrene oxide generated in the previous step at 50 °C and 0.5 MPa for 48 h. As shown in Table 1 (Entry 1b), the results exhibited excellent reaction efficiency (90%
efficiency with 90.6% yield, which reveals the successful execution of our design. The control experiment demonstrated no detectable conversion for the model reaction in the absence of CuMo-BPY. The control experiment employing Na2MoO4 as catalyst gives a conversion of 35%, which is far lower than that obtained with CuMo-BPY as catalyst. The higher conversion in the case of the CuMo-BPY system is due to the suitable distribution of the oxidant catalyst MoO42−. In contrast to the smooth reaction of substrate (Entries 1a-5a), the epoxidation catalysis reaction in the presence of the bulky olefin 3,5-di-tert-butyl-4′-vinyl 15
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Fig. 6. (a) Recyclability tests of CuMo-BPY catalyst for the heterogeneous assisted tandem catalysis of styrene. All reactions were carried out with 0.01 mmol catalyst, 0.1 mmol TBHP (70% in decane), 20 mmol TBABr, and 0.5 MPa CO2 at 50 °C for 96 h in total. After every run, the catalysts were separated and accumulated by centrifugal separation followed by washing three times with methanol. (b) PXRD pattern of CuMo-BPY (a- Simulated, b- Experimental, c- Recovery of catalyst after three runs).
Scheme 2. The diagram of potential mechanisms of the assisted tandem catalytic process. Step 1: the epoxidation of styrene (blue arrows); Step 2: the cycloaddition of CO and epoxides (black arrows).
activated CO2, enabling cyclic carbonate ring formation. In comparison, one-pot autotandem catalysis was examined in the presence of 10 mmol styrene, 20 mmol TBHP, 0.1 mol% CuMo-BPY and the cocatalyst TBABr [22]. However, only a 54.8% conversion rate was obtained after 120 h at 50 °C (Table 1, Entry 7). We postulated that the entire system was destroyed by the excess TBABr, since the bromide ion can catalyze the unwanted and nonproductive decomposition of tertbutyl hydroperoxide to generate tert-butyl alcohol. Thus, separated
yield) for phenyl(ethylene carbonate). The results demonstrated that almost all of the styrene oxide generated in the previous step can be transformed into phenyl(ethylene carbonate). The control experiments demonstrated that traces of conversions were obtained for the model reaction in the absence of CuMo-BPY or the ammonium salt cocatalyst. It is postulated that the μ3-OH tricopper (II) core not only enhanced the reaction rate by increasing the concentration of CO2 substrate around its reactive center but also increased the electron cloud density of 16
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center but also increased the electron cloud density of activated CO2, enabling cyclic carbonate ring formation [32–34].
tandem catalysis was viewed as a more attractive and practical route for the oxidative carboxylation of styrene and CO2 catalyzed by the CuMoBPY catalytic system. The above results indicated that assisted tandem catalysis in the CuMo-BPY/TBHP/TBABr system showed much better activity for the direct synthesis of styrene carbonate from styrene and CO2 than did the autotandem catalysis procedure. We further evaluated the CuMo-BPY/TBHP/TBABr system for the assisted tandem catalysis of other substrates. CuMo-BPY exhibited its superiority by the highly efficient conversion of most olefins to cyclic carbonates (Table 1, Entries 2–5). However, due to the spatial resistance, the large-ring oxides are not successfully transformed into cyclic carbonates in the second process (Table 1, Entries 4b, 5b). These results strongly indicated that this system combined with the assisted tandem catalysis protocol could efficiently promote aromatic olefins, which is probably ascribed to the avoidance of interactions between the intermediate product and the catalysts themselves at different stages.
4. Conclusions In summary, we achieved a molybdenum (VI) oxide-incorporated cooperative catalyst and developed a highly efficient and atom-economical assisted tandem process to synthesize cyclic carbonates from olefins and CO2. The oxidation catalysis sites of Mo(VI) and alkaline μ3OH tricopper (II) cores, exhibiting orderly distribution and spatial matching in the framework, provide further compatibility with the Mo = Ot activated epoxidation intermediate and drive the tandem catalytic process. As a heterogeneous catalyst, CuMo-BPY displays excellent properties, such as unambiguous X-ray crystallography, readily available chemical reagents, a highly stable crystalline framework, repeated catalyst recyclability and a mild catalytic environment, which is promising for the preparation of practical chemical materials.
3.4. Heterogeneity and recyclability of CuMo-BPY catalyst Acknowledgements To verify heterogeneity, CuMo-BPY was removed by filtration in the step of styrene epoxidation after 24 h, and the filtrate afforded nearly no additional conversion after stirring for another 24 h. A leaching test showed that the resulting filtrate retained a yield of approximately 57% after filtration of the solid over the next 24 h. Furthermore, an identical test was executed for the step of the cycloaddition of CO2. Specifically, to make the experimental phenomena more prominent, we chose pure styrene epoxide as the substrate for the cycloaddition reaction. For all cycloaddition reactions, when the experiment was carried out for 24 h, the catalyst was filtered out after 24 h, and then stirring continued for another 24 h. The leaching test showed that the resulting filtrate retained an approximate 63% yield over the ensuing 24 h after filtration of the solid. The recyclability test for the CuMo-BPY catalyst was performed at 50 °C with 0.5 MPa carbon dioxide in the heterogeneous assisted tandem catalysis of styrene. The catalysts could be reused at least three times with moderate loss of activity from 90.0 to 87.6% yield (Fig. 6a, Table S9). Solids of CuMoBPY could be isolated from the reaction suspension by simple filtration alone. The filtrate was further analyzed by ICP analysis, and the results displayed almost negligible detection of Co or Mo ions. These observations suggested that CuMo-BPY was a true heterogeneous catalyst. The PXRD patterns of the initial and recovered samples after three cycles also further indicate the high stability of this catalyst (Fig. 6b). As a result, high stability and recyclability will make this bifunctional catalyst attractive for practical applications [38,39].
