Photocatalytic organic syntheses using a glass-milled microchip

Article abstract of DOI:10.1016/j.jphotochem.2016.02.020

Photocatalytic reactions, typically known as difficult to control due to secondary reactions and involvement of multiple radicals, were demonstrated to be controlled well by optimizing the reaction time and the microchip design. The glass-milled microchip has a good resistivity for the organic solutions and withstands multiple usage more than 100 times. The oxidization and reduction reactions were demonstrated and the flow considerably improved the conversion and yield. Furthermore, an imine synthesis was demonstrated as a multi-step reaction by combination of photocatalytic oxidation and reduction reactions.

Full text of DOI:10.1016/j.jphotochem.2016.02.020

Journal of Photochemistry and Photobiology A: Chemistry 322 (2016) 35–40
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photocatalytic organic syntheses using a glass-milled microchip
1
1
*
A. Nakamura , K. Yoshida , S. Kuwahara, K. Katayama
Department of Applied Chemistry, Chuo University, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551, Japan
A R T I C L E I N F O
A B S T R A C T
Article history:
Photocatalytic reactions, typically known as difficult to control due to secondary reactions and
involvement of multiple radicals, were demonstrated to be controlled well by optimizing the reaction
time and the microchip design. The glass-milled microchip has a good resistivity for the organic solutions
and withstands multiple usage more than 100 times. The oxidization and reduction reactions were
demonstrated and the flow considerably improved the conversion and yield. Furthermore, an imine
synthesis was demonstrated as a multi-step reaction by combination of photocatalytic oxidation and
reduction reactions.
Received 28 November 2015
Received in revised form 18 February 2016
Accepted 22 February 2016
Available online 26 February 2016
Keywords:
Photocatalytic organic synthesis
Microchip
Nitrobenzene
Aniline
ã 2016 Elsevier B.V. All rights reserved.
Imine
1. Introduction
different radical species are involved and/or the direct and
secondary reactions are mixed in the process. Thus, it is necessary
Photocatalytic reaction processes are classified into direct [1–3]
and secondary reactions [4–7]; the former corresponds to the
reaction between the adsorbed species and photoexcited electrons
or holes, while the latter is the one induced by active oxygen
species generated in the solvent due to the reaction between
photoexcited holes and solvent or ambient molecules. Since there
have been no specific methods for separating the two processes,
the reactions have been usually understood on the phenomeno-
logical average, and the actual reaction mechanism is still difficult
to understand for various conditions.
In most of the applications using photocatalytic reactions, only
the oxidation ability, especially total oxidation from organics to
carbon dioxide and water, has been paid attention to. However,
photocatalyt also has the abilities such as partial or selective
oxidation [8–13], and reduction [14–16] for various aromatic
compounds [17,18]. Selective oxidation of the alcohol functional
groups gave a high yield [19]. The reduction of nitrocompounds was
investigated well on the reaction mechanism [14,20,21]. and imine
formation was reported after reacting with alcohols [22]. Further-
more, complicated reactions following the photocatalytically
generated intermediate radical species were found, too [11,23–25].
However, in many of the organic conversions, the yields or
selectivities are not high enough to be eligible for the replacement
of the organic reactions [18,19]. This is probably because several
to understand the reaction process, and to control it.
We have studied the mechanism of the photocatalytic reactions
such as the formation of the active oxygen species [26], separation of
the direct and secondary reactions [27], and degradation of various
dyeswithdifferentmainstructures[28]. Furthermore,we foundthat
photocatalytic reactions proceeded quickly with high selectivity by
combining photocatalyst and microreactors, which has a high
surface/volume ratio, preferable for the photocatalytic reactions
[29]. There have been similar approaches on the photocatalytic
microreactors, and it was reported that the reaction time became
shorter, and the selectivities for specific reactions were improved
[30–33]. It was also demonstrated that the microreactor is suitable
for general photochemical syntheses [34,35].
