Whole ceramic-like microreactors from inorganic polymers for high temperature or/and high pressure chemical syntheses

Article abstract of DOI:10.1039/c3lc51191j

Two types of whole ceramic-like microreactors were fabricated from inorganic polymers, polysilsesquioxane (POSS) and polyvinylsilazane (PVSZ), that were embedded with either perfluoroalkoxy (PFA) tube or polystyrene (PS) film templates, and subsequently the templates were removed by physical removal (PFA tube) or thermal decomposition (PS). A POSS derived ceramic-like microreactor with a 10 cm long serpentine channel was obtained by an additional "selective blocking of microchannel" step and subsequent annealing at 300 °C for 1 h, while a PVSZ derived ceramic-like microreactor with a 14 cm long channel was yielded by a co-firing process of the PVSZ-PS composite at 500°C for 2 h that led to complete decomposition of the film template leaving a microchannel behind. The obtained whole ceramic-like microfluidic devices revealed excellent chemical and thermal stabilities in various solvents, and they were able to demonstrate unique chemical performance at high temperature or/and high pressure conditions such as Michaelis-Arbuzov rearrangement at 150-170°C, Wolff-Kishner reduction at 200°C, synthesis of super-paramagnetic Fe₃O₄ nanoparticles at 320°C and isomerisation of allyloxybenzene to 2-allylphenol (250°C and 400 psi). These economic ceramic-like microreactors fabricated by a facile non-lithographic method displayed excellent utility under challenging conditions that is superior to any plastic microreactors and comparable to glass and metal microreactors with high cost.

Full text of DOI:10.1039/c3lc51191j

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Whole ceramic-like microreactors from inorganic
polymers for high temperature or/and high
pressure chemical syntheses†
Cite this: Lab Chip, 2014, 14, 779
Wurong Ren,‡§a Jayakumar Perumal,§b Jun Wang,a Hao Wang,a Siddharth Sharmac
c
*
and Dong-Pyo Kim
Two types of whole ceramic-like microreactors were fabricated from inorganic polymers, poly-
silsesquioxane (POSS) and polyvinylsilazane (PVSZ), that were embedded with either perfluoroalkoxy
(PFA) tube or polystyrene (PS) film templates, and subsequently the templates were removed by physical
removal (PFA tube) or thermal decomposition (PS). A POSS derived ceramic-like microreactor with a
10 cm long serpentine channel was obtained by an additional “selective blocking of microchannel” step and
subsequent annealing at 300 °C for 1 h, while a PVSZ derived ceramic-like microreactor with a 14 cm
long channel was yielded by a co-firing process of the PVSZ–PS composite at 500 °C for 2 h that led to
complete decomposition of the film template leaving a microchannel behind. The obtained whole
ceramic-like microfluidic devices revealed excellent chemical and thermal stabilities in various solvents,
and they were able to demonstrate unique chemical performance at high temperature or/and high pres-
sure conditions such as Michaelis–Arbuzov rearrangement at 150–170 °C, Wolff–Kishner reduction at
200 °C, synthesis of super-paramagnetic Fe3O4 nanoparticles at 320 °C and isomerisation of allyloxybenzene
to 2-allylphenol (250 °C and 400 psi). These economic ceramic-like microreactors fabricated by a facile
non-lithographic method displayed excellent utility under challenging conditions that is superior to any
plastic microreactors and comparable to glass and metal microreactors with high cost.
