Bin
Huang
a,
Wei-Ling
Jiang
b,
Jie
Han
c,
Yan-Fei
Niu
a,
Hai-Hong
Wu
a and
Xiao-Li
Zhao
*a
aShanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry, East China Normal University, 3663 North Zhongshan Road, Shanghai 200062, P. R. China. E-mail: xlzhao@chem.ecnu.edu.cn; Fax: +86-21-62233179
bKey Laboratory of Synthetic and Natural Functional Molecule of the Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an 710127, P. R. China
cDepartment of Science, School of Science and Technology, Hong Kong Metropolitan University, Hong Kong SAR, P. R. China
First published on 23rd June 2022
A newly functionalized MIL-101(Fe) type MOF, namely NaPT@MIL-101-NH-PNIPAM, decorated with a thermo-responsive poly(N-isopropylacrylamide) (PNIPAM) brush outside and loaded with sodium phosphotungstate particles (NaPT) inside has been successfully synthesized. The new thermally responsive amphiphilic NaPT@MIL-101-NH-PNIPAM powder can well serve as a highly efficient Pickering emulsifier to stabilize the H2O2-in-oil emulsion. The resultant H2O2-in-oil Pickering emulsion displayed good catalytic performance in the epoxidation of olefins at room temperature, promoting a greener reaction with good yield and selectivity. Notably, the constitutes (H2O2, olefin) of the Pickering emulsion is also the substrate of the epoxidation of the olefin reaction, which will facilitate further expansion of the reaction scale. Moreover, the catalyst of the Pickering emulsion featured intriguing thermo-responsive properties of swelling/shrinking behavior that can trigger emulsification at room temperature (25 °C) and accomplish demulsification at an elevated temperature (50 °C), mainly benefitting from the thermally responsive amphiphilic nature of the attached PNIPAM polymer. This thermo-responsive feature of the Pickering emulsion catalyst renders the catalyst–product separation and catalyst recycling feasible during the epoxidation of alkenes. This work reveals new insights for the design of a highly efficient heterogeneous catalytic system in H2O2-based epoxidation and may have potential for application in industry in the future.
In recent years, environmentally responsive Pickering emulsions have attracted great focus as a highly efficient catalytic system due to their excellent catalytic performance and ease of product separation and recyclability.27–33 It has been reported that MOFs can be employed as suitable emulsifiers for Pickering emulsion after post-synthetic modification (PSM) and covalent incorporation with functional polymers.34–39 For example, light-, pH-, and thermo-responsive Pickering emulsions provide efficient solutions for various catalytic reactions.40–45 So far, the reported Pickering emulsions with MOFs as emulsifiers are all oil-in-water systems. However, the water core of the water-in-oil emulsion is considered to be an ideal reactor for the encapsulation of nanoparticles.46 Therefore, the development of water-in-oil Pickering emulsions with MOF emulsifiers is important for improving its catalytic performance. Based on the above-mentioned considerations, using POM-loaded MOFs as emulsifiers to stabilize emulsion catalytic systems can realize the convenient separation and recycling of POM-based catalysts. In addition, the water phase and oil phase in most of the emulsion catalyst systems currently reported only serve as solvents for the substrate and do not participate in the reaction, which greatly limits the scale and efficiency of the reaction. In our opinion, it is highly anticipated to overcome the above shortcomings by using the components involved in the reaction (oxidizing or reducing agent, substrate, etc.) as constituent phases of the emulsion.
Herein, we report the first example of a thermo-responsive MOF-based H2O2-in-oil type Pickering emulsion for improving the catalytic performance toward olefin epoxidation. The target MOF of NaPT@MIL-101-NH-PNIPAM (NaPT: sodium phosphotungstate particles, PNIPAM: poly(N-isopropylacrylamide) as an emulsifier can not only stabilize the Pickering emulsion, but also significantly increase the conversion and selectivity of olefin epoxidation at room temperature. NaPT@MIL-101-NH-PNIPAM bearing PNIPAM chains has a thermally responsive amphiphilic nature and thus can well serve as a highly efficient Pickering emulsifier to stabilize the H2O2-in-oil emulsion. To the best of our knowledge, this is the first time that a H2O2-in-oil system is realized for efficient olefin epoxidation, i.e., H2O2 as an oxidant in this reaction is also used as one of the emulsion phases (aqueous phase), and reactant olefin is used as part of the oil phase, leading to a “Pickering emulsion in situ catalytic system”. In addition, the scale of the olefin epoxidation reaction in this catalytic system is successfully expanded with excellent yield and selectivity compared with other emulsion catalytic systems based on MOFs. Moreover, demulsification can be achieved by adjusting the temperature to separate the oil and aqueous phase, and the aqueous phase with NaPT@MIL-101-NH-PNIPAM can be reused directly in the next catalytic cycle. Therefore, this work has demonstrated a successful example of an MOF-based H2O2-in-oil Pickering emulsion system for the highly efficient olefin epoxidation.
