A bifunctional mechanism for ethene dimerization: Catalysis by rhodium complexes on zeolite HY in the absence of halides

Article abstract of DOI:10.1002/anie.201008086

No ligands needed: Rhodium complexes supported on HY zeolite catalyze the formation of C - C bonds by a new mechanism involving cooperation between the metal species and Bronsted acid sites of the zeolite support (see picture). The catalyst operates in the absence of ligands such as halides and shows high selectivity to n-butenes, even in an excess of H₂. Copyright

Full text of DOI:10.1002/anie.201008086

Communications
DOI: 10.1002/anie.201008086
Catalytic Alkene Dimerization
A Bifunctional Mechanism for Ethene Dimerization: Catalysis by
Rhodium Complexes on Zeolite HY in the Absence of Halides**
Pedro Serna and B. C. Gates*
À
Complexes of numerous transition metals catalyze C C bond
formation, and an enormous industrial production of oligo-
mers and polymers from alkenes is carried out with complexes
of early transition metals such as Cr, Ti, and Zr, some of them
supported.[1–4] Late transition metals such as Ni, Pd, or Rh
also catalyze these reactions, typically relying on halides,
which allow the metal to shuttle between oxidation states in
the catalytic cycle.[5,6] Active alkene oligomerization catalysts
have also been prepared from complexes of late transition
metals by incorporation of ligands such as those with neutral
multidentate nitrogen donor groups.[7]
species expected to be intermediates in the hydrogenation
reaction[11] are not involved in the formation of the C C
À
bonds. Thus, the mode of action of the new dimerization
catalyst differs both from that pertaining to halide-containing
rhodium complexes[12–17] and that pertaining to strongly
Brønsted acidic solids.[18–24]
The observation that site-isolated rhodium complexes
supported on zeolite HY, initially present as [Rh(C2H4)2]
(Table 1), catalyze the dimerization of ethene in a once-
through plug-flow reactor at 1 bar and 303 K (with or without
H2 in the feed—the reaction is faster with H2) is contrasted
À
The formation of C C bonds by dimerization of small
alkenes such as ethene can take place on supported catalysts
including reduced cobalt oxides,[8] nickel oxides,[9] and ruthe-
nium clusters[10] in the presence of H2, provided that the ratio
of the H2 to alkene partial pressures is low—but only with
poor selectivities to n-butenes, typically < 1% (the dominant
Table 1: EXAFS data at the Rh K edge characterizing the sample prepared
by reaction of [Rh(C2H4)2(acac)] with the surface of zeolite DAY in gases
flowing at 303 K.[a]
Gas in contact
with sample
Shell
N
R [ꢀ]
103 ꢁDs2
DE0
[eV]
=
reaction is hydrogenation of the C C bond).
[ꢀ2]
Now we show that site-isolated rhodium complexes
supported on dealuminated zeolite HY (DAY zeolite),
[b]
[b]
[b]
[b]
À
helium
Rh Rh
À
Rh O
–
–
–
–
2.1
3.7
1.1
2.15
2.08
3.02
3.1
3.4
6.7
7.1
prepared
from
[Rh(C2H4)2(acac)]
(acac =
À
Rh C
À2.0
CH3COCHCOCH3), catalyze the dimerization of ethene in
the absence of halides, with high selectivities to n-butenes (ca.
75%), even when high partial pressures of H2 are employed
(H2/C2H4 = 4, molar). The selective performance of this
À
Rh Al
À2.5
À
C2H4 +H2
Rh Rh
À
Rh O
–
–
–
–
(1:4)[c]
2.0
2.1
0.7
1.8
2.13
2.03
3.09
3.05
2.4
2.5
2.4
1.8
2.6
2.8
3.8
9.2
À
Rh C
À
catalyst for C C bond formation in the absence of labile
À
Rh Al
ligands such as halides is a new and unanticipated result.
Characterization of the catalyst in the working state by IR
and extended X-ray absorption fine structure (EXAFS)
spectroscopies (even during catalysis with flowing mixtures
of C2H4 + H2 at 303 K and 1 bar) coupled with H/D exchange
reaction experiments demonstrated that the catalysis involves
a cooperation between mononuclear rhodium complexes and
Brønsted acid sites of the zeolite support. H2 promotes the
dimerization reaction, which is accompanied by a slower
À
Rh Clong
[a] Notation: N, coordination number; R, distance between absorber and
backscatterer atoms; Ds2, Debye–Waller factor; DE0, inner potential
correction. Error bounds (accuracies) characterizing the structural
parameters obtained by EXAFS spectroscopy are estimated to be as
follows: coordination number N, Æ20%; distance R, Æ0.02 ꢀ; Debye–
Waller factor Ds2, Æ20%; and inner potential correction DE0, Æ20%.