In the development of the photocatalytic microreactor, we
noticed that many of the photocatalytic reactions gave a high yield
and selectivity within several minutes if we could appropriately
stop the reaction and take out the products. However, there were
several difficulties for the implementation of the combination of
the photocatalytic reactions and the microreactor. We could not
use the polymer microreactor, which are subject to deterioration
during the photocatalytic reactions and also cannot be heated for
the deposition of photocatalytic materials. This made us difficult to
use
a flexible microreactor, which is advantageous for the
operation at high flow rate and for easy irradiation of light. Thus,
we need to use a glass microreactor, which loses the above-
mentioned feature of the polymer microreactors. Furthermore, it is
difficult to get a large amount of products because the channel
depth is typically on the order of hundreds of microns at most, by
* Corresponding author.
E-mail address: kkata@kc.chuo-u.ac.jp (K. Katayama).
These authors contributed equally.
1
http://dx.doi.org/10.1016/j.jphotochem.2016.02.020
1010-6030/ã 2016 Elsevier B.V. All rights reserved.

36
A. Nakamura et al. / Journal of Photochemistry and Photobiology A: Chemistry 322 (2016) 35–40
Fig. 1. A picture of a photocatalytic microchip (left) and that with an aluminium holder and the connecting tubes. A UV-LED was irradiated from the top. (right).
wet etching typically used for fabrication of a glass microchip. Also,
we could not use the microchannels with heterogeneous structure
for our purpose, which has a large distribution of the residence
time in the reactors, and they are not favorable for the control of
the reaction time.
To solve these problems, we fabricated the photocatalytic
microchip by milling the glass slide, which is preferable for making
a microchannel with larger width and depth with high flexibility.
We demonstrated several organic reactions with high yield and
selectivity by optimizing the reaction time and flow condition,
which could be roughly estimated by the in-situ reaction
monitoring [28]. Surprisingly, in most of the reactions, the product
yields were improved under the continuous-flow condition,
compared with the same residence time under the stop-flow
condition.
with a diameter of 2 mm. The channel width, and length were 2,
and 250 mm, respectively, which corresponds to the inside volume
of 250 mL. The milled glass was thermally bonded with another
slide glass in a furnace for 9 h at 650 ꢀC. The top glass was drilled to
have inlet holes. The microchannel was filled with a TiO2 paste
three times and furnaced at 450 ꢀC for 1 h after it was washed
3 times with an alkaline solution (5 M NaOH) and water. TiO2
powder (P25), average diameter (D: 21 nm) was used for the paste.
It was mixed with water (3.33 mL) and acetylacetone (0.33 mL) and
mixed in a mortar for 30 min. The TiO2 amount deposited for a
microchip was estimated to 4–8 mg. As shown in Fig. 1, the
microchip was sandwiched with an aluminium holder and the
inlet was connected to a syringe pump (YSP-202, YMC), and the
outlet was tubed into a vial for the analysis by gas chromatography
(GC). To start the photocatalytic reactions, a UV LED with a center
wavelength of 365 nm (EXECURE-LH-1 V, HOYA) was irradiated
over the whole area of the channel with an intensity of 16 mW/cm2,
and the transmittance was >90% for the glass slide. The experi-
ments were made after checking no reactions proceeded without
TiO2 coating in microchannels. Unless otherwise stated, the
concentrations of reactants were 5 mM.
Furthermore, we applied this microchip for more complicated
reaction, imine synthesis, which is
a useful chemical for
agrichemical and pharmaceutical purposes [36]. In recent years,
several groups demonstrated the imine synthesis by using
photocatalysis [22,37–39]. In this reaction, both the oxidation
and reduction reactions were utilized at the same time, which is
favorable for photocatalytic reactions. Since this reaction is a
multi-step reaction; photocatalytic and condensation reaction, we
accomplished this reaction by separating the photocatalytic
microchip and a vial.
For the estimation of the reaction time, each reaction was
monitored with the in-situ spectroscopic monitoring device
developed recently [29] (Fig. S1) . Briefly, a photocatalytic reaction
inside a fused silica capillary (I.D. 500 mm) was monitored, in which
TiO2 was coated with the same procedure as for the microchip. The
photocatalyticreactionwasinitiatedby thelightirradiationfromthe
top side with a UV-LED at 365 nm (45 mW/cm2). Perpendicularly
with it, the UV spectral range in 220–400 nm was monitored. In any
reactions we performed here, the necessary time was estimated less
than 20 min. A typical analysis made for nitrobenzene (NB) is shown
in Fig. S1.
