Unveiling the Anti-Trap Effect for Bridging Intermediates on ZnAlOx/AlPO-18 Bifunctional Catalysts to Boost Syngas to Olefin Conversion

The migration of bridging intermediates (methanol) from oxides to zeolites is crucial in the direct conversion of syngas to light olefins via the tandem process. As the initiation step of this migration, the adsorption and desorption effect of methanol on oxide components remains elusive. Herein, we systematically compared the catalytic performances of the syngas conversion on a series of ZnAlOx oxides with different Zn/Al ratios and the corresponding bi-component catalysts integrated with the AlPO-18 zeolite component. The structures and surface acidity of ZnAlOx oxides were characterized by XANES, STEM, and py-IR, and the adsorption behavior of methanol on ZnAlOx oxides was characterized by in situ infrared spectroscopy. It is confirmed that the ZnAl2O4 spinel surface is the intrinsic active site for CO activation while the strong Lewis acid sites of the oxide surface adsorb methanol strongly, serving as traps to inhibit methanol desorption. The additional Zn-containing phase in ZnAlOx oxides with the Zn/Al ratio higher than the stoichiometry of spinel ZnAl2O4 is identified as the ZnO component to cover the strong acid sites of the oxide surface and thus enhance the desorption of methanol, benefiting toward the higher catalytic performance of the syngas conversion. A high CO conversion of 41.6% with a C2- C4 olefin selectivity of 79.3% is achieved when the Zn/Al ratio of oxide is 1, and the C2-C4 olefin space time yield reaches 147.0 g kgcat-1 h-1. The anti-trap effect for bridging intermediates on oxide components is thus proposed in bifunctional catalysis, which serves as an effective strategy to exploit and design more efficient multifunctional catalytic systems.

Automated summary

This study systematically compared the catalytic performances of ZnAlOx oxides with different Zn/Al ratios and corresponding bi-component catalysts in the syngas conversion. It was found that the ZnAl2O4 spinel surface is the active site for CO activation while the strong Lewis acid sites of the oxide surface adsorb methanol strongly, inhibiting its desorption. The additional Zn-containing phase in ZnAlOx oxides with a higher Zn/Al ratio acts as a ZnO component to cover the strong acid sites and enhance methanol desorption, leading to higher catalytic performance. The anti-trap effect for bridging intermediates on oxide components is proposed as an effective strategy in bifunctional catalysis.