NO2-tagged pyrazolate-based MOFs: Efficient CO2 sorbents at ambient conditions

Organic linkers modification in MOFs is a valuable approach to prepare materials with targeted functionalities. Following this concept and given our past experience with pyrazolate-based MOFs [1-4], we have prepared, by solvothermal routes, the three novel MOFs [M(BPZNO2)] (M = Co, Cu and Zn; BPZNO2 = 3-nitro-4,4’-bis(pyrazolate)). Their crystal structures have been solved from powder X-ray diffraction (PXRD). They show 3D (4,4)-connected networks with 1D squared (Co, Zn; Figure 1) or rhombic (Cu) channels decorated by (BPZNO2)2- ligands and accounting for an empty volume in the range 14-35%. They are isotypical to the analogues [M(BPZ)], [M(BPZMe2)] and [M(BPZMe4)] (M = Co, Zn) containing the 4,4’-bis(pyrazolate), [2] 3,3’-dimethyl-4,4’-bis(pyrazolate) [3] and 3,3’,5,5’-tetramethyl-4,4’-bis(pyrazolate) [4] spacers, respectively. The thermal stability of the [M(BPZNO2)] MOFs (Tdec ~ 470, 370 and 310 °C for M = Zn, Co and Cu, respectively) was evaluated by combining thermogravimetric analyses and variable-temperature PXRD. Their textural properties (BET specific surface area, pore size distribution) and their ability as CO2 adsorbents were investigated through N2 and CO2 adsorption. After thermal activation, [Zn(BPZNO2)] adsorbs 22 wt % CO2 at the mild conditions of 298 K and 1.2 bar, hence ranking among the best performing MOFs in this context to date [5]. Moreover, at 298 K [Zn(BPZNO2)] shows the remarkable CO2/N2 selectivity of 25 and a [H(ads)]CO2 of 20.3 kJ/mol.