This work was supported by the National Natural Science Foundation of China (No. 21601048, 21671055), the China Postdoctoral Science Foundation (2015M580626), the Natural Science Foundation Project of Henan province (162300410012), the State Key Laboratory of Fine Chemicals (KF1602). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2018.10.003. References [1] D.E. Fogg, E.N. dos Santos, Tandem catalysis: a taxonomy and illustrative review, Coord. Chem. Rev. 248 (2004) 2365–2379, https://doi.org/10.1016/j.ccr.2004.05. 012. [2] S. Choi, V. Srinivasulu, S. Ha, C.M. Park, Synthesis of Carbazoles based on goldcopper tandem catalysis, Chem. Commun. 53 (2017) 3481–3484, https://doi.org/ 10.1039/C7CC00103G. [3] T. Miao, Z.Y. Tian, Y.M. He, F. Chen, Y. Chen, Z.X. Yu, Q.H. Fan, Asymmetric hydrogenation of in situ generated isochromenylium intermediates by Copper/ Ruthenium tandem catalysis, Angew. Chem. 129 (2017) 4199–4204, https://doi. org/10.1002/anie.201611291. [4] M. Bakos, Á. Gyömöre, A. Domján, T. Soós, Auto-tandem catalysis with frustrated lewis pairs for reductive etherification of aldehydes and ketones, Angew. Chem. Int. Ed. 56 (2017) 5217–5221, https://doi.org/10.1002/anie.201700231. [5] M. Arisawa, Y. Fujii, H. Kato, H. Fukuda, T. Matsumoto, M. Ito, S. Shuto, One-pot ring-closing Metathesis/1,3-Dipolar cycloaddition through assisted tandem ruthenium catalysis: synthesis of a dye with isoindolo [2,1-a] quinoline structure, Angew. Chem. Int. Ed. 52 (2013) 1003–1007, https://doi.org/10.1002/anie.201206765. [6] A. Beyer, T. Castanheiro, P. Busca, G. Prestat, Copper(I)/Copper(II)-assisted tandem catalysis: the case study of Ullmann/Chan–evans–lam N1,N3-Diarylation of 3-aminopyrazole, ChemCatChem 7 (2015) 2433–2436, https://doi.org/10.1002/cctc. 201500510. [7] Y. Yang, W.M. Shu, S.B. Yu, F. Ni, M. Gao, A.X. Wu, Auto-tandem catalysis: synthesis of 4H-pyrido[1,2-a]pyrimidin-4-ones via copper-catalyzed aza-Michael addition–aerobic dehydrogenation–intramolecular amidation, Chem. Commun. 49 (2013) 1729–1731, https://doi.org/10.1039/C3CC38131E. [8] M.Z. Wang, C.Y. Zhou, C.M. Che, A silver-promoted auto-tandem catalysis for the synthesis of multiply substituted tetrahydrocarbazoles, Chem. Commun. 47 (2011) 1312–1314, https://doi.org/10.1039/c0cc04383d. [9] N. Shindoh, H. Tokuyama, Y. Takemoto, K. Takasu, Auto-tandem catalysis in the synthesis of substituted quinolines from aldimines and electron-rich olefins: cascade povarov-hydrogen-Transfer reaction, J. Org. Chem. 73 (2008) 7451–7456, https:// doi.org/10.1021/jo8009243. [10] A.R. Venning, M.R. Kwiatkowski, J.E. Roque Peña, B.C. Lainhart, A.A. Guruparan, E.J. Alexanian, Palladium-catalyzed carbocyclizations of unactivated alkyl bromides with alkenes involving auto-tandem catalysis, J. Am. Chem. Soc. 139 (2017) 11595–11600, https://doi.org/10.1021/jacs.7b06794. [11] C. Robert, C.M. Thomas, Tandem catalysis: a new approach to polymers, Chem. Soc. Rev. 42 (2013) 9392–9402, https://doi.org/10.1039/c3cs60287g. [12] A. Aillerie, V. Rodriguez-Ruiz, R. Carlino, F. Bourdreux, R. Guillot, S. BezzenineLafollée, J. Hannedouche, Asymmetric assisted tandem catalysis: hydroamination followed by asymmetric friedel-crafts reaction from a single chiral N,N,N’,N’Tetradentate pyridylmethylamine-based ligand, Chem. Cat. Chem. 8 (2016)
3.5. Mechanism of the assisted tandem catalytic process From a mechanistic perspective, during the epoxidation process, the generation of peroxide molybdenum intermediates oxidized by TBHP ensured the smooth progress of the reaction. Subsequently, the peroxide molybdenum intermediates act as electrophilic reagents to attract the styrene, thus forming the epoxide product (Scheme 2). In the process of cycloaddition of CO2 with epoxide, the CO2 molecules are fixed and activated by the alkaline μ-OH tricopper (II) cores of CuMo-BPY, which enable the formation of a bidentate bridge or tridentate linear adsorption state [32–34]. At the same time, the epoxide oxygen atom that is coordinated with the n-Bu4N+ cation would promote the nucleophilic attack of the Br− anion on the less hindered carbon atom of the epoxide, thus resulting in the ring opening of the epoxide and generation of an oxygen anion [22]. Then, the activated CO2 reaction with epoxide by nucleophilic attack converges into one vital intermediate. Subsequently, after intramolecular rearrangement and cycloaddition, the styrene cyclic carbonates are released from the catalytic cycles, and the active species of TBABr are also regenerated. It is postulated that the μ3OH tricopper (II) core in channels not only enhanced the reaction rate by increasing the concentration of CO2 substrate around its reactive 17
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Molecular Catalysis journal homepage: www.elsevier.com/locate/mcat
