PH control of the structure, composition, and catalytic activity of sulfated zirconia

Article abstract of DOI:10.1016/j.jssc.2012.11.022

We report a detailed study of structural and chemical transformations of amorphous hydrous zirconia into sulfated zirconia-based superacid catalysts. Precipitation pH is shown to be the key factor governing structure, composition and properties of amorphous sulfated zirconia gels and nanocrystalline sulfated zirconia. Increase in precipitation pH leads to substantial increase of surface fractal dimension (up to ～2.7) of amorphous sulfated zirconia gels, and consequently to increase in specific surface area (up to ～80 m ²/g) and simultaneously to decrease in sulfate content and total acidity of zirconia catalysts. Complete conversion of hexene-1 over as synthesized sulfated zirconia catalysts was observed even under ambient conditions.

Full text of DOI:10.1016/j.jssc.2012.11.022

Journal of Solid State Chemistry 198 (2013) 496–505
Contents lists available at SciVerse ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
pH control of the structure, composition, and catalytic activity of
sulfated zirconia
Vladimir K. Ivanov a,b, Alexander Ye. Baranchikov a,c,*, Gennady P. Kopitsa d, Sergey A. Lermontov e,
Lyudmila L. Yurkova e, Nadezhda N. Gubanova a,d, Olga S. Ivanova a, Anatoly S. Lermontov a,
Marina N. Rumyantseva c, Larisa P. Vasilyeva f, Melissa Sharp g, P. Klaus Pranzas g, Yuri D. Tretyakov b
a Kurnakov Institute of General and Inorganic Chemistry of Russian Academy of Sciences, Leninsky ave., 31, Moscow 119991, Russian Federation
b Materials Science Department, Moscow State University, Moscow 119991, Russian Federation
c Department of Chemistry, Moscow State University, Moscow 119991, Russian Federation
d Petersburg Nuclear Physics Institute, Orlova Roscha, Gatchina 188300, Russian Federation
e Institute of Physiologically Active Compounds of the Russian Academy of Sciences, Chernogolovka 142432, Russian Federation
f Institute of Problems of Chemical Physics, Russian Academy of Sciences, Chernogolovka 142432, Russian Federation
g GKSS Forschungszentrum, Geesthacht 21502, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a detailed study of structural and chemical transformations of amorphous hydrous zirconia
into sulfated zirconia-based superacid catalysts. Precipitation pH is shown to be the key factor
governing structure, composition and properties of amorphous sulfated zirconia gels and nanocrystal-
line sulfated zirconia. Increase in precipitation pH leads to substantial increase of surface fractal
dimension (up to ꢀ2.7) of amorphous sulfated zirconia gels, and consequently to increase in specific
surface area (up to ꢀ80 m2/g) and simultaneously to decrease in sulfate content and total acidity of
zirconia catalysts. Complete conversion of hexene-1 over as synthesized sulfated zirconia catalysts was
observed even under ambient conditions.
Received 30 July 2012
Received in revised form
12 October 2012
Accepted 18 November 2012
Available online 30 November 2012
Keywords:
Sulfated zirconia
Small-angle neutron scattering
Mesostructure
& 2012 Published by Elsevier Inc.
Fractal properties
1. Introduction
The most common route to prepare fine zirconia powders
includes precipitation of amorphous hydrous zirconia from aqu-
Superacids including sulfated titania, zirconia, and stannia are
among the most promising heterogeneous catalysts [1]. Their
acidity is much higher even than that of concentrated H2SO4, with
their surface sulfate groups acting as Lewis and Brønsted sites [2].
The main advantages of solid superacid oxide catalysts include
high stability at elevated temperatures, resistance to deactivation,
and easy regeneration. The SO4/ZrO2 is the most comprehensively
studied system because its acidity is one of the highest among the
sulfated oxides [2,3]. The Hammett acidity functions H0 for SO4/
ZrO2 and SO4/SnO2 fall within the range from ꢁ16 to ꢁ18; SO4/
TiO2 has a slightly lower value of ꢁ14.6 [4]. The above indicated
values can vary because of the lack of common procedures for
analysis of the properties of sulfated oxides [5]. Reproducibility of
synthetic procedures used to prepare sulfated oxides is still also
rather poor [1].
eous or alcoholic solutions and its further treatment (e.g., drying,
milling and annealing). Typical procedure for a sulfated zirconia
catalyst synthesis includes impregnation of thus obtained amor-
phous ZrO2 ꢂ xH2O or crystalline ZrO2 with sulfate-containing
compounds (e.g., sulfuric acid, ammonium persulfate) [3,6]. Thus
characteristics of nanocrystalline zirconia-based materials should
depend on the composition and structure of intermediate amor-
phous phases formed during hydrolysis of Zr(IV) compounds.
Recently [7] we have demonstrated that variation in the pre-
cipitation pH indeed considerably changes the mesostructure,
fractal dimension and specific surface area of hydrous zirconia
and hafnia gels. Thermal decomposition of these gels actually
gives rise to ZrO2 and HfO2 nanopowders having considerably
different particle sizes and specific surface areas.
In this paper, we aimed at establishing the probable interrela-
tion between the composition and the mesostructure of amor-
phous zirconia gels and phase composition, structure and
catalytic activity of sulfated zirconia prepared thereof. To synthe-
size sulfated zirconia we have used a simple procedure which
includes only two steps: one-pot precipitation of sulfated hydrous
* Corresponding author at: Kurnakov Institute of General and Inorganic
Chemistry of Russian Academy of Sciences, Leninsky ave., 31, Moscow 119991,
Russian Federation. Fax: þ7 495 9541279.
E-mail address: a.baranchikov@yandex.ru (A.Ye. Baranchikov).
