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Citation: Misol, A.; Jiménez, A.;
Labajos, F.M. Use of Ethylamine,
Diethylamine and Triethylamine in
the Synthesis of Zn,Al Layered
Double Hydroxides. ChemEngineering
2022,6, 53. https://doi.org/10.3390/
chemengineering6040053
Academic Editor: Dmitry Yu. Murzin
Received: 3 June 2022
Accepted: 4 July 2022
Published: 6 July 2022
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chemengineering
Article
Use of Ethylamine, Diethylamine and Triethylamine in the
Synthesis of Zn,Al Layered Double Hydroxides
Alexander Misol , Alejandro Jiménez and Francisco M. Labajos *
GIR-QUESCAT, Departamento de Química Inorgánica, Facultad de Ciencias Químicas,
Universidad de Salamanca, 37008 Salamanca, Spain; alex_aspa6@usal.es (A.M.); alejm@usal.es (A.J.)
*Correspondence: labajos@usal.es
Abstract:
Amines with two carbon atoms in the organic chain [ethylamine (EA), diethylamine (DEA),
triethylamine (TEA)] have been used as precipitant agents to obtain a hydrotalcite-like compound
with Zn (II) and Al (III) as layered cations and with nitrate anions in the interlayered region to balance
the charge. This Layered Double Hydroxide was prepared following the coprecipitation method, and
the effect on the crystal and particle sizes was studied. Also, the effect of submitting the obtained
solids to hydrothermal post-synthesis treatment by conventional heating and microwave assisted
heating were studied. The obtained solids were exhaustively characterized using several instru-
mental techniques, such as X-ray diffraction, Thermal Analysis (DTA and TG), Chemical Analysis,
Infrared Spectroscopy (FT-IR), determination of Particle Size Distribution and BET-Surface area. Well
crystallized solids were obtained showing two possible LDH phases, depending on the orientation of
the interlayer anion with respect to the brucite-like layers. The results indicated that there is a certain
influence of the amine, when used as a precipitating agent, and as a consequence of the degree of
substitution, on the crystallinity and particle size of the final solid obtained. The LDHs obtained
using TEA exhibited higher crystallinity, which was improved after a long hydrothermal treatment by
conventional heating. Regarding the shape of the particles, the formation of aggregates in the former
solid was detected, which could be easily disintegrated using ultrasound treatments, producing solid
powder with high crystallinity and small particle size, with homogeneous size distribution.
Keywords: LDHs; hydrotalcite; amines; coprecipitation method; hydrothermal treatment; crystallinity
1. Introduction
The term Layered Double Hydroxides (LDHs) is used to name synthetic or natural
hydroxides with a layered structure and with at least two types of metal cations in the main
layers, which are positively charged, and contain anionic species in the interlayer space.
This large family of compounds is also called anionic clays, in comparison with cationic
clays, which, in their interlayer region, contain cations to balance the negative charge of the
layers [
1
]. They are also known as hydrotalcite-like compounds, because hydrotalcite is the
most abundant mineral with this layered structure in Nature. These materials are not as
abundant in Nature as the analogous cationic clays, but they are very easy to synthesize
with a tuned composition and are generally not very expensive.
The layered structure of the LDH consists of brucite-like layers with divalent cations
occupying octahedral spaces formed by OH
−
ions [M(OH)
6
], in which an isomorphic,
and partial, substitution of divalent cations by trivalent cations has taken place, leaving
the layers positively charged. The electroneutrality of the compound is achieved by the
incorporation of anions in the interlayer space [
2
]. The chemical composition of LDHs is
described by the chemical formula:
hMII
1−xMIII
x(OH)2ix+Am−x
m·nH2O
, where M
II
and
M
III
are the divalent and trivalent metal cations, respectively, and A
−
is the interlayer anion,
with
x
defined as the
MIII/MII +MIII
ratio [
1
,
3
]. Both organic and inorganic anions
ChemEngineering 2022,6, 53. https://doi.org/10.3390/chemengineering6040053 https://www.mdpi.com/journal/chemengineering
ChemEngineering 2022,6, 53 2 of 19
can be incorporated into the interlayer space, in a wide range of sizes and charges
[1,4]
.
In addition, in the interlayer space there are randomly arranged water molecules, and
this interlayer region has quasi-liquid behavior. The wide range of anions and divalent
and trivalent cations that can be used to prepare LDH, provide them with a diversity
of compositions.
In recent decades, LDHs have established themselves as promising materials, due to
their properties and applications in a number of fields, such as water remediation [
5
–
7
],
catalysis [
8
–
10
], drug delivery [
11
,
12
], electroactivity [
13
], biomedicine [
14
,
15
], and others.
Their applications as anion exchanging solids depend on the interlayer anion and its affinity
for the LDH layers. Thus, Miyata et al. [
16
] have reported an order of anionic selectivity
for MgAl hydrotalcite-like materials, which could be applicable to other combinations
of elements [
17
]. According to this, for applications involving anion exchanging, LDHs
containing nitrate have a higher ion exchange facility than those containing carbonate in
the interlayer space.
As above mentioned, the synthesis of LDHs in the laboratory is relatively simple
and cheap, and they could be synthesized by many different methods. The method se-
lected has an impact on the properties of the final solid and, therefore, on its subsequent
application. Among the different methods described, the most widely used is the copre-
cipitation method, due to its great ease of use and reproducibility [
3
]. This method is
based on precipitation by the slow dropwise addition of a solution containing the mix-
ture of M
II
and M
III
salts solution in a fixed ratio and the anion to an alkaline solution,
working at constant pH. The addition of a second alkaline solution allows the pH of the
precipitation medium to be maintained during the precipitation of the cations [
3
]. Many
parameters are relevant to monitoring the process, such as the type and concentration of
cations and anions, the precipitation medium, the pH and the temperature [
18
,
19
]. The
optimal pH depends on the nature of the cations to be incorporated into the structure; thus,
Kloprogge et al. [20]
reported that the best crystallinity for samples of Zn/Al LDH were
exhibit in the
pH range 11–12
. Moreover, when the coprecipitation method is carried out,
the incorporation of the carbonate anion into the interlayer space is very difficult to avoid,
due to the fact that it is the anion with the highest affinity for the LDH layers. However, its
incorporation can be prevented by using decarbonated water and bubbling N2.
LDH synthetized by the coprecipitation method generally exhibits low crystallinity
with a high degree of aggregation and a wide particle size distribution. The most common
way to obtain more uniform particle properties, with an improved crystallinity is by an
aging process. For this reason, the coprecipitation method is generally followed by a
long aging period, from 10 to 80 h, and often longer [
19
]. Also, post-synthesis treatment
heating at moderate temperatures is used; for instance, by gentle-to-gentle reflux or by
hydrothermal treatment. The aging presumably occurs through the Ostwald ripening
process, in which larger and more perfect crystallites grow at the expense of smaller
particles in solution by dissolution/precipitation processes [
21
]. Therefore, microwave-
assisted hydrothermal treatment, or hydrothermal treatment by conventional heating, have
been widely used with the aim of improving structural and textural properties [22–26].
In our previous work [
27
], Zn/Al-LDH in molar ratio 2:1 was synthesized using amines
with one carbon atom in the organic chain [methylamine (MMA), dimethylamine (DMA)
and trimethylamine (TMA)], such as precipitant agents. Furthermore, the evolution on the
properties caused by hydrothermal treatment of samples synthetized with amines using the
following two ways of heating was studied: conventional heating and microwave assisted
heating. Highly crystalline LDH with nitrate anion in the interlayer was obtained using
DMA as the precipitant agent and, after submitting the solid to conventional hydrothermal
treatment. In the present work, we reported the use of amines with two carbon atoms in the
organic chain [ethylamine (EA), diethylamine (DEA), triethylamine (TEA)] as precipitant
agents in the synthesis of Zn,Al-LDH in molar ratio 2:1. Furthermore, the solids were treated
hydrothermally by conventional heating and microwave-assisted heating to improve the
ChemEngineering 2022,6, 53 3 of 19
crystallinity and properties of the solids. The effect of the treatment methods was analyzed
for each amine-assisted synthesis condition.
2. Materials and Methods
2.1. Materials
Zn(NO
3
)
2·
6H
2
O (98–102%), Al(NO
3
)
3·
9H
2
O (98–102%) and NaOH (98%) were pur-
chased from Panreac and used as received. An aqueous solution of ethylamine (70% in
H
2
O) was purchased from Alfa Aesar (Ward Hill, MA, USA). Diethylamine (99.5%) was
purchased from Panreac (Barcelona, Spain). Triethylamine (
≥
99%) was purchased from
Sigma Aldrich (Burlington, MA, USA).
