Defective NH2-UiO-66 (Zr) effectively converting CO2 into cyclic carbonate under ambient pressure,solvent-free and co-catalyst-free conditions

Xueting Liu,Chunhui Hu,Jingjing Wu,Peng Cui,Fengyu Wei

School of Chemistry and Chemical Engineering,Hefei University of Technology,Anhui Key Laboratory of Controllable Chemical Reaction &Material Chemical Engineering,Hefei 230009,China

Keywords:CO2 conversion Metal-organic frameworks Acetic acid modulation Defect Mild conditions

ABSTRACT In this study,a series of metal–organic frameworks(MOFs)NH2-UiO-66-xHAc catalysts were synthesized by solvothermal method using acetic acid(HAc)as a modulator,and were applied to the cycloaddition of CO2 and epichlorohydrin (EPIC) under ambient pressure.Influences of the modulation by HAc on morphologies and structures of the MOFs are demonstrated via PXRD,FESEM,FTIR,N2 adsorption–desorption,XPS and 1H NMR characterizations.The results show that the MOFs containing mesoporous pores can be prepared by adjusting the concentration of HAc.By optimizing the amount of HAc added,the specific surface area of NH2-UiO-66-8HAc is as high as 879.17 m2.g-1,which is 28.3% higher than that of the original MOFs.And the evaluation of catalytic performance showed that HAc modulation enhanced the activity of NH2-UiO-66-xHAc under mild conditions.The exposure of Lewis acid sites,increased specific surface area and porosity via the modulation of HAc defective ligand can be supposed the key factors to determine the enhanced catalytic activities.In addition,considering the influence of gas concentration on the reaction,the concept of TOP (Turnover of Pressure,defined as the mass of conversions of a unit mass catalyst under unit pressure and unit time) was first proposed in this article.

1.Introduction

In the past decades,CO2is a kind of exhaust gas with the largest emission,and it has not only affected human health,but also caused a series of problems such as changes in plant and animal species,greenhouse effect and global warming [1,2].However,as a component of C1,CO2is also a kind of carbon source with rich storage,which is non-toxic,harmless,renewable,cheap,easy to obtain and environmentally friendly,providing a favorable basis for the production of high value-added chemicals,thus saving limited fossil resources [3].Therefore,carbon capture and storage(CCS)and its chemical transformation have become a hot research topic [4,5].Nevertheless,due to the inherent thermodynamic stability and kinetic inertia of CO2,it is bound to be a new challenge for the efficient conversion and resource utilization of CO2[6].

For the past few years,many catalysts and conversion reactions have been reported in ‘‘carbon dioxide chemistry”.Their high atomic economy and remarkable application prospects have successfully attracted extensive attention [7,8].In particular,the coupling reaction of CO2and epoxide to form cyclic carbonate is a high value utilization of CO2.Cyclic carbonate is a kind of chemical intermediate in the production process of pharmaceutical products,organic solvents and fine chemicals[9].At present,some homogeneous catalysts for CO2cycloaddition have been reported,such as ionic liquid[10],quaternary phosphonium and ammonium salt [11],transition metal complexes [12] and alkali metal halides[13].Whereas,homogeneous catalysts are easily dissolved in the system,and it is difficult to separate them from the reaction phase,resulting in the loss of catalysts and leading to the problem that catalysts can not be recycled[14].Therefore,it is urgent to develop efficient recyclable catalysts to absorb CO2and convert it into more valuable energy,materials and chemical products [15,16].In this regard,heterogeneous catalysts have the great advantage of acting as recyclable catalysts,because they can be recovered from the reaction system via facile filtration or precipitation,although their catalytic efficiency may be lower than that of homogeneous catalysts.

Metal organic frameworks (MOFs) are a kind of unique porous materials.Compared with traditional porous materials,its controllable porous structure,high specific surface area,large porosity and high adsorption capacity have attracted wide attention [17–22].Many studies show that,due to its structural diversity and chemical stability,MOFs have prominent performance in different application fields,such as gas storage and separation [23],chemical sensing [24],heterogeneous catalysis [25],drug delivery [26] and so on.Over the last few decades,MOFs,such as NH2-UiO-66 [27],MOF-5 [28],ZIF-90 [29],HKUST-1 [30],etc.,as heterogeneous catalysts,have been used in the coupling reaction of CO2and epoxide.However,most of these catalysts can achieve ideal catalytic effects only under the co-catalysis of organic salts or under high pressure,which is neither environmental benign nor economic.Hence,it is of great significance to design a catalyst for the cycloaddition of CO2and epoxides under the conditions of ambient pressure and co-catalyst free.

