Microporous organic networks bearing metal-salen species for mild CO2 fixation to cyclic carbonates

Abstract

This work shows that porous solid systems for catalytic carbon dioxide fixation can be developed by a direct assembly of metal-salen containing building blocks with organic connectors through a carbon–carbon bond formation reaction. For use as a dihalo building block, Al, Cr, Co-salen building blocks with two iodo groups were prepared. Sonogashira coupling of these building blocks with tetra(4-ethynylphenyl)methane resulted in microporous organic networks (MONs) bearing Al, Cr, and Co-salen species (Al-MON, Cr-MON, Co-MON). Scanning electron microscopy (SEM) showed that the materials had granular or spherical shapes. Brunauer–Emmett–Teller (BET) analysis revealed surface areas of up to 522–650 m² g⁻¹, and microporosity (<2 nm). The thermal stability of the materials was dependent on the degree of networking. Their chemical components were characterized by solid-phase ¹³C-nuclear magnetic resonance spectroscopy (NMR) and X-ray photoelectron spectroscopy (XPS). The materials containing metal-salen (M-MON) showed excellent chemical conversion of carbon dioxide with epoxide to cyclic carbonates under mild conditions (60 °C and 1 MPa CO₂). Among the M-MONs, the Co-MON showed the best reactivity for carbon dioxide conversion to cyclic carbonates with 1400–1860 TON and 117–155 h⁻¹ TOF. The size effect of epoxides in CO₂ fixation was observed due to the microporosity of Co-MON.