2.2. Synthesis
The coprecipitation method was used to prepare the desired solids [
3
]. In order to
avoid the intercalation of carbonate anions, decarbonated water solutions and nitrogen
atmosphere were used during the synthesis. In order to prepare the solution of the metal
cations, 0.3 L in 2.5 M concentration of their nitrate salts in a M
II
/M
III
molar ratio 2:1
was prepared. For the precipitation medium, an aqueous solution of 4.5 M concentration
of the desired amine was prepared. The metal cation solution was added dropwise to
the amine solution, which was maintained under vigorous magnetic stirring. The pH of
the precipitation media was kept at a preselected pH value of 10 by adding the required
amount of a 2 M NaOH solution using a 240 CRISON pH-burette. After complete addition,
the aqueous suspension of the precipitated solid was stirred for 1 h at room temperature
and, then, the slurry obtained was subjected to different ageing treatments: (i) a portion
of the sample without any hydrothermal treatment was kept as a reference; (ii) a portion
was subjected to a hydrothermal treatment by microwave-assisted heating (MW) for 60 or
300 min at 90
◦
C; (iii) a portion was subjected to a hydrothermal treatment by conventional
heating (HT) for 1 or 7 days at 90
◦
C using a home-made stainless-steel bomb lined
with Teflon. Hydrothermal treatment by microwave-assisted heating was carried out
in a MILESTONE ETHOS PLUS microwave oven, where the aqueous suspension was
placed in Teflon digestion vessels, sealed and mounted on a turntable inside the oven. The
programmed temperature was controlled by a thermocouple immersed in a reference vessel
and software provided by the manufacturer. The different portions of the solid suspensions
were separated and washed by centrifugation with distilled water until reaching a pH close
to 7, in order to eliminate the equilibrium ions of the starting salts. Finally, the solids were
dried at 40 ◦C in an oven under air atmosphere.
The samples were labelled according to the preparation procedure: reaction medium,
hydrothermal treatment method and time period to which they had been subjected. So, a
sample labeled as ZA2XYt, ZA2 represents the cations (Zn–Al, molar ratio 2/1); X represents
the precipitation media (depending on the amine used EA, DEA or TEA); Y represents the
aging treatment (STH for the reference sample, HT or MW); and t the treatment duration
(in minutes for MW, in days for HT).
2.3. Characterization
A Yobin Ivon Ultima II apparatus at NUCLEUS (University of Salamanca, Salamanca,
Spain) was used for elemental chemical analysis of Zn and Al by ICP-OES.
A Siemens D-5000 instrument was used to record Powder X-ray diffraction (PXRD)
patterns using Cu-K
α
radiation (
λ
= 1.54050 Å) with a scanning rate of 2
◦
/min from 5
◦
to
70
◦
(2
θ
). The Scherrer equation was used to calculate the crystallite sizes from the FWHM
(Full Width at Half Maximum) of the diffraction maximum (00l). The Warren correction
for instrumental line broadening was taken into account, but the possible contribution of
disorder effects and/or lattice strains to the peak broadening was ignored.
A Perkin-Elmer Spectrum One instrument was used to record the FT-IR spectra by
transmission with a nominal resolution of 2 cm
−1
from 4000 cm
−1
to 450 cm
−1
, using KBr
pressed pellets.
ChemEngineering 2022,6, 53 4 of 19
SDT Q600 equipment from TA Instruments was used to carried out the thermogravi-
metric (TG) and differential thermal analyses (DTA). The thermal analyses were carried
out by heating from room temperature to 900 ◦C at a rate of 10 ◦C/min under continuous
oxygen (L’Air Liquide, 99.995%) flow (50 mL/min).
A Micromeritics Gemini VII 2390t apparatus was used to record the nitrogen (
L’Air Liquide
,
99.999%) adsorption–desorption isotherms at
−
196
◦
C, and to calculate the specific surface
area and porosity data. The apparatus was calibrated with He (L’Air Liquide, 99.999%).
Before measurements, the samples were pretreated at 110
◦
C for 2 h under a stream of N
2
in a Micromeritics FlowPrep 060 Sample Degass System.
A Diffraction Mastersizer 2000 equipment from Malvern Instruments was used to
determine the particle size distribution (PSD) by Laser Diffraction. Using the dispersion
unit Hydro 2000 from Malvern Instruments, the solid was dispersed in water at 25
◦
C
(approx. 0.05 vol.%), and, after measuring the PSD for the dispersed samples, ultrasounds
were applied in situ to disaggregate the particles.
3. Results and Discussion
3.1. Element Chemical Analysis
Table 1gives the Zn/Al molar ratio values and chemical formulae of the samples
synthesized in the presence of the different amines used as precipitant agents. The chemical
formulae were deduced from the results of elemental chemical analysis and thermogravi-
metric analysis (see below).
Table 1. Element chemical analysis results and the chemical formulae of each sample.
Sample Al aZn aZn/Al bxcFormulae
ZA2EASTH 8.32 40.27 2.00 0.33 [Zn0.67Al0.33(OH)2](NO3)0.33 ·0.44 H2O
ZA2EAMW60 7.98 38.63 2.00 0.33 [Zn0.67Al0.33(OH)2](NO3)0.33 ·0.48 H2O
ZA2EAMW300 7.96 38.30 1.99 0.33 [Zn0.67Al0.33(OH)2](NO3)0.33 ·0.50 H2O
ZA2EAHT1 9.84 46.51 1.95 0.34 LDH * + ZnO + Al2O3
ZA2EAHT7 11.05 52.01 1.94 0.34 LDH * + ZnO + Al2O3
ZA2DEASTH 7.51 38.44 2.11 0.32 [Zn0.68Al0.32(OH)2](NO3)0.32 ·0.57 H2O
ZA2DEAMW60
7.67 38.87 2.09 0.32 [Zn0.68Al0.32(OH)2](NO3)0.32 ·0.55 H2O
ZA2DEAMW300
7.64 38.31 2.07 0.33 [Zn0.67Al0.33(OH)2](NO3)0.33 ·0.55 H2O
ZA2DEAHT1 7.73 38.47 2.05 0.33 [Zn0.67Al0.33(OH)2](NO3)0.33 ·0.51 H2O
ZA2DEAHT7 8.37 41.88 2.07 0.33 [Zn0.67Al0.33(OH)2](NO3)0.33 ·0.42 H2O
ZA2TEASTH 7.64 37.56 2.03 0.33 [Zn0.67Al0.33(OH)2](NO3)0.33 ·0.55 H2O
ZA2TEAMW60
7.62 37.62 2.04 0.33 [Zn0.67Al0.33(OH)2](NO3)0.33 ·0.51 H2O
ZA2TEAMW300
7.60 37.39 2.03 0.33 [Zn0.67Al0.33(OH)2](NO3)0.33 ·0.52 H2O
ZA2TEAHT1 7.67 37.82 2.04 0.33 [Zn0.67Al0.33(OH)2](NO3)0.33 ·0.50 H2O
ZA2TEAHT7 8.13 41.16 2.09 0.32 [Zn0.68Al0.32(OH)2](NO3)0.32 ·0.43 H2O
a
Mass percentage.
b
Molar ratio.
c
Al/(Al + Zn) molar ratio. * It was not possible to determine the chemical
formula of the LDH phase.
In all cases, the Zn/Al molar ratio approached the value of 2, suggesting a complete
precipitation of the existing cations in the synthesis medium. In some cases, a small
deviation could be observed, but never more than 5%.
For the determination of the chemical formula of each of the samples, the amount
of nitrate anion was calculated from the Al/(Al + Zn) molar ratio, assuming that it was
the only interlayered anion, as observed by FT-IR spectroscopy, neutralizing the positive
charge excess of the layers.
Unlike samples synthesized without amines or in the presence of methylamine, for
dimethylamine or trimethylamine samples synthesized in the presence of EA, DEA or
TEA have water content per chemical formula which is generally lower, as reported in
our previous work [
27
] The more regular stacking of the octahedral layers, as observed by
PXRD, leads to a decrease in the number of water molecules per unit formula, and this fact
is more evident in the samples having HT hydrothermal treatment.
ChemEngineering 2022,6, 53 5 of 19
On the other hand, samples ZA2EAHT1 and ZA2EAHT7 were mostly or completely
formed by ZnO, as deduced from the PXRD analysis. However, after chemical analysis,
the presence of Al in the solid sample could be determined, indicating the formation of an
amorphous phase containing aluminum [
28
], not observed by X-ray diffraction. Therefore,
as there was a mixture of phases in the solid sample, it was not possible to determine the
amount of Zn and Al forming the LDH structure (if its collapse was not complete); thus,
making it difficult to determine its chemical formula.
