Preparation supported heteropoly (acid)/polyaniline catalysts and catalytic synthesis of tributyl citrate

Article abstract of DOI:10.1039/c9ra06071e

A series of polyaniline supported heteropoly acids were prepared through a simple method at room temperature. The obtained heterogeneous catalysts were comprehensively characterized by powder FTIR spectroscopy, UV-vis spectra, NH₃ temperature programmed desorption (TPD) and scanning electron microscopy (SEM). The influence of various process parameters such as heteropoly loading (10 to 25 wt%), catalyst amount (3-5%), molar ratio of n-butanol to citric acid (3 to 5), and reaction time (3.5-12 h) have been investigated over heteropoly/polyaniline catalysts with the aim to maximize citric acid conversion and tributyl citrate selectivity. The different catalytic tests has shown that the catalyst exhibits high conversion and selectivity by using the as-prepared heteropoly/polyaniline catalysts for esterification under appropriate conditions. The present method of using 20% heteropoly/polyaniline catalyst for the synthesis of tributyl citrate would be environmentally benign in the reusability of catalyst.

Full text of DOI:10.1039/c9ra06071e

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Preparation supported heteropoly (acid)/
polyaniline catalysts and catalytic synthesis of
Cite this: RSC Adv., 2019, 9, 33124
tributyl citrate
a
Limin Wang,b Bin Dingb and Miao Zhang
*
A series of polyaniline supported heteropoly acids were prepared through a simple method at room
temperature. The obtained heterogeneous catalysts were comprehensively characterized by powder
FTIR spectroscopy, UV-vis spectra, NH3 temperature programmed desorption (TPD) and scanning
electron microscopy (SEM). The influence of various process parameters such as heteropoly loading (10
to 25 wt%), catalyst amount (3–5%), molar ratio of n-butanol to citric acid (3 to 5), and reaction time
(3.5–12 h) have been investigated over heteropoly/polyaniline catalysts with the aim to maximize citric
acid conversion and tributyl citrate selectivity. The different catalytic tests has shown that the catalyst
exhibits high conversion and selectivity by using the as-prepared heteropoly/polyaniline catalysts for
esterification under appropriate conditions. The present method of using 20% heteropoly/polyaniline
catalyst for the synthesis of tributyl citrate would be environmentally benign in the reusability of catalyst.
Received 5th August 2019
Accepted 27th September 2019
DOI: 10.1039/c9ra06071e
rsc.li/rsc-advances
product, corrosion of equipment, complicated manufacturing
process and environmental protection issues. The sulfuric acid
Introduction
has one of the important disadvantage that the sulfuric acid
cannot be reused. The titanate also has the disadvantages of
higher cost, difficult separation from products, and high energy
consumption.3 The production process of TBC synthesis mainly
included the complex processes such as reactive distillation,2,5
three stage batch system7 and continuous water removal.10
Therefore, there was an urgent need to develop environmentally
friendly and economical processes.
Recently, the plasticizers market is growing worldwide, as pro-
jected by latest statistical data at about 11 billion pounds per
year of which, 2.4 billion pounds is shared by the United States.1
Tributyl citrate (TBC) has been applied in polymer industries
such as solvents for synthesis of polyvinyl chloride (PVC) and its
copolymers.2 The copolymers of TBC are widely applied in food
wrapping lms because TBC is a vital thermally stable and non-
toxic plasticizer.3 TBC does not discolour when processed in
compounded resins because of the excellent thermal stability of
TBC.4 TBC as a non-toxic plasticizer is also extensively applied
in toys, medical products, printing ink coatings, biodegradable
polymers, cosmetics and food additives.5–8 Moreover, TBC is
a kind of environmental friendly plasticizer because the mate-
rials used in its synthesis are available from renewable
resources by fermentation processes.4 TBC plasticizer would be
an entirely bio-renewable and sustainable option because the
reaction of citric acid (CA) and butanol from renewable sources
has been extensively studied.1 Recent biotechnology and bio-
processing advancements on the production of CA and n-
butanol have been reported in the literature.9
The solid acid catalysts have been used widely in many
reactions because of the obvious advantages such as high
acidity, no corrosion of the reactor, lower cost, ease of recovery
and reuse.11,12 Recently, the solid acid catalysts have been
applied in the synthesis of TBC such as ionic liquids,4 HZSM-5,8
SA/MCM-41 (ref. 13) and USY.1 It is well known that heteropoly
acids (HPAs) as solid acid has been applied widely in various
reaction because HPAs as economically and environmentally
solid acid have the advantages of acidity and redox proper-
ties.14,15 However, industrial use of HPA is limited because of its
solubility in polar solvents and small surface area.16,17 Hence,
when used in heterogeneous catalysis, there is a need to support
HPA on suitable solid, which will improve dispersion, acidity
and stability.18 In the past few decades, polyaniline (PANI) as
one of the most important conducting polymers has been
extensively studied. Polyaniline (PANI) as one of the important
classes of conjugated polymers has been applied widely in many
elds because of interesting electrochemical, optical properties
and environmental stability.19 PANI can be obtained easily by
chemical or electrochemical polymerization of aniline in
aqueous or non-aqueous media. PANI doped with
The traditional catalysts used in the synthesis of TBC were
mainly sulfuric acid, titanate and solid acid.3,4,8 The sulfuric
acid has many disadvantages such as formation additional by
aShandong Applied Research Center of Nanogold Technology (Au-SDARC), School of
Chemistry
& Chemical Engineering, Yantai University, Yantai 264005, China.
