(196e) Efficient Fixation of CO2 into Cyclic Carbonate Catalyzed By Ionic Liquids in Microchannels

Carbon dioxide (CO₂) is a greenhouse gas, the increase of CO₂ concentration will not only lead to global warming, but will also cause frequent extreme weather events.¹ However, CO₂ is still an abundant C1 resource, and converting CO₂ into chemicals with high added value is an attractive approach to carbon-neutral. The conversion of CO₂ with epoxide to cyclic carbonate, which can be used as lithium ion battery components, is an atom-economic and effective CO₂ utilization route.² Ionic liquids (ILs) have gained widespread attention as green solvents and new catalytic systems due to their excellent chemical and thermal stability.³ However, the cycloaddition of CO₂ with epoxides is a typical gas-liquid multiphase reaction involving both gas-liquid mass transfer and chemical reaction, which is commonly carried out in batch reactors for a long time under high temperature and pressure conditions with low efficiency.⁴ In recent years, micro-reactor technology has been developed as a novel approach for process intensification, and could hopefully enhance this CO₂ fixation reaction. The interfacial area (a) and the mass transfer coefficient of liquid-phase (k_L) are enhanced in microreactors due to the small channel size.⁵

Cyclic carbonate was synthesized by CO₂ cycloaddition with ionic liquid as catalyst in a 2mm microchannel reactor. The impact of mass transfer and intrinsic kinetics in the microchannel reactor on the ionic liquid-catalyzed coupling reaction of carbon dioxide (CO₂) and propylene oxide (PO) for the synthesis of propylene carbonate (PC) were investigated by examining the influence of various catalysts, catalyst concentration, reaction temperature, CO₂/PO molar ratio, operating pressure, and residence time on the system performance.

A series of imidazole based ionic liquids catalysts were screened for the synthesis of PC, and the discoveries revealed that 1-hexyl-3-methylimidazolium bromide (HMIMBr) with longer alkyl chains and stronger nucleophilic ability had the best catalytic activity. At 125°C and 2.5 MPa, PC yields of more than 99% can be achieved in a 2mm micro-reactor with a residence time of 5 minutes. Subsequently, the kinetics of CO₂ fixation into cyclic carbonate catalyzed by [Hmim]Br were investigated in the microchannel reactor. The activation energy for [Hmim]Br at the temperature range from 65°C to 125°C was calculated as 56.86KJ/mol and 62.55 kJ/mol for 1mm and 2mm micro-reactor, respectively, and the reaction rate showed a first-order dependence on PO. The decrease in reactor size from 2 mm to 1 mm inner diameter resulted in an approximately double improvement in catalytic performance and a slight decrease in reaction activation energy, which was mainly attributed to the increase in mass transfer efficiency. And the specific area of microchannel was found to range from 1000 to 10000 m²/m³, and the volumetric mass transfer coefficient (k_La) was 1 to 2 orders of magnitude higher than that of ordinary gas-liquid contactors. The present work was compared with the results reported in the literature and is summarized in Fig.1(c). In general, the reactions of CO₂ and epoxide are usually carried out in batch reactors, either under stringent operating conditions or with poor results. Thus, compared to conventional reactors, micro-reactors weaken the mass transfer limitations, but for the typical fast inversion process of CO₂ transition to cyclic carbonate, the reaction performance is still mainly controlled by mass transfer. Increases in operating temperature, pressure and catalyst concentration optimize the reaction, and residence time can be drastically reduced from several hours in a conventional batch reactor to about a few minutes in a microreactor.

In brief, despite the mass transfer limitations in microchannel reactors can be significantly attenuated compared to conventional reactors, the reaction performance of CO₂ and PO is still considerably affected by mass transfer. This provides new opportunities for enhanced gas-liquid reactions with typical mass transfer control and for CO₂ utilization to achieve carbon-neutral, but the development of more efficient microreactors remains challenging. For highly efficient microreactor development, more dedicated research is needed to reach a compromise between mass transfer and intrinsic kinetics.

Fig1. (a) Schematic of the experimental setup for the cycloaddition of PO and CO₂. (b) Effect of the catalyst on the yield and STY. (c) Diagram of the Arrhenius curve of ln k_obs versus 1/T for 1 mm and 2 mm microchannel reactors. Reaction conditions: 125℃, Catalyst/H₂O/PO(mol)=0.14/0.25/1, 2.5 MPa, Q_G=176ml/min(STP), Q_L=0.5ml/min. (d) Comparison of microchannel reactor with other traditional reactors.

This work was supported by the National Key Projects for Fundamental Research and Development of China (2020YFA0710202).

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