Preparation of glycerol monostearate from glycerol carbonate and stearic acid

Article abstract of DOI:10.1039/c6ra02912d

The chemical equilibrium for the preparation of glycerol monostearate (GMS) from glycerol carbonate (GC) and stearic acid (SA) was investigated. The chemical equilibrium constant K of the base-catalyzed synthesis of GMS from GC and SA was much smaller than that of the acid-catalyzed synthesis of (2-oxo-1,3-dioxolan-4-yl) methyl stearate (ODOMS) from GC and SA. In other words, it was thermodynamically difficult to obtain GMS with a high yield from GC and SA catalysed by basic catalysts. To prove this argument, we used magnesium oxide (MgO) as a catalyst to synthesize GMS from GC and SA. As expected, the yield of GMS was quite low. To increase the yield of GMS, a two-step procedure was proposed. First, pure ODOMS was synthesized by the esterification of GC with SA using copper p-toluenesulfonate (CPTS) as the catalyst. The conversion of SA reached 96.14% under the following conditions: reaction temperature, 140 °C; catalyst amount, 3% CPTS (based on the SA weight); reaction time, 3 h; GC-to-SA molar ratio, 1.5:1. Second, GMS was produced at a yield of 64.4% by the hydrolysis of ODOMS in the presence of triethylamine. The syntheses of ODOMS and GMS were confirmed by ¹H- and ¹³C-NMR, FTIR and LC-MS analysis.

Full text of DOI:10.1039/c6ra02912d

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PAPER
Preparation of glycerol monostearate from glycerol
carbonate and stearic acid
Cite this: RSC Adv., 2016, 6, 34137
*
Li Han and Tao Wang
The chemical equilibrium for the preparation of glycerol monostearate (GMS) from glycerol carbonate
(GC) and stearic acid (SA) was investigated. The chemical equilibrium constant K of the base-catalyzed
synthesis of GMS from GC and SA was much smaller than that of the acid-catalyzed synthesis of
(2-oxo-1,3-dioxolan-4-yl) methyl stearate (ODOMS) from GC and SA. In other words, it was
thermodynamically difficult to obtain GMS with a high yield from GC and SA catalysed by basic
catalysts. To prove this argument, we used magnesium oxide (MgO) as a catalyst to synthesize GMS
from GC and SA. As expected, the yield of GMS was quite low. To increase the yield of GMS, a two-step
procedure was proposed. First, pure ODOMS was synthesized by the esterification of GC with SA using
copper p-toluenesulfonate (CPTS) as the catalyst. The conversion of SA reached 96.14% under the
following conditions: reaction temperature, 140 ꢀC; catalyst amount, 3% CPTS (based on the SA
weight); reaction time, 3 h; GC-to-SA molar ratio, 1.5 : 1. Second, GMS was produced at a yield of
64.4% by the hydrolysis of ODOMS in the presence of triethylamine. The syntheses of ODOMS and
Received 1st February 2016
Accepted 28th March 2016
DOI: 10.1039/c6ra02912d
1
www.rsc.org/advances
GMS were confirmed by H- and 13C-NMR, FTIR and LC-MS analysis.
