Abstract

Cost-effective separation of xylene isomers is challenging owing to the similarity in their molecular structures, kinetic diameters and boiling points. Recently, the MIL-53 class of metal-organic frameworks (MOFs) has generated interest as potential adsorbents for xylene mixture separations. Here we report a systematic experimental and computational study of xylene isomer adsorption in MIL-53 adsorbents, focusing particularly on the effects of different metal centers, determination of separation properties with industrially relevant quaternary liquid-phase C8 aromatic feeds, and a predictive molecular simulation methodology that accounts for all relevant modes of MIL-53 framework flexibility. Significant scale-up of MIL-53 synthesis was carried out to produce high-quality materials in sufficient quantities (300-500 g each) for detailed measurements. Single-component adsorption simulations incorporating the MIL-53 ‘breathing’ and linker flexibility effects showed good agreement with experimental isotherms. Based upon these results, three materials – MIL-53(Al), MIL-53(Cr) and MIL-53(Ga) – were selected for detailed quaternary liquid breakthrough measurements. High o-xylene quaternary selectivity was obtained from all the MIL-53 materials, with MIL-53(Al) being the most selective. Better packing efficiency of o-xylene and its preferred interactions with the MIL-53 framework are hypothesized to lead to high selectivity. Predictions from flexible-structure multicomponent adsorption simulations showed good overall agreement with experiment. This is, to the best of our knowledge, the first experimental report on the xylene separation performance of MIL-53 adsorbents under industrially relevant operating conditions. In addition, it is also the first attempt to develop computational methods that account for various flexibility modes in MIL-53 materials for adsorption simulations. This has significant broader applications for the successful prediction of adsorption properties of larger molecules (such as C8 aromatic isomers) in flexible MOFs.

Mayank Agrawal

Postdoctoral Research Associate

My research include atomistic simulation of nano-porous materials for separation and catalysis.