J. Phys. Chem. C 2007, 111, 10073-10081

10073

Selective Butanol Synthesis over Rhodium-Molybdenum Catalysts Supported in Ordered
Mesoporous Silica
Yasuo Izumi,* Kazushi Konishi, Masayasu Tsukahara, Diaa Mosbah Obaid, and Ken-ichi Aika
Department of EnVironmental Chemistry and Engineering, Interdisciplinary Graduate School of Science andEngineering, Tokyo Institute of Technology, Nagatsuta 4259-G1-16, Midori-ku, Yokohama 226-8502, Japan
ReceiVed: April 3, 2007; In Final Form: May 11, 2007

Rhodium catalysts were synthesized in ordered mesoporous silica FSM-16 using Rh/Al heteropolyacid anions
and/or [Rh(COD)2]+ complex. The hydroformylation activity of propene was compared to impregnated Rh/
FSM-16 catalysts prepared from rhodium chloride. The catalysts were very selective to produce butanols dueto the effect of two-dimensional mesoporous reaction space (effective internal pore diameter 27 Å). The
selectivity to butanols (n-butanol and i-butanol) was as much as >98% over [RhMo6O18(OH)6]3-/FSM-16
catalysts and 73% over RhCl3/FSM-16 catalysts. Under the hydroformylation reaction conditions at 433 K
and 60 kPa, 22-28 Å of supported nanoparticles were present based on Rh K-edge extended X-ray absorption
fine structure (EXAFS) and high-resolution transmission electron microscopy (HR TEM) measurements. Rh
metallic nanoparticles atomically mixed with Mo atoms and distorted heteropolyacid species coexisted in the[RhMo6O18(OH)6]3-/FSM-16 catalysts based on Rh and Mo K-edge EXAFS and HR TEM measurements.
The population of metallic nanoparticles increased as the metal loading amount decreased from 5.2 to
0.22 wt % Rh. Thus, metallic nanoparticles in FSM-16 were essential for the butanol synthesis and Mo
played an additional promoter role to increase the selectivity further. A reaction mechanism was proposed inwhich the metallic nanoparticle surface was predominantly adsorbed with CO and multiple-step hydrogenationsof oxygenate intermediates proceeded to form butanols during slow diffusion in the two-dimensional
mesoporous space.

Introduction
The homogeneous-phase hydroformylation of propene is oneof the major industrial processes to produce butanal using
rhodium catalysts ligated with P(C6H5)3 group(s).1 Major
practical problems of homogeneous catalytic processes are
separation and stability. Recently, technical solutions for the
separation problem were reported to utilize a polymer ligand
soluble in the reactants but insoluble in the product. Supercritical
CO2 and ionic liquids dissolve the reactants and Rh catalysts,
but the produced aldehyde was separated when the density of
CO2 decreased from supercritical conditions.1
Heterogeneous hydroformylation reactions of hexene andoctene were reported to proceed over the HRhI(CO)(PPh3)3
complex supported in/on a micro-/mesoporous NaY zeolite,
MCM-41, and MCM-48 crystallines to produce aldehydes.2 The[RhICl(CO)2]2 complex was anchored via silane groups terminated with phosphine, amine, or a thiol group grafted to MCM41.3 The Rh catalysts anchored via an amine were stable andrecyclable in the cyclohexene hydroformylation to produce
cyclohexane carboxaldehyde. The Rh(CO)2(acac) (acac )acetylacetonate) complex anchored to trisulfonated triphenylphosphine and the phosphinated [RhCl(1,5-COD)]2(COD ) cyclooctadiene) complex were immobilized in MCM41 and used for hexene hydroformylation.4,5
In this study, the Anderson-type, flat pillbox-shaped heteropolyacid anion molecule6 [AlMo6O18(OH)6]3- was used as
an anchor for the [RhI(COD)2]+ cation in FSM-16 [effective
pore diameter: 27 Å (10 Å ) 1 nm)] for propene hydroformy* To whom correspondence should be addressed. Phone and Fax: +8145-924-5569. E-mail: izumi.y.ac@m.titech.ac.jp.

lation. Rh complexes were immobilized in/on MCM-417 and a
NaY zeolite8 using Keggin-type H3PW12O40 molecules.7-9 The
heterogenized catalysts were applied for the styrene derivatives
hydroformylation7 and enantioselective hydrogenation reactions.8 The Rh complexes were also immobilized using heteropolyacid anions on alumina,10 carbon,10 or montmorillonite11
for selective hydrogenation reactions. The activity loss was
minimal with the supporting Rh complexes anchored withheteropolyacid when compared to the direct impregnation in/on meso-/microporous supports.
A binary oxide [NH4]+3[RhIIIMo6O18(OH)6]3- crystal6 1 wasalso supported in/on FSM-16. Catalytic performance of the RhMo catalysts was compared to conventional, impregnated Rh/
FSM-16 derived from RhCl3 and [RhI(COD)2]+ directly immobilized in/on FSM-16. Promoted butanol formation wasreported over Rh-Mo/SiO2 catalysts in the propene hydroformylation.12,13 The requisites for selective butanol synthesis
were suggested to be the combination of two-dimensional
mesoporous space and Rh nanoparticles atomically mixed withMo.
Methods
Catalysts Synthesis/Preparation. Crystal 1 was synthesized
by adding Rh(NO3)3 (Kanto, 1.6 mmol) to an aqueous solutionof [NH4]+6[Mo7O24]6- (Wako, special grade, 2.1 mmol) at
373 K.6 The reaction mixture was cooled to 290 K and filtered.
The obtained crystal powder was dried in air. FSM-16 was fromFuji Silysia Chemical Ltd. and was used as received. The crystal
1 was dissolved in a minimal amount of distilled, deionizedwater and reacted with FSM-16 at 290 K for 12 h using amagnetic stirrer. Then, the solvent was evaporated. The obtainedorange powder is denoted as [RhMo6O18(OH)6]3-/FSM-16.

10.1021/jp072616h CCC: $37.00 © 2007 American Chemical Society
Published on Web 06/19/2007

10074 J. Phys. Chem. C, Vol. 111, No. 27, 2007

Izumi et al.

