Sulphuric acid-functionalized siliceous zirconia as an efficient and reusable catalyst for the synthesis of glycerol triacetate

Article abstract of DOI:10.1007/s11696-020-01189-z

In the present study, a sulphated siliceous zirconia catalyst (SSZ-550) has been prepared and characterized by XRD and FTIR analysis to indicate the incorporation of sulphate group over the matrix. X-ray photoelectron spectroscopy also revealed that sulphur group was incorporated over the matrix to impart the Br?nsted acidity to the catalyst which is vital for the acetylation activity. To optimize the reaction parameters, viz. reagent molar ratios (3–12; glycerol/acetic acid), catalyst amount (1–5?wt%; catalyst/glycerol), reaction duration (20–50?min) and reaction temperature (30–100?°C) have been varied to obtain the optimum catalyst activity for the maximum glycerol triacetate yield. Finally, under the optimized reaction parameters of 9:1 glycerol/acetic acid molar ratio, 3?wt% catalyst, 80?°C reaction temperature and 40?min of reaction duration, a 93% glycerol triacetate yield was obtained. The catalyst was recovered from the reaction mixture and reused during six consecutive reaction runs while retaining 50% glycerol triacetate selectivity in the last cycle. A plausible mechanism suggests the heterogeneous catalyst-assisted protonation of carbonyl group of acetic acid to initiate the stepwise esterification of the hydroxyl groups of glycerol.

Full text of DOI:10.1007/s11696-020-01189-z

Chemical Papers
https://doi.org/10.1007/s11696-020-01189-z
ORIGINAL PAPER
Sulphuric acid‑functionalized siliceous zirconia as an efcient
and reusable catalyst for the synthesis of glycerol triacetate
Km Abida1 · Amjad Ali1
Received: 9 November 2019 / Accepted: 7 May 2020
© Institute of Chemistry, Slovak Academy of Sciences 2020
Abstract
In the present study, a sulphated siliceous zirconia catalyst (SSZ-550) has been prepared and characterized by XRD and FTIR
analysis to indicate the incorporation of sulphate group over the matrix. X-ray photoelectron spectroscopy also revealed that
sulphur group was incorporated over the matrix to impart the Brønsted acidity to the catalyst which is vital for the acetyla-
tion activity. To optimize the reaction parameters, viz. reagent molar ratios (3–12; glycerol/acetic acid), catalyst amount
(1–5 wt%; catalyst/glycerol), reaction duration (20–50 min) and reaction temperature (30–100 °C) have been varied to obtain
the optimum catalyst activity for the maximum glycerol triacetate yield. Finally, under the optimized reaction parameters
of 9:1 glycerol/acetic acid molar ratio, 3 wt% catalyst, 80 °C reaction temperature and 40 min of reaction duration, a 93%
glycerol triacetate yield was obtained. The catalyst was recovered from the reaction mixture and reused during six consecu-
tive reaction runs while retaining 50% glycerol triacetate selectivity in the last cycle. A plausible mechanism suggests the
heterogeneous catalyst-assisted protonation of carbonyl group of acetic acid to initiate the stepwise esterifcation of the
hydroxyl groups of glycerol.
Keywords Glycerol · Glycerol triacetate · Acetic acid · HPLC
Introduction
having a variety of applications in pharmaceuticals, fuel,
cosmetic and food industry (Goscianska and Malaika 2019).
As a bio-additive, GD and GT could be utilized to enhance
the cold fow properties and to improve the viscosity of the
BD fuel (Malaika and Kozłowski 2018). These molecules
have also been reported to improve the octane number of
the gasoline fuel as well (Patel and Singh 2014). Acetins
could be produced by employing acetic acid (AcA) or acetic
anhydride (AcAn) as acetylating agents for GL (Setyaning-
sih et al. 2018; Bedogni et al. 2017). Sandesh et al. (2015)
and Han et al. (2019) reported that acetylation of GL with
AcAn could be performed even at low reaction temperature
of 30 °C; however, GL acetylation with AcA demands rela-
tively higher temperature of up to 120 °C. Although AcAn-
mediated GL acetylation demands milder reaction condi-
tions, at the same time, it yielded AcA as a corrosive side
product. Further, AcAn above 49 °C forms the explosive
mixture with air, and hence, it is not considered suitable for
the large-scale manufacturing of GT (Kong et al. 2016). On
the other hand, utilization of AcA as acetylating agent would
form harmless water as a side product. In the literature, GT
has been primarily produced via homogeneous acid (H2SO4,
H3PO4 or HCl)-catalysed GL acetylation with AcA or AcAn
The demand and cost of the biodiesel (BD) have been con-
stantly increased over the past few years (Edward et al.
2014). The production of global BD is expected to reach
almost 39 billion litres by 2024, and accordingly the glycerol
(GL) production, a by-product from BD industry, is bound
to increase many fold (FAO, OECD 2015; Khadijeh et al.
2014). GL is considered as a nontoxic molecule having a
variety of application in the synthesis of monoglyceride,
diglyceride (Fregolente et al. 2010), glycerol carbonate
(Jiahui et al. 2018), glycerol monoacetate (GM), glycerol
diacetate (GD) and glycerol triacetate (GT) (Malaika and
Kozłowski 2018). GT is an important derivative of GL
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-020-01189-z) contains
supplementary material, which is available to authorized users.
