Studies On Conversion Of Syngas Into Liquid Fuels Over Carbon Nanotubes Supported Co Catalysts

The utilization of non-petroleum carbon resources to obtain liquid fuels via syngas patform has become a hot topic in the energy field in recent years.Fischer-Tropsch synthesis is a process which converts syngas into liquid fuels.However,the products selectivity of Fischer-Tropsch synthesis is limited by Anderson-Schulz-Flory distribution and the selectivity of specific products is still not ideal.Hence,design the highly efficient catalysts for controlling the selectivity of syngas conversion is still a great challenge.In this dissertation,we focused on regulating of product selectivity,and studied the effect of promoters of the Co/CNT catalysts on catalytic performances of Fischer-Tropsch,and further investigate the relationship between product selectivity of Fischer-Tropsch and hydrogenolysis reaction.For the purpose of improving the selectivity of the C10-20 hydrocarbons(diesel fuel)and reducing the selectivity of CH4,we studied the effect of promoters on the Co/CNT catalysts for the Fischer-Trospch synthesis.The results showed that the addition of promoter of Ir or Re could reduce the selectivity of low carbon hydrocarbons such as CH4,and increase the selectivity of diesel fractions.We found that the introducing of Ir into Co/CNT catalyst could improve theconversion of CO and suppress the formation of CH4.The Ir0.04-Co/CNT catalyst with a Ir/Co ratio of 0.04 exhibited better catalytic performance,in which the CO conversion was 46%,and the selectivity of CH4 and C10-20 hydrocarbons were 2.7%and 58%,respectively.The catalyst also showed excellent stability.The catalytic performances did not change significantly during 1100 h test.Simultaneously,the effects of Ir/Co ratio of Ir-Co/CNT catalysts on the hydrogenolysis reaction of linear alkanes were investigated.It was found that the addition of Ir into Co/CNT could inhibit the activity of hydrogenolysis reaction,and thus reduced the formation methane during Fischer-Tropsch synthesis.The catalsts characterization results show that the addition of Ir into Co nanoparticles could form the Co-Ir alloy,and the particles size of Co gradually decrease with increase in Ir/Co ratio while the reduction degree of catalyst increases.Meanwhile,the CO chemisorption amounts increase.We speculate that the improvement on reduction degree of Co and the decrease in Co particles size are responsible for the increase in the CO conversion,and the increase in CO adsorption would inhibit the hydrogenolysis reaction and then reduce the methane selectivity in Fischer-Tropsch synthesis.We further study the effect of Re promoter over Co/CNT and the results reveal that the catalyst with a Re/Co ratio of 0.04 also show a higher catalytic reactivity,in which the CO conversion reach 40%,and the selectivity of C10-20 hydrocarbons and C21+ hydrocarbons is 64%and 8.0%,respectively.After 200 h ttest,the catalytic performance remains stably.We found that the particle size gradually decreased with the increase of Re/Co ratio.The degree of reduction of the catalyst increased.Thus,the decrease in the particle size of metallic Co is responsible for the increase in CO conversion.Through studies the alkanes hydrogenolysis reaction of Re-Co/CNT catalyst,it was found that the additon of Re promoter could inhibit the hydrogenolysis activity of Co/CNT.Hence,the decrease in selectivity of CH4 was considered due to the suppression on hydrogenolysis of heavier hydrocarbons during the Fischer-Tropsch synthesis.