ZIF-8 Organometallic Frame Catalyst Composite and Carbon Dioxide Conversion Method Using the Same

Abstract

The present invention relates to a ZIF-8 metalorganic framework catalyst composite, comprising: a ZIF-8 metalorganic framework; and a water molecule provided within the ZIF-8 metalorganic framework.

Description

Technical Field

[0001] The present invention relates to a ZIF-8 metal-organic framework catalyst composite and a method for carbon dioxide conversion using the same.

Background Art

[0002] Carbon dioxide is one of the main culprits of global warming, and research is actively being conducted not only on methods to reduce carbon dioxide but also on methods to utilize it in industry. Among them, as methods for utilizing carbon dioxide, there are methods of storing or converting it in various ways such as lipid storage, mineral storage, and chemical conversion.

[0003] The carbon dioxide cycloaddition reaction, which is one of various chemical conversion methods, is a reaction in which carbon dioxide is added into an epoxy compound to produce a cyclic carbonate. At this time, various forms of cyclic carbonates (chloropropylene carbonate, propylene carbonate, styrene carbonate) can be produced depending on the alkyl group type (chloromethyl group, methyl group, phenyl group) of the epoxy compound used, and the produced cyclic carbonate has high use value in various industrial fields such as polar solvents, fuel additives, and battery electrolytes.

[0004]

Chemical Formula

[0005] In addition, MOFs composed of the combination of metal cations and organic ligands have high porosity, structural flexibility, and various functionalities, and thus have high potential in various fields such as sensing, drug delivery, and catalysis. One of the MOFs, ZIF-8, consists of zinc (Zn) and 2-methylimidazolate, and due to the defects present on the external surface, it can simultaneously exhibit Lewis acid-base properties. Therefore, the cyclization addition reaction of various epoxy compounds and carbon dioxide can be promoted using ZIF-8.

Summary of the Invention

Problems to be Solved by the Invention

[0006] An object of the present invention is to provide a ZIF-8 organometallic framework catalyst composite containing water molecules.

[0007] Another object of the present invention is to provide a method for promoting the conversion of carbon dioxide using a ZIF-8 organometallic framework catalyst composite.

Means for Solving the Problems

[0008] According to one aspect of the present invention, an embodiment of the present invention may be a ZIF-8 organometallic framework catalyst composite including, in an organometallic framework, a ZIF-8 organometallic framework; and water molecules provided inside the ZIF-8 organometallic framework.

[0009] In one embodiment, the water molecules may dissociate at least a part of the bonds of the ZIF-8 organometallic framework to form reaction sites, and may include a ZIF-8 organometallic framework with reaction sites formed and a ZIF-8 organometallic framework without reaction sites formed.

[0010] In one embodiment, the reaction sites may include a first reaction site containing a Zn-OH bond; and a second reaction site containing an N-H bond.

[0011] In one embodiment, the formation ratio of the ZIF-8 metal-organic framework with the reaction sites formed may be such that the weight ratio of the ZIF-8 metal-organic framework with the reaction sites formed to the ZIF-8 metal-organic framework without the reaction sites formed is 0.05 to 1.0.

[0012] In one embodiment, at least one of the first reaction site and the second reaction site may promote the cycloaddition reaction of carbon dioxide and an epoxide compound to form a carbonate.

[0013] In one embodiment, the epoxide compound may be at least one of epichlorohydrin, ethylene oxide, styrene oxide, and propylene oxide.

[0014] In one embodiment, the higher the content of the water molecules, the more the yield of the carbonate may increase.

[0015] In one embodiment, the carbonate is a cyclic carbonate and may be at least one of chloropropene carbonate, ethylene carbonate, styrene carbonate, and propylene carbonate.

[0016] In one embodiment, the yield of the carbonate by the cycloaddition reaction may be calculated by the following formula 1.

[0017]

Number

[0018] In one embodiment, the water molecules may be contained in the ZIF-based organometallic frame catalyst composite at a weight ratio of 0.005 to 0.35.

[0019] In one embodiment, the BET surface area is 1300 m 2 / g~1600m 2 / g.

[0020] In one embodiment, the ZIF-8 organometallic framework in which the reaction site is formed may exhibit, in X-ray photoelectron spectroscopy (XPS) analysis, a first peak at 1022.3 eV to 1022.5 eV and a second peak at 531.8 eV to 520 eV, indicating the formation of Zn—OH.

[0021] In one embodiment, in an X-ray photoelectron spectroscopy (XPS) analysis, the ratio of the XPS peak area of the ZIF-8 organometallic framework showing the Zn-N bond energy to the XPS peak area of the ZIF-8 organometallic framework in which the reaction site is formed may be 0.4 to 0.8.

[0022] In addition, according to one aspect of the present invention, an embodiment of the present invention is a method for converting carbon dioxide using a ZIF-8 organometallic framework catalyst composite having at least one of the above characteristics, comprising the steps of: placing a material containing the ZIF-8 organometallic framework catalyst composite and an epoxide-based compound in an autoclave; supplying carbon dioxide into the autoclave; heating the inside of the autoclave to react the ZIF-8 organometallic framework catalyst composite, the epoxide-based compound, and carbon dioxide to obtain carbonate; and quenching the product; A method for converting carbon dioxide using a ZIF-8 organometallic framework catalyst composite.

[0023] In one embodiment, the inside of the autoclave may be heated to 40°C to 200°C.

[0024] In one embodiment, the carbon dioxide may be supplied until the pressure reaches 1 bar to 30 bar.

[0025] In one embodiment, the yield of the carbonate can be calculated by the following formula (2).

[0026]

Equation

Advantages of the Invention

[0027] As described above, the present invention provides a ZIF-8 organometallic framework catalyst complex that promotes the conversion reaction of carbon dioxide, and a method for promoting carbon dioxide conversion using the same can be provided.

Brief Description of the Drawings

[0028]

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Mode for Carrying Out the Invention

[0029] Specific matters of other embodiments are included in the detailed description and drawings.

[0030] The advantages and features of the present invention and the methods for achieving them should become clear by referring to the embodiments described in detail below together with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below and may be embodied in various different forms. Unless otherwise specified in the following description, all numbers, values, and / or expressions representing components, reaction conditions, and component contents in the present invention are approximate values reflecting various uncertainties in measurements that occur when obtaining such values among such essentially different numbers, and should be understood to be modified by the term "about" in all cases. Also, when a numerical range is disclosed in this description, such a range is continuous and includes all values from the minimum value to the maximum value including the maximum value thereof unless otherwise indicated. Further, when such a range refers to a constant, unless otherwise indicated, it includes all constants from the minimum value to the maximum value including the maximum value thereof.

[0031] Also, when the scope is described with respect to a variable in the present invention, it should be understood that the variable includes all values within the described scope including the described end points of the scope. For example, the range of "5 to 10" includes not only the values of 5, 6, 7, 8, 9, and 10, but also any sub-ranges such as 6 to 10, 7 to 10, 6 to 9, 7 to 9, etc., and any values between constants that are appropriate within the scope of the described range such as 5.5, 6.5, 7.5, 5.5 to 8.5, and 6.5 to 9. For example, the range of "10% to 30%" includes not only the values such as 10%, 11%, 12%, 13%, etc. and all constants up to 30%, but also any sub-ranges such as 10% to 15%, 12% to 18%, 20% to 30%, etc., and any values between constants that are appropriate within the scope of the described range such as 10.5%, 15.5%, 25.5%, etc.

[0032] FIG. 1 is a schematic diagram showing the process in which the Zn-N bond is dissociated (hydrolyzed) by the ZIF-8 organometallic framework catalyst composite during the cycloaddition reaction of carbon dioxide in one embodiment according to the present invention. It shows that the Zn-N bond on the outer surface of the ZIF-8 organometallic framework can be dissociated and converted into pyrrolic N, pyridinic N, and N-Zn-OH structures. The double Zn-OH structure can provide a Lewis acid / base moiety in the cycloaddition reaction of carbon dioxide and can be used as a catalyst.

[0033] The ZIF-8 organometallic framework catalyst composite according to one embodiment of the present invention can include, in the organometallic framework, a ZIF-8 organometallic framework; and water molecules provided inside the ZIF-8 organometallic framework.

[0034] The catalyst composite according to one embodiment of the present invention can include reaction sites on the outer surface of the ZIF-8 organometallic framework, and these reaction sites can have catalytic activity for promoting the reaction between CO2 and an epoxide compound.

[0035] The water molecules present inside the ZIF-8 metal-organic framework may dissociate at least a part of the bonds of the ZIF-8 metal-organic framework to form reaction sites. Here, the water molecules can be expressed as being occluded inside the ZIF-8 metal-organic framework.

[0036] Among the N-Zn-N bonds present in the structure of the ZIF-8 metal-organic framework, the water molecules can escape from the ZIF-8 metal-organic framework structure while hydrolyzing (dissociating) the Zn-N bond. In this process, the Zn-N bond can be divided into a Zn-OH bond and an N-H bond to form reaction sites. Therefore, the reaction sites can include at least one of a first reaction site containing a Zn-OH bond and a second reaction site containing an N-H bond.

[0037] The water molecules can hydrolyze at least a part of the N-Zn-N present in the ZIF-8 metal-organic framework. The formation ratio of the ZIF-8 metal-organic framework with the reaction sites formed as a result may be formed at a weight ratio of ZIF-8 metal-organic framework with reaction sites formed / ZIF-8 metal-organic framework without reaction sites formed of 0.05 to 1.0. Or, it may be formed at a weight ratio of ZIF-8 metal-organic framework with reaction sites formed / ZIF-8 metal-organic framework without reaction sites formed of 0.05 to 0.9. Or, it may be formed at a weight ratio of ZIF-8 metal-organic framework with reaction sites formed / ZIF-8 metal-organic framework without reaction sites formed of 0.05 to 0.69. When the ratio of ZIF-8 metal-organic framework with reaction sites formed / ZIF-8 metal-organic framework without reaction sites formed is less than 0.05, the catalytic activity for the cycloaddition reaction may not be sufficient.

[0038] At least one of the first reaction site and the second reaction site may promote the cycloaddition reaction of carbon dioxide and an epoxide compound to efficiently form a carbonate. The first reaction site can act simultaneously as a Lewis acid and a Lewis base in the cycloaddition reaction. Specifically, at the first reaction site, the Zn moiety can act as a Lewis acid, and the OH moiety can act as a Lewis base.

[0039] The water molecule may be contained in the ZIF-based organometallic framework catalyst complex at a weight ratio of 0.005 to 0.35. The water molecule may be present within the pore structure of the ZIF-based organometallic framework. Therefore, the above range may be present in the minimum amount that allows the water molecule to cause dissociation of the N-Zn-N bond in the ZIF-based organometallic framework, or may be present in the maximum amount that can be present inside the ZIF-based organometallic framework.

