Nanosized TiO2—A promising catalyst for the aldol condensation of furfural with acetone in biomass upgrading

Article abstract of DOI:10.1016/j.cattod.2015.11.027

Nanosized TiO₂ catalyst was successfully prepared by a simple green procedure and used in liquid phase aldol condensation of furfural with acetone, a key step in bio-fuel processing. In order to determine the effect of calcination temperature on catalytic properties of TiO₂, the as-prepared TiO₂ and calcined TiO₂ (150–900?°C) were studied by XRD, BET, TPD-CO₂/NH₃, TGA/DTG and FTIR evaluation. The catalytic performance of TiO₂ samples in aldol condensation of furfural with acetone was evaluated and compared with that of Mg–Al hydrotalcites and a BEA zeolite. These experiments showed that uncalcined TiO₂ possessed reasonable activity in aldol condensation of furfural to acetone and resulted in commonly produced condensation products. The observed catalytic behavior of TiO₂ could be competitive with that reported for other inorganic solids. The calcination of TiO₂ resulted, however, in a decrease in its catalytic activity due to extensive dehydration and surface dehydroxylation as well as due to changes of textural properties resulting in a decrease in the amount of accessible active sites. Thanks to its advanced properties, nanosized TiO₂ is a promising catalyst for aldol condensation of furfural with acetone and could broaden possibilities for optimizing conditions for bio-fuel production.

Full text of DOI:10.1016/j.cattod.2015.11.027

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Nanosized TiO2—A promising catalyst for the aldol condensation of
furfural with acetone in biomass upgrading
Dong Nguyen Thanha, Oleg Kikhtyanina, Ruben Ramosa, Maadhav Kotharia,
Pavel Ulbrichb, Tasnim Munshic, David Kubicˇkaa,∗
a Research Institute of Inorganic Chemistry, RENTECH-UniCRE, Chempark Litvínov, Záluzˇí, Litvínov 43670, Czech Republic
b Institute of Chemical Technology, Technická 5, 166 28, Prague 6, Czech Republic
c School of Chemistry, College of Science, University of Lincoln, Brayford Pool, Lincoln, Lincolnshire, LN6 7TS, United Kingdom
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 30 July 2015
Received in revised form 16 October 2015
Accepted 23 November 2015
Available online xxx
Nanosized TiO2 catalyst was successfully prepared by a simple green procedure and used in liquid phase
aldol condensation of furfural with acetone, a key step in bio-fuel processing. In order to determine
the effect of calcination temperature on catalytic properties of TiO2, the as-prepared TiO2 and calcined
TiO2 (150–900 ◦C) were studied by XRD, BET, TPD-CO2/NH3, TGA/DTG and FTIR evaluation. The catalytic
performance of TiO2 samples in aldol condensation of furfural with acetone was evaluated and compared
with that of Mg–Al hydrotalcites and a BEA zeolite. These experiments showed that uncalcined TiO2
possessed reasonable activity in aldol condensation of furfural to acetone and resulted in commonly
produced condensation products. The observed catalytic behavior of TiO2 could be competitive with
that reported for other inorganic solids. The calcination of TiO2 resulted, however, in a decrease in its
catalytic activity due to extensive dehydration and surface dehydroxylation as well as due to changes of
textural properties resulting in a decrease in the amount of accessible active sites. Thanks to its advanced
properties, nanosized TiO2 is a promising catalyst for aldol condensation of furfural with acetone and
could broaden possibilities for optimizing conditions for bio-fuel production.
Keywords:
Aldol condensation
Furfural
Acetone
Titanium oxide
Calcination
Biofuel
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
categorized based on their source such as industrial waste (saw-
dust, paper mill discards, food industry residues, etc.), forestry
The world’s energy system needs to be adapted to a more sus-
tainable one, based on a diverse mix of energy resources, addressing
the pressing challenges of energy security supply and climate
change [1]. Nowadays, there is a change in the biomass energy uses
from traditional and non-commercial ones, such as simple open
combustion to produce heat, to modern ones, such as advanced pro-
cesses to produce electricity and bio-fuels integrated in food and
biomaterials industries [2]. The technical and economic potentials
of biomass are higher than the current world energy consumption
[1,2], thus, the main challenge is in its viable and sustainable use.
The most abundant biomass resource is lignocellulosic biomass
consisting of cellulose, hemicellulose and lignin [3]. Some other
materials such as ash, proteins, pectin, etc. are also found in the
lignocellulosic biomass in different proportion depending on the
biomass source and origin [4]. In addition to dedicated biomass-
for-energy plantations, waste biomass holds enormous potential
with a high degree of sustainability. These waste materials can be
waste (branches, hard and soft wood residues, etc.), agricultural
residues (straw, stover, peelings, cobs, stalks, nutshells, non-food
seeds, etc.), domestic waste (kitchen wastes, sewage, waste papers,
etc.), and municipal solid waste [4,5]. These materials are consid-
ered as one of the most promising bio-based feedstocks for the
production of fuels and other chemical products as they are not in
direct competition with food crops, fodder, and natural habitat [6].
Furanic compounds (e.g. furfural, 5-hydroxymethlyfurfural) and
acetone are readily available chemicals, which can be produced
by the hydrolysis of biomass [7]. Additionally, acetone is avail-
able as a by-product in phenol production. Aldehydes and ketones
can react by aldol condensation and result in the production of
higher added value products from simple and cheap ones. In order
to exploit the full potential of furfural as a platform chemical, its
transformation by aldol condensation with acetone followed by
hydrogenation/deoxygenation has been proposed to afford hydro-
carbons, namely C8 and C13 alkanes [8] (see Scheme 1). Aldol
condensation proceeds in the presence of either basic or acidic cat-
alysts [9,10]. The most effective industrially available methods rely
on using liquid bases (NaOH or KOH) or mineral acids (e.g. H2SO4)
as catalysts [9]. However, these catalysts pose a major environmen-
∗
Corresponding author. Fax: +420 476768476.
E-mail address: david.kubicka@vscht.cz (D. Kubicˇka).
http://dx.doi.org/10.1016/j.cattod.2015.11.027
0920-5861/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: D. Nguyen Thanh, et al., Nanosized TiO2—A promising catalyst for the aldol condensation of furfural
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Scheme 1. Reaction scheme of aldol condensation of furfural with acetone.
tal threat due to waste water production and equipment corrosion.
