Decoration of chitosan microspheres with Br?nsted heteropolyacids and Lewis ion Ti: Trifunctional catalysts for esterification to biodiesel

Article abstract of DOI:10.1039/c7ra07479d

H₃PW₁₂O₄₀ is a commonly used Br?nsted acid catalyst in esterification and transesterification reactions to produce biodiesel, whose homogeneous form and single acid sites lead to difficulties in separation and relatively less activity. Herein, the water-insoluble and multifunctional active sites based on H₃PW₁₂O₄₀, chitosan and Ti⁴⁺ had been fabricated giving H₃PW₁₂O₄₀/Ti/chitosan tri-functional hybrids. Such hybrids exhibited higher activity in esterification reactions due to the existence of Br?nsted acid from H₃PW₁₂O₄₀, Lewis acid from Ti⁴⁺, and base sites from the -NH₂ group of chitosan, and all also due to the generation of pores in chitosan through introduction of the Ti ions. Furthermore, H₃PW₁₂O₄₀/Ti/chitosan acted as heterogeneous catalysts and could be separated for reuse at least six times without significant loss of activity and with little leaching of Ti⁴⁺ and H₃PW₁₂O₄₀ from the support chitosan.

Full text of DOI:10.1039/c7ra07479d

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PAPER
Decoration of chitosan microspheres with Brønsted
heteropolyacids and Lewis ion Ti: trifunctional
catalysts for esterification to biodiesel†
Cite this: RSC Adv., 2017, 7, 42422
Tong Tong, Yiming Li, Rui Hou, Xiaohong Wang * and Shengtian Wang
H
3
PW12
produce biodiesel, whose homogeneous form and single acid sites lead to difficulties in separation and
relatively less activity. Herein, the water-insoluble and multifunctional active sites based on H PW12
40/Ti/chitosan tri-functional hybrids. Such hybrids
O40 is a commonly used Brønsted acid catalyst in esterification and transesterification reactions to
3
40
O ,
4+
3
chitosan and Ti had been fabricated giving H PW12O
exhibited higher activity in esterification reactions due to the existence of Brønsted acid from
Received 7th July 2017
Accepted 28th August 2017
4+
H
3
PW12
O
40, Lewis acid from Ti , and base sites from the –NH
2
group of chitosan, and all also due to
PW12 40/Ti/
chitosan acted as heterogeneous catalysts and could be separated for reuse at least six times without
the generation of pores in chitosan through introduction of the Ti ions. Furthermore, H
3
O
DOI: 10.1039/c7ra07479d
4+
rsc.li/rsc-advances
3
significant loss of activity and with little leaching of Ti and H PW12O40 from the support chitosan.
bifunctional catalysts in transesterication and esterication
reactions. Nevertheless, there is still plenty room for fabrication
Introduction
Heteropolyacids (HPAs) are discrete early transition metal-oxide different bifunctional HPAs with spatially isolated multiple
clusters with a variety of compositions and structures, which active sites and easy regeneration, which can promote simul-
1
have been applied in many elds of science. HPAs are well- taneous reactions.
known super-strong acids for both homogeneous and hetero-
Biodiesel is a viable alternative to replace petrodiesel, and it
2
geneous acid-catalyzed reactions. The free acidic forms of has become more attractive due to the environmental
8
HPWs with Keggin structure, especially 12-tungstophosphoric concerns. The second-generation biodiesel is normally
acid (H PW O , HPW), have attracted much attention for considered to be obtained from nonedible oils such as castor,
3
12 40
3
chemical conversions because of their strong Brønsted acidity Jatropha, microalgae, and animal fats (e.g. tallow and yellow
and higher stability. During these years, fabrication of multi- grease). By now, an interesting alternative for low cost biodiesel
functional HPAs have attracted much attention due to their production is the utilization of low quality raw materials as
wide applications in one-pot transformation or tandem reac- feedstocks, especial waste cooking oil (WCO) obtained from
4,5
tions of organic substrates. Articial HPA materials func- restaurants or from houses with higher contents of free fatty
9
–11
tionalized by acid–base synergy and cooperative effects are acids and water.
