Design of efficient bifunctional catalysts for direct conversion of syngas into lower olefins via methanol/dimethyl...

The direct conversion of syngas into lower olefins is a highly attractive route for the synthesis of lower olefins. The selectivity of lower olefins via the conventional Fischer-Tropsch (FT) synthesis is restricted to ∼60% with high CH₄ selectivity due to the limitation by the Anderson-Schulz-Flory (ASF) distribution. Here, we report the design of bifunctional catalysts for the direct conversion of syngas into lower olefins with selectivity significantly breaking the ASF distribution. The selectivity of C₂-C₄ olefins reached 87% at a CO conversion of 10% and was sustained at 77% by increasing CO conversion to 29% over a bifunctional catalyst composed of Zn-doped ZrO₂ nanoparticles and zeolite SSZ-13 nanocrystals. The selectivity of CH₄ was lower than 3% at the same time. It is demonstrated that the molar ratio of Zn/Zr, the density of Brønsted acid sites of SSZ-13 and the proximity of the two components play crucial roles in determining CO conversion and lower-olefin selectivity. Our kinetic studies indicate that methanol and dimethyl ether (DME) are key reaction intermediates, and the conversion of syngas to methanol/DME is the rate-determining step over the bifunctional catalyst. Formate and methoxide species have been observed on Zn-doped ZrO₂ surfaces during the activation of CO in H₂, and the formed methanol/DME are transformed into lower olefins in SSZ-13.