Molybdenum containing surface complex for olefin epoxidation

Article abstract of DOI:10.1039/b206013b

Silica-supported molybdenum surface complexes were prepared by the reaction between (N≡)Mo(OtBu)₃ and silica via displacement of the tert-butoxy ligands for siloxyls from the silica surface. The structure of the surface molybdenum complexes was well defined by in-situ FT-IR, elemental analysis, ¹H NMR and ¹³C CP/MAS NMR techniques. The surface complexes could undergo alcoholysis reaction with CD₃OD and CH₃OH in the same way as free (N≡)Mo(OtBu)₃ and they show high catalytic activity and selectivity in olefin epoxidation. Initial rates up to 24.9 mmol epoxide (mmol Mo)^-1 min^-1 were achieved in the epoxidation of cyclohexene using TBHP as oxidant.

Full text of DOI:10.1039/b206013b

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Molybdenum containing surface complex for olefin epoxidation
a
b
a
Qihua Yang,ab Christophe Coperet, Can Li* and Jean-Marie Basset
´
a
´
Laboratoire de Chimie Organometallique de Surface, CPE, F-69626, Villeurbanne Lyon,
France. E-mail: basset@cpe.fr; Fax: 33(0)472431795; Tel: 33 (0)472431794
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics,
Chinese Academy of Sciences, Dalian 116023, China. E-mail: canli@dicp.ac.cn;
Fax: (86) 4114694447; Tel: (86) 4114379070http://www.canli.dicp.ac.cn
b
Received (in London, UK) 21st June 2002, Accepted 26th September 2002
First published as an Advance Article on the web 12th December 2002
=
Silica-supported molybdenum surface complexes were prepared by the reaction between (N )Mo(OtBu) and
=
3
silica via displacement of the tert-butoxy ligands for siloxyls from the silica surface. The structure of the surface
1
molybdenum complexes was well defined by in-situ FT-IR, elemental analysis, H NMR and 13C CP/MAS
NMR techniques. The surface complexes could undergo alcoholysis reaction with CD3OD and CH3OH in the
=
same way as free (N )Mo(OtBu) and they show high catalytic activity and selectivity in olefin epoxidation.
3
=
Initial rates up to 24.9 mmol epoxide (mmol Mo)À1 minÀ1 were achieved in the epoxidation of cyclohexene
using TBHP as oxidant.
material thus formed were investigated as disclosed herein.
Introduction
The catalytic activity of the surface complexes obtained was
tested by olefin epoxidation using TBHP as oxidant, and an
initial rate as high as 24.9 mmol epoxide (mmol Mo)À1 minÀ1
was achieved.
Epoxides are important building blocks in organic synthesis.
In fine chemical processes, epoxides are still mostly produced
from the oxidation of olefins with peracids; these processes
usually lead to low selectivities for epoxide due to a variety
of secondary nucleophilic ring-opening reactions and they gen-
erate large amounts of waste. Direct epoxidation of olefins
using more environmentally benign oxidants such as hydrogen
peroxide, alkyl hydroperoxide, and oxygen is probably of the
most importance in terms of economical processes.1 Many
kinds of catalysts have appeared in this field in the past dec-
ades including zeolitic materials substituted with transition
metals,2 transition-metal containing amorphous materials,3
homogeneous catalysts based on Re or W,4 tungsten-based
heteropolyacids, etc.5 Molybdenum-containing catalysts are
widely used in the field of selective oxidation, and one of the
most famous example is probably the Halcon (homogeneous)
process for the industrial manufacture of propylene oxide.6
Because of the advantages of heterogeneous catalysts over
homogeneous ones, tremendous effort has been directed
towards the heterogenization of molybdenum onto different
kinds of supports, such as silica, polymers and mesoporous
materials.7 One problem associated with the heterogenization
of the catalyst is the difficulty to obtain a truly well-defined
Experimental
General procedure
All experiments were carried out in the strict absence of oxygen
and water. Standard Schlenk techniques under Ar were used
for organometallic syntheses. Pentane was distilled over
Na/K amalgam and stored under Ar over Na. The
=
(N )Mo(OtBu) complex and the MCM-41 were prepared
3
=
according to
a
literature procedure.13 Silica (Aerosil1
Degussa, 200 m2 gÀ1) was calcined at 500 ꢀC in air for 2 h, trea-
ted at 500 ꢀC under vacuum (10À5 mmHg) for 12 h (support
referred to as SiO2-500) and then at 700 ꢀC for 4 h (support
referred to as SiO2-700). MCM-41 (1000 m2 gÀ1) was treated
at 500 ꢀC under vacuum (10À5 mmHg) after removing the tem-
plate at 550 ꢀC (support referred to as MCM-41-500). For the
catalytic tests, all substrates were purchased from Aldrich and
distilled under Ar before use. TBHP (5.5 M in decane, Aldrich)
was degassed and stored over MgSO4 . GC analyses were per-
formed on a HP6890 equipped with an HP-1 column (25 m,
0.32 mm).
