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Metal–Organic Frameworks with Reduced Hydrophilicity for Postcombustion CO2 Capture from Wet Flue Gas Yuxiang Wang, Zhigang Hu, Tanay Kundu, Youdong Cheng, Jinqiao Dong, Yuhong Qian, Linzhi Zhai, and Dan Zhao ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b02173 • Publication Date (Web): 02 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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ACS Sustainable Chemistry & Engineering

Metal–Organic Frameworks with Reduced Hydrophilicity for Postcombustion CO2 Capture from Wet Flue Gas Yuxiang Wang, Zhigang Hu, Tanay Kundu, Youdong Cheng, Jinqiao Dong, Yuhong Qian, Linzhi Zhai, and Dan Zhao* Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive 4, 117585, Singapore *Email Address: [email protected]

ABSTRACT: Postcombustion CO2 capture from wet flue gas is a daunting challenge that metal–organic frameworks (MOFs) based adsorbents need to address, because the moisture in the gas stream may not only hydrolyze the coordination bonds of MOFs but also be competitively adsorbed in MOFs leading to compromised CO2 capture performance. In this study, two isostructural water-stable MOFs decorated with alkyl groups, namely UiO-66(Zr)(OAc)2 and UiO-66(Zr)-(OPr)2, are synthesized from UiO-66(Zr)-(OH)2 via a facile postsynthetic esterification strategy and evaluated for their water affinity and CO2 capture performance. The increased water contact angle and reduced water vapor capacity at 60% relative humidity indicate the positive role of propionyl group of UiO-66(Zr)-(OPr)2 in reducing material hydrophilicity. When the adsorbent beds are not fully saturated with water,

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breakthrough experiments using simulated wet flue gas reveal that UiO-66(Zr)-(OPr)2 possesses a CO2/N2 adsorption selectivity of 76.6, which is 229% of that of UiO-66(Zr)-(OH)2 (33.4). Our study successfully demonstrates a scalable material modification approach to engineer MOF adsorbents toward practical CO2 capture processes under wet conditions.

Keywords: Postcombustion CO2 capture, Pressure swing adsorption, Metal-organic frameworks, Post-synthetic modification, Wet gas breakthrough

INTRODUCTION Global warming because of escalating anthropogenic greenhouse gas emission has aroused worldwide concerns. Owing to the slow-pacing development of sustainable energy supplies, CO2 capture and utilization is considered as a promising solution for the dilemma between ascending energy demand and pressing deterioration of environment.1-2 Despite being the most commonly applied method for CO2 capture in industry, amine scrubbing suffers from equipment corrosion, solvent loss, amine degradation, and high energy consumption to regenerate strongly absorbed CO2 from the absorbents.3-5 On the contrary, adsorption-based CO2 capture techniques such as pressure/temperature swing adsorption are attracting increasing interests due to their lower operational cost, less toxicity, and milder corrosivity.1,6 Ideally, adsorbents used for CO2 capture should possess high working capacity (defined as the CO2 uptake capacity difference between adsorption and desorption stages in the process), high gas selectivity of CO2 over N2, decent chemical resistivity and mechanical strength, minimalized water affinity, and low material costs.7-9 Among many adsorbents being studied, metal–organic frameworks (MOFs), a category of porous crystalline materials constructed from inorganic clusters and organic bridging ligands,


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have demonstrated enormous potential in gas separation processes, especially in CO2 capture.2,1013

Because of the hygroscopic nature of CO2 binding sites in MOFs (e.g., unsaturated open metal

sites, polar functional groups, and metal clusters), an inevitable challenge that many MOF adsorbents are facing is the competitive adsorption of water over CO2 in wet flue gas which typically contains 5−10 vol% of water vapor. This competitive adsorption of water will not only compromise the adsorbent working capacity but also increase the energy required for adsorbent regeneration.14-16 In addition, the hydrothermal stability of MOFs is another issue handicapping the practical applications of MOF-based CO2 capture.17 For example, the CO2 uptake capacities of MOF-74(Mg) and SIFSIX-3-Ni are 5.34 and 2.48 mmol g−1, respectively, at 303 K and 0.15 atm under dry conditions. Nevertheless, these values shrink significantly to 1.55 and 1.73 mmol g-1, respectively, when the 15/85 CO2/N2 gas mixture at 1 atm is saturated with moisture at room temperature.15 This striking decline in CO2 adsorption capacity is likely due to the competitive adsorption of water along with the structural evolutions of the MOFs, namely, partial structure collapse in MOF-74(Mg) and gradual phase transition in SIFSIX-3-Ni.17-18 To overcome the adverse effects of moisture on CO2 adsorption in MOFs, enhancing CO2/H2O adsorption selectivity by the introduction of chemisorption sites such as amine groups into the nanospace of MOFs has been extensively studied.19-23 A series of diamines have been loaded into MOFs with open metal sites such as M2(dobpdc) (M = Mg, Fe, Co, Zn, Mn, dobpdc4- = 4,4’dioxidobiphenyl-3,3’-dicarboxylate)

