The influence of long-term exposure of Mg–Al mixed oxide at ambient conditions on its transition to hydrotalcite: The long-term aging of Mg–Al mixed oxide

Article abstract of DOI:10.1016/j.jssc.2021.122556

The paper is focused on the study of long-term aging of Mg–Al mixed oxides, which causes the rebuilding of the hydrotalcite layered structure in the presence of humidity. The novelty consists in the description of influence during the long-term aging (6 m

Full text of DOI:10.1016/j.jssc.2021.122556

Journal of Solid State Chemistry 304 (2021) 122556
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Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
The influence of long-term exposure of Mg–Al mixed oxide at ambient
conditions on its transition to hydrotalcite
a
b,
*
a
ꢀ
ꢁ
ꢁ
ꢁ
ꢀ a
ꢀ a
Jaroslav Kocík , Martin Hajek , Zdenek Tisler , Katerina Strejcova , Romana Velvarska ,
ꢀ
ꢀ a
Monika Babelova
a
ꢀ
ꢀ
ꢁ
Research Department, ORLEN UniCRE, Inc., Areal Chempark 2838, 436 70, Litvínov-Zaluzí 1, Czech Republic
b
ꢀ
Department of Physical Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentska 573, 532 10, Pardubice, Czech Republic
A R T I C L E I N F O
A B S T R A C T
Keywords:
The paper is focused on the study of long-term aging of Mg–Al mixed oxides, which causes the rebuilding of the
hydrotalcite layered structure in the presence of humidity. The novelty consists in the description of influence
during the long-term aging (6 months) on the phase transition of mixed oxides to hydrotalcites at ambient
conditions. The structure throughout aging has been determined by several techniques, such as X-ray diffraction,
DRIFT and thermogravimetric analysis. The diffraction lines of mixed oxides were observed for 91 days of aging
(after that the lines of hydrotalcites started to occur). However, the specific surface area rapidly decreased after
only two aging days. The change was caused by an adsorption of water and especially of carbon dioxide from the
air (formed carbonates). After 166 days, the hydrotalcite structure was fully recovered. Moreover, all aged ma-
terial was applied as the heterogeneous catalyst for the aldol condensation with furfural as acid-base reaction. The
conversion decreased from 72% (immediately after the thermal treatment) to 27% after 4 aging days. The sta-
tistical evaluation of results was also carried out and relation between variables described.
Mg-Al hydrotalcites
Mg–Al mixed Oxides
Long-term aging
Aldol condensation
the spinel phase formation [7]. The hydrotalcites and especially mixed
oxides are broadly used in many applications: (i) adsorption [8], (ii) the
heterogeneous catalyst for acid-base reactions, such as transesterification
[9], etherification of glycerol [10] or oxidation of toluene [11] or CO
1. Introduction
The hydrotalcites (known as layered double hydroxides – LDH)
belong to group of anion clays with a layered structure which is derived
from brucite – Mg(OH)2 [1]. In the hydrotalcites structure, the part of
magnesium ions is substituted by trivalent cations and this cation layer
has positive chards, which is compensated by anions in the interlayer
space (anion layer) [2]. The compensated anions are mostly carbonates,
but other ions, such as chloride, nitrate adipic, etc., can be also used. The
[12], cycloaddition of nitrilimines [13], Henry reaction [14] or (iii) used
material with electronic [15,16] and microwave properties [17,18]. The
aldol condensation of furfural by acetone was chosen as acid-base het-
erogeneous catalyst reaction, because it consists of several consecutives
steps (Scheme 1), which are catalyzed by different active sites [19,20].
The reaction produces 4-(2-furyl)4-hydroxybuthane-2-on (FAc-OH),
which subsequently reacts to 4-(2-furyl)-3-buthene-2-on (FAc) and 1,4-
pentandiene-3-on-1,5-di-2-furanyl (F2Ac) [21]. The products could be
used after hydrogenation as a biofuel of the second generation [22].
The mixed oxides can be transformed back to the layered structure of
hydrotalcite [23] by (i) only hydration (so called memory effect [23,24])
or (ii) addition of water solution of Na2CO3, where the reconstruction
takes only 5 min [25]. The transformation is also possible by flow of
water vapor, but the final structure is different from transformation by
carbonated water [26]. The mixed oxides adsorb carbon dioxide, which
changes the structure back to hydrotalcites at high pressure (4.4 MPa)
[27]. Hanif et al. published the adsorption of carbon dioxide of mixed
Â
Ã
xþ
formula of hydrotalcite is M2þ
M
ðOHÞ2
A
ꢁ zH2O, where M rep-
3þ
nꢀ
1ꢀx
x
x=n
resents bivalent (Mg, Ca and Zn) or trivalent metallic cations (Al and Fe),
An- is an anion (inorganic or organic) and x is the stoichiometric coeffi-
cient of M3þ [3]. The type and molar ratio of metallic cation and the way
of synthesis influence the properties and structure of formed hydro-
talcite. The combination of different compounds can lead to new mate-
rials including the composite materials [4].
The mixed oxides as mesoporous materials are formed by a thermal
treatment (calcination) of hydrotalcites [5], where the Brønsted active
sites are transformed to Lewis active sites [6]. The temperature of
calcination depends on the type of metals and higher temperature caused
* Corresponding author.
ꢀ
E-mail address: martin.hajek@upce.cz (M. Hajek).
https://doi.org/10.1016/j.jssc.2021.122556
Received 23 July 2021; Received in revised form 30 August 2021; Accepted 31 August 2021
Available online 1 September 2021
0022-4596/© 2021 Elsevier Inc. All rights reserved.

J. Kocík et al.
Journal of Solid State Chemistry 304 (2021) 122556
65 ꢂC (hydrotalcite before calcination – HT0 was formed). The mixed
oxide was prepared by thermal heating (calcination) at 450 ꢂC
(5 ꢂC.minꢀ1) for 3 h in the air flow.
