Amino-decorated bis(pyrazolate) metal-organic frameworks for carbon dioxide capture and green conversion into cyclic carbonates

The novel Metal-Organic Frameworks (MOFs) M(BPZNH 2 ) (M = Zn, Ni, Cu) were prepared from the reaction of the corresponding metal acetates M(OAc) 2 ·nH 2 O and the organic linker 3-amino-4,4′-bipyrazole (H 2 BPZNH 2 ) under solvothermal conditions. H 2 BPZNH 2 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 BPZNH 22- anions defining the vertices and edges of square channels. The isostructural Ni(ii) and Cu(ii) MOFs show square-planar metallic nodes and bridging BPZNH 22- 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 m 2 g -1 range. Zn(BPZNH 2 ), showing the highest SSA (395 m 2 g -1 ) and a prevalently microporous texture (micropore area = 69% of the accessible SSA), has been exploited as a CO 2 capture material: at T = 298 K and pCO 2 = 1 bar, the total gas uptake equals 3.07 mmol g -1 (13.5 wt% CO 2 ). Its affinity for CO 2 (isosteric heat of adsorption Q st = 35.6 kJ mol -1 ; CO 2 /N 2 Henry selectivity = 17; CO 2 /N 2 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(BPZNH 2 ) was tested as a heterogeneous catalyst in the reaction of CO 2 with activated epoxides bearing a -CH 2 X pendant arm (X = Cl: epichlorohydrin; X = Br: epibromohydrin) to give the corresponding cyclic carbonates at T = 393 K and pCO 2 = 1 bar under green (solvent- and co-catalyst-free) conditions. A good conversion of 47% and a TOF of 3.9 mmol(carbonate) (mmol Zn ) -1 h -1 were recorded with epibromohydrin.