An efficient catalyst COK-15b for the catalytic synthesis of lubricating ester oils

Article abstract of DOI:10.1016/j.catcom.2019.01.007

The new material MOF of (CTA)_1/3[Cu₄₆(C₉H₃O₆)₂₄(OH)₁₂] (PW₁₂O₄₀)₃·xH₂O (COK-15b) was synthesized via a dual-templating approach and exhibited high activity and selectivity for the esterification of pentaerythrotol with fatty acids at stoichiometric ratio to the corresponding pentaerythrotol tetra-esters with up to 98% yields under solvent-free conditions. The COK-15b was characterized by TEM, SEM, TG, BET, TPD and Py-IR. Moreover, the stability of COK-15b was investigated, and it can be reused for 5 running cycles without significant deactivation.

Full text of DOI:10.1016/j.catcom.2019.01.007

Accepted Manuscript
An efficient catalyst COK-15b for the catalytic synthesis of
lubricating ester oils
Yanan Wang, Rui Ma, Cheng Jiang, Wenjing Lou, Xiaobo Wang
PII:
S1566-7367(19)30007-X
DOI:
https://doi.org/10.1016/j.catcom.2019.01.007
CATCOM 5591
Reference:
To appear in:
Catalysis Communications
Received date:
Revised date:
Accepted date:
18 November 2018
2 January 2019
7 January 2019
Please cite this article as: Yanan Wang, Rui Ma, Cheng Jiang, Wenjing Lou, Xiaobo Wang
,
(
An efficient catalyst COK-15b for the catalytic synthesis of lubricating ester oils. Catcom
2019), https://doi.org/10.1016/j.catcom.2019.01.007
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ACCEPTED MANUSCRIPT
An Efficient Catalyst COK-15b for the Catalytic Synthesis of
Lubricating Ester Oils
a,b
a,b
a,b
a,
a,
Yanan Wang , Rui Ma , Cheng Jiang , 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 Centerof 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
Abstract
The new material MOF of (CTA)1/3[Cu46(C9H3O6)24(OH)12] (PW12O40)3·xH2O (COK-15b) was synthesized via
a dual-templating approach and exhibited high activity and selectivity for the esterification of pentaerythrotol
with fatty acids at stoichiometric ratio to the corresponding pentaerythrotol tetra-esters with up to 98% yields
under solvent-free conditions. The COK-15b was characterized by TEM, SEM, TG, BET, TPD and Py-IR.
Moreover, the stability of COK-15b was investigated, and it can be reused for 5 running cycles without
significant deactivation.
Keywords: MOF catalyst,Esterification, Stoichiometric ratio, Pentaerythrotol

