Kinetics of the oxygenation of unsaturated organics with singlet oxygen generated from H2O2 by a heterogeneous molybdenum catalyst

Article abstract of DOI:10.1021/ja065849f

A heterogeneous catalyst containing MoO₄^2- exchanged on layered double hydroxides (Mo-LDHs) is used to produce ¹O ₂ from H₂O₂, and with this dark ¹O₂, unsaturated hydrocarbons are oxidized in allylic peroxides. The oxidation kinetics are studied in detail and are compared with the kinetics of oxidation by ¹O₂, formed from H ₂O₂ by a homogeneous catalyst. A model is proposed for the heterogeneously catalyzed ¹O₂ generation and peroxide formation. The model divides the reaction suspension in two compartments: (1) the intralamellar and intragranular zones of the LDH catalyst; (2) the bulk solution. The 2-compartment model correctly predicts the oxidant efficiency and peroxide yield for a series of olefin peroxidation reactions. ¹O ₂ is generated at a high rate by the heterogeneous catalyst, but somewhat more ¹O₂ is lost by quenching with the heterogeneous catalyst than using the homogeneous catalyst. Quenching occurs mainly as a result of collision with the LDH hydroxyl surface, as is evidenced by using LDH supports containing strong ¹O₂ deactivators such as Ni²⁺. A total of 15 organic substrates were peroxidized on a preparative scale using the best Mo-LDH catalyst under optimal conditions.

Full text of DOI:10.1021/ja065849f

Published on Web 05/08/2007
Kinetics of the Oxygenation of Unsaturated Organics with
Singlet Oxygen Generated from H2O2 by a Heterogeneous
Molybdenum Catalyst
Bert F. Sels,* Dirk E. De Vos, and Pierre A. Jacobs
Contribution from the Centre for Surface Chemistry and Catalysis, Katholieke UniVersiteit
LeuVen, Kasteelpark Arenberg 23, 3001 HeVerlee, Belgium
Received August 11, 2006; E-mail: bert.sels@agr.kuleuven.ac.be
2-
Abstract: A heterogeneous catalyst containing MoO4
exchanged on layered double hydroxides
(Mo-LDHs) is used to produce 1O2 from H2O2, and with this dark 1O2, unsaturated hydrocarbons are oxidized
in allylic peroxides. The oxidation kinetics are studied in detail and are compared with the kinetics of oxidation
by 1O2, formed from H2O2 by a homogeneous catalyst. A model is proposed for the heterogeneously
catalyzed 1O2 generation and peroxide formation. The model divides the reaction suspension in two
compartments: (1) the intralamellar and intragranular zones of the LDH catalyst; (2) the bulk solution. The
2-compartment model correctly predicts the oxidant efficiency and peroxide yield for a series of olefin
peroxidation reactions. 1O2 is generated at a high rate by the heterogeneous catalyst, but somewhat more
1O2 is lost by quenching with the heterogeneous catalyst than using the homogeneous catalyst. Quenching
occurs mainly as a result of collision with the LDH hydroxyl surface, as is evidenced by using LDH supports
1
containing strong O2 deactivators such as Ni2+. A total of 15 organic substrates were peroxidized on a
preparative scale using the best Mo-LDH catalyst under optimal conditions.
Introduction
the success of photochemical procedures at laboratory scale,
they have inherent shortcomings for large-scale production.9 The
application of “dark” reactions for the production of O2 has
therefore attracted increasing interest. The principal reaction is
the heterolytic disproportionation of two molecules of H2O2 into
1O2 and water. Molybdate, calcium, and lanthanide salts have
been reported to catalyze this decomposition very efficiently.10-12
Singlet molecular oxygen is a highly selective and reactive
oxidant which is increasingly used in the synthesis of fine
chemicals, particularly in the industrial preparation of perfumes
and medicinal compounds.1-4 For instance, 1O2 is employed in
the chemical synthesis of artemisisin, the newest type of
antimalarial drug.5 Most procedures for 1O2 generation rely on
photochemistry, in which light energy is transferred to molecular
dioxygen with the aid of suitable organic sensitizers.6-8 Despite
1
Recently we focused on the design of heterogeneous versions
of these dark 1O2 generators. The development of cheap,
1
recyclable, and on-to-go catalysts will greatly extend the O2
chemistry from today’s laboratory scale application to multigram
scale processes. While previous immobilization attempts mainly
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9
6916
J. AM. CHEM. SOC. 2007, 129, 6916-6926
10.1021/ja065849f CCC: $37.00 © 2007 American Chemical Society

Oxygenation of Unsaturated Organics with 1O2
A R T I C L E S
focused on polystyrene resins13 and zeolite solid supports,8 we
succeeded in creating a stable inorganic solid catalyst, viz.,
molybdate exchanged on layered double hydroxides (Mo-
LDH).14 LDHs are hydroxide-type materials with a lamellar
structure and an exceptionally high anion exchange capacity.15
This catalyst is free of metal leaching, easy to handle, and can
be prepared in large quantities with standard laboratory equip-
ment.
DMBO2 was based on the external standard technique using 2-
heptanol. Note that, owing to its pronounced stability, this hydroper-
oxide can easily be analyzed by GC without thermal reduction into
the corresponding 2,3-dimethyl-1-buten-3-ol. A precolumn was not used
to avoid any peroxide decomposition. Upon contact of the reaction
mixture with a reducing agent such as Me3P, the alcohol is formed,
which confirms the presence of the hydroperoxide.
General Procedure for Dioxygenation of DMB. A 0.4 mmol
amount of DMB (0.1 M) and Mo-LDH are mixed in 4 mL of MeOH
2-
While the kinetics of peroxide formation are fairly well
understood in homogeneous systems, there is a lack of in-depth
and stirred in closed glass bottles. The MoO4 concentrations used
are specified in the captions of the figures and explained in Table SI1
(Supporting Information). The bottles are placed in a thermostatic bath
to keep the reaction temperature constant at 23 ( 0.5 °C throughout
the whole experiment. The reaction is initiated after approximately 15
min stirring by addition of 110 µL of 35% H2O2 (0.28 M). With regular
intervals, minimal samples are withdrawn and quickly centrifuged prior
to GC analysis.
1
knowledge on the behavior of O2 in solid-liquid mixtures.16
Data on the lifetime of 1O2 in such heterogeneous mixtures have
been reported only sporadically and mainly in the context of
oxygen diffusion and adsorption experiments. Despite the
importance of 1O2 in organic oxidation, the kinetic information
has only scarcely been translated and discussed with regard to
reaction productivity. There is general agreement that porous
materials, e.g., zeolites and silicas, tend to shorten the lifetime
Typical Procedure for Preparative Oxygenation. To a stirred
suspension of Mo-LDH-4 catalyst and the substrate (compounds 1-15
in Table 2) in 20 mL of solvent, 300 µL of 35 wt % H2O2 (or 55 µL
for compound 1) is added. The mixture is stirred at 500 rpm at 30 °C.
As soon as the red-brown suspension fades into yellow, a new portion
of the oxidant is added. Substrate concentrations, oxidant concentrations,
and reaction times are mentioned in Table 2. Product analysis was done
with GLC (30 m Chrompack CP-Sil 5 column) after reduction or
without reduction. Suitable reducing agents are Na2SO3 and (CH3)3P.
