вход по аккаунту


Designing a Solid Catalyst for the Selective Low-Temperature Oxidation of Cyclohexane to Cyclohexanone.

код для вставкиСкачать
metrical peripheral group substitution on mesophase behavior
in order to understand the factors which govern organization of
phenylacetylene dendrimers with an interest toward the preparation of low-symmetry columnar phases.
Received: January 31, 1997 [Z 10062IEj
German version: Angew. Chem. 1997, 109, 1709- 1712
Keywords: dendrimers - liquid crystals * mesophases * self-organization
[I] For recent rewews see: a) G. R. Newkorne, C. N . Moorefield, F. Vb;gtle, Dendrilic Molecules: Concepts. Synfheses,Perspectives, VCH, Weinheim, 1996; b)
J. M. J. Frechet, C. 3. Hawker, I. Gitsov, J. W Leon, J: Macromol. Sci. Pure
Appl. Chem. 1996, A33, 1399-1425; c) D. A. Tomalia, P. R. Dvornic, Curr.
Opin. Colloid Interface Sci. 1996, t , 221 -235.
[2] J. S . Moore, Curr. Opin. Solid Srare Mater. Sci. 1996, 1 , 798-805.
[3] a) C . J. Hawker, K. L. Wooly, J. M. J. Frechet, J: Am. Chem. Sac. 1993, 115,
4375-4376; b) A. M. Naylor, W. A. Goddard 111, G. E. Kiefer, D. A. Tomalia,
ibid. 1989, 1 1 1 . 2339-2341. c) D. Farin, D. Avnir, Angew. Chem. 1991, 103,
1409-1410; Angew. Chem. In(. Ed. Engl. 1991,30,1379-1380.
141 A hyperbranched polymer refers to a dendritic macromolecule prepared by a
one-step polymerization of an AB, monomer (x 22).
[5] Y. H. Kim, J Am. Chem Sac. 1992, 114,4947-4948.
16) a) V. Percec, M. Kawasumi, Macromolecules 1992,25, 3843-3850; b) V. Percec, P. Chu, M. Kawasumi, ibid. 1994, 27, 4441-4453.
[7] V. Percec, P. Chu, G. Ungar, J. Zhou, J. Am. Chem. Sac 1995, lf7, 1 1 4 4 1 1454.
(81 a) S . A. Ponomarenko, E. A. Rebrov. A. Y. Bobrovsky, N. I. Boiko, A M.
Muzafarov, V. P. Sbibaev, Liq. Cryst. 1996,Zl.l- 12. b) K. Lorenz, D. Holter,
B. Stuhn, R. Miilhaupt, H. Frey, Adv. Mater. 1996,8,414-416; c) U. Stebani,
G. Lattermann, M. Wittenberg, I. H. Wendorff, Angew. Chem. 1996, 108,
1941-1943; Angeu. Chem. bit. Ed. Engl. 1996.35, 1858-1861.
[9] V. Percec, G Johansson, G. Ungar, J. Zhou, J Am. Chem. Soc. 1996, 118,
9855-9866. and references therein.
[lo] T. Plesnivy, H. Ringsdorf, P. Schuhmacher, U. Niitz, S . Diele, Liq. Cryst. 1995,
18, 185-190.
[ll] a) Z. Xu. M. Kahr, K. L. Walker, C. L. Wilkins, J. S . Moore, J Am. Chem. Soc.
1994, 116, 4537-4550; b) T. Kawaguchi, K. L. Walker, C. L. Wilkins, J. S .
Moore, ihid. 1995, 117. 2159-2165.
[12] By "intrinsically mesogenic" we mean that liquid crystallinity is a property of
the molecule as a whole rather than the result of catenation of individual
mesogenic monomer units.
113) C. F. van Nostrum, Adv. Mater. 1996, 8, 1027-1030.