Nomenclature
a
cation–cation distance in the brucite-like layer (Å)
c
thickness of one brucite-like layer and one interlayer (Å)
2.2. Exposure of Mg–Al mixed oxides
SBET
k
m
t
CWP
DRIFT
specific surface area (m2 gꢀ1
)
rate coefficient (timeꢀ1
Avrami exponent (ꢀ)
aging time (day)
)
The formed mixed oxide was spontaneously aged (166 days) in the air
at the real laboratory conditions: temperature 20–23 ꢂC, relative hu-
midity 20–40% and the content of carbon dioxide 430–445 ppm (the
track during the aging time is depicted in Fig. S1 in the Supplementary
materials). After 166 days, the material was calcined again (HTc_2).
components weight plot
Diffuse Reflectance Infrared Fourier Transform
spectroscopy
F2Ac
FAc
1,4-pentandiene-3-on-1,5-di-2-furanyl
4-(2-furyl)-3-buthene-2-on
2.3. Characterization of materials
FAc-OH 4-(2-furyl)4-hydroxybuthane-2-on
X-ray diffraction (XRD): The crystallographic structure of the catalysts
was determined by examining the X-ray diffraction patterns of the
powder samples obtained using a D8 Advance ECO (Bruker) applying
HT0
hydrotalcite before calcination
HTc_2
calcined hydrotalcite after end of aging
TGA-MS thermogravimetric analysis with mass spectrometer
XRD X-ray diffraction patterns
CuK
α radiation (λ ¼ 1.5418 Å) with the help of JCPDS sheets. The step
size of 0.04ꢂ and a step time of 1 s were used. The patterns were collected
over the 2θ range from 5ꢂ to 70ꢂ and evaluated using the Diffrac. Eva
software with the Powder Diffraction File database (PDF-2 2002, Inter-
national Centre for Diffraction Data).
oxides with silica at temperature 300–400 ꢂC and pressure at 1–14 bar for
50 min (capacity of mixed oxides was filled) [28]. However, these
transformations are not spontaneous and usually takes minutes or hours.
The influence of long-term spontaneous exposition on air at laboratory
conditions was not described.
Thermogravimetric analysis with mass spectrometer (TGA-MS) of
dried materials was obtained using TA Instruments TGA Discovery series
(USA) operating at heating ramp 10 ꢂC/min from temperature of
40 ꢂC–900 ꢂC in flowing of nitrogen (20 ml minꢀ1, Linde 5.0). Approx-
imately 15 mg of the sample was heated in an open alumina crucible
The novelty of the present work consists of the explanation and
description of the long-term aging of Mg–Al mixed oxides to air exposi-
tion at laboratory conditions. This can have an impact on environmental
area because oxides are applied as catalysts for a biofuel preparation.
During aging, the change of the structure was determined by several
techniques (X-ray diffraction, thermogravimetric analysis, DRIFT).
Moreover, the specific surface area (SBET), the content of carbonates and
water were determined. The aged material was also used as a catalyst for
the aldol condensation and the influence of catalyst was activity
described. The results were statistically evaluated and the relationship
was described.
(70 μl). The mass spectrometer detection Pfeiffer Vacuum OmniStarTM
GSD 320 was used.
Elemental analysis: Carbon, hydrogen and nitrogen content in the
materials were ascertained by elemental analysis of the catalyst powder
using a Flash2000 (Thermo Scientific, USA). The sample (approximately
2.5 mg) was mixed with oxidant (V2O5) and burned in oxygen flow at
temperature 950 ꢂC. The resulting gas was reduced to CO (N2 and H2O),
separated with chromatic column and detected by the TCD detector.
The specific surface area of the catalysts was determined by N2
adsorption/desorption at ꢀ196 ꢂC using an Autosorb iQ (Quantachrome
Instruments, Boynton Beach, FL, USA). All samples were dried before the
analysis in a glass-cell at 110 ꢂC under vacuum for 16 h.
2. Materials and methods
Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFT)
of dried sample was obtained using Nicolet iS 10 (Thermo Scientific,
USA). The sample (15 mg) was mixed with KBr (300 mg) and measured
2.1. Preparation of Mg–Al mixed oxides
with following parameters: number of scans 128 and resolution 2 cmꢀ1
.
The co-precipitation method was used to synthesize the Mg–Al
hydrotalcite. The cation solution was prepared by dissolution of
Mg(NO3)2∙6H2O and Al(NO3)3∙9H2O in deionized water (molar ratio
Mg:Al ¼ 3:1). The total concentration of cations was 1 mol dmꢀ3. The
alkali solution was prepared by dissolution of KOH (2 mol dmꢀ3) and
K2CO3 (0.2 mol dmꢀ3) in deionized water. The synthesis was carried out
in the precipitation devices (Syrris Globe Atlas) with directed pH, tem-
perature and flow of solutions. The conditions durinꢀg1precipitation were:
60 ꢂC, pH ¼ 9.5, flow of cation solution 50 ml min and flow of alkali
solution 50 ml minꢀ1. After precipitation, the suspension of hydrotalcite
was filtered over the filter press and the filter cake was dried overnight at
2.4. Aldol condensation
The aldol condensation of furfural by acetone was carried out in a
batch jacket glass reactor (100 ml) at 50 ꢂC. The amount of 29.5 g of
acetone (dried with molecular sieve 3A) was mixed with 4.8 g of furfural
(molar ratio 10:1 acetone to furfural) and was heated to the reaction
temperature 50 ꢂC. Then, 0.75 g of material – catalyst (grain of
0.25–0.5 mm) was added to furfural and the reaction proceeded at 50 ꢂC
for 4 h. The conversion of furfural was determined throughout the re-
action time. After reaction time, the catalyst was filtered off from the
Scheme 1. Reaction scheme of aldol condensation of furfural.