Received 20th October 2013,
Accepted 20th November 2013
DOI: 10.1039/c3lc51191j
www.rsc.org/loc
silicon,5 and ceramics6 are the most commonly employed
materials for the fabrication of microfluidic devices. Despite
Introduction
Microfluidic devices, including lab on a chip systems, have
attracted a great deal of interest in various areas of chemistry
due to their unique characteristics such as miniaturized reac-
tion volume, extremely large surface to volume ratio and effi-
cient mass and heat transfer capability.1–3 With the advent of
this technology, it has also been considerably challenging to
develop new structural materials and convenient fabrication
processes for these microfluidic tools. To date, metals,4
their high mechanical stability and robust performance, fab-
rication of microfluidic devices using these materials requires
sophisticated facilities with expensive equipment to fabricate
channels through wet/dry etching or micromachining and to
seal the devices via fusion, adhesive or anodic bonding.5 In
the meantime, microfluidic devices made of polydimethylsi-
loxane (PDMS) have also been widely used due to their low
cost and easy fabrication process based on soft lithographic
technique.7 Recently, alternative materials such as poly-
methyl methacrylate (PMMA),8 polycarbonate,9 fluoro-
polymers,10,11 thiol-ene resins,12 polystyrene elastomers13 and
polyimide14 have also been used to fabricate microfluidic
devices. Despite their successful applications in certain areas
such as biology, most of these microfluidic devices suffer
from mechanical, chemical and thermal stabilities that
restricts their applications for chemical syntheses.15 Recently,
our group has demonstrated alternative technologies to par-
tially overcome the swelling problem in PDMS microfluidic
devices by employing protective coating of the inner walls of
the channels with inorganic polymers such as sol–gel
hybrids,16 polyvinylsilazane17,18 and polycarbosilane.19 However,
at present, there is an increasing demand to develop durable
a Science and Technology on Advanced Ceramic Fibers and Composites Laboratory,
College of Aerospace Science and Engineering, National University of Defense Technology,
Changsha 410073, People's Republic of China
b Bio-Optical Imaging Group, Singapore Bioimaging Consortium (SBIC), Agency for
Science and Research (A*STAR), 11 Biopolis Way, #02-02 Helios, Singapore
138667, Singapore
c National Center of Applied Microfluidic Chemistry, Department of Chemical
Engineering, Pohang University of Science and Technology, Environ. Eng. Bldg.,
San31, Hyoja-dong, Nam-gu, Gyungbuk, Pohang, 790-784, Korea.
E-mail: dpkim@posetch.ac.kr
† Electronic supplementary information (ESI) available: Molecular structure of
POSS and PVSZ, characterization results of SEM, TEM, XRD, VSM, CA. See
DOI: 10.1039/c3lc51191j
‡ Wurong Ren worked at Professor Kim's lab under a co-advisor program.
§ Wurong Ren and Jayakumar Perumal contributed equally to this work.
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and economic microfluidic devices, which can retain and
exhibit high mechanical and chemical stabilities even under
extremely harsh conditions. Unlike most of the organic
polymers that undergo ready decomposition accompanied by
high weight loss at elevated temperatures, silicon-containing
inorganic polymers are thermally resistant and gradually trans-
form into an opaque Si-based ceramic phase, often with severe
volume shrinkage. In particular, polyvinylsilazane and poly-
silsesquioxane have been used as precursors for SiCN and
silica ceramics.20 Furthermore, it would be anticipated that a
transparent transient phase between polymer and ceramic
phases with negligible or affordable shrinkage can be achieved
under certain conditions, which would have significantly
enhanced chemical and thermal stabilities like glass devices in
conjunction with convenient and cost effective fabrication
processes like organic polymers. In addition, the inorganic
polymer derived reactors may outperform any reactor fabri-
cated from organic polymers.
Herein, we report a simple template based method for
fabrication of novel, robust and cost effective ceramic-like
microreactors embedded in preceramic polymers such as
polysilsesquioxane (POSS) or polyvinylsilazane (PVSZ), that are
capable of operating under high temperature or/and high pres-
sure conditions for unaccommodating inorganic and organic
chemical syntheses. The presented simple non-lithographic
fabrication process does not require a troublesome sealing
step, which is a highly complex task to be performed on consol-
idated materials by conventional processes. The two types of
whole ceramic-like matrix microreactors were fabricated by
taking advantage of the intrinsic processability of viscous
polymers without any use of sophisticated facilities; the
microreactors exhibited excellent chemical and thermal dura-
bility comparable to a glass microreactor, and with much better
performance than any plastic microreactors.
in the PDMS boat, in which the POSS precursor solution was
introduced and subsequently cured by UV (Heraeus Amba
Ltd., 365 nm) irradiation for 5 min. Finally, the template
tubes were carefully removed from the aforementioned solidi-
fied PFA tubing/POSS composite to create the interconnected
microfluidic channels.
In order to make a serpentine microfluidic channel from
the interconnected open channel structure, the POSS precursor
solution was re-filled into the channel driven by capillary force.
