Hollow MOF capsule encapsulated amino-functionalized ionic liquid for excellent CO2 catalytic conversion

Bo-Yun Liu,Min-Jie Chen,Liang Yang,Bo Zhao,*,Tao Xia,Gang-Gang Chang,*

1 School of Power Engineering,Naval University of Engineering,Wuhan 430033,China

2 School of Chemistry,Chemical Engineering and Life Science,Wuhan University of Technology,Wuhan 430070,China

Keywords:MOFs Catalyst Hollow capsule CO2 capture Ionic liquids

ABSTRACT The engineering of highly efficient and stable heterogeneous catalysts for catalytic conversion of CO2 to high value-added products is highly desirable but presents a great challenge.Herein,we reported the synthesis of a series of multifunctional IL@H-Zn/Co-ZIF composite catalysts with a unique porous hollow capsule structure and encapsulated amino-functionalized ionic liquids (ILs).The unique hollow capsule structure of IL@H-Zn/Co-ZIF provides sufficient space for loading active ILs ([C2NH2Mim+][Br-]) and fast mass transfer of substrate molecules during catalysis.Furthermore,the microporous Zn-ZIF shell can effectively avoid the leaching of active ILs.Benefiting from the unique hollow structure,the resultant IL@H-Zn/Co-ZIF demonstrated excellent catalytic performance (＞95% yield),and good recyclability (still remained about 90% activities after 5 cycles) when applied in the CO2 cycloaddition reaction under solvent and co-catalyst free conditions.

1.Introduction

Anthropogenically discharged carbon dioxide (CO2),as a major component of greenhouse gas in the atmosphere,is triggering a series of serious problems all over the world,such as the ocean acidification,global warming and even the extinction of species[1-4].The Intergovernmental Panel on Climate Change (IPCC) has predicted that atmospheric CO2concentration could reach 950 ppm by 2100 if no necessary action was taken,which will result in unexpected consequence [5].In this regard,decreasing CO2emissions seems imperative and an immediate attention.The carbon capture and storage (CCS) technologies by using various adsorbents have been regarded as a very effective strategy to overcome these issues [6-10].However,their high regeneration temperatures and/or limited CO2adsorption capacities still hinder this technology.Alternatively,from the perspective of ‘‘green chemistry”,the direct chemical insertion of CO2(as a plentiful C1 resource) into epoxides for affording organic cyclic carbonates(OCCs)beyond CCS is considered as a promising protocol,not only due to its 100% atom-economy but also because of widespread application of OCCs in industry as degreasers,electrolytes,and so on [11-15].

To facilitate high reactivity and selectivity,various homogeneous molecule catalysts with high affinity to CO2,like ionic liquids[16-19],Schiff bases [20,21],and metal complexes [22-25] have been intensively investigated.In particular,task-specific ionic liquids have shown outstanding performance on the fixation and conversion of CO2[26-29].Despite of the high catalytic activities exhibited,these homogeneous catalysts,however,suffer from intrinsic difficulty in the separation and recovery from the solution mixture.To overcome the above issues,the encapsulation of these homogeneous catalysts in porous supports might be an effective strategy [30].However,two key points rise when fabricating the supported composite catalysts:firstly,in most cases,due to the weak interaction between the support and the guest molecule,the catalytic activities usually decrease gradually due to the significant leaching of the active sites from the support.Secondly,mostly crystalline porous supports,like zeolites and metalorganic frameworks (MOFs) are confined to the micropore regime[31-36].Their microporous structures,unfortunately,usually lead to low loading amount of guest species and cause mass transfer resistance during catalytic reactions.Thus,a robust composite catalyst with abundant pore space for high loadings of guest molecule and fast mass transfers is highly desirable for achieving excellent performance yet with big challenge.

