Titanium-silica catalyst derived from defined metallic titanium cluster precursor: Synthesis and catalytic properties in selective oxidations

Article abstract of DOI:10.1016/j.ica.2017.06.059

A class of titanium-grafted mesoporous silica catalysts has been designed and prepared starting from molecularly defined metal clusters. The organosol mixture of zerovalent Ti₁₃ clusters was impregnated onto the surface of ordered mesoporous silica molecular sieves (MCM-41 and MMM-2) and, after high-temperature calcination, an evenly dispersed non-single-site Ti(IV)_nO_x-like silica-supported catalyst was obtained. The catalytic solids, fully characterized by microscopic, spectroscopic and porosimetric techniques, showed standard performance in the liquid-phase epoxidation of a cyclic alkene, as limonene, but remarkably high selectivity values in the oxidative carboxylation of styrene, with tert-butylhydroperoxide and carbon dioxide in the presence of tetrabutylammonium bromide as a cocatalyst. Unprecedented high yields, up to 67%, in styrene carbonate were achieved after 24 h, under solvent-free conditions. The catalysts displayed also a noteworthy stability of the performance to repeated recovery and reuse cycles.

Full text of DOI:10.1016/j.ica.2017.06.059

Accepted Manuscript
Research paper
Titanium-Silica Catalyst derived from Defined Metallic Titanium Cluster Pre-
cursor: Synthesis and Catalytic Properties in Selective Oxidations
Claudio Evangelisti, Matteo Guidotti, Cristina Tiozzo, Rinaldo Psaro, Nataliya
Maksimchuk, Irina Ivanchikova, Alexandr N. Shmakov, Oxana Kholdeeva
PII:
S0020-1693(17)30555-8
http://dx.doi.org/10.1016/j.ica.2017.06.059
ICA 17708
DOI:
Reference:
Inorganica Chimica Acta
To appear in:
Received Date:
Revised Date:
Accepted Date:
21 April 2017
25 June 2017
27 June 2017
Please cite this article as: C. Evangelisti, M. Guidotti, C. Tiozzo, R. Psaro, N. Maksimchuk, I. Ivanchikova, A.N.
Shmakov, O. Kholdeeva, Titanium-Silica Catalyst derived from Defined Metallic Titanium Cluster Precursor:
Synthesis and Catalytic Properties in Selective Oxidations, Inorganica Chimica Acta (2017), doi: http://dx.doi.org/
10.1016/j.ica.2017.06.059
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Titanium-Silica Catalyst derived from Defined Metallic Titanium Cluster
Precursor: Synthesis and Catalytic Properties in Selective Oxidations
Claudio Evangelistia, Matteo Guidottia, Cristina Tiozzoa, Rinaldo Psaroa,
Nataliya Maksimchukb,c, Irina Ivanchikovab, Alexandr N. Shmakovb,c,d, Oxana Kholdeevab,c
a Istituto di Scienze e Tecnologie Molecolari (CNR-ISTM), C. Golgi 19, 20133 Milano, Italy
b Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk, 630090, Russia
c Novosibirsk State University, Pirogova 2, Novosibirsk, 630090, Russia
d Budker Institute of Nuclear Physics, Lavrentieva 11, Novosibirsk, 630090, Russia
Abstract
A class of titanium-grafted mesoporous silica catalysts has been designed and prepared starting
from molecularly defined metal clusters. The organosol mixture of zerovalent Ti13 clusters was
impregnated onto the surface of ordered mesoporous silica molecular sieves (MCM-41 and MMM-
2) and, after high-temperature calcination, an evenly dispersed non-single-site Ti(IV)nOx-like silica-
supported catalyst was obtained. The catalytic solids, fully characterized by microscopic,
spectroscopic and porosimetric techniques, showed standard performance in the liquid-phase
epoxidation of a cyclic alkene, as limonene, but remarkably high selectivity values in the oxidative
carboxylation of styrene, with tert-butylhydroperoxide and carbon dioxide in the presence of
tetrabutylammonium bromide as a cocatalyst. Unprecedented high yields, up to 67%, in styrene
carbonate were achieved after 24 h, under solvent-free conditions. The catalysts displayed also a
noteworthy stability of the performance to repeated recovery and reuse cycles.
dedicated to Dr. Carlo Mealli
1

Keywords
Titanium clusters; heterogeneous catalysis; alkene epoxidation; oxidative carboxylation;
mesoporous silica
Highlights
•
•
•
Heterogeneous titanium-grafted silica catalysts obtained from molecularly defined
zerovalent Ti13 clusters
Uniformly dispersed non-single-site titanium oxide-like catalytic sites supported onto
ordered mesoporous molecular sieves
TixOn-MCM-41 and TixOn-MMM-2 solids active as oxidation catalysts with tert-
butylhydroperoxide
•
•
Conventional performance in the liquid-phase epoxidation of limonene
Promising performance and high yields in oxidative carboxylation of styrene with tert-
butylhydroperoxide and carbon dioxide
•
Remarkable stability of the catalytically-active sites to repeated recycles under reaction
conditions
Graphical Abstract
O
CO2
O
O
+ TBABr
mesoporous SiO2
25°C
O2(g)
TinOx
Ti13-THF
+
TBHP
500°C
SiO2
O