In recent years,with the development of crystal defect engineering,the use of crystal defects method to construct multilevel pores has attracted more and more attention.Crystal defect method has the advantages of simple preparation,no need to change the synthesis conditions of the original MOFs,and being easy to form defect structures in situ.It is expected to become a practical method for large-scale preparation of porous MOFs [31].For MOFs with perfect structure,metal atoms are commonly coordinately saturated,which makes MOFs lack the exposed Lewis acid active center for catalyzing cyclic addition of CO2and epoxide.Furthermore,several studies reported that frame structure can generate defects by introducing acid modulators into MOFs.It also has a great influence on its physical and chemical properties,such as gas adsorption and catalytic performance [32,33].Therefore,the preparation of MOFs with mesopores and abundant active sites is of great significance for the design of MOFs-based catalysts with ultra-high adsorption capacity.Compared with other common MOFs,NH2-UiO-66 has high chemical,structural and thermal stability,so it has caused great concern.In addition,it was reported that NH2-UiO-66 can be modified to form defective structures[34].

In this article,acetic acid (HAc) is selected as the modulator to synthesize NH2-UiO-66-xHAc by solvothermal method.The x is the volume (ml) of HAc added in the synthesis process,in which the structural defects can be effectively adjusted,and the pore size of the material can be facilely modified by changing the concentration of HAc.Compared with the initial NH2-UiO-66,the MOFs with acetic acid as the crystal regulator produced local coordination unsaturated during the crystallization process.The partial loss of organic linkers made the difference in the pore structure between the skeleton structure and the perfectly coordinated crystal.The obtained micro-meso porous NH2-UiO-66-8HAc not only maintains the original crystal structure,but also exhibits excellent adsorption and catalytic capacity.Finally,based on the proposed performance evaluation level of TOP,it has excellent catalytic performance for the cycloaddition of epichlorohydrin(EPIC)and CO2under ambient pressure,solvent-free and co-catalyst-free conditions.

2.Experimental

2.1.Materials

Zirconium chloride (ZrCl4),N,N-dimethyl formamide (DMF),Acetic acid (HAc),Ethanol (EA),Epichlorohydrin (EPIC),Chloroform-d (CDCl3),Hydrochloric acid (HCl) and 2-aminoterephthalic acid(H2BDC–NH2)are of analytical grade without further purification.All reagents used in this study were supplied by Aladdin reagent (Shanghai) Co.,Ltd.

2.2.Synthesis of acetic acid-modified NH2-UiO-66

The synthesis of defect-free NH2-UiO-66 is according to the traditional solvothermal method [35].0.466 g of ZrCl4(2 mmol) and 0.363 g of H2BDC-NH2(2 mmol) were fully dissolved in 50 ml DMF.After 30 min of ultrasound,the mixed solution was transferred to an autoclave.Subsequently,crystallization was finished at constant temperature of 150 °C for 22 h,then the mixture was naturally cooled to room temperature and filtered to obtain yellow solid.DMF was used to soak and dissolve the unreacted metal salt or ligand crystal,and then the resulted solid was washed and filtered three times with ethanol solution.The collected product was finally dried in a vacuum dryer at 150 °C for 12 h for further tests and characterization.

The preparation process of acetic acid-modified NH2-UiO-66 is similar to that of aformentioned defect-free NH2-UiO-66,except that HAc was added.The amount of HAc added was denoted as x(x=0 ml,2 ml,4 ml,6 ml,8 ml,10 ml and 12 ml).Finally,the resulting modified sample was labeled as NH2-UiO-66-xHAc(Fig.1).

Fig.1.Catalyst preparation and cycloaddition reaction process.

2.3.General procedure for the cycloaddition of CO2 and epichlorohydrin

In a typical reaction,NH2-UiO-66-xHAc (0.1 mmol) and EPIC(20 mmol) were added into 100 mL reaction flask.Then,it was transferred to a constant temperature oil bath connected with a condensing device.Before the reaction,CO2gas with the purity of 99.9%(vol)was poured into the reaction system and maintained for 20 min to remove the air in the device,and then the reaction was carried out at ambient pressure and 80 °C for 10 h (Fig.1).The product was achieved by a disposable syringe and examined by1H NMR to calculate conversion rate,selectivity and yield,and analyze resultant structure.The catalyst was recovered by centrifugation,washed twice with DMF and CH2Cl2,and dried at 150 °C for 24 h in vacuum for the next reuse.

