1 Introduction
Metal-organic framework materials (MOFs) are a kind of organic-inorganic hybrid porous materials synthesized by metal ion clusters and organic ligands through coordination1, 2. They are good adsorbents owing to easy synthesis, diverse structures, large specific surface area, and easy functional modification of pore surface3, 4. In addition, the structure and pore properties of MOFs can be precisely tuned according to the property of the adsorbate, which shows great potential in adsorption. However, most MOFs reported only have a microporous structure5, 6. Thus, the macromolecules as adsorbate can hardly reach the adsorption active sites at the internal cave of the MOFs and the diffusion of adsorbate could be weakened, which limits the application of MOFs7, 8. It has been reported that the fabrication of lattice vacancies can produce the mesopores and macrospores in the bulk MOFs9. The MOFs with hierarchical porous structure can improve the diffusion of adsorbate in MOFs and provide more active sites for adsorption.
Currently, the hierarchically porous MOFs (HP-MOFs) is widely fabricated by template synthesis, in which the template agents are applied during the synthesis of MOFs and the mesoporous structure can be achieved by removing the template agents10. Compared with other methods, the template synthesis can conveniently regulate the size, morphology and structure of HP-MOFs, owing to the spatial confinement and the adjustment of the template agent11. For instance, Cai et al. used organic acids with long alkyl chains as soft template agents to prepare HP-UiO-66 and better adsorption performance for dye adsorption could be achieved12.
The separation and recovery process are required, if the powdered MOFs is expected to applied in industry13. Porous membrane is the good carrier for MOFs immobilization, since it has a tortuous pore structure and uniform pore size distribution, as well as diverse preparation methods14. Immobilizing MOFs in membrane pores to fabricate membrane adsorber can uniformly disperse MOFs and the aggregation beyond the membrane pores can be prevented. Besides, the curved pore structure of membrane can improve the stability of MOFs and the detaching of the MOFs particles from the membrane pores can be avoided15. Furthermore, from the perspective of adsorption kinetics, enhanced mass transfer can be achieved owing to the confined space effect of membrane pores during the adsorption process with membrane adsorber as flowing through mode, since the distance of mass transfer is reduced to micro or nano scale. Therefore, the improvement of adsorption rate can be expected by membrane adsorber16.
In our previous study, flowing synthesis has been developed to immobilize the MOFs particles in membrane pores16-22. During this process, the precursor solution of the MOFs particles is flowing through the membrane by an external force and the MOFs particles can be fabricated and immobilized in membrane pores simultaneously. As a result, enhanced catalytic or adsorption performance can be achieved by this membranes23. HKUST-1, with regular octahedron structure, good water, chemical stability and rich active adsorption active sites, is a kind of adsorbent with good performance24. Enhanced adsorption performance can be expected by hierarchically porous HKUST-1 (HP-HKUST-1). In this study, HP-HUST-1 will be immobilized in membrane pores for the assembly of membrane adsorber by combing flowing synthesis and template agent removal. The adsorption for MB and CR is carried out to evaluate the adsorption performance of the membrane adsorber. The effect of the concentration of dyes and the flowrate and the mass of MOFs on adsorption are also studied.