GC analyses of the reaction mixtures were compared with those of
authentic samples prepared photochemically or in dark conditions, e.g.,
via the Kasha-Khan reaction. The identity of the organic peroxides
1
of O2.8f
We here report on the production of 1O2 from H2O2 by
MoO42--exchanged layered double hydroxides (Mo-LDHs) and
on the usefulness of the 1O2 produced by this system in organic
transformations. On the basis of the catalytic results for olefin
peroxidation, a general kinetic model is proposed that adequately
describes the yields for olefin peroxidation in the heterogeneous
catalytic system. A key assumption in the model is the
compartmentalization of the reaction suspension in terms of a
compartment close to the catalyst, i.e., the intralamellar and
intragranular zones, and a second compartment formed by the
bulk solution. On the basis of the given amounts of reagents,
the model predicts the feasibility of olefin peroxidation in terms
of oxidant efficiency and organic peroxide productivity. The
model is validated in the preparative peroxidation of 15
substrates, using the best Mo-LDH catalyst in optimal condi-
tions.
1
was confirmed by 300 MHz H and 13C NMR.
Evolution of H2O2 Conversion. The evolution of H2O2 was followed
by cerimetry. A 150 µL sample of the reaction mixture was quickly
diluted into 17 mL of water, acidified with H2SO4 (7% of the total
volume). The titration was performed with Ce(SO4)2·4H2O (0.1 M) using
an automatic 725Dosimat (Metrohm).
Determination of 1O2 Yield and Values of âAPP (Wilkinson Plot).
A 4 mL volume of MeOH was used to dissolve various amounts of
DMB: e.g., for homogeneous catalysis, 65.3, 48.3, 34.2, 23.2, 13.4,
10.2, and 4.6 mg or 189, 140, 99, 67, 39, 30, and 14 mM; e.g., for
heterogeneous catalysis, 91.9, 61.4, 39.5, 38.5, 13.1, 12.0, 7.6, 6.9, and
5.2 mg or 265.8, 177.5, 114.2, 111.1, 37.9, 34.8, 22.0, 19.9, and 15.1
mM. The heterogeneous reactions were carried out with 0.055 g of
Mo-LDH-2 (or 2.5 mM Mo) in 4 mL of pure MeOH, whereas the
homogeneous reactions were performed with 2.5 mM Na2MoO4·2H2O
in 0.01 M NaOH in a H2O/MeOH mixture (15/85 vol %). The reaction
was started by adding 110 µL of 35% H2O2 (0.28 M) and was stirred
magnetically at 25 °C. Each solution was sampled after exactly 68 min
and quickly centrifuged (∼1 min) in the case of heterogeneous catalysis.
To 100 µL of this solution (or supernatant) was added 50 µL of a 2.0
× 10-3 M standard solution of 2-heptanol in MeOH. The yields of
DMBO2 were determined by GC analysis. For both the homogeneous
and heterogeneous set of reactions, an extra reaction was carried out
in absence of DMB. After the same reaction time, the amount of H2O2
consumed was determined from the latter reactions by following the
procedure described earlier. As will be explained, a “Wilkinson plot”
can be constructed based on such a data set. Similar sets of data were
also gathered for the DMB hydroperoxidation with other Mo-LDH
catalysts (Mo-LDH-1, -2, -4, and -7) having different Mo loadings and
for other substrates (see Table 1).
Experimental Section
Materials. All materials were used as received. 2,3-Dimethyl-2-
butene (DMB), 2-heptanol, and methanol were from Acros, Acros, and
Merck, respectively. H2O2 was used as a 35% aqueous solution.
Magnesium and aluminum salts were purchased from commercial
sources in the highest grade and were used as such.
Instrumentation for Analysis. The reaction product 2,3-dimethyl-
3-hydroperoxo-1-butene (DMBO2) was analyzed by gas chromatogra-
phy using a HP 5890 gas chromatograph fitted with a FI detector and
a Chrompack CP-Sil 5 column (WCOT, 40 m). Quantification of
(13) (a) Blossey, E. C.; Neckers, D. C.; Thayer, A. L.; Schaap, A. P. J. Am.
Chem. Soc. 1973, 95, 5820. (b) Schaap, A. P.; Thayer, A. L.; Blossey, E.
C.; Neckers, D. C. J. Am. Chem. Soc. 1975, 97, 3741. (c) Neckers, D. C.;
Blossey, E. C.; Schaap, A. P. Photosensitized reactions utilizing polymer
bound photosensitizing catalysts. U.S. Patent 4 315 998, Feb 1982. (d)
McGoran, E. C.; Wyborney, M. Tetrahedron Lett. 1989, 30, 783.
(14) (a) van Laar, F.; De Vos, D.; Vanoppen, D.; Sels, B. F.; Jacobs, P. A.; Del
Guerzo, A.; Pierard, F.; Kirsch-De Mesmaeker, A. Chem. Commun. 1998,
267. (b) Sels, B. F.; De Vos, D.; Pierard, F.; Kirsch-De Mesmaeker, A.;
Jacobs, P. A. J. Phys. Chem. B 1999, 103, 11114. (c) De Vos, D.; Wahlen,
J.; Sels, B. F.; Jacobs, P. A. Synlett 2002, 3, 367. (d) De Vos, D.; Sels, B.
F.; Jacobs, P. A. CATTECH 2002, 6, 14. (e) Wahlen, J.; De Vos, D. E.;
Sels, B. F.; Nardello, V.; Aubry, J.-M.; Alsters, P. L.; Jacobs, P. A. Appl.
Catal, A: Gen. 2005, 293, 120.
Synthesis and Characterization of the Mo-LDHs. The preparation
of small LDH crystallites is based on the precipitation of the nitrate
(15) (a) Sels, B. F.; De Vos, D.; Jacobs, P. A. Catal. ReV. 2001, 43, 443. (b)
Trifiro`, F; Vaccari, A. Hydrotalcite-like Anionic Clays (Layered Double
Hydroxides). In ComprehensiVe Supramolecular Chemistry; Alberti, G.,
Bein, T., Eds.; Pergamon, Elsevier Science: Oxford, U.K., 1996; Vol. 7,
p 251. (c) Rives, V.; Ulibarri, M. A. Coord. Chem. ReV. 1999, 181, 61.
(16) Wilkinson, F.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1995,
24, 663.
salts under alkaline conditions at pH ) 10 for the Mg0.7Al0.3(OH)2-
-
{NO3
}
0.3·mH2O sample and at pH ) 8.5 for the Mg0.7-xNixAl0.3(OH)2-
-
{NO3 }0.3·mH2O samples in slight supersaturation. A detailed procedure
can be found elsewhere.14,17 In a typical anion exchange procedure,
the hydrated LDH powder (1 g) is contacted with a solution of Na2-
9
J. AM. CHEM. SOC. VOL. 129, NO. 21, 2007 6917

A R T I C L E S
Sels et al.
Figure 1. Kinetic profile of the hydroperoxidation of 0.1 M DMB by Mo-
LDH-3 (or 3.6 mM MoO42-) plus H2O2 (0.28 M) in MeOH at 296 K.
MoO4 (100 mL) overnight under vigorous stirring. The suspension is
then subjected to several centrifugation-washing cycles until salt free.
The wet cakes were lyophilized to dryness. Analysis for Mo in the
centrifugate and in the collected washing waters showed in all cases a
more than 95% uptake of the MoO42-. This illustrates the known high
affinity of the LDH ion-exchanger for MoO42-. The techniques used
for the characterization of Mo-LDH have been published before.18
Semiquantitative ESR Study of Reaction Suspensions. Aliquots
of the suspension are taken after 5 min from a typical reaction of
hydrogen peroxide (110 µL, 35 wt % in H2O) with 0.05 g of Mo-LDH
in 2 mL of methanol. The samples are immediately frozen with liquid
N2 in ESR tubes. ESR measurements were carried out at 130 K using
a Bruker ESP 300 E spectrometer (1 G modulation amplitude; 7-10
dB; 9.5 GHz).
