Grafted non-ordered niobium-silica materials: Versatile catalysts for the selective epoxidation of various unsaturated fine chemicals

Article abstract of DOI:10.1016/j.cattod.2014.02.027

Two kinds of niobium(V)-silica catalysts for the selective epoxidation were synthesised by post-synthesis modification of non-ordered mesoporous silica supports, starting from niobocene dichloride via solvent-less organometallic precursor dry impregnation

Full text of DOI:10.1016/j.cattod.2014.02.027

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Grafted non-ordered niobium-silica materials: Versatile catalysts for
the selective epoxidation of various unsaturated fine chemicals
a
a,b
b
a,∗
Cristina Tiozzo , Chiara Bisio , Fabio Carniato , Matteo Guidotti
a
Istituto di Scienze e Tecnologie Molecolari, ISTM-CNR, via C. Golgi 19, 20133 Milano, Italy
Dipartimento di Scienze e Innovazione Tecnologica and Nano-SiSTeMI Interdisciplinary Centre, Università del Piemonte Orientale “A. Avogadro”, viale T.
Michel 11, 15121 Alessandria, Italy
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Two kinds of niobium(V)-silica catalysts for the selective epoxidation were synthesised by post-synthesis
modification of non-ordered mesoporous silica supports, starting from niobocene dichloride via solvent-
less organometallic precursor dry impregnation or conventional liquid-phase grafting technique. Grafted
Nb/SiO2 solids were used as catalysts, in the presence of aqueous H2O2, for the epoxidation of unsaturated
cyclic and terpenic compounds of interest for fine and specialty chemistry, in particular: cyclohexene,
Received 6 December 2013
Received in revised form 11 February 2014
Accepted 14 February 2014
Available online xxx
1
-methylcyclohexene, limonene, carveol, ␣-terpineol, isopulegol, carvotanacetol, carvone, as well as
Keywords:
squalene and isopulegyl acetate. These catalysts showed high yields (up to 73%) and excellent chemose-
lectivities to the desired epoxides (up to 98%), also in short reaction times (down to 1 h).
Sustainable epoxidation
Hydrogen peroxide
Heterogeneous catalyst
Niobium-silica catalyst
Grafted catalyst
©
2014 Elsevier B.V. All rights reserved.
Unsaturated terpenes
1
. Introduction
Several authors have proposed the strategy of heterogenisa-
tion via covalent anchoring of homogeneous catalytic systems with
a proven activity and selectivity (such as organometallic species,
metal-Schiff base complexes or metal silsesquioxanes), in order
to obtain effective heterogeneous catalysts for the epoxidation of
heavy and bulky substrates [8–12]. However, such approach is not
always easily feasible and requires a delicate know-how of surface-
chemistry modifications [13].
Starting from the first syntheses of transition metal-containing
mesoporous silicates in the early 1990s, many research teams
have directed their efforts in developing innovative catalysts for
liquid-phase oxidation reactions and have tested these solids in
the selective epoxidation of bulky intermediates of interest for the
fine chemical industry [1]. In fact, the optimal performance of the
titanium silicalite-1 (TS-1) zeotype, obtained by incorporation of
Ti centres within the structure of a MFI zeolite [2], is still currently
unbeaten in terms of activity, selectivity, robustness and recyclabil-
ity [3–5]. The remarkable efficiency of TS-1 for selective oxidation
with aqueous hydrogen peroxide is attributed to: 1) the isolation of
titanium sites, preventing the undesired and useless decomposition
of H O , and 2) the hydrophobic character of the lattice, enabling
the preferential adsorption of the hydrophobic substrates within
the micropores, even in the presence of water [6]. However, the
main drawback of such microporous zeotypes is due to the limited
dimensions of their channels (typically ca. 0.5 nm × 0.6 nm max-
imum). So, only simple and less bulky reactants, whose kinetic
diameter is smaller than 0.55 nm, can be easily epoxidised in their
porous network [7].
Mesoporous metal-containing silicates were expected to over-
come these problems, opening the way to the agile oxidation of
sterically demanding (and high added-value) substrates. Unfortu-
nately, these materials showed intrinsically poorer activity than
TS-1 when used with aqueous H O , because of the presence,
2
2
on their surface and mesopores, of a large number of silanols,
which promote the preferential adsorption of water at the expenses
of non-polar alkene substrates [14]. On these systems, organic
hydroperoxides, such as cumyl hydroperoxide (CHP) or tert-
butylhydroperoxide (TBHP), are the most suitable oxidants, since
they can be used under anhydrous conditions, hence avoiding (or
reducing) the problems linked to the hydrolysis and the leaching
of surface Ti species [15–17]. Nevertheless, it is worth highlighting
that hydrogen peroxide was used successfully for the epoxidation
of bulky olefins over Ti-silicates in few notable examples. This was
possible thanks to the peculiar morphology and/or robustness of
the porous material, as in the epoxidation of cyclododecene over
Ti-MCM-48 [18] and of caryophyllene over Ti-MMM-2 [19], or by
2
2
∗ Corresponding author.
