1 INTRODUCTION
Membrane
separation technology with high energy efficiency and low carbon
footprints has drawn increasing attention in a wide range of
applications including petroleum refining, water purification and carbon
capture, etc .[1-5] Two-dimensional (2D)
lamellar membranes with controllable microstructure and well-aligned
nanochannels make excellent candidates for devising fast molecule
permeation and precise sieving.[6-8] Typically,
membranes assembled by porous nanosheets offer continuous transfer
highways in vertical direction, which further shortens mass transfer
distance and permits fast transport.[9-11]Molecule transport through porous
lamellar membranes should firstly
dissolve into pore entrances on
membrane surface and then diffuse through inner pores. It has been
demonstrated that the resistance of molecular dissolution process
accounts for 20% ~ 40% of the total transport
resistance, and even 70% for membranes with sub-10 nm
thickness.[12-14]Therefore, molecular dissolution
process exerts huge and even decisive impact on molecular transport
efficiency, which deserves the investigation of underlying molecular
dissolution behaviors on membrane surface.
In general, experimental researches
have confirmed that enhancing the affinity of membrane surface to
molecules by grafting active sites or attaching compatible films could
obviously improve molecular dissolution rate and hence permeation
ability.[15-17] As a noteworthy example,
water-capture capacity of over 300% was achieved for graphene oxide
(GO) lamellar membrane by modifying tannic acid on membrane surface,
which highly improve water permeance from 175 to 320 L
m-2 h-1.[18]These observations are generally attributed to the strong
molecule-membrane interactions that enrich molecules on membrane surface
and promote them to enter into inner channels. In contrast, recent
studies discovered that due to the strong interactions between water
molecules and –COO– groups on the pore rim of
aquaporins, water permeance was 6 times lower than in carbon nanotube
porins (CNTPs) which possessed similar channel size but hydrophobic pore
rim.[19] This phenomenon parallels findings by
simulation where the addition of four –COO– groups
to the rims of a 1.1-nm-diameter CNTPs reduced the water flux by 3 folds
due to the enhanced water-rim interactions. These controversial
viewpoints suggest that molecular dissolution behaviors need deep
investigation, espeically for porous materials. On the other hand,
expect for molecule-membrane interaction, other factors such as
molecule-molecule interaction should also count much due to the
confinement effect of molecules on pore entrances. However, the pores in
current porous lamellar membrane are usually
artificially created with random
distribution and wide pore size
distribution.[20-22]And the chemical groups on pore
entrances are heterogeneous, which hampers the control of molecule-pore
interactions on molecule level. More importantly, lamellar membranes
generally possess homogeneous structure, and the high resistance of
molecular diffusion in inner pores exerts huge impact on molecule
permeance. This conceals the contribution of dissolution process on
molecule permeance, hence impeding the exploration of underlying
molecular dissolution behaviors.[20,23,24]Therefore, constructing hierarchical porous lamellar membranes with
independently manipulated surface and support layers, and especially
regular and controllable pore entrances on surface layer, should provide
desired platform.
Metal-organic frameworks (MOFs) show
great expectations as next-generation materials for porous membranes
because of intrinsic characteristics including large porosity,
well-defined pore structures, and tunable molecule-/ion- specific
functional groups.[25-29] Significantly, the
intrinsic pores (usually < 2 nm) in MOF nanosheets can be
subtly tailored by controlling
metallic node and organic linkers.[30-32] And the
topology and chemical composition of pores are periodically distributed
within the frameworks, which permits subtle manipulation of
molecule-pore interaction on molecule level. Additionally, some MOFs can
be synthesized in the form of ultrathin nanosheets with large lateral
size (> 2 μm), holding promise for constructing porous
lamellar membranes.[9,33,34] On the other hand,
electrostatic atomization technology provides a novel strategy to evenly
spray atomized nanodrops that carry desired nanosheets onto receiver to
assemble ultrathin lamellar architectures.[35,36]In this way, hierarchical lamellar membranes with independently
controlled surface and support layer can be facilely constructed.
Herein, hierarchical MOF lamellar
membranes with ultrathin (~ 7 nm) surface layer and
identical support layer (~ 553 nm) were preparedvia double-needled electrostatic atomization technology.
Specifically, MOF nanosheets with hydrophobic (–CH3)
pores were sprayed on Nylon substrate to construct support layers. And
MOF nanosheets with hydrophilic (–NH2) or hydrophobic
(–CH3) groups on pores were assembled to create surface
layers, respectively. Based on these platforms, molecular dissolution
behaviors on porous membrane surface was investigated in detail.
Dissolution performances demonstrate that molecular dissolution
behaviors are highly affected by molecule-molecule and molecule-pore
interactions. Furthermore, the corresponding dissolution model equations
were established in term of dissolution activation energy
(E S) on
membrane surface with hydrophilic and hydrophobic pore entrances.
Importantly, membrane with hydrophilic pores on surface exerts strong
interactions with polar solvents, contributing to lowE S and high permeance of over 270 L
m-2 h-1 bar-1 for
polar solvents. In addition, the regular pores within MOF lamellar
membranes bring precise sieving with rejection of 99% for reactive red
(1.6 × 1.4 nm). The elucidation of molecular dissolution behaviors on
membrane surface is paramount to develop highly permeable and selective
membranes for separation applications.
2 MATERIALS AND METHODS
2.1 Materials
NiCl2·6H2O (99.9%) and
CoCl2·6H2O (99.9%) were purchased from
Aladdin Reagent. Triethylamine (TEA), benzenedicarboxylic acid (BDC),
2-aminoterephthalic acid (NH2-BDC),
2-methylterephathalic acid
(CH3-BDC) and diiodomethane were bought from Macklin
Reagent. Organic solvents (methanol, ethanol, dimethyl sulfoxide (DMSO),
N,N-dimethylformamide (DMF), acetonitrile, acetone, n-octane,
cyclohexane, n-hexane, n-pentane and toluene) were obtained from Tianjin
Kemiou Chemistry Reagent Co., Ltd. Nylon microfiltration substrates (0.2
μm pore size, 50 mm diameter) were bought from Tianjin Jinteng
Experimental Equipment Co., Ltd. Polyethylene glycol (PEG, 200, 300,
400, 600, 800, 1000), Brilliant blue (BB), reactive red (RR), rhodamine
B (RB), crystal violet (CV), and methylene blue (MB) were supplied by
Aladdin Chemical Co., Ltd. These were used as received without further
treatment, and deionized water was
used throughout the experiment.