Highly ordered mesoporous functionalized pyridinium protic ionic liquids framework as efficient system in esterification reactions for biofuels production

Article abstract of DOI:10.1016/j.mcat.2020.111238

Polysiloxane acidic ionic liquids containing pyridinium trifluoroacetate salts (PMO-Py-IL) were synthesized from pyridine containing organosilane precursors. Characterization by SEM, XRD, TGA, and nitrogen porosimetry confirmed that both pyridinium cation and trifluoroacetate anion were successfully incorporated within the organosilica network. The resulting organic-inorganic hybrid nanomaterial (PMO-Py-IL) was studied as nanocatalyst in free fatty acids esterification into biodiesel-like compounds. Remarkably, the synergistic hydrophilic/hydrophobic effect of pyridinium and trifluoroacetate ionic liquid in the well-ordered channels of PMO-Py-IL nanomaterial enhanced the activity toward sustainable biodiesel-like esters production. More importantly, PMO-Py-IL nanocatalyst also exhibited an exceptional activity and stability. The catalyst could be easily separated to reuse at least in ten reactions runs preserving almost intact its catalytic activity under otherwise identical conditions to those employed for the fresh catalysts.

Full text of DOI:10.1016/j.mcat.2020.111238

Molecular Catalysis 498 (2020) 111238
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.journals.elsevier.com/molecular-catalysis
Highly ordered mesoporous functionalized pyridinium protic ionic liquids
framework as efficient system in esterification reactions for
biofuels production
a,
b,c
Fatemeh Rajabi *, Rafael Luque
a
Department of Science, Payame Noor University, P. O. Box: 19395-4697, Tehran 19569, Iran
Departamento de Química Org a´ nica, Universidad de C ´o rdoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, Cordoba 14014, Spain
Peoples Friendship University of Russia (RUDN University), 6 Miklukho Maklaya St., 117198, Moscow, Russia
b
c
A R T I C L E I N F O
A B S T R A C T
Polysiloxane acidic ionic liquids containing pyridinium trifluoroacetate salts (PMO-Py-IL) were synthesized from
pyridine containing organosilane precursors. Characterization by SEM, XRD, TGA, and nitrogen porosimetry
confirmed that both pyridinium cation and trifluoroacetate anion were successfully incorporated within the
organosilica network. The resulting organic-inorganic hybrid nanomaterial (PMO-Py-IL) was studied as nano-
catalyst in free fatty acids esterification into biodiesel-like compounds. Remarkably, the synergistic hydrophilic/
hydrophobic effect of pyridinium and trifluoroacetate ionic liquid in the well-ordered channels of PMO-Py-IL
nanomaterial enhanced the activity toward sustainable biodiesel-like esters production. More importantly,
PMO-Py-IL nanocatalyst also exhibited an exceptional activity and stability. The catalyst could be easily sepa-
rated to reuse at least in ten reactions runs preserving almost intact its catalytic activity under otherwise identical
conditions to those employed for the fresh catalysts.
To the memory of Prof. Mohammad Reza Saidi,
colleague and inspiration for this work, who
passed away in 2020.
Keywords:
Nanomaterial
Pyridinium
Protic ionic liquids
Biodiesel
Free fatty acid
PMO
Esterification
1
. Introduction
Global awareness of energy and environmental issues related to the
several issues including product separation, installation corrosion and
undesired waste production. Therefore, improved catalytic systems with
operational simplicity and economic viability are essential for the green
and sustainable preparation of target chemical products. Immobilization
of homogeneous catalysts with easy handling and possible recyclability
can circumvent current issues, providing more sustainable catalyst al-
ternatives. A wide variety of porous supported acid and base catalysts
with various organic and inorganic supports including polymers, mes-
oporous materials and zeolites have been utilized in esterification re-
actions [10–25]. Several supported catalysts have shown promising
catalytic performance in the esterification reaction with an effective
separation step. However, the majority exhibited temperature
-dependent activities, thus poor recyclability. The weak catalyst sup-
port interaction caused catalytic site leaching from the support surface
at elevated temperatures.
burning of non-renewable fuels including petrol oil, natural gas and coal
inspired scientists to explore the use of sustainable alternative feed-
stocks for renewable energy including biofuels [1]. Among renewable
biofuel resources, biodiesel comprises long-chain fatty acid esters pro-
duced from biomass-derived oils and fats [2–4]. Conventional produc-
tion of biodiesel is base-catalyzed transesterification of triglycerides
contained in oils or fats with methanol or ethanol [5,6]. Despite its
utility, soap formation of free fatty acids content in oil feedstock results
in low biodiesel yield and difficulty in its separation from glycerol.