[0040] At this time, the higher the content of the water molecule present in the ZIF-8 organometallic framework, the higher the yield of the carbonate can be. The higher the content of the water molecule, the more the N-Zn-N bond of the ZIF-8 organometallic framework can be dissociated, and the higher the catalytic activity can be. For example, as the content of the water molecule decreases to 8 wt%, 4.2 wt%, 2.1 wt%, 0.5 wt%, and 0.1 wt%, the yields of the carbonate corresponding thereto can decrease to 20.5%, 9.9%, 4.5%, 2.6%, and 1.38%, respectively.

[0041] As a result of the cycloaddition reaction promoted by the ZIF-8 organometallic framework, the yield of the carbonate can be calculated by the following formula 3.

[0042]

Equation

[0043] In the above cyclization addition reaction, the epoxy compound can be used without limitation as long as it contains an epoxy structure. For example, the epoxy compound may be at least one of epichlorohydrin, ethylene oxide, styrene oxide, and propylene oxide.

[0044] Also, in the cyclization addition reaction, the carbonate can be formed according to the type of the epoxy compound used. For example, the carbonate may be a cyclic carbonate. Also, for example, the cyclic carbonate may be at least one of chloropropene carbonate, ethylene carbonate, styrene carbonate, and propylene carbonate.

[0045] The ZIF-8 organic metal framework may have a BET surface area of 1300 m 2 / g to 1600 m 2 / g.

[0046] At least a part of the ZIF-8 organic metal framework catalyst complex may show a first peak at a binding energy indicating the formation of Zn-OH in X-ray photoelectron spectroscopy (XPS) analysis at 1022.3 eV to 1022.5 eV and a second peak at 531.8 eV to 520 eV.

[0047] Also, in the ZIF-8 organic metal framework catalyst complex, in X-ray photoelectron spectroscopy (XPS) analysis, the ratio of the XPS peak area of the ZIF-8 organic metal framework including the reaction site to the XPS peak area of the ZIF-8 organic metal framework showing the binding energy of Zn-N may be 0.4 to 0.8.

[0048] According to another aspect of the present invention, an embodiment of the present invention is a method for converting carbon dioxide using a ZIF-8 metal-organic framework catalyst composite including at least any one of the above-described features, the method comprising: putting a substance containing the ZIF-8 metal-organic framework catalyst composite and an epoxide compound into an autoclave; supplying carbon dioxide into the autoclave; heating the inside of the autoclave to react the substance containing the ZIF-8 metal-organic framework catalyst composite, the epoxide compound, and carbon dioxide to obtain a product; and rapidly cooling the product to obtain a carbonate. The method can include a method for converting carbon dioxide using a ZIF-8 metal-organic framework catalyst composite.

[0049] The inside of the autoclave may be heated to 40°C to 200°C. Alternatively, the inside of the autoclave may be heated to 60°C to 80°C. When the heating temperature inside the autoclave is less than 40°C, the temperature may be too low and the reaction between carbon dioxide and the epoxide compound may not occur properly. Also, when the heating temperature inside the autoclave exceeds 200°C, the compound may be thermally decomposed and the desired reaction may not occur.

[0050] The carbon dioxide supplied to the autoclave can be used to react with the epoxide compound to form a carbonate. The carbon dioxide may be supplied until the pressure reaches 1 bar to 30 bar. Alternatively, the carbon dioxide may be supplied until the pressure reaches 7 bar to 9 bar. Inside the autoclave, the pressure of the carbon dioxide may be maintained during the reaction. When the pressure of the carbon dioxide is less than 1 bar, the amount of carbon dioxide may be small and the cycloaddition reaction may not occur properly. Also, when the pressure of the carbon dioxide exceeds 30 bar, side reactions may occur and the yield of the carbonate may decrease.

[0051] The yield of the carbonate can be calculated by the following formula 4.

[0052]

Equation

[0053]

Equation

[0054] Hereinafter, examples and comparative examples of the present invention will be described. However, the following examples are merely preferred examples of the present invention, and the scope of rights of the present invention is not limited by the following examples.

[0055] The terms in this specification and drawings are as follows. ZIF-8 can be a concept that includes normal ZIF-8 and all types of ZIF-8 used herein. ZIF-8W can mean that which is synthesized using water as a solvent during the synthesis of ZIF-8. ZIF-8W_S can mean the state after using ZIF-8W as a catalyst. Fresh ZIF-8W can mean the state immediately after synthesis and before the Zn-N decomposition reaction by water occurs. ZIF-8M can mean that which is synthesized using methanol as a solvent during synthesis. ZIF-8C may mean a common product. DP can mean a new dense phase generated by ZIF-8. ZIF-8W_Tx can mean that obtained by removing water from fresh ZIF-8W. x can mean the time of heat treatment for removing water. In addition, terms used in the drawings in the following description can be defined.

[0056] All chemical substances used in the following examples and experimental examples were used as they were without additional purification.

[0057] [Production Example] To synthesize ZIF-8 containing water inside its structure, a zinc precursor and an aqueous solution of 2-methylimidazole were prepared respectively. The zinc precursor solution was prepared by dissolving 1.7 g of zinc nitrate (Zn(NO3)3·6H2O, 98%, Sigma-Aldrich; product number: 228737) in 18 mL of DI water. The 2-methylimidazole solution was prepared by dissolving 2-methylimidazole (22.7 g, 99%, Sigma-Aldrich; product number: m50850) in 70 mL of DI water. Next, the zinc precursor solution was added to the 2-methylimidazole solution. This mixture was reacted while stirring at room temperature for about 20 minutes. Thereafter, a cycle of centrifugation, decantation, and washing three times with DI water was performed, and finally, it was dried at 70 °C for at least 12 hours to recover the solid product.

[0058] [Example] ZIF-8W, which contains water molecules in the synthesis method of the production example, was synthesized.

[0059] [Comparative Example] 1. Production of Heat-Treated ZIF-8W In addition to fresh ZIF-8W, the synthesized ZIF-8W was heat-treated at 100 °C for various times (6, 12, 18, 24 hours) to obtain heat-treated ZIF-8W. For convenience, the heat-treated ZIF-8W is denoted as ZIF-W_Ty, where y represents the heat treatment duration (h).

[0060] [Table 1] 2. ZIF-8M Synthesis To synthesize ZIF-8M containing methanol inside the structure of ZIF-8, a zinc precursor and a 2-methylimidazole solution were prepared respectively. Zinc nitrate (2.93 g, 9.87 mmol) and 2-methylimidazole (6.49 g, 79.0 mmol) were each dissolved in 200 mL of methanol (99.8%, Sigma-Aldrich; product number: 322415). These two solutions were mixed and reacted while stirring at room temperature for about 2 hours. Then, centrifugation, decantation, and washing three times with methanol were carried out, and finally, the solid product was recovered by drying at 70 °C for at least 12 hours.

[0061] 3. ZIF-8C was purchased and used as commercially available ZIF-8.

[0062] 4. DP (Dense Structure) Synthesis In addition to the ZIF-8 phase, DP (dense phase) was synthesized to compare the reaction of ZIF-8W. For this purpose, zinc nitrate (2.1 g, 7.06 mmol) and 2-methylimidazole (0.6 g, 7.21 mmol) were dissolved in 180 mL of dimethylformamide (DMF, 99%, Sigma-Aldrich; product number: 227056). 100 mL of DI water was added to this mixture. After stirring the mixture until it reached approximately room temperature, it was transferred to a Teflon liner. The sealed Teflon-lined autoclave was reacted at 140 °C for approximately one day while rotating (about 60 rpm). Then, the reaction was quenched using tap water. The product was recovered through cycles of centrifugation, decantation, and washing three times with DMF, and finally, it was dried at 70 °C for at least 12 hours to recover the resulting solid product. For convenience, the recovered product was named DP, which refers to those having a dense phase. , quenching) it. The product was recovered through cycles of centrifugation, decantation, and washing three times with DMF, and finally, it was dried at 70 °C for at least 12 hours to recover the resulting solid product.

[0063] [Experimental method] 1. Perform the CO2 cycloaddition reaction The CO2 cycloaddition reaction with the ZIF-8 catalyst was carried out in a specially fabricated Teflon®-lined stainless-steel autoclave (internal volume of the Teflon-liner: 250 mL). ZIF-8 (0.72 g) and epichlorohydrin (ECH, 10 mL, 99%, Sigma-Aldrich; product number: 45340) were placed in the Teflon® liner. Then, the Teflon® liner containing the reactants and the catalyst was positioned inside the autoclave. Next, the autoclave was filled with CO2 gas until a pressure of 7 bar was reached, and the reactor was heated to the desired reaction temperature (70 °C) using an electric heating jacket. For consistency, the reaction time was measured after the temperature of the reactor reached the reaction temperature, and the reaction was carried out for about 4 hours. After the specified time elapsed, the reaction was quenched with tap water. Then, the residual gas was released, and the used catalyst and the liquid product were centrifuged separately. The used ZIF-8 catalyst was recovered by performing cycles of centrifugation, decantation, and washing three times with DI water and drying at 70 °C for at least 12 hours. For convenience, the used catalyst is designated as ZIF-8x_S, where x corresponds to the fresh catalyst (ZIF-8x) and thus indicates M, W, or C, and S indicates the used catalyst. The obtained liquid product was analyzed using a gas chromatograph (YL6500, Young In Chromass, South Korea) equipped with a flame ionization detector and a capillary column (DB-5, 30 × 0.25 mm, Agilent). Toluene was used as an internal standard to ensure accurate measurement of the catalytic activity. Chloropropene carbonate (CC) was measured by multiplying the conversion rate of ECH by the selectivity of CC.The selectivity of CC was assumed to be 100% because all liquid-phase and gas-phase products consisted of only chloro-1,2-propanediol (diol) and 2,5-bis(chloromethyl)-1,4-dioxane (dimer), with no other products. The following Equation 6 was used to calculate the yield of chloropropene carbonate and can be similarly applied to other carbonates. Also, Equation 7 was utilized to calculate the conversion rate of epichlorohydrin and can be similarly applied to other epoxy compounds.

[0064]

Eq.

Eq.

