Acetone condensation over sulfated zirconia catalysts

Article abstract of DOI:10.1007/s10562-013-1020-8

The aldol condensation reaction over sulfated zirconia led to the production of diacetone alcohol, which was further dehydrated forming mesityl oxide. The sulfated zirconia was obtained from zirconium acetate ethane sulfonate as a single source precursor. Oxides were obtained by calcinations of the precursors at 550-650 C, while the self-condensation reaction of acetone was carried out at 150 C. The precursor and the produced oxides were characterized using various characterization techniques. The precursors were synthesized with different acetate to ethane sulfonate ratio, ranging from 1 to 3. The major products obtained from the condensation reaction over the resulted oxide were mesityl oxide, mesitylene isophorone, naphthalene, and pentamers. The selectivity of mesityl oxide was approximately 100 % at the initial time-on-stream.

Full text of DOI:10.1007/s10562-013-1020-8

Catal Lett (2013) 143:705–716
DOI 10.1007/s10562-013-1020-8
Acetone Condensation Over Sulfated Zirconia Catalysts
•
Mohammed H. Al-Hazmi YongMan Choi
•
Allen W. Apblett
Received: 25 February 2013 / Accepted: 30 April 2013 / Published online: 15 May 2013
Ó Springer Science+Business Media New York 2013
Abstract The aldol condensation reaction over sulfated
zirconia led to the production of diacetone alcohol, which
was further dehydrated forming mesityl oxide. The sulfated
zirconia was obtained from zirconium acetate ethane sul-
fonate as a single source precursor. Oxides were obtained
by calcinations of the precursors at 550–650 °C, while the
self-condensation reaction of acetone was carried out at
150 °C. The precursor and the produced oxides were
characterized using various characterization techniques.
The precursors were synthesized with different acetate to
ethane sulfonate ratio, ranging from 1 to 3. The major
products obtained from the condensation reaction over the
resulted oxide were mesityl oxide, mesitylene isophorone,
naphthalene, and pentamers. The selectivity of mesityl
oxide was approximately 100 % at the initial time-on-
stream.
bond formation. The self-condensation of ketone is a well-
established process, and important for the production of a,b-
unsaturated carbonyl compounds [6]. Such reactions are
performed homogenously over a liquid base catalyst (i.e.,
soda or potash) [4, 7, 8] or a liquid acid catalyst (i.e., sulfuric
acid) [9]. In addition, the reactions ordinarily require very
long, complex, and hazardous procedures. Recently, efforts
have been directed toward replacing the catalysts with more
environmentally friendly heterogeneous solid catalysts [10–
23]. For example, solid base catalysts, such as MgO and CaO,
have been used [11–14, 24], while conventional solid acid
catalysts used for the aldol condensation reaction include
aluminum oxide, aluminum alkoxides, and zeolites [25, 26].
The aldol condensation of ketone is a complex reaction
producing many products via the self-condensation of two
ketone molecules or the cross condensation of one ketone
with other ketone products formed. The reaction network and
product distribution are controlled by catalyst’s properties,
reaction conditions, and the chemical nature of ketone.
Acetone condensation has been widely studied over several
solid acid catalysts along with accurate characterization using
13C nuclear magnetic resonance (NMR) and infrared (IR)
spectroscopies [27–29]. The results from the condensation
reaction of acetone over zeolites and alumina indicated that
Lewis acid sites are mainly responsible active sites for the
initial activation of acetone. It is known that base catalysts in
adol condensation reactions are preferred [30], Hino and
Arata [31–33] reported that remarkable enhancement of
catalytic activities were observed by adding sulphate ions,
leading to an increase in the surface acidity. Side reactions
involved double bond migration, hydride transfer, oligo-
merization, and cracking can occur on catalyst surfaces.
The influence of several factors (i.e., preparation methods,
surface acidity, and physico-chemical properties of sulfated
zirconia) on the catalytic activity and productivity was
Keywords Sulfated zirconia Á Ethane sulfonic acid Á
Aldol condensation Á Acetone Á Mesityl oxide
1 Introduction
The aldol condensation is an important reaction in fine
chemicals and organic synthetic chemistry [1–5]. It involves
the production of b-hydroxy aldehyde or b-hydroxy ketone
by a condensation of aldehyde or ketone via a carbon–carbon
M. H. Al-Hazmi (&) Á Y. Choi
SABIC Technology Center, Riyadh 11551, Saudi Arabia
e-mail: HazmiMH@sabic.com
A. W. Apblett
Chemistry Department, Oklahoma State University, Stillwater,
OK 74078, USA
123

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M. H. Al-Hazmi et al.
addressed. To the best of our knowledge, the transition metal
coordination chemistry by sulfonates is poorly understood.
The anions of sulfonic acids are believed to be bonded
covalently with a zirconium metal forming salts (i.e.,
[Zr(SO3R)n]?(4/n)). It is well known that sulfonate ions are an
inefficient ligand, and they are weakly bound or non-bonded
to most of the transition metals [34]. Generally, sulfated
zirconia is synthesized by a precipitation of zirconium
hydroxide from a variety of zirconium salts by aqueous
ammonia solution, followed by sulfation using ammonium
sulfate or sulfuric acid solutions [32, 33, 35–37]. The only
single precursor source utilized for the synthesis of sulfated
zirconia in the literature is zirconium sulfate. The thermal
decomposition of zirconium sulfate leads to the formation of
zirconium oxide and sulfur trioxide, which can be retained on
the zirconia surface producing sulfated zirconia [38, 39].
