Enhanced CO2 Adsorption and Selectivity of CO2/N-2 on Amine@ZIF-8 Materials

The ZIF-8 crystals were successfully postsynthetically modified using methylamine (MA), ethylenediamine (ED), and N, N & PRIME;-dimethylethylenediamine (MMEN) to improve their adsorption performance toward CO2. Results showed that, compared with the original ZIF-8, the BET specific surface area of MA-ZIF-8, MMEN-ZIF-8, and ED-ZIF-8 has increased by 118.2%, 92.0%, and 29.8%, respectively. In addition, their total pore volume increased separately by 130.8%, 100%, and 48.7%. The adsorption capacities of CO2 on the amine-modified ZIF-8 samples followed the order MA-ZIF-8 > MMEN-ZIF-8 > ED-ZIF-8 > ZIF-8. The CO2 adsorption capacities at 298 K on MA-ZIF-8, MMEN-ZIF-8, and ED-ZIF-8 were increased by 118.2%, 90.2%, and 29.8%, respectively. What is more, the CO2/N-2 selectivities calculated using an IAST model of the amine@ZIF-8 samples at 0.01 bar and 298 K were also significantly improved and followed the order MA-ZIF-8 31.4 > ED-ZIF-8 25.1 > MMEN-ZIF-8 14.1 > ZIF-8 11.5, which increased by 173.0%, 121.4%, and 22.6%, respectively. The isosteric heat of CO2 adsorption (Q(st)) on the MA-ZIF-8, MMEN-ZIF-8, and ED-ZIF-8 all becomes higher, while Q(st) of N-2 on these samples was slightly lower in comparison with that on the ZIF-8. Furthermore, after six recycle runs of gravimetric CO2 adsorption-desorption on MA-ZIF-8, the adsorption performance of CO2 is still very good, indicating that the MA-ZIF-8 sample has good regeneration performance and can be applied into industrial CO2 adsorption and separation.