[0065] In addition to the above CO2 cycloaddition reaction conditions (standard conditions), experiments with different reaction times and catalyst weights were additionally conducted for the case of ZIF-8W. Specifically, the reactions carried out with 0.72 g of ZIF-8W were performed with the reaction durations set to 1, 2, 3, 6, 10, and 16 hours respectively, and the weight of ZIF-8W was set with a reaction time of 4 hours, 0It was diversified while increasing it by 0.18 g from 0 g to 0.54 g. Also, a mixture of ZIF-8W and DP with a total catalyst mass of 0.72 under standard conditions was carried out with the weight (g) ratio of the ZIF-8 / DP mixture set to 0 / 0.72, 0.18 / 0.54, 0.36 / 0.36, 0.54 / 0.18. Finally, the reaction of the completely dried ZIF-8W (i.e., ZIF-8W_T24) was carried out under reaction conditions containing water, where the ratio of water added to ZIF-8W_T24 (0.72 g) was 8 wt% (the same as the water absorbed by the synthesized ZIF-8W), 20 wt%, and 40 wt% of the catalyst. Also, for convenience, ZIF-8W_T24 used under reaction conditions containing water (8 wt%) is denoted as ZIF-8W_T24_S_H, where the letter H was added to indicate the humid reaction condition.

[0066] 2. Characterization To confirm the structure of ZIF-8, an X-ray diffraction pattern of the ZIF-8 sample was obtained using a Rigaku model D / MAX-2500V / PC (Japan) equipped with a RINT2000 vertical goniometer (40 kV, 100 mA and λ = 1.54 Å). The crystallographic information file (CIF) for ZIF-8 was downloaded from the Cambridge Crystallographic Data Centre website (CCDC, www.ccdc.cam.ac.uk: deposition number 602542), and mercury software (also available on the CCDC website) was used to generate the simulated XRD pattern of ZIF-8. For reliable XRD analysis, α-alumina powder was used as an internal standard physically mixed with ZIF-8x or ZIF-8x_S (x = W, M and C) at the same weight ratio (w / w = 1). Also, XRD analysis was used to evaluate the ratio of active ingredients possible as a function of reaction time. First, XRD patterns of pure ZIF-8 or DP particles mixed with α-alumina powder at various weight ratios (0.1, 0.5 and 1) were obtained. Next, a good linear correlation between the weight ratio of the sample and the peak area ratio was provided by a calibration curve, obtained for both ZIF-8 and DP. Thereafter, the ratio or phase of the intermediate derivative (neither ZIF-8 nor DP) could be measured indirectly by tracking the ratios of 100% ZIF-8 and DP.

[0067] Hitachi S-4800 field emission scanning electron microscope (SEM, Japan) and FEI Tecnai G 2The F30ST field-emission transmission electron microscope (TEM, USA) was used to obtain SEM and TEM images of ZIF-8x and ZIF-8x_S (x = W, M, and C), respectively. The N2 adsorption / desorption isotherms of ZIF-8x, ZIF-8x_S (x = W, M, and C), and the example were obtained at 77 K using ASAP 2020 (Micromeritics Inc., USA). Prior to measurement, all samples were degassed at 80 °C for approximately 4 hours under vacuum. Subsequently, such isotherms were used to calculate the specific surface area by the Brunauer-Emmett-Teller (BET) equation and to calculate the micropore and mesopore size distributions using the Horvαth-Kawazoe (H-K) and Barrett-Joyner-Halenda (BJH) methods, respectively. Thermogravimetric analysis (TGA) was performed to measure the water content in ZIF-8W using a Q50 instrument (TA Instruments, USA). In TGA, the powder sample was under nitrogen with a flow rate of 100 mL·min -1 and heated at 5 °C·min -1It was heated from room temperature at a ramp rate of

[0068] In addition, to evaluate the ZIF-8x, ZIF-8x_S (x = W, M, and C) and the acid-base degree of the DP, temperature-programmed desorption analysis (TPD analysis) for various probe molecules (basic CO2 and acidic NH3) was performed using BELCAT II (MicrotracBEL Corp., Japan). Prior to the TPD measurement, ZIF-8x, ZIF-8x_S (x = W, M, and C), and for example, to remove the absorbed molecules, it was heated at 300 °C for 1 hour under a helium stream, and then exposed to a stream of CO2 or NH3 at 0 °C for 1 hour. After sufficient adsorption, the sample was at 10 °C·min under a helium stream of 30 mL·min -1 of helium gas flow -1It was heated from 0 °C to 300 °C at a ramping rate of

[0069] The chemical compositions of ZIF-8x, ZIF-8x_S (x = W, M, and C), and DP were determined by performing Fourier transform infrared (FT-IR) spectroscopy using a Nicolet TM iS50 FT-IR spectrometer (Thermo Scientific TM , USA). Specifically, ZIF-8W, ZIF-8W_S, DP, and 2-methylimidazole pellets prepared by pressing a sample mixture with KBr were placed in a specially fabricated in-situ cell equipped with KBr windows on both sides. The KBr pellets deposited with ZIF-8 were thermally treated at 150 °C for approximately 120 minutes under vacuum and then cooled to room temperature. Finally, not only ZIF-8x, ZIF-8x-S (x = W, M, and C), and DP but also the chemical compositions of ZIF-8x, ZIF-8x-S (x = W, M, and C), and DP adsorbed with NH3 were irradiated using the attenuated total reflection (ATR) mode. For the FT-IR analysis of the NH3-adsorbed samples, the samples were prepared using BELCATII. The samples were heated at 150 °C for 1 hour under a helium gas flow to remove the adsorbed molecules and then exposed to an NH3 gas flow at 30 °C for 1 hour. After sufficient adsorption, the NH3-adsorbed samples were under a helium gas flow of 30 mL·min -1 at a rate of 10 °C·min -1At a ramping rate of, heated from 30 °C to x °C ((1) for ZIF-8W, ZIF-8W_S, and DP, x = 50, 100, and 150 °C; (2) for ZIF-8M_S and ZIF-8C_S, x = 50 °C). Such adsorption processes were carried out with diverse amounts of NH3. Finally, X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES) were performed using an X-tool (Ulvac-PHI, Japan) equipped with a monochromatic Al Kα X-ray source (hν = 1486.6 eV, 15 kV, and 24.1 W) to detect the species of Zn, N, and O in ZIF-8x, ZIF-8x-S (x = W, M, and C), and DP. All measured binding energies were referenced to the C 1s peak at 284.5 eV. The software Magic Plots Student2.5.1 was used for the stable deconvolution of the FT-IR and XPX spectra.

[0070] [Experimental Results] 1. Characterization Analysis of ZIF-8 (1) Water Adsorption Characteristics of ZIF-8 In Figure 2, (a) shows the TGA profiles of ZIF-8W and ZIF-8W_T24_Im, and (b) shows the water vapor adsorption isotherms of ZIF-8W_T24 and ZIF-8x (x = M and C) at 70 °C. The region in the dotted box in Figure 2(b) is enlarged and inserted for display. For comparison, the amount of water trapped in ZIF-8W is shown by a dotted line below 5 on the y-axis. The TGA profile of ZIF-8W_T24 (ZIF-8W_T2_Im) immersed in water for one day showed negligible water adsorption characteristics (Figure 2(a)). Also, at 70 °C, the water adsorption isotherm of ZIF-8W_T24 showed a very low H2O adsorption capacity (Figure 2(b)), which is consistent with previously reported results. Such results are important in that the water molecules trapped in the synthesized ZIF-8W generate active sites for the CO2 cycloaddition reaction, as will be explained later, and strongly support the fact that completely dried ZIF-8 cannot form active sites for the CO2 cycloaddition reaction because water molecules cannot be adsorbed at intermediate pressures.

[0071] (2) Textural properties of ZIF-8 (a) and (b) of Fig. 3 show the N2 physical adsorption isotherms of ZIF-8W and ZIF-8M degassed at 80 °C and 200 °C for 4 hours, respectively. Table 2 shows the N2 physical adsorption amount and the corresponding BET surface area. In Table 2, the physical adsorption isotherm data were calculated using the Brunauer-Emmett-Teller equation. For the three types of ZFI-8 for N2 physical adsorption, they were degassed at 80 °C, and the water molecules trapped inside ZIF-8W remained to some extent even after degassing. Nevertheless, the unique change stage of ZIF-8W was observed. The degassing temperature similar to the reaction temperature (70 °C) in the cycloaddition reaction was applied to avoid the unfavorable diversification of the assembly characteristics due to the conventional higher degassing temperature, while investigating the change in the assembly characteristics of ZIF-8 before and after the reaction (especially the unique stage change). When ZIF-8W and ZIF-8M were all degassed at a higher temperature (200 °C instead of 80 °C), their N2 physical adsorption amounts and the corresponding BET surface areas were similar to each other, indicating that the water molecules in ZIF-8W were completely removed.

[0072] [Table 2] (3) Chemical species analysis on the external surface of ZIF-8 and DP Fig. 4 shows the XPS analysis results for ZIF-8W, ZIF-8W-S, and DP. In Fig. 4, for each compound, (a1)-(a3) show the XPS spectra of N 1s, (b1)-(b3) show Zn 2p 3 / 2 , and (c1)-(c3) show the XPS spectra of O 1s. In contrast, (a1)-(c1) are for ZIF-8W, (a2)-(c2) are for ZIF-8W_S, and (a3)-(c3) are for DP, each having a deconvoluted curve. For comparison, the curve obtained by summing the deconvoluted curves is shown as "SUM". To show appropriate fitting, the residual plot was displayed below the experimentally obtained XPS spectrum. For the residual plot, the normalized residual (R N) was calculated by subtracting the XPS spectrum experimentally measured with the intensity integrated from the deconvoluted curve, and further dividing by the value of the experimentally measured XPS spectrum. Figure 5 shows the (a) N 1s and (b) Zn 2p XPS spectra of ZIF-8x and ZIF-8x_S (x = W, M, and C). For comparison, the results for DP are added below. Figure 6 shows the XAES spectra of Zn LMM for (a) fresh ZIF-8x and (b) used ZIF-8x_S (x = W, M, and C). In Figure 6, for comparison, the Auger electron spectra of Zn LMM for DP are added below, and the peaks corresponding to Zn 0 and Zn 2+ are indicated by dotted lines.

[0073] Figures 4(b1)-(b3) and 5(b) show the XPS results for three types of ZIF-8x (x = W, M, and C) and DP, and in Figure 6, the presence of Zn metal was further verified by the said XAES.

[0074] Also, Zn metal has been revealed to be present in very low content by XPS and XAES analyses for intact ZIF-8 in previous studies. Even though some metal Zn species can be formed during the synthesis process with the help of the positive charge supplied by 2-methylimidazole, the presence of ZnO implies facile oxidation. Next, considering that ZnO single crystals (purity 99.99%) show a certain degree of Zn metal, we hypothesized that the ZnO portion that can be inevitably exposed to ambient conditions and formed on the surface might reduce Zn metal during the preparation of XPS measurements. Nevertheless, the ratios of metal Zn species and ZnO were almost the same for all four samples, regardless of whether they were in fresh or used form. Therefore, it is considered that neither metal Zn nor ZnO contributes to the catalytic activity of the CO2 cycloaddition reaction.

