TBN-mediated regio- and stereoselective sulfonylation & oximation (oximosulfonylation) of alkynes with sulfonyl hydrazines in EtOH/H2O

Article abstract of DOI:10.1039/c8gc02708k

An alkyne difunctionalization cascade reaction was developed to generate α-sulfonylethanone oximes under metal-free conditions. The reaction features environmentally benign EtOH/H₂O as a solvent, a broad substrate scope and easily scaled up for gram-scale synthesis. The reaction mechanism was illustrated with UPLC, GC-MS and UPLC-tof/MS.

Full text of DOI:10.1039/c8gc02708k

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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ARTICLE
2D-Dual-Spacing Channel Membranes for High Performance
Organic Solvent Nanofiltration
Shaofei Wang, Dinesh Mahalingam, Burhannudin Sutisna, Suzana P. Nunes*
Received 00th January 20xx,
Accepted 00th January 20xx
Two-dimensional (2D) lamellar membranes are praised as being highly advantageous in molecular separations. However,
the permeance-rejection trade-off is alwasys a major chanllenge, since the permeant transport mostly occurs in single-
spacing channels with undesired microenvironments. Inspired by the structure of aquaporins, we design alternating dual-
spacing channel graphene oxide (GO) membranes, with locally tailored chemical microenvironment, that give high
permeance, high rejection and high stability in organic solvent nanofiltration. This unique structure is easily constructed by
in-situ intercalating and cross-linking scattered sub-5 nm silica nanoparticles in the GO interlayers. The hydrophilic
nanoparticles locally widen the interlayer channels to enhance the solvent permeance. In the alternating nanoparticle-free
areas, the GO layers simultaneously bend and the π-π interactions keep the narrow and hydrophobic channel, promoting a
high solute rejection. With a 10-fold increase in water permeance and unaffected rejection, the dual-spacing channel
membranes exhibit one of the best performances for organic solvent nanofiltration. The methanol permeance reaches 290
L m-2 h-1 bar-1, with more than 90% rejection of dyes larger than 1.5 nm. This new approach of designing hierarchical channels
in 2D materials can be used for a wide spectrum of applications.
DOI: 10.1039/x0xx00000x
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performances. Ideally, drilling uniform pores in a single layer or
vertically aligning of 2D materials could form the mostly desired
channel structure.5, 17-19 However, these preparation processes
Introduction
Membranes have a growing impact in water desalination,
chemical and pharmaceutical industrial separations.1
synthetic membrane with nanometer-size channels,
characterized by high flux, precise size sieving ability and
excellent durability, is the key component in these processes.
are challenging and would be hardly scalable. Multilayered
stacked membranes, referred also as lamellar membranes, offer
a technically practical pattern for translating 2D materials into
membranes. In lamellar membranes, the penetrants flow in a
tortuous way, first passing through in-plane gaps, then through
the nanochannels between the adjacent layers. Due to the
irregularity of the in-plane gaps, the interlayer nanochannel
becomes the path with predominant control of the separation
efficiency.20 With this regard, major efforts have been devoted
to increase the regularity of these channels by tuning the
stacking efficiency,21 adopting rigid nanosheets,22 or
crosslinking them with different segments23, 24 to obtain single-
spacing channels that can execute precise size-sieving.
However, considering the long and tortuous in-plane channels,
a regular and tight channel size weakens the penetrant
permeance. A widely adopted solution to enhance the
permeance of the lamellar membranes is to intercalate nano-
A
The separation performance of
a membrane is largely
dependent on the dimensions, as well as on the physical and
chemical microenvironments of the transmembrane channels.2
The development of advanced membranes with engineered
nanochannel structures is urgently required to satisfy the
industrial demands for filtrating large volumes of organic
solvents.3, 4
In the past decades, the demands for ultra-thin membranes
have been fulfilled by the exfoliation of various two-
dimensional (2D) materials, including graphene,5 graphene
oxide (GO),6-8 Mxene,9, 10 BN,11 C3N4,12, WS2,13 2D metal-organic
framework,14 etc. These 2D material membranes, either
processed in the form of single porous layers, multilayered
sized spacers in the interlayers,20 such as copper hydroxide,6
stacks, or mixed matrix membranes have been featured with
ultimate separation capabilities.
