Highly selective oxidation of alkylphenols to benzoquinones with hydrogen peroxide over silica-supported titanium catalysts: Titanium cluster site versus titanium single site

Article abstract of DOI:10.1002/adsc.200900109

Titanium-silica catalysts have been prepared by supporting titanium(IV) precursors with different nuclearity {mononuclear titanocene dichloride Ti(Cp)₂Cl₂, dinuclear titanium diethyl tartrate and the tetranuclear titanium peroxo complex (NH₄)₈[Ti ₄(C₆H₄O₇)₄(O ₂)₄].8H₂O} onto the surface of silica materials with different textural characteristics. The supported catalysts have been explored as highly active and reusable catalysts for the oxidation of 2,3,6-trimethylphenol (TMP) and 2,6-dimethylphenol (DMP) to 2,3,5-trimethyl-l,4- benzoquinone (TMBQ, vitamin E key intermediate) and 2,6-dimethyl-l,4- benzoquinone (DMBQ), respectively, using aqueous hydrogen peroxide as green oxidant. Catalysts prepared by grafting mononuclear Ti(Cp)₂Cl ₂ revealed a strong dependence of the product selectivity on the surface concentration of titanium active centers. Mesoporous materials with titanium surface concentration in the range of 0.61.0 Ti/nm² were identified as optimal catalysts for the transformation of alkylphenols to benzoquinones. Catalysts having <0.6Ti/nm² produced a mixture of benzoquinones and dimeric by-products. Conversely, when di-/tetranuclear titanium precursors were employed for the catalyst preparation, a diminution of the titanium surface concentration had no impact on the benzoquinone selectivity, which was typically as high as 96-99%. DR-UV spectroscopic studies revealed that the catalysts capable of producing alkylbenzoquinones with nearly quantitative yields possess titanium dimers and/or subnanometer-size clusters homogeneously distributed on a silica surface. On the contrary, catalysts with isolated titanium sites give a considerable amount of dimeric by-products. This is the first example which clearly demonstrates the advantages of titanium cluster-site catalysts over titanium single-site catalysts in hydrogen peroxidebased selective oxidation reaction.

Full text of DOI:10.1002/adsc.200900109

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DOI: 10.1002/adsc.200900109
Highly Selective Oxidation of Alkylphenols to Benzoquinones
with Hydrogen Peroxide over Silica-Supported Titanium
Catalysts: Titanium Cluster Site versus Titanium Single Site
Oxana A. Kholdeeva,a,* Irina D. Ivanchikova,a Matteo Guidotti,b,*
Claudio Pirovano,c Nicoletta Ravasio,b Marina V. Barmatova,a
and Yurii A. Chesalova
a
Boreskov Institute of Catalysis, Lavrentieva 5, Novosibirsk 630090, Russia
Fax : (+7)-3833-309-573; e-mail: khold@catalysis.nsk.su
CNR-ISTM and IDECAT Research Unit, via G. Venezian 21, 20133 Milano, Italy
b
Fax : (+39)-02-5031-4405; e-mail: m.guidotti@istm.cnr.it
Universitꢀ di Milano, Dip. Chimica IMA “L. Malatesta”, via Venezian 21, 20133 Milano, Italy
c
Received: February 17, 2009; Published online: July 27, 2009
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200900109.
Abstract: Titanium-silica catalysts have been pre- Catalysts having <0.6 Ti/nm2 produced a mixture of
pared by supporting titanium(IV) precursors with benzoquinones and dimeric by-products. Conversely,
different nuclearity {mononuclear titanocene dichlor- when di-/tetranuclear titanium precursors were em-
ide Ti(Cp)2Cl2, dinuclear titanium diethyl tartrate ployed for the catalyst preparation, a diminution of
and the tetranuclear titanium peroxo complex the titanium surface concentration had no impact on
ACHTUNGTRENNUNG(NH4)8ACHTUNGTRENNUNG[Ti4ACHTUNGTRENNUNG(C6H4O7)4(O2)4]·8H2O} onto the surface of the benzoquinone selectivity, which was typically as
silica materials with different textural characteristics. high as 96–99%. DR-UV spectroscopic studies re-
The supported catalysts have been explored as vealed that the catalysts capable of producing alkyl-
highly active and reusable catalysts for the oxidation benzoquinones with nearly quantitative yields pos-
of 2,3,6-trimethylphenol (TMP) and 2,6-dimethylphe- sess titanium dimers and/or subnanometer-size clus-
nol (DMP) to 2,3,5-trimethyl-1,4-benzoquinone ters homogeneously distributed on a silica surface.
(TMBQ, vitamin E key intermediate) and 2,6-di- On the contrary, catalysts with isolated titanium sites
methyl-1,4-benzoquinone (DMBQ), respectively, give a considerable amount of dimeric by-products.
using aqueous hydrogen peroxide as green oxidant. This is the first example which clearly demonstrates
Catalysts prepared by grafting mononuclear the advantages of titanium cluster-site catalysts over
Ti(Cp)2Cl2 revealed a strong dependence of the titanium single-site catalysts in hydrogen peroxide-
product selectivity on the surface concentration of ti- based selective oxidation reaction.
tanium active centers. Mesoporous materials with ti-
tanium surface concentration in the range of 0.6– Keywords: alkylphenols; heterogeneous catalysis; hy-
1.0 Ti/nm2 were identified as optimal catalysts for the drogen peroxide; quinones; selective oxidation; tita-
transformation of alkylphenols to benzoquinones. nium-silica catalysts
Introduction
which would employ heterogeneous catalysts and
clean oxidants, such as O2 and H2O2, has become a
The selective catalytic oxidation of organic com- widely accepted strategy in fine chemicals synthesis.[2]
pounds with environmentally benign, cheap and read- However, progress in this direction is obstructed by
ily available oxidants is the most economic and eco- the limited availability of efficient and stable hetero-
logical route to a wide variety of valuable oxygen- geneous catalysts for selective oxidations in the liquid
containing products and intermediates.[1] In past de- phase.[3] Much attention was focused on the elabora-
cades, the replacement of stoichiometric and homoge- tion of different approaches, leading to spatially well-
neous processes by environmentally friendly ones, separated active sites, uniform in composition and dis-
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Oxana A. Kholdeeva et al.