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1
. Introduction
Currently, the energy-waste and unnecessary environmental emission caused by high friction and severe wear
stemfrom the shortage of task-specified lubricant. With fast development of advanced technique and emerging
in large numbers of moving mechanism lubrication requirement, the exploration of new classes of energy-
efficient and environmentally -compatible lubricant is thus becoming increasingly important [1,2].
Pentaerythrotol fatty acid ester (C5-C11) is drawing great attentions as one kind of biodegradable lubricants, due
to their high viscosity indexes and thermal stability. The tranditionally explored homogeneous acid catalysts
could’t achieve high selectivity of tetraester even though it had a high conversion, which would also cause
corrosion of equipment and pollution of environment. The recently developed enrivonment-friendly catalysts
(resins, solid acids, etc.) could overcome the above mentioned problems but their low mass transfer capacity
and deactivation would hinder the sufficient reaction process under the theoretic stoichiometric ratio of
reagents. Thus, the residual fatty acids would have a negative effect on the lubricity of as -synthesized esters
[3-6]. Most recently, a green way had been given in our research work to synthesize lubricating ester (C5-C11)
oils for the esterification of pentaerythrotol with fatty acid at stoichiometric ratio by using the newly -
+
−
synthesized ionic liquid catalyst supported with mesopous silica ([BHSO3MMIm] [HSO4] /SiO2) under
solvent-free conditions. This catalyst could show excellent reusability and maintain stability by means of
catalysis function of ionic liquid and high diffusion and transmission capacity of mesopous silica, respectively
[2]. Nevertheless, it will be much more economic to develop one efficient candidate substance with both
catalysis and mass transfer capacity to replace the abovementioned complex component combination to reduce
the praperation time and steps based on our recent research achievement.
Recently, the significant progress has been made on porous materials owing to their multifunction
characterization. Metal-organic frameworks (MOF) is considered as a new class of highly crystalline porous
solids which has attracted great research interests over last decade due to its potential applications, including gas
storage, purification and separation and catalysis. Compared to conventionally-used microporous and
mesoporous inorganic materials, the MOF, as catalyst, has many advantages, such as the large surface area, high
customizability and controllable porous structure with uniform pore size. Thus, it can be easily modified and
fabricated [6-13]. The MOF of (CTA)1/3[Cu (C H O ) (OH) ] (PW O ) ·xH O (COK-15b), as firstly
46
9
3
6 24
12
12 40 3
2
reported by Kirschhock, could be stabilized by Keggin type phosphotungstate (HPW) that is systematically
occluded in the cavities constituting the walls between the mesopores [9]. In the synthesis procedure, the Keggin
ions serve as molecular template for the structure motif of the COK-15, and the cetyltrimethylammonium
bromide (CTAB) directs these units in an ordered mesoporous structure demonstrating excellent catalytic
activity under mild reaction conditions. Therefore, it inspires us to achieve this kind of superior morphology
with the large contact area and substantially wide permanent mesopores by adjusting the content of CTAB to
further expand synthesis reaction.
Herein, we report an excellent MOF catalyst stabilized by Keggin polyoxo metallate ions for the
esterification of pentaerythritol with C5-C11 fatty acid. The Keggin ions act as template species during
synthesis and stabilization of the mesoporous structure via the synergism between metal and Keggin
ions. We overca me the challenge of synthesis tetraesters from pentaerythritol to C -C11 fatty acids with
5
stoichiometric ratio over MOF catalyst: the selectivity of tetraesters reached up to 99% with almost
complete conversion of C5-C11 fatty acids, meanwhile the catalytic performance still kept stable.
2
. Experiment details

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2.1. Catalyst preparation
The new MOF was coined COK-15b which were prepared according to reference [7]. 5.76 g phosphotungstic
acid hydrate (H3PW12O40·nH2O) and 8.7 g copper nitrate trihydrate Cu(NO3)2·3H2O were dissolved in 72 mL
deionized water. A total of 4.2 g 1,3,5-benzenetricarboxylate (BTC) and 0.24 g cetyltrimethylammonium
bromide (CTAB) were dissolved in the other 96 mL absolute ethanol, which were further combined and mixed
with the 72 mL deionized water. The mixed solution were stirring for 30 minutes and then statically aged for 5
days without heating. The synthesised solid was washed by using deionized water via filtration and then dried in
oven at 60 °C for 1 day. CTAB leaching was performed by using 1.5 mL acetic acid in 250 mL ethanol under
reflux at 77 °C for 1 hour. After that, it was filtrated and washed with deionized water/ethanol (50:50 (v/v))
solution and dried at 60 °C for 1 hour.
2.2. Catalyst characterization
The morphology of COK-15b was characterized by using a field emission scanning electron microscopy
(
FESEM, JSM-6701F). XRD patterns (Low-angle X-ray diffraction included) of COK-15b were obtained on a
STADI P automated transmission diffractometer instrument equipped with an incident beamcurved germanium
monochromator selecting Cu Kα1 radiation (40 kV and 40 mA). The precipitated particles were dried in air and
pressed on a glass slide for analysis. For the sample preparation for the analysis of high resolution transmission
electron microscopy (HRTEM), the COK-15b particle dispersion was further diluted by ethanol, 10 µL of which
was cast on the Cu grid. TEM images were obtained on a Tecnai G2 F30 S-Twin operating at 300 kV. Nitrogen
(N2) adsorption-desorption isotherms were measured at 77 K using Micromeritics 2010 instrument. The pore-
size distribution was calculated by Barrett, Joyner and Halenda (BJH) method from desorption isotherm. The
thermal decomposition temperatures and weight loss were assessed by employing a Pyris Diamond analyzer at a
o
heating rate of 10 C/min under N2 atmosphere.
The acidity type of the COK-15b catalyst was investigated by using the pyridine adsorption Fourier-
transform infrared (Py-IR), as described in Supplementary material. The test of temperature programmed
desorption of NH (NH -TPD) adsorbed on the COK-15b catalyst was carried out by using a Quanta chrome
3
3
Autosorb-iQ-C chemisorption analysis-meter: 80 mg catalyst was placed into the quartz reactor and pretreated
o
o
under 150 C in a N2 flow (50 mL/min) condition for1 h, after which the sample was cooled to 50 C, followed by
the saturation for 1 h in a flow of 10% NH3/N2 (50 mL/min) condition. Then, the sample was further flushed
o
o
with a N2 flow for 1 h and TPD was performed by ramping the temperature at 10 C/min up to 800 C. The
desorption behavior of NH was recorded by a thermal conductivity detector(TCD).
3
2.3. Activity test
The esterification reaction conditions were similar with our previous report [7]. In a 100 mL three-necked
round-bottomed flask, the esterification of pentaerythrotol with C5-C11 straight-chain fatty acid was carried out
with magnetic stirring. 0.2g COK-15b catalyst and 0.013 mol pentaerythrotol, a certain amount of fatty acid (4:1
of the molar ratio of pentaerythrotol with straight-chain fatty acid) were added into the flask to react for 6 hours
at the given temperature, which saved 4 hours as compared with our recently reported catalyst
+
−
[
BHSO3MMIm] [HSO4] /SiO2 [2]. Meanwhile, 2 mL toluene was also put in the flask to remove product water.
The reaction mixture was separated by centrifugation as the flask cooled down to room temperature. The
products were concentrated by rotational evaporation method to get the corresponding ester, in which the by-
product of water and unreacted substrates would be removed. The above procedure would be repeated several

