Optimized CO₂ Capture of the Zeolitic Imidazolate Framework ZIF-8 Modified by Solvent-Assisted Ligand Exchange

Abstract

Zeolitic imidazolate frameworks, like ZIF-8 and related structures, have shown great potential for the capture of carbon dioxide. Modifying their structure by exchanging part of the constituent organic ligands is a proven method for enhancing the capacity to absorb CO₂. In this work, we performed solvent-assisted ligand exchange (SALE) on nanosized ZIF-8 (nZIF-8) with a series of functionalized imidazole derivatives (exchange percentages, after 24 h): 2-bromoimidazole (19%), 2-chloroimidazole (29%), 2-trifluoromethylbenzimidazole (4%), 2-mercaptobenzimidazole (4%), and 2-nitroimidazole (54%). The sodalite topology and porosity of nZIF-8 were maintained with all SALE modifications. Low-pressure CO₂ adsorption of nZIF-8 (38.5 cm³ g^-1) at STP was appreciably enhanced with all mixed-linker SALE products. Using halogenated (-Cl, -Br, and -CF₃) imidazole derivatives in a 24 h SALE treatment resulted in increases between 11 and 22% in CO₂ adsorption, while the thiol (-SH)- and nitro (-NO₂)-functionalized SALE products led to 32 and 100% increases in CO₂ uptakes, respectively. These CO₂ uptakes were further optimized by varying the SALE treatment time. The SHbIm- and NO₂Im-exchanged SALE products of nZIF-8 show 87 and 98 cm³ g^-1 of CO₂ uptakes after 60 and 120 h of SALE, respectively. These are record high CO₂ adsorptions for all reported ZIF derivatives at low-pressure conditions.