construction is of vital importance in achieving the high
26
carbon nanotubes,25 nanoparticles,9,
and organic
15, 16
The rational channel
28
molecules.27,
Recently, covalent cross-linking is proved
effective in achieving high stability of GO membranes in organic
solvents. 16, 28 An expanded interlayer spacing always leads to a
considerably enhanced penetrant permeance. However, in
some previous studies, the spacer size, varying from 5 nm to
even hundred nanometers, are always larger than that of the
rejected molecules. Also, a high spacer distribution density
leads to the entire membrane channel expansion to undesired
size range, weakening their size sieving ability.29 Therefore, a
a. Biological and Environmental Science and Engineering Division (BESE), Advanced
Membranes and Porous Materials Center (AMPM), King Abdullah University of
Science and Technology (KAUST), Thuwal, 23955-6900, Saudi Arabia. Email:
suzana.nunes@kaust.edu.sa
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
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single spacing channel can hardly provide a solution to
overcome the trade-off between permeance and rejection.
In nature, biological membranes possess delicate hierarchical
channels with a perfectly tuned chemical microenvironment
that provides an ultrahigh permselectivity for molecules and
ions. Aquaporins are known for promoting a highly efficient
water transport in biological membranes. The protein
molecules arrange themselves forming a hourglass channel with
a large and hydrophilic part to confer the high water
permeance, and a narrow hydrophobic site to confer the high
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DOI: 10.1039/C8TA10872B
30
ion rejection. The discovery of aquaporin has inspired the
design of highly efficient liquid separation membranes.31
Learning from the unique structure of aquaporins, we believe
the design of multi-spacing channels with locally tuned
microenvironments would be a potential solution to overcome
the critical restrictive permeance-rejection trade-off. While
Scheme 1 The concept of designing alternating dual-spacing
channel with tailored chemical miroenvironment in 2D material
there has been considerable work regarding the hierarchical membranes to overcome the permeance-rejection trade-off.
32
pore construction in polymeric membranes,3,
analogous
On the bottom: the green colour indicates a hydrophilic domain;
the yellow colour indicates the hydrophobic sectors.
reports on laminate 2D material membranes are quite deficient.
Meanwhile, though there has been quite a lot work on
intercalated 2D membranes, the local channel affinity towards
targeted penetrants has attracted less attention than the size
control.23, 33 The difficulty lies in the lack of effective methods to
locally tailor the physical and chemical microenvironments in
the specific nanospace. Recently, Abozar et al34 demonstrated
that the interactions between the polar groups along the edges
of the GO sheets and solvent plays a dominant role in the
solvent permeance.
In this study, we propose a novel and simple strategy to
fabricate GO membranes with alternating dual-spacing
channels, enabled by the in-situ generation of silica
nanoparticles at specific domains between the GO layers.
Different from other post intercalating GO membranes, the in-
situ method could help the formation of small and more
scattered nanoparticles, which would contribute new
structures. The expanded interlayer spacing around the
hydrophilic nanoparticles contributes to a high permeance.
Experimental section
Synthesis of GO sheets:
All the chemicals were used as received from Sigma-Aldrich
without further purification, unless otherwise specified. The GO
sheets were synthesized according to the improved synthesis
method reported by Tour et al.36 with minor modifications.
Typically, 3.0 g graphite flakes (>100 mesh) and KMnO4 (18.0 g,
Fisher Scientific) were added to a three-necked flask in an ice
bath. Subsequently, a 9:1 mixture of concentrated H2SO4/H3PO4
(360:40 mL) was added to the mixture. The reaction medium
was then heated to 50 °C and stirred at 300 rpm for 24 h. The
color changed from black to brown. After cooled to room
temperature, the mixture was poured into a beaker containing
400 mL ice with 30% H2O2 (3 mL, VWR Chemicals). The mixture
was kept undisturbed help the deposition of graphite oxide.
After decanting, the bottom deposits was centrifuged (10000
rpm for 1 h) and washed in succession with HCl solution (9:1
water: HCl by volume), water and ethanol till the pH reached 7.
Then the obtained solid was vacuum-dried at room
temperature for 48 h. The GO sheets were exfoliated in ethanol
by sonication in a water bath for 1 h. To obtain uniform-sized
single layers of GO, the dispersions were further centrifuged at
3000 rpm for 10 min, removing the unexfoliated and excessively
large GO sheets. The concentration of the GO dispersion in
ethanol was 0.1 mg/mL.