A molybdate-incorporated cooperative catalyst: High efficiency in the assisted tandem catalytic synthesis of cyclic carbonates from CO2 and olefins ⁎
Zhuolin Shia, Guiqin Niua, Qiuxia Hana,b, , Xiaoyun Shia, Mingxue Lia,
T
⁎
a
Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan, 475004, China b State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian, Liaoning, 116024, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Assisted tandem catalysis Bifunctional catalyst Molybdate Metal-organic frameworks Cyclic carbonates
Currently, substantial interest is focused on developing assisted tandem catalysis to optimize the reaction conditions for each step in the process. By combining a molybdate oxidation catalyst and alkaline μ3-OH tricopper (II) cores into a metal-organic framework (MOF), a bifunctional catalyst, Cu3(μ3-OH)2(4,4′-BPY)(MoO4)2 (CuMoBPY, BPY = 4,4′-bipyridine), was achieved and successfully explored for the assisted tandem catalytic conversion of olefins into value-added cyclic carbonates. CuMo-BPY displayed a three-dimensional (3D) extended network of two-dimensional (2D) inorganic CuMo layers interconnected with 4,4′-bipyridine groups. Two potential catalytic sites, Mo(VI) oxide (an oxidant for the epoxidation of olefins) and the alkaline μ3-OH of tricopper (II) cores (for the capture and activation of CO2), exhibit an orderly distribution and spatial matching in the framework, which can provide further compatibility with the Mo=Ot-activated epoxidation intermediate to drive tandem catalysis in a single workup stage.
1. Introduction Tandem catalysis is an ideal catalysis process that involves two or more mechanistically distinct reactions promoted by a single catalyst. Currently, this approach has aroused much interest within the synthetic community in an effort to enhance the efficiency of chemical synthesis [1–3]. Tandem catalysis includes two general subcategories: autotandem catalysis and assisted tandem catalysis [4–7]. Autotandem catalysis can proceed without the addition of other reactants or alteration of the reaction conditions [8–10]. Assisted tandem catalysis, by contrast, can be a useful approach for positioning two or more incompatible ingredients within one catalyst through high precision and control, thus promoting the generation of the desired transition state. In particular, additional reagents may be required for a specific catalytic cycle to perform this function [11–13]. Cyclic carbonates are a class of important chemical intermediates that are widely applied in urea and polymer synthesis, as gasoline additives, as electrolytes in lithium battery devices and for other applications [14–17]. A tandem catalytic procedure that processes an olefin through two discrete catalytic steps is the most desirable method for the environmentally benign conversion of CO2 to cyclic carbonates [18–20]. Recently, several efficient catalytic systems for the tandem
transformation of cyclic carbonates from olefins have been developed. In 2009, Sun’s group reported the direct synthesis of styrene cyclic carbonates from styrene and CO2 with a composite system of Au/Fe (OH)3-ZnBr2/[Bu4N]Br under 4 MPa CO2 at 80 °C [21]. In 2011, Hu’s group explored a method to directly synthesize cyclic carbonates from olefins and CO2 by a MoO2(acac)2-quaternary ammonium salt system [22]. In 2015, Wang’s group presented the direct synthesis of cyclic carbonates with Mn-salen catalysis [23], and Gao’s group investigated the one-pot synthesis of propylene carbonate from propylene and CO2 with a system of H3PW12O40-TBABr under 3 MPa CO2 at 140 °C [24]. In 2016, Kholdeeva’s group described the direct synthesis of cyclic carbonates with a Ti-containing catalyst under 0.8 MPa CO2 at 50–70 °C [25]. Although the explorations above have performed excellently in the preparation of cyclic carbonate, most of these approaches include composite systems and require multiple components and high temperatures and pressures. Therefore, there is still a strong imperative to search for new types of heterogeneous catalysts that can unite the active ingredients of multicomponent systems and various catalytic sites into one single framework. Furthermore, the achievement of compatibility between the reaction intermediates and synergy among multiple catalytic cycles continues to be demanding and challenging.
⁎ Corresponding authors at: Henan Key Laboratory of Polyoxometalate Chemistry, Institute of Molecular and Crystal Engineering, School of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan, 475004, China. E-mail addresses: [email protected] (Q. Han), [email protected] (M. Li).
https://doi.org/10.1016/j.mcat.2018.10.003 Received 27 July 2018; Received in revised form 27 September 2018; Accepted 1 October 2018 2468-8231/ © 2018 Elsevier B.V. All rights reserved.
