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

The promotional effects of a cesium promoter on higher alcohol (C2+OH) synthesis from syngas over Cs2O-Cu/ZnO/Al2O3 catalysts were investigated using a combined experimental and density functional theory (DFT) calculation method. In the presence of a cesium promoter, the C2+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 Cs2O, the stabilities of key intermediates such HCO and H2CO are enhanced, which facilitates both HCO + HCO and HCO + H2CO 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 H2CO for the initial C C bond formation.