Ultrafast and continuous synthesis of unaccommodating inorganic nanomaterials in droplet- and ionic liquid-assisted microfluidic system

Article abstract of DOI:10.1021/ja2054429

Despite many efforts on the synthesis of inorganic nanomaterials with uniform structure and narrow size distribution in a fast and continuous way, it is still a critical challenge in the chemistry research community due to the uncontrollable mass and heat

Full text of DOI:10.1021/ja2054429

ARTICLE
pubs.acs.org/JACS
Ultrafast and Continuous Synthesis of Unaccommodating
Inorganic Nanomaterials in Droplet- and Ionic Liquid-Assisted
Microfluidic System
Phan Huy Hoang,† HoSeok Park,*,§ and Dong-Pyo Kim*,†,‡
†National Creative Research Center of Applied Microfluidic Chemistry and ‡Graduate School of Analytical Science and Technology,
Chungnam National University, Daejeon 305-764, South Korea
§Department of Chemical Engineering, College of Engineering, KyungHee University, 1 Seochon-dong, Giheung-gu, Youngin-si,
Gyeonggi-do 446-701, Korea
S Supporting Information
b
ABSTRACT: Despite many efforts on the synthesis of inorganic nanomaterials
with uniform structure and narrow size distribution in a fast and continuous way,
it is still a critical challenge in the chemistry research community due to the
uncontrollable mass and heat transfer and the harsh experimental conditions of
high temperature and pressure. Here we report a droplet- and ionic liquid-
assisted microfluidic (DIM) synthesis method, which takes full advantage of both
ionic liquids and droplet-assisted microreaction systems, for an ultrafast, mild,
and continuous synthesis of various inorganic nanomaterials that takes only tens
of minutes rather than days that are usually needed to synthesize. In particular,
unaccommodating inorganic nanomaterials that are difficult to produce, such as nanoporous ZSM-5, γ-AlOOH, and β-FeOOH
nanorods, were synthesized in only “20 minutes” of reaction time even with simple instrument. The DIM method delineated herein
would offer a breakthrough synthetic approach for functional but unaccommodating inorganic nanomaterials in a continuous and
mild manner.
’ INTRODUCTION
although there were problems associated with outgassing and
clogging phenomena.2a,8b In particular, droplet-assisted micro-
reactors offered rapid mixing of reactants and easy handling of
solid reagents resulting in high-quality inorganic and hybrid
nanoproducts.2,5À7,8a
At the heart of nanoscience and nanotechnology are nanos-
tructured inorganic materials with unique properties and func-
tions that can be used for a variety of applications.1 Many efforts
have been directed toward synthesizing nanostructured inorgan-
ic materials efficiently and obtaining their uniformity in bulk
solution phase for wide applications.1 Despite these intensive
efforts, effective means for mass and heat transfer that are
necessary to facilitate synthesis of inorganic nanomaterials with
uniform shape and narrow size distribution2 are yet to arrive.
More problematic with crystalline inorganic nanoproducts, such
as zeolite, is a long reaction time that typically lasts up to 1À2
days, even under hydrothermal conditions of high autogenous
pressures up to 15 bar and high temperature in discontinuous
batch processes.3,4 Therefore, a breakthrough is needed for the
synthesis of inorganic nanomaterials that allows fast process in a
continuous manner under mild reaction conditions and yet re-
sults in uniform shape and narrow size distribution. The break-
through would lead to wide applications in various fields by both
academia and industry.
Ionic liquids (ILs) have been shown to be very useful for
applications involving solvents, catalysis, electrochemistry, se-
paration, etc.10 In particular, use of ILs as thermally stable,
nonvolatile solvents, and/or templates was essential in preparing
nanomaterials with simple reaction apparatus under mild condi-
tions. For instance, nanoporous zeolite analogues and inorganic
oxide nanoparticles were synthesized by bulk process in the
presence of ILs under ambient pressure,4,11À15 without the com-
plications associated with high hydrothermal pressure in auto-
clave and postaddition of molecular or organic solvents. Although
microwave synthesis in ILs is an efficient method to rapidly
synthesize inorganic materials,14e,f delicate control in the burst
heating and degradation of reactants is required.
