(629h) Computer-Assisted Screening of Ordered Crystalline Nanoporous Adsorbents for Separation of Alkane Isomers

The separation of linear, mono-branched, and di-branched isomers of alkanes is of significant importance in the petrochemical industry, in particular for producing high-octane gasoline. Using computational techniques to screen a large variety of adsorbent materials, ZIF-77 emerges as the best adsorbent. It has the unique ability to fractionate isomer mixtures according to the degree of branching. 

We have rationalized the adsorption behavior of hexane (and heptane) isomers and our screening study provides a thorough understanding. First, it is an illadvised endeavor to use large-pore MOFs for alkane separations. Above a certain threshold (depending on the alkane chain-length) the excess pore volume just goes to waste. Below the threshold for selectivity, both branching hierarchies are possible, but the one corresponding to the largest pore volume (the normal hierarchy) is probably most efficient, also because adsorption and diffusion selectivity go hand-in-hand. To properly evaluate a structure at least a 5- component mixture is needed, with both the 2,2-di-branched and the 2,3-di-branched included, as these have usually very different behaviors. The simulated breakthrough curves provide the best way of assessing the efficiency of a structure. The efficiency depends on a) the selectivity, and b) the accessible pore volume. CoBDP provides a very good compromise between the two, but the selectivity of ZIF-77 is so high that this sieve fractionates to yield individual pure components. The screening strategy we present⁽¹⁾ can be applied to other problems, such as, for example, separating alkane mixtures with C10–C18 hydrocarbons and xylene isomers.

(1) Dubbeldam, D., Krishna, R., Calero, S., Yazaydin, A.O., "Computer-Assisted Screening of Ordered Crystalline Nanoporous
Adsorbents for Separation of Alkane Isomers", Angewandte Chemie International Edition, 2012, 51, 11867-11871.