Study On The Selective Oxidation Of Ethyl Lactate To Ethyl Pyruvate Over Silver-based Catalysts

Pyruvic acid is an important organic chemical material.It was a trend to use ethyl lactate as raw materials for the vast production of pyruvic acid.The key technology lies in the effective conversion of ethyl lactate to ethyl pyruvate.The current thesis focused on the preparation and modification of silver catalyst for the vapor phase catalytic oxidation of ethyl lactate to ethyl pyruvate,which could provide some basic scientific support for the development of commercial process.A series of silver electrolysis catalysts,including supported silver catalysts and Raney-Ag catalysts,were prepared.Its physical-chemical properties were measured by ICP-AES,XRD,BET,NH3-TPD,O2-TPD,H2-TPR,UV-Vis DRS,XPS,SEM and TGA analysis.The structure-activity relationship of the catalysts was elucidated.Lifetime experiments of the catalysts for 100 h were evaluated.Deactivation mechanism and regeneration methods of the catalysts were suggested.Conclusions are as following.（1）The calcination temperature of electrolytic silver catalysts had significant impact on their activity.The optimal calcination temperature was found to be 600℃.With increasing calcination temperature,surface silver crystallites tend to agglomerate and the particle size would increase,resulting in the reduced reactivity of the catalyst.（2）Optimal reaction conditions of electrolytic silver catalyst were:reaction temperature of 380℃,molar ratio of oxygen to ester of 1.4,and liquid hourly space velocity（LHSV）of 0.6 h^-1.The 100 h lifetime appraising of electrolytic silver catalyst was performed.The conversion of ethyl lactate and yield of ethyl pyruvate form 96.95%and 85.97%to 55.00%and 22.70%,respectively,The XRD,SEM,DTG and H2-TPR analysis showed that the deposition of coke was the main reason for catalyst deactivation.After regeneration with in-situ air calcination,activity is basically restored.（3）Compared with SiO₂ andγ-Al₂O₃ support,theα-Al₂O₃ supported silver catalyst showed the best catalytic performance.Results of characterization showed that the surface of Ag/α-Al₂O₃ catalyst can enrich more highly dispersed Ag⁺ions and silver clusters Ag_n^δ+with positive charge and the acid strength of Ag/α-Al₂O₃ was weak and the number of acid sites was low,which inhibited hydrolysis side reactions,surface coking,and possible deep oxidations.（4）When silver loading is 8%for Ag/α-Al₂O₃ and the calcination temperature is 600℃,the performance of catalyst was optimal with a conversion of 96.80%,and yield of 87.80%.The slight deposition of coke on the surface was found to be the main cause of the decrease in catalytic performance during the life of the catalyst.（5）The optimal conversion and yield were 97.80%and 88.02%,respectively,on 2%ZrO₂-8%Ag/α-Al₂O₃ under the following reaction conditions:reaction temperature of 340℃,molar ratio of oxygen to ester of 1.4,and LHSV of 0.6 h^-1.The characterization results show that the apparent morphology and electronic structure of Ag/α-Al₂O₃ modified by ZrO₂ were changed,also reduced the intensity and quantity of acid centers on the surface,improving the selectivity of the reaction.The results of lifetime tests for 2%ZrO₂-8%Ag/α-Al₂O₃ revealed that the introduction of ZrO₂ can improve the stability of the catalyst,delay the rates of coke depositions,and keep the catalytic activity almost unchanged within 100 h.（6）The optimal preparation conditions of Raney-Ag catalyst were as follows:the ratio of silver and aluminum alloy being 1:1 kg/kg,the leaching concentration of NaOH solution being 30 wt.%,and the soaking time being 120 min.The conversion and yield were 90.00%and 76.50%,respectively,under the conditions of 340℃,molar ratio of oxygen to ester being 1.4,and LHSV of 0.6 h^-1.The UV-Vis DRS results showed that there was a large number of dispersed Ag⁺ions and silver clusters Ag_n^δ+enriched on the surface of Raney-Ag catalyst,providing a large number of catalytic active sites for the reaction that dramatically reduced the reaction temperature.（7）In conclusion,comparison of the above three different kinds of silver-based catalysts with regard to catalytic performance indicates that the optimal reaction temperatures were 380℃,340℃and 320℃,respectively,when electrolytic silver,Ag/α-Al₂O₃ and Raney-Ag catalysts exhibited equivalent catalytic activity.Therefore,Ag/α-Al₂O₃ and Raney-Ag catalysts have better catalytic activity compared with electrolytic silver catalysts.