Article Cite This: Inorg. Chem. 2017, 56, 14005-14012
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Coordinatively Unsaturated Metal−Organic Frameworks M3(btc)2 (M = Cr, Fe, Co, Ni, Cu, and Zn) Catalyzing the Oxidation of CO by N2O: Insight from DFT Calculations Sombat Ketrat,† Thana Maihom,*,‡,⊥ Sippakorn Wannakao,§ Michael Probst,∥,⊥ Somkiat Nokbin,† and Jumras Limtrakul*,§
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†
Department of Chemistry, Faculty of Science and Center for Advanced Studies in Nanotechnology for Chemical, Food and Agricultural Industries, KU Institute for Advanced Studies, Kasetsart University, Bangkok 10900, Thailand ‡ Department of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom 73140, Thailand § Department of Materials Science and Engineering, School of Molecular Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong 21210, Thailand ∥ Institute of Ion Physics and Applied Physics, University of Innsbruck, 6020 Innsbruck, Austria ⊥ Department of Chemical and Biomolecular Engineering, School of Energy Science and Engineering, Vidyasirimedhi Institute of Science and Technology, Rayong, 21210, Thailand S Supporting Information *
ABSTRACT: The oxidation of CO by N2O over metal−organic framework (MOF) M3(btc)2 (M = Fe, Cr, Co, Ni, Cu, and Zn) catalysts that contain coordinatively unsaturated sites has been investigated by means of density functional theory calculations. The reaction proceeds in two steps. First, the N−O bond of N2O is broken to form a metal oxo intermediate. Second, a CO molecule reacts with the oxygen atom of the metal oxo site, forming one C− O bond of CO2. The first step is a rate-determining step for both Cu3(btc)2 and Fe3(btc)2, where it requires the highest activation energy (67.3 and 19.6 kcal/mol, respectively). The lower value for the iron compound compared to the copper one can be explained by the larger amount of electron density transferred from the catalytic site to the antibonding of N2O molecules. This, in turn, is due to the smaller gap between the highest occupied molecular orbital (HOMO) of the MOF and the lowest unoccupied molecular orbital (LUMO) of N2O for Fe3(btc)2 compared to Cu3(btc)2. The results indicate the important role of charge transfer for the N−O bond breaking in N2O. We computationally screened other MOF M3(btc)2 (M = Cr, Fe, Co, Ni, Cu, and Zn) compounds in this respect and show some relationships between the activation energy and orbital properties like HOMO energies and the spin densities of the metals at the active sites of the MOFs.
1. INTRODUCTION Metal−organic frameworks (MOFs) are porous coordination polymers that have recently attracted attention as a promising material for catalysis because of their extremely high surface areas and controllable pore sizes. Moreover, the organic linkers and open metal centers of MOFs can be customized to be active sites for reactions.1−12 From a catalysis point of view, MOFs that contain coordinatively unsaturated sites (CUSs) show an especially high potential because the CUSs are able to act as Lewis acid sites that can easily undergo reactions with probe molecules. HKUST-1 or Cu3(btc)2 (btc = benzene-1,3,5tricarboxylate), reported first by Chui et al.,12 is a representative MOF containing a CUS. The metal site of HKUST-1 has a paddlewheel-shaped unit of metals connected by carboxylate groups, as illustrated in Figure 1. Copper (Cu) can be © 2017 American Chemical Society
substituted by other metals, including chromium (Cr), iron (Fe), nickel (Ni), zinc (Zn), molybdenum (Mo), and ruthenium (Ru) to form an isostructural series.13−18 Their members are frequently used for gas adsorption and separation19−21 as well as active catalysts for various Lewis acid catalyzed reactions; examples are cyanosilylation of benzaldehyde, various Diels−Alder reactions, esterification, and isomerization of α-pinene oxide, and the Friedlander reaction.22−24 Carbon monoxide (CO) and nitrous oxide (N2O) are harmful gases produced from the partial combustion of fuel especially in automobiles. They contribute to global warming Received: August 19, 2017 Published: October 30, 2017 14005
