Mechanochemical synthesis of CuO/MgAl2O4 and MgFe2O4 spinels for vanillin production from isoeugenol and vanillyl alcohol

Article abstract of DOI:10.3390/molecules24142597

CuO/MgAl₂O₄ and CuO/MgFe₂O₄ catalysts were successfully synthesized with the use of spinel supports by a very simple and low-cost mechanochemical method. High-speed ball-milling was used to synthesize these catalyst supports for the first time. Materials were subsequently characterized by using XRD, FESEM, TEM, EDS-Dot mapping, XPS, BET-BJH, and Magnetic Susceptibility to investigate the physical-chemical characteristics of the catalysts. Acidity evaluation results indicated that the catalyst with the Mg-Al spinel support had more acid sites. XRD results showed a successful synthesis of the catalysts with large crystal sizes. Both catalysts were used in isoeugenol oxidation and vanillyl alcohol to vanillin reactions, with the CuO/MgAl₂O₄ showing optimum results. This catalyst provided 67% conversion (74% selectivity) after 2 h and this value improved to 81% (selectivity 100%) with the second reaction after 8 h. The CuO/MgFe₂O₄ catalyst in the first reaction after five hours revealed 53% conversion (47% selectivity) and after eight hours with the second reaction, the conversion value improved to 64% (100% selectivity). In terms of reusability, CuO/MgAl₂O₄ showed better results than the CuO/MgFe₂O₄ catalyst, for both reactions.

Full text of DOI:10.3390/molecules24142597

molecules
Article
Mechanochemical Synthesis of CuO/MgAl2O4 and
MgFe2O4 Spinels for Vanillin Production from
Isoeugenol and Vanillyl Alcohol
Behgam Rahmanivahid 1,2 , Maria Pinilla-de Dios 2 , Mohammad Haghighi 3 and
Rafael Luque 2,4,
*
1
Esfarayen University of Technology, Esfarayen 96619-98195, Iran
Departamento de Qu mica Org nica, Facultad de Ciencias, Universidad de C
Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, E14014 Córdoba, Spain
Chemical Engineering Faculty, Sahand University of Technology, P.O. Box 51335-1996, Sahand New Town,
Tabriz 53318-11111, Iran
2
3
4
í
á
órdoba, Campus de Rabanales,
Peoples Friendship University of Russia (RUDN University), 6 Miklukho Maklaya str.,
117198 Moscow, Russia
*
Correspondence: rafael.luque@uco.es; Tel.: +34-957211050
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editors: Vjekoslav Štrukil and Matej Baláž
Received: 4 June 2019; Accepted: 15 July 2019; Published: 17 July 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: CuO/MgAl2O4 and CuO/MgFe2O4 catalysts were successfully synthesized with the use of
spinel supports by a very simple and low-cost mechanochemical method. High-speed ball-milling was
used to synthesize these catalyst supports for the first time. Materials were subsequently characterized
by using XRD, FESEM, TEM, EDS-Dot mapping, XPS, BET-BJH, and Magnetic Susceptibility to
investigate the physical-chemical characteristics of the catalysts. Acidity evaluation results indicated
that the catalyst with the Mg-Al spinel support had more acid sites. XRD results showed a successful
synthesis of the catalysts with large crystal sizes. Both catalysts were used in isoeugenol oxidation and
vanillyl alcohol to vanillin reactions, with the CuO/MgAl2O4 showing optimum results. This catalyst
provided 67% conversion (74% selectivity) after 2 h and this value improved to 81% (selectivity
100%) with the second reaction after 8 h. The CuO/MgFe2O4 catalyst in the first reaction after five
hours revealed 53% conversion (47% selectivity) and after eight hours with the second reaction,
the conversion value improved to 64% (100% selectivity). In terms of reusability, CuO/MgAl2O4
showed better results than the CuO/MgFe2O4 catalyst, for both reactions.
Keywords: high speed ball-milling; mechanochemistry; spinel; vanillin; vanillyl alcohol; isoeugenol
1. Introduction
Vanillin is a chemical compound from the aromatic aldehydes family with many industrial
applications [
and other chemical industries [
been used for extracting it in so many different ways [
1
]. This valuable substance is used in perfumes, food, cosmetics, pharmaceuticals,
]. Vanillin naturally exists in some plant families which have
]. According to the multiple uses of vanillin,
2–4
5
the annual required amount of this material has been estimated at 12,000 tons. Nowadays, due to the
lack of natural resources (less than 1% is provided from them) and considering the low price of synthetic
vanillin, this substance is produced by industrial methods [
to produce vanillin. One of them is the bioconversion of materials such as vanillyl alcohol, vanillic
acid, creosol, phenolic stilbenes, glucose, lignin, isoeugenol, and eugenol [ 10]. Another method to
produce vanillin is using biocatalysts because the marginal cost of the product then becomes very
low [11 14]. Chemical synthesis of vanillin from lignin, guaiacol, coniferin, ferulic acid as raw materials
6,7]. There are various industrial methods
8–
–
Molecules 2019, 24, 2597; doi:10.3390/molecules24142597
www.mdpi.com/journal/molecules