Subsequently, the channel was selectively blocked by 1 min of
UV curing using a spot UV laser source (Hamamatsu Photonics
K.K., 365 nm wave length, beam diameter 1 mm). Finally, the
POSS microfluidic device with a serpentine channel (10 cm
length, 360 μm diameter) was thermally treated at 300 °C for
1 h in an inert atmosphere after flushing out the uncured POSS
solution from the channel. The inlet and outlet connection of
the prepared POSS microreactor were made by inserting a silica
capillary into the channel and subsequently filling it using a
polyvinylsilazane (HTT 1800, Clarient) adhesive with 1% DMPA
as a photoinitiator, followed by serial UV exposure for 5 min
and thermal treatment at 200 °C for 1 h.
Fabrication of a polyvinylsilazane (PVSZ) microreactor by
employing a polystyrene (PS) film template
An alternative ceramic-like microreactor was fabricated from
polyvinylsilazane (PVSZ, HTT 1800, Clarient) embedded with
a patterned polystyrene (PS) film as the thermally sacrificial
template. In order to achieve greater control over the accuracy
and precision of the designed channels, a PS film template
was designed using CAD software (AutoCAD 2008), loaded in
a lab view program and subsequently cut by laser ablation
process using femto-second laser pulses (150 fs, 1 kHz,
810 nm). The resultant PS template pattern (25 μm thickness ×
500 μm width × 14 cm length, Goodfellow Cambridge Ltd.)
was fixed onto a PDMS boat (0.5 × 3 × 4 cm) coated with a
mold releasing agent (Clarient). A mixture of PVSZ and 0.5%
dicumyl peroxide thermal initiator (Sigma Chemicals) was
then poured into the PDMS boat with the PS template pattern.
The boat was kept on a hot plate and slowly heated to 100 °C
at a rate of 1 °C min−1 and held for 2 h to yield a cross linked
solid. Subsequently, the free standing PVSZ–PS composite,
after demolding from the PDMS boat, was slowly heated again
to 500 °C at a rate of 1 °C min−1 and held for 2 h. During the
co-firing treatment, the PS film was completely decomposed,
while the PVSZ was thoroughly cross-linked to produce a
solvent resistant ceramic-like material that was still transparent
but slightly yellowish in color. The inlet and outlet connection
of the microreactor was made of a stainless steel capillary
(od = 1/16 inch) and a ceramic SiC paste (Aremco Products)
that were consolidated by annealing at 70 °C for 3 h.
Experimental
Fabrication of polysilsesquioxane (POSS) microreactor by
employing PFA tubing template
POSS precursor solution was prepared by mixing methacrylate
functionalized polysilsesquioxane (MA0735, Hybrid Plastics)
with 5 wt% methacrylate functionalized silica filler solution
and 1 wt% 2,2-dimethoxy-2-phenyl-acetophenone (DMPA, Aldrich)
photoinitiator in methyl ethyl ketone solvent and stirring the
mixture for 5 h at 80 °C; subsequently, the solvent was removed
using a rotary evaporator. The methacrylate functionalized
silica filler was made by mixing a silica filler (MEK-ST, Nissan
Chemical Industries, Ltd.) solution with 5 wt% 3-(trimethoxysilyl)
propyl methacrylate (MPTMS, Aldrich) and stirring it for one day
at room temperature. In order to fabricate the POSS microreactor
with interconnected microchannel structures, a PDMS (Sylgard
184, Dow Corning) support boat (50 × 20 × 5 mm) was made by
molding a PDMS prepolymer against a stainless steel block.
Then, the embedded template framework was assembled using
commercially available perfluoroalkoxy (PFA) tubing (360 μm
OD, Health & Science Co.) as a physically sacrificial template
High temperature or/and high pressure reactions of ceramic-
like microreactors
The ceramic-like microreactor derived from POSS was used
for high temperature reactions such as Michaelis–Arbuzov
rearrangement at 150–170 °C and Wolff–Kishner reduction at
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200 °C, whereas the PVSZ derived ceramic-like microreactor
was employed for high temperature or/and high pressure
reactions such as synthesis of super-paramagnetic Fe3O4
nanoparticles at 320 °C and isomerisation of allyloxybenzene
to 2-allylphenol in aqueous phase under conditions of
230–250 °C and 400–450 psi. Firstly, dibromopropane
(Aldrich) and triethylphosphate (Aldrich) were mixed together
at a molar ratio of 1 : 10 to be employed as the reactant
solution for Michaelis–Arbuzov rearrangement. The microchemical
reaction was carried out by injecting the reactant solution
through the inlet of the POSS microreactor (10 cm length,
360 μm diameter) and by heating the microreactor at 150–170 °C
by placing it on a hot plate at various flow rates using a syringe
pump (Harvard). The product solution was collected from the
outlet and the conversion was analyzed by GC-MS (Agilent
5975C, Agilent Tech). Secondly, 0.001 M benzophenone hydrazone
(Aldrich) and 0.005 M potassium hydroxide (Aldrich) were added
into 10 ml of ethylene glycol (Aldrich) to be used as the reactant
solution for Wolff–Kishner reduction at 200 °C, and the product
was analyzed by GC-MS.
