A Metal-Organic Framework Designed for Separation of Liquid Organic Hydrogen Carriers

As implied by their name, liquid organic hydrogen carriers (LOHCs) have been proposed as liquid-phase hydrogen storage and transportation media, circumventing the cryogenic conditions and metal embrittlement associated with molecular hydrogen while enabling utilization of existing liquid hydrocarbon infrastructure. A common LOHC example is the methylcyclohexane-toluene system (MTS), where toluene is hydrogenated to afford methylcyclohexane for transport, while the reverse reaction recovers toluene and releases hydrogen at the point of use. Round-trip efficiency of the MTS is low because high conversion from methylcyclohexane to toluene consumes significant energy. Lower conversions produce a mixture of both compounds and separation of LOHC pairs is particularly difficult, owing to the similarity of their molecular sizes. Here, we detail the separation of aromatics and naphthenes with a unique metal-organic framework (MOF). By changing the aromatic ring in the Zr-based MOF UiO-66 to an aliphatic, cubane-based moiety, we found that the resulting MOF, Zr(cub), had consistent aromatic selectivity across a broad pressure range while retaining the high stability of Zr-based MOFs, demonstrated by exposure to both water and toluene. The capacity for LOHC mixture separation avoids the energy demands of high-conversion LOHC dehydrogenation, thereby improving the feasibility of hydrogen as an energy vector.