and

Mg2(dobdc)

(dobdc4-

=

2,5-dioxido-1,4-

benzenedicarboxylate) via the coordination between amino groups and metal cations, resulting in MOFs capable of performing effective CO2 capture at elevated temperatures in the presence of moisture.19,24-26 The second general strategy is endowing hydrophobicity to MOFs to weaken the interactions between MOFs and water so that the competition adsorption of water can be



relieved.27 For instance, alkyl chains and perfluoroalkyl groups have be immobilized into MOFs by postsynthetic acylation, thiol-ene click reaction, or liquid phase chemisorption to provide excellent hydrophobicity.28-30 In addition, hybridizing MOFs with hydrophobic polymers can also modify the surface properties of the MOFs.31 Apart from the commonly used polydimethysiloxane (PDMS) as the protective hydrophobic layer, reduced graphene oxide (rGO) and microporous organic networks (MONs) have also been employed to prepare MOF/polymer composites with enhanced water stability.32-35 Recently, multivariate (MTV) MOFs incorporating hydrophobic ligands have been demonstrated to counteract the moisture interference effectively in MOF-based CO2 capture from wet flue gas. MTV zeolitic imidazole frameworks (ZIFs) containing 2-mthylimidazolate and a series of benzimidazolate derivatives exhibit equally effective CO2 separation from N2 in both the presence and absence of water.36 In a recent study, our group synthesized MTV UiO-66(Zr) incorporating aminoterephthalate (NH2BDC) and tetrafluoroterephthalate (TFBDC) by a triphasic modulated hydrothermal (MHT) synthesis method.37 Dynamic breakthrough experiments show that the MTV MOF UiO-66(Zr)NH2-F4-0.53 only loses 30% of its CO2 uptake capacity in simulated wet flue gas mixture at 298 K, outperforming the almost 100% loss of zeolite materials and other hydrophilic adsorbents under similar conditions. This study highlights the importance of engineering water affinity of MOFs for moisture-resistant CO2 capture applications. Opt-UiO-66(Zr)-(OH)2 is a water-stable Zr MOF developed in our lab with a high CO2 uptake capacity (2.52 mmol g-1 in a 15/85 CO2/N2 gas mixture at an equilibrium pressure of 1.3 bar, 298 K), outstanding CO2/N2 adsorption selectivity (~45 based on dynamic column breakthrough experiments), and excellent hydrothermal stability.38 However, up to 85% loss in CO2 uptake capacity was observed when this MOF was saturated with moisture under the atmosphere of 80%


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relative humidity (RH), which undesirably impedes its practical CO2 capture applications. To overcome this drawback, we hypothesize that the hygroscopic nature of this MOF could be altered by converting its hydrophilic hydroxyl groups to less polar ester groups along with the introduction of aliphatic chains so that the adverse effects of moisture on CO2 adsorption might be alleviated. Herein, we show that postsynthetic esterification of UiO-66(Zr)-(OH)2 using aliphatic carboxylic anhydride can indeed reduce the hydrophilicity of the MOF precursor without damaging its intrinsic stability. As demonstrated by the breakthrough experiments using adsorption beds which are not saturated with water beforehand, one of the esterified MOFs, UiO66(Zr)-(OPr)2, can capture CO2 from simulated wet flue gas with a higher CO2/N2 co-adsorption selectivity than that of UiO-66(Zr)-(OH)2. Our proof-of-concept study suggests the promising direction of water affinity engineering in MOFs for enhanced CO2 capture performance under wet conditions.

