Fabrication of hierarchically porous MIL-88-NH2(Fe): a highly efficient catalyst for the chemical fixation of CO2 under ambient pressure

Jintu Francis Kurisingal, Yadagiri Rachuri, Yunjang Gu, Youngson Choe and Dae-Won Park*
Division of Chemical and Biomolecular Engineering, Pusan National University, Busan, 46241, Korea. E-mail: dwpark@pusan.ac.kr

Received 10th September 2019 , Accepted 13th October 2019

First published on 14th October 2019

A hierarchically micro- and mesoporous MIL-88-NH2 metal organic framework was prepared through an easy template directed methodology. Hierarchically porous MIL-88-NH2 promoted CO2 fixation reactions with epoxides to yield cyclic carbonates at atmospheric pressure and low reaction temperature and under solventless conditions. Significant conversion of epoxides was achieved under atmospheric pressure, with excellent selectivity toward the desired five-membered cyclic carbonates. The porous nature and weak Lewis acidic–basic centers of the hierarchical MIL-88-NH2 played a crucial role in exploiting its synergism with halide nucleophiles and metal centers. The catalyst was totally recyclable up to five times without compromising the activity, and the extent of heterogeneity was also studied. On the basis of experimental inferences and our previous DFT studies, a reaction mechanism for the CO2-epoxide cycloaddition reaction has been proposed. Hierarchically porous MIL-88-NH2(Fe) tuned using a structure-directing agent demonstrated its catalytic efficacy for the first time in cyclic carbonate synthesis.


Carbon dioxide (CO2) is not only the main prevalent greenhouse gas that causes global warming, ocean acidification and weather fluctuation, but also a candidate having potential chemical applications as an inexhaustible, inexpensive, abundant, available and nontoxic C1 building block, which can be employed in an alternative sustainable approach in the synthesis of fine chemicals.1–6 Numerous studies have been devoted to constructing a variety of catalytic systems for the chemical fixation of CO2.7–11 Efficient conversion of CO2 with epoxides into value-added cyclic carbonates has attracted interest from researchers and industries due to its potential economic impacts.12–24 Five-membered cyclic carbonates have a wide range of applications as green solvents, electrolytes in Li ion batteries, precursors for polycarbonates, degreasing agents, resins, intermediates in the synthesis of ethylene glycol, and personal care products.25–29 Notably, it is necessary to develop more effective heterogeneous catalytic systems under low temperature conditions with a shorter reaction time in order to reduce the production costs and energy consumption. However, an easily synthesizable potent heterogeneous catalyst containing both acidic and basic sites to materialize a CO2-epoxide cycloaddition reaction under ambient pressure still remains rarely addressed.

Metal–organic frameworks (MOFs) are a relatively well-known class of versatile porous materials that combine the advantageous properties of both organic and inorganic branches.30–33 MOFs are constructed by joining metal nodes with multifunctional ligand units and have attracted tremendous interest due to their structural and topological diversities including uniform porosity, ultrahigh specific surface area, creation of nano-sized spaces and both chemical and structural tunability, all of which arise from their structural features.34–40 Because of these unique properties, MOFs have the potential to enable disruptive technologies as reported in a large body of research across several disciplines.41–47 By modifying the building blocks of a MOF, i.e., varying the metal nodes or ligand units, it can be tuned for a specific application. For instance, improvement in the catalytic efficiency mainly requires high pore volumes and large surface areas.

To date, porous MOFs still have a negative impact on the microporous regime (pore diameter smaller than 2 nm) due to the mass transfer limitation, which may limit catalytic and separation performances.48 For example, MOFs have abundant acidic and basic sites, but most of them are largely restricted because of narrow pores which do not allow the anchoring of bulky reactant molecules. It is well known that mesopores are beneficial for catalytic reactions because of the elimination of internal diffusion issues.49–51 Therefore, the augmentation of pore size to over 2 nm would offer several advantages in catalysis, but has been challenging to date. Thus far, specially designed elongated organic linkers have been mainly used for creating mesoporous MOFs, but only a few such successful examples are found in the literature. The danger is that elongated linkers often result in the collapse of the framework upon the removal of guest molecules or the framework itself may cause the formation of interpenetrated structures or undesired topologies, which restricts the pore size and porosity.52–54

