Comparative study of physico-chemical properties of laboratory and industrially prepared layered double hydroxides and their behavior in aldol condensation of furfural and acetone

Article abstract of DOI:10.1016/j.cattod.2014.03.010

In this article, properties of laboratory and industrially prepared Mg-Al layered double hydroxides (LDH) with Mg:Al molar ratios varied in the range from 2 to 4 were compared. Physico-chemical properties of the studied materials were investigated with XR

Full text of DOI:10.1016/j.cattod.2014.03.010

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Comparative study of physico-chemical properties of laboratory and
industrially prepared layered double hydroxides and their behavior in
aldol condensation of furfural and acetone
a
a
b
c
a,∗
ˇ
Lukásˇ Hora , Oleg Kikhtyanin , Libor Capek , Oleg Bortnovskiy , David Kubicˇka
a Research Institute of Inorganic Chemistry, UniCRE-RENTECH, Areál Chempark Záluzˇí, 436 70 Litvínov, Czech Republic
b Department of Physical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 57, 532 10 Pardubice, Czech Republic
c Eurosupport Manufacturing Czechia, Areál Chempark Záluzˇí, 436 70 Litvínov, Czech Republic
a r t i c l e i n f o
a b s t r a c t
Article history:
In this article, properties of laboratory and industrially prepared Mg–Al layered double hydroxides (LDH)
with Mg:Al molar ratios varied in the range from 2 to 4 were compared. Physico-chemical properties of
the studied materials were investigated with XRD, SEM, N2 physisorption, TGA and CO2-TPD methods.
Catalytic behavior of the LDH samples was examined in the liquid phase aldol condensation of furfural and
acetone at 20 and 60 ◦C. It was found that LDH from the small-scale preparation have different physico-
chemical characteristics than the LDH materials originating from the large-scale (industrial) preparation
which is also reflected in their catalytic behavior. As-synthesized laboratory LDH catalysts are phase-
pure crystalline samples with hydrotalcite structure while the industrial materials also possess MgO as
admixture. The laboratory samples possess larger BET surface area and mesopore volume in comparison
with the industrially prepared ones. Despite the similarity in basic properties of the studied materials, the
samples with molar ratio Mg:Al = 3 in both groups of LDH using different method of preparation possess
the best activity in aldol condensation. Generally, the calcined laboratory samples are more active than
the industrial materials in the reaction. This is explained by the better textural properties of the former
ones and their susceptibility to in-situ reconstruction in the presence of water formed during the reaction.
Received 24 October 2013
Received in revised form 2 March 2014
Accepted 5 March 2014
Available online xxx
Keywords:
Aldol condensation
Layered double hydroxides
Hydrotalcites
Furfural
Acetone
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
the connected adverse environmental impacts of the use of these
non-renewable resources [5,6]. Among them, aldol condensation
Layered double hydroxides (LDH) are formed by substitution
of atoms of the matrix crystalline framework on atoms of elements
with different valence. Among them, hydrotalcites (HTC) are mostly
known, which possess a brucite-like [Mg(OH)2] network wherein
isomorphous substitution of Mg2+ ion by a trivalent cation M3+
occurs and the excess positive charge is compensated by anions
which are located in the interlayer along with water molecules.
Consequently, they exhibit basic properties and could be used in
a number of reactions of organic synthesis, including C–C bond
formation, isomerization, cyclization, etc. [1–4].
is an important reaction as it allows obtaining compounds with
larger number of C atoms starting from simple organic molecules
[1,7,8] that are typical intermediates in biomass processing. More-
over, it makes possible the evaluation of the catalysts from the
viewpoint of the peculiarities of their physicochemical character-
istics. As a result, the attention is focused on the search of efficient
catalysts for aldol condensation due to the prospect of creating
new technologies for the synthesis of fine chemicals, pharma-
ceuticals, plasticizers and fragrances from renewable chemicals
[3,9,10]. Until now the industrial aldol condensation technologies
rely on the use of NaOH as a catalyst, which results in high opera-
tion costs and serious environmental problems [11,12]. Hence, the
development of basic heterogeneous catalysts, for example HTC,
for this field is essential. A lot of publications accessible in open
literature deal with different methods of preparation of HTC mate-
rials on laboratory scale [13–16], as well as with study of their
physico-chemical and catalytic properties [17,18]. Nevertheless,
Synthesis of fine chemicals from biomass by heterogeneously
catalyzed reactions is a recent topic of great importance partic-
ularly in the view of the finite reserves of fossil resources and
∗
Corresponding author. Tel.: +420 476163735; fax: +420 476768476.
