Generation of acid sites on silica-supported rare earth oxide catalysts: Structural characterization and catalysis for α-pinene isomerization

Article abstract of DOI:10.1039/a900742c

Silica-supported rare earth oxide catalysts (Ln/SiO₂; Ln = La, Ce, Pr, Sm, Eu, Tb, Yb and Y), loading amounts of which were 3.4 mmol g (support)^{- 1}, were characterized by α-pinene isomerization, temperature-programmed desorption (TPD), Fourier transform infrared (FTIR), X-ray diffraction (XRD), X-ray absorption fine structure (XAFS), thermogravimetric-differential thermal analysis (TG-DTA) and Raman spectroscopy. In the lanthanoid series, the catalytic activity increased with atomic number from ⁵⁷La to ⁷⁰Yb, except for Ce. All the Ln/SiO₂ catalysts, except for Ce, were amorphous. On the surface of the catalyst, Ln-O-Si and Ln-O-Ln linkages formed, the ratio of which varied with the loaded element. The ratio of Ln-O-Si linkage increases with stronger affinity among LnO(n) units and SiO₄ tetrahedra, and the affinity depends on the size of Ln³⁺. With increasing ratio of Ln-O-Si to Ln-O-Ln linkage, the catalytic activity increases. Silica-supported yttrium oxide catalyst, the trivalent ion radius of which is quite similar to that of ytterbium, exhibited the same activity as that of Yb/SiO₂. Raman spectroscopic characterization revealed that excess loading of Yb atoms on SiO₂-support block Yb-O-Si linkage to form Yb₂O₃ fine particles. When Yb/SiO₂ was pretreated at 1273 K, fine ytterbium silicate crystallites formed. Ln-O-Si linkage without a long-ranged ordering structure was the active site for α-pinene isomerization.

Full text of DOI:10.1039/a900742c

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Generation of acid sites on silica-supported rare earth oxide catalysts:
Structural characterization and catalysis for a-pinene isomerization
Takashi Yamamoto,* Takahiro Matsuyama, Tsunehiro Tanaka,* Takuzo Funabiki and Satohiro
Yoshida
Department of Molecular Engineering, Kyoto University, Kyoto 606-8501, Japan.
E-mail: yamamoto=dcc.moleng.kyoto-u.ac.jp; Fax: ]81 75 753 5925
Received 27th January 1999, Accepted 16th April 1999
Silica-supported rare earth oxide catalysts (Ln/SiO ; Ln \ La, Ce, Pr, Sm, Eu, Tb, Yb and Y), loading
2
amounts of which were 3.4 mmol g (support)~1, were characterized by a-pinene isomerization,
temperature-programmed desorption (TPD), Fourier transform infrared (FTIR), X-ray di†raction (XRD),
X-ray absorption Ðne structure (XAFS), thermogravimetric-di†erential thermal analysis (TG-DTA) and
Raman spectroscopy. In the lanthanoid series, the catalytic activity increased with atomic number from 57La
to 70Yb, except for Ce. All the Ln/SiO catalysts, except for Ce, were amorphous. On the surface of the
2
catalyst, LnÈOÈSi and LnÈOÈLn linkages formed, the ratio of which varied with the loaded element. The ratio
of LnÈOÈSi linkage increases with stronger affinity among LnO units and SiO tetrahedra, and the affinity
n
4
depends on the size of Ln3`. With increasing ratio of LnÈOÈSi to LnÈOÈLn linkage, the catalytic activity
increases. Silica-supported yttrium oxide catalyst, the trivalent ion radius of which is quite similar to that of
ytterbium, exhibited the same activity as that of Yb/SiO . Raman spectroscopic characterization revealed that
2
excess loading of Yb atoms on SiO -support block YbÈOÈSi linkage to form Yb O Ðne particles. When
2
2 3
Yb/SiO was pretreated at 1273 K, Ðne ytterbium silicate crystallites formed. LnÈOÈSi linkage without a
long-ranged ordering structure was the active site for a-pinene isomerization.
2
From X-ray absorption Ðne structure (XAFS) character-
ization, we have concluded that Yb atoms are supported on
Introduction
The addition of rare earth elements to catalysts has been per-
formed mainly to enhance thermal stability of the catalysts
themselves.1,2 In contrast, there are few studies concerning
catalyses of supported rare earth oxides. Capitan et al. investi-
gated the Sm O ÈAl O catalyst for correlations between
catalysis for oxidative coupling of methane and the surface
SmÈAlÈO phases.3 They concluded that the oxide-like struc-
ture shows better selectivity toward C species than the
silica in a highly dispersed form as a YbO octahedron. A
6
YbO octahedron strongly interacts with SiO tetrahedra,
6
4
rather than other YbO octahedra. The YbÈOÈSi linkage was
6
concluded to be the active sites for a-pinene isomerization.
