New bio-nanocomposites based on iron oxides and polysaccharides applied to oxidation and alkylation reactions

Article abstract of DOI:10.3762/bjoc.13.194

Polysaccharides from natural sources and iron precursors were applied to develop new bio-nanocomposites by mechanochemical milling processes. The proposed methodology was demonstrated to be advantageous in comparison with other protocols for the synthesis of iron oxide based nanostructures. Additionally, mechanochemistry has enormous potential from an environmental point-of-view since it is able to reduce solvent issues in chemical syntheses. The catalytic activity of the obtained nanocatalysts was investigated in both the oxidation of benzyl alcohol to benzaldehyde and in the alkylation of toluene with benzyl chloride. The microwave-assisted oxidation of benzyl alcohol reached 45% conversion after 10 min. The conversion of the alkylation of toluene in both microwave-assisted and conventional heating methods was higher than 99% after 3 min and 30 min, respectively. The transformation of benzyl alcohol and toluene into valuable product in both the oxidation and alkylation reaction reveals a potential method for the valorization of lignocellulosic biomass.

Full text of DOI:10.3762/bjoc.13.194

New bio-nanocomposites based on iron oxides and
polysaccharides applied to oxidation and
alkylation reactions
Daily Rodríguez-Padrón, Alina M. Balu, Antonio A. Romero and Rafael Luque*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 1982–1993.
Departamento de Química Orgánica, Grupo FQM-383, Universidad
de Cordoba, Campus de Rabanales, Edificio Marie Curie (C-3), Ctra
Nnal IV-A, Km 396, E14014, Cordoba, Spain
doi:10.3762/bjoc.13.194
Received: 23 May 2017
Accepted: 30 August 2017
Published: 21 September 2017
Email:
Rafael Luque* - q62alsor@uco.es
This article is part of the Thematic Series "Mechanochemistry".
Guest Editor: J. G. Hernández
* Corresponding author
Keywords:
alkylation; benzyl alcohol; benzyl chloride; iron oxide;
mechanochemistry; microwave-assisted oxidation; polysaccharide;
toluene
© 2017 Rodríguez-Padrón et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
Polysaccharides from natural sources and iron precursors were applied to develop new bio-nanocomposites by mechanochemical
milling processes. The proposed methodology was demonstrated to be advantageous in comparison with other protocols for the
synthesis of iron oxide based nanostructures. Additionally, mechanochemistry has enormous potential from an environmental point-
of-view since it is able to reduce solvent issues in chemical syntheses. The catalytic activity of the obtained nanocatalysts was in-
vestigated in both the oxidation of benzyl alcohol to benzaldehyde and in the alkylation of toluene with benzyl chloride. The micro-
wave-assisted oxidation of benzyl alcohol reached 45% conversion after 10 min. The conversion of the alkylation of toluene in both
microwave-assisted and conventional heating methods was higher than 99% after 3 min and 30 min, respectively. The transformat-
ion of benzyl alcohol and toluene into valuable product in both the oxidation and alkylation reaction reveals a potential method for
the valorization of lignocellulosic biomass.
Introduction
Heterogeneous catalysis has played a crucial role in the devel- to the difficulty in separation and recovery of the latter. Hetero-
opment of the chemical industry. It has allowed the design of geneous catalytic systems, as a priority of research activity in
more efficient processes, both in an economical and environ- the field of green chemistry, open up new possibilities for
mental way, thanks to the higher activity and selectivity of further development of environmentally friendly, catalyzed pro-
heterogeneous catalysts [1-3]. These systems, in particular, are cesses [4]. In this sense, metal oxide nanoparticles have been
preferred over the use of catalysts in a homogeneous phase due extensively studied in recent decades because of their high ac-
1982

