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Highly Selective Epoxidation Catalysts Derived from Intrazeolite Trimethyltriazacyclononane-Manganese Complexes.

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Highly Selective Epoxidation Catalysts Derived
from Intrazeolite TrimethyltriazacyclononaneManganese Complexes**
Dirk E. De Vos, Julia L. Meinershagen, and
Thomas Bein*
Dedicated to Professor Gaien D . Stucky
on the occasion of his 60th birthday
Over the last few years the development of zeolite-based catalysts for the liquid-phase oxidation of hydrocarbons has been
a field of active research, and several interesting systems have
been discovered.['
However, the selectivities of these systems
in the epoxidation of olefins may be limited by acid-catalyzed or
other side reactions. This is particularly true with Ti-B and
[ M n ( b ~ y ) ~ ]-*N+a y catalysts (bpy = 2,T-bypridine). Furthermore, the epoxides are frequently subject to solvolysis,['731 or
large amounts of less desirable products are formed, for example phenylacetaldehyde and benzaldehyde in the case of the
oxidation of styrene.[*]
We now report on a novel zeolite-based catalyst, a manganese
faujasite containing the cyclic triamine 1,4,7-trimethyl-l,4,7-triazacyclononane (tmtacn). This material catalyzes the oxidation
of numerous olefins with H,O,, yielding epoxides with only
minimal by-product formation. We also report on the characteristics of the corresponding homogeneously catalyzed reaction,
which allows us to outline the effect of the heterogenization on
the catalytic activity.
The zeolite-based oxidation catalysts were prepared by reacting a Mn'INaY zeolite (0.8 wt % Mn, or about one metal ion per
four supercages), dehydrated at 250 "C, with the polyamine ligand for 10 h at 150 "C under an inert atmosphere and occasional
shaking. A polyamine to Mn molar ratio of 2.2 was found to be
optimal for an efficient diffusion of the ligand into the zeolite
pores. Homogeneous catalysts were prepared in situ by adding
the ligand, dissolved in the organic reaction solvent, and
MnSO;H,O, dissolved in water, to the reaction mixture. All
chemicals used were commercial products, with the exception of
tmtacn and 1,4,7-triazacyclononane (tacn), which were prepared by known p r o c e d ~ r e s .Numerous
epoxides were synthesized as reference compounds through the peracid method
and identified through gas chromatograpy/mass spectrometry
(GC/MS) and 'H NMR spectroscopy.
Before the adsorption of the ligand by the Mn zeolite, X-band
ESR spectroscopy shows the characteristic six-line signal of
Mn" with a total width of about 500 G, which is typical for an
oxygen-coordinated Mn ion in zeolites (Fig. la).['O1After sorption of the ligand into the zeolite host, the intensity of this signal
strongly decreases at the expense of a new Mn" signal, which
extends over a much wider field range, and displays a recognizable fine splitting (Fig. lb).'"' We recently observed similar
spectra for Mn" ions in different A zeolites.['z] From the spectrum, the zero-field splitting parameter D can be estimated to
amount to 0.06i0.015 cm- '. Such a fine splitting is characteristic for a departure of the crystal field around Mn from cubic
symmetry. with preservation of an at least threefold symmetry
axis." 31 As the formation of complexes with two tmtacn ligands
I")Prof. Dr T. Bein. Dr. D. E. De Vos. J. L. Meinershagen
Purdue University
Department of Chemistry
West Lafayette. I N 47907 (USA)
Fax: Int. code +(317)494-0239
e-mail' tbeinxt
This work was supported by a grant from the U . S . Department of Energy
Angen . Chrm. lor. Ed. Engl. 1996. 35, No. 19
Fig. 1. X-band ESR spectra (9.44 GHz) of Mn in a) Mn"NaY, before ligand adsorption, 298 K ; b) [Mn"(tmtacn)12' -Nay, under a dry N, atmosphere, 298 K ; c)
[Mn(tmtacn)I2' -Nay. exposed for 2 h to H 2 0 2 in the concentrations of catalytic
experiments (see Table 1). recorded at 8 K. In a) only the central M s= 1/2 +
- 1/2 transition of the Mn fine structure is resolved. indicating that the symmetry
of the all-oxygen Mn environment is close to cubic. The reaction wlth the tmtacn
ligand in b) reduces the local symmetry t o axially symmetric; the non-central fine
transitions (involving M , = k 3/2 and M , = i- 5/2) now also contribute to the spectrum. The resulting five fine lines (AMs = 1) are roughly spaced by the distance
D and have been marked ( V ) in the spectrum. At lower magnetic field in b),
half-field transitions ( A M s = f 2 ) are observed (*). In the H,O, oxidized sample c),
a sixteen-line signal of the Mn"'-Mn'" dinuclear complex is observed. dpph = l , l diphenyl-2-picrylhydrazyl (as reference).
per Mn ion is unlikely for steric reasons,['4] we ascribe the new
ESR signal to the formation of the [Mn(trntacn)]'+ complex.