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times for the reuse process of COK-15b catalyst, and the detailed procedure was provided in Supplementary
material.
3
. Results and discussion
3.1. Catalytic activity study
The catalytic performance of COK-15b was investigated by the esterification of fatty acid with
different –R1 to examine the scope limitations and the results were listed in Table 1. The NMR and HR-
MS spectra were given in Supple mentary material. As –R1 =C4, the esterification rate is over 98% and
the yield of the corresponding tetra-substituted ester reaches up to 91% (Table 1, entry 1). With the
increase of both esterification te mperature fro m 155°C to 170°C and carbon chain of –R1 from C5 to C9, the
yields of the corresponding tetra-substituted esters go up from 94% to 98% with the constantly high
esterification rate of higher than 98% (Table 1, entry 2-6).
We also investigated the stability of catalyst COK-15b and the results were present in Fig. 1. The
catalyst was employed for promoting the esterification reaction of pentaerythrotol with caproic acid.
th
The catalytic activity decreases gradually but the 90% yield is maintained after the 4 running cycles,
indicating that the catalyst COK-15b had high stability during the esterification reaction. With further
th
recycle of running (5 ), the yield of pentaerythrotol tetrahexanoate still keeps at the level of 85%.
3.2. Characterization of COK-15b
The synthesized catalyst COK-15b was characterized by using FESEM, and the images were shown in
Fig. 2(a). It can be seen the octahedral crystal planes are decorated by obvious holes, and also the
formed crystals of COK-15b are smaller than that reported by Kirschhock [7]. The small size would
increase the special surface of this catalyst crystal. With careful control of reagent concentration and
reaction conditions, the invalid hexagonal crystal for catalysis of esterification was not formed. It can be
verified from sharp reflections at low angle of XRD pattern (the left one of Fig.2 (b)) that the
synthesized crystal possessed high degree of ordering. Moreover, the close investigation of XRD pattern
at low angle range (the right one of Fig.2 (b)) clearly showed the [100], [001] and [201] diffraction
peaks typical for COK-15 crystals.
In many circumstances, most of MOFs are extremely sensitive to the electron bea m and always
collapse caused by irradiation. Thus, the TEM analysis of MOF material is big challenge. While in this
research, to some extent, the copper benzene tricarboxylate metal-organic framework with wide
permanent mesopore stabilized by Keggin Polyoxo metallate ions could allow a close investigation of
the nanostructure of COK-15b by using HRT EM. Both the sharp outline of nanoparticles in HETEM
(the left one of Fig. 3) and the clear dotted circle of selected area diffraction pattern (SADE) (the right
one of Fig. 3) indicated that catalyst COK-15b had polycrystalline structure, in good accordance with
the XRD analysis results. Also, this material is plentiful of pores.
The basic physicochemical property of COK-15b was presented in Fig. 4 (left). It can be found that the
2
material has superior mesoporous nanostructure, large specific surface area (16.71 m /g), average pore
3
volume (0.31 cm /g) and average pore radius (17.06 n m). According to synergistic effect theory of
catalysis reaction, the catalytic activity of the catalyst COK-15b could be significantly improved
because the superior structure was benefit to transmission and diffusion of substrates and products [14].

