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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Mimic Carbonic Anhydrase Using Metal−Organic Frameworks for CO2 Capture and Conversion Chaonan Jin,⊥,† Sainan Zhang,⊥,† Zhenjie Zhang,*,‡,§ and Yao Chen*,†,∥ †

State Key Laboratory of Medicinal Chemical Biology, Nankai University, Tianjin 300071, People’s Republic of China Key Laboratory of Functional Polymer Materials, Ministry of Education, Nankai University, Tianjin 300071, People’s Republic of China § College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China ∥ College of Pharmacy, Nankai University, Tianjin 300071, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Carbonic anhydrase (CA) is a zinc-containing metalloprotein, in which the Zn active center plays the key role to transform CO2 into carbonate. Inspired by nature, herein we used metal−organic frameworks (MOFs) to mimic CA for CO2 conversion, on the basis of the structural similarity between the Zn coordination in MOFs and CA active center. The biomimetic activity of MOFs was investigated by detecting the hydrolysis of para-nitrophenyl acetate, which is a model reaction used to evaluate CA activity. The biomimetic materials (e.g., CFA-1) showed good catalytic activity, and excellent reusability, and solvent and thermal stability, which is very important for practical applications. In addition, ZIF-100 and CFA-1 were used to mimic CA to convert CO2 gas, and exhibited good efficiency on CO2 conversion compared with those of other porous materials (e.g., MCM41, active carbon). This biomimetic study revealed a novel CO2 treatment method. Instead of simply using MOFs to absorb CO2, ZIF-100 and CFA-1 were used to mimic CA for in situ CO2 conversion, which provides a new prospect in the biological and industrial applications of MOFs.

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INTRODUCTION The increase of greenhouse gases such as carbon dioxide (CO2) in the atmosphere causes serious climate problems. The release of CO2 by anthropogenic activity may lead to a rise in global temperature over the past several hundred years.1 Hence, effective methods to capture CO2 and mitigate CO2 emissions are urgently demanded. Several strategies have been attempted to reduce CO2, including physical adsorption,2 and chemical sequestration of CO2.3 However, many challenges still remain. For example, CO2 adsorption or sequestration usually requires high cost input for equipment regeneration, and the commercial techniques possess the limitations of high costs and secondary pollution. Therefore, in situ conversion of the captured CO2 into useful product could be the most effective method for CO2 treatment.4−6 In nature, CO2 can be hydrated by carbonic anhydrase (CA),7−9which is a metalloenzyme with a Zn active site (Figure 1). The active site of CA consists of Zn centers with distorted Td geometry coordinated by three imidazole nitrogens from three different histidines and one oxygen from either an aqua water or hydroxyl group depending on pH. A water molecule plays an important role in the CO2 hydration process by supplying an OH− unit to furnish the bicarbonate ion (HCO3−) from CO2 (Scheme 1).10 Unfortunately, the high cost, thermal and chemical instability, sensitivity to environment, and fragile nature of © XXXX American Chemical Society

Figure 1. Active site of carbonic anhydrase.

CA hinder it from wide application.12,13 Nature inspires us in that the biomimetic catalysts that imitate the active site of CA are likely sufficiently stable to surmount these problems.14−18 As a new type of functional porous materials, metal−organic frameworks (MOFs) are highly crystalline inorganic−organic hybrids that are constructed by assembly of metal ions or metal-containing clusters with organic ligands via coordination bonds.19−21 The structural versatility, high surface area, and tunable porosity endow MOFs with great potential in plenty of fields such as gas separation,22,23 gas storage,24−27 drug delivery,28,29 catalysis,30−33 sensors,34−37 etc. In addition, Received: December 2, 2017

