Surface structures and catalytic hydroformylation activities of Rh dimers attached on various inorganic oxide supports

Article abstract of DOI:10.1021/jp953124k

trans-[(RhCp*CH₃)2(μ-CH₂)₂] (Cp* = pentamethylcyclopentadienyl) was chemically attached to inorganic oxides such as SiO₂, Al₂O₃, TiO₂, and MgO to prepare Rh dimer catalysts. The catalytic activities of the obtained supported Rh dimers for ethene hydroformylation depended on the kinds of supports. The SiO₂-attached Rh dimers showed highest activity and selectivity for the hydroformylation. Surface structure and behavior of the Rh sites during CO insertion process were examined by in situ FT-IR and EXAFS techniques. Similarly to the previous observation that Rh-Rh metal bonding in the Rh dimers on SiO₂ alternately broken and formed in the course of the catalytic hydroformylation, Rh dimers on TiO₂ showed a reversible bond break formation, but CO-bridged Rh dimers inactive for the hydroformylation were produced unlike the case of Rh dimer/SiO₂. On Al₂O₃ most of Rh dimers were degraded to monomers which exhibited no activity for the hydroformylation. The Rh dimers on MgO were aggregated to metal clusters. These structural behaviors of Rh sites are discussed in terms of the interaction between the Rh dimer and the support.

Full text of DOI:10.1021/jp953124k

13636
J. Phys. Chem. 1996, 100, 13636-13645
Surface Structures and Catalytic Hydroformylation Activities of Rh Dimers Attached on
Various Inorganic Oxide Supports
†
,⊥
‡
§
|
Kyoko Kitamura Bando, Kiyotaka Asakura, Hironori Arakawa, Kiyoshi Isobe, and
Yasuhiro Iwasawa*
,
†
Department of Chemistry, Graduate School of Science, UniVersity of Tokyo,
Hongo, Bunkyo-ku, Tokyo 113, Japan; Research Center for Spectrochemistry, Faculty of Science,
UniVersity of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan; National Institute for Material and
Chemical Research, Higashi, Tsukuba, Ibaraki 305, Japan; and Department of Material Science,
Faculty of Science, Osaka City UniVersity, Sugimoto, Sumiyoshi, Osaka 558, Japan
X
ReceiVed: October 23, 1995; In Final Form: May 28, 1996
trans-[(RhCp*CH
oxides such as SiO
obtained supported Rh dimers for ethene hydroformylation depended on the kinds of supports. The SiO
3
)2(µ-CH
2
)
2
] (Cp* ) pentamethylcyclopentadienyl) was chemically attached to inorganic
2
, Al
2
O
3
, TiO , and MgO to prepare Rh dimer catalysts. The catalytic activities of the
2
2
-
attached Rh dimers showed highest activity and selectivity for the hydroformylation. Surface structure and
behavior of the Rh sites during CO insertion process were examined by in situ FT-IR and EXAFS techniques.
Similarly to the previous observation that Rh-Rh metal bonding in the Rh dimers on SiO
and formed in the course of the catalytic hydroformylation, Rh dimers on TiO showed a reversible bond
break formation, but CO-bridged Rh dimers inactive for the hydroformylation were produced unlike the case
of Rh dimer/SiO . On Al most of Rh dimers were degraded to monomers which exhibited no activity for
2
alternately broken
2
2
2 3
O
the hydroformylation. The Rh dimers on MgO were aggregated to metal clusters. These structural behaviors
of Rh sites are discussed in terms of the interaction between the Rh dimer and the support.
1
. Introduction
CO insertion during the course of the hydroformylation,
demonstrating an important role of the adjacent Rh atoms in
In supported metal catalyst systems, reaction mechanisms
the CO insertion (ethyl migration) reaction.
generally involve more than one metal atom at catalyst surfaces.
Ethene hydroformylation on supported Rh catalysts shows a
maximum activity with an optimum particle size, suggesting
contribution of metal ensemble to the catalytic performance,
while Rh catalysis in homogeneous systems where alkene
hydroformylation proceeds on mononuclear Rh complexes. It
may be of importance to know the role of metal-metal bonding
in metal catalysis. The role of the adjacent metal atom in
catalytic reactions has also been reported in homogenous
systems, though the homogeneous systems often deal with
irreversible reactions, because irreversible decomposition of the
complex occurs. To stabilize the metal centers to work
catalytically and to prepare new active metal structures, there
have been many attempts to attach di- and multinuclear metal
complexes on oxide supports.3
5
In this paper we have extended the above work to other oxide
supports to examine metal support interaction or support effects
on metal catalysis. TiO2, Al2O3, and MgO were used as
supports, which are representative among oxide supports. As
already reported, supported Ru catalysts derived from Ru3-
6
- 7
(
CO)12 and [Ru6C(CO)16(CH3)] showed different structures
on these supports, which caused the unique catalytic behaviors.
The purpose of this work is to obtain information on essential
factors which control surface structure and catalytic activity of
Rh dimers chemically attached on the oxide supports.
1
,2
2
. Experimental Section
trans-[(RhCp*CH3)2(µ-CH2)2] (1) was synthesized according
,4
1
to the literature. Two kinds of deuterium-labeled complexes,
trans-[(RhCp*CH3)2(µ-CH2)2] (1) (Cp* ) pentamethylcy-
clopentadienyl) has CH3 and µ-CH2 ligands which are regarded
as important intermediates for C-C bond chain growth. From
this point of view, the reactivity of complex 1 has been
intensively studied in homogeneous systems.1 We succeeded
in attaching complex 1 to SiO2 surface with retention of the
Rh-Rh bond, which were characterized by EXAFS and FT-
trans-[(Rh(C5(CD3)5)(CH3))2(µ-CH2)2] (2) and trans-[(Rh(C5-
(
CD3)5)(CD3))2(µ-CH2)2] (3), were also synthesized for IR study.