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tribution, on the surface, in the framework or within TMBQ selectivity and Ti surface concentration has
the cages of inorganic solids – the so-called heteroge- been noted during the analysis of the data on TMP
neous single-site catalysts.[4] Such catalysts are expect- oxidation acquired over various titanium silicate cata-
ed to combine the main advantages of homogeneous lysts.[17]
(activity and selectivity) and heterogeneous catalysts
In this paper, we report the synthesis of titanium-
(simplicity of recovering and recycling). The key strat- silica catalysts by supporting titanium precursors with
egies for their design involve insertion of an active different nuclearity onto the surface of ordered and
center into an inert matrix by grafting, framework non-ordered silica supports, their characterization and
substitution, tethering, encapsulation, intercalation, systematic study of their catalytic performance in
and some other techniques.[4,5]
H2O2-based oxidation of two representative alkylsub-
The remarkable properties of Ti,Si catalysts in se- stituted phenols, TMP and DMP.
lective oxidations with clean oxidants, such as H2O2
and t-BuOOH, are well-documented.[4,5a,6] Mesopo-
rous titanium silicates possess active Ti centers acces-
sible to large molecules (>1 nm kinetic diameter),
which makes them attractive for catalytic applications
in the transformation of bulky and richly functional-
ized fine chemicals. So far, the most studied reaction
catalyzed by mesoporous Ti,Si catalysts was the epox-
idation of alkenes.[6,7] The selective oxidation of phe-
nols is a less investigated topic although quinones
with different functional groups are powerful inter-
mediates in the production of fine chemicals, includ-
ing the synthesis of vitamins E and K, coenzyme Q,
and other valuable products.[2,8]
Pinnavaia and co-workers revealed that bulky 2,6-
di-tert-butylphenol (DTBP) can be selectively oxi-
dized to a mixture of the corresponding benzoquinone
and diphenoquinone using aqueous H2O2 as oxidant
and mesoporous Ti-HMS as catalyst.[9] Catalysts pre-
pared by post-synthesis methodology via grafting tita-
nium precursors on to the surface of MCM-41, KIT-1
We discuss the crucial factors that govern the reac-
and MCM-48 were also found to be active in the tion selectivity, address the question on the structure/
liquid phase oxidation of DTBP with aqueous selectivity relationships, and suggest a reaction mech-
H2O2.[10] However, the question about the ratio be- anism. Special attention is paid to the issue of catalyst
tween the monomeric and dimeric quinone products reusability, which is crucial for heterogeneous liquid
was not addressed, and the factors governing the phase oxidation processes.
product selectivity were not studied.
In 2000, some of us found that oxidation of 2,3,6-
trimethylphenol (TMP) to 2,3,5-trimethyl-1,4-benzo- Results and Discussion
quinone (TMBQ, a key intermediate in the synthesis
of vitamin E) can be accomplished with the yield of Catalyst Synthesis and Characterization
75–80% using aqueous H2O2 as oxidant and meso-
structured titanium silicate Ti-MMM as catalyst.[11] The supports used in this work were commercial non-
Similar results were obtained over Ti-MCM-41[12] and ordered silicas from different suppliers and meso-
later, over hydrothermally stable Ti-MMM-2.[13] Sur- structured materials MCM-41 and Ti-MMM-2 pre-
prisingly, TiO2-SiO2 aerogels appeared to be even pared by hydrothermal synthesis. The textural proper-
more selective catalysts for TMP oxidation, yields of ties of the supports are presented in Table 1 along
the target quinone as high as 95–98% being at- with the data on the concentration of OH groups on
tained.[14] In 2003, Tuel and Hubert-Pfalzgraf claimed the surface acquired by TGA method. The size of
high selectivities (98%) to TMBQ obtained using mesopores spanned in the range of 2.6 to 16.0 nm.
nanometric monodispersed titanium oxide particles The concentration of OH groups on the surface cov-
supported on mesostructured silicates, SBA-15, ered the range of 2.9–5.1 OH per square nanometer
MCM-41 and HMS.[15] Recently, we found that some (Table 1). In accord with literature,[13,18] low angle
catalysts prepared by grafting titanium(IV) onto com- XRD patterns of MCM-41 and Ti-MMM-2 exhibited
mercial mesoporous silica produce TMBQ with a a sharp primary (100) reflex as well as less intensive
nearly quantitative yield.[16] A correlation between higher order (110) and (200) diffraction reflexes, in-
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Table 1. Textural and surface properties of the silica supports.
Support
BET surface area S [m2 gÀ1]
Pore volume V [cm3 gÀ1]
Mean pore diameter D [nm]
OH/nm2[a]
Davicat
295
291
529
679
972
976
1.48
1.28
0.88
0.66
0.65
0.54
16.0
13.7
5.4
3.8
2.6
5.10
4.74
3.87
3.10
2.90
n.d.[b]
Davisil A
Davisil C
Nippon–Kasei
MCM-41
Ti-MMM-2
3.1
[a]
From TGA data, considering that 1 molecule of H2O forms from 2 OH groups.
Not determined.
[b]
dicating that the materials have an ordered hexagonal not able to introduce by grafting more than 0.5
structure of mesopore channels.
Ti/nm2 into MCM-41 channels. Note that both values
The use of post-synthesis techniques displays some are much less than the value of 5.5 Ti/nm2 for the sur-
advantages with respect to a conventional direct syn- face Ti density of an anatase 010 plane or the value
thesis of in-framework or in-matrix titanium silicates. of ca. 4 Ti/nm2 estimated as experimental monolayer
In particular, the amount of Ti sites loaded onto a dispersion of surface titanium oxide species on
support can be easily tuned, and a wide series of cata- SiO2.[19] In terms of silica-supported titanium species,
lysts with variable surface Ti concentrations can be the loading of 3 Ti/nm2 was estimated by Srinivasan
obtained. The textural and surface properties of some et al. to be the upper limit for dispersed titania on
representative Ti/SiO2 catalysts, prepared using the silica, which corresponds to monolayer coverage.[20]
mononuclear precursor Ti(Cp)2Cl2, with high Ti load- However, the authors could not achieve Ti loading
ings are presented in Table 2. The specific surface greater than 1 atom Ti/nm2 on the high surface area
area, pore volume and mean mesopore diameter had silicas using room temperature impregnation with
some tendency to decrease after the grafting proce- Ti(IV) isopropoxide.[20] In fact, maximal loadings
dure, as already reported.[7f,15]
around 1.4 Ti/nm2 are widely accepted as the upper
It is evident that the surface density of Ti atoms limit in the case of the mesoporous Merck silica func-
should increase with increasing Ti loading and/or de- tionalized with titanium isopropoxide.[21]
creasing surface area of the support. The Ti surface
The surface density of hydroxy groups (OH/nm2) in
concentration is roughly estimated from the specific the catalysts obtained by grafting Ti(Cp)2Cl2 was esti-
surface area of the support and Ti content (deter- mated by TGA, and the values are summarized in
mined by elemental analysis), and some values are Table 2. The hydroxy density of Ti-containing cata-
given in Table 2. Note that for grafted titanium cata- lysts is lower than that of the initial supports (Table 2
lysts, the preparation method excludes the occlusion vs. Table 1). However, the Ti-grafted catalysts showed
of Ti species in the bulk (within the walls) of the a higher concentration of hydroxy groups than expect-
silica solid, which justifies the correctness of such esti- ed, if the grafting reaction led only to isolated TiOH
mations.