E-mail address: m.guidotti@istm.cnr.it (M. Guidotti).
http://dx.doi.org/10.1016/j.cattod.2014.02.027
0
920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: C. Tiozzo, et al., Grafted non-ordered niobium-silica materials: Versatile catalysts for the selective
epoxidation of various unsaturated fine chemicals, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.027

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2
◦
applying a drop-wise addition of H O to minimise the detrimental
were calcined under dry oxygen at 500 C for 2 h to obtain the final
2
2
effects of water on the mesoporous catalyst, as in the epoxidation
of cyclohexene and methyl oleate over Ti-MCM-41 [20–22].
In the search for mesoporous silicates containing metals other
than Ti, that are active and truly heterogeneous, the metals of
Group 4, i.e. Zr and Hf, showed scarce performances [23], whereas
promising results were obtained by using metals of Group 5.
Vanadium silicates suffer from extensive leaching, mainly due to
extra-framework hexa-coordinated V(V) species that are readily
hydrolysed in the presence of protic solvents [24–26]. Very few
reports about tantalum-containing mesoporous silica materials
show interesting data, but a deeper investigation has not been
carried out so far [27,28].
The study of niobium-containing silica catalysts for liquid-phase
oxidation is, on the contrary, a field in restless expansion [29–31].
These solids have shown promising catalytic performances in
water-containing liquid-phase oxidation reactions and a higher
stability and robustness towards metal leaching and hydrolysis,
with respect to Ti-silica ones. Mesoporous niobium-silica materi-
als are effective systems for the epoxidation of cycloalkenes and
their catalytic activity is found to be influenced by the disper-
sion of niobium in SiO2 matrix and by the synthesis conditions
Nb(V)-silica catalysts.
Conversely, Nb/SiO -liq was prepared via liquid-phase grafting
on the same support. In this case, Nb(Cp) Cl2 was grafted onto the
surface of the silica support by adapting and applying the graft-
ing protocol developed by Maschmeyer et al. [42] and modified by
some of us [43].
2
2
2.2. Catalyst characterisation
The Nb elemental content of the prepared samples was
determined by inductively coupled plasma optical emission
spectroscopy (ICAP 6300 Duo, Thermo Fisher Scientific) after min-
eralisation of the samples in a microwave digestion apparatus
(Milestone MLS 1200; maximum power 500 W) with a mixture of
hydrofluoric (aq. 40%) and fuming nitric acid.
X-ray diffractograms (XRD) were collected on unoriented
ground powders with a Thermo ARL ‘XTRA-048 diffractometer
using Cu K␣ (ꢁ = 1.54 A˚ ) radiation. Diffractograms were recorded at
◦
◦
−1
room temperature with a step size of 0.02 and a rate of 1 2ꢂ min .
N2 physisorption measurements were carried out at 77 K in
−6
the relative pressure range from 1 × 10
to 1 P/P0 by using a
[
32]. Mesoporous materials, in which niobium centres are homoge-
Quantachrome Autosorb 1MP/TCD instrument. Prior to analysis,
neously dispersed into the silicate matrix, catalysed efficiently the
epoxidation of unsaturated terpenes and alkenes of interest for the
industry of the intermediates [33–38]. On such Nb-SiO2 based sys-
tems, aqueous H O can be used as a direct oxidant, added in one
◦
the samples were outgassed at 100 C for 3 h (residual pressure
−
6
lower than 10 Torr). Apparent surface areas were determined
by using Brunauer–Emmett–Teller equation (BET), in the relative
2
2
pressure range from 0.01 to 0.1 P/P . Pore size distributions were
obtained by applying the BJH (Barret–Joyer–Halenda) approach to
the desorption branch.
Diffuse reflectance UV–vis (DR UV–vis) spectra were recorded at
room temperature using a PerkinElmer Lambda 900 spectrometer
equipped with an integrating sphere accessory, and using a custom-
made quartz cell. Before the analysis, the samples were dispersed
in anhydrous BaSO4 (5 wt.%).
0
aliquot since the beginning of the reaction and no slow drop-wise
addition of the oxidant is therefore needed.
