(134a) Computational Screening of Metal-Organic Frameworks for Hexane Isomer Separation

Authors: 
Chung, Y. G., Northwestern University
Bai, P., University of Minnesota
Haranczyk, M., Lawrence Berkeley National Laboratory
Siepmann, J. I., University of Minnesota
Snurr, R. Q., Northwestern University

Hexane isomers differ greatly in their octane number, and better separation of these isomers could lead to better processes for the production of high-quality gasoline.  Current adsorbents can differentiate linear alkanes from mono- and di-branched alkanes via molecular sieving. However, the desired operation is to efficiently separate the di-branched isomers, which have the highest octane number, from the linear and mono-branched alkanes. Recently, a highly stable metal-organic framework (MOF) with triangular pores was synthesized by Long and co-workers, and it was demonstrated that the separation of hexane isomers could be done efficiently based on their shape1.

It has been hypothesized that the exceptional separation capability of the MOF with triangular pores for hexane isomers arises from the shape and size of the pore, which can differentiate between different isomers. In order to test the generality of this hypothesis and to investigate the molecular-level details, we have created and computationally screened a database of thousands of existing metal-organic frameworks.  The database contains a structurally diverse set of MOFs with various pore sizes, pore shapes, and other structural properties. The screening was carried out using a hierarchical strategy.  First, thousands of MOFs were characterized by a descriptor-based classification of their pore shapes, and the Henry’s constants were calculated for 5 different hexane isomers in all MOF structures.  Next, grand canonical Monte Carlo simulations were performed to calculate the full adsorption isotherms for a subset of structures, and ideal adsorbed solution theory was used to predict the mixture behavior. Finally, full mixture simulations were performed for the most promising structures.  The results were used to find correlations among structural properties, Henry’s constants, and the selectivity and capacity in mixtures of hexane isomers. 

References

1. Herm, Z. R.; Wiers, B. M.; Mason, J. A.; van Baten, J. M.; Hudson, M. R.; Zajdel, P.; Brown, C. M.; Masciocchi, N.; Krishna, R.; Long, J. R., Separation of Hexane Isomers in a Metal-Organic Framework with Triangular Channels. Science 2013, 340 (6135), 960-964