Meanwhile, the combination of sub-5 nm spacers and flexible
22, 35
GO sheets
lead to the local formation of narrow
hydrophobic necks, contributing to a high rejection of small
molecules Scheme ). Compared with unmodified GO
(
1
membranes, the GO-Si membranes exhibited 10 times higher
water permeance with negligible loss of dye rejection. These
membranes show excellent performance in organic solvent
nanofiltrations, with a high methanol permeance of 290 L m-2 h-
bar-1, and >90% rejection of dyes larger than 1.5 nm.
1
Furthermore, the spacers also act as cross-linkers and bring
excellent stability to the GO membrane in organic solvents.
Synthesis of GO-Si membranes:
The GO-Si membranes were fabricated by the vacuum filtration
method. A 5 mL of GO dispersion in stock was diluted to 0.01
mg/mL, by adding ethanol. After adding 20 μL (3-
aminopropyl)triethoxysilane (APTES), the mixture was stirred
for 20 min at room temperature to help the attachment of
APTES on GO. Then the mixture was filtrated through a nylon
microfiltration membrane (0.22 μm pore size, 47 mm diameter,
supplied by GVS Filtration Inc) or anodic aluminum oxide (AAO)
membrane (0.2 μm pore size, 47 mm diameter, supplied by
Whatman). After the filtration, 200 mL NaOH solutions (pH=11)
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was added. The filtration was terminated when about 10 mL
ARTICLE
The dye rejection was determined by Equation 2, where Cp and
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solution had passed through. In this case, the membrane would Cf represent the concentration of theDOpeI:r1m0.1e0a3t9e/Ca8nTAd10f8e7e2dB
be soaked in NaOH solutions. The system was maintained in this solutions, respectively.
way for 8 h, and then the filtration continued to eliminate the
NaOH. The membrane was then filtrated with 100 mL ethanol
to wash the unreacted APTES. These membranes are referred
as GO-Si2 membrane. GO-Si2 membranes with different
thickness were also prepared, by proportionally increasing the
GO and APTES amount. Other membranes were also prepared
by following the same procedure without further filtration of
NaOH, the membranes were denoted as GO-Si1 membrane.
Cp
R = (1 - ) ´100%
Cf
(2)
Results and discussion
GO sheets, the building blocks, were synthesized by the
exfoliation of graphite oxide prepared by the mild improved GO
synthesis method. By successive centrifugation steps, we
removed the large or non-exfoliated GO unities and obtained
GO nanosheets with a thickness of ~1 nm (Fig. 1a) and relatively
uniform lateral size, around 500-1000 nm (atomic force
microscopy (AFM) image in Fig. 1b; scanning electron
microscope (SEM) image in Fig. S1, ESI). The nano- rather than
micrometer-sized GO sheets are expected to lead to
membranes with more in-plane pores, which favor a faster
solvent transport, as reported by Zhu et al.37, and Wang et al.9
The Fourier-transform infrared spectroscopy (FTIR) spectra of
GO (Fig. S2, ESI) shows the characteristic peaks of C-O, C-OH,
C=O vibration, confirming the successful oxidation of GO.36 The
C/O ratio calculated from X-ray photoelectron spectroscopy
(XPS) (Fig. S3, ESI) is about 7/3, which indicates a medium
degree of oxidation. The four deconvoluted peaks in Fig. 3c
show the four kinds of carbon bonds and their percentage,
which is a good indication of epoxy, carbolic and hydroxyl
oxygen on the GO. In addition, the X-ray diffraction (XRD) curves
in Fig. 1d demonstrate the interlayer spacing were expanded
from graphite 0.34 nm to graphene oxide 0.86 nm. All the
evidences confirm the successful exfoliation and oxidation of
GO.
Characterization:
The size of the GO sheets and the cross-sectional morphology
of the composite membranes were analyzed by scanning
electron microscopy, SEM (FEI Magellan™ XHR SEM, equipped
with energy-dispersive X-ray spectroscopy (EDS)). Samples for
cross-section images were prepared by freeze fracturing the
membranes in liquid nitrogen, and then coating with a 3 nm thin
film of Ir. Transmission electron microscopy (TEM) images of the
GO layer were obtained on an FEI Titan G2 80-300 ST, operated
at 300 kV. For that, the thin layer of GO-Si (~60 nm) was
fabricated on an anodic alumina (AAO) porous support. After
dissolving the support with HCl, the GO layer was gently
transferred to a TEM carbon grid. The thickness of the GO
sheets and membranes was measured by AFM (Bruker
Dimension Icon SPM microscope). For that, the samples were
filtrated on an AAO support and transferred to a clean silicon
wafer, after dissolving the support with HCl. The X-Ray
Diffraction (XRD) investigation was carried out using a powder
XRD system (Bruker D8 advance) with CuKα (λ = 0.154 nm),
operating at 40 keV with a current of 20 mA. The chemical
analysis was performed using Attenuated Total Reflectance
(ATR) Fourier Transform Infra-Red Spectroscopy (FTIR) (FTIR-
iS10) in the wavenumber range of 500-4000 cm-1 as an average
of 32 scans with 4 cm-1 resolution. X-ray Photoelectron
Spectroscopy (XPS) was performed on an Amicus equipment to
measure the elemental composition of the surface of the
synthesized GO sheets and membranes.
