Hydrogen Storage in the Expanded Pore Metal-Organic Frameworks M-2(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn)

The hydrogen storage properties of a new family of isostructural metal organic frameworks are reported. The frameworks M-2(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc(4-) = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) are analogous to the widely studied M-2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn; dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) family of materials, featuring the same weak-field oxo-based ligand environment for the M2+ metal centers, but with a larger pore volume resulting from the extended length of the dobpdc(4-) linker. Hydrogen gas adsorption isotherms measured at 77 and 87 K indicate strong H-2 binding at low pressures, corresponding to the adsorption of one molecule per M2+ site. Isosteric heats of adsorption indicate adsorption enthalpies ranging from -8.8 to -12.0 kJ/mol, with the trend Zn < Mn < Fe < Mg < Co < Ni. Room-temperature high-pressure adsorption isotherms indicate enhanced gravimetric uptakes compared to the M-2(dobdc) analogues, a result of the higher surface areas and pore volumes of the expanded frameworks. Indeed, powder neutron diffraction experiments performed on Fe-2(dobpdc) reveal two additional secondary H2 adsorption sites not observed for the nonexpanded framework. While displaying higher gravimetric capacities than their nonexpanded counterparts, the larger pore volumes result in lower volumetric capacities. Upon comparison with other promising frameworks for hydrogen storage, it becomes evident that in order to design future materials for on-board hydrogen storage, care must be placed in achieving both a high surface area and a high volumetric density of exposed metal cation sites in order to maximize gravimetric and volumetric capacities simultaneously.

Metal-organic frameworks for on-board storage of hydrogen and natural gas

Miguel Gonzalez, Wendy Lee Queen

Materials exhibiting reversible hydrogen adsorption with high gravimetric and volumetric capacities are sought for use in on-board storage systems of hydrogen fuel cell-powered vehicles. Microporous metal-org. frameworks with high internal surface areas have been shown to display excellent storage properties, but only at cryogenic temps. Methods for synthesizing frameworks contg. coordinatively-unsatd. metal centers are therefore being developed as a means of increasing the H2 adsorption enthalpy. In particular, we seek to synthesize thermally-robust, high-surface area materials with a high concn. of open metal coordination sites. By adjusting the electronic structure of the metal ions, it is expected that an optimal H2 adsorption enthalpy in the range -15 to -20 kJ/mol can be achieved, leading to a high storage capacity at room temp. and pressures of up to 100 bar. Such materials could similarly be of utility in boosting the methane storage capacity in the tanks of natural gas-powered cars operating at reduced pressures of up to 35 or 65 bar. Particular attention will be devoted to recent results involving efforts to achieve new materials in which multiple H2 or CH4 mols. can adsorb at a single metal site, and in which the thermal management requirements assocd. with gas adsorption/desorption can be reduced.

American Chemical Society2014

Metal-organic frameworks for on-board storage of hydrogen and natural gas

Miguel Gonzalez, Wendy Lee Queen

Materials exhibiting reversible hydrogen adsorption with high gravimetric and volumetric capacities are sought for use in on-board storage systems of hydrogen fuel cell-powered vehicles. Microporous metal-org. frameworks with high internal surface areas have been shown to display excellent storage properties, but only at cryogenic temps. Methods for synthesizing frameworks contg. coordinatively-unsatd. metal centers are therefore being developed as a means of increasing the H2 adsorption enthalpy. In particular, we seek to synthesize thermally-robust, high-surface area materials with a high concn. of open metal coordination sites. By adjusting the electronic structure of the metal ions, it is expected that an optimal H2 adsorption enthalpy in the range -15 to -20 kJ/mol can be achieved, leading to a high storage capacity at room temp. and pressures of up to 100 bar. Such materials could similarly be of utility in boosting the methane storage capacity in the tanks of natural gas-powered cars operating at reduced pressures of 35 or 65 bar. Particular attention will be devoted to recent results involving efforts to achieve new materials in which multiple H2 or CH4 mols. can adsorb at a single metal site, and in which the thermal management requirements assocd. with gas adsorption/desorption can be reduced.

American Chemical Society2015

Systematic Tuning and Multifunctionalization of Covalent Organic Polymers for Enhanced Carbon Capture

Johanna Maria Huck, Rocio Mercado, Berend Smit, Hui Wang

Porous covalent polymers are attracting increasing interest in the fields of gas adsorption, gas separation, and catalysis due to their fertile synthetic polymer chemistry, large internal surface areas, and ultrahigh hydrothermal stabilities. While precisely manipulating the porosities of porous organic materials for targeted applications remains challenging, we show how a large degree of diversity can be achieved in covalent organic polymers by incorporating multiple functionalities into a single framework, as is done for crystalline porous materials. Here, we synthesized 17 novel porous covalent organic polymers (COPs) with finely tuned porosities, a wide range of Brunauer Emmett Teller (BET) specific surface areas of 430-3624 m(2) g(-1), and a broad range of pore volumes of 0.24-3.50 cm(3) g(-1), all achieved by tailoring the length and geometry of building blocks. Furthermore, we are the first to successfully incorporate more than three distinct functional groups into one phase for porous organic materials, which has been previously demonstrated in crystalline metal organic frameworks (MOFs). COPs decorated with multiple functional groups in one phase can lead to enhanced properties that are not simply linear combinations of the pure component properties. For instance, in the dibromobenzene-lined frameworks, the bi- and multifunctionalized COPs exhibit selectivities for carbon dioxide over nitrogen twice as large as any of the singly functionalized COPs. These multifunctionalized frameworks also exhibit a lower parasitic energy cost for carbon capture at typical flue gas conditions than any of the singly functionalized frameworks. Despite the significant improvement, these frameworks do not yet outperform the current state-of-art technology for carbon capture. Nonetheless, the tuning strategy presented here opens up avenues for the design of novel catalysts, the synthesis of functional sensors from these materials, and the improvement in the performance of existing covalent organic polymers by multifunctionalization.

American Chemical Society2015

Systematic Tuning and Multifunctionalization of Covalent Organic Polymers for Enhanced Carbon Capture

Johanna Maria Huck, Rocio Mercado, Berend Smit, Hui Wang

American Chemical Society2015