Novel porous and hydrophobic POSS-ionic liquid polymeric hybrid as highly efficient solid acid catalyst for synthesis of oleate

Article abstract of DOI:10.1016/j.catcom.2016.05.004

A novel porous and hydrophobic Br?nsted acidic solid catalyst (POSS-[VMPS][H₂SO₄]) was successfully synthesized by copolymerization of polyhedral oligomeric vinylsilsesquioxanes (POSSs) and sulfonic acid-functionalized imidazole ionic liquids. Catalytic tests for the esterification of oleic acid with methanol have shown that this newly obtained polymeric hybrid exhibits very high catalytic activity and selectivity, which was more active or comparable to those of the common solid acid Amberlite-732 and liquid acid H₂SO₄. Moreover, the catalyst has a good recyclability without significant loss in the activity.

Full text of DOI:10.1016/j.catcom.2016.05.004

Catalysis Communications 83 (2016) 27–30
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Novel porous and hydrophobic POSS-ionic liquid polymeric hybrid as
highly efficient solid acid catalyst for synthesis of oleate
Dan Lu, Jiwei Zhao, Yan Leng ⁎, Pingping Jiang, Chenjun Zhang
The Key Laboratory of Food Colloids and Biotechnology, Ministry of Education, School of Chemical and Material Engineering, Jiangnan University, Wuxi 214122, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel porous and hydrophobic Brønsted acidic solid catalyst (POSS-[VMPS][H SO ]) was successfully synthe-
2
4
Received 26 February 2016
Received in revised form 27 April 2016
Accepted 10 May 2016
sized by copolymerization of polyhedral oligomeric vinylsilsesquioxanes (POSSs) and sulfonic acid-functional-
ized imidazole ionic liquids. Catalytic tests for the esterification of oleic acid with methanol have shown that
this newly obtained polymeric hybrid exhibits very high catalytic activity and selectivity, which was more active
Available online 12 May 2016
2 4
or comparable to those of the common solid acid Amberlite-732 and liquid acid H SO . Moreover, the catalyst has
a good recyclability without significant loss in the activity.
Keywords:
©
2016 Elsevier B.V. All rights reserved.
Solid acid catalyst
Esterification
Poly(ionic liquid)
Polyhedral oligomeric vinylsilsesquioxanes
(POSSs)
1
. Introduction
sustainable chemistry for catalyst recovery, operation and reuse. In par-
ticular, ILs supported on porous inorganic materials and organic poly-
mers are efficient catalysts for the production of fine chemicals and
have good recyclability [16–18]. Nevertheless, several factors such as
low loading, tendency to deactivation, high mass transfer resistance,
or hydrophilic frameworks retard the widespread implementations of
many of these acidic catalysts. This has stimulated the development of
strategies for preparation of novel IL-based solid catalysts with the
high catalytic activity and steady reusability.
Poly(ionic liquids) (PILs) resulting from the combination of ionic liq-
uid (IL) monomers or copolymerization of ILs with other monomers
were shown to constitute an interesting system to develop IL-based
solid catalysts [19,20]. A topic of interest in this context is the design
of mesoporous PILs (MPILs), and many efforts have been devoted to
controlling the pore structure, morphology, chemical composition, and
surface functionality for various MPIL materials [21–23]. Previously,
we designed and synthesized a family of porous polyhedral oligomeric
silsesquioxanes (POSSs)-containing PILs by copolymerization of POSS
and ILs, followed by ion exchange with polyoxometalates (POMs), re-
vealing that the resulting POM-based PILs were highly efficient solid
catalysts in epoxidations of alkenes [24–26]. Taking into account that
Due to the growing concerns of the exhaustion of fossil fuel re-
sources and degradation of the environment, the production of eco-
friendly fuels from abundant and renewable biomass resources has
attracted much attention [1,2]. Biodiesel, a mixture of C12–C22 fatty
acid monoalkyl esters (FAMEs), obtained from acid- or base-catalyzed
transesterification of triglycerides (TGs) with short-chain alcohols or
esterification of fatty acid with alcohols, is a sustainable, non-toxic,
and biodegradable petrol fuel replacement [3–8]. However, the
transesterification of TGs is often accompanied by the production of un-
avoidable side product glycerol. Esterification is an important class of
organic reaction, and various mineral acids including H
H
Nevertheless, these acids are extremely corrosive, contaminative and
need to be neutralized at the end of the reaction. Furthermore, it is
very difficult to separate and reuse the mineral acid catalysts.