2
SiO2 (Aerosil 300, surface area ∼300 m /g, Davison No. 57,
2
surface area ∼280 m /g), TiO2 (Aerosil titanium oxide P-25,
2
surface area ∼50 m /g) , Al2O3 (Aerosil aluminum oxide C,
2
surface area ∼100 m /g), and MgO (Ube High Purity 100A,
2
surface area 100-170 m /g) were used as supports. The
5
IR. The obtained Rh dimer catalyst showed efficient catalytic
inorganic oxides were pretreated in situ before use for the
attachment of the Rh dimer complex; SiO2 was treated at 673
K for 1 h in vacuum to control the concentration of the surface
performance for the catalytic hydroformylation of ethene. It
was found that Rh-Rh bonds in the attached Rh dimers were
reversibly broken by CO adsorption and reformed by subsequent
-2 3
OH groups at about 2 OH nm , TiO2 was treated at 673K for
-
2 3
1
h (1∼2 OH nm ), Al2O3 was treated at 473K for 1 h (8∼10
*
To whom correspondence should be addressed.
Department of Chemistry, University of Tokyo.
Research Center for Spectrochemistry, University of Tokyo.
National Institute for Material and Chemical Research.
Osaka City University.
-2 3 -2 3
OH nm ), MgO was treated at 673K for 2 h (6∼8 OH nm ).
†
‡
§
|
⊥
The loading of Rh was regulated to be 1-2 wt %. Thus the
surface concentration of the Rh dimer was estimated to be 0.1-
-2
-2
0
0
.2 dimers nm for SiO2, 0.6-1.2 dimers nm for TiO2, 0.3-
Present address: Institute for Research and Innovation, Takada,
.6 dimers nm- for Al2O3, and 0.2-0.6 dimers nm for MgO,
2
-2
Kashiwa, Chiba 277, Japan.
X
Abstract published in AdVance ACS Abstracts, July 15, 1996.
on average.
S0022-3654(95)03124-8 CCC: $12.00 © 1996 American Chemical Society

Surface Structures of Rh Dimers
J. Phys. Chem., Vol. 100, No. 32, 1996 13637
TABLE 1: Curve-Fitting Analysis for the EXAFS Data of
Reference Rh Complexes by Use of Theoretical Parameters
discussed later. Sj in eq 1 is the amplitude reduction parameter
that arises from many-body effects and inelastic losses in the
scattering process. This value can be regarded as independent
5
distance/nm
1
0
of k because the two factors have an opposite k dependency.
sample
bond
CN
EXAFS
X-ray
Sj for each scatterer was determined by the model compound
when a theoretical amplitude function was used. Sj is unity
when an empirically derived amplitude function is used. Nj,
σj, and rj are coordination number, Debye-Waller factor, and
interatomic distance, respectively. The fitting parameters are
usually Nj, rj, ∆Ej (difference between the origin of the
photoelectron wave vector and that conventionally determined),
and σj. Nidp (independent number of fitting parameters) given
Rh metal
Rh-Rh
Ru-O
Rh-C
Rh-Cl
Rh---O
12
6
2
2
2
0.266 ( 0.002
0.203 ( 0.003
0.183 ( 0.002
0.233 ( 0.003
0.298 ( 0.002
0.268
0.205
0.181
0.2355
0.298
Rh
Rh
2
O
3
2
(CO)
4
Cl
2
Incipient attached Rh dimers were prepared by reaction of
trans-[(RhCp*CH3)2(µ-CH2)2] (1) with the surface OH groups
of oxide supports in pentane or deuterium-labeled benzene at
ambient temperature under high-purity inert gas (99.9999% He
or 99.9999% Ar). Handling of the sample in the inert gas was
necessary because the obtained attached Rh dimer species were
air sensitive unlike the original Rh dimer complex (1). Pentane
used as the solvent for the complex 1 was purified before use
by distillation over Na wire or CaH2. The solvent was removed
by evacuation or under a flow of high-purity He. Commercially
available deuterium-labeled benzene (Aldrich, 99.5 atom % D)
was also used as the solvent without further purification for IR
study.
FT-IR spectra were measured by JEOL JIR-100 spectrometer.
trans-[(Rh(C5(CD3)5)(CH3))2(µ-CH2)2] (2) and trans-[(Rh(C5-
CD3)5)(CD3))2(µ-CH2)2] (3) were employed to separate the IR
absorption peaks for methyl and methylene ligands from those
of the pentamethylcyclopentadienyl (Cp*) ligand in the ν(CH)
region. The oxide powder was pressed into a form of a disk
11
by 2∆k∆r/π + 2 is nearly 8 in this work. We used four
independent parameters for one-shell fitting. But we reduced
them to half by fixing ∆E and σ to the values of reference
compounds when we carried out two- and three-shell fitting.
Because structures derived from EXAFS are reduced to be
of one dimension from three-dimensional real structures, the
curve fitting often converges in completely different structures
from the real ones at the same level of fitting accuracy.
Therefore, in this work, we first gave the possible structures
from TPD and IR data which could provide quantitative
information about ligands and homogeneity of a sample. Based
on these inputs, we conducted the curve-fitting analysis to
determine bond distances and coordination numbers. The errors
were estimated according to the Report on Standards and
(
12
Criteria in XAFS Spectroscopy. We carried out two- or three-
shell fitting on a single Fourier transform peak because other
techniques indicate the presence of two or more bondings and
because we did not fit well with one shell. In these cases, due
to the correlation problem between fitting parameters, the
determined parameters had a little larger error bars. We
additionally compared the fitting results such as bond length
with those of the compounds with similar structures in the
literature and confirmed that the determined structures were
chemically reasonable. By doing the procedure of EXAFS
analysis explained above, we could deduce a surface structure
with less ambiguity.
(
60 mg/20 mm diameter), and was pretreated in an IR cell. Then
the dimer solution was added dropwise to the disk under the
flow of high-purity He. The IR cell used in this work was a
stainless steel IR cell connected to a flow system.