1141 a) C. J. Hawker, J. M J. Frichet, J Chem. Sac. Chem. Commun. 1990, 10101013. b) C. J. Hawker, J. M. J. Frechet, J Am. Chem Sac. 1990, ff2, 76387647. c) K L. Wooly, C. J. Hawker, J. M. J. Frechet, J. Chem. Sac. Perkin
T r a m 1 1 9 9 1 , 1059-1076, d) T. M. Miller, T. X. Neenan, Chem. Mater. 1990,
2, 346-349. e) E. W. Kwock, T. M. Miller, T. X. Neenan, ibid. 1991, 3, 775777; f) T. M. Miller, T. X. Neenan, R. Zayas, H. E. Bair, J. Am. Chem. SOC.
1992, 114, 1018-1025; g) J. S . Moore, Z. Xu, Macromolecules 1991,24,58935894.
[IS] K. L. Walker, M. S . Kahr, C. L. Wilkins, 2.Xu, J. S . Moore, J. Am. Soc. Mass
Specrrom. 1994, 9, 731 -739.
[16] A detailed description of the synthesis is in preparation.
[17] Enthalpies of isotropization of other discotic mesogens range from 0.8 to
20.1 kJmo1-I. a) S . Chandrasekhar, B. K. Sadashiva, K. A. Suresh, Pramana
1977, 9 , 471 --480; b) C. Destrade, M. C. Mondon, J. Malthete, J. Phys.
( P a r i s ) ,Colloy. C3 1979,40, 17-21;c) J. Billard, J. C. Dubois, H. H. Tihn,A.
Zann, Nouv. J. Chim. 1978,2, 535-540.
[18] Preliminary microscopy studies under crossed polarizers of the fourth generation (46mer) show an initial birefringent texture. When heated, the sample
shows a clearing point; however, upon cooling, no birefringent texture reforms
down to room temperature. The original birefringence could be caused by
mechanical stress within the sample. Differential scanning calorimetry (DSC)
shows no observable phase transitions. X-ray diffraction shows only amorphous characteristics. This behavior should be viewed as preliminary since the
MALDI mass spectrum reveals the presence of appreciable amounts of defective dendritic species which may affect possible mesophase behavior.
[I 91 While compound 2 does possess a phase transition at about 30 "C, it is unlikely
that the X-ray diffraction data is a composite of two phases. DSC shows that
a sample of 2 heated from room temperature does undergo this phase transition. thus X-ray diffraction data is of the D, phase.
[20] We have initially assigned D, (1 and 2) and D (3) to the ordered hexagonal
discotic phase (Dh,,) and D, to the disordered hexagonal discotic phase (DhJ
X-ray diffraction data of the D, phase is needed, however to verify the assignment. C Destrade, H. H. Tinh, H. Gasparoux, J. Malthete, A.M. Levelut,
Mol. Crysf. 1981, 7 t , 111-135.
[21l Molecular modeling was performed on a Silicon Graphics Indigo workstation
using MSI's Quanta and Cerius2 software packages.
Angeu Chem. Int. Ed. Engl. 1997, 36, No 15
Designing a Solid Catalyst for the Selective
Low-Temperature Oxidation of Cyclohexane to
Thomas Maschmeyer,* Richard D. Oldroyd,
Gopinathan Sankar, John M. Thomas,* Ian J. Shannon,
John A. Klepetko, Anthony F. Masters,
James K. Beattie, and C . Richard A. Catlow
The catalytic activation, especially partial oxidation, of alkanes constitutes one of the major challenges of present-day chemistry; and the conversion of cyclohexane to cyclohexanone is
among the principal target reactions since the latter is used as
feedstock in several industrial processes including the production of nylon from ecaprolactam and adipic acid. There have
been numerous fundamental studies of the role of cobalt acetate
as catalysts for such processes and['] it is now recognized that
cobalt(1rr) acetate exists as a number of distinct species,['] both
in solution and in the solid-state. Although stable alkylperoxo
complexes of Co"' are formed in these systems there is also
unmistakable evidence for free radicals such as RO; and RO' in
the reaction, the occurrence of which is symptomatic of autooxidation. Homogeneous catalytic processes with Co(OAc), suffer
from the generation of significant quantities of unwanted byproducts such as aIkyl acetates, other ketones, cyclohexylidene
diacetate, alkyl chlorides, and occasionally, cyclohexyl3J To confer greater stability upon the homogeneous Co"' acetate catalyst it is also necessary to employ, for
reasons that are not entirely understood, either acetic acid or
trifluoro- (or trichloro-) acetic acid as solvent for the reactant
mixture and bromides are often used as promoters.
