Structure–Activity Relationships of WOx-Promoted TiO2–ZrO2 Solid Acid Catalyst for Acetalization and Ketalization of Glycerol towards Biofuel Additives

Article abstract of DOI:10.1007/s10562-021-03733-2

Abstract: WO_x-promoted TiO₂–ZrO₂ solid acid catalyst was prepared and applied in the catalytic acetalization and ketalization of glycerol with carbonyl compounds to produce biofuel additives. The presence of WO_x promoter and TiO₂ remarkably improved the catalytic activity of ZrO₂. Approximately, 100% glycerol conversion was evidenced with non-bulky aliphatic aldehydes and ketones like, propanol and cyclohexanone. The physical characterization of WO_x-promoted TiO₂–ZrO_2, revealed a higher formation of tetragonal crystalline phase of ZrO₂, over monoclinic. The total surface acidity and the ratio of Br?nsted to Lewis acidic site concentrations were determined by NH₃-TPD and pyridine-chemisorbed FTIR spectroscopy, respectively. A considerably higher concentration of Lewis acidic sites, ~ 213.29?μmol/gm, was evidenced on the WO_x-promoted TiO₂–ZrO₂ catalyst surface. Catalytic activity study revealed a direct correlation between the surface Lewis acidic site concentration and the activity of catalyst. This significant observation indicated the key role of Lewis acidic sites in this catalytic process. The WO_x-promoted TiO₂–ZrO₂ catalyst was also considerably stable and showed good performance in the acetalization/ketalization of glycerol with other substituted carbonyl compounds. Graphic Abstract: The WO_x-promoted TiO₂–ZrO₂ solid acid catalyst exhibits superior catalytic performance for acetalization and ketalization of glycerol with carbonyl compounds to produce biofuel additives. [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-021-03733-2

Catalysis Letters
https://doi.org/10.1007/s10562-021-03733-2
Structure–Activity Relationships of WOx‑Promoted TiO2–ZrO2
Solid Acid Catalyst for Acetalization and Ketalization of Glycerol
towards Biofuel Additives
Mallesham Baithy1,2 · Deboshree Mukherjee1 · Agolu Rangaswamy1 · Benjaram M. Reddy1
Received: 5 January 2021 / Accepted: 8 July 2021
© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021
Abstract
WOx-promoted TiO2–ZrO2 solid acid catalyst was prepared and applied in the catalytic acetalization and ketalization of glyc-
erol with carbonyl compounds to produce biofuel additives. The presence of WOx promoter and TiO2 remarkably improved
the catalytic activity of ZrO2. Approximately, 100% glycerol conversion was evidenced with non-bulky aliphatic aldehydes
and ketones like, propanol and cyclohexanone. The physical characterization of WOx-promoted TiO2–ZrO2, revealed a higher
formation of tetragonal crystalline phase of ZrO2, over monoclinic. The total surface acidity and the ratio of Brønsted to
Lewis acidic site concentrations were determined by NH3-TPD and pyridine-chemisorbed FTIR spectroscopy, respectively. A
considerably higher concentration of Lewis acidic sites, ~213.29 μmol/gm, was evidenced on the WOx-promoted TiO2–ZrO2
catalyst surface. Catalytic activity study revealed a direct correlation between the surface Lewis acidic site concentration
and the activity of catalyst. This signifcant observation indicated the key role of Lewis acidic sites in this catalytic process.
The WOx-promoted TiO2–ZrO2 catalyst was also considerably stable and showed good performance in the acetalization/
ketalization of glycerol with other substituted carbonyl compounds.
Graphic Abstract
The WOx-promoted TiO2–ZrO2 solid acid catalyst exhibits superior catalytic performance for acetalization and ketalization
of glycerol with carbonyl compounds to produce biofuel additives.
Extended author information available on the last page of the article
Vol.:(0123456789)
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M. Baithy et al.
Keywords WOx/TiO2−ZrO2 · Solid acid catalyst · Biofuel additives · Lewis acidic sites · Bioglycerol · Aldehydes · Ketones
times. Some heterogeneous catalysts studied and reported
for glycerol acetalization include sulfonic mesostructured
silica [27], zeolite H-BEA (SAR 19) [28], Ni–Zr supported
on mesoporous activated carbon [29], Zr- and Hf-TUD-1 and
Sn-MCM-41 [30]. ZrO2-based solid acid catalysts are active
for glycerol acetalization, and exhibit outstanding physico-
chemical properties [31], including, low thermal conductiv-
ity, ample acidic sites, high melting point, high corrosion
resistance power, and redox (oxidizing and reducing) proper-
ties [32–34]. However, these catalysts show lower rates of
reaction than homogeneous catalysts. Improving the reaction
rate and product yield can help to design improved substi-
tutes for homogeneous catalysts for this process [7, 15, 25].