2. Experimental
A normal glass slide (S1112, Matsunami) with the dimension of
75 Â 25 Â1 mm was rinsed before fabrication with acetone in a
ultrasonic bath for 30 min and flowing water for 5 min and and was
machined by a miller (RD300, Original Mind) with a diamond mill
Fig. 2. A section image of the microchannel measured by SEM (left) and the expanded images at the top and bottom layer are shown on the right.

A. Nakamura et al. / Journal of Photochemistry and Photobiology A: Chemistry 322 (2016) 35–40
37
100
90
80
70
60
50
100
80
60
40
20
0
conv
Conversion under flow
40
30
20
10
0
yield
Yield
under flow
conv
Conversion under stop-flow
yield
Yield
under stop flow
0
5
10
15
Surface / Volume (1/mm)
0
2
4
6
8 10
Irradiation time (min)
Fig. 3. The microchip depth dependence of the conversion (blue diamond) and
yield (red square) of the reduction reaction of NB is shown. The conversion and the
yield are defined as the molar ratio between the consumed reactant and the original
amount of the reactant, and the molar ratio between the product and the consumed
reactant, respectively. The horizontal axis indicates the division of the surface area
by the volume of the microchannel. In this reaction, the NB solution was introduced
inside the microchip and a UV was irradiated after stopping the flow. The irradiation
time was 3 min. (For interpretation of the references to colour in this figure legend,
the reader is referred to the web version of this article.)
Fig. 4. Light irradiation time dependence of the conversions of NB and yields of ANI
under the flow and stop-flow conditions in the reduction reaction from NB to ANI in
a photocatalytic microchip.
good enough volume for the purpose of syntheses under the flow
condition.
Next, we tried each organic synthesis in a photocatalytic
microchip. To decide the irradiation time for the reaction, we put
the solution into the microchip and irradiated light for different
times after stopping the flow, and the solution was analyzed after
taking the solution out. For the reduction reaction of NB, the
irradiation time dependence is shown in Fig. 4, and the conversion
and yield were saturated around 6 min (69%). To elucidate the
effect of the flow purely, we varied the irradiation time under the
flow by changing the flow rate, namely the irradiation time is equal
to (Volume of microchannel)/(Flow rate). The irradiation time
dependence is shown in Fig. 4, too. Unexpectedly, we found a
tremendous improvement of the conversion and yield even under
the same irradiation time and same microchip conditions. To
clarify the result, the GC charts for the optimized irradiation time
under the flow and stop-flow conditions are shown in Fig. 5. The
improvement by the flow was obvious, and the conversion reached
100% after 6 min, and the corresponding yield was over 80%. The
reason why the flow made the reaction efficient is that the product
species sometimes blocks the active reaction site but that they
were carried away by flow. A main by-product in this reaction was
nitrosobenzene, an intermediate species in this reaction, whose
yield was less than a few percent.
3. Results and discussion
3.1. Oxidation and reduction reactions
We investigated organic conversions in the following: the
reduction of NB to aniline (ANI) [14,20,21], reduction of
Benzaldehyde (BAld) to Benzylalcohol (BAlc) and oxidation of
BAlc to BAld [8,11]. The reactions of BAlc and BAld are the reverse
reactions each other, and they can be controlled just by changing
the solvent; acetonitrile for oxidation and alcohol for reduction. In
the oxidation, holes are used as an oxidant. In the reduction,
because alcohol is a good scavenger of holes, preventing the
recombination of electrons, the electrons are efficiently used for
reduction [20,40]. In most of the reduction reaction, ethanol is used
as a solvent. However, in the reduction of NB, 2-propanol was used
as a solvent because otherwise ANI was further changed to an
imine compound for the ethanol as a solvent. Ethanol was oxidized
to acetaldehyde, which easily reacts with ANI to generate the
corresponding imine compound [22,37]. The solvents were used
without degassing, because dissolved oxygen did not affect the
result, indicating oxygen is not a main electron scavenger in
organic solvents.