We were, therefore, motivated that the breakthrough could be
made by taking fully the advantages that are offered by both ILs
and microfluidic droplet synthesis. Such a synthesis could open a
new route for ultrafast and versatile synthesis of functional but
unaccommodating inorganic nanomaterials that are difficult to
Recently, microfluidic synthesis has received much attention
because of the advantages it offers, such as high surface area to
volume ratio at the microscale, fast mass and heat transfer, and
short diffusion distance.5 Microreactors with continuous or dis-
continuous flow have been shown to efficiently produce mono-
dispersed coreÀshell oxide, Au, or quantum dot nanoparticles,2,5À9
Received: June 16, 2011
Published: August 10, 2011
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synthesize, such as silicate-based zeolites that have not yet been
attempted by either ILs or microfluidic process.
compositions. Finally, the resultant products was converted into the
H-form by ion exchanges by treating three times with a solution of
NH4NO3 1 M at 60 °C and followed by washing with DI water, drying at
100 °C, and calcination in air at 550 °C for 6 h.
Herein, we report an ultrafast, mild, and continuous synthesis
of unaccommodating inorganic nanomaterials that takes only
tens of minutes rather than days that are usually needed to syn-
thesize. For the purpose, we devised and fabricated a droplet and
IL-assisted microfluidic (DIM) system that does not require any
sophisticated instruments and equipment. The basic strategy
delineated in this work is to integrate ILs into droplet-based
microreactors: the ILs as templates for the construction of
nanostructures and also as thermally stable, nonvolatile solvents
to resolve problems of microfluidic synthesis, and the droplet-
based microreactors for rapid synthesis of uniform inorganic
nanomaterials. Nanoporous zeolite and aluminum and iron
hydroxide nanoparticles are chosen as target inorganic materials
because of inability of the conventional technologies to construct
the nanostructures in a short reaction time with uniform morphol-
ogy and narrow size distribution, which would be a delightful
innovation to the inorganic materials chemistry. The excellent
quality of the materials thus synthesized is illustrated with ZSM-5
zeolite by testing the catalytic activity of the as-obtained zeolite
for acetal formation reaction.
In order to examine effectiveness of DIM process on aging step, the
DIM system for continuous aging and reaction step in PFA capillary tube
were fabricated by assembling a lab-on-a-chip part for droplet generation
at T-junction and second part of PFA capillary tube for aging step at
room temperature and reaction step at high temperature (see Figure S1,
Supporting Information). The synthetic solution was prepared similarly
to the above-mentioned procedure except without aging step.
DIM Synthesis of Aluminum Hydroxide (γ-AlOOH). C4MimCl
(1.4 g) was mixed with 3 mL of an aqueous solution of 0.1 M HCl. After
that 1.96 g of aluminum chloride hexahydrate was added to above
solution. The resultant mixture was stirred at 60 °C for 2 h, followed by
aging at 60 °C for several hours. To complete crystallization and induce
the formation of boehmite structure, the mixture was synthesized at
ambient pressure through a DIM process by following the same
procedure as ZSM-5 except for the temperature set at 130 °C. The
synthesized products were collected at the outlet of the PFA tube,
purified by centrifuging, and washed with acetonitrile and deionized
water. The obtained γ-AlOOH product was transformed into γ-Al2O3 at
550 °C in air for 3 h.
DIM Synthesis of Iron Hydroxide (β-FeOOH). In 1 mL of
deionized water, 1.222 g of C4MimCl was mixed. After 1.89 g of ferric
chloride hexahydrate was added to the mixture at the molar ratio of
’ EXPERIMENTAL DETAILS
FeCl3 6H2O to C4MimCl of 1:1, 3 mL of 0.1 M HCl was added to the
DIM Synthesis of Silicate-Based ZSM-5 Zeolite. The ZSM-5
zeolite particles were synthesized by the DIM method. The zeolite
precursor solution was prepared by adding sodium aluminate (NaAlO2)
to a homogeneous solution of mixture of 1-butyl-3-methylimidazolium
chloride (C4MimCl) and deionized water. The solution of tetrapropy-
lammonium hydroxide (TPAOH, 25% aqueous solution) and potas-
sium hydroxide was added to the precursor solution. This solution was
stirred for 1 h until completely dissolved. Then, tetraethyl orthosilicate
(TEOS) as a silica source was added to the aforementioned aluminum
precursor solution to prepare a solution with a mole ratio of the com-
position TEOS/TPAOH/NaAlO2/KOH/H2O/C4MimCl = 8:1.05:0.1:
0.85:64:20. The synthetic solution was obtained after aging for several
hours (0À15 h) by stirring at room temperature, and used as a precursor
solution of zeolite synthesis.