The main goal of the present work is to describe the use
of grafted niobium non-ordered mesoporous silica materials as
water-tolerant catalysts for the epoxidation of a broad series of
unsaturated substrates. Actually, promising results have already
obtained by applying these catalysts for the epoxidation of cyclo-
hexene, limonene and some fatty acid methyl esters (FAMEs)
Thermogravimetric analysis (TGA) was performed on a Setaram
SETSYS Evolution instrument, under dry air flow in the temperature
[
39–41]. Here, this kind of grafted Nb-SiO2 catalysts are applied
to a wide series of cyclic alkenes, terpenes and terpenoids in order
to study the influence of C C double bond position and of addi-
tional functional groups (e.g., alcoholic –OH moiety) on the catalytic
performance. The catalysts were prepared according to a post-
synthesis approach following two routes: (i) liquid-phase grafting
and (ii) an alternative solventless synthesis method based on dry
impregnation of niobocene dichloride (organometal dry impregna-
tion; OM-DI), that proved to be versatile, cheap, environmentally
convenient and straightforward.
◦
range 25–800 C.
2.3. Catalytic tests
◦
All catalysts were pre-treated under dry air at 500 C and
cooled to room temperature under vacuum prior to use. The epox-
idation tests on the alkenes, namely, cyclohexene (Aldrich 99%),
1-methyl-1-cyclohexene (Aldrich 97%), R-(+)-limonene (Aldrich
9
7%), (−)-carveol (Aldrich 97%), ␣-terpineol (Aldrich 90%), isop-
ulegol (Aldrich techn.), carvotanacetol (prepared by selective
hydrogenation of carveol, as previously described [44,45]), R-
(−)-carvone (Aldrich 98%), isopulegyl acetate (Aldrich 96%) and
squalene (Aldrich ≥98%) were carried out in a round-bottom glass
2
. Experimental details
◦
2.1. Catalyst preparation
batch reactor in an oil bath at 90 C equipped with magnetic stirring
(ca. 800 rpm) under inert atmosphere. The substrate (1.0 mmol)
Niobium-silica catalysts were all prepared by grafting
was dissolved in acetonitrile (Aldrich, HPLC grade; 5.0 mL) and
aqueous hydrogen peroxide (H O ; aq. 50% Aldrich; 2.0 mmol) was
bis(cyclopentadienyl)niobium(IV) dichloride (Nb(Cp) Cl ; 95%
2
2
2
2
Aldrich) onto Grace Davison, Davisil SiO2 LC60A, 60–200 ␮m, as
used as oxidant. The samples were taken after reaction times of 1, 2
and 4 h and analysed by gas-chromatography (Agilent 6890 Series;
HP-5 column, 30 m × 0.25 mm; FID detector). Mesitylene (Fluka,
puriss. ≥99%) was used as internal standard. GC-peaks were iden-
tified by comparison with peaks of genuine samples of reference
standards and/or by means of GC-MS analysis. In the tests of squa-
lene epoxidation, the reaction was followed by 1H and C NMR
analysis at room temperature (Bruker UXNMR, 400 MHz) [46,47].
After all tests, the presence of residual hydrogen peroxide was
checked and confirmed by iodometric assays and titrations. In none
of the cases hydrogen peroxide was the limiting reagent. Specific
activity, SA, of the catalyst is defined as the amount (moles) of
described previously in detail [40]. All supports were pre-treated
◦
◦
in dry air at 500 C for 1 h, then left for 1 h at 500 C under vacuum
and finally cooled to room temperature under vacuum.
For the catalyst obtained via dry impregnation (Nb/SiO -DI),
2
the silica support was hydrated with high purity deionised water
13
(
MilliQ Academic, Millipore, 18 Mꢀ cm) for 2 h, dried at rotary
◦
evaporator and then pre-treated in air at 300 C for 1 h and under
vacuum overnight at 300 C. Nb(Cp) Cl was finely ground and
◦
2
2
mixed, under inert atmosphere in solid phase, to the silica. The
so-obtained samples were stirred overnight under steady vacuum
◦
at room temperature (20 C). The resulting light brown mixtures
Please cite this article in press as: C. Tiozzo, et al., Grafted non-ordered niobium-silica materials: Versatile catalysts for the selective
epoxidation of various unsaturated fine chemicals, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.027

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converted alkene per amount (moles) of total Nb in the unit of time
1 h).
In order to check the possible leaching of Nb species, the solid
(
catalyst was rapidly removed from the liquid mixture by centrifu-
gation and the resulting solution was tested for further reaction
[
48,49]. In the tests for the recovery of the catalyst, the solid was
separated by filtration and thoroughly washed with fresh acetoni-
trile and then with methanol (Fluka, HPLC grade). The filtered solid
◦
◦
was dried gently at 110 C, weighed, activated again at 500 C in dry
air and then reused in a new test as described above.