0022-4596/$ - see front matter & 2012 Published by Elsevier Inc.
http://dx.doi.org/10.1016/j.jssc.2012.11.022

V.K. Ivanov et al. / Journal of Solid State Chemistry 198 (2013) 496–505
497
zirconia gels at various pHs and their further annealing. We hope
that such an approach would favour reproducibility of structure
and composition of sulfated zirconia catalysts and thus their
catalytic properties.
F1 Jupiter thermal analyzer equipped with mass-spectrometer
(heating rate 5 1C/min).
Transmission electron microscopy images were taken using
Leo 912 AB Omega electron microscope operating at 100 kV.
Microstructure of the samples was also studied using Carl Zeiss
NVision 40 scanning electron microscope (micrographs were
obtained at 1 kV acceleration voltage) equipped with Oxford
Instruments X-MAX energy-dispersive X-ray (EDX) analyzer oper-
ating at 30 kV acceleration voltage. The samples were not spe-
cially prepared (e.g., coated with conducting material) for TEM
and SEM measurements. Before EDX analysis samples were
coated with ꢀ5 nm Au/Pd.
2. Experimental
2.1. Synthesis of samples
All starting materials used in the experiments were of analy-
tical grade. Zirconium oxynitrate hydrate (ZrO(NO3)2 ꢂ xH2O, 99%
Aldrich) was dissolved in distilled water and then ammonium
sulfate (99%, Chimmed, Russia) was added so that the final
concentrations of ZrO(NO3)2 and (NH4)2SO4 were 0.25 M and
1.0 M, respectively. The resulting solution was kept overnight
and then filtered. To obtain hydrous sulfated zirconia, aqueous
ammonia (2.7 M, 99%, Chimmed, Russia) was added dropwise to
the starting zirconium-containing solution under vigorous stir-
ring until the desired pH value was reached (3.98, 6.98 or 8.98).
pH measurements were made using Crison GLP-22 pH-meter.
Obtained suspensions were additionally stirred for 30 min in
mother liquor while pH value was carefully adjusted with aqu-
eous ammonia. The resultant white precipitates were washed
four times by redispersion in distilled water followed by centri-
fugation (8000 minꢁ1). All the samples were further dried in an
oven under air flow at 50 1C overnight and carefully grounded in
an agate mortar. Hereafter the as-synthesized sulfated hydrous
zirconia xerogels precipitated at pH 3.98, 6.98 or 8.98 are named
Z-4S, Z-7S and Z-9S, respectively.
Thermal decomposition of the xerogels was conducted in a
muffle furnace in air at 500 1C, 550 1C, 600 1C and 700 1C for 5 h.
Heating rate was 10 1C/min. After the annealing samples were
cooled down to ambient temperature within the furnace. Result-
ing sulfated zirconia powders are named hereafter Z-4S–T, Z-7S–
T, Z-9S–T (here T is a temperature of thermal treatment—500 1C,
550 1C, 600 1C or 700 1C), respectively.
Throughout this paper the characteristics of sulfated zirconia
are compared with the properties of zirconia samples synthesized
using the same procedure but without addition of ammonium
sulfate to the starting solution [7].
Low temperature nitrogen adsorption measurements were con-
ducted using ATX-6 analyzer (Katakon, Russia) at ꢁ196 1C. Before
measurements the samples were outgased at 200 1C for 30 min
under dry helium flow. Determination of the surface area was
carried out by 8-point Brunauer–Emmett–Teller (BET) method.
To study the influence of precipitation conditions on the
mesostructure of amorphous zirconia xerogels a small angle
neutron scattering (SANS) technique was used. SANS measure-
ments were performed on SANS-2 setup (FRG-1 neutron reactor,
GKSS Research Centre, Geesthacht, Germany). The experiments
˚
were performed at neutron wavelength
l
¼5.8 A with Dl/l¼10%
and for four sample-detector distances SD¼1 m, 3 m, 7 m, and
20.7 m, which allowed to perform the measurements of the
neutron scattering intensity for momentum transfers in the range
ꢁ1
2.5 ꢄ 10ꢁ3oqo2.6 ꢄ 10ꢁ1
. The scattered neutrons were
˚
A
detected by a two-dimensional position-sensitive 3He detector.
The samples of amorphous xerogels of sulfated hydrous ZrO2
were placed in a 1 mm thick quartz cells. Apparent density rH of
each sample was calculated as a weight of a powder divided by its
volume. The initial spectra for each q range were corrected using
the standard procedure [9] taking into account the scattering
from the setup equipment and cell, as well as background from
incoherent scattering of neutrons on hydrogen atoms which are
present in samples in the form of physically and chemically
bounded water. Resulting 2D isotropic spectra were averaged
azimuthally and their absolute values were determined by nor-
malizing to the incoherent scattering cross section from vana-
dium with inclusion of the detector efficiency and apparent
density (rH) for each sample. All measurements were done at
room temperature.
The SANS intensity analyzed hereafter was defined as
2.2. Methods of analysis
ISðqÞ ¼ IðqÞꢁT ꢄ I0ðqÞ,
ð2Þ
X-ray powder diffractograms (XRD) were obtained using
where I(q) and I0(q) are the momentum-transfer distributions of
scattered neutrons behind the sample and beam without the
Rigaku D/MAX 2500 diffractometer (Cu K radiation) over a 2
range of 10–851 with an increment 0.021/step at the rate 2 1/min.
y
a
sample, respectively. T¼I/I0¼exp(ꢁ
coefficient of the neutrons passing through the sample, where
sa is the integral scattering cross section which includes
nuclear scattering ss and absorption a, and L is the sample
thickness. The setup resolution function was approximated by a
Gaussian and was calculated separately for each SD distance with
the use of the standard procedure [10].
