Mechano-chemical versus co-precipitation for the preparation of Y-modified LDHs for cyclohexene oxidation and Claisen-Schmidt condensations

Article abstract of DOI:10.1016/j.apcata.2020.117797

Y-modified LDHs with atomic Mg²⁺/(Al³⁺+Y³⁺) of 3 and Al³⁺/Y³⁺ ratios of 0.5, 1 and 1.5 were prepared following two preparation methods, i.e. the co-precipitation and mechano-chemical one. The substitution of Al by Y in the brucite-type layer was less effective for the samples prepared by co-precipitation compared to those prepared via mechano-chemical route. In spite the fact yttrium has a larger ionic radius (0.9?) the structural characterizations of these solids confirmed that the layered structure incorporates part of it in the octahedral positions. Further, the reconstruction of the layered structure after an exposure to water for 1 h was more effective for the solid prepared by co-precipitation. The yttrium modified LDHs showed better catalytic activities for cyclohexene oxidation to the corresponding epoxide than the un-modified LDH sample. Then, mixed oxides derived from yttrium-LDH showed very high conversions and selectivities for the synthesis of chalcone.

Full text of DOI:10.1016/j.apcata.2020.117797

AppliedCatalysisA,General605(2020)117797
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Mechano-chemical versus co-precipitation for the preparation of Y-modified
LDHs for cyclohexene oxidation and Claisen-Schmidt condensations
Octavian Dumitru Pavela,*, Alexandra-Elisabeta Stamatea, Rodica Zăvoianua,
Ioana Cristina Bucurb, Ruxandra Bîrjegac, Emilian Angelescua, Vasile I. Pârvulescua,*
a University of Bucharest, Faculty of Chemistry, Department of Organic Chemistry, Biochemistry and Catalysis, 4-12 Regina Elisabeta Av., S3, 030018 Bucharest, Romania
b National Institute of Materials Physics, Department of Surfaces and Interfaces, Atomistilor 405 A, Măgurele, Ilfov 077125, Romania
c National Institute for Lasers, Plasma and Radiation Physics, 409 Atomistilor Street, PO Box MG-16, 077125, Măgurele, Romania
A R T I C L E I N F O
A B S T R A C T
Keywords:
Y-modified LDHs with atomic Mg2+/(Al3++Y3+) of 3 and Al3+/Y3+ ratios of 0.5, 1 and 1.5 were prepared
following two preparation methods, i.e. the co-precipitation and mechano-chemical one. The substitution of Al
by Y in the brucite-type layer was less effective for the samples prepared by co-precipitation compared to those
prepared via mechano-chemical route. In spite the fact yttrium has a larger ionic radius (0.9Å) the structural
characterizations of these solids confirmed that the layered structure incorporates part of it in the octahedral
positions. Further, the reconstruction of the layered structure after an exposure to water for 1 h was more
effective for the solid prepared by co-precipitation. The yttrium modified LDHs showed better catalytic activities
for cyclohexene oxidation to the corresponding epoxide than the un-modified LDH sample. Then, mixed oxides
derived from yttrium-LDH showed very high conversions and selectivities for the synthesis of chalcone.
LDH
Mechano-chemical method
Yttrium
Memory effect
Oxidation
Condensation
1. Introduction
washing and drying. In this method, water is provided by the hydration
molecules coming from the employed inorganic salts.
Besides the interest for the synthesis of base-type materials and
controlled mixed oxides, Layered Double Hydroxides (LDH) have at-
tracted an increased attention due to the facile preparation mechanism,
i.e. shorter reaction times and low amounts of wastes [1–4]. To this
respect, most of the reported investigations focused on the control of
the factors affecting the kinetics of the process, the purity of the crys-
tallographic phases, the homogeneous distribution of the cations in the
structure and the diminution of the operation steps. The general for-
Taking into account these advantages, a consistent number of re-
ports already utilized this method for the LDH preparation. They de-
monstrate the role of the anion (carbonate [12], nitrate [13], sulfate
[14]), the effect of the peptization [15], pre-milling [16,17] and reac-
tion time [18] in these syntheses. The substitution of the cations like
Al3+ by Fe3+ [19–21] or Mg2+ by Ca2+, Cu2+, Zn2+, Ni2+, Cd2+ or
Co2+ was successfully achieved [21–23]. Mechano-chemistry was also
reported for the synthesis of LDH containing monovalent species like
Li+ [24–27], including Li-Al-B4O7-LDH [28], tetravalent like Sn4+ [29]
but also for LDHs containing intercalated organic moieties such as
aminoacids [30,31] or dodecyl sulfate [32]. The same method allowed
the production of LDH nanoparticles [33–37]. However, to date, the
successful substitution of large cations, like lanthanides, was scarcely
reported. Most of the above structures demonstrated catalytic activities,
especially after a thermal treatment when the LDH structure has been
destroyed.
