Subscriber access provided by Northwestern Univ. Library Article Framework Reduction and Alkali-Metal Doping of a Triply Catenating Metal#Organic Framework Enhances and Then Diminishes H2 Uptake Karen L. Mulfort, Thea M. Wilson, Michael R. Wasielewski, and Joseph T. Hupp Langmuir, 2009, 25 (1), 503-508• DOI: 10.1021/la803014k • Publication Date (Web): 10 December 2008 Downloaded from http://pubs.acs.org on February 11, 2009 More About This Article Additional resources and features associated with this article are available within the HTML version: • Supporting Information • Access to high resolution figures • Links to articles and content related to this article • Copyright permission to reproduce figures and/or text from this article Langmuir is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Langmuir 2009, 25, 503-508 503 Framework Reduction and Alkali-Metal Doping of a Triply Catenating Metal-Organic Framework Enhances and Then Diminishes H2 Uptake Karen L. Mulfort,†,‡ Thea M. Wilson,† Michael R. Wasielewski,† and Joseph T. Hupp*,† Department of Chemistry & Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208, and DiVision of Chemical Sciences and Engineering, Argonne National Laboratory, 9700 South Cass AVenue, Argonne, Illinois 60439 ReceiVed September 14, 2008 A permanently microporous metal-organic framework compound with the formula Zn2(NDC)2(diPyTz) (NDC ) 2,6-naphthalenedicarboxylate, diPyTz ) di-3,6-(4-pyridyl)-1,2,4,5-tetrazine) has been synthesized. The compound, which features a triply catenating, pillared-paddlewheel structure, was designed to be easily chemically reduced (diPyTz sites) by appropriate channel permeants. Reduction was achieved by using the naphthalenide anion, with the + + + accompanying metal cation (Li ,Na or K ) serving to dope the compound in extraframework fashion. H2 uptake at 1 atm and 77 K increases from 1.12 wt % for the neutral material to 1.45, 1.60, and 1.51 wt % for the Li+-, Na+-, and K+-doped materials, respectively. The isosteric heats of adsorption are similar for all four versions of the material despite the large uptake enhancements for the reduced versions. Nitrogen isotherms were also measured in order to provide insight into the mechanisms of uptake enhancement. The primary mechanism is believed to be dopant- facilitated displacement of catenated frameworks by sorbed H2. More extensive cation doping decreases the H2 loading. 1. Introduction by adjusting ligand the length and shape9,10 or through framework catenation.11 Because H contains only two electrons, these Permanently microporous, crystalline, metal-organic frame- 2 dispersion interactions are relatively weak. Furthermore, they work materials (MOFs) are being considered for a wide range decrease with 1/r6 where r is the distance between H and the of chemical applications that can capitalize on their high internal 2 framework. Under the storage temperatures and pressures surface areas, uniform pore sizes, and enormous potential diversity in composition and structure.1-3 Among the most intriguing is envisioned for vehicular applications, H2 is well above its critical hydrogen gas storage.4-6 MOFs have the capacity to revolutionize point. Significant sorption excesses, therefore, can be achieved gas storage methods and materials because of their ultralow only by framework adsorption, not by pore condensation. Thus, densities and their crystalline micropore and/or ultramicropore there is no incentive to create huge pores (i.e., pores that can structures; in principle, these structures can promote ordered and accommodate more than a monolayer of H2 molecules). The therefore exceptionally high density guest packing. There have second approach attempts to create exceptionally attractive surface sites within the MOF via the formation of unsaturated metal been several recent reports of large H2 uptake in MOFs at 77 K and high pressure,7,8 but none yet satisfy the proposed capacity centers typically through the removal of coordinated solvent benchmarks for commercially viable and safe hydrogen storage molecules at nodes (metal ions or clusters comprising framework 12-15 at noncryogenic temperatures. Although there is an immediate structural sites). Neutron-scattering experiments have veri- fied that H binds at these sites first, most likely through Kubas- need to improve H2 sorption, equally important for long-term 2 success is the need to understand fully the factors that affect H2 type interactions. Whereas this approach has been shown to impact uptake and binding