Adsorption of NH3 for temperature-programmed desorption
experiments (NH3-TPD) was performed using Chemisorb 2750
(Micromeritics) in NH3:N2¼5:95 (v/v) gas mixture. Before the
experiments, the samples were pretreated for 1 h at 300 1C under
helium flow (30 ml/min), then for 1 h at 300 1C under
O2:N2¼20:80 (v/v) gas mixture (30 ml/min) and finally for
30 min at room temperature under O2:N2¼20:80 (v/v) gas mix-
ture (30 ml/min). NH3-TPD was carried out under helium flow
after purging the sample at 50 1C for 20 min to decrease the
amount of physisorbed ammonia.
S
ꢄ L) is the transmission
Particle size was estimated using Scherrer equation
K
l
S¼ssþ
D ¼
ð1Þ
½
b yÞꢁsꢃcosy0
ð2
Here, y0 is the peak position,
wavelength (0.154,056 nm), hkl(2
sponding peak, s is the instrumental broadening (is equal to 0.11).
The value of K (shape factor) was set equal to 1. The value was
s
l
is the Cu K radiation
a
b
y) is the FWHM of a corre-
b
determined by subtraction of background followed by fitting
profiles of (1 1 1) and (1 1 1) reflections of monoclinic zirconia
and (1 1 1) reflection of tetragonal zirconia to the Voigt pseudo-
functions. Volume fractions of monoclinic (m-ZrO2) and tetra-
gonal (t-ZrO2) phases in zirconia samples were estimated using
relations proposed by Toraya et al. [8].
Thermal analysis (TGA/DTA) of the samples was performed in
air using Pyris Diamond thermoanalyzer (Perkin–Elmer) in the
temperature range 20–1100 1C (heating rate 10 1C/min). Chemical
composition of gases evolved during thermal decomposition
under argon flow was established by means of Netzsch STA 449
Studies of catalytic activity of Z-4S-600, Z-7S-600, Z-9S-600
samples in hexene-1 oligomerization were performed as follows.

498
V.K. Ivanov et al. / Journal of Solid State Chemistry 198 (2013) 496–505
Before a reaction each sample was heated in air at 600 1C for 2 h
and then cooled in dry atmosphere. A 1 g portion of catalyst and
5 ml (3.4 g) of hexene-1 were placed into a thermostated glass
flask and kept at constant temperature in 20–60 1C range under
vigorous stirring. Samples of the reaction mixtures (0.2 ml) were
taken for 1H–NMR and GC–MS analyses. 1H NMR spectra were
recorded using a Bruker DPX-200 spectrometer in CDCl3. NMR
spectra gave a possibility to monitor a vinyl group protons
disappearance. Gas chromatography (GC) and GC–MS were per-
formed using a LHM-2000 chromatograph (3% Dexsil 300—Chrom
W–AW column) and Perkin–Elmer Clarus 500 (SE-30 column)
spectrometer, respectively. These data were used to determine
oligomers content in reaction mixtures.
The exponent n value for the Z-4S sample found from the slope
of the straight-line section of the experimental d (q)/d plotted
in log–log scale is equal to 3.68. Thus scattering from the sample
occured on the fractal surface with the dimension 2rDS¼6ꢁ
n¼2.32o3. In the q4qc1 region, the scattering curve exhibits a
shoulder indicating the presence of small inhomogeneities with
S
O
˚
the effective gyration radius rg¼3372 A.
Thus, the Z-4S xerogel contains two types of scattering
inhomogeneities with strongly different characteristic sizes
which correspond to relatively large aggregates with a fractal
surface and small monomer particles.
The absence of the Guinier region on the scattering curve for
low q-values means that the gyration radius rg or, for the fractal
systems, the upper self-similarity boundary is larger than the
maximum size Rmax of the inhomogeneities that can be detected
in the experiment with a given resolution. In turn, the lower self-
similarity boundary is likely to be determined by the size rc of
small inhomogeneities, which usually lies in the range from 2rg to
3rg (approximately 2.6rg for spherical particles) [12].
3. Results and discussion
Fig. 1 shows the experimental curves of differential neutron cross
section dS(q)/dO for amorphous sulfated zirconia xerogels precipi-
SANS cross-section for the samples Z-7S and Z-9S as distin-
tated at different pHs. According to these data, increase in the
precipitation pH from 3.98 to 8.98 leads to notable (approximately
two orders of magnitude) increase in SANS intensity as well as to
changes in scattering pattern. This clearly indicates that the nuclear
guished from Z-4S has higher intensity and possesses an extra
region in the range of small q-values (qoqc2E0.005 Aꢁ1), where
˚
the deviation of dS(q)/dO
from power-law qꢁn is observed. This
deviation is due to the transition to Guinier regime, where cross-
˚
density homogeneity of xerogels in the range 10–1000 A is
section is governed by relatively large scattering inhomogeneities
decreased with the increase in the precipitation pH.
with characteristic size rc (aggregates). As for the region q4qc1
,
Sample synthesized in acidic media (Z-4S) shows two various
ranges which significantly differ by the behavior of the
the scattering curves also exhibit a shoulder due to the scattering
on small inhomogeneities, while this shoulder appears at larger q-
values compared to the spectrum of Z-4S sample. This indicates
that precipitation of sulfated hydrous zirconia at higher pH leads
to decrease in the primary particles size rc. Moreover, despite we
cannot estimate the upper limit of self-similarity range for all the
samples, the widest self-similarity range can be ascribed to the
xerogel synthesized at the lowest pH. Values of n parameter as
calculated from linear slopes of spectra for samples Z-7S and Z-9S
are equal to 3.43 and 3.17, respectively.
q
SANS crossꢁs1ection d
S(q)/dO. For low values qoqc1 (where
˚
qc1E0.06 A
is the transition point between one scattering
regime and another), the scattering cross-section dS(q)/dO satis-
fies the power law qꢁn. Such a power-law dependence is typical
for systems with a wide size distribution (Rmax cRmin) if the
condition
ꢁ1
ꢁ1
R
o oqo oR
ð3Þ
max
min
SANS from sulfated hydrous zirconia xerogels is in a good
accordance with our previously reported data [7,13] indicating
that scattering from non-sulfated zirconia precipitated at pH46
can be described within the model of porous structure with
fractal surface.
