Microwave-Assisted Synthesis and Characterization of Stearic Acid Sucrose Ester: A Bio-Based Surfactant

Article abstract of DOI:10.1002/jsde.12280

Sugar esters of fatty acids are bio-based surfactants that were synthesized by transesterification of table sugar (sucrose) with stearic acid methyl ester (SAME) in high yield and purity using an optimized microwave-assisted method. The maximum surfactant product yield of 88.2% was obtained by combining reactants with potassium methoxide and irradiating the mixture with microwaves to achieve an ideal reaction temperature of 132 °C over 21 min. The synthetic procedure described herein minimized undesirable sugar pyrolysis as evidenced by retention of white color characteristic of a pure bio-based surfactant product. The experiment implemented Box–Behnken design for response surface methodology to refine reaction parameters for optimal product yield. The following parameters were augmented: the irradiation time, the mole ratio of catalyst to sucrose, and temperature.

Full text of DOI:10.1002/jsde.12280

J Surfact Deterg (2019)
DOI 10.1002/jsde.12280
ORIGINAL ARTICLE
Microwave-Assisted Synthesis and Characterization of Stearic
Acid Sucrose Ester: A Bio-Based Surfactant
1
1
Narasimharao Kondamudi · Owen M. McDougal
Received: 14 July 2018 / Revised: 27 January 2019 / Accepted: 26 February 2019
2019 AOCS
©
Abstract Sugar esters of fatty acids are bio-based sur-
factants that were synthesized by transesterification of
table sugar (sucrose) with stearic acid methyl ester
Introduction
Sucrose is one the most abundantly produced organic com-
pounds in the world (Koo and Taylor, 2013). It is one of
the principal precursors for industrially significant com-
pounds including sugar esters (SE) of fatty acids (Chang
and Shaw, 2009). SE are used extensively in the food
industry, and as components of cosmetics, detergents (Neta
et al., 2015), oral-care products, and medical supplies
(SAME) in high yield and purity using an optimized
microwave-assisted method. The maximum surfactant
product yield of 88.2% was obtained by combining reac-
tants with potassium methoxide and irradiating the mix-
ture with microwaves to achieve an ideal reaction
ꢀ
temperature of 132 C over 21 min. The synthetic proce-
dure described herein minimized undesirable sugar pyroly-
sis as evidenced by retention of white color characteristic
of a pure bio-based surfactant product. The experiment
implemented Box–Behnken design for response surface
methodology to refine reaction parameters for optimal
product yield. The following parameters were augmented:
the irradiation time, the mole ratio of catalyst to sucrose,
and temperature.
(
Heirwegh et al., 1989; Szuts and Szabo-Revesz, 2012). SE
are tasteless, odorless, nontoxic, nonirritating to the eyes
and skin, and biodegradable. Especially, in the surfactant
industry, SE have been used to form oil in water (o/w)
emulsions, enhance solubility, and improve penetration of
drugs through skin, because of their wide ranging hydro-
phobic lyophobic balance (HLB) values (1–16), which can
be controlled by varying the fatty acid (Benito et al., 2010;
Nakayama et al., 2015; Okamoto et al., 2011; Szuts and
Szabo-Revesz, 2012; van Kempen et al., 2014).
Keywords Sugar ester ꢁ Microwave-assisted synthesis ꢁ
Bio-based surfactant ꢁ Response surface methodology
Reported methods for SE synthesis are either traditional
oil bath syntheses or enzymatic conversion in combination
with ionic liquids or supercritical CO . These synthetic
J Surfact Deterg (2019).
2
methods suffer from prolonged reaction times and/or low
product yield (Habulin et al., 2008; Sabeder et al., 2006;
Tai and Brunner, 2009). An additional ubiquitous problem
with current approaches to synthesize SE is discoloration of
the products due to sugar pyrolysis. Crude ester products
are often bleached to obtain a white-colored or less-colored
SE (Glasser and Jain, 1993). The undesirable dark colora-
tion is attributable to thermal instability of the product,
excess reaction heat, and the residual catalyst (Wang,
Supporting information Additional supporting information may be
found online in the Supporting Information section at the end of the
article.