In the feld of catalysis, use of binary oxides such as
TiO2–ZrO2 have gained more interest in the recent past, due
to their superior acidic strength and improved physicochemi-
cal properties compared to the individual component oxides
[15, 16]. Literature reports also evidence improvement of
the acidic properties of ZrO2-based materials on addition
of various promoters such as SO42−, MoO3, WO3, and other
[7, 15, 35]. Therefore, in the present study the catalytic
acetalization/ketalization of glycerol was performed with
acetone (Scheme 1), benzaldehyde (Scheme 2), and fur-
fural (Scheme 3) under solvent-free conditions, in the pres-
ence of ZrO2-based acid catalysts, namely, WOx-promoted
TiO2–ZrO2, WOx-promoted ZrO2, TiO2–ZrO2, and pure
ZrO2. The WOx-promoted TiO2–ZrO2 and WOx-promoted
ZrO2 catalysts were prepared via deposition-coprecipitation
method. Pure ZrO2 and TiO2–ZrO2 samples were synthe-
sized via simple coprecipitation methods. All the prepared
catalysts were characterized in detail through powder X-ray
difraction, N2 adsorption–desorption, Raman spectroscopy,
and X-ray photoelectron spectroscopy. Additionally, the
total surface acidity and the concentration ratio of Brønsted
to Lewis acidic sites of all the catalysts were determined
employing NH3-TPD and Py-FTIR spectroscopy, respec-
tively. This study helped to justify the role of diferent types
of surface acid sites in the catalytic acetalization/ketalization
of glycerol. The infuence of changeable reaction parameters
such as molar ratio of glycerol to carbonyl compounds, and
reaction time were also investigated. The WOx-promoted
1 Introduction
Biodiesel is a promising sustainable alternate of fossil feed-
stock to overcome the various energy and environmental
crises related to the usage of fossil fuels [1–3]. However, the
commercial synthesis of biodiesel via transesterifcation of
vegetable oil or animal fat leads to the production of 10 wt%
glycerol byproduct. This enormous amount of bioglycerol
produced is expected to cause a sharp decrease in its price,
which can afect the economics of biodiesel industries and
the environment [4–6]. Therefore, the regular availability
of excess and ecofriendly bioglycerol has instigated the
development of industrially important products utilizing
the bioglycerol to improve its commercial value. Glycerol
can be used as a versatile raw material for the synthesis of
various fne chemicals and fuel additives leading to cleaner
emissions [3]. Several catalytic conversion processes of bio-
glycerol are reported in the literature, including acetaliza-
tion/ketalization, carbonylation, etherifcation, esterifcation,
hydrogenolysis, oxidation, dehydration, and other [7–13].
The glycerol acetals/ketals are used in other valorization
strategies, such as production of favors, surfactants, efec-
tive fuel additives, bases, protecting agents, scents, and low-
toxic solvents for various pharmaceutical and veterinarian
formulations [14–20]. Therefore, the acetalization/ketaliza-
tion of bioglycerol with carbonyl compounds has received
much attention [21–24].
Five and six membered cyclic acetals and ketals are pro-
duced via condensation of two alcohol functional groups of
glycerol with aldehydes and ketones [25, 26]. Convention-
ally, solketal is produced using homogeneous acid catalysts
such as p-toluenesulfonic acid and sulfuric acid. Application
of these catalysts leads to an enhanced production cost due
to the requirement of downstream separation of homog-
enous catalyst and the damage caused to the equipment
by the corrosive acids. Some homogenous acid catalysts,
such as SnF2 and SnCl2 can be easily recovered, but still
cause the problem of corroding the reactor and pipes. Het-
erogeneous acid catalysts can ofer partial solution to these
problems and are more environment friendly than homog-
enous catalysts, as they can be recycled and reused several
Scheme 1 Acetalization
of glycerol with acetone to
(2,2-dimethyl-1,3-dioxolan-
4-yl)- methanol (solketal) and
2,2-dimethyl-1,3-dioxan-5-ol
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Structure–Activity Relationships of WOx-Promoted TiO2–ZrO2 Solid Acid Catalyst for…
Scheme 2 Catalytic acetaliza-
tion of glycerol with ben-
zaldehyde to cyclic acetals,
(2-phenyl-1,3-dioxolan-4-yl)-
methanol [5-membered acetal]
and 2-phenyl-1,3-dioxan-5-ol
[6-membered acetal]
Scheme 3 Catalytic acetaliza-
tion of glycerol with furfural
to cyclic acetals, (2-(furan-
2-yl)-1,3-dioxolan-4-yl)-meth-
anol [5-membered acetal] and
2-(furan-2-yl)-1,3-dioxan-5-ol
[6-memebered acetal]
TiO2–ZrO2 catalyst that was found to be most efcient was
further subjected to the acetalization/ketalization of glycerol
with other substituted carbonyl compounds. This investiga-
tion revealed the infuence of steric and electronic factors
of the substrates on the reaction. Recyclability test of the
WOx-promoted TiO2–ZrO2 catalyst was also performed to
assess the catalytic stability of the material.
required amount of (NH4)6H2W12O40·xH2O (Aldrich, AR
grade) was dissolved in deionized water, followed by the
addition of fnely powdered ZrO2 and/or TiO2–ZrO2. The
excess water was evaporated on a hot plate at 353 K under
vigorous stirring. The resulting samples were oven dried at
393 K for 12 h and fnally calcined at 923 K for 5 h with a
heating ramp of 5 K/min in atmospheric air.
