Self-assembly of the custom-designed octatopic porphyrin ligand of tetrakis(3,5-dicarboxybiphenyl)porphine with the in situ generated Cu₂(CO₂)₄ paddlewheel moieties afforded a porous metal–metalloporphyrin framework, MMPF-9, which features a high density of Cu(II) sites confined within nanoscopic channels and demonstrates excellent performances as a heterogeneous Lewis-acid catalyst for chemical fixation of CO₂ to form carbonates at room temperature under 1 atm pressure.