In addition, solid catalysts with acid-base character are recognized
as a promising alternative to homogeneous catalysts for aldol con-
densation of furfural with acetone. Among solid catalysts, double
layered hydroxides (hydrotalcites) with basic character are consid-
ered as the most promising catalysts for this reaction [8,11–15]. The
essential disadvantages of such basic catalysts are their high sen-
sitivity to ambient CO2, which transforms them into catalytically
inactive carbonates, and the lack of reliable methods for recover-
ing their catalytic properties after regeneration [16–18]. Recently,
zeolites have been reported as catalysts for aldol condensation of
furfural with acetone [19]. It was shown that by using zeolites the
disadvantages of basic catalysts i.e. susceptibility to CO2 and poor
regenerability could be avoided. On the other hand, their activity
in aldol condensation of furfural with acetone is lower than that of
hydrotalcites. Moreover, they are rapidly deactivated due to coke
formation during the reaction [19]. Therefore, along with the opti-
mization of the properties of the solid basic catalysts, attention
is currently being focused on alternative solid catalysts for aldol
condensation of furfural with acetone.
Titanium dioxide (TiO2) is one of the most common materi-
als in our daily life; in addition to its application in water and air
purification and in pigments, it has emerged as an excellent pho-
tocatalyst for environmental purification applications [20]. TiO2
exists mainly in three different crystalline phases: rutile (tetrago-
nal), anatase (tetragonal) and brookite (orthorombic), out of which
rutile is the most thermodynamically stable phase [21]. The main
advantages of TiO2 are its high chemical stability when exposed to
acidic and basic compounds, nontoxicity, relatively low cost and
high oxidizing power, which make it a competitive candidate for
many photocatalytical applications [22].
p.a.) were used for synthesis of TiO2. Mg–Al hydrotalcites (HTC)
with Mg to Al molar ratio of 3 was prepared according to [23].
BEA zeolite (Si/Al = 12.5) was received from Zeolyst Int. (CP 814-
E). Commercial sample TiO2 P25 (Aeroxide® P25, Titanium (IV)
oxide nanopowder) was obtained from Sigma–Aldrich. Acetone
(Lach:Ner, p.a.) and furfural (Acros Organics, 99%) used for the cat-
alytic experiments were dried using a CaA molecular sieve prior to
the experiments.
2.2. TiO2 synthesis
The nanosized TiO2 was prepared by hydrolysis of TiOSO4 in
aqueous solutions using urea as the precipitation agent. In a typ-
ical process, TiOSO4 was dissolved in distilled water. The pellucid
solution was heated to 95 ◦C for 1 h, then mixed with urea and con-
tinuously heated at 100 ◦C for 5 h under stirring until pH reached
7.02. The formed precipitate was decanted, filtered and dried at
100 ◦C and the obtained white powder was denoted as “as-prepared
TiO2” sample. In order to study the effect of calcination tempera-
ture on the catalytic performance, the as-prepared TiO2 sample was
calcined in air at different temperatures in the range 150–900 ◦C for
2 h. All the samples were denoted as “TiO2–T”, where T represents
the calcination temperature.
2.3. Physico-chemical characterization
The catalysts were characterized by several techniques to assess
their structure, composition, surface area and pore volume. The
crystallographic structures of TiO2 catalysts were determined by
X-ray powder diffraction using a Philips MPD 1880, working with
the Cu-K␣ line (ꢀ = 0.154 nm) in the 2ꢁ range of 5–70◦ at a scanning
rate of 2ꢁ of 2.4 ◦/min. The XRD crystal size was calculated by using
Scherrer equation as follows [24]:
To the best of our knowledge, no research study has exam-
ined the reaction of aldol condensation of furfural with acetone
over nanosized titanium oxides up to date. The aim of this work
is to investigate a reusable solid TiO2 catalyst suitable for aque-
ous phase environment having high activity in aldol condensation.
The research is focused on the following key objectives: (1) prepa-
ration of TiO2 catalysts by co-precipitation from environmentally
friendly and low-cost raw materials; (2) in-depth characterization
of the prepared materials; and finally (3) a description of the effect
of pretreatment temperature of the TiO2 catalysts on their activity
and selectivity in aldol condensation of furfural with acetone.
kꢀ
D =
␤cosꢁ
where k is a constant equal to 0.89; ꢀ is the X-ray wavelength, ␤
is the full width at half maximum intensity (FWHM) and ꢁ is the
half diffraction angle. The phase composition of the samples was
calculated by using the following equation:
100
Rutile phase % =
( )
1+0.8 IA/IR
)]
[
(
2. Experimental and materials
where IA and IR are integrated intensities of the anatase (1 0 1) and
rutile (1 1 0) diffraction peaks, respectively.
2.1. Materials
Chemical analysis of as-prepared TiO2 sample was performed
with WDXRF spectrometer Philips PW 1404 with rhodium tube. The
measurement was aimed to control the content of residual sulfate
groups on TiO2 surface after hydrolysis and washing stages.
All chemicals were reagent grade and used as received. Titanium
(IV) oxysulfate TiOSO4 (Precheza a.s.) and urea CH4N2O (Lach:Ner,
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A full isotherm of each sample was obtained under standard
conditions at 77 K with nitrogen gas as the adsorbate using an
Autosorb-IQ (Quantachrome). Each sample was outgassed under
vacuum for 4 h at a lower temperature than the pre-prepared
material. The specific surface area was calculated by applying
the Brunauer–Emmett–Teller (BET) equation within the range of
0.05–0.2 P/P0. The overall pore volume and the average pore size
were determined by the BJH method to the adsorption branch.
FTIR spectra were collected using a Thermo Scientific Nicolet
iS10 FT-IR Spectrometer using the ATR technique. The IR spectra of
TiO2 samples were obtained as dry samples. All IR measurements
were carried out at room temperature.
Thermogravimetric analysis (TGA/DTG) of the dried TiO2 cata-
lysts was performed using a TA Instruments TGA Discovery series
equipment operated at a heating ramp of 10 ◦C/min from room
temperature to 900 ◦C in flowing nitrogen (20 mL/min, Linde 3.0).
Approximately 15 mg of sample was heated in an open alumina
crucible.
Analytical measurements of temperature programmed desorp-
tion (TPD) were performed in order to evaluate the acidity and
basicity of the synthesized TiO2 samples, as well as some refer-
ence materials (Beta zeolite, commercial TiO2 and hydrotalcite).
The analysis was carried out using a Micromeritics Autochem 2950
instrument, supplied with streams of NH3 and CO2 (10 vol.% in He).
Prior to the chemisorption step, each parent sample (0.1 g) was
heated (10 ◦C min−1) under helium (25 cm3 min−1) up to 500 ◦C.
In case of NH3 TPD analyses, the samples were cooled down to
180 ◦C, saturated with ammonia for 30 min (25 cm3 min−1) and
followed by removal of the physically adsorbed molecules by He
stream (25 cm3 min−1) for 60 min. Subsequently, the TPD curve was
obtained by increasing the temperature with a heating ramp of
15 ◦C min−1 up to 500 ◦C under He (25 cm3 min−1), and maintaining
this temperature for 30 min. In the case of the CO2 TPD analyses, the
chemisorption was carried out at 50 ◦C for 30 min (50 cm3 min−1).
