Cyclohexanone Ammoximation Over Carbon Modified TS-1

Cyclohexanone-oxime is an important intermediate for the production of caprolactam,and is mostly synthesized by the cyclohexanone hydroxylamine method.In 1988,Italian Enichem company developed a liquid phase ammoximation process that overcome the shortcomings of the hydroxylamine process and made caprolactam production more environmentally friendliness.However,many researchers have found that the catalyst,TS-1 molecular sieve suffers from a problem of fast deactivation during the cyclohexanone ammoximation reaction.In order to improve the stability of TS-1,researchers conducted a large number of modification studies on TS-1.Porous carbon have large specific surface area,chemical stability in both acidic and alkaline media,tunable porosity and abundant surface chemistry,which makes it been extensively studied in a series of catalytic reactions.Based on this,we speculate that introduction of on TS-1 molecular sieve could improve its stability in alkaline system.In this work,TS-1 molecular sieve was prepared via one-step hydrothermal method by using tetraethyl orthosilicate as the silicon source,tetrabutyl titanate as the titanium source,and tetrapropylammonium hydroxide as the template.The as-synthesized TS-1 was calcined in oxygen gas,nitrogen gas,nitrogen and hydrogen mixed gas,respectively,which resulted in the carbon-modified TS-1.XRD,N2 physical adsorption,DR UV-Vis,Raman,FT-IR and other methods were used to characterize the crystal structure,specific surface area,titanium content in the molecular sieve framework and physicochemical properties of different titanium species.The catalytic performance of carbon modified TS-1 was investigated in the ammoximation of cyclohexanone.The main conclusions are as follows:(1)According to the results of XRD,it can be known that the calcination atmosphere has little effects on the skeleton structure of TS-1 molecular sieve.The effects of calcination temperature,calcination time and nitrogen flow rate on crystallinity are as follows: calcination temperature > nitrogen flow rate > calcination time,and the optimum calcination condition is 450?-30 m L/min-8 h.(2)According to the results of DR UV-Vis,the effects of calcination temperature,calcination time and nitrogen flow rate on the framework titanium are as follows: nitrogen flow rate > calcination temperature > calcination time.The best calcination condition is 500?-10 m L/min-8 h.(3)In the intermittent and continuous reaction of cyclohexanone ammoximation,the carbon modified TS-1 has better stability and longer service life than the unmodified and commercially purchased TS-1,which is attributed to the introduction of carbon prevents the erosion of the framework structure of the molecular sieve in the alkaline environment and extends the service life of the catalyst.When the carburization content and skeleton titanium content were considered,the modified TS-1 had the best stability when the carburization degree was 1.0% and the skeleton titanium content was about 14%.(4)The reason for the deactivation of TS-1 is the dissolution of silicon in continuous cyclohexanone amoxime reaction system leading to the loss of the active center of the framework titanium.