Design of a highly active silver-exchanged phosphotungstic acid catalyst for glycerol esterification with acetic acid

Article abstract of DOI:10.1016/j.jcat.2013.06.026

A series of highly active, selective, and stable silver-exchanged phosphotungstic acid (AgPW) catalysts were prepared, characterized, and evaluated for bio-derived glycerol esterification with acetic acid to produce valuable biofuel additives. The structures, morphologies, acidities, and water tolerance of these samples were determined by FTIR, Raman, XRD, SEM-EDX, FT-IR of pyridine adsorption, and H₂O-TPD. Several typical acidic catalysts were also performed for comparison. Among them, partially silver-exchanged phosphotungstic acid (Ag₁PW) presented exceptionally high activity, with 96.8% conversion within just 15 min of reaction time and remarkable stability, due to the unique Keggin structure, high acidity as well as outstanding water-tolerance property. A plausible reaction mechanism was also proposed. In addition, this Ag₁PW catalyst exhibited universal significance for esterification, holding great potential for a wide range of other acid-catalyzed reactions.

Full text of DOI:10.1016/j.jcat.2013.06.026

Journal of Catalysis 306 (2013) 155–163
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Design of a highly active silver-exchanged phosphotungstic acid catalyst
for glycerol esterification with acetic acid
c
a,c
Shanhui Zhu a,b, Xiaoqing Gao c, Fang Dong a,b, Yulei Zhu a,c, , Hongyan Zheng , Yongwang Li
⇑
a State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China
b Graduate University of Chinese Academy of Sciences, Beijing 100039, PR China
c Synfuels China Co. Ltd., Taiyuan 030032, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of highly active, selective, and stable silver-exchanged phosphotungstic acid (AgPW) catalysts
were prepared, characterized, and evaluated for bio-derived glycerol esterification with acetic acid to
produce valuable biofuel additives. The structures, morphologies, acidities, and water tolerance of these
samples were determined by FTIR, Raman, XRD, SEM-EDX, FT-IR of pyridine adsorption, and H2O-TPD.
Several typical acidic catalysts were also performed for comparison. Among them, partially silver-
exchanged phosphotungstic acid (Ag1PW) presented exceptionally high activity, with 96.8% conversion
within just 15 min of reaction time and remarkable stability, due to the unique Keggin structure, high
acidity as well as outstanding water-tolerance property. A plausible reaction mechanism was also pro-
posed. In addition, this Ag1PW catalyst exhibited universal significance for esterification, holding great
potential for a wide range of other acid-catalyzed reactions.
Received 26 April 2013
Revised 24 June 2013
Accepted 25 June 2013
Available online 27 July 2013
Keywords:
Glycerol
Esterification
Biofuels
Ag
Ó 2013 Elsevier Inc. All rights reserved.
Phosphotungstic acid
1. Introduction
monoacylglycerol (MAG), diacylglycerol (DAG), and triacylglycerol
(TAG) because of their versatile uses as biofuel additives directly or
As a consequence of the diminishing fossil resources, significant
efforts have been taken worldwide to convert renewable biomass
into fuels and value-added compounds [1,2]. In this context, bio-
diesel derived from the lipid fraction of biomass is currently
attracting considerable attention for its sustainability, reduced tox-
icity and carbon neutral alternative of fossil fuels [2,3]. Biodiesel is
manufactured by the transesterification of triglycerides with meth-
anol or ethanol, which concurrently yields large amounts of glyc-
erol equivalent to about 10 wt% of the total biodiesel production
[3]. With the steady growth of biodiesel industry, tremendous sur-
plus of glycerol has been produced, and evidently, a glycerol ‘‘lake’’
is being formed, which makes its commercial value depreciate
sharply. Due to its nontoxic, edible, biodegradable properties as
well as multifunctional structure, glycerol holds the potential of
being an extremely important building block for the biorefinery.
Consequently, a variety of catalytic processes have been envisaged
for the valorization of glycerol by hydrogenolysis, reforming, ether-
ification, esterification, oxidation, dehydration, and so on [4–9]. As
such, one of the most promising potential approaches is catalytic
esterification of glycerol with acetic acid to the formation of
as precursors in the synthesis of polyesters [10].
The esterification of alcohols with organic acids is a conversion
of both environmental and industrial importance, being employed
extensively in the synthesis of various esters from bio-derived
feedstocks. Among them, the selective esterification of glycerol
with acetic acid is probably one of the most studied processes in
the literature of esterification, and so can be taken as a model reac-
tion in this type of catalysis [11]. Conventionally, the esterification
of glycerol with acetic acid is performed in the presence of mineral
acids catalysts. However, the inherent disadvantages related with
homogeneous system lie in separation and purification of the prod-
ucts, resulting in the environmental problems and economical
inconveniences. In order to tackle these problems, a great number
of heterogeneous solid acid catalysts have been developed in re-
cent works, such as hydroxylated magnesium fluorides [12], sul-
fated activated carbon [13], SO3H-functionalized ionic liquids
[14], mesoporous silica with sulfonic acid groups [15], zeolites
[16], Amberlyst-15 [17], and heteropolyacids [18–20]. Despite
the relative efficiency of these catalysts in glycerol esterification,
many of them have low densities of effective acid sites, tedious
preparation protocols, and rapid loss of catalytic activity. Further-
more, the unavoidable generation of water as a byproduct of ester-
ification can have a serious impact on the catalytic performance of
some solid acid catalysts such as supported heteropolyacids (HPAs)
⇑
Corresponding author at: State Key Laboratory of Coal Conversion, Institute of
Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China. Fax: +86
351 7560668.
E-mail address: zhuyulei@sxicc.ac.cn (Y. Zhu).