3+
n−
mula of the LDH corresponds to [M2+
M
(OH)2]x+[A ]∙mH2O,
1-x
x
x/n
where M2+ and M3+ are divalent and trivalent cations in the brucite-
type layers, and A is the interlayer anion which balances the exceeding
charge resulting after the isomorphic substitution of M2+ by M3+; x is
usually 0.20−0.33 and m accounts for the water of crystallization [4].
Co-precipitation is the traditional preparation method of these
materials. It consists in the mixing of aqueous salts solutions containing
the targeted cations with an alkaline solution [5–11] at appropriate pH
values. Although it is a reliable procedure, it has some disadvantages
such as multitude steps, different specific equipments and high energy
consumption. As a more simplistic alternative, the mechano-chemical
method requires only a ball-milling of the reactants followed by
The base properties of the LDH and derived oxides also re-
commended these as catalysts for organic reactions. In this respect,
Claisen-Schmidt condensations represent an important base catalyzed
carbon‐carbon bond formation reaction [38]. Using this route
⁎ Corresponding authors.
E-mail addresses: octavian.pavel@chimie.unibuc.ro (O.D. Pavel), vasile.parvulescu@chimie.unibuc.ro (V.I. Pârvulescu).
https://doi.org/10.1016/j.apcata.2020.117797
Received 14 July 2020; Received in revised form 13 August 2020; Accepted 17 August 2020
Availableonline19August2020
0926-860X/©2020ElsevierB.V.Allrightsreserved.

O.D. Pavel, et al.
AppliedCatalysisA,General605(2020)117797
compounds such as chalcone (or chalconoids) and flavone can be pre-
pared. They are subsets of compounds known as flavonoids exhibiting
anti‐oxidant, anti‐inflammatory and anti-cancer activity properties with
great potential applications in medicine [39]. Flavones have proven to
be effective for several diseases associated to an oxidative stress, such as
arteriosclerosis, diabetes, cancer, Alzheimer or Parkinson [40,41].
Benzylidene acetophenone (1,3-diphenyl-2-propen-1-one), the parent
member of the chalcone series, can be prepared via a Claisen-Schmidt
condensation between benzaldehyde and acetophenone in one-pot
synthesis. Flavone can be obtained in the same manner as the chalcone,
by substituting acetophenone with its derivative 2′-hydro-
xyacetophenone. Accordingly, various basic solid catalysts have already
been investigated in these reactions. However, to-date they were less
effective than the layered double hydroxides [42,43].
460 °C for 18 h in air flow led to the mixed oxide (cLDHY-m). The
corresponding reconstructed sample was named hyLDHY-m.
2.2. Characterization of the catalysts
The chemical analysis was performed by Atomic Absorption
Spectrometry on Pye-Unicam AAS Spectrometer to determine the metal
content, while N, H and C were determined by elemental analysis on a Carlo
Erba automatic analyzer. Powder X-ray diffraction patterns were recorded
with a Shimadzu XRD 7000 diffractometer using the Cu Kα radiation
(λ = 1.5418 Å, 40 kV, 40 mA) at a scanning speed of 0.10° min−1 in the
5−80° 2θ range. DRIFTS spectra obtained from accumulation of 400 scans
in the domain 400–4000 cm−1 were recorded with a NICOLET 4700
spectrometer. N2 adsorption - desorption isotherms were determined using
a Micromeritics ASAP 2010 instrument. Prior to the nitrogen adsorption,
samples were out gassed under vacuum for 24 h. The base properties of the
catalysts were determined by in situ UV–vis titration with organic molecules
with different pKa values, e.g. acrylic acid, pKa = 4.2, and phenol, pKa = 9.9
[48–51] (the detailed working procedure is described in a previous pub-
lication [50]). The number of weak and medium base sites was calculated
as the difference between the chemisorbed acrylic acid and phenol. XPS
measurements were performed at normal angle emission with a Specs
spectrometer, using the Al Kα monochromatic radiation (hν = 1486.7 eV)
of an X-ray gun, operating with 300 W (12 kV / 25 mA) power. A flood gun
with an electron acceleration at 1 eV and electron current of 100 μA was
used in order to avoid charging effects. The energy of the photo ejected
electrons was measured using a Phoibos 150 analyzer, operating with a pass
energy of 30 eV. The XP spectra were fitted using Voigt profiles combined
with their primitive functions for inelastic backgrounds.
The partial substitution of Al3+ by Y3+ may provide the basicity
required by these reactions as a result of the lower electronegativity of
Y3+ (1.22) compared to Al3+ (1.61) [5,44–46]. This advantage has
already been confirmed for both the cyanoethylation of ethanol
[5,46,47] and epoxidation of styrene [45]. However, there are no re-
ports concerning the utilization of these base-solids as catalysts for
Payne-oxidation of cyclohexene or Claisen-Schmidt condensation.
This work aimed: i) a selection of the beneficial Al3+/Y3+ ratio for
the partial substitution of aluminum (ionic radius 0.535 Å) by yttrium
(ionic radius 0.9 Å) in the octahedral positions of the LDH lattice
through samples prepared by co-precipation with Mg2+/(Al3+
+Y3+) = 3 and Al3+/Y3+ = 0.5, 1 and 1.5 molar ratios; ii) the com-
parative study of yttrium modified LDH catalysts Mg2+/(Al3+
+Y3+) = 3 and Al3+/Y3+ = 1 molar ratios prepared via co-precipa-
tion and mechanical routes, thermal treatment and reconstruction; and
iii) the behavior of these catalysts for cyclohexene oxidation with H2O2
and Claisen-Schmidt condensation.