VI
Mo center covalently bonded to the support. Surface
Organo-Metallic Chemistry (SOMC) can provide in principle
an opportunity to construct a uniform distribution of active
sites on the surface of the support, and thus help in getting
the required structure/activity relationship.8 This approach
has already been used to study, understand and improve the
Shell catalyst,9 and also to design an efficient heterogeneous
Sharpless epoxidation catalyst.10 A crucial point in generating
a well-defined system is to use a monomeric precusor and try
to graft it via one or several covalent bonds. Molybdenum alk-
oxides typically exist as dinuclear compounds, [Mo2(OR)6].11
=
Grafting of [(N )Mo(OtBu) ] onto support by gas–solid reaction
=
3
This was carried out using standard techniques for the various
supports.14 Typically, the support as a pellet (20 mg) was
heated at 500 ꢀC under vacuum (10À5 mmHg) for 12 h and
cooled to room temp. The Mo molecular complex was sub-
limed onto the silica pellet under vacuum, and this support
was heated to 80 ꢀC for 1 h. The excess of molecular complex
was removed by reverse sublimation.
=
In contrast [(N )Mo(OtBu) ] is monomeric in solution, stable,
3
=
easily prepared and possesses alkoxy ligands which may poten-
tially be substituted by surface siloxyl ligands.12 Therefore its
grafting onto SiO2 and MCM-41 and the reactivity of the
DOI: 10.1039/b206013b
New J. Chem., 2003, 27, 319–323
319
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Ligand exchange experiment of SiO2–Mo-500 with alcohol
=
After sublimation of the [(N )Mo(OtBu) ] onto a silica pellet
=
3
as described above (pretreated at 500 ꢀC under vacuum at
10À5 mmHg for 12 h), CD3OD was introduced into the reac-
tor; after 5 minutes, the reactor was placed under vacuum at
70 ꢀC for 1 h to remove the excess CD3OD, then CH3OH
was introduced, the reactor was placed under vacuum after 5
minutes to remove the excess CH3OH at 70 ꢀC for 1 h. This
procedure was repeated twice and the ligand exchange reaction
was followed by in-situ FT-IR spectroscopy.
=
Grafting of [(N )Mo(OtBu) ] onto support by liquid–solid
=
3
reaction
Preparation of Mo–Si-500. Grafting was typically carried
=
out by stirring a mixture of [(N )Mo(OtBu) ] (164 mmg, 0.5
=
3
mmol) and SiO2-500 (1.00 g) (pretreated at 500 ꢀC under
vacuum at ꢁ10À5 mmHg for 12 h) in pentane (10 mL) at
20 ꢀC for 3 h. After filtration, the solid was washed three times
with pentane and dried under vacuum to yield 1.1 g of a white
powder. Elemental analysis: C, 1.09%wt; N, 0.28%wt and Mo,
1.74%wt. Solid state 1H NMR d 0.84 ppm. CP/MAS 13C
NMR d 29.4 ppm. IR: 2976, 2945, 2902, 2871, 1471, 1451,
Fig. 1 In situ IR spectroscopy before and after sublimation of
=
[(N )Mo(OtBu) ] onto an SiO -700 pellet; inset figure is the gas phase
=
evolved during the sublimation.