However, the acid strength and the catalytic activity of the
oxide obtained via this approach may be lower than those
prepared by conventional procedures. This may be due to a
decomposition of the oxide materials, which facilitates a
rapid loss of sulfur species and reduces the surface acidity. To
improve the catalytic activity and the acid strength of the
oxide, it is imperative systematically to examine physical and
chemical properties (i.e., specific surface areas, active sites,
crystalline structures, and phase compositions) which are
strongly influenced by preparation procedures, thermal
treatment, starting materials, and sulfation agents [40, 41]. To
date, no comprehensive studies describing the synthesis and
characterization of sulfated zirconia from zirconium sulfo-
nate complexes are available in the literature, which is crucial
to design a better catalyst. In this study, we present a route to
synthesize sulfated zirconia from zirconium acetate with
different concentrations of the ethanesulfonic acid as a single
source precursor. The synthesized zirconium oxide was
evaluated for the aldol condensation reaction of acetone.
yield was [95 %. The precursor is denoted as ZS-n(x:y),
where ZS is a zirconium sulfonate complex, n is a serial
number, x is a mole ratio of zirconium acetate, and y is a
mole ratio of the ethanesulfonic acid. Oxides were obtained
by calcinations of the precursors at 650 °C.
2.2 Catalyst Characterization
The prepared precursors and the oxides were characterized
using various techniques. The specific surface area was
obtained via a conventional Brunauer–Emmett–Teller (BET)
multilayer nitrogen adsorption method using a Quantachrome
Nova 1200 instrument. Thermogravimetric analyses (TGA)
were performed using a Seiko EXSTAR 6000 TG/DTA 6200
instrument. A scanning electron microscope (JEOL JXM
6400 SEM) was used to examine the morphology of catalyst
surfaces. Infrared spectra in the 4,000–400 cm-1 region were
collected by diffuse reflectance of a ground powder diluted
with potassium bromide on a Nicolet Magna-IR 750 spec-
trometer. Elemental analyses of carbon, hydrogen, and sulfur
were performed by Desert Analytics. X-ray powder diffrac-
tion patterns were obtained by a Bruker AXS D8 advance
diffractomer using copper Ka radiation with a wavelength of
˚
1.5418 A. XRD patterns were collected at ambient temper-
ature, and the phases were identified using the ICDD database
(ICDD# 42–1164 and #37–1484). The mean crystallite sizes
of oxide samples were estimated using a line broadening
method and Scherer’s equation [42]. 13C NMR was also used
for the characterization of the prepared zirconium complexes.
13C spectra were obtained with a Chemagnetics CMX-II
solid-state NMR spectrometer operating at 75.694 MHz for
13C and a Chemagnetics 5 mm double resonance magic-
angle spinning probe. 13C cross-polarization/magic-angle
spinning (CP/MAS) was carried out with a quasi-adiabatic
sequence using two pulse phase modulation (TPPM) decou-
pling at 50–75 kHz. The 13C CP contact pulse of 1 ms length
was divided into 11 steps of equal length with ascending
radiofrequency field strength, while the 1H contact pulse had
constant radiofrequency field strength. At least 3,600 scans
were acquired with a delay of 1.0 s between scans. The MAS
sample spinning frequency was 6.0 kHz, maintained to
within a range of 5 Hz or less with a Chemagnetics speed
controller.
2 Experimental
2.1 Catalyst Preparation
All chemicals were purchased from Aldrich and used
without further purification. The precursors were prepared
by the reaction of zirconium acetate ((CH3CO2)xÁZr(OH)y)
and zirconium oxychloride (ZrOCl2Á8H2O) with ethane-
sulfonic acid (CH3CH2SO3H) at variant mole ratios of the
zirconium salt to ethanesulfonic acid, followed by drying.
The ethanesulfonic acid solution was added slowly to the
zirconium acetate solution. The ethyl sulfonate group
replaced the acetate ligands and produced acetic acid into
the solution. The precursor was obtained by evaporation of
water and the formed acetic acid, followed by drying under
vacuum for 12 h. Based on the zirconium salt, the reaction
The acid strength of prepared sulfated zirconia powders
was estimated using Hammett organic base indicators [43].