In view of this circumstance, to analyze scattering from Z-7S
and Z-9S amorphous xerogels over the entire q range, we used the
expression [13,14]:
is satisfied. In addition, the power scattering law means that
inhomogeneities making the dominant contribution to scattering
are sufficiently large (qmin ꢂ Rc 1). Bale and Schmidt [11] pro-
posed a more exact criterion for determination of a certain
characteristic sizeꢁo1f inhomogeneities, qmin ꢂ RE3.5. In our case,
qmin¼2.5 ꢂ 10ꢁ3
A
and consequently the characteristic size of
˚
˚
the inhomogeneities is RE1250 A.
!
!
q2R2g
q2r2g
d
S
ðqÞ
A2ðDSÞ
¼ A1 ꢄ exp
ꢁ
þ
þA3 ꢄ exp
ꢁ
ð4Þ
ð5Þ
ꢀ ꢁ
n
dO
3
3
^
q
Here,
ꢂ
ꢃ
1=2
3
^
q ¼ q=½erf q ꢄ Rg=6
ꢃ
is the momentum q transferred, normalized to an error function
erf(x). Such a procedure allows the cross-section of scattering
dS(q)/dO to be described correctly in the intermediate region
between q ꢂ R0o1 (Guinier approximation) and q ꢂ R0c 4 41
(qꢁn asymptotics), where scattering from both surfaces and
aggregates of characteristic size R0 contributes [14]. The values
of A1 and A3 are directly proportional to the number and the
volume of inhomogeneities (aggregates and monomers, respec-
tively) as well as to the density of their neutron scattering
amplitudes [15]. Amplitude A2(DS) depends on the fractal dimen-
sion of the system [11] as
Fig. 1. SANS differential cross-section dS(q)/dO for amorphous sulfated zirconia,
ꢀ
ꢁ
precipitated at pH¼8.98 (1), 6.98 (2), and 3.98 (3) versus momentum transfer q.
A2ðDSÞ ¼ pr2Gð5ꢁDSÞsin½ðDSꢁ1Þ
p
=2 ꢃN0
ð6Þ
Continuous lines represent results of experimental data fitting.

V.K. Ivanov et al. / Journal of Solid State Chemistry 198 (2013) 496–505
499
where
G
is the gamma function, and
r
for a compound containing
further increase in pH will result in formation of hydrous zirconia
samples possessing even higher surface fractal dimension.
Fig. 2 shows SANS data for amorphous sulfated zirconia in
comparison with our recent data for hydrous zirconia xerogels
synthesized using precisely the same procedure excluding addi-
tion of ammonium sulfate to the starting solution [7]. Surface
fractal dimension for both sulfated and non-sulfated zirconia
xerogels increases with the increase in the precipitation pH
(Fig. 2A). These data are in line with our earlier supposition that
precipitation conditions influence strongly the mesostructure of
zirconia xerogels and this effect is due to change in colloid
particles aggregation mechanism upon change of acidity of the
media and surface charge of colloid particles. On the other hand,
DS of sulfated zirconia is notably higher than DS of non-sulfated
zirconia within the whole pH range. This difference is probably
due to the presence of doubly charged sulfate ions, which strongly
adsorb onto the surface of colloid ZrO2 ꢂ xH2O particles, while
nitrate ions have less sorption ability.
several elements is defined as
X
rhNA
M
r
¼
biNi
:
ð7Þ
i
Here, NA is the Avogadro constant, M is the molar mass, bi is
the scattering length for the ith element in the compound, Ni is
the number of atoms of this element, and rH is the solid-phase
density. The constant N0 is related to the specific surface of the
surface fractal as S0¼N0 ꢂ r2ꢁDs, where r2ꢁDs is determined by the
length of the yardstick. For smooth surfaces, DS¼2 and N0¼S0.
Taking into account that for the Z-4S sample no deviation of
scattering spectrum from power-law dependence in small q range
was observed, expression (4) was simplified to:
!
q2r2g
d
S
ðqÞ
A2ðDSÞ
¼
þA3 ꢄ exp
ꢁ
ð8Þ
d
O
qn
3
It is worth saying that reproducibility of structural parameters
of hydrated zirconia gels as derived from small-angle neutron
scattering data is very high. For instance, the fractal dimension of
amorphous zirconia gels precipitated at given pH is always the
same (namely, variation of DS value is less then 0.02 from one
experiment to another). In fact, from our point of view, such a
reproducibility is a very amazing phenomenon. It clearly demon-
strates that the ‘‘soft matter’’ is in fact not so ‘‘soft’’.
To obtain the final results, expressions (4, 8) were convolved
with the setup resolution function. The experimental curves of
the differential cross section d (q)/d versus q were processed by
S
O
the least mean squares method over the entire q range under
investigation. The results of this analysis are given in Table 1.
These data evidently show that both surface fractal dimension
and A2(DS) values increase with the increase in the precipitation
pH. The value of DS for the sample Z-4S is the lowest, i.e., in this
case monomer particles form fractal aggregates with the smooth-
est surface. Upon precipitation from neutral (pH¼6.98) and
alkaline (pH¼8.98) media the values of surface fractal dimension
are higher (2.6 and 2.8, respectively). In other words, increase in
the precipitation pH results in notable changes in monomer
aggregation behavior.
