Efficient Olefins Epoxidation on Ultrafine H2O-WOx Nanoparticles with

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Efficient Olefins Epoxidation on Ultrafine H2O-WOx Nanoparticles with Spectroscopic Evidence of Intermediate Species mengrui zhang, Vikram Singh, Xuefu Hu, Xinyi Ma, Jingkun Lu, chao zhang, Jingping Wang, and Jingyang Niu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b01226 • Publication Date (Web): 17 Jul 2019 Downloaded from pubs.acs.org on July 20, 2019

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ACS Catalysis

Efficient Olefins Epoxidation on Ultrafine H2O-WOx Nanoparticles with Spectroscopic Evidence of Intermediate Species Mengrui Zhang,† Vikram Singh,† Xuefu Hu,‡ Xinyi Ma,† Jingkun Lu,† Chao Zhang,*,† Jingping Wang,† and Jingyang Niu*,†

†Key

Laboratory of Polyoxometalate Chemistry of Henan Province

Institute of Molecular and Crystal Engineering College of Chemistry and Chemical Engineering Henan University, Kaifeng, Henan 475004 (P. R. China) ‡Collaborative

Innovation Center of Chemistry for Energy Materials, College of

Chemistry and Chemical Engineering of Xiamen University, Xiamen 361005, P. R. China

KEYWORDS: space-confinement, post-modification, epoxidation, polyoxometalate, vacant active site, reactive intermediates

ACS Paragon Plus Environment

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ABSTRACT: Design and synthesis of high-performance heterogeneous catalysts are of immense interest in chemical industries. Herein, ultrafine H2O coordinated WOx nanoparticles (H2O-WOx, average diameter 2.24 nm) embedded into porous carbon matrix have been obtained via space-confinement pyrolysis with post-modification oxidation approach. The synthesized H2O-WOx nanoparticles exhibit high activities towards olefin epoxidation reaction using H2O2 as oxidant. In epoxidation of ciscyclooctene, the turnover frequency (TOF) of the catalyst can achieve as high as 949 h1,

an activity comparable to the highest of all reported early transition-metal (TM) based

catalysts. UV-vis spectra and controlled catalytic experiments have been performed to ascertain the possible active site in the catalyst: compared with traditional WO3 and peroxide coordinated W-(η2-O2) species, the water coordinated H2O-WOx can transform to hydroperoxide HOO-WOx in the presence of H2O2, and the HOO-WOx species can be further activated by proton acid to enhance its catalytic activities.

INTRODUCTION


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Epoxidation of olefins has wide applications in pharmaceuticals, epoxy resins, plastics and other high value-added chemical products.1–3 For the sake of economy and environment, design and synthesis of high performance epoxidation catalysts with H2O2 as oxidant have become the research priority.4,5 Early transition-metal (TM) based catalysts, especially involving tungsten, are particularly concerned for olefin epoxidation reactions, because of their favorable electron affinities, ease of accessibility and modifiability on various inorganic/ organic supports.6,7 However, the contradiction between catalytic activity and practical usability of early TM based catalysts still exists in its infancy in the current state: as on one hand, to fully utilize the electron affinities of the catalysts, at least one unoccupied coordination site needs to be exposed under reaction system. Nevertheless, most of these unoccupied early TM species are homogeneous catalysts.8,9 They are usually hydroscopic and can be deactivated by aqueous solution of H2O2 or even in moist air. Thus, their practical usabilities are limited.10,11 On the other hand, with the aim of improving the accessibility of active sites, these homogenous catalysts can be immobilized on solid supports, forming heterogeneous catalysts.12 Unfortunately, their activities often decrease dramatically after immobilizations due to


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the unfavorable interactions between the unoccupied active sites and the supports, not to mention the aggregation effect of the homogenous species during such processes.13– 15

Over the last decades, early TM oxides, from homogeneous polyoxometalates (POMs) to heterogeneous WO3/MoO3, have been employed as efficient epoxidation catalysts. One obvious merit of early TM oxide species is that they are commonly water stable, thus eco-friendly H2O2 could be used as oxidant directly. In addition, the versatility of early TM oxides can further boost their catalytic activities in different ways. For example, POMs, with tunable structures and coordination environments, can be used to explore the structure-activity relationships in epoxidations;16 WO3 can be modified into nanoparticles (NPs)17 and decorated onto different supports with various additives to enhance their activities.18 Nevertheless, despite all these achievements in fabricating early TM oxides-based catalysts, the identification of catalytic active sites and intermediates of early TM oxides has been rarely reported. Previous studies tend to consider peroxide M-(η2-O2) as the most active intermediate in early TM based epoxidation catalysts.19 For example, both spectroscopic evidences and structural data


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have proved that the terminal oxygen of POMs (M=O) can be substituted by peroxide

η2-O2, thus forming peroxide M-(η2-O2) as intermediate species. However, no explanation was made for the fact that not all catalysts bearing M-(η2-O2) possess satisfying epoxidation activities.20 Recently, Flaherty and coworkers found that the hydroperoxide M-OOH species, together with M-(η2-O2), possesses great activities for epoxidation in Ti, Nb and Ta based catalysts, showing an alternative active intermediate for such catalytic reactions,21 but the question still to answer for early TM based catalysts. Polyoxometalates (POMs), as early TM-oxo (Mo, W, V, Nb etc.) clusters, with tunable molecular size, well-defined structures and high solubility, are suitable for being used as catalyst precursors.22 Under appropriate conditions, POMs can be easily converted to carbides, sulfides, oxides, and other active species with corresponding resources and maintain their sizes as POM precursors in the nanoscale.23,24 Zeolitic imidazolate frameworks (ZIFs), which possess highly ordered porous structures with adjustable crystal morphologies, have been considered to be promising precursors for synthesizing well-defined nanocatalysts.25 As a representative of ZIFs, ZIF-8 can be