species tripodally bound to three SiOH moieties of
Using the grafting methodology, we managed to the silica support (Scheme 1a). This allowed us to
achieve a high Ti concentration (1.1 Ti/nm2) on the suggest that dimers or higher oligomers (clusters) con-
surface of Davicat silica. On the contrary, we were taining OH bridges and similar to the diACTHNUTRGNE(NUG poly)nuclear
Table 2. Textural and surface properties of grafted Ti/silica catalysts.
Catalyst (wt% Ti)
BET surface area
Pore volume
Mean pore diameter
D [nm]
Ti/nm2[a]
OH/nm2[b]
S [m2 gÀ1]
V [cm3 gÀ1]
Ti/SiO2 Davicat (1.93)
Ti/SiO2 Davisil A (2.19)
Ti/SiO2 Davisil C (3.13)
Ti/SiO2 Nippon–Kasei (3.63)
Ti/MCM-41 (3.87)
286
301
479
592
922
548
1.46
1.26
0.80
0.50
0.63
0.25
15.4
12.8
4.9
3.3
2.6
0.82
0.94
0.74
0.70
0.50
0.95
3.58
2.78
2.64
2.02
2.01
n.d.[c]
Ti/Ti-MMM-2 (6.13)
3.0
[a]
Estimated based on BET surface area of the support and Ti content.
From TGA data, considering that 1 molecule of H2O forms from 2 OH groups.
Not determined.
[b]
[c]
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Scheme 1. a) Isolated Ti single-site species and b) possible
dinuclear Ti species supported on silica surface.
species, supposed earlier by Marchese et al.,[22] are
present in the samples of the grafted catalysts with
high Ti surface concentrations (Scheme 1b).
The presence of such species was confirmed by
DR-UV spectroscopy, which is a useful and available
technique to characterize the local geometry and
bonding environment of the titanium ions. It is widely
accepted in the literature that catalysts containing
site-isolated Ti atoms show a ligand-to-metal charge
transfer band with a maximum centered in the range
of 208–230 nm.[6,22,23] The position of the maximum de-
pends on the coordination number of Ti(IV) and re-
veals a red-shift upon interaction with H2O and other
coordinating molecules due to expansion of the titani-
um coordination sphere.[23] A shift of the maximum to
wavelengths ꢀ230 nm is usually attributed to the for-
[22,23]
À À
mation of Ti O Ti connectivities.
However, as
noted by Gao and Wachs,[23c] it is often not easy to
distinguish whether a change in the position of DR-
UV band is related to the alteration in the coordina-
tion number of Ti(IV) or in the nature of ligands.
DR-UV spectra of MCM-41-grafted titanium cata-
lysts are shown in Figure 1a. One can see that cata-
lysts with Ti loading ꢁ2 wt% exhibit a rather narrow
band with a maximum at 212 nm, which is typical of
isolated titanium sites, presumably, in tetrahedral co-
ordination.[6a,23] The increase in Ti loading to 3.9 wt%
leads to the broadening and red-shift of the DR-UV
band (Figure 1a). When 2 wt% of Ti was grafted onto
the non-ordered Nippon–Kasei silica (a support with
Figure 1. DR-UV spectra of Ti catalysts prepared by graft-
ing Ti(Cp)2Cl2 on a) MCM-41, b) Nippon-Kasei silica and c)
Davisil silicas.
a lower surface area than MCM-41; see Table 1), a sphere.[23] Although the nature of Ti in such catalysts
sample with a higher Ti surface density was evidently is not completely understood, most of authors attri-
produced, compared to the Ti/MCM-41 sample with bute broad bands centered at 240–250 nm to dimeric
the same Ti loading. This was manifested in the DR- and/or small oligomeric (clustered) titanium spe-
UV spectrum (Figure 1b) which showed a broad band cies.[22,23] Further band shifting to 260–290 nm is usual-
centered around 240 nm. As mentioned above, the ly interpreted in the literature as yn indication of a
band broadening and shifting to wavelengths gradual increase in the degree of oligomerization of
ꢀ230 nm indicate both an increase of the coordina- Ti atoms and formation of subnanometric titanium
tion number of Ti and the presence of species having oxide clusters.[15,23c,24] Note that DR-UV spectra of all
at least one Ti atom in the second coordination the samples with high Ti surface concentration (0.60–
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Highly Selective Oxidation of Alkylphenols to Benzoquinones with Hydrogen Peroxide
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1.0 Ti/nm2) reveal the characteristic broad band in the content (Figure 1b and c) and show a broad absorp-
range of 260–290 nm (Figure 1a–c). Some authors at- tion band centered around 250–280 nm, suggesting
tributed such bands to highly hydrated six-coordinat- the presence of titanium dimers/oligomers. Again, the
ed isolated Ti species.[6d] However, the red-shift of the DRS-UV band shifted to longer wavelengths with in-
band that we and other authors observed with in- creasing Ti loading in the samples, indicating a further
creasing Ti content in the sample strongly supports condensation of Ti species on the silica surface.
that the broad feature is mainly due to associated
Importantly, the DR-UV band edge in the spectra
rather than isolated Ti species. Note that the band at of all supported titanium catalysts here described was
215–220 nm, corresponding to isolated Ti sites, can always observed below 350 nm. Stein and co-workers
also be distinguished in the DR-UV spectra of the estimated titania cluster diameters of 1.2–1.6 nm (30–
catalysts with high Ti concentration (Figure 1a–c). Its 70 TiO2 units) for a UV band edge of 355 nm.[24b]
intensity is a bit higher for the ordered catalysts than Given in mind this estimation and the position of the
for the non-ordered ones, but generally it is rather DR-UV band edge in our catalysts, we may suggest
low compared to the broad feature centered at 260– that the number of Ti atoms in the clusters formed on
290 nm. Therefore, we may conclude that a cluster- the silica surface at high Ti loadings is, at least, lower
ized state of Ti predominates on the surface of the than 30 and the size of the clusters, most likely, does
grafted Ti/SiO2 catalysts with Ti density in the range not exceed 1 nm.
of 0.60–1.0 Ti/nm2.
