Computational Screening of Metal Catecholates for Ammonia Capture

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Computational screening of metal catecholates for ammonia capture in metal-organic frameworks Ki Chul Kim, Peyman Z. Moghadam, David Fairen-Jimenez, and Randall Q. Snurr Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie504945w • Publication Date (Web): 23 Mar 2015 Downloaded from http://pubs.acs.org on March 28, 2015

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Industrial & Engineering Chemistry Research

Computational screening of metal catecholates for ammonia capture in metal-organic frameworks Ki Chul Kim,a,+ Peyman Z. Moghadam,a David Fairen-Jimenezb and Randall Q. Snurra,* a

Department of Chemical and Biological Engineering, Northwestern University, 2145 Sheridan Rd., Evanston, IL 60208, USA b Department of Chemical Engineering and Biotechnology, University of Cambridge, Pembroke St., Cambridge, CB2 3RA, UK [+] Current address: School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Dr., GA 30332, USA

∗

Corresponding author. Email: [email protected]

Abstract A series of metal catecholate candidates, which could potentially be incorporated into metalorganic frameworks (MOFs), have been screened using quantum chemical methods to identify the most promising metals for ammonia capture in the presence of water. Binding energies and free energies of binding were calculated first for ammonia and water separately with a series of metal catecholates. Further analysis was performed to understand the competitive binding of ammonia and water with the metal catecholates. Among the metals studied, Pt- and Cucatecholates are the best candidates for ammonia capture under humid conditions, while Cucatecholate is especially promising under dry conditions.

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1. Introduction Ammonia is a severe irritant to the eyes, nose, throat, and lungs, and its effective capture can mitigate its potential hazards.1 Chemical, biological, radiological, and nuclear (CBRN) filters are required by the National Institute for Occupational Safety and Health (NIOSH) to provide protection against a wide variety of gases including ammonia.2, 3 Therefore the development of novel purifying filters for the efficient adsorption of ammonia from air, particularly under real conditions in the presence of humidity, is a very important task.4 A variety of techniques have been studied, including catalytic oxidation,5,

6

acidic removal,7,

8

and adsorption on porous

solids.9-13 In the case of catalytic oxidation, zirconia catalysts have been widely used to decompose ammonia in the gas phase.5,

6

However, this method requires high operating

temperatures, which makes it economically inefficient and non-practical in mobile applications.5, 6

On the other hand, acidic removal of ammonia with mineral acids such as sulfuric acid is a

simple method but it is still non practical.7, 8 Compared to these techniques, ammonia adsorption on porous materials such as activated carbon is an attractive method to capture ammonia due to its simplicity and economic operation.9-12 However, the adsorption capacity of ammonia using activated carbons,14 the most widely used porous materials, is still very low (up to 1.8 mg NH3/g activated carbon at 40˚C from humid air).11 Thus, capture of low ammonia concentrations in the presence of water vapor is still an important challenge, which requires the development of new selective and efficient adsorbent materials. Metal-organic frameworks (MOFs)15-23 are a class of porous crystalline materials consisting of metal clusters coordinately linked by organic ligands. They have attracted great attention in the fields of gas storage,24-34 separations35-43 and catalysis44-47 due to their tunable surface chemistry, as well as high surface area and pore volume. However, only a few studies of ammonia capture on MOFs have been reported to date.48-57 For example, Bandosz and coworkers studied the ammonia adsorption properties of MOF-5- and HKUST-1-graphite oxide (GO) composites as well as their stability.48-53 The adsorption capacities for these composites were improved compared to the parent MOFs due to the increased pore volume synergy between the MOF and the GO. However, the MOFs collapsed during the adsorption process due to the attack of ammonia on the bonds between the carboxylic linkers and the metal corners. Saha and Deng also reported the collapse of MOF-5 and MOF-177 after exposure to ammonia.54 Yaghi and coworkers examined the dynamic ammonia capacities of a series of MOF-74 analogues (Co, Mg, 2 ACS Paragon Plus Environment