2

J. Kocík et al.
Journal of Solid State Chemistry 304 (2021) 122556
Fig. 1. The diffractograms of material during aging time and dependency of degree of reconstruction (
α) on aging time for hydrotalcites (time 91–166 days).
reaction mixture and the products were analyzed by the gas chromato-
used for synthesis.
graph (Agilent 7890A with a flame ionization detector, USA) using a HP
5 capillary column (30 m/0.32 mm, ID/0.25
μm). The furfural (fur)
3.1.1. X-ray diffraction
conversion and selectivity to FAc-OH, FAc and F2Ac were determined by
equations:
The diffractogram of synthesized hydrotalcite before calcination
(HT0) contained typical lines (11.6, 23.2, 34.1, 38.2, 59.9 and 60.9ꢂ) for
the hydrotalcite structure (Fig. 1). After calcination (aging time 0), the
layer structure collapsed and changed to mixed oxides, which was proved
by two diffraction lines (43.0 and 62.5ꢂ) attributed to the crystallite of
MgO phase [30,31]. Through the aging 4–14 days, the lines for MgO
were no longer so noticeable, which can be associated with an adsorption
of carbon dioxide (see later paragraphs) and the formation of more
amorphous structure. The mixed oxide was stable for 91 days (typical
two lines were still presented). After 91 days of aging, the diffraction
lines of hydrotalcites started to occur (especially the most intensive line
at 11.6ꢂ) and lines for mixed oxides disappeared, i.e., the crystallisation
of HT begins. The relative humidity increased approximately from 20%
(40 aging day) to 40% (110 aging days), Fig. S1. However, the mixed
oxides were stable for 91 days, i.e. not only water was caused the change
to HT. The lines, typical for HT, increased with increasing aging time, i.e.,
the structure was more restored to HT. However, the lines after 166 aging
days were not as intensive as for pure hydrotalcites at the beginning. In
the end, the hydrotalcite was calcined again (HTc_2) and the mixed ox-
ides were formed (typical lines were presented at the diffractogram). Qin
at al. Studied the influence of the structure change on (i) the length of the
carbon chain as the material in the interlayer space (added during
co-precipitation) and (ii) aging time only 15 days at ambient conditions.
The change of the structure depends on the length of carbon chain and
almost no structure change was found with increasing aging time [32].
However, no other characterization of the material (depending on aging
time) was not published.
product
before
fur
n
ꢀ nfur
Xfur
¼
before
fur
n
nFAcꢀOH
ꢀ nfur
nFAc
2n
F2Ac
product
SFAcꢀOH
¼
SFAc
¼
SF
2Ac
¼
before
n
fur
product
product
before
fur
before
fur
n
n
ꢀ nfur
ꢀ nfur
where Xfur is the furfural conversion, nbfuerforeand nfpurroduct is the amount of
furfural in feedstock and in the product; Si is the selectivity to in-
termediates (FAc-OH and FAc) and main products (F2Ac) and nFAcꢀOH
,
n
FAc, nF Acare the amounts of the products. The amount of furfural, in-
2
termediates and the product were determined on the base of calibration
curves.
2.5. Statistical evaluation
The principal component analysis including the components weight
plot (CWP) was carried out to find the relations between the dependent
variables (Statistica 12). The explanation of the CWP: (i) variables that
are opposite to each other have a negative correlation, (ii) variables close
together have a positive correlation and (iii) right angle between vari-
ables means any correlation. The variable distance from the centre shows
how much the concrete variable contributes to individual main compo-
nents [29].
For hydrotalcites, the lattice parameters a and c were calculated on
the base of d003 and d110 values originated from the diffraction lines at
11.6 and 59.9ꢂ (a ¼ 2 d110, c ¼ 3 d003). The parameter a means cati-
on–cation distance in the brucite-like layer. The parameter c is a thick-
ness of one brucite-like layer and one interlayer, which contains water
and anions [33]. The parameter a was constant (3.054 ꢃ 0.001 Å) during
the aging time 119–166 days (HT was presented), i.e., the distance be-
tween cations did not depend on the aging time. The parameter c
increased from 22.77 Å (day 91) to 23.42 Å (day 166), because the
amount of carbonates and water increased and so increased the interlayer
distance.
3. Results and discussion
3.1. Aging of Mg–Al mixed oxides
Throughout aging time, the material was sampled and analyzed by
various techniques such as, XRD, TGA, diffuse reflectance infrared
Fourier transform spectroscopy (DRIFT). The SBET and the content of
metals, hydrogen and carbon were also determined.
The true molar ratio of Mg–Al was 3.28 ꢃ 0.07:1 for the whole aging
time (determined by inductively coupled plasma). The small differences
(ꢃ0.07) were caused by non-stoichiometric amount of water in nitrates
3

J. Kocík et al.
Journal of Solid State Chemistry 304 (2021) 122556
Fig. 2. The TGA of material during aging (A) and dependency of mass loss on aging time (B).
The crystallite size of mixed oxides was calculated from the diffrac-
tion line at 43ꢂ (0–152 aging days) and crystallite size of HT was calcu-
lated from 11.6ꢂ (91–166 aging days). For mixed oxides, the crystallite
size was approximately constant (47.4 ꢃ 1.2 Å) throughout the whole
aging time 0–91 days. After this time, it increased to 60 Å, because the
hydrotalcites started to form. For formed HT, the crystalline size linearly
increased with increasing aging time from 158 (95 day) to 217 Å (166
day). The results showed that the formed HT had a larger crystalline size
than the original HT0 (194 Å) – the HT was recrystallized with different
crystalline size. A decrease in crystallite size increases the contribution of
surface distortions and the electrical conductivity is significantly reduced
[34,35].
The calculated amount of crystalline phase for hydrotalcites before
calcination (HT0) was 79.4%, after calcination it decreased to 42% and
until the 112th day of aging this value remained approximately the same.
From the 112th day, the amount of crystalline phase gradually increased
to 79.9%, which was almost the same value as for HT0.
Therefore, the mixed oxide was stable approximately for 3 months
and then the restoration to hydrotalcite started, which took almost next 3
months at ambient conditions. The hydrotalcite was formed after the
166th day of aging with the following results: (i) the same amount of
crystalline phase as for HT0 was present, (ii) the lines were not so
intensive as for HT0 and (iii) the crystallite size was higher.
Fig. 3. The dependence of the specific surface area and content of water and
carbonates on aging time.
and could process in a different way.
