High surface area, nanostructured boehmite and alumina catalysts: Synthesis and application in the sustainable epoxidation of alkenes

Article abstract of DOI:10.1016/j.apcata.2018.12.017

We report a new, straightforward and inexpensive sol-gel route to prepare boehmite nanorods [γ-AlO(OH)-NR] with an average length of 23 nm ± 3 nm, an average diameter of 2 nm ± 0.3 nm and a high specific surface area of 448 m²/g, as evidenced by TEM and N₂-physisorption, respectively. The boehmite was converted to γ-alumina with preserved nanorod morphology (γ-Al₂O₃-NR) and high surface area upon calcination either at 400 or 600 °C. These nanostructured materials are active and selective heterogeneous catalysts for the epoxidation of alkenes with the environmentally friendly H₂O₂. The best catalyst, γ-Al₂O₃-NR-400, showed to be versatile in the scope of alkenes that could be converted selectively to their epoxide and displayed enhanced reusability compared to previously reported alumina catalysts. Furthermore, the catalytic performance of the material was enhanced by optimising the reaction conditions such as the solvent and the type of hydrogen peroxide source. Under the optimised reaction conditions, the γ-Al₂O₃-NR-400 catalyst displayed 58% cyclooctene oxide yield after 4 h of reaction at 80 °C with full selectivity towards the epoxide product. The correlation between the catalytic activity of these materials and their physicochemical properties such as surface area, hydrophilicity and number and type of acid sites was critically discussed based on a detailed characterisation study.

Full text of DOI:10.1016/j.apcata.2018.12.017

Accepted Manuscript
Title: High surface area, nanostructured boehmite and alumina
catalysts: synthesis and application in the sustainable
epoxidation of alkenes
Authors: Warunee Lueangchaichaweng, Bhawan Singh,
Dalmo Mandelli, Wagner A. Carvalho, Sonia Fiorilli, Paolo P.
Pescarmona
PII:
S0926-860X(18)30613-6
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.12.017
APCATA 16921
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
24 October 2018
11 December 2018
15 December 2018
Please cite this article as: Lueangchaichaweng W, Singh B, Mandelli D, Carvalho WA,
Fiorilli S, Pescarmona PP, High surface area, nanostructured boehmite and alumina
catalysts: synthesis and application in the sustainable epoxidation of alkenes, Applied
Catalysis A, General (2018), https://doi.org/10.1016/j.apcata.2018.12.017
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High surface area, nanostructured boehmite and alumina catalysts: synthesis
and application in the sustainable epoxidation of alkenes
Warunee Lueangchaichaweng, Bhawan Singh,a,b,c Dalmo Mandelli, Wagner A. Carvalho, Sonia Fiorilli,
a
c
c
d
Paolo P. Pescarmona b,*
a
Centre for Surface Chemistry and Catalysis, KU Leuven, Celestijnenlaan 200F, 3001 Heverlee, Belgium
Chemical Engineering group, Engineering and Technology institute Groningen (ENTEG), University of
Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
b
c
Centre of Natural and Human Sciences, Federal University of ABC, 166 Santa Adélia Street, Bangu,
Santo André, SP 09210-170, Brazil
d
Department of Applied Science and Technology, Politecnico di Torino, Corso Duca degli Abruzzi 24,
10129 Torino, Italy
*
Corresponding author: E-mail: p.p.pescarmona@rug.nl
Graphical abstract
O
H
H
O
R1
R2
O

-Al O -NR-400
2 3
R1
R2
R3 R4
R4
R3
Nanostructured catalysts
for the sustainable
epoxidation of alkenes
1

Highlights


Novel route to synthesise nanostructured boehmite and alumina with high surface area.
The best nanostructured alumina catalyst displayed high activity, excellent selectivity and
versatility in terms of substrate scope in the epoxidation of alkenes with H
oxidant.
2
2
O as green

Enhanced reusability compared to previously reported alumina catalysts.
Abstract
We report a new, straightforward and inexpensive sol-gel route to prepare boehmite nanorods
[
γ-AlO(OH)-NR] with an average length of 23 nm ± 3 nm, an average diameter of 2 nm ± 0.3
2
nm and a high specific surface area of 448 m /g, as evidenced by TEM and N
2
-physisorption,
respectively. The boehmite was converted to -alumina with preserved nanorod morphology
-Al -NR) and high surface area upon calcination either at 400 or 600 °C. These
nanostructured materials are active and selective heterogeneous catalysts for the epoxidation of
alkenes with the environmentally friendly H . The best catalyst, -Al -NR-400, showed to
(
2
O
3
2
O
2
2
O
3
be versatile in the scope of alkenes that could be converted selectively to their epoxide and
displayed enhanced reusability compared to previously reported alumina catalysts. Furthermore,
the catalytic performance of the material was enhanced by optimising the reaction conditions
such as the solvent and the concentration of hydrogen peroxide. Under the optimised reaction
conditions, the -Al
2
3
O -NR-400 catalyst displayed 58 % cyclooctene oxide yield after 4 h of
reaction at 80 °C with full selectivity towards the epoxide product. The correlation between the
catalytic activity of these materials and their physicochemical properties such as surface area,
hydrophilicity and number and type of acid sites was critically discussed based on a detailed
characterisation study.
2