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Ni, and Zn MOF-74) under both dry and humid conditions.55 They reported good performance under dry conditions, where Mg-MOF-74 and Co-MOF-74 had the highest ammonia capacities, but all of the capacities were significantly decreased under humid conditions. Britt et al. evaluated the potential of MOF-5, MOF-177, MOF-74, MOF-199, IRMOF-3 and IRMOF-62 for ammonia capture using breakthrough measurements and reported that the MOFs showed better performance than BPL activated carbon under dry air conditions.56 Peterson et al. prepared UiO66-NH2 samples containing amine pendant groups on benzene dicarboxylate linkers and pressed the particles into pellets to investigate the effect of pelletization pressure on the ammonia removal performance under dry and humid conditions.2 Their breakthrough measurements on the samples showed that ammonia uptake decreased with increasing pelletization pressure due to the reduced surface area and pore volume, which may result from partial amorphization, something that has been observed before.58 Gas capture processes, including ammonia but also CO2, have also been studied computationally. Some studies have focused on the calculation of the binding properties and the adsorption of different molecules on organic ligands with a range of functional groups that could be incorporated into MOFs or other adsorbents.59-62 For example, Torrisi et al. used density functional theory (DFT) to calculate the interactions between a carbon dioxide molecule and a range of functionalized aromatic MOF linkers.59, 60 They reported that functional groups such as methyl-, lone-pair donating- and acidic-groups enhanced the intermolecular interactions through an inductive effect, Lewis acid-base interactions and hydrogen bonding, respectively. In a similar study, we examined the binding properties of ammonia with 21 different functional groups attached to naphthalene to evaluate their ability for ammonia capture under humid conditions.62 Among the functional groups examined, copper and silver carboxylates were the most promising. Yu et al. included the functional groups –OH, –C=O, –Cl, and –COOH on the organic linkers of several MOFs to study the adsorption of ammonia using grand canonical Monte Carlo (GCMC) simulations.61 Since generic force fields such as the Universal Force Field (UFF) or Dreiding cannot describe accurately the strong interactions between polar ammonia molecules and polar functional groups63-65, the force field was parameterized by using a simulated annealing algorithm to fit a hybrid Morse-Lennard-Jones + Coulomb potential to the quantum mechanical potential energy surface for ammonia interacting with the different functional groups. Overall, computational studies indicate that well-chosen functionalized linkers can significantly improve 3 ACS Paragon Plus Environment


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the selective adsorption of molecules in MOFs. In particular, functional groups containing open metal sites seem to be promising for the selective capture of ammonia. One strategy for incorporating open metal sites into MOFs is via post-synthesis modification. For example, Mulfort et al.66 and Himsl et al.67 have incorporated lithium alkoxides into MOFs. Gadzikwa et al. post-synthetically functionalized free carboxylic acid sites in a MOF with copper cations to generate COO-Cu-COO groups.68 More recently, Cohen and coworkers have successfully incorporated an isolated metal (Fe, Cr)-monocatecholato moiety into the highly robust Zr(IV)-based MOF UiO-66 by two different post-synthesis strategies.69 We postulated that the electron deficient metal atoms in metal catecholates (i.e. metal alkoxides) should interact strongly with ammonia via acid-base interactions. In particular, our previous report on the topperforming functional groups including Cu and Ag open metal sites encouraged us to systematically investigate the binding affinity of ammonia to different open metal sites in this interesting class of materials. In addition, the oxygen atoms in the M-catecholates are potential candidates for hydrogen bonding with ammonia’s hydrogen atoms. Given the large number of metals that could be introduced into metal catecholates, we have performed computational screening to estimate the binding properties of ammonia and water with a series of metal catecholates to evaluate their potential for ammonia capture under both dry and humid conditions. We have chosen 18 different metals including alkali, alkaline earth, and transition metals and calculated the binding energies and free energies of binding (∆G) for ammonia and water with the metal catecholates using quantum chemical methods. In addition, we have examined the competitive adsorption of ammonia and water for the top candidates.

