Synthesis and characterization of a bifunctional nanomagnetic solid acid catalyst (Fe3O4@CeO2/SO42?) and investigation of its efficiency in the protection process of alcohols and phenols via hexamethyldisilazane under solvent-free conditions

Article abstract of DOI:10.1002/jccs.201800056

In this research, Fe₃O₄@CeO₂ (FC) was synthesized using the coprecipitation method and functionalized by an ammonium sulfate solution to achieve a heterogeneous solid acid Fe₃O₄@CeO₂/SO₄^2? (FCA) catalyst. The synthesized bifunctional catalyst was used in the protection process of alcohols and phenols using hexamethyldisilazane (HMDS) at ambient temperature under solvent-free conditions. Due to its excellent magnetic properties, FCA can easily be separated from the reaction mixture and reused several times without significant loss in its catalytic activity. Excellent yield and selectivity, simple separation, low cost, and high recyclability of the nanocatalyst are outstanding advantages of this procedure. The characterization was carried out using different techniques such as Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and vibrating sample magnetometry (VSM).

Full text of DOI:10.1002/jccs.201800056

Received: 11 February 2018
DOI: 10.1002/jccs.201800056
Revised: 8 April 2018
Accepted: 9 April 2018
A R T I C L E
Synthesis and characterization of a bifunctional nanomagnetic
2−
solid acid catalyst (Fe3O4@CeO2/SO4 ) and investigation of its
efficiency in the protection process of alcohols and phenols via
hexamethyldisilazane under solvent-free conditions
Esmaeel Mohammadiyan1 | Hossein Ghafuri2 | Ali Kakanejadifard1
1Faculty of Science, Department of Chemistry,
In this research, Fe3O4@CeO2 (FC) was synthesized using the coprecipitation
Lorestan University, Khorramabad, Iran
2Catalysts and Organic Synthesis Research
Laboratory, Department of Chemistry, Iran
University of Science and Technology,
Tehran, Iran
method and functionalized by an ammonium sulfate solution to achieve a heteroge-
neous solid acid Fe3O4@CeO2/SO42− (FCA) catalyst. The synthesized bifunctional
catalyst was used in the protection process of alcohols and phenols using hexam-
ethyldisilazane (HMDS) at ambient temperature under solvent-free conditions. Due
to its excellent magnetic properties, FCA can easily be separated from the reaction
mixture and reused several times without significant loss in its catalytic activity.
Excellent yield and selectivity, simple separation, low cost, and high recyclability
of the nanocatalyst are outstanding advantages of this procedure. The characteriza-
tion was carried out using different techniques such as Fourier transform infrared
spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersive X-
ray spectroscopy (EDX), X-ray diffraction (XRD), and vibrating sample magne-
tometry (VSM).
Correspondence
Hossein Ghafuri, Catalysts and Organic Synthesis
Research Laboratory, Department of Chemistry,
Iran University of Science and Technology,
P.O. Box 16846-13114, Tehran, Iran.
Email: ghafuri@iust.ac.ir
KEYWORDS
Fe3O4@CeO2/SO42 −, heterogeneous solid acid, HMDS, magnetic nanocatalyst
acid sites. Recently, both of Lewis and Brønsted acid cites
have been assembled in one compound with interesting syn-
1
| INTRODUCTION
Today, improvement of the research in the catalyst sciences
has a significant role in solving the environmental problems.
Catalysts can be designed to give higher activity and better
selectivity to the reactions.[1–4] Functionalization of catalysts
is considered as the main technique to devolop their activi-
ties. Recently, to increase the surface area and the efficiency
of catalysts, sulfation and phosphatation treatments of cata-
lysts are carried out.[5,6] Some reports have shown that the
sulfation and phosphatation treatments are used to produce
solid superacid catalysts with high thermal stability (more
ergistic effects to improve the catalytic activity of cata-
lysts.[9] Sulfated zirconia (ZrO2.SO42−),[7] sulfated titania
(TiO2.SO42−),[10] sulfated tungstate,[11] and some other sul-
fated transition metal oxides are obtained from these bifunc-
tional acids[12] (Scheme 1). Also, the magnetization of these
catalysts allows easy separation from the reaction mixture
using an external magnet.[15]
Among the transition metal oxides, CeO2 has a high oxy-
gen storage capacity and high oxygen ion conductivity. The
high oxygen mobility of ceria causes changes in its oxidation
state between Ce3+ and Ce4+ facilitating oxygen buffer cali-
bration for catalytic redox reactions.[16,17] Ceria has also
shown catalytic activity in other reactions such as oxidation
than 600 C).[7] There are different reactions such as esterifi-
ꢀ
cation and isomerization that are catalyzed by these functio-
nalized catalysts.[8] Solid acids can have Lewis or Brønsted
© 2018 The Chemical Society Located in Taipei & Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
J Chin Chem Soc. 2018;1–8.
http://www.jccs.wiley-vch.de
1

