Interaction of Small Gases with the Unsaturated Metal Centers of the

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Interaction of Small Gases with the Unsaturated Metal Centers of the HKUST-1 Metal Organic Framework Barbara Supronowicz, Andreas Mavrandonakis, and Thomas Heine J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4018037 • Publication Date (Web): 19 Jun 2013 Downloaded from http://pubs.acs.org on June 25, 2013

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The Journal of Physical Chemistry

Interaction of Small Gases with the Unsaturated Metal Centers of the HKUST-1 Metal Organic Framework Barbara Supronowicz, Andreas Mavrandonakis,* and Thomas Heine School of Engineering and Science, Jacobs University Bremen, 28759 Bremen, Germany

Keywords: MOF, harmful gases, adsorption, CuBTC, IR, DFT

Abstract

The interactions of CO, CO2, OCS, SO2, NO, NO2, N2O, NH3, PH3 and other small molecules with the undercoordinated metal centers of the HKUST-1 metal organic framework are studied by means of Density Functional Theory. These molecules are potentially harmful for humans and environment and are widely studied because of their spectroscopic properties. In this work, the energetic and vibrational characteristics of the adsorbed species are calculated. Adsorption energies on the Cu2+ sites of the paddlewheel have been calculated and the order is: NH3 > H2O > PH3 > H2S > SO2 > CO ~OCS ~CO2 ~NyOx > N2 > O2. The results show that the interactions can be classified in three categories: 1) weak physisorption, 2) polarization and electrostatics, and 3) strong acid – base. Moreover, interesting vibrational * To whom correspondence should be addressed: e-mail: [email protected]

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properties are calculated especially for carbonyl sulfide and dinitrogen monoxide, which can be bound via two different configurations on the metal atoms. The vibrational modes are shifting in different directions depending on the binding way of the molecule, e.g. the symmetric stretching of OCS is shifted by +17 or -16 cm-1 when bound via the oxygen or the sulfur atom, respectively.

1. Introduction: Porous coordination materials such as metal/covalent organic frameworks (MOFs/COFs) have attracted considerable interest for their potential use in catalysis and gas storage or gas separation.1-4 Of particular interest are dihydrogen, methane and carbon dioxide due to their environmental and economic importance. Dihydrogen is considered to be the most promising energy carrier for the substitution of fast diminishing liquid fuel resources in mobile applications. In particular, H2 is a fully renewable energy source, environmentally friendly and suitable as an automobile fuel. Carbon dioxide emissions from burning of the fossil fuels are considered to have the biggest contribution in the greenhouse effect and subsequently in climate change. Of significant importance is the carbon dioxide capture and storage process. A large number of MOFs are reported to have CO2 storage abilities.5, 6 Besides storage, the CO2/CH4 separation is very important. The contamination of CH4 with CO2 from various sources, such as natural gas and landfill gas, can decrease the energy density and cause equipment corrosion.7It was shown that a Co(II) based MOF can preferentially adsorb carbon dioxide over methane8, thus indicating the applicability of MOFs to gas storage and separation. Another evnvironmentally important potential application is to separate N2 and CO2 from exhaust gases, e.g. for CO2 free coal power plants, where CO2 is planned to be pumped into exploited natural gas deposits.


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The removal of sulphur content in fuels is an important environmental issue, because upon combustion sulphur is converted to sulphur oxides (SOx) that have negative impact on human health and environment. SOx and NOx are involved in the formation of photochemical smog and acid rain. Moreover, S- and N-containing compounds poison the catalytic converters for the treatment of exhaust emissions. As because the catalytic converters are mainly composed of noble metals, there is a strong economic incentive to prevent this poisoning. Nitrogen monoxide and carbon monoxide are some of the molecules widely studied due to their importance in biology. There is particular interest in NO delivery materials for medical devices to prevent thrombosis formation.9 However, in increased concentrations it is an asphyxiant for humans. Similarly, CO is also an asphyxiant at very small concentrations. On the other hand, these two molecules along with H2S are important gasotransmitter molecules.10 Despite their harmfulness for humans and environment, they are vital for life as signalling molecules and are produced endogenously by the body. Their adsorption has been studied in HKUST-1 and other MOF materials.9-12 The main conclusions from these studies are that the coordination of these molecules to the metal centers causes changes in the electronic and vibrational spectra. Many investigations exist on the adsorption of various gases in MOFs and on the spectroscopic properties of the adsorbed species. Apart from CO2 and CH4, the storage capacity of CO, NO and N2 has been studied on MOFs with coordinatively unsatured metal centers9, 12-19. Additionally, the spectroscopic properties of the adsorbed species have been reported with special attention paid to their vibrational and electronic spectroscopy. The interaction of these gases with the metal sites of the MOFs has been investigated by characterizing the changes in the vibrational (IR) and electronic (UV – Vis and XANES)