The zeolite precursor solution was forced into the continuous phase
at the T-junction of the channel at flow rate of Qd μL minÀ1 (flow rate
2À8 μL minÀ1) to form the disperse phase. For the continuous phase,
fluorocarbon oil (FC oil, 3M, St. Paul, MN) was introduced from the
horizontal inlet at a flow rate of Qc μL minÀ1 (flow rate 12À48 μL minÀ1).
Both the dispersed and continuous phases were injected into the
microfluidic device using a syringe pump (PHD 2000, Harvard Instru-
ments, Holliston, MA). Each droplet reactor was then flowed in the
perfluoroalkoxyalkane (PFA) tube (i.d. 508 μm), which was immersed
in a silicon oil bath at 150 °C, with different delay loop lengths of the
PFA tube. The experiments were carried out at various flow rates of
dispersed phase (Qd) and continuous phases (Qc) with constant ratio
Qd:Qc = 1:6. After 5À20 min of reaction, the synthesized products were
collected at the outlet of the PFA tube by a cooled vial at low tem-
perature to avoid further crystallization. Then, the lower FC oil phase
(continuous phase) was separated, and the synthesized products were
collected by centrifuging (11 000 rpm, 15 min). After washing the as-
obtained samples with acetonitrile and deionized water by three repeti-
tions to remove the excess ionic liquids, the resultant products were
dried at 100 °C for several hours for analysis of samples by scanning
electron microscopy (SEM), transmission electron microscopy (TEM),
and dynamic light scattering (DLS). Owing to the fast heat release of
microfluidic system for quenching of reaction, the as-collected samples
were not gel-like but in a colloidal state with respect to the chemical
3
resulting mixture dropwise to control to a pH of 3. The resultant mixture
was stirred at room temperature for 2 h in order to obtain a homo-
geneous solution. To induce the formation of nanorod iron hydroxide
via a green ionothermal process, the mixture was synthesized at ambient
pressure through a DIM process by following the same procedure as the
boehmite synthesis in droplet. The synthesized products were collected
at the outlet of the PFA tube, purified by centrifuging, and washed with
acetonitrile and deionized water.
Catalytic Reaction. The catalytic reactions were carried out in a
three-necked flask equipped a reflux condenser under N2 atmosphere.
The molar ratios of benzaldehyde with ethylene glycol and 1-butanol
were 1:2 and 1:4, respectively. In particular, benzaldehyde (1.06 g,
0.05 mol), ethylene glycol (1.24 g, 0.1 mol), or n-butyl alcohol (2.96 g,
0.2 mol) and the calcined ZSM-5 catalyst (0.05 g) were mixed under
continuous stirring. The mixture was kept stirring at reaction tempera-
ture of 80 °C for several hours. In the acetal formation reaction of
benzaldehyde with glycol or n-butyl alcohol, reactions were continued
until the equilibrium. The samples were analyzed by NMR to calculate
the conversion of reactants.
’ RESULTS AND DISCUSSION
Set up For a DIM System. Figure 1 illustrates the synthetic
procedure of inorganic nanomaterials using a DIM system. The
fluoropolymer lab-on-a-chip with multisized channels was fabri-
cated by a simple, cost-effective single-step method as previously
reported by our group.16 The DIM system was assembled with a
solvent resistant and thermally stable fluoropolymer (FP) lab-on-
a-chip part for droplet generation at T-junction and a capillary
perfluoroalkoxyalkane tube (PFA, 60À180 cm, i.d. 508 μm) part
for the extended reaction at high temperature. Inorganic pre-
cursors were dissolved in ILs and then were encapsulated in
nanoliter droplets (see Figure S2, Supporting Information) at the
T-junction. Droplets passed through the PFA tube (i.d. 508 μm)
of a microreactor that is immersed in a silicon oil bath, which
was heated up to the desired temperature (130À150 °C).
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Figure 1. Illustration of the DIM system for synthesis of various in-
organic nanomaterial.
