Selective esterification of glycerol to bioadditives over heteropoly tungstate supported on Cs-containing zirconia catalysts

Article abstract of DOI:10.1016/j.apcata.2010.07.046

Esterification of glycerol with acetic acid was carried out over tungstophosphoric acid (TPA) supported on Cs-containing zirconia. The catalysts were prepared by impregnation method and characterized by FT-infrared spectroscopy, X-ray diffraction and temperature program desorption of NH ₃. The catalysts exhibited more than 90% conversion within a short reaction time. The catalytic activity depends on the amount of exchangeable Cs with TPA on zirconia, which is in tern related to the acidity of the catalysts. The acidity of the catalysts varied with the presence of residual protons of TPA. The effects of various parameters, such as reaction temperature, catalyst concentration and molar ratio of glycerol to acetic acid, were studied and optimized reaction conditions are established.

Full text of DOI:10.1016/j.apcata.2010.07.046

Applied Catalysis A: General 386 (2010) 166–170
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Selective esterification of glycerol to bioadditives over heteropoly tungstate
supported on Cs-containing zirconia catalysts
K. Jagadeeswaraiah, M. Balaraju, P.S. Sai Prasad, N. Lingaiah∗
Catalysis Laboratory, I&PC Division, Indian Institute of Chemical Technology, Hyderabad 500607, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Esterification of glycerol with acetic acid was carried out over tungstophosphoric acid (TPA) supported
on Cs-containing zirconia. The catalysts were prepared by impregnation method and characterized by
FT-infrared spectroscopy, X-ray diffraction and temperature program desorption of NH3. The catalysts
exhibited more than 90% conversion within a short reaction time. The catalytic activity depends on the
amount of exchangeable Cs with TPA on zirconia, which is in tern related to the acidity of the catalysts.
The acidity of the catalysts varied with the presence of residual protons of TPA. The effects of various
parameters, such as reaction temperature, catalyst concentration and molar ratio of glycerol to acetic
acid, were studied and optimized reaction conditions are established.
Received 23 February 2010
Received in revised form 14 July 2010
Accepted 26 July 2010
Available online 3 August 2010
Keywords:
Esterification
Glycerol
Acetic acid
Tungstophosphoric acid
Cesium
© 2010 Elsevier B.V. All rights reserved.
Zirconia
1. Introduction
heterogeneous phases [15]. The major disadvantages of HPAs
as catalysts lie in their low thermal stability, low surface area
Glycerol is the main by-product in biodiesel production by
transesterification of oil with methanol or ethanol. The increasing
use and production of biodiesel has resulted in substantial amount
of glycerol accumulation. This has allowed researchers to look for
new usages of glycerol [1,2]. In future, glycerol will be a cost-
effective raw material for the preparation of valuable chemicals
and fuel additives. Different methods for the conversion of glycerol
to value-added chemicals such as hydrogenolysis to propane diols
and for the preparation of different oxidation products have been
explored [3–12]. The uses of glycerol-based additives to improve
properties of biodiesel are also being explored [13]. The ester-
ification of glycerol with acetic acid can be a good application
for glycerol utilization. The products of glycerol esterification are
monoacetin, diacetin and triacetin, which have great industrial
applications. The products like triacetin have different applications,
going from cosmetics to fuel additives. The mono and diacetylated
esters are also have applications, particularly in cryogenics and syn-
theses of biodegradable polyesters [14]. Acetylation of glycerol has
been carried using solid acid catalysts such as zeolites. The main
aspect of the catalyst is its selectivity for this reaction.
(1–10 m2/g) and solubility in polar media. HPAs can become eco
friendly insoluble solid acid catalysts with high thermal stability
and high surface area by exchanging their protons with metal/alkali
metal ion and supporting them on suitable supports. The support
not only increases the surface area but also its stability. Supported
HPAs were also studied for simple esterification reactions and were
found to be highly active [16]. The support plays an important role
in the dispersion of HPA and in the acidic nature of the final catalyst.
The activity of the HPAs can be exchanged by supporting them on
acidic supports and by exchanging the proton partially with alkali
metal ions like Cs+. It is known that the Cs salts of heteropoly acids
are more acidic than parent HPAs [17]. Partial substitution of H+
by Cs+ changes the number of available surface acid sites and the
resulting materials exhibit significantly higher activities than the
parent acid in acid-catalyzed reactions [18]. Cs-containing phospho
tungstates are well-known as a water-insoluble strong Brønsted
acid and a versatile solid acid catalyst possessing high thermal sta-
bility (≥500 ◦C) and water tolerance [19–22]. Even though the Cs
salts possess more surface area compared to HPAs, their overall
surface area and stability is minimum. It is not possible to pre-
pare directly Cs salts of HPA dispersed on support. The usual way
to prepare is firstly dispersing Cs on support and then support-
ing HPA on it by anticipating the partial exchange of Cs present
on support with HPA. There are few studies where the support
is doped with Cs and heteropoly acid is supported on it. Yang et
al. reported the heteropoly tungstate supported on Cs modified
Heteropoly acids (HPAs) are typical strong Bronsted acids and
catalyze a wide variety of reactions in both homogeneous and
∗
Corresponding author. Tel.: +91 40 2719 3163; fax: +91 40 2716 0921.
E-mail address: nakkalingaiah@iict.res.in (N. Lingaiah).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.07.046