N2 purging. The ceramic-like microreactors fabricated from
POSS and PVSZ were tested to evaluate their chemical, pressure
and thermal stabilities. Firstly, the solvent compatibility of the
ceramic-like material was tested with the annealed POSS blocks
at 300 °C for 1 h and with the PVSZ blocks (5 × 5 × 5 mm)
at 500 °C for 2 h under N2, respectively by soaking it in
diverse solvents for 24 h. The swelling ratio, W/Wo, where W
and Wo are the weight of the sample in solvent for 24 h and
that of the dried sample, respectively, was examined by
following the reported method.13 The linear shrinkage of
POSS or PVSZ was calculated by measuring the length of the
polymer bar before curing (L0) in a PDMS boat and after
curing and annealing at 300 °C or 500 °C (L), and the (1 − L/L0)
was denoted as the shrinkage ratio. The pressure tolerance of
the fabricated PVSZ microreactor was investigated by connecting
the inlet of the microreactor to a HPLC pump (SP-930D,
Younglin, Korea) and the outlet to a back pressure regulator.
Water contact angles were measured using a Ramé-Hart
200-F1 goniometer.
Monodisperse Fe3O4 (magnetite) nanocrystals were synthe-
sized from an iron–oleate complex as reported earlier.21 The
iron–oleate complex was obtained by dissolving 4 mmol of
iron chloride (FeCl3·6H2O) and 12 mmol of sodium oleate
into 28 ml of the mixed solvent (8 ml of ethanol + 6 ml of dis-
tilled water + 14 ml of hexane). Subsequently, the upper
organic layer (hexane) containing the iron–oleate complex
was separated after completion of reaction at 70 °C for 4 h,
washed with distilled water and evaporated under reduced
pressure to yield the iron–oleate complex in the form of a
waxy solid. Then, 4 mmol of the complex product dissolved
in 20 g of 1-octadecene with oleic acid (2 mmol) and oleyl
amine (2 mmol) was injected into the PVSZ microreactor
(25 μm × 500 μm × 14 cm) at a flow rate of 0.5 to 2 μl min−1,
and heated at 320 °C by placing the Al foil wrapped
microreactor in a sand bath. The collected product solution
from the outlet was centrifuged to separate the Fe3O4
nanoparticles and to be analyzed using TEM (JEM-2100,
JEOL), XRD (D/MAX-2500/PC, RIGAKU) and a vibrating sam-
ple magnetometer (3900 VSM, MicroMag™ Magnetometers)
after washing with ethanol and drying. Finally, in order to
illustrate the HTHP performance of the fabricated ceramic-
like PVSZ microreactor, isomerization of allyloxybenzene to
2-allylphenol was conducted by injecting an aqueous suspen-
sion of allyloxybenzene (0.1 M) into the microreactor using a
HPLC pump (Iocratic Model 501, Analytical Scientific Instru-
ments) at two sets of temperatures and pressures (230 °C – 400 psi,
250 °C – 450 psi) with a back pressure regulator. The product,
2-allylphenol, was extracted in ethyl acetate and the yield was
determined by GC/MS using anisole as an internal standard.
Results and discussion
One of the advantages of the template or scaffold method
to fabricate microfluidic devices is that no bonding step
is required, as the monolithic microreactor is formed by
physical or thermal removal of the template.22 An appropriate
combination of templates and matrix materials is critical for
fabricating the microchannel structures without mechanical
damage during removal of the templates. In this work, we
selected the assembly structures of low surface energy
perfluoroalkoxy (PFA) tubing that can be physically pulled
out and the patterned polystyrene (PS) thin film by laser
ablation, which is thermally decomposable (refer to ESI,†
Scheme S1).