DR-UV spectra of Ti4/SiO2 and Ti2/SiO2 catalysts techniques, which enables detection of early stages of
prepared using (NH4)8ACHUTTNGERNNNUG[Ti4ACTHNURTGENNGU(C6H4O7)4(O2)4]·8H2O and the emergence of TiO2 anatase microcrystallites in ti-
Raman spectroscopy is one of the most sensitive
titanium diethyl tartrate complex, respectively, are tanium silicates.[6a,23c,24,25] We used Raman spectrosco-
shown in Figure 2. Even for catalysts with a rather py to check the presence of TiO2 extra-phase species
low Ti loading (about 1 wt% of Ti), the spectra are in the supported Ti-catalysts obtained in this work.
very similar to those of Ti/SiO2 catalysts with high Ti No band at 140–145 cmÀ1 characteristic of anatase
was found (Figure 1S in the Supporting Information),
thus pointing out that titanium species are well dis-
persed and no TiO2 microcrystallites formed on the
silica surface.
Catalytic Oxidation
The results of the catalytic tests performed for the ox-
idation of TMP with 30% H2O2 over the catalysts pre-
pared by grafting Ti(Cp)2Cl2 onto various silica sup-
ports are summarized in Table 3. One can see that
practically for all catalysts TMP conversion was close
to 100% when a 75% molar excess of H2O2 with re-
spect to the stoichiometric ratio (2:1 for TMP conver-
sion to TMBQ) was employed. Previously, some of us
found that the use of a lower H2O2/TMP ratio results
in uncompleted TMP conversion because unproduc-
tive decomposition of the oxidant occurs along with
its consumption in the phenol oxidation reaction. As
a result, H2O2 efficiency in the TMP oxidation was
moderate (ca. 56%) over Ti single-site catalysts like
Ti-MMM-2.[13,17] Surprisingly, we revealed that H2O2
efficiency can be improved using grafted Ti/SiO2 cata-
lysts. Thus 95% TMP conversion and 94% TMBQ
yield were achieved with only 25% excess of H2O2
(that is 76% H2O2 efficiency) over Ti/SiO2 Davicat
(1.93 wt% Ti) catalyst.
The MCM-41-supported catalysts with Ti content
ꢁ2 wt% Ti (entries 8 and 9 in Table 3), which possess
isolated Ti centers according to their DR-UV spectra
(see Figure 1a), revealed the highest activity ex-
pressed as TOF values (2.7 minÀ1). This is close to the
Figure 2. DR-UV spectra of Ti catalysts prepared using a)
(NH4)8A[Ti4A(C6H4O7)4(O2)4]·8H2O and b) diethyl tartrate
Ti(IV) complex.
G
CHTUNGTRENNUNG
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Table 3. TMP oxidation with H2O2 over titanocene-derived grafted Ti/SiO2 catalysts.[a]
Entry
Catalyst
Ti loading [wt%]
TMP conversion [%]
TMBQ selectivity [%]
TOF[b] [minÀ1]
1
2
3
4
5
6
7
8
Ti/SiO2 Davicat
Ti/SiO2 Davicat
Ti/SiO2 Davicat
Ti/SiO2 Nippon–Kasei
Ti/SiO2 Nippon–Kasei
Ti/SiO2 Davisil A
Ti/SiO2 Davisil C
Ti/MCM-41
0.92
1.93
2.50
2.09
3.63
2.19
3.13
0.95
2.00
3.87
1.89
6.13
100
100
95
79
97
95
91
99
98
99
47
77
89
76
96
2.0
1.9
1.4
1.4
1.4
2.1
1.8
2.7
2.7
2.0
3.0
1.9
97
100
100
100
97
98
99
9
Ti/MCM-41
10
11
12
Ti/MCM-41
Ti-MMM-2[c]
100
99
Ti/Ti-MMM-2
[a]
Reaction conditions: TMP, 0.1M; catalyst, 0.006 mmol of Ti, H2O2, 0.35M, CH3CN 1 mL, 808C, 30 min.
Catalyst turnover frequency, TOF=(moles of TMP consumed)/(moles of Tiꢂtime), determined by GC from the initial
rates of TMP consumption.
[b]
[c]
Mesostructured Ti-MMM-2 catalyst prepared by hydrothermal synthesis, given for comparison with Ti/Ti-MMM-2.
activity found for the Ti-MMM-2 catalyst (entry 11) selectivity is rather moderate and usually does not
prepared by hydrothermal synthesis, which enclosed exceed 75–80%. Moreover, it decreases with further
mostly isolated Ti atoms in the framework.[13] The decreasing surface Ti density and reaches only 47%
constant activity of the Ti/MCM-41 samples with 0.95 for Ti/MCM-41 with 0.12 Ti/nm2 (entry 8). Figure 3
and 2.0 wt% of Ti implies that all the Ti centers shows TMBQ selectivity as a function of Ti surface
remain accessible for reactants and have the same concentration. Excellent selectivity (95–99%) is typi-
nature. In turn, the activity of the catalysts containing cally observed for catalysts having Ti densities in the
Ti dimers/clusters (those showing DRS-UV absorp- range of 0.6–1.0 Ti/nm2, which corresponds to cluster-
tion maxima at 240–290 nm) is a bit lower compared ized Ti centers as indicated by the DRS-UV study.
to the catalysts with isolated Ti atoms. Such a trend is An additional increase in titanium concentration
evident if we compare the TOF values of samples leads to a decrease in TMBQ selectivity (Figure 3),
with different Ti loadings on the same silica support. most likely, due to further aggregation of titanium
Indeed, the activities within the Ti/SiO2 Davicat and clusters, which finally produces TiO2-like extra-phase
Ti/MCM-41 series decrease after Ti loading reaches a species. The maximal selectivity, close to 100%, is
certain level (Table 3; entries 1 through 3 and 8 achieved around 0.7 Ti/nm2. Importantly, introducing
through 10). This might be explained either by some an additional amount of titanium by grafting
gradual reduction of the number of accessible Ti sites Ti(Cp)2Cl2 onto the surface of the Ti-MMM-2 in-
in the supported clusters and/or by changes in the
active site nature (e.g., altering coordination number
of Ti(IV) from 4 in a single site to 5 in a dimer, etc.).
