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J. Phys. Chem. C 2008, 112, 3152-3154
COMMENTS Comment on “Kinetics and Mechanistic Model for Hydrogen Spillover on Bridged Metal-Organic Frameworks” Andreas Mavrandonakis† and Wim Klopper*,†,‡ Institut fu¨r Nanotechnologie, Forschungszentrum Karlsruhe, Postfach 3640, D-76021 Karlsruhe, Germany, and Lehrstuhl fu¨r Theoretische Chemie, Institut fu¨r Physikalische Chemie, UniVersita¨t Karlsruhe (TH), D-76128 Karlsruhe, Germany ReceiVed: June 19, 2007; In Final Form: NoVember 9, 2007 In a recent paper in this journal, both experimental and theoretical work has been reported on the kinetics and a mechanistic model of hydrogen adsorption on the bridged IRMOF-8 isoreticular metal organic framework.1 Unfortunately, in these calculations, an improper model system was chosen. The carboxylate unit (-CO2-) of the MOF had been terminated with two hydrogen atoms (or rather, with a H atom and a proton). Furthermore, the structures were optimized at the Hartree-Fock (HF) level, followed by single-point energy calculations at the B3LYP level of density functional theory (DFT). The approach chosen in ref 1 is not correct, because an unpaired electron is present in the model for the IRMOF. Hence, the ground state of the model system is a doublet spin state, which renders geometry optimizations at the HF level cumbersome. Furthermore, the para and ortho adsorption sites were strongly but artificially favored over the meta site due to the resonance structures of the doublet state. Finally, but less importantly, the calculations were performed on a model for IRMOF-1 rather than IRMOF-8. However, this mistake will not change the adsorption energies significantly, since the structures of IRMOF-1 (benzene dicarboxylate) and IRMOF-8 (naphthalene dicarboxylate) are very similar. In order to provide useful energetic data for the adsorption of H atoms on the isoreticular metal organic frameworks, we have reinvestigated this adsorption on various models of IRMOF-1 and -8. Two proper models were chosen. The first model (A) is almost the same in as the work of Li et al., but the anionic carboxylate group -CO2- is terminated with a Li+ cation (Figure 1a), as done before in other works.2-4 In the second model (B), a benzene dicarboxylate dianion is coordinated to two metal clusters (Figure 1b), as also done before.5 In order to reduce computational cost, the other ligands are chosen as formiate ions HCO2- in model B. For the simulation of the IRMOF-8 cluster, a model (D) similar to model A for IRMOF-1 is used with the benzene dicarboxylate linker replaced by naphthalene dicarboxylate (Figure 1d). For completeness, the improper model of Li et al.1 is given in Figure 1c. All calculations were performed with the TURBOMOLE program package.6a,6b All structures were optimized at the DFT6c level, using either the B3LYP or the PBE functional. The resolution of identity (RI)6d approximation was used in conjunction with the non-hybrid PBE functional. In all calculations, * Corresponding author. E-mail:
[email protected]. † Forschungszentrum Karlsruhe. ‡ Universita ¨ t Karlsruhe (TH).
the def2-svp6e basis set was used together with the corresponding auxiliary basis set for the RI approximation in the PBE calculations. In a first step, the structures were optimized with the PBE functional, using the RI approximation. The adsorption energies of the hydrogen atom at various binding sites are summarized in Table 1. We use the same atom numbering as in the work of Li et al.1 In the second step, the most important sites with binding energies larger than a few kcal/mol were reoptimized at the B3LYP/def2-SVP level, either using the converged PBE structures or the initial ones, which led to the same minima. The B3LYP results are also summarized in Table 1. First of all, chemisorption does not take place on Zn or O1 (the central O atom of the OZn46+ cluster). Here, the hydrogen