Formation of zirconia surface fractal aggregates and increase in
their surface fractal dimension upon increase in pH can be
explained using well known data on the mechanism of condensa-
tion of hydrous silica monomers [16]. Under acidic conditions
condensation occurs mainly between silanol groups located at the
ends of polymeric chains leading to the formation of linear
polymers. Contrariwise, under basic conditions, the condensation
reaction occurs preferentially between the ends and the interior
of polymeric chains thus leading to formation of ramified aggre-
gates with a higher fractal dimension. Such an approach explains
the increase in the surface fractal dimension value of hydrous
zirconia upon gradual increase in the precipitation pH from 4 to 9.
These suppositions are also in agreement with well established
Clearfield theory [17], which states that slow hydrolysis of
zirconyl salts leads to formation of regular sheets consisting of
tetrameric [Zr4(OH)8] units, while fast hydrolysis (for instance,
under basic conditions) promotes rapid polymerization occurring
in many directions at once. The degree of polymerization
increases with an increase in pH [18]. Such a mechanism of
polymerization can be also the main reason of fractal formation in
course of hydrous sulfated zirconia precipitation. In this case,
Table 1
Parameters of mesostructure of amorphous sulfated zirconia according to SANS
measurements.
Zr-4S
Zr-7S
Zr-9S
A1 ꢄ 10ꢁ6 (cm2 gꢁ1
)
–
31,40072000
710770
25,40072,000
6907100
Z1,250
˚
R0 (A)
A2 ꢄ 10ꢁ6 (cm2 gꢁ1
)
)
0.870.1
2.3270.09
0.5770.05
3372
100725
210760
2.8370.08
5.370.08
15.170.9
DS¼6ꢁn
2.5770.08
4.0870.07
17.370.8
A3 ꢄ 10ꢁ1 (cm2 gꢁ1
Fig. 2. Surface fractal dimension, DS (A) and primary particle size, rc (B) of
amorphous zirconia xerogels versus precipitation pH. 1 – non-sulfated zirconia
according to [7], 2 – sulfated zirconia synthesized in this work.
˚
rg (A)

500
V.K. Ivanov et al. / Journal of Solid State Chemistry 198 (2013) 496–505
The rise of the precipitation pH from 3.98 to 8.98 leads to
These data are also in a good agreement with our recent results
on thermal decomposition of non-sulfated hydrous zirconia [7].
It is well known that hydrous oxides generally possess extremely
approximately twofold decrease in the characteristic size rc (for
spheres, rc¼(5/3)1/2rg) of primary sulfated ZrO2 ꢂ xH2O particles
˚
˚
(from ꢀ43 A to ꢀ20 A, see Fig. 2B). For non-sulfated zirconia
high sorption capacity. Below the point of zero charge (pHpzc)
synthesized under the same conditions a slight increase in
their surface is positively charged and this charge is compensated
by preferential adsorption of negatively charged ions. Thus
sedimentation of hydrous zirconia under acidic conditions is
accompanied by considerable adsorption of NO3ꢁ and SO42ꢁ ions
which are present in mother liquor. However, higher bonding
strength of sulfate ions results in preferred adsorption of SO24ꢁ. These
species cannot be removed completely even by repeated washing.
Temperature corresponding to maximum rate of sulfates
decomposition decreases with the increase in the precipitation
pH and is equal to 715–725 1C, 685–695 1C, 650–660 1C for Z-4S,
Z-7S and Z-9S samples, respectively. This effect is probably caused
by the stronger bonding between SO24ꢁ-ions and hydrous zirconia
particles when precipitation is conducted under acidic conditions.
Differential TGA curve of Z-4S sample shown in Fig. 3B exhibits
a distinct decomposition stage at 430–500 1C which is absent for
the other samples. According to TGA–MS data this stage corre-
sponds to decomposition of nitrate-containing zirconium com-
pounds. Thus, hydrous zirconia precipitated under acidic
conditions contains not only high amount of sulfate species, but
also a certain amount of NO3ꢁ ions. Two peaks in the range of
600–750 1C are attributed to two-stage decomposition of sulfate
groups. This evidences the presence of at least two types of
surface sulfate species.
˚
primary particle size (from ꢀ18 to ꢀ22 A) is observed upon the
rise of pH.
According to XRD data (see Fig. S1 in the Supporting Informa-
tion) all the initial xerogels are completely amorphous giving two
extremely broad peaks at ꢀ30 and ꢀ50–6012
y. Basing on short-
range structural model proposed by Southon et al. [19], repro-
ducible extremely broadened peaks on diffractograms of hydrous
zirconia xerogels can be caused by short range ordered zirconium
hydroxide polymers consisting of cyclic tetramer species. Accord-
ing to Moroz et al. hydrous zirconia gels precipitated at pH 7 and
pH 9 are indeed X-ray amorphous [20] but possess short-range
ordering [21]. Amorphous nature of sulphated zirconia xerogels
was also confirmed by TEM and SAED patterns of the samples.
According to TGA (Fig. 3A) and TGA–MS (see Fig. S2 in the
Supporting Information) data the first stage of thermal decom-
position of sulfated zirconia xerogels at temperatures up to
approximately 200 1C corresponds to elimination of physisorbed
water, and the second one (at 600–750 1C) corresponds to
decomposition of sulfates.
Thermal analysis shows that precipitation conditions substan-
tially affect the chemical composition and thermal behavior of
sulfated zirconia gels. Weight loss at high temperature decom-
position stage decreases notably with the increase in the xerogel
precipitation pH. This effect is obviously due to the less content
of sulfates in hydrous zirconia samples precipitated at higher
pH values and corresponds well to observations made in [22].
