Catalytic Upgrading Of Ethanol To Butanol Over Metal-Cerium Oxide Based Catalysts

The one-step upgrading of bio-ethanol to n-butanol fuel or chemicals has been increasingly concerned in catalysis field,in which homogeneous noble metal catalysts and heterogeneous metal catalysts have been widely used for this kind of reaction due to their excellent catalytic properties.However,there may still exist some of problems to be resolved in terms of industrial applications:1)The yield to n-butanol need to be improved significantly.2)The ethanol coupling reaction needs to be performed in milder reaction conditions.3)The earth-abundant metals should be considered preferentially to be used in this reaction compared to expensive noble metals.The Cu-CeO₂/AC catalysts were prepared by simple wetness-impregnation method and used for the upgrading of ethanol to butanol in milder reaction(250?,2 MPa,LHSV=2 h^-1 and N₂/ethanol=500:1 v/v).As a result,the Cu-CeO₂/AC catalysts exhibited up to 20%of butanol yield with 46.2%of ethanol conversion and 41.3%of butanol selectivity,especially without any carbon loss.To the best of our knowledge,it is the first time that Cu-CeO₂/AC catalysts are reported to show such excellent catalytic performance in milder reaction conditions.Investigations on relationships between catalyst structure and their catalytic behaviors were based on a wide range of characterization technologies including N₂-adsorption,XRD,TEM,H₂-TPR,H₂-TPD,CO₂-TPD etc.The characterization results suggested that the excellent catalytic performance of Cu-CeO₂/AC catalysts is mainly attributed to high dispersion of Cu and CeO₂ on the surface of active carbon supports,which provides a large number of metal and Lewis basic active centers for ethanol dehydrogenation,acetaldehyde aldol condensation or crotonaldehyde hydrogenation reaction,while the synergy of Cu,CeO₂and AC or the special structure of Cu nanoparticles partly covered by CeO₂nanoparticles is responsible for good stability of Cu-CeO₂/AC catalysts.The reaction kinetics experiments for ethanol dehydrogenation,aldol condensation of acetaldehyde and crotonaldehyde hydrogenation were performed over Cu-CeO₂/AC catalysts as well,which revealed that the apparent activation energies for ethanol dehydrogenation,aldol condensation of acetaldehyde and crotonaldehyde hydrogenation are 32.5,70.1,12.4 KJ/mol,respectively,indicating aldol condensation of acetaldehyde is the decisive step in these tandem steps over Cu-CeO₂/AC catalysts.However,the activation energy of aldol condensation of acetaldehyde is much lower than that reported recently over typical heterogeneous acid-base catalysts.This result could explain the high efficiency of Cu-CeO₂/AC catalysts for upgrading of bio-ethanol to n-butanol or significance of introduction of CeO₂ species into metal catalyst system.Based on previous investigations on Cu-CeO₂/AC catalysts,other metal?Co,Fe,Ni,Pd?modified CeO₂/AC catalysts were examined as well for upgrading of ethanol to n-butanol by catalytic tests and a series of characterization technologies.The results suggested an order of Ni>Co>Fe>Pd>Cu for capacity of cleaving C-C bonds,Cu>Co>Ni>Pd>Fe for dehydrogenation activity,and Pd>Ni>Co>Cu>Fe for hydrogenation activity.It is noticeable that the metal Cu is more active for ethanol dehydrogenation,while metal Pd or Ni possesses the highest hydrogenation activity of crotonaldehyde.However,metal Ni is more liable to crack C-C bonds,which therefore results in a lower yield of liquid products compared to other metals incorporated.