First, we checked the surface of the microchannels. Just after
the milling, the surface roughness was on the order of several
microns, but it was smoothed out after thermal bonding of the
microchip. After coating of TiO2, the section image was confirmed
by scanning electron microscope (SEM), which was about 1 mm.
(Fig. 2). The adhesion of TiO2 was strong enough that it could be
used at least 300 times.
ANI
After 6 min
(flow)
To optimize the depth of the microchip, its dependence was
investigated. By decreasing the depth, the surface/volume ratio is
increased, which is preferable for photocatlytic reactions, on the
contrary, the solution amount inside the reactor is reduced. We
prepared microchips with their depth of 0.4, 0.8, 1.2, 1.6 mm,
respectively, and the NB reaction was performed inside the
microchip under the same light intensity and irradiation time, and
the conversion and yield of ANI were compared in Fig. 3. The
conversion and yield were saturated between 0.4 and 0.8 mm in
depth. Then, we decided to use the depth of 0.5 mm in the
After 6 min
NB
(stop-flow)
Before irradiation
0
5
10
15
Retention time (min)
Fig. 5. GC charts before and after the reduction reaction from NB to ANI in a
photocatalytic microchip. The results for the optimized irradiation time were
compared under the flow and stop-flow conditions.
following experiments. The internal volume was 250 mL, which is

38
A. Nakamura et al. / Journal of Photochemistry and Photobiology A: Chemistry 322 (2016) 35–40
Table 1
O
Summary of the reaction conditions, conversions and yields for the reduction
reactions of NB and BAld and the oxidation reaction of BAlc.
H2N
H
Substrate
Flow
Residence time (min)
Conversion (%)
Yield (%)
NB
NB
BAld
BAld
BAlc
BAlc
Off
On
Off
On
Off
On
6
6
10
10
20
29
73
100
72
93
57
72
69
92
68
89
51
72
H+
NH
NH
OH+2
OH
The same comparisons were made for the reactions of BAld and
BAlc (Figs. S2 and S3 for BAld, Figs. S4 and S5 for BAlc in SI). In the
reduction of BAld, the reaction was saturated about 10 min under
the stop-flow condition, and the conversion and yield were 72 and
68%, while they reached 93 and 89%, just by adding the flow. The
main by-product in this reaction was benzoic acid, an oxidized
product of BAld, whose yield was less than a few percent. In the
oxidation of BAlc, the saturation time was shorter, which was 60 s,
but the conversion and yield were as low as 49 and 32% under the
stop-flow condition, and they were to 43 and 25% even under the
flow condition, which was reported previously [29].
We investigated the dependences on the concentration to
improve the oxidation reaction of the BAlc, but they were not
effective to increase the yield (21% for 1 mM, 25% for 5 mM), while
it was effective by adding one of electron scavengers. (nitor-
obenzene and acetophenone) Since we knew that NB is a good
electron scavenger, we added it in the reaction. The irradiation
time dependence and the optimized GC charts are given in
Figs. S6 and S7 in SI. The best result was obtained when two
equivalent amount of NB to BAlc was added. Even under the stop-
flow condition, the conversion and yield reached 57% and 51%, and
they were improved to 72% and 72% under the flow condition.
Thus, the reason why the oxidation reaction was saturated was due
to the electrons accumulated inside TiO2, preventing the further
oxidization of BAlc. In this reaction, the saturation time (20 min)
was longer compared with those for the reduction reactions. It is
well known that alcohol molecules are adsorbed on a TiO2 surface
[41], and their oxidization would be quick at the adsorption site,
which was proved by the fact that the saturation time only for BAlc
was as short as 60 s. Thus, the long saturation time when NB was
added is dominated by either the electron capture process by NB or
the desorption process of BAld.
N
+ H2O
Scheme 2. Mechanism of the condensation reaction of BAld and ANI.