The body of TGA–MS and differential TGA data gave us the
possibility to estimate the chemical composition of sulfated
zirconia xerogels: Zr-4S—ZrO1.46(NO3)0.12(SO4)0.48 ꢂ 2.2H2O; Zr-
7S—ZrO1.76(SO4)0.24 ꢂ 2.6H2O;
Zr-9S—ZrO1.88(SO4)0.12 ꢂ 2.7H2O.
Mass spectroscopy data shows no signal corresponding to sulfur
oxides at temperatures below 500–550 1C since SO24ꢁ groups are
chemically bonded to zirconia particles and no other sulfur-
containing compounds (like ammonium sulfates or sulfuric acid)
are present in samples.
DTA data are presented in Fig. 4. Strong endothermic peak
below ꢀ200 1C which is observed for all the samples corresponds
to elimination of physisorbed water from zirconia xerogels. At
higher temperatures a notable difference in samples behavior is
observed. An exothermic peak (maximum at 590–600 1C) for Z-9S
sample does not correspond to release of any gaseous products
and so it is probably caused by crystallization of zirconia from
Fig. 3. Thermal analysis (A) and differential thermal analysis (B) curves of sulfated
zirconia xerogels precipitated at different pH (1—Z-4S, 2—Z-7S, 3—Z-9S).
Fig. 4. Differential thermal analysis of sulfated zirconia xerogels (1—Z-4S, 2—Z-7S,
3—Z-9S).

V.K. Ivanov et al. / Journal of Solid State Chemistry 198 (2013) 496–505
501
X-ray amorphous state. In turn, non-sulfated zirconia synthesized
under the same conditions crystallizes at 415–435 1C. Thus,
introduction of sulfate ions favors additional stabilization of
amorphous zirconia and inhibits crystallization process. This
conclusion is also in line with [22].
DTA curves for Z-7S and Z-4S samples show no evident
exothermic effects corresponding to crystallization of ZrO2. On
the contrary, DTA curve for Z-7S sample shows endothermic
effect at 700–705 1C which corresponds to decomposition of
sulfates, while Zr-4S sample exhibits two close endothermic
effects with maxima at 630–635 and 720–725 1C which are also
due to the elimination of sulfur oxides. The absence of exothermic
crystallization peaks for Z-4S and Z-7S xerogels is probably
because of superposition of two opposite sign thermal effects
caused by decomposition of sulfates and crystallization of ZrO2.
Stabilization of amorphous zirconia by sulfate species was
confirmed by X-ray diffraction of ZrO2 ꢂ xH2O samples subjected to
long term (5 h) annealing at 500–700 1C (Fig. 5). While Z-9S
xerogel which contains minimum amount of sulfates crystallizes
upon annealing at 500 1C, Z-4S and Z-7S samples treated under
the same conditions remain completely or almost completely
amorphous (Z-4S-500 sample contains trace amounts of crystal-
line zirconia in addition to amorphous phase). Increase in anneal-
ing temperature to 550 1C results in notable decrease in the
amount of amorphous phase in the samples. Further temperature
rise up to 600 1C results in formation of crystalline ZrO2 in all the
Fig. 5. X-ray diffraction patterns of sulfated zirconia xerogels upon thermal treatment at (a) 500 1C, (b) 550 1C, (c) 600 1C, (d) 700 1C during 5 h (1—pH 4, 2—pH 7, 3—pH 9).
Solid black vertical lines correspond to tetragonal zirconia, dotted red ones—to monoclinic zirconia.

502
V.K. Ivanov et al. / Journal of Solid State Chemistry 198 (2013) 496–505
cases while small amounts of amorphous zirconia still remain in
Z-4-600 and Z-7-600 samples. Complete crystallization occurs
only after 5 h treatment at 700 1C.
media [7]. In turn, to explain unusual behavior of sulfated zirconia
we had to take into consideration the second possible stabiliza-
tion mechanism of tetragonal ZrO2 described elsewhere [24]
which deals with surface excess energy of sub-30 nm zirconia
particles.
Actually, mean particle size calculated using Scherrer formula
notably depends on the precipitation pH. Particle size of tetra-
gonal zirconia obtained upon thermal treatment of sulfated
zirconia xerogels precipitated at low pH values (23–28 nm) is
notably higher compared with particle size of t-ZrO2 synthesized
in neutral and alkaline media (14–16 nm). This effect is in a good
accordance with transmission electron microscopy data (Fig. 7).
Upon the rise of the temperature from 500 1C to 700 1C the
particle size of tetragonal zirconia does not change significantly.
The same effect is observed for monoclinic ZrO2 forming upon
thermal decomposition of Z-4S xerogel—particle size of mono-
clinic ZrO2 is always about 15–18 nm (see Table 4).
Phase composition (content of metastable tetragonal and
stable monoclinic zirconia) depends on a number of factors
including chemical composition of zirconium-containing amor-
phous precursors. Thermal treatment of Z-7S and Z-9S samples
leads to formation of metastable tetragonal zirconia with no
marked traces of monoclinic phase. On the contrary, powders
obtained upon annealing of Z-4S sample at 500–700 1C contain
notable amount of monoclinic zirconia (tetragonal to monoclinic
zirconia volume fraction ratio is 1:3–1:4).
Phase composition of sulfated and non-sulfated zirconia
obtained under the same conditions differs dramatically. An
illustration of this difference is given in Fig. 6 and in Table 4.
In non-sulfated samples tetragonal zirconia content decreases
with increase in the precipitation pH, while sulfated ones show an
opposite tendency.
