Study On Co-based Fischer-Tropsch Synthesis Catalyst By Wet Chemical Method

Fischer-Tropsch synthesis?FTS?technology applied to coal-to-liquids is an efficient solution to crude oil shortage and a significant response to the clean energy development.This study,focusing on Co-based catalyst for F-T synthesis,mainly includes two parts.Part one investigates the effect of promoters on the structure and performance of Co/SiO₂ catalysts,including the varieties and contents of the promoters.Part two study the Co/ZnO catalysts prepared by chemical precipitation method,which focus on the effect of precipitation temperature,precipitants and Ti promoter.The main results can be summarized as follows:1)The SiO₂ is decorated by nitrites of Zr,A1 and Ti and then used as supports of the Co catalysts.The Zr-modified catalysts turn out to be most active,while the Ti-modified catalysts have lowest selectivity for CH₄ and highest selectivity for C5+.Characterization results indicate that Zr modification reduces the interaction between Co and the carrier,which elevates the reducibility of the catalysts.Some reduction-resistant cobalt aluminates are produced when A1 is added to the catalysts,resulting in a suppression of the catalyst activity.In the case of Ti modification,though the reduction peak moves towards higher temperature,the dispersion of Co3O4 on the carrier surface significantly improve.2)The impact of Ti content on CO₁₅/SiO₂ catalyst has been further clarified.A small amount of Ti can effectively enhance the conversion of CO but also reduces the selectivity for CH₄ and CO₂;however,excess Ti content will lead to a decrease of the catalyst activity.The reason is that a small amount of Ti can enhance the interaction between active constituents and carrier and impro've the dispersion of active metal.Excessive addition of Ti,however,leads to a strong interaction,which greatly undermines the reducibility of the catalyst.3)Ru promoter has been introduced to Co₁₅/Ti_2.5-SiO₂ catalyst.A small amount of Ru will obviously increase the catalyst activity while maintain the selectivity for CH₄ and C5+.Nevertheless,the byproduct of CH₄ distinctly increases with Ru addition increasing to 1%,while the selectivity for C5+ decreases.The stability test results ascertain that the activity of the optimized catalysts greatly reduced after 30 h reaction.XRD characterization reveals that the deactivation is resulted by aggregation and sintering of active constituents,which transforms CoO into CoSi2O4 and leads to the loss of active sites.4)Co/ZnO catalysts prepared with chemical precipitation method and the effect of precipitation temperature and precipitants have been investigated.Experimental results show that the catalysts prepared at 90 ? yield highest conversion while those prepared at 70 ?exhibit lowest selectivity for CH₄ but highest selectivity for C5+.Characterization results prove that precipitation temperature has no bearing on the interaction between the carrier and active constituents,while a higher precipitation temperature results in a smaller pore size.The activity of catalysts prepared with?NH₄?2CO3 is higher than those prepared with CO?NH2?2,while those prepared with Na2CO3 distinctly reduce the CO conversion in F-T synthesis.The presence of Na+ may account for this as it can seriously inhibit the catalytic activity.5)The influence of Ti on Co₁₀/ZnO catalyst has been studied in the same way.The results demonstrate that proper amount of Ti can promote the catalytic activity.When Ti totals 0.5%,the catalyst reaches the highest catalytic activity and C5+selectivity.Characterization results indicate that Ti is able to increase the specific surface area of the Co₁₀/ZnO catalysts,which strengthens the interaction between active constituents and the carrier,and enlarges the dispersion of active constituents;however,excess Ti will lead to diminish of Co3O4 reducibility.Moreover,reaction conditions of the Co₁₀/Ti0.5-ZnO catalyst have been optimized,low GHSV enables reactants and intermediate to stay longer at the surface of catalysts,which improves the CO conversion and extends the alkyl chain of hydrocarbon products.High temperature enhances the catalytic activity but leads to desorption of intermediate at the catalyst surface,results in a disadvantage of chain elongation.The stability test proves that small specific surface area and poor heat dissipation may cause aggregation and sintering of active constituents,which is detrimental to the reactive stability of the catalysts.