1. Introduction
The deposition of well-defined metal-containing precursors via post-synthesis techniques onto
structured porous inorganic oxides is a versatile and convenient method to prepare heterogeneous
catalysts with tailored catalytic performances [1,2,3]. It is possible to select the support with the
most suitable characteristics (elemental composition, presence of dopant species,
hydrophilic/hydrophobic character, morphology, topology and hydrothermal stability) and the most
promising catalytic metal centres in terms of chemical nature, loading, redox properties and/or acid-
base character [4]. By this approach, active metal sites can be added to an oxidic porous support by
grafting, i.e. by irreversible deposition of a precursor metal species (typically an inorganic or
organometallic complex) and formation of covalent bonds between the metal centre and the support.
As a final result, the metal site presents different chemical features with respect to the pristine
precursor species it is derived from, since the chemical surroundings around the metal centre is
partially (or totally) modified during the chemical chemisorption of the precursor and, in most
cases, during the further final thermal treatment [5,6].
The deposition and grafting strategies have been widely explored for the preparation of
titanium-silica catalysts [7,8]. For instance, the catalyst for alkene epoxidation developed by Shell
for the styrene monomer-propylene oxide (SMPO) process is considered a milestone in the
preparation of grafted titanium(IV)-silica systems and it is obtained by a multistep vapour-phase
process by deposition of the titanium precursor (TiCl4 or an organotitanium compound), heating the
obtained material, followed by steaming and silylation [9]. In this case, the Ti(IV)/SiO2 catalyst is
active in epoxidation in the presence of alkylhydroperoxides as oxidants thanks to the formation of
site-isolated Ti species on the surface of the support and the increased Lewis acidity of the Ti(IV)
sites due to electron withdrawing effect by the siloxy ligands [10]. Since then, an exponential
growth of Ti(IV)-silica catalysts obtained via post-synthesis grafting has been observed. Among the
most used precursor, one can mention inorganic precursors as well as organometallic complexes
3

with an increasing level of complexity in order to direct as best the isolation, dispersion and
nuclearity expected for the metal sites in the final catalysts: for instance, TiCl4 [11], Ti(OiPr)4
[12,13,14], Ti(OBu)4 [15] or Ti(OEt)4 [16], TiF4 [17] or Ti(triethanolaminate)-isopropoxide [18],
Ti(η5-C5H5)2Cl2 [19,20], [{Ti(OiPr)2(OMenth)}2] and [Ti(OMenth)4] (OMenth = 1R,2S,5R-(-)-
menthoxo) [21], [(tBuO)2Ti{µ-O2Si[OSi(OtBu)3]2}]2 [22], [Ti6(µ3-O)6(µ-O2CC6H4OPh)6(OEt)6]
[23] or (NH4)8[Ti4(C6H4O7)4(O2)4]·8H2O [24,25]. These species can be grafted either from solutions
in liquid phase or directly from vapour phase onto either ordered (such as MCM-41, MCM-48 and
SBA-15 molecular sieves) or non-ordered mesoporous silicas. Alternatively, Ti(IV) species can be
introduced into the silica matrix in one step, e.g., by direct synthesis [26] or by evaporation-induced
self assembly (EISA) [27]. All these titanium-containing silica systems showed interesting results in
selective oxidation reactions, such as the epoxidation of alkenes and unsaturated alcohols [28,29] or
oxidative carboxylation of alkenes in the presence of tert-butylhydroperoxide (TBHP) [30,31,32],
as well as oxidation of substituted phenols to quinones with aqueous hydrogen peroxide (for recent
reviews, see ref. 8)
However, to our best knowledge, the use of metallic titanium clusters as precursors to be
deposited on silica supports has not been explored yet. In fact, most of the synthetic strategies
reported so far aimed at obtaining single-site heterogeneous catalysts in which the Ti centres could
be dispersed as evenly as possible, avoiding the formation of titanium oxide-like domains that may
be detrimental for the preparation of efficient oxidation catalysts [33]. However, it has been
observed in several reports that the site isolation of Ti(IV) atoms is not mandatory to have an
effective and selective system and that the formation of nanosized TiO2 domains at the surface
could give rise to good catalysts as well [13,23,24,27,33,34]. For these reasons, in the present work,
a multinuclear titanium(0) metallic cluster has been chosen as a starting precursor for the
preparation of a mesoporous grafted titanium-silica catalyst to be tested in the selective oxidation of
two substrates with different characteristics. The following calcination at high temperature gives
rise to the catalytically active Ti(IV) sites with particular properties that are worth to be studied as
4