*
Owen M. McDougal
owenmcdougal@boisestate.edu
2
007). A major challenge for SE synthesis is the choice of
1
solvent that will simultaneously dissolve both the nonpolar
fatty acid and the polar sugars. Examples of solvents that
Department of Chemistry and Biochemistry, Boise State
University, 1910 University Drive, Boise, ID 83725, USA
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fit these criteria include pyridine, dimethyl sulfoxide
The goal of the current investigation was to optimize
reaction conditions to balance the solvent properties with
ideal reaction temperature to minimize pyrolysis, maximize
yield, and ensure desired reactant miscibility. Here, we
describe utilization of microwave-assisted SE synthesis in
two consecutive reactions, i.e., converting stearic acid into
stearic acid methyl ester (SAME), which is then reacted
with sucrose to obtain SASE (Scheme 1). The process
parameters were optimized using Box–Behnken design.
(DMSO), and dimethyl formamide (DMF).
Microwave-assisted chemical reactions in food chemistry
are common (Jensen et al., 2007; Shanker et al., 2011).
Microwaves are a form of nonionizing electromagnetic
energy at frequencies between 300 MHz and 300 GHz.
This energy is transmitted as waves, which can interact
with polar molecules to generate heat (Gregory et al.,
1981). Conventional heating uses conduction, convection
phenomena to transfer heat, whereas microwaves generate
internal heat, thereby minimizing heat loss to the environ-
ment (Wang, 2007). Microwave-assisted processes are
closed systems; therefore, heat loss due to conduction is
minimized. The unique feature of heating by microwaves
significantly reduces the reaction time as compared to con-
ventional processes, such as hot plate or jacketed heating
methods (Bekdeser et al., 2014).
Experimental
Materials
Stearic acid (C H O , ACS grade, Sigma-Aldrich, St. Louis,
18 36 2
MO, USA), methanol (CH OH, Sigma-Aldrich, HPLC grade),
3
Response surface methodology is a compilation of statis-
tical and mathematical methods that are useful for model-
ing and analyzing engineering problems (Hinkelmann and
Jo, 1998). Various process parameters can be optimized
using one of the many response surface techniques.
Response surface methodology computes the relationship
between the convenient input parameters and the achieved
response surfaces (Aslan and Cebeci, 2006; Ferreira
et al., 2007).
sulfuric acid (H SO , 99.999%, Sigma-Aldrich, USA), calcium
2
4
chloride (CaCl , ACS grade, Sigma-Aldrich, USA), table sugar
2
(WinCo Foods, Boise, ID, USA), potassium methoxide
(KOCH , Sigma-Aldrich, 95%), dimethyl formamide (DMF,
3
Fluka, Morristown, NJ, 99%), acetonitrile (HPLC grade,
Fischer Scientific, Pittsburgh, PA), butanol (ACS grade,
Fluka), and acetone (ACS grade, Fluka) were used as received.
Methods
In this work, we have utilized the Box–Behnken method
to study the combined effects of time, temperature, reactant
ratio, and amount of potassium methoxide on stearic acid
sucrose ester (SASE) product yield. The Box–Behnken
method has superior accuracy due to, a spherical, revolving
and linear trend free design. A second-order model is gen-
erated from the experimental responses by response surface
methodology in central composite design (CCD). More-
over, this method is used for all the systems with an effec-
tive curvature in the response.
SAME and SASE were synthesized using a 400 W capac-
ity microwave reactor (MARS 5, CEM Corporation, USA).