TABLE 1: Basic Physicochemical Data for Various Rhodium-Molybdenum Catalysts Supported in/on FSM-16, Reference Rh
Catalysts, and FSM-16
particle
entry
a
b
c
d
e
f
g
h

specificsurfacearea (m2g-1)

poreradius (Å)b

porevolume(mm3 g-1)bd100spacing
(Å)c

0.22
3.0

1126 (1234)e923 (930)e12.1 (12.1)e11.2 (11.7)e873 (889)e786 (813)e37.8 (37.8)e
37.7

5.2
1.6
0.5
3.0
33

814
642

10.8
10.7

715
678

37.8
37.5

738e
633e
1274

11.4e
10.2e
13.5

170e
145e
904

38.8
37.4
36.8

Rh
(wt %)[RhMo6O18(OH)6]3-/FSM-16[Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16
RhCl3/FSM-16

Cl(wt %)a0.28 ((0.14)
0.22 ((0.13)

FSM-16mean
size (Å)df
24 ((0.4)
28 ((5.5)e22 ((0.4)e28 ((9.3)eaBased on EPMA measurements, after pretreatment in H2 (673 K). b Based on DH plots. c Based on XRD measurements. d Based on highresolution TEM measurements. e Data for catalyst after hydroformylation reaction (433 K, 5 h). f Metal particles were difficult to detect.

[NH4]+3[AlIIIMo6O18(OH)6]3- crystal 2 was synthesized6 ina similar manner to the route for crystal 1 and impregnated in/on FSM-16 similar to the route for [RhMo6O18(OH)6]3-/FSM16. The [RhI(COD)2]+[BF4]- crystal 3 was synthesized from
[RhCl(COD)]2 (Wako), 1,5-COD (Wako, >98%), and AgBF4(Wako, >97%).14 The obtained dark red crystal powder was
dissolved in distilled ethanol in an argon atmosphere and mixed
with [AlMo6O18(OH)6]3-/FSM-16 at 290 K for 12 h. The
mixture was filtered, and the powder was washed several times
until the filtrate became colorless, i.e., the UV-visible absorption
peak derived from compound 3 disappeared. The obtained
yellow powder is denoted as [Rh(COD)2]+-[AlMo6O18(OH)6]3-/
FSM-16.
The ethanol solution of crystal 3 was mixed with FSM-16 in
an Ar atmosphere similar to that for [RhI(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16. The obtained sample was denoted as [RhI(COD)2]+/FSM-16. RhCl3‚3H2O (Wako, >95%) was impregnated in/on FSM-16 from the aqueous solution. This catalyst isdenoted as RhCl3/FSM-16. Rh loading was varied between 0.22
and 5.2 wt % in all the catalysts above, except that 33 wt % of
Rh was impregnated from RhCl3 to check the maximum Rh
loading in the mesopore of FSM-16.
Propene Hydroformylation Measurements. Hydroformylation reactions of propene were carried out in a Pyrex glass
closed circulation system (effective internal volume 132 cm3).
The typical catalyst volume used was 0.20 g. All of the catalyst
characterization was performed with sample treatments/kinetic
measurements in this system followed by transfer to each cell
utilizing the Schlenk technique without contact to air except
for sample loading on the TEM and XRD apparatus.
[RhMo6O18(OH)6]3-/FSM-16, [Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16, and [Rh(COD)2]+/FSM-16 catalysts were
evacuated (<10-6 Pa) at 290 K for 2 h prior to tests. RhCl3/
FSM-16 catalysts were heated in hydrogen (27 kPa) at 673 K
for 1 h and evacuated at 673 K for 30 min before tests.
Relatively severe pretreatment for the latter catalysts was done
to remove chlorine (0.22-0.28 wt % of Cl after pretreatment,
Table 1). A total pressure of 60 kPa of C3H6, CO, and H2 (molar
ratio 1:1:1) was introduced to the reaction system at 413453 K. The flow rate was 90 cm3 min-1. The conversion andproduct distribution did not change significantly when the ratevaried in the range 60-120 cm3 min-1. Produced propane,n-butanal, i-butanal, n-butanol, and i-butanol were analyzed by
a TCD gas chromatograph (Shimadzu GC-8A) equipped withseparation columns of Porapak Q (3 m) and active carbon (2 m).
XRD, Sorption/Desorption Isotherms, TEM, and EPMA
Measurements. X-ray diffraction (XRD) data were obtainedusing the Multiflex-S diffractometer (Rigaku). Cu KR emission
was used with a Ni filter. The diffractions were monitored in

the 2θB range of 2-70° (θB, Bragg angle). The BET specificsurface area (SA) was measured at 77 K using BELSORP Mini
(Bell Japan) with N2 as an adsorbate. The samples were
evacuated at 383 K for 1 h before measurements. Transmission
electron microscope (TEM) and electron probe microanalysis(EPMA) observations were performed using the field-emissiontype JEM-2010F at an accelerating voltage of 200 kV (JEOL)
and JXA-8100 (JEOL), respectively, at the Center of Advanced
Materials Analysis (CAMA), Tokyo Institute of Technology (Dr.
A. Genseki). The sample powder was dispersed in carbon
tetrachloride using ultrasound and mounted on a carbon-coated
Cu grid (TEM) or pressed as a disk (EPMA).
EXAFS Measurements and Analyses. The incipient catalyst
samples were evacuated (<10-6 Pa) at 290 K for 2 h andtransferred to a Pyrex glass cell. The windows were sealed with
Kapton film of 12.5 µm in thickness. Sample thickness wascontrolled to give a Rh and Mo K absorption edge jump of 1
for Rh and Mo K-edge measurements.
The Rh and Mo K-edge extended X-ray absorption finestructure (EXAFS) spectra were measured at 30-290 K intransmission mode at beamlines 10B of the Photon Factory (PF)and NW10A of the Photon Factory advanced ring (PF-AR) at
the High Energy Accelerator Research Organization (KEK). The
PF and PF-AR storage ring energy was 2.5 and 6.5 GeV,
respectively, and the ring current was 310-260 mA and 5733 mA, respectively. A Si(311) double crystal monochromator
was used. The detail of the focusing cylindrical mirror system
of NW10A was in ref 15. Ar and Kr gases were purged in theI0 and It ion chambers, respectively. The rising edge energy ofRh and Mo metal was calibrated at 23 219.8 and 20 003.9 eV,
respectively.16 The scan steps were ≈9, ≈0.5, and 1.0-1.5 eV
for the preedge, near edge, and EXAFS regions, respectively.
The accumulation time for each data point was 1-10 s.
The EXAFS data analysis was performed using XDAP
version 2.2.7 (XAFS Services International) based on the worksof M. Vaarkamp, H. Linders, and D. Koningsberger. Multiple
shell analyses were performed for the Fourier-filtered k3weighted EXAFS data in k- and R-space using empiricalparameters extracted from EXAFS for Rh2O3, crystal 3, Rh
metal, crystal 1, K2MoO4, and crystal 1 for Rh-O, Rh-C, Rh(-C-)C, Rh-Rh, Rh(-O-)Mo, Mo-O, and Mo(-O-)Mo
bonds, respectively. The bond distances and coordination
numbers (N) of these references were summarized in Table 2
based on crystal structure data for Rh metal, Rh2O3, crystals 1
and 3, and K2MoO4.17-20 The model parameters for the MoRh bond were theoretically generated using FEFF version 8.2(Table 2).21 The σ2 values were relative to those for references.
The many-body reduction factor S02 was assumed to be equal
for the sample and reference. The goodness of fit was given asButanol Synthesis over Rh-Mo Catalysts in FSM-16