0021-9517/$ - see front matter Ó 2013 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2013.06.026

156
S. Zhu et al. / Journal of Catalysis 306 (2013) 155–163
because of their extremely hydrophilic nature. Consequently, how
to design a highly active, inexpensive, robust, and water-tolerant
heterogeneous solid acid catalyst that can be employed by simple
preparation protocol is still a great challenge.
Wet ion-exchanged resins Amberlyst-15 (30 nm of average pore
diameter, 50 m2 gꢁ1) and Amberlyst-30 (30 nm of average pore
diameter, 53 m2 gꢁ1) were dried overnight at 110 °C prior to the
catalytic tests. ZSM-5 (Si/Al = 25, 0.56 nm of average pore diame-
ter, 50 m2 gꢁ1) was activated at 500 °C in static air for 4 h before
the test. ZrO2 (59.7 m2 gꢁ1) supplied from Jiangsu Qianye Co., Ltd
was used as the support. MoO3/ZrO2, WO3/ZrO2, Nb2O5/ZrO2, and
SO24ꢁ=ZrO2 were prepared by incipient wetness impregnation
method by using (NH4)6Mo7O24ꢀ4H2O (SCRC), (NH4)6W7O24ꢀ6H2O
(SCRC), Nb(OH)5 (King-Tan Tantalum Industry Ltd.), H2SO4 (SCRC)
as precursors. Specifically, these catalysts were prepared by
impregnation of ZrO2 with the calculated amount of aqueous solu-
tion of desired precursors and then dried overnight at 110 °C fol-
lowed by calcination at 600 °C in static air for 4 h. Appropriate
amount of oxalic acid dehydrate (SCRC) was added to the solution
of Nb(OH)5 to improve the solubility during the preparation of
Nb2O5/ZrO2. The nominal loadings of acid components (MoO3,
HPAs, possessing characteristics of strong Brønsted acidity and
easily tunable acidity, have been demonstrated to display out-
standing catalytic performance in a wide range of acid-catalyzed
reactions [21–24]. However, the lack of thermal stability and high
solubility in polar media of HPAs has limited their further applica-
tions in heterogeneous acid-catalyzed reactions. In contrast to
grafting HPAs onto porous supports, it is more effective to ex-
change protons of HPAs with different cations (e.g., K+, Cs+, Ag+)
to form insoluble salts, which can tune and amplify HPAs reactivity
or even result in the appearance of bifunctional or multifunctional
catalysis [25]. Such a conceptual strategy would lead to offer a new
class of tunable and recoverable HPA salts catalysts with high effi-
ciency and heterogeneity, providing versatile applications in sus-
tainable chemistry. Recently, Borghèse et al. [26] have developed
a series of exceedingly effective and reusable silver-exchanged sil-
icotungstic acid catalysts for the rearrangement of alkynyloxiranes
to furans. Compared to H4SiW12O40 and other HPAs such as H3-
PMo12O40, H3PW12O40 (HPW) presents stronger Brønsted acidity
and thermal stability, because of the weak interaction between
acidic protons and large Keggin anion. Thereby, in this work, we
have focused on designing silver modified HPW catalysts prepared
by an ion-exchanged method, exhibiting unprecedented catalytic
activity and superior stability for glycerol esterification. To the best
of our knowledge, this is the first report on the catalytic perfor-
mance of heterogeneous silver-exchanged HPA catalysts for the
esterification.
WO3, Nb2O5, and SO2ꢁ) in the corresponding catalysts were 15%.
4
Cs-exchanged HPW catalyst (Cs2.5PW) was prepared with the same
procedure as the aforementioned Ag1PW, wherein 2.5 protons in
one HPW molecule can be replaced by Cs atoms.
2.2. Catalyst characterization
Powder X-ray diffraction (XRD) patterns of the catalysts were
recorded on D2/max-RA X-ray diffractometer (Bruker, Germany)
using Cu K
a radiation at 30 kV and 10 mA. The measurements
were obtained in the step of 0.04° with account time of 0.5 s and
in the 2h rang of 5–90°.
Raman spectroscopy was obtained on a Renishaw–UV–vis Ra-
man System 1000 equipped with a CCD detector at room temper-
ature. The air-cooled frequency doubled Nd–Yag laser operating at
532 nm was employed as the exciting source with a power of
30 MW.
Scanning Electron Microscopy (SEM) was conducted on a Quan-
ta 400F microscope. EDX spectra were obtained using 20 kV pri-
mary electron voltages to determine the composition of the
samples.
Accordingly, in the present investigation, Ag-exchanged HPW
catalysts with varying Ag contents were prepared, characterized,
and evaluated for glycerol esterification with acetic acid. The cata-
lyst features were characterized using various spectroscopy tech-
niques and correlated with the observed catalytic performance of
glycerol esterification. The esterification of different alcohols with
organic acids was performed to check the scope of this catalyst.
The IR spectra were measured on a Vertex 70 (Bruker) FT-IR
spectrophotometer, equipped with a deuterium triglycine sulfate
(DTGS) detector. The powder samples were mixed with KBr
(2 wt%) and pressed into translucent disks at room temperature.
2. Experimental
2.1. Catalyst preparation
The spectra were recorded in the range of 400–4000 cmꢁ1
.
All the chemicals were obtained commercially and used with-
out any further purification. H3PW12O40ꢀxH2O was purchased from
Sinopharm Chemical Reagent Co., Ltd. (SCRC). AgNO3 was supplied
by Tianjin Damao Chemical Co., Ltd. Prior to the preparation, the
water content of H3PW12O40ꢀxH2O was checked by TG-MS. The
Ag-exchanged HPW catalysts were synthesized by an ion-ex-
changed method, according to the procedure described previously
[26,27]. Firstly, 10.25 g HPW was dissolved in 20 ml deionized
water at room temperature under vigorous stirring. Then, the
appropriate amount of AgNO3 (0.1 mol/L) aqueous solution was
added dropwise to the former solution with continuous stirring.
