Copper(II)-porphyrin functionalized titanium(IV) metal-organic aerogels for the visible-light driven conversion of CO2 to alcohols

Abstract

This manuscript covers the synthesis and characterization of a series of titanium based metal-organic aerogels (MOAs) functionalized with copper(II)-metalated porphyrins for their application in visible-light-driven carbon dioxide (CO2) conversion to alcohols. A thorough characterization is performed using a set of spectroscopic and microstructural analysis techniques to reveal the structural and microstructural features that can aid in establishing structure-activity relationships. The parent MOAs consist of metal-organic nanoparticles (5–10 nm) crosslinked into a highly porous microstructure (surface area: 600–800 m2·g−1). The post-synthetic reaction with copper(II) enables the metalation of the tetrapyrrole ring of porphyrin, which is confirmed by analyzing the absorption and luminescence spectra. High-angle annular dark-field electron microscopy imaging demonstrates a uniform distribution of the metalation throughout the nanoparticles that compose the material. The CO2 photoreduction experiments performed in an optofluidic microreactor show that the metalation markedly invigorates the total alcohol (methanol and ethanol) production rates and apparent quantum yields (AQY), from 21 to 367 μmol·g−1·h−1 (AQY: 0.4–7%) prior to metalation to values of 356–642 μmol·g−1·h−1 (AQY: 11–20%). Additionally, the metalation inverts the selectivity towards ethanol, increasing from 0–12% in the parent MOA to 67–76% after incorporating Cu(II).