Structural susceptibility of gas inclusion crystal to external gas pressure and temperature: force guide role of channel

Abstract

In order to investigate the methane-included condition in porous materials and its response to external stimuli, the methane adsorption state of a single-crystal adsorbent, [Cu(II)₂(bza)₄(pyz)]_n, was studied by single-crystal X-ray analysis and adsorption measurement as functions of temperature (90–298 K) and pressure (3.2–42 MPa). The included methane molecule was encapsulated into the channel of the adsorbent and stabilized through the interactions between the surrounding aromatic rings. The single-crystal host readily adsorbed methane gas and easily achieved the saturated condition with the included amount of 2 methane molecules per Cu₂ unit. The results from the gas adsorption measurements were consistent with the results from the crystallographic structures. Single-crystal X-ray analysis showed that the methane-saturated crystal has a critical temperature of the crystal phase transition from the C2/c to the P space group between 150 and 200 K. In temperature swinging, the thermal factors of the atoms of the guest methane and host skeleton monotonically decreased as the temperature decreased. In contrast, in pressure swinging at 298 K, the thermal factors gradually decreased as the pressure increased, after passing 11 MPa only the thermal factor of the guest methane decreased in response to an increase in gas pressure while those of the host skeleton remained almost constant. It is suggested that the channel acts as “force guide,” propagating the outside gas pressure into the crystal inside through a guest–guest interaction in the included gas array. In addition, the difference in the pressure dependencies of the thermal factors on the host and guest suggested that the structural susceptibility to external gas pressure and temperature is different between open porous solids and non-porous solids.