Enhanced Selective CO2 Capture upon Incorporation of

Dec 18, 2015 - During the sample preparation or after an exchange process, compound 1 could trap guest molecules [dimethylformamide (DMF)] into the ...
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Enhanced Selective CO2 Capture upon Incorporation of Dimethylformamide in the Cobalt Metal−Organic Framework [Co3(OH)2(btca)2] Hai-Yun Ren and Xian-Ming Zhang* School of Chemistry and Material Science, Shanxi Normal University, Linfen, Shanxi 041004, People’s Republic of China S Supporting Information *

ABSTRACT: As society’s demand for energy increases and more CO2 emissions are produced, it becomes imperative to selectively capture CO2 and decrease the amount emitted to the atmosphere. Here, we present a flexible and porous metal− organic framework (MOF), [Co3(OH)2(btca)2] (1) (where btca = benzotriazole-5-carboxylic acid), which exhibits excellent CO2 uptake (223.7 mg g−1 at 273 K and 104.7 mg g−1 at 298 K) and better selectivity for CO2/N2 (46.3 at 273 K) . During the sample preparation or after an exchange process, compound 1 could trap guest molecules [dimethylformamide (DMF)] into the channels and then yield an isostructural product [Co3(OH)2(btca)2]·0.5DMF (1·0.5DMF). 1·0.5DMF hardly adsorbs N2, O2, and H2 but favorably captures CO2 in the entire pressure range measured, responded by highly selective CO2/N2 gas separation with Henry’s law selectivity of 79.6.

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INTRODUCTION The sharply rising level of atmospheric carbon dioxide from fossil fuel combustion (accounting for ca. 80% of CO2 emissions worldwide) is one of the greatest environmental concerns facing our civilization today.1 Developing effective technologies/processes for CO2 capture, storage, and utilization is especially important.2 Among them, physisorption-based technologies and processes account for the majority of these research activities, which could require much less energy to allow for conveniently reversible processes to capture CO2 gas.3 Activated carbon, carbon molecular sieves, and zeolites have been extensively studied as adsorbents for CO2 gas.4 However, the common shortfalls of these traditional adsorbents are either low capacities or difficult regeneration processes. Metal−organic frameworks (MOFs) as promising materials could serve as an ideal platform for the development of nextgeneration CO2 capture materials, owing to their large capacity for the adsorption of gases and their structural and chemical tunability. However, the selective capture of CO2, in particular at ambient temperature and pressure, from transportation emissions or direct air capture still remains challenging. In recent years, worldwide efforts have been devoted to improve the selectivity of MOFs toward CO2 capture,2 which result in a lot of efficient approaches, such as tuning of pore size contribution via controlling the size of the ligand,5 functionalization of the pore wall via modifying organic linkers or unsaturatedly coordinated metal sites,6 introduction of an electrostatic field with charged frameworks or counterion species,7,2e and modification of the large pores or open channels with functional small molecules, such as organic amine and inorganic salts.8 Most notably, the incorporation of N,N-dimethylethylenediamine into a triazolate-bridged framework drastically enhances CO2 adsorption to 2.38 mmol g−1 (9.5 wt %).2f In contrast, it is much more convenient to incorporate functional small molecules into the large pores or open channels. © 2015 American Chemical Society

We previously reported desolvation within a {Co3(OH)2} chain-based MOF resulted in one-dimensional (1D) rhombic channels in dehydrated [Co3(OH)2(btca)2] (1) and induced the magnetic transition from single-chain magnet-like behavior to field-induced metamagnetism.9 However, the lower N2 isotherm of compound 1 in that work was not enough to display its higher porosity (39.1%). Therefore, we investigated in detail the adsorption behavior of compound 1 on various gases, especially those related to energy and material, aiming at revealing its adsorptive performance and selectivity. Additionally, the efficacy of incorporation of dimethylformamide (DMF) into the 1D channels was also discussed.