E-mail address: david.kubicka@vuanch.cz (D. Kubicˇka).
http://dx.doi.org/10.1016/j.cattod.2014.03.010
0920-5861/© 2014 Elsevier B.V. All rights reserved.
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ally prepared layered double hydroxides and their behavior in aldol condensation of furfural and acetone, Catal. Today (2014),
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an objective assessment of the prospects of a new technology is
hardly possible without estimation of the possibility to move from
the laboratory synthesis of catalysts to the large-scale industrial
preparation. However, publications on comparing the properties
of laboratory and industrially prepared HTC samples in aldol con-
densation are missing. Therefore, the main objective of the present
study is to provide information about differences between lab-
oratory and industrially prepared hydrotalcites with respect to
their physico-chemical and catalytic properties. Both groups of the
materials were characterized by a number of methods (XRD, N2
physisorption, ICP, TGA, CO2-TPD and SEM). Their catalytic proper-
ties were compared in aldol condensation of furfural and acetone
which was chosen as a commonly used test reaction for evaluating
basic properties of solids.
2.2. Physico-chemical methods
The crystallographic structures of dried LDH 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 2ꢂ of 2.4◦/min. The size and shape of hydro-
talcite crystals were determined by scanning electron microscopy
(Jeol, JSM-5500LV). The elemental composition was determined
by ICP-OES equipment (HPST, Agilent 725). The textural proper-
ties of the catalysts (specific surface area and pore volume) were
measured by nitrogen physisorption at 77 K using a Micromeritics
TRISTAR 3000 surface area and porosity analyzer. Thermogravi-
metric analysis (TGA) of dried LDH catalysts were obtained using
a TA Instruments TGA Discovery series operating at heating ramp
10 ◦C/min from room temperature to 900 ◦C in flowing of nitrogen
(20 ml/min, Linde 5.0). Approximately 15 mg of sample was heated
in an open alumina crucible.
2. Experimental
Temperature programmed desorption of carbon dioxide as
probe molecule (TPD-CO2) was carried out on AutoChem equipped
with a TPD detector and mass spectrometry using quadrupole spec-
trometer (MS OmniStar). Approximately 100 mg of catalyst (grain
of 0.25–0.5 mm) was placed in quartz reactor. Before TPD exper-
iments the catalysts were outgassed at 450 ◦C for 4 h in a flow of
helium. Subsequently the catalysts were cooled down to 0 ◦C and
treated with a CO2 flow (99% purity) for 30 min. Weakly adsorbed
CO2 was removed by flushing with He at 0 ◦C for 30 min. The desorp-
tion of CO2 was measured by heating the catalyst from 0 to 900 ◦C at
a heating rate of 10 ◦C/min in He flow. The desorbed products were
analyzed by mass spectrometry. The number of basic sites was cal-
culated from the CO2 peaks (the molecular ion, m/z = 44) with help
of calibration using a known amount of CO2 desorbed from CaCO3
[19].
2.1. Preparation of the samples
2.1.1. Preparation of catalysts precursor
Industrially prepared Mg–Al hydrotalcites with Mg/Al molar
ratio ranging from 2:1 to 4:1 were provided by Eurosupport Man-
ufacturing Czechia (ESMC) as dried samples at temperature 150 ◦C.
They were denoted HTC Mg/Al molar ratio-I; e.g. HTC 3:1-I denotes
industrially prepared hydrotalcite sample having Mg/Al molar ratio
equal approximately to 3:1.
Laboratory prepared Mg–Al hydrotalcites with Mg/Al the same
molar ratio ranging from 2:1 to 4:1 were synthesized by co-
precipitation at constant pH value. They were denoted HTC Mg/Al
molar ratio-L; e.g. HTC 3:1-L denotes laboratory prepared hydrotal-
cite sample having Mg/Al molar ratio equal approximately to 3:1.
The preparation procedure involves mixing of an aqueous solution
of magnesium nitrate Mg(NO3)2·6H2O, p.a. (Lach-Ner) and alu-
minum nitrate Al(NO3)2·9H2O, p.a. (Lach-Ner), solution A, and a
basic solution of potassium carbonate K2CO3, p.a. (Lachema) and
potassium hydroxide KOH, p.a. (Lach-Ner), solution B. Solution A
contained always the same concentration of Mg(NO3)2 8.2 wt% and
the appropriate amount of Al(NO3)3 in order to obtain hydrotalcite
with the desired Mg/Al molar ratio. Solution B used as precipita-
tion agent was prepared by dissolving relevant amount of K2CO3
and KOH in order to obtain content 5.9 wt% of K2CO3 and 14.4 wt%
of KOH.