EXPERIMENTAL SECTION Modulated Hydrothermal (MHT) Synthesis of UiO-66(Zr)-(OH)2. UiO-66(Zr)-(OH)2 was synthesized according to the previously reported method.38 Briefly, 2,5-dihydroxyterephthalic acid (H4DOBDC, 2 g, ~10 mmol) and ZrOCl2·8H2O (3.4 g, ~10.4 mmol) were added in a 100 mL water/acetic acid (20/30, v/v) mixture and heated under reflux (~105 °C) for 1 day to afford the dark khaki powder. The powder was washed with water for three times before soaking in water and methanol each for 3 days at room temperature, during which the solvent was refreshed every day. After the final removal of methanol by decanting, the sample was activated under vacuum at 120 °C overnight for further modifications and characterizations.



Postsynthetic Esterification of UiO-66(Zr)-(OH)2. A suspension of activated UiO-66(Zr)(OH)2 (100 mg, ~0.65 mmol) in 5 mL of aliphatic carboxylic anhydride [acetic anhydride for UiO-66(Zr)-(OAc)2 and propionic anhydride for UiO-66(Zr)-(OPr)2] was heated to 55 °C before adding three drops of concentrated H2SO4. After 5 hours of heating, dark khaki powder was isolated by centrifugation and washed with tetrahydrofuran (THF) for three times. The powder was further soaked in THF for 3 days, during which the solvent was refreshed every day. Finally, the sample was activated under vacuum at 120 °C overnight for further characterizations. Breakthrough Experiments. The breakthrough experiments were conducted using a homebuilt set up shown in Scheme S1. Gas composition at the exit of the column was determined by a mass spectrometer (Hiden QGA quantitative gas analysis system). The flow rate of each component gas was calculated by an internal Ar flow reference that has a fixed flow rate of 5 ± 0.01 sccm (standard cubic centimeter per minute). Before the breakthrough experiments, CO2/N2 (15 ± 1) / (85 ± 1) binary mixture gas with a flow rate of 5 ± 0.1 sccm was introduced through the bypass line with various resistances (pressure drop) controlled by a needle valve. The corresponding gas residence time was calculated for the calibration of system dead volume. A 7 cm stainless steel column (internal diameter: 0.46 cm) filled with MOF powders (~0.6 g depending on the adsorbent density) was activated by purging a constant He flow (8 ± 0.1 sccm) through the column at 60 °C for 1 day until no solvent or moisture signal could be detected by the mass spectrometer. For dry gas breakthrough experiments, a (15 ± 1)/(85 ± 1) mixture of CO2 and N2 with a total flow rate of 5 ± 0.1 sccm was stabilized for 15 minutes before being introduced into the column. As for experiments of wet gas breakthrough in the MOF-packed columns, a (15 ± 1)/(85 ± 1) CO2/N2 mixture with a relative humidity (RH) around 60% was directly introduced into the system after stabilization for 15 minutes. Pre-saturation of packed


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columns with water vapor was conducted by purging a (15 ± 1)/(85 ± 1) He/N2 mixture (RH ~60%) through the column after activation described above. Mild activation was conducted by purging the packed column with a constant He flow (8 ± 0.1 sccm) at room temperature for 3 hours. Methods to calculate gas co-adsorption capacity and selectivity based on breakthrough experiments are detailed in the supporting information.

RESULTS AND DISCUSSION Structure and Porosity Characterization. There are two paths to introduce alkyl chains into the framework of UiO-66-(OH)2 by esterification of the hydroxyl groups (Scheme 1). The first path can be regarded as a direct approach, namely, synthesizing esterified ligand first and subsequently using the modified ligand to grow corresponding MOFs by solvothermal synthesis.39 The second path is the postsynthetic approach where esterification to immobilize alkyl chains is conducted after MOF formation. The direct approach was initially adopted because this method was expected to afford a higher esterification ratio than the MOFs obtained by postsynthetic approach which may have undesirable diffusion limitations of the reactants. Nevertheless, as shown in Figure S1, the MOF prepared by this approach displays a Fouriertransform infrared spectroscopy (FTIR) spectrum similar to that of UiO-66(Zr)-(OH)2 with no band at about 1750 cm-1 from ester carbonyl groups, suggesting that the ester bonds of the ligands were hydrolyzed during the process of solvothermal synthesis. Therefore, the postsynthetic approach was used instead in this study.



