DOBDC MOF with Amines to Enhance CO2

Apr 11, 2012 - ... is competitive with the best-known adsorbents based on amine–oxide composites. .... ACS Applied Materials & Interfaces 2016 8 (43...
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Modification of the Mg/DOBDC MOF with Amines to Enhance CO2 Adsorption from Ultradilute Gases Sunho Choi,†,# Taku Watanabe,# Tae-Hyun Bae,‡ David S. Sholl,* and Christopher W. Jones* School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, United States S Supporting Information *

ABSTRACT: The MOF Mg/DOBDC has one of the highest known CO2 adsorption capacities at the low to moderate CO2 partial pressures relevant for CO2 capture from flue gas but is difficult to regenerate for use in cyclic operation. In this work, Mg/ DOBDC is modified by functionalization of its open metal coordination sites with ethylene diamine (ED) to introduce pendent amines into the MOF micropores. DFT calculations and experimental nitrogen physisorption and thermogravimetric analysis suggest that 1 ED molecule is added to each unit cell, on average. This modification both increases the material’s CO2 adsorption capacity at ultradilute CO2 partial pressures and increases the regenerability of the material, allowing for cyclic adsorption− desorption cycles with identical adsorption capacities. This is one of the first MOF materials demonstrated to yield significant adsorption capacities from simulated ambient air (400 ppm CO2), and its capacity is competitive with the best-known adsorbents based on amine−oxide composites. SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis capture from dilute gases (i.e., flue gas) or ultradilute streams (i.e., ambient air). Magnesium dioxybenzenedicarboxylate (Mg/DOBDC or Mg-MOF-74) is a MOF built of Mg(II) ions linked by 2,5dioxido-1,4-benzenedicarboxylate (DOBDC) ligands, where the coordination sphere of the Mg(II) ions is also associated with solvent molecules such as H2O or DMF upon synthesis.23,24 The hexagonal, one-dimensional pore structure of Mg/DOBDC becomes accessible to gases after removing the solvent molecules, which also generates coordinatively unsaturated metal sites that can strongly adsorb gases such as CO2. Mg/DOBDC has been reported to have the highest CO2 adsorption capacity, 0.38 g of CO2/g of sorbent at 25 °C using dry 100% CO2, of the series of MOFs synthesized from the DOBDC ligand (where M = Zn, Mg, Ni, or Co) as well as relative to other MOFs reported in the literature to date.25 However, this material has proven to be difficult to regenerate. For example, the Mg\DOBDC material has been reported to require 5 h of desorption at 250 °C for full regeneration of the adsorbent.25 Others have shown the Mg/DOBDC MOF to recover only 87% of its capacity following a room-temperature purge with dry CH423 or that thermal regeneration of Mg/ DOBDC under humid conditions yielded only ∼16% of the initial CO2 adsorption capacity.26 Mason et al. have recently highlighted the difficulty in regenerating this seemingly promising framework (from a CO2 capacity perspective) for use in cyclic gas adsorption cycles.13 Another recent study

M

etal−organic frameworks (MOFs) are an emerging class of nanoporous crystalline solids built of metal coordination sites linked by organic molecules.1−4 Threedimensional organic/inorganic hybrid networks formed by multiple metal−ligand bonds offer well-defined porosity, high surface area, and tunable chemical functionalities, with demonstrated applications in catalysis,5−8 separations,9−14 and gas storage.15,16 Despite the wide range of available structures, most of the MOFs are prepared in a similar manner, via selfassembly of metal precursors and ligands under hydrothermal or solvothermal conditions, followed by the removal of the solvent molecules that remain in the pores. Removal of the solvent molecules renders the frameworks nanoporous and in some cases generates open metal coordination sites, endowing the materials with an ability to adsorb gases such as CO2. However, due to the fact that most MOFs adsorb gases such as CO2 via weak physisorption interactions, most MOFs exhibit unfavorable adsorption capacities and moderate N2/CO2 selectivity at low CO2 partial pressures, such as those found in flue gases (PCO2 = 0.05−0.2 bar) or in ultradilute gas streams such as ambient air (PCO2 = 0.0004 bar).17 Furthermore, use of MOFs for CO2 capture from dilute gases may require partial or complete drying of the gas stream because some MOFs are hydrophilic and thus can adsorb water competitively with CO2.12,17 Moreover, for some materials, the strong affinity of open coordination sites in MOFs for H2O can lead to structural degradation, during which H2O displaces framework ligands and generates defects in the crystal lattice.18−22 Thus, generation of MOF materials that preferentially adsorb CO2 over water and are structurally stable in humid gas streams can help to increase the variety of MOFs that are available for CO2 © 2012 American Chemical Society