J. Phys. Chem. C, Vol. 111, No. 27, 2007 10075

TABLE 2: Empirical (A) and Theoretical (B) Fit Parameters Information for Rhodium and Molybdenum K-edge EXAFS
Analyses
model
shell

compound

Rh-O
Rh-C
Rh(-C-)C
Rh-Rh
Rh(-O-)Mo
Mo-O
Mo(-O-)Mo

Rh2O3[Rh(COD)2]+[BF4][Rh(COD)2]+[BF4]Rh metal[NH4]+ 3[RhMo6O18(OH)6]3K2MoO4[NH4]+ 3[RhMo6O18(OH)6]3-

Mo-Rh(FEFF 8.2)

data range usedk (Å-1)R (Å)

reference(A) Empirical
2.05
6
2.228
8
3.043
8
2.689
12
3.355
6
1.76
4
3.366
3

3.56-18.32
3.45-15.50
3.45-15.50
2.58-20.38
2.57-12.88
3.37-19.30
3.45-13.70

1.37-1.95
1.58-2.10
2.30-2.84
2.12-2.67
2.75-3.37
1.25-1.63
2.48-3.38

17
18
18
17
19
20
19

(B) Theoretical
2.689
12

1.72-19.65

2.13-2.68

21

R (Å)

N

TABLE 3: Results of Propene Hydroformylation Reaction at 433 Ka for 5 h over Various Rhodium-Molybdenum Catalysts
Supported in/on FSM-16 and Reference Rh Catalysts
(A)
entry
[RhMo6O18(OH)6]3-/FSM-16

a
b
c
d
e
f
g
h
i

[Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16[Rh(COD)2]+/FSM-16
RhCl3/FSM-16

selectivity (mol %)Rh(wt %)

Mo/Rh
atomic ratiob

conversion
(%)

C3H8

n-C3H7CHO

i-C3H7CHO

n-C4H9OH

i-C4H9OH

0.22
1.0
3.0
5.2
1.6
1.1
0.50
3.0
33

6
6
6
6
9

0.27
0.13
0.52
0.40
1.33
1.55
0.30
0.91
1.61

<0.7
19
3.0
2.7
29
91
10
1.2
40

<0.7
<0.7
<0.7
16
34
5.8
<0.7
23
11

<0.7
<0.7
<0.7
22
8.0
3.6
16
24
2.9

62
81
97
60
<0.7
<0.7
73
39
25

38
<0.7
<0.7
<0.7
29
<0.7
<0.7
13
21

(B)
specific reaction rate (10-3 molproduct molRh-1 h-1)c
entry
a
b
c
d
e
f
g
h
i

[RhMo6O18(OH)6]3-/FSM-16[Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16[Rh(COD)2]+/FSM-16
RhCl3/FSM-16

Rh (wt %)C3H8

n-C3H7CHO

i-C3H7CHO

n-C4H9OH

i-C4H9OH

0.22
1.0
3.0
5.2
1.6
1.1
0.50
3.0
33

<0.5
1.3
0.3
0.1
13
70
3.3
0.2
1.1

<0.5
<0.1
<0.1
0.7
15
4.5
<0.2
3.8
0.3

<0.5
<0.1
<0.1
0.9
3.6
2.8
5.4
3.9
0.1

41
5.6
9.2
2.5
<0.1
<0.1
24
6.5
0.7

26
<0.1
<0.1
<0.1
13
<0.1
<0.2
2.1
0.6

a Initial total gas pressure 60.0 kPa; propene/CO/H ) 1:1:1. b Evaluated based on the edge jump values of Rh and Mo K-edge X-ray absorption
2
using a spline function. c Average reaction rate for 5 h of reaction.

requested by the Committee on Standards and Criteria in X-ray
Absorption Spectroscopy.
Results
Propene Hydroformylation. The catalytic test results inpropene + CO + H2 at 433 K were summarized in Table 3 andthe time courses were depicted in Figure 1 for supported RhMo catalysts and reference Rh catalysts. For the RhCl3/FSM16 catalyst with a Rh loading of 33 wt %, the major productwas propane (Table 3, part A, entry i).22 As the Rh loading
decreased from 33 to 0.50 wt %, the specific reaction rates bothfor propane and for n-butanol formation increased (part B of
Table 3) and the latter increase was greater. As a balance, theselectivity to butanols increased to 73 mol % (entries g-i inpart A of Table 3).
When compound 1 was used as a catalyst precursor, butanols
selectivity was even greater (60-98 mol %, Table 3, entriesa-d). The specific reaction rates [(2.5-67) × 10-3 molproduct
molRh-1 h-1] to butanols were comparable to those over RhCl3/
FSM-16 catalysts [(1.3-24) × 10-3 molproduct molRh-1 h-1]. The
nature of using the precursor 1 anion suppressed the simple
hydrogenation of propene: 0.7-19 mol % and, accordingly,

