Abstract
Currently, metal-organic frameworks (MOFs) are receiving significant attention as part of an international push to use their special properties in an extensive variety of energy applications. In particular, MOFs have exceptional potential for gas storage especially for methane and hydrogen for automobiles. However, using theoretical approaches to investigate this important problem presents various difficulties. Here we present the outcomes of a basic theoretical investigation into methane adsorption in large pore MOFs with the aim of capturing the unique features of this phenomenon. We have developed a pseudo one-dimensional statistical mechanical theory of adsorption of gas in a MOF with both narrow and large pores, which is solved exactly using a transfer matrix technique in the Osmotic Ensemble (OE). The theory effectively describes the distinctive features of adsorption of gas isotherms in MOFs. The characteristic forms of adsorption isotherms in MOFs reflect changes in structure caused by adsorption of gas and compressive stress. Of extraordinary importance for gas storage for energy applications, we find two regimes of Negative gas adsorption (NGA) where gas pressure causes the MOF to transform from the large pore to the narrow pore structure. These transformations can be induced by mechanical compression and conceivably used in an engine to discharge adsorbed gas from the MOF. The elements which govern NGA in MOFs with large pores are identified. Our study may help guide the difficult program of work for computer simulation studies of gas storage in MOFs with large pores.
Highlights
In the few years, a revolution is expected to occur in energy storage that will modify the way energy is used and the impact it has on climate change [1,2]
As we will discuss in detail below, we find two regions of Negative adsorption of gas (NGA) in Metal-organic frameworks (MOFs) with large pores
The compressive stress σ and the chemical potential μa are assumed to be independent variables. We develop this transfer matrix treatment of a pseudo-one-dimensional statistical mechanical Osmotic Ensemble (OE) theory of pressure and adsorption-induced structural transformations in MOFs with large pores and in this way predict that NGA is common in such solids
Summary
In the few years, a revolution is expected to occur in energy storage that will modify the way energy is used and the impact it has on climate change [1,2]. MOFs are hybrid organic-inorganic nanoporous materials with remarkable adsorption properties in which organic units link metal framework centres which afford structural flexibility, thereby allowing transformations to occur between different pore systems. Coudert and co-workers [27] identify two successive transformations, from a large pore (LP) to a narrow pore (NP) state and back again to the LP state which they classify as guest-induced structural transformations termed breathing and opening of gates These depend on applied conditions such as temperature and pressure [23,24,26], as well as adsorption [20,21,22,23,24,25] of gas molecules.
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