Received: March 16, 2012 Accepted: April 11, 2012 Published: April 11, 2012 1136

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the experimental value in the unit cell volume is slightly larger with the LDA (−2.8%) than with the GGA (−1.9%). Because of the better agreement of the unit cell structure, PW91-GGA with the D2 dispersion correction was used in the rest of our DFT calculations. Functionalization of the framework was carried out by grafting of ED onto dehydrated Mg/DOBDC in anhydrous toluene under reflux for 12 h.50 The structures potentially generated after grafting were simulated using DFT calculations, where each open coordination site in the unit cell of Mg/ DOBDC was allowed to bind up to 1 ED molecule. Thus, we examined three possible configurations of ED-functionalized Mg/DOBDC, that is, 1, 3, 6, and 18 ED molecules per unit cell, which corresponds to 2.7, 7.6, 14.2, and 33.1 wt % ED loading in the hybrids, respectively. Figure 1 illustrates the unit cell

regarding the stability of Mg/DOBDC showed that even longterm storage under dry conditions leads to a reduction of unsaturated metal centers and makes the Mg/DOBDC structure unstable, causing a significant decrease in its CO2 adsorption capacity.27 These results suggest that although Mg/ DOBDC initially exhibits outstanding CO2 adsorption capacity during a single operation of adsorption under dry conditions, cyclic CO2 capture processes under either dry or humid conditions can cause a degradation in the working capacity and overall material stability. Amine sites are known to be highly effective for CO2 adsorption and to be amenable to use under dry or humid conditions.18,28 Here, we present the use of postsynthesis amine functionalization of Mg/DOBDC to create a functional adsorbent with a regenerative CO2 adsorption capacity and improved material stability under practical operating conditions simulating CO2 capture from ambient air. The ability to extract CO2 from ultradilute gases such as ambient air (0.0004 bar of CO2) is a stringent test for a CO2 adsorbent, with only materials that strongly adsorb CO2 leading to significant adsorption capacities.29 Recently, we30−35 and others36−40 have demonstrated that amine-modified oxides effectively adsorb CO2 from air with pseudoequilibrium capacities that approach those obtained at flue gas conditions (0.1 bar CO2), but to date, no MOF materials have been reported that do this effectively (although MOFs modified with amines in various ways are known).41−48 In this work, ethylenediamine (ED) was employed as a grafting reagent to functionalize the open coordination sites of the MOF, Mg/DOBDC. ED has been used before to facilitate selective functionalization of metal coordination sites by direct ligation, while the other amine group remains in the pore space of the MOF.49,50 This approach has been used to endow other MOFs with Lewis basic catalyst sites in prior studies.49,50 Mg/ DOBDC was synthesized following a reported procedure under solvothermal conditions at 125 °C in a dimethylformamide (DMF)/ethanol/H2O mixture.51 Activation of the Mg/ DOBDC removed the solvent molecules by solvent exchange with methanol, followed by evacuation at 250 °C. X-ray diffraction (XRD) patterns reveal characteristic peaks of Mg/ DODBC at 2Θ ≈ 6.89 and 11.87° (Supporting Information, Figure S1), which are consistent with those reported previously. The crystalline structure of the Mg/DOBDC was also modeled by optimizing the crystal structure using plane wave density functional theory (DFT) calculations using Ceperley-Alder LDA (CA-LDA) and PW91-GGA functionals. These calculations included Grimme’s D2 dispersion correction52 because LDA or GGA calculations do not correctly account for dispersive interactions. Table 1 shows lattice parameters and unit cell volumes of Mg/DOBDC calculated from these DFT calculations, which show good agreement with those determined experimentally from XRD.51 The deviation from

