Improving the Stability of CeO₂ Catalyst by Rare Earth Metal Promotion and Molecular Insights in the Dimethyl Carbonate Synthesis from CO₂ and Methanol with 2-Cyanopyridine

Abstract

The unmatched efficiency of the direct dimethyl carbonate (DMC) synthesis from CO2 and methanol over CeO2 catalysts in the presence of an organic dehydrating agent (2cyanopyridine, 2-CP) was recently reported with high DMC yield (>90%) in both batch and continuous operations. However, the CeO2 catalyst gradually deactivates in the timescale of days due to suggested surface poisoning by 2-picolinamide (2-PA) produced by hydration of 2-CP. This work seeks for active and stable CeO2-based catalysts and aims to understand the material factors influencing the catalytic performance. Surface modification of CeO2 by the addition of rare earth metal (REM) was found effective to improve the catalyst stability. Surface basicity and reducibility of the Ce 4+ species play important roles in preventing catalyst deactivation by stabilizing the reactive methoxy species in comparison to the poisoning species (2-PA or species alike). This has been evidenced by in situ ATR-IR spectroscopy. CeO2 materials promoted with 1 wt% rare earth metals (La, Gd, and Pr) greatly enhanced the catalyst stability while retaining the high catalytic activity of CeO2. Among them, 1 wt% Pr promotion to CeO2 was the most effective, affording 35% higher DMC yield in comparison to bare CeO2 after 150 h time on stream under the optimized reaction condition of 30 bar and 120 °C.

CO2 Conversion with Alcohols and Amines into Carbonates, Ureas, and Carbamates over CeO2 Catalyst in the Presence and Absence of 2‐Cyanopyridine

“…Figure summarizes mechanistic views on the active sites and deactivation of CeO 2 during the DMC synthesis from CO 2 and methanol in the presence of 2‐cyanopyridine . Methanol molecules adsorb over CeO 2 as t ‐OCH 3 and b ‐OCH 3 species (Figure a).…”

“…As shown in Figure , all the more basic catalysts with the basicity/acidity ratio larger than 1 such as CeO 2 and REM‐promoted CeO 2 have high initial catalytic activity. On the other hand, more acidic catalysts with basicity/acidity ratio <1 such as CeO 2 −ZrO 2 solid solutions, ZrO 2 , and PM (Precious metal)‐CeO 2 series exhibited lower activity …”

“…Furthermore, it has been recently reported that surface modification of CeO 2 by the addition of rare earth metals (REM) at relatively small modifier amount such as 1 wt % rare earth metals (La, Gd, and Pr) is effective for improving the catalyst stability while retaining the high catalytic activity (Figure ) …”

“…In situ ATR‐IR study was also conducted by alternatingly passing a vapor containing 2‐cyanopyridine and methanol and that of methanol over CeO 2 and 1 wt % Pr−CeO 2 at 393 K to monitor surface chemical processes . For CeO 2 , the methoxy bands appeared strongly at the initial stage of the introduction of 2‐cyanopyridine+methanol, and the intensity of the bands characteristic of 2‐picolinamide anion as the surface intermediate in the hydrolysis of 2‐cyanopyridine to 2‐picolinamide decreased gradually with the time on stream.…”

Direct Phenolysis Reactions of Unactivated Amides into Phenolic Esters Promoted by a Heterogeneous CeO2 Catalyst

“…This catalytic system not only offers a facile means to cleave the amide C−N bond, but also significantly promotes the use of amides and phenols as important building blocks for the synthesis of valuable esters. Moreover, the obtained results provide a better understanding of the catalytic behavior of CeO 2 , which has been of significant interest recently …”