This chelation is not observed with smaller pore MnNa-ZSM-5
or MnNa-A zeolites that have pore openings of 0.53 and
0.41 nm, respectively, which implies that the chelation in the
large-pore NaY zeolite is essentially intraporous. This [Mn(tmtacn)]'+-NaY material is used as the catalyst precursor in the
catalytic reactions.
The selective epoxidation activity of the [Mn(tmtacn)]' NaY zeolite is somewhat surprising. Complexes of manganese
and tmtacn ligands have been observed to activate H,O, at
room temperature, for example for bleaching and for epoxidations.['" 1 6 ] However, bleaching is often associated with a) the
production of reactive particles such as (OH)', (OOH)', or singlet oxygen (lo2),
which may cause the formation of side products in epoxidations, and with b) disproportionation of the peroxide to molecular oxygen and water, which leads to a reduction
of the selectivity of the oxidant.["' It is therefore understandable that the reported epoxidation of styrenes with homogeneous Mn-tmtacn catalysts requires an at least 100-fold excess
of the oxidant.["]
In our initial experiments with the zeolite-based [Mn(tmtacn)]'+ - N a y catalyst, we adopted the same conditions as
used for the homogeneous epoxidation (aqueous methanol,
VCH Verlagsgesellschafi mbH, 0.69451 Weinheim, 1996
0570-0833/96/3519-2211S 15.00+ .25/0
221 1
room temperature)['51except that the oxidant to substrate ratio
was 1 : 1. The epoxide yields of these reactions were very low
( < 1 YO).
However, three major modifications of the procedure
resulted in highly improved epoxide yields, even at close to
stoichiometric H,O,:olefin ratios (Table 1): a) cooling the reacTable 1 . Effect of solvents and temperature on the epoxidation of styrene and
cyclohexene with H,O,, catalyzed by the [ M n ( t m t a ~ n ) ] ~~' N a Yzeolite o r by
homogeneous Mn- tmtacn complexes [a].
T[K] Solvent
Olefin conversion ["YO]
['//.I [bl
84 [dl
sition of the H,O, oxidant is observed, resulting in the formation of the products in extremely low yields. While the methylation might protect the amine ligand against oxidation, an alternative explanation for this dramatic difference is that the methyl
groups provide a hydrophobic wrapping around the Mn ion
(Fig. 2). This lipophilic shield around the M n ion probably
favors the adsorption of an olefin at the peroxide-activated
Mn ion, retarding the approach of new peroxide molecules and
thus the catalytic H,O, decomposition.
[Mn(tmtacn)12 - NaY
Mn" +tmtacn
[a] Reaction conditions: olefin ( I mmol), H,O, ( 1 mmol. as a 30 wt O/U aqueous
solution, gradually added), solvent (1 g). [Mn(tmtacn)12+- N a y solid catalyst
(0.025 g). For homogeneous catalysis, tmtacn (22 pmol) and Mn" (10 pmol) were
consecutively added in organic solvent and water (0.1 mL). respectively. The homogeneous reaction conditions were chosen to match the heterogeneous experiments:
they are not optimized (see ref. 1211). H 2 0 , consumption was usually complete
within 12 h o r less. [b] Remaining products: benzaldehyde (in case of styrene),
(E)-1,2-cyclohexanediol (in case of cyclohexene). [c] A second aliquot of H,O,
( I mmol) was added after 6 h. [d] Allylic cyclohexene oxidation products: 7 % .
tion mixture to 273 K or below, b) gradual peroxide addition,
and c) use of acetone as a solvent. The latter produces a dramatic effect, not only for the zeolite-catalyzed reaction but also for
the homogeneous reaction. For a discussion of the effect of
acetone see reference [21]. The combined effects of these modifications result in a marked reduction of the undesired oxidant
disproportionation, and a n increase of the epoxide yield by a
factor of 50.
This yield increase does not affect the selectivity of the oxidation process. With the [Mn(tmtacn)]" - N a y system, side products in the reactions of styrene, cyclohexene, and cis-2-hexene
(not listed) are minimal, if observed at all (Table 1). Products of
double-bond cleavage, such as benzaldehyde, account for less
than 3 OO/ of the products in the case of the oxidation of styrene.