2.4.Characterization

The crystal structures and phase states were determined by Powder X-ray diffractometry (PXRD) using PANalytical X-Pert PRO MPD with Cu Kɑ(λ=0.154 nm)radiation at an operating voltage of 40 kV and an operating current of 40 mA.The morphologies of catalysts were observed by a SU8020 field emission scanning electron microscope (FESEM) with an accelerating voltage of 3 kV.N2adsorption–desorption (BET) was performed on a 3H-2000PS2 specific surface and aperture analyzer at 77 K.The specific surface area of the samples was estimated by the BET multipoint method.The infrared absorption spectra were investigated using a Fourier transform infrared (FTIR) spectrophotometer (Nicolet 67,America) in the wave number range from 400 to 4000 cm-1.Superconducting nuclear magnetic resonance (NMR) spectra were performed on VNMRS600 to detect the composition of catalyst and conversion rate of cycloaddition reaction.X-ray photo-electron spectroscopy (XPS) measurements were performed on ESCALAB250Xi spectrometer to identify the elemental compositions and chemical states.

3.Results and Discussion

3.1.Catalyst characterization

Fig.2 exhibits the Powder X-ray diffractometry (PXRD) spectra of NH2-UiO-66-xHAc.The positions of the diffraction peaks of NH2-UiO-66-xHAc synthesized with different acetic acid additions were also completely consistent with those of the standard spectra.We can clearly observe the diffraction peaks at 7.42°,8.79°,12.36°,17.51°,22.50° and 25.34°,which can be indexed to the (1 1 1),(0 0 2),(0 2 2),(0 0 4),(1 1 5)and(0 0 6)crystal planes of the standard NH2-UiO-66 crystal,respectively [36].Moreover,the PXRD patterns display that the peaks intensity of NH2-UiO-66-xHAc progressively increased with the increase of HAc amount,indicating that the HAc modulation is beneficial for increasing the crystallinity of framework materials.CH3COOH has a certain coordination capacity to Zr4+,and can compete with H2BDC-NH2to bind Zr4+.With the increase of acetic acid content,the coordination reaction rate between H2BDC-NH2and [Zr6O4(OH)4] will be reduced,and the nucleation rate of NH2-UiO-66 will also decrease,which is beneficial to the formation of MOFs with good crystallization[37,38].In addition,in the small-angle region below 5°,we can see the X-ray diffraction peak(Fig.3).It can be speculated that this peak is caused by the presence of defect sites in the crystal [39],indicating that the addition of HAc in the synthesis process can induce the formation of defect sites in the material.Besides,after soaking in hydrochloric acid solution with pH of 3 for 24 h,the PXRD pattern of NH2-UiO-66-8HAc had no significant variation(Fig.S1 in Supplementary Material) [40].This means that NH2-UiO-66-8HAc has excellent acid stability.

Fig.2.Wide-angle PXRD patterns of a series of NH2-UiO-66-xHAc.

Fig.3.Small-angle PXRD patterns of NH2-UiO-66-0HAc and NH2-UiO-66-8HAc.

Fig.4 is a field emission scanning electron microscope (FESEM)image of NH2-UiO-66-xHAc.It can be seen that the addition of defect ligand HAc has a great influence on the morphology of MOFs formed by ZrCl4and H2BDC-NH2.The morphology of the sample without HAc defect ligand is irregular.When the amount of HAc added increases to 8 ml,the sample begins to show regular octahedral morphology.With the increase of the volume of HAc,the samples will show more perfect octahedral morphology.This agrees with the results of PXRD analysis.As seen,in the synthesis of MOFs,the size and morphology of NH2-UiO-66 crystals can be controlled by changing the content of HAc.Furthermore,the particle size of all materials is ca.200–300 nm.

Fig.5 reveals Fourier transform infrared (FTIR) spectra of NH2-UiO-66-xHAc (x=0,2,4,6,8,10 and 12).The absorption peaks of the prepared samples are consistent with the results reported in previous literature [41–44].Two adjacent shoulder peaks,3470 cm-1and 3360 cm-1,are attributed to the characteristic peaks of primary amine stretching and contraction vibration.The absorption peak at 1673 cm-1originates from the expansion vibration of C=O on the carboxyl group.The absorption band between 1573 cm-1and 1473 cm-1is ascribed to the C=C stretching vibration of the benzene ring skeleton on the organic ligand.1227 cm-1belongs to the absorption peak of C-N,which proves that the organic ligand has an amino group.1152 cm-1can be assigned to the characteristic absorption peak of Zr-O single bond vibration.The absorption bands between 888 cm-1and 665 cm-1are the out-of-plane bending vibrations of the C-H bond on unsaturated carbon.The peak at 568 cm-1is corresponded to the asymmetric contraction of Zr-OOC.The absorption band at about 2900 cm-1can be ascribed to the C-H vibration of methyl group in Ac-.Therefore,HAc is successfully inserted into NH2-UiO-66 to act as a defective ligand to modify the structure of MOFs.

