Molybdate Stabilized Magnesium‐Iron Hydrotalcite Materials: Potential Catalysts for Isoeugenol to Vanillin and Olefin Epoxidation

Article abstract of DOI:10.1016/j.apcata.2021.118292

A series of molybdate-intercalated and stabilized magnesium‐iron hydrotalcite (HMFeMo) materials with different molybdate loadings were successfully prepared by an in-situ hydrothermal method. The prepared HMFeMo materials were systematically characterized using Fourier-transform infrared spectroscopy (FT-IR), powder X-ray diffraction (XRD), Ultraviolet-visible spectroscopy, scanning electron microscopy, thermo-gravimetric analysis, nitrogen adsorption-desorption and X-ray photoelectron spectroscopy (XPS) experiments. The XRD results demonstrated the successful intercalation of molybdate ions in the interlayer space of magnesium-iron hydrotalcite and the stabilization of the layered structure. In addition, the XPS spectra of the HMFeMo materials revealed the presence of molybdenum in a higher-valent oxidation state. The calcination of HMFeMo materials led to the formation of solid solution of mixed metal oxides. Both the as-prepared and calcined HMFeMo catalysts showed promising activity for the epoxidation of cyclooctene, as a model reaction. Furthermore, the performance of the as-prepared and calcined HMFeMo catalysts for the oxidation of a biomass model compound, namely isoeugenol to vanillin, was evaluated. The isoeugenol conversion over the as-prepared HMFeMo catalysts under solvent-free conditions and using tertiary-butyl hydroperoxide in decane as the oxidant was good. Moreover, the isoeugenol conversion and selectivity toward vanillin of HMFeMo0.1, with a molybdate loading of 0.1 mol %, were the highest (86.2% and 83.1%, respectively) of all HMFeMo catalysts in this study at 80 °C for 5 hr. HMFeMo0.1 presented the best catalytic activity for both the epoxidation of cyclooctene and oxidation of isoeugenol to vanillin, and its activity remained unchanged after several runs.

Full text of DOI:10.1016/j.apcata.2021.118292

Applied Catalysis A, General 623 (2021) 118292
Contents lists available at ScienceDirect
Applied Catalysis A, General
journal homepage: www.elsevier.com/locate/apcata
Molybdate Stabilized Magnesium-Iron Hydrotalcite Materials: Potential
Catalysts for Isoeugenol to Vanillin and Olefin Epoxidation
P.P. Neethu, A. Sreenavya, A. Sakthivel*
Inorganic Materials & Heterogeneous Catalysis Laboratory, Department of Chemistry, School of Physical Sciences, Central University of Kerala, Kasaragod, Kerala,
671320, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Molybdenum
Vanillin
A series of molybdate-intercalated and stabilized magnesium-iron hydrotalcite (HMFeMo) materials with
different molybdate loadings were successfully prepared by an in-situ hydrothermal method. The prepared
HMFeMo materials were systematically characterized using Fourier-transform infrared spectroscopy (FT-IR),
powder X-ray diffraction (XRD), Ultraviolet-visible spectroscopy, scanning electron microscopy, thermo-
gravimetric analysis, nitrogen adsorption-desorption and X-ray photoelectron spectroscopy (XPS) experiments.
The XRD results demonstrated the successful intercalation of molybdate ions in the interlayer space of
magnesium-iron hydrotalcite and the stabilization of the layered structure. In addition, the XPS spectra of the
HMFeMo materials revealed the presence of molybdenum in a higher-valent oxidation state. The calcination of
HMFeMo materials led to the formation of solid solution of mixed metal oxides. Both the as-prepared and
calcined HMFeMo catalysts showed promising activity for the epoxidation of cyclooctene, as a model reaction.
Furthermore, the performance of the as-prepared and calcined HMFeMo catalysts for the oxidation of a biomass
model compound, namely isoeugenol to vanillin, was evaluated. The isoeugenol conversion over the as-prepared
HMFeMo catalysts under solvent-free conditions and using tertiary-butyl hydroperoxide in decane as the oxidant
was good. Moreover, the isoeugenol conversion and selectivity toward vanillin of HMFeMo0.1, with a molybdate
loading of 0.1 mol %, were the highest (86.2% and 83.1%, respectively) of all HMFeMo catalysts in this study at
80 ◦C for 5 hr. HMFeMo0.1 presented the best catalytic activity for both the epoxidation of cyclooctene and
oxidation of isoeugenol to vanillin, and its activity remained unchanged after several runs.
Iron
Oxidation
Olefin
Iso-eugenol
Hydrotalcite
1. Introduction
Friedel-Crafts alkylation [6,7], Baeyer-Villiger oxidation [8], and aro-
matic nitro compound reactions [9].