Ionic liquids (ILs) have been revealed as green reaction media owing
to their negligible volatility, remarkable solubility, and the variety of
structures available [10–12]. Many acid-catalyzed organic reactions
based on acidic ILs have been reported, among which esterifications
are a hot topic [13–15]. However, ILs usually cause homogeneous catal-
ysis systems, which suffer from the high content of ILs (20–50 mol%
with respect to substrate) needed in the reaction media. The replace-
ment of homogeneous liquid catalysts by heterogeneous solid catalysts
has received much attention because of the interest in green or
2 4
SO , HF, and
3
PO , have been employed as catalysts for biodiesel production [9].
4
the hydrophobicity of the inorganic core (Si
8
O12) in POSS can also be
used to adjust hydrophobic features of the hybrid materials, which
could separate out the produced water from the organic phase in time
to shift the reaction to products side [27,28]. In this work, we have dem-
onstrated a successful preparation of solid acid catalyst by copolymeri-
zation of octavinyl POSS with SO
liquids (Scheme 1). The obtained hybrid POSS-[VMPS][H
proved to be porous material with excellent hydrophobicity and to
3
H-functionalized imidazole ionic
SO ] was
4
⁎
Corresponding author.
E-mail address: lengyan1114@126.com (Y. Leng).
2
http://dx.doi.org/10.1016/j.catcom.2016.05.004
566-7367/© 2016 Elsevier B.V. All rights reserved.
1

28
D. Lu et al. / Catalysis Communications 83 (2016) 27–30
2 4
Scheme 1. Synthesis of the POSS-ionic liquid polymeric hybrid POSS-[VMPS][H SO ].
behave as highly efficient and recyclable heterogeneous catalyst for
esterification of oleic acid with alcohols.
appeared distinctively with the appearance of a broad band at
1113 cm , which is attributed to the asymmetric vibration of Si–O–Si
in the POSS. Therefore, the FT-IR spectra confirm the successful combi-
−
1
2
2
2
. Experimental
nation of SO
tion route. TG analysis in Fig. S3 in SM shows that the prepared POSS-
[VMPS][H SO ] has superior thermal stability with decomposition
starting at about 250 °C. The significant weight loss in the range 250–
400 °C is due to the decomposition of organic groups and the destruc-
tion of the polymer network.
3
H functionalized IL with POSS through the copolymeriza-
.1. Catalyst preparation
2
4
.1.1. Preparation of octavinyl POSS and 1-vinyl-3-propane sulfonate
imidazolium (VMPS)
Octavinyl POSS was prepared according to the previous literatures
The composition of POSS-[VMPS][H
emental analysis. According to the CHNS elemental analysis results in
Table 1, the actual molar ratio of [VMPS][H SO ] to POSS is approxi-
mately 8.2, which is very near to the theoretical values of 8. Notably,
the acid density of this type of polymeric solid acid catalysts solely de-
pends on the charge ratio during the catalyst preparation, and the sulfur
2 4
SO ] was examined by CHNS el-
[
29]. 29Si NMR (400 MHz, CDCl
3
, TMS) δ (ppm) = 80.19 (See Fig. S1 in
Supplementary Material (SM)). VMPS was prepared according to the
2
4
1
previous literature [30]. H NMR (400 MHz, CDCl
3
, TMS) δ (ppm) =
), 4.43 (t, 2H, –CH ), 5.32 (d, 1H,
CH), 5.68 (d, 1H, –CH), 7.04 (m, 1H, –CH), 7.60 (d, 2H, –CH ), 9.05 (s,
2
–
1
.25 (m, 2H, –CH
2
), 2.85 (m, 2H, –CH
2
2
2
H, –CH) (Fig. S2, SM).
content of POSS-[VMPS][H
.63 mmol/g, which is higher than those of the supported IL catalysts
obtained by post modification method [31,32].
