Amino-decorated bis(pyrazolate) metal–organic frameworks for carbon dioxide capture and green conversion into cyclic carbonates
The novel Metal–Organic Frameworks (MOFs) M(BPZNH2) (M = Zn, Ni, Cu) were prepared from the reaction of the corresponding metal acetates M(OAc)2·nH2O and the organic linker 3-amino-4,4′-bipyrazole (H2BPZNH2) under solvothermal conditions. H2BPZNH2 was obtained straightforwardly from the reduction of the related nitro-compound using hydrazine as a reducing agent. The Zn(II) polymer is characterized by a 3D porous network featuring tetrahedral metallic nodes and bridging BPZNH22− anions defining the vertices and edges of square channels. The isostructural Ni(II) and Cu(II) MOFs show square-planar metallic nodes and bridging BPZNH22− spacers at the vertices and edges of the rhombic channels of a 3D porous framework. All the MOFs were characterized in the solid state [(VT)-PXRD, IR, TGA-DTG]. The textural property analysis revealed that they are micro–mesoporous materials with BET specific surface areas (SSAs) falling in the 100–400 m2 g−1 range. Zn(BPZNH2), showing the highest SSA (395 m2 g−1) and a prevalently microporous texture (micropore area = 69% of the accessible SSA), has been exploited as a CO2 capture material: at T = 298 K and pCO2 = 1 bar, the total gas uptake equals 3.07 mmol g−1 (13.5 wt% CO2). Its affinity for CO2 (isosteric heat of adsorption Qst = 35.6 kJ mol−1; CO2/N2 Henry selectivity = 17; CO2/N2 IAST selectivity = 14) is higher than that of its nitro-functionalized analogue and comparable to that of other amino-decorated MOFs from the literature. Finally, Zn(BPZNH2) was tested as a heterogeneous catalyst in the reaction of CO2 with activated epoxides bearing a –CH2X pendant arm (X = Cl: epichlorohydrin; X = Br: epibromohydrin) to give the corresponding cyclic carbonates at T = 393 K and pCO2 = 1 bar under green (solvent- and co-catalyst-free) conditions. A good conversion of 47% and a TOF of 3.9 mmol(carbonate) (mmolZn)−1 h−1 were recorded with epibromohydrin.