Low temperature mechanism of adsorption of methane: Comparison between homogenous and heterogeneous pores

• Mechanism of layer-by-layer adsorption of methane in carbon slit pores has been compared with phase-transition-like adsorption in IRMOFs.
• The mechanism of adsorption has been related to the heterogeneity of sorbents walls.
• The mechanism of adsorption in IRMOF-1 and IRMOF-16 show non-continuous reconstruction of the methane density as a function of pressure.
• The variation of the methane density includes changes of the symmetry.
• Methane adsorbed in IRMOF-1 show typical characteristics of structural phase transformation.

The mechanisms of methane adsorption in (i) homogeneous carbon slit pores of widths between 1 nm and 2 nm and (ii) heterogeneous MOF pores of similar unit cell sizes have been compared. We discuss the mechanism of layering transition in subcritical conditions, for temperatures between 80 K and 180 K. The layer formation is strongly temperature-dependent. In slit pores it varies from a sharp adsorption at low temperatures to a more continuous uptake at higher temperatures. The pore size defines the number of adsorbed layers: the 1 nm pore allows adsorption of 2 layers while the 2 nm pore allows adsorption of 5 layers of methane molecules. We compare this behavior with the mechanism of adsorption in two MOFs, IRMOF-1 and IRMOF-16, with strongly heterogeneous walls (both structurally and energetically). This comparison allows us to discuss separately the influence of wall topology and intermolecular interactions on the mechanism of layering.

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