[b] Contribution not detectable. Details of the EXAFS fitting are provided
in the Supporting Information. [c] Molar ratio.
=
hydrogenation of the C C bond, and we infer that the ethyl
[*] Dr. P. Serna, Prof. B. C. Gates
with our observation that the isostructural rhodium com-
plexes supported on highly dehydroxylated MgO (Table SI-1
in the Supporting Information) are 100% selective for
Department of Chemical Engineering and Materials Science
University of California
One Shields Avenue, Davis, CA 95616 (USA)
Fax: (+1)530-752-1031
E-mail: bcgates@ucdavis.edu
Homepage: http://www.chms.ucdavis.edu/research/web/catalysis
=
hydrogenation of the C C bond in the presence of H2 and
inactive under our conditions when C2H4 is used in the
absence of H2 (Table 2). The behavior of the MgO-supported
catalyst agrees with reports demonstrating that RhI species
require the participation of ligands (e.g., halides) to catalyze
the alkene oligomerization.[12–14] For example, [Rh(C2H4)2-
(acac)], the precursor used to synthesize our zeolite- and
MgO-supported catalysts, can be transformed into an active
alkene dimerization catalyst by treatment with HCl.[12]
[**] We acknowledge DOE Basic Energy Sciences for support (Contract
No. FG02-04ER15513) and acknowledge beam time and support of
the DOE Division of Materials Sciences for its role in the operation
and development of beam line MR-CAT at the Advanced Photon
Source at Argonne National Laboratory.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201008086.
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Angew. Chem. Int. Ed. 2011, 50, 5528 –5531

Table 2: Catalytic results characterizing DAY zeolite-supported rhodium
species for conversion of ethene at 303 K and 1 bar.
Predominant form
of rhodium
TOF[b]
[sÀ1
Product selectivity,
molar ratio of
]
in catalyst[a]
dimers/ethane[e]
none[c]
–
–
[Rh(C2H4)2][c]
[Rh(C2H4)2]
0.075
0.012[d]
0.26[e]
0.93[f]
3.55
8.83
2.63
0.12
[Rh(C2H4)2]
Rh clusters
(2–4 atoms each, on average)
[Rh(C2H4)2][g]
0.025
0
[a] As determined by EXAFS and IR spectroscopies (details provided in
the Supporting Information). [b] Turnover frequencies determined at
time t=0 from the corresponding TOF vs. time-on-stream curves (see
Supporting Information). [c] Reaction performed at 0.4 mbar of C2H4,
0.1 mbar of H2, and 0.5 mbar of helium. [d] Reaction performed in the
absence of H2. [e] Reaction performed at 0.1 mbar of C2H4, 0.4 mbar of
H2, and 0.5 mbar of helium. [f] Constant once the composition of the
feed had stabilized, approximately 5 min after the start of reactant flow.
[g] Rhodium complexes supported on MgO.
The potential role of the zeolite itself as the catalyst for
À
the C C bond formation reaction is ruled out because its
activity was too low to measure at 303 K (Table 2), as
expected.[25,26] In the flow reactor containing the DAY zeolite
without rhodium, C2H4 + H2 were observed to form traces of
dimers (together with C1, C2, and C3 hydrocarbons), but only
at temperatures > 433 K.
À
À
Figure 1. IR spectra in the OH (top) and CH (bottom) stretching
regions characterizing the catalyst formed by reaction of [Rh(C2H4)2-
(acac)] with the surface of dealuminated HY zeolite after contact with
a) flowing helium for 5 min and b) a reacting C2H4 +H2 mixture at
303 K and 1 bar. Spectrum (b) (bottom) was recorded with the sample
in helium after removal of ethene from the gas phase by evacuation.
Time-resolved IR spectra of the catalyst in the working state are shown
in the Supporting Information.
IR spectra show that the alkene interacted, on the one
hand, with the zeolite through the acidic Al-OH groups, as
À
indicated by the reduction in intensity of the O H stretching
bands at 3630 and 3565 cmÀ1 (Figure 1, top), which became
strongly broadened and shifted to lower frequencies (Fig-
ure SI-1)[18,19] and, on the other hand, with the rhodium sites
as well, as indicated by the disappearance of the bands at
3084, 3060, and 3016 cmÀ1 characterizing the C H stretching
À
À
vibrations in the ethene initially p-bonded to the Rh atoms
working state show that the initial Rh C coordination
(Figure 1, bottom).[27]
number of nearly 4 (corresponding to two p-bonded ethene
ligands per Rh) decreased to 2.1 Æ 0.4 upon introduction of
C2H4 + H2 (Table 1), accompanied by the appearance of a
The C2H4–zeolite interaction is inferred to be weak, in
agreement with the results of previous investigations,[25,28] as
=
À
the C C bond was retained upon ethene adsorption, as
second Rh C contribution at a greater absorber–backscat-
terer distance, consistent with the formation of ethyl or butyl
ligands. Furthermore, the EXAFS data gave no evidence of
indicated by the appearance of an IR band at 1615 cmÀ1
=
(Figure SI-2), assigned to the C C stretching vibration of
ethene p-bonded to the acidic sites of the zeolite.[29] This
result is consistent with the observation that the alkene
readily desorbed from the acidic sites when helium flowed
over the sample at room temperature, with full recovery of
Rh Rh contributions and thus no evidence of rhodium
À
clusters, indicating the stability of the mononuclear rhodium
complexes.[30]
The observation that neither rhodium complexes sup-
ported on MgO nor the DAY zeolite alone was catalytically
active for ethene dimerization under our conditions (303 K,
1 bar), together with the spectroscopic data demonstrating
the interactions of ethene with both the rhodium complexes
the initial Al OH bands at 3630 and 3565 cmÀ1 (Figure SI-1).
