Construction Of High-efficiency Mesoporous Nanocatalysts And Their Selective Catalytic Oxidation Properties Of Hydrocarbons

In fine organic chemistry,catalytic oxidation is one of the most effective and convenient ways for C-H bonds functionalization.Directly converting cheap hydrocarbon materials into highly economic value products such as alcohols,aldehydes,phenols,acids,ketones are important research topic in the field of catalytic chemistry.The greatest challenge in oxidation hydrocarbon is the high C-H bond dissociation energy.Many industrial hydrocarbon oxide processes require high temperature environments and excessive energy input,which often results in uncontrolled product selectivity and excess coke production.Therefore,how to achieve selective oxidation of C-H bonds under relatively mild conditions has become a key challenge for the petroleum industry.The design and synthesis of high efficiency catalysts are the key to catalytic oxidation.Mesoporous metal oxides have been found to be the particularly suitable catalysts and supports because of their adjustable pore size and structure,low density,good adsorption,high specific surface area and well-developed pore structure.On the one hand,the porous structure is advantageous for reducing the mass transfer resistance between the reactants and product,on the other hand,it is advantageous for the dispersion of the active component on the surface,thereby greatly improving the performance of the catalyst.In this thesis,we designed and synthesized a series of high-efficiency mesoporous heterogeneous catalysts for the oxidation of hydrocarbons;revealing the correlation between the physicochemical properties of materials and their catalytic activity;proposed and realized the synthesis strategy of improving the structural stability and activity of the catalyst;deeply studied the reaction mechanism of catalyst in the oxidation process of hydrocarbons.The aim of this paper is to provide technical accumulation for the application of C-H bond sp3 activation system for hydrocarbons.The major achievements are described as following.We successfully encapsulated and stabilized ultra-small gold nanoparticles in the mesopores of colloidal metal oxides nanospheres by a simple and efficient phase transfer in-situ self-assembly method.Firstly,organic-ligand-protected gold nanoparticles with a certain size were synthesized.Then,the oil-phase gold nanoparticles were encapsulated into the hydrophobic core of CTAB micelles to form an aqueous phase gold@CTAB solution,followed by adding to the mesoporous oxide synthesis system.In finally,the CTAB serves as a bridge for the in-situ formation of mesoporous metal nanospheres around gold nanoparticles.The ultra-small gold nanoparticles firmly anchored in the inner wall of the mesopores nanospheres under the interaction of the interface effect and the space confinement.Different from the traditional impregnation method,the stabilized gold nanoparticles obtained by the phase transfer method showed much higher stability,dispersion and activity.The gold nanoparticles inside nanospheres exhibit excellent thermal stability and can be subjected to long-term high temperature treatment?up to 500°C?,which effectively solving the sintering problem of small-sized precious metal nanoparticles.In term of catalysis,our catalysts show excellent catalytic activity for aerobic oxidation of hydrocarbons.In addition,we further studied the effect of gold nanoparticle sizes and the types of support on catalytic activity,and found that with a decrease in gold nanoparticle size,the catalytic conversion efficiency of indane oxidation increased.In order to further increase the catalytic activity of the gold-based catalyst,we have attempted to synthesize gold cluster@mCeO₂ catalyst with a sub-nano size to catalyze the oxidation of the inert aromatic hydrocarbon C-H bond.Firstly,we stabilize the gold cluster(Au₂₅,Au₁₄₄)into the pores of mesoporous CeO₂ by phase transfer method.Then the organic ligand and surface surfactant of the gold cluster were removed by calcination,so that the active sites of the gold cluster are exposed.Gold clusters were encapsulated and confined in the intersectional channels of ceria due to the strong interfacial force between the carrier and the gold cluster and the spatial confinement of the mesopores structure.As for the oxidation reaction of toluene and ethylbenzene,these gold cluster catalysts exhibited especially excellent catalytic activities and afforded record turnover frequencies.Furthermore,we try to study the mechanism of high activity of the gold cluster catalyst by a series of experimental characterization and Density functional theory calculations,and found that the encapsulated positive gold clusters can effectively reduce the dissociation barrier for the oxygen molecules.Although the above-mentioned gold-based catalyst possesses high catalytic activity in hydrocarbon oxidation reaction,the economic cost of the catalyst is relatively high,and the selectivity of the hydrocarbon oxidation product is still unideal.In order to decrease the cost of the catalyst and further increase the selectivity of the product,we have attempted to synthesize other mesoporous metal oxide catalyst.A cost-effective mesoporous Co₃O₄ material with large pore volumes and high crystallinity was successfully synthesized by a simple ligand-assisted self-assembly method.In term of catalytic reaction,the mCo₃O₄-350 sample shows highly efficient catalytic activity for solvent-free oxidation of hydrocarbons in with molecular oxygen as oxidant,especially for the solvent-free selective oxidation of ethylbenzene.Furthermore,a series of experiments and characterizations were carried out to gain insights into the origin of the catalytic mechanism of the ethylbenzene oxidation process.We found that high surface area might result in the wreaking of the Co-O bond and the exposure of more surface vacancy defects.Abundant surface oxygen vacancies have advantages in reducing the chemisorption energy of O₂,then the adsorbed oxygen molecules accept delocalized electrons transform to reactive oxygen species.The synergistic effects between O/Vo and Co³⁺/Co²⁺redox can validly accelerate the oxygen transfer during ethylbenzene oxidation process.Notably,acetophenone direct transformation from the 1-phenylethyl hydroperoxide in our catalytic system,which may open new doors for selective oxidation alkanes.