heterogeneous catalysts in the liquid-phase epoxidation of cyclic alkenes and in the oxidative
carboxylation of styrene, in the presence of carbon dioxide.
2. Experimental
2.1. Materials
All operations involving Tin-THF colloidal solution were performed under a dry argon atmosphere
with the use of the standard Schlenk techniques. TiCl4⋅2THF (97%), THF (puriss. p.a., ACS
reagent, RPE ≥ 99.9%, distilled under inert atmosphere prior to use), K[BBu3H] (K-Selectride, 1.0
M in THF solution), (R)-(+)-limonene (97%; 98% e.e.), CH3CN (HPLC grade) and TBHP (5.5 M in
n-decane solution) were all Aldrich products and used as received. Other reactants were obtained
commercially and used without additional purification.
2.2. Synthesis of the siliceous supports
MCM-41 support was prepared as previously reported, by using Aerosil 200 (Degussa) as a silica
source and cetyltrimethyl ammonium bromide (CTAB) as a surfactant. The molar ratio of the
synthesis mixture was: 1 SiO2/ 0.1 CTAB, 0.25 NaOH, 20 H2O. The slurry was maintained at
115°C for 24 h before being filtered, washed, dried at 80°C and calcined at 550°C for 8h.[35].
The mesoporous mesophase MMM-2 silica support was prepared by hydrothermal synthesis
under moderately acidic conditions (pH 3.0) following a two-step procedure described previously
[36], using sodium silicate as silicon source and cetyltrimethylammonium bromide (CTAB) as a
template.
2.3. Preparation of the titanium-silica catalysts
1.0 g of TiCl4·2THF was added to a three neck flask at 40 mL of THF and the solution was heated
up to 40°C. 12 mL of a solution of K[BBu3H] 1.0 M in THF were added dropwise to the solution,
5

under stirring. The addition of the borohydride salt was completed in 1 h and, after that, the solution
was left under magnetic stirring for the following 30 min. During the reaction the formation of a
white solid (KCl) was observed and it was removed by filtration under argon with a D4 fritted glass
filter. BBu3 and THF were removed from the brown filtrate under vacuum (10-4 mbar) and the
residue was dissolved in 80 mL of THF. The Tin-THF colloidal solution was thus stored in
refrigerator (-20°C) under inert atmosphere.
1.5 g of the siliceous support (MCM-41 or MMM-2) were pretreated at 150 °C under inert
atmosphere for 2.5 h, cooled and then treated in vacuo. The solid was suspended in 20 mL of dry
THF under argon atmosphere at 25°C and then 20 mL of the Tin-THF solution were added leaving
the mixture under stirring for 20 min. After that time, the stirring was then stopped and the
colourless supernatant liquid was removed by filtration. The obtained powder was dried under
vacuum and then calcined under dry oxygen at 500°C (heating temperature programme 10°C min-1)
for 2.5 h.
2.4. Characterisation
The titanium content in the catalysts was analyzed by inductively coupled plasma optical emission
spectroscopy ICP-OES (ICAP 6300 Duo, Thermo Fisher Scientific) after mineralization of the
sample with a 1:1 aqueous HF/HNO3 mixture.
Nitrogen adsorption measurements were carried out at 77 K using a NOVA 1200 instrument
(Quantachrome) within partial pressure in the range 10-4–1.0. The catalysts were degassed at 150°C
for 24 h prior to measurements. Surface areas of the samples were determined by BET analysis of
the low-temperature N2 adsorption data. Pore size distributions were calculated from the adsorption
branches of the nitrogen isotherms by means of the regularization procedure, using reference local
isotherms calculated in a cylindrical silica pore model, according to the density functional theory
(DFT) approach. Special software provided by Quantachrome Corp. was used for this purpose.
Mean pore diameters were calculated as mathematical expectation values from these distributions.
6

Transmission electron microscopy (TEM) analysis were performed by a ZEISS
LIBRA200FE microscope equipped with a 200 kV FEG source. Energy-dispersive X-ray
spectroscopy (EDS – Oxford INCA Energy TEM 200) and elemental mapping were collected along
with HAADF-STEM (high angular annular dark field scanning electron microscopy) images.
Powders of the samples were ultrasonically dispersed in isopropyl alcohol and then each suspension
were dropped onto a holey carbon coated copper grid (300 mesh) and evaporating the solvent.
XRD measurements were performed with a high-precision X-ray diffractometer mounted on
the beamline No. 2 of the VEPP-3 storage ring at the Siberian Synchrotron Radiation Centre
(SSRC). The state of titanium in the catalysts was monitored by DR UV–vis spectroscopy under
ambient conditions, by using a Shimadzu UV–VIS 2501PC spectrophotometer. FT-Raman spectra
(3600-100 cm-1, 300 scans, resolution 4 cm−1, 180° geometry) were recorded using a RFS 100/S
spectrometer (Bruker). Excitation of the 1064-nm line was provided by a Nd-YAG laser (100 mW
power output).
2.5. Catalytic Studies
GC analyses were carried out on an Agilent 6890 Series; HP-5 column, 30 m - 0.25 mm; FID
detector (for limonene oxidation) or a gas chromatograph Tsvet-500 equipped with FID and a
quartz capillary column (30 m×0.25 mm) filled with Agilent DB-5MS (for styrene oxidative
carboxylation). GC–MS analyses were performed by using an Agilent GC System 7890 interfaced
Inert MSD Network 5975C (HP-5MS column 30 m - 0.25 mm - 0.25 µm). GC peaks were
identified by comparison with peaks of genuine samples of reference standards and by means of
GC-MS. Oxidation products were quantified by GC using internal standards.
2.5.1. Epoxidation of R-(+)-limonene
The catalyst was pre-treated under dry air at 500°C (heating temperature programme 10°C min-1)
for 1 h prior to use. The solid was then cooled down under inert atmosphere. The oxidation
7