2.3. Catalytic tests
The catalytic performances of the synthesized LDHs were evaluated
in i) the oxidation of cyclohexene with hydrogen peroxide in acetoni-
trile (Scheme 1), and ii) synthesis of chalcone and flavones (Schemes 2
and 3). The oxidation was performed in the presence of 3% (w/w)
catalyst reported to the weight of the reaction admixture consisting of 4
mmoles cyclohexene and 32 mmoles acetonitrile. This mixture was
dissolved in 20 mL of solvent made of equal volumes of acetone and
water. The reactions were carried out at 60 °C, for 5 h reaction time in a
stirred flask. An amount of hydrogen peroxide (30 wt. % H2O2) re-
specting the overall molar ratio H2O2/cyclohexene of 32/1 was step
wisely added in the reaction mixture in the first 4 h. The reaction was
monitored hourly by the analysis of the organic phase extracted in
diethyl ether using a GC K072320 Thermo Quest Chromatograph
equipped with a FID detector and a capillary column of 30 m length
with DB5 stationary phase. The reaction products were identified by
mass spectrometer coupled chromatography, using a GC/MS/MS
Varian Saturn 2100 T equipped with a CP-SIL 8 CB Low Bleed/MS
column of 30 m length and 0.25 mm diameter. The H2O2 consumption
was determined by iodometric titration at the end of the reaction.
The condensation reactions were performed in a 50 mL stirred flask,
under reflux conditions, for 4 h reaction time using a catalyst con-
centration of 5% (w/w) reported to the weight of the reaction ad-
mixture. Benzaldehyde, acetophenone and 2′-hydroxyacetophenone
were purchased from Merck. The catalytic tests were performed at three
different benzaldehyde/methylenic compound molar ratios, i.e. 1/1, 5/
1, 10/1. The analysis of the organic reaction products extracted in
ethanol was performed using the above mentioned equipments.
2. Experimental
2.1. Preparation of the catalysts
Yttrium-modified LDH (LDHY-cp) was prepared via: i) the co-pre-
cipitation, and ii) mechano-chemical route. In the route i) the synthesis
was carried out at a pH of 10 under low supersaturation conditions
using a solution A containing 1.5 M of nitrates of Mg2+, Al3+ and Y3+
(0.2 mol Mg2+, 0.0333 mol Al3+ and 0.0333 mol Y3+ for Al3+
/
Y3+ = 1) in bi-distilled water, and an equal volume of solution B
containing 0.18 mol of Na2CO3 and 0.44 mol of NaOH in bi-distilled
water. Both solutions, A and B, were simultaneously added in a batch
reactor at a feed flow of 60 mL h−1 with a Gilson peristaltic pump and
mixed at room temperature under vigorous stirring of 600 rot∙min−1
.
The obtained gel was then aged 18 h at 75 °C, cooled to room tem-
perature, filtered and washed with bi-distilled water until the neutral
pH of the washing water was reached. The drying of the LDH gel was
performed at 90 °C for 24 h in an air flow. The corresponding mixed
oxide (cLDHY-cp) was obtained via the calcination of LDHY-cp at
460 °C for 18 h in an air flow. The reconstruction of the layered struc-
ture (partial recovering the original LDH structure) by a “memory ef-
fect” was performed by impregnation of the mixed oxides (samples
calcined at 460 °C) with a volume of bi-distilled water exceeding with
10 vol% the volume of pores. After impregnation the samples were kept
for 24 h at 25 °C under a pure nitrogen flow in order to avoid any
contamination with carbonate anions. The reconstructed solids were
then separated by filtration and dried at 90 °C for 24 h in the same N2
flow (hyLDHY-cp).
In the route ii) the required amounts of nitrates, Na2CO3 and NaOH
were mixed directly in a Mortar Grinder RM 200 for 1 h without the
addition of water or another compound. The obtained white paste was
then washed with bi-distillated water until the neutral pH value. The
drying of the obtained gel was performed at 90 °C for 24 h in air flow,
yielding the solid LDHY-m. The calcination of the dried sample at
3. Results and discussion
3.1. Catalysts characterization
The chemical analysis of the synthesized LDH, Table 1, showed the
absence of nitrogen confirming that nitrate ions were completely
2

O.D. Pavel, et al.
AppliedCatalysisA,General605(2020)117797
Scheme 1. Mechanism of cyclohexene oxidation.
removed during the washing step. The content of carbon was lower for
mechano-chemical prepared samples compared to the co-precipitated
ones suggesting that the first method allows the incorporation of more
OH− than CO32−. Then, the comparison of the initial materials with the
reconstructed ones revealed a decrease of the content of C. During the
calcinations, a part of carbonate was lost (the entire amount being re-
moved at temperatures higher than 650−700 °C). Also, due to the lack
of carbonate in the bi-distilled water, the content of OH embedded in
the rebuilt material was higher as a result of the increase of the H
content.