2
MOHAMMADIYAN ET AL.
alcohols and phenols. The Lewis and Brønsted acidity of the
synthesized nanocatalyst was measured and reported in the
characterization section (Scheme 1). The reaction is carried
out under solvent-free conditions with very simple recovery
of the catalyst using an external magnet. High selectivity
and environmentally friendly nature are remarkable advan-
tages of this procedure.
SCHEME 1 Bifunctional structure of the synthesized nanocatalyst
2− [13,14]
(Fe3O4@CeO2/SO4
)
2
| RESULTS AND DISCUSSION
Based on the results obtained, this nanocatalyst can be con-
sidered as an effective bifunctional acid catalyst with
Brønsted and Lewis acid sites in its structure (Scheme 1).
According to our measurement, the Brønsted acidity value
of the catalyst is 1.51 and Lewis acid value is 1.21.
Based on the considerable acidity value and magnetic
properties, it can be an interesting replacement for some
toxic, homogenous, and expensive catalysts in the protection
process of alcohols.
SCHEME 2 The protection of alcohols and phenols in the presence of
Fe3O4@CeO2/SO42− as an effective magnetic nanocatalyst
and reforming reactions.[18,19] In addition, the high thermal
stability of ceria is appropriate for some reactions at high
temperatures. Because of these interesting capabilities, ceria
has been widely utilized as catalysts and in fuel cells.[20–29]
Many methods were reported in recent years for protect-
ing of alcohols. This reaction has been carried out using dif-
ferent reagents like alkylsilanes[30–32] and bis (trimethylsilyl)
amine (HMDS)[33] in the presence of an appropriate cata-
lyst.[34] HMDS as a cheap reagent is a conventional com-
pound for the protection of a hydroxyl functional group that
can be used as an alternative silylating agent.[35,36] However,
low efficiency, moisture sensitivity, long reaction times, and
hard workups are considered as the main disadvantages of
The trimethylsilylation of hydroxyl groups of alcohols
was simply carried out under the solvent-free and room tem-
2−
perature conditions in the presence of Fe3O4@CeO2/SO4
catalyst (Scheme 2).
The possible mechanism for the protection reaction is
shown in Scheme 3.
According to our suggested mechanism, this catalyst can
facilitate the reaction in two steps. First, the amine group of
HMDS can be protonated by the Bronsted sites of the cata-
lyst causing easy separation of ROSiMe3. Next, the Lewis
sites can interact with Ce facilitating the nucleophilic attack
of alcohols on the complex. Finally, with separation of the
NH3 group and the catalyst, silylether is prepared.
To optimize the reaction condition, different conditions
were tested and the results are summarized in Table 1. The
reaction was done by mixing benzyl alcohol, HDMS, and
the various protection methods using a HMDS reagent.[37]
.
Recently, application of some effective catalysts such as
ZnCl4,[38] LiClO4,[39] and MgBr2 resulted in suitable
improvements in these methods, but still they should be
developed.
In this research, we introduced Fe3O4@CeO2/SO42− as a
new heterogeneous solid acid catalyst for the protection of
ROSiMe3
(Me3Si)2NH
H
R-O-SiMe3
(Me3Si)2NH2
NH3
(Me3Si)NH3
ROSiMe3
ROH
2-
Fe3O4@CeO2/SO4
2−
SCHEME 3 The proposed mechanism of protection of alcohols and phenols in the presence of Fe3O4@CeO2/SO4