[0075] 2. Characteristics of ZIF-8 before / after the cycloaddition reaction Figures 7 to 9 are drawings showing the analysis of the properties of ZIF-8.

[0076] In Figure 7, (a) shows the CC yields of ZIF-8x (x = W, M, and C) when the cycloaddition reaction of carbon dioxide was carried out for 4 hours with the CC yields of ZIF-8W at various reaction times (inserted drawing). (b) shows the external surface composition of nitrogen in ZIF-8x. (c1)-(c2) show the SEM images of ZIF-8W, where (c1) is a low-magnification and (c2) is a high-magnification image. (c3) is the TEM image of ZIF-8W. (d1)-(d2) show the SEM images of ZIF-8W_S, where (d1) is a low-magnification and (d2) is a high-magnification image. (d3) is the TEM image of ZIF-8W_S. (e) shows the XRD patterns of ZIF-8 simulated by ZIF-8W, ZIF-8W_S, and dense phase (DP) particles. For comparison, the highest peak in ZIF-8 or DP was used after normalizing the XRD pattern. In the inserted drawing of (a), the dotted line indicates the maximum yield (55.6%) under this reaction condition. For understanding, a schematic diagram of 2-methylimidazole is added to the upper corner of (b). For clarity, descriptions of the images are provided in (c1)-(c3) and (d1)-(d3). In (c1)-(d1), the dotted rectangles indicate the high-magnification SEM image regions in (c2)-(d2) respectively. In (d1)-(d2), the long-tailed arrows indicate the newly appeared particles in ZIF-8W_S. The dotted rectangles in the TEM images of (c3)-(d3) emphasize the morphological changes after the reaction. In (e), the arrow with a tail indicates the newly formed XRD peak, and the inverted triangle indicates the XRD peak of α-alumina used as an internal standard. In (e), for the cases of ZIF-8W and ZIF-8W_S, the ratio of ZIF-8 (indicated by A Z / A α ) to α-alumina was obtained using the XRD peak regions of 7.3° and 35.1° corresponding to ZIF-8 and α-alumina respectively.

[0077] In Figure 8, (a1)-(b1) are SEM images of ZIF-8M, and (a2)-(b2) are SEM images of ZIF-8M_S. (a1)-(a2) show low-magnification and (b1)-(b2) show high-magnification images. (c1) shows a TEM image of ZIF-8M, and (c2) shows a TEM image of ZIF-8M_S. (d) shows the XRD patterns of ZIF-8M and ZIF-8M_S based on the simulated XRD pattern of ZIF-8. The inverted triangle display in (d) indicates α-alumina used as an internal standard.

[0078] In Figure 9, (a1)-(b1) are SEM images of ZIF-8C, and (a2)-(b2) are SEM images of ZIF-8C_S. (a1)-(a2) show low-magnification and (b1)-(b2) show high-magnification images. (c1) shows a TEM image of ZIF-8C, and (c2) shows a TEM image of ZIF-8C_S. (d) shows the XRD patterns of ZIF-8C and ZIF-8C_S based on the simulated XRD pattern of ZIF-8. The inverted triangle display in (d) indicates α-alumina used as an internal standard.

[0079] Figure 10 shows the size distribution of each catalyst particle. For each catalyst particle, the size distributions of (a) ZIF-8W, (b) ZIF-8M, and (c) ZIF-8C are shown for the fresh (hatched bars) and used (solid black bars) states. The size measurements were made on 100 particles for each of the three types of ZIF-8, excluding the longest one. For consistency, particles that were too large in ZIF-8W_S (i.e., newly formed particles) were not measured.

[0080] Figure 11 shows SEM images of ZIF-8W and ZIF-8W_S. (a1)-(a3) are for ZIF-8W, and (b1)-(b3) are for ZIF-8W_S. (a1)-(b1) are low-magnification, and (a2)-(b2) are high-magnification images. The images in (a2)-(b2) and (a3)-(b3) are magnifications of the dotted rectangle parts in (a1)-(a2) and (b1)-(b2), respectively.

[0081] Figure 12 shows the XRD patterns of ZIF-8W, ZIF-8W_S and DP according to the simulated XRD pattern of ZIF-8. In Figure 12, the XRD patterns for ZIF-8W and ZIF-8W_S are identical to Figure 7(e) except for the intensities normalized here. The intensities normalized here are such that the XRD peaks of ZIF-8-W and ZIF-8W_S are normalized against the α-alumina peak at 35.1°. The triangular representation indicates the XRD peaks for α-alumina used as an internal standard.

[0082] When completely integrated into the ZIF-8 framework, Zn or N atoms were thought to be saturated and show no catalytic activity. However, in Figure 7(a) showing the characteristics among three types of ZIF-8x (x = W, M and C), only ZIF-8W showed remarkable catalytic characteristics in the cycloaddition reaction of ECH and CO2, showing a CC yield as high as about 20.3% after reacting for 4 hours. Surprisingly, the CC yields of the other two types of ZIF-8 (ZIF-8M and ZIF-8C) appeared to be 1.2% and 1.5% respectively, with little or no catalytic characteristics shown. Also, the CC yield of ZIF-8W increased slowly with the increase in reaction time and reached a maximum yield of 55.4% after 16 hours of reaction (inserted drawing in Figure 7(a)). In previous studies, the catalytic activity mainly contributed from coordinatively unsaturated Zn or unbonded N species present on the surface of the ZIF-8 structure. According to this, XPS analysis was performed to confirm the dissociated Zn or N species on the surface of ZIF-8x (x = W, M and C) (Figure 7(b)). In particular, the contributions of Zn and N species in the three fresh ZIF-8 catalysts were similar, suggesting that if there were changes in physicochemical properties, they occurred during the CO2 cycloaddition reaction.

[0083] Considering the similar characteristics and distributions of chemical species on the surfaces of the three types of ZIF-8, we investigated other physicochemical properties of the catalysts to find significant differences. The morphology / size and crystal structure of ZIF-8M (Figs. 8(a1)-(c1) and (d)) are similar to those reported in previous studies, while the commercially available ZIF-8C had the same physicochemical properties (Figs. 9(a1)-(c1) and (d)). Also, the morphology / size of ZIF-8W, which shows remarkable catalytic performance (Fig. 7(a)), was similar to that reported in previous studies (Figs. 7(c1)-(c2)). To confirm the angular morphology of ZIF-8W perceived at SEM resolution (Fig. 7(c2)), TEM analysis was additionally performed. In Fig. 7(c3), the dotted rectangle clearly shows the angular morphology of ZIF-8W. Finally, the particle sizes of ZIF-8x (x = W, M, and C) were measured to be 65 ± 15 nm, 40 ± 6 nm, and 206 ± 68 nm, respectively (Fig. 10). Also, XRD analysis shows the pure ZIF-8 crystal structure of ZIF-8W (Fig. 7e). TEM results of all three types of ZIF-8x (x = W, M, and C) showed some other microscopic features (e.g., regardless of the plane more defined in ZIF-8M (Fig. 8(c1))), indicating high crystalline perfection of ZIF-8M. In this study, we suggest that the cycloaddition reaction of CO2 to ECH should mainly occur preferentially on the external surface of ZIF-8, and the three types of ZIF-8x (x = W, M, and C) with the aforementioned particle sizes and simultaneously determined external surface areas can be examples for understanding the intrinsic catalytic activity of ZIF-8 based on their notable differences in physicochemical properties. Considering that the size and crystallinity of ZIF-8W are larger or similar to those of ZIF-8M, other physicochemical properties should be crucial in explaining the remarkable catalytic activity of ZIF-8W. The surface areas for each catalyst are tabulated in Table 3. In Table 3, a(S BET ) was calculated from N2 physical adsorption isotherm data using the Brunauer-Emmett-Teller equation, and b(S external ) was calculated using the improved t-plot method, and c(Sinternal ) was calculated using Equation 8.

[0084]

Table 3

[0085]

Number

[0086] According to the prior literature, the ZIF-8 structure has been reported to be deformable in the presence of CO2 and water vapor, and similar deformations occurred in this study. Considering that water is not utilized in the CO2 cycloaddition reaction, the deformation of ZIF-8W occurs due to its inherent properties. In particular, when compared with ZIF-8M and ZIF-8C, only ZIF-8W contains a solvent (i.e., water) inside its pore structure. To confirm this, we performed TGA and FT-IR analyses on the three types of ZIF-8 (Fig. 13). FT-IR spectroscopy showed the presence of hydroxyl groups in ZIF-8W, revealing distinct features (Fig. 13(a)). Also, TGA (Fig. 13(b)) showed that weight loss occurred only in ZIF-8W as the temperature increased from 100 °C to 200 °C. Not only the inference based on the use of water in the synthesis of ZIF-8W, but also these two characteristics indicate that only ZIF-8W contains water molecules inside its pore structure. The water molecules trapped inside ZIF-8W are thought to be related to the deformation of the ZIF-8 structure, and the accompanying formation of active sites can explain the prominent activity of ZIF-8W towards the CO2 cycloaddition reaction.

[0087] 3. Cause of the Activity of ZIF-8W towards the CO2 Cycloaddition Reaction Figure 14 shows the results of experiments in which ZIF-8W was heat-treated to remove water molecules or mixed with DP to confirm the catalytic properties of ZIF-8W. (a) shows the CC yields when ZIF-8W (black bars), and ZIF-8W and DP were mixed at different ratios (hatched bars). In this experiment, 0, 0.18, 0.36, 0.54, and 0.72 g of ZIF-8W were used respectively, and the mixing ratios of ZIF-8W and DP were 0 / 0.72, 0.18 / 0.54, 0.36 / 0.36, 0.54 / 0.18, and 0.72 / 0 g respectively. (b) shows the CC yields and water contents of ZIF-8W heat-treated at 100 °C for various heat-treatment times. (c) shows the CC yield of ZIF-8W heat-treated at 100 °C for 24 h (ZIF-8W_T24) under reaction conditions in which a specific amount of water (up to 40%) was present. In Figure 14, all reactions except (c) were carried out according to the following reaction conditions: (1) reactants: ECH (epichlorohydrin, 10 mL) and CO2 (7 barg); (2) total weight of the catalyst used: 0.72 g; and (3) reaction temperature and time: 70 °C, 4 h. In (c), water was added to the reactants to meet a predetermined ratio of water for ZIF-8W_T24 (0.72 g). In (b)-(c), the water inside the catalyst was determined by TGA.

