Acid–base bifunctional catalysis of ceria- and zirconia-based oxides for non-reductive CO2 conversion

Abstract

CeO2- and ZrO2-based oxides have bifunctionality consisting of both relatively weak acidic and basic properties, and such acid–base bifunctionality leads to unique catalytic functions in the non-reductive CO2 conversion with alcohols and/or amines to corresponding organic carbonates, organic carbamates, and urea derivatives, as well as other related reactions. The conversion and product yield in these reactions are generally governed by equilibrium limitation, and the degree of such limitation is strongly influenced by substrates. These reactions have usually been conducted under highly pressurized CO2 conditions, where CeO2 has been demonstrated to be an effective heterogeneous catalyst for these reactions. In the case of reactions with severe equilibrium limitation, high yield of target product can be achieved by combining CeO2 catalysts with the hydration of nitriles to corresponding amides as a means of in situ removal of water and shift of equilibrium toward the product side. Recently, the utilization of low-pressure CO2 contained in the exhaust gas and air has garnered significant attention because pressurization of CO2 is rather energy consuming. In some cases, the reaction order with respect to CO2 pressure over CeO2 tends to be negative, which makes reaction operation under low CO2 pressure favorable in the presence of CeO2 catalysts. As an example, the conversion of CO2-absorbed aliphatic amines without external CO2 is mentioned. Another exemplified reaction introduced here is the reaction of long-chain diols with atmospheric pressure CO2, whose equilibrium limitation is quite severe. For this reaction, the gas-stripping method with CO2 flow to remove water is effective, and polycarbonate diols can be obtained in high yield. In these exemplified reactions operated under low-pressure CO2 (or even without CO2), CeO2 works as an effective catalyst owing to its unique acid–base bifunctionality.

Affiliated Institutions

• Tohoku University JP

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