MOHAMMADIYAN ET AL.
3
2−
TABLE 1 Optimization of the reaction of benzyl alcohol (1 mmol) and
HMDS (.7 mmol) in the presence of Fe3O4@CeO2/SO4
2.2 | XRD patterns of Fe3O4@CeO2/SO4
2−
X-ray diffraction (XRD) patterns of Fe3O4 and Fe3O4@-
CeO2/SO4 are shown in Figure 2. The diffraction peak
Entry
Temp. (ꢀC)
Solvent
—
Cat. (mg)
Yield (%)
2−
1
2
3
4
5
6
7
8
9
25
25
25
25
65
65
25
40
60
3
89
91
95
95
—
80
99
95
90
labeled “a” represents the Fe3O4 phase and the CeO2 phase
is denoted by “b”. As shown in the XRD patterns of
—
5
—
7
Fe3O4@CeO2 and final catalyst Fe3O4@CeO2/SO42− (C and
—
10
5
[34]
D), there are similar peaks for Fe3O4 and CeO2
obvi-
CH3CN
CH2Cl2
—
ously. Disappearance of some peaks demonstrates that this
modification has a low effect on the Fe3O4 phase. In addi-
tion, the results have indicated that the crystalline structure
of the Fe3O4 and CeO2 compounds is maintained in the
nanocatalyst structure.
5
5
—
5
—
5
2.3 | FE-SEM analysis
different amounts of catalysts. According to the results, the
best results were achieved with 7 mg of the catalyst under
solvent-free conditions at room temperature.
After optimization of the reaction conditions, to show
the generality of the procedure, the reactions were carried
out with various alcohols and the results are summarized in
Table 2.
The achieved results show that electron-withdrawing
groups can improve the reaction yield (entry: 1a, 1h). In
addition, the results display that aliphatic alcohols have less
yield than aromatic alcohols (entry: 1l, 1r, 1s, and 1t). Thus
according to Table 2, among the different alcohols some of
them such as benzyl, primary, secondary, and phenolic
show the best results on the yields and reaction time (entry:
1a, 1g, 1h, 1k). The reaction progress was monitored by
TLC and FT-IR was used to demonstrate complete protec-
tion of hydroxyl groups. The results show that all reactions
have well to high yields for all alcohols under the solvent-
free conditions at room temperature. Also, it was deter-
mined that the catalyst has high selectivity for protection of
hydroxyl groups in the presence of amine groups (entry: 1j
and 1m).
To gain information about external morphology and struc-
ture of synthesized nanocatalyst, scanning electron micros-
copy (SEM) was used. SEM images of Fe3O4@CeO2/SO4
are illustrated in Figure 3. According to the size distribution
chart, the distribution is narrow and the size of the majority
of the particles of the synthesized catalyst is nearly
40–50 nm. However, the catalyst particle size is a suitable
proof of successful synthesis of the catalyst.
2−
2.4 | EDS patterns of Fe3O4@CeO2 and Fe3O4@CeO2/
2−
SO4
The EDS spectrum of Fe3O4@CeO2 (Figure 4a) displays the
peaks of iron, cerium, and oxygen. In the spectrum of
2−
Fe3O4@CeO2/SO4 (Figure 4b), in addition to these three
elements, the peaks of sulfur and carbon were observed. The
content of sulfur was shown to be 15.4 wt%. Also, the
reduction of iron peak intensity demonstrates that Fe3O4@-
CeO2 was coated by sulfate, which confirms the successful
2−.
synthesis of Fe3O4@CeO2/SO4
2.5 | Back titration in aqueous media catalyst
2−
(Fe3O4@CeO2/SO4
)
2.1 | FT-IR analysis
Back titration method was used to determine the Bronsted
acidity [H+] of the synthesized catalyst. First, 0.5 g of the
catalyst, 0.5 g of NaCl, and 10 mL of NaOH (0.1 M) were
added to 20 mL distilled water and stirred for 24 hr. Next,
drops of phenolphthalein indicator were added, which results
in the change of the solution color to bright brown. Then,
the mixture was titrated with solution of HCl (0.1 M) until
reaching the end point (colorless solution). The pH value of
the catalyst was calculated to be 1.51.
2−
FT-IR spectra of Fe3O4, CeO2, and Fe3O4@CeO2/SO4
samples are shown in Figure 1. In the CeO2 spectra, the
presence of a sharp peak at 508 cm−1 is related to the
stretching mode of Ce-O bonds.[40] Two well-defined bands
appeared at 1,618 and 3,400 cm−1 that are ascribed to the
bending vibration of absorbed water and surface hydroxyl
and OH stretching mode, respectively. In Fe3O4, there is a
peak at 574 cm−1 that shows the Fe-O bonding (peaks at
588 and 574 cm−1 are assigned to the Fe3+ O Fe3+ and Fe2+
O Fe2+ symmetrical stretching vibrations, respectively).[33]
The peaks at 1,012, 1,124, and 1,260 cm−1 in the Fe3O4@-
2.6 | Surface acidity studies
This synthesized catalyst as a bifunctional solid acid catalyst
has two acid groups whose Bronsted acidity value was mea-
sured with back titration, and in this section, the measure-
ment of Lewis acidity value is reported.
2−
CeO2/SO4 spectra are attributed to the O=S=O asymmet-
ric groups, and S-O vibrations of the (SO4) groups,
respectively.

