Controlling the strength of interaction between carbon dioxide and nitrogen-rich carbon materials by molecular design

Thermal treatment of hexaazatriphenylene-hexacarbonitrile (HAT-CN) in the temperature range from 500 °C to 700 °C leads to precise control over the degree of condensation, and thus atomic construction and porosity of the resulting C 2 N-type materials. Depending on the condensation temperature of HAT-CN, nitrogen contents of more than 30 at% can be reached. In general, these carbons show adsorption properties which are comparable to those known for zeolites but their pore size can be adjusted over a wider range. At condensation temperatures of 525 °C and below, the uptake of nitrogen gas remains negligible due to size exclusion, but the internal pores are large and polarizing enough that CO 2 can still adsorb on part of the internal surface. This leads to surprisingly high CO 2 adsorption capacities and isosteric heat of adsorption of up to 52 kJ mol −1 . Theoretical calculations show that this high binding enthalpy arises from collective stabilization effects from the nitrogen atoms in the C 2 N layers surrounding the carbon atom in the CO 2 molecule and from the electron acceptor properties of the carbon atoms from C 2 N which are in close proximity to the oxygen atoms in CO 2 . A true CO 2 molecular sieving effect is achieved for the first time in such a metal-free organic material with zeolite-like properties, showing an IAST CO 2 /N 2 selectivity of up to 121 at 298 K and a N 2 /CO 2 ratio of 90/10 without notable changes in the CO 2 adsorption properities over 80 cycles. Size selective adsorption of carbon dioxide over nitrogen is maximized in a nitrogen-rich carbon material prepared from a preorganized molecular precursor.