3
. Results and discussion
3.1. Catalyst preparation and characterisation
Niobocene(IV) dichloride species were deposited on the silica
support either by base-induced liquid-phase covalent grafting or by
solvent-less deposition under reduced pressure onto the activated
silica surface. In all catalysts, the desired expected Nb loading was
2
wt.%, according to the amount of metallocene precursor added
Fig. 1. DR UV–vis spectra of Nb/SiO2-liq (a) and Nb/SiO2-DI (b) and Nb/SiO2-DI, after
at the beginning of the dry impregnation or grafting procedure.
Indeed, 2 wt.% is the optimal loading for the epoxidation of a large
variety of alkenes [36,39]. Nevertheless, the measured Nb content
showed that only a part of the expected metal was present (Table 1).
In particular, via liquid-phase grafting, a remarkable part of the
precursor was not deposited onto the silica surface, thus resulting
in a sample with a low final Nb content (0.76 wt.%). In this case, in
fact, Nb was deposited on the silica surface by chemisorption only
and the loosely physisorbed niobocene dichloride precursor was
removed by washing with fresh chloroform. At this step, part of
the initial expected niobium loading was lost.
five catalytic cycles (c).
to the presence of Nb O5-like nano-aggregates formed during the
dry impregnation process (Fig. 1b) [41].
The presence and the nature of these aggregates was confirmed
by XRD analysis. In fact, the diffractogram of Nb/SiO2 prepared
by dry impregnation approach showed, in addition to the broad
signal typical of the amorphous silica observed in the 15–35 2ꢂ
range, three weak and wide reflections at ca. 23 , 29 and 37 2ꢂ,
typical of nanometre-sized crystalline Nb O5 aggregates (data not
shown for the sake of brevity), which were not present in the XRD
2
◦
◦
◦
◦
2
The samples underwent physico-chemical characterisation in
order to investigate the surface properties of the catalysts and the
nature and dispersion of the niobium sites on the support.
The textural properties of Davisil SiO2 and derived catalysts,
pattern of Nb/SiO -liq sample [41]. These results confirm that the
2
dry impregnation methodology led to the formation of nano-sized
Nb O5 aggregates on the surface of the support, whereas the liquid-
2
phase grafting approach led to a uniformly dispersed sample, with
a very large amount of surface single-site Nb atoms on the silica.
prepared by both dry impregnation (Nb/SiO -DI) and liquid-phase
2
grafting (Nb/SiO -liq), were studied by N physisorption analysis
2
2
at 77 K. An isotherm of type IV with an H2 hysteresis loop, typi-
cal for solids with disordered structural pore size distribution, was
observed for all the samples (Table 1). The presence of Nb metal
centres led to a reduction of the specific surface area (estimated
3
.2. Catalytic tests
The two catalysts, Nb/SiO -liq and Nb/SiO -DI, were tested in
2
2
the selective epoxidation, under heterogeneous conditions, of a
broad range of alkenes of interest for fine and flavours and fra-
grances (F&F) chemistry: cyclohexene 1, 1-methylcyclohexene 2,
limonene 3, carveol 4, ␣-terpineol 5, isopulegol 6, carvotanacetol
by BET method, SBET), passing from 580, for pure SiO , to 435 and
2
2
−1
4
44 m g for Nb/SiO -liq and Nb/SiO -DI samples, respectively.
2
2
The pores size distribution resulted broad for all samples, cover-
ing a wide range of values from 4.0 to 10.0 nm. Finally, the pores
7
, carvone 8, isopulegyl acetate 9 and squalene 10.
All catalysts were active in the presence of hydrogen peroxide
volume of SiO samples decreased as a consequence of Nb introduc-
2
3
−1
3
−1
tion, passing from 0.97 cm g for SiO2 to 0.78 and 0.79 cm g
for Nb/SiO -DI and Nb/SiO -liq samples, respectively. The coor-
as oxidant and no detectable leaching of metal species in solution
was recorded. Tests on cyclohexene and limonene either with Nb-
free silica or without catalyst showed that there was no significant
auto-oxidation or support-catalysed contribution to epoxidation:
the epoxide amount resulted under the chromatographic detec-
tion limit. This confirms that niobium is the active site over which
oxidation takes place.
2
2
dination geometry and the chemical environment of the Nb sites
dispersed on the silica surface were evaluated by using DR UV–vis
spectroscopy (Fig. 1).