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals were analytically pure from commercial sources and used without further purification. Elemental analyses were performed on a Vario EL-II analyzer. Fourier transform infrared (FTIR) spectra were recorded from KBr pellets in the range of 4000−400 cm −1 on a PerkinElmer spectrum BX FTIR spectrometer. Powder X-ray diffraction (PXRD) data were collected in a Rigaku Ultima IV X-ray diffractometer. The purity of all compounds was confirmed by comparison of experimental PXRD patterns to the simulated pattern derived from the X-ray single-crystal data compound. The thermogravimetric analyses (TGAs) were carried out in an air atmosphere using SETARAM LABSYS equipment at a heating rate of 10 °C/min. Gas Sorption Measurements. Gas adsorption measurements were performed using a Micromeritics ASAP 2020HD88 surface area and pore size analyzer. Samples 1 and 1·0.5DMF were degassed at 180 and 40 °C, respectively, for 12 h under conditions of dynamic vacuum before analysis. The N2, O2, and H2 sorption isotherms were acquired in the pressure range of P/P0 from 0.0001 to 1.0 at 77 K in a liquid nitrogen bath. The gas sorption experiments of CO2 were conducted Received: October 12, 2015 Revised: December 6, 2015 Published: December 18, 2015 526

DOI: 10.1021/acs.energyfuels.5b02393 Energy Fuels 2016, 30, 526−530

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Energy & Fuels

C7.75H3Co1.5N3.25O3.25 (1·0.5DMF): C, 33.01; H, 1.07; and N, 16.14. Found: C, 33.06; H, 1.08; and N, 16.12. Infrared (IR) data (KBr, cm−1): ν = 3615(w), 3440(vs), 2918(s), 2848(m), 1662(s), 1560(m), 1400(s), 1269(w), 1168(w), 1093(w), 785(m), 702(w), 604(w), and 401(w). Simulation Model and Methods. Grand canonical Monte Carlo (GCMC) simulations were carried out for CO2 adsorption in samples 1 and 1·0.5DMF to identify the adsorption sites in gas-loaded structures. The experimentally determined crystal structures of samples 1 and 1·0.5DMF were used in the simulations. One unit cell was used during the simulations. All parameters for CO2 molecules and atoms of samples 1 and 1·0.5DMF were modeled with the universal force field (UFF) embedded in the MS modeling package. A cutoff distance of 15.5 Å was used for Lennard−Jones (6−12) interactions, and the Coulombic interactions were calculated using Ewald summation. For each run, the 5 × 105 maximum loading steps, 1 × 106 production steps, automated temperature control in the annealing cycles, and 15 temperature cycles were employed.

in an ice−water bath (273 K) and a room-temperature water bath (298 K), respectively. Additionally, the N2 sorption isotherms of samples 1 and 1·0.5DMF at 273 K were also measured in an ice−water bath (P/P0 from 0.0001 to 1.0). The obtained N2 adsorption− desorption isotherms at 77 K were evaluated to give the pore parameters, including Brunauer−Emmett−Teller (BET) surface area, pore size, and pore volume. Crystallographic Studies. X-ray single-crystal diffraction data of 1·0.5DMF were collected on an Agilent Technologies Gemini EOS diffractometer at 293 K using Mo Kα radiation (λ = 0.710 73 Å). The program SAINT was used for integration of diffraction profiles, and the program SADABS was used for absorption correction. The structure was solved with the SHELXS structure solution program by direct methods and refined by the full-matrix least-squares technique using SHELXL.10 All non-hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were generated theoretically onto the specific carbon atoms and refined isotropically with fixed thermal factors. Further details for structural data are summarized in Tables S1 and S2 of the Supporting Information. Synthesis of [Co3(OH)2(btca)2]·0.5DMF (1·0.5DMF). As shown in Figure 1, [Co3(OH)2(btca)2] (1) was prepared according to our

