Reaction Integration Of N-butanal Self-condensation And Selective Hydrogenation

2-Ethylhexanal is an important organic chemical raw material,which is mainly used for producing 2-ethyhexanoic acid with high added value.The industrial production of 2-ethylhexanal from n-butanal mainly includes two reaction stpes:n-butanal self-condensation and 2-ethyl-2-hexenal selective hydrogenation.In view of the problems existing in the production of 2-ethylhexanal such as using liquid alkali catalyst and long process line,this thesis is intended to develop a new process for synthesis of 2-ethylhexanal through reaction integration of n-butanal self-condensation and selective hydrogenation catalyzed by heterogeneous catalyst.The catalytic performance of anatase TiO₂ prepared by hydrothermal,precipitation and sol-gel methods for n-butanal self-condensation was separately evaluated and the results show TiO₂ prepared by hydrothermal method is the best.Subsequently,the effects of hydrothermal temperature,molar ratio of H₂O/Ti,and p H of the mixture on the crystal structure,physical structure,acid-base properties and catalytic performance of TiO₂ were investigated,and the TiO₂ was characterized by XRD,BET,NH₃-TPD and CO₂-TPD.The results demonstrate that both the specific surface area and strong acid center of TiO₂ are positively correlated with the conversion of n-butanal,while the yield and selectivity of 2-ethyl-2-hexenal are positively correlated with the total acid amount and total base amount,respectively.Then,the relationship between the crystal facet and catalytic performance of TiO₂ was elucidated.Based on the result of characterization and activity evaluation,it can be concluded that the higher the exposure ratio of{101}crystal faces is,the greater the amount of acid and base will be,which corresponds to higher yield and selectivity of 2-ethyl-2-hexenal,respectively.The product distribution of n-butanal self-condensation catalyzed by TiO₂ with different exposure ratios of{101}crystal facet was analyzed by GC-MS and it was found that increasing the exposure ratio of in TiO₂{101}facet could effectively inhibit the occurrence of side reactions.On this basis,the suitable reaction conditions for n-butanal self-condensation were determined by single factor experiment as follows:a reaction temperature of 180℃,a reaction time of 8 h and a catalyst dosage of 10 wt.%.Under this reaction conditions,the conversion of n-butanal reached 92.1%,and the selectivity of 2-ethyl-2-hexenal was 99.9%.The catalytic activity and crystal structure of TiO₂ remained unchanged after recycled five times.The catalytic performance of TiO₂-supported metal catalysts on the reaction integration of n-butanal self-condensation and selective hydrogenation was evaluated on an autoclave and the results show that Pd/TiO₂ has a higher product selectivity.Subsequently,the effects of different impregnation methods and preparation conditions on the catalytic performance of Pd/TiO₂ were investigated.Pd/TiO₂ was characterized by XRD,CO-pulse adsorption,TEM,NH₃-TPD and CO₂-TPD,and the results show that crystal size of Pd and the amount of acid and base are the main factors affecting its catalytic performance:the smaller the particle size of Pd is,the larger the amount of acid and base of the catalyst is,the better its catalytic performance will be.The suitable preparation conditions of Pd/TiO₂ were obtained as follows:a Pd loading amount of 0.5 wt.%,a calcination temperature of 300℃,a calcination temperature increasing rate of 5℃/min,a reduction time of 4 h,and a reduction temperature of 400℃.The conversion of n-butanal was 95.4%,and the selectivity of 2-ethylhexanal reached 99.9%.The rection integration of n-butanal self-condensation and selective hydrogenation catalyzed by Pd/TiO₂ was qualitatively analyzed by GC-MS.Based on the analysis results,the side reactions in the reaction system was speculated and a reaction integrated mechanism was also established.Based on the excellent catalytic activity of Pd/TiO₂ exhibited on the autoclave,the reaction integrated operation conditions of n-butanal self-condensation and selective hydrogenation were optimized on a fixed bed by single factor experiment and response surface methodology,respectively.Combined with the optimization analysis of each factor by response surface methodology,it is found that the influence order of each factor on the 2-ethylhexanal yield is:liquid hourly space velocity（LHSV）>H₂pressure>reaction temperature;and the interaction between H₂ pressure and reaction temperature is the most significant.The optimal operating conditions were determined as follows:a LHSV of 7.2 h^-1,a reaction temperature of 190°C,a H₂ pressure of 3.2 MPa and a hydrogen flow rate of 100 m L/min.Under these reation conditions,the yield and selectivity of 2-ethylhexanal were 81.9%and 89.8%,respectively.The n-butanal conversion decreased slightly at first and then stabilized,and the selectivity of the target product 2-ethylhexanal remained unchanged during the continuous operation for 300 h under the optimal conditions.