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

Abstract

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 C₂N-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 CO₂ can still adsorb on part of the internal surface. This leads to surprisingly high CO₂ 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 C₂N layers surrounding the carbon atom in the CO₂ molecule and from the electron acceptor properties of the carbon atoms from C₂N which are in close proximity to the oxygen atoms in CO₂. A true CO₂ molecular sieving effect is achieved for the first time in such a metal-free organic material with zeolite-like properties, showing an IAST CO₂/N₂ selectivity of up to 121 at 298 K and a N₂/CO₂ ratio of 90/10 without notable changes in the CO₂ adsorption properities over 80 cycles.