Mesoporous SBA-15 Supported Catalyst For The Cycloaddition Of CO₂ And Epoxides

In connection with high concentration of CO₂ in the atmosphere and a range of environmental degradation,the chemical conversion of CO₂ and epoxides to form various cyclic carbonates by a typical cycloaddition reaction becomes a potential and efficeient strategy,which contribute to the reduction of CO₂ emission and synthesize high-value compounds.Taking aim at the chemical inactivity of CO₂,we prepared two efficient catalytic systems for the cycloaddition of CO₂ to epoxides on the basis of mesoporous SBA-15.Foremost,mesoporous SBA-15 incorporated with citric acid?CA?by simple condensation reaction between abundant carboxyl groups of CA with Si-OH on the surface of SBA-15,and successfully synthesized heterogeneous catalyst SBA-15-CA containing many hydrogen bond donor groups,which was confirmed by a series of characteristic.Under the same reaction conditions,the catalytic performance of SBA-15-CA/Bu₄ NBr was significantly improved compared to the reported SBA-15/Bu₄ NBr catalytic system.The PC?propylene carbonate?yield up to 99% under the reaction conditions of 110 °C and 2 MPa CO₂.Besides,several reaction parameters were investigated detailedly,including reaction temperature,reaction time,the type and mass of co-catalyst.The SBA-15-CA/Bu₄ NBr catalytic system showed excellent recyclability and easily reused 5 times without obvious catalytic activity decrease.Moreover,the catalytic system also presented good catalytic activity for various epoxides with different substituents,such as propylene oxide,1,2-epoxidebutane,epichlorohydrin,styrene oxide,etc.Based on the presented theories and experimental data,a possible catalytic mechanism was proposed.Despite the fact that the SBA-15-CA/Bu₄ NBr catalytic system show good catalytic activity and selectivity for the cyclic carbonates synthesis from CO₂ and various epoxides,the addition of co-catalyst Bu₄ NBr is necessary.To achieve efficient catalytic performance under co-catalyst free condition,an imidazole ionic liquid was introduced into the mesoporous structure of SBA-15.First of all,mesoporous materials SBA-15 with a stable metal Zn loading?SBA-15-Zn?was synthesized according to the reported researches,and imidazole type ionic liquid [?CH2COOH?Mim]Br was obtained by using N-methylimidazole and2-bromoacetic acid,then forming Zn-N bonds to achieve successful loading of imidazole ionic liquid on the mesoporous material SBA-15?SBA-15-Zn-IL?,which was characterized by N2 adsorption-desorption,FT-IR analysis,etc.The SBA-15-Zn-IL was further applied to the cycloaddition reaction of CO₂ and propylene oxide to form propylene carbonate.SBA-15-Zn-IL could catalyze the cycloaddition reaction of CO₂ to propylene oxide underco-catalyst free condition.With single SBA-15-Zn-IL?0.2?as a catalyst,the PC yield reached98% at the reaction temperature of 130 °C,CO₂ pressure of 2 MPa and 4 h,the versatility and recyclability were further explored under the same conditions.Besides,the influence of critical reaction parameters,such as reaction temperature and reaction time,has been discussed detailedly.Catalytic mechanism for the cycloaddition reaction of propylene oxide and CO₂ over single SBA-15-Zn-IL catalyst was proposed:?1?reaction substrates were fully adsorbed and activated at abundant active sites in the mesoporous structure,?2?the halogen anion Br-nucleophilic attack of the ionic liquid facilitated ring-opening process,?3?the insertion of activated CO₂ molecules and the occurrence of a closed-loop reaction.