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further carbonized into carbon matrix and can act as a high specific surface area carbon support.25–27 Moreover, it is reported that carbon support can enhance epoxides selectivity in epoxidation due to the induced hydrophobicity.28–30 In order to balance the contradiction between activity and usability of early TM based catalysts, and to investigate the active sites in W based catalyst, we prepared ultrafine H2O coordinated H2O-WOx NPs as a stable and efficient heterogeneous epoxidation catalyst. Its TOF can be reached as high as 949 h-1 in cis-cyclooctene epoxidation with H2O2 as oxidant. The conversion and selectivity of linear 1, 2epoxyoctane can also achieve 88.5 % and 81.2 % respectively. More importantly, spectroscopic results indicated that H2O-WOx species can transform into hydroperoxide HOO-WOx when using H2O2 as oxidant. The catalytic activity of HOO-WOx was then carefully compared with that of traditional WO3 and peroxide coordinated W-(η2-O2) species, and the results indicated that HOO-WOx is the most active epoxidation intermediate in W based catalysts.

MATERIAL AND METHODS


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Materials and Instrumentation. All chemicals were acquired from commercial suppliers and used without further treating unless specified otherwise: 2-methylimidazole (98 %, Aladdin), zinc nitrate hexahydrate (99 %, Aladdin), acetonitrile (CH3CN, 99.5 %, Aladdin), propionic acid (99.5 %, Aladdin), cis-cyclooctene (95 %, Alfa Aesar), cyclododecene (95 %, TCI), isopentenyl alcohol (96 %, Alfa Aesar), dicyclopentadiene (98 %, Adamas), cinene (98 %, Adamas), 3,7-Dimethyl-2,6-Octadien-1-ol (98 %, Adamas), crotyl alcohol (96 %, Alfa Aesar), trans-2-Hexen-1-ol (95 %, Alfa Aesar), 2methyl-2-propen-1-ol (96 %, Alfa Aesar), norbornylene (99 %, Adamas), cyclohexene (99 %, Acros), 1-octene (99 %, Adamas), trans-2-octene (98 %, Acros), metallic W particles (99.5 %, Adamas). The manufacturer-added stabilizer (100 to 200 ppm irganox 1076) in ciscyclooctene was removed as follows:31,32 The aqueous potassium hydroxide (10 mL, 3 mol L-1) was added to cis-cyclooctene (10 mL) under vigorous stirring for 10 min at room temperature, and the cis-cyclooctene layer was separated by repeated washing with deionized water. To further remove all impurities, distillation of cis-cyclooctene was also


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performed using an oil bath at around 190 °C while collecting the sample at 145 °C. The first fraction of the distillate was discarded. X-ray diffraction (XRD) was measured on a Bruker D8 Advance X-ray diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 1.54 Å). Liquid ultraviolet-visible (UV-vis) spectra were operated on UV-4100 spectrometer and solid UV-vis spectra were performed on HITACHI UH4150 spectrometer. Infrared (IR) spectra were performed on a PerkinElmer spectrum instrument with KBr tabletting, and the range was 400-4500 cm-1. The contents of C, N, and H were quantified by elemental analysis on Vario EL cube elemental analyzer. The contents of Zn and W were quantified on Perkin-Elmer Optima 2100 DV inductively coupled plasma atomic emission spectroscopy (ICP-AES). X-ray photoelectron spectroscopy (XPS) was operated on AXIS ULTRA X-ray photoelectron spectrometer. Scanning electron microscopy (SEM) images were carried on JSM-7610F. Transmission electron microscopy (TEM) images were carried out on JEM-2100 with an electron energy of 200 kV. Thermogravimetric analysis (TGA) curves were acquired on Mettler-Toledo TGA/SDTA 851e instrument in air atmosphere, temperature ranges 25~800 °C with a


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heating rate of 10 °C·min-1. Brunauer-Emmett-Teller (BET) surface area and pore size measurements were operated on Quadrasorb SI-4. Electrospray Ionization Mass Spectrometry (ESI-MS) was performed on a Triple TOF 4600 mass spectrometer from AB SCIEX. The ESI-MS spectra were collected and analyzed using the PeakView software. Catalysts Synthesis. Preparation of ZIF-8: The synthesis of ZIF-8 was based on previous report.33 To the 15 mL of deionized water Zn(NO3)2·6H2O (1.17 g, 3.93 mmol) was slowly added to prepare a suspension at room temperature. A mixture of 2methylimidazole (2-mIm, 22.70 g, 276.83 mmol) with 80 mL deionized water was added to the above solution with vigorous stirring for 1 h. The product was separated by centrifugation and washed thoroughly with pure water for five times. The precipitates were dried in vacuum at 75 °C overnight to obtain ZIF-8. Yield: 0.53 g, 59.14 %. Preparation of K5BW12O40 (BW12): BW12 was synthesized according to the literature.34 The Na2WO4·2H2O (100 g) and H3BO3 (5 g) were dissolved in water (100 mL) with vigorous stirring, and then HCl (60 mL, 6 mol·L-1) was added slowly to the above solution. The solution (pH ~6) was heated to boil for 4 h (water was added from time to


