Environmentally Friendly, Co-catalyst-Free Chemical Fixation of CO2 at Mild Conditions Using Dual-Walled Nitrogen-Rich Three-Dimensional Porous Metal-Organic Frameworks

Highly porous, polyhedral metal-organic frameworks (MOFs) of Co(II)/Ni(II), {[M-6(TATAB)(4)(DABCO)(3)(H2O)(3)]center dot 12DMF. 9H(2)O}(n) (where M = Co(II) (1)/Ni(II) (2), H(3)TATAB = 4,4',4 ''-s-triazine-1,3,5-triyl-tri-p-aminobenzoic acid, and DABCO = 1,4-diazabicyclo[2.2.2]octane) have been synthesized solvothermally. Both MOFs 1 and 2 show a 2-fold interpenetrated 3D framework structure composed of dual-walled cages of dimension similar to 30 angstrom functionalized with a high density of Lewis acidic Co(II)/Ni(II) metal sites and basic -NH-groups. Interestingly, MOF 1 shows selective adsorption of CO2 with high heat of adsorption (Q(st)) value of 39.7 kJ/mol that is further supported by theoretical studies with computed binding energy (BE) of 41.17 kJ/mol. The presence of the high density of both Lewis acidic and basic sites make MOFs 1/2 ideal candidate materials to carry out co-catalyst-free cycloaddition of CO2 to epoxides. Consequently, MOFs 1/2 act as excellent recyclable catalysts for cycloaddition of CO2 to epoxides for high-yield synthesis of cyclic carbonates under co-catalyst-free mild conditions of 1 bar of CO2. Further, MOF 1 was recycled for five successive cycles without substantial loss in catalytic activity. Herein, rational design of rare examples of 3D polyhedral MOFs composed of Lewis acidic and basic sites exhibiting efficient co-catalyst-free conversion of CO2 has been demonstrated.