High catalytic efficiency of nanostructured molybdenum trioxide in the benzylation of arenes and an investigation of the reaction mechanism

Article abstract of DOI:10.1002/chem.200801153

The synthesis and characterization of nanostructured MoO₃ with a thickness of about 30 nm and a width of about 450 nm are reported. The composition formula of the MP (precipitation method) precursor was estimated to be [(NH₄)₂O]_0.169·MoO ₃· (H₂O)_0.239. The calcination of the precursor in air afforded nanostructured pellets of the α-MoO₃ phase. The nano-structured MoO₃ catalyst exhibited high efficiency in catalyzing the benzylation of various arenes with substituted benzyl alcohols, which were strikingly different to common bulk MoO₃. Most reactions offered >99% conversion and >99% selectivity to monoalkylated compounds. MoO₃ is a typical acid catalyst. However, the benzylation reaction over nanostructured MoO₃ does not belong to the acid-catalyzed type or defect site-catalyzed type, since the catalyst has no acidity and defect site on surface. Characterization with thermal, spectroscopic, and electronic techniques reveal that the catalyst contains fully oxygen-coordinated MoO ₆ octahedrons on the surface but partially reduced species (Mo ⁵⁺) within the bulk phase. The terminal oxygen atoms of Mo=O bonds on the (010) basal plane resemble oxygen anion radicals and act as active sites for the adsorption and activation of benzyl alcohols by electrophilic attack. Such sites are indispensable for catalytic reactions since the blocking of these sites by electron acceptors, such as tetracyanoethylene (TCNE), can greatly decrease catalytic activity. This work represents a successful example of combining a heterogeneous catalysis study with nanomaterial synthesis.

Full text of DOI:10.1002/chem.200801153

DOI: 10.1002/chem.200801153
High Catalytic Efficiency of Nanostructured Molybdenum Trioxide in the
Benzylation of Arenes and an Investigation of the Reaction Mechanism
Feng Wang*[a, b] and Wataru Ueda*[b]
Abstract: The synthesis and characteri-
zation of nanostructured MoO3 with a
thickness of about 30 nm and a width
of about 450 nm are reported. The
composition formula of the MP (pre-
cipitation method) precursor was esti-
mated to be [(NH4)2O]0.169·MoO3·
(H2O)0.239. The calcination of the pre-
cursor in air afforded nanostructured
pellets of the a-MoO3 phase. The nano-
structured MoO3 catalyst exhibited
high efficiency in catalyzing the benzy-
lation of various arenes with substitut-
ed benzyl alcohols, which were strik-
ingly different to common bulk MoO3.
Most reactions offered >99% conver-
sion and >99% selectivity to mono-
G
ly reduced species (Mo5+) within the
bulk phase. The terminal oxygen atoms
of Mo=O bonds on the (010) basal
plane resemble oxygen anion radicals
and act as active sites for the adsorp-
tion and activation of benzyl alcohols
by electrophilic attack. Such sites are
indispensable for catalytic reactions
since the blocking of these sites by
electron acceptors, such as tetracyano-
ethylene (TCNE), can greatly decrease
catalytic activity. This work represents
a successful example of combining a
heterogeneous catalysis study with
nanomaterial synthesis.
Introduction
niques has been developed to control the architectures and
morphological patterns of molybdenum oxide. Examples in-
clude hydrothermal synthesis, template or host molecule-in-
volved synthesis, and electrochemical processes. The mor-
phologies of rods,[2–4] wires,[5] tubes,[6] belts,[7,8] ribbons,[9]
rings,[10] have been prepared. However, little attention was
paid to catalytic performance. A search we undertook
through the SciFinder Scholar database (keyword: molybde-
num oxide) returned more than 20000 papers, most of
which were devoted to material synthesis. Regarding, the
surface science, research on the structure of bulk MoO3 with
perfect crystalline phase by microscopy,[11] spectroscopy,[12,13]
and calculations[14–17] has provided significant insight into the
atomic surface chemistry governing the catalytic activity.