À
On the other hand, the adsorption of ethene on the site-
isolated rhodium species led to an almost instantaneous
formation of alkyl species on the rhodium, indicated by the
disappearance of the initially p-bonded ethene ligands and
the concomitant growth of bands at 2960, 2935, and 2875 cmÀ1
assigned to stretching vibrations of CH, CH2, and CH3
groups, as shown in Figure 1, bottom (consistent with this
interpretation, no such change was observed when ethene
flowed over the zeolite without rhodium). In agreement with
these results, EXAFS data characterizing the catalyst in the
À
and the acidic Al OH groups of the support, indicates that
both the rhodium and the Al OH groups participate in the
À
À
À
À
À
catalytic C C bond-forming reaction.
To provide further understanding of the bifunctional
reaction mechanism of the dimerization and the role of H2,
experiments were carried out with D2 and with mixtures of D2
and ethene. IR spectra of the zeolite-supported catalyst in
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Communications
À
flowing D2 show that the OH bands of the zeolite initially
present at 3630 and 3565 cmÀ1 readily disappeared—as new
bands grew in the range of 2550–2780 cmÀ1 (Figure SI-3),
classes of active sites are required to be close enough to each
other to allow a cooperative mechanism, consistent with our
observation that a physical mixture of the bare H-form zeolite
and the MgO-supported rhodium complex (5:1 mass ratio)
led to the formation of ethane as the only observed reaction
product, corresponding to the performance of the rhodium
complex anchored to the basic support. Thus, we rule out the
possibility that the dimerization occurs as a result of action of
the acidic zeolite sites, even when hydrogen may spill over
onto this support. However, we do not discard the possibility
that the reaction takes place by activation of one of the ethene
molecules on the same Al site where the rhodium complex is
bonded, as it is known that the Al sites can be partially
À
À
corresponding to the formation of Si OD and Al OD groups
resulting from HD exchange.[29] Upon introduction of C2H4 to
À
the IR cell, the Al OD bands at 2677 and approximately
2600 cmÀ1 declined drastically in intensity (Figure SI-3)—just
as the dimerization reaction started. Mass spectra of the gas-
phase products show that the butenes were characterized by
C3H5 and C4H8 fragments (m/z 41 and 56, respectively,
corresponding to the typical fragmentation of n-butenes) with
no deuterated dimers. This result demonstrates that C2H5
species, either as ethyl ligands formed on the rhodium after a
partial hydrogenation of the initially p-bonded ethene
ligands[31] or as carbenium ions on the acidic zeolite sites,[20]
are not reaction intermediates in the formation of n-butenes,
in contrast to the accepted mechanisms for dimerization of
alkenes on halide-containing rhodium complexes[12,13] and on
strong solid Brønsted acids.[18–24] (Scheme SI-1 illustrates the
reported reaction mechanisms for ethene dimerization on
these catalysts.) The results of the D2 exchange experiments
are thus consistent with the observation that ethene retained
removed from the zeolite framework under some conditions
[32]
À
to yield another type of acidic Al OH species, which we
have observed for our catalyst in the presence of H2 at 303 K
and 1 bar, leading to the appearance of a new band in the
OH stretching region at 3710 cmÀ1 that readily disappeared
upon contact with C2H4.
