Nanoparticle cages as microreactors for producing acrolein from glycerol in the liquid phase

Article abstract of DOI:10.1039/d0nj05125j

Liquid phase dehydration of glycerol to acrolein commonly suffers from a low acrolein yield because of severe side reactions of chemically active acrolein with itself or glycerol in the reaction system. Rapid separation of acrolein from the reaction system is one of the effective strategies to increase the yield of acrolein. Herein, a renewable water-in-oil Pickering emulsion with silica nanoparticle cages as microreactors was designed for liquid phase glycerol dehydration to acrolein. The influences of the oil phase types on the droplet size of the Pickering emulsion and the yield of acrolein were investigated in a microreactor. The smaller Pickering emulsion droplet showed a superior ability to intensify the separation of chemically unstable acrolein molecules from the nanoparticle cages and thus improved the yield of acrolein. A droplet size of about 11 μm was obtained with peony seed oil as the oil phase, in which 84.6% of acrolein yield was achieved. This journal is

Full text of DOI:10.1039/d0nj05125j

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Nanoparticle cages as microreactors for
producing acrolein from glycerol in the
Cite this:DOI: 10.1039/d0nj05125j
liquid phase†
Jiaojiao Zhang,‡ab Jie Zhao,‡c Xiaojing Cui,bd Xianglin Hou,ab Lijuan Su,ab
ab
Hongliang Wange and Tiansheng Deng
*
Liquid phase dehydration of glycerol to acrolein commonly suffers from a low acrolein yield because of
severe side reactions of chemically active acrolein with itself or glycerol in the reaction system. Rapid
separation of acrolein from the reaction system is one of the effective strategies to increase the yield of
acrolein. Herein, a renewable water-in-oil Pickering emulsion with silica nanoparticle cages as microreactors
was designed for liquid phase glycerol dehydration to acrolein. The influences of the oil phase types on the
droplet size of the Pickering emulsion and the yield of acrolein were investigated in a microreactor. The
smaller Pickering emulsion droplet showed a superior ability to intensify the separation of chemically
unstable acrolein molecules from the nanoparticle cages and thus improved the yield of acrolein. A droplet
size of about 11 mm was obtained with peony seed oil as the oil phase, in which 84.6% of acrolein yield was
achieved.
Received 18th October 2020,
Accepted 17th November 2020
DOI: 10.1039/d0nj05125j
rsc.li/njc
derivatives of glycerol, the dehydration product acrolein can be
used as a starting material to synthesize acrylic acid, DL-methionine,
1. Introduction
With the development of green chemistry and the consumption 1,3-propanediol and polymers.10,11 Thereby, acrolein produced
of fossil energy, biomass has been widely used as a renewable from glycerol by a dehydration reaction has received extensive
alternative and a sustainable resource for the production of attention.
biofuels, materials and chemicals.1 Glycerol, as the main by-
The liquid phase glycerol dehydration to acrolein is mainly
product in a biodiesel production process, accounts for 10 wt% conducted in sub- or supercritical water systems under the
of biodiesel.2 With the increase of annual biodiesel production, reaction conditions of 250–400 1C and 25–35 MPa and with
the price of glycerol decreased continuously due to the increase ZnSO4, H2SO4, and WO3/TiO2 as catalysts, affording an acrolein
of glycerol production. Therefore, the development and utiliza- yield of 40–70%.12–14 The extremely high temperatures and
tion of glycerol can not only solve the problem of glycerol pressures make the industrial production difficult. Though
surplus, but also increase the market competitiveness of mild reaction conditions were studied, the obtained acrolein
biodiesel.3 The conversion of glycerol to high-value-added fine yield was low.15,16 It is highly desirable to design efficient liquid
chemicals, such as acrolein, glyceric acid, lactic acid, hydroxy- phase systems for obtaining a high acrolein yield under mild
acetone and so on, has been a great concern.4–9 Among the conditions. Because acrolein molecules are highly chemically
active and can easily form humus with themselves or glycerol
a Shanxi Engineering Research Center of Biorefinery, Institute of Coal Chemistry,
Chinese Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, China.