Hydrotalcite (HT) materials are well known as potential catalyst
precursors and supports for several organic reactions [1]. Compared
with bulk oxides as support materials, HT materials present attractive
properties, such as a good anion exchange capacity, tunable acid base
and redox properties, remarkable activity, low waste formation, and
facile product recovery [2]. The general formula of HTs is [M(II)(1-x)M
(III)x(OH)2]x+[An-x/n]⋅mH2O, where M(II) and M(III) are divalent and
trivalent metals, respectively, and A is an interlayer anion [3,4] and
tailor-made HT-based catalysts with different compositions, viz., cations
in the brucite layer, and anions in the interlayer region can be fabri-
cated. Iron-based catalysts have invariably contributed to mankind’s
progress owing to their involvement in the Haber-Bosch and Fischer--
Tropsch processes. Iron-based catalysts can be used for several organic
transformations [5] and, particularly, magnesium–iron HTs are used for
Stabilization of the interlayer spaces of HTs using anions, such as
silicate, molybdate, vanadate and phosphate enhance the number of
exposed active sites and improves the catalytic activity of HTs [1, 10-
14]. Furthermore, the chemical, magnetic, electronic, and optical
properties of the host HT can be tuned using different intercalated an-
ions [1]. The intercalation of different poly-oxometallates facilitated the
development of catalysts with attractive acid bases and redox properties
[10–14]. Poly-oxometallate-intercalated layered double hydroxides
have been used for epoxidation, esterification, photo-degradation, per-
oxidation, and alkoxylation reactions [10–12].
To date, the limited studies are known on catalytic activity of
molybdate-intercalated HT materials for organic processes, such as
propane dehydrogenation [13], tertiary butanethiol oxidation [14], and
styrene oxidation with air [15]. The typical oxidation state of
* Corresponding author.
E-mail address: sakthiveldu@gmail.com (A. Sakthivel).
https://doi.org/10.1016/j.apcata.2021.118292
Received 7 April 2021; Received in revised form 12 July 2021; Accepted 17 July 2021
Available online 21 July 2021
0926-860X/© 2021 Elsevier B.V. All rights reserved.

P.P. Neethu et al.
Applied Catalysis A, General 623 (2021) 118292
molybdenum varies from -2 to +6, and the coordination number of
molybdenum ranges from 4 to 6; moreover, molybdenum species pre-
sent different stereo-chemistries, and can form bi- and poly-nuclear
compounds with many organic and inorganic ligands. Owing to the
aforementioned properties, molybdenum can be used to develop new
catalysts with attractive potential applications. Molybdenum-containing
catalysts are widely used for organic reactions, such as epoxidation,
metathesis, hydroformylation, and C–H activation [16,17]. Several
cyclopentadienyl molybdenum complexes have been used for various
oxidation reactions. Researchers have grafted molybdenum complexes
on supports, such as MCM-41, MCM-48, and SBA-15, and the catalytic
activity of molybdenum toward oxidation reactions has been well
established [17]. However, to date, the introduction of molybdate ions
into HT materials and evaluation of their catalytic activity has been
limited. Molybdenum–iron centers are known as potential
nitrogen-fixation cofactors in biological systems. Therefore, the devel-
opment of molybdenum–iron-based HT materials and exploration of
their redox properties for oxidation of biomass-derived model compo-
nents are attractive research topics. Among biomass-derived model
components, isoeugenol has received increasing attention because the
selective oxidation of isoeugenol to vanillin is an important process in
the fine chemical industry.
evaluated. To the best of our knowledge, this is the first report on the
conversion of isoeugenol to vanillin over a HT-like material.
2. Experimental
2.1. Materials
Magnesium nitrate hexahydrate (Mg(NO3)2⋅6H2O (Merck)), Ammo-
nium heptamolybdate tetrahydrate ((NH4)6Mo7O24⋅4H2O (SRL)), Ferric
nitrate nonahydrate (Fe(NO3)3⋅9H2O (LOBA)), aqueous ammonia solu-
tion (25 %; SRL) were used as starting material for synthesis. For car-
rying out the reaction, cyclooctene (Spectrochem), tertiary-Butyl
hydroperoxide solution 5.0-6.0 M in decane (Sigma Aldrich), isopropyl
alcohol (LOBA), isoeugenol (Avra), acetonitrile (Merck) were used.
2.2. Catalyst preparation
Molybdate-intercalated and stabilized magnesium–iron HT
(HMFeMo) catalysts were prepared using an in situ hydrothermal
method, as follows [4,27]. Solution 1 was prepared by adding Mg
(NO3)2⋅6H2O (45.54 mmol) and Fe(NO3)3⋅9H2O (15 mmol) to deionized
water (120 mL). Solution
2
was prepared by adding
Vanillin, which is an important flavor and aroma compound, is
present in vanilla and is mainly used in the food, cosmetic, perfume, and
pharmaceutical industries [18]. Moreover, vanillin has been used to
manufacture thermoplastics [19]. Vanillin is produced worldwide using
different sources, such as oil (85%), biomass (15%), and orchid pods
(<1%) [20]. Because vanillin natural resources are increasingly scarce,
many cost-effective methods are used for the industrial production of
vanillin. Recently, one of the primary methods for the production of
vanillin from lignin-derived feed-stocks, such as isoeugenol, has
attracted increasing attention, and several reports regarding the syn-
thesis of vanillin from isoeugenol have been published. Isoeugenol was
oxidized to vanillin using graphene oxide-supported copper oxide with a
vanillin yield of 53% under mild reaction conditions [21].