This determination is based on the ability of the oxide
surface to change the organic base indicator into its con-
jugate acid due to proton transfer from Brønsted acid site
on the surface to the indicator. The lower the Hammett
acidity function (H0) value is, the higher the acidity of the
oxide surface. If the surface sites have an H0 value less than
the pKa value of the indicator, its color is changed as a
result of the acid base reaction on the surface. The indicator
123

Acetone Condensation
707
solutions were prepared in 1 % (wt./wt.) concentration by
dissolving of 0.5 g of the organic indicator in 50 ml of dry
benzene since water can affect the results by rapid
adsorption on the surface oxide. Before performing tests,
sample surfaces were cleaned by heating under vacuum at
80 °C for 2 h. The acidity strength was experimentally
determined by the addition of 0.1 g of the solid to 2 ml of
dry benzene in a 20 ml test tube followed by an addition of
1 ml of the indicator solution. The tube was covered and
left for 1 h to reach equilibrium before recording changed
color. The pKa values of the conjugate acid of the indi-
cators can be found in Ref. [38]. The total acidity mea-
surement of catalyst samples was measured using the NH3
temperature programmed desorption technique (NH3–
TPD) (Micromeritics AutoChem). Prior to NH3 adsorption,
the catalyst sample of 0.2–0.5 g was pre-treated in He
(50 ml/min) at 500 °C for 45 min to remove pre-adsorbed
species. Then it was cooled down to 100 °C, leading to a
saturation with NH3 (15 vol.% in He). Loosely bound NH3
species were purged out in a He stream until the baseline of
the integrator was stable. The samples were then heated to
550 °C at a heating rate of 10 °C/min in He (50 ml/min).
The profile of ammonia evolved was monitored and
recorded by the thermal conductivity detector (TCD)
detector [44–46].
Fig. 1 TGA curves for the prepared zirconium ethane sulfonate
hydroxide precursors of ZS-3(1:1) and ZS-4(1:2)
decomposition was observed between 150 and 350 °C,
which can be attributed to the decomposition of the organic
sulfate ligands and the hydroxy group. The weight loss
between 350 and 600 °C is attributed to the decomposition
of the acetate ligands. The TGA curve for the ZS-4(1:2)
sample, however, showed a sharp weigh loss between 350
and 450 °C due to the loss of the additional ethane sulfo-
nate ligands. Table 1 shows the CHS elemental analyses
and detail information of the precursors obtained from the
reaction of zirconium acetate and zirconium oxychloride
with the ethanesulfonic acid in different mole ratios. The
possible proposed formulas were made based on the ele-
mental analyses and the TGA data. Table 2 and Fig. 2
show the stretching frequencies of the carboxylate carbonyl
group mCOO(sym.) and mCOO(asym.) and the splitting (Dm)
between the asymmetric and symmetric carbonyl stretching
frequencies of several synthesized zirconium acetate ethyl
sulfonate complexes, which were prepared by the reaction
of zirconium acetate with a variant amount of the ethane-
sulfonic acid. The carboxylate’s stretching frequencies of
zirconium acetate complexes are between 1,400 and
1,700 cm-1. The results clearly show that the asymmetric
carboxylate absorption stretching frequencies mCOO(asym.)
and the Dm values decrease dramatically with an increase of
the number of ethylsulfonates on the precursors. This is
indicative of a change of the coordination mode of the
bridging acetate group to either a wider angle or a higher
delocalization to the double bond. Additionally, 13C NMR
spectra of the zirconium sulfonate precursors are shown in
2.3 Catalytic Testing
The self-condensation reaction of acetone was carried out
in an autoclave Teflon-lined batch reactor [47] in a constant
temperature oil bath with a magnetic stirring. The reaction
was carried out at 150 °C for 1–9 h. All of the experiments
were carried out at ambient pressure. Samples were col-
lected from the reactor at different reaction times for
analysis. GC/MS (Hewlett Packard G1800A) equipped
with a 30 m/0.25 mm HP5 column (crosslinked 5 % PhME
silicone) was used to measure products.
3 Results and Discussions
3.1 Characterization of Synthesized Precursors
and Sulphated Zirconium Oxides
TGA profiles of zirconium sulfonate precursors derived
from the reaction of zirconium salts with different con-
centrations of the ethanesulfonic acid are shown in Fig. 1.
The TGA curve of the ZS-3(1:1) sample was slightly dif-
ferent from the ZS-4(1:2) sample. The TGA profile of ZS-
3(1:1) exhibited a stepwise thermal decomposition with
three distinct regions. A weight loss observed between
room temperature and 150 °C is due to the evolution of the
coordinated water molecules on the surface. A gradual
Fig. 3. The NMR spectra show that the intensity of the 13
C
acetate peaks at 24 and 178 ppm is decreasing, while that
of the ethylsulfonate at 8.8 and 45.7 ppm is increasing with
an increasing amount of ethylsulfonate until the acetate
peaks are completely disappeared. This observation sup-
ports the findings using IR spectroscopy, and clearly sug-
gests that the ethane sulfonate gradually replaces the
123