E-mail: dts117@sxicc.ac.cn
molecules under reaction conditions, their rapid separation
from the liquid phase system is crucial to suppress the side
reactions. However, the high viscosity of glycerol inevitably
slows down the diffusion of acrolein drastically out of the
reaction system, which aggravates the occurrence of side reac-
tions and thus is detrimental to the acrolein yield. Therefore,
the key to increase the acrolein yield is to separate the gener-
ated acrolein as soon as possible.
To enhance the separation efficiency of the produced
acrolein, there is a need to (1) shorten the separation path
of acrolein and (2) increase the separation area of acrolein,
such as by forming an emulsion. However, forming emulsion
b Center of Materials Science and Optoelectronics Engineering, University of Chinese
Academy of Sciences, Beijing 100049, China
c Institute of Coal Chemistry,
Chinese Academy of Sciences, Taiyuan 030001, China
d State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese
Academy of Sciences, 27 South Taoyuan Road, Taiyuan 030001, China
e Center of Biomass Engineering/College of Agronomy and Biotechnology, China
Agricultural University, Beijing, 100193, China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj05125j
‡ These authors contributed equally to this work.
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needs addition of some surfactants, which are chemically active added on the right of the formula (1). The higher the value of JA,
additives so that they have an adverse effect on the production the faster the diffusion rate. DAW represents the diffusion
of acrolein from glycerol. Pickering emulsion that does not coefficients of acrolein in the water phase, whose value is
need surfactants is an excellent emulsion stabilized by solid constant at a specific temperature and pressure. CAI and CAO
nanoparticles.17,18 The nanoparticles in a Pickering emulsion mean the concentration of acrolein inside and outside of the
can form nanoparticle cages as microreactors, not only sharply droplet, respectively. The difference between CAI and CAO is
shorten the single reactor scale, but the large contact area of mainly determined by the amount of acrolein produced upon
the water and oil phases will be realized to accelerate obviously the dehydration of glycerol in the droplet. The amount of
the mass transfer between the two phases.19
produced acrolein is the same at the same time interval under
In our previous work, a water/oil Pickering emulsion with a fixed reaction conditions (such as temperature, pressure,
smallest droplet size of 30 mm was developed for producing catalyst, etc.). r is the radius of the droplet. A smaller r caused
acrolein from glycerol in the liquid phase.20 Compared with the a bigger JA, which means the produced acrolein can diffuse
conventional aqueous reaction system, the mass transfer inten- rapidly outside the droplet. Thus, a reduction of the droplet
sification of Pickering emulsions promoted the acrolein yield radius can improve the diffusion efficiency of acrolein, which
from a low value of 20% to a moderate value of 48%. Notice- effectively inhibited the side reactions and improved the yield
ably, this yield was obtained with a 100% conversion of of acrolein. Hence, the fabrication of emulsion systems with
glycerol, indicating the presence of acrolein side reactions. droplets of small size would favor a high acrolein yield.
Hence, it is still troublesome to suppress largely the side
reactions of acrolein to obtain a high acrolein yield.
However, it remains a great challenge to prepare small-sized
emulsion systems. A common method to produce small-sized
The generated acrolein diffused from the microreactor into Pickering emulsions is to add organic surfactants as co-
the oil phase of the Pickering emulsion is shown in Scheme 1. emulsifiers to stabilize the emulsions.21–23 However, the intro-
The acrolein generated by the reaction inside the droplet duction of typical organic surfactants in a Pickering emulsion is
entered the oil phase through the oil–water interface, and then troublesome for the separation and recycling of the emulsifiers.
