ZnO [sbnd]Cr 2O 3 + ZSM-5 catalyst with very low Zn/Cr ratio for the transformation of synthesis gas to hydrocarbons

Abstract

Three Zn Cr mixed oxide plus ZSM-5 compound catalysts with different compositions covering a wide range of Zn/Cr atomic ratios (0.064 to 1.913), aimed at the direct conversion of synthesis gas into hydrocarbons, were tested under different experimental conditions: temperature ranged from 356 to 410°C, pressure from 3.60 to 4.49 MPa and space velocity from 0.2 to 3.0 mmol reactants/(g cat min). Reaction products included carbon dioxide, water and hydrocarbons with methanol conversion (through which hydrocarbons are formed) being complete. The catalyst with the least content of zinc gave the highest yields of liquid hydrocarbons (up to 74% of total hydrocarbons). Different crystalline phases (ZnO, ZnCr 2O 4 and Cr 2O 3) were found in the methanol synthesis component as a function of Zn/Cr ratio. The low Zn/Cr catalyst was characterized by X-ray diffraction, differential thermal analysis, nitrogen adsorption, temperature-programmed reduction, Infrared and X-ray photoelectron spectroscopy. The only phases observed in this catalyst were ZnCr 2O 4 and Cr 2O 3. Calcination temperature had an influence in both physical and chemical catalyst properties. After calcination, Cr VI and Cr III species could be seen on the catalyst surface, but only Cr III species were observed after reaction or reduction. The evidences gathered suggest that Cr 2O 3 is mainly responsible for methanol synthesis, while ZnCr 2O 4 contributes to increase the specific surface area of the catalyst and influences gas product distributions. Compound catalysts with the mixed oxide (Zn Cr) as the methanol synthesis component showed to be more active than those with the individual (Cr or Zn) oxides.