Waste cooking oil, which is available
relatively less addressed and stimulate a real challenge. The rst cheaply, is an attractive starting material that can help in
report on bifunctional HPAs catalysis with acidic and basic sites improving the economic feasibility of biodiesel especially for
12
was reported by N. R. Shiju and co-workers, demonstrating that China or India. However, there are some drawbacks in
site-isolated amine and phosphotungstic acid groups were conversion of such low-quality feedstocks — impossibility of
6
loaded on mesoporous silica, but the Brønsted acidity of HPAs converting free fatty acid (FFA) to FAME and soap formation
and the base property of the host were suppressed by each catalyzed by base, and unavailability for mineral acid due to the
13
other. Other bifunctional HPAs had been prepared using existence of the high contents of water. In this concept, the
layered double hydroxides as the base source and HPAs as the heterogeneous acid catalysis process is expected to be an
7
acidic one. During our research work on biomass conversion effective biodiesel production process with some benets:
catalyzed by HPAs, we designed and prepared acid–base HPAs insensitivity to FFA and water contents, simultaneous occur-
4
using amino acids as base groups, which were used as rence of esterication and transesterication, no further
washing treatment for generated soup, easy separation of the
catalyst from the reaction medium, low contamination of the
Key Lab of Polyoxometalate Science of Ministry of Education, Northeast Normal
product, recyclability of the catalyst, and no corrosion
University, Changchun 130024, P. R. China. E-mail: wangxh665@nenu.edu.cn; Tel:
14
problem. However, their application usually suffer from infe-
rior activity due to some disadvantages: declining acid strength,
non-uniform dispersion of acid or base sites on the solid
+
†
1
86-431-85099759
Electronic supplementary information (ESI) available. See DOI:
0.1039/c7ra07479d
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surface, mass transferring difficulty at the interface, or over- was separated through centrifuge and dried at 303 K for 24 h to
15
lapped exposure of the acid sites. Therefore, an ideal solid acid give HPW functionalized chitosan (HPW/Ti/chitosan) with yield
catalyst for biodiesel production should have high stability, of 78% as HPW (29 wt%)/Ti/chitosan.
isolated acid sites, large pores, hydrophobic surface, multi-
Other solid catalysts containing different amount of HPW
functional sites with coordination effect, recyclability and was prepared using the same procedure except with various
15,16
reusability.
In addition, the existence of Lewis acid on the usage of HPW as 0.142 g, 0.166 g, 0.250 g and 0.334 g,
solid catalysts could provide synergistic effect on esterication respectively.
1
7,18
reactions.
From the literature, it is noted that catalysts with
19
Lewis acid sites such as Zn, Ag, Sn could gave the higher Determination of the acidic and basic properties
5
ꢀ
conversion of ester. In our previous work, PW TiO
40
anion
had been prepared and showed promoting effect on esterica-
1
1
Two titration methods were put into use to gure up the total
acid content of the catalysts. Firstly, 0.05 g catalyst was put into
20
tion reaction. Recently, some methods have been developed to
design multifunctional solid catalysts to achieve esterication
and transesterication occur simultaneously.
Herein, we designed to synthesize bifunctional HPAs
through loading Brønsted HPW H 40 on chitosan to form
45 mL distilled water and stirring the mixture for 3 h. The
concentration of total acid was measured by titration with
diluted n-butylamine in distilled water (0.05 M) with the indi-
cator anthraquinone (pKa ¼ ꢀ8.2). And then IR spectra of
pyridine adsorption worked for recognizing the Lewis and
Brønsted acid sites. Pyridine was adsorbed by catalysts speci-
15
3
PW12O
acid–base hybrids HPW/chitosan. Chitosan has been proposed
to be an ideal support material with weak basic properties due
ꢁ
mens for 12 h under the condition of vacuum and 60 C. The
21
the existence of free amino groups. HPW reacted with some of
the amino groups, hence were loaded on the matrix of chitosan.
And other amino groups without interacting with HPW were
contribution to the basic property for HPW/chitosan. The
acidity and basicity could be controlled through the component
of HPW and chitosan. Furthermore, Lewis centres were intro-
duced by introduction of Ti ion into the matrix of chitosan.
Hence, trifunctional active sites based on heteropolyacid HPW/
Ti/chitosan were fabricated through the electrostatic interaction
between amino group in chitosan and HPW molecules, coor-
24
capacity of acidity was calculated by Lambert–Beer formula:
A
ꢀ1
¼
(3 ꢂ W ꢂ c)/S. (A: absorbance, cm , 3: extinction coefficient,
2
ꢀ1
m mol , W: the sample weight, kg, c: the concentration of
ꢀ1
2
acid, mmol g , S: the sample disk area, m ). Base content of the
catalysts was also obtained by titration and the pretreatment
was the same as acid. Then the titration was proceeded with
acetic acid in distilled water (0.05 M) with the 2-(4-dimethyla-
minophenylazo) benzoic acid (pK
In addition, another titration method with automatic
potentiometric titrator was done as 0.02 g catalyst was put into
a
¼ 4.76).