Figure 2. Arrhenius plot for the hydroperoxidation of 0.1 M DMB in the
presence of Mo-LDH-3: (9) experimental data points corresponding to
results of Figure SI1 (Supporting Information).
1
BO2, according the ene reaction (eq 1), loss of O2 due to
physical quenching with solvent or substrate molecules can be
neglected at sufficient DMB concentrations. Thus, the rate of
DMB oxidation, νo, equals the rate of 1O2 formation, νs. More
1
detailed information concerning the kinetics of O2 trapping
in homogeneous systems can be found in the Supporting
Information. Since both DMB and DMBO2 are easily analyzed
by GC, chemical trapping experiments with DMB provide a
1
convenient tool to study the production of O2 from homoge-
neous catalysts.
Results
Characterization of Catalysts. Table SI1 (Supporting In-
formation) summarizes the composition of the catalysts syn-
thesized, along with their lattice parameters, sorption charac-
teristics, and acid-base properties. The catalysts have been
extensively characterized by IR, Raman, ICP, XRD, SEM, and
BET in previous papers.18 In general, electron micrograph
pictures show that the Mo-LDH catalyst can be considered as
small plateletlike crystallites agglomerated into large porous
granules. The pores are typically in the mesoporous range. The
volume of these pores depends on various parameters such as
the MII/Al ratio in the Mo-LDH catalyst; in the series of catalysts
discussed here, the pore volume, e.g., decreases as the amount
When 0.28 M H2O2 is added to a suspension of Mo-LDH-3
in MeOH containing 0.1 M DMB, the concentration of DMB
typically falls as shown in Figure 1. For all sampling points,
the hydroperoxide DMBO2 is the principal product, with a
selectivity exceeding 94% in all cases. No significant consump-
2-
tion of DMB is observed when H2O2 or MoO4 is omitted
from the reaction suspension.
Figure 1 shows that the peroxidation rate is constant until at
least 80% of the substrate consumption, and this rate can be
calculated from the slope of the plot. The order of the oxidation
with respect to the trapping agent DMB is thus zero, implying
that, at least in the initial phase of the reaction, the DMB
concentration is sufficiently high to trap any 1O2 that is available
2-
of exchanged MoO4 increases (Table SI1). The crystallinity
of the catalysts was confirmed by X-ray diffraction analysis.
The pattern of the reflections in the diffractogram and the lattice
parameters deduced from it are characteristic for LDH-like
phases. The speciation of the Mo has been studied with a
combination of FT-IR and Raman spectroscopy. Irrespective
of the Mo surface concentration, the exchanged Mo mainly
occurs in the monomeric MoO42- form. Characteristic absorp-
tion bands are observed near 820-850 cm-1 in IR, which can
be ascribed to the antisymmetric Mo-O stretching vibration,
and at 898, 842, and 317 cm-1 in the Raman spectra; the latter
are due to the symmetric and antisymmetric Mo-O stretching
and the O-Mo-O deformation vibrations, respectively.
in the solution. Thus, when 0.28 M H2O2 is added to 2.5 mM
MoO4 on LDH in MeOH at 296 K, 11.8 ( 0.7 × 10-6
M
2-
1O2/s is released into the reaction solution and is trapped by
DMB.
Influence of the Temperature. Using the procedure de-
scribed earlier, values of the oxidation rate Vo were calculated
from experiments in which the temperature was varied between
2-
0 and 30 °C. Initial H2O2, DMB, and MoO4 concentrations
1
were kept constant at 0.28 M, 0.1 M, and 3.6 mM, respectively.
The time profiles for the reactions are given in the Supporting
Information (Figure SI1). For all reaction temperatures, hydro-
peroxide selectivity was over 94% at total H2O2 conversion.
Again, DMB linearly disappears with time. Figure 2 shows the
Determination of the O2 Formation Rate. Since DMB
1
(2,3-dimethyl-2-butene) reacts rapidly with O2 yielding DM-
(17) Sels, B. F.; De Vos, D.; Jacobs, P. A. J. Am. Chem. Soc. 2001, 123, 8350.
(18) Sels, B. F.; De Vos, D.; Grobet, P.; Jacobs, P. A. Chem.sEur. J. 2001, 7,
2547.
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Oxygenation of Unsaturated Organics with 1O2
A R T I C L E S
Figure 3. Influence of the Mo concentration on the hydroperoxidation rate
(0.1 M DMB): (b) with variation of the total weight of added catalyst
2-
using catalyst Mo-LDH-3; (0) by variation of the degree of MoO4
exchange of the LDH support.
Figure 4. Influence of the initial [H2O2]0 on the hydroperoxide formation
rate Vo in the presence of (b) Mo-LDH-2 and (0) Na2MoO4 (0.1 M DMB,
[Mo]T ) 1 mM).
corresponding Arrhenius plot. From the slope, the apparent
activation energy is calculated, yielding Ea ) 66.5 ( 7.5
kJ‚mol-1
.
Influence of Variable [Mo]T. Experiments in which the total
concentration of Mo ([Mo]T) was varied in the range of 1.5-
16 mM were conducted in two ways: (a) The oxygenation was
higher for the Mo-LDH/H2O2 system than for the soluble
catalyst. For the homogeneous system, the maximum value for
Vo equals (3.4 ( 0.2) × 10-6 M s-1 at [H2O2]0 ) 0.030 M,
whereas for the heterogeneous system Vo is (5.9 ( 0.3) × 10-6
M s-1 at [H2O2]0 ) 0.020 M.
2-
studied by changing the amount of catalyst with fixed MoO4
loadingsin this experiment a Mo-LDH-3 was used. (b) The
2-
oxygenation was studied using Mo-LDHs with varying MoO4
Influence of the Substrate Concentration. To examine the
effect of the initial DMB concentration [DMB]0 on νo, a series
of experiments was carried out with [H2O2]0 ) 0.28 M and
[Mo]T ) 2.5 mM at 296 K, with homogeneous and heteroge-
neous catalysts. In this series, the concentration of DMB was
varied from 0.01 to 0.3 M. All reactions were analyzed after
exactly 68 min. Figure 5 displays the plot of the yield of DMBO2
versus [DMB]0. The curves of Figure 5 display a clear plateau,
which is characteristic for trapping of 1O2 using reactive
loading (Mo-LDH-1 to -8). The concentrations of H2O2 and
DMB were kept constant at 0.28 and 0.1 M, respectively, and
the reactions were performed in MeOH at 296 K. The
dependence of Vo on [Mo]T for experiments a and b is
summarized in Figure 3.
The results of experiment a show that Vo increases almost
linearly when increasing the amount of Mo-LDH, indicating a
first-order dependence on MoO42-. The slope gives the oxidation
rate (M‚s-1) per mole of MoO4 and equals 3.6 ( 0.4 10-3
2-
1
acceptors. Indeed, when sufficient DMB is available, all O2
s-1 for the conditions used (i.e., in MeOH, 296 K, and [H2O2]
) 0.28 M).
produced is consumed in the chemical reaction.
Even more important is the quantitative information that can
be extracted when the data of Figure 5 are plotted in a way
analogous to equation SI8 (Supporting information). Dividing
equation SI 8 by the total amount of H2O2 consumed results in
the following expression, in which Y is the yield of the DMB
oxidation based on the oxidant used:
By variation of [Mo]T by method b, a similar linearity was
observed with respect to [Mo]T for concentrations in the range
of 1.8-4.8 mM, i.e., for MoO42- loadings between 8% and 22%
of the total anion exchange capacity (AEC ∼ 3.3 mequiv·g-1).
