Electrochemical Fixation Of Carbon Dioxide With Alcohls To Synthesize Of Organic Carbonates

CO2is one of the main greenhouse gases. Also, CO2is well believed to be a nontoxic, cheap, abundant, renewable C1source. Utilization of CO2not only can solve the environmental problems caused by greenhouse effect, but also could alleviate the increasingly serious problem of energy depletion. But CO2is highly thermodynamic stable and kinetically inert. How to activate CO2effectively has become a key problem. Electrochemical fixation of CO2is one of the effective routes of CO2chemical fixation which could be achieved under room temperature and normal pressure.One of the effective routes that utilize CO2is the synthesis of organic carbonates. Organic carbonates are a very important class of compounds whose versatility allows their applications in several fields of the chemical and pharmaceutical industry. Many catalysts such as metal alkoxylate, organic/inorganic alkali, acid salt and metal oxide can catalyze CO2reaction with alcohols to synthesize of organic carbonates, under high temperature and high pressure. But these processes generally have several drawbacks, such as poor catalytic activity, low target product yield, harsh reaction conditions and so on. Therefore, more green synthesis route is highly desirable.Recently, room temperature ionic liquids (RTILs) are widely used in electrochemical research for their special physical and chemical properties. It has also been reported that CO2is remarkably soluble in room temperature ionic liquids, which, therefore, makes RTILs become good reaction medium for electrochemical fixation of CO2. N-heterocyclic carbenes (NHCs), which can be use as molecules catalyst for a variety of organic reactions without metal participation and be friendly to environmental, could be generated by cathode reduction of RTILs. Recent studies have found that NHCs could effectively activate CO2to form stable NHC-CO2adducts which can be used to activate CO2transformation. Under mild conditions, electrochemical fixation of greenhouse gas CO2to synthesis of useful organic carbonates has important significance. Especially, using ionic liquids as green reaction medium and precursors of NHCs for synthesis organic carbonates have certain innovation significance and practical value.The details are given as follows:(1) Direct electrosynthesis of organic carbonates from CO2with alcohols under mild conditionsElectrosynthesis of organic carbonates from CO2with alcohols has been carried out directly under galvanostatic condition at room temperature and normal pressure in DMF/CH3CN-TBAI (0.1mol L-1) without any additional catalyst. For the model compound methanol, the effects of solvents, electrode materials, current densities, charge amount, CH3OH concentration and temperature have been investigated. Under the optimized conditions, the effectiveness and generality of the proposed methodology has been examined. Primary and second alcohols are converted into corresponding linear organic carbonates with moderate and low yields, whereas tertiary alcohol and phenol are unreactive. Moreover, cyclic carbonates could be synthesized from CO2and diols. According to our experiment results and literature, the reaction process has been discussed. This work studies direct electrosynthesis of organic carbonates from CO2in organic solvent completely, which has a certain reference value.(2) Synthesis of cyclic carbonates from CO2and diols via electrogenerated cyanomethyl anionElectrosynthesis of cyclic carbonates from CO2and diols has been carried out under galvanostatic condition at room temperature and normal pressure in a divided cell via electrogenerated cyanomethyl anion. The reported methodology has been carried out under mild conditions, avoiding any addition of classical bases, catalysts. The effects of reaction conditions, such supporting electrolyte, cathode material, current densities, charge amount, diol concentration, have been investigated to optimize the carbonates yield. Moreover, the effectiveness and generality of the proposed methodology has been examined. (3) Electrosynthsis of cyclic carbonates from CO2and diols in ionic liquids under mild conditionsAnew electrochemical process for synthesis of cyclic carbonates from CO2and diols has been tested in CO2-saturated room temperature ionic liquids (RTILs) for the first time. The synthesis has been carried out under mild and safe condition in an undivided cell without any additional catalysts. The influence of reaction conditions, such as electrode materials, electrolysis potential, temperature, diol concentration, composition of RTILs and ultrasound, have been investigated. The recyclability of RTILs also has been confirmed. Moreover, the electrochemical reduction behavior has been studied by cyclic voltammetry. This work provides a green synthesis approach for cyclic carbonate.(4) Synthesis of cyclic carbonates from CO2and diols via electrogenerated N-heterocyclic carbenesN-heterocyclic carbenes, which could react with CO2to form NHC-CO2adducts, could be generated by cathodic reduction of ionic liquid in a divided cell. The formation of NHC-CO2has been supported by electrochemical analysis. This work mainly describes the utilization of NHC-CO2which transfers of CO2to diols for the synthesis of cyclic carbonates under mild conditions. For the model compound styrene glycol, various conditions such as cathode materials, charge amounts, current densities, temperature, stirring times were studied to optimize conditions. Under optimized conditions,60%yield of styrene carbonate was obtained. The effectiveness and generality of the proposed electrochemical methodology has been examined under optimized conditions. Moreover, the reaction mechanism has been discussed. This study provides a new method for the synthesis of cyclic carbonates of chemical and pharmaceutical interest, as well as expands on the applications of NHCs in CO2fixation.(5) Synthesis of dialkyl carbonates from CO2and alcohols via electrogenerated N-heterocyclic carbenesThe monoelectronic cathodic reduction of an imidazolium cation leads to the formation of N-heterocyclic carbenes (NHCs). Stable NHC-CO2adducts formed from CO2and NHCs was used in transcarboxylation reactions. Based on the previous work about synthesis of cyclic carbonates from CO2and diols via electrogenerated N-heterocyclic carbenes, here, we studied the synthesis of linear carbonates via electrogenerated N-heterocyclic carbenes from CO2and alcohols. For the model compound benzyl alcohol, various conditions were studied to optimize conditions. Under optimized conditions,90%conversion and96%selectivity were obtained. Ionic liquid plays a dual role of green solvent and NHCs precursor in the reaction process, avoiding the use of toxic volatile organic solvent and supporting electrolyte. This work provides a new synthetic route for linear carbonate, but also further expands the application of NHCs in CO2fixation.