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(hv = 1486.6 eV) achromatic X-ray source. The vacuum inside the analysis chamber was maintained at 1 × 10−9 Pa during analysis. Thermogravimetric (TG) analysis was conducted on a Mettler-Toledo TGA/SDTA 851e instrument, with a heating rate of 10 °C min–1, heated from 25 to 800 °C under nitrogen. The morphology and structure of sample catalysts were characterized using a field-emission scanning electron microscope (SEM) (ZEISS, MERLIN Compact) at an accelerating voltage of 15.0 keV. The N2 absorption-desorption isotherms for determining the Brunner − Emmet − Teller (BET) surface area and Barrett − Joyner−Halenda (BJH) pore width distribution were obtained with an ASAP 2460 Micromeritics instrument, using N2 as the adsorbent at 77 K. 1H nuclear magnetic resonance (NMR) spectra were recorded on a Varian INOVA-400 MHz type (1H, 400 MHz; 13C, 400 MHz) spectrometer. The chemical shifts are reported in ppm relative to CDCl3 (d = 7.26) for 1H NMR.
Homogeneous molybdate species have demonstrated high conversion and selectivity for the synthesis of epoxides [26–29]. However, their applicability is limited by poor catalyst separation and reusability. The design of active, environmentally benign and recyclable heterogeneous molybdate-incorporated catalysts is expected to have a major impact on applications of oxidation catalysts. In 2016, Hupp’s group designed a Mo(VI) oxide-deposited MOF for cyclohexene epoxidation that exhibited excellent catalytic activity, with a yield eventually reaching 93% [30]. Nevertheless, the structure and net content were only simulated and computationally determined. The lack of direct single-crystal X-ray diffraction analyses will limit the speculation and verification of the catalytic mechanism. Furthermore, the method to modify molybdate active sites on MOF by solvothermal deposition is more complex than a one-pot procedure. Herein, by incorporating simple MoO42−, copper (II) ion, and 4,4′bipyridine (BPY) bridging links in a mixed solution of H2O and CH3CN under hydrothermal conditions, we report the design and synthesis of the MOF-based material CuMo-BPY. Although the structure of CuMoBPY was reported by Nocera’s group in 2005, only the magnetic properties were described in detail [31]. In this work, we synthesized CuMo-BPY from different precursors and conducted primary research on the catalysis of the material. We envisioned that the incorporation of a Mo(VI) oxide as an oxidation catalyst and the fixing and activation of μ3-OH tricopper (II) cores for CO2 molecules into an MOF would be a powerful approach to devise a tandem catalytic process for the chemical transformation of olefins to cyclic carbonate (Scheme 1). In addition, CuMo-BPY possesses intrinsic crystalline properties, which could provide precise knowledge about the nature and distribution of catalytically active sites as well as the potential interactions between the catalytic sites and the adsorbed substrates.
2.2. Procedure for assisted tandem catalysis from olefins to cyclic carbonate in the CuMo-BPY/TBHP/TBABr system The assisted tandem catalytic reaction was carried out heterogeneously with 10 mmol olefin, 0.01 mmol catalyst, and 20 mmol tbutylhydroperoxide (TBHP, 70% in decane), without extra solvent, under stirring in a 25 mL stainless steel autoclave equipped with a magnetic stirrer. The mixture was then placed in an oil bath and heated to the desired temperature for a predetermined time. After completion of the reaction, 0.1 mmol tetrabutylammonium bromide (TBABr) was added to the reactor without separating the above mixture, CO2 was charged into the autoclave, and the pressure (0.5 MPa) was kept constant during the reaction. The autoclave was placed into an oil bath and heated to the desired temperature. After the desired time, the excess gases were vented. The remaining mixture was degassed and fractionally distilled under reduced pressure. The organic extracts were dried over Na2SO4, evaporated to dryness and purified by column chromatography on silica gel with eluent solvents (dichloromethane/petroleum ether = 4/1) to obtain pure cyclic carbonate. The yields were directly determined by 1H NMR analysis of the reaction solution.
2. Experimental 2.1. Materials and methods All solvents and reagents used in this study were reagent grade and used without further purification. All chemicals were of reagent-grade quality, obtained from commercial sources and used without further purification. The elemental analysis (EA) of C, H, and N was performed on a Vario EL III elemental analyzer. Inductive coupled plasma (ICP) emission spectrometry was performed on a JarrelAshJ-A1100 spectrometer. The infrared (IR) spectra were recorded from a solid sample pelletized with KBr on a JASCO FT/IR-430. The powder X-ray diffractometry (PXRD) spectra were obtained on a Rigaku D/Max-2400. The X-ray photoelectron spectroscopy (XPS) analyses were performed on an ESCALAB 250 XI (Thermo Fisher, USA) spectrometer with an Al Ka
3. Results and discussion 3.1. Structural description An asymmetric unit of CuMo-BPY consists of a BPY ligand, one tetrahedral {MoO4} group, and two crystallographically distinct Cu(II) atoms (Fig. S1). As shown in Fig. 1a, both crystallographically independent Cu(II) ions adopt distorted octahedral geometries. Cu(1) is coordinated to one nitrogen atom from a BPY ligand, two μ3-OH oxygen atoms from {MoO4}, one μ2-OH oxygen atom from {MoO4} and two cis
Scheme 1. Synthetic procedure of CuMo-BPY, showing the pattern for tandem catalytic synthesis of cyclic carbonates from CO2 and olefins. Color code: Cu (sky blue), Mo (green), N (blue), O (red), C (gray); hydrogen atoms are omitted for clarity. 11
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Fig. 1. (a) The coordination environment of the two crystallographically independent atoms Cu(1) and Cu(2). (b) The connection scheme of Cu(II) and Mo(VI) in the inorganic layer along the c-axis. (c, d) The 3D open network of CuMo-BPY with two particular pores. The hydrogen atoms are omitted for clarity. all hydrogen atoms are omitted for clarity.