Earlier, we found a very low activity for micropo-
rous TS-1 in the title reaction.[13] From the TOF
values given in Table 3, it is clearly seen that even a
large increase in mesopore diameter from 2.6 nm (Ti/
MCM-41) to 15.4 nm (Ti/SiO2 Davicat) does not
affect markedly the reaction rate. Therefore, internal
diffusion limitations do not play a primary role in
TMP oxidation over catalysts with mesopores larger
than 2.6 nm. In addition, the structure ordering has
no pronounced effect on the catalytic activity in this
reaction, as one can judge comparing the data given
in Table 3 for the catalysts with comparable Ti surface
concentrations.
It is worth noting that selectivity to TMBQ, in gen-
eral, does not correlate with catalytic activity
(Table 3). While the activity is high for catalysts
Figure 3. TMBQ selectivity versus Ti surface concentration
in the silica-grafted Ti catalysts. Reaction conditions as in
having isolated Ti sites (ca. 0.3 Ti atoms/nm2), their Table 3.
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Highly Selective Oxidation of Alkylphenols to Benzoquinones with Hydrogen Peroxide
FULL PAPERS
matrix catalyst (hence passing from 1.89 to 6.13 wt% (86%) for Ti/MCM-41 (0.5 Ti/nm2) and it reduced fur-
Ti, that is 0.25 and 0.95 Ti/nm2) results in a significant ther and attained only 69% for the catalyst having
increase of the quinone selectivity from 76 to 96%.
0.12 Ti/nm2, the main by-product being 2,2’,6,6’-tetra-
It is evident from the data in Table 3 and Figure 3 methyl-4,4’-diphenoquinone.
that no considerable effect of the nature of the meso-
porous silica support on the reaction selectivity was
found for a wide series of Ti/SiO2 catalysts. In fact, Active Site and Reaction Mechanism
the bell-shaped curve presented in Figure 3 is valid
for any sort of the grafted titanium catalyst, notwith- The structure-selectivity relationships established in
standing the morphology, the shape, the order, the this work allowed us to suggest that Ti dimers or
pore size or the specific surface area of the mesopo- small oligomers (clusters) are the active species re-
rous material. Hence, the surface Ti density is the key sponsible for the highly selective transformation of al-
parameter to be carefully controlled in order to kylphenols to alkylbenzoquinones. To verify this hy-
obtain an active and highly selective catalyst for TMP pothesis, we intentionally prepared and investigated
oxidation.
the catalysts using dinuclear and tetranuclear titanium
In order to extend these observations to other sub- complexes. With such precursors, the proximity of the
stituted phenols, we studied the oxidation of DMP Ti sites and the formation of surface polynuclear spe-
too and found a similar dependence of the quinone cies should be enhanced with respect to the use of
selectivity on the surface Ti density, as we recorded mononuclear precursors.
for TMP oxidation. The results are shown in Table 4.
The catalytic performances of the Ti4/SiO2 and Ti2/
As one can see, the selectivity to DMBQ achieved SiO2 catalysts (Table 5 and Table 6, respectively) were
97% over the Ti/SiO2 Davicat sample having an opti- very similar to the catalytic performance of the Ti/
mal Ti density (0.82 Ti/nm2). The selectivity was lower SiO2 catalysts with optimal Ti surface density (0.6–
Table 4. DMP oxidation with H2O2 over grafted Ti/SiO2 catalysts.[a]
Catalyst
Ti loading [wt%]
DMP conversion [%]
DMBQ selectivity [%]
TOF[b] [minÀ1]
Ti/SiO2 Davicat
Ti/MCM-41
Ti/MCM-41
1.93
3.87
0.95
93
90
90
97
86
69
1.6
1.7
1.9
[a]
Reaction conditions as in Table 3, with the only difference that DMP was used instead of TMP.
Catalyst turnover frequency, TOF=(moles of TMP consumed)/(moles of Tiꢂtime), determined by GC from the initial
[b]
rates of TMP consumption.
(C6H4O7)4(O2)4]·8H2O.[a]
Table 5. TMP oxidation with H2O2 over Ti4/SiO2 catalysts obtained using (NH4)8ACHTNUGRTNENUG[Ti4AHCTUNGTRENNGUN
Catalyst
Ti loading [wt%]
TMP conversion [%]
TMBQ selectivity [%]
TOF[b] [minÀ1]
Ti4/SiO2 Nippon–Kasei
Ti4/SiO2 Nippon–Kasei
Ti4/SiO2 Davisil C
1.19
1.50
1.26
96
95
99
99
99
96
1.6
1.5
2.0
[a]
Reaction conditions as in Table 3.
Catalyst turnover frequency, TOF=(moles of TMP consumed)/(moles of Tiꢂtime), determined by GC from the initial
[b]
rates of TMP consumption.
Table 6. TMP oxidation with H2O2 over Ti2/SiO2-catalysts obtained using Ti diethyltartrate complex.[a]
Catalyst
Ti loading [wt%]
TMP conversion [%]
TMBQ selectivity [%]
TOF[b] [minÀ1]
Ti2/SiO2 Nippon–Kasei
Ti2/SiO2 Nippon–Kasei
Ti2/SiO2 Davisil C
0.85
1.72
0.53
1.11
97
98
100
95
96
98
97
99
1.5
1.7
1.9
1.9
Ti2/SiO2 Davisil C
[a]
Reaction conditions as in Table 3.
Catalyst turnover frequency, TOF=(moles of TMP consumed)/(moles of Tiꢂtime), determined by GC from the initial
[b]
rates of TMP consumption.
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1.0 Ti/nm2). First, comparable TOF values were ob-
served for the Ti4/SiO2, Ti2/SiO2 and Ti/SiO2 catalysts,
thus pointing out a similar nature of the active centers
in all these catalysts. Secondly, both TMP conversion
and selectivity to TMBQ were very high (>95%)
over the Ti4/SiO2 and Ti2/SiO2 catalysts. In particular,
excellent selectivity (98–99%) was achieved even with
low Ti loading, such as 1.19% Ti4/SiO2 Nippon–Kasei
or 1.72% Ti2/SiO2 Nippon–Kasei, to be compared
with the inferior performance of 2.09% Ti/SiO2
Nippon–Kasei (Table 3; entry 4). So, in sharp contrast
with Ti/SiO2 catalysts, no decrease in TMBQ selectivi-
ty was observed with decreasing Ti loading when di-
nuclear or tetranuclear Ti precursors were used for
the catalyst preparation.