After the removal of physisorbed CO2 (by He 25 cm3 min−1 for
60 min), the chemically absorbed CO2 was desorbed by heating the
sample to 900 ◦C (heating rate of 15 ◦C min−1) under continuous
flow of He gas (25 cm3 min−1) and keeping the final temperature for
30 min. The concentration of desorbed ammonia and carbon diox-
ide in helium was recorded continuously by means of a thermal
conductivity detector (TCD).
The samples of reaction mixtures were withdrawn from the
reactor during the experiment at certain reaction times, filtered,
and analyzed by an Agilent 7890A gas chromatograph equipped
with a flame ionization detector (FID), using a HP 5 capillary column
(30 m/0.32 mm ID/0.25 ␮m). The obtained products were identi-
fied based on standard reference compounds as well as additional
GC–MS analyses.
Catalytic results of aldol condensation of furfural and acetone
were described by conversion and selectivity parameters that were
calculated as follows:
reactant conversion (t) (mol%) = 100 × (reactantt=0 − reactantt)/reactantt=0
;
where selectivity to product i = (mole of reactant converted to prod-
uct i)/(total moles of reactant converted).
Carbon balance was monitored in all experiments as the total
number of carbon atoms detected in each organic compound with
Cn atoms (where n = 3, 5, 8, . . ., etc.) divided by the initial number
of carbon atoms in F + Ac feed:
ꢀ
ꢁ
3molC3 + 5molC5 + ...nmolCn
3molC3 t=0 + 5molC5 t=0
Cbalance % =
( )
.
(
)
(
)
3. Result and discussion
3.1. Catalyst structure
In the present study, anatase TiO2 was prepared by thermal
hydrolysis of aqueous TiOSO4. According to Ahmed et al. [25],
the TiOSO4 solution contains chains of Ti and O atoms which are
2−
interconnected by SO4
groups. The Ti2+
O
Ti2+
O
zigzag
chains or oligomers have different chain lengths depending on the
TiOSO4 concentration. At low concentrations, it has been shown
that monomeric titanyl species with 2+ charge, as well as dimers or
trimmers are present. However, the number of monomeric titanyl
decreases as the concentration of TiOSO4 increases. At high concen-
trations of titanyl, the percentage of monomeric form is negligible
and titanium is mainly in the zigzag chains of Ti2+
O O .
Ti2+
The hydrolysis of urea generally proceeds in two steps (Eqs. (1) and
(2)), the formation of ammonium cyanate (NH4CNO) being the rate
determining step, with subsequent fast hydrolysis of the cyanate
to ammonium carbonate, resulting in a pH value of 9. However, in
the synthesis of TiO2 in this study, the final pH for the suspension
was about 7.02. The pH of 7–8 is supposed to be suitable for the
complete precipitation of Ti(IV) cations because the precipitation
of Ti(IV) occurs even in an acidic or neutral aqueous medium.
For catalyst re-activation, Bandelin DT 100 ultrasonic bath (fre-
quency of 35 kHz, nominal power 320 W and heating (30–80 ◦C))
and a normal UV lamp (Nail Art Gel 36W) were used for the ultra-
sonic irradiation and/or UV radiation, respectively.
CO NH
→ NH4CNO
(1)
(2)
(
)
2
2
2−
NH4CNO+2H2O → 2NH4++CO3
2.4. Catalytic experiments
The catalytic properties of the prepared TiO2 samples were
investigated in the aldol condensation of furfural with acetone. The
properties of Mg-Al HTC and BEA zeolite samples were evaluated in
comparative catalytic runs as well. The catalytic experiments were
carried out in a 100 ml stirred batch reactor (a round bottomed glass
flask reactor with magnetic stirring) at T = 50 ◦C. For each experi-
ment, 1 g of catalyst was taken and calcined, if necessary. Before
the start of the catalytic runs at 50 ◦C, the weighed amount of a
catalyst was mixed together with a stirred mixture consisting of
19.7 g acetone and 6.5 g of furfural (acetone/furfural molar ratio
5/1), pre-heated to the desired reaction temperature and kept at
the reaction temperature for 0–180 min under intensive stirring.
In specially performed catalytic runs with Mg–Al HTC taken as a
reference sample it was established that under the chosen reaction
conditions the reaction was limited neither by external nor inter-
nal mass transfer by changing the stirring rate and catalyst particle
size.
Under these hydrolysis conditions OH groups attach to the
Ti2+
Ti2+
O
Ti2+
Ti2+
O
chains. The solubility of the partially hydrolysed
O chains is lower than that of their precursors.
O
Consequently, these particles agglomerate to secondary and ter-
tiary particles of titania. The hydrolysis of TiOSO4 results in the
formation of white precipitate 3TiO2·4H2O according to the fol-
lowing equation:
ꢂ
ꢃ
Ti2+
O
Ti2+
O
n + 4nOH → [ Ti(OH)2
O Ti(OH)2 O ]n
→ 2/3n 3TiO · 4H O ↓ .
[
]
2
2
The primary 3TiO2·4H2O particles are produced in non-
equilibrium conditions (high oversaturation, quick hydrolysis) and
do not have a pronounced crystal shape. According to XRF measure-
ment the as-prepared TiO2 material did not contain any residual
sulfate groups, so their possible influence could be excluded from
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Table 1
4
Crystallite size, phase contenta of TiO2 nanoparticles calcined at different
temperatures.
Catalysts
Crystal size (nm)b
Anatase (1 0 1) plane
Rutile (1 1 0) plane
TiO2
14.8
14.8
20.7
17.2
51.6
68.4
203.8
TiO2 150
TiO2 250
TiO2 350
TiO2 450
TiO2 550
TiO2 900
629.4
a
Calcination in the temperature range of 100–550 ◦C resulted in materials in the
anatase form only, while calcination at 900 ◦C resulted in both anatase (68%) and
rutile (32%) phases. TiO2 P25 consists of anatase (87%) and rutile (13%).
b
The estimation of crystal size was carried out using a Scherrer equation.
Table 2
Textural properties of the TiO2 samples calcined at different temperatures deter-
mine by N2 adsorption-desorption isotherms at 77 K.
Catalysts
Specific surface
(m2/g)a
Pore volume
(cm3/g)b
Pore size (nm)b
Fig. 1. XRD patterns of (a) TiO2 and TiO2 calcined at (b) 150, (c) 250, (d) 350, (e) 450,
TiO2
216
189
187
175
120
78
1.4
77
179
652
0.438
0.416
0.298
0.297
0.301
0.209
–
0.136
0.188
0.151c
3
(f) 550 and (g) 900 ◦C.
