Unusual Moisture-enhanced CO2 Capture within Microporous PCN

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Surfaces, Interfaces, and Applications

Unusual Moisture-enhanced CO2 Capture within Microporous PCN-250 Frameworks Yongwei Chen, Zhiwei Qiao, Jiali Huang, Houxiao Wu, Jing Xiao, Qibin Xia, Hongxia Xi, Jun Hu, Jian Zhou, and Zhong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14400 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 16, 2018

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ACS Applied Materials & Interfaces

Unusual Moisture-enhanced CO2 Capture within Microporous PCN-250 Frameworks Yongwei Chen,a+ Zhiwei Qiao,a, c+ Jiali Huang,b Houxiao Wu,a Jing Xiao, a Qibin Xia,*a Hongxia Xi, a Jun Hu,b Jian Zhou,*a Zhong Lia a

School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, P. R. China

b

Key Laboratory for Advanced Materials, School of Chemistry and Molecular Engineering, East China University of Science and Technology, Shanghai 200237, P. R. China

c

School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou, 510006, P. R. China

ABSTRACT: Developing metal-organic frameworks (MOFs) with moisture-resistant feature or moisture-enhanced adsorption is challenging for the practical CO2 capture under humid conditions. In this work, under humid conditions, the CO2 adsorption behaviors of two iron-based MOF materials, PCN-250(Fe3) and PCN-250(Fe2Co), were investigated. An interesting phenomenon is observed that the two materials demonstrate an unusual moisture-enhanced adsorption of CO2. For PCN-250 frameworks, H2O molecule induces a remarkable increase in the CO2 uptake for the dynamic CO2 capture from CO2/N2 (15:85) mixture. For PCN-250(Fe3), its CO2 adsorption capacity increases by 54.2% under the 50%

+

These authors contributed equally to this work.

* Corresponding authors. Address: School of Chemistry and Chemical Engineering, South China University of Technology, P. R. China. E-mail addresses: [email protected] (Q. B. Xia), [email protected] (J. Zhou). 1

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RH humid condition, compared with that under dry conditions (from 1.18 to 1.82 mmol/g). Similarly, the CO2 adsorption uptake of PCN-250(Fe2Co) increases from 1.32 to 2.23 mmol/g, exhibiting a 68.9% increase. Even up to 90% RH, for PCN-250(Fe3) and PCN250(Fe2Co), obvious increases of 43.7% and 70.2% in the CO2 adsorption capacities are observed in comparison with those under dry conditions, respectively. Molecular simulations indicate that the hydroxo functional groups (μ3-O) within the framework play a crucial role in improving CO2 uptake in the presence of water vapor. Besides, partial substitution of Fe3+ by Co2+ ions in the PCN-250 framework gives rise to a great improvement in CO2 adsorption capacity and selectivity. The excellent moisture stability (stable even after exposure to 90% RH humid air for 30 days), superior recyclability as well as moisture-enhanced feature make PCN-250 as an excellent MOF adsorbent for CO2 capture under humid conditions. This study provides a new paradigm that PCN-250 frameworks can not only be moisture-resistant but also subtly convert the common negative effect of moisture to a positive impact on improving CO2 capture performance. KEYWORDS: PCN-250, iron-based MOFs, moisture-enhanced CO2 adsorption, gas adsorption, separation 1. INTRODUCTION Given the continuous combustion of fossil fuels for the world energy needs, the everincreasing CO2 emissions have created an environmental crisis in the 21st century.1-3 Massive CO2 emissions can bring a series of environmental problems such as global warming and ocean acidification.4-6 To mitigate CO2 emissions, CO2 capture and 2


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sequestration (CCS) technique has increasingly attracted interest for CO2 reduction.7,8 It is generally known that power plants are one of the largest contributors for increasing atmospheric CO2 concentration, which discharges about 60% of the global CO2 emissions.9 Consequently, efficient CO2 capture from flue gas emitted by power plants becomes urgent. Currently, chemical absorption using amine solutions has been employed in industrial settings.10 Although amine solutions can react with CO2 from flue gas with exceptional selectivity, there are several tricky problems of equipment corrosion, amine degradation, solvent loss, and most severely, energy-intensive regeneration.11 In contrast, adsorption using porous solid sorbents is a viable alternative method for CO2 capture with low-energy consumption.12 Under such circumstance, developing new sorbents with superior properties for CO2 capture has become an imperative challenge to the worldwide researchers.13,14 Among various types of adsorbent materials, metal-organic frameworks (MOFs), as a special family of highly promising materials, are being studied for related CO2 capture applications owing to their structural advantages, including designable and modifiable pore surface, high porosity, structural diversity and so on.15-23 For example, Caskey et al. reported that the CO2 uptake of Mg-MOF-74 was found to high up to 8.0 mmol/g at 1 atm and 296 K, recording as one of the highest CO2 uptake at low pressure to date.24 For pillared square-grid type MOFs, SIFSIX-3-Zn exhibited an exceptionally high CO2/N2 (10:90) selectivity of over 1000 at 1 bar and 298 K, yet it had moderate CO2 uptake of 2.5 mmol/g.25 In practice, water vapor is ever-present in various industrial applications.26 Although 3



