A green way to synthesize lubricating ester oils: The esterification of pentaerythrotol with fatty acids at stoichiometric ratio over [BHSO3MMIm]+[HSO4]?/SiO2

Article abstract of DOI:10.1016/j.catcom.2018.03.015

A green way was provided to produce lubricating ester oils under solvent-free conditions over an ionic liquid catalyst supported with mesoporous silica, which was prepared by using sol-gel method and exhibited high activity and selectivity for the esterification of pentaerythrotol with fatty acid at stoichiometric ratio to the corresponding pentaerythrotol tetra-esters. The catalyst was systematically characterized by TEM, TG/DTA, BET and FT-IR. Moreover, the stability of the catalyst was investigated, and it can be recycled for several times without significant deactivation.

Full text of DOI:10.1016/j.catcom.2018.03.015

Accepted Manuscript
A green way to synthesize lubricating ester oils: The esterification
of pentaerythrotol with fatty acids at stoichiometric ratio over
[BHSO3MMIm]+[HSO4]−/SiO2
Yanan Wang, Wenjing Lou, Xiaobo Wang
PII:
S1566-7367(18)30107-9
DOI:
doi:10.1016/j.catcom.2018.03.015
CATCOM 5356
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
25 December 2017
25 February 2018
14 March 2018
Please cite this article as: Yanan Wang, Wenjing Lou, Xiaobo Wang , A green way to
synthesize lubricating ester oils: The esterification of pentaerythrotol with fatty acids
at stoichiometric ratio over [BHSO3MMIm]+[HSO4]−/SiO2. The address for the
corresponding author was captured as affiliation for all authors. Please check if
appropriate. Catcom(2018), doi:10.1016/j.catcom.2018.03.015
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A Green Way to Synthesize Lubricating Ester Oils: The Esterification of Pentaerythrotol
+
-
with Fatty Acids at Stoichiometric Ratio over [BHSO MMIm] [HSO ] /SiO
2
3
4
a,b
a,
a,
Yanan Wang, Wenjing Lou * and Xiaobo Wang *
a
State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,
No.18 Tianshui Middle Road, Lanzhou, 730000, P.R. China.
b
Qingdao Center of Resource Chemistry & New Materials, No. 36 Jinshui Road, Qingdao, 266100, P.R. China.
*
Corresponding Authors: Email: wjlou@licp.cas.cn (W.J. Lou); wangxb@licp.cas.cn (X.B. Wang).
Fax: +86-931-4968019

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Abstract
A green way was provided to produce lubricating ester oils under solvent-free conditions over an ionic
liquid catalyst supported with mesoporous silica, which was prepared by using sol-gel method and exhibited
high activity and selectivity for the esterification of pentaerythrotol with fatty acid at stoichiometric ratio to
the corresponding pentaerythrotol tetra-esters. The catalyst was systematically characterized by TEM,
TG/DTA, BET and FT-IR. Moreover, the stability of the catalyst was investigated, and it can be recycled
for several times without significant deactivation.
Keywords: Esterification, Stoichiometric ratio, Ionic liquid, Pentaerythrotol, Green way

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1
. Introduction
With the growing of environmental concerns and stricter legislations throughout the world,
pentaerythrotol fatty acid esters (tetra) as a kind of bio-degradable lubricants are drawing the great attention
in industry. Pentaerythrotol fatty acid esters (tetra) possess excellent physical properties such as high
lubricity and viscosity indexes, high thermal stability, and biodegradability, etc. [1]. Note that the
esterification of pentaerythrotol with straight chain fatty acid (C -C ) is the most effective way to produce
5
11
this kind of environmental biolubricant in industry. Conventionally, the homogeneous acidic catalysts, such
as p-Toluenesulfonic acid, hydrofluoric acid and phosphoric acid, were employed to catalyze this
esterification reaction with a high conversion but the low selectivity of tetraester (less 98%). Furthermore,
the intrinsic disadvantages of homogeneous acids catalysts (tedious neutralization, time-consuming of its
purification and separation procedure from product, corrosion of equipment, and pollution of environment)
hindered the wide application of them in esterification reaction processes [2,3]. Most recently, to overcome
the above mentioned problems, much effort had been made on the development of efficient and
environment-friendly catalysts such as resins, solid acids, zeolites and molecular sieves, whereas other
drawbacks of high mass transfer resistance and easy deactivation of these catalysts reappeared [4,5].
Therefore, there is an urgent need to develop the highly efficient, eco-friendly, and recyclable catalyst for
this esterification reaction of pentaerythrotol fatty acid esters (tetra) to address the need of high-performance
ester oils in industry.
There is a way out of dilemma: Ionic liquids (ILs) have received significant attention in the field of
catalyst research in the last few years due to their broad range of potential properties. Especially, the high
design ability of them provides much more chance to achieve suitable ILs based on the given reactions [6-9].
Most notably, in practical application, their drawbacks of high viscosity minimizing the reaction kinetics
and homogeneous reaction making product separation and catalyst recovery difficulty could be well
overcome and advantageous characteristics of ILs could also be highlighted by immobilizing ILs on solid
acids [10-14].
Mesoporous material has a large specific surface area, pore volume and rich nanopores, which are
benefit to the diffusion and transmission of substrates and products. The catalytic activity of catalyst
supported by it is expected to be improved greatly [15, 16]. Therefore, the mesoporous SiO was expected
2
to be the support to enhance the catalytic activity of ILs. Herein, we document the first demonstration of
high-efficiency IL catalyst supported by mesoporous silica for the esterification of pentaerythrotol (C -C ).
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This supported IL catalyst exhibited good generality to the esterification of pentaerythrotol with straight
chain fatty acid (C -C ) and high yields of the products. Moreover, the introduction of SiO made a
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2
convenient separation of the IL catalyst from the reaction system.
2
. Experimental details
2
.1 Catalyst preparation