[0088] Figure 15 shows the XRD patterns measured by mixing ZIF-8W or DP with α-alumina at various ratios to confirm the state of ZIF-8W having catalytic activity for the carbon dioxide cycloaddition reaction. (a) shows the XRD patterns for the DP / α-alumina and ZIF-8W / α-alumina mixtures mixed at different ratios (0.1, 0.5, and 1), respectively. (b) is a linear regression for the weight ratio of this mixture, where (b1) is for DP / α-alumina and (b2) is for ZIF-8W / α-alumina. (c) shows the XRD patterns of ZIF-8W recovered after performing the CO2 cycloaddition reaction at different reaction times (1, 2, 3, 4, 6, 10, and 16 h). The catalyst used for convenience is denoted as ZIF-8W_Sx, where x is 1, 2, 3, 4, 6, 10, or 16 h, indicating the reaction time. In (a) and (c), the inverted triangle indicates α-alumina used as an internal standard. For comparison, (c) includes the XRD patterns for pure ZIF-8W and DP.

[0089] Figure 16 shows a graph estimating the weight ratio of the ZIF-8 intermediate derivative from the weight ratios of ZIF-8 and DP in the catalyst after the reaction and the weight ratio of the ZIF-8 intermediate derivative and the CC yield as a function of the reaction time. (a) shows the weight ratios (w / wα) for ZIF-8 (black bars), the ZIF-8W intermediate derivative (estimated value, hatched bars), and DP (unfilled bars) for ZIF-8W_S recovered after reacting at different reaction times (1, 2, 3, 4, 6, 10, and 16 h), respectively. (b) shows the weight ratio of the ZIF-8 intermediate derivative (estimated value) in ZIF-8W_S and the CC yield (shown in Fig. 7(a)) plotted as a function of the reaction time.

[0090] Figure 17 shows the TGA results of heat-treated ZIF-8W. It is denoted as ZIF-8W_Tx for each heat treatment time, where Tx indicates that hydrothermal treatment was performed for x hours. The heat treatment was carried out at 100 °C for (a) 0 h, (b) 6 h, (c) 12 h, (d) 18 h, and (e) 24 h.

[0091] As an initial attempt to reveal the unique catalytic activity of ZIF-8W, we investigated the catalytic ability of newly formed particles from ZIF-8W (Figure 14(a)). To simulate the particles shown in Figures 7(d1)-(d2), particles named dense phase (DP) that could be explained by the newly emerged XRD peaks (Figure 7(e)) mentioned above were synthesized and utilized. In particular, this DP is similar to that reported in the prior literature, and DP particles can be formed when ZIF-8 is exposed to an environment containing CO2 and gaseous-phase water simultaneously, or when ZIF-8 is immersed in liquid-phase water. For illustration, the weight fractions of ZIF-8W and DP were systematically varied, but the total weight was fixed at 0.72 g, and the weight of this ZIF-8x (x = W, M, and C) was used to carry out the reactions shown in Figure 7(a). Apparently, the CC yield was almost the same as the result of reacting with the same amount of ZIF-8W. Therefore, the catalytic activity was related to the amount of ZIF-8W rather than the amount of DP. Thus, it did not appear that the same DP when ZIF-8W was completely converted could explain any catalytic activity for the CO2 cycloaddition reaction. Therefore, the catalytic activity of ZIF-8W in the CO2 cycloaddition reaction is mostly related to the intermediate derivative. As inferred from the XRD pattern of ZIF-8W (Figure 7(e)), the ratios of DP and ZIF-8 should increase and decrease respectively after the reaction. In particular, the use of α-alumina as an internal standard is preferable for quantitative analysis. Referring to such an approach, we attempted to measure the ratio of the ZIF-8 intermediate derivative as a function of the reaction time (Figure 15). Figure 16 shows that the CC yield and the ratio of the ZIF-8 intermediate derivative follow a similar trend as a function of the reaction time. This indicates that in this study, the possible active component of the ZIF-8 intermediate derivative was derived from the CO2 cycloaddition reaction.

[0092] As the next step to clarify the cause of the catalytic activity of ZIF-8W, we investigated the deformation of ZIF-8W during the CO2 cycloaddition reaction. Considering that ZIF-8W can induce the formation of DP particles during the CO2 cycloaddition reaction, we performed heat treatment at about 100 °C with different times to reduce the amount of water molecules trapped inside ZIF-8W. The final water amount was accurately determined considering the weight change between 100 °C and 170 °C in the TGA profile (Figure 17). For convenience, the heat-treated ZIF-8 is denoted as ZIF-8W_Ty, where y represents the heat treatment time (h). The catalytic activity corresponding to each heat-treated ZIF-8W was revealed by the fact that the CC yield decreased gently with the increase in the duration of heat treatment (i.e., water content decrease) (Figure 14(b)). Also, the same amount of water as the amount of water inside the synthesized ZIF-8W was added to the reactants by supplying it to the completely dried ZIF-8 (i.e., ZIF-8W_T24). Surprisingly, the CC yield showed a catalytic activity similar to that of ZIF-8W_T24, regardless of the added water content (Figure 14(c)). Even when up to 40 wt% of water was added (vs. about 8 wt% of water inside the synthesized ZIF-8W), the catalytic activity was kept not so low as that of ZIF-8W_T24. Considering that it is very difficult to put water molecules into the pores of ZIF-8 at pressures below 19 MPa clearly due to its hydrophobic property, water molecules outside the ZIF-8 pore structure do not contribute to forming the active sites of the desired catalytic activity. More information on negligible water adsorption by completely dried ZIF-8x (x = W, M and C) is shown in the graph of Figure 2 and explained above. This strongly indicates that water molecules trapped in the ZIF-8 pore structure are the key to forming the active sites of the CO2 cycloaddition reaction using the catalyst. Also, the fact that the CC yield decreased monotonically with the decrease in water molecules (Figure 14(b)) indicates that the generation of active sites depends on the water content. Therefore, the other two types of ZIF-8 without water molecules do not show remarkable catalytic activity for the CO2 cycloaddition reaction.

[0093] 4. Active Sites of ZIF-8W Acting as a Catalyst in the CO2 Cycloaddition Reaction Figure 18 shows the results of measuring the N2 adsorption / desorption isotherms, pore size distributions, and FT-IR of each sample to investigate the sites that act as catalysts in ZIF-8W. Here, (a) shows the N2 adsorption / desorption isotherms of ZIF-8x, and (b) shows those of ZIF-8x_S (x = W, M, and C). (c) shows the N2 adsorption / desorption isotherms of ZIF-8x and ZIF-8x_S (x = W and M) with a logarithmic x-axis, where the results for DP are added for comparison. (d) shows the pore size distributions of ZIF-8x and ZIF-8x_S (x = W and m). (e) shows the FT-IR spectra of ZIF-8W, ZIF-8W_S, DP, and 2-methylimidazole (2-mim). For C=N stretching, C-N stretching, bending of the imidazole ring, and Zn-N bond, respectively, the peaks are tagged with an inverted triangle. In (e), the samples were prepared in pellet form by pressing a mixture of the sample and KBr. For a fair comparison, in (e), all samples were normalized by sample weight, and the FT-IR for ZIF-8W / ZIF-8W_S and ZIF-8W_S / DP is shown together. 。( (f) shows the FT-IR spectra of ZIF-8W, ZIF-8W_S, and their heat-treated samples. In (f), one side of the sample was vapor-deposited with a pure KBr pellet.

[0094] In Figure 19, (a) shows the N2 adsorption / desorption isotherms of ZIF-8W_S, DP, and a mixture of ZIF-8W and DP with a logarithmic X-axis. (b) shows the N2 adsorption / desorption isotherms of ZIF-8C and ZIF-8C_S with a logarithmic X-axis. (c) shows the micropore size distribution of ZIF-8C and ZIF-8C_S in the range of 0.4 nm to 2.0 nm.

[0095] Figure 20 shows the mesopore size distributions of (a) ZIF-8W, (b) ZIF-8M, (c) ZIF-8C in the range of 2 nm to 50 nm, and their respective catalysts used (ZIF-8x_S; x = W, M, and C). Each of the samples is tagged.

[0096] Figure 21 shows the FT-IR spectra of each sample. (a) shows the FT-IR spectra of ZIF-8W, ZIF-8W_S, DP, and 2-methylimidazole (2-mim). (b) shows the FT-IR spectra of ZIF-8M, ZIF-8C, and their used catalyst states. (c) shows the FT-IR spectra of ZIF-8M, ZIF-8C, and their used catalyst states in the range of 2000~650 cm -1 range 。( All the FT-IR spectra in (a)-(c) are normalized with respect to the peak at 684 cm -1 corresponding to the bending of the imidazole ring 。

[0097] The effect of water molecules trapped in the ZIF-8 pore structure was revealed, but the chemical cause of the catalytic activity remained unclear other than the phase change. Since the nitrogen and zinc of the Zn-N bond are hardly active in this reaction because the CO2 cycloaddition reaction can be carried out under the conditions where a Lewis acid, a base, or both are present, we hypothesized that the active site is derived from the Zn-N bond and is related to newly formed N and / or Zn species that clearly involve the structural deformation of ZIF-8. For this purpose, we investigated the N2 adsorption-desorption isotherms of ZIF-8x and ZIF-8x_S (x = W, M, and C) to show the structural differences (Figs. 18(a)-(b)). ZIF-8x shows typical type1 adsorption behavior, and regardless of the synthesis route, at low P / P0 values (6.0×10 -3 and 4.0×10 -2A unique step change appeared in the range (shown by the dotted square in Fig. 18(a)). ZIF-8 exhibits structural flexibility through imidazolate (IM) linked to zinc when gas molecules are adsorbed, which is a swing effect phenomenon reflected by a low P / P0 value and a remarkable increase in N2 adsorption at 77K. Such a step change in N2 adsorption can be utilized to confirm the existence of Zn-IM linkages in ZIF-8. Even though the hysteresis loops in ZIF-8W and ZIF-8M are closed at different relative pressures, it seems to start from apparent non-specific particle aggregation and thus should not affect any physicochemical or catalytic properties. Also, it was noted that there was a slight discrepancy with respect to the BET surface area of ZIF-8W (see Table 2) in the N2 adsorption isotherm (obtained after degassing at 80°C). However, at a higher degassing temperature of 200°C, it was confirmed that the micropores were similar between ZIF-8W and ZIF-8M (Figs. 3 and 2). Together with the previously mentioned XRD results, N2 physical adsorption shows a highly crystalline nature comparable among the three types of ZIF-8 used in this study (Tables 2 and 3).