Synthesis of SAME Using Microwave-Assisted Reactor
Stearic acid was converted to SAME using a microwave-
assisted, acid-catalyzed esterification reaction according to
the method reported by Reilly (et al., 2014). Stearic acid
(14.2 g, 50 mM) was added to an excess of methanol
H SO
2
4
C COOH + CH OH
C COOCH
3
1
7
3
17
1
20 °C, 100 W, 3 minutes
Stearic Acid Methyl Ester (SAME)
Sucrose
Stearic Acid Sucrose Ester (SASE)
Scheme 1 Steps involved in microwave assisted synthesis of stearic acid sucrose ester
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(
15 mL) in a 100 mL Ultra Prep Teflon microwave vessel
equipped with a stir bar. Concentrated sulfuric acid
0.25 mL) was cautiously added before the reaction vessel
Table 2 Experimental design and response value
Run no.
X
1
X
2
X
3
Yield
(
1
2
−1
−1
1
−1
0
0
−1
0
18.5
16.4
68.4
41.6
61.4
41.3
81.6
76.3
77.6
37.8
39.2
81.6
72.1
39.4
81.4
was securely capped. The reaction vessel was heated from
ꢀ
room temperature to 120 C over 3 min. The temperature
ꢀ
3
−1
0
was held at 120 C for 3 min using a power of 100 W. A
4
1
−1
1
subsequent 20 min cool-down period resulted in a solution
ꢀ
5
0
−1
1
temperature of less than 55 C. The reaction mixture was
6
−1
0
0
allowed to cool to room temperature providing separation
of the SAME, as a floating solid on aqueous methanol solu-
tion. The solid was recovered by filtration and gently
7
0
0
8
0
1
1
ꢀ
9
1
0
1
heated to 65 C to liquefy. The SAME was washed with
ꢀ
10
0
1
−1
−1
0
warm water (40–50 C) to remove any traces of sulfuric
11
12
13
14
15
0
−1
0
acid and methanol. The purified SAME was dried over
anhydrous calcium chloride followed by stearic acid SE
synthesis as described below.
0
1
1
0
−1
0
0
1
0
0
Synthesis of SASE Using a Microwave-Assisted Reactor
analysis was performed using UPLC coupled to a charged
aerosol detector (CAD). The purified monosubstituted
SASE was used as a standard to quantify the yield under
different experimental conditions. The term “yield” used
here is defined according to Eq. (1).
A typical batch reaction mixture consisting of 6 mM
sucrose and 2 mM SAME was dissolved in 20 mL of
DMF. The reaction mixture was transferred to a sealed
100 mL Ultra Prep Teflon microwave vessel equipped
with a stir bar for the synthesis of sucrose monoesters of
stearic acid (Osipow et al., 1956). Reaction parameters
were optimized for the weight percent of the catalyst with
respect to sucrose, time, and temperature (Table 1 and
Table 2). The reaction vessel was heated in the microwave
reactor, with a 4 min ramp period to reach the desired reac-
tion temperature, followed by a 20 min cool-down period
Yield of SASE
ꢀ
ꢁ
Amount of SASE formed during the experiment
=
Theoritical amount of SAME for 100%conversion
*100
ð1Þ
ꢀ
to less than 55 C. The reaction mixture was washed with
three portions of hexane to remove unreacted SAME. DMF
was removed by a rotatory evaporator. The SASE residue
was dissolved in a two phase mixture of 20 mL water and
Fourier Transform Infra-Red Spectroscopy (FTIR)
2
0 mL butanol. The butanol layer containing the dissolved
FTIR spectra were recorded on a Perkin Elmer Spectrum
100 spectrometer equipped with a universal attenuated total
reflectance (ATR) sampling accessory. The dry SAME,
SASE, sugar, and stearic acid samples were analyzed in the
solid form on the ATR device. FTIR spectra were obtained
SASE was collected and dried over calcium chloride. Buta-
nol was evaporated using a rotary evaporator. The residue
was further extracted with methanol to recover exclusively
monosubstituted SASE (Kurzin et al., 2007).