3
2
1362 cmÀ1
.
agreement with a grafting via the displacement of the tert-
butoxy ligands for siloxyls from the silica surface, which is clo-
sely related to the alcoholysis reaction reported by Chisholm
et al.11
Usually, the structure of the product obtained from the reac-
tion of the organometallic compound and triphenylsilanol was
a reliable reference to determine the structure of the surface
complex. In our previous study of the reaction between
Preparation of Mo–Si-700. SiO2-700 (pretreated at 700 ꢀC
under vacuum at ꢁ10À5 mmHg for 12 h) was used as support,
the preparation procedure is similar to that described above.
Elemental analysis: C, 2.13%wt; N, 0.30%wt and Mo,
1.90%wt. Solid state 1H NMR d 0.74 ppm. CP/MAS 13C
NMR d 27.4 ppm. IR: 2976, 2945, 2902, 2871, 1471, 1451,
1362 cmÀ1
.
=
[(N )Mo(CH tBu) ] and silica, we found that protonation of
=
nitride occurred to give an imido surface complex [( SiO)-
2
3
=
Preparation of Mo–MCM-41. A mixture of [(N )Mo-
=
=
=
(OtBu)3] (492 mmg, 1.5 mmol) and MCM-41-500 (1.00 g) (pre-
treated at 500 ꢀC under vacuum at ꢁ10À5 mmHg for 12 h) in
pentane (15 mL) was stirred at 20 ꢀC for 3 h. After filtration,
the solid was washed three times with pentane and dried in
vacuo to yield 1.4 g of a white powder. Elemental analysis:
C, 4.18%wt; N, 1.26%wt and Mo, 6.69%wt. Solid state 1H
NMR d 1.45. CP/MAS 13C NMR d 29.6. IR: 2977, 2943,
=
Mo( NH)(CH2tBu)3]. Using triphenylsilanol in place of silica
=
provided the same reactivity and yielded [Ph3SiO–Mo( N-
H)(CH2tBu)3].16 The reaction of (N )Mo(OtBu) with tri-
=
=
3
=
phenylsilanol gave a mixture of [(Ph SiO) –Mo( N)(OtBu) ]
=
3
x
3Àx
(observed by liquid H and 13C NMR) as in the case of the
alcoholysis reaction.11 This result is consistent with the FT-
IR data, which support the reaction pathway between
1
2902, 2871, 1471, 1451, 1361 cmÀ1
.
=
[(N )Mo(CH tBu) ] and siloxyls of the silica surface. The dif-
2
=
ferent reactivity of the two analogous molybdenum complexes
with OH groups is probably related to the difference in Mo–N
3
Epoxidation of olefins using various surface Mo-complexes
˚
bond strengths, which is 1.66 and 1.72 A for [(N )Mo(OtBu) ]
=
=
3
The Mo-containing solid (18 mmol of Mo) was introduced into
a two-necked 50 mL glass reactor fitted with a reflux condenser
under argon. After addition of the substrate (50 mmol) and
CH2Cl2 (5 mL), the reaction mixture was heated to reflux
and TBHP (17.5 mmol) was added via a syringe. The reaction
was monitored at certain time intervals by GC analysis using
dodecane as an internal standard. The amount of remaining
TBHP was measured by iodometric titration at the end of
the reaction.
=
and [(N )Mo(CH tBu) ], respectively.
2
=
3
The surface complex shows a free ligand exchange property
as exhibited by the reaction with CD3OD and CH3OH. The
FT-IR shows the disappearance of the n(C–H) vibration asso-
ciated with the OtBu groups for n(C–D) vibrations corre-
sponding to CD3O–M when the surface complex is reacted
with CD3OD (Fig. 2). When it was further reacted with
CH3OH, the n(C–D) vibration associated with CD3O–M was
replaced by the n(C–H) vibration of CH3OH. The same pro-
cess could be repeated several times. This further corroborates
the proposed reaction pathway.