Bulk MoO3 is known to be catalytically inactive. Much re-
search has focused on Mo-based complex oxide catalysts. In
combination with other elements, such as bismuth, vanadi-
um, cobalt, or aluminum, Mo-based catalysts can be active
and selective for many reactions. Selective oxidation,[18–21]
hydrogenation, and dehydrogenation[22–24] are a few exam-
ples of important applications of these catalysts. The general
character of Mo-catalyzed reactions is that certain reaction
Molybdenum oxide is widely used in catalysts, sensors, lubri-
cants, and fuel cell materials. Specifically, the catalysis of
MoO3 greatly relies on the crystal phase, surface structure,
and particle size. To develop a highly active and selective
catalyst, it is necessary to couple the synthesis of a nano-
structured material with the understanding of material sur-
face structures, like local geometry and electronic properties
of active sites.[1] Regarding the synthesis, a variety of tech-
[a] Dr. F. Wang
CREST, Japan Science and Technology Corporation (CREST-JST)
Kawaguchi, Saitama 332–0012 (Japan)
Fax : (+81)11-706-9164
E-mail: wangfeng@cat.hokudai.ac.jp
[b] Dr. F. Wang, Prof. Dr. W. Ueda
Catalysis Research Center, Hokkaido University, N-21, W-10
Sapporo 001–0021 (Japan)
Fax : (+81)11-706-9164
E-mail: ueda@cat.hokudai.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801153.
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FULL PAPER
steps need oxygen species from the catalyst surface or the
transfer of hydrogen species to the catalyst surface.[25] In the
complicated system, the supports or co-catalysts play a key
role in providing high specific surface area and additional
acidity or basicity. Thus, the real role of MoOx in catalysis is
difficult to investigate. Furthermore, it is known that MoO3
can exhibit pronounced crystallographic anisotropy. As a
result, different exposed crystal facets have different catalyt-
ic behaviors (so-called structure sensitivity).[26] Despite
many efforts in experiments and calculations, the active
facet of MoO3 still remains debatable.[27,28] Therefore, pure
nanostructured MoO3 is more favorable than complex
oxides for studying active site and sensitive facets given that
MoO3 is active and selective.
high efficient at catalyzing the benzylation of various arenes
with substituted benzyl alcohols. Most reactions offer
>99% conversion and >99% selectivity to monoalkylated
compounds. More interestingly, the reaction is not catalyzed
by defect sites, since the surface is defect-free. Moreover,
the reaction is not catalyzed by acid sites, since nanostruc-
tured MoO3 contains no acid sites. We will show that the ter-
minal oxygen of the Mo=O bond on the (010) basal plane,
which resembles an anion radical, is the active site for the
adsorption and activation of substrate. Although the cataly-
sis of an oxygen anion radical has been previously discussed
on complex oxides,[48] such phenomena and the excellent ef-
ficiency in catalysis is completely new for a nanostructured
material like MoO3. The present research will contribute
considerably to the understanding of the surface of the
nanomaterial and its heterogeneous catalysis.
Nanostructured oxides, compared to their bulk counter-
parts, can exhibit unexpected catalysis. Some examples are
[36]
MgO,[29–33] CeO2,[34] TiO2,[35] and WO3. The active site of
these catalysts is typically an oxygen vacancy or a defect
center. To the best of our knowledge, there has been no
report on utilizing nanostructured MoO3 in a catalytic reac-
tion. Recently, we found that the nanostructured MoO3 con-
sisting of unsupported platelets exhibits excellent catalytic
activities in benzylation reactions.[37] By treating benzyl alco-
hol in toluene with a catalytic amount of nanostructured
MoO3, we obtained benzyl toluene isomers in a quantitative
yield. Benzylation is a traditional and very important Frie-
del–Crafts alkylation reaction and represents a satisfactory
route to synthesize diphenylmethane derivatives [Eq. (1)].[38]
In comparison with processes using H2SO4, AlCl3, metal
complexes,[39–41] supported solid acids,[42,43] metal ion-incor-
porated zeolite catalysts,[44] complex oxides,[45,46] and Nafion
membranes,[47] our method possesses the advantages of
facile catalyst preparation, excellent catalytic performances
and easy catalyst recycling and reuse.