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RESULTS AND DISCUSSION Effect of DMF Incorporation. As shown in Figure 2, the biggest difference in the PXRD between samples 1 and 1· 0.5DMF observed that the intensity of the peak at 2θ = 9.7° is considerably higher than the peak at 12.1° in 1·0.5DMF but it would be exactly the opposite in compound 1. The PXRD patterns of DMF-soaked compound 1 and single-crystal simulated pattern of 1·0.5DMF are in excellent agreement, indicating incorporation of DMF into the channels of compound 1. The extent of noise in the PXRD patterns might be due to minor loss of crystallinity as a result of the long-term soaking in DMF. Single-crystal X-ray analysis of 1· 0.5DMF shows that the three-dimensional (3D) framework remains intact upon dehydration. The phase purity is confirmed by PXRD. In comparison to compound 1, the Co1 atom of 1· 0.5DMF shows an elongated octahedral geometry (shown in Figure S1 of the Supporting Information), in which the equatorial Co1−L (L = N and O) bond lengths are slightly contracted by 0.013 Å and the axial Co1−O (O3 and O3a) bond lengths stretch about 0.031 Å. The changes in the Co− O−Co angles are limited to 2.6° [Co−O−Co angles of 106.3(2)°, 115.5(3)°, and 101.5(2)° in 1·0.5DMF]. A calculation with PLATON11 reveals that the free volume of the channels in 1·0.5DMF is contracted to 294 Å3 per unit cell or 12.1% of the total volume, which may show promising properties for selective gas capture. N2, H2, O2, and CO2 Sorption of Compound 1. Gas adsorption studies were conducted up to a relative pressure (P/ P0) of 1.0 on the activated frameworks at standard temperature and pressure (STP). The N2 and O2 isotherms of compound 1 (Figure 3) show rapid gas uptake at low relative pressures (P/ P0 < 0.02), which is typical for microporous materials. Pore size distributions were calculated from the adsorption branch of the N2 isotherm by the density functional theory (DFT) (Figure S2 of the Supporting Information), which shows major pore widths centered at 1.12 nm. The result also confirmed that compound 1 was a typical crystalline microporous material. Meanwhile, it has a moderate absorption of H2 (uptake capacity of 1.12 wt % at 1.1 atm and 77 K). Because of the considerable porosity and electron richness, we were interested in assessing its performance in CO2 uptake. We first collected CO2 isotherms, as shown in Figure 4. Compound 1 shows excellent CO2 uptake of 223.7 mg g−1 (273 K) and 104.7 mg g−1 (298 K) at 1.12 atm. In principle, nitrogen-rich porous MOFs exhibit high CO2 adsorption

Figure 1. Synthesis routes toward samples 1 and 1·0.5DMF. previous paper, and its purity was confirmed by the PXRD pattern (Figure 2). The crystal samples were immersed in 15 mL of DMF,

Figure 2. PXRD patterns of compound 1 in various processes and assynthesized 1·0.5DMF: violet, adsorption/desorption of 1·0.5DMF; pink, 1·0.5DMF via incorporation of DMF in compound 1; blue, assynthesized 1·0.5DMF; orange, simulated 1·0.5DMF; red, dehydrated compound 1; and blank, simulated dehydrated compound 1). which was replaced with fresh DMF every 3 h for 1 week, thus ensuring a complete incorporation of DMF. During this course, the color of the samples changes from red to dark violet. The final products were separated by filtration and dried in a vacuum oven at 40 °C for 12 h. Dark-violet powder of 1·0.5DMF was recovered. Singlecrystal samples of 1·0.5DMF can be obtained via solvothermal treatment of a mixture of H2btca, Co(NO3)2·6H2O, DMF, CH3CN, and H2O in a molar ratio of 1:2:26:19:222 at 150 °C in a Teflon-lined stainless autoclave for 5 days. Anal. Calcd (%) for 527