Liquid–solid reaction provides the same material according
to IR spectroscopy. The supported surface complexes prepared
by liquid–solid reaction were characterized by elemental analy-
sis (Table 1). The results show that the pretreatment tempera-
ture of the support has an important effect on the amount of
the molybdenum species grafted. With increasing pretreatment
temperature, the amount of grafted molybdenum species
decreased, this is mainly due to the decrease in the amount
of reactive hydroxyl groups when the support was pretreated
at higher temperature. Elemental analysis of the supported sur-
face complexes gave C/N/Mo ratios of 5/1/1 and 9/1/1 for
the surface complexes obtained with SiO2-500 and SiO2-700,
respectively. Theoretical values in elemental analysis for a
Results and discussion
Preparation and characterization of the surface complexes
=
The reaction of [(N )Mo(OtBu) ] with SiO partially dehy-
=
3
2
droxylated at either 500 ꢀC or 700 ꢀC (respectively named
Mo–Si-500 and Mo–Si-700) was performed using both gas–
solid and liquid–solid reactions.15 When [(N )Mo(OtBu) ] is
=
=
3
sublimed at 85 ꢀC onto a silica disc, the intensity of the
À1
=
=
n( Si–OH) band at 3745 cm decreases, while bands in the
range of 3000–2860 cmÀ1 and 1500–1350 cmÀ1, attributed to
n(C–H) and d(C–H) respectively, appear (Fig. 1). The evolu-
tion of 2,2-dimethylpropanol in the gas phase was detected
by IR spectroscopy (Fig. 1 inset). These observations are in
320
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=
Scheme 1 Proposed reaction pathway between [(N )Mo(OtBu) ]
=
3
and silica.
Fig. 2 In situ IR spectroscopy. a: Mo–Si-500; b: after reaction with
CD3OD at room temperature and desorption at 70 ꢀC for 1 h; c: fur-
ther reaction with CH3OH and desorption at 70 ꢀC for 1 h; d: further
reaction with CD3OD and desorption at 70 ꢀC for 1 h; e: further reac-
tion with CH3OH and desorption at 70 ꢀC for 1 h.
bis-, 1, or a monografted species, 2, via reaction of
[(N )Mo(OtBu) ] with a silanol group of silica give C/N/
=
=
3
Mo ratios of 8/1/1 and 4/1/1, respectively (Scheme 1). This
supports that a monografted species is obtained on a SiO2-
700, while the reaction with a SiO2-500 gives a bisgrafted spe-
cies and suggests that the nitrido ligand is retained around the
Mo center.
The surface complex obtained by grafting on MCM-41 is
structurally similar to those obtained on SiO2-500 according
to elemental analysis and spectroscopic evidence (Fig. 3).
The C/N/Mo ratio is 5/1/1, suggesting the formation of bis-
grafted species. The mesostructural nature of MCM-41 has a
tendency for the formation of mutigrafted species because
the hexagonal channels in the mesoporous silica could induce
strong steric effects. The MCM-41 supported surface complex
shows a much higher Mo content in relation to the higher spe-
cific surface area and therefore higher silanol contents.15
Solid state 1H NMR and CP/MAS 13C NMR of all the sup-
ported surface species show signals assigned to CH3 of OtBu
groups, which also provide good evidence for the presence of
tert-butoxy ligands remaining around the metal center in all
cases (Fig. 4).
Fig. 3 In situ IR spectroscopy before and after sublimation of
=
[(N )Mo(OtBu) ] onto MCM-41.
=
3
than 99% and 20 mol epo (mol Mo)À1 minÀ1 (initial rate),
respectively. In contrast, the molecular precursor showed a
much lower initial rate as well as a poor selectivity for epoxide;
this is probably associated with its rapid transformation under
reaction conditions. The high activity and selectivity of the sur-
face complex could be attributed to the isolated molybdenum
site.
