Quaternary phosphonium salt-functionalized Cr-MIL-101: A bifunctional and efficient catalyst for CO2 cycloaddition with epoxides

Journal of CO₂ Utilization 36 (2020) 295–305

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Quaternary phosphonium salt-functionalized Cr-MIL-101: A bifunctional and efficient catalyst for CO2 cycloaddition with epoxides

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Weili Dai*, Pei Mao, Ying Liu, Shuqu Zhang, Bing Li, Lixia Yang*, Xubiao Luo, Jianping Zou Key Laboratory of Jiangxi Province for Persistent Pollutants Control and Resources Recycle, Nanchang Hangkong University, Nanchang 330063, Jiangxi, China

A R T I C LE I N FO

A B S T R A C T

Keywords: CO2 cycloaddition Cyclic carbonate Epoxide Quaternary phosphonium salt Metal-organic framework

Integration of synergic Lewis acid sites and nucleophilic anions that can facilitate ring-opening of epoxide is a good way to develop efficient catalyst for cycloaddition of CO2 with epoxides. Herein, we prepared a novel metal-organic framework (MOF) catalyst (Cr-MIL-101-[BuPh3P]Br) through grafting quaternary phosphonium salt ionic liquid (IL) on Cr-MIL-101 by a facile post-synthetic approach. The introduction of phosphonium salt IL highly improves the catalytic activity and stability compared with the parent Cr-MIL-101-NH2. In the absence of any solvent and co-catalyst, Cr-MIL-101-[BuPh3P]Br showed excellent catalytic activity for the cycloaddition reaction under moderate reaction conditions. Under optimized conditions, the yield and TOF of propylene carbonate can be achieved 97.8% and 1086.7 h−1, respectively. This is because the synergetic interaction of dual functional sites including Cr3+ as Lewis acid sites in MOF and Br- as nucleophile in IL could promote the ringopening of epoxide through the coordination of Cr3+ sites with O atom and the nucleophilic attack of Br on the less sterically hindered β-carbon atom of epoxide, respectively. Comparison with other reported ILs-functionalized Cr-MIL-101 or other kinds of MOFs catalysts reveals that Cr-MIL-101-[BuPh3P]Br has superior catalytic performance and potential. Moreover, Cr-MIL-101-[BuPh3P]Br as a heterogeneous catalyst also showed good chemical stability and reusability.

1. Introduction CO2 as the primary anthropogenic gas involved in the culprit for climate change has attracted much attention [1]. Nevertheless, CO2 is also under the spotlight as C1 building block and has been viewed as an abundant, economical, nontoxic, and renewable carbon source to produce useful organic compounds [2]. Among various CO2 fixation routes, the cycloaddition with epoxides into cyclic carbonates is promising in both academia and industry, due to high atom efficiency of this reaction and the wide applications of cyclic carbonates. The cyclic carbonates are valuable compounds, and widely employed as polar aprotic solvents, electrolytes in lithium secondary batteries, precursors for the formation of polycarbonates, and intermediates in the production of pharmaceuticals and fine and agricultural chemicals [3,4]. However, due to the high inherent thermodynamic stability and kinetic inertness of CO2, the catalyst is an essential factor for its conversion to proceed. In order to be more efficient in the synthesis of cyclic carbonates through the cycloaddition reaction, plenty of catalysts have been developed, including homogeneous and heterogeneous catalysts of various type ranging from alkali metal salts [5], metal oxides [6], organometallic complexes [7], ionic liquids (ILs) [8] and most recently

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metal organic frameworks (MOFs) [9]. Of particular interest is that ILs have been regarded as an effective and selective catalyst for such catalytic transformation, because of their uniquely tunable structure and polarity, good thermal stability, and acid-basic property [10]. Since 2000s, variety of ILs catalyst systems, such as imidazolium [11], quaternary ammonium [12], quaternary phosphonium [13], pyridinium, and guanidinium [14] salts were developed and extensively studied. Unlike imidazolium-based ILs, there are few works about the quaternary phosphonium salts, attributing to the less tailored structures and relatively low catalytic activity. For examples, polyfluoroalkyl phosphonium iodides [15] and triphenylalkyl phosphonium halides [16], were all evaluated as catalysts for the cycloaddition reaction, but suffer from unsatisfactory activity and harsh reaction conditions. To overcome this shortcoming, we reported the synthesis and evaluation of task-specific triphenylalkylphosphonium halides ILs that are functionalized with hydroxyl and carboxyl groups, which showed higher catalytic activity than traditional ones, attributing to the polarization between –OH and epoxy ring [17]. Additionally, Shirakawa’s [18] and Werner’s groups [19] reported several bifunctional phosphonium salt systems that showed good catalytic activity for the cycloaddition reaction. However, for commercial application of the phosphonium salts

Corresponding authors. E-mail addresses: [email protected], [email protected] (W. Dai), [email protected] (L. Yang).

https://doi.org/10.1016/j.jcou.2019.10.021 Received 2 July 2019; Received in revised form 19 September 2019; Accepted 29 October 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

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Scheme 1. Preparation process and the structure model of Cr-MIL-101-[BuPh3P]Br.

efficient cycloaddition catalysts by combination of Cr-MIL-101 and quaternary phosphonium salt IL. In this work, we prepared one-component bifunctional catalyst through one-step post-synthetic functionalization of Cr-MIL-101 using triphenylbutylphosphonium bromide, which was found to have excellent activity, good recyclability and stability for the synthesis of cyclic carbonates from CO2 and epoxides.

