Journal of Oleo Science
Copyright ©2023 by Japan Oil Chemists’ Society
doi : 10.5650/jos.ess22340
J. Oleo Sci. 72, (2) 233-243 (2023)
Ultrasound-assisted Pickering Interfacial Catalysis
for Transesterification: Optimization of Biodiesel
Yield by Response Surface Methodology
Siyuan Zou, Hao Zhang＊, and Jianli Wang＊
State Key Laboratory Breeding Base of Green Chemistry-Synthesis Technology, Zhejiang Province Key Laboratory of Biofuel, Biodiesel
Laboratory of China Petroleum and Chemical Industry Federation, College of Chemical Engineering, Zhejiang University of Technology,
Hangzhou 310014, PR CHINA
Abstract: Recently, Pickering interfacial catalyst (PIC) was widely applied for liquid-liquid reactions, in
view of not only intensifying the mass transfer through significant reducing both the drop sizes and the
diffusion distance, but also supplying a flexible platform for the immobilization of valuable active sites.
However, the restriction of the mobility of catalyst somehow decreases the activity of a catalyst. To obtain a
promise reaction efficiency, we firstly report a synergistic method to enhance the biphasic reaction by
Pickering emulsion and ultrasound concepts, targeted at efficient production of biodiesel. Response surface
methodology based on Box-Behnken design was applied to optimize the reaction conditions, such as
composition of catalyst, reaction temperature, ultrasound power, methanol to oil molar ratio and catalyst
amount. An over 98% yield of biodiesel could be achieved within 2.5 hours by ultrasound assisted Pickering
interfacial catalysis, which is over two times higher than that of ultrasound assisted homogeneous
transesterification system. Besides, the ultrasound assisted Pickering emulsion shortened the reaction time
by 3.6 fold when compared to mechanical stirring assisted Pickering emulsion system.
Key words: transesterification, biodiesel, Pickering emulsion, ultrasound, response surface methodology
1 Introduction
Liquid-liquid interaction somehow lies everywhere in a
chemical engineering process. To achieve an efficient mass
transfer, tremendous efforts were applied to modify the
dispersion behavior of one phase in a continuous phase, for
example by varying the structure of stir and reactor1−3）,
using external fields or even microchannel technology4−6）.
As a valuable candidate of renewable liquid fuel, biodiesel
could be produced by transesterification of methanol and
plant oil or animal fat under the existence of homogeneous
7, 8）
. The immiscibility of
alkali catalysts
（KOH, NaOH, etc.）
raw materials, methanol and triglycerides, makes transesterification a typical biphasic reaction featuring high
mass transfer resistance9−12）. Besides, the use of homogeneous catalysts is unfavorable for environment protection,
owing to the generation of large amount of waste pollutants
during catalysts removing and neutralization process13, 14）.
Therefore, intensifying mass transfer of methanol-triglycerides biphasic systems and developing recyclable catalyst
become two key points for efficient and green production
of biodiesel15, 16）.
In recent years, solid particle stabilized emulsion
（Pickering emulsion）has been developed as an innovative and
green platform to intensify mass transfer in biphasic
systems 17−20）. Different from traditional intensification
techniques like mechanical agitation21）, Pickering emulsions do not rely on intensive and continuous energy input
to maintain the dispersion state of emulsion droplets22, 23）.
And the preparation of Pickering emulsions do not need
reactors with complicated structures. In a Pickering emulsion, amphiphilic solid particle（nano or micro scale）
emulsifiers adsorb at the interface between the two immiscible
phases, forming single or multiple protective layers that
prevent droplet coalescence through steric and viscosity
effects24, 25）. These highly dispersed droplets with small size
can prominently increase the interfacial contact area and
significantly shorten mass transfer distance22, 26）.