Rh K-edge EXAFS measurements were conducted under in
situ conditions in a transmission mode at BL 10B of Photon
Factory in the National Laboratory for High-Energy Physics
(
KEK-PF) with a Si(311) channel-cut monochromator. The
estimated energy resolution is 7 eV. The spectra were measured
at room temperature. To measure the in situ EXAFS spectra,
we used a Pyrex glass EXAFS cell with two thin glass windows
in which no Rh impurity was detected. The in situ EXAFS
cell was connected to a closed circulating system through a stop
The catalytic hydroformylation reaction was conducted in a
closed circulating system. The products were collected by a
dry ice-acetone trap (179 K) and detected by a gas chromato-
graph using VZ-10 and DOS columns. VZ-10 column was used
for the detection of ethane untrapped and DOS column was used
for the detection of oxo compounds collected in the trap. The
components of reactants were CO:C2H4:H2 ) 1:1:1 (total
pressure ) 40.0 kPa) and CO:C2H4:H2 ) 0.2:1:1 (total pressure
valve. The following formula was used for the curve-fitting
analysis of EXAFS data8,9
2
2
2
ø(k′ ) ) S N F (k′ ) exp(-2k′ σ ) sin(2k′ r + φ (k′ ))/k′ r
j j
j
∑
j
j
j
j
j
j
j j
j
j
(
1)
)
33.3 kPa). An impregnated Rh/SiO2 catalyst with 30%
2
dispersion conventionally prepared from RhCl was used as
3
k′ ) k - 2m∆E /p
j
x
j
reference. It showed the highest activity among series of
where k is the photoelectron wave number, F(k) is the back
scattering amplitude function, and φ(k) is the phase shift
function. For φ(k) and F(k), either theoretical or empirical form
was chosen depending on the kind of system. Model com-
pounds for Rh-Rh and Rh---O (carbonyl) were Rh metal and
impregnation catalysts for propanal formation in ethene hydro-
formylation. The impregnation Rh/SiO2 catalyst was calcined
at 637 K for 1 h followed by reduction with H2 at 773 K for 1
h. The reactivity was compared by the turnover frequencies
(product molecules/Rh atom/min). The number of active Rh
[
Rh(CO)2]2(µ-Cl)2, respectively. Theoretical parameters deter-
sites in attached Rh dimer catalysts was assumed to be equal to
the number of the Rh dimers deposited.
mined by Teo and Lee were used for the analysis of the first
9
,10
shell.
These theoretical parameters are not recommended
by the international XAFS committee now. So we carefully
checked the validity of these parameters using reference
compounds as shown in Table 1.5 The interatomic distances
could be determined within the error of 0.003 nm. Thus, the
theoretical parameters derived from Teo and Lee did not seem
to cause serious problems in our system. However, we will
only discuss the change of the structure but not the absolute
structure for safety. Moreover, we have paid much attention
to the independent number of fitting parameters which will be
3
. Results and Discussion
3.1. Catalytic Hydroformylation of Ethene. In general,
Rh catalysts are known to have high activity for CO insertion
(alkyl migration). Alkene hydroformylation reaction can be
sometimes regarded as a reaction to reflect the ability of CO
insertion for metal catalysts. We conducted hydroformylation
of ethene by the attached Rh dimer catalysts prepared by using
various supports. First the reaction was carried out in a closed

13638 J. Phys. Chem., Vol. 100, No. 32, 1996
Bando et al.
TABLE 2: TOF and Selectivity of Ethene Hydroformylation
a
at 413 K
TOF/10- min-1
4
catalyst
total
ethane
propanal
selectivity (%)
impreg Rh/SiO
2
22.8
36.9
6.3
6.8
56
21.5
4.1
6.3
6.8
56
1.3
32.8
5.6
88.9
Rh
Rh
Rh
Rh
2
2
2
2
/SiO
/TiO
/Al
/MgO
2
2
2
O
3
a
2 2 4
CO:H :C H ) 1:1:1; total pressure ) 40.0 kPa.
circulating system (CO:H2:C2H4 ) 1:1:1, total pressure ) 40.0
kPa). The results obtained for the attached Rh dimer catalysts
and a conventional SiO2-supported Rh catalyst are shown in
Table 2. The Rh dimer catalysts were all inactive except for
the SiO2-attached Rh dimer catalyst (Rh2/SiO2), which exhibited
much higher activity and selectivity for propanal formation than
the impregnated Rh/SiO2 catalyst.5 This result is opposite to
the supported Rh clusters catalysts which showed higher activity
when Rh4(CO)12 and Rh6(CO)16 were deposited on basic
supports like MgO, ZnO, and La2O3 compared with the ones
1
3
supported on acidic supports like SiO2 and Al2O3. Thus, the
possibility of another mechanism and support effect which works
in the Rh dimer catalyst system is suggested. The catalytic
ethene hydroformylation reaction will be discussed again in
section 3.6 after the surface Rh structures are described.
3.2. Transformation of the Surface Structure. To eluci-
date the mechanism which controls performance of the attached
Rh dimer catalysts, we investigated the transformation of the
surface structure of the attached Rh dimers under various
conditions.
3
.2.1. SiO2-Attached Rh Dimers (Rh2/SiO2). In our pre-
Figure 1. IR spectra of the SiO
2
-attached Rh dimers (2): (a) after
vious work, we investigated the transformation of surface
structure of the SiO2-attached Rh dimers by means of in situ
EXAFS, in situ FT-IR, and TPD.5 Figure 1 partly reproduced
the IR spectra for the transformation of the SiO2-attached Rh
dimer complex.5 In order to clarify the discussion, we briefly
summarize the results obtained in our previous work. The
assignment of ν(CH) peaks was based on the IR spectra for the
deuterated Rh dimer complexes. A peak at 2935 cm- is
assigned to the antisymmetric ν(CH) of µ-CH2. Both symmetric
ν(CH) of µ-CH2 and asymmetric ν(CH) of Rh-CH3 appear at
attachment at 313 K, (b) treatment at 373 K, (c) CO adsorption at 313
K, (d) treatment at 473 K, and (e) hydrogenation at 423 K.