2.3. Reaction studies
The experiments were carried out in a 100 ml stirred batch reac-
tor (a glass flask reactor) at temperatures 20 and 50 ◦C. Before the
start of an experiment, 2 g of catalyst was mixed together with a
stirred mixture of 39.5 g acetone and 6.5 g furfural (acetone/furfural
molar ratio 10:1, dried with molecular sieve 3A, pre-heated to the
desired reaction temperature) and kept at this temperature for 6 h
under intensive stirring. Before performing the series of exper-
iments, it has been established that under the chosen reaction
conditions the reaction is limited neither by external nor internal
mass transfer by changing the stirring rate and catalyst particle size.
As synthesized and calcined catalyst agglomerates with particle
size less than 350 ␮m were used in the experiments. Samples were
withdrawn from the reactor during the experiment (one sample
per 1 h), filtered, and analyzed by Agilent 7890A gas chromato-
graph equipped with a flame ionization detector (FID), using a
HP 5 capillary column (30 m/0.32 mm ID/0.25 ␮m). Reaction path-
way of aldol condensation of furfural with acetone is shown in
Scheme 1.
In the synthesis procedure, 500 ml of demineralized water was
placed in a 4000 ml baker and heated to temperature 60 ◦C. This
temperature was stayed constant for the duration of the precip-
itation procedure. Solution A was dropped via membrane pump
(STEPDOS FEM 08) into the baker under vigorous stirring. Flow rate
of solution A was 10 ml per minute. At the same time, solution B
was added into the baker via membrane pump (KNF Liquidport NF
100 FT.18RC) connected with pH controller (OMEGA INFCPH PHCH-
37) which set up flow rate of solution B for constant pH value 9.5
maintaining.
After that, the obtained suspension was stirred at 60 ◦C for 1 h.
Then the solid LDH was separated by filtration, washed with dem-
ineralized water and dried at temperature 60 ◦C for 12 h.
Catalytic results of aldol condensation of furfural and acetone
have been described by conversion and selectivity parameters that
have been calculated as follows:
2.1.2. Catalyst activation
(reactantt=0–reactantt)
reactant conversion t mol% = 100 ×
( ) (
;
)
The following calcination procedure was applied to transform
the as supplied LDH materials into catalysts. As-prepared LDH
materials were calcined at 450 ◦C in air and mixed Mg–Al oxide
catalysts were obtained. The temperature during calcination was
raised at the rate of 5 ◦C/min to reach 450 ◦C and maintained for
16 h. The calcined mixed oxides were used as catalysts in aldol
condensation reaction directly.
reactantt=0
selectivity to product i = mole of reactant converted to product i
(
)
.
total number of mole of reactant converted
(
)
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3
OH
O
O
O
O
O
H3C
CH3
O
base
CH3
CH3
+
O
-H2O
Acetone
Furfural
(F)
4-(2-furyl)-3-buten-2-one
(FAc)
4-(2-furyl)-4-hydroxy-butan-2-one
(C8 alcohol)
(Ac)
O
O
O
O
O
O
base
-H2O
O
CH3
+
1,4-pentadien-3-one, 1,5-di-2-furanyl
(F2Ac)
Scheme 1. Mechanism of the condensation reactions of furfural and acetone.
The carbon balance calculated as the carbon atoms detected in
each sample (C3, C5, C8 and C13) divided by the initial carbon atoms
(C3 and C5 fed) has been preserved during all experiments:
indicate presence of more than one crystalline phase. Along with
the presence of XRD peaks corresponding to the HTC structure,
there are also reflexes from magnesium oxide and magnesium
hydroxide. It may be assumed that in the course of large scale
preparation an incomplete interaction of the initial reactants has
occurred. As a result the content of the desired HTC structure
decreased at the expense of the simultaneous formation of MgO
and Mg(OH)2 admixture phases. It is clearly seen that the intensity
of the characteristic peaks of the HTC structure is lower in the
industrial samples in comparison with for the laboratory ones.
Among the large-scale prepared materials, the intensity of the
peaks attributed to both HTC and MgO phases is the highest for
the HTC 2:1-I sample while the intensity of the peaks of Mg(OH)2
phase is the highest for HTC 4:1-I and HTC 3:1-I samples.
