ACCEPTED MANUSCRIPT
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