New concept of N₂-phobicity for post-combustion CO₂ separation

Novel azo-linked covalent organic polymers were synthesized using a catalyst-free synthetic approach. Unlike any other porous materials, azo-polymers showed increasing CO₂/N₂ selectivity with rising temperature, mainly due to the entropic loss of N₂ gas molecules upon their interaction with azo-groups (N₂-phobicity), thereby leading to the highest CO₂/N₂ selectivity reported to date at 50℃.

Article  |  Fall 2014

Although post-combustion CO₂ capture and air separation for N₂ production are integral parts of today’s energy industry, the available technologies remain inefficient, resulting in costly energy penalties. Porous solids offer reversible capture/release and safe operations at high temperatures while retaining their structural integrity, thermal and chemical stability, and gas selectivity. Most existing porous materials, however, have the disadvantage of water instability (e.g., metal-organic frameworks, MOFs), not to mention the inability to tune their structures (e.g., zeolites) or the lack of sustainable synthesis, owing to the use of rare earth catalysts (e.g., conjugated microporous polymers, CMPs).

In this collaborative work, the research groups of Profs. Ali Coskun, Cafer T. Yavuz, and Yousung Jung report azo-bridged, nitrogen-rich, aromatic, nanoporous covalent organic polymers (azo-COPs) that can be synthesized by catalyst-free direct coupling of aromatic nitro and amine moieties under basic conditions. Azo-COPs have been found to be extremely stable at up to 350^oC in air as well as in boiling water for a week. Unlike other porous materials, azo-COPs exhibit an unprecedented increase in CO₂/N₂ selectivity with a rise in temperature, reaching the highest value (288 at 323 K) reported to date. More recently, same research team reported the CO₂/N₂ selectivity of azo-COPs can be further increased up to 308 by incorporating CO₂-philic building blocks into the polymer backbone.

The research groups report that azo groups reject N₂ totally, in thereby making the framework N₂-phobic. A larger entropy loss associated with N₂ adsorption onto the azo groups of the framework as compared to CO₂ explains the high selectivity as well as its temperature dependence.

Any gas separations that require the efficient exclusion of N₂ gas would benefit from the utilization of azo units in the sorbent chemistry. It is not unlikely that these N₂-phobic azo-COPs will provide design principles for the development of next-generation porous polymers for highly selective gas separations, especially in natural gas industry.

This paper (entitled “Unprecedented high-temperature CO₂ selectivity in N₂-phobic nanoporous covalent organic polymers”) was published on January 15 in Nature Communication and was also highlighted in both C&EN News (January 21^st) and Chemistry World (January 17^th).

Reference: Hasmukh Patel, Sang Hyun Je, Joonho Park, Dennis Chen, Yousung Jung, Cafer T. Yavuz &Ali Coskun. Unprecedented High-Temperature CO₂ Selectivity in N₂-phobic Nanoporous Covalent Organic Polymers. Nature Commun. 4, 1357 (2013)

Unprecedented high-temperature CO₂ selectivity in N₂-phobic nanoporous covalent organic polymers
[Hasmukh Patel, Sang Hyun Je, Joonho Park, Dennis Chen, Yousung Jung, Cafer T. Yavuz &Ali Coskun, Nature Commun. 2013].

Additional links for more information:

http://www.nature.com/ncomms/journal/v4/n1/abs/ncomms2359.html

http://alicoskun.kaist.ac.kr

http://yavuz.kaist.ac.kr/p/welcome.html