Hydrogen storage in microporous metal-organic frameworks with exposed metal sites

The work herein describes the design, synthesis, and characterization of a series of metal-organic frameworks with unsaturated metal centers, with an emphasis on their hydrogen storage properties. Chapter 1 presents an extensive survey of the literature in the area of hydrogen storage in metal-organic frameworks, and discusses the influence of metal-H2 binding on the enthalpy of hydrogen adsorption in these materials. The current synthetic strategies and potential new directions for increasing the H2 binding energy in physisorptive materials are presented.;Chapter 2 addresses the possibility of using light metals, such as magnesium, for synthesizing lighter materials with increased H2 gravimetric uptake. The synthesis of Mg3(NDC)3 (NDC2- = 2,6-naphthalenedicarboxylate), the first magnesium-based microporous metal-organic framework is described. It is shown that this material is isostructural with a zinc-based framework and with a series of other first-row transition metal-based materials. Upon desolvation, Mg3(NDC)3 selectively adsorbs O2 and H2, showing potential for separating these two gases from mixtures with N2 and CO.;Chapter 3 introduces a new class of bridging ligands: the tetrazolates. Use of 1,4-benzeneditetrazolate (BDT2-) for the synthesis of a series of two- and three-dimensional frameworks, some of which are isostructural with carboxylate frameworks, shows that this ditopic ligand can function as an analogue of 1,4-benzenedicarboxylate. All four three-dimensional frameworks: Zn3(BDT)3, Mn3(BDT)3, Mn 2(BDT)C12, and Cu(BDT) exhibit permanent microporosity and show relatively large enthalpies of H2 adsorption, hinting to the possibility that H2 binds directly to unsaturated metal sites in these materials.;In Chapters 4 and 5, powder neutron diffraction is used to demonstrate unequivocally that H2 binds to unsaturated Mn2+ and Cu2+ sites within Mn3[(Mn4Cl) 3(BTT)8(CH3OH)10]2 (BTT 3- = 1,3,5-benzenetristetrazolate) and HCu[(Mn4Cl) 3(BTT)8]•3.5HCl, two metal-organic frameworks with a sodalite-like topology. It is shown that the strong interaction of H 2 with exposed metal sites in these frameworks contributes to very high H2 uptakes of up to 6.9 wt % and 60 g/L, and H2 binding energies of up to 10.1 kJ/mol, among the highest thus far for any microporous material.;The unique anionic character of Mn3[(Mn4Cl) 3(BTT)8(CH3OH)10]2 is exploited in Chapter 6, where a series of cation exchange reactions are shown to produce a series of related materials with the general formula M3[(Mn 4Cl)3(BTT)8(CH3OH)10] 2 (M = Fe2+, Co2+, Ni2+, Cu2+, Zn2+. Li+, Cu+). Hydrogen sorption measurements revealed significant differences among the enthalpies of adsorption in these materials, with the C2+-exchanged framework displaying the largest enthalpy of adsorption reported thus far for any microporous material: 10.6 kJ/mol. The possibility of using similar ion exchange reactions for catalytic applications is also discussed here.;Chapter 7 addresses the phenomenon of catenation in the synthesis of metal-organic frameworks. For the first time, it is demonstrated that catenation can be controlled by introducing minute modifications in the backbone of an organic ligand. This strategy is employed to synthesize two related metal-organic frameworks, Cu3[(Cu4Cl)3 (TPB-3tz)8]2 (TPB-3tz3- = 1,3,5-tris( p-tetrazolylphenyl)-benzene) and Cu3[(Cu4Cl) 3(TPT-3tz)]2 (TPT-3tz3- = 2,4,6-tris( p-tetrazolylphenyl)-triazine), and it is shown that interpenetration confers stability to the latter, which consequently exhibits higher surface area and increased H2 uptake compared to the former.;This work concludes with Chapter 8, wherein the synthesis of H4 TTPM, a novel tetrahedral tetrazolate-based ligand is described. The new ligand used to synthesize two highly-connected three-dimensional frameworks, Mn6(TTPM)3 and Cu[(Cu4Cl)(TTPM)2] 2, which exhibit very rare topologies identical to those of garnet and fluorite. It is shown that the anionic Cu-based framework displays a large surface area, high H2 uptake, and is amenable to a post-synthetic treatment that transforms it into a neutral framework, a phenomenon that has not been reported for a metal-organic framework thus far.