Bearing above considerations in mind,we trust that if the guest molecules could be encapsulated in to a porous hollow capsule,in which,the hollow hole can provide enough room for loading sufficient catalytic sites,and fast transfer kinetics.Moreover,the unique hollow capsule that with micropore wall can also effectively prevent the catalytic sites from leaching,while the transport of substrate molecules was not affected.Herein,based on the above concept,we successfully fabricated a series of multifunctional IL@H-Zn/Co-ZIF composites with a unique porous hollow capsule structure,in which,the amino functionalized ILs ([C2NH2-Mim+][Br-]) acted as catalytic sites,while the hollow Zn-ZIF with microporous wall(aperture 3.4 ?)used as support for ILs.Besides,the existence of abundant Lewis acid and base sites within Zn-ZIF can also enhance the catalytic performance.As a result,the resultant catalyst IL@H-Zn/Co-ZIF showed excellent catalytic performance with almost completed conversion and yield towards the OCCs under solvent and co-catalyst free conditions.Furthermore,IL@H-Zn/Co-ZIF also exhibited good reusability outperforming the micropore IL/Zn-ZIF catalyst system.

2.Experimental

2.1.Materials

Zinc nitrate hexahydrate (Zn(NO3)2·6H2O),cobalt nitrate hexahydrate (Co(NO3)2·6H2O),propylene oxide and methanol were purchased from Sinopharm Chemical Reagent Co.,Ltd.Epichlorohydrin,1-bromo-2,3-epoxypropane,1,2-epoxybutane,styrene oxide,cyclohexene oxide and glycidyl phenyl ether was obtained from Aladdin Industrial Co.,Ltd. 1-aminoethyl-3-methylimidazolium bromide([C2NH2Mim][Br],＞98%)was obtained from Lanzhou Institute of Chemical Physics.All of the reagents were obtained from commercial suppliers and used without further purification.

2.2.Synthesis of IL@H-Zn/Co-ZIF

The synthesis of IL@H-Zn/Co-ZIF refers to the previously reported synthesis method,but is slightly different [37].In a normal procedure,Co(NO3)2·6H2O (1.092 g),Zn(NO3)2·6H2O(1.116 g),2-methylimidazole (1.232 g) and [C2NH2Mim][Br](1.000 g) were dissolved in 15 ml,15 ml,30 ml and 20 ml methanol,respectively.After forming homogeneous solution,Co(NO3)2-·6H2O in methanol solution was mixed with ligand solution(2-methylimidazole) slowly in 1 min by syringe.Then,xml of[C2NH2Mim][Br]/methanol solution (x=0.4,1,2) was introduced immediately and the mixed solution was further sonicated at 40°C for 15 min.Subsequently,Zn(NO3)2·6H2O in methanol solution was added in the above-mentioned solution and kept ultrasound for additional 30 min.The resulting suspension was transferred to 100 ml Teflon-lined stainless-steel autoclaves and then heated at 120°C for 2 h.Finally,the mixture were washed via centrifugation at 9000 r·min-1for 5 min with methanol and the obtained powders were dried overnight at 80 °C under vacuum,the simple is named as IL@H-Zn/Co-ZIF-m (where m represents the amount of ionic liquid added,m=20 mg,50 mg,100 mg).

2.3.Synthesis of IL/Zn-ZIF

Zn(NO3)2·6H2O (0.558 g),2-methyl imidazole (0.616 g) and[C2NH2Mim][Br] (1.000 g) were dissolved in 15 ml,15 ml and 20 ml methanol,respectively.After forming homogeneous solution,Zn(NO3)2·6H2O in methanol solution was mixed with ligand solution (2-methyl imidazole) slowly in 1 min under ultrasound for 15 min at room temperature.Then,1 ml of [C2NH2Mim][Br]/methanol solutionwas introduced immediately and the mixed solution was further sonicated at 40 °C for 15 min.Subsequently,Zn(NO3)2·6H2O in methanol solution was added in the abovementioned solution and kept ultrasound for additional 30 min.Finally,the mixture were washedviacentrifugation at 9000 r·min-1for 5 min with methanol and the obtained powders were dried overnight at 80 °C under vacuum.

2.4.Synthesis of Zn/Co-ZIF.

The procedure is the same as its H-Zn/Co-ZIF without solvothermal procedure.