Fig. 2. (a) The TG curve of CuMo-BPY. (b) The IR spectra of fresh CuMo-BPY catalyst (top line) and the catalyst CuMo-BPY (bottom line) impregnated with TBHP. (c) TPD−CO2 curve of the CuMo-BPY catalyst. (d) The IR spectra of fresh CuMo-BPY (top line) catalyst and the catalyst CuMo-BPY (bottom line) impregnated in an atmosphere of 0.5 MPa CO2. 12
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Fig. 3. SEM images of CuMo-BPY display the morphology and structure of CuMo-BPY nanorods with high resolution.
chemical and thermal stability and meets most of the prerequisites as an ideal platform for heterogeneous catalysis. The results show excellent consistency with the X-ray structure.
bridging oxygen atoms from tricopper (Fig. S2). O(1) and O(4) occupy the axial positions of the elongated octahedron; other atoms build the equatorial plane. Cu(2) is coordinated to four μ3-OH oxygen atoms from {MoO4} and two trans μ3-OH oxygen atoms from tricopper (II); two O (1) atoms occupy the axial positions of the elongated octahedron (Fig. S3). The relevant bond lengths and bond angles are listed in Table S1. The Cu(II) octahedra form complex chains of alternating corner- and edge-sharing Cu3(μ3-OH) triangles. The chains are then linked by three oxygen atoms of the molybdate anion to form Cu3(OH)2(MoO4)2 layers with {MoO4} groups distributed on two sides of the one-dimensional chains (Fig. 1b). The layers are bridged by BPY ligands to form a 3D framework with 1D channels along the b axis (Fig. 1c, S4). The most unique structural feature of CuMo-BPY is that two types of pores are arranged alternately in a square cross section with the approximate dimensions of 4.16 × 7.13 Å for pore A and 5.55 × 9.48 Å for pore B (Fig. 1d). Pore A is a pure inorganic pore with hydrophilicity conferred by the μ-OH copper-molybdenum cores, in which the alkaline μ-OH possesses the function of the capture and activation of CO2, as other studies in the literature have reported [32–34]. Pore B is alternately constituted by hydrophilic μ-OH copper-molybdenum cores and hydrophobic organic BPY ligands, as well as Mo6+ catalytic sites, which are exposed fully and point toward the inside of the pore, favoring contact with styrene and TBHP substrates. The orderly distribution and spatial matching in the framework of the oxidation catalytic sites of Mo (VI) and the alkaline μ-OH copper core can provide further compatibility with the Mo = Ot-activated epoxidation intermediate, driving the tandem catalytic process in a single workup stage.
3.2.2. IR spectra The IR spectra of CuMo-BPY are collected and shown in Fig. 2b (top line). The characteristic bands of ν(Mo = Ot) and ν(Mo–O–Mo) vibrations in CuMo-BPY are in the ranges of 927 and 772 cm–1. The resonances at 1615 and 1321 cm–1 are assigned to the ν(CeN) and ν(NeH) stretching vibrations of BPY, respectively. These characteristic vibration resonances confirm the existence of BPY ligands and {MoO4} in CuMo-BPY. The results of the IR spectra are quite consistent with those of X-ray diffraction. Additionally, the IR spectrum of the catalyst impregnated in TBHP revealed a new ν(OeO) band at 887 cm−1, which indicated that the active intermediate peroxymolybdate was formed during the epoxidation process (Fig. 2b). Additionally, the IR spectrum of the catalyst CuMo-BPY impregnated in an atmosphere of 0.5 MPa CO2 exhibits a weak peak in the vicinity of 1659 cm−1 (Fig. 2d), which indicates the fixation of CO2 molecules by the μ3-OH tricopper (II) core of CuMo-BPY and the formation of a tridentate linear adsorption state. The intense peak in the vicinity of 1098 cm−1 represents the fixation of CO2 molecules by the μ3-OH dicopper (II) core of CuMo-BPY and formation of a bidentate bridge state [32–34]. 3.2.3. Surface base properties of CuMo-BPY samples (CO2-TPD) CO2-TPD (temperature-programmed desorption of CO2) techniques have been widely used to probe the surface base properties of solid catalysts [35] and were therefore employed here to correlate the structure of the base sites with catalytic performance. A strong desorption peak for CO2 is observed at approximately 314.5 °C (Fig. 2c). Based on this result, the high temperature peak could be ascribed to strong basic sites generated by the existence of μ3-OH tricopper (II) cores in CuMo-BPY. Based on the results of CO2-TPD and IR analysis, we speculated that CO2 was adsorbed and activated by μ3-OH in the channels of CuMo-BPY.