such that the essentially limitless potential H2 uptake within MOFs appreciably, it is sometimes difficult to variety of MOF materials can be used to full advantage. synthesize frameworks that can resist collapse upon complete desolvation of metal sites. A further challenge is to introduce Improving MOF H2 adsorption capacity through structural means has been pursued by two general strategies. The first is enough such sites to make a practical difference under high- to manipulate the MOF structure to increase H2-framework loading conditions. dispersion interactions and decrease unused pore space, either We recently communicated preliminary results for a distinct but related third approach: chemical reduction of framework * Author to whom correspondence should be addressed. E-mail: struts. For a representative doubly interpenetrating MOF, we [email protected]. observed a 75% increase in H2 uptake at 77 K and 1 atm † Northwestern University. ‡ Argonne National Laboratory. (1) James, S. L. Chem. Soc. ReV. 2003, 32, 276–288. (9) Lin, X.; Jia, J.; Zhao, X.; Thomas, K. M.; Blake, A. J.; Walker, G. S.; (2) Kitagawa, S.; Kitaura, R.; Noro, S. Angew. Chem., Int. Ed. 2004, 43, Champness, N. R.; Hubberstey, P.; Schroeder, M. Angew. Chem., Int. Ed. 2006, 2334–2375. 45, 7358–7364. (3) Rowsell, J. L. C.; Yaghi, O. M. Microporous Mesoporous Mater. 2004, (10) Dybtsev, D. N.; Chun, H.; Yoon, S. H.; Kim, D.; Kim, K. J. Am. Chem. 73, 3–14. Soc. 2004, 126, 32–33. (4) Collins, D. J.; Zhou, H.-C. J. Mater. Chem. 2007, 17, 3154–3160. (11) Ma, S. Q.; Sun, D. F.; Ambrogio, M.; Fillinger, J. A.; Parkin, S.; Zhou, (5) Latroche, M.; Surble, S.; Serre, C.; Mellot-Draznieks, C.; Llewellyn, P. L.; H. C. J. Am. Chem. Soc. 2007, 129, 1858–1859. Lee, J. H.; Chang, J. S.; Jhung, S. H.; Ferey, G. Angew. 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Soc. 2007, 129, 12680–12681. 10.1021/la803014k CCC: $40.75 2009 American Chemical Society Published on Web 12/10/2008 504 Langmuir, Vol. 25, No. 1, 2009 Mulfort et al. (0.93f1.63 wt %) as well as significant increases in loading- dependent heats of adsorption, following partial reduction with lithium metal.16 We suggested that framework reduction could potentially enhance the uptake of H2 by MOFs by at least three mechanisms: (i) greatly enhanced strut polarizability, potentially resulting in stronger induced-dipole/induced-dipole interactions 6 between the strut and H2 (i.e., 1/r interactions); (ii) the introduction of essentially completely unsaturated metal centers in the form of charge-compensating cations (potential sites for charge/quadrupole interactions with dihydrogen or Kubas interactions if transition-metal ions are used); and (iii) the displacement of interwoven networks to create pores and channels of more optimal size for H2 sorption. On the basis of several lines of evidence, including an unusual and highly hysteretic N2 isotherm, we concluded that (at least) mechanism iii was important for the particular system examined. We were unable to draw conclusions about the significance of mechanism ii, in part because of ambiguity in the degree of residual solvation of the dopant metal ions and in part because only a single dopant (Li+) was examined. In a follow-up study pursued in parallel with the present study,17 we concluded that mechanism ii was likely not operative for the doubly interpenetrating system. The follow-up work Figure 1 Chemical structure of dipyridyl ligand diPyTz, structure of + + . entailed an extension of the investigation of Na and K as 1. (A) Single-crystal structure of 1. For clarity, two levels of dopants. interpenetration are omitted. The yellow polyhedra represent the zinc In this article, we report further on the idea of the chemical ions; gray, carbon; blue, nitrogen; and red, oxygen. Hydrogens are omitted for clarity. (B) Packing diagram of 1. reduction of organic struts as a strategy for enhancing H2 uptake by metal-organic framework compounds. As in the initial report, (Norcross, GA). Powder X-ray diffraction (PXRD) patterns were we employ a mixed-strut material featuring a pillared-paddlewheel recorded with a Rigaku XDS 2000 diffractometer using nickel- filtered Cu KR radiation (λ ) 1.5418 Å) over a range of 5° < 2θ structure. MOFs of this kind consist of metal pairs (generally < 40° in 0.1° steps witha1scounting time per step. Powder samples Zn(II) or Cu(II)) bridged by linear dicarboxylates that create were placed in the diffractometer mounted on a stainless steel holder paddlewheel sheets in two dimensions.