As a general feature, both the UV–vis spectra of Nb/SiO -liq
2
and Nb/SiO -DI samples show a main absorption with maximum
2
2−
5+
centred at ca. 245 nm, tentatively assigned to O → Nb charge-
transfer bands of isolated Nb(V) centres (Fig. 1) [50]. In addition,
especially for Nb/SiO -DI sample, the band at 245 nm appears
3.2.1. Performance in the epoxidation of cyclic alkenes
2
asymmetric and slightly broadened towards low frequency side as a
consequence of the expansion of the Nb coordination sphere (from
tetrahedral to octahedral) promoted by the presence of coordinated
water molecules.
Cyclohexene 1 and 1-methylcyclohexene 2 were readily epox-
idised to epoxycyclohexane and 1,2-epoxy-methylcyclohexane,
respectively, in 1 h, with a very high selectivity (Table 2). Nb/SiO2-
liq, in particular, showed a remarkably higher activity than
No absorption bands were detected above 400 nm, indicating
Nb/SiO -DI, if one considers that the former contains only 43% of
2
that no large Nb O5 domains are present on both catalysts surface
Nb with respect to the latter (Fig. 2). Such behaviour is consistent
with the one observed on other epoxidation catalysts [52–55] and is
attributed to a high fraction of evenly dispersed sites on the surface
2
(Fig. 1) [51]. Nevertheless, the spectrum of the sample prepared by
dry impregnation shows a shoulder centred at ca. 340 nm, ascribed
Please cite this article in press as: C. Tiozzo, et al., Grafted non-ordered niobium-silica materials: Versatile catalysts for the selective
epoxidation of various unsaturated fine chemicals, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.027

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Table 1
Textural features of SiO2 support and Nb-silica catalysts.
Metal loading (wt.%)a
SBETb (m2
g
−1
)
PVc (cm3
g
−1
)
PDd (nm)
Sample
SiO2
Nb/SiO2-DI
Nb/SiO2-liq
–
1.75
0.76
580
444
435
0.97
0.78
0.79
4.0–10.0
4.0–10.0
4.0–10.0
a
Nb content measured by ICP–MS.
BET specific surface area.
Specific pore volume.
Mean pore diameter.
b
c
d
influence of the C C double bond position and the presence of an
alcoholic moiety (if any) on the catalytic performances.
With all cyclic terpenes (limonene 3, carveol 4, ␣-terpineol
5
, isopulegol 6, carvotanacetol 7; Scheme 1) Nb/SiO -liq showed
2
higher conversion and higher specific activity values than Nb/SiO2-
DI, as observed above with simpler cyclic alkenes (Fig. 2). The good
and uniform dispersion of the Nb(V) sites obtained by liquid-phase
grafting is an advantage on these substrates too.
Most of the epoxidation activity took place within the first hour
of reaction. For longer reaction times, the rate of advancement of
the reaction decreased due to the gradual formation of oxidised side
products on the surface of the catalyst, leading to a gradual deac-
tivation of Nb sites [32] (with the notable unexpected exception of
Nb/SiO2-liq in the presence of carvotanacetol 7).
Fig. 2. Specific activity (h−1) after 1 h of Nb/SiO2-DI (grey) and Nb/SiO2-liq (black)
in the epoxidation of alkenes 1–7.
Taking into account the effect of a hydroxyl moiety in allylic,
homoallylic or bishomoallylic position (for carvotanacetol, isopule-
gol and ␣-terpineol, respectively), the presence and the proximity
of the OH-group did not favour necessarily the epoxidation step
and no neat trend in reactivity can be described: carvotanacetol
was less easily epoxidised than ␣-terpineol; limonene and carveol
showed fully similar conversion values. Such behaviour differs
from the trend observed in the epoxidation of the same series of
terpenes over Ti(IV)-silica catalysts with tert-butylhydroperoxide
and, hence, to a better availability of Nb(V) centres in the cata-
lysts prepared via liquid-phase grafting [40]. In fact, in the sample
Nb/SiO -DI, in which some niobia-like aggregates were detected
2
by spectroscopic investigation (v. supra and [41]), a fraction of Nb
sites can be buried within the aggregate and thus inaccessible, or
less accessible, for the reactant. However, a partial contribution by
the small domains of niobia to the epoxidation reaction cannot be a
priori excluded, since hydrogen peroxide can interact cooperatively
with crystalline niobia to generate oxidising species [30,56]. There-
fore, the presence of such niobia aggregates cannot be considered
fully detrimental.