Esterification of free fatty acids accounts for a promising alternative for
the production of biodiesel-like biofuels [7,8]. The well-known synthesis
method for ester products, Fischer esterification, as atom economy fa-
vors the reaction between carboxylic acids and alcohols for the prepa-
ration of esters [9]. Esterification reactions are rather slow in the
absence of catalysts, requiring harsh conditions to reach equilibrium.
Hence, highly efficient solid acid catalysts with higher catalytic
stability and activity, tunable acidity and surface polarity for sustainable
biofuels production are needed.
Homogeneous mineral acid catalysts (i.e., H
SO
2 4
, HCl or H
3 4
PO ) exhibit
The use of suitable supports is the most effective approach to
*
Corresponding author.
E-mail addresses: f_rajabi@pnu.ac.ir (F. Rajabi), q62alsor@uco.es (R. Luque).
https://doi.org/10.1016/j.mcat.2020.111238
Received 20 August 2020; Received in revised form 24 September 2020; Accepted 27 September 2020
468-8231/© 2020 Published by Elsevier B.V.
2

F. Rajabi and R. Luque
Molecular Catalysis 498 (2020) 111238
Fig. 2. Pore size distribution (BJH) of a) PMO-Py and b) PMO- Py-IL.
and technology [34–38]. The unique physical and chemical properties of
ILs can be tuned by altering nature, size and shape of anions and cations.
A sub-class of ILs are protic ionic liquids (PILs), prepared via proton
transfer between a Br o¨ nsted acid and base. Their ease of preparation and
functionalization make them particularly beneficial in thermo-
electromechanical devices, gas absorption and separation, organic and
inorganic synthesis, biological applications, fuel cells and chromatog-
raphy. PILs have gained significant attention as acid catalysts in various
chemical reactions [39–43]. However, recyclability and separation of
PILs from the reaction media is a time and energy consuming process. An
alternative is the heterogenization of PILs on solid supports using several
approaches such as grafting, impregnation, polymerization, and
anchoring. Compared to the considerable research efforts on homoge-
neous PILs, there is so far little research on supported PILs, despite their
obvious importance in green and sustainable chemical process [44–50].
Nevertheless, several challenges such as low catalyst activity, high
catalyst amount, poor recyclability and harsh operating conditions
remain un- addressed. This work is aimed to develop a stable and
reusable supported solid catalyst with high catalytic performance
through the simple combination of trifluoroacetic acid and
pyridine-based PMO materials. Following previous studies on the
preparation of supported catalytic systems for organic transformations
2
Fig. 1. N physisorption isotherms of a) PMO-Py and b) PMO-Py-IL.
Table 1
Nitrogen adsorption data for PMO-Py and PMO-Py-IL.
2
3
Material
Surface Area (m
Pore Volume (cm
Pore diameter
(nm)
ꢀ
1
ꢀ 1
g
)
g
)
PMO-Py
PMO-Py-
IL
610
514
0.69
0.61
6.9
6.1
overcome these drawbacks. Among solid supports, periodic mesoporous
organosilicas (PMOs) with various bridge organo bis-silan are promising
in the development of functionally advanced materials. This generation
of well-ordered mesoporous materials combines a stable inorganic
support together with the presence of homogeneously distributed
organic moieties, leading to hybrid organic-inorganic frameworks. Due
to excellent characteristics and properties including tunable surface
properties, high surface areas and pore sizes, high thermal stabilities,
appropriate hydrophobicity and the potential to incorporate a range of
organic functionalities into their walls, PMOs have been proved to be
excellent systems with a broad variety of potential applications in
physics, chemistry, engineering, medicine, biology, and advanced ma-
terial science [26–34].