entered the gas phase through the oil–air interface, and was Moreover, auxiliary surfactants have a deleterious effect on
condensed and collected by a condenser. In this process, the the active substances, especially for the enzyme catalysts. To
entry of acrolein from the water phase into the oil phase was overcome this problem, an efficient strategy was developed by
the key to improving the reaction effect. There was no real T. Ngai and co-workers, in which the Pickering emulsion was
diffusion equilibrium state in the actual reaction process, but pretreated with tetraethyl orthosilicate to prepare a submicron-
research on the diffusion behavior of products in the droplet sized emulsion by alkali treatment.24 However, the presence of
should be carried out in a steady state. In a very short time, the base is detrimental to the acid-catalyzed reaction systems. More
diffusion of acrolein through the oil–water interface and the importantly, a common problem with small-sized Pickering
oil–gas interface can be considered to be in a steady state. emulsions is their poor thermal stability, which are commonly
Therefore, according to Fick law, the diffusion flux of acrolein unstable at temperatures above 100 1C.19,25,26 Thus, the design
diffusing into the oil phase in the droplet is:
of a small-sized and thermally stable Pickering emulsion sui-
table for acid-catalyzed reactions remains a great challenge.
Noticeably, the huge impact of the viscosity of the oil phase
on the aqueous droplet size, and further the mass transfer
of product(s), was not reported in the studies of water-in-oil
Pickering emulsions. In this work, a novel strategy for the
fabrication of a small-sized Pickering emulsion system was
developed by tuning the viscosity of oil phases. Besides, cycle
of the Pickering emulsion was carried out. The average droplet
size of the Pickering emulsion decreased from 80 to 10 mm
upon varying the types of oil phases. Accordingly, the yield of
acrolein increased largely from 61.8% to 84.6%, a value much
higher than those reported in the liquid phase acrolein produc-
tion from glycerol under similar reaction conditions.
dCA
JA ¼ ꢀDAW
dr
(1)
the diffusion flux of acrolein can be obtained by directly
integrating the above formula:
DAW
JA
¼
ðCAI ꢀ CAO
Þ
(2)
r
where JA means the diffusion flux of acrolein. The direction of
JA is opposite to the concentration gradient, so a minus sign is
2. Experimental details
2.1. Modification of hydrophilic silica nanoparticles
The commercially available hydrophobic silica, named SN–C,
was used directly as an emulsifier without any treatment.
Emulsifiers with different surface properties were prepared by
silylation of hydrophilic SNs (silica nanoparticles). (MeO)3
Si(CH2)2CH3, (MeO)3Si(CH2)7CH3, (MeO)3Si(CH2)11CH3 and
Scheme 1 Schematic diagram of the generation and transfer path of
acrolein in the Pickering emulsion.
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(MeO)3Si(CH2)15CH3 were applied as silylation reagents. 1.0 g of acrolein product was determined by GC (gas chromatography).
hydrophilic SNs were dried at 125 1C for 4 h. The dried SNs were For the cycle of the Pickering emulsion, 10 mL of 30 wt%
dispersed into 15 mL of toluene. Then 1.0 mmol of the given glycerol aqueous were added and re-emulsified for each cycle.
silylation reagent (1.5 mmol of (MeO)3Si(CH2)2CH3) was added The reaction time for each cycle run was 0.5 h, except for 1 h in
into the mixture and heated under reflux at 110 1C for 4 h under the last cycle run.
an N2 atmosphere. The modified hydrophilic silica nano-
2.3. Characterization of the emulsifier
particles were collected by centrifugation. Then, the obtained
sample was dried at 80 1C under vacuum for 6 h after being
washed with toluene five times. The resulting samples were
The contact angle (water–solid–air) of the SiO2 nanoparticles
was measured on a Drop Shape Analyzer DSA-100. Transmis-
sion electron microscopy (TEM) imaging of SiO2 nanoparticle
samples was performed on a JEM-2100 microscope operated at
200 kV. Thermogravimetric (TG) analysis of the SiO2 nano-
particles was implemented on a SETSYS EVOLUTION TGA 16/18
(France) under a nitrogen atmosphere with the temperature
designated as SN–C3–1.5, SN–C8–1.0, SN–C12–1.0 and SN–C16–1.0
respectively.