Results and Discussion
Several MoO3 catalysts were prepared and these samples
are designated as follows: MP, precipitation method; MH,
hydrothermal method; MM, calcination of molybdate salt;
and MC, commercial product. All catalysts were tested in
the benzylation of toluene with benzyl alcohol. The results
of the reactions are listed in Table 1. The reaction does not
occur when the catalyst is not present (Table 1, entry 1). The
commercial MoO3 (MC-MoO3, Table 1, entry 2), the MM
(Table 1, entry 3) and the MH (Table 1, entry 4) catalyzed
the reaction to give the desired product, but also produced
benzyl diether (BDE) as a byproduct. In contrast, the MP
catalyst gave the best results (Table 1, entries 5, 6), with
Herein, we will demonstrate that nanostructured MoO3
containing oxygen-saturated MoO6 octahedrons on the sur-
face, which is strikingly different to common bulk MoO3, is
Table 1. Alkylation of toluene over molybdenum oxides.[a]
Entry
Cat[b]
Color
Crystal
S[d]
[m2 gÀ1
Crystallite size [nm][e]
t
Conv.
[%][f]
Major product
Sel. [%][g] IY [%][h]
BDE
[%][i]
system[c]
G
]
020
110
021
ACHTUNGTNER[NUNG min]
1
2
3
4
5
6
7
8
9
10
11
12
13
–
–
–
–
–
–
–
30
30
40
30
10
20
30
30
20
30
40
60
0
54
90
82
76
>99
>99
>99
>99
0
–
–
–
–
–
–
95
–
93
92
–
–
26
10
5
0
0
0
0
0
–
MC-MoO3
MM
MH
light gray
light gray
white
orth
orth
orth
orth
2.9
6.5
7.7
94.7
48.7
44.3
41.0
91.6
29.1
26.8
24.3
102.0
81.9
78.4
70.7
44:9: 47
44:9:47
44:14:42
44:10:46
44:10:46
44:10:46
44:10:46
44:8:48
–
MP
light blue
14.6
[j]
[k]
MP-NC
MM-m15
b-MoO3
dark blue
light gray
bright yellow
brown
orth + mono
orth
mono
17.0
20.5
21.5
2.0
35.6
34.0
–
20.2
18.5
–
48.1
45.6
–
98
0
11
46:7:47
–
48:6:46
–
–
–
42
–
55
MC-MoO2
mono
–
–
–
260
[a] Alcohol 0.24 mL, arene 15.0 mL, hexadecane (internal standard) 0.1 mL, reaction temperature 1108C, catalyst 0.3 g, Ar protection. [b] MP: precipita-
tion method; MH: hydrothermal method; MM: calcination of molydate salt; MC: commercial product. [c] orth: orthorhombic; mono: monoclinic.
[d] Specific surface area by N2 adsorption (BET method). [e] Calculated by Sherrerꢁs formula. [f] GC conversion of benzyl alcohol. Isomeric ratio of
o:m:p. [h] Yield of isolated product. [i] The selectivity of benzyl diether determined by the integration of GC peak area. [j] Catalyst results over different
batch of catalyst MP. [k] PhCH2Cl was alkylating agent.
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F. Wang and W. Ueda
>99% conversion of benzyl alcohol and a 95% yield of the
isolated alkylated product in 20 min at 1108C (heated to
reflux in toluene). The regioselectivity was maintained at
44:10:46 (ortho:meta:para). Extending the reaction time to
30 min did not lead to any byproduct (Table 1, entry 7), such
as polymer.[49] Different batches of catalysts gave very simi-
lar results (Table 1, entry 8), suggesting that the catalyst
preparation can be reproduced. Similar good results employ-
ing benzyl chloride as the alkylating agent to benzyl alcohol
were achieved over the MP catalyst (Table 1, entry 9). Be-
cause of less poisoning and easy handling, benzyl alcohol
was preferred to benzyl chloride in this research. The
sample calcined in a N2 atmosphere (MP-NC, Table 1,
entry 10) was totally inactive in the 30 min reaction. The
milling of MM, intended to increase the surface area of
MM, resulted in higher conversion but worse selectivity
compared to unmilled MM (Table 1, entry 11). Although b-
MoO3 was claimed to be an active catalyst in selective oxi-
dation, the catalyst was totally inactive in alkylation
(Table 1, entry 12). The catalysis over MoO2 is sluggish and
less selective (Table 1, entry 13).