glycidol, but the industrialization of this process is restricted by
the high cost of glycidol. The reaction of glycerol acetonide with
stearic acid ester to produce GMS had also been reported.16
However, the hydrolysis procedure of the intermediate “ester-
ketal” to remove the acetone is tedious. Other synthetic routes
for preparing GMS have utilized lipase-catalyzed reactions17 and
microwave irradiation.18
1. Introduction
Glycerol monostearate (GMS), which is the glycerol mono-
ester of stearic acid, and features a specic hydrophilic head
and a hydrophobic tail, is an important non-ionic surfactant
with low hydrophile–lipophile balance (HLB). This
compound is particularly valuable for use in environmentally
benign “water-in-oil” emulsions.1 Therefore, GMS is widely
used as an emulsiers, emollients, lubricants and disper-
sants in such applications as food and feedstuff,2,3
cosmetics,4 pharmaceutical formulations,5 plasticizers6 and
drug delivery systems.7,8
GMS is conventionally prepared by the esterication of
glycerol (Gly) with stearic acid (SA)9 and transesterication of
Gly with natural fats and oils10,11 or stearic acid methyl ester.12 In
most cases, these reactions are usually catalyzed by homoge-
neous catalysts, which are either basic (such as metal hydrox-
ides, methyl oxides and inorganic carbonates)13 or acidic
(such as sulfuric acid, phosphoric acid and organic sulfonic
acids).14 The main drawback of these conventional synthetic
routes is their low GMS selectivity. Their products are mixtures
of mono-, di- and triglycerides. In recent years, more selective
routes have been explored. For example, glycidol was reacted
with stearic acid anhydride at 85 ꢀC for 5 h to synthesize GMS at
a yield of 62%.15 The yield of GMS can be improved by using
As a potential chemical intermediate, glycerol carbonate
(GC) has many desirable properties, such as low toxicity, low
volatility and low ammability.19 Researchers have applied GC
to synthesize GMS by triethylamine-catalyzed reaction with SA
at 140 ꢀC for 10 h (Scheme 1), claiming that this method
produces pure GMS.20 Although this claim has been cited in
a number of review articles,21–25 no further study had been re-
ported. Mhanna et al.23 noted that the reaction of GC and SA
utilizing tetrabutylammonium iodide as a basic catalyst
produced GMS at only 14% yield aer 24 h of reaction. The
preparation of (2-oxo-1,3-dioxolan-4-yl) methyl stearate
(ODOMS) from GC and SA in the presence of acidic catalyst has
also been reported.26 ODOMS possesses excellent physico-
chemical properties, such as good thermal stability and
oxidation stability, good surfactant properties and high
biodegradability.27 Based on a mechanism similar to that of the
hydrolysis of GC in alkaline solutions,28,29 we supposed that
GMS might be formed by the hydrolysis of ODOMS in alkaline
solutions.
To the best of our knowledge, the chemical equilibrium for
the synthesis of GMS from GC and SA has not yet been char-
acterized. In this work, we determined the chemical
State Key Laboratory of Chemical Engineering, Department of Chemical Engineering,
Tsinghua University, Beijing, 100084, China. E-mail: taowang@tsinghua.edu.cn;
Fax: +86 10 62784877; Tel: +86 10 62784877
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2. Calculation of the chemical
equilibrium constant
The chemical equilibrium constants at standard condition
(T ¼ 298.15 K, P ¼ 101 325 Pa) were calculated as follows.
Firstly, the functional group method of Benson30 was adopted to
estimate the gas phase standard molar enthalpy of formation
DfHqm,g (eqn (1)) and molar entropy Smq ,g (eqn (2)) of GC, SA,
ODOMS and GMS. The standard molar enthalpy of evaporation
DVHqm (eqn (3)) was obtained by using the functional group
method of Ducros.31 Subsequently, the liquid phase standard
molar enthalpy of formation DfHqm,l and liquid phase standard
molar entropy Sqm,l can then be acquired by combing eqn (4)
Scheme 1 GMS formation from GC and SA.
and (5).
X
Df Hmq ;g
¼
niDf Hiq;g
(1)
(2)
(3)
i
X
Smq ;g
¼
niSiq;g ꢁ R ln d þ R ln h þ Scqorrection
i
Scheme 2 Esterification of GC with SA to form ODOMS.
X
DVHmq
¼
niDVHiq
i
DfHqm,l ¼ DfHqm,g ꢁ DVHqm
DVHmq
(4)
(5)
Smq ;l ¼ Smq ;g
ꢁ
T
where, R is the mole gas constant; d ¼ dindext is the symmetry
number of the molecule; din is the internal symmetry number of
the molecule; dext is the external symmetry number of the
molecule; h is the number of enantiomers; Sqcorrection, the cyclic
correction factor, is obtained from the literature.32 The data for
H2O and CO2 were acquired from http://webbook.nist.gov/
chemistry.