DOI: 10.1021/acs.inorgchem.7b02143 Inorg. Chem. 2017, 56, 14005−14012
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Inorganic Chemistry
Figure 1. Unit cell of M3(btc)2 and the cluster model used in this work. transition-metal atoms (Cr, Fe, Co, Ni, Cu, and Zn). This combination of methods and basis sets has been shown to be very useful for studying hydrocarbon adsorption and reaction mechanisms in zeolites39−47 and also for MOFs48−54 in which van der Waals interactions predominate. During geometry optimization, the interacting molecule and entire M3(btc)2 structure except the terminating H atoms were allowed to relax. Partial charges were determined by the natural bond orbital (NBO) methods.55 To verify and confirm the transition states, frequency calculations were performed at the same level of theory to classify the stationary points along the reaction coordinates. We also calculated the rate constants using classical transition-state theory (TST) according to the following equation:
and ozone destruction. Therefore, the transformation of CO and N2O to less harmful dinitrogen (N2) and carbon dioxide (CO2) is necessary. A variety of catalysts are active for this reaction especially ones utilizing transition-metal cations with their respective support materials.25−28 The CUS−MOFs have been verified to catalyze N2O decomposition and CO oxidation.29,30 For example, Long and co-workers demonstrated the reactivity of the CUS in Fe-MOF-74 toward N2O activation. In the activation step, an iron(IV) oxo species is formed that is capable of oxidizing strong C−H bonds. For CO oxidation, Xu and co-workers applied MOF-based heterogeneous catalyst.31 They showed the excellent catalytic activity of the stable copper MOF, which includes a CUS Lewis acid site toward the CO oxidation reaction.32 Ye and Liu demonstrated that Cu3(btc)2 is an excellent catalyst for CO oxidation.33 They found that the activity of the MOF can be improved by adding noble metals such as palladium (Pd) and silver (Ag). Moreover, activity in the framework walls of Co-MOF-74 due to the Lewis acidic CUS has been reported by Kim et al.34 These studies confirm that the Lewis acidic CUS in MOFs indeed participates in the catalytic CO oxidation reaction. The oxidation of CO by N2O, despite seemingly natural, has not previously been studied in the CUS of Cu3(btc)2 and its isostructural systems. In the present study, we employ density functional theory (DFT) calculations with the M06-L functional to study CO oxidation by N2O as the oxidant on MOFs containing CUSs. The reaction mechanism, relative energies and structures of adsorption, transition state, and product are investigated. The ramifications of the different transition metals in the isostructural series M3(btc)2 (M = Cr, Mn, Fe, Co, Ni, Cu, and Zn) are also analyzed. We also propose informative chemical descriptors to predict the reaction rate-determining step of N−O bond breaking to be used to quickly screen promising catalytic materials for this reaction.
k=
kBT exp(−ΔG⧧/RT ) h
where k is the reaction rate, kB is Boltzmann’s constant, h is Planck’s constant, T is the absolute temperature, R is the universal gas constant, and ΔG⧧ is the free energy difference between the initial and transition states. The rate constants were derived for a reaction temperature of 298.15 K. All calculations were performed with Gaussian 09.56