2.2 Catalyst Characterization
2 Experimental Section
The X-ray diffraction patterns of the samples were
obtained using a Rigaku Multifex instrument equipped
with nickel‐filtered Cu Kα radiation source. The wide-
angle scan was made from 12 to 80° with a 2θ step size
of 0.02° and using a counting time of 1 s per point. The
Brunauer−Emmett−Teller (BET) surface area of catalysts
was measured using a SMART SORB‐92/93 instrument. All
samples were degassed at 393 K for 2 h prior to analysis.
The surface area was calculated from adsorption−desorp-
tion of nitrogen at 77 K using the multipoint BET method.
NH3-TPD of the samples was carried out in a Micromerit-
ics AutoChem 2910 instrument (USA). A total of 100 mg of
sample was taken in a quartz tube and was pre-treated in the
instrument. The sample was heated to 473 K and held at that
temperature under the fow of helium for 30 min to remove
moisture on the surface of the sample. The TPD analysis
was performed using a 3% NH3/He (v/v) gas mixture at
a fow rate of 20 mL/min. The heating rate was 10 K/min
from room temperature to 1073 K. A DILOR XY Raman
spectrometer equipped with a laser having a wavelength of
632 nm excitation line from an argon ion laser was used
to record Raman spectra of all the synthesized samples at
ambient temperature. The XPS analyses of the prepared
samples were performed using Shimadzu (ESCA 3400) XP
2.1 Catalyst Preparation
The TiO2–ZrO2binary oxide with a molar ratio of unity was
synthesized by a simple co-precipitation method from ultra-
high dilute aqueous solutions. In brief, the desired amounts
of ZrOCl2 and TiCl4 (Fluka, AR grade) were separately dis-
solved in deionized water and mixed together under vigorous
stirring. The titanium tetrachloride precursor was frst dis-
solved in cold concentrated HCl and then diluted with deion-
ized water. Subsequently, 20 wt% of aqueous NH3 solution
was gradually added dropwise into the above mixture solu-
tion until the pH of the solution reached~8.5. The obtained
precipitate slurry was fltered of, washed several times with
double distilled water, until completely freed from chloride
ions, detected by adding silver nitrate to the fltrate. The
precipitate was oven‐dried at 393 K for 12 h. Some portions
of the dried sample were calcined at 923 K for 5 h with a
heating rate of 5 K/min in atmospheric air. Pure ZrO2 was
also prepared by implementing the identical procedure under
same conditions.
The 10 wt% WOx-promoted ZrO2 and/or TiO2–ZrO2 solid
acids (WOx/ZrO2 and WOx/TiO2–ZrO2) were prepared by an
incipient wet-impregnation method. In this procedure, the
1 3

M. Baithy et al.
spectrometer with an Mg Kα radiation (1253.6 eV) X-ray
excitation source at a pressure<10–7 torr. Every spectrum
of the samples was calibrated with the binding energy of
adventitious carbon C 1 s peak at 284.6 eV.
The pyridine chemisorbed FT-IR (Py-FTIR) spectra were
recorded on a Nicolet 740 FT-IR spectrometer with KBr
pellets at a resolution of 4 cm−1 and averaging 256 scans
for every sample. Firstly, the catalyst was dried at 373 K for
1 h and the pyridine was directly added to the dried sample.
Further, the pyridine adsorbed sample was kept in an oven
at 393 K for 1 h for desorption of the physisorbed pyridine.
Then, cooling to room temperature and recorded the IR
spectrum with a nominal resolution of 4 cm−1 in the spectral
range of 1900–1400 cm−1.
2.3 Activity Studies
The condensation reaction of glycerol with carbonyl com-
pounds (acetone, benzaldehyde and furfural) was performed
over the solid acid catalysts in a 25 mL round bottom fask,
with vigorous stirring (~ 1000 rpm), at 313 K of reaction
temperature, under solvent-free conditions. In a typical run,
the catalyst (100 mg), carbonyl compound, and glycerol
(1:1 molar ratio) were placed in the round bottom fask and
the reaction mixture was then heated to 313 K temperature
under continuous stirring for 90 min. Upon completion of
the reaction, the catalyst was separated from the reaction
mixture by centrifugation and subsequently analyzed by
GC equipped with a BP-20 (Wax) capillary column and a
Flame ionization detector (FID). The obtained products 5-
and 6-membered ring acetal/ketals were also identifed by
GC–MS equipped with a DB-5 capillary column and an FID.
The obtained glycerol conversions and products selectivity
were calculated as done in our previous reports [36].