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spectra. Furthermore, the strength of the interaction has been determined by calculating the adsorption enthalpies. A plethora of these studies have been performed on HKUST-120 (also known as Cu3BTC2 or MOF-19921). It is constructed from a binuclear copper paddlewheel unit connected with four benzene tricarboxylate ligands. One special characteristic of HKUST-1 is that the solvent molecules contained in the channels after the synthesis can be removed by thermal treatment, producing an activated MOF. The vacant coordination sites at the metal ions are the primary coordination sites for the guest molecules. However, it has been shown that defects are essentially always present in HKUST-1, even in high-quality epitaxial films they account for about 5% of the Cu adsorption sites.11 IR measurements of CO adsorption indicate the defects are due to the presence of Cu+ centers. The effect of defect sites on the adsorption energies of guest molecules is not clear, because it depends on the molecule studied. Carbon monoxide binds strongly to Cu+, whereas nitrogen monoxide prefers binding to Cu2+ as two recent studies for HKUST-1 have shown.22, 23 In this work we concentrate on the majority of free copper sites, the regular sites of HKUST-1, i.e. perfect Cu2+/Cu2+ dimers in the paddlewheel structure. A later work will be dedicated to the role of defects for the adsorption of gas molecules. In the present work, ab initio methods are used to investigate the interactions between small molecules and the dicopper paddlewheel. We do not intend to study the adsorption on the periodic structure of HKUST-1, but we want to model the adsorption on the coordinatively unsaturated Cu2+ centers of the paddlewheel structure. This will allow us a better understanding of the interactions of the molecules with the metal atoms and to extract useful conclusions about the interactions occurring in framework. The objectives of this work are: 1) the energetics of the adsorption, 2) the vibrational characteristics of the adsorbed species and 3) the nature of the interaction. We focus on small molecules, such as


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CO, CO2, NO, NO2, N2O, N2, OCS, SO2, H2S, NH3, not only because of their environmental and biological significance, but also because of their spectroscopic interest. Vibrational spectroscopy is an important tool in catalysis and adsorption, as it can provide detailed information about the nature of the catalytic/adsorption sites, the binding mode, the oxidation state of the metal centers, the presence of defects and the strength of the solid– gas interactions.24 For comparison reasons, the interaction of H2O with the metal centers is also studied, because under normal conditions water vapours are always present. The main objective of this work is to conclude if any of the abovementioned small gas molecules can be bound strongly and selectively on the metal centers of HKUST-1.

2. Computational Details: Density Functional Theory (DFT) calculations were performed on molecular models of the HKUST-1. The DFT calculations were carried out using the hybrid B3LYP functional25, 26 with a damped 1/r6 term added as proposed by the latest London dispersion correction scheme of Grimme (denoted as D3)27, as implemented in the Turbomole program, in combination with the all electron TZVP basis set (denoted as ‘def-TZVP’).28 The TZVP basis set contains (17s11p6d) functions contracted to [6s4p3d] for Cu, [14s9p1d] contracted to (5s4p1d) for S/P, (11s6p1d) contracted to [5s3p1d] for C/N/O and (5s1p) contracted to [3s1p] for H29. The convergence criterion for the SCF cycle was set to 10-7 Hartrees and the 'm4' grid was used. The geometries of all structures were optimized with the STATPT module of Turbomole, until the Cartesian gradients were smaller than 10-4 Hartrees/Bohr and the energy change smaller than 10-6 Hartrees. No symmetry constraints were applied. The optimized minimum-energy structures were verified as stationary points on the potential energy surface by performing analytical harmonic vibrational frequency calculations, using the



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AOFORCE module of Turbomole. The thermal contributions to enthalpy are calculated using harmonic approximations at a temperature of 298 K with the FREEH module of Turbomole. The HKUST-1 connector, the paddle-wheel, is composed of Cu2+ cations, spin polarized centers with an electronic state that can be described by either a parallel or antiparallel coupling of the unpaired electrons of the two copper atoms. This dicopper paddlewheel is typically modeled to be in the ferromagnetic high-spin electronic state (triplet), although the ground state is an antiferromagnetic low-spin, open-shell singlet, with their difference being less than 2 kJ mol-1 30, 31. The reason for choosing the lower-energy high spin state is that the accurate treatment of the Cu2 paddlewheel in the antiferromagnetic ground state would require the use of multi-reference methods, which is computationally demanding and beyond the scope of this study. In a previous work, it has been proven from multi-reference MP2 calculations that the Singlet-Triplet gap of the Cu2 paddlewheel unit did not change significantly upon adsorption of one H2O or CO molecules.23,32, 33 This indicates that the interaction energy depends only on the electronic density distribution around the Cu2+ centers. The electron density is equivalent in both magnetic states and does not change if the Cu2+ centers are ferro- or antiferromagnetically coupled.34 In all calculations in this work, the Cu2 paddlewheel is always modeled to be in the highspin triplet state. In the case of radical guest molecules, such as O2, NO, N2O and NO2, all possible spin states have been investigated, but in all cases the spins of the two copper atoms are in parallel configuration with respect to each other.