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Ionic liquids (IL ): 1.00 mol 1-methylimidazole and 1.00 mol 1,4-butanesultone was mechanically stirred in three-
1
o
necked round-bottomed flask containing 100 mL acetone for 72 hours under 25 C, after which the liquid in the flask
o
was heated at 50 C for 72 hours. The chemical reaction was stopped until a large amount of white solid of
BMImSO precipitated. Another small amount of BMImSO could be also obtained by distilling the filtrate.
3
3
Afterwards, the collected white solid was washed by acetone for three times. 1.00 mol BMImSO was added drop-
3
wise to the flask with 1.02 mol concentrated sulfuric acid (98%, wt) after which the obtained liquid was evaporated
+
-
to remove water to get the colorless and thick liquid [BHSO MIm] [HSO ] (labeled as IL ), and the yield of
3
4
1
+
-
1
[
BHSO MIm] [HSO ] was up to 97% and the H NMR was provided in Supplementary material.
3 4
o
IL /SiO -5%: 0.066 g IL and 4.0 mL anhydrous ethonal was stirred in in round-bottomed flask at 25 C for 1 hour.
1
2
1
o
5
.0 mL silicon tetraacetate was added drop-wised to the flask until the solution was clear in a water bath at 60 C.
Afterwards, 2.5 mL 6 mol/L hydrochloric acid solution was added into the flask drop by drop and stirred for 1 hour.
o
The colloidal solution was aged for 24 hours which further was dried in vacuum for 4 hours at 80 C.
The preparation of other catalysts and the characterization of those catalysts and the catalytic products were
provided in Supplementary material.
2
.2 Activity test
The esterification of pentaerythrotol with straight-chain fatty acid was carried out in a 100 mL three-necked
round-bottomed flask with magnetic stirring. 0.013 mol pentaerythrotol, a certain amount of fatty acid (the molar
ratio of pentaerythrotol with straight-chain fatty acid being 4:1) and catalyst ( IL content : 0.032 mmol) were
introduced, and 2 mL toluene was also added as a water-removing agent to react for 10 hours at the given
temperature. As flask was cooled to room temperature the reaction mixture was separated by centrifugation. The
obtained oil phase was concentrated by rotational evaporation to give the corresponding ester. For the reuse process
of IL /SiO -5% catalyst, the above procedure would be repeated again and again, and the detailed procedure was
1
2
provided in Supplementary material.
3
. Results and discussion
3
.1 Catalytic activity study
By applying the esterification of pentaerythrotol with caproic acid as model reaction, the influence of
different supports, ILs, and the amount of IL on the catalytic activity of catalysts were investigated, and the
results were listed in Table 1. The NMR spectra are given in Supplementary material. Comparing the acid
value and the yield of pentaerythritol tetra-hexanoate of catalysts with different support, the catalyst
IL /SiO -5% showed the best catalytic activity, and the yield of pentaerythritol tetra-hexanoate was up to 99%
1
2
with the lowest acid value (Table 1. Entry 3-6). Thus, the SiO was chosen as the catalyst support to further
2
investigate the influence of different ILs on the catalytic activity of catalysts for the esterification of
pentaerythrotol with caproic acid. It can be seen that both the yields of pentaerythritol tetra-hexanoate
catalyzed by IL /SiO -5% and IL /SiO -5% were much lower than that catalyzed by IL /SiO -5%, and even
2
2
3
2
1
2
worse, their acid values of the products for the esterification were much higher than that of the product for
the esterification catalyzed by IL /SiO -5% (Table 1. Entry 3, 7, 8). Then the influence of the amount of IL1
1
2