The resultant mixture was aged 2 h at room temperature, and
the excess water was evaporated in a rotary evaporator. The
remaining powder was dried at 80 °C overnight and then calcined
at 250 °C in static air for 4 h. Analogously, the number of Ag atom,
i.e., x in AgxH3ꢁxPW12O40 (x = 1, 2, 3), can be conveniently con-
trolled by varying the amount of AgNO3 aqueous solution. These
as-prepared catalysts are designated as Ag1PW, Ag2PW, and Ag3-
PW, wherein the number implies the number of Ag ions ex-
changed. The formation of AgxH3ꢁxPW12O40 reaction undergoes
based on the following equations:
IR spectra of adsorbed pyridine (Py-IR) were recorded with the
same apparatus as above. The samples were pressed into self-sup-
porting wafers, degassed in a vacuum at 300 °C for 1 h, and subse-
quently exposed to the pyridine vapor after cooling down to 30 °C.
The Py-IR spectra were then recorded at 200 °C after applying vac-
uum for 30 min. The quantification of acidity was calculated by
Lambert–Beer equation,
e
ꢀ W ꢀ c
A ¼
S
where A is the absorbance (area in cmꢁ1),
e the extinction coeffi-
cient (m2/mol), W the sample weight (kg), c the concentration of
acid (mol/kg or mmol/g) and S is the sample disk area (m2).
The amount of Brønsted and Lewis acid sites was estimated
from the integrated area of the adsorption bands at ca. 1540 and
1450 cmꢁ1, respectively, using the extinction coefficient values
based on the previous report [28].
H2O-TPD was performed in an Auto Chem.II 2920 equipment
(Mircromeritics, USA). Prior to each run, 0.3 g catalyst was first
pretreated in flowing He at 250 °C for 1 h and then cooled to
50 °C followed by saturating with water using pulse model until
saturation. After being purged with He for 30 min, the catalyst
xAgNO3 þ H3PW12O40 ! xHNO3 þ AgxH3ꢁxPW12O40 ðx ¼ 1; 2; 3Þ

S. Zhu et al. / Journal of Catalysis 306 (2013) 155–163
157
bed was heated to 500 °C at a rate of 10 °C/min, and the desorbed
water was monitored by a TCD detector.
ICP optical emission spectroscopy (Optima2100DV, PerkinEl-
mer) was employed to analyze the remaining solution after glyc-
erol esterification with acetic acid.
other triads through bridging oxygens [29]. FTIR spectra of parent
HPW and Ag-salts catalysts are illustrated in Fig. 1. In the case of
parent HPW, the characteristic bands of typical Keggin anions
can be clearly observed as follows: 1080 cmꢁ1
(
mas PAO),
mas WAOAW inter-octahedral),
mas WAOAW intra-octahedral) [30]. All silver-ex-
982 cmꢁ1
( (
mas W@O), 890 cmꢁ1
and 797 cmꢁ1
(
changed HPW catalysts exhibited these typical bands at similar
frequencies, close to those of parent HPW, indicating the whole
retention of Keggin structure on the as-prepared Ag-salts catalysts.
Raman spectroscopy is uniquely suited for check Keggin struc-
ture of HPAs to verify the presence and integrity of HPAs structure.
Fig. 2 shows Raman spectra of parent HPW and its Ag-salts cata-
lysts. It has been identified and attributed Raman bands at
1010 cmꢁ1 and a shoulder at 990 cmꢁ1 to the symmetric and asym-
metric stretching modes of W@O [30]. The bands at 900 and
550 cmꢁ1 are ascribed to the bending modes of WAOAW and
OAPAO, respectively [30]. These bands were all present and clearly
identified in the as-prepared silver-exchanged HPW catalysts, sug-
gesting that the Keggin structure was intact even after exchange of
acidic protons with silver.
2.3. Catalytic tests
All the catalytic tests were conducted at atmospheric pressure.
Esterification of glycerol with acetic acid was carried out in a
100 ml stainless steel with Teflon liner autoclave under vigorous
stirring. In a typical run, 0.05 g catalyst, 5.0 g glycerol, and 32.6 g
acetic acid were introduced into the reactor. Then, the reaction
mixture was stirred and heated at 120 °C for an appropriate reac-
tion time. After reaction, the reactor system was rapidly cooled
to room temperature in an ice-water bath. Subsequently, the solid
catalyst was separated by centrifugation, and the liquid products
were analyzed by a gas chromatography (Ruihong chromatogram
analysis Co., Ltd, China) with a flame ionization detector using a
DB-WAX capillary column. Concurrently, the assignment of the
products was identified by a gas chromatography–mass spectrom-
etry (Agilent, USA) with a DB-WAX capillary column. The conver-
sion of glycerol and selectivity of products were calculated based
on the following equations:
Fig. 3 displays the XRD patterns of parent HPW and Ag-salts cat-
alysts. The parent HPW exhibited all of typical X-ray diffracto-
grams of body-centered cubic secondary structure of Keggin
anion, with characteristic diffraction peaks at 10.3°, 25.3°, and
34.6° [30]. Although the as-prepared Ag-salts samples exhibited
very similar diffraction patterns as that of HPW, a slight shift to-
ward higher 2h values was observed, and no diffraction peaks of
pure HPW were detected, suggesting the presence of only one
Ag-salt phase with good crystallinity. A detailed study of the pre-
dominant 25.0–27.0° reflections confirmed this effect, showing
the continuous shift toward lower lattice parameters with the in-
crease in silver content, as reported previously [25,31]. It is specu-
lated that silver-exchanged HPW catalysts possess the same
symmetry as parent HPW but with a contracted unit cell. This con-
traction of unit cell can be explained by the exchange of protons
present in the secondary Keggin structure in the form of dihydro-
moles of glycerol ðinÞ ꢁ moles of glycerol ðoutÞ
Conversion ð%Þ ¼
moles of glycerol ðinÞ
ꢂ 100
moles of one product
Selectivity ð%Þ ¼
ꢂ 100
moles of all products
3. Results and discussion
nium ions H2Oþ for hydrated silver cation. The same behavior has
3.1. Catalyst characterization
2
been observed by Borgh‘ese et al. in the case of silver-exchanged
silicotungstic acids catalysts [26,27]. Accordingly, our XRD pat-
terns revealed that only one phase of Ag-salt with uniform crystal-
line was formed, whatever the content of Ag+ incorporated. It
should be noted that the observed fact from XRD is in support of
the results from FTIR and Raman, which has demonstrated the suc-
cessful incorporation of Ag+ into HPW clusters and the formation of
good crystalline with the remaining of Keggin structure.