Rh catalysts. We designated this Rh-Rh interaction by a bro-
ken line, to represent noncovalent bonding interaction. By sub-
sequent treatment of species 5 at 473 K, the peaks of the dicar-
bonyls almost disappeared, new peaks at 1710 and 1394 cm-
(Figure 1d) appeared, and the ν(CH) peaks changed, which
implies that CO insertion into an ethyl ligand occurred (species
6). The peak corresponding to ν(CH) of µ-CH2 group disap-
peared possibly by reaction with surface OH groups. Interest-
ingly, Rh-Rh bond was regenerated at this stage. When species
6 was exposed to CO again at 313 K, dicarbonyl species was
formed, while Rh-Rh bond was cleaved (species 7). Trans-
formation between 6 and 7 was found to reversibly occur by
both IR and EXAFS. CO insertion from 7 to 6 proceeded under
CO-deficient conditions. Ordinarily in homogeneous systems,
CO insertion proceeds more favorably under CO-rich (high-
pressure CO) conditions or in the coexistence of cocatalysts
like PPh3 because excess CO or PPh3 occupies the vacant site
1
1
-1
-1
2
879 cm . A peak at 2790 cm is assigned to symmetric
ν(CH) of Rh-CH3. When the Rh dimer complex (2) was
attached to the SiO2 surface and treated at 373 K, the peak of
1
-1
2
879 cm- and the peak of Cp* around 2200 cm decreased
-1
to half of the original ones and the peak at 2790 cm almost
disappeared as shown in Figure 1b. The surface OH peak at
1
3
740 cm- reduced in intensity upon supporting complex 2 on
SiO2 at 313 K and treating the species at 373 K. Thus, it was
suggested that a methyl ligand and a Cp* ligand reacted with
the surface OH groups, making Rh-O bonds which were
5
17
characterized separately by EXAFS as shown in Scheme 1.
on the Rh atom to stabilize the produced acyl species. On
At this stage a Rh-Rh bond was observed by EXAFS analysis.
Rh2/SiO2 the regenerated Rh-Rh bond played the similar role
as an additional ligand. We proposed a novel CO insertion
mechanism which was promoted by the formation of metal-
By exposure to CO, dicarbonyls were formed, which appeared
-
1
at 2032 and 1969 cm in Figure 1c. From both volumetric
12
12
12
13
13
13
5
analysis and IR spectra ( CO- CO, CO- CO, CO- -
CO), it was concluded that the dicarbonyl was attached on the
Rh atom without Cp* ligand as shown in Scheme 1, species 5.
At the same time, the cleavage of Rh-Rh bond was observed
by EXAFS. Although the Rh-Rh bond cannot be seen due to
the effect of static or thermal disorder, two Rh atoms may still
metal bonding. This mechanism was also rationalized by ki-
15
netic data of CO hydroformylation. Anderson and co-workers
carried out theoretical calculation on this Rh dimer system and
found that a stable Rh-Rh bond is present in the Rh acyl species
1
6
(6). They also calculated decarbonylation mechanism. The
Rh-Rh bond in species 6 was stretched to 0.566 nm by the
adsorption of CO to make CO bridging species (C2H5)(CO)-
RhCO---RhCp*. But IR spectra did not reveal any low-
frequency peak assignable to bridging CO. Furthermore, if
species 7 really had a side-on CO ligand, four independent peaks
1
4
be located nearby similar to the case of Rh2(CO)4Cl2.
However, there should be a kind of Rh-Rh interaction between
two Rh atoms because the frequencies of two CO adsorbates
were lower than those observed in conventional SiO2-supported

Surface Structures of Rh Dimers
J. Phys. Chem., Vol. 100, No. 32, 1996 13639
SCHEME 1: Transformation of the Surface Rh Dimers Attached on SiO25
TABLE 3: Temperature-Programmed Desorption Data for
the TiO -Attached Rh Dimers
d-Cp* ligands took place in conjunction with decrease of the
peak for the surface OH groups of TiO2. Accordingly, it is
most likely that the methyl and Cp* ligands reacted with the
surface OH groups to form Rh-O bonds. According to the
TPD measurement in Table 3, one methane molecule per Rh
dimer was desorbed at 293-343 K, which is in agreement with
the IR results. The methylene-methyl recombination to form
an ethyl ligand on the Rh dimer was also suggested by
appearance of the peak at 2917 cm- similarly to the case of
SiO2 support.
2
quantity of evolved gas per Rh dimer
totala
temp/K
CH
4
C
2
H
6
2 4
C H
2
93-303
03-343
43-383
83-423
23-463
0.35
0.44
0.10
0.06
0.02
0.00
0.00
0.01
0.06
0.00
0.00
0.00
0.11
0.07
0.01
0.35
0.44
0.34
0.29
0.04
3
3
3
4
1
a
Total ) CH
4
2 6 2 4
+ 2(C H ) + 2(C H ).
After the chemical attachment was completed at 373 K, CO
was admitted to the catalyst at 313 K. New peaks attributed to
Rh dicarbonyls appeared at 2021 and 1953 cm- (Figure 2c′).
According to the volumetric measurement, 1.9 molecules of CO
per Rh dimer adsorbed at 313 K. They disappeared together
by the following treatment at 473 K and reappeared together
by CO admission under ambient pressure.
should be observed by 13CO substitution, but we observed three
couples of six peaks as reported previously, which indicated
1
5
the existence of dicarbonyl species, where the two equivalent
CO ligands were strongly coupled. If RhsCO---Rh was
arranged in a collinear form, we could observe the Rh-Rh
distance by EXAFS due to the strong multiple scattering effect
but it was not the case. The discrepancy between the experi-
mental and calculated structures might occur from the fact that
they used the mobile OH groups to optimize the structure though
the position and the orientation of OH groups on a rigid SiO2
surface cannot freely be changed.
The ν(CH) for the remaining methylene completely disap-
peared at 473 K in Figure 2d. Comparing the peak at 1814
-1
cm (Figure 2d′) with bridged CO peaks found in the literature
-1
listed in Table 4, we can assign the peak at 1814 cm as a
bridged carbonyl. The volumetric analysis revealed that about
0
.6 CO per Rh dimer desorbed by treatment at 473K, which
3.3. TiO2-Attached Rh Dimers (Rh2/TiO2). 3.3.1. IR and
means that the formation of bridged CO species at 473 K was
accompanied with desorption of about one molecule of CO for
the TiO2-attached Rh dimer species. There was another weak
TPD Analyses. Table 3 shows the integrated temperature-
programmed desorption (TPD) products, and Figure 2 shows
FT-IR spectra for the TiO2-attached deuterated Rh dimer (2)
catalyst under various conditions. Figure 2, a and a′, shows
the spectra observed right after attachment at 313 K, and they
were almost the same as the spectra observed for the authentic
Rh dimer complex 2 itself. The catalyst was subsequently
treated at 373 K under He to complete the attachment reaction.