DOI: 10.1021/acs.energyfuels.5b02393 Energy Fuels 2016, 30, 526−530

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Energy & Fuels

to the secondary amine group in triazole as a Lewis base site, which is expected to induce a strong interaction between triazole and CO2. The higher adsorption enthalpy (Qst) at zero coverage for CO2 (42.9 kJ mol−1) gives more insight into the interaction of the adsorbate with the framework, which is evaluated using the Clausius−Clayperon equation13 from the isotherms obtained at 273 and 298 K (Figure S3 of the Supporting Information). Effect of DMF Incorporation on the Gas Sorption. The incorporation of DMF into the channels of compound 1 results in a material with specific CO2 adsorption characteristics. As shown in the inset of Figure 5, 1·0.5DMF hardly adsorbs N2,

Figure 3. N2, O2, and H2 at 77 K sorption isotherms of dehydrated compound 1.

Figure 5. CO2 sorption isotherms of 1·0.5DMF at 273 and 298 K (inset: N2, O2, and H2 sorption isotherms at 77 K).

Figure 4. CO2 at 273 and 298 K sorption isotherms of dehydrated compound 1.

O2, and H2 at 77 K in the entire pressure range measured but favorably displays excellent CO2 uptake of 119.8 mg g−1 at 273 K and 63.4 mg g−1 at 298 K, respectively. The much poorer N2 sorption at 273 K suggests its highly selective sorption toward CO2 (Figure 7b). The measured adsorption isotherm for CO2 at 273 K is 6.8 molecules per unit cell, which is lower than 12 molecules per unit cell in compound 1 at 273 K. GCMC simulations (see the Supporting Information) agree fairly well with the experiments. The sorption in 1·0.5DMF (about 6

capacities as a result of the strong dipole−quadrupole interactions between CO2 and nitrogen sites. The amount of adsorbed CO2 in compound 1 at 273 K is significantly higher than those nitrogen-rich MOFs,2b,e−g,12a−k,n,o just below the MOFs with large surface areas, such as MAF-6612l (140 cm3 g−1) and [CuL]·DMF·H2O12m (151 cm3 g−1) (Table 1), which represent the best CO2 adsorption property among the MOFs with low surface areas. Higher adsorbed CO2 can be attributed

Table 1. Comparison of Some Microporous MOFs for CO2 Storage (at 273 K and 1 atm) structure

BET surface area (m2/g)

adsorption for CO2 (cm3/g)

heat of adsorption (kJ/mol)

1 1·0.5DMF A-B2III2b [Zn3L2(HCOO)1.5][(CH3)2NH2]1.5·xDMF2e mmen-CuBTTri2f [Zn(L)·H2O]·DMA2g MnIII2MnII4O2(pyz)2(C6H5CH2COO)1012a ZIF-812b CMP-1-NH212c [Zn2(L)(bpb)2]·(NO3)·(DMF)3·(H2O)412d Zn2(adb)2(dabco)12e Cu-TBA-112f Cz-POF-312g [Co2(DBIBA)3]·Cl·9H2O12h UTSA-15a12i UTSA-1612j [Cd(Tipa)Cl2]·2(DMF)·H2O12k MAF-6612l [CuL]·DMF·H2O12m MAF-2512n U-Liu312o

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114 61 60.7 58 89.6 20.8 38 22.4 35.8 54.3 30 68.9 106.9 52.1 56 71.6 48.17 140 151 29 33

42.9 60.1 33.4

589 569.5 870 175 1758 710 135.9 960 616

553 939 348.8 1014 810 325 528

96 43.7 29 17.5 29.5 23.5

27.8 40.6 32 26.0 26.3 31 DOI: 10.1021/acs.energyfuels.5b02393 Energy Fuels 2016, 30, 526−530

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Energy & Fuels molecules per unit cell) is smaller than the result in compound 1 (8 molecules per unit cell). Furthermore, it is noted that no obvious change in the PXRD pattern was observed, except that the peak at 2θ = 9.7° shifted slightly as a result of the flexibility and shrinkage of the framework, which shows that the crystalline structure of 1·0.5DMF was stable throughout adsorption/desorption (Figure 2). TGA revealed that the DMF molecules in 1·0.5DMF are not released upon adsorption/desorption, which is in line with the stable framework of 1·0.5DMF (Figure 6).