2. Computational Methods We used cluster models to calculate the interactions of ammonia and water with a variety of metal catecholates. Metal catecholates were created by substituting either two hydrogen atoms in a catechol molecule with a divalent metal (Be, Mg, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo, Pd, W and Pt) or one of the two hydrogen atoms with a monovalent metal (Li, Na and K). All atoms in the cluster were allowed to relax to find the minimum-energy configuration. Geometry optimizations were performed using the GAUSSIAN 09 software at the MP2 level of theory.70 The LANL2DZ effective core potential was used for the transition metal atoms, and a 6311+G(d,p) basis set was used for all other atoms.71 Geometry optimization calculations were 4 ACS Paragon Plus Environment

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performed until the maximum atomic force and rms force were less than 4.5 × 10−4 Ha·Å−1 and 3.0 × 10−4 Ha·Å−1, respectively. Basis set superposition errors were corrected in the binding energy calculations using the counterpoise method.72 Frequency calculations were performed at the same level of theory to determine the vibrational and entropic contributions to the free energy.70

3. Results Adsorption of ammonia and water in a microporous material is affected not only by the chemical properties of the material but also by the textural properties such as pore size distribution and surface area, as well as the operating temperature and partial pressures of the adsorbates. We focus here, however, only on the chemical properties delivered by the selected functional groups. We first examined the binding energies (BE) of ammonia and water separately with 18 metal catecholates. To find different possible preferential adsorption sites for ammonia and water on the linkers, we considered five different initial configurations for the adsorbates with respect to each M-catecholate. These initial configurations, depicted in Figure 1, are based on the possible Lewis acid-base interactions, hydrogen bonding, and electrostatic interactions that could take place in these systems. Four of the initial configurations consider possible interactions between the electron deficient metal atom in the M-catecholate and the nitrogen or oxygen atom of ammonia or water, respectively, as well as possible hydrogen bonding. In the fifth initial configuration, the adsorbate molecule was located on top of the benzene ring, considering possible electrostatic interactions between the benzene π orbitals and the hydrogen adsorbate atoms. After geometrically optimizing the cluster, we calculated the BE BEmolecule = EM-catecholate + molecule – EM-catecholate – Emolecule

(1)

where E is the electronic energy, and the molecule is either ammonia or water. Table 1 lists the lowest BE of ammonia and water with the M-catecholates. Table S1 shows the full list of the calculated BEs for all of the initial configurations. Among the metals considered, Cu-catecholate is the most promising candidate under dry conditions, having the strongest ammonia BE: -340 kJ/mol. For ammonia, 9 of the 18 M-catecholates have BE stronger than -150 kJ/mol, whereas only 4 catecholates have water BE stronger than -150 kJ/mol.



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Figure 1: Five initial configurations of ammonia on a metal catecholate. Grey, white, red, blue, and green atoms correspond to C, H, O, N, and the metal, respectively. Table 1: Lowest binding energies (in kJ/mol) of ammonia (BENH3) and water (BEH2O) on different Mcatecholates. The last column represents the difference between the lowest BE of water and of ammonia. A positive value indicates that the binding of ammonia is stronger than that of water. Metal Pt Zn Cu Be Fe Ni W Co Mn Mo Cr Mg V Li Ca Na Pd K

BENH3 -291 -153 -340 -203 -185 -185 -211 -175 -140 -126 -142 -156 -142 -93.7 -107 -82.8 -97.6 -72.5

BEH2O -213 -105 -292 -156 -142 -142 -171 -137 -104 -89.8 -111 -127 -127 -86.7 -102 -79.4 -104 -85.7

BEH2O – BENH3 77.9 48.7 48.6 46.4 43.8 43.0 40.3 38.5 36.2 35.8 31.1 28.9 14.4 7.0 5.2 3.4 -6.9 -13.2

Figure 2 highlights different trends in the results. In the case of the alkaline earth metals, both ammonia and water binding energies weaken with increasing atomic number. However, for the 6 ACS Paragon Plus Environment

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alkali metals, the ammonia binding weakens with increasing atomic number, while the water binding is non-monotonic. For the first-row transition metals, the binding energies of both ammonia and water follow approximately the Irving-Williams series73, although the binding energies for Fe, Co, and Ni are roughly equal. This trend Mn2+ < Fe2+ < Co2+ < Ni2+ < Cu2+ > Zn2+ is observed in a variety of other contexts for ligand binding to these metals.74-76 Interestingly, the two noble metal Pt- and Pd-catecholates exhibit different binding behaviors: the Pt-catecholate shows much stronger BE for both ammonia and water than does the Pdcatecholate. In addition, Pt-catecholate shows stronger BE for ammonia than water in contrast with the Pd-catecholate, where the water BE is slightly stronger than the ammonia BE. Similar observations on the different binding behaviors of Pt and Pd based complexes have been reported.77,