Compared to traditional emulsifiers（surfactants）which
are difficult to be separated, the solid particles in Pickering
emulsions can be easily recycled by centrifugation27−29）, fil-
＊
Correspondence to: Jianli Wang, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, PR
CHINA. Hao Zhang, College of Chemical Engineering, Zhejiang University of Technology, Hangzhou 310014, PR CHINA
E-mail: wangjl@zjut.edu.cn (JW), haozhang2019@zjut.edu.cn (HZ) ORCID ID: https://orcid.org/00000002-7525-465X (JW)
Accepted November 8, 2022 (received for review October 12, 2022)
Journal of Oleo Science ISSN 1345-8957 print / ISSN 1347-3352 online
http://www.jstage.jst.go.jp/browse/jos/
http://mc.manusriptcentral.com/jjocs
233
S. Zou, H. Zhang, and J. Wang
tration or magnetic attraction30, 31）. Hence, one can simultaneously achieves the goals of reaction intensification and
catalyst recycling by integrating catalytic active sites to the
solid particles. These catalytically active particles can be
termed as Pickering interfacial catalyst（PIC）32−34）, which
was widely applied for biphasic catalytic reactions（Pickering interfacial catalysis）35, 36）. For instance, amphiphilic
silica immobilized with Pd nanoparticles was employed as
PIC for hydrogenation, and five fold reaction rate enhancement was achieved in comparison to the conventional biphasic system37）. Xu et al. developed phenyl sulfonic functionalized activated charcoal and utilized these particles to
construct dodecylaldehyde/ethylene glycol Pickering emulsions for accelerating acetalization reaction. The charcoal
catalysts can be filtrated and resued for five consecutive
runs 31）. 1,1,3,3-tetramethylguanidine（ TMG）is a strong
organic base that has been utilized as a catalyst for transesterification reaction16）. Compared to alkali hydroxides,
TMG exhibits higher affinity for organic substrates（methanol and triglyceride）and has the advantage of no soap
forming in the product. To simultaneously intensify and
catalyze transesterification reaction, our group have constructed two kind of PICs by immobilizing organic base
TMG or lipase on microspheres composed of Fe3O4 core
and polystyrene（PS）shell, and these PICs can be conveniently recycled by a magnet26, 38）.
Despite the excellent performance of PIC in biphasic reactions, it should be noted that the restriction of the mobility of catalyst decreases the activity of a catalyst at some
extent3）. Ultrasound was found to be an efficient intensification technique for heterogeneous reaction systems39−41）.
In the process of ultrasonic treatment, cavities are created,
which would grow to be micro fine bubbles. The radial
motion of these cavitation bubbles would emulsify immiscible reactants owing to microturbulence effect. Besides, the
collapse of these cavitation bubbles produce intense local
heating, high pressures, as well as micro jets. The formation of micro jets not only further enhance mass and heat
transfer, but also help to maintain the catalytic activity by
refreshing the surface of a heterogeneous catalyst42, 43）. In
our previous work, we have compared the effect of power
ultrasonic and conventional mechanical stirring on an alkali-catalyzed transesterification reaction, and found that
power ultrasonic gave shorter reaction time and less
energy consumption 44）. Therefore, it is promising to
combine the effect of Pickering emulsion and ultrasound
for effectively enhancing biphasic reactions.
In this work, for the first time, we explored the synergistic effect of Pickering emulsion and ultrasound on the efficiency of an immiscible biphasic reaction, aiming at efficient
and green production of biodiesel. The transesterification
of methanol and soybean oil was selected as a model reaction. The magnetic recyclable catalysts loading with TMG
（Fe 3O 4@PS-TMG）were utilized as PICs. The response
surface methodology（RSM）based on Box-Behnken design
（BBD）was used to optimize reaction parameters of the ultrasound assisted Pickering emulsion systems. At last, the
optimized result was compared to mechanical agitation assisted Pickering emulsion and ultrasound assisted homogeneous transesterification systems, which demonstrates the
advantage of Pickering emulsion-ultrasound synergistic
method.
2 Experimental Procedures
2.1 Materials
Refined soybean oil
（molecular weight of 882 g/mol, acid
value of 0.12 mg KOH/g）
was purchased from Zhejiang Yihai
Kerry Food Industry Co., Ltd. Methanol（AR, 99.5％）was
purchased from Adamas-beta. Other materials and reagents used for the synthesis of Fe3O4@PS-TMG were obtained from Aladdin Co. Ltd. The catalysts were synthesized according to the previously reported method 26）.
shell（PS）
structured catalyst supporter
Briefly, core
（Fe3O4）
Fe3O4@PS-CH2Cl was firstly prepared through miniemulsion polymerization. Then, TMG was grafted onto the supporter by reacting with benzyl chloride groups. Molecular
structure of TMG, schematic diagram of Fe3O4@PS-TMG
and typical TEM image of the catalyst particles were shown
in Fig. 1.