3.1.2. Thermogravimetric analysis
The rate of crystallisation of reconstructed hydrotalcites was calcu-
lated in a time interval from 91 to 166 days of aging. The evolution of the
The aging of mixed oxides was also studied by the thermogravimetric
analysis (Fig. 2A). The TGA of hydrotalcite contained typical steps for a
double layer hydroxide material. The decomposition of HT usually takes
a place in two or three steps, which are characterized by different tem-
peratures [38]. At the first step (40–200 ꢂC), water absorbed on the
surface and in the anion layer of hydrotalcite was released [39]. At the
second step (200–450 ꢂC), the dehydroxylation and decarboxylation
proceeded, which was determined by the release of water and carbon
dioxide [40]. The last step (400–600 ꢂC), which is not always present,
was assigned to the decomposition of the remaining layer structure [31]
and formation of mixed oxides. The mass loss was calculated only in the
first and second step (Fig. 2B), because the third step was not present.
Immediately after calcination (aging time 0), the total mass loss was
2.5%, but only after two aging days the mass loss rapidly increased to
12% (i.e., increased almost 5 times). The reason is the adsorption of
carbon dioxide and water from the air. The mass loss increased from 3.3
to 4.6 wt% for carbonates and from 7.9 to 10.6 wt% for water (deter-
mined by elemental analysis, Fig. 3). The formation of carbonates was
confirmed by DRIFT (3.1.4). Water and carbonates were desorbed and
decomposed by heating at both steps during TGA. The increasing con-
centration of water and carbonates caused increasing of oxygen content
in the material. The difference of oxygen stoichiometry in oxides leads to
a strong change in electronic parameters (dc- and ac-resistivity and band
gap), and in particular, electric transport [41,42].
degree of reconstruction (
line at 11ꢂ, the height at the time was divided by the hight of HT0
(inserted figure in Fig. 1). The dependency of on the time was fitted
α) was calculated from the height of diffraction
α
using simple empirical nucleation growth model [36] (1):
m
α
¼ 1 ꢀ exp½ꢀðktÞ ꢄ
(1)
where
α
is the degree of reconstruction, k is the rate coefficient, t is the
(aging) time and m is the Avrami exponent. This equation has been
widely applied to model crystallisation and phase transformations in
solid–state chemistry, especially for in-situ XRD [37]. After linearization
of equation (1) (Fig. 1), the rate coefficient k ¼ 7.08⋅1016 dayꢀ1
(2.12⋅1055
range of
s
ꢀ1) and exponent m ¼ 0.13 (R2 ¼ 0.996) were calculated for a
α
from 0.15 to 0.6 (note: the range of α applicability is different
in different published papers). The results from calculation were very
different from the values in other papers, e.g., J. Ramírez at al. published
for 30 ꢂC k ¼ 2.26⋅10ꢀ5 sꢀ1 and the exponent m ¼ 1.39 [37] and A. Fogg
published for 65–90 ꢂC rate coefficient in the range of (1–4)⋅10ꢀ4 sꢀ1 and
exponent in the range of 0.9–1.8 [36]. In the both published papers, the
Mg–Al mixed oxides were rehydrated with water within several hours,
which is very short time in comparison with 75 days (1800 h) of for-
mation of hydrotalcites in this study. Different rehydration times could
be the reason for such different results (especially exponent m, which
influence the rate coefficient), because the crystallisation was very slow
The increase of mass loss in the first step was not as high as for the
4

J. Kocík et al.
Journal of Solid State Chemistry 304 (2021) 122556
Fig. 4. DRIFT of material during aging.
second step. This indicated an adsorption especially of carbon dioxide
(determined by mass spectrometer) and formation of carbonates, which
were decomposed in the second step. During aging time 30–91 days, only
a negligible change of mass was observed, so the material was saturated
by water and CO2. In the aging time 91–166, the mass loss rapidly
increased from 8 to 15.3% for the first step and from 10 to 22.0% for the
second step. This increase was caused by other adsorption of water and
CO2: water concentration increased from 15.3 to 32.5 wt% (more than
twice) and carbonates from 7.2 to 9.8 wt% (confirmed by elemental
analysis). The reason of change of adsorption speed was the formation of
hydrotalcites with the layer structure (confirmed by XRD), which
absorbed water and carbon dioxide faster. After 166 aging days, the total
mass loss was 37.3 wt%, which is typical value for hydrotalcites [9].
The water and carbon dioxide, releasing during TGA measurement,
were determined by mass spectrometer at the first 16 aging days (Fig. S2
in the Supplementary Material), because the catalyst activity changed
most significantly. Water was released at 100 ꢂC, which means that it was
absorbed only on the surface and no structure change (rebuilding of the
layer structure) proceeded. The carbon dioxide was released at two
temperatures: 120 and 300 ꢂC. However, only 10–18% from the total
amount of carbon dioxide leached at 120 ꢂC. The main part (82–90%)
was released at higher temperatures, which corresponds to the strong
acid sites [43].
The SBET of hydrotalcites and especially mixed oxides depends on the
type of metals and the way of synthesis, therefore the published SBET
values of hydrotalcites vary in the range of about 20–200 m2 gꢀ1 [9,44].
The correlations of SBET of mixed oxides with other parameters, such as
molar ratio of metal or calcination temperature, are not usually published
[45,46]. This could be caused by the fact that the determination of SBET is
not usually carried out immediately after calcination, but after some time
(not specified in publications).