708
M. H. Al-Hazmi et al.
Table 1 The structures and properties of synthesized zirconium sulfonate complexes obtained from the reaction of zirconium salts and the
ethanesulfonic acid
Samplea
Elemental analysis (%)
ZrO2 (%)b
Proposed precursor formulac
Expt. MW
Theo. MW
C
H
S
ZS-1(1:0.25)
ZS-2(1:0.5)
ZS-3(1:1)
ZS-4(1:2)
ZS-5(1:3)
a
12.3
13.5
12.1
14.6
15.7
3.29
3.40
3.54
3.61
3.49
2.62
55.4
50.5
44.5
34.0
26.4
[Zr(O)0.9(OH)1(OAC)1(ESA)0.2]Á0.8H2O
[Zr(O)1.25(OAC)1(ESA)0.5]Á0.5H2O
[Zr(O)1.3(OAC)0.6(ESA)0.8]Á2H2O
[Zr(O)0.9(OAC)0.4(ESA)1.8]Á2.2H2O
[Zr(O)0.6(ESA)2.8]Á3H2O
222
243
276
361
465
219
247
272
367
464
6.33
9.52
16.91
18.50
Zirconium sulfonate complexes with different concentrations of the ethanesulfonic acid (ESA)
b
c
Calcination was done at 650 °C
Based on the results of TGA and elemental analysis
Table 2 IR stretching frequencies of the asymmetric and symmetric
carbonyl groups in the synthesized zirconium acetate ethyl sulfonate
complexes containing a different amount of the ethanesulfonic acid
Sample
mCOO (cm-1
)
Dm (cm-1 b
)
Asym.
Sym.
ZrAa
1573
1575
1557
1540
1450
1456
1455
1459
123
119
102
81
ZS-2(1:0.5)
ZS-3(1:1)
ZS-4(1:2)
a
Zirconium acetate hydroxide
b
Dm = mCOO(asym.) - mCOO(sym.)
Fig. 3 13C NMR spectra of the zirconium precursors obtained from
the reaction of 1 mol of zirconium acetate with different mole ratios
of ethanesulfonic acid (ESA). a zirconium acetate, b ZS-3(1:1), c ZS-
4(1:2), and d ZS-5(1:3)
acetate ligands. The complete replacement was observed
when the zirconium acetate precursor reacts with three
mole equivalents of the ethanesulfonic acid (ZS-5(1:3)).
The XRD pattern of the precursor derived from ZS-3(1:1)
precursor at different temperatures is shown in Fig. 4. It is
known that hydroxyl ions available on the surface of zir-
conium oxide stabilize the tetragonal phase and delay the
transformation to the monoclinic phase [48]. As shown in
Fig. 4, the tetragonal phase started to appear at approxi-
mately 500 °C. As temperature increases, more crystalline
tetragonal phases are developed without the appearance of
the monoclinic phase until about 950 °C. The comparison
of these data with that from zirconium acetate shown in
Fig. 4 revealed that the zirconium sulfonate precursors
produce a thermally stable tetragonal phase due to the
Fig. 2 IR spectra of the zirconium sulfonate precursors obtained
from the reaction of 1 mol of zirconium acetate with a different mole
ratio of ESA. a zirconium acetate, b ZS-2(1:0.5), c ZS-3(1:1), d ZS-
4(1:2), and e ZS-5(1:3). The arrow corresponds to an asymmetric
stretching of COO
123

Acetone Condensation
709
presence of the sulfate ions. Sulfate ions are believed to
increase the thermal energy required to remove the
hydroxyl ions during the thermal dehydroxylation process
[49]. The phase composition results also show a degree of a
dependency on the nature of the sulfated zirconia single
precursor and on the amount of sulfur species formed on
the surface. The crystallite size measurements obtained
from the X-ray diffraction for the thermally treated single
precursor (ZS-5(1:3)) at different calcination temperatures
are shown in Fig. 5. For the sample calcined at 550 °C, a
small average crystallite size of about 4 nm was observed.
The crystallite size is increased by raising the temperature,
and reached a 12 nm average size when the sample was
calcined at 850 °C due to a sintering and an agglomeration
of the particles during the thermal treatment. IR spectra of
the oxide from the ZS-5(1:3) precursor are shown in Fig. 6.
Three peaks in the IR region from 1,000 to 1,250 cm-1
were observed (1,029 to 1,069 cm-1, 1,143 cm-1, and
1,222 to 1,243 cm-1). These bands are considered as
characteristic peaks for the S–O stretching modes of the
2-
coordinated SO4 species on the surface. They are char-
acteristic peaks for the chelating bidentate sulfate species
coordinated to the zirconium metal ion [50]. Furthermore,
the peak observed at about 1,389 cm-1 is corresponding to
the SO stretching frequency of sulphate species adsorbed
on the metal oxide surface [50, 51]. The broad peak
observed at 3,375 cm-1 is attributed to the hydrogen
bonding of adsorbed water on the zirconia surface. Addi-
tionally, the peak at 1,627 cm-1 is attributed to the OH
bending mode of adsorbed water [51]. Based on the IR
studies (Fig. 6), the oxide obtained from the ZS-3(1:1) and
ZS-4(1:2) precursors are similar to each other with slight
differences in the relative intensities of the peaks. This
implies that the sulfate group adsorbed on the surface have
similar structures. The characteristic peak for the mS=O of
the ZS-3(1:1) sample was observed at 1,376 cm-1, but it is
shifted to a higher position (1,389 cm-1) than that of ZS-
4(1:2). This increase may occur due to the increase of the
surface sulfate group. As shown in Fig. 6, the oxide with
high sulfate contents (ZS-5(1:3)) has a different IR spec-
trum in the mS–O region with appearance of new strong
bands at about 1,065 cm-1 and 900–1,000 cm-1. This
change may result from the formation of polynuclear
Fig. 4 X-ray diffraction pattern for the zirconium sulfonate single
precursor derived from the reaction of 1 mol of zirconium acetate
with 1 mol of ethanesulfonic acid (ZS-3(1:1)) at a 550 °C, b 700 °C,
c 850 °C, d 950 °C, e 1,050 °C, and f zirconium acetate at 800 °C
Fig. 5 Average crystallite size of the ZS-5(1:3) precursor calcined at
different temperatures
Fig. 6 a Sulfated zirconia from
pyrolysis of zirconium sulfonate
precursors at 650 °C of ZS-
3(1:1), ZS-4(1:2), and ZS-
5(1:3). b IR spectra of the
a zirconia obtained from
zirconium acetate heated at
720 °C, b zirconium sulfonate
precursor ZS-1(1:3) dried at
100 °C, and c sulfated
zirconium oxide obtained from
calcination of ZS-5(1:3)
calcined at 650 °C
123