Molecular Models: Two molecular models of the CuBTC with increasing size have been chosen for this work: i) a dicopper tetra-formate Cu2(HCOO)4 and ii) a dicopper tetra-Benzenetricarboxylate Cu2BTC4 (see Fig.1). The models have been cut from the periodic structure and saturated


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with protons, so that the overall charge remains zero. This approximation has been used many times and is able to reproduce the binding and spectroscopic properties of small molecules interacting with the HKUST-1.23, 32, 34, 35 The interaction energy of the adsorbed molecules was calculated according to the formula: ΔΕ = (E[M-MOF] – E[MOF] – E[M]) (1) All interaction energies are corrected for the basis set superposition error (BSSE) by the standard counterpoise correction of Boys and Bernardi.36 3. Results: i) Accuracy of the method The accuracy of the method (B3LYP-D3/TZVP) is evaluated against several structural, energetic and vibrational criteria. The optimized structures for empty and hydrated Cu2BTC4 show very good agreement with the simulated data from the EXAFS data for HKUST-1.37 The computed Cu – Cu bond lengths of 255 and 265 pm for the empty and hydrated paddlewheel agree well with the experimental values of 258 and 265 pm, whereas the Cu – Owater distance is computed 221 pm versus 224 pm from the EXAFS. The second criterion is the H2O interaction energy with the Cu2(HCOO)4 model calculated from the CCSD(T)/CBS reference value.32 The highly symmetric structure from the above model was optimized with B3LYP-D3/TZVP and the interaction energy is -49.5 kJ mol-1 that differs less than 2 kJ mol-1 from the best estimate of -51.2 kJ mol-1. We also checked the dependence of the binding energy on the size of the basis set, by adding polarization or diffuse functions. Single point calculations with larger basis sets were performed on the structure for the smaller formate model that was optimized with the B3LYP-D3/def-TZVP method. The interaction energies of the paddlewheel with CO changed by less than 2 kJ mol-1, from -30.9 for def-TZVP, to -29.3 for def2-TZVP, to -29.4 for def2-



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TZVPD, to -29.5 for def2-TZVPP and -29.1 for def2-QZVPP. However, the calculation time is increasing significantly by a factor of 3 for the def2-TZVP and a factor of 25 for the def2QZVPP basis set. Same trend was calculated for the CO2 case. Single point calculations with larger basis sets yield interaction energies of -22.1 for def2-TZVP, -22.0 for def2-TZVPD, 22.3 for def2-TZVPP and -22.3 for def2-QZVPP, in comparison to -24.0 for the def-TZVP. In this work, we are not interested in the accurate calculation of the interaction energies but are mainly interested in the relative binding energies of the various molecules with the paddlewheel structure. For this reason, we believe that the def-TZVP basis set is able to reproduce the relative binding strength of the various molecules with a good accuracy to the available experimental data or the best theoretical estimates. The third criterion is the shift of the CO and NO stretching frequency upon interacting with the Cu2+ centers, due to the plethora of the available experimental data.11, 13, 34 According to these studies, the CO and NO vibrational mode is shifted by Δṽ=+35 and +11 cm-1 respectively with respect to the free gaseous molecule. According to the B3LYPD3/TZVP calculations, the corresponding shifting are Δṽ=+47 and +20 cm-1 respectively, which is in reasonable agreement with the experimental data. Based on these benchmark criteria, we conclude that the B3LYP-D3/TZVP method is sufficient to describe the energetic and spectroscopic properties of the guest molecules adsorbed on the HKUST-1 MOF.

ii) Interaction energies and main distances. As a first step, we calculated the H2O adsorption on the Cu2(BTC)4 model. This is also done in order to compare to the interaction energies of the other guest molecules. This is important because for real system applications water vapors are always present due to the humidity of the air and would compete with the other guest molecules. The results for all