All these results strongly support the hypothesis
that the catalytic site necessary for the efficient and
selective 4-electron oxidation of alkylphenols into al-
kylbenzoquinones does enclose, at least, two Ti
atoms. The formation of doubly hydroxo-bridged Ti
dimers and higher oligomers upon grafting Ti(Cp)2Cl2
onto silica support was suggested by Marchese and
co-workers.[22] The formation of dinuclear Ti species
in silica-grafted Ti catalysts and high activity of dimer-
ic Ti species (both homogeneous and supported) in
alkene epoxidation were reported by a few authors.[26]
Recently, the efficiency of a binuclear Ti(IV) dihydr-
oxide site in catalyzing the epoxidation of olefins with
hydrogen peroxide was demonstrated by Panas et al.
using density functional theory calculations.[26e] Never-
theless, the majority of researchers still view isolated
Ti species as the species responsible for high yields of
epoxides. The latter point of view is, likely, valid for
reactions which proceed via heterolytic oxygen atom
transfer mechanisms (alkene epoxidation, hydroxyl-
Figure 4. The effect of different treatments on the DR-UV
spectrum a) and catalytic performance b) of Ti/SiO2 (Davi-
cat, 1.93 wt% Ti).
ation of aromatics), but it certainly cannot be applied the state of Ti in the grafted catalyst and the TMP ox-
to alkylphenol oxidation, the reaction which has been idation process are shown in Figure 4. It is clearly
established to occur through a homolytic mecha- seen that exposure of the preliminary calcined cata-
nism.[27] The results obtained in this work clearly dem- lyst to humid air resulted in a red shift of the DR-UV
onstrate the advantage of polynuclear Ti sites over Ti band, indicating increasing amount of 6-coordinated
single sites in the oxidation of alkylphenols to alkyl- Ti species upon adsorption of water. In turn, subse-
benzoquinones with hydrogen peroxide.
quent catalyst evacuation led to a blue shift and en-
It has been reported that as far as the Ti content in- hancing intensity of the 215 nm feature, thus pointing
creases, Brønsted acid sites appear and their relative out the increase of isolated and/or tetrahedrally-coor-
concentration increases with the amount of titanium dinated Ti species.[23] Importantly, both treatments re-
in the materials.[6] This implies that Ti dimers and sulted in decreasing selectivity to TMBQ compared to
oligomers formed on the catalyst surface are connect- the freshly calcined catalyst (Figure 4). Therefore, we
ed through OH bridges. Unfortunately, the modern may conclude that only calcination gives rise to the
level of spectroscopic techniques does not allow one formation of optimal active sites on the silica surface,
to distinguish unequivocally between Ti dimers and which most likely includes 5-coordinated (unsatu-
small oligomers (e.g., tetramers) and to determine the rated) hydroxo-bridged Ti dimers/clusters similar to
structure of the polynuclear Ti species which operate those shown in Scheme 1b.
in our catalytic system. Meanwhile, some suggestions
might be made based on the following experiments.
This conclusion is also supported by the unusual
effect of water on the TMP oxidation rate in the pres-
First, we found that a proper catalyst pre-treatment ence of the grafted Ti catalysts. Indeed, we found that
is extremely important to achieve high yields of qui- the reaction rate increases with decreasing water con-
none. The effects of different pre-treatments on both centration in the reaction medium (Figure 5). This is
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Highly Selective Oxidation of Alkylphenols to Benzoquinones with Hydrogen Peroxide
FULL PAPERS
and phenol compete for the adsorption site. Hence,
unsaturation of the coordination sites of titanium and
the presence of TiOH groups are crucial for the cata-
lyst efficiency in the title reaction.
To get a deeper insight into the reaction mechanism
over clusterized Ti centers, we performed a careful
analysis of the TMP oxidation by-products. Under the
reaction conditions shown in Table 3, the main identi-
À
À
fied by-products were the C C and C O coupling
dimers, 2,2’,3,3’,5,5’-hexamethyl-4,4’-biphenol and
2,3,6-trimethyl-4-(2,3,6-trimethyl)phenoxyphenol.
When we increased the concentrations of reactants
keeping the molar ratios constant, the selectivity to
TMBQ decreased markedly (see Table S1 in the Sup-
porting Information) and some other by-products
(Scheme S1 in the Supporting Information), also con-
sistent with a homolytic oxidation mechanism, were
found. Additionally, the formation of phenoxyl radi-
cals during TMP oxidation over the Ti/SiO2 catalyst
was confirmed by EPR techniques using the spin-trap
agent, 3,5-dibromo-4-nitrosobenzene-sulfonic acid
(DBNBS) (Figure S2 in the Supporting Information).
The observed EPR spectrum was very close to the
EPR spectra of DBNBS adducts with TMP phenoxyl
radicals acquired earlier for TiO2-SiO2 aerogel.[27]
All these facts collectively corroborate a homolytic
mechanism for alkylphenol oxidation to quinone,
which is tentatively shown in Scheme 2. A dinuclear
active site may undergo chemisorption of two H2O2
molecules and ensure a close proximity of two
TiOOH species. Provided TMP concentration is kept
Figure 5. The effect of water concentration on TMP con-
sumption a) and TMBQ accumulation b) rates over Ti/SiO2
(Davicat, Ti 1.97 wt%; reaction conditions as in Table 3).
not a typical behaviour for Ti,Si catalysts.[28] It has
been found that water addition to a reaction solvent
accelerated the catalytic oxidation of phenol with hy-
drogen peroxide over TS-1.[28a] For TiO2-SiO2 aero-
gels, some of us observed a bell-shape dependence of
the reaction rate on the amount of water in the cata-
lytic system.[28b] The initial increase of the oxidation
rate is usually explained by the necessity to hydrolyze
À À
À
the Ti O Si bond to produce Ti OH, which easily
reacts with H2O2 with the formation of an active hy-
droperoxo titanium species. The different perfor-
mance that we observed for the grafted Ti catalysts is
caused, most likely, by the specificity of the prepara-
tion method.[4] Since the formation of more than
À À
three Si O Ti linkages to the mesoporous walls
would be sterically difficult to achieve, each Ti is ex-
À
pected to have at least one Ti OH group resulting
from the conversion of the remaining h5-cyclopenta-
À
dienyl-Ti group into Ti OH group during calcination.
