High alkylation activities of ball-milled synthesized low-load supported iron oxide nanoparticles on mesoporous aluminosilicates

Article abstract of DOI:10.1016/j.cattod.2012.02.028

Low-load iron oxide nanoparticles on Al-SBA-15 prepared using a novel dry milling approach exhibited excelling activities in the microwave-assisted akylation of toluene with benzyl chloride (Lewis acids promoted reaction) and benzyl alcohol (Br?nsted acids promoted reaction) as compared to the parent Al-SBA-15 and similar iron oxide nanoparticles supported on Al-MCM-41 materials. Materials prepared using the milling protocol possessed remarkably low iron loadings (<0.1 wt%) but featured highly accessible sites and small nanoparticle sizes that seemed to be related to the observed differences in activities in the systems.

Full text of DOI:10.1016/j.cattod.2012.02.028

Catalysis Today 187 (2012) 65–69
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
High alkylation activities of ball-milled synthesized low-load supported iron
oxide nanoparticles on mesoporous aluminosilicates
Antonio Pineda, Alina M. Balu, Juan Manuel Campelo, Rafael Luque∗, Antonio Angel Romero,
Juan Carlos Serrano-Ruiz
Departamento de Quimica Organica, Universidad de Cordoba, Campus de Excelencia Internacional Agroalimentario CeiA3, Edificio Marie Curie (C-3), Ctra Nnal IV-A, Km 396, E-14014
Cordoba, Spain
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2011
Received in revised form 11 January 2012
Accepted 8 February 2012
Available online 14 March 2012
Low-load iron oxide nanoparticles on Al-SBA-15 prepared using a novel dry milling approach exhib-
ited excelling activities in the microwave-assisted akylation of toluene with benzyl chloride (Lewis acids
promoted reaction) and benzyl alcohol (Brönsted acids promoted reaction) as compared to the parent
Al-SBA-15 and similar iron oxide nanoparticles supported on Al-MCM-41 materials. Materials prepared
using the milling protocol possessed remarkably low iron loadings (<0.1 wt%) but featured highly acces-
sible sites and small nanoparticle sizes that seemed to be related to the observed differences in activities
in the systems.
Keywords:
Supported nanoparticles
Iron oxide nanoparticles
Ball milling
© 2012 Elsevier B.V. All rights reserved.
Mechanochemistry
Alkylations
1. Introduction
including co-precipitation [13], microwave irradiation [1,4,14],
ultrasounds [15] and others [16].
Supported nanoparticles on porous materials have attracted a
great deal of attention in past years due to their interesting proper-
ties compared to bulk metals [1]. These include high surface areas
and specificities which make them particularly suitable for various
applications including sensors [2], biomedicine [3] and catalysis
[1,4]. In terms of catalytic applications, transition metal and metal
oxide nanoparticles have been reported to be highly active and
selective in a number of processes including redox [5–7] and C–C
and C-heteroatom couplings [8,9]. In particular, supported iron
oxide nanoparticles have been the subject of most research endeav-
ours from our group over the past years [8–12].
The most promising feature of such nanoentities is the
bifunctional oxidative and acidic nature which in turn can be
fine tuned to design highly active materials for both oxidation
(e.g. alcohols, alkenes) [10] and acid-catalysed processes (e.g.
alkylations) [11], as well as potentially in tandem acid/redox
catalysed processes such as isomerisation–cyclisation/oxidation
reactions. Apart from conventional impregnation/deposition pro-
tocols, several methodologies have been reported to prepare
the aforementioned supported transition metal nanoparticles
We recently devised a novel dry milling approach to achieve
the desired iron oxide nanoparticle decoration on the surface
of mesoporous aluminosilicates [17]. This methodology involves
simultaneous milling of the metal precursor (e.g. FeCl2) and
the mesoporous support (e.g. aluminosilicate), both in the solid
phase, using a planetary ball mill. By means of this methodology,
metal precursors undergo hydrolysis generating the corresponding
hydroxides, which upon calcination subsequently form highly dis-
persed iron oxide nanoparticles on the surface of the support [17].
Despite their minimum Fe content (<0.2 wt% Fe), these materials
were found to be highly active in the microwave-assisted selective
oxidation of benzyl alcohol to benzaldehyde. Preliminary results
were also obtained in the microwave-assisted alkylation of toluene
with benzyl chloride [17]. Comparatively, the aluminosilicate sup-
port was poorly active in both processes.
