Synthesis And Characterization Of Micro-mesoporous Metal-organic Framework HKUST-1

Metal-organic frameworks（MOFs） are a new generation of nano-porous materials, due to their high specific surface area, high porosity and excellent structure adjustment properties, they have great appplication prospect in gas storage, separation, catalysis and so on. Currently synthesized MOFs are mainly microporous strutured. The narrow channels make it difficult to have rapid diffusion and transfer of small molecules, which also limit MOFs materials’ s appplication in many reactions involving large molecules. Hierarchically MOFs containing microporous, mesoporous even macroporous, which can selectively integrated the advantages of microporous, mesoporous and macroporous material. The micropore walls can ensure that the materials have huge surface area, however, mesoporous or macroporous can allow large molecule’s diffusion mass transfer. Therefore, the development of hierarchically MOFs material containing stable mesoporous and macroporous is very important. In this paper, micro-mesopous HKUST-1 was synthesized by using templating method and crystal defects method. The impact of the adding amount of template, reaction solvent and other parameters on the pore structure of MOFs material was studied, at the same time, the toluene adsorption properties of micro-mesopous HKUST-1 was researched and the synthesis mechanism of micro-mesopous HKUST-1 was briefly discussed.In this work, cationic surfactant Dodecyl trimethyl ammonium bromide（DTAB） acts as the template, 1,3,5-trimethylbenzene（TMB） act as an expanding agent, meso-micropous HKUST-1 was successfully syntheiszed by using solvent hot method. The influence of water and alcohol volume ratio,the adding amount of template and the alkyl chain length of template on the pore structure of MOFs material was discussed. The experimental results indicated that all the three factors have an effect on mesoporous structure of the micro-mesopous HKUST-1:When the water and alcohol volume ratio VH₂O/VEtOH gradually increased from 0 to 1, the specific surface area of each sample and total pore volume were gradually reduced. When increased the adding amount of templating agent and expanding agent, the specific surface area of the samples was gradually reduced, while the total pore volume and mesopore volume of the sample will first increases and then decreases. When VH₂O/VEtOH=0.2, Cu²⁺/X/TMB molar ratio is 1:0.3:0.3, the as-systhesized MOFs display rich, regular, worm-like mesoporous structures at about 3.89 nm, accompanied with BET specific surface area of 950 m2/g and total pore volume of 0.53 cm3/g. Increasing the alkyl chain length of the surfactant, the pore size of mesoporous increases, indicating that the surfactants play a template role. At 25 ℃, we tested the toluene adsorption property of meso-microporous HKUST-1, when P/P0 =0.99, the toluene adsorption amount of micro-mesopous HKUST-1 is 18.8% higher than conventional HKUST-1.In this work, meso-microporous HKUST-1 with rich mesostructures was successfully synthesized by using organosilan surfactant Dimethyloctadecyl[^3-（trimethoxysilyl）propyl] ammonium chloride（TPOAC） as templates. As the research results show, When molar ratio of Cu²⁺and TPOAC is 1: 0.24, the as-systhesized MOFs display rich regular mesoporous structures at about 3.5 nm. When TPOAC adding amount is small, it has little impact on HKUST-1’s morphology. However,when Cu²⁺ and TPOAC molar ratio reaches 1: 0.40, the octahedral crystal morphology becames irregular, there are many depressions and protrusions on its octahddra surface. Computer simulations about the track distribution of TPOAC show that, LUMO orbital of TPOAC is mainly contributed by silicon atoms and three hydroxyl groups contribute connected to it, while the LUMO+1 orbital is mainly contributed by the quaternary amino functional groups, it proves the synthesis mechanism of micro-mesopous HKUST-1 by using templating method from the molecular level, that the micelle with a positive charge attract btc^3- in the reaction solution, btc^3- reacts with Cu²⁺ and generate microporous crystalline framework of HKUST-1, after TPOAC is removed, mesopore structures built by the microporous crystalline framework can be formed.In this work, by adding some benzoic acid in the reaction solvent, HKUST-1 wtih rich micro- and mesoporous structure can be well syntheiszed by using crystal defects method. All samples syntheiszed by adding benzoic acid have mesoporous structure. The best molar ratio of benzoic acid and trimesic acid is 0.5, the as-systhesized MOFs display rich regular mesoporous structures at 3.9 nm and 5.^3-25.4 nm, the mesoporous volumeis 0.25 cm³/g, BET specific surface area is 564 m²/g and the total pore volume is 0.46 cm³/g. Change the mixed reaction solvent to ethanol, the as-systhesized MOFs display mesoporous structures at 6.4 and 11 nm. Using DMF as the reaction solvent, mesoporous HKUST-1can not be synthesized. By a simple inquiry, we suggest that the synthesis mechanism of meso-microporous HKUST-1 by crystal defect method is as follows: because benzoic acid only has one carboxyl group on its benzene ring, when it coordinates with copper ion, some “defects” or “void site” may appears inside the HKUST-1 crystal, these “defects” or “void site” can be connected to form mesoporous or macroporous channels.