Scheme 1. Schematic illustration of the strategy combing modulated hydrothermal (MHT) synthesis and postsynthetic modification to prepare esterified MOFs. The MOF precursor UiO-66(Zr)-(OH)2 was synthesized based on a previously reported protocol of modulated hydrothermal (MHT) synthesis.38,40-41 The phase purity and quality of the MOF were examined by powder X-ray diffraction (PXRD, Figure 1a), thermogravimetric analysis (TGA, Figure S2), and Ar sorption at 87 K (Figure 2a), all confirming the successful repetition of the reported method. Calculation based on the TGA data shows that about 1.44 linkers are missing for each Zr6 cluster, leaving vacant sites capped by acetate anions as indicated by the proton signal of acetyl at around 1.7 ppm in the 1H-NMR spectra (Figure S3).42 After activation, UiO-66(Zr)-(OH)2 was subsequently added into aliphatic carboxylic anhydride and the mixture was heated to about 55 °C for 10 hours under the catalysis of concentrated sulfuric acid. The PXRD patterns of esterified MOFs are consistent with that of the precursor MOF (Figure 1a). In addition, there is no obvious change in the particle size or morphology of the MOFs before and after esterification (Figure S4), confirming the structural integrity of UiO66-Zr-(OH)2 in the highly corrosive reaction environment of postsynthetic esterification. For the MOFs treated with acetic anhydride and propionic anhydride, which are denoted as UiO-66(OAc)2 and UiO-66-(OPr)2, respectively, new FTIR peaks at ~1740 cm-1 corresponding to the


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ester carbonyl groups can be observed (Figure 1b).43 Besides, the intensity of C−O stretching in the ligands at 1231 cm-1 decreases noticeably compared to the intensity of carbonyl groups in the carboxylate, suggesting that part of the hydroxyl groups have been converted during the esterification process.43 The FTIR spectrum of UiO-66(Zr)-(OiBu)2 obtained by treating the MOF precursor with isobutyric anhydride, however, does not exhibit the characteristic band of ester carbonyl groups. Instead, there is a new band at about 1646 cm-1 indicating the presence of uncoordinated aromatic acid, which can be attributed to the partial decomposition of the MOF.44 This unsuccessful esterification using isobutyric anhydride is likely due to the steric effect of the reactants and diffusion limitations in the confined nanospace of UiO-66(Zr)-(OH)2. 1H-NMR spectra of the digested esterified MOFs were collected to estimate the conversion ratio of phenolic hydroxyl groups (Figure S3). For digested UiO-66(Zr)-(OAc)2, the acetyl proton signal has an integration area of ca. 3.7 with reference to the phenyl proton signal (integration area assumed to be 1), which suggests 100% esterification yield of the phenolic hydroxyl groups if the capping acetates in the MOF are taken into consideration. On the other hand, the acetyl methyl proton signal almost disappears for digested UiO-66(Zr)-(OPr)2, suggesting that the capping acetates were replaced by propionates after the postsynthetic modification. Assuming that the amount of the defects remained unchanged, the esterification yield is estimated to be 44% for UiO-66(Zr)-(OPr)2.



(a)

UiO-66(Zr)-(OiBu)2

(b)

UiO-66(Zr)-(OPr)2

Transmittance / a.u.

UiO-66(Zr)-(OAc)2 UiO-66(Zr)-(OH)2

Intensity / a.u.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1740 cm-1 1646 cm-1 UiO-66(Zr)-(OH)2 UiO-66(Zr)-(OAc)2 UiO-66(Zr)-(OPr)2

1231 cm-1

UiO-66(Zr)-(OiBu)2

5

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35

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Wavenumber / cm

2 Theta / degree

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-1

Figure 1. (a) PXRD patterns and (b) FTIR spectra of UiO-66(Zr)-(OH)2 and its esterified derivatives. Ar sorption tests at 87 K were conducted to probe the changes in the internal texture such as porosity and pore size distribution of the MOFs after postsynthetic modification. As shown in Figure 2a, the MOFs exhibit typical type I isotherms with minor desorption hystereses at high P/P0 values. The BET surface area of UiO-66(Zr)-(OH)2 drops from 929.7 m2 g-1 to 552.0 m2 g-1 for UiO-66(Zr)-(OAc)2 and 574 m2 g-1 for UiO-66(Zr)-(OPr)2. The modified MOFs possess comparable surface area even though propionyl groups in UiO-66(Zr)-(OPr)2 are bulkier than acetyl groups in UiO-66(Zr)-(OAc)2, which is possibly due to the lower alkyl chain loading in UiO-66(Zr)-(OPr)2 indicated by 1H-NMR. According to the pore size distribution (Figure 2b) calculated by non-local density functional theory (NLDFT) model, the esterification process reduces the total amount of pores, especially those with pore sizes around 5.3 Å, suggesting that the alkyl carbonyl moieties mainly reside in the octahedral pores of the MOFs.