With the aim of arriving at a cleaner, more efficient catalytic
system for the production of cyclohexanone from cyclohexane,
and also of gaining deeper insights into the nature of the catalytically active centers by in-situ EXAFS studies (EXAFS =
extended X-ray absorption fine structure), we have designed a
new catalyst that entails harnessing the advantages of homogeneous organometallic catalysis on the one hand and of a higharea, well-defined (MCM-41) mesoporous silica support on the
Although there is still considerable debate15] about the
mechanism of the oxidation of cyclohexane in the presence of
Co"', Mn"', and Ce" salts in acetic acid or other solvents, and
even more about the nature of the active sites, our point of
departure in this work is the earlier observationC6]that several
0x0-centered trimeric cobalt(rr1) acetates (coordinated with
exhibit much greater activity in selectively oxidizing
the tertiary C-H bond in adamantane than their dimeric analogues.
Dr. T. Maschmeyer, Prof. Sir J. M. Thomas, Dr. R. D. Oldroyd, Dr. G. Sankar,
1. J. Shannon, Prof. C. R. A. Catlow
Davy Faraday Research Laboratory
The Royal Institution of Great Britain
21 Albemarle Street, London Wl X 4BS (UK)
Fax: Int. code +(171)629-3569
J. A. Klepetko, Prof. A. F. Masters, Prof. J. K. Beattie
School of Chemistry, Inorganic Division
The University of Sydney
NSW 2006 (Australia)
[**I This work was supported by a ROPA award and a rolling grant from the
Engineering and Physical Science Research Council (UK) and grants from the
Australian Research Council and ANSTO (Australia). We thank the CLRC
for the synchrotron beam time and other facilities at Daresbury Laboratory.
We also gratefully acknowledge assistance with the solid-state NMR spectoscopy by Dr. Abil Aliev and useful initial discussions with Dr. Fernando
VCH Verlagsgesellschaft mbH, 0-69451 Weinheim. 1997
Table 1. Catalyses with the cobalt complex 1 immobilized on MCM-41.
Catalyst support
[wt %]
MCM 41, glycine-functionahzed through
bromoalkyl tethers
["/.I [a1
TOF [b]
[%I Icl
time [h]
< 48
> 96 [d]
[a1 Cyclohexane (9.5 mL), TBHP (10 mL), and mesitylene (0.5 mL; internal standard) at 70 "C are required for 150 mg of catalyst. [b] In mol cyclohexane/(mol catalyst per
hour). [c] Based on cyclohexanone and cyclohexanol. [d] The experiment was stopped after four days.
The apertures of the MCM-41 samples prepared by
close to 30 A (determined by high-resolution scanning transmission electron microscopy and adsorption isotherms) and readily
permits ingress of the monohydroxycobalt complex 1 (known
[Co,(fi,-O)(OAc), (fit,-OH)(py),lPF,
to substitute the hydroxyl group for other l i g a n d ~ ) which
' ~ ~ we
selected for immobilization inside the channels of the MCM-41
mesoporous silica.["] It is possible to employ various routes to
effect immobilization; anchoring to the surface directly or using
surfaces functionalized with, for example, alkylcarboxylic acids.
Each of these routes yields a catalyst for the selective oxidation
of cyclohexane to cylcohexanone. However, distinct differences
are observed in their respective turnover frequencies (TOF),
lifetimes, and selectivities (Table 1 ) . The catalysis reported here
was effected in an essentially solvent-free fashion : the sacrificial
oxidant tert-butyl hydroperoxide (TBHP) and the reactant
cyclohexane form a miscible liquid and each is consumed to
yield cyclohexanone, cyclohexanol, other minor amounts of
by-products (dicyclohexyl ether, dicyclohexyl peroxide, and
tert-butyl cyclohexyl peroxide), and tert-butyl alcohol (from
In our first attempt at immobilization we reacted the cobalt
complex 1 with partially dehydrated siliceous MCM-41. The
EXAFS spectrum after immobilization showed that there was
virtually no change in the structure of the cobalt complex, indicating either substitution of the bridging hydroxide by a surface
silanol or physisorption brought about by interaction of the
surface with the cationic cluster. Catalytic tests showed that
after 24 h 7.6 % of the available cyclohexane was converted with
a selectivity of about 89 YOto cyclohexanol and cyclohexanone.
