Position-dependent carboxyl functionalization in covalent organic frameworks for selective photocatalytic CO₂...

Area of Science:

• Materials Science
• Catalysis
• Green Chemistry

Background:

• Photocatalytic reduction of carbon dioxide (CO2) is a key strategy for converting greenhouse gases into valuable solar fuels.
• Controlling product selectivity in CO2 photoreduction, particularly towards shallow (CO) or deep (CH4) products, remains a significant challenge.
• Covalent organic frameworks (COFs) offer tunable structures for photocatalysis, but understanding structure-activity relationships is crucial.

Purpose of the Study:

• To investigate how subtle differences in carboxyl group spatial arrangement within isomeric COFs influence CO2 photoreduction pathways and product selectivity.
• To compare the performance of two specific COFs, TpBdda and TpBdad, in photocatalytic CO2 reduction.
• To elucidate the underlying mechanisms responsible for selective CO2 conversion in COFs.

Main Methods:

• Synthesis of two carboxyl-position isomeric COFs: TpBdda and TpBdad.
• Photocatalytic CO2 reduction experiments to quantify product yields and selectivity (CO and CH4).
• In situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) to analyze reaction intermediates.

Main Results:

• TpBdda predominantly produced carbon monoxide (CO) with high selectivity (approx. 5.2 μmol g⁻¹ h⁻¹ CO evolution rate).
• TpBdad favored deep reduction, yielding methane (CH4) as the main product with high selectivity (approx. 1.8 μmol g⁻¹ h⁻¹ CH4 evolution rate, ~90% selectivity).
• In situ DRIFTS revealed that TpBdda favors CO-related intermediate desorption, while TpBdad promotes the stabilization and hydrogenation of methoxy-related intermediates.

Conclusions:

• The spatial arrangement of carboxyl groups in COFs significantly modulates intermediate evolution and product selectivity during photocatalytic CO2 reduction.
• This study provides a molecular-level design strategy for developing highly selective organic photocatalysts for CO2 conversion.
• Isomeric COFs offer a powerful platform for fine-tuning photocatalytic activity and selectivity in solar fuel production.