An ultramicroporous hydrogen-bonded organic framework for efficient CO2/CH4 and CO2/N2 separation

Efficient capture and separation for carbon dioxide is of great significance for improving energy utilization and addressing environmental concerns such as the upgrading of natural gas, and greenhouse effect. Hydrogen-bonded organic frameworks (HOFs) have great potential for efficient capture and separation of CO2 due to their mild synthesis conditions, ease of regeneration and good solution processability. However, there is a great challenge in fully activation of HOFs for permanent porosity given that hydrogen bonds are relatively weak. In this study, an ultramicroporous hydrogen-bonded organic framework, HOF-GS-10, has been reported to achieve efficient CO2 capture of CH4 and N-2. That HOF is assembled by guanidinium cations and 1,5-naphthalene disulfonate anions through charge-assisted hydrogen bonds, showing two-dimensional double-layered networks with honeycomb-like hydrogen bonding units. The HOF structure contains 1D pore channels with pore aperture size of 2.67 A. By virtue of mild activation involving solvent exchange by acetone and further heating at relatively lower temperature, this HOF can be fully desolvated as verified by its experimental pore volumes. The measured pore volume of micropores in that HOF is 0.16 cm(3) g(-1), which is consistent with the theoretical pore volume of 0.15 cm(3) g(-1) from crystal structures. At 298 K and 100 kPa, the HOF material can adsorb 1.23 mmol g(-1) CO2, which is higher than that for CH4(0.37 mmol g(-1)) and N-2(0.09 mmol g(-1)), resulting in IAST (ideal adsorbed solution theory) selectivity of 19.1 and 5.1 for equimolar mixture of CO2/N-2 and CO2/CH4, respectively. Additionally, the isosteric heat of that HOF material for CO2 adsorption at low coverage is only about 27.1 kJ mol(-1). The modelling study by grand canonical Monte Carlo simulations indicates that CO2 molecules interact with the host framework through weak intermolecular interactions, which are stronger than those for CH4 and N-2 molecules. Breakthrough experiments validated the separation performance of HOF for CO2/N-2 (v/v, 50/50) and CO2/CH4 (v/v, 50/50). After flowing through the fixed-bed HOF material, the purity of N-2 reaches to 98.52% when the cumulative injection amount of CO2 is 10.90 cm(3) g(-1). On the other hand, the maximum purity of CH4 from CO2/CH4 mixture is up to 99.60%, when the cumulative injection amount of CO2 reaches to 9.06 cm(3) g(-1). In addition, that HOF can maintain its crystallinity upon heating at elevated temperature and exposure to various solvents. Overall, this work illustrates a successful example of challenging HOF activation, and also demonstrates that HOF has a certain potential application in carbon capture and separation.