Keywords: aluminium oxide; boehmite; nanorods; heterogeneous catalyst; epoxidation;
hydrogen peroxide.
1. Introduction
One-dimensional nanomaterials such as nanorods and nanowires are of great interest
commercially and scientifically. They display larger surface area compared to their counterpart
with larger size and can also show unique and improved mechanical [1], magnetic [2] and
electronic properties [3]. Aluminas (Al
2
O
3
) are a class of versatile materials with widespread
is one of
applications as adsorbents, catalysts and catalyst supports [4], [5]. Particularly, γ-Al
2
O
3
the most employed supports for heterogeneous industrial catalysts [6]. The asset of alumina
include low cost, relatively high surface area and good chemical and thermal stability [7].
Recently, also the aluminium oxyhydroxide known as boehmite [γ-AlO(OH)] has found
application as support for platinum-tungsten catalysts active in the hydrogenolysis reaction of
glycerol to 1,3-propanediol [8]. Boehmite and γ-Al
2
3
O are closely related, as the latter can be
obtained by dehydration of the former upon thermal treatment [9]. Boehmite and alumina
themselves have also been reported to be active heterogeneous catalysts in the epoxidation of
alkenes with H
2
2
O [10], [11], The activity is strongly dependent on the synthesis route used in
their preparation [12]. Boehmite and alumina can be prepared by many synthetic routes such as
sol-gel [13], hydrothermal [14] and solvothermal methods [15], or by the hydrolysis of
aluminium metal [16]. The synthetic parameters involved in sol-gel methods can be tuned, thus
allowing a certain degree of control of the structural and textural properties of the resultant
aluminum oxide [17]. Also the acidity and hydrophilicity of alumina can be tuned by varying the
type of aluminium precursor, the ratio of water to precursor, the pH and the solvent used in the
sol-gel synthesis [18]. An important advantage of these low temperature synthesis methods is the
tendency to afford materials with large specific surface area [19]. Many efforts have been
3

focused on the preparation of high surface area one-dimensional alumina nanostructures, e.g. by
using surfactants or other structure directing agents [20], [21], [22], or sophisticated techniques
such as chemical etching of porous anodic alumina [23], electrochemical synthesis [24] or normal
and lateral stepwise anodisation [25]. However, a sol-gel synthesis without employing templating
or chelating agents, a straightforward, highly reproducible and mild route, would still be a
rewarding achievement. Therefore, we focussed our attention on the discovery of a sol-gel
synthesis procedure that does not require the use of expensive reagents or surfactants, with the
goal of providing a simple and efficient approach for the synthesis of nanostructured boehmite
and alumina with very high surface areas, a well-defined structure with uniquely small
dimensions and a large population of coordinatively unsaturated Al sites at the surface. These
physicochemical features would be desirable for application as heterogeneous catalysts for the
sustainable epoxidation of alkenes using the environmentally benign aqueous hydrogen peroxide
as the oxidant, with the target of providing a cheaper alternative to the highly active and selective
gallium oxide nanorods, which represent the state-of-the-art transition-metal-free heterogeneous
epoxidation catalyst [26].
2
2
. Experimental
.1. Synthesis of Al
2
O
3
·xH O nanorods
2
The protocol for the synthesis of boehmite nanorods [γ-AlO(OH)-NR] was developed by
adapting the methodology reported by some of us for the synthesis of gallium oxide nanorods
[
Error! Bookmark not defined.], and by employing the Al source (aluminium tri-sec-butoxide)
that provided an alumina catalyst with optimum epoxidation activity in a previous study [27].
First, aluminium tri-sec-butoxide (1.23 g, 5 mmol) was dissolved by dropwise addition of 2-
butanol (1.60 g) within 10 min, and the obtained clear solution was stirred for 30 min. Then, an
ammonia solution (25 % aqueous ammonium hydroxide) diluted with absolute ethanol (1:1 v/v,
~1.6 mL) was added dropwise until a pH ≈ 9-10 was reached. The white turbid gel was stirred for
4

1
.5 h and then heated at 70 °C for 23 h. The obtained material was aged for 3 days at ambient
temperature while stirring. Finally, the white powder was collected by centrifugation, washed at
least three times with ethanol and then dried overnight in an oven at 80 °C in air. This protocol
for the synthesis of boehmite nanorods is highly reproducible and efficient, leading to a typical
yield of approximately 85 %, assuming the product formula to be AlO(OH). The synthesis of the
γ-alumina nanorods (γ-Al
2
3
O -NR-T, where T is the temperature of the thermal treatment
expressed in °C) was carried out by thermal treatment of the prepared γ-AlO(OH)-NR in a
calcination oven in air at 400 or 600 °C for 10 h (heating rate: 3 °C/min in both cases).
2.2. Characterisation
Powder X-ray diffraction (XRD) patterns were recorded on a STOE Stadi P Combi diffractometer
with position sensitive detector (IP PSD) in the region 2θ = 4.26 to 82°, with internal resolution
of 0.03° The measurements were performed in Debye-Scherrer mode at room temperature using
CuKα1 radiation (λ = 1.54056 Å, selected by means of a Ge (111) monochromator). The ICDD
database was employed to assign the phases identified by XRD.
2
N -physisorption measurements were performed on a Micromeritics Tristar 3000 equipment. The
samples were outgassed under reduced pressure for 12 h at 120 °C (heating rate: 10 °C/min) prior
to the measurement. The Brunauer-Emmet-Teller (BET) method was applied to calculate the
specific surface area [28].
To evaluate the level of hydrophilicity of the synthesized Al
analyses (TGA) were carried out under N atmosphere with a heating rate of 10 °C/min on a TGA
Q500 from TA Instruments. The samples were pre-treated overnight in a desiccator containing a
2 3 2
O ·xH O materials, thermogravimetric
2
saturated aqueous solution of NH
4
Cl to reach the maximum degree of water adsorption. The
2
number of the water molecules adsorbed per nm of surface of the alumina (nH2O) was estimated
from the weight loss between 25 and 150 °C using the following equation [29]:
Δm
m
NA
n
H2O
=
x
i
5
A
BET
M
H2O

