Study On Hydrogenation Of Carbon Dioxide-Oligomerization To Long Chain Hydrocarbons

The preparation of liquid hydrocarbon fuel from CO₂ hydrogenation has important military significance and can be used as energy storage technology of renewable energy,which is of great significance for energy technology and sustainable development.Generally,iron-based catalysts can produce more small molecular olefins,but the selectivity of long-chain hydrocarbons is poor.In this paper,based on the study of preparation methods of iron-based catalyst and suitable CO₂ hydrogenation conditions in the fixed-bed reactor,the possibility of relay catalysis between iron-based catalyst and molecular sieve catalyst was explored to improve the selectivity of long chain hydrocarbons in the final product through the series reaction of"CO₂ hydrogenation-oligomerization of small molecular olefins".In this paper,Fe-Mn-A/Al₂O₃（A:alkali metal）was prepared by mechanical mixing method and co-precipitation method.The effects of different alkali metal promoters（Na and K）and different iron precursors（Fe₂O₃ and Fe₃O₄）on CO₂ hydrogenation performance were studied.The experimental results show that the addition of alkali metal promoter is beneficial to improve the CO₂ adsorption capacity and hydrogenation conversion rate of iron-based catalyst,and the performance of potassium（K₂CO₃）and sodium（Na OH）is almost the same;Fe₃O₄ is more suitable as the precursor of active iron in iron-based catalyst than Fe₂O₃,and more Fe₇C₃ can be generated in the reduction process,which is beneficial to improve CO₂conversion rate and the selectivity of long-chain hydrocarbons in hydrogenation products.The co-precipitation method combined with special drying treatment for preparaing iron-based catalyst is better than the simple mechanical mixing method.The prepared catalyst has high specific surface area and is easy to be reduced,which can improve the catalytic performance for CO₂ hydrogenation and selectivity for long chain hydrocarbons.The suitable content of Fe-Mn coprecipitates in the Fe-Mn-K/Al₂O₃ catalyst is 40wt.%（Fe,Mn and K contents are17wt.%,12wt.%,11wt.%,respectively）,the suitable CO₂ hydrogenation conditions are:catalyst reduction temperature of 400℃,the reduction time of 8h,the CO₂ hydrogenation reaction temperature of 320℃,the reaction pressure of 3MPa,and the molar ratio of hydrogen to CO₂ in the reactant of 3:1.In order to improve the yield of long chain hydrocarbons in CO₂ hydrogenation products,the possibility of"CO₂ hydrogenation-oligomerization of small molecular olefins"was preliminarily explored by using iron-based catalysts and molecular sieves for relay catalysis.The effects of different types of molecular sieves（Y and ZSM-5）,Si/Al ratio of ZSM-5molecular sieves and nickel loading on oligomerization of small molecular olefins were investigated.The experimental results show that the suitable reaction temperature for oligomerization of small molecular olefins is lower than that of CO₂ hydrogenation.ZSM-5zeolite with appropriate Si/Al ratio and relatively strong surface acidity is suitable for oligomerization of small molecular olefins.When the ZSM-5 zeolite is loaded with 2wt.%Ni,the catalytic activity of oligomerization could be improved.When the Fe-Mn-K/Al₂O₃catalyst prepared by co-precipitation method and ZSM-5（Si/Al:130）catalyst loaded with2wt.%Ni are used for relay catalysis,it can achieve a CO₂ conversion of 68%,a CO selectivity of 5%,and a liquid hydrocarbon C₅⁺ yield of up to 80%under the same CO₂hydrogenation conditions and oligomerization temperature of 240°C.