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

Abstract

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 p_CO2 = 1 bar, the total gas uptake equals 3.07 mmol g⁻¹ (13.5 wt% CO₂). Its affinity for CO₂ (isosteric heat of adsorption Q_st = 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 p_CO2 = 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.