* Amjad Ali
amjadali@thapar.edu
1
School of Chemistry and Biochemistry, Thapar Institute
of Engineering and Technology, Patiala 147004, India
Vol.:(0123456789)
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Chemical Papers
(Khayoon and Hameed 2011). Even the homogeneous cata-
lyst-assisted and AcA-mediated acetylation demands higher
temperature (up to 135 °C) and prolonged reaction duration
(up to 3 h) to yield the partial GT selectivity (Khayoon and
Hameed 2011). Moreover, such catalysts are not only toxic
and corrosive but also non-reusable and difcult to separate
from the reaction mixture. In order to circumvent the issues
of homogeneous catalysts, GT production with AcA in the
presence of acidic heterogeneous catalyst was been explored
in the literature. The reactivity of the few literature-reported
heterogeneous catalysts with that studied in the present work
is compared in Table 1.
acetylation (Bhorodwaj and Dutta 2011); nevertheless, they
sufer the drawbacks of having the low surface area, low
thermal stability and solubility in polar media. In order to
immobilize the HPAs, they have been supported over the
MCM-41 (e.g. HPAs/ZrO2–MCM-41) to impart them large
surface area, higher thermal stability and insolubility in
reaction medium (Kaya Ekinci and Oktar 2019).
Along with the catalytic sites, GT selectivity was also
found to be a function of reaction duration, reagent ratio and
reaction temperature (Reddy et al. 2010). Hence, an optimal
combination of all these parameters is essential to achieve
the maximum selectivity of GT.
As evident from Table 1, most of the reported catalysts
sufer the drawback of poor GT selectivity as well as poor
reusability. These catalysts also required more than 100 °C
reaction temperature to yield the acceptable conversion
levels. However, the catalyst reported in the present study
(SSZ-550) not only provides the excellent triacetin selectiv-
ity (93%) at 80 °C, but also able to demonstrate the reus-
ability during 6 catalytic runs.
With an aim to develop the reusable heterogeneous cata-
lyst for the GL acetylation with AcA, (Scheme 1) sulphate
groups have been appended over the siliceous zirconia
matrix, in present study. Further, the reaction conditions
have been optimized to achieve the maximum GT selectiv-
ity during the acetylation reaction. The catalyst stability and
reusability have been evaluated with the recovered catalyst,
and the reason for the decline of the catalyst activity has also
been investigated.
In order to impart the acidity to the heterogeneous
catalysts, either Lewis acid sites (e.g. Mn2+On2−) or Brøn-
−
sted acid sites (e.g. –SO3 H+) have been introduced into
the matrix. A variety of the heterogeneous acid catalysts
such as metal oxide-based or sulphated one, viz. sulphated
STA–ZrO2–MCM-41 (Kaya Ekinci and Oktar 2019),
Amberlyst (Kale et al. 2015; Rastegari et al. 2015), WO/
TiO2–ZrO2, MoO3/TiO2–ZrO2 (Reddy et al. 2010,) MP(10)/
SBA-15 (Trejda et al. 2012) and SO42−/CeO2–ZrO2 (Reddy
et al. 2012), have been explored for the GL acetylation.
Heteropolyacids (HPAs) are also strong Brønsted acids and
could catalyse a wide variety of reactions including glycerol
Experimental section
Materials and methods
Zirconium (IV) oxychloride octahydrate and tetraethox-
ysilane (TEOS 98%) were procured from Sigma-Aldrich
(USA). Ammonia solution, sulphuric acid, acetic acid (99%)
and glycerol (99%) used for the acetylation were obtained
from Loba Chemie Ltd. (India).