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
tivity, specificity of interaction and advantageous properties in- tic polymers in the preparation of nanomaterials, in
cluding a high surface/volume ratio combined with their small order to ameliorate environmental issues [34]. Therefore,
size [5-7]. Moreover, metal oxide nanoparticles have the addi- one of the objectives of this work was to investigate the
tional advantage of easy recycling and reuse, which is an essen- catalytic behavior of nanocomposites based on iron oxide
tial and desired property in many applications such as catalysis, and the polysaccharide S4, obtained from Lentinus Tigrinus
sensors and even medicine [2,6,8,9]. Our research group has (PS4).
recently prepared different types of metal and metal oxide nano-
particles which have several applications in heterogeneous ca- The most promising feature of such nanoentities based on iron
talysis [10-14]. Transition metal and metal oxide nanoparticles oxide and polysaccharides is the bifunctional, oxidative [20]
have been reported to be highly active and selective in several and acidic nature [21], which in turn can be fine-tuned to design
processes, such as redox [15-17], C–C and C–heteroatom highly active materials for both oxidation and acid catalyzed
couplings [18,19]. In particular, iron oxide nanoparticles have processes.
been the object of most research from our group over the past
years [10,20-22].
Among all the known oxidative transformations, the oxidation
of alcohols to ketones and aldehydes have gained a lot of atten-
One of the main challenges in the field of catalysis is the prepa- tion for the research community due to its broad range of indus-
ration of new materials to replace the traditional catalysts trial applications [38,39]. Nonetheless, the scale up of the oxi-
quickly, cheaply and efficiently [5]. In this regard, mechano- dation reactions has been very restricted due to the use of heavy
chemical synthesis has become one of the most advantageous metals, the limited selectivity for highly functionalized com-
and environmentally friendly alternatives compared to the tradi- pounds, and the thermal hazards posed [40]. Consequently, cat-
tional routes [5,23]. This novel approach offers the possibility alytic reactions should be further investigated in order to find
of a solvent-free process, avoiding environmental problems new alternatives to conventional oxidation methods that require
related to toxicity and the use thereof [24,25]. Moreover, the stoichiometric amounts of inorganic oxidants, which are highly
mechanochemical protocols have potential applicability due to toxic and polluting. Aiming to minimize chemical waste in
the extreme simplicity, cleanliness, reproducibility and versa- these catalytic processes, the scientific community is moving
tility, haven been already demonstrated to be highly useful for towards the use of clean oxidants ("green oxidants"), such as
the development of a range of advanced nanomaterials includ- molecular oxygen or H2O2 [39]. Thus, the use of clean oxidants
ing metal-organic frameworks (MOFs), supported metal and with heterogeneous catalysts such as Fe2O3 nanoparticles,
metal oxide nanoparticles and nanocomposites with diverse ap- Ag nanoparticles supported on hydrotalcites, Au nanoparticles
plications in catalysis, sensing, drug delivery and adsorption supported on metal oxides, and Pd nanoparticles supported on
[25-28]. In addition, mechanochemical protocols have also been SBA-15 has been developed [41-44]. In this regard, both unsup-
employed to functionalize the surfaces of magnetic nanoparti- ported “free” iron oxide nanoparticles [45] and supported iron
cles (MNPs) with monosaccharides [29] and to obtain bio-nano- oxide based catalytic systems [46] have been extensity reported
composites based on proteins and dopamine (DA)-coated metal to be active, stable and selective catalysts for the oxidation of
oxide MNPs [30,31].
alcohols with hydrogen peroxide. Specifically, the oxidation of
benzyl alcohol to benzaldehyde has generated great interest in
On the other hand, nature has inspired many scientists to inno- order to study the oxidation of substituted benzyl alcohols. Al-
vate and design new materials. The miniaturization and effi- though benzyl alcohol is industrially produced by reduction of
ciency achieved by entities in nature for energy production, benzaldehyde, this aldehyde is considered as the second most
biometabolite, photo-processing and resource maximization has important flavoring molecule after vanillin, due to its variety of
always been an attractive option to imitate based on a funda- applications in cosmetics, perfumes, food, dyes, agrochemicals
mental and rational understanding [28,32]. In that sense, poly- and pharmaceuticals [41]. Regarding the acid-catalyzed pro-
saccharides extracted from fungal organisms can be used both cesses, aromatic alkylation reactions are among the most versa-
as nanoparticle carriers and sacrificial templates due to their tile and widely investigated reactions which can grant access to
highly functionalized structure. Although such carbohydrates a wide range of compounds as important intermediates,
have been widely reported for the preparation of nanocompos- fragrances, agrochemicals and pharmaceuticals [47-49]. In this
ites with a great range of applications, due to their low cytotox- sense, the benzylation of benzene or other aromatic substrates is
icity and notable biocompatibility and stability [33-37], their well-known to be an important step in the preparation of rele-
catalytic application is still lacking. In addition, these natural vant building blocks in organic synthesis, such as diphenyl-
products are easily and inexpensively produced by microbes, methane and substituted diphenylmethanes [50]. Therefore,
plants, and animals, and constitute a green alternative to synthe- many studies have been focus on the preparation of novel Lewis
1983

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
acid catalysts, such as mesostructured zeolitic materials. In par- Results and Discussion
ticular, in this study, our research group has focused attention In the present study, we prepared and analyzed three different
on the alkylation of toluene with benzyl chloride, since is catalysts based on iron oxides and polysaccharides, in particu-
promoted by the presence of Lewis acids such as iron oxides lar: iron oxide–polysaccharide 4 magnetic nanoparticles
[49].
(Fe2O3-PS4-MNP), iron oxide–polysaccharide 4 (Fe2O3-PS4)
and titanium oxide–iron oxide–polysaccharide 4 (TiO2-Fe2O3-
These two reactions in particular (oxidation and alkylation of PS4) nanocomposites. The materials were successfully ob-
benzyl alcohol and toluene, respectively) could find current ap- tained using the proposed solvent-free methodology, which is
plication in the valorization of lignocellulosic biomass with depicted in Figure 1. The materials were characterized by the
heterogeneous catalysis.
techniques presented below. The catalytic activity of these
systems has been assessed in the alkylation reaction of toluene
However, the use of heterogeneous catalysts in the aforemen- with benzyl chloride and the selective oxidation of benzyl
tioned reactions usually requires a filtration or centrifugation alcohol to benzaldehyde.
step to recover the catalyst. In order to simplify the recovery
and reuse of the catalytic system, a magnetically separable X-ray diffraction
nanocomposite could represent a breakthrough in the scientific The structure and arrangement of the synthesized materials
community [51]. Therefore, our research group has focused on were analyzed by X-ray diffraction. The XRD pattern of the
the investigation of the aforementioned oxidation and alkyl- Fe2O3-PS4 nanomaterial exhibited a series of distinctive
ation reactions, using heterogeneous catalysts with magnetic diffraction lines that could be correlated to the hematite diffrac-
properties.
tion pattern. A characteristic broad band in the 20° to 40° range,
Figure 1: Overview of the preparation of the nanocomposites based on iron oxide and polysaccharide.
1984