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molecules studied are summarized in Table 1 and the optimized structures are presented in Figure 2. Firstly, the highly symmetric structure from reference 32 with two water molecules was taken. One water molecule was removed from the axial position and the geometry was optimized. However, this D2h symmetric structure is not a minimum on the potential energy surface, as indicated by many imaginary frequencies after the vibrational analysis. By distorting the structure along the motion of the imaginary mode, a less symmetric structure with Cs symmetry is obtained, which is a minimum on the potential energy surface. The Cu – O distance is 224 pm. The interaction energy is calculated -58.7 kJ mol-1. Enlargement of the molecular model, increases the interaction energy by only 2.2 kJ mol-1 to the value of -60.9 kJ mol-1and the conformation of H2O with respect to the dicopper padlewheel remains almost unchanged. The Cu – Owater distance decreases to 221 pm, which agrees very well with the value of 224 pm from EXAFS data.37 Similarly, good agreement is observed with the Cu – Cu distance that is elongated to 260 pm (265 pm from EXAFS data). Our values differ slightly from those obtained for periodic model within the DFT/CC method. 32 The interaction energy deviates by 5.6 kJ mol-1 in comparison to -55.3 kJ mol-1, and the Cu-Owater distance is only 5 pm longer (219 pm versus 224 pm in this work). Comparison for the formate and BTC model can be found in table S3 of the Supporting Information. The initial H2S structure is prepared by substitution of the O atom with S in the MOF-H2O complex. After optimization, a similar structure as with water is obtained, with a Cu – S distance of 260 pm. Hydrogen sulfide is bound less strongly than water, with an interaction energy of -52.2 kJ mol-1. Carbon monoxide is bound in an end-on way via the carbon atom,giving rise to an almost linear configuration with the copper atoms. It is weakly bound with an interaction energy of -30.9 kJ mol-1, what is in good agreement with the CCSD(T)/CBS estimate of -32.0 kJ mol-1.34



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Enlargement of the model increases the interaction energy to -32.4 kJ mol-1. The B3LYPD3/TZVP Cu – C distance of 234 pm agrees very well with the best theoretical estimate of 237 pm. Our calculated interaction energy is in good agreement with the PBE/CC value of 35.8 kJ mol-1 for the periodic structure.23 The PBE/CC approach is a densityfunctional/coupled cluster method that can correct density-functionals for the missing dispersion interactions. The contribution from dispersions is significant, because a conventional periodic PBE calculation yields a significantly lower value of -26.8 kJ mol-1. As the next case, the CO2 molecule was studied. For the smaller model system, the O – C – O molecular axis remains almost linear deviating by only 1.1°. The Cu – O – C angle is 117° and the Cu – O distance 246 pm. The interaction energy is calculated as -24.0 kJ mol-1, which overestimates the CCSD(T)/CBS value of -18.8 kJ mol-1.38 For the bigger paddlewheel model, this value increases to -27.0 kJ mol-1, in very close agreement to the PBE/CC value of -28.2 kJ mol-1 38 and vdW-DFT values of -27.3 kJ mol-1 39 for the periodic structure. This value is also in reasonable agreement with the calculated value of ~-22 kJ mol-1 of Ding et al., who used a smaller dicopper benzenecarboxylate model.40 They used the M06 density functional, which can account for dispersive interactions, with a smaller basis set of double-zeta quality (631G*). The main reason for the underestimation by ~5 kJ mol-1 is the usage of a smaller basis set. However, a small fraction (~1 kJ mol-1) can be attributed to the fact that the framework atoms were kept fixed during the optimization in their calculations and were not allowed to relax. According to our calculations, the deformation energy of the paddlewheel upon interaction with the CO2 is very small and is computed to be ~1 kJ mol-1. Binding on the metal site is stronger than on the organic linkers. According to previous computational studies, the interaction energy of CO2 with various N-containing organic heterocycles has been computed to have a value of ~-25 kJ mol-1, whereas with the amino group of aniline, which is a substituted benzene, has been estimated as ~-10 kJ mol-1, which


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is approximately 3 times weaker than on the metal site.41, 42 The geometry of CO2 changes very little upon interacting with the metal atom; the O – C – O molecular axis deviates only 0.8 degrees from linearity, the Cu – O – C angle is 120 degrees and the Cu – O(CO2) distance is 245 pm. The conformation of CO2 around the Cu–Cu axis is flexible. This is in agreement with experiments for a Zn substituted analogue of HKUST-1 with the formula Cu0.97Zn0.03(BTC)2.43 According to DFT calculations, the barrier for CO2 rotation around the Cu–Zn axis is very low. This agrees with the EPR measurements that the conformation of CO2 around the paddlewheel axis cannot be well defined. Next, the carbonyl sulfide (OCS) was studied. It can be bound in two different ways: either through the S or the O atom via an η1 binding mode. Both configurations are almost isoenergetic with interaction energies of -21.9 kJ mol-1 (η1-O mode) and -24.7 kJ mol-1 (η1-S mode). The calculated distances from the copper atom are 240 and 281 pm for oxygen and sulfur respectively. Enlargement of the model causes a significant change in the these energies to values of -30.5 and -29.0 kJ mol-1 respectively. However, the relative energy difference of the two conformations remains almost the same; that is the O-binding mode is slightly more stable than the S-binding by approximately 2 kJ mol-1. The distance of the guest molecule from the copper atom is elongated to 244 and 283 pm respectively. The geometry of the carbonyl sulfide is almost linear and deviates by 0.5° for the O-binding mode and 1.0° for the S- conformation. Similarly to OCS, two adsorption configurations were considered for the SO2 molecule, either through the O or the S atom. According to our calculations, adsorption via S does not occure as the Cu – S interaction is repulsive. For the oxygen binding mode, the interaction energy of -38.6 kJ mol-1 is stronger than in the OCS case. The molecular angle is distorted slightly with respect to free SO2, being reduced from 117.7° to 116.0°, upon interaction with the copper atom. The binding configuration of SO2 is similar to the work of Yu et al.44, who