Table 1 Comparison of acetylation activity of few literature-reported catalysts
Catalyst
Tem (°C) AcA/GL Time (h) wt%* GM% GD% GT% GLC% Catalyst reus-
References
ability (# of
cycles)
STA–ZrO2–MCM-41 200
6:1
6:1
8:1
6:1
6:1
7:1
9:1
10:1
6:1
16:1
6:1
6:1
9:1
4
5
–
5
5
5
3
1.5
3
5
2
5
5
3
37
25
–
42
15
–
21
4
100
97
NR
4
10
2
3
3
4
4
5
3
Kaya Ekinci and Oktar (2019)
Malaika and Kozłowski (2018)
Karnjanakom et al. (2018)
Kale et al. (2015)
CS–H2SO4
80
100
105
105
100
150
120
120
120
120
120
80
24
2.5
10
10
25
NR
4
1
3
3
3
SO3H/G–C
100 100
Amberlyst 15
Amberlyst-70
pAmberlyst-36
MP(10)/SBA-15
HSiW/ZrO2
–
12.3 83.9 100
51.7 45.8 100
44
51
2.5
43
9
Kale et al. (2015)
13
40
100
78
Rastegari et al. (2015)
Trejda et al. (2012)
Zhu et al. (2013)
Reddy et al. (2012)
Ferreira et al. (2011)
Reddy et al. (2010)
Reddy et al. (2010)
PR
28.3 60.5 11.2 96.4
25.8 57.7 16.5 100
25
53.21 40
52
–
SO42−/CeO2–ZrO2
PW2_AC
63
11
6.78 99
40.45 7.52 100
93 100
86
WO/TiO2–ZrO2
MoO3/TiO2–ZrO2
SSZ-550
NR
5
6
0.7
7
*Catalyst wt% with respect to glycerol, S sulphonic acid-functionalized, AC activated carbon, G–C glycerol–carbon, GL glycerol, GM glycerol
monoacetate, GD glycerol diacetate, GT glycerol triacetate, GLC glycerol conversion, NR not reported, P pressure (1 bar), PR present report
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Chemical Papers
Scheme 1 Stepwise glycerol acetylation with acetic acid in the presence of catalyst
Powder X-ray diffraction data were collected by
PANalytical X’Pert Pro using Cu-Ka radiation (operated
at 45 kV) in 2θ range of 20° to 70°. Fourier transform
infrared spectra (FTIR) of samples were recorded in KBr
matrix on Agilent Cary-660 spectrophotometer. Surface
area was measured by Brunauer, Emmett and Teller (BET)
method and pore size by BJH method using Micro Trac-
BEL mini-II instrument. Prior to analysis, samples were
heated at 100 °C for 3 h under vacuum to remove any
adsorbed molecule from the catalyst surface. The sul-
phated siliceous zirconia (SSZ-550) surface acidity was
calculated by conducting temperature-programmed des-
orption of NH3 (NH3-TPD) analysis using the Micro Trac-
Belcat II instrument. The Brønsted and Lewis acidic sites
of the catalyst were demonstrated by Pyridine adsorption
method. Catalyst sample under investigation was initially
saturated with pyridine, dried at 50 °C for 2 h, further
heated for 10 min at 300 °C to desorb the pyridine and
fnally subjected to the difuse refectance FTIR analysis.
Scanning electron microscopy coupled with energy-
dispersive X-ray spectrometry (SEM–EDX) was performed
over JEOL JSM 6510LV instrument, and transmission elec-
tron microscope (TEM) images were recorded on Hitachi
7500 instruments. X-ray photoelectron spectra (XPS) were
determined by ESCA+instrument (omicron nanotechnol-
ogy, Oxford Instrument, Germany) equipped with mono-
chromator aluminium source (Al K radiation of 1486.7 eV
energy).
Preparation of siliceous zirconia
The Si(OH)4Zr(OH)4 matrix has been prepared by sol–gel
method using ZrOCl2·8H2O as precursor of zirconia. In
a representative preparation method, ZrOCl2·8H2O (5 g)
was dissolved in 20 mL deionized water under constant
stirring at room temperature (35 °C). To this, aqueous
ammonia solution was added dropwise until 10 pH was
obtained and the resulted mixture was stirred for 15 min.
To this, tetraethoxysilane (TEOS, 98%) was added (cor-
responding to theoretical 1:1 molar ratio of Si/Zr) slowly,
with constant stirring for 1 h at room temperature. Finally,
the mixture was fltered and washed with deionised water
multiple times to remove the chloride ions and then dried
at 100 °C for 12 h to obtain the matrix Si(OH)4Zr(OH)4
which is referred as SZ.
To incorporate the sulphate ion over the matrix, 1 g
of prepared matrix (SZ) was powdered and suspended in
20 mL deionized water. To this, 1 molar solution (3 mL)
of H2SO4 was also added. The resulted mixture was stirred
for 24 h at room temperature, then dried for 12 h at 100 °C
and fnally calcined at 550 °C for 4 h in a mufe furnace.
The catalyst (referred as SSZ-550) preparation is shown
in Scheme 2.
Acetylation of glycerol with acetic acid
TA in the reaction mixture was detected and quantifed
by high-performance liquid chromatography (HPLC) which
was performed over Agilent Infnity 1200 instrument. Prior
to the analysis, catalyst from the reaction mixture was fl-
tered out and the liquid phase, thus obtained, was dissolved
in 60/40 (v/v) hexane/isopropanol (IPA) solvent system. A
fxed sample volume of 20 μL was injected into the HPLC
instrument which was analysed at 35 °C column temper-
ature. During the analysis, 60/40 (v/v) hexane/IPA was
employed as mobile phase with a fow rate of 0.6 mL/min
and RX-SIL column (250 ×4.6 mm, 5 μm) as a stationary
phase. The retention time of the molecules, present in the
sample under investigation, was detected with the help of
refractive index (RI) detector.
The optimum reaction condition for the GL acetylation
was established by varying the temperature (30–100 °C),
AcA-to-GL molar ratio (3–12) and catalyst amount
(1–5 wt% of GL). To evaluate the catalyst reusability, it
was recovered from the reaction mixture by centrifugation,
washed with methanol and calcined at 550 °C. The catalyst
thus regenerated was employed in six successive cycles
under the same experimental and regeneration conditions.