2.2 Preparation of biodiesel
The experimental device is showed in Fig. 2. A certain
proportion of soybean oil, methanol and Fe3O4@PS-TMG
were added into a 25 mL round-bottomed flask, which was
placed in a water bath（LANYI-1000D, Shanghai LanYi）.
The volume of distilled water in the water bath is maintained at 2/3 of the total volume. Then, the reaction
mixture was mechanical stirred at 300 rpm for 2 min to
Fig. 1 M
olecular structure of
（a）
1,1,3,3-tetramethylguanidine
（TMG）
,（b）
schematic diagram and（c）typical TEM
image of the magnetic recyclable catalyst Fe3O4@
PS-TMG.
234
J. Oleo Sci. 72, (2) 233-243 (2023)
Ultrasound-assisted Pickering Interfacial Catalysis for Biodiesel Production
the yield becomes lower with 21:1 ratio. This is because
excessive methanol reduce the concentration of the catalyst. For response surface design, five factors was selected,
（A）, reaction temperaincluding Fe3O4 content of catalyst
ture（B）
, ultrasound power（C）, methanol to oil molar ratio
（D）and catalyst amount（E）. The factors and levels are
shown in Table 1. The experimental data obtained were
analyzed by Design Expert 10.0.7 using second-order polynomial model. The equation is as follows:
m
m
m
m
i
i
i>j
j
Y＝β 0＋ Σβ i Xi＋ Σβ ii Xi2＋ Σ Σ β ij Xi Xj
（1）
Where Y is the yield of FAME, Xi and Xj are different
factors, B0, Bi, Bj, Bii, Bij, are the coefficients of constant
term, linear term, quadratic term and interaction term, and
m is the number of factors.
Fig. 2
3 Results and Discussion
3.1 Model and analysis of variance
According to Box-Behnken design, a total of 46 experiments were conducted. The experimental results of BBD
are shown in Table 2. According to multiple regression
fitting based on Design Expert, a quadratic regression
model of selected factors was obtained:
E xperimental device for ultrasound assisted
Pickering interfacial catalysis. 1. support stand, 2.
ultrasonic water bath, 3. ultrasonic transducer, 4.
ultrasonic operating, 5. temperature controller, 6.
reactor, 7. condenser.
form Pickering emulsion. Soon afterwards, the transesterification reaction was carried out under certain temperature
and ultrasound power for a period of time. During the reaction, a small amount of sample was taken out regularly and
gas chromatography
（GC-2104C, Shimadzu Co., Ltd. Japan）
equipped with Column rtx-5
（30 m×250 μm×0.25 μm）
was
used to analyze the content of fatty acid methyl esters
（FAME）
in samples. The injector and the detector temperature are set as 240 and 250℃, respectively. Meanwhile, the
initial column temperature is 50℃.
2.3 Experiment design
According to single factor experiment results（Fig. S1 to
S4）, the FAME yield of ultrasound assisted Pickering
emulsion changed little after 150 min. So the reaction time
was fixed at 150 min. In Fig. S1, the FAME yield reaches
the highest when methanol to oil molar ratio is 18:1, and
Table 1
Yield＝89.79−9.75A＋4.36B＋4.42C＋2.90D＋4.37E−
0.75AB−0.70AC−0.17AD−0.30AE−0.22BC＋
0.47BD＋0.53BE＋1.03CD＋0.47CE＋0.18DE−
6.81A2−7.09B2−2.87C2−1.68D2−5.40E2. （2）
Table 3 listed the analysis of variance（ANOVA）results.
The accuracy and reliability of model and examining the
effects of factors on the FAME yield depends on these
results. The model F and P value represents the significance of the model. The F value of 186.53 and P value
（＜
0.0001）indicates the model have high significance. The P
value in regression model, including linear terms and quadratic terms（A, B, C, D, E, CD, A2, B2, C2, D2, E2）, are all
less than 0.05, which shows that these terms has statistical
significance at 95％ confidence interval. The lack of fit F
value is 1.12, which indicates that the model has strong
Independent variables and levels used for response surface design.