710
M. H. Al-Hazmi et al.
sulfate compounds [51, 52]. Furthermore, the degree of
hydration of the crystalline sulfate groups may also influ-
ence the structure of the adsorbed sulfate group on the
surface [51]. Figure 7 shows the influence of the sulfate
concentration on the phase composition and thermal sta-
bility. All samples were calcined at 950 °C for 8 h. It was
reported that the acetate/zirconium stoichiometry in the
zirconium acetate complex precursors has a negligible
effect on crystallite phases and crystallization temperatures
[53]. Therefore, the observed variation in the phase com-
position may be directly attributed to the presence of the
sulfate group on the final oxide surface. Zirconium acetate
itself showed a complete transformation of the tetragonal
phase to the monoclinic phase at this temperature. How-
ever, the results showed that by the addition of 0.1 mol
ratio of ESA to zirconium acetate (Fig. 7a), the tetragonal
phase was highly stable and resisted the transformation into
a monoclinic phase. The presence of the tetragonal phase
was *56 %. Even a small amount of sulfate may create
defect sites on the zirconium oxide crystals, affecting the
stability of the tetragonal phase. By increasing the mole
ratios of ESA, the structure retained more in the tetragonal
phase. It has been reported that sulfates stabilized the
tetragonal phase by a contribution to the rigidity of the
structure, leading to an elongation of the Zr–Zr bond dis-
tance [40]. Active sites and vacancies on the zirconium
oxide surface that adsorbs oxygen species can facilitate the
transformation from tetragonal to monoclinic phases [37].
However, the sulfate ions linked to the surface cover the
sites and impede the phase development from both amor-
phous to crystalline phases and from tetragonal to mono-
clinic phases. Interestingly, beyond the 1:1 mol ratio of
zirconium acetate to ESA (Fig. 7e), a decrease in the sta-
bility of the tetragonal phase was observed. It may reflect a
very different decomposition pathway leading to sulfated
zirconia particles with varying microstructures, resulting in
the tetragonal to monoclinic phase transformation. This
may also be due to the observed change of the nature of the
sulfate group coordinated to the zirconium surface. For
example, zirconia with a 91.9 % monoclinic phase was
obtained from the three mole equivalents of the ESA pre-
cursor (Fig. 1g). Therefore, based on the characterization,
we found that there is a maximum in the amount of the
sulfate group necessary for the stabilization of the tetrag-
onal phase at high temperature, and its maximum phase
stability is obtained when the oxide surface has a certain
2-
ratio of SO4 /OH. Beyond that ratio, the sulfate group
¯
replaces a large amount of the hydroxyl group from the
surface, and appears to be abundant on the surface. This
affects its coordination nature and facilitates the evolution
of sulfate species in the form of SO2 gas at elevated tem-
peratures. Figure 8 shows the specific surface area of sul-
fated zirconium oxides obtained from different precursors
by varying the ESA mole ratios. The mole ratio of the ESA
is proportional to the amount of the sulfate group on the
surface area of final oxides. All samples were calcined at
650 °C for 8 h. We observed a dramatic increase in the
oxide surface area as the sulfur content arises. This can be
attributed to the interaction of the sulfate group on the
oxide surface. It is assumed that the surface area increase
2-
may be due to the presence of bridging sulfate SO4 ions
on the surface. The bridging sulfate ions replace hydroxyl
species on the surface, resulting in a more rigid and stable
structure. Furthermore, the bridging sulfate ions elongate
˚
the Zr–Zr separation from about 3.4–4.3 A [39, 54], which
facilitates the dispersion of the oxide particles, and
accordingly, increases the surface area. Figure 9 shows
representative SEM images of the zirconium acetate
Fig. 7 Effect of the sulfate concentration on the phase composition
and thermal stability. a Zirconium acetate, b with 0.1 ESA, c ZS-
1(1:0.25), d ZS-2(1:0.5), e ZS-3(1:1), f ZS-4(1:2), and g ZS-5(1:3).