Fig. 1 Powder X–ray difraction profles of pure ZrO2, TiO2–ZrO2,
WOx/ZrO2, and WOx/TiO2–ZrO2 catalysts calcined at 923 K
3 Results and Discussion
3.1 Characterization of Catalysts
The typical XRD patterns of all the samples are shown in
Fig. 1. The XRD pattern of pure ZrO2 exhibited peaks at
2θ values of 28.3, 24.46, and 31.58° due to the monoclinic-
ZrO2 phase and at 30.27 and 49.21° due to tetragonal-ZrO2
phase [37–39]. The monoclinic-ZrO2 phase dominated
over the tetragonal-ZrO2 phase in the pure ZrO2 sample. In
contrary, the XRD analysis of WOx/ZrO2, TiO2–ZrO2, and
WOx/TiO2–ZrO2 catalysts revealed the tetragonal zirconia
as the major phase. This observation indicates that WOx
and TiO2 stabilize the tetragonal phase over the mono-
clinic phase. Additional peaks corresponding to Zr(WO4)2
(JCPDS #21–1496) were observed in the XRD pattern of
WOx/ZrO2. In the difraction pattern of TiO2–ZrO2, the
Fig. 2 Visible Raman spectra of ZrO2, TiO2–ZrO2, WOx/ZrO2, and
WOx/TiO2–ZrO2 catalysts calcined at 923 K
formation of a new ZrTiO4 phase (JCPDS #7–290) was
also observed. In the XRD pattern of WOx/TiO2–ZrO2,
both the crystalline ZrTiO4 and Zr(WO4)2 phases were pre-
sent. Noticeably, there were no XRD peaks corresponding
to crystalline WOx and TiO2, which could be due to the
inclusion of the Ti-ions in the ZrO2 crystals or the exist-
ence of strong interaction between the fnely dispersed
WOx and TiO2 on the ZrO2 and TiO2–ZrO2supports.
1 3

Structure–Activity Relationships of WOx-Promoted TiO2–ZrO2 Solid Acid Catalyst for…
Figure 2 shows the Raman spectra of the catalysts. The
main Raman active characteristic bands of monoclinic or
tetragonal ZrO2 are observed in the 100–650 cm−1 region.
The complex spectrum of pure ZrO2 as observed in Fig. 2
can be explained by the presence of both tetragonal and
monoclinic phases. The bands appeared at 180, 188, 380,
and 476 cm−1 are related to monoclinic phase and the
bands at 290, 316, 454, and 647 cm−1 correspond to the
tetragonal phase [19, 39]. In line with the XRD patterns,
the Raman bands of pure ZrO2 showed higher intensity of
the monoclinic ZrO2 bands than the tetragonal ZrO2 bands.
The TiO2–ZrO2 and promoted TiO2–ZrO2 samples exhibited
Raman bands at approximately 168, 280, 338, 412, 640 and
803 cm−1 which are attributed to ZrTiO4 compound [40,
41]. The WOx/TiO2–ZrO2 and WOx/ZrO2 samples exhibited
Raman bands at 329 and 440 cm−1 for the Zr(WO4)2 phase.
In addition, the Raman bands appeared at higher frequency
region (700–1100 cm−1) in the spectra of WOx/TiO2–ZrO-
2catalyst, which could be attributed to the formation of
mono- and polytungstate (WOx) species on the surface of
the support matrix [42].
were in the order: ZrO2 < TiO22ZrO2< WOx/ZrO2 < WOx/
TiO2–ZrO2. The NH3–TPD results evidenced the key role
of the WOx promoter towards the improvement in the con-
centration of acidic sites. Signifcant rise in the acidic site
concentration was found on the WOx/TiO2–ZrO2 sample.
The XPS analysis was performed to study the changes in
the bonding environment of elements present on the cata-
lyst surface. Figure 3a shows Zr 3d core level XP spectra of
pure and promoted ZrO2 samples. In every case, the peaks
were symmetric and showed peak maxima at around~182.2
and~184.6 eV. These peaks correspond to 3d5/2 and 3d3/2,
respectively [40, 41, 43, 44]. Additionally, the estimated
value of spin–orbit splitting was 2.4 eV, which confrm
the+4 oxidation state of Zr in the pure as well as promoted
ZrO2 samples. However, no peak was observed for metal-
lic Zr (~ 178 eV) in the samples [45, 46]. The Zr 3d peak
position of the promoted ZrO2 sample was slightly shifted
towards the lower binding energy side in comparison with
pure ZrO2. The binding energy shift may be attributed to
the Zr ions bound to the surrounding metal ions by Zr-O-
M (M=W/Ti) linkages at the interface. These results were
consistent with XRD and Raman analysis which indicated
the presence of ZrTiO4 and Zr(WO4)2 phases.
The estimated specific surface areas and amounts of
acidic sites obtained from N2 adsorption–desorption and
NH3–TPD results of all the samples are presented in Table 1.