Level
Symbols
Factors
Units
Min Value
(–1)
Centre
Point (0)
Max Value
(+1)
A
Fe3O4 content of catalyst
%
30
38
46
B
reaction temperature
℃
50
55
60
C
ultrasound power
D
alcohol to oil molar ratio
E
catalyst amount
W
90
105
120
mol/mol
12
15
18
wt%
5
6.5
8
235
J. Oleo Sci. 72, (2) 233-243 (2023)
S. Zou, H. Zhang, and J. Wang
Table 2 BBD matrix and the responses of FAME yield.
RUN
Fe3O4
content/%
Reaction
temperature/℃
Ultrasound
power/W
Methanol to
oil molar ratio
Catalyst
amount/wt%
Experimental
Yield/%
Predicted
Yield/%
1
38
55
120
18
6.5
94.11
93.61
2
38
55
90
18
6.5
82.17
82.70
3
30
60
105
15
6.5
91.24
90.75
4
38
50
120
15
6.5
79.38
80.12
5
38
55
105
15
6.5
89.71
89.79
6
38
55
105
15
6.5
90.57
89.79
7
46
55
105
18
6.5
74.11
74.28
8
46
55
105
12
6.5
68.83
68.82
9
38
55
120
12
6.5
86.28
85.74
10
38
55
105
15
6.5
89.13
89.79
11
38
55
90
12
6.5
78.47
78.95
12
38
55
105
15
6.5
90.91
89.79
13
30
55
105
15
5.0
82.35
82.66
14
38
60
90
15
6.5
79.84
80.00
15
38
60
105
15
8.0
85.12
86.56
16
38
50
105
15
5.0
70.95
69.09
17
30
55
105
18
6.5
93.85
94.13
18
46
55
105
15
5.0
63.31
63.76
19
46
55
120
15
6.5
74.14
74.09
20
46
55
90
15
6.5
66.33
66.64
21
38
55
105
12
8.0
84.05
84.01
22
46
60
105
15
6.5
71.20
69.75
23
38
55
105
15
6.5
89.97
89.79
24
38
55
105
12
5.0
75.45
75.62
25
38
55
105
18
5.0
80.73
81.08
26
46
50
105
15
6.5
61.93
62.52
27
38
55
90
15
5.0
72.95
73.21
28
38
55
105
18
8.0
90.03
90.17
29
46
55
105
15
8.0
71.91
71.90
30
38
55
120
15
8.0
91.25
90.79
31
38
60
120
15
6.5
86.95
88.40
32
38
50
105
18
6.5
79.35
79.10
33
30
50
105
15
6.5
78.96
80.52
34
38
60
105
12
6.5
82.34
82.02
35
38
55
90
15
8.0
81.55
81.00
36
38
50
105
15
8.0
77.15
76.78
37
38
60
105
18
6.5
89.49
88.76
38
30
55
120
15
6.5
95.98
94.99
39
30
55
90
15
6.5
85.37
84.74
40
30
55
105
12
6.5
87.87
87.97
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J. Oleo Sci. 72, (2) 233-243 (2023)
Ultrasound-assisted Pickering Interfacial Catalysis for Biodiesel Production
Table 2
Continued.
RUN
Fe3O4
content/%
Reaction
temperature/℃
Ultrasound
power/W
Methanol to
oil molar ratio
Catalyst
amount/wt%
Experimental
Yield/%
Predicted
Yield/%
41
38
60
105
15
5.0
76.82
76.77
42
38
50
105
12
6.5
74.07
74.22
43
38
55
105
15
6.5
88.47
89.79
44
30
55
105
15
8.0
92.15
92.00
45
38
50
90
15
6.5
71.37
70.82
46
38
55
120
15
5.0
80.76
81.11
Table 3 Variance analysis of the quadratic regression model.