several MOFs hold a great promise for CO2 capture applications, such as Cu-BTC,27 MOF74 series,2,6,13,28,29 and SIFSIX series,25,30 these MOFs are sensitive to moisture and an irreversible hydrolysis reaction occurs once being exposed to water vapor owing to the dissociation of weak metal-oxygen coordination bonds.26,31 Thus, their CO2 capture application is largely impeded by their poor moisture stability. For example, Matzger’s group systematically investigated the effect of humidity on M-MOF-74 series performance (where M = Zn, Ni, Co, and Mg), among which all MOF-74 series showed an obvious decrease in corresponding CO2 adsorption capacity at 70% relative humidity (RH), particularly for Mg-MOF-74 merely about 16% of its initial CO2 capacity recovered.32 Thus, when selecting a MOF for CO2 capture, the stability of MOF adsorbents plays an important role under humid conditions, similar conditions including the high CO2 adsorption capacity and CO2/N2 selectivity. Although many moisture-stable MOFs have been confirmed, such as ZIF-8,33 UiO66,34,35 and MIL-101,36 their CO2 capture performance generally cannot meet the industrial requirements due to the weak CO2 affinity. Most importantly, their CO2 adsorption behaviors of these moisture-stable MOFs are generally largely compromised due to the H2O competitive adsorption against CO2 under humid conditions, particularly for postcombustion capture from humid flue gas containing 5-7% water.37 For UiO-66(Zr)NH2, its CO2 working capacity showed a decrease of 88% under humid condition (70% RH) in comparison with that under dry condition (from 0.96 to 0.11 mmol/g).34 In this context, successful paradigms of MOFs capable of CO2 capture under humid conditions 4


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without compromise of performance via overcoming H2O competitive adsorption remain scarce, much less an unusual increase in CO2 uptake in the existence of moisture.13,38-40 As far as we know, only few of MOFs are found to enhance CO2 adorption performance in the existence of H2O so far, including Cu-BTC,41 MIL-100(Fe),38,39 InOF-1,42 NOTT400(Sc),43 NOTT-401(Sc),44 MIL-53(Al),45 MIL-96(Al)46 and Mg-CUK-1.47 For Cu-BTC, Snurr and coworkers firstly found that hydrated (4 wt%) Cu-BTC showed an interesting increase of CO2 uptake compared with dehydrated Cu-BTC, due to the specific interaction between the quadrupole moment of CO2 and the electric field created by H2O.41 With respect to InOF-1,42 NOTT-400(Sc),43 NOTT-401(Sc),44 MIL-53(Al),45 MIL-96(Al),46 and Mg-CUK-1,47 Ibarra et al. concluded that the hydroxo functional groups (μ2-OH) in their frameworks could form hydrogen bond between μ2-OH and H2O, and thus was responsible for the augmented CO2 uptake via efficient packing.40 Llewellyn et al. and our group, respectively, observed the moisture-enhanced feature of improving CO2 uptake in MIL100(Fe) explained by two different mechanisms.38,39 It is well known that most flue gas streams under consideration of postcombustion CO2 capture are highly humid. However, it is stressed that the humidity in these previous works mainly focuses on low humid conditions, generally below 50% RH. To date, no MOF material of moisture-enhanced CO2 adsorption under high humid conditions (above 50% RH) has been reported. To expand their utilizations, it is highly challenging but desirable to explore new MOF materials that their CO2 uptakes remain unchanged or enhanced under high humidity. In this work, we utilize the strategy of bimetallic MOFs to improve the CO2 capture 5



performance, more importantly, subtly converting the negative effect of H2O molecules into an uncommonly positive impact on CO2 capture from the simulated flue gas at RH= 50% and 90% conditions. It is well-known that, iron-based MOFs are emerging candidate materials owing to its non-toxicity, natural abundance, exceptional thermal and chemical stability. Thus, we choose iron-based MOFs, PCN-250(Fe3) and bimetallic PCN250(Fe2Co) constructed from Fe3(μ3-O)(CH3COO)6 or Fe2Co(μ3-O)(CH3COO)6 metal cluster and H4ABTC organic linker (H4ABTC=3,3',5,5'-azobenzenetetracarboxylic acid) for the separation of CO2/CH4 and CO2/N2 mixtures. Herein, we demonstrate that PCN250(Fe2Co) has better CO2 capture performance with respect to CO2 uptake, CO2/CH4 and CO2/N2 selectivities than that of PCN-250(Fe3), due to the introduction of Co2+ ions into the framework. Particularly, both materials have excellent moisture-resistance capability and can be reused without declining in CO2 adsorption capacity. Gratifyingly, dynamic experiments of mimicked postcombustion capture from simulated flue gas confirm that both materials show obvious enhancement of CO2 uptake under humid conditions, despite up to 90% RH. 2. EXPERIMENTAL SECTION 2.1. Materials. All reagents and solvents without further purification were bought by the trade and commerce. The organic ligand 3,3',5,5'-azobenzenetetracarboxylic acid (H4ABTC, 99.0%) was got from Beijing HWRK Chem Co Ltd. Iron nitrate nonahydrate [Fe(NO3)3·9H2O, 98.5%] and cobalt nitrate hexahydrate [Co(NO3)2·6H2O, 99.0%] were obtained from Tianjin Damao Chemicals Co Ltd. Sodium acetate anhydrous (CH3COONa, 6