Molecules 2019, 24, 2597
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is also an important way to produce this substance [
3
,15
,
16]. In recent years, vanillin production by
the oxidation of some materials such as isoeugenol and vanillyl alcohol has been considered [7,17,18].
In this method, raw materials with an oxidizer such as hydrogen peroxide in the presence of a catalyst
with the ability of oxidation produce vanillin [19]. There are several catalysts with oxidation ability
which are used in various industries. Some of these catalysts such as Cu-Ti composite, Mn-doped ceria,
and Co3O4 were used in the vanillin production reaction [17
temperature and pressure or a significant time for this reaction. Also other catalysts such as Pd [20
Mn2O3 [22 24], and Au [25 27] have been used in oxidation reactions of CH4, CO, NO, and CH4O.
In several references, CuO was used as a powerful catalyst for oxidation reactions [28 31]. It seems
–19]. However, these catalysts need high
,
21],
–
–
–
that these metal oxides, due to their low cost and high oxidation power, are suitable catalysts for the
oxidation of isoeugenol and vanillyl alcohol. In this work, CuO was employed as active phase for the
synthesis of CuO-based catalysts in view of their application in the selective oxidation of isoeugenol to
vanillin. Mg-Al and Mg-Fe spinels (MgAl2O4 and MgFe2O4) were selected as supports due to their
previously reported convenient catalytic properties [32–35].
In recent years, mechanochemistry emerged as a relevant methodology for the synthesis,
preparation, and design of nanomaterials [36]. MgFe2O4 and MgAl2O4 spinels were previously
synthesized via a mechanochemical method by other researchers, but, with low speed, lengthy,
and energy intensive ball-milling conditions (e.g., over 12 h) [37,38]. In this work, high speed
ball-milling mechanochemistry was employed as a simple and low-cost methodology, for the first time,
in the synthesis of these spinels.
Various analyses, such as X-ray Powder Diffraction (XRD), Field Emission Scanning Electron
Microscopy (FESEM), Transmission Electron Microscopy (TEM), BET Surface Area and BJH Pore Size,
Energy Dispersive X-Ray Spectroscopy (EDS)-Dot mapping, X-Ray Photoelectron Spectroscopy (XPS),
and Magnetic Susceptibility, were used to determine the characteristics of the synthesized catalysts.
2. Results and Discussion
2.1. Characterization of the Nanocatalysts
2.1.1. XRD Analysis
XRD analyses of the synthesized samples are shown in Figure 1. Comparing the analysis of
the CuO/MgAl2O4 sample with the standard peaks of MgAl2O4 spinels (JCPDS: 01-086-0083; Cubic,
2θ = 19.0, 31.3, 36.8, 44.8, 55.6, 59.3, 65.2, and 77.3), the successful synthesis of this spinel as catalyst
support could be easily recognized. However, comparing the analysis of this sample with the standard
patterns of CuO (JCPDS: 01-080-1268; 2θ = 35.5, 35.7, 38.8, 39.0, 48.7, 53.5, 58.4, 61.6, 65.9, 66.3, and 68.2),
no significant peak was observed in these samples.
This phenomenon represents an appropriate and uniform distribution also with small particles of
CuO on the surface of the catalyst support [32]. This occurs due to the use of the mechanochemical
method in the distribution of the active phase on the catalyst support. Furthermore, peaks of
CuO/MgFe2O4 corresponding to the standard patterns of MgFe2O4 (JCPDS: 01-073-2211; Cubic,
2θ = 30.2, 35.6, 43.2, 53.6, 57.2, 62.8, and 74.3) were observed, which confirms the successful synthesis
of this spinel. Also, studying the analysis of this sample and standard peaks of CuO, no peak was
observed. This phenomenon can be attributed to the low ratio of the active phase to the support
(0.5 wt%), to the proper and uniform dispersion, and also to the small particle size due to the use
of a mechanochemical method. Crystal sizes of these two samples are shown in Table 1, which are
calculated by the Scherrer equation [39]. These results showed sizes of 25.9 nm and 78 nm for the
CuO/MgAl2O4 and CuO/MgFe2O4 samples, respectively. Larger crystal sizes of the synthesized sample
with magnetic support indicate that this sample has a lower surface size than the other sample.