The XRD patterns of the calcined samples (Fig. 1B) exhibit
peaks at 2ꢃ ≈ 37◦, 43◦ and 62◦ which correspond to diffraction
by the (1 1 1), (2 0 0) and (2 2 0) planes of periclase MgO phase
[14]. The change of diffractograms reveals that the layered HTC
structure was completely transformed after the heat treatment
of the samples. It is believed that after the calcination the alu-
minium compounds should be well-dispersed in the MgO phase
or should form a separate amorphous phase [22]. It is also seen
that peaks reflections of MgO structure are higher and sharper for
(3 mol C3 + 5 mol C5 + 8 mol C8 + 13 mol C13)
C balance % =
(3 mol C3t=0 + 5 mol C5t=0
)
3. Results and discussion
3.1. XRD study
Fig. 1 depicts XRD patterns of the samples under study. Three
laboratory prepared samples, HTC 2:1-L, HTC 3:1-L and HTC 4:1-L
(Fig. 1A), possess the typical diffractograms of hydrotalcite-like
materials. The intensive peaks at 2ꢃ ≈ 11◦, 23◦ and 34◦ corre-
spond to the (0 0 3), (0 0 6) and (0 0 9) planes [14,20], indicating
a well-formed crystalline layered structure. A small shift of these
characteristic peaks of HTC structure toward low-angle region is
observed with the increasing Mg/Al molar ratio. This shift may
suggest the basal spacing expansion due to e.g. a difference in
nature of the intercalated anions which are presented in the
interlayer space [21]. Unlike the laboratory samples, the XRD
patterns of the materials prepared in an industrial large-scale basis
Fig. 1. XRD patterns of the laboratory and industrially prepared samples with different Mg/Al molar ratio. (A) Raw materials, (B) calcined materials.
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Table 1
Chemical composition of the laboratory and industrially prepared samples and crystallite size of the calcined materials.
Parameter/sample
Unit
HTC 2:1-I
HTC 3:1-I
HTC 4:1-I
HTC 2:1-L
HTC 3:1-L
HTC 4:1-L
Mg
Al
wt%
wt%
11.3
5.6
13.2
4.5
14.9
4.0
10.5
5.7
14.8
5.4
11.8
3.2
Si
Ca
Fe
K
Mg:Al
L200
wt%
wt%
wt%
wt%
mol/mol
nm
0.1
0.06
0.08
0.03
<0.02
3.28
15.5
0.05
0.11
<0.02
<0.02
4.14
21.4
0.04
0.02
<0.02
0.03
2.06
7.1
0.04
0.02
<0.02
0.02
3.03
7.1
<0.02
0.01
<0.02
0.08
4.09
7.1
0.12
0.05
<0.02
2.24
17.1
the calcined industrially prepared samples in comparison with the
laboratory ones. The crystallite size of the mixed oxides was esti-
mated from the plane (2 0 0) (2ꢃ ≈ 43◦) using the Debye–Scherrer
equation [23], and the calculated values are presented in
Table 1.
3.2. SEM
Fig. 2 shows SEM images of the prepared samples. It is seen that
the large-scale prepared samples represent large agglomerates of
10–15 ␮m, which consist of tightly compacted sheets of a layered
Fig. 2. SEM images of laboratory and large-scale prepared HTC samples with different Mg:Al molar ratio.
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Table 2
Textural properties of laboratory and industrially prepared LDH samples.
Sample
BET surface area (m2/g)
Pore volume* (cm3/g)
Pore volume1–4 nm (cm3/g)
Pore volume4–10 nm (cm3/g)
Pore volume10–50 nm (cm3/g)
HTC 2:1-I
HTC 3:1-I
HTC 4:1-I
HTC 2:1-L
HTC 3:1-L
HTC 4:1-L
167
164
159
247
213
200
0.41
0.34
0.33
1.18
1.08
0.96
0.09
0.11
0.1
0.03
0.02
0.05
0.16
0.13
0.14
0.07
0.10
0.10
0.15
0.08
0.07
0.96
0.91
0.74
*
BJH method.
material. At the same time, small particles of micron size also pos-
sessing a layered structure can be determined. These are likely to
be fragments of the large agglomerates. Laboratory samples also
represent large agglomerates up to 10 ␮m, which consist of small
shapeless particles of 1–2 ␮m in size. A more detailed analysis of
the images of the laboratory prepared samples reveals that these
small particles possess a well-developed layered structure which
is characteristic of the LDH materials [24].
the laboratory samples occasionally contain a detectable amount
of potassium.
