Theoretical Studies In Catalytic Reduction Of CO₂ To Formamides And Methanol By Iron Complexes

The conversion of CO₂ into valuable industrial products or precursors has attracted widespread attention due to the severe environmental impact caused by high CO₂ emissions.Conversing CO₂ into small,useful molecules such as carbonates,formamide,methanol,formic acid,etc.by using transition metal homogeneous catalysts is now a hot topic which has developed rapidly in recent years.So far,several transition metals homogeneous catalysts have been using in experiment as well as theoretically studies for CO₂ hydrogenation.However,most active catalysts are based on noble metals.Conversing CO₂ into organic small molecules has great developed by using non-noble iron based catalysts in recent years.In this work,Density functional theory?DFT?study has been carried out on the hydrogenation of CO₂ to formamides and cyclic carbonates hydrogenation to methanol,catalyzed by a series of aliphatic PNP pincer Fe complexes.First,we calculated the mechanism of PNP bifunctional iron catalyst which containing N-H ligand and monofunctional iron catalyst with methylation of the N-H functionality for the formation of formamide through CO₂ hydrogenation,respectively.The whole mechanistic pathway is divided into three parts:?i?Precatalyst activation,?ii?hydrogenation of CO₂ to generate formic acid?HCOOH?,?iii?amine thermal condensation to formamide with HCOOH.We propose that the interaction between CO₂ and bifunctional catalyst could inhibit the generaction of active species,which may go against the whole catalytic cycle.We found that CO₂ hydrogenation to generate HCOOH mainly includes hydride transfer and heterolytic H₂cleavage.Comparing these two catalysts,we could make a decision that the N-H bond was not required for stabilizeing the intermediate.We hypothesize that the carbamate complex could originate from the reaction of active catalyst with the ammonium carbamate salt.When studying the catalytic activity of PNP based on Fe,Mn and Ru homogeneous catalysts for CO₂hydrogenation,we found that the apparent activation energy of Fe and Mn catalysts are much closer to the precious metal Ru,which indicates that non-precious metal catalysts have a bright research future.In addition,we found that catalyst with a strong electron donor has lower apparent activation energy,but higher potential barrier may be detrimental to the hydrogenation reaction of CO₂,which provides a theoretical idea for future catalyst design.Secondly,we studied the detailed mechanism of the iron-based PNP catalyst containing N-H functional group for transfer hydrogenation of cyclic carbonates to methanol.The reaction mainly follow these parts:?i?precatalyst deprotonation,?ii?dehydrogenation of isopropanol to form an active catalyst,?iii?continuous hydrogenation of the cyclic carbonate to methanol.After deprotonation of the precatalyst,we found that two types of intermediates were formed with similar energies.In the process of cyclic carbonate hydrogenation,we found that the N-H functional group plays an important role in the hydrogenation process,and the substitution process of H atom on N is conducive for transfer hydrogenation of carbonate.In addition,we compared the performance of Fe and Mn PNP catalysts in the hydrogenation reaction,and proposed that the 16-electron intermediate produced by Mn-PNP deprotonation may responsible for the inferior catalytic activity.