Following these preliminary results, herein we report a detailed
study of the activity of ball-milled prepared supported iron oxide
nanoparticles on mesoporous aluminosilicates (Al-doped SBA-15
and MCM-41 with Si/Al ratio of 20 prepared as reported else-
where [10]) in acid catalysed processes such as the alkylation of
toluene with both benzyl chloride (Lewis acid-promoted) and ben-
zyl alcohol (Bronsted acid-catalysed) under microwave irradiation
and conventional heating conditions. These reactions generally
require the utilisation of highly acidic materials as catalysts [18].
The catalytic properties of the ball-milled prepared materials in
∗
Corresponding author.
E-mail address: q62alsor@uco.es (R. Luque).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2012.02.028

66
A. Pineda et al. / Catalysis Today 187 (2012) 65–69
the aforementioned reactions were compared to those of similar
materials prepared by microwave methods.
Diffuse Reflectance Fourier-Transform Infrared (DRIFT) spec-
tra of adsorbed PY were carried out in an ABB IR-ATR instrument
equipped with an environmental chamber. PY was adsorbed at
room temperature for a certain period of time (typically 1 h) to
ensure a complete saturation of the acid sites in the catalyst and
then spectra were recorded at different temperatures according to
the previously reported methodology [20]. With this purpose, the
different types of acid sites in the materials (Brönsted and Lewis)
could be measured and quantified.
2. Experimental
2.1. Ball-milled assisted preparation of iron oxide nanoparticles
In a typical synthesis of ball-milled materials, 0.2 g Al-SBA-15
support was grinded with the adequate amount of iron precur-
sor (FeCl2·4H2O) to reach a theoretical 0.5 wt% iron loading in a
Retsch PM-100 planetary ball mill using a 125 mL reaction chamber
and eighteen 10 mm stainless steel balls. Optimised milling condi-
tions were 10 min at 350 rpm. The resulting materials (Fe/Al-SBA)
were then subjected to two different conditioning methodologies
to ensure the removal of all unreacted precursor species: (i) soaking
with 2 mL water followed by a microwave treatment for 5 + 5 min
in a domestic LG MS 19296/00 (maximum power 800 W). The final
material (denoted hereafter as Fe/Al-SBA-W) was eventually thor-
oughly washed with 50 mL water and then calcined at 400 ◦C under
air for 4 h. (ii) Soaking with 2 mL H2O2, microwaved (5 + 5 min at
the same conditions as with H2O2), thoroughly washed with 50 mL
water and calcined at 400 ◦C for 4 h (Fe/Al-SBA-HP). Fe content
of the catalysts was estimated by EDX. We note that Al-MCM-41
was not selected as support in ball-milling experiments due to its
reduced stability under the ball milling preparation conditions as
compared to Al-SBA-15. Consequently, an alternative microwave-
based route was used to prepare the Fe/Al-MCM-41 as described
below.
Iron content in the materials was quantified by TEM-EDX and
AAS or ICP/MS as previously reported [10,17], showing consistent
results between materials and samples.
2.4. Catalytic activity
2.4.1. Conventional heating
In a typical experiment, 10 mL of toluene and 1 mL of benzyl
alcohol or benzyl chloride were pre-heated at 110 ◦C in a round
bottomed flask for a few minutes and then 0.1 g of catalyst was
added to the reaction mixture, which was further stirred at 110 ◦C
for 10–12 h until reaction completion. The filtrate was analysed by
GC and GC/MS Agilent 6890N fitted with a capillary column HP-5
(30 m × 0.32 mm × 0.25 ␮m) and a flame ionisation detector (FID).
The identity of the products was confirmed by GC–MS and 1H and
13C NMR. The blank reaction showed the thermal effects in the reac-
tion were negligible (less than 5% conversion was obtained after
24 h). Response factors of the reaction products were determined
with respect to the substrates from GC analysis using standard
compounds in calibration mixtures of specified compositions.
2.2. Microwave synthesized materials
2.5. Microwave-assisted reactions
These materials were prepared following a methodology pre-
viously reported [10,19]. Briefly, the aluminosilicate support
(Al-MCM-41, 0.2 g) and the desired quantity of iron precursor
(FeCl2·4H2O) to achieve a 1 wt% loading dissolved in a ace-
tone/ethanol mixture (1:1, v:v) were placed on a pyrex vial and
microwaved at two different power settings (150 and 300 W) for
15 min in a CEM-DISCOVER microwave reactor in closed-vessel
(pressure controlled) and power controlled method (maximum
temperature reached 100–120 ◦C). The resulting coloured materi-
als was then filtered off, thoroughly washed with acetone, ethanol
and water, dried overnight at 100 ◦C and calcined at 400 ◦C prior to
its utilisation in the investigated reaction. The two materials pre-
pared were denoted as Fe/Al-MCM-MW-150 (150 W power) and
Fe/Al-MCM-MW-300 (300 W power).