3.2. Powder X-ray Diffraction (PXRD)
The samples synthesized using EA, DEA and TEA as modifiers of the precipitation
medium were also obtained in the form of microcrystalline powder. Figure 1shows the
PXRD diagrams of these samples without hydrothermal treatment. The positions and
relative intensities of the recorded diffraction peaks revealed a layered structure of the
solids, characteristic of an ordered 3R
1
polytype of solids with the LDH structure (JCPDS:
22-0700) [
29
–
31
]. In all cases, the most intense diffraction peak, attributed to the diffraction
plane (003) of the crystal structure, was recorded at a position 10.0
◦
(2
θ
), with a spacing
of 8.93 Å. This spacing is in agreement with the values reported by Miyata et al. [
16
] for
LDH with nitrate as the interlayer anion arranged in a perpendicular orientation to the
brucite-like layers and with a M
2+
/M
3+
molar ratio close to 2. Confirming the layered
structure, diffraction peaks corresponding to crystallographic planes (006) and (009) were
recorded at values close to 19.9
◦
(2
θ
) and 30.0
◦
(2
θ
), respectively, and with spacings of
4.46 Å and 2.98 Å. Reflections corresponding to diffraction planes (110) and (113) were
recorded at 60.3◦(2θ) and 61.3◦(2θ), with spacings of 1.53 Å and 1.51 Å, respectively.
ChemEngineering 2022, 6, x FOR PEER REVIEW 5 of 19
Unlike samples synthesized without amines or in the presence of methylamine, for
dimethylamine or trimethylamine samples synthesized in the presence of EA, DEA or
TEA have water content per chemical formula which is generally lower, as reported in
our previous work [27] The more regular stacking of the octahedral layers, as observed by
PXRD, leads to a decrease in the number of water molecules per unit formula, and this
fact is more evident in the samples having HT hydrothermal treatment.
On the other hand, samples ZA2EAHT1 and ZA2EAHT7 were mostly or completely
formed by ZnO, as deduced from the PXRD analysis. However, after chemical analysis,
the presence of Al in the solid sample could be determined, indicating the formation of an
amorphous phase containing aluminum [28], not observed by X-ray diffraction. There-
fore, as there was a mixture of phases in the solid sample, it was not possible to determine
the amount of Zn and Al forming the LDH structure (if its collapse was not complete);
thus, making it difficult to determine its chemical formula.
3.2. Powder X-ray Diffraction (PXRD)
The samples synthesized using EA, DEA and TEA as modifiers of the precipitation
medium were also obtained in the form of microcrystalline powder. Figure 1 shows the
PXRD diagrams of these samples without hydrothermal treatment. The positions and rel-
ative intensities of the recorded diffraction peaks revealed a layered structure of the solids,
characteristic of an ordered 3R1 polytype of solids with the LDH structure (JCPDS: 22-
0700) [29–31]. In all cases, the most intense diffraction peak, attributed to the diffraction
plane (003) of the crystal structure, was recorded at a position 10.0° (2θ), with a spacing
of 8.93 Å. This spacing is in agreement with the values reported by Miyata et al. [16] for
LDH with nitrate as the interlayer anion arranged in a perpendicular orientation to the
brucite-like layers and with a M2+/M3+ molar ratio close to 2. Confirming the layered struc-
ture, diffraction peaks corresponding to crystallographic planes (006) and (009) were rec-
orded at values close to 19.9° (2θ) and 30.0° (2θ), respectively, and with spacings of 4.46
Å and 2.98 Å. Reflections corresponding to diffraction planes (110) and (113) were rec-
orded at 60.3° (2θ) and 61.3° (2θ), with spacings of 1.53 Å and 1.51 Å, respectively.
Figure 1. Powder X-ray diffraction diagrams of the samples prepared in the presence of EA, DEA
and TEA with no hydrothermal treatment.
Figure 1.
Powder X-ray diffraction diagrams of the samples prepared in the presence of EA, DEA
and TEA with no hydrothermal treatment.
As can be seen in Figure 1, the solids prepared using DEA or TEA as precipitant agents
showed a single crystallographic phase. However, in the case of the sample prepared in the
presence of EA, diffraction peaks corresponding to a second LDH phase could be distin-
guished in the PXRD diagram. Thus, it is possible to distinguish the most characteristic
peak of this secondary phase, which was recorded close to 11.33
◦
(2
θ
) and ascribed to
the diffraction peak (003), with a spacing of 7.81 Å. In this case, the diffraction angle for
this diffraction peak was slightly higher than that found for the sample synthesized in the
ChemEngineering 2022,6, 53 6 of 19
absence of amines [
27
], where, unlike the shoulder observed for the sample prepared in the
absence of amines, a well-defined maximum could be clearly distinguished. In this second
phase with a spacing of 7.81 Å for the peak, due to the (003) planes, the interlaminar nitrate
anions were arranged with their molecular plane parallel to the plane of the brucite-like
layers [32–34].
Observing the width and profile of the peaks ascribed to the diffraction plane
(003) close to 10
◦
(2
θ
), the crystallinity of these samples decreased in the order:
ZA2TEASTH > ZA2DEASTH > ZA2EASTH
. Again, it could be observed how the use
of amines (and the nature of these) in the precipitation medium modifies the crys-
tallinity of the solids, to the point of two phases coexisting, with nitrate anion in the
interlayer space depending on the amine used.
The samples synthesized in the presence of EA, DEA and TEA were also subjected
to hydrothermal treatments, using two heating routes: microwave-assisted heating (MW)
and conventional oven heating (HT). Figure 2includes the PXRD plots of these samples
with different MW hydrothermal treatment periods. Observing the profile of the diffrac-
tion peaks and their relative intensities as the treatment time increased, it can be seen
how, unlike the samples prepared in our previous work [
27
], the application of MW hy-
drothermal treatment increased the crystallinity of the solids as the MW treatment time
increased. Comparing the width of the diffraction peaks (003) of the samples subjected
to a longer treatment time, that is, the samples with 300 min of treatment, the following
decreasing order of crystallinity could be established as a function of the amine used:
Z
A2TEAMW300 > ZA2DEAMW300 > ZA2EAMW300
. It is noteworthy that, as the MW
hydrothermal treatment time was prolonged on the samples synthesized with EA, the
relative intensity of the diffraction peak (003) of the phase with the nitrate anion parallel
to the brucite-like layers did not increase as the treatment time increased, going from
being a well-defined peak in sample ZA2EASTH to being a shoulder of the peak (003) in
sample ZA2EAMW300.
ChemEngineering 2022, 6, x FOR PEER REVIEW 6 of 19
As can be seen in Figure 1, the solids prepared using DEA or TEA as precipitant
agents showed a single crystallographic phase. However, in the case of the sample pre-
pared in the presence of EA, diffraction peaks corresponding to a second LDH phase
could be distinguished in the PXRD diagram. Thus, it is possible to distinguish the most
characteristic peak of this secondary phase, which was recorded close to 11.33° (2θ) and
ascribed to the diffraction peak (003), with a spacing of 7.81 Å. In this case, the diffraction
angle for this diffraction peak was slightly higher than that found for the sample synthe-
sized in the absence of amines [27], where, unlike the shoulder observed for the sample
prepared in the absence of amines, a well-defined maximum could be clearly distin-
guished. In this second phase with a spacing of 7.81 Å for the peak, due to the (003) planes,
the interlaminar nitrate anions were arranged with their molecular plane parallel to the
plane of the brucite-like layers [32–34].
Observing the width and profile of the peaks ascribed to the diffraction plane (003)
close to 10° (2θ), the crystallinity of these samples decreased in the order: ZA2TEASTH >
ZA2DEASTH > ZA2EASTH. Again, it could be observed how the use of amines (and the
nature of these) in the precipitation medium modifies the crystallinity of the solids, to the
point of two phases coexisting, with nitrate anion in the interlayer space depending on
the amine used.
The samples synthesized in the presence of EA, DEA and TEA were also subjected to
hydrothermal treatments, using two heating routes: microwave-assisted heating (MW)
and conventional oven heating (HT). Figure 2 includes the PXRD plots of these samples
with different MW hydrothermal treatment periods. Observing the profile of the diffrac-
tion peaks and their relative intensities as the treatment time increased, it can be seen how,
unlike the samples prepared in our previous work [27], the application of MW hydrother-
mal treatment increased the crystallinity of the solids as the MW treatment time increased.
