Engineering of an Isoreticular Series of CALF-20 Metal–Organic Frameworks for CO 2 Capture

Abstract

A series of linker-substituted ultramicroporous CALF-20 metal−organic frameworks (MOFs) were built in silico, and their CO 2 capture performances over N 2 in flue gas conditions were systematically computationally explored. Among the various linker substitutions explored, squarate-linker-incorporated CALF-20 (SquCALF-20) was demonstrated to show a larger CO 2 uptake at 0.15 bar (3.6 mmol/g) and higher CO 2 /N 2 selectivity (500) in dry conditions compared to pristine CALF-20. Interestingly, this MOF was shown to maintain a high level of CO 2 capture performance even in the presence of humidity, although it starts to adsorb H 2 O at lower relative humidity compared to CALF-20. Because squaric acid is a semiconductor industry feedstock and the few-already published squarate-based MOFs are chemically robust, this engineered SquCALF-20 offers a promising avenue for cost-effective CO 2 capture via physisorption, with potential applications in addressing environmental concerns associated with CO 2 emissions. ■ INTRODUCTION CO 2 emissions into the atmosphere warm the planet, causing climate change and serious related health issues. 1 Aqueous amine-based CO 2 absorption technology is mature for postcombustion capture of CO 2 emitted from power plant exhaust gases; however, it raises concerns in terms of toxicity and implies a high energy-cost regeneration process. 2 Physisorption-based processes using porous sorbents are a valuable alternative strategy to achieve energetically effective CO 2 capture. 3 Beyond the emblematic porous sorbents, including activated carbons, 4 mesoporous silica, 5 and zeolites, 6 metal−organic frameworks (MOFs), 7 one of the most recent classes of porous crystalline solids, have attracted tremendous interest for diverse adsorption/separation applications due to their unprecedented chemical versatility and high tunability of their pore size/shape. A myriad of MOFs has been proposed over the last 2 decades with promising performance for CO 2 capture 7−12 that paves the way toward alternative solutions to the standard CO 2 zeolite sorbents. 6 The ultramicroporous SIFSIX-3-M (M = Zn and Cu) is one of the first prominent CO 2 sorbent MOFs exhibiting very high CO 2 /N 2 selectivity, i.e., SIFSIX-3-Cu (10500) and SIFSIX-3-Zn (7250) associated with relatively large CO 2 uptakes (1.24 and 0.13 mmol/g, respectively) at 400 ppm and 298 K. 13 The hydrolytically stable fluorinated MOF NbOFFIVE-1-Ni (KAUST-7) derived from the same MOF platform was further demonstrated to be an excellent candidate for CO 2 capture directly from air (direct air capture) combining high CO 2 amount adsorbed at traces (1.3 mmol/g at 400 ppm and 298 K) and reasonable regeneration energy cost. 14 Long et al. reported the N,N-dimethylethylenediamine-functionalized Mg 2 (dobpdc) MOF structure for CO 2 capture from air/flue gas (CO 2 , N 2 , and O 2). 15 This MOF was demonstrated to show very high CO 2 uptake of 2.0 mmol/g at 0.00039 bar and 298.15 K (air capture) and 3.1 mmol/g at 0.15 bar and 313.15 K (flue gas) associated with very high CO 2 /N 2 selectivity of up to 49000. However, some of these potential CO 2 sorbent MOFs suffer from competitive adsorption of H 2 O, which leads to a substantial drop of CO 2 sorption capacity under the operating humidity conditions and/or a costly regeneration process with the use of relatively high-temperature treatment and still some issues in terms of long-term stability. 16−18 To overcome these shortcomings, Shimizu et al. 19 recently proposed a highly thermal and chemically robust zinc triazolate MOF made of 1,2,4-triazolate-bridged zinc(II) layers pillared by the oxalate ligand [Zn 2 (1,2,4-triazolate) 2 (oxalate)], namely, CALF-20 (CALF stands for Calgary Framework), seen as the current