Hence, these catalysts have already Ti OH species on
the catalyst surface to accomplish the reaction with
H2O2, and the adsorption of water results in expan-
sion of Ti coordination sphere, which is indicated by
the DRS-UV study (see Figure 4). This process leads
À
Scheme 2. Tentative mechanism of alkylphenol oxidation
to reduction of the TMP oxidation rate because water with H2O2 over a dimeric Ti site.
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low enough and the H2O2 concentration is oppositely
kept high (these are exactly the conditions which
favour TMBQ formation[11]), one phenol molecule
can be adsorbed by the reactive center that involves
two adjacent hydroperoxo titanium groups. Interac-
tion with the first TiOOH group produces a phenoxyl
radical which is oxidized immediately by the second
TiOOH, giving rise to an intermediate quinol product,
which is, in turn, oxidized rapidly to the final quinone.
As a consequence, recombination of phenoxyl radi-
À
À
cals leading to the undesired C C and C O dimeric
by-products is minimized or, in the best cases, sup-
pressed over the dimeric Ti site, while the yield of the
target quinone is enhanced compared to a single Ti
site.
Catalyst Stability and Re-usability
The question about the stability of a heterogeneous
catalyst is crucial for liquid phase oxidations.[2,3] Thus,
chemical analysis of the mother liquor after reaction
of DTBP oxidation over grafted Ti-MCM-48 showed
that about 1.6% of Ti had been leached.[10b] It is a
matter of common observation that monopodal titani-
um species are more prone to be leached and hence
the use of a support with higher silanol density in-
creases the stability of the catalyst. Elemental analysis
data confirmed that no titanium leaching from the Ti/
SiO2, Ti2/SiO2 and Ti4/SiO2 catalysts occurs under the
conditions used for TMP oxidation. Following the
methodology suggested by Sheldon et al.,[3a] we found
neither TMP conversion nor TMBQ accumulation in
the filtrate after fast hot catalyst filtration at about
40% TMP conversion (Figure S3 in the Supporting
Information), which proved a true heterogeneous
nature of catalysis over Ti/SiO2.
Figure 6. Catalyst recycling in TMP oxidation with 30%
H2O2 over Ti/SiO2 (Nippon–Kasei): a) Ti 2.09 wt% and b) Ti
3.63 wt%.
To assess the propensity of the materials to be re-
covered and reused, the Ti/SiO2, Ti2/SiO2 and Ti4/SiO2
catalysts were recycled in several consecutive catalytic
runs. We found that the Ti/SiO2 catalysts with a
higher Ti concentration are more stable to deactiva-
tion than the catalysts with a lower Ti loading
(Figure 6). The recycling behaviour of the Ti2/SiO2
and Ti4/SiO2 catalysts was similar to that of Ti/SiO2
with high Ti surface density (Figure S4 in the Sup-
porting Information). The catalyst re-usability can be
further improved by using more concentrated H2O2.
Figure 7 demonstrates that with 70% H2O2, the opti-
mal Ti/SiO2 catalyst can be used repeatedly for at
least four catalytic runs without a loss in both activity
and selectivity. Therefore, the grafted Ti catalysts re-
vealed one more advantage in comparison with in-
matrix Ti,Si catalysts, for which the TMP oxidation
rate decreased significantly when concentrated H2O2
was employed and, as a result, no complete substrate
conversion could be achieved.[6f,11]
Figure 7. Catalyst recycling in TMP oxidation with 70%
H2O2 (0.5M) over Ti/SiO2 (Nippon–Kasei, Ti 3.63 wt%).
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Highly Selective Oxidation of Alkylphenols to Benzoquinones with Hydrogen Peroxide
FULL PAPERS
ed 4 ꢃ molecular sieves. All other reactants were obtained
commercially and used without further purification.
Conclusions
Commercial amorphous silica supports, herein mentioned
as SiO2 Davicat, SiO2 Davisil and SiO2 Nippon–Kasei, were
received directly from the suppliers (Grace Davison and
Nippon–Kasei). Nippon–Kasei silica was used without any
further pre-treatment. Davicat and Davisil silicas were
washed in 1M aqueous HNO3, thoroughly rinsed and dried
in oven prior to use. MCM-41 was prepared using cetyltri-
methylammonium bromide (CTAB) as surfactant following
the literature procedure.[18]
The investigation of the relationships between compo-
sition, structure and selectivity of the grafted titani-
um-silica catalysts in alkylphenol oxidation to benzo-
quinones with hydrogen peroxide revealed an out-
standing performance of the mesoporous solids with
Ti surface concentration in the range of 0.6–1.0 Ti/
nm2. Using such catalysts, TMBQ (vitamin E precur-
sor) and other benzoquinones can be obtained with
nearly quantitative yields. The DRS-UV study identi-
fied dimers and/or subnanometer-size clusters homo-
geneously distributed on a silica surface as the opti-
mal catalytic sites for this reaction.
Ti/SiO2 catalysts were synthesized by supporting a titani-
um precursor onto the surface of various silicas. Titanocene
dichloride Ti(Cp)2Cl2, dinuclear titanium diethyl tartrate
complex [Ti
ACHTUGTNRENNU(G tartrate)ACHUTNGTREN(NUGN i-PrO)2]2 and peroxo complex (NH4)8
A
CHTUNGTRENNUNG
The silica-supported Ti catalysts intentionally pre-
pared to obtain Ti dimers and/or clusters via immobi-
lization of peroxocitrate or tartrate di/tetranuclear Ti
precursors displayed excellent properties in terms of
selectivity to quinones regardless of Ti loading, thus
confirming the superior performance of clusterized Ti
sites with respect to isolated Ti single sites. This phe-
nomenon can be explained by the particularly favour-
able interaction between the phenolic substrate and
(at least) two contiguous Ti hydroperoxo groups, as
indicated by the suggested oxidation mechanism.
Yet, the results of this study show that the catalysts
with di-/polynuclear Ti active sites have advantages of
a better H2O2 efficiency and improved re-usability in
alkylphenol oxidation compared to single-site Ti cata-
lysts.
