(395h) Propane/Propylene Separation in Ion-Exchanged ZMOFs

Separation of propane/propylene mixtures formed during catalytic or thermal cracking of hydrocarbons is an important process in petrochemical industry. Conventional separation processes use widely cryogenic distillation, which requires high energy-consumption due to close relative volatilities of propane and propylene. Separation of propane/propylene mixtures via adsorption, such as PSA or TSA, is a less-energy demanding and environmentally friendly alternative. Conventional solid adsorbents such as zeolites are widely used in adsorption processes, but they suffer low capacity and selectivity in the case of propane/propylene separation. Metal organic frameworks (MOFs), on the other hand, can be good candidate as solid adsorbents. Zeolite-like MOFs (ZMOFs), a subclass of MOFs, have anionic structure and charge compensating extraframework cations. ZMOFs of rhombic and sodalite topologies, rho- and sod-ZMOFs, respectively, may exhibit customized gas separation performances due to their ion exchanging capabilities. [1]  

In this study, propane/propylene separation performances of rho- and sod-ZMOFs were investigated at different temperatures and pressures via Grand Canonical Monte Carlo simulation method. Since the extraframework cation type is an important parameter in gas separation properties of ZMOFs, separation performances of Li⁺, Na⁺ K⁺, Ca²⁺ and Mg²⁺-exchanged structures of  ZMOFs at different ratios were also evaluated by estimating adsorption selectivities and working capacities at different pressures.

Results showed that ZMOFs have strong potential for propane/propylene separation and their separation properties can be tuned through ion exchange. The extraframework cation type and ion-exchange ratio are other important parameters determining the adsorption capacity and selectivity of the materials for propane/propylene separation. Both the adsorption selectivity and the working capacity may be increased substantially by increasing the ion-exchange ratio. Furthermore, comparison with the separation performance of zeolites, such as LTA and 13X, suggests that the adsorption selectivities of ZMOFs may be as higher as an order of magnitude than those of the zeolites.

[1] Liu, Y.; Kravtsov, V. C.; Larsen, R.; Eddaoudi, Chem. Commun.2006, 1488-1490