Recyclable solid catalysts for epoxidation of alkenes: Amino- and oniumsilica-immobilized [HPO4{W2O2(μ-O2)2(O2)2}]2- anion

Article abstract of DOI:10.1016/j.jcat.2007.03.018

We designed solid catalysts for liquid-phase epoxidation based on functionalized silica {triple bond, long}Si(CH₂)₃Q⁺ [Q: {single bond}NH₃, {single bond}NEt₃, {single bond}NC₅H₅, {single bond}PPh₃] and [HPO₄{W₂O₂(μ-O₂)₂(O₂)₂}]^2-. The approach that we adopted allowed us to avoid the use of chlorocarbon solvent and enabled catalyst recycling. By using supports with 4 different linking chains between the anion and silica and different surface lipophilicities, we followed their influence on catalyst activity in the epoxidation of cyclooctene and (R)-limonene by H₂O₂ in t-BuOH. All solids were active in cyclooctene epoxidation (conversion up to 100%; epoxide selectivity 100%; TOF 2-4 h^-1 anion^-1). The degree of surface coverage by organic functions was crucial for recycling performance. Catalysts with low densities of organic functions and hydrophilic surfaces were easily deactivated. End-capping improved their stability but decreased their activity. Catalysts with dense coverage of onium groups and the active site in a hydrophobic chloropropyl environment demonstrated high activity and excellent recycling stability. Less promising results were obtained in the epoxidation of (R)-limonene.

Full text of DOI:10.1016/j.jcat.2007.03.018

Journal of Catalysis 249 (2007) 1–14
www.elsevier.com/locate/jcat
Recyclable solid catalysts for epoxidation of alkenes: Amino- and
2
−
oniumsilica-immobilized [HPO {W O (μ-O ) (O ) }] anion
4
2 2
2 2 2 2
a
b,c
a
d,e,∗
T. Kovalchuk , H. Sfihi , V. Zaitsev , J. Fraissard
a
National Taras Chevchenko University, Chemistry Department, 64 Vladimirskaya str., Kiev 01033, Ukraine
b
Laboratoire de Physique Quantique, UMR CNRS 7142, ESPCI, 10 rue Vauquelin, 75231 Paris cedex 05, France
c
Département de Physique, UFR SMBH, Université Paris 13, 74 rue Marcel Cachin, 93012 Bobigny cedex, France
d
Laboratoire de Physique Quantique, ESPCI, 10 rue Vauquelin, 75231 Paris cedex 05, France
e
Université Pierre et Marie Curie, 4 place Jussieu, 75005 Paris, France
Received 3 January 2007; revised 15 March 2007; accepted 16 March 2007
Available online 17 May 2007
Abstract
+
We designed solid catalysts for liquid-phase epoxidation based on functionalized silica ≡Si(CH ) Q [Q: –NH , –NEt , –NC H , –PPh ]
2
3
3
3
5
5
3
2−
and [HPO {W O (μ-O ) (O ) }] . The approach that we adopted allowed us to avoid the use of chlorocarbon solvent and enabled catalyst
4
2
2
2 2 2 2
recycling. By using supports with 4 different linking chains between the anion and silica and different surface lipophilicities, we followed their
influence on catalyst activity in the epoxidation of cyclooctene and (R)-limonene by H O in t-BuOH. All solids were active in cyclooctene epoxi-
2
2
−
1
−1
dation (conversion up to 100%; epoxide selectivity 100%; TOF 2–4 h anion ). The degree of surface coverage by organic functions was crucial
for recycling performance. Catalysts with low densities of organic functions and hydrophilic surfaces were easily deactivated. End-capping im-
proved their stability but decreased their activity. Catalysts with dense coverage of onium groups and the active site in a hydrophobic chloropropyl
environment demonstrated high activity and excellent recycling stability. Less promising results were obtained in the epoxidation of (R)-limonene.
©
2007 Elsevier Inc. All rights reserved.
31
Keywords: HPA; Immobilized; Anion exchanger; Silica; Epoxidation; Peroxocomplex; H O ; Cyclooctene; Limonene; P NMR
2
2
1
. Introduction
Beiles et al. [14], and the binuclear complex [HPO4{W2O2(μ-
O2)2(O2)2}]2 [15]. Various other transition-metal-substituted
heteropolyoxometalates (HPAs) have been reported to be ac-
tive oxidation/epoxidation catalysts, including tetrameric tri-
−
The epoxidation of alkenes is widely used for the pro-
duction of oxiranes, valuable industrial products providing
an access to various fine chemicals. This reaction is cat-
alyzed by a number of transition metal compounds, titanium
silicalites, and titanium-containing molecular sieves [1–4]
and is performed in homogeneous or liquid biphasic me-
dia, with the latter affording easy product separation [5].
Among the most efficient catalysts suggested for alkene epox-
idation are anionic Re-, Mo-, and W-containing compounds
titanium(IV)-substituted Wells–Dawson anion [(P W Ti -
2
15
3
36−
O6 ) ]
0.5 4
[16] and sandwich-type transition-metal-substitu-
ted polyoxometalates [Na M Zn W O ], M = Ru, Mn, Zn,
x
2
3
19 68
Pd, Pt, Co, Fe, and Rh [17].
The epoxidation of alkenes in a two-phase system in-
volves an organic solution of substrate (often chlorocarbon)
and an aqueous solution of H O , although some other oxygen-
2
2
[
6–13], covered in a review by Kozhevnikov up to 1998 [9].
transfer reagents, such as alkylhydroperoxides [18,19], io-
dosylbenzene [18–20], and the environmentally and econom-
The most studied and characterized species are the so-called
Venturello complex, [PO4{W2O2(μ-O2)2(O2)2}2]3 [6,7], an
−
ically significant O [21–23], can be used as oxidants. The
2
analogous molybdenum-containing anion, first reported by
reaction proceeds in the presence of phase-transfer reagents
(PTRs) [24].
*
The need for sustainable chemistry and clean technologies
Corresponding author.
E-mail address: jfr@ccr.jussieu.fr (J. Fraissard).
has stimulated the search for new, environmentally friendly
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.03.018