In terms of reactivity of the two cyclohexenes, 1-
methylcyclohexene was more readily epoxidised than
unsubstituted cyclohexene. The trisubstituted alkene showed
indeed a higher nucleophilic character with respect to cyclohex-
ene, thus favouring the electrophilic transfer of the oxygen from
hydrogen peroxide to the alkene [36,57,58].
(TBHP), where the reactivity trend followed the clear order:
allylic > homoallylic > bishomoallylic [58].
Nevertheless, by comparing the reactivity of isopulegol 6 and
isopulegyl acetate 9 (i.e. the same molecule backbone, bearing
either an OH- or an acetyl-group; Scheme 2), some promotion effect
by the hydroxyl moiety was evident and conversion values on the
terpenic alcohol resulted twice as higher as its acetylated derivative
(Fig. 3).
From these results, there is not a sharp evidence of the role of the
OH-group in enhancing epoxidation rate, as it has been observed,
conversely, on several titanium-containing homogeneous or het-
erogeneous epoxidation catalysts [45,58,60]. For sure, in some
examples, the presence of an alcoholic function on the alkene
molecule is an advantage for epoxide formation, but such enhance-
ment is largely substrate-dependent and is not valid on a general
basis. This feature is therefore a remarkable difference in the epox-
idation behaviour of Nb-based systems with respect to Ti-based
ones.
In all cases, the selectivity to the desired epoxide was very
high, with a good oxidant efficiency due to a limited use-
less decomposition of H O2 (oxidant efficiency in all tests was
2
around 35–40 mol%). At longer reaction times (4 h), the selectiv-
ity decreased, as part of the epoxide underwent acid-catalysed
rearrangement or ring-opening. Nb(V) centres on the silica surface,
indeed, possess intrinsic acidic character [51], leading to the forma-
tion of secondary products obtained by subsequent acid-catalysed
transformations (cyclohexanone and methylcyclohexanone, cyclo-
hexanediol and methylcyclohexanediol, derived from cyclohexene
oxide and methylcyclohexene oxide, respectively). No appreciable
yield of allylic oxidation products, such as cyclohexenol or methyl-
cyclohexenol, was detected. This means that the oxidation reaction
follows mainly a heterolytic pathway, since the presence of Nb sites
is able to address the reaction towards a non-radical mechanism
Interestingly, a direct comparison between the performances
obtained with carveol 4 and carvone 8, showed that Nb-SiO2 cat-
alysts are active in the epoxidation of ␣,␤-unsaturated ketones
too (Fig. 4). Carvone was indeed less easily epoxidised with H O2
2
than carveol (36% vs. 51% of conversion after 1 h, respectively) and
such behaviour was reasonably expected, taking into account the
poorer nucleophilic character of the C C bond in ␣,␤-unsaturated
ketones than in allylic systems. However, carvone showed sur-
prisingly better performances than carvotanacetol 7 (in Fig. 4
for the sake of comparison) and this is a remarkable result. In
fact, electron-deficient ␣,␤-unsaturated ketones usually need a
completely different approach to be epoxidised in the presence of
H2O2 with good yields in epoxide, such as heterogeneous catalysts
with a neat basic character [61]. So, this is a further noteworthy
[
59].
3
.2.2. Performance in the epoxidation of cyclic unsaturated
terpenes
Both catalysts were also active in the epoxidation of a broad
series of cyclic unsaturated terpenes with a menthane-like skele-
ton (Table 2). In particular, such screening aims at studying the
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epoxidation of various unsaturated fine chemicals, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.027

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Table 2
Performance of Nb/SiO2-liq and Nb/SiO2-DI in the catalytic epoxidation of cyclic alkenes and unsaturated terpenes.
Alkene
Catalyst
Ca 1 h (%)
Sb 1 h (%)
Ca 4 h (%)
Sb 4 h (%)
Cyclohexene 1
Nb/SiO2-DI
Nb/SiO2-liq
53
53
94
90
82
65
87
74
1-Methylcyclohexene 2
Nb/SiO2-DI
Nb/SiO2-liq
59
71
85
94
99
86
53
56
98c
62
d
97c,d
>98
Limonene 3
Nb/SiO2-DI
Nb/SiO2-liq
58
64
>98c
74
d
c,d
Carveol 4
Nb/SiO2-DI
Nb/SiO2-liq
51
78
73c
46
65
80
60c
57
c
d
c,d
OH
␣-Terpineol 5
Nb/SiO2-DI
Nb/SiO2-liq
52
77
51
89
66
83
44
48
OH
Isopulegol 6
Nb/SiO2-DI
Nb/SiO2-liq
42
71
84
89
58
76
86
74
OH
Carvotanacetol 7
Nb/SiO2-DI
Nb/SiO2-liq
23
26
66
59
37
61
63
57
OH
Conditions: Dry CH3CN solvent; 100 mg cat; 1.0 mmol limonene; 2.0 mmol aq. H2O2; reflux temperature; batch reactor.
a
Conversion of alkene.