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Table 1. Structural properties of CuO/MgAl2O4 and CuO/MgFe2O4 nanocatalysts.
Surface Area
Pore Volume
Mean Pore Size
(nm)
Crystallite Size
(nm)
Spinel
(m2/g)
(cm3/g)
CuO/MgAl2O4
CuO/MgFe2O4
20
0.09
18.2
25.9 a
78.0 b
<5
0.03
51.4
a
b
Crystallite size estimated by Scherrer’s equation at 2
at 2θ = 35.6◦.
θ
= 36.8◦. Crystallite size estimated by Scherrer’s equation
Figure 1. XRD patterns of prepared nanocatalysts: (a) CuO/MgAl2O4, (b) CuO/MgFe2O4
.

Molecules 2019, 24, 2597
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2.1.2. TEM and SEM Analysis
TEM analyses of the synthesized samples are shown in Figure 2. TEM images of CuO/MgAl2O4
(Figure 2a) indicate that the average particle size of this catalyst is smaller than 100 nm and, with more
detail, crystalline forms of the particles can be seen. On the contrary, the analysis of CuO/MgFe2O4
(Figure 2b) shows that the particle size is around 200 nm and also the formation of large crystals can be
seen in this sample. The minimum, maximum, and average particle size of both samples are shown in a
particle size distribution plot. The average particle sizes obtained of CuO/MgAl2O4 and CuO/MgFe2O4
were 39 nm and 121 nm, respectively, which is in good agreement with results of crystal size.
Due to the magnetic properties of MgFe2O4, the associated TEM images are not more transparent
than the other sample. SEM analysis of CuO/MgAl2O4 and CuO/MgFe2O4 catalysts which are shown in
Figure 3 confirm the results of TEM analysis. These analyses show that the particles of CuO/MgAl2O4
(Figure 3a) are smaller than the particles of CuO/MgFe2O4 (Figure 3b). Particle sizes with a magnetic
support are large so the crystal regular form can be easily observed.
Figure 2. TEM images and size distribution of (a) CuO/MgAl2O4 catalyst, (b) CuO/MgFe2O4 catalyst.
Both catalysts were found to be non-porous materials. However, due to the small particle size of
the CuO/MgAl2O4 catalyst, it can be predicted that its surface area is higher than the other catalyst
area. It is noteworthy that the results of this analysis confirmed the XRD analysis results.

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Figure 3. SEM images of synthesized nanocatalysts: (a) CuO/MgAl2O4, (b) CuO/MgFe2O4
.
2.1.3. EDX Analysis
EDX analysis was carried out to determine the distribution of the elements on each catalyst surface
and its impact on the activity of the catalysts in the reaction [40 41]. EDX-dot mapping analysis was
,
used for both CuO/MgAl2O4 and CuO/MgFe2O4 catalysts, respectively. Results are shown in Figure 4.
Mg, Al, and O elements follow a uniform distribution in the CuO/MgAl2O4 sample which results in
a proper and uniform formation of this spinel. Dot mapping of Cu element in this catalyst shows a
highly proper and uniform distribution of CuO on the support surface, as the active phase. On the
other hand, CuO/MgFe2O4 catalyst follows a uniform distribution of Mg, Fe, and O elements and
shows also a good distribution of Cu on the support surface. Both element weight distributions are
consistent with the theoretical data. It can be said that these uniform distributions are attributable
to the mechanochemical synthesis method. This method forces a proper mix of the components of
the spinel support synthesis and also an appropriate distribution of the active phase (CuO) on the
supports (MgAl2O4 and MgFe2O4) [42,43].

