Direct conversion of syngas to aromatics via a two-stage C–C coupling over MnZr/HZSM-5 bifunctional catalysts employing OX-ZEO strategy

Abstract

Even though the oxide–zeolite (OX-ZEO) strategy is generally applied for the direct conversion of syngas into aromatics (STA), previous researches have rarely studied the evolution of the surface species. In this work, MnZrOx binary oxides were synthesized and bifunctional catalysts were prepared by physically mixing powders of the binary oxides with H-ZSM-5. In situ diffuse reflection Fourier transform infrared spectroscopy was performed, and surface C–C coupling between the alkyl and formate groups over the surface of the oxides was observed, which was named as primary C–C coupling. As products of the primary C–C coupling, the higher alkyl carboxylate groups could be hydrogenated to surface aldehydes, and both the species could diffuse into the zeolite for secondary C–C coupling, thereby generating aromatics. The reaction pathway of CO was shortened by the insertion of surface formate, thus promoting CO conversion. Under optimal reaction conditions, the CO conversion reached 55% with the selectivity of aromatics exceeding 80%. Coke deposition was also observed on the external surface of H-ZSM-5, which might be the reason for the further decline in CO conversion.