The mechanism of the cycloaddition reaction of propylene oxide with CO2 catalyzed by[MOBMIM][Gly]
The mechanism of the cycloaddition reaction of CO2 with propylene oxide(PO)catalyzed by the amino acid ionic liquid 1-(4-methoxybutyl)-3-methylimidazolium glycinate([MOBMIM][Gly])was investigated for the first time by utilizing density functional theory.In addition,the reactivity of the catalyst with the reaction substrate was explored using the frontier molecular orbital theory and main influence factors of the catalytic activity for the amino acid ionic liquids were analyzed in combination with the natural bond orbital(NBO)theory.The calculations show that the optimal reaction pathway is a three-step reaction mechanism catalyzed by[MOBMIM][Gly],i.e.,ring-opening of the epoxide,insertion of CO2 and closed-ring to generate the product.The energy barrier of the ring-opening step is similar to the energy barrier of the closed-ring step and becomes a competing step.Although[MOBMIM][Gly]is capable of absorbing CO2 to produce the carbamate[MOBMIM][CGly]in the catalytic reaction system,it weakens the nucleophilic activation ability of the amino acid anion,leading to an increased catalytic energy barrier,which is not beneficial to the ring-opening of PO,and it is thus infeasible that[MOBMIM][CGly]becomes a catalyst.The results of the NBO analyses indicate that nucleophilic activation of the[Gly]-anion plays a critical role in promoting the PO ring-opening process.
amino acid ionic liquidcarbon dioxidepropylene oxidedensity functional theory
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