The sulfated zirconium oxides were calcined at 950 °C
Fig. 8 Effect of ESA mole ratios on the surface area of the sulfated
zirconium oxide calcined at 650 °C
123

Acetone Condensation
711
starting material and the zirconium ethyl sulfonate pre-
cursor (ZS-5(1:3)). They were recorded for both the pre-
cursor itself and the oxide obtained from the pyrolysis of
the precursor. The SEM image of zirconium acetate shows
smooth spherical particles with holes on the particles,
whose sizes are in the range from 2 to about 20 lm
(Fig. 9a). Heating of the zirconium acetate at 650 °C
yielded the zirconium oxide (Fig. 9b), and the SEM images
clearly shows that it became more polished spherical par-
ticles. The morphology of the zirconium sulfonate pre-
cursor (ZS-5(1:3)) (Fig. 9c) shows uniformly shaped
crystals sticks with an average diameter of about 0.5 lm.
By calcination at 650 °C, the morphology was completely
changed to form rod-like zirconia with an average diameter
of about 200 to 500 nm and with several microns in length.
This is due to the sintering of the particles during the
thermal transformation of the precursor to the oxide. The
uniaxial growth may be a reflection of the formation of the
tetragonal phase with preferred growth in the C axis.
The general proposed pathway for the thermal decom-
position of the zirconium sulfonate precursors to form
sulfated zirconium oxide is shown in Scheme 1. To study
the decomposition behaviour of the precursor, the precur-
sor derived from the reaction of zirconium acetate with
ethanesulfonic acid was heated in a sealed glass tube using
a tube furnace at various temperatures in the range of
250–450 °C. The decomposition process was initiated at
about 200 °C–300 °C by the b-hydride elimination reac-
tion to evolve ethylene. Ethylene presumably further
undergoes the dimerization and trimerization over the acid
surface at high temperatures generating a mixture of C4–C6
olefins. Sulfur dioxide (SO2) was also formed at this stage.
Hydrogen sulfide (H2S) and more SO2 were also released
as by-products at higher decomposition temperatures
(above 350 °C). The C4 and C6 olefins directly reacted
with H2S via a cyclization reaction and produced a mixture
of thiophene. It is well known that the reaction of alkanes
or olefins with SO2 or H2S at high temperature in an inert
gas environment can produce thiophene [55, 56]. In our
experiments, the formation of thiophene and H2S started at
about 400 °C. At \400 °C, the reaction mainly produced
SO2, water, and ethylene. Figure 10 shows a schematic
diagram for the possible products formed from the thermal
decomposition of ZS-5(1:3). Due to a smaller amount of
O2 than that for zirconium acetate, thiophene was pro-
duced when a larger amount of the sample was placed in
the sealed tube. Thiophene may also be formed as a result
of the reaction of sulfur with ethylene producing a
Fig. 9 SEM images of
a zirconium acetate before
heating, b zirconium acetate
prepared at 650 °C, c ZS-5(1:3)
before heating, and d ZS-5(1:3)
prepared at 650 °C. The scale
bar is 1 lm
Scheme 1 Thermal
decomposition of zirconium
sulfonate precursors to sulfated
zirconium oxides
123

712
M. H. Al-Hazmi et al.
Fig. 10 Schematic of possible
products formed from the
thermal decomposition of ZS-
5(1:3)
conjugated thiodiethylene intermediate, which further
reacts with ethylene followed by an intramolecular cycli-
zation. The formation of diethyl sulfide and dithiolanes
also suggests that the reaction follows a free radical
mechanism. Other products obtained, such as elemental
sulfur (S8), carbonyl sulfide (COS), diethyl disulfide, 1,4-
dithiane, and 2-methyl-1,3-dithiolane were also detected as
a result of the reduction pyrolysis of the precursor. Ele-
mental sulfur, which was observed at a high decomposition
temperature (above 450 °C), apparently was formed as a
result of the decomposition of H2S or thiophene under the
reduction conditions. The sulphur was, then, deposited on
the surface of the decomposed precursor. When the pyro-
lysis was conducted in a larger volume sealed tube with
more O2, CO2 and SO2 were produced more than thio-
phene and other products. This implies that thiophene,
alkylated thiophene products, and other possible aromatic
products are oxidized to SO2 and CO2. When the precursor
was pyrolyzed in O2 rich environment in an open vessel at
450 °C followed by extraction with methylene chloride, no
S8 was deposited, indicating that all the sulfur species were
oxidized to SO2 or the surface sulfate group [55]. Three
main steps were observed during the thermal decomposi-
tion of the zirconium acetate sulphonate precursors: (i) the
evolution of SO2 at a temperature below 300 °C, (ii) the
evolution of the H2S and thiophene at a temperature range
between 300 and 450 °C, and (iii) the formation of ele-
mental sulfur at a temperature above 450 °C. The amount
of SO2, S8, and H2S purely depend on the pyrolyzed pre-
cursor. This clearly implies that upon the formation of the
zirconia lattices as a result of the thermal treatment, dif-
ferent sulfate group’s structures may be present on the
surface and be decomposed at different temperatures. The
preparation methods and the nature of the zirconium
sulfonate precursor strongly influence the sulfate content,
the nature and the structure of the sulfate group on the
surface.