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
typical of amorphous materials, was observed in the Fe2O3-PS4 C 1s XPS spectra of the obtained materials exhibited two differ-
nanomaterial (Supporting Information File 1, Figure S1A). The ent contributions associated to the presence of C–C/C=C and
X-ray diffraction patterns of the magnetic material showed a C–O bonds. Also, the O 1s XPS spectra for the Fe2O3-PS4 and
mixture of maghemite and hematite phases. In this case, a simi- Fe2O3-PS4-MNP nanomaterials displayed two different peaks
lar XRD pattern could be in principle associated to magnetite attributed to O–C and O–Fe, while for the TiO2-Fe2O3-PS4
(Fe3O4) over the maghemite phase, since these two phases are nanocomposite contained three contributions related to O–C,
difficult to clearly distinguish by XRD analysis. However, the O–Ti and O–Fe (see also Supporting Information File 1, Figures
absence of Fe2+ species (see the following XPS analysis) and S2–S4 for the XPS spectra).
the reddish color are consistent with a maghemite magnetic
phase (Supporting Information File 1, Figure S1B) [51,52]. On Nitrogen physisorption
the other hand, the crystal structure of the material TiO2-Fe2O3- The textural properties of the materials have been studied with
PS4 turned out to be a mixture of ilmenite and pseudobrookite N2 absorption–desorption isotherms analysis. The TiO2-Fe2O3-
phases (Supporting Information File 1, Figure S1C).
PS4 nanomaterial presents a mesoporous structure with a pore
size of 20 nm and a surface area of 58 m2/g. However, in the
other two catalysts, a particular macroporosity was found at
X-ray photoelectron spectroscopy
X-ray photoelectron spectroscopy (XPS) measurements were p/p0 > 0.98 (isotherms of type III), which are clearly dissimilar
consistent with XRD data, where the main peaks were found to to those of conventionally ordered mesoporous materials [53]
correspond to Fe2O3 species. In the three nanocomposites, the having a sharp increase in p/p0 from 0.85 to 0.90 (see Support-
presence of Fe3+ species could be also inferred from the ing Information File 1, Figure S5 for all adsorption–desorption
Fe 2p3/2 and Fe 2p1/2 peaks around 710 eV and 725 eV, respec- isotherms). Thus, the Fe2O3-PS4 and the Fe2O3-PS4-MNP ma-
tively (Figure 2). These results are in good agreement with terial are macroporous solids with interparticle pores. The sur-
previous studies and did not show the characteristic peak asso- face area was found to be 33 and 6 m2/g for Fe2O3-PS4-MNP
ciated with the presence of Fe(II) or Fe(0) species in the materi- and for Fe2O3-PS4 nanomaterials, respectively. The pore
als [51,52]. Concerning the TiO2-Fe2O3-PS4 nanocatalyst, the volumes were found to be in the range of 0.30–0.40 mL/g for
XPS experiments results revealed a band at 462 eV (Ti 2p3/2), the three materials (Table 1). The materials exhibited, in
which confirmed the presence of TiO2 on the surface of the general, satisfying surface areas and pore volumes, particularly
nanocomposite (Figure 2C). Additionally, the deconvoluted taking into account their preparation methodology.
Figure 2: XPS spectra of A: Fe2O3-PS4, B: Fe2O3-PS4-MNP and C: TiO2-Fe2O3-PS4 nanohybrids.
Table 1: Textural properties of iron oxide/polysaccharide nanohybrids.
Catalyst
TiO2-Fe2O3-PS4
Fe2O3-PS4-MNP
Fe2O3-PS4
SBET (m2/g)
DBJH (nm)
58
33
6
20.9
0.32
41.2
0.36
171.2
0.40
VBJH (mL/g)
1985

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
Electron microscopy
Fe2O3-PS4, Fe2O3-PS4 and Fe2O3-PS4-MNP, respectively
The morphology of the nanomaterials was determined by scan- (Figure 3B,D,F).
ning electron microscopy (SEM) and transmission electron
Diffuse reflectance infrared Fourier transform
microscopy (TEM). The micrographs show a homogeneous dis-
tribution of iron oxide nanoparticles for the three catalysts spectroscopy
(Figure 3A,C,E). The analysis of the SEM images reveals the The acidic properties of the Fe2O3-PS4-MNP and TiO2-Fe2O3-
tendency of the constituent particles of the magnetic material to PS4 materials were studied by diffuse reflectance infrared
form agglomerates due to their nanometer size (Figure 3E). Fourier transform spectroscopy (DRIFT) experiments. The
When these agglomerates are observed at higher magnification, TiO2-Fe2O3-PS4 nanocomposite has well-marked acidic char-
they can be seen as independent particles. The three acteristics. This can be deduced from the intense and well-
materials displayed a similar particle-size distribution defined bands observed at 1449 and 1600 cm−1, which can be
average of around 9 nm, 12 nm and 10 nm for the TiO2- attributed to Lewis acid centers (Figure 4A). Additionally, in
Figure 3: A and B: SEM and TEM images of TiO2-Fe2O3-PS4; C and D: SEM and TEM images of Fe2O3-PS4. E and F: SEM and TEM images of
Fe2O3-PS4-MNP. Inset: Particle-size distribution of the obtained nanohybrids.
Figure 4: DRIFT spectra of A: TiO2-Fe2O3-PS4 and B: Fe2O3-PS4-MNP nanohybrids.
1986