Comparing the width of the diffraction peaks (003) of the samples subjected to a longer
treatment time, that is, the samples with 300 min of treatment, the following decreasing
order of crystallinity could be established as a function of the amine used:
ZA2TEAMW300 > ZA2DEAMW300 > ZA2EAMW300. It is noteworthy that, as the MW
hydrothermal treatment time was prolonged on the samples synthesized with EA, the
relative intensity of the diffraction peak (003) of the phase with the nitrate anion parallel
to the brucite-like layers did not increase as the treatment time increased, going from be-
ing a well-defined peak in sample ZA2EASTH to being a shoulder of the peak (003) in
sample ZA2EAMW300.
Figure 2.
PXRD of the samples prepared in the presence of EA, DEA and TEA with 60 and 300 min of
MW treatment.
The application of a conventional hydrothermal treatment (HT) resulted in a greater
increase in the crystallinity of the solids synthesized in the presence of DEA and TEA.
ChemEngineering 2022,6, 53 7 of 19
However, the same effect is not observed for the samples synthesized in the presence of
EA, where the application of HT treatment resulted in the collapse of the layered structure,
as can be observed in Figure 3. Thus, the PXRD recorded for samples synthetized using EA
showed the formation of a zinc oxide (ZnO) like zincite phase (JCPDS: 00-036-1451 [
29
]).
This ZnO phase is identified mainly by the reflections recorded in the range of 28
◦
–38
◦
(2
θ
). While with 1 day of HT treatment the collapse of the LDH phase was not complete, as
can be seen from its corresponding PXRD diagram, after 7 days of treatment the collapse
was complete. So, the main diffraction peaks of sample ZA2EAHT7 corresponded to the
ZnO phase and, with very little intensity, the diffraction peaks at 11.6
◦
and 23.4
◦
(2
θ
)
corresponded to the crystallographic planes (003) and (006), respectively, of the LDH phase
with the interlayer nitrate arranged parallel to the plane of the brucite-like layers.
ChemEngineering 2022, 6, x FOR PEER REVIEW 7 of 19
Figure 2. PXRD of the samples prepared in the presence of EA, DEA and TEA with 60 and 300 min
of MW treatment.
The application of a conventional hydrothermal treatment (HT) resulted in a greater
increase in the crystallinity of the solids synthesized in the presence of DEA and TEA.
However, the same effect is not observed for the samples synthesized in the presence of
EA, where the application of HT treatment resulted in the collapse of the layered struc-
ture, as can be observed in Figure 3. Thus, the PXRD recorded for samples synthetized
using EA showed the formation of a zinc oxide (ZnO) like zincite phase (JCPDS: 00-036-
1451 [29]). This ZnO phase is identified mainly by the reflections recorded in the range of
28°–38° (2θ). While with 1 day of HT treatment the collapse of the LDH phase was not
complete, as can be seen from its corresponding PXRD diagram, after 7 days of treatment
the collapse was complete. So, the main diffraction peaks of sample ZA2EAHT7 corre-
sponded to the ZnO phase and, with very little intensity, the diffraction peaks at 11.6° and
23.4° (2θ) corresponded to the crystallographic planes (003) and (006), respectively, of the
LDH phase with the interlayer nitrate arranged parallel to the plane of the brucite-like
layers.
Figure 3. PXRD of the samples prepared in the presence of EA, DEA and TEA with 1 and 7 days of
HT treatment. (·) ZnO phase.
In the case of the samples synthetized using DEA or TEA, segregation of a small
amount of the zincite phase (ZnO) could also be observed as the HT treatment time in-
creased. Thus, from the profile of the diffraction peaks (003) and their relative intensities
for these two series of samples, it can be observed how with 1 day of treatment, solids of
high crystallinity were obtained without segregation of the ZnO phase when TEA was
used as the precipitant agent, and with a very small amount of this phase when DEA was
used. Therefore, the segregation of ZnO decreased as the degree of substitution of the
amino group in the compound used as precipitating agent increased. Comparing the
width of the diffraction peaks (003) of the samples subjected to a longer treatment time
(samples with 7 days of treatment) the following decreasing order of crystallinity can be
established as a function of the amine used: ZA2TEAHT7 > ZA2DEAHT7. Table 2 shows
the amount of zincite phase present in the sample calculated from the calibration line with
the ratio of the areas of the characteristic peaks of the zincite phase and the LDH phase
[35]. Thus, it is shown how the amount of zincite phase increased as the HT treatment
time increased, finding an amount of approximately 2% of ZnO in the sample
ZA2DEAHT7. By extrapolation of the calibration line to samples synthetized using EA,
Figure 3.
PXRD of the samples prepared in the presence of EA, DEA and TEA with 1 and 7 days of
HT treatment. () ZnO phase.
In the case of the samples synthetized using DEA or TEA, segregation of a small
amount of the zincite phase (ZnO) could also be observed as the HT treatment time
increased. Thus, from the profile of the diffraction peaks (003) and their relative intensities
for these two series of samples, it can be observed how with 1 day of treatment, solids
of high crystallinity were obtained without segregation of the ZnO phase when TEA was
used as the precipitant agent, and with a very small amount of this phase when DEA
was used. Therefore, the segregation of ZnO decreased as the degree of substitution of
the amino group in the compound used as precipitating agent increased. Comparing the
width of the diffraction peaks (003) of the samples subjected to a longer treatment time
(samples with 7 days of treatment) the following decreasing order of crystallinity can be
established as a function of the amine used: ZA2TEAHT7 > ZA2DEAHT7. Table 2shows
the amount of zincite phase present in the sample calculated from the calibration line with
the ratio of the areas of the characteristic peaks of the zincite phase and the LDH phase [
35
].
Thus, it is shown how the amount of zincite phase increased as the HT treatment time
increased, finding an amount of approximately 2% of ZnO in the sample ZA2DEAHT7. By
extrapolation of the calibration line to samples synthetized using EA, for the ZA2EAHT1
sample, 67% was ZnO and for longer treatments it was close to 100% (Table 2).
ChemEngineering 2022,6, 53 8 of 19
Table 2.
ZnO content in the Zn and Al samples prepared in the presence of EA, DEA and TEA with
HT treatment.
Sample (101) Peak
Area (ZnO) a(003) Peak
Area (LDH) aArea Ratio
(101)/(003) ZnO Content b
ZA2EAHT1 1807.0 1424.0 1.268961 67
ZA2EAHT7 2776.0 - - ≈100
ZA2DEAHT1 122.1 18,339.8 0.006659 0.32
ZA2DEAHT7 790.7 19,232.1 0.041114 2.14
ZA2TEAHT1 - - - -
ZA2TEAHT7 502.6 26,627.0 0.018876 0.97
aa.u. bMass percentage.
The lattice parameters of the prepared solids were calculated from the positions of the
diffraction peaks due to the (003) and (110) planes [
30
]; being, c= 3
·
d(003)
≈
26.6–26.8 Å,
and a= 2
·
d(110)
≈
3.069–3.077 Å. In addition, from the value of the Full Width at Half
Maximum (FWHM) of reflection 003, the crystallite size (D) in the cdirection was calculated
using the Scherrer equation, D = k
λ
/
β
cos
θ
[
36
,
37
], where k is a constant, taken in this
case as 0.9;
λ
is the wavelength of the radiation used;
β
the FWHM and
θ
the diffraction
angle; correction due to instrumental broadening was not applied. The values calculated
for the samples obtained are included in Table 3, together with the calculated values for the
number of stacked layers.
Table 3.
Lattice parameters cand a, average crystal size D and number of stacked layers for the
samples obtained.
Sample c(Å) a(Å) D (Å) Number of
Stacked Layers
ZA2EASTH 26.80 3.0697 111 12
ZA2EAMW60 26.80 3.0697 118 13
ZA2EAMW300 26.80 3.0720 135 15
ZA2EAHT1 - - - -
ZA2EAHT7 - - - -
ZA2DEASTH 26.67 3.0673 126 14
ZA2DEAMW60 26.80 3.0697 148 17
ZA2DEAMW300
26.80 3.0720 210 23
ZA2DEAHT1 26.67 3.0743 284 32
ZA2DEAHT7 26.80 3.0766 334 37
ZA2TEASTH 26.80 3.0697 150 17
ZA2TEAMW60 26.80 3.0697 142 16
ZA2TEAMW300 26.80 3.0697 218 24
ZA2TEAHT1 26.80 3.0766 319 36
ZA2TEAHT7 26.67 3.0743 322 36
The similarity of the lattice parameter avalue, with differences of less than 1%, are
coherent with the homogeneity of metals composition in the samples.