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time to avoid parch). The solid paratungstate Na10W12O41·xH2O was filtered off after boiling. Subsequently, the filtrate was acidified by 6 mol·L-1 HCl to pH ~2 and heated to boil for another 0.5 h, the precipitates generated after the addition of 20 g KCl. After filtration and washing with Et2O, 60 g crude product was collected. The recrystallized product was collected from 50 mL of crude product solution at 60 °C. Yield: 47 g, 44.70 %. Preparation of [email protected]: A solution of Zn(NO3)2·6H2O (1.17 g, 3.93 mmol) and BW12 (0.60 g, 0.197 mmol) in deionized water (15 mL) was added to a solution of 2-mIm (22.70 g, 276.83 mmol) in deionized water (80 mL). The mixture solution was stirred for 1 h at room temperature. Milky-white precipitates were then collected by centrifugation which is subsequently washed with distilled water until no BW12 could be detected by UV-vis spectra in the solution (see Figure S1). The resulting mixtures were dried in vacuum at 75 °C overnight (yield: 0.87 g, 73.10 %). ICP-AES result confirmed the doping amount of W in [email protected] is 18.7 wt%. Preparation of BW12@ZIF-x: By changing the dosage of the BW12 in the synthesis procedure, BW12@ZIF-x could be obtained (where -x indicates the mass fraction of W in


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ACS Catalysis

BW12@ZIF): [email protected] (BW12: 0.24 g, 0.078 mmol, yield: 0.71 g, 70.5 %); [email protected] (BW12: 0.12 g, 0.039 mmol, yield: 0.59 g, 62.7 %); [email protected] (BW12: 0.06 g, 0.020 mmol, yield: 0.43 g, 46.2 %). Preparation of pyrolyzed ZIF-8: ZIF-8 (0.30 g) was transferred in a temperatureprogrammed tube furnace under Ar flow (40-60 mL min-1), subsequent heat treated at 200 °C and at 900 °C for 3 h and 2 h respectively with a heating rate of 5 °C min-1. The sample was then cooled to room temperature naturally under Ar flow to obtain pyrolyzed ZIF-8. Yield: 0.12 g, 40.00 %. Preparation of W@C-x: The synthesis procedure is similar to that of pyrolyzed ZIF-8, except that ZIF-8 was replaced by BW12@ZIF-x (where -x indicates the mass fraction of W in BW12@ZIF). Yield: 0.07-0.09 g, 25.00 - 30.00 %. Preparation of H2O-WOx@C-x: 0.20 g W@C-x was dispersed in deionized water (1 mL), then 30 % H2O2 (2 mL) was added into the solution under vigorous stirring. The mixture solution was heated to boil until all liquid was evaporated. Then, the resulting black powder was thoroughly washed with deionized water and dried in vacuum at 75 °C overnight. Yield: 0.21 -0.26 g, 105% - 130% (the yield was calculated based on the


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masses. As the metallic W was oxidized to oxide H2O-WOx, thus the yield has been found to be > 100%. See Experimental details section in supporting information for more details). Preparation of WO3@C: 0.2 g H2O-WOx@C was transferred in a temperatureprogrammed tube furnace under Ar flow (40 mL·min-1), heat-treated at 500 °C for 3 h with a heating rate of 2 °C min-1. The samples were then cooled to room temperature naturally under Ar flow to obtain WO3@C. Yield: 0.17 - 0.18 g, 85.00 – 90.00 %. Preparation of H2O-WOx: 0.09 g metallic W particles were dispersed in deionized water (0.5 mL), then 30 % H2O2 (1 mL) was added into the above solution. The mixture solution was heated to 70 °C until the liquid was evaporated. Beige powder was collected and dried. Yield: 0.112 g, 123 %.

Catalytic Reaction. Typically, cis-cyclooctene (0.26 mL, 2 mmol), catalysts (2.20 mol % W), 30 % H2O2 (H2O2: substrate molar ratio = 1.175:1), CH3CN (3 mL) were added into a test tube under vigorous stirring at 85 °C. Then the tube was placed in a cylindrical reaction vessel which equipped with a water-cooling condenser. The


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conversion and selectivity were determined by GC-MS at stipulated times (5, 10, 15, 25, 35, 45 min). The epoxide products were identified by Proton Nuclear Magnetic Resonance (1H NMR) and quantified by gas chromatography-mass spectrometer (GCMS). The scope and limitations of the [email protected] were evaluated in the epoxidation with a variety of olefins. The reactions were carried out in the presence of olefins (1-2 mmol), 30 % H2O2 (2-2.35 mmol), catalysts (2.20 mol % W) and solvent (3 mL). A detailed reaction condition can be found in Table S6.

RESULTS AND DISCUSSION

Catalysts Synthesis and Characterization. The porous carbon matrix supported ultrafineH2O-WOx NPs (denoted as H2O-WOx@C) were prepared as follows (Scheme 1): first, Keggin-type K5BW12O40 (BW12) were in-situ encapsulated into ZIF-8’s cavities (BW12@ZIF-8); Subsequently, the BW12@ZIF-8 was pyrolyzed under the protection of Ar, immobilizing ultrafine metallic tungsten NPs on porous carbon supports (denoted as W@C); Then, the metallic W NPs were post modified to amorphous H2O coordinated WOx NPs (H2O-WOx@C) by H2O2. Our POMs@ZIF synthesis strategy is similar to


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previous reports but more strict.35,36 The choice of BW12 and ZIF-8 was crucial in the synthesis: First, single BW12 anion (0.9 nm) could fit into the hollow cage of a ZIF-8 (1.1 nm), but it could not leach out from its windows (0.3 nm). Second, BW12 is stable in the procedure of fabricating BW12@ZIF (note that many saturated Keggin type anions are only stable in strong acid solutions but 2-mIm solution is alkaline). Third, the counter ions of BW12 (mainly K+) could reside at the adjacent cages where BW12 was encapsulated and thus guaranteed the homogenous distribution of BW12.