the selectivity to butanols further increased for [RhMo6O18(OH)6]3-/FSM-16. The conversion was greatest when the Rh
loading was 3.0 wt % (Table 3, part A, entry c). The n- andi-butanal formation was negligible over [RhMo6O18(OH)6]3-/
FSM-16 catalysts with a Rh loading of 0.22-3.0 wt %, similar
to refs 12 and 13. Repeated hydroformylation tests (3 times)
for 5 h were performed for the [RhMo6O18(OH)6]3-/FSM-16
catalyst with 3.0 wt % Rh by evacuating the catalyst at 433 K
between each test. Significant change was not found in theconversion and selectivity to each product.
The trend of time course was in contrast between [RhMo6O18(OH)6]3-/FSM-16 and RhCl3/FSM-16 catalysts. The catalytic
performance of supported Rh heteropolyacid catalysts was
persistent. Typically, the formation rates of each product wereconstant for the catalyst of 5.2 wt % Rh (Figure 1c). In contrast,
products switching was observed at 1.5-2 h from n-butanol
and i-butanol to n-butanal and/or i-butanal over RhCl3/FSM16 catalysts (Figure 1f,g).
When compound 3 cation was heterogenized using compound
2 anion as an anchor to FSM-16, the major products were
n-butanal, propane, and i-butanol all in a similar selectivity(Table 3e). Over [Rh(COD)2]+/FSM-16 in the absence of10076 J. Phys. Chem. C, Vol. 111, No. 27, 2007

Izumi et al.

Figure 1. Time courses of propene hydroformylation reaction at 433 K over [RhMo6O18(OH)6]3-/FSM-16 [(a) 0.22, (b) 3.0, and (c) 5.2 wt % Rh],(d) [Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16, (e) [Rh(COD)2]+/FSM-16, and RhCl3/FSM-16 catalysts [(f) 3.0 and (g) 33 wt % Rh]. Products: ×
propane, 9 n-butanal, [ i-butanal, 2 n-butanol, and 1 i-butanol.

compound 2, propane was the major product (91 mol %) in
addition to 9.4 mol % of butanals (Table 3, entry f).
Basic Physicochemical Characterization of Rh[-Mo]
Catalysts. Basic physicochemical data were summarized inTable 1 for supported Rh-Mo catalysts and reference Rh
catalysts. The specific surface area (SA) for FSM-16 (1274 m2g-1) decreased by 12-36% upon supporting crystal 1 (Table
1, entries a-c). The decrease for the sample of 0.22 wt % Rh
may be trivial because the specific SA increased by 10% after
the hydroformylation reaction (Table 1, entry a). The pore radiusand volume decreased by 10-20% and 1.7-21%, respectively,
upon supporting crystal 1 and/or the hydroformylation tests. The
specific SA decreased to 50-58% upon supporting crystals
3 + 2 or RhCl3. The pore volume detrimentally became only
16-19% for RhCl3/FSM-16 catalysts compared to data for
FSM-16.
The change of d100 spacing values was monitored by XRD
based on the two-dimensional hexagonal symmetry of FSM16. The d100 value increased by 2.4-2.7, 1.9, and 1.6-5.4%
upon supporting compound 1, compounds 3 + 2, and RhCl3,
respectively (Table 1). The (100) peak height (diffracted X-ray
counts per second) decreased by 8.1-72% upon supporting
crystal 1. The FSM-16 matrix may swell up during the
supporting procedure in aqueous solution. The gradual decreasesof specific SA, pore radius, and pore volume as the metalloading increased should be the geometrical (blocking) effectof compounds 1, 3 + 2, or RhCl3.

The pore radius and d100 spacing values based on theadsorption/desorption method and XRD, respectively, werecompared to mean metallic particle size based on TEM
measurements (Table 1 and Figures 2 and 3). Obtained values
from TEM were 24-28 and 22-28 Å for [RhMo6O18(OH)6]3-/
FSM-16 (3.0 wt % Rh, Figure 3A,B) and RhCl3/FSM-16
catalysts (0.5-33 wt %, Figure 3C). Thus, the major part of
supported particles can be accommodated in the effective poreof 27 Å of FSM-16 (Figure 2) when the Rh loading was lessthan 3 wt % and before catalysis. The mean nanoparticle size
increased by 4 Å after hydroformylation tests (Figure 3B and
Table 3, entry b) for the [RhMo6O18(OH)6]3-/FSM-16 catalyst.
As the Rh loading decreased from 33 to 0.5 wt %, the mean
nanoparticle size decreased by 6 Å for the RhCl3/FSM-16
catalysts. In the TEM image, particles exterior to the FSM-16
mesopore were minor (Figure 3) except for the RhCl3/FSM-16
catalyst with 33 wt % Rh.
Rh and Mo K-edge EXAFS for Supported Rh[-Mo]
Catalysts. Rh and Mo K-edge EXAFS spectra were measured
for Rh[-Mo]/FSM-16 catalysts before and after propenehydroformylation tests. The best-fit values were summarizedin Tables 4 and 5, respectively. Relative Debye Waller factors
and fit errors were listed in more detailed versions (Tables S1and S2).
a. Rh K-Edge. For [RhMo6O18(OH)6]3-/FSM-16 catalysts, anintense peak was observed at 1.4-1.6 Å (Supporting Information, Figure S1, part B, spectra a1, b1, and c1) in the Fourier

Butanol Synthesis over Rh-Mo Catalysts in FSM-16

J. Phys. Chem. C, Vol. 111, No. 27, 2007 10077

Figure 2. Pore size distribution base on DH-plot method for the sorption and desorption isotherms for (a) FSM-16, [RhMo6O18(OH)6]3-/FSM-16
catalysts [(b, c) 0.22 and (d, e) 3.0 wt % Rh], before (b, d) and after (c, e) the propene hydroformylation at 433 K for 5 h, and (f) RhCl3/FSM-16
catalyst (3.0 wt % Rh) after propene hydroformylation.