[
51–53], a porous hydrophobic PMO nanomaterial based on pyridinium
ionic liquid organosilica was synthesized and employed as a highly
stable and efficient system for the esterification of various free fatty
acids and methanol under mild conditions.
Ionic liquids (ILs), a class of low-melting organic salts, have attracted
considerable attention as green media and promising catalysts in science
Fig. 3. XRD patterns corresponding to the PMO-Py (left) and PMO-Py-IL (right).
2

F. Rajabi and R. Luque
Molecular Catalysis 498 (2020) 111238
Fig. 4. SEM images of PMO-Py-IL.
Scanning electron microscopy (SEM) images were recorded in a Jeol
JSM 6490 LA field emission apparatus, recorded at an acceleration
voltage of 15 kV.
◦
Thermogravimetric analysis (TGA) was performed from 25 to 800 C
◦
ꢀ 1
using a heating rate of 10 C⋅min under static air atmosphere by a
NETZSCH STA 409 PC/PG Instrument.
2
.2. Synthesis of diethyl pyridine-2,6-dicarboxylate
Diethyl pyridine-2, 6-dicarboxylate, employed as a bridging group,
was prepared following a previously reported protocol [50]. Briefly, 50
mmol of pyridine-2,6-dicarboxylic acid and 1 mmol of p-toluenesulfonic
acid (p-TsOH) in 100 mL ethanol were refluxed under stirring for 2 h,
then diethyl pyridine- 2,6-dicarboxylate was obtained after dissolving
the solid residue obtained in about 30 mL of diethyl ether (subsequently
washed with sodium bicarbonate solution and water), followed by
drying (magnesium sulfate), filtration and solvent evaporation to yield
Fig. 5. Thermogravimetric analysis/differential thermogravimetric (TGA/
1
DTG) curves for PMO-Py-IL.
pure diethyl pyridine-2,6-dicarboxylate as a white solid (95 % yield). H
NMR (300 MHz, CDCl
4 H), 8.01 (t, J = 8.5 Hz, 1 H), 8.26 (d, J = 8.4 Hz, 2 H) ppm. C NMR
75 MHz, CDCl
): δ, 167.8, 149.6, 142.2, 131.0, 65.7, 15.7 ppm.
3
): δ, 1.46 (t, J = 8.7 Hz, 6 H), 4.49 (q, J = 8.7 Hz,
13
2
. Materials and methods
(
3
2
.1. Material characterization
2
.3. Synthesis of Bis(3-(Triethoxysilyl)Propyl)Pyridine-2,6-
Textural properties of design nanomaterials were determined by
Dicarboxamide
using nitrogen physisorption measurements (77 K) using a Micro-
meritics ASAP 2000 instrument (Micromeritics, Norcross, GA, USA).
Bis(3-(Triethoxysilyl)Propyl)Pyridine-2,6-Dicarboxamide was also
◦
ꢀ 4
Materials were degassed for 24 h at 100 C under vacuum (10 mbar),
prior to analysis. Surface area was calculated using the BET (Bru-
nauer–Emmet–Teller) equation, while pore size distributions (DBJH) and
pore volumes (VBJH) were worked out using the BJH method (Barrett,
◦
obtained as previously reported [50] via heating at 170 C (in a sealed
tube under nitrogen atmosphere for 5 h) a mixture of sodium ethoxide
(
0.1 mmol), diethyl pyridine-2,6-dicarboxylate (10 mmol), and 3-(trie-
thoxysilyl)propyl amine (20.5 mmol). Chloroform was added for
extraction purposes and the final suspension was filtered (solvent
removed under vacuum) to obtain pure bis(3-(triethoxysilyl)propyl)
Joyner, and Halenda) from the N
Powder X-ray diffraction patterns were obtained in a Bruker-AXS
diffractometer using Cu K
2
adsorption branch.
