Synthesis of benzaldehyde by swern oxidation of benzyl alcohol in a continuous flow microreactor system

Article abstract of DOI:10.3906/kim-1704-42

Preparation of benzaldehyde by Swern oxidation of benzyl alcohol was carried out in a continuous flow microreactor system. Dimethyl sulfoxide (Me 2 SO) was used as oxidizing agent and oxalyl chloride or p-toluenesulfonyl (p-TsCl) chloride was used as the activating agent. Benzyl alcohol was oxidized to benzaldehyde by the Me 2 SO-activating agent mixture in the continuous flow microreactor system. The optimized reaction conditions of the Swern oxidation were as follows: oxalyl chloride was used as the activating agent; the mole ratio of Me 2 SO, oxalyl chloride, and benzyl alcohol was 4:2:1; the flow rate of Me 2 SO was 1.5 mL/min; the reaction temperature was 15 ^? C; length of delay loop was 1.5 m; a Caterpillar Split-Recombine Micro Mixer was used; and all of the experiments were completed at atmospheric pressure. The yield of benzaldehyde can reach 84.7% with selectivity of 98.5%. Due to the small reactor volume and short residence times, the Swern oxidation of benzyl alcohol in a continuous flow microreactor system can be operated at nearly room temperature (5–19 ^? C) instead of –70 ^? C in a batch reaction, with residence time of reactants in microreactors in milliseconds instead of several hours in a batch reaction.

Full text of DOI:10.3906/kim-1704-42

Turk J Chem
2018) 42: 75 – 85
Turkish Journal of Chemistry
(
http://journals.tubitak.gov.tr/chem/
¨
˙
⃝
c TUBITAK
Research Article
doi:10.3906/kim-1704-42
Synthesis of benzaldehyde by Swern oxidation of benzyl alcohol in a continuous
flow microreactor system
1
,2
1,2
1,2,∗
Lin ZHU , Xiaohui XU , Fuping ZHENG
1
Beijing Advanced Innovation Centre for Food Nutrition and Human Health, Beijing Technology and
Business University, Beijing, P.R. China
2
Beijing Laboratory for Food Quality and Safety, Beijing Technology and Business University, Beijing, P.R. China
Received: 22.04.2017
•
Accepted/Published Online: 29.08.2017
•
Final Version: 08.02.2018
Abstract: Preparation of benzaldehyde by Swern oxidation of benzyl alcohol was carried out in a continuous flow
microreactor system. Dimethyl sulfoxide (Me SO) was used as oxidizing agent and oxalyl chloride or p-toluenesulfonyl
p-TsCl) chloride was used as the activating agent. Benzyl alcohol was oxidized to benzaldehyde by the Me SO-
activating agent mixture in the continuous flow microreactor system. The optimized reaction conditions of the Swern
2
(
2
oxidation were as follows: oxalyl chloride was used as the activating agent; the mole ratio of Me
and benzyl alcohol was 4:2:1; the flow rate of Me
2
SO, oxalyl chloride,
◦
2
SO was 1.5 mL/min; the reaction temperature was 15 C; length of
delay loop was 1.5 m; a Caterpillar Split-Recombine Micro Mixer was used; and all of the experiments were completed
at atmospheric pressure. The yield of benzaldehyde can reach 84.7% with selectivity of 98.5%. Due to the small reactor
volume and short residence times, the Swern oxidation of benzyl alcohol in a continuous flow microreactor system can be
◦
◦
operated at nearly room temperature (5–19 C) instead of –70 C in a batch reaction, with residence time of reactants
in microreactors in milliseconds instead of several hours in a batch reaction.
Key words: Swern oxidation, microreactor, benzaldehyde, benzyl alcohol
1. Introduction
Benzaldehyde, which occurs naturally in almond, makes a characteristic almond-like odor. It is commonly
used as a flavor in foods and drinks, and as a fragrance in cosmetics and household chemicals. Moreover,
benzaldehyde also acts as a crucial intermediate in chemical and pharmaceutical production,1 and it is used to
prepare herbicides and plant growth regulators when it comes to agriculture.2 Various oxidants were applied
to synthesize benzaldehyde in the past several years, like transition-metal derivatives (Collins, pyridinium
chlorochromate, and pyridinium dichromate reagents),3
−5
hypervalent iodine reagents (2-iodoxybenzoic acid
and Dess–Martin periodinane),6,7 and hypervalent manganese reagents (KMnO4). They performed well in
efficiency, but usually the transition metallic and halide elements also caused severe environment contamination.
Moreover, the high activity of these oxidants easily leads to overoxidizing, so a lot of byproducts are inevitable.
In order to overcome these shortcomings, heterogeneous catalysts (NaOCl in PTC),8 mild oxidants (hydrogen
peroxide, O2), and the Me2 SO-activator system (Swern oxidation)9,10 have been widely used and developed
recently. The Me2 SO-activator system, which is known as Moffatt–Swern oxidation, consists of dimethyl
∗
Correspondence: zhengfp@btbu.edu.cn
7
5