Membranes nanofiltration:
The membrane nanofiltration performances were tested in
dead-end stainless-steel cells with a driving force of 1 bar at
room temperature. All the dyes used in this study were supplied
by Sigma-Aldrich, except for brilliant blue R250, supplied by
Fisher Scientific. The membranes were pre-soaked in the testing
solvents for 1 h before testing. The analysis of the dye
concentration was conducted with
a NanoDrop UV–vis
spectrophotometer. All the collected data are averages of three
parallel tests. The pure solvent permeance (F) was calculated by
Equation 1:
Fig. 1 Characterization of graphene oxide. (a) AFM image of the
synthesized GO sheets. The inset shows the corresponding
height profile of the green line traced on the AFM image. (b) Size
distribution of the GO sheets. (c) XPS (C1s) of graphene oxide.
(d) XRD of graphene and graphene oxide powder.
The membranes were prepared by the simple and
commonly used vacuum filtration method (Fig. 2a). In an
Dw
rADpDt
F =
(1)
Where Δw refers to the permeate weight increase during the
filtration time Δt; A is the separation area of the cell; ρ is the
density of permeate solvent.
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optimized procedure, we mixed a dilute concentration of GO of different membranes are shown in Fig. 2b, in which pristine
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DOI: 10.1039/C8TA10872B
(0.01 mg/mL) with (3-aminopropyl)triethoxysilane (APTES). In GO membranes shows light brown color, the GO-Si1 and GO-Si2
the mixtures, APTES may attach on the GO through hydrogen membranes exhibits darker color. The pristine nylon support
bonding. Ethanol, rather than water, was applied as the has a highly porous structure (Fig. 2c inset and Fig. S4 in ESI).
dispersing solvent to avoid the APTES hydrolysis. The mixture With a GO loading of 440 mg/m2, the nylon support is uniformly
was filtrated through a highly porous nylon support with covered without defects (GO-Si2 in Fig. 2c). The intercalation of
average pore size of 0.22 μm. The membranes, referred here as APTES and the further treatment do not affect the integrity of
GO-Si1, have the APTES uniformly distributed through all GO the GO membranes (Fig. S5a-h, ESI). For some characterizations,
interlayer nanochannels. By further treatment with NaOH we also prepared membranes by using anodic aluminum oxide
aqueous solution, the GO-Si2 membranes were obtained. In this (AAO) membranes as the support. As seen in the photograph in
step, the SiO2 nanoparticles should be produced between the Fig. S6 in ESI, due to the low thickness of the GO layer, the
GO layers, by hydrolysis and condensation reactions. APTES membrane is transparent. The SEM cross-section image of the
modified GO have been reported in applications, such as composite membranes display a laminar structure with the
thermally conductive materials,38 reinforcing components,39 thickness of 180 nm (Fig. 1d), which can be confirmed by the
and gas separation membranes.40 In our case, APTES AFM height profile in Fig. 1e and f. The thickness could be
modification could greatly improve the mechanical stability of feasibly tuned from less than 100 nm to large than 1μm, by
the membranes. Moreover, APTES works a silica precursor that increasing the GO loading amount on the support (Fig. S7a-e,
endows the in-situ formation of nanoparticles. The photographs ESI).
Fig. 2 Dual-spacing channel GO membranes intercalated with silica nanoparticles. a) Preparation procedure. b) Photograph of
different membranes. c) SEM image of the GO-Si2 membrane surface. The inset shows the surface of the nylon support. d) Cross-
sectional SEM image of the GO-Si2 membrane, prepared on the nylon support. The inset shows a lower magnification of the whole
membrane cross section. e) AFM image of the GO-Si2 membrane. f) The corresponding height profile of e.