α
radiation (λ =1.5409 Å). Low-angle X-ray
pyridine-2,6-dicarboxamide as
11Si (778.17): the calculated composition was C: 52.48, H:
.68, N: 5.40, while the actual one was C: 53.01, H: 8.24, N: 5.14.
NMR (300 MHz, CDCl
): δ, 0.07 (m, 4 H), 1.16 (t, J = 7.02 Hz, 18 H),
a
white solid (92
%
yield).
◦
◦
diffractograms were recorded in the 0.5 ⩽ 2θ ⩽ 5 range (step size
C
34
H
67
N
3
O
3
◦
0
.01 , 5 s time per step). Wide-angle diffraction patterns were collected
1
8
H
◦
◦
in the 10 ⩽ 2θ ⩽ 80 range at a scan time of 1.3 s (step size 0.017).
3
1
8
1
.72 (m, 4 H), 3.44 (m, 4 H), 3.72 (m, 12 H), 7.97 (d, J = 8.4 Hz, 1 H),
13
.31 (d, J = 8.4 Hz, 2 H) ppm. C NMR (75 MHz, CDCl ): δ, 166.3,
3
51.5, 141.3, 127.1, 60.7, 44.4, 25.5, 20.6, 10.3 ppm.
2
.4. Synthesis of PMO nanomaterials bearing pyridinedicarboxamide
(
PMO-Py)
Following our previously reported protocol [50], 12.5 g of 2 M HCl
solution was employed to dissolve 0.4 g of Pluronic P123 (Aldrich,
◦
average Mw ≅ 5800) under stirring at 35 C, followed by the addition of
2
.3 g of bis(3-triethoxysilyl)propyl)pyridine-2,6-dicarboxamide, keep-
◦
ing the mixture under stirring for 20 h and subsequent aging (90 C,
overnight without stirring). The final material was filtered off and dried
◦
Fig. 6. Reusability of PMO-Py-IL nanocatalyst in the esterification of stearic
at 50 C overnight, removing the template via solvent extraction in a
acid and methanol.
solution of 1.0 mL 37 % HCl and 100 mL ethanol for 12 h. The
3

F. Rajabi and R. Luque
Molecular Catalysis 498 (2020) 111238
Scheme 1. Synthesis of protic pyridinium ionic liquid loaded periodic mesoporous nanomaterials (PMO-Py-IL).
template-free material was eventually washed thoroughly with a
3. Results and discussion
◦
mixture water/ethanol (24 h, Soxhlet apparatus) and dried at 80
overnight prior to its use. Finally, it was dried in an oven at 80
overnight.
C
C
◦
Novel PMO-Py materials were recently reported according to a two-
step approach depicted in Scheme 1. PMO-Py exhibited a high catalytic
activity in the Knoevenagel condensation reaction [35]. Herein, the
synthesis of an acidic hybrid organosilica with pyridinium ionic liquid
framework as a solid-acid nanocatalyst is described. The synthesis of the
pyridinium-based ionic liquid hybrid nanomaterial (PMO-Py-IL) was
achieved by the addition of trifluoroacetic acid to PMO-Py. The obtained
porous solid-acid pyridinium nanomaterial (PMO-Py-IL) was charac-
terized by SEM, XRD, and nitrogen porosimetry analysis experiment.
2
.5. Synthesis of PMO Materials bearing protic pyridinium ionic liquid
(
PMO-Py-IL)
2
g of PMO materials bearing Pyridinedicarboxamide (PMO-Py) were
mixed with 10 mL of dry acetonitrile 3 mmol of trifuloroacetic acid were
subsequently poured into the mixture under vigorous stirring for 5 h (at
room temperature). The resulted protic pyridinium ionic liquid silica
was filtered, washed with acetonitrile and dried under vacuum.