Two types of inorganic polymers, POSS and PVSZ (refer to
ESI,† Fig. S1) were selected as structural materials for fabrica-
tion of high temperature or/and high pressure tolerable
ceramic-like microreactors. Our first goal was to authenticate
our assumption that a ceramic-like transparent transient
phase existed between the polymer and ceramic phases of
POSS and PVSZ with affordable shrinkage under certain con-
ditions and would possess excellent chemical and thermal
stabilities like glass devices. Also, it was important to confirm
if the selected inorganic polymers were comparable with the
cost-effective and convenient fabrication process. Thus, we
firstly studied the ceramization process of the two polymers
to obtain information about the transparent transient phase
followed by solvent resistant tests to demonstrate their robust
stabilities. Unlike most of the solid POSS, which contain
other functional groups, the methacrylate functionalized
POSS (in a liquid state) can readily infiltrate into void gaps of
the template assembly and consolidate via free radical poly-
merization of methacrylate groups by UV exposure to generate
a monolithic solid.23 Various thermal treatments of the UV
cured POSS at temperatures between 200–400 °C for 1 h in an
inert atmosphere have been implemented to ensure the
Characterization and device tolerance test
Thermal decomposition of the PS film was investigated by
carrying out thermogravimetric analysis (SDT-Q600, TA
Instruments) at a ramping rate of 2 °C min−1 under constant
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presence of a fully cross-linked ceramic-like phase,24 and it is
remarkable to find the existence of a glassy transparent state
of the polymer upon heating at 300 °C, which however
became opaque with cracks at 400 °C, presumably due to
appreciable thermal decomposition of organic parts (refer to
ESI,† Fig. S2). In the case of organic polymers with identical
functional groups such as poly(ethylene glycol)dimethacrylate
(PEGDMA), severe decomposition was observed even only at
200 °C leading to formation of a black residue. Hence, it is
obvious that the thermal stability of inorganic polymers is
superior to that of organic polymers due to the presence of
inorganic parts, which increases the stability of organic parts.
Shrinkage, which commonly occurs during the curing stage
of polymers, may cause defects or cracks in the polymeric
products. In the present study, 5 wt% addition of a methacry-
late functionalized silica filler into the POSS matrix led to
affordable shrinkage (~1.1% linear shrinkage), with an
improvement from 2.3% shrinkage of a filler free POSS
sample upon annealing at 300 °C. The solvent compatibility
of the filler containing POSS annealed at 300 °C was tested by
measuring the swelling ratio after immersion into the respec-
tive solvents at room temperature and at elevated tempera-
tures (3–5 °C below boiling point of the tested solvents) for
24 h, respectively. Table 1 provides details on the chemical
stability of the inorganic POSS material against a series of
organic solvents with different polarity such as ethanol,
DMSO, DMF, IPA, acetonitrile, acetone, dichloromethane,
chloroform, toluene, hexane, THF and CCl4. This excellent
solvent resistance property is comparable to glass and is
much better than organic polymers such as PDMS or PDMS
coated with inorganic polymers. In addition, the water contact
angle of the annealed POSS block at 300 °C was 93°, which
was moderately hydrophobic (refer to ESI,† Fig. S3).
or/and heating at 150–250 °C via inter- and intra-molecular
hydrosilylation of vinyl groups attached to silicon. Thermal
annealing at 500 °C for 2 h could generate a transparent tran-
sient phase between the polymer and ceramic phases with
affordable shrinkage (~4.1% linear shrinkage), but higher
temperature treatment (over 500 °C) rendered opacity to
PVSZ with cracks due to ceramization accompanied by severe
chemical decomposition (refer to ESI,† Fig. S4).25 The solvent
compatibility of PVSZ annealed at 500 °C that was also
measured using the swelling ratio after immersion in the
aforementioned solvents revealed the excellent chemical
resistance of PVSZ (Table 1). In addition, the contact angle of
100° of the annealed PVSZ at 500 °C indicated a moderately
hydrophobic surface (refer to ESI,† Fig. S3). Based on the
above discussion, it is obvious that when POSS and PVSZ
polymers were annealed at 300 °C and 500 °C, respectively,
the transition states of these two inorganic polymers retained
the intrinsic transparency of the polymer and revealed
enhanced chemical robustness as good as the ceramic phase.