Next, eqn (6) and (7) were applied to calculate the change of
the standard molar enthalpy DrHqm and entropy DrSmq of the
reaction.
Scheme 3 Hydrolysis of ODOMS to form GMS.
equilibrium constants about the preparation of GMS from GC
and SA by two synthetic routes. In the rst route, GMS was
directly synthesized by GC and SA using a basic catalyst
(Scheme 1). The chemical equilibrium constants calculated
for GMS synthesis from GC with SA at different temperatures
revealed that the production of GMS by this route (Scheme 1)
was not thermodynamically favorable. Moreover, magnesium
oxide (MgO) was used as a basic catalyst to synthesize GMS
from GC and SA for the experimental verication. The second
route was a new two-step procedure that we proposed. In this
procedure, ODOMS was rst produced as an intermediate by
the esterication of GC with SA using copper p-toluenesul-
fonate (CPTS) as a catalyst (Scheme 2) in the solvent-free
system. The subsequent hydrolysis of ODOMS could
produce GMS in the presence of a basic catalyst (Scheme 3).
The chemical equilibrium constants calculated for the
formation of ODOMS from GC and SA (Scheme 2) at different
temperatures indicated that this synthesis was feasible when
catalyzed by an acidic catalyst. Moreover, the synthesis was
conducted to verify the results derived from the calculated
chemical equilibrium constants. The syntheses of ODOMS
and GMS were conrmed by 1H- and 13C-NMR, FTIR and
LC-MS analyses.
X
X
DrHmq
¼
niDf Hmq ;i
ꢁ
niDf Hmq ;i
(6)
reactant
product
X
X
DrSmq
¼
niSmq ;i
ꢁ
niSmq ;i
(7)
reactant
product
Then, the change of the molar Gibbs free energy DrGqm and
the chemical equilibrium constant K of the reaction can be
obtained from DrHqm and DrSqm by eqn (8) and (9).
DrSmq
DrGmq ¼ DrHmq ꢁ T ꢂ
DrGqm ¼ ꢁRT ln Kq
(8)
(9)
1000
The liquid-phase heat capacities Cp,m,l of GC, SA, GMS and
ODOMS were estimated by the functional group method of
Ruzicka–Domalski (eqn (10)).33 This method requires the cyclic
alkylene carbonate correction factor for Cp,m,l because GC and
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ODOMS are cyclic compounds. Because the correction factor for basic catalyst for the GMS synthesis. For the ODOMS synthesis
cyclic alkylene carbonate has not yet been reported, it was according to Scheme 2, the catalyst was CPTS, which was
replaced by that of 1,3-dioxa-cyclopentane. DrCp,m can be prepared with the procedure reported in the literature.34
counted by eqn (11). DrHm for different temperatures can then
be obtained by DrHqm and eqn (12). Finally, eqn (13) and (14) can
3.3 Hydrolysis of ODOMS
be used to calculate DrGm and the chemical equilibrium
The same experimental setup was utilized for the hydrolysis of
constant K at different temperatures under atmospheric pres-
ODOMS. ODOMS (1 g), ultra-pure water (20 g), and 6.0% trie-
sure can be calculated.
thylamine (based on the water weight) were placed into the ask
(
)
ꢀ
ꢁ
ꢀ
ꢁꢀ
ꢁ
2
​
​
​
X
X
X
and heated to the reaction temperature. Aer the termination of
the reaction by cooling, the hot petroleum ether was added into
the reaction mixture, which was then reuxed and allowed to
stand, which led to the separation of two liquid phases. The
white powder product was obtained by the crystallization from
the upper liquid phase.