To thoroughly recognize the structure transformation induced
by silver doping and morphologies of as-prepared Ag-exchanged
HPW catalysts, a combination of FTIR, Raman, XRD, and SEM-EDX
was employed. Infrared spectra are known to be an informative
fingerprint of Keggin structure at molecular level [29]. The Keggin
anion of HPW is composed of a tetrahedral PO4 surrounded by 12
octahedral WO6, sharing edges in W3O13 triads and corners with
Ag3PW
Ag2PW
Ag3PW
Ag2PW
Ag1PW
HPW
Ag1PW
HPW
400
600
800
1000
1200
1400
200
400
600
800
1000
1200
Wavenumber (cm-1)
Raman shift (cm-1)
Fig. 1. FTIR spectra of bulk HPW and Ag-salts catalysts.
Fig. 2. Raman spectra of bulk HPW and Ag-salts catalysts.

158
S. Zhu et al. / Journal of Catalysis 306 (2013) 155–163
Table 1
Energy dispersive X-ray analysis data for Ag and W on Ag-salts of HPW.
Catalyst W/Ag nominal
ratio
W/Ag determined by W/Ag average value
Ag3PW
Ag2PW
EDX
from EDX
Ag1PW
12
14.8; 11.8; 12.2;
10.2; 9.3
11.66
Ag2PW
Ag3PW
6
4
7.3; 5.9; 4.4; 6.5; 5.8
4.6; 3.1; 4.3; 3.0; 3.8
5.98
3.76
Ag1PW
HPW
B
L
Ag3PW
Ag2PW
Ag1PW
20
40
60
80
26
(o)
(o)
2
2
Fig. 3. XRD patterns of bulk HPW and Ag-salts catalysts.
To gain further insights, SEM was conducted to directly image
HPW
the as-prepared catalysts. A representative selection of SEM images
of Ag1PW and Ag3PW samples is presented in Fig. 4. It can be found
that these as-synthesized materials displayed well-shaped crystal-
line particles, particularly for the fully substituted Ag3PW. With the
increase in Ag content, the size of crystalline particles evidently
grew, resulting in the formation of bigger sizes of Ag3PW than
those of Ag2PW and Ag1PW. Furthermore, EDX was examined to
determine the elemental compositions of these samples. The val-
ues of W/Ag atomic ratios calculated by EDX analysis are listed
in Table 1. For all the Ag-exchanged HPW samples, the W/Ag ratios
determined by EDX analysis were well consistent with the nominal
value. In sum, the above observation from SEM-EDX acted in accor-
dance with the XRD results, revealing the presence of only one
crystalline phase for all the Ag-exchanged catalysts and not a mix-
ture of different stoichiometric Ag-salts.
For the acid-catalyzed esterification of glycerol, both the surface
acid density and the nature of acid sites play the key roles in deter-
mining the catalytic performance. Therefore, FT-IR spectra of pyri-
dine absorption were recorded to probe accessible surface acid
sites, which is a powerful tool for identifying the nature of acid
sites. Additionally, in situ pretreatments can be available to the so-
lid acid catalysts. As shown in Fig. 5, all the samples presented typ-
ical bands corresponding to strong Brønsted bound pyridine, at
around 1540 and 1639 cmꢁ1 [32]. The bands at around 1450 and
1610 cmꢁ1 were assigned to the coordinated pyridine adsorbed
on Lewis acid sites, while the band at 1489 cmꢁ1 was originated
1400
1450
1500
1550
1600
1650
1700
Wavenumber (cm-1)
Fig. 5. FTIR spectra of pyridine adsorption of bulk HPW and Ag-salts catalysts.
from the combination of pyridine on both Brønsted and Lewis acid
sites [33]. The concentrations of Brønsted acid and Lewis acid sites
are obtained from the bands at 1540 and 1450 cmꢁ1 based on Lam-
bert–Beer equation, and the calculated results are listed in Table 2.
In comparison with pure Brønsted acid of parent HPW, Lewis acid
sites emerged as a result of the exchange of Ag with protons of
HPW, originating from the coordinately unsaturated Ag species
in the catalyst. Among them, the Ag1PW catalyst presented the
maximum acidity, even more than the parent HPW, owing to the
enhanced Lewis acid sites. In comparison with the fully Ag-ex-
changed HPW sample, partially Ag substituted HPW catalyst in-
creased available acid sites, due to the presence of residual
protons capable of mobility and inducing new strong acid sites
[29]. Similar behavior has been reported in the cases of Cs- and
Sn-exchanged HPW catalysts [29,34]. It was interesting to find that
the fully Ag-exchanged HPW sample presented 1.10 mmol/g
Brønsted acids, which may originate from the adsorbed surface
OH groups or even remaining trace protons.