E-mail: zhangmiao@ytu.edu.cn; Fax: +86 535 6911732; Tel: +86 535 6911732
bInstitute of Petrochemical Technology, Jilin Institute of Chemical Technology, Jilin
132022, China
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heteropolyacids as the catalysis has been used in many
researchers due to the surface of the active phase of PANI
supported catalysts exhibits properties signicantly different
from those of crystalline catalysts.20–23
Catalytic performance
The esterication reaction was performed under vigorous stir-
ring in a three-neck glass ask with a water-cooled jacket. 0.45 g
citric acid, 0.42 g n-butanol and 0.4 g catalyst were added to the
reactor. The reaction temperature was slowly raised to reaction
temperature and kept constant for reaction time. The liquid
reaction feed and product were analyzed by using gas chroma-
tography (GC), GC-7890, capillary column, SPB-5 (30 m length,
0.25 mm width, 0.25 mm diameter) with nitrogen as a carrier gas
and a ame ignition detector (FID) in the programmable
temperature range of 353 to 553 K. The reaction products were
also conrmed by GC-MS (Agilent 6890, HP-5 MS capillary
column, 30 m ꢂ 0.25 mm ꢂ 0.25 mm).
In this paper, we prepared supported HPA/PANI catalysts,
which possesses high active and stable solid acid catalysis,
and used them in the synthesis of TBC by esterication of
citric acid and butanol to develop sustainable, industrial
benign and environmentally friendly catalytic process. The
inuencing factors of reaction such as HPA loading, the
amount of catalyst, molar ratio of citric acid and butanol,
reaction temperature time will investigated systematically.
Through orthogonal experiment, the optimization of reac-
tion conditions was studied. The reusability of the catalysts
was also investigated.
Results and discussion
Experimental
Preparation of PTA/PANI
The FTIR spectra of PTA, PANI and 20% PTA/PANI were shown
in Fig. 1(a). The unsupported PTA shown characteristic
absorption bands at 1072, 972 and 900 cmꢀ1 those could be
interrelated to P–O stretching, terminal W]O stretching and
W–O–W asymmetric stretching of corner sharing bridged
oxygens, respectively.18,24,25 Fig. 1 shown the FT-IR spectrum of
PANI. The peak at 802 cmꢀ1 belonged to the C–H out-of-plane
bending vibration of para-disubstituted benzene rings. The
characteristic band of the dopant anion (HCl-PANI) appeared
Aniline was purchased from Shanghai Sinopharm Chemical
Reagent Co. and was puried by distillation under reduced
pressure before used. All other chemicals such as phospho-
tungstic acid (H3PW12O40) was purchased from Shanghai
Sinopharm Chemical Reagent Co. and was used without further
purication.
The PANI support was prepared by interfacial polymeriza-
tion. At rst, 1.86 g aniline monomer was added in a 100 mL
round-bottomed ask. Then, 4.65 g ammonium persulfate
dissolved in 50 mL hydrochloric acid solution (1 mol Lꢀ1) was
added slowly into the above system. The reaction was kept
standing for 24 h under N2 protection in dark conditions.
Finally, the product was washed with ethanol and water and
around 1140 cmꢀ1 26 The peak centered at 1302 cmꢀ1 was due to
.
the C–N stretching mode in Ar–N.27 The peak at 1495 and
1576 cmꢀ1 belonged to the benzenoid and quinoid C]C
stretching vibration of PANI, respectively.28 All the above peaks
suggest that the HCl doped PANI was successfully synthesized.
