Photodimerization of maleic anhydride in a microreactor without clogging

Article abstract of DOI:10.1021/op900306z

Photodimerization of maleic anhydride (MA) gives insoluble precipitated products that can be a trigger to clog a conventional microreactor. To avoid this problem, we devised a microreactor that uses liquid/gas slug flow and ultrasonication. Inert N2 gas introduced into the reaction solution swept through the reactor tube and transported precipitated products in the liquid segments.Ultrasound vibrations inhibited the adhesion and sedimentation of precipitate in the reactor tube. The combination of gas and ultrasound prevented the tube from clogging. Fluorinated ethylene propylene (FEP) tubes of various sizes were investigated to use as a tube reactor. The tubes were wound around a high-pressure Hg lamp with a Pyrex immersion well which has been using generally as a light source of photoreaction, and the reaction solution was then passed through the tube and irradiated through the tube wall. The slug flow microreactor could be operated for more than 16 h continuously without clogging. Compared to using a batch reactor, this method achieves better product quality, improved conversion, and reduced waste.

Full text of DOI:10.1021/op900306z

Organic Process Research & Development 2010, 14, 405–410
Photodimerization of Maleic Anhydride in a Microreactor Without Clogging
Tomoaki Horie,*,† Motoshige Sumino,‡ Takumi Tanaka,‡ Yoshihisa Matsushita,§ Teijiro Ichimura,⊥ and Jun-ichi Yoshida¶
The Research Association of Micro Chemical Process Technology (MCPT), Katsura-int’tech Center Room 305, Kyoto
UniVersity, Nishikyo-ku, Kyoto 615-8530, Japan, Wako Pure Chemical Industries, Ltd., 1633 Matoba, Kawagoe,
Saitama 350-1101, Japan, Department of Chemistry and Department of Chemistry and Materials Science, Tokyo Institute of
Technology, 2-12-1 Ohokayama, Meguro-ku, Tokyo 152-8551, Japan, and Department of Synthetic Chemistry and Biological
Chemistry, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku, Kyoto 615-8105, Japan
Abstract:
actors that can handle various reactions.2 When the reaction
product is insoluble, clogging of the microreactor can be a
serious problem, and it is highly desirable to be able to transport
precipitated products out of the reactor continuously. A method
for particle formation and handling in capillary flow was
reported by Lilly (Indianapolis, U.S.A.).3
Several methods to transport precipitated products and avoid
clogging have been reported. These include using the sheath
flow technique to produce titania nanoparticles,4 using droplet
formation with the carrier phase to synthesize indigo,5 using
liquid/liquid segmented flow with immiscible solvent to produce
inorganic nanoparticles,6 and using a microreactor with slug
flow to crystallize proteins.7 Combinations of two solvents with
different polarity are typically used to achieve liquid/liquid slug
flow, but the risk with this type of flow is that some products
might change to undesired byproducts (for example, by hy-
drolysis of anhydrides). Thus, liquid/gas slug flow achieved by
introducing inert gas as a spacer is an attractive option.
Ultrasonication8 and mechanical vibration9 are also effective
in inhibiting adhesion and sedimentation of precipitates.
Photodimerization of maleic anhydride (MA) gives insoluble
precipitated products that can be a trigger to clog a conventional
microreactor. To avoid this problem, we devised a microreactor
that uses liquid/gas slug flow and ultrasonication. Inert N2 gas
introduced into the reaction solution swept through the
reactor tube and transported precipitated products in the
liquid segments.Ultrasound vibrations inhibited the adhe-
sion and sedimentation of precipitate in the reactor tube.
The combination of gas and ultrasound prevented the tube
from clogging. Fluorinated ethylene propylene (FEP) tubes
of various sizes were investigated to use as a tube reactor.
The tubes were wound around a high-pressure Hg lamp
with a Pyrex immersion well which has been using generally
as a light source of photoreaction, and the reaction solution
was then passed through the tube and irradiated through
the tube wall. The slug flow microreactor could be operated
for more than 16 h continuously without clogging. Com-
pared to using a batch reactor, this method achieves better
product quality, improved conversion, and reduced waste.