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Fig. 3 Chemical and physical structures of the membranes. a) XPS (C 1s), b) TEM image from the top surface, c) Cross sectional SEM
image of GO-Si2 membranes. The orange arrows show the SiO2 nanoparticles. d) XRD diffractogram of the membranes, showing
the different interlayer d-spacings. Peaks from 13-26° are from the nylon support membrane. e) Schematic diagram to illustrate
the different GO membrane structure.
As for the chemical structures, based on the FTIR spectrum
in Fig. S8 in ESI, the newly formed C-Si (1354 cm-1) and Si-O-Si
(1040 cm-1 and 860 cm-1) bonds reveal the chemical
functionalization of APTES on GO. The XPS analysis was
conducted to confirm the completion of the reactions to form
the GO-Si2 membrane (Fig. 3a; Fig. S9, ESI). The curve for the
GO-Si2 membrane has N and Si peaks at 401.3 eV and 102.8
eV, respectively. From the deconvoluted C 1s peaks (Fig. 3a),
the epoxy C-O ratio is considerably reduced compared with
pure GO. Therefore, in alkaline conditions, these silane
moieties are grafted onto the GO sheets via the SN2
nucleophilic displacement reaction between epoxy groups of
GO and amino moieties of APTES.39, 41
a thin (~60 nm) GO-Si2 film onto a copper grid. As seen from
Fig. 3b, from the top view of the membrane, the particle size
is about 3-5 nm. The difference in the nanoparticles darkness
is attributed to the different layer location in GO multilayers.
The overlap of nanoparticles in different layers may indicate
an apparent aggregation. TEM and SEM images in Fig. S12 in
ESI further confirm this. The high-resolution cross- sectional
SEM image of the GO-Si2 membrane displays scattered white
dots, which are attributed to the intercalated SiO2
nanoparticles. These evidences prove the successful formation
of sub-5 nm SiO2 nanoparticles between the interlayers. The Si
mapping performed by EDS on the membrane surface
indicates that there is no aggregation of SiO2 particles (Fig. S12,
Supporting Information). Compared with the pristine brown-
colored GO membrane in Fig. 2b, the GO-Si1 and GO-Si2
membranes are clearly darker. This is an indication of the
progressed GO reduction.44 The partial reduction of GO can be
confirmed by analyzing the deconvoluted part of the C1s peak
in the XPS spectra that corresponds to C-O-C. The C-O-C ratio
decreases from 40.4% in the pristine GO to 16.9% in the GO-
Si2 membranes. During the membrane preparation, the APTES
molecules are first uniformly distributed, presumably
occupying the GO interlayer spaces. With the NaOH solution
For the physical structures, the AFM membrane images in
Fig. S10 display a wrinkled surface, which can be attributed to
the in-plane interactions of GO nanosheets during stacking.42,
43
By zooming the flat region of the membranes, it can be
observed that the GO and GO-Si1 membranes have a smooth
morphology. In contrast, the surface of the GO-Si2 membrane
is decorated with ultra-small nanoparticles. The size of the
nanoparticles was further estimated by transmission electron
microscopy (TEM), for which the samples were prepared by
peeling off the sample from an AAO support and transferring
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treatment, two kinds of reaction take place: the APTES grafting
on the GO original epoxy sites and the sol-gel reaction around
the grafted sites to form the SiO2 nanoparticles. Since the
interlayer space is confined with only 1-2 nm, the formed SiO2
particles are smaller than other intercalated membranes. In
addition to their role in expanding the layers, the intercalated
SiO2 nanoparticles are covalently bonded to the epoxy groups
in different GO layers, and act as crosslinkers, bringing stability
to the membranes.
evaluated (Table S1, ESI). Due to the expanded channel sizes,
the GO-Si2 membrane has almost two-fold higher gas
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DOI: 10.1039/C8TA10872B
permeance than the pristine GO membrane. This indicates
that the presence of SiO2 nanoparticles create more free
spaces around the nanoparticles rather than blocking some
pathways for molecular transport. The membrane
performance for liquid separation was then investigated. The
water permeances and rose bengal rejections (Fig. 4a) were
tested. Due to the uniform stacking and small interlayer
The interlayer d-spacings govern the separation
performances of the membranes. The diffraction peaks were
obtained by XRD and the d-spacing further calculated by the
Bragg's law. As shown in Fig. 3d, the peaks ranging from 13 to
26° are relative to the nylon support membranes.45 The XRD
pattern for the unmodified GO membrane indicates a single d-
spacing of 0.85 nm. After treating with APTES (GO-Si1
membrane), the high content of APTES helped to expand
single interlayer d-spacing to 1.03 nm. While, the XRD pattern
for the GO-Si2 membrane indicates a dual d-spacing, with
peaks corresponding to d values of 1.31 and 0.89 nm. Although
there have been dual d-spacing reports for GO membranes
before,27, 46 they refer to regions with distinct degree of partial
reduction. In most of the cases, single d-spacings are observed
in the nanoparticle intercalated GO membranes6, 9, 26 relative
to narrow original GO interlayers or enlarged modified ones.