Qualitative analysis of the product was conducted using
ultrahigh performance liquid chromatography–mass spec-
troscopy (UPLC–MS) equipped with an atmospheric pres-
sure chemical ionization (APCI) detector. A quantitative
−1
by recording 64 scans between 4000 and 650 cm with a
−
1
resolution of 4 cm .
Ultrahigh Performance Liquid Chromatography
Table 1 Parameter levels and coded values used in the experimental
design
The UPLC analyses were performed using UPLC (Dionex
Ultimate 3000, Thermo Scientific, USA) equipped with a
Acclaim surfactant plus column (3 μm particle size with
column dimensions 2.1 μm × 150 mm), and a two phase
solvent system (mobile phase A: 0.1 M ammonium acetate;
mobile phase B: acetonitrile) at a flow rate of 0.3 mL min.
The injection volume was 5 μL and the eluent (reaction
mixture) was analyzed by CAD (Dionex Corona veo RS) at
Variables
Symbol
Range and levels
0
−1
+1
Time (minutes)
X
X
X
1
2
3
12
120
5
18
130
7.5
24
140
10
ꢀ
Temperature ( C)
%
Catalyst w.r.t. sucrose
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ꢀ
2
1
9
5 C. The HPLC gradient method was as follows: 0 to
Results and Discussion
Synthesis
min at 5% B, 1 to 10 min at 50% B, 10 to 23 min at
0% B, and 23 to 25 min at 5% B. Yield calculations were
based on the area under the SAME peaks at
Rt = 19–22 min.
The field of microwave-assisted chemistry has experienced
a growth due to energy efficiency (Cho et al., 2014), reac-
tion selectivity (Khan and Jhung, 2015), and benefits of
uniform reactant heating (Singh et al., 2015). Moreover,
microwave-assisted syntheses prevent undesirable side
reactions and increase the purity of the final product. Syn-
thesis of fatty acid methyl esters (biodiesel) using
microwave-assisted technology is well documented
(Kumar et al., 2012; Muley and Boldor, 2013). The syn-
thetic process reported here provides a superior method for
the conversion of stearic acid into SAME. The purity of
1
Proton–Nuclear Magnetic Resonance ( H NMR)
Spectroscopy
1
H NMR data were obtained on a Bruker Avance III
600 MHz spectrometer and processed with TopSpin soft-
ware. NMR solvents were D O (Sigma-Aldrich, 99.8%),
DMSO-D6 (Sigma-Aldrich, 99.9%), or CDCl3 (Acros
Organics, 99.8%).
2
1
SAME was confirmed using H NMR spectroscopy
(
Fig. S1) and segued well into the raw material SE synthe-
Experimental Design Using Response Surface
Methodology (RSM)
1
sis. The H NMR spectrum of sucrose, prior to SASE syn-
thesis, is provided in Fig. S2.
A 3:1 M ratio of sugar to SAME was fixed based on the
literature reports. Higher amounts of sugar closer to its sol-
ubility limit led to undesirable discoloration of the final
SASE product (Fig. S3). Newly formed product (sugar
esters) molecules, being more soluble in the solvent, further
decrease sugar solubility. When the sugar concentration
exceeds its solubility limit, the sugar precipitates. At higher
temperature, the precipitated sugar caramelizes, causing the
dark brown coloration to the SE products. Sugar concentra-
A three-level Box–Behnken design with categorical factor
was employed to optimize the microwave reactor condi-
tions for SASE synthesis. The design was composed of
three levels (low, medium, and high) and a total of 15 runs
were executed to optimize the variables of time, tempera-
ture, and weight percent of the catalyst with respect to
sucrose (Table 2). For the purpose of statistical calcula-
tions, the three variables were denoted as X , X , and X ,
respectively. The range for parameter settings was derived
from the results of preliminary experiments. The speed of
the magnetic stirrer in the reactor system was maintained
constant and the reaction was conducted under autogenous
pressure, therefore, these effects were not considered in the
model. The mathematical relationship between the three
variables and response was approximated by the second-
order polynomial equation (Eq. 2):
1
2
3
tions were kept to <50% of their solubility point (14.1 g at
ꢀ
3
0 C) to eliminate discoloration of the SAME products.
The maximum yield of SASE obtained using microwave-
assisted synthesis was 88.2% at an optimum irradiation
time of 21 min and 8.78 wt.% potassium methoxide cata-
lyst. A comparison of the method presented here for the
SAME synthesis with procedures based on the enzymatic
and chemical processes in literature is shown in Table 3.