4
MOHAMMADIYAN ET AL.
2− a
TABLE 2 HMDS protection of alcohols and phenols in the presence of Fe3O4@CeO2/SO4
Entry
Substrate
Product
Time (min)
Yield (%)
1
15
93
2
15
91
3
4
15
25
91
91
5
30
89
6
7
25
35
91
95
8
9
45
50
95
90
10
60
90
11
12
30
35
95
85
13
14
15
35
40
30
90
89
90
16
17
20
25
80
80
18
19
50
30
80
80
20
25
85
a
Reaction conditions: alcohol (1 mmol), catalyst (10 mg), and HMDS (0.7 mmol) at room temperature.
The acidity strength of an acid in organic solvents can be
are previously determined and can be obtained from the liter-
ature (for example the pK(I)aq value of 4-nitroaniline is
0.99). According to the Lambert–Beer's Law, the value of
[I]s/[IH+]s can be calculated using the UV–visible spectrum.
In this test, 4-nitroaniline was used as the basic indicator and
CCl4 was selected as the solvent. The maximal absorbance
of the unprotonated form of 4-nitroaniline was monitored at
determined using the Hammett acidity function (H0).[41,42] It
can be calculated using the equation: H0 = pK[I]aq + log
([I]s/[IH+]s). In this equation, “I” represents the indicator
base, [IH+] is the molar concentrations of the protonated
forms of the indicator, and [I]s is the molar concentrations of
the unprotonated forms of the indicator. The pK(I)aq values