K. Jagadeeswaraiah et al. / Applied Catalysis A: General 386 (2010) 166–170
167
mesoporous silica catalysts and their catalytic activity towards n-
butane isomerization [23]. In another study cesium salts of TPA
supported on dealuminated ultra-stable Y zeolite catalysts are
reported. The Cs salt of TPA was generated by first dispersing Cs on
Y zeolite, followed by impregnation of TPA [24]. Yadav and George
reported the novelty of Cs substituted HPA supported on clay for
acid-catalyzed reactions [25].
In the present study, tungstophosphoric acid supported on Cs-
containing zirconia catalysts are prepared and evaluated for the
acetylation of glycerol with acetic acid. This reaction is tested under
different reaction parameters to yield the desired product. The cata-
lyst performance is discussed with the observed physico-chemical
properties derived from different characterization methods. It is
interesting to know the activity of the catalysts when Cs present on
zirconia is partially exchanged with TPA.
2. Experimental
Fig. 1. XRD profiles of TPA supported on Cs-containing zirconia catalysts. (o) TPA,
(#) Monoclinic phase of ZrO2, (*) Tetragonal phase of ZrO2.
2.1. Preparation of the catalysts
Initially Cs is doped on zirconia and this material is used as sup-
port to disperse TPA. The chemicals CsNO3 and H3PW12O40 are
purchased from Aldrich Chemicals. The support hydrous zirconia
was prepared by hydrolyzing the aqueous solution of ZrOCl2·8H2O
with ammonium hydroxide at a pH of 10. The precipitate was
filtered off and thoroughly washed with deionized water several
times until the chloride content was negligible. The precipitate is
dried at 120 ◦C for 36 h. The hydrous zirconia after thorough dry-
ing was used as support. Firstly, the required amount of CsNO3 was
dissolved in aqueous solution and added to the hydrous zirconia.
The solution was allowed to sit for 3 h and then the excess water
was evaporated on a water bath. The dried catalyst masses were
kept for further drying in an air oven and calcined at 500 ◦C for 2 h.
Later TPA supported on Cs-containing zirconia was prepared in the
same fashion by taking calculated amounts of TPA in aqueous solu-
tion. The final catalysts were calcined at 350 ◦C for 4 h in air. The
quantity of Cs is varied such that Cs can be exchangeable with 1,
2 and 3 protons of TPA. A catalyst without Cs also prepared in the
similar way by dispersing TPA on zirconia. The active component
TPA is kept constant at 20 wt% for all the catalysts. These catalysts
are denoted as 20%TPA/Csx-ZrO2 where x = 0, 1, 2 and 3.
3. Results and discussion
3.1. Catalyst characterization
The XRD patterns of the catalysts are shown in Fig. 1. The cat-
alysts showed main patterns related to the support zirconia. Both
tetragonal and monoclinic phases of ZrO2 are present, monoclinic
being predominant. The characteristic diffraction peaks related to
Keggin ions of TPA are observed at 2Â of 10.3◦ and 24.4◦ [26]. XRD
analysis suggests the presence of intact Keggin ion structure of TPA
on Cs-ZrO2.
FT-IR spectra give an informative fingerprint about the presence
of Keggin structure of heteropoly tungstates. The FT-IR spectra of
the catalysts are presented in Fig. 2. The catalysts mainly exhibited
bands at 1081, 990, 887 and 798 cm−1 related to the asymmetric
vibrations of (P–O), (W Ot), (W–Oc–W) and (W–Ob–W) modes,
respectively [27]. These results endorse the existence of Keggin
structure of TPA on support. The FT-IR results support the observa-
tions made from XRD analysis.
2.2. Characterization of the catalysts
X-ray diffraction (XRD) patterns of the catalysts were recorded
on
a Rigaku Miniflex diffractometer using CuK␣ radiation
˚
(1.5406 A) at 40 kV and 30 mA. The measurements were obtained
in steps of 0.045◦ with a account time of 0.5 s and in the 2Â range
of 10–80◦. FT-IR spectra were recorded on Biorad Excalibur series
using KBr disc method.
Temperature programmed desorption of ammonia (TPA) was
carried out on a laboratory-built apparatus equipped with a gas
chromatograph using a TCD detector. In a typical experiment about
0.05 g of the oven dried sample was taken in a quartz tube. Prior
to TPD studies, the catalyst sample was treated at 300 ◦C for 1 h
by passing pure He gas (99.9%, 50 ml/min). After pretreatment,
the sample was saturated with anhydrous ammonia (10% NH3) at
100 ◦C at a flow rate of 50 ml/min for 1 h and was subsequently
flushed with He at the same temperature to remove physisorbed
ammonia. The process was continued until a stabilized base line
was obtained in the gas chromatograph. Then the TPD analysis was
carried out from ambient temperature to 700 ◦C at a heating rate
of 10 ◦C/min. The amount of NH3 evolved was calculated from the
peak area of the already calibrated TCD signal.
Fig. 2. FT-IR of TPA supported on Cs-containing ZrO2 catalysts.