(a)

(b) 250 200

Vads / cm3stp g-1

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150 100 UiO-66(Zr)-(OH)2 UiO-66(Zr)-(OAc)2

50

UiO-66(Zr)-(OPr)2

0 0.0

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Incremental Pore Volume / cm3 g-1 Å-1

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0.6

UiO-66(Zr)-(OH)2 UiO-66(Zr)-(OAc)2 UiO-66(Zr)-(OPr)2

0.4

0.2

0.0 4

Relative Pressure P/P0

6

8

Pore Width / Å

Figure 2. (a) Ar sorption isotherms at 87 K and (b) pore size distributions of UiO-66(Zr)-(OH)2, UiO-66(Zr)-(OAc)2, and UiO-66(Zr)-(OPr)2. Assessment of Water Affinity of MOFs. There are two potential water adsorption sites in UiO-66(Zr)-(OH)2: zirconium-oxo clusters and hydroxyl groups from the ligands. Postsynthetic esterification is expected to convert the hydroxyl adsorption sites to less hydrophilic moieties to alleviate the competitive sorption of water during CO2 capture processes. Besides, replacing the node-capping acetates by propionate may also weaken the water affinity of UiO-66(Zr)-(OPr)2. Water vapor sorption at 293 K and water contact angle measurement were performed to assess water affinity in the esterified MOFs and the MOF precursor. As shown in Figure 3a, the initial slopes of water isotherms of the three MOFs are close to each other, indicating small changes in the thermodynamic water affinity of the MOFs after the introduction of alkyl chains. These results suggest that the introduced alkyl chains are not long enough to block the exposed hydrophilic metal clusters, which are probably the major water adsorption sites at low pressure ranges. At higher RHs (e.g., 60%), UiO-66(Zr)-(OH)2 exhibits a higher water adsorption capacity (15.5 mmol g-1) than that of UiO-66(Zr)-(OAc)2 (10.7 mmol g-1) and UiO-66(Zr)-(OPr)2 (9.0 mmol g-1), which is likely due to the larger porosity and the presence of hydroxyl sorption sites in the precursor MOF. Interestingly, although UiO-66(Zr)-(OPr)2 has a surface area



comparable to that of UiO-66(Zr)-(OAc)2, the water sorption capacity of the former is noticeably lower than that of the latter, suggesting that the less polar propionyl group can endow the MOF with weaker hydrophilicity. Furthermore, UiO-66(Zr)-(OPr)2 exhibits larger contact angles (mean value, 73.6°) than that of UiO-66(Zr)-(OH)2 (mean value, 36.4°) and UiO-66(Zr)-(OAc)2 (mean value, 36.3°) as shown in Figure 3b, 3c and Figure S5, confirming the effective role of propionyl groups in reducing the hydrophilicity of the MOF.

(a) 15 Gas Uptake / mmol g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(b)

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6 UiO-66(Zr)-(OH)2

3

(c)

UiO-66(Zr)-(OAc)2 UiO-66(Zr)-(OPr)2

0 0.0

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Figure 3. (a) Water vapor sorption isotherms at 293 K of UiO-66(Zr)-(OH)2, UiO-66(Zr)(OAc)2, and UiO-66(Zr)-(OPr)2 (solid symbols, adsorption; open symbols, desorption). Snapshots of water contact angle measurements of (b) UiO-66(Zr)-(OPr)2 and (c) UiO-66(Zr)(OH)2. CO2 Capture Performance Based on Isotherms. Low pressure CO2 and N2 sorption properties of the esterified MOFs were studied by measuring gas sorption isotherms at 298 K and 273 K (Figure 4a, Figure S6). At 298 K, UiO-66(Zr)-(OH)2 shows a CO2 uptake capacity of 1.92 mmol g−1 at 0.15 bar (0.15 bar is the partial pressure of CO2 in flue gas). This value is reasonably lower than the reported one (2.50 mmol g-1) because the BET surface area of UiO-66(Zr)-(OH)2 (929.7 m2 g-1) has not been optimized to the value in the literature (1230 m2 g-1).38 Although the