The slope (3.4 ( 0.2 10-3 s-1) of the curve agrees within
experimental error with that obtained via method a. However,
at higher Mo loadings, the linearity is disturbed and Vo becomes
independent of [Mo]T. This inflection point corresponds to a
critical Mo loading of 20-25% of the total AEC (∼3.3
[DMB]0 - [DMB]t
[H2O2]0 - [H2O2]t
kr
[1O2]total
Y )
)
-
(
)(
)
kr + kq
[H2O2]0 - [H2O2]t
[DMB]0
ln
mequiv·g-1) and a maximum Vo of about 20 × 10-6 M‚s-1
.
(
)
Influence of [H2O2]0. To study the influence of the initial
hydrogen peroxide concentration [H2O2]0, experiments were
conducted with 0.1 M DMB at 296 K in MeOH, with a constant
[Mo]T of 1 mM. Both the soluble Na2MoO4 and the heteroge-
neous Mo-LDH-3 catalysts were investigated. Note that the
experiments with Mo-LDH-3 were conducted in MeOH,
whereas the reactions with Na2MoO4 were performed in aqueous
MeOH (85/15 vol % for MeOH/H2O), because of the poor
solubility of Na2MoO4 in neat MeOH. The dependence of Vo
on [H2O2]0 up to 0.30 M is depicted in Figure 4. As can be
seen, the kinetics with regard to H2O2 are complex, showing
an asymmetric bell shape for both systems. By the increase of
[H2O2]0, the rate of disappearance of DMB (Vo) initially rises,
then reaches a well-defined maximum, and finally gently
decreases. Note that the peroxidation rates are significantly
kd
[DMB]t
{
)
}
(
kr + kq
[H2O2]0 - [H2O2]t
Y ) γY0 - âX
(2)
Here [1O2]total is the cumulative concentration of 1O2 after time
t, i.e., the 1O2 concentration that would be present in the reaction
1
mixture if O2 were not quenched at all by the substrate DMB
or by the solvent and Y0 ) [1O2]total/([H2O2]0 - [H2O2]t).
According to eq 2, Y ) ([DMB]0 - [DMB]t)/([H2O2]0
-
[H2O2]t) can be plotted versus X ) ln([DMB]0/[DMB]t)/([H2O2]0
- [H2O2]t). In these so-called Wilkinson plots, a straight line
is normally obtained, with γY0 as intercept with the vertical
axis and â as slope. For homogeneous systems, the meaning of
the slope and intercept in eq 2 is particularly well understood.
9
J. AM. CHEM. SOC. VOL. 129, NO. 21, 2007 6919

A R T I C L E S
Sels et al.
Table 1. Wilkinson Plot Data for Mo-LDH Catalysts with Varying
2-
MoO4 Content and Various Substrates and Conditions as in
Figures 5 and 6
substrate
catal
wt (g)
Y0,app
âapp (M)
R2
2-
2,3-dimethyl-2-butene
MoO4
0.46 0.0075
0.99
0.98
0.97
0.98
0.95
Mo-LDH-1 0.110 0.38 0.011
Mo-LDH-2 0.055 0.36 0.011
Mo-LDH-4 0.041 0.31 0.015
Mo-LDH-7 0.020 0.21 0.025
2,3-dimethyl-2-butene
2-methyl-2-butene
2-methyl-2-pentene
1-methyl-1-cyclohexene
Mo-LDH-2 0.055 0.36 0.011 (0.0035)a 0.97
0.34 0.15 (0.11)a
0.34 0.18 (0.15)a
0.29 0.54 (0.53)a
0.95
0.95
0.94
Figure 5. DMBO2 yields as a function of initial alkene concentration for
2-
Mo-LDH-2 (left) and MoO4 (right).
a â values in homogeneous solution.16
M, while Y0,app ) 0.38 ( 0.03. Since this âapp value is
significantly higher than that reported for pure MeOH (âMeOH
) 0.004 M),16 physical quenching is apparently more pro-
nounced in the solid-liquid system. The lower oxidant ef-
ficiency Y0 for the heterogeneous Mo-LDH system implies that
1
considerably less than 0.5 mol of O2 is available in solution/
mol of H2O2 that is disproportionated. This is a clear difference
2-
with the common idea on H2O2 decomposition by MoO4
,
namely that 1O2 formation is almost quantitative. This anomaly,
along with the lower apparent lifetime of 1O2, will be taken in
hand in the Discussion.
Figure 6. Wilkinson plots for DMB peroxidation in methanolic solvent
conditions. The singlet oxygen is produced from 0.28 M H2O2, with the
2-
catalysts Mo-LDH-2 (left) or MoO4 (right) at 296 K with [Mo]T ) 2.5
The characteristic Wilkinson parameters for the hydroper-
oxidation of substrates other than DMB are gathered in Table
1. Again, the efficiencies Y0 for 1O2 production are smaller than
0.5. The same table also summarizes data for Wilkinson plots
obtained for DMB in presence of various Mo-LDH catalysts
having various concentrations of MoO42-. Thus, it appears that
the least-squares linear fit of eq 2 not only holds for the soluble
catalyst and Mo-LDH-2 but also for the catalysts having higher
mM. Samples are taken after 68 min and immediately analyzed with GC.
H2O2 is titrated with 0.1 N Ce4+. X and Y are clarified in the text.
The ratio γ ) kr/(kr + kq) expresses the contribution of the
chemical reaction to the overall deactivation by the substrate.
Since physical quenching is negligible in the peroxidation of
DMB, i.e., kq , kr, γ can be set to unity.16 The ratio â ) kd/(kr
+ kq), known as the “Foote reactivity index”, gives the minimum
DMB concentration required so that the physical and chemical
Mo loadings. The apparent efficiency of the oxidant (Y0,app
)
decreases with increasing Mo content, whereas the apparent
Foote index âapp tends to increase with increasing Mo content.
1
interaction of O2 with DMB dominates over the deactivation
by the solvent. Since this value is characteristic for a given
trapping agent and a given solvent, it is often used to verify
1O2 involvement. Reference lists with values for â, kr, and kq
have been reported.16 Finally, Y0 ) [1O2]total/([H2O2]0 - [H2O2]t)
Semiquantitative ESR Spectroscopy. According to Nardello
et al.,10d the quantitative conversion of H2O2 into 1O2 can only
be rationalized on the basis of heterolytic pathways. Indeed, if
the decomposition involved homolytic routes, the reactive
oxygen radicals would undergo an intersystem crossing with
1
equals the O2 yield on the basis of the oxidant used, or the
1
overall oxidant efficiency for O2 production, defined as the
3
total molar amount of 1O2 available in solution (for interaction
with substrate or solvent) per mol of H2O2 consumed. Obviously,
as one molecule of 1O2 is formed from two molecules of H2O2,
the maximum theoretical value for Y0 is 0.5.
formation of ground-state dioxygen O2. Hence, if the decom-
2-
position of H2O2 by MoO4 or Mo-LDH exclusively occurs
via nonradical pathways, the reaction suspension must be ESR
silent.
Figure 7 shows, for various Mo-LDH catalysts, the X-band
ESR spectra of the frozen suspensions during the initial stage
of the H2O2 decomposition. As was demonstrated earlier,14b the
decomposition of H2O2 by homogeneous MoO42- produces only
traces of radicals, which is in agreement with the high efficiency
of 1O2 generation with soluble MoO42-. For Mo-LDH-1, which
has the lowest Mo content, the signal is of a comparable
The kinetic data for DMB peroxidation and H2O2 consump-
tion after 68 min (Figure 5) were converted to a Wilkinson plot,
not only for the MoO4 reaction but also for the heteroge-
2-
neously catalyzed LDH-MoO42- reaction (Figure 6). From these
plots, values can be calculated for the parameters Y0 and â. For
the homogeneous system, a â value of 0.0075 ( 0.002 M was
obtained. This value is in good agreement with the calculated
â value of DMB for this particular solvent mixture.16 For Y0, a
value equal to 0.46 ( 0.02 was found. This value compares
well with reported values10 and confirms that practically 1 mol
weakness, proving that the selective heterolytic generation of
2-
1O2 from H2O2 is not disturbed by heterogenization of MoO4
.