Figure 1. Unit cell structures of Mg/DOBDC with (a) 0 (bare), (b) 1, (c) 3, (d) 6, and (e) 18 EDs per unit cell.

structures of the ED-functionalized Mg/DOBDC for each case. The distortion in the MOF’s geometry that is visible in Figure 1 with 3 and 6 EDs/unit cell is unlikely to occur in real materials where the ED molecules would not have the highly ordered structure dictated by the periodic nature of our calculations. DFT calculations show that the diamine molecule spontaneously coordinates with the open coordination sites on MG if it is placed in the vicinity of the sites, suggesting that the reaction forming a dative bond between Mg and the amine is thermodynamically favored. One way to gather information on the possible loadings of ED that can be realized experimentally is to assess the binding energy of ED molecules as a function of loading with DFT.

Table 1. Lattice Parameters and Unit Cell Volume from DFT Calculations and XRD measurements parameters

experimental51

CA-LDA

PW91-GGA

a,b (Å) c (Å) α,γ (deg.) β (deg.) volume (Å3)

26.02 6.72 90.0 120.0 3941

25.63 6.74 90.0 120.0 3831

25.79 6.73 90.0 120.0 3864 1137

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volume calculated in this way to 66% of the bare material. A higher loading of 3 ED/unit cell reduces the calculated pore volume to only 53% of that of the bare material. The reduction of the pore volume by ED incorporation was measured experimentally by volumetric N2 adsorption isotherms measured at 77 K. The pore volume of the ED-Mg/DOBDC was lowered compared to that of the bare Mg/DOBDC by a factor of 0.57, to 0.402 cm3/g.25 The surface area dropped from 1094 to 469 m2/g after functionalization. The decrease in pore volume is in between those calculated for our DFT structures with 1 and 3 ED/unit cell. Combining our experimental data and DFT-based results suggests that the ED-functionalized Mg/DOBDC prepared in this work has an ED loading intermediate between the 1 and 3 ED/unit cell configurations discussed above. The CO2 adsorption capacity and regenerability of the Mg/ DOBDC and the ED-Mg/DOBDC MOF were evaluated by monitoring pseudoequilibrium adsorption uptakes of ∼30 mg samples over four adsorption−desorption cycles using a TGA instrument, in which the CO2 adsorption experiments were carried out with dry simulated air (400 ppm CO2 balanced with Ar) for 12 h.33,34 Results attained from the multicycle stability tests are illustrated in Figure 2, along with those of benchmark

Table 2 summarizes the binding energies of ED per molecule in the Mg/DOBDC framework computed from DFT. These Table 2. Binding Energies of an EDA/molecule in Mg/ DOBDC (eV/molecule) Computed from the DFT Calculation Using the PW91-GGA Functional with D2 Correction number of ED molecules in a unit cell binding energy (eV/molecule)