78

Chung et al. employed DFT to examine the electronic properties of transition

metal-monoxide complexes (PtO vs. PdO).77 They reported a stronger Pt-monoxide bond compared to the Pd-monoxide one. The explanation for this observation was linked to the contraction and stabilization of the atomic Pt 6s orbital in Pt-O, as well as the destabilization of the 5p shell by the contraction of the outer s and p core orbitals, a situation that does not arise for Pd-O.79 They claimed that these effects not only favor the 5d-6s mixing in Pt-O but also a shorter Pt-O distance and a stronger overlap with the 2p oxygen levels with subsequent enhancement of the covalent bonding contribution in Pt-O. The bond orbitals in the transition metal-based complexes are intimately related to their bonding properties. We therefore investigated the changes in the orbital occupancies using the Natural Bond Orbital (NBO) analysis implemented in the GAUSSIAN 09 software to better understand the difference in the binding properties of ammonia and water between the Pt- and Pd-catecholates (see Figure 4c).80-82 The NBO analysis verifies that stronger binding of ammonia and water with Pt compared to Pd results from larger electron transfer to the Pt-catecholate than to the Pd-catecholate. In addition, the Pt-catecholate has higher electron transfer with ammonia than water while the Pd-catecholate reversely has higher electron transfer with water than ammonia. These results correlate with the binding energies.



a)

-250

BE (kJ/mol)

-200

-150

-100

-50

0

Li

b)

Na

K

Be

Mg

Ca

-400

BE (kJ/mol)

-300

-200

-100

0

V

c) BE (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Cr Mn Fe Co Ni Cu Zn

-300

-200

-100

0

Cr

Mo

W

Ni

Pd

Pt

Figure 2: Lowest binding energies of ammonia (blue) and water (red) for the different M-catecholates: (a) alkali and alkaline earth metals, (b) first row transition metals, and (c) group 6 and 10 transition metals.

The chemical similarity of ammonia and water makes it difficult to selectively capture ammonia under humid conditions. However, our calculations show that some M-catecholates present a higher binding strength for ammonia compared to water. Note that a positive value of BEH2O – BENH3 in Table 1 indicates stronger binding affinity with ammonia than water. Among the metals studied, Cu-catecholate shows the highest interaction with NH3 (-340 kJ/mol), which makes it especially promising under dry conditions. On the other hand, Pt-catecholate shows the 8 ACS Paragon Plus Environment

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highest value for BEH2O – BENH3 (77.9 kJ/mol) and is therefore a promising candidate under humid conditions. Zn-, Cu- and Be-catecholates are also promising candidates under humid conditions. On the other hand, Pd- and K-catecholates, with negative values for BEH2O – BENH3, have stronger binding with water than ammonia and therefore are not suitable for humid conditions. To understand the preference for ammonia vs. water molecules in the most promising catecholates, we examined the binding geometries at the global minimum-energy configurations. Figure 3 shows the most stable positions of ammonia and water in the Be-, Cu-, Zn-, and Ptcatecholates; Pd-catecholate has been included as well for comparison. Table 2 shows the binding distances. As expected, the main catecholate-ammonia and catecholate-water interaction is a Lewis acid-base interaction between the electron deficient metal and the free pair of electrons from the nitrogen and oxygen atoms of ammonia and water, respectively. Additionally, Cu-, Pt-, and Pd-catecholates show hydrogen bonding between a catecholate oxygen and a hydrogen atom of ammonia or water, giving rise to an asymmetric position of the molecule. The binding distances and O-H-N(O) bond angles in the hydrogen bonding ranged from 2.9 – 3.4 Å and 93 - 101°, respectively, which loosely satisfy the criteria (2.0 – 3.0 Å and 90 – 180°) for weak hydrogen bonding.83 We previously observed a similar phenomenon with other functional groups for ammonia binding.62 The binding distances are directly related to the ionic radii of the divalent metal cations: Pd, the largest metal, exhibits the largest distance whereas Be, the smallest metal, exhibits the shortest distance.