atom is only weakly physisorbed, and after the geometry optimization, it is located at 3.7 to 3.9 Å from the O1 atom. Both PBE and B3LYP functionals predict that the C1 site of all models is energetically unfavorable. In models A and B, the reaction is predicted to be endothermic while in the improper model C the hydrogen atom is transferred from C1 to C2 when the geometry is optimized. Chemisorption on the C2 site of models A and B is favored by 23.7-25.8 kcal/mol. However in the case of model C, the interaction energies computed by us are almost two times larger than in ref 1. In all cases, the hydrogenated structure is very distorted, as shown in Figure 2i. As a consequence, we suppose that adsorption on C2 will be much less favorable in the full 3D solid-state structure. Sites C3 and C4 are almost equivalent in model A and equivalent in model B. The adsorption energies of the hydrogen atom are estimated at about 30 kcal/mol. In the improper model C, however, these two sites are not equivalent. In this model, the adsorption energy on C3 is about 12-13 kcal/mol (B3LYP value). However, the singlet spin state is not the ground state. Due to the wrong termination of model C, a triplet state (〈S2〉 ) 2) is found to be the ground state. Also a broken-symmetry singlet state (〈S2〉 ≈ 1) is lower in energy than the closed-shell singlet. The triplet state is energetically favored by 18.5 kcal/ mol with respect to the closed-shell singlet, yielding a binding energy of 31 kcal/mol (B3LYP value). Concerning adsorption on C4, a significantly higher energy of about 56 kcal/mol is obtained. This latter value is in contrast with the value of 10.0 kcal/mol obtained by Li et al.1 Concerning adsorption on O2, large distortions occur on the metal cluster, as can be seen in Figure 2ii. Calculations on the models A and B yield binding energies from 13 to 14 kcal/ mol. In model C, however, a value of 30 kcal/mol is obtained, in contrast with the value of 9.7 reported by Li et al.1 We believe that the difference between the interaction energies at sites C3 and C4 occurs due to the resonance structures of model C, which is not terminated properly. After performing Mulliken and natural population analyses, the unpaired electron of model C is found to be mainly localized on C2, C4, and the C of the -C(OH)2 group. This is the main reason why high adsorption energies are obtained for these sites. While the results of our calculations are very plausible, we have not been able to reproduce the results obtained by Li et al.,1 which are displayed in the last column of Table 1. B3LYP
10.1021/jp074758h CCC: $40.75 © 2008 American Chemical Society Published on Web 02/02/2008
Comments
J. Phys. Chem. C, Vol. 112, No. 8, 2008 3153
Figure 1. (A-C) Various models of IRMOF-1 and (D) model of IRMOF-8. See text for more detailed explanation.
Figure 2. Hydrogen adsorption on (i) C2 site (left side, models B and C) and (ii) O2 site (right side, models B and C). The chemisorbed hydrogen atoms are shown in black color for clarity reasons.
TABLE 1: Adsorption Energies (De) of a Hydrogen Atom on Various Sites and Modelsa model A (Figure 1a)
model B (Figure 1b)
riPBE/ B3LYP/ riPBE/ B3LYP/ def2-svp def2-svp def2-svp def2-svp C1 C2 C3 C4 O1 O2 Zn
-2.2 23.7 30.4 29.1 0.4 13.5 0.4
n.c.b 24.7 30.9 29.8 n.c. 12.6 n.c.
-2.0 25.0 30.5 30.5 0.4 15.6 0.4
n.c. 25.8 31.3 31.3 n.c. 14.6 n.c.
model C of ref 1 (Figure 1c) riPBE/ def2-svp not stablec 52.3 19.0/29.4d 55.5 n.c. 30.1 n.c.
B3LYP/ def2-svp
Cs symmetry, we could calculate adsorption energies similar to those reported by Li et al.1
MOF + 1/2H2 f MOF-H ref 1
not stable 1.8 53.0 3.6 12.9/31.3d 11.9 56.8 10.0 n.c. 0.9 28.9 9.7 n.c 0.8
a All values are in kcal/mol. b n.c.: value has not been calculated. Not stable: adsorption on the C1 site is not favorable and under geometry optimization the H atom is transferred to the C2 site. d Values in bold refer to the triplet spin state.
