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 (CO2) than for other gases; in addition to its involvement in climate change, CO2 is an impurity in natural gas, biogas (natural gas produced from biomass), syngas (CO/H2, 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 CO2 separation and capture, as exemplified by zeolites 5A and 13X (ref. 2), as well as metal-organic materials (MOMs). Here we report that a crystal engineering or reticular chemistry strategy that controls pore functionality and size in a series of MOMs with coordinately saturated metal centres and periodically arrayed hexafluorosilicate (SiF(2-)(6)) anions enables a 'sweet spot' of kinetics and thermodynamics that offers high volumetric uptake at low CO2 partial pressure (less than 0.15 bar). Most importantly, such MOMs offer an unprecedented CO2 sorption selectivity over N2, H2 and CH4, even in the presence of moisture. These MOMs are therefore relevant to CO2 separation in the context of post-combustion (flue gas, CO2/N2), pre-combustion (shifted synthesis gas stream, CO2/H2) and natural gas upgrading (natural gas clean-up, CO2/CH4).

A previously known class of porous coordination polymer (PCP) of formula [Cu(bpy-n)(2)(SiF(6))] (bpy-1 = 4,4'-bipyridine; bpy-2 = 1,2-bis(4-pyridyl)ethene) has been studied to assess its selectivity toward CO(2), CH(4), N(2), and H(2)O. Gas sorption measurements reveal that [Cu(bpy-1)(2)(SiF(6))] exhibits the highest uptake for CO(2) yet seen at 298 K and 1 atm by a PCP that does not contain open metal sites. Significantly, [Cu(bpy-1)(2)(SiF(6))] does not exhibit particularly high uptake under the same conditions for CH(4), N(2), and, H(2)O, presumably because of its lack of open metal sites. Consequently, at 298 K and 1 atm [Cu(bpy-1)(2)(SiF(6))] exhibits a relative uptake of CO(2) over CH(4) of ca. 10.5:1, the highest value experimentally observed in a compound without open metal sites. [Cu(bpy-2)(2)(SiF(6))] exhibits larger pores and surface area than [Cu(bpy-1)(2)(SiF(6))] but retains a high CO(2)/CH(4) relative uptake of ca. 8:1.

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The potential energy surface for [Zn(pyz) 2 SiF 6 ] consists of van der Waals repulsion/dispersion (modeled using the Lennard-Jones 12-6 potential), atomic partial point charges, and an explicit polarizability model. The complete list of force field parameters is given in Table 3. The Lennard-Jones parameters for the C, H, and N atoms were taken from the Optimized Potentials for Liquid Simulations -All Atom (OPLS-AA) force field, 1 while those for the Zn, Si, and F atoms were taken from the Universal Force Field (UFF). 2 The remaining parameterization is described in the following subsections.

Square grid coordination polymers (CPs) based upon four-connected metal centres linked by linear bifunctional ligands such as 4,4'-bipyridine were first reported in 1990 and the study of their pillared variants began in 1995. It was quickly realized by crystal engineers that the modularity of such CPs creates families of related compounds or platforms which in turn affords opportunities for systematic study of structure/function relationships in the context of catalysis, magnetism and porosity. This review covers the historical development of this important class of CPs before addressing recent studies of variants which incorporate 4,4'-bipyridine and related linkers to facilitate control over pore size and inorganic anion pillars to enable strong interactions with polarizable molecules such as CO2. Such pillared CPs offer relatively low cost, high stability and modularity. When these features are coupled with superior performance vs . other classes of porous materials in the context of carbon capture and other gas separations involving CO2, they are likely to gain increased attention in the future.