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Figure 3. Most stable positions of ammonia and water binding with Be-, Cu-, Zn-, Pt-, and Pd -catecholates. Grey, white, red, blue, green, brown, yellow, violet, and dark green atoms correspond to C, H, O, N, Be, Cu, Zn, Pt, and Pd, respectively. Values in dark green represent the binding distances in Å. Table 2. Binding distances between selected metals with ammonia and water. Metal

Ionic Radius (Å)

H2O

Distance (Å)

2+

0.59

1.710

1.636

2+

0.87

1.973

2.007

2+

0.88

2.072

2.054

2+

0.94

2.089

2.176

2+

1.00

2.193

2.281

Be

Cu Zn Pt

NH3 84

Pd

To study further the trends in the binding strengths of ammonia and water with the different 10 ACS Paragon Plus Environment

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catecholates, we examined the changes in the orbital occupancies using the NBO analysis. Figure 4 shows the changes in the orbital occupancies of the metal atoms in the M-catecholates. By comparing Figures 2 and 4, we can see that, in general, the changes in the orbital occupancies increase when the binding is stronger. This indicates that charge transfer plays an important role in the BE. The general relationship can be particularly applied to the two noble metal catecholates. As seen in Figure 4, the changes in the orbital occupancies when binding ammonia or water are much higher in the Pt-catecholate than the Pd-catecholate. Moreover, in the case of the Pt-catecholate, the change in the orbital occupancies when binding ammonia is higher than that when binding water, in contrast with the Pd-catecholate. As stated earlier, the different binding behaviors between the two noble metal catecholates therefore result from the difference in the charge transfer. In contrast with the general trend, Co- and Zn-catecholates do not follow the rest of the metals. Both Co and Zn show exceptionally high changes in the orbital occupancies even though their binding is not very strong. However, the overall intermolecular interactions arise not only from the charge transfer due to the intermolecular relaxation of the molecular orbitals but also from the geometric distortions and intramolecular relaxation of the molecular orbitals as described recently, for example, by Head-Gordon and co-workers.85-87



a) Orbital Occupancy change

0.3

0.2

0.1

0

Li

Na

K

Be

Mg

Ca

1

Orbital Occupancy change

b)

0.8

0.6

0.4

0.2

0

V

c)

Cr Mn Fe Co Ni Cu Zn

1.6

Orbital Occupancy change

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1.2

0.8

0.4

0

Cr

Mo

W

Ni

Pd

Pt

Figure 4: Orbital occupancy changes of metals in the M-catecholates, when binding ammonia (blue) and water (red): (a) alkali and alkaline earth metals, (b) first row transition metals, and (c) group 6 and 10 transition metals.

Our discussion so far has analyzed the optimal functional groups for ammonia capture under humid conditions based on the BE at 0 K. However, for a complete thermodynamic analysis, the vibrational and entropic contributions need to be considered at a finite temperature. We have therefore calculated the free energy changes for ammonia and water binding to the Mcatecholates at 298 K: 12 ACS Paragon Plus Environment

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∆G = ∆H – T∆S = (∆U0 + ∆Uvib + ∆Urot + ∆Utrans + ∆(PV)) – T∆S

(2)

where ∆G, ∆H, and ∆S are the changes in the Gibbs free energy, enthalpy, and entropy, respectively. The enthalpy change consists of the electronic binding energy (∆U0), the changes of the vibrational (∆Uvib), rotational (∆Urot), and transitional (∆Utrans) energies, and the PV work, which under the ideal gas assumption can be considered as RT. All terms were obtained from the electronic and frequency calculations of cluster models involved in Eq. (1). Note that the resultant vibrational frequencies for all the cluster models were positive, indicating that the species are thermodynamically stable. The calculated ∆G values of ammonia and water binding with the M-catecholates are listed in Table 3. Figures 5a and b show the comparisons between the calculated BE and ∆G for ammonia and water, respectively. In all cases, the calculated ∆G values are lower in magnitude than the calculated BE values, but the ranking is generally maintained with some minor variations. Table 3 also shows the difference between the binding free energies of water and ammonia with the M-catecholates, ∆GH2O – ∆GNH3, and Figure 5c shows the comparison between BEH2O – BENH3 listed in Table 1 and ∆GH2O – ∆GNH3 listed in Table 3. Both BE and ∆G consistently show that Cu-catecholate is the best candidate for ammonia capture under dry conditions, whereas Pt-catecholate is the best candidate under humid conditions followed by Cu, Be and Ni. In addition, all of the catecholates except Pd- and Kcatecholates have positive values for ∆GH2O – ∆GNH3, suggesting a more stable binding with ammonia than water. This observation is in agreement with our previous electronic BE result. An interesting observation is that, although the difference between the BE for water and ammonia in the Zn-catecholate case suggested the usefulness of this particular group, ∆G predictions indicate a much lower performance for this candidate. On the other hand, Pt-, Cu-, and Be-catecholates are still promising candidates, with ∆GH2O – ∆GNH3 values higher than 45 kJ/mol. In conclusion, despite a few minor inconsistencies between the two analyses, they indicate that a simple screening based on the binding energies, without further computationally expensive frequency calculations, would identify the top candidates.