2
dination bond between –NH and Ti ions. The strong interac-
40 mL acetonitrile solution with stirring intensely for 3 h and
tion between HPW, Ti and chitosan conrmed the less leaching
of active sites from the support. Furthermore, this chitosan
matrix did not dissolve in methanol, which could be easily
separate from the reaction mixture for reuse. As-prepared HPW/
Ti/chitosan was evaluated in esterication of FFAs for produc-
tion of biodiesel.
then the obtained suspension was titrated with diluted n-
butylamine in acetonitrile (0.1 M). The titration end-point could
be received from potentiometric titration curve so that the total
acid content of the catalysts could be calculated. In a similar
way, the base content was measured. Putting 0.02 g catalyst into
ethanol solution and stirring for 3 h. Then the mixture was
titrated with benzoic acid in ethanol (0.02 M).
Experimental
Materials
Esterication reaction
Esterication reaction was carried out in a 50 mL three-necked
glass ask charged with cooled condenser and a mechanical
stirrer at 65 C. In a typical experiment, the reactor was loaded
H PW O $23H O was prepared according to ref. 22. All
chemicals of AR grade were attained from commercial suppliers
without further purication.
3
12 40
2
ꢁ
with 2.5 g of palmitic acid, a certain amount of anhydrous
methanol, and a certain amount of catalyst. Samples were taken
periodically, and tested to determine the conversion of palmitc
acid in gas chromatography (GC). Aer nishing reaction, the
catalyst was separated by centrifugation, washed with methanol
to remove the remaining reactants and water, then dried in air
and reused for the next experiment. And the leaching of the
solid catalyst into the mixture was measured the W contents by
ICP.
Preparation of catalysts
In a 3-necked ask, urea (0.1 mol) and titanium sulfate (0.0125
mol) were dissolved in 100 mL of distilled water and stirred for
1
h at 333 K. Acetic acid solution (5%, 50 mL) containing chi-
tosan (2%) was subsequently added to the ask. The pH was
adjusted to 7 using ammonia, and then the mixture was stirring
for about 3 h. The powder was centrifuged to be separated and
washed with water to separate unreacted urea and Ti(SO
4 2
) .
Instrument
Finally, the powder was dried at 353 K for 6 h to give Ti func-
23
tionalized chitosan (Ti/chitosan) with yield of 84%.
The acidic and basic properties were measured on a ZDJ-4B
Ti/chitosan (0.48 g) and H PW O (0.2 g) were dissolved in automatic potentiometric titrator. FTIR spectra (4000–
3
12 40
ꢀ
1
dilute hydrochloric acid solution (1 M, 20 mL) together with 500 cm ) were recorded on a Nicolet Magna 560 IR spec-
vigorous stirring at room temperature for 6 h. Then the powder trometer. A Leeman corporation inductively coupled plasma
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ICP) emission spectrometer and a PE 2400 CHN elemental Characterization of catalysts
Paper
(
analyzer was used to determine the contents of elements in
catalysts. XPS was performed by an Escalab-MK II photoelec-
From the result of elemental analysis, the loading amounts of
H PW12O40 on chitosan were 23 wt%, 26 wt%, 29 wt%, 34 wt%
3
3
1
tronic spectrometer with Al Ka (1200 eV). P MAS NMR spectra
of the catalysts were obtained using a Bruker AM500 spec-
trometer at 202.5 MHz. Nitrogen adsorption–desorption
isotherms and Brunauer–Emmet–Teller (BET) surface area
were measured on an V-Sorb2800P surface analyzer. X-ray
diffraction (XRD) patterns of the catalysts were obtained on
Rigaku Dmax 2000 X-ray diffractometer with Cu Ka radiation (l
and 41 wt%, respectively (Table 1). And the amount of Ti was
about 19 wt%, 18 wt%, 17 wt%, 16 wt% and 15 wt%,
respectively.
The IR spectra of HPW/Ti/chitosan were investigated (Fig. 1).
