Synthesis Of Dimethyl Carbonate From Supercritical CO₂ And Methanol Catalyzed By Transition Metal Oxide Through One-step Method

In this paper,the selection of catalysts and the reaction mechanism about one-step synthesis for Dimethyl carbonate?DMC?using supercritical CO₂ and methanol?CH₃OH?as reactants are studied.CO₂ is more than a reactant,which could also provide a supercritical CO₂ atmosphere to better contact and react with CH₃OH.The advantage of supercritical fluid technology can be combined with this traditional one-step synthesis DMC process route,at the same time,the transition metal oxide which shows good catalytic activity loaded on the supporter is added into this reaction system in order to improve the production rate of the DMC synthesis.Firstly,the Gibbs free energy thermodynamic calculation of this reaction is carried out,the results show that the reaction under atmospheric pressure and room temperature cannot occur spontaneously,and thus to change the reaction temperature and pressure conditions in order to prompt this reaction.Then,the catalyst that has the best catalytic performance is screened out based on the yield of DMC.The experimental results show that Y₂O₃ as the active component loaded on the coconut shell charcoal represents the highest catalytic activity.The activity of the catalyst is evaluated by the specific surface area test?BET?,X-ray diffraction?XRD?,SEM characterization,CO₂/NH₃-TPD test and thermogravimetric analysis?TGA?.The conclusions are as follows:the surface of coconut shell charcoal increased after calcination;The SEM shows that the porous structures are opened after calcination,and the pores are arranged in a more orderly way.According to XRD,Y₂O₃ crystal is formed while Y?NO₃?₃·6H₂O is calcined under a high temperature;at the same time,according to the CO₂/NH₃-TPD test results,Y₂O₃ has the acid/base sites that can activate CO₂ and CH₃OH.Through thermogravimetric data analysis,XRD,CO₂/NH₃-TPD tests of Y?NO₃?₃·6H₂O,and the comparison of DMC yields,the best calcination temperature of Y?NO₃?₃·6H₂O is 700 ^oC.Considering methyl iodide?CH₃I?and potassium methanol?CH₃OK?can directly provide methyl positive ion?CH₃⁺?and methoxyl anion?CH₃O^-?that participate in the synthesis progress,we introduce them into the reaction system.When the loading of Y₂O₃ is 15%,the input of CH₃I and CH₃OK is 2 g respectively,and the yield of DMC is up to 21.7%under optimal operating conditions.In addition,Y₂O₃/coconut shell charcoal has barely catalytic activity after undergoing 5 repeated experiments.