Scheme 3. Synthesis of flavone.
apparently there are insignificant or no substitution of Al3+ by Y3+ in
the brucite-like layer. For the solid prepared with an Al3+/Y3+ molar
ratio of 1 an increase of the a-lattice parameter is observed, indicating a
partial substitution of Al3+ by Y3+. Additionally, the above samples
obtained by co-precipitation in the presence of large amount of yttrium,
exhibit significant larger and less intense lines compared to the re-
ference Y-free HT-cp solid (Fig. 2) revealing a decline of their overall
crystallinity with the increase of yttrium content. The same effect at-
tributed to the distortions induced in the layers by the large difference
between the ionic radii of Y3+ and Al3+ was observed also by Fer-
nandez et al [44]. The influence of the increasing Y amount on the
crystallinity could be surveyed by the reduction of their crystallites
sizes. The D003 crystallites size were extracted via the Scherrer formula
from the broadness (full width at half maximum, FWHM) of the (003)
reflection. The values for Y-modified LDHs are proportional with the
yttrium content and significant smaller (Table 2) compared to the Y-
free reference sample of 84 Å (Table 3). The D003 crystallite size is re-
lated to the brucite-like layers stacking distance, therefore one could
assume that larger yttrium amount impeded the long distance packing
of the layers along the c-axis. For small Y loading, we observed and
described the slight increase of the c-lattice parameter [45,53], as
The XRD patterns of Y-modified solids at high yttrium contents
(Al3+/Y3+ = 0.5; 1 and 1.5) and Mg2+/(Al3++Y3+) = 3 are displayed
in Fig. 1. Their structural data were gathered in Table 2. The unit cell
parameter a is related to the distance between two metallic cations
present in the brucite–like layer and is regard as an index of the che-
mical composition of LDH phase [4,52]. Considering the octahedral
arrangement of the cations, a=√2·d(M–O), where d(M–O) is the mean
metal-oxygen distance and depends on the proportion of the metal
components and their ionic radii. Although there is an important dif-
ference in their Shannon ionic radii (Y3+/Al3+=0.9/0.535), for very
low Y concentration (Al3+/Y3+ molar ratio of 11.5), it is most likely
not to observe an effect on the a-lattice parameter as we had found and
reported [53]. Starting from Al3+/Y3+ = 4 and at a higher amount of
Al3+/Y3+ = 1, which is ten times higher, the slight increase of the a-
lattice parameter due to the partial substitution of Al3+ with Y3+ was
confirmed [45].
For the solids obtained by co-precipitation, i.e. for Al3+/Y3+ ratios
of 0.5 and 1.5, the a-parameter does not change significantly compared
to that of the unmodified reference Y-free hydrotalcite Mg2+/Al3+ = 3
LDH (e.g. 3.06 [52], Table 3), consequently for these samples,
Scheme 2. Claisen-Schmidt condensation for
the synthesis of benzylidene acetophenone.
3

O.D. Pavel, et al.
AppliedCatalysisA,General605(2020)117797
Table 1
Elemental analysis of the samples.
Sample
Chemical composition, wt% (moles %)
Mg2+
/
Al3+/Y3+
;
(Al3++Y3+) ratio(a)
[CO32−/ (Al3++Y3+)] ratiosa
C
H
Mg2+
Al3+
Y3+
HT-cp [52]
HT-m [52]
LDHY-cp
2.1(1.9)
1.9(1.7)
1.8(1.8)
0.5(0.5)
1.7(1.7)
0.4(0.4)
4.2(44.3)
4.3(44.9)
3.6(42.9)
4.5(48.9)
3.8(44.4)
4.2(47.1)
24.1(10.4)
24.2(10.4)
22.5(11.0)
22.3(10.0)
22.4(10.8)
22.1(10.2)
8.9(3.5)
8.8(3.4)
4(1.8)
-(-)
3.008
3.055
3.097
2.985
3.001
3.023
-; [0.531]
-(-)
-; [0.486]
13.4(1.8)
13.5(1.6)
13.8(1.8)
13.9(1.8)
0.983; [0.502]
1.024; [0.136]
0.978; [0.461]
0.924; [0.111]
hyLDHY-cp
LDHY-m
4.2(1.7)
4.1(1.8)
3.9(1.6)
hyLDHY-m
a
As determined from the results of AAS analysis.
Fig. 1. The XRD patterns of the LDHs with at Mg2+/(Al3++Y3+) and Al3+
/
Fig. 2. The XRD patterns of LDH with Mg2+/(Al3++Y3+) = 3 and Al3+
/
Y3+ = 0.5; 1 and 1.5 respectively, prepared by co-precipitation. On top the
XRD pattern of the reference Y-free LDH (Mg2+/Al3+) (denoted HT-cp in [52])
is included.