3.2. Characterization of CuMo-BPY 3.2.1. Thermogravimetric analysis (TG) The TG curve of a pure sample of CuMo-BPY indicates one weightloss step between 270 and 446 °C, which is associated with the loss of BPY, with a total loss of 24.23% (calcd. 22.87%)(Fig. 2a). The entire structure collapsed completely beyond 446 °C. CuMo-BPY exhibits high 13
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Fig. 4. Corresponding EDS mappings with Cu (KA) and Mo (LA) of CuMo-BPY.
[36]. Two other peaks at 231.8 and 234.9 eV are attributed to Mo6+(3d5/2) and Mo6+(3d3/2), respectively [37]. All of these results further confirm the structural analyses. The porosity and specific surface area of the samples can be determined from N2 adsorption-desorption isotherms (Fig. 5d). The pore width of CuMo-BPY is generally ca. 13.88 nm. According to the N2 adsorption-desorption isotherms, the BET surface area and BJH pore volume of CuMo-BPY are approximately 635.76 m2 g−1 and 1.85 cm3 g−1, respectively.
3.2.4. Morphology characterization The morphology and structure of a CuMo-BPY sample were investigated with a field-emission scanning electron microscope (FESEM). As shown in Fig. 3, CuMo-BPY shows nanorod-like structure with widths in the range of 140–150 nm and lengths of up to 1.3–1.4 micrometers. As shown in the typical SEM images of some representative single rods, most of the nanorods are very straight and uniform with smooth surfaces, although some of the nanorods are slightly agglomerated. Additionally, energy-dispersive spectroscopy (EDS) presents the distribution of the individual elements in the CuMo-BPY sample (Fig. 4a–e). Elemental mapping collected from the same nanorod region identified the homogeneous distribution of Cu (red) and Mo (green). The different intensities of the green and red colors were assumed to be proportional to the metal composition in each sample. It can be concluded that both copper and molybdenum were homogeneously distributed in the framework of CuMo-BPY.
3.3. Assisted tandem catalysis in the CuMo-BPY/TBHP/TBABr system A two-step catalytic process for the epoxidation of olefins and the cycloaddition of CO2 with epoxide was studied separately in order to understand the compatibilities of these catalysts in terms of chemical stabilities in the process utilizing one-pot assisted tandem catalysis. To explore the CuMo-BPY/TBHP/TBABr assisted tandem-catalysis system, various reaction conditions were scanned; the reaction optimization results are presented in Table S3-8. Initially, we examined epoxidation using styrene and TBHP as the oxidant, along with CuMoBPY (0.1% mol ratio), in a heterogeneous reaction at 50 °C for 48 h. As shown in Table 1 (Entry 1a), the results exhibit excellent reaction
3.2.5. XPS spectra and BET analysis The XPS spectra of the Cu 2p and Mo 3d orbitals of the CuMo-BPY sample are displayed in Fig. 5. The peaks observed at 934.4 and 954.0 eV are assigned to Cu2+ (2p3/2) and Cu2+ (2p1/2), respectively 14
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Fig. 5. (a) Survey-scan XPS of the studied membranes and high-resolution XPS spectra of Mo 3d (b) and Cu 2p (c) of a CuMo-BPY sample. (d) N2 adsorptiondesorption isotherms of a CuMo-BPY sample. Table 1 Conversion in the two-step catalytic process: epoxidation of the olefins and cycloaddition of CO2 with epoxide by the CuMo-BPY/TBHP/TBABr system[a].
Yield (%)[c]
Entry
1a[b]
90.6
1b[b]
90.0
2a
86.5
2b
85.4
3a
91.6
3b
90.8
4a
88.2
4b
–
5a
89.5
5b
–
6a
20.5
7
Entry
Substrate
Product
Substrate
Product
Yield (%)[c]
– – 54.8
[a]Entries 1–6: Conditions: olefin: 10 mmol; CuMo-BPY: 0.01 mmol; TBHP (70% in decane): 20 mmol; TBABr: 0.1 mmol; CO2: 0.5 MPa; 50 °C, 96 h in total. [b] Twostep catalytic process for olefin epoxidation (1a) and the cycloaddition of CO2 with epoxide (1b). [c] The yield was determined by 1H NMR spectroscopy of the crude products.
biphenyl generated less than 20.5% conversion under the same reaction conditions (Table 1, Entry 6a). This fact revealed that substrate 6a is too large to be adsorbed into the channels. The results suggest that epoxidation indeed occurred in the channels of the MOF, not on the external surface. Subsequently, the coupling reaction was performed by adding the cocatalyst TBABr (0.1 mmol) and CO2 to the styrene oxide generated in the previous step at 50 °C and 0.5 MPa for 48 h. As shown in Table 1 (Entry 1b), the results exhibited excellent reaction efficiency (90%
efficiency with 90.6% yield, which reveals the successful execution of our design. The control experiment demonstrated no detectable conversion for the model reaction in the absence of CuMo-BPY. The control experiment employing Na2MoO4 as catalyst gives a conversion of 35%, which is far lower than that obtained with CuMo-BPY as catalyst. The higher conversion in the case of the CuMo-BPY system is due to the suitable distribution of the oxidant catalyst MoO42−. In contrast to the smooth reaction of substrate (Entries 1a-5a), the epoxidation catalysis reaction in the presence of the bulky olefin 3,5-di-tert-butyl-4′-vinyl 15
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Fig. 6. (a) Recyclability tests of CuMo-BPY catalyst for the heterogeneous assisted tandem catalysis of styrene. All reactions were carried out with 0.01 mmol catalyst, 0.1 mmol TBHP (70% in decane), 20 mmol TBABr, and 0.5 MPa CO2 at 50 °C for 96 h in total. After every run, the catalysts were separated and accumulated by centrifugal separation followed by washing three times with methanol. (b) PXRD pattern of CuMo-BPY (a- Simulated, b- Experimental, c- Recovery of catalyst after three runs).