Following on from this, we prepared several kinds of silica-
supported rare earth oxide catalysts to elucidate the speciÐc
character of each rare earth element. Here, we will report on
the catalytic properties of supported rare earth oxides and
their structural characterization. As a model reaction of acidÈ
base properties of each catalyst, a-pinene isomerization was
adopted, as in our previous work.9 It is known that the di†er-
ence in acidity of each catalyst inÑuences the catalysis for a-
pinene isomerization concerning its selectivity.11h15 E†ective
acid strength for a-pinene isomerization was proposed as
2
3
2 3
2
SmAlO phase. Shi et al. reported that dispersed La O on
3
2 3
alumina exhibits higher activity for reduction of NO with
CH in the presence of oxygen than unsupported La O ,
while the catalytic activity of La/SiO was signiÐcantly lower
4
2 3
2
than that of La O .4
2
3
According to the hypothesis for acidity prediction proposed
by Tanabe et al., binary oxides among silica and rare earth
oxides, such as La O ÈSiO and Y O ÈSiO are expected to
exhibit solid acidity.5,6 Shen et al. investigated acidÈbase
properties of silica- and alumina-supported europium oxide
with spectroscopic methods. They found that alumina-
supported europium oxide possesses basic sites stronger than
that of Eu O , and silica-supported europium oxide exhibits
Lewis acid properties.7 To our knowledge, this report is the
only study so far to investigate acidÈbase properties of sup-
ported rare earth oxides. Recently, the catalysis of mono-
ethanolamine synthesis over SiO ÈY O was reported, while
H O ]3.3.15,16
0
2
3
2
2
3
2
Experimental
Material
SiO gel (661 m2 g~1) was synthesized from tetraethyl ortho-
2
3
2
silicate (Nacalai, EP-grade, singly distilled) by hydrolysis in a
waterÈethanol mixture at the boiling point, followed by cal-
cination at 773 K for 5 h in dry air stream.17 Before calcina-
tion, a dried sample was grounded to a powder under 100
mesh. The silica-supported rare earth oxide catalyst was pre-
2
2 3
the precise acidity remained unknown.8
In a previous study, we reported that silica-supported ytter-
bium oxides exhibit solid acidity and catalyze a-pinene isom-
erization at 323 K.9,10 The activity depends on the loading
amounts of ytterbium and pretreatment temperatures. The
maximum activity was exhibited when the loading amount
pared by impregnation of SiO gel with an aqueous solution
2
of Ln(NO ) É xH O at 353 K. The elements used were La
3 3
2
(Nacalai, 99.9%), Ce, Eu (Rare Metallic, 99.9%), Pr, Gd, Tb
(Rare Metallic, 99.99%), Sm (Wako, 99.5%), Yb (Mitsuwa,
99.9%), and Y (Wako, 99.9%). The impregnated sample was
dried at 363 K for 12 h, followed by calcination at 773 K for 5
was 3.4 mmol Yb g (SiO )~1, and was pretreated at 1073 K.
2
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h. Other supports used were c-Al O (JRC-ALO-4), Mg(OH)
GeV, and the stored current was 17È20 mA. The X-ray
2
3
2
(Rare Metallic, 99%), and Zr(OH) (obtained by hydrolysis of
absorption spectra were recorded in the transmission mode
with a Si(111) two-crystal monochromator. Higher harmonics
were eliminated with Rh-coated mirrors (1.5 mrad for YÈK
4
ZrOCl 18,19). The loading amount of Ln atom per one gram
2
of support was 3.4 mmol in each catalyst.
edge, and 5 mrad for LnÈL edge XAFS measurements). The
III
dispersive X-ray was collimated by a total reÑection mirror, at
Catalysis
an upper stream 32.9 m from an X-ray source as a parallel
ray. The height of the X-ray size was 1.0 mm. X-ray absorp-
tion spectra were recorded every 0.3 eV in the XANES region
of each L edge. Data reduction was performed using a
FACOM M1800 computer at the Kyoto University Data
a-Pinene isomerization was carried out under a dry N atmo-
sphere using a stirred batch reactor at 323 K. Prior to each
reaction, the catalyst was evacuated at 1073 K for 0.5 h and
calcined under 6.66 kPa of O for 1 h, followed by evacuation
2
III
2
at the same temperature for 1 h. For an each experiment, the
Processing Center. The normalization method has been pre-
reactor was loaded with 2 mL (12.6 mmol) of a-pinene
(Nacalai, EP, 99.8%) and 50 mg of catalyst. Products were
analyzed by FID gas chromatography (GC-14A; Shimadzu)
with a CBP20-M25-025 capillary column (Shimadzu).
viously reported in detail.20
Results
Catalysis
Characterization
Table 2 shows results of a-pinene isomerization over 3.4 mmol
Ln/SiO catalysts pretreated at 1073 K. Only Ce/SiO was
The BET speciÐc surface area measurement was carried out
2
2
inert for this reaction in the series of Ln/SiO catalysts, and
with BELSORP 28SA (BEL Japan, Inc.) using a N adsorp-
2
Yb/SiO exhibited the highest activity. The activity of
2
2
tion isotherm at 77 K. Prior to the measurement, each sample
La/SiO was much lower than that of Yb/SiO . With increas-
was outgassed at 1073 K for 3 h. The analyzed results are
summarized in Table 1.
X-ray di†raction (XRD) patterns were obtained with a
Rigaku GeigerÑux di†ractometer using Ni-Ðltered Cu-Ka
radiation (averaged as 1.5418 Ó).
FTIR spectra of adsorbed pyridine were recorded using a
Perkin-Elmer Paragon 1000 spectrometer with the resolution
of 4 cm~1. Each sample (42È79 mg) was pressed into a self
supporting wafer (20 mm in diameter), and was mounted in an
in situ IR cell equipped with NaCl windows. A wafer was
pretreated in the same way as for the catalytic reaction. The
pretreated wafer was exposed to 1.0 kPa of pyridine vapor at
423 K for 5 min, followed by evacuation at the same tem-
perature for 1 h. After cooling to room temperature, each
spectrum was recorded in the transmission mode.
