Computational Screening of Functional Groups for Ammonia Capture

Article pubs.acs.org/Langmuir

Computational Screening of Functional Groups for Ammonia Capture in Metal−Organic Frameworks Ki Chul Kim, Decai Yu, and Randall Q. Snurr* Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) containing functional groups that strongly bind ammonia could be promising candidates for ammonia capture from air. To identify functional groups that preferentially bind ammonia versus water, we used quantum chemical methods to calculate the binding energies of ammonia and water with 21 different functional groups attached to aromatic rings, such as are common in MOF linkers. Among the functional groups studied, R−COOCu and R−COOAg are the top two candidates for ammonia capture under both dry and humid conditions. Orbital and charge analyses were performed to provide additional insight into observed behavior and trends. For Bronsted acid functional groups, increasing acidity and dielectric constant promote protonation of ammonia, as expected.

1. INTRODUCTION Metal−organic frameworks (MOFs) are porous, crystalline materials, consisting of metal clusters connected by organic ligands.1−9 Because of their tunable pore size, large pore volume, high surface area, and tailorable surface chemistry, they have attracted tremendous attention in the fields of gas storage10−16 and gas separation17−25 over the past few years. A better understanding of the adsorption of gas molecules in MOFs is required to select or design MOFs for particular applications. To date, many reports on adsorption in MOFs have concentrated on the simple molecules hydrogen,10−12,26 methane,13,27 and carbon dioxide17−20,24,28−30 for fuel storage or gas separation. Another potentially promising application of MOFs is in capturing toxic gases such as ammonia from air. Removing small amounts of ammonia from humid air is a challenging task, and only a few studies have reported on the adsorption of ammonia to date.31−39 Yaghi and co-workers examined the dynamic loading capacity of ammonia in the Co, Mg, Ni, and Zn analogues of MOF-74 under both dry and humid conditions.31 They reported that Mg-MOF-74 and Co-MOF74 had the highest capacities under dry conditions and all analogues had capacities equivalent to or higher than traditional zeolites. However, the capacities were reduced under humid conditions. Bandosz and co-workers studied ammonia © 2013 American Chemical Society

adsorption on MOF/graphite oxide (GO) nanocomposites, where the MOF was MOF-5 or HKUST-1.32−37 They reported enhanced ammonia adsorption capacity for the nanocomposites compared to the parent MOFs. They suggested that the enhancement is due to increased porosity, located between the MOF and the GO. They also observed the collapse of the MOFs due to the interaction between ammonia and the metallic centers of the MOFs. Saha and Deng also reported that the exposure of MOF-5 and MOF-177 to ammonia resulted in the destruction of the MOFs due to hydrogen bonds between ammonia and the Zn4O clusters of the MOFs.38 Britt et al. used several MOFs including MOF-5, MOF-177, MOF-74, MOF199, IRMOF-3, and IRMOF-62 for the adsorptive removal of ammonia.39 They reported that the MOFs outperformed BPL activated carbon in kinetic breakthrough measurements. Recently, several computational studies have discussed the adsorption of molecules in MOFs containing functionalized ligands.40−43 Torrisi et al. used DFT to calculate the intermolecular interactions between a CO2 molecule and a range of functionalized aromatic molecules in the context of designing MOF linker molecules.40,41 They reported that Received: November 13, 2012 Revised: January 9, 2013 Published: January 10, 2013 1446

dx.doi.org/10.1021/la3045237 | Langmuir 2013, 29, 1446−1456

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Figure 1. Variants of MOF-177, containing a 1,3,5-tris(4-carboxyphenyl)benzene ligand, can be synthesized in principle with a variety of functional groups in the organic linkers.

Table 1. Twenty-One Functional Group Candidates Examined for Ammonia Capture; the Bare Naphthalene without Any Functional Group Is Included As a Reference System

hybrid Morse/Lennard-Jones + Coulomb potential to the quantum mechanical potential energy surface for NH3 interacting with the four different functional groups at various distances and angles. They reported that the incorporation of functional groups in the MOFs not only led to increased ammonia uptake but also gave rise to changes in the isotherm shapes. These studies illustrate the potential of well-chosen functionalized linkers to significantly improve the adsorption of ammonia in MOFs. The objective of this study is to computationally explore the binding of ammonia with a variety of functional groups that could be incorporated into MOF ligands. Twenty one different functional groups were chosen, including halogens, acidic groups, metal carboxylates, hydroxyls, and amines. Using