3. RESULTS AND DISCUSSION 3.1. Reaction Mechanism of CO Oxidation by N2O. First, we studied the reaction mechanism on Fe3(btc)2 and Cu3(btc)2, two well-known paddlewheel MOFs that are widely used for gas adsorption and in catalysis.21−24 Because the spin state crossing might play a role in the catalytic reactions, we analyzed the reaction pathways for two possible spin states of Cu3(btc)2 and Fe3(btc)2 systems, which are singlet−triplet states for Cu3(btc)2 and septet−nonet states for Fe3(btc)2, respectively (Figures S1 and S2). We validated that the CO oxidation catalytic process proceeds entirely in the high-spin state of the system. In the case of Cu, we additionally considered the open-shell singlet state. The corresponding calculations were performed with unrestricted DFT using the M06-L functional and applying the so-called broken-spatialsymmetry approach.57 For the singlet state, the activation energy for N−O bond breaking is higher than that for the triplet state. The energy profiles for CO oxidation on the triplet surface seem therefore to be more important than the singlet ones. Hence, in the following sections, only the high-spin state is considered for all elementary steps along the reaction coordinate. In Cu3(btc)2, the Cu−Cu bond distance is 2.47 Å and the average Cu−O bond length is 1.96 Å. These values are in good agreement with extended X-ray absorption fine structure (EXAFS) data of 2.58 and 1.93 Å, respectively.12 The Cu1 and Cu2 partial atomic charges are 0.97 and 0.98|e|, respectively. For Fe3(btc)2, the Fe−Fe bond distance is 2.74
2. MODELS AND METHOD The cluster model used to represent M3(btc)2 is depicted in Figure 1. For Cu3(btc)2,12 Fe3(btc)2,13 and Cr3(btc)2,14 structural data are available, while for the other species, hypothetical M3(btc)2 frameworks were constructed by substituting Fe in Fe3(btc)2. The models contain one M−M paddlewheel unit connected by four benzene-1,3,5tricarboxylate (btc) organic linkers. This simple model can reasonably mimic the active site of the M3(btc)2 MOF, and it has been successfully used in previous works.35,36 The dangling bonds are terminated with hydrogen atoms. All calculations were performed with the M06-L density functional37 and the 6-31G(d,p) basis set for the C, O, and H atoms and the Stuttgart effective core potential (ECP) basis set38 for the 14006
DOI: 10.1021/acs.inorgchem.7b02143 Inorg. Chem. 2017, 56, 14005−14012
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Figure 2. Energy profile and selected geometric parameters of transition states and intermediates involved in the oxidation of CO by N2O on Cu3(btc)2 (a) and Fe3(btc)2 (b).
(see Table S1) because of the negative partial charge located on the C atom. These results agree well with CO adsorption on metal-exchange zeolites.58 For the N2O molecule, also two adsorption modes of the N- and O-bound configurations are found, as shown in Figure S3. The O-bound adsorption complex has a slightly stronger bond for both MOFs (cf. Table S1). CO adsorbs more strongly than N2O in each of its adsorption modes. Subsequently, CO might block the active site of MOFs for N2O adsorption and N−O bond breaking. Therefore, experimentally N2O should be allowed to react toward N−O bond breaking and the formation of active O, followed by the CO oxidation reaction.
Å and the average Fe−O bond length is 2.01 Å. These values again agree well with the experimental results of 2.98 Å for the Fe−Fe bond and 2.10−2.12 Å for the Fe−O bond.13 The partial atomic charges on the Fe2 paddlewheel are 1.15 and 1.17|e| for Fe1 and Fe2 atoms, respectively. We first consider adsorption of the reactants N2O and CO on the MOF catalyst. The CO molecule can adsorb on the MOFs in two different configurations in which either a C or O atom binds to the metal active site (cf. Figure S3). The adsorption energies for the C-bound configuration are −9.4 and −20.3 kcal/mol for Cu3(btc)2 and Fe3(btc)2, respectively. This mode is almost two times more stable than the O-bound one 14007
DOI: 10.1021/acs.inorgchem.7b02143 Inorg. Chem. 2017, 56, 14005−14012
Article
