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binding strength varies, the isosteric heat of CO2 adsorption revealed that the higher content of nitrogen in BILP-19 did not
(Qst) of the two materials were analysed. BILP-19, as predicted, provide a critically stronger bindingDOtoI: 10C.1O0239/cDa0rCbCo0n0s982aBs
showed higher values than BILP-5. However, in general, they demonstrated by almost equal Qst values from the two
were not dramatically different (Fig. 3d) or it could be stated polymers and this was due to the lower nucleophilic nature of
that the distinction was negligible. This indicates that the the conjugated nitrogens in BILP-19. The higher CO2/N2
addition of a nitrogen atom in BILP-19 did not increase separation potential of BILP-19 was indeed the combination of
significantly the binding strength but rather the captured the higher retention of N2 in BILP-5 network through stronger
amount of CO2. It is hypothetically the conjugation of the lone π-π interaction and the minimal enhance in CO2 capture of BILP-
pair to the benzene rings that restrains the nucleophilicity of the 19 owing to its smaller pore size. Above all, the uniform
tertiary nitrogen and hence diminishes its chemisorptive morphology and porosity of the studied materials have enabled
interaction with the CO2 carbon. At this point, the greater CO2 these informative conclusions to be drawn.
seizing ability of BILP-19 could be traced back to its smaller pore
size that allowed a more efficient trapping effect and slightly
more favourable binding to CO2. Such significance of pore size
Conflicts of interest
in gas confinement, in particular CO2, can also be found in the
literature. An example is the case of chemically stable β-
ketoenamine-linked covalent organic frameworks. TPPa-117,
the first of the class, with a BET surface area of 535 m2/g and
pore size ranging from 0.8 to 1.5 ppm could capture CO2 as
much as 3.5 mmol/g (273K, 1bar) while its analogous structure,
azoTP18,19 , due to the longer azo-linkers possessed a larger pore
size of 3.5 nm and hence exhibited a lower CO2 uptake capacity
at 2.4 mmol/g despite the much higher surface area (1,552
m2/g).
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP)
(No. NRF-2017M3A7B4042140, and NRF-
2017M3A7B4042235).
Notes and references
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Fig. 3 Gas uptake properties of BILP-5 and BILP-19. (a) CO2 adsorption-desorption
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In summary, as a rare occasion, two isoporous, structurally
similar bisimidazole polymers were serendipitously obtained.
The slight difference in the core structure appeared to affect the
CO2 as well as N2 capture ability. BILP-19, built on a nitrogen-
centred trialdehyde, exhibited higher CO2 uptake capacities but
lower nitrogen capture capabilities than BILP-5, which was
constructed on a benzene core. This, in turn, led to greater
CO2/N2 selectivities from BILP-19. Qst values determination
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