T
T
Cp;l
¼
niai þ
nibi
þ
nici
ꢂ R
(10)
100
100
X
X
DrCp;m
¼
niCp;m;i
ꢁ
niCp;m;i
(11)
reactant
product
dDrHm
dT
3.4 Analysis
¼ DrCp;m
(12)
(13)
The acid value of the solid sample was determined according to
a previously reported procedure.35 The difference in the acid
value of the solid sample before and aer the reaction was
measured to determine the conversion of SA, as:
ꢂ
ꢃ
dðDrGm=TÞ
dT
DrHm
T2
¼ ꢁ
P
ꢀ
ꢁ
acid value after reaction
acid value before reaction
DrGm(T,P) ¼ ꢁRT ln K
(14)
SA conversion ¼ 1 ꢁ
ꢂ 100%
(15)
where, DrCp,m is the change of the molar heat capacity of the
reaction; DrHm is the change of the molar enthalpy of the
reaction; DrGm is the change of the molar Gibbs free energy of
the reaction.
The purity of GMS was determined according to a previously
reported procedure.36 The melting points of ODOMS and GMS
3. Experimental section
3.1 Chemicals and materials
Table 1 Standard molar enthalpy of formation and standard mole
entropy of the substances involved in the reactions of GC with SAa
Glycerol carbonate (>90%) was purchased from Tokyo Chemical
Industry Co. Ltd. Stearic acid (98%) was purchased from TCI
Shanghai Co. Ltd. Petroleum ether (analytical grade), sodium
chloride (NaCl, analytical grade), triethylamine (analytical
grade) were provided by Beijing Tongguang Fine Chemical
Company. Magnesium oxide (MgO, analytical grade) was
purchased from Tianjin Guangfu Fine Chemical Research
Institute. Methanol was purchased from Anhui Fulltime Special
Solvents & Reagents Co., Ltd. All chemicals were used without
further purication.
Phase
DfHqm/(kJ molꢁ1
)
Sqm/(J Kꢁ1 molꢁ1
)
GC
SA
GMS
ODOMS
H2O
CO2
Liquid
Liquid
Liquid
Liquid
Gas
ꢁ839.16
ꢁ896
257.65
433.2
493.27
710.03
188.84
213.78
ꢁ1298.82
ꢁ1456.57
ꢁ241.83
ꢁ393.5
Gas
DfHqm, the change of the standard molar enthalpy of formation;
Sqm, standard molar entropy. The values for GC, SA, GMS and ODOMS
were obtained by the functional group method of Benson.30 The
values for H2O and CO2 were acquired from the literature.37
a
3.2 Synthesis of GMS and ODOMS from GC and SA
SA (0.025 mol) and 3.0% catalyst (based on SA weight) were
placed in a 100 mL, three-neck, round-bottom ask, which was
immersed in an oil bath and equipped with a thermometer and
a pressure-equalizing addition funnel. GC (0.0375 mol) was
added drop-wise to the molten SA at the required temperature.
Aer the reaction was terminated by cooling to 60 ꢀC, the
saturated NaCl solution and petroleum ether, which had been
heated to 60 ꢀC, were added. The reaction mixture was then
reuxed and allowed to stand for resulting two liquid layers.
Table 2 Thermodynamic data for the reactions of GC with SA at the
standard condition (T ¼ 298.15 K, P ¼ 101 325 Pa)a
DrHqm/
DrSqm/
DrGqm/
(kJ molꢁ1
)
(J Kꢁ1 molꢁ1
)
(kJ molꢁ1
)
Kq
Scheme 1
Scheme 2
42.84
36.76
16.20
208.02
38.01
ꢁ25.31
2.19 ꢂ 10ꢁ7
2.72 ꢂ 104
DrHqm, the change of the standard molar enthalpy of reactions;
a
The upper layer, which is the petroleum ether solution con- DrSqm, the change of the standard molar entropy of reactions;
DrGqm, the change of the standard of molar Gibbs free energy of
taining the product, was then separated by decantation.