MOHAMMADIYAN ET AL.
5
FIGURE 1 FT-IR spectra of (a) CeO2, (b) Fe3O4, (c) Fe3O4@CeO2, and
2−
(d) Fe3O4@CeO2/SO4
FIGURE 2 XRD patterns of bare Fe3O4 (a), CeO2 (B), Fe3O4@CeO2 (C),
and Fe3O4@CeO2/SO42− (D)
FIGURE 3 SEM images of catalyst Fe3O4@CeO2/SO42− (a) and the
distribution chart of particle size (b)
329 nm in CCl4. According to these results, the indicator
was [IH+] (the absorbance of the unprotonated form of the
indicator in Fe3O4@CeO2/SO4 was weak as compared to
2.8 | Product characterization
2−
FT-IR spectroscopy is the best technique for characterization
of the products in the protection reaction. First, the reagents
have a sharp peak at 3,200–3,400 cm−1 for the OH group of
alcohol and after the protection, this peak completely disap-
peared and a sharp peak appeared at 1,100–1,200 cm−1
(1,182 cm−1), which is ascribed to the Si–O bond. The spec-
tra related to the protection of octanol and the disappearance
of the OH peak clearly show the completion of the protec-
tion reaction. (Figure 7)
the sample of the indicator in CCl4). The results of acidity
2−
strength of Fe3O4@CeO2/SO4 are summarized in Table 3
and the absorption spectra of 4-nitroaniline and Fe3O4@-
CeO2/SO42− are shown in Figure 5.
2−
2.7 | Magnetic properties of Fe3O4@CeO2/SO4
The vibrating sample magnetometry (VSM) measurement
was used to study the magnetic properties of the synthesized
nanomagnetic catalyst. The hysteresis loop of Fe3O4@CeO2/
SO42− obtained using limited field from −1,000 to 1,000 Oe
is shown in Figure 6. According to this figure, the amount of
saturation magnetization decreased from 7 emu/g for
3
| EXPERIMENTAL
3.1 | Materials and methods
2−
Fe3O4@CeO2 to 2 emu/g for Fe3O4@CeO2/SO4 due to
All chemicals were purchased from Merck and utilized without
further purification. FT-IR spectra were recorded using KBr
pellets on a Shimadzu-8400s in the range 400–4,000 cm−1.
The XRD pattern was recorded using a STOE powder diffrac-
tometer with Co kα (λ = 1.789 Å) irradiation, and the mor-
phology of the catalyst was studied using SEM. The images
the coating of surface of Fe3O4@CeO2 by sulfate. Also, the
results show that the calcination of the prepared magnetic
nanocatalyst should be done in a N2 atmosphere and in an
air atmosphere (O2), and the magnetic properties were per-
ished through reduction of Fe3O4 nanoparticles (blue).

6
MOHAMMADIYAN ET AL.
FIGURE 6 VSM results for Fe3O4@CeO2 and Fe3O4@CeO2/SO42− in air
and N2 atmospheres
2−
FIGURE 4 EDX analysis of (a) Fe3O4@CeO2 (b) Fe3O4@CeO2/SO4
TABLE 3 Calculation of the Hammett acidity function (H0) of
FIGURE 7 IR spectra of protected octanol in the presence of
Fe3O4@CeO2/SO42− as a catalyst
2− a
Fe3O4@CeO2/SO4
Entry
Catalyst
Amax
2.147
1.255
[I]s (%)
100
[IH+]s (%)
H
1
2
—
0
—
2−
Fe3O4@CeO2/SO4
62.95
37.05
1.21
a
Conditions for UV–visible spectrum measurement: solvent, CCl4; indicator,
4-nitroaniline (pK(I)aq: (0.99), 1.44 × 10−4 mol/L, catalyst, Fe3O4@CeO2/
2−
SO4 , 20 mg, 25 ꢀC.
were taken using a Hitachi S4160. Magnetic properties were
tested using a VSM (lake shore 7,410). The protection products
were studied using a gas chromatograph (Shimadzu) equipped
with a HP-5 capillary column.
FIGURE 8 Recyclability of the Fe3O4@CeO2/SO42− catalyst for the
4-iodobenzyl alcohol protection
3.2 | Preparation of Fe3O4 nanoparticles
Ferric acid and ferrous salts were employed as precursors for
the synthesis of Fe3O4 nanoparticles. Briefly, FeCl3.6H20
(12.2 g, 0.04 mol) and FeCl2.4H20 (4.7 g, 0.02 mol) were
dissolved in 100 mL distilled water under vigorous stirring.
ꢀ
After 10 min, the solution was heated at 50 C under a nitro-
gen atmosphere. Consequently, the ammonium hydroxide
solution (25%) was added dropwise to the mixture to main-
tain the reaction pH about 9. Then, the mixture was cooled
at room temperature and the black precipitate was collected
FIGURE 5 Absorption spectra of 4-nitroaniline (indicator; curve a) and
Fe3O4@CeO2/SO42− (catalyst; curve b) in CCl4