2
2
ing atomic number from 57La of light rare earth elements to
70Yb of heavy rare earth elements, the activity increased
except for Ce/SiO . In all the catalysts, a-pinene was selec-
tively converted to limonene (ca. 68%) and camphene (ca.
2
20%) when the conversion was above 1%. Because La/SiO
2
and Pr/SiO were poorly active at 323 K, the obtained selec-
2
tivities were di†erent from those of other catalysts. When the
reaction was performed at 353 K, La/SiO and Pr/SiO gave
2
2
identical selectivities to the others. Ce/SiO exhibited quite
2
low activity for a-pinene isomerization even at 353 K. Y/SiO
2
exhibited almost the same activity and selectivity as those of
Yb/SiO , although yttrium is not a lanthanoid but an element
2
of the sixth period.
The e†ect of support was examined with a supported ytter-
bium oxide catalyst. The supports used were SiO , c-Al O ,
Pyridine temperature-programmed desorption (TPD)
experiments were performed at a heating rate of 10 K min~1
and a quadrupole-type mass spectrometer (MASSMATE-100,
ULVAC) was used as a detector. Prior to TPD measurement,
200 mg of sample was pretreated in the same way as for a-
pinene isomerization. The pretreated sample was exposed to
2.0 kPa of pyridine at 473 K for 10 min, followed by evac-
uation at the same temperature for 1 h. The amount of
desorbed pyridine was normalized to that of introduced Ar
(m/z \ 40) as an internal standard.
2
2 3
MgO and ZrO . The pretreatment temperature was 1073 K
2
for each catalyst. As summarized in Table 3, only a silica-
supported ytterbium oxide catalyst exhibited catalytic activity.
No active sites catalyzing this reaction formed on the other
supports.
XRD
Fig. 1 shows Cu-Ka XRD patterns of 3.4 mmol Ln/SiO cata-
2
lysts and SiO gel pretreated at 1073 K. SiO gel exhibited
2
2
Thermogravimetric-di†erential thermal analysis (TG-DTA)
was carried out with a Rigaku ThermoÑex TG 8110. Each
only one halo around 2h \ 22. The other catalysts possess a
halo around 2h \ 29È31¡ beside the halo due to amorphous
proÐle was recorded under a dry N stream (20 mL min~1) at
silica. XRD characterization demonstrates that only Ce/SiO
2
2
crystallized to form CeO , and the other catalysts were
2
a heating rate of 5 K min~1. a-Al O crystal was utilized as
2
3
the standard material for DTA analysis.
amorphous. It shows that all of the other rare earth oxides
The laser Raman spectra were recorded with a JASCO
NRS-2000 spectrometer using the 514.5 nm line of Ar` laser
emission. The incident laser power was 20 mW at sample
position, and scan time was 60È120 s for a single spectrum.
The spectral resolution was 4 cm~1.
were supported on silica as an amorphous state. The origin of
amorphous phases could not be identiÐed, because both crys-
talline rare earth silicates and rare earth oxides exhibit typical
reÑections in this region where the new halo appeared. The 3.4
mmol Yb/SiO sample pretreated at 1273 K, the catalytic
2
X-ray absorption experiments were carried out on the
BL01B1 at SPring-8 (Hyogo, Japan). The ring energy was 8
activity of which was signiÐcantly lower than that of one
pretreated at 1073 K,9 was amorphous as well.
Table 1 SpeciÐc surface area of catalysts pretreated at 1073 K
3.4 mmol Ln/SiO
2
SiO
57La
58Ce
59Pr
62Sm
63Eu
65Tb
70Yb
39Y
2
wt.% a
È
601
601
36
159
248
37b
301
477
36
216
337
37
179
284
37
164
260
38
210
338
40
180
300
28
246
341
Area/m2 g~1 c
S /m2 g~1 d
0
a As Ln O . b As CeO . c BET speciÐc surface area estimated with a N adsorption isotherm at 77 K. d Surface area of g (SiO )~1 carrier (ref. 9).
2
3
2
2
2
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Table 2 Results of a-pinene isomerization over 3.4 mmol Ln/SiO at 323 K a
2
Selectivityb (%)
Conversion
Element
La
(%)
1
2
3
4
5
6
7
8
0.4
15.3c
0
10
1
22
17
1
2
50
66
5
6
5
3
3
3
4
3
Ce
1.0c
0.7
30.1c
3.7
6.7
8.2
9
4
Trd
1
1
1
27
19
16
18
18
18
23
3
1
2
1
1
2
2
43
58
67
66
69
67
67
7
6
5
5
5
5
4
2
4
3
3
2
2
2
4
3
3
2
2
3
2
5
5
4
4
2
2
Trd
Pr
Sm
Eu
Tb
Yb
26.5
Trd
(SiO )c
Y
0
28.3
2
Trd
17
2
63
4
2
2
Trb
a a-Pinene, 2 mL (12.6 mmol); catalyst, 50 mg; pretreatment temperature, 1073 K; reaction time, 3 h. b 1: b-Pinene, 2: camphene, 3: a-fenchene,
4: limonene, 5: terpinolene, 6: a-terpinene, 7: c-terpinene, 8: others. c Catalyst, 100 mg; reaction temperature, 353 K. d Trace.