reactions were performed in a 10 mL-glass batch reactor at 90°C under inert atmosphere. To 100
mg of the catalyst, 5 mL of acetonitrile were added and then 0.162 mL (1.0 mmol) of R-(+)-
limonene and 0.220 mL (1.1 mmol) of TBHP (5.5 M in n-decane solution). The resulting mixture
was vigorously stirred (800 rpm). The n-decane present as a solvent in the solution of TBHP was
used also as internal standard. For each test, conversion of R-(+)-limonene and selectivity to
limonene monoepoxide (sum of limonene endocyclic 1,2-epoxide plus exocyclic limonene 8,9-
epoxide; Scheme 2) were recorded. Each test was reproduced at least 2 times. Neither significant
auto oxidation nor support-catalysed contributions to epoxidation were recorded with titanium-free
siliceous supports, limonene conversion being less than 5% with no remarkable epoxide formation.
The catalyst was recovered and reused in further catalytic tests after filtration, rinsing with fresh
solvent and calcination under dry air at 500°C.
2.5.2. Oxidative carboxylation of styrene
Styrene (0.2 mmol), tetrabutylammonium bromide (TBABr, 0.02 mmol), TBHP (0.3 mmol), the
solid catalyst (19-20 mg, corresponding to 0.008 mmol of Ti), biphenyl (internal standard for GC)
and acetonitrile (2 mL) were added into a 50 mL high pressure/high temperature stainless steel Parr-
4792 reactor with PTFE insert.
For solvent-free conditions, the catalyst loading was 300 mg for TinOx-MMM-2 (0.11 mmol
Ti) at styrene/TBHP/TBABr molar ratio equal to 4.8/7.2/0.48 mmol/mmol. CO2 was introduced
into the reactor, and the pressure was adjusted to 8 bar. The mixture was stirred continuously while
the pressure (8 bar) and temperature (70 °C) were kept constant during the reaction. After a
specified reaction time (24 or 48 h), the reactor was cooled down by immersion in ice/water bath
and depressurized slowly before opening. Each experiment was reproduced at least 2 times.
After the catalytic run, the solid catalyst was separated by filtration and washed with
solvent, dried in air at room temperature overnight, calcined under dry air at 350°C for 2 h and at
8

510°C for 4 h, and then reused. The catalyst was characterized by DR UV–vis spectroscopy, N2
adsorption measurements and elemental analysis before and after reuse.
Treatment of TinOx-MMM-2 with styrene carbonate was carried out in CH2Cl2 solution at
70°C. Styrene carbonate (350 mg, 2.1 mmol), TinOx-MMM-2 catalyst (200 mg, i.e. 0.075 mmol Ti),
and CH2Cl2 (3 mL) were added into a Parr-4792 reactor with PTFE insert. The mixture was stirred
continuously for 24 h, while the temperature (70°C) was kept constant.
3. Results and discussion
3.1 Preparation and characterization of the titanium-silica catalyst
The titanium(0) cluster precursor was synthesized following a previously reported procedure
[37]. Briefly, TiCl4·2THF was reduced under inert atmosphere with a trialkyl borohydride salt
in THF solution leading to organosol titanium clusters. Detailed X-ray absorption
spectroscopy (XAS) studies performed on the isolated titanium species revealed the presence
of a regular zerovalent Ti13 clusters stabilized by 6 intact THF molecules possibly in the
octahedral configuration [38]. The Ti13-THF organosol mixtures were used as precursors of
supported titanium species by simple impregnation of the mesoporous silica support at room
temperature (25°C). Subsequent exposure to atmospheric air and calcination of the obtained
powder gave rise to the TinOx-silica catalysts (Scheme 1; sketch for MCM-41). The
calcination step turned the catalyst from the yellowish-brownish colour of the initial solid into
a pure white, suggesting that all of the deposited Ti centres were converted into oxidic Ti(IV)
species.
Scheme 1. Preparation sequence for TinOx-MCM-41
9

Tin
TinOx
O2(g)
MCM-41
25°C
Ti13-THF
500°C
MCM-41
MCM-41
ICP-OES investigations performed on the final TinOx-silica catalysts revealed a quantitative
deposition of the Ti clusters onto the support leading to a titanium metal loading values
ranging from 1.9 to 2.1 wt.%, that is always close to the expected value of 2.2 wt.% (Table 1).
In terms of textural properties, the deposition and subsequent calcination of the Ti
species onto the siliceous support gives rise to a drastic diminution of specific surface area and
pore volume (Table 1). This is consistent with a gradual and remarkable lining of the inner
surface of the mesopore channels by the TinOx moieties. A similar behavior was previously
observed with the dry impregnation of titanocene dichloride molecules onto mesoporous silica
supports, where a moderate shrinkage of the porosity was observed, although without pore
blocking [39,40,41]. In the present case, the species to be deposited is polynuclear, instead of
mononuclear (as Ti(C5H5)Cl2 was), and the resulting oxidic supported species is accordingly
bulkier. For this reason, a more marked decrease in specific surface area and pore volume was
recorded, especially for the TixOn-MCM-41 sample.
Table 1.Textural features and titanium content of the mesoporous silica supports and the
related TinOx-silica catalysts.
a
Sample
Ti content
/ wt.%
n.d.
SBET
/ m2g-1
960
Vb
Dc
/ cm3g-1
0.61
0.24
0.60
0.28
0.06d
/ nm
3.7
MCM-41
TixOn-MCM-41
MMM-2
2.07
260
3.2
n.d.
940
3.39
2.78
9.33d
TixOn-MMM-2
1.90
1.90d
580
36d
a: SBET. N2-adsorption BET specific surface area; b: V. total specific pore volume; c: D. average mesopore
diameter; d: after 5th catalyst reuse in styrene oxidative carboxylation (reaction conditions as in Tab. 2).
10