Hierarchically micro–mesoporous systems have gained much attention owing to their high pore volumes and large surface areas.55–58 The mesopores allow improved mass transfer through the framework, whereas the micropores provide high surface areas. Perturbation-assisted synthesis and template-directed synthesis are the commonly used approaches to develop hierarchically porous materials.59–61 The template-directed method utilizes surfactants, block copolymers or metal–organic assemblies as sacrificial templates, which can create large cavities inside the MOF frameworks once the templates are removed. In this work, we have followed the template-directed method, using cetyltrimethylammonium bromide (CTAB) as a structure-directing agent (surfactant) and Fe3+ and 2-aminoterephthalate ions as framework-building blocks to prepare hierarchically porous MIL-88-NH2(Fe) MOFs [HP-MIL-88-NH2(Fe)]. The synthesized HP-MIL-88-NH2(Fe) possessed a higher surface area and a higher pore diameter as compared to the parent MIL-88-NH2. Furthermore, the synthesized HP-MIL-88-NH2(Fe) was evaluated in the cycloaddition of CO2 and epoxide in order to study its catalytic properties, including its efficiency and reusability (Scheme 1). To the best of our knowledge, this work reports the synthesis of hierarchically porous MOFs using the template directed method and demonstrates their catalytic potential for the first time in the field of cyclic carbonate synthesis.

image file: c9qi01163c-s1.tif
Scheme 1 Template directed synthesis of HP-MIL-88-NH2(Fe) and cycloaddition of CO2 with epoxide forming cyclic carbonates.

Results and discussion

The solvothermal treatment of ferric chloride hexahydrate and amine functionalized terephthalic acid (NH2-BDC) in a solution of 30 mL DMF at 120 °C for 24 h resulted in microporous MIL-88-NH2(Fe) nanooctahedra.62 As expected, HP-MIL-88-NH2(Fe), with the same molecular formula, was obtained by the reaction of ferric chloride hexahydrate and amine functionalized terephthalic acid in the presence of cetyltrimethylammonium bromide micelles under similar reaction conditions (Table S1). Firstly, the morphology and particle size of both the parent and CTAB treated catalysts were verified using the FE-SEM image (Fig. 1). The parent MIL-88-NH2(Fe) has nanooctahedral crystals with a uniform size of ∼500 nm, whereas the CTAB treated MIL-88-NH2(Fe) has microsized octahedra with a uniform size of ∼2 μm. Due to the CTAB surfactant effect, an increase in the particle size has been observed and once the encapsulated templates from the framework are removed, large cavities may be formed inside the framework. Furthermore, FE-TEM was used to verify the morphology and EDX elemental analysis was performed to map the distribution of different elements in both catalysts (Fig. 2 and Fig. S1). FE-TEM images indicated that the synthesized HP-MIL-88-NH2(Fe) and MIL-88-NH2(Fe) have uniform octahedra with smooth surfaces, while elongation of the octahedra has been observed for HP-MIL-88-NH2(Fe).
image file: c9qi01163c-f1.tif
Fig. 1 FE-SEM image of MIL-88-NH2(Fe) (a) and (b) and HP-MIL-88-NH2(Fe) (c) and (d).

image file: c9qi01163c-f2.tif
Fig. 2 FE-TEM image of HP-MIL-88-NH2(Fe) and MIL-88-NH2(Fe) (a) and (b), elemental composition data of HP-MIL-88-NH2(Fe) (c) and TEM-EDX elemental mapping of HP-MIL-88-NH2(Fe) (d).