TiO2 150
TiO2 250
TiO2 350
TiO2 450
TiO2 550
TiO2 900
TiO2 P25
Mg-Al HTC
H-BEA
4.3
4.8
6.6
9.6
12.4
–
further considering physico-chemical and catalytic properties of
titania samples.
The XRD patterns shown in Fig. 1 indicated that the as-prepared
powder (without further calcination) was already in the form of
anatase TiO2. The reflections at 25.57, 38.08, 48.02, 54.20 and 62.90
corresponded to the (1 0 1), (0 0 4), (2 0 0), (1 0 5) and (2 0 4) planes
of anatase (JCPDS file n◦. 21–1272). Upon calcination in the range of
300–550 ◦C (Fig. 1), the diffraction reflection intensities increased,
indicating enhanced crystallinity of the anatase phase. Phase trans-
formation from anatase to rutile occurred at 900 ◦C as shown in
Fig. 1. The reflections at 27.73, 36.36, 41.51, 55.36, 56.89 and 62.98
corresponded to the (1 1 0), (1 0 1), (1 1 1), (2 1 1), (2 2 0) and (0 0 2)
planes of rutile with JCPDS file n◦. 21–1276.
In most studies, TiO2 is synthesized by hydrolysis and condensa-
tion of titanium alkoxide precursors such as titanium isopropoxide
and titanium butoxide. Using these alkoxide precursors normally
results in amorphous TiO2 and thus subsequent calcination at ele-
vated temperature (300–500 ◦C) has to be carried out to obtain
anatase TiO2 [26,27]. In addition, the resulting anatase TiO2 is sus-
ceptible to rutile transformation at relatively low temperatures
(500–700 ◦C) [26–28]. On contrary, the XRD results in this study
indicated that pure anatase was already formed without further
calcination. In addition, the anatase-to-rutile transformation tem-
perature was observed only at 900 ◦C, indicating the high thermal
stability of the sample. The results suggested that thermal hydroly-
sis of aqueous TiOSO4 as presented in this study could be considered
as a simple synthesis of thermally stable anatase TiO2. The hydroly-
sis of TiOSO4 has previously been used to prepare TiO2 supported on
sintered glass by Hidalgo and Bahnemann [29]. Unfortunately, the
crystallinity and thermal phase transformation of the as-prepared
3.4
3
0.7c
a
BET nitrogen isotherm at 77 K.
BJH method adsorption branch.
Micropore volume and BEA framework pore size.
b
c
material has not been discussed. Sakthivel et al. [30]. synthesised
TiO2 via the forced hydrolysis of TiOSO4 at room temperature by
using base hydrolysing agents (NaOH, Na2CO3 and NaHCO3). How-
ever, the TiO2 obtained therein was not as thermally stable as the
one obtained in this work because 83 wt.% rutile was already found
at 800 ◦C, while only 32 wt.% of rutile phase was observed after cal-
cination at 900 ◦C in this study. All the other prepared TiO2 samples
were in the anatase phase.
The results in Fig. 1 indicate that with increasing calcination
temperature, the intensity of peaks increases and the diffraction
peaks become sharper and narrower. This suggests the enhance-
ment of the crystallinity which originated from the increment of the
crystalline volume ratio due to the size enlargement of the nuclei
[31]. The broad diffraction reflections observed for the TiO2 calcined
at low temperature indicate the presence of crystalline particles
with nanoscale size. In contrast, sharp diffraction peaks, indicating
large crystallite size, can be seen for the sample calcined at high
temperature. The crystallite size and phase composition of the TiO2
calcined at different temperatures are reported in Table 1. It can be
clearly seen that an increase in the calcination temperature from
Table 3
OH groups weight percentage of TiO2 treated at different temperature and TiO2—P25 determined by TGA.
Catalysts
Total weight loss, % (w/w)
Humidity-physical adsorbed water % (w/w)
OH weak % (w/w)
OH strong % (w/w)
OH total % (w/w)
TiO2
10.72
7.77
5.57
3.94
2.98
2.21
0.25
1.00
4.20
2.42
1.83
1.24
0.94
0.71
0.03
0.40
4.49
3.38
2.14
1.61
1.22
0.86
0.06
0.47
1.71
1.71
1.27
0.78
0.43
0.28
0.10
0.09
6.20
5.09
3.41
2.39
1.65
1.14
0.16
0.56
TiO2 150
TiO2 250
TiO2 350
TiO2 450
TiO2 550
TiO2 900
TiO2-P25
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Fig. 2. TEM micrographs of TiO2 (a) and TiO2 calcined at (b) 250, (c) 350, (d) 450, (e) 550 and (f) 900 ◦C.
100 to 550 ◦C resulted in larger anatase crystallite size from 14.8
to 68.4 nm. Raising the temperature further to 900 ◦C produced a
mixed phase of 68 wt.% anatase and 32 wt.% rutile with the anatase
and rutile crystallite sizes of 203.8 and 629.4 nm, respectively.
materials show lower pore volume and pore size than the synthe-
sized titania materials. Nevertheless, Beta zeolite has much larger
specific surface and smaller pore size due to its microporous frame-
work, what is expected to lead to significant differences in the
catalytic activity.
3.2. Textural properties
3.3. TEM images
Based on the analysis of the experimental data from N2
physisorption, the increase of average crystal sizes as the cal-
cination temperature increased was also accompanied with a
significant decrease of BET specific surface area. According to
Table 2, the specific surface area was drastically decreased from
216 m2/g (for TiO2) to 78 m2/g (for TiO2 550). Similar BET val-
ues and tendency to decrease with the growth of temperature
were observed for titania materials in [32,33]. The variation in BET
surface area was also accompanied by a reduction in pore vol-
ume values, which for the same samples decreased from 0.438
to 0.209 cm3/g (≈52%). These results may be correlated with the
above discussed increase in crystal sizes with increasing calcination
temperature, since according to literature [34–36], the crystallite
growth of the anatase phase, along with the sintering and grain
growth of the rutile phase, usually involve a reduction in the over-
all specific surface and pore volume. In contrast, pore size values
showed an increasing trend with calcination temperature, likely
associated with the transformation of anatase into rutile struc-
ture [36,37]. As soon as the anatase-to rutile transformation was
occurred at T = 900 ◦C, BET surface of final sample decreased to
1.4 m2/g with hardly determined pore volume value. Additionally,
the nitrogen adsorption–desorption isotherm of TiO2 samples cal-
cined at T ≤ 550 ◦C exhibited a noticeable adsorption at high relative
pressures (P/P0 ᮀ 0.8) which corresponded to interparticle voids
filling.
The size of the primary particles was dependent upon the calci-
nation temperature as observed from the TEM images (Fig. 2c and
d). The particle size was 14.8 nm at 100 ◦C (Fig. 2c) and increased to
51.6 nm at 450 ◦C (Fig. 2d) what is in accordance to results published
elsewhere [32,33] The particle size increased further to 203.8 and
629.4 nm for anatase and rutile, respectively, due to calcination at
900 ◦C. The results of TEM analysis of TiO2 samples are in agreement
with the XRD results.
