Precise Construction And Catalytic Performance Of Hierarchical Polyoxometalate-Based Metal-Organic Frameworks

NENU-n series polyoxometalate-based metal-organic frameworks（POM@MOFs）are inorganic-organic hybrids built from Keggin POMs loading in[Cu₃（BTC）₂]（BTC=trimesic acid）framework.Because of their porosity,amphiphilicity,and double acid sites（B and L acid）,they show excellent catalytic activity in numerous reactions and have become a representative novel class of POM-based solid catalytic materials.However,the inherent microporous nature of MOFs results in substrate diffusion barrier or poor accessibility to active POM sites inside,thus limiting catalytic applications of POM@MOFs.In order to break microporous restrictions,a series of innovative works around constructing hierarchical POM@MOFs are carried out,and the main research achievements are as follows:1.For the first time,the fractal growth mechanism is found in POM@MOF system,and it is used to construct hierarchical POM@MOF fractal crystals.Fractal growth is a kinetic process far from equilibrium.In the double solvent of chloroform/methanol（v/v=4/1）,NENU-3a([Cu₁₂（BTC）₈][H₃PW₁₂O₄₀])spontaneously assembles into hierarchical structure with ordered layered surface,and the morphology gradually evolves with time during the crystallization process and finally appears as the regular cuboctahedron.The kinetic-controlled nonrandom crystallization process is typical characteristic of fractal growth.Because the fractal growth is controlled by the crystal structure symmetry,and the Keggin POM as a structure directing agent has cuboctahedral structure symmetry,the NENU-3a fractal crystal exhibits a cuboctahedral morphology.The hierarchical layered surface structure effectively promotes mass transfer and enhances active center exposure,thus greatly improving catalytic activity in Biginelli reactions.The POM@MOF fractal growth,as a solvent-assisted self-modulated process,turns the disadvantage of morphology control in complex system into an advantage,and may be applied to the construction of other hierarchical POM@MOFs.2.A chelation-assisted selective etching strategy is proposed to construct macro-,meso-and microporous POM@MOF.Acid etching can locally break MOF coordination bonds to bring defects and thus construct hierarchical pores.However,excessive acidity can reduce etching selectivity or even entirely destroy MOFs.Effective control of acidity is the key to selective etching.We propose a coordination-assisted acid etching strategy,in which a chelator that can bind and release protons is added to maintain appropriate acidity for selective etching.We use H₃PO₄ as the etchant and ethylenediaminetetraacetic acid disodium（Edta H₂Na₂）as the aided chelator for etching NENU-3a.After adding the etchant,Edta H₂^2-binds protons to avoid excessive etching caused by strong acidity;during the etching process,the chelator captures the liberated Cu²⁺and thus releases protons.The on-demand storage and release of protons by chelators enables dynamic proton concentration equilibrium.The POM-exposed{100}facets are selectively etched by appropriate acidity,affording cubic NENU-3a with macroporous voids and internal mesopores.As the hierarchical pores effectively facilitate substrate transport and contact to active sites,the associated catalytic activity is significantly enhanced.3.A hard template method is used to prepare ordered macro-microporous POM@MOF single crystals.The methodology uses close-packed polystyrene（PS）nanospheres as a template,and a precursor solution is injected into the template gaps.Then in-situ POM@MOF crystallization is induced.Finally,removal of the PS template results in ordered macroporous POM@MOF single crystals.The key of this method lies in stabilizing the precursors from assembly before filling into the PS template interstices.We propose a solvent stabilizing precursor strategy,taking NENU-3a as an example,which utilizes the solvent effect（hydrogen bond）on POM by polar solvents such as methanol to weaken the coordination between POM surface oxygen and Cu²⁺,thereby inhibiting premature NENU-3a nucleation.Then the weak alkaline HCOONa regulates the in-situ nucleation and growth of NENU-3a through ligand deprotonation as well as coordinating modulation,affording ordered macroporous and intrinsic microporous NENU-3a single crystals after template removal.The ordered macroporous single-crystalline structure not only greatly improves the mass transfer efficiency,but also endows the catalyst with high stability,and circumvents prolonged diffusion channels and poor stability in disordered cross-linking or polycrystalline-stacking hierarchical architectures.4.A pseudo-homoepitaxial growth strategy is used to directly construct hollow POM@MOF single crystals.The strategy ingeniously exploits favorable characteristics of well-matched lattice/structure but acid stability difference between MOF and POM@MOF to construct a pseudo-homoepitaxial structure.To be specifical,the[Cu₃（BTC）₂]single crystals are used as seeds to perform epitaxial growth of the lattice/structure matched NENU-3a under appropriate conditions.Interestingly,the[Cu₃（BTC）₂]seeds collapse spontaneously during epitaxial growth,affording regular hollow NENU-3a single crystals in a short time（only 1.5min）.Because the acid stability of NENU-3a is much higher than that of[Cu₃（BTC）₂],it is speculated that the hollow structure formation may be related to a small amount of acid generated from the epitaxial NENU-3a growth.The hollow structure effectively shortens the microporous diffusion distance and facilitates active site accessibility.