Scheme 2. The diagram of potential mechanisms of the assisted tandem catalytic process. Step 1: the epoxidation of styrene (blue arrows); Step 2: the cycloaddition of CO and epoxides (black arrows).
activated CO2, enabling cyclic carbonate ring formation. In comparison, one-pot autotandem catalysis was examined in the presence of 10 mmol styrene, 20 mmol TBHP, 0.1 mol% CuMo-BPY and the cocatalyst TBABr [22]. However, only a 54.8% conversion rate was obtained after 120 h at 50 °C (Table 1, Entry 7). We postulated that the entire system was destroyed by the excess TBABr, since the bromide ion can catalyze the unwanted and nonproductive decomposition of tertbutyl hydroperoxide to generate tert-butyl alcohol. Thus, separated
yield) for phenyl(ethylene carbonate). The results demonstrated that almost all of the styrene oxide generated in the previous step can be transformed into phenyl(ethylene carbonate). The control experiments demonstrated that traces of conversions were obtained for the model reaction in the absence of CuMo-BPY or the ammonium salt cocatalyst. It is postulated that the μ3-OH tricopper (II) core not only enhanced the reaction rate by increasing the concentration of CO2 substrate around its reactive center but also increased the electron cloud density of 16
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center but also increased the electron cloud density of activated CO2, enabling cyclic carbonate ring formation [32–34].
tandem catalysis was viewed as a more attractive and practical route for the oxidative carboxylation of styrene and CO2 catalyzed by the CuMoBPY catalytic system. The above results indicated that assisted tandem catalysis in the CuMo-BPY/TBHP/TBABr system showed much better activity for the direct synthesis of styrene carbonate from styrene and CO2 than did the autotandem catalysis procedure. We further evaluated the CuMo-BPY/TBHP/TBABr system for the assisted tandem catalysis of other substrates. CuMo-BPY exhibited its superiority by the highly efficient conversion of most olefins to cyclic carbonates (Table 1, Entries 2–5). However, due to the spatial resistance, the large-ring oxides are not successfully transformed into cyclic carbonates in the second process (Table 1, Entries 4b, 5b). These results strongly indicated that this system combined with the assisted tandem catalysis protocol could efficiently promote aromatic olefins, which is probably ascribed to the avoidance of interactions between the intermediate product and the catalysts themselves at different stages.
4. Conclusions In summary, we achieved a molybdenum (VI) oxide-incorporated cooperative catalyst and developed a highly efficient and atom-economical assisted tandem process to synthesize cyclic carbonates from olefins and CO2. The oxidation catalysis sites of Mo(VI) and alkaline μ3OH tricopper (II) cores, exhibiting orderly distribution and spatial matching in the framework, provide further compatibility with the Mo = Ot activated epoxidation intermediate and drive the tandem catalytic process. As a heterogeneous catalyst, CuMo-BPY displays excellent properties, such as unambiguous X-ray crystallography, readily available chemical reagents, a highly stable crystalline framework, repeated catalyst recyclability and a mild catalytic environment, which is promising for the preparation of practical chemical materials.
3.4. Heterogeneity and recyclability of CuMo-BPY catalyst Acknowledgements To verify heterogeneity, CuMo-BPY was removed by filtration in the step of styrene epoxidation after 24 h, and the filtrate afforded nearly no additional conversion after stirring for another 24 h. A leaching test showed that the resulting filtrate retained a yield of approximately 57% after filtration of the solid over the next 24 h. Furthermore, an identical test was executed for the step of the cycloaddition of CO2. Specifically, to make the experimental phenomena more prominent, we chose pure styrene epoxide as the substrate for the cycloaddition reaction. For all cycloaddition reactions, when the experiment was carried out for 24 h, the catalyst was filtered out after 24 h, and then stirring continued for another 24 h. The leaching test showed that the resulting filtrate retained an approximate 63% yield over the ensuing 24 h after filtration of the solid. The recyclability test for the CuMo-BPY catalyst was performed at 50 °C with 0.5 MPa carbon dioxide in the heterogeneous assisted tandem catalysis of styrene. The catalysts could be reused at least three times with moderate loss of activity from 90.0 to 87.6% yield (Fig. 6a, Table S9). Solids of CuMoBPY could be isolated from the reaction suspension by simple filtration alone. The filtrate was further analyzed by ICP analysis, and the results displayed almost negligible detection of Co or Mo ions. These observations suggested that CuMo-BPY was a true heterogeneous catalyst. The PXRD patterns of the initial and recovered samples after three cycles also further indicate the high stability of this catalyst (Fig. 6b). As a result, high stability and recyclability will make this bifunctional catalyst attractive for practical applications [38,39].
