Study On The Performance Of Carbon Modified Alumina Supported Cobalt Catalyst For Fischer Tropsch Synthesis

The rapid increase in the demand for oil and the depletion of oil have promoted the conversion and utilization of non-oil carbon resources such as coal,natural gas,shale gas,coalbed methane and biogas.However,direct and efficient conversion of these non-petroleum carbon sources into liquid fuels or valuable chemicals remains difficult.One efficient way to convert non-petroleum carbon resources is to produce syngas first and then convert it into liquid fuels or chemicals.Fischer-tropch synthesis（FTS）technology can convert syngas（CO+H₂）to produce hydrocarbons and oxygen-containing compounds with different carbon numbers,which is an important route to produce chemicals and liquid fuels from natural gas,coal and biomass energy.Co/Al₂O₃ as a common catalyst for Fischer-Tropsch synthesis industry has serious deactivation problem.The water,which was one of the product of Fischer-Tropsch reaction.Cobalt nanoparticles may be oxidized to Co O under hydrothermal conditions,forming metal-carrier mixed oxides,or agglomerating into larger particles.Understanding the structure change of Co/Al₂O₃ catalyst deactivation process in Fischer-Tropsch synthesis reaction is helpful to design more stable industrial catalyst or optimize the process conditions.Carbon supports are often used to solve the problem of hydrothermal stability of catalysts because of their hydrophobicity,excellent stability and adsorption capacity.In this thesis,the influence of calcination temperature on the structure and catalyst performance of carbon modified alumina carrier was studied.The stability of cobalt catalyst supported by alumina without carbon layer was compared with that modified by carbon layer under hydrothermal condition.On the basis of hydrothermal stability,the synthesis performance of Fischer-Tropsch catalyst was significantly improved by optimizing carbon content.The detailed research contents of this thesis are as follows:（1）A high specific surface flake alumina carrier was prepared by hydrothermal method.High specific surface flake alumina carrier was prepared by hydrothermal method.Glucose solution was impregnated into the prepared alumina support,and a series of carbon modified alumina carrier materials were obtained by calcination at different temperatures of 600℃,800℃,1000℃and 1200℃（which the carbon content is about 10%.）.The catalyst was obtained by loading Co₃O₄ particles onto a carbon-coated alumina carrier by ultrasonic impregnation.TEM characterization results indicated that cobalt was uniformly dispersed on the carrier.Performance tests showed that the calcination carrier-supported cobalt catalyst at 1000℃had higher CO conversion and heavy hydrocarbon selectivity,but lower methane selectivity.H₂-TPR and XPS characterization results showed that the catalyst calcined at 1000℃had the highest reducibility.It was easier to reduce the active metal cobalt and its catalytic performance was better.（2）The Co/Al₂O₃@C-1000 catalyst with the best catalytic performance in the previous chapter was selected to test the hydrothermal stability with the traditional Co/Al₂O₃ catalyst.Nitrogen physical sorption-desorption,XRD and TEM characterization showed that the structure of alumina collapsed,leading to the migration and sintering of cobalt metal.The catalytic performance of Co/Al₂O₃@C-1000 catalyst is basically constant when a small amount of water vapor is introduced,but it shows a slow deactivation trend with the increase of water vapor.The XRD and TEM characterization of the catalyst after the reaction showed that the structure of the alumina carrier was changed greatly and the cobalt on the surface was partially oxidized to form a composite oxide with the carrier.In order to improve the hydrothermal stability,Co/6C-Al₂O₃-350 catalyst with carbon content of 6%was prepared.Previous studies have shown that too much carbon coating will cover part of the active sites and reduce the catalytic performance.The experimental results show that there is little difference in the catalytic performance of the catalyst modified by carbon layer before water permeation,and the catalytic activity of Co/6C-Al₂O₃-350 catalyst is basically unchanged after water vapor permeation.However,compared with the traditional Co/Al₂O₃ catalyst,the catalytic activity of the catalyst is low.Thus the performance of the catalyst still needs to be improved.（3）In view of the previous part of the study,the carbon content is adjusted to 4%,2%and 1%on the basis of Co/6C-Al₂O₃-350.The results show that the catalyst with carbon content of 2%has the best catalytic performance.At the same time,H₂-TPR characterization of the catalyst is carried out.It can be seen from the results that the reduction temperatures of 4%and 6%are higher than that of Co/Al₂O₃ catalyst,the reduction temperatures of 1%and 2%are lower than those of Co/Al₂O₃ catalyst,indicating that the introduction of a small amount of carbon can reduce the interaction between metal and support and improve the reducibility of the catalyst,while the introduction of excessive carbon will cover part of the cobalt,making it difficult to reduce the catalytic performance.