Aluminosilicate-supported transition metal catalysts have also been
used to produce vanillin. The conversion and selectivity for the oxida-
tion of isoeugenol using H2O2 as a green oxidizing agent over
(NH4)6Mo7O24⋅4H2O (4.54 mmol) and aqueous ammonia (NH3aq.10
mL) to deionized water (100 mL). Solution 1 was added to solution 2
dropwise at 60 ◦C under vigorous stirring. Precipitation was observed at
a final gel pH of 9.0. After precipitation was completed, the slurry was
transferred to a stainless-steel autoclave and aged at 100 ◦C for 4 d. The
precipitate was filtered and washed with deionized water until the pH of
the filtrate reached 7, followed by drying at 80 ◦C. The catalysts with
ammonium hepta-molybdate loadings of 0.02, 0.05, 0.1 and 0.15 mole
with respect to 1 mole of magnesium in the gel are denoted as HMFeMo
0.02, HMFeMo 0.05, HMFeMo 0.1, and HMFeMo 0.15, respectively. For
comparison, a carbonate intercalated magnesium–iron HT (HMFe) ma-
terial was also prepared using the same procedure without molybdenum
source. The synthesized HMFeMo catalysts were calcined at different
temperatures (T), and the calcined samples are hereafter denoted as
HMFeMoC-T, where T represent the calcination temperature.
niobium-incorporated catalysts yielded only 46
%
vanillin [22].
2.3. Material characterization
Gusevskaya et al. reported the oxidation of isoeugenol to vanillin over a
n-Bu4NVO3/pyrazine-2-carboxylic acid catalyst combination [23]. The
typical vanillin synthesis method from eugenol involves two steps:
eugenol isomerization to isoeugenol and isoeugenol oxidation to
vanillin. In contrast, a direct one-step method was developed for the
oxidation of eugenol to vanillin using a cobalt-based catalyst [24].
Among the various catalysts used for isoegenol oxidation the catalyst
prepared by Shimazu et al. showed the highest conversion for iso-
eugenol. They synthesized a cobalt porphyrin intercalated into lithium
taeniolite clay to oxidize isoeugenol using molecular oxygen as the
oxidant and achieved a highest vanillin yield of 72 % [25]. But, the
preparation of the catalyst involved complex synthetic procedure.
Recently, cerium-containing zeolite, viz., MCM-22 and molybdenum
incorporated MCM-22 are explored as catalysts for the oxidation of
isoeugenol [26]. However, the selectivity towards vanillin always
limited and the development of catalyst for selective oxidation of iso-
eugenol to vanillin is an important area of research in recent years. The
present catalyst showed improved selectivity with good conversion of
isoeugenol.
The powder X-ray diffraction (XRD) patterns of the synthesized
catalysts were recorded on a MiniFlex (300/600) (Rigaku, Japan) XRD
instrument with Cu K
α
radiation (λ =1.54059 Å) in the 2θ range of 3-80◦
at a scan speed of 0.05 and step size of 0.5◦. The Fourier-transform
infrared (FT-IR) spectra of the catalysts were obtained using a FT/IR-
4700 (JASCO, Japan) spectrometer in the wavenumber range of 4000-
400 cm-1. The diffuse reflectance ultraviolet–visible (UV–vis) spectra
of the catalysts were recorded (200–800 nm), by using a UV-visible
spectrophotometer (UV2600 Shimadzu, Japan), and BaSO4 as a refer-
ence. Thermo-gravimetric analysis (TGA) of the catalysts was performed
using a STA 6000 (Perkin Elmer, Germany) simultaneous thermal
analyzer. About 2–5 mg of the sample was taken in the silica crucible
and heated under a nitrogen atmosphere with a heating rate of 10 ◦C/
min, ranging from 40 to 900 ◦C. The scanning electron microscopy
(SEM) images of the catalysts were obtained using a SEM (Phillips
Technai G2 T30 SEM) operated at 300 Kv. The X-ray photoelectron
spectroscopy (XPS) profiles of the samples were obtained using a
photoelectron spectrometer (Prevac, Poland) that was equipped with a
VG Scienta’s R3000HP analyser and a MX650 monochromator. Nitrogen
adsorption–desorption experiments were performed at ꢀ 196 ◦C using an
ASAP 2020 (Micromeritics USA) automatic micropore physisorption
analyzer after the samples were degassed at 200℃ for at least 8 h under
10-3 Torr pressure prior to each run.
Therefore, designing a highly active, selective, and cost-effective
catalyst for the conversion of isoeugenol to vanillin is of potential in-
terest for researchers. In this study, we evaluated the structural prop-
erties and catalytic activity of magnesium–iron HT-like materials with
molybdate anions in the interlayer region. Furthermore, the catalytic
activity of molybdate-intercalated magnesium–iron HT-like materials
for isoeugenol oxidation to vanillin was investigated. In addition, the
activity of the synthesized catalysts for cyclooctene epoxidation was
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P.P. Neethu et al.
Applied Catalysis A, General 623 (2021) 118292
2.4. Cyclooctene oxidation
Catalytic oxidation of cyclooctene using tertiary-butyl hydroperoxide
(TBHP) in decane as the oxidizing agent was performed in a 25 mL two-
necked round-bottom flask. For a typical procedure, 2 mmol of cyclo-
octene and 0.025 g of catalyst were added to the flask and magnetically
stirred at the desired reaction temperature. Thereafter, 2 mmol of TBHP
in decane was added dropwise to the reaction mixture. After the reaction
was completed, the products were extracted by adding 1 mL of iso-
propanol to the reaction mixture and were subsequently analyzed using
agas chromatography (GC; Mayura Analytical GC 1100, India) instru-
ment with a flame ionization detector connected with AB-Innowax
column (30 m length and 0.25 mm internal diameter).