Fabrication of ceramic-like microreactors
The POSS derived ceramic-like microreactor was fabricated
by following a three step process. In the first step, the PFA
template framework was assembled in a PDMS boat by crossing
the aligned tubes with each other (refer to ESI,† Fig. S5), and
subsequently the mixed POSS polymer was infiltrated into the
PDMS boat. In the second step, the PFA tubing template was
manually removed from the PFA tubing/POSS composite to
obtain the interconnected POSS microchannel structures after
solidification by UV exposure. Subsequently, the liquid POSS
mixture was re-filled into the entire microchannel, and then
the selected parts of the channels were blocked by UV spot
curing to form a serpentine type, 10 cm long channel after
removing the uncured POSS. Finally, the POSS microreactor
was annealed at 300 °C in an inert atmosphere to achieve a
ceramic-like phase with enhanced chemical and thermal
stabilities for high temperature organic reactions. The entire
fabrication process is represented in Fig. 1(a) and S6 (refer to
Another inorganic polymer employed in this work was
PVSZ. The solidification chemistry of PVSZ as a precursor
of SiCN ceramics has been extensively studied either in
the presence or absence of curing initiators.24 It is well
documented that PVSZ could be consolidated by UV radiation
Table 1 Comparative analysis of chemical stability of POSS annealed at 300 °C for 1 h and PVSZ annealed at 500 °C for 2 h under nitrogen
atmosphere on the basis of changes in sample weight before and after immersion in various solvents at different temperatures for 24 ha
Swelling ratio of POSS
At room temp.
Swelling ratio of PVSZ
At room temp.
Solvents
At elevated temp.
At elevated temp.
Acetone
DCM
MeOH
Hexane
THF
AN
Toluene
CB
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
—
—
—
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
1.00
—
—
1.00 (60 °C)
1.00 (60 °C)
1.00 (60 °C)
1.00 (80 °C)
1.00 (105 °C)
1.00 (125 °C)
1.00 (130 °C)
1.00 (150 °C)
1.00 (200 °C)
—
1.00 (60 °C)
1.00 (60 °C)
1.00 (60 °C)
1.00 (80 °C)
1.00 (105 °C)
1.00 (125 °C)
1.00 (130 °C)
1.00 (150 °C)
1.00 (200 °C)
1.00 (320 °C)
Xylene
DMF
EG
OD
a
MeOH (methanol), DCM (dichloromethane), AN (acetonitrile), CB (chlorobenzene), DMF (dimethylformamide), EG (ethylene glycol),
OD (octadecene).
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ESI†). The POSS derived ceramic-like reactor retained optical
transparency with a pale yellowish tint and the label ‘POSTECH’
can be easily visualized through the microreactor set (Fig. 1(b)).
The scanning electron microscopy (SEM) image (cross section
view) of the POSS derived ceramic-like microchannel reveals a
round shape with a diameter of ~360 μm (refer to ESI† Fig. S7
for the SEM image and S8 for the connecting method).
Alternatively, the PVSZ derived ceramic-like microreactor
was fabricated using a PS film template pattern made by a
laser cutting process. As shown in Fig. 2(a), the PS film
template was completely submerged into the preceramic
polymer, PVSZ, followed by co-firing at 500 °C for 2 h to
completely burn off the PS film template, while the PVSZ
matrix was converted into a ceramic-like phase, leaving
behind the microchannel structure. In order to confirm the
thermal removal of the template, the thermal stability of PS
was investigated by TGA (refer to ESI,† Fig. S9). The analysis
revealed complete decomposition of the template into
gaseous products in the temperature range 370–425 °C in the
presence of inert atmosphere. The PVSZ derived ceramic-like
reactor (shown in Fig. 2(b)) displayed glass-like optical trans-
parency and the microchannel was also clearly visible (25 μm ×
500 μm × 14 cm, refer to ESI,† Fig. S10). The pressure tolerance
test exhibited resistance of the PVSZ reactor up to ~3100 psi
(~213 bar) indicating the feasible utility of the microreactor in
chemical applications requiring conditions of high tempera-
ture and high pressure (refer to ESI,† Fig. S11).