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The T G characterization of COK-15b was shown in Fig.4 (right). The TG profile for catalyst COK-15b
presents a weight loss (4%) from 40°C to 120°C corresponding to evaporation of physically absorbed
water. The chemical decomposition of catalyst started at around 290°C and completed at 413°C with 17%
weight loss. As compared with the esterification conditions, the high decomposition temperature, i.e.
high thermal stability of catalyst COK-15b, would enable the high stability during repeated
esterification reaction process.
The Py-IR spectrum of the COK-15b catalyst (as shown in Fig. S1) has the characteristic absorption
-
1
+
band around 1450 cm referred to the C-C(N) bond vibration of PyH , which was formed from pyridine
with BrØnsted acid [15]. It can be also found that the characteristic absorption band of C-C(N) bond
-
1
around 1540 cm originating from the vibration of C-C(N) bond of PyL [15]. Generally, the amount of
the acid sites on catalyst surface has a positive effect on the esterification reaction. Thus, to further
investigate the stability of the catalyst, the surface acidities of the catalyst before and after the reaction
was exa mined by NH3 -TPD, and the NH3 -TPD spectra were shown in Fig. 5. It could be seen that each
o
sample has more than two peaks: the peak in the range of 100~300 C could be ascribed to the
desorption behavior of physisorbed NH3 and BrØnsted acid sites, and those located in the range of
o
3
00~700 C were originated from the desorption behavior of NH3 bound to Lewis acid sites [16, 17]. By
comparison of the spectra integral areas of the catalysts before and after catalysis reaction (as listed in
Table. S1), it can be stated that both of them have almost the same area values after the deduction of
blank spectru m, indicating that acidity of the catalyst kept unchanged after the catalysis reaction. The
high stability could enable the high catalytic performance of COK-15b catalyst even after several cycles.
4
. Conclusion
In summary, a practical MOF, i.e. COK-15b with highly octahedral crystalline structure was
synthesized by using a combined te mplate and surfactant method at room temperature. This catalyst
could enable to promote the esterification of pentaerythrotol with fatty acids (C -C ) go smoothly at
5
11
stoichiometric ratio and achieve high esterification rate over 98% and high yields of the corresponding
tetra-substituted esters from 94% to 98%. Besides, COK-15b also showed an e xcellent reusability and
maintained stability after 5 running cycles of catalysis reaction. It is considered to provide a green and
economic way to produce lubricating esters.
Acknowledgements
Financial support was provided by the National Natural Science Foundation of China (21303228 and
21073208.)
Supplementary material
Supplementary data to this article can be found online at http://
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4
[
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Figure and Table Captions
Fig. 1. The reusability test of COK-15b in the esterification reaction of pentaerythrotolwith caproic acid.
Fig.2. (a) FESEM images of representative COK-15b crystal and (b) XRD patterns of COK-15b (the
right one at low-angle range included).
Fig.3. HRTEM image and selected area diffraction pattern (SADE) of COK-15b.
Fig.4. The N2 gas adsorption-desorption isotherm plot with inset of BET surface area, pore volume and
pore radius (the left one), and TG characterization curve of COK-15b (the right one).
Fig.5. NH3-TPD spectra of catalysts: blank, before reaction and after reaction.
a
Table 1. Catalytic properties of COK-15b for the reaction of synthesizing lubricating ester .

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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
R1
O
O
O
OH
OH
O
O
O
O
R1
OH
OH
COK-15b
4
R1
OH
R1
4H2O
O
R1
O
b
c
d,e
Entry
R1
T /°C
Est /%
98
Y
/%
1
2
3
4
5
6
135
91
94
97
97
96
98
155
99
160
99
160
> 99
98
165
170
> 99
a
Reaction conditions: 0.2 g COK-15b, 0.013 mol Pentaerythro-tol, 0.052 mol acid, 2 mL
toluene; reaction time: 6 h.
b
T: reaction temperature.
Est: esterification.
c
d
1
The products were identified with H NMR and HR-MS.
The yields of products were determined by H NMR with triphenylmethane as internal
e
1
standard (Supplementary material).

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Highlights




A feasible and green way was provided to efficiently synthesize the ester oil;
Esterification was catalyzed by MOF (COK-15b) underthe solvent-free conditions;
Esterification of pentaerythrotolwith fatty acidy was with stoichiometric ratio;
This catalyst presented high activity, selectivity and maintained high stability.