Fig. 2 shows the N adsorption–desorption isotherms and the corre-
sponding pore size distributions of the prepared polymeric hybrids, re-
spectively. The sample POSS-[VMPS][H SO ] displays a typical IV type
isotherm, giving a steep increase at a relative pressure of 0.8 b P/
b 1, indicating the presence of porous structures, including
mesopores and macropores. Additionally, POSS-[VMPS][H SO ] has a
2 4
SO ] indicates a high acid concentration of
2
2
2 4
.1.2. Preparation of POSS-[VMPS][H SO ]
First, POSS-[VMPS] was synthesized through free radical copolymer-
2
ization of octavinyl POSS with VMPS. 0.63 g of octavinyl POSS was added
into 18 mL THF, followed by addition of 23 mL of methanol, then 1.73 g
VMPS was introduced. After stirring at 60 °C for 1 h, 0.05 g of AIBN was
added, the mixture was then reacted at 65 °C for 24 h. The formed solid
product POSS-[VMPS] was filtered, washed with THF and methanol for
three times, dried under vacuum at 60 °C for 10 h, which was further
2
4
P
0
2
4
2
3
specific surface area of 43.56 m /g and pore volume of 0.271 cm /g
treated with sulfuric acid (H
into 20 mL of ethanol, then 2 mL of 10 M H
ous stirring. After stirring for 24 h at room temperature, the sample was
treated by centrifugation and washed with ethanol until the superna-
2
SO
4
). 1.0 g POSS-[VMPS] was dispersed
SO was added with vigor-
(Table 1, entry 1). However, the surface area and pore volume of the
2
4
POSS-free sample Poly([VMPS][H
2
SO
7.55 m /g and 0.039 cm /g, respectively (curve b in Fig. 2A and B,
Table 1, entry 2). Poly([VMPS][H SO ]) shows artifact at about 4 nm
4
]) are significantly reduced to
2
3
2
4
tant was neutral. After drying at 60 °C for 12 h, POSS-[VMPS][H
was obtained. POSS-free sample Poly([VMPS][H SO ]) was prepared in
the same way without the adding of POSS units.
2
SO
4
]
which may be interpreted as partial blocking of the pores. These results
demonstrate that the POSS plays important roles in the pore formation.
2
4
2
.2. Procedure for catalytic performance
The esterification reactions were performed using oleic acid with
different alcohols (methanol, ethanol, propanol and butanol) at differ-
ent temperatures. Taking the esterification of oleic acid with methanol
as an example, 0.01 mol oleic acid, 0.2 mol methanol, and 0.2 mmol cat-
alyst were added into a 50 mL round-bottomed flask equipped with a
reflux condenser. The mixture was reacted at 70 °C for 3 h without the
addition of any solvents. Notably, the reactions performed at high tem-
peratures (100 °C and 120 °C) were carried out in a 50 mL autoclave.
After the reaction, the solid catalyst can be easily recovered by centrifu-
gation, and the reaction product was determined by GC–MS.
3
. Results and discussion
3
.1. Preparation and characterization of catalysts
Fig. 1 shows the FT-IR spectra of POSS-[VMPS][H
sample Poly([VMPS][H SO ]). Obviously, Poly([VMPS][H
the featured peaks at 1553, 1187 and 1037 cm
stretching vibration of ν(C_N), ν(O_S_O), and ν(S\\O) in
VMPS][H SO ] IL, respectively. For POSS-[VMPS][H SO ], these peaks
2
SO
4
] and POSS-free
SO ]) shows
assigned to the
2
4
2
4
−
1
Fig. 1. FT-IR spectra of (a) Poly([VMPS][H
2 4 2 4
SO ]), (b) fresh POSS-[VMPS][H SO ], and (c)
[
2
4
2
4
reused POSS-[VMPS][H SO ].