Epoxidation of various olefins on Mo–Si-700. The epoxida-
tion of representative olefins was then performed using
Mo–Si-700 as catalyst (Table 3). Methylcyclohexene and cyclo-
hexene have the highest reactivities followed by styrene. Ter-
minal olefins like 1-hexene or 1-octene showed much slower
reactivities, and trans olefins do not seem to react under the
reaction conditions. Cis olefins however are converted with
initial rates of 6 to 14 mol epo (mol Mo)À1 minÀ1 depending
on how sterically crowded the double bond is. The data show
Epoxidation of olefins using TBHP as oxidant
Epoxidation of cyclohexene using various supported surface
VI
complexes. With these isolated heterogenized Mo centers on
various supports in hand, their catalytic activity was tested
in the epoxidation of cyclohexene using TBHP as oxidant
(Table 2). All supported surface complexes exhibited high
selectivities and activities as expected, which were greater
=
Table 1 Elemental analysis for the surface complexes obtained by liquid–solid reaction of [(N )Mo(OtBu) ] with partially dehydroxylated silica
=
3
and MCM-41
wt% Mo
on support
wt% C
on support
mmol Mo/g
of support
Support
wt% N on support
C/Moa
N//Moa
Mo–Si-500
Mo–Si-700
Mo–MCM-41
1.74
1.90
6.69
1.09
2.13
4.18
0.28
0.30
1.26
5.0 (4.0)
8.9 (8.0)
5.0 (4.0)
1.1 (1.0)
1.3 (1.0)
1.3 (1.0)
0.18
0.20
0.70
a
The number in parentheses (italic) represents the corresponding theoretical numbers (see Scheme 1).
New J. Chem., 2003, 27, 319–323
321

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Fig. 4 1H and 13C CP/MAS NMR spectra of the supported surface complexes.
that the surface complex catalyzed reaction system is typical of
electrophilic epoxidation.
Table 2 Epoxidation of cyclohexene on the surface organometallic
complexes using TBHP as oxidanta
Initial rateb /mmol epo
(mmol Mo)À1 minÀ1
c
Sepo (%)
Heterogeneity of the supported surface complex. The hetero-
geneity of Mo–Si-700 was tested by the epoxidation of cyclo-
hexene. The supported surface complex was reused three
times. An obvious loss of activity for each cycle was observed
when the supported surface complex was cycled (Fig. 5). How-
ever, the filtrate of the reaction mixture of the epoxidation of
cyclohexene obtained after one cycle (that is after conversion
of 99% of TBHP) was treated with an additional 1000 equiv.
of TBHP and showed no or little catalytic activity under
Catalyst
Mo–SiO2-500
Mo–SiO2-700
Mo–MCM-41
22.8
24.9
21.4
6.6
99.1
99.0
96.3
90.30
—
=
(N )Mo(OtBu)
=
3
1.5d
VI
Homogeneous Mo
a
Reaction conditions: Mo (0.018 mmol), cyclohexene (50 mmol),
CH2Cl2 (5 mL), TBHP 5.5 M in decane (17.5 mmol) heated under
b
reflux in a glass reaction vessel for 2 h. The initial rate is based on
the epoxide formation in the first 20 min. c The epoxide selectivity cor-
d
responds to the formation of epoxide on the oxidant added. See
ref. 17.
Table 3 Epoxidation of representative olefins on Mo–Si-700a
Initial rateb /
mol epo
(mol Mo)À1
minÀ1
Sepo
(mol%)c
Conv. TBHP
(mol%)d
Olefin
Cyclohexene
24.9
31.4
18.0
3.6
98.9
97.3
92.1
96.7
97.2
97.0
97.8
93.2
99.5 (2 h)
95.8 (1 h)
89.0 (7 h)
72.4 (12 h)
73.8 (12 h)
90.1 (3 h)
92.1 (5 h)
90.3 (5 h)
1-Methyl-1-cyclohexene
Styrene
1-Octene
1-Hexene
1.5
Cis-3-heptene
Cis-4-methyl-2-pentene
2-Ethyl-1-butene
13.7
6.3
5.7
a
Reaction conditions: Mo (0.018 mmol), substrate (119 mmol),
CH2Cl2 (5 mL), TBHP 5.5 M in decane (17.5 mmol) heated under reflux
in a glass reaction vessel. b The initial rate is based on the the epoxide
Fig. 5 Different runs of epoxidation of cyclohexene on Mo–Si-700.