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modeled the adsorption on a dicopper acetate model by using the PBE functional with a triple-zeta (6-311++G**) basis set. Their interaction energies are significantly lower compared to this work; -23.4 versus -38.6 kJ mol-1, although their Cu–S distance is significantly shorter (225 versus 232 pm). This difference can be attributed to the method used in their study, because it cannot take into account the London dispersion forces, and not on the slightly different molecular model. A series of nitrogen containing molecules have also been taken into account. We have calculated the adsorption of N2, NH3, NO2, NO and N2O. Their results are also presented in Table 1. Interestingly, ammonia yields the highest interaction energy, which is calculated to be -99.4 kJ mol-1. This suggests that it will be preferably adsorbed in comparison with water. Such a result might explain some experimental observations, according to which the ammonia adsorption on HKUST-1 decomposes the framework.45, 46 However, in a recent work the authors did not observe decomposition of the framework under adsorption of NH3 in dry conditions and the crystallinity of the material is kept.37 Ammonia is strongly chemisorbed on copper sites, resulting in a distorted framework. XAS, IR and UV spectroscopy suggest that the interactions are analogous to the H2O case, but the perturbation is significantly bigger. This is reflected in the higher interaction energy, which is almost 30 kJ mol-1 stronger than for H2O. According to the EXAFS data, the Cu – Cu and Cu – N distances are estimated 280 and 231 pm respectively, whereas in the hydrated case the Cu – Cu and Cu – Owater distances are measured 265 and 224 pm respectively. Our structures do not show such a large elongation of the Cu – Cu distance, i.e. the optimized distance is 263 pm and the Cu – N 218 pm. In the case of N2O, two conformations have also been taken into account. They are almost isoenergetic: -26.6 and -27.8 kJ mol-1 for the O- and N- modes respectively and their


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interaction energies increase to -28.4 and -24.8 kJ mol-1 for the bigger model. The molecular axis is linear in both cases and deviates by less than 0.5°. Interesting cases are the NO, NO2 and O2 radicals. We have calculated several spin states, but the most stable is always the highest spin multiplicity. There is no pairing of the electrons between the copper atom and the guest molecule; all unpaired electrons are arranged in parallel. The ground state is a quartet (total spin S=3/2) for NO, NO2 and a quintet (S=4/2) for O2. NO and O2 are bound to the metal in an end-on (η1-) configuration with an angle of 126.7° and 122.5° respectively, and their interaction energies are -21.7 and -8.7 kJ mol-1 respectively. Increasing the model changes the corresponding angles to and 125° and 123.3° and the interactions to -21.3 and -9.6 kJ mol-1 respectively. The molecular bond does not change at all upon interacting with the metal center. Nitrogen dioxide is bound via the oxygen bond, binding via the nitrogen atom is not observed. The molecular angle changes by only 0.3° upon binding. Its interaction energy is calculated -23.4 kJ mol-1. Also in this case, the binding is much stronger compared to the organic linker. The interactions of NO and NO2 with various benzene molecules modified with polar groups do not exceed 8 and 10 kJ mol-1 respectively.42 The case of phospane is also studied. The configuration is similar to ammonia. The Cu–P and Cu–Cu distances are 257 pm and 262 pm respectively. Phospane interacts strongly with an energy of -46.9 kJ mol-1 for the smaller model. This value increases to -50.8 kJ mol-1 for the bigger model.

iii) Vibrational spectroscopy After completing harmonic vibrational analysis of the host-guest complexes, we performed an analysis on the vibrational modes of the guest molecules to determine how they change upon complexation. The results are presented in Table 2. The frequencies have