Source
Sum of
squares
Degrees of
freedom
Mean square
F-value
P-value
Model
3344.75
20
167.24
186.53
＜0.0001
A
1521.20
1
1521.20
1696.65
＜0.0001
B
304.85
1
304.85
340.01
＜0.0001
C
313.29
1
313.29
349.42
＜0.0001
D
135.02
1
135.02
180.60
＜0.0001
E
305.29
1
305.29
340.50
＜0.0001
AB
2.27
1
2.27
2.53
0.1245
AC
1.96
1
1.96
2.19
0.1518
AD
0.12
1
0.12
0.14
0.7148
AE
0.36
1
0.36
0.40
0.5321
BC
0.20
1
0.20
0.23
0.6387
BD
0.87
1
0.87
0.98
0.3329
BE
1.10
1
1.10
1.23
0.2780
CD
4.26
1
4.26
4.76
0.0388
CE
0.89
1
0.89
1.00
0.3278
DE
0.12
1
0.12
0.14
0.7148
2
405.23
1
405.23
451.97
＜0.0001
B2
439.32
1
439.32
489.99
＜0.0001
2
71.72
1
71.72
79.99
＜0.0001
2
24.49
1
24.49
27.31
＜0.0001
2
E
254.25
1
254.25
283.58
＜0.0001
Residual
22.41
25
0.90
Lack of fit
18.34
20
0.95
1.12
0.4938
0.82
A
C
D
Pure Error
4.08
5
Total
3367.16
45
fitting ability and is not remarkable compared with the
2
of determination）is a statistical
pure error. R（coefficient
measure of the degree of the regression line approaching
the actual data points. The R2 value（0.9933）implied second-order quadratic model can explain more than 99.33％
of the output response changes. At the same time the pre-
dicted R2 value（0.9765）is almost consistent with the adjusted R 2 value（ 0.9882）, which demonstrated that the
model has a good fitting degree. The Adeq Precision value
is based on the signal-to-noise ratio（S/N）
. The S/N（50.748）
is greater than 4, indicating that the signal is sufficient.
Figure 3 shows the predicted value and experimental value
237
J. Oleo Sci. 72, (2) 233-243 (2023)
S. Zou, H. Zhang, and J. Wang
sion, because emulsion stability directly affect the interfacial area. The higher mean square value of factor A relative
to C indicate that the stability of Pickering emulsion is the
major influence factor, while ultrasound plays an assistant
role in the reaction system.
Fig. 3
Experimental and predicted value of FAME yield.
of FAME yield. It can be seen that they are close to each
other. The results of ANOVA showed the obtained model is
accurate and reliable.
The mean square value of each factor can reflect its influence on FAME yield. Among the five factors, the mean
square value of factor A is the largest, followed by C, E, B,
and D, which means that Fe3O4 content of catalyst is the
most critical factor. In PIC particles, the Fe3O4 nanoparticles do not act as catalytic active sites, but as a magnetic
responsive component and density controller. Previously,
we demonstrated that PIC density plays pivotal role in the
stability of static triglyceride/methanol Pickering emulsions
（especially at elevated temperature）45）, owing to non-negligible effect of gravity on catalyst adsorption at the triglyceride-methanol interface. Here, the Fe3O4 content influence
the FAME yield by affecting the stability of Pickering emul-
Fig. 4
3.2 Interaction effects between parameters
Figure 4a shows the effects of ultrasound power and
Fe3O4 content of the catalyst on FAME yield. When Fe3O4
content lies in the range of 30％ to 42％, the yield of FAME
increases rapidly with the increase of ultrasound power.
When the Fe3O4 content further increases, the contour line
becomes almost parallel to the ultrasound power axis,
which indicates that the ultrasound power has little effect
on the FAME yield. Such phenomenon can be due to that
the density of the PIC is too high when the Fe3O4 content
is higher than 42％. In this case, the influence of low power
ultrasound on the adsorption stability of catalyst particles
might be much smaller than that of gravity field, which will
cause destabilization of the Pickering emulsion, resulting in
a substantial decrease of the interface area of triglyceridemethanol biphasic systems. Figure 4b exhibits the interaction effect of temperature and Fe3O4 content of the catalyst. At low Fe3O4 content, the yield of FAME increases
with the increase of temperature and the decrease of Fe3O4
content. The most optimized reaction temperature and
Fe3O4 content is 56.82℃ and 31.61％ respectively. Further
reducing the Fe3O4 content can not improve the yield of
FAME, but make the recovery of the catalyst more difficult.
At high Fe3O4 content, the effect of temperature on the
FAME yield significantly reduced, which is consistent with
that of the interaction of ultrasound power and Fe 3O 4
content.
