Sometimes you have to cool down to become ‘HOT’

Metal-organic frameworks (MOF) are typically microporous materials (i.e. with pore diameter < 2 nm), and their properties in respect to gas adsorption and separation processes have been much investigated. One of the most interesting MOF materials for a number of sorption processes, e.g. hydrogen purification or storage at low temperature, methane storage at high pressures, or carbon dioxide capture, is the CPO-27 series of materials, in whose development Prof. Pascal Dietzel from our department had a pioneering role. One of his specialties is the use of in situ X-ray diffraction measurements to study the host-guest structures of gases adsorbed in the porous materials. The research group had studied CO2 adsorption on CPO-27 in the past, by cooling samples in 1 bar CO2 from 453 to 195 K. Usually, one wouldn’t cool below the sublimation temperature of CO2 in the experiment because the pores are full with CO2 at that temperature. However, the CO2 adsorbed on the metal cation of the MOF is known to perform a precession movement that affects the crystal structure determination, so Dietzel and co-workers decided to cool the sample further to see whether the adsorbed molecule gets locked in a more defined position at lower temperatures. This wasn’t the case, but instead they observed a phase transition in CO2 loaded CPO-27-Co and Zn below 110 K, in which the inorganic chains and the organic ligands of the framework collectively rearrange into a distorted superstructure of the original honeycomb-resembling structure. The paper is being featured with free access for 6 weeks in the quarterly HOT article collection of CrystEngComm, which represents the top 10% of interesting and significant research for this community. We congratulate!