Inorganic Chemistry
Article
between the imidazolium cation and the MOF framework. A
slight shift to lower binding energies could be observed in S 2p
(Figure 3B) and Cr 2p (Figure 3C) spectra, which also
addition of carbon dioxide to epoxides. When using only O2 or
H2O2 as the oxidizing agents, the selectivity to styrene
carbonate was negligible and most of the raw material
transformed to benzaldehyde. Comparatively, for the blank
experiment, the tandem reaction could not happen at all
without the addition of catalysts and oxidizing agent. The
−
confirmed the interaction between −SO3 anion and
imidazolium cation as well as the interaction between Cr3+
and imidazolium, respectively.49 Meanwhile, the high-reso-
lution Au 4f spectra (Figure 3D) depicted the presence of Au
above results demonstrated that Au@[IM+]/[MIL-101-SO3 ]
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element in the Au@[IM+]/[MIL-101-SO3 ]. The XPS
was excellent catalyst in catalyzing the tandem synthesis of
styrene carbonate, during which pure O2 and TBHP as the
initiator were necessary to guaranty the high yield.
spectrum showed two peaks at a binding energy of 84.00
and 87.67 eV which could be attributed to the formation of
Au0 4f7/2 and 4f5/2, respectively. Note that the peak intensity of
Au was very weak due to the low loading within the composite
catalyst, which is consistent with ICP-AES analysis. Compared
to the samples without doped ILs, about a ca. 0.13 eV (Figure
3E) slight shift to lower binding energies could be attributed to
the strong electron-donating effect of nitrogen atoms to the Au
NPs, resulting in more electron-rich Au species.50,51
As previously reported,12 the solvent type, especially the
polarity, might have a significant effect on the product
distribution. In this regard, we further investigate the effect
of solvent in the catalytic oxidative carboxylation of olefins, and
various solvents with different polarities were tested. As shown
in Figure S8, an excellent conversion of olefin was realized in
all the solvents, indicating the high catalytic activity of
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To systematically evaluate the catalytic performance of the
single catalyst in the challenging autotandem synthesis of
OCCs, we separately tested the individual oxidation of olefins
and CO2 cycloaddition prior to performing the tandem
catalysis. The individual catalytic results are shown in Figure
Au@[IM+]/[MIL-101-SO3 ] in the oxidation of olefin. The
highest yield of styrene carbonate was obtained in the case of
DMF (74.5% yield), that with the strongest polarity, whereas
the yield toward styrene carbonate decreased sharply with the
decrease of solvent polarity; no styrene carbonate was observed
when using the nonpolar cyclohexane as the solvent, resulting
in benzaldehyde as the undesirable byproduct. Because of the
very hydrophilic nature of MIL-101-SO3H, the reaction that
performed in polar solvents exhibited better performance than
that in nonpolar solvents. This solvent-driven selectivity
control has also been reported in the recent studies.55,56
Additionally, to identify the crucial role of ILs and Au NPs that
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4. We could find that Au@[IM+]/[MIL-101-SO3 ] exhibited
excellent catalytic performance toward the oxidation of styrene
with complete conversion and >85% yield. While for the
substituted styrene, the reaction process was more compli-
cated; it showed relatively lower selectivity but was still
completely reacted. Meanwhile, Au@[IM+]/[MIL-101-SO3 ]
−
was further evaluated for the individual CO2 cycloaddition.
Prior to performing the reactions, we measured the adsorption
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has been played by Au@[IM+]/[MIL-101-SO3 ] in enhancing
capacity of [IM+]/[MIL-101-SO3 ] toward CO2. Delightedly,
the one-pot tandem reactions, two contrast catalysts in the
absence of ILs or Au NPs were separately synthesized and
utilized in the one-pot catalytic reaction. For comparison,
when using Au@MIL-101-SO3H (Table 1 entry 4) and Au@
ILs/MIL-101 (Table 1 entry 5) as the catalyst, the yield of
styrene carbonate decreased drastically, which demonstrated
that the imidazole cation and −SO3H group played an
important role in activating the CO2 molecules. The MOF
framework was also essential to obtain a high yield of cyclic
carbonate by providing a confined reaction room and in
stabilizing the whole catalytic sites (Table 1 entry 6). Similarly,
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[IM+]/[MIL-101-SO3 ] showed a much enhanced absorption
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capacity of CO2 compared to the initial MIL-101-SO3H
despite having much decreased surface areas (Figure 3F),
which could be attributed to the strong affinity of the
imidazolium-based ILs to CO2. The catalytic performance of
CO2 cycloaddition was displayed in Figure 4B, which exhibited
excellent catalytic performance toward all the substrate
molecules at moderate reaction condition. Subsequently, the
tandem catalytic synthesis of styrene carbonate was inves-
tigated by using the styrene and CO2 as the starting material,
and the results were shown in Table 1. First, the Au@[IM+]/
when this reaction was catalyzed by [IM+]/[MIL-101-SO3 ]
−
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[MIL-101-SO3 ] catalysts with varied Au content were
(Table 1 entry 7), the tandem reaction could not work at all,
indicating the necessity of Au NPs in olefin oxidation.
Furthermore, when only a physical mixture of Au NPs, ILs,
and MIL-101-SO3H (Table 1 entry 8) was employed to
catalyze this reaction, the selectivity to styrene carbonate was
negligible in spite of the complete conversion, demonstrating
the synergetic effects of the integrated composites than the
physical mixture. The catalytic applicability of Au@[IM+]/
evaluated. As shown in Table 1 (entry 1), 0.30 wt %
Au@[IM+]/[MIL-101-SO3 ] possessed the best catalytic
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activity with higher than 99% conversion of styrene and
achieves 74.5% yield toward styrene carbonate. Note that most
previously reported catalysts exhibited less than 50% yield to
the target product in the oxidative carboxylation of olefins.52,53
Besides, the main byproducts produced during the aerobic
oxidation of olefins were the benzaldehyde due to the
oxidation of CC bond cleavage in accordance with
previously published work,54 whereas the phenylacetaldehyde
generated by the rearrangement of styrene oxide was not
detected in the current reaction system. Second, various green
and common oxidizing agents, e.g., H2O2, O2, and air
atmosphere were evaluated to screen out the best oxidizing
agents (Figure S7). The pure O2 showed much higher
selectivity to the styrene carbonate than air atmosphere
(only 16.8% yield), and the utilization of TBHP as the
initiator is a must in order to afford a higher yield of epoxides
due to the activation of molecular O2 by TBHP for olefin
epoxidation, which could further promote the next cyclo-
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[MIL-101-SO3 ] toward styrene bearing different functional
groups and two cycloolefins were also investigated with the
optimized conditions in DMF. As shown in Table 1, all olefins
bearing different functional groups were converted completely,
which further demonstrated the excellent catalytic performance
of Au NPs in olefin oxidation. However, except for 4-
methoxystyrene (10% yield) (Table 1 entry 9) and cyclo-
hexene (60% yield) (Table 1 entry 14), other substrates (Table
1 entries 10−13) exhibited negligible yield of corresponding
carbonates, resulting in the substituted benzaldehyde as the
major byproduct.
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Finally, the recyclability of the Au@[IM+]/[MIL-101-SO3 ]
catalyst in one-pot oxidative carboxylation of olefins was also
F
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