High resolution transmission electron microscopy (HR-TEM) and scanning
transmission electron microscopy (STEM) analysis carried out on both freshly prepared
calcined samples (i.e. TixOn-MCM-41 and TixOn-MMM-2) indicated the absence of detectable
metal oxide aggregates. This suggests the presence of very small size of TiOx clusters, with
sizes ranging from few Ti-O-Ti units up to approx. 0.8 nm (Figure 1). STEM-EDX element
maps of some representative grains of the two samples revealed a very high and uniform
dispersion of titanium clusters on the mesoporous silica excluding the presence of larger
isolated titanium aggregates (Figure 2 and Figure 3). The resulting EDX spectrum showed an
elemental compositions very similar to the one recorded by ICP-OES analysis, for the of
titanium loading (TixOn-MCM-41: Si = 97.6 at.%, Ti = 2.4 at.%; TixOn-MMM-2: Si = 97.1
at.%, Ti = 2.9 at.%). Interestingly, STEM-EDX map of a Tin-MCM41 sample, before
calcination with air, showed a dispersion of Ti atoms on the silica support that is fully
comparable to the one observed on the calcined system, indicating that the calcination step did
not affect the size of titanium clusters (Figure S1, Supplementary Material).
Figure 1. HR-TEM micrographs of a representative grain of: A) TixOn-MCM-41 and B)
TixOn-MMM-2.

Figure 2. STEM measurements of TixOn-MCM-41: A) HAADF image of a catalyst grain; B-
C) STEM-EDX mapping of the catalyst showing the titanium dispersion: B) silicon map (red),
C) titanium map (blue).
Figure 3. STEM measurements of TixOn-MMM-2: A) HAADF image of a catalyst grain; B-
C) STEM-EDX mapping of the catalyst showing the titanium dispersion: B) silicon map (red),
C) titanium map (blue).
These evidences were further supported by DR UV-vis and Raman spectroscopic
techniques. Figure 4 shows the DR UV-vis spectra of the two TinOx-silica catalysts. The

absence of any characteristic intense absorption around 330-350 nm suggests the absence of
TiO2 anatase microcrystallites [42,43,44]. This observation is also consistent with the absence
of the characteristic band at 145 cm-1 in the Raman spectra (Figure S2) [7,45,46] and with the
above reported electron microscopy analysis. The DR UV-vis spectra of the two catalysts
indeed reveal that the Ti(IV) centres are well dispersed on the surface. Both catalysts show a
maximum at ca. 225-230 nm with a shoulder in the range 260-290 nm, which is more
pronounced in the spectrum of TinOx-MCM-41. The former absorption feature can be
attributed to oxygen–to–metal charge transfer of distorted tetrahedrally coordinated isolated
Ti(IV) sites while the latter is an evidence of octahedrally coordinated titanium centres
[47,48,49] and may belong to either 6-coordinated Ti(IV) dimers/small oligomers or hydrated
5/6-coordinated Ti(IV) species [7 and references cited therein]. The presence of hydrated
Ti(IV) species is less probable as DR UV-vis spectra are similar after calcination and vacuum
treatments (cf. Figure S3).
5
Ti O -MCM-41
n
x
4
3
2
1
0
Ti O -MMM-2
n
x
200
250
300
350
400
450
λ/ nm
Figure 4. DR UV–vis spectra of TinOx-catalysts.
3.2 Catalytic Studies
3.2.1 Epoxidation of R-(+)-limonene
The catalytic behaviour of the mesoporous TinOx-silica systems has been evaluated in the liquid-
phase epoxidation of limonene (Scheme 2) in the presence of TBHP as an oxidant. Well-dispersed
13

Ti(IV) sites on mesoporous silica supports are indeed classical good catalysts for the epoxidation of
bulky alkenes in the presence of organic hydroperoxides [1,7,8]. For this reason, TinOx-MCM-41
and TinOx-MMM-2 were first tested in the epoxidation of a cyclic terpenic olefin. Limonene
epoxide is a promising intermediate for the production of polylimonene carbonate and bio-based
polymers [50,51] as well as a wide range of products, such as insect repellents [52] or as herbicide
components [53].
O
oxidant
+
O
endocyclic
exocyclic
Scheme 2. Epoxidation of R-(+)-limonene
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
Time / h
A)