Inorganic Chemistry The first step of the reaction is breaking of the N2O O−N bond, which is systematically investigated, followed by the formation of N2. As displayed in Figure 2, it starts with adsorption of N2O on the MOFs with the O-bound configuration via interaction between its O lone pair and the metal active site. These interactions slightly change the lengths of the Cu−Cu and Fe−Fe bonds in the MOFs and the O−N bond of N2O (cf. Figure 2). The distances between the active metal site and the O atom of the adsorbed N2O are 2.48 and 2.43 Å for Cu···O in Cu3(btc)2 and Fe···O in Fe3(btc)2, respectively. The adsorption energies are calculated as −7.4 and −13.4 kcal/mol for Cu3(btc)2 and Fe3(btc)2, respectively. Then N2O reacts to form the metal oxo intermediate via the transition state TS1. At the transition state, the N−O bond is broken, leaving one O atom to attach to the active site with a new bond. In Cu3(btc)2, the N−O bond is lengthened from 1.19 to 1.75 Å and the Cu···O distance is contracted to 1.98 Å. This transition state has been identified by one imaginary frequency at 783.0i cm−1, related to the N−O bond lengthening and Cu−O bond formation. The activation energy is 67.3 kcal/mol. The geometries of the transition states (TS1) on Fe3(btc)2 and Cu3(btc)2 resemble each other. The imaginary frequency of the transition state of the ironcontaining MOF is found at 536.2i cm−1, and the activation energy required for N−O bond breaking is 19.6 kcal/mol. This is in the range of activation energies previously found for such transition states on the Fe-ZSM-5 zeolite and on the Fe-activesite-based MOF.59−62 The O deposited on the metal site of the MOFs with nearby N2 molecule is formed after TS1 with relative energies of +35.0 and −22.0 kcal/mol for Cu3(btc)2 and Fe3(btc)2, respectively, compared to the isolated molecule. The metal oxo species are formed after N2 desorption. Their Cu−O and Fe−O distances are 1.92 and 1.61 Å in Cu3(btc)2 and Fe3(btc)2, respectively. The Fe−O bond length is similar to the one found in iron oxo heme63 and nonheme enzymes.64 It is also consistent with the bond length of [Fe−O]+ calculated from CCSD(T) (1.66 Å).65 The spin densities located on the O atom of the Cu−O and Fe−O sites are 1.63 and 0.39|e|, respectively. This indicates the free-radical character of the O atoms, which makes them active for attaching the C atom of the CO molecule in the next step of the reaction. Subsequently, CO oxidation occurs when one CO molecule is adsorbed on the metal oxo site of the MOFs. The adsorption energies of CO are −5.9 and −4.6 kcal/mol for Cu3(btc)2 and Fe3(btc)2, respectively. At the transition state TS2, the O atom stemming from N2O attacks the C atom of the CO molecule to form CO2. In this step, the intermolecular distance of the O···C bond decreases from 2.44 to 2.10 Å for Cu3(btc)2, while the Cu−O bond lengths are elongated to 1.91 and 1.92 Å. A corresponding TS2 structure in Fe3(btc)2 is also found but with shorter O···C and metal−oxo distances than those in the case of Cu3(btc)2. A normal-mode analysis of TS2 reveals one imaginary frequency each at 173.0i and 345.7i cm−1 of Cu3(btc)2 and Fe3(btc)2, respectively. Those vibrations correspond to the reaction coordinate in which the C−O bond is formed. The energy barriers for this step are calculated to be 0.4 and 3.1 kcal/mol for reaction in Cu3(btc)2 and Fe3(btc)2, respectively. The smaller activation energy in Cu3(btc)2 is due to a higher spin density located on the O atom of the Cu−O site. The energy for C−O bond formation on Fe3(btc)2 is comparable with the one previously calculated
on Fe-doped graphene, which is more favorable for the reaction than other metals in the literature.66 CO2 is finally formed and bound on the active metal site with intermolecular distances of 2.49 Å for O···Cu and 2.46 Å for O···Fe. The complexation energies with respect to the isolated molecule are −91.7 and −97.7 kcal/mol for Cu3(btc)2 and Fe3(btc)2, respectively. CO2 desorption from Cu3(btc)2 and Fe3(btc)2 requires energies of 7.2 and 13.2 kcal/mol, respectively. The desorption energies are in good agreement with the values obtained from previous calculations for Cu3(btc)2 (∼8 kcal/mol).67,68 The overall transformation of N2O and CO to CO2 and N2 is exothermic by −84.5 kcal/mol. These values are, as expected, close to the experimental heat of reaction.69 The energetic profiles for CO oxidation by N2O on Cu3(btc)2 and Fe3(btc)2 MOFs are placed over each other in Figure 3. It can be seen that the rate-determining step of the
Figure 3. Energy profiles of the catalytic oxidation of CO by N2O on Cu3(btc)2 and Fe3(btc)2.