3.1.4. DRIFT
The structure change of mixed oxides was determined by DRIFT
(Fig. 4). Several bands were detected: the broad band with a maximum at
3460 cmꢀ1 is attributed to hydroxyl group, 3050 and 1640 cmꢀ1 is
attributed to interlayer of molecule water [47]. The intensive band with
maximum at 1380 cmꢀ1 is usually attributed to the presence of carbon-
ates as compensating anions [48] to cations in the anion layer in the
reconstructed materials [25], i.e., the carbonates were formed from
carbon dioxide adsorbed from the air. For HT0, all bands were intensive,
because hydrotalcite contained water and carbonates as compensating
anions. All bands (for hydroxides, water and carbonates) decreased after
calcination, because of the dehydration and decarboxylation during
calcination. Only after 4 aging days, the bands of carbonate anions and
water molecules increased and remained approximately the same to the
91st aging day, which corresponded with the constant amount of carbon
and water in the material (determined by elemental analysis). After this
time, the bands rapidly increased because of adsorption of carbon dioxide
and water, which led to the formation of double layer of hydrotalcites. In
the end, the material was again calcined (HTc_2) and so the bands
decreased, because of the dehydration and decarboxylation.
3.1.3. Specific surface area
The SBET of formed mixed oxides rapidly decreased from 215 m2 gꢀ1
(aging time 0) to 120 m2 gꢀ1 only after 2 days of aging, i.e., decreased
about 45% (Fig. 3). This rapid decrease was related to an increase of
water and carbon dioxide (determined by elemental analysis – Fig. 3).
The SBET did not change from the 8th to 119th day of aging (the value of
SBET was approximately 95 m2
g
ꢀ1). However, after the 119th day of
3.2. Aldol condensation of furfural
aging, SBET gradually decreased to 55 m2
g
ꢀ1 in the 166th day of aging.
This decrease was caused by the adsorption of CO2 and water from the air
and also corresponded with the formation of hydrotalcites after the 119th
day of aging. In the end of aging (166th day), the water content was
approximately the same as for HT0, while the content of CO2 was higher.
After subsequent calcination (HTc_2), the SBET increased to
150 m2 gꢀ1 which was lower than after first calcination at the beginning
The basic properties were not possible to determine by the temper-
ature programme desorption of CO2, because CO2 and water were
already adsorbed on the surface. The solid material would have to be
heated to approximately 450 ꢂC, which would cause the desorption of
CO2 and water and so the change of the basic properties. Therefore, the
aldol condensation as a typical acid-base reaction (the aged mixed oxides
(218 m2 ꢀ1), probably due to different crystallite size.
g
were applied as
a
heterogeneous catalyst) was applied as
5

J. Kocík et al.
Journal of Solid State Chemistry 304 (2021) 122556
Fig. 5. The dependency of furfural conversion and selectivity of products (isoconversion data Xfur ¼ 18%) on the aging time.
“characterization” methods for basic properties.
The reaction mechanism consisted of several consecutive steps
(Scheme 1). The dependence of (i) the furfural conversion, (ii) the
selectivity of intermediates (FAc-OH and FAc) and (iii) the selectivity of
main product (F2Ac) on the aging time were determined (Fig. 5). Note:
the selectivity was calculated at constant furfural conversion (iso-
conversion), which had to be the same for all aging times – the highest
conversion was 18% for all aging times.
The furfural conversion rapidly decreased from 72 to 38% after two
aging days and subsequently decreased to 27% after another two days,
which corresponded with increasing carbonates content. Then, the con-
version gradually decreased to approximately 20% within 40 days of
aging and after this time stood approximately constant. The reason of the
rapid decrease was blocking of strong active sites by the adsorption of
carbon dioxide (formed of carbonates), which were not accessible for
aldol condensation, which proceeds on the strong basic sites (isolated
O2ꢀ) [49]. The adsorption of CO2 was confirmed by determination of
desorbed CO2 during TGA (end of section 3.1.2) for aging 0–16 days,
because the furfural conversion decreased most. It was found that the
CO2 was desorbed almost around 300 ꢂC only, which corresponds to the
strong basic sites and their amount increases with increasing aging time.
The conversion also decreased with decreasing SBET, because the reaction
proceeds also on the catalyst surface [43]. After subsequent calcination
(HTc_2), the conversion was almost the same as after first calcination
(aging time 0), but the SBET decreased about 30% (Fig. 3). This can be
explained by the fact that the activity depends on other parameters and
so the decrease of SBET has not significant influence.
Fig. 6. The components weight plot for properties of material during
aging time.
3.3. Statistical evaluation
The selectivity of intermediate alcohol (FAc-OH) increased, while the
selectivity of intermediate FAc and the main product F2Ac decreased
during aging time. The reason was water, which was the side product of
reactions form FAc and F2Ac (the second and the last reaction step (1)).
The increasing water amount within aging time in the material decrease
content of FAc and F2Ac in reaction mixture, because shifted the equi-
librium composition to reactants (FAc-OH). Moreover, Smolakova et al.
published, that (i) selectivity of FAc-OH was slightly correlated with
density of strong basic sites and (ii) F2Ac was highly positively correlated
with density of strong basic sites [43]. The content of basic sites
decreased within aging time due to the adsorption of CO2, which also
The CWP (Fig. 6) was used to determine the relations between the
dependent variables such as crystallite size, mass loss of the first and
second step at TGA, SBET, the conversion and selectivity of aldol
condensation (the explanation of plot is in section 2.4).
There were two groups with positive correlation: (i) furfural con-
version, selectivity of FAc, F2Ac and specific surface area and (ii) mass
loss 1 and 2, selection of FAc-OH and content of carbonates, which was
slightly positively correlated with water content. These two groups were
negatively correlated to each other. The crystallite size (calculated from
the line at 43ꢂ), was not correlated with other variables, i.e., it was not
influenced by content of carbonates and water. The exposition of mixed
oxides in the air at laboratory conditions caused the adsorption of water
decrease the SBET
.
6

J. Kocík et al.
Journal of Solid State Chemistry 304 (2021) 122556
[6] J. Kocik, K. Frolich, I. Perkova, J. Horacek, Pyroaurite-based Mg-Fe mixed oxides
and their activity in aldol condensation of furfural with acetone: effect of oxide
composition, J. Chem. Technol. Biotechnol. 94 (2) (2019) 435–445. https://
10.1002/jctb.5787.
[7] A. Kozlovskiy, K. Egizbek, M.V. Zdorovets, M. Ibragimova, A. Shumskaya,
A.A. Rogachev, Z.V. Ignatovich, K. Kadyrzhanov, Evaluation of the efficiency of
detection and capture of manganese in aqueous solutions of FeCeO(x)
Nanocomposites doped with Nb2O5, Sensors-Basel 20 (17) (2020).
