| Metal organic frameworks MOFs with high surface area,high porosity,ordered pore structure,adjusted pore size,high stability and other advantages,have a good prospect in adsorption separation,energy storage,catalysis,sensing,drug delivery and other fields.Up to now,researchers have successfully synthesized more than 70 thousand kinds of MOFs materials,but most of the MOFs are microporous.This not only is not conducive to the mass transfer of small molecules,but also greatly reduces the reaction rate,and it also limits the role of MOFs in the adsorption and catalysis of macromolecules,which greatly limits the practical application of MOFs materials.Therefore,it is of great importance to synthesize stable mesoporous or macroporous structure MOFs.In recent years,with the development of crystal defect engineering,the use of crystal defect method to construct hierarchical MOFs has attracted people’s attention.Crystal defect method is expected to be a practical method for hierarchical MOFs preparation because of its simple operation,less raw materials and no need to change the synthetic conditions of the original microporous MOFs.In this paper,the hierarchical Cu-BTC and Mg-MOF-74 materials are successfully synthesized by the crystal defect method.The influence of the ligand ratio,the reaction solvent,the activated solvent and the type of carboxylic acid on the pore structure and morphology of the materials are studied,and the synthesis mechanism of the crystal defect method is briefly discussed.At the same time,taking hierarchical Cu-BTC as an example,the rapid synthesis of crystal defect method of hierarchical MOFs is discussed,which provides the basic data for the practical application of hierarchical MOFs material.The specific research content and results of this paper are as follows:In this paper,a series of hierarchical Cu-BTC with different mesoporous pore sizes have been successfully synthesized using a mono carboxylic acid as a defect agent.The synthesis time of each sample is about 12 hours(not excluding the pre and post processing).The results show that:the samples prepared after the addition of acetic acid contain mesoporous structure,The optimum molar ratio of ligand to benzoic acid is 1:1.The pore size of the sample is concentrated at 2.7 nm,8.82 nm and 34 nm at the optimum ratio.The mesoporous pore volume of the sample is 0.22 cm~3/g,the BET specific surface area is 833 m~2/g,and the total pore volume is 0.50 cm~3/g.On the basis of the process parameters,a series of hierarchical Cu-BTC with different mesoporous pore diameter has been successfully synthesized by using a mono carboxylic acid as a defect agent.The synthesis time of each sample is about 10minutes(not excluding the pre and post processing).The results show that:the optimum molar ratio of benzoic acid and hexanoic acid to ligand ligand is 1:1,and the pore size of the sample is concentrated at 2.05 nm,2.32 nm,6.3 nm and 10.7 nm at the optimum ratio.The mesoporous pore volume of the sample is 0.16 cm~3/g,the BET specific surface area is 1425m~2/g,and the total pore volume is 0.72 cm~3/g.It is speculated that the larger pore size of acetic acid is due to the smaller molecular size of acetic acid than that of hexanoate.When the reaction solvent is the mixed solutions of DMF,water,ethanol and the activated solvent is ethanol,the material is the best.On the basis of the above work,a series of hierarchical porous Cu-BTC series with different mesoporous pore sizes have been successfully synthesized in this paper by using polycarboxylic acid terephthalic acid and citric acid as defect agents in about 10 minutes(not including the pre and post processing).The results show that:the samples prepared after the addition of terephthalic acid contain mesoporous structure,and the optimum molar ratio of benzoic acid to benzoic acid and terephthalic acid is 4:1,and the pore size of the synthesized samples is concentrated at 2.05 nm,2.54 nm and 3.88 nm at the optimum ratio.The mesoporous pore volume of the sample is 0.16 cm~3/g,the BET specific surface area is 1233m~2/g,and the total pore volume is 0.6 cm~3/g.When terephthalic acid is replaced with citric acid,the mesoporous structure is rich,and the optimum molar ratio of ligand to benzoic acid and citric acid is 6:1.The pore size of the sample is concentrated at 2.05 nm,2.32 nm and3.88 nm at the optimum ratio.The mesoporous pore volume of the sample is 0.16 cm~3/g,the specific surface area of BET is 1176 m~2/g,and the total pore volume is 0.72 cm~3/g.Compared with monobasic acid,the mesoporous pore size of polycarboxylic acids is smaller,probably because the space occupied by polycarboxylic acids in space is less than that of mesopores.In this paper,with acetic acid as a defect agent,the hierarchical porous Mg-MOF-74material was synthesized for the first time,and the optimum organic ligand to the defect agent molar ratio and the reaction mechanism of the synthetic process were investigated.The results show that:the optimum molar ratio of organic ligand 2,5-two hydroxy terephthalic acid and the defect agent t is 1:0.2,and the pore size of the synthesized samples is concentrated at 2.7nm,6.8 nm and 10.8 nm.The mesoporous pore volume of the sample is 0.12 cm~3/g,the BET specific surface area is 979 m~2/g,and the total pore volume is 0.41 cm~3/g.It is speculated that the synthesis mechanism of hierarchical Mg-MOF-74 is that there is no additional carboxyl group and magnesium ion because of the only one carboxyl group of the addition of the defect agent,and it will form a regular distribution of"vacancy"defect in the crystal because of the loss of the ligand,and then the mesoporous pore is formed locally.Further speculating that polycarboxylic acid is a defect agent,the reason for the formation of mesopore is that the added polycarboxylic carboxylic group has many carboxyl groups,but the molecular size is smaller than that of the ligand.The multiple carboxyl groups in the MOFs framework constructed by the macromolecule ligand acid can not all be paired with the magnesium ions,which will also be due to the loss of some ligands and in the crystal.The"vacancy"defect is formed in the interior,and then the mesoporous channel is formed locally.Moreover,the smaller the size of the defect acid is,the larger the mesopores are. |
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