LinYuan Fan,
YaYu Hong,
Jie Cao* and
ChangWen Hu*
Key Laboratory of Cluster Science, Ministry of Education of China, Beijing Key Laboratory of Photoelectronic/Electrophotonic Conversion Materials, School of Chemistry, Beijing Institute of Technology, Beijing 100081, P. R. China. E-mail: jcao@bit.edu.cn; cwhu@bit.edu.cn; Fax: +86-10-81381380; Tel: +86-10-68912631
First published on 6th June 2016
The reaction process of cyclooctene epoxidation was real-time monitored by ESI-MS using H3PW12O40 and H2WO4 as catalysts. By observing the species changes of the catalysts in the reaction solutions and the combination of catalysts with the substrate, a catalytic reaction mechanism of polyoxometalate clusters was proposed from the aspect of aggregation and dissociation. When H3PW12O40 was used as the catalyst, it was degraded into minor peroxo-species [PW4O16−x(O2)x]3− (x = 0, 2, 4, 6, 8) which performed as the active center of the epoxidation of cyclooctene. When H2WO4 was used as the catalyst, it reacted with H2O2 and generated mononuclear tungsten peroxo-species [HWO2(O2)2]− and [HWO2(O2)3]−. After the addition of cyclooctene, the peroxo-species catalyzed the epoxidation of cyclooctene, generating 1,2-epoxycyclooctane and combining with 1,2-epoxycyclooctane to form the intermediate [HW2(O2)2O5(C8H14O)2]−.
Tungsten-based epoxidation systems with hydrogen peroxide have attracted much attention because of their high reactivity for olefins and inherent poor activity for the decomposition of hydrogen peroxide.12 In the catalytic process, polyoxotungstates act as catalyst precursors, which decompose into the monomeric, dimeric and tetrameric peroxo species by reaction with hydrogen peroxide.14 The [PO4{WO(O2)2}4]3−7,15,16 and [W2O3(O2)4(H2O)2]2−
17,18 peroxotungstates, which can catalyze the epoxidation, have been isolated and characterized crystallographically. Although the peroxo-species in the TBA2[W2O3(O2)4(H2O)2]/H3[PW12O40]-based epoxidation systems with H2O2 have been investigated by 31P/17O NMR and Density Functional Theory (DFT) calculations,19–21 the intermolecular oxidation reaction providing further proof that peroxo-species are reactive species has not yet been probed.
Mass spectrometry, as a new characterization method for the observation of active species,22,23 has been previously applied to the study of catalytic mechanisms. For example, Cronin observed the generation of an intermediate Fe(v)O species within a synthetic non-haem complex and its reaction with an olefin by variable temperature mass spectrometry.24 In this paper, we investigate the reaction mechanism for the epoxidation of cyclooctene with H2O2 catalyzed by H3PW12O40 and H2WO4 with mass spectroscopy for the aspects of decomposition and aggregation and explored the effects of reaction time, ratio of catalyst in the catalytic system.
Entry | Catalyst | Concentration [mM] | Conv.b [% mol] | Yieldb [% mol] | |
---|---|---|---|---|---|
[POM] | [W] | ||||
a Reaction condition: refer to the Experimental section.b Determined by GC with area normalization. | |||||
1 | H3PW12O40 | 0.005 | 0.06 | 86.71 | 76.30 |
2 | H3PW12O40 | 0.1 | 1.2 | 84.63 | 20.41 |
3 | H2WO4 | 0.06 | 0.06 | 80.52 | 71.21 |
4 | H2WO4 | 1.2 | 1.2 | 80.37 | 35.29 |
5 | None | 0 | 0 | 0 | 0 |
To further probe the formula of the [HW2(O2)2O5(C8H14O)2]− anions generated from the different catalysts, CID experiments on [HW2(O2)2O5(C8H14O)2]− were performed (Fig. 2). The CID spectra of [HW2(O2)2O5(C8H14O)2]− with H3PW12O40 as the catalyst (Fig. 2A) and H2WO4 as the catalyst (Fig. 2B) were similar, proving that [HW2(O2)2O5(C8H14O)2]− anions have the same structure in different catalytic systems. Besides, the CID spectra of the [HW2(O2)2O5(C8H14O)2]− from cyclooctene and 1,2-epoxycyclooctane with H3PW12O40 (Fig. S5A and B†) and H2WO4 (Fig. S5C and D) as the catalyst are almost the same.
Firstly, the reaction solutions were real-time monitored using ESI-MS by one-step (Fig. S6 and S7†). First, the catalyst, H2O2 (1 mmol) and cyclooctene (1 mmol) were dissolved in CH3CN. Then the samples were taken from the solution at intervals. The real-time monitoring of the solution of catalyst 1 (Fig. S6†) showed that the peaks assigned to [HW2(O2)2O5(C8H14O)2]− appeared at the 5th min and its abundance increased progressively. At last, it became the most intense peak at about the 60th min. The corresponding GC spectra showed that cyclooctene was completely converted to 1,2-epoxycyclooctane at about the 60th min (Fig. S9A†). The ESI-MS spectra of the real-time monitoring of the solution containing catalyst 2 (Fig. S7†) were similar to that with catalyst 1. The peaks assigned to [HW2(O2)2O5(C8H14O)2]− appeared at the 5th min and became the most intense peaks for the low solubility of H2WO4. The corresponding GC spectra showed that cyclooctene was converted to 1,2-epoxycyclooctane completely at about the 3rd hour (Fig. S10A†).
To further investigate how the [HW2(O2)2O5(C8H14O)2]− formed, the catalytic reactions were real-time monitored using ESI-MS by two-step. H2O2 and the catalyst were mixed previously and reacted for 4 h, and cyclooctene was added to the solution.