,
2.2. Preparation of the Pickering emulsion and the reaction
A detailed description of the reaction test runs in this study
is provided elsewhere.20 In brief, in a typical test run, 0.05 mol
of KHSO4 as a catalyst was dissolved in 10 mL of 30 wt%
glycerol aqueous solution (designated as solution 1) at 80 1C.
Then, 20 mL of oil and 3.5 wt% (relative to the volume of
increasing from 25 1C to 800 1C at a rate of 20 1C minꢀ1
.
3. Results and discussion
3.1. Characterization of the emulsifier
the water phase) of the emulsifier (SN–C, SN–C3–1.5, SN–C8–1.0
,
SN–C12–1.0 or SN–C16–1.0) were added to the solution 1. The
resulting mixture was strongly stirred at 6500 rpm for 5 min to
prepare the Pickering emulsion. Then, the obtained Pickering
emulsion was heated promptly to 180 1C for 1 h to evaluate the
production of acrolein from glycerol at 1 atm. Notably, the
reaction took place in a distillation unit, where acrolein pro-
duced in the liquid phase was directly separated from the
system by distillation and collected; a schematic diagram of
the acrolein formed and diffused in the Pickering emulsion is
shown in Fig. 1(A). After the reaction, the conversion of glycerol
was almost complete, which was determined by HPLC (high
performance liquid chromatography), while the yield of the
The water contact angle of SN–C was as high as 148.21 (Fig. S1, ESI†).
The strong hydrophobicity of the emulsifier benefits the for-
mation of water-in-oil Pickering emulsion. The hydrophobic
SiO2 nanoparticles were spherical with an average diameter of
20 nm, as evidenced by TEM (Fig. S2, ESI†). TG (thermogravi-
metric) analysis illustrated that the nanoparticles have good
stability upon heating at 500 1C (Fig. S3, ESI†).
3.2. Effect of oil on the Pickering emulsion
Microscopy images of the Pickering emulsions formed in
different oil phases are shown in Fig. 1(B) and Fig. S4, ESI.†
High internal phase Pickering emulsions27 were prepared
from decane, hexadecane, ethyl oleate and peony seed oil.
The former had a volume fraction of about 80% and the
latter had a volume fraction of over 95%. As the viscosity of
the oil phase increases, the droplet size of the Pickering
emulsions gradually decreased, from 80 to under 11 mm. The
ꢀ
average droplet size D was calculated by using the equation
n
P
ꢀ
D ¼
xiDi, where xi is the ratio of droplets with diameter Di as
i¼1
shown in Fig. S4 (ESI†). With the decreased emulsion droplet
size, the relative Pickering emulsion interface area increased
from 0.76 even to 5.45 m2. A much smaller droplet size below
11 mm was achieved when the peony seed oil was used as the oil
phase of the Pickering emulsion. Vegetable oils have a higher
viscosity and their greater shear stress at the same stirring
speed (during the preparation of the Pickering emulsion) than
long-chain alkanes confers on them a higher ability to produce
and stabilize the water-in-oil Pickering emulsion to produce
smaller droplets.
3.3. Catalytic reaction in the Pickering emulsion
The small droplet in the SiO2 nanoparticle cage of the Pickering
emulsion acted as a microreactor. The micron scale size and large
oil–water interface area of the microreactor greatly improved the
substance transfer rate in water and oil. Therefore, this could be a
Fig. 1 (A) A schematic diagram of acrolein formed in Pickering emulsion
and its diffusion path and (B) shows the optical microscopy images (a–d) of
decane, hexadecane, ethyl oleate and peony seed oil, respectively. The
scale bar is 100 mm. The kinematic viscosity of oils is tested at 40 1C.