ꢀ1
ꢀ1
For Ti/chitosan, the band at 1620 cm and 590 cm were
29
30
assigned to vibration of Ti–NH2 bond, and Ti–O bond,
respectively, determining that Ti ions were loaded on the matrix
of chitosan through coordinate interaction between Ti and –OH
¼
0.154178 nm) (Rigaku Corporation). The energy dispersive X-
ray spectrometry (EDS) elemental mapping was given on
a HITACHI SU8010 Field-emission scanning electron micros-
copy. The concentrations of esters were determined periodi-
cally on Agilent Technologies 7820A GC system tted with
capillary column (30 m ꢂ 0.25 mm) and ame ionization
detector.
groups and Ti and –NH
vibration of O–H in chitosan was revealed at 1020 cm and in
Ti/chitosan was presented at 1068 cm . And some of –NH
2
as well in chitosan. In addition, the
ꢀ1
ꢀ1
2
ꢀ1
ꢀ1
vibration peak also shied from 1545 cm
to 1522 cm
+
belongs to –NH . Such shis also conrmed the existence of
3
4+
coordinate interaction between Ti and –NH2 and –OH in
chitosan. For H PW O , the IR stretching vibrations found in
3
12 40
ꢀ
1
the range of 700–1100 cm were assigned to the Keggin unit of
3
ꢀ
ꢀ1
PW12
W]O
O
40 , which correspond to vibration of P–O
a
(1073 cm ),
Results and discussion
ꢀ
1
ꢀ1
d
(957 cm ), W–O
C
–W and W–O
b
–W (877 and 745 cm ),
Preparation of trifunctional HPW/Ti/chitosan
respectively. Aer being loaded on chitosan, these four peaks
were also found with some shis of the peak positions. The
Chitosan is a kind of polysaccharide macromolecules as hier-
archical supports for catalysts, which was used to fabricate
peaks for W]O , W–O –W and W–O –W shied to higher
d
C
b
25–27
wavenumbers, indicating the existence of electrostatic interac-
acid–base bifunctional catalysts.
M. Yamada had used the
31
tion between HPW anion and chitosan. Same results
conrmed the structural integrity of HPW and stability of HPW
and Ti in chitosan.
amino groups in chitosan to load heteropolyacids H
3
3ꢀ
PW12O
40
+
through the interaction of –NH
3
and PW12O40 , which
28
exhibited catalytic activity in epoxidation of allylic alcohols.
The structural integrity of the Keggin unit in the HPW
And strong electrostatic interaction between HPW anion and
amino group in chitosan conrmed no leaching of HPW from
support. Using the same synthetic strategy, here we fabricated
trifunctional HPW/Ti/chitosan containing Brønsted acid, Lewis
acid and base sites. Firstly, Ti metal ions were loaded on chi-
tosan matrix through coordinate interaction following the
equation:
3
1
(
29 wt%)/Ti/chitosan hybrids was further conrmed by P MAS
NMR (Fig. 2a). One peak with a shoulder for HPW/Ti/chitosan
was observed at ꢀ16.92 ppm, which was attributed to the
resonance of PO unit within the bulk H PW O similar to its
4
3
12 40
32
parent (ꢀ15.6 ppm). The shoulder and some shi were
assigned to interaction between chitosan and H
which conrmed less leaching of H 40 from the support
chitosan during the catalytic reactions.
3
40
PW12O ,
3
PW12O
2 4 2 2
Chitosan–NH + Ti(SO ) / chitosan–NH /Ti
The XPS analysis mainly reects the composition and
chemical elementary state of the surface and the inferior
33
Secondly, HPW molecules were anchored on chitosan matrix
by mixing the Ti/chitosan and HPW solutions through electro-
surface of sample. XPS survey spectrum and the correspond-
ing high-resolution of the catalysts were shown in Fig. 3a.
Compared to chitosan, new absorption peaks at 458.6 eV and
3
ꢀ
static interaction between PW O
2 40
+
and NH3 as HPW/
1
+
NH –chitosan–NH /Ti (Scheme 1).
34
3
2
464.3 eV correspond to Ti 2p3/2 and Ti 2p1/2 in Ti/chitosan were
found due to the introduction of Ti into the chitosan matrix
4
+
(
3
Fig. 3c), which existed as Ti . And the N 1s gave two peaks at
99.70 eV and 401.51 eV, respectively (Fig. 3b), which the peak at
around 399.70 eV was originated from NH from chitosan. And
another peak at 401.51 eV was due to the coordinated N to Ti
2
35
(
NH2 / Ti). Therefore, there were two species of nitrogen
species existence, i.e. coordinated N and unreacted N. This
conrmed that Ti cation was successfully introduced into chi-
tosan through coordination interaction. For HPW/Ti/chitosan
(
4
Fig. 3d), the N 1s also gave two peaks at 400.33 eV and
01.86 eV, respectively, which were attributed to NH from
, respectively. This indicated
2
+
2 3
chitosan and –NH to Ti or NH
the formation of HPW/Ti/chitosan through electrostatic
Scheme 1 The preparation of HPW/Ti/chitosan hybrids.
