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.