For the two-step catalytic system 1 (H3PW12O40 as catalyst), the ESI-MS monitoring data (Fig. 3) was illustrated as follows. Firstly, when H3PW12O40 (0.005 mmol) was dissolved in CH3CN, all major peaks in the mass spectrum can be assigned to [PW12O40]3− (m/z 958.7) and [HPW12O40]2− (m/z 1438.6), which means the {PW12} unit is stable in CH3CN at this step (Fig. 3A). After adding 1 mmol H2O2 to the previous solution with stirring at 50 °C for 5 min, some new species formed during this process. As the spectrum (Fig. 3B) shows, the new peaks at m/z 340.9–383.6 can be assigned to [PW4O16−x(O2)x]3− (x = 0, 2, 4, 6, 8), with m/z 340.9 for [PW4O16]3−, m/z 351.6 for [PW4O14(O2)2]3−, m/z 362.2 for [PW4O12(O2)4]3−, m/z 372.9 for [PW4O10(O2)6]3−, m/z 383.6 for [PW4O8(O2)8]3−, meaning that the parent {PW12} can react with H2O2, yielding the degraded peroxo-species [PW4O16−x(O2)x]3−. Upon continuous heating of the solution for 4 h, the intensity of the peaks assigned to [PW4O16−x(O2)x]3− (x = 0, 2, 4, 6, 8) gradually increased and became the main peaks (Fig. 3C). After adding cyclooctene (1 mmol) to the former reaction solution of {PW12} + H2O2 with stirring at 50 °C for 5 min, the new species related to [HW2(O2)2O5(C8H14O)2]− (m/z 765.1) emerged accompanied with a decline of [PW4O16−x(O2)x]3− (Fig. 3D). After continuous heating of the solution for 4 h, the intensity of [HW2(O2)2O5(C8H14O)2]− increased (Fig. 3E), indicating that the [HW2(O2)2O5(C8H14O)2]− intermediate was a combination of peroxo-species {W2} and product 1,2-epoxycyclooctane. As 1,2-epoxycyclooctane increased in the system, the amount of [HW2(O2)2O5(C8H14O)2]− gradually increased. Besides, the corresponding GC spectra showed that cyclooctene quickly completed the conversion to 1,2-epoxycyclooctane at about the 10th min (Fig. S9B†), which supported that [PW4O16−x(O2)x]3− (x = 0, 2, 4, 6, 8) were active species in the catalytic system.
For the catalytic system 2 (H2WO4 as catalyst), the real-time ESI-MS monitoring data (Fig. 4) by two-step was similar to system 1. Firstly, when the catalyst H2WO4 (0.06 mmol) mixed with CH3CN, there are only weak peaks assigned to [HWO4]− as a result of the low solubility of H2WO4 in CH3CN (Fig. 4A). After adding 1 mmol H2O2 to the H2WO4 solution with stirring at 50 °C for 5 min, a remarkable change occurred in the mixed solution. As the spectrum (Fig. 4B) shows, the new peaks at m/z 280.9–296.9 can be assigned to [HWO2(O2)2]− (m/z 280.9) and [HWO(O2)3]− (m/z 296.9). This result indicates that H2WO4 can react with H2O2, producing soluble peroxo-species [HWO4−x(O2)x]− (x = 2, 3). Besides, the reaction was performed under heating, which also increases the solubility of H2WO4. Upon continuous heating of the solution for 4 h, the intensity of the peaks assigned to [HWO(O2)3]− gradually increased and became the main peak (Fig. 4C). Then, after adding cyclooctene (1 mmol) to the mixed reaction solution of H2WO4 + H2O2 with stirring at 50 °C for 5 min, a new species belonging to [HW2(O2)2O5(C8H14O)2]− (m/z 765.1) emerged accompanied with a decrease of [HWO4−x(O2)x]− (Fig. 4D). After continuous heating of the solution for 4 h, the intensity of [HW2(O2)2O5(C8H14O)2]− increased (Fig. 4E). The corresponding GC spectra showed that cyclooctene converted to 1,2-epoxycyclooctane completely at about the 30th min (Fig. S10B†), which supported the speculation that [HWO4−x(O2)x]− (x = 2, 3) were active species in the catalytic system.
In summary, using Keggin type heterpolyoxotungstates H3PW12O40 and mononuclear tungstate H2WO4 as catalysts, the reaction process of cyclooctene epoxidation was real-time monitored by ESI-MS (Scheme 1). When H3PW12O40 was used as catalyst, it was degraded into minor peroxo-species [PW4O16−x(O2)x]3− (x = 0, 2, 4, 6, 8) which performed as the active center of the epoxidation of cyclooctene. After the addition of cyclooctene, the peroxo-species and 1,2-epoxycyclooctane formed intermediate [HW2(O2)2O5(C8H14O)2]−. When H2WO4 was used as the catalyst, it reacted with H2O2 and generated mononuclear tungsten peroxo-species [HWO2(O2)2]− and [HWO2(O2)3]−, which catalyzed the epoxidation of cyclooctene generating 1,2-epoxycyclooctane. After the addition of cyclooctene, the peroxo-species and 1,2-epoxycyclooctane formed intermediate [HW2(O2)2O5(C8H14O)2]−. In the presence of a large amount of catalyst, the non-conservation of olefin-epoxide in GC analysis also confirmed the existence of a stable intermediate. Future work will be focused on the structure and effect of the intermediate [HW2(O2)2O5(C8H14O)2]− in the catalytic system.
Footnote |
† Electronic supplementary information (ESI) available: Additional GC spectra of the epoxidation reaction of cyclooctene; comparison of calculated and observed mass spectra; CID mass spectra; real-time monitoring on the cyclooctene epoxidation by one-step; conversion-time curve of cyclooctene epoxidation. See DOI: 10.1039/c6ra09561e |
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