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Fig. 1 IR spectra of (a) chitosan, (b) Ti/chitosan, (c) HPW (50 wt%)/
chitosan, (d) HPW (23 wt%)/Ti/chitosan, (e) HPW (26 wt%)/Ti/chitosan,
(
(
f) HPW (29 wt%)/Ti/chitosan, (g) HPW (34 wt%)/Ti/chitosan, (h) HPW
41 wt%)/Ti/chitosan, and (i) H PW12
3
40
O .
31
Fig. 2
P MAS NMR spectra of HPW (29 wt%)/Ti/chitosan before (a)
and after catalytic reaction (b).
interactions. Meanwhile, two new peaks at 35.38 eV and
3
W
7.45 eV were found, which the new peaks were attributed to the
6
+
36
from HPW. As result, HPW/Ti/chitosan triple solid had
+
been prepared as HPW/NH –chitosan–NH /Ti.
3
2
Nitrogen sorption analysis for chitosan, Ti/chitosan, HPW
and HPW (29 wt%)/Ti/chitosan were done to analyze the surface
area and pore characters (Table 1 and Fig. S1†). It can be seen
that native chitosan and HPW exhibited lower BET surface area
37
and no porous characters. However, the surface areas were
2
ꢀ1
found to be 133.18 and 92.26 m
g
corresponding to Ti/
chitosan and HPW (29 wt%)/Ti/chitosan, respectively. And
pore diameters were 4.08 nm and 4.38 nm, respectively. These
new generations for surface area and porous characters were
4
+
supposed to the introduction of Ti to chitosan matrix.
The acidic and basic properties for chitosan, Ti/chitosan and
a series of HPW/Ti/chitosan were measured using different
methods (Table 1). And these two methods did not give signif-
icant difference for total acidity and basicity. It can be seen that
native chitosan only exhibited basic property with basic
ꢀ1
strength as 0.69 mmol g . For Ti/chitosan, the basicity was
ꢀ
1
decreased to 0.29 mmol g and Lewis acidic strength was
ꢀ
1
enhanced to 0.56 mmol g due to the introduction of Lewis
acid metal Ti.
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resulted in tri-functional acid–base catalysts, and the acidic and
basic distribution could be controlled by varying the different
loading amount of HPW on Ti/chitosan.
Catalytic activity
Stimulated by the presence of both basic primary amino groups
(
NH ) from chitosan, Brønsted acidity from HPW, and Lewis
2
acidity by Ti, we anticipated that this coexistence might lead to
synergic and cooperative effect in catalysis. We selected an
organic transformation (esterication), which is considered as
sensible to the acid/base synergy.
In order to determine the different inuence of active sites
on esterication reaction of palmitic acid with methanol, the
comparison experiments (Fig. 5) had been done using a various
of catalysts including chitosan, Ti/chitosan, H PW O , HPW
3
12 40
(
50 wt%)/chitosan, HPW (23 wt%)/Ti/chitosan, HPW (26 wt%)/
Fig. 3 XPS surveys and the corresponding high-resolution (a), N 1s (b),
Ti 2p (c), and W 4f (d) core level spectra of the chitosan, Ti/chitosan and
HPW (29 wt%)/Ti/chitosan.
Ti/chitosan, HPW (29 wt%)/Ti/chitosan, HPW (34 wt%)/Ti/
chitosan, HPW (41 wt%)/Ti/chitosan under the reaction condi-
ꢁ
tions as 65 C, molar ratio of methanol/oil ¼ 13 : 1, 5 wt%
catalyst, 7 h. It can be seen that without any catalysts, palmitic
acid was esteried with methanol only giving 8.7% conversion.
Using chitosan and Ti/chitosan the conversion of palmitic acid
increased to 33.9% and 63.2%, respectively. The TOF values
HPW/Ti/chitosan exhibited both acidic and basic properties,
as well as double Brønsted acidity and Lewis acidity. The total
acidity depended on the different weight ratios between
(
TOF ¼ (conversion% ꢂ amount of palmitic acid)/[actual active
HPW : Ti : chitosan
as
HPW
(41
wt%)/Ti/chitosan
ꢀ1 ꢀ1
sites (acidity + basicity) ꢂ h] g mmol
h
) based on chitosan
ꢀ
1
ꢀ1
(1.06 mmol g ) > HPW (34 wt%)/Ti/chitosan (0.98 mmol g
)
ꢀ1 ꢀ1
and Ti/chitosan were 1.40 and 2.12 g mmol
h , respectively.