ZHU et al./Turk J Chem
sulfoxide (Me2 SO) and an electrophilic species at extremely low temperatures. Because of its totally metal-free
quality, it is thought to be environmentally benign.
The Swern experimental conditions are challenging because activation of Me2 SO is usually accompanied
by Pummerer rearrangement.11,12 Therefore, the temperature of the reaction is severely restricted between
–
60 C and –10 C13 as only in that case could the side reaction be reduced to an acceptable level. Control
◦ ◦
of the reaction conditions of Swern oxidation is thus of great importance.
In order to achieve that, the dropwise addition of the reagent should be done and the reaction system
should be stirred for a very long time in a macroscale batch reactor.10,14 Moreover, for such kinds of extremely
exothermic reactions or reactions that proceed through highly reactive and unstable intermediates, the use of
batch reactors has more disadvantages that are related to temperature control and safety problems.
A continuous flow microreactor system, in which the reactor volume is so small that the characteristic
dimensions are in millimeter or even micrometer range, has excellent mixing and heat transfer ability. Only small
amounts of reactants are consumed at the same time so that less heat is given off and then potentially dangerous
situations can be avoided.15 As a result, Swern oxidation, which must be conducted under extremely low
temperatures, could be safely performed in the continuous microreactor system at nearly ambient temperature.
In addition, the residence time of the reactant in the continuous microreactor system is much less than that in
batch systems and leads to only a few intermediates accumulating, which is beneficial to improve the yield and
selectivity of Swern reaction.11,16
Therefore, we performed our research into conducting Swern oxidation with a microreactor to prepare
benzaldehyde, employing a Me2 SO-activator system as the oxidant, benzyl alcohol as the substrate, and
triethylamine as the base. The parameters of Swern oxidation were also investigated and screened in order
to find the optimal reaction parameters, which include the type of micromixers, type of activators, temperature,
flow rate residence time, and the mole ratio of Me2 SO, oxalyl chloride, and benzyl alcohol. This research has
not been published before and, at the same time, the method is environmentally benign, safe, and efficient.
◦
Compared with the existing data in the literature, which reported the yield of benzaldehyde of 78% at 0
C
and 75% at 20 C, the optimized yield of 84.7% with a selectivity of 98.5% is relatively considerable.16
◦
2
2
. Results and discussion
.1. Selection of activator
The result of Swern oxidation is greatly influenced by the type of activators. In 1978, Mancuso et al. found
that the Me2 SO-oxalyl chloride system showed the best activity and led to much better yields when compared
with Me2 SO-trifluoroacetic anhydride,17 pyridinium chlorochromate, and Me2 SO-pyridine-SO3 . On this basis,
oxalyl chloride is widely used as the activator in many recent studies of Swern oxidation.
However, oxalyl chloride will decompose in a humid environment, react violently with water, and emit
gases CO, CO2 , and HCl, known to be toxic and corrosive. Taking green chemistry and the safety of industrial
production into account, p-toluenesulfonyl chloride (p-TsCl) is often used as the activator instead of oxalyl
chloride.10 Compared with oxalyl chloride, p-toluenesulfonyl chloride tends to be less toxic and safe. At the
same time, the activity of p-toluenesulfonyl chloride is decreased, too. Consequently, it is necessary to find a
method with low toxicity and high effect. Both oxalyl chloride and p-toluenesulfonyl chloride were chosen as
activators and parameters were screened in later experiments.
76

