Construction Of Bi-,Sn-Modified Pd/MCM-41 Catalyst For Selective Catalytic Oxidation Of 1,2-Propanediol To Pyruvic Acid

Fine chemicals such as pyruvic acid and lactic acid can be selectively produced by catalytic oxidation of 1,2-propanediol,which belongs to the biomass derivative.The catalytic oxidation of 1,2-propanediol to pyruvic acid is using heavy metals?eg.Pb,Bi,Sn?promoted noble metals?eg.Pt or Pd?as catalysts has been reported.However,the doping method of modified element is simple and the modified elements can easily leak from the catalyst,displaying low activity and poor recycling performance.The efficiency of pyruvic acid production from 1,2-propanediol over these catalysts is far from satisfactory.Therefore,it is significant to enhance catalytic performance for converting 1,2-propanediol to pyruvic acid by constructing alternative catalysts.In this study,Pd@Bi/M and Pd/SnM catalysts were constructed by using MCM-41 to support Bi and Pd bimetals and Sn-doped MCM-41 loading with Pd nanoparticles respectively,which are used for the selective catalytic oxidation of1,2-propanediol to pyruvic acid.The structure-activity relationship between the catalyst structure and the catalytic activity,the role of the modified element in the reaction,and the reaction kinetics were investigated.The main research results are as follows:1.The modified catalyst Pd@Bi/MCM-41 was prepared by doping the Bi element into the active component Pd of Pd/MCM-41.By analyzing catalytic oxidation of 1,2-propanediol and the chacracterizations of XRD,TEM,XPS,it was found that the mesoporous MCM-41 can well disperse the active component Pd@Bi nanoparticles.Since the incorporation of Bi changes the electronic arrangement of the surface of the active component Pd,the selectivity of the catalyst is changed and the activity of the catalyst is also improved.Under the same conditions,the selectivity of the bimetallic?Pd₄@Bi₁?₅/M catalyst to pyruvic acid was 3 times higher than the single metal?Pd?₅/M catalyst.The reaction activation energies calculated by fitting the reaction kinetic equation were 26.05 kJ mol^-11 and 18.65 kJ mol^-11 for?Pd?₅/M and?Pd₄@Bi₁?₅/M,respectively.The results show that the modified doping of Bi element effectively improves the catalytic activity and pyruvate selectivity as compared with the original catalyst.2.Pd/SnM?Si/Sn?catalyst was prepared by in-situ doping modified?Sn?MCM-41 molecular sieve as carrier supported Pd.Through a series of tests such as XRD and TEM,it is found that when the doping amount is appropriate?Si/Sn is greater than 6?,the Sn element can be well fixed in the MCM-41 framework by direct hydrothermal synthesis method.The as-prepared Sn doping MCM-41 still maintains the well mesoporous structure.The particle size of the active metal Pd can be optimized by changing the addition amount of the promoter Sn,which can also enhance the basicity strength and surface charge of the catalyst.In the process of catalytic oxidation of 1,2-propanediol over Pd/SnM catalysts,the area-specific rates?TOF?of these catalysts displayed the volcano-type tendency with the Sn doping amount.The Pd₂/SnM?12?catalyst presented the highest catalytic performance with about 1.5 times higher TOF value than the Pd₂/MCM and 3 times than Pd₂/SnO₂,along with the highest pyruvic acid selectivity because of relatively lower activation energy?Ea?and bigger pre-exponential factor?A?.In addition,the Pd₂/SnM?12?catalyst exhibited good recycling performance in the experiment of catalytic oxidation of 1,2-propanediol to pyruvic acid.