Scheme 1. Illustration of the synthesis procedure for H2O-WOx@C.


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Figure 1. a), b) & e) TEM, HRTEM and EDX mapping image of [email protected]. c), d) & f) TEM, HRTEM and EDX mapping image of [email protected]. The successful fabrication of BW12@ZIF-8 was confirmed by UV-vis, IR spectra, XRD spectra, N2 sorption isotherms experiments, ICP-AES analysis and TGA (See supporting information, SI, Table S1 and Figure S1-S5). As shown in Figure S1, the characteristic O→W absorption peak pointed at 266 nm cannot be found in the


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supernatant solution of BW12@ZIF-8, indicating the adsorbent BW12 have been thoroughly washed. Whereas the IR spectrum of BW12@ZIF-8 (Figure S2) shows three characteristic vibration peaks this clearly demonstrates the existence of BW12. The IR peaks shift of BW12 can be explained by the interactions between BW12 surface and interior sites of ZIF-8. Further, the similar XRD patterns of BW12@ZIF-8 and ZIF-8 (Figure S3) revealed that the ZIF-8 framework was remain intact and no crystalline aggregation of BW12 was observed, indicating the homogenous doping of BW12 in ZIF8. N2 sorption isotherms experiments confirmed that the BET surface area of BW12@ZIF-8 was obviously lower than that of ZIF-8 (844.25 m2 g-1 vs 1755.98 m2 g-1 Figure S4). The pore size distribution of ZIF-8 uniformly centered at 1.12 nm, whereas the pore size distribution of BW12@ZIF-8 decreased to less than 1 nm. Considering the diameter of BW12 is similar to the pore size of ZIF-8, the uneven pore size distribution of BW12@ZIF-8 could be attributed to the encapsulation of smaller counter K+ ions. TGA curves showed that the quality loss of BW12@ZIF-8 was lower than ZIF-8 (Figure S5), which further indicates the successful encapsulation of BW12. Based on TGA results and ICP-AES analysis, the W loading amount in BW12@ZIF-8 is 18.7 wt% (Table S1).


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Interestingly, the as-synthesized BW12@ZIF-8 shares the same morphology with simple ZIF-8 (Figure S6&S7) thus confirmed about the encapsulation condition which is moderate and not destroying the nanocrystalline morphology of ZIF-8 during the entire process. The BW12@ZIF-8 was further pyrolyzed under the protection of argon at 900 oC for 2 hours (see experimental section). In the pyrolysis process, the framework of ZIF-8 will prevent the BW12 from aggregation, meanwhile the W(VI) from BW12 could be reduced by metallic Zn and carbon from ZIF-8 at high temperature. Because the calcination temperature had reached up to the boiling point of zinc,37 thus the Zn species in POM@ZIF would volatilize in the end, leaving porous carbon supported W species (denoted as W@C-x, where the -x indicates the mass fraction of W in BW12@ZIF-8). Although the fine structure of ZIF-8 was destroyed by pyrolysis, the porous carbon matrixes could still protect W NPs from detachment or aggregation.26 XRD pattern of [email protected] showed that there was only metallic W phase in the porous carbon. On the other hand, XPS spectrum of [email protected] revealed that not all the W(VI) species can be reduced to metallic W during the pyrolysis procedure. The four obvious peaks and one


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weak peak centered at 31.6, 33.7, 35.5, 37.8 and 32.3 eV can be attributed to W(0) 4f7/2, W(0) 4f5/2, W(VI) 4f7/2, W(VI) 4f5/2 and W(0) 4f7/2 (W-C), respectively (Figure S8a). The proportion of the metallic W in all W species is 43.6 % based on the areas of these XPS peaks. It should be noted that the proportion of metallic W in our system is much larger than other similar pyrolysis procedures.23,38 The existence of W(VI) species (probably WO3) can be explained by two reasons: First, pyrolysis procedure cannot fully reduce the W(VI) (BW12) to metallic W; or/and second, the surface of metallic W NPs had been oxidized by air before the XPS tests, and the XPS method can only detect surface layers of the samples. No peak of Zn 2p could be found in the XPS spectrum of [email protected], indicating the successful removal of Zn in the pyrolysis process (Figure S8b). Based on XPS and XRD results, we conclude that the W(VI) species can be partly reduced to crystalline metallic W and the remained W(VI) species (probably WO3) is amorphous. The TEM images of [email protected] revealed that the metallic W NPs were uniformly embedded into the carbon matrixes and the average particle diameter of W NPs in [email protected] is only 2.28 nm (Figure 1a, S9&S10a). The lattice fringes with an interplanar distance of 0.223 nm can be attributed to the (110) plane of metallic W


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(Figure 1b). Energy dispersive X-ray (EDX) elemental mapping showed that C, W and trace amount O were uniformly distributed on the surface of [email protected] (Figure 1e). Corresponding to the XPS result, Zn of W@C could hardly be observed in EDX mapping, indicating the successful removal of Zn in the pyrolysis process (Figure S11&S12).

c) Transmittance (%)

b) O 1s

[email protected] (after catalysis)

W-O-W

Intensity

Intensity

[email protected] (as-synthesized)

[email protected]

W-OH2 fitted curve

experimental data

Metallic W PDF#04-0806

25

d)

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65

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Na2WO4 Na2WO4 + H2O2 Na2WO4 + H2O2&PAc

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550

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e)

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H2O-WOx H2O-WOx + H2O2 H2O-WOx + H2O2&PAc

Absorbance

5

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Wavelength (nm)

[email protected] (after catalysis) 967.93 897.11

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[email protected] (as-synthesized) 967.64 896.81

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WO3@C (as-synthesized) 877.26

