Study On The Catalysts For Dehydration Of Lactic Acid To Acrylic Acid

Acrylic acid(AA),a kind of chemical products,is produced from petrochemical industry by two-step gas-phase oxidation of propylene currently.In order to reduce the dependence on fossil fuel,especially petroleum,much research on biorenewable feedstocks transformation has been reported.Lactic acid,the most prevalent product of fermentation,can be dehydrated to acrylic acid.Molecular sieves,due to adjustable solid acid/base properties,have been used as catalysts for biorenewable molecular transformation such as dehydration,hydronenolysis,hydrogenation and so on.In this dissertation,a series of NaY molecular sieves modified by rare earth metals and different potassium salts were synthesized by the impregnation method,and characterized by XRD,TG,CO2-TPD,NH3-TPD.Their catalytic performance was tested using lactic acid dehydration as probe reaction.Conclusions were listed as followed:Firstly,NaY,H?,ZSM-5,SAPO-34 and SAPO-11 were used as the catalysts for the dehydration of lactic acid to acrylic acid and the results demonstrated that NaY had a best catalytic dehydration performance.And on this basis,the effects of reaction temperature,flow rate of gas carrier N2,the ratio of water/lactic acid in the feeding stock,the ratio of Si/Al and the Na+ source on the conversion and yield were investigated.The NaY catalyst,exchanged by NaN03,exhibited the best catalytic performance for 39%lactic acid dehydration,over which 34.7%acrylic acid selectivity could be gained at 598 K when the flow rate of gas carrier N2 was 30ml/min and the feed rate was 4.5ml/min.Then,NaY molecular sieves modified by rare earth metals(lanthanum,cerium,neodymium and yttrium)were synthesized by impregnation method and used as catalysts for the dehydration of lactic acid to acrylic acid.The results demonstrated that the rare earth metals were introduced into NaY molecular sieve framework and 2%La/NaY has a least influence on the catalyst crystal structure.The total acid amounts of RE-modified samples decreased,but the weak acid site ratios and medium basic site amounts increased with an increase in loading rare earth elements' diameter except the heavy rare earth Y,which might contribute to higher acrylic acid selectivity.RE-modified samples improved the yield of acrylic acid and inhibited the formation of acetaldehyde in some degree.2%La/NaY showed the highest yield of acrylic acid at up to 54.2%(39%lactic acid solution,LHSV = 3 h-1,325?)while the yield of acrylic acid at unmodified NaY was only 34.7%.However,the coke was increased,thereby the catalyst life was limited.Finally,the selectivity of acrylic acid was improved significantly and acetaldehyde was inhibited with the NaY modified by La and K.Due to the appropriate acid amount,higher weakly acidic ratio and more moderate-intensity basic sites,the KBrLa/NaY exhibited the best catalytic performance,over which 61.7%acrylic acid selectivity could be obtained.The selectivity to acrylic acid was enhanced,and the catalyst coke was reduced at the same time after zeolites modified by La and K.