The addition of WOx promoter enhanced the specifc surface
area of ZrO2 due to the inhibition of crystal growth caused
by the promoter ions. On the other hand, the incorporation
of TiO2 lowered the specifc surface area of ZrO2. Notice-
ably, the addition of WOx promoter signifcantly reduced
the surface area of TiO2–ZrO2. This observation could be
explained by the role of promoter species in blocking the
pores of the support and/or solid state reaction between the
promoted metal oxide and the support matrix [15]. The con-
centration of acidic sites on the synthesized catalyst surfaces
The Ti 2p core level XP spectra of TiO2–ZrO2 and
WOx/TiO2–ZrO2 samples are shown in Fig. 3b. As can be
noticed from the Fig. 3b, the binding energy of the Ti 2p3/2
is observed at~459.3 eV for both the samples, confrming
the existence of Ti in + 4 oxidation state [47]. Generally,
the Ti 2p3/2 peak of pure TiO2 appears at ~ 458.5 eV. An
increase in the binding energy of the Ti 2p3/2 for the pro-
moted TiO2 sample is due to the formation of ZrTiO4 com-
pound. As well, the intensity of the Ti 2p3/2 peak was drasti-
cally decreased after incorporation of WOx promoter into the
TiO2–ZrO2. This noticeable observation could be explained
Table 1 Physicochemical properties of all the synthesized ZrO2, TiO2–ZrO2, WOx/ZrO2, and WOx/TiO2–ZrO2 catalysts
Catalyst
SA (m2/g)a XRD phasesb
Acidic
sites
BAS (μmol/g)d LAS (μmol/g)d B/Ld Selectivity (%)
Sel. 5/Sel. 6e Sel. 5/Sel. 6f Sel. 5/Sel. 6g
(μmol/g)c
ZrO2
TiO2–ZrO2
WOx/ZrO2
42
30
60
14
m-ZrO2, t-ZrO2 210
63.83
71.97
89.40
21.75
37.48
45.15
2.93 96/4
1.92 98/2
1.98 98/2
1.74 99/1
60/40
62/39
64/36
68/32
57/43
58/42
60/40
62/38
ZrTiO4
260
390
700
Zr(WO4)2
WOx/TiO2–
ZrTiO4,
213.29
129.22
ZrO2
Zr(WO4)2
aBET analysis
bXRD analysis
cNH3-TPD analysis
dPyridine-FTIR analysis
eKetalization of the glycerol with acetone reaction
fAcetalization of the glycerol with furfural reaction
gAcetalization of the glycerol with benzaldehyde reaction
1 3

M. Baithy et al.
Fig. 3 a Zr 3d XP spectra of
pure ZrO2, TiO2–ZrO2, WOx/
ZrO2, and WOx/TiO2–ZrO2
catalysts. b Ti 2p XP spectra
of TiO2–ZrO2 and WOx/TiO2–
ZrO2 catalysts
Table 2 Binding energy (eV)
values of pure ZrO2, TiO2–
ZrO2, WOx/ZrO2, and WOx/
TiO2–ZrO2 samples
Catalyst
O 1 s
Ti 2p3/2
Zr 3d3/2
Zr 3d5/2
W 4f7/2
ZrO2
530.4 0.05
530.4 0.05
531.2 0.05
531.0 0.05
–
184.6 0.02
184.5 0.02
184.4 0.02
184.4 0.02
182.2 0.02
182.1 0.02
182.0 0.02
182.0 0.02
–
–
TiO2–ZrO2
WOx/ZrO2
WOx/TiO2–ZrO2
459.3 0.02
–
459.3 0.02
35.6 0.02
35.6 0.02
on the basis of strong interaction of the tungsten ions with
TiO2–ZrO2 mixed oxide. From Table 2, the binding energies
of W 4f7/2 XP peaks are observed to be ∼35.6 eV for WOx/
ZrO2 and WOx/TiO2–ZrO2 samples and these results are in
well agreement with the reported values in the literature for
tungsten compounds [36, 47, 48]. After deconvolution, there
were three binding energy peaks observed for the promoted
samples and the binding energies of W 4f7/2 peaks were
observed at 35.6, 36.54, and 34.5 eV for WOx/ZrO2 and
WOx/TiO2–ZrO2 catalysts. This result indicates the presence
of diferent oxidation states of tungsten (i.e., W6+, W5+, and
W4+) present in the corresponding promoted catalysts [48].
These results clearly indicated that the Zr(WO4)2 compound
was present on the catalyst surface [32].
The obtained binding energies of O 1 s core level XPS
analysis of the pure and promoted ZrO2 samples are listed
in the Table 2. A major O 1 s peak is found at~530.4 eV for
all the samples, which can be attributed to oxygen species in
the lattice oxygen of mixed metal oxides [49]. Table 2 shows
that the binding energy of all the samples is very sensitive to
the nature of promoted metal ions. In case of promoted sam-
ples, the binding energy of O 1 s is slightly shifted towards
higher binding energy side compared with that of pure ZrO2,
which could be due to the formation of respective mixed
metal oxides (ZrTiO4 and Zr(WO4)2).
Fig. 4 Pyridine chemisorbed FTIR spectra of ZrO2, TiO2–ZrO2,
WOx/ZrO2, and WOx/TiO2–ZrO2 catalysts calcined at 923 K
The concentration of Lewis and Brønsted acidic sites on
the investigated catalyst surfaces are estimated by employ-
ing pyridine chemisorbed FTIR (Py-FTIR) spectroscopy.
The Py-FTIR profles of the prepared catalysts are shown
1 3

Structure–Activity Relationships of WOx-Promoted TiO2–ZrO2 Solid Acid Catalyst for…
in Fig. 4. The quantity of Brønsted acidic sites (BAS) and
Lewis acidic sites (LAS) on the surface of ZrO2, TiO2–ZrO2,
WOx/ZrO2, and WOx/TiO2–ZrO2 catalysts could be esti-
mated from the integral intensities of the typical IR bands
centred at 1446 and 1545 cm−1, respectively, using the total
surface acidity obtained from the NH3-TPD experiments.
The results are furnished in the Table 1. The absorption
bands centred at wavelength of 1446 cm−1 represents the
adsorbed pyridine chemically coordinated on the LAS of
the catalyst surface. The IR bands centred at wavelength
of 1545 cm−1 are the characteristics of surface protonated
pyridine or pyridinium ion (Py+) attached to the BAS. As is
apparent from Table 1, all the catalysts exhibited more BAS
than LAS. The B/L signifes the BAS-to-LAS ratio for the
prepared catalysts. A remarkable lowering of B/L ratio was
observed for WOx/TiO2–ZrO2 catalyst, which indicates that
the combination of WOx, ZrO2, and TiO2 is benefcial for
improving the quantity of surface Lewis acidic sites.
