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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!