2
T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
epoxidation systems. The conventional epoxidation system can
be improved by avoiding harmful chlorocarbon solvents. Water-
soluble substrates, such as oleic acid, can be epoxidized in
aqueous 35–60% hydrogen peroxide solution under catalysis by
unsuccessful [42]. Isomorphously substituted Cr-MCM-41 has
been shown to be an active recyclable catalyst in catalytic epox-
idation of aromatic olefins and for the oxidation of aromatics
with t-BuOOH in C6H6 [43].
PO4{W2O2(μ-O2)2(O2)2}2]3 without the addition of chloro-
−
3. Entrapping of HPA-anions in a porous inorganic or hy-
brid organic–inorganic matrix, via sol–gel technology. This
approach is more promising than the above-described inclusion
of redox cations, because it applies strong electrostatic or com-
plexing interactions between the functional group of silica and
the HPA. Many successful examples of incorporation of HPAs
via the sol–gel technique have been reported [44], with most of
the research in this field performed by the groups of Neumann
et al. [17–19,38,45–47] and Schroeder et al. [49]. The initial
work dealt with the incorporation of [PO4{W2O2(μ-O2)2-
[
carbons [25]. Less water-soluble substrates can be epoxidized
by aqueous H2O2 in the presence of amphiphilic carbohydrate
additives, enhancing their solubility [26].
The difficulty of catalyst recycling has inspired the develop-
ment of recyclable and suitable fixed-bed heterogeneous cata-
lysts [27,28]. Recently, the number of publications devoted to
heterogeneous oxidation catalysts has grown enormously. The
replacement of homogeneous oxidation catalysts with hetero-
geneous oxidation catalysts is not simple. The use of hetero-
geneous catalysts in liquid-phase oxidation is associated with
leaching of metal catalysts from the surface, because highly po-
lar molecules are generated in the course of the reaction [28,29].
Heterogenization of redox-active elements in solid matrices can
be achieved via several approaches, such as sol–gel methods,
including framework substitution of molecular sieves, grafting
or tethering to the inner walls of mesoporous molecular sieves,
encapsulation by the “ship-in-a-bottle” technique, ion exchange
in layered double hydroxides, and others [30–33].
(O2)2}2]4 and [WZnMn2(H2O)2–(ZnW9O34)2]
−
12−
species
into silicate xerogels modified with phenyl and quaternary am-
+
monium groups {(EtO)3Si[CH2]3N Me3} and {(MeO)3SiPh-
+
CH2N Me2[(CH2)nMe]} (n = 7, 9) [46]. This catalyst gave
efficient oxidation of nonbulky alkenes by 30% H2O2 at room
temperature in the absence of organic solvents, but decreased
with substrate bulkiness. Catalyst performance was improved
by the presence of phenyl groups.
II
12−
-
Cohen et al. [47] described [ZnWMn –(ZnW9O34)2]
2
Heterogeneous HPA-catalysts are prepared via four ap-
proaches:
containing silicates combining hydrophilic polyethylene oxide
(PEO), hydrophobic poly-propylene oxide (PPO), and cationic
+
+
1
. Deposition on porous, usually inorganic, supports, such
quaternary ammonium (Q ) groups (SiO2–PhCH2N (CH3)2-
−
as silica [30,32,33], silica–titania [34], basic oxides such as alu-
mina, MgO [35], alumina and silica–alumina [36]. Achieved
via adsorption from, for example, an organic solution of
(CH2)7CH3Cl ) prepared via the sol–gel technique. The active
species was introduced into silicate at the sol–gel stage and ei-
+
ther electrostatically bound to Q –PE(P)O–SiO2 or dissolved
II
12−
(
Bu4N)2[HPO4{W2O2(μ-O2)2(O2)2} [33] or impregnation
in PE(P)O–SiO2. Anion [ZnWMn –(ZnW9O34)2] , bound
by a Q moiety, produced an assembly with excellent recycling
2
+
with an aqueous solution of H2WO4/H2O2/H3PO4 [31]. No
strong chemical interaction is involved when silica is used,
making catalyst leaching and activity loss unavoidable, ex-
cept for basic oxides, which provide electrostatic interac-
tion. Such catalysts, e.g. 10 wt% on silica K5[PW11TiO40],
K7[PW10Ti2O40], K2[Bu4N]5[PW10Ti2O40], K7[PW10Ti2O40]
can be used in nonpolar media or in gas-phase oxidations [37].
properties, active in the epoxidation of alkenes with aqueous
30% H2O2. Compared with the electrostatically bound species,
adsorption of the anion by complexation of its quaternary am-
monium salt to tethers did not result in strong retention.
A “quasi”-covalent heterogenization of lacunary Keggin
HPA into a polymeric, siliceous, or hybrid matrix has been
proposed by Mayer and Thouvenot, who incorporated the func-
tionalized lacunary anion {γ -SiW10O36[HC=C(Me)C(O)OPr-
2
. Incorporation of “HPA precursors,” redox cations Mo and
W in the walls of microporous and mesoporous molecular sieves
or amorphous composite oxides via a sol–gel procedure in alco-
holic [38,39], aqueous [32,33,40], or hydrogen peroxide media
SiO]2O}4 into the polymeric matrix via copolymerization
with ethylmethacrylate [48]. A similar procedure for the in-
corporation of this species into a siliceous matrix with a good
dispersion was used by Schroeder et al. in 2001 [49]. Lacu-
nary γ -decatungstosilicate was reacted in acidic solution with
tetraethoxysilane with and without addition of the polyfunc-
tional linking group 1,2-bis(triethoxysilyl)ethane, followed by
condensation around polystyrene as a templating agent. Re-
moval of the polystyrene by extraction produced porous hy-
brid materials with intact HPA clusters molecularly dispersed
in the walls. These materials exhibited moderate activity in
epoxidation with anhydrous H2O2/t-BuOH and were resistant
to leaching. A similar technique was applied to the prepara-
tion of composite silica-HPA films containing several mono-
−
[
32,33,40,41], with the use of Si(OEt)4 and either alkoxides
or inorganic precursors of redox cations. The redox cation dis-
persion and activity depend on the preparation procedure, and
although some groups report good distribution of active species
and stable performance, others are less optimistic. Most studies
report high activity and selectivity in epoxidation with the use
of H2O2 or t-BuOOH, but with leaching of the cation and ho-
mogeneous reaction. Sol–gel microporous Mo-containing sili-
cas were active, selective and stable in epoxidation with the use
of t-BuOOH, if prepared using Mo(V)isopropoxide, contrast-
ing molybdenyl–acetylacetonate [39]. Metallosilicates contain-
ing TiO2, MoO3 and WO3 were active in the oxidation of
alkenes and alcohols with 30% aqueous H2O2 if prepared from
the dichlorodialkoxy, but not tetraalkoxy, derivatives, with the
best efficiency at low metal loadings [38]. Incorporation of
molybdenum into silicalite and vanadium into Y-zeolite was
n+
12−n −
)
vacant Keggin-type anions, [X W11O39](
(X = Si,
Ge, P) [50]. These films were active in the photocatalytic degra-
dation of aqueous formic acid and were claimed to not be
subject to leaching.

T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
3
The heterogenization of an active species via incorporation
in hybrid materials is a very good choice for catalyst prepa-
ration. High degrees of functionalization, strong bonding, and
activity can be obtained for the hybrid materials. However,
achieving good distribution and accessibility of active sites
often requires strict control of the synthetic procedure at all
stages.
lization of HPA by electrostatic interaction with grafted silicas,
as an extension of earlier preparations of epoxidation cata-
lysts with silica and polymers [44–47,51,53–55]. A binuclear
2−
[HPO {W O (μ-O ) (O ) }] anion (designated as “PW2”
4
2
2
2 2 2 2
in what follows) with a well-documented activity in homoge-
neous media was chosen as the active species [15,33].
Catalysts were tested in the epoxidation of cyclooctene and
(R)-limonene by H O in t-BuOH solution. The former reac-
4
. Chemical immobilization of HPA via ion exchange or co-
2
2
ordinative bonding to functional polymers and functionalized
tion allowed us to monitor the overall activity of the catalyst,
whereas we used the latter to study the accessibility of the
active site, and the correlation between their structure and reac-
tion selectivity. Using various supports (some hydrophobic), we
evaluated the influence of the linking chain between the anion
and silica [protonation and anion exchange with different onium
silicas was shown to have considerable potential. Heterogeniza-
2−
tion of the [HPO4{W2O2(μ-O2)2(O2)2}] on Amberlyst A26,
2
−1
with high porosity and moderate surface area (30 m g ), via
ion exchange was studied by Brégeault et al. [31]. Epoxidation
of (R)-limonene in a three-phase system, comprising catalyst
and an aqueous solution of H2O2 and CH2Cl2, afforded high
conversion and selectivity (epoxide yield, 68–94%) at room
temperature. Diols and diepoxides were formed as byproducts.
The reaction was nearly 3 times slower than the correspond-
ing reaction in solution using Arquad 2HT-[HPO4{W2O2(μ-
O2)2(O2)2}; this was considered evidence of catalysis occur-
ring on the surface, not in the solution. Performance of the
recycled catalyst and the extent of leaching were not discussed.
Vassilev et al. [51] described epoxidation of styrene with
hydrogen peroxide using quaternary ammonium salts of per-
oxo W(VI) complexes grafted on polymers: poly(2-N,N-
dimethylaminoethyl)methacrylate and a mixed network of
poly(oxyethylene) and poly(4-vinylpyridine). The catalytic sys-
tem comprised a water-immiscible solvent, aqueous H2O2, and
Na2WO4/H3PO4. The activity of the polymeric ammonium
salts depended on the support, the length of the hydrocarbon
radicals, and the nature of the anion. Increased reaction selec-
tivity was observed with the use of a heterogeneous catalyst
rather than a homogeneous one.
+
+
salts ≡Si(CH ) Q (with Q as –NEt , –NC H , or –PPh )],
2
3
3
5
5
3
as well as the degree of surface lipophilicity (i.e., surface pro-
portions of silanol, chloropropyl, and trimethylsilyl groups), on
catalyst performance.
2
. Experimental
2
.1. Characterization techniques
Tungsten concentrations in as-prepared samples and spent
samples were determined by X-ray fluorescence measurements.
◦
The solids were dried at 100 C for 1 h (10 wt%) and mixed
with boronic acid (90 wt%). W concentrations were determined
using a standard calibration set.
DRIFTS spectra with in situ thermal treatment were recorded
on a Bruker Vector 22 spectrometer equipped with a Harrick
diffuse reflectance cell and a DTGS detector, using KBr as
background. Samples were ground and diluted with KBr (1:10).
◦
The in situ thermal treatment was performed by heating at 60 C
Up to now, the most outstanding catalytic performance
of heterogeneous catalysts for the epoxidation of alkenes
by hydrogen peroxide was demonstrated previously [52],
using tungsten complexes immobilized on organophospho-
ryl macroligands. Catalysts were prepared via complexation
of peroxotungstic acid to phosphorus on organophosphoryl
macroligands related to phosphonic, phosphinic, or phosphoric
under vacuum for 1 h; to avoid catalyst decomposition, higher
temperatures were not used.) The spectrum of the precursor salt
(
NBu ) [HPO {W O (μ-O ) (O ) }] was recorded as non-
4 2 4 2 2 2 2 2 2
treated.
31P single-pulse excitation (SPE) magic-angle spinning
MAS) NMR measurements were made at 202.46 MHz on a
(
ꢀ
Bruker ASX 500 NMR spectrometer, operating in a static field
of 11.7 Teslas at room temperature. The air-dried sample was
placed in a 4-mm double-bearing zirconia rotor and spun at 7–
10 kHz for all measurements. Chemical shifts, including the
cited literature data, were referenced with respect to external
85% H3PO4. The recycle delay was 30 s for all measurements,
in accordance with the 31P spin-lattice relaxation times. The
number of accumulations (NS) is given on the figures. The rel-
ative intensities of the peaks were estimated by decomposition
of the spectra with mixed Gauss–Lorentz functions using WIN-
NMR software.
acid amides, R-P(O)(NRR)2, RR > P(O)NR2, and PO(NR2)3,
and introduction of these amides in hydrophobic polystyrene
and hydrophilic polymethacrylate resins. Whereas the phos-
phine oxides were shown to bind the metal insufficiently, the
phosphamides grafted onto polymethacrylates formed the best
catalysts from the standpoint of reactivity, epoxide selectivity,
and stability, allowing recycling of the catalyst. The authors
−
1
claim very good performance (TOF = 30–1200 h ), which is
better than that obtained with soluble analogues, thus represent-
ing a unique example of an immobilized metal complex with
significantly higher selectivity and activity than in solution.
In our work we describe the design and performance of
new heterogeneous epoxidation catalysts based on functional-
ized silica and HPA. The approach that we adopted allowed
us to avoid the use of chlorocarbon solvent and enabled cat-
alyst recycling, in accordance with the trend toward “green”
reagents and processes. For this, we studied the strong immobi-
Temperature-programmed desorption (TPD) analysis was
−
3
performed by heating the sample in high vacuum (P < 10 Pa)
◦
and a rate of 0.150 C/s. Evolving products were analyzed by
mass spectroscopy (MS) on a Selmi MX 7304 A instrument.
Before TPD-MS measurements, samples were activated by pre-
−
3
evacuation at 10 Pa for 20 min.