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Table 1 Preparation of acrolein under different Pickering emulsions
long-chain alkanes, which could be due to the smaller-sized
droplets. However, for the ethyl oleate as the oil phase of the
emulsion, the droplet size was similar to that of the long-chain
alkanes, and the yield of acrolein in the Pickering emulsion
formed by ethyl oleate was higher than that of the long-chain
alkanes. Therefore, there may be other factors promoting the
yield of acrolein in the Pickering emulsion. Hence, the solubi-
lity of acrolein in different oils was considered as an important
factor affecting the yield of acrolein and was investigated. As
shown in Table 1, the distribution coefficient of acrolein was
the highest in ethyl oleate, the second in vegetable oil, and the
poorest in long chain alkanes. In particular, though the size of
the droplet formed with ethyl oleate showed no obvious differ-
ence to tetradecane and hexadecane (50 mm vs. 56 and 52 mm),
the great difference in acrolein solubility (0.794 vs. 0.271 and
0.255) makes the generated acrolein transfer from the water
phase to the oil phase quickly, thus obtaining a better yield of
acrolein. As a result, the acrolein solubility in the oil phase
showed a secondary significance in improving the mass trans-
fer of acrolein in the Pickering emulsion.
Distribution Kinematic
coefficient viscosity (cSt) conv. (%) yield (%)
Glycerol Acrolein
Entry Oil phase
1a
Decane
0.360
0.295
0.271
0.255
0.794
0.97
1.45
2.10
2.93
4.89
26.57
25.51
33.12
39.02
499.0
499.0
499.0
499.0
499.0
499.0
499.0
499.0
499.0
—
51.8
55.6
62.9
69.9
79.2
84.6
80.9
79.7
79.1
0
2a
Dodecane
Tetradecane
Hexadecane
Ethyl oleate
Peony seed oil 0.693
Flaxseed oil
Soybean oil
3a
4a
5b
6c
7c
0.654
0.593
0.562
8c
9c
Olive oil
10d
11d
12d
13d
14e
15e
16e
17f
18f
19g
20g
21g
Peony seed oil
Flaxseed oil
Soybean oil
—
—
—
0
0
0
Olive oil
Peony seed oil
Peony seed oil
Peony seed oil
Peony seed oil
Peony seed oil
Peony seed oil
Peony seed oil
Peony seed oil
499.0
499.0
499.0
499.0
499.0
499.0
499.0
499.0
56.0
40.2
74.1
72.4
61.3
78.8
75.1
70.3
Reaction conditions: 0.05 mol KHSO4, 10 mL of 30 wt% glycerol aqueous,
20 mL of oil phase, 3.5 wt% SN–C, 180 1C, 1 h. a Oil phase is long-chain
alkanes. b Oil phase is ethyl oleate. c Oil phase is vegetable oils. d Control
experiments of vegetable oils. e 0.05 mol of H2SO4, H3PO4 and NaHSO4,
respectively. f 0.04 and 0.03 mol of KHSO4. g First, second and third recycling
of the Pickering emulsions formed in peony seed oil. For each recycle, the
Pickering emulsion was re-emulsified after adding 10 mL of 30 wt% glycerol
aqueous. The reaction time was 0.5 h for each cycle run, except for 1 h in the
last cycle run. The kinematic viscosity of oil phases was measured at 40 1C.
3.4. Mass transfer of acrolein in the Pickering emulsion
In order to understand the factors affecting the yield of acrolein in
the Pickering emulsion, the transfer effects of acrolein molecules
in the Pickering emulsion formed by hexadecane, ethyl oleate and
peony seed oil were studied, respectively. As shown in Fig. 2, the
acrolein concentration in the oil phase of the Pickering emulsion
increased with the prolonged reaction time. At the same reaction
time, the acrolein concentration in the oil phase followed the
order of Cpeony seed oil 4 Cethyl oleate 4 Chexadecane. The droplet size
of the Pickering emulsion formed in peony seed oil, 11 mm, was
much smaller than those formed in ethyl oleate and hexa-
decane, 50 and 52 mm. Thus, the droplet size was the main
reason for enhancing the transfer of acrolein. This result was
consistent with above demonstration that small-sized emulsion
droplets can promote the transfer of acrolein from water to oil
phases (formula (2)). But for ethyl oleate and hexadecane, the
small difference in the emulsion size resulted in a large difference
great advantage for the preparation of acrolein from glycerol in
the Pickering emulsion microreactor.