Fig.4.SEM images of NH2-UiO-66-0HAc (a and b),NH2-UiO-66-2HAc (c),NH2-UiO-66-4HAc (d),NH2-UiO-66-6HAc (e),NH2-UiO-66-8HAc (f),NH2-UiO-66-10HAc (g),and NH2-UiO-66-12HAc (h).

Fig.5.FTIR spectra of a series of NH2-UiO-66-xHAc.

Fig.6.Nitrogen adsorption–desorption isotherms of a series of NH2-UiO-66-xHAc.

Fig.6 displays the N2adsorption–desorption isotherms of NH2-UiO-66-xHAc series materials.At the low end of P/P0,there is a very obvious adsorption capacity,which is a typical feature of Type I adsorption isotherms.This is mainly due to the enhancement of the interaction between adsorbent and adsorbate in the presence of micropores.As a result,the micropores are filled,and the gas adsorption amount developed rapidly in the low relative pressure region.Afterwards,the isothermal line reaches the near-horizontal platform,indicating that the micropores are filled.Also,the NH2-UiO-66 etched with different amounts of HAc showed different trends in the ability to absorb and desorb N2.With increasing of HAc amount,the adsorption capacity of the material can be seen to increase significantly in the low-pressure zone,indicating that the addition of acetic acid will increase the microporous adsorption capacity of the material.When the added amount is increased to 8 ml,the adsorption capacity of the micropores reaches the best state.What’s more,it can be seen that the adsorption curve of NH2-UiO-66-8HAc and NH2-UiO-66-4HAc does not overlap with the desorption curve in the range of 0.8–1.0.This is an obvious hysteresis phenomenon,which can be explained by the capillary condensation theory,indicating that there are mesopores in the defect structure.From the structure point of view,the reason for the formation of mesopores is that the acidity of H2BDC-NH2linker is higher than that of CH3COOH defect ligand.Therefore,H2BDCNH2will replace part of CH3COOH during the reaction.This incomplete exchange will cause the disappearance of coordination chains around some metal clusters,leading to structural defects to form micro-mesopore structures [45–48].As seen in Fig.S2 (Supplementary Material),great amount of the pore diameters are in the micropore range(less than 2 nm),and a small part is in the mesoporous region:2–5 nm.As the amount of HAc increases,the portion of mesoporous structure increases,but excessive addition of HAc may cause the specific surface area,total pore volume and average pore size to decrease.This can explain why the content of HAc can influence the catalytic efficiency of the cycloaddition reaction:the size of mesopores is larger than that of micropores,which is conducive to the diffusion and transfer of epichlorohydrin and its product molecules inside and outside the pores,but the reduction of the specific surface area will not be conducive to the progress of the reaction,because both the mesopores and the specific surface area play a role in the reaction.Table S1 lists the detailed Langmuir specific surface area,total pore volume and average pore size.The specific surface area of the prepared series of materials showed a trend of first increasing and then decreasing.Among them,NH2-UiO-66-8HAc has the largest total pore volume(1.0590 cm3.g-1),the highest Langmuir specific surface area of 1292.84 m2.g-1and the largest average pore size (4.8182 nm),which can be explained that the nano-defect area of NH2-UiO-66 continues to expand to the periphery and causes the micropores to form a large number of mesopores.Therefore,NH2-UiO-66-8HAc may have the best catalytic effect (vide infra),because large pore size,high surface area and large pore volume can promote reactant to diffuse and access the catalytic active sites of NH2-UiO-66,thus raising the reaction yield.

