Cyclohexanone Ammoximation On Ti（OⁱC₃H₇）_n/SAPO-5 Molecular Sieves

Cyclohexanone oxime is an important intermediate for the synthesis of ε-caprolactam which is the main raw material of nylon-6 fiber and polyamide engineering plastics. Due to the mild reaction conditions, atom economy and environmental benign of liquid ammoximation of cyclohexanone, it has attracted the interest of academia and industry and has been industrialized. As the industrial catalyts of cyclohexanone ammoximation, the deficiencies of TS-1 are obvious:high cost, fast activity decline and difficult recovery due to its small particle size, etc. In view of these shortcomings, the design and preparation of high efficient catalysts to displace TS-1, the research on the relationship between the structure and performance of new catalysts as well as catalytic mechanism of these new catalysts are still the focus of the scientific and industrial field.Tetrahedral Ti4+ of TS-1 is the active site for the catalytic ammoximation of cyclohexanone. Silicoaluminophosphate sieve SAPO-5 has the advantages of uniform constitute, regular structure, abundant surface hydroxyls and easy doping of transition metal atoms, etc. Shall the incorporation of tetrahedral Ti4+ into SAPO-5 also catalyze the ammoximation of cyclohexanone? However, there is no report on this work yet. Based on the above analysis, a series of hierarchical SAPO-5 molecular sieves with different silicon loadings were prepared by hydrothermal synthesis. Tetraisopropyl titanate as the titanium source was grafted onto the SAPO-5 with different silicon contents under different grafting temperatures and calcination temperatures. The resulting samples were designated as Ti(O’C3H7)n/SAPO-5. The crystal structure, pore structure, specific surface area, coordination of titanium, the types of surface hydroxyls, the content of titanium, hydrophilicity and acidity of these samples were characterized by XRD, N2-physical absorption, diffuse reflectance UV-vis spectrum, Vacuum FTIR spectrum, elemental analysis and surface contact angle, etc. The catalytic performance on cyclohexanone ammoximation of these samples was evaluated. The main conclusions of this thesis are as follows:(1) The characterization results of the supports showed that:with the increase of silicon contents, the crystallinity of hierarchical SAPO-5 molecular sieve was decreased because of the polycondensation of silicates which haven’t enter into SAPO-5 skeleton. When the silicon content was 0.33, the sample was no longer AFI structure. There was no significant alteration of the pore diameter with the silicon content increase, but the pore volume increased; the total acid amount presented a trend of increase at first and then decrease, the total amount of acid reached the largest when the silicon content approached 0.2.(2) Comparing the physic-chemical properties of catalysts prepared under different conditions, it was found that:all the catalysts were AFI structure, titanium presented mainly as the oligomer or TiO2 nano clusters. With the increase of the silicon contents of supports, the specific surface area and pore volume decreased, while the average pore diameter increased; titanium contents and the hydrophobicity of the catalysts were reduced first, then reached the largest when the silica content is 0.13. With the increase of grafting temperature, more and more isopropoxys reacted with the surface hydroxyls of the support, which resulted in the decrease of isopropoxys on the catalyst. With the increase of calcination temperature, the specific surface area of the samples decreased due to the shrinkage of the pores caused by the dehydration of surface hydroxyls. Furthermore, higher calcination temperature resulted in the decomposition of the residual isopropoxys bonded to titanium, which in turn led to the decrease of the hydrophobicity of catalyst. The main reason of the reaction differences is that the common influence between hydrophilicity/hydrophobicty and activity.(3) The optimal preparation condition for Ti(O’C3H7)n/SAPO-5 is:the silica content of support is 0.13, grafting temperature is 50℃, and calcination temperature is 300 ℃. The selectivity toward cyclohexanone oxime on the catalyst prepared under optimal condition was as high as ca.96.1% which almost equaled to that of TS-1.(4) The mechanism of cyclohexanone ammoximation on Ti(OiC3H7)n/SAPO-5 was studied by designing two simple experiments. It can be deduced from the results of the experiments that the cyclohexanone ammoximation took place on Ti(OiC3H7)n/SAPO-5 via both cyclohexylimine imine and hydroxylamine mechanisms, which needs to be confirmed in the future by advanced instruments and theoretical calculations.