(2) Some recent examples: (a) Nagaki, A.; Togai, M.; Suga, S.; Aoki,
N.; Mae, K.; Yoshida, J. J. Am. Chem. Soc. 2005, 127, 11666. (b)
Kawaguchi, T.; Miyata, H.; Ataka, K.; Mae, K.; Yoshida, J. Angew.
Chem. 2005, 117, 2465. Angew. Chem., Int. Ed. 2005, 44, 2413. (c)
He, P.; Watts, P.; Marken, F.; Haswell, S. J. Angew. Chem. 2006,
118, 4252. Angew. Chem., Int. Ed. 2006, 45, 4146. (d) Uozumi, Y.;
Yamada, Y.; Beppu, T.; Fukuyama, N.; Ueno, M.; Kitamori, T. J. Am.
Chem. Soc. 2006, 128, 15994. (e) Tanaka, K.; Motomatsu, S.; Koyama,
K.; Tanaka, S.; Fukase, K. Org. Lett. 2007, 9, 299. (f) Sahoo, H. R.;
Kralj, J. G.; Jensen, K. F. Angew. Chem. 2007, 119, 5806. Angew.
Chem., Int. Ed. 2007, 46, 5704. (g) Hornung, C. H.; Mackley, M. R.;
Baxendale, I. R.; Ley, S. V. Org. Process Res. DeV. 2007, 11, 399.
(h) Fukuyama, T.; Kobayashi, M.; Rahman, M. T.; Kamata, N.; Ryu,
I. Org. Lett. 2008, 10, 533. (i) Nagaki, A.; Takizawa, E.; Yoshida, J.
J. Am. Chem. Soc. 2009, 131, 1654. (j) Nagaki, A.; Takizawa, E.;
Yoshida, J. Chem. Lett. 2009, 38, 486.
Introduction
Chemical reactions in microchannels are important in the
chemical, pharmaceutical, and analytical fields.1 Therefore,
much effort is devoted to the development of versatile microre-
* Author for correspondence. E-mail: horie@mcpt.mbox.media.kyoto-u.ac.jp.
† The Research Association of Micro Chemical Process Technology (MCPT),
Kyoto University.
‡ Wako Pure Chemical Industries, Ltd.
§ Department of Chemistry, Tokyo Institute of Technology.
⊥ Department of Chemistry and Materials Science, Tokyo Institute of
Technology.
(3) Eli Lilly and Company (Indiana). WO/2009/023515, 2009.
(4) Takagi, M.; Maki, T.; Miyahara, M.; Mae, K. Chem. Eng. J. 2004,
101, 269.
¶ Department of Synthetic Chemistry and Biological Chemistry, Graduate
School of Engineering, Kyoto University.
(5) Poe, S. L.; Cummings, M. A.; Haaf, M. P.; McQuade, D. T. Angew.
Chem., Int. Ed. 2006, 45, 1544.
(1) Reviews for microreactor: (a) Ja¨hnisch, K.; Hessel, V.; Lo¨we, H.;
Baerns, M. Angew. Chem. 2004, 116, 410. Angew. Chem., Int. Ed.
2004, 43, 406. (b) Doku, G. N.; Verboom, W.; Reinhoudt, D. N.; van
den Berg, A. Tetrahedron 2005, 61, 2733. (c) Watts, P.; Haswell, S. J.
Chem. Soc. ReV. 2005, 34, 235. (d) Yoshida, J. Chem. Commun. 2005,
4509. (e) Geyer, K.; Codee, J. D. C.; Seeberger, P. H. Chem.sEur. J.
2006, 12, 8434. (f) deMello, A. J. Nature 2006, 442, 394. (g) Song,
H.; Chen, D. L.; Ismagilov, R. F. Angew. Chem. 2006, 118, 7494.
Angew. Chem., Int. Ed. 2006, 45, 7336. (h) Kobayashi, J.; Mori, Y.;
Kobayashi, S. Chem. Asian J. 2006, 1, 22. (i) Mason, B. P.; Price,
K. E.; Steinbacher, J. L.; Bogdan, A. R.; McQuade, D. T. Chem. ReV.
2007, 107, 2301. (j) Ahmed-Omer, B.; Brandtand, J. C.; Wirth, T.
