Design Of FeAlO Catalysts And Its Catalytic Performance For CO Hydrogenation To Olefins

Light olefin(C2=-C4=)is one of the most important basic raw materials in modern chemical industry.The direct production of light olefins from syngas by Fischer-Tropsch Synthesis(FTS)reaction is a promising technique.In recent years,the one-step selective production of light olefins and linearα-olefin(LAOs)by FTS has been one of the research hot topics in the field of C1 chemistry.The key technology is to control product distribution effectively through catalyst design to achieve high selectivity and stability of catalyst.Due to the different properties of catalysts and the different process conditions,the products obtained by FTS reaction are of great difference.The key technology is to obtain olefin products with high selectivity by adjusting the structure and properties of catalysts.Iron-based catalysts are cheap and tend to generate light olefins in FTS reaction.Iron-based catalysts are usually prepared by precipitation method.Alkali metal hydroxides are commonly used as precipitators throughout the preparation process.Alkali metals,such as residual Na,are frequently left in the precipitation,weakening olefin adsorption,inhibiting olefin secondary hydrogenation,and improving the selectivity of olefins,all of which have a substantial impact on FTS performance.In view of inhibiting the olefin secondary hydrogenation reaction,improving product selectivity and how residual Na affects the CO hydrogenation reaction performance,Fe-Al catalysts prepared by different preparation methods and their effects on FTS performance were investigated in this paper,and alkali metal Na was added.The modulation mechanism of residual Na and added Na on FTS performance of Fe-Al catalyst was investigated.Combined with XRD,SEM,TEM,XPS,H2-TPR,CO-TPD,CO2-TPD and other characterization methods,the influence of FeAlO catalyst on product distribution was explored through systematic study of CO hydrogenation performance and physicochemical properties of the catalysts before and after reaction.The main research contents and results are as follows:(1)FeAlO catalyst was prepared by coprecipitation,hydrothermal and ball milling methods,respectively.SEM and BET characterizations showed that the preparation method affected the crystallization of the catalysts,resulting in different morphology and texture properties of the catalysts.Catalysts with large specific surface area were obtained by ball milling and coprecipitation method.For coprecipitation method,precipitant Na is often difficult to remove,providing alkaline environment for the catalyst.The surface basicity characterized by CO2-TPD proved that residual Na improved the surface basicity of the catalyst and thus the CO adsorption ability was improved significantly,which then improved the catalyst activity and chain growth ability.In CO hydrogenation,the CO conversion is up to 98%,with 57%for LAOs(O/P=6.19)and 13%for CH4 in the hydrocarbon product distribution.(2)The effects of Na loading prepared by impregnation method on the distribution of CO hydrogenation products were investigated.Characterizations showed that residual Na had significant effects on physicochemical properties of iron-based catalysts,such as structural properties,reduction behavior,and catalytic activity and hydrocarbon distribution of FTS reaction.According to H2-TPR,the introduction of impregnated alkali metal Na inhibits the adsorption of H2 on the catalysts,and the residual Na catalyst is easier to reduce due to its small microcrystalline size.According to CO2-TPD results,there are more basic sites in the bulk phase,enhancing the surface basicity,in turn,improving the CO adsorption ability and obtaining stronger competition for CO and H2 adsorption,so that the modified catalyst has lower methane selectivity,high O/P ratio and strong ability of chain growth.The Fischer-Tropsch reaction performance of the catalysts can be significantly improved by adding an appropriate amount of sodium additives.However,when the Fe/Na mole ratio is higher than 20:1,the Fe phase of the catalyst agglomerates on the surface of the catalyst,which led to a significant decrease in catalyst activity.The stability of the catalyst also decreases significantly.Compared with the same loading content of Na,the product distribution in FTS was significantly different from that of the added sodium catalyst.The selectivity of CH4 decreased from 18.7%to 12.8%,the selectivity of C2-C40 decreased from 13.34%to 5.98%,and the O/P value increased from 2.43 to 6.19.The selectivity of linear a-olefin is increased from 40.7%to 57%.