3.4. Textural properties
Table 2 presents data on textural properties of the samples. The
obtained results show that in the two groups of differently pre-
pared samples both BET specific surface area (BET SSA) and the
total pore volume increase as Mg/Al molar ratio decreases. BET SSA
for the laboratory samples increases from 200 m2/g for HTC 4:1-L
to 247 m2/g for HTC 2:1-L while the total pore volume increases in
the same direction from 0.89 cm3/g to 1.06 cm3/g. BET SSA and the
total pore volume of the industrially prepared samples are lower
being in the range of 159–167 m2/g and 0.31–0.4 cm3/g, respec-
tively. The pore size distribution provides additional information
about the difference in the textural properties of the two groups of
3.3. Chemical analysis
Chemical composition of all the materials in the as-dried form
was determined by the ICP method. Results presented in Table 1
show that Mg/Al molar ratio in the samples is close to the claimed
content of these elements. The main impurities in the industrially
prepared samples include small amounts of Fe, Ca and Si, while
Fig. 3. TGA/DTG curves of the fresh laboratory and industrially prepared LDH catalysts.
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Table 3
Weight loss at different stages of heat treatment of the laboratory and industrially prepared LDH samples.
Sample
Temperature range (◦C)
Maximum of peak (◦C)
Weight loss (%)
HTC 2:1-L
20–220
220–500
≥500
116, 193
309, 379
650
106, 193
335, 390
–
130
375
530
160, 219
382
–
125
412
555
117
406
563
16.5
25.1
1.9
17.3
26.3
0
15.7
25.7
2.2
11.8
21.5
0
7.5
24.1
1.8
6.7
24.3
1.7
HTC 3:1-L
HTC 4:1-L
HTC 2:1-I
HTC 3:1-I
HTC 4:1-I
20–220
220–500
≥500
20–220
220–500
≥500
20–250
250–500
≥500
20–240
240–500
≥500
20–240
240–500
≥500
HTC samples (Table 2). For the industrial materials the pore size dis-
tribution in the range of 1–50 nm is quite uniform and amounts to
0.07–0.16 cm3/g. Apparently, a relatively large contribution of the
small pores of 1–4 nm in the industrially prepared samples could
be a result of close packing of the layers which make up the large
agglomerates, as evidenced by the SEM images (Fig. 2–left column).
In contrast, most of the pores in the laboratory samples are in the
range of 10–50 nm, which is a consequence of the specific morphol-
ogy of the hydrotalcite materials (Fig. 2—right column) [24].
3.5. Thermal decomposition of LDH materials
The process of the transformation of a hydrotalcite structure into
mixed oxides was investigated by thermal analysis of the samples.
TGA/DTG profiles of decomposition of the laboratory and industri-
ally prepared materials are presented in Fig. 3. All of the studied
samples show three general decomposition steps that are accom-
panied by weight loss. The first step occurs at 50–200 ◦C and is
ascribed to the loss of physically adsorbed and interlayer water
molecules [20,24]. The second step takes place in the range of
200–500 ◦C and corresponds to the loss of layer OH− groups and
decomposition of the interlayer carbonates [24]. Due to this stage
a transformation of the layered structure to the mixed oxide takes
place [25]. At temperatures above 500 ◦C, the DTG curves of sev-
eral samples contain an additional peak which could be possibly
attributed to removal of the residual interlayer species.
Fig. 4. CO2-TPD spectra of laboratory (HTC 2:1-L (1), HTC 3:1-L (2), HTC 4:1-L (3))
and industrially (HTC 2:1-I (4), HTC 3:1-I (5), HTC 4:1-I (6)) prepared samples.
temperature in the range of 375–412 ◦C. When comparing the
obtained results of the thermal analysis it may be concluded that
the difference between the samples originates not only from the
amount of the desorbed species but also from their nature.
Table 3 summarizes the weight losses observed at each step of
the thermal treatment of LDH samples. The amount of desorbed
water is higher for the laboratory samples in comparison with the
industrially prepared ones, 15.7–17.3% and 6.7–11.8%, respectively.
The observed difference in water amount may be attributed to dif-
ferent HTC content in the samples what is proved by XRD data
(Fig. 1A). The asymmetrical shape of the curve at this stage observed
for several samples could be explained by the co-existence of two
modes of the water adsorption, namely loosely surface-adsorbed
and bound interlayer water. The weight loss at the second step
amounts to 25.1–26.3% for the laboratory samples and 21.5–24.3%
for the industrial samples. It is seen that in the case of HTC 2:1-
L, and to a lesser degree, HTC 3:1-L, the signal in the range of
T = 300–400 ◦C is splitted to two different peaks. According to [14],
the loss of OH− groups is observed at lower temperatures than CO2
removal. So, the observed splitting of the signal at the second step
may be attributed to the dehydroxylation of the interlayer space
followed by decarbonation. On the other hand, the thermal decom-
position of the industrially prepared samples (Fig. 3) as well as
of the HTC 4:1-L sample exhibits only one dominant peak at the
3.6. CO2-TPD
The results of the investigation of basicity of the LDH samples by
means of CO2-TPD method are presented in Fig. 4 and Table 4. CO2-
TPD spectra of the samples indicate presence of at least two types
of basic sites located in low temperature (T = 20–500 ◦C) and high
temperature (T ≥ 500 ◦C) regions. It is known that the basic sites
can be classified according to their different strengths (i.e. different
Table 4
Concentration of basic sites in the samples.