Remarkably, in the oxidation of cyclohexene with zeolite catalysts, no cyclohexenyl-2-hydroperoxide, cyclohex-2-enol, o r
cyclohex-2-enone was detected. This indicates that under these
conditions, (HOO)' or (HO)' radicals, or other reactive 0 species, such as lo,, play no major role in the formation of cyclohexene oxidation products.['*] Epoxide solvolysis is also minimal. While in the related homogeneous systems an alkaline
aqueous buffer may play a role in preventing this side reaction>l51the excess amine adsorbed on the zeolite catalyst seems
to quench effectively any acidity of the zeolite surface. This is a
marked advantage of our material over catalysts such as Ti@
and [Mn(bpy),l2+ -faujasite; in these materials the surface pH
is intrinsically acidic and further reactions of epoxides are the
rule rather than the exception.['. 31
The structure of the tmtacn ligand seems to fulfill an essential
role in making the Mn ion suited for the selective epoxidation.
With the unmethylated tacn as ligand, only vigorous decompo221 2
VCH Vcrlig.~gP.srll.~~liofi
mhH. D-69451 Wi~mhriiii.1996
2, Computer model (Cerius2. MolecularSimulations
of a
plex of Mn and tmtacn i n the supercage of a
zeolite (Mn = green;
= blue;
= white).
While with the homogeneous catalyst the reaction starts almost instantaneously, an induction period is observed with
[Mn(tmtacn)I2+- N a y In the ESR spectrum (Fig. Ic), this period corresponds to the appearance of a 16-line signal with a
hyperfine constant A = 78 G, which is typical for a Mn"'-MnlV
dinuclear c ~ m p l e x .91~ Although
other ESR-silent species could
have formed, the correlation between the induction period and
the appearance of the Mn"'-Mn"' dinuclear complex suggests
that catalysis occurs in conditions that promote oxidative formation of dinuclear M n complexes in the cavitities of the Y
zeolite. Analogous spectra are not observed when MnNa-ZSM5 or MnNa-A zeolites treated with tmtacn are contacted with
H 2 0 2. This failure to observe dinuclear complexes in smaller
pore zeolites suggests that for the Y zeolite, the dinuclear complexes are largely confined to the inner surface. Such complexes
can contain manganese in its + IV oxidation state, in which state
the metal is known to have the capacity to act as an oxygentransferring species.
The spatial interaction between a NaY supercage and a dinuclear Mn-tmtacn complex, such as [(tmtacn)Mn"L-02(OH)Mn'"(tmtacn)12+, was studied by molecular modeling.[201These
complexes appear to have the appropriate dimensions to reside
in the large faujasite cage, suggesting their steric entrapment
inside these supercages (Fig. 2). The proposed intrazeolitic complexation of the Mn" ion by tmtacn, and the consecutive formation of the larger dinuclear complexes upon addition of H,02,
are schematically represented in Scheme 1. The zeolite-confined
character of the [Mn(tmtacn)]" - N a y catalysis was ultimately
confirmed by a filtration experiment. Two styrene oxidations
were run under identical conditions, in one case, however, the
catalyst powder was removed from the reaction after about 5 %
conversion. While in the other reaction an oxide yield of 20%
0570-08331Y613519-2212S 15.001- 2510
Angew Chem Int Ed Engl 1996 35 N o IY
[19] K. Wieghardt, U. Bossek, L. Zsolnai. G. Huttner, G. Blondin. 1-J. Girerd, F.
Babonneau. J. Chem. Soc. Chem. Commun. 1987. 651; R. Hage, B. Krijnen,
J. B. Warnaar, F. Hartl, D. J. Stufkens, T. L. Snoeck, Inorg. Chem. 1995, 34,
1201 Molecular modeling was performed with the Cerius' Software from Molecular
Simulations Inc. Plausible coordinates for a dimeric complex of Mn and tmtacn were obtained from K. Wieghardt, U. Bossek. B Nuber, J. Weiss. J.
Bonvoisin. M. Corbella. S. E. Vitols. J. J. Girerd, J Am. C%em.SOC.1988,110,
[21] D. E. De Vos, T. Bein, Chem. Commun. 1996, 917; J Orgunomel. Chem. in
Scheme 1. Proposed reactions of Mn ions and tmtacn ligands in zeolite Y. Reaction
a: In the chelation reaction, the Mn" ion leaves an all-zeolite coordination, for
example on a six-membered ring, to form a mononuclear [ M n " ( t m t a ~ n ) ]chelate
complex. Reaction b: The interaction of [Mn"(tmta~n)]~'-NaY with H,O, in a
wateriacetone medium leads to intrazeolite formation of Mnii'-MniV dinuclear
complexes or other dimerization products (X = 0'-, OH-, or other oxygen
was eventually obtained, the increase of the oxide yield stopped
in the vial without catalyst.
In summary, the intrazeolite assembly of manganese complexes with cyclic amine ligands leads to novel catalytic materials for highly selective epoxidation. Dramatic effects of the ligand structure and of the solvent on the reactivity, as well as the
patterns of substrate and product selectivity,r211support the
emerging picture of a sterically crowded, ligand-embedded active site in the cages of zeolite Y.