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
the spectrum of Fe2O3-PS4-MNP, having bands at 1440 and
1618 cm−1, indicates the peculiar Lewis acidity of this material
(Figure 4B). Furthermore, in both materials, a band of lesser in-
tensity can be seen around 1490 cm−1, which is due to the pres-
ence of both Brønsted and Lewis centers.
Table 3: Elemental composition of the TiO2-Fe2O3-PS4 nanohybrid
material.
Element
ICP–MS (wt %)
Ti
12.8
38.3
Fe
These materials maintained a remarkable acidity, even at high
temperatures (200 and 300 °C) with visible acid centers distin-
guishable from noise. This behavior has a high value for acid-
catalyzed processes such as alkylation. Furthermore, the Fe2O3-
PS4 sample does not show appreciable acidity [43].
Table 4: Magnetic susceptibility of the Fe2O3-PS4-MNP nanohybrid
material.
Catalyst
Milling time
(min)
Magnetic susceptibility
(10−6 m3 kg−1)
Pyridine (PY) and 2,6-dimethylpyridine
Fe2O3-PS4-MNP
15
30
420
337
(DMPY) titration
The acidic properties of these materials have also been deter-
mined by the chromatographic method of pulses. Pyridine, due
to low steric hindrance, adsorbs nonspecifically in both types of
centers, while dimethylpyridine adsorbs specifically on Catalytic activity
Brønsted acid centers, due to the high steric hindrance of the The catalytic activity of these materials has been investigated in
methyl groups [54]. It is noticeable that the TiO2-Fe2O3-PS4 two reactions: 1) the oxidation of benzyl alcohol to benzalde-
catalyst possesses both Lewis and Brønsted acid sites with a hyde and the 2) alkylation of toluene with benzyl chloride.
more marked Lewis acidity. The Fe2O3-PS4-MNP material
presents instead only Lewis acid sites, while the Fe2O3-PS4 The oxidation reaction of benzyl alcohol was carried out using
does not show appreciable acidity to be quantized (Table 2). the three nanomaterials as heterogeneous catalysts (Scheme 1).
The results of conversion and selectivity are reported in Table 5
Acidity measurements from both methodologies (PY DRIFT, and Figure 5. After 10 min, the conversions were 32 and 45%
PY and DMPY pulse chromatography titration data) were gen- for TiO2-Fe2O3-PS4 and Fe2O3-PS4 nanomaterials, respective-
erally in good agreement, supporting the validity of our assump- ly, while for the Fe2O3-PS4-MNP catalyst the conversion
tion on DMPY adsorbing selectively on Brønsted acid sites.
reaches just 10%. Remarkably, the selectivity to benzaldehyde,
employing Fe2O3-PS4 nanocatalysts, was higher than 90% for a
reaction time of 5 and 10 min. Since the best results were ob-
tained with the Fe2O3-PS4 nanocomposite, the latter was em-
Inductively coupled plasma–mass
spectrometry (ICP–MS)
The elemental composition of the TiO2-Fe2O3-PS4 material ployed to carry out the reaction for 30 min in order to improve
was determined by ICP–MS. The content of iron and titanium the obtained results. However, the conversion increased only to
was 38 and 12 wt %, respectively (Table 3). These values
corroborate the incorporation of titanium in the material and
confirm the results obtained by XPS.
Magnetic susceptibility
The magnetic susceptibility of Fe2O3-PS4-MNP is consistent
with the XRD data and confirms the magnetic characteristics of
the material. Such values make this a material with attractive
Scheme 1: Oxidation of benzyl alcohol to benzaldehyde.
feature for magnetic separation (Table 4) [40].
Table 2: Surface acidity of iron oxide/polysaccharide nanohybrids.
Catalyst
Total acidity PY (µM/g)
Brønsted acidity DMPY (µM/g)
Lewis acidity (µM/g)
TiO2-Fe2O3-PS4
Fe2O3-PS4-MNP
81
14
25
_
56
14
1987

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
Table 5: Conversion and selectivity of the oxidation reaction of benzyl alcohol.
Catalyst
TiO2-Fe2O3-PS4
Ca (%) Sb (%)
Fe2O3-PS4
Sb (%)
Fe2O3-PS4-MNP
Time (min)
Ca (%)
Ca (%)
Sb (%)
5
30
32
76.6
78.1
18
45
94.4
97.7
10
10
24.4
24.7
10
aConversion (%); bselectivity (%) to benzaldehyde.
Figure 5: Conversion and selectivity of the oxidation of benzyl alcohol for the three catalytic systems.
47%, which does not compensate the energy consumption by
extending the reaction time from 10 to 30 min.
The three synthesized catalysts showed high activity in the
alkylation of toluene with benzyl chloride, either via micro-
wave-assisted or with conventional heating (60 °C). For the
microwave-assisted reaction (Scheme 2), after three minutes,
the conversion was higher than 99% for all of the three materi-
als (Figure 6). For the Fe2O3-PS4-MNP nanocatalyst, even after
just 1 min, the reaction showed a conversion higher than 99%.
In this reaction, the three corresponding isomers (ortho, meta
and para substituted) were obtained. In particular, the synthesis
Scheme 2: Microwave-assisted alkylation of toluene with benzyl
chloride.
of the para-isomer can be achieved with high selectivity, em-
ploying the TiO2-Fe2O3-PS4 and Fe2O3-PS4 nanomaterials for
1 and 2 min, respectively (Table 6).
Reusability studies prove the high inherent stability and activi-
The alkylation reaction with conventional heating (Scheme 3) ty of Fe2O3-PS4-MNP and TiO2-Fe2O3-PS4 nanomaterials
was followed by gas chromatography. After 30 min, the conver- (Figure 8). However, the Fe2O3-PS4 nanocatalyst loses its ac-
sion was greater than 99% for all of the three materials and the tivity after the first use, which can be due to the loss of residual
selectivity values were slightly higher compared with the micro- acidity, which might happen to the material after the synthesis
wave-assisted reaction during 3 min (Table 7, Figure 7).
process.
1988