The resident time of droplet was controlled by the loop length
(60À180 cm) of the PFA tube. In particular, neither back
pressure nor clogging in microfluidic system occurred even in
the absence of any instrument, such as pressure regulator and
high-pressure pump, because of the negligible vapor pressure and
thermal stability of ILs. Moreover, the stable device operation
was achieved even at high temperature (≈150 °C). In this regard,
the DIM synthesis method provides a convenient, safe, fast, and con-
trollable approach for the preparation of inorganic nanomaterials.
Ultrafast and Continuous Synthesis of ZSM-5 Zeolite in a
DIM System. The DIM method is first demonstrated here by
applying it to the synthesis of silicate-based ZSM-5 zeolite. Although
there were significant reports on successful bulk syntheses of
aluminophosphate zeolite analogues using ILs in the form of
powder and film,4,11,12 there were very few examples for alumi-
nosilicate- or silicate-based zeolite synthesis due to the limited
solubility of silica precursors (TEOS) in the common ILs.4 The
enhanced solubility of TEOS, attributable to the use of water as
cosolvent and the presence of ion hydroxide in reaction media,4a
enabled to synthesize uniform ZSM-5 products via a solution
crystallization in a DIM process.11 Hydrophilic C4MimCl IL was
used as a cosolvent and a suppressor of vapor pressure of water.
In addition, compartmentalization of synthetic solution in dro-
plets assisted the prevention of water evaporation due to the
confinement effect.7a The ZSM-5 was successfully synthesized
with the DIM method only within 15 min of reaction time, much
faster than several hours of bulk reaction as previously
reported.17 Along with the ultrafast synthesis, ZSM-5 revealed
uniform morphology and narrow size distribution, with a dia-
meter of 380 ( 20 nm, as shown in Figure 2. Furthermore, high
crystallinity and purity of ZSM-5 was confirmed by XRD
patterns, which were assigned to the orthorhombic structure
(JCPDS 44À0002) (see Figure S3, Supporting Information).
The crystalline structure of ZSM-5 was observed in an HR TEM
image, showing interplanar spacings of 3.838 Å corresponding to
the (501) plane. The crystalline size of 386 nm, calculated by the
Scherrer’s equation, was reasonably consistent with the average
particle size observed from SEM image. The crystalline size and
crystallinity of ZSM-5 was controlled by manipulating the
reaction time with loop length or flow rate. With respect to the
textural properties, the ZSM-5, calcined at 550 °C in air, has a
pore volume of 0.46 cm3.gÀ1, a surface area of 408 m2.gÀ1, and a
pore size of ≈3 nm as determined by Brunauer-Emmertt-Teller
(BET) analysis (see Figure S4, Supporting Information). It is
postulated that the hydrophilic ILs interact with the primary
Figure 2. (A) SEM (2 μm bar) images of ZSM-5, (B) TEM (200 nm
bar) and (C) HRTEM (20 nm bar) images of ZSM-5 obtained by DIM
process in 20 min of residence time, (D) XRD patterns of ZSM-5 for
four different residence times of (a) 5, (b) 10, (c) 15, and (d) 20 min.
aluminosilicate crystals through the hydrogen bonding-co-πÀπ
stacking and as a result, the interactions lead to aggregation and
organization into mesoporous zeolite crystals.15 Consequently,
ZSM-5 with uniform size, high crystallinity, and large surface area
was very rapidly and readily synthesized through the DIM
synthesis.
In order to assess the advantages offered by the DIM method,
three control experiments were performed: experiment with the
DIM system without IL, the same experiment only with capillary
reactor, and that with flask or bulk phase synthesis. In addition to
the role of cosolvent played by IL, it is also capable of retaining a
sufficient amount of water at high temperature by suppressing
the vapor pressure.4b In the absence of IL, the DIM device did not
work due to the occurrence of high back pressure and clogging.
These findings indicate that the use of IL is crucial for over-
coming the operation problem of microfluidic system in synthe-
sizing nanoporous ZSM-5. For comparison, ZSM-5 was also
synthesized by capillary continuous microreactor and bulk phase
reaction, under the same chemical composition and reaction
temperature as in ZSM-5 prepared through the DIM method. In
the case of continuous microreactor systems without droplet
generators, ZSM-5 showed irregular shape, wider size distribu-
tion and lower crystallinity. Besides, the crystallinity of ZSM-5
was comparable to that of ZSM-5 prepared through bulk reaction
that took 24 h but with much higher uniformity (see Figure
S5ÀS7, Supporting Information). It is worth noting that a DIM
process integrated by ILs and droplet microfluidics induces the
uniform and high crystalline zeolite nanoparticles by effectively
regulating their growth even with simple instrumentation.