The catalyst lifetime, however, was less than 48 h (Table I).['*'
This limited lifetime is most likely due to mobility of the cobalt
clusters on the surface, giving rise to cluster-cluster and other
unfavorable interactions at the interior surface as well as some
leaching indicated by slight discoloration of the solution.
Working on the principle that immobilization at a carboxylfunctionalized surface (that is envisaging the replacement of the
bridging hydroxyl by a carboxyl group) may produce a potentially more stable catalyst, we proceeded to functionalize the
MCM-41 surfaces with a 3-bromopropyltrichlorosilane tether
which we then derivatized by reaction with glycine. The virtual
lack of solubility of glycine in solvents other than water means
that the efficiency of the reaction between the amine part of the
glycine and the bromoalkyl group could be expected to be low
owing to the interaction of water with the amine.1' 31 However,
this would as a side-effect provide a means to ensure a high
dispersion of the cobalt complex which might aid stability (and,
hence, longevity) by reducing possible interactions between
cobalt clusters. Additionally, the nonderivatized bromoalkyl
groups might act as a potential promoter echoing the use of
bromide in commercial processes.
VerlagsgesellschaftmbH, 0-69451 Weinheim, 1997
Catalytic tests established that after an initial TOF of around
352 mol cyclohexane/(mol catalyst per hour) a steady TOF of
around 216 could be sustained for at least four days. The true
heterogeneity of the catalyst was also examined by separating it
by filtration and establishing that the filtrate displayed no further activity under the same reaction conditions. Additionally,
no leaching could be detected (within experimental error) when
comparing the elemental analysis results of the catalyst before
and after reaction. No induction period could be observed (at a
time resolution of 5 min); after an initial exponential behavior,
the catalyst continued to produce cyclohexanone essentially in a
linear fashion. At the outset more cyclohexanol than cyclohexanone is produced, but after about six hours cyclohexanone
production is predominant, an equilibrium between cyclohexanol generation and consumption to form the ketone being
reached after about 12 h (Figure 1).
Figure 1. Plot of the percentage conversion x of cyclohexane into cyclohexanol (e)
and cyclohexanone (n) against time t with 150 mg catalyst.
As expected, addition of extra TBHP after 12 h increased the
oxidation rates, thus showing the potential for rate control (that
is stabilization) by drip-feeding the reactants. Small amounts of
the oxygenated by-products already mentioned (totalling no
more than 5 % of the converted cyclohexane) are also generated;
however, this compares well with the 11 % for the nonfunctionalized support. Hence, there is a clear advantage associated with
the use of functionalized MCM-41 in terms of lifetime, TOF,
and ultimate sele~tivity.~'~]
We believe this may be explained in
terms of the improved isolation of the active centers achieved by
the functionalization of the surface.
Additionally, in-situ X-ray absorption spectroscopy using
(in what is to our knowledge the first
report of such a type of experiment) enabled the detailed atomic
8 17.50tS O / O
Angew. Chem. Inl. Ed. Engl. 1997, 36, No. 15
environment of the active cobalt comDlex to be determined both
prior to and during the course of the solid-liquid heterogeneous
catalysis at realistic temperatures under continuous flow using a
novel liquid/solid cell.[lsblThe liquid collected was analyzed by
GC/MS to confirm catalytic activity at the point of measurernent. Figure2 shows the EXAFS spectrum (and associated
Fourier transform) of the active catalyst and Table 2 contains
the relevant structural parameters.