for the synthesis of cyclic carbonate from CO2 and epoxides, more efficient catalysts should be further developed. It is notable that combination of metal salts as Lewis acid (such as zinc halides, KI, etc.) [20] as co-catalyst with phosphonium-based ILs exhibits superior catalytic activity than the task-specific ones. However, the solubility of the components of the above catalyst system in the strong polar reaction mixture is an issue during catalyst recovery after the completion of the reaction. Obviously, if we can fabricate a heterogeneous catalyst that with both active metal sites and phosphonium-based IL, it will be an excellent strategy for development of efficient and promising cycloaddition catalysts. Metal organic frameworks (MOFs), as a class of new emerging porous crystalline materials, have accessible pore volumes, high surface area, high adsorption capacity, and diverse means of functionalization that promising applications in catalysis, gas storage/sorption, proton conductivity, drug delivery, and so on [21–26]. In addition to the above advantages, what is more important that the coordinatively unsaturated metal sites of MOFs can be used as the active Lewis acid center for the corresponding catalytic reaction [27,28]. Consequently, in recent years, extensive research efforts have been dedicated to use of MOFs materials for the CO2-epoxide cycloaddition reaction. Unfortunately, a very few of MOFs can be directly used as the cycloaddition reaction by itself. Either co-catalyst or solvent is essential for most of the MOFs catalyst systems, which are uneconomical and complicated. For example, tetrabutylammonium bromide (TBAB), a typical quaternary ammonium IL, is an extensively used and promising co-catalyst for promoting the catalytic activity of MOFs. However, the inherent homogeneous property of TBAB makes it difficult to separate from the products. Hence, the fabrication of MOFs functionalized with ILs that can be used as onecomponent bifunctional catalyst for conversion of CO2 is highly desirable. Chromium terephthalate MIL-101 (Cr-MIL-101) is a mesoporous MOF material with many outstanding physical and chemical properties. It has extremely high surface area and pore volume, and good chemical resistant to air, water, common solvents, as well as good thermal stability, which is highly desirable and most promising combination for catalytic applications [29,30]. More important for the cycloaddition reaction, Cr-MIL-101 possesses numerous potentially unsaturated chromium sites (up to 3.0 mmol g−1) with Lewis acid properties upon removal of the terminal water molecules [31]. To date, only few studies about imidazolium [32], quaternary ammonium [33], or guanidinium [34] salts functionalized Cr-MIL-101 as heterogeneous catalysts for the fixation of CO2 with epoxides have been reported. In terms of previous reported and our studies, it will be a good strategy for developing

2. Experimental 2.1. Chemicals Propylene oxide, triphenylphosphine, 1,4-dibromobutane, and chromium(III) nitrate nonahydrate were purchased from Shanghai Jingchun Industry Co., Ltd. 2-Aminoterephthalic acid (2-ATPA) and the other epoxides were purchased from Alfa Aesar China Co., Ltd. All chemicals were used as received. The CO2 (99.5% purity) purchased from Nanchang Guoteng Gas Co., Ltd was used without any purification treatment. 2.2. Synthesis of catalysts 2.2.1. Synthesis and activation of Cr-MIL-101-NH2 The amino-functionalized Cr-MIL-101 (Cr-MIL-101-NH2, structure shown in Scheme 1a) was prepared according to the literature procedures [35] with a little modification. 180 mg 2-ATPA, 400 mg Chromium(III) nitrate nonahydrate, and 100 mg NaOH were dissolved in 7.5 mL deionized water and transferred to a 25 mL Teflon-lined stainless-steel autoclave. Then the autoclave was heated at 150 °C for 24 h. After the hydrothermal reaction, the obtained green solid product was collected by filtration. To remove the unreacted 2-ATPA, the rude CrMIL-101-NH2 was purified by double treatment with DMF at 60 °C for 1 h and then with ethanol at 70 °C for 3 h. Activation of Cr-MIL-101NH2 was performed by drying under vacuum at 100 °C for 24 h. 2.2.2. Synthesis of (4-bromobutyl)triphenylphosphonium bromide In a typical reaction (Scheme 1b), a solution of triphenylphosphine (5 mmol), 1,4-dibromobutane (5.1 mmol) in 20 mL toluene was heated at 110 °C and subject to reflux for 24 h. After the reaction, the mixture was cooled down to room temperature, and the resultant crude solid was filtered out and washed with diethyl ether and dichloromethane, respectively. The product was dried at 60 °C under vacuum for 24 h to give (4-bromobutyl)triphenylphosphonium bromide (denoted as [BrBuPh3P]Br) as a white powder. 1H NMR (500 MHz, DMSO-d6): δ (ppm) 7.91–7.79 (m, 15 H), 3.64–3.60 (m, 2 H), 2.51-2.50 (m, 2 H), 296

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2.02–1.97 (m, 2 H), 1.71–1.65 (m, 2 H). 13C NMR (125 MHz, DMSO-d6): δ(ppm) 135.5, 134.1, 130.8, 119.2, 34.3, 33.1, 20.8, 19.6. 2.2.3. Synthesis of [BuPh3P]Br-functionalized Cr-MIL-101 In a typical procedure (Scheme 1c), a solution of 460 mg Cr-MIL101-NH2, 2 mmol [BrBuPh3P]Br, 100 mg 4-dimethylaminopyridine (DMAP) and 20 mL DMF were added to a 100 mL three-necked flask. The mixture was stirred at 70 °C for 24 h under a nitrogen atmosphere. After the reaction, the mixture was cooled to room temperature, and the resultant solid was filtered out and washed three times with dichloromethane. The [BuPh3P]Br-functionalized Cr-MIL-101 (denoted as Cr-MIL-101-[BuPh3P]Br) as product was obtained after purification by washing with hot ethanol for three times, and drying at 60 °C for 24 h under vacuum. 2.3. Characterization Field Emission Scanning Electron Microscope (FESEM) observations were carried out over a Nova Nano SEM 450 microscope. Energy dispersive X-ray spectroscopy (EDS) was performed using accessory (INCA 250) of the Nova Nano SEM 450 instrument. X-ray diffraction (XRD) patterns were obtained on a Rigaku D/Max 2200PC operating at 40 kV and 40 mA using Ni-filtered Cu Kα radiation (λ = 1.542 Å). The XRD patterns were recorded in the 2θ range of 4°–30°. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis was measured on Agilent 720 spectrometer. The Fourier transformed infrared (FT-IR) spectra were recorded on a Bruker vertex 70 FT-IR spectrophotometer. Thermogravimetric analysis (TGA) was performed on a SDT Q600 (TA Instruments-Waters LLC) at a heating rate of 15 °C/min from 50 °C to 700 °C in nitrogen flow. X-ray photoelectron spectroscopy (XPS) was conducted using a Kratos Axis Ultra DLD (delay line detector) spectrometer with a monochromatic Al Kα radiation line source. BrunauerEmmett-Teller (BET) analysis was carried out by a Micromeritics TriStar II 3020 nitrogen adsorption apparatus at 77 K. The specific surface area of the samples was estimated by the BET method. The pore size distribution plots were obtained by the Barret–Joyner–Halenda (BJH) model.