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WO3 WO3 + H2O2 WO3 + H2O2&PAc

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250

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Wavelength (nm)

Figure 2. a) XRD patterns of different samples. b) XPS spectrum of O 1s for [email protected]. c) IR spectra of different samples. UV-vis spectra of d) Na2WO4, e) H2OWOx, and f) WO3 with different treatment methods. In the post-modification process, H2O2 was employed to oxidize the as-synthesized W NPs in W@C to form H2O coordinated H2O-WOx@C. XRD result further authenticates the amorphous nature of the final product (Figure 2a). Whereas XPS


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revealed some important structural details: as illustrated in Figure S13d, the W 4f7/2 peak in XPS confirmed the complete transformation of W from metallic W(0) to W(VI), indicating the oxidation process had accomplished. The O 1s peak can be fitted with two different species (Figure 2b): the common μ2-bridged W-O-W (530.3 eV) and water coordinated H2O-W (532.3 eV).39,40 Further, the IR peak positioned at 813.23 cm-1 could also be attributed to the characteristic vibration mode of H2O-W (Figure 2c).41 No peroxy -O-O- vibration peak was observed from the XPS and IR spectra.19 A close comparison of TEM images between W@C and H2O-WOx@C proofed that H2O-WOx@C maintained its morphology after post-modification oxidation. The H2O-WOx NPs still uniformly resided in the carbon matrixes with particle sizes almost unchanged (avg. 2.28 nm of metallic W vs. avg. 2.24 nm of H2O-WOx, Figure 1c, d, f, S9, S10b&S14). Compared with W@C sample, the surface area of [email protected] decreased from 361.50 m2·g-1 to 221.66 m2·g-1, but the pore size remained at ca. 3.81 nm (Figure S15a&b). It is also worthy to note that H2O2 cannot destroy the covalent bonds between W and C, as the peak at 32.2 eV on the W 4f7/2 XPS spectrum of H2O-WOx@C can be assigned to tungsten of W-C bonds (Figure S13d).23,42 We infer the single-particle-to-


20

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ACS Catalysis

single-particle transformation of W NPs during the post-modification oxidation has correlations with the reservation of the W-C covalent bonds and the pores on carbon matrixes.

Catalytic Performance. Cis-cyclooctene was first employed as substrate to investigate the catalytic activity of H2O-WOx@C. H1 NMR and GC-MS were employed to identify and quantify the final cyclooctene oxide (Figure S16). The optimized results showed that there are mainly four factors that affect the catalytic performance of H2OWOx@C: First, the H2O-WOx@C exhibited superior conversion and selectivity only when using H2O2 as the oxidant (Table S2), we thus conclude the catalytic active sites on H2O-WOx@C can only be activated by H2O2. Second, the H2O2 dosage in the catalytic reactions also influenced the catalytic performance (Table S3): Low H2O2 dosage could improve H2O2 utilization efficiency (UH2O2 up to 97 %) but reduce the conversion, whereas large H2O2 dosage would reduce the UH2O2, conversion and selectivity at the same time. By comprehensive consideration of TOF, conversion, selectivity and UH2O2, the optimum H2O2: substrate ratio is 1.175:1, and the UH2O2 is ca.


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85 %, which is close to the theoretical value and comparable to that of the best early TM-based catalysts (SI experimental details and Table S4); Third, proper reaction temperature and solvent are also essential for the catalysis (Table S3), and the assynthesized catalysts worked best at 85 oC in CH3CN solution. Fourth, the catalytic activities of H2O-WOx@C enhanced with the increase of W loading amount (Figure S17, Table 1 entries 1&3-5). It is noteworthy to say that [email protected] exhibited the highest activity with the conversion of 99.7 % and selectivity of 98.9 %. In fact, cyclooctene oxide forms rapidly once H2O2 was added into the catalytic reaction system, and the catalysis reaction almost finished within 25 min (Figure 3a). Conversion/time profile showed that the catalytic reaction confirms to pseudo-first-order kinetics based on linear fit (R2 = 0.99) (Figure S18). Moreover, the TOF of [email protected] in the first 5 min reached to 325.3 h-1, which is about 2.3-fold increase to former reported WO3 NPs (140 h-1).17 Controlled experiments with no catalysts or only pyrolyzed ZIF-8 as catalysts were employed to confirm that the catalytic active sites indeed come from W-based species (Table 1, entries 13&14).


22

60

61.6

97.8

97.7

96.6

97.0

20

96.7

40

conversion selectivity

0.0 0

10

20

30

40

60

1st (C:99.1 %,S:99.9 %) 2nd (C:98.5 %,S:99.9 %) 3rd (C:97.9 %,S:99.8 %) 4th (C:97.1 %,S:99.4 %)

20

Remove catalyst by hot filtration

20 Catalyst-free

0

d) 100

80

40

40

50

Time (min)

c) 100

Original reaction

60

0

Conversion (%)

0

80

10

20

80 60

10

20

30

Time (min)

40

WO3@C Original reaction

+ NH4SCN

0

40

10 20 30 40 50 Time (min)

20 0

0

40

Original reaction

+ NH4SCN

0

30

Time (min)

0

10

20

O O W O O O

30

Time (min)

SCN

83.1

96.0

89.6

b) 100

99.2

98.3

Conversion (%)

80

96.6

Percentage (%)

a) 100

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

Conversion (%)

Page 23 of 44

+ SCN-

40

O O W O O O

50

Figure 3. a) Reaction kinetics of [email protected] in the epoxidation of cis-cyclooctene with 30 % H2O2. b) [email protected] hot filtration experiment with cis-cyclooctene (5 mmol) as substrate. c) The recycling tests of [email protected] for the kinetic reaction rates of ciscyclooctene epoxidation. d) SCN- poisoning experiment of [email protected] for the epoxidation of cis-cyclooctene (insert: SCN- poisoning experiment of WO3@C).