4
T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
Table 1
Characteristics of supports and catalysts. Functional (onium or aminopropyl groups) and hydrophobic (chloropropyl or trimethylsilyl) group concentrations, CL,
−
1
2
−1
3
−1
µmol g ; specific surface area, SBET, m g ; pore volume VBJH, cm g ; pore diameter DBJH, Å, C(PW ), concentration of active anions, as immobilized;
2
ꢀ
and C (PW ) after last catalysis run
2
−
1
−1
ꢀ
−1
Support
Abbreviation CL, µmol g (function)
SBET; VBJH; DBJH Catalyst
C(PW ), µmol g
C (PW ), µmol g
2
2
Propyltriethyl ammonium
EA-1
77 (EA), 143 (chloropropyl)
267.2; 0.66; 72
EA-1-PW
60
55
2
EA-1H
EA-2
77 (EA), 110 (–SiMe )
155 (EA), 535 (chloropropyl) 272.0; 0.64; 69
267.2; 0.66; 72
EA-1H-PW
55
85
50
85
3
2
EA-2-PW
2
Propylpyridinium
Py-1
253 (Py), 112 (chloropropyl)
272.4; 0.69; 77
Py-1-PW
145
130
2
Py-2
480 (Py), 450 (chloropropyl)
115 (PP), 25 (chloroethyl)
275.6; 0.61; 72
Py-2-PW
280
55
285
60
2
Alkyltriphenyl phosphonium PP-1
297.2; 0.58; 56.5
PP-1-PW
2
PP-1H
PP-2H
115 (PP), 750 (–SiMe )
100 (PP); 900 (–SiMe )
3
282.0; 0.52; 54.3
215.8; 0.50; 62
PP-1H-PW
PP-2H-PW
50
40
–
–
3
2
2
Propylammonium
Am-1
580 (amine)
240; nonporous
Am-1-PW
255
245
2
(method 1)
Am-2H
350 (amine) /820 (–SiMe )
276; 0.54; 52.0
Am-2H-PW
335
–
3
2
(method 2)
2
2
.2. Catalyst preparation
Solid (NBu4)2[HPO4{W2O2(μ-O2)2(O2)2}] was synthe-
sized as described previously [33] from (NBu4)[HSO4] and
an aqueous solution of H [HPO {W O (μ-O ) (O ) }], pre-
.2.1. Choice of the supports
Functionalized silica supports (Table 1) were described
2
4
2
2
2 2 2 2
pared as follows. A mixture of H2WO4 (2.5 g, 10 mmol) and
aqueous hydrogen peroxide 30 wt% (7 ml, 69 mmol) was re-
previously [56]. We tested as supports two aminopropyl-
silica: hydrophilic Am-1 and hydrophobic end-capped Am-
◦
acted at 60 C for 40 min. The unreacted WO3 was removed
2
H. We tested two sets of anion-exchanging supports: hy-
from solution by centrifugation. Na2HPO4 (0.71 g, 5 mmol),
dissolved in H2O (2 ml), was added to the resulting solu-
tion to form H2[HPO4{W2O2(μ-O2)2(O2)2}]. To this solution,
drophilic EA-1 and PP-1 (prepared starting from chloroalkyl-
silicas and containing residual silanols and small amounts of
chloroalkyl groups) and hydrophobic EA-1H and PP-1H (pre-
pared by end-capping of the two previous silicas), and supports
with improved hydrophobicity, EA-2, Py-1, and Py-2, which,
along with a target onium salt, contain significant amounts of
chloroalkyl groups to increase the lipophilicity of the environ-
ment of the active site. Py-2 was prepared to favor formation of
a polymerized bonded layer.
(NBu4)[HSO4] (6.78 g, 20 mmol) dissolved in water (10 ml)
was slowly added under magnetic stirring. After 10 min of stir-
ring, the white precipitate was filtered off, washed with 10 ml
of H2O and Et2O, and air dried.
2
.2.3. Immobilization of active anion on supports surface
For immobilization of PW2 on onium-modified silicas,
(
(
NBu4)2[HPO4{W2O2(μ-O2)2(O2)2}] was dissolved in MeCN
reagent grade) to give a ca. 5 × 10 M solution, purified by
2
.2.2. Preparation of active anion
An aqueous solution of H2[HPO4{W2O2(μ-O2)2(O2)2}]
−
2
centrifugation. To 10 ml of this solution (0.5 mmol of active
compound) was added an onium-modified silica (1 g, activated
in air at 120–140 C before the experiment) and the resulting
(
≈0.5 M) was prepared from WO3, H2O2, and H3PO4 as
described previously [12]. Tungstic acid (H2WO4) (2.5 g,
◦
1
1
0 mmol) was added to 30% H2O2 (7 ml, 69 mmol). After
◦
◦
h stirring at 60 C, followed by centrifugation to remove the
suspension was stirred for 6 h at 25 C. The solid was re-
unreacted H2WO4, 6 M H3PO4 (0.85 ml, 5.1 mmol) was added
to the supernatant liquid. The resulting mixture was stirred for
covered by filtration, and the anion-exchange procedure was
repeated three times with fresh portions of the solution of
(NBu4)2[HPO4{W2O2(μ-O2)2(O2)2}]. Finally, the solid was
thoroughly washed with MeCN to completely remove the phys-
3
0 min at ambient temperature to produce H2[HPO4{W2O2(μ-
O2)2(O2)2}]. This acid was used as prepared for immobilization
on aminopropylsilicas.
◦
ically adsorbed salt, then air-dried at ca. 60 C.