Then, in order to test the effect of the reaction, acrolein from
glycerol dehydration was obtained in the Pickering emulsion
microreactor with different oil phases (Fig. S5, ESI†). As shown
in Table 1 (entries 1–9), the yields of acrolein in the Pickering
emulsions with low viscosity long-chain alkanes as the oil
phase ranged from 52% to 70%, while they were all higher
than 75% when the high viscosity ethyl oleate and renewable
vegetable oils were used as the oil phases. This result proved
that the high viscosity oil phase was beneficial to increase the
yield of acrolein, which may be related to the formation of
small Pickering droplets. However, with the increase of oil
phase viscosity under the same preparation conditions, the
droplet size cannot be reduced indefinitely. High viscosity
vegetable oils had the smallest droplet sizes, about 10 mm
(Fig. S5, ESI†). Particularly, the highest acrolein yield of
84.6% was obtained when peony seed oil was used as the oil
phase, which was higher than the most reported value (less
than 80%, Table S1, ESI†). It was also possible that vegetable oil
was hydrolyzed to glycerol, and thus the acrolein yield increased.
To rule out this possibility, control experiments were carried out
under the same reaction conditions but without glycerol. No
formation of acrolein was observed (Table 1, entries 10–13). Thus,
acrolein in the Pickering emulsions with vegetable oils as the oil
phase was formed from glycerol, and not from the vegetable oils.
The Pickering emulsions formed from the vegetable oils had
a better effect of improving the yield of acrolein than the
Fig. 2 Acrolein concentration as a function of reaction time in the
Pickering emulsions with peony seed oil and hexadecane as the oil phase.
Reaction conditions: 0.05 mol KHSO4, 10 mL of 30 wt% glycerol aqueous,
20 mL of oil phase, 3.5 wt% SN–C, 180 1C.
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in the yield of acrolein, which was mainly caused by the ability to
dissolve acrolein of the oil phase. In a word, the Pickering
emulsion system with smaller-sized droplets and the oil with a
high ability to dissolve acrolein can enhance the transfer rate of
acrolein out of the aqueous phase to improve the acrolein yield.
3.5. Effect of the catalyst
The production of acrolein by dehydration of glycerol catalyzed
by Brønsted acid has been widely accepted. The catalytic effects
of several mineral acids were studied and the results are shown
in Table 1 (entries 6 and 14–16). Although strong acids can
promote glycerol conversion into acrolein, due to the oxidation
of H2SO4 at a high concentration, the water in the solution in
the reaction process increases the acid concentration of the
reaction system with evaporation in the reaction process,
increasing the possibility of humus formation. For H3PO4 as
a catalyst, the weaker acid may cause a lower yield of acrolein.
In order to obtain more efficient catalytic effects, the acid salts
of polybasic mineral acids of strongly acidic nature were used
in the preparation of acrolein from glycerol. Compared with
NaHSO4, KHSO4 had a better catalytic effect, possibly due to the
influence of metal ions on acids.
In addition to the type of acid, the amount of catalyst also
had a significant influence on the catalytic effect. With the
increase of KHSO4 concentration (Table 1 entries 6, 17 and 18),
the yield of acrolein increased gradually. However, due to the
limitation of KHSO4 solubility, the amount of catalyst cannot be
increased indefinitely. The yield of acrolein was the highest
when 0.05 mol KHSO4 was used as the catalyst.
Fig. 3 (A) Effect of the silica emulsifiers in Pickering emulsions on the
yield of acrolein. (B) Microscopy images of the Pickering emulsion with
different silica emulsifiers. Reaction conditions: 3.5 wt% emulsifier, 180 1C
for 1 h. P-1 to P-4: peony seed oil as the oil phase, the emulsifiers were
SN–C3–1.5, SN–C8–1.0, SN–C12–1.0 and SN–C16–1.0, respectively. T-1 to T-4:
3.6. Effect of the emulsifier
It was noticed that the silica emulsifier was located at the
interface of the water and oil phases. Its surface properties tetradecane as the oil phase, the emulsifiers were SN–C3–1.5, SN–C8–1.0
,
SN–C12–1.0 and SN–C16–1.0, respectively. The scale bar is 100 mm. Error bar
represents the standard deviation of three measurements.
may affect the properties of the Pickering emulsion, and
consequently influence the yield of acrolein. Silica with the
surface modified by different silylation reagents were applied
as emulsifiers, and the effect of the emulsifier surface on the
acrolein yield was investigated in the Pickering emulsions with
tetradecane and peony seed oil as the oil phase, respectively.