ZHU et al./Turk J Chem
2.2. Mechanistic considerations and microreactor setup
In the Me2 SO-oxalyl chloride system, activator 2 interacts with Me2 SO 1 or 1’ to give the sulfonium ion, which
contains a leaving group, chlorodimethylsulfonium salt 4. The first intermediate 4 quickly reacts with benzyl
alcohol 5 to form the key phenylmethoxydimethylsulfonium salt 6. The additions of triethylamine deprotonate
the phenylmethoxydimethylsulfonium ion to give intermediate 7, which is alternatively converted to dimethyl
sulfide (DMS) and benzaldehyde 8 (Scheme 1).
Scheme 1. The proposed mechanism of the Swern oxidation with oxalyl chloride.
The Me2 SO-p-TsCl system is similar to the Me2 SO-oxalyl chloride system. The Me2 SO is activated by
p-TsCl to form key intermediates tosyloxydimethylsulfonium chloride 9 or chlorodimethylsulfonium tosylate 10.
Then the activated intermediates quickly react with benzyl alcohol 5 to give the phenylmethoxydimethylsulfo-
nium salt 6 and an equivalent p-toluene sulfonic acid. Finally the triethylamine takes away an equivalent proton
of intermediate 6 to give intermediate 7, which is alternatively converted to dimethyl sulfide and benzaldehyde
8
(Scheme 2).
Scheme 2. The proposed mechanism of the Swern oxidation with p-TsCl.
Figure 1 shows a schematic drawing of the microreactor setup. Me2 SO, oxalyl chloride, and benzyl alcohol
were pumped into the system through three HPLC pumps (P1, P2, and P3). Me2 SO and oxalyl chloride were
premixed with a T-shaped joint (M1) and reacted in a reactor loop (Rl) with release of CO and CO2 to give
7
7

ZHU et al./Turk J Chem
intermediate 4, and then intermediate 4 was well blended with benzyl alcohol in the micromixer (M2) and
reacted with it in a reactor loop (R2) to form another intermediate 6. An appropriate length of reactor loop
(
R2) was of great use to control the reaction progress and product selectivity, avoiding too many byproducts.
M1, M2, Rl, R2, and most of the tubes were sunk in the constant-temperature bath. The effluent was collected
in an Erlenmeyer flask containing continuously magnetic stirred triethylamine. A separating funnel that could
clearly show the state of the effluent was between the effluent tube and Erlenmeyer flask.
Figure 1. Schematic drawing of the continuous microreactor system.
2.3. Optimization of reaction parameters
The molar ratio of n (Me2 SO): n (activator): n (alcohol) = 3:1.5:1 was found to be the best according to the
works conducted in batch systems,10 and it was chosen as the molar ratio of most experiments except the ones
related to optimizing the molar ratio itself. Linden et al. investigated the conversion of the Swern reaction at
◦ ◦
11
–
20 C to 30 C in a continuous microreactor system The results showed that the conversion was considerable
◦
◦
from –20 to 20 C but sharply decreased at 20–30 C. Considering the aim of creating a mild and effective
◦
◦
experimental method, all experiments were performed between 0 C and 20 C, at atmospheric pressure.
2.3.1. Optimization of the temperature and flow rate
The temperature and flow rate were investigated under the condition of n (Me2 SO): n (oxalyl chloride): n
◦
(
benzyl alcohol) = 3:1.5:1 and L (R2) = 1.5 m. The reaction temperature (5, 10, 13, 15, 17, and 19 C) and
the flow rate of Me2 SO (1.2, 1.5, 1.8, and 2.0 mL/min) were studied separately.
2.3.1.1. Oxalyl chloride as the activator
The results are shown in Figures 2 and 3. The highest yield at which oxalyl chloride acted as the activator was
◦
7
0.0%, and the best selectivity was 97.3% at 15 C and Me2 SO flow rate of 1.5 mL/min. In this case, 1.024 g
78