2.5. Isoeugenol oxidation
The isoeugenol oxidation reaction (Scheme 1) using TBHP in decane
as the oxidizing agent was performed in a 25 mL round-bottom flask.
Isoeugenol (2 mmol) and catalyst (0.05 g) were added to the flask and
magnetically stirred at the desired reaction temperature. TBHP in
decane was added dropwise to the reaction mixture. After the reaction
was completed, the products were extracted by adding 4 mL acetonitrile
to the reaction mixture and were subsequently analyzed using the
aforementioned GC instrument.
Fig. 1. Fourier-transform infrared spectra of as-prepared (a) HMFeMo0.15, (b)
HMFeMo0.10, (c) HMFeMo0.05, (d) HMFeMo0.02, and (e) HMFe.
modes of α-FeOOH [29]. The vibrational bands at 973, 835, and 547
cm-1 in the FT-IR spectra of the calcined samples corresponded to ter-
minal M = O stretching, antisymmetric Mo–O–Mo, and Fe2O3, respec-
tively [30].
The powder XRD patterns of the as-synthesized HMFeMo samples are
presented in Fig. 2 and confirmed the layered hydrotalcite structure of
the synthesized materials. The broad peaks at 2θ value of 11.5◦, 23.1◦,
34.7◦, 59.5◦, and 61.2◦ in the XRD patterns of the as-synthesized
HMFeMo samples corresponded to the (003), (006), (012), (110), and
(113) planes, respectively, of layered hydrotalcite materials [15]. As
molybdate loading increased, the aforementioned peaks broadened,
confirming that molybdate ions were intercalated in the interlayer space
3. Results and discussion
3.1. Catalyst characterization
The FT-IR spectra of the as-synthesized and calcined HMFeMo ma-
terials are illustrated in Fig. 1 and Fig. S1, respectively. The broad vi-
bration band at approximately 3260 cmꢀ 1 corresponded to the
stretching of hydrogen-bonded hydroxyl (–OH) groups of the surface-
adsorbed and interlayer water molecules present in the HMFeMo ma-
terials. The bands at approximately 1635 cmꢀ 1 were ascribed to the
bending vibrations of water molecules. The sharp bands at 1412 and
1484 cm-1 in the FT-IR spectrum of HMFe were attributed to the
vibrational modes of the interlayer carbonate anions. The band at 913
cm-1 in the FT-IR spectra of the HMFeMo materials was characteristic of
the vibrations of the Mo=O bonds of the interlayer molybdate ions in the
catalyst. In addition, the band at approximately 845 cmꢀ 1 was charac-
teristic of the Mo–O–Mo stretching vibrations of the molybdate ions in
the interlayer region [10,15]. The absorption band at 1386 cm-1 in the
FT-IR spectra of HMFeMo corresponded to the asymmetric stretch of
carbonate ions, indicating that traces of carbonate ions were present in
the interlayer region. The presence of a shoulder band at approximately
1386 cmꢀ 1 with increasing molybdate loading was attributed to the
disordered nature of the interlayer region; however, it can also be
ascribed to the decrease in symmetry of carbonate ions [28]. With
increasing molybdate loading, the intensity of the band corresponding to
–OH stretching decreased. This was attributed to the intercalation of
molybdate anions in the interlayer region, which can lead to a decrease
in the number of hydrogen bonds. The bands at 795 and 883 cm-1 in the
FT-IR spectra of the HMFeMo samples corresponded to the OH bending
Fig. 2. Powder X-ray diffraction patterns of as-synthesized (a)HMFe, (b)
HMFeMo0.02, (c) HMFeMo0.05, (d) HMFeMo0.10, and (e) HMFeMo0.15.
Scheme 1. Isoeugenol oxidation to vanillin over HMFeMo catalysts.
3

P.P. Neethu et al.
Applied Catalysis A, General 623 (2021) 118292
of the layered structures [4]. The peaks in the XRD pattern of
HMFeMo0.15 with a high molybdate loading were very broad. The XRD
pattern of pure HMFe revealed that HMFe presented a HT structure with
minor (Fe2O3 orthorhombic) impurity phases. The XRD patterns of the
HMFeMoC-T samples are presented in Fig. S2. The XRD patterns of the
samples calcined at 800 ◦C included characteristic peaks of MgFe2O4
spinel (30.1◦, 35.3◦, 43.02◦, 57.03◦, and 62.4◦) [31], Mo2O5 and Fe2O3
(hematite) phases.
layer hydroxyl groups. The total weight loss of HMFe was 28 %. The
HMFeMo0.1 sample presented a single-stage decomposition with a
maximum weight loss of 18 %. The smaller weight loss of the
HMFeMo0.1 sample indicated that molybdenum intercalation increased
thermal stability [4,14,35].