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99.0%), N,N-dimethylformamide (DMF, 99.5%), ethanol (EtOH, 99.7%), methanol (MeOH, 99.5%), dichloromethane (CH2Cl2, 99.5%) and glacial acetic acid (CH3COOH, 99.5%) were purchased from Guangdong Guanghua Sci-Tech Co Ltd. 2.2. Synthesis of Fe3(μ3-O)(CH3COO)6 and Fe2Co(μ3-O)(CH3COO)6 Clusters. The Fe3(μ3-O)(CH3COO)6 cluster was synthesized using the same procedure in the literature.48 Typically, CH3COONa (25.43 g, 0.31 mol) and Fe(NO3)3·9H2O (8.08g, 0.02 mol) were added in 50 mL deionized water and then kept simultaneously intense stirring. Then, CH3COONa solution was slowly added into Fe(NO3)3 solution under stirring. The resulting mixed solution was kept stirring for overnight at the room temperature. After stirring, the brown precipitate was filtered and thoroughly washed with large amounts of deionized water and EtOH. Finally, the product was dried at 70 °C for 6 h under vacuum. The preparation of Fe2Co(μ3-O)(CH3COO)6 cluster was similar to that of Fe3(μ3-O)(CH3COO)6 cluster, except that Co(NO3)2·6H2O (29.103 g, 0.1 mol) was added in the Fe(NO3)3 solution. 2.3. PCN-250(Fe3) and PCN-250(Fe2Co) Synthesis. Synthesis of PCN-250(Fe3): PCN250(Fe3) was synthesized using a previously published solvothermal approach.48 In detail, H4ABTC (200 mg), Fe3(μ3-O)(CH3COO)6 cluster (300 mg) and CH3COOH (28 mL) in 40 mL DMF were mixed in a 100 mL Pyrex vial. This mixture was ultrasonicated for 5 min, further heated in a 140 °C oven for 12 h. After cooling to 25 °C, brown crystal PCN250(Fe3) were separated and washed using DMF. Preparation of PCN-250(Fe2Co) was similar to that of PCN-250(Fe3), except that the Fe3(μ3-O)(CH3COO)6 cluster (300 mg) was replaced by Fe2Co(μ3-O)(CH3COO)6 cluster (300 mg). 7



The activation treatment of two MOF materials was conducted as follows: first, to separate redundant reactants, as-synthesized PCN-250(Fe3) and PCN-250(Fe2Co) were soaked in 100 mL DMF for 2 days. Then, fresh MeOH replaced DMF and two MOFs were immersed in MeOH for 2 days. Thereafter, MeOH was discarded and the two MOFs were soaked in CH2Cl2 for 3 days. Each solvent exchange was updated using a cycle of 8 h and this whole procedure was heated at 60 °C. After decanting the CH2Cl2, two activated MOFs were dried at 150 °C under vacuum for 8 h. 2.4. Characterization. Powder X-ray diffraction (PXRD) experiments of PCN-250(Fe3) and PCN-250(Fe2Co) were performed on a Bruker D8 Advance X-ray diffractometer at 25 °C, using Cu-Kα radiation at 40 kV and 40 mA in the range of 5-50°. Morphologies of two MOFs materials and X-ray spectroscopy (EDX) analysis of the distribution of elements on the surface were performed by scanning electron microscopy (SEM, Hitachi S-4800; EDX, Hitachi SU-70). Prior to the observation, to improve the conductivity, we sprayed a thin gold layer on the MOFs as a coating. Thermogravimetric analysis (TGA) was used on a TGA Q500 instrument by rising in the temperature of sample at 5 oC/min from 25 to 800 oC

in a N2 gas environment. Micromcritics ASAP-2460 system was used to determine the

N2 isotherms of sample at 77 K. Before each measurement, the activated MOFs were desorbed at 150 oC for 6 h. The pore size distribution was simulated by density functional theory (DFT) and the pore structural features were measured by Brunauer-Emmett-Teller (BET) equation. 2.5. Static Gas Adsorption Measurements. CO2, CH4 and N2 adsorption isotherms at 8


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273 and 298 K were measured by using a 3Flex Surface Characterization Analyzer (Micromeritics, USA). A water bath (298 K) or ice water bath (273 K) method was used to control the adsorption temperature. Before the measurement, 80-100 mg activated MOF samples were degassed at 423 K for 6 h. High-purity gases CO2, CH4 and N2 with 99.999% were performed to examine in all of adsorption measurements. 2.6. Moisture Stability Tests. To examine moisture stability, PCN-250(Fe3) and PCN250(Fe2Co) samples were placed to sealed moist chamber (90% RH) for 30 d at room temperature. Then the resulting samples were measured immediately by PXRD and N2 adsorption without any extra treatment to confirm their structure and porosity after humid treatment. 2.7. Dynamic Gas Adsorption Measurements. Dynamic CO2 adsorption performance of PCN-250(Fe3) and PCN-250(Fe2Co) was evaluated by a simulated flue gas (CO2/N2, 15:85) under dry and humid conditions. We used a quantitative method by the combination of thermogravimetric (TG) analysis and mass spectrometry (MS), in which TG analysis was conducted on a Netzsch STA 449 F3 Jupiter and MS was performed on a Netzsch QMS 403 D Aëolos. Firstly, the freshly activated PCN-250(Fe3) and PCN-250(Fe2Co) samples were purified with a N2 stream of 20 mL/min at 110 oC for 60 min, where the samples were loaded in the TGA oven. After the samples cooled to 25 oC, the simulated flue gas was introduced through the oven with the entire flow rate of 20 mL/min (CO2 3 mL/min, N2 17 mL/min) for the exposure of samples to the flue gas, in which 50% and 90% RH were regulated by a modular humidity generator and introduced into the mixed 9



