The Study On Chemical Conversion Of Carbon Dioxide Catalyzed By Bifunctional Niobium Complex Or EDTA

Carbon dioxide is one of the main gases that cause the greenhouse effect.Excessive carbon dioxide which was emitted by human beings will cause the global warming and have a great impact on the environment.However,carbon dioxide is also C1resource with abundant reserves,which can be converted into high value-added chemical products.Those compounds are widely used in industry,medicine,food and other fields.Therefore,chemical conversion of carbon dioxide is an effective method to rationally utilize carbon dioxide and reduce carbon dioxide emissions.Cycloaddition reaction of carbon dioxide and epoxides to synthesize cyclic carbonate is one of the ways to carry out chemical conversion of carbon dioxide.And the catalyst of the reaction is an important factor influencing the reaction.Because the bifunctional catalyst contains Lewis acid center and Lewis base center,it can promote the activity of reaction through synergistic effect.Therefore,a series of bifunctional niobium catalyst was synthesized and applied to the cycloaddition reaction of carbon dioxide and epoxides.The structures of the catalysts were characterized by NMR,FTIR and elemental analysis.Research shows that the catalyst system can efficiently catalyze the cycloaddition reaction of propylene oxide?PO?and carbon dioxide to generate propylene carbonate?PC?with high selectivity?>99%?without adding any solvent and cocatalyst.The research results of the influence of the catalyst structure on the reaction activity showed that the order of the catalyst activity with different substituents is methyl>ethyl>butyl,and the order of the catalyst activity with different halogen ions is Cl^—<Br^—<I^—.The effects of reaction parameters such as reaction temperature,carbon dioxide pressure,reaction time and catalyst dosage on the reaction were systematically studied.The optimum reaction conditions were screened as follows:100 ^oC,1 MPa CO₂,2 h,and the ratio of catalyst to alkylene oxide was 1:100.Under these conditions,the conversion rate of epoxide was 96%,and the selectivity of PC was nearly 100%.Moreover,under the optimal reaction conditions,the catalyst 3a can catalyze the cycloaddition reaction of carbon dioxide with various epoxides.More importantly,the catalyst 3a can catalyze the coupling reaction of carbon dioxide with epoxide under atmospheric pressure.Additionally,the catalyst could be easily reused only with slight loss of activity after five recycles.Moreover,kinetic studies were carried out preliminarily for three metal?3a,3b,3c?catalysts and the formation activation energies?E_a?of cyclic carbonate were obtained.The apparent activation energy E_a catalyzed by 3c is only 37.4 kJ/mol,while the E_a for 3b is 68.2 kJ/mol and Ea of 3a is 96.2 kJ/mol.The sequence of Ea agrees well with the catalytic activity.Finally,a possible mechanism was proposed,in which the one-component bifunctional catalyst could activate epoxide via the interreaction of M-O and open the ring of epoxide by nucleophilic attack with X^—ions.Meanwhile,ethylenediamine tetraacetic acid?EDTA?,which is cheap and easily available,is used as a catalyst to catalyze the formylation reaction of carbon dioxide and amines to generate formamides with high selectivity.Carboxyl and nitrogen atoms in EDTA play an important role in the catalytic reaction,which has been proved by the research on organic matters containing different functional groups.The effects of reaction parameters such as reaction temperature,carbon dioxide pressure,reaction time,catalyst dosage,reductant type and solvent were systematically studied.The optimal reaction conditions were as follows:catalyst EDTA 50 mol%,substrate 5mmol,PhSiH₃ 5mmol,DMSO 1 mL,reaction temperature 80 ^oC,CO₂ pressure 2 MPa,reaction time 6 h.Under optimal reaction conditions,the conversion rate was 96%.Under these conditions,the conversion rate of the reaction of carbon dioxide and N-methylaniline was 96%,which was catalyzed by EDTA.Furthermore,the formylation activity of EDTA on different amines was studied.The results showed that EDTA could catalyze the formylation of carbon dioxide and various amines,including para-nitro-substituted aromatic amines which were difficult to formylate with carbon dioxide.It is especially worth pointing out that EDTA can catalyze the formylation of cyclic amines with carbon dioxide,and the corresponding formylation products can be obtained with high yield and high selectivity under normal temperature and pressure?catalyst EDTA 50 mol%,substrate 2.5 mmol,PhSiH₃2.5mmol,DMSO 2 mL,reaction temperature 25 ^oC,CO₂ pressure 0.1 MPa,reaction time 12 h?.The EDTA can be simply separated and reused by adding solvent?dichloromethane?,and can be reused for five times without obvious activity loss.Finally,according to the experimental results and relevant literature,the possible reaction mechanism is proposed.