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Table 1. Catalytic activity of different samples for epoxidation of cis-cyclooctene with H2O2a.

Catalysts, 30% H2O2 CH3CN, 85 oC, 45 min

O

Entry

Catalyst

Cb (%)

Sc (%)

TOF (h-1)

1

[email protected]

99.7

98.9

325.3

2d

[email protected]

98.6

99.0

948.9

3

[email protected]

75.3

96.6

152.6

4

[email protected]

52.1

96.7

86.6

5

[email protected]

17.2

97.2

40.9

6

WO3@C

33.7

99.8

26.5

7d

WO3@C

29.3

99.7

24.7

8

commercial WO3

8.0

90.5

7.1

9d

commercial WO3

6.7

88.6

6.6

10

Na2WO4 (W-(η2-O2))

7.0

52.2

6.3

11d

Na2WO4 (W-(η2-O2))

4.3

99.2

5.1

12e

Na2WO4 (W-(η2-O2))

54.3

99.5

46.9

13

none

0.3

-

-

14

pyrolyzed ZIF-8

2.1

99.3

-

15d

propionic acid (PAc)

-

-

-

16

H2O-WOx

96.7

99.0

144.6

aReaction

conditions unless specified otherwise: substrate: cis-cyclooctene (entries 1 & 3-16: 0.26

mL, 2 mmol; entry 2: 0.65 mL, 5 mmol ); catalyst: entries 1-5 & 14, 18 mg (2.20, 2.20, 0.95, 0.40, 0.28, and 0.00 mol % W respectively); entries 6-12&16, 2.20 mol % W; oxidant: 30 % H2O2 in aqueous solution (H2O2: substrate molar ratio = 1.175:1); solvent: CH3CN (3 mL); temperature (85 oC);

reaction time (entries 1 & 3-16: 45 min; entry 2: 10 min). bC (conversion) and cS (selectivity)

were determined by GC-MS analysis based on the initial concentration of cis-cyclooctene. PAc was added. dPAc (0.4 mL), and ePAc (2 mL).


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ACS Catalysis

The heterogeneous nature of H2O-WOx@C was examined through a hot filtration test (Figure 3b&S19). Once [email protected] was removed from the reaction system after 5 min of reaction, the catalysis reaction stopped immediately, and no significant change of substrate conversion could be observed in the next time. Furthermore, no W species were traced from the filtrate by ICP-AES or ESI-MS (Figure S20). This result indicates the active sites of H2O-WOx@C stay firmly on the carbon matrixes. To further confirm the reusability of the catalyst, the recovered [email protected] was reused in a recycling experiment (Figure 3c&S21). After four runs of reactions, the conversion of

cis-cyclooctene only slightly decreased from 99.1 % to 97.0 %, and the selectivity of cyclooctene oxide was nearly unchanged (99.9 % to 99.4 %). After 4 reaction cycles, the activity of [email protected] catalyst declined to about 5.4 % in the first 5 minutes. However, this performance gap gradually disappeared after 25 minutes of reaction completion. No obvious change of IR and XRD patterns could be observed from the used catalyst (Figure 2a&c), indicating a good reusability and stability of H2O-WOx@C.


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Moreover, its catalytic activity remains unchanged even after being exposed to air for at least 60 days (Figure S22). TEM images of the used samples showed that H2O-WOx NPs still homogeneously dispersed on the carbon matrixes, no obvious detachment or aggregation was observed (Figure S23).

Identification of Active Sites and Intermediates. Although the as-synthesized catalyst exhibits excellent activity in epoxidation reactions, the question still needs to answer: Is the active intermediate in H2O-WOx@C different from other W based catalyst? To address this issue, we first compared H2O-WOx@C with WO3. This is because WO3 has been demonstrated to be the most active phase for olefin epoxidation.17 Fortunately, the coordinated H2O of H2O-WOx@C can be easily removed by heating. After pyrolyzed at 500 oC under Ar flow for 3 hours, its characteristic H2O-W vibration peak of H2O-WOx@C disappeared (Figure 2c), and the amorphous WOx has been successfully transformed into crystalline WO3 (Figure S24). TEM image confirmed the average particle size of the WO3 is about 2.67 nm (Figure S25). The as-synthesized sample was then named as WO3@C and utilized as control catalyst. Surprisingly, the


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ACS Catalysis

WO3@C exhibited a poorer activity in epoxidation of cis-cyclooctene, the conversion of

cis-cyclooctene is 33.7 % and TOF is only 26.5 h-1 (Table 1, entry 6). It should be noted that commercial WO3 also showed a conversion of only 8.0 % and selectivity of 90.5 % (Table 1, entry 8). From the structural viewpoint, all the W atoms in WO3 are coordinated with 6 μ2-O bridges, which means that there is no extra coordination site for the interactions between W and H2O2 (Figure S26). By contrast, the coordinated H2O have been proved to exits in H2O-WOx@C. Considering the coordinated H2O in H2O-WOx@C could fall off from WOx and leave an unoccupied coordination site, we infer this site is vital to high catalytic performance of H2O-WOx@C. The assumption was further supported by SCNpoisoning experiments. As illustrated in Figure 3d, the catalytic process was obviously impeded after the addition of NH4SCN into reaction system. Meanwhile, the characteristic H2O-W coordinated IR vibration peak of [email protected] disappeared after the SCN- poisoning experiment (Figure S27). By comparison, SCN- ions was invalid for WO3@C or commercial WO3 (Figure 3d insert and Figure S28): the activity of WO3 did not change in poisoning experiments, which is consistent with its saturated