Following crystallization, the product was obtained as white
reactions; Kq, the chemical equilibrium constant at the standard
condition.
solid powder. According to Scheme 1, MgO was used as the solid
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Table 3 Heat capacity of all substances involved in the reactionsa
Cp,m/(J Kꢁ1 molꢁ1) ¼ a + b(T/K) + c(T/K)2
(FT-IR Tensor 27, Bruker, Germany) over the wavenumber range
of 400 to 4000 cmꢁ1 at room temperature. The 1H- and 13C-NMR
spectra were recorded on nuclear magnetic resonance (NMR,
JNM-ECA 300, JEOL, Japan). The molecular weight was
measured by liquid chromatography-mass spectrometer
(LC-MS, QTrap 4500, AB SCIEX, USA, mobile phase: 90%
methanol and 10% water, ow rate: 1.0 mL minꢁ1).
a
b ꢂ 102
c ꢂ 104
GC (l)
SA (l)
GMS (l)
ODOMS (l)
H2O (g)
CO2 (g)
6.972
ꢁ99.04
ꢁ2.914
ꢁ47.64
ꢁ47.08
56.32
43.02
17.63
30.42
43.05
450.1
630.9
400.6
29.16
26.75
ꢁ0.02022
4.226
ꢁ0.1425
4. Results and discussion
4.1 Thermodynamic analysis
a
Cp,m is the molar isobaric heat capacity. a, b, c are coefficients in the
equation for Cp,m. The heat capacities Cp,m of H2O (g) and CO2 (g)
were obtained from the book.37 The heat capacities of other
substances mentioned in the above table were obtained by Ruzicka–
Domalski functional group method.34
The standard molar enthalpies of formation DfHqm and standard
molar entropies Sqm of the substances involved in the reactions
of GC with SA are listed in Table 1. The changes of the standard
molar enthalpy DrHmq , entropy DrSqm as well as the standard
chemical equilibrium constants K of the reactions calculated
according to eqn (6) and (7) are listed in Table 2. As can be seen
from Table 2, the standard chemical equilibrium constant at
298.15 K, 101325 Pa for the reaction of GC with SA to form GMS
(Scheme 1) in the presence of a basic catalysts is the fairly low
value of 2.19 ꢂ 10ꢁ7, and the change of the molar Gibbs free
energy DrGqm (38.01 kJ molꢁ1) is greater than zero. This nding
indicates that the synthesis of GMS from GC and SA (Scheme 1)
is unfeasible under standard conditions. Meanwhile, the stan-
dard chemical equilibrium constant for the formation of
ODOMS from GC and SA (Scheme 2) in the presence of an acid
catalyst is the extremely high value of 2.72 ꢂ 104, and the
Table 4 Chemical equilibrium constant K for Schemes 1 and 2 at
different temperature
T/(K) (P ¼ 101 325 Pa)
383.15
393.15
403.15
413.15
Scheme 1
Scheme 2
1.28 ꢂ 10ꢁ5
6.51 ꢂ 105
1.87 ꢂ 10ꢁ5
8.49 ꢂ 105
2.65 ꢂ 10ꢁ5
10.9 ꢂ 105
3.68 ꢂ 10ꢁ5
13.6 ꢂ 105
change of the molar Gibbs free energy DrGqm is ꢁ25.31 kJ molꢁ1
.
were measured by using differential scanning calorimetry (DSC,
Q 2000, TA, USA). Samples were sealed in an alumina pan and
heated from 0 to 100 ꢀC at a rate of 5 ꢀC minꢁ1 in puried
nitrogen atmosphere. The functional groups were identied by
mixing the samples with KBr, pressing them into pellets, and
recording their Fourier transmittance infrared spectrometer
This nding indicates that the synthesis of ODOMS from GC
and SA is thermodynamically favorable under standard
conditions.
