Conversion of syngas to methanol and DME on highly selective Pd/ZnAl_(2)O_(4) catalyst

Supported Pd catalysts with varied Pd loadings(x=0.5 wt%, 2.0 wt%, 5.0 wt%, 7.5 wt%, 15.0 wt%) were prepared by the incipient wetness impregnation method using a ZnAl_(2)O_(4) spinel support.We found that ZnAl_(2)O_(4) supported Pd catalysts with low Pd loadings(e.g., 0.5 wt%) are very selective in syngas conversion to methanol and dimethyl-ether(DME).XRD and TEM characterization shows that,after reduction at350℃,PdZnβ phase with Pd:Zn molar ratio of 1:1 is favored to form predominantly on the spinel support at relatively low Pd loadings,i.e.less than 5.0 wt%, while Pd-rich PdZnα alloy phase exists at Pd loadings above 5.0 wt%.A higher reduction temperature such as 500℃ can facilitate the transformation from PdZnα to PdZnβ phase in those catalysts with high Pd loading.We further found that catalysts with predominant PdZnβ phase are selective in the methanol and DME production from syngas,while the presence of PdZnα phase leads to the notable formation of alkanes byproducts,resulting in reduced methanol and DME selectivity.DME formation from dehydration of methanol depends on the acidity of catalysts,which was found to increase with Pd loading,probably due to the formation of isolated Al_(2)O_(3) as a result of Zn migrating from ZnAl_(2)O_(4) spinel phase to form the PdZn phases with Pd.