Role of Zn-Al oxide structure and oxygen vacancy in bifunctional catalyst for syngas conversion to light olefins

Metal oxide plays a vital role in bifunctional tandem-catalyzed CO hydrogenation to light olefins. Herein, a series of Zn-Al metal oxides were prepared and then physically mixed with SAPO-18, the prepared bifunctional catalyst was applied in direct conversion of syngas to light olefins. The effects of structure and oxygen vacancy of Zn-Al oxide were studied by changing the preparation method and the sequence of precipitation. The sequentialprecipitated ZA-SP oxide exhibits the ZnAl2O4 spinel structure and no obvious crystalline phase of gamma-Al2O3 or ZnO. Compared with the reverse- or co-precipitated or the impregnated Zn-Al oxides, ZA-SP oxide possesses the highest oxygen vacancy content and amounts of desorbed CO, which facilitates the CO adsorption and activation. The adsorbed and activated CO on the surface of Zn-Al oxide converted to *CH3O intermediates with the dissociated H species on -Zn-O- sites, and then formed reaction intermediates as in methanol to olefins reaction. After optimizing the calcination temperature of ZA-SP oxide, weight ratio of ZA-SP to SAPO-18, reaction temperature, and gas hourly space velocity, the ZA-SP/SAPO-18 bifunctional catalyst exhibits the highest light olefins space time yield of 10.0 mmol center dot gcat- 1 center dot h- 1 at a CO conversion of 40.2 %, which outperforms the previous reported values.