The crystallite size (D) values highlight how the conventional hydrothermal treatment
led to a greater increase in the crystallinity of the solids. When the MW treatment was
applied, it was observed that, as the treatment time increased the crystal size increased,
resulting in the obtaining of, in both the DEA and TEA series, crystal sizes around 200 Å.
However, when HT treatment was applied, it could be observed that the samples synthe-
sized in the presence of TEA with only one day of treatment reached a crystal size close to
320 Å, which was practically maintained, even if the HT treatment time increased. In the
case of samples prepared in the presence of DEA, with one day of HT treatment a crystal
size higher than 280 Å was obtained; reaching a crystal size of 334 Å when the treatment
was extended to 7 days.
ChemEngineering 2022,6, 53 9 of 19
3.3. FT-IR Spectroscopy
FT-IR spectra of the synthesized solids are plotted in Figure 4. The broad band at
3460 cm
−1
is ascribed to the stretching vibration modes of the hydroxyl groups and the
water molecules of the interlayer space. At lower wavenumbers it was possible to observe
a bands at 615 and 556 cm
−1
caused by M-OH vibration modes. The vibration ascribed to
the bending mode of the interlayer water molecules was recorded at 1624 cm
−1
[
18
,
38
,
39
].
The bands at 2396 and 2428 cm
−1
could be attributed to atmospheric CO
2
weakly bonded
on the LDH surface.
ChemEngineering 2022, 6, x FOR PEER REVIEW 9 of 19
synthesized in the presence of TEA with only one day of treatment reached a crystal size
close to 320 Å, which was practically maintained, even if the HT treatment time increased.
In the case of samples prepared in the presence of DEA, with one day of HT treatment a
crystal size higher than 280 Å was obtained; reaching a crystal size of 334 Å when the
treatment was extended to 7 days.
3.3. FT-IR Spectroscopy
FT-IR spectra of the synthesized solids are plotted in Figure 4. The broad band at 3460
cm−1 is ascribed to the stretching vibration modes of the hydroxyl groups and the water
molecules of the interlayer space. At lower wavenumbers it was possible to observe a
bands at 615 and 556 cm−1 caused by M-OH vibration modes. The vibration ascribed to the
bending mode of the interlayer water molecules was recorded at 1624 cm−1 [18,38,39]. The
bands at 2396 and 2428 cm−1 could be attributed to atmospheric CO2 weakly bonded on
the LDH surface.
Figure 4. FT-IR spectra of the samples prepared in the presence of EA, DEA and TEA with no hy-
drothermal treatment and with MW and HT treatments.
In all cases, the characteristic bands of the vibrational modes of the nitrate anion
molecules were recorded, confirming its presence as an interlayer anion. Thus, bands at
1385 cm−1 and 826 cm−1 can be observed, assigned to the ν3(E’) and ν2 (A2’’) vibrational
modes, respectively, of NO
with a D3h symmetry [19,38]. At 1763 cm−1 a narrow band can
be observed, corresponding to the combination of the vibrational modes ν1(A1′) at 1068
cm−1 and ν4(E’) at 692 cm−1 of nitrate, the latter not clearly observed in the infrared spectra.
For sample ZA2EASTH, a shoulder can be seen at 1356 cm−1 (Figure 4), which could be
due to nitrate anions in parallel orientation in the second LDH phase observed by PXRD.
As observed by PXRD for the samples synthesized in the presence of EA after MW
hydrothermal treatment, a crystallinity increase of the phase with the anions in
perpendicular orientation took place in detriment of the phase with the anions in parallel
orientation to the layers. This effect was reflected in the FT-IR spectra, where the shoulder
at 1356 cm−1 became less evident (Figure 4). On the other hand, when DEA or TEA were
used as precipitaton agents, such a shoulder was also not observed at 1356 cm−1.
When the sample synthesized in the presence of EA was subjected to a HT
hydrothermal treatment process, as observed in the PXRD studies, the structure collapsed
segregating the zinc oxide in its zincite phase, being almost complete with long treatment
Figure 4.
FT-IR spectra of the samples prepared in the presence of EA, DEA and TEA with no
hydrothermal treatment and with MW and HT treatments.
In all cases, the characteristic bands of the vibrational modes of the nitrate anion
molecules were recorded, confirming its presence as an interlayer anion. Thus, bands at
1385 cm
−1
and 826 cm
−1
can be observed, assigned to the
ν
3(E’) and
ν
2 (A2”) vibrational
modes, respectively, of
NO−
3
with a D
3h
symmetry [
19
,
38
]. At 1763 cm
−1
a narrow band
can be observed, corresponding to the combination of the vibrational modes
ν
1(A1
0
) at
1068 cm−1and ν4(E’) at 692 cm−1of nitrate, the latter not clearly observed in the infrared
spectra. For sample ZA2EASTH, a shoulder can be seen at 1356 cm
−1
(Figure 4), which
could be due to nitrate anions in parallel orientation in the second LDH phase observed
by PXRD. As observed by PXRD for the samples synthesized in the presence of EA after
MW hydrothermal treatment, a crystallinity increase of the phase with the anions in
perpendicular orientation took place in detriment of the phase with the anions in parallel
orientation to the layers. This effect was reflected in the FT-IR spectra, where the shoulder
at 1356 cm
−1
became less evident (Figure 4). On the other hand, when DEA or TEA were
used as precipitaton agents, such a shoulder was also not observed at 1356 cm−1.
When the sample synthesized in the presence of EA was subjected to a HT hydrother-
mal treatment process, as observed in the PXRD studies, the structure collapsed segregating
the zinc oxide in its zincite phase, being almost complete with long treatment times; this
caused the FT-IR spectra of these samples to change slightly. In Figure 4the bands at-
tributed to the vibrational modes of the hydroxyl groups and water molecules (band
positions already mentioned above) can be identified. In addition, the vibration band at
ChemEngineering 2022,6, 53 10 of 19
1385 cm
−1
, attributed to the presence of the nitrate anion of the LDH phase recorded with
low intensity in the PXRD diagrams of the sample, can be clearly observed. As a result
of the formation of ZnO in the HT-treated samples, a band at the limit of the spectrum,
around 470 cm
−1
, attributed to the stretching vibrational mode of ZnO, was observed in
the FT-IR spectra [40].
In the spectra of the samples synthesized in the presence of DEA and TEA, no different
bands were observed for the hydrothermally treated samples, both MW and HT, with
respect to that of the samples that received no treatment. It should be noted that the higher
crystallinity of the samples resulted in a better ordering of both the interlaminar anions
and the layered structure, giving rise to a regularity that was reflected in a subtle increase
in intensity and narrowing of the vibration bands.
Neither in the FT-IR spectra recorded for the samples without hydrothermal treat-
ment, nor for the samples subjected to MW or HT hydrothermal treatments, were bands
corresponding to the vibrational modes of the amines used during the synthesis observed.
3.4. Thermal Analysis
The thermal analysis of the samples prepared using EA, DEA or TEA as precipitan
agents were carried out to determine their stability and evolution to mixed oxides. During
the thermal analysis, mass spectrometry (MS) of the gases and vapors formed during
the process (EGA, evolved gas analysis) was carried out. To identify the masses of the
generated species, a complete mass spectrum was initially recorded and, in a second step,
the MS analysis was performed by fixing these masses and following the change in their
intensities throughout the analysis. The reference MS of the expected evolved gases was
also taken into account. The signals monitorized corresponded to H
2
O (m/z= 18), N
2
and
CO (m/z= 28), NO (m/z= 30), N
2
O (m/z= 44), NO
2
(m/z= 45), EA (fragment at m/z= 30,
44 and 45), DEA (fragments at m/z= 58 and 73) and TEA (fragments at m/z= 58 and 101).
TG curves of the samples without hydrothermal treatment (STH) are included in
Figure 5, together with the tracked masses of the gases generated during the process. In
all curves, the typical decomposition stages of LDH compounds can be identified. Three
decomposition stages could be identified in all TG curves. First, below 180
◦
C, water
removal was observed, as shown by the MS peak at m/z= 18. The second stage of
decomposition was observed up to around 300
◦
C, and corresponded to the release of
water from the condensation of hydroxyl groups of the brucite-like layers. Finally, a steady
mass loss was observed between 300 and 700
◦
C, which corresponded to the removal of
interlayer nitrate species. The MS signals recorded in this temperature range corresponded
to formation of species such as NO, NO
2
, and N
2
O, from nitrate decomposition. In all
the curves, the process of elimination of the hydroxyl groups practically overlapped with
the last stage of decomposition/removal of the interlaminar anion. However, there was
better differentiation of the first stage from the second decomposition stage as the degree
of substitution of the amino group in the compound used as precipitating agent increased.