In a typical reaction, 2 mL toluene, 0.2 mL benzyl alcohol or ben-
zyl chloride and 0.025 g catalyst were added to a pyrex vial and
microwaved in a pressure-controlled CEM-Discover microwave
reactor for 3 min at 300 W (110–120 ◦C, maximum temperature
reached) under continuous stirring. Samples were then withdrawn
from the reaction mixture and analysed in a similar way to that
reported above. The microwave method was generally tempera-
ture controlled (by an infra-red probe) where the samples were
irradiated with the required power output (settings at maximum
power, 300 W) to achieve the desired temperature.
3. Results and discussion
As indicated in Section 2, the acid properties of the materials
were measured using a pulse adsorption chromatographic mode
using pyridine (PY) and 2,6-dimethyl pyridine (DMPY) as probe
molecules [20]. A recent report by Anderson et al. discloses that
DMPY has the ability to titrate some Lewis acid sites (mainly
those on edges and imperfections originated by a low tempera-
ture calcination—200 ◦C) in silica–alumina systems [22]. However,
the nature and synthetic protocol of the current systems is likely
to minimise the presence of such claimed defects thus maximising
DMPY adsorption on Brönsted sites and validating our protocol.
Results included in Table 1 show, in general, the remarkable
differences in acidity of the synthesized materials (as compared
to the parent aluminosilicate support) upon iron incorporation,
mostly in terms of Lewis acidity (2–4 times the Lewis acidity of
the support). Interestingly, Brönsted acidity also increased in some
cases as compared to the parent support, especially for microwave
materials in which a larger Fe content was present (ca. 0.5 wt%).
Nevertheless, the observed increase in both Brönsted and Lewis
acidity for extremely low loaded materials (e.g. Fe/Al-SBA-W con-
taining only a 0.04 wt% Fe) is a remarkable feature of the proposed
synthetic methodology which points to a synergetic effect Fe/Al in
2.3. Material characterisation
Pyridine (PY) and 2,6-dimethylpyridine (DMPY) titration exper-
iments were conducted at 200 ◦C via gas phase adsorption of the
basic probe molecules utilising a pulse chromatographic titration
methodology [20,21]. Briefly, probe molecules (typically 1–2 ␮L)
were injected in very small amounts (to approach conditions of
gas-chromatography linearity) into a gas chromatograph through
a microreactor in which the solid acid catalyst was previously
placed. Basic compounds are adsorbed until complete saturation
from where the peaks of the probe molecules in the gas phase are
detected in the GC. The quantity of probe molecule adsorbed by the
solid acid catalyst can subsequently be easily quantified. In order
to distinguish between Lewis and Brönsted acidity, the assump-
tion that all DMPY selectively titrates Brönsted sites (methyl groups
hinder coordination of nitrogen atoms with Lewis acid sites) while
PY titrates both Brönsted and Lewis acidity in the materials was
made. Thus, the difference between the amounts of PY (total acid-
ity) and DMPY (Brönsted acidity) adsorbed should correspond to
Lewis acidity in the materials.

A. Pineda et al. / Catalysis Today 187 (2012) 65–69
67
Table 1
Surface acid properties measured via adsorption of PY and DMPY (200 ◦C) of sup-
ported iron oxide nanoparticles on various aluminosilicates.
Catalyst
Fe loading PY
(wt%)
DMPY
Lewis aciditya
(␮mol g−1
)
(␮mol g−1
)
(␮mol g−1
)
Al-MCM-41
Fe/Al-MCM-MW-150 0.60
Fe/Al-MCM-MW-300 0.63
–
135
352
269
80
88
169
165
36
47
183
104
44
Al-SBA-15
–
Fe/Al-SBA-W
Fe/Al-SBA-HP
0.04
0.08
162
101
37
52
125
49
a
Lewis acidity obtained as the difference between DMPY and PY data.
the materials, similar to that observed in previously reported oxi-
dation reactions [10]. Ball milling and/or microwave irradiation of
the aluminosilicate support did not originate any structural or acid-
ity changes in the materials so that the Fe/Al synergetic effect in the
materials (due to the deposition of the iron oxide nanoparticles) is
believed to be the main reason for the enhanced acidity.
