ZnOx overlayer confined on ZnCr2O4 spinel for direct syngas conversion to light olefins

ZnCrOx oxides coupled with zeolites (OXZEO) allow direct conversion of syngas into light olefins, while active sites in the composite oxides remain elusive. Herein, we find that ZnO particles physically mixed with ZnCr2O4 spinel particles can be well dispersed onto the spinel surfaces by treatment in syngas and through a reduction-evaporation-anchoring mechanism, forming monodispersed ZnOx species with uniform thickness or dimension on ZnCr2O4 up to a dispersion threshold ZnO loading of 16.0 wt% (ZnCr2O4@ZnOx). A linear correlation between CO conversion and surface ZnO loading clearly confirms that the ZnOx overlayer on ZnCr2O4 acts as the active structure for the syngas conversion, which can efficiently activate both H2 and CO. The obtained ZnCr2O4@ZnOx catalyst combined with SAPO-34 zeolite achieves excellent catalytic performance with 64% CO conversion and 75% light olefins selectivity among all hydrocarbons. Moreover, the ZnOx overlayer is effectively anchored on the ZnCr2O4 spinel, which inhibits Zn loss during the reaction and demonstrates high stability over 100 hours. Thus, a significant interface confinement effect is present between the spinel surface and the ZnOx overlayer, which helps to stabilize ZnOx active structure and enhance the catalytic performance.