Received: March 19, 1996 [Z8945IE]
German version: Angeiv. Chem. 1996. 108.2355-2357
Keywords: catalysis * complexes with nitrogen ligands epoxidations zeolites
[l] A. Corma. M. A. Camblor. P. Esteve. A. Martinez, J. Perez-Pariente, J Curd.
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Toward the Design of Porous Organic Solids:
Modular Honeycomb Grids Sustained by Anions
of Trimesic Acid * *
Rosa E. Melendez, C. V. Krishnamohan Sharma,
Michael J. Zaworotko,* Carey Bauer, and
Robin D. Rogers
The concept of design or crystal engineering of functional
solids relies upon controlling the topology of crystal packing
through exploitation of forces that range in the continuum from
weak hydrogen bonds to covalent
In the context of
porosity, a particularly relevant functional property given the
commercial importance of zeolites, it is becoming increasingly
clear that even noncovalent interactions can be directional and
strong enough to generate and sustain porosity.[31Several research groups have recently demonstrated that a modular strategy to crystal engineering can propagate molecular symmetry
into crystalline architecture or even space group ~ y m m e t r y . [ ~ - ~ ]
Unfortunately, two- (2D) and three-dimensional (3D) frameworks typically fail to realize their full potential for porosity
because of interpenetration or self-inclusion of identical framew o r k ~ . [7a*~91~ In
. such a context, trimesic acid (1,3,5-benzenetricarboxylic acid, H,TMA) is prototypal since it predictably selfassembles through the well-known carboxylic acid dimer motif
into an interpenetrating honeycomb grid with 11 A cavities.16.l o , l'] We report herein on two compounds that illustrate
two alternate modular strategies to propagate the trigonal symmetry of H,TMA: the ammonium salt 1forms a neutral honeycomb grid and the ammonium salt 2 has an anionic honeycomb
grid based upon hydrogenbiscarboxylate hydrogen bonds.
[7] M. G. Clerici, P. Ingallina. J Cutul. 1993, 140. 71
[8] S . B. Kumar. S P. Mirajkar, G. C. G. Pais, P. Kumar, R. Kumar, J. Curul.
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(121 D. E. De Vos. B. M. Weckhuysen. T. Bein, J. Am. Chem. Soc. in press.
[13) A. Abragam. B. Bledney, Electron Purumugnefic Resononce of Trunsition Ions,
Clarendon Press. Oxford, 1970.
[I41 P. Chaudhuri, K. Wieghardt. Prog. Inorg. Chem. 1987, 35, 329.
[I51 R. Hage, J E Iburg. J. Kerschner, J. H. Koek, E. L. M. Lempers, R. J.
Martens, U. S. Racherla, S. W. Russell. T. Swarthoff, M. R. P. van Vliet. J. B.
Warnaar. L. van der Wolf. B. Krijnen. Nuture 1994, 369. 637.
[I61 S. H. Jureller. J L. Kerschner. R. Humphreys. (Unilever). US Patent 5329024.
Compounds 1 and 2, which contain the fully and 50 % deprotonated forms of H,TMA, respectively, predictably self-assemble into honeycomb grids because of their trigonal symmetry
[*] Prof. M. Zaworotko, R. E. Melendez, Dr. C. V. K. Sharma
Department of Chemistry
Saint Mary's University
Halifax, Nova Scotia. B3H 3C3 (Canada)
e-mail : mzaworot(i
Fax: Int. code +(902) 420 5261
[17] K. M. Thompson. W. P. Griffith. M. Spiro, J. Chem. SOC.Furudoy Trans. 1993.
89. 1203, ibid. 1993,89, 4035.
[18] A. A. Frimer. Singlet O.s.rgen, Vol. 2, CRC Press, Boca Raton. Florida, 1985;
R. A. Sheldon, 1. K. Kochi. Merul-Curul~~=edOxirlalion
of Organic Compounds,
Academic Press. New York, 1981.
Angun.. Chem. Int. Ed Engl. 1996. 35. No. 19
Dr. C. Bauer. Prof. R. D. Rogers
Department of Chemistry, Northern Illinois University
DeKalb, IL, 601 15-2862 (USA)
We gratefully acknowledge financial support from the National Science and
Engineering Research Council (NSERC) and the Environmental Science and
TechnologyAlliance of Canada (ESTAC). Trimesic acid = 1.3.5-benzenetrlcarboxylic acid.
Q VCH Verlugsgesellschuft mbH, 0-69451 Wemheim, 1996
$15.00 f ,2510
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intrazeolite, trimethyltriazacyclononane, selective, epoxidation, complexes, derived, catalyst, highly, manganese
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