Computational Identification and Experimental Evaluation of Metal–Organic Frameworks for Xylene Enrichment

Abstract

We report the computational discovery and experimental evaluation of nanoporous materials targeted at the adsorptive separation of p-xylene from a C 8 aromatics mixture. We first introduce a computational method that is capable of efficiently predicting the p-xylene selectivities and capacities for a large database of porous materials. We then demonstrate the application of this method to screen a database of several thousand metal−organic framework (MOF) structures. Our computational screening methodology predicted that two MOFs with good solvothermal stability and commercially available linkers give comparable performance to the state-of-the-art zeolite BaX currently used in industrial p-xylene separations. The bestperforming MOFs are then synthesized, and their xylene separation characteristics are evaluated in detail through breakthrough adsorption experiments and modeling. We find that the selectivities obtained in these materials are higher than that of any MOF previously reported in the literature and in some cases exceed the measured performance of zeolite BaX. In the case of the pxylene selective material MOF-48, we use calculated free energy profiles to show how the presence of methyl substituents on the linkers allows the inversion of selectivity from the equivalent MOF with no methyl substituents (MIL-47, which is o-xylene selective). This combined computational and experimental methodology is a useful step in the development of MOFs for separation of aromatic hydrocarbons and can also be applied to other chemical separations and other classes of porous materials as long as the appropriate intermolecular force fields are available.

Efficiently Exploring Adsorption Space to Identify Privileged Adsorbents for Chemical Separations of a Diverse Set of Molecules

“…Previous efforts to identify promising MOFs for chemical separations by using large libraries of materials have focused almost exclusively on specific pairs of molecules of practical interest, such as CO 2 /N 2 , xylene mixtures, olefin/paraffin mixtures, Xe/Kr, and hexane/heptane . A question that cannot be answered by studies of this kind is whether MOFs (or other nanoporous materials) exist that are “privileged” for molecular separations.…”

Efficient separation of xylene isomers by a guest-responsive metal–organic framework with rotational anionic sites

“…Porous materials with designable pore topology and selective binding sites open up unexpected possibilities for recognition of xylene isomers [13][14][15][16][17][18][19][20] . Different kinds of metal-organic frameworks (MOFs) or porous coordination polymers, including MIL (Materials of Institut Lavoisier) series and MOFs with coordinatively unsaturated metal sites (CUSs), have been investigated for the separation of xylene isomers, both in vapor and liquid phases [21][22][23][24][25][26][27][28][29][30][31] . However, MIL-47 and MIL-125-NH 2 can hardly discriminate pX and mX, and the pX/mX selectivity of MIL-47 is 1.1 (calculated from breakthrough data) 23 .…”

The Role of Molecular Modeling & Simulation in the Discovery and Deployment of Metal-Organic Frameworks for Gas Storage and Separation

“…12 However, MOFs often exhibit complicated pore geometries, which, from the perspective of a configurational integral in a partition function, 65 dictate the ensemble of configurations of the adsorbate(s) within the pores and the energetics of its interactions with the pore walls; subtle differences in pore geometry can have large effects on adsorption when the pore size is commensurate with the molecule. 281 Also, interactions with e.g., coordinatively unsaturated metal sites in MOFs are difficult to predict. 326 Moreover, some MOFs harbor flexible backbones or rotating/wobbling constituents 189 and undergo structural changes upon adsorption of gas, 327 sometimes involving delicate competition between entropy and energy.…”