Molecules 2019, 24, 2597
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a) CuO/MgAl2O4
Element
wt %
Al
O
Mg
Cu
35.33 46.99 15.88 1.80
b) CuO/MgFe2O4
Element
wt %
Fe
O
Mg
Cu
56.26 31.99 10.54 1.22
Figure 4. EDX elemental dot mapping analysis of (a) CuO/MgAl2O4 and (b) CuO/MgFe2O4
nanocatalysts.
2.1.4. XPS Analysis
The surfaces of both synthesized samples were studied by XPS analysis and the results are shown
in Figures 5 and 6. Before analysis, the peak locations for both samples were calibrated according to
the peak of C1s (adsorbed species CO and CO2) which appears at 284.6 eV. Figure 5 shows peaks at
724.6 and 711.1 eV which are related to Fe2p3/2 and Fe2p1/2 respectively. They confirmed the existence
of Fe3+ species in the CuO/MgFe2O4 catalyst.

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Figure 5. XPS data of CuO/MgFe2O4 nanocatalyst.
The presence of the Mg2p peak at 48.9 eV is related to Mg2+ in the sample. So, it is possible to
conclude the successful formation of MgFe2O4 spinel [44 46]. The O1s peak at 529.8 eV belongs to the
Mg–O, Fe–O bond or is related to absorbed oxygen of various species on the catalyst surface [44 46].
–
–
In this case, the peak of CuO was not observed correctly which may be due to the very low amount
(0.5 wt%) and also XPS is a local/superficial analysis [47].
The XPS image corresponding to the CuO/MgAl2O4 catalyst is shown in Figure 6. Peaks at 50.3 eV
and 74.7 eV which are related to Mg2p and Al2p, indicate the presence of Mg2+ and Al3+ species in
this sample which proves the successful synthesis of MgAl2O4 spinel [47–49]. Also various species of
O1s detected in the synthesized samples at 531.5 and 532.5 eV peaks are related to Al–O and Mg–O
bonds, respectively and also the peak at 530.2 eV, because of the oxygen of absorbed species on the
catalyst surface or it is related to Cu–O bond in the active phase [49,
50]. For Cu2+ species, the peaks at
934.0 and 954.0 eV are related to Cu2p in this sample which shows the existence of CuO on the catalyst
surface [51]. It should be noted that a partial change in peak locations of some species, with decreasing
or increasing bonding energy, is due to a change in the energy level of various species in different
environments [45,52–54]. Results from this analysis support the results from XRD and EDS analysis.

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Figure 6. XPS data of CuO/MgAl2O4 nanocatalyst.
2.1.5. BET-BJH Analysis
The surface area and pore diameter of the catalyst are two key characteristics to be evaluated in all
catalysts. Synthesized catalysts were evaluated by BET-BJH analysis and the results are shown in Table 1
and Figure 7. According to the table, the surface areas of CuO/MgAl2O4 and CuO/MgFe2O4 catalysts
were 20 m2/g and 3 m2/g, respectively. The reason for the very small surface area of the CuO/MgFe2O4
catalyst is its large particle and crystal size which is demonstrated in XRD and FESEM analysis.
From Figure 7, which shows the adsorption and desorption isotherms of the synthesized samples,
these catalysts can be easily categorized into type III of the IUPAC classification. The hysteresis forms
of the samples indicate that both synthesized catalysts are non-porous and have plate structures, so the
samples have an inter-particle surface. However, according to Table 1, the obtained pore diameters
for CuO/MgAl2O4 and CuO/MgFe2O4 catalysts are 51.4 nm and 18.2 nm, respectively. These results
confirm previous analyses, such as XRD and SEM.

