Prparation Of Co₃O₄@SiO₂ Core-shell Catalyst And Its Application In The Decomposition Of Cyclohexyl Hydroperoxide

Cyclohexanone is an important chemical raw material and the main intermediate for the synthesis of caprolactam and adipic acid.At present,a large proportion of cyclohexanone is produced by cyclohexane non-catalytic oxidation method,which controls the conversion rate of cyclohexane at about 3.4%in the oxidation stage to obtain the important intermediate cyclohexyl hydroperoxide（CHHP）,and then uses the alkali addition process to catalyze the decomposition of CHHP.The total selectivity of cyclohexanol and cyclohexanone is 87%.There are many by-products and post-treatment of alkali liquor is easy to cause environmental pollution.Therefore,it is of great significance to develop a catalyst that can decompose CHHP efficiently without alkali for improving the competitiveness of cyclohexane oxidation method.Supported cobalt-based catalysts have a good catalytic effect and application prospect in CHHP decomposition,but the problem of active component loss is common in the recovery of the catalysts.In this paper,a cobalt-based core-shell catalyst was applied to the decomposition of CHHP in order to reduce the loss of metal components by using the core-shell coated double-layer structure.Firstly,Co₃O₄ core was prepared by the pyrolysis of cobalt nitrate.Cetyltrimethylammonium bromide（CTAB）was used as the template,and SiO₂ was deposited on the surface of Co₃O₄ particles through the hydrolysis and condensation reaction of tetraethyl orthosilicate（TEOS）.Co₃O₄@SiO₂was obtained after the template was removed by the method of calcination and extraction washing,respectively.The structure of the catalysts was characterized by various techniques and the catalytic performance in decomposition of CHHP was investigated.The results show that the catalyst has a spherical double-layer core-shell structure,and the Co₃O₄ core possess diameter of 110-280 nm.The shell is amorphous SiO₂ material with mesoporous channels,and the shell thickness is between 20-50 nm.The method of calcination to remove template agent would lead to the agglomeration of Co₃O₄nanocrystals and reduce the catalytic activity.The method of extraction washing could effectively remove template agent and maintain the catalytic activity of Co₃O₄ core.The optimal dosage of Co₃O₄@SiO₂ in the CHHP decomposition reaction is 2 mg Co/g reaction solution,and the conversion rate of CHHP is 64.20%,lower than the core Co₃O₄ catalyzed conversion rate of 86.18%,indicating that the coated SiO₂ shell increased the diffusion resistance,slowed down the reaction rate.Secondly,a series of Co₃O₄@SiO₂ catalyst samples were prepared by changing the ratio of ethanol to water,the concentration of CTAB and the adding amount of TEOS in the SiO₂ shell preparation process.The structure and composition of the catalysts were characterized by various techniques,and the catalytic performance and stability were evaluated in the decomposition reaction of CHHP.The results show that when the water content in ethanol aqueous solution is more than 10%,the pore size of disordered mesopores can be increased,but it is easy to cause the agglomeration of Co₃O₄ core and the instability of shell structure.The amount of CTAB does not affect the thickness of SiO₂ shell and the size of pores in the shell,but the specific surface area and pore volume of the catalyst will be increased with the increase of the amount of CTAB.When the amount of TEOS increases,the thickness of SiO₂ shell increases.At the same time,the drop acceleration rate of TEOS should be controlled to ensure the uniformity of shell thickness.The best performance sample Co₃O₄@SiO₂(V_TEOS=0.8 m L)catalyzed the reaction at 80℃for 90 min,the conversion rate of CHHP decomposition was 81.68%,the selectivity of cyclohexanol and cyclohexanone was70.60%and 34.06%,respectively.This is related to the thinner shell thickness and the higher porosity in the shell.Different degree of shell breakage occurred in the recovery process of these catalyst samples.Thirdly,the effects of the reaction time of thermal decomposition of cobalt nitrate in n-octanol on the particle size and activity of Co₃O₄ core,as well as the effects of template agent with different carbon chain lengths on the pore size of shell and the activity of Co₃O₄@SiO₂ were investigated.The results show that the particle size of Co₃O₄ increases and the monodispersity decreases with the increasing of pyrolysis time.The activity of Co₃O₄（1.5h）was the best,and it can still keep high activity after one recovery.The activity of Co₃O₄@SiO₂(C₁₈-TAB)in CHHP decomposition reaction is higher than Co₃O₄@SiO₂(C₁₆-TAB).The content of cobalt in the catalytic reaction solution of Co₃O₄@SiO₂(C₁₈-TAB)is significantly lower than that in the reaction solution of Co₃O₄,which indicates that SiO₂ coating can effectively slow down the loss of cobalt element.Finally,in order to improve the stability of the catalyst,polyvinylpyrrolidone（PVP）was used to modify Co₃O₄@SiO₂.The structural composition and morphology of the catalysts before and after modification were analyzed by various characterization methods.At the same time,the catalysts before and after modification of PVP were applied to the decomposition reaction of CHHP and the stability was investigated.The results show that the free hydroxyl groups on the surface of SiO₂ combine with the C=O bond in PVP to form hydrogen bond,which realizes the preparation of the modified sample Co₃O₄@SiO₂-PVP.The catalytic activity of Co₃O₄@SiO₂-PVP did not decrease compared with that before modification,and the catalytic performance remained stable after being recycled for five times,what’s more,PVP modification is helpful to further reduce the loss of cobalt element.