T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
5
Immobilization of PW2 on aminopropylsilicas was done
through one of two methods. In method 1, a suspension of
◦
aminopropylsilica (1 g, activated under vacuum at 140 C be-
fore the experiment) in 10 ml of an aqueous 0.5 M solution
of H2[HPO4{W2O2(μ-O2)2(O2)2}], (a 5- to 6-fold excess) was
◦
stirred for 24 h at 25 C. The solid material was filtered off and
washed with t-BuOH or i-PrOH or water to remove unreacted
Scheme 1. Preparation of catalysts via anion exchange: treatment of oniumsili-
acid (until the elute was neutral in pH), and the solid was dried
cas with MeCN solution of (NBu ) [HPO {W O (μ-O ) (O ) }].
4
2
4
2
2
2 2 2 2
◦
at 25 C. In method 2, a suspension of aminopropylsilica (1 g,
◦
activated under vacuum at 140 C before the experiment) in
an aqueous solution of H2[HPO4{W2O2(μ-O2)2(O2)2}] (mo-
◦
lar ratio, 1:1) was stirred for 24 h at 25 C. The solvent was
◦
removed from the reaction mixture at 25 C.
Only the freshly prepared solids were used in catalysis (kept
from several days to two weeks). Before use, the catalysts were
kept in closed vials in a refrigerator, whereas no special care
was taken to avoid contact with air. The catalysts are identi-
fied using the abbreviations for the supports (Table 1) and PW2
to designate the anion. Sample compositions are given in Ta-
ble 1.
Scheme 2. Preparation of catalysts via protonation: treatment of amino-silicas
with aqueous solution of H [HPO {W O (μ-O ) (O ) }].
2
4
2
2
2 2 2 2
of the formation of these species is high for silica supports with
dense coverage (samples EA-2, Py-1, and Py-2). For exam-
ple, sample Py-2-PW consists of the species propyl chloride
2
−
1
≡
SiC3H6Cl (480 µmol g ), ≡Si–C3H6Py–[PW2]-NBu4 (B,
−
1
−1
1
10 µmol g ), and (≡SiC3H6Py)2[PW2] (C, 170 µmol g ).
For samples with a low density of grafted groups (EA-1, EA-
H, and PP-1, PP-1H), formation of a 2:1 species is doubtful.
2
.3. Catalysis testing
1
Epoxidation was performed with an anhydrous solution of
In this case, the 2:1 cation:anion ratio suggests that under the
experimental conditions, some of the grafted onium groups
were not involved in ion exchange.
hydrogen peroxide in t-BuOH, prepared by drying a mixture of
50 ml of t-BuOH and 45 ml of 30 wt% H2O2 with MgSO4
1
(
≈30 g) for 24 h [12]. To a mixture of oxidant solution (5 ml
or ca. 12 mmol of H2O2) and a catalyst (generally 0.3 g),
3
.1.2. PW anion, immobilised on amino silicas
2
placed in a Schlenk vessel, was added 0.5 ml of cis-cyclooctene
Immobilization of HPA is best performed in organic sol-
(
3.73 mmol) or 0.3 ml of (R)-limonene (2.8 mmol). The sus-
◦
vents, because the high polarity of water favors its leaching and
hydrolysis. Because the acid H2[HPO4{W2O2(μ-O2)2(O2)2}],
designated as H2[PW2], is available in aqueous solution only,
a freshly prepared concentrated solution was used for im-
mobilization to reduce the risk of its decomposition. Cata-
pension was stirred at 25 or 60 C in the tightly closed Schlenk
vessel for 24 h (or as indicated in the table). Conversion versus
reaction time was followed by gas chromatographic analysis of
the organic phase. These experiments were performed using a
Delsi 30 gas chromatograph equipped with a 0.25-mm × 50-m
OV 1707 capillary column and a flame ionization detector,
linked to a Delsi Enica 10 electronic integrator using the fol-
lyst Am-1-PW was prepared following the equilibrium ad-
2
sorption method (1) from a solution containing an excess of
◦
2 2
the H [PW ] reagent. According to the concentrations of the
lowing parameters: injector temperature, 180 C; detector tem-
−
4
−1
◦
◦
[PW ] anion (2.55×10 molg ) and of aminopropyl groups
perature, 200 C; oven temperature, 70–200 C; and a linear
temperature gradient. For the reuse studies, the spent catalysts
were recovered by centrifugation, washed several times with
2
−
4
−1
(
5.80 × 10 molg ), this sample consists of mainly neutral
salt, designated type E (Scheme 2). Catalyst Am-2H-PW was
2
◦
prepared via impregnation with the calculated amount of acid
warm t-BuOH (ca. 15–20 ml), and dried in air at 60 C.
solution (method 2) to produce an acid salt, designated D (ca.
−
4
−1
1
:1 ratio of H2[PW2] to amino groups; 3.35 × 10 molg
3
3
3
. Results and discussion
of anion). Washing the prepared solids with t-BuOH afforded
active catalysts; thus, we followed this approach to create cata-
lyst samples and reactivate spent samples. Washing as-prepared
.1. Composition of catalysts based on elemental analysis
catalysts (samples Am-1-PW and Am-2H-PW ) with water
.1.1. PW anion, immobilised on onium silicas
2
2
2
resulted in poisoning of the active sites and absence of catalytic
activity.
Immobilization of the PW2 anion by treatment of onium
silicas with MeCN solution of (NBu4)2[HPO4{W2O2(μ-O2)2-
O2)2}], designated salt A, affords a mixture of two types of
(
species, designated salt B [(≡SiR–Q–(PW2)–(NBu4)] and salt
C [((≡SiR–Q)2(PW2)] (Scheme 1). The ratio of the concentra-
tion of the onium cation (CL) to that of the immobilized oxoper-
3.2. Catalyst structure based on DRIFTS study
According to previously published work [15,33], the precur-
oxopolyanions [C(PW )] for all samples is in the range 1.5:1–
sor (NBu4)2[HPO4{W2O2(μ-O2)2(O2)2}] is characterized by
2
−
1
2
:1 on the basis of the elemental analysis (wt% of C, H, N, Cl, P,
the following IR bands at (in cm ): 1018 and 995 (ν PO4);
−
1
and W), suggesting the prevalence of species C. The probability
962 (ν W=O); 883, 852–854, and 843–847 cm (ν O–O of

6
T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
Fig. 1. IR spectra of [n-(C H ) N] [(HPO ){W O (μ-O ) (O ) }] (1);
4
9 4
2
4
2
2
2 2 2 2
+
precursor silica support Py-2 [≡Si(CH ) N C H (2); catalyst Py-2-PW
2
3
5
5
2
+
[
≡Si(CH ) N C H –PW ] (3); precursor silica support EA-2 [≡Si(CH ) -
2
3
5
5
2
2 3
+
+
N Et ] (4); catalyst EA-2–PW [≡Si(CH ) N Et –PW ] (5); starting sil-
3
2
2 3
3
2
ica (6).
the peroxo ligands); 616(w) (ν W2–O)s, 569 [or 573(m) and
62(m)] (νas W(O2)), and 517 (νsym W(O)2). The IR spec-
5
Fig. 2. Thermal decomposition of catalyst Py-2-PW , followed by mass spec-
2
trometry (I–peak intensity, arbitrary units).
trum reported in this work (Fig. 1) confirms the precursor
structure [1021(s), 996(m), 964(s), 883(m), 853(m), 844(s),
−
1
5
(
75(s), and 519(s) cm ]. As reported previously for solid
NBu4)2[HPO4{W2O2(μ-O2)2(O2)2}], the spectrum also con-
tains bands at 1085(s), 1053(m), 975(w), 800(w), 739(m),
−
1
6
48(s), 624(w), 549(w), 488(w), and 457(w) cm , which may
be indicative of the parallel formation of a PW4 anion [15].
The DRIFTS spectra of samples with immobilized PW2
species are similar to those of the supports (Fig. 1). They give
little information in the region of W–O vibrations, because of
the relatively small concentration of PW2 and a strong absorp-
tion of the silica lattice (Fig. 1, spectra 3, 5). In the spectrum
of the Py-2-PW with the greatest amount of grafted anions
2
−
1
(
Fig. 1, spectrum 3), a shoulder appears at around 880 cm
that can be easily assigned to the O–O vibration of the peroxo
ligand.
In the spectra of all oniumsilica-based catalysts, new C–H
−
1
absorption bands (2961, 2935, 2874, 1469, and 1381 cm
Fig. 1, compare spectra 3 and 1, and 5 and 4]) are observed.
[
These bands correspond to the C–H vibration mode for the
+
(
NBu4) cation (Fig. 1, compare spectrum 1 with 3 and 5),
+
providing evidence for the presence of (NBu4) on the surface.
Because no physically adsorbed salt can be present as a result
of the preparation procedure applied, this cation is included in
the structure of the ≡SiR-Q-(PW )–(NBu ) species (active site
Scheme 3. Scheme of thermal decomposition of Py-2-PW .
2
2
4
(m/z 28), H O (m/z 18), pyridine (m/z 79, 80), aziridine (m/z
2
of type B; Scheme 1).
43), butene (m/z 56), propene (m/z 42, 41), ethylene (m/z 28,
accompanied by a peak with m/z 27).
◦
◦
3
.3. Thermal stability by TPD-MS
In stage 1 (160–240 C, a maximum around 200 C), si-
multaneous evolution of peaks of pyridine (m/z 79), aziridine
(m/z 43), butene (m/z 56, accompanied by a peak of propene
fragment with m/z 42), H2O (m/z 18) provides evidence for
the decomposition of ≡Si–C3H6Py–PW2–NBu4 (B species)
(Scheme 3-1).
The thermal stability of one sample, Py-2-PW , was moni-
2
tored by MS (Fig. 2). Decomposition of the catalyst proceeds in
four main stages, accompanied by the evolution of the follow-
ing fragments: HCl (m/z 36, 38, and Cl with m/z 35, 37), CO