The heat capacity, Cp,m of the various substances calculated
by the functional group method of Ruzicka–Domalski33 (eqn
Fig. 1 FTIR spectra of ODOMS.
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Fig. 2 1H NMR spectra of ODOMS.
(10)) are listed in Table 3. The chemical equilibrium temperature, 140 ꢀC; catalyst amount, 3% MgO (based on the SA
constants K calculated by eqn (11)–(14) are listed in Table 4. It weight); reaction time, 10 h; GC-to-SA molar ratio, 1.5 : 1.
can be seen from Table 4 that the chemical equilibrium Mhanna et al.23 reported that the synthesis from GC and SA in
constant K of the reaction of GC with SA to form GMS the presence of tetrabutylammonium iodide produced GMS at
(Scheme 1) increased with increasing temperature. However, only 14% yield aer 24 h of reaction. Our experimental results
the value of K remained quite small, even at temperature of thus agree with both this previous nding and our thermody-
up to 413.15 K. From a thermodynamics perspective, it was namics calculations.
difficult to obtain high yields of GMS from GC and SA by
SA conversion of up to 96.14% was obtained for the
using a basic catalyst. This nding was also consistent with synthesis of ODOMS according to Scheme 2 under the
our experimental results, too. In contrast, the chemical following reaction conditions: reaction temperature, 140 ꢀC;
equilibrium constant K of the reaction to form ODOMS from catalyst amount, 3% CPTS (based on the SA weight); reaction
GC and SA (Scheme 2) was considerably high, ranging from time, 3 h; GC-to-SA molar ratio, 1.5 : 1. The product, white
6.51 ꢂ 105 to 13.6 ꢂ 105 as the temperature increased from solid powder with a melting point of 75 C, was identied as
ꢀ
1
383.15 to 413.15 K. This nding indicated that the synthesis pure ODOMS by LC-MS, FTIR, and H and 13C NMR analyses.
of ODOMS from GC and SA catalyzed by an acidic catalyst was This experimental result, namely, that pure ODOMS could be
feasible and that higher temperatures were more favorable prepared in high yield from GC and SA in the presence of an
for the reaction.
acid catalysts was consistent with the thermodynamic
analysis.
The results of molecular weight analysis of ODOMS by
+
4.2 Experimental results
LC-MS analysis were as follows: LC-MS calculated for C22H40O5
¼ 385; found: 385. Fig. 1 displays the FTIR spectrum of ODOMS.
GMS was prepared in a yield of 10.0% from GC and SA (Scheme
1) under the following reaction conditions: reaction
IR (KBr): nmax ¼ 2919.6, 2850.3, 1783.8, 1736.2, 1471.5 cmꢁ1
.
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Fig. 3 13C NMR spectra of ODOMS.
Fig. 4 FTIR spectra of GMS.
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Fig. 5 1H NMR spectra of GMS.
The FTIR spectra of ODOMS includes the characteristic
Fig. 4 shows the FTIR spectrum of GMS. IR (KBr): nmax
¼
absorption peaks at 1784 cmꢁ1 and 1736 cmꢁ1, which 3315.7 (OH), 2918.7 and 2850.88 (CH), 1734.8 (C]O), 1179.6 and
correspond to the stretching vibrations of endocyclic 1050.0 (C–O) cmꢁ1. Fig. 5 and Fig. 6 present the 1H and 13C NMR
ester functions and exocyclic ester functional group,27 spectra of GMS, respectively. 1H NMR (300 MHz, CDCl3): d ¼ 0.88
respectively.