The microwave-assisted process presented here is 3 to
X
X
X
2
Y = a0 +
aixi +
aiix +
aijxixij
ð2Þ
i
6
7 times faster and provides superior yields of SASEs in
where Y is the predicted response by the model, i.e., the
nearly all cases reported. Additional advantages of the
microwave-assisted method were selectivity to produce
solely the monoester of SASE and the elimination of unde-
sirable product discoloration.
yields of SASE; X , X , and X are the independent vari-
1
2
3
ables, temperature, time, and catalyst weight percent with
respect to sucrose, respectively; a is an equation constant;
0
and a , a , and a are regression coefficients of the model.
i
ii
ij
The accuracy and general ability of the second-order multi-
ple regression models were then evaluated by the coeffi-
cient of determination, a.k.a. the R value. The coefficients,
Characterization
2
FTIR spectroscopy was used to establish the existence of
an ester linkage. The IR spectrum of stearate is labeled 1 in
Fig. 1. Stearic acid when deprotonated will have character-
istic absorbance bands for sp hybridized C-H stretching
between 2860 and 2960 cm , and a carbonyl absorbance
at around 1560 cm . Spectrum 2 represents SAME
(Reactant 1), which has a nearly identical spectrum to the
parent stearate with a slight shift in carbonyl absorbance to
a , and two interaction parameters a , and a were esti-
i
ii
ij
mated from the experimental results. The MINITAB (ver-
sion 17.2.1) software was used for regression analysis of
the experimental data and the response. The quality of the
multiple regression model fit was represented by the coeffi-
3
−
1
−
1
2
cient of regression R and an F-test was used to check the
statistical significance.
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Table 3 Comparison of microwave-assisted sugar ester synthesis with reported processes
ꢀ
Methods
Reaction time
Temperature ( C)
Catalyst
Yield (%)
Supercritical CO
2
+ Enzymatic (Habulin et al., 2008)
24 h
72 h
72 h
80
60
60
lipase
67
82
60
Enzymatic (Sabeder et al., 2006)
Candida Antarctica B
CAL B, ionic liquid
Ionic liquid + Enzymatic (Ganske and
Bornscheuer, 2005)
High pressure CO
2
acetone system
4 h
50
Novozyme 435
24
(Enzymatic) (Tai and Brunner, 2009)
Heating (Osipow et al., 1956)
Heating (Chortyk et al., 1996)
Ultrasound (Huang et al., 2010)
Heating (Kurzin et al., 2007)
Microwave (this work)
3 h
1 h
60
65
Potassium methoxide
pyridine
25
85–90 (all types of SE)
2 h
70
K
2
CO
CO , KOH
Potassium Methoxide
3
≥72
72
20 h
21 min
90
K
2
3
130
88.2
−
1
three absorbance bands at 1739, 1560, and 1466 cm , cor-
responding to the C O stretching of the methylated car-
boxyl groups, and to asymmetric and symmetric stretching
4
3
. SASE (product)
−
modes of carboxylate (COO ), respectively. For SASE, the
. Sucrose (Reactant 2)
. SAME (Reactant 1)
. Stearic acid
appearance of a new band in the C O ester stretching
−
1
region (1739 cm ), which overlaps with the SAME ester
−
1
2
1
band (1741 cm ), was observed. UPLC analysis of the
reaction mixture (Fig. 2) showed formation of a new peak,
and the APCI-MS spectrum shows the principle signal at
6
05 m/z (Fig. 3) consistent with SASE monoester. Forma-
tion of the monosubstituted ester was further confirmed
