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Comment on “How Well Do Metal–Organic Frameworks Tolerate Flue Gas Impurities?” Kuang Yu, and Jordan R. Schmidt J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp3126413 • Publication Date (Web): 19 Jan 2013 Downloaded from http://pubs.acs.org on January 22, 2013
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Comment on “How Well Do Metal–Organic Frameworks Tolerate Flue Gas Impurities?” Kuang Yu and J.R. Schmidt* Theoretical Chemistry Institute and Department of Chemistry, University of Wisconsin, Madison, WI 53706 ABSTRACT In the title paper, Ding et al. reported interesting electronic structure results regarding the tolerance of metal-organic framework materials to flue gas impurities. Specifically, they reported binding energies of nearly 140 kJ/mol for NO binding to the coordinately unsaturated “open metal” Mg2+ sites of Mg/MOF-74, and corresponding binding energy of nearly 220 kJ/mol for SO2. These binding energies are strikingly large, and would imply essentially irreversible poisoning of the MOF by even trace amounts of these species. In contrast, we conclude that these large binding energies are an artifact of using a non-ground state spin configuration for the MOF (quintet vs. singlet), and that employing the singlet ground state dramatically reduces the associated binding energies of these species. This alters some of the conclusions of the title paper. In particular, NO can no longer be considered a potential poison, and (accounting for the low flue gas concentration) the impact of SO2 is likely strongly mitigated.
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Recently, Ding et al. reported interesting electronic structure results regarding the tolerance of metal-organic framework materials to flue gas impurities.1 Specifically, they reported binding energies of nearly 140 kJ/mol for NO binding to the coordinately unsaturated “open metal” Mg2+ sites of Mg/MOF-74, and corresponding binding energy of nearly 220 kJ/mol for SO2. These binding energies are strikingly large, and would imply essentially irreversible poisoning of the MOF by even trace amounts of these species. In contrast, our previously published data suggested far more modest binding energies enthalpies: 33 and 73 kJ/mol for NO and SO2, respectively.2 Given the massive discrepancies between these two results and the associated implications for MOFs under flue gas conditions, it is important to determine the origin of these differences. Both the title manuscript and our prior work utilize very similar cluster models to represent the Mg/MOF-74 bulk. Based on the published methodological details, the principle difference appears to be the choice of computational method: Ding et al. utilized the M06 density functional (which offers a good description of medium range dispersion), while we employed the range-separated and dispersion-corrected ωB97X-D functional (although we arrive at qualitatively identical conclusions using MP2). Using our prior model system and simply changing to the M06 functional, we obtained results in qualitative agreement with our previous reported results – with binding energies over 100 kJ/mol less strong than those reported in the title manuscript. After contacting, Ding et al., who kindly provided us with their optimized geometry and input file, we find that the difference lies in the choice of the spin state for the MOF. Although not noted explicitly in their original manuscript, Ding et al. modeled the Mg/MOF-74 as a quintet. However, the closed-shell nature of both the cationic metal (Mg2+) and the anionic linker
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The Journal of Physical Chemistry
groups suggests that the ground state will be a singlet; our computational results confirm that the singlet state is over 600 kJ lower in energy. As such, we conclude that the singlet (and not the quintet) is the most faithful representation of the bulk MOF. As expected, the quintet state binds species significantly more strongly than does the singlet ground state, presumably due to the presence of additional unpaired electrons. Recalculating the singlet binding energy at their optimized geometry yields a BSSE-corrected binding energy of 64 kJ/mol for SO2, as compared to 219 kJ/mol for the quintet. The former number compares favorably to our reported binding enthalpy of 73 kJ/mol, despite the methodological differences. We also repeated the M06 calculation for NO, using our (previously optimized) cluster model. Here, we also find a far more modest binding energy of 35 kJ/mol for the singlet, as compared to ~140 kJ/mol for the quintet; the former compares favorably to our previously binding enthalpy of 33 kJ/mol. Employing results from the singlet (ground state) alters some of the conclusions of the title paper. In particular, NO can no longer be considered a serious contaminant for Mg/MOF-74, and evaluation of the effects of SO2 need to be carefully evaluated using thermodynamic arguments due to its low partial pressure and more modest binding energy; given that the corrected (singlet) binding energy of SO2 is quite similar to our previously reported numbers, our prior analysis suggests that SO2 itself is unlikely to poison Mg/MOF-74 (particularly at putative regeneration temperatures of 383K), although hydrates (H2SO3) may pose a more significant concern with respect to irreversible poisoning.2 AUTHOR INFORMATION Corresponding Author
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[email protected] ACKNOWLEDGMENT This work was supported by Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy, under award DEFG02-09ER16059. REFERENCES
1. Ding, L.; Yazaydin, A. O., How Well Do Metal–Organic Frameworks Tolerate Flue Gas Impurities? Journal of Physical Chemistry C 2012, 116 (43), 22987-22991. 2. Yu, K.; Kiesling, K.; Schmidt, J. R., Trace Flue Gas Contaminants Poison Coordinatively Unsaturated Metal-Organic Frameworks: Implications for CO2 Adsorption and Separation. Journal of Physical Chemistry C 2012, 116 (38), 20480-20488.
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