Letter pubs.acs.org/JPCL
Site Partition: Turning One Site into Two for Adsorbing CO2 Ziqi Tian,† Sheng Dai,‡,§ and De-en Jiang*,† †
Department of Chemistry, University of California, Riverside, California 92521, United States Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6201, United States § Department of Chemistry, The University of Tennessee, Knoxville, Tennessee 37996-1600, United States ‡
S Supporting Information *
ABSTRACT: We propose the concept of site partition to explain the role of guest molecules in increasing CO2 uptake in metal−organic frameworks and to design new covalent porous materials for CO2 capture. From grand canonical Monte Carlo simulations of CO2 sorption in the recently synthesized CPM-33 MOFs, we show that guest insertion divides one open metal site into two relatively strong binding sites, hence dramatically increasing CO2 uptake. Further, we extend the site partition concept to covalent organic frameworks with large free volume. After insertion of a designed geometry-matching guest, we show that the volumetric uptake of CO2 doubles. Therefore, the concept of site partition can be used to engineer the pore space of nanoporous materials for higher gas uptake.
C
CO2 uptake to MOF-74-Ni at ambient temperatures such as 298 K. In comparison with the idea of pore-space partition proposed, we show in this work that a more important factor underlying the excellent CO2-uptake performance of CPM-33 MOFs is site partition. More excitingly, we demonstrate that the site-partition concept can be broadened to design new guest-inserted framework materials for doubling CO2 uptake. This paper is organized as follows. First, we focus on simulation of CO2 absorption in CPM-33a [Ni3OH(bdc)3tpt], the basic structure of CPM-33 family MOFs. From the experimental crystal structure, we performed spin-polarized DFT calculation using Vienna ab initio simulation package (VASP),24 to relax its geometry and obtain the electrostatic potential that was used to generate atomic partial charges by the REPEAT method.25 Next, we carried out grand canonical Monte Carlo (GCMC) simulation with the MUSIC code26 to reproduce CO 2 absorption isotherms at 273 and 298 K, based on the REPEAT-derived atomic charges and Dreiding force field for the framework27 and the three-site model for CO2.28 The simulation details are provided in the Supporting Information (SI). Figure 1 shows the GCMC-simulated CO2 adsorption isotherms (Figure 1a) in comparison with the experiment (Figure 1b) for CPM-33a (which contains the tpt guest ligands in the framework) at 273 and 298 K. One can see that the simulated and experimental isotherms are in good agreement, even though the simulation slightly overestimates CO2 uptakes
O2 capture from exhaust streams of fossil fuel combustion is required to reduce greenhouse gas emission and control climate change. Due to their intrinsic porosity and abundant functionality, metal−organic frameworks (MOFs) are promising materials for efficient CO2 separation from other gases.1−4 It has been shown that gas adsorption uptake depends on binding site, surface area, and free volume at low, moderate, and high pressures, respectively.5 Therefore, creating strong binding site is a commonly used strategy to maximize gas uptake at ambient condition, which can be realized by introducing functional groups or open metal sites (OMS) into the framework. Most CO2-philic functional groups physisorb CO2 with binding energy in a range from 15 to 25 kJ/mol.6 In comparison, interaction between CO2 and OMS is generally more than 30 kJ/mol,7−9 leading to high CO2 uptake at ambient condition.10−12 So far, the MOF-74 family with OMS are one of the best CO2 sorbents at room temperature and low pressure among nanoporous materials. However, OMS may result in decreased stability and competitive sorption from undesired sorbates,13−15 such as water and other Lewis bases. Another strategy to increase CO2 uptake is by controlling pore space.16−20 As such, Bu and Feng proposed the idea of pore space partition.21,22 They selected the flexible MIL-88-Ni [Ni3OH(bdc)3, bdc = benzene-1,4-dicarboxylate] as the parent framework, and then inserted symmetry-matching secondary ligand, such as 2,4,6-tri(4-pyridinyl)-1,3,5-triazine (tpt), to obtain a series of new MOFs, labeled as the CPM-33 family.23 The guest ligands occupied all the open-metal sites in the parent MOF, rigidified the MIL-88-Ni framework, and further partitioned the one-dimensional channel into multiple compartments. By further introducing functional groups, they demonstrated that some CPM-33 MOFs have comparable © XXXX American Chemical Society
Received: May 25, 2016 Accepted: June 18, 2016
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Figure 1. (a) Simulated isotherms of CO2 uptake in CPM-33a and CPM-33-open at 273 and 298 K; (b) experimental isotherms23 of CO2 uptake in CPM-33a at 273 and 298 K.
