The electron-donating effect of pyridinic N sites in covalent triazine frameworks on the molecular ruthenium catalyzing CO2 hydrogenation to formate

Abstract

In this study, molecular ruthenium (Ru3+) was immobilized onto pyridine-functionalized covalent triazine frameworks (CTFs) to catalyze the hydrogenation of CO2 to formate efficiently. The energy barrier for this process on N3-Ru, coordinated by both pyridinic N and triazinic-N, was found to be 29.91 kcal/mol, significantly lower than the 38.07 kcal/mol on N2-Ru, coordinated only by triazinic-N. Increasing the content of pyridine ring in the catalyst (RuCl3@CTF-1<RuCl3@CTF-2< RuCl3@CTF-3) led to a higher ratio of N3-Ru to N2-Ru. The higher the N3-Ru content, the denser the electron cloud density of Ru species, favoring Ru-H nucleophilic attack on C atoms in CO2, as confirmed by XPS. RuCl3@CTF-3, with the highest content of N3-Ru structure, exhibited the best performance (TOF=1274 h−1 at 120 °C, 6 Mpa) and the lowest apparent activation energy (Ea=36.53 KJ·mol−1) in CO2 hydrogenation, consistent with DFT calculations.