T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
7
◦
In stage 2 (200–320 C), two processes can be distinguished:
3.04 ppm (PW3), and 6.15 ppm (PW4). A CDCl3 solution used
in epoxidation and containing the oxoperoxophosphatotungstic
system showed several peaks in the 31P NMR spectrum; two
of these, at 0.5 and 3.5 ppm, were assigned to PW and PW ,
decomposition of species C, (≡SiC3H6Py)2PW2 (evidenced by
the simultaneous evolution of the peaks of water, which is a
part of PW2 anion [m/z 18] and pyridine [m/z 79]), and de-
composition of propyl chloride (supported by the evolution of
the peak of HCl [m/z 35]) (Scheme 3-2 and 3-3). The pyri-
2
4
respectively [24].
The chemical shifts are also affected by immobilization on
◦
31
dine peak (maximum at ca. 300 C) appears somewhat earlier
than that of propene (maximum at 360 C; the very first peak of
solid supports, and the assignment becomes more complex.
P
◦
SPE-MAS NMR spectra recorded for some of the catalysts pre-
pared in this work are shown in Figs. 3 and 4.
propene is a fragment of butene), indicating that species C and
B can first transform into silica-bonded fragments, ≡Si–C3H5,
which subsequently decompose. HCl is evolved in two steps
3.4.1. Am-1-PW2
◦
(
maxima at 280 and 440 C), and these two maxima precede
those of propene (340 and 540 C). Therefore, we assume that
The corresponding 31P SPE-MAS NMR spectrum contains
◦
two broad peaks at ca. 6 and −8 ppm (Fig. 3a). The peak at
ca. 6 ppm (ꢀν1/2 600 Hz, 64% of the total intensity) can be
tentatively assigned to the PW2 anions, taking into account the
fact that the solution contains mainly PW2 species. Compar-
ison of our data with the values reported for the PW2 anion
the propyl chloride is first transformed into ≡Si–C3H5.
◦
◦
In stage 3 (280–400 C, maximum at 340 C) and stage 4
◦
◦
(
480–600 C, maximum at 540 C), decomposition of ≡Si–
C3H5 fragments occurs, accompanied by evolution of propene
(
a peak with m/z 42) (Scheme 3-4). The need for ≡Si–OH
(
2.95 ppm in MeCN-CDCl3 solution [15], 0.9 ppm in the solid
to react with ≡Si–C3H5 affording ≡Si–O–Si≡ explains why
some of the ≡Si–C3H5 groups decompose at higher tempera-
tures, when silanol migration is favored.
state [33], and 0 ppm adsorbed on SiO2 [33]) indicates an up-
field shift of ca. 3 ppm. This shift probably results from the
strong chemical interactions of the anion with the support sur-
face. This conclusion is not definite, however. Such a deviation
from the reported data also can suggest the formation of a more
stable PW4 species on the support surface as a result of the im-
mobilization procedure. For the latter, the reported values are
scattered between 3.1 to 6.15 ppm.
◦
At around 520 C, peaks of CO (m/z 28) and H2O (m/z
8) also appear (Fig. 2). These peaks correspond to the in-
1
depth decomposition of an organic residue, with the partic-
ipation of WO3 formed from the immobilized anion and to
water evolution due to silanol condensation. Therefore, the
TPD-MS study shows evidence of both of the structures pro-
posed for the onium-grafted salts, ≡Si–C3H6Py–PW2–NBu4
and (≡SiC3H6Py)2PW2. The thermal stability of these salts is
sufficiently high to allow their use in epoxidation reactions; de-
composition starts at about 160 C in the former and at around
00 C in the latter. Obviously, a disturbance of the structure of
the anion and loss of peroxoligands, occurring at lower temper-
ature, would not be detected by this technique.
In addition, the main peak contains a shoulder at around
2
ppm (600 Hz, 11% of the whole intensity), which can origi-
nate from other polynuclear oxoperoxo species (PW2 or PW1)
produced in the course of immobilization. A second peak at
around −8 ppm, (ꢀν1/2 1700 Hz, 25% of the whole intensity)
◦
◦
3
(Fig. 3a) is too upfield and thus cannot be assigned to any of the
peroxoligand-containing complexes. This peak indicates for-
mation of other polynuclear oxotungstophosphate complexes
with a greater number of tungsten atoms (Keggin-derived la-
cunary oxotunstophosphate complexes, PWx, x > 4), and no
peroxoligands. The 31P SPE-MAS NMR spectrum of sample
3
.4. Structure of the anion in immobilised catalysts: 31P MAS
NMR results
Am-2H-PW (not shown here) is similar to that of Am-1-PW .
It is well established that several oxoperoxoanions with the
general formula [PWx], (x = 1–4) are formed in aqueous per-
oxide solutions of H3PW12O40, used as a homogeneous epox-
idation catalyst. In the presence of PTRs, the organic phase
contains a solution of the resultant onium salts. The 31P NMR
chemical shifts of these salts are affected by solvent, ion pair-
ing, the degree of protonation, and the nature of the cation.
Consequently, the values reported by different authors can vary
to some extent. For the isolated salt with the Venturello an-
ion (PW4), the following chemical shifts have been reported:
2
2
The observed line width may be indicative of a “pseudoliquid
phase” behavior of the immobilized anion, as suggested for the
adsorbed species in [33]. The mobility of HPA-anion is favored
by the hydrophilic surface of the support.
The spectrum of Am-1-PW2 thoroughly washed with water
(
Fig. 3b) shows two peaks. The first peak, due to the grafted
PW2 or PW4 oxoperoxo species, is observed at ≈6 ppm (ꢀν1/2
00 Hz). Its relative intensity is reduced to 44% after an aque-
9
ous wash (Fig. 3b), compared with 64% in the spectrum of
the unwashed sample (Fig. 3a). The second peak is observed
at around −2 ppm (ꢀν1/2 2600 Hz, 56%, overlapped with the
peak at ≈6 ppm). As in the spectrum of initial Am-1-PW2,
this peak arises mainly from the Keggin-derived lacunary com-
plexes (PWx), which do not bear peroxo ligands. Its relative in-
tensity increases from 25% in the initial Am-1-PW2 to 56% in
the same sample thoroughly washed with water. If irreversible,
such a loss of peroxoligands could be at the origin of the lack
of catalytic activity of the latter sample.
3
.16 ppm (Arquad 1 cation, solution in CDCl3), 4.13 (tetra-n-
hexylammonium cation, solution in CDCl3), and 3.8 ppm (the
same cation, solution in CD3CN) [57]. For the isolated salt
with the PW2 anion, the following chemical shifts were found:
0
0
.9 ppm (tetra-n-hexylammonium salt in solid state) [33],
.5 ppm (Arquad-1 salt in the solid state) [15] and 2.95 ppm
(
[n-(C4H9)4N] cation, solution in MeCN/CDCl3 [15]. For a so-
+
lution containing a mixture of anions and the [(nC10H21)4N]
cation in CD3CN, Duncan et al. [57] reported 1.50 ppm (PW2),