However, when the Mo loading of the catalyst is increased,
the intensity is remarkably amplified. The shape of the signal
is indicative of the axial symmetry of superoxo-type radicals.
On the basis of the calculation of the g parameters (g ) 2.023,
g| ) 2.063), it can be stated, however, that this species is not
a free superoxo radical anion O2-° (g ) 2.023 and g| ) 2.092).
A MoV-OO° type radical, which might be formed by homolysis
1
of O2 is formed from 2 mol of H2O2.
Parameters were also calculated from the plot for the
heterogeneous catalyst Mo-LDH-2. Since the physical meaning
of these parameters is not necessarily the same as for the
homogeneous system, these parameters are denoted as apparent
â and apparent Y0 values. From Figure 6, âapp is 0.011 ( 0.003
9
6920 J. AM. CHEM. SOC. VOL. 129, NO. 21, 2007

Oxygenation of Unsaturated Organics with 1O2
A R T I C L E S
the exact mechanism for the radical decomposition is unknown.
Anyway, it is clear that a radical decomposition process
1
competes with O2 formation in the heterogeneous system, at
least if Mo-LDH with high Mo loadings is used.
Preparative Peroxidation. To demonstrate the usefulness
of the model, oxygenations of several reactive organic substrates
were carried out on a preparative scale. The substrates include
polycyclic aromatics 1 and 2, one conjugated diene 3, several
olefins 4-7 and 12-14, allylic alcohols 8-11, and one phenolic
compound 15. Some of them have been used in industry since
years in the fine chemicals production.1 The oxidation reactions
1
cover the two most important reaction modes of O2: the ene
hydroperoxidation (entries 4-14) and the [2 + 4] cycloaddition
(entries 1-3). The results of the preparative experiments are
tabulated in Table 2. As can be seen, the oxidation is reasonably
fast and leads to useful yields for many substrates. The
separation of the Mo-LDH catalyst from the reaction products
is easily performed by a simple centrifugation or filtration. A
specific problem often encountered in heterogeneous catalysis
is leaching of the metal ions into the solution. For the Mo-
LDH, leaching was quantified by measuring the amount of Mo
present in the liquid phase by means of plasma emission
spectroscopy (ICP). Experimental results show that the con-
centration of Mo in the filtrate is systematically lower than 2
ppm. The degree of leaching is therefore below 0.4%.
Since the heterogeneous system operates with maximal rates
in less concentrated peroxide solutions, the total amount of H2O2
was added in portions, as described in the Experimental Section.
Each portion contains 50 equiv of peroxide with respect to Mo.
On the other hand, to keep the oxidant efficiency as high as
possible, Mo-LDHs having not too high Mo loadings are
preferably used. The experiments in the Table 2 have been
carried out with Mo-LDH-4.
Figure 7. Top: X-band ESR spectra (130 K) of quickly frozen suspensions
of LDH-MoO42- in MeOH/H2O2. Conditions: 0.05 g of LDH catalyst with
MoO42- content of 66% (Mo-LDH-8), 44% (Mo-LDH-7), 32% (Mo-LDH-
6), 16% (Mo-LDH-3), and 8% (Mo-LDH-1) of total AEC (3.3 mequiv g-1);
2 mL of MeOH; 110 µL of 35% H2O2; 300 K, 5 min. Bottom: (9) Relative
intensities of ESR signals as obtained by double integration, for suspensions
containing 0.05 g of LDH, and (O) ESR signal intensity obtained with 0.15
g of Mo-LDH-3.
Discussion
Reliability of the Data Analysis. The straightforward and
correct nature of our experimental approach using DMB is
confirmed by the close agreement of the results with known
data on the kinetic properties of 1O2. Indeed, from the kinetics
of the soluble MoO42- catalyst (Figure 5), a lifetime value of τ
of a side-on bound peroxo ligand in a peroxomolybdate, can
also be discarded, because this reaction would in addition
produce a signal of MoV. A plausible explanation for the ESR
signal is that O2-° is formed and is subsequently stabilized by
LDH lattice cations, e.g., as Mg2+-O2-°, via ligand exchange
of a hydroxyl anion for a superoxo radical anion. Such a process
is in line with the upfield shift of the parallel component of the
g tensor; a similar g| parameter is observed for the superoxo
1
) 4.1 ( 0.6 µs can be calculated for O2 in the MeOH:H2O
mixture. This value τ ) kd-1 is obtained from â ) kd/(kr + kq)
) 0.0075 (Table 2), with kr ) 3.3 × 107 M-1 s-1 and kq ∼ 103
M-1 s-1 16
. Within experimental error, this is in agreement with
radical bound to, e.g., Ba2+ 19
.
the expected values (e.g., τ ) 3.5 µs)16 for the solvent mixture
used. Moreover, the oxidant efficiency Y0 ) 0.46 ( 0.02 implies
that the production of 1O2 is more or less quantitative, which is
consistent with experimental findings from trapping and NIR
spectroscopy.10 Therefore, it may be stated that trapping with
DMB is an experimentally sound method to study 1O2 kinetics.
Kinetic Model for 1O2 Generation in a Suspension.
Equations SI1-SI5 (Supporting Information) adequately de-
Double integration of the ESR signals gives an idea of the
relative radical concentrations in the suspensions (Figure 7,
lower part). It appears that the intensity of the radical signal
2-
increases spectacularly with the MoO4 loading. In contrast,
tripling the amount of Mo-LDH-3 increases the signal intensity
only by a factor of about 3. Thus, the signal obtained for the
frozen H2O2-exposed Mo-LDH-3 (0.15 g solid) suspension is
much weaker than that for the suspension with Mo-LDH-8 (0.05
g of solid but with a 4-fold higher Mo loading). Therefore, it is
tentatively proposed that the radical decomposition route
involves at least two Mo ions, in contrast to the unimolecular
decomposition in the 1O2 formation. At this moment, however,
1
scribe the kinetic behavior of O2 in a homogeneous liquid
system. However, to understand the physical meaning of the
data for the solid-liquid system, a new kinetic model is needed.
The pictorial representation of this model is given in Scheme
1. In the model, the LDHs are considered as porous granules,
which consist of aggregated LDH crystals. As LDHs are strongly
hydrophilic and charged materials, a hydrophobic organic
substrate A would rather reside in the bulk of the liquid than
(19) Rabo, J. A. Salt Occlusion in Zeolites in Zeolite Chemistry and Catalysis;
Rabo, J. A., Ed.; American Chemical Society: Washington, DC, 1976;
Vol. 5, p 332.
9
J. AM. CHEM. SOC. VOL. 129, NO. 21, 2007 6921

A R T I C L E S
Sels et al.
Table 2. Oxygenation of Organic Compounds by the
1O2-Producing System Mo-LDH-4/H2O2 at 30 °C
Scheme 1. Schematic Representation of the Solid-Liquid System
Used to Peroxidize Organic Substrates with H2O2 as the Oxidant
1
substrate concentration than in the bulk ([A]in = 0). O2 is
chemically generated in the inner compartment from H2O2. As
1
soon as O2 is formed, it may decay in the inner compartment
(kdin) or it may diffuse into the bulk (kout). In the bulk, 1O2 can
out
either be quenched by the solvent (kd ) kd for the solvent
composition) or by the substrate (kq); it can oxygenate the
substrate A to form AO2 (kr), or it can reenter an LDH granule
(kin). Note that because of its hydrophilic nature, H2O2 will be
abundantly available in the inner compartment.
