Study On Preparation And Property Of Hierarchically Porous Metal–organic Frameworks

Metal-organic frameworks(MOFs) are recognized as a novel series of porous materials which exhibit zeolite-like structures, with a total reported structure number of over 40,000. Due to their giant surface area, ultrahigh porosity and diverse structures, MOFs have shown versatile applications such as adsorptive separation, gas storage, catalyst, drug delivery and sensing. Nonetheless, only microporous structure presents in most conventional MOFs, which greatly hinders the mass transfer and diffusion process of macro molecules. Therefore, the active sites inside MOFs are not accessible for macromolecules, leading to poor performances in many applications of MOFs. In this work, supramolecular surfactants were acted as templates to introduce mesoporous and macroporous structures into conventional MOFs to prepare the hierarchically porous MOFs. The synthetic conditions, for example, the content of templates and the species of swelling agent, were investigated to investigate their influence on pore structures and crystal morphology. Moreover, the synthetic mechanism was discussed based on the previous research theories of hierarchically porous zeolites or computer simulation. The synthesis of hierarchically porous MOFs not only solves the mass transfer problems of microporous MOFs, but also provides the base on the further development on future hierarchically porous MOFs. Meanwhile, hierarchically porous MOFs will enrich the family of porous materials and extend the application fields of MOFs, which demonstrates great significance for the development of MOFs.Micro-mesoporous HKUST-1 was successfully hydrothermally synthesized using P123 as a template with decane or TMB as swelling agents. The porosity properties and morphology of prepared micro-mesoporous HKUST-1were characterized by XRD, adsorption of nitrogen, SEM and TEM. The influence of the content of templates and the addition of swelling agents on the porosity properties and morphology was also discussed. Additionally, the catalytic performance of prepared micro-mesoporous HKUST-1 was estimated, respectively. The results indicated the as-synthesized MOFs displayed uniform morphology and rich regular mesoporous structures at pore diameter of 3.85 nm when the content of templates was 0.65 g, accompanied with a high BET surface area of 1452 m2·g-1 and a total pore volume of 0.74 cm3.g-1. When the amount of templates further increased, the size and shape of the as-synthesized crystal lost its regularity gradually. Both swelling agents led to similar and more regular octahedral crystal structures with diameter of 200 ~ 300 nm. The BJH distribution results indicated both swelling agents could further enlarge the pores, and TMB demonstrated better performances with a relatively focused distribution in pore diameters at about 3.5 nm. The as- synthesized micro-mesoporous HKUST-1 catalyst was tested in the acetalization of benzaldehyde and ethylene glycol, and the conversion rate reached as high as 82% after 10 hours and has increased compared with the conversion rate of traditional HKUST-1(about 50%).Another type of micro-mesoporous HKUST-1 with ordered mesopores was synthesized using organic silane surfactants γ-APS as templates by a hydrothermal method with mixed solvents of H2O、ethanol and DMF in this work. The influence of the content of templates to the structure and morphology to the as-synthesized MOFs was investigated through XRD, N2 adsorption-desorption tests, SEM and TEM. In addition, the Synthesis mechanism of the micro-mesoporous HKUST-1 was examined by computer simulation. The results indicated that the micro-mesoporous HKUST-1 contained rich mesoporous structure inside its structure and a largest BET surface area of 1276 m2.g-1. The result of Density functional theory(DFT) calculation demonstrated that amino group of γ-APS may play an electrophilic role can interact with the metal ion by electrostatic interaction and then the γ-APS serve as a template for constructing mesopores.The hierarchically porous MIL-101 was successfully obtained using Organosilane γ-APS surfactants as templates in this work. The impact of the content of the templates and the mixing order of the reactants to the structure and morphology of the hierarchically porous MIL-101 was investigated through characterization of XRD, N2 adsorption-desorption, SEM and TEM. Meanwhile, the water adsorption performances of the as-synthesized MIL-101 was examined. The results indicated the best mole ratio of Cr3+/γ-APS was 1:0.1 when γ-APS was used as a template to hierarchically porous MIL-101. Under this circumstance, the as-synthesized MIL-101 contained rich macroporous structure, exhibiting a hierarchically micro-/meso-/macroporous structure, with a high BET surface area of 3491 m2.g-1 and a large pore volume of 2.27 cm3.g-1, which exceeds the normal MIL-101 material to a great extent, and acceptable water and thermal stability was observed. The macroporous structure was richer when Cr(NO3)3.9H2 O and γ-APS was mixed first, followed by the addition of organic linkers, compared to the direct mix of all three raw materials. The latter exhibited poor porosity results, with its BET surface area of 2563 m2.g-1, a total pore volume of 1.45 cm3.g-1 including the mesopore volume of 1.14 cm3.g-1. The as-synthesized hierarchical MIL-101 showed better water adsorption performance in comparison with normal MIL-101. At 20 ℃, the water adsorption amount of the hierarchical MIL-101 could reach 1.41 g/g, high than 1.21 g/g for the normal MIL-101.