Org. Biomol. Chem. 2007, 5, 733. (k) Fukuyama, T.; Rahman, M. T.;
Sato, M.; Ryu, I. Synlett 2008, 151. (l) Yoshida, J.; Nagaki, A.;
Yamada, T. Chem.sEur. J. 2008, 14, 7450.
(6) (a) Jongen, N.; Donnet, M.; Bowen, P.; Lemaˆıtre, J.; Hofmann, H.;
Schenk, R.; Hofmann, C.; Aoun-Habbache, M.; Guillemet-Fritsch, S.;
Sarrias, J.; Rousset, A.; Viviani, M.; Buscaglia, M. T.; Buscaglia, V.;
Nanni, P.; Testino, A.; Herguijuela, J. R. Chem. Eng. Technol. 2003,
26 (3), 303. (b) Vacassy, R.; Lemaˆıtre, J.; Hofmann, H.; Gerlings,
J. H. AIChE J. 2000, 46 (6), 1241. (c) Shestopalov, I.; Tice, J. D.;
Ismagilov, R. F. Lab Chip. 2004, 4, 316.
(7) Gerdts, C. J.; Tereshko, V.; Yadav, M. K.; Dementieva, I.; Collart,
F.; Joachimiak, A.; Stevens, R. C.; Kuhn, P.; Kossiakoff, A.; Ismagilov,
R. F. Angew. Chem., Int. Ed. 2006, 45, 8156.
(8) Preventing a clogging of nozzle hole by ultrasonication. Patent of Japan
No. 2006-239502, 2006.
(9) Preventing a clogging of nozzle hole by mechanical vibration. Patent
of Japan No. 2009-122173, 2009.
10.1021/op900306z  2010 American Chemical Society
Published on Web 01/28/2010
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Scheme 1. Photodimerization of maleic anhydride (MA) to
Wako Pure Chemical Industries, Ltd., and used without further
purification. The product was confirmed by 1H NMR on Varian
MERCURYplus-400 (400 MHz). A 400 W high-pressure Hg
lamp with a Pyrex immersion well (UVL-400HA) was pur-
chased from Rico-Kagaku Sangyo Co. Various sizes of FEP
tubes were purchased from Junkosha Inc. and were connected
with union fittings and T-shaped fittings purchased from
Swagelok. An ultrasonic bath (TUS-820: 100 W, 39 kHz) was
purchased from Asone Co., and an external cooling unit (TRL-
117NHF) was purchased from Thomas Kagaku Co., Ltd. A
quartz beaker (outer diameter (OD) 64 mm, height (h) 300 mm)
around which to wind the tube reactor was manufactured by
Eiko CO., Ltd. A plunger pump (LC-10Ai) was purchased from
Shimadzu Co. The needle valve (MF-33; 0-5 mL/min) was
purchased from GL Sciences. A back-pressure regulator was
purchased from Upchurch Scientific. A filter unit was purchased
from Sanplatec Co., Ltd.