The total acidity measurements of several sulfated zir-
conium oxides obtained from different zirconium sulfonate
precursors are listed in Table 3. The total acidity was
measured using the NH3–TPD technique, while the acid
strength data was estimated using different Hammett
indicators. Table 3 indicates that the surface acidity of the
sulfated zirconia obtained from different precursors has a
wide variety of total acidities and acid strengths. The dis-
similar acidic properties can be mainly attributed to: (i) the
variations in specific surface area of the oxide, (ii) the
availability and concentration and distribution of the acid
Table 3 Acidity measurements of sulfated zirconia catalytic systems utilized for acetone condensation reaction
Catalysta
SBET (m2/g)
Total acidity NH3 desorption (mmol/g)
Acidity strength (pKa)
400–800 K
800–950 K
Total
Acid density (lmol/m2)
ZrO2
2.5
–
–
–
–
[ ?3.3
ZS-2(1:0.5)
ZS-3(1:1)
ZS-4(1:2)
ZS-5(1:3)
a
20.1
23.9
27.5
49
1.87
2.72
3.33
3.85
0.28
0.46
0.52
0.62
2.65
3.18
3.85
4.47
131.8
133.0
140
From –5.7 to –8.2
From –8.2 to –11.4
\ –11.4
91.2
\ –11.4
Sulfated zirconia obtained from precursors that were synthesized from zirconium acetate with different mole ratio of ethane sulfonic acid. All
samples were calcined at 650 °C. (For samples description, please refer to Table 1)
123

Acetone Condensation
713
sites, and (iii) the structure of sulfate species on the surface.
This confirms that the coordinated surface sulfate group
affects the strong acidity. Moreover, the total acidity
measurements of the oxide series obtained from the reac-
tion of zirconium acetate with different mole ratios of
ethanesulfonic acid demonstrated that the acidity increased
with the increase of the sulfate concentration on the sur-
face. As shown in Table 3, the acid strength measured by
Hammett indicators showed that the strength increased
with an increase of the sulphonate group to reach the
maximum at pKa higher than -11.4, which then protonated
4-nitrotouluene. The surface of the oxide turned to yellow.
This may be attributed to the generation of more Brønsted
acid sites. It is expected that the initial decomposition of
the zirconium sulfonate precursor during the thermal
treatment in air at low temperature may proceed by the b-
hydride elimination forming ethylene and water. Adsorbed
water may then react with the adjacent sulfate groups to
sample with high sulfur species undergoes a rapid trans-
formation of the tetragonal to monoclinic phases upon cal-
cination at higher temperature. This may be either due to the
different microstructure (nanocrystalline) of zirconia or a
non-facile loss of sulfate due to the different mode of
coordination. On the other hand, the total acidity measured
by the NH3–TPD adsorption technique showed that there are
two regions (400–800 K and 800–950 K), representing the
strength of mild and strong acids, respectively. The total
acidity density showed an increase by a rise of ethane sul-
phonate in the oxide precursor to a maximum density at ZS-
4(1:2), and then, was declined. In addition, we found that the
strong acid site’s density was also increased by increasing
the sulphone group on the surface at an optimum value, and
then, a dramatic decline was observed (ZS-5(1:3)).
3.2 Acetone Condensation Reaction
¯
yield adsorbed bisulfates (HSO4) and the hydroxyl group.
The reaction of acetone over the solid acids follows several
pathways leading to numerous products. The acetone
condensation reaction was performed at 150 °C with 10
wt./wt.% (catalyst/ketone). It is well known that the ace-
tone condensation reaction starts with a protonation of
acetone over the solid surface to form a conjugate acid. The
next step involves an electrophilic addition of a carbonium
ion of the conjugate acid with the enol form of another
acetone molecule via a well-known aldol reaction to yield
diacetone alcohol (DAA). Dehydration of DAA occurs
readily over sulfated zirconia acid sites to form mesityl
oxide (MO). Another acetone enol reacts further with the
mesityl oxide following the similar mechanism mentioned
above to produce linear phorone. The cyclization of pho-
rone to isophorone mainly arises through the Michael
addition mechanism which involves the conjugate addition
of the enolate nucleophile anion to the b-carbon of an a,b-
unsaturated carbonyl electrophile double bond [57, 58].
Mesitylene [59] may be formed as a result of the dehy-
dration and rearrangement of isophorone over acid sites.
As proposed by Ecormier et al. [57], at low sulfate
concentration the surface may be rich in hydroxyl and
bridge the hydroxyl group with the bisulfate group. Upon
further heating to higher temperature ([600 °C), the bisul-
fate undergoes a condensation reaction with the adjacent
hydroxyl group evolving water and forms a bridging
bidentate sulfate group on the surface with a concurrent
formation of weak and strong Lewis acid sites. Water
molecules were evolved during the thermal decomposition
of the precursor, or it could be re-adsorbed on Lewis acid
sites to form weak Brønsted acid sites. The formation of the
tetragonal phase with a low sulfate concentration and the
thermal stability of this phase at higher temperature may be
delayed by the presence of a high concentration of the sta-
bilized hydroxyl group on the surface. In addition, by a
further increase in the concentration of sulfur species, the
bisulfate group on the oxide surface can be increased, and
polynuclear sulfate compounds (i.e., pyrosulfate) may be
formed. As discussed above, XRD data showed that the
Scheme 2 Schematic for the
formation of pentamer product
from acetone. Bullet is the
position of 13C
123