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Table 3: Calculated free energies of binding (in kJ/mol) at 298 K for ammonia (∆GNH3) and water (∆GH2O) on different metal catecholates using their lowest BE listed in Table 1. The last column represents the difference between the binding free energies of water and ammonia. A positive value indicates that the binding of ammonia is thermodynamically more favorable than that of water. Metal

∆GNH3

∆GH2O

∆GH2O – ∆GNH3

Pt

-242

-172

70.2

Cu

-274

-228

45.6

Be

-154

-108

45.8

Ni

-130

-89.0

41.1

W

-167

-128

39.4

Fe

-139

-102

37.2

Co

-160

-126

35.0

Mo

-73.6

-38.6

35.0

Zn

-128

-94.8

33.6

Mn

-98.8

-66.1

32.7

Cr

-90.0

-60.2

29.8

Mg

-115

-89.3

25.9

V

-100

-75.1

25.1

Li

-45.4

-36.3

9.1

Ca

-59.4

-51.1

8.3

Na

-36.8

-32.2

4.6

Pd

-51.7

-61.4

-9.7

K

-27.6

-39.0

-11.4


a)

-300

-200

-100

0

b)

Pt Zn Cu Be Fe Ni W Co Mn Mo Cr Mg V Li Ca Na Pd K

Energy (kJ/mol)

-400

Energy (kJ/mol)

-300

-200

-100

0

c)

100

80

60

40

20

0

Pt Zn Cu Be Fe Ni W Co Mn Mo Cr Mg V Li Ca Na

Energy (kJ/mol)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Pt Zn Cu Be Fe Ni W Co Mn Mo Cr Mg V Li Ca Na Pd K

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-20

Figure 5: Comparison between the lowest BE listed in Table 1 (black) and the ∆G listed in Table 3 (white) for (a) ammonia and (b) water. (c) Comparison between BEH2O – BENH3 (black) and ∆GH2O – ∆GNH3 (white).

Some of the optimal candidates identified here have recently attracted the attention of experimentalists. Weston et al. have synthesized different porous organic polymers (POPs) for toxic industrial chemical (TIC) capture, including two POPs where the catechols contained in the 15 ACS Paragon Plus Environment


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original structure were metallated by Cu and Zn atoms.88 They studied the retention and saturation capacities for NH3, CNCl and SO2 under dry and humid (80% relative humidity) conditions using micro-breakthrough measurements. The Cu-metallated POP presented higher uptake than the Zn-metallated material under both dry and humid conditions. Specifically, Cuand Zn-metallated POPs showed ammonia capacities of 2.10 and 1.36 mmol/g, respectively, under dry conditions and 4.32 and 3.32 mmol/g, respectively, under humid conditions. The thermodynamic analysis summarized in Table 3, showing that Cu-catecholate has higher preferential interaction with ammonia compared to Zn-catecholate in both dry and humid conditions, qualitatively agrees with the experimental data. The discussion above describes large-scale screening of functional group candidates for ammonia capture under humid conditions based on the BE or ∆G values for separate binding of ammonia and water. However, the mechanism for simultaneous binding of ammonia and water may be more complex. To provide some insights about this, we studied the competitive binding of ammonia and water in the two top M-catecholates identified above: Cu and Zn. We considered two different approaches: i) the addition of a water molecule to an optimized cluster of ammonia binding with a M-catecholate and ii) the addition of an ammonia molecule to an optimized cluster of water binding with a M-catecholate. For each approach, four different initial positions of ammonia and water were considered to cover different possible adsorption sites. Remarkably, both approaches provided the same global minimum-energy configuration, shown in Figure 6. Both Cu- and Zn-catecholates bind ammonia via Lewis acid-base interaction. In the Cu-catecholate case, water binds to both the Cu-catecholate and ammonia via two hydrogen bonds, while, in the Zn-catecholate case, water does not show any hydrogen bonding with ammonia.