In supported 20% PTA/PANI catalysts, absorption bands were
observed compared with PTA and PANI indicating that sup-
ported PTA/PANI catalysts could not change the topological
structure of PTA and PANI. The XRD patterns of PTA, PANI and
20% PTA/PANI were shown in Fig. 1(b). PANI exhibited char-
acteristic diffraction pattern of amorphous. Aer supporting
PTA, the XRD patterns of 20% PTA/PANI exhibited diffraction
pattern of PANI and PTA indicating the structure was preserved.
ꢁ
dried in a vacuum oven at 60 C overnight.
0.2 g of phosphotungstic acid was dissolved in 20 mL of
water solution. 0.8 g of PANI was slowly added to water solution
with string. The resulted mixture was kept overnight for stirring
and then washed and dried at 80 ꢁC for 4 h. The supported PTA
catalysts prepared in this manner were labelled as 20 wt% PTA/
PANI.
Characterization
Powder X-ray diffraction (XRD) patterns were recorded at
ambient temperature on a D/MAX-2400 diffractometer at 40 kV,
ꢀ
100 mA for Cu Ka (l ¼ 1.5418 A). The metal contents in the
catalysts were determined by inductively coupled plasma-
atomic emission spectroscopy (ICP-AES) (PerkinElmer Optima
2000DV). The shapes of catalysts were observed by scanning
electron microscopy (SEM). FT-IR spectra were characterized by
VERTEX70 spectrometer with KBr pellets. In the case of NH3-
ꢁ
TPD experiments, all catalysts were outgassed in He at 150 C
for 60 min, and nally saturated at 50 ꢁC in a 10% NH3/He
stream (50 mL minꢀ1) for 1 h. Aer removing most weakly
physisorbed NH3 by owing He (50 mL minꢀ1) for 30 min, the
chemisorbed amꢀm1onia was determined by using TCD by heat-
Fig. 1 FTIR spectra of PTA, PANI and 20% PTA/PANI (a) and XRD
spectra of PTA, PANI and 20% PTA/PANI (b).
ꢁ
ꢁ
ing at 10 C min up to 550 C under the same ow of He.
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Fig. 2 UV-vis spectra of PTA, PANI and PTA/PANI (a) and N2 adsorp-
tion results (b) for PTA/PANI catalysts.
Fig. 3 SEM images of PANI (a) and (b) and 20% PTA/PANI (c) and (d).
UV-vis spectra of the prepared PTA, PANI and PTA/PANI were
shown in Fig. 2(a). Charge transfer absorption bands of PTA
were generally observed between 200–400 nm in UV-vis spec-
trum.29–31 The PTA had strong absorption bands at 260 nm
agreement with the previous research.18 These bands at 260
could be related to ligand (O) to metal (W) charge transfer
transitions involved in edge-sharing and corner-sharing
Fig. 4 NH3-TPD of 10% PTA/PANI (a), 20% PTA/PANI (b) and 25% PTA/
PANI (c).
W–O–W bridges present in the Keggin units respectively.30,31
A
typical absorption spectrum of the polyaniline had distinct
absorption bands located 370 nm depending on preparation
and/or processing of PANI which the presence of bands in this
region had been reported in many researches.32 The absorption
band at 260 nm was retained in both the supported PTA/PANI
samples which shows this particular transition was not
affected while supporting PTA on the PANI. The supported PTA
catalysts could not affect the structure of PTA which had been
reported in previous works.18 The N2 adsorption isotherms of
PTA/PANI were shown in Fig. 2(b) and the specic surface area
and porosity are summarized in Table 1. Type I isotherms
according to IUPAC classication were observed, a typical
feature for materials with microporous structures. The as-
synthesized PANI exhibited a Brunauer–Emmett–Teller (BET)
surface area of 31 m2 gꢀ1 and a pore volume of 0.157 cm3 gꢀ1. A
gradual decrease of surface area and pore volume with loaded
PTA suggested that the porous PANI were occupied by highly
dispersed PTA.
oen been reported in the literature.33 The presence of PANI
nanotubes was also observed in the products prepared in
solutions of acid in Fig. 3. Nanotubes of 1 mm diameter were
produced. The diameter within a single nanotube was relatively
uniform but various nanotubes had different thicknesses. Some
of the nanotubes were several micrometres long; others were
shorter than 300 nm. A similar observation had also been re-
ported for PANI nanotubes.34 The morphology of 20% PTA/PANI
was consisted with PANI indicating the supported catalysts
could not inuent the morphology and size of PANI.