Molecules 2019, 24, 2597
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A
a
d
)
so
C
rb
u
si
O
on/MgAl2O4
180
160
140
120
100
80
D-Mg-Al
b) CuO/MgFe2O4
60
40
20
0
0
0.2
0.4
0.6
0.8
1
Relative Pressure (P/P°)
Figure 7. Adsorption/desorption isotherms of the nanocatalysts (a) CuO/MgAl2O4 and (b) CuO/MgFe2O4
.
2.1.6. Acidity
◦
Pyridine (PY) and 2,6-dimethylpyridine (DMPY) absorption methods were used at 200 C
(pulse chromatographic titration methodology) to investigate the acidity of CuO/MgAl2O4 and
CuO/MgFe2O4 catalysts [55–57]. Through the PY absorption, it is possible to obtain the total amount
of acid sites. By the DMPY absorption method, Brönsted acid sites can be determined. Lewis acid
sites are calculated by subtracting these values from each other [58]. The results of this method can be
seen in Table 2. According to the results, the total amount of acid sites of the CuO/MgAl2O4 catalyst
(69
µmol PY/g) is higher than the amount of the CuO/MgFe2O4 catalyst (38
µmol PY/g). However,
the amount of Lewis acid sites is almost the same for both samples (26
µmol PY/g). The reason for the
difference in the value of Brönsted acid sites and the coincidence on the value of Lewis acid sites is
the magnetic support of MgFe2O4 which is related with the surface area. The effect of acid sites was
further determined by examining the catalyst activity.
Table 2. Acidity and magnetic properties of CuO/MgAl2O4 and CuO/MgFe2O4 nanocatalysts.
Magnetic
Total Acidity
(µmol PY/g)
Brönsted Acidity
Lewis Acidity
Nanocatalyst
Susceptibility
(µmol DMPY/g) 1
(µmol PY/g) 2
(10−6 m3/Kg)
CuO/MgAl2O4
CuO/MgFe2O4
-
69
38
43
12
26
26
416
1
DMPY: 2,6-dimethylpyridine; 2 PY: Pyridine.
2.1.7. Magnetic Susceptibility Analysis
Magnetic properties of CuO/MgAl2O4 and CuO/MgFe2O4 catalysts are shown in Table 2. Due to
the use of Mg-Fe spinel in the synthesis of CuO/MgFe2O4 as catalyst support, this sample showed a
highly desirable magnetic susceptibility of 416
×
10−6 m3/kg. On the contrary, the synthesized catalyst
with Mg-Al spinel does not have magnetic susceptibility. According to the magnetic behavior of the
CuO/MgFe2O4 catalyst, this sample has a great ability to be separated easily from the reaction mixture
which can be considered an advantage for this catalyst.

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2.2. Catalytic Performance Study toward Vanillin Production
In order to evaluate the synthesized catalysts activity, CuO-based spinel systems were employed
in oxidation reactions of isoeugenol and vanillyl alcohol to vanillin. To carry out these reactions,
an oxidizer (hydrogen peroxide) and a solvent (acetonitrile) were used. Both reactions were carried out
at a temperature of 90 ◦C. After using GC to obtain the amount of the reaction products, the conversion
and selectivity of each reaction were calculated. Results are shown in Figures 8 and 9. To determine
the behavior of each catalyst in the reaction, first both vanillin production reactions were performed
at 90 ◦C (isoeugenol oxidation) and 40 ◦C (vanillyl alcohol oxidation) over 24 h without any catalyst
(Tables 3 and 4, respectively). Using the synthesized catalysts, the results showed that CuO/MgAl2O4
catalyst has a very good activity in the isoeugenol oxidation reaction. After two hours of reaction,
conversion reached 67%. The selectivity of vanillin production was 74%, at the same time. After 8 h
with the same catalyst, conversion was 81% and the selectivity in the oxidation reaction of vanillyl
alcohol to vanillin was raised to 100%. The performance of CuO/MgFe2O4 was not so efficient. With this
catalyst, after 2 h, the conversion was 36% with a selectivity of 39% in the vanillin production reaction
from isoeugenol. After 5 h, the conversion increased to 53% with a selectivity of 46%. After 8 h of the
oxidation reaction of vanillyl alcohol, the conversion was 64% with a selectivity of 100%.
Taking into account the results, it is possible to determine that the Mg-Al spinel support exhibited
a comparably superior performance to that of Mg-Fe spinel. This may be due to the greater surface
area and pore volume of CuO/MgAl2O4. Active sites for this reaction were previously reported to be
moderate acidic and/or redox metal sites, in this case being a synergistic combination between the
CuO phase and the spinel support. Another important characteristic of heterogeneous catalysts is their
reusability. The CuO/MgAl2O4 catalyst was used four consecutive times in the isoeugenol and vanillyl
alcohol oxidation reaction over 2 and 8 h, respectively and CuO/MgFe2O4 catalyst was used over 5 and
8 h. For this purpose, CuO/MgAl2O4 and CuO/MgFe2O4 catalysts were separated once finished from
the reaction mixture with filter paper and by a magnet, respectively. Then, they were oven-dried for
24 h at 110 ◦C to use again in reactions. From Figure 9, the first reuse of CuO/MgAl2O4 catalyst in
the vanillin production reaction from isoeugenol, showed a good performance with a 7% decrease in
the conversion and a 13% reduction in selectivity. After the second and third reuse, the conversion
changed to 40% and 29%, respectively and the selectivity varied to 42% for the second reuse and 28%
for the third one. With the same catalyst, in the oxidation reaction of vanillyl alcohol the conversion
gradually decreased to 79% after the first reuse.
For the next iterations, conversion decreased to 56% and 29%, respectively with 100% selectivity.
On the other hand, the CuO/MgFe2O4 catalyst showed different results. In both reactions, drastic
reductions were observed after their reuse. The conversion of this catalyst in the isoeugenol oxidation
reaction after the first reuse was 29% with a selectivity of 23%. After the second and third iterations the
conversion was almost constant at 17% with a selectivity of 8%. The conversion of this catalyst after
the first reuse in the vanillyl alcohol oxidation reaction was 42%. This value decreased to 26% and
25% after the second and third reuse, respectively. According to the results, it is possible to conclude
that both catalysts CuO/MgAl2O4 and CuO/MgFe2O4 have good ability to produce vanillin from
vanillyl alcohol and isoeugenol. Based on the analysis and the values from conversion, selectivity and
reusability, it can be said that the MgAl2O4 spinel is the most appropriate support for the active phase
of CuO to perform both reactions.