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Page 28 of 44

coordination condition. We thus can conclude that the catalytic active site of H2OWOx@C is different from WO3, and the key to identify the active intermediate in H2OWOx@C is to understand the interactions between unoccupied W site and H2O2. To further verify the catalytic intermediate, Na2WO4 was employed as reference substance. This is because Na2WO4 can be easily transformed into peroxide coordinated Na2W(η2-O2)4 in the presence of H2O2 (Figure S29&S30),43 and the W-(η2O2) species is usually considered to be the catalytic active site in POMs and other TM oxides;2,19 Moreover, W(η2-O2)42- is soluble in catalytic solution to form a homogeneous reaction system, which means its catalytic potential can be fully released in a homogenous solution. Surprisingly, the H2O2 treated Na2WO4 in solution demonstrated the poorest activity compare with [email protected] and WO3 (Table 1, entry 10) in the identical catalytic condition. We thus strongly suspect that H2O-WOx turns into peroxide coordinated W-(η2-O2) species as the Na2WO4 does in the presence of H2O2. The active intermediates in W based catalysts were further identified by solid UVvis spectra as Flaherty and co-workers have investigated before.7,21 Because it has been proved that the WO42- can be turned into peroxide coordinated W(η2-O2)42- in the


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ACS Catalysis

presence of H2O2 as mentioned above, we first investigated the UV-vis spectra change of Na2WO4 in the presence of H2O2. As showed in Figure 2d, a distinct red shift of Na2WO4’s absorption threshold edge can be found after Na2WO4 was mixed with H2O2, indicating Na2WO4’s structure has transformed into Na2W(η2-O2)4. Due to the low W content and the high absorptivity of carbon matrix in [email protected], all the characteristic UV-vis absorption peaks of W species were overlapped by carbon’s (Figure S31). Considering our aim is to confirm the W intermediate species, we replace the H2O-WOx@C with H2O2 modified metallic W powder to further investigate the active intermediate (SI, experimental section). The modified W powder (denoted as H2O-WOx) was fully characterized to make sure it contains amorphous H2O-WOx species (Figure S32&S33). Analogously to Na2WO4, the absorption threshold edge of H2O-WOx shifted to longer wavelength region after the addition of H2O2 as well, indicating the structural transformation of H2O-WOx. Nevertheless, we noticed that the absorption edge of Na2W(η2-O2)4 is at the lower wavenumber region than that of H2O2 modified H2O-WOx. As shown in Figure S34, the wavelength difference of absorption edges between H2O2 modified H2O-WOx and Na2W(η2-O2)4 is ca. 50 nm, indicating the structure of H2O2


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Page 30 of 44

modified H2O-WOx should not be the peroxide coordinated W-(η2-O2). Considering two or more adjacent H2O coordination sites will greatly increase the instability of H2O-WOx, each H2O-W site can supply only one vacant site for H2O2 and only one oxygen atom from H2O2 could coordinate with W, we thus infer it probably forms a hydroperoxide HOO-WOx species. Meanwhile, the hydroperoxide coordination process is reversible as the UV-vis absorption curve recovered back to the original H2O-WOx pattern after immersing HOO-WOx species into water (Figure S35). We also investigated the catalytic performance of H2O-WOx. Surprisingly, the H2O-WOx exhibits a superior catalytic activity compared with WO3@C. Its conversion, selectivity and TOF in ciscyclooctene epoxidation are 96.7 %, 99.0 % and 144.6 h-1 respectively (Table 1, entry 16), despite the fact that the commercial metallic W powder (1-3 μm) is much larger than the elaborate WO3@C (2.67 nm). It has been reported that the addition of protons can cause the proportion change of catalytic intermediates in H2O2-early TM based catalyst system.7,21 To verify the intermediate changes, we first introduced propionic acid (PAc) to H2O2-Na2WO4 system. As demonstrate in Figure S34, its absorption edge continued shifting to the right until it


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ACS Catalysis

almost overlapped to the curve of H2O2 modified H2O-WOx, strongly indicating the W(η2-O2) can be induced to form the active species in H2O-WOx. More importantly, the addition of proton acid to the catalytic system can remarkably improve the epoxidation catalytic activation of Na2WO4 (Note that PAc itself is unserviceable for epoxidation, Table 1, entry 15). In epoxidation of cis-cyclooctene, TOF of Na2WO4 was raised from 6.3 h-1 to 46.9 h-1, and the conversion and selectivity also increased to 7.7- and 1.9-fold respectively with adequate PAc supplement (Table 1, entries 10-12). Although no POMs formed in the acidified H2O2-Na2WO4 system (the main W-oxo species in the system was [WO2(O2)(OH)(H2O)4]-, see Figure S36), we also investigated the influence of POMs ions in acidified catalytic system. A series of POMs (BW12, H4SiW12O40·xH2O (SiW12), H6P2W18O62 (P2W18) and Na10H2W12O40·xH2O (W12)) were employed as catalysts in epoxidation reactions. However, all these polyoxometalates exhibited poor catalytic activities, and they were also immune from acid inducing process (see Table S5). Based on these above-mentioned investigations, we concluded that the W-(η2-O2) species can be transformed to a more catalytic active species in the presence of protons, and the most active species is probably HOO-WOx.


