Synthesis,Formation Mechanism And Adsorption Performance Of Hierarchically Porous Metal-Organic Frameworks

Hierarchically porous metal-organic frameworks?MOFs?are a novel class of porous function materials,which organically combine the merits of MOFs and porous materials,such as high surface area,connected pore channel structure,abundant active sites and various topologies.Especially,the introduction of hierarchical pore can effectively improve the mass transfer performance of conventional microporous materials,which greatly extends its application in the adsorption and separation of complex gases and the transfer and transportation of macromolecular gases.Therefore,hierarchically porous MOFs have been widely used in the field of gas storage,adsorption/separation,drug delivery,sensing,and catalysis.To realize large-scale and commercial application of hierarchically porous MOFs,the first problem is to realize large-scale synthesis and control energy consumption.However,some problems are hindering the synthesis of hierarchically porous MOF materials.First,existing methods on hierarchically porous MOFs preparation are still in an infancy stage,and the preparation process is still in the"black box"state.Furthermore,the synthesis mechanism remains unclear and the stability and pore structure of materials are difficult to be guaranteed.Second,the limited types of structure-directing agents and complex synthesis process will greatly restrict the type of hierarchically porous MOFs generated.Third,the synthesis conditions of hierarchically porous MOFs is harsh,which conventionally requires high temperature and high pressure.Fourthly,hierarchically porous MOF synthesis requires to a long crystallization times.However,maintaining high temperature and pressure for a long time not only consumes huge amounts of energy,but also leads to low production rate.Due to that,based on the reviewing of existing synthesis methods of hierarchically porous MOFs,this paper aims to exploiting simple,facile and rapid strategies to synthesize hierarchically porous MOF materials.Furthermore,the synthesis mechanism and adsorption performance of material have been investigated.In this paper,a series of neutral organic molecules and amphiphilic quaternary ammonium salt were applied as templates,hierarchically porous Cu-BTC were successfully synthesized under hydrothermal conditions.The porosity properties of Cu-BTC can be easily tuned by controlling the amount of template.The synthesis of structure-directing agent greatly expands the types of template agents that can be used in the preparation of H-MOFs by soft template method.In particular,it is proved that neutral organic molecules have similar functions to the amphiphilic quaternary ammonium salt.Furthermore,the organic molecules are more cheap,which provides a new strategy for the preparation of hierarchically porous MOFs.In this paper,p-benzoquinone was innovatively employed as the functional additive to reduce the energy barrier of MOFs crystallization,and p-phthalic acid was introduced as the mixed ligand to generate defect structure.Hierarchically porous MIL-100?Fe?with micro-meso-macropores was synthesized at room temperature and pressure.The introduced p-benzoquinone and p-phthalic acid had a synergistic effect.The saturation adsorption capacity of toluene and p-xylene by hierarchically porous MIL-100?Fe?was 523.3 mg·g^-1 and560.2 mg·g^-1,respectively.This facile synthesis strategy provides a new possibility for low-energy preparation of hierarchically porous MOFs.In this paper,double salt intermediates formed from zinc oxide and metal ions was innovatively employed to accelerate the crystallization of MOFs,and the soft template agent was introduced as structure-directing agents to generate mesopores.Hierarchically porous Cu-BTC and ZIF-8 were successfully prepared at room temperature within 30 minutes.The space-time-yields?STYs?of Cu-BTC was up to 2366 kg·m^-3·d^-1.The maximum adsorption capacity of uranium?VI?on hierarchically porous Cu-BTC were 839.7 mg/g,which provides a new possibility for the application of hierarchically porous MOFs in the field of nuclide adsorption and separation.Using cationic surfactant?tetrabutylammonium hydroxide?as deprotonation agent and structure-directing agent simultaneously,hierarchically porous ZIF-67 was rapidly synthesized within 8 minutes at room temperature,which greatly reduced the energy consumption.The as-synthesized ZIF-67 had high total pore volume(V_t=1.46 cm³·g^-1)and mesopore volume(V_meso=0.9 cm³·g^-1),excellent thermal stability,solvent stability and alkali stability.The results of theoretical calculation show that the ligand defect and pore blockage defect of hierarchically porous ZIF-67 were 56.7%and 29.1%,respectively.The saturation adsorption capacity of toluene by hierarchically porous ZIF-67 was 296.7 mg·g^-1,which is12.7%higher than that of microporous ZIF-67.In this paper,a series of organic amines were used as dual-function template to rapidly synthesize hierarchically porous MOFs?ZIF-8?ZIF-61 and ZIF-90?within 1 minute.The introduced organic amines not only acted as deprotonation agents to deprotonate the organic ligands rapidly,but also served as structure-directing agents to direct the formation of mesopores.The maximal space-time-yield?STY?of ZIF-8 was 1.29×10⁴ kg·m^-3·d^-1.The maximum adsorption capacity of CO₂ and CH₄ by hierarchically porous ZIF-8 was 9.0mmol/g and 4.1 mmol/g,respectively.This facile,rapid,and versatile synthetic route not only reduces the energy-consumption of hierarchically porous MOFs,but also greatly improves the production rate.Thus,it is expected to provide theoretical guidance and reference for the large-scale industrial production and commercial application of hierarchically porous MOFs.