Solventless Catalytic Etherification of Glycerol Using Acetate Salts as Efficient Catalysts

Article abstract of DOI:10.1002/bkcs.11460

Diglycerol (DG) and triglycerol (TG) were synthesized via dehydrative etherification reaction of glycerol without solvent. The catalytic activities of alkali metal-acetate salts (LiOAc, NaOAc, and KOAc) for the etherification reaction of glycerol were investigated. Compared to the stronger basic catalysts, the less basic catalysts showed higher selectivity of DG and TG. A half mol % of LiOAc exhibited excellent selectivities of DG and TG as 52.5 and 30.4%, at 260 °C and 6 h. The dependence of reaction on the relative basicity and loading amount of catalyst, reaction time, and reaction temperature were investigated to elucidate the characteristics of etherification reaction of glycerol.

Full text of DOI:10.1002/bkcs.11460

Article
DOI: 10.1002/bkcs.11460
BULLETIN OF THE
J. H. Lee et al.
KOREAN CHEMICAL SOCIETY
Solventless Catalytic Etherification of Glycerol Using Acetate Salts as
Efficient Catalysts
Ji Hyun Lee,† Seo Kyoung Park,† Jung-Bok Ryu,‡ Hyunjoo Lee,§ and Je Seung Lee†,
*
†Department of Chemistry and Research Institute of Basic Sciences, Kyung Hee University, Seoul 02447,
Republic of Korea. *E-mail: leejs70@gmail.com
‡R&D Center, C-tri, 48 Geodudanji2-gil, Dongnae-myeon, Chuncheon 24398, Republic of Korea
§Clean Energy Center, Korea Institute of Science and Technology, Seoul 02792, Republic of Korea
Received February 13, 2018, Accepted March 29, 2018
Diglycerol (DG) and triglycerol (TG) were synthesized via dehydrative etherification reaction of glycerol
without solvent. The catalytic activities of alkali metal-acetate salts (LiOAc, NaOAc, and KOAc) for the
etherification reaction of glycerol were investigated. Compared to the stronger basic catalysts, the less
basic catalysts showed higher selectivity of DG and TG. A half mol % of LiOAc exhibited excellent
selectivities of DG and TG as 52.5 and 30.4%, at 260 ꢀC and 6 h. The dependence of reaction on the rela-
tive basicity and loading amount of catalyst, reaction time, and reaction temperature were investigated to
elucidate the characteristics of etherification reaction of glycerol.
Keywords: Glycerol, Dehydrative etherification, Diglycerol, Triglycerol, Acetate
Introduction
In this work, we report the improved yields and selectiv-
ity of DG and TG in the etherification reaction of glycerol
using sodium acetate (NaOAc) or potassium acetate
(KOAc) as a catalyst. The key feature of this work is con-
trolling the reactivity of the etherification reaction by
adjusting the basicity of the catalyst to obtain the enhanced
selectivity of DG and TG. We also investigated the effect
of reaction temperature, reaction time and the amount of
catalyst in the reaction to enhance the yields and selectivity
of DG and TG.
Glycerol is one of the plentiful renewable resources for var-
ious chemical intermediates. Recently, glycerol has been
produced in large quantities as a side-product during the
production of biodiesel which could be prepared by the
transesterification of triglyceride and methanol in the pres-
ence of catalyst.1–3 Various methods for the transformation
of surplus glycerol to more value-added materials such as
epichlorohydrine,4,5 glycerol carbonate,6,7 glycidol,8,9 and
ally alcohol10–12 have been reported.