where Δm is the weight loss between 25 and 150 °C (g); m
i
is the initial mass of the sample at 25
2
3
-1
2
°
C (g); N
A
is the Avogadro constant (6.022·10 mol ); ABET is the surface area (nm /g); MH2O is
the molecular mass of water (18.0135 g/mol).
Transmission electron microscopy (TEM) images were obtained on a Philips FEG CM 200
operating at 200 kV. The samples for TEM analysis were prepared by dispersing the solid in few
drops of absolute ethanol in an agate mortar followed by deposition on 300 mesh carbon-coated
copper grids. For each set of nanorods, ca. 30 particles were counted and measured to determine
the nanorod dimensions.
For Fourier-Transform Infrared (FT-IR) spectroscopy study of adsorbed ammonia, approximately
20 mg of the boehmite or alumina powders were pressed into thin, self-supporting wafers. The
FT-IR measurements were performed on a Bruker Tensor 27 spectrometer equipped with a liquid
-
1
nitrogen-cooled mercury–cadmium–telluride (MCT) detector, operating at 2 cm resolution. Pre-
treatments were carried out using a standard vacuum frame, in an IR cell equipped with KBr
windows. The wafers were outgassed for 1 h at 250 °C before adsorption of ammonia at room
temperature (note that this thermal treatment does not affect the boehmite phase in -AlO(OH)-
NR, as proven by the XRD pattern in Figure S2). Adsorption of ammonia was studied in the
pressure range 0.01-20.0 mbar: the reversible fraction of the adsorbate was then removed by
prolonged outgassing at room temperature.
2.3. Catalytic tests
The epoxidation reactions were carried out in a 50-wells parallel reaction block, equipped with a
heating and stirring unit. The liquid reagents and the solvent were dispensed through fixed metal
tips using a Tecan Genesis RSP 100 liquid-handling high-throughput workstation coupled with a
personal computer supplied with Gemini software enabling to program the workstation [Error!
Bookmark not defined.]. The catalytic epoxidation reactions were performed by employing: the
selected alkene (2 mmol), di-n-butyl ether (1 mmol) as GC internal standard, 50 wt% aqueous
6

H
2
2
O (4 mmol), 2.0 g of solvent (i.e. ethyl acetate in most of the cases). First, a solution
containing the alkene and the GC internal standard in the solvent was added. Next, the aqueous
solution of hydrogen peroxide was added. The reaction mixtures were stirred for 4 h at 800 rpm
and 80 °C in capped vials placed in the 50-wells reaction block. Selected catalytic tests were
performed on three different batches of γ-Al
2
3
O -NR-400, and showed high reproducibility of the
results (deviation in the epoxide yield value within ± 2%). For these experiments, the average
value of the epoxide yield is reported. The studies of the effect of the type of hydrogen peroxide
were carried out by comparing the catalyst performance with 50 wt% aqueous H
that with 70 wt% aqueous H (only available in Brazil, supplied by Solvay for academic
research) and 24.5 wt% anhydrous H in ethyl acetate, which was prepared by removing water
from the homogeneous solution of 20 mL of 70 wt% aqueous H and 188 mL of ethyl acetate
2
2
O (Aldrich) to
2
O
2
2
O
2
2
O
2
by means of a Dean-Stark trap at 110 ˚C. Important safety note: the anhydrous solution of
hydrogen peroxide in flammable organic solvents should be considered as dangerous and
explosive [30]. For safe handling, the preparation should be performed in fume hood with the
proper ventilated preparation set-up to release the oxygen generated by hydrogen peroxide
decomposition and the possible explosive gas mixtures of solvent and oxygen.
Regardless of the type of hydrogen peroxide solution that was employed, the final concentration
of H
2
O
2
was the same (ca. 5 wt%) in all catalytic tests. The use of much higher concentration of
H
2
O
2
in combination with relatively high reaction temperature (≥ 80 °C) is not advised for safety
reasons, as the rate of decomposition of hydrogen peroxide into oxygen and water increases with
rising reaction temperature, thus increasing the risks of explosions.
Conversion, epoxide yield and selectivity were determined by gas chromatography (GC) analysis
on a Finnigan Trace GC Ultra chromatograph from Interscience, equipped with an RTX-5 fused
silica column (5 m; 0.1 mm) [31] or on an Agilent 7890A gas chromatograph, equipped with an
HP 5 capillary column (30 m; 0.25 mm). The identification of the by-products of the epoxidation
of the various alkenes was performed by gas chromatography-mass spectrometry (GC-MS) on an
7

Agilent 6890N gas chromatograph coupled to an Agilent 5973 MSD mass 4 spectrometer. The
GC was equipped with a WCOT fused silica column (30 m; 0.25 mm) coated with a 0.25 mm thick
HP-5 MS film.
For the recycling tests, after the reaction in ethyl acetate at 80 °C for 4 h, the sample was
centrifuged for 10 min at 3500 rpm to deposit the solid catalyst. Next, the reaction solution was
removed with a pipette, 5 mL of ethanol were added and the sample was stirred for at least 5 min.
The sample was again centrifuged to deposit the catalyst and then the supernatant ethanol
solution was removed. The washing procedure was repeated three times. Finally, the samples
were dried overnight at 120 °C in air and reused in a new catalytic test.
3
. Results and discussion
Boehmite nanorods, -AlO(OH)-NR, with distinctly small size and high surface area, were
synthesised by a simple and straightforward method via a base-catalysed sol-gel method
employing aluminium tri-sec-butoxide as precursor. The XRD pattern of the prepared material
(Fig. 1.a) can be indexed as boehmite phase [-AlO(OH)] with orthorhombic structure (JCPDS
021-1307). TEM images reveal that the prepared boehmite displays nanorod morphology with an
average length of 23 nm ± 3 nm and an average width of 2 nm ± 0.3 nm (Fig. 2.a and Fig. S3.a).
These images also suggest the possible presence of some nanoflakes with similar dimensionality
to the length of the nanorods. The nanostructuring of boehmite led to a material with remarkably
2
high surface area (448 m /g), as revealed by N
2
physisorption analysis. The hydrolysis and
condensation rates of aluminium precursors are mainly governed by the type of precursor, the
solvent, the water content and the pH [32]. Alkoxides are reactive precursors and generally need
complex solvent mixtures or chelating agents to control the hydrolysis and condensation rates
[
Error! Bookmark not defined.]. In our synthesis protocol, the chosen molar ratios, Al:
H
2
O:NH
3
:2-BuOH = 1: 6.7: 2.4: 7.3, the use of 2-butanol as solvent and the dropwise addition of
8