1
According to the proposed model, the rate equation for O2
in the two different compartments is given by eqs 3 and 4 for
the inner and outer compartments, respectively:
d[1O2]in
d[H2O2]
R
2f
)
- (kdin + kout)[1O2]in + kin[1O2]out (3)
dt
dt
d[1O ]
f
2 out ) -{kdout + (kr + kq)[A]out}[1O2]out + k
out (1 - f)
dt
f
[1O2]in - k
[1O2]out (4)
in (1 - f)
The parameter f ) Vin/(Vin + Vout) takes into account the ratio
between the volumes Vin and Vout of the inner and outer
compartments. As will be demonstrated, exact knowledge of f
is not required in applying the model. Concentrations with the
suffixes “out” or “in” refer to the inner or outer compartments;
if no suffix is given, the concentration is given over the whole
reaction volume (Vin + Vout). In eq 3, the decomposition rate
d[H2O2]/dt can be defined as VHOOH; this rate was studied in
detail earlier.18 Possible formation of radicals is accounted
for by introducing R, which is defined as the fraction of H2O2
a Based on 1H NMR, 13C NMR and GC-MS. b 1O2 is produced by a
solid Mo-LDH catalyst. c The value presents the selectivity of the trans-
pinocarveyl hydroperoxide, as illustrated in the table. The total selectivity
for hydroperoxides is larger than 90%. c 50% aqueous H2O2 is used instead
of 35%.
1
that is disproportionated into O2. Thus, in a heterogeneous
close to the solid. Due to such partitioning, the equilibrium
concentration of a substrate A inside a porous LDH granule is
lower than in the surrounding liquid. For convenience, it is
assumed that the concentration of A ([A]) follows a step profile
as shown in Scheme 1. This implies that the liquid system is
divided into two compartments: (1) the bulk solution (“out”)
and (2) an intragranular compartment (“in”), with a much lower
system, equation SI1 (Supporting Information) is better replaced
by eq 5:
2-
LDH-MoO4
2H2O2
8 R1O2 + (1 - R)3O2 + 2H2O (5)
Hence, Vs ) RVHOOH/2, with Vs the rate of singlet oxygen
formation.
9
6922 J. AM. CHEM. SOC. VOL. 129, NO. 21, 2007

Oxygenation of Unsaturated Organics with 1O2
A R T I C L E S
Taking into account the simplifying assumption that only 1O2
in the outer compartment is involved in the peroxidation, eqs 3
and 4 can be simplified into eq 6, in which the escape ratio q
) kout/(kd + kout) expresses the fraction of O2 that actually
arrives in the bulk without being quenched inside the LDH
granules:
is also analogous to that of the H2O2 decomposition rate. That
peroxidation rates are higher with MoO42- immobilized on the
LDH than with dissolved MoO42- is ascribed to the labilization
of the peroxocomplexes at the LDH surface.
in
1
Second, the formation of singlet oxygen and the ensuing
peroxidation seem less efficient with respect to H2O2 using LDH-
2-
1
MoO4 than using dissolVed MoO42-, i.e. the apparent O2
yield Y0,app is smaller for the heterogeneous than for the
homogeneous system. According to the model, Y0,app ) (qR)/2.
The lower 1O2 yield in the heterogeneous system can therefore
[DMB]0 - [DMB]
[H2O2]0 - [H2O2]t
kr
t ) Rq
-
1
(
)
2 kr + kq
[DMB]0
[DMB]t
ln
kdout(1 - f) + (1 - q)kinf
kr + kq
1
(
)
partly be explained by a less efficient production of O2 with
the immobilized MoO42-. Homolytic side reactions decrease
(6)
{
}
(
)
[H2O2]0 - [H2O2]t
the factor R, particularly at high MoO42- loadings (νs ) RνHOOH
/
Using the parameter γ, and with the same X and Y parameters
2). However, since even at low MoO42- contents less than 80%
of the H2O2 is eventually used for peroxidation (Table 1), there
as before, eq 6 can be simplified into eq 7:
1
must be additional losses of O2. A q value smaller than one
accounts for these supplementary O2 losses. Physically, the q
1
2
1
Y ) qγR - âappX
value expresses the probability that a 1O2 molecule reaches the
solution containing the target substrate instead of being quenched
inside a LDH granule. Hence, the Wilkinson plots for a Mo-
LDH with low Mo content show that about 20-30% of the
produced 1O2 molecules are deactivated before they are able to
peroxidize the substrate (q ) 0.76 with R ∼ 1). Quenching of
1O2 is probably promoted by the high anion and surface hydroxyl
concentration.20 A quantitative study of quenching by surface
hydroxyls has been performed by Thomas and Iu, using
sensitizers adsorbed at mesoporous amorphous silica gel.20a-c
with
kdout(1 - f) + (1 - q)kinf
âapp
)
(
)
kr + kq
kout
q )
(7)
in
(kout + kd )
This expression bears a more than superficial similarity to
that for the homogeneous reactions (eq 2). Likewise, a plot of
Y against X should give a straight line with 1/2γqR or γY0,app
as intercept and with âapp, instead of â, as the slope. Since the
data for the heterogeneously catalyzed peroxidation of DMB
show adequate linearity in the Y-X Wilkinson-like plots (Figure
6, left), the model may well be valid, despite the assumptions
made.
1
They discovered that reduction of the O2 lifetime is more
pronounced in small pores as a consequence of a higher rebound
1
frequency of diffusing O2 molecules against the walls. Ad-
sorbed water and silanol surface groups were identified as the
effective deactivators. In the proposed model, the decrease of
Y0,app is only determined by catalyst-specific effects, as expressed
in R and q. This implies that irrespective of the substrate used,
the same Y0,app should be obtained for the same catalyst.
Wilkinson plots for oxidation of 2,3-dimethyl-2-butene, 2-meth-
yl-2-butene, or 2-methyl-2-pentene using Mo-LDH-2 result in
closely similar Y0,app values, which corroborates the validity of
the model. Only for 1-methyl-1-cyclohexene, a lower Y0,app value
is found, but this might be due to the poor reactivity of this
substrate and, consequently, the larger errors in determining
peroxidation yields.
Interpretation of the Kinetic Data. If one looks at the
peroxidation kinetics with 1O2 in heterogeneous conditions (Mo-
LDH in methanolic solvents), three observations merit attention.
First, the kinetic data of the peroxidation accord reasonably
well with the kinetics of the decomposition of H2O2 (see ref
18): The activation energy for the peroxidation of DMB (66.5
( 7.5 kJ‚mol-1) is close to the 72.7 ( 6.7 kJ‚mol-1 obtained
earlier for the decomposition of H2O2. This close agreement is
1
in line with the fact that peroxidation of organics by O2 is
generally fast. The rate-determining step in the overall catalytic
process is therefore the disproportionation of the peroxomo-
lybdate complexes, with 1O2 formation, rather than the peroxi-
dation. While the peroxidation rate νo is proportional with the
catalyst concentration [MoO42-] at low Mo content of the solid,
the order in MoO42- becomes zero when the Mo concentration
is raised by further increasing the Mo content of the solid (Figure
3). The similar behavior of the decomposition rate νHOOH was
explained in terms of a rate-limiting diffusion of H2O2 to the
intercalated Mo anions. Moreover, it can be assumed that even
Third, the apparent lifetime of 1O2 is smaller in the
heterogeneous conditions, i.e., âapp > kd/(kr + kq). This means
that the heterogeneous reaction medium contains extra deactiva-
tion pathways in comparison with the homogeneous solution.