-1
peak at 1737 cm , and it was assigned to acyl, judging from
its position. But the intensity was about 13% of that of the
acyl observed on Rh2/SiO2. Nearly 90% of the Rh2/TiO2 were
converted to the species with bridged CO unlike the case of
Rh2/SiO2. Other peaks marked with + in Figure 2d′ are
At this stage, the intensity of the peak at 2879 cm- for both
1
regarded as the adsorbed species on TiO support judging from
2
their intensity change during CO adsorption at 313 K, the
the methyl ligand and the methylene ligand and that at 2065
-1
treatment at 473 K, and the reduction by H at 573 K.
cm for ν(CD) of d-Cp* decreased to half of the original peak
intensity (Figure 2, b and b′). The shape of the peak at 2935
2
It was observed that the transformation between the Rh
dicarbonyl species and the CO-bridged Rh species occurred
reversibly as proved by FT-IR.
-1
cm attributed to antisymmetric ν(CH) became broad, and the
-1
top of the peak red-shifted about 14 cm . These results suggest
that the original Rh dimer structure changed at 373 K. The
decrease in intensity of the absorption peaks for the methyl and
3.3.2. EXAFS Analysis. Figure 3 shows the Fourier trans-
3
form of the k -weighted EXAFS data for the Rh dimers

13640 J. Phys. Chem., Vol. 100, No. 32, 1996
Bando et al.
2
Figure 2. IR spectra of the TiO -attached Rh dimers (2): (a, a′) after
attachment at 313 K; (b, b′) treatment at 373 K; (c, c′) CO adsorption
at 313 K; and (d, d′) treatment at 473 K.
TABLE 4: C-O Stretching Frequencies for Adsorbed CO
Species
ν(C-O)/cm-
1
ref
sample
Rh
Rh
2
/SiO
/TiO
2
2032, 1969 (dicarbonyl)
2021, 1953 (dicarbonyl)
5
2
2
1814 (bridged CO)
this work
this work
Rh
Rh
2
/Al
2
O
3
2017, 1953 (dicarbonyl)
2013 (linear CO)
2
/MgO
1
1
938 (linear CO)
884 (bridged CO)
3
Figure 3. Fourier transform of k -weighted EXAFS oscillations for
the TiO -attached Rh dimers: (a) after attachment at 373 K, (b) CO
this work
2
[
Rh(Cp*)(CO)]
2
(µ-CH
2
)
1933 (linear CO)
901 (linear CO)
adsorption at 313 K, (c) treatment at 473 K, (d) CO adsorption at 313
K, and (e) treatment at 473 K.
1
18
19
20
21
[Rh(Cp)(CO)]
2
(µ-CH
2
)
1984 (linear CO)
2000, 1950 (dicarbonyl)
1966 (linear CO)
Cp*Rh(CO)
2
[
RhCO(PPh
3
)
2
Cl]
(µ-CO)
in Rh dimer species 8. The intensity of the first shell in Figure
a increased as compared with that for the unsupported Rh dimer
[Rh(Cp*)(CO)]
2
1947 (linear CO)
3
1
794 (bridged CO)
2101, 2031 (dicarbonyl)
855-1870 (bridged CO)
22
23
5
complex. This is well explained by the Rh-O (surface)
formation by the reaction between the surface OH group and
the Rh dimer complex as in the case of Rh2/SiO2. From the IR
study, an ethyl ligand, a methylene ligand, and a Cp* ligand
were suggested to be present in the surface species (8), but four-
shell fitting did not give any reasonable result because of
correlation problem between the parameters for EXAFS analysis.
So two-shell fitting (Rh-O, Rh-C) was carried out instead and
gave the bondings at 0.220 ( 0.003 nm and 0.207 ( 0.006 nm
for Rh-O (surface) and Rh-C (µ-CH2), respectively, as in the
case of Rh2/SiO2.
2 3
Rh/Al O
1
on TiO2 under various conditions. Table 5 shows the results
3
of curve fitting for the k -weighted Fourier filtered EXAFS
oscillations. Figure 3a was observed with treatment of the
incipient attached Rh dimers at 373 K in vacuum. It shows no
significant peak at 0.22-0.29 nm. Therefore, at this stage the
adjacent two Rh atoms were not definitely bonded each other
at fixed bond distance though the two Rh atoms had a bridged
methylene ligand. This may be due to a large thermal disorder

Surface Structures of Rh Dimers
J. Phys. Chem., Vol. 100, No. 32, 1996 13641
new peak at 2919 cm- appeared similarly to the cases of Rh2/
SiO2 and Rh2/TiO2. It is likely that the Rh dimer complex
reacted with the surface OH groups on the methyl ligand and
the Cp* on Al2O3 surface.
1
TABLE 5: Curve-Fitting Results of the EXAFS Data for the
2
TiO -Attached Rh Dimers
sample
assigned bond
Ru-O
Rh-C(µ-CH
CN
r/nm
σ/nm
8
1.8 ( 0.7 0.220 ( 0.003 0.005 ( 0.003
1.5 ( 1.2 0.207 ( 0.006 0.005 ( 0.003
2
)
By introduction of CO at 313 K, intense carbonyl peaks
appeared at 2017 and 1953 cm (Figure 5c). These two peaks
9
Ru-O(dicarbonyl) 0.8 ( 0.3 0.300 ( 0.002 0.005 ( 0.002
-1
Ru-O(surface)
Rh-C(µ-CH
1.7 ( 0.3 0.220 ( 0.002 0.005 ( 0.003
1.5 ( 1.2 0.206 ( 0.007 0.005 ( 0.003
behaved together under various conditions like those of dicar-
bonyls on Rh2/SiO2 and Rh2/TiO2. The low frequencies of the
carbonyl peak suggested that there should be a kind of
interaction between two Rh atoms. Besides the dicarbonyl peak,
a peak at 2084 cm- was also observed, suggesting a hetero-
geneous property of the Rh sites. The antisymmetric stretching
peak of dicarbonyls on Rh(I) monomers supported on Al2O3
has been reported to appear at similar wavenumbers as shown
in Table 4. The higher frequency peak of 2084 cm may be
due to isolated Rh sites which were formed by decomposition
of the Rh dimer complex at the surface.
