Preparation Of Lacunary Keggin-structure Phosphotungstic Salts And Its Intercalated Hydrotalcite For Ammoximation Of Cyclohexanone

Cyclohexanone ammoximation reaction is an important process for industrial production ofcaprolactam, our previous studies showed Keggin type phosphotungstic acid had similar catalyticperformance with TS-1in cyclohexanone ammoximation, but it could not be recycled because ofdissolution in the reaction system. Long-chain pyridine, imidazole and quaternary ammoniumcations phosphotungstate could recycled and reused by changing solubility, while long carbonchain was unexpected to break down and saturated Keggin-structure phosphotungstic saltdecomposed into lacunary one. Solid supported Phosphotungstic acid catalyst was still with thedisadvantage such as active component easy to loss, small amount of solid loading resulting in thelarge amount of catalyst usage. In this work, lacunary Keggin-structure phosphotungstic salts withbetter activity and stability were applied in the ammoximation of cyclohexanone system for thefirst time. Single and multi lacunary, transition metal ions and peroxides substitution wereadoppted to adjust the structure of Lacunary phosphotungstic salts, through which therelationship between structure and properties of the catalyst were achieved. The stability andreusability of the obtained compounds were also studied, This work was aimed to providetechnology accumulation for the new catalytic system of cyclohexanone ammoximation.Firstly,1:11and1:9two series of phosphotungstic acid potassium salt, ammonium salt andcesium salt were successfully synthesized with phosphotungstic acid and transition metal salts asraw material at pH4.0~5.0. They were characterized by methods of FT-IR、UV-Vis、XRD、TG-DSC、 X-ray diffraction, cyclic voltammetry and their catalytic performance inammoximation of cyclohexanone were investigated. The characterization results revealed thatlacunary phosphotungstic salts had the typical Keggin-structure. The reaction tests indicated that1:11and1:9series of phosphotungstic acid had similar catalytic activity in ammoximation ofcyclohexanone, while1:9series of phosphotungstic acid salt had a higher catalyst recovery rate.Ammonium salt showed better activity compared with potassium salt and cesium salt. Thecatalytic activity order of transition metal substituted lacunary phosphotungstic salt was as follows:Zr2+> Co2+> Ni2+> Cu2+. Because different transition metal could promote a certain extent ofthe decomposition of hydrogen peroxide, Ni2+, Co2+, Cu2+substituted lacunary phosphotungsticsalt could not effectively catalyze the cyclohexanone ammoximation with strong alkalineconditions; monolacunary phosphotungstate salt exhibited better performance compared withtransition metal ions and peroxides substituted lacunary phosphotungstic salt, of which NH4PW9catalyzed the reaction with the conversion of cyclohexanone87.1%and the selectivity of cyclohexanone oxime99.8%, recovery rate of NH4PW9was79.8%. FT-IR spectra and XRDcharacterization of recovering catalyst showed that their Keggin type primary structure still keepintegrated while the secondary structure were destroyed in a certain degree after ammoximation ofcyclohexanone reaction.Secondly, lacunary phosphotungstic salts and nitrate hydrotalcite as a carrier got intercalationcompounds. The obtained compounds were characterized by FT-IR、 XRD、 N2Physicaladsorption、ICP and their catalytic performance in ammoximation of cyclohexanone wereinvestigated. The characterization results revealed that lacunary phosphotungstic heteropoly anionssubstituted some NO3-between the layers of hydrotalcite and its Keggin structure kept integrated.Surface area of intercalation compounds increased obviously and the catalyst immobilizationamount can be up to0.85g/g carrier. Different kind of lacunary phosphotungstic salts intercalatedhydrotalcite compound had significant different catalytic properties in cyclohexanoneammoximation. Magnalium hydrotalcite carrier has a higher catalytic activity and recovery ratethan zinc-aluminum hydrotalcite, which MgAl-NH4ZrPW11intercalation compound with the massratio of1:2to MgAl-NO3and NH4ZrPW11as a catalyst exhibited better performance with theconversion of cyclohexanone90.4%and the selectivity of cyclohexanone oxime98.0%, recoveryrate of the catalyst79.3%.