aqueous ammonia causing a gradual increase in the pH of solution, are believed to allow the
hydrolysis and condensation reactions to occur slowly, thus leading to the formation of nanorods
with small and uniform size and with high surface area [Error! Bookmark not defined.].
Boehmite has an intrinsic tendency to form in nanorod morphology [33] without need for a
structure directing agent. This behaviour is probably related to the orthorhombic crystal lattice of
boehmite, in which one cell parameter is much larger than the other two [a = 3.700 Å, b = 12.227
Å, c = 2.868 Å].
*
(
c)
*
*
(
b)






(
a)
10
20
30
40
50
60
70
80
2θ
2 ( °)
Figure 1. XRD patterns of (a) as prepared γ-AlO(OH)-NR (b) γ-Al
2
O
3
-NR-400 and (c) γ-Al
2
3
O -
NR-600. The circles indicate the position of the diffraction peaks of boehmite; the asterisks
denote the position of the reflection of the (400) and (440) planes of γ-Al
2
3
O .
(b)
(c)
(a)
20nm
20nm
20nm
9

Figure 2. TEM images of: (a) boehmite -AlO(OH)-NR (b) -Al
2
O
3
-NR-400 and (c) -Al
2
3
O -
NR-600.
The nanostructured boehmite -AlO(OH)-NR was converted to γ-Al
2
3
O -NR by phase transition
induced by calcination, which occurs via the removal of physically- and chemically-bound water.
The thermal treatment was carried out either at 400 or 600 °C, and the resulting materials were
characterised by XRD, TEM, N
analysis (Fig. 1.b) showed that the calcination treatment at 400 °C was sufficient to convert the
boehmite into a -Al phase with tetragonal structure, with the observed broad peaks matching
2
-physisorption, TGA and FT-IR spectroscopy. Powder XRD
2
O
3
with the JCPDS powder diffraction file 01-074-4629 [34] The thermal treatment at higher
temperature (600°C, see Fig. 1.c) led to a material with the same diffraction pattern, and thus the
same structure corresponding to a -Al
2
O
3
phase. The morphology of the two -Al
2
3
O materials
was investigated by TEM. The size and morphology of γ-Al
2
O
3
-NR-400 are very similar to those
of the boehmite precursor (Fig. 2.b and Fig. S3.b), whereas the thermal treatment at 600 °C led to
a change in the aspect ratio of the nanorods (width: 3 ± 0.6 nm, length: 13 ± 1 nm). Moreover,
particles with less defined morphology are also observed in the latter material (Fig. 2c and Fig.
S3c). The phase transition from boehmite to alumina leads to a densification of the materials with
a gradual decrease of surface area as a function of increasing calcination temperature.
Nevertheless, the surface area of the γ-alumina obtained upon calcination at 400 °C is still
2
remarkably high (376 m /g). This high value is ascribed to the small size of these nanorods
compared to previous reports of alumina nanorods or nanofibers [35]. The N
adsorption/desorption isotherms of the boehmite [-AlO(OH)-NR] and of the two γ-Al -NR
samples (Figure S1) are of type II according to IUPAC classification, which are typically
2
2
O
3
1
0

encountered when adsorption occurs on non-porous materials, in agreement with the open
structure of these materials. The hydrophilicity of the materials was estimated by calculating the
number of H
All three materials display similar, relatively low degree of hydrophilicity per surface unit
Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark not
2
O molecules adsorbed per square nanometre of surface by means of TGA (Table 1).
[
defined.]. This can be explained with their analogous open, non-porous structure, which prevents
the entrapment of water molecules that typically occurs in porous aluminas [36]. The materials
were further characterised by FT-IR spectroscopy (Figure 3.A). In the high frequency region of
the FT-IR spectrum of -AlO(OH)-NR outgassed at 250 °C (curve a), the intense bands at 3324
-1
and 3099 cm are assigned to the asym(Al)O-H and sym(Al)O-H stretching vibrations in the
-1
bohemite lattice, whereas the narrower band centred at 3668 cm is attributed to free or quasi-
-1
free surface OH groups [37]. In the low frequency region, the intense band at 1070 cm and the
-1
shoulder at 1150 cm are assigned to the δsym Al-O-H and δasym Al-O-H modes of boehmite
-1
lattice, respectively [Error! Bookmark not defined.]. The region below 700 cm (not shown) is
dominated by the strong stretching mode of 'condensed' AlO octahedra [Error! Bookmark
not defined.]. The two weak bands observed at 2094 and 1973 cm can be assigned to
combination bands [Error! Bookmark not defined.]. The OH region of γ-Al -NR-400 (curve
b) and γ-Al -NR-600 (curve c) is markedly different from that shown by -AlO(OH)-NR
curve a): the intense bands at 3324 and 3099 cm assigned to the stretching vibrations of bulk
6
-1
2
O
3
2
O
3
-1
(
OH groups disappeared, due to the transformation of the boehmite phase into an alumina phase.
In the high frequency region, the intense and broad absorption is attributed to H-bonded surface
-1
-1
OH groups, whereas the bands observed at around 3580 cm , 3675 cm and the shoulder around
-1
3
730 cm are ascribed to various types of hydroxyls species, differing in the coordination
-1
number of the OH group [38]. The weak signals observed in the 1800-1300 cm region of all
spectra are most likely due to carbonate species formed by the interaction of atmospheric CO
2
1
1