1

3

6

18

0.74

0.98

1.02

1.30

GGA-D2 calculations showed the binding energy increasing monotonically with loading, from 0.74 eV at 1 ED/unit cell to 1.30 eV at 18 EDs/unit cell, due to favorable dispersion interactions between ED molecules. The results presented below, however, indicate that loadings this high were not achieved in our experiments. Our DFT calculations show little change in the unit cell volume of the MOF for ED loadings up to 3 EDs/unit cell, but loadings of 6 (18) EDs/unit cell lead to a significant reduction (increase) in the unit cell volume (see Table S1, Supporting Information). The amount of ED in the experimentally synthesized EDMg/DOBDC sample was determined experimentally by comparing the weight loss of the bare Mg/DOBDC and EDMg/DOBDC in the temperature range of 100−300 °C, chosen considering the boiling point of ED of ∼120 °C. The TGA results (Supporting Information, Figure S2) indicate that the ED amount is ∼5.5 wt %, which suggests that the structure of the ED-Mg/DOBDC most closely approaches the 3 ED configuration shown in Figure 1c. The ED content can also be estimated by assessing the pore volume in the functionalized material. Table 3 lists the pore limiting diameter (PLD), the Table 3. Changes of the Pore Characteristics in Mg/DOBDC Frameworks As a Function of the Number of the ED Molecules Incorporated number of ED molecules in a unit cell pore limiting diameter (Å) largest cavity diameter (Å) pore volume fractiona a

0

1

3

10.6 11.3 1

10.5 11.1 0.66

6.1 6.9 0.53

Figure 2. Changes in the CO2 adsorption capacity of different hybrid adsorbents as a function of the number of adsorption−desorption cycles. The adsorption capacities of the Mg/DOBDC and ED-Mg/ DOBDC was determined from multicycle TGA adsorption experiments in which dry 400 ppm CO2 in Ar was used as the test gas at room temperature. The capacities of the PEI/silica and diamine/silica were reported previously,33,34 where the PEI/silica corresponds to the class (i) solid amine adsorbent prepared by the conventional PEIimpregnation method and the diamine/silica is the class (ii) material made by the silane-grafting of N-(3-(trimethoxysilyl)-propylethylenediamine onto mesoporous silica.

Ratio of the pore volume relative to that of the bare Mg/DOBDC.

largest cavity diameter (LCD), and the pore volume relative to unfunctionalized Mg/DOBDC calculated for the DFToptimized molecular configuration with 0 (bare Mg/ DOBDC), 1, and 3 EDs, respectively. The geometry of the DFT-optimized single ED in Mg/DOBDC is given in Figure S3 and Table S2 in the Supporting Information. The PLD and LCD are geometric parameters that estimate the pore spaces available for molecular adsorption and diffusion.53 Adding 1 ED/unit cell reduces these parameters relative to the bare material, but there is little additional reduction in increasing the ED loading to three molecules/unit cell. The pore volume for these structures was estimated computationally by determining the fraction of locations inside of each material where a probe particle of fixed diameter could be placed without overlap with atoms in the pore structure. The probe size was set to 3.64 Å to approximately match the kinetic diameter of the N2 molecule. The presence of 1 ED molecule/unit cell reduces the pore

materials, PEI-impregnated silica, and diamine-grafted silica reported previously.33,34 Despite having the highest capacity in the first cycle among all of the adsorbents tested here, the conventional impregnated-amine adsorbent (PEI/silica) revealed a significant loss in the adsorption capacity over successive operations. For example, the adsorption capacity of the PEI/silica decreased by 0.70 mmol/g after four cycles, from 2.36 to 1.65 mmol/g. An observed substantial capacity degradation of amine-impregnated or amine-grafted materials was previously attributed to the loss of physically adsorbed amines or amine degradation by urea formation.54−56 In contrast, the silica surface-grafted amine adsorbent (diamine/ 1138