ZHU et al./Turk J Chem
benzaldehyde was obtained in 1 h (1.024 g/h). Obviously, the optimal temperature is much higher than the
◦
one (–60 C) in the conditional batch reactor. That could be attributed to the excellent heat transfer ability
−1
−2
−2
−1
in batch reactors).18
K
(
heat transfer coefficients reached 60,000 W m
K
, compared to 100 sW m
Consequently, reaction exotherms can be easily dissipated, enabling highly energetic materials to be synthesized
in a safe manner.
Figure 2. Influence of temperature on yield when oxalyl
Figure 3. Influence of temperature on selectivity when
chloride acted as the activator.
oxalyl chloride acted as the activator.
The selectivity of benzaldehyde was very considerable (more than 85.0%), as shown in Figure 3. In other
words, high-purity products can be obtained in most cases. The microreactor has a very small internal volume;
for example, the internal volume of the Caterpillar Split-Recombine Micro Mixer (CPMM) is 30 µL and that
of the Slit Interdigital Micro Mixer (SIMM) is 5.625 µL. This means that only a small amount of reactants
interacted with each other at a moment and accumulation of the labile intermediate was minimized. In addition,
because of the short residence time, the side reactions of the labile intermediate were limited.
In the microreactor, temperature and flow rate are the key factors that can decide the conversion of the
reactants, yield, and selectivity of products. The temperature and flow rate interacted with each other in the
progress. When the flow rate became slower, the optimal temperature for yield was gradually decreased; on
the other hand, when the flow rate became faster, the optimal temperature increased. This is easy to explain
by basic physical knowledge. When the flow rate stays at a relatively low level, the reactants are well mixed
because of the sufficient contact time in tubes at low temperatures. As the temperature increases, byproducts
are accumulated, which leads to low yield. Otherwise, when the flow rate is at high levels, the residence time
would be too short to mix the reactants well. At this point, increasing the temperature could make the molecules
of reactants move faster, thus improving the rate of mass transfer and conversion of reaction. We can conclude
that the velocity of reaction, which was intensively exothermic and accompanied by a number of side reactions,
could be improved by increasing the temperature, but the yield was not always improved.
2.3.1.2. p-TsCl as the activator
Figures 4 and 5 show the results of applying different temperatures and flow rates when keeping n (Me2 SO):
n (oxalyl chloride): n (benzyl alcohol) to 3:1.5:1 and L (R2) to 1.5 m. The highest yield of benzaldehyde was
only 15.0%, 302 mg/h, which was much lower than oxalyl chloride activating Swern reactions. At the same
◦
time, however, the selectivity was obviously considerable, which reached 95.0% at 17 C and 1.2 mL/min. This
phenomenon also proved the ability of the microreactor in increasing selectivity and controlling the accumulation
of byproducts.
When the flow rate of Me2 SO was less than 1.5 mL/min, we got little benzaldehyde in the system even
if the residence time was increased. The yields were not significantly improved although the temperature rose.
7
9

