UC Berkeley

Metal-organic frameworks (MOFs) are known to facilitate energy-efficient separations of important industrial chemical feedstocks. Here, we report how a class of green MOFs-namely CD-MOFs-exhibits high shape selectivity toward aromatic hydrocarbons. CD-MOFs, which consist of an extended porous network of γ-cyclodextrins (γ-CDs) and alkali metal cations, can separate a wide range of benzenoid compounds as a result of their relative orientation and packing within the transverse channels formed from linking (γ-CD)6 body-centered cuboids in three dimensions. Adsorption isotherms and liquid-phase chromatographic measurements indicate a retention order of ortho- > meta- > para-xylene. The persistence of this regioselectivity is also observed during the liquid-phase chromatography of the ethyltoluene and cymene regioisomers. In addition, molecular shape-sorting within CD-MOFs facilitates the separation of the industrially relevant BTEX (benzene, toluene, ethylbenzene, and xylene isomers) mixture. The high resolution and large separation factors exhibited by CD-MOFs for benzene and these alkylaromatics provide an efficient, reliable, and green alternative to current isolation protocols. Furthermore, the isolation of the regioisomers of (i) ethyltoluene and (ii) cymene, together with the purification of (iii) cumene from its major impurities (benzene, n-propylbenzene, and diisopropylbenzene) highlight the specificity of the shape selectivity exhibited by CD-MOFs. Grand canonical Monte Carlo simulations and single component static vapor adsorption isotherms and kinetics reveal the origin of the shape selectivity and provide insight into the capability of CD-MOFs to serve as versatile separation platforms derived from renewable sources.