Fig. 4. SEM micrographs of Ag1PW (a) and Ag3PW (b) catalysts.

S. Zhu et al. / Journal of Catalysis 306 (2013) 155–163
159
Table 2
Amount of acid sites on bulk HPW and Ag-salts catalysts determined by FT-IR spectra
of pyridine absorption.
100
80
60
40
20
0
Catalyst Brønsted acidity
(mmol/g)
Lewis acidity
(mmol/g)
Total acidity
(mmol/g)
HPW
1.61
1.38
1.10
1.02
0.04
0.54
0.53
0.41
1.65
1.92
1.63
1.43
Ag1PW
Ag2PW
Ag3PW
HPW
Ag1PW
Ag2PW
Ag3PW
To explore the water-tolerant property of Ag-salts, the H2O-TPD
test was performed. As shown in Fig. 6, the Ag-salts catalysts
showed good water tolerance as indicated by its only one small
low-temperature desorption peak, particularly for Ag3PW wherein
the protons were entirely substituted by Ag+. Contrarily, the bulk
HPW presented two broad peaks at about 270 °C and 455 °C,
respectively, suggesting that water was strongly absorbed on this
sample. The hydrophobic property decreased in the following or-
der of Ag3PW > Ag2PW > Ag1PW > HPW, consistent well with the
increasing proton numbers.
0
10
20
30
40
50
Time (min)
Fig. 7. Glycerol conversion as a function of reaction time over HPW and Ag-salts
catalysts. Reaction conditions: 120 °C, glycerol/acetic acid = 1:10 (molar ratio),
1 wt% catalyst loading relative to glycerol.
3.2. Catalytic reaction of Ag-salts catalysts
that at the same reaction conditions, the Ag2PW and Ag3PW cata-
lysts needed longer time to attain equilibrium compared to Ag1PW.
This may be related to the concentrations of Brønsted and Lewis
acid sites, which were lower on Ag2PW and Ag3PW. Although Ag2-
PW and Ag3PW possessed better water tolerance, Ag1PW exhibited
superior catalytic performance. The role of acid sites seems to be
more significant for glycerol esterification on these Ag-salts cata-
lysts. In all the cases, the conversion of glycerol improved rapidly
within the initial 10 min of reaction time, suggesting the rapid con-
sume of glycerol in the initial reaction stage. Moreover, the reac-
tion order on glycerol concentration in this stage was probably
zero. Nevertheless, the reaction rate declined significantly in the
latter reaction stage. The above results revealed that the glycerol
concentration (acetic acid was excess) has significant impact on
the kinetic behavior of glycerol esterification. Additionally, large
amount of by-product water was formed during the reaction,
which can inhibit the conversion of glycerol and thus decrease
the reaction rate, particularly in the latter stage. Therefore, it can
be inferred that the initial high reaction rate was switched off,
and then, the reaction rate declined remarkably, which was the
combined effect of decreased glycerol concentration and the accu-
mulation of by-product water.
Fig. 7 displayed the profiles of glycerol conversion as a function
of reaction time for glycerol esterification with acetic acid. The par-
ent HPW catalyst, which is well known for its strong Brønsted acid-
ity for versatile acid-catalyzed reactions, was employed to make a
comparison with Ag-salts catalysts. The glycerol conversion over
homogenous HPW catalyst rapidly reached 70.3% even after
15 min and then increased slowly with extended reaction time.
Compared to HPW, all the Ag-exchanged catalysts presented
superior performance, despite the nature of heterogeneity of these
samples. As shown in Fig 7, at all reaction time, the activity
of the catalysts decreased in the following order of Ag1PW >
Ag2PW > Ag3PW > HPW. The conversion of glycerol was up to
100% within only 45 min on Ag1PW, while the glycerol did not
reach complete conversion on Ag2PW and Ag3PW within 45 min.
The difference in catalytic activity was mainly attributed to the
variant concentrations of Brønsted and Lewis acid sites as well as
water tolerance. It should be mentioned that although the initially
high rate on Ag2PW and Ag3PW declined and further reaction pro-
gress at lower rate, complete conversion of glycerol can be still
achieved with prolonged reaction time. The above results indicated
Table 3 listed the representative product distributions over
HPW and Ag-salts catalysts. The obtained products of glycerol
esterification were MAG, DAG, and TAG, while no dehydration
products of glycerol were detected in the product mixture, proba-
bly because of the relatively mild reaction conditions [24]. Among
them, DAG and TAG are the most preferred products owing to their
versatile applications as fuel additives, which can be utilized to im-
prove the cold and viscosity properties of biodiesel as well as antik-
nocking properties of gasoline [10]. It is identified that glycerol
esterification with acetic acid is a consecutive reaction including
three continuous steps (glycerol + acetic acid ? MAG + water;
MAG + acetic acid ? DAG + water; DAG + acetic acid ? TAG +
water) [35]. Thereby, to maximize production of the valuable
DAG and TAG, it is highly favorable to perform this reaction with
extended reaction time. Since the Ag1PW catalyst afforded the best
reactivity, the following study would focus on the investigation of
catalytic behavior of Ag1PW. As displayed in Fig. 8, Ag1PW catalyst
presented exceptional activity even just 15 min and achieved com-
plete conversion within 45 min. The selectivity of MAG is high at
the beginning of the reaction. Nevertheless, as the reaction pro-
ceeded, the selectivity of DAG and TAG enhanced at the expense
Ag3PW
Ag2PW
Ag1PW
HPW
100
200
300
400
Temperature (oC)
Fig. 6. H2O-TPD profiles of bulk HPW and Ag-salts catalysts.