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Scheme 2 Reaction scheme
for the formation of sulphated
siliceous zirconia (SSZ-550)
catalyst
the amorphous nature of the prepared matrix. On incor-
porating the sulphate group over the matrix, SSZ-550 has
been formed which was found to exist in crystalline phase
as evident by the appearance of the sharp peaks in the
XRD patterns (Fig. 1b). The crystalline nature of the mate-
rial was due to the formation of ZrSiO2 as minor phase
(JCPDF = 01-081-0590) and Zr(SO4)2 as major phase
(JCPDF = 00-024-1492), as shown in Fig. 1b. The crystal-
lite size of the Zr(SO4)2 phase, calculated by the Scherrer
equation, was found to be 13.2 nm.
FTIR spectroscopy analysis
The FTIR spectra of the prepared SZ and SSZ-550 are
compared in Fig. 2 to indicate the sulphate group anchor-
ing over the siliceous zirconia surface. Both the samples
show a strong band at~ 802 cm−1 due to the vibrations of
Si–O–Si bond. In the literature, the presence of a band at
808 cm−1 in the FTIR spectra has been reported to indicate
the presence of Si–O–Si bonding (Hu et al. 2013). In the
FTIR spectra, new bands in the range of~1028–1266 cm−1
were observed (Fig. 2b), due to the stretching vibrations cor-
responding to S–O and S=O bonds to support the incorpo-
ration of sulphate group over the siliceous zirconia matrix.
Similar observation was reported by Sun and Garcia (Garcia
et al. 2008; Sun et al. 2005) where the bands in the range of
1045–1223 cm−1 have been reported to indicate the anchor-
ing of sulphate ion over the siliceous zirconia matrix.
Fig. 1 Powder XRD spectra of the a SZ and b SSZ-550
Results and discussion
Catalyst characterization
XRD analysis
The X-ray difraction (XRD) patterns for SZ demonstrate
a broad hump (2θ = 22), as shown in Fig. 1a, to support
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0.430 cm2 g−1) when sulphated zirconia was impregnated
over the mesoporous silica surface (Ward et al. 2011).
NH3 temperature‑programmed desorption study
The acidic sites present over the catalyst surface were quanti-
fed by the temperature-programmed desorption of ammonia
(NH3-TPD). The TPD profles of the desorbed NH3 from SZ
and SSZ-550 catalysts are shown in Fig. 4. A broad peak in
the range of 233–243 °C, for both the samples, was observed
owing to the ammonia desorption from weak acidic sites.
However, sulphate incorporation over the siliceous zirconia
nanoparticles demonstrates an additional desorption peak at
473 °C (4.96 mmol g−1) to indicate the formation of strong
acidic sites. Chen et al. (2007) have been reported that sul-
phated silica–zirconia (SiO2–SZ) nanoparticles showed des-
orption peaks in the range of 210–290 °C and 460–600 °C
to indicated the presence of weak and strong acidic sites,
respectively, over the catalyst surface (Ganpat and Gbadebo
2013).
Fig. 2 FTIR spectra of the a SZ and b SSZ-550
BET analysis
The BET surface area of SZ and SSZ-550 was measured
by N2 adsorption desorption isotherms as represented in
Fig. 3. All the samples were found to exhibit type IV curves
of IUPAC classifcation, which indicates the mesoporous
nature of the samples. In case of the siliceous zirconia
(SZ), matrix surface area and pore volume were found to
be 197.1 m2 g−1 and 0.31 cm3 g−1, respectively, as shown
in Fig. 3b. However, upon sulphate group impregnation
over the SZ matrix, surface area (69.9 m2 g−1) as well as
pore volume (0.10 cm3 g−1) was found to decrease. The
drop in the surface area could be ascribed to the pore block-
age of the matrix upon sulphate group attachment. Ward
et al. also observed the reduction of surface area (from
709 to 342 m2 g−1) as well as pore volume (from 0.819 to
Pyridine adsorption study
To study the nature of the acidic sites present over the cata-
lyst surface, pyridine-saturated matrix and catalyst were sub-
jected to FTIR spectroscopy. Through this method, Brønsted
and Lewis acid sites present over the catalyst could be identi-
fed and diferentiated (Parry 1963). As shown in Fig. 5, the
siliceous zirconia matrix shows a single band at 1636 cm−1
owing to the presence of Brønsted (B) acidic sites on its
surface. Upon SO42− incorporation over the SZ matrix, new
bands at 1445, 1490, 1545 and 1613 cm−1 were observed
in addition to the band observed in the matrix. Thus, incor-
poration of the sulphate group over the matrix leads to the
formation of the Brønsted as well as Lewis acidic sites. The
Fig. 3 a N2 adsorption–desorption isotherms and b pore distribution branch of the nitrogen isotherm by the BJH
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Brønsted sites interacts with pyridine which leads to the for-
mation of pyridinium ions, and same could be identifed due
to the presence of bands at 1490 and 1545 cm−1 in the FTIR
spectrum. In the literature, similar bands were also reported
at 1450, 1487 and 1540 cm−1, to support the formation of
pyridinium cations, owing to the interaction of pyridine with
Brønsted acids (Kansedo and Lee 2012; Yang and Kou 2014;
Massam and Brown 1995). Chomel et al. (2006) concluded
that sulphate group is mainly responsible for the generation
of strong acidic sites which impart the acetylation activity
to the catalyst.