MOHAMMADIYAN ET AL.
7
TABLE 4 Comparison of the obtained results for Fe3O4@CeO2/SO42− with yields obtained using the recently reported catalysts
Entry
Catalyst
Cat. (mg)
T (min)
Solvent
CH2Cl2
Neat
Yield (%)
Temp. (ꢀC)
Ref.
[38]
1
2
3
4
5
6
7
Trichloroisocyanuricacid (TCCA)
24
29
5
240
23
1
90
90
95
92
91
90
95
r.t.
[45]
[46]
[45]
[47]
H3PW12O40
55–60
r.t.
ZrCl4
CH3CN
CH3CN
CH2Cl2
CH2Cl2
Neat
ZrO(OTf )2
20
49
20
7
1
r.t.
LaCl3
180
90
6
r.t.
Poly(N-bromobenzene-1,3-disulfonamide)
r.t.
2−
Fe3O4@CeO2/SO4
r.t.
This work
using an external magnet and washed several times with eth-
anol and distilled water. In the final step, nanoparticles were
dried at 60 C in an oven.
Table 4, this reaction will be considered as a most suitable reac-
tion among others.
ꢀ
4
| CONCLUSIONS
3.3 | General procedure for the synthesis of
Fe3O4@CeO2/SO4
2−
In this study, Fe3O4@CeO2/SO42− was synthesized as a het-
erogeneous solid acid catalyst and used for the protection of
alcohols and phenols via HMDS under solvent-free condi-
tions. The synthesized nanomagnetic catalyst was character-
ized using FT-IR, SEM, energy dispersive X-ray
spectroscopy (EDX), XRD, and VSM techniques. Our syn-
thesized bifunctional nanocatalyst as a new strong solid acid
catalyst can be used in a wide range of reactions that are cat-
alyzed by acid catalysts. The existence of two Bronsted and
Lewis acid sites cause to carry out the reaction under differ-
ent conditions. Also, easy separation of the nanocatalyst
from the reaction mixture is possible with its excellent mag-
netic properties. In addition, the application of this nanocata-
lyst for a wide range of alcohols such as aliphatic, aromatic,
and benzylic alcohols shows its potential to be employed in
the protection reactions. High yields of product, simple
recovery, easy workup, solvent-free conditions, environmen-
tally benign, inexpensive catalyst, and high selectivity
between –OH and –NH groups are outstanding advantages
of this method.
Initially, 12 g of Ce(NO3)3 were dissolved in 100 mL solu-
tion (Ethylene Glycol) EG: H2O (1:1) for 5 min to form a
homogeneous solution. Next, aqueous ammonia (25%) was
added dropwise under vigorous stirring until the pH of the
solution reached 4. Then, 1 g of Fe3O4 was added to the
mixture under sonication and aqueous ammonia (28%) was
added dropwise until pH reached 9.7. After 24 hr, the solu-
tion was filtered and the obtained solid was dipped in
(3 mol/L) aqueous solution of (NH4)2SO4 at room tempera-
ꢀ
ture for 24 hr. After filtering and drꢀying at 100 C, the calci-
nation was carried out at 550 C for 2 hr under N2
atmosphere conditions.
3.4 | General procedure for the protection of alcohol
(preparation of trimethyl silyloxy group)
Alcohols or phenols (1 mmol) were added to a stirred mix-
2−
ture of Fe3O4@CeO2/SO4
(10 mg) and HMDS
(0.7 mmol) at room temperature. The reaction progress was
monitored using TLC. After the completion of the reaction,
the nanocatalyst was simply separated from the reaction
mixture using an external magnetic field. The product was
characterized using IR and 1H-NMR spectroscopy.
ORCID
Hossein Ghafuri
http://orcid.org/0000-0001-6778-985X
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ZrO(OTf )2,[45] sulfonic acid@nanoporoussilica,[46] and
LaCl3.[47] The results are given in Table 4. According to

8
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