Study Of The Design,Construction And Catalytic Performance Of Polyoxometalate-based Heterogeneous Catalysts With Hollow/Core-shell Nanostructure

Polyoxometalates（POMs）,a class of discrete anionic metal-oxygen clusters,has been widely used in various catalytic applications due to their strong Br（?）nsted acidity,high proton mobility,excellent redox properties,and high thermal stability.However,homogeneous POMs suffer from the low surface area,high solubility in polar solvents,and difficulty in separation and recovery.The heterogenization of POMs is an effective strategy to fabricate POM-based heterogeneous catalysts by immobilizing POMs on supports or encapsulating POMs into the cavities.Furthermore,hollow/core-shell nanostructures always endow heterogeneous supports with high surface area,convenience for functionalization,and high accessibility of active sites.Therefore,it is of great significance to fabricate POM-based heterogeneous catalyst with hollow/core-shell nanostructure.In the design and construction of heterogeneous catalysts,the keys are ensuring the uniform distribution and exposing of POMs,improving the accessibility of active sites,the synergistic effect between POMs and supports,and the catalytic performance in specific catalytic reactions.In this dissertation,porous materials with large surface area,as well as adjustable morphology and composition were selected as porous materials,such as organosilica hollow nanospheres,layered double hydroxides（LDHs）,and metal-organic frameworks（MOFs）.By immobilization or encapsulation of POMs on these supports,a series of POM-based heterogeneous catalysts with hollow/core-shell nanostructures were successfully fabricated.The morphologies,compositions,and structures of these heterogeneous catalysts were carefully characterized,the interaction and assembly mechanism between POMs and supports were investigated,and the catalytic performance and structure-activity relationship were further studied.The specific research contents of this dissertation are mentioned as following:1.By using sol-gel strategy,POMs were immobilized on the surface of sulfonic acid-based ionic liquids functionalized organosilica hollow nanospheres by the electrostatic interaction between POMs anions and N⁺in imidazole groups,and a series of novel PW₁₂/Si（Et）Si-Dim-SO₃H solid acid catalysts were successfully fabricated.In the esterification of cellulose to cellulose acetate propionate（CAP）,PW₁₂/Si（Et）Si-Dim-SO₃H exhibited excellent catalytic performance,which produced high value-added CAP products with substitution degree（DS）of 2.69,viscosity of 470 m Pa·s,and molecular weight（Mw）of 102882,and can be recycled for at least 6 cycles Furthermore,two-dimensional nuclear magnetic resonance（2D NMR）spectra indicated that in CAP products,the hydroxyl groups at C2 and C3 in anhydroglucose units were mainly replaced by acyl groups.The excellent performance can be attributed to the large surface area of hollow nanospheres,which is beneficial to exposing acidic sites,shorten the mass transfer path and improve the accessibility between substrate and active sites.In addition,PW₁₂and sulfonic acid active site facilitate the controllable esterification of cellulose.2.A“pre-encapsulation-coating-transformation”strategy was proposed,which involves the first preparation of PW₁₂@UiO-66 to achieve the pre-encapsulation of PW₁₂,followed by the uniformly coating of ZIF-8 to obtain PW₁₂@UiO@ZIF-8 core-shell nanostructure.With the subsequent in situ transformation from ZIF-8 to Zn Co-LDH,PW₁₂@UiO@Zn Co-LDH acid-base bifunctional catalysts with core-shell nanostructure were successfully fabricated.Within the core-shell nanostructure,octahedral PW₁₂@UiO-66 with acid sites acted as the core and interlaced Zn Co-LDH nanosheets with base sites acted as the shell.In cascade deacetalization-Knoevenagel condensation,PW₁₂@UiO@Zn Co-LDH exhibited excellent>99%conversion of BDMA and>99%yield of final product ECC at 80 ^oC within 6 h due to the synergistic effect between acid-base sites,and can be recycled for more than 10 cycles.The excellent performance can be attributed to the unique core-shell nanostructure of PW₁₂@UiO@Zn Co-LDH with acidic core and basic shell,which is beneficial to shortening the mass transfer path,promoting the rapid conversion of intermediates,and facilitating the cascade reaction.3.An“in situ encapsulation-reassembly”strategy was proposed for the construction of PMo₁₂@NiCo-LDH hollow nanocages.The evolution process included the etching of PMo₁₂@ZIF-67 precursor,the releasing of PMo₁₂anions,the in situ formation of NiCo-LDH hollow nanocages and the reassembly between PMo₁₂ and NiCo-LDH.Different from traditional POM-LDH composites in which POMs intercalated into LDHs,PMo₁₂@NiCo-LDH hollow nanocage was consisted of interlaced NiCo-LDH nanosheets with PMo₁₂ encapsulated in.In photocatalytic CO₂ reduction（CO₂PR）,PMo₁₂@NiCo-LDH exhibited excellent CH₄ selectivity of 86.2%and suppressed H₂ selectivity of only 3.3%underλ>500 nm irradiation,and can be recycled for at least 5 cycles.Such excellent photocatalytic performance can be attributed to the facilitated efficient generation,transfer and separation of photo-induced electron-hole pairs by the incorporation of PMo₁₂ into NiCo-LDH,and the presence of crucial intermediates CH₃O*and CHO*which are beneficial to the highly selective generation of CH₄.4.The“in situ encapsulation-reassembly”strategy was extended to PW₁₂clusters and PW₁₂@NiCo-LDH acid-base bifunctional catalysts with hollow nanocage morphologies were successfully fabricated.The reassembly mechanism and assembly form between PW₁₂ and NiCo-LDH layers during the evolution process were deeply studied,and the catalytic performance was carefully investigated in cascade deacetalization-Knoevenagel condensation.Most importantly,in-depth investigation of fine structure by XAFS analysis and DFT theoretical calculation suggested that the terminal oxygen of PW₁₂preferred to assemble with oxygen vacancies on NiCo-LDH layers by strong electrostatic interaction.Furthermore,PW₁₂ species endowed PW₁₂@NiCo-LDH with moderately strong acid sites,and NiCo-LDH species endowed with weak,medium and strong base sites.Compared with physical mixture of PW₁₂and NiCo-LDH,PW₁₂@NiCo-LDH achieved product ECC yield of 99%in a shorter reaction time,and can be recycled for 10 cycles in cascade deacetalization-Knoevenagel condensation.The excellent catalytic performance is attributed to the stable assembly between PW₁₂ and NiCo-LDH,which is beneficial to shortening the mass transfer path of intermediates.In addition,the encapsulation of PW₁₂ in hollow nanocages endows bifunctional catalysts with excellent stability.