Unlike MOFs, COF structures are entirely composed of light elements (H, B, C, and O) that are linked by strong covalent bonds (B-O, C-C, and B-C) to make a highly porous class of materials. Indeed, one member of this class has the lowest density ever reported for a crystalline solid (0.17 g cm^-3 for COF-108). This has led us to investigate the potential use of COFs in the storage of some gases relevant to clean energy. Here we report the first adsorption studies of hydrogen, methane, and carbon dioxide in COFs and show that COFs rank among the highest performing materials in terms of their gas storage capacities.

—Furukawa and Yaghi (2009)

Furukawa and Yaghi classified seven different COFs into three groups based on their structural dimensions and corresponding pore sizes:

• Group 1: 2D structures with 1D small pores (9 Å for each of COF-1 and COF-6)
• Group 2: 2D structures with large 1D pores (27, 16, and 32 Å for COF-5, COF-8, and COF-10, respectively)
• Group 3: 3D structures with 3D medium-sized pores (12 Å for each of COF-102 and COF-103)

In dihydrogen, methane, and carbon dioxide isotherm measurements performed at 1-85 bar (0.1 to 8.5 Mpa) and 77-298 K (-196 to 25°C), the researchers found that Group 3 COFs outperform group 1 and 2 COFs, and rival the best metal-organic frameworks and other porous materials in their uptake capacities.

At 35 bar, COF-102 showed excess gas uptake of 72 mg g^-1 at 77 K for hydrogen; 187 mg g^-1 at 298 K for methane, and 1,180 mg g^-1 at 298 K for carbon dioxide).

Hydrogen. For hydrogen, the Group 3 COFs demonstrate one of the best performances in the class of physisorption materials, approaching the 2010 DOE system target at 77 K (6.0 wt % and 45 g of H₂ L^-1). More importantly, the researchers notes, the comparable H₂ uptake capacities of the group 3 COFs and MOFs indicate that H₂ uptake capacity is independent of the composition of the structure’s backbone and that design of high-affinity sites by metal doping is a promising pathway for enhancing hydrogen storage performance at ambient temperature.

Methane. The current storage target set by DOE is 180 cm³(STP) cm^-3 at 35 bar, which is comparable to the energy density of compressed natural gas at 250 bar.

Remarkably, [COF-102] CH₄ uptake at 35 bar is roughly 4 times larger than bulk CH₄ density at the same temperature and pressure. The values in cm³ cm^-3 units for COF-102 are well within the realm of the DOE target of 180 cm³ cm^-3 at 35 bar. It should be noted that the contribution of the packing factor of COF samples is important to determine practical uptake in a canister. The packing density is influenced by both shape and size of the materials and usually is below unity, although these numbers for COFs are not available here. Indeed, the actual volumetric uptake is 20-30% smaller compared to the present data if the packing density is 0.7.

—Furukawa and Yaghi (2009)

(Yaghi’s group has a long-standing collaboration with BASF to expand the use of methane as an automobile fuel; the company contributes to the research funding and has licensed MOF technology and is moving forward on commercialization.)

CO₂. Furukawa and Yaghi found that the relationship between the pore diameter and saturation pressure of COFs for storing CO₂ is similar to that of MOFs, indicating that gas adsorption behavior in COFs is substantively the same as that in MOFs. The high CO₂ storage capacity of COFs could be applicable to the short-term CO₂ storage and transport of CO2, although it is lower than certain other materials.

Resources

• Hiroyasu Furukawa and Omar M. Yaghi (2009) Storage of Hydrogen, Methane, and Carbon Dioxide in Highly Porous Covalent Organic Frameworks for Clean Energy Applications. J. Am. Chem. Soc., Article ASAP doi: 10.1021/ja9015765