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Present Addresses

silica) presents robust stability under these bone dry conditions, showing relatively stable CO2 uptakes during the same number of cycles, with only a slight decrease.33,34 However, the relatively small number of amine adsorption sites in this material leads to the lowest adsorption capacity under the conditions used. The initial CO2 uptake of the Mg/DOBDC was higher than that of the grafted-amine adsorbent, measured as 1.35 mmol/g in the first cycle, but decreased considerably under recurring operations. For instance, the adsorption capacities of the Mg/ DOBDC were lowered by 20% after four cycles, from 1.35 to 1.06 mmol/g. Significant degradation of the adsorption capacities found in the Mg/DOBDC can be possibly ascribed to the material stability26,27 or the activation/desorption method.23,25 Among those factors, the latter may be better associated with the capacity reduction observed in this work, as the desorption conditions employed in this work (Ar purge, 3 h, 110 °C) are milder than those required for full regeneration (high vacuum, 5 h, 250 °C).25 Previous reports indicate that moderate desorption processes depreciate the regenerability of the Mg/DOBDC significantly.9,23 This implies that the bare Mg/DOBDC demands more energy input for its regeneration than applied in this work, possibly making a CO2 capture process with this adsorbent significantly more energy-intensive. In contrast, the ED-Mg/DOBDC marked the second highest capacity among the samples studied, 1.51 mmol/g in the first cycle, which is higher than that of the parent Mg/DODBC MOF. Moreover, this adsorbent provides excellent stability and becomes fully regenerable under the moderate adsorption− desorption operations used here, showing CO2 adsorption capacities of 1.50, 1.54, and 1.55 mmol/g over successive cycles. These observations suggest that the new ED-Mg/DOBDC material synthesized in this work is a more stable adsorbent compared to the parent Mg/DOBDC because it can completely regain its adsorption capacity under mild regeneration conditions. In summary, we have demonstrated a simple modification of the well-known MOF, Mg/DOBDC, with ED to substantially improve its CO2 adsorption properties at low CO2 partial pressures. The modified MOF, ED-Mg/DOBDC, had both improved CO2 adsorption capacity at ultradilute CO2 partial pressures and increased stability/regenerability, being cycled four times with identical adsorption capacities. This is the first MOF material demonstrated to yield significant adsorption capacities from simulated ambient air (400 ppm CO2),57 and its capacity is competitive with the best known adsorbents based on amine−oxide composites.31,33,34,36,37 Future studies will need to address stability to repeated, long-term adsorption cycling,54,56stability to contaminants, and oxidation.58

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†

Department of Chemical Engineering, Northeastern University, Boston, MA 02115, United States. ‡ Department of Chemistry, University of California, Berkeley, Berkeley, CA 94720, United States. Author Contributions #

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank DOE-NETL for funding under Contract DE-FE0002438. T.W. and D.S.S. acknowledge support from the DOE ARPA-E IMPACCT program under Grant DEAR0000074.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental and simulation procedures and details. This material is available free of charge via the Internet at http:// pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (D.S.S.); cjones@chbe. gatech.edu (C.W.J.). 1139

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The Journal of Physical Chemistry Letters

Letter

(55) Hicks, J. C.; Drese, J. H.; Fauth, D. J.; Gray, M. L.; Qi, G. G.; Jones, C. W. Designing Adsorbents for CO2 Capture From Flue Gas− Hyperbranched Aminosilicas Capable of Capturing CO2 Reversibly. J. Am. Chem. Soc. 2008, 130, 2902−2903. (56) Sayari, A.; Belmabkhout, Y. Stabilization of Amine-Containing CO2 Adsorbents: Dramatic Effect of Water Vapor. J. Am. Chem. Soc. 2010, 132, 6312−6313. (57) Long and coworkers have applied a similar approach, and their publication has recently appeared: McDonald, T. M.; Lee, W. R.; Mason, J. A.; Wiers, B. M.; Hong, C. S.; Long, J. R. Capture of Carbon Dioxide from Air and Flue Gas in the Alkylamine-Appended Metal− Organic Framework mmen-Mg2(dobpdc). J. Am. Chem. Soc. 2012, DOI: 10.1021/ja300034j. (58) Bollini, P.; Choi, S.; Drese, J. H.; Jones, C. W. Oxidative Degradation of Aminosilica Adsorbents Relevant to Postcombustion CO2 Capture. Energy Fuels 2011, 25, 2416−2425.

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