2
4

D. Lu et al. / Catalysis Communications 83 (2016) 27–30
29
Table 1
Elemental analysis and BET surface areas of the catalysts.
2
2
3
Entry
Catalyst
C (%)
H (%)
N (%)
S (%)
Acid site (mmol/g)
S
BET (m /g)
S
external (m /g)
Vp (cm /g)
1
2
3
Poly([VMPS][H
POSS-[VMPS][H
POSS-[VMPS][H
2
SO
SO
4
])
30.36
30.31
30.29
4.38
4.05
3.94
8.46
7.07
6.89
19.75
16.16
15.67
3.22
2.63
2.55
7.55
43.56
–
5.64
43.26
–
0.039
0.271
–
2
4
]
]
a
2
SO
4
a
2 4
Recycled catalyst POSS-[VMPS][H SO ].
Also notably, the external surface areas of POSS-[VMPS][H
2
SO
4
] and
reaction system, and exhibited 94.1% conversion with 100% selectivity,
which is comparable to that of the homogeneous H SO . Notably, with-
Poly([VMPS][H SO ]) shown in Table 1 are very close to their BET sur-
2
4
2
4
face areas, and thus the catalytic reactions will be mostly carried out
on the external surface of the catalysts.
out water removal, the reaction will not proceed to 100% conversion be-
cause of the equilibrium reaction. In contrast, the non-porous and
The SEM image of POSS-[VMPS][H
coral-shaped morphology with micrometer size and nanoscale hollow
structure, while the morphology of Poly([VMPS][H SO ]) is almost amor-
phous blocks with a relative smooth surface (Fig. 3B). In the TEM image of
POSS-[VMPS][H SO ] (Fig. 3C), the observed dark nano-particles with
small size of about 5–10 nm indicate that the introduced POSS units
have been uniformly dispersed in the polymer network. The previous re-
searches have demonstrated that the hydrophobicity of inorganic nuclear
in POSS units can adjust the surface wettability of polymer materials [33,
2
SO
4
] (Fig. 3A) shows the irregular
2 4
hydrophilic POSS-free Poly([VMPS][H SO ]) gave a low conversion of
79.2% although it also acted as a heterogeneous catalyst. Furthermore,
the typical solid catalyst acidic ion exchange resin Amberlite-732 with
good hydrophilicity (CA test in Fig. S7, SM) offered a low conversion
2
4
2
4
68.3% as well. Additionally, POSS-[VMPS][H
catalytic rate than Amberlite-732, which was almost similar to that of
SO (Fig. S4 in SM). Based on these comparisons, we thus proposed
that the porous structure and the hydrophobic frameworks of POSS-
[VMPS][H SO ] may be responsible for the high catalytic activity [27,35].
2 4
SO ] showed much higher
H
2
4
2
4
3
4]. Accordingly, we carried out the contact angle (CA) tests to study the
wettability of this solid acid catalyst. As can be seen in Fig. 3, when a water
droplet is brought in contact with the surface of the POSS-[VMPS][H SO ],
it yields a range of 94–97° for CA, indicating a good hydrophobicity. How-
ever, the CA for POSS-free sample Poly([VMPS][H SO ]) is measured to be
6–41°. This phenomenon indicating that the POSS units endow the
POSS-[VMPS][H SO ] with hydrophobic frameworks, which perhaps is
The reaction temperature plays an important role in influencing the
product yield. Therefore, the esterification of oleic acid with methanol
was further tested at different temperatures (30 °C, 70 °C, 100 °C, and
120 °C) (Fig. S5 in SM). It is found that the conversion of oleic acid in-
creased with the increase in the reaction temperature, and very high
conversion of 98% could be observed at 120 °C. As esterification is an en-
dothermic reaction and the reaction rate usually increases with the in-
crease of temperature, thus high temperature is advantageous to the
equilibrium conversion. Additionally, up to 51% conversion of oleic
acid was obtained even at a low temperature (30 °C) only 3 h. Encour-
aged by the above results, the esterification of oleic acid with other alco-
hols, such as ethanol, 1-propanol, and 1-butanol were also investigated
2
4
2
4
3
2
4
essential for the esterification reactions, where the produced byproduct
water will be easily removed from the acid centers, resulting in accelerat-
ing the reaction to products side.