Ytterbium oxide and zirconium oxide form a solid solu-
similar wavenumbers, and it is difficult to distinguish between
tion.21 In the XRD pattern of 3.4 mmol Yb/ZrO pretreated
H-pyridine and L-pyridine. However, the spectrum of SiO
2
2
at 1073 K, only a cubic solid solution ZrO ÈYb O phase was
had no peak around 1490 cm~1 (19a mode), and had the
2
2
3
detected. As shown in Table 3, the supported species crys-
typical wavenumber of the 8a mode below 1600 cm~1. In
tallized on c-Al O and MgO to form Yb O (JCPDS Ðle No.
addition, it was reported that SiO gave H-pyridine at 1446
2
3
2
3
2
18-1463). Any binary oxides present were not detected in the
XRD patterns of all of the supported ytterbium oxide samples.
FTIR
Fig. 2 shows FTIR spectra of adsorbed pyridine on catalyst
samples pretreated at 1073 K. The peak assignments were
carried out based on studies by Connell and Dumesic.22,23 On
3.4 mmol Yb/SiO , pyridine adsorption gave three distinct
2
bands bonded to a Lewis site at 1448, 1492 and 1608 cm~1.
The spectrum on Yb O displayed Ðve bands at 1442, 1474,
2
3
1488, 1574 and 1602 cm~1, and a similar spectrum of
adsorbed pyridine was obtained on La O .24 Bands at 1442,
2
3
1488 and 1602 were attributed to pyridine adsorbed on a
Lewis acid site (L-pyridine). The residual two peaks at 1474
and 1574 were due to carboxylate species formed as cracking
products.24 In contrast, only hydrogen-bonded pyridine (H-
pyridine) was observed on SiO pretreated at 1073 K.
2
The 19b mode of L-pyridine on Yb/SiO (1448 cm~1) and
2
19b mode of H-pyridine on SiO (1446 cm~1) gave quite
2
Table 3 Results of a-pinene isomerization over supported ytterbium
oxides and the physical properties of catalyst samples pretreated at
1073 K
Supporta
Conversionb (%)
S
/m2 g~1
Phase
BET
SiO
2
MgO
26.5
Trc
Trc
Trc
Trc
180
76
39
25
16
Amorphous
c-Al O
c-Al O , Yb O
2
3
2
3
2
3
3
MgO, Yb O
2
c-ZrO ss d
2
ZrO
2
(Yb O )
Yb O
2
3
2 3
a Loading amount of Yb; 3.4 mmol g (support)~1. b a-Pinene, 2 mL;
catalyst, 50 mg; pretreatment temperature, 1073 K; reaction tem-
perature, 323 K; reaction time; 3 h. c Traces. d Cubic-ZrO solid solu-
tion.
2
Fig. 1 Cu-Ka XRD patterns of 3.4 mmol Ln/SiO pretreated at
2
1073 K.
Phys. Chem. Chem. Phys., 1999, 1, 2841È2849
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from Ce/SiO was due to the CeO crystal. An evident di†er-
2
2
ence in acidic properties of Ln/SiO was not found from TPD
2
experiments. It clearly shows that part of the pyridine adsorp-
tion sites catalyze a-pinene isomerization, the ratio to total
amounts of which we were not able to estimate.
XANES
Fig. 4 shows X-ray absorption near-edge structure (XANES)
spectra of rare earth oxides and 3.4 mmol Ln/SiO catalysts
2
pretreated at 1073 K. In the L -edge XANES spectra, a
III
strong white line due to the atomic like transition of 2pÈ5d
appeared. XANES spectra of CeO and Tb O exhibit
2
4 7
doublet white lines due to the coexistence of Ln3` and
Ln4`.27 The ground state of CeO should be described by a
2
mixing of two electron conÐgurations (4f )0 and (4f )1L, where
L indicates a hole in the oxygen 2p valence bond,28 the same
spectrum was recorded by other research groups.27h29 Those
of the other rare earth oxides exhibit a single white line. A
Fig. 2 FTIR spectra of adsorbed pyridine at 423 K: 3.4 mmol
Yb/SiO (46 mg) (a), Yb O (79 mg) (b), and SiO (42 mg) (c) pretreat-
ed at 1073 K.
double white line was observed for that of Ce/SiO , which
2
2
3
2
2
was identical to that of CeO . In contrast, single white lines
2
were observed for those of other Ln/SiO samples. The single
2
white lines and the photon energies of the Ln/SiO catalysts
and 1598 cm~1.22 Therefore, we conclude that Yb/SiO pos-
2
indicate that all Ln species supported on SiO were trivalent.
2
2
sesses Lewis acid sites, and pretreated SiO has no acid sites
2
Terbium atoms supported on silica were trivalent only,
to give the L-pyridine. Yb O possesses Lewis acid sites of
2
3
although the average composition of terbium oxide obtained
almost as much as 3.4 mmol Yb/SiO , however, the acid sites
2
by calcination of terbium salts at 1073 K is Tb 3`Tb 4`O .30
do not catalyze a-pinene isomerization at 353 K.
2
2
7
The height of the white line for each supported rare earth
oxide is higher than that of each rare earth oxide crystal, as
well as in the case of Yb/SiO ,10 Ln/Al O (Ln \ La, Sm,
TPD
2
2 3
Fig. 3 shows pyridine TPD proÐles of catalysts pretreated at
1073 K. The pyridine adsorption procedure was carried out at
Lu),31,32 and Si : Er O .33
2
3
473 K to avoid e†ects of physisorption. SiO seldom desorbed
2
pyridine. The proÐles of all Ln/SiO catalysts possessed a
2
single desorption peak around 600 K, indicating that certain
amounts of acid sites exist on the surface of Ln/SiO . In con-
2
trast to the catalytic activity for a-pinene isomerization, the
di†erence in each TPD proÐle among the supported rare
earth elements was relatively small.