ꢀ1
>
HPW (29 wt%)/Ti/chitosan (0.92 mmol g ) > HPW (26 wt%)/
It can be concluded that for chitosan only showed lower activity
in esterication, while Ti/chitosan with Lewis acid sites resulted
in the increasing of TOF. This indicated that the Lewis acid
metals played a positive effect on esterication reactions, while
the base sites showed a little activity in esterication. Compared
to chitosan, HPW/Chitosan presented increasing TOF as
ꢀ
1
Ti/chitosan (0.88 mmol g ) > HPW (23 wt%)/Ti/chitosan
ꢀ1
(
0.84 mmol g ). It can be concluded that higher amount of
HPW in hybrids, higher acid capacity would be owned for HPW/
Ti/chitosan materials. Meanwhile, the basic capacity presented
the opposite trend to acidic strength. All HPW/Ti/chitosan
4
+
exhibited Lewis acidic sites due to the existence of Ti , which
were determined by IR spectra of pyridine adsorption (Fig. 4). It
ꢀ1
ꢀ1
2
.20 g mmol
h
under the same reaction conditions,
showing that the introduction of Brønsted acid played a positive
effect on esterication. And the esterication efficiency was
ꢀ1
can be seen that characteristic peaks at around 1540 cm and
ꢀ
1
1
450 cm were attributed to the materials with Brønsted and
24
4+
Lewis acidic sites. Introduction of Ti and HPW to chitosan
Fig. 5 The influence of total acidity or basicity on conversion by
different catalysts. (a) Empty, (b) chitosan, (c) Ti/chitosan, (d)
3
H PW12O40, (e) HPW (50 wt%)/chitosan, (f) HPW (23 wt%)/Ti/chitosan,
(g) HPW (26 wt%)/Ti/chitosan, (h) HPW (29 wt%)/Ti/chitosan, (i) HPW
Fig. 4 IR spectra of pyridine adsorption of (a) HPW (23 wt%)/Ti/chi- (34 wt%)/Ti/chitosan, and (j) HPW (41 wt%)/Ti/chitosan. Reaction
ꢁ
tosan, (b) HPW (26 wt%)/Ti/chitosan, (c) HPW (29 wt%)/Ti/chitosan, (d) conditions: 65 C, molar ratio of methanol/oil ¼ 13 : 1, 5 wt% catalyst,
HPW (34 wt%)/Ti/chitosan, and (e) HPW (41 wt%)/Ti/chitosan.
7 h.
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increased signicantly as adding HPW/Ti/chitosan catalysts
with ranging as H PW O < HPW (41 wt%)/Ti/chitosan < HPW
3
12 40
(
(
34 wt%)/Ti/chitosan < HPW (23 wt%)/Ti/chitosan < HPW
26 wt%)/Ti/chitosan < HPW (29 wt%)/Ti/chitosan.
It can be seen that all HPW/Ti/chitosan materials present
higher activity than their parent H
3
PW12O40. HPW as strong
Brønsted acid presented 83.5% conversion with TOF of 1.34 g
ꢀ
1
ꢀ1
mmol
h , which did not give higher conversion of palmitic
ꢀ
1
ꢀ1
acid than HPW/chitosan (TOF ¼ 2.20 g mmol
h
) with
smaller acidic capacity but with basicity. Such increase was
contributed to the synergistic effect of multifunctional sites
including Brønsted acid and base belonging to HPW/chitosan.
Compared with HPW/chitosan, Ti/chitosan only exhibited
Lewis acidity and almost the same basicity, which gave 63.2%
ꢀ1
ꢀ1
conversion and 2.12 g mmol
existence of Brønsted acid sites was rather essential for esteri-
cation between acid and methanol than that of Lewis acid
h . This result showed that the

sites. Aer combination of triple sites of Brønsted acid,
Lewis acid and base, the acid conversion and TOF was
enhanced signicantly as 97.6% conversion with TOF as
ꢀ1
ꢀ1
2.56 g mmol
h
being catalyzed by HPW (29 wt%)/Ti/
chitosan. Therefore, it can be concluded that synergistic effect
of basicity on esterication would help more palmitic acid
convert to ester. Furthermore, the catalytic activity for HPW/Ti/
chitosan materials depended on the loading amount of HPW on
chitosan. The catalytic activity increased as the loading amount
decreased from 23 wt% to 29 wt%, which was attributed to
increasing of HPW on chitosan resulted in increasing of Fig. 6 The effect of different (a) acid and (b) alcohol in esterification
ꢁ
Brønsted acid strength, hence the conversion of acid. Further reaction. Reaction conditions: 65 C, molar ratio of alcohol/palmitic
acid ¼ 13 : 1, 5 wt% catalyst, 7 h.
increasing of HPW amount from 34 wt% to 41 wt%, the effi-
ciency decreased due to the large aggregating existence of HPW
molecules on chitosan.