3.4. FTIR
FTIR spectra of TiO2 samples calcined at different tempera-
tures are shown in Fig. 3. The bands observed in the range of
3100–3400 cm−1 can be attributed to the stretching vibrations of
structural hydroxyl groups, which is very similar to that observed
for the hydrotalcites [8]. The intensity of this band decreases with
the increase in calcination temperature from 100 to 550 ◦C until
its total disappearance at 900 ◦C (anatase and rutile phase mix-
ture). The bands observed at 1440 cm−1 can be associated with
the presence of OH groups related to the bound water or free
water adsorbed on the surface of TiO2 [12]. This band is present
only for the samples thermally treated at temperatures below
250 ◦C (Fig. 3). Also, in analogy with the hydrotalcites, the peak
at 1632 cm−1 could be attributed to water molecules which results
in a bending vibration in the TiO2 mesopores [8].
Compared to the textural properties showed by the commercial
TiO2, the synthesized samples possess a higher pore volume for any
calcination temperature, and a larger specific surface for calcination
temperatures below 250 ◦C. Likewise, an important improvement
is observed with respect to TiO2 P25 pore size (3.4 nm), increas-
ing especially for the samples calcined above 350 ◦C (6.6–12.4 nm).
Regarding the textural properties of Mg–Al HTC and H-BEA, both
The high hydration level of samples calcined at low tempera-
tures is probably also responsible for broad absorption bands in
the spectral range of 1000–1380 cm−1, assigned to the ␦(Ti
O H)
deformation vibrations [38]. Data from Fig. 3 clearly show that the
intensity of the peaks associated with the presence of hydroxyl
groups decreased (or the peaks even disappeared) when increas-
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1440 cm-1
100
98
96
94
92
90
88
3194 cm-1
1381cm-1
1119 cm-1
a)
1632 cm-1
TiO2 900
(g)
(f)
TiO2 550
TiO2 450
TiO2 350
(e)
TiO2 250
(d)
(c)
TiO2 150
TiO2 100
(b)
(a)
0
200
400
600
800
1000 1200
Temperature (oC)
4000 3500 3000 2500 2000 1500 1000
Wavelength (cm-1)
0.10
0,10
b)
TiO2 100
TiO2 150
TiO2 250
TiO2 350
TiO2 450
TiO2 550
Fig. 3. FTIR spectra of TiO2 P25 (a), TiO2 (b) and TiO2 calcined at (c) 250, (d) 350, (e)
0,08
0.08
450, (f) 550 and (g) 900 ◦C.
0,06
0.06
ing the temperature in the range of 100–900 ◦C, especially at 900 ◦C
there is no evidence of hydroxyl groups being present at all.
FTIR spectra of commercial TiO2 P25 shows the absence of the
band at 1440 cm−1 due to the presence of OH groups related to the
bound water what makes this material similar to calcined TiO2 sam-
ples prepared in the study. Nevertheless, TiO2 P25 contains both
structural hydroxyl groups and water molecules in the mesopores,
what is confirmed by the presence of IR bands at 3100–3400 and
1632 cm−1, respectively.
0,04
0.04
0,02
0.02
0.00
0,00
0
200
400
600
800
Temperature (oC)
Fig. 4. TGA/DTG curves of TiO2.
3.5. TGA/DTG
Table 4
Results from the CO2 and NH3 temperature programmed desorption of the studied
samples.
The number of surface OH groups can be determined by vari-
ous methods such as infrared spectroscopy [39–42], ion-exchange
reaction [41], titration procedures [42–45] and nuclear magnetic
resonance [46,47]. In addition, thermal gravimetric analysis is often
used to estimate the number of OH groups on the surface as it
is a simple and fast method compared to the other techniques
[40,43,47,48]. The TGA–DTG peaks and data obtained from the ther-
mal analysis studies of the TiO2 samples are presented in Fig. 4
and Table. 3. Total weight loss of the samples with anatase struc-
ture decreased from 10.72% to 0.25% as the calcination temperature
increases to 900 ◦C (Fig. 4a, Table 3). The course of the TGA curves
(Fig. 4a) exhibited a gradual weight loss process with four main
events that were reflected as peaks in the DTG curves (Fig. 4b).
According to Mueller et al [48]. the weight loss of hydroxylated TiO2
samples in the 30–120 ◦C range is due to the physically adsorbed
water. In line with this, for all TiO2 samples calcined at different
temperatures, the first step involved the loss of adsorbed water
that occurred between ambient temperature and about 75–120 ◦C
and decreased from 4.20 to 0.034% with the increase in calcination
temperature. The second weight loss was observed in the range
of 150–300 ◦C (Fig. 4b). It decreased from 4.49% for as-prepared
TiO2 sample to 0.86% for TiO2 500 and 0.06% for TiO2 900. Samples
thermally treated at T = 100–350 ◦C had also an additional weight
loss at ca. 450–550 ◦C, while another wide signal in the range of
750–809 ◦C was present on DTG curves, especially for the sam-
ples calcined at high temperature. Taking into account FTIR data
(Fig. 3), it can be concluded that DTG signals in the range of 150–350
and 450–550 ◦C are related to the dehydroxylation of TiO2 surface
and prove the presence of weakly/medium and strongly bonded
OH groups, respectively. On the other hand, when combined with
the XRD data, the TGA results indicate that the high temperature
signal above 700 ◦C on DTG curve can be attributed to the anatase
to rutile phase transformation.
Acid-basic properties
Catalysts
NH3 uptake
Tmax (◦C) CO2 uptake
Tmax (◦C)
(␮mol/g)
(␮mol/g)
TiO2 500
TiO2 P25
Mg-Al HTC
H-BEA
256
140
–
274
286
–
344
223
836
–
184
183
180
–
572
278
3.6. TPD of NH3 and CO2
Fig. 5 shows the CO2 (5a) and NH3 (5b) temperature pro-
grammed desorption (TPD) of the synthesized TiO2 material after
being calcined at 500 ◦C. In order to evaluate its acid-basic proper-
ties, TPD curves of the reference basic material, such as hydrotalcite,
and acid catalyst, like Beta zeolite, are also represented. Likewise,
this figure includes the TPD results obtained from the commercial
TiO2 P25. The overall measured amount and strength of acid/basic
sites of the samples are summarized in Table 4. It is worthy to note
that CO2/NH3 adsorption was performed after an outgassing treat-
ment at 500 ◦C under helium. Consequently, some artefacts may
occur altering the features of the samples with respect to the aldol
condensation conditions.