2.5.Characterization

The powder X-ray diffraction(XRD)studies were recorded in an X-ray diffractometer(Model D8 Avance,Bruker).Sample morphology observation was performed on a field-emission scanning electron microscopy (FESEM) system (Model S-4800,Hitachi).The morphology and size of the Sample composites were performed on a transmission electron microscopy (TEM) system (JEM-2100F,JEOL).The N2adsorption isotherms measurements were recorded on a Micromeritics ASAP 3020 instrument.X-ray photoelectron spectroscopy (XPS) measurements were performed with a PHI Quantera II instrument.Fourier transform infrared (FT-IR) spectra were obtained on an AVATAR 370 Thermo Nicolet spectrophotometer with a resolution of 4 cm-1.

2.6.Catalytic tests

All the catalytic reactions were carried out in a 25 ml stainlesssteel batch reactor.In a typical batch reaction process,40 mg of catalyst and 20 mmol of the epoxide was charged into the reactor,which was sealed and purged with CO2for 8 times.Then,the reactor was pressurized to a pressure of 0.8 MPa with CO2and kept at 80°C in an oil bath for 24 h under magnetic stirring.After the completion of the reaction,the reactor was cooled to room temperature(zero degree in case of propylene oxide) and the products were identified by a gas chromatograph (Agilent HP 7890B) equipped with a capillary column (HP-5) using a flame ionized detector.

3.Results and Discussion

3.1.Preparation and characterization of IL@H-Zn/Co-ZIF

Fig.1. Schematic illustration of the synthesis of IL@H-Zn/Co-ZIF.

Fig.2. (a) PXRD patterns,(b) N2 adsorption-desorption isotherms at 77 K,(c) infrared spectra and (d) XPS spectra for N 1s of IL@H-Zn/Co-ZIF-50 and H-Zn/Co-ZIF.

To demonstrate the proof of concept design,the H-Zn/Co-ZIF was selected as the support matrix referred to reported protocol[38,39].The synthetic method for the composite IL@H-Zn/Co-ZIF is briefly illustrated in Fig.1.The Co-ZIF nanocrystal ([Co(MeIm)2]n)core was firstly synthesized,and then coated with the Zn-ZIF([Zn(MeIm)2]n)shell,during which,the amino-functionalized ionic liquid ([C2NH2Mim+][Br-]) was introduced.Because of the similar zeolite SOD topology,the delicate core/shell of Zn/Co-ZIF can be finely constructed.Secondly,the H-Zn/Co-ZIF capsule with immobilized guest ILs was successfully fabricated by selective destruction of Co-ZIF cores in core/shell Zn/Co-ZIF crystal via a solvothermal transformation process in methanol.Powder X-ray diffraction (XRD) analysis was carried out to identify the crystal structure.As shown in Fig.2a,the fabricated core-shell H-Zn/Co-ZIF and resulted IL@H-Zn/Co-ZIF composites still maintain similar zeolite SOD topology except for the observation of broadening of PXRD peaks for the characteristic peaks for Zn-ZIF,indicating the crystal structure might transform from the solid crystal to hollow shell layers.Besides,the encapsulation of ILs has no obvious influence on the crystal structure.The hierarchically porous hollow structure of IL@H-Zn/Co-ZIF was further confirmed by the nitrogen adsorption/desorption measurement.As displayed in Fig.2b,all of the H-Zn/Co-ZIF composites displayed type IV isotherms with distinct and large hysteresis loops,indicating the sufficient mesostructured voids within the Zn-ZIF layers.By contrast,the solid Zn-ZIF only exhibited a type I curve and no hysteresis loop was observed,revealing the existence of only micropore.Besides,after the introduction of ILs,the specific surface areas and pore volume compared to H-Zn/Co-ZIF were slightly reduced,but an abundant pore void was still remained,which showed the surface areas of 1102 m2·g-1,1050 m2·g-1and 922 m2·g-1,respectively for IL@H-Zn/Co-ZIF with varied loading amount of ILs.The successful encapsulation of [C2NH2Mim+][Br-] in H-Zn/Co-ZIF was verified by FT-IR analysis.As displayed in Fig.2c,the newly generated peaks of IL@H-Zn/Co-ZIF at 1670 cm-1and 3028 cm-1compared to the H-Zn/Co-ZIF could be ascribed to in-plane N-H bending vibrations and the stretching vibrations of N-H,respectively,confirming the existence of amine (-NH2) group on the IL@H-Zn/Co-ZIF.The elemental valence states of the IL@H-Zn/Co-ZIF were also measured by X-ray photoelectron spectroscopy(XPS).The little difference of the two N 1s peak positions may result from the different chemical states of N in the two specimens,where the N(Fig.2d) in IL@H-Zn/Co-ZIF-50 is similar to the pyridinic N(398.8 eV) and amine N (399.3 eV),while that in H-Zn/Co-ZIF can be assigned to pyridinic N(398.8 eV),which also demonstrated the successful doping of ILs.