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gas. After the adsorption of the respective dry and humid CO2/N2 gas mixture, a pure N2 stream of 20 mL/min was purged for the desorption process for 60 min, and then the samples were desorbed with the temperature up to 110 oC by a rate of 10 oC/min. The signals of weight loss and MS of H2O (mass/charge ratio= 18) were together monitored during the desorption. The detailed data of their adsorption capacities of CO2 and H2O is based on the previous method reported in the literature,49 where the whole measured conditions and apparatus remain the same. 2.8. Simulation Models and Methods. For PCN-250(Fe3) and PCN-250(Fe2Co), their atomic structures were refined from the experimental data after the removal of nonframework molecules.48 The atoms of MOFs were described by Lennard-Jones (LJ) and Coulombic interactions:

  4   ij  rij   ij

12

   ij      rij

  

6

 qq  i j 4 0 rij  

(1)

where εij and σij refer to the potential strength and diameter; rij is denoted as the distance of atom i with atom j; qi represents the charge of atom i; ε0 is the permittivity of vacuum. The force field parameters were derived from the universal force field (UFF).50 Previous simulation studies have demonstrated that this force field is sufficient to describe N2, CH4 and CO2 adsorption for different MOFs.51-54 The charges of MOFs were evaluated by MEPO-QEq,55 which is a rapid and accurate method to calculate electrostatic potentials of MOFs. The N2, CH4 and CO2 molecules were denoted by the TraPPE force field.56 The Lorentz-Berthelot equations were used to predict the parameters between different atoms. 10


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The single component adsorption of three gases in PCN-250(Fe3) and PCN-250(Fe2Co) MOFs were simulated by grand canonical Monte Carlo (GCMC) ensemble. All the GCMC simulations were carried out using the RASPA package.57 During all of calculations, each adsorbent was regarded as a rigid structure. A cut-off of 12.0 Å was employed for the calculation of LJ potentials, while the Coulombic interactions were simulated through the Ewald summation method. Each GCMC calculation was carried out for 20000 cycles inlcuding 10000 cycles for equilibration and 10000 cycles for production. Every cycle comprised N GCMC moves (N: the number of adsorbed gas molecules), such as translation, rotation, reinsertion and swap. The calculation of CO2 interactions with PCN-250(Fe2Co) under humid conditions used the Forcite module in Materials Studio software. The LJ potential parameters were derived from the same force field, UFF. The atomic charges of the PCN-250(Fe2Co), H2O and CO2 were calculated using the Qeq method.58 3. RESULTS AND DISCUSSION 3.1. Characterization of PCN-250(Fe3) and PCN-250(Fe2Co). The purity and crystallinity of both PCN-250 materials are confirmed by PXRD. In Figure S1, the PXRD patterns of both as-synthesized PCN-250 samples are similar to the simulated ones obtained from their single-crystal structures,48 confirming that the two materials are successfully synthesized with high purity. Upon solvent exchange treatment, no additional diffraction peaks appear in the resulting activated PXRD patterns, indicating that the PCN250 frameworks remain intact after removing guest solvent molecules trapped in the channels. Meanwhile, the activated PCN-250(Fe2Co) is isostructural to the pristine PCN11



250(Fe3) and means that the introduction of Co2+ ions into the framework of PCN-250(Fe3) does not destroy the pristine framework by partial substitution with Co2+ ions. The permanent porosity of PCN-250(Fe3) and PCN-250(Fe2Co) is determined by N2 adsorption isotherms collected at 77 K. Figure S2 shows that the N2 isotherms of two PCN250 structures are found to be typical type I, indicating that the structures are microporous. However, it is observed that the pore size distribution of PCN-250(Fe2Co) mainly centers in the range of 6.8-9.3 Å, whereas PCN-250(Fe3) possesses much smaller pores centered at 5.9 Å besides in the range of 6.8-9.3 Å, as illustrated in Figure S3. The BET surface area of PCN-250(Fe3) is 1470 m2/g, which is slightly lower than that of PCN-250(Fe2Co) (1653 m2/g) upon the introduction of Co2+ ions. In addition, the micropore volume of PCN250(Fe2Co) is also enhanced compared with that of PCN-250(Fe3), 0.573 vs 0.506 cm3/g. Furthermore, thermal analysis of the two MOFs was also performed to assess their thermal stability. Figure 1 illustrates that the weight loss of the measured MOFs includes several distinct stages in the range from 25 to 800 oC and both materials are thermally stable up to 400 oC. For the first stage before 100 oC, the sharp weight loss of PCN-250(Fe3) and PCN-250(Fe2Co) are about 18.9 and 17.6 wt%, respectively, which reasonably originates from the removal of adsorbed water and guest molecules on the surfaces. Secondly, a slight weight loss of about 11.3 wt% is observed below 400 oC for both cases, associated with the volatilization of solvents molecules inside the pores. Before 400 oC, the TGA profiles of pristine PCN-250(Fe3) and PCN-250(Fe2Co) are very similar, indicating that both frameworks have similar thermal stability. The partial collapse of two MOFs 12


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structures occurs above 400 oC and a plateau is observed before 530 oC for PCN-250(Fe2Co) and 620 oC for PCN-250(Fe3), respectively, followed by the complete framework collapse. Particularly, it is stressed that the introduction of Co2+ ions into the PCN-250 framework obviously decreases the thermal stability, despite the fact that both MOFs have exceptional thermal stability. Considering the difference in the metal-oxygen bond strength (Fe-O 468.3 kJ/mol vs Co-O 456.3 kJ/mol),59 the weaker Co-O coordination bond strength in the PCN-250(Fe2Co) should be reasonable to explain this observation.

