Size Effects of ZnO Nanoparticles in Bifunctional Catalysts for Selective Syngas Conversion

The oxide-zeolite (OX-ZEO) bifunctional catalyst design concept has shown promising potential for selectivity control in syngas conversion by separating CO activation and C-C coupling onto oxides and zeolites, respectively. However, the structural effects of the oxides are far from being understood. Herein, we take a single component, ZnO, as a probe oxide in combination with SAPO-34 as a composite catalyst for light olefins synthesis from syngas. The space time yield of hydrocarbons significantly increases as the ZnO crystal size decreases from 79 to 23 nm, which can be attributed to more active centers available over smaller particles with a higher specific surface area. The apparent turnover frequency (TOF) for formation of light olefins (TOFfight olefins) increases over smaller ZnO particles, whereas TOFlight paraffins decreases, leading to an increased ratio of light olefins/light paraffins, which indicates that smaller particles benefit the formation of olefins. These results demonstrate that it is crucial to control the particle size of oxides to design more active and selective OX-ZEO catalysts for syngas conversion to olefins.