(t, J ¼ 6.8 Hz, 3H, –CH3), 1.26 (m, 28H, CH2), 1.63 (m, 2H, CH2),
Fig. 2 and 3 present the 1H and 13C NMR spectra of ODOMS, 2.36 (t, J ¼ 7.5 Hz, 2H, CH2), 3.60 (dd, J ¼ 6.0, 9.0 Hz, 1H, CHOH),
respectively. 1H NMR (300 MHz, CDCl3): d ¼ 0.89 (t, J ¼ 7.1 Hz, 3.69 (dd, J ¼ 6.0, 12.0 Hz, 1H, CHOH), 3.93 (m, 1H, CH), 4.19 (m,
3H, –CH3), 1.29 (t, J ¼ 7.6 Hz, 2H, –CH2CO–), 4.29 (dd, J ¼ 4.1, 2H, CH2O); 13C NMR (300 MHz, CDCl3): d ¼ 14.1 (CH3), 22.7
12.6 Hz, 1H, –CH2OCO2–), 4.35 (dd, J ¼ 7.3 Hz, 1H, –CH2OCO–), (CH2), 24.9 (CH2), 29.1–29.7 (12, CH2), 31.9 (CH2), 34.2 (CH2),
4.39 (dd, J ¼ 3.3, 12.6 Hz, 1H, –CH2OCO2–), 4.58 (t, J ¼ 8.6 Hz, 63.4 (CH2O), 65.2 (CH2O), 70.3 (CHOH), 174.4 (C]O).
1H, –CH2OCO–), 4.93 (m, 1H, CH); 13C NMR (300 MHz, CDCl3):
The result of the molecular weight analysis of GMS by LC-MS
+
d ¼ 14.1, 22.7, 24.8, 29.1–29.7, 31.9, 33.9, 62.8, 66.0, 73.8, 154.3, analysis was as follows: LC-MS calculated for C21H42O4 ¼ 359;
173.3 ppm. The results were consistent with those reported by found: 359.
K. Mojgan.21 Thus, it was reasonable to conclude that the
product is ODOMS.
The white solid product has a GMS mass fraction of 67% and
a melting point of 78 ꢀC. The yield of GMS based on SA was
GMS was obtained by the hydrolysis of ODOMS (Scheme 3) 64.4%. The yield was not high under the present conditions
ꢀ
under the following conditions: hydrolysis temperature, 80 C; because the hydrolysis was conducted in a liquid–liquid two
catalyst amount, 6% triethylamine (based on water weight); phase system. Because ODOMS is insoluble in water, the yield of
hydrolysis time, 1.5 h; ODOMS-to-water weight ratio, 1 g : 20 g. GMS may be limited by interphase mass transfer. The use of
The obtained GMS was identied by LC-MS, FTIR, and 1H- and co-solvents or phase transfer catalysts might improve the yield
13C-NMR.
of GMS.
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Fig. 6 13C NMR spectra of GMS.
acidic catalyst (Scheme 2) and then hydrolyzed to yield GMS in
the presence of basic catalyst (Scheme 3).
5. Conclusions
In this work, the chemical equilibrium about the synthesis of
GMS from GC and SA was studied. The chemical equilibrium
constant K for the reaction of GC and SA to synthesize GMS
(Scheme 1) increased with the increase of temperature, but it
remained quite small, even at temperatures as high as 413.15 K.
From a thermodynamics perspective, it is difficult to obtain
a high yield of GMS from GC and SA using a basic catalyst
(Scheme 1). This result was veried experimentally. The yield of
GMS obtained by the direct reaction of GC with SA (Scheme 1)
was quite low when magnesium oxide (MgO) was used as the
catalyst. However, the chemical equilibrium constant K for the
reaction of GC with SA to synthesize ODOMS (Scheme 2) was
considerably high, reaching 13.6 ꢂ 105 at 413.15 K. This nding
indicated that the formation of ODOMS from GC and SA was
thermodynamically very favorable, and was veried experi-
mentally. Pure ODOMS and a high conversion of SA could be
obtained using CPTS as the catalyst. ODOMS could be further
hydrolyzed to form GMS when triethylamine was used as the
catalyst. A two-step procedure for synthesizing GMS from GC
and SA was proposed, in which ODOMS as an intermediate was
rstly produced by the esterication of GC with SA using an
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