The reactive adsorption of CO2 over metal-oxide materials
(Fig. 5a), leads to the formation of carbonates species which are
desorbed at different temperatures [49–52]. Thus, bicarbonate
anions, whose formation is associated with the presence of sur-
face hydroxyl groups, desorb typically in the range 90–110 ◦C.
Hence, these species are usually attributed to the interaction of
CO2 with sites of weak basic strengths. The peaks at 190–230 ◦C
correspond to the desorption of the bidentate carbonates, formed
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Mg-Al HTC
H-BEA
b)
a)
TiO 500
TiO 500
TiO22 P25
TiO22 P25
0
100
200
300
400
500
600
700
800
900
150
200
250
300
350
400
450
500
Temperature (ºC)
Temperature (ºC)
Fig. 5. CO2 (a) and NH3 (b) temperature programmed desorption curves of synthesized TiO2 500 and the commercial catalysts: TiO2 P25, Mg–Al hydrotalcite (Mg–Al HTC)
and Beta zeolite (H-BEA).
90
30
20
10
0
TiO2
Mg-Al HTC
H-BEA
Mg-Al HTC-nc
TiO2- P25
b)
a)
80
70
60
50
40
30
20
10
0
FAc
FAc-OH
F2Ac
(FAc)2
0
50
100
150
200
250
nc
1
O 2
-
i
5
C
TC
T
BEA
-
P2
HT
Time, min.
Al H
g-
TiO 2
Al
H
g-
M
M
Fig. 6. Properties of different solids as catalysts for aldol condensation of furfural and acetone. (a). Furfural conversion. (b). Selectivity to reaction products at t = 240 min.
30
25
20
15
10
5
25
20
15
10
5
b)
a)
TiO
Mg-Al HTC
TiO
2
2
0
0
250
150
200
100
50
450
50
250
350
500
25
100
Temperature of thermal treatment, oC
Temperature of thermal treatment, oC
Fig. 7. Effect of thermal treatment on furfural conversion over TiO2 (a) and Mg–Al hydrotalcite (b) in aldol condensation of furfural and acetone. T = 50 ◦C, time of
experiment—240 min.
on metal–oxide pairs and related to basic sites of medium strength.
Likewise, monodentate species formed on low-coordination oxy-
gen anions appears around 320 ◦C and are related to the presence of
strong basic sites. On the other hand, according to previous works
[53,54], the CO2 desorbed above 500 ◦C arises from the remain-
ing carbonates that were not removed during the calcination and
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25
20
15
10
5
0
1
Fig. 8. Comparison of catalytic properties of TiO2 samples before and after calcination with.
Table 5
both H-BEA and titania materials have more than one kind of acid
sites which could differ in their strength. This observation is in
agreement with results from [58] where NH3 TPD signal for TiO2
was observed in the range of T = 100–500 ◦C. Additionally it was
suggested that the acid strength of TiO2 may be affected by the for-
mation of titania suboxides (coordinatively unsaturated Ti cations)
under the influence of elevated temperatures [55].
To summarize, the obtained TPD results reveal that the synthe-
sized titania sample (TiO2 500) possess a higher amount of both
base and acid sites than the commercial titania material, with a
similar basic/acid strength. This fact may is of importance for the
studied process as the number and accessibility of these active
sites determine the activity and selectivity of aldol condensation
reactions.
Effect of pre-rehydration with aqueous acetone mixture (10 wt.% of water) on the
properties of inorganic solids in aldol condensation of furfural and acetone.
Catalyst
Furfural Conversion, %
HTC-450
HTC-450 (H2O)
HTC-50
HTC-50 (H2O)
TiO2-50
TiO2-50 (H2O)
TiO2-500
23.7
98.9
9.8
0
24.7
0
3.3
4.9
TiO2-500 (H2O)
outgassing steps (500 ◦C). Thus, these residual carbonates were not
considered to be accounted for as basic sites (Table 4), since, they
were not derived from chemisorbed CO2, which was confirmed by
a blank test.
3.7. Catalysis
The total amount of basic sites determined by the integration of
the TPD curves for both titania samples were considerably lower
than the value showed by Mg–Al HTC (836 ␮mol g−1). The pro-
file of hydrotalcite CO2 TPD is in accordance with the well-known
3.7.1. Aldol condensation of furfural and acetone with different
samples
Fig. 6 depicts the results on the catalytic performance of differ-
ent catalysts in aldol condensation of furfural with acetone. In all
cases furfural conversion increased with the increase in experiment
duration (Fig. 6a) but it also showed an obvious dependence on the
type of material used. Hydrotalcites with different chemical com-
position are solids which are most often used as heterogeneous
catalysts for aldol condensation. Fig. 6a shows that furfural con-
version over uncalcined Mg–Al HTC was about 10% after 240 min
of the reaction. The calcination of this material at 450 ◦C resulted
in a significant increase in its activity in the reaction so that fur-
fural conversion reached 24%. According to previous studies, such
increase in activity in base-catalyzed reactions observed for cal-
cined Mg–Al materials is due to the decomposition of carbonate
groups during the heat treatment of the as-prepared samples and
formation of strong basic sites [15]. The presence of strong basic-
ity in calcined Mg–Al mixed oxides was confirmed by CO2-TPD
(see above). Previously we have shown [14] that zeolites can also
be used as acid catalysts for aldol condensation. Indeed, Fig. 6a
shows that furfural conversion in the presence of BEA zeolite is 12%
after 240 min of the reaction. Experiments performed with TiO2
samples as catalysts showed that catalytic performance of these
materials substantially depends on the method of their prepara-
tion. The industrially available commercial TiO2 exhibited a very
low activity in the reaction, furfural conversion being only 3% after
240 min. In contrast, TiO2 samples prepared in the present study
basic character of this material, and, equally, with the important
2−
presence of CO3
species acting as compensation anions in the
interlayer region (peak ≈550 ◦C). Titania samples exhibited sim-
ilar CO2 TPD profiles with three different desorption peaks at
90 ◦C, 185 ◦C and 290–320 ◦C, due to the presence of basic sites
of different strengths (bicarbonate, bidentate and monodentate
carbonates, respectively). In both cases, the maximum desorption
rate corresponded to the bidentate species (185 ◦C), indicating the
prevalence of intermediate-strength basic sites. Nevertheless, for
all the adsorbed carbonates the synthesized TiO2 500 showed a
higher contribution than the commercial TiO2 P25, reaching overall
values of basic sites of 344 and 223 ␮mol g−1, respectively (Table 4).
According to NH3 TPD method, H-BEA zeolite possessed the
highest acid character with the total concentration of acid sites
as high as 572 ␮mol g−1 (Fig. 5b and Table 4). Nevertheless, tita-
nia samples under the study also exhibited reasonable acidity.