Figure 1. Catalyst separation at 10 min and triggering at 25 min.
finished in 5–10 min at a slightly higher temperature with
93–95% isolated yield. The benzylation of nonactivated
arenes such as benzene (Table 2, entry 6, 94% yield) and
naphthalene (Table 2, entry 7, 85% yield) proceeded
smoothly. The more challenging reaction of the benzylation
of acetophenone (Table 2, entry 8) gave the corresponding
product in 15% yield with 88% selectivity for the para
isomer. We noted that substituent effects greatly influenced
the reactivity of arenes. For example, the reaction rate of
benzyl alcohol with toluene was 12 times faster than that
with acetophenone. Trifluoromethyl benzene, the least
After the reaction, the MP catalyst was filtered off,
washed with toluene, and reused several times. As a result,
the conversions and product distributions over the reused
catalyst were comparable to fresh MP (Figure S1 in the Sup-
porting Information). Removal of the MP catalyst in the
course of the reaction completely stopped the conversion of
benzyl alcohol, and the addition of the catalyst clearly trig-
gered the reaction (Figure 1),
suggesting that the reaction was
heterogeneously catalyzed. The
inductively couple plasma
Table 2. Reaction of benzyl alcohol with different arenes.[a]
Entry Arene
Major product
T [8C] t [min] Conv. [%] Sel. [%]
IY [%][b]
95
(ICP) mass analysis revealed
that no Mo species leached into
the reaction mixture.
1
2a 110
2b 110
2c 135
20
20
10
>99
>99
>99
47:9:44 (p:m:o)
51:49 (p:o)
>99
2
3
94
93
Having
established
the
method of the benzylation of
toluene, we were interested in
the alkylation of other substi-
tuted arenes with benzyl alco-
hol (Table 2) over the MP cata-
lyst. As expected, a variety of
electron-rich arenes, such as
anisole (Table 2, entry 2, yield
4
2d 135
2e 135
5
>99
46:12:42 (p:m:o) 93
5
6
5
>99
>99
>99
>99
95
94
94%),
p-xylene
(Table 2,
2 f
80
90
entry 3, yield 93%), ethylben-
zene (Table 2, entry 4, yield
93%), and mesitylene (Table 2,
entry 5, yield 95%) gave the
7[c]
2g 100
120
95
74:26 (1-:2-)
85
corresponding
benzylated
arenes in excellent yield. Due
to the activated benzene ring in
anisole, the benzylating reac-
tion primarily occurred in para
and ortho positions to the me-
thoxy group. For entries 3–5 in
8
9
2h 110
2i 110
60
25
0
88:12 (p:o)
15
–
120
0
[a] Reaction condition, see Table 1, footnote a. [b] Yield of isolated product. [c] 0.78 mmol naphthalene,
Table 2, these reactions were 26 mmol benzyl alcohol, 2.7 mL hexadecane, 30 mg catalyst, heated to reflux in 2 mL 1,4-dioxane.
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Nanostructured Molybdenum Trioxide
FULL PAPER
active species we studied (Table 2, entry 9), did not react
with benzyl alcohol in a 120 min run.
Moreover, the MP catalyst could efficiently catalyze intra-
molecular benzylation (Table 3). The five-, six-, and seven-
membered rings were formed by connecting two phenyl
methanol (Table 4, entry 5), quantitative yields of benzylat-
ed arenes were obtained. The benzylation with electron-
poor alcohols also proceeded smoothly by increasing reac-
tion time and temperature (Table 4, entries 6 and 7). More
interestingly, the reaction of chlorobenzyl alcohol with p-
xylene resulted in a spectacular yield (93%). It is worthy
noting that the latter reaction is one of the few successful
Friedel–Crafts benzylation reactions that worked well with
deactivated arenes. However, for the p-CF3-substituted
benzyl alcohol (Table 4, entry 8), the reaction did not take
place under these conditions.
Table 3. Intramolecular benzylation[a]
Arene
Product
t [h]
Conv. [%][b]
40
IY [%]
–
3a
4
Kinetic runs were carried out in the benzylation of tolu-
ene at temperatures ranging from 90 to 1108C. The initial
concentration of benzyl alcohol was 0.15 molLÀ1. The reac-
tion pressure was kept constant at 1 atm. The stirring speed
was over 500 rotations per minute (RPM) to eliminate mass
transfer limitations. The concentration of toluene is consid-
ered to be constant. Thus, the rate equation is simplified as:
10
4
97
88
92
3b
3c
>99
4
42
95
–
Àd
ACHTGNURETNN[UNG alcohol]=kCAHUTTNGRENNUG[cat]ACHUTNGERTG[NNNU alcohol]dt=kobsACHTNUGTRNE[UNGN alcohol]dt. The kobs
10
91
values at 363, 373, and 383 K are based on data from Fig-
ure 2a and are calculated to be 8.4ꢂ10