Synthesis Of Hierarchical Cu-BTC And Hierarchical ZIF-8 By Template Method

Metal-organic framework materials（MOFs）are a crystalline material with a three-dimensional network structure formed by metal ions or metal clusters and organic ligands through a self-assembly process.In recent years,MOFs have attracted wide attention from researchers because of their high specific surface area,large pore volume,and diversified structures.However,most of the MOFs reported in the literature are microporous structures.The microporous structure makes it difficult for the reactant molecules,product molecules,and reaction intermediates with large dimensions to enter the interior of the channel,and it is also not conducive to the rapid diffusion and transport after entering the tunnel,which greatly limits the practical application of MOFs materials.As a result,the research of hierarchical MOFs has emerged.At present,the template method is considered to be the most potential method for preparing hierarchically porous MOFs,because the preparation of hierarchical MOFs by the template method has the advantages that the pore structure can be highly adjustable,the reaction conditions are mild,and there is no network penetration.This paper mainly focuses on the following issues:the adjustment of mesoporous pore size in hierarchical MOFs,the poor generality of templating agents,the high price of most templating agents,the pollution to the environment,and the difficulty in removing the templating agent,and carried out the following work.For hierarchical MOFs,it is important to adjust the mesoporous pore size flexibly according to actual needs.In response to this practical requirement,the cationic surfactant cetyltrimethylammonium bromide（CTAB）was used as a template and the linear-chain alkane C_nH_2n+2（5<n<12）was used as a pore-enlarging agent.The effect of linear alkanes on the mesoporous pore size of hierarchical Cu-BTC was studied systematically.Studies have shown that the addition of the template agent CTAB can only form small mesopores.The mesoporous structure cannot be formed by adding the expanding agent decane alone.Only the addition of CTAB and alkane can form mesoporous structures with large pore diameters.1:0.556:0.30:0.30 of Cu²⁺/H₃BTC/CTAB/swelling agent molar ratio is the optimum synthesis condition.Under this condition,the BET surface area of the prepared hierarchical Cu-BTC is846 m²·g^-1,the total pore volume is 0.52 cm³·g^-1,the mesopore volume is 0.21 cm³·g^-1.Alkanes can be used as swelling agents to enlarge the pore size of mesoporous materials,but the hole-expanding effect of pore-enlarging alkanes on the material is not infinite,and the mesoporous pore size of the material is concentrated at 4¹0nm.The optimal swelling effect of alkane introduced as expanding agent is decane,and the mesoporous pore size is 10.49 nm.From pentane to decane,as the number of carbon atoms of the introduced alkane increases,the pore size increases from 4.51 nm to 10.49 nm.For dodecane,the swelling effect starts to weaken,dodecane has a mesoporous pore diameter of only 9.35 nm.The mechanism of alkane as a swelling agent is as follows:the surfactant forms micelles in an aqueous solution,non-polar organic substances long-chain alkane molecules penetrate into the hydrophobic part of the micelle center（solubilization）increases the apparent diameter of the rod-shaped micelle,the volume of the micelles also increases,and the soluble MOFs precursors are then molecularly assembled on the surface of the micelles to form mesoporous MOFs with increased pore size.In view of the great success of the neutral organic amine templating agent as a structure-directing agent in the synthesis of hierarchical molecular sieves,this paper tried to hydrothermally synthesize two kinds of hierarchical MOFs—Cu-BTC and ZIF-8,using the neutral organic amine tetradecyldimethyl tertiary amine as the template,expanded the types of templating agents that can be used for the preparation of hierarchical MOFs.The experimental results showed that the mesopore volume can be adjusted by changing the dosage of the template.With the increase of the amount of templating agent,the mesopore volume first increases and then decreases.When molar ratio of Cu²⁺and template is 1:0.5,the synthesized hierarchically porous Cu-BTC has the most abundant meso-macroporous pore volume.Under this condition,the meso-macropore volume is up to 0.20 cm³·g^-1;When molar ratio of Zn²⁺and template is 1:0.7,the synthesized hierarchically porous ZIF-8 has the most abundant meso-macroporous pore volume.Under this condition,the meso-macropore volume is up to0.22 cm³·g^-1;The mesoporous pore size of Cu-BTC has a tendency of increasing with the increase of the alkyl chain length.Therefore,the pore size can be adjusted by changing the chain length of the anionic surfactant.Since the tetradecyldimethylamine contains a nitrogen atom and easily forms a hydrogen bond with the carboxyl group in the organic ligand,the hydrogen bond interaction is weak.In this paper,a low temperature evaporation method was used to remove the template agent.The evaporation method and conventional extraction methods for removing MOFs template have little difference in the pore structure parameters.The evaporation method has the potential as a new method for removing the template.One of the biggest problems in the synthesis of MOFs with cationic templating agents and neutral templating agents mentioned above is that the precursors of the MOFs and the templating agents exert weak forces,which makes the tendency of metal ions and organic ligands to bind to each other more.In view of this,the paper use of non-toxic,inexpensive anionic surfactant sodium dodecylbenzene sulfonate（SDBS）as a template to change the mode of action of the template agent and the metal ion or organic ligand of the synthetic MOFs material,and reduce the tendency of metal ion to bind to organic ligand,The hierarchical Cu-BTC and ZIF-8 with rich mesopores was successfully synthesized by hydrothermal method.The results show that the mesopore size varies from 7 to 21 nm by changing the amount of the templating agent.1:0.35 of Cu²⁺/SDBS molar ratio is the optimum synthesis condition.Under this condition,the BET surface area of the prepared hierarchically porous Cu-BTC is 659 m²·g^-1,the total pore volume is 0.55 cm³·g^-1,micropore volume is 0.21 cm³·g^-1,and the mesopore volume is up to 0.31 cm³·g^-1.With the increase of the amount of templating agent,the mesopore volume of the layer ZIF-8 increased gradually.When the molar ratio of Zn²⁺to templating agent SDBS is 1:1.0,the mesopore volume is as high as 0.28 cm³·g^-1;The adding order of the templating agent and the reaction precursor also has an effect on the material pore properties.The order Adding SDBS to the aqueous solution of copper nitrate trihydrate and then mixing with the trimesic acid ethanol solution is superior to that of directly adding SDBS to the mixed solution of copper nitrate trihydrateand trimesic acid,The samples prepared under the former order have higher crystallinity,the larger number of mesopores,and more concentrated mesopore size distribution.The as-synthesized hierarchical Cu-BTC was tested in the Herry reaction—the oxidation of p-nitrobenzaldehyde and nitromethane,and shows more excellent catalytic activity than the traditional microporous Cu-BTC.After 36 hours of reaction,the conversion rate of p-nitrobenzaldehyde is up to 47.3%,which is 30%higher than that of conventional microporous Cu-BTC.