An Iron Separator: Petroleum processing initially yields a mixture of saturated and unsaturated hydrocarbons—the feedstocks for fuels, plastics, pharmaceuticals, and a wide range of other commercial products. At present, distillation is the primary means of separating the components of this mixture. A sorbent or membrane-based approach to separation could reap substantial energy savings. Bloch et al. (p. 1606 ) found that an iron-based metal organic framework material shows promise for very efficient sorption-based separation of ethane and ethylene, propane and propylene, and several other light hydrocarbon mixtures. Neutron diffraction directly revealed the binding motifs at the iron centers that selectively pinned down the olefins while the saturated hydrocarbons passed by.

Separating one organic from another: Separating closely related organic molecules is a challenge (see the Perspective by Lin).The separation of acetylene from ethylene is needed in high-purity polymer production. Cui et al. developed a copper-based metal-organic framework with hexafluorosilicate and organic linkers designed to have a high affinity for acetylene. These materials, which capture four acetylene molecules in each pore, successfully separated acetylene from mixtures with ethylene. Propane and propylene are both important feedstock chemicals. Their physical and chemical similarity, however, requires energy-intense processes to separate them. Cadiau et al. designed a fluorinated porous metal-organic framework material that selectively adsorbed propylene, with the complete exclusion of propane. Science , this issue pp. 141 and 137 ; see also p. 121

A preference for ethane: Industrial production of ethylene requires its separation from ethane in a cryogenic process that consumes large amounts of energy. An alternative would be differential sorption in microporous materials. Most of these materials bind ethylene more strongly that ethane, but adsorption of ethane would be more efficient. Li et al. found that a metal-organic framework containing iron-peroxo sites bound ethane more strongly than ethylene and could be used to separate the gases at ambient conditions. Science , this issue p. 443

Two representative metal–organic frameworks, Zn₄O(BTB)₂ (BTB^{3 −} = 1,3,5-benzenetribenzoate; MOF-177) and Mg₂(dobdc) (dobdc⁴⁻ = 1,4-dioxido-2,5-benzenedicarboxylate; Mg-MOF-74, CPO-27-Mg), are evaluated in detail for their potential use in post-combustion CO₂ capture via temperature swing adsorption (TSA). Low-pressure single-component CO₂ and N₂ adsorption isotherms were measured every 10 °C from 20 to 200 °C, allowing the performance of each material to be analyzed precisely. In order to gain a more complete understanding of the separation phenomena and the thermodynamics of CO₂ adsorption, the isotherms were analyzed using a variety of methods. With regard to the isosteric heat of CO₂ adsorption, Mg₂(dobdc) exhibits an abrupt drop at loadings approaching the saturation of the Mg²⁺ sites, which has significant implications for regeneration in different industrial applications. The CO₂/N₂ selectivities were calculated using ideal adsorbed solution theory (IAST) for MOF-177, Mg₂(dobdc), and zeolite NaX, and working capacities were estimated using a simplified TSA model. Significantly, MOF-177 fails to exhibit a positive working capacity even at regeneration temperatures as high as 200 °C, while Mg₂(dobdc) reaches a working capacity of 17.6 wt % at this temperature. Breakthrough simulations were also performed for the three materials, demonstrating the superior performance of Mg₂(dobdc) over MOF-177 and zeolite NaX. These results show that the presence of strong CO₂ adsorption sites is essential for a metal–organic framework to be of utility in post-combustion CO₂ capture via a TSA process, and present a methodology for the evaluation of new metal–organic frameworks via analysis of single-component gas adsorption isotherms.

Carbon dioxide capture and separation are important industrial processes that allow the use of carbon dioxide for the production of a range of chemical products and materials, and to minimize the effects of carbon dioxide emission. Porous metal-organic frameworks are promising materials to achieve such separations and to replace current technologies, which use aqueous solvents to chemically absorb carbon dioxide. Here we show that a metal-organic frameworks (UTSA-16) displays high uptake (160 cm³ cm⁻³) of CO₂ at ambient conditions, making it a potentially useful adsorbent material for post-combustion carbon dioxide capture and biogas stream purification. This has been further confirmed by simulated breakthrough experiments. The high storage capacities and selectivities of UTSA-16 for carbon dioxide capture are attributed to the optimal pore cages and the strong binding sites to carbon dioxide, which have been demonstrated by neutron diffraction studies.