À
To account for the observed high selectivity to butenes
(Table 2), we infer that the interaction between two adsorbed
=
ethene molecules is faster than the hydrogenation of the C C
=
À
À
the C C bond upon interaction with the zeolite Al OH
groups.
bond. The data indicate that the formation of the C C bond,
which is the rate-determining step when the reaction is
catalyzed by rhodium halide complexes[12] and is quite slow on
acidic zeolites,[20,21] is facilitated by a cooperation between the
RhI species and the acidic sites of the support. In contrast, the
accumulation of relatively large amounts of hydrocarbons on
the surface of the catalyst during reaction (evidenced by
intense IR bands in the range 2960–2875 cmÀ1, Figure 1,
bottom), together with the observed deactivation of the
catalyst over time (Figure SI-6), suggests that product
desorption might be rate-determining (notwithstanding the
deactivation, more than 2000 turnovers of the catalyst were
achieved in 5 h on stream, demonstrating that the reaction is
catalytic). This observation, together with the fact that H2
accelerates the dimerization reaction (Table 2) without being
incorporated into the product butenes, suggests that the role
of hydrogen is to facilitate the regeneration of the active sites
by accelerating the desorption of the products.
Moreover, the mass spectra characterizing the product
observed with the zeolite-supported rhodium catalyst in the
presence of flowing C2H4 + D2 demonstrate the formation of
C2H5D and C2H4D2 in low concentrations, resulting from a
=
minor, parallel hydrogenation of the C C bond. Accordingly,
we infer that this hydrogenation, in contrast to the dimeriza-
tion process, proceeds through ethyl ligands on the rhodium,
as expected.[31]
On the basis of these results, we propose a mechanism
(Scheme 1) for the dimerization of ethene on the zeolite
incorporating the rhodium complex. In this scheme, an ethene
À
molecule weakly adsorbed on an Al OH site of the zeolite
(which is not reactive enough to couple with another ethene
from the gas phase as occurs with stronger Brønsted acids)
reacts with a second ethene molecule activated on a nearby
rhodium site (Figure SI-7). According to this model, the two
To understand the role of the H2 activation
in the dimerization process, as well as in the
=
competitive hydrogenation of the C C bond, we
compared the performance of the rhodium
complex catalyst supported on zeolite DAY
with that of a sample consisting of extremely
small rhodium clusters on the same support,
prepared by treatment of the former catalyst
with H2 at 303 K. EXAFS spectra taken at the
Rh K edge after 1.2 h in flowing H2 indicate a
À
Rh Rh coordination number of nearly
2
(Table SI-1), which corresponds to an average
cluster nuclearity of 3, and thus to the presence
of extremely small rhodium clusters together
with unconverted rhodium complexes.[30]
The catalytic performance of the zeolite
containing the rhodium clusters in flowing
mixtures of C2H4 + H2 at 303 K and 1 bar
indicates that the hydrogenation of the double
Scheme 1. Simplified reaction mechanism for the conversion of ethene on a zeolite-
supported rhodium complex catalyst in the presence of H2. The reaction intermediates
are proposed on the basis of IR and EXAFS data, results of H2/D2 exchange
experiments, and catalyst performance data.
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bond swamped the dimerization reaction kinetically—the
overall reaction rate was more than three times that observed
with the catalyst incorporating the site-isolated rhodium
complexes, and the selectivity to ethane was high (Table 2).
IR spectra characterizing the catalyst incorporating rhodium
clusters in mixtures of C2H4 + H2 demonstrate the appearance
Keywords: alkene dimerization · bifunctional catalysis ·
.
À
C C bond formation · rhodium complexes · zeolites
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À
À
À
(i.e., Al OH + C2H4 Q Al OH C2H4) was shifted to the left
as the rate of the H2 dissociation increased when rhodium
clusters were present.
Thus, the activation of H2 plays a key role in the catalytic
behavior of the zeolite-supported rhodium species. On the
catalyst containing the mononuclear rhodium complexes, H2
is inferred to boost the dimerization reaction by facilitating
the desorption of the products. Paradoxically, a too-fast
activation of H2, such as was observed when small rhodium
clusters were present, largely impedes the activation of the
ethene on the acidic sites of the zeolite (thereby inhibiting the
dimerization according to the mechanism proposed in
Scheme 1)—as demonstrated by the IR results—instead
favoring the competitive hydrogenation on the rhodium
sites. We emphasize, however, that the capability of the
catalyst to produce butenes is not controlled merely by the H2
À
dissociation rate, but is influenced by the presence of Rh/Al
OH pairs acting in concert, as the data show that rhodium
complexes on MgO (which are relatively low in activity for
hydrogenation (Table 2)) do not catalyze dimerization of the
alkene under our conditions.
In summary, to our knowledge the results provide the first
evidence of a cooperation between metal complexes and solid
À
Brønsted acids in the formation of C C bonds, demonstrating
the activity of supported rhodium complexes for alkene
dimerization in the absence of halides—with a high selectivity
to n-butenes in an excess of H2. The EXAFS and IR spectra
and the results of the H/D exchange experiments indicate that
the mode of action of the zeolite-supported rhodium com-
plexes acting in concert with the adjacent acidic sites differs
fundamentally from that pertaining to halide-containing
rhodium complexes and strong Brønsted acids alone.
Received: December 21, 2010
Published online: April 29, 2011
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