Selectivity to alkene monoepoxide.
Selectivity to monoepoxide (sum of exocyclic + endocyclic isomers).
Values after 3 h of reaction.
b
c
d
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O
Nb-SiO2
aq. H O2
2
+
a)
CH CN
3
O
3
4
O
Nb-SiO2
OH
OH
OH
O
aq. H O2
2
+
+
b)
c)
CH CN
3
O
O
Nb-SiO2
Nb-SiO2
O
aq. H O2
2
CH CN
3
OH
OH
OH
5
6
7
Nb-SiO2
aq. H O2
2
+
d)
e)
OH
OH
OH
OH
O
CH CN
3
O
O
Nb-SiO2
O
aq. H O2
2
+
CH CN
3
Scheme 1. Epoxidation of cyclic unsaturated terpenes 3–7.
difference in the epoxidation behaviour of Nb-based systems with
respect to Ti-based ones.
from epoxide rearrangement, etc.), the lower is the selectivity
to the target molecule (Scheme 1). So, limonene was epoxi-
dised with excellent selectivity (limonene epoxide was practically
the only product) and no side products, due to parallel oxida-
tion pathways (e.g., allylic oxidation or hydroperoxidation) were
formed. Carveol, carvotanacetol and isopulegol, on the contrary,
suffered from the concurrent oxidation of the OH-group and ca.
In terms of selectivity to desired epoxides, values spanned
from >98% with limonene epoxide to less than 50% with ␣-
terpineol epoxide. Such variability in chemoselectivity values is
largely affected by the reactivity of the terpenic substrates: the
higher the number of possible side products (ketones, by-products
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7
O
O
O
8
9
Scheme 2. Carvone 8 and isopulegyl acetate 9.
Fig. 4. Conversion profile vs. time for carveol 4 (♦), carvone 8 (ꢂ) and carvotanacetol
7
(ꢃ) over Nb/SiO2-DI.
Nb-SiO
aq. H O
2
2
10
3
CH CN
2
Fig. 3. Conversion profile vs. time for isopulegol 6 (ꢀ) and isopulegyl acetate 9 (ꢁ)
over Nb/SiO2-DI.
O
terminal epoxide(s)
internal epoxide(s)
2
0–30% of the products were due to the formation of carvone,
carvotanacetone and isopulegone, respectively, in the reaction mix-
ture.
O
O
Finally, ␣-terpineol 5 was converted into the epoxide, which
underwent rapidly acid-catalysed intermolecular ring closure,
leading to 2-hydroxy-1,8-cineol [58,62]. After 4 h of reaction, in fact,
the selectivity to ␣-terpineol epoxide dropped down to 44–48%, the
rest being mainly composed by the bicyclic cineol (Scheme 1).
Finally, in terms of regioselectivity, when doubly unsaturated
terpenes were studied (limonene 3 and carveol 4), the forma-
tion of the monoepoxide occurred with unexpected regioselectivity
towards the exocyclic isomer. In fact, exocyclic 7,8-limonene
epoxide and 7,8-carveol epoxide were formed preferentially
from limonene and carveol over both catalysts, with exo-
cyclic/endocyclic ratios of ca. 75:25 and ca. 60:40, respectively. In
this particular case, the expected preferential epoxidation of the
endocyclic unsaturation (more nucleophilic than the exocylic one
Scheme 3. Epoxidation of squalene 10.
with relevant biological activity [46,47]. Unfortunately, no marked
regioselectivity was observed on this substrate and the formation
of the more stable internal epoxides was regularly observed over
both catalysts at various reaction times (Fig. 5).
All these evidences confirm that the regioselective behaviour
recorded on limonene and carveol was not due to intrinsic elec-
tronic reasons, but, rather, to some special interactions among this
kind of substrates, the heterogeneous catalyst and the oxidant.
[
55]) did not occur and the terminal exocyclic epoxide is the major
product.
Such peculiar behaviour, however, is to be attributed mainly to
steric factors and this statement can be confirmed by a set of spe-
cific tests. In fact, by using TBHP as oxidant, instead of hydrogen
peroxide, the epoxidation of limonene was slower, but, in partic-
ular, exocyclic/endocyclic ratio was the opposite (i.e., 24:76 with
Nb/SiO -liq, at 29% conversion after 1 h).
