ZnO overlayer confined on ZnCrO spinel for direct syngas conversion to light olefins.

ZnCrO 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 ZnCrO spinel particles can be well dispersed onto the spinel surfaces by treatment in syngas and through a reduction-evaporation-anchoring mechanism, forming monodispersed ZnO species with uniform thickness or dimension on ZnCrO up to a dispersion threshold ZnO loading of 16.0 wt% (ZnCrO@ZnO). A linear correlation between CO conversion and surface ZnO loading clearly confirms that the ZnO overlayer on ZnCrO acts as the active structure for the syngas conversion, which can efficiently activate both H and CO. The obtained ZnCrO@ZnO catalyst combined with SAPO-34 zeolite achieves excellent catalytic performance with 64% CO conversion and 75% light olefins selectivity among all hydrocarbons. Moreover, the ZnO overlayer is effectively anchored on the ZnCrO 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 ZnO overlayer, which helps to stabilize ZnO active structure and enhance the catalytic performance.