aromatic molecules functionalized by methyl groups, lone-pair donating groups, or groups containing acidic protons had enhanced interactions with CO2 through the inductive effect, acid−base interactions, or hydrogen bonding, respectively. Amrouche et al. examined the change of CO2 uptake due to ligand functionalization of zeolitic imidazolate framework (ZIF) materials using a combination of grand canonical Monte Carlo (GCMC) simulations and experiments.42 They reported that the isosteric heat of adsorption of CO2 at zero coverage increased with the ligand dipole moment. Yu et al. examined the adsorption of ammonia in four MOFs modified with −OH, −CO, −Cl, or −COOH functional groups using GCMC simulations with a parametrized force field.43 The force field was obtained by using a simulated annealing algorithm to fit a 1447


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modification of hydroxyl or carboxylate groups in MOFs to generate metal alkoxides. Mulfort et al. incorporated lithium alkoxides (−O−Li+) into a MOF by postsynthesis modification of hydroxyl groups and investigated their ability to bind hydrogen.52 Similarly, Himsl et al. prepared a lithium-alkoxidemodified MIL-53 by treating the glycol linkers of the MOF with a lithium-containing base.53 Gadzikwa et al. experimentally demonstrated that free carboxylic acid sites in a Zn-based, mixed-ligand (pillared paddlewheel) metal−organic framework could be postsynthetically functionalized by copper cations to generate COO−Cu−OOC.54 The electron-deficient metal atoms can be expected to interact strongly with the ammonia nitrogen atom. All three categories contain hydrogen atoms either in the functional groups or in the naphthalene backbone, and thus each of them also has a possibility of hydrogen bonding with the nitrogen atom in ammonia. We examined the binding energies of ammonia and water with each of the functional groups. We considered five different initial positions for the adsorbate molecule with respect to each functional group. The five initial positions were based on chemical intuition for possible strong interactions, such as hydrogen bonding or Lewis acid−Lewis base interactions. Specifically, for three of the five different initial positions, the ammonia molecule was placed to foster the interaction between an electron-rich atom in the functional group and a hydrogen atom of ammonia, the interaction between an electron-deficient metal in the functional group and the nitrogen atom of ammonia, or both of these interactions. For the remaining two initial positions, we placed the ammonia molecule on top of the benzene ring near the functional groups, with hydrogen atoms either downward or upward. A similar strategy was used for water. After a geometry optimization starting from each initial position, we calculated the binding energy:

quantum chemical methods, we calculated the binding energies of both ammonia and water with these functional groups to evaluate the potential for preferential adsorption of ammonia under humid conditions. Additionally, we analyzed the electronic structure and modes of binding for the most promising cases.

2. COMPUTATIONAL METHODS The interactions between ammonia or water with the different functional groups were calculated using cluster models. The functional groups were attached to aromatic rings, since such rings are common in MOF linkers. All atoms in the cluster were allowed to relax to find the minimum-energy configurations. The geometry optimizations were performed using the GAUSSIAN 0944 software at the MP2 level of theory. The LanL2DZ effective core potential was used for the silver atom and a 6-31+G(d,p) basis set was used for all other atoms.44 The basis set superposition errors were corrected in the binding energy calculations using the counterpoise method.45 All electronic structure calculations were performed self-consistently, and the electronic structures were converged until the changes in the rms density matrix, the maximum density matrix, and the electronic energy during the iterations became less than 10−8 Å−3, 10−6 Å−3 , and 10−6 Ha, respectively. The geometry optimization calculations were performed until the maximum atomic force, rms force, maximum displacement, and rms displacement were less than 4.5 × 10−4 Ha·Å−1, 3.0 × 10−4 Ha·Å−1, 1.8 × 10−3 Å, and 1.2 × 10−3 Å, respectively.