By treatment at 453 K, the peak at 2017 and 1953 cm-
2
)
Rh-C(dicarbonyl) 1.2 ( 0.8 0.188 ( 0.005 0.005 ( 0.003
1
1
0
1
Rh-Rh
Ru-O(surface)
Rh-C(µ-CO)
1.0 ( 0.6 0.267 ( 0.002 0.007 ( 0.003
1.4 ( 0.4 0.218 ( 0.002 0.005 ( 0.003
1.2 ( 0.8 0.205 ( 0.006 0.005 ( 0.003
1
Ru-O(dicarbonyl) 1.2 ( 0.4 0.300 ( 0.003 0.007 ( 0.002
Ru-O(surface)
1.4 ( 0.3 0.219 ( 0.002 0.005 ( 0.003
Rh-C(dicarbonyl) 0.8 ( 0.6 0.185 ( 0.005 0.005 ( 0.003
0a
1
Rh-Rh
Ru-O(surface)
Rh-C(µ-CO)
1.0 ( 0.6 0.267 ( 0.003 0.007 ( 0.003
1.4 ( 0.4 0.218 ( 0.002 0.005 ( 0.003
1.2 ( 0.8 0.205 ( 0.006 0.005 ( 0.003
2
3
-1
a
After heating species 11 at 473 K again.
1
decreased in intensity (Figure 5d). A weak peak appeared at
Upon CO adsorption on the Rh species treated at 373 K, a
second peak in Figure 3b appeared at 0.22-0.29 nm. Curve-
fitting analysis for the second peak was carried out for two
models for bonding as Rh-Rh or Rh---O (carbonyl). The best
fit was obtained by Rh---O rather than Rh-Rh, and the distance
and the coordination number were determined to be 0.300 (
-1
1
714 cm , which was assigned to the acyl species, compared
with the absorption of the acyl observed on Rh2/SiO2. Acyl
species formed on Rh2/Al2O3 was less than 23% of total Rh
sites, estimated from its intensity. Unlike Rh2/SiO2, it seems
that the majority of the adsorbed CO desorbed without acyl
formation on Rh2/Al2O3. The adsorption and desorption of
adsorbed CO occurred reversibly.
0.002 nm and 0.8 ( 0.3, respectively, in Table 5. There was
no Rh-Rh bond when CO was adsorbed on the Rh dimers.
Combining the EXAFS data with the IR data and the volumetric
analyses, it is concluded that two CO molecules per Rh dimer
were adsorbed. For the first shell of Figure 3b, Rh-C(alkyl)
and Rh-C(Cp*) were neglected and three-shell fitting was
performed. The characterized bonds at 0.220 ( 0.002, 0.188
3.4.2. EXAFS Analysis. Figure 6a-c shows the k-weighted
EXAFS oscillations for the Al2O3-attached Rh dimers, after
treatment at 373 K, exposure to CO at 313 K, and treatment at
473K under vacuum, respectively. The EXAFS oscillation in
the high-energy region was weak for all the samples on Al2O3.
This may be ascribed to the fact that the Rh-Rh was not
retained to Rh2/SiO2 and Rh2/TiO2. The curve fitting analysis
was carried out and the results were given in Table 6. No Rh-
Rh distance was found in 0.2-0.3 nm. The proposed trans-
formation of the Rh dimers on Al2O3 is depicted in Scheme 3.
The species for Rh2/Al2O3 are proposed mostly to be monomer
pairs without Rh-Rh bonding where there should be interaction
between with two Rh atoms because the dicarbonyl species
showed low-frequency peaks in IR than those of the dicarbonyls
(
0.005, and 0.206 ( 0.007 nm are attributable to Rh-O
(
surface), Rh-C (carbonyl), and Rh-C (µ-CH2), respectively,
from the comparison with species 8 as shown in Table 5. Thus,
the first shell of species 9 with the dicarbonyls has a similar
local structure to species 5 in Rh2/SiO2. Based on the IR and
EXAFS analyses, structure 9 was proposed as shown in Scheme
2
(9).
Species 9 was subsequently treated at 473 K. The second
peak still appeared in the Fourier transform as shown in Figure
c. The second peak was curve-fitted on the assumption of
2
3
3
on Rh(I) monomers often reported in the literature.
Rh-Rh bonding. The curve-fitting result is shown in Figure
3.5. MgO-Attached Rh Dimers (Rh /MgO). MgO did not
easily react with the Rh dimer complex, and the characteristic
2
4
0
a, and the obtained distance and coordination number were
.267 ( 0.002 nm and 1.0 ( 0.6, respectively, as shown in
IR absorption bands of the Rh dimer complex did not change
until 473 K as shown in Figure 7d. The species obtained at
473 K was exposed to CO at 313 K, and three kinds of ν(CO)
at 2013, 1938, and 1884 cm- were observed by IR (Figure
7e). The behaviors of these peaks were independent of one
another because the 1884 cm- peak disappeared by treatment
Table 5. This result implies that Rh-Rh bond was reproduced
again at the surface like Rh2/SiO2.5 Two-shell analysis for
species 10 treated at 473 K gave good fitting results for the
first shell as 0.218 ( 0.002 and 0.205 ( 0.006 nm for Rh-O
1
1
(surface) and Rh-C (bridged CO), respectively (Table 5).
-1
After CO adsorption on species 10 at 313 K again, curve-
at 373 K and then the 2013 cm peak disappeared at 473 K,
while the 1938 cm- peak remained. This behavior of the
adsorbed carbonyls is entirely different from that of the
dicarbonyls adsorbed on the Rh dimers discussed above. But
these peaks appear at low frequencies like the dicarbonyls on
Rh2/SiO2, Rh2/TiO2, and Rh2/Al2O3. Acyl formation was not
observed under any conditions. Two kinds of peaks around
2600-2800 cm- appeared at 473 K. They were assigned to
Mg-OD groups generated by HD exchange between d-Cp* and
Mg-OH groups.