The hydrophilic silica was chosen as the precursor for surface
silylation, and the silylated SN samples were designated as
SN–C3–1.5, SN–C8–1.0, SN–C12–1.0 and SN–C16–1.0. As shown in
Fig. S1 (ESI†), the nanoparticles of the silylated SN samples
as the oil phase had a smaller droplet size compared with that
with tetradecane as the oil phase, leading to a higher yield of
acrolein. Thus, for the Pickering emulsions, a hydrophobic
silica emulsifier was beneficial to the acrolein yield.
3.7. Recycling of the Pickering emulsion
were spherical with an average diameter of 20 nm. TG analysis The separation and recovery of catalysts were the main problems
indicated that the silylated SN samples showed good stability faced by liquid phase catalytic systems. Pickering emulsions
upon heating at 450 1C (Fig. S3, ESI†). As shown in Fig. 3A, the also faced the problem of recycling the emulsifiers. What’s more,
yield of acrolein was gradually enhanced with the increase of for producing acrolein from glycerol, the glycerol transformed
the water contact angle of the emulsifiers. This enhancement gradually into humus with the prolonged reaction time, mak-
resulted from the improved stability of the Pickering emulsion. ing it difficult to recover the emulsion system. Besides, the acid
With SN–C3–1.5 as the emulsifier, the Pickering emulsion with catalyst was dissolved in glycerol aqueous, making the catalyst
either tetradecane or peony seed oil as the oil phase showed separation and recycling more difficult. To tackle the emulsifier
poor stability (Fig. 3B) so that the acrolein yield was relatively separation and catalyst recycling problems, an additional
low (Fig. 3A). When the surface of the emulsifier became more glycerol solution was added, and the whole system was re-
hydrophobic, the Pickering emulsions were more stable, which emulsified before each recycling run. For each cycle, the reac-
facilitated the formation of smaller droplets and thus enhanced tion time was set to 0.5 h to reduce the formation of humus as
the acrolein yield. The Pickering emulsion with peony seed oil much as possible. The first, second and third recycling results
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recycling number, the yield of acrolein gradually decreased.
This decrease may result from the deposition of the formed
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In summary, a small droplet of water-in-oil Pickering emulsion
prepared by a high viscosity oil phase can efficiently promote
the production of acrolein from glycerol due to the mass
transfer intensification effect. Moreover, the high solubility of
the oil phase for acrolein promoted the transfer of acrolein
from the aqueous phase to the oil phase. This combined mass
transfer intensification effect was the most obvious on the
Pickering emulsion with peony seed oil as the oil phase, leading
to the highest acrolein yield of 84.6%. Furthermore, the effects
of the surface properties of the silica emulsifier on the transfer
of acrolein at the water–oil interface and consequently on the
acrolein yield were investigated. When the surface of the silica
emulsifier became hydrophobic via silylation treatment, both
the transfer rate and yield of acrolein were enhanced. The
recycling of the Pickering emulsion was investigated, and the
acrolein yield changed from 84.6% to 70.3% after three recy-
cling runs, indicating that the Pickering emulsion in this work
is a sustainable and green system for efficient acrolein produc-
tion from glycerol dehydration.
Conflicts of interest
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There are no conflicts to declare.
Acknowledgements
We gratefully acknowledge the financial support from the
National Key R&D Program of China (no. 2018YFB1501600) and
the Shanxi Province Platform Base and Talent Special Fund
(no. 201705D211023).
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New J. Chem.
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