Fig. 1. XRD patterns of Cr-MIL-101-NH2 and Cr-MIL-101-[BuPh3P]Br.

Fig. 2. FT-IR spectra of (a) Cr-MIL-101-NH2, (b) Cr-MIL-101-[BuPh3P]Br, and (c) [BuPh3P]Br.

2.4. General procedure for the cycloaddition of CO2 with epoxides In a typical reaction, the 50 mL high-pressure stainless-steel autoclave was added with epoxides (35.7 mmol), an appropriate amount of catalyst and biphenyl (0.97 mmol, as the internal standard for GC analysis). After sealing, the reactor was fed with CO2 to a desired pressure, then the autoclave was heated to a selected temperature and stirred for a designated period of time. After reaction, the reactor was rapidly cooled to room temperature in an ice water bath, the unreacted CO2 was released slowly. The catalyst was separated by centrifugation, and the liquid products were quantitatively analyzed on a gas chromatograph (Agilent 7890A) equipped with a TCD detector and a DBwax capillary column (30 m × 0.53 mm × 1.0 μm).

remained intact during the post-synthesis processes. To confirm the successful modification of quaternary phosphonium salt IL on Cr-MIL-101, the FT-IR spectra of [BuPh3P]Br, Cr-MIL-101NH2 and the stepwise functionalized MOF were collected and shown in Fig. 2. The double peaks at 3473 and 3345 cm−1 appears in the spectrum of Cr-MIL-101-NH2 (Fig. 2a), which are ascribed to the symmetrical and asymmetrical stretching vibration adsorption of the –NH2 groups [35]. After bonding with alkyl-bromine of [BrBuPh3P]Br, the peaks of primary amino groups in the spectrum of Cr-MIL-101[BuPh3P]Br weakened (Fig. 2b). But a new peak bond at 1659 cm−1 was observed, ascribing to the bending vibration of as-formed secondary amine NeH. In addition, Cr-MIL-101-[BuPh3P]Br also exhibits the characteristic bands of the phosphonium salt IL. The spectra of [BuPh3P]Br (Fig. 2c) and Cr-MIL-101-[BuPh3P]Br both exhibit bands at 700 cm−1, which can be associated with the characteristic of stretching vibration of PeC bond [38]. Furthermore, the CeH stretching vibration of alkyl chain also can be observed at 2883 cm−1 in their spectra [39]. The FT-IR results evidently indicate that the [BuPh3P]Br has been successfully introduced on Cr-MIL-101 surface through the substitution reaction between the alkyl-bromine of [BrBuPh3P]Br and the -NH2 group of Cr-MIL-101-NH2. SEM and EDS were carried out to investigate the surface morphologies and elemental composition of Cr-MIL-101-NH2 before and after IL functionalization, and the results were shown in Fig. 3. The SEM image of Cr-MIL-101-NH2 (Fig. 3a) shows presence of irregularly particle like

3. Result and discussion 3.1. Catalyst characterization The XRD diffraction was performed to investigate the crystal structure of the as-prepared MOF samples. As shown in Fig. 1, Cr-MIL101-NH2 basically maintains the fundamental crystalline structure compared with the parent framework (Cr-MIL-101), although rather broad Bragg reflections were detected. This may be due to the fact that the NH2-functionalized MOF consists of small nanoparticles, which is consistent with other reports [36,37]. After further modification of CrMIL-101-NH2 with phosphonium salt IL through the formation of covalent bond between -NH2 and alkyl bromine groups, the crystalline structure of the resulted Cr-MIL-101-[BuPh3P]Br was unchanged and 297

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Fig. 3. SEM images and EDS spectra of (a, c) Cr-MIL-101-NH2, and (b, d) Cr-MIL-101-[BuPh3P]Br; EDS Mapping images of (e-h) Cr-MIL-101-[BuPh3P]Br.

grafted phosphonium salt IL. Additionally, the EDS mapping of Cr, N, P and Br atoms in Cr-MIL-101-[BuPh3P]Br (Fig. 3e–h) also proves the presence and homogeneous distribution of the phosphonium salt IL on the MOF surface. Those results also indicate the successful functionalization of quaternary phosphonium salt. According to the P element content of EDS result, the IL loading amount on MOF surface is estimated to be 0.43 mmol/g. Moreover, the content of phosphonium salt IL in the framework was also characterized by ICP-OES. It was found that the IL amount in MOF bulk is estimated to be 0.16 mmol/g, which

morphology with average size ca. 50 nm, not the typical octahedral geometry. Similar result was also reported by other works [35]. Compared with parent Cr-MIL-101-NH2, the Cr-MIL-101-[BuPh3P]Br exhibits similar morphology and particle size (Fig. 3b), suggesting the post-synthetic modification did not fundamentally affect the morphology and structure. EDS of Cr-MIL-101-NH2 (Fig. 3c) shows the presence of C, N, O and Cr from the ligand and metal center. It is noted that new characteristic peaks of elements P and Br are observed in the spectrum of Cr-MIL-101-[BuPh3P]Br (Fig. 3d), which assigns to the 298

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Fig. 4. (a) XPS survey spectra of Cr-MIL-101-NH2 and Cr-MIL-101-[BuPh3P]Br; XPS spectra of (b) N 1s in Cr-MIL-101-NH2 and Cr-MIL-101-[BuPh3P]Br; (c) P 2p and (d) Br 3d in [BrBuPh3P]Br and Cr-MIL-101-[BuPh3P]Br.