with the oxygen atoms of the boehmite and alumina. FT-IR spectroscopy was also used to
monitor the adsorption of NH on the surface of the three materials, with the purpose of analysing
3
the type and relative amount of surface acid sites. The difference FT-IR spectra in the low
frequency region (the spectrum recorded before ammonia dosage was subtracted for each sample)
were obtained after ammonia dosage and upon prolonged outgassing at room temperature (Figure
-1
-1
3
.B). Two positive bands at about 1620 cm and 1240 cm are irreversibly formed for all three
materials and are attributed, respectively, to the anti-symmetric and symmetric bending vibrations
coordinated to unsaturated Al3+ ions in tetrahedral position, acting as Lewis acid sites
of NH
3
[
39]. The intensity of these signals is higher for γ-Al
concentration of Lewis acid centres at the surface of this material. The lower intensities observed
for -AlO(OH)-NR (curve a) and for γ-Al -NR-600 (curve c) are ascribable to two different
2
O
3
-NR-400 (curve b), indicating a higher
2
O
3
reasons: for the sample treated at 600 °C, the decrease of probed Lewis acid sites is consistent
with the observed reduction of surface area upon thermal treatment (Table 1), whereas in the case
of the boehmite, the surface is fully hydroxylated, as shown by the related FT-IR spectrum (Fig.
3.A.a), thus leading to a relatively low amount of exposed Al (III) cations. The signal at ca. 1465
-1
+
cm is ascribed to the bending vibration of NH
surface hydroxyls, acting as mild Brønsted sites [40] The intensity of the signal is substantially
higher for -AlO(OH)-NR (a) and γ-Al -NR-400 (b) compared to γ-Al -NR-600 (c),
4
species formed by proton-transfer reaction with
2
O
3
2
O
3
evidencing a higher concentration of mild Brønsted sites for samples treated at lower
temperature, in agreement with their higher population of surface hydroxyl groups.
1
2

0
.5
0
.2
L
L
B
(a)
(
a)
b)
(
b)
c)
(
(
(c)
A
B
3500
3000
2500
2000
1500
1000
1800
1600
1400
1200
-
1
wavenumbers (cm-1)
wavenumbers (cm )
Figure 3. (A) FT-IR spectra of: (a) -AlO(OH)-NR, (b) -Al
-Al -NR-600, outgassed at 250 °C. (B) FT-IR spectra of: (a) -AlO(OH)-NR, (b) -Al
NR-400 and (c) -Al -NR-600, outgassed at 250 °C and in contact with 20 mbar (equilibrium
2
3
O -NR-400 and (c)

2
O
3
2
3
O -
2
O
3
pressure) of ammonia [L: Lewis acid site; B: Brønsted acid site].
The physicochemical properties of the prepared nanostructured boehmite and γ-aluminas defined
in the characterisation study discussed above are highly attractive for application as catalysts for
the epoxidation of alkenes with hydrogen peroxide. Particularly, the nanorod morphology brings
about: (i) a high specific surface area; (ii) an open structure with fully accessible surface that
intrinsically avoids the diffusion limitations characteristic of high surface area porous catalysts;
(iii) a population of surface acid sites that can play a role as active sites; (iv) a relatively low
hydrophilicity that can facilitate the approach of the apolar alkene molecules to the catalyst
surface and also prevent strong bonding of water, which could poison the active sites [41].
Similar features were identified as the responsible for the notable catalytic performance of
gallium oxide nanorods [Error! Bookmark not defined.]. An important asset of this
heterogeneous catalyst is the ability to activate hydrogen peroxide towards epoxidation reactions.
Aqueous hydrogen peroxide is a highly attractive oxidant for a number of reasons: it is cheap,
1
3

safely stored and handled, mild and an environmentally benign reagent with a high content of
active oxygen, and forms water as only by-product. The initial development of transition-metal-
free heterogeneous catalysts for epoxidation using aqueous H
2
2
O revolved heavily around metal
oxides of group 13. Though their catalytic activity was promising, the poor reusability of the
most active among these systems was a main drawback [Error! Bookmark not defined.,Error!
Bookmark not defined., [42], [43]. In this context, a breakthrough was represented by the
discovery of gallium oxide nanorods exhibiting significantly enhanced catalytic activity and
reusability in the epoxidation reaction [Error! Bookmark not defined.]. Here, as part of our
continued interest in developing this type of catalysts, the nanostructured boehmite [-AlO(OH)-
NR] and alumina (-Al
2
O
3
-NR-400 and -Al
2
O
3
-NR-600) materials were tested as heterogeneous
as the oxidant, with the aim of improving the
catalysts for the epoxidation of alkenes with H
2
O
2
catalytic performance, especially in term of reusability, compared to previously reported
aluminas [Error! Bookmark not defined.,Error! Bookmark not defined.,Error! Bookmark
not defined.,Error! Bookmark not defined.].
In analogy to what has been found for gallium oxide nanorods, the active sites of these
nanostructured boehmite and -alumina catalysts should be provided by coordinatively
unsaturated Al atoms (Lewis acid sites) and mildly acidic surface Al-OH (Brønsted acid sites),
which upon interaction with H
2
2
O can form surface hydroperoxide species [Error! Bookmark
not defined.]. For our initial catalytic tests, we choose cis-cyclooctene as model substrate and
compared the catalytic activity of boehmite and alumina nanorods with two types of
commercially available alumina (Table 1). Our boehmite and alumina with nanorod morphology
displayed much higher activity compared to the commercial aluminas, while achieving complete
selectivity (> 99 %) towards the epoxide product in all cases. This confirms the anticipated
advantages of the rod-like morphology of our catalysts. The slightly higher activity of γ-Al
2
3
O -
NR-400 compared to γ-Al -NR-600 could be attributed, at least in part, to the larger surface
2
O
3
1
4