The NH3-TPD proles of PTA/PANI with difference loading
of PTA was shown in Fig. 4 and the results were summarized
in Table 2. The actual PTA loadings were also summarized in
Table 2 which were determined by ICP-OES. The broad
desorption peak emerged at 100 ꢁC belonged to weak acid
sites. The broad desorption peaks at 300–400 ꢁC corre-
sponded to mediate strong acid sites. The broad desorption
peaks at 450 ꢁC belonged to strong acid sites. These acid sites
were mainly formed by protons in catalysts. The strong
The representative scanning electron microscope images of
PANI and 20% PTA/PANI were presented in Fig. 3. Polymeriza-
tion in acid led to the granular morphology in Fig. 3 that had
Table 1 N2 physisorption results of PTA/PANI catalysts
Sample
SAa (m2 gꢀ1
)
MAb (m2 gꢀ1
)
EAc (m2 gꢀ1
)
MVd (cm3 gꢀ1
)
PANI
31
23
17
18
13
10
11
10
18
13
6
0.157
0.098
0.079
0.09
10% PTA/PANI
20% PTA/PANI
25% PTA/PANI
8
a
BET surface area. b Micropore area. c External surface area. d Micropore volume.
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Table 2 NH3-TPD data of the catalysts
Sample
HPA loadinga (%)
TAb (mmol gꢀ1
)
WAc (mmol gꢀ1
)
MAd (mmol gꢀ1
)
SAe (mmol gꢀ1
)
10% PTA/PANI
20% PTA/PANI
25% PTA/PANI
9.2
17.6
20.8
0.043
0.053
0.061
0.009
0.014
0.011
0.005
0.003
0.008
0.029
0.036
0.042
a
e
ICP measurement. b Total acidity. c Weak acidity. d Medium acidity. Strong acidity.
acidity increased resulting from a change in the bonding temperature of 140 ꢁC and reaction time of 3.5 h. The inuence
structure where the nitrogen and oxygen bonding inuence of 20% PTA/PANI catalyst amounting on CA conversion and the
the acidity and basicity. The lower dispersion leads mono- selectivity of different ester products (MBC, DBC and TBC) was
layer coverage of PTA on the support. However the acid investigated by varying the catalyst amounting from 3% to 5%.
strength was found to increase when the loading was The conversion of CA increased from 63 to 68% with increase
increased due to the formation of multilayer of PTA. Overall catalyst amounting from 3% to 4%. The conversion of CA
a signicant increase in total acidity and increase in the decreased to 62.3% when catalyst amounting further increased
intensity of high temperature desorption peaks were from 4% to 5%. The conversion increase could be attributed to
observed. With increasing PTA loading, the acid amount and proportionally increase in active sites which increases rate of
acidic strength of acid site increased signicantly.35,36 Espe- esterication.37 However, the conversion decreased with
cially, the PTA/PANI catalyst acidity increased from 0.043 to increase the catalyst amounting which could be attributed that
0.061 mmol gꢀ1 while those of the increasing the loading of a large number of catalysts leaded to the agglomeration of
PTA from 10% to 20% showed a moderate increase in acidity catalysts in the reaction system increasing the diffusion resis-
aer increasing the loading of PTA. It was known that the tance because the density of PANI was light. The selectivity of
acidity modied shown an important affect the synthesis of TBC increased with increase the catalyst amounting. Mean-
TBC.1,4,8,13
while, the selectivity of MBC decreased with increase the cata-
The catalytic performance of different loading PTA/PANI lyst amounting. This might be reasoned that a high catalyst
catalysts were researched which the model reaction of amounting provided a large number of active catalytic sites.
synthesis TBC by esterication of CA with n-butanol was chose. Hence, 4% catalyst amounting would be optimum and an
The reaction conditions were molar ratio of CA to n-butanol of appropriate amount at the studied reaction conditions in
1 : 5, CA of 0.2 mol, reaction temperature of 170 ꢁC and reaction present study.
time of 6 h. The reaction results were listed in Fig. 5. The
The catalytic performance of different molar ratio (n-buta-
reaction products of ester were monobutyl citrate (MBC), nol : CA) were investigated for synthesis of TBC by esterication
dibutyl citrate (DBC) and tributyl citrate (TBC). The conversion of CA with n-butanol in Fig. 7. The reaction conditions were
of CA found to be increased from 67 to 78% with increase PTA catalyst amounting of 3%, CA of 0.2 mol, reaction temperature
loading from 10% to 20%. The conversion of CA increased to of 140 ꢁC and reaction time of 3.5 h. The results obtained at
80% when PTA loading was further increased up to 25%. The different molar ratio (n-butanol : CA) were illustrated in Fig. 7.