The shift of P 2p and Br 3d peaks possibly due to the interaction of exposed secondary amino groups and Cr atom with electron-withdrawing P atom and electron-rich Br- anion, respectively. Those results further demonstrate the [BuPh3P]Br moiety successfully grafted onto the Cr-MIL-101 frameworks, which are well agreement with the structures of the as-synthesized MOF materials as shown in Scheme 1. The N2 physisorption measurements were used to investigate the pore structure and specific surface area of the as-synthesized MOF materials. As shown in Fig. 5a, the N2 adsorption-desorption isotherms of all samples feature the type-I isotherm, which is similar to that reported previously [45]. However, compared with pure Cr-MIL-101NH2, the IL functionalized catalyst (Cr-MIL-101-[BuPh3P]Br) shows a significant decrease in the adsorption amount of N2. Furthermore, it was found that the mesoporous size of Cr-MIL-101-NH2 and Cr-MIL101-[BuPh3P]Br (Fig. 5b) is decreased from 2.40 to 2.24 nm. In addition, the pure Cr-MIL-101-NH2 shows a BET surface area and pore volume of 1429 m2 g−1 and 1.32 cm3 g−1, whereas Cr-MIL-101-[BuPh3P] Br has corresponding values of 925 m2 g−1 and 0.83 cm3 g−1. It is easy to see that the surface area, pore size, and pore volume declined after loading of phosphonium salt species. This is because that the pores of the carrier were partially occupied by [BuPh3P]Br moiety after modification. Those results further demonstrate that the phosphonium salt IL was successfully grafted onto Cr-MIL-101 through the reaction between –NH2 and bromobutyl groups. It is noteworthy that the as-synthesized Cr-MIL-101-[BuPh3P]Br still remains open cavities and high surface area, which is of benefit in the diffusion of reactants and products during the catalytic reaction process. Moreover, the thermal property of Cr-MIL-101-NH2 and Cr-MIL101-[BuPh3P]Br was examined by TGA analysis. As depicted in Fig. 6, there is no significant difference in the thermal stability before and after functionalization of Cr-MIL-101-NH2. It is clearly that both MOF samples are thermally stable up to 170 °C, which is enough for its use as a heterogeneous catalyst for the cycloaddition reaction in this study.

is lower than that on MOF surface. This is because most of the interaction between NH2 groups of MOF and IL are happening on the MOF surface, due to the large molecular structure of phosphonium salt and/ or small pore size of Cr-MIL-101-NH2. Actually, this phenomenon is benefit for the interaction between the active sites and substrates, thus promoting the cycloaddition reaction. The chemical composition and states of the samples were further analyzed by XPS. In Fig. 4a, the XPS survey profile of Cr-MIL-101[BuPh3P]Br shows new characteristic peaks of P and Br elements in contrast to Cr-MIL-101-NH2, which is consistent with the EDS results. Fig. 4b shows the high-resolution spectra of N 1s of abovementioned MOF samples. In the spectrum of Cr-MIL-101-NH2, the N 1s peak can be fitted into two peaks at 399.6 and 401.2 eV, which are attributed to N species in the NH2 group and the protonated amines in the form of ammonium salts, respectively [40,41]. As for the Cr-MIL-101-[BuPh3P] Br, a peak at 400.3 eV assigned to the N species of secondary amine newly appears [42,43], while the peak attributed to NH2 group shows a relative decrease in the intensity compared with Cr-MIL-101-NH2. This result is mainly ascribe to the substitution reaction between the NH2 groups of Cr-MIL-101-NH2 and butyl-bromine of [BrBuPh3P]Br in the post-synthetic modification process. In addition, the high-resolution P 2p (Fig. 4c) and Br 3d (Fig. 4d) spectra of both [BrBuPh3P]Br and CrMIL-101-[BuPh3P]Br were also recorded. In the P 2p spectra of [BrBuPh3P]Br, two peaks at binding energy of 132.5 and 133.3 eV were observed, which are ascribed to P 2p3/2 and 2p1/2 of quaternary phosphonium salt, respectively [44]. Nevertheless, corresponding peaks of Cr-MIL-101-[BuPh3P]Br shift to higher binding energies of 133.3 and 134.1 eV. Similar result was also observed in the spectra of Br 3d. For [BrBuPh3P]Br, the peaks of Br 3d appear at 69.7 and 70.7 eV, ascribing to Br 3d5/2 and 3d3/2 of covalently bonding bromine of the bromobutyl group. Whereas, the other two peaks located at 66.9 and 67.9 eV belongs to Br 3d5/2 and 3d3/2 of ionic bromine of Br- counter anions. However, the Br 3d spectrum of Cr-MIL-101-[BuPh3P]Br only shows one peak which can be fitted into two peaks at 68.0 and 69.0 eV, assigning to Br 3d5/2 and 3d3/2 of ionic bromine, respectively. The peaks of ionic bromine have 1.1 eV of upshift compared with [BrBuPh3P]Br. 299

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Fig. 5. (a) N2 adsorption-desorption isotherms, and (b) pore size distribution profiles from BJH method of Cr-MIL-101-NH2 and Cr-MIL-101-[BuPh3P]Br.