3
.2. Catalytic activity of the catalyst
The polymeric hybrid POSS-[VMPS][H
2 4
on the catalyst POSS-[VMPS][H SO ] (Fig. S6 in SM), and the results
demonstrate that the present catalyst can be applied to the esterifica-
tion of oleic acid with various alcohols, and optimum results of activities
and selectivity were obtained as well with a relative high reaction tem-
perature of 120 °C.
2 4
SO ] and various control solid
acid and liquid acid were tested as the catalysts for the esterification of
oleic acid with methanol under heating temperature of 70 °C for 3 h.
The esterification process and the results are summarized in Fig. 4. In
the absence of catalyst, only 11.7% conversion of oleic acid was obtained.
3.3. Catalyst reusability
2 4
Although the liquid H SO (98%) exhibited a high conversion of 98.3%
with 100% selectivity, it caused a homogeneous catalysis, and the cata-
lyst can't be easily recovered and reused. Interestingly, the synthesized
porous and hydrophobic POSS-[VMPS][H SO ] was insoluble in the
2 4
Finally, the reusability of the catalyst POSS-[VMPS][H
2
4
SO ] was eval-
uated in the esterification of oleic acid using methanol at 70 °C and 120 °
C, respectively. After each cycle, the catalyst was separated from the re-
action system by filtration or centrifugation, washed with ethanol, dried
and reused for the next run without adding any fresh catalyst. As shown
in Fig. 5, the POSS-[VMPS][H
with a slight decrease in catalytic activity after four-run tests. The IR
spectrum for the recovered POSS-[VMPS][H SO ] in Fig. 1 was well con-
2 4
SO ] exhibits a relative steady reusability
2
4
sistent with that of the fresh one, demonstrating a durable catalyst
structure accounting for the relative steadily catalytic reuse. Further-
more, content of S in the recovered POSS-[VMPS][H SO ] was measured
2 4
to be 15.67% (Table 1, entry 3), which is relative lower than that of the
fresh catalyst 16.16%. Therefore, the small leaching of S in the catalyst
may be account for the slight decreasing of the activity.
4. Conclusions
In summary, we have demonstrated the successful application of
octavinyl POSS and SO
constructing porous and hydrophobic solid acid catalyst POSS-
VMPS][H SO ] via free radical polymerization. The obtained hybrid
3
H-functioned ionic liquid as building blocks for
[
2
4
was found to be highly efficient catalyst for the esterification of oleic
acid with alcohols. And the catalyst was recovered easily and reused
Fig. 2. Nitrogen adsorption–desorption isotherms and BJH pore size distributions of (a)
POSS-[VMPS][H SO ] and (b) Poly([VMPS][H SO ]).
2
4
2
4

30
D. Lu et al. / Catalysis Communications 83 (2016) 27–30
2 4 2 4 2 4
Fig. 3. SEM and water droplets on the surface of (A) POSS-[VMPS][H SO ] and (B) Poly([VMPS][H SO ]), and TEM of (C) POSS-[VMPS][H SO ].
for four runs with just a small loss in catalytic activity. The POSS units
are revealed to be essential for the pores formation and the hydropho-
bicity of the catalyst, which are responsible for the great catalytic perfor-
Acknowledgements
The authors thank the National Natural Science Foundation of China
(No. 21206052), MOE & SAFEA for the 111 Project (No. B13025), and the
Fundamental Research Funds for the Central Universities (No.
JUSRP51507).
2 4
mance of POSS-[VMPS][H SO ].
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2016.05.004.
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Fig. 5. Catalytic reusability of POSS-[VMPS][H SO ] for the esterification of oleic acid with
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