area of the former (Table 1). Nevertheless, γ-AlO(OH) is less active than γ-Al
2
3
O -NR-400,
despite its larger surface area. Indeed, other factors are known to contribute to the catalytic
behaviour of these materials. All of them present an open and accessible surface, thus preventing
diffusion limitations during the approach of the reactants to the active site. Moreover, in previous
studies of alumina as epoxidation catalysts, it was highlighted that the presence of adsorbed water
at the surface of the catalyst is detrimental as it would disfavour the approach of the apolar alkene
molecules [Error! Bookmark not defined.]. The materials reported here display similar, low
water-adsorption behaviour (Table 1). These features contribute in explaining the observed
promising performance of these catalysts, but still do not account for the observed slightly higher
activity of γ-Al
into account the results of the FT-IR study of the adsorption of NH
catalysts, which indicated that γ-Al -NR-400 has similar concentration of Brønsted acid sites
2
O
3
-NR-400 compared to γ-AlO(OH)-NR. For this purpose, it is necessary to take
3
on the surface of the three
2
O
3
and higher concentration of Lewis acid sites compared to γ-AlO(OH)-NR.
Importantly, recycle tests show that both γ-AlO(OH)-NR and the two γ-Al
2
3
O -NR catalysts can
be reused in four consecutive runs without significant loss of activity (Table 1). This is a much
superior reusability compared to previous reports of the best performing alumina catalysts, whose
activity dramatically decreased during successive runs [Error! Bookmark not defined.,Error!
Bookmark not defined.,Error! Bookmark not defined.]. This observation is in line with the
finding that Ga
2
3
O nanorods show significantly enhanced recyclability compared with previous
report on the use of gallium oxides as epoxidation catalysts [Error! Bookmark not
defined.,Error! Bookmark not defined.,Error! Bookmark not defined.]. Accordingly, we
attribute the excellent reusability of γ-AlO(OH)-NR, γ-Al
2
O
3
-NR-400 and γ-Al
2
3
O -NR-600 to the
well-defined structure of the nanorods, providing enhanced chemical stability compared to
conventional, unstructured aluminas [Error! Bookmark not defined.,Error! Bookmark not
defined.,Error! Bookmark not defined.].
1
5

The versatility of the best catalyst for the epoxidation of cis-cyclooctene, i.e. -Al
2
3
O -NR-400,
was investigated by expanding the substrate scope to more challenging unsaturated compounds,
including a natural product as (R)-(+)-limonene (Table 2). The catalyst was able to convert all
tested alkenes, with nearly complete selectivity towards the respective epoxide in the case of
cyclooctene, cyclohexene, 1-octene and limonene. This is remarkable particularly in the case of
cyclohexene, which is generally prone to undergo side reactions as allylic oxidation or further
reaction of cyclohexene epoxide to the corresponding diol [44], [45]. The high epoxide selectivity
obtained here is ascribed to the well-known lack of strong Brønsted acid sites in alumina [46]
which implies that ring-opening reactions of the formed epoxide with consequent generation of
by-products become unlikely [Error! Bookmark not defined.]. In the epoxidation of styrene,
the selectivity toward styrene oxide was lower (64 %) compared to the other epoxides but
nevertheless higher compared to state-of-the-art catalysts as gallium oxide nanorods or TS-1
[
Error! Bookmark not defined.]. Benzaldehyde (7 % yield), which formed by the oxidative
cleavage of the vinyl group of the styrene oxide product [47], was determined as the only side
product. The relative activity in the conversion of different alkenes follows the same trend
previously observed for gallium oxide nanorods under the same reaction conditions [Error!
Bookmark not defined.], with the latter being consistently more active (both in terms of epoxide
yield and productivity). On the other hand, aluminium is a much more abundant and cheaper
element compared to gallium (compared to Ga, Al is ca. 4000 times more abundant in the earth
crust and ca. 300 times cheaper) [48], [49], making this catalyst an attractive alternative for the
sustainable epoxidation of alkenes with hydrogen peroxide.
1
6

100
cyclohexene oxide yield
8
6
4
2
0
0
0
0
0
cyclohexene oxide selectivity
%
0
4
8
12
16
20
24
28
reaction time (h)
Figure 4. Kinetic test for the epoxidation of cyclohexene with 50 wt% H
Al -NR-400. Conditions: 2 mmol of cyclohexene, 4 mmol of H (as 50 wt% aqueous
solution), 40 mg of catalyst, 1 mmol of dibutyl ether, 2.0 g of ethyl acetate, 4 h at 80 °C.
2
2
O catalysed by -
2
O
3
2
O
2
In the case of cyclohexene and limonene, the epoxidation over γ-Al
2
3
O -NR-400 was monitored
over the course of time. With cyclohexene as substrate, an epoxide of yield of 52 % was reached
after 28 h (Fig. 4), while retaining a very high selectivity (97 %) towards the epoxide with only
minor amounts of 2-cyclohexene-1-one and 1,2-cyclohexanediol, thus confirming the very low
tendency of this catalyst to promote allylic oxidation or hydrolysis of the epoxide, respectively.
The epoxidation of (R)-(+)-limonene was studied in detail because there is growing interest for
the utilisation of this bio-based product, which can be obtained from waste peel generated as
abundant side products of juice extraction from orange and other citrus fruits [50]. Particularly,
the selectivity between the three possible epoxide products (internal-, external- and di-epoxides,
see Fig. 5) is worth being studied, as each of these compound can find different applications. For
example, the internal limonene oxide has been employed as monomer for copolymerisation with
CO to produce fully renewable polycarbonates [51], whereas limonene di-epoxide is used as
2
reactive diluent in cationic UV-curing applications [52] and employed in the synthesis limonene
dicarbonate, which can be used as monomer for the production of polyurethanes [53]. The
1
7