5
50
1050
1550
2050
2550
3050
3550
4050
4550
-
1
Wave number (cm )
4
Fig. 1 FTIR comparison of the reactants and product
3
2
1
0
.5
3
Sucrose
a higher wavenumber and appearance of a C-O stretch
.5
2
−
1
circa 1100 cm . The difference between sucrose
Spectrum 3) and SASE (Spectrum 4) is easier to see, due
(
1. Stearic acid
. Pure stearic acid methyl ester
.5
1
to the appearance of the ester carbonyl stretch observed at
around 1600 cm− for the SASE (Fig. 1). The spectral data
were studied for characteristic absorbance in the following
regions: O H stretch of free hydroxyl in sucrose
2
1
.5
0
3
. Reaction mixture
10
0
5
15
20
25
−
1
−1
(
3420–3500 cm ); ester C O (1740–1750 cm ); glyco-
Time (minutes)
−
1
sidic bond stretch for sucrose (995 cm ); and C O,
−1
C O C stretch (1052, 1105 cm ). The infrared analysis
of the SASE revealed the main changes occurred in spectral
regions corresponding to chemical modifications intro-
duced to sucrose and SAME. In addition, a decrease of the
O H stretching band (3500–3200 cm ) was observed as
expected due to the decrease in intensity for the hydroxyl
group upon reaction with SAME. The reduction in the
O H peak intensity was complemented by a corresponding
increase in peak absorbance in the C H stretching region
and the appearance of well-defined bands at 2850 and
Fig. 2 UPLC chromatograms of reactants and products
605.92
−1
−
1
2
917 cm , indicative of the hydrocarbon chain from stea-
5
00
550
600
650
700
750
800
850
900
950
1000
ric acid. The region featuring the ester carbonyl groups
m/z
−1
(
1750–1350 cm ) is where more diagnostic information
can be found. The FTIR spectrum of the SAME showed
Fig. 3 APCI-MS spectrum of reaction products
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2
8.00
HOD
5
.69
DMSO-d
6
3
.24
1
.02
1
.01
1
.95
1
.13
1.08
1.19
1
.04
1.02
2
.2
1
Fig. 4 H NMR spectrum of SASE
1
2
using H NMR integration analysis (Fig. 4). The NMR sig-
nals of the purified product were assigned by comparing
the spectra of individual SAME and sucrose reactants
The quality of the fitted model was expressed by an R
value, which gives the proportion of the total variation in
the response predicted by the model; a high R value
2
(Fig. S1 and Fig. S2).
(closer to 1) is desirable. Eq. (2) demonstrates that the
Process Optimization
Table 5 Analysis of variance model regression on SASE yield
The optimum conditions for synthesis of SASE were deter-
mined by means of the Box–Behnken diagram under RSM.
The results are displayed in Table 4. In this way, the rela-
tion among individual parameters for the yield of SASE
was expressed using the following equation (Eq. (3)):
Source
DF
Adj. SS
MS
F
P
Regression
Linear
9
3
7502.69
4586.61
2716.20
199.88
58.92
833.63
1528.87
905.40
66.63
11.78
19.63
0.01
70.74
128.73
76.83
5.65
0.000
0.000
0.000
0.046
—
Square
3
Y = − 2240 + 32:41x1 + 28:70x2 + 21:9x3 − 0:5748x2
Interaction
Residual error
Lack-of-fit
Pure error
Total
3
1
5
—
2
2
2
3
−
0:1077x − 2:735x − 0:0796x1x2 + 0:217x1x3
3
58.90
1472.44
—
0.001
—
+
0:1630x x3
2
2
0.03
ð3Þ
14
7561.62
—
—
—
2
2
R = 99.22%, (R adj) = 97.82%.
where, X , X , and X represent time, temperature, and cata-
lyst weight percent with respect to sucrose, respectively.