by about 10%. The comparison between the simulation and the experiment for the other two members of the CPM-33 family shows good agreement as well (see Figures S1 and S2 in the SI), confirming the good quality of the force-field parameters used in our GCMC simulations. Figure 1a also compares simulated CO2 adsorption isotherms for CPM-33a and CPM33-open (where the tpt guest molecule was removed); one can see that after guest insertion, CO2 uptake more than doubles. Indeed, our simulation confirms that insertion of the guest ligands dramatically increases CO2 uptake. To understand the role of the guest ligands in the pore space of the framework, we further examined where CO2 are adsorbed in CPM-33a. From CO2 distribution in CPM-33a (Figure 2), we can locate two binding positions: Site I around
Figure 3. DFT-D3-optimized structures of CO2 adsorption in CPM33a: (a) first and (b) second binding positions around one Ni cation. The key distances (in angstrom) between C of CO2 and the carboxylate O atoms in the framework are shown.
adsorbed complexes. The guest insertion divides the space into two independent areas; as a result, adsorption of the first CO2 has little influence on the second one. The pair of binding sites have almost identical DFT-D3 binding energies of 29.9 and 29.7 kJ/mol, respectively. Some recent studies showed that neglect of zero-point energy and other thermal correction in calculation might reduce binding energy by about 5 kJ/mol.30 Thus, our theoretical calculation is in consistent with the experimental isosteric heat of 22.5 kJ/mol.23 Binding site II has a moderate computed binding energy of 20.5 kJ/mol, mainly due to electrostatic attraction from hydrogen in the framework to oxygen in CO2. At relatively low temperature, such as 273 K, site II can also contribute to CO2 capture, which results in more than two adsorbed gas molecules per Ni2+. The above analysis of the binding sites suggests that site I around the Ni cation is the dominant location for CO2 adsorption. In addition, this site is partitioned into two symmetric binding positions. We think that this is the key reason for the significantly increased CO2 uptake after the guest ligand insertion. This point is different from the pore-space partition assumed previously.21−23 One can see from Figure 2 that there is still much empty space not occupied by CO2 even after the compartmentization of the pore space, supporting our explanation of site partition. This site partition is best illustrated in Scheme 1: partitioning one open metal site into two relatively strong binding sites, leading to the remarkable CO2 uptake at low temperature.
Figure 2. CO2 distribution (gray dots) in CPM-33a from GCMC simulation at 273 K and 100 kPa. Color code: C in the framework, dark gray; H, white; O, red; N, blue; Ni, green.
the Ni atoms and site II around the bdc linker. We determined their binding energies with dispersion-corrected DFT calculation (DFT-D3):29 the calculated binding energies of site I and site II are 29.9 and 20.5 kJ/mol, respectively. Site I with the higher binding energy dominates CO2 adsorption at ambient condition and the optimized structure with adsorbed CO2 at site I is shown in Figure 3. There are two symmetric binding positions around the Ni cation. Binding is mainly originated from carboxylate groups with a negative partial charge. C−O nonbonded distances range from 3.0 to 3.1 Å in the CO22569
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framework (COF) system, another type of promising material for CO2 capture.31−34 Because of its high symmetry and rigid structure, triptycene has been widely used as the building blocks for COF construction.35−39 Based on the triptycene derivative, El-Kaderi et al. prepared a COF named TDCOF-5 (Figure 4),35 which has high crystallinity, high porosity, and
Scheme 1. Partition of the Open-Metal Site by Guest Insertion
To further increase CO2 uptake, Feng et al. introduced hydroxyl groups on the bdc linker in CPM-33a to enhance the CO2 affinity, leading to CPM-33b [Ni3OH(dhbdc)3tpt, dhbdc =2,5-dihydroxlbenzene-1,4-dicarboxylate].23 We performed GCMC simulations of CO2 uptake in CPM-33b (Table 1). Table 1. Comparison of Simulated CO2 Uptakes (mmol/g) of Two CPM-33 MOFs at 100 kPa with the Experiment23 experiment
simulation
MOF
273 K
298 K
273 K
298 K
CPM-33a CPM-33b
6.09 7.76
3.29 5.64
6.65 8.21
3.87 5.70
The simulated CO2 uptakes are in good agreement with experiment. More important, hydroxyl-functionalized CPM-33b greatly increases CO2 uptake than CPM-33a, especially at room temperature, as confirmed in our GCMC simulations. Carboxylate group on the dhbdc linker can form intramolecular hydrogen bond and polarize the neighboring hydroxyl group, thereby enhancing interaction of hydroxyl to CO2, as represented in Scheme 2. DFT optimization finds that the
Figure 4. Structures of (a) TDCOF-5 (ref 35) and (b) a designed framework comprising TDCOF-5 and size-matching guests. Color code: C, dark gray; H, white; O, red; N, blue; B, pink.