3
.1. Catalyst characterization
3
.1.1. BET-BJH analysis
N
2
physiosorption analysis of the organic-inorganic hybrid materials
was performed to study their porosity. According to Fig. 1, PMO-Py
support and PMO-Py-IL nanocatalyst exhibited N -adsorption/desorp-
2
.6. General procedure for biodiesel production using PMO-Py-IL
materials
2
tion isotherms with hysteresis loop as IV type isotherm curves, classic of
mesoporous materials. Hysteresis loop shape pointed to 2D hexagonal
structures, so that the uniform dispersion of active trifluoroacetate exists
within the pores of the material. As expected, after addition of tri-
fluoroacetic acid and formation of pyridinium salt with trifluroacetate
counter ion, the average pore diameter and surface area of the modified
materials (PMO-Py-IL) decreased (Table 1) (Fig. 2).
In a typical reaction, fatty acids (10 mmol), methanol or ethanol (5
mL), and PMO-Py-IL (0.1 g) as nanocatalyst were mixed in a 50 mL
single-necked round bottomed flask and stirred under reflux conditions
for 4 h. Upon reaction completion, the mixture was cooled down at RT,
PMO-Py-IL was recovered and washed with ethyl acetate for the next
run. The combined filtrate and ethyl acetate washings were washed with
water and the organic layer was separated and dried over sodium sul-
fate, filtered, and concentrated under vacuum to provide the desired
methyl esters as pure products.
Scheme 2. Esterification of stearic acid catalyzed by PMO-Py- IL.
4

F. Rajabi and R. Luque
Molecular Catalysis 498 (2020) 111238
Table 2
acid addition, with PMO-Py being stable under these conditions.
Optimisation of reaction conditions in the esterification of stearic acid with
methanol.
3.1.3. SEM analysis
◦
b
Fig. 4 depicts scanning electron microscopy (SEM) images of PMO-
Py-IL, consisting of uniform cylindrical/spheroidal shape structures as
similarly reported for PMO-Py materials [50].
Entry
PMO-Py-IL (g)
Time (h)
T ( C)
Yield(%)
1
2
3
4
5
6
7
8
–
10
6
reflux
reflux
reflux
reflux
reflux
reflux
reflux
45
10
96
95
90
95
72
0.2
0.1
0.1
0.1
0.05
–
6
3
3
.1.4. TG/DTG analysis
4
To assess the thermal stability of supported protic ionic liquid PMO-
4
c
Py-IL thermogravimetric analysis/differential thermogravimetric anal-
ysis (TGA/DTGA) was performed (Fig. 5). TG and DTG curves displayed
in Fig. 4 indicate three clear weight losses at different temperatures. A
10
6
70
0.1
55
a
All reactions were carried out with 10 mmol of stearic acid and 5 mL of MeOH.
◦
b
ca. 10 % mass loss could be observed up to 150 C due to physisorbed
Isolated yield.
c
water and ethanol (additional water loss via condensation of residual
silanol and ethoxy groups). A further distinct weight loss (ca. 50 %) was
PMO-Py (0.2 mol%) was used as catalyst.
◦
found to be present in the 400ꢀ 600 C range, related to the removal of
pyridinium moieties and remaining organics in the PMO structure. As-
synthesized PMO-Py-IL materials exhibited a rather stable behavior to-
wards temperature, keeping PMO-Py-IL framework thermal stability up
3
.1.2. XRD analysis
Fig. 3 depicts low angle X-ray diffraction (XRD) patterns for PMO-Py
◦
to 400 C.
materials, clearly showing a sharp signal corresponding to the (100)
reflection and two additional less intense signals assigned to the (110)
and (200) diffraction lines, suggesting a well- ordered 2D-hexagonal
structure corresponding to the P6mm space group. By comparing the
XRD data of PMO-Py-IL with PMO-Py, it was found that the diffraction
peaks with lower intensity in PMO-Py-IL were observed at the same
region and show no differences in XRD patterns. This evidence indicates
that the ordered mesostructure is well preserved after trifluoroacetic
3.2. Catalytic activity for the esterification of free fatty acids to produce
biodiesel
After characterization of PMO-Py-IL, the esterification of stearic acid
using different alcohols (methanol) was subsequently performed, with
model reaction results shown in Scheme 2. Reaction parameters
including quantity of catalyst, reaction temperature and times were
Table 3
Esterification of various fatty acids and alcohols catalyzed by PMO-Py-IL.
a
Entry
1
Substrate
Alcohol
MeOH
Yield (%)
95
2
3
4
5
MeOH
MeOH
MeOH
Et OH
95
92
92
96
6
7
EtOH
96
90
MeOH
8
MeOH
MeOH
92
90
9
a
Isolated yield.