2
Then, by using hydrogen peroxide as oxidant and a non-
porous Nb-SiO2 catalyst, prepared by grafting Nb(V) centres onto
a pyrogenic nanosized (Aerosil380) silica [39], the unusual regio-
selectivity towards exocyclic limonene epoxide was no longer
observed.
Finally, some efforts were devoted to exploit such particular
regioselectivity towards terminal unsaturations for the epoxida-
tion of squalene 10 (Scheme 3). The regiospecific preparation of
terminal squalene epoxides would be extremely useful as a precur-
sor for the synthesis of a variety of polycylic steroidic derivatives
Fig. 5. Internal (grey) to terminal (black) epoxide ratio, at different reaction times,
in the epoxidation of squalene 10.
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4. Conclusions
Mesoporous non-ordered niobium-silica solids proved to be
versatile, rather robust and efficient catalysts for the epoxidation,
in the presence of aqueous hydrogen peroxide, of a broad series
of unsaturated cyclic and terpenic compounds of interest for fine
and specialty chemistry. These heterogeneous catalysts can be pre-
pared via liquid-phase grafting of niobocene dichloride or via an
organometallic dry-impregnation approach from the same precur-
sor. High yields (up to 73%) and excellent chemoselectivity to the
desired epoxides (up to 98%) were obtained in short reaction times
(1 h). It is evident, however, that these performances can be fur-
ther optimised by tuning carefully the experimental conditions,
adapting them to the target substrate of interest. Interestingly,
Nb/SiO2 systems showed an unexpected peculiar regioselectivity
towards the epoxidation of the less electron-rich exocyclic C
C
double bond of limonene 3 and carveol 4. In addition, carvone 8,
an ␣,␤-unsaturated ketone, was efficiently epoxidised on the same
catalysts as well.
Fig. 6. Conversion profile vs. time for limonene 3 epoxidation over Nb/SiO2-DI (♦)
1
st run, (ꢂ) 2nd run, (ꢃ) 3rd run, (ꢀ) 4th run, (᭹) 5th run.
Finally, niobium-silica systems showed some advantages, in
terms of catalytic performance, with respect to widely-used and
deeply-studied titanium-silica solids. In particular, they did not
suffer, under reaction conditions here tested, from severe rapid
deactivation by water and non-anhydrous reagents can thus be
employed. They can lose, on the contrary, some catalytic activ-
ity whenever they are reused and recycled extensively in several
catalytic runs. For these reasons, mesoporous niobium(V)-silica
materials can deserve a deeper study as heterogeneous catalysts to
be applied in the selective epoxidation of bulky and richly function-
alised unsaturated molecules for high added-value applications.
3.3. Recover and recycling
The catalysts were easily recovered by filtration, washed,
calcined and reused in subsequent catalytic runs. In order to
recover the pristine activity, the organic deposits on the cata-
lyst surface had to be removed only by thorough calcination of
the spent catalyst. Such treatment converted back the yellow-
ish colour of the spent catalyst to the pure white of the fresh
one.
Detailed recovery and reuse tests were performed for limonene
Acknowledgements
epoxidation. The recycled Nb-SiO -DI catalyst underwent five cat-
2
alytic runs overall (Fig. 6). By thermogravimetric analysis (TGA),
it was possible to quantify the amount of carbonaceous products
adsorbed on the silica surface after the catalytic tests. After the
C.T. thanks the Italian Ministry of Education, University and
Research through the Project “ItalNanoNet” (prot. no. RBPR05JH2P)
for a fellowship. M.G. thanks Regione Lombardia for funding
through the Sus-ChemLombardia project (no. 3667, 29/04/2013).
NATO-Science for Peace and Security Project “Nanostructured
Materials for the Catalytic Abatement of Chemical Warfare Agents
fifth run, for Nb/SiO -DI, the carbonaceous deposits resulted to be
2
3
.2 wt.%. The catalyst kept its epoxidation activity during the first
reuse. Then, it decreased gradually to about one fourth of the ini-
tial activity at the fifth run. The selectivity to limonene epoxide,
on the contrary, was very high in all cases until the fourth run and
only in the last run some decrease in selectivity was observed. The
promising stability of Nb-silica catalysts has been already reported
in previous works [36,63], but it is important to evaluate the cat-
alyst behaviour over repeated runs, if some further exploitation at
larger scale is envisaged.
(NanoContraChem)” (no. 984481) is acknowledged.
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Please cite this article in press as: C. Tiozzo, et al., Grafted non-ordered niobium-silica materials: Versatile catalysts for the selective
epoxidation of various unsaturated fine chemicals, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.02.027