Nanocrystallization,hierarchical Structure Construction And Catalysis Of POM-based MOF

Polyoxometalate-based metal-organic framework hybrids?POM@MOFs?combining the structural characteristics and performance advantages of POMs and MOFs,are a new kind of solid catalytic materials and have developed rapidly in recent years.In this paper,we take NENU-n series?composing of Keggin-type POMs loading in a MOF of Cu ions bridging with H₃BTC?POM@MOFs as the research object.A variety of nano/micro-sized POM@MOFs hybrid materials have been obtained by constructing hierarchical pores,regulating morphology and shaping films.Their catalytic performances have been systematically studied and the main innovative achievements are as following:1.We propose a gel-like cooperated with freeze-drying strategy to construct hierarchically porous POM@MOF hybrids.And hierarchically micro-and mesoporous NENU-9([Cu₁₂?BTC?₈?H₂O?₁₂][H₅PV₂Mo₁₀O₄₀])is obtained by this strategy.Firstly,Cu?OAc?₂ and H₅PV₂Mo₁₀O₄₀0 quickly react with H₃BTC to form NENU-9 nanoparticles in ethanol.And then ethanol is exchanged for water by distillation.These nanoparticles with sol character have a high surface energy and easily interconnect.Take into consideration of the nature of volume expansion and expelling impurities in water,NENU-9 nanoparticles are squeezed,further cross-linked and then trapped between the layers of ice crystals during the process of water solidificating into ice.After removing ice crystals by freeze-drying process,stable mesopores are formed.This strategy is simple and probably extended to other POM@MOF systems to construct hierarchical structures.The hierarchically porous structure breaks the micropore size limit of MOF materials,having wider diffusion channels and more accessible catalytic active sites of POMs.Therefore,the formed hierarchically porous NENU-9 shows significantly improved catalytic activities for the oxidation of alcohols with large size compared to the microporous catalyst.2.A simple strategy is proposed to construct POM@MOF hybrids with controlled morphology.By using the ability of Keggin-type POMs to oxidize Cu to Cu²⁺as metal sources,POM@MOF?NENU-n?nano/micro crystals which encapsulated different Keggin-type POMs are prepared in situ on Cu substrates?including Cu foil,wire or mesh?.It is found that NENU-n crystals exhibit different morphology in the absence of any modulators,achieving the morphology-controlled growth of POM@MOFs.It is well known that NENU-n series exhibit an octahedral morphology under normal conditions and cubic crystals are obtained by adding a modulator.In this work,although Keggin-type POMs have the same structure,cubic NENU-n crystals are obtained when polyoxoanions have a higher charge number?5 or 6?.Whereas,NENU-n crystals show an octahedral morphology when polyoxoanions have a lower charge number?3 or 4?.We speculate that the morphology of NENU-n nano/micro crystals is closely dependent on the interaction between POMs and Cu substrate.The higher negative charge of polyoxoanion results in a stronger interaction with Cu substrate,which will affect the growth of{100}crystal facets which exposing POMs,leading to cubic crystals.Otherwise,the lower negative charge of polyoxoanion results in a weak interaction with Cu substrate,the growth of the{100}crystal facets is not affected by the interaction between POMs and Cu substrate,leading to an octahedral morphology.The synthesis of cubic NENU-n nano/micro crystals by a simple and fast method is of great significance for catalytic research,because the cubic morphology exposes more POMs active sites on surface compared with the octahedral morphology.The formed cubic catalyst exhibites high catalytic activity and aldehyde selectivity for both solvent-free oxidation of benzyl alcohol and g-s phase oxidation of ethanol compared with octahedral catalyst.3.NENU-9 thin film is formed in situ on Cu substrates by solvent evaporation.It is observed that the thin film is composed by closely coexisted NENU-9 crystals by SEM images.The formed film showed the high stability,due to the strong affinity between the Cu substrate and in situ formed NENU-9.We delved into the growth conditions of NENU-9 film.It is found that a low-volatile solvent and a low evaporation temperature are beneficial to the formation of a highly uniform,dense and crystalline cubic NENU-9 film.The NENU-9 film formed by this method can be used as a film catalytic reactor,which exhibits high catalytic activity and stability for the phenol oxidation reaction.