Selective Carbon Dioxide versus Nitrous Oxide Adsorption in Cerium(IV) Bithiazole and Bipyridyl Metal-Organic Frameworks

The two CeIV MOFs [Ce6O4(OH)4(TzTz)6] (Ce_TzTz) and [Ce6O4(OH)4(TzTz)4(PyPy)2] (Ce_TzTz_PyPy; H2TzTz = [2,2′-bithiazole]-5,5′-dicarboxylic acid, H2PyPy = 2,2′-bipyridine-5,5′-dicarboxylic acid) are prepared starting from a pre-formed [Ce6] glycinate-capped cluster. N2 isotherms collected at 77 K revealed BET specific surface areas of 1136 and 238 m2 g−1, respectively. Their CO2 and N2O adsorption capacity is assessed at T = 273 and 298 K and p = 1 bar. Ce_TzTz shows the highest gas uptake (7.9 and 9.7 wt% at 298 K and 11.5 and 12.5 wt% at 273 K for CO2 and N2O, respectively). More interestingly, this homo-linker MOF possesses a higher capacity, thermodynamic affinity [(Qst)CO2 = 18.2 kJ mol−1 vs (Qst)N2O = 25.4 kJ mol−1] and selectivity (IAST SN2O/CO2 = 1.6 at T = 273 K) toward N2O than the mixed-linker sample. At variance, Ce_TzTz_PyPy shows a slightly higher capacity, thermodinamic affinity [(Qst)CO2 = 29.5 kJ mol−1 vs (Qst)N2O = 26.4 kJ mol−1] and selectivity (IAST SCO2/N2O = 1.4 at T = 298 K) toward CO2. DFT optimizations carried out on the [N2O@Ce_TzTz] and [CO2@Ce_TzTz_PyPy] systems revealed that the primary adsorption sites are the cerium ions of the [Ce6] metallic node for N2O and the thiazole N-atoms on the TzTz linker for CO2, respectively.