Finally, the bell-shaped curve displaying the de-
pendence of the selectivity on surface Ti density ob-
tained in this work can be used as a predictive tool
for the preparation of catalysts with optimized perfor-
mance for the oxidation of substituted phenols to re-
lated quinones with hydrogen peroxide. Using this
curve, one can evaluate the optimal amount of Ti pre-
cursor necessary for grafting onto the surface of silica
with known textural properties.
hereinafter referred to as Ti/SiO2, Ti2/SiO2 and Ti4/SiO2, re-
spectively. Grafting of Ti(Cp)2Cl2 was accomplished by
adapting the method developed by Maschmeyer, et al.[4a,7f]
Ti2/SiO2 catalysts were prepared as follows. Nippon–Kasei
and Davisil C silicas were pre-treated at 3008C for 1 h in
open air and 1 h under vacuum, then cooled down under
vacuum. A solution of an equimolar mixture of l-(+)-dieth-
yl tartrate and TiACTHNUTRGNE(UNG O-i-Pr)4 in anhydrous dichloromethane
was added to the pre-treated silica under argon atmosphere
using standard Schlenk techniques. The suspension was
maintained under stirring for 3 h and then the solvent was
removed by evaporation under reduced pressure and dried
under vacuum overnight. The samples were calcined at
5508C for 3 h under oxygen flow (100 mLminÀ1). Ti4/SiO2
catalysts were prepared by impregnation at the rotary evap-
orator from aqueous solution of the complex
AHCNUTRTGEGNU(NN NH4)8AHCTUNRTGENG[NUN Ti4ACTHUNGTRNE(NGUN citrate)4(O2)4]·8H2O onto the Nippon–Kasei and
Davisil C pre-treated silicas. The impregnated samples were
calcined at 5508C for 3 h under an oxygen flow. Ti-MMM-2
was prepared by hydrothermal synthesis under moderately
acidic conditions following a slightly modified procedure de-
scribed earlier.[13] All the catalysts were calcined at 5608C
for 5 h in air directly prior to use in catalytic tests or before
physical measurements. The catalysts were characterized by
elemental analysis, N2 adsorption, thermogravimetric analy-
sis (TGA) and DRS-UV measurements.
General Procedure for Catalytic Oxidation of
Alkylphenols
Catalytic oxidations with H2O2 were performed under vigo-
rous stirring (500 rpm) in thermostated glass vessels. The
rate of phenol substrate consumption remained constant
when the stirring rate varied in the range of 200–1000 rpm,
indicating no external diffusion limitation. Typically, the re-
actions were initiated by adding 0.35 mmol H2O2 to a mix-
ture, containing 0.1 mmol of TMP (or DMP), 8–28 mg of Ti
catalyst (to introduce 0.006 mmol of Ti), internal standard
(biphenyl), and 1 mL of a solvent (anhydrous CH3CN). The
reaction time was 15–40 min. Samples of the reaction mix-
ture were withdrawn periodically during the reaction by a
syringe through a septum. Each experiment was reproduced
4–6 times.
Experimental Section
Reagents and Catalysts
2,3,6-Trimethylphenol (TMP) and 2,6-dimethylphenol
(DMP) were purchased from Fluka and used without addi-
tional purification. Reference 2,2’,3,3’,5,5’-hexamethyl-4,4’-
biphenol (BP) was prepared as described elsewhere.[29] The
tetranuclear ammonium citratoperoxotitanate(IV) complex,
(NH4)8ACHTUNGTRENNUNG[Ti4ACHTUNGTRENNUNG(C6H4O7)4(O2)4]·8H2O, was synthesized according
to the literature protocol.[30] Dinuclear titanium diethyl tar-
trate complex was prepared from l-(+)diethyl tartrate (Al-
drich) and TiACHTUNGTRENNUNG(O-i-Pr)4 (Fluka) taken in equimolar amounts
following the protocol proposed by Sharpless et al.[31] Hy-
drogen peroxide (ca. 30%) was determined iodometrically
prior to use. Acetonitrile was dried and stored over activat-
The oxidation products were identified by GC-MS and
1H NMR (see the Supporting Information). The TMBQ and
DMBQ yields as well as TMP and DMP conversions were
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Oxana A. Kholdeeva et al.
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quantified by GC using internal standard (diphenyl). Turn-
over frequency values (TOF) were determined from the ini-
tial rates of substrate consumption. In order to isolate qui-
none product, the reaction mixture was diluted with water
followed by extraction with CH2Cl2. After concentration
under vacuum, pure TMBQ (or DMBQ) was isolated by
preparative thin-layer chromatography on silica gel using
gradient elution with hexane and hexane/ethyl acetate.
1H NMR spectra of the isolated quinone products were
identical to those of the authentic samples.
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Instrumentation
Gas chromatographic analyses were performed using a gas
chromatograph “Tsvet-500” equipped with a flame ioniza-
tion detector and a quartz capillary column (30ꢂ0.25) filled
with Supelco MDN-5S. GC-MS analyses were carried out
using a gas chromatograph Agilent 6890 (quartz capillary
column 30 mꢂ0.25 mm/HP-5ms) equipped with a quadru-
pole mass-selective detector Agilent MSD 5973. 1H NMR
spectra of the reaction products were recorded on an AM
250 Bruker spectrometer. Nitrogen adsorption measure-
ments were carried out at 77 K using an ASAP-2400 instru-
ment (Micromeritics) instrument within the partial pressure
range 10À6–1.0. Before measurements, the samples were de-
gassed at 908C during 48 h. Thermogravimetric analysis
(TGA) was performed using a Perkin-Elmer Pyris appara-
tus. The analyses were run under an oxygen flow
(25 mL minÀ1) in the temperature range between 508C and
10008C. DRS-UV measurements were performed on a Shi-
madzu UV-VIS 2501PC spectrophotometer. Raman spectra
were recorded on a RFS 100/S Bruker spectrometer with
1064 nm excitation line of an Nd-YAG laser operating at a
power of 100 mW. EPR spectra were recorded at room tem-
perature using a Bruker ER-200D spectrometer. Experi-
ments in the presence of the spin trap DBNBS were carried
out as described previously.[27]
Acknowledgements
The authors are grateful to V. A. Rogov for GC-MS analyses,
O. V. Zalomaeva for EPR, and E. Gavrilova for TGA meas-
urements. OAK thanks CNR for the Short Term Mobility
Program grants (Nos. 30212, year 2006 and 50558, year
2007).
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