As a signicant inuencing factor, different substrate has
crucial effect on the efficiency of the esterication reaction. For
this purpose, a series of acids and alcohols was used as
substrate and the efficiency for the conversion was shown in
Fig. 6. A phenomenon could be observed that palmitic acid and
methanol had the greatest conversion efficiency, while cin-
namic acid and n-amyl alcohol performed with worst efficiency.
That might be due to the steric-hindrance effect of the mole-
cules. Superabundant branched chain and unsaturated bond
obstructed free rotation of single bond, therefore, it impeded
the contact of reactant so that displayed low conversion.
Generally, in order to obtain a higher conversion of esters,
excess methanol is necessary because of the reverse reaction for
esterication. From Fig. 7, it can be seen that changing the
methanol/acid from 11 : 1 to 16 : 1 resulted in a signicant
effect of the ester yield. Obviously, at the methanol/acid ratio of
Fig. 7 The effect of methanol/palmitic acid molar ratio. Reaction
conditions: 65 C, 5 wt% catalyst, 7 h.
ꢁ
1
9
3 : 1, the conversion of the reaction reached the maximum of
7.6%. By increasing the molar ratio continually, no improve-
38
ꢁ
ment was observed. The best selection of the result was 65 C. Fig. 8 showed the effect of the catalyst amount to the
methanol/acid ratio of 13 : 1 being catalyzed by HPW (29 wt%)/ conversion. It can be noticed that the conversion increased
Ti/chitosan.
from 76.5% to 97.6% through increasing the catalyst from
During the esterication process, the mass of catalyst rep- 3 wt% to 5 wt%, then the conversion remained the same by
39
resented a critical parameter to attain a high yield. Experiments further increasing the amount of the catalyst. When 5 wt%
were carried out by changing the catalyst amounts from 3 wt% amount of catalyst was used, the acid–based catalysis process
to 7 wt% and keeping a methanol/oil ratio of 13 : 1 for 7 h at reached a maximum conversion of 97.6%.
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Fig. 10 In esterification the run times of catalyst. (a) Conversion, (b)
4+
the loss of Ti in HPW (29 wt%)/Ti/chitosan, and (c) the loss of HPW in
HPW (29 wt%)/Ti/chitosan. Reaction condition: 65 C, 5 wt% catalyst,
molar ratio of methanol/oil ¼ 13 : 1, 7h.
Fig. 8 The effect of mass of catalysts (wt%). Reaction conditions:
ꢁ
ꢁ
6
5 C, molar ratio of methanol/oil ¼ 13 : 1, 7 h.
then being dried in the air for reuse. Aer six reactions cycles,
there was slightly change in the catalytic activity of the catalyst
4
+
(
5
Fig. 10). The leaching ratio of Ti and HPW was only 6.2% and
42
.8% aer six cycles. The HPW kept the Keggin structure
which can be proved by P MAS NMR (Fig. 2). Furthermore, the
3
1
4
+
IR spectra (Fig. S2†) with obvious feature peaks of Ti and HPW
aer esterication reaction also demonstrated the stability of
catalyst. The X-ray diffraction (XRD) and the Energy Dispersive
X-ray spectrometry (EDS) elemental mappings of HPW were also
used to check the stability and components. From XRD
(
Fig. S3†), it can be seen that fresh HPW/Ti/Chitosan gave peaks
ꢁ
ꢁ
ꢁ
ꢁ
at 9.63 (110), 25.42 (222), 34.42 (332) and 37.73 (510) with
little shi corresponding to their parent HPW (JCPDS no. 75-
ꢁ
2
125), while intense peak at 25.76 (101) was due to a charac-
Fig. 9 The effect of reaction temperature and time. Reaction condi- teristic peak of O–Ti–O (JCPDS no. 75-1537) and peaks of chi-
ꢁ
ꢁ
tions: 5 wt% catalyst, molar ratio of methanol/oil ¼ 13 : 1.
tosan at 10.43 and 21.00 was also observed. Aer reaction,
these peaks were found, indicating the existence of HPW and Ti
on chitosan and their stability during the reaction. From the
To explore the effect of temperature, the reaction was carried EDS (Fig. S4†), it can be seen that fresh HPW (29 wt%)/Ti/
ꢁ
ꢁ
ꢁ
40,41
out at 55 C, 60 C and 65 C.