DRIFTs spectra of selected materials (microwave vs ball milled
samples) are depicted in Fig. 1. An increase in Brönsted (B) and
particularly Lewis (L) acidity in the characteristic peaks at 1543
and 1454 cm−1, respectively, can be clearly spotted in the DRIFTs
spectra even at remarkably low Fe loadings (0.04 wt%, Fe/Al-SBA-
W). These results were in general in good agreement with those
obtained in the PY titration experiments (Table 1). Remarkably,
materials possessed noticeable acidities (both Brönsted and Lewis)
even at temperatures as high as 200–300 ◦C (Fig. 1, bottom, spectra
d) for which the presence of these sites is still marginally dis-
tinguishable from the background noise. This enhanced acidity is
highly valuable for acid catalysed processes such as the alkylation
of toluene.
Importantly, acidity measurements from both methodologies
(PY DRIFTs and most importantly PY and DMPY pulse chromatog-
raphy titration data) were generally in good agreement between
them, supporting the validity of our assumption on DMPY adsorb-
ing selectively on Brönsted acid sites.
Aromatic alkylation processes are among the most versatile and
widely investigated processes which can grant access to a wide
range of compounds as important intermediates, fragrances, agro-
chemicals and pharmaceuticals [23,24]. In particular, the alkylation
of toluene with benzyl chloride or benzyl alcohol has been reported
to be promoted by Lewis and Brönsted acid sites, respectively [18].
In this way, this test reaction is a promising approach to distinguish
between Brönsted and Lewis sites in the materials.
Fig. 1. DRIFTs of (top figure) PY adsorption on Al-SBA-15 (dashed line, bottom
spectrum) and Fe/Al-SBA-W (solid line, top spectrum) and (bottom figure) Fe/Al-
MCM-MW-300 material at different temperatures: (a) 100 ◦C; (b) 150 ◦C; (c) 200 ◦C;
(d) 300 ◦C.
Initially, the activity of the synthesized supported iron oxide
nanoparticles in the alkylation of toluene with benzyl alcohol was
screened under conventional heating. In general, all materials pos-
sessed comparatively improved activities compared to the parent
support, with complete conversions obtained after 2–3 h of reac-
tion (data not shown) regardless of the method of preparation (ball
milling or microwaves). The comparison of activity with time of
reaction for the particular case of microwave prepared materials
has been included in Fig. 2.
Table 2
Activity of Fe/Al-SBA-15 materials in terms of total conversion of starting material
(XT, mol%) and selectivity to 1-benzyl-2-methylbenzene (SF2, mol%) in the alkylation
of toluene with benzyl chloride and benzyl alcohol under microwave irradiation.
Catalyst
Benzyl chloride
XT SF2
Benzyl alcohol
a
a
XT
SF2
However, the striking differences in terms of activity were
observed for microwave-assisted experiments. Microwaves have
been reported as a highly useful tool to speed up rates of reactions
in heterogeneously catalysed processes as well as often improv-
ing selectivities to the target products [25]. Reactions run under
microwave irradiation are summarised in Table 2.
b
Blank
Al-MCM-41
–
–
–
–
27
62
65
<20
47
–
49
46
50
52
50
47
–
81
60
<10
>99
85
Fe/Al-MCM-MW-150
Fe/Al-MCM-MW-300
Al-SBA-15
Fe/Al-SBA-W
Fe/Al-SBA-HP
53
52
50
52
45
52
In general, both ball-milling and microwave prepared materials
provided remarkably superior activities to those of their respec-
tive supports in both the alkylation of toluene with benzyl chloride
(Lewis promoted) and benzyl alcohol (Brönsted promoted). Never-
theless, ball-milled materials exhibited an unusual activity under
microwave irradiation conditions despite their extremely low Fe
Reaction conditions: 2 mL toluene, 0.2 mL alkylating agent, 0.025 g catalyst,
microwave irradiation, 300 W, 110–120 ◦C, 3 min reaction.
a
The remaining selectivity to 100 corresponds to the formation of 1-benzyl-4-
methylbenzene.
b
No reaction.

68
A. Pineda et al. / Catalysis Today 187 (2012) 65–69
synthesized materials does not particularly correlate well with
their measured relatively low acidity, so there seem to be other
important factors influencing the catalytic activity in these mate-
rials.
Acidity may in fact influence the activity in the materials but
there are two key properties on ball milled materials that make
them unique as compared to those obtained under microwave
irradiation: the accessibility of the active sites (which has been
maximised due to the presence of the iron oxide nanoparticles
in the external surface of the supports [10,17]) and the small
nanoparticle sizes (typically 2–3 nm) of the iron oxide supported
nanoparticles. These two properties have been already proved
to be crucial in other related chemistries for the transformation
of bulky molecules [17,26]. The presence of uniform and small
nanoparticles supported in the aluminosilicates and the previously
reported synergy observed between Fe and Al [10] may prevent
the loss of metals as well as their sintering, therefore leading to
an increase in activity. The small nanoparticle size distribution
previously reported in ball-milling materials (2–3 nm) [17] as com-
pared to that obtained for microwave prepared materials (4–6 nm)
[10–12] might as well contribute to an uniform performance of the
catalyst.