Sample
CBS* 20–485 ◦C [␮mol/CO2/gcat
]
HTC 2:1-L
HTC 3:1-L
HTC 4:1-L
HTC 2:1-I
HTC 3:1-I
HTC 4:1-I
640
642
753
718
549
547
*
CBS—concentration of basic sites.
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carbon dioxide desorption temperature). In accordance with this
dependence peaks located in the two above-mentioned tempera-
ture regions indicate presence of weak/moderate and strong basic
sites, respectively. However, as pointed out in [26], information
obtained from CO2 desorbed at temperatures above the calcination
temperature of the solids should be carefully considered since
structural transformations could occur on the samples. Since the
samples of the present work were calcined at T = 450 ◦C, the high
temperature peak at T ≥ 500 ◦C in the CO2-TPD curve was excluded
from the discussion of the obtained results. CO2-TPD spectra
presented in Fig. 4 show that the shape of all the low-temperature
peaks in the range of 25–485 ◦C is essentially the same for different
samples and does not reveal any significant dependence neither
on the preparation method nor on the chemical composition of the
samples. Table 4 gives total amount of desorbed CO2 detected in
TPD-CO2 profiles of Mg–Al mixed oxides in the range of 25–485 ◦C.
The obtained results show that the highest concentration of
basic sites in the laboratory samples is observed for HTC 4:1-L
(753 ␮mol CO2/g), while this value for HTC 2:1-L and HTC 3:1-L is
approximately the same and amounts to 640–642 ␮mol CO2/g. For
industrially prepared samples a reverse tendency is observed. The
highest concentration of basic sites is observed for HTC 2:1-I with
the highest Al content (718 ␮mol/CO2/g), while for HTC 3:1-I and
HTC 4:1-I this value is 547–549 (␮mol CO2/g).
more active in furfural conversion than the industrially prepared
ones. To explain this result it is necessary to take into account in
addition to chemical composition also the collected data on the
physico-chemical characterization of the samples.
The XRD patterns of the as-prepared dried laboratory sam-
ples show the characteristic peaks corresponding to HTC structure.
After calcination this layered material is completely transformed
into highly disordered structure of periclase (MgO) as proved by
XRD. According to N2 physisorption this disordered mixed oxide,
independently on chemical composition, possesses high degree of
mesoporosity. Contrary to that, the industrially prepared samples
before calcination consist of a mixture of at least two crystalline
phases, HTC and MgO, but after calcination the XRD patterns
of these materials contain only peaks characteristic for highly-
crystalline MgO phase which is made of large crystals (Table 1). As
a consequence, these materials possess small mesopore volume, as
proved by N2 adsorption measurements (Table 2). Apparently, the
small dimensions of the particles and high mesopores volume are
very favorable for easy access of reactant molecules to the active
sites of the disordered calcined LDH, i.e. the mixed oxide. Indeed,
the improved textural properties of the laboratory samples show
good correlation with the high activity of these samples in aldol
condensation. So, generation of transport pores with dimensions
≥4 nm formed during the calcination process ensures improved
transport of both reactants and reaction products, which is essen-
tial for preparing highly active catalysts. Despite this consideration,
there is a disagreement between the textural characteristics of the
prepared materials and their catalytic properties. Fig. 5 shows that
activity of HTC 3:1-I sample is almost similar to that of HTC 4:1-L
sample. At the same time, the textural data presented in Table 2
prove that BET SSA and mesopores volume is significantly higher
for HTC 4:1-L when compared with HTC 3:1-I. It means that other
factor(s) that affect the catalytic behavior of the studied catalysts
need to be taken into account.
Among these factors basicity of the materials is an obvious
parameter that might affect significantly the catalytic behavior of
HTC materials in aldol condensation. It is known [27] that calcined
Mg–Al hydrotalcites can contain basic sites with pKa values up to
16.5. These sites act, in fact, as weak Lewis acid/strong Lewis base
catalysts due to the presence of O2− basic sites. Indeed, Fig. 4 and
Table 4 confirm the presence of basic sites in all the studied sam-
ples. Their concentration measured by CO2-TPD is in the range of
547–753 ␮mol/g. Additionally, it seems that their strength is also
approximately the same, as the temperature at the CO2 desorption
peak maximum as well as the shape of the signals are approxi-
mately the same and show neither dependence on the chemical
composition of the samples nor the method of their preparation.