Figure 3. High-resolution TEM image and particle size distribution histogram for [RhMo6O18(OH)6]3-/FSM-16 catalyst (3.0 wt % Rh) before (A)and after (B) propene hydroformylation reaction at 433 K for 5 h and for the RhCl3/FSM-16 catalyst (0.5 wt % Rh) after hydroformylation (C).

transform of the Rh K-edge EXAFS before catalytic tests. This
peak was due to the Rh-O bonds at 1.88-2.03 Å (Table 4,part A). The central hetero Rh atom constitutes octahedral
coordination with six bridging oxygen atoms in crystal 1. The

N value for Rh-O bonds was 6.2-7.0. The Rh(-O-)Mo peakintensity appeared at 3.0 Å and progressively decreased as theRh amount decreased from 5.2 to 0.22 wt % (Figure S1, part B,
spectra a1, b1, and c1). The Rh-O and Rh(-O-)Mo bond

10078 J. Phys. Chem. C, Vol. 111, No. 27, 2007

Izumi et al.

TABLE 4: Best Fit Results to Rhodium K-edge EXAFS for Various Rhodium-Molybdenum Catalysts Supported in/on
FSM-16 and Reference Rh Catalysts
Rh-O
Rh wt %

N

Rh-C

R(Å)

N

R(Å)

Rh(-C-)C

Rh-Rh or Rh-Mo

Rh(-O-)Mo

N

N

N

R(Å)

goodness of fit

4.2
5.3

3.34
3.35

12350
1138
5829

R(Å)

R(Å)

(A) Before Hydroformylation[RhMo6O18(OH)6]3-/FSM-16
0.22
6.6
1.88
3.0
6.2
2.014
5.2
7.0
2.030
[Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16
1.6
1.8
2.05
8.0b
2.092
[Rh(COD)2]+/FSM-16
b
1.1
2.0
2.04
8.0
2.09

8.0b
8.0

b

2.962

140

2.99

404

(B) After Hydroformylation (433 K, 5 h)a[RhMo6O18(OH)6]3-/FSM-16
0.22
3.0
5.2
2.028
5.2
4.0
2.041
[Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16
1.6
RhCl3/FSM-16
0.5
33
crystal 1
crystal 3

6

8.4
1.6
1.3

2.679
2.709
2.676

8.2

2.688

1567

7.3
7.7

2.686
2.685

2074
2431

2.021

0.8
1.0

3.37
3.364

6
8

2.228

8

3.355

3.043

1674
48
187

ref 19ref 18a Initial total gas pressure 60.0 kPa; propene/CO/H ) 1:1:1. b Equalized to the values for [Rh(COD) ]+[BF ]- because no loss of COD ligand
2
2
4
was found during catalyst synthesis (UV-visible absorption) and pretreatment (GC).

TABLE 5: Best Fit Results to Molybdenum K-edge EXAFS for Various Rhodium-Molybdenum Catalysts Supported in/on
FSM-16 and Reference Rh Catalysts
Mo-O
Rh wt %

N

R(Å)

Mo-O
N

Mo-Rh

R(Å)

N

R(Å)

Mo(-O-)Mo or Mo(-O-)Rh
N

R(Å)

goodness of fit

2.07
2.00

4.0
2.5

3.41
3.356

1680
1955

1.99

2.2

3.324

425

(A) Before Hydroformylation[RhMo6O18(OH)6]3-/FSM-16
0.22
6.1
1.60
1.3
3.0
0.6
1.67
1.1
[Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16
1.6
1.6
1.664
1.9

(B) After Hydroformylation (433 K, 5 h)a[RhMo6O18(OH)6]3-/FSM-16
0.22
1.0
1.664
5.3
3.0
1.8
1.66
2.4
5.2
0.7
1.67
4.7
[Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16
1.6
1.7
1.66
2.8
crystal 1

4

1.825

2

1.98
1.99
1.94

0.9
1.5
1.4

2.74
2.59
2.56

1.4

3.185

0.8

3.35

17
3.0
993

1.89

0.2

2.563

1.3

3.286

203

3

3.366

ref 192.289

a Initial total gas pressure 60.0 kPa; propene/CO/H

2 ) 1:1:1.

distances were comparable to the values for crystal 1 (Table
4). No Rh(-O-)Mo peak was observed for the sample of 0.22
wt % Rh.
For the catalysts after the hydroformylation reaction, a newpeak at 2.4 Å appeared commonly in Figure S1, part B, spectra
a2, b2, and c2. The peak was fit well as a metallic Rh-Rh bondat 2.676-2.709 Å with N values of 1.3-8.4 (Table 4, part B).
Note that metallic Rh-Rh and Rh-Mo (see the Mo K-edge
EXAFS section) bonds were not separated in the Rh K-edge
EXAFS, and thus the peak may include the contribution of
metallic Rh-Mo bonds. These peak positions were essentially
identical to those for a single peak due to Rh-Rh bonding in
the Fourier transform for RhCl3/FSM-16 after hydroformylation
tests (2.685-2.686 Å, Figure S1, part B, spectrum f2 and Table
4, part B). Metallic Rh nanoparticles were formed under thecatalytic conditions.
For [RhMo6O18(OH)6]3-/FSM-16 (3.0 and 5.2 wt % Rh)
catalysts after catalysis, two peaks were observed at 1.6 and3.1 Å in the Fourier transform (Figure S1, part B, spectra b2

and c2) in addition to the Rh-Rh (and/or Rh-Mo) bonding.
The two peaks were fit well as Rh-O and Rh(-O-)Mo bonds
at 2.028-2.041 Å (N ) 4.0 - 5.2) and 3.36-3.37 Å (N )0.8-1.0) (Table 4, part B), comparable to values before catalysis(Table 4, part A). The N values decreased to 57-84% for Rh-O
bonds and 19% for Rh(-O-)Mo bonds compared to those
before catalysis. Thus, a part of compound 1 anions remainedin samples of 3.0-5.2 wt % Rh after catalytic tests. The
heteropolyacid molecules (and/or distorted ones) and aggregatedmetallic nanoparticles coexisted.
For [Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16 before thecatalytic test, the bonds of Rh-C (2.092 Å) and Rh(-C-)C(2.962 Å) corresponded to values for crystal 3 (Table 4). The
two COD coordination was distorted but intact upon supporting.
New Rh-O bonds were formed at 2.05 Å with an N value of
1.8. For [Rh(COD)2]+/FSM-16 before hydroformylation (Table
4, part A), two COD ligands seem retained based on Rh-C
(2.09 Å) and Rh(-C-)C bonds (2.99 Å). In addition, two
Rh-O bonds were formed at 2.04 Å (Table 4, part A). A clear