The proÐles of Yb/SiO and Tb/SiO were quite similar to
2
2
each other, the tailing of which ranged to slightly higher tem-
peratures than those of the other Ln/SiO . Although catalytic
2
activities of La/SiO and Eu/SiO di†ered by one order of
2
2
magnitude, their TPD proÐles exhibited identical features
above 700 K. The total desorbed amount of pyridine from
La/SiO was 1.5 times larger than that of Pr/SiO . Further-
more, the Ce/SiO catalyst desorbed about half as much pyri-
dine as the Yb/SiO catalyst. It was reported that CeO
possesses Lewis acid sites, which was conÐrmed by the IR
spectra of adsorbed pyridine25 and adsorption micro-
calorimetry of ammonia.26 A large part of desorbed pyridine
2
2
2
2
2
Fig. 3 Pyridine TPD proÐles of 3.4 mmol Ln/SiO and SiO
Fig. 4 XANES spectra of 3.4 mmol Ln/SiO pretreated at 1073 K
2
2
2
pretreated at 1073 K.
(solid curves) and rare earth oxide crystal (broken curves).
2844
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Furthermore, the shapes of YÈK edge XANES spectra of
Y O and Y/SiO are di†erent from each other. If the Y atom
2
3
2
(Y3`; (4d)0) was supported on silica in a tetrahedral coordi-
nation, a pre-edge or shoulder peak due to the 1sÈ4d tran-
sition should be observed, as is the case for Nb34,35 and
Mo.36,37 In the YÈK edge XANES spectra of the catalyst, no
pre-edge and/or shoulder peaks were observed. The coordi-
nation sphere around Y on silica is concluded not to be tetra-
hedral. These results indicate that the geometry and electric
conÐguration of each supported rare earth oxide is di†erent
from that of the rare earth oxide itself.
EXAFS
The extraction of the CeÈL edge EXAFS function of
III
Ce/SiO was not carried out because Ce species on silica
2
consist of tri- and tetravalent cations with di†erent threshold
energy, and the formation of a crystalline CeO phase has
2
already been conÐrmed by XRD characterization. Fig. 5
shows k3-weighted EXAFS spectra of 3.4 mmol Ln/SiO
2
pretreated at 1073 K. YbÈL and YÈK edge EXAFS spectra
III
of the catalysts were quite similar to each other. Because the
phase shift of the K and L edge EXAFS function di†er by p
III
radians,38 those of Y/SiO and Yb/SiO are fundamentally
2
2
the same. This result indicates that local structures around the
Y and Yb atoms of supported catalysts are similar to each
other. The TbÈL edge EXAFS spectrum of the Tb/SiO
III
2
catalyst was quite similar to that of Yb/SiO , while the oscil-
2
lation amplitude of Tb/SiO below k \ 5 Ó~1 was slightly
2
smaller than that of Yb/SiO . With decreasing atomic number
2
from 70Yb to 57La, the complicated oscillations of each
EXAFS spectrum of the catalyst changed. Furthermore, each
amplitude decreased as well. One possible reason for the
decrease in the amplitude is that many kinds of scatters with
di†erent bond length counteract the EXAFS oscillations.
The radial structure functions (RSFs) obtained by Fourier
transformation of k3-weighted EXAFS functions are shown in
Fig. 6. The Fourier-Ðltered range of each EXAFS spectrum
Fig. 6 Fourier transforms for k3-weighted EXAFS spectra of 3.4
mmol Ln/SiO pretreated at 1073 K.
2
di†ers by the element because of the di†erent limitation of
each L -absorption edge. In the RSFs of Y/SiO , Yb/SiO
II
2
2
and Tb/SiO , distinct two peaks were observed around 1.8
2
and 2.8 Ó. In the Yb/SiO catalysts, loading amounts of which
were 280 lmolÈ8.4 mmol g (SiO )~1, we have assigned the
peak around 1.8 Ó due to oxygen, and that around 2.8 Ó due
2
2
to Si atoms of the supports.9,10 Analogous with Yb/SiO ,
2
LnÈOÈSi linkages are present on Tb/SiO and Y/SiO . On
2
2
the other hand, another peak was observed above 3 Ó in the
RSFs of Eu/SiO , Sm/SiO , Pr/SiO , and La/SiO . These
2
2
2
2
distances were similar to the second peaks observed in the
RSFs of rare earth oxide itself. We assigned the peaks
observed in the RSFs of the catalysts above 3 Ó due to
LnÈOÈLn linkages. Therefore, the coexistence of LnÈOÈLn
and LnÈOÈSi linkages with di†erent bond lengths was con-
cluded, especially in light rare earth oxides supported on
silica.
All the samples remained in the amorphous state and each
loading amount of the rare earth element was very large,
30È40 wt.% as Ln O . The complicated EXAFS spectra of
2
3
Ln/SiO catalysts show that the coordination sphere around
2
each Ln atom does not always consist of a single component.
Therefore, we did not carry out curve Ðtting analyses for these
Ln/SiO catalysts.