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
Figure 6: Conversion and selectivity of the microwave-assisted alkylation of toluene for the three catalytic systems.
Table 6: Conversion and selectivity of the microwave-assisted alkylation of toluene for the three catalytic systems.
Catalyst
TiO2-Fe2O3-PS4
Ca (%) S-pb (%)
Fe2O3-PS4
S-pb (%)
Fe2O3-PS4-MNP
Ca (%) S-pb (%)
Time (min)
Ca (%)
1
2
3
14.6
>99
>99
97.9
49.1
46.8
–
4
–
>99
>99
>99
46.3
46.9
48.4
90.1
46
>99
aConversion (%); bselectivity (%) with respect to the para-isomer.
Reference experiments for the two investigated reactions were
carried out in the absence of catalyst, demonstrating that the
nanocomposites play a crucial role in order to accelerate the
reaction rates (Supporting Information File 1, Table S1). It can
be concluded that for the oxidation of benzyl alcohol, the
Fe2O3-PS4 presents a better employability outlook, whereas for
the alkylation reaction, the Fe2O3-PS4-MNP showed promising
conversion and selectivity values.
Conclusion
The three bio-nanocomposites TiO2-Fe2O3-PS4, Fe2O3-PS4
and Fe2O3-PS4-MNP, based on iron oxide and polysaccharide
S4 were synthesized by mechanochemical processes. The mag-
netic susceptibility measurements show attractive magnetic
Scheme 3: Alkylation of toluene with benzyl chloride with conven-
tional heating.
Table 7: Conversion and selectivity of the alkylation of toluene with conventional heating.
Catalyst
Time (min)
30
TiO2-Fe2O3-PS4
Ca (%) S-pb (%)
>99 51.6
Fe2O3-PS4
S-pb (%)
50.5
Fe2O3-PS4-MNP
Ca (%) S-pb (%)
>99 49.5
Ca (%)
>99
aConversion (%), bselectivity (%) with respect to the para-isomer.
1989

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
Figure 7: Conversion and selectivity of the alkylation of toluene with conventional heating for the three catalytic systems.
Figure 8: Reusability of the iron oxide/polysaccharide nanohybrids.
characteristics for recovery and reusability of the Fe2O3-PS4- selective oxidation of benzyl alcohol and alkylation of toluene
MNP nanocomposite. Furthermore, the nanomaterials showed with benzyl chloride.
promising activity in the oxidation reaction of benzyl alcohol to
Experimental
benzaldehyde, with conversions of 32–45%. The three synthe-
Synthesis of bio-nanocomposites based on
iron oxide and polysaccharide S4
sized nanocomposites have proved to be highly active and
selective catalysts in the alkylation reaction of toluene with
benzyl chloride, due to the exceptional surface acidic properties
of the nanoparticles. Both microwave irradiation and conven-
tional heating exhibited high conversion and selectivity to the
main product of the reaction in extremely short reaction times.
Moreover, reusability studies showed high stability and activity
of the nanohybrids TiO2-Fe2O3-PS4 and Fe2O3-PS4-MNP,
establishing these catalysts as potential candidates in both the
A simple, reproducible and environmentally friendly protocol
has been developed for the synthesis of the three catalysts
Fe2O3-PS4, Fe2O3-PS4-MNP, and TiO2-Fe2O3-PS4. The three
materials were synthesized using a 2:1 metal precursor/polysac-
charide ratio (2 g of polysaccharide S4, 4 g of FeCl2·4H2O), in
a ball mill (Retsch PM100 ball mill model), at 350 rpm for
30 min, using a 125 mL reaction chamber and 18 10 mm stain-
1990