By comparing the obtained CO2-TPD data and catalytic results it
is hardly possible to introduce a definite correlation between the
basic properties of the mixed oxides and their behavior in aldol
condensation. It may be concluded that in this case basicity of the
samples is an important but not a determining factor which could
unambiguously explain the obtained experimental results.
3.7. Catalytic experiments
Fig. 5 depicts catalytic behavior of the calcined laboratory and
industrially prepared samples. Experiments at two different reac-
tion temperatures, T = 20 ◦C and 50 ◦C, show that all the samples
possess good activity in aldol condensation of furfural and acetone
(Fig. 5A and B). Already after 1 h of the experiment furfural conver-
sion reaches 10–17% at T = 20 ◦C and 40–83% at T = 50 ◦C. After 6 h of
the reaction furfural conversion is 17–36% and 79–100% at T = 20◦ C
and T = 50◦ C, respectively. As a result of reaction between furfural
and acetone the formation of C8 alcohol, FAc and F2Ac takes place,
which is in full agreement with the commonly accepted scheme of
aldol condensation of F and Ac over basic catalysts (see Scheme 1).
The selectivity toward C8 alcohol (Fig. 5C and D), FAc (Fig. 5E and
F) and F2Ac (Fig. 5G and H) shows a dependence on both furfural
conversion and the type of the used catalyst. The compounds
formed due to self-condensation of acetone to diacetone alcohol
with its further transformation are also observed among the reac-
tion products, but here we exclude them from the consideration
as their overall concentration among all reaction products does
not exceed 8 wt% and furfural is not involved in their formation.
A steep increase of FAc selectivity at high F conversion (>80%)
is observed at Treac .= 50 ◦C (Fig. 5F). The comparison of FAc and
Fac-OH selectivities indicates that with the increase of the TOS
the ability of the catalyst to dehydrate the intermediate alcohol is
enhanced. It can assumed that consecutive changes in the catalytic
properties of the catalyst are the result of the partial reconstruction
of HTC structure due to the interaction of the catalyst with water
released during the reaction, as discussed below. Fig. 5 shows that
at 50 ◦C the overall selectivity to (FAc + FAc-OH) decreases gradu-
ally with the increasing TOS, while the selectivity to F2Ac increases
showing an almost linear dependency on the F conversion.
Investigating influence of basic properties of HTC materials on
their catalytic behavior in aldol condensation of furfural and ace-
tone it is necessary to consider conformity of the interlayer distance
in HTC materials with dimensions of reactant molecules. A distance
˚
In both groups of HTC catalysts, i.e. laboratory and industrially
prepared HTC catalysts, the samples with molar ratio Mg:Al = 3:1
are the most active in aldol condensation. However, there is not any
obvious dependence of the catalytic behavior of the samples on
their chemical composition. For the laboratory prepared samples
the conversion of furfural decreases in the order: HTC 3:1-L > HTC
2:1-L > HTC 4:1-L, whereas for the industrially prepared samples
it decreases in a different order: HTC 3:1-I > HTC 4:1-I > HTC 2:1-
I. Nevertheless, Fig. 5 shows that laboratory samples are clearly
between layers of HTC structure is 2.9 A [28], and kinetic diameter
˚
of CO2 is 3.9 A [29]. Taking into account a linearity of CO2 molecule
it could be assumed that determination of basic sites in the inter-
layer space of HTC with CO2-TPD is reasonable. However, molecular
˚
diameter of acetone is larger, 6.16 A [30], and it is hardly possi-
ble that this molecule could interact with interlayer active sites.
So, concentration of basic sites measured by CO2-TPD should be
higher than amount of active sites accessible to acetone molecules.
Consequently, one should be careful in finding correlation between
Please cite this article in press as: L. Hora, et al., Comparative study of physico-chemical properties of laboratory and industri-
ally prepared layered double hydroxides and their behavior in aldol condensation of furfural and acetone, Catal. Today (2014),
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Fig. 5. Catalytic properties of calcined LDH samples at T = 20 ◦C (left) and T = 50 ◦C (right). (A) and (B) Furfural conversion in dependence on reaction time. C–H-products
selectivity in dependence on furfural conversion ((C) and (D) FAc-OH, (E) and (F) FAc, (G) and (H) F2Ac).
basic properties of HTC materials and their catalytic properties in
reactions between large organic molecules.
to the experiments using a molecular sieve). Simultaneously, the
interaction of O2− basic sites present in the calcined material with
water formed due to aldol condensation results in the formation
of OH− Brønsted basic sites which provide higher activity to the
reconstructed LDH materials in aldol condensation in comparison
with the calcined ones [18,31]. Thus, the presented XRD results
prove that samples HTC 2:1-L and HTC 3:1-L are the most suscepti-
ble catalytic materials to the presence of water formed during the
reaction. As a result they exhibit higher catalytic activity in aldol
condensation than the other studied materials.