Indeed, the presence of different acid sites in TiO2 was reported
repeatedly [55–58]. It was suggested that the Lewis and Brønsted
acid sites responsible for the catalytic activity of these materials
could be generated from adsorbed water molecules [57]. Among
two titania samples, the TiO2 500 uptakes a higher amount of
ammonia (256 ␮mol g−1) than the TiO2 P25 (140 ␮mol g−1), imply-
ing a greater presence of active acid sites [57] in the synthesized
material. The shape of NH3 TPD curves (Fig. 5b) also indicate that
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Table 6
Different treatments applied for calcined TiO2-500 sample.
N◦
1
Sample name
TiO2 500(WV)
Sample preparation
Catalytic procedure
Usual
Treatment with water vapors for 5 h, filtration
and drying at 100 ◦C for 12 h
2
3
4
5
6
TiO2 500(WV + UV)
TiO2 500(BW)
Treatment with water vapors for 5 h, filtration
UV irradiation of the catalyst for 1 h before the
catalytic experiment
Usual
and drying at 100 ◦C for 12 h
Treatment with boiling water for 5 h, filtration
and drying at 100 ◦C for 12 h
TiO2 500(BW + UV)
TiO2 500(USonic)
TiO2 500(USonic + UV)
Treatment with boiling water for 5 h, filtration
UV irradiation of the catalyst for 1 h before the
catalytic experiment
Usual
and drying at 100 ◦C for 12 h
Treatment in a sonicator for 3 h, filtration and
drying at 100 ◦C for 12 h
Treatment in a sonicator for 3 h, filtration and
UV irradiation of the catalyst for 1 h before the
catalytic experiment
drying at 100 ◦C for 12 h
possessed activity in the reaction which was comparable with that
observed for calcined Mg–Al sample. Fig. 6a shows that furfural
conversion over nanosized TiO2 reached 25% after 240 min. The pre-
sented results show that, depending upon the preparation method,
TiO2 material may have sufficient amount of active centers active
in aldol condensation furfural with acetone to compete with the
well-known materials used in the reaction. The composition of
the aldol condensation reaction products allowed us to assess the
importance of different active sites which are present in TiO2.
Selectivity to the main reaction products after 240 min of the
reaction are shown in Fig. 6b. FAc-OH was the most abundant com-
pound observed over as-prepared Mg–Al HTC. An increase in the
activity of this catalyst (which was achieved by its calcination)
decreased selectivity to FAc-OH and increased the selectivity to the
products formed either by its dehydration (yielding FAc) or interac-
tion with another furfural molecule (affording F2Ac). FAc-OH, as the
initial product of aldol condensation, was easily dehydrated in the
presence of strong acid centers of H-BEA to form FAc with selectiv-
ity exceeding 80%. According to [14,19], the presence (FAc)2, which
is formed by the dimerization of FAc, is a distinctive feature of aldol
condensation of furfural with acetone in the presence of acidic cat-
alysts. Characterization of the TiO2 materials by the TPD method
using NH3 and CO2 indicated that these solids exhibited ampho-
teric character, i.e. they possessed both basic and acidic sites. Thus,
the question arised which centers were responsible for the aldol
condensation activity of TiO2 materials.
Fig. 6b shows that (FAc)2 was absent in reaction products formed
in the presence of TiO2 catalysts regardless of their origin. Conse-
quently, it can be deduced that the reaction between furfural and
acetone to aldol condensation products over TiO2 occurred with the
participation of basic rather than acidic sites. However, the effect of
acidic centers on the composition of the reaction products cannot
be completely excluded. Fig. 6b shows that even for the commer-
cial TiO2 sample possessing low activity, the selectivity to FAc, i.e.
a dehydrated reaction product, was significantly higher in compar-
ison with the more active Mg–Al HTC. Moreover, in the case of the
more active TiO2, i.e. the nanosized TiO2, the selectivity to the main
dehydrated product, FAc, reached 72%. The strength of basic sites
in Mg-Al HTC is higher than of those in TiO2 (Fig. 6a), but the ability
of the former to promote dehydration reaction is lower at the same
conversion level. Therefore it can be proposed that the enhanced
selectivity to FAc observed for TiO2 is due to the presence of acid
sites. In summary, the catalytic results suggest that the properties
of the acid sites in TiO2 were not sufficient to promote aldol con-
densation or dimerization yielding (FAc)2, but were sufficient to
dehydrate FAc-OH to FAc.
material was investigated in more detail. Fig. 7 shows the effect of
calcination temperature on the furfural conversion over TiO2.
The increase in the temperature of thermal pretreatment of TiO2
resulted in a gradual decrease in its activity in aldol condensation.
Furfural conversion over the as-prepared TiO2 reached 27% after
240 min, but even a gentle drying at 100 ◦C decreased the furfural
conversion to 24%. Subsequently, furfural conversion decreased to
15% and as low as 3% after 240 min of the reaction when the TiO2
catalyst was pretreated (calcined) at T = 250 ◦C and 500 ◦C, respec-
tively. Considering the changes of physico-chemical characteristics
of the TiO2 sample in the dependence on its thermal pretreatment
allows understanding the reasons of the observed changes in the
catalytic performance.
Tables 1 and 2 provide evidence that as a result of thermal pre-
treatment of TiO2 sample in the range from room temperature to
350 ◦C, the crystal size of TiO2 gradually increased whereas the BET
surface area reduced. Moreover, the increase in the temperature of
the thermal pretreatment from room temperature to 350 ◦C caused
an increase in pore volume of the resulting samples. Therefore,
the observed reduction of the catalytic activity with the increas-
ing pretreatment temperature is unlikely to be solely associated
with the found changes in the textural properties of TiO2 materials.
On the other hand, TGA data show that with increasing calcination
temperature an irreversible decrease of weight loss due to dehydra-
tion and surface dehydroxylation were observed. Moreover, TGA
experiments show that both the amount of surface hydroxyl groups
which were removed at T ≥ 100 ◦C and the amount of physisorbed
water removed at lower temperatures decreased when the pre-
treatment temperature was subsequenlty increased. The results of
thermal analysis thus clearly showed that even a small increase
in the pretreatment temperature may influence the external sur-
face properties of TiO2, despite the catalysts have similar textural
chacracteristics. Data in Fig. 8 suggest that samples with the largest
amount of both physically adsorbed water and hydroxyl groups
which, according to TGA, are removed at T = 100–300 ◦C exhib-
ited the highest activity in aldol condensation. Combined with TPD
results, it can be concluded that basic sites in TiO2 catalyzing aldol
condensation consist of weak hydroxyl groups which are probably
formed at the defect sites of anatase nanocrystals. However, the
positive effect of physically adsorbed water on the catalytic activ-
ity of inorganic oxides in aldol condensation cannot be completely
excluded.