Powder X-ray diffraction analysis was conducted to verify the phase purity and framework integrity of the synthesized catalysts. HP-MIL-88-NH2(Fe) is isostructural with the parent MIL-88-NH2(Fe); hence their powder X-ray diffraction (PXRD) patterns matched perfectly with each other (Fig. 3a). The compositional similarity of both parent and hierarchical MIL-88-NH2 was proven by Fourier-transform infrared (FT-IR) analysis (Fig. 3b). The presence of amine functionality in both catalysts can be confirmed by a peak in the range of 3450–3470 cm−1. The strong peaks observed at 1650 and 1577 cm−1 correspond to the coordinated carboxylate groups. The resemblance of the observed peaks for both catalysts confirmed that chemical integrity is preserved even after treatment with CTAB. It also verified the absence of CTAB in the framework of HP-MIL-88-NH2. Furthermore, the high thermal stability of the catalysts was evident from the thermogravimetric analysis (TGA) results (Fig. 3c). Both HP-MIL-88-NH2(Fe) and MIL-88-NH2(Fe) showed material stability to be as high as 300 °C. The thermal stability of HP-MIL-88-NH2(Fe) is much higher and expected to be suitable for cycloaddition reactions and other various applications at moderately high temperatures. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of the Fe 2p3 peak at a binding energy of around 712.4 eV for HP-MIL-88-NH2(Fe) (Fig. 3d) and 710.71 eV for MIL-88-NH2(Fe) (Fig. S2). The main N 1s peak at 399.16 eV and 396.35 eV confirmed the presence of amine functionality in the synthesized MIL-88-NH2(Fe) and HP-MIL-88-NH2(Fe) catalysts, respectively. Additionally, the C 1s spectrum can be deconvoluted into three peaks at 284.4, 285.2 and 288.4 eV, assigned to the binding energies of C[double bond, length as m-dash]C (benzene ring of the BDC ligand), C–N (amine functionality) and C[double bond, length as m-dash]O (carboxylate groups of the BDC ligand), respectively (Fig. 4a). The spectra of N 1s and O 1s can be deconvoluted into two characteristic peaks that are assigned to N–C, N–H and C[double bond, length as m-dash]O, Fe–O, respectively (Fig. 4b and c). As shown in Fig. 4d, the spectral peak of Fe 2p for HP-MIL-88-NH2(Fe) showed two peaks Fe 2p3/2 and Fe 2p1/2 at 712.4 and 725.8 eV, respectively, which indicates the presence of Fe(III).

image file: c9qi01163c-f3.tif
Fig. 3 (a) PXRD patterns, (b) FT-IR spectra, (c) TGA curves and (d) XPS spectra of both MIL-88-NH2(Fe) and HP-MIL-88-NH2(Fe) catalysts.

image file: c9qi01163c-f4.tif
Fig. 4 XPS deconvolution spectra of HP-MIL-88-NH2(Fe) (a) C 1s, (b) N 1s, (c) O 1s and (d) Fe 2p.

After careful removal of CTAB from the catalyst, we performed the N2 adsorption–desorption isotherm analysis (Fig. 5a). The presence of a hysteresis loop in the N2 sorption isotherms indicated the presence of newly formed mesopores and micropores and the well-retained structural integrity of HP-MIL-88-NH2(Fe) after the removal of the template (CTAB). In addition to the micropores with a diameter of 1 nm, the modified catalyst contained mesopores with an average pore diameter of 8.8 nm. Notably, the Brunauer–Emmett–Teller (BET) surface area measured from the N2 isotherms showed a gradual increase for HP-MIL-88-NH2(Fe) (478.8 m2 g−1) as compared to the parent MIL-88-NH2(Fe) (125.7 m2 g−1). All these results revealed that the reaction of Fe3+ and 2-aminoterephthalate ions in the presence of CTAB micelles produced the hierarchically micro- and mesoporous MOFs.

image file: c9qi01163c-f5.tif
Fig. 5 (a) N2 adsorption–desorption isotherms at 77 K, (b) NH3-TPD plot, (c) CO2-TPD plot and (d) CO2 adsorption–desorption isotherms at 298 K.