3.2. Multi-step reaction
Since we could optimize the oxidation and reduction reaction,
we tried more complicated reaction, imine synthesis. In this
reaction as shown in Scheme 1, (1) a nitro-compound is reduced
and (2) an alcohol compound is oxidized by using the photo-
catalytic reactions, and both the products were condensed to give
an imine compound. Here, we used NB and BAlc as reactants. Since
we had already known the optimized conditions for the reactions
(1) and (2), the reactions were performed in a photocatalytic
microchip for the residence time of 25 min, but the yield of the
imine was quite low (<5%), and we investigated the optimal
condition of the condensation reaction (3).
We put ANI and BAld in a vial for different conditions, and the
reaction took more than 1 h to reach the end of the reaction, and
also the yield was better for an acidic solvent such as alcohol. This
indicates that the dehydration reaction of the intermediate species
in the condensation reaction is the rate-limiting reaction, which
could be promoted by addition of proton as shown in Scheme 2.
Thus, we studied the effect on the yield of the reaction (3) by
adding an acid (oxalic acid), which could reduce the reaction time
(<20 min) and improve the yield of reaction (3).
By optimizing the reaction, we separated the photocatalytic
oxidation and reduction reactions, (1) and (2), and the condensa-
tion reaction (3); the former was made in the photocatalytic
microchip and the latter was performed in a vial with an addition
of an oxalic acid (2.4–3 mg). The summary result is given in Table 2.
The main by-product in this reaction was diphenylmethane, whose
yield was <5%.
We compared the result with the reaction made in a batch
reactor; a test tube including the same TiO2 particle (4 mg) in
10 mM NB in BAlc (700 uL). The analysis of the products revealed
the yield was as low as 14%, which indicates that the reaction in the
microchip improved the product yield.
Finally, the optimized conditions for the above three reactions
were summarized in Table 1. In every case, the flow improved the
result and we could get almost >80% yield in the reduction
reactions, and >70% in the oxidation reaction on the order of
minutes.
We studied the substituent effect of nitrobenzene derivatives
on this reaction, which is shown in Table 3. Most of the substituent
led lower yields. This is probably because the nucleophilicity of
aniline derivatives was lowered by the substituent, causing the
ability to attack carbonyl of BAld lower [42]. Since the yields of the
NO
NH
2
TiO2, hν
2
(1)
(2)
CHO
TiO , hν
CH2OH
2
Table 2
Optimized reaction condition of imine synthesis by combination of photocatalytic
reactions in a microchip and a condensation reaction in a vial.
CHO
NH2
Entry
Oxalic
acid
Residence time
(min)
Vial reaction time
(min)a
Yield(%)
+
(3)
1
2
3
–
–
O
25
25
25
0
180
0
0.3
2.3
62
N
a
Scheme 1. Imine synthesis by combination of photocatalytic oxidation and
The meaning 0 min for the vial reaction time is that the product was taken just
after passing through the microchip.
reduction reactions.

A. Nakamura et al. / Journal of Photochemistry and Photobiology A: Chemistry 322 (2016) 35–40
39
Table 3
Substituent effects of NB on the multi-step reaction of Scheme 1. The residence time
in a microchip a was 25 min for the flow rate of 10 L/min.
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ÀR
H
Conversion (%)
Selectivity (%)
Yield (%)
100
100
100
87
62
40
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45
27
62
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24
Cl
CH3
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4. Conclusions
We developed the photocatalytic microchips optimized for the
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minutes with a volume of hundreds of microliters. By milling the
glass slide, its surface had good adhesion of photocatalytic
materials and the volume is much larger than a conventional
glass microchip. We demonstrated a few organic syntheses of the
reduction of nitro and aldehyde aromatics and the oxidation of
alcohol aromatics, and the yields for the reduction and oxidation
reached >90% and >70%, respectively. Furthermore, it was applied
for a multi-step reaction and imine was synthesized successfully.
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Author contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Acknowledgments
This study was supported by a research grant from the Tokyo
Ohka Foundation for the Promotion of Science and Technology, and
the Institute of Science and Engineering, Chuo University.
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Appendix A. Supplementary data
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Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.
jphotochem.2016.02.020.
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