NaPT@MIL-101-NH-PNIPAM was prepared as a red-brown powder (Fig. S3, ESI†) by mixing NaPT (100 mg) and MIL-101-NH-PNIPAM (100 mg) in DI water at room temperature for 72 h (Scheme 1). The loading amount of tungsten (W) was up to 4.63 wt%, which was examined by ICP-MS. In addition, FT-IR spectra and 31P-NMR analysis collectively suggested that NaPT was successfully doped inside the pores of MOFs. Specifically, the bands at 1102 cm−1 were assigned to the stretching frequency of P–O, while peaks at 934 cm−1, 896 cm−1 and 817 cm−1 were assigned to WO, WOb–W and W–Oc–W, respectively. The presence of one peak of 31P NMR spectra suggested NaPT was doped inside the MOFs successfully (Fig. S4, ESI†). Other characterization results including thermogravimetry analysis (TGA) are shown in the ESI† (Fig. 1b and Fig. S5 and S6, ESI†). PXRD and SEM showed that the crystalline nature and structural integrity of MOFs were well maintained after the PSM process and NaPT loading (Fig. 1c–g).
In addition, as MIL-101-NH2 was modified step by step, part of the modified chains grew inside the pores, resulting in a decrease in the total pore volume which was proved by the isotherm study of N2 sorption (Fig. S7, ESI†). The adsorption capabilities of the MOFs declined significantly after PNIPAM grafting and NaPT loading. Moreover, dynamic light scattering (DLS) revealed that the sizes of MOF particles slightly increased after continuous PSM procedures, and centered at ca. 345.2, 348.2, 354.2, and 361.3 nm, for MIL-101-NH2, MIL-101-NH-Met, MIL-101-NH-PINIPAM and NaPT@MIL-101-NH-PINIPAM, respectively (Fig. 2a). It is noteworthy that SEM-EDX elemental mapping confirmed the uniform texture of NaPT@MIL-101-NH-PNIPAM by showing a homogeneous distribution of Fe, W, P, O, N and C elements in the composite material (Fig. 2b).
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Fig. 2 (a) DLS results of MIL-101-NH2, MIL-101-NH-Met, MIL-101-NH-PINIPAM and NaPT@MIL-101-NH-PINIPAM at room temperature. (b) SEM-EDX elemental mapping of NaPT@MIL-101-NH-PINIPAM. |
In order to evaluate the changes from hydrophilicity to hydrophobicity of the PNIPAM chains below and above the LCST (32 °C), the water contact angle (WCA) of the MOF particles was measured at different temperatures. The photographs of water-drop profiles on the MOF tablets at 25 and 50 °C are shown in Fig. S9 (ESI†). These results exhibited that the surface properties of MIL-101-NH-PNIPAM changed from hydrophilicity with a WCA of 18° ± 2° at 25 °C to hydrophobicity with a WCA of 82° ± 2° at 50 °C. Similarly, the WCA of NaPT@MIL-101-NH-PNIPAM increased from 38 ± 2° to 92° ± 2° upon heating from 25 to 50 °C. This indicated an obvious phase transition of PNIPAM from hydrophilic (25 °C) to hydrophobic (>50 °C). In contrast, the WCAs of MIL-101-NH2 and MIL-101-NH-Met without the PNIPAM decoration were unaffected by temperature.
The co-solution of the two liquids was known to be related to the interfacial tension (IFT). The “barrier” between the two phases gradually disappears to reach the state of co-solution when the interfacial tension decreased. Thus, to further understand the thermo-responsive behaviour of MIL-101-NH-PNIPAM and NaPT@MIL-101-NH-PNIPAM, IFT between cyclohexane and H2O2 was measured using a spinning drop tensiometer from 20 to 55 °C (Fig. 3c). The IFT value for H2O2/cyclohexane had no significant changes with an increase in temperature (Fig. S10, ESI†). Upon addition of MIL-101-NH-PNIPAM, the IFT was down to 8.24 mN m−1 at 20 °C, and then it went through a sharp increase from 8.25 to 8.94 mN m−1 in the range of 30–45 °C, which was similar to the change of H2O2/cyclohexane with PNIPAM. Finally, the IFT decreased to 8.82 mN m−1 when the temperature was up to 55 °C (Fig. 3c and Fig. S10, ESI†). Meanwhile, NaPT@MIL-101-NH-PNIPAM had a similar temperature-dependent change with a lower ITF value of 7.62 mN m−1 at 20 °C, while the IFT data of MIL-101-NH2 and MIL-101-NH-Met had no significant change from 20 °C to 55 °C (Fig. S10, ESI†). Therefore, both MIL-101-NH-PNIPAM and NaPT@MIL-101-NH-PNIPAM could decrease the interfacial tension between two solvents, so that two solvents realized the co-solution state.