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Figure 6: Most stable positions of ammonia binding to (top) Cu- and (bottom) Zn-catecholates in the presence of a water molecule. Grey, white, red, blue, brown and yellow atoms are C, H, O, N, Cu and Zn, respectively. Values in dark green represent the binding distances in Å.

The binding energy for the co-binding of both molecules was calculated according to: co-BENH3 = EM-catecholate + NH3 + H2O – EM-catecholate + H2O – ENH3

(3)

where E is the electronic energy. Table 4 shows the calculated co-BE and co-∆G values. In the presence of water, the co-BENH3 and co-∆GNH3 values are -84.6 kJ/mol and -29.2 kJ/mol, respectively, for Cu-catecholate. These values are far from the BE and ∆G obtained in dry conditions (-340 kJ/mol and -274 kJ/mol, respectively). Similarly, co-BENH3 and co-∆GNH3 for the Zn-catecholate are -116 kJ/mol and -58.0 kJ/mol, respectively, which once more are lower in magnitude than those obtained in dry conditions (-153 and -128 kJ/mol). The results indicate that, in the presence of water, the binding of ammonia with the Cu- and Zn-catecholates is weakened due to the competitive adsorption. To test the influence of additional water molecules on the competitive adsorption between ammonia and water, we also looked explicitly at the competitive binding behavior of ammonia in the presence of up to five water molecules for Zn-catecholate. We considered a scenario where multiple water molecules bind first to the catechol and then ammonia binds. Computationally, this means that we optimize the geometry for the water/Zn-catecholate for various numbers of water molecules and then bring in an ammonia molecule and re-optimize the system. Using MP2 calculations as discussed above, for each case, a minimum of four initial positions of ammonia or 17 ACS Paragon Plus Environment


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water were considered. Figure S1 shows the relaxed conformations of ammonia in the presence of different numbers of water molecules and compares the ammonia binding energies. The largest decrease in ammonia’s binding energy occurs when a single water molecule is adsorbed first with the Zn-catecholate (i.e. from -153 kJ/mol to -115 kJ/mol). Adsorption of two to five water molecules leads to a decrease of 10-20 kJ/mol in ammonia binding energy per water molecule. As water molecules are added to the system, they start to hydrogen bond to both the catechol oxygen and surrounding water molecules. It should be emphasized that ammonia interacts with the zinc ion even when five water molecules are pre-adsorbed before ammonia is introduced (Figure S1). It is also observed that water molecules preferentially form hydrogen bonds with other water molecules rather than ammonia. The above observations appear to be contradictory to the experimental observation from Weston et al.88, where the saturation uptake of ammonia was improved under humid conditions. However, we studied here the interaction of a single ammonia molecule with the M-catecholate. In a real situation, the capture mechanism will involve the clustering of multiple ammonia and water molecules around the primary adsorption sites (i.e. the M-catecholates in the functionalized POPs), as well as confinement effects in the micropore space, a situation that is not fully covered in our quantum mechanics cluster approach. Apart from that, our calculations also indicate that the presence of water has a much worse effect on Cu- than on Zn-catecholate (see Table 4). This difference can be explained from the differences in the global minimum-energy configurations in the absence of water and in the presence of water between the Cu- and Zn-catecholate cases (see Figures 3 and 6). In the Cucatecholate case, a hydrogen bond exists between ammonia and the Cu-catecholate in the absence of water, while the hydrogen bond is broken in the presence of water. On the other hand, in the Zn-catecholate case, the presence of water only results in a small distortion of the bonding between ammonia and the Zn-catecholate without any broken bonding. The above findings suggest that the binding of ammonia in the presence of water follows a more complex mechanism than the isolated binding of ammonia and water.