XPS analysis was performed to study the surface chemical state of NH2-UiO-66-xHAc,and the high-resolution Zr 3d spectrum of NH2-UiO-66-xHAc is shown in Fig.7(a).It can be seen that the spectrum of Zr 3d can be deconvolved into two peaks.Peaks at about 185.28 eV and 182.90 eV for NH2-UiO-66-0HAc can be attributed to Zr 3d3/2and Zr 3d5/2of zirconium atoms in Zr6cluster,respectively [49–50].In comparison with NH2-UiO-66-0HAc,Zr 3d3/2and Zr 3d5/2of NH2-UiO-66-8HAc synthesized by adding defective ligand HAc have new peaks at 185.53 eV and 183.19 eV,respectively,which corresponds to two forms of Zr in the coordination process.The first form is ascribed to coordination of Zr with a monocarboxylic acid (HAc),and the second form is assigned to coordination of Zr with solvent molecules such as DMF and H2O,and these coordinated solvent molecules can be removed by heating and vacuum activation,so that coordination unsaturated sites are generated in the metal center.For the second form,the decrease of electron density around zirconium reduces the shielding effect,and finally increases the electron binding energy [51].In addition,the optical spectrum of O 1s can be fitted into three peaks [52],as shown in Fig.7(b).The area of the Zr-O peak in NH2-UiO-66-8HAc is lower than that in the original NH2-UiO-66-0HAc,which suggests that the number of Zr-O coordination bonds is reduced,and this will induce missing joints in the synthetic MOFs [53,54].

Fig.S3 is a H-NMR diagram of the NH2-UiO-66-8HAc.The catalyst was digested with 1 mol.L-1NaOH(in D2O),then measured by nuclear magnetic analysis.The chemical shift of 1.69 can be assigned to 3H of CH3-in HAc.The three consecutive signal peaks between 7 and 8 correspond to benzene ring structure.Moreover,the peak occurring at the chemical shift around 8.22 is the formic acid produced by DMF decomposition.The1H NMR results indicate that the structure of the catalyst has the insertion of acetic acid,further verifying the generation of the defective structure.

3.2.Catalytic performance

Taking the cycloaddition reaction of epichlorohydrin(EPIC)and CO2as a model,the catalytic performance of NH2-UiO-66-xHAc was studied under mild conditions.The results are summarized in Table 1 where values of conversion,selectivity,yield and TOP of NH2-UiO-66-xHAc were calculated based on Fig.S4-S10.As shown,without the participation of catalyst,no cycloaddition reaction occurred(entry 9,Table 1),which is due to the inherent thermodynamic stability of CO2.Also,it can be seen that the linear relationship between x and catalytic performance is inverted Vshaped.From x=0–8 ml,the catalytic effect increases with the growth of the amount,and the opposite is true for x=8–12 ml.We can get a conclusion that the catalyst with x=8 ml has the best effect.This conclusion is exactly the same as the result of the BET analysis.Due to the modification of HAc,the defect structure and mesoporous pores are formed.The defect structure makes the clusters not closed,and is conducive to the diffusion of CO2gas molecules into the mesopores of MOFs,so that the reaction substrate can contact more metal catalytic active acid sites,as a result,the catalytic activity is enhanced.However,the excessive amount of acetic acid added will lead to the decrease of surface area,pore size and pore volume of MOFs as shown in Table S1.This may be due to the fact that,although acetic acid can improve the crystallinity of MOFs via decreasing the growth rate of crystals,and produce mesopores as defect structure,it may also block the tunnel of mesopores when excessive acetic acid was added,and cannot be washed out completely in preparation.Thus,excessive acetic acid will pose the negative effect on the catalytic activity of MOFs for cycloaddition reaction of EPIC and CO2as evidenced in Table 1.To normalize the effect of gaseous pressure on the catalytic performance,this article first proposed the concept of TOP (Turnover of Pressure),which is defined as the mass of conversions of a unit mass catalyst under unit pressure and unit time.Usually,when calculating the catalytic efficiency,the pressure is often ignored,but as one of the reactants,the pressure of CO2input to the reaction system directly determines the concentration of the gas in the reaction system,and is directly related to the quality of the catalytic effect.In general,under the same conditions,the more pressure of CO2input,the more concentration of CO2,thus,the more catalytic efficiency of the CO2-involved reaction.Thus,the proposed TOP index may be more accurate than TOF index (defined as the number of conversions of a single active site under unit time)to evaluate the role of the synthesized catalysts for the reaction of CO2with epoxide under the various reaction pressure.

Table 1 Comparison of the conversion rate and selectivity obtained under the same reaction conditions

Table 2 Comparison of NH2-UiO-66-8HAc with other MOFs for cycloaddition of CO2 and epoxide

Fig.7.XPS spectra of (a) Zr 3d and (b) O 1s of NH2-UiO-66-0HAc and NH2-UiO-66-8HAc.