Obviously, quenching inside the LDH granules (kdin) is such a
1O2 deactivation route. The impact of this quenching is more
1
pronounced as more O2 fails to escape to the solution, i.e., as
the escape ratio q decreases. This is also clarified by rewriting
the expression for âapp
:
1
if O2 is formed by intercalated Mo anions, it will be rapidly
out
kd
kin
deactivated in the confinement of the interlayers. Finally, it was
found that, at high Mo loadings, H2O2 is not only converted
âapp
)
(1 - f) + (1 - q)f
) â(1 - f) +
(
)
(
)
kr + kq
kr + kq
1
into O2. Considerable amounts of radicals are also produced,
kin
as proven by ESR (Figure 7). The complex kinetic dependence
of the peroxidation rate on the hydrogen peroxide concentration
(1 - q)f
(8)
(
)
kr + kq
9
J. AM. CHEM. SOC. VOL. 129, NO. 21, 2007 6923

A R T I C L E S
Sels et al.
Figure 9. Graphical representation of the kinetic model. The Y axis gives
the estimated concentration of the oxidant as a function of the required
substrate conversion in the peroxidation of substrate A using catalyst Mo-
LDH-2 in MeOH. Symbols used in the X and Y axis are explained in the
text.
Figure 8. Efficiency of 1O2 trapping by DMB (0.1 M) during the
decomposition of H2O2 in the presence of various Ni-containing MgAl LDHs
exchanged with molybdate. Y0,app is the oxidant efficiency extrapolated for
high DMB concentration in the heterogeneous conditions as determined
from Wilkinson plots.
from Figure 8, 1O2 is quenched more efficiently when more Ni
is available in the LDH structure. Thus, successful peroxidation
is governed by the efficiency of diffusion of O2 through a
The first term corresponds to the contribution of the solution
compartment, which is similar in the heterogeneous and
homogeneous systems. Additional quenching originates in the
second term and increases for a larger (1 - q) value, i.e., for a
lower escape ratio. A plot of the âapp values obtained for
different substrates in presence of Mo-LDH-2 (Table 1) versus
reported â values gives a straight line (R2 ) 0.998) in accordance
to eq 8 (figure not shown). A volume fraction f of 1.7% and a
kin of (3.6 ( 0.4) × 103 s-1 can be calculated from the slope
and from the intercept, respectively.
1
deactivating zone between its origin at the surface and the bulk
solution. In conclusion, deactivation results not only from
quenching with ions and water molecules present in the polar
double layer but also from frequent collisions with the LDH
surface.
Simulation of the Peroxidation Reactions. A convenient
way to study the peroxidation of organics with the LDH-
MoO42-/H2O2 system consists in transforming eqs 6 and 7 into
9, under the assumption that all H2O2 has been disproportionated
([H2O2]t ) 0):
Notice also that the data of Table 1 prove that, at high Mo
content, 1O2 escapes even less efficiently from its locus of origin
to the bulk of the solution. This drop in oxidant efficiency is
easily explained taking into account the presence of intercalated
molybdate for the molybdate-rich samples in addition to surface
adsorbed molybdate. Singlet oxygen generated in the interior
of the interlayers is obviously quenched very effectively.
To assess the surface quenching of singlet oxygen and its
effect on the overall oxidant efficiency, LDH supports were
doped with various amounts of Ni+2 (Mo-LDH-9 to Mo-LDH-
12; see Table SI1 in Supporting Information). Ni2+ with the
characteristic absorption at 8500 cm-1 due to 3Ag f 3T2g d-d
electron transition16 is known to deactivate 1O2 in solution with
a bimolecular rate constant comparable to the oxygenation rate
of DMB by 1O2 (viz. kqNi ) 3.3 × 107 M-1s-1, whereas krDMB
A
γ[H2O2]tot
kd,app
2
2
1
1
)
X +
ln
(9)
( )
( )(
)( ) {
}
Rq
Rq kr + kq
1 - X
[A]0
[A]0
A
Here X is the conversion of the organic substrate A and kd,app
) âapp(krA + kqA). Values of âapp were reported in Table 1 for
various substrates and catalysts. [H2O2]tot is the total concentra-
tion of oxidant that would be accumulated if the H2O2 had not
been decomposed. Note that for a given catalyst/solvent system
(e.g., Mo-LDH-2 in MeOH), the value for Rq ) 2Y0,app is
directly obtained from Table 1. Finally, values for the rate
constants of the substrate A, namely kr + kq, and for γ are known
for many organic substances and were compiled in the review
of Wilkinson.16 The kinetic model can then be translated into a
graphical form such as Figure 9, with different curves for
different values of the substrate conversion X.
) 3 × 107 M-1 -1
s
).16 DMB peroxidation with the Ni-containing
LDH-MoO42- hardly showed the formation of DMBO2, despite
the gradual consumption of H2O2 and the formation of the
2-
This graph allows estimating the required amount of oxidant
to obtain a certain substrate conversion. As an example, the
results of the peroxidation of 0.1 M DMB with 0.28 M H2O2
using Mo-LDH-2 in MeOH can be simulated. Using Figure 9,
the conversion of DMB at complete consumption of H2O2 is
calculated at 88%, which is not so far from the 86% obtained
experimentally (Figure 1).
typical red-brown color of the tetraperoxomolybdate Mo(O2)4
.
Since the concentration of soluble Ni2+ does not exceed the
ppb level, deactivation by solubilized Ni2+ does not compete
with the hydroperoxidation of DMB, i.e., [DMB]kr
.
[Ni2+]kq. Hence Ni2+ in the LDH structure is able to deactivate
1O2. The observation that almost all 1O2 is quenched on
1
Ni-containing LDH thus supports the idea that O2 molecules
A parity plot in Figure 10 illustrates the relatively small
deviations between the observed data and the calculated data
for many organic compounds. The experimental data were col-
lected from experiments with various olefins such as presented
in Table 2 and in ref 14b. Hence, the compartmentalization
model enables a fair approximation of the experimental findings.
frequently collide with the LDH surface before they diffuse into
the bulk solution where they react with DMB. As can be derived
(20) (a) Iu, K.-K.; Thomas, J. K. J. Photochem. Photobiol., A: Chem. 1993,
71, 55. (b) Iu, K.-K.; Thomas, J. K. J. Am. Chem. Soc. 1990, 112, 3319.
(c) Krasnansky, R.; Koike, K.; Thomas, J. K. J. Phys. Chem. 1990, 94,
4521. (d) Clennan, E. L.; Chen, M. F. J. Org. Chem. 1995, 19, 6004.
9
6924 J. AM. CHEM. SOC. VOL. 129, NO. 21, 2007

Oxygenation of Unsaturated Organics with 1O2
A R T I C L E S
liquid system are the simple recovery of the solid Mo catalysts
and the use of halogen-free solvents. One has to concede,
however, that the use of the oxidant is somewhat less efficient
due to competitive decay routes in the interior of the LDH
granules.
Entries 1 and 2 focus on the nonradical bleaching of
polyaromatic hydrocarbons in ambient conditions. Rubrene (1)
and diphenylisobenzofuran (2) are frequently used as model
substrates for such reactions. Both colored aromatics are cleanly
converted into the colorless desired products.
Apart from these bleaching reactions, 1O2 plays also an
important role in the synthesis of some naturally occurring
endoperoxides. Ascaridole (entry 3) was used as a remedy
against mawworm and is the first endoperoxide which is
1
produced by using O2 on a preparative scale. Nowadays, if
readily accessible, the endoperoxide is a promising reactive
intermediate for the synthesis of monoterpene derivatives.1
Ascaridole is also the pharmacologically active principle in a
Peruvian medicinal plant, which has been used as nervine and
antirheumatic drug.22 The Mo-LDH/H2O2 is able to transform
R-terpinene (3) almost completely into its 1,4-endoperoxide,
ascaridole.