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CO2 uptake capacities of UiO-66(Zr)-(OAc)2 and UiO-66(Zr)-(OPr)2 are 1.28 mmol g-1 and 1.19 mmol g-1, 33% and 38% lower than that of their precursor, respectively, these values are still among the best of all UiO-66 type materials.45-46 The decline in CO2 uptake capacity of the MOFs can be attributed to lower surface area and fewer polar hydroxyl groups in the modified MOFs. To probe the interactions between CO2 molecules and MOFs, the CO2 isosteric heat of adsorption (Qst) of the MOFs were calculated using the Clausius-Clapeyron equation based on isotherms at 298 K and 273 K (Figure 4b and Figure S7).47 As expected, UiO-66(Zr)-(OH)2 exhibits a zero-coverage CO2 Qst of 34.5 kJ mol-1, which is comparable to the previously reported value (33 kJ mol-1).38 Although the zero-coverage CO2 Qst of UiO-66(Zr)-(OPr)2 is close to that of its precursor, UiO-66(Zr)-(OAc)2 possesses a larger zero-coverage CO2 Qst of about 36.2 kJ mol-1. We presume that this slightly intensified CO2 interaction with UiO-66(Zr)(OAc)2 may result from the enhanced van der Waals forces experienced by CO2 molecules due to the shrinkage of pore size in the MOF, while the loading of propionyl groups in UiO-66(Zr)(OPr)2 is not high enough to manifest this effect.48-50 Ideal adsorbed solution theory (IAST) was employed to predict the ideal CO2/N2 separation performance of the MOFs in this study for a 15/85 CO2/N2 gas mixture at 298 K. As shown in Figure 4c, UiO-66(Zr)-(OH)2 has an IAST CO2/N2 adsorption selectivity of 66 at 298 K and 1 bar, which falls between the previously reported values (34.2 and 105).38,40 As for the esterified MOFs, UiO-66(Zr)-(OAc)2 and UiO66(Zr)-(OPr)2 exhibit ideal CO2/N2 selectivities of 42.5 and 39.1, respectively, which are lower than that of UiO-66(Zr)-(OH)2 but still higher than most Zr MOFs such as UiO-66(Zr) (19.4),51 UiO-66(Zr)-NH2 (32.3),52 and UiO-66(Zr)-(COOH)2 (35.4).40



(a)

5

Gas Uptake / mmol g-1

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UiO-66(Zr)-(OAc)2 UiO-66(Zr)-(OPr)2

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UiO-66(Zr)-(OAc)2

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Figure 4. (a) CO2 sorption isotherms of UiO-66(Zr)-(OH)2, UiO-66(Zr)-(OAc)2, and UiO66(Zr)-(OPr)2 at 298 K (solid symbols, adsorption; open symbols, desorption). (b) Qst values of CO2 in the MOFs. (c) IAST selectivities of a 15/85 CO2/N2 gas mixture in the MOFs at 298 K.


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Stability Tests of MOFs. Excellent material robustness is expected for MOFs to be used in industrial applications under harsh conditions.7 In this study, the UiO-66(Zr)-(OPr)2 was selected as the representative esterified MOF and subjected to a series of stability tests including 1-day soaking in boiling water (100 °C), 70 °C water, highly acidic (pH = 1), and basic (pH = 11) aqueous solutions. UiO-66(Zr)-(OPr)2 demonstrates excellent water stability comparable to its MOF precursor, which is reflected in the unchanged PXRD patterns and gas sorption isotherms (Figure 5a-c).38 Nevertheless, it should be noted that some of the ester bonds in the modified MOFs were hydrolyzed after soaking in boiling water as indicated by the reduced band intensity at 1740 cm-1 in FTIR spectrum (Figure 5d). Reducing temperature to 70 °C helps to preserve the ester bonds, suggesting mild activation conditions to be adopted in further studies. (a)

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Vads / cm3stp g-1

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100ºC water treated for 1 day 70°C water treated for 1 day pH = 11 treated for 1 day pH=1 treated for 1 day UiO-66(Zr)-(OPr)2

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Figure 5. Evaluation of the hydrothermal and chemical stability of UiO-66(Zr)-(OPr)2: (a) PXRD patterns, (b) N2 sorption isotherms at 77 K, (c) CO2 sorption isotherms at 298 K, and (d) FTIR spectra.



