Effect of Al_2O_3/SiO_2 ratio on iron-based catalysts for Fischer-Tropsch synthesis

摘要

A systematic study was undertaken to investigate the effects of Al_2O_3/SiO_2 ratio on reduction, carburization and catalytic behavior of iron-based Fischer-Tropsch synthesis (FTS) catalysts promoted with potassium and copper. The catalysts were characterized by N_2physical adsorption, CO_2 temperature-programmed desorption (TPD), H_2temperature-programmed reduction (TPR) and Mossbauer effect spectroscopy (MES). CO_2-TPD indicated that Al_2O_3 binder has stronger acidity than SiO_2 binder and weakens the surface basicity of the catalysts. H_2-TPR profiles suggested that the lower Al_2O_3/SiO_2 ratio promotes the reduction of Fe_2O_3- > Fe_3O_4. With further increasing Al_2O_3/SiO_2 ratio, the transformation of Fe_2O_3 - > Fe_3O_4 shifts to higher temperatures. The MES results showed that the increase of Al_2O_3/SiO_2ratio leads to the relatively large crystallite size of alpha-Fe_2O_3 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 Al_2O_3/SiO_2 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 Al_2O_3/SiO_2 ratio. The oxygenate selectivity in total products increases with increasing Al_2O_3/SiO_2 ratio.