Study On Co/ZnO Catalysts For Fischer-Tropsch Synthesis

Fischer-Tropsch synthesis （FT synthesis） is an effect method toconvert syngas derived from coal, natural gas and biomass to clean liquidfuels and high value chemicals. It is considered to be the most effectivetechnology to solve the shortage of liquid fuel. Cobalt-based catasyst isone promising catalyst for FT synthesis, because of its high activity, highselectivity to linear paraffin and low water–gas shift activity.This paper mainly investigated the influences of preparation method,precipitation parameter, promoter Al₂O₃on the performance of catalystCo/ZnO in FT synthesis. Analysis methods of X-ray diffraction （XRD）,Temperature-programmed reduction （TPR）, low temperature N2physicaladsorption, scanning electron microscope （SEM）, infrared spectroscopy（FT-IR） and laser Raman spectroscopy （LRS） have been used tocharacterize the physicochemical properties and study the relationshipbetween catalyst structure and its reaction performance. The results are asfollows：1、Precipitation and hydrolysis method were compared for preparingZnO support. It is found that the interaction between Co and ZnO preparedby hydrolysis method is much stronger, decreasing the reduction degree ofcobalt and leading to the lower activity for FT synthesis. So it is better to ultilize precipitation method to prepare the support. The influences of fourpreparation methods, including impregnation, co-precipitation, sequentialprecipitation and homogeneous precipitation method, were studied on thecatalyst activity. It is found that the preparation method has large impacton the crystallite, pore structure and BET surface area of ZnO, leading tothe different strength of Co and ZnO interaction and property of thecorresponding catalysts. The result shows that the catalyst prepared bysequential precipitation method has the best activity and selectivity. Inaddition, it was investigated about the influence of reduction temperatureon the reactivity of the catalyst prepared by impregnation. When cobaltloading was5wt%, the activity of catalyst is increased first and thendecreased with the increased reduction temperature in the range of230-320C. The optimum reduction temperature is260C.2、The influences of precipitation pH value and aging time had beeninvestigated on the performance of Co/ZnO catalysts. These two factorsare stronger to influence the activity and selectivity of catalyst than theselectivity of C₅+product. It is concluded that the best precipitation pHvalue is6.8, and the best aging time is1h.3、The promotional effect of Al₂O₃has been studied on the propertyof Co/ZnO. The added Al₂O₃has two effects. One is to increase thesurface area which is beneficial to disperse the cobalt, and another is toenhance the interaction of cobalt with support which can stabilise catalyst structure but decrease its reducibility. TPR results prove that theinteraction between cobalt and Al₂O₃is much stronger than that betweencobalt and ZnO. When Al₂O₃content is below5%, catalyst activity doesnot change significantly due to the comparable but opposite influences ofAl₂O₃on the dispersion and reduction of cobalt species. When Al₂O₃content is above5%, reducibility becomes a major factor to influence FTsynthesis. CH₄selectivity of catalysts increases with the adding of Al₂O₃due to the incomplete reduction. Catalyst Co/ZnO-2Al₂O₃shows the bestperformance in FT synthesis, especially its stability.