Metal Organic Complex Catalyzed Carbon Dioxide And Epoxide Synthetic Cyclic Carbonate And Mechanism Research

In recent years,excessive consumption of fossil fuels has directly led to the massive accumulation of carbon dioxide（CO₂）in the atmosphere.Since the beginning of the 19th century,the concentration of CO₂in the atmosphere has increased rapidly and continuously.So far,the average growth rate of atmospheric CO₂concentration is 2.5 ppm every year.By March 2022,the total CO₂concentration has reached 417.97 ppm,the highest level in the past million years.As a typical greenhouse gas,the increase of CO₂concentration has led to climate change,such as thermal anomaly,drought,severe cold,etc.At the same time,the excessive consumption of fossil fuels has also caused energy shortage,which has restricted the rapid development of society.Under the strategic goal of"carbon neutrality",various countries and local governments have issued a series of policies to promote energy transformation,improve energy utilization and reduce carbon emissions.We designed and prepared a metal complex catalyst using pyridine amide as the ligand raw material for catalytic conversion of CO₂to cyclic carbonate.We further explored the activation mechanism of carbon dioxide in the cycloaddition reaction and the ring opening of epoxides by means of online infrared and other means,and finally deduced the possible mechanism of the cycloaddition reaction.The main research contents are as follows:First of all,we designed and prepared the metal complex catalyst Al-PPPA.Through a series of characterization,we determined the structure of the catalyst,optimized the optimal reaction conditions,and evaluated the applicable substrate range and recycling capacity of the catalyst.The catalyst ligand PPPA was synthesized by pyridine-2-carboxylic acid hydrochloride and 1,3-propanediamine,supported with different metals to obtain catalyst M-PPPA,preferably Al-PPPA had the best catalytic activity,and its chemical composition and structure were determined by UV-Vis,FT-IR,XPS,1H NMR and other means.The catalyst Al-PPPA reacted at 120^oC and 2.0 MPa CO₂pressure for 8 h to obtain 95.8%conversion,close to 99%selectivity and good substrate universality.Compared with the relevant catalyst systems reported in the literature,there are certain advantages in reaction conditions and catalytic performance.Finally,the recycling performance of the catalyst was tested,and it was found that its catalytic activity remained at a high level after 6 times of recycling.After that,we designed and prepared the optimized metal complex catalyst Fe-PYPA,determined the structure of the catalyst,screened the reaction conditions and the range of substrates applicable to the catalyst,and finally evaluated the stability and circulation ability of the catalyst.Pyridine-2-carboxylic acid hydrochloride and 4-aminopyridine were used to synthesize catalyst ligand PYPA,which was supported with different metals to obtain catalyst M-PYPA.Preferably,Fe-PYPA had the best catalytic activity.Its chemical composition and structure were determined by UV-Vis,FT-IR,XPS,1H NMR and other means.Under the condition of no cocatalyst added,after 1mol%of the catalyst reacted for 8h at 100℃and 1.5MPa CO₂pressure,the conversion was up to 95.7%and the selectivity was up to 99%.At the same time,in the presence of tetrabutyl ammonium iodide,the conversion of propylene oxide can reach 98.2%.The catalyst system showed good substrate universality and cyclic catalytic ability for more than ten epoxides.Based on the above research results,we propose a possible mechanism by using online infrared spectroscopy.It is believed that the metal center（M）activates the epoxide,the X^-assisted ring opening of the epoxide,and then the closed loop generates the cyclic carbonate.Using Al-PPPA and Fe-PYPA,we explored the mechanism of the cycloaddition reaction,monitored the reaction process using online infrared,intuitively saw the consumption of raw materials,the process of intermediate from generation to consumption and the final production of products,and proposed a possible mechanism.It is believed that the metal center（M）can effectively activate the epoxide.At this time,X^-nucleophilic attack the side of epoxide with less resistance,and assist epoxide ring opening,Then CO₂is added into the reaction system and undergoes intramolecular cyclization with epoxide to produce cyclic carbonate.This research has important theoretical significance for the comprehensive utilization of environmental pollutants,energy and other fields.