2
TG-DTA
The preparation of each catalyst was carried out accompanied
by calcination of the Ln(NO ) ÈSiO precursor at 773 K. We
3 3
2
have concluded from XAFS analysis that a strong interaction
between Yb and Si atoms, which are the active sites for a-
pinene isomerization, had formed in the sample preparation
step.10 The transformation of each precursor upon thermal
decomposition was examined with TG-DTA up to 1200 K. If
Fig. 5 k3-weighted EXAFS spectra of 3.4 mmol Ln/SiO pretreated
2
at 1073 K.
Phys. Chem. Chem. Phys., 1999, 1, 2841È2849
2845

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some crystalline phase formed or phase transition was
brought about upon thermal treatment, exothermic or endo-
thermic heat Ñow should be observed. Fig. 7 shows TG-DTA
proÐles of Yb(NO ) É xH O, non-calcined 3.4 mmol Yb/SiO ,
3 3
2
2
SiO and Si(OH) . Yb(NO ) É xH O decomposed to Yb O
2
4
3 3
2
2 3
over many steps, including ytterbium hydroxide nitrate
salts.39 SiO and Si(OH) desorbed a large amount of water
2
4
below 373 K, and then continuously released water with
ramping temperatures. The exothermic peak was not observed
in the DTA proÐle of SiO , indicating crystallization did not
2
occur below 1200 K. The TG-DTA proÐle of non-calcined
Yb/SiO was quite di†erent from those of Yb(NO ) É xH O,
2
3 3
2
Si(OH) and SiO . In contrast to the confusing DTA proÐle
4
2
of Yb(NO ) É xH O, that of the non-calcined sample was
3 3
2
uncomplicated in the temperature range of 473È773 K. This
result shows that the decomposition mechanism of the ytter-
bium nitrate salt on silica was di†erent from the nitrate salt
only. Any signiÐcant heat Ñow was not observed in the proÐle
of Yb/SiO sample up to 1200 K. The DTA characterization
suggests that some ytterbium species, which are di†erent from
2
Fig. 8 Raman spectra of Yb O , 8.4 mmol Yb/SiO , 3.4 mmol
2
3
2
Yb/SiO , and SiO .
Yb O crystal, had formed during the catalyst sample prep-
2
2
2
3
aration step. In addition, excess loading amounts of non-
calcined sample (15 mmol Yb loading g (SiO )~1) exhibited a
2
similar DTA proÐle to that of the Yb(NO ) salt.
3 3
shows laser Raman spectra of SiO , Yb O , 3.4 mmol
2
2 3
Yb/SiO and 8.4 mmol Yb/SiO . The spectrum of Yb O
2
2
2 3
exhibited strong Raman bands at 360 and 612 cm~1, and a
Raman spectra
small band at 310 cm~1. That of SiO gel exhibits character-
2
To clarify the structure of each catalyst in more detail, Raman
istic Raman bands at 490 and 605 cm~1.40,41 The band at 977
spectroscopy was applied to the Yb/SiO catalysts. Fig. 8
cm~1 is due to silanol groups.42 The Raman spectrum of 3.4
2
mmol Yb/SiO was quite similar to that of SiO gel, and
2
2
exhibited no other bands except for those due to SiO . The
lack of characteristic Raman bands due to crystalline oxides
2
indicates that the metal oxides were well dispersed and that
no crystallites formed on the support.43 Raman spectroscopic
characterization shows that the crystalline phase of Yb O
2
3
was not present on 3.4 mmol Yb/SiO . In the spectrum of 8.4
2
mmol Yb/SiO , a tiny but distinct peak appeared at 364
cm~1 marked by an arrow, besides the peaks due to SiO .
This tiny peak was assigned to the Yb O crystal. Although
the YbÈL edge EXAFS spectrum of 8.4 mmol Yb/SiO did
not indicate any presence of YbÈOÈYb bonding,9 quite a
small part of the Yb species on 8.4 mmol Yb/SiO form the
Yb O phase. The loading amount of Yb on 8.4 mmol
Yb/SiO is very high, 62 wt.% Yb O . Because EXAFS char-
acterization gave the averaged information against a target
element, trace amounts of Ðne Yb O crystallite could not be
detected by EXAFS analysis. We have proposed that a large
2
2
2
3
III
2
2
2
3
2
2 3
2
3
part of the Yb species in 8.4 mmol Yb/SiO constitute
2
YbÈOÈSi linkages, and the trace residues deposit over
YbÈOÈSi linkages to reduce the catalytic activity.9 The depos-
ited excess Yb species, which were YbO , formed a Ðne
6
Yb O crystallite and was detected with Raman spectroscopy.
2
3
To examine the structural transformation of catalysts upon
pretreatment temperature, in-situ Raman spectra of 3.4 mmol
Yb/SiO pretreated at various temperatures were recorded.
2
The results are shown in Fig. 9. The sample pretreated below
1073 K exhibited an identical spectrum to that of SiO gel,
2
while many Raman bands appeared on the sample pretreated
at 1273 K. The new band positions were quite di†erent from
those of the Yb O crystal. One possible assignment of them
2
3
is due to formation of crystalline SiO polymorphism. In the
2
temperature range 1140È1743 K, the stable phase of SiO is
2
tridymite.44 It is well known that the phase transition of SiO
2
polymorphism occurs rapidly and reversibly.44 We did not
quench the sample immediately after the pretreatment pro-
cedure, and we measured the Raman spectra at room tem-
perature. If this was the case, the observed crystalline phase of
SiO should be a-quartz. However, there were only two
2
observed Raman bands for a-quartz (Nacalai, GR) at 206 and
Fig. 7 TG-DTA proÐles of Yb(NO ) É xH O, non-calcined 3.4
3 3
2
mmol Yb/SiO , Si(OH) and SiO .