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K. Jagadeeswaraiah et al. / Applied Catalysis A: General 386 (2010) 166–170
Fig. 4. Esterification of glycerol over TPA supported on Cs-containing ZrO2 catalysts.
maximum conversion within 2 h. Initially, at the start of the reac-
tion, the selectivity towards monoacetin was high; as the reaction
prolonged, the selectivity towards di and triacetin increased at the
expense of that toward monoactin. The increase in selectivity for
di and triacetin with time is mainly due to the further acetylation
of monoacetin as the glycerol conversion is nearly completed with
in 2 h of reaction time. The high activity of the catalysts mainly
related to the presence of intact Keggin ions on the support. Dur-
ing the preparation, there was a possibility for the formation of Cs
exchanged TPA on ZrO2. The highest activity showed by TPA/Cs2-
ZrO2 catalyst might be due to the partial exchange of Cs, which
results in the presence of residual protons. The TPD results of the
catalysts suggest the presence of a maximum amount of acidity for
this catalyst. The catalyst with Cs amount equal to three protons of
TPA showed the lowest activity as expected due to the absence of
residual protons.
Fig. 3. TPD of NH3 pattern of the catalysts.
Ammonia adsorption–desorption technique permits the deter-
mination of the strength of acid sites present on the catalyst
surface, together with total acidity. The TPD profiles of the cata-
lysts are shown in Fig. 3. The TPD profiles of the catalysts without Cs
(TPA/ZrO2 catalyst) are also included for the sake of comparison. All
samples showed broad TPD profiles, revealing that the surface acid
strength is widely distributed. Three desorption peaks centered at
240, 430 and 570 ◦C, are observed; these are related to the presence
of weak, moderate and strong acid sites. The catalyst TPA/Cs2-ZrO2
showed the highest amount of total acidity with more strong acidic
sites than other catalysts. The variation in number of acidic sites
depended on the amount of the cesium on zirconia. The cesium
present on zirconia is exchangeable with the protons of TPA dur-
ing the impregnation. The partial exchange of TPA protons with Cs
results in enhanced surface acidic sites due to the presence of resid-
ual protons [17,18]. Generally these residual protons are mobile
and lead to generation of strong acidic sites. The catalyst with-
out Cs showed the presence of more weak to moderate acidic sites
compared to the Cs-containing catalysts.
The conversion and selectivity during glycerol acetylation not
only depends on the nature of the catalyst but also on the reaction
parameters. The influence of different reaction parameters such as
reaction temperature, catalyst weight and glycerol to acetic acid
molar ratio are studied.
3.2. Glycerol acetylation
The acetylation of glycerol over TPA/Csx-ZrO2 catalysts leads to
the formation of monoacetin, diacetin and triacetin. Fig. 4 compares
the activity of the catalysts for the acetylation of glycerol with acetic
acid. The TPA/ZrO2 catalyst reaction profile is also included for com-
parison. The catalytic activity varied with the exchangeable amount
of Cs on ZrO2. The catalyst without Cs showed relatively less activity
compared to all other catalysts. The catalyst with Cs content equal
to 2 moles exchangeable with two protons of TPA showed the high-
est activity. The glycerol conversion is increased with reaction time
for all the catalysts and a attained maximum conversion within 3 h
of reaction time.
As the catalyst TPA/Cs2-ZrO2 showed excellent esterification
activity, the reaction profile over this catalyst on varying the reac-
tion time was studied and the results are presented in Fig. 5. The
conversion of glycerol was very high even at 30 min and attained
Fig. 5. Reaction profile of TPA/Cs2-ZrO2 catalysts during glycerol esterification.