Table 2. Local structural parameters of the complex 1 derived by Co K-edge
Cobalt cluster
u2 [A']
before immobilization [a]
0 [b]
0 [b]
0 [b]
before catalysis
during catalysis
[a] Deviation from distances derived from the crystal structure analysis are within
experimental error (k0.03 A). [b] Oxygen and nitrogen shells were averaged and
treated as one shell.
-x n I
rlAFigure 2. a) EXAFS of the active catalyst immobilized on glycine-functionalized
MCM-41. b) Associated Fourier transform; solid line: experimental data, broken
line: calculated data.
Furthermore, in-situ EXAFS measurements during the catalysis of the glycine-immobilized catalyst revealed noteworthy
structural changes (Scheme 1). The major feature is a contraction in the Co-Co distances from 3.13 A to 2.84 A, indicating
a significant structural rearrangement in the complex. Dimitrou
et al. have reported an 0x0-centered trimeric cobalt acetate system that displaysvery similar Co-Co distances (average 2.81 A)
to the ones found in our active catalyst.[171In their complex,
however, three of the acetate groups have been displaced by
bridging hydroxide groups, giving rise to the shorter distances.
This may imply the presence of similarly bridging oxygen atoms
in the catalytic species, which could arise from rearrangement of
the pz-acetate groups due to the interactions with substrate molecules or, more likely, from the peroxide present in the reaction
mixture (Scheme 1). The simultaneous presence of bridging hydroxide and some bridging peroxide or other organic oxide is
consistent with the slightly higher than expected coordination
number of the shell at 3.07 A. For a trimer in which three of the
acetate groups have been replaced by hydroxyl groups, only two
oxygen atoms at around 3.1 8, would be expected; however,
about three are observed. Since the distance to a carbon atom,
for example in a bridging alkoxide group, is also about 3.1 A,
the larger coordination number may indicate the presence of
such a species (Figure 3).
29Si MAS NMR spectroscopy (MAS = magic angle spinning) revealed a significant perturbation in the "Si NMR spectrum of nonderivatized MCM-41 after immobilization of 1, indicating a strong interaction between the complex and the
surface. However, cross polarization (CP) MAS NMR spec-
Upon reaction of 1 with the glycine-functionalized MCM-41,
the asymmetric Co, unit rearranged into a symmetric one, that
is the Co -Co distances changed
from 2.808, and 3.178, to
3.13A for both Co-Co pairs
(Table 2). This is readily explicable by postulating the replacement of the bridging OH
group in 1 with the COOH
group of the glycine tether,
yielding essentially three equal
coordination environments for
the cobalt atoms. The structure
of a homogeneous analogue has
recently been determined by usI
ing single-crystal X-ray diffracRtion data and compares ex- Scheme 1. Structure of the cobalt complex before catalysis (left) and
tremely we11.[161
tBuOO, alkyl; R = alkyl tether.
Angew Chem Int. Ed. Engl 1997,36, No. 15
Verlagsgesellschaft mbH, 0-69451 Weinhelm, 1997
Rof the active catalyst (right). py
= pyridine,
0S70-0833~971361S-1641$17.50+ SO10
Figure 3. Proposed structure of the active catalyst (center) as well as the starting
materials (top) and end products (bottom) of the oxidation of cyclohexane with
tert-butyl hydroperoxide and Co'"-modified MCM41 as catalyst.
troscopy could not detect the presence of the cobalt acetate (the
acetate resonances) for either catalyst, but this may well be due
to a reduction of one of the diamagnetic Co"' centers to a paramagnetic Co" center; a fact which could, incidentally, explain
the fate of the PF; ion.
The identification of the active trimeric cobalt allows not only
further rational design of improved catalysts, but also provides
a well-defined starting point for experimental and computational studies on the mechanism.
Experimental Section
Preparation of glycine-functionaliredMCM-41: MCM-41 (1 g) was dehydrated under vacuum at 200°C for 2 h and was then suspended in diethyl ether (30 mL).