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Table 4: Lowest binding energies at 0 K and ∆G at 298 K (in kJ/mol) for simultaneous ammonia and water binding with Cu and Zn-catecholates. Cu-catecholate

Zn-catecholate

co-BENH3

-84.6

-115.6

co-∆GNH3

-29.2

-58.0

Another scenario that may take place when ammonia interacts with a M-catecholate in the presence of water is the protonation of ammonia to form NH4+OH-. No ionic complexes were observed in Figure S1, but if they were to form they might have a much stronger affinity with the M-catecholates, so the binding energies reported above where ammonia and water remain as neutral molecules may be only local minima. To test this further, we checked the possibility of forming the ionic complex as described by Figures S2 and S3. We repeated the energy minimizations for the Cu- and Zn-catecholate cases with a proton transferred from water to ammonia in the starting configuration. The initial cluster models for the energy minimizations thus contained the Cu- or Zn-catecholate with NH4+ and OH- ions. Again, the initial cluster models were created using two different approaches: i) the addition of an OH- ion to an optimized cluster of ammonium ion binding with a M-catecholate and ii) the addition of an ammonium ion to an optimized cluster of an OH- ion binding with a M-catecholate. With all of these starting configurations, the proton transferred back to water, forming neutral ammonia and water molecules in their final configuration.89 We note that all of the calculations described so far were performed under the assumption of isolated, gas-phase clusters. This assumption seems reasonable for large-scale screening of a large number of functional group candidates. However, if the functional groups are incorporated into MOFs, different surrounding polar groups can create a dielectric polarization in the pores, which can in turn affect the formation of the ionic complex. Therefore, we considered various dielectric constants to understand the effect on the formation of the ionic complex as shown in Figures S2 and S3. (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.) However, our examination showed that ammonia was not protonated at any dielectric constant for both the Cu- and Zn-catecholate cases and thus the ionic complex is not predicted to form. Our investigation of the relative adsorption strength of ammonia over water on a large number 19 ACS Paragon Plus Environment


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of metal catecholates provides insight into the ability of the metal catecholates to serve as binding sites in MOFs for ammonia capture under humid conditions. However, it should be noted that this information is meaningful mainly as a prescreening process to identify promising functional groups for MOF ligands. To ultimately identify optimal MOFs for selective ammonia capture under humid conditions, other parameters such as the pore size, pore volume, surface area, and pore geometry may also need to be tuned in conjunction with the binding properties of the functional groups.

4. Conclusions We calculated the binding energies and free energies of adsorption for ammonia and water with a series of metal catecholates to identify the most promising candidates for ammonia capture under humid conditions. Although selectively capturing ammonia is challenging due to competitive binding of ammonia and water, this thermodynamic study shows that 16 of the 18 proposed functional groups have a preferential binding towards ammonia vs. water. In particular, Pt-, Cu-, and Be-catecholates are promising candidates to capture ammonia in humid conditions. Further analysis of the competitive binding mechanism of ammonia and water showed that the presence of water weakens the binding of ammonia with the M-catecholates. Our study also revealed that there is no reaction between ammonia and water when co-adsorbed on the metal catecholates under a wide range of dielectric constants and thus the ionic complex, NH4+OH-, is not formed.

Acknowledgements: This work was supported by the Army Research Office (grant W911NF-12-1-0130). D. F-J. thanks The Royal Society (U.K.) for a University Research Fellowship. This work was also supported through the computational resources and staff contributions provided for the Quest high performance computing facility at Northwestern University, which is jointly supported by the Office of the Provost, the Office for Research, and Northwestern University Information Technology (NUIT).

Supporting Information Available: The full version of the binding energies of ammonia and water with the M-catecholates; binding 20 ACS Paragon Plus Environment

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energies and geometrically optimized configurations of ammonia in the presence of multiple water molecules in Zn-catecholate; starting and final configurations of Cu(Zn)-catecholate with a proton transferred from water to ammonia under four different dielectric polarization conditions. This information is available free of charge via the Internet at http://pubs.acs.org/.



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88. Weston, M. H.; Peterson, G. W.; Farha, O. K.; Hupp, J. T.; Nguyen, S. T., Removal of airborne toxic chemicals by porous organic polymers containing metal-catecholates. Chem. Comm. 2013, 49, 2995-2997. 89. We use 1.2 Å for the criterion of N-H bond length for whether ammonia is protonated or not. Thus, if there are 4 N-H bond lengths smaller than 1.2 Å, we determine that the ammonia molecule is protonated.


Computational Screening of Metal Catecholates for Ammonia Capture

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