8
T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
ponents (Fig. 4b) of quasi-equal intensities at 6, −0.9, −8,
and −13 ppm (ꢀν1/2 1000–1300 Hz). These are assigned to
oxoperoxoditungstophosphate anions PW2, phosphate groups
PO4, and other tungstophosphate anions (PWx, x > 4), respec-
tively. These lines are somewhat broader than those observed in
the spectra of aminopropylsilica-immobilized species, suggest-
ing a stronger fixation of these species on the surface and/or
relatively lower symmetry of the grafted complexes with PW2
anions.
(a)
3.4.3. PP-1-PW2
The corresponding 31P SPE-MAS spectrum (Fig. 4a) pres-
ents a strong peak at 25 ppm arising from the grafted alkyltriph-
enylphosphonium cations; and a broad peak of low intensity.
The latter was perfectly fitted with three components (Fig. 4a),
attributed to different immobilized tungstophosphates species:
PW2, PO4, and PWx. These are located at 3, 1, and −1 ppm
and have relative intensities of 40, 20, and 40%, respectively.
3
.4.4. Influence of the immobilisation on the structure of
(b)
immobilised PW2-species
3
1
To understand the reason for the large number of peaks
observed in the 31P SPE-MAS NMR spectra of immobilized
catalysts and their different chemical shifts, several factors, in-
cluding the different degree of protonation and the number of
peroxo bridges, should be considered. However, we consider
the main reason to be the formation from the initial PW2 anion
in solution of various polynuclear species PWx (x = 1, 3, 4, and
higher) and their subsequent immobilization. For the system
studied here, 31P MAS NMR spectra suggest that immobiliza-
tion by means of both precursors, either an aqueous solution
of H2[(HPO4){W2O2(μ-O2)2(O2)2}] or an acetonitrile solu-
tion of [n-(C4H9)4N]2[(HPO4){W2O2(μ-O2)2(O2)2}], yields a
mixture of grafted PWx species (x = 1–4).
Fig. 3.
P SPE-MAS NMR spectra of (a) Am-1-PW , “as-prepared”
2
NS = 304; (b) Am-1-PW , thoroughly washed with water, NS = 16 (at
2
4
000 Hz); asterisks denote spinning side-bands. Experimental spectrum (—),
fitted peaks (- - -), sum of the fitted lines (···).
For aminopropylsilica-based catalysts, these species are
mostly the target PW /PW (their attribution cannot be de-
(a)
2
4
finitive), whereas for oniumsilica, there is a mixture of equal
proportions of the target anions PW2/PW4 together with other
anionic species, including phosphate ions and PWx (x =
1
, 3). Important parts of these species (peaks at −8 ppm for
the aminopropylsilica-based catalyst and −13 ppm for the
oniumsilica-based catalyst) are attributed to the Keggin-derived
lacunary oxotungstophosphate complexes with fewer peroxo
ligands or without such ligands. These results are not surprising
if one considers the equilibrium existing between PWx anions
in aqueous peroxide solution [57]. On contact with the support,
the equilibrium can be changed in favor of the polynuclear com-
plexes with a greater number of tungsten atoms and larger size,
because of the stronger interaction between these anions with
“soft” onium cations. As a result, these anions may be preferred
for adsorption by the support.
(
b)
3
1
Fig. 4.
P SPE-MAS NMR spectra of oniumsilica with immobilized
[
(
n-(C H ) N] [(HPO ){W O (μ-O ) (O ) }]: (a) PP-1-PW , NS = 3200;
4
9 4
2
4
2
2
2 2 2 2
2
b) Py-1-PW , NS = 64. Experimental spectrum (—), fitted peaks (- - -), sum
2
of the fitted lines (···).
One previously reported experiment of immobilization of
oxoperoxo anions on siliceous supports gave results consistent
with those reported herein. Hoegarts et al. attached the PWx
3
.4.2. Py-1-PW2
The corresponding 31P SPE-MAS spectrum (Fig. 4b) pres-
ents a broad peak and, in contrast to the previously discussed
spectra (Fig. 3a), intense spinning side bands. Decomposition
of the central line allowed us to distinguish four distinct com-
anion (presumably a PW species) to the surface of MCM-
2
2−
41 with grafted amino groups by reaction with WO4 /H PO
3
4
solution and, as supposed by the author, formation of a phos-

T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
9
phoramide linkage. The resulting catalyst sample displayed
three 31P NMR peaks, at 6, 0, and −10 ppm [53]. These values
are very close to those observed in the present work. In contrast
to these and our results, [n-(C H ) N] [(HPO ){W O (μ-
O2)2(O2)2}], physically adsorbed on silica, displayed a single
peak at 0 ppm, which could be indicative of adsorption of
phosphate anion [33]. Even more dissimilar results were ob-
activity of the catalysts. Epoxidation of a bulkier substrate, (R)-
limonene (Scheme 4.2), allowed us to test the accessibility of
the active sites. The selectivity of the latter reaction is very
sensitive to the Brønsted acidity of catalysts. Indeed, in acidic
media epoxide, a is hydrolyzed to a mixture of diastereomeric
diols c; moreover, in the case of overoxidation, diepoxides b can
be obtained. For this substrate, the two immobilization routes
[adsorption of H2PW2 and ion exchange with (NBu4)2PW2]
gave very different results in terms of activity and selectiv-
ity.
4
9 4
2
4
2
2
tained when heterogenization of PW anions was performed
2
by sol–gel insertion of its precursor into MCM-41. Such im-
mobilization resulted in the formation of at least three species
with chemical shifts of ca. 0, −10, and −20 ppm [33]. We
assume that in this case, immobilization resulted in a mix-
4.1. Epoxidation of cyclooctene
ture containing not only PW (PW or PW ) species, but also
x
2
4
several polynuclear oxotungstophosphates without peroxo lig-
ands.
4.1.1. Activity of PW2 catalysts immobilized on oniumsilica
Results of cyclooctene epoxidation with the use of the
oniumsilica-based catalysts are presented in Table 2. To com-
pare the samples, average turnover frequencies (TOFs) and
turnover numbers (TONs) were calculated as the number of
4
. Catalysis results and discussion
moles of substrate converted per mole of PW anion per hour
2
Epoxidation of cyclooctene and (R)-limonene is described in
Scheme 4. Epoxidation of cis-cyclooctene to 1,2-epoxycyclo-
octene (Scheme 4.1) is a model reaction that reflects the general
(
TOF) and per total reaction time (TON).
In the first run, most of the immobilized catalysts exhib-
ited high activity and selectivity toward epoxidation of cy-
clooctene with H O /t-BuOH. Complete (100%) conversion
2
2
was achieved with EA-1-PW and Py-2-PW . High conversion
2
2
was obtained with EA-2-PW (50%) and Py-1-PW (66%).
2
2
Other catalysts turned out to be less active. To make experimen-
tal conditions regular, we used equal catalyst weights; however,
this resulted in unequal PW /substrate ratios (0.5–2.5%), be-
2
cause the solid catalysts have different concentrations of PW2
anion per gram of catalyst). Therefore, we compare them on the
basis of average TOF, per anion per hour.
Scheme 4. Epoxidation of cyclooctene (1); epoxidation of (R)-limonene (2).
Table 2
◦
Catalytic activity of oniumsilica-based catalysts in epoxidation of cyclooctene. Catalysis conditions: 60 C, 3.73 mmol of cyclooctene, 12 mmol of H O /in t-BuOH,
2
2
0
.3 g of catalyst (substrate/“PW ” ratio varied depending on the catalyst, 50–250), reaction time 24 h. In all cases, selectivity was around 100%
2
ꢀ
−1
−1
TON, ac
N
C(PW ), ± 5
C (PW ), ± 5
Run
Y, %
TOF, h
2
2
a
EA-1-PW
60
55
1
2
3
100 (6 h)
34
7
35
3.0
0.6
175
72
16
2
EA-1H-PW
55
85
50
85
1
2
12
19
1.2
1.8
27
43
2
EA-2-PW
1
2
49
50
2.6
3.2
74
76
2
Py-1-PW
145
130
1
2
3
66
78
75
2.6
3.1
2.7
63
75
65
2
a
Py-2-PW
280
55
285
60
1
2
100 (5 h)
8.8
4.4
44
44
2
a
100 (14 h)
PP-1-PW
1
2
3
60
34
23
5.6
3.2
2.2
135
77
52
2
PP-1H-PW
PP-2H-PW
50
40
–
–
1
2
37
25
3.8
2.6
92
62
2
1
2
12
17
1.5
2.2
37
54
2
−
1
ꢀ
−1
Note. N, catalyst reference; C(PW ), initial concentration of anions, µmol g ; C (PW ), concentration in spent sample after last run, µmol g ; Y , yield of
2
2
epoxycyclooctane in 24 h; TOF, average turnover frequency per hour per anion; TON, turnover number (total yield) per immobilized anion.
a
Time of complete conversion when the reaction is over before 24 h.