100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
Time / h
B)
Figure 5. Curves of conversion (A) and selectivity to monoepoxide (B) for TinOx-MCM-41 (ꢀ and
ꢀ) and TinOx-MMM-2 (ꢀ) catalyst in the liquid-phase epoxidation of (R)-(+)-limonene.
Conditions: dry CH3CN solvent; 90°C, 6h, batch reactor; 100 mg catalyst; 1.0 mmol limonene;
anhydrous TBHP (1.1 mmol ꢀ and ꢀ, 2.0 mmol ꢀ). Blank test: no catalyst (ꢁ).
Both catalysts were active in limonene epoxidation and limonene 1,2-epoxide was the main
observed product, in particular for short reaction times (Figure 5A and B). As expected, the
electron-richer endocyclic C=C bond was epoxidised more readily than the exocyclic one over these
systems and this is fully in line with previous observations on Ti-containing mesoporous silicate
solids [12,54]. The conversion profiles for TinOx-MCM-41 and TinOx-MMM-2 are quite similar, the
former being slightly more active at shorter times (1-2 h) and the latter at the end of the tests
(Figure 5A). Nevertheless, with an oxidant to alkene molar ratio of 1.1, the conversion values of
34% and 24%, after 1 h of reaction, for TinOx-MCM-41 and TinOx-MMM-2, respectively,
correspond to specific activities (expressed as moles of converted limonene per mole of Ti in 1 h) of
7.1 h-1 and 6.1 h-1 after 1h. These values are notably lower than the one (21 h-1) recorded, under the

same conditions, over a reference single-site Ti-MCM-41 catalyst obtained by grafting a titanocene
dichloride precursor [55]. Considering such modest activity values, any contribution of direct
oxidation of TBHP on limonene (in the absence of the catalyst) can be excluded, as shown in the
blank test in Figure 5A. The selectivity to limonene monoepoxide, after 1h, on the contrary, was
practically the same over TinOx-MCM-41 and the reference Ti-MCM-41 (73% vs. 75%,
respectively), whereas it was remarkably lower (56%) for TinOx-MMM-2. At longer reaction times,
however, the reaction proceeds, but with a non-negligible decrease in selectivity (down to 56% after
6h), a gradual formation of limonene diepoxide (at both 1,2 and 8,9 C=C positions) and the
important formation of several by-products with high molecular weight (> 300 g mol-1).
The comparable behaviour of the two catalysts, at least as far as the conversion is concerned,
was expected, as the patterns of the DR UV-vis spectra were very similar for the two solids (Figure
4). However, in this set of tests, a new aspect was recorded, with respect to previous observations
on previous single-site Ti-silicates: the oxidant efficiency [56] was far lower than usual (< 40%,
whereas typical values for TBHP in epoxidation reactions over Ti-silicates are higher than 90%)
and TBHP proved thus to be the limiting agent in these tests. Actually, although TBHP was in slight
excess (TBHP : alkene = 1.1 : 1 mol/mol), no residual oxidant was detected by iodometric
evaluation at the end of the 6h of reaction.
In order to circumvent this limitation and to push further the production of limonene
epoxide, an additional aliquot of TBHP (1 mmol) was added at the end of the 6h of reaction (Figure
S4). A very high limonene conversion value of 96% has been reached over TinOx-MCM-41, but
with a rather low selectivity to the desired monoepoxide, because of the formation of remarkable
amounts of side products. So, in the presence of a larger excess of oxidant (TBHP : alkene = 2 : 1
mol/mol), higher conversion values (up to 77% after 6h) were attained over TinOx-MCM-41.
In specific tests with a radical trap species (hydroquinone in a quasi-equimolar amount with
respect to the total Ti sites in the solid catalyst), no inhibition of the epoxidation was observed and
16

this indicated that the role of a free-radical pathway is limited over these catalysts, under these
conditions.
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
Time / h
A)
100
90
80
70
60
50
40
30
20
10
0
0
1
2
3
4
5
6
Time / h
B)
Figure 6. Curves of conversion (A) and selectivity to monoepoxide (B) for TinOx-MCM-41 catalyst
in the liquid-phase epoxidation of (R)-(+)-limonene. Conditions: dry CH3CN solvent; 90°C, 6h,

batch reactor; 100 mg catalyst; 1.0 mmol limonene; 2.0 mmol anhydrous TBHP. Fresh catalyst: 1st
run (ꢀ). Recycled catalyst: 1st recycle (ꢀ); 2nd recycle (◊); 3rd recycle (ꢂ).
The catalyst could also be recycled and a series of tests was performed over TinOx-MCM-41,
as a significant catalyst (Figure 6A and 6B). Interestingly, when TinOx-MCM-41 was recovered and
reused in the second catalytic test (first recycle), it even showed an improved performance with
conversions as high as 56 % after 1h (with a TBHP : alkene ratio of 2 : 1 mol/mol) and hence a
specific activity of 13 h-1. Such improved performance was kept for the second recycle too. Then,
the performance, in terms of conversion and selectivity, started to decrease after the third recycle.
The catalyst is therefore able to keep most of its epoxidation capability after the recovery and
intermediate re-calcination steps, at least for three catalytic cycles.
3.2.2. Oxidative carboxylation of styrene
Synthesis of cyclic carbonates from olefins and CO2 through oxidative carboxylation is one of the
challenging technologies for the sustainable utilization of CO2 and production of valuable cyclic
organic carbonates [57,58,59,60], which are widely used as a raw material in the synthesis of
polycarbonates, fuel additives, electrolyte solvents for lithium batteries, etc. [61]. Recently, some of
us have found that one mesoporous titanium-silicate Ti-MMM-E, prepared by evaporation-induced
self-assembly, is an efficient heterogeneous catalyst for the one-step oxidative carboxylation of
styrenes with TBHP and CO2, in the presence of TBABr as a cocatalyst, under mild conditions (50-
70 °C, 8 bar CO2) [30]. It was suggested that di(oligo)meric titanium species are more preferable
active centers for oxidative carboxylation of alkenes than isolated Ti atoms, although the latter
exhibit high activity and selectivity in epoxidation of alkenes with hydroperoxides. This prompted
us to evaluate the catalytic performance of elaborated and designed TinOx-based catalyst for this
reaction (Scheme 3).
18

