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 make 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; dobdc4– = 2,5-dioxidobenzene-1,4-dicarboxylate)
and M2(m-dobdc) (m-dobdc4– = 4,6-dioxidobenzene-1,3-dicarboxylate), 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 the liquid phases
via breakthrough and batch adsorption experiments.