Dry Gas Breakthrough Experiments. Based on the comprehensive consideration of water affinity, CO2 uptake, and chemical stability, UiO-66(Zr)-(OPr)2 was selected as the representative esterified adsorbent for further evaluation of its postcombustion CO2 capture performance by dynamic breakthrough experiment using simulated dry flue gas. As shown in Figure S8, the MOF precursor UiO-66(Zr)-(OH)2 exhibits similar breakthrough curves using dry simulated flue gas in five consecutive breakthrough runs, indicating high material quality and good consistency with our previous results.38 The average CO2 uptake amount in these runs is 1.83 mmol g-1 (Table S1), which is slightly lower than the value obtained from single component gas sorption experiment. This trend is reasonable considering the competitive adsorption of N2 during breakthrough conditions. The CO2/N2 co-adsorption selectivity was further determined to be 29 based on the uptakes of CO2 and N2 calculated by Equation S15, which is comparable to the previously reported value (~40 depending on the gas pressure).53 Similarly, the packed bed containing UiO-66(Zr)-(OPr)2 powder exhibited decent recyclability of up to eight cycles with mild activation process in between (purging He gas through the column at room temperature for 3 hours), showing an average CO2 uptake capacity of 1.25 mmol g-1 (Figure 6, Table S1). Although the specific CO2 uptake capacity of UiO-66(Zr)-(OPr)2 is about 32% smaller than that of UiO-66(Zr)-(OH)2, the CO2 mean residence time in the column containing UiO-66(Zr)-(OPr)2 is actually only 14% lower than that of the column containing UiO-66(Zr)-(OH)2 (Table S1), suggesting that the esterification step has only a small effect on the volumetric CO2 uptake capacity which is a more important criterion for process design of postcombustion CO2 capture.7


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(a) 1.0 0.8

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Figure 6. (a) Breakthrough curves obtained by passing a 15/85 CO2/N2 mixture gas with a flow rate of 5 sccm through a column (length: 7 cm; diameter: 0.46 cm) packed with UiO-66(Zr)(OPr)2 at 298 K with an equilibrium pressure of 1.11 bar. (b) Cyclic dry gas breakthrough performance of the above column at 298 K with an equilibrium pressure of 1.11 bar. Wet Gas Breakthrough Experiments. A straightforward strategy to exclude moisture effect on CO2 capture is to remove water in flue gas before introducing it into adsorption columns.54 The cost and energy consumed by drying flue gas can be waived if CO2 capture performance of the adsorbents is unaffected in wet conditions. Previously, we have reported that the CO2 uptake capacity of UiO-66(Zr)-(OH)2 decreased by 85% after it was pre-saturated by water vapor at 80% RH.38 However, the experimental condition of the previous study is different from practical pressure swing adsorption (PSA) processes where multiple gas components (e.g., N2, CO2 and



H2O) in the frontal loading compete with each other for the limited sorption sites of the unsaturated adsorbents.55-56 In this study, we conducted simulated wet flue gas breakthrough experiments without any pre-treatment of the column to mimic the industrial PSA processes, and studied how moisture in the gas stream would affect CO2 capture performance of the MOF-filled column. Inspiringly, UiO-66(Zr)-(OH)2 exhibits a CO2 uptake capacity of 1.67 mmol g-1 after two consecutive breakthrough runs in the presence of moisture with mild activation in between (Figure 7a and 7b, Table S2). The CO2 uptake capacity of MOF-filled column can be easily restored to 1.78 mmol g-1 after overnight purging of He gas at 60 °C. These values are slightly lower than that obtained from the single-component CO2 sorption isotherm (1.98 mmol g-1) or that predicted by IAST (1.96 mmol g-1). The deviation from the IAST prediction could be rationalized by the competitive adsorption of N2 and H2O together with the adsorbate-adsorbate interaction between CO2 and H2O.57 As for UiO-66(Zr)-(OPr)2, it shows a CO2 separation performance similar to that of UiO-66(Zr)-(OH)2 with CO2 uptake capacity decreasing from 1.13 mmol g-1 to 0.94 mmol g-1 in three consecutive breakthrough runs, slightly lower than the values obtained by gas sorption measurement (1.25 mmol g-1) and IAST calculation (1.20 mmol g-1). The capacity can be restored to 1.23 mmol g-1 after overnight activation (Figure 7c and 7d, Table S2). Although the CO2 uptake capacity of UiO-66(Zr)-(OPr)2 is still lower than that of UiO66(Zr)-(OH)2, it is interesting to find that the average CO2/N2 co-adsorption selectivity of UiO66(Zr)-(OPr)2 for simulated wet flue gas becomes as high as 76.6, which is more than two times of that of UiO-66(Zr)-(OH)2 (Figure 7b and 7d, Table S2). Although the propionyl groups in UiO-66(Zr)-(OPr)2 are expected to alleviate the competitive adsorption of water against CO2, we could not detect any water signal at the exit of the column during breakthrough tests, suggesting that almost all the water molecules remain trapped within UiO-66(Zr)-(OPr)2. This could be


