Synthesis Of UiO-66 Composites And Their Catalytic Properties For Hydrogen Production And Methane Oxidation

Metal-organic frameworks（MOFs）,a kind of new porous crystalline materials assembled from metal ions（or metal clusters）and organic ligands,possessing tailorable and diversified structure as well as high specific surface area and large porosity,have gained widespread interest in catalysis,chemical sensing,pharmaceutical chemistry,gas adsorption and other fields.The accesible pores of MOFs facilitate the diffusion of substrate molecules to the catalytic active sites.Furthermore,the uniform pore size facilitates screening reactive substrates through size-matching effect to afford the superior selectivity while achieving high reactivity.UiO-66,as an important representative MOFs,has been used as a platform for various catalytic reactions owing to its variable ligand structure and high chemical stability.In this paper,based on the structural characteristics of UiO-66,different kinds of catalytic active sites,such as metal nanoparticles,metal single-atom,and metal complexes were introduced into UiO-66 for variable reactions.This dissertation focuses on the controllable synthesis,structural properties,and catalytic performance of UiO-66 composites in photocatalytic water decomposition and thermal catalytic methane oxidation.The influence of the microenvironment in MOFs on catalytic activity was discussed.The key research results are as follows:1.Single-atom catalysts（SACs）,identified as the ultimate limit of the small size of metal nanoparticles in supported catalysts,have been extensively studied over the last few years.Featuring the maximized atomic usage of metal catalysts and similar microenvironment,SACs show superior activity and selectivity to diverse catalytic reactions.However,isolated single metal atoms are apt to aggregate during reaction because of the high surface free energy,which severely handicaps its application.Therefore,the development of the synthesis strategy for stabilizing single metal atoms is highly desired.Herein,a general approach,using SnO₂ as a mediator onto metal-oxo clusters via microwave-assisted modification,has been developed to immobilize several single metal atomic sites in various MOFs.The M1/SnO₂/UiO-66-NH₂ present 5 times higher photocatalytic H2 production rate than the corresponding metal nanoparticles（-2.5 nm）stabilized by SnO₂/UiO-66-NH₂.The Pt1/SnO₂/UiO-66-NH₂ catalyst possesses the best hydrogen evolution rate,compared to the Cu1/SnO₂/UiO-66-NH₂ and Ni1/SnO₂/UiO-66-NH₂,which is attributed to the differentiated hydrogen binding free energies,as supported by density-functional theory（DFT）calculations.2.Direct oxidation of methane（CH4）into high value-added liquid organic oxygenates with high selectivity at low temperature and pressure remains a huge challenge due to the high energy barrier of CH4 activation.Herein,the AuPd@Cu-UiO-66x composites,AuPd alloy nanoparticles encapsulated inside a series of Cu-modified metal-organic frameworks（MOFs）UiO-66,realize highly efficient and direct catalytic conversion of CH4 to CH3OH at room temperature in aqueous solution with H2O2 as oxidant.The selectivity of CH3OH is effectively modulated by the amount of Cu species.Mechanistic studies reveal that Cu as the auxiliary component can regulate both the electronic state of AuPd nanoparticles and the reaction pathway of H2O2,which consequently resulted in a volcano-type dependency of the CH3OH selectivity on the amount of Cu.This work offers important insights into the understanding of the complex role of co-catalyst in the direct partial oxidation of CH4 to CH3OH.3.Soluble methane monooxygenases（SMMO）is a water-soluble protein with a nonheme iron active site that can efficiently convert methane to methanol in methanotrophic bacteria.Mimicking the catalytic function of enzymes by nanocatalysts can reduce the cost of catalysts while retaining the high catalytic activity of natural enzymes and improving the stability of catalysts.Iron porphyrin,possessing similar active sites to cytochrome P450,whose Fe sites can convert H2O2 to ·OH,is a suitable candidate for mimicking soluble methane monooxygenases（sMMO）.Fe-TCPP（TCPP=tetrakis（4-carboxyphenyl）porphyrin）has been incorporated into UiO-66,to achieve the direct conversion of methane to methanol in an aqueous solution at a mild temperature of 50℃.Long-chain saturated fatty acid was further introduced into the catalyst to convert the catalyst from hydrophilic to hydrophobic,further mimicking the hydrophobic pocket in sMMO.Remarkably,the hydrophobic modification around Fe-TCPP with fatty acids bearing long alkane chains effectively boosts and tunes the selectivity towards CH3OH in the CH4 oxidation.This hydrophobic modification not only regulates the electron state of the Fe active site but also tunes the concentration of reactive oxygen species around Fe active site,resembling the microenvironment regulating mechanism found in sMMO.