Table 2 shows the catalytic performance of some previously reported catalysts for catalyzing the cycloaddition reaction of CO2and EPIC.The TOP value was calculated according to the method proposed in this article.As seen,the pressure of the reaction system has a crucial impact on the catalytic effect.When the pressure is normalized,the calculated TOP value for the catalyst with excellent yield is slightly lower.According to previous literature reports,the co-catalyst TBAB(Tetrabutylammonium bromide)plays a role for the cycloaddition reaction,so the TOP of entry 2,4,5 can be partly the contribution of the promoter[62,63].Besides the absence of co-catalysts and solvents,the reaction temperature adopted in this article is also mild by comparison.Although the yield of the reaction system in this article is relatively low,the reaction is carried out under ambient pressure.In terms of the TOP values calculated for the catalysts listed in Table 2,the as-synthesized NH2-UiO-66-8HAc has the highest value.Regretfully,according to our study,its catalytic effect on larger epoxides including styrene oxide is not ideal,which may be due to the limitation of the pore channels of the catalyst itself and the mild reaction conditions of ambient pressure.After a series of catalytic reactions for EPIC were studied,BET specific surface area was greatly reduced,which also indicated the limitation of the pore channels of the catalyst itself.It can be suggested that adopting modulators with larger size than HAc may be helpful to construct larger pores to promote the catalytic efficiency of larger epoxides in the future research.

3.3.Reusability study

In order to evaluate the reusability of the catalyst,the catalyst recycling experiment was carried out,and the results are shown in Fig.8.The yields of three cycles are calculated based on Figs.S11–S13.In general,the yields showed a downward trend in repeated use.The possible reason may be that EPIC is blocked in the pores of the catalyst during the reaction,and it is difficult to be completely washed out,which can be proved by the decrease of gas adsorption–desorption performance of the catalyst after the reaction (Fig.S14),and its specific surface area drops from 1292.84 m2.g-1to 337.59 m2.g-1.The more efficient regeneration method needs to be explored in the future such as adopting longer washing time and higher regeneration temperature,etc.

Fig.8.Recycling experiment with NH2-UiO-66-8HAc.

3.4.Catalyst stability

In order to further identify the stability of the MOFs catalyst involved in the cyclic addition reaction,the fresh catalyst and the recycled catalyst were analyzed by PXRD and FTIR (Fig.S15).It can be observed that there was no significant difference between the fresh catalyst and the recycled catalyst,indicating that the basic lattice structure was well preserved after catalytic reaction.These results show that the catalyst has good structural stability,proving its great potential in industrial applications.

Fig.9.Proposed reaction mechanism for the cycloaddition of CO2 and epichlorohydrin (EPIC) over NH2-UiO-66-xHAc catalyst.

3.5.Possible reaction mechanism

According to previous reports,a possible reaction mechanism for the cycloaddition of CO2and epichlorohydrin (EPIC) over NH2-UiO-66-xHAc catalyst was proposed as presented in Fig.9[64–68].The O-atom of EPIC and the Zr Lewis acid site are combined with each other,so that the epoxy group is activated.CO2is polarized by the amine group of NH2-UiO-66-xHAc.The Oatom of the polarized CO2molecule nucleophilically attacks the β-carbon of the epoxide,thereby opening the epoxy ring to form an intermediate.This is followed by the interaction of C atom from CO2with the oxygen anions of the opened epoxy ring.Finally,it is converted into cyclic chloropropene carbonate through a ringclosing step,and by coordinating with the next epoxide molecule,the regenerated MOFs participate in the next cycloaddition cycle.

4.Conclusions

In conclusion,NH2-UiO-66-xHAc MOFs with regular octahedral morphology and mesoporous structure were successfully prepared using solvothermal method with HAc as a modulator.The effects of HAc amount on the crystallinity,morphology,adsorption capacity,and performance of the as-prepared MOFs for the coupling reaction of CO2and epoxide were studied in detail.The adding of HAc in the synthesis process decreased the nucleation rate of NH2-UiO-66,which is beneficial to the formation of MOFs with good crystallization,moreover,the addition of HAc can induce the formation of mesoporous defect sites in the MOFs.This allows the reaction substrate EPIC to access the Lewis acid catalytic activity site on the surface and in the mesopores,thereby improving the catalytic performance of MOFs.This work provides a simple method for the design and preparation of mesoporous MOFs,and the obtained MOFs may find potential application in the field of catalyzing CO2conversion reaction under mild conditions.

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 financially supported by the Anhui Provincial Natural Science Foundation (1908085MB42),the National Natural Science Foundation of China (51372062).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.02.016.