160
S. Zhu et al. / Journal of Catalysis 306 (2013) 155–163
Table 3
the previous report [35], glycerol esterification with acetic acid is
equilibrium limited and a consecutive reaction including three
continuous steps, which can determine the final product distribu-
tion. An effective technique to address the thermodynamic limita-
tion is to remove the generated water from the reaction system by
reactive distillation.
Catalytic performance of glycerol esterification with acetic acid over various
catalysts.a.
Entry
Catalyst
Conversion (%)
Selectivity (%)
MAG
DAG
TAG
1
2
3
4
5
6
7
8
HPW
Ag1PW
Ag2PW
Ag3PW
Blank
ZSM-5
Amberlyst-15
Amberlyst-30
MoO3/ZrO2
WO3/ZrO2
Nb2O5/ZrO2
SO24ꢁ=ZrO2
Cs2.5PW
70.3
96.8
82.5
75.7
21.3
22.9
33.8
39.8
25.6
23.1
24.7
25.5
59.3
48.4
52.7
59.7
83.5
83.8
65.8
60.3
70.7
63.6
83.1
83.3
37.7
46.4
43.4
37.1
16.3
14.8
32.0
37.7
28.3
35.5
15.5
15.3
3.0
5.2
3.9
3.2
0.2
1.4
2.2
2.0
1.0
0.9
1.4
1.4
3.3. Comparision with other typical catalysts
A number of typical solid acid catalysts have been selected and
evaluated for glycerol esterification in comparison with the best
Ag1PW catalyst at 120 °C within 15 min of reaction time. As shown
in Table 3, glycerol conversion was up to 21.3% without addition of
any catalyst, suggesting that glycerol esterification with acetic acid
was a self-catalysis reaction wherein the acidic protons from acetic
acid were capable of catalyzing the reaction itself [35]. ZSM-5 did
not present any promotion of this reaction with such low amount
and short reaction time, which was likely ascribed to the diffusion
limitations caused by the narrow pore structure. Contrarily,
Amberlyst-15 and Amberlyst-30 acid resins promoted this reaction
mildly in terms of glycerol conversion and the selectivity of DAG
and TAG. Our previous report [17] revealed that these acid resins
presented better performance of glycerol esterification than ZSM-
5 due to their large pore diameter. With respect to commonly
exploited mixed metal oxides such as MoO3/ZrO2, WO3/ZrO2, and
Nb2O5/ZrO2, their reactivity was remained rather low, which might
be related to their low acidity. Furthermore, other heterogeneous
acid catalysts SO24ꢁ=ZrO2 and Cs2.5PW also gave low activity in
comparison with the present Ag1PW catalyst.
9
10
11
12
13
32.4
72.2
27.7
0.1
a
Reaction conditions: 120 °C, glycerol/acetic acid = 1:10 (molar ratio), 1 wt%
catalyst loading relative to glycerol, 15 min.
100
Glycerol
Monoacetin
Diacetin
Triacetin
80
60
40
20
0
It should be noted that our as-prepared Ag-salts catalysts pre-
sented an extremely high activity for glycerol esterification, de-
spite with very low catalyst amount (1 wt% relative to glycerol)
as well as mild reaction conditions. As far as we know, the Ag1PW
catalyst is the most active catalyst reported in the literature
(Table S1). Actually, compared with the reported catalysts, the Ag1-
PW catalyst showed exceptionally high specific rate and turnover
number (TON), or even one order of magnitude more active
0
50
100
150
200
250
(Table S1). The TON of Ag1PW was up to 2189.9 hꢁ1
.
Time (min)
3.4. Reusability of Ag1PW catalyst
Fig. 8. Effect of reaction time on glycerol esterification with acetic acid over Ag1PW
catalyst. Reaction conditions: 120 °C, glycerol/acetic acid = 1:10 (molar ratio), 1 wt%
catalyst loading relative to glycerol.
To assess the reusability of Ag1PW catalyst for glycerol esterifi-
cation, the spent catalyst was separated from the reaction system
by centrifugation after the completion of each run. The spent sam-
ple was washed with ethanol, dried at 80 °C overnight prior to its
reuse for further reaction cycle under the identical reaction condi-
tions. As illustrated in Fig. 9, Ag1PW catalyst exhibited similar
activity for glycerol esterification until five reaction cycles without
showing distinct decline in catalytic ability. The above results have
demonstrated that the Ag1PW catalyst is rather durable and holds
the potential for practical applications. Furthermore, the inherent
heterogeneous nature of Ag1PW catalyst can favor to recover the
catalyst as well as the reaction products. The XRD and Raman spec-
troscopy were employed to check the state of spent Ag1PW cata-
lyst. As illustrated in Fig. S2, the spent catalyst displayed similar
characteristic bands as the fresh one, revealing that the Keggin
structure of Ag1PW catalyst remained intact during this reaction.
With respect to Ag2PW and Ag3PW, it was anticipated that both
of them presented good reusability without obvious loss in the
activity (Fig. S3), due to their better water tolerance than Ag1PW.
As indicated by XRD and Raman spectroscopy of spent Ag2PW
and Ag3PW in Fig. S2, both of them were rather durable.
of MAG. The combined selectivity of DAG (58.2%) and TAG (31.9%)
reached the highest value of 90.1% with the complete conversion of
glycerol within 4 h of reaction time, which is superior or at least
comparable to the best catalysts [14,17,18,36].