The dual peaks in our study should be assigned to the different
flake regions with and without nanoparticles. By considering
the low atomic ratio of Si (~4%), the number of nanoparticles
in the membrane is quite small. This explains the low peak
intensity at d =1.31 nm. Since the uncrosslinked APTES is
washed way (confirmed by the lower Si content in GO-Si2
compared with GO-Si1 membrane), the silica is formed only
around scattered GO epoxy sites. Due to the flexible nature of
GO nanosheets,35, 47 sections of the pristine GO interlayer is
preserved with the d-spacing increasing only slightly to 0.89
nm (Fig. 3e). Therefore, the result is a hierarchical structure
containing 2D channels with dual interlayer spacings. The
change in d-spacing also contributes to different total
membrane thickness with the same GO loading (Fig. S13, ESI).
The pristine GO membrane exhibits a thickness of 170 nm.
With localized spacers, the thickness of the GO-Si2 membrane
slightly increased to 180 nm, while the GO-Si1 membrane with
uniformly distributed molecular spacers has a much thicker GO
layer of 216 nm. The requirements to achieve such a dual-
channel structure are schematically illustrated in Fig. S15.
Besides these factors, the in-situ method helps to reserve the
membrane structure, which leads to the different structure
from other particle intercalated membranes.
spacing, the pristine GO membrane exhibits
a water
permeance of 18.7 Lm-2 h-1 bar-1, and rose bengal rejection of
99.3%. After intercalating APTES, the channel expands and the
water permeance increases to 225 L m-2 h-1 bar-1, whereas the
rose Bengal rejection declines to 48.6%. Following Geim and
co-workers,48 we used the Hagen–Poiseuille equation
(Equation 3) for 2D laminar membranes to explain the
permeation increase:
h4Dp
Flux =
2
12LhDx
(3)
where h is d-spacing of adjacent layers; L is the average
lateral length of sheet; η is the viscosity of solvent, Δp is the
applied pressure difference and Δx is the membrane thickness.
Considering the frictionless graphite regions and slip flow
theory, the tested flux are always a few orders of magnitude
higher than predicted for GO membranes.6, 46, 48 However, in
addition to that, the Hagen–Poiseuille equation shows that the
increase in interlayer spacing would bring a substantial
enhancement of the permeance. Since the interlayer channels
are responsible for the solute rejection, the enlarged channel
would inevitably lead to a decreased rejection. In contrast, the
GO-Si2 membrane still has a superior rejection. The water
permeance of the GO-Si2 membrane is 204 L m-2 h-1 bar-1,
which is more than 10-times higher than that of the GO
membrane. In the pristine GO membranes, the non-oxided
hydrophobic graphite domains provide a nearly frictionless
flow of molecules, which helps to greatly promote the
48, 49
molecular transport.23,
Disturbing the non-oxided
domains could break the continuous flow and decrease the
permeance. GO-Si1 and most other membranes are
intercalated with uniformly distributed spacers.12, 26, 50 In the
case of the GO-Si2 membranes, however, the intercalation is
rather diluted and localized only in certain positions. The
hydrolysis and polycondensation of APTES is initiated around
the GO epoxy groups, nucleating the nanoparticles formation.
Their growth continues by reacting with other APTES
molecules around without affecting the frictionless graphite
domains, creating expanded hydrophilic channels and narrow
hydrophobic channels.
Having the intercalated nanoparticles in the interlayer
channels could potentially block molecular pathways through
the channels. To check whether the intercalated nanoparticles
could hinder any molecular transport, the permeances of the
single gases, N2, CH4 and CO2, through the membranes were
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Fig. 4 Membrane nanofiltration performance. a) Water permeance and rose bengal rejection of GO, GO-Si1 and GO-Si2
membranes. b) Pure solvent permeance as a function of their inverse viscosity for GO-Si2 membranes. The used solvents are
numbered and named on the left. The blue dash line indicates the hypothetical linear relationship. c) Dye rejection of the GO-Si2
membranes. The dyes were dissolved in methanol. The dyes used were: methyl orange (MO), indigo carmine (IC), congo red (CR),
brilliant blue R250 (BB), rose bengal (RB), chrysoidine G (CG), methylene blue (MB), crystal violet (CV). The corresponding sizes (in
nm) are mentioned under their abbreviations.