Me,SiC1, (0.017 mL) was added and the suspension was left to stir for one hour. The
solvent was then removed under vacuum. The solid was resuspended in dry chloroform (30 mL) and the suspension cooled in an isopropyl alcohol/liquid nitrogen
slurry bath. CI,Si(CH,),Br (0.6 mL) was slowly added, and the mixture left to stir
until it had warmed to room temperature. The resulting suspension was left to stir
for a further 16 h. The solid was then separated by filtration, washed with chloroform (8 x 25 mL), suspended in 30 mL of an aqueous solution containing glycine
(0 1 g) and stirred for 16 h after which it was separated by filtration, washed with
water (150 mL) and then diethyl ether (100mL).
Immobilrzation of I : The same conditions were used for both supports (nonderivatized and glycine-derivatized MCM-41). The support (0.5 g) was suspended in a
solution of 1 ( S O mg) in dichloromethane (30 mL). The suspension was left to stir
for 16 h after which the solid was separated by filtration and washed until the
washings were colorless (usually around 200 mL dichloromethane). The catalysts
were then washed by Soxhlet extraction in dichloromethane for 8 h.
EXAFS measurements The EXAFS data were recorded at the Daresbury SRS
facility on station 8.1 and analyzed with the XFIT (WIN95) and EXCUR V92 suites
of programs [18]. The station was equipped with a Si(220) monochromator, and
data were collected in fluorescence mode using a thirteen-element Canberra fluorescence detector. A powdered sample of each catalyst studied was pressed into a
self-supporting wafer, and mounted into an in-situ reaction cell through which the
reactant mixture (ofcyclohexaneand tert-butyl hydroperoxide) was passed by using
a syringe driver. Heating to the required temperature was achieved by a jet of hot
air directed onto the center of the sample. Details of the cell design have been
published [15b]. Spectra were recorded both before and during the catalysis, in
which the sample was heated to about 60 "C and the reactants were passed through
the cell at a rate of 0.1 mLmin-' and the system allowed to stabilize for one hour
before measurement (normal X-ray absorption spectroscopy, aquisition time per
scan: 45 min). The reaction products were in all cases collected for subsequent
analysis by GCIMS.
Received: September 23, 1996
Revised version: March 14, 1997 [Z9580IE]
German version: Angew. Chem. 1997, 109, 1713-1716
[I] a) N. M. Emanuel, 2. K. Maims, I. P. Skibida, Angew. Chem. 1969, 81, 91;
Angew. Chem. Int. Ed. EngIl969,8,91; b) G. W Parshall, HomogeneousCatalysis: The Application of Catalysis by Soluble Transition Metal Complexes, Wiley, New York, 1980; c) E. P. Talsi, V. D. Chinakov, V. P. Balenko, V. N.
Sideinikov, K. I. Zamaraev, Molecular Catalysis, Wiley, New York, 1980; d)
R. A. Sheldon, J. K Kochi, Metal Catalysed Oxidation of Organic Compounds,
Academic Press, New York, 1981.
[2] a) A. B. Blake, J. R. Chipperfield, S. Lau, D. E. Webster, J: Chem. SOC.Dalton
Trans. 1990, 3719; b) C. F. Hendriks, H. C. A. van Beek, P. M. Heertjes, Ind.
Eng. Chem. Prod. Res. Dev. 1979, 18, 43; c) W. Partenheimer, R. K. Gipe in
Cafalytic Selectrve Oxidation (Eds.: S . T. Oyama, J. W. Hightower), ACS
Symp. Ser. 523, 1993, 81.
[3] a) A. Onopchenko. J. G. D Schultz, J: Org. Chem. 1975, 40, 3338; b) J. K.
Kochi, Organometallir Mechanisms and Catalysi.s, Academic Press, New York,
[4] Mesoporous materials readily permit the combination of organometallic surface chemistry with the principle of shape-selectivity generally associated with
microporous zeolites and the results from this initial study of cyclohexane
might possibly be extended to applications where shape-selectivity is important.
[ S ] a) J. K. Kochi, R. T. Tang, T. Bernath, J: Am. Chem. Soc. 1973,95,7114; b) P. J.