ZHU et al./Turk J Chem
Figure 4. Influence of temperature on yield whenp-TsCl
acted as the activator.
Figure 5. Influence of temperature on selectivity when
p-TsCl acted as the activator.
However, the yields reached a peak for almost the whole temperature range at the flow rate of Me2 SO of 1.5
mL/min. When the flow rate continued to increase, regardless of whether the temperature was increased or
not, the yield decreased. This could be explained by the flow rate directly influencing the residence time with
◦
too short of a residence time for sufficient mixing. Especially at higher temperatures than 15 C a mass of
byproducts accumulated, which caused a great decrease in the selectivity of benzaldehyde.
2.3.2. Optimization of the residence time
Residence time is an important reaction parameter of the microreactor and also reflects the production capacity
of the microreactor. It could be controlled by two parameters, the length of the reactor loop (R2) and the flow
rate. In the previous experiment, 1.5 mL/min was chosen as the optimum flow rate of Me2 SO, and then the
length of R2 was studied.
The residence time was investigated under the condition of n (Me2 SO): n (oxalyl chloride): n (benzyl
◦
alcohol) = 3:1.5:1, T = 15 C, flow rate of Me2 SO = 1.5 mL/min. The effects of residence time were
investigated by setting the length of R2 to 1.0 m, 1.5 m, 2.0 m, and 2.5 m, respectively.
2
.3.2.1. Oxalyl chloride as the activator
The results are shown in Table 1. The highest yield at which oxalyl chloride acted as the activator was 70.0%
1.024 g/h) and the best selectivity was 97.3%. Increasing the length of R2 slightly from 1.0 m to 1.5 m appeared
(
to have a negative impact on the selectivity of benzaldehyde (decreased from 97.3% to 94.1%), whereas a positive
impact was shown on the yield of benzaldehyde (increased from 55.5% to 70%, 0.846 g/h to 1.024 g/h). However,
when we continued to lengthen R2 from 1.5 m to 2.5 m, both the selectivity of benzaldehyde and the yield
decreased, and leakage was observed at the joints between the micromixer (M1, M2) and tubes. The selectivity
of benzaldehyde always decreased with the lengthening of R2. In other words, short residence time seems to
play a crucial role in improving the selectivity.
2.3.2.2. p-TsCl as the activator
The results of p-TsCl activating Swern reactions are shown in Table 2. The highest yield at which p-TsCl
acted as the activator was 15% (302 mg/h) and the best selectivity was 95.1%. In the system benzyl chloride
was found as a byproduct, which was formed by halogenation of benzyl alcohol (Scheme 3). The involved
chlorine in Scheme 3 was inferred to have come from HCl, which was formed by intermediate 4 (Scheme 2) or
the decomposed oxalyl chloride. When we extended the length of R2 from 1.0 to 1.5 m, the yield increased by
80

ZHU et al./Turk J Chem
Table 1. Influence of the length of R2 to Swern oxidation when oxalyl chloride acted as the activator. All of the yields
and selectivities were determined by GC-MS with the normalization method of peak area.
The length The yield of
The selectivity of The yield of
of R2 (m) benzyl chloride (%) benzaldehyde (%) benzaldehyde (%)
1
1
2
2
.0
.5
.0
.5
2.3
4.8
7.5
13.4
97.3
94.1
85.6
72.0
55.5 (0.846 g/h)
70.0 (1.024 g/h)
45.1 (0.655 g/h)
39.7 (0.563 g/h)
5%, and the highest yield of 15% (302 mg/h) appeared at 1.5 m. When we continued to extend the length of
R2 from 1.5 to 2.5 m, the yield decreased by 7%. The selectivity always decreased with the extending of R2,
from 95.1% to 70.6%. Leakage was observed after lengthening R2 over 2.0 m.
Table 2. Influence of the length of R2 to Swern oxidation when p-TsCl acted as the activator. All of the yields and
selectivities were determined by GC-MS with the normalization method of peak area.
The length The yield of
The selectivity of
The yield of
of R2 (m) byproduct (%) benzaldehyde (%) benzaldehyde (%)
1
1
2
2
.0
.5
.0
.5
0.52
2
95.1
88.3
84.2
70.6
10.0 (0.196 g/h)
15.0 (0.302 g/h)
9.2 (0.179 g/h)
7.5 (0.143 g/h)
1.82
3.2
Scheme 3. The proposed formation mechanism of benzyl chloride in the system.
2.3.3. Optimization of the type of micromixers
Micromixers are main components of continuous microreactor systems and directly affect the mixing efficiency
and the yield of products. The CPMM and SIMM were both investigated in this paper.
The internal structures of the CPMM and SIMM are shown in Figure 6. The CPMM has channel size of
6
00 µm × 600 µm and internal volume of 30 µL. It has a three-dimensional tooth profile structure, and the
fluids will rotate into a vortex, which facilitates the mixing of materials. Because of its property of stainless
steel, CPMM performs well in reactions of high pressure or high flow rate. There are 30 vertical channels in the
SIMM and the internal volume is 5.625 µL. Instead of stainless steel, the SIMM is made of special glass, so the
pressure durability of the SIMM is weaker than that of the CPMM.
The effect of the SIMM and CPMM were investigated under the condition of n (Me2 SO): n (oxalyl
◦
chloride): n (benzyl alcohol) = 3:1.5:1, 15 C, and L (R2) =1.5 m. The flow rate of Me2 SO was set to 1.2
mL/min, 1.5 mL/min, 1.8 mL/min, and 2.0 mL/min, respectively.
8
1