Thus, for the ZA2TEASTH sample a small plateau could be observed around 200 ◦C.
From the tracking of the m/zsignals of each amine, the absence of amine residues
in the final solids could be concluded, as could also be observed by FT-IR spectroscopy.
Only in the case of the sample prepared in the presence of EA, could the mass at 45 m/zbe
attributed to that amine. However, some of the masses associated with EA overlap with
the signals of the decomposition products of the nitrate anion (N
2
O, NO and NO
2
). The
absence of EA was confirmed because the mass tracking curves had the same profile as
those of NOx gas formation in the solids synthesized in the presence of DEA and TEA.
For the hydrothermally treated samples, both MW and HT, had similar TG curves
recorded, in which the aforementioned plateau was more evident as a result of the increase
in the crystallinity of the samples.
ChemEngineering 2022,6, 53 11 of 19
ChemEngineering 2022, 6, x FOR PEER REVIEW 11 of 19
Figure 5. TG curves, in O2 atmosphere, of the samples prepared in the presence of EA, DEA and
TEA with no hydrothermal treatment and tracking of the characteristic m/z signals.
From the tracking of the m/z signals of each amine, the absence of amine residues in
the final solids could be concluded, as could also be observed by FT-IR spectroscopy. Only
in the case of the sample prepared in the presence of EA, could the mass at 45 m/z be
attributed to that amine. However, some of the masses associated with EA overlap with
the signals of the decomposition products of the nitrate anion (N2O, NO and NO2). The
absence of EA was confirmed because the mass tracking curves had the same profile as
those of NOx gas formation in the solids synthesized in the presence of DEA and TEA.
For the hydrothermally treated samples, both MW and HT, had similar TG curves
recorded, in which the aforementioned plateau was more evident as a result of the
increase in the crystallinity of the samples.
On the other hand, it can also be observed that, as the crystalline regularity of the
solids increases, the total mass loss decreased (Table 4), due to the lower amount of water
retained in the solids. Table 4 shows that as the hydrothermal treatment time increased,
the total mass loss was lower, being in all cases between 30 and 40%. Only in the case of
the samples synthesized in the presence of EA and with HT hydrothermal treatment was
a total mass loss less than 30% observed. This was due to the formation of ZnO in these
samples, where after 7 days of treatment practically the whole structure had collapsed
and the mass loss observed in the TG curve was due to the small amount of the LDH
phase, the retained water and the decomposition of the nitrate anion.
Table 4. Total weight loss and H2O molecules per chemical formula calculated for each sample.
Sample Weight Loss (%)
H2O Molecules Per Chemical For-
mula (n)
ZA2EASTH 36.6 0.44
ZA2EAMW60 36.7 0.48
ZA2EAMW300 38.0 0.50
ZA2EAHT1 22.6 -
ZA2EAHT7 14.6 -
ZA2DEASTH 38.1 0.57
ZA2DEAMW60 37.8 0.55
ZA2DEAMW300 38.2 0.55
ZA2DEAHT1 37.3 0.51
ZA2DEAHT7 31.0 0.42
ZA2TEASTH 38.7 0.55
ZA2TEAMW60 38.3 0.51
ZA2TEAMW300 38.5 0.52
ZA2TEAHT1 37.9 0.50
ZA2TEAHT7 32.8 0.43
Figure 5.
TG curves, in O
2
atmosphere, of the samples prepared in the presence of EA, DEA and TEA
with no hydrothermal treatment and tracking of the characteristic m/zsignals.
On the other hand, it can also be observed that, as the crystalline regularity of the
solids increases, the total mass loss decreased (Table 4), due to the lower amount of water
retained in the solids. Table 4shows that as the hydrothermal treatment time increased,
the total mass loss was lower, being in all cases between 30 and 40%. Only in the case of
the samples synthesized in the presence of EA and with HT hydrothermal treatment was
a total mass loss less than 30% observed. This was due to the formation of ZnO in these
samples, where after 7 days of treatment practically the whole structure had collapsed and
the mass loss observed in the TG curve was due to the small amount of the LDH phase, the
retained water and the decomposition of the nitrate anion.
Table 4. Total weight loss and H2O molecules per chemical formula calculated for each sample.
Sample Weight Loss (%) H2O Molecules Per
Chemical Formula (n)
ZA2EASTH 36.6 0.44
ZA2EAMW60 36.7 0.48
ZA2EAMW300 38.0 0.50
ZA2EAHT1 22.6 -
ZA2EAHT7 14.6 -
ZA2DEASTH 38.1 0.57
ZA2DEAMW60 37.8 0.55
ZA2DEAMW300 38.2 0.55
ZA2DEAHT1 37.3 0.51
ZA2DEAHT7 31.0 0.42
ZA2TEASTH 38.7 0.55
ZA2TEAMW60 38.3 0.51
ZA2TEAMW300 38.5 0.52
ZA2TEAHT1 37.9 0.50
ZA2TEAHT7 32.8 0.43
The DTA curves of the samples both without hydrothermal treatment and with hydrother-
mal treatments, MW or HT, are included in Figure 6. In all cases, endothermic minima associated
with the different decomposition processes of the samples can be observed.
The minimum recorded at 120–130
◦
C in the DTA curves was associated with the
process of release of the water retained in the interlayer space. However, in the case of the
samples synthesized in the presence of EA, another minimum could be observed at 176
◦
C,
which could correspond to the elimination of water molecules retained more strongly in
the structure of the layered solid. On the other hand, at temperatures above 200
◦
C a
minimum associated with the process of elimination of hydroxyl groups in the form of
water vapor and the decomposition of the nitrate anion was found. In many cases this
minimum presented a shoulder at lower temperature, or was even dissociated into two
ChemEngineering 2022,6, 53 12 of 19
clearly distinguishable minima. This dissociation and, therefore, differentiation in the
decomposition processes, became more evident as the crystalline regularity in the solids
increased after the application of a hydrothermal treatment. On the other hand, it is worth
mentioning that in all cases the main minimum was found at higher temperatures than in
the case of the solids synthesized in our previous work using methylamine, dimethylamine
or trimethylamine as precipitant agents, and even in the case of the solids synthesized in
the absence of amines [
27
]. Thus, for the sample synthesized in the presence of DEA, this
minimum was found at 260
◦
C, while for the sample synthesized in the presence of TEA,
the minimum shifted to 272 ◦C.
ChemEngineering 2022, 6, x FOR PEER REVIEW 12 of 19
The DTA curves of the samples both without hydrothermal treatment and with
hydrothermal treatments, MW or HT, are included in Figure 6. In all cases, endothermic
minima associated with the different decomposition processes of the samples can be
observed.
Figure 6. DTA curves, in O
2
atmosphere, of the samples prepared in the presence of EA, DEA and
TEA with no hydrothermal treatment and with MW and HT treatments.
The minimum recorded at 120–130 °C in the DTA curves was associated with the
process of release of the water retained in the interlayer space. However, in the case of the
samples synthesized in the presence of EA, another minimum could be observed at 176
°C, which could correspond to the elimination of water molecules retained more strongly
in the structure of the layered solid. On the other hand, at temperatures above 200 °C a
minimum associated with the process of elimination of hydroxyl groups in the form of
water vapor and the decomposition of the nitrate anion was found. In many cases this
minimum presented a shoulder at lower temperature, or was even dissociated into two
clearly distinguishable minima. This dissociation and, therefore, differentiation in the
decomposition processes, became more evident as the crystalline regularity in the solids
increased after the application of a hydrothermal treatment. On the other hand, it is worth
mentioning that in all cases the main minimum was found at higher temperatures than in
the case of the solids synthesized in our previous work using methylamine,
dimethylamine or trimethylamine as precipitant agents, and even in the case of the solids
synthesized in the absence of amines [27]. Thus, for the sample synthesized in the presence
of DEA, this minimum was found at 260 °C, while for the sample synthesized in the
presence of TEA, the minimum shifted to 272 °C.
3.5. Specific Surface Area and Porosity
The textural properties of the synthesized solids were studied from the N
2
adsorption–desorption isotherms at −196 °C. Table 5 includes the values of the specific
surface areas calculated by the BET (S
BET
) method [41,42], the pore volume (V
pore
) and the
average pore diameter calculated by the BJH method [42,43] for the synthesized samples.
For the samples prepared in the presence of DEA and TEA, both without hydrothermal
treatment and with MW treatment, the adsorption capacity was below the confidence
limit of the equipment used. Only for the samples with HT hydrothermal treatment (and
sample ZA2DEAMW300) did the adsorption measurements present confidence values.
This behavior was not observed for samples prepared in the presence of EA, although the
Figure 6.