Higher numbers of acidic–basic sites, especially weak and medium sites, were observed forHP-MIL-88-NH2(Fe), which is analyzed by NH3 and CO2 temperature programed desorption (TPD) analyses (Fig. 5b and c). Even though both the catalysts showed almost the same acidic–basic strength, HP-MIL-88-NH2(Fe) showed a higher number of weak (11.88/5.95 mmol g−1) and medium (17.14/18.01 mmol g−1) acidic–basic sites than the parent MIL-88-NH2(Fe) (weak acidic/basic sites – 1.3/2.65 mmol g−1 and medium acidic/basic sites – 18.1/11.41 mmol g−1), which was vital for catalysis (Table 1). By analyzing CO2 adsorption–desorption isotherms of both catalysts as shown in Fig. 5d, an interestingly high CO2 adsorption capacity of 26.7 cm3 g−1 was observed for HP-MIL-88-NH2(Fe). Herein, the larger pore size and pore volume support the availability of a higher number of amine functionalities and a diffusion pathway for the CO2 molecules. Meanwhile, the parent MIL-88-NH2(Fe) accommodates a smaller number of amine groups which caused a reduction in the CO2 uptake (18.6 cm3 g−1).

Table 1 Details of the acidic and basic sites in MIL-88-NH2(Fe) and HP-MIL-88-NH2(Fe)
Catalyst Acidic/basic site concentration (mmol g−1 of catalyst)
Acidity/basicity at A Acidity/basicity at B Acidity/basicity at C Total acidity/basicity
A = weak acidic/basic site, B = medium acidic/basic site, and C = strong acidic/basic site.
MIL-88-NH2(Fe) 1.3/2.65 18.1/11.41 19.48/13.54 38.88/27.60
HP-MIL-88-NH2(Fe) 11.88/5.95 17.14/18.01 16.86/8.30 45.88/32.26

The catalytic potential of the hierarchically porous catalyst towards the cycloaddition reaction of various epoxides with CO2 was investigated under solvent-free conditions and at a low reaction temperature (45 °C) and atmospheric CO2 pressure (Table 2). The use of atmospheric pressure and lower reaction temperatures in the cycloaddition is of great interest, yet challenging. 15 mmol of the model substrate epichlorohydrin (ECH) did not result in any conversion in the absence of any catalyst at 0.1 MPa CO2 pressure and a reaction temperature of 45 °C for 12 h (entry 1, Table 2). For 0.5 mol% of hierarchically porous material alone, ECH conversion was low but better than that of parent MIL-88-NH2(Fe) (entries 2 and 3, Table 2). In order to promote the cycloaddition reactions, it is necessary to use tetra-n-butylammonium halide (TBAX, X = Cl, Br, I) as the co-catalyst (entries 4–6, Table 2). We used a smaller amount of co-catalyst (0.5 mol%) and as expected, the combination of HP-MIL-88-NH2(Fe) and TBAB exhibited high ECH conversion (95%) with >99% ECH carbonate selectivity (entry 7, Table 2). In contrast to the order of nucleophilicity (actual order I > Br > Cl), Br ions showed a higher activity than chloride and iodide ions (entries 7–9, Table 2). Due to its mesoporous nature, HP-MIL-88-NH2(Fe) with tetra-n-butylammonium iodide (TBAI) also exhibited high ECH conversion (73%), while the parent MIL-88-NH2(Fe) exhibited lower ECH conversion (55%) with TBAI probably due to the mass transfer limitation. The higher catalytic potential of HP-MIL-88-NH2(Fe) in comparison with that of the parent MIL-88-NH2(Fe) may be attributed to the hierarchical nature and the active Lewis acidic–basic centers of the material. The porous nature of the material facilitated easier diffusion of epoxides, CO2 and nucleophilic anions. The hierarchical nature and Lewis acidic–basic centers of HP-MIL-88-NH2(Fe) played a critical role in exploiting its synergism with halide nucleophiles.