K. Jagadeeswaraiah et al. / Applied Catalysis A: General 386 (2010) 166–170
169
Fig. 6. Effect of reaction temperature on glycerol acetylation with acetic acid.
Fig. 8. Effect of catalyst weight on glycerol acetylation with acetic acid.
3.3. Influence of reaction temperature
increased with increase in the amount of acetic acid and maximum
conversion was attained at a mole ratio of 1:5. A marginal increase
is noticed as the results are taken after 4 h of reaction time. The
conversion did not varied above the molar ratio of 1:5. However,
the selectivity varied with glycerol to acetic acid molar ratio. More
monoacetin formed at lower glycerol to acetic acid ratio and its
selectivity decreased with increase in acetic acid amount. At higher
acetic acid concentrations, one expects to get more di and triacetin
due to the availability of more acetylating agent.
The influence of reaction temperature on the esterification of
glycerol over TPA/Cs2-ZrO2 was evaluated by varying the reac-
tion temperature from 60 to 120 ◦C; results are shown in Fig. 6. As
expected, the conversion of glycerol increased with temperature.
At the same time, the selectivity also varied with temperature. The
selectivity to di and tri acetin increased with increase in tempera-
ture from 60 to 120 ◦C. The present catalyst showed considerable
activity even at low reaction temperature.
3.4. Effect of glycerol to acetic acid molar ratio
3.5. Effect of catalyst weight
Glycerol conversion and selectivity during acetylation of glyc-
erol also depends on glycerol to acetic acid molar ratio. The reaction
was carried out for the glycerol to acetic acid molar ratios from 1:4
to 1:7; the results are shown in Fig. 7. The conversion of glycerol
does not vary drastically with change in molar ratio. The conversion
Fig. 8 shows the effect of catalyst weight on glycerol conver-
sion. The increase in catalyst loading from 0.1 to 0.4 g results in
a marginal variation in glycerol conversion. When the catalyst
amount increased from 0.1 to 0.2 g, only a moderate increase in
glycerol conversion was observed. Thus, it is clear that the increase
in catalyst loading above 0.2 g did not help to improve the initial
rate of the reaction. The results suggest that a small amount cata-
lyst is sufficient to attain maximum conversion. The selectivity of
catalyst did not vary much with the change in catalyst weight.
4. Conclusions
Heteropoly tungstate supported on Cs-containing ZrO2 cata-
lysts was prepared with retention of Keggin ion structure. The
characterization results suggest the presence of strong acidic sites,
where acid strength depends on the exchangeable Cs content on
zirconia. The esterification activity is related to the acidity of the
catalysts. The Cs-containing catalysts possess stronger acidic sites
compared to the catalyst without Cs. The presence of Cs substan-
tially enhanced the activity. The glycerol esterification activity and
selectivity not only depended on the nature of the catalyst but also
on some reaction parameters such as temperature, time and mole
ratio of glycerol to acetic acid.
Acknowledgement
The authors KJ and MB are thankful to CSIR for the awards of
junior and senior research fellowships, respectively.
Fig. 7. Influences of glycerol to acetic acid molar ratio on glycerol acetylation with
acetic acid.

170
K. Jagadeeswaraiah et al. / Applied Catalysis A: General 386 (2010) 166–170
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