Furthermore, the presence of such small and homogeneously
distributed nanoparticles on the external surface of the supports
(being the ball-milling approach mainly a surface phenomenon)
also improves the accessibility of the reactants to the active sites
and may contribute to the observed increase in activity. In any
case, the combination of these features turned ball-milling materi-
als into highly active and better performing catalysts as compared
to “classical” catalysts prepared by other conventional methods.
Fig. 2. Activity comparison of Al-MCM-41 (white symbols) and Fe/Al-MCM-41
(black symbols) (conversion vs time of reaction) in the alkylation of toluene with
benzyl alcohol under conventional heating. Reaction conditions: 10 mL toluene,
1 mL benzyl alcohol, 0.1 g catalyst, 110 ◦C.
loadings (<0.1 wt% Fe). Thus, an almost quantitative conversion
of starting material was obtained for Fe/Al-SBA-W in the alky-
lation of toluene with benzyl chloride after 3 min reaction, as
compared to a moderate 45% observed using benzyl alcohol as
the alkylating agent (Table 2). These results are in good agree-
ment with those previously reported in our group with similar
ball-milled Fe/Al-SBA materials in the microwave-assisted alkyla-
tion of toluene with benzyl alcohol (50% conversion to alkylation
product) [17]. Remarkably, as shown in Table 2, activities obtained
for ball-milling materials for both alkylation reactions were at
least comparable to those obtained for materials prepared under
microwave irradiation, despite their significantly lower Fe content
(0.5 wt% vs 0.04 wt%). Also, when compared with materials pre-
pared by conventional impregnation techniques with ten times
higher iron content, ball-milled Fe/Al-SBA materials showed higher
activities in the alkylation of benzene with benzyl alcohol [17]. Ball-
milled materials also showed superior activities compared with
other SBA-15 based materials such as AlGa-SBA-15. The Ga-doped
mesoporous materials showed low activities after 3 h (conversion
<10%) in the conventional heating alkylation of benzene with ben-
zyl chloride [27]. Reused Zr-SBA15 materials showed, however,
higher activities than their Ga-counterparts (complete conversion
after 1 h) at similar conditions [28]. Nevertheless, longer reaction
times are generally required under conventional heating conditions
(6–12 h) as compared to the 3–5 min required to achieve almost
quantitative conversion of the starting material under microwave
irradiation over Fe/Al-SBA (Table 2). This represents a great advan-
tage in terms of time and energy consumption that, together with
the simplicity of preparation of ball milled materials as compared
with other protocols, give this approach a valuable green character.
The activity of microwave-prepared Fe materials generally cor-
related well with the acid properties of the materials (Table 1), in
good agreement with the Lewis and Brönsted acid sites of the mate-
rials promoting the alkylation with benzyl chloride and alcohol,
respectively. Comparatively, the high activity obtained (especially
in the reactions run under microwave irradiation) for ball-milling
4. Conclusion
The alkylation of toluene with benzyl chloride and alcohol
as alkylating agents has been investigated using different iron
oxide nanoparticles supported on aluminosilicate materials (Al-
MCM-41 and Al-SBA-15) prepared using two novel methodologies,
namely microwave-assisted and dry milling deposition. Unex-
pected activities in the investigated reactions were obtained for
low Fe content (<0.1 wt%) ball-milled materials, particularly under
microwave irradiation. In spite of their low acidity as compared
to other microwave-prepared materials, the presence of small and
homogeneously distributed nanoparticles on the external surface
of the supports is believed to contribute to the observed unexpected
activities which were in all cases at least comparable to those of
higher Fe content (>0.5 wt%).
Acknowledgments
RL gratefully acknowledges Ministerio de Ciencia e Innovacion,
Gobierno de Espan˜a for the concession of a Ramon y Cajal con-
tract (ref. RYC-2009-04199) and Consejeria de Ciencia e Innovacion,
Junta de Andalucia for funding under the project P10-FQM-6711.
Funding from projects CTQ-2010-18126 (MICINN) and P09-FQM-
4781 (Consejeria de Ciencia e Innovacion, Junta de Andalucia) are
also gratefully acknowledged.
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