3.2 Catalytic Activity Studies
The acetalization of glycerol with carbonyl compounds
like, acetone, benzaldehyde, and furfural were carried out
in the presence of ZrO2, TiO2–ZrO2, WOx/ZrO2, and WOx/
TiO2–ZrO2 catalysts. The standard reaction conditions
employed in all cases were 313 K temperature, 1000 rpm
stirring speed, 10 wt% with respect to glycerol, 1:1 molar
ratio of glycerol to carbonyl compound, and 90 min of reac-
tion time under solvent-free conditions.
3.2.1 Product Distribution and Activity Order
In all the cases, formation of two isomeric fve-membered
and six-membered cyclic ketals/acetals were observed
(Fig. 5), as reported in the earlier literature [25, 27]. The
main observations of the activity study are as follows: (i)
The activity order of the catalysts in terms of glycerol con-
version in all the reactions was WOx/TiO2–ZrO2 > WOx/
ZrO2 >TiO22ZrO2>ZrO2; (ii) the activity order of the cata-
lysts was not in relevance with their specifc surface area;
(iii) a direct correlation was observed between the activ-
ity order and the concentration of total acidic sites on the
catalyst surface (presented in Table 1); and (iv) irrespective
of the catalyst used, the fve membered cyclic product was
majorly formed with acetone and furfural. In contrast, the
acetalization of glycerol with benzaldehyde resulted in the
formation six-membered 1,3-dioxane as the major product.
Assimilation of these observations indicated the important
role of acidic sites in the glycerol conversion and activity of
the catalysts. However, there was no signifcant change in
the product distribution pattern for diferent catalysts used.
Fig. 5 Acetalization of glycerol over ZrO2, TiO2–ZrO2, WOx/ZrO2,
and WOx/TiO2–ZrO2 catalysts with a acetone, b furfural, and c
benzaldehyde. Reaction conditions: molar ratio of glycerol to ace-
tone=1:1; reaction time=90 min; reaction temperature=313 K; stir-
ring speed=1000 rpm; catalyst amount=10 wt% (w.r.t. glycerol)
3.2.2 Infuence of Variation of Glycerol to Carbonyl
Compounds Molar Ratio
The glycerol conversion percentage in the presence of best
performing WOx/TiO2–ZrO2 catalyst was studied against
time at 1:1 and 1:2 molar ratio of glycerol to carbonyl com-
pounds, i.e., acetone, benzaldehyde, and furfural, leaving
the other reaction conditions unchanged, as shown in Fig. 6.
An increase in the glycerol conversion was observed with an
increase in the molar ratio of glycerol to carbonyl compound,
1 3

M. Baithy et al.
Fig. 6 The efect of molar
ratio of a glycerol to acetone,
b glycerol to furfural, and c
glycerol to benzaldehyde as a
function of reaction time in the
acetalization of glycerol with
acetone, furfural, and benzal-
dehyde over WOx/TiO2–ZrO2
catalyst. Reaction conditions:
reaction temperature=318 K;
stirring speed=800 rpm;
catalyst amount=10 wt% (w.r.t.
glycerol)
as reported in the earlier studies [36]. The product selectiv-
ity was unchanged. Presence of excess carbonyl compound
favoured its chemisorption over the acid catalyst surface,
which is the frst step of the reaction, and thus pushes the
equilibrium towards the product side, resulting in higher
glycerol conversion [20].
there were no signifcant changes in the glycerol conver-
sion. However, the selectivity to the fve and six-membered
ring acetal/ketals was independent of the catalyst loading.
Thus, catalyst loading plays a major role in the conversion
of glycerol until a saturation point.
3.2.4 Acetalization of Glycerol with Various Aldehydes
and Ketones
3.2.3 Efect of Catalyst Loading
The efect of catalyst loading on the ketalization of glycerol
with acetone, benzaldehyde, and furfural was studied over
the WOx/TiO2–ZrO2 catalyst, as presented in Fig. 7. The
catalytic reactions were examined by a varying the weight
of the catalyst from 2.5 to 15 wt% with respect to the initial
glycerol amount, for 90 min time, maintaining other reaction
conditions unchanged. It is clearly observed from Fig. 7 that
the glycerol conversion gradually increased with increasing
the catalyst loading until 10 wt%, in all cases. The attain-
ment of optimum conversion at 10 wt% was connected to the
availability of maximum active sites on the catalyst surface
at this percentage. At higher catalyst loading till 15 wt%,
The best performing WOx/TiO2–ZrO2 catalyst was fur-
ther explored for the reaction using various aliphatic and
aromatic aldehydes and ketones. The glycerol conversion
and product selectivity are presented in Table 3. Entry
1–7 represent the acetalization of glycerol with selected
aliphatic and aromatic aldehydes. It is apparent from the
results that electron withdrawing efect and absence of bulky
substituents near the carbonyl carbon favored the glycerol
conversion. These electronic and steric factors also infu-
enced the product selectivity. With anisaldehyde and ben-
zaldehyde substrates (Entry 4 and 6) higher selectivity for
the 6-membered ring acetal was observed, which could be
1 3

Structure–Activity Relationships of WOx-Promoted TiO2–ZrO2 Solid Acid Catalyst for…
Fig. 7 The efect of catalyst loading on acetalization of glycerol with a acetone, b furfural, and c benzaldehyde over WOx/TiO2–ZrO2 catalyst.
