Promotional Effects of Cesium Promoter on Higher Alcohol Synthesis from Syngas over Cesium-Promoted Cu/ZnO/Al₂O₃ Catalysts

Abstract

The promotional effects of a cesium promoter on higher alcohol (C 2+ OH) synthesis from syngas over Cs 2 O-Cu/ZnO/Al 2 O 3 catalysts were investigated using a combined experimental and density functional theory (DFT) calculation method. In the presence of a cesium promoter, the C 2+ OH productivity increases from 77.1 to 157.3 g kg cat –1 h –1 at 583 K due to the enhancement of the initial C–C bond formation. A detailed analysis of chain growth probabilities (CGPs) confirms that initial C–C bond formation is the rate-determining step in the temperature range of 543–583 K. Addition of a cesium promoter significantly increases the productivities of 2-methyl-1-propanol, while the CGP values (C 3 * to 2-methyl-C 3 *) are almost unaffected. With the assistance of a cesium promoter, the CGPs of the initial C–C bond formation step (C 1 * to C 2 *) increase from 0.13 to 0.25 at 583 K. DFT calculations indicate that the initial C–C bond formation during syngas synthesis over the ZnCu(211) model surface is mainly due to the HCO + HCO coupling. In the presence of Cs 2 O, the stabilities of key intermediates such as HCO and H 2 CO are enhanced, which facilitates both HCO + HCO and HCO + H 2 CO coupling steps with lower activation barriers. In addition, Bader charge analysis suggests that the presence of cesium ions could facilitate nucleophilic coupling between HCO and H 2 CO for the initial C–C bond formation.