Y3+ = 1 produced via co-precipitation (cp) and mechano-chemical (m) routes.
a slight increase of c-parameter [58,59] or no modification of both
lattice parameters [60] for relatively low amounts of yttrium. In the
case of these as-prepared samples Y-modified LDHs, with high amounts
of yttrium, we observed a decrease of c-parameter, in particular for the
Al3+/Y3+ = 0.5 sample. This result could be associated with the sig-
nificant distortion effect produced by the high amount of Y generating
an important degree of stacking disordering along the c-axis, with a
modification of anions composition and orientation, e.g. high amount of
Table 2
The structural data of the LDH samples Mg2+/(Al3++Y3+) = 3 produced by
co-precipitation method.
Al3+/Y3+ molar ratio
Lattice parameters
IFS (Å)a
D003 (Å)
a (Å)
c (Å)
1.5
1
3.058
3.076
3.058
22.244
22.213
21.894
2.61
2.60
2.50
22
17
12
OH− and lower amount CO3 groups along with the formation of
2-
0.5
amorphous material containing yttrium, more likely basic yttrium
carbonate Y(OH)CO3 or Y(OH)3 [61]. The sample obtained by co-pre-
cipitation at Al3+/Y3+ = 1 for which the increase of the a-parameter
endorsed an Al3+ partial substitution by Y3+ and the c-parameter a
a
IFS = c/3–4.8 Å, where 4.8 Å is the thickness of the brucite-like layer [54].
Table 3
Structural data of the investigated samples.
Samples
2-
reduced CO3 amount is labeled catalyst LDHY-cp. Indeed, the ele-
mental analysis for this sample, presented in Table 1, revealed a smaller
CO32-/(Al3++Y3+) ratio of 0.502 for this sample compared to the ratio
CO32-/Al3+ of 0.531 for Y-free HT-cp. The Al3+/Y3+ = 1 molar ratio
with prospects to secure a partial substitution of Al3+ by Y3+ in the
“brucite-like” layer was selected for the preparation of the Y-modified
LDH catalysts tested for their catalytic performance in the oxidation of
cyclohexene with H2O2 and Claisen-Schimdt condensation.
a (Å) c (Å)
IFS* (Å) FWHM
2θ (003) Crystallite
size
(003)1/
(200)2
(003) (Å)1/
(200) (Å) 2
HT-cp [52] 3.060 23.357 2.99
HT-m [52] 3.053 23.022 2.87
0.9917(1)
0.9134(1)
3.68(1)
11.3549 84(1)
11.5963 91(1)
12.1536 22(1)
LDHY-cp
3.076 22.213 2.60
cLDHY-cp
4.210
–
–
2.5933(2)
1.8667(1)
0.355(1)
2.2100(2)
1.5200(1)
–
28(2)
The XRD patterns of the Y-modified LDHs as-prepared either by co-
precipitation or mechanical routes, thermally treated and reconstructed
via memory effect are presented in Fig. 2. The structural data of the
LDHY catalysts are included in Table 3 alongside with the corre-
sponding data for Y-free reference LDHs (Mg/Al = 3 molar ratio) that
were already reported [52].
hyLDHY-cp 3.051 22.415 2.67
11.8017 43(1)
LDHY-m
3.098 21.894 2.50
4.230
12.6092 41(1)
cLDHY-m
–
–
–
28(2)
hyLDHY-m 3.101 21.136 2.25
13.1896 53(1)
* IFS represents the interlayer free distance.
1
2
Corresponds to LDH-phases.
The survey of the structural data included in Table 3 exposed some
aspects associated with yttrium incorporation. It is to be observed that
the mechanical route leads to a higher a-parameter thus, to a higher
partial substitution of Al3+ by Y3+ compared to the co-precipitation
route. Its c-lattice parameter is considerable smaller than for LDHY-cp,
indicative of a change of the interlayer anionic composition namely, a
Corresponds to the mixed oxides phases.
expected, due to the larger size and lower polarizing ability of Y3+
compared to Al3+, causing the decrease of the Coulombic attraction
force between the brucite-like layers [55,56]. Similar results were re-
ported also by Rives and coworkers [44,57]. Swirk et al. reported either
4

O.D. Pavel, et al.
AppliedCatalysisA,General605(2020)117797
2−
reduction of the number of the larger CO3
anions. It is worth to
notice that the c-parameter of the reference HT-m sample is smaller
than that of the HT-cp sample. The observations are consistent with the
analytical measured CO32−/(Al3++Y3+) and CO32−/Al3+ ratios re-
spectively, which indicate a decrease of these ratios for the mechanical
route as compared with the co-precipitation route e.g. from 0.502 to
0.461 for the LDHY catalysts and from 0.531 to 0.486 for the reference
HT samples, respectively (Table 1).