The interaction of catalyst amount and temperature on
FAME yield is shown in Fig. 5a. With the increase of catalyst amount, the yield of FAME first increased and then
slightly decreased. The excess PIC particles might hinder
mass transfer due to multi layer adsorption at triglyceridemethanol interface. For low catalyst amount, the yield of
Response 3-D surface plots showing the interaction effect of（a）ultrasound power and Fe3O4 content of the catalyst,
（b）
temperature and Fe3O4 content of the catalyst.
238
J. Oleo Sci. 72, (2) 233-243 (2023)
Ultrasound-assisted Pickering Interfacial Catalysis for Biodiesel Production
Fig. 5
Response 3-D surface plots showing the interaction effect of（a）
catalyst amount and temperature,（b）methanol to oil
molar ratio and temperature.
Fig. 6
Response 3-D surface plots showing the interaction effect of
（a）
methanol to oil molar ratio and ultrasound power,（b）
catalyst amount and ultrasound power.
FAME is very low even if increasing the reaction temperature. On the one hand, less amount of catalyst means less
active sites. On the other hand, a low PIC particles amount
normally leads to a larger droplet size and lower emulsion
stability, which will reduce the interfacial area for the reaction. Figure 5b represents the interaction of alcohol to oil
molar ratio and temperature on FAME yield. The FAME
yield first increase and then decrease with the increase of
temperature. At low temperature, FAME yield change little
even if increasing the methanol to oil molar ratio, which indicates that temperature is crucial for the reaction. When
the molar ratio is higher than 16:1, the contour line is
nearly parallel to the molar ratio axis, suggesting that the
methanol to oil molar ratio is high enough. Further improving the amount of methanol contribute little to improving
reaction rate.
Figure 6a illustrates the interaction of methanol to oil
molar ratio and ultrasound power on FAME yield. The
higher the methanol to oil molar ratio and ultrasound
power, the more significant the increase of FAME yield. Increasing the molar ratio of methanol to oil and ultrasound
power are beneficial to the formation of more cavitation
bubbles, thus enhancing mass transfer43）. When the methanol to oil molar ratio is large enough, partial methanol vaporization caused by higher ultrasound power has little
effect on the state of emulsion and mass transfer rate of
the whole system, thus the reaction system can maintain a
high reaction rate. Figure 6b shows the effect of catalyst
amount and ultrasound power on FAME yield. At low catalyst amount, increasing ultrasound power can not obtain a
high FAME yield, which can be due to few active sites and
unstable Pickering emulsion. When catalyst amount is large
enough, the synergistic effect of Pickering emulsion and
ultrasound greatly improves the transesterification rate,
leading to a high yield of FAME.
3.3 Optimum reaction conditions
By using the numerical optimization method of Design
Expert, the optimum reaction conditions were predicted as
follows: Fe3O4 content of 31.61％, reaction temperature of
56.82℃, ultrasound power of 119.33 W, methanol to oil
molar ratio of 16:1, and catalyst amount of 7.24 wt％.
Under these conditions, the maximum theoretical yield of
FAME was 98.69％. Considering the convenience of operation, the actual conditions for transesterification were
chosen as follows: Fe3O4 content of 32％, reaction temperature of 57℃, ultrasound power of 120 W, methanol to oil
molar ratio of 16:1, and catalyst amount of 7.24 wt％.
Under this condition, three experiments were carried out
and the average FAME yield is 98.10％, which is close to
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J. Oleo Sci. 72, (2) 233-243 (2023)
S. Zou, H. Zhang, and J. Wang
the predicted value, indicating that the regression model is
reliable for predicting FAME yield.
3.4 Ultrasound assisted versus stirring assisted Pickering
interfacial catalysis
To compare the transesterification efficiency of ultrasound assisted Pickering interfacial catalysis and traditional
stirring assisted Pickering interfacial catalysis systems, the
yield of FAME over time using both methods were depicted
in Fig. 7. The yield of FAME using PIC and ultrasound is
over 98.10％ in 2.5 h, while it needs 9 h to obtain a comparable yield when PIC was combined with mechanical stirring. The higher reaction efficiency in the case of ultrasound assisted Pickering emulsion can be ascribed to that
ultrasound is more efficient relative to mechanical stirring
in terms of promoting mass transfer44, 46）. The growth and
burst of cavitation bubbles during ultrasonic treatment
generate shockwaves, local turbulence and microjets 47）,
which can destroy the phase interface and increase the interfacial area, thus reducing the mass transfer resistance.
