Rational Synthesis Of Supported Noble Metal Catalysts And Their Catalytic Application In Gas-phase Oxidative Dehydrogenation Of Cvclohexanol

With the rapid development of nano-science,supported metal nanoparticle catalysts have received increasing attention and have been applied in industrial production of many bulk chemicals.Cyclohexanone is a very important intermediate in current chemical industrial practices as it is frequently used in the manufacture of nylon.Meanwhile,cyclohexanone is also a widely used solvent because many organics such as paints,pesticides and dyes can dissolve in it.Nowadays,cyclohexanone is mainly produced from separation of KA-oil(a mixture of cyclohexanol and cyclohexanone),which comes from oxidation of cyclohexane,and direct dehydrogenation(DDH)of cyclohexanol.However,the DDH process is limited by thermodynamic equilibrium and needs to be operated at a relatively high temperature.Thus,this process is energy-intensive and catalyst deactivation caused by coke is frequent.For the purpose of simplifying operations and saving energy,it is of great significance to develop gas-phase oxidative dehydrogenation(ODH)of cyclohexanol and KA-oil.Aimed at the aforesaid reaction,in this thesis,ordered mesoporous silicas with very large pores(denoted as LP-FDU-12)were synthesized and used as supports for gold nanoparticles(AuNPs).The prepared Au/LP-FDU-12 catalyst with optimized gold particle size exhibits excellent and stable catalytic performance in gas-phase ODH of cyclohexanol.Besides,we also studied the effect of surface property and morphology of Pt nanoparticles(PtNPs)on their catalytic selectivity and activity in direct gas-phase ODH of KA-oil.The main works in this thesis are as follows:(1)Ordered mesoporous silicas with very large pores(up to 21.2 nm)were successfully synthesized at room temperature(25 ?)using F108(EO132PO50EO132)as a template,breaking the limitation of traditional low-temperature strategy.Compared with the more commonly used template F127(EO106PO70EO106),F108 possesses a larger molecular weight and a higher critical micelle temperature(CMT).By studying the relationship between the unit-cell size of ordered mesoporous silica FDU-12 and the initial synthesis temperature,we found that the temperature range,in which the unit-cell size rapidly increases,shifts to a higher temperature in the case of F108.Therefore,when the initial synthesis temperature is maintained at 25 ?,the swelling agent can penetrate into the micelle formed by F108 more easily,resulting in subsequent size expansion of unit-cell and mesopores.In addition,the sizes of mesopores and entrances of LP-FDU-12 are tunable by changing hydrothermal temperature.At elevated hydrothermal temperature,both of pore size and entrance size will increase.(2)We systematically studied the effect of entrance size of LP-FDU-12 on the anti-sintering properties of Au/LP-FDU-12 catalysts and then applied these gold catalysts in gas-phase ODH of cyclohexanol.Excellent anti-sintering property was observed for LP-FDU-12 with an entrance size of 3-5 nm over a wide Au loading concentration(1.0-8.3 wt%).LP-FDU-12 possessing a larger entrance size(7 nm)shows a poor anti-sintering property at low loading amounts because Au atoms or small Au particles can easily migrate and aggregate through entrances between the mesopores.Smaller entrance size(<3 nm)prevents ingress of 3-nm AuNPs to the mesopores and the unique mesoporous structure of LP-FDU-12 cannot play a role,finally resulting in sintering.When LP-FDU-12 with suitable entrance size(3-5 nm)is used as the support,its 3D large mesopores and relatively narrow entrance dimension can enhance inter-particle distances and meanwhile function as physical barriers,and therefore,the AuNPs can be stabilized within the range of 4.5-5.0 nm after calcination at 550 ? in air for 5 h.The gas-phase ODH of cyclohexanol is proved to be sensitive to the particle size of AuNPs.By choosing suitable LP-FDU-12 support and calcination temperature,thermally stable and size-optimized AuNPs are obtained which exhibit superior catalytic activity up to 1544 mmol gAu-1 h-1 and can avoid particle growth and obvious deactivation in 100 h.(3)Aimed at direct gas-phase ODH of KA-oil,we studied the effect of different promoter metals on the catalytic performance of 3-nm PtNPs(1 wt%on SiO2).The result showed that by adding 0.1-0.2 wt%Bi,the cyclohexanone selectivity can be enhanced from 16%to 76%,while the cyclohexanol conversion is almost maintained the same.Furthermore,we analyzed the generating pathways of by-product CO2 and found that the poor selectivity of 3-nm PtNPs is caused by the excessively strong adsorption of cyclohexanone on the surface of the catalyst.The introduction of Bi can weaken the adsorption strength of cyclohexaone to make the product molecules desorb from active sites more easily and also to suppress the adsorption of gas-phase cyclohexanone molecules.Thus,the total oxidation of cyclohexanone is avoided to a certain extent and the selectivity is obviously enhanced.Combining the results of XPS,HAADF-STEM EDS mapping and DFT theorectical calculations,we excluded the electronic effect and proposed that the geometrical effect due to the deposition of Bi on the surface of PtNPs is the main reason for the higher cyclohexanone selectivity of PtBi catalysts.This work highlights the significance of weakening the adsorption of target products to enhance their selectivities.(4)We successfully regulated the morphology of PtNPs by tuning the calcination atmosphere and applied them in gas-phase ODH of KA-oil.We found that calcinating H2PtCl6/SiO2 in H2 atmosphere will result in spherical PtNPs.However,in O2 atmosphere,many cube-like PtNPs can be obtained due to the strong interaction between O2 and Pt(100)facets.Experimental and DFT theorectical calculation results indicate that cubic PtNPs possess stronger adsorption ability to cyclohexanol molecules and therefore the Pt/SiO2 catalyst calcined in O2 shows excellent catalytic performance up to 78.1%cyclohexanone yield at 170 ?,exceeding most of the reported catalysts.