To investigate the kinetic behavior conveniently, the product
yield of glycerol esterification as a function of time over Ag1PW
catalyst was illustrated in Fig. S1. As mentioned above, the glycerol
reaction rate declined quickly after 10 min of reaction time, reveal-
ing the rapid consumption of glycerol and accumulation of by-
product water at the initial stage. Then, the conversion of glycerol
was restricted. As can be seen in Fig. S1, it took 35 min to convert
the remaining glycerol (<20%) and much more time to attain the
product distribution equilibrium. The consecutive reaction mecha-
nism of glycerol esterification can be observed from the product
distribution. The yield of MAG increased firstly with the reaction
time, while that of MAG passed a maximum value before its de-
cline. Nevertheless, the yield of DAG and TAG increased gradually
with the reaction time, and only a relatively low amount of TAG
was formed in the initial stage. When much MAG converted to
DAG at the initial stage, DAG was yielded rapidly. However, DAG
formation rate declined remarkably with the time due to its abun-
dant consumption, while the TAG increased steadily. According to
In order to explore the possible leaching of active component,
an additional experiment of glycerol esterification with acetic acid
was conducted by using Ag1PW catalyst at 120 °C for 15 min. Sub-
sequently, to avoid the adsorption of possible leached metal ions,

S. Zhu et al. / Journal of Catalysis 306 (2013) 155–163
161
conversion of glycerol esterification with various carboxylic acids
decreased gradually with increasing chain length of carboxylic
acids (entries 1–3) in the following order: acetic acid > propionic
acid > 1-butanoic acid. Analogously, the conversion of primary
alcohol esterification with acetic acid decreased gradually with
increasing chain length of primary alcohols (entries 4–8) in the fol-
lowing order: ethanol ꢃ propanol > 1-butanol > 1-pentanol > 1-
hexanol. This similar trend has also been observed in the case of
myristic acid esterification with different alcohols over sulfated
zirconia and cetyl alcohol esterification with various acids over
ZrOCl2/MCM-41, which can be explained by the steric hindrance
effects of the larger alkyl chains either of acids or alcohols
[38,39]. Additionally, the esterification of acetic acid with various
polyols including ethylene glycol, 1,2-propanediol, 1,4-butanediol,
and 1,5-pentanediol was also exhibited in the presence of Ag1PW
catalyst. The esterification of acetic acid with polyols resulted in
corresponding monoacetate and diacetate in good conversion de-
spite the gradual decrease in the reactivity with increasing chain
length of polyols. The above results suggested that the Ag1PW cat-
alyst exhibited superior performance in the esterification of vari-
ous alcohols and acids, being a potential catalyst for the selective
esterification of glycerol with fatty derivatives which is an attrac-
tive pathway for the transformation of biomass-derived substrates.
100
80
60
40
20
0
1
2
3
4
5
Reaction cycle
Fig. 9. Reusability test preformed for Ag1PW catalyst in glycerol esterification with
acetic acid. Reaction conditions: 120 °C, glycerol/acetic acid = 1:10 (molar ratio),
1 wt% catalyst loading relative to glycerol, 15 min.
the catalyst was removed from the reaction system at reaction
temperature followed by continuing this reaction with the remain-
ing solution for another 60 min [37]. The solution after reaction
was checked by ICP, and it was verified that there was no leaching
of Ag1PW during glycerol esterification. However, the product dis-
tributions did change marginally after 60 min of reaction time,
which can be explained by the self-catalyzed characteristic of
esterification. Therefore, the above results demonstrated unambig-
uously the inherent heterogeneous nature of Ag1PW catalyst, in
well agreement with the properties of Ag-exchanged silicotungstic
acid catalyst described previously [37].
3.6. Reaction mechanism of glycerol esterification
It is well known that the acidity of catalyst has significant im-
pact on the catalytic cycle and is necessary for optimal perfor-
mance in glycerol esterification, particularly for Brønsted acid
sites [14,15,18,20]. To elucidate the superior performance of Ag1-
PW catalyst, a plausible reaction mechanism of glycerol esterifica-
tion with acetic acid was proposed in Scheme 1. Initially, the
interaction of acetic acid with a proton site derived from Brønsted
acid site results in the protonation of the carbonyl group in the ace-
tic acid molecule possessing a higher positive charge. Subse-
quently, the carbon atom of the carbonyl can be attacked by the
hydroxyl group of glycerol, which results in generating a new
CAO bond between the carbon atom of carbonyl and the oxygen
atom of hydroxyl group of glycerol. With the elimination of water,
a new MAG molecule is formed. Owing to the steric constrains, the
3.5. Influence of carboxylic acids and alcohols for glycerol esterification
Finally, the catalytic properties for esterification with various
carboxylic acids and alcohols were examined to investigate the
universal significance of Ag1PW. As shown in Table 4, the
Table 4
Esterification of various alcohols with carboxylic acids over Ag1PW catalyst.a.
Entry
Alcohol
Glycerol
Glycerol
Glycerol
Carboxylic acid
Conversion (%)
Selectivity
Product distribution
(%)
1
2
3
Acetic acid
96.8
70.9
64.3
MAG
DAG
TAG
48.4
46.4
5.2
55.0
43.1
1.9
53.5
46.4
0.1
100
100
100
100
100
38.9
61.1
61.7
38.3
71.5
28.5
95.8
4.2
Propionic acid
1-Butanoic acid
Glyceryl monopropionate
Glyceryl dipropionate
Glyceryl tripropionate
Glyceryl momobutyrate
Glyceryl dibutyrate
Glyceryl tributyrate
Acetic ether
Propyl acetate
1-Butyl acetate
1-Pentyl acetate
1-Hexyl acetate
Ethylene glycol monoacetate
Ethylene glycol diacetate
1,3-Propanediol monoacetate
1,3-Propanediol diacetate
1,4-Butanediol monoacetate
1,4-Butanediol diacetate
1,5-Pentanediol monoacetate
1,5-Pentanediol diacetate
4
5
6
7
8
9
Ethanol
Propanol
1-Butanol
1-Pentanol
1-Hexanol
Ethylene glycol
Acetic acid
Acetic acid
Acetic acid
Acetic acid
Acetic acid
Acetic acid
99.5
99.1
95.2
93.2
82.4
99.2
10
11
12
1,3-Propanediol
1,4-Butanediol
1,5-Pentanediol
Acetic acid
Acetic acid
Acetic acid
98.9
98.0
81.2
a
Reaction conditions: 120 °C, alcohol/acid = 1:10 (molar ratio), 1 wt% catalyst loading relative to alcohol, 15 min.

