philic Groups - American Chemical Society

Article pubs.acs.org/JPCA

Ab Initio Screening of CO2‑philic Groups Ziqi Tian,† Tomonori Saito,‡ 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 37831-6210, United States

‡

ABSTRACT: Ab initio calculations were used to identify CO2-philic groups. Over 55 neutral molecules were screened for CO2 affinity via binding energetics. It is found that poly(ethylene oxide)s (PEO) oligomers with more than three repeating units are good CO2-binding groups, consistent with the highperformance of PEO-based materials for CO2/N2 separation. More interestingly, two triazole groups linked with a methylene chain are also excellent for CO2 binding with a favorable interaction of more than 28 kJ/mol, indicating that polymers or covalent-organic frameworks (COFs) with triazoles may be utilized for CO2 capture. This work provides a useful guide to introduce promising organic groups into polymeric membranes and COFs for CO2/N2 separation media.

1. INTRODUCTION Fossil fuel combustion and many other industrial processes generate large amounts of CO2, which contributes significantly to greenhouse effect and global warming. Carbon capture is critical to carbon emission reduction and recycling. Various materials have been utilized in industrial applications or explored in laboratories for separation of CO2 from other gases, including amines,1 ionic liquids,2−7 polymers,8−12 microporous metal−organic frameworks (MOFs),13−17 covalent organic frameworks (COFs),18−22 and amorphous porous materials.23−25 In the majority of adsorption and membrane-based processes for CO2 separation, nonbonded interactions including electrostatic and van der Waals interactions between CO2 and the separation media underlie CO 2 capacity/solubility and selectivity. These nonbonded interactions can be manifested as dipole interaction,26 acid−base interaction,27 and hydrogen bonding,28 which all can improve the separation performances (higher capacity/solubility and selectivity for CO2). Thus, numerous oxygen/nitrogen-containing organic compounds and carbonaceous materials are synthesized as practical adsorbents. Polymer membranes are important materials for gas separation in both industrial applications and fundamental investigations. To characterize membrane performance, the Robeson upper bound is often employed as a criterion, which states that highly selective membranes tend to have low permeability, and vice versa.29,30 The upper bound indicates the limit of traditional materials for separating gas mixtures based on physical interaction. To overcome the bound, membrane materials have been functionalized with CO2-phillic groups, such as polar O-containing and N-containing groups, to improve solubility. To gain insight into CO2 affinities of various structures and to guide the design of novel materials, binding energies of a series of O-containing and N-containing groups have been © 2015 American Chemical Society

obtained with ab initio calculations in this work. Especially, interaction between CO2 and oligomer model of poly(ethylene oxide) is studied to find out the relationship between CO2capture ability and chain lengths. Furthermore, tens of compounds containing nitrogen heterocyclic rings are investigated systematically. Several molecules show great potential for binding CO2.

2. COMPUTATIONAL METHODS Various CO2-philic groups and their complexes with CO2 were optimized with the RI-MP2 method in Turbomole 6.5 software package.31 To accurately capture the van der Waals interaction between CO2 and the functional groups, second-generation triple-ζ valence basis sets with heavy polarization (def2TZVPP) were employed for simple organic groups. For large molecules, a quick screening of poly(ethylene oxide)s oligomers and some other nitrogen-containing ligands was performed at the low-level def2-SVP basis set, to find out binding geometries and general trends. At the low-precision level (RI-MP2/def2-SVP), we used −25 kJ/mol as a screening criterion for binding energy (BE for short): BE = E(complex) − E(CO2) − E(ligand). For the promosing ligands (|BE| > 25 kJ/mol), the structures and binding energies were recalculated at the higher level (RI-MP2/def2-TZVPP). Binding energies calculated at the lower level were usually 5−6 kJ/mol larger than those at the higher level, mainly originated from basis set superposition error. In consideration of zero point energy (ZPE), BE would decrease by 1 kJ/mol systematically. In the following discussion, BEs without ZPE corrections would be employed for simplicity. Received: February 26, 2015 Revised: March 30, 2015 Published: March 31, 2015 3848