10
T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
Table 3
Catalytic activity of Am-1-PW in epoxidation of cyclooctene. Catalysis con-
2
ditions: Tests were made with 3.73 mmol of cyclooctene and 12 mmol of H O .
2
2
The weight of catalyst varied from 0.15 to 0.095 g, giving substrate/PW ratios
2
(run): 89 (first), 155 (second), 190 (third)
◦
−1
Run Substrate/“PW ” ratio t, h T , C Y, %
TOF, h
TON
2
1
2
2
3
89
155
155
190
24
24
5
22
22
60
60
59.5
24.0
51.0
22.0
2.3
1.9
9.0
8.2
57
40
a
40 + 45
5
41
Y , yield of cycloepoxyoctane; TOF, turnover frequency; TON, turnover num-
ber. In all cases, the selectivity was close to 100%.
a
Run 2 is continuation of Run 2 after the temperature was increased.
Scheme 5. Suggested pathways for deactivation and regeneration of active
species in homogeneous and heterogeneous reactions.
Comparison of the oniumsilica-based catalysts with the
same cation but prepared by different silanization procedures
fore and after catalysis run (last run, Table 2) suggests that the
marked reduction of the activity (observed for catalysts EA-1-
(
e.g., series EA-1-PW , EA-1H-PW , EA-2-PW ; series PP-
2 2 2
1
-PW , PP-1H-PW , PP-2H-PW ; and series Py-1-PW ,
2 2 2 2
PW and PP-1-PW ) should be attributed to some other factor
2
2
Py-2-PW ), and with different onium salts but made via a sim-
2
than the simple leaching of active species, even though a slight
decrease in tungsten concentration occurred.
Several groups have studied the Venturello–Ishii system, but
little is known about the mechanism. Study of the kinetics of
ilar silanization route (e.g., series EA-1-PW , PP-1-PW and
2
2
series EA-2-PW , Py-1-PW ), suggests the following conclu-
2
2
sions:
a) The structure of the onium salt does not determine cata-
(
epoxidation by a PW complex by Hill et al. [57] revealed that
4
lyst activity (expressed as TOF value of the immobilized PW2
the reaction is first-order in both alkene and [PO4{W2O2(μ-
anion). Most of the catalysts (EA-1H-PW , EA-2-PW , Py-1-
2
2
3−
O2)2(O2)2}2] . The reaction follows the rate equation ν0 = k
PW , PP-1-PW PP-1H-PW , PP-2H-PW ) exhibit consis-
2
2,
2
2
3−
−1
[[PO4{W2O2(μ-O2)2(O2)2}2] ] [alkene].
Epoxidation, starting from the Ishii precursor, (PW12O40)
begins with the generation of [PO4{W2O2(μ-O2)2(O2)2}2]
tent TOF (1.5–5.6 h ), especially in the second and third runs
2.2–3.3 h ), with the exception of much greater values for
EA-1-PW , Py-2-PW (35 and 8.8 h ; Table 3).
3
−
−
−
1
,
,
(
3
−
1
2
2
which transfers oxygen to alkene, and is followed by the for-
mation of subsequent peroxo PW species (predominantly PW
(
b) The method of preparation of the siliceous support and
x
4
the composition of the grafted layer (presence of hydrophilic
silanols and hydrophobic –SiMe3 and propylchloride groups;
see Table 1) has a considerable effect on the catalytic activ-
ity (Table 2). In particular, the performance of the recycled
catalysts differs significantly. Supports EA-1 and PP-1 have a
small concentration of grafted onium cations and a hydrophilic
support surface [56]. The corresponding catalysts EA-1-PW2,
and PW3 complexes) rapidly regenerated into [PO4{W2O2(μ-
O2)2(O2)2}2]3 by the reaction with H2O2 [9]. Species [PO4-
−
{
W2O2(μ-O2)2(O2)2}2]3 is proposed as the kinetically most
−
significant epoxidizing agent.
In a homogeneous system and in the presence of an oxidant
solution, this process is reversible, with transformation between
PW complexes and reconstruction of the active species pro-
x
PP-1-PW are rapidly deactivated. It must be emphasized that
2
−
1
ceeding readily (Scheme 5). In contrast to what occurs in a
homogeneous system, the surface of the catalyst support can
EA-1-PW displays very high TOF (35 h ) in the first run,
2
−
1
but is quickly deactivated (TOF 3.0 and 0.6 h in subsequent
runs). These supports, when end-capped, were used for prepara-
2
−
entrap the subsequent PW species and also inactivate (PO )
x
4
and (WO4)2 anions via adsorption on silanols. Strong reten-
−
tion of samples EA-1H-PW , PP-1H-PW , and PP-2H-PW .
2
2
2
tion by polar silanols can prevent reconstruction of the active
End-capping gave catalysts that were less active but relatively
more stable in the consecutive runs. Supports EA-2 and Py-1
species by reaction with H O and result in a loss of activ-
2
2
ity.
Fast deactivation of active species in the Venturello–Ishii
system is one of its major drawbacks. Hill et al. [57] pointed
(
obtained by direct silanization with a mixture of target onium-
silane with chloropropyltriethoxysilane) had a higher degree of
surface coverage and a large proportion of chloropropyl groups.
Catalysts EA-2-PW , and Py-1-PW based on these supports
out the unavoidable drop in catalytic activity of PW -based
x
2
2
complexes in the Venturello–Ischii system. 31P NMR spec-
troscopy revealed that the catalyst is irreversibly inactivated
by the epoxide produced. A maximum TON of 500 was found
are quite stable in two or three consecutive runs (TOF 2.6–
−
1
−1
3
.1 h for Py-1-PW and 2.6–3.2 h for EA-2-PW2).
2
for the [PO4{W2O2(μ-O2)2(O2)2}2]3 anion, whereas the re-
action rate decreased even before the catalytic activity disap-
peared.
−
4
.1.2. Recycling performance. Influence of structure of
catalyst supports
As follows from the IR study, absorption bands due to the
grafted organic moieties of catalysts are not altered for the
spent samples (after 3 catalysis cycles). This demonstrates suf-
ficient stability of the bonded organic layer toward the oxidant
solution. Analysis of the W concentration for the catalysts be-
We believe that particularly in the case of heterogeneous
catalysts, the degree of such poisoning can be influenced by
the hydrophilic/hydrophobic nature of the silica and the struc-
ture of the interfacial layer. High surface polarity may favor

T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
11
Table 4
Catalytic activity of oniumsilica- and aminopropylsilica-based catalysts in epoxidation of limonene. Catalysis conditions: Tests were made at 22 or 60 C using
◦
2
.8 mmol of limonene and 12 mmol of H O ; the substrate/W ratio was 113 (EA-2-PW ), 33 (Py-2-PW ), 29 (Am-2H-PW )
2 2 2
2 2 2
◦
−1
−1
TON, ac
Catalyst
Run
T , C
C, %
t, h
S, %
TOF, h
EA-2-PW
1
1
2
22
60
60
0
33
9
24
48
24
–
88
78
0
1.8
0.5
0
60
12
2
Py-2-PW
1
2
3
22
22
22
86
50
12
22
25
24
81 (100 in 1 h)
93
94
1.3
0.66
0.2
29
17
4
2
a
Am-2H-PW
1
22
22
36
31
0.3
0.6
4
32
40
14
–
–
2
a
2
Note. Run, number of catalysis run; t, time of reaction; C, conversion of limonene; S, selectivity to epoxide; Y , yield of epoxide; TOF, average turnover frequency;
TON, turnover number.
a
Reaction was monitored over a short period of time, so TON was not calculated.
stronger retention of polar epoxides near the active sites and
lead to faster catalyst deactivation.
4.2. Epoxidation of (R)-limonene with PW2-aminopropylsilica
and PW2-oniumsilica
Because the hydrophilic surface of PP-1-PW and EA-1-
2
PW can contribute to the poisoning by adsorption of epoxides
To test epoxidation of a more sterically demanding sub-
strate, (R)-limonene, two most active onium-based catalysts,
Py-2-PW2 and EA-2-PW2, were chosen. Use of these catalysts
for epoxidation of the limonene gave low activity (Table 4).
Py-2-PW2 at room temperature resulted in 86, 50, and 12%
conversion in 3 consecutive runs, with 80–90% selectivity. Cat-
alyst EA-2-PW2 was not active at room temperature, although
2
and deactivated species, reducing the concentration of silanols
should result in greater stability of PW anions toward poi-
2
soning. Indeed, EA-1H-PW and PP-1H-PW , prepared from
2
2
end-capped EA-1 and PP-1 and PP-2H-PW , displayed more
2
similar TOF values in two runs, though the activity of the ac-
tive site is lowered by the end-capping [EA-1H-PW (TOFs,
2
−
1
−1
◦
1
3
1
–
.2 h in run 1 and 1.8 h in run 2), PP-1H-PW (TOFs,
it catalyzed the reaction at 60 C. The conversion reached 33%
2
−
1
−1
.8 h in run 1 and 2.6 h in run 2), and PP-2H-PW (TOFs,
in 24 h. The selectivity to epoxides fell from 88% (after 1 h)
to 76% (after 24 h). A subsequent test done with the spent
catalyst gave only 9% conversion. Thus, the activity of both cat-
alysts decreased considerably when they were reused (TOF fell
from around 1.8 to around 0.5 and from around 1.3 to around
2
−1
−1
.5 h in run 1 and 2.2 h in run 2). Possibly, end-capping
SiMe3 fragments on the catalyst surface reduce the accessibil-
ity of the active sites to some degree.
Very stable performance, together with high activity, was
−
1
−1
displayed by the catalysts EA-2-PW , (TOF 2.6–3.2 h ) and
0.2 h ).
In both cases of (R)-limonene oxidation, selectivity to epox-
2
−
1
Py-1-PW (TOF 2.6–3.1 h ), with high grafted layer den-
2
sities, prepared by one-step silylation with a mixture of two
silanes. The higher density and the increased lipophilicity of
the grafted layer, due to the grafted propylchloride, may be
responsible for the stabilization of the catalyst performance.
Consequently, the variations in the performance of the immo-
bilized catalysts with different grafted layer structures can be
ascribed to inherent deactivation of active species, which hin-
ders their regeneration due to silanols, and to poisoning and
different resistance toward poisoning with epoxide.
ide was 78–94%, and highly reactive nucleophilic diols were
formed as side products. This suggests that strong deactivation
of catalyst can result from diol adsorption on the catalyst sur-
face. Marked catalyst poisoning by diols is in contrast to what
was observed with cyclooctene as substrate. Moreover, the po-
lar diols can induce transfer of the active species from the sur-
face to the solution. According to elemental analysis, leaching
of active species, not observed in cyclooctene epoxidation, was
around 15%.
Catalyst Am-2H-PW (prepared by method 2 to obtain acid
2
4
.1.3. Activity of PW -aminopropylsilica catalysts
salt, 1:1 amine-to-acid ratio) was active at room temperature:
2
−
1
The results obtained with PW2-aminopropylsilica catalysts
(Table 4; 36% conversion in 0.3 h, TOF 40 h ); however, the
selectivity to epoxide was poor (4%). The relatively high activ-
ity (approaching homogeneous conditions) and low selectivity
of this sample can be explained by the high residual acidity
of the supported anion and its strong leaching into the solu-
tion. Apparently, the reaction is homogeneous in the first run.
◦
are presented in Table 3. At 22 C, Am-1-PW exhibited high
selectivity (100%) and a TOF, similar to the oniumsilica-based
catalysts (1.9–2.3 h ). Its activity was stable in the two sub-
sequent runs with TOF 2.3 h (run 1) and 1.9 h (run 2).
2
−1
−
1
−1
◦
Increasing the temperature to 60 C using the reaction mixture
−
1
of the second run increased the TOF to 9 h , and this value
was not significantly changed in the subsequent run (run 3)
using spent catalyst. The concentration of PW2 anions in the as-
In the second run, Am-2H-PW gave lower activity (Table 4;
2
−
1
31% limonene conversion in 0.6 h, TOF 14 h ) but better se-
lectivity (32%). Its activity was close to that obtained in the
epoxidation of cyclooctene (sample Am-1-PW ). Am-1-PW
−
4
−1
prepared sample was 2.6–2.8 × 10 molg ; after the fourth
2
2
−
4
−1
run, the catalyst contained 2.4–2.6 × 10 molg of PW2 an-
ion, according to the X-ray fluorescence analysis. This result
indicates slight leaching of the immobilized anion.
and Am-2H-PW , thoroughly washed with water (to remove
2
the leaching species and reduce catalyst acidity), exhibited no
catalytic activity. According to their 31P NMR spectra, these