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not been scaled. Comparison with the available experimental data shows that the DFT overestimates the frequencies. If we want to make an accurate estimation, the frequencies should be scaled by an appropriate factor.34 However, in this work we are mainly interested in the relative trends, therefore scaling the frequencies was not necessary. The calculated frequencies of the guest molecules (free and adsorbed) are shown in the Table S5 of Supporting Information. The IR spectrum of H2O shows significant changes upon adsorption. The three vibrational modes of water are shifted by +9, -11 and -20 cm-1 with respect to the gas phase respectively. Comparison with the experimental data can be done only for the low frequencies region (600 – 50 cm-1), which is more informative due to the presence of the vibrational modes involving the Cu atoms. According to the experimental data, the mode at 193-177 cm-1 is shifted to 228 cm-1 by approximately 43 cm-1, when the sample is activated.47 The DFT results indicate a similar shift of 34 cm-1 for the Cu-Cu mode from the hydrated to the empty paddlewheel model. In the case of H2S, the changes in the IR spectrum are much less pronounced than in the case of water. The H2S modes are shifted by only -3 and -5 cm-1. For CO2, a minor change is calculated, since the geometry is distorted only slightly. The bending mode upon binding is no longer degenerated and areshifted by -7 and -3 cm-1. Interesting cases are the carbonyl sulfide and dinitrogen monoxide, since they can be bound in two ways. The modes are shifted to different signs for the two different conformers. For OCS, the most pronounced difference is in the symmetric and asymmetric stretching. When the η1-O isomer is formed, these two modes are shifted by +17 and -38 cm1,

respectively. However, in the η1-S conformation, they are shifted by -16 and +14 cm-1,

respectively. A similar trend is computed for the two different isomers of the paddlewheel with N2O. The bending mode is not degenerate anymore and is shifted by -19 and -1 cm-1 for


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the η1-O conformation, whereas by -1 and +17 cm-1 for the η1-N conformation. The bending modes related to the shifts of -19 and +17 cm-1 are coupled with some vibrations from the framework atoms. A similar observation is made for the symmetric stretching mode, that shifts by -18 cm-1 for the former case and +25 cm-1 for the latter. The asymmetric stretching mode is shifted in the same direction, by +18 and +12 cm-1, respectively. This is an interesting finding, since the two binding modes show different vibrational behaviour upon binding and could be identified by the experiment. For sulfur dioxide, the scissoring mode is shifted by +10 cm-1, whereas the symmetric and asymmetric modes to -11 and -19 cm-1, respectively. For the radical molecules, the largest shift is computed for the stretching mode of nitrogen monoxide. It is shifted by +20 cm-1, and such large shift can be attributed to the π back-donation.9 The shift is negligible for dioxygen, being only -2 cm-1. This change is also reflected in the weak interaction energy of -8.7 kJ mol-1, which can be attributed to London dispersion forces. For nitrogen dioxide, the scissoring mode changes by -7 cm-1 and the symmetric/asymmetric stretching by -10 and +10 cm-1. Among all compounds studied, the largest changes are observed in case of NH3 (+151 cm-1 for wagging and -18/-17 cm-1 for asymmetric stretching) and PH3 (-23 cm-1 for wagging, +57 cm-1 for symmetric and +64 cm-1 for asymmetric stretching). This results from the Lewis acid-base interactions and the strong electrostatics with the metal atoms.

vi) Effect of the model size There is a small increase in the interaction energies when the molecular model is enlarged from Cu2(HCOO)4 to Cu2BTC4. The difference is small and varies between 1 and 4 kJ mol-1. The only exception is the OCS molecule, for which the interaction energy differs 8.5 kJ mol-1 between the two models.



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The vibrational characteristics of the guest molecules show only a weak dependence on the size of the model. For the diatomic molecules, there is almost no dependence. However, there is a small dependence on the guest molecules that show a weak interaction with the organic linker, i.e. for the OCS and N2O molecules. Main shifts for the big and small model are summarized in Table 2.

v) Thermodynamic properties The thermodynamic properties for the large model are summarized in Table 1, while, for comparison, the data for the smaller formate model are given in Table S1 of the Supporting Information. The contributions of the zero point energies in the adsorption enthalpies are varying from 2 kJ mol-1 for the diatomic molecules (O2, N2, NO) to 8 kJ mol-1 (for H2O and NH3). Despite some overestimation, comparison with the available experimental data for isosteric heat of adsorption shows reasonable agreement. However, we are not interested in the absolute values, but only in the relative difference of the adsorption enthalpies. For CO, our results predict an isosteric heat of adsorption of 28.1 kJ mol-1, in good agreement with the experimental value of 29 kJ mol-1.23 There is a plethora of experiments for CO2, yelding values varying from 12 to 35 kJ mol-1, 5, 38, 48, 49 with a mean value of 24.3 kJ mol-1 that falls close to the calculated value of 20.1 kJ mol-1 from this work. The available experimental value for N2 is ~5 kJ mol-1 from gravimetric measurements.48 However this specific experimental work also shows lower values for CO2 (12 kJ mol-1), so the value for N2 is likely to be an underestimate with respect to the calculated value of 15.5 kJ mol-1. There is no available experimental data for the other cases in the present literature. The order of the isosteric heats of adsorptions is following the same order as for the interaction energies. Ammonia shows the largest (83.7 kJ mol-1), followed by water (53.8), hydrogen sulfide (46.2), phospane (44.7) and sulfur dioxide (34.2). The weakest heat of adsorption is