The conversion of palmitic acid chitosan gave the molar ratio of W : P : Ti : C as
at different reaction temperatures was shown in Fig. 9. It can be 12 : 1 : 35 : 100, indicating HPW kept Keggin structure aer
seen that the conversion of palmitic acid increased with being loaded on chitosan. Aer the reaction, the molar ratio of
ꢁ
ꢁ
temperature from 55 C to 65 C, which reached the maximum W : P : Ti : C was 12 : 1 : 34 : 102 for reused HPW/Ti/chitosan,
ꢁ
value at 65 C. Under catalyzing, the esterication of acid to fatty showing little leaching of active HPW and Ti from chitosan
ꢁ
acid ester can get the maximum conversion 97.6% at 65 C.
aer reaction and further demonstrating the stability of the
In order to determine the effect of reaction time, esterica- catalyst.
tion reactions were carried out ranging from 5 h to 9 h. It was
In addition, the leaching test had been done just as the
found (Fig. 9) that the conversion of fatty acid increased before following: palmitic acid (0.01 mol) and HPW (29 wt%)/Ti/
7
h and reached the maximum 97.6% at 7 h. Therefore, the chitosan (0.125 g) was added into absolute methanol solu-
ꢁ
optimum reaction time was reacted at 7 h.
tion (0.13 mol) with stirring 1 h at 65 C, then separated the
catalyst and continued to react for further 6 h. It can be found
that the conversion was only increased to 35.6% from 31.4%,
which proved little leaching of HPW or Ti from HPW (29 wt%)/
Ti/chitosan into solution during esterication reaction. This
indicated that HPW/Ti/chitosan behaved as heterogeneous
catalysts and little leaching of the active sites from the
support.
Reusability of the catalyst
From the viewpoint of practical applications of the catalyst, an
important advantage of the solid catalyst is its recyclable usage
and catalyst stability. The HPW/Ti/chitosan catalyst was easily
separated from the mixture by centrifugation, and was washed
with methanol for a few times to remove the polar compounds
42428 | RSC Adv., 2017, 7, 42422–42429
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1
5 N. Narkhede, S. Singh and A. Patel, Green Chem., 2015, 17,
Conclusion
89–107.
We synthesized a series of triple-functional catalyst materials 16 A. I. Tropec ˆe lo, M. H. Casimiro, I. M. Fonseca, A. M. Ramos,
4
+
HPW/Ti/chitosan containing H
Such HPW/Ti/chitosan hybrids exhibited Brønsted acid from
HPW, Lewis acid from Ti and base from –NH
san, which presented synergetic effect on catalyzing of esteri-
cation reaction. HPW (29 wt%)/Ti/chitosan showed the highest 18 S. H. Zhu, X. Q. Gao, F. Dong, Y. L. Zhu, H. Y. Zheng and
conversion due to the suitable amount of HPW on chitosan. The
Y. W. Li, J. Catal., 2013, 306, 155–163.
7.3% conversion of palmitic acid to ester was achieved by HPW 19 R. S. Nunes, F. M. Altino, M. R. Meneghetti and
29 wt%)/Ti/chitosan. And little leaching of active sites of HPW
S. M. P. Meneghetti, Catal. Today, 2017, 289, 121–126.
and Ti from chitosan permitted it could be reused for at least six 20 J. Zhao, H. Y. Guan, W. Shi, M. X. Cheng, X. H. Wang and
times. The loading of Ti and HPW on chitosan could provide
S. W. Li, Catal. Commun., 2012, 20, 103–106.
a useful for fabrication of triple-functional and heterogeneous 21 V. Belessi, R. Zboril, J. Tucek, M. Mashlan, V. Tzitzios and
3
PW12
O
40, Ti and chitosan.
J. Vital and J. E. Castanheiro, Appl. Catal., A, 2010, 390, 183–
189.
group in chito- 17 K. Jagadeeswaraiah, C. R. Kumar, P. S. S. Prasad and
N. Lingaiah, Catal. Sci. Technol., 2014, 4, 2969–2977.
2
9
(
HPA catalysts for organic transformation.
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2
2
Conflicts of interest
There are no conicts to declare.
2
4 C. A. Emeis, J. Catal., 1993, 141, 347–354.
2
5 A. E. Kadib, K. Molvinger, C. Guimon, F. Quignard and
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6 P. K. Sahu, P. K. Sahu, S. K. Gupta and D. D. Agarwal, Ind.
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Acknowledgements
2
2
This work was supported by the National Natural Science Foun-
dation of China (No. 51578119), the major projects of Jilin
Provincial Science and Technology Department (20140204085GX).
7 K. Khan and Z. N. Siddiqui, Ind. Eng. Chem. Res., 2015, 54,
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