Carborane-Based Metal–Organic Framework with High Methane and Hydrogen Storage Capacities

Abstract

A Cu−carborane-based metal−organic framework (MOF), NU-135, which contains a quasi-spherical para-carborane moiety, has been synthesized and characterized. NU-135 exhibits a pore volume of 1.02 cm 3 /g and a gravimetric BET surface area of ca. 2600 m 2 /g, and thus represents the first highly porous carborane-based MOF. As a consequence of the unique geometry of the carborane unit, NU-135 has a very high volumetric BET surface area of ca. 1900 m 2 /cm 3. CH 4 , CO 2 , and H 2 adsorption isotherms were measured over a broad range of pressures and temperatures and are in good agreement with computational predictions. The methane storage capacity of NU-135 at 35 bar and 298 K is ca. 187 v STP /v. At 298 K, the pressure required to achieve a methane storage density comparable to that of a compressed natural gas (CNG) tank pressurized to 212 bar, which is a typical storage pressure, is only 65 bar. The methane working capacity (5−65 bar) is 170 v STP /v. The volumetric hydrogen storage capacity at 55 bar and 77 K is 49 g/L. These properties are comparable to those of current record holders in the area of methane and hydrogen storage. This initial example lays the groundwork for carborane-based materials with high surface areas. ■ INTRODUCTION Nanoporous materials such as metal−organic frameworks (MOFs) with tailorable pore volumes, high internal surface areas, 1,2 and chemical diversity form an important emerging class of materials that are potentially useful in a wide range of applications including ion exchange, 3 catalysis, 4,5 gas separation , 6−8 sensing, 9 and storage. 10−12 In particular, MOFs have attracted great interest for onboard hydrogen and methane storage in vehicles, which is a long-standing goal for those interested in energy independence and the protection of the environment. Over the past decade, significant progress has been made toward building new classes of MOFs with desired material characteristics. This has been achieved via, for example, the tailoring of pore geometry, volume, surface area, and density, and by the incorporation of unsaturated metal centers. 13,14 Control over these parameters is crucial for realizing materials with high gas uptake capacity. Polyhedral boranes and heteroboranes are a unique class of cluster compounds with properties and geometries that are not accessible with carbon-based systems. 15 Among the polyhedral boranes, the family of isomeric icosahedral dicarbaboranes (closo-C 2 B 10 H 12) 15 and their metallo-derivatives 16 have received much attention because of their varied chemistry, 17 redox properties, 18 thermal stability, and their potential for applications in the areas of medicine 19 and materials chemistry. 15 Recently we reported a series of MOFs that are derived from closo-1,12-C 2 B 10 H 12 (para-carborane). The carborane-based MOFs displayed exceptional thermal stability and large CO 2 /CH 4 and CO 2 /N 2 adsorption selectiv-ities. 7,8,20−22 However, the pore volumes and internal surface areas of these carborane-based MOFs were relatively small, thus...

Carboranes in the chemist's toolbox

“…136 For example, under only 65 bar pressure, the CH 4 storage capacity of this MOF is comparable to that of a compressed natural gas tank at 212 bar. 2600 m 2 g −1 ) has been prepared and demonstrated to have extremely high CH 4 and H 2 storage capacities, which compare favorably with current methane and hydrogen storage technology.…”

Evolution of the Design of CH4 Adsorbents

“…(a) Carborane-bis(isophthalic acid) ligand (LH4); (b) NU-135, which is a MOF which has a p-carborane subunit and Cu metal sites. Adapted with permission from Ref [107]…”

Porous Metal–Organic Polyhedral Frameworks with Optimal Molecular Dynamics and Pore Geometry for Methane Storage

“…At 35 bar, MFM-112a and MFM-115a show high total gravimetric CH 4 uptakes of 322 and 304 cm 3 (STP) g –1 , respectively, comparable to those measured for the best-behaving MOFs under the same conditions. 31 − 34 In addition, MFM-115a shows a high volumetric total CH 4 capacity of 186 v/v when compared to other high-performing CH 4 uptake materials such as MOF-519, 35 PCN-14, 36 and NU-125 24a , 31 , 37 at 35 bar and 298 K. In comparison, MFM-112a has a lower total volumetric CH 4 uptake of 162 v/v at 35 bar. Although MFM-132a shows lower total gravimetric CH 4 uptake of 249 cm 3 (STP) g –1 than MFM-112a, it has the same volumetric uptake of 162 v/v at 35 bar and 298 K owing to the higher crystal density of MFM-132a.…”