Computational Screening of Nanoporous Materials for Hexane and Heptane Isomer Separation

Computational
high-throughput screening was carried out to assess
a large number of experimentally reported metal–organic frameworks
(MOFs) and zeolites for their utility in hexane isomer separation.
Through the work, we identified many MOFs and zeolites with high selectivity
(SL+M > 10) for the group of n-hexane, 2-methylpentane, and 3-methylpentane (linear and
monobranched
isomers) versus 2,2-dimethylbutane and 2,3-dimethylbutane (dibranched
isomers). This group of selective sorbents includes VICDOC (Fe2(BDP)3), a MOF with triangular pores that is known
to exhibit high isomer selectivity and capacity. For three of these
structures, the adsorption isotherms for a 10-component mixture of
hexane and heptane isomers were calculated. Subsequent simulations
of column breakthrough curves showed that the DEYVUA MOF exhibits
a longer process cycle time than VICDOC MOF or MRE zeolite, which
are previously reported, high-performing materials, illustrating the
importance of capacity in designing MOFs for practical applications.
Among the identified candidates, we synthesized and characterized
a MOF in a new copper form with high predicted adsorbent capacity
(qL+M > 1.2 mol/L) and moderately high
selectivity (SL+M ≈ 10). Finally,
we examined the role of pore shape in hexane isomer separations, especially
of triangular-shaped pores. We show through the potential energy surface
and three-dimensional siting analyses that linear alkanes do not populate
the corners of narrow triangular channels and that structures with
nontriangular pores can efficiently separate hexane isomers. Detailed
thermodynamic analysis illustrates how differences in the free energy
of adsorption contribute to shape-selective separation in nanoporous
materials.