Thermodynamic\nSeparation of 1‑Butene from 2‑Butene\nin Metal–Organic Frameworks with Open Metal Sites

Abstract

Most C₄ hydrocarbons are obtained as byproducts of ethylene\nproduction or oil refining, and complex and energy-intensive separation\nschemes are required for their isolation. Substantial industrial and\nacademic effort has been expended to develop more cost-effective adsorbent-\nor membrane-based approaches to purify commodity chemicals such as\n1,3-butadiene, isobutene, and 1-butene, but the very similar physical\nproperties of these C₄ hydrocarbons make this a challenging\ntask. Here, we examine the adsorption behavior of 1-butene, <i>cis</i>-2-butene, and <i>trans</i>-2-butene in the\nmetal–organic frameworks M₂(dobdc) (M = Mn, Fe,\nCo, Ni; dobdc^4– = 2,5-dioxidobenzene-1,4-dicarboxylate)\nand M₂(<i>m</i>-dobdc) (<i>m</i>-dobdc^4– = 4,6-dioxidobenzene-1,3-dicarboxylate), which all\ncontain a high density of coordinatively unsaturated M²⁺ sites. We find that both Co₂(<i>m</i>-dobdc)\nand Ni₂(<i>m</i>-dobdc) are able to separate\n1-butene from the 2-butene isomers, a critical industrial process\nthat relies largely on energetically demanding cryogenic distillation.\nThe origin of 1-butene selectivity is traced to the high charge density\nretained by the M²⁺ metal centers exposed within the M₂(<i>m</i>-dobdc) structures, which results in a\nreversal of the <i>cis</i>-2-butene selectivity typically\nobserved at framework open metal sites. Selectivity for 1-butene adsorption\nunder multicomponent conditions is demonstrated for Ni₂(<i>m</i>-dobdc) in both the gaseous and the liquid phases\nvia breakthrough and batch adsorption experiments.