[0098] Also, the general adsorption isotherm trend was maintained for the catalysts used (i.e., ZIF-8x_S, x = M and C). However, ZIF-8W_S showed significantly decreased adsorption. Therefore, the BET surface area of ZIF-8W decreased to 530 m 2 ·g -1 from 1390 m 2 ·g -1 after the reaction, which is a considerable decrease. For reference, ZIF-8M decreased from 1592 m 2 ·g -1 to 1501 m 2 ·g -1 and ZIF-8C decreased from 1626 m 2 ·g -1 to 1473 m 2 ·g -1decreased. Also, the unique stepwise change mentioned previously was not visible in ZIF-8W_S (shown by the dotted rectangle in Fig. 18(b)). In Fig. 18(c), for easy comparison, the N2 adsorption isotherms of ZIF-8W and ZIF-8W_S are shown together with those for ZIF-8M, ZIF-8M_S, and DP. The plots in the exponential range are for ZIF-8W, ZIF-8M, and ZIF-8M_S at approximately 6.0×10 -3 and 4.0×10 -2With P / P0, a two-step change was clearly shown. In contrast, ZIF-8W_S showed not only a decrease in adsorption but also the presence of stepwise adsorption (Figure 18(c)). Considering the negligible N2 adsorption capacity of DP generated from the complete structural deformation of ZIF-8W during the CO2 cycloaddition reaction, the significant decrease in N2 adsorption in ZIF-8W_S is mainly due to the collapse of the original structure that induces a decrease in micropores during the formation of DP (Figure 7(e)). Conversely, ZIF-8x (x = M and C), whose structures are retained even after the reaction without the formation of DP (Figure 8(d) and 9(d)), can retain their original N2 adsorption capacity (Figure 18(a)-(b)). Nevertheless, considering that the physical mixture of ZIF-8W and DP has stepwise adsorption behavior (Figure 19(a)), the mere presence of DP cannot explain the absence of stepwise absorption behavior in ZIF-8W_S. The micropore size distribution was considered to investigate the presence and absence of stepwise adsorption behavior before and after the reaction (Figure 18(d)). ZIF-8W, ZIF-8M, and ZIF-8M_S have three peaks at 0.6, 0.9, and 1.2 nm. Among them, the first peak at 0.6 nm is related to the micropore structure of ZIF-8 and N2 adsorption, and the two peaks at 0.9 and 1.2 nm are related to additional N2 adsorption due to the swing effect. However, as expected, the two peaks at 0.9 and 1.2 nm were not present in ZIF-8W_S. Also, the adsorption behaviors of ZIF-8C and ZIF-8C_S (Figure 19(b)-(c)) were similar to those in ZIF-8M and ZIF-8M_S. Only ZIF-8W was converted to a new dense phase, and the formed ZIF-8W did not show stepwise absorption after the CO2 cycloaddition reaction. These results indicate that the structural deformation of ZIF-8W to an intermediate phase before the DP phase is related to the generation of active sites through the dissociation of Zn-IM linkages. Also, after the reaction, the fact that no additional micropores and mesopores were formed in any of the three types of ZIF-8 (Figure 18(d), Figure 19(c), and Figure 20) means that the structural decomposition started with the dissociation of Zn-N bonds inside the ZIF-8 structure.

[0099] In addition to the physical pore characteristics, we also refer to the FT-IR analysis of ZIF-8W, ZIF-8W_S, and DP to investigate the chemical property discrepancies (Figs. 18(e) and 21(a)). The FT-IR spectra of 2-methylimidazole (2-mim) are also shown in Fig. 18(e). For comparison, all the FT-IR spectra shown in Fig. 18(e) are normalized by the weight of the samples. Also, the FT-IR shown in Fig. 21, which is collected in ATR mode, is normalized to the peak intensity of the out-of-plane bending vibration of the imidazole ring indicated by the inverted triangle shown at the reference peak (see drawing tag, 684 cm -1 ), since the imidazole ring has no relation to the dissociation of the Zn-N bond. This is because after the CO2 cycloaddition reaction, the intensity of the C=N stretching peak (shown by the inverted triangle at 1578 cm in Fig. 18(e)) increases, while the C-N stretching peak (shown by the inverted triangle at 1180 cm in Fig. 18(e)) and the Zn-N bond peak (shown by the inverted triangle at 420 cm in Fig. 18(e)) -1 decrease. -1 -1 ​It is noted that the intensity of the inverted triangle display decreased. In contrast to ZIF-8W, the FT-IR spectra of ZIF-8M and ZIF-8C did not change after the CO2 cycloaddition reaction (Figs. 21(b)-(c)). Although the possibility of an increased peak of C=N stretching emerging from DP particles in ZIF-8W_S cannot be completely excluded, the FT-IR spectra of ZIF-8W_S and DP overlapping in Fig. 18(e) suggest that the increased C=N stretching peak mainly originated from the formation of additional C=N bonds at the expense of the dissociation of Zn-N bonds in ZIF-8W. Otherwise, a prominent broad bump corresponding to DP (indicated by the arrow with a tail in Fig. 18(e)) should be observed in ZIF-8W_S. Therefore, FT-IR analysis indicates that during the reaction time, N species became pyridine-based N containing C=N bonds through Zn-IM linkages. Although the decrease in the FT-IR Zn-N bond peak was not significantly reduced for ZIF-8W_S (Fig. 18(e)), the peak definitely increased due to the C=N bond, which explains that the Zn-N bond was broken. Moreover, such a trend was completely consistent with the XRD and N2 physisorption results that appeared when the ZIF-8 structure was damaged in ZIF-8W_S.

[0100] After the reaction, the appearance of an additional broad peak at approximately 3400 cm -1 for ZIF-8W_S in Fig. 21 is considered to be due to N-H and O-H groups, indicating the formation of pyrrole-based N species. In particular, pyrrole-based N species are mainly formed after the dissociation of Zn-N bonds and subsequent protonation, suggesting a certain association with some proton species. Considering that the conversion of ZIF-8W to the dense phase is caused by the dissociation of Zn-N bonds in the presence of water, the additional protons that generated pyrrole-based N species are closely related to the occluded water molecules. For pyrrole-based N species, the protons seem to start from the water molecules occluded inside ZIF-8W. Therefore, other hydroxyl groups clearly generated from water molecules are expected to be found in the FT-IR spectrum of ZIF-8W_S. From 4000 to 2000 cm -1The broad FT-IR spectrum revealed the presence of additional hydroxyl groups in ZIF-8W_S (Figure 18(f)), and these were present along with the peaks corresponding to N-H bonds. For a rigorous analysis, the FT-IR spectra of heat-treated ZIF-8W and ZIF-8W_S were compared in Figure 18(f). In particular, heat-treated ZIF-8W_S still contained hydroxyl groups that could not be removed by heat treatment. Through a comprehensive characterization of fresh and used ZIF-8x (x = W, M, and C), it was concluded that during the CO2 cycloaddition reaction, the only structural deformation of ZIF-8W occurred from the dissociation of the Zn-N bond by water molecules trapped within the ZIF-8W pore structure. Also, the dissociation of the Zn-N bond induced the simultaneous formation of Zn or N species, which brought about the catalytic activity of ZIF-8W during the process of being structurally deformed into DP.

[0101] 5. Chemical species on the external surface of ZIF-8W Figure 22 shows the XPS results for O 1s of (a) ZIF-8M, ZIF-8M_S, and DP, and (b) ZIF-8C, ZIF-8C_S, and DP.

[0102] Figure 23 investigated the elemental composition on the surface of each sample. For ZIF-8x and ZIF-8x_S (x = W, M, and C), the compositions of the external surface for (a1) nitrogen, (a2) zinc, and (a3) oxygen are shown. In (a1)-(a3), for comparison, the results for DP are added. For ZIF-8x, ZIF-8x-S, and DP, the compositions of the external surface for nitrogen, zinc, and oxygen were obtained by deconvoluting the XPS spectra for N 1s, Zn 2p 3 / 2 and O 1s, respectively.

[0103] Acid-base sites can exist on the external surface of ZIF-8 or at their defect sites. Catalytic reactions such as the cycloaddition reaction of CO2 and transesterification with respect to ZIF-8 or ZIF-67 are promoted at the active sites on the external surface of the catalyst (not in their micropores). The prominent catalytic activity of ZIF-8W contributes to the generation of new Zn and N species by the dissociation of Zn-N bonds, but the actual active component responsible for the catalysis of the CO2 cycloaddition reaction is unknown. As mentioned above, the CO2 cycloaddition reaction with respect to ECH should mainly occur preferentially on the external surface of ZIF-8, and research on the chemical species on the external surface of ZIF-8 by XPS analysis is required. In particular, the resulting properties should be understood as complementary and consistent with the physicochemical properties of the various characterizations described above. First, to confirm the active sites of ZIF-8W for the CO2 cycloaddition reaction, we utilized the XPS analysis of ZIF-8W and ZIF-8W_S to refer to the chemical species on the external surface (Figure 4). In particular, we focused on tracking the XPS spectra of N 1s, O 1s, and Zn 2p. For comparison, XPS analyses of DP and two other fresh and used ZIF-8s (ZIF-8x and ZIF-8x_S; x = M and C) were performed (Figures 4, 5, and 22). In particular, the excellent deconvolution of the XPS spectra of ZIF-8W, ZIF-8W_S, and DP, which are supported by almost zero residual values, shows the quantitative ratios of the chemical species associated with Zn, N, and O atoms. The corresponding fit parameters and standard deviations are summarized in Table 4 below. The chemical compositions of the external surfaces of ZIF-8x, ZIF-8x_S (x = W, M, and C), and DP are summarized in Figures 23(a1)-(a3).

[0104]

Table 4-1

Table 4-2

[0105] In Fig. 23(a1), the fine peak deconvolution revealed for ZIF-8W shows that the ratios of pyrrolic N and pyridinic N species increased after the reaction, while the ratio of saturated N species decreased. This trend also agreed with the FT-IR spectra of ZIF-8W and ZIF-8W_S (Figs. 18(e) and 21(a)). Such complementary features indicate a significant dissociation of the Zn-N bonds that occur only in ZIF-8W, which is related to the change in the ratio of N species after the CO2 cycloaddition reaction. Therefore, the Zn species distributions of ZIF-8W and ZIF-8W_S also differed. They contain three types of Zn species (metallic zinc, zinc linked to nitrogen, and zinc oxide), but ZIF-8W_S has another minor Zn species with a higher binding energy (~1022.4 eV) (Figs. 4(b2) and 23(a2)). In Fig. 4(b2), the deconvolution of the main peak shows the ratio of Zn species corresponding to N-Zn-N decreased after the reaction, while those of zinc oxide and metallic zinc were maintained almost constant (Fig. 23(a2)). Therefore, it is reasonable to consider that the newly formed Zn species corresponding to the ~1022.4 eV peak are derived from the dissociation of the Zn-N bonds in ZIF-8W during the reaction. In this part, we speculated that the coexistence of metallic Zn and ZnO species on the external surface could contribute to the decrease in the ratio of ZnO when forming a vacuum state during XPS measurement. This speculation started from the presence of a small amount of metallic Zn species in the XPS and XAES spectra of ZnO single crystals (99.99%).