Figure 1. TGA plots of PCN-250(Fe3) and PCN-250(Fe2Co). To confirm the microstructure and morphology of PCN-250(Fe3) and PCN-250(Fe2Co), SEM is also performed. As illustrated in Figure 2 (a) and (b), the crystal of PCN-250(Fe3) exhibits dodecahedral shape with additional four small rhombic surfaces, whereas the shape of PCN-250(Fe2Co) crystal is a well-defined cubo-octahedron. The only difference in PCN-250(Fe2Co) structure is that there exist two additional square surfaces in the two vertices positions, which is possibly due to the introduction of Co2+ ions. However, the average particle size of both crystals is almost unchanged and around 50 µm. Furthermore, the signals of Fe and Co elements in PCN-250(Fe2Co) are clearly observed in Figure 2 (d) 13



and (e). As depicted, the elemental mapping images of PCN-250(Fe2Co) demonstrate the homogenous distribution of Fe and Co elements. It implies that successful incorporation of Co2+ ions in the PCN-250(Fe2Co) framework. Furthermore, the atomic ratio of Fe : Co is 2 : 1 by the analysis of single-crystal structure, previously confirmed by Zhou’s group.48

Figure 2. SEM images of (a) for PCN-250(Fe3); (b) and (c) for PCN-250(Fe2Co). (d) and (e) elemental mapping images of PCN-250(Fe2Co). 3.2. Adsorption of CO2, CH4 and N2 in PCN-250(Fe3) and PCN-250(Fe2Co). 3.2.1. Static Adsorption and Separation Performance. For exploring the potential of CO2/CH4 and CO2/N2 gas separations, CO2, CH4 and N2 isotherms are measured at two temperatures of 298 and 273 K and pressures of 0.1-100 kPa. As shown in Figure 3, at 100 kPa, CO2 uptakes of PCN-250(Fe2Co) and PCN-250(Fe3) are 3.87 and 3.02 mmol/g at 298 K, and 7.33 and 5.60 mmol/g at 273 K, respectively. However, PCN-250(Fe2Co) and PCN250(Fe3) adsorbs much smaller amounts of CH4 (0.89 and 0.84 mmol/g at 298 K, 1.53 and 1.46 mmol/g at 273 K) and negligible amounts of N2 (0.26 and 0.20 mmol/g at 298 K, 0.46 14


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and 0.40 mmol/g at 273 K) under the same conditions. Indeed, a further comparison with other MOFs listed in Table S1 manifests that the CO2 adsorption loadings of PCN250(Fe2Co) and PCN-250(Fe3) in this study are far higher than those of most reported MOFs such as ZIF-8 (0.82 mmol/g at 298 K),60 MOF-5 (1.70 mmol/g at 273 K),61 UiO-66 (1.70 mmol/g at 298 K),62 MIL-101(Cr) (2.45 mmol/g at 298 K),63 and MOF-505 (2.87 mmol/g at 298 K).4 For both temperatures, an obvious increase in CO2 uptake is apparently observed upon the incorporation of Co2+ ions into the PCN-250 framework under the same conditions, whereas only slight increase in CH4 and N2 adsorption capacities is observed. The significant difference means that the Co2+ ions have stronger affinity toward CO2 within PCN-250 framework compared with Fe3+ ions. Furthermore, the differences in uptakes between CO2 and CH4 or N2 are significant, suggesting that the two materials can be highly potential adsorbents for selectively capture CO2 from the postcombustion flue gas and the natural gas.

Figure 3. CO2, CH4 and N2 adsorption isotherms of PCN-250(Fe2Co) (solid symbol) and PCN-250(Fe3) (open symbol): (a) 298 K and (b) 273 K. 15



To investigate the selective CO2 adsorption over CH4 and N2, the ideal adsorbed solution theory (IAST)64,65 is used to predict the multicomponent adsorption separation of CO2/CH4 and CO2/N2. Before applying IAST, the dual-site Langmuir-Freundlich (DSLF) equation is chosen to fit the data of experimental single-component isotherms and then the fitting parameters are used to obtain selectivity of binary gas mixtures. Table S2 shows that the DSLF model can accurately predict the isotherms at 298 K, giving all R2 value greater than 0.999. After above-mentioned procedure, the IAST selectivities of CO2/CH4 and CO2/N2 for PCN-250(Fe2Co) and PCN-250(Fe3) are calculated. In Figure 4, CO2/CH4 and CO2/N2 selectivities of both MOFs significantly decrease in low pressure region, finally reach a plateau despite the increase of pressure. The highest selectivities of PCN-250(Fe2Co) are 47 and 11 for CO2/N2 and CO2/CH4, respectively; the corresponding values are 40 and 7 for PCN-250(Fe3). At 100 kPa, the selectivities are 19 and 5 for PCN-250(Fe2Co), and 15 and 4 for PCN-250(Fe3). Evidently, both CO2/CH4 and CO2/N2 selectivities of PCN250(Fe2Co) are much higher than those of PCN-250(Fe3) under the same conditions, indicating that the introduction of Co2+ ions into the PCN-250 framework results in an obvious increase in both CO2 uptake and CO2/CH4 and CO2/N2 selectivities. This could be ascribed to the strengthened affinity of CO2 with PCN-250(Fe2Co) framework.