c
single-point energy calculations in combination with the LanL2DZ or SVP basis sets using the Gaussian 03 package7 did not change the results significantly. Only by restricting our structures to
Furthermore, in the present work, calculations were carried out on the correct linker of IRMOF-8, which is modeled by model D. The results lie in the same range as for models A and B. Chemisorption is unfavorable on the C1 site. On the Zn and O1 sites, only physisorption takes place with energies of ca. 0.5 kcal/mol. Furthermore, binding energies of 25 and 14 kcal/ mol are found for the sites C2 and O2, respectively, but large distortions occur on the metal cluster. Finally, adsorption on the aromatic carbon atoms yields interaction energies of 30 and 35 kcal/mol except for C5, where a value of 12 kcal/mol is obtained. All values reported so far are pure electronic energies (De) that have neither been corrected for the basis-set superposition
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Comments
error (BSSE) nor for zero-point vibrational energies (ZPVEs). Counterpoise corrections showed that the BSSE is negligible, since the corrections were smaller than 0.5 kcal/mol, but the calculation of ZPVEs is required for a meaningful comparison with experimental data. In the following, we report computed enthalpies of reaction (at 0 K) including ZPVEs at the riPBE/ def2-svp level. We first consider the reaction where MOF and MOF-H are the pure and the hydrogenated MOF models. Analytical harmonic frequency calculations were performed on model B. The enthalpies of reaction at 0 K for the most important sites C2, C3, and O2 are ∆rH (0 K) ) +28.6, +23.1, and +38.1 kcal/mol, respectively. Furthermore, the inclusion of thermal corrections at T ) 298.15 K changed the results only by a few tenths of a kcal/mol. Hence, the spillover effect cannot be explained by the De and ∆rH (0 K) data for the adsorption of one H atom on our model B. Trying to explain the effect, the addition of two hydrogen atoms on IRMOF-1 and -8 was considered by describing the hydrogenation reaction with the following chemical equation:
MOF + H2 f H-MOF-H In a first approach, simplified MOF models were used consisting of benzene and naphthalene dicarboxylates. Geometry optimizations were followed by analytical harmonic frequency calculations in order to verify the stationary point and to estimate the ZPVE. All possible combinations of binding sites were studied and those yielding negative reaction enthalpies were further investigated using the larger MOF model depicted in Figure 3A. Only for the naphthalene dicarboxylate system, the 2-fold addition to the C3 and C4 as well as to the C4 and C6′ sites gave a negative reaction enthalpy (-5.4 and -2.1 kcal/mol respectively; for the numbering of the carbon atoms, see Figure 3A). The corresponding values for the larger model were ∆rH (0 K) ) -5.3 and -1.8 kcal/mol. Moreover, the structures of the MOF models remained largely undistorted (Figure 3B,C). Also a 4-fold hydrogenation on sites C3, C4, C3′, and C4′ was studied. Also in this case, the structure remained largely undistorted and the overall reaction was exothermic (∆rH (0 K) ) -2.0 kcal/mol). In conclusion, the spillover effect on IRMOF-1 and -8 has been reinvestigated using a proper model. All interaction energies are calculated by performing full optimizations at the PBE and B3LYP levels. Our results show that adsorption occurs on the aromatic carbon atoms with electronic binding energies De of 25-35 kcal/mol for both functionals and MOFs. The interaction of atomic hydrogen with Zn and O1 is mainly due to physisorption. Furthermore, we do not think that hydrogen spillover occurs via the simple mechanism involving the addition of one H atom, for which all reactions are predicted to be endothermic. The spillover effect may be explained by a double hydrogenation of the organic linker of IRMOF-8. The corresponding enthalpy of the reaction (at 0 K) is in agreement with the experimental results (∆rH (0 K) ) -5.3 kcal/mol versus D0(experimental) ) 5-6 kcal/mol). The further addition of two more hydrogen atoms is also exothermic. Acknowledgment. The present study has been supported by the Deutsche Forschungsgemeinschaft through the Center for Functional Nanostructures (CFN, Project No. C3.3). It has been further supported by a grant from the Ministry of Science, Research and the Arts of Baden-Wu¨rttemberg (Az: 7713.14300). The Chemistry Department of the University of Crete is
Figure 3. (A) Model of IRMOF-8. Double hydrogenation on: (B) C3 and C4, and (C) C4 and C6′ sites (views perpendicular on and along the naphthalene plane).
gratefully acknowledged for allowing us to perform the Gaussian 03 calculations. References and Notes (1) Li, Y.; Yang, F. H.; Yang, R. T. J. Phys. Chem. C 2007, 111, 34053411. (2) Sagara, T.; Klassen, J.; Ganz, E. J. Chem. Phys. 2004, 121, 1254312547. (3) Hu¨bner, O.; Glo¨ss, A.; Fichtner, M.; Klopper, W. J. Phys. Chem. A 2004, 108, 3019-3023. (4) Sagara, T.; Ortony, J.; Ganz, E. J. Chem. Phys. 2005, 123, 214707. (5) Lee, T. B.; Kim, D.; Jung, D. H.; Choi, S. B.; Yoon, J. H.; Kim, J.; Choi, K.; Choi, S.-H. Catal. Today 2007, 120, 330-335 (6) (a) Turbomole, version 5.9. See http://www.turbomole.com. (b) Ahlrichs, R.; Ba¨r, M.; Ha¨ser, M.; Horn, H.; Ko¨lmel, C. Chem. Phys. Lett. 1989, 162, 165. (c) Treutler, O.; Ahlrichs, R. J. Chem. Phys. 1995, 102, 346. (d) Eichkorn, K.; Treutler, O.; O ¨ hm, H.; Ha¨ser, M.; Ahlrichs, R. Chem. Phys. Lett. 1995, 242, 652. (e) Weigend, F. Phys. Chem. Chem. Phys. 2006, 8, 1057. (7) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.02; Gaussian, Inc.: Pittsburgh PA, 2003.