Large ion Y3+ induces a similar distortion effect in the sample ob-
tained via the mechanical route as observed by the its XRD curve profile
but, apparently the order along the c-axis is less affected in comparison
with its corresponding LDHY obtained by co-precipitation: the D003
crystallite size is 41 nm for LDHY-m and only 22 nm for LDHY-cp. The
products obtained by their calcination are mixed oxides Mg(Al/Y)O of
periclase-type structure (ICDD card no.045-0946) and are Miller in-
dexed consequently in Fig. 2. In the cLDHY-m pattern a very small line,
assignable to the most intense line of cubic Y2O3 (ICDD card no.041-
1105) is detectable. The cubic lattice parameters a of the references
cHT-cp and cHT-m had smaller values than the standard value of
4.2112 Å of MgO-pure (ICDD card no. 045-0946) as expected due to the
difference between Shannon ionic radii of Al3+ and Mg2+ (0.535Å/
0.72Å). The lattice a-parameter of the cLDHY solids are larger, in
particular for the cLDHY-m sample, preserving the difference between
the brucite-like layer related a-parameter of their parent LDHYs. The
crystallites sizes for cLDHY-m and cLDHY-cp are equal and smaller than
their reference Y-free samples due to the distortion effect of Y insertion.
The reconstruction occurred for both hyLDHY-cp and hyLDHY-m sam-
ples with the same characteristics as for Y-free LDH [51], namely: a
decrease of the c-parameter marking the predominance of OH− anions
replacing the CO32- anions and, an improved layers stacking along the c-
axis revealed by larger crystallites sizes. The differences observed be-
tween the two routes are preserved in their “reconstructed” formed:
slight larger a-lattice parameter, smaller c-parameter, thus lower
Fig. 3. The DRIFT of the synthesized catalysts.
Table 4
Textural characteristics and basicity of the investigated samples.
No. Samples
Surface
Pore
Total
Distribution of base sites
area
volume
(cm3‧g−1
)
number of
base sites*
(mmol
(m2‧g−1
)
Strong Weak and
base sites* medium
AA‧g−1
)
(mmol
strength
PhOH‧g−1
)
base sites#
(mmol‧g−1
2-
amount of CO3 and larger crystallite size. The analytically measured
)
CO32-/(Al3++Y3+) ratios of the hyLDHY catalysts corroborate the XRD
results: 0.136 for hyLDHY-cp and 0.111 for hyLDHY-m (Table 1). The
(222) line of the Y2O3 are less detectable in the XRD pattern of hyLDHY-
m compared to cLDHY-m. Therefore, it may be inferred that the sam-
ples prepared via the mechanical route are structurally promising in
view of their ability to accommodate Y3+ in the brucite-like layer and a
slightly better crystalline order.
1
2
3
4
5
6
LDHY-cp
84
0.12
0.26
0.06
0.10
0.22
0.05
8.00
8.25
8.06
8.12
8.34
8.17
0.39
0.42
0.36
0.61
0.63
0.59
7.61
7.83
7.70
7.51
7.71
7.58
cLDHY-cp
190
hyLDHY-cp 44
LDHY-m
70
cLDHY-m
163
hyLDHY-m 38
* AA = acrylic acid; PhOH = phenol.
The DRIFTS spectra of the yttrium modified LDHs are shown in
Fig. 3. The band at 3700−3400 cm−1 corresponds to the OH group
vibration, ν(OeH), in the LDH structure while that at 3070 cm−1 relates
to the hydrogen bonds between water and carbonate in the interlayer of
the hydrotalcite. The band at 1640−1650 cm−1 is due to the H2O
bending vibration of interlayer water in the hydrotalcite structure.
#
Equal to the difference: Total number of base sites-Strong base sites.
lower density of anions (such as OH− groups) in the catalyst structure.
This result may be also attributed to a partial release of the CO2 during
the milling of the reagents, as an effect of heating.
2-
The bands at 1400 cm−1 and 680 cm−1 were assigned to the CO3
The basicity measurements results were compiled in the same
Table 4. Mixed oxides possessed a stronger basicity than the LDH pre-
cursor or reconstructed layered double hydroxides regardless the pre-
paration route. The stronger basicity is associated to surface O2− spe-
cies [62], the weak base sites to surface OH- species and the medium
strength basic ones to the oxygen bridging Mg2+, Al3+ and Y3+ cations.
These results indicated that the materials obtained via the mechano-
chemical route present an enhanced basicity compared to the co-pre-
cipitated ones that is accordance to the higher concentration of the OH
groups. Very important, the addition of yttrium induced a slightly more
increased basicity compared to the reference LDH material [52].
Y has the lowest electronegativity in the series Y (1.22) < Mg
(1.31) < Al (1.61) that corresponds to an increased basic character
[63]. This fact has also been confirmed by our basicity measurements
(Table 4) and agrees with the literature reports [64–67].
group vibration while those at 600−500 cm−1 and below 500 cm−1
correspond to MgeO and AleO bonds. The intensity of these bands is
well correlated with the results of XRD and chemical analyses. By cal-
cination, part of OH- and CO32- groups are removed from the structure
as CO2 and H2O leading to a decrease of the intensity of the bands
located at 3070 and 1640−1650 cm−1, respectively. However, a
complete removal of CO2 has been achieved only after calcination at
temperatures higher than 650 °C. The hydration of the mixed oxides
restored the bands at 3070 and 1640−1650 cm−1. Unfortunately, such
a treatment led to an additional segregation yttrium in the LDH as Y2O3
(new characteristic bands at 440−442 cm−1).
