Effect of Al2O3/SiO2 ratio on iron-based catalysts, for Fischer-Tropsch synthesis

A systematic study was undertaken to investigate the effects of Al2O3/SiO2 ratio on reduction, carburization and catalytic behavior of iron-based Fischer-Tropsch synthesis (FFS) catalysts promoted with potassium and copper. The catalysts were characterized by N-2 physical adsorption, CO2 temperature-programmed desorption (TPD), H-2 temperature-programmed reduction (TPR) and Mossbauer effect spectroscopy (MES). CO2-TPD indicated that Al2O3 binder has stronger acidity than SiO2 binder and weakens the surface basicity of the catalysts. H-2-TPR profiles suggested that the lower Al2O3/SiO2 ratio promotes the reduction of Fe2O3 --> Fe3O4. With further increasing Al2O3/SiO2 ratio, the transformation of Fe2O3-->Fe3O4 shifts to higher temperatures. The MES results showed that the increase of Al2O3/SiO2 ratio leads to the relatively large crystallite size of alpha-Fe2O3 and inhibits carburization of the catalyst. During reaction tests in a fixed bed reactor it was found that a maximum in catalyst activity is noted at the Al2O3/SiO2 ratio of 5/20 (weight basis). The selectivity to olefins shows a rapid decrease and the formations of methane and light hydrocarbons are promoted with increasing Al2O3/SiO2 ratio. The oxygenate selectivity in total products increases with increasing Al2O3/SiO2 ratio. (C) 2006 Elsevier Ltd. All rights reserved.