SEM, EDX, elements mapping and TEM analysis
The morphology and elemental distributions of the cata-
lyst particles were examined by SEM–EDX technique, and
corresponding elemental mapping is shown in Fig. ESM-1.
The SEM images of the siliceous zirconia (Fig. ESM-1a)
and sulphated siliceous zirconia (Fig. ESM-1b) show the
formation of agglomerated nanoparticles in irregular geom-
etries. Elemental mapping clearly demonstrates that all the
elements (O, Si, Zr and S) are uniformly distributed over the
catalyst surface.
Fig. 4 Ammonia TPD analysis of a SZ and b SSZ-550
TEM image of SSZ-550 was studied in order to get the
information about the particle size and geometry as shown
in Fig. 6. The TEM analysis of SSZ-550 revealed the forma-
tion of spherical agglomerated particles (~16 nm particles
size), which is close to the crystallite size (15.2 nm) cal-
culated from XRD data. The smaller particle size could be
correlated with the higher catalyst surface exposure to the
reactants which may facilitate the higher catalyst activity.
XPS study
The XPS spectra of silicon, sulphur, zirconia and oxygen
are presented in Fig. 7 for determining the electronic state
of the elements present in the catalyst. The band observed
Fig. 5 FTIR spectra of pyridine-absorbed SZ and SSZ-550
Fig. 6 TEM images of SSZ-550
a low and b high resolution
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Fig. 7 XPS spectra of the various elements present in SSZ-550: a Si2p, b S2p, c Zr3p and d O1s
at 103.6 eV and 102 eV could be attributed to 2p1/2 and
2p3/2 level of Si4+ as shown in Fig. 7a. Observed values are
very close to the literature-reported values of 103.1 eV and
102 eV for Si4+2p state present in SiO2 material (Chernavs-
kii et al. 2006; Malhotra and Ali 2019). The presence of
sulphate functional group over the siliceous zirconia matrix
could be attributed to the presence of two peaks at 168.4
and 170.1 eV corresponding to the S+62p3/2 and S+62p1/2
states, respectively (Fig. 7b). These binding energy values
also support the linkages of sulphate group with zirconia/
silica matrix as the reported binding energies for the sul-
phate group anchored over the ZrO2 matrix were found
to be at 168.9 and 169.2 eV (Hino et al. 2006). The XRD
analysis of the same material also indicates the formation of
Zr(SO4)2; however, owing to the amorphous nature of the
silica, no peak due to the silica sulphate could have been
observed. Anchoring of the sulphate group over the matrix
was also supported by the FTIR analysis of the catalyst as
discussed in earlier section. Furthermore, the XPS spectrum
shows (Fig. 7c) peaks at 185.1 and 182.7 corresponding to
the Zr4+3d3/2 and Zr4+3d5/2 states, respectively. This study
could be correlated with the literature, where the presence of
Zr4+ has been ascribed to the appearance of peaks at 182.9
and 185.3 eV for Zr4+3d5/2 and Zr4+3d3/2 states, respectively,
in ZrO2 phase (Desmartin-Chomel et al. 2006). The peaks
(Fig. 6d) at 528.7, 533.1 and 531.4 eV may be due to the
presence of O2−1s state of lattice oxygen and could also be
ascribed to the various linkages of oxygen present in the
catalyst, viz. Si–O–Si, S–O–Zr and Zr–O–Si, respectively.
Similar observations were reported in the literature, where
the presence of O2−1s in the sulphated siliceous zirconia
nanoparticles was supported by the presence of peaks at
532.4 and 532.5 eV corresponding to Si–O–Si or Zr–O–Si
and S–O–Zr linkage (Rosenberg et al. 2002).
Product analysis
FTIR spectroscopy
Formation of the GT could be supported with the help of
FTIR spectroscopic technique. In the FTIR spectra of glyc-
erol, two bands at 1042 and 3300 cm−1 were observed due to
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the C–O and O–H group stretching frequencies, respectively,
as shown in Fig. ESM-2a. Upon glycerol acetylation, a new
band at 1737 cm−1 (Fig. ESM-2b), due to –C=O group,
was observed to support the formation of ester bond. The
absence of any band in –OH region further supports the
acetylation of all three glycerol hydroxyl groups.
GD (34%), GT (12%) and unreacted GL. On increasing the
molar ratio up to 9, GT yield was found to increase up to
93% within 40 min of reaction duration. In the literature,
at 3:1 AcA/GL molar ratio, SO42−/CeO2–ZrO2 catalyst was
found to demonstrate the poor GT selectivity of 5% along
with GM yield of 25%. Even on increasing the molar ratio
up to 6, the GT selectivity was found to increase margin-
ally (from 5 to 16%) that too at higher reaction temperature
of 120 °C and after a prolonged duration of 3 h (Trejda
et al. 2012).
HPLC analysis
The major product, GT, formed during the reaction was con-
frmed by comparing its retention time (6.30 min) with that
of standard GT sample, as shown in the HPLC chromato-
gram in Fig. ESM-3. The HPLC chromatogram (Fig. ESM-
3c) of product shows no peak corresponding to the glycerol
(8.90 min) to confrm its 100% conversion into the product
molecules. The quantitative analysis of the product mixture
further supports the GT selectivity of 93% while remaining
7% being the glycerol diacetate (GD).