DF, degrees of freedom; SS, sum of squares; MS, mean of squares; F,
F-value; P, significance level of P values (a significance level
is ˂0.5).
1
2
3
Table 4 Estimated regression coefficients and the corresponding sta-
tistical t- and P- values for the SASE yield
100
Term
Coefficient factor
t
P
8
6
4
0
0
0
Constant
81.53
18.01
5.00
41.14
14.84
4.12
0.000
0.000
0.009
0.000
0.000
0.002
0.000
0.039
0.117
0.064
X
X
X
1
2
3
14.96
−20.69
−10.77
−17.09
−4.78
3.25
12.33
−11.58
−6.03
−9.57
−2.78
1.89
X
X
X
X
X
X
1
*X
2
*X
3
*X
1
*X
1
*X
2
*X
1
2
3
2
3
3
20
0
0
20
40
60
80
100
Yield (Experimental)
4.07
2.37
Fig. 5 Plot of the experimental and predicted responses
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(
a)
(b)
(c)
Fig. 6 Response surface plot showing the predicted values of the yield of SASE: (a) effect of time and wt.% catalyst, (b) time and temperature,
and (c) temperature and wt.% catalyst on the yield. Other variables are held at constant level
2
model is well-fitted, as evidenced by an R value equal
to 0.97.
relationship between experimental and predicted values for
the response, and the underlying assumptions of the above
analysis were appropriate.
The surface plots of the response functions were useful in
understanding both the main interaction effects of the fac-
tors. The response surface and contour plots given in Figs. 6
and 7 show the relative effects of any two variables when
the other variables are kept constant. From this model, the
optimum values for individual parameters were deduced.
The ANVOA table was used to test the statistical signifi-
cance of the ration of mean square due to regression and
mean square to residual error. The estimated effects and
coefficients for the model are listed in Table 5. Model
terms were evaluated by a P-value (probability) with
9
7.82% confidence. The P-values were used to estimate
whether F was large enough to indicate statistical signifi-
cance and used to check the importance of each coefficient.
The P-values lower than 0.05 indicated that the model and
the model terms were statistically meaningful. All factors
and their square interactions (P < 0.05) were significant at
the 97% confidence level.
Conclusion
Data points were well distributed close to a straight line
R = 0.99) between experimental and predicted values
Fig. 5). The corresponding data for the experimental and
predicted values are tabulated and given in the supporting
information (Table S1). These results suggest the excellent
A microwave-assisted synthetic procedure for obtaining
fatty acid SE was described. Stearic acid SE were used as a
model compound. The reported procedure uses microwave
energy for activation, offers cleaner, more efficient, and
faster processes for SE fatty acid synthesis with respect to
2
(
(
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(
a)
(b) 140
2
2
2.5
0
1
1
35
30
1
1
7.5
5
1
1
25
20
1
2.5
5
6
7
8
9
10
125
15
17.5
20
22.5
Wt. % catalyst
Time (min)
(
c) 140
1
1
35
30
1
1
25
20
5
6
7
8
9
10
Wt. % catalyst
Fig. 7 Contour plot showing the predicted yield of SASE: (a) effect of time and wt.% catalyst, (b) time and temperature, and (c) temperature and
wt.% catalyst on the yield. Other variables are held at constant level
traditional oil bath synthesis. Moreover, the microwave
irradiation, allowing a uniform heating throughout the sam-
ple, leads to less by-products and reagent (sucrose) decom-
position, thus increasing the yield of the final products.
The linear, square, and interactive effects of process
parameters, namely, the reaction time, the catalyst to
sucrose mole ratio, and temperature on the yield of stearic
acid sucrose ester were optimized. A graphical response
surface method was used to locate optimum reaction condi-
tions. A fractional factorial design and CCD were applied
to establish a second-order model relating to the transesteri-
fication of SAME with sucrose. F-tests and P-values show
the parameters have a significant influence on the yield of
SASE. The optimum parameters for the batch reaction of
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