ZHU et al./Turk J Chem
Figure 6. Internal structure and path of fluids in CPMM and SIMM.
2.3.3.1. Oxalyl chloride as the activator
The result is shown in Figure 7. When the flow rate of Me2 SO was increased from 1.2 mL/min to 1.5 mL/min,
the yields of both of the two micromixers increased dramatically, and both the yields decreased at the same
time when the flow rate of Me2 SO increased over 1.5 mL/min. When the flow rate of Me2 SO was between
1.5 and 2.5 mL/min, the yields of the CPMM were better than those of the SIMM, and the highest yield of
benzaldehyde reached 70.0% (1.024 g/h) at 1.5 mL/min.
2.3.3.2. p-TsCl as the activator
Figure 8 shows the results of reactions applying the CPMM and SIMM micromixers. The yields with the CPMM
increased massively when the flow rate of Me2 SO was increased from 1.2 mL/min to 1.5 mL/min. When we
continued to increase the flow rate of Me2 SO, the yield decreased by 4/5. In contrast to the CPMM, the SIMM
obviously acted well at low flow rate. When the flow rate of Me2 SO was 1.2 mL/min, the yield of the SIMM
was higher than that of the CPMM, but as the flow rate of Me2 SO increased, the yield of the SIMM always
decreased. There was almost no benzaldehyde in the system when the flow rate of Me2 SO was 2.0 mL/min.
This could be attributed to the special internal structure (Figure 6), which caused the SIMM to act better at
low flow velocity.
2.3.4. Optimization of the mole ratio of Me2 SO, oxalyl chloride, and benzyl alcohol
Based on the investigation above, the mole ratio of Me2 SO, oxalyl chloride, and benzyl alcohol (1:1:1, 2:1:1,
◦
3
:1.5:1, 4:2:1, 5:2.5:1, and 6:3:1) was optimized under the condition of L (R2) = 1.5 m, 15 C, CPMM
micromixer, oxalyl chloride as activator, and Me2 SO flow rate of 1.5 mL/min. In this study, the reaction
time and the concentration of Me2 SO was fixed at 40 min and 14 mmol/mL, so the concentration of oxalyl
chloride and benzyl alcohol can be easily calculated according to the mole ratio. In addition, it was ensured
that the excess amount of triethylamine was used for the consumption of the acid produced in reaction media
and for the efficient conversion of intermediate 6 to 7 (Scheme 1). The results are shown in Table 3. When the
82