Table 4 presents the surface areas of the investigated samples. The
presence of the interlayer yttrium determined a quite drastic decrease
of the surface area [4]. Also, the reconstructed samples (hyLDHY-cp
and hyLDHY-m) had smaller surface areas confirming the fact that the
restoration of the LDH structure is only partial.
The XPS spectrum of dried LDHY-cp and LDHY-m samples in the
region of Y3d level (Fig. 4 a,b) has been deconvoluted into two doublets
assigned to the binding energy of this element in hydroxide and car-
bonate, respectively (Table 5) [68,69]. Taking as reference the 3d5/2
Mechano-chemical route led as well to smaller surface areas com-
pared to LDH produced by co-precipitation proving the presence of a
5

O.D. Pavel, et al.
AppliedCatalysisA,General605(2020)117797
Fig. 4. XPS core level spectra: Y3d of LDHY-cp (a) and LDHY-m (b); C1s of LDHY-cp (c) and LDHY-m (d); O1s of LDHY-cp (e) and LDHY-m (f); Mg2p of LDHY-cp (g)
and LDHY-m (h); Al2p of LDHY-cp (i) and LDHY-m (j).
6

O.D. Pavel, et al.
AppliedCatalysisA,General605(2020)117797
Table 5
attributable to MgCO3 [72]. For LDHY-m the XPS spectrum had only
one component at 50.2 eV corresponding to Mg in magnesium hydro-
xycarbonate [73].
The XPS binding energies for the analyzed levels.
XPS level/
Binding energies/bond
The XPS spectrum corresponding to the Al2p level (Fig. 4i,j) showed
a broad band that has been deconvoluted in two components at 74.2 eV
corresponding to Al2O3 and second one at 75.1 attributed to AlOOH
(boehmite) [74].
LDH
Y3d
Y-OH− 3d5/2 Y-OH− 3d3/2 Y-CO32− 3d5/2 Y- CO32− 3d3/2
LDHY-cp
LDHY-m
C1s
157.9 eV
157.9 eV
C-O-H
286.5 eV
–
160.2 eV
159.8 eV
C-O
159.6 eV
158.7 eV
O-C = O
289.6 eV
–
162.0 eV
160.7 eV
291.5 eV
291.5 eV
–
LDHY-cp
LDHY-m
O1s
287.7 eV
–
3.2. Catalytic tests
Y-OH
O=C-O
532.2 eV
532.5 eV
MgCO3
51.8 eV
–
O-C-O
O = C-O
534.6 eV
–
In the absence of the catalyst, after 5 h, the decomposition of H2O2
(determined by the iodometric titration) was in the range of 5–7 % (w/
w) from the total introduced amount.
LDHY-cp
LDHY-m
Mg2p
531.6 eV
530.9 eV
Mg(OH)2
49.6 eV
–
533.5 eV
533.6 eV
Hydroxycarbonate
LDHY-cp
LDHY-m
Al2p
–
The oxidation of cyclohexene may lead to several products, Scheme
1. Without any catalyst only cyclohexanol and cyclohexenone were
produced with very small conversions. Without the solvent the oxida-
tion occurred with high conversions of cyclohexene but with yields in
cyclohexene epoxide below 10 %. Only the presence of the catalyst
conducted the oxidation of cyclohexene towards cyclohexene oxide.
Also, the presence of water enables the extraction of the acetamide by-
product in the aqueous phase thus affording an easier separation of the
epoxide from the reaction mixture. The water–acetone as solvent is
appropriate for this oxidation reaction due to its requiring of a smaller
amount of organic solvent, being more eco-friendly [50].
50.2 eV
Al2O3
AlOOH (boehmite)
75.1 eV
LDHY-cp
LDHY-m
74.2 eV
74.1 eV
76 eV
component, the ratio between the YeOH and Y−CO32− was of 1.45 for
LDHY-m and 0.89 for LDHY-cp. This result fits the conclusions resulted
from the X-ray analysis of these samples. The deconvolution of the C1s
and O1s spectra also provided arguments for its association to hydroxyl
and carbonate groups (Fig. 4c–f). For C1s this is reflected by the bands
at 286.5 eV (CeOeH), 287.7 eV (CeO), and 289.6 eV (OeC]O) while
for O1s 532.2-532.5 eV (O]CeO) [70], 533.5 eV (OeCeO) and
534.6 eV (O]CeO). In a good connection to the XPS spectrum of the
Y3d level, for LDHY-m the intensity of the bands assigned to the
O]CeO bonds was also very small.
Fig. 5 presents the variation of the conversion vs. time. The catalytic
activity was influenced by the basic active sites strength, resulting the
following variation trendlines: calcined sample (O2−) > hydrated
sample (OH- majority) > dried sample (mixture of OH-/CO32−).