Andrulis, M. J. S . Dewar, T. Dietz, R. L. Hunt, ibid. 1966, 88, 5473; c) W. S.
Trahanovsky, D. W. Brixius, ibid. 1973, 95, 6778.
[6] A. F. Masters, J. K Beattie, personal communication.
[7] S. Uemura, A. Spencer, G. Wilkinson, J: Chem. Soc. Dalton Trans. 1973,2565.
[S] F. Rey, G. Sankar, T. Maschmeyer, J. M. Thomas, R. G. Bell, G. N. Greaves,
Top. Catal. 1996, 3, 121
[9] C. E. Sumner, Jr., G. R. Steinmetz, Inorg. Chem. 1989, 28, 4290.
[lo] a) J. S. Beck, J. C. Vartuli, Curr. Opm. SolidState Muter. Sci. 1996,1,76; b) Q.
Huo, R. Leon, P. M. Petroff, G . D. Stucky, Science 1995,268, 1324.
[ l l ] a) This approach (results were calibrated by using mesitylene as internal standard) was deemed necessary for the in-situ X-ray absorption spectroscopy
(XAS) experiments as the TOF using the flow system were anticipated to be low
and appropriate chlorinated solvents interfere with the XAS measurements.
b) Reaction conditions: TBHP (10 mL), cyclohexane (9.5 mL), mesitylene
(0.5 mL; internal standard), catalyst (150 mg) stirred in a batch reactor at
70°C under argon. Up to 20 aliquots of 0.1 mL were withdrawn for analysis
during a catalysis run. The MCM-41 was initially exposed to a small amount
of the reactive dichlorodimethylsilane with the aim of functionalizing the majority of the silanol groups on the external surface, assuming a faster rate of
reaction for the external compared to the internal surface. Hence, to ensure
that the catalyst was present predominantly on the inside of the MCM-41
channels, the MCM-41 was treated with the various alkyl tethers only after this
pretreatment. This procedure has been shown by scanning transmission electron microscopy to yield the desired effect in related systems (unpublished
[12] The kinetic profile showed no detectable induction period at a resolution of 5
minute time steps and behaved in a typical exponential fashion
1131 From the microanalysis the ratio of bromide tether to tethered cobalt is 100:1,
NMR experiments aimed at trying to identify the glycine-derived tether were
not sensitive enough to conclusively pick up the giycine-part, as can be expected at such low loadings.
[I41 The increase in the selectivity of the catalyst system with time can be explained
by the observation that the by-products dicyclohexyl peroxide and cyclohexyl
tert-butyl peroxide might both decompose to cyclohexanone and that the byproducts might react further, disappearing in the base-line. Due to the solventfree conditions no full mass balance was attempted.
[I51 a) J. M. Thomas, G. N. Greaves, Science 1994, 265, 1675; b) J. M. Thomas,
I. J. Shannon, G. Sankar, T. Maschmeyer, M. Sheeky, D. Madill, A. Waller,
Catal. Lett. 1997, 44, 23.
[16] J K. Beattie, T. W. Hambley, J. A. Klepetko, A. F. Masters, P. Turner, Polyhedron 1996, 15, 2141.
[I71 K. Dimitrou, K. Folting, W. E. Streib, G. Christou, J: Am. Chern. Sac. 1993,
115, 6432.
1181 a) XFIT (WIN95): P. J. Ellis, H. C. Freeman, J Synchrotron Radiaf. 1995,2,
190; b) EXCURV92-Program: N. Binstead, J. W. Campell, S. J. Gurman,
P. Stephenson, SERC Daresbury Laboratories, 1992.
Keywords: cobalt * EXAFS spectroscopy heterogeneous catalysis mesoporosity * oxidations
0 VCH Verlagsgesellschaft mbH, 0-69451
Weinheim, 1997
0570-0833/97/3615-1642$ f 7.50+ SOjO
Angew. Chem. Int. Ed. Engl. 1997, 36, No. 15
Без категории
Размер файла
691 Кб
oxidation, solis, low, temperature, selective, cyclohexane, cyclohexanone, designing, catalyst
Пожаловаться на содержимое документа