Abstract
Metal–organic frameworks have received significant attention as a new class of adsorbents for natural gas storage; however, inconsistencies in reporting high-pressure adsorption data and a lack of comparative studies have made it challenging to evaluate both new and existing materials. Here, we briefly discuss high-pressure adsorption measurements and review efforts to develop metal–organic frameworks with high methane storage capacities. To illustrate the most important properties for evaluating adsorbents for natural gas storage and for designing a next generation of improved materials, six metal–organic frameworks and an activated carbon, with a range of surface areas, pore structures, and surface chemistries representative of the most promising adsorbents for methane storage, are evaluated in detail. High-pressure methane adsorption isotherms are used to compare gravimetric and volumetric capacities, isosteric heats of adsorption, and usable storage capacities. Additionally, the relative importance of increasing volumetric capacity, rather than gravimetric capacity, for extending the driving range of natural gas vehicles is highlighted. Other important systems-level factors, such as thermal management, mechanical properties, and the effects of impurities, are also considered, and potential materials synthesis contributions to improving performance in a complete adsorbed natural gas system are discussed.
Highlights
Metal–organic frameworks have received significant attention as a new class of adsorbents for natural gas storage; inconsistencies in reporting high-pressure adsorption data and a lack of comparative studies have made it challenging to evaluate both new and existing materials
Even with compression to 250 bar, the energy density of compressed natural gas (CNG) is only 26% that of gasoline,2a leading to a signi cant reduction in the driving range of a vehicle
It is possible to improve the thermal conductivity by incorporating an additive such as graphite, but this will lead to a decrease in both the gravimetric and volumetric CH4 capacities.72b To better understand these tradeoffs, there is a clear need for thermal conductivity and heat capacity measurements on a much wider range of metal–organic frameworks, especially with experiments designed to identify structural and chemical features that are likely to lead to frameworks with higher intrinsic thermal conductivities and heat capacities
Summary
Metal–organic frameworks have received significant attention as a new class of adsorbents for natural gas storage; inconsistencies in reporting high-pressure adsorption data and a lack of comparative studies have made it challenging to evaluate both new and existing materials. To illustrate the most important properties for evaluating adsorbents for natural gas storage and for designing a generation of improved materials, six metal–organic frameworks and an activated carbon, with a range of surface areas, pore structures, and surface chemistries representative of the most promising adsorbents for methane storage, are evaluated in detail. Other important systems-level factors, such as thermal management, mechanical properties, and the effects of impurities, are considered, and potential materials synthesis contributions to improving performance in a complete adsorbed natural gas system are discussed
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