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Conversion (%mol)
Vanillin Selectivity (%)
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
time (h)
Conversion (%mol)
Vanillin Selectivity (%)
100
90
80
70
60
50
40
30
20
10
0
110
100
90
80
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
time (h)
Figure 8. Catalytic activity at 90 ◦C of CuO/MgAl2O4 catalyst in the oxidation of isoeugenol to vanillin
top image) and CuO/MgAl2O4 catalyst in the oxidation of vanillyl alcohol to vanillin (bottom image).
(

Molecules 2019, 24, 2597
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Conversion (%mol)
Vanillin Selectivity (%)
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
time (h)
Conversion (%mol)
Vanillin Selectivity (%)
110
100
90
80
70
60
50
40
30
20
10
0
110
100
90
80
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
time (h)
Figure 9. Catalytic activity at 40 ◦C of CuO/MgFe2O4 catalyst in the oxidation of isoeugenol to vanillin
top image) and CuO/MgFe2O4 catalyst in the oxidation of vanillyl alcohol to vanillin (bottom image).
(
3. Conclusions
CuO/MgAl2O4 and CuO/MgFe2O4 catalysts with spinel supports were synthesized for the first
time by using a mechanochemical method (high-speed ball-milling) as a quick and efficient method.
The active phase of CuO was distributed on the catalyst surface using conventional (low speed)
ball milling. With this method, the synthesis time and the costs were significantly reduced. Results
from different analyses showed that CuO/MgFe2O4 catalyst had higher particle and crystal size than
CuO/MgAl2O4 catalyst which is related to the small surface area. Larger particle size caused a smaller
area. Both catalysts used in the oxidation reaction of isoeugenol and vanillyl alcohol showed a good
ability to produce vanillin. CuO/MgFe2O4 catalyst showed high magnetic susceptibility and therefore
easy separation from the reaction mixture. In both reactions comparing the conversion, selectivity,
and reusability of the catalysts, CuO/MgAl2O4 presented better characteristics for these reactions,
explainable because of its surface area and acid sites.

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Table 3. Catalytic activity of CuO/MgAl2O4 and CuO/MgFe2O4 catalysts in the oxidation of isoeugenol
to vanillin.
O
O
CH3
CH3
H
Conventional Heating
90 ºC
OH
OH
OCH3
Selectivity
Entry
Catalyst
Time (h)
Conversion (%mol)
Vanillin
Others
1
2
3
4
5
6
7
8
9
Blank
2
2
2
2
2
2
5
5
5
5
5
5
18
67
70
63
40
29
20
53
50
29
17
17
7
93
26
24
36
58
72
89
54
55
77
91
92
CuO/MgAl2O4
1 Reuse CuO/MgAl2O4
2 Reuse CuO/MgAl2O4
3 Reuse CuO/MgAl2O4
4 Reuse CuO/MgAl2O4
Blank
74
76
64
42
28
11
46
45
23
9
CuO/MgFe2O4
1 Reuse CuO/MgFe2O4
2 Reuse CuO/MgFe2O4
3 Reuse CuO/MgFe2O4
4 Reuse CuO/MgFe2O4
10
11
12
8
Reaction Conditions: 8 mL acetonitrile, 5 mmol isoeugenol, 20 mmol H2O2, 100 mg catalyst, 90 ◦C.
Table 4. Catalytic activity of CuO/MgAl2O4 and CuO/MgFe2O4 catalysts in the oxidation of vanillin
alcohol to vanillin.
OH
O
H
Conventional Heating
CH3
O
HO
40 ºC
OH
OCH3
Selectivity
Entry
Catalyst
Time (h)
Conversion (%mol)
Vanillin
Others
1
2
3
4
5
6
7
8
9
Blank
8
8
8
8
8
8
8
8
8
8
8
8
21
81
84
79
56
29
21
64
65
42
26
25
0
100
-
-
-
-
-
CuO/MgAl2O4
1 Reuse CuO/MgAl2O4
2 Reuse CuO/MgAl2O4
3 Reuse CuO/MgAl2O4
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CuO/MgFe2O4
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Reaction Conditions: 8 mL acetonitrile, 5 mmol vanillin alcohol, 20 mmol H2O2, 100 mg catalyst, 40 ◦C