Butanol Synthesis over Rh-Mo Catalysts in FSM-16
difference in the presence/absence of compound 2 was not foundin the Rh K-edge EXAFS.
After hydroformylation, the Rh K-edge EXAFS spectra forthe [Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16 catalyst (Figure
S1, spectra d2) became similar to that for [RhMo6O18(OH)6]3-/
FSM-16 (0.22 wt % Rh, Figure S1, spectra a2) or RhCl3/FSM16 (Figure S1, spectra f2). The best-fit values for the Rh-Rh(and/or Rh-Mo) bonds were at 2.688 Å with the N value of
8.2 (Table 4, part B). No Rh-O or Rh(-O-)Mo peaks weredetected in contrast to data for [RhMo6O18(OH)6]3-/FSM-16
catalysts. Therefore, the Al sites of [AlMo6O18(OH)6]3-/FSM16 were not replaced with the Rh cations of [Rh(COD)2]+.
b. Mo K-Edge. The Mo K-edge EXAFS data for [RhMo6O18(OH)6]3-/FSM-16 (0.22-3.0 wt % Rh) before catalysis required
three shells (two Mo-O and one Mo(-O-)Mo) for reasonable
fit. The best-fit data were shorter Mo-O bonds at 1.601.67 Å with N values of 0.6-6.1, longer Mo-O bonds at 2.002.07 Å with N values of 1.1-1.3, and Mo(-O-)Mo (or Mo(-O-)Rh) bonds at 3.36-3.41 Å with N values of 2.5-4.0.
The deviations for Mo-O bonds were relatively greatercompared to the corresponding data for crystal 1 (Table 5).
For [RhMo6O18(OH)6]3-/FSM-16 (0.22-5.2 wt % Rh) after
catalytic tests, two Mo-O peaks remained at 1.66-1.67 and1.94-1.99 Å (Table 5, part B). The peak due to Mo(-O-)Moor Mo(-O-)Rh bonds (3.19-3.35 Å) became weaker (0.22
and 5.2 wt % Rh; Figure S2, part B, spectra a2 and c2) orundetectable (3.0 wt % Rh, Figure S2, part B, spectrum b2). In
addition, a new peak appeared at 3.3-3.6 Å (Figure S2, part B,
spectra a2, b2, and c2). The peak was fit well as metallic MoRh bonds at 2.56-2.74 Å (Table 5, part B) with N values 0.91.5.
The Mo K-edge EXAFS spectra for [Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16 catalyst before hydroformylation (Figure S2,
spectra d1) were similar to those for crystal 2 and incipient[RhMo6O18(OH)6]3-/FSM-16 (Figure S2, spectra b1). The data
was best fit with three shells of two Mo-O and one
Mo(-O-)Mo (Table 5, part A). The bond distances werecomparable to those for crystal 1 and the N value was 4095% of the corresponding values for crystal 1.
After the catalysis, a new peak appeared at 2.3 Å (Figure
S2, part B, spectrum d2) due to metallic Mo-Rh bonds in addition
to three bonds derived from a Mo oxide species. The bonddistance was determined to 2.563 Å with an N value of 0.2(Table 5, part B).
Discussion
Selective Butanol Synthesis. The propene hydroformylation
reaction has been studied over supported Rh catalysts. Under
101 kPa at 400 K over Rh/silica-gel prepared from rhodium
chloride, the major product was propane (46%) in addition to
i-butanal (35%) and n-butanal (19%).22 At 400-423 K over
Rh-Y zeolite catalysts, the products were propane (64-97%)
and minor butanals.22,23 Under 101 kPa at 418 K over MoRh/SiO2 catalysts prepared via subsequent impregnation ofrhodium chloride and ammonium heptamolybdate, the major
products were butanols (27-39%) and propane (61-67%).12
A similar product distribution was reported for the catalyst
prepared from Cp3RhMo2(CO)5 (Cp ) cyclopenetadiene) on
SiO2.13
The formation of n-butanol was superior over [RhMo6O18(OH)6]3-/FSM-16 (60-97 mol %) and RhCl3/FSM-16 catalysts(25-73 mol %) to i-butanol 0-38 and 0-21 mol %, respectively. Selection to the n-form was in accord with refs 12 and
13. Selective alcohol formation (94-100%) was reported in the