By taking advantage of the high stability of GO, our goal is
to use the dual-spacing channel GO-Si2 membranes for
separations in organic solvents. The permeances of various
solvents were evaluated by using a dead-end filtration setup
under a pressure of 1 bar. In Fig. 4b, we plot the permeance of
different solvents as a function of the reciprocal of the
viscosity (η). For most solvents, as the viscosity decreases, the
GO-Si2 membrane exhibits an increase of permeance. Notably,
the membrane has an acetone permeance of 444 L m-2 h-1 bar-
potential, such as GO, the synergy of solution and diffusion is
more effective in achieving high molecular transport.
Besides the permeance of solvents, molecular selectivity is
essential for the application in nanofiltration. To evaluate the
membranes ability to reject low molecular weight solutes, the
rejection of different dyes was evaluated in methanol, a
commonly applied organic solvent for pharmaceuticals. Both
positively and negatively charged dyes with different
molecular sizes were tested. The chemical structure,
molecular weight and molecular dimensions of the dyes are
displayed in Fig. S16, as estimated using a Molecular
Mechanics 2 method in Chem3D. The values are agree with
other previous reports11 53, 54 and have slight differences with
those reported by Shi et al45. The dye rejections are plotted in
Fig. 4c, and the corresponding sizes are mentioned below.
Colored feed and permeate samples are shown in Fig. S17 in
ESI and the corresponding UV-vis spectra are shown in Fig. S19.
Theoretically, three major phenomena could contribute to the
dyes rejection: absorption on the membrane surface, physical
sieving by the 2D nanochannels, and electrostatic interactions.
In all rejection tests, we used 80 mL of dye solutions with a
concentration of 100 ppm (Cf) as feed. After filtering 40 mL,
we measured the concentration of both permeate and
retentate, Cp and Cr, respectively. In Table S3, we calculated
the expected theoretical Cr/Cf values to fulfill the mass
balance, assuming that the dyes do not absorb on the
membranes. For both positively and negatively charged dyes
the actual and theoretical values are quite close, indicating low
dyes absorption. Also, the thin membrane thickness will allow
only small amount absorption. The channel size is therefore
the predominant parameter to achieve the dye rejection.
The GO-Si2 membrane rejects more than 90% of large
molecules, such as brilliant blue R250 (BB, 2.2 nm×1.7 nm,
99.2% rejection), congo red (CR, 2.3 nm×0.7 nm, 95.8%
rejection) and rose bengal (RB, 1.5 nm×1.2 nm, 91.9%
rejection). In contrast, the rejection of dyes smaller than 1.5
nm is quite low. For instance, the rejection of chrysodine G
(CR) with size of 1.1 nm×0.5 nm is only 23.3%. For methylene
blue (MB) with size of 1.4 nm×0.6 nm, the rejection is 45.8%,
1
and a methanol permeance of 290 L m-2 h-1 bar-1, which are
quite high values compared with the pristine GO and other
45, 51
types of membranes.21,
There is a linear correlation
between the methanol permeance and 1/thickness of the GO
layer in the GO-Si2 membrane (Fig. S15, ESI), indicating the
uniform structure along the thickness axis. Unlike the GO
membrane reported by Nair and co-workers,21 for which the
solvent permeances linearly dependent on 1/η, our GO-Si2
membrane shows some deviation of the linear correlations
(the dash line in Fig. 4b). The solvent properties including the
viscosity, relative polarity, kinetic diameter, total Hansen
solubility parameter are listed in Table S2 in Supporting
Information. In the GO-Si2 membrane, the SiO2 nanoparticles
are distributed in the membrane channels with abundant
hydrophilic -OH groups. The interaction with hydrophilic or
polar solvents is therefore favored.52 Therefore, we assume
that while the viscosity plays the predominant role, the
relative polarity also contributes to the solvent permeance.