reaction in both MOFs is the first step where the metal oxo intermediates are formed with activation barriers of 67.3 and 19.6 kcal/mol for Cu3(btc)2 and Fe3(btc)2, respectively. Subsequent formation of a C−O bond has a much lower barrier because of the highly reactive O atom in the metal oxo species [0.4 and 3.1 kcal/mol for Cu3(btc)2 and Fe3(btc)2, respectively]. Hence, we now focus on the rate-determining step, which is expected to relate to the catalytic performance of the MOFs. Previous studies of N2O decomposition suggest that electron density transfer from a catalyst to N2O is essential for N−O bond breaking.70 For example, the lower TS1 in Fe3(btc)2 compared to Cu3(btc)2 can be explained by charge transfer from the MOF to the N2O molecule. The charge difference between the adsorption and transition-state complexes (qTS − qads) in Fe3(btc)2 is 0.20e higher than that in Cu3(btc)2. This can be traced back to the energy difference between the highest occupied molecular orbital (HOMO) of Fe3(btc)2 and the lowest unoccupied molecular orbital (LUMO) of N2O (HOMOMOFs − LUMON2O) and the corresponding difference in Cu3(btc)2, as shown in Figure 4. The electron density transfer from the occupied d orbitals of the metals to the unoccupied antibonding π orbital of the O−N bond is higher in Fe3(btc)2 than in Cu3(btc)2. This weakens the N−O bond and facilitates its dissociation, which is quantified in a much lower activation barrier for Fe3(btc)2 than for Cu3(btc)2 (Figure 4). 14008
DOI: 10.1021/acs.inorgchem.7b02143 Inorg. Chem. 2017, 56, 14005−14012
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Figure 4. Energy levels and shapes of the frontier molecular orbitals of MOFs and N2O (energies in kcal/mol).
3.2. Computational Screening of the Active Sites of M3(btc)2 MOFs. We investigated the oxidation of CO on the M3(btc)2 MOFs with M = Cr, Co, Ni, and Zn additionally to the species with Fe and Cu that have been discussed above. Formation of the metal oxo intermediate via breaking of the N2O N−O bond is rate-determining, and we analyzed the electronic structure at this transition state. The optimized structures for the reaction on M3(btc)2 are displayed in Figure S4. The adsorption and transition states are similar to the ones found for Cu3(btc)2 and Fe3(btc)2 (Figure 2). The activation energies and reaction rates are given in Table 1. We also Table 1. Activation Energies and Reaction Rates of the N−O Breaking Step of N2O M3(btc)2
ΔEact (kcal/mol)
Cr3(btc)2 Fe3(btc)2 Co3(btc)2 Ni3(btc)2 Zn3(btc)2 Cu3(btc)2
17.1 19.6 42.0 51.6 58.8 67.3
k298.15 (s−1) 2.16 3.85 1.52 2.69 5.18 1.02
× × × × × ×
101 101 10−14 10−19 10−29 10−34
provide the activation enthalpies and free energies in Table S2. We investigated the relationship between, on the one hand, the charge transfer qTS − qads, which is related to N−O bond activation (see above), and, on the other hand, the activation energy (Ea; Figure 5a). One sees that higher charge transfer leads to a smaller activation energy, as expected, and a linear correspondence can be approximated. One must keep in mind, though, that the rather cumbersome calculation of the transition structures is still needed to obtain this parameter. We therefore further investigate the relationship between the HOMOMOF−LUMON2O gap and the activation energy. A linear regression can approximate the activation energy quite well (see Figure 5b). As a third descriptor, we correlate the spin density (ρ) on the metal sites to the activation barrier. The spin density is calculated by the summation of the value on two metal atoms of the MOFs. We also obtain a good linear correlation between ρ and the activation energy, with higher ρ leading to lower activation energies, as shown in Figure 5c. When the HOMO− LUMO gap and spin density are combined as descriptors in a linear equation, naturally the best model obtained is shown in Figure 6. The calculations suggest that the spin density ρ and HOMO energy can, alone or combined, be used as activity descriptors for this reaction. We also correlated the activation energies to other parameters such as the second-order interaction energies and spin density on the oxo O atom, as
Figure 5. Relationship of the activation energy and (a) charge-transfer difference between the adsorption and TS states, (b) the HOMO− LUMO gap, and (c) the spin density differences.