1
fitting analysis of the second peak of the Fourier transformed
spectrum was performed. We obtained the best fitting result
when Rh---O (carbonyl) is assumed as shown in Figure 4b,
indicating breakage of the Rh-Rh bond by dicarbonyl formation
(Rh---O (carbonyl) at 0.300 nm). By subsequent treatment at
4
73 K, Rh-Rh bond was reproduced accompanied by µ-CO
1
formation again as shown in Scheme 2 (10).
In summary, reversible formation-breaking of Rh-Rh bond
accompanied by reversible µ-CO formation-dicarbonyl forma-
tion was observed on Rh2/TiO2 by EXAFS.
Figure 8a shows k-weighted EXAFS oscillation observed after
3
.4. Al2O3-Attached Rh Dimers (Rh2/Al2O3). 3.4.1. IR
treatment at 473 K. Amplitude of the oscillation did not
decrease till the high-energy region. Fourier transform for k -
weighted EXAFS oscillation is shown in Figure 8b. The peak
at 0.25 nm is attributed to Rh-Rh bonding which is stronger
as compared with other attached Rh dimers. Curve-fitting
3
Study. Figure 5 shows in situ FT-IR spectra observed for the
Al2O3-attached Rh dimers (3) . After treatment at 373 K, the
intensity of ν(CH) peaks at 2937 and 2869 cm- for the
methylene ligand and ν(CD) peaks for Cp* decreased and a
1

13642 J. Phys. Chem., Vol. 100, No. 32, 1996
Bando et al.
SCHEME 2: Transformation of the Surface Rh Dimers Attached on TiO2
Figure 4. Curve-fitting results of the second peak of Figure 5. The
solid curves represent observed oscillations and the dashed curves
represents calculated ones: (a) treatment at 473 K; (b) CO adsorption
at 313 K.
Figure 5. IR spectra of the Al
attachment at 313 K, (b) treatment at 373 K, (c) CO adsorption at 313
K, and (d) treatment at 453 K.
2
O
3
-attached Rh dimers (3); (a) after
analysis of the Fourier-filtered spectra (0.20-0.27 nm) revealed
that the bond length and coordination number for Rh-Rh did
not change significantly under the examined conditions as shown
in Table 7. The coordination number was around 1.3 ( 0.5,
and the bond length was around 0.264 ( 0.002 nm. From these
results we speculate an ensemble of Rh sites (Rh4 like species),
but its structure cannot be drawn.
showed activity and selectivity for hydroformylation at reduced
pressures, and other catalysts were all inactive. It corresponds
well to the CO insertion reaction promoted by Rh-Rh bond
observed by IR and EXAFS. On Rh /SiO , CO insertion
proceeded in conjunction with formation of the Rh-Rh bond
as shown in Scheme 1.
2
2
On Rh /TiO the Rh-Rh bond was also regenerated by
2
2
It is concluded that the dimer structure was decomposed and
clustered to species with Cp* ligands on MgO. CO insertion
did not take place on this catalyst.
heating the dicarbonyl species, but it was accompanied by
formation of the CO-bridged species, where one of the dicar-
bonyl ligands was merely desorbed. Thus CO insertion
promoted by Rh-Rh bond formation did not take place on Rh2/
TiO2 as shown in Scheme 2.
3.6. Catalytic Performance Related to Structure of Rh
Species. Ethene hydroformylation activity was compared
among Rh2/SiO2, Rh2/TiO2, Rh2/Al2O3, and Rh/MgO. Com-
pared with a conventional impregnated catalyst, only Rh2/SiO2
We could not observe Rh dimer structure by EXAFS as main
species on Rh2/Al2O3. Rh atoms were attached as monomers

Surface Structures of Rh Dimers
J. Phys. Chem., Vol. 100, No. 32, 1996 13643
SCHEME 3: Transformation of the Surface Rh Dimers
Attached on Al2O3
Figure 6. k-weighted EXAFS oscillations for the Al
dimers: (a) after attachment at 373 K, (b) CO adsorption at 313 K,
and (c) treatment at 473 K.
2
O
3
-attached Rh
3.7. Metal-Support Interactions. The different structures
of Rh dimers on several kinds of support materials can be
explained by the different reaction patterns between the Rh
dimers and support surfaces. The acid-base property of the
supports may be a key factor for the different reactivity of the
support. Acidity/basicity depends on the electronegativity of
TABLE 6: Curve-Fitting Results of the EXAFS Data for
Al
2 3
O -Attached Rh Dimers
sample
assigned bond
CN
r/nm
σ/nm
4+
4+
support metal ions which is in the following order: Si > Ti
1
1
2
3
Ru-O (surface)
1.7 ( 1.0 0.215 ( 0.004 0.005 ( 0.004
3+
2+
>
Al > Mg . The basicity becomes stronger as the
Ru-O (dicarbonyl) 1.0 ( 0.3 0.301 ( 0.004 0.008 ( 0.002
electronegativity of metal ions becomes weaker. For the oxides
with basic property, as the electron density of surface oxygen
becomes higher, the coordination ability of the oxygen atom
increases, and therefore the metal-support interaction becomes
stronger. The interaction between metal and support can be
estimated by ν(CO) for adsorbed carbonyl species in similar
structures. When the basicity of support is stronger, more
electrons transfer to Rh through Rh-O (surface) bonding, and
the electron back-donation from Rh to CO increases and
ν(CO) shifts to lower frequency. As shown in Table 4, the
order of frequency of the Rh dicarbonyls is SiO > TiO >
Ru-O (surface)
Ru-O (surface)
1.9 ( 1.0 0.215 ( 0.004 0.006 ( 0.004
1.7 ( 1.0 0.214 ( 0.003 0.004 ( 0.003
1
1
4
5
Ru-O (dicarbonyl) 1.1 ( 0.3 0.300 ( 0.001 0.006 ( 0.002
Ru-O (surface)
1.9 ( 1.0 0.215 ( 0.004 0.004 ( 0.004
shown in Scheme 3. The Rh monomer species showed little
CO insertion ability, which agrees with the work of Ichikawa
on Rhn/ZnO (n ) 1, 2, 4, 6, 7, 13, infinity) where n ) 1 is the
least active.3
,13
For Rh2 on both Al2O3 and TiO2, a small
production of acyl was observed after desorption of CO from
Rh species in IR spectrum. These acyl groups were produced
on minor components of Rh species. The reason for little
production of hydroformylated species in the gas phase on these
supports may be due to the stronger CO adsorption on Rh
species on Al2O3 and TiO2 through larger back-donation as was
proved by the IR spectra discussed later. Acyl was formed
accompanied by the desorption of CO. Thus the little CO
insertion occurred during the reaction conditions probably
because CO desorption was unlikely to occur.