Ti-catalyst
+ TBABr
8 bar CO 2
TBHP
70°C
Styrene
Benzaldehyde
(BA)
Styrene carbonate Styrene oxide
(SC) (SO)
Benzoic
acid (BAc)
Scheme 3. Oxidative carboxylation of styrene over Ti-catalysts.
Both of the prepared TinOx-silica catalysts were active in the oxidative carboxylation of
styrene in the presence of TBABr as cocatalyst (Table 2). However, in the absence of TBABr the
yield of carbonate was insignificant (Table S1). The results obtained using TinOx-catalyst are
superior to the results previously reported for Ti-MMM-E under the same reaction conditions (24 h
at 70 °C): 36 or 41% styrene carbonate (SC) selectivity at 80 or 71% styrene conversion (Table 2;
entries 1 and 2) vs. 33% selectivity at 58% conversion (entry 4). After 48 h of reaction, the
selectivity to SC and the alkene conversion over TinOx-MMM-2, i.e., the most efficient catalyst,
reached 51 and 81%, respectively (entry 2).
Table 2.Styrene oxidative carboxylation with TBHP and CO2over TinOx-silica catalysts.
Entry Catalyst
[Styrene]
/ M
Time
/h
Styrene
Product selectivitya (%)
conversion (%)
80
BA
SO
BAc SC
1
2
3
TinOx-MCM-41
0.1
24
21
20
21
35
12
20
4
36
40
41
51
70
68
33
67
70
48
24
48
24
48
24
24
48
89
71
81
94
99
58
67
92
13
21
16
19
14
42
16
15
11
21
9
TinOx-MMM-2
0.1
2.4b
6
3
13
-
4
5
Ti-MMM-E [30] 0.1
Ti-MMM-E [30] 2.4b
25
16
10
-
-
Conditions: styrene 0.2 mmol, TBHP 0.3 mmol, TBABr 0.02 mmol, TinOx-catalyst 0.008 mmol Ti,
CH3CN 2 mL, p(CO2) 8 bar, 70 °C.
19

a
b
GC yield based on substrate consumed; Solvent-free conditions: styrene/TBHP/TBABr molar
ratio = 1/1.5/0.1, TinOx-catalyst 0.11 mmol Ti, p(CO2) 8 bar, 70 °C.
Unlike the reaction catalyzed by Ti-MMM-E, where the only side products were benzaldehyde
(BA) and styrene oxide (SO) (entry 4), significant amounts of benzoic acid (BAc) were formed in
the presence of TinOx-silica catalysts. It is worth noting that the oxidation of styrene with TBHP (in
the absence of CO2) over TinOx-silicates gave BA as the main product (43-62% selectivity), along
with SO (32-38%) and BAc (6-10%), at 43-49% alkene conversion after 5 h at 50 °C (Tab. 3). The
addition of TBABr to the reaction system promoted the oxidative C=C bond cleavage and led to a
slight increase in selectivity of both BA and BAc (Table 3, cf. entries 2 and 3). On the other hand,
by running the oxidation reaction under CO2 atmosphere (8 bar), the formation of styrene oxide was
favoured (Table S1, entry 1).
Table 3. Styrene oxidation with TBHP over TinOx-silica catalysts.
Entry Catalyst
Time
/h
Styrene
Product selectivitya (%)
conversion (%)
43
BA
SO
BAc
1
2
TinOx-MCM-41
5
62
32
6
TinOx-MMM-2
5
49
58
50
71
43
54
45
24
38
28
32
26
10
14
19
47
24
5
3
TinOx-MMM-2 + TBABr
24
Conditions: styrene 0.1 mmol, TBHP 0.15 mmol, TinOx-catalyst 0.005 mmol Ti, TBABr 0.01 mmol
(if any), CH3CN 1 mL, 50 °C.
aGC yield based on substrate consumed.
In order to obtain a higher volume yield of the desired carbonate, a larger loading of
reactants was used in the reactor. As previously observed over Ti-MMM-E, by increasing the initial
concentration of styrene up to solvent-free conditions (i.e., 2.4 M), a high selectivity to SC (70%)
was attained at 94 % conversion after 24 h, over TinOx-MMM-2 (Tab. 2, entry 3). This means that
the volume yield of SC can be as high as 0.3 kg L-1 for the reaction mixture. Interestingly, 24 h of
20

