(50j) Impacts of Entropic Separation Effects

Adsorption processes using metal-organic frameworks ­– or nanoporous materials in general – are capable of replacing unsustainable separation processes like distillation or extraction to isolate hydrocarbons. Besides the well-known separation mechanisms based on enthalpic, kinetic or steric (i.e., sieving) differences, there is a further mechanism based on entropic differences. Those entropic effects describe the efficiency with which guest molecules can be arranged within the channels of nanoporous materials. In some cases, they lead to astonishing selectivities [1-3]. Unfortunately, these effects are seldom taken into account and seem to be virtually unknown.

The motivation behind this project is to contribute towards a better understanding of those entropic separation effects and indicating potentially interesting metal-organic frameworks, which are capable to efficiently solve a separation problem by means of entropic effects. To that end, grand-canonical Monte Carlo simulations were conducted to compute the adsorption isotherms of several hydrocarbon mixtures for a wide range of metal-organic frameworks and to visualise the arrangement of the investigated guest molecules within the channels of those materials.

For instance, in the event of separating hexane isomers ­– which is of great importance to the petrochemical industry ­– several metal-organic frameworks have been identified, whose selective adsorption of dibranched isomers is caused by entropic effects. Building upon these results, entropic effects seem to be capable of solving some of the major separation problems in energy and chemical engineering.

[1] Z. R. Herm et al., Science, 340, p. 960 ff. (2013)
[2] A. Torres-Knoop, J. Heinen, R. Krishna, D. Dubbeldam, Langmuir, 31, p. 3771 ff. (2015)
[3] R. Krishna, J. M. van Baten, Sep. Purif. Technol., 55, p. 246 ff. (2007)