[8] J. Xie, T. Yamaguchi, J.M. Oh, Synthesis of a mesoporous Mg-Al-mixed metal oxide
with P123 template for effective removal of Congo red via aggregation-driven
adsorption, J. Solid State Chem. 293 (2021).
and especially of carbon dioxide. It was statistically proved, that it led to
(i) an increase of mass loss at the first and second steps during TGA, (ii)
decrease of SBET and (iii) decrease of furfural conversion and selectivity
of FAc, F2Ac, because the CO2 blocked the active sites. On the other hand,
the selectivity of FAc-OH increased with the increasing amount of
adsorbed carbon dioxide. The decrease in conversion caused the decrease
in selectivity of FAc, F2Ac and the increase in the selectivity of FAc-OH.
4. Conclusions
[9] M. Hajek, J. Kocik, K. Frolich, A. Vavra, Mg-Fe mixed oxides and their rehydrated
mixed oxides as catalysts for transesterification, J. Clean. Prod. 161 (2017)
1423–1431. https://10.1016/j.jclepro.2017.05.199.
The spontaneous long-term aging of Mg–Al mixed oxide at laboratory
conditions (exposition to humidity and carbon dioxide) was described.
The change of the structure was determined by many methods such as X-
ray diffraction, thermogravimetric analysis, DRIFT and specific surface
area. The structure of mixed oxides was stable for 91 days, but the ac-
tivity as a catalyst for aldol condensation rapidly decreased only after 4
aging days. The reason was the adsorption of water and especially carbon
dioxide, which blocked the strong active sites and partly decreased the
specific surface area. After 91st day, the structure started to change to
hydrotalcites (determined by XRD), which proceeded to the end of aging
(166 days). The subsequent calcination formed the mixed oxides, but
with a different crystallite size. However, the conversion and selectivity
has recovered to approximately the same as for mixed oxides after first
calcination, i.e., the active sites were restored by calcination.
[10] P. Sangkhum, J. Yanamphorn, A. Wangriya, C. Ngamcharussrivichai, Ca-Mg-Al
ternary mixed oxides derived from layered double hydroxide for selective
etherification of glycerol to short-chain polyglycerols, Appl. Clay Sci. 173 (2019)
79–87. https://10.1016/j.clay.2019.03.006.
[11] C. Huang, X.Y. Gu, X.Y. Su, Z.C. Xu, R. Liu, H.J. Zhu, Controllable synthesis of Co-
MOF-74 catalysts and their application in catalytic oxidation of toluene, J. Solid
State Chem. 289 (2020), 121497, https://doi.org/10.1016/J.Jssc.2020.121497.
[12] A. Rastegarpanah, Y.X. Liu, J.G. Deng, L. Jing, W.B. Pei, X. Zhang, Z.Q. Hou,
M. Rezaei, H.X. Dai, Influence of preparation method on catalytic performance of
three-dimensionally ordered macroporous NiO-CuO for CO oxidation, J. Solid State
Chem. 297 (2021), 122091, https://doi.org/10.1016/J.Jssc.2021.122091.
[13] T.S. Saleh, K. Narasimharao, N.S. Ahmed, S.N. Basahel, S.A. Al-Thabaiti,
M. Mokhtar, Mg-Al hydrotalcite as an efficient catalyst for microwave assisted
regioselective 1,3-dipolar cycloaddition of nitrilimines with the enaminone
derivatives: a green protocol, J. Mol. Catal. Chem. 367 (2013) 12–22. https://10.10
16/j.molcata.2012.11.009.
[14] M.H. Abdellattif, M. Mokhtar, MgAl-layered double hydroxide solid base catalysts
for Henry reaction: a green protocol, Catalysts 8 (4) (2018), 133, https://doi.org/
10.3390/Catal8040133.
CRediT authorship contribution statement
[15] S.V. Trukhanov, A.V. Trukhanov, V.G. Kostishin, L.V. Panina, I.S. Kazakevich,
V.A. Turchenko, V.V. Oleinik, E.S. Yakovenko, L.Y. Matsui, Magnetic and absorbing
properties of M-type substituted hexaferrites BaFe12-x Ga (x) O-19 (0.1 < x < 1.2),
J. Exp. Theor. Phys. 123 (3) (2016) 461–469. https://10.1134/S1063776116
090089.
Jaroslav Kocík: Conceptualization, Visualization, Formal analysis.
ꢀ
Martin Hajek: Methodology, Writing – original draft, Writing – review &
[16] M.V. Zdorovets, A.L. Kozlovskiy, D.I. Shlimas, D.B. Borgekov, Phase transformations
in FeCo - Fe2CoO4/Co3O4-spinel nanostructures as a result of thermal annealing
and their practical application, J. Mater. Sci. Mater. Electron. 32 (12) (2021)
16694–16705. https://10.1007/s10854-021-06226-5.
ꢁ
ꢁ
ꢁ
ꢀ
editing. Zdenek Tisler: Investigation. Katerina Strejcova: Investigation.
ꢀ
ꢀ
ꢀ
Romana Velvarska: Investigation. Monika Babelova: Investigation.
[17] M.A. Almessiere, Y. Slimani, A.V. Trukhanov, A. Baykal, H. Gungunes,
E.L. Trukhanova, S.V. Trukhanov c, V.G. Kostishin, Strong correlation between Dy3
þ concentration, structure, magnetic and microwave properties of the
[Ni0.5Co0.5](DyxFe2-x)O4 nanosized ferrites, J. Ind. Eng. Chem. 90 (2020)
251–259, https://doi.org/10.1016/j.jiec.2020.07.020.