due to the intrinsic thermodynamic stability of CO2. While using the sole phosphonium salt ([BuPh3P]Br) and MOF (Cr-MIL-101-NH2) as catalysts, although remarkable yields (24.3% and 42.8%, respectively) of propylene carbonate (PC) can be obtained (entries 2 and 3, Table 1), it is far from the requirement for the application. To our delight, the phosphonium salt-functionalized MOF material, Cr-MIL-101-[BuPh3P] Br, exhibited much superior activity than its ingredients (entries 4 vs 2 and 3, Table 1), which is attributed to synergistic effects of bifunctional active sites Cr3+ and Br-. While using Cr-MIL-101-[BuPh3P]Br as catalyst, a high efficiency with PC yield of 97.8%, and TOF value of 1086.7 h−1 could be achieved (Table 1, entry 4). Furthermore, a good isolated yield of PC (91.2%) also can be obtained, which is important for the industrial application. Additionally, it is also observed that the simple physical mixture of [BuPh3P]Br and Cr-MIL-101-NH2 can also give good PC yield and high TOF value (entry 5, Table 1), but the soluble [BuPh3P]Br is difficult to separate and recycle in the reaction system, which is the main shortcoming of the physical mixture compared to Cr-MIL-101-[BuPh3P]Br. It is notable that the physical mixture of Cr-MIL-101-NH2, Ph3P and BuBr shows similar activity with pristine Cr-MIL-101-NH2 (entry 6 vs 3, Table 1), but much lower than that of CrMIL-101-[BuPh3P]Br and Cr-MIL-101-NH2/[BuPh3P]Br (entries 6 vs 4 and 5, Table 1). This illustrates the important role of the ionic Br- for the enhancement of reaction conversion. In our previously work, we prepared a chitosan-grafted [BuPh3P]Br catalyst (CS-[BuPh3P]Br), which showed good catalytic activity for the cycloaddition reaction [46]. To make a direct comparison, we evaluated CS-[BuPh3P]Br under the conditions adopted in this work. It displays PC yield of 5.7% and TOF of 63.3 h−1, much lower than that obtained over Cr-MIL-101[BuPh3P]Br. This may be attributed to the stronger Lewis acidity and higher specific area of Cr-MIL-101 than that of chitosan, respectively, which could be in favor of the adsorption and activation of epoxides. Those results also demonstrate that the Cr3+ as Lewis acid sites is also important for the excellent activity of Cr-MIL-101-[BuPh3P]Br. Moreover, it is noted that almost 100% of PC selectivity can be achieved during the cycloaddition reaction. There was only a minute amount of 1,2-propanediol generated due to PO hydrolysis detected as by-product in the cycloaddition reaction, when the selectivity was below 100%. To further identify the potential of the as-fabricated Cr-MIL-101[BuPh3P]Br for the synthesis of cyclic carbonates, we compared its catalytic efficiency with the earlier reported IL-supported Cr-MIL-101 or other typical MOF materials in the CO2-PO cycloaddition reaction. The comparative data are summarized and listed in Table 2. Because turn over frequency (TOF) is the most efficient and valuable index to illustrate the performance of a catalyst, the TOFs of the reported ILsupported MOFs are compared with that of the as-fabricated Cr-MIL101-[BuPh3P]Br. Additionally, considering that the nucleophilic halide anion is more efficient than the metal center in promoting the reaction, the supported IL always regarded as active component of the catalyst

Fig. 6. TGA curves of (a) Cr-MIL-101-NH2, and (b) Cr-MIL-101-[BuPh3P]Br.

3.2. Catalytic performance For the cycloaddition of epoxides and CO2 to produce cyclic carbonates, propylene oxide (PO) was selected as the model substrate to evaluate the activity of the catalysts. All the reactions were carried out at 120 °C and 2.0 MPa for 2 hours by using 0.045 mol% of catalyst, and the results are summarized in Table 1. Obviously, there is no cycloaddition reaction took place in the absence of catalyst (entry 1, Table 1),

Table 1 Catalytic performance of various catalystsa. Entry

Catalyst

1 2 3 4

Blank [BuPh3P]Br Cr-MIL-101-NH2c Cr-MIL-101-[BuPh3P]Br

5 6 7

Cr-MIL-101-NH2/[BuPh3P]Bre Cr-MIL-101-NH2/Ph3P/BuBrf CS-[BuPh3P]Brg

Catalytic results Yield (%)

Sel. (%)

TOFb

— 24.3 42.8 97.8 (91.2d) 92.5 47.3 5.7

— 100 99.5 99.7

— 270.0 — 1086.7

100 100 98.4

1027.8 525.6 63.3

a Reaction conditions: PO 35.7 mmol, catalyst 0.045 mol% (refer to phosphonium salt), CO2 pressure 2.0 MPa, temperature 120 °C, time 2 h. b TOF (Turn over frequency): yield/(IL amount in mol%)/time. c Cr-MIL-101-NH2 0.10 g. d Isolated yield. e Cr-MIL-101-NH2 0.092 g, [BuPh3P]Br 0.016 mmol. f Cr-MIL-101-NH2 0.092 g, Ph3P 0.016 mmol, BuBr 0.016 mmol. g CS-[BuPh3P]Br was synthesized acording to the literature [46].

