Pore engineering in highly stable hydrogen-bonded organic frameworks for efficient CH4 purification

Abstract

The unsatisfactory stability of hydrogen-bonded organic frameworks (HOFs) arising from the weakness and flexibility of hydrogen-bonding (H-bonding) has generally hindered their practical applications. By synergistically employing three key strategies: (Ⅰ) introducing charge-assisted H-bonding to increase the polarity of the framework, (Ⅱ) optimizing the length of the building unit by extending the core structure of the linker molecule, and (III) incorporating functional sites containing methoxy and trifluoromethyl, to increase the stability of HOFs and the adsorption affinity for gas molecules, we successfully synthesized three charge-assisted hydrogen-bonded HOFs (NKM-HOF-3, NKM-HOF-4, and NKM-HOF-5; NKM = Nankai Materials). These HOFs exhibit distinct space configurations of nonporous packing, discrete cavities or continuous one-dimensional channels. Notably, NKM-HOF-5 exhibits commendable thermal and chemical stability and can be readily regenerated by re-solvothermal reaction. The activated NKM-HOF-5a exhibits a low affinity for CH4 but captures C2H6 and C3H8 with a relatively high affinity. According to theoretical calculations, the suitable pore size, together with functionalized and polarized surface environments in NKM-HOF-5a contribute to the excellent selective gas adsorption performance of NKM-HOF-5a. Furthermore, the dynamic breakthrough experiments demonstrate the highly efficient separation performance of NKM-HOF-5a for C2H6/CH4, C3H8/CH4, and C3H8/C2H6/CH4.