Fig.3. SEM images of (a) Zn/Co-ZIF,(b) IL@H-Zn/Co-ZIF-50,(c) STEM image of IL@H-Zn/Co-ZIF-50 and (d-g) the corresponding elemental mappings.

Fig.4. (A)The catalytic performance of(a)H-Zn/Co-ZIF,(b)IL@H-Zn/Co-ZIF-20,(c)IL@H-Zn/Co-ZIF-50,(d)IL@H-Zn/Co-ZIF-100 in 60°C and(e)IL@H-Zn/Co-ZIF-50 in 80°C,and (B) recycling test of IL/Zn-ZIF-50 and IL@H-Zn/Co-ZIF-50.

Table 1 Cycloaddition of CO2 catalyzed by various catalysts.

The surface morphology and formation of hollow structure of IL@H-Zn/Co-ZIF were further demonstratedviaScanning electron microscope (SEM) and transmission electron microscope (TEM)assisted energy-dispersive X-ray (EDX).As shown in Fig.3a,a well-defined and compact rhombic dodecahedron shape with uniform size (about 500 nm) was observed for the core-shell Zn/Co-ZIF before solvothermal transformation,while after the solvothermal transformation in methanol at 120 °C for 2 h,we found that the solid interior gradually dissolved and generated a hollow structure with some open holes on the surface (Fig.3b).Note that,the solvothermal condition was critical for yielding a high-quality hollow structure,without proper control (temperature or time)the hollow structure might decompose to the discrete nanoplates (see Supplementary Material Fig.S2).The porous structure within IL@H-Zn/Co-ZIF-50 was demonstrated by the TEM images.As shown in Fig.3c,the original morphology and symmetry of Zn-ZIF shell was still preserved to hold the hollow structure,while a large amount of pore space was existed in the inner space,thus forming a unique hollow capsular structure.Besides,the uniform distribution of Co,Zn and Br elements were confirmed by the EDX results in Fig.3d-g,the overlap image of Zn and Co also obviously revealing the Zn-ZIF shell of IL@H-Zn/Co-ZIF.It should be noted that the hollow structure can provide abundant pore void for ILs to mimic homogeneous catalytic environment,and the micropore shell can prevent their leaching,while permits the transfer of the small substrate molecules.Moreover,encapsulated ILs can effectively anchor the CO2molecule in this confined pore space and work harmony with the Zn-ZIF shell to promote the catalytic process.

