Thermodynamic Separation of 1-Butene from 2-Butene in Metal–Organic Frameworks with Open Metal Sites

Most C₄ hydrocarbons are obtained as byproducts of ethylene production or oil refining, and complex and energy-intensive separation schemes are required for their isolation. Substantial industrial and academic effort has been expended to develop more cost-effective adsorbent- or membrane-based approaches to purify commodity chemicals such as 1,3-butadiene, isobutene, and 1-butene, but the very similar physical properties of these C₄ hydrocarbons makes this a challenging task. Here, we examine the adsorption behavior of 1-butene, cis-2-butene and trans-2-butene in the metal–organic frameworks M₂(dobdc) (M = Mn, Fe, Co, Ni; dobdc²⁻ = 2,5-dioxidobenzene-1,4-dicarboxylate) and M₂(m-dobdc) (m-dobdc⁴⁻ = 4,6-dioxido-1,3-benzenedicarboxylate), which all contain a high density of coordinatively-unsaturated M²⁺ sites. We find that both Co₂(m-dobdc) and Ni₂(m-dobdc) are able to separate 1-butene from the 2-butene isomers, a critical industrial process that relies largely on energetically demanding cryogenic distillation. The origin of 1-butene selectivity is traced to the high charge density retained by the M²⁺ metal centers exposed within the M₂(m-dobdc) structures, which results in a reversal of the cis-2-butene selectivity typically observed at framework open metal sites. Selectivity for 1-butene adsorption under multicomponent conditions is demonstrated for Ni₂(m-dobdc) in both the gaseous and liquid phases via breakthrough and batch adsorption experiments.