Functionalized Organosilica Nanotube-based Heterogeneous Catalysts For Catalytic Energy Conversion

Metal complexes and ultrasmall metal particle catalysts exhibit extremely high catalytic performance due to the complete exposure of active sites during the reaction.However,these catalysts often tend to decompose or agglomerate due to intermolecular or interparticle interactions,leading to the lost catalytic activity.Therefore,how to build an efficient and stable catalyst and prevent intermolecular or interparticle agglomeration is a key scientific issue in this research field.Among them,the spatial isolation of catalytically active sites is one of the effective strategies to solve this problem.Organosilica nanotubes are a new type of organic-inorganic composite mesoporous materials,whose framework and surface structure is easy to functionalize,which provides a platform for the heterogenization of homogeneous catalyst.This thesis focuses on the synthesis of functionalized organosilica nanotubes to construct heterogeneous catalysts for energy-catalyzed reactions.The main research contents include:Bipyridine-functionalized phenyl-bridged organosilica nanotubes（BPy-NT）were controlledly synthesized.The length of nanotubes can be effectively controlled by adjusting the ratio of bipyridine to phenyl bridged precursor,and the shortest is about40 nm.Using bipyridine as the anchor,two metal iridium-based complexes[IrCp*-and Ir（cod）-]were successfully immobilized on nanotubes to construct molecular heterogeneous catalysts,and applied to the C-H oxidation of heterocyclic（such as tetrahydrofuran,etc.）and cycloalkanes and C-H boronation of aromatics.The catalytic performance indicates that the two molecular heterogeneous catalysts exhibit high catalytic activity comparable to that of analogous homogeneous catalysts,which is mainly attributed to the nanotube with large pore and short lengths,accelerating the diffusion of reactants and products.In addition,the heterogenization process achieves spatial isolation of active sites and effectively inhibits agglomeration and decomposition of molecular complexes.Therefore,the two heterogeneous catalysts showed significantly improved reusability.In order to expand the catalytic reaction in the water phase,bipyridine-functionalized ethenyl-bridged organosilica nanotubes were controlledly synthesized.Subsequently,the metal iridium-based complex（IrCp*-）was also successfully coordinated to the nanotubes to obtain a molecular heterogeneous catalyst,and used for the photocatalytic hydrogen generation from formate or aldehyde-water solution.Compared with the inorganic silica catalysts,the organosilica nanotube heterogeneous catalyst containing ethenyl groups in the skeleton can accelerate the desorption process of the hydrophilic products on the active sites during the reaction,thereby accelerating the release of hydrogen.Compared with the analogous homogeneous catalyst,heterogeneous catalysts effectively inhibit the agglomeration and decomposition of molecular complexes due to the spatial isolation of the active sites,which greatly improves the catalyst’s activity and cycle stability.It is worth mentioning that this novel and efficient heterogeneous photocatalytic system can be performed under relatively mild conditions（visible light,weak alkaline solution and room temperature）.More importantly,the purity of H₂ can be effectively adjusted by adjusting the pH value of the reaction system.Above the pH of 8,the H₂ purity was about 100%.This reaction system has more obvious advantages for meeting the criteria of hydrogen production from the the angle of safety and purity,especially for proton exchange membrane fuel cells.In order to further improve the efficiency of photocatalytic hydrogen production,and considering that amino groups have a good stabilizing effect on metal nanoparticles.Novel amino-functionalized phenyl-bridged organosilica nanotubes（AM-NT）were controlledly synthesized.Using this as a carrier,nanotube-limited Au Pd nanoclusters（～1 nm）were successfully prepared and further applied for the hydrogen evolution from formaldehyde aqueous solution.These bimetallic AuPd nanocatalysts exhibited remarkably improved catalytic activity under visible light than in the dark,and the highest initial TOF value of 241.7 h^-1 could be achieved.Performance experiments and optoelectronic characterization show that the high catalytic performance is mainly due to the unique nanocluster structure.In particular,electrons can be efficiently transferred from the Au sites with localized surface plasmon resonance（LSPR）effect to active Pd sites under visible light irradiation,making the electron density of the Pd active site increased greatly,which in turn brings high hydrogen production performance.In addition,the stabilization and confined effects of the amino and nanopore channels effectively inhibited the agglomeration of the nanoclusters,resulting in the significantly higher reusability.Utilizing the anchoring effect of silica and the controllable functional modification characteristics,molecular Re-based heterogeneous catalyst（Re-BPy-NT）was constructed by coordinating molecular rhenium complexes onto organosilica nanotubes via the"post-metallation method",and used to study the photocatalytic CO₂reduction performance in water-acetonitrile system.By carefully adjusting the bipyridine content in the nanotube framework,the heterogeneous molecular-catalyst-based nanotubes could effectively increase the adsorption capacity of CO₂ according to the results of CO₂ isothermal adsorption and in-situ infrared.In order to further inhibit protons from reaching the active sites to produce H₂,the remaining accessible surface silanols groups（Si-OH）of the organosilica nanotubes were capped using trimethylsilyl[-Si（CH₃）₃]to increase the hydrophobicity.Relative to that of the unmodified catalyst and analogous homogeneous catalyst,the prepared Re-BPy_0.3-NT-Me catalyst exhibits higher catalytic activity,CO product selectivity（>94%）and reusability in a high concentration water-containing system,and combined with density functional theory calculations（DFT）and in-situ characterization techniques,a reasonable reaction mechanism was proposed.