large surface area. However, owing to lack of strong binding site, its CO2 uptake was low at even low pressure. We designed and introduced a geometry-matching guest molecule into TDCOF-5. Guest insertion increases the number of corners in each channel from 6 (Figure 5a) to 12 (Figure 5b), leading to a volumetric uptake increase from 0.110 mmol/mL to 0.250 mmol/mL at 273 K and 100 kPa, doubling the CO2 capacity (Figure 6). Hence, the site-partition concept is a very useful strategy to increase gas adsorption in nanoporous materials. It is of interest to compare the concept of pore space partition proposed previously21−23 based on the experimental data of the CPM-33 family and the idea of site partition proposed here based on molecular simulations. The concept of pore space partition is more general but less detailed, while site partition focuses on space partition at the binding sites. So site partition can be viewed as a specific manifestation of pore space partition. In other words, they are not mutually exclusive but complementary concepts. In sum, our simulations reproduced the outstanding CO2 uptake of CPM-33 family MOFs at ambient conditions, in good agreement with experiment. From CO2 distribution and calculated binding energies, we showed that guest ligand insertion divides one open metal site into two relatively strong CO2 binding sites, leading to much higher CO2 uptake. From this site partition concept, we designed rigid guest molecules for triptycene-based COF materials. We showed that the COF material with the size-matching guest can double the volumetric CO2 uptake. Therefore, site partition can be a very useful strategy to increase CO2 uptake of porous material at ambient conditions.
Scheme 2. Intramolecular Hydrogen Bond (Indicated by the Solid Arrow) between the Carboxylate Group and the Hydroxyl Group on the dhbdc Linker of CPM-33b Helps Polarize the O−H Bond That in Turn Helps Binding with CO2 (Curved Arrow)
distance between oxygen in the hydroxyl group and CO2 is 2.805 Å, resulting a binding energy of 34.5 kJ/mol, 5.6 kJ/mol higher than the binding energy at site I in CMP-33a. This stronger interaction in CPM-33b is consistent with the higher experimental isosteric heat of adsorption of CO2 in CPM-33b than in CPM-33a.23 The large binding energy improves CO2 capacity at room temperature. The guest insertion and site partition as shown above for the CPM-33 family of MOFs can be a general strategy to engineer pore space for doubling gas uptake. To show that it is indeed the case, we applied the concept to a covalent organic 2570
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ACKNOWLEDGMENTS This work was supported by the Division of Chemical Sciences, Geosciences and Biosciences, Office of Basic Energy Sciences, U.S. Department of Energy. We thank Dr. Xianhui Bu and Dr. Pingyun Feng for very helpful discussions. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
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Figure 5. GCMC-simulated CO2 distribution (gray dots) in (a) TDCOF-5 and (b) the designed TDCOF-5-guest framework (Figure 4b) at 273 K and 100 kPa. Color code: C in the framework, dark gray; H, white; O, red; N, blue; B, pink.
Figure 6. Comparison of volumetric CO2 uptakes at 273 K for TDCOF-5 (red triangle) and the designed TDCOF-5-guest framework (black solid circle).
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpclett.6b01141. Computational details and comparisons of simulated and experimental isotherms of CPM-33a, CPM-33b, and CPM-33c at 298 and 273 K (Figures S1−S2) (PDF)
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AUTHOR INFORMATION
Corresponding Author
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[email protected]. Telephone: +1-951-827-4430. Notes
The authors declare no competing financial interest. 2571
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