DOI: 10.1021/acs.jpca.5b01892 J. Phys. Chem. A 2015, 119, 3848−3852

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3. RESULTS AND DISCUSSION We first examine small organic molecules and then move to PEOs and finally to large nitrogen-containing groups. 3.1. Binding Sites of Different CO2-philic Ligands. The carbon atom of the CO2 molecule has positive charge, while its oxygen atoms possess negative charge. Interaction between CO2 and the CO2-philic ligand mainly originates from the attraction between carbon in CO2 and the center of negative charge on heteroatoms (N, O, F, and so on). Thus, typical CO2-philic ligands are often Lewis bases. In addition, the electrostatic attraction between the oxygen of CO2 and atoms with positive charge in the ligand (such as hydrogen in alcohol) can strengthen the interaction further. Table 1 lists binding energies (BEs) between CO2 and various small organic molecules containing heteroatoms. One can see that in similar chemical environments, interaction between CO2 and oxygen is slightly weaker than that between CO2 and nitrogen. For example, BE of alcohol is −14.4 kJ/mol, whereas that of methylamine is −15.2 kJ/mol. (Here we note that we did not consider the chemical reaction between amines and CO2, as this paper focuses on the functional groups for physical sorption of CO2.) The binding energy of the −F group with CO2 is even weaker. This sequence of interaction strength follows basicity. Likewise, the electron-donating group attached to a heteroatom (O or N), such as an alkyl, can result in increased basicity and enhance the weak interaction; conversely, an electron-withdrawing group may weaken the interaction obviously: for instance, |BE|(CH3OCH3) > |BE| (CH3CH2OH) > |BE|(CF3CH2OH) ≈ |BE|(H2O). Hybridization of the oxygen or nitrogen atom is another important factor that affects the interaction intensity. The sp3hybridized O or N has a stronger attraction to CO2 than the sp2-hybridized heteroatom. The sp-hybridized N (nitrogen in acetonitrile) has almost no interaction with CO2, as indicated by the distance (3.170 Å) between −CN and CO2. |BE| of pyrrolidine with sp3-hybridized N (−21.2 kJ/mol) is larger than that of pyrrole with sp2-hybridized N (−16.3 kJ/mol); similarly, |BE| of piperidine (−19.9 kJ/mol) is larger than that of pyridine (−18.0 kJ/mol). For ethers with alkyl groups, |BE| of methoxymethane (−16.3) is smaller than that of tetrahydrofuran (−19.3). Dimethylamine, piperidine, and pyrrolidine are all secondary amines, whereas the interaction of CO2 with dimethylamine is the weakest in the three. Furthermore, two simple amides are included in Table 1. Besides electrostatic attraction between carbon in CO2 and sp2-O in the ligand, there is a hydrogen bond in the complex between a H atom connected to N and an O atom of CO2. Compared with aliphatic ketones, binding energies of amides strengthen by about 6 kJ/mol. Thus, polyamides compounds may be a potential class of material for CO2 absorption. 3.2. Binding Behavior of PEO and Methyl-Terminated PEO. Based on the favorable interaction between CO2 and ethers from above, we investigated poly(ethylene oxide)s [CH3(OCH2CH2)nOH, n = 1−4, PEO for short], methylterminated PEO [CH3(OCH2CH2)nOCH3, n = 1−4, Me-PEO for short]. In addition, we considered more diverse ligands with N-containing heterocyclic units.32 In PEO oligomers, there are several oxygen atoms which can bind with CO2. Binding energies of eight PEO-based oligomers, labeled as PEO-n and Me-PEO-n in Figure 1 (n = 1−4) at the RI-MP2/def2-SVP level are shown in Figure 1a.

Table 1. Binding Energy of CO2 with Some Organic Molecules Calculated at the RI-MP2/def2-TZVPP Level

*

There is no notable interaction between CO2 and sp2-N with connectivity of 3 in the conjugated ring. 3849

DOI: 10.1021/acs.jpca.5b01892 J. Phys. Chem. A 2015, 119, 3848−3852

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Figure 1. (a) Binding energies (BEs) of PEO-n and Me-PEO-n calculated at the RI-MP2/def2-SVP level. (b) Structures of two promising oligomers, Me-PEO-3 and Me-PEO-4. BEs at the RI-MP2/ def2-TZVPP level are listed in parentheses.