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Figure 4. Adsorption selectivity of CO2/N2 (15:85) and CO2/CH4 (50:50) mixtures for PCN-250(Fe3) and PCN-250(Fe2Co) at 298 K. To further understand the affinity of PCN-250(Fe3) and PCN-250(Fe2Co) frameworks toward CO2 molecules, the isosteric heat of adsorption (Qst) is calculated by the ClausiusClapeyron equation. As shown in Figure 5, the resulting Qst of CO2 for both materials decrease until a platform is obtained with the increase of loading, suggesting that the adsorption sites for two MOFs are energetically heterogeneous.66 At the initial adsorption process, CO2 molecules preferentially occupy the stronger binding sites due to the fact that these sites are vacant and available on the pore surfaces that induce stronger forces between CO2 molecules and the framework. Therefore, the value of Qst is maximum at the initial loading. With continuous adsorption, the limited stronger binding sites are fully occupied, thus resulting in weaker interactions between the framework and guest molecules, and consequently the Qst slightly decreases.67 Furthermore, it is observed that the overall values of Qst for PCN-250(Fe2Co) are higher than those of PCN-250(Fe3), indicating stronger interactions between CO2 and PCN-250(Fe2Co). Besides, the moderate Qst values of both materials are still below 30.0 kJ/mol, meaning that mild desorption condition is sufficient 17



to regenerate the MOFs.

Figure 5. Isosteric heat of CO2 adsorption on PCN-250(Fe3) and PCN-250(Fe2Co). 3.2.2. Moisture Stability and Recyclability of PCN-250(Fe2Co) and PCN-250(Fe3). Although increasing MOF materials have the potential capability of CO2 capture,68,69 most of them are moisture-sensitive and decomposed when exposed to humid conditions,26,39,70 which certainly hinders their practical applications. To estimate the moisture stability, a stability test is performed, in which PCN-250(Fe2Co) and PCN-250(Fe3) are exposed to humid condition (90% RH) for 30 days. The comparison of PXRD patterns of two MOFs after humid treatment is shown in Figure 6. It is clearly found that the resulting PXRD patterns of humid treated PCN-250(Fe2Co) and PCN-250(Fe3) are almost identical with those of their parent materials, indicating that the structural frameworks of both MOFs remain still intact and no framework collapse takes place during the period of 30 days exposure to high humidity. Furthermore, the N2 isotherms of both materials are also measured to more thoroughly characterize their stability. As illustrated in Figure S4, there is only slightly reduction in N2 uptake of both materials after humid treatment compared with those of freshly made MOFs, indicating excellent moisture stability. This can be 18


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explained by the reason that Fe3+ ions in their frameworks have stronger coordination bonds with carboxylate groups.59,71

Figure 6. PXRD patterns of (a) PCN-250(Fe3) and (b) PCN-250(Fe2Co) after moisture stability tests at 90% RH for 30 days. Besides excellent separation performance, the recyclability of an adsorbent is a critical property determining the potential of practical applications.72 Thus, CO2 adsorption (298 K, 100 kPa) and adsorbent regeneration (423 K, 120 min, vacuum) are performed. The adsorption-desorption is carried out for consecutive 10 cycles, in which the CO2 uptakes of both MOFs are almost similar without no noticeable loss, as presented in Figure 7. It indicates that PCN-250(Fe3) and PCN-250(Fe2Co) can be recycled under mild regeneration conditions; both MOFs have excellent recyclability of CO2 adsorption.

19



Figure 7. Cycling adsorption isotherms of CO2 on (a) PCN-250(Fe3) and (b) PCN250(Fe2Co) at 298 K. 3.2.3. Dynamic Adsorption Performance. Because water vapor is always ubiquitous under practical conditions,45 however, water vapor often generates detrimental effect on CO2 adsorption performance since H2O molecules generally give rise to competitive adsorption during the CO2 adsorption.73,74 From an industrial perspective, a perfect MOF for CO2 capture should have not only high CO2 adsorption capacity and selectivity, but also excellent dynamic CO2 capture property that can overcome the competitive adsorption of H2O. Therefore, dynamic CO2 adsorption under dry and humid conditions was performed to investigate the effect of water vapor on their CO2 adsorption performances of PCN-250(Fe3) and PCN-250(Fe2Co) from the flue gas (CO2/N2, 15:85) at 298 K. As discussed previously,49 the designed experimental device in this work is a quantitative determination method and detailed explanation and calculation are provided in the Supporting Information. Figure 8 exhibits the dynamic adsorption experiments of dry and humid flue gas at 298 K. For both MOFs, a gradual increase in weight is observed until 120 min both MOFs reach 20


