1 | Introduction
Carbon dioxide capture is an effective strategy to mitigate greenhouse effect and other related challenges. CO2/N2 separation technology is pivotal for post-combustion CO2 capture since the flue gas from power plants is mainly composed of N2(70%~75%) and CO2(10%~15%) 1. Besides, CO2 is also a harmful component for natural gas, especially under moisture condition 2. Therefore, separation of CO2 from CH4 is challenging but of great significance to methane upgrading3. Compared with traditional amine-scrubbing, adsorptive separation employing suitable porous materials is sought-after as a result of the ease of operation and low energy expenditure.
Metal-organic frameworks (MOFs) are promising adsorbents for their diversified chemical compositions and porous skeletons4-6. The advancement of MOFs has motivated their utility in gas separation and purification 7-13. Selective adsorption on MOFs underpins isolating gases in high purity. The core is to significantly accentuate the adsorption discrepancy of a pair of gases (e.g., CO2/N2, CO2/CH4) on MOFs, which is critical for selectivity in dynamic adsorption-based separation and membrane separation 14-18. Cavity-occupying is an efficient solution to narrow the pore size, and thereby to reinforce the molecular recognition and selective adsorption of MOFs while retaining structural integrity. This operation would be more effective in MOFs with flexibility or with pore size significantly larger than molecule dimensions. Ban et al. demonstrated that the confinement of ionic liquid [bmim][Tf2N] into the sodalite nanocage of ZIF-8 can tailor the cage size of ZIF-8, impeding the passage of the larger molecule and appreciably improve CO2/N2selectivity from 19 to 100 19. Liao et al. decorated hydrazine into the pore of a MOF. The composite also excels in selective adsorption of CO2 at low pressure 20. Lin et al. loaded molecular-level polyethyleneimine into the nanopore of MIL-101, engendering lower CO2 capacity but higher CO2/N2 selectivity in the constrained inner pore space 21.
MOF-74 possessing abundant open metal sites take up significant saturable CO2 capacity. However, the large pore of MOF-74 displays moderate CO2 selectivity over N2 and CH4 22. It is worthy to explore the possibility of improving CO2selectivity by cavity-occupying. Nitrogen-containing agents show good CO2 affinity in terms of Lewis acid-base interactions but weaker binding with CH4 and N2 23. Therefore, introduction of the suitable nitrogen-containing agent as a cavity-occupant could be helpful for reducing the effective pore size of MOF-74, and simultaneously providing compensation sites for CO2 trapping. In this work, we seek to demonstrate a platform based on MOF-74 for outstanding CO2 selective adsorption. Pyrazine with dual “para-nitrogen” atoms was introduced into the pore of MOF-74 via a post vapor modification method (Scheme 1). One nitrogen atom of pyrazine is bonded to the open metal ions in MOF-74 with a minimal steric hindrance, and the other nitrogen atom of pyrazine provides potential affinity to CO2, resulting in a stable pyrazine-interior-embodied MOF-74 composite for boosting CO2/N2 and CO2/CH4 adsorption selectivity through varying the pyrazine loading content. Given the pore environment of MOF-74 after pyrazine modification, the selective adsorption of pyrazine-interior-embodied MOF-74 for C4olefine/paraffine was also investigated.