Synergistic Effect Of Nitrogen-doped Carbon-nanotubes-supported Cu-Fe Catalyst For The Synthesis Of Higher Alcohols From Syngas

Syngas(H2 + CO)can be readily generated from carbon based materials including coal,natural gas,shale gas,biomass and even the organic waste.It is the starting material and intermediate of C1 chemistry,playing an important role in chemical industry.Catalytic conversion of syngas to higher alcohols(C2+-OH)is one.of important subjects in C1 chemistry,owing to the potential use of higher alcohols as value-added chemical feedstock,alternative fuels and gasoline additives.Over the past few decades,many valuable experiences and insights have been accumulated in catalyst design,preparation,and structure in order to improve the CO conversion and the yield of higher alcohols.However,there are still many scientific problems to be solved,for example,easy sintering of catalyst,and low selectivity and yield of higher alcohols.A large number of studies show that,use of a carries with suitable acidity and basicity will modify the interactions between the active species and the carrier,so as to influence the adsorption and desorption properties,the catalyst reducibility and the dispersion of active species,thereby improving the catalyst performance.The present dissertation prepared a kind of nitrogen-doped carbon-nanotubes-supported Cu-Fe catalyst xCun-Fem/yN-CNT(x and y represent the weight percentage of the metal and nitrogen;n and m represent the atomic ratio of Cu and Fe)for the synthesis of higher alcohols from syngas.The effects of N species on the performance of the catalysts and the correlation of catalyst structure-performance were intensively studied to elucidate the promotional mechanism of N doping.The N-CNT supports were synthesized through annealing the mixture of CNTs and melamine.Then the catalysts were prepared by a conventional co-impregnation method.The results indicated that the N-CNTs that were obtained firstly by purification with concentrated HNO3(68 wt%)and then calcination with melamine were suitable for the Cu-Fe/N-CNT catalyst preparation.The catalyst showed improved catalytic activity for the synthesis of higher alcohols compared with the N-free catalyst Cu-Fe/CNT under identical conditions.The evaluating conditions for the catalyst performance was obtained through the results of optimization experiment,which was determined as below:catalyst reduction temperature:400 ?;catalytic reaction condition:P = 2.0 MPa,T = 220 ?,GHSV =6000 mL g-1 h-1,and n(H2)/n(CO)= 2.Under the above optimal evaluating conditions,the performance of the Cu-Fe/N-CNT catalysts with different N contents showed a volcanic type variation.Among them,the 15Cu1-Fe1/1.3N-CNT catalyst displayed the highest CO conversion of 20.9%,and the selectivities to total alcohols and C2+-OH of 27.2%and 68.8%,respectively.The 15Cu1-Fe1/1.3N-CNT catalyst also exhibited excellent stability compared with 15Cu1-Fe1/CNT,sustaining higher CO conversion and C2+-OH selectivity for 300 h.A series of characteristic studies revealed that the doping N species could change the electronic property and surface structure of the CNTs,providing the promotional effects mainly on two aspects.(1)The doping N species could enhance the surface basicity of the support and then promote the chemisorption of CO on the catalysts;(2)The electronic interactions between the lone pair electrons of N species and metal species could strengthen the interactions of active metals and support,and promote the CO dissociate adsorption on the catalyst surfaces.These effects are believed to benefit the alcoholic formation in the process of CO hydrogenation.Therefore,N doping is an effective approach to improve the catalytic performance of CNT-supported Cu-Fe catalyst for CO hydrogenation to synthesize higher alcohols.