Table 2 Catalyst screening for cycloaddition reactions of epichlorohydrin and CO2
Entry Catalyst Conversiona (%) Selectivitya (%)
Reaction conditions: Epichlorohydrin (ECH) = 15 mmol, catalyst = 0.5 mol%, TBAX = 0.5 mol%, pressure = 0.1 MPa CO2, temperature = 45 °C, time = 12 h, semi batch reaction.a Determined by GC.
1 None
2 MIL-88-NH2(Fe) 29 >99
3 HP-MIL-88-NH2(Fe) 37 >99
4 MIL-88-NH2(Fe)/TBAB 78 >99
5 MIL-88-NH2(Fe)/TBAC 69 >99
6 MIL-88-NH2(Fe)/TBAI 55 >99
7 HP-MIL-88-NH2(Fe)/TBAB 95 >99
8 HP-MIL-88-NH2(Fe)/TBAC 90 >99
9 HP-MIL-88-NH2(Fe)/TBAI 73 >99

Reaction parameters such as reaction time, reaction temperature, catalyst amount, and CO2 pressure played a crucial role in the cycloaddition reaction of epoxide and CO2. The effects of varying reaction times were examined in the range of 4–14 h with the use of 0.5 mol% HP-MIL-88-NH2(Fe). A noticeable conversion of ECH started only beyond 4 h and reached 95% by the end of 12 h, which was estimated to be the optimum time. The highest ECH conversion was achieved at a reaction temperature of 45 °C with 0.5 mol% of catalyst and at 0.1 MPa CO2 pressure for 12 h. The HP-MIL-88-NH2(Fe) system was found to be applicable for the insertion of CO2 into various epoxides providing the corresponding cyclic carbonates in nearly >99% selectivity (Scheme 2). The aliphatic epoxides (epichlorohydrin, propylene oxide and allyl glycidyl ether) exhibited excellent conversion with high selectivity to the corresponding cyclic carbonates at 0.1 MPa CO2 pressure and a reaction temperature of 45 °C for 12 h (Scheme 2a–c). The combined effects of the hierarchical porous nature and an appropriate number of Lewis acidic–basic centers may be the reason for the higher conversion of aliphatic epoxides. Because of the bulky nature of styrene oxide, it showed a comparatively lower conversion (Scheme 2d). Meanwhile, the internal epoxide cyclohexene oxide exhibited poor conversion due to the steric crowding caused by the cyclic ring (Scheme 2e).63–65

image file: c9qi01163c-s2.tif
Scheme 2 Insertion of CO2 into various epoxides, providing the corresponding cyclic carbonates. Reaction conditions: Epichlorohydrin (ECH) = 15 mmol, catalyst = 0.5 mol%, TBAB = 0.5 mol%, pressure = 0.1 MPa CO2, temperature = 45 °C, time = 12 h, semi batch reaction.

Easy separation and reusability for several cycles are crucial criteria to be considered when choosing a catalyst for industrial processes to minimize the production cost. Herein, we separated HP-MIL-88-NH2(Fe) simply from the reaction mixture by using the centrifugation method and thoroughly washed it with methanol and dried. The dried HP-MIL-88-NH2(Fe) was subjected to reusability experiments under the reaction conditions of 45 °C and 0.1 MPa CO2 pressure for 12 h. The HP-MIL-88-NH2(Fe) catalyst was recycled without any obvious loss in its catalytic activity (Fig. 6a). The recovered HP-MIL-88-NH2(Fe) was further employed for physio-chemical characterization to prove the stability of the catalyst. The PXRD analysis results of the recycled HP-MIL-88-NH2(Fe) were exactly the same as those of the fresh HP-MIL-88-NH2(Fe) (Fig. 6b), which showed the structural stability possessed by HP-MIL-88-NH2(Fe) under the employed reaction conditions. The chemical integrity of the recovered HP-MIL-88-NH2(Fe) was analyzed using FT-IR analysis, which showed that both the fresh and recovered HP-MIL-88-NH2(Fe) showed similarity in their spectra (Fig. 6c). The recovered HP-MIL-88-NH2(Fe) possessed exactly the same degradation temperature as that of the fresh catalyst, while the recovered framework showed an earlier weight loss at around 100 °C, attributed to the reactant or product molecules occluded during the reusability experiment (Fig. 6d). EA analysis was carried out with the recovered HP-MIL-88-NH2(Fe) and the same elemental composition was observed as that of the fresh catalyst. The catalytic efficiency of HP-MIL-88-NH2(Fe) in the cycloaddition reactions is compared with recently reported MOF catalysts. Table 3 summarizes the comparison data of the reported MOFs in the coupling reactions under the conditions of ambient CO2 pressure. HP-MIL-88-NH2(Fe) showed promising catalytic activity compared to other recently reported MOFs at a low reaction temperature.