Single Pass Flow Microreactor. The single pass flow
reactions were carried out using a setup shown in Figures 1
and 2. An FEP tube (outer diameter (OD): 1.0-3.2 mm, inner
diameter (ID): 0.5-1.6 mm), was wound around the outside
of a quartz beaker (Figure 2a). Inside the beaker was placed a
400 W high-pressure Hg lamp with a Pyrex immersion well.
A Pyrex immersion well works as a filter for a shorter
wavelength of UV (∼<280 nm), which is not suitable for this
reaction. The beaker was set in an ultrasonic bath. The bath
and lamp were maintained at less than 15 °C by an external
cooling unit and a refrigerated water circulator, respectively,
because the yield decreased when the temperature became
higher than 20 °C. We used ethyl acetate (bp 76.8 °C) as
solvent. A solution of MA (10 w/w % in ethyl acetate) was
fed into the tube reactor by plunger pump (0.1-2.0 mL/min)
through a back-pressure regulator, which reduces the effect of
the pulsation of the pump. N2 gas was fed into the tube reactor
through a T-shaped connector and mixed with the reaction
solution to achieve liquid/gas slug flow (Figure 2b). N2 flow
rate was controlled by a needle bulb to maintain the length of
each liquid segment at about 2-5 cm and each gas segment at
about 0.5-1.0 cm. Reaction solution, separated by N2 segments,
passes through the tube and was irradiated through the tube
wall. The residence time was calculated on the basis of the
measured velocity of the N2 segments (Figure 2c). After the
operation lasted for 1-2 h, the suspension was collected and
condensed under reducing pressure (160 mmHg, 50 °C) to
remove volatile materials and then cooled to room temperature
(20-25 °C), filtered, and dried in a vacuum drying oven (60
°C). The conversion was estimated by the weight of CBTA.
Continuous Recycle Flow Microreactor. The continuous
recycle flow reactions were carried out using the setup shown
in Figure 3. Most of the apparatus was the same as in the case
of the single pass flow microreactor except for a magnetic stirrer
and a filter unit installed for recycling. A solution of MA (10
w/w% in ethyl acetate) was stirred by a magnetic stirrer to
maintain a homogeneous concentration. Precipitated CBTA was
filtered out continuously by a filter unit containing a paper filter
installed above the reaction solution vessel, through which the
homogeneous solution was recycled. The recycle number was
calculated as the ratio of the weight of a solution (300 or 100 g)
form cyclobutane tetracarboxylic dianhydride (CBTA)
When using a microreactor to produce a substance that
precipitates in the microchannnel, the following points are
important: (1) we must be able to control the quantity of solids
and their particle size, (2) precipitates must be continuously
transported, (3) and adhesion and sedimentation of precipitates
must be prevented. Phototransparent materials are advantageous
for observing liquid/gas slug flow in a microreactor, and this
feature of the material is adequate for photochemical reactions.10
Thus, we chose as a model reaction photodimerization of maleic
anhydride (MA) to produce cyclobutane tetracarboxylic dian-
hydride (CBTA) (Scheme 1), which is practically insoluble in
common organic solvents and precipitates during the course of
the reaction, and we used a microtube reactor made of
phototransparent fluorinated ethylene propylene (FEP).
Conventional photoreactions are usually performed in im-
mersion well reactors (batch reaction). Most photoreactions
occur within a short radius of the lamp, so conversion and
irradiation time are much influenced by scale-up factors.
Vigorous stirring is required to avoid this problem, but the
cylindrical shape of the reactor prohibits effective stirring, and
the reaction solution is thus exposed to a heterogeneous
illumination field. In contrast, microreactions are expected to
be homogeneously irradiated because of the short optical pass
length involved.
Photodimerization of MA is usually performed in a con-
ventional batch reactor. However, a batch reactor is not ideal
for this reaction because precipitated product goes into suspen-
sion, adheres to the lamp cooling jacket, and scatters the
irradiated light, causing the conversion to be very small even
for long reaction times. To make matters worse, surplus UV
irradiation may cause undesired side reactions and/or product
decomposition. We envisioned that these problems could be
overcome using a slug flow microreactor.
In this contribution, we present a valuable method for
transporting precipitates through a microreactor without clog-
ging, and we also describe the relative merits of the method
compared to batch reaction systems, using the photodimerization
reaction of maleic anhydride.
Experimental Section
General. Maleic anhydride, ethyl acetate, 4,4′-diamino-
diphenylmethane, N-methylpyrrolidone were purchased from
(10) (a) Gorges, R.; Meyer, S.; Kreisel, G. J. Photochem. Photobiol. A:
Chem. 2004, 167, 95. (b) Matsushita, Y.; Kumada, S.; Wakabayashi,
K.; Sakeda, K.; Ichimura, T. Chem. Lett. 2006, 35, 410. (c) Sugimoto,
A.; Sumino, Y.; Takagi, M.; Fukuyama, T.; Ryu, I. Tetrahedron Lett.
2006, 47, 6197. (d) Sakeda, K.; Wakabayashi, K.; Matsushita, Y.;
Ichimura, T.; Suzuki, T.; Wada, T.; Inoue, Y. J. Photochem. Photobiol.