162
S. Zhu et al. / Journal of Catalysis 306 (2013) 155–163
O
C
O
HO
HO
CH3
H3C
OH
O
H3C
OH
HO
O
CH3
C
O
O
H
O
H3C
OH
O
AgH2PW12040
O
H
O
CH3
C
O
AgH2PW12040
O
H
O
HO
AgH2PW12040
O
C
HO
H3C
O
C
O
CH3
HO
OH
O
HO
O
C
O
H3C
C
O
H3C
H3C
CH3
C
O
O
H
C
O
O
O
CH3
C
O
AgH2PW12040
O
CH3
O
C
AgH2PW12040
HO
O
HO
OH
H3C
O
H3C
C
O
CH3
O
C
C
O
H2O
H3C
C
O
O
O
O
C
H
O
OH
AgH2PW12040
CH3
AgH2PW12040
AgH2PW12040
H3C
C
O
O
O
O
H
C
AgH2PW12040
CH3
H2O
H2O
Scheme 1. The proposed reaction mechanism of glycerol esterification with acetic acid.
secondary hydroxyl group of glycerol proceeds less nucleophilic at-
tack on acetic acid than that of primary hydroxyl group, which
leads to form less 2-MAG than 1-MAG, as evidenced by our GC–
MS and the literature [14]. Similarly, the formed MAG proceeds
nucleophilic attack on acetic acid and leads to the formation of
DAG. Finally, DAG undergoes further esterification with acetic acid
to yield TAG. On the other hand, Lewis acid sites may also play a
role in the catalytic performance, as claimed by Troncea et al.
[12]. The Lewis acid sites can act as active catalytic sites involved
in the formation of reactive nucleophilic intermediate and interact
with the by-product water to generate new Brønsted acid sites
[12,40]. Thus, the high activity of Ag1PW catalyst can be related
to the most acidic sites in terms of Brønsted acid and Lewis acid
sites. It is speculated that both Brønsted and Lewis acid sites can
activate carbonyl group of acetic acid and hydroxyl group of glyc-
erol, which might be a rate-determining step.
However, there is not a consistent correlation between glycerol
activity and total acidity over these HPW and Ag-salts catalysts, as
shown in Tables 2 and 3. In comparison with HPW, Ag2PW and Ag3-
PW possessed less acidic sites, but exhibited superior activity.
These results might be related to the excellent water tolerance of
Ag-salts catalysts, as indicated by H2O-TPD results. The primary
structure (heteropolyanion) of HPW is pretty stable, but the sec-
ondary structure (heteropolyanion as well as protons) is fairly mo-
bile, which can interact with polar water molecules; even the
water molecules can enter into the bulk crystallite of HPW and
form protonated clusters [41]. The good solubility of HPW in water
originating from the esterification can affect acid strength and
cause deactivation. Contrarily, the insoluble Ag-salts are highly
hydrophobic and capable of inhibiting the diffusion of water mol-
ecules into the secondary structure, where located the acidic pro-
tons and exchanged Ag+ sites. Additionally, the generated water
can be released rapidly from the secondary structure, which can re-
main the integrity of Keggin structure as well as acid strength and
impair the reverse reaction. This behavior is characteristic of insol-
uble HPA salts and has already been reported in other water-sen-
sitive reactions, such as hydrolysis of ester and dehydration
[40,42]. On the basis of above discussions, it is concluded that
the excellent performance of Ag1PW catalyst can be ascribed to
the unique Keggin structure, high amount of Brønsted and Lewis
acid sites, and outstanding water tolerance.
4. Conclusions
This study has demonstrated that a partially silver-exchanged
HPW salt catalyst (Ag1PW) using simple preparation procedure,
presented exceptionally high activity for glycerol esterification
with acetic acid. The high Brønsted acid and Lewis acid sites, out-
standing water tolerance, strong stability in polar reaction environ-
ment, and remaining of unique Keggin structure are responsible for
the excellent performance of Ag1PW catalyst in this reaction. As far
as we know, Ag1PW catalyst exhibited the best catalytic activity
ever reported for glycerol esterification. Concurrently, this catalyst
gave high selectivity to the desired DAG and TAG with a combined
selectivity of 90.7%. Furthermore, Ag1PW catalyst did not suffer
from leaching and deactivation in five consecutive reaction cycles.
A plausible reaction mechanism was uncovered for further study
and as references.
This Ag1PW catalyst derived from HPA could be a promising
candidate to substitute conventional mineral acids for the catalytic
esterification of glycerol with acetic acid and for the synthesis of
valuable biofuel additives. In addition to be suitable for esterifica-
tion, such an environmentally friendly alternative catalyst holds
great potential for a wide range of other acid-catalyzed reactions.
Acknowledgments
The authors gratefully acknowledge the financial support of the
Natural Science Foundation of China (No. 20976185). This work
was also supported by the Major State Basic Research Development
Program of China (973 Program) (No. 2012CB215305).
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2013.06.026.

S. Zhu et al. / Journal of Catalysis 306 (2013) 155–163
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