[18] A.V. Trukhanov, N.A. Algarou, Y. Slimani, M.A. Almessiere, A. Baykal,
D.I. Tishkevich, D.A. Vinnik, M.G. Vakhitov, D.S. Klygach, M.V. Silibin, T.I. Zubar,
S.V. Trukhanov, Peculiarities of the microwave properties of hard-soft functional
composites SrTb0.01Tm0.01Fe11.98O19-AFe(2)O(4)(A ¼ Co, Ni, Zn, Cu, or Mn),
RSC Adv. 10 (54) (2020) 32638–32651. https://10.1039/d0ra05087c.
[19] O. Kikhtyanin, P. Chlubna, T. Jindrova, D. Kubicka, Peculiar behavior of MWW
materials in aldol condensation of furfural and acetone, Dalton Trans. 43 (27)
(2014) 10628–10641. https://10.1039/c4dt00184b.
[20] R. Xing, A.V. Subrahmanyam, H. Olcay, W. Qi, G.P. van Walsum, H. Pendse,
G.W. Huber, Production of jet and diesel fuel range alkanes from waste
hemicellulose-derived aqueous solutions, Green Chem. 12 (11) (2010) 1933–1946.
https://10.1039/c0gc00263a.
[21] L. Hora, V. Kelbichova, O. Kikhtyanin, O. Bortnovskiy, D. Kubicka, Aldol
condensation of furfural and acetone over Mg-Al layered double hydroxides and
mixed oxides, Catal. Today 223 (2014) 138–147. https://10.1016/j.cattod.2013.0
9.022.
Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
Acknowledgements
This work was supported by the Czech Science Foundation, Project
No. 19-00669S. The result was achieved using the infrastructure included
in the project Efficient Use of Energy Resources Using Catalytic Processes
(LM2018119) which has been financially supported by MEYS within the
targeted support of large infrastructures.
Appendix A. Supplementary data
[22] D.M. Alonso, J.Q. Bond, J.A. Dumesic, Catalytic conversion of biomass to biofuels,
Green Chem. 12 (9) (2010) 1493–1513. https://10.1039/c004654j.
[23] G. Mascolo, M.C. Mascolo, On the synthesis of layered double hydroxides (LDHs) by
reconstruction method based on the "memory effect, Microporous Mesoporous
Mater. 214 (2015) 246–248. https://10.1016/j.micromeso.2015.03.024.
[24] K.L. Erickson, T.E. Bostrom, R.L. Frost, A study of structural memory effects in
synthetic hydrotalcites using environmental SEM, Mater. Lett. 59 (2–3) (2005)
226–229. https://10.1016/j.matlet.2004.08.035.
[25] F. Delorme, A. Seron, M. Bizi, V. Jean-Prost, D. Martineau, Effect of time on the
reconstruction of the Mg4Al2(OH)(12)CO3 center dot 3H(2)O layered double
hydroxide in a Na2CO3 solution, J. Mater. Sci. 41 (15) (2006) 4876–4882. http
s://10.1007/s10853-006-0304-4.
[26] M. Mokhtar, A. Inayat, J. Ofili, W. Schwieger, Thermal decomposition, gas phase
hydration and liquid phase reconstruction in the system Mg/Al hydrotalcite/mixed
oxide: a comparative study, Appl. Clay Sci. 50 (2) (2010) 176–181. https://10.1016
/j.clay.2010.07.019.
[27] S.I. Garces-Polo, J. Villarroel-Rocha, K. Sapag, S.A. Korili, A. Gil, Adsorption of CO2
on mixed oxides derived from hydrotalcites at several temperatures and high
pressures, Chem. Eng. J. 332 (2018) 24–32. https://10.1016/j.cej.2017.09.056.
[28] A. Hanif, S. Dasgupta, A. Nanoti, High temperature CO2 adsorption by mesoporous
silica supported magnesium aluminum mixed oxide, Chem. Eng. J. 280 (2015)
703–710. https://10.1016/j.cej.2015.06.018.
Supplementary data to this article can be found online at https://do
i.org/10.1016/j.jssc.2021.122556.
References
[1] I. Pausch, H.-H. Lohse, K. Schürmann, R. Allmann, Syntheses of disordered and Al-
rich hydrotalcite-like compounds, Clay Clay Miner. 34 (1986) 507–510.
[2] Z.P. Xu, H.C. Zeng, Interconversion of brucite-like and hydrotalcite-like phases in
cobalt hydroxide compounds, Chem. Mater. 11 (1) (1999) 67–74, https://doi.org/
10.1021/Cm980420b.
[3] G.W. Brindley, Lattice-parameters and composition limits of mixed Mg, Al hydroxy
structures, Mineral. Mag. 43 (332) (1980), https://doi.org/10.1180/
minmag.1980.043.332.14, 1047-1047.
[4] P.J. Wang, D. Zhou, J. Li, L.X. Pang, W.F. Liu, J.Z. Su, C. Singh, S. Trukhanov,
A. Trukhanov, Significantly enhanced electrostatic energy storage performance of P
(VDF-HFP)/BaTiO3-Bi(Li0.5Nb0.5)O-3 nanocomposites, Nanomater. Energy 78
(2020).
[5] M.C.D. Mourad, M. Mokhtar, M.G. Tucker, E.R. Barney, R.I. Smith, A.O. Alyoubi,
S.N. Basahel, M.S.P. Shaffer, N.T. Skipper, Activation and local structural stability
during the thermal decomposition of Mg/Al-hydrotalcite by total neutron
scattering, J. Mater. Chem. 21 (39) (2011) 15479–15485. https://10.1039/c
1jm11530h.
7

J. Kocík et al.
Journal of Solid State Chemistry 304 (2021) 122556
[29] I.T. Jolliffe, J. Cadima, Principal component analysis: a review and recent
developments, Philos T R Soc A 374 (2065) (2016), 20150202, https://doi.org/
10.1098/Rsta.2015.0202.
[30] M. Hajek, P. Kutalek, L. Smolakova, I. Troppova, L. Capek, D. Kubicka, J. Kocik,
D.N. Thanh, Transesterification of rapeseed oil by Mg-Al mixed oxides with various
Mg/Al molar ratio, Chem. Eng. J. 263 (2015) 160–167. https://10.1016/j.cej.201
4.11.006.
