Study On The Catalytic Performance Of Co(Salen) And Supported Co(Salen) In The Oxidative Carbonylation

Co(Salen) complexes are widely used as a kind of efficient catalysts in the fields of many organic synthesis reactions, such as asymmetric oxidation of olefin, ring-opening hydrolysis of epoxides, polymerization and Diels-Alder reaction.With the public environment awareness and the compulsive implement of environment legislation, chemists have focused their interest on highly effective, environment friendly and recyclable "green" clean catalysis recently. Among those, homogeneous catalysts immobilized onto inorganic materials are the important research fields.In this work, three kinds of Co(Salen) complexes were synthesized and applied to the oxidative carbonylation of ethanol to diethyl carbonate(DEC). It was found that the activity of Co(Salophen) is higher than that of Co(Salen) and Co(Salcyen), which is rationally explained by the quantum chemistry calculation. The electron density of Co in Co(Salophen) is higher than the other Co complexes, which helps reactants combine with Co and increases the activity of Co(Salophen) catalyst. The reaction conditions, such as temperature, gas pressure, catalyst concentration and reaction time, are also investigated. The conversion of ethanol and the selectivity to DEC in the optimal condition are 15.8% and 99%, respectively.Co(Salen) complexes are active homogeneous catalysts and less expensive compared with noble metal catalysts. However, Co(Salen) complexes are easily prone to formμ-oxo dimmer to lead to lose their activity, especially in the oxidative carbonylation reaction. The heterogeneous catalysis of Co(Salen) complexes is an effective solution to this problem.Four Co(Salen) complexes were successfully encapsulated in the Y zeolite using "ship-in-a-bottle" method. The Co(Salen) complexes encapsulated in Y zeolite are characterized by FT-IR, UV-Vis, BET, XRD and thermal analysis (TG/DTA). The immobilized catalysts and their homogeneous analogues were evaluated in the oxidative carbonylation of methanol to dimethyl carbonate (DMC). Compared with the homogeneous catalysts, the encapsulated Co(Salen) complexes are more active. The content and leaching of Co are determined by atomic absorption spectrometry (AAS). It was found that the Co content in the Co(Salen)-Y and the activity of the catalysts have no change during the reaction, which make sure that the immobilized Co(Salen) complexes were stable and recyclable. The corrosion tests of the catalysts show that Co(Salophen) and Co(Salophen)-Y are noncorrosive to the stainless steel, which is vital to the selection of reactor material in the oxidative carbonylation.The electrochemistry performance of Co(Salophen) and Co(Salophen)-Y was investigated by the cyclic voltammetry (CV). Based on the electrochemistry results, the reaction mechanism of oxidative carbonylation was provided, which was different from the mechanism catalyzed by CuCl system. The mechanism is involved in several steps: Co(Salophen) is first combined with oxygen to form -O-O- species, then combined with methanol, CO insert into the complex, finally another methanol molecule nucleophilic attack reaction to form dimethyl carbonate (DMC).In the further research work, nine Co(Salen) complexes were synthesized to the application of oxidative carbonylation of ethylenediamine to 2-imidazolidinone. It was found that the activity of Co(Salophen)(OH)2 is highest. The -OH substituted salicylaldehyde complexed with o-phenylene diamine help to enhance the electron density of the Co(Salophen)(OH)2. The reaction conditions, such as reaction temperature, catalyst concentration and reaction time, were also studied. The conversion of ethylenediamine and the yield of 2-imidazolidinone at the optimal condition are 54.4% and 49%, respectively. The reaction mechanism of oxidative carbonylation of ethylenediamine catalyzed by Co(Salophen)(OH)2 was provided.In a word, the immobilized Co(Salen) catalysts and their homogeneous analogues are efficient, environmental friendly, noncorrosive and recyclable catalytic systems to oxidative carbonylation reaction, and have wide applications in the fields of organic synthesis.