Efect of catalyst amount on product selectivity
In order to optimize the catalyst amount, acetylation of GL
was studied in the presence of SSZ-550 catalyst using AcA
as acetylating agent. Upon acetylation of GL with AcA
(9:1 molar ratio) in the presence of 1–5 wt% catalyst (with
respect to GL) at 80 °C, the selectivity of GT signifcantly
increases due to the gradual increase in the active sites in
the reaction mixture. It can be seen that at lower catalyst
amount (up to 2 wt%), 90% GL conversion was observed
but yielding the lower glycerol triacetate selectivity (46%)
along with the formation of some GM (14%) and GD (30%)
by-products. Maximum GT selectivity of 93% was obtained,
at the cost of GM and GD, when 3 wt% of catalyst was
employed, and a further increase in catalyst amount (up to
5 wt%) was neither found to improve the selectivity not able
to reduce the reaction duration (Fig. 8b).
Catalyst screening
Prior to the catalyst screening, in blank runs, acetylation of
GL with AcA was performed in the absence of catalyst, or
in the presence of bare siliceous zirconia matrix. It is evi-
dent from Fig. ESM-4 that in the absence of catalyst merely
11.5% GL conversion with negligible GT selectivity was
obtained, while the matrix material, bare siliceous zirco-
nia, was able to yield 69% GL conversion with 1.6% GT
selectivity. Thus, sulphate group anchoring over the matrix
is required to generate the acidic sites which is primarily
responsible for catalysing the glycerol acetylation. Reddy
et al. (Trejda et al. 2012) also suggested that sulphate-
impregnated CeO2–ZrO2 and CeO2–Al2O3 catalysts dem-
onstrate better GT selectivity (90%) at 120 °C reaction tem-
perature, while carrying out the reaction at 6:1 AcA-to-GL
molar ratio and in the presence of 5 wt% of catalyst amount.
In order to demonstrate the efect of reaction parameters
on the catalyst activity, in the present study, the reagent ratio,
catalyst amount, reaction temperature and reaction time have
been sequentially varied as discussed in subsequent section.
The same phenomenon is also reported in the literature
for the acetylation of GL with AcA (1:7 molar ratio) at
100 °C reaction temperature in the presence of acidic cata-
lyst (Lewatit catalyst), where an increase in catalyst amount
from 2 to 3 wt% was also found to improve the GT selectiv-
ity from 56.28 to 66.91% (Setyaningsih et al. 2018).
Efect of reaction temperature on product selectivity
Acetylation of GL in the literature has been reported to fol-
low the endothermic pathway and hence, external source of
heat is essential to push the reaction in the forward direc-
tion (Patel and Singh 2014). In order to study the efect of
temperature on GL conversion as well as product selectivity,
a series of reaction was performed in the presence of SSZ-
550 catalyst by varying the temperature in the range of 30
to 100 °C, as shown in Fig. 8c. A signifcant increase in GT
selectivity from 0 to 93% was observed when the reaction
temperature was raised from 30 to 80 °C, respectively. A
further increase in the reaction temperature was not resulted
in any signifcant increase in GT selectivity. Hence, reac-
tion should be performed at 80 °C to achieve the optimum
catalyst activity.
Efect of AcA/GL molar ratio on product selectivity
Conversion of GL as well as GT selectivity over SSZ-
550 at 80 °C on 3 wt% of catalyst (with respect to GL)
with varying AcA/GL molar ratio (from 3 to 12) is shown
in Fig. 8a. Theoretically, every molecule of GL required
three molecules of AcA to achieve the 100% TA selectiv-
ity. However, to push the equilibrium in the forward direc-
tion, the reaction is usually performed in the presence of
excess amount of AcA. During the present study, the use
of lower AcA/GL molar ratio (up to 3) was found to yield a
mixture of products which is dominated by the GM (41%),
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Fig. 8 Infuence of the a molar ratio of AcA to GL [reaction condi-
tions: reaction time=40 min, catalyst amount=3 wt% (with respect
to GL) and reaction temperature=80 °C], b catalyst amount with
respect to GL (reaction conditions: AcA/GL=9:1 molar ratio, reac-
tion time=40 min and reaction temperature=80 °C), c reaction
temperature [reaction conditions: AcA/GL=9:1 molar ratio, reac-
tion time=40 min and catalyst amount=3 wt% (with respect to
GL)] and d reaction time, on the products selectivity during GL
acetylation [reaction conditions: AcA/GL=9:1 molar ratio, cata-
lyst amount=3 wt% (with respect to GL) and reaction tempera-
ture=80 °C]
Efect of reaction time on product selectivity
supported the stepwise acetylation of GL and an increase in
GT selectivity on increasing the reaction duration.