Notably, when the ratio of oil to H2O2 was 1:
1, there was a brown solution, not emulsion at the bottom of the cuvette, indicating that H2O2 containing hydrophilic NaPT@MIL-101-NH-PNIPAM was excessive in it. But the transparent oil layer appeared when the ratios of oil and H2O2 were 3
:
1 and 4
:
1, which suggested that the oil was excessive at this time, while the 2
:
1 oil–H2O2 ratio avoided the above two situations (the content of NaPT@MIL-101-NH-PNIPAM was 2.0 wt%). Therefore, we adopted this ratio for establishing the relationship between the mass percentage of MOFs and the effect of the emulsion. It could be found from Fig. 4b that when the content of NaPT@MIL-101-NH-PNIPAM was 0.5 wt%, the emulsification is not uniform and some small bubbles were visible, indicating that the amount of MOFs was insufficient to stabilize the emulsion. The same situation appeared when the ratio was increased to 1.0 wt%. The size of the H2O2 droplets was uniform roughly until the ratio reached 2.0 wt%. When the proportion of MOFs was further increased to 3.0 wt%, although a stable emulsion could be formed, it became too viscous, so that the emulsion still could not flow after the cuvette was turned upside down (Fig. S11, ESI†), which would not be conducive to the subsequent catalytic reaction. In summary, we could conclude the optimal conditions for preparing a stable and roughly homogeneous emulsion: (1) oil
:
H2O2 ratio = 2
:
1; (2) 2.0 wt% NaPT@MIL-101-NH-PNIPAM. Furthermore, the resulting emulsion could remain stable for at least a week without conspicuous demulsification (Fig. S12, ESI†).
The thermo-responsivity behaviour of the prepared Pickering emulsion was further investigated. As shown in Fig. 5a, the Pickering emulsion generated from a cyclohexene & cyclohexane–H2O2 mixed solvent system with 2.0 wt% NaPT@MIL-101-NH-PNIPAM was very stable at 25 °C, and it became unstable with a temperature increase under stirring. When the temperature reached 50 °C, the phase separation occurred, i.e., the separated oil and H2O2 phases became clearer and the MOF particles gradually aggregated during the heating process, and finally remained in the oil phase. A reasonable explanation for this phenomenon was that the attached PNIPAM chains compacted on the MOF surface and turned to be hydrophobic above the LCST (32 °C), which led NaPT@MIL-101-NH-PNIPAM to be incompatible with H2O2 (50 °C). When the temperature was cooled down to 25 °C, NaPT@MIL-101-NH-PNIPAM completely moved to the H2O2 phase since PNIPAM chains were outspread and miscible with H2O2 below the LCST. This observation was also supported by WCA measurements at different temperatures (Fig. S9, ESI†). In addition, confocal microscopy images of the upper and lower layers of the system at different temperatures showed that when the temperature exceeded the LCST, demulsification occurred and the H2O2 droplets gradually transferred to the lower layer, while the cyclohexane transferred to the upper layer. Complete demulsification occurred at 50 °C (Fig. S13, ESI†). Upon re-homogenization, a stable emulsion was formed again at 25 °C (Fig. 5b). Most importantly, this thermo-responsivity process was highly reversible. Even after 5 cycles, Pickering emulsions could be formed again steadily. In addition to the cyclohexene & cyclohexane–H2O2 system, NaPT@MIL-101-NH-PNIPAM could also be applied to stabilize the emulsion when other organic solvents were employed as the oil phases, such as n-pentane, n-hexane, benzene, and toluene (Fig. S14, ESI†).
