Porous materials with optimal adsorption thermodynamics and kinetics for CO2 separation

A series of porous crystalline materials known as metal–organic materials are prepared, and a full sorption study shows that controlled pore size (rather than large surface area) coupled with appropriate chemistry lead to materials exhibiting fast and highly selective CO 2 sorption. Pore performance good for energy storage Metal organic frameworks are porous crystalline materials widely studied as potential gas separation and storage materials for clean energy applications. A general trend in this field has been the development of materials with the largest possible surface area with the aim of maximizing uptake of gases. In this paper the authors generate a series of metal organic frameworks and carry out sorption experiments that suggest that surface area may not be as important as was thought. Rather, pore size, coupled with appropriate chemistry, are the keys to fast CO 2 uptake and strong CO 2 sorption. Materials designed on these principles attain high selectivity for CO 2 over nitrogen, oxygen, methane and hydrogen even in the presence of moisture. The energy costs associated with the separation and purification of industrial commodities, such as gases, fine chemicals and fresh water, currently represent around 15 per cent of global energy production, and the demand for such commodities is projected to triple by 2050 (ref. 1 ). The challenge of developing effective separation and purification technologies that have much smaller energy footprints is greater for carbon dioxide (CO 2 ) than for other gases; in addition to its involvement in climate change, CO 2 is an impurity in natural gas, biogas (natural gas produced from biomass), syngas (CO/H 2 , the main source of hydrogen in refineries) and many other gas streams. In the context of porous crystalline materials that can exploit both equilibrium and kinetic selectivity, size selectivity and targeted molecular recognition are attractive characteristics for CO 2 separation and capture, as exemplified by zeolites 5A and 13X (ref. 2 ), as well as metal–organic materials (MOMs) 3 , 4 , 5 , 6 , 7 , 8 , 9 . Here we report that a crystal engineering 7 or reticular chemistry 5 , 9 strategy that controls pore functionality and size in a series of MOMs with coordinately saturated metal centres and periodically arrayed hexafluorosilicate (SiF 6 2− ) anions enables a ‘sweet spot’ of kinetics and thermodynamics that offers high volumetric uptake at low CO 2 partial pressure (less than 0.15 bar). Most importantl