464 cm~1 in the range 200È1000 cm~1. It was concluded that
2
4
2
2846
Phys. Chem. Chem. Phys., 1999, 1, 2841È2849

View Article Online
Yb/SiO catalysts. The di†erence in TPD proÐle among sup-
2
ported rare earth elements was small, and the existence of acid
sites on Ce/SiO was demonstrated. The pyridine TPD pro-
2
Ðles and IR spectra of adsorbed pyridine gave information not
only about the real active sites, but also pyridine adsorption
sites which did not participate in catalyses. It is impossible to
distinguish them. Pyridine desorption sites do not always
directly reÑect on the real active sites for catalyses. From TPD
measurements, we could not Ðnd out the e†ective di†erence in
acid strengths and the amounts among Ln/SiO catalysts.
2
XRD characterization shows that all of the Ln/SiO cata-
2
lysts are amorphous, except for Ce/SiO . It was reported that
2
the monolayer structures of transition metal oxides hardly
formed on SiO with a simple impregnation method, and the
2
oxides were easily crystallized.43,48 However, we could
prepare amorphous silica-supported rare earth oxides by
impregnation with nitrate aqueous solutions, followed by cal-
cination. There are some reports that X-ray amorphous rare
earth oxides supported on silica are prepared with the same
preparation methods, such as 22 wt.% La O /SiO 49 and 31
Fig. 9 Raman spectra of 3.4 mmol Yb/SiO and SiO pretreated at
2
2
2
3
2
various temperatures. \: laser plasma lines.
wt.% Eu O /SiO .7 Rare earth oxides easily spread on the
2
3
2
SiO surface, in contrast to other transition metal oxides.
2
crystalline SiO polymorphism had not formed on Yb/SiO
Only ceria hardly spread on silica, and the crystalline phase
2
2
pretreated at 1273 K. The other possible assignment of the
new Raman bands is due to the formation of crystalline ytter-
bium silicates. As shown in the phase diagram of the
Yb O ÈSiO system Yb O É SiO and Yb O É 2SiO are
was detected even at a low loading amount of 5.8 wt.%.50
In the Yb/SiO system, the loading amount of which ranged
2
from 280 lmolÈ8.4 mmol Yb atom
g
(SiO )~1, each
2
YbÈL edge EXAFS spectra were identical and no YbÈOÈYb
contributions were observed.9 We have concluded that ytter-
bium atoms were widely spread on the SiO surface as a
YbO octahedron. Each YbO octahedron strongly interacted
with SiO tetrahedra, rather than with other YbO octahedra.
The YbÈOÈSi species exhibit solid acidity to catalyze a-pinene
2
3
2
2
3
2
2
3
2
III
stable binary oxides.45 Although we have not prepared ytter-
bium silicate crystals and do not have any information about
their Raman spectra, we presume that Ðne crystalline particles
of Yb O É 2SiO and/or Yb O É SiO formed on 3.4 mmol
2
6
6
2
3
2
2
3
2
4
6
Yb/SiO pretreated at 1273 K. The size and proportion of
YbÈsilicate crystallite to the total contents of Yb were too
2
isomerization.9,10 In a series of Ln/SiO catalysts, the selec-
2
small to detect by XRD and EXAFS measurements.
tivity for various products from a-pinene was similar to each
other. The di†erence in acidity of each catalyst inÑuences the
selectivity of the produced compounds from a-pinene. Thus,
we propose that the structures of active sites of all the
Discussion
In a series of lanthanoid oxides supported on silica, Yb/SiO
exhibited the highest activity for a-pinene isomerization. The
Ln/SiO catalysts are LnÈOÈSi species with similar acidity.
2
2
In the case of other Ln/SiO catalysts, EXAFS spectra indi-
2
catalytic activities of Yb/SiO and La/SiO were quite di†er-
cate that a contribution of LnÈOÈLn appeared, besides that of
2
2
ent from each other by Ðfty times. The activities changed by
the loaded elements in the order of atomic number, but the
selectivities were almost the same for all the catalysts. It is
very interesting that Yb/SiO and Y/SiO exhibited not only
LnÈOÈSi. Especially in case of the light rare earth elements,
LnÈOÈSi contribution was small. The decrease of LnÈOÈSi
contribution and the increase of LnÈOÈLn contribution
directly related to the catalytic performance. The LnO poly-
2
2
n
similar catalytic activities but also similar selectivities. The
hedron connected with other SiO tetrahedra and/or LnO
4
n
selectivity for b-pinene was relatively high over La/SiO and
polyhedra. In the case of the heavy rare earth element of Yb,
almost all of the YbO units strongly connected with SiO
2
Pr/SiO catalysts when the reaction was carried out at 323 K.