3. RESULTS MOFs containing functional groups within the organic linkers can be obtained by incorporating the functional group into the linker before MOF synthesis or, in some cases, in a postsynthesis modification.46−51 As an example, Figure 1 shows schematically how variants of MOF-177, which contains a 1,3,5-tris(4-carboxyphenyl)benzene ligand, could be obtained with functional groups such as −COOH, −OH, and −COOCu. In some cases, the functional group may interfere with the MOF synthesis (by binding to metal nodes) making the synthesis difficult using prefunctionalized linkers. We postpone consideration of synthetic issues in this work and focus first on identifying promising functional groups that can facilitate ammonia capture if the synthetic challenges can be addressed. To identify functional group candidates that strongly interact with ammonia, we first selected twenty one functional groups that, based on chemical intuition, could interact via either hydrogen bonding or an acid−base interaction. The functional groups chosen are listed in Table 1. All functional groups were added to a naphthalene molecule (R in Table 1), as a representative aromatic backbone. The chosen functional groups can be organized into three main categories. The first category contains R−F, R−Cl, and R−CH2−F. Each of these groups contains a halogen atom that could form a hydrogen bond or an acid−base interaction with a hydrogen atom in an ammonia molecule. The second category is characterized by electron-rich atoms such as oxygen, nitrogen, or sulfur. This category is composed of R−OOH, R−OH, carbonyl, R− COOH, R−SO3H, R−C(O)−H, R−NH2, R−CH2−NH2, R−OP(O)(OH)2, R−P(O)(OH)2, R−HSO4, R−NO3, and R−NCO. For these groups, one might expect interactions between the lone-pair electrons on oxygen, nitrogen, or sulfur atoms with the hydrogen atoms of ammonia. The third category is made up of metal-containing functional groups, namely R−COONa, R−COOLi, R−COOK, R−COOCu, and R−COOAg. There are several reports on the postsynthesis

E bind = Efunctional group + molecule − Efunctional group − Emolecule (1)

where, E is the electronic energy. The lowest calculated binding energies are presented in Table 2. For both ammonia and water, the binding energies with the functionalized naphthalene are higher in magnitude than those with the bare naphthalene, indicating that all of the functional groups examined improve the binding strengths of ammonia and water. Most importantly, R−COOCu is the most promising candidate, having exceptionally strong binding (−161.2 kJ/mol) with ammonia. Notably, 18 of the 21 functional groups have ammonia binding energies lower than 70 kJ/mol in magnitude. Ten of them even have binding energies lower than 20 kJ/mol in magnitude. For water, 19 of the 21 functional groups have binding energies lower than 70 kJ/mol in magnitude. However, R−COOCu and R−COOLi have strong binding with water in the range of 70− 100 kJ/mol in magnitude. Because of the chemical similarity of NH3 and H2O, it is difficult to find functional groups that selectively adsorb NH3 over H2O, but our calculation results indicate that there are a few promising candidates. The relative binding strength of ammonia versus water, which is obtained from the difference between the lowest binding energy of water and the lowest binding energy of ammonia, is listed in the fourth column of Table 2 for each functional group. A positive value indicates that the binding with ammonia is stronger than with water. It can be seen in Table 2 that 10 functional groups have stronger binding with ammonia than water (positive values in last column). In particular, R−COOCu and R−COOAg have a 1448