714
M. H. Al-Hazmi et al.
Furthermore, another possible route for the formation of
mesitylene is the condensation of three acetone molecules
to produce the trialcohol intermediate, followed by a rapid
dehydration on the acid sites to release three water mole-
cules and mesitylene. Among all these products, mesityl
oxide, isophorone, and mesitylene are commercially
important products obtained from the acetone condensation
reaction [18, 60]. These products are mainly applicable in
polymerization and separation of heavy metals. Naphtha-
lene products such as pentamers are also formed in con-
siderable amount via a proposed mechanism shown in
Scheme 2. The pentamer was identified using mass spec-
trometry, which indicated a molecular mass fragmentation
at m/e = 200 involving five 13C atoms. This suggests that
it is formed as a result of the condensation of five acetone
molecules. Most likely, this product was formed directly
from the reaction of isophorone with mesityl oxide pro-
ducing an intermediate, followed by a rapid dehydration to
evolve two water molecules and products. Figure 11 shows
the conversion of acetone versus a reaction time over the
sulfated zirconia samples. The results clearly showed that
the conversion is enhanced by the oxide obtained from the
increase of the ethane sulfonic acid/acetate molar ratio in
the precursor. ZS-1(1:0.25) shows the least conversion over
time. On the other hand, the conversion over ZS-5(1:3) was
Fig. 11 Acetone conversion over sulfated zirconia samples at 150 °C
Fig. 12 Product distribution resulted from acetone condensation over the sulfated zirconia samples calcined at 650 °C. After a 1 h, b 3 h, and
c 9 h of reaction time. In the x axes, only mole ratios were shown for clarity (Table 1)
123

Acetone Condensation
715
rapidly increased to its maximum after only 3 h of the
reaction time (about 80 % conversion). Then, the conver-
sion remained constant. Obviously, the high initial activity
of the ZS-5(1:3) sample is attributed to its high surface area
and dispersion of the active sites on the surface. The
product distribution results are summarized in Fig. 12,
which demonstrates that the aldol condensation reaction of
acetone initially produced mesityl oxide during the first 3 h
of the reaction time with a high selectivity. However,
mesityl oxide was not observed in this condition due to the
high acidity of the catalyst surface. It was reported that the
alcohol dehydration process requires low acidic strength
sites with ?0.8 of pKa [61]. However, when the reaction
was performed at room temperature for 12 h with 10 wt.%
(catalyst/acetone), primary condensation products were
observed (i.e., diacetone alcohol and mesityl oxide). The
acetone conversion in this condition was very low
(approximately 92 % selectivity for diacetone alcohol and
8 % selectivity for mesityl oxide). It clearly indicates that
the selectivity toward the diacetone alcohol is sensitive to
the reaction temperature. The high selectivity for diacetone
alcohol observed at room temperature is presumably owing
to dehydration steps, and a further condensation of ketone
to higher molecular-weight products has a high activation
energy and occurs quite slowly at room temperature. As
shown in Fig. 12, more condensation products can be
formed when the reaction was performed at 150 °C for a
longer time. These products are generally formed as a
result of self and cross condensation of mesityl oxide with
acetone. When the reaction proceeds for a longer time or
over stronger acidic surfaces, the concentration of mesityl
oxide was decreased since it was consumed during the
cross-condensation reaction with acetone producing
isophorone, mesitylene, and pentamer. The selectivities
towards these products were increased by the reaction time.
The selectivity of isophorone is much lower than other
products due to its instability, resulting in the decomposi-
tion over strong acid sites producing mesitylene or a further
reaction with mesityl oxide forming pentamers. It was
found that more self- and cross-condensation reactions take
place on a stronger acid site.
zirconium sulfonate single source precursors exhibited a
strong surface acidity and relatively high surface areas. On
the basis of various characterization techniques, we verified
that the surface sulfate structure and the surface acidity
strongly depend on the concentration of the sulfates on the
surface. The total surface acidity, acid strength, and surface
area showed an increase by the arising of the amount of
sulfate groups on the surface.
The zirconium sulfonate complexes used in this study
yielded the highly stabilized tetragonal phase sulfated zir-
conia with a small crystallite size after pyrolysis.
The obtained sulfated zirconium oxides catalyzed acetone
self-condensation reactions. The catalytic activity and prod-
uct selectivity were strongly influenced by catalyst properties
and the surface acidity of the employed sulfated zirconia
catalyst. Our systematic examination showed that acidity is
critical in the dehydration of the intermediate alcohol to the
corresponding a,b-unsaturated carbonyl compounds, and
accordingly, mesityl oxide was produced with a high selec-
tivity from the acetone self-condensation reaction.
To support our proposed mechanism based on ex situ
techniques, in situ experiments (i.e., time-resolved XRD
approaches) may need to obtain more detailed information
to understand the mechanism. In addition, more funda-
mental studies on acid sites at the molecular level may
provide more detailed information about the elemental
steps to more accurately elucidate the reaction mechanism.
Acknowledgments We highly appreciate the financial support by
SABIC.
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