Fig. 2 Fabrication of a PVSZ derived ceramic-like microreactor from a
polystyrene (PS) film template. a) Schematic illustration of the fabrication
process: (I) the PS template processed by a fs laser, (II) introduction and
curing of PVSZ, (III) the ceramic-like microreactor after co-firing at
500 °C for 2 h. b) Optical image of a ceramic-like PVSZ microreactor.
ultimate microfluidic devices by bonding two consolidated
pieces with apparently no leakage. In addition, the channels
with different sizes can be fabricated by employing the
template with the required size.
Microchemical performance at high temperature and/or high
pressure conditions
The miniaturized microreactor provides advantages of
controlled fluid transport, rapid chemical reactions and cost
efficiency when compared to conventional reactors. It is
hypothesized that the developed ceramic-like microreactors
with excellent thermal and chemical stabilities could be quite
useful for organic or inorganic chemical syntheses under
high temperature or/and high pressure conditions, when
compared to PDMS and any plastic microreactors. In the
following discussion, four types of model reactions were
demonstrated to explore the utility of the two ceramic-like
microreactors in a comparative manner to the results in bulk
reactions (refer to ESI,† Fig. S12 for experimental set-up).
The POSS derived ceramic-like microreactor (10 cm length,
360 μm diameter) was firstly tested for Michaelis–Arbuzov
rearrangement reaction that is one of the versatile pathways
for the formation of carbon–phosphorus bonds to produce
various phosphonates, phosphinates and phosphine oxides.
Generally this reaction requires high temperatures and
longer reaction times to complete the conversion.26 The result
presented in Fig. 3(a) illustrates that the continuous flow reac-
tion could be accelerated by elevating the temperature in the
range of 150–170 °C and by extending the residence time in
the range of 10–120 min. An increase in the conversion of
dibromopropane from 21% (150 °C) to 63% (170 °C) at
25 min of retention time was observed, with further improve-
ment to 72% at 170 °C for 42 min, and eventually 100% con-
version at 170 °C for 120 min (refer to GC-MS evidence
provided in ESI,† Fig. S13). The observed enhancement in the
rate of conversion was obviously superior to the bulk systems
that required 20 h to obtain the comparable result.26
In the present work, two types of whole ceramic-like
microreactors with robust durability have been successfully
fabricated by molding the silicon-based polymers, POSS or
PVSZ, into the sacrificial templates, with a subsequent heat
treatment step to obtain a transient phase between the poly-
mer and ceramic phases. This simple non-lithographic fabri-
cation method avoids a troublesome sealing step in the
ceramic chip, which makes it cumbersome to obtain the
Fig. 1 Fabrication of a POSS derived ceramic-like microreactor using
the PFA tubing template method. a) Schematic illustration of fabrication
process for the serpentine microchannel: (I) framework assembly of
PFA tubing template, (II) infiltration and UV curing of POSS, (III) pullout
of tube template, (IV) re-infiltration of POSS into channel, (V) selective
curing of POSS to block the channel, (VI) removal of uncured POSS.
In addition, Wolff–Kishner reduction reaction involving
the formation of hydrocarbons from carbonyl compounds
b) Optical image of
microreactor, inset shows the channel design of 10 cm length.
a transparent and serpentine ceramic-like
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synthesis of the super-paramagnetic iron oxide (Fe3O4) nano-
crystal that is usually carried out at high temperatures.28 The
Fe3O4 nanocrystals were synthesized by flowing a solution of
iron(III) oleate in 1-octadecene at 320 °C as shown in
Fig. 4(a). A consistent increase in the size of nanoparticles
was observed with increasing retention times: 5 1 nm at
12 min, 10 2 nm at 24 min, and 20 2 nm at 48 min when
mixed surfactants of oleic acid and oleyl amine were
employed (refer to ESI,† Fig. S16). Moreover, the shape
of nanoparticles could be conveniently controlled by appro-
priate selection of dispersion agents in the continuous flow
microchemical synthesis: spherical shape when oleic acid was
employed and rod shaped in the presence of a mixture of
oleic acid and oleyl amine (refer to Fig. 3(b) and (c)). The
super-paramagnetic behaviour of highly crystalline Fe3O4
nanoparticles was confirmed using a vibrating sample magne-
tometer (VSM) (refer to ESI,† Fig. S17) and X-ray diffraction
(XRD) measurements (refer to ESI,† Fig. S18), respectively, the
results of which were in good agreement with those of the
earlier reported batch process product.21
Eventually, the PVSZ derived ceramic-like microreactor
was tested for its performance under high temperature and
high pressure (HTHP) conditions by performing the isomeri-
zation of allyloxyphenol to 2-allylphenol as a typical Claisen
rearrangement reaction that is useful for syntheses of natural
Fig.