reaction was enough to get the almost complete (94%) conversion of styrene in the presence TinOx-
MMM-2, while only 67% conversion was reached over Ti-MMM-E catalyst under the same
conditions (Table 2, entry 5). The higher activity of the grafted TinOx-silica materials is, most
likely, due to a better accessibility of titanium active centres in these materials, since all of the Ti
sites are exposed on the surface of the catalyst and are not buried within the walls of silicate matrix,
as it happens in the case of Ti,Si-materials synthesized via direct synthesis, including EISA
technique.
In this case too, an adequate stability of the solid catalysts is a crucial factor for
heterogeneous catalysis in liquid phase. The XRD study confirmed the preservation of the MCM-41
ordered structure under turnover conditions of styrene oxidative carboxylation in acetonitrile
solution (Figure S5, curves A and B). A slight shift of the XRD reflex is indeed likely due to some
shrinkage of the material subjected to turnover conditions and repeated calcinations during the
recycling.
A set of recycling tests was carried out on TinOx-MMM-2. The catalyst was recovered
from the reaction mixture after 24 h by simple filtration and then used repeatedly in several
consecutive runs under solvent-free conditions. Figure 7 demonstrated that TinOx-MMM-2 could be
reused without notable deterioration of the catalytic properties. After 6 recycles, a total volume
yield of SC as high as 1.5 kg L-1 could be achieved.
21

Styrene conversion
SC yield
100
80
60
%
40
20
0
1
2
3
4
5
6
Ti O -MMM-2 reuse
n
x
Figure 7. Reuse of TinOx-MMM-2 in solvent-free styrene oxidative carboxylation. Conditions as in
Table 2, entry 3; reaction time 24 h.
4
A
B
3
C
D
2
1
0
200
250
300
350
400
λ/ nm
Figure 8. DR UV–vis spectra of TinOx-MMM-2: (A) fresh catalyst, (B)-(D) after one, three and six
runs of styrene oxidative carboxylation, respectively. Conditions as in Table 2, entry 3; reaction
time 24 h.
In terms of textural properties, a severe loss in specific pore volume was detected after the fifth
consecutive recycle of styrene oxidative carboxylation (Table 1). The DR UV-vis spectroscopic
analysis showed some changes in the state of the Ti centres after the catalyst reuse (Figure 8). The
22

observed shift of the absorption apparently suggests that a transformation of di(oligo)meric titanium
species into isolated ones takes place. This might be caused by a gradual leaching of titanium
during the recycling steps. However, the content of titanium in the solid catalysts remained intact
after six reuses (1.9 wt%; Table 1) and really negligible amounts of titanium (1.0 ppm Ti) were
found in the liquid-phase reaction mixtures during the recycle tests.
In order to get a deeper insight into the reason for such a change in the state of the active
metal along the catalytic reactions, the catalyst was treated with styrene carbonate. The DR UV
spectra of TinOx-MMM-2 material after the first run of oxidative carboxylation of styrene and after
the treatment with SC alone are almost identical (Figure S6). Therefore, since no significant
leaching of Ti species out of the solid was detected, the TinOx-like moieties on the silica surface
likely undergo a local restructuring of the active sites upon exposure to the polar carbonate
products. On the other hand, the reference Ti-MMM-E catalyst showed a gradual agglomeration of
the Ti sites upon recycling after the carboxylation of styrene [30], according to a typical process
widely observed in other liquid-phase oxidation over titanium-silicate catalysts [7,62]. These data
suggest that, although the mesoporous structure of the molecular sieve itself is not fully stable to the
hard recycling conditions (especially in repeated solvent-free tests), the chemical environment, the
total metal content and the catalytic properties of the Ti(IV) sites are kept and stable even after
prolonged reaction periods.
Conclusions
Starting from a molecularly defined zerovalent Ti13 cluster, it has been possible to obtain a novel
series of titanium oxide-like supported catalysts. The deposition of the metal clusters led to a
uniform dispersion and thorough deposition of the Ti sites on the surface of the mesoporous silica
molecular sieves. The subsequent calcination transformed the pristine Ti(0) centres into TinOx
catalytically active species with enhanced properties for the selective oxidative conversion of
23

alkenes. In the presence of TBHP as an oxidant, moderate results were obtained in the liquid-phase
epoxidation of limonene. On the other hand, unprecedented high selectivity (up to 70% to styrene
carbonate) and total volume yield (up to 1.5 kg L-1 of the total reaction mixture) values were
achieved in the oxidative carboxylation of styrene with TBHP and CO2 in the presence of
tetrabutylammonium bromide as cocatalyst, under solvent-free conditions.
The obtained solid catalysts showed a remarkable catalytic stability, along repeated recovery and
reuse cycles, not only in epoxidation reactions, but also under the harsh conditions of oxidative
carboxylation. These highly dispersed and evenly-distributed TinOx-like catalysts may therefore
deserve further attention, since they can also be optimal systems for other Ti(IV)-catalysed
transformations by exploiting their potentially enhanced and tunable redox / Lewis-acid /
photocatalytic properties.
Acknowledgments
The authors thank Dr. Yu. A. Chesalov, Dr. T. V. Larina and Dr. I. Y. Skobelev for the Raman,
DRS UV–vis, and nitrogen adsorption measurements, respectively. This work was conducted
within the framework of the budget project No. 0303-2016-0005 for the Boreskov Institute of
Catalysis. The research activity was partially supported by the Russian Foundation for Basic
Research (grant N 15-33-20225) and the Ministry of Education and Science of the Russian
Federation.
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