J. Phys. Chem. C, Vol. 111, No. 27, 2007 10079
hydroformylation of 1-hexene using homogeneous RhH(PEt3)3or Rh2OAc4(PPh3)3 in ethanol at 3.0-6.6 MPa and 393 K.24
The butanols selectivity over [RhMo6O18(OH)6]3-/FSM-16(60-98 mol %) was high under milder condition at 60 kPa and433 K (Table 3). The butanols selectivity (46-73 mol %) over
RhCl3/FSM-16 was comparable to Mo-Rh binary catalysts on
SiO2.12,13 In this context, mesoporous reaction space seems theprimary control factor to selectively form butanols. As the Rh(and Mo) loading amount decreased, butanol selectivity increased over both [RhMo6O18(OH)6]3-/FSM-16 and RhCl3/
FSM-16 catalysts. In the comparison of identical Rh loading
amounts of [RhMo6O18(OH)6]3-/FSM-16 and RhCl3/FSM-16
catalysts (3.0 wt %), the butanols selectivity was 97 and52 mol %, respectively. Thus, the presence of Mo additionallyaffected the formation to be more selective to butanol.
Active Sites for Selective Butanol Synthesis. a. Incipient
Supported Structure. The mesoporous framework of FSM-16
is basically retained upon supporting crystals 1-3 and after thepropene hydroformylation reaction at 433 K for 5 h based onthe (100)-reflection XRD peak and TEM images (Table 1 andFigures 2 and 3). The d100 spacing expanded by 1.6-5.4% forRh[-Mo]/FSM-16 catalysts compared to native FSM-16 (Table
1). A hexagonal symmetric structure was observed (parts A-Cof Figure 3).
The incipient supported structure of [RhMo6O18(OH)6]3- in/on FSM-16 was studied by Rh and Mo K-edge EXAFS. On
the basis of the Rh-O bond distances and the N values (Table
4, part A), the octahedral central Rh cation was retained. The
presence of Rh(-O-)Mo bonds demonstrated that the heteropolyacid framework was retained in the cases of 3.05.2 wt % Rh. The peak was not observed for [RhMo6O18(OH)6]3-/FSM-16 with 0.22 wt % Rh. A smaller amount of theanions may interact more effectively with the surface of FSM16 and distort more. The deviation of fit data (bond distanceand the N value) for Mo-O bonds were relatively greater thanthose for Rh-O, Rh(-O-)Mo, or Mo(-O-)Mo/Rh bonds
compared to data for crystal 1 (Tables 4 and 5). The hydroxyl
and/or oxo group(s) bound to the Mo atoms of compound 1
anion may be interacting with the surface of FSM-16.
On the basis of Mo K-edge EXAFS, the framework of[AlMo6O18(OH)6]3- was demonstrated to remain; however, local
structure around the Mo-O bonds was more distorted (Table
5, part A) than for Mo(-O-)Mo bonds, similar to the case of
supported [RhMo6O18(OH)6]3-. On the basis of Rh K-edge
EXAFS, two COD ligands per one Rh cation were distorted
upon supporting and the Rh cation was bound via about twooxygen atoms for the [Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM16 catalyst (Table 4, part A). Because very similar fit results
were obtained for [Rh(COD)2]+/FSM-16 and the Rh loading
was greater for [Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16(1.6 wt % Rh) than 1.1 wt % Rh for [Rh(COD)2]+/FSM-16,
the [Rh(COD)2]+ cations should be immobilized with both
surface O/OH groups of FSM-16 and [AlMo6O18(OH)6]3dispersed in/on FSM-16.
b. Structure during/after Hydroformylation. After the propenehydroformylation at 433 K for 5 h, the Rh site structure changed
in all the Rh[-Mo]/FSM-16 catalysts studied. The Rh cations
aggregated to form metallic Rh nanoparticles based on the RhRh bonds for the [RhMo6O18(OH)6]3-/FSM-16 catalyst with
0.22 wt % Rh (Table 4, part B). As the Rh loading amountincreased to 5.2 wt %, the N values for Rh-Rh bonding
gradually decreased and those for Rh-O and Rh(-O-)Mo
derived from the remaining/distorted Rh heteropolyacid anions
increased.

10080 J. Phys. Chem. C, Vol. 111, No. 27, 2007

Figure 4. Proposed mechanism of selective n-butanol synthesis over
[RhMo6O18(OH)6]3-/FSM-16 catalysts.

In the Mo K-edge EXAFS, metallic Mo-Rh bonds wereobserved at 2.56-2.74 Å with N values of 0.9-1.5 in addition
to Mo-O and Mo(-O-)Mo/Rh bonds. Because the Mo(VI)site is difficult to be reduced to metallic Mo nanoparticles inpropene + CO + H2 at 433 K, the Mo-Rh bonds should bederived from alloylike Mo sites included in/on metallic Rh
nanoparticle lattices. The face-centered cubic and body-centeredcubic crystal phases were reported for the MoxRh100-x alloy(x ) 10, 26, 35, and 65).25 The MoRh3 alloy on SiO2 wasdetected by XRD,26 and Mo-Rh alloy crystal phases wereobserved by TEM.27 In summary, Rh sites constituted metallic
nanoparticles mixed with Mo atoms and the rest of the Mo sites
constituted Mo oxide and/or Mo-Rh binary oxide phases. The
ratio of Mo[-Rh] oxide phase(s) increased as the metal loading
increased.
For [Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16 and RhCl3/
FSM-16 catalysts after catalytic tests, the Rh nanoparticles werepredominantly formed based on a single Rh-Rh peak in the
Rh K-edge EXAFS (Table 4). The interaction between Rh andthe MoOx species should be weak based on the absence of
Rh-O and Rh(-O-)Mo bonds. A Mo-Rh bond(s) was
observed in the Mo K-edge EXAFS for [Rh(COD)2]+[AlMo6O18(OH)6]3-/FSM-16 (Table 5), but the N value was only
0.2 compared to 0.9-1.5 for [RhMo6O18(OH)6]3-/FSM-16
catalysts. Therefore, the population of alloylike Rh-Mo particle
sites were smaller in the catalyst. Rh and Mo atoms wereeasier to mix as alloylike nanoparticles starting from
[RhMo6O18(OH)6]3- rather than from separate molecules of[Rh(COD)2]+ and [AlMo6O18(OH)6]3-.
Implications to Butanol Synthesis Mechanism. Aldehydes
are predominantly produced in homogeneous-phase hydroformylation1 because the Rh centers are often coordinated withmultiple CO ligands to preferably proceed CO insertion.28
Metallic nanoparticles were formed in all the catalysts in thiswork (Table 4). The relatively small Rh[-Mo] nanoparticle
confined in the FSM-16 mesopore may be predominantly
adsorbed with CO to be selective toward oxygenates (Figure4). The diffusion of reactants and intermediates was relatively
slow in the two-dimensional mesospace compared to that over
the amorphous silica gel surface. Thus, multiple-step hydrogenations of intermediate aldehydes should proceed for Rh/FSM16 catalysts in this work (Figure 4). Similar diffusion effects in
microporous space were reported for the regioselective hydrogenation over Rh clusters in NaY and chemo-selective hydrogenation over Pt or Rh clusters in NaY.29
Selective n-butanol synthesis (Table 3, entries a-d) was inaccord with the predominant n-butanol among oxygenate
compounds over Mo-Rh/SiO2 catalysts.12,13 However, propane
formation was not promoted by the addition of Mo in Table 3
in clear contrast to promoted formation of n-butanol and propane
over Mo-Rh/SiO2 catalysts.12,13First, the Rh[-Mo] particle size is considered. The size was
24-28 Å for [RhMo6O18(OH)6]3-/FSM-16 catalysts affected