MgO-supported Rh species was aggregated to form a small
Rh cluster with direct Rh-O bonding. The Rh-Rh bonding
in the cluster was stable and not cleaved by exposure to CO.
The metal-assisted CO insertion was not observed with the
Rh2/MgO catalyst. In conventional supported Rh catalyst
systems the basic supports promote the CO insertion, because
electronic contribution from basic supports makes the acyl
species stable.13 This support effect was not observed on
Rh2/MgO.
2
2
Al O , which is the same order as the electronegativity of
2
3
support metal ions. In other words, the order of interaction
between Rh dimer and support can be represented by the ν-
(CO) order. The strength of Rh-O bonding may be estimated
by the bond length as in the case of Ru on Al O and K-Al O ,
2
3
2
3
and MgO where distances of Ru-O and basicity of supports
6
have good correlation. In the present case the error bars for
Rh-O bonding are too large to distinguish small differences in
bond distance due to the multishell fitting.
If the attachment reaction in principle occurs in the following
manner the reaction easily proceeds on the support with
CH2
CH2
Cp*
CH3
Cp*
CH3
Cp*
H
Rh
Rh
+ 2O
Rh
Rh
+ CH4 + HCp*
CH3
O
M
O
M
M
CH2
Brønsted acidity. Thus the attachment reaction occurs on SiO2,

13644 J. Phys. Chem., Vol. 100, No. 32, 1996
Bando et al.
Figure 8. (a) k-weighted EXAFS oscillation and (b) Fourier transform
3
of the k -weighted EXAFS oscillation for the MgO-attached Rh species
treated at 473 K.
TABLE 7: Curve-Fitting Results of the EXAFS Data for the
a
MgO-Attached Rh Dimers
sample assigned bond
CN
r/nm
σ/nm
16
17
18
Rh-Rh
Rh-Rh
Rh-Rh
1.3 ( 0.5 0.264 ( 0.002 0.006 ( 0.002
1.2 ( 0.8 0.262 ( 0.003 0.006 ( 0.002
1.2 ( 0.5 0.265 ( 0.002 0.006 ( 0.002
a
1
6: after attachment at 473 K. 17: exposure to CO at 313 K;
1
8: subsequent heating at 473 K.
reactivity cannot be explained simply by difference of basicity.
The reactivity is thought to be influenced by many factors like
Ru-O bond strength, the spacing between oxygen atoms that
fix Rh atoms, or electronic state of Rh atoms. However, we
think that a main factor may arise from the strength and
flexibility of Ru-O bonding. We are now conducting further
experiments to clarify this point.
4
. Conclusion
Ethene hydroformylation by attached Rh catalysts was
investigated in relation to the surface structure of Rh sites. Rh2/
SiO2 had high activity and selectivity, but other catalysts did
not show any activity for this reaction. The reason why the
activity varies so much according to the kind of supports was
that the surface structure properties with the kinds of supports
which cause significant difference in reactivity. The Rh dimer
complex was attached on SiO2 keeping its dimer structure. The
Rh-Rh bond has such a flexible character which makes CO
insertion prompted by Rh-Rh bond formation possible. On
TiO2, the Rh dimer was attached, keeping its original structure
to some extent. Rh-Rh bond was broken at once, but
regenerated by treatment at 473 K. Unlike Rh2/SiO2, the
formation of Rh-Rh bond did not promote CO insertion; instead
µ-CO was merely formed. On Al2O3, metal-support interaction
became stronger and the Rh dimer was attached as monomers,
and formation of Rh-Rh bond was prohibited. Again, metal-
metal assisted CO insertion did not occur on Al2O3 support.
On MgO, the Rh dimer was converted to clusters during heating
the surface Rh dimers. No hydroformylation reaction occurred.
On Rh2/TiO2, Rh2/Al2O3, and Rh2/MgO promotion of CO
insertion by the adjacent Rh atom was not observed possibly
because the interaction between Rh dimers and supports were
too strong.
Figure 7. IR spectra of the MgO-attached Rh dimers (2): (a) treatment
at 313 K, (b) treatment at 373 K, (c) treatment at 423 K, (d) treatment
at 473 K, (e) CO adsorption at 313 K, (f) treatment at 473 K, and (g)
CO adsorption at 313 K.
Al2O3, and TiO2 under mild conditions. On MgO, the Rh dimer
reacted with the surface oxygen atoms or OH- groups under
more severe conditions and the dimer structure was destroyed
to form clusters with higher nuclearity.
The property of the attached Rh species depend on the nature
of Ru-O bonding. Since Si-O bonding was the strongest
among three, Ru-O bonding on Rh2/SiO2 became weak and
restriction of the motion of Rh at the surface weakened.
Therefore, “Rh-Rh bond-assisted CO insertion” occurred. On
the other hand, on Al2O3 Ru-O bonding became stronger due
to the stronger basicity, and the Rh was tightly bound to the
Al2O3 surface and behaved like a Rh monomer species. On
TiO2 surface Rh showed intermediate behavior. Rh-Rh bond
was cleaved with the CO adsorption and was regenerated by
CO desorption. However, the Rh-Rh bond-assisted CO
insertion did not take place. The acyl species was formed on
Rh2/SiO2, while bridge CO was simply formed on Rh2/TiO2.
The difference between the two catalysts in CO insertion

Surface Structures of Rh Dimers
J. Phys. Chem., Vol. 100, No. 32, 1996 13645
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