image file: c9qi01163c-f6.tif
Fig. 6 (a) Reusability study of HP-MIL-88-NH2(Fe) at 45 °C and 0.1 MPa CO2 pressure for 12 h (>99% selectivity towards ECH carbonate), (b) PXRD patterns, (c) FTIR spectra, and (d) TGA curves of the reused HP-MIL-88-NH2(Fe) catalyst.
Table 3 Comparison study of the catalytic activity of HP-MIL-88-NH2(Fe) with those of recently reported catalysts for the cycloaddition reaction
No Catalysts Epoxide T (°C) PCO2 (MPa) t (h) Yield (%) Ref.
1 Ni-MOF-1a ECH 60 0.1 06 97 66
2 Zn(BPZ-NH2) ECH 120 0.1 24 41 67
3 Zn(BPZ-NO2) ECH 120 0.1 24 67
4 [{Co(TCPB)0.5(H2O)}·DMF]n ECH 80 0.1 09 99 68
5 {(Me2NH2)2·[Zn8(Ad)4(DABA)6O]·7DMF}n ECH 100 0.1 24 99 69
6 JLU-MOF58 PO 80 0.1 24 95 70
7 HP-MIL-88-NH2(Fe) ECH 45 0.1 12 95 This work
8 HP-MIL-88-NH2(Fe) PO 45 0.1 12 94 This work

A plausible reaction mechanism based on experimental observations and previous computational studies27,63 on the cycloaddition of epoxide in the presence of HP-MIL-88-NH2(Fe) co-catalyzed by TBAB is demonstrated in Scheme S1. In HP-MIL-88-NH2(Fe), the Fe Lewis acidic site interacts with the O atom of epoxide (epoxide activation). Afterwards, the Br ion of the co-catalyst attacks the least hindered carbon atom of epoxide, resulting in a negative charge on the oxygen atom of the epoxide (ring opening of the epoxide). Subsequently, the O of the epoxide ring attacks the carbon atom of the adsorbed carbon dioxide (polarization), forming a carbonate complex. The O atom of the polarized CO2 molecule attacks the β-carbon of the ring opened epoxide, forming an intermediate. Finally, ring closure takes place to generate a cyclic carbonate with the elimination of the Br ion.


In summary, the present work demonstrates the fabrication of a hierarchically micro- and mesoporous metal organic framework through a simple template directed method. The use of a structure-directing agent (cetyltrimethylammonium bromide) enables the synthesis of a porous material having both micro- and mesopores. This HP-MIL-88-NH2(Fe) shows outstanding catalytic performance in the cycloaddition of CO2 and epoxide under atmospheric pressure and at a low reaction temperature. The hierarchal nature and Lewis acidic–basic centers of MIL-88-NH2(Fe) played a critical role in exploiting its synergism with halide nucleophiles from the co-catalyst. HP-MIL-88-NH2(Fe) possessed good thermal and chemical stability, and it was employed as an excellent reusable catalyst. A comparison of the reaction conditions used by us with those of the recently reported MOF catalysts showed that the HP-MIL-88-NH2(Fe) reported here is on par with or a step ahead of the other reported MOFs. To the best of our knowledge, HP-MIL-88-NH2(Fe) tuned using a structure-directing agent (CTAB) demonstrated its catalytic efficacy for the first time in cyclic carbonate synthesis.

Conflicts of interest

There are no conflicts to declare.


This study was supported by the National Research Foundation of Korea through the Basic Research Program (2019-057644).

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Electronic supplementary information (ESI) available: Synthesis procedure of catalysts, instrumental characterization, elemental analysis and ICP-OES, TEM-EDX elemental mapping, and NMR spectra of the cyclic carbonates. See DOI: 10.1039/c9qi01163c

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