shown in Figure S5. However, in these cases, no good linear correlations were obtained. Moreover, from the series of activation energies, we can also calculate the reaction rates (Table 1) using simple TST. They show the reaction rates k to be in the order Cr3(btc)2 > Fe3(btc)2 > Co3(btc)2 > Ni3(btc)2 > Cu3(btc)2 < Zn3(btc)2. The results correlate with the Irving−Williams series. We think that Cr3(btc)2 might be a catalyst for the reaction studied.
4. CONCLUSION DFT calculations with the M06-L method have been performed to investigate CO oxidation by N2O on MOFs M3(btc)2 (M = Fe, Cr, Co, Ni, Cu, and Zn). The reaction mechanism begins with breaking of the N−O bond of adsorbed N2O to form a metal oxo intermediate. The O atom of metal oxo then reacts to the coming CO molecule via C−O bond formation to produce the product CO2. The first step is rate-determining with activation energies of 67.3 and 19.6 kcal/mol for Cu3(btc)2 and Fe3(btc)2, respectively. The activation energies of the following step are only 0.1 and 3.1 kcal/mol for Cu3(btc)2 and Fe3(btc)2, respectively, because of the radical character of the O 14009
DOI: 10.1021/acs.inorgchem.7b02143 Inorg. Chem. 2017, 56, 14005−14012
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ACKNOWLEDGMENTS
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REFERENCES
This research was supported, in part, by grants from the Thailand Research Fund (Grant TRG5880248) and the Postdoctoral Fellowship from Vidyasirimedhi Institute of Science and Technology to T.M. The National Science and Technology Development Agency (NANOTEC Center for Nanoscale Materials Design for Green Nanotechnology funded by the National Nanotechnology Center), the Commission on Higher Education, Ministry of Education (the “National Research University Project of Thailand”), the Kasetsart University Research and Development Institute, and the Graduate School of Kasetsart University are also acknowledged.
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Figure 6. Comparison between the activation energy from DFT and model-predicted activation energy from the model derived from a linear combination of the HOMO−LUMO gap and spin density.
atom and the small difference between the HOMO energy of the MOFs and the LUMO energy of N2O. We further evaluated the relationship between the activation energy, on the one hand, and descriptors like charge transfer, the HOMO energy, and spin density ρ on the metals, on the other hand. With good correlation, the HOMO energy and ρ might be used as activity descriptors for the reaction without the need to locate the transition state. The reaction rate is also in the order Cr3(btc)2 > Fe3(btc)2 > Co3(btc)2 > Ni3(btc)2 > Cu3(btc)2 < Zn3(btc)2.
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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.inorgchem.7b02143. Energy profiles for the catalytic oxidation of CO by N2O (Figures S1 and S2), optimized structures of the adsorption complexes (N2O and CO molecules; Figure S3), optimized structures of the adsorption and transition states (Figure S4), relationships of the electronic activation energy (Figure S5), calculated adsorption energies (Table S1), and activation energies of the N−O breaking step (Table S2) (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Thana Maihom: 0000-0002-8180-1218 Sippakorn Wannakao: 0000-0003-2613-5184 Notes
The authors declare no competing financial interest. 14010
DOI: 10.1021/acs.inorgchem.7b02143 Inorg. Chem. 2017, 56, 14005−14012
Article
Inorganic Chemistry
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DOI: 10.1021/acs.inorgchem.7b02143 Inorg. Chem. 2017, 56, 14005−14012
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DOI: 10.1021/acs.inorgchem.7b02143 Inorg. Chem. 2017, 56, 14005−14012