Subsequently, different types of olefins were further explored for epoxidation using H2O2 as an oxidant (Table 1 and Fig. S16–S27, ESI†). On the one hand, it could be found that different ring sizes of cyclopentene, cyclohexene and cyclooctene had little influence on the reaction results (Table 1, entries 1-3). Therefore, the reaction system was suitable for cycloalkenes with different ring tensions. On the other hand, we also investigated aliphatic olefins. Fortunately, the reaction system worked well with aliphatic olefins of different alkyl chain lengths (Table 1, entries 8 and 9). We speculated that the conjugation effect of aromatic substituents would reduce the reactivity of olefins. Surprisingly, the yield and selectivity of aryl substituted olefins were still higher than 95% (Table 1, entries 10–12). To illustrate the effect of the type and position of the substituent on the reaction efficiency, we further explored methylcyclohexene with electron-donating methyl groups at the α-position and γ-position, cyclohexene substituted with the electron-withdrawing carbonyl group at the β-position, and norbornene (Table 1, entries 4–7). Interestingly, the substitution of carbonyl groups significantly reduced the epoxidation yield of olefins, indicating that the reaction system had a negative effect on the epoxidation of electron-deficient olefins (Table 1, entry 6). As a whole, such a “Pickering emulsion in situ catalytic system” exhibits excellent yield and selectivity of versatile olefin epoxidations under mild conditions with excellent regeneration ability over traditional biphasic catalytic systems (Table S2, ESI†). It is noteworthy that the emulsion could be recycled easily after the first cycle of the catalytic reaction. When the reaction was complete, the temperature was increased to 50 °C. Then the system was cooled to room temperature after the demulsification was complete. After that, we took out the oil phase and re-added the organic solvent and substrate to start a new cycle of catalysis (Scheme 2). Remarkably, the NaPT@MIL-101-NH-PNIPAM-based Pickering emulsion showed excellent catalytic efficiency and selectivity up to 97% even after five recycles (Fig. 6c). This corresponds to the full contact between the active species [PO4{WO(O2)2}4]3− and the substrate, the former was the active species for H2O2-based epoxidations. When H2O2 completely converted to H2O, [PO4{WO(O2)2}4]3− converted to [PW12O40]3−. After the oxidation was complete, [PO4{WO(O2)2}4]3− regenerated and entered the next catalytic cycle, and FTIR and 31P NMR had proved the mechanism (Fig. S28 and S29, ESI†). With an increase of the recycle times, it was found that the particle size of the water in the emulsion became less uniform, possibly due to the shedding effect of PNIPAM chains (Fig. S30, ESI†). The PXRD results showed that the structural features of the MOF component within the system were well maintained during the first four recycles (Fig. 6d) and only a little bit loss of NaPT particles occurred after the reuse of NaPT@MIL-101-NH-PNIPAM three times (Fig. 6f and Fig. S31, ESI†), leading to the decrease in yields (Fig. 6c). In addition, XPS measurement provided the direct evidence of the remained structural integrity of NaPT. We found that there was no valence change of W species observed after recycling, which may be attributed to the protection of W species by MOF cores (Fig. 6e).
Entry | Substrate | Product | t (h) | Yieldb (%) | Selectivityb (%) |
---|---|---|---|---|---|
a Reaction conditions: olefins (2.0 mmol), H2O2 (10 mmol), NaPT@MIL-101-NH-PINIPAM-based Pickering emulsion (olefins & cyclohexane: H2O2 = 2![]() ![]() |
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1 |
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6 | 99 | 99 |
2 |
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6 | 99 | 99 |
3 |
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6 | 96 | 97 |
4 |
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8 | 98 | 96 |
5 |
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6 | 95 | 96 |
6 |
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6 | 88 | 99 |
7 |
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8 | 97 | 99 |
8 |
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8 | 95 | 97 |
9 |
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10 | 99 | 99 |
10 |
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8 | 96 | 92 |
11 |
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10 | 95 | 99 |
12 |
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8 | 95 | 99 |
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Scheme 2 A schematic model for the thermo-responsive NaPT@MIL-101-NH-PINIPAM-based Pickering emulsion catalytic system. |
Three control experiments were designed to demonstrate the catalytic effect of the NaPT@MIL-101-NH-PNIPAM-stabilized Pickering emulsion: (1) the reaction was carried out without NaPT@MIL-101-NH-PNIPAM; (2) NaPT@MIL-101-NH-PNIPAM was added into the oil–H2O2 system without emulsification; and (3) the solid catalyst of NaPT@MIL-101-NH-PNIPAM was separated from the catalytic emulsion system after reacting for a given time (2 h). The reaction was continued with the filtrate in the absence of NaPT@MIL-101-NH-PNIPAM. The results of the above experiments were all monitored by 1H NMR and GC-MS.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2ma00245k |
This journal is © The Royal Society of Chemistry 2022 |