A: Chem. 2007, 192, 166. (e) Meyer, S.; Tietze, D.; Rau, S.; Scha¨fer,
B.; Kreisel, G. J. Photochem. Photobiol. A: Chem. 2007, 186, 248.
(f) Matsushita, Y.; Ohba, N.; Kumada, S.; Suzuki, T.; Ichimura, T.
Catal. Commun. 2007, 8, 2194. (g) Matsushita, Y.; Ohba, N.; Suzuki,
T.; Ichimura, T. Catal. Today 2008, 132, 153.
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Figure 1. Single pass flow microreactor and continuous transportation of precipitates.
Figure 2. Slug flow microreactor: (a) FEP tube reactor, wound around the outside of a quartz beaker; (b) N2 gas feed, via T-shaped
connector; (c) liquid/gas slug flow before and after irradiation.
N2 bubbling through a gas inlet tube, the suspended solution
was worked up as described above. CBTA (15.8 g, 26%) was
obtained.
The Viscosity Measurement of Polyamic Acid. Preceding
polymerization, CBTA was treated with a solvent mixture of
toluene (10 w/w) and acetic anhydride (10 w/w) at 120 °C for
2 h, and then was cooled to room temperature, filtered, and
dried in a vacuum drying oven (60 °C).
4,4′-Diaminodiphenylmethane 0.7931 g (4.000 mmol) was
placed in a 100 mL round-bottomed flask which was substituted
with Ar, and 11.392 g of N-methylpyrrolidone was added. After
1 min of Ar bubbling, CBTA 0.7844 g (4.000 mmol) was added
into the solution. The solution was stirred at 25 °C under Ar
for 24 h. The resulting polyamic acid was measured by a
rotational type viscometer at 25 °C.
Figure 3. Continuous recycle flow microreactor with filter unit.
Results and Discussion
and flow rate (2.5-5.0 mL/min) compared to that for a single
pass flow reaction. CBTA was filtered and worked up as
described above.
Batch Reactor. A cylindrical glass vessel (ID 75 mm; h
300 mm) covered with aluminum foil was used as a reactor
and set in cooled brine (0 °C). A 400 W high-pressure Hg lamp
in a Pyrex immersion well was set into a solution of MA (10
wt/wt % in ethyl acetate: 600 g). After irradiation for 6 h with
Photodimerization of MA is a well-known reaction yielding
precipitated compound, CBTA. CBTA, which is used as a raw
material of polyimide and an alignment film for liquid crystal
display (LCD), has been produced in commercial chemical
plants in relatively large scales. Because the amount of
production is increasing, much effort has been devoted to the
optimization of reaction conditions. During the course of the
optimization of the reaction, ethyl acetate was found to be
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407

Figure 4. Change of conversion with various MA concentra-
tions and residence time (tube OD ) 1.6 mm, ID ) 1.2 mm, L
) 11.2 m).
Figure 6. Change of conversion with increase of tube ID (MA
concentration ) 10%, residence time ) 5 min).
Figure 5. Change of conversion with various tube sizes and
residence time (MA concentration ) 10%). Inset shows tube
size (OD (mm) × ID (mm) × length (m)).
Figure 7. Solution color after irradiation reaction followed by
filtration. After batch reaction for 6 h (left); after flow reaction
for 10 min (right).
the best solvent for the production of CBTA. Therefore, in this
study we used only ethyl acetate as solvent. MA exhibits an
intense UV absorption band at 250 nm and a broad absorption
up to 350 nm. However, it was better to use a Pyrex filter, which
cuts the UV wavelength short (∼<280 nm).
attempted to use a liquid/gas slug formed by introduction of
inert N2 gas. The role of N2 gas is to separate the liquid into
segments that transport the precipitated product, and each
segment acts as a very small “micro batch reactor”. Precipitated
product in all segments can be transported without transferring
to the following segment and the risk of clogging becomes
scarce. The combination of segments of different states could
transport CBTA with neither decomposition nor clogging. No
connector was used after photoirradiation to avoid clogging
there. Ultrasonic irradiation is better to use for reliability and
long-time operation. We achieved continuous operation for more
than 16 h with this system (tube ID ) 1.2 mm, L ) 11.2 m).