Condensed glycerols including diglycerol (DG) and tri-
glycerol (TG) which have been used as additives in phar-
maceuticals, foods, cosmetics and also used as plasticizer,
lubricant, dispersant, stabilizer, and wetting agent13–16 in
various fields were prepared by the reaction of glycerol and
epichlorohydrine or glycidol industrially. However, the pro-
duction of DG and TG using epichlorohydrine or glycidol
has suffered by generating large amounts of salt as a by-
product or high cost of raw material. To overcome the
drawbacks of commercial methods, the etherification reac-
tion of glycerol using base catalysts such as NaOH, KOH,
CsOH, and Na2CO3 has been reported to convert the glyc-
erol to DG and TG, directly.3,17–19 Although, however, the
etherification reaction of glycerol is a very simple reaction,
the direct condensation reaction of glycerol needs harsh
reaction condition. Moreover, this reaction is not easy to
control because it is difficult to cease at two or three glyc-
erol molecules.20,21 For these reasons, DG and TG prepared
by the direct etherification reaction of glycerol showed poor
yield and selectivity due to the production of various side-
products including oligomeric and polymeric compounds.22
Experimental
Materials.
Glycerol, NaOH, KOH, lithium acetate
(LiOAc), sodium acetate (NaOAc), potassium acetate
(KOAc), t-butanol, sulfuric acid (98 wt.%, HPLC grade),
and H2O (HPLC grade) were purchased from Sigma-
Aldrich Co. (Yongin, Korea). Water (HPLC grade) was
purchased from J. T. Baker Co. (Seoul, Korea). All the che-
micals were used as obtained without further purification.
Etherification Reaction of Glycerol. The etherification
reactions of glycerol were performed usinꢀg the proper
amount of catalyst at the range of 220–280 C and atmo-
spheric pressure. Glycerol (50 g) was put into a round bot-
tomed flask (100 mL) with magnetic stirring bar. The flask
was placed on a heating mantle equipped with a tempera-
ture controller. The water generated during the etherifica-
tion reaction was removed using a Dean-Stark apparatus.
Analysis of Reaction Mixtures. The reaction mixtures
were analyzed using the high performance liquid chroma-
tography (HPLC) equipped with an Aminex HPX-87H
(Bid Rad) column and the reflective index (RI) detector.
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KOREAN CHEMICAL SOCIETY
The aqueous solution of H2SO4 (5 mM) was used as
mobile phase with a flow rate of 0.6 mL min−1. To quanti-
tatively analyze the amount of glycerol, DG, and TG in the
reaction mixture, each compound were calibrated using the
aqueous solution of respective authentic sample with
known concentration in the presence of t-butanol as an
internal standard.
Results and Discussion
The direct condensation reactions of glycerol were per-
formed in the presence of base catalyst at an ambient pres-
sure. The water generated as a by-product during the
reaction was removed by Dean-Stark apparatus. A compari-
son of base catalysts used in the etherification reaction of
glycerol including the previously reported results performed
by other groups was shown in Table 1. The catalysts which
have relatively strong basicity, LiOH, NaOH, KOH, CsOH,
and Na2CO3 (pKb = 3.67), showed high conversions of
glycerol but low selectivities of DG and TG. Meanwhile,
the catalysts which have relatively weak basicity,23,24
NaHCO3 (pKb = 10.33), LiOAc (pKb = 6.79), NaOAc
(pKb = 6.58), and KOAc (pKb = 6.10), exhibited lower
conversion of glycerol and higher selectivities of DG and
TG than which have stronger basicity implying that the fur-
ther etherification reaction of DG or TG was not effectively
proceeded by the weak bases.17,25,26
The effects of reaction temperature in the presence of
catalyst were investigated by ꢀvarying the reaction tempera-
ture in the range of 200–280 C. As shown in Figure 1, all
catalysts showed very low conversion of glycerol but high
ꢀ
selectivity of DG at 200 and 220 C. With elevating the
reaction temperature, however, the conversion of glycerol
drastically increased and the selectivity of DG decreased
with increasing that of TG and other products. The selectiv-
ities of TG showed maximum as 30.4, 31.9, and 25.1% for
LiOAc, NaOAc, and KOAc, respectively, at 260 ꢀC and
the selectivity of other products kept increasing indicating
that DG was produced at early stage of the reaction. TG
Figure 1. The dependence of reaction temperature on the etherifi-
cation reaction of glycerol in the presence of (a) LiOAc,
(b) NaOAc, and (c) KOAc (
Selectivity of DG, : Selectivity of TG,
others, the amount of catalyst: 0.5 mol% and reaction time: 6 h).