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Table 2 Comparison of the catalytic activities of the Cr-MIL-101-[BuPh3P]Br catalysts with previously reported MOF catalyst system. Entry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 a b c d e f g h i j

Catalyst

Cr-MIL-101/TBAB IL/MIL-101-NH2c MIL-101-IMBrd IL@MIL-101-SO3He MIL-101(Cr)-TSILf MIL-101-tzmOH-Brg MIL-101-N(n-Bu)3Br (I-)Meim-UiO-66 IL-ZIF-90h FJI-C10 Cr-MIL-101-[BuPh3P]Bri Cr-MIL-101-[BuPh3P]Bri Cr-MIL-101-[BuPh3P]Bri Cr-MIL-101-[BuPh3P]Bri Cr-MIL-101-[BuPh3P]Bri Cr-MIL-101-[BuPh3P]Brj Cr-MIL-101-[BuPh3P]Brj

Epoxidea

PO PO PO ECH SO PO PO ECH PO ECH PO SO ECH PO PO PO PO

Reaction conditions

Catalytic results

Ref. b

T (oC)

P (MPa)

t (h)

Yield (%)

Sel. (%)

TOF (h−1)

25 120 80 90 110 80 80 120 120 80 120 120 120 120 120 50 120

0.8 1.3 0.8 0.1 2.0 1.0 2.0 0.1 1.0 0.1 2.0 2.0 2.0 1.0 0.1 2.0 0.1

24 1 4 24 6 10 8 24 3 12 2 2 2 2 24 12 24

82.0 91.0 93.4 98 93.1 93.0 99.1 93.0 95.0 87.0 97.8 68.5 98.3 87.5 36.3 84.1 89.0

90.0 — 95.8 — 98.0 — — 93.0 98.0 — 99.7 100 99.3 99.4 97.9 98.2 98.9

2.0 209.2 28.3 9.5 235.1 37.0 13.8 5.2 211.1 21.0 1086.7 761.1 1092.2 972.2 33.6 51.9 27.5

[30] [48] [32] [50] [49] [51] [33] [52] [53] [54] This work This work This work This work This work This work This work

Epoxides: ECH (epichlorohydrin), PO (propylene epoxide), SO (styrene oxide). TOF (Turn over frequency): yield/(IL amount in mol%)/time. IL: 1-carboxyethyl-3-methylimidazolium chloride. IMBr: 1-methylene-3-hexylimidazolium bromide. IL: 1-methylene-3-ethylimidazolium bromide. TSIL: 1-methylene-3-(4-carboxylbenzyl)imidazolium bromide. tzmOH-Br: hydroxyl-functionalized 1,2,3-triazolium bromide. IL:1-aminopyridinium iodide. Catalyst loading amount: 0.045 mol% (refer to phosphonium salt). Catalyst loading amount: 0.135 mol% (refer to phosphonium salt).

(entry 15, Table 2). Similar results were also reported by Diaz Diaz and colleagues [55]. However, while increasing the catalyst loading content to 0.135 mol%, good PC yields of 84.1% and 89.0% can be obtained at low temperature (50 °C, 2.0 MPa, 12 h; entry 16, Table 2) and low pressure (0.1 MPa, 120 °C, 24 h; entry 17, Table 2), respectively, which demonstrates that Cr-MIL-101-[BuPh3P]Br also exhibits good catalytic activity under mild condition compared to previously reported catalysts. From the comparative results, it is noteworthy that although the catalyst systems, including Cr-MIL-101-[BuPh3P]Br, could be operated at low temperature and/or low pressure, large catalyst loading, and/or cocatalyst, and/or long reaction time are needed for the satisfactory yield of cyclic carbonate, which is not beneficial for the industrial production of cyclic carbonates. On the whole, Cr-MIL-101-[BuPh3P]Br would be a valuable heterogeneous catalyst for the CO2 cycloaddition reaction.

[47]. Hence, the TOF values are calculated upon the molar ratio of epoxide to the amount of IL moiety. Zalomaeva et al. [30] first reported the use of physical mixture of Cr-MIL-101 and TBAB as catalyst system for the cycloaddition reaction. Although the cyclic carbonates can be synthesized under mild reaction conditions (25 °C and 0.8 MPa), a long reaction time (24 h) and extra separation process of TBAB were needed (entry 1, Table 2). After that, several imidazolium-based ILs supported Cr-MIL-101 catalysts were synthesized by different post-synthetic approaches (entries 2–5, Table 2). It can be seen that the catalyst IL/MIL101-NH2 [48] and MIL-101(Cr)-TSIL [49] showed much lower TOF values compared with Cr-MIL-101-[BuPh3P]Br (entries 2 and 5 vs 11 and 12, Table 2) under the similar reaction conditions. Over the catalyst MIL-101-IMBr [32] and IL@MIL-101-SO3H [50], satisfactory yield of cyclic carbonate can be obtained under relative mild reaction temperature (80 and 90 °C) and low pressure (0.8 and 0.1 MPa), but it showed much lower TOF values (entries 3 and 4, Table 2). Furthermore, the quaternary ammonium and triazolium salt-based IL were also used as the active components to promote the activity of Cr-MIL-101. However, the resulted catalyst systems, MIL-101-tzmOH-Br and MIL101-N(n-Bu)3Br, still needed long reaction time and large catalyst loading to complete the cycloaddition reaction (entries 6 and 7, Table 2). Except Cr-MIL-101, other MOF materials were also widely used as support to graft ILs and thus evaluated as catalyst for the cycloaddition reaction. Hence, several imidazolium-based IL functionalized Uio-66, ZIF-90 and FJI-C10 are also compared. Under the same reaction pressure and temperature, Cr-MIL-101-[BuPh3P]Br showed much higher catalytic activity than IL-ZIF-90 (entry 9 vs 14, Table 2). Like IL@MIL-101-SO3H (entry 4, Table 2), (I-)Meim-UiO-66 and FJIC10 also needed long reaction time to convert epichlorohydrin (ECH) to corresponding cyclic carbonate (entries 8 and 10 vs 13, Table 2). Actually, to make direct comparison, the catalytic activity of Cr-MIL-101[BuPh3P]Br was also performed under low pressure or temperature. Disappointedly, only 36.3% of PC yield was obtained over Cr-MIL-101[BuPh3P]Br under 0.1 MPa, even prolonging the reaction time to 24 h