We examined the effect of MA concentration on conversion
using a tube of ID ) 1.2 mm (Figure 4). Conversion increased
with an extension of the residence time. A 5% solution gave
the highest conversion with 22 min of residence time, because
at low MA concentration, transparency is sufficient even for a
tube of large ID.
We examined the effect of tube size on conversion using a
10% MA solution (Figure 5). Conversion became higher with
decreasing tube ID, because narrower tubes have shorter optical
pass lengths and the solution in the reactor were irradiated
homogeneously. The operation with a small tube ID (0.5 mm)
required smaller flow rate (0.10 mL/min) for long residence
time. It caused eventual clogging. A tube of ID ) 0.8 mm was
found to be the smallest and most favorable reactor to give the
satisfied conversion (70%). The particles of CBTA were
analyzed by SEM. Their shape was hexagonal, and the average
length of long axis was about 14 µm (see the Supporting
Our first decision for the reactor concerned the reactor
material. Different materials have been used for the reactor of
a
photoreactionsincluding glass,11 polydimethylsiloxane
(PDMS),12 and polystyrol13 plates and FEP tubes.14 In terms of
transparency, glass is superior. But most of the glass reactors
are plate form which cannot be irradiated homogeneously by a
cylindrical lamp. Glass tubing is vulnerable to breakage and
cannot be wound tightly around the lamp. Our final choice was
FEP tubing, which is flexible enough to wind around the lamp
tightly and smooth enough to inhibit adhesion of the precipitate.
FEP tubing has good UV transparency (250 µm of thickness:
80.4%, 300 µm: 75.6% at 330 nm).
Our second decision for the reactor concerned the manner
of the flow because of the precipitated product. When we
attempted the flow reaction with only one liquid and no
segment, clogging occurred within an hour, and continuous
operation was impossible. If we used a liquid/liquid slug flow
formed by combining two mutually insoluble solvents, the
segment transported precipitated product out of the tube.
However, if one of the solvents was water, both MA and CBTA
hydrolyzed and converted to carboxylic acids. Thus, we
(11) Fukuyama, T.; Hino, Y.; Kamata, N.; Ryu, I. Chem. Lett. 2004, 33
(11), 1430.
(12) Maeda, H.; Mukae, H.; Mizuno, K. Chem. Lett. 2005, 34, 66.
(13) Ueno, K.; Kitagawa, F.; Kitamura, N. Lab Chip 2002, 2, 231.
(14) Hook, B. D.; Dohle, W.; Hirst, P. R.; Pickworth, M.; Berry, M. B.;
Booker-Milburn, K. I. J. Org. Chem. 2005, 70, 7558.
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Scheme 2. Polycondensation of CBTA and diamine compound to form polyamic acida
a Note the difference in viscosity achieved for flow and batch reactions.
Information). Though it was reported that CBTA produced
in 1,4-dioxane was a mixture of cis and trans isomers, in
the present study only the trans isomer was obtained. The
Table 1. Conversion and the amount of MA at various
recycle conditions (OD ) 1.6 mm, ID ) 1.2 mm, L )
11.2 m)
1
cis isomer was not detected by H NMR.15
single recycle recycle recycle
passb
1c
2d
3e
The power efficiency (W/mmol) of each tube was estimated
by lamp power (W) and CBTA (mmol) (see the Supporting
Information). The reaction with a shorter residence time gave
higher efficiency, but the narrower tube was not better. We think
all differences need to be considered, because the tube thickness
and reflectance of light were also altered as the tube size
changed.
We also examined the effect of tube size on conversion at
a constant residence time of 5 min with a 10% MA solution
(Figure 6). As before, better conversion was obtained with
decreasing tube ID. At ID > 2.4 mm, the conversion was quite
small. At ID ) 3.6 mm, regular liquid/gas slug flow was not
achieved, because the gas segment moved individually regard-
less of the flow rate control. N2 segments in a large space were
not controllable, and the precipitate could not be transported
through the tube at a constant rate. Thus, it is clear that formation
of continuous slug flow, and hence continuous sweeping of
precipitate product, is achieved only when tube IDs are from a
micrometer to a millimeter in size.
