(674d) Highly Selective, High-Capacity Metal–Organic Frameworks for Olefin Production

Metal­–organic frameworks have enormous potential to enable new technologies in energy and petrochemical production. For example, hydrocarbon separations are traditionally conducted using decades-old distillation techniques that require high thermal and capital costs. The use of adsorption technology can significantly improve process efficiency, but has not been widely adopted due to the low selectivities and capacities of adsorbent materials. We have demonstrated that the metal–organic frameworks M₂(m-dobdc) (M = Mn, Fe, Co, Ni; m-dobdc^4– = 4,6-dioxido-1,3-benzenedicarboxylate) have an exceptional ability to separate olefins from gas mixtures, potentially unlocking an alternative to the costly conventional processes. By tuning the metal-site and thus the electronic structure, the metal–adsorbate interactions can be tailored. Specifically, Fe₂(m-dobdc) is the ideal material for adsorptive olefin/paraffin separations, given its record physisorptive selectivity of 55 for propylene/propane and 25 for ethylene/ethane. Further, Fe₂(m-dobdc) displays high olefin capacity (> 7 mmol/g), facile regeneration, and a low-cost production route.

Beyond olefin/paraffin separations, we have explored the use of M₂(m-dobdc) in a novel separation for an alternative ethylene production process. While ethylene is currently derived primarily from naphtha and ethane cracking, it can also be generated from the oxidative coupling of methane. As methane is a cheap and potentially renewable feedstock, this process has enormous commercial potential. However, costly separations in this process restrict feasibility for widespread deployment. We have shown that Mn₂(m-dobdc) has the unique ability to separate ethylene from a complex effluent mixture, including ethane, CO, CO₂, CH₄, and H₂, owing to its unique balance of electropositivity and Ï€-backbonding character. This novel separation has the potential to greatly reduce the barriers to deployment of the oxidative-coupling of methane process, potentially disrupting the conventional ethylene industry.