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on the catalytic activity of catalysts for the same reaction process was further investigated in details. The
yield of the product increased when the amount of IL increased from 3% to 5% (Table 1. Entry 2, 3).
1
While the yield of product was kept almost unchanged with the increasing amount of IL when it was more
1
than 5%. On the contrary, the acid values of products increased with the increasing amount of IL (Table 1
.
1
Entry 3, 9-11). Therefore, the IL /SiO -5% was supposed to be optimum design of catalyst for esterification
1
2
of pentaerythrotol with fatty acids.
The esterification of fatty acid with different -R was systematically tested over IL /SiO -5% catalyst to
1
1
2
examine the scope limitations, and the results were shown in Table 2. The NMR spectra are given in
Supplementary material. As the R was C , the esterification carried out smoothly and the corresponding
1
4
pentaerythrotol tetra-substituted ester with was up to 97% yields (Table 2. Entries 1-3). The yield of the
corresponding product declined with the increasing length of carbon chain of R under raised temperature
1
conditions (Table 2. Entries 4-6).
To further investigate the stability of the IL /SiO -5% catalyst, the reusability experiment was performed
1
2
and the results were shown in Fig. 1. The catalyst was used for six cycles in the esterification of
pentaerythrotol with caproic acid and the catalytic activity decreased slightly with the increasing of cycle
times before 6 cycles. As the catalyst was further recycled, the yield of pentaerythrotol tetrahexanoate was
also maintained at the level of 82%, indicating that the catalyst was relatively stable during the reaction.
3
.2 Characterization of IL /SiO
1 2
The basic physicochemical properties of SiO and IL /SiO -5% catalyst are shown in Fig. 2. It can
2
1
2
be seen that both of them have typical mesoporous structure, and there was almost no influence on the
structure of SiO support by introduce of IL catalyst. The large specific surface area and pore volume of
2
1
mesoporous structure SiO could be benefit to the diffusion and transmission of substrates and products,
2
and thus the catalytic activity of the catalyst was improved greatly [15,16]. The result of esterification
reaction in this research work is accordance with this synergistic effect theory.
The TEM images are presented in Fig. 3 to illustrate the IL dispersion and the morphology of SiO and
1
2
IL /SiO -5%. It can be seen that the material with plentiful pores and the IL dispersed homogeneously on
1
2
1
the SiO support, which is in good accordance with the BET results (as shown in inset of Fig. 2). The well
2
dispersion of IL on silica support should be in favor of creating more catalyst site modifying the catalyst
1
activity. The images also revealed that both of them are amorphous, and the incorporation of IL had no
1
side effect on the crystal form of SiO support itself.
2
The FT-IR spectra of IL and IL /SiO -5% composite are portrayed in Fig. 4. The adsorption peaks of anti-
1
1
2
-
1
symmetrical and symmetrical stretching vibration of CH - can be observed at 2964, 2880 cm , and the peak
3
-
1
at 1565 cm can be assigned to the skeleton stretching vibration of imidazolium ring of IL [17], as shown
1
-
1
in Fig. 4 (a). In Fig. 4 (b) the peaks at 3003, 2904 cm are assigned to the anti-symmetrical and
symmetrical stretching vibration of IL /SiO -5%, and the adsorption peak of the skeleton stretching
1
2
-
1
vibration of imidazolium ring is located at 1575 cm . The above results revealed that all the adsorption
peaks of IL in the catalyst have been significantly influenced by modifier silica: peak positions shift to the
1