Methyl termination oligomers are more preferred for CO2 binding than the corresponding hydroxyl-terminated ones, because of electron-donating property of methyl as previously discussed. With the increase in the number of repeating units and molecular weight (MW for short), binding interaction strengthens. From the optimized binding structures in Figure 1b, one can see that CO2 interacts with multiple ether oxygen atoms in long oligomers, similar to binding of crown ethers with alkali metal cations. In the Me-PEO-3-CO2 complex, there are at least two weak bonds between carbon in CO2 and oxygens in oligomer; in the Me-PEO-4-CO2 complex, an additional oxygen atom coordinates with CO2, leading to 5 kJ/ mol stronger binding. It is considered that Me-PEO oligomers with more than two repeating units are good CO2-binding molecules, in agreement with the fact that PEO-based polymeric membranes have been widely used for CO 2 absorption and separation.33 3.3. Performance of Nitrogen-Containing Compounds. We examined various nitrogen-containing ligands (shown in Figure 2) for CO2 binding. The optimized complex structures and their corresponding BEs for CO2 are shown in Figure 3. The quick screening was performed at the RI-MP2/ def2-SVP level; BE = −25 kJ/mol was used as a criterion to select the most promising ligands for more accurate BE calculation at the RI-MP2/def2-TZVPP level. In ionic liquids and membrane materials, single rings with sp2 nitrogen are more commonly used than rings containing sp3 nitrogen. In Table 1, we have tested binding energies of several nitrogen-containing single rings. Triazole (also labeled as L1 in Figure 2) has the largest binding energy (−19.9 kJ/mol) in the sp2-N containing compounds, because there is not only electrostatic interaction between N of the functional group

Figure 2. Screened N-containing ligands.

and C of CO2 but also hydrogen bonding between −NH of the ring and O of CO2, as depicted in Figure 3. If the H atom in −NH is replaced by methyl (L2), |BE| will decrease by 3.5 kJ/ mol. We designed more groups on the basis of the triazole structure (L3−L23). The typical distance between N of the functional group and C of CO2 is approximately 2.85 Å. If CO2 interacts with two nitrogen atoms, both N−C distances will be elongated. For the two neighboring nitrogen atoms, as in L5 and L12, both N−C distances are longer than 3.00 Å. Two Ncontaining rings such as two triazole groups can be connected by the flexible −(CH2)n− linker to chelate CO2, thereby promoting CO2 binding, such as in the cases of L15 and L17− L22. When n = 0 (L12, L13), there is no chelate effect. Overall, L11 and L22 are the two best ligands in the screened Ncontaining molecules for binding with CO2: BEs are −29.5 and −28.5 kJ/mol, with three and six CH2 units, respectively. 3.4. Implications. The greater |BE| implies stronger physical (van der Waals and electrostatic) interaction between CO2 and the functional groups. PEO-based units have been of choice for polymeric membranes for CO2 separation. The BE data above support this choice. More interestingly, we have shown that two triazole groups with a flexible linker as in L11 and L22 can also be used in a polymer or a part of a framework 3850

DOI: 10.1021/acs.jpca.5b01892 J. Phys. Chem. A 2015, 119, 3848−3852

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Figure 3. Binding energies of 23 nitrogen-containing rings at the RIMP2/def2-SVP level (more accurate calculations at the RI-MP2/def2TZVPP level shown in parentheses for selected groups). Bond lengths are in units of angstroms. Key: C, yellow; O, pink; N, purple; H, cyan.

material. Polymers or covalent-organic frameworks having such groups could have superior performance for carbon capture.

4. CONCLUSIONS We have screened tens of organic groups for their CO2-binding energies by quantum chemical calculations at the MP2 level, to help design materials with greater CO 2 solubility or physisorption capacity. We found that the heteroatom type, hybridization, and flexibility of the linked groups all can influence the CO2-binding energetics. PEO-based systems were found to be most effective in CO2 binding. Moreover, triazole groups with a flexible linker were also found to be a good candidate for CO2 binding. Therefore, this work suggests some new groups for functionalizing polymers or covalent materials for CO2 absorption and separation, which experimentalists may find useful.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Telephone: +1-951-827-4430. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Laboratory Directed Research and Development Program of Oak Ridge National Laboratory, 3851

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DOI: 10.1021/acs.jpca.5b01892 J. Phys. Chem. A 2015, 119, 3848−3852