3.3. Reaction parameter optimization After witnessing the high catalytic activity of Cr-MIL-101-[BuPh3P] Br for the cycloaddition reaction, the effect of reaction parameters (catalyst loading amount, time, temperature and CO2 pressure) on the PC synthesis was studied to optimize these reaction variables. The impact of the catalyst loading amount is shown in Fig. 7a. It was found that the PC yield sharply increased from 62.6 to 97.8% when the catalyst concentration increasing from 0.023 to 0.045 mol%, attributing to the increase of active sites introduced into the reaction system. Thereafter, further increasing the catalyst loading amount to 0.068 mol % slightly promoted the PC yield. Additionally, the selectivity of PC is independent of the catalyst loading amount, which always keeps at almost 100%. The influence of reaction temperature on the yield and selectivity of PC is shown in Fig. 7b. It can be observed that the temperature has a greatly positive effect on the synthesis of PC generally. This is because the CO2-PO cycloaddition with high innate activation energy needs 301

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Fig. 7. Effect of different reaction parameters on the yield and selectivity of PC. (Reaction conditions: PO 35.7 mmol, (a) temperature 120 °C, CO2 pressure 2.0 MPa, time 2 h; (b) catalyst 0.045 mol%, CO2 pressure 2.0 MPa, time 2 h; (c) catalyst 0.045 mol%, temperature 120 °C, time 2 h; (d) catalyst 0.045 mol%, temperature 120 °C, CO2 pressure 2.0 MPa.)

reusability and stability of Cr-MIL-101-[BuPh3P]Br are very important parameters to determine if it is a real ‘heterogeneous’ catalyst. To test the stability of Cr-MIL-101-[BuPh3P]Br, a hot filtration experiment was performed firstly. The catalyst was removed after the reaction proceeded for 1.5 h, and the filtrate was further heated under the adopted reaction conditions. The PC yield was monitored at given time intervals and the results are shown in Fig. 7d (dashed curve). It was found that there is only slight increase of PC yield (from 85.0% to 87.9%) was produced with further increasing the reaction time from 1.5 to 2.5 h, which is consistent with the result of recycling experiments. Additionally, the leaching of phosphonium salt was investigated by examining the P content in the filtrate by ICP-OES. It was found that there is only 1.8% of phosphonium salt was leached from the solid catalyst into the liquid product, indicating good stability of Cr-MIL-101[BuPh3P]Br. The reusability was further examined through the recycle experiments under the optimized conditions. After each cycle, the catalyst was recovered by simple filtration and subject to washing and drying. As depicted in Fig. 8, the Cr-MIL-101-[BuPh3P]Br could be used

thermal energy input and suitable active species to overcome the energy barrier, and form the desired cyclic carbonates [56]. The PC yield remarkably increases from 49.8 to 97.8% with the temperature rising from 90 to 120 °C. A further increase in temperature up to 130 °C showed slightly increase in the yield but decrease in the selectivity of PC. At higher temperature (140 °C), both the yield and selectivity of PC distinctly decreased. The reason for this phenomenon could be ascribed to the acceleration of side reactions of PO such as hydrolysis, isomerization and/or thermal polymerization at high temperature (> 130 °C) [57–60]. Hence, the suitable temperature for the cycloaddition reaction using Cr-MIL-101-[BuPh3P]Br as catalyst is 120 °C. The dependence of PC synthesis on the CO2 pressure is also investigated. As depicted in Fig. 7c, the selectivity of PC is always above 99.4% and can be regarded as being independent of CO2 pressure. Nevertheless, the variation of CO2 pressure has a significant effect on the reaction conversion. In the lower pressure region (0.5 MPa to 2.0 MPa), an increase in CO2 pressure led to a rapid increase in PO conversion, whereas higher pressure (2.0–3.0 MPa) made it moderately decreased. This reaction system has gas-liquid-solid three phase and the cycloaddition reaction primarily occurs through the interaction between substrates and active sites in the liquid phase. Hence the increased pressure can enhance the CO2 concentration and CO2-epoxidecatalyst interaction in liquid phase, thus resulting the increase of PC yield. However, higher pressure could extract more PO into the gas phase, leading to the reduction of PO concentration in the vicinity of catalyst in the liquid phase, which causes a decrease of PC yield. Similar trend of PC yield dependents on CO2 pressure was also observed in other studies [61,62]. In addition, the influence of reaction time on the PC yield and selectivity was investigated in Fig. 7d. It can be seen that the PC yield quickly increased from 64.6 to 97.8% when the reaction time increased from 0.5 to 2.0 h. Further prolonging the time to 2.5 h has no obvious promotion for the PC yield. Furthermore, the PC selectivity reaches almost 100 % when the reaction proceeds after a period of 1.5 h. 3.4. Reusability study

Fig. 8. Recycle test of Cr-MIL-101-[BuPh3P]Br and Cr-MIL-101-NH2. Reaction conditions: PO 35.7 mmol, catalyst 0.045 mol%, CO2 pressure 2.0 MPa, temperature 120 °C, time 2 h.

