J. Phys. Chem. C 2008, 112, 3155-3156
Reply to “Comment on ‘Kinetics and Mechanistic Model for Hydrogen Spillover on Bridged Metal-Organic Frameworks’” Yingwei Li, Frances H. Yang, and Ralph T. Yang* Department of Chemical Engineering, UniVersity of Michigan, Ann Arbor, Michigan 48109 ReceiVed: July 10, 2007; In Final Form: December 3, 2007 We welcome the comments of Mavrandonakis and Klopper and the comparison of different results obtained from different model systems. We employed the model system (terminated by 2 hydrogens) that is consistent with the 3-dimenional XRD crystal structure data (see Table 1 below). This is important since there are significant constraints on the structure that are a consequence of the extended 3-D framework of the IRMOF. Simply stated, full relaxation of one building block does not yield a structure that is consistent with the measured 3-D structure. In addition to the good agreement between our calculated and the measured geometries, the results obtained with our model were consistent with the experimental results. For example, our calculated binding energies are below11.9 kcal/ mol for all sites, suggesting that the spiltover hydrogen atoms can desorb readily at room temperature. On the contrary, the binding energies from the calculations of Mavrandonakis and Klopper are in the range of 30-56 kcal/mol for most of the sites (see their Table 1). At such high binding energies, it is not possible to desorb at room temperature. An important experimental result from our work as well as from others is that the spiltover H can desorb at room temperature, i.e., the isotherm is reversible.1 Unfortunately, the results of Mavrandonakis and Klopper contradict this important experimental fact. Moreover, Mavrandonakis and Klopper concluded that “the spillover effect may be explained by a double hydrogenation of...”. This conclusion fails to corroborate the experimental storage capacity. Spillover is a far more complex phenomenon than such a simple-minded argument could explain (more details given in Comment 6). Several serious simulations efforts on spillover are in progress and will be published shortly.2 What follows are our point-to-point comments on each of their criticisms. As we stated in our paper that “the computational model used for IRMOF shown in Figure 1 is the smallest repeating unit found in IRMOF-1, with the -CO2 unit terminated with two hydrogen atoms”. (I) Mavrandonakis and Klopper claim that we had chosen an improper model system, because an unpaired electron is present in our model for the IRMOF. They further stated that a doublet spin state renders geometry optimizations at HF level cumbersome. Comment 1. They seem to imply that an unpaired electron is created when a -CO2 unit is terminated with two hydrogen atoms. The fact is this electron on the carbon atom of -CO2 is present in IRMOF whether the -CO2 unit is connected to two hydrogen atoms or to two lithium atoms or even to two zinc atoms as in the actual IRMOF. The electron is delocalized in the π-electrons system of 1,4-benzenedicarboxylate (BDC) in our IRMOF model, shown as (I) in Figure 2. If we had * Corresponding author. E-mail:
[email protected].
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TABLE 1: Bond Lengths [Å] and Angles [°] for IRMOF-1 C1-C2 C2-C3 C3-C4 C1-O2 XRD data our model
1.486 1.480
1.392 1.416
1.388 1.397
1.301 1.274
C-O C1C3(H) C2-C3 C2-C3 NA 1.385
120.0 120.0
119.9 120.4
terminated -CO2 with one hydrogen or one lithium as proposed in their models, then this mobile electron will be localized on one of the two oxygen atoms on -CO2, forming carboxylate anions, shown as (II) and (III), thus changing completely the chemical nature of our IRMOF model. It is interesting to note that, when we examine only the terminating BDC part of the IRMOF model, there is actually no unpaired electron in (I); however, there is an unpaired electron in both (II) and (III). The reason we have a doublet spin state for our IRMOF model is because there are six benzenes in the model and only one benzene has a -CO2 on the para position of that benzene. If we had included another -CO2 unit on one of the five remaining benzenes, and also terminated it with two hydrogen atoms, then we would no longer have a doublet spin state. Thus, the doublet spin state is unrelated to hydrogen termination. Furthermore, Gaussian calculations for different spin states are rather routine, not at all “cumbersome”. We believe this delocalized state of π-electrons in the BDC part of IRMOF is essential, and it is these delocalized π-electrons that can explain at least in part why IRMOF, like graphite and carbon nanotube, has high capacity in adsorbing hydrogen. (II) Mavrandonakis and Klopper claim that our approach is not “correct” when the structures were optimized at the HartreeFock (HF) level, and then binding energies were performed at the density functional theory (DFT) level. Comment 2. When we compare the various bond lengths and bond angles generated from our optimized structure of IRMOF model to the published values from single-crystal XRD data,3 our values are rather close to the published values, as can be seen from the comparison in Table 1. (III) Mavrandonakis and Klopper claim the para and ortho adsorption sites were strongly but artificially favored over the meta site due to the resonance structures of the doublet state. Comment 3. When they use a different type of bond system for our model, one would of course expect them to get results quite different from ours. (IV) Mavrandonakis and Klopper claim the calculations were performed on a model for IRMOF-1 rather than IRMOF-8.