This work was supported by the National Natural Science Foundation of China (No. 21601048, 21671055), the China Postdoctoral Science Foundation (2015M580626), the Natural Science Foundation Project of Henan province (162300410012), the State Key Laboratory of Fine Chemicals (KF1602). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.mcat.2018.10.003. References [1] D.E. Fogg, E.N. dos Santos, Tandem catalysis: a taxonomy and illustrative review, Coord. Chem. Rev. 248 (2004) 2365–2379, https://doi.org/10.1016/j.ccr.2004.05. 012. [2] S. Choi, V. Srinivasulu, S. Ha, C.M. Park, Synthesis of Carbazoles based on goldcopper tandem catalysis, Chem. Commun. 53 (2017) 3481–3484, https://doi.org/ 10.1039/C7CC00103G. [3] T. Miao, Z.Y. Tian, Y.M. He, F. Chen, Y. Chen, Z.X. Yu, Q.H. Fan, Asymmetric hydrogenation of in situ generated isochromenylium intermediates by Copper/ Ruthenium tandem catalysis, Angew. Chem. 129 (2017) 4199–4204, https://doi. org/10.1002/anie.201611291. [4] M. Bakos, Á. Gyömöre, A. Domján, T. Soós, Auto-tandem catalysis with frustrated lewis pairs for reductive etherification of aldehydes and ketones, Angew. Chem. Int. Ed. 56 (2017) 5217–5221, https://doi.org/10.1002/anie.201700231. [5] M. Arisawa, Y. Fujii, H. Kato, H. Fukuda, T. Matsumoto, M. Ito, S. Shuto, One-pot ring-closing Metathesis/1,3-Dipolar cycloaddition through assisted tandem ruthenium catalysis: synthesis of a dye with isoindolo [2,1-a] quinoline structure, Angew. Chem. Int. Ed. 52 (2013) 1003–1007, https://doi.org/10.1002/anie.201206765. [6] A. Beyer, T. Castanheiro, P. Busca, G. Prestat, Copper(I)/Copper(II)-assisted tandem catalysis: the case study of Ullmann/Chan–evans–lam N1,N3-Diarylation of 3-aminopyrazole, ChemCatChem 7 (2015) 2433–2436, https://doi.org/10.1002/cctc. 201500510. [7] Y. Yang, W.M. Shu, S.B. Yu, F. Ni, M. Gao, A.X. Wu, Auto-tandem catalysis: synthesis of 4H-pyrido[1,2-a]pyrimidin-4-ones via copper-catalyzed aza-Michael addition–aerobic dehydrogenation–intramolecular amidation, Chem. Commun. 49 (2013) 1729–1731, https://doi.org/10.1039/C3CC38131E. [8] M.Z. Wang, C.Y. Zhou, C.M. Che, A silver-promoted auto-tandem catalysis for the synthesis of multiply substituted tetrahydrocarbazoles, Chem. Commun. 47 (2011) 1312–1314, https://doi.org/10.1039/c0cc04383d. [9] N. Shindoh, H. Tokuyama, Y. Takemoto, K. Takasu, Auto-tandem catalysis in the synthesis of substituted quinolines from aldimines and electron-rich olefins: cascade povarov-hydrogen-Transfer reaction, J. Org. Chem. 73 (2008) 7451–7456, https:// doi.org/10.1021/jo8009243. [10] A.R. Venning, M.R. Kwiatkowski, J.E. Roque Peña, B.C. Lainhart, A.A. Guruparan, E.J. Alexanian, Palladium-catalyzed carbocyclizations of unactivated alkyl bromides with alkenes involving auto-tandem catalysis, J. Am. Chem. Soc. 139 (2017) 11595–11600, https://doi.org/10.1021/jacs.7b06794. [11] C. Robert, C.M. Thomas, Tandem catalysis: a new approach to polymers, Chem. Soc. Rev. 42 (2013) 9392–9402, https://doi.org/10.1039/c3cs60287g. [12] A. Aillerie, V. Rodriguez-Ruiz, R. Carlino, F. Bourdreux, R. Guillot, S. BezzenineLafollée, J. Hannedouche, Asymmetric assisted tandem catalysis: hydroamination followed by asymmetric friedel-crafts reaction from a single chiral N,N,N’,N’Tetradentate pyridylmethylamine-based ligand, Chem. Cat. Chem. 8 (2016)
3.5. Mechanism of the assisted tandem catalytic process From a mechanistic perspective, during the epoxidation process, the generation of peroxide molybdenum intermediates oxidized by TBHP ensured the smooth progress of the reaction. Subsequently, the peroxide molybdenum intermediates act as electrophilic reagents to attract the styrene, thus forming the epoxide product (Scheme 2). In the process of cycloaddition of CO2 with epoxide, the CO2 molecules are fixed and activated by the alkaline μ-OH tricopper (II) cores of CuMo-BPY, which enable the formation of a bidentate bridge or tridentate linear adsorption state [32–34]. At the same time, the epoxide oxygen atom that is coordinated with the n-Bu4N+ cation would promote the nucleophilic attack of the Br− anion on the less hindered carbon atom of the epoxide, thus resulting in the ring opening of the epoxide and generation of an oxygen anion [22]. Then, the activated CO2 reaction with epoxide by nucleophilic attack converges into one vital intermediate. Subsequently, after intramolecular rearrangement and cycloaddition, the styrene cyclic carbonates are released from the catalytic cycles, and the active species of TBABr are also regenerated. It is postulated that the μ3OH tricopper (II) core in channels not only enhanced the reaction rate by increasing the concentration of CO2 substrate around its reactive 17
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