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mol stronger than for R−OH, which forms only a single hydrogen bond. The last two cases, R−COOCu and R− COOAg, display Lewis acid-Lewis base interactions with ammonia. The electron-deficient transition metal ions in the two functional groups interact with the lone pair electrons of the nitrogen atom of ammonia with binding distances of 1.88 Å and 2.263 Å, respectively. These transition metals also have Lewis acid-Lewis base interactions with water. In the case of R−COOAg interacting with water, there is also a hydrogen bond. An interesting feature from Figure 2 is that the copper and silver atoms, which are symmetrically located with respect to the two oxygen atoms in R−COOCu and RCOOAg, move to asymmetric positions after binding with ammonia or water. To better understand the asymmetric positioning of the copper atom in R−COOCu upon ammonia or water adsorption, we first examined the changes in the orbital occupancies with a natural bond orbital (NBO) analysis.55−57 The changes in the orbital occupancies of Cu and O in R− COOCu as well as the N atom of ammonia are listed in Table 3, where a positive value means an increase in the orbital occupancy (more electrons). Interestingly, the orbital occupancy of the oxygen atom labeled O1 in R−COOCu increases by 0.027 (0.037) after the binding of ammonia (water), whereas the orbital occupancy of the other oxygen atom O2 decreases by 0.104 (0.097). In the presence of ammonia or water, the electron-deficient copper atom moves closer to the O1 atom, which is more electron-rich than O2. We also examined the changes in the atomic charge distribution in the clusters as shown in Figure 3 by calculating the CHELPG (charges from electrostatic potentials using a grid-based method) charges of the clusters before and after the binding of ammonia or water.58 As shown in Figure 3, the atomic charges of O1 and O2 are −0.81 and −0.80 before the binding of a molecule respectively indicating that the electronic density is almost equally distributed between the two oxygen atoms. However, after the binding of ammonia (water) the values changed to −0.85 (−0.84) and −0.76 (−0.76), respectively, again indicating that the electron density becomes higher on O1 than O2. Additionally, the change of the copper atom’s charge from 0.74 to 0.61 or 0.68 can be partially understood by electron transfer from ammonia or water to the copper atom. Table 3 also shows that the copper atom in R−COOCu gains s and p electrons but loses d electrons from adjacent atoms as R−OOCu binds with ammonia or water. The electron donation and back-donation between the copper atom and the nitrogen atom of ammonia or oxygen atom of water result in a strong interaction. A similar interaction is reported for π complexation between a transition metal atom and a C−C double bond.59,60 Yang and co-workers computationally examined the adsorption of simple alkenes on Ag+- and Cu+exchanged sulfonic acid resins (CuSO3C6H5, AgSO3C6H5) using the extended Hückel molecular orbital (EHMO) method and reported that electron donation from the π orbital of the ethylene or propylene molecule to s and d orbitals of the copper or silver atom was combined with electron backdonation from d orbitals of the copper or silver atom to antibonding π orbital (i.e., π*) of the ethylene or propylene molecule.59 Lamia et al. also reported that copper sites in the MOF HKUST-1 interact strongly with propylene due to the interaction between the transition metal and π-bonding orbital in propylene.60 Watanabe and Sholl computationally examined the interactions between a copper dimer of HKUST-1 and various adsorbate molecules using the PW91 and B3LYP

Table 2. Calculated Lowest Binding Energies in kJ/mol of Ammonia and Water on Bare Naphthalene and the 21 Functional Groups Listed in Table 1; the Last Column Represents the Difference between the Lowest Binding Energy of Water and the Lowest Binding Energy of Ammonia for Each Case; a Positive Value Indicates That the Binding of Ammonia Is Stronger than That of Water functional group R−COOCu R−COOAg R−HSO4 R−COOLi R−OOH R−SO3H R−OP (O) (OH)2 R−P (O) (OH)2 R−OH R−COOH R−CI R−NCO R−NO3 R−F R−NH2 R (Napthalene) R−COONa R−CH2−F R−C(O)−H Carbonyl R−CH2−NH2 R−COOK

lowest binding energy of H2O − lowest binding energy of NH3

NH3

H2O

−161.2 −84.9 −65.9 −81.1 −37.2 −52.1 −54.2

−97.8 −51.5 −46.9 −70.2 −27.7 −42.7 −44.9

63.4 33.4 19.0 10.9 9.5 9.4 9.3

−50.8

−42.7

8.1

−34.3 −41.8 −11.2 −15.6 −14.7 −12.9 −18.8 −8.2 −62.3 −13.9 −16.4 −16.1 −18.3 −48.0

−26.5 −37.2 −11.9 −16.8 −16.7 −15.2 −21.2 −10.7 −65.0 −19.5 −23.3 −23.4 −28.1 −61.8

7.8 4.6 −0.7 −1.2 −2.0 −2.3 −2.4 −2.5 −2.7 −5.6 −6.9 −7.3 −9.8 −13.8

strong preference (>30 kJ/mol) for ammonia over water. It should be noted that these two functional groups also have the strongest binding energies of ammonia among all of the functional groups. In our study, R−COOCu and R−COOAg are therefore the top two functional group candidates for ammonia capture under both dry and humid conditions. We now examine the binding geometry of ammonia and water with six selected functional groups, specifically two functional groups (R−Cl and R−NH2) that bind water more strongly than ammonia, two functional groups (R−OH and R− COOH) that bind ammonia more strongly than water, and the top two functional group candidates for ammonia capture (R− COOCu and R−COOAg). Figure 2 shows the initial and optimized geometry of the lowest binding energy state for each of the selected functional groups. We highlight a few interesting results. First, R−Cl binds with ammonia and water via a combined hydrogen bond and electrostatic interaction. The binding distances between the functional group and ammonia (water) are in the range of 2.4−3.1 Å (2.3−2.7 Å). Second, the interaction between R−NH2 and ammonia results mainly from a hydrogen bond between a hydrogen atom of R−NH2 and the nitrogen atom of ammonia, whereas the interaction between R−NH2 and water results mainly from a hydrogen bond between the nitrogen atom of R−NH2 and a hydrogen atom of water. Third, R−OH has a strong interaction with ammonia (water) via an H−N (H−O) hydrogen bond. The binding distances are 1.859 Å and 1.877 Å for ammonia and water, respectively. R−COOH forms two hydrogen bonds with ammonia or water, and the binding energies are 7−10 kJ/ 1449