internal limonene oxide is the major product throughout the whole reaction (Fig. 5, left plot). The
total yield of epoxides reached 78 % after 26 h of reaction, with a ratio internal: external: di-
epoxide = 90: 7: 3. On the other hand, the selectivity towards the internal epoxide gradually
decreased as the reaction proceeded, while the selectivity towards the external epoxide, the di-
epoxide and the minor side products (mainly diols) gradually increased (Fig. 5, right plot). This
indicates that the di-epoxide is mainly formed by further reaction of the internal limonene oxide,
which is possibly also more prone to hydrolysis of the epoxide ring with formation of the
corresponding diol.
O
O
H O
2
2
+
+
O
O
(R)-limonene
internal epoxide
external epoxide di-epoxide
100
100
limonene conversion
80
60
40
20
0
8
0
60
Selectivity
%)
internal epoxide
external epoxide
di-epoxide
%
(
40
20
0
yield of internal epoxide
yield of external epoxide
yield of di-epoxide
side products
yield of side products
0
4
8
12
16
20
24
28
0
4
8
12
16
20
24
28
reaction time (h)
reaction time (h)
Figure 5. Kinetic test for the epoxidation of (R)-(+)-limonene with 50 wt% H
2
O catalysed by
2

-Al -NR-400. Conditions: 2 mmol of (R)-(+)-limonene, 4 mmol of H (as 50 wt% aqueous
2
O
3
2
O
2
solution), 40 mg of catalyst, 1 mmol of dibutyl ether, 2.0 g of ethyl acetate, 4 h at 80°C. The ratio
between the cis and trans isomers was estimated to be 1:1.1 for the internal epoxide and 1:1.2 for
the external epoxide. For the di-epoxide, the GC peaks were not sufficiently resolved to allow
quantification of the RSS:SRS:RSR:SRR ratio. The yields reported in the figure are the sum of the
yields of the isomers.
1
8

Hydrogen peroxide is generally available as an aqueous solution and in this work most
experiments were performed with commercially available 50 wt% aqueous H . The nature and
amount of solvent in which H is diluted can play an important role on the catalytic
2
O
2
2
O
2
performance in epoxidation reactions, as water can compete with hydrogen peroxide for
adsorption on the catalyst surface. In this context, it is relevant to investigate the catalytic
behaviour of -Al
2
O
3
-NR-400 employing hydrogen peroxide sources with a lower water content,
and 24.5 wt% anhydrous H in ethyl acetate. This study was
and namely 70 wt% aqueous H
2
O
2
2
O
2
carried out on three different substrates: cis-cyclooctene, cyclohexene, and 1-octene (Figure 6 and
Table S1). With all the tested alkenes, higher epoxide yields were obtained switching from 50 to
70 wt% aqueous H
2
2
O , while retaining analogous very high selectivity towards the epoxide
product. A further, though less marked, improvement in epoxide yield was achieved when using
the 24.5 wt% anhydrous hydrogen peroxide in ethyl acetate as the oxidant, also in this case
without loss of epoxide selectivity. It is worth noting that the increase in epoxide yield was more
marked in the epoxidation of cyclohexene and, particularly, 1-octene, reaching more than a two-
and three-fold increase, respectively. The enhancement in epoxide yield upon decrease of the
water content in the hydrogen peroxide source confirms the detrimental role of water molecules,
which can interact with the active sites at the catalyst surface and thus decrease their availability
for activating hydrogen peroxide towards the epoxidation reaction [Error! Bookmark not
defined., [54].
1
9

1
00
5
7
2
0 wt% (aqueous)
0 wt% (aqueous)
4.5 wt% (anhydrous)
80
60
40
20
0
Epoxide
yield
(%)
Cyclooctene
Cyclohexene
Alkene
1-Octene
Figure 6. Effect of the hydrogen peroxide source on the epoxidation of cyclooctene,
cyclohexene, and 1-octene using -Al -NR-400 as catalyst. Conditions: 2 mmol of alkene, 4
mmol of H , 40 mg of catalyst, 1 mmol of dibutyl ether, 2.0 g of ethyl acetate, 4 h at 80 °C.
2
O
3
2
O
2
Finally, we investigated the effect of the nature of the solvent in which the epoxidation reaction is
carried out. The performance of a catalyst in liquid-phase reactions can be strongly influenced by
the solvent. Identifying the most suitable solvent can drastically accelerate the rate of reaction,
selectivity of a product, and stability of the catalyst by facilitating the access of the reagents to the
active sites of the catalyst and stabilising the transition state [55], [56]. When selecting potential
solvent candidates, the use of environmentally friendly compounds with low toxicity is preferred,
in accordance with the Green Chemistry principles. Additionally, the solvent should be an
inexpensive, available compound. Based on these considerations and taking into account that the
solvent should be able to dissolve both the relatively apolar alkenes and the polar hydrogen
peroxide, we selected a set of relatively polar green solvents, both protic and aprotic (2-propanol,
dimethyl carbonate, 1,4-dioxane and 1,3-dioxolane) and we screened them as alternative to ethyl
acetate in the epoxidation of cis-cyclooctene with 50 wt% aqueous H
2
O
2
over -Al
2
3
O -NR-400
as catalyst (Figure 7). Poor epoxide yield and selectivity (with 2-cyclooctene-1-one and 1,2-
cyclooctene diol as side products) were observed when 1,4 -dioxane, dimethyl carbonate or 2-
2
0