2
6
4
The selective isomerization of a-pinene toward b-pinene is one
tetrahedra only. In the case of the light rare earth element of
La, LaO units connected with other LaO and SiO units.
of the characteristic base-catalyzed reactions and other com-
pounds hardly formed over a solid base.46 However, the for-
mation of b-pinene is catalyzed by both acids and
bases.15,46,47 The equilibrium constant of b-pinene to a-
pinene is quite low and produced b-pinene converting to a-
pinene again. The selectivity of b-pinene decreased with
decreasing total concentration of a-pinene in the reaction
system. The high selectivity for b-pinene observed in the cases
of La/SiO and Pr/SiO was due to the low conversion. In
n
n
4
The fraction di†ered in order of atomic number of each
element. In the case of CeO , the interaction among SiO
2
4
tetrahedra and CeO polyhedra are particularly weak. We
conclude that the Ce/SiO catalyst is the mixture of CeO and
SiO . The low affinity among ceria and SiO might be due to
the high redox ability of CeO . It was reported that Ce sili-
cate formed only if Ce/SiO was reduced by hydrogen at 1073
K in the presence of Rh.51 As a result, the Ce/SiO catalyst
was inert for the acid catalyzed reaction, and the speciÐc
n
2
2
2
2
2
2
2
2
2
fact, identical selectivity was obtained over La/SiO and
2
Pr/SiO when each reaction was performed at 353 K. The
2
surface area was much larger than the others, especially in the
similarity of each selectivity indicates that the properties and
surface area of g (SiO )~1 carrier.
2
structures of active sites are similar among all of the Ln/SiO
catalysts.
Quite similar EXAFS spectra were observed at the YÈK
2
and YbÈL edge. As shown in Fig. 6, YÈOÈSi contributions
III
Pyridine and/or ammonia adsorption experiments are
general methods to characterize acid properties of catalysts.
As shown in Figs. 2 and 3, however, no distinct di†erence in
were present in the Y/SiO catalyst, whereas YÈOÈY contri-
2
butions were absent. The similarity of the YÈK edge EXAFS
spectrum of Y/SiO to that of Yb/SiO revealed that all of the
2
2
acid properties of Ln/SiO catalysts was obtained by charac-
Y atoms located at a center of oxygen-octahedron connected
2
terization with pyridine. Ce/SiO and Yb O did not catalyze
with SiO tetrahedra by sharing oxygen.
2
2
3
4
a-pinene isomerization very e†ectively. On the other hand, IR
Two reasons why the catalysis for a-pinene isomerization
spectra of adsorbed pyridine showed that Yb O possesses
certain amounts of Lewis acid sites, as much as 3.4 mmol
changes continually with atomic number are considered. The
Ðrst possible reason is concerning electronegativity. Tanabe et
2
3
Phys. Chem. Chem. Phys., 1999, 1, 2841È2849
2847

View Article Online
exhibited the same activity as Yb/SiO . All the Ln/SiO cata-
al. found the correlation between maximum acid strengths of
binary metal oxides and the averaged electronegativities of
each ion.6,52 Maximum acid strengths of La O ÈSiO and
2
2
lysts, except for Ce/SiO , are amorphous, and form LnÈOÈSi
2
and/or LnÈOÈLn linkages with di†erent compositions in each
2
3
2
Y O ÈSiO have been estimated to H O [4.0 and H O
catalyst. The affinity among the LnO units and SiO tetra-
2
3
2
0
0
n
4
[5.6, respectively.6,52 The acid strength of 3.4 mmol Yb/SiO
hedra depends on the size of Ln3`. With increasing ratio of
LnÈOÈSi to LnÈOÈLn linkages, the catalytic activity
increases.
2
was evaluated with Hammet indicators as [3.0 O H (max)
\ [5.6. Various deÐnitions of electronegativity have been
0
proposed, such as Pauling, Mulliken, Allred and Rochow,
Sanderson, and so on. Because all electronegativities of the
rare earth elements are quite similar to each other, the contin-
uous and drastic change in the catalytic activity could not be
explained by electronegativity e†ects. This was supported by
the result that each selectivity of produced compounds from
a-pinene was independent of the supported elements.
Excess loading of Yb atoms to SiO blocks the YbÈOÈSi
2
linkages of the active site to form Yb O Ðne particles. When
2
3
Yb/SiO was pretreated at 1273 K, Ðne ytterbium silicate
crystallites formed and the crystalline ytterbium silicate did
2
not exhibit catalysis. The LnÈOÈSi linkage in Ln/SiO having
2
no long-ranged ordering structure is the active site for a-
pinene isomerization.
The other possible reason for this continuous change in
catalyses is the e†ect of ion radius. It is well known that the
ion radius of lanthanides continually decrease with atomic
number from 57La to 71Lu. The largest ion radius of the
lanthanoid ion employed in this work is 1.061 Ó for La3`, and
the smallest one is 0.858 Ó for Yb3`.53 The second smallest
lanthanide ion used in the present study is 0.923 Ó for Tb3`.
The size of Y3` is 0.88 Ó, which is close to that of Yb3`. It
clearly shows that a close relationship between the ion radius
of Ln3` and the catalytic activity exists. It is obvious that the
Acknowledgements
We thank Drs S. Emura (Osaka University), H. Tanida (Japan
Synchrotron Radiation Research Institute, SPring-8) and T.
Uruga (Japan Synchrotron Radiation Research Institute,
SPring-8) for making the XAFS beam line of SPring-8
BL01B1 available. The X-ray absorption experiments have
been performed under the approval of the JASRI (Proposal
No. 1998A0258). This work was partially supported by a
grant-in-aid (No. 08405052) from the Japan Ministry of Edu-
cation, Science, Sports, and Culture.
size of the LnO unit would change with the center element.
n
We propose that a suitable size of LnO unit strongly inter-
n
acts with SiO units rather than the other LnO units. As a
4
n
result, remarkable catalytic activity for a-pinene isomerization
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