KI/NH_3协同催化CO_2与环氧丙烷合成碳酸丙烯酯的反应机理

Reaction Mechanism for Propylene Carbonate Prepared by KI/NH_3 Catalysis from Propylene Oxide and CO_2

采用密度泛函理论(DFT)研究了NH3/KI、KI及无催化剂条件下,CO2与环氧丙烷(PO)合成碳酸丙烯酯(PC)的反应机理.在B3LYP/6-311++G**基组水平上(I采用MIDIX基组)优化了反应过程中的反应物、中间体、过渡态和产物,通过振动分析及内禀反应坐标(IRC)确定中间体和过渡态的真实性.同时,在相同基组水平应用自然键轨道(NBO)理论和分子中的原子(AIM)理论分析了这些化合物的轨道间相互作用和成键特征.研究结果表明:在无催化剂条件下非协同反应通道PO+CO2→M0a→TS0c→M0c→TS0c′→PC为最有利通道,其活化能为200.65kJ·mol-1;KI催化下活化能降低至187.40kJ·mol-1,反应速率较小;在KI/NH3协同催化下,除KI的催mo化l-1作,大用幅外度,提NH高3中了的PO氢与原C子O还2环能合与生C成O产2或物PPOC中的的反应氧速原率子,形理成论氢计键算,与活实化验反结应果,一活致化.能降低至154.64kJ·

The reaction mechanisms for the preparation of propylene carbonate （PC） from propylene oxide （PO） and CO2 in the absence of a catalyst or by catalysis using KI or KI/NH3 were studied in detail using density functional theory （DFT） at the B3LYP/6-311＋＋G＊＊ level （I atom using the MIDIX basis set）. The geometric configurations of the reactants, intermediates, transition states, and products were optimized. Vibration analysis and the intrinsic reaction coordinate （IRC） of the reactions proved that the intermediates and transition states predicted were present. The natural bond orbital （NBO） and atoms in molecules （AIM） theories were used to determine the orbital interactions and the bond nature at the same level. The results reveal that PO＋CO2→M0a→TS0c→M0c→TS0c′→PC is the main reaction channel in the absence of the catalyst and it has a high energy barrier of 200.65 kJ·mol-1. The energy barrier is reduced to 187.40 kJ·mol-1 in the presence of KI, and it has a slow reaction rate. However, the energy barrier is reduced to 154.64 kJ·mol-1 and the reaction rate increases considerably upon promotion by KI/NH3, possibly because of the formation of hydrogen bonds between H in NH3 and O in CO2 or PO, which is in good agreement with the experimental results.