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Figure 2. continued

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Figure 2. Initial and optimized clusters for the lowest binding energy cases of (a) ammonia and (b) water on selected functional groups. The atoms in gray, white, red, green, orange, blue, and violet are carbon, hydrogen, oxygen, chlorine, copper, silver, and nitrogen, respectively. The values in green represent binding distances in Å.

Table 3. Changes in the Orbital Occupancies of R−COOCu upon Binding of Ammonia or Water, the Values in White Represent the Sum of the Changes of the s, p, and d Orbitals for Each Atom, the Values in Green Represent Binding Distances in Å; the Atoms in Gray, White, Red, Orange, and Violet Are Carbon, Hydrogen, Oxygen, Copper, and Nitrogen, Respectively

functionals with a 6-311++G** basis set.61 They reported that ammonia and pyridine showed the strongest binding energies with the open metal site due to the Lewis acid−Lewis base interaction. However, they did not perform any atomic charge or orbital analysis. We also examined the changes in the orbital occupancies and the atomic charges in R−COOAg, which also exhibits an asymmetry around the silver atom upon binding with ammonia or water. The resulting orbital occupancy changes are listed in Table 4, which again shows an asymmetry for O1 and O2. The orbital occupancy of O1 in R−COOAg decreases (increases) by 0.004 (0.014) after the binding of ammonia (water), whereas the orbital occupancy of O2 in R−COOAg decreases by 0.047 (0.012). The unbalanced distribution of the electron density and asymmetric position of the silver atom are also described by the atomic charge distributions of the related clusters as illustrated in Figure S1 of the Supporting Information. Tables 3 and 4 also show that the relative binding strengths between the two functional groups and the molecules are

related to the amount of donation and back-donation between the orbitals of the transition metals and those of ammonia or water. The relative amount of electron donation to s and p orbitals of the transition metals in the four cases has the following order: R−COOCu−NH3 (0.288) > R−COOCu− H2O (0.225) > R−COOAg−NH3 (0.152) > R−COOAg−H2O (0.023). Similarly, the relative amount of the electron backdonation from the d orbitals of the transition metals has the same order. As listed in Table 2, the relative binding strengths between the two functional groups and the molecules in the four cases also have the same order, indicating that the electron transfer associated with the electron donation and backdonation is the main source of the binding interaction. The results above are meant to suggest functional groups that can be incorporated into MOFs to selectively interact with ammonia in the presence of water. Because we did not look at a specific MOF linker, an important question is the transferability of the results. That is, will the conclusions remain the same for other organic backbones? To check the transferability of the results, we examined the binding energies of ammonia or water 1451


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Figure 3. CHELPG-based charge analysis of the lowest binding energy geometries for ammonia and water with R−COOCu. The atoms in gray, white, red, orange, and violet are carbon, hydrogen, oxygen, copper, and nitrogen, respectively. The values in white represent the atomic charges, and values in green represent binding distances in Å.

Table 4. Changes in the Orbital Occupancies of R−COOAg upon Binding of Ammonia or Water; the Values in White Represent the Sum of the Changes of the s, p, and d Orbitals for Each Atom, the Values in Green Represent Binding Distances in Å; the Atoms in Gray, White, Red, Blue, and Violet Are Carbon, Hydrogen, Oxygen, Silver, and Nitrogen, Respectively

with −OH and −COOLi connected to two other backbones that are common in MOFs, benzene and imidazole, as R. The resulting binding energies for ammonia or water with the −OH functional group on three different backbones including naphthalene are listed in Table 5 for five different initial