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
less steel balls. Additionally, in the case of the TiO2-Fe2O3- samples were degassed for 24 h at 140 °C under vacuum
PS4, 4.08 mL of titanium isopropoxide was added to obtain the (p < 10−2 Pa). The surface areas were calculated according to
desired nanomaterial. Subsequently, the materials were oven- the linear equation of BET in the 0.05 < p0 < 0.22 range. The
dried at 100 °C for 24 h, and finally calcined at 600 °C for 3 h pore size distributions (PSDs) were obtained from the N2 de-
in air atmosphere.
sorption branch.
The Fe2O3-PS4-MNP catalyst was obtained using 2 g of poly- Electron microscopy
saccharide S4, 4 g of Fe(NO3)3·9H2O and 1.5 mL of propionic SEM images and the elemental composition were recorder
acid. The milling process was carried out at 350 rpm for 15 and using the JEOL JSM-6490 LV microscope. The samples were
30 min, respectively. The resulting material was oven-dried at Au/Pd-coated on a high-resolution sputter SC7640 at a sput-
100 °C for 24 h and slowly heated up (1 °C/min) to 300 °C tering rate of 1.5 kV per minute, up to 7 nm thickness. TEM
under air and kept at that temperature for 30 min.
micrographs were obtained in the FEI Tecnai G2 system,
equipped with a charge coupling device camera. Prior to analy-
sis, the samples were suspended in ethanol and directly
Material characterization
In order to characterize the synthesized materials, several tech- deposited on a copper grid.
niques have been employed, including XRD, XPS,
absorption–desorption of N2, SEM, TEM, DRIFT and titrations Diffuse reflectance infrared Fourier transform
with pyridine and dimethylpyridine. In addition, the magnetic spectroscopy
susceptibility of Fe2O3-PS4-MNP was measured and the The DRIFT spectra of the materials were recorder on an infra-
elemental composition of TiO2-Fe2O3-PS4 was determined by red spectrophotometer (ABB MB3000 with Horizon MBTM
ICP–MS.
software), equipped with an ATR PIKE MIRacleTM sampler,
with a ZnSe window using 256 scans at a resolution of 8 cm−1.
During the measurements, the sample was purged with a
X-ray diffraction
X-ray diffraction has been used for the structural study of the nitrogen flow (20 mL min−1, dehydrated and deoxygenated).
synthesized nanocatalysts. The diffraction patterns were ob- The spectra were recorded at room temperature in the
tained on a Bruker D8 Discover diffractometer, equipped with a 4000–600 cm−1 wavenumber range. The materials were heated
goniometer Bragg Brentano θ/θ of high precision, and with a Cu at 300 °C for 3 h prior to acquiring the reference spectra. Thus,
X-ray tube. Scans were performed in the 0.5 to 80° range at a the temperature was decreased to 200 °C, and after 10 min the
step size of 0.02° with a counting time per step of 20 s.
reference spectrum was again recorded. Similarly, the reference
spectra at 150 and 100 °C were acquired. Once the references
were obtained, the acquisition of the spectra was carried out
X-ray photoelectron spectroscopy
XPS measurements were performed at the Central Service of starting with the lowest temperature.
Research Support (SCAI) of the University of Cordoba, in an
ultrahigh vacuum (UHV) multipurpose surface analysis system Pyridine (PY) and 2,6-dimethylpyridine (DMPY)
(SpecsTM model, Germany), operating at pressures of titration
<10−10 mbar, using a conventional X-ray source (XR-50, Specs, Pyridine (PY) and 2,6-dimethylpyridine (DMPY) titration ex-
Mg Kα, hv = 1253.6 eV, 1 eV = 1.603 × 10−19 J) in a "stop and periments were carried out at 300 °C, via gas phase adsorption
go" mode. Powdered samples were deposited on a sample of the basic probe molecules applying a pulse chromatographic
holder using double-sided adhesive tape and subsequently evac- titration methodology. The catalyst used (≈0.025 g) was fixed
uated overnight under vacuum (<10−6 Torr). The spectra were inside a tubular stainless steel microreactor (4 mm internal di-
taken at room temperature (pass energy: 25 and 10 eV, step ameter) by Pyrex glass wool. A cyclohexane solution of titrant
size: 1 and 0.1 eV, respectively) with a Phoibos 150-MCD (0.989 M in PY and 0.686 M in DMPY, respectively) was
energy detector. For the deconvolution of the obtained curves, injected into a gas chromatograph through a microreactor in
the XPS software CASA was used.
which the catalyst was previously sited. The injected base was
analyzed by gas chromatography with a flame ionization
detector and using an analytical column of 0.5 m length, con-
N2 physisorption
The Brunauer–Emmett–Teller (BET) surface area and pore taining 5 wt % of polyphenylether in the Chromosorb
volume measurements were obtained from N2 adsorption–de- AW-DMCS in 80/100. The quantity of probe molecule
sorption isotherms at liquid nitrogen temperature (77 K) in a adsorbed by the solid acid catalyst can subsequently be easily