In comparative experiments, we have investigated the effect
of mild heat treatment on the catalytic activity of Mg–Al HTC in
aldol condensation. Fig. 7b depicts that the increase in the pretreat-
ment temperature from 50 to 150 ◦C decreased furfural conversion
from 10 down to 0.3%. In this temperature range only the removal
of physically adsorbed water from HTC was possible, so the cat-
alytic activity of the as-prepared Mg–Al HTC in its carbonate form
could be attributed to the presence of water physically bound to
the surface of HTC layered structure. The presented example of
the dependence of HTC properties on its thermal pretreatment is
3.7.2. The properties of thermally treated TiO2 and Mg–Al HTC
samples
Since nanosized TiO2 exhibited good activity in aldol conden-
sation of furfural with acetone, the catalytic performance of this
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aimed to illustrate that hydrated layered materials could possess
activity in basic catalyzed reactions even not being calcined. This is,
however, outside the topic of the present study and hence we will
discuss this research topic in our forthcoming publication. How-
ever, the comparison of catalytic results for uncalcined HTC and
titania materials show definite similarities in their behavior: their
activity in aldol condensation could be attributed to the presence of
water molecules either as adsorbed species in mesoporous space or
as terminal hydroxyls attached to the defective sites of a crystalline
framework.
the most successful, ensuring an increase in furfural conversion
from 3.3% for TiO2-500 to 5.3–6.5% for the treated catalysts after
240 min of the reaction. Nonetheless, compared with the uncal-
cined TiO2 catalyst the activity of the re-activated calcined TiO2
materials is significantly lower. Moreover, the results show that
the effect of UV irradiation is not apparent, i.e. the photocatalytic
properties of the TiO2 surface are clearly not important for the
studied reaction. So, despite the promising catalytic properties of
TiO2 materials as catalysts for aldol condensation of furfural and
acetone the question of their regeneration allowing at least main-
taining of their initial activity is still open to discussion and could
be considered as a challenge for future studies.
3.7.3. Re-activation of calcined TiO2
Taking into account above suggestion, either the excessive
hydroxylation or hydration of TiO2 surface seems to be of great
importance for the aldol condensation of furfural with acetone to
take place. As a result of thermal pretreatment, TiO2 samples lost
both surface water and hydroxyl groups and as a consequence their
activity in the reaction decreased. Such behavior of TiO2 catalysts
could have serious negative implications for practical applicabil-
ity of these materials as catalysts for aldol condensation of furfural
with acetone, as thermal regeneration of catalysts after the reaction
would inevitably result in a significant decrease in their activity in
following catalytic runs. Hence, the next step of the present study
was concerned with evaluating the possibility to rehydrate ther-
mally treated TiO2 materials. For these experiments, we have used
our own method developed during studies of the in situ rehydra-
tion of calcined Mg–Al mixed oxides. According to this method,
the calcined catalyst is contacted with acetone containing known
amount of water for a specific time, followed by furfural addition
to start aldol condensation reaction. Table 5 evidences that such
in situ rehydration method could be really effective, since the pre-
treatment of calcined Mg–Al mixed oxide with aqueous acetone
mixture containing 10% of water resulted in a dramatic increase in
the activity of rehydrated Mg–Al hydrotalcites in aldol condensa-
tion. The observed improvement in the catalytic activity is caused
by the transformation of Lewis basic sites which are present in cal-
cined Mg–Al material to Brønsted basic sites which possess higher
activity in different organic reactions in comparison with Lewis
basic sites [19,23].
4. Conclusions
In the present work, thermal hydrolysis of TiOSO4 aqueous solu-
tion has successfully been used to prepare nanocrystalline anatase
TiO2. The method provides an advantage over the hydrolysis of
titanium alkoxide precursors, as the crystalline anatase is read-
ily formed in a one-step process. The presence of rutile phase was
found at 900 ◦C in only 32 wt.% suggesting a higher thermal stability
of the nanosized TiO2 compared with that prepared from alkoxide
precursors.
The catalytic performance of the prepared TiO2 sample in aldol
condensation of furfural with acetone was evaluated and com-
pared with those for Mg–Al hydrotalcites and BEA zeolite. These
experiments showed that uncalcined TiO2 possesses good activity
which could be competitive with that reported for other inorganic
solids. The absence of (FAc)2 compound in the reaction prod-
ucts confirmed that the aldol condensation between aldehyde and
ketone over TiO2 proceeded with the participation of basic rather
than acidic sites. However, the presence of acidic sites could be
advantage for more facile dehydration of condensation products
as indicated by the comparison of the product distribution over
TiO2 and hydrotalcites. The calcination of TiO2 results, however, in
a decrease in its catalytic activity due to extensive dehydration and
surface dehydroxylation, as well as due to the change in textural
properties resulting in a decrease in the amount of accessible active
sites. The re-hydration/re-hydroxylation of calcined TiO2 sample by
different methods resulted in only a slight increase in the catalytic
activity.
However, the contact of Mg-Al HTC heated at T = 50 ◦C with the
same aqueous acetone mixture completely suppressed the catalytic
activity of the catalyst. A similar result was also obtained when
the same treatment procedure was used for TiO2 sample thermally
treated at T = 50 ◦C: the activity of this overhydrated sample in aldol
condensation was negligible. These results show that the nature of
active sites catalyzing aldol condensation in calcined Mg–Al mixed
oxide and in as-prepared Mg–Al HTC is different and that the active
sites in TiO2 which possess the activity in aldol condensation mostly
resemble those which are present in the as-prepared Mg–Al hydro-
talcites. In both cases the nature of such sites could be related to
either hydration or hydroxylation of the external surface of these
inorganic materials.
Acknowledgements
The authors acknowledge the Czech Science Foundation for
the financial support (Center of Excellence—P106/12/G015). The
project (P106/12/G015) is being carried out in the UniCRE cen-
ter (CZ.1.05/2.1.00/03.0071) in the Research Institute of Inorganic
Chemistry whose infrastructure was used. The authors also wish to
ˇ
thank to P. Rysˇánek, K. Cerná and P. Kovárˇ for XRD, FTIR measure-
ments and chemicals supply, respectively.
The unsuccessful attempts on the direct hydroxylation of
calcined TiO2 with aqueous acetone solution indicate that the
agglomeration of TiO2 nanoparticles due to calcination and the
related decrease in BET surface could be contributing factor to the
decrease in catalytic activity. To de-agglomerate the TiO2 particles
in the calcined TiO2 500 sample and re-hydroxylation its surface,
several methods were used (Table 6). In these experiments the
possible influence of UV irradiation on catalytic activity of the pre-
pared samples was investigated; such additional pre-treatment of
TiO2 materials would result in the further growth of their catalytic
activity provided that photocatalytic effect took place.
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with acetone in biomass upgrading, Catal. Today (2016), http://dx.doi.org/10.1016/j.cattod.2015.11.027