3 High temperature chemical syntheses by a POSS derived
microreactor. a) Michaelis–Arbuzov rearrangement reaction at 150–170 °C,
and b) Wolff–Kishner reduction reaction at 200 °C.
was chosen to test the performance of the POSS derived
ceramic-like microreactor under more challenging condi-
tions. It is reported that this reaction involves heating crude
hydrazone derivatives with alkali in a high-boiling ethylene
glycol solvent, typically at temperatures of 195–200 °C for one
day.27 The POSS derived ceramic-like reactor was tested for
its performance under continuous flow at 200 °C. A strong
increase in conversion from 28% for 3 min to 83% for 25 min
of retention time was observed as can be seen in Fig. 3(b)
(refer to GC-MS evidence provided in ESI,† Fig. S14). The con-
version was comparable with bulk reaction for 24 h,27 while
the reaction time was significantly decreased. In addition, the
chemical resistance of the POSS derived ceramic-like reactor
under the reaction conditions of a corrosive alkali solution
was investigated by observing the changes in surface mor-
phology when the tested sample (POSS block annealed at
300 °C) was immersed in ethylene glycol with 1 wt% KOH at
200 °C for various time periods. The surface retained a
smooth texture after 1 h exposure to alkali and was still stable
even after 24 h of exposure, except for the presence of little
scratches, indicating the excellent thermal and chemically
stable nature of the microreactor (refer to ESI,† Fig. S15).
An alternative PVSZ derived ceramic-like microreactor
(25 μm × 500 μm × 14 cm) was used for continuous flow
Fig. 4 High temperature and/or high pressure chemical syntheses
using
a PVSZ derived ceramic-like microreactor. a) Synthesis of
super-paramagnetic Fe3O4 nanoparticles at 320 °C, b) formation of
spherical 10 nm sized nanocrystals from a mixture of surfactants, oleic
acid and oleyl amine (1 : 1) after 24 min, inset shows 20 nm cluster after
48 min, c) rod shaped Fe3O4 nanocrystals after 24 min by employing
oleic acid as a surfactant, and d) isomerization of allyloxybenzene to
2-allylphenol at 230–250 °C and 400–450 psi.
784 | Lab Chip, 2014, 14, 779–786
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products or pharmaceutical intermediates.29,30 The pressure
during the reaction was generated by heating the aqueous
mixture over 200 °C, which is the desirable reaction condi-
tion. The reaction was performed by flowing an aqueous solu-
tion of allyloxybenzene (0.1 M) using a HPLC pump and a
back pressure regulator at the outlet, and two HTHP condi-
tions were achieved by heating at 230 °C and at 250 °C and
developing 400 psi and 450 psi pressure, respectively. It can
be seen in Fig. 4(d) that better conversion is achieved at a
higher temperature even during a very short reaction time
(3–14 s); 70% of 2-allylphenol was formed at 250 °C (450 psi)
for 14 s of residence time, whereas in the bulk, the reaction
was usually performed at a lower temperature for one day in
the presence of a catalyst.30 On the contrary, in the PVSZ
derived microreactor fabricated by bonding two pieces at
150 °C for 3 h the performance was limited only at low
temperatures (<60 °C) and ambient pressure as reported
earlier.31 Based on the obtained results, it is stated that both
the ceramic-like microreactors made from templates by the
bonding free method truly demonstrated the realistic use for
continuous flow chemical reactions under high temperature
or/and high pressure conditions with reliable durability when
compared to the microreactors fabricated by the two piece
bonding method, which also exhibited weak interface and
poor pressure resistance.
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In conclusion, we have developed whole ceramic-like micro-
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template method without experiencing a troublesome bonding
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Acknowledgements
This study was supported by the National Research Foundation
of Korea (NRF) grant funded by the Korea government (MSIP)
(no. 2008-0061983).
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