Monolithic CoO@Cu₂o/Oxides/Al-fiber Catalyst:Fabrication For The Gas Phase Oxidation Of Ethanol To Acetaldehyde

The gas-phase aerobic oxidation of ethanol to acetaldehyde is a green and sustainable alternative route for ethylene oxidation,but needs an ideal catalyst with both low-temperature activity/selectivity and intensified heat/mass transferring.The particulate oxide supported catalysts possess several problems of "low heat/mass transferring" and "containing toxic metals",etc.In this paper,the monolithic metal fiber（such as Al-fiber,50 μm in diameter）,which has thin-sheet and large area structure,open porous system,unique form factor,and excellent heat/mass transferring,was employed as support.The Al-fiber was firstly modified,and then the Cu-Co active components were supported on the as-modified Al-fiber.Finally,the monolithic catalyst was obtained with the combination of high activity/selectivity,high heat/mass transferring,and good stability/regenerability for the gas-phase oxidation of ethanol to acetaldehyde.The detailed results are as follows:（1）The ns-AlOOH/Al-fiber substrate was initially prepared by oxidizing Al-fiber with water stream to endogenously grow 2D boehmite nanosheets（ns）onto Al-fiber surface.After calcination at 550 ℃,the ns-AlOOH/Al-fiber was transformed into γ-Al₂O₃/Al-fiber.The Cu（NO3）2 and Co（NO3）2 were supported on γ-Al₂O₃/Al-fiber,and the as-obtained catalyst was used in the ethanol oxidation.The optimum total Cu plus Co loading is 10 wt%with Cu:Co weight ratio of 2:3;the optimum reaction conditions are as following:O₂/ethanol molar ratio of 0.6,and weight hourly space velocity of 6 h-1.At 300 ℃,ethanol conversion is 89%and acetaldehyde selectivity is 89%.However,the ethyl acetate selectivity is 9%,which is likely caused by the γ-Al₂O₃ in the catalyst.（2）In order to reduce the ethyl acetate selectivity,the Mg promoter was introduced via in situ growing Mg-Al hydrotalcites onto the Al-fiber surface to prepare the Mg-Al-LDHs/Al-fiber substrate.After calcination at 550 ℃,Mg-Al-LDHs/Al-fiber was transformed into MgO-Al₂O₃/Al-fiber.The Cu（NO3）2 and Co（NO3）2 were supported on MgO-Al₂O₃/Al-fiber,and the as-obtained catalyst was used in the ethanol oxidation.The large surface area and high mechanical strength of the MgO-Al₂O₃/Al-fiber support ensure the high dispersion of the Cu-Co components.The results indicate that Mg introduction can not only decline the ethyl acetate selectivity,but also increase the catalyst activity:at 280 ℃,ethanol conversion is 93%and acetaldehyde selectivity is 96%,while ethyl acetate selectivity is reduced to 2%.In 150 h running,the ethanol conversion and acetaldehyde selectivity are well maintained,indicating the good catalyst stability for the titled reaction.（3）The systematic and deep characterizations were conducted on the Cu-Co/MgO-Al₂O₃/Al-fiber catalyst,and the results show that the Cu（NO3）2 and Co（NO3）2 are in situ induced by the reaction itself to be the optimum structure of "CoO@Cu₂O"（i.e.,large CoO nanoparticle partially covered by small Cu₂O nanoparticles）,and large amount of CoO-Cu₂O interface is generated.In addition,the CoO-Cu₂O interactions not only endow the catalyst with high low-temperature activity,but also stabilize Cu₂O in the reaction stream to offer good catalyst stability.