Figure 10. Parity plot for the kinetic model on Mo-LDH-2. Linear least-
squares fit gives R2 ) 0.988. AO2 ) peroxide of substrate A.
1
The following general characteristics of O2-mediated per-
oxidations in the solid-liquid system can be extracted from
the graph in Figure 9: (1) To maximize the oxidant efficiency,
it is important to work with substrate concentrations as high as
possible. (2) The oxidant efficiencies are higher for the more
reactive substrates. (3) The maximum oxidant efficiency is
determined by the type of catalyst used. The most efficient
catalysts are those with the lower MoO42- loading. For instance,
for Mo-LDH-3, the maximum efficiency for complete conver-
sion can be estimated from Figure 9 by extrapolation to ∞
substrate concentration, giving a value of about 76%. Note that
Singlet oxygen is also the oxidant of choice for the synthesis
of allylic hydroperoxides. This route is even more promising
than the classical autoxidation because of selectivity reasons.
Entries 4-14 summarize the results for several cyclic and
1
aliphatic olefins. Typically, the affinity for O2 increases with
the degree of alkyl substitution of the double bond (com-
pare entries 4 with 5). Consequently, 2,3-dimethyl-2-butene (3)
needs less H2O2 than 2-methyl-2-pentene (4) for complete
reaction.
1
this value is considerably lower than the 92% obtained with
The typical regioselectivity of the O2 hydroperoxidation is
2-
2-
the soluble MoO4
.
well preserved with the LDH-MoO4 catalyst. For instance,
1-methyl-1-cyclopentene (6) gives the three hydroperoxide
products in the proper ratio of about 4:43:53. Regio- and
stereoselectivity of the singlet oxygen reaction are demonstrated
in entries 13 and 14. R-Pinene (13) forms only the trans-
pinocarveyl hydroperoxide. The cis-compound is not formed
due to steric hindrance of the bulky isopropylidene group toward
Finally, it is important to note that the applicability of the
Mo-LDH/H2O2 for synthetic peroxidations is restricted to
sufficiently reactive organic compounds. This is easily derived
from the graph. Suppose that the demanding requirements are
at least 95% consumption of the starting material with at least
20% efficiency of the oxidant; it then follows that one has to
work in the gray triangle in Figure 9. This means that (kr +
kq)[A]0 must be larger than 3.4 × 105, in case Mo-LDH-3 is
used as catalyst. Taking into account realistic substrate con-
centrations of 2 M, suitable substrates have rate constants kr
toward 1O2 of at least 1.7 × 105 s-1 (and kq ∼ 0). The oxidation
of less reactive substrates is practically not recommended with
the LDH-Mo/H2O2 catalytic system, since these substrates
require too much H2O2 to be fully converted.
1
attack of O2. Note that by autoxidation at least three more
hydroperoxides would be formed along with some carbonylic
compounds.1d Likewise, only the trans-hydroperoxides of
2-carene (14) have been found. The product distribution, namely
24% endocyclic and 76% exocyclic allylic hydroperoxide, is
essentially identical with those of photooxidized reaction
mixtures.23
Reactions 8-12 and 15 are examples of industrial interest.
The corresponding allylic alcohols of geraniol (8), nerol (9),
citronellol (10), and linalool (11), which are easily obtained after
reduction of the hydroperoxides, are useful precursors for
substances in the fragrance chemistry (e.g., rose oxides from
citronellol).1 The hydroperoxide of terpinolene (12) is used for
the manufacture of terpineol, whereas the hydroperoxide of
mesitol (15) is a valuable intermediate in the synthetic route to
vitamin E.24
Peroxidation of Unsaturated Organics on a Preparative
Scale. In the batch reactions of Table 2, 1 g of Mo-LDH-4
typically produces 1-10 g of the desired product, or 30-280
mol of product can be obtained/mol of exchanged MoO42-. This
2-
highlights that the peroxidations are indeed catalytic in MoO4
.
As a comparison, the MoO42--exchanged resin of McGoran and
Wyborney13d produces at most 0.5 mol of products/mol of Mo.
Besides the high productivity, the high activity of the Mo-
LDH is also an appreciable advantage. For instance in the case
of citronellol (10), 8.6 mol of product/mol of catalyst can be
obtained/h, which is slightly higher than the activity (5.6
mol‚(mol-1 h-1)) of the microemulsion catalytic system of
Aubry et al.22 Moreover, important advantages of the solid-
(21) Lever A. B. P. Inorganic Electronic Spectroscopy; Elsevier: London, 1984.
(22) Okuyama, E.; Umeyama, K.; Saito, K.; Yamazaki, M.; Motoyoshi, S. Chem.
Pharm. Bull. 1993, 41, 1309.
(23) Gollnick, K. AdV. Photochem. 1968, 6, 78.
(24) Ullmann’s Encyclopedia of Industrial Chemistry, 6th ed.; Electronic Release,
1998.
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J. AM. CHEM. SOC. VOL. 129, NO. 21, 2007 6925

A R T I C L E S
Sels et al.
It is important to notice that, in the latter case (entry 15), the
use of Mo-LDH is not an attractive option due to the extremely
low oxidant efficiency (about 4%). Nevertheless, on the basis
of calculations with our kinetic model, mesitol belongs to the
group of organic compounds that should be oxidized completely
using at most 10 equiv of H2O2. This deviation can be
rationalized if we take into account the basic properties of the
LDH support. Indeed, due to this basicity, mesitol mainly exists
in the suspension in its deprotonated form. Since it is known
that the rate of physical quenching by phenolates is larger than
A model has been elaborated that allows the estimation of
the peroxide production in terms of reaction rates and oxidant
efficiency. The model is based on Wilkinson’s approach, and,
in comparison with this approach, needs only two additional
parameters in order to be practically useful for the heterogeneous
case: The R value expresses the fraction of H2O2 that is
converted to O2. The q value expresses the fraction of O2
that diffuses out of the LDH catalyst particle before being
quenched.
1
1
The efficiency of singlet oxygen trapping in the Mo-LDH/
1
3
their oxygenation rate, most of the O2 is lost as O2.16
2-
H2O2 system seems to depend on the location of the MoO4
in the LDH granule. To improve activity and oxidant efficiency,
work is in progress to manipulate the density and the polarity
of the LDH powder and the location of the molybdate within
the catalyst, e.g., by competitive exchange procedures.
Conclusion
The kinetics of oxygenation by singlet oxygen are well-known
for homogeneous media but less for heterogeneous systems. For
multiphasic media, the kinetics of 1O2 reactions in microemul-
sions have been studied. The present work is the first detailed
Acknowledgment. We are grateful to the Belgian Federal
Government for support in the frame of a IAP action on
Supramolecular Chemistry and Catalysis. CECAT is also
acknowledged.
1
kinetic study of oxidation of organics by O2, generated by a
heterogeneous catalyst. As the solid catalyst, we have used the
Mo-LDH material, which generates 1O2 from H2O2. Especially
for the production of hydroperoxides and endoperoxides derived
from reactive substrates, i.e., with low Foote-index â values,
this catalytic system is highly applicable. Although the yields
based on H2O2 are somewhat lower in comparison with soluble
molybdate, higher productivities are obtained as a result of the
improved disproportionation rates.
Supporting Information Available: Physicochemical data for
the catalysts and a full mathematical derivation of the com-
partmentalization model. This material is available free of charge
via the Internet at http://pubs.acs.org.
JA065849F
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6926 J. AM. CHEM. SOC. VOL. 129, NO. 21, 2007