propanol were employed as solvent. On the other hand, the use of 1,3-dioxolane as solvent led to
a significant increase in epoxide yield (49 %) compared to ethyl acetate (35 %). The yield could
be further improved by employing anhydrous hydrogen peroxide instead of the 50 wt% aqueous
H
2
2
O , reaching 58 % cyclooctene oxide yield with nearly full selectivity. The higher activity by
using a 1,3-dioxolane as solvent can be ascribed to the observed good dispersion of the catalysts
in the monophasic reaction mixture.
1
00
epoxide yield
epoxide selectivity
80
60
40
20
0
%
1
,4-dioxane
dimethyl
carbonate
2-propanol ethyl acetate 1,3-dioxolane 1,3-dioxolane*
Solvent
Figure 7. Effect of the nature of the solvent on the epoxidation of cis-cyclooctene with 50 wt%
aqueous H using -Al -NR-400 as catalyst. Conditions: 2 mmol of alkene, 4 mmol of H
0 mg of catalyst, 1 mmol of dibutyl ether, 2.0 g of solvent, 4 h at 80 °C. *Anhydrous H
ethyl acetate) was used as oxidant instead of 50 wt% aqueous H
2
O
2
2
O
3
2
2
O ,
4
2
2
O (in
2
O
2
4
. Conclusions
A facile and cost effective method was developed for the synthesis of high surface area
γ-AlO(OH) (boehmite) nanorods, through a very reproducible sol-gel process in which aqueous
NH
structure templating agent or other additive. γ-Al
transformation upon thermal treatment which led a slight reduction in specific surface area
3
is dosed into a solution of aluminium tri-sec-butoxide in 2-butanol without the need of any
2
O
3
nanorods were obtained through phase
2
1

relative to the as-prepared boehmite nanorods. A characterisation study by means of XRD, TEM,
-physisorption, TGA and FT-IR spectroscopy demonstrated that these nanostructured materials
N
2
display a combination of properties that are suitable for their application as epoxidation catalysts,
and namely a high specific surface area, an open, fully accessible structure, a population of
surface acid sites; and a relatively low hydrophilicity. Accordingly, the γ-AlO(OH)-NR and γ-
Al
alkenes using the environmentally friendly H
00, achieved good activity, excellent selectivity, broad substrate applicability and enhanced
reusability compared to previously reported alumina epoxidation catalysts. The superior activity
of -Al -NR-400 is ascribed to the best balance between surface area and nature and
concentration of surface acid sites. Furthermore, the activity of -Al -NR-400 could be
substantially enhanced by using anhydrous H instead of 50 wt% aqueous H as the oxidant
2
O
3
-NR were found to be active and selective heterogeneous catalysts for the epoxidation of
2
O
2
as the oxidant. The best catalyst, -Al -NR-
2
O
3
4
2
O
3
2
O
3
2
O
2
2
O
2
and by optimising the solvent in which the epoxidation is carried out. Though the maximum
achieved activity is still inferior compared to that of previously reported gallium oxide nanorods,
which represents the state-of-the-art among transition-metal-free oxide catalysts, these alumina
nanorods are a promising alternative considering that aluminium is much more abundant and
cheaper than gallium. Additionally, the high surface area and highly accessible open surface of
these distinctively small boehmite and -alumina nanorods are desirable features for various other
catalytic applications, and particularly for the widespread use of aluminas as support for metal
nanoparticles [57][58].
Acknowledgements.
2
2

The authors acknowledge sponsoring in the frame of the following research programs: START1
from KU Leuven and project G0D9313N from the Flemish Science Foundation (FWO). We
thank Sonia Parres-Esclapez and Gina Vanbutsele for assistance in the measurement of the N -
2
physisorption isotherms. We thank Damiano Cani for the TEM measurements, the Flemish
Hercules Stichting for its support in HER/08/25and the MTM Department for technical support.
2
3

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[
5
58] N. Nagaraju, A. Fonseca, Z. Konya and J. B. Nagy, J. Mol. Catal. A: Chem., 181, (2002),
7-62.
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6

Table 1. Physicochemical properties and epoxidation activity of the nanostructured boehmite and
γ-alumina catalysts and of two types of commercial γ-alumina.
b,c
Epoxide yield (%)
BET surface
area (m /g)
nH
/nm
2
O
Dimensions
(nm)
Catalyst
2
2 a
Fresh
29
Rec1
29
Rec2
28
Rec3
27
w: 2 ± 0.3
l: 23 ± 3
w: 2 ± 0.2
l: 16 ± 1
w: 3 ± 0.6
l: 13 ± 1
γ-AlO(OH)-NR
448
376
305
155
9
9
8
-
γ-Al
γ-Al
2
O
O
3
-NR-400
-NR-600
36
32
9
36
33
-
33
32
-
33
29
-
2
3
d
Commercial γ-Al
2
O
3
-
-
Commercial γ-Al
O
2 3
1
71
-
16
-
-
-
e
nanopowder
a
b
Estimated by TGA. Conditions: 2 mmol of cis-cyclooctene, 4 mmol of H
2
O
2
(as 50 wt% aqueous solution), 40 mg
c
of catalyst, 1 mmol of dibutyl ether (as GC internal standard), 2.0 g of ethyl acetate as solvent, 4 h at 80°C. The
selectivity towards the epoxide was higher than 99% with all catalysts. d Activated, neutral, Brockmann I , particle
e
size ~150 mesh. γ-Al
2
O
3
nanopowder, particle size < 50 nm (TEM).
2
7

Table 2. Epoxidation of selected alkenes catalysed by -Al
as the oxidant.
2
3
O -NR-400 using 50 wt% aqueous
H
2
O
2
Substrate
Cyclooctene
Cyclohexene
Convalkene (%)
Y
epoxide (%)
S
epoxide (%)
>99
Productivity(a)
36
23
6
19
42
36
23
6
2.3
1.1
0.4
0.7
3.1
>99
>99
1
-Octene
Styrene
Limonene
12
64
(b)
(b)
41
97
2 2
Conditions: 2 mmol of alkene, 4 mmol of H O (as 50 wt% aqueous solution), 40 mg of catalyst, 1 mmol of dibutyl
ether (as GC internal standard), 2.0 g of ethyl acetate as solvent, 4 h at 80°C. (Y = yield; S = selectivity). (a)
Productivity is defined as gepoxide/gcatalyst, (b) total epoxides selectivity, the details of the distribution between the
internal-, external- and di-epoxides is presented in Figure 5.
2
8