It is reported that the aging step of the synthetic solution at
room temperature is useful for reducing the crystallization time.18
The bulk reaction for 24 h without aging step produced nearly
amorphous ZSM-5 with a broad XRD peak (Figure S8a, Sup-
porting Information). Then the effectiveness of DIM process on
aging step was examined by performing continuous aging and
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Figure 3. (A) SEM image (500 nm bars) of γ-AlOOH obtained by
DIM process in 17 min of residence time, and TEM images of γ-AlOOH
(B) scale bar of 100 nm, (C) scale bar of 20 nm (inset is the XRD pattern
of γ-AlOOH), and (D) HR-TEM image with scale bar of 2 nm.
Figure 4. TEM image of β-FeOOH nanocrystals obtained by DIM
process in 15 min of residence time: (A) scale bar of 200 nm, (B) scale
bar of 50 nm, (C) scale bar of 60 nm (inset is XRD pattern of β-FeOOH
nanocrystals), and (D) HRTEM image with scale bar of 5 nm.
reaction step in PFA capillary tube (Figure S1, Supporting
Information), in comparison to different aging times in bulk phase.
The comparative X-ray diffraction (XRD) spectra of Figure S8b
and c, Supporting Information reveal that 20 min aging in the
droplet corresponds to 5 h aging in bulk phase. It indicates that
droplet was also effective to promote the nucleation during the
aging step as well as the growth of zeolite nanoparticle.
(γ-AlOOH) as a precursor of γ-Al2O3 and iron hydroxyde
(β-FeOOH) as a precursor of iron-based materials were synthe-
sized by the DIM process. Although γ-AlOOH is regarded as an
advanced material owing to its excellent catalytic and textural
characteristics, it is very difficult to form the nanostructure due to
fast reaction rates of precursors and harsh synthetic conditions.19
As was the case with the synthesis of ZSM-5, γ-AlOOH with
uniform nanostructures was rapidly synthesized only in 17 min,
which is the fastest synthetic time ever reported. As shown in
Figure 3, γ-AlOOH was assembled into well-aligned bundles
consisting of nanofibers with diameters of 1À3 nm and lengths of
120À188 nm. The crystalline structures of γ-AlOOH were identi-
fied to be the typical orthorhombic boehmite phase (JCPDS 21-
1307). The broad XRD patterns of γ-AlOOH are attributed to
the small size of the nanocrystalline domains, as shown in the
high-resolution transmission electron microscopy (HR-TEM)
image (Figure 3D). The observation of the lattice resolved fringes of
γ-AlOOH, which displayed a constant spacing of about 1.85 Å
corresponding to that of the (200) planes, indicates the existence
of a crystalline framework and the preferential orientation of
specific crystalline planes. The orientation of the bunches of
nanofibers was derived from the interactions in play on the
surface of the individual nanofibers as previously reported.13d In a
manner analogous to the IL-templated system in bulk phase, ILs
played the role of cosolvent and templating agent during the
thermal treatment in microreactors for the construction of γ-
AlOOH nanostructures. Furthermore, it was confirmed that the
topochemical transformation of γ-AlOOH into γ-Al2O3 oc-
curred during dehydration and deoxohydration at 550 °C.13c,d
Another unaccommodating nanomaterial of β-FeOOH nano-
crystals was also synthesized by the DIM method using the
identical C4MimCl IL. The structures and the crystalline phases
of β-FeOOH are shown in the TEM images and XRD pattern
given in Figure 4. Nanorods of β-FeOOH with a diameter of
The reaction mixture confined in discrete droplets on a nanoliter
scale that move along the channel gets homogenized quickly in
chemical composition and temperature through efficient heat
transfer and chaotic advection in the droplets that enhances
greatly the degree of mixing. Moreover, high surface area to
volume ratio of droplet would result in rapid heat and mass
transfer in crystallization.7a,16 It is apparent that the crystal-
lization in droplets occurred very fast, within a short residence
time of minutes, and this fast crystallization could lead to
narrow particle-size distribution with high crystallinity. The
synthesis of high-quality ZSM-5 is ultrafast in the sense that
the synthesis by DIM takes 15 min, whereas it takes 1 day or 24 h
with bulk phase reaction. And it is well-known that the crystal-
lization process is strongly influenced by kinetics in nucleation
and growth steps. Considering that the crystallization described
herein is a diffusion controlled system rather than a reaction
controlled system due to the fast reaction rates of precursors used
in these syntheses of inorganic nanomaterials,19 the crystalliza-
tion in the nanoliter-scale droplet was facilitated by the enhanced
mass transfer with a short diffusion distance. In other words, the
chaotic advection in the droplet flow might promote efficiently
the probability of a solute molecule to come into contact with an
existing crystal. In addition, it is believed that the fast heat transfer
in a DIM system enabled to supply rapidly thermal energy for the
burst nucleation and the subsequent crystal growth with less
lattice defects.