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a plateau under dry and humid conditions. At 120 min, the maximum adsorbed amounts of PCN-250(Fe3) and PCN-250(Fe2Co) are 5.18 and 5.81 wt% under dry conditions, respectively. Based on the premise that the adsorbed amount of N2 is marginal and can be negligible; therefore, the increase of weight is ascribed to CO2 adsorption and calculated to be 1.18 and 1.32 mmol/g under dry conditions. This is consistent with the static results that the introduction of Co2+ ions into PCN-250 framework causes an increase of CO2 uptake in comparison with PCN-250(Fe3). In the presence of H2O, however, an interesting observation appears, both MOFs exhibit a substantial increase in total adsorbed uptake at 120 min and an obviously faster weight increase starting at 15 min compared with those under dry conditions. Under humid conditions of 50% RH, the maximum adsorbed amounts of PCN-250(Fe3) and PCN-250(Fe2Co) are increased to 8.42 and 10.85 wt%, respectively. More importantly, the dynamic adsorption curves of both materials remain almost similar, despite that the humidity is high up to 90% RH. After the whole process of dynamic adsorption and desorption illustrated in Figure S6 and S7, the detailed dynamic uptakes of H2O and CO2 can be obtained. Figure 9 shows the comparison of dynamic uptakes of CO2 for PCN-250(Fe3) and PCN-250(Fe2Co) under dry and humid conditions. By comparison with dry conditions, the CO2 uptakes of PCN-250(Fe3) and PCN250(Fe2Co) are 1.82 and 2.23 mmol/g at 50% RH, having 54.2% and 68.9% increase in CO2 uptake, respectively. Even up to 90% RH, both MOFs also show 43.7% and 70.2% increase. This suggests that water vapor can cause an extraordinary increase in CO2 uptake within iron-based MOFs of PCN-250(Fe3) and PCN-250(Fe2Co). Gratifyingly, H2O 21



molecules induce a positive effect on the CO2 capture from humid flue gas.

Figure 8. Dynamic adsorption of the simulated flue gas (CO2/N2, 15: 85) at 298 K (a) PCN-250(Fe3) and (b) PCN-250(Fe2Co) under dry and humid conditions.

Figure 9. Measured dynamic CO2 uptakes from the simulated flue gas (CO2/N2, 15: 85) at 298 K for PCN-250(Fe3) and PCN-250(Fe2Co) under various RH. 3.2.4. Revealing the Adsorption Mechanism by Molecular Simulations. Firstly, Figure S8 shows that the simulated isotherms of three gases at 273 K and 298 K are good agreement with the experiment, although there still exist slight differences. The slightly lower adsorption uptakes of N2 and CH4 are resulted from the existence of non-framework solvents in the experiment, reducing the volume of pore and then giving low adsorption loadings. Furthermore, the density contours indicate that the first adsorption positions (red 22


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cycles) for CO2 are near the metal centers in Figure 10, especially for PCN-250(Fe2Co), because of strong interactions (van der Waals and electrostatic interactions) between CO2 and the open metal sites.48 After the Co-doping, the higher performance of PCN-250(Fe2Co) is ascribed to the comparatively higher ionic character of the Co-O bond than Fe-O bond between oxygen atoms of CO2 and metals. CO2 is not chemisorbed by the Co-O or Fe-O bond in PCN-250, although the ionic character of this bond forms stronger interaction with CO2, leading to higher adsorption loadings of CO2 in any pressure. This phenomenon is similar to the CO2 adsorption in different M-MOF-74 materials (M=Mg, Co, Ni, Zn) in Yazaydın et al.’s work.75 CO2 adsorption loading increases with the decrease of the M-O bond lengths in PCN-250(Fe2Co) and PCN-250(Fe3). The bond lengths between metal and oxygen atoms are plausibly an indication of the interaction of metal with O.

Figure 10. Density contours of CO2 at room temperature and 100 kPa in (a) PCN-250(Fe3) and (b) PCN-250(Fe2Co).

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Furthermore, to provide microscopic insight to prove that the existence of H2O in PCN250(Fe2Co) leads to a positive impact on CO2 capture, theoretical calculation was employed to reveal the adsorption mechanism again, it was worth noting that we calculate the istheroms of CO2 under humid condition by GCMC, but the CO2 adsorption loadings were significantly unchanged, thus the computer was used to predict the binding energies among CO2, H2O and MOFs. Figure 11 shows the CO2 adsorption positions in pore of PCN-250(Fe2Co) with and without H2O, the adsorbed water molecules can coordinate with open-metal sites in the framework. Based on the trigonal prismatic [Fe2Co(μ3O)(CH3COO)6] building block within the crystal structure of PCN-250(Fe2Co), hydroxo functional groups (μ3-O) are constructed by adsorbing a small amount of H2O, which is consistent with the previous works of MOFs including open metal sites.40,41 The adsorbed H2O molecule occupies the space, but simultaneously it leads to the CO2 molecule closer to the metal atom in the other side. The μ3-O groups within the framework can interact strongly with H2O molecules adsorbed inside the pores due to the strong polarity of H2O, and subsequently act as a directing agent for H2O molecules to induce enhanced CO2 adsorption, allowing for more CO2 molecules being efficiently adsorbed due to the confinement effects. As shown in Figure 11, the distance is shortened from 3.261 to 2.544 Å. In view of the isotherms of CO2, we find that the adsorption of CO2 is unsaturated at 298 K and 100 kPa in PCN-250(Fe2Co), thus the adsorption sites of CO2 are sufficient despite a handful of H2O molecules are adsorbed. Additionally, the binding energies between CO2 and MOFs with and without H2O are -14.82 and -12.62 kcal 24




