Construction of a 3D porous Co(ii) metal–organic framework (MOF) with Lewis acidic metal sites exhibiting selective CO2 capture and conversion under mild conditions

Abstract

A porous, 3D Co(II) metal–organic framework, [{Co(TCPB)_0.5(H₂O)}·DMF]_n (MOF1), (where H₄TCPB = 1,2,4,5-tetrakis(4-carboxyphenyl)benzene) constituted by a rigid, tetratopic, H₄TCPB ligand was synthesized under solvothermal conditions. Single crystal X-ray structure determination revealed the 3D framework structure of MOF1 with 1D channels of dimensions 10.5 × 10.5 Ų decorated with Co(II) ions in the crystallographic a-axis. Gas adsorption studies showed the selective adsorption property of MOF1 for CO₂ over other (N₂, Ar and H₂) gases with an isosteric heat of adsorption (Q_st) value of 35.4 kJ mol⁻¹. The considerably high value of Q_st has been ascribed to the stronger interaction of CO₂ molecules with the Co(II) metal sites lined in the 1D channels of MOF1. Interestingly, the coordinated water molecules at the Co(II) centers can be removed by activating MOF1 at a temperature of 120 °C to generate a framework with pores lined with unsaturated Lewis acidic Co(II) ions. The activated MOF1 shows a very good catalytic activity for highly selective, solvent-free, heterogeneous conversion of CO₂ into cyclic carbonates under mild conditions of an atmospheric pressure of CO₂. Further, the MOF1 catalyst was facilely recycled and reused for several cycles without substantial loss of catalytic activity.