Versatile Synthesis of Inorganic Nanomaterials in a DIM
System. In order to demonstrate the versatility of this approach
for the synthesis of unaccommodating inorganic materials, boehmite
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40 ( 5 nm and a length of 400 ( 20 nm were constructed only in
15 min of reaction time. The XRD peaks of β-FeOOH nanorods
were assigned to the tetragonal crystalline phase (JCPDS 34-
1266). The lattice resolved fringes of β-FeOOH nanorods with a
constant spacing of 2.60 Å correspond to the (400) planes, as
confirmed by TEM and XRD results, which means that β-FeOOH
nanorods were preferentially grown along a specific direction of
the Z axis with the minimization of surface energy. The findings
attest to the solvent and templating effects of ILs on the for-
mation of nanostructures in droplet-based microreactors. These
results confirm that the DIM method is an innovative synthetic
route to nanostructured inorganic oxides, which is ultrafast and
versatile without any rate-controlling agents.
Catalytic Reaction. ZSM-5 is one of the most important
catalysts in organic synthesis and in oil refining process, owing to
its high-surface area, shape selectivity, and strong acidity. There-
fore, acetal formation reaction was chosen to test the quality of
the ZSM-5 zeolite catalyst synthesized by DIM. As can be seen in
Table S1, Supporting Information, the as-prepared ZSM-5
shows 53.6% and 71.8% conversion yields of benzaldehyde
when reacted with 1-butanol and glycol, respectively, which
are significantly higher than 30.4% and 52.9% yielded by the
conventional ZSM-5. The highly improved catalytic perfor-
mance of the zeolite obtained from DIM could be ascribed to
the large number of active sites on the high surface area as well
as the presence of mesopore that is an important factor in the
acetal formation with large size reactant molecules, such as
benzaldehyde.17b
’ ACKNOWLEDGMENT
This study was funded by the 2008 Creative Research Initiative
Program [R16-2008-138-01000-0(2008)] administered by the
Korean Ministry of Education, Science, and Technology.
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’ CONCLUSION
In summary, an ultrafast and continuous synthesis method has
been presented for those unaccommodating nanomaterials that
are difficult to synthesize because of a long reaction time that can
last up to days even at high pressure and temperature. The droplet-
and ionic liquid-assisted microfluidic (DIM) system devised and
fabricated in this work enables a reduction of reaction time from
days to tens of minutes, allows mild reaction conditions, and yet
produces narrow size distribution and excellent crystalline qua-
lities. Three unaccommodating nanomaterials have been synthe-
sized with the DIM system to fully demonstrate the advantages
offered by the synthesis method with excellent results. In
particular, ZSM-5 zeolite nanomaterial, which is an important
catalyst, was synthesized by the method and delivered a perfor-
mance superior to the conventionally synthesized catalyst. The
synergistic combination of ionic liquid and droplet microfluidic
process allows for ultrafast and continuous synthesis of various
inorganic nanomaterials, which greatly expands the utility and
the versatility of this approach for the synthesis of alternative
nanomaterials.
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’ ASSOCIATED CONTENT
S
Supporting Information. Experimental details, Table S1,
b
and Figure S1ÀS8. This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
dpkim@cnu.ac.kr; phs0727@khu.ac.kr
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