Izumi et al.
by the regular pore opening of 27 Å for FSM-16 (Table 1 andFigure 3). The mean size varied between 19 and 42 Å for MoRh/SiO2 catalysts as a function of Mo/Rh atomic ratio (0.21), but the butanols selectivity did not significantly change (2739%)12 compared to 60-98% for [RhMo6O18(OH)6]3-/FSM16 (Table 3, part A). Thus, a different trend to propane was not
ascribed only to the Rh[-Mo] particle size difference.
Next, the contact between Mo and Rh is considered. The Mo
atoms were demonstrated in/on the Rh nanoparticle lattice basedon the Mo-Rh peak at 2.56-2.74 Å (Table 5, part B). Direct
metallic Mo-Rh bonding was confirmed for the first time for
Mo-promoted Rh nanoparticle catalysts. Mo-Rh alloy dispersedon SiO2 was already demonstrated by XRD26 and TEM.27 A
reference [AlMo6O18(OH)6]3-/FSM-16 catalyst exhibited negligible catalytic activity in propene hydroformylation and no
Mo-Al peak was observed in the Mo K-edge EXAFS after
the catalytic test. For [Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM16, the N value of Mo-Rh bonds was only 0.2. Thus, Rh
nanoparticles atomically mixed with Mo atoms were active, andinert Mo oxide species coexisted.
We propose that CO insertion predominantly proceeded on
the Rh-Mo nanoparticles dominated with adsorbed CO, andhydrogenations of alkyl and aldehyde intermediates were
suppressed on the Rh ensemble segregated by Mo atoms (Figure4). In sacrifice of total conversion (0.13-0.52%, Table 3, partA) over [RhMo6O18(OH)6]3-/FSM-16, butanols were selectively
produced during slow, rate-limiting diffusion in a twodimensional mesopore. The propyl intermediate may quickly
react with CO to form an acyl intermediate (Figure 4). Hence,
propane formation was negligible on the catalysts (Table 3).
An alloylike site may be easier to be formed with the greater
Mo/Rh atomic ratio 6 for [RhMo6O18(OH)6]3-/FSM-16 compared to 0.2-2 for Mo-Rh/SiO2 catalysts.12,13 Atomically
mixed Mo sites on/in Rh nanoparticles were more easily formed
starting from compound 1 rather than compounds 2 + 3 (Tables
4 and 5). In fact, both propane and butanols were formed on[Rh(COD)2]+-[AlMo6O18(OH)6]3-/FSM-16 (Table 3, entry e).
The alloylike Zn-Rh ensemble was suggested on SiO2 basedon infrared absorption spectroscopy for adsorbed CO. CO
insertion was enhanced in contrast to the remarkable suppressionof hydrogenation over segregated Rh sites by Zn atoms.30,31
Similarly, Rh sites were segregated by Sn atoms to be selective
for hydrogenation and hydrogenolysis.32
Patchlike transition metal suboxide or cationic sites (Ti,33
V,12,34 Mn,35-37 Fe,12,38 Zr,30 Nb,39 or Mo12,13,35,40) were
proposed to cover Rh nanoparticles dispersed on SiO2. The Rh
sites covered with patchlike MoOx may retain hydrogenation
activity of alkyl and aldehyde intermediates in Mo-Rh/SiO2
catalysts.12,13 The promotion of both propane and alcohol
synthesis was also reported for Rh nanoparticles in contact with
Feδ+.38
In summary, two control factors were proposed in combination for selective butanol synthesis: the two-dimensional spaceof FSM-16 to lead to slower diffusion of oxygenate intermediates and Rh nanoparticles atomically mixed with Mo. The
selectivity to butanols versus propane increased in the order[RhMo6O18(OH)6]3-/FSM-16 (2-D diffusion and atomically
mixed Rh-Mo) > RhCl3/FSM-16 (2-D diffusion) > Mo-Rh/
SiO212,13 . Rh/SiO2. The effect of the remaining Cl (0.220.28 wt %, Table 1) cannot be excluded for RhCl3/FSM-16.
The reason is unclear why the butanol selectivity increased
as the Rh loading amount decreased both in the [RhMo6O18(OH)6]3-/FSM-16 and in the RhCl3/FSM-16 catalysts. The
mixing ratio of atomic Mo with Rh may change as a function

Butanol Synthesis over Rh-Mo Catalysts in FSM-16
of metal loading, as suggested by the variation of N values for
Mo-Rh bonds (0.9-1.5, Table 5, part B).
Finally, the population of Rh[-Mo] metal particles andMo[-Rh] oxide particles inside and outside the FSM-16
framework should be mentioned. On the basis of the relatively
narrow particle size distribution centered at 22-28 Å (Figure
3 and Table 1), these particles could grow up to the internal
size limit of 27 Å in the catalyst impregnation steps (Figure
3A). A particle size increase of 4-6 Å as the metal loading
increases or after hydroformylation suggests that extra Rh/Mo
species may grow also on the external surface of the FSM-16
crystalline. These trends are consistent with catalytic results thatthe multiple-step product butanols were more selectively formed
as the metal loading progressively decreased and the populationof Rh/Mo inside mesopore was greater (Table 3).
Conclusions
Selective butanol synthesis catalysts (60-98 mol %) were
found in propene hydroformylation at 433 K and 60 kPa
prepared from [RhMo6O18(OH)6]3- crystals and FSM-16. Rh
nanoparticles (≈23 Å) in the mesopores were essential for theselective synthesis. Because of the slow diffusion in themesopore, oxygenate intermediates were subject to multiplestep hydrogenations to convert finally to n-butanol. Additionally,
Mo atomically mixed in/on Rh nanoparticles was suggested tosegregate the surface Rh ensemble and be further selective for
CO insertion. The Mo-Rh coordination based on Mo K-edge
EXAFS was the key to indicate the Mo-Rh alloy formationand selective formation of n-butanol. The coordination number
was smaller/zero for catalysts prepared from [AlMo6O18(OH)6]3and/or [Rh(COD)2]+ crystals over FSM-16, and the catalysis
was not selective to butanol.
Acknowledgment. The authors are thankful for research
grants from the Grant-in-Aid for Fundamental Scientific Research (Y.I., Grant C17550073) and the Research Foundation
for Opto-Science and Technology (Y.I., 2005-2006). The
EXAFS measurements were performed under the approval of
the Photon Factory Proposal Review Committee (Grants
2005G220, 2004G278, and 2003G074).
Supporting Information Available: Data and best fit resultsof Rh K-edge and Mo K-edge EXAFS. This material is available
free of charge via the Internet at http://pubs.acs.org.
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