Micromeritics ASAP 2000 instrument. The weight of the sam- quantified. In order to distinguish between Lewis and Brønsted
ples ranged between 0.15–0.20 g. Prior to the analysis, the acidity, it was assumed that all DMPY selectively titrates
1991

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
Brønsted sites (methyl groups hinder coordination of nitrogen where cinitial and cfinal are the concentrations of the reagents
atoms with Lewis acid sites) while PY titrates both Brønsted before and after the reaction, respectively, and cproduct is the
and Lewis acid sites in the materials. Thus, the difference be- concentration of the product, as determined by gas chromatog-
tween the amounts of PY (total acidity) and DMPY (Brønsted raphy (GC).
acidity) adsorbed should correspond to Lewis acidity in the ma-
terials.
The samples were analyzed with a HP5890 Series II gas chro-
matograph (60 mL min−1 N2 carrier flow, 20 psi column top
head pressure) using a flame ionization detector (FID). A
ICP–MS
The metal content in the TiO2-Fe2O3-PS4 catalyst was deter- HP-101 capillary column (25 m × 0.2 mm × 0.2 μm) was em-
mined by ICP–MS in an Elan DRC-e (PerkinElmer SCIEX) ployed. All calculations were based on the use of benzyl chlo-
spectrometer. The sample (≈25 mg) was previously digested ride and benzyl alcohol as limiting reagents for the studied
using an acid mixture of HF/HNO3/HCl 1:1:1. Dilutions were alkylation and oxidation reaction, respectively.
made with miliQ water (double distilled) up to a maximum of
1% of HF2− in acid solution.
Supporting Information
Magnetic susceptibility
The magnetic susceptibility was measured at room temperature
at low frequency (470 Hz) using a Bartington MS-2 instrument.
Supporting Information File 1
Additional spectra.
[http://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-13-194-S1.pdf]
Catalytic experiments
The oxidation of benzyl alcohol to benzaldehyde was per-
formed using 25 mg of catalyst, 0.2 mL of benzyl alcohol,
0.3 mL of hydrogen peroxide, and 2 mL of acetonitrile as the
solvent, for 5 and 10 min, respectively.
References
1. White, R. J.; Luque, R.; Budarin, V. L.; Clark, J. H.; Macquarrie, D. J.
Chem. Soc. Rev. 2009, 38, 481–494. doi:10.1039/B802654H
2. Astruc, D.; Lu, F.; Aranzaes, J. R. Angew. Chem., Int. Ed. 2005, 44,
7852–7872. doi:10.1002/anie.200500766
The microwave-assisted alkylation of toluene and oxidation of
benzyl alcohol was carried out in a CEM-Discover microwave
reactor, equipped with a PC-controlled interface. The alkyl-
ation reactions were carried out by the standard "open vessel"
method, while for oxidation reactions, the "discover" method
was used under pressure, allowing us to control the irradiation
power, temperature and pressure.
3. Grunes, J.; Zhu, J.; Somorjai, G. A. Chem. Commun. 2003,
2257–2260. doi:10.1039/b305719b
4. Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice;
Oxford University Press: New York, 1998.
5. Xu, C.; De, S.; Balu, A. M.; Ojeda, M.; Luque, R. Chem. Commun.
2015, 51, 6698–6713. doi:10.1039/C4CC09876E
6. Astruc, D., Ed. Nanoparticles and catalysis; John Wiley & Sons, 2008.
7. Polshettiwar, V.; Luque, R.; Fihri, A.; Zhu, H.; Bouhrara, M.;
Basset, J.-M. Chem. Rev. 2011, 111, 3036–3075.
doi:10.1021/cr100230z
The alkylation reaction of toluene with benzyl chloride was per-
formed under conventional heating, too. In both alkylation ex-
periments, 2 mL of toluene, 0.2 mL of benzyl chloride and
25 mg of catalyst were used. The microwave-assisted reaction
was conducted for 1, 2 and 3 min, while the reaction under
conventional heating was carried out for 30 min until the
maximum conversion was reached. The temperature in both
cases was kept at around 60 °C.
8. Cai, D.; Mataraza, J. M.; Qin, Z. H.; Huang, Z.; Huang, J.; Chiles, T. C.;
Carnahan, D.; Kempa, K.; Ren, Z. Nat. Methods 2005, 2, 449–454.
doi:10.1038/nmeth761
9. Hu, X.; Dong, S. J. Mater. Chem. 2008, 18, 1279–1295.
doi:10.1039/b713255g
10.Balu, A. M.; Pineda, A.; Yoshida, K.; Campelo, J. M.; Gai, P. L.;
Luque, R.; Romero, A. A. Chem. Commun. 2010, 46, 7825–7827.
doi:10.1039/c0cc02015j
11.Garcia-Olmo, A. J.; Yepez, A.; Balu, A. M.; Romero, A. A.; Li, Y.;
Luque, R. Catal. Sci. Technol. 2016, 6, 4705–4711.
doi:10.1039/C6CY00249H
The conversion and selectivity were calculated from the chro-
matograms by:
12.Ouyang, W.; Kuna, E.; Yepez, A.; Balu, A. M.; Romero, A. A.;
Colmenares, J. C.; Luque, R. Nanomaterials 2016, 6, 93.
doi:10.3390/nano6050093
13.Colmenares, J. C.; Ouyang, W.; Ojeda, M.; Kuna, E.; Chernyayeva, O.;
Lisovytskiy, D.; De, S.; Luque, R.; Balu, A. M. Appl. Catal., B: Environ.
2016, 183, 107–112. doi:10.1016/j.apcatb.2015.10.034
14.Yepez, A.; De, S.; Climent, M. S.; Romero, A. A.; Luque, R. Appl. Sci.
2015, 5, 532–543. doi:10.3390/app5030532
1992