Figure 1. Computational model for IRMOF.
10.1021/jp075382a CCC: $40.75 © 2008 American Chemical Society Published on Web 02/02/2008
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Figure 2. BDCs with 3 types of terminations. (I) When terminate a BDC with 2 hydrogen atoms. (II) When terminate a BDC with one hydrogen atom. (III) When terminate a BDC with one lithium atom.
Comment 4. As we stated in our paper, the model in our Figure 1 was for IRMOF-1, and it was the simplest model for MOF. Our calculations indicated that IRMOF-1 and IRMOF-8 were Very similar, so we used the results on IRMOF-1 for discussion. We should have made this point more clear. The use of the word “mistake” by these authors is a bit too aggressive and not called for. (V) Mavrandonakis and Klopper claim that in the “improper model C” the hydrogen atom is transferred from C1 to C2 when the geometry is optimized.
Comments Comment 5. We did not observe such hydrogen transfer in our model, which further indicated that their “improper model C” cannot be identical to our model. It is entirely possible that the differences were caused by their use of the TURBOMOLE code while we used Gaussian 03. Further work is ongoing to see the differences between TURBOMOLE and Gaussian 03. They employed zero-point energy corrections (ZPEC). The major reason for the significant decrease of their calculated energy by the addition of H2 to their model is not due to the ZPEC; it is due to the bond energy of H-H, 104 kcal/mol. (VI) In order to lower their 0 K enthalpies for adsorption to get close to our experimental result, they attempted to bind twin H atoms (“2-fold addition”) on certain sites (C3, C4, ...) and claimed much lower values were obtained. Comment 6. Another important experimental result is that the isotherm is nearly linear at 298 K in the entire pressure range (0-100 atm) and that the storage capacity reached nearly 4 wt % at 100 atm and 298 K and could increase further at still higher pressures. The proposal of “2-fold addition” on certain sites would result in only 1.3 wt % storage at saturation. This is in clear contradiction to the experimental result. The spiltover H atoms do not migrate and adsorb in twins or H2 (as evidenced by numerous experimental results in the literature as well as our recent deuterium isotope tracer results); they migrate and adsorb in indiVidual atoms, not H2. The spillover process involves hydrogen dissociation on the metal surface, migration (or spillover) of H atoms to the carbon substrate, migration of H atoms over the carbon bridges, migration of H atoms to the IRMOF particle, and finally binding on the IRMOF sites. Such a complex process cannot be described by such a simple-minded argument made by these authors. References and Notes (1) Li, Y.; Yang, F. H; Yang, R. T. J. Phys. Chem. C 2007, 111, 34053411. (2) For example, Chen, L.; Cooper, A. C.; Pez, G. P.; Cheng, H. J. Phys. Chem. C 2008, in press. (3) Eddaoudi, M.; Kim, J.; Rosi, N.; Vodak, D.; Wachter, J.; O’Keeffe, M.; Yaghi, O. M. Science 2002, 295, 469-472.