Molecules 2019, 24, 2597
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4. Materials and Methods
4.1. Materials
Magnesium oxide (MgO 99%, Aldrich, St. Louis, MO, USA), aluminum oxide (Al2O3 99%,
Panreac, Barcelona, Spain), iron oxide III ( -Fe2O3 99%, Merck, Kenilworth, NJ, USA) and hydrated
α
copper chloride (CuCl2.2H2O 99.8%, Sigma-Aldrich, St. Louis, MO, USA) were used to synthesize
CuO/MgAl2O4 and CuO/MgFe2O4. Isoeugenol (C10H12O2 98%, Aldrich), acetonitrile (C2H3N
99.8%, Panreac), vanillyl alcohol (C8H10O3 98%, Aldrich), and hydrogen peroxide (H2O2 50 wt% in
H2O, Sigma-Aldrich) were used for vanillin production reactions. All reagents were used without
further purification.
4.2. Preparation and Procedure of the Nanocatalysts
The synthesis procedure of CuO/MgAl2O4 and CuO/MgFe2O4 nanocatalysts is shown in Figure 10.
As can be seen, for the synthesis of MgAl2O4 spinel, stoichiometric amounts of MgO and Al2O3 were
mixed by high speed ball milling (Emax model, Retsch, Haan, Germany) at 900 rpm for 1 h. Then,
the obtained mixture of oxides was calcined in a furnace with air flow at 900 ◦C for 3 h. After preparing the
spinel support for the deposition of CuO active phase on its surface, specific amounts of hydrated copper
chloride and MgAl2O4 (to achieve a 0.5 wt% CuO in the final material) were mixed by conventional ball
milling (PM 100 model, Retsch, Germany) at 350 rpm for 10 min. Finally, to synthesize the CuO/MgAl2O4
catalyst, ball milled samples were calcined at 400 ◦C for 2 h (Figure 10).
(a) Preparation of support precursors
Precursors
Al2O3 Or Fe2O3
MgO
Weighing with 1:1 molar ratio
(b) Mechanochemical synthesis of support
High Speed ball milling with 900 rpm for 1 h
Mix Oxide:
MgO/Al2O3 , MgO/Fe2O3
(c) Support post treatment
Calcination at 900°C for 3 h in air ambient
Spinel forming:
MgAl2O4 , MgFe2O4
(d) Active phase loading
Precursor
Usual ball milling
with 350 rpm for 10 min
CuCl2.2H2O
(e) Post treatment
Calcination at 400°C for 2 h in air ambient
Nanocatalysts forming:
CuO/MgAl2O4 , CuO/MgFe2O4
Figure 10. Mechanochemical synthesis of CuO/MgAl2O4 and CuO/MgFe2O4 nanocatalysts.