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calculated for dioxygen (5.7 kJ mol-1). For the remaining cases, values lie in the region between 18 and 25 kJ mol-1.

vi) Classifying the interactions The interaction energies of the molecules studied here, vary from ~10 to 100 kJ mol-1. This energy scale is quite broad, so a classification of the interactions is necessary. For this reason, a Bader charge analysis has been performed and the results are summarized in Table S6 of the Supporting Information. As a first step, the contributions from the London dispersions are calculated and presented in Table 1. They vary from 10 kJ mol-1 for the diatomic molecules (CO, O2, N2, NO) to ~20 kJ mol-1 for the tri- and tetra-atomic ones. They only depend on the molecular size and not on the molecular composition. Dioxygen, which shows the weakest binding, interacts via London forces with the paddlewheel structure. 90% of the total interaction energy isdue to dispersion (9.0 out of 9.6 kJ mol-1). For the other diatomic molecules (CO, NO and N2), the dispersion contributions are between 30 and 50%. This implies that binding is an effect of electrostatics and σ and π back-donation. In fact, a charge transfer of 0.06 e- has been calculated for the CO case, which shows the strongest binding of the abovementioned diatomic molecules. For the other cases, which are less strongly bound, a very small charge transfer of 0.02 e- has been calculated. The second group of molecules is CO2, OCS, NO2, N2O and H2S. No significant charge transfer towards the paddlewheel has been calculated, except in the case of H2S (0.10 e-). The contributions from London dispersions to the total interaction energies are between 40 and 70%. This suggests that apart from dispersions, also electrostatics play an important role.



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Ammonia, water and phosphane can be classified to a third group. Dispersion interactions contribute 20, 20 and 35% respectively to the total interaction energies. A significant charge transfer of 0.11 and 0.14 e- has been calculated for NH3 and PH3 respectively, but for H2O only; 0.05 e-. The charge transfer provides a reasonable explanation for the increased shifts of the vibrational modes of NH3 and PH3 upon binding with the paddlewheel. However, electrostatics is not the main reason for the increased binding. The strong binding can be explained by an acid – base type of interaction between the Cu sites and the three basic molecules. In fact, the binding strength (NH3 > H2O > PH3) follows the basicity order, which supports our acid – base explanation.

Conclusions The aim of this work is to show the importance of the undercoordinated Cu2+ centers of HKUST-1 on the adsorption of small gases important for environmental processes. Interaction energies, enthalpies of adsorption and harmonic frequencies of several small molecules have been calculated. The interactions with the undercoordinated metal centers are significantly stronger than with the organic ligand. The results suggest that water is adsorbed strongly on the metal centers in a Lewis acid-base type interaction. Only ammonia is adsorbed more strongly on the metal sites than water, exceeding the water-framework interaction by 30 kJ mol-1. Hydrogen sulfide and phosphane are less strongly bound by 10 kJ mol-1 compared to water. Interactions for these four cases are significantly stronger than physisorption and fall in the regime of chemisorption. These findings are in agreement with experimental observations that ammonia and hydrogen sulfide decompose the framework.45, 46 The type of interactions can be classified in three categories: 1) weak physisorption, 2) polarization and electrostatics, and 3) strong acid – base.


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The interaction energies and enthalpies suggest that the MOF has high affinity towards the three basic molecules: NH3, H2O, PH3. The paddlewheel does not show strong binding of the sulfur and nitrogen oxides. The results suggest that under humid conditions the majority of the metal sites would be occupied by water molecules and no other gas molecule would be adsorbed except ammonia. It has been shown that the size of the MOF model has minor influence on the interaction energies, since the interactions are mainly localized on the metal site. However, for some of the gases (OCS, N2O) it might cause changes on the IR spectra, due to the interactions with the organic linkers. The frequency shifts with respect to free gaseous molecules have been calculated for both, Cu2(HCOO)4 and Cu2BTC4 models, and the major differences upon adsorption have been pointed out in Tables 2 and S5 Interesting vibrational properties are calculated for carbonyl sulfide and dinitrogen monoxide, which can be bound via two different configurations on the metal atoms. The shift of the vibrational modes depends on the binding way of the molecule. An indicative example is the symmetric stretching mode of N2O. This particular mode is shifted by -18 or +25 cm-1, when the molecule interacts via the oxygen or nitrogen atom with the Cu2+ sites of the paddlewheel.