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a)

O O

H 2O 2

O

W

O

O W

O

O

b)

Proton acid (excess) O

O

2H+

O W

O

O

+

OH2 O O

H 2O

O

O

H

Page 32 of 44

OH

W O

O

O

O

O

O O

O

O

O O

H 3O +

O

W

O

O

O

O

O

O

O W

H 2O 2

R O

H+

OH

R

O

W

O O

R R

O

R

H 2O

R O O

O

O W O

O

H 2O

O

Scheme 2. Proposed mechanism for the epoxidation of olefins with H2O2 over a) W-(η2O2) and b) H2O-WOx@C in the presence of proton acid. The proton can also greatly boost the catalytic activity of H2O-WOx@C. In ciscyclooctene epoxidation, the TOF of [email protected] increased from 325.3 h-1 to 948.9 h-1 after the addition of PAc to the system, meanwhile the conversion and selectivity kept at high level (Table 1, entry 2). However, one should note that the addition of acid will not change the UV-vis curve, XRD pattern or IR spectrum of H2O2 modified H2O-WOx (Figure 2e, S37&S38), indicating the HOO-WOx was preserved in acid environment. It is also worthy to note that in order to achieve noticeable catalytic improvement, the acid dosage in Na2WO4 system is five times as high as H2O-WOx@C-

18.7. We infer the role of acid in HOO-WOx system is different from W-(η2-O2) system. In the latter case, as mentioned protons were employed to induce the W-(η2-O2) to form


32

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ACS Catalysis

HOO-WOx. Because W-(η2-O2) is quite stable and the Lewis acid sites of W from W-(η2O2) would compete against protons to capture the O of peroxides, substantial proton acid is needed to trigger the reaction (Figure S39, Scheme 2a). On the other hand, in the epoxidation catalysis process of HOO-WOx from H2O-WOx@C, protons can directly induce HO- in the HOO-WOx species to form water and leave the active sites in the presence of olefins, and thus greatly improved the catalytic performance (Scheme 2b). Because extra protons are needed in former process, once the W-(η2-O2) has been successfully transformed into HOO-WOx, the excess protons can directly active the formed HOO-WOx and further improved the catalytic activity of Na2WO4. Different from these two systems, there is no unoccupied Lewis acid sites in WO3, thus WO3@C NPs and commercial WO3 were immune from acid inducing (Figure 2f, S26, Table 1, entries 7&9).

Scope of the [email protected] for olefins epoxidation. Encouraged by the excellent catalytic activity of [email protected] in cis-cyclooctene epoxidation, we further expanded substrates to investigate its compatibility. As shown in Table S6, H2O-


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Page 34 of 44

[email protected] demonstrated good epoxidation activity in a broad range of olefins, and the addition of proton can significantly increase products’ conversion and selectivity. The olefinic alcohols (Table S6, entries 1-5) were successfully converted to the corresponding epoxides with preferable conversion and selectivity at room temperature. For example, isopentenyl alcohol and geraniol were transformed into the corresponding epoxides with >95.0 % conversion and 88.0 % selectivity within 5 h (Table S6, entries 12). A conversion between 60.0 % and 70.0 % was obtained in the epoxidation of crotyl alcohol, trans-2-Hexen-1-ol and 2-methyl-2-propen-1-ol (Table S6, entries 3-5). The epoxidation of cyclic olefins afforded moderate conversion and selectivity (Table S6, entries 6-9). Norbornylene was oxidized to norbornylene epoxide with 95.0 % conversion and 64.2 % selectivity (Table S6, entry 6). The epoxidation of dicyclopentadiene afforded approximately 85.0 % conversion and 98.0 % selectivity (Table S6, entry 7). Cyclododecene and cyclohexene were converted into the corresponding epoxides with 71.0 % conversion and 66.0 % selectivity (Table S6, entries 8-9). The addition of protons can also boost the catalytic activities for linear olefins. After adding PAc to reaction system, the 2-octene (Table S6, entry 11) exhibits


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ACS Catalysis

a desirable conversion (80.2 %) and selectivity (96.1 %). The conversion and selectivity of 1-octene reached 88.4 % and 81.2 % respectively (Table S6, entry 13).

CONCLUSIONS

In summary, based on space-confinement pyrolysis and post-modification oxidation strategy, we have successfully fabricated a H2O coordinated WOx epoxidation catalyst H2O-WOx@C. The unique H2O coordinated site can be directly turned into hydroperoxide coordinated HOO-WOx in the presence of H2O2 and exhibits a higher activity in epoxidation compared with W-(η2-O2) species and WO3. The as-synthesized catalyst also demonstrates remarkable stability/reusability and broad compatibility of substrate in epoxidation. More importantly, by adding protons to the system, its catalytic activities were largely enhanced. The successful construction of HOO-WOx species in W based catalyst will be helpful for further studies in olefins epoxidations and can be used as a guide to improve the catalytic activities in the future.

ASSOCIATED CONTENT


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Page 36 of 44

Supporting Information

The Supporting Information is available free of charge.

Additional information on experimental and computational procedures and additional figures.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected] [email protected] ORCID Chao Zhang:0000-0002-7400-5803 Jingyang Niu:0000-0001-6526-7767

Author Contributions Dr. Chao Zhang conceived of the presented idea. Mengrui Zhang and Dr. Chao Zhang carried out the main experiments and wrote the manuscript. Dr. Vikram Singh revised the manuscript. Xinyi Ma and Xuefu Hu carried out the TEM experiments. Jingkun Lu analyzed part of the data. Prof. Jingping


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ACS Catalysis

Wang provided the necessary lab equipment and reagents. Prof. Jingyang Niu supervised the project.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was supported by the Natural Science Foundation of China (Grants 21701040, 21573056 and 21771053). The authors are grateful to young talent promote program of chemistry and chemical engineering of Henan University and key projects of science and technology research of Henan provincial department of education (No. 18B150002).

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Efficient Olefins Epoxidation on Ultrafine H2O-WOx Nanoparticles with

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