ZHU et al./Turk J Chem
Figure 7. Influence of the types of microreactors on yield
when oxalyl chloride acted as the activator.
Figure 8. Influence of types of microreactors on yield
when p-TsCl acted as the activator.
mole ratio of Me2 SO, oxalyl chloride, and benzyl alcohol was 4:2:1, the yield reached 84.7% (1.800 g/h) and
the selectivity reached 98.5%, which were the best in all conditions (Table 3).
Table 3. Influence of the mole ratio of reactants (Me
2
SO: oxalyl chloride: benzyl alcohol) to Swern oxidation. All of
the yields and selectivities were determined by GC-MS with the normalization method of peak area.
The mole ratio The yield of
The selectivity of The yield of
of reactants benzyl chloride (%) benzaldehyde (%) benzaldehyde (%)
1
2
3
4
5
6
:1:1
2.9
3.5
4.8
6.3
9.1
11.3
88.5
90.4
94.1
98.5
95.9
96.8
58.9 (0.811 g/h)
63.1 (0.893 g/h)
70.0 (1.024 g/h)
84.7 (1.800 g/h)
70.9 (1.466 g/h)
69.4 ( 1.343 g/h)
:1:1
:1.5:1
:2:1
:2.5:1
:3:1
2.4. Conclusions
The oxidation of benzyl alcohol to benzaldehyde was performed by Swern oxidation in a continuous flow
microreactor system. The method was tested to be practical and the setup of the continuous microreactor
system seems to be reasonable. Moreover, it could be applied to similar reactions that were accompanied by
liable intermediates and a great deal of heat. Parameters such as reaction temperature, flow rates of reactants,
types of micromixers, length of delay loops, and mole ratio of reactants were optimized and Swern oxidations
activated by oxalyl chloride and p-TsCl were discussed, respectively. It was found that the optimized parameters
of the Swern oxidation of benzyl alcohol were as follows: oxalyl chloride was applied as activating agent; the
mole ratio of Me2 SO, oxalyl chloride, and benzyl alcohol was 4:2:1; the flow rate of Me2 SO was 1.5 mL/min; the
◦
reaction temperature was 15 C; the length of the delay loop was 1.5 m; the CPMM served as the micromixer;
and all of the experiments were conducted at atmospheric pressure. The yield of benzaldehyde can reach 84.7%
(
1.800 g/h) with a selectivity of 98.5%.
3
3
. Experimental
.1. Material and apparatus
Dichloromethane was distilled so that the dissolved water could be removed. The other chemicals were used
as received. The related components of the continuous flow microreactor system include three LC 3000 HPLC
8
3

ZHU et al./Turk J Chem
pumps made by Beijing Chuangxintongheng Science and Technology Co., Ltd. A 1/8 T-shaped joint (M1), a
CPMM or a SIMM (M2), and several pieces of 1/12 316 stainless steel tube (R1, R2) were used, made by IMM.
A water bath with constant circulation temperatures and an 85-2 type thermostat magnetism agitator from
Gongyikehua Instrument Co., Ltd. were applied to control the temperature. Reactions were monitored offline
by GC-MS.
3.2. Experimental procedure
The continuous flow microreactor system was assembled as shown in Figure 1. The Me2 SO, activator, and
benzyl alcohol were loaded into HPLC pumps with appropriate flow rates. After a stable period of 10 min, the
effluent was collected in a separation funnel through which we could observe the color and layering by controlling
the valve. Then effluent was finally collected into a continuously stirred flask containing an excess amount of
triethylamine. The temperatures were kept constant by a water bath with constant circulation temperatures
and a thermostat magnetism agitator. All of the reactions were monitored offline by GC-MS; the yields and
selectivities were calculated by the method of area normalization.
Upon completion of the reaction, the mixture of triethylamine and effluent was washed with 5% HCl
and the organic layer was separated. Then the organic layer was washed 2 or 3 times until the mixture in
the separation funnel reached neutral. In the process, the aqueous layers were always extracted twice with
dichloromethane. At last, the organic layers were collected together and dried by anhydrous MgSO4 for 4 h.
The final product, benzaldehyde, was obtained after filtration and reduced pressure distillation.
3.3. Quantitative method
Reactions were quantified with the normalization method of peak area by GC-MS. The GC method was
optimized so that all of the components were separated well and the peak of each compound appeared in
the chromatogram. The samples were injected and ran according to the following method. An Agilent HP-5MS
◦
column (30 m × 0.25 mm, 0.25 µm) was used. Oven temperature was started at 50 C and then increased
◦
◦
to 250 C at a rate of 15 C/min with a final stand of 2 min. The temperature of the injection port was
◦
2
50 C and the sample size was 0.2 µL. The split ratio was set at 100:1. An EI ion source was used and the
◦
ionizing voltage and temperature of ion source was set respectively at 70 eV and 230 C. The temperature of
◦
the quadrupole was 150 C. The scan mode was set as Scan and the range of mass scan was 33 to 300 m/Z.
Acknowledgment
This work was financially supported by the National Key Technology R & D Program, P.R. China (2011BAD23B01).
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