mol-1,

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respectively. Again, this indicates a stronger interaction of CO2 under humid conditions, thus water molecules can promote the adsorption of CO2. The interaction is enhanced due to the introduction of coordinated water molecules with the open metal sites in PCN250(Fe2Co) structure, the coordinated H2O molecule seems to clamp the CO2 molecule on open metal sites in the other sides, like a plier. The plier-effect among CO2, H2O and MOFs can more effectively use most of adsorption sites in the unsaturated conditions, which are responsible for the increased CO2 adsorption at 100 kPa.

Figure 11. Binding sites of CO2 in PCN-250(Fe2Co) structure (a) with and (b) without H2O. 4. CONCLUSIONS In this work, we have successfully demonstrated the strategy of pre-synthesis of bimetallic Fe2Co(μ3-O)(CH3COO)6 clusters to prepare bimetallic PCN-250(Fe2Co) with improved CO2 adsorption performance compared with the parent PCN-250(Fe3) due to the partial Co2+ ions substitution in PCN-250 framework. Furthermore, both PCN-250(Fe3) and PCN250(Fe2Co) exhibit excellent moisture stability, as well as good recyclability. For the first time, an unusual and important phenomenon of the enhancement of CO2 uptake with 25



moisture is highlighted here in the case of PCN-250 frameworks. Molecular simulations reveal the hydroxo functional groups (μ3-O) within the famework giving the plier-effect which can improve CO2 uptake in the presence of water vapor. A paradigm of moistureenhanced MOF with high performance of postcombustion CO2 capture under humid conditions is designed, attributed from the positive impact of H2O on CO2 adsorption. Our findings pave a new way for designing functional MOFs for postcombustion CO2 capture under practical humid conditions. ASSOCIATED CONTENT Supporting Information PXRD patterns of PCN-250(Fe3) and PCN-250(Fe2Co); N2 adsorption/desorption isotherms at 77 K; pore size distributions; comparison of CO2 uptake of several MOFs; definition of dual site Langmuir Freundlich (DSLF) model; fitting parameters of the DSLF model; definition of adsorption selectivity and Clausius-Clapeyron equation; N2 isotherms at 77 K after moisture stability test; the detailed calculation of dynamic adsorption capacities of CO2 and H2O; dynamic adsorption and desorption of the simulated flue gas at 298 K; MS signal curves of desorbed H2O; comparison simulation and experiment data. ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 21576092, 21436005, 21676094 and U1662136), the Guangdong Science Foundation (2014A030312007), the Guangdong Province Science and Technology Project (No. 2016A020221006) and Fundamental Research Funds for the Central Universities 26


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Figure 1. TGA plots of PCN-250(Fe3) and PCN-250(Fe2Co). 64x48mm (300 x 300 DPI)



Figure 2. SEM images of (a) for PCN-250(Fe3); (b) and (c) for PCN-250(Fe2Co). (d) and (e) elemental mapping images of PCN-250(Fe2Co). 43x22mm (300 x 300 DPI)


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Figure 3. CO2, CH4 and N2 adsorption isotherms of PCN-250(Fe2Co) (solid symbol) and PCN-250(Fe3) (open symbol): (a) 298 K and (b) 273 K. 31x11mm (300 x 300 DPI)



Figure 4. Adsorption selectivity of CO2/N2 (15:85) and CO2/CH4 (50:50) mixtures for PCN-250(Fe3) and PCN-250(Fe2Co) at 298 K. 64x49mm (300 x 300 DPI)


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Figure 5. Isosteric heat of CO2 adsorption on PCN-250(Fe3) and PCN-250(Fe2Co). 66x52mm (300 x 300 DPI)



Figure 6. PXRD patterns of (a) PCN-250(Fe3) and (b) PCN-250(Fe2Co) after moisture stability tests at 90% RH for 30 days. 31x12mm (300 x 300 DPI)


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Figure 7. Cycling adsorption isotherms of CO2 on (a) PCN-250(Fe3) and (b) PCN-250(Fe2Co) at 298 K. 29x10mm (300 x 300 DPI)



Figure 8. Dynamic adsorption of the simulated flue gas (CO2/N2, 15: 85) at 298 K (a) PCN-250(Fe3) and (b) PCN-250(Fe2Co) under dry and humid conditions. 31x11mm (300 x 300 DPI)


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Figure 9. Measured dynamic CO2 uptakes from the simulated flue gas (CO2/N2, 15: 85) at 298 K for PCN250(Fe3) and PCN-250(Fe2Co) under various RH. 65x49mm (300 x 300 DPI)



Figure 10. Density distributions of CO2 at 298 K and 100 kPa in (a) PCN-250(Fe3) and (b) PCN-250(Fe2Co). 36x15mm (300 x 300 DPI)


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Figure 11. Binding sites of CO2 in PCN-250(Fe2Co) structure (a) with and (b) without H2O. 37x16mm (300 x 300 DPI)



Table of Contents 35x18mm (300 x 300 DPI)


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Unusual Moisture-enhanced CO2 Capture within Microporous PCN

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