It is important to note that the flow reactions gave
better-quality product than batch reactions. Precise control
of residence time led to fewer byproducts resulting from
decomposition of product and/or undesirable side reac-
tions. It was visually evident by changes of solution color.
For the same conversion (26%), the batch reaction solution
(reaction time ) 6 h) turned red, but the flow-reaction
solution (reaction time ) 10 min) remained almost
colorless (Figure 7). The flow reaction gave fewer
byproducts because the irradiation time required for the
reaction is shorter. The quality of the product is very
important for commercial production of CBTA, but the
impurities were difficult to identify by 1H NMR, GC, and
HPLC analyses because their amounts were too small.
Thus, quality of the product was evaluated by measuring
the viscosity of polyamic acid, which is important from
an industrial point of view. The flow-reaction sample
showed higher viscosity (Scheme 2), indicating higher
purity.
operation time (h)
residence time (min)
recycle number (times)
conversion (%)
6
11.6
1
29
2.4
6
11.8
3
60
0.67
6
5.9
6
52
0.92
9
11.8
4.5
69
0.44
waste (g)a
a The amount of wasted MA(g) per obtained CBTA(g). b 10% MA solution
(300 g) was used. c MA concentration ) 10% (100 g); flow rate is the same as for
the single pass method. d MA concentration ) 10% (100 g); flow rate is 2 times
faster than that for recycle 1. e Operation time was extended with the conditions of
recycle 1.
conversion and reducing MA waste. In such a system,
precipitated CBTA can be filtered out and reduce some
shading and scattering of light (Figure 3). The use of a
convectional reactor for the dimerization of MA using self-
filtering with low-density particles has already been
reported.16
A flow reaction system is useful to test a recycling effect
to improve conversion and to reduce the amount of
unreacted raw materials. The results of recycling are
shown in Table 1. When one-third the amount of MA
solution was fed at the same flow rate as for the single
pass operation, the conversion improved greatly compared
to that for the single pass operation with the same
operation time (6 h). However, when one-third the amount
of the MA solution flowed at 2 times the flow rate for the
conditions of recycle 1, the conversion decreased. Judging
from the difference of irradiation time with the two
conditions, a residence time of >10 min is required to
improve conversion. A longer operation time resulted in
higher conversion (recycle 3: 69%). The amount of MA
that goes to waste to produce 1 g of CBTA was 2.4 g for
single pass operation but just 0.44 g for recycle 3
conditions. This continuous circulation and recycle slug
flow system can be useful when raw material is expensive
or contamination of raw material into a product would be
troublesome problem.
When environmental friendliness is considered in a
chemical process, the reduction of disposal of raw
materials is important. Accordingly, we investigated the
effectiveness of the slug flow microreactor, adapted for
continuous circulation and recycle operation, in improving
Conclusions
Use of liquid/gas slug flow in a microreactor enables
operations of long times without any risk of clogging.
Higher conversion than that of a batch system was
achieved for photodimerization of maleic anhydride in an
(15) Nagahiro, I.; Nishihara, K.; Sakota, N. J. Polym. Sci., Polym. Chem.
Ed. 1974, 12, 785.
(16) Boule, P.; Jeandrau, J. P.; Gramain, J. C.; Lemaire, J. J. Photochem.
1981, 16, 67.
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FEP tube with an ID of about a millimeter. It is also
noteworthy that the flow reaction system gave better-
quality CBTA than the batch system. Reduced waste of
raw materials by recycling is easily realized. We hope
that this methodology will be useful for reactions in which
the risk of clogging is a factor.
Energy and Industrial Technology Development Organiza-
tion (NEDO), Japan.
Supporting Information Available
This material is available free of charge via the Internet at
http://pubs.acs.org.
Acknowledgment
This research was financially supported by the Develop-
ment of Microspace and Nanospace Reaction Environment
Technology for Functional Materials project of the New
Received for review November 17, 2009.
OP900306Z
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