In order to obtain additional information to explain the obtained
catalytic results, we investigated the properties of the materials
after the reaction. It is seen (Fig. 6) that the XRD patterns of the
spent laboratory samples, HTC 2:1-L, HTC 2:1-L and HTC 4:1-L, con-
tain very small peaks characteristic to the layered HTC structure.
This observation indicates that during the reaction a partial recon-
struction of the highly disordered mixed oxide to HTC phase takes
place for these samples. As this can be achieved only by rehydra-
tion, the local reconstruction of the HTC structure is made possible
only by water released during the reaction which reacts with the
surface of the mixed oxide (as both raw materials were dried prior
At present it is difficult to reveal the underlying reason of the
observed discrepancy. The reason could be that the aldol conden-
sation reaction over HTC materials occurs with the participation of
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ally prepared layered double hydroxides and their behavior in aldol condensation of furfural and acetone, Catal. Today (2014),
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9
with CO2-TPD showed that their basic properties did not reveal
any dependence on the method of preparation. Besides, within
each group of the materials no correlation between basic proper-
ties and chemical composition was observed. Catalytic experiments
showed that all the samples exhibited good activity in aldol con-
densation of acetone and furfural. Catalysts with a molar ratio of 3:1
were the most active for both, laboratory and industrial, groups of
the materials. Nevertheless, when comparing the samples of the
same chemical composition, it was seen that laboratory samples
possessed better properties in aldol condensation. The observed
difference could be attributed to the increased mesoporosity of
the laboratory samples. It was also proved by XRD method that
in reaction conditions the calcined laboratory materials showed
an increased susceptibility to reconstruction of mixed oxide phase
back to hydrotalcite structure. As a consequence, this reconstruc-
tion resulted to a change in the structure of the active sites in
the samples what could be also a reason of increased activity
of the laboratory LDH materials. However, the received experi-
mental results have showed that other factors that influence the
catalytic properties of LDH materials may also exist, and elicita-
tion of such factors is considered as a task for the forthcoming
study.
Fig. 6. XRD patterns of the laboratory and industrially prepared spent catalysts with
different Mg:Al molar ratio.
Acknowledgments
Financial support from the Ministry of Industry and Trade of the
Czech Republic (project FR-TI3/327) and Czech Science Foundation
(P106/11/0773) is gratefully acknowledged. This publication was
created in connection with the project Unipetrol research and edu-
cation center Reg. CZ.1.05/2.1.00/03.0071, which is funded through
the Operational Programme for Research and Innovation Develop-
ment of the Structural Funds (specifically the European Regional
Development Fund) and the state budget of the Czech Repub-
lic.
those basic OH− sites present in the reconstructed HTC which are
readily accessible for the reactants, i.e. with the hydroxyl groups
located at the edges of the platelets. Apparently, under the reac-
tion conditions the water molecules interact only with the most
accessible O2− basic sites, i.e. with those located at the edges of the
platelets. This limited interaction between these sites and water
molecules may explain the observed low degree of reconstruction
of the calcined material. As pointed out in [27], the amount of such
accessible sites is connected with the presence of smaller and more
irregular platelets which exhibit a higher proportion of the edges.
Therefore, the possibility of formation of OH− ions is determined
by the degree of irregularity of the morphology of the materi-
als. Hence, the samples 2:1-L and HTC 3:1-L should possess an
increased degree of irregularity of the structure, which is indirectly
confirmed by the low intensity of the reflections in the XRD patterns
of the calcined laboratory prepared materials (Fig. 1). However,
similar effect of the partial re-hydration is not observed for the
industrially prepared samples, possibly due to decreased meso-
porosity and larger crystal size of these materials, thus limiting the
number of the sites available for interaction with water molecules.
The laboratory sample HTC 4:1-L exhibits worse catalytic behavior
than the other laboratory HTC sample which can be explained only
by performing additional experiments. A detailed study of the re-
hydration process in a dependence on the method of preparation
of the samples and their chemical composition will be the subject
of our forthcoming research.
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Please cite this article in press as: L. Hora, et al., Comparative study of physico-chemical properties of laboratory and industri-
ally prepared layered double hydroxides and their behavior in aldol condensation of furfural and acetone, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.03.010