The decrease in particle size of t-ZrO2 with the increase in the
precursor xerogels precipitation pH is in line with our previously
reported data [7,25]. As far as thermal decomposition of hydrous
sulfated zirconia can be judged as a topochemical process,
structure of initial xerogel should affect the structure and proper-
ties of the product of its thermal decomposition. In particular,
xerogel precipitated in acid media has higher apparent density
compared to xerogels precipitated in neutral and alkaline media.
This was also the case for non-sulfated zirconia. Denser xerogels
precipitated at low pH values transform into larger ZrO2 particles
while xerogels possessing more ramified structure decompose
upon annealing with formation of relatively small ZrO2 particles.
Thermal behavior of Z-4S-600, Z-7S-600, and Z-9S-600 samples
(Fig. S3 in the Supporting Information) is in line with the above
outputs. Weight loss corresponding to elimination of sulfur oxides
decreases with increase in the precipitation pH of initial xerogel.
Furthermore, the temperature corresponding to the maximum rate
of sulfate decomposition falls with the increase in the precipitation
pH and is equal to 695–700 1C, 655–665 1C, and 625–635 1C for
Z-4S-600, Z-7S-600, and Z-9S-600, respectively.
This result is quite unusual. It is well known that stabilization
of tetragonal zirconia is caused by the presence of various anionic
and cationic impurities [23]. ZrO2 ꢂ xH2O precipitated in acid
media (below the point of zero charge) indeed contains the
highest amount of impurities. Thus products of the thermal
decomposition of hydrous zirconia precipitated in acid media in
the absence of sulfate species contain higher fraction of tetragonal
zirconia in comparison with zirconia precipitated in alkaline
Chemical composition of xerogels upon annealing at 500 1C
and 600 1C according to energy dispersive X-ray spectroscopy
data is given in Table 2 and in Table 4. These data also confirm
Table 2
Sulfur content (wt% S) in sulfated zirconia samples.
Precipitation pH
Temperature
500 1C
600 1C
3.98
6.98
8.98
8.1
8.5
4.7
7.7
5.0
2.8
Fig. 6. Relative content of tetragonal zirconia in ZrO2 samples precipitated at
various pH and then annealed at 600 1C for 5 h.
1 – non-sulfated zirconia
according to [7], 2 – sulfated zirconia synthesized in this work.
Fig. 7. TEM images of sulfated zirconia samples Z-4S-600 (a), Z-7S-600 (b), Z-9S-600 (c).

V.K. Ivanov et al. / Journal of Solid State Chemistry 198 (2013) 496–505
503
that sulfur content in sulfated nanocrystalline zirconia decreases
with the rise of hydrous zirconia precipitation pH and agree with
the thermal analysis data presented in Fig. 3 and S3. Temperature
rise from 500 1C to 600 1C does not substantially affect the sulfur
content in powders derived from Z-4S, while lead to 1.5 and 2.0-
fold decrease in sulfur content in ZrO2 powders derived from Z-7S
and Z-9S samples, respectively. This is in a good agreement with
the above conclusion that raise in the precipitation pH facilitates
sulfates decomposition and lowers the temperature of sulfur
oxides elimination.
Table 4. Specific surface area rises with the increase in the
precipitation pH as well as with the raise of the temperature of
thermal treatment. Close results on the influence of pH on the
amount of sulfate sorbed as well as effect of calcination temperature
on surface area of sulfated zirconia was reported earlier by [26].
Actually specific surface area (SBET) of samples originated from Z-4S
xerogel is very low and do not exceed 20 m2/g. At the same time
increase in the precipitation pH up to the point of zero charge and
higher leads to dramatic increase in corresponding specific surface
area values. These data give additional evidence that initial amor-
phous hydrous zirconia mesostructure influences the microstructure
of nanocrystalline sulfated zirconia. The highest surface area of
zirconia can be reached when starting hydrous zirconia is precipi-
tated under alkaline conditions (pH4pHpzc). These data correlates
well with the Arata’s pioneering work [26]. Increase in specific
surface area values with the raise of annealing temperature is
presumably caused by elimination of gaseous products from sul-
fated zirconia causing formation of microcracks. SEM images
(see Fig. 9) support surface area measurements and vividly present
the difference between dense glass-like structure of Z-4S-600
sample and porous structure of Z-7S-600 and Z-9S-600 samples.
Fig. 10 represents NH3 temperature-programmed desorption
(TPD) spectra normalized per 1 m2 surface area for sulfated
zirconia samples annealed at 600 1C for 5 h. The surface of all
samples contains acid sites of two different types. Low tempera-
ture peaks at 150–200 1C in NH3-TPD spectra correspond to weak
acid centers. Extended shoulders with no pronounced maxima
which appear at 250–600 1C correspond to strong acid sites.
Increase in initial xerogel precipitation pH leads to the raise of
the ratio between weak and strong acid centers. As weak acid
sites are commonly assigned to the surface hydroxyl groups, the
surface of sulfated zirconia remains hydroxylated even upon
annealing at 600 1C. Distribution of weak acid sites over ammo-
nium desorption activation energy becomes narrower in a row of
Z-4S-600, Z-7S-600, and Z-9S-600. This can be explained by
higher phase uniformity of samples obtained upon annealing of
xerogels precipitated at higher pH (see Fig. 5c). Note that Z-4S-
600 sample possesses the highest concentration of strong acid
sites. When adsorbed on oxide surfaces sulfate species generate
strong acidity [3,4] through both Lewis and Brønsted superacid
centers [2]. Accordingly sulfated zirconia synthesized from xer-
ogel precipitated at the lowest pH possess the highest sulfur to
zirconium molar ratio.