Besides, the impact of shockwaves or microjets on PIC particles can refresh phase interface and create active
surface42）. The catalyst particles may also slip at the triglyceride-methanol interface under impact force, which
can promote the contact of reactants with catalyst active
sites.
Fig. 7
C omparison of FAME yield using（ a）PIC and
ultrasound,（b）PIC and mechanical stirring,（c）
TMG and ultrasound respectively. The amount of
catalyst is 7.24 wt％ for PIC and 0.415 wt％ for
TMG, the other conditions are as followed: reaction
temperature of 57℃, ultrasound power of 120 W,
stirring rate of 300 rpm, methanol to oil molar ratio
of 16:1.
3.5 Pickering emulsion-ultrasound synergistic method
versus ultrasound
In order to further confirm the synergistic effect of Pickering emulsion and ultrasound, we also performed ultrasound assisted homogeneous transesterification using TMG
（Fig. 7c）. The amount of TMG is controlled to be equivalent to that of active sites of PIC
（Fe3O4@PS-TMG）
. Generally, the catalytic activity of homogeneous catalyst is better
than that of heterogeneous catalyst because of its good
dispersion in reactants. However, as shown in Fig. 7, the
FAME yield of PIC in 2.5 hours is over two times higher
than that of homogeneous catalyst TMG. The main reason
is due to that ultrasound alone is unable to maintain the
emulsified state of the triglyceride-methanol biphasic
system, especial when the ultrasound power is relatively
low and there is no extra emulsifier. In this case, stratification（TMG is soluble in methanol but insoluble in soybean
oil）
of the biphasic system leads to the low reaction rate. As
for PIC-ultrasound system, the formation of Pickering
emulsion can increase the interfacial area for hundreds of
thousands of times, and shorten the diffusion distance of
reactants22）. The ultrasound treatment not only help to
maintain a stable Pickering emulsion, but also improve activity of The PIC and further accelerate mass and heat
transfer. Therefore, the synergistic effect of Pickering
emulsion and ultrasound can significantly promote the reaction rate of triglyceride-methanol biphasic system.
4 Conclusion
In summary, we have demonstrated a synergistic intensification method for biodiesel production by Pickering
emulsion and ultrasound. The magnetic recyclable
nanoparticles loading with TMG were utilized as PICs for
transesterification between methanol and soybean oil. The
reaction conditions, such as composition of catalyst, reaction temperature, ultrasound power, methanol to oil molar
ratio and catalyst amount, were optimized by response
surface methodology（RSM）based on Box-behnken design
（BBD）. A quadratic multiple regression mathematical
model with FAME yield as response value was established.
And the analysis of variance（ANOVA）implied the model
can make good prediction on fatty acid methyl ester
（FAME）yield. The optimal transesterification conditions
are as follows: Fe3O4 content of 31.61％, reaction temperature of 56.82℃, ultrasound power of 119.33 W, methanol to
oil molar ratio of 16:1, and catalyst amount of 7.24 wt％.
Under the optimal conditions, a FAME yield over 98％
could be achieved in 2.5 h, which shortened the reaction
time by 3.6 fold when compared to mechanical stirring assisted Pickering emulsion system. Furthermore, the synergistic effect makes the catalytic efficiency of ultrasound
assisted Pickering emulsion higher than that of ultrasound
240
J. Oleo Sci. 72, (2) 233-243 (2023)
Ultrasound-assisted Pickering Interfacial Catalysis for Biodiesel Production
assisted homogeneous transesterification using TMG. This
work provides a novel strategy for green and efficient production of biodiesel, which is also believed to be useful in
other immiscible biphasic reaction systems.
Acknowledgment
This work was supported by National Nature Science
Foundation of China（Grants 22178317 and 22109138）,
Zhejiang Provincial Natural Science Foundation of China
（LY18B040004）and China Postdoctoral Science Foundation（2020M671790）
.
Author Contributions
Siyuan Zou: Investigation, validation, writing-original
draft. Hao Zhang: Supervision, data analysis, writingreview and editing. Jianli Wang: Conceptualization, resources, funding acquisition.
Conflict of Interest
The authors declare no conflict of interest.
Supporting Information
This material is available free of charge via the Internet
at doi: 10.5650/jos.ess22340
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