Study Of Surface Silanol On The Catalytic Oxidation Of Cyclohexylamine By Silica

Silica is an inorganic solid material with porous structure,it is usually used as catalyst carrier in chemical reactions,and owing to the rich silanol groups on the surface,it is occasionally used to catalyze ester condensation and olefin isomerization reactions.Recently,it has been found that metal-free SiO₂ has good catalytic effect on the selective oxidation of cyclohexylamine.The active silanol on the surface has attracted great interest from researchers,which is especially used to explore the research and development of new caprolactam processes.In this paper,a two-dimensional hexagonal mesoporous SBA-15pure silicon material with high specific surface area was selected,and molecular oxygen was used as green oxygen source to explore the selective oxidation of cyclohexylamine to cyclohexanone oxime.At the same time,the structure-activity relationship between the intrinsic structure and catalytic activity of the active sites of silanol groups was revealed from the molecular level by combining with various spectral characterization and DFT theoretical calculations.The rule of surface silanol active sites in the selective activation of guest molecules was elucidated,and a possible amination mechanism was proposed.The main contents are as follows:?1?Mesoporous SBA-15 catalytic material was prepared by hydrothermal synthesis,and then the surface silanol of SBA-15 was modifiedbysilylationreagents?trimethylchlorosilane,triphenylchlorosilane?,hydrochloric acid,ammonia and different temperatures.The obtained catalyst used air as a green oxygen source to study the effect of the change of the surface silanol active point on the oxidation of cyclohexylamine.The results showed that the activity of the catalyst decreased significantly after modified with silylation reagent,and the conversion rate was lower than 1%after the removal of silanol in SBA-15 channel by trimethylchlorosilane with small molecular dynamics radius.The triphenylchlorosilane with a larger molecular dynamic radius only desiliconized the surface of the catalyst,and the reduction of conversion of cyclohexylamine was relatively low?4%?,but the selectivity of cyclohexanone oxime was lower than 80%,indicating that the silanol group in SBA-15 channel had reaction-limited catalytic effect.There was no significant change in the activity of the surface silanol treated with hydrochloric acid?8.63%?,but the selectivity of cyclohexanone oxime decreased?68.28%?.After the treatment with ammonia,the conversion of cyclohexylamine was significantly reduced?3.31%?and the selectivity of oxime was also significantly decreased?78.54%?,this showed that the microenvironment of surface silanol affected the activity of silanol.The silanol groups on the surface of the catalyst were treated at different temperatures,and the catalytic activity gradually decreased with the increase of dehydroxylation.The structure of the catalyst was characterized by in-situ infrared spectroscopy,Py-IR spectroscopy and solid-state NMR;the results confirmed that the surface silanol with neutral or weakly acidic microenvironment was the active center of the catalyst.?2?Based on the above results,four different surface silanol structures were constructed,which were isolated silanol,adjacent silanol,geminal silanol and“hydrogen-bonded silanol”,and the silanol structures optimized by quantitative calculation.Meanwhile,the adsorption activation of the guest reactant molecules by four silanol structures was calculated and predicted theoretically.The results showed that the adjacent silanol and“hydrogen-bonded silanol”can form strong intermolecular force with cyclohexylamine molecules,which was beneficial to the activation of N-H bond of cyclohexylamine and the rearrangement of the intermediate nitrosocyclohexane.On this basis,the possible catalytic mechanism for the silanol to participate in the oxidation of cyclohexylamine to cyclohexanone oxime was proposed.?3?In this study,a new process for the preparation of caprolactam by selective oxidation isomerization of cyclohexylamine with air as green oxygen source and isoamyl alcohol as diluent was also designed,in which low titanium species were loaded onto SBA-15 surface by impregnation method.At the same time,the conditions such as reaction temperature,titanium loading and reactant ratio were optimized to determine the optimal reaction conditions.The results showed that when 5%Ti/SBA-15was used to catalyze the oxidation of cyclohexylamine,the conversion rate was 16.65%,and the selectivity of caprolactam reached 53.05%.Finally,the structure of the catalyst was characterized by FT-IR spectroscopy and UV-vis spectroscopy.