The purpose of this study is to fabricate an active and heterogeneous catalyst for the cycloaddition of CO2 and epoxides. Hence the 302

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Fig. 9. (a) XRD patterns and (b) FT-IR spectra of fresh and recovered Cr-MIL-101-[BuPh3P]Br. Table 3 Synthesis of various cyclic carbonates over Cr-MIL-101-[BuPh3P]Bra. Entry

Epoxide

Cyclic carbonate

Time (h)

Catalytic results Yield (%)

Sel. (%)

TOF (h−1)

1

2

97.8

99.7

1086.7

2

2

98.3

99.3

1092.2

3

2

98.5

99.6

1094.4

4

2

87.3

99.7

970.0

5

2

78.9

99.7

876.7

6

2

68.5

100

761.1

7

24

70.5

98.7

65.3

a

Reaction conditions: PO 35.7 mmol, catalyst 0.045 mol% (refer to phosphonium salt), CO2 pressure 2.0 MPa, Temperature 120 °C, Time 2 h.

at least five consecutive runs with only a minor decrease in the yield of PC. It is noted that there is no further decrease in the PC yield after three runs, indicating the good reusability of Cr-MIL-101-[BuPh3P]Br. Furthermore, the leaching of phosphonium salt in the filtrate after five runs was also investigated by ICP-OES. This time the P element was not detectable in the leached liquid product. In contrast, the parent Cr-MIL101-NH2 showed poor reusability. After three runs, it lost 85.7% of catalytic activity. This phenomenon demonstrates that the introduction of IL could also stabilize the structure of Cr-MIL-101-NH2 framework, besides promote the catalytic activity. To further identify the stability of Cr-MIL-101-[BuPh3P]Br, the comparative XRD and FT-IR analysis of the fresh catalyst and the recovered one after last run was collected (Fig. 9). It was observed that there is no significant difference between the fresh and recovered catalyst, illustrating that the basic lattice structure and phosphonium salt as active species were well-preserved. Those results indicate that the Cr-MIL-101-[BuPh3P]Br shows good stability and reusability, which demonstrates its high potential for the industrial application.

3.5. Catalytic activity towards different epoxides Due to the outstanding catalytic performance of Cr-MIL-101[BuPh3P]Br for the cycloaddition reaction of CO2 with PO, we further investigated the CO2 cycloaddition with various epoxides to determine the versatile applicability of the catalyst. These experiments were carried out under the above optimized conditions, and the results are summarized in Table 3. Obviously, excellent conversions of a variety of mono-substituted terminal epoxides were achieved. Among them, epichlorohydrin (entry 2, Table 3) and epibromohydrin (entry 3, Table 3) showed the highest activity due to the electron-withdrawing effect of its substituent, which facilitated nucleophilic attack to open the epoxide ring [63]. Additionally, the activity decreased with the alkyl length of epoxide increasing: methyl (PO; entry 1, Table 3) > ethyl (1,2-epoxybutane; entry 4, Table 3) > butyl (1,2-hexene oxide, entry 5, Table 3) > phenyl (styrene oxide; entry 6, Table 3) > cyclohexyl (cyclohexene oxide; entry 7, Table 3). This is because the steric hindrance at the β-carbon atom of epoxide restricts the nucleophilic attack of bromine anion on the epoxy ring, causing the decrease of ringopening rate. On the other hand, the bigger hindrance of the bulky substrate makes a difficult mass transfer in the channels of the 303

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Scheme 2. Proposed mechanism for the cycloaddition of CO2 and epoxide over Cr-MIL-101-[BuPh3P]Br.

obtained while using PO as the substrate. By comparing with other previously reported IL-functionalized Cr-MIL-101 or other MOF materials, Cr-MIL-101-[BuPh3P]Br shows superior catalytic activity. The synergistic effect of Cr3+ as Lewis acid sites in MOF and Br- as nucleophile in IL could be the main reason for the promotion of cycloaddition reaction by facilitating the ring-opening of epoxides. Furthermore, the catalyst could be recovered simply and reused at least five times. By characterizing the fresh and recovered catalyst using XRD and FT-IR analysis, it was found that the catalyst also has good chemical and structural stability. In this study, we provide a new sight and approach for designing IL-functionalized MOF material, which is expected to be a potential cycloaddition catalyst for the industrial application.

framework, which also inhibits the reaction proceeding. Especially for cyclohexene oxide, a much longer reaction time was needed to complete the conversion due to the highest hindrance originated from the two rings (entry 7, Table 3). 3.6. Possible reaction mechanism According to the above results and previous reports [33,51], a tentative reaction mechanism was proposed for the cycloaddition of CO2 to PO over the catalyst Cr-MIL-101-[BuPh3P]Br. As shown in Scheme 2, the PO molecules approach into the catalyst pores and binding with Cr3+ as Lewis acid sites. The epoxide is adsorbed and polarized by the coordination between Cr3+ sites and O atom of epoxide, which makes a role of activating the epoxy ring. Simultaneously, the Br- anion generated from IL makes a nucleophilic attack on the less sterically hindered β-carbon atom of epoxide, promoting the epoxy ring opening and affords an intermediate Cr-coordinated alkoxide. This follows with the insertion of CO2 into the ring-opening intermediate by the interaction of CO2 with the oxygen anion of the opened epoxy ring. Finally, the corresponding cyclic carbonate is formed and the catalyst regenerates by the simultaneous intramolecular ring-closure and release of Br-. Moreover, the DFT studies on the mechanism of the cycloaddition reaction show that the ring-opening of epoxide is the ratedetermining step [64–66]. Accordingly, the synergistic effect of Cr3+ as Lewis acid site and Br- as nucleophilic center is the main reason for the high catalytic activity.

Declaration of Competing Interest The authors declared that they have no conflicts of interest to this work. We declare that we do not have any commercial or associative interest that represents a conflict of interest in connection with the work submitted. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 51572119 and 51662031), and Distinguished Young Scientists program of Jiangxi Province (Grant Nos. 20162BCB23040). References

4. Conclusions

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In summary, we proposed a new strategy of combining MOF (CrMIL-101) with quaternary phosphonium salt to construct bifunctional heterogeneous catalyst (Cr-MIL-101-[BuPh3P]Br) for efficient cycloaddition of CO2 to epoxides. In the absence of solvent and co-catalyst, the catalyst Cr-MIL-101-[BuPh3P]Br exhibited excellent catalytic activity for the synthesis of cyclic carbonate from CO2 and various epoxides. For example, high yield (97.8%) and TOF (1086.7 h−1) of PC can be 304

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