Acknowledgements Financial support by DFG SPP 1362 (HE 3543/7-2, MA 532/1) and the European Research Council through FP7-IDEAS-ERC-StG-256962 is gratefully acknowledged.

Supporting Information Available



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Cartesian coordinates of all structures, tables with bond distances, interaction energies, adsorption enthalpies, harmonic frequencies and charge analysis can be found in the Supporting Information. The information is available free of charge via the Internet at http://pubs.acs.org


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Table 1: Important distances, total interaction energies (ΔEint), contributions from London dispersion to interaction energies (ΔEdisp), adsorption enthalpies at 0 and 298 K of various molecules on the undercoordinated Cu2+ centers of the HKUST-1. Values correspond to the Cu2BTC4 molecular model. Distances are in pm and energies in kJ mol-1.

(X)a R(Cu–X)

R(Cu–Cu)

ΔEint

ΔEdisp

-ΔH(0 K)

-ΔH(298 K)

H2O

O

221

260

-60.9

-15.6

55.3

53.8

H2S

S

260

260

-52.2

-22.2

45.4b

46.2b

CO

C

234

259

-32.3

-11.0

27.6

28.1

CO2

O

245

256

-27.0

-17.2

21.2

20.1

OCS

O

244

257

-30.5

-22.0

26.0

24.5

S

283

257

-29.0

-23.5

24.4

22.7

O

234

258

-39.2

-22.8

36.9

34.2

254

255

-9.6

-9.0

7.9

5.7

SO2 O2 NO

N

242

257

-21.3

-10.9

19.4

19.2

NO2

O

243

256

-23.4

-20.2

20.7

18.4

N2O

O

249

256

-28.4

-21.1

23.0

22.1

N

247

256

-24.8

-19.4

19.3

18.1

240

257

-19.3

-11.3

17.3

15.5

N2 NH3

N

218

262

-91.5

-21.4

83.3b

83.7b

PH3

P

257

262

-50.8

19.3

43.9b

44.7b

a

X represents the binding mode.



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For H2S, NH3 and PH3 a small imaginary frequency is always calculated. This mode corresponds to the relative rotation of the guest molecule with respect to the Cu–Cu axis. This imaginary frequency could not be eliminated after performing various geometry optimizations. The contributions to the enthalpies and for the adsorption on the big Cu2BTC4 model have been estimated by the corresponding contribution from the smaller formate model. b


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Table 2: Characteristic shifts of motions of gases from IR analysis for the smaller formate model Cu2(HCOO)4 (in parenthesis) and for the larger Cu2BTC4 molecular model (bold).

Binding mode (X)

Changes in the vibrational modes of the guest molecules upon adsorption [cm-1]

H2O

O

(-19, -29, -43) / +9, -11, -20

H2S

S

(0, +4, -5) / -7, +3, +1

CO

C

(+44) / +48

CO2

O

(-8, 0, 0, +3) / -7, -3, 0, 0

OCS

O

(-3, +7, +18, -23) / 0, +10, +17, -38

S

(-8, +4, -18, +22) / -8, +4, -16, +14

O

(+19, -9, -18) / +10, -11, -19

SO2

(-2) / -2

O2 NO

N

(+20) / +20

NO2

O

(-6, -10, +6, +17) / -7, -10, +10

N2O

O

(-16, +2, -19, +21) / -19, -1, -18, +18

N

(+4, +19, +30, +23)/ -1, +17, +25, +12 (+19) / +19

N2 NH3

N

(+151, -6, -1, -2, -12, -9) / +165, -5, +1, +1, -18, -17

PH3

P

(-23, -8, -7, +57, +63, +64) / -19, -9, -8, +58, +65, +66



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Figure 1: Molecular cluster models used for the calculations: dicopper tetra-formate Cu2(HCOO)4 model (side view – a , top view - b) and dicopper tetra-Benzenetricarboxylate Cu2BTC4 (side view – c , and top view - d). (orange- Copper, grey- Carbon, red – Oxygen, white – Hydrogen).


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Figure 2: Optimized structures of guest molecules interacting with the undercoordinated metal sites of the Cu2BTC4 model, a) CO, b) CO2, c) O2, d) OCS(O), e) OCS(S), f) PH3, g) NH3, h) N2, i) NO, j) N2O(O), k) N2O(N), l) NO2



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Graphical TOC 79x48mm (300 x 300 DPI)


Interaction of Small Gases with the Unsaturated Metal Centers of the

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