DTA curves, in O
2
atmosphere, of the samples prepared in the presence of EA, DEA and
TEA with no hydrothermal treatment and with MW and HT treatments.
3.5. Specific Surface Area and Porosity
The textural properties of the synthesized solids were studied from the N
2
adsorption–
desorption isotherms at
−
196
◦
C. Table 5includes the values of the specific surface areas
calculated by the BET (S
BET
) method [
41
,
42
], the pore volume (V
pore
) and the average
pore diameter calculated by the BJH method [
42
,
43
] for the synthesized samples. For the
samples prepared in the presence of DEA and TEA, both without hydrothermal treatment
and with MW treatment, the adsorption capacity was below the confidence limit of the
equipment used. Only for the samples with HT hydrothermal treatment (and sample
ZA2DEAMW300) did the adsorption measurements present confidence values. This behav-
ior was not observed for samples prepared in the presence of EA, although the samples
presented low S
BET
values, close to the detection limit. Figures 7and 8include the corre-
sponding adsorption–desorption isotherms, which correspond to type II according to the
IUPAC classification [
44
,
45
], corresponding to adsorption on non-porous or mesoporous
adsorbents, where adsorption can occur without monolayer-multilayer restrictions. More-
over, it can be observed how all of them presented a hysteresis cycle, corresponding to the
H3 type according to the IUPAC classification [
42
], indicating that adsorption took place in
slit-shaped pores formed by layer-like particles.
ChemEngineering 2022,6, 53 13 of 19
Table 5.
BET specific surface area, pore volume and average pore diameter of the samples prepared.
Sample SBET (m2/g) Vpore (mm3/g)
BJH Desorption
Average Pore
Diameter (nm)
ZA2EASTH 6.1 14.9 9.1
ZA2EAMW60 3.4 9.8 9.6
ZA2EAMW300 4.5 11.2 9.6
ZA2EAHT1 74.3 73.7 4.6
ZA2EAHT7 118.5 83.2 3.0
ZA2DEASTH - - -
ZA2DEAMW60 - - -
ZA2DEAMW300 6.4 16.0 6.9
ZA2DEAHT1 15.3 32.2 7.6
ZA2DEAHT7 28.2 42.2 5.3
ZA2TEASTH - - -
ZA2TEAMW60 - - -
ZA2TEAMW300 - - -
ZA2TEAHT1 9.0 23.3 9.9
ZA2TEAHT7 21.0 32.4 5.7
ChemEngineering 2022, 6, x FOR PEER REVIEW 14 of 19
Figure 7. Nitrogen adsorption-desorption isotherms for HT treated samples prepared in the pres-
ence of EA, DEA and TEA.
Figure 8 shows the adsorption–desorption curves for the samples without
hydrotermal treatment and with MW treatment. While the sample without hydrothermal
treatment had an SBET value close to 6 m2/g, for samples synthetized using EA as
precipitant agent, when a MW hydrothermal treatment was applied the surface area
decreased. In the three curves a similar behavior against the desorption process can be
observed, with slightly larger pore sizes when MW treatment was applied. On the other
hand, when DEA or TEA were used as precipitant agents, only the adsorption–desorption
curve for sample ZA2DEAMW300 was recorded. The SBET value for this sample was 6.4
m2/g, higher than that found for the analogous sample synthesized in the presence of EA.
Figure 8. Nitrogen adsorption-desorption isotherms for MW treated samples prepared in the pres-
ence of EA and DEA.
3.6. Particle Size Distribution
Figure 9 shows the particle size distribution curves of the samples synthesized in the
presence of EA, DEA and TEA without hydrothermal treatment and after the application
of the longest periods of both hydrothermal treatments: MW and HT. For each of the
samples two distribution curves are represented: (i) the distribution curve of the sample
in aqueous suspension, black curve, and (ii) the distribution curve of the sample in
aqueous suspension subjected to sonication treatment for 15 min, directly in the particle
Figure 7.
Nitrogen adsorption-desorption isotherms for HT treated samples prepared in the presence
of EA, DEA and TEA.
ChemEngineering 2022, 6, x FOR PEER REVIEW 14 of 19
Figure 7. Nitrogen adsorption-desorption isotherms for HT treated samples prepared in the pres-
ence of EA, DEA and TEA.
Figure 8 shows the adsorption–desorption curves for the samples without
hydrotermal treatment and with MW treatment. While the sample without hydrothermal
treatment had an SBET value close to 6 m2/g, for samples synthetized using EA as
precipitant agent, when a MW hydrothermal treatment was applied the surface area
decreased. In the three curves a similar behavior against the desorption process can be
observed, with slightly larger pore sizes when MW treatment was applied. On the other
hand, when DEA or TEA were used as precipitant agents, only the adsorption–desorption
curve for sample ZA2DEAMW300 was recorded. The SBET value for this sample was 6.4
m2/g, higher than that found for the analogous sample synthesized in the presence of EA.
Figure 8. Nitrogen adsorption-desorption isotherms for MW treated samples prepared in the pres-
ence of EA and DEA.
3.6. Particle Size Distribution
Figure 9 shows the particle size distribution curves of the samples synthesized in the
presence of EA, DEA and TEA without hydrothermal treatment and after the application
of the longest periods of both hydrothermal treatments: MW and HT. For each of the
samples two distribution curves are represented: (i) the distribution curve of the sample
in aqueous suspension, black curve, and (ii) the distribution curve of the sample in
aqueous suspension subjected to sonication treatment for 15 min, directly in the particle
Figure 8.
Nitrogen adsorption-desorption isotherms for MW treated samples prepared in the presence
of EA and DEA.
ChemEngineering 2022,6, 53 14 of 19
In all cases S
BET
values were higher than those obtained for the samples obtained with
the amines used in our previous work and, also, the values were higher than those found
for the samples synthesized in the absence of amines [
27
]. On the contrary, smaller pore
diameter sizes were recorded.
In view of the results included in Table 5, the segregation of ZnO in the samples
synthesized in the presence of EA with HT treatment led to a substantial increase in
the specific surface area. Solids with higher porosity were obtained, where the pore
volume increased, with smaller pore diameter sizes with respect to the sample without
hydrothermal treatment.
In the case of the solids synthesized in the presence of DEA and TEA, after the
application of a HT hydrothermal treatment, according to the PXRD results, an increase
in the crystallinity took place, as a consequence of better ordering of the brucite-like
layers. Together with the increase in the crystallinity of the solids, an increase in the S
BET
could be observed and adsorption–desorption curves could be recorded in both cases.
Moreover, the prolongation of the HT treatment resulted in higher crystallinity linked
to an increase of the S
BET
value, where, in both series of samples, the S
BET
value for
sample with 7 days of treatment was twice that for the sample with one day of treatment.
However, it is important to remember the presence of approximately 2% of ZnO in sample
ZA2DEAHT7, which could justify the difference in specific surface area with respect to that
of sample ZA2TEAHT7.
Figure 8shows the adsorption–desorption curves for the samples without hydrotermal
treatment and with MW treatment. While the sample without hydrothermal treatment had
an S
BET
value close to 6 m
2
/g, for samples synthetized using EA as precipitant agent, when
a MW hydrothermal treatment was applied the surface area decreased. In the three curves
a similar behavior against the desorption process can be observed, with slightly larger pore
sizes when MW treatment was applied. On the other hand, when DEA or TEA were used
as precipitant agents, only the adsorption–desorption curve for sample ZA2DEAMW300
was recorded. The S
BET
value for this sample was 6.4 m
2
/g, higher than that found for the
analogous sample synthesized in the presence of EA.
3.6. Particle Size Distribution
Figure 9shows the particle size distribution curves of the samples synthesized in the
presence of EA, DEA and TEA without hydrothermal treatment and after the application
of the longest periods of both hydrothermal treatments: MW and HT. For each of the
samples two distribution curves are represented: (i) the distribution curve of the sample in
aqueous suspension, black curve, and (ii) the distribution curve of the sample in aqueous
suspension subjected to sonication treatment for 15 min, directly in the particle size analyzer,
red curve. Sonication treatment is often used to disaggregate primary particles, changing
the distribution curves.
The samples without hydrothermal treatment presented a monomodal size distribu-
tion centered between 300 and 400
µ
m. Although, as the degree of amino group substitution
increased, a small shoulder could be observed at lower size values, approximately at 20
µ
m,
as observed in Figure 9a–c. The application of ultrasound for 15 min did not have a great
impact on the distribution curves; only in the case of sample ZA2TEASTH was the afore-
mentioned shoulder accentuated at 20
µ
m. This fact indicates that samples prepared in the
presence of EA and DEA give rise to robust particles that are more difficult to disaggregate
than particles obtained using TEA as the precipitant agent.
