Visible-light-driven sustainable conversion of carbon dioxide to methanol using a metal-free covalent organic framework as a recyclable photocatalyst

Abstract

A new triazine-based organized 2D covalent organic framework (COF), namely, TRITER-2, was built through combinatorial copolymerization between 4,4′-biphenyldicarbaldehyde and a triamine 1,3,5-tris-(4-aminophenyl) triazine (TAPT). This highly crystalline organic polymeric material displayed a superior BET surface area (1260 m² g⁻¹) and exceptional photocatalytic activity for the synthesis of methanol. The metal-free catalytic reduction to methanol has emerged as a sustainable avenue due to the ambient reaction conditions by which CO₂ is reduced to methanol. The catalytic reduction of CO₂ under metal-free conditions offered the synthesis of methanol at a low price, in a non-toxic way with earth-abundant reagents, and a low carbon footprint. Using this COF TRITER-2, we have carried out the photocatalytic reduction of CO₂ into high-value methanol as the active photo-catalyst under atmospheric pressure CO₂. The highest turnover number (TON = 141) for visible-light-accelerated metal-free CO₂ reduction to methanol (major product) using crystalline TRITER-2 (10 mg) was achieved under 20 W white LED irradiation. The reaction did not proceed without any applied potential (i.e., a white LED) and can be readily monitored by altering the light intensity. We demonstrate that a TON of about 10 can be reached with sunlight under sustainable reaction conditions. This catalyst is efficient, selective, and can be recycled several times, retaining its catalytic performance.