12
T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
catalysts retained immobilized PWx anions; however, these an-
oniumsilica-based ones. Moreover, for the acid-sensitive epox-
ides, Hoegaerts et al. reported diol formation, which we also
noted in our study.
31
ions were mostly the non-peroxo ligand-containing species ( P
NMR peak at −4 ppm). This finding could be at the origin of
the lack of activity of the washed catalysts.
Another related approach to heterogeneous epoxidation
3
−
Thus, in contrast to the epoxidation of cyclooctene, the cata-
lysts based on grafted silicas were found to be inappropriate for
the epoxidation of the sterically demanding and acid-sensitive
substrate. The aminopropylsilica-based catalysts were active
but gave insufficient selectivity in this reaction, due to their high
acidity, and favored formation of diols as side products. The
nonacidic onium-based catalysts were selective and prevented
formation of diols, but turned out to be relatively weakly active
as well as insufficiently stable.
catalysts is based on the use of {PO4[WO(O2)2]4}
and
[WZnMn2(H2O)2–(ZnW9O34)2]1
2−
anions immobilized in
composite silica containing phenyl and quaternary ammo-
nium groups [46]. The oxidation of cyclooctene with aque-
−1
ous H2O2 proceeds with a TOF of around 1–5 h
per
tungsten atom, which is again consistent with our results.
+
The previously reported ≡Si–Ph–CH2N Me2(C8H17)-and-
II
2
[ZnWMn –(ZnW9O34)2]12-based catalyst displayed the high-
est activity, which was significantly improved by the pres-
ence of hydrophobic phenyl groups in silica–gel. Epoxida-
tion of alkenes with bulky substituents was much less suc-
cessful [47], similar to our results on (R)-limonene. Further
investigation by the same research group showed very high
activities and excellent recycling properties of a catalyst con-
4
.3. Comparison of catalytic activity with published results
Comparison of our results with those reported for ho-
mogeneous systems—namely epoxidation under phase trans-
fer conditions (PTC), using Q2[HPO4{W2O2(μ-O2)2(O2)2}],
where Q = Arquad 2HT = (C18H37)2 75% + (C16H33)2 25%]
1
2−
sisting of [WZnMn2(H2O)2–(ZnW9O34)2]
(compared with
{PO4[WO(O2)2]4}3 ) and silica–gel containing polyethylene,
−
◦
N(CH3)2Cl, (21 C, 10% H2O2, CHCl3)—shows much greater
polypropylene, and quaternary ammonium groups.
−
1
activity of the catalyst in the latter case (TOF 100 h ) [12].
This is not surprising, because immobilization of active HPA
anions has been previously reported to cause reduced activ-
ity [31]. For instance, a ca. 3-fold rate reduction, compared
with the corresponding reaction in solution, was reported for
Amberlyst-supported [HPO4{W2O2(μ-O2)2(O2)2}] catalyst
This analysis led us to the conclusion that the catalytic per-
formance of the heterogeneous catalysts reported here is com-
parable to that of previously reported related solids. We have
shown that the degree of surface coverage by organic functions
is a crucial factor in determining catalyst stability and recycling
behavior. We confirm the importance of hydrophobization of
the catalyst surface on its dispersion in reaction media as well
as its catalytic activity.
(
used in a triphase system, comprising catalyst, aqueous H2O2,
and CH2Cl2, at room temperature). In this system, moderate
to high selectivities (epoxide yields 68–94%) were obtained
for (R)-limonene epoxidation at 70–95% conversion, which is
close to those observed with the Py-2-PW2 catalyst in our study
5. Conclusion
(
81–94%) [31].
The protonation of aminopropylsilica with H2[HPO4{W2O2-
(μ-O2)2(O2)2}] and anion exchange on the onium-modified
silica with (NBu4)2[HPO4{W2O2(μ-O2)2(O2)2}] allows us to
strongly immobilize peroxoophosphoditungstate PW2 anions
and prepare heterogeneous catalysts for liquid-phase epoxida-
tion. Our 31P SPE-MAS NMR study revealed that the immobi-
lized species in fact consists of several PWx anions, including
mainly the starting PW2 and PW4 anions as well as some oth-
ers containing more than four tungsten atoms and a reduced
number of peroxo ligands.
The activity of catalysts prepared in much the same way
as our immobilization approach was described by Hoegaerts
et al. Three types of catalysts were studied: mononuclear
and binuclear tungsten oxoperoxo anions on organic resins,
Venturello anions on triethylpropylammonium-MCM-41, and
a tungsten complex with phosphoramide-MCM-41. Resin-
immobilized mononuclear and binuclear anions had very low
activity, whereas tetranuclear anions exhibited high activity
−1
(
TOF 5.3 h per W site), selectivity, and efficiency in terms
of H2O2 consumption, along with no leaching (epoxidation in
The catalysts were active in epoxidation using H2O2/t-
BuOH oxidant solution in the absence of chlorocarbon solvent.
In the case of cyclooctene, high conversion (up to 100%) and
selectivity to epoxide (100%) were achieved. The oniumsilica-
based catalysts perform similarly to the previously reported
related heterogeneous catalysts based on HPA, with TOF val-
◦
MeCN with aqueous H2O2 [35%] at 50 C). The tetranuclear
anion {PO4[WO(O2)2]4}3 on tetraalkylammonium-MCM-
−
4
1, immobilized from a solution of H3{PO4[WO(O2)2]4},
−
1
was less active (TOF 1.5 h per W site). Interestingly, we
−
1
found similar TOF values (1–2 h per W site) with differ-
ent precursors, silica supports, and epoxidation conditions [53].
Hoegaerts et al. reported the formation of diols as a result
of the residual acidity of the immobilized oxoperoxo anion,
which we avoided in our work by using a nonacidic pre-
cursor, (n-Bu4N)[HPO4{W2O2(μ-O2)2(O2)2}] salt. The third
type, phosphamide-linked PW2 catalyst (on aminopropylated
MCM-41), led to the epoxidation of cyclooctene with much
−
1
ues per anion of 2–4 h . Aminopropylsilica-based catalysts
are somewhat more active (TOF values per anion of around
−
1
8 h ) but less stable toward leaching.
The nature of the grafted onium cations (triethylammonio-
propyl, pyridiniopropyl, and triphenylphosphonioalkyl) was not
important for the catalytic activity. The degree of surface cov-
erage by organic functions (onium cation, chloropropyl, or
–SiMe3 groups) and thus the hydrophobicity of the environ-
ment of the catalytic site are the crucial factors in determining
−
1
greater TOF (around 84 h ). In our work, we also found
greater TOF for aminopropylsilica-based catalysts than for

T. Kovalchuk et al. / Journal of Catalysis 249 (2007) 1–14
13
catalyst stability and recycling behavior. The performance of
onium-based catalysts depends greatly on the procedure used
for grafting the onium cation on the surface.
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