Specific surface area values of Z-4S, Z-7S, and Z-9S xerogels
treated at 500 1C, 550 1C, and 600 1C are given in Fig. 8 and in
Table 3
Catalytic isomerization and oligomerization of hexene-1 over sulfated zirconia.
Run
Catalyst
t, 1C/t, h
Conversion, %
Products content, %
C6
C12
C18
C24
C30
1
2
Z-4S-600
Z-7S-600
Z-9S-600
20/1
6
54
20/24
40/1
60/1
20/1
20/24
40/1
60/1
20/1
20/24
40/1
60/1
3
60
4
67
78a
18b
16
49
6
0
7
0
2
5
58
6
100
100
100
74
7
24
8
9
10
11
12
100
100
100
13b
45
26
10
4
a
ꢀ30% of hexene-1.
b
Only hexene-2þhexene-3.
Surface concentration of acid sites can be determined by
integrating the NH3-TPD curves, supposing that each NH3 mole-
cule desorbs from one site. Z-4S-600 sample showed the highest
specific concentration of acid sites (18.6
values for Z-7S-600 and Z-9S-600 are quite close and are equal
to 6.5 and 8.9
mol/m2, respectively (see Table 4).
m
mol/m2), while the
m
To estimate the catalytic activity of sulfated zirconia we have
studied hexene-1 oligomerization catalyzed by Z-4S-600, Z-7S-
600, and Z-9S-600 samples. During each experiment a portion of
pure hexene-1 was stirred with a catalyst at a predetermined
temperature. Analysis of reaction mixtures was performed by 1H
NMR and GC–MS methods. The catalytic transformation of
Fig. 8. Specific surface area of sulfated zirconia against precipitation pH and
temperature of thermal treatment (1–500 1C, 2–550 1C, 3–600 1C).
Table 4
Properties of sulfated zirconia catalysts.
Sample
Surface area,
m2/g
Particle size (XRD), nm
Volume fraction
Sulfur
Specific acidity,
mol/m2
Total acidity,
mol/g
Conversion of hexene-1
of t-ZrO2, %
content, wt%
m
m
at 40/1 run (see Table 3), %
t-ZrO2
m-ZrO2
Z-4S-600
Z-7S-600
Z-9S-600
1773
4574
7676
2672
1571
1671
1772
2473
100
7.770.4
5.070.3
2.870.3
18.670.6
6.470.4
8.970.4
320750
290750
680750
6073
100
–
–
100
100

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V.K. Ivanov et al. / Journal of Solid State Chemistry 198 (2013) 496–505
Fig. 10. NH3-TPD data normalized per 1 m2 of surface area of sulfated zirconia
(1—Z-4S-600, 2—Z-7S-600, 3—Z-9S-600).
Scheme 1. Chemical transformations catalyzed by sulfated zirconia. .
(18.6
what gives the value of total acidity approx. 320
Z-9S-600 sample with 8.9
mol/m2 specific acidity and 76 m2/g
surface area gives more than a two times higher total acidity
value—approx. 680 mol/g. This means that 1 g of Z-9S-600
m
mol/m2) but the lowest specific surface area (17 m2/g),
mol/g. The
m
m
m
catalyst will be at least two times more active in catalytic
transformation of hexene-1 than an equivalent amount of Z-4S-
600 catalyst. The difference in catalytic activity must be even
larger because of the surface structure of the samples—glass-like
for Z-4S-600 and porous for Z-9S-600 (Fig. 9). The total acidity of
Z-7S-600 sample (approx. 290 mmol/g) is close to Z-4S-600, but
the porous surface structure makes it more active.
4. Conclusions
Fig. 9. SEM images of Z-4S-600 (a), Z-7S-600 (b), and Z-9S-600 (c) samples.
In this work, we have established how mesostructure and
composition of hydrous sulfated zirconia gels precipitated from
ZrO(NO3)2/(NH4)2SO4 aqueous solutions at different pHs govern
structure, chemical and phase composition, and catalytic activity
of nanocrystalline sulfated zirconia formed via thermal decom-
position of these gels. We have shown for the first time that the
raise in precipitation pH from 4 to 9 leads to dramatic increase in
the surface fractal dimension (DS) of sulfated zirconia xerogels as
well as to a decrease in the size of primary particles (rc) forming
zirconia fractal aggregates. Sulfated nanocrystalline zirconia sam-
ples prepared from gels precipitated in neutral and alkaline media
have relatively low sulfur content but they possess much higher
catalytic activity due to the smaller particle size, higher specific
surface area and higher total acidity. We hope this paper makes a
contribution to reproducible synthetic methods of sulfated zirco-
nia superacid catalysts preparation.
hexene-1 under acid conditions leads to a double bond shift
and/or to oligomerization resulting in formation of higher olefins.
Catalytic reaction scheme is given in Scheme 1 and the results are
summarized in Tables 3 and 4. The influence of precipitation pH
on the catalytic activity of sulfated zirconia in oligomerization as
well as on side isomerisation reaction of hexene-1 can be easily
observed. The data obtained correlate well with earlier short
communication of Arata et al. devoted to catalytic isomerization
of n-pentane [27].
Table 4 summarizes properties of sulfated zirconia catalysts.
The results of catalytic experiments can be explained from the
point of view of surface acidic properties of the samples (see
Table 4). Z-4S-600 sample has the highest specific acidity

V.K. Ivanov et al. / Journal of Solid State Chemistry 198 (2013) 496–505
505
Acknowledgments
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Authors are grateful to Dr. Tatyana I. Vetrova for technical
assistance.
V.I., A.B. and G.K. would like to thank GKSS Forschungszentrum
for hearty welcome.
The work was partially supported by the Russian Ministry of
Science and Education (project ]14.740.11.0281) and Russian
Foundation for Basic Research (projects 09-03-01067 and 11-03-
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