Reaction conditions: reaction time=90 min; reaction temperature=318 K; stirring speed=800 rpm; molar ratio of glycerol to acetone=1:1
explained by the steric hindrance on the carbonyl carbon
and the higher stability of the 6-membered cyclic intermedi-
ate, as discussed in the later section. In case of ketalization
(Entry 8–13) of glycerol, lower conversion was observed
than that of acetalization reaction, which can be attributed
to the higher electron donating efect and presence of steric
hindrance at the carbonyl carbon of the keto groups. The
ketalization of glycerol with acetone, cyclopentanone, and
cyclohexanone showed high glycerol conversion with more
selectivity towards 5-membered cyclic ketal. Noticeably,
steric hindrance severely afected the conversion percent-
ages, as presented in Entry 11–13.
WOx/TiO2–ZrO2 catalyst indicated the benefcial role of
WOx, ZrO2, and TiO2 for improving the quantity of surface
Lewis acidic sites. It can also be assumed that the presence
of large Ti4+ species (ZrTiO4) act as the Lewis acidic sites
on the WOx/TiO2–ZrO2 catalyst surface [50, 51]. The B/L
ratio of the synthesized catalysts was found to be in the order
ZrO2 > WOx/ZrO2 >TiO2–ZrO2> WOx/TiO2–ZrO2.
The catalytic activities of the prepared materials were
calculated for the acetalization/ketalization reaction by
normalizing the catalytic reaction rates measured at glyc-
erol conversion to the catalyst surface areas, using ace-
tone, benzaldehyde, and furfural substrates. For all three
reactions, the catalytic activity followed the order WOx/
TiO2–ZrO2> TiO2ZrO2 > WOx/ZrO2 > ZrO2, as presented
in Fig. 8. This fgure clearly shows an apparent correlation
between the increasing trend of activity and the decreas-
ing order of the B/L ratio of the catalysts studied. Even
though, the WOx/ZrO2 sample exhibited higher number
of total acidic sites than TiO2–ZrO2 sample, but, a lower
3.3 Structure–Activity Relationships
We already signifed the crucial role of surface acid sites in
the catalytic acetalization/ ketalization reaction. The nature
of the active catalytic sites was established by the Py-FTIR
spectroscopy. The remarkably lower value of B/L ratio for
1 3

M. Baithy et al.
Table 3 The acetalization of bioglycerol with various carbonyl compounds over WOx/TiO2–ZrO2 catalyst
Reaction conditions: molar ratio of furfural to glycerol=1:2; reaction temperature=318 K; stirring speed=800 rpm; catalyst amount=10 wt%
(w.r.t. glycerol)
1 3

Structure–Activity Relationships of WOx-Promoted TiO2–ZrO2 Solid Acid Catalyst for…
of glycerol, forming a hemiacetal/hemiketal intermediate.
Dehydration of the intermediate leads to the fnal 5- and
6-membered ring acetal/ketal. Formation of the 5-membered
ring product involves bond formation between the carbonyl
oxygen and the β-carbon of glycerol, accompanied by dehy-
dration. In contrary, formation of the 6-membered ring
requires bond formation between the carbonyl oxygen and
the primary carbon of glycerol. For the aliphatic aldehydes
and ketones, the 5-membered cyclic compound was the
major product, whereas, for aromatic aldehydes and ketones,
the 6-membered cyclic products were majorly formed. This
observation can be rationalized in terms of two-stage reac-
tion, as proposed by Deutsch et al. [53], and supported by
other research groups [19, 54]. The frst stage is kineti-
cally controlled resulting in the prevalent formation of the
5-membered cyclic product, due to the shorter life-time of
the 5-membered cyclic intermediate [55]. In the thermody-
namically controlled second stage, ring transformation takes
place resulting in the fnal 5- and 6-membered cyclic product
mixture. The higher stability of the benzyl cation leads to
the major formation of the 6-memmbered cyclic acetal/ketal
through the thermodynamically controlled second stage in
the case of aryl aldehydes and ketones.
Fig. 8 Structure–activity relationship between B/L ratio and areal
activity of ZrO2, TiO2–ZrO2, WOx/ZrO2, and WOx/TiO2–ZrO2 cata-
lysts for acetalization of glycerol with acetone, furfural, and benzal-
dehyde
B/L ratio was observed for TiO2–ZrO2. In the correlation,
the TiO2–ZrO2 showed a higher activity than WOx/ZrO2.
These results provide a clear indication of the vital role of
Lewis acidic sites in the catalytic acetalization/ketalization
of glycerol.
