Preparation, Characterization And Catalytic Properties Of Imidaze Phosphotungstate For Ammoximation Of Cyclohexanone

Heteropoly acid (salt) is a kind of environment-friendly catalyst which has thecharacteristic of redox and acidity. It also has a wide prospect of research and application forits advantages of high catalytic activity, good selectivity and adjustablity. The first patentabout using heteropoly acid as catalyst for ammoximation of cyclohexanone was reported bythe German Toa Gosei company in1967. The lastest studies were carried out using the theAl2O3-based heteropoly acid as the catalyst for ammoximation of cyclohexanone,but it wasconstrained because of the off-bond of active ingredient and the difficulty of reusing catalysts.The recovery of catalyst can be improved by synthesising a new multifunctional compoundby leading organic groups into the secondary structure of phosphotungstic acid. But theresearch on the structure change of catalyst in the ammoximation has not been found. Thisthesis aims at synthesize imidaze phosphotungstate with high affinity of oil-water two phasesby joint imidazolium cation with different length. The catalystic properities are investigatedand the changes of the structure of catalyst are characterized.Firstly phosphotungstic acid was used as the catalyst in the reaction of cyclohexanoneammoximation and the effects of reaction solvent, reaction time, the way of adding ammonia,the concentration of H2O2and catalyst on the reaction were studied The results indicate thatthe optimal reaction condition are: without solvent for5h, the concentration of catalyst is3g/15g cyclohexanone, ammonia is once joined, the H2O2(30%) dropped in for4h, Under theabove conditions the conversion of cyclohexanone is90.12%and the selectivity ofcyclohexanone oxime can reach97.16%.Then imidazolium cation with different carbon chain length were leaded into thesecondary structure of phosphotungstic acid to prepare a series of organic-inorganicheteropolyacid salt. The catalysts exhibited not only strong acidity and redox,but also lowsolubility in polar solution. The catalysts are characterized by means of FT-IR spectroscopy,TG/DSC, XRD, NMRand so on. The results show the imidazolium cation take place of the H+of H3PW12O40,and the catalysts preserve the primary keggin structure.The catalysticalproperities of the phosphotungstic acid salt with different carbon chain length on the reactionof cyclohexanone ammoximation were compared. It is found that the the selectivity ofcyclohexanone oxime are decreasing gradually with the increasing of the carbon chain lengthof the cation. Using butyl imidazole phosphotungstate as catalyst, the conversion ofcyclohexanone and the selectivity of cyclohexanone oxime can reach89.66%and95.52%,respectively. The catalysts were separated by vacuum distillation and the recovery of all thecatalysts were over70%. The ammoximation results show that the recycled catalysts exhibitegood reusability without significant loss of its catalytic activity.The recycled caralysts werealso characterized by the means of FT-IR, TG-DSC, UV and ICP and the composition andstructure were compared with those of the fresh catalysts. The result show the structure of therecycled catalysts has changed which the carbon chain is broken and the keggin structure ofheteropoly anion is also changed. The catalysts are pretreated with NH3and H2O2. It is provedthat the mainly reason of the change of the catalysts is the strong alkaline environment withNH3, in which the keggin structure is destroyed.