Synthesis, Characterization And Catalytic Activity Of Phosphotungstic Acid Encapsulated In Y Zeolite For Ammoximation Of Cyclohexanone

Cyclohexanone oxime is one of the most important intermediate in the industrial caprolactam technology. Currently, in the industry the cyclohexanone oxime was synthesized in a step which used the titanium silicalite TS-1 as catalyst with cyclohexanone, ammonia and hydrogen peroxide as the reactants. Although this liquid ammoximation process can proceed in mild reaction condition, reach high selectivity, and environmental friendliness. However, TS-1 is very expensive and with small-granularited, which causes difficulties in industrial production and makes the high cost of catalysts. Thus, the development of new catalysts for cyclohexanone ammoximation has attracted wide interest in scientific research personnel. Phosphotungstic acid is one of new kind catalysts, which showed considerable catalytic activity and selectivity with TS-1. However, phosphotungstic acid has small surface area, easily soluble in water and polar organic solvents, which made it difficult to use as a solid catalyst in polar reaction system. So, the preparation of heteropoly acid complexs with different carries has become an important means to solve this problem.In this paper, phosphotungstic acid encaged in Y zeolites compounds was prepared and characterized with a variety of physical and chemical methods. These compounds were applied in the ammoximation of cyclohexanone system for the first time. And the catalytic activity and reusability of the compouds were also studied, which designed to develop a new catalytic system for the cyclohexanone ammoximation.Firstly, the aluminum type NaY zeolite, which possessed high crystallinity, excellent thermal stability. It was selected as a carrier precursor, and modified to dealumination. Using the oxalic acid as the dealumination agent, the impaction of the concentration of oxalic acid, the joining orders of dealumination agent and the hydrothermal treatment were studied. The results showed that: ammonium sulfate and oxalic acid added at the same time, kept the concentration of oxalic acid C((C₂O₄)^2-)=0.1mol/L, and for hydrothermal treatment, can obtain modified zeolite DY1 which has a lager surface area and better dealumination degree. Then the phosphotungstic acid and Y zeolites compounds was synthesized by the"ship in the bottle"method. From the results of the characterization, we can see the phosphotungstic acid encaged in the compounds has been realized, but the package was very small. And the phosphotungstic acid had a strong interaction with the carriers in the compounds, resulting the crytallinity of carriers decreased.Secondly, using the ultra-stable Y zeolite (USY) which has higher degree of dealunination as a carrier, by inverstigating the factors such as pH value, the adding orders of phosphorus source and tungsten source etc, optimized and determined the conditions of the"ship in the bottle"synthesis. The obtained catalysts were characterized with FT-IR, ICP, UV-vis, N2-adsorption, XRD and SEM techniques. The results showed that encapsulation of Keggin-type phosphotungstic acid in the supercage of USY zeolite was realized. In the ammoximation of cyclohexanone system with H₂O₂ as oxygen source, PW-USY showed considerably catalytic performance of pure phosphotungstic acid. The PW-USY after using 5 times, the catalytic performance was not significantly reduced, and have better reusability than the supported catalyst (PW/USY), which can solve the problem of wastage of active ingredients, showing potential applications .Finally, using the zeolite synthesis method first time to prepare the phosphotungstic acid encaged in Y zeolites compounds. The impactions of the kind of phosphotungstic acid, the solvents of phosphotungstic acid and the phosphotungstic acid sequence accession were studied. It was found that the aqueous solution of phosphotungstic acid added in the reaction system after the Si-Al gel had formed, by mixing adequately, and after crystallization, filtration and washing process, the phosphotugstic acid encapsulated in Y zeolite (PW-Y(ZS)) can be obtained.The PW-Y(ZS) were characterized by FT-IR , nitrogen adsorption-desorption, elemental analysis, SEM and XRD etc. The results showed that despite the mount of phosphotugstic acid was low in the PW-Y(ZS), but still relatively large. At the same time, the crystal structures of PW-Y(ZS) have been changed. In the ammoximation of cyclohexanone system, when the active component of phosphotungstic acid in the same, PW-Y(ZS) had a considerable catalytic activity with pure phosphotungstic acid. But the repeated experiment results showed that phosphotungstic acid in the PW-Y (ZS) was easily lossed and had poor reproducibility.