The nitrogen adsorption–desorption isotherms of the HMFe and
HMFeMo samples are illustrated in Fig. 6. The HMFe and HMFeMo0.05
(Fig. 6 (d) and (c) respectively) materials showed continuous adsorption
with distinct H3 type hysteresis loops at p/p0 range of 0.8 to 0.9, char-
acteristic of layered hydrotalcite with inter-particles mesoporous
structure [36]. The surface areas and pore volumes of the samples are
summarized in Table 1. The BET surface area of HMFe and HMFeMo0.05
were found to be 77 m2/g. The surface area of the more molybdate
intercalated HT samples (HMFeMo0.1and HMFeMo0.15) is higher than
those of the HMFe and HMFeMo0.05 samples. The increase in concen-
tration of molybdate anions (HMFeMo0.1 and HMFeMo0.15) in the
synthesis gel facilitate uniform condensation of molybdate ions in the
interlayer domain of layered HT structure and results an increase in
surface area and micropore volume as evident from sharp uptake in the
p/p0 range of below 0.1. The behavior is analogues to the hard-silicate
anion intercalation on HT layer [4] and making the interlayer surface
more accessible [4,37]. A slight decrease in the surface area (177 m2/g)
for the HMFeMo0.15sample was a result of the filling of the pore sur-
faces and HT interlayer by molybdenum polyoxoanions. The decrease in
surface area of the calcined sample (HMFeMoC0.1-800) was attributed
to the formation of crystalline bulk metal oxide [38,4].
The coordination and oxidation states of iron and molybdenum
species present in the HMFe and HMFeMo samples were analyzed using
DRUV–Visible spectroscopy, and the results are presented in Fig. 3. The
HMFe material showed two distinguish absorption bands appeared
around 220 nm and 305 nm, can be assigned as ligand to metal charge
transfer bands of trivalent iron species present in the octahedral coor-
dination of the layered hydrotalcite framework. Further an additional
broad band above 450 nm is arising from the presence of bulk iron oxy-
hydroxy species present in the uniform layer of hydrotalcite framework
of HMFe sample [32,33]. The introduction of molybdate ions in the
interlayer of HMFe results overlapping of charge transfer peaks due to
Fe3+ ions and showed broad absorption band in the wavelength region
of 210-285 nm, which was assigned to the charge transfer from O2- to
Mo6+ ions in isolated molybdate (MoO24-) species present in the inter-
layer space of hydrotalcite. As molybdate content increased
(HMFeMo0.15), the absorption bands become much broader and shifts
to higher wavelength, indicating the presence of octahedrally coordi-
nated molybdenum polyoxoanions[15,34] present on the interlayer
space.
The oxidation state of the iron and molybdenum species present on
the surface of the HMFeMoC0.1-800 sample was analyzed using XPS,
and the deconvoluted results are presented in Fig. 7. The Fe 2p XPS
profile of HMFeMoC0.1-800 (Fig. 7a) showed well distinguished peaks
at the binding energies of 710.7 and 709.1 eV indicates the presence of
iron in +3 and +2 oxidation states [39,40]. Based on the literature, the
energy difference between the iron +2 and +3 levels should be for ~2
eV [40]. The present studies showed the difference in binding energy
value of 1.6 eV was observed (710.7 - 709.1 = 1.6 eV) (Fe 2p3/2), the
deviation may be due to the Fe-O-Mo species interaction. The Mo 3d
core-level XPS profile of HMFeMoC0.1-800 (Fig. 7b) exhibited a Mo
3d5/2 peak at the binding energies of 231.1 and 231.7 eV, which was
The SEM images of the HMFeMo catalysts (Fig. 4) were used to
analyze catalyst morphology. The as-prepared HMFe sample presented a
semi-crystalline morphology. The molybdate-intercalated HMFeMo0.1
catalyst consisted of ordered particles with sizes in the micrometer
range. The HMFeMoC0.1-800 sample comprised tiny particles with a
uniform spherical morphology, indicating that particle size decreased
upon calcination. The calcined HMFe sample presented a flower-like
morphology.
The behavior of HMFe and HMFeMo0.1 during thermal decompo-
sition was analyzed using TGA, and the TGA curves are presented in
Fig. 5. The thermal behavior of the HMFe and HMFeMo 0.1 samples was
showed different thermal decomposition behavior. The HMFe sample
exhibited three major weight loss stages. The first weight loss stage
(<130 ◦C) corresponded to the removal of physically adsorbed water.
The second weight loss stage (130-370 ◦C) was attributed to the removal
of chemically adsorbed water present in the internal and external sur-
faces of HT. The last decomposition stage (370-450 ◦C) corresponded to
the removal of interlayer carbonate anions and dehydroxylation of intra-
characteristic of the molybdenum present in the higher valent (Mo5+/6+
)
oxidation state [41–44]. The XPS results indicates the formation of solid
solution of metal oxides of magnesium-iron-molybdate was obtained
after calcination, which was in good agreement with the aforementioned
FT-IR data. The surface chemical composition of HMFeMoC0.1 sample
was found to be 4 wt. % of molybdenum, 24.4 wt. % of iron and 7 wt. %
of magnesium respectively and support the presence of surface exposed
molybdenum and iron species and can be potential for catalytic
activities.
3.2. Catalytic activity
The catalytic activity of HMFeMo catalysts was analyzed for the
liquid-phase epoxidation of cyclooctene and oxidation of isoeugenol to
vanillin, and the results are summarized in Sections 3.2.1 and 3.2.2,
respectively.