Molecules 2019, 24, 2597
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To synthesize the CuO/MgFe2O4 catalyst, the whole synthesis process was performed as the
previous catalyst, but in the synthesis of the magnetic spinel support MgFe2O4, the MgO, and -Fe2O3
α
were mixed by high speed ball milling. For other parameters such as stoichiometric ratio, calcination
time and temperature, the ball milling speed and time were quite similar to the synthesis process of
CuO/MgAl2O4 catalyst.
4.3. Characterization Techniques of the Nanocatalysts
Analysis by X-Ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM),
Transmission Electron Microscopy (TEM), Energy-dispersive X-ray spectroscopy (EDS) Dot mapping,
X-ray Photoelectron Spectroscopy (XPS), Brunauer-Emmett-Teller (BET)/Barrett-Joyner-Halenda (BJH)
and Magnetic Susceptibility techniques were used to evaluate the characteristics of the synthesized
nanocatalysts. Catalyst acidity was determined by the titration method of pyridine and dimethyl
pyridine through a gas chromatograph with flame ionization detector (FID) and a packed column
Chromosorb AW-MCS 80/100 of 0.5 m. The surface morphology and particle size of the synthesized
catalysts were analyzed by using TEM (Transmission Electron Microscopy, JEM-1400 (JEOL, Peabody,
MA, USA) analyser) and SEM (Scanning Electron Microscopy, JSM-7800F Prime (JEOL, Peabody, MA,
USA) analyzer). Considering the importance of the element composition on the catalyst surface,
an XPS analyzer Ultra High Vacuum (UHV) multipurpose surface analysis (Specst model, Berlin,
Germany) operating at pressures <10−10 mbar using a conventional X-ray source—XR spectra were
recorded on a Bruker D8 X ray diffractometer (10–80◦ 2
θ range) in the Bragg-Brentano geometry
and in reflection mode, using a Cu X-ray tube, a rotating platform, a monochromatic primary beam,
and a high sensitivity detector—and an EDX analyzer X-MaxN (OXFORD Instruments, Abingdon,
UK) were applied to obtain the surface elements composition and the material types. The specific
surface area and pore diameter of the catalysts were measured by using the BET-BJH method with
ASAP 2000 (Micromeritics Instrument, Norcross, GA, USA) device. A MS2 A Magnetic Susceptibility
Meter (Bartington Instruments, Witney, Oxon, UK) device was used to determine the magnetic power
of the MgFe2O4 spinel.
4.4. Experimental Set-Up for the Catalytic Performance Test
Synthesized catalysts were used in the vanillin production reaction from vanillin alcohol and
isoeugenol, to gain an appropriate assessment of their catalytic activity.
The reaction was carried out in a carousel with a Pyrex tube. For the oxidation reaction of
isoeugenol, 8 mL acetonitrile, 1.2 mL H2O2 (hydrogen peroxide) solution (20 mmol H2O2), and 0.8 mL
isoeugenol (5 mmol) were poured into a Pyrex tube with 0.1 g of catalyst. Products were analyzed
at different time intervals by a Gas chromatograph (7890A model, Agilent Technologies, Santa Clara,
CA, USA) fitted with a capillary column Petrocol 100 m
detector (FID). The results were finally confirmed by GC-MS.
×
0.25 mm and 0.5 µm and a flame ionization
Also, for the vanillin alcohol oxidation reaction, 8 mL of acetonitrile was poured into a carousel
tube and the temperature was increased to 40 ◦C to provide suitable conditions for complete dissolution
of vanillyl alcohol. Then 0.77 g (5 mmol) of vanillyl alcohol was added to the solvent and after complete
dissolution of vanillyl alcohol in acetonitrile, 1.2 mL of hydrogen peroxide with 0.1 g catalyst were
added to the reaction mixture. The carousel was adjusted to 90 ◦C with a magnetic stirrer speed of
800 rpm. To evaluate the progress of the reactions, the reaction mixture was sampled at different times.
Sampling was done by a syringe with a filter. To study the reusability of the samples, catalysts were
separated after reaction by a paper filter and washed by acetonitrile. Then, they were oven dried at
110 ◦C for 24 h and re-used in the vanillin production reaction. To obtain conversion and selectivity of
the reaction, products were analyzed by the same gas chromatograph and also, results were confirmed
by GC-MS.

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Author Contributions: Conceptualization, B.R., M.H. and R.L.; methodology, B.R.; validation, M.H. and R.L.,
investigation, B.R..; resources, R.L.; data curation, B.R. and M.P.d.D.; writing—original draft preparation, B.R. and
M.P.d.D.; writing—review and editing, M.H. and R.L.; supervision, M.H. and R.L.
Funding: The publication was prepared with support from RUDN University Program 5-100.
Acknowledgments: The authors gratefully acknowledge Ministry of Science Research and Technology of Iran for
the financial support of the research as well as Iran Nanotechnology Initiative Council and Iran Science Elites
Federation for complementary financial supports.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds CuO/MgAl2O4 and CuO/MgFe2O4 are available from the authors.
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2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
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