Study On The Mechanism Of Oxygenates Formation In Fischer-tropsch Synthesis

Fischer-Tropsch synthesis can be briefly defined as the means used to convert synthesis gas containing hydrogen and carbon monoxide to a mixture of hydrocarbon products,among the products,higher alcohols has drawn much attention due to its potential application.The Cu-Fe based catalyst has obvious advantages in the synthesis of higher alcohols from syngas for the existence of two kinds of active sites for generating alcohols and promoting carbon chain growth.The formation mechanism of oxygenates on the Cu-Fe based catalyst has not yet reached a consensus because there are too many side reactions in the process.This thesis conductes in situ DRFTIR,chemical trapping,TPR and TPD to investigate the effect of manganese on the adsorbed species on the catalytic surface and the individual steps of oxygenates formation from syngas on iron-based catalysts during Fischer-Tropsch synthesis.The results showed that the addtion of manganese would promote the CO adsorption and the formation of acyl;while the CO adsorption would be suppressed fiercely with much more added Mn.The adsorbed CO on iron carbide would contribute to the production of methylene,which increased the probability of the carbon chain growth in the hydrogenation process.There were two routes for the gtowth of carbon chain:one was the methanol oxidized to generate formate species,which would be inserted by methylene and resulted in the synthesizing of the acid and the C2+ alcohols that had more carbon atoms with the anhydride species as the intermediates;the other one was CO insertion between the surface alkyl species and the catalyst,leading to the formation of surface acyl species,which finally undergo hydrogenation to form C2+ alcohols.Mn would promote the first route.The addtion of manganese would increase the activity of the catalysts and the amount of C2+ alcohols,while the selectivity to alcohols would be decreased.