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because propionyl groups are not long enough to block the pore entrance of the MOF so that water molecules could still access the hydrophilic metal clusters and get adsorbed. In the extreme scenario where the packed column was pre-saturated with 60% RH 15/85 He/N2 mixture, UiO66(Zr)-(OPr)2 only retained a minimal CO2 uptake of 0.04 mmol g-1 (Figure S9), suggesting that

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CO2 and H2O molecules still share the same sorption sites even after postsynthetic esterification.

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Figure 7. Breakthrough curves of the columns (length, 7 cm; diameter, 0.45 cm) packed with (a) UiO-66(Zr)-(OH)2 and (c) UiO-66(Zr)-(OPr)2 using wet 15/85 CO2/N2 mixture (60% RH) with a flow rate of 5 sccm at 298 K and a pressure drop of 0.17 and 0.24 bar across the column, respectively. Cyclic wet gas breakthrough performance of the above columns packed with (b) UiO-66(Zr)-(OH)2 and (d) UiO-66(Zr)-(OPr)2.

Conclusion



In summary, we adopted a scalable and facile approach combining the MHT approach and postsynthetic modification to reduce the hydrophilicity of UiO-66(Zr)-(OH)2 for postcombustion CO2 capture under wet conditions. PXRD patterns and Ar sorption isotherms reveal the intact crystallinity of the MOFs after being treated in acidic conditions during the postsynthetic esterification. Moreover, a series of stability tests show that the esterified MOF UiO-66(Zr)(OPr)2 preserves the excellent water stability of the precursor MOF UiO-66(Zr)-(OH)2. Water contact angle tests and water sorption measurements further confirm the repressed hydrophilicity of the esterified MOFs compared to UiO-66(Zr)-(OH)2. During the breakthrough experiments using simulated wet flue gas, although UiO-66(Zr)-(OPr)2 exhibits lower CO2 uptake capacity (1.08 mmol g-1) than that of UiO-66(Zr)-(OH) (1.73 mmol g-1), the CO2/N2 co-adsorption selectivity of UiO-66(Zr)-(OPr)2 (76.6) is higher than that of the latter (33.4), showing a tradeoff balance between gas uptake capacity and selectivity. Despite the continuous loss in CO2 uptake capacity during three consecutive breakthrough experiments using simulated wet flue gas, both MOFs can be easily regenerated by prolonged activation at 60 °C. It is noteworthy that esterification and the concomitant replacement of capping molecules on the Zr6 clusters cannot eliminate water adsorption in the MOFs as shown in the breakthrough experiments, suggesting that moderate hydrophobicity of MOFs cannot fully repel water molecules. Alternative esterification approaches using carbonyl chloride or anhydrides with long aliphatic chains may be promising solutions, as the aliphatic chains may not only further increase surface hydrophobicity of the MOFs but also impede water molecules from entering the internal pores of MOFs by pore channel blockings with the bulky alkyl groups. Further studies related to these approaches are undergoing in our lab to pursue water repellant adsorbents with huge potentials for practical postcombustion CO2 capture.


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Supporting Information. Materials and instrumental information, data processing methods, the breakthrough setup scheme, and additional characterization results. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work is supported by National University of Singapore (CENGas R-261-508-001-646), Ministry of Education - Singapore (MOE AcRF Tier 1 R-279-000-472-112, R-279-000-540114), and Agency for Science, Technology and Research (PSF R-279-000-475-305, IRG R-279000-510-305).

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Synopsis Esterified zirconium metal–organic frameworks with increased hydrophobicity can enhance the CO2/N2 co-adsorption selectivity during breakthrough experiments using simulated wet flue gas, suggesting their huge potential in practical postcombustion CO2 capture.


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