3.5 Recyclability Study
The stability of the WOx/TiO2 −ZrO2 catalyst was checked
for acetalization of glycerol with furfural by applying the
used catalyst (Fig. 9). The study was carried out for fve con-
secutive cycles. In every cycle, the catalyst was tested under
optimized reaction conditions. After each run, the used cata-
lyst was separated from reaction mixture and washed with
methanol several times to remove adsorbed species, followed
by drying at 393 K for 12 h. Insignifcant decrease of the
conversion percentage was observed up to fourth cycle,
which could be attributed to the small loss of the catalyst
during the purifcation stage. After fourth run, the activity
3.4 Mechanism
Based on our experimental results and previous literature
reports we propose the reaction mechanism, as presented
in Scheme 4 [52]. The acetalization/ketalization of glycerol
over the acid catalysts take place through the initial acti-
vation of the carbonyl carbon through chemisorption over
the catalyst active sites. The activated carbonyl carbon then
undergoes nucleophilic attack by the primary –OH group
Scheme 4 Plausible mechanistic pathway of acetalization/ketalization of glycerol over WOx/TiO2–ZrO2 solid acid catalyst (R1 represents alkyl/
aryl group and R2 represent H/alkyl/aryl group)
1 3

M. Baithy et al.
Fig. 9 Reusability study of
WOx/TiO2–ZrO2 catalyst for
acetalization of glycerol with
furfural. Reaction condi-
tions: molar ratio of glycerol
to furfural=1:1; reaction
time=90 min; reaction
temperature=318 K; stir-
ring speed=800 rpm; catalyst
amount=10 wt% (w.r.t. glyc-
erol)
was decreased to 62%. These results indicate that the WOx/
TiO2 − ZrO2 catalyst was stable and reusable without any
considerable loss of efciency up to fourth run.
propanal and cyclohexanone with more than 80% selec-
tivity for the 5-membered ring acetal/ketal. Further, the
WOx/TiO2–ZrO2 catalyst was subjected to the acetaliza-
tion/ketalization reaction using diferent substituted car-
bonyl compounds. The glycerol conversion and product
selectivity were controlled by the electronic and steric
factors on the carbonyl carbon. A plausible mechanism
for the acetalization/ketalization of glycerol involving the
Lewis acidic sites has been proposed from our experimen-
tal observations and based on earlier literature reports.
The WOx/TiO2–ZrO2 catalyst was also considerably stable
and reusable without any signifcant loss of activity up to
four cycles.
4 Conclusions
In summary, ZrO2-based solid acid catalysts, such as, pure
ZrO2, WOx/ZrO2, TiO2–ZrO2, and WOx/TiO2–ZrO2 were
synthesized by coprecipitation and deposition-coprecip-
itation procedures. These materials were successfully
applied for catalytic acetalization/ketalization of glycerol
with carbonyl compounds, i.e., acetone, benzaldehyde, and
furfural. The WOx/TiO2–ZrO2 catalyst showed superior
catalytic performance among all the prepared materials.
Various spectroscopic and non-spectroscopic techniques
were applied to study the physicochemical properties
of the prepared catalysts. The presence of WOx clusters
and incorporation of Ti4+ ions stabilized the crystalline
tetragonal phase of zirconia. The formation of ZrTiO4
and Zr(WO4)2 species were also confrmed. The cata-
lytic activity study indicated that the addition of WOx
promoter stabilized the stronger acidic sites of ZrO2 and
TiO2–ZrO2 mixed oxides, which played an important role
in the activation of the carbonyl compounds in the frst
step of the reaction. The important fnding was the direct
correlation between the activity of the catalysts and the
increasing surface Lewis acidic sites concentration (i.e.,
decreasing order of B/L ratio). The WOx/TiO2–ZrO2
catalyst showed an approximately 100% glycerol conver-
sion for non-bulky aliphatic aldehydes and ketones like,
Acknowledgements MB thanks Department of Science and Technol-
ogy (DST)-SERB, New Delhi for the award of National Post-Doctoral
Fellowship (PDF/2018/002244). RA thanks the Council of Scientifc
and Industrial Research (CSIR), New Delhi for senior research fel-
lowship. DM thanks the University Grants Commission (UGC), New
Delhi, for senior research fellowship. BMR thanks the Department of
Atomic Energy (DAE), Mumbai for the award of the Raja Ramanna
Fellowship.
Author Contributions MB and AR carried out all acetalization/ket-
alization experiments and characterized the catalysts by various tech-
niques, DM helped in the interpretation of the results and manuscript
preparation and BMR supervised the research.
Declarations
Conflict of interest The authors declare that they have no known com-
peting fnancial interests or personal relationships that could have ap-
peared to infuence the work reported in this paper. The authors declare
that they have no confict of interest.
1 3

Structure–Activity Relationships of WOx-Promoted TiO2–ZrO2 Solid Acid Catalyst for…
28. Rossa V, Pessanha YSP, Diaz G, Diogenes L et al (2017) Ind Eng
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Authors and Afliations
Mallesham Baithy1,2 · Deboshree Mukherjee1 · Agolu Rangaswamy1 · Benjaram M. Reddy1
1
* Mallesham Baithy
Catalysis and Fine Chemicals Department, CSIR-
baithy.m@gmail.com
Indian Institute of Chemical Technology, Uppal Road,
Hyderabad 500 007, India
* Benjaram M. Reddy
2
bmreddy@iict.res.in
Chemical Engineering Department, Indian Institute
of Technology Hyderabad, Sangareddy, Kandi 502 285, India
1 3