positions. Full results are listed in Table S2 of the Supporting Information. The initial and optimized clusters for each case listed in Table S2 are shown in Figures S2−S13 of the Supporting Information. Each of the cases 1−5 has the same initial position regardless of R. As shown in Figures S2−S4 of the Supporting Information; in cases 1, 2, 3, and 5, the positions of ammonia in the optimized clusters with the benzene and imidazolate backbones are similar to the position of ammonia with the naphthalene backbone. This configuration corresponds to the global energy minimum for cases 1, 3, and 5 and to a local minimum for case 2 (Table 5). In case 4, ammonia again finds the global minimum for the benzene and imidazolate backbones, whereas it finds another local minimum with naphthalene. Thus, in all cases the position of ammonia interacting with functionalized benzene or imidazolate corresponds to the same (general) position for interactions with functionalized naphthalene backbone or to the global minimum energy position. The similar positions and energies for the global minima indicate good transferability for NH3 and H2O with R−COOLi (Table S2 of the Supporting Information). However, for NH3

Table 5. Comparison of the Binding Energies of Ammonia with R−OH for Three Different R’s: Naphthalene, Benzene, Imidazole; Each Case 1−5 Has the Same Initial Position Regardless of R; the Values in Red Represent the Lowest Binding Energy Results for the Respective Backbone

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Table 6. Observation of the Protonation of Ammonia (Yes or No) on R−SO3H, R−HSO4, R−COOH, R−OOH, and R−OH under Various Dielectric Values; the Name of the Solvent Corresponding to Each Dielectric Constant Is in the First Column, the Binding Energy (Based on Eq 1) of Ammonia with Each Functional Group at Different Dielectric Constants Is Given in kJ/ mol NH3 protonation solvent

dielectric constant

gaseous phase

0

chloroform

4.7113

pentanal

10

2-hexanone

14.136

acetone

20.493

nitroethane

28.29

R−SO3H (pKa = 0.27) No −52.1 Yes −104.6 Yes −114.0 Yes −116.4 Yes −118.3 Yes −119.3

R−HSO4 (pKa = 1.92) No −65.9 Yes −142.6 Yes −151.5 Yes −154.0 Yes −155.9 Yes −151.5

R−COOH (pKa = 4.17) No −41.8 No −58.1 No −58.1 Yes −54.3 Yes −55.3 Yes −56.0

R−OH (pKa = 9.51) No −34.3 No −50.8 No −50.8 No −50.6 No −50.4 No −50.3

R−OOH No −37.2 No −55.0 No −57.3 No −55.7 No −55.7 No −55.6

Information. (The specific solvents do not have any particular relevance to ammonia capture or MOFs; they were simply chosen to provide a wide range of dielectric constants.) Table 6 shows the range of the dielectric constant at which the protonation of ammonia occurs for each functional group. For example, ammonia on R−SO3H and R−HSO4 is protonated when the dielectric value is higher than a transition point between 0 and 4.71. For R−OH and R−OOH, protonation is not observed for any dielectric value that we examined between 0 and 28.29. As expected, ammonia is more easily protonated on more acidic functional groups. For example, the minimum dielectric constants required for the protonation of ammonia on R−SO3H and R−HSO4, which are the most acidic among the functional groups in Table 6 according to their pKa values (0.27 and 1.92, respectively), are lower than the values required for R−COOH, R−OH, and R− OOH. Thus, the order of the acidity (R−SO3H > R−HSO4 > R−COOH > R−OH, R−OOH) is intimately related to the order of the minimum dielectric constant required for the protonation of ammonia. Note that the pKa value of R−OOH is unknown.64,65 The dielectric constant of a MOF can be estimated using the Clausius-Mossotti approach.66−69 Zagorodniy et al. reported the dielectric constants of an isoreticular metal−organic framework (IRMOF) series, ZIF-8, and Cu1,3,5-benzenetricarboxylate (Cu3(BTC)2) calculated using this approach. Interestingly, all of the MOFs examined have dielectric constants less than 2. Our results, therefore, suggest that ammonia will not be protonated if MOFs are functionalized with −COOH, −OH, or −OOH groups. Another interesting feature of Table 6 is that the protonation of ammonia does not increase the binding strength. Note that the sudden increase in the binding energy of ammonia with the functional groups listed in Table 6 is observed when the positive dielectric constant is introduced into the system. For example, under dielectric polarization conditions, the binding energies of ammonia with R−COOH are similar regardless of whether ammonia is protonated or not. This is somewhat unexpected. However, our observation is in line with other work on ammonia adsorption in zeolites.70,71 Solans-Monfort et al. computationally examined the adsorption of ammonia in an acidic chabazite zeolite with the B3LYP method using both a cluster approach and periodic calculations.70 They found that both protonated ammonia (−82.32 kJ/mol) and neutral ammonia (−81.48 kJ/mol) were local minima when using