3.2.1. Cyclooctene epoxidation
3.2.1.1. Catalyst screening. The catalytic activities of all the as-
synthesized and 800 ◦C calcined catalysts were evaluated for cyclo-
octene epoxidation, and the results are presented in Fig. 8. All catalysts
selectively yielded cyclooctene epoxide as the product. The cyclooctene
yield of the pure HMFe sample was approximately 45%. The HMFeMo
samples presented good cyclooctene epoxidation activity. Moreover, as
the molybdate loading of the HMFeMo samples increased, the epoxide
yields also increased. The HMFeMo0.1 sample presented the highest
Fig. 3. Diffuse reflectance ultraviolet–visible spectra of as-synthesized (a)
HMFe
(b)
HMFeMo0.02,
(c)HMFeMo0.05,
(d)HMFeMo0.10,
and
(e)HMFeMo0.15.
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P.P. Neethu et al.
Applied Catalysis A, General 623 (2021) 118292
Fig. 4. Scanning electron microscopy images of as-prepared (a) HMFeMo0.1 and (b) HMFe and calcined (c) HMFeMoC0.1-800 and (d) HMFeC-800.
activity in the cyclooctene epoxidation. Whereas upon higher molyb-
denum loading, the polyoxoanions species [34] are formed and results in
a slightly decreased yield of cyclooctene epoxide. The reaction without
catalyst was also studied, and its conversion was found to be below 10%.
3.2.1.2. Effect of calcination temperature. The catalytic activity of the
HMFeMo0.1 samples calcined at different temperatures were explored
for cyclooctene epoxidation, and the results are presented in Fig. S3. The
increase in calcination temperature facilitated the increase in cyclo-
octene epoxide yield. The highest cyclooctene epoxide yield was ach-
ieved over the HMFeMoC0.1-800 sample (94 %). A further increase in
calcination temperature (HMFeMoC0.1-1000) led to a significant
decrease in cyclooctene conversion (48%) may be due to sintering of
active sites.
3.2.1.3. Effect of reaction temperature. The effect of reaction tempera-
ture on the epoxidation of cyclooctene is presented in Fig. 9. Cyclo-
Fig. 5. Thermo-gravimetric analysis profiles of (a) HMFe and (b) HMFeMo0.1.
octene epoxide yield increased with increasing reaction temperature. At
a
low temperature (50 ◦C), the cyclooctene conversion over
HMFeMoC0.1-800 was only 20 %. The increase in reaction temperature
cyclooctene conversion steadily increases and reached 94 % at 70 ◦C. A
complete conversion of cyclooctene to cyclooctene epoxide was ach-
ieved at 70 ◦C after 24 h.
cyclooctene epoxide yield (86%) among all HMFeMo samples. Further
increasing the molybdenum loading (HMFeMo0.15) led to a slight
decrease in cyclooctene conversion (80%). The corresponding calcined
samples followed the same trend for cyclooctene epoxidation, and the
cyclooctene epoxide yield of HMFeMoC0.1-800 was 94%. These results
suggested that HMFeMo0.1 was the most active catalyst for cyclooctene
epoxidation among all as-prepared and calcined catalysts in this study
may be due to uniform distribution of molybdenum species on the
interlayer surface. Therefore, all subsequent experiments were per-
formed using the HMFeMoC0.1-800 catalyst. The HMFe catalysts with
Mg-O-Fe site shows comparable activity towards epoxidation reaction.
The introduction of molybdenum results the presence of surface Mo-O-
Fe-O species, which is the active site for the oxidation reactions [45].
Thus, the presence of surface exposed iron and molybdenum oxo species
and synergistic effect of magnesium–iron can account for the observed
3.2.1.4. Catalyst recyclability testing. The stability of the HMFeMoC0.1-
800 catalyst during the cyclooctene epoxidation reaction was evaluated
(Fig. 10). The recyclability test was performed by separating the catalyst
from the reaction mixture via centrifugation, followed by solvent (iso-
propyl alcohol) washing, and drying at 80 ◦C overnight before using the
catalyst for the next cycle. The conversion of cyclooctene (more than 90
%) remained unchanged (within the error limit) after four consecutive
reaction cycles, indicating the high stability of the catalyst under the
reaction conditions. The observed better activity during the recycle
studies may be due to presence of molybdenum as oxo-species, which is
5

P.P. Neethu et al.
Applied Catalysis A, General 623 (2021) 118292
Fig. 6. Nitrogen adsorption–desorption isotherms of (1) (a) HMFeMo0.15, (b) HMFeMo0.1, (c) HMFeMo0.05, (d) HMFe and (2) HMFeMoC0.1-800.
conversion and selectivity toward vanillin also increased steadily. The
Table 1
presence of surface-exposed molybdenum oxo species and synergistic
effect of magnesium–iron can account for the observed activity. The
isoeugenol conversion and selectivity toward vanillin of HMFeMo0.1
were 86% and 83%, respectively. Upon further increasing molybdenum
loading (HMFeMo0.15), a similar conversion was obtained; however,
selectivity toward vanillin was lower (66%). A higher molybdenum
loading can decrease material basicity [46], which can lead to a decrease
in selectivity toward vanillin. When the isoeugenol oxidation reaction
was performed using acetonitrile as the solvent, isoeugenol conversion
was even lower (35%). This may be due to solvation of active species.