The particle size distribution curves for the samples subjected to MW hydrother-
mal treatment for 300 min are included in Figure 9d–f. While the sample synthesized
in the presence of EA presented the same profile as its version without hydrothermal
treatment, broader distribution curves were observed for samples ZA2DEAMW300 and
ZA2TEAMW300. After the application of ultrasound, practically no changes in the distribu-
tion were observed for sample ZA2EAMW300, indicating the difficulty in desaggregating
its particles, showing the same robustness as the STH sample. In the case of samples pre-
ChemEngineering 2022,6, 53 15 of 19
pared in the presence of DEA and TEA, after the application of ultrasound, disaggregation
to smaller particle size took place, resulting in bimodal distribution curves with maxima
at 40
µ
m. Moreover, after the application of ultrasound, it can be observed how these
distributions became wider.
ChemEngineering 2022, 6, x FOR PEER REVIEW 15 of 19
size analyzer, red curve. Sonication treatment is often used to disaggregate primary
particles, changing the distribution curves.
Figure 9. Particle size distribution before (black) and after (red) sonication in water suspension of
synthetized samples (a) ZA2EASTH, (b) ZA2DEASTH, (c) ZA2TEASTH, (d) ZA2EAMW300, (e)
ZA2DEAMW300, (f) ZA2TEAMW300, (g) ZA2EAHT7, (h) ZA2DEAHT7, (i) ZA2TEAHT7.
The samples without hydrothermal treatment presented a monomodal size
distribution centered between 300 and 400 µm. Although, as the degree of amino group
substitution increased, a small shoulder could be observed at lower size values,
approximately at 20 µm, as observed in Figure 9a–c. The application of ultrasound for 15
min did not have a great impact on the distribution curves; only in the case of sample
ZA2TEASTH was the aforementioned shoulder accentuated at 20 µm. This fact indicates
that samples prepared in the presence of EA and DEA give rise to robust particles that are
more difficult to disaggregate than particles obtained using TEA as the precipitant agent.
The particle size distribution curves for the samples subjected to MW hydrothermal
treatment for 300 min are included in Figure 9d–f. While the sample synthesized in the
presence of EA presented the same profile as its version without hydrothermal treatment,
broader distribution curves were observed for samples ZA2DEAMW300 and
ZA2TEAMW300. After the application of ultrasound, practically no changes in the
distribution were observed for sample ZA2EAMW300, indicating the difficulty in
desaggregating its particles, showing the same robustness as the STH sample. In the case
of samples prepared in the presence of DEA and TEA, after the application of ultrasound,
Figure 9.
Particle size distribution before (black) and after (red) sonication in water suspension
of synthetized samples (
a
) ZA2EASTH, (
b
) ZA2DEASTH, (
c
) ZA2TEASTH, (
d
) ZA2EAMW300,
(e) ZA2DEAMW300, (f) ZA2TEAMW300, (g) ZA2EAHT7, (h) ZA2DEAHT7, (i) ZA2TEAHT7.
The distribution curves for the samples subjected to HT hydrothermal treatment for
7 days are included in Figure 9g–i, where it is observed that the samples synthesized
in the presence of DEA and TEA presented the same behavior as the samples with MW
hydrotermal treatment over 300 min, obtaining monomodal particle size distributions
that covered a wide range of sizes. However, when an ultrasound treatment was applied,
the particles disaggregated, obtaining monomodal distributions at smaller particle sizes
with a maxima between 20 and 30
µ
m, which presented, in both cases, a shoulder at even
smaller particle sizes, around 5
µ
m. Furthermore, in both samples it was observed how the
size distributions became narrower. For example, the particle size distribution of sample
ZA2DEAHT7 decreased from a maximum at 130
µ
m to a maximum at 30
µ
m, with a
shoulder at 6
µ
m. In contrast, the curve of sample ZA2EAHT7 presented slightly different
ChemEngineering 2022,6, 53 16 of 19
behavior, where a multimodal distribution was obtained over a wide range of particle
sizes, with maxima at 0.6, 2.7, 10 and 300
µ
m. After the application of ultrasound, the
disintegration of the larger particles occurred, also resulting in a multimodal distribution
with maxima at 0.6, 2.5, 12 and 225
µ
m, with a higher percentage of smaller particles.
The different behavior of sample ZA2EAHT7 was due to fact that in this sample the
hydrotalcite-type structure had completely collapsed and the only phase detected by PXRD
was ZnO (Figure 3).
Table 6includes the volume-weighted mean particle diameter, D[4,3], values for
each of the samples, both before and after ultrasound treatment [
46
,
47
]. In general, the
lowest particle size values were obtained for the samples synthesized in the presence of
TEA, obtaining values even lower than those of the samples obtained without amines
in the reaction medium (reported in our previous work [
27
]). For all series of samples,
similar D[4,3] values were found, regardless of the MW treatment time, whereas, when
HT hydrothermal treatment was applied, as the treatment time increased, the D[4,3] value
slightly decreased. The smallest particle size, 114
µ
m, was found for sample ZA2EAHT7,
where, as previously commented, the structure had collapsed to zinc oxide.
Table 6. Volume-weighted mean particle diameter, D[4,3], values of prepared samples.
Sample D[4,3]
Before Sonication
D[4,3]
After Sonication
ZA2EASTH 428 399
ZA2EAMW60 280 258
ZA2EAMW300 298 248
ZA2EAHT1 327 230
ZA2EAHT7 114 16
ZA2DEASTH 347 324
ZA2DEAMW60 268 263
ZA2DEAMW300 275 210
ZA2DEAHT1 272 94
ZA2DEAHT7 232 22
ZA2TEASTH 274 212
ZA2TEAMW60 221 219
ZA2TEAMW300 201 112
ZA2TEAHT1 253 79
ZA2TEAHT7 231 36
The application of a sonication treatment had a great impact on the D[4,3] value of
the samples subjected to HT hydrothermal treatment, as can be deduced from the data
collected in Table 6. Thus, the lowest value was obtained for sample ZA2DEAHT7, with a
mean particle diameter of 36 µm.
It can be concluded that the samples subjected to MW treatment were formed by
tightly bound particle agglomerates, deduced by the low disintegration after the sonication
treatment. Only for samples with long periods of MW treatment was a slight disinte-
gration of the agglomerates observed, obtaining the lowest D[4,3] value for the sample
ZA2TEAMW300, with 112 µm.
4. Conclusions
The effect on the properties of Zn-Al-NO
3
LDHs, prepared using amines with two car-
bon atoms in the organic chain, together with the application of hydrothermal treatments
using different energy sources, conventional or microwave heating, was studied. Well
crystallized compounds were obtained, the crystallinity of which improved a lot after
prolonged conventional hydrothermal treatment when diethylamine or triethylamine were
used as precipitant agents. When ethylamine was used in the synthesis media, two LDH
phases were obtained, in which the nitrate anion had two different orientations in the
interlayer space. Furthermore, the LDH structure of solids prepared using ethylamine
ChemEngineering 2022,6, 53 17 of 19
collapsed easily with a conventional hydrothermal treatment to form zinc oxide. When
the samples were subjected to conventional hydrothermal treatment, the formation of ZnO
was also observed when DEA or TEA were used, although to a smaller amount. So, the
ZnO content is lower as the degree of substitution of the amino group of the compound
used as precipitating agent increases. The results also showed the formation of aggregates
which could be disaggregated by sonication. So, it was possible to obtain solids with high
crystal sizes and low particle size distributions when a conventional treatment was used.
Author Contributions:
Conceptualization, F.M.L.; methodology, A.M., A.J. and F.M.L.; investigation,
A.M.; resources, F.M.L.; writing—original draft preparation, A.M.; writing—review and editing, A.M.
and F.M.L.; visualization, A.M. and A.J.; supervision, F.M.L.; funding acquisition, F.M.L. All authors
have read and agreed to the published version of the manuscript.
Funding:
Financial support from Universidad de Salamanca (Plan I-B3) and Proyect RTC-2014-1908-3,
Ministerio de Economía y Competitividad.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Not applicable.
Acknowledgments:
A. Misol thanks Junta de Castilla y León and ERDF for a predoctoral contract.
A. Jiménez thanks Universidad de Salamanca and Banco Santander for a predoctoral contract. The
authors want to express the great pleasure and privilege in collaborating with this paper for this
special issue of the ChemEngineering in honor of our mentor and friend Vicente Rives, Professor of
Inorganic Chemistry at University of Salamanca.
Conflicts of Interest: The authors declare no conflict of interest.
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