[0106] FT-IR analysis revealed that there were some hydroxyl groups in ZIF-8W_S (Figure 18(f)). In addition to N and Zn species, we further investigated the presence of O atoms by XPS analysis. In ZIF-8W_S, O species existed in the states of zinc oxide (~530.8 eV) and hydroxyl groups (~531.9 eV). The presence of hydroxyl groups in the XPS spectrum of O 1s was consistent with Zn species linked to hydroxyl groups (Figures 4(b1)-(b2), 4(c1)-(c2) and 23(a2)-(a3)), which pointed to an additional ~1022.4 eV peak in the Zn 2p XPS spectrum of ZIF-8W_S described above. In XPS analysis, the binding energy of chemical species in the same oxidation state is related to the electronegativity of other bonding atoms. Therefore, Zn species bonded to atoms with higher electronegativity have higher binding energies (i.e., N-Zn-N, ZnO and Zn(OH)2 have binding energies of ~1021.1 eV, ~1022.1 eV and ~1022.7 eV, respectively), and the newly formed Zn species with an XPS peak of ~1022.4 eV can correspond to N-Zn-OH.

[0107] From the FT-IR and XPS analyses of ZIF-8W and ZIF-8W_S, we propose the phenomena that can occur in ZIF-8W during the CO2 cycloaddition reaction process (Figure 1). The Zn-N bond in ZIF-8W is mostly dissociated by water molecules trapped inside the ZIF-8W structure. This results in the formation of dissociated N and Zn species. The dissociated N species can become pyrrole-based and pyridine-based, while the dissociated Zn species can exist in the form of N-Zn-OH. Since no additional micropores / mesopores are formed after the reaction, such species should mainly exist on the external surface (Figures 18(d), 19(c) and 20). In particular, the N-Zn-OH species are consistent with the species that can exist by the hydrolysis of ZIF-8. Consequently, the unique catalytic activity of ZIF-8W is manifested by being derived from the dissociated N or Zn species. In this part, we assume that the water molecules trapped inside ZIF-8W are in an unstable state.

[0108] 6. Role of water molecules in the formation of active sites in ZIF-8W Figure 24 is a diagram for examining how ZIF-8W and water molecules form active sites. For used ZIF-8W (ZIF-8W_S) and used ZIF-8W_T24, respectively, (a) XRD patterns, (b) XPS spectra of O 1s, (c1) N2 adsorption–desorption isotherms, and (c2) micropore size distributions in the size range of 0.4–2.0 nm are shown. Here, for ZIF-8W_T24, the same amount of water molecules as the water molecules trapped inside ZIF-8W (8 wt%) was additionally added to the reaction (denoted as ZIF-8W_T24_S_H). For comparison, the analysis results of ZIF-8W and DP are added in (a)–(d). The inverted triangle in (a) indicates α-alumina as an internal standard.

[0109] Figure 25 shows XPS spectra for (a) N 1s and (b) Zn 2p and the composition of the external surface in ZIF-8W, ZIF-8W_S, ZIF-8W_T24_S_H, and DP.

[0110] The completely dried ZIF-8W_T24 did not show remarkable catalytic activity in the CO2 cycloaddition reaction in the presence of water (water supplied up to 40 wt%) (Fig. 14(c)). To understand the role of water blocked by the unique catalytic activity of ZIF-8W, we investigated the physicochemical properties of ZIF-8W (containing 8 wt% water inside) and ZIF-8W_T24 (having 8 wt% water outside) after the CO2 cycloaddition reaction. For convenience, the used ZIF-8W_T24 under the reaction conditions containing water (8 wt% water) is denoted as ZIF-8W_T24_S_H, where the letter H is added to indicate the high humidity reaction conditions. In Fig. 24(a), the XRD patterns of ZIF-8W_S and ZIF-8W_T24_S_H show that the conversion from ZIF-8 to DP is inevitable when CO2 and water are supplied to the reaction. However, the degree of conversion varies depending on the position of the water molecules. Assuming that the same amount of water is applied to the CO2 cycloaddition reaction containing ZIF-8W and ZIF-8W_T24, during the conversion from the ZIF-8 structure to DP, the generation of active sites was insensitive to the water molecules present outside ZIF-8.

[0111] Therefore, we showed that Zn and N species dissociated in ZIF-8W during the reaction, which is related to the catalytic activity and involves the conversion from ZIF-8 to DP. The conversion from ZIF-8 to DP was also observed for ZIF-8W_T24 after the reaction (Figure 24(a)), but ZIF-8W_T24 did not show remarkable catalytic activity (Figure 14(c)). Therefore, to understand the effect of water on the conversion of the ZIF-8 structure, we characterized the external surfaces of ZIF-8W_S and ZIF-8W_T24_S_H. The N 1s and Zn 2p XPS results showed that the external surface composition of ZIF-8W_T24_S_H was similar to that of DP, but quite different from that of ZIF-8W or ZIF-8W_S (Figure 25). In addition, the XPS results of O 1s showed that ZIF-8W_T24_S_H had additional O species at ~533.4 eV, which was consistent with DP (Figure 24(b)). Therefore, according to the XPS analysis, despite the lower degree of ZIF-8 structure conversion from ZIF-8W_T_S_H, the external surface of ZIF-8W_T_S_H was similar to that of DP (Figures 24(a)-(b) and Figure 25). More importantly, the Zn species linked to the hydroxyl groups shown in ZIF-8W_S were not present in ZIF-8W_T24_S_H (Figure 25(b)). Therefore, if ZIF-8W_T24_S_H has an external surface composition similar to that of DP, it is considered that the deformation of ZIF-8W_T24 started and proceeded on the external surface (Figures 24(a)-(b)).

[0112] Also, as shown in Figs. 24(c1)-(c3) and (d), the comparison of the N2 physisorption and NH3 TPD results of ZIF-8W_S and ZIF-8W_T24_S_H corroborates the results obtained from XPS analysis. The N2 adsorption-desorption isotherms show that, despite the deformation of the ZIF-8 structure, ZIF-8W_T24_S_H, in contrast to ZIF-8W_S, retains all of the adsorption capacity and the unique swing-effect associated with the adsorption behavior of ZIF-8 (Fig. 24(c1)). As expected, Fig. 24(c2) shows that the micropore volume distributions of ZIF-8W, ZIF-8W_S, and ZIF-8W_T24_S_H retain their inherent micropore structures despite the deformation of ZIF-8W_T24 to DP after reaction. The deformation of the ZIF-8 structure from ZIF-8W_T24_S_H is limited to the external surface. The NH3 TPD results show that ZIF-8W_T24_S_H deviates from the pattern of ZIF-8W_S and has a desorption pattern similar to that of DP (Fig. 24(d)). Considering that DP showed no catalytic activity in the CO2 cycloaddition reaction, the formation of a similar DP phase on the external surface of ZIF-8W_T24_S_H did not produce active components suitable for the CO2 cycloaddition reaction. Thus, the prominent discrepancy between ZIF-8W_S and ZIF-8W_T24_S_H may be due to the unstable state of the occluded water molecules. Under the reaction conditions, such water molecules tend to exit the ZIF-8 pore structure but do not have sufficient energy to escape (note that the synthesized ZIF-8W was dried at 70 °C for at least 12 h but still contains water (8 wt%) inside the molecule). Instead, at a reaction temperature of 70 °C, the hydrolysis of the Zn-N bond appears to produce active components, such as those in the aforementioned ZIF-8 intermediate derivatives and on the external surface. However, if there is too much structural conversion to DP, having such derivatives or phases is not tolerated. In the case of ZIF-8W_T24_S_H, hydrolysis through water molecules does not appear to be activated at the reaction temperature.

[0113] 7. Confirmation of the Active Sites of ZIF-8W in the Cycloaddition Reaction Using a Catalyst Figure 26 shows the TPD (Temperature Programmed Desorption) profiles for each sample. For ZIF-8x and ZIF-8x_S (x = W, M, and C), the TPD profiles of (a) CO2 and (b) NH3 are shown. In (a) and (b), the arrow with a tail indicates the additional peak that appears in ZIF-8W_S.

[0114] Figure 27 shows the results of various experiments to confirm the active sites in ZIF-8W. (a) shows the TPD profiles of NH3 for ZIF-8W, ZIF-8W_S, and DP. (b) shows the FT-IR spectra of ZIF-8W, ZIF-8W_S, and DP before and after NH3 adsorption (NH3 is tagged in the graph). (c) shows the FT-IR spectra of NH3-adsorbed ZIF-8W_S at the respective tagged desorption temperatures (50, 100, and 150 °C), and (d) shows the expanded FT-IR spectra in the range from 1800 cm -1 to 1500 cm -1 For (b)-(d), for the FT-IR measurement, after performing NH3 adsorption at 30 °C for 1 hour, the NH3-adsorbed samples were heated at different temperatures (50 °C in (b), 50, 100, and 150 °C in (c)-(d)). All FT-IR spectra were normalized to the peak at 684 cm -1 corresponding to the bending of the imidazole ring (shown as an inverted triangle in (b)). Detailed information on the deconvolution curve in (d) is shown in Table 5 and Figure 29.

[0115] Figure 28 shows the FT-IR spectra before and after NH3 adsorption. (a) shows the FT-IR spectra of ZIF-8M_S and ZIF-8C_S before and after adsorption (NH3 is tagged in the sample label). (b) shows the expanded FT-IR spectra of ZIF-8M_S and ZIF-8C_S before and after NH3 adsorption in the range of 1800 - 1500 cm -1 For (c), it shows the expanded FT-IR spectra of ZIF-8M_S and ZIF-8C_S before and after NH3 adsorption in the range of 1800 - 1500 cm -1It shows the expanded FT-IR spectra of DP before and after NH3 adsorption within the range. For the FT-IR measurement, after executing NH3 adsorption at 30 °C for 1 hour, the NH3-adsorbed ZIF-8M_S, ZIF-8C_S, and DP were heated to the set temperature in a He flow. Here, (a)-(b) show ZIF-8M_S and ZIF-8C_S respectively, heated up to 50 °C, and (c) shows DP, heated up to 50, 100, and 150 °C. The desorption temperature is shown at the end of each sample.

[0116] Figure 29 shows information regarding the deconvolution curves. Along the deconvolved curves at different temperatures (50, 100, 150 °C) respectively, the FT-IR spectra of (a) ZIF-8W_S and (b)-(d) NH3-adsorbed ZIF-8W_S are shown, which are the same as those shown in Figure 27(d). For the deconvolution, the region indicated by the dotted line (1540~1700 cm -1 up to) was considered. For the demonstration of an appropriate fit, the residue plot where the experimentally obtained FT-IR spectrum is shown below. In the case of the residue plot, the normalized residue (R N ) was calculated by subtracting the intensity of the experimentally measured FT-IR spectrum from the intensity summed from the deconvolved curve and further dividing by the intensity of the experimentally measured FT-IR spectrum.