To establish the reaction duration in order to achieve the
maximum GT selectivity, a series of GL acetylation was per-
formed at 80 °C in the presence of 3 wt% catalyst at constant
AcA/GL molar ratio of 9:1 (Fig. 8d). It is evident from the
plot that during frst 20 min of reaction period GD (25%) is
the leading product and remains predominant till 30 min of
reaction duration. However, after 40 min, GT became the
exclusive product (93%) to support that GL acetylation took
place in a step wise fashion. Reddy et al. (Trejda et al. 2012)
have also studied the extent of GL acetylation at various
time intervals over SO42−/CeO2–ZrO2 catalyst at 120 °C and
observed 63% and 11% selectivity of GD and GT, respec-
tively, within 0.5 h of reaction duration in the presence of
6:1 AcA/GL molar ratio. Same study reported a maximum
GT selectivity of 90%, nevertheless after prolonged reaction
duration of 4 h. Thus, our study as well as literature report
The reusability and stability
The reusability of SSZ-550 catalyst was evaluated under the
optimized reaction conditions, viz. AcA/GL molar ratio of
9:1, 3 wt% of catalyst (with respect to GL), 80 °C reac-
tion temperature and 40 min of reaction duration. As shown
in Fig. 9, the SSZ-550 catalyst was employed during six
catalytic runs under the identical reaction conditions and
after every run the catalyst was recovered from the mix-
ture through fltration washed with methanol to remove the
reactants/products from the catalyst surface, dried at 100 °C
for 12 h and fnally calcined at 550 °C for 4 h. The results
revealed that during frst 3 catalytic cycles up to 80% GT
selectivity which was found to decline gradually to 54% in
6th cycle.
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Chemical Papers
band associated with the TG or GL in the FTIR of reused
catalyst further maintained that organic molecules have not
collected over the catalyst facade to hinder the active sites.
As evident from the FTIR study, sulphate group has
been detached from the catalyst surface during the repeated
use. The leached active sites from the matrix may catalyse
the reaction similar to the homogeneous catalyst. In order
to quantify the homogeneous contribution in the catalyst
activity, a hot fltration test was performed. During the test,
acetylation of GL was performed under optimized reaction
parameters for 20 min and after that catalyst was separated
from the reaction mixture with the help of simple fltration.
Now, the reaction mixture, without catalyst, was allowed
to react for additional 20 min. As evident from Fig. 11, no
signifcant change in the GL conversion level was observed
when the reaction was allowed to continue in the absence
of catalyst. Thus, it is safe to assume that dissolved catalyst
contents, if any, were not able to catalyse the reaction to
signifcant extent and heterogeneous catalyst is primarily
responsible for almost entire catalytic activity.
Fig. 9 Study of the SSZ-550 reusability during the GL acetylation
with AcA. Reaction conditions: AcA/GL=9:1 molar ratio, reaction
time=40 min, catalyst amount=3 wt% (with respect to GL) and
reaction temperature=80 °C
Proposed mechanism for the acetylation of GL with acetic
acid
In order to establish the reason(s) behind the loss in cata-
lytic activity, NH3-TPD profle of fresh and reused catalysts
is compared in Fig. 10a. Fresh catalyst exhibited the strong
acid sites, as revealed from the desorption peak at 473 °C,
with total acidity of 4.96 mmol g−1 in comparison with
1.36 mmol g−1 for the spent catalyst at the same desorption
temperature. Thus, a decrease in catalyst acidity could be
attributed to loss of acidic strength which might be due to
the loss of sulphate species from the catalyst surface.
As indicated by the blank experiments (“Catalyst screen-
ing” section) in the absence of catalyst, merely 11.5% GL
conversion with negligible GT selectivity was obtained.
On the other hand, bare siliceous zirconia was able to yield
merely 1.6% GT selectivity with higher GL conversion level
of 69%. These experiments clearly indicate the catalytic role
−
of strong Brønsted acid cites (SO4 H+) during the synthe-
In order to evaluate the loss of sulphate ions from the cat-
alyst surface, the FTIR spectra of fresh and spent catalyst are
compared in Fig. 10b. The partial loss of the SO42− moiety
from the spent catalyst is evident due to the loss of bands at
1028, 1140 and 1266 cm−1. The absence of any vibrational
sis of GT. However, relatively higher GL conversion levels
were obtained with bare catalyst, due to the presence of sur-
face Brønsted acidic sites (O−H+) (structure I in scheme 3);
nevertheless, these sites are not strong enough to yield the
higher GT selectivity. Sulphate group anchoring over the
Fig. 10 Comparison of a TPD and b FTIR plot of fresh and reused catalyst
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Chemical Papers
frst-order kinetic model (Setyaningsih et al. 2018; Zhou
et al. 2012). Consequently, in the present study, sulphate
groups have been proposed as the reaction centres for initiat-
ing the GT synthesis.
As discussed earlier (“Efect of reaction time on product
selectivity” section), the GM selectivity was found to be
maximum in the beginning of the reaction which gradually
got converted into the GD and GT to support the stepwise
acetylation of the GL. During the present study (“Efect of
AcA/GL molar ratio on product selectivity” section), when
the AcA concentration was varied, the rate of GT formation
was also found to change. However, same was not found to
be efected when GL concentration was varied.