It is worth to note that the initial activity of the mechano-chemical
catalysts was superior to that of the co-precipitated ones (after 1 h re-
action time, the conversions were almost double). This result is in line
with the information provided by both XPS (showing an increased
population of YeOH compared to YeCO32−) and titration with organic
molecules with different pKa values. It is also consistent to the XPS
The bands at 531.6 and 530.9 eV accounts for the O1s level attri-
butable to an oxygen directly connected to Y [71]. The XPS spectrum of
the Mg2p level (Fig. 4g,h) indicated a broad band in the range
44−54 eV for LDHY-cp that could be deconvoluted in a component at
49.6 eV attributable to Mg in Mg(OH)2 and a second one at 51.8 eV
Fig. 5. The catalytic activity of the LDHY catalysts for the oxidation of cyclohexene (blue points [52]) (For interpretation of the references to colour in this figure
legend, the reader is referred to the web version of this article).
7

O.D. Pavel, et al.
AppliedCatalysisA,General605(2020)117797
Fig. 6. The variation of the yield to cyclohexene oxide vs. the sum of weak and
medium strength basic sites.
Fig. 7. The cyclohexene oxide yield of LDHY-cp and LDHY-m after fourth re-
action cycles.
Fig. 9. Catalytic performances for the synthesis of chalcone and flavone using
cLDHY-cp and cLDHY-m as catalysts and different benzaldehyde/methylenic
compound ratios: 1/1 (A), 5/1 (B) and 10/1 (C).
Fig. 8. Comparison between the XRD patterns of the reconstructed samples in
water after 24 h by memory effect synthesis (hyLDHY-cp and hyLDHY-m) and
layered samples reconstructed during 1 h reaction time (hyLDHY-cp-1 h and
hyLDHY-m-1 h).
the weak and medium strength base sites is presented in Fig. 6. This
dependence was linear irrespective of the preparation route.
patterns showing a better insertion of Y in the LDH lattice for LDHY-m.
Longer reaction times had as an effect the diminution of these dif-
ferences. The variation of the yield to cyclohexene oxide vs. the sum of
Indeed, the interaction of H2O2 with base surface sites led to active
peroxo species, Scheme 1 [52]. However, the stronger base sites of
8

O.D. Pavel, et al.
AppliedCatalysisA,General605(2020)117797
mixed oxides, for a higher conversion level, change the selectivity in the
favor of large by-products.
CRediT authorship contribution statement
Recycling experiments demonstrated the stability of the parent
LDHY-m and LDHY-cp catalysts. After the fourth reaction cycles there
were no sensible changes in the initial yield of cyclohexene-epoxide
(Fig. 7).
Octavian Dumitru Pavel: Conceptualization, Methodology,
Supervision, Funding acquisition. Alexandra-Elisabeta Stamate:
Investigation, Writing
- review & editing. Rodica Zăvoianu:
Conceptualization, Investigation, Supervision. Ioana Cristina Bucur:
Investigation. Ruxandra Bîrjega: Investigation, Writing - review &
editing, Visualization. Emilian Angelescu: Conceptualization,
Methodology, Supervision. Vasile I. Pârvulescu: Conceptualization,
Methodology, Supervision.
The stability presented by the parent materials over several cycles is
not valid in the case of mixed oxides. The XRD patterns of calcined
catalysts, cHTY-cp and cHTY-m, recovered by filtration from the reac-
tion mixture after 1 h were quasi identical to those obtained by
synthesis involving the memory effect, hyHTY-cp and hyHTY-m, Fig. 8.
The reconstruction of the parent layered structure during the che-
mical reaction was also reported in the literature for the mixed oxides
derived from the calcinations of LDH at a temperature not exceeding
460 °C [52].
Declaration of Competing Interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influ-
ence the work reported in this paper.
Moving to a more challenging reaction, i.e. Claisen-Schmidt con-
densation to chalcone and flavone, the catalytric tests confirmed both
co-precipitation and mechano-chemical prepared LDH as very selective
catalysts (Fig. 9).
Acknowledgments
Under the investigated conditions, the selectivity to chalcone was
higher than 90 %, while that to flavone varied in the range of 57.4–65
%. The conversion of the methylenic compound increased significantly
for reaction mixtures containing an excess of benzaldehyde. Besides
acting as a reactant, benzaldehyde can also play a role in the solubili-
sation of the condensation products thus facilitating their removal from
the surface of the solid catalyst. However, a further increase of the
benzaldehyde/methylenic compound molar ratio beyond 5/1 does not
bring a significant improvement of the performances. Except for an
equimolar benzaldehyde/2′-hydroxyacetophenone ratio, for both the
chalcone and flavone synthesis, the catalyst prepared from the co-pre-
cipitated precursor was barely more active than the one obtained via
the mechano-chemical route, due to the presence of diffusion hin-
drances as a result of the lower specific porosity and specific surface
areas in the case of materials prepared by the mechano-chemical
method and the larger dimension of the condensation reaction pro-
ducts.
This work was supported by a grant of the Romanian Minister of
Research and Innovation, CCCDI – UEFISCDI, project number PN-III-P1-
1.2-PCCDI-2017-0387 / 80PCCDI - 30.03.2018, within PNCDI III
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