Textural properties of molybdate-intercalated magnesium-iron hydrotalcite
materials.
Sl. no.
Sample
BET (m2/g)
Pore volume (cm3/g)
1
2
3
4
5
HMFe
77
77
0.26
0.26
0.09
0.08
0.04
HMFeMo0.05
HMFeMo0.1
HMFeMo0.15
HMFeMoC0.1-800 ◦
182
177
5
C
formed in the presence of peroxide.
3.2.2. Isoeugenol oxidation to vanillin
The as-prepared and calcined HMFeMo and pure HMFe samples were
further used for liquid-phase oxidation of isoeugenol to vanillin, and the
results are summarized herein. The formation of vanillin from iso-
eugenol involves the oxidative cleavage of C = C to C–C bonds, followed
by the formation of an aldehyde functionality. The anti-Markovnikov
oxidation product, guaiacyl acetone and isomerised product eugenol,
were formed as a side product (Scheme 1). In the presence of HMFeMo
catalysts, isoeugenol oxidation yielded vanillin as the major product and
guaiacyl acetone and eugenol as the minor products. The reaction did
not proceed in the absence of a catalyst, and the formation of sticky
products in the reactor was attributed to the formation of isoeugenol
dimers and trimers.
3.2.2.1. Effect of molybdenum loading. HMFeMo samples with different
molybdenum loadings were evaluated for isoeugenol oxidation, and the
results are illustrated in Fig. 11. The isoeugenol conversion and selec-
tivity toward vanillin of pure HMFe were 66 % and 72 %, respectively.
As the molybdenum loading of HMFeMo increased, the isoeugenol
Fig. 8. Catalytic activity of the as-synthesized HMFeMo samples for cyclo-
octene epoxidation.
Fig. 7. (a) Fe 2p and (b) Mo 3d X-ray photoelectron spectroscopy profiles of HMFeMoC0.1-800.
6

P.P. Neethu et al.
Applied Catalysis A, General 623 (2021) 118292
are represented in Fig. 12. It is clear from the figure that the conversion
of isoeugenol was increase from 48 % at 200 rpm to 83 % at 300 rpm. At
lower stirring speed (200 rpm), under solid-liquid mass transfer limi-
tations, the reaction is under diffusion control [47]. The conversion of
isoeugenol and vanillin selectivity does not significantly change for
agitation speed larger than 300 rpm and remain about 80 % might be
due to better dispersion of reactant and oxidant on the surface and re-
action becomes kinetic control [47].
3.2.2.3. Effect of catalyst amount. The effect of the substrate-to-catalyst
ratio on the isoeugenol oxidation reaction was evaluated using the
HMFeMo0.1 sample by varying the catalyst amount in the range of
0.025 to 0.1 g using 2 mmol of isoeugenol and 4 mmol of TBHP in
decane as the reactant and oxidant, respectively, and the results are
summarized in Fig. 13. When a small amount of catalyst (0.025 g) was
used, the isoeugenol conversion and selectivity toward vanillin were
70% and 63%, respectively. Upon increasing the catalyst amount from
0.025 to 0.050 g, the isoeugenol conversion and selectivity toward
vanillin increased and achieved a maximum. As the catalyst amount was
further increased from 0.050 to 0.1 g the isoeugenol conversion and
selectivity toward vanillin decreased to 67% and 64%, respectively. The
use of excess catalyst results in polymerization of iso-eugenol and yield
sticky-mass. The polymeric-sticky by-product formed using excess
catalyst, deposit on the active surface and cause a decrease in the cat-
alytic activity. This also yielded in low vanillin selectivity; thus, all
subsequent experiments were performed using 0.05 g of catalyst.
Fig. 9. Effect of reaction temperature on cyclooctene epoxidation over
HMFeMoC0.1-800.
3.2.2.4. Effects of different oxidants. The effects of different oxidants,
namely TBHP in decane, TBHP in water (70%), cumene hydroperoxide,
and H2O2 in water (30%) on the oxidation of isoeugenol to vanillin were
studied (Fig. 14). The isoeugenol conversion and selectivity toward
vanillin were 28% and 42.5% when TBHP in water (70%) was used as
the oxidant. The isoeugenol conversion when H2O2 in water (30%) was
used as the oxidant was only 18.5%. The isoeugenol conversion and
selectivity toward vanillin were higher (62.9% and 56.3%, respectively)
when cumene hydroperoxide was used as the oxidant. The highest iso-
eugenol conversion and selectivity toward vanillin (86% and 83%,
respectively) were achieved when TBHP in decane was used as the
oxidant. The use of aqueous peroxide, the water competitively adsorbed
on the active surface and leads to poor activity. The use of organic ox-
idants and non-polar solvents (TBHP in decane) facilitate to bring effi-
ciently the substrate and oxidant on the active surface and enhance the
activity.
Fig. 10. Recyclability of HMFeMoC0.1-800 for cyclooctene epoxidation.
Therefore, all subsequent experiments were performed in the absence of
solvents.