[0117]

Table 5-1

Table 5-2

[0118] The TPD results of NH3 showed that only ZIF-8W_S had an additional desorption peak at 225 °C after the CO2 cycloaddition reaction (Figs. 27(a) and 26(b)). Also, the amount of NH3 desorbed at all desorption peaks was more than that of ZIF-8W_S. In particular, the amount of NH3 desorbed by ZIF-8W_S was almost four times more than that of ZIF-8W. The NH3 molecules desorbed by ZIF-8W_S can titrate the acidic sites that may arise from the dissociation of Zn-N bonds, although these can also occur by physical adsorption. At low temperatures (<100 °C), the amount of NH3 desorbed by ZIF-8W_S was much more than that of ZIF-8W, and such an increase was also shown in the DP. The increase in desorption of ZIF-8W_S at low temperatures is related to the formation of DP as the final structure instead of forming the desired acidic sites in the CO2 cycloaddition reaction. The additional peak at 225 °C (indicated by an arrow in Fig. 26(b)) is not clearly understood, but this may be due to the acidic sites derived from the dissociation of Zn-N bonds during the CO2 cycloaddition reaction. ZIF-8M and ZIF-8C (Figs. 21, 22 and 5), which preserve the original Zn-N bonds after the CO2 cycloaddition reaction, did not show additional peaks in their used forms (Fig. 26(b)).

[0119] The CO2 cycloaddition reaction occurs at Lewis acid sites, not Bronsted acidic sites. Since the TPD results of NH3 only provide information on all acidic sites, it is necessary to investigate the acid properties of ZIF-8W_S. Therefore, to distinguish the acidic sites of ZIF-8W_S, after the adsorption of NH3, FT-IR measurements of DP, ZIF-8W, and ZIF-8W_S were carried out, which were interpreted using the TPD curve of NH3 at a high temperature (>100 °C) (Figure 27(a)). The Zn-OH group can act as a Bronsted acidic site, and the corresponding NH3 adsorption peak is known to appear at about 200 - 300 °C. Since the Zn-OH group is generated by the dissociation of Zn-N during the CO2 cycloaddition reaction (Figure 4(b1)-(b2)), the additional peak in the TPD result of NH3 for ZIF-8W_S is due to the Bronsted acidic site. The FT-IR results of NH3 adsorption shown in Figure 27(b) and Figure 28(a) indicate that only ZIF-8W_S has an additional FT-IR peak, which corresponds to the N-H peak of ammonium ions that are Bronsted acidic sites (indicated by the arrow tagged with ammonium ions in Figure 27, ~2850 cm -1 ). The desorption of NH3 from ZIF-8W_S was carried out at different temperatures (50, 100, and 150 °C), and the corresponding FT-IR spectra show that the FT-IR peak intensity of N-H stretching (~2850 cm -1 ) is similar regardless of the desorption temperature (Figure 27(c)). Therefore, considering that the TPD curve of NH3 for ZIF-8W_S has four main peaks around 60, 95, 140, and 225 °C (Figure 27(a)), the desorption peak at 150 °C is related to the Bronsted acidic site and is clearly due to the formation of the Zn-OH group. The intensity of the broad peak in the FT-IR spectrum in the range of 1650 cm -1 ~1600 cm -1 increased after the adsorption of NH3. The FT-IR peaks at 1645 cm -1 and 1600 cm -1 are NH4 +It corresponds to the symmetric and NH3 asymmetric bending modes. The low-coordinated Zn species act as Lewis acids, and ZIF-8W_S contains Lewis acidic sites for the Zn species. Based on such facts, the FT-IR spectra of ZIF-8W_S and NH3-adsorbed ZIF-8W_S (desorbed at 50, 100, and 150 °C) are NH4 + In order to distinguish the NH and NH3 bending modes, a Gaussian distribution was utilized for deconvolution (Figure 27(d)). The deconvolution results show NH4 that indicates Bronsted acidic sites + symmetric bending mode (~1645 cm -1 ) was shown to be similar regardless of the desorption temperature. This result was in good agreement with the trend in the N-H stretching peak. In contrast to the NH4 + bending peak, the NH3 asymmetric bending mode (~1600 cm -1 ) indicating Lewis acidic sites changed gradually corresponding to the desorption temperature. The intensity of the NH3 bending peak decreased when the desorption of NH3 was carried out at 150 °C in ZIF-8W_S, while the peak intensities at 50 °C and 100 °C were similar (Figure 27(d)). According to XPS analysis, the effective deconvolution of FT-IR shown in Figure 29 provides trends regarding the NH4 + and NH3 bending modes. Therefore, the complementary NH3 TPD and NH3-adsorbed FT-IR analysis results indicate that at 225 °C, the TPD desorption of NH3 originated from Bronsted acidic sites, while in the case of 150 °C, it originated from Lewis acidic sites. In summary, the formation of low-coordinated Zn species is clearly linked to OH groups, which are provided as Lewis acids during the reaction and are accompanied by structural changes in ZIF-8 during the CO2 cycloaddition reaction.

[0120] In contrast to ZIF-8W_S, the FT-IR spectra of NH3-adsorbed DP showed that no acidic sites due to NH3 were found in the DP particles regardless of the desorption temperature (Fig. 28(c)), although there were TPD desorption peaks of NH3. Moreover, ZIF-8M_S and ZIF-8C_S, which experienced some desorption according to the TPD results of NH3, did not show NH3 absorption peaks (Figs. 28(a)-(b)). From the TPD results of NH3 for ZIF-8W_S and DP, the NH3 molecules desorbed at 100 °C were related to the structural changes of ZIF-8 accompanying the CO2 cycloaddition reaction (related to physically adsorbed NH3 molecules) and were clearly not related to acidic sites.

[0121] Those with ordinary knowledge in the technical field to which the present invention pertains should understand that the present invention can be implemented in other specific forms without changing its technical idea and essential features. Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive. The scope of the present invention is indicated by the claims rather than the above detailed description, and all changes or modified forms derived from the meaning and scope of the claims and their equivalent concepts should be construed as being included in the scope of the present invention.

Claims

1. In an organometallic framework, a ZIF-8 organometallic framework; and water molecules provided inside the ZIF-8 organometallic framework; A ZIF-8 organometallic framework catalyst complex comprising: The water molecules dissociate at least a part of the bonds of the ZIF-8 organometallic framework to form reaction sites, A ZIF-8 organometallic framework catalyst complex comprising a ZIF-8 organometallic framework with reaction sites formed and a ZIF-8 organometallic framework without reaction sites formed.

2. The reaction sites are A first reaction site containing Zn-OH bonds; and A second reaction site containing N-H bonds; The ZIF-8 organometallic framework catalyst complex according to claim 1, comprising:

3. The formation ratio of the ZIF-8 organometallic framework with reaction sites formed is The ZIF-8 organometallic framework catalyst complex according to claim 1, wherein the ZIF-8 organometallic framework with reaction sites formed / the ZIF-8 organometallic framework without reaction sites formed is formed at a weight ratio of 0.05 to 1.

0.

4. At least one of the first reaction site and the second reaction site promotes the cycloaddition reaction of carbon dioxide and an epoxide compound to form a carbonate, and the ZIF-8 organometallic framework catalyst complex according to claim 2.

5. The epoxide compound is at least one of epichlorohydrin, ethylene oxide, styrene oxide, and propylene oxide, and the ZIF-8 organometallic framework catalyst complex according to claim 4.

6. The higher the content of the water molecules, the higher the yield of the carbonate, and the ZIF-8 organometallic framework catalyst complex according to claim 4.

7. The carbonate is a cyclic carbonate, The ZIF-8 organometallic framework catalyst complex according to claim 4, which is at least one of chloropropene carbonate, ethylene carbonate, styrene carbonate, and propylene carbonate.

8. The yield of the carbonate is represented by the following formula 1; 【Mathematics 1】 The ZIF-8 organometallic framework catalyst complex according to claim 4, calculated by

9. The ZIF-8 organometallic framework catalyst complex according to claim 8, wherein the yield of the carbonate by the cycloaddition reaction is 20% to 99%.

10. The ZIF-8 organometallic framework catalyst complex according to claim 1, wherein the water molecules are contained in a weight ratio of 0.005 to 0.35 with respect to the ZIF-8 organometallic framework catalyst complex.

11. The BET surface area is 1300 m ２ / g to 1600 m ２ / g, and the ZIF-8 organometallic framework catalyst composite according to claim 1.

12. The ZIF-8 organometallic framework catalyst complex according to claim 2, wherein the ZIF-8 organometallic framework having the reaction site formed shows a first peak at a binding energy of 1022.3 eV to 1022.5 eV and a second peak at 531.8 eV to 520 eV for the formation of Zn-OH in X-ray photoelectron spectroscopy (XPS) analysis.

13. In X-ray photoelectron spectroscopy (XPS) analysis, The ratio of the XPS peak area of the ZIF-8 organometallic framework having the reaction site formed to the XPS peak area of the ZIF-8 organometallic framework showing the binding energy of Zn-N is 0.4 to 0.

8. The ZIF-8 organometallic framework catalyst complex according to claim 2.

14. A method for promoting the cycloaddition reaction of carbon dioxide using the ZIF-8 organometallic framework catalyst complex according to any one of claims 1 to 13, Putting a substance containing the ZIF-8 organometallic framework catalyst complex and an epoxy compound into an autoclave; Supplying carbon dioxide into the autoclave; Heating the inside of the autoclave to react the ZIF-8 organometallic framework catalyst complex, the epoxy compound, and carbon dioxide to obtain a carbonate; and Quenching the carbonate; Including, A method for promoting the cycloaddition reaction of carbon dioxide using a ZIF-8 organometallic framework catalyst complex.

15. The method for promoting the cycloaddition reaction of carbon dioxide using the ZIF-8 organometallic framework catalyst complex according to claim 14, wherein the inside of the autoclave is heated to 40°C to 200°C.

16. The method for promoting the cycloaddition reaction of carbon dioxide using the ZIF-8 organometallic framework catalyst complex according to claim 14, wherein the carbon dioxide is supplied until the pressure reaches 1 bar to 30 bar.

17. The yield of the carbonate is given by the following formula 2; 【Number 2】 A method for promoting the cycloaddition reaction of carbon dioxide using the ZIF-8 organometallic framework catalyst complex according to claim 14, calculated by

18. The method for promoting the cycloaddition reaction of carbon dioxide using the ZIF-8 organometallic framework catalyst complex according to claim 17, wherein the yield of the carbonate is 20% to 99%.

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