overall trend of TBC selectivity obtained was 10% PTA < 20% The conversion of CA increased from 70 to 75% and selectivity
PTA < 25% PTA which the selectivity of TBC increased with of TBC increased from 25 to 35% with increase the molar ratio
increase the PTA loading. However, the selectivity of MBC
decreased with increase the PTA loading. With the increase of
PTA loading, MBC might further undergo esterication to
produce TBC. All PTA/PANI catalysts showed higher activity and
selectivity which might be attributed to the effect of total acidity
because the previous research found that the increase of acidity
was benecial to esterication.1,7,8 In view to maximize CA
conversion, TBC selectivity, the utilization of PTA and the
economical of catalyst, the most favourable parameters for
esterication were achieved by process optimization over 20%
PTA/PANI catalyst because with increase in PTA loading from
20% to 25% on PANI the conversion of CA increased only from
78 to 80%. Therefore, 20% PTA/PANI catalyst was applied in
later reactions.
The catalytic performance of different catalyst amounting
were investigated in Fig. 6 which the reaction conditions were
molar ratio of CA to n-butanol of 1 : 5, CA of 0.2 mol, reaction
Fig. 5 Effect of PTA loading on the conversion of citric acid and
selectivity of different products.
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Fig. 8 Effect of reaction time on the conversion of citric acid and
selectivity of different products.
Fig. 6 Effect of catalyst amounting on the conversion of citric acid
and selectivity of different products.
The different catalytic tests had shown high conversion and
selectivity by using the as-prepared PTA/PANI catalysts for
esterication under appropriate conditions. Recyclability, the
other essential requirement for industrial catalysts in industrial
applications, was evaluated for PTA/PANI with esterication of
CA with n-butanol. The catalysts can be reused without any
reactivation treatments except for a two-time washing with H2O
between two recycle. The catalytic performance of catalyst
recycle were investigated at identical set of reaction conditions:
molar ratio of CA to n-butanol of 1 : 4, CA of 0.2 mol, reaction
temperature of 140 ꢁC and reaction time of 3.5 h in Fig. 9. The
recyclability of 20% PTA/PANI for esterication was shown in
Fig. 9, which shown that the 20% PTA/PANI exhibited excellent
recyclability up to 5 successive cycles. It could be observed that
the conversion of citric acid and selectivity of TBC both decrease
with increase catalyst recycle number. The conversion of citric
acid decreased from 74% to 63% and the selectivity of TBC
decreased only from 43% to 39%. The experiment results were
agreement with previous research.8 This was attributed to the
longer catalyst recycle led the catalyst loss decreasing the
activity sites.
from 3 to 4. The higher conversion of CA and selectivity of TBC
might be attributed that CA occupied the active sites on catalyst
surface and the availability of n-butanol molecules were utilized
for further esterication.38 The conversion of CA and selectivity
of TBC were observed to be decreased with the further increase
of molar ratio of n-butanol to CA from 5 to 7. The decrease of
TBC selectivity might be due to the n-butanol molecules occu-
pied the large active sites on catalyst surface.1 The experiment
results were agreement with the previous research.1
The catalytic performance of different reaction time were
investigated in Fig. 8 which the reaction conditions were molar
ratio of CA to n-butanol of 1 : 4, CA of 0.2 mol, and reaction
temperature of 170 ꢁC. The results obtained at different reaction
time from 3.5 h to 12 h were illustrated in Fig. 8. The conversion
of CA increased with longer reaction time and the selectivity of
TBC was also found to be increased with increasing the reaction
time. Meanwhile, the selectivity of MBC and DBC decreased
with increase in reaction time which could be attributed to the
longer reaction time facilitated MBC and DBC successive
esterication formed TBC.1
Fig. 7 Effect of molar ratio on the conversion of citric acid and
selectivity of different products.
Fig. 9 Catalyst recycle studies for 20% PTA/PANI.
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different catalytic tests have shown that the catalyst shown high
conversion and selectivity by using the as-prepared PTA/PANI
catalysts for esterication under appropriate conditions. The
20% PTA/PANI catalyst exhibited excellent recyclability up to 5
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