with R−OH, the energies of the global minima differ by about 8 kJ/mol, and for H2O with R−OH the energies differ by 3.5 kJ/ mol depending on the R backbone as shown in Table S2 of the Supporting Information. In both cases, naphthalene and benzene exhibit very similar behavior (energies within 1 kJ/ mol), but imidazole exhibits stronger binding. Nevertheless, the trend of stronger binding of ammonia versus water is maintained even when the naphthalene backbone is changed to imidazole. When ammonia interacts with a Bronsted acid, one might expect that it could be protonated to form an ammonium cation. For example, Johnson et al. experimentally examined ammonia capture on functionalized organosilicate materials and reported that a material functionalized with an isocyanate group reacts with ammonia under humid conditions to form ammonium isocyanate.62 In our study, the protonation of ammonia might lead to stronger binding of ammonia with the functional groups than reported above, where ammonia remained as a neutral molecule in all cases. We wondered if the results above were simply local minima, so we repeated the energy minimizations for selected cases with a hydrogen transferred to the ammonia molecule in the starting configuration. We specifically performed this examination for R−SO3H, R−HSO4, R−COOH, R−OH, and R−OOH. The initial clusters for the energy minimizations contained R−SO3−, R−SO4 −, R−COO −, R−O −, and R−OO− with NH4 +, respectively. Even with these starting configurations, the hydrogen transferred back to the functional group, forming a neutral ammonia molecule in the optimized structure. See the first row of Table 6. We should note here that all of the calculations reported so far were performed for isolated, gasphase clusters. This simplifying assumption seems reasonable for a first-pass screening effort that examines a large number of systems. However, if these functional groups are incorporated into MOFs, the surrounding atoms can create a dielectric polarization in the pores, and this dielectric polarization could affect whether or not ammonia is protonated. Therefore, we considered various dielectric constants to understand the effect on the protonation of ammonia for the selected functional groups. The resulting protonation (yes or no)63 of ammonia for each functional group is listed in Table 6 for a variety of dielectric constants representative of common solvents, along with the binding energies. The initial and optimized clusters for each case are shown in Figures S14−S18 of the Supporting 1453


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the cluster model, with a negligible difference in binding energy. However, for the periodic calculations, only the protonated ammonia case could be located. Similarly, Liu et al. calculated binding energies of ammonia with an acidic zeolite cluster model with and without proton transfer from the zeolite to ammonia at the MP2 level of theory.71 They reported that the binding energy of the protonated ammonia was very close to that of the nonprotonated ammonia with a difference of only 0.5 kJ/mol.

4. CONCLUSIONS The binding energies of ammonia and water with 21 functional groups were calculated to screen promising candidates for ammonia capture under humid conditions. Ammonia and water bind in a similar fashion with the functional groups, through hydrogen bonding or acid−base interactions making it challenging to selectively bind ammonia. Our examination reveals that 10 functional groups have stronger binding with ammonia than water. Among them, R−COOCu and R− COOAg are the top two promising candidates for ammonia capture under humid conditions. Under dry conditions, R− COOCu binds ammonia most strongly among the 21 functional groups examined. The adsorption of ammonia or water on R−COOCu and R−COOAg gives rise to an asymmetric positioning of the copper and silver atoms. NBO and charge analyses reveal that the orbitals and charge distributions between two oxygen atoms of the R−COOCu and R−COOAg are asymmetrically changed after the adsorption of ammonia or water, with the electron-deficient copper and silver atoms moving toward the electron-rich oxygen atom. For Bronsted acid functional groups, increasing acidity and dielectric constant can promote protonation of ammonia, as expected.

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ASSOCIATED CONTENT

S Supporting Information *

Full list of the calculated binding energies, the CHELPG-charge analysis, and initial and optimized configurations of selected cluster models. This material is available free of charge via the Internet at http://pubs.acs.org.

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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 Defense Threat Reduction Agency and the Army Research Office. We also acknowledge XSEDE for computational resources on Steele at Purdue University under Project Number TG-CTS080002. We thank Gregory Peterson for helpful discussions.

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REFERENCES

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