Synthesis,characterization And Catalytic Properties Of MnAPO-36 Molecular Sieve

MeAPO-36 is one of the member of aluminophosphates families, which has a unique microporous structure. Because of its side pockets in the one-dimensional channels, the surface area is very large . In possession of both Bronsted active sites and redox active sites, the application of MeAPO-36 is very popular. For example,the sieve performs high catalytic activity on the selective oxidation of cyclohexane in air produce cyclohexanol, cyclohexanone and adipic acid, n-alkanes in the air oxidation, Baeyer-Villiger oxidation of cyclohexanone, olefin epoxidation, one-step production of caprolactam from cyclohexanone, and other reactions. These reaction systems are important industrial reaction process, whose chemical products are unable to meet the demands, therefore the research on the synthesis of MnAPO-36 molecular sieve is of great significance.Manganese aluminophosphate of type 36 (MnAPO-36) molecular sieve was hydrothermally synthesized in situ using pseudo-boehmite, phosphoric acid(85 % ), tetrahydrate manganese acetate as raw material and tripropylamine as templating agent. Characterization of the synthesis products was performed using FT-IR, SEM, XRD, TG-DSC, NH3-TPD , N2 adsorption-desorption , XPS and other characterization methods.Firstly, the influence of two step crystallization , crystallization time and template dosage on the crystallinity of MnAPO-36 molecular sieve were investigated. The results showed that small amounts of MnAPO-5 always crystallinzed parallel to the MnAPO-36 material under hydrothermal conditions when tripropylamine was used as organic template. The dosage of template is a critical factor in the preparation process. Highly crystalline MnAPO-36 molecular sieve was crystallized in two temperature steps, namely at 373K for 50 hours and subsequently at 423K for 24 hours, when the material composition is 1.5C9H21N:0.171MnO: 1.22Al2O3:1.028P2O5 :40～80H2O.The effects of three different calcination atmosphere(oxygen, air, nitrogen) on acid strength and the valence of Mn in MnAPO-36 were also considered in the research. The XPS and NH3-TPD analysis results suggested that: oxygen is the best calcination atmosphere. The MnAPO-36 molecular sieve have both strong acid sites and weak acid sites, and one friction of MnII raised to MnIII ,when it is calcinated in oxygen and air. And the quatitity of acid sites and MnIII calcined in oxygen is much than the one calcinated in air. But only weak acid sites exisit without MnIII when calcinated in nitrogen atmosphere.The catalytic performance of MnAPO-36 molecular sieves in the oxidation of cyclohexane have been studied as well, using pure oxygen as the oxygen source.The results showed that: the higher the crystallinity of MnAPO-36 is, the easier the cyclohexane convert, the easier the CHHP decompose, the higher the total selectivity of cyclohexanol,cyclohexanone,CHHP and adipic acid is.The investigation also made some research on following the trail of another reaction—one step production ofε-caprolactam from cyclohexanone with MnAPO-36 molecular sieves. A large quantity of products is discovered: cyclohexenyl cyclohexanone and its isomer, cyclohexenyl cyclohexanone oxime and isomers, cyclohexenyl nitrocyclohexane and its isomers. The main reason is that, the strong alkalinity damage the adipic active sites in the catalyst, making no further conversion of cyclohexanone oxime to caprolactam, aldol condensation occurs easily in strong alkalinity environment.