5

F. Rajabi and R. Luque
Molecular Catalysis 498 (2020) 111238
optimized (Table 2). Blank runs carried out without catalyst addition
provided ca. 10 % of methyl stearate yield under reflux conditions in 10
h. Optimum conditions were achieved using 0.1 g PMO-Py-IL nano-
catalyst under methanol reflux in 4 h (Table 2, entry 5) due to the high
efficiency of the nanocatalyst. Compared to PMO-Py (Table 2, entry 7),
higher product yield with less amount of catalyst was obtained by PMO-
Py-IL which is attributed to the synergistic effect between pyridinium
and trifluoroacetate ionic liquid in enhancing the activity of the catalyst.
To assess catalyst stability and leaching of active species from the
support, a hot filtration test was conducted for the model reaction
Credit author statement
FATEMEH RAJABI
Experiments, funding, writing first draft
RAFAEL LUQUE
Revisions, discussion, writing final article and submission
Declaration of Competing Interest
(
esterification of stearic acid and methanol) under optimized conditions.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
The catalyst was removed via hot filtration after 1 h and the filtrate
system proceeded to react for 5 h after catalyst removal. No esterifica-
tion reaction progress was observed, pointing to the absence of any
leached active species in the filtrate. These results confirmed strong
incorporation and interaction of trifluoroacetate and pyridinium groups
in the organosilica network not only could suppress leaching of active
species, but also could enhance the catalytic activity of the catalyst via
synergistic and cooperative effects.
Acknowledgments
F. R. would like to acknowledge Payame Noor University and Iran
National Science Foundation (INSF) for support of this work. The pub-
lication has been prepared with support from RUDN University Program
These promising results were followed by detailed studies on sub-
strate scope, using a broad range of renewable fatty acids (Table 3). As
shown in Table 3, the isolated yields of esters ranged from 90 % to 96 %
and the required time for the reaction completion was 4 h. Linear fatty
acids such as palmitic, myristic, lauric, and stearic acid were reacted
with methanol or ethanol to produce corresponding methyl or ethyl
esters in high yields (Table 3, entries 1–6). Furthermore, this approach
was amenable to the presence of other functionalities in the substrates
such as carbonyl and alkene moieties, that remain completely unaffected
for which biomass- derived substrates including oleic, levulinic and
linoleic acid could be employed without any protection of their acid-
sensitive functional groups under acid-catalyzed esterification reaction
conditions (Table 3, entries 7–9). Most significantly, these excellent
results provided strong evidence that the synergetic effects of pyr-
idinium cation and trifluoroacetate anion in the mesoporous organo-
silica support were the key factors in catalyst performance to produce
high yield of methyl esters under mild conditions.
5
-100.
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. Conclusions
[
[
Protic pyridinium based ionic liquid hybrid mesoporous materials
(
PMO-Py-IL) were successfully prepared by introduction of a 3-propyl
triethoxysilane side chain to pyridine ring and formation of pyridine
polysiloxane network followed by trifluoroacetic acid addition and
subsequent ionic liquid organosilica formation. SEM, BET, TGA, and
XRD patterns confirmed the interaction between the pyridinium cation
and trifluoroacetate anion in the mesoporous material. After full char-
acterization and structural confirmation, the resulted solid PMO-Py-IL
could be utilized in esterification reaction between a broad range of
free fatty acids and short chain alcohols. PMO-Py-IL hybrid materials
showed the strong ability and activity toward the desired ester products
and proved to be active and stable in the esterification reaction.
Remarkably, an eco-friendly and feasible method for sustainable pro-
duction of methyl esters has been accomplished using mesoporous
supported protic ionic liquid with highly well-ordered surface area as a
highly efficient and robust nanocatalyst.
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