: Conversion of glycerol,
:
: Selectivity of
Table 1. Etherification reactions of glycerol with base catalysts at 260 ꢀC.
Selectivity (%)
Catalyst
Amount of catalyst (mol %)
Reaction time (h)
Conversion of glycerol (%)
DG
TG
Others
Ref.
25
Na2CO3
NaHCO3
LiOHa
CsOH
NaOH
KOH
1.74
0.22
7.7
0.18
0.50
0.50
0.50
0.50
0.50
8
8
6
8
6
6
6
6
6
96
75
100
74
83.8
91.9
66.7
72.8
82.6
24
27
33
32
24.5
18.9
52.5
38.7
32.8
35
12
ND
21
19.5
14.5
30.4
31.9
25.1
41
61
ND
47
26
3
17
56
this work
this work
this work
this work
this work
66.6
17.1
29.4
42.1
LiOAc
NaOAc
KOAc
a
Reaction temperature = 240 ꢀC.
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Solventless Catalytic Etherification of Glycerol
KOREAN CHEMICAL SOCIETY
would be generated by the etherification reaction of pro-
duced DG and unreacted glycerol. Other products including
tetra- and longer oligomeric ethers were resulted from the
further etherification reaction of produced DG or TG with
glycerol, DG or TG existing in the reaction mixture as
reported.3,22
observed as 31.9 and 26.6%at 6 and 4 h, respectively. The
selectivity of other products also increased slowly with
increasing the reaction time and the increasing rate were
corresponding to the basicity of catalyst. The conversion of
glycerol was inversely proportional to the selectivity of DG
because the increased concentration of DG in the reaction
mixture could be subjected on further dehydration reaction
which gave oligomeric ethers. As shown in Figures 1 and
2, the catalyst which had higher basicity showed the higher
conversion of glycerol but exhibited the lower selectivities
of DG and TG than the catalyst which had lower basicity
suggesting that the further dehydrative etherification reac-
tion of produced DG was proceeded efficiently in the pres-
ence of catalyst which had stronger basicity.
The effect of reaction time was investigated by varying
ꢀ
the etherification reaction time from 1 to 8 h at 260 C. As
showed in Figure 2, increasing the reaction times led to
increase the conversion of glycerol and to decrease the
selectivity of DG. The selectivity of TG and other products
increased slowly with increasing the reaction time. The
selectivity of TG in the presence of LiOAc kept increasing
in the reaction condition. However, in the presence of
NaOAc and KOAc, the maximum selectivities of TG were
Figure 2. The dependence of reaction time on the etherification
Figure 3. The dependence of amount of (a) LiOAc, (b) NaOAc,
reaction of glycerol in the presence of (a) LiOAc, (b) NaOAc, and
and (c) KOAc on the etherification reaction of glycerol (
version of glycerol, : Selectivity of DG, : Selectivity of
TG, : Selectivity of others, reaction time: 6 h, reaction temper-
ature: 260 ꢀC).
: Con-
(c) KOAc (
: Selectivity of TG,
catalyst: 0.5 mol% and reaction temperature: 260 ꢀC).
: Conversion of glycerol,
: Selectivity of DG,
: Selectivity of others, the amount of
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Solventless Catalytic Etherification of Glycerol
KOREAN CHEMICAL SOCIETY
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The etherification reactions of glycerol were executed for
6 h at 260 ꢀC with varied amount of each catalyst. As
shown in Figure 3, the conversion of glycerol steeply
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The dehydrative etherification reactions of glycerol have been
performed without solvent using LiOAc, NaOAc, or KOAc
as the catalyst at an ambient pressure. The conversion of
glycerol and the selectivities of DG, TG, and other oligo-
meric compounds could be controlled by varying the basicity
and amount of catalyst. The dependences of the etherification
reaction of glycerol on the reaction time and reaction temper-
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Acknowledgments. This work was financially supported by
the Ministry of Trade, Industry & Energy (MOTIE), Korea
Institute for Advancement of Technology (KIAT) through
the Encouragement Program for The Industries of Economic
Cooperation Region and by grants from Korea Ministry of
Environment as “The Eco-process based technologies”.
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