3.2.The catalytic performance of IL@H-Zn/Co-ZIF

The above characterizations demonstrated the successful construction of multifunctional IL@H-Zn/Co-ZIF composites.Prior to performing the cycloaddition reactions,we measured the adsorption capacity of IL@H-Zn/Co-ZIF towards CO2.Delightedly,IL@H-Zn/Co-ZIF showed much enhanced absorption capacity of CO2compared to the initial H-Zn/Co-ZIF despite of much decreased surface areas (Fig.S1),which could be attributed to the strong affinity of the amino-functionalized ILs to CO2.Further,we tried to evaluate their catalytic performance on the CO2cycloaddition reaction.Currently,most CO2cycloaddition reaction are still performed at relatively higher temperature (＞100 °C) and moderate pressure (＞1 MPa),and large amount of organic solvents and cocatalysts are required to achieve higher conversion and selectivity[40,41],although solvent free circumstance at relative mild reaction condition is urgently needed.Firstly,the optimum doping amount of ILs was investigated.Taking the cycloaddition of CO2with epichlorohydrin as an example,as shown in Fig.4A,the doping of ILs can significantly improve the catalytic activities.For example,without ILs and co-catalyst,only ＜10% conversion was obtained at the reaction conditions,while introduction of ILs(50 mg) showed a much enhanced conversion (40%),which was about four times than that of H-Zn/Co-ZIF.Besides,we found that the conversion was slightly improved with increased loadings of ILs,but no significant enhancement was achieved when the loading exceeding 50 mg,thus the IL@H-Zn/Co-ZIF-50 was selected as the optimum loadings.Moreover,when slightly increase the reaction temperature to 80°C,an excellent catalytic performance with＞95% yields could be obtained.The controlling experiments by using the neat IL and other hybrid strategies were studied(Table 1).Compared with single ILs,ZIF and other hybrid strategies,IL@H-Zn/Co-ZIF-50 exhibited the highest catalytic activity.Meanwhile,other catalysts show a lower selectivity of main product (＜99%).The main byproducts produced during the cycloaddition of CO2were the diols of the epoxide and dimers of epichlorohydrin in accordance with previously published work [42].To demonstrate the leaching resistance of active sites,the catalytic recyclability of IL@H-Zn/Co-ZIF-50 was further investigated and compared to IL/Zn-ZIF (Fig.4B).Apparently,the IL@H-Zn/Co-ZIF-50 exhibitedexcellent stability and there was no significant loss(still remained about 90%)of catalytic activity even after successive 5 cycles usage.Although the microporous Zn-ZIF shell can effectively avoid the leaching of active ILs,the leaching of active sites is inevitable after multiple cycles.Comparatively,the conversion of IL/Zn-ZIF significantly reduced,and it was only ＜10% after reused for 5 cycles.In this hollow capsule structure,the microporous Zn-ZIF shell can effectively avoid the leaching of active ILs,while the free transfer of small substrate molecule was not affected.Besides,the PXRD pattern(Fig.S3)and SEM image(Fig.S4)after usage was remained as before.These results demonstrate that IL@H-Zn/Co-ZIF composite catalysts possess outstanding catalytic performance and stability,which is promising for practical applications.

Table 2 Cycloaddition of CO2 with different epoxides over IL@H-Zn/Co-ZIF-50.

Encouraged by the superior catalytic performance of IL@H-Zn/Co-ZIF,various epoxides with different functional groups were further investigated (Table 2).Delightedly,good to excellent conversions (＞90%) could be obtained for all the epoxides,except for entry 5-7,indicating the great substrate tolerance of IL@H-Zn/Co-ZIF catalyst.The poor catalytic performance of entry 5-7,might be due to the steric hindrance of large substituents,which restricted their accessible to active sites.Finally,based on the catalytic results,a possible reaction mechanism was provided [43-46],which was shown in Fig.S6.The epoxides are first activated by the lewis acid sites on the MOFs.The less-hindered carbon atom of the activated epoxide is then attacked by the Br-generated from[C2NH2Mim+][Br-]to open the epoxy ring.Subsequently CO2interacts with the oxygen anion of the opened epoxy ring to form an alkylcarbonate anion,cyclization via an intramolecular nucleophilic attack leads to the cyclic carbonate product and regeneration of the catalyst.

4.Conclusions

In summary,a series of multifunctional IL@H-Zn/Co-ZIF composite catalysts with a unique porous hollow capsule structure were successfully fabricated.The unique hollow capsule structure of IL@H-Zn/Co-ZIF provides sufficient space for loading active ILs([C2NH2Mim+][Br-])and fast mass transfer for substrate molecules during catalysis,while the microporous Zn-ZIF shell can effectively avoid the leaching of active ILs.The resultant IL@H-Zn/Co-ZIF demonstrated excellent catalytic performance,and good recyclability when applied in the CO2cycloaddition reaction under solvent and co-catalyst free conditions.Finally,this study provides an effective strategy to heterogenize homogenous molecule catalysts to integrate respective advantages of homogeneous and heterogeneous catalysts.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by National Natural Science Foundation of China (21706199,U1662134,5181101338).This study was also supported by National Key Research and Development Program of China (2017YFC1103800).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2020.12.019.