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relative high waveband and the peak intensity became weak both of which should be due to the low amount
of IL loaded on SiO support. Meanwhile, there is no new peak in the FT-IR spectra ( Fig. 4 (b) ),
1
2
indicating that no new type chemical bond was formed after the IL packaged in SiO .
1
2
The TG/DTA of IL and IL /SiO -5% was tested, and the results are shown in Fig. 5. The decomposition
1
1
2
o
o
of pure IL was started around 310 C and completed at 383 C. While for the IL in the catalyst IL /SiO -
1
1
1
2
o
o
5
%, both the start and completion decomposition temperature raised up to 320 C and 417 C, respectively.
The TG/DTA results verified that the thermal stability of IL could be enhanced by being loaded on the
1
mesoporous SiO support. So this high-efficiency 5% IL catalyst supported with silica would exhibit high
2
1
stability and high reusability even after several runs.
4
. Conclusion
In summary, a simple and practical IL /SiO -5% catalyst for the esterification of pentaerythrotol
1
2
with straight chain fatty acid (C -C ) has been introduced under solvent-free conditions. Pentaerythrotol
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reacted with straight chain fatty acid at stoichiometric ratio, which offered a green and economical route to
produce lubricated esters, and the esterification was carried out smoothly to the corresponding
pentaerythrotol tetra-substituted ester with good to excellent yields. In addition, this IL /SiO -5% showed
1
2
excellent reusability and maintained stability for at least 6 runs.
Acknowledgments
Financial support was provided by the National Natural Science Foundation of China (51775536,
5
1475445 and 51605471).
Supplementary material
Supplementary data to this article can be found online at http://

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Figure and Table Captions
Fig. 1. The reusability test of IL /SiO in the esterification of pentaerythrotol with caproic acid.
1
2
Fig. 2. The N gas adsorption-desorption isotherm plots of SiO and IL /SiO -5% with inset of BET surface
2
2
1
2
area and pore volume and pore radius.
Fig. 3. TEM images and diffraction patterns of SiO and IL /SiO -5%.
2
1
2
Fig. 4. FT-IR spectra of (a) IL and (b) IL /SiO .
1
1
2
Fig. 5. TG/DTA characterizations of IL and IL /SiO -5%.
1
1
2
Table 1. Catalytic properties for the estrification of pentaerythrotol with caproic acid with different
a
catalysts .
a
Table
2
.
Catalytic properties of IL /SiO -5% for the synthetic esters lubricating base oil .
1
2

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Fig. 1

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Fig. 2

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Fig. 3

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Fig. 4

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Fig. 5

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Table 1.
O
OH
OH
O
O
O
O
catalyst
HO
O
O
4
OH
2
4H O
O
HO
O
b
c
d
Entry
Catalyst
SiO2
IL /SiO -3%
Acid value (mg KOH/g)
Yield /%
72
TOF
34
34
41
36
38
37
38
38
40
40
40
1
2
3
4
5
6
7
8
9
58
53
2
43
22
32
19
18
3
76
98
76
88
76
87
87
97
1
2
IL /SiO -5%
IL /Al O 5%
IL /ZrO -5%
IL /TiO -5%
IL /SiO -5%
IL /SiO -5%
IL /SiO -10%
IL /SiO -15%
1
2
1
2
3
1
2
1
2
2
2
3
2
1
2
1
1
0
1
4
4
96
96
1
2
IL /SiO -20%
1
2
a
Reaction conditions: 0.032 mmol (IL content) catalyst, 0.013 mol pentaerythrotol, 0.052 mol caproic acid,
o
b
2
mL toluene; reaction temperature : 158 C; reaction time: 10 h; The procedure for the synthesis of ILs
c
and catalysts were presented in Supplementary material; The value was tested after the reaction and used
for the conversion calculation: Conversion (%) = [(1-Acid value(after reaction)/Acid value(before reaction))
×
(
d
1
nacid/nalcohol/4]
×100%; The yield was determined by H NMR with triphenylmethane as internal standard
Supplementary material)

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Table 2.
O
R1
OH
OH
O
O
IL /SiO -5%
O
1
2
4H2O
HO
4
R1
O
O
O
R1
R1
OH
HO
O
O
R1
o
b,c
Entry
-R1
T/ C
Yield /%
1
2
3
4
5
6
155
158
160
160
165
170
98
98
97
93
94
96
a
Reaction conditions: 0.2 g IL /SiO -5%, 0.013 mol pentaerythrotol, 0.052 mol fatty acid, 2 mL toluene;
reaction time: 10 h; The products were identified with H NMR and HR-MS, and other side product was
pentaerythrotol tri-substituted ester; The yield was determined by H NMR with triphenylmethane as
internal standard (Supplementary material).
1
2
b
1
c
1

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Graphical Abstract

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Highlights




A green and feasible way was provided to efficiently synthesize ester oil
Esterification reaction was catalyzed by Ionic Liquid (IL) under solvent-free conditions
IL supported with mesoporous SiO has high activity and selectivity for esterification
2
This supported catalyst possesses extended reusability and maintained stability