Figure 6. TGA curve of compound 1 (red), as-synthesized 1·0.5DMF (green), and adsorption/desorption for 1·0.5DMF (blue) in air at the heating rate of 10 °C min−1.

Figure 7. Virial analyses of the CO2 and N2 sorption data for (a) 1 and (b) 1·0.5DMF.

These results indicate that 1·0.5DMF displays significant CO2 selectivity over N2, O2, and H2, despite the reduction in CO2 capacity upon DMF grafting. It is possible that the amide functionalities endow the channel with a higher polarity, which interacts with CO2 molecules and, thus, increases the CO2 binding14 by virtue of the strong dipole−quadrupole interactions. However, in comparison to the special quadrupole moment of CO2 (−1.4 × 10−39 C m2),15 N2 and H2 have small quadrupole moments (4.7 × 10−40 and 2.2 × 10−40 C m2, respectively)16 and, thus, cannot be adsorbed on the pore surface via this interaction. Qst at zero coverage for CO2 (60.1 kJ mol−1) is higher than that observed in compound 1, which shows very good agreement with the stronger CO2 binding (Figure S4 of the Supporting Information). However, Qst of CO2 gradually decreases with increasing loading, indicating the preferable adsorption sites being occupied gradually. CO2/N2 Gas Selectivity Studies. The above-mentioned results demonstrated that samples 1 and 1·0.5DMF have the ability to selectively adsorb CO2 over N2. To evaluate their CO2/N2 adsorption selectivity, Henry’s law selectivity was calculated with estimated Henry’s constants of singlecomponent adsorption isotherms (Figure 7). ln(N /P) = A 0 + A1N + A 2 N 2 , ...

Henry’s law selectivity for gas component i over j is calculated on the basis of eq 3.

Sij = KHi /KHj

On the basis of the above isotherms, a higher CO2/N2 selectivity of 79.6 for 1·0.5DMF was obtained at 273 K, which is greater than 46.3 for compound 1. The satisfying high selectivity of CO2 was attributed to the synergistic effect of the pore size effect and the enhanced host−guest interaction caused by DMF incorporated into the channel, in agreement with increased Qst at zero coverage. Therefore, their application in CO2 capture and separation, especially CO2/N2 separation from flue gas, can be energetically expected.

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CONCLUSION In conclusion, compound 1 exhibits an excellent CO2 uptake (223.7 mg g−1 at 273 K and 104.7 mg g−1 at 298 K) and better selective CO2/N2 gas separation, with Henry’s law selectivity of 46.3. Incorporation of DMF into its channels results in an interesting sorption that only CO2 molecules could be adsorbed in the channel of 1·0.5DMF, with a better selectivity for CO2/ N2 (79.6). The greatly improved selectivity for CO2/N2 suggests an efficient direction for synthesis of high-selectivity CO2 adsorbents. Future work will focus on evaluating the efficacy of small molecular amide, such as formamide, in improving the separation performance of the material.

(1)

After being converted to exponential form, the whole of eq 1 becomes P = N /exp(A 0 + A1N + A 2 N 2 , ...)

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(2)

Henry’s constant (KH) can be obtained by virial fitting based on eq 2, in which P is the pressure in Pa, N is the adsorbed amount in mmol g−1, and A0, A1, A2, etc. are virial coefficients. A0 is related to adsorbate−adsorbent interactions, whereas A1 and A2 describe adsorbate−adsorbate interactions. Henry’s law constant (KH) can be obtained. KH = exp(A 0)

(4)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.5b02393. Crystal structural data for 1·0.5DMF, experimental details, and additional figures and tables (PDF) CCDC1412257 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the 973 Program (2012CB821701) and the Ministry of Education of China (IRT1156).

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DOI: 10.1021/acs.energyfuels.5b02393 Energy Fuels 2016, 30, 526−530