Thus, plausible mechanism assumes the sulphate group-
assisted protonation of the acetic acid carbonyl group in the
frst step of the reaction to form the carbocation (II) as an
intermediate (Kale et al. 2015; Zhou et al. 2012; Venkatesha
et al. 2016) as shown in Scheme 3. Now the carbocation (II)
may react with the –O–H of GL to form another intermediate
(III). A water molecule has been given away by the interme-
diate (III) to generate a new carbocation (IV). Carbocation
(IV) ultimately regenerates the catalyst and leads to the for-
mation of the GM (V). Now, one of the remaining two –OH
groups of GM would further react with AcA following the
similar mechanism as described above to form GD which
may further react with AcA to form the GT molecule.
Fig. 11 Homogeneous contribution analysis during the GL acetyla-
tion (catalyst amount—3 wt%, AcA/GL molar ratio—9:1, reaction
temperature 80 °C)
siliceous zirconia matrix imparted the strong Brønsted acid-
ity, and consequently, the SSZ-550 catalyst was able to yield
93% GT selectivity with overall 100% GL conversion within
40 min of reaction duration. Homogeneous sulphuric acid
has also been reported to catalyse the acetylation of mono-
as well tri-alcohol (GL) and found to follow the (pseudo)
Scheme 3 Proposed mechanism
of glycerol acetylation with
acetic acid in the presence of
SSZ-550 catalyst
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Chemical Papers
Ferreira P, Fonseca IM, Ramos AM, Vital J, Castanheiro JE (2011)
Acetylation of glycerol over heteropolyacids supported on
activated carbon. Catal Commun 12:573–576. https://doi.
org/10.1016/j.catcom.2010.11.022
Conclusion
Glycerol acetylation with acetic acid was performed in
the presence of the SSZ-550 heterogeneous acid catalyst
to yield the 93% glycerol triacetate during a quick succes-
sion of 40 min. The presence of Brønsted acidic sites, in the
form of sulphate group, was found to be accountable for the
higher catalytic activity. The catalyst was recovered from
the reaction mixture and reused during six catalytic runs
albeit, with partial loss in activity owing to the fractional
loss of sulphate group from the catalyst surface. Such a high
glycerol triacetate yield under ambient reaction conditions in
the presence of reusable heterogeneous catalyst has not been
frequently reported in the literature. Presently the work is in
progress, in our laboratory, to improve the catalyst stability
in order to achieve the better catalyst reusability.
Fregolente PBL, Pinto GMF, Wolf-Maciel MR, Filho RMI (2010)
Monoglyceride and diglyceride production through lipase-cat-
alyzed glycerolysis and molecular distillation. Appl Biochem
Biotechnol 160:1879–1887. https://doi.org/10.1007/s1201
0-009-8822-6
Ganpat V, Gbadebo Y (2013) Mesoporous nanocrystalline sulfated
zirconia synthesis and its application for FFA esterifcation in
oils. Appl Catal A Gen 463:196–206. https://doi.org/10.1016/j.
apcata.2013.05.005
Garcia CM, Teixeira S, Marciniuk LL, Schuchardt U (2008) Trans-
esterification of soybean oil catalyzed by sulfated zirconia.
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ech.2007.09.092
Goscianska J, Malaika A (2019) A facile post-synthetic modifcation
of ordered mesoporous carbon to get efcient catalysts for the
formation of acetins. Catal Today. https://doi.org/10.1016/j.catto
d.2019.02.049
Han X, Zhu X, Ding Y, Miao Y, Wang K, Zhang H, Wang Y, Liu
S (2019) Selective catalytic synthesis of glycerol monolaurate
over silica gel-based sulfonic acid functionalized ionic liquid
catalysts. Chem Eng J 359:733–745. https://doi.org/10.1016/j.
cej.2018.11.169
Acknowledgements We acknowledge DST-SERB for the fnancial sup-
port (Ref. No. EMR/2014/000090) and DST-FIST (Ref. No. SR/FST/
CSI-217/2010) for funding the instrumentation facility to the School of
Chemistry and Biochemistry. We are also thankful to SAI Lab (Thapar
Institute of Engineering and Technology, Patiala, India) for NMR and
XRD studies.
Hino H, Kurashige M, Matsuhashi H, Arata K (2006) The surface
structure of sulfated zirconia: studies of XPS and thermal analy-
sis. Thermochim Acta 441:35–41. https://doi.org/10.1016/j.
tca.2005.11.042
Compliance with ethical standards
Hu X, Hao X, Wu Y, Zhang J, Zhang X, Wang PC, Zou G, Liang
XJ (2013) Multifunctional hybrid silica nanoparticles for con-
trolled doxorubicin loading and release with thermal and pH dual
response. J Mater Chem B 1:1109–1118. https://doi.org/10.1039/
c2tb00223j
Conflict of interest The authors declare that there is no confict of in-
terest.
Jiahui Z, Yuanfeng W, Xianghai S, Siquan X, Shuai L, Yanli Z, Lijing
G, Jin Z, Guomin X (2018) Thermodynamic and kinetic studies
for synthesis of glycerol carbonate from glycerol and diethyl car-
bonate over Ce–NiO catalyst. Chem Pap 72(11):2909–2919. https
://doi.org/10.1007/s11696-018-0518-3
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