Shape-sorting of aromatic molecules by porous organic cages
File(s)
Author(s)
Jackson, Edward
Type
Thesis
Abstract
Porous organic cages are organic molecules containing discrete internal cavities, which can self-assemble to form extended pore networks in the solid-state. Numerous uses have been proposed for these molecules, including in the separation of gases and chiral aromatic molecules. Amorphous membranes have also been formed from porous organic cages, a process which takes advantage of the cages' solution processability. In this project, selected porous organic cages are studied to see whether they can be used for the shape-selective separation of para-xylene from its structural isomers meta-xylene, ortho-xylene and ethylbenzene. Many widely used polymers have para-xylene as a precursor, so its separation is a crucial industrial process, for which membrane separations are an active area of research.
Two porous organic cages which have been successfully used to formulate amorphous membranes, CC3 and CC13, were initially studied. Molecular dynamics simulations were used to gain a qualitative idea of the C8 aromatics' diffusion rates, and metadynamics in single cages was used to quantify the most significant energetic barriers to this diffusion. The metadynamics calculations indicated that both cages would be able to separate para-xylene from meta- and ortho-xylene and CC3 may also be able to separate para-xylene and ethylbenzene with lower selectivity. This difference in selectivity was rationalised based on the greater flexibility of the window of CC13.
A model separation process using a CC3 membrane was designed and analysed, in order to estimate the purity of the para-xylene produced if the membrane performs as predicted. The model separation process indicated that, while the membrane would not be able to achieve sufficient purity of para-xylene by itself, combining the membrane with the currently used separation processes could result in significant capital and utility savings. Additionally, the cost of the CC3 raw material is predicted to be a negligible cost in the production of such a membrane, although the membrane's mechanical stability and lifetime are still not known.
Finally, an evolutionary algorithm was used to search for other porous organic cages that could carry out the separation, based on the factors that gave CC3 a higher predicted performance than CC13. The top candidates produced by the evolutionary algorithm included CC3, which indicated that the setup was valid, and synthesis of the top two cages has already been reported in the literature. These results provide a starting point for future research in this area.
Two porous organic cages which have been successfully used to formulate amorphous membranes, CC3 and CC13, were initially studied. Molecular dynamics simulations were used to gain a qualitative idea of the C8 aromatics' diffusion rates, and metadynamics in single cages was used to quantify the most significant energetic barriers to this diffusion. The metadynamics calculations indicated that both cages would be able to separate para-xylene from meta- and ortho-xylene and CC3 may also be able to separate para-xylene and ethylbenzene with lower selectivity. This difference in selectivity was rationalised based on the greater flexibility of the window of CC13.
A model separation process using a CC3 membrane was designed and analysed, in order to estimate the purity of the para-xylene produced if the membrane performs as predicted. The model separation process indicated that, while the membrane would not be able to achieve sufficient purity of para-xylene by itself, combining the membrane with the currently used separation processes could result in significant capital and utility savings. Additionally, the cost of the CC3 raw material is predicted to be a negligible cost in the production of such a membrane, although the membrane's mechanical stability and lifetime are still not known.
Finally, an evolutionary algorithm was used to search for other porous organic cages that could carry out the separation, based on the factors that gave CC3 a higher predicted performance than CC13. The top candidates produced by the evolutionary algorithm included CC3, which indicated that the setup was valid, and synthesis of the top two cages has already been reported in the literature. These results provide a starting point for future research in this area.
Version
Open Access
Date Issued
2019-09
Date Awarded
2020-03
Copyright Statement
Creative Commons Attribution NonCommercial NoDerivatives Licence
Advisor
Jelfs, Kim
Song, Qilei
Sponsor
Engineering and Physical Sciences Research Council (EPSRC)
British Petroleum Company
Publisher Department
Chemistry
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)