This assumption can be proved by the lower permeance of low
polarity
solvents,
such
as
tetrahydrofuran,
dimethylformamide and N-methyl-2-pyrrolidone, being
located under the dash line, while highly polar solvents, such
as methanol, water and ethanol, have permeance values well
above the dash line. In this regard, the hydrophilic
nanoparticles domains act like a pump that efficiently gathers
the polar molecules and promotes the fast transport through
the hydrophobic graphite channel sectors. For a channel with
uniform chemical potential, the single spacing channel could
achieve high permeance and rejection for molecular
separations.22 But for 2D materials with non-uniform chemical
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Journal Name
and for crystal violet (CV, 1.4 nm×1.3 nm) is just below 70%.
Therefore, the channels in GO-Si2 membrane are highly size-
selective, with a sharp limit for permeation of dyes only
smaller than 1.5 nm. Nevertheless, charge is also an important
factor in this size range. For the much higher rejection of rose
bengal, compared to crystal violet of similar size, the
negatively charged carboxylic groups46 might play an auxiliary
role, leading to the dye repulsion by the negatively charged
GO-Si2 membrane.
highly charged surfaces can be easily re-exfoliated in polar
View Article Online
solvents.28 While Si-GO2 membranesDOkI:e1p0t.103t9h/eC8TuAn1i0f8o7r2mB
structure. (Fig. S19, Supporting Information). The stability
arises from the cross-linking and partially reduction in the
alkaline treatment process, which enhance the covalent force
and π-π stacking between adjacent layers. Due to the presence
of SiO2 nanoparticles, the membranes may not be stable in
high pH solvents, which needs to be addressed in future work.
However, bearing the advantages of high permeance, high
rejection, and excellent stability, our GO-Si2 membrane shows
great potential in OSN.
In our system, we used a vacuum filtration method to obtain
flat sheet GO composite membranes. Vacuum filtration is the
most common and straightforward route for the large-scale
fabrication of GO membranes.56 Besides flat sheet
membranes, a more potential scalable configuration may be
hollow fiber composite membranes prepared by vacuum
filtration method. The challenges may lie in achieving suitable
support surface with optimized surface roughness and
wettability to obtain a homogeneous and highly ordered
laminate GO membrane. And these issues can be resolved by
pre-modification on the support membrane surface22 or more
delicate filtration method.51
400
3000
a
350
2500
Fig. 5. Comparison of the separation performance of the GO-
Si2 membrane versus other reported membranes in methanol
media. Detailed information can be found in Table S4.
300
250
2000
200
1500
By considering the permeance and rejection properties, the
alternating dual-spacing channel structure of the GO-Si2
membranes, with small-sized GO building blocks, enable an
excellent performance for nanofiltration in organic solvents.
As illustrated in Fig.5 and detailed in Table S4 in Supporting
Information, the performance of our membrane is highly
competitive with most GO membranes in the literature, being
among the best reported data for organic solvent resistant
ones.
The stability of membranes in organic solvents is another
major issue for OSN membranes. The flux, exemplified for
methanol in Fig. 6a linearly increases with the applied
pressure. The permeance, defined by flux/pressure, showed
only slight decrease. This indicates that the membrane is
mechanically stable, does not suffer any compaction and the
fluidic channels in the GO layer are less affected by pressure,
at least up to the tested value of 10 bar. This is quite different
from previously reported GO membranes, which frequently
150
1000
500
100
50
0
0
0
2
4
6
8
10
Pressure (bar)
700
600
500
400
300
100
Acetone
Methanol
Dimethylformamide
b
50
0
suffer an elastic deformation and consequent permeance loss
55
under higher pressure.6,
The stability is given by the
intercalated and cross-linked SiO2 nanoparticles, which helps
to maintain a stable channel size and makes the GO membrane
quite robust. In a long-term test the GO-Si2 membrane
demonstrated excellent stability with the different solvents
permeance being constant after 72 h operation (Fig. 6b). For
unmodified GO membrane, after 72 h testing, the integrity was
partially destroyed. This may be because GO membranes with
10
20
30
40
50
60
70
80
Testing time (h)
Fig. 6 Membrane stability. a) Methanol flux and permeance of
the GO-Si2 membrane at different pressures. The blue dash
line shows the trend for some other GO membranes, in which
8 | J. Name., 2012, 00, 1-3
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flux slowly increases as the pressure increases. b) Long-term
solvent permeance of GO-Si2 membranes.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
The authors thank King Abdullah University of Science and
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Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C8TA10872B
Dual-spacing-channel graphene oxide membrane with multi-hydrophilicity domains gives
high permeance and high rejection in organic solvent nanofiltration