Beilstein J. Org. Chem. 2017, 13, 1982–1993.
15.Juárez, R.; Parker, S. F.; Concepción, P.; Corma, A.; García, H.
Chem. Sci. 2010, 1, 731–738. doi:10.1039/c0sc00336k
16.Navalon, S.; Martin, R.; Alvaro, M.; Garcia, H. Angew. Chem., Int. Ed.
2010, 49, 8403–8407. doi:10.1002/anie.201003216
17.Mendes, D.; Garcia, H.; Silva, V. B.; Mendes, A.; Madeira, L. M.
Ind. Eng. Chem. Res. 2009, 48, 430–439. doi:10.1021/ie8010676
18.Gonzalez-Arellano, C.; Luque, R.; Macquarrie, D. J. Chem. Commun.
2009, 1410–1412. doi:10.1039/b818767c
41.Rajabi, F.; Naresian, S.; Primo, A.; Luque, R. Adv. Synth. Catal. 2011,
353, 2060–2066. doi:10.1002/adsc.201100149
42.Yang, J.-C. E.; Yuan, B.; Cui, H.-J.; Wang, S.; Fu, M.-L.
Appl. Catal., B: Environ. 2017, 205, 327–339.
doi:10.1016/j.apcatb.2016.12.046
43.Huang, X.; Wang, X.; Wang, X.; Wang, X.; Tan, M.; Ding, W.; Lu, X.
J. Catal. 2013, 301, 217–226. doi:10.1016/j.jcat.2013.02.011
44.Rak, M. J.; Lerro, M.; Moores, A. Chem. Commun. 2014, 50,
12482–12485. doi:10.1039/C4CC04749D
19.Budarin, V. L.; Clark, J. H.; Luque, R.; Macquarrie, D. J.; White, R. J.
Green Chem. 2008, 10, 382–387. doi:10.1039/B715508E
20.Gonzalez-Arellano, C.; Yoshida, K.; Luque, R.; Gai, P. L. Green Chem.
2010, 12, 1281–1287. doi:10.1039/c003410j
45.Shi, F.; Tse, M. K.; Pohl, M.-M.; Brückner, A.; Zhang, S.; Beller, M.
Angew. Chem., Int. Ed. 2007, 46, 8866–8868.
doi:10.1002/anie.200703418
21.Yepez, A.; Lam, F. L. Y.; Romero, A. A.; Kappe, C. O.; Luque, R.
ChemCatChem 2015, 7, 276–282. doi:10.1002/cctc.201402802
22.Ozin, G. A.; Arsenault, A.; Cademartiri, L. Nanochemistry: A chemical
approach to nanomaterials; Royal Society of Chemistry: Cambridge,
UK, 2009.
46.Campelo, J. M.; Luna, D.; Luque, R.; Marinas, J. M.; Romero, A. A.
ChemSusChem 2009, 2, 18–45. doi:10.1002/cssc.200800227
47.Wilson, K.; Adams, D. J.; Rothenberg, G.; Clark, J. H. J. Mol. Catal. A
2000, 159, 309–314. doi:10.1016/S1381-1169(00)00185-0
48.Bastock, T. W.; Clark, J. H. Speciality chemicals; Elsevier Applied
Science: London, New York, 1991; p 383.
23.Tsuzuki, T.; McCormick, P. G. J. Mater. Sci. 2004, 39, 5143–5146.
doi:10.1023/B:JMSC.0000039199.56155.f9
49.Pineda, A.; Balu, A. M.; Campelo, J. M.; Luque, R.; Romero, A. A.;
Serrano-Ruiz, J. C. Catal. Today 2012, 187, 65–69.
doi:10.1016/j.cattod.2012.02.028
24.Mei, K.-C.; Guo, Y.; Bai, J.; Costa, P. M.; Kafa, H.; Protti, A.;
Hider, R. C.; Al-Jamal, K. T. ACS Appl. Mater. Interfaces 2015, 7,
14176–14181. doi:10.1021/acsami.5b03577
50.Gracia, M. J.; Losada, E.; Luque, R.; Campelo, J. M.; Luna, D.;
Marinas, J. M.; Romero, A. A. Appl. Catal., A 2008, 349, 148–155.
doi:10.1016/j.apcata.2008.07.023
25.Ding, J.; Tsuzuki, T.; McCormick, P. G. J. Mater. Sci. 1999, 34,
5293–5298. doi:10.1023/A:1004736602847
26.Koch, C. C. Nanostruct. Mater. 1993, 2, 109–129.
doi:10.1016/0965-9773(93)90016-5
51.Ojeda, M.; Balu, A. M.; Barrón, V.; Pineda, A.; Coleto, Á. G.;
Romero, A. A.; Luque, R. J. Mater. Chem. A 2014, 2, 387–393.
doi:10.1039/C3TA13564K
27.Ding, J.; Shi, Y.; Chen, L. F.; Deng, C. R.; Fuh, S. H.; Li, Y.
J. Magn. Magn. Mater. 2002, 247, 249–256.
52.Bourlinos, A. B.; Simopoulos, A.; Boukos, N.; Petridis, D.
J. Phys. Chem. B 2001, 105, 7432–7437. doi:10.1021/jp010286+
53.Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity;
Academic Press Inc.: London, 1982.
doi:10.1016/S0304-8853(02)00173-7
28.Xu, C.; Ojeda, M.; Arancon, R. A. D.; Romero, A. A.; Domingo, J. L.;
Gómez, M.; Blanco, J.; Luque, R. ACS Sustainable Chem. Eng. 2015,
3, 2716–2725. doi:10.1021/acssuschemeng.5b00568
29.Herea, D. D.; Chiriac, H.; Lupu, N.; Grigoras, M.; Stoian, G.;
Stoica, B. A.; Petreus, T. Appl. Surf. Sci. 2015, 352, 117–125.
doi:10.1016/j.apsusc.2015.03.137
54.Luque, R.; Campelo, J. M.; Luna, D.; Marinas, J. M.; Romero, A. A.
Microporous Mesoporous Mater. 2005, 84, 11–20.
doi:10.1016/j.micromeso.2005.05.013
30.Rodriguez-Padrón, D.; Puente-Santiago, A. R.; Balu, A. M.;
Romero, A. A.; Luque, R. Chem. Commun. 2017, 53, 7635–7637.
doi:10.1039/C7CC03975A
License and Terms
31.Rodríguez-Padrón, D.; Puente-Santiago, A. R.; Caballero, A.;
Benítez, A.; Balu, A. M.; Romero, A. A.; Luque, R. J. Mater. Chem. A
2017, 5, 16404–16411. doi:10.1039/C7TA04135G
This is an Open Access article under the terms of the
Creative Commons Attribution License
(http://creativecommons.org/licenses/by/4.0), which
permits unrestricted use, distribution, and reproduction in
any medium, provided the original work is properly cited.
32.Huebsch, N.; Mooney, D. J. Nature 2009, 462, 426–432.
doi:10.1038/nature08601
33.Salgueiriño-Maceira, V.; Correa-Duarte, M. A. Adv. Mater. 2007, 19,
4131–4144. doi:10.1002/adma.200700418
34.Zheng, Y.; Monty, J.; Linhardt, R. J. Carbohydr. Res. 2015, 405,
23–32. doi:10.1016/j.carres.2014.07.016
The license is subject to the Beilstein Journal of Organic
Chemistry terms and conditions:
35.Majewski, P.; Thierry, B. Crit. Rev. Solid State Mater. Sci. 2007, 32,
203–215. doi:10.1080/10408430701776680
(http://www.beilstein-journals.org/bjoc)
36.Tallury, P.; Payton, K.; Santra, S. Nanomedicine 2008, 3, 579–592.
doi:10.2217/17435889.3.4.579
The definitive version of this article is the electronic one
which can be found at:
37.Kim, J.; Piao, Y.; Hyeon, T. Chem. Soc. Rev. 2009, 38, 372–390.
doi:10.1039/B709883A
doi:10.3762/bjoc.13.194
38.Pineda, A.; Balu, A. M.; Campelo, J. M.; Romero, A. A.; Carmona, D.;
Balas, F.; Santamaria, J.; Luque, R. ChemSusChem 2011, 4,
1561–1565. doi:10.1002/cssc.201100265
39.Matsumoto, T.; Ueno, M.; Wang, N.; Kobayashi, S. Chem. – Asian J.
2008, 3, 196–214. doi:10.1002/asia.200700359
40.Dey, S. K.; Mukherjee, A. Coord. Chem. Rev. 2016, 310, 80–115.
doi:10.1016/j.ccr.2015.11.002
1993