Reaction conditions for run 1: Mo, 0.018 mmol; cyclohexene, 50
mmol; TBHP, 9 mmol; the reaction mixture is heated under reflux in
CH2Cl2 (5 mL). The subsequent runs were performed by using the
solid after filtration and the same quantities of the various reagents
were added.
c
formation in the first 20 min. The epoxide selectivity corresponds to
d
the formation of epoxide on the oxidant added. The conversion in
TBHP is obtained by its titration by iodometry at the end of the reac-
tion; the number in parentheses corresponds to the reaction time.
322
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3
4
H. P. Wulff, Br. Pat., 1971, 1 249 079.
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similar reaction conditions. These data indicate that the high
catalytic activity of the surface complexes can probably be
attributed to the isolation of molybdenum active sites resulting
from controlled grafting on a support. Molybdenum elemental
analysis of the supported complex showed that there is about
15 mol% leaching of molybdenum from the catalyst for each
cycle. The leaching of molybdenum caused the decrease of
the catalytic activity.
5
6
7
8
D. C. Sherrington, Catal. Today, 2000, 57, 87.
J.-M. Basset, J. P. Candy, A. Choplin, D. Didillon, F. Quignard
and A. Theolier, in Perspectives in Catalysis, eds. J. A. Thomas
and K. I. Zamaraev, Blackwell Scientific, Oxford, 1992, p. 125.
M. Crocker, R. H. M. Herold, A. Guy Orpen and M. T. A.
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9
Conclusion
´
10 C. Vallantin-Rosier, PhD Thesis, Universite Claude Bernard
Lyon 1, 1999; A. O. Bouh, G. L. Rice and S. L. Scott, J. Am.
Chem. Soc., 1999, 121, 7201; D. Meunier, A. Piechaczyk, A. de
Mallmann and J.-M. Basset, Angew. Chem., Int. Ed., 1999, 38,
3540.
11 A. H. Chisholm, F. A. Cotton, C. A. Murillo and W. W. Reichert,
Inorg. Chem., 1997, 16, 1801.
12 D. M. T. Chan, M. H. Chisholm, K. Folting, J. C. Huffman and
N. S. Marchant, Inorg. Chem., 1986, 25, 4170.
In conclusion, silica supported molybdenum containing surface
complexes with well-defined structure could be synthesized
=
using the reaction of [(N )Mo(OtBu) ] and the hydroxyl group
3
=
of the silica. The surface complexes with the highly isolated
molybdenum centers show high activity and selectivity for
the epoxidation of olefins.
13 M. Chatterjee, T. Iwasaki, Y. Onodera, T. Nagase, H. Hayashi
and T. Ebina, Chem. Mater., 2000, 12, 1654.
Acknowledgements
14 L. Lefort, M. Chabanas, O. Maury, D. Meunier, C. Cope´ret, J.
Thivolle-Cazat and J.-M. Basset, J. Organomet. Chem., 2000,
96, 593.
´
15 X. X. Wang, PhD Thesis, Universite Claude Bernard Lyon 1,
1999.
This work is a project of the joint Laboratory of France–China
on Catalysis (LFCC). Q. H. Yang is grateful for the support of
the K.-C. Wong Foundation. We are also grateful to the
CNRS and the ESCPE Lyon for financial support.
16 W. A. Herrmann, A. W. Stumpf, T. Priermeier, S. Bogdanovic, V.
Dufaud and J.-M. Basset, Angew. Chem., Int. Ed., 1996, 35, 2803.
17 R. A. Sheldon and J. A. van Door, J. Catal., 1973, 31, 427 . For a
comprehensive list of initial rates in the epoxidation of cyclohex-
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through Heterogeneous Catalysis, eds. R. A. Sheldon and H. van
Bekkum, Wiley-VCH, p. 473.
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