3.2.2.2. Effect of stirring rate. In order to understand the role of agita-
tion on isoeugenol oxidation reaction, the conversion of isoeugenol and
selectivity of vanillin was followed at different stirring rate. The reaction
was studied at 200 rpm, 300 rpm, 500 rpm and 600 rpm and the con-
version of iso-eugenol and vanillin selectivity was followed. The results
3.2.2.5. Effect of substrate-to-oxidant ratio. The effect of substrate-to-
oxidant ratio (1:1, 1:2, 1:3, and 1:4) on the oxidation of isoeugenol to
vanillin was analyzed, and the results are presented in Fig. 15. At a
Fig. 11. Catalytic activity of the as-synthesized HMFeMo samples for iso-
eugenol oxidation.
Fig. 12. Effect of stirring rate on isoeugenol oxidation.
7

P.P. Neethu et al.
Applied Catalysis A, General 623 (2021) 118292
temperature on the isoeugenol oxidation reaction was evaluated at
different temperatures for 90 min and the results are illustrated in
Fig. S4. The isoeugenol conversion and selectivity toward vanillin
increased steadily with increasing reaction temperature. At a high
temperature of 110 ◦C, the isoeugenol conversion and selectivity toward
vanillin were found to be 84 % and 83 %, respectively. A further increase
in reaction temperature (at 120℃) resulted in the formation of a sticky
bulky mass [48] owing to the oxidative polymerization of isoeugenol,
which strongly bonded to the reactor walls, hindering product separa-
tion. Similarly, when the reaction was carried out for long duration at
100-110 ℃ considerable polymerization occurs. To improve the activity
of the catalyst at lower temperature, we have chosen 80℃ as the reac-
tion temperature and the reaction was followed at different duration.
The results are displayed in Fig. S5. When the reaction time around 1.5
hr, the isoeugenol conversion was 63 % with vanillin selectivity of 81%.
As the reaction time increases the steady improvement in the isoeugenol
conversion observed and reaches maximum of 86% and vanillin selec-
tivity of 83% is achieved at 5 hr. To the best of our knowledge the
catalyst chosen in the present study (HMFeMo0.1) showed maximum
vanillin selectivity (83 %) compared to other literature reported cata-
lysts such as zeolite and cobalt based catalysts [24–26].
Fig. 13. The role of catalyst amount on isoeugenol oxidation.
3.2.2.7. Effect of calcination temperature. The HMFeMo0.1 catalyst was
the most active catalyst under optimized reaction conditions. Further-
more, the effect of calcination temperature of the HMFeMo0.1 catalyst
was studied for the selected catalytic reaction. The results (Fig. 16)
indicated that as the catalyst calcination temperature increased, the
isoeugenol conversion and selectivity toward vanillin decreased
considerably. The increase in calcination temperature resulted in the
collapse of the layered structure of the catalyst and the formation of
Fe2O3 and Mo2O5, as confirmed by the FT-IR and XRD data, which can
account for the observed decrease in isoeugenol conversion.
3.2.2.8. Recyclability study. Reusability is the most important property
of heterogeneous catalysts. The recyclability of HMFeMo0.1 for iso-
eugenol oxidation under optimal reaction conditions was evaluated over
four reaction cycles. The catalyst recovered after each cycle was washed
with acetonitrile, dried, and calcined at 400 ◦C for 4 h to remove all
adsorbed organic molecules before the next use. The isoeugenol con-
version slightly decreased after the first run, whereas the selectivity
toward vanillin remained almost unchanged for all reaction cycles
(Fig. 17). Overall, the present catalyst HMFeMo0.1 catalyst showed
maximum isoeugenol conversion of 86 % with the excellent vanillin
selectivity of 83 % achieved under optimum conditions with comparable
recyclability.
Fig. 14. Effect of several oxidants on isoeugenol oxidation.
substrate-to-oxidant ratio of 1:1, the isoeugenol conversion (63.6%) and
selectivity toward vanillin (68.2%) were comparatively low. An iso-
eugenol conversion of 86% and a selectivity toward vanillin of 83% were
achieved at a substrate-to-oxidant ratio of 1:2. Further increasing the
amount of oxidant did not improve the isoeugenol conversion signifi-
cantly. Therefore, the substrate-to-oxidant ratio for all subsequent ex-
periments was selected to be 1:2.
3.2.2.6. Effect of temperature and time. The effect of reaction
Fig. 16. Effect of calcinations temperature on isoeugenol oxidation
Fig. 15. Effect of substrate-to-oxidant ratio on isoeugenol oxidation.
over HMFeMo0.1.
8

P.P. Neethu et al.
Applied Catalysis A, General 623 (2021) 118292
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molybdate intercalated magnesium-iron hydrotalcite (HMFeMo) mate-
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layered HT structure and molybdate successfully intercalated and sta-
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A. Sakthivel: Conceptualization, Formal analysis, Writing – review
and editing, Funding acquisition, Supervision. P.P. Neethu: Methodol-
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manuscript. A. Sreenavya: Synthesis and reaction methodology, prep-
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Declaration of Competing Interest
[22] A. Franco, S. De, A.M. Balu, A.A. Romero, R. Luque, Selective oxidation of
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The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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Oxidation of Eugenol to Vanillin and Vanillic Acid, J Chin Chem Soc. 63 (2016)
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Acknowledgement
The author thanks DST-SERB-CRG(Project No: CRG/2019/004624)
for financial support. Neethu and Sreenavya are grateful to CSIR (File
No. 09/1108(0036)/2019-EMR-1) and CUKfor the fellowship and Lab
facilities.
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