Preparation And Characterization Of Potassium Nitrate Supported Solid Superbases And Their Application In The Synthesis Of Dimethyl Carbonate

Dimethyl carbonate（DMC）is an important organic chemical intermediate,which is widely used in organic synthesis,solvent,polymer synthesis and other fields.In this thesis,solid superbase catalyst was studied and used to replace the conventional homogeneous base catalyst for the transesterification between ethylene carbonate（EC）and methanol to synthesize DMC.The potassium nitrate loaded solid superbase catalysts supported on La₂O₃-MgO,La₂O₃-MgO@SBA-15 and ZIF-67 were prepared,and the catalytic performance for transesterification was investigated.The conclusions obtained are as follows:Firstly,the La₂O₃-MgO bimetallic oxide support was prepared by the co-precipitation method,and then the base precursor KNO₃ was supported on the support by wet impregnation method.A batch of catalysts with different base precursor loadings were prepared,and it was found through experimental screening that the optimal loading of KNO₃ on La₂O₃-MgO was 15%.By setting different calcination temperatures,it was finally determined that the conversion temperature of the base precursor on the support is 700°C.The catalyst has good catalytic activity for the transesterification reaction between EC and methanol,and the maximum yield of DMC could reach about 50%.Secondly,the mesoporous silica-based support SBA-15 was first prepared,and then La₂O₃-MgO bimetallic oxide was coated in the pores of SBA-15 by the ammonia/water induced hydrolysis method.A large specific surface area support with ordered pore structure was prepared.Thermogravimetric-mass spectrometry（TG-MS）results showed that the base precursor began to transform at about 500℃on this support,which was lower than that on the pure La₂O₃-MgO.Due to the abundant pores,the active components are relatively difficult to lose.The catalytic activity of the catalyst was investigated by the transesterification reaction between EC and methanol,and the maximum yield of the product could reach 47.6%.Finally,Co（NO₃）₂·6H₂O and 2-methylimidazole were used as precursors to prepareZIF-67,a metal-organic framework material with reducibility.And then the base precursor KNO₃ was loaded on it by wet impregnation method.Through TG-MS characterization,it was proved that the base precursor was converted on the support at a temperature of about 335℃,and the conversion was completed at about 380℃,which was much lower than the temperature required on La₂O₃-MgO and La₂O₃-MgO@SBA-15.This is because the Co²⁺in the support has reducibility and can promote the conversion of base precursors through oxidation-reduction.The XPS results confirm the above point of view.The prepared catalyst was used to catalyze the transesterification reaction to prepare DMC,and the maximum yield of the product could reach about 60%.In addition,the catalyst has good reusability,and experiments have proved that its catalytic activity remains at a high level after being used repeatedly for 4 times.