[31] L. Hora, O. Kikhtyanin, L. Capek, O. Bortnovskiy, D. Kubicka, Comparative study of
physico-chemical properties of laboratory and industrially prepared layered double
hydroxides and their behavior in aldol condensation of furfural and acetone, Catal.
Today 241 (2015) 221–230. https://10.1016/j.cattod.2014.03.010.
[32] Q.Q. Qin, J.Y. Wang, T.T. Zhou, Q.W. Zheng, L. Huang, Y. Zhang, P. Lu, A. Umar,
B. Louis, Q. Wang, Impact of organic interlayer anions on the CO2 adsorption
performance of Mg-Al layered double hydroxides derived mixed oxides, J Energy
Chem 26 (3) (2017) 346–353. https://10.1016/j.jechem.2017.01.003.
[33] M.G. Alvarez, R.J. Chimentao, F. Figueras, F. Medina, Tunable basic and textural
properties of hydrotalcite derived materials for transesterification of glycerol, Appl.
Clay Sci. 58 (2012) 16–24. https://10.1016/j.clay.2012.02.004.
[34] S.V. Trukhanov, A.V. Trukhanov, C.E. Botez, A.H. Adair, H. Szymczak, R. Szymczak,
Phase separation and size effects in Pr0.70Ba0.30MnO3þdelta perovskite
manganites, J Phys-Condens Mat 19 (26) (2007).
[40] W.S. Yang, Y. Kim, P.K.T. Liu, M. Sahimi, T.T. Tsotsis, A study by in situ techniques
of the thermal evolution of the structure of a Mg-Al-CO3 layered double hydroxide,
Chem. Eng. Sci. 57 (15) (2002) 2945–2953, https://doi.org/10.1016/S0009-
2509(02)00185-9.
[41] I.O. Troyanchuk, S.V. Trukhanov, H. Szymczak, K. Baerner, Effect of oxygen content
on the magnetic and transport properties of Pr0.5Ba0.5MnO3-gamma, J Phys-
Condens Mat 12 (7) (2000) L155–L158, https://doi.org/10.1088/0953-8984/12/
7/103.
[42] D.I. Shlimas, A.L. Kozlovskiy, M.V. Zdorovets, Study of the formation effect of the
cubic phase of LiTiO2 on the structural, optical, and mechanical properties of
Li2ꢃxTi1ꢃxO3 ceramics with different contents of the X component, J. Mater. Sci.
Mater. Electron. 32 (6) (2021) 7410–7422. https://10.1007/s10854-021-05454-z.
[43] L. Smolakova, K. Frolich, J. Kocik, O. Kikhtyanin, L. Capek, Surface properties of
hydrotalcite-based Zn(Mg)Al oxides and their catalytic activity in aldol
condensation of furfural with acetone, Ind. Eng. Chem. Res. 56 (16) (2017)
4638–4648. https://10.1021/acs.iecr.6b04927.
[44] J. Wang, M. Feng, H. Zhang, X. Xu, Catalytic decomposition of N2O over Mg-Fe
mixed oxides, J. Fuel Chem. Technol. 42 (2014).
[45] D.P. Debecker, V. Hulea, P.H. Mutin, Mesoporous mixed oxide catalysts via non-
hydrolytic sol-gel: a review, Appl Catal a-Gen 451 (2013) 192–206. https://10.10
16/j.apcata.2012.11.002.
[35] I.Z. Zhumatayeva, I.E. Kenzhina, A.L. Kozlovskiy, M.V. Zdorovets, The study of the
prospects for the use of Li0.15Sr0.85TiO3 ceramics, J. Mater. Sci. Mater. Electron.
31 (9) (2020) 6764–6772. https://10.1007/s10854-020-03234-9.
[36] A.M. Fogg, S.J. Price, R.J. Francis, S. O'Brien, D. O'Hare, Determination of the
kinetics of crystallisation of gibbsite using time resolved in situ energy dispersive
powder X-ray diffraction, J. Mater. Chem. 10 (10) (2000) 2355–2357, https://
doi.org/10.1039/B005198p.
[46] M. Hajek, A. Tomasova, J. Kocik, V. Podzemna, Statistical evaluation of the mutual
relations of properties of Mg/Fe hydrotalcites and mixed oxides as
transesterification catalysts, Appl. Clay Sci. 154 (2018) 28–35. https://10.1016
/j.clay.2017.12.039.
[47] F. Cavani, F. Trifiro, A. Vaccari, Hydrotalcite-type Anionic clays: preparation,
properties and applications, catal, Today Off. 11 (2) (1991) 173–301, https://
doi.org/10.1016/0920-5861(91)80068-K.
[37] J. Perez-Ramirez, S. Abello, N.M. van der Pers, Memory effect of activated Mg-Al
hydrotalcite: in situ XRD studies during decomposition and gas-phase
reconstruction, Chem. Eur J. 13 (3) (2007) 870–878. https://10.1002/chem.
200600767.
[48] Z.Y. Sun, L. Jin, S. He, Y.F. Zhao, M. Wei, D.G. Evans, X. Duan, A structured catalyst
based on cobalt phthalocyanine/calcined Mg-Al hydrotalcite film for the oxidation
of mercaptan, Green Chem. 14 (7) (2012) 1909–1916. https://10.1039/c2gc
16462k.
[38] S. Miyata, Physicochemical properties of synthetic hydrotalcites in relation to
composition, Clay Clay Miner. 28 (1) (1980) 50–56, https://doi.org/10.1346/
Ccmn.1980.0280107.
[49] V.K. Diez, C.R. Apesteguia, J.I. Di Cosimo, Aldol condensation of citral with acetone
on MgO and alkali-promoted MgO catalysts, J. Catal. 240 (2) (2006) 235–244. https
://10.1016/j.jcat.2006.04.003.
[39] V.V. Dabholkar, K.S. Badhe, S.K. Kurade, Calcined Mg/Fe hydrotalcite catalysed
synthesis of pyrano-pyrazoles at ambient temperature, Int J Res Eng Appl Sci 6
(2016) 176–186.
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