A

DOI: 10.1021/acs.inorgchem.7b03021 Inorg. Chem. XXXX, XXX, XXX−XXX

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there is a large cage of a diameter around 35.6 Å and cage windows of ∼4 Å in ZIF-100. CFA-1’s secondary building block (SBU) possesses five Zn atoms.37,44 The central Zn is octahedrally coordinated with six triazole nitrogens, and four peripheral Zn2+ ions are five-coordinated with three triazole nitrogens and two terminal acetate oxygens (Figure 3 and Figure S2). Because the terminal acetate ligands could be replaced by hydroxyl ligand via solvent exchange, CFA-1 can be used to mimic the active site of CA, and demonstrated excellent catalytic activity. In addition, Eu−Zn is isostructural to Gd−Zn, in which there are four crystallographically independent Zn(II) ions (Figure S2).45 Among them, there is a three-coordinated Zn with a distorted trigonal geometry occupied by three nitrogen atoms from three distinctive ligands. Thus, Eu−Zn and Gd−Zn are potential options to mimic CA as well. Our results revealed that MOFs can not only capture and store CO2, but also be used as catalysts to hydrolyze CO2 with high efficiency by imitating the features of the structural and catalytic properties of CA. In addition, these biomimetic catalysts possess enhanced stability and ease of preparation and recyclability. This study paves the way for highly efficient CO2 treatment and conversion through biomimetic catalysis.

Scheme 1. Mechanism of CO2 Hydrolysis Reaction Catalyzed by CA11

MOFs can be designed to create a similar environment (such as metal coordination, hydrophobic pocket, etc.) as the metalloenzymes’ catalytic active site. Therefore, its potential as advanced biomimetic materials has attracted increasing attention.38−40 The well-defined shape and chemical environments of the cavities or channels of MOFs offer a suitable environment akin to that of enzymes, which will improve the catalytic performance (such as stability and reusability) and demonstrate selectivities that cannot be expected in enzymes. However, the biomimetic studies of MOFs are still in its infancy compared to other applications of MOFs.27,41,42 Herein, inspired by the CO2 transformation property of CA, we selected and prepared a series of MOFs (ZIF-100, CFA-1, Eu−Zn, and Gd−Zn, etc.) that possess a suitable chemical environment similar to the active site of CA to mimic its biocatalysis (Figure 2). Some of the materials indicated

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EXPERIMENTAL SECTION

Materials and Methods. Required chemicals, including zinc(II) trifluorome-thanesulfonate (Zn(O3SCF3)2, 99%), 5-chlorobenzimidazole (C7H5ClN2, 97%), 3,3′-diaminobenzidine ((NH2)2C6H3C6H3(NH2)2, 97%), zinc acetate dihydrate (Zn(oAc)2· 2(H2O), 99.99%), gadolinium(III) nitrate hexahydrate (Ga(NO3)3· 6H2O, 99.99%), were obtained from commercial sources. ZIF-100, CFA-1, Eu−Zn, and Gd−Zn were synthesized according to the literature method.43−45 PXRD was used to determine the final structure of the synthesized material. Activation of MOFs. ZIF-100, CFA-1, Eu−Zn, and Gd−Zn were activated before being used as catalysts. The as-synthesized ZIF-100 sample was immersed into anhydrous methanol for 3 days. During the exchange process, anhydrous methanol was refreshed for 3 days (methanol was refreshed 3 times per day). The solvent-exchanged sample was then centrifuged and dried at 50 °C under vacuum for 10 h and then at 100 °C for 12 h. CFA-1 was immersed in HEPES buffer (50 mM, pH = 8) for 3 days (buffer was refreshed 3 times per day). The solvent-exchanged sample was then centrifuged and dried at 250 °C under vacuum for 20 h to remove solvent molecules. Eu−Zn and Gd−Zn were immersed in HEPES buffer (50 mM, pH = 8) for 3 days (buffer was refreshed 3 times per day). The solvent-exchanged sample was then centrifuged and dried at 120 °C under vacuum for 10 h to remove solvent molecules. Biomimetic Catalysis Study. The catalytic reactions (Scheme 2) were conducted at room temperature and monitored by detecting the

Figure 2. Structures of CFA-1 (left) and ZIF-100 (right).

excellent catalytic activity, stability, and reusability. ZIF-100 (Figure 2 right) was first reported by Yaghiet al.43 In its unit cell, there are 10 independent Zn centers. Among them, nine Zn centers are connected to four nitrogens from four different 5-chlorobenzimidazolate (cbIM) linkers and the remaining one Zn center possess a coordination environment similar to that of the Zn center in CA, which is connected to three nitrogens and one oxygen from a hydroxyl group (Figure 3). In addition,

Scheme 2. Reaction of p-NPA Conversion

para-nitrophenol (p-NP), which is the hydrolysis product of paranitrophenyl acetate (p-NPA). In order to determine the concentration of p-NP, the calibration curve (402 nm) was drawn by scanning various known concentrations (5−90 μM) of product (Figure S12) by UV−vis (ultraviolet spectrophotometer, UH5300).46 To mimic the biocatalysis of CA, the reactions catalyzed by various MOFs were conducted under the following conditions: p-NPA was first dissolved in 2.5 mL of acetonitrile to form a 10 mM solution, and then added into 47.5 mL HEPES buffer (50 mM, pH = 8) together with a certain amount of MOFs (the final molar % of MOFs catalyst is

Figure 3. Coordination environments of Zn active centers in CA (left), CFA-1 (middle), and ZIF-100 (right). Active Zn sites are in cyan with stick−ball mode. B

DOI: 10.1021/acs.inorgchem.7b03021 Inorg. Chem. XXXX, XXX, XXX−XXX

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5). However, as a control, the metal ions (Zn2+) and the ligands of MOFs demonstrated no catalytic activity (Figure S8). In addition, Eu−Zn exhibited very low catalytic activity toward pNPA, and Gd−Zn demonstrated almost no activity. The results revealed that CFA-1 and ZIF-100 can effectively catalyze the hydrolysis of p-NPA (Scheme 2), which is the model reaction used to evaluate CA activity (Figure 5, left). The catalytic reaction rates of CFA-1 and ZIF-100 are 2.61 and 2.14 μM/min, respectively, while the rates of Eu−Zn and Gd− Zn are 0.75 and 0.53 μM/min, respectively, which are quite close to the self-decomposition of substrate which is 0.28 μM/ min. In addition, the 2 h conversions of p-NPA for CFA-1 and ZIF-100 are 41.8% and 19.0%, respectively, while all the other materials demonstrated no significant conversion of the substrate close to blank reaction (Figure 5, right). The kinetic data and the conversion results revealed that CFA-1 and ZIF100 have good biomimetic catalytic activity, while Gd−Zn and Eu−Zn demonstrated no activity. On the basis of the structural analysis of these materials, the pore sizes of CFA-1 and ZIF-100 are the largest among all the tested materials: the cage of ZIF10043 is 35.6 Å with a window of ∼4 Å; CFA-1 has two channels with the sizes of 6.2 and 3.4 Å, respectively.44 The other two MOF materials (Gd−Zn and Eu−Zn) possess pore sizes that are even smaller (∼3 Å), which is not sufficient for the ingression of the substrate (p-NPA molecular size: 4.4 × 1.9 × 10.5 Å3). This explained why CFA-1 and ZIF-100 exhibited significantly higher activity than Gd−Zn and Eu−Zn. Other factors may also contribute to their catalytic performance: pNPA is transformed into p-NP during this catalysis, and the hydroxyl group in p-NP increases the hydrophilicity of the molecule. Therefore, the hydrophobic environment of the catalyst (e.g., CFA-1) may accelerate the efflux of the product, which facilitates the reaction. Among all the materials, CFA-1 has the highest catalytic rate as well as conversion rate compared to other materials (Figure 5), which may be due to its highly active center density. CFA-1 has eight active centers in one unit, which may increase the catalytic efficiency. On the other hand, because the cage window size of ZIF-100 is too close to the dimensions of the substrate, at least part of the catalysis happened on the surface of ZIF-100, which decreased its catalytic efficiency compared to that of CFA-1. The low conversion rate observed in ZIF-100s catalysis may also be attributed to its relatively low buffer stability. Moreover, as a representative material, the reusability of CFA-1 was evaluated for up to 5 catalytic cycles. As shown in Figure 6, CFA-1 can be reused for 5 cycles without a significant drop in the reaction rate or substrate conversion. In addition, CFA-1 can retain its structural integrity, as identified by PXRD studies (Figure S9), highlighting its stable feature. In order to evaluate if the biomimetic catalysts can be used under harsh conditions for potential industrial applications, we used CFA-1 as a representative to investigate the influences of temperature, pH, and solvent stability toward catalytic activity. It is reported that CFA-1 can be stable up to 300 °C,44 which provides the possibility to catalyze reactions under high temperature conditions. We also observed that CFA-1 is stable at various pH values (pH = 6.5, 7.0 and 8.0) and solvents (DMF, DMA, DEF, methanol, and NMP) (Figures S10 and S11). These properties can greatly expand the scope of reactions that can be catalyzed by MOFs. MOFs have been demonstrated to have high CO2 storage capacity. However, storage and conversion of CO2 have not

14%). The formation of product was measured at 402 nm using UV− vis. The blank control is conducted under the same conditions in the absence of catalyst, with the aim of eliminating the effect of selfdecomposition of p-NPA.9,47 To test the reusability of the biomimetic catalysts (e.g., CFA-1), we conducted 5 cycles of the catalysis using the same catalytic conditions. Biomimetic Conversion of CO 2. One of the potential applications of mimicking CA is to eliminate CO2 which is an important greenhouse gas. The biomimetic conversion of CO2 is carried out under the following conditions: 50.0 mg of MOFs materials first absorbed CO2 at 1 atm for 2 h, and then MOFs were added into 5.0 mL of HEPES buffer (50 mM, pH = 8) to initiate the biomimetic reaction of CO2 conversion, so that CO2 will convert into HCO3− under the catalysis of MOFs (see Scheme 3). In order to quantify

Scheme 3. Conversion Reactions of CO2

HCO3−, 5 mL of CaCl2 (400 mM, HEPES buffer) was added into the system to form CaCO3 precipitate with HCO3− after 2 h of reaction. The formed CaCO3 was then filtrated, dried, and weighed. In order to illustrate whether the reaction was catalyzed by MOFs or not, control reactions using active carbon and MCM-41 which adsorbed CO2 at 1 atm were conducted under the same reaction conditions.

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RESULTS AND DISCUSSION CFA-1, ZIF-100, Eu−Zn, and Gd−Zn were prepared according to the reported methods43−45 (see Experimental Section and Supporting Information). PXRD was used to identify the crystal structures of the materials (Figures S2−S5) and verify the successful synthesis of the materials. In order to mimic CA, the hydrolysis of p-NPA (Scheme 2) was first carried out as a model reaction to evaluate the biomimetic activities of MOFs.9 The reactions were conducted in HEPES buffer to create the same environment as the enzymatic catalysis, so the buffer stability of the synthesized MOFs was first tested. These results indicated good stability of CFA-1 and moderate stability for ZIF-100, Gd−Zn, and Eu−Zn in buffer (Figures S3−S7). Catalytic activities of MOFs were tested by monitoring the formation of p-NP (UV−vis, 402 nm) at room temperature. CFA-1 and ZIF-100 exhibited high catalytic performance compared to those of the other tested MOFs (Figures 4 and

Figure 4. Kinetic traces of the biomimetic catalysis of p-NPA using different MOFs as catalysts. C

DOI: 10.1021/acs.inorgchem.7b03021 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 5. Biomimetic catalysis of the p-NPA using different MOFs as catalysts: the reaction rate (left) and the 2 h conversion of p-NPA (right).

Figure 6. Reusability of CFA-1: relative reaction rate at different cycles (left) and relative conversion at different cycles (right).

Figure 7. CO2 adsorption isotherms of porous materials at 298 and 273 K: (a) active carbon, (b) MCM-41, (c) ZIF-100, and (d) CFA-1.

CFA-1 had a CO2 uptake of 2.1 mmol/g at 298 K and 3.8 mmol/g at 273 K and 1 atm. ZIF-100 had a CO2 uptake of 0.7 mmol/g at 298 K and 0.9 mmol/g at 273 K and 1 atm. Although active carbon exhibited higher CO2 uptake than CFA-1 and ZIF-100 (Figure 8), the final conversion of CO2 for the active carbon is very low (9.4%), and the final conversion for MCM-41 is even lower (7.5%), which is probably because active carbon and MCM-41 possess no catalytic active sites. By

been combined together yet. To be specific, after CO2 absorption, MOFs can be used as catalysts to convert CO2 into valuable products instead of being discarded. In this study, CO2 sorption data were first collected to quantify the amount of CO2 adsorbed by the materials. As shown in Figure 7, the control materials, active carbon and MCM-41, can absorb CO2 of 2.2 and 0.7 mmol/g at 298 K and 1 atm, respectively, and CO2 of 3.6 and 0.8 mmol/g at 273 K and 1 atm, respectively. D

DOI: 10.1021/acs.inorgchem.7b03021 Inorg. Chem. XXXX, XXX, XXX−XXX

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhenjie Zhang: 0000-0003-2053-3771 Author Contributions ⊥

C.J. and S.Z. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Notes

The authors declare no competing financial interest.

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Figure 8. Final conversion of CO2 using different porous materials as catalysts.

ACKNOWLEDGMENTS The authors acknowledge the Young 1000-Plan program and financial support from the National Natural Science Foundation of China (21601093).

contrast, CFA-1 and ZIF-100 can efficiently catalyze CO2 into HCO3−, and the final conversion of CO2 catalyzed by ZIF-100 is about 62.0%. Meanwhile, CFA-1 can convert as high as 87.0% of CO2. As summarized in Table 1, the turnover number

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Table 1. Turnover Number (TON) of Catalysts material

active site (mmol−1)

N (mmol)

TON (mmol/mmol)

ZIF-100 CFA-1

1 8

0.0282 0.0395

1.56 0.52

(TON) of ZIF-100 is 1.56 mmol/mmol and the TON of CFA1 is 0.52 mmol/mmol. This difference may be due to the high active site density of CFA-1 (eight active centers per unit cell). These results indicated that CFA-1 and ZIF-100 not only can store CO2 but also can mimic CA to efficiently hydrolyze CO2.

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CONCLUSION In summary, we synthesized some Zn-based MOFs (such as CFA-1 and ZIF-100) that possess similar active Zn sites as carbonic anhydrase (CA), and evaluated their biomimetic activity. Our study revealed that CFA-1 and ZIF-100 have similar catalytic performance of CA by detecting the hydrolysis reaction of para-nitrophenyl acetate. In addition, these biomimetic catalysts demonstrated excellent reusability, and solvent/thermal stability. Moreover, for the first time, we combined CO2 storage and conversion in one MOF system. CFA-1 and ZIF-100 not only can store CO2, but also can mimic CA to efficiently hydrolyze CO2. These attempts point out a new approach to convert waste gases into valuable products in situ. Furthermore, it is possible to prepare columns with biomimetic MOFs to continuously convert CO2 into HCO3− via flow reactions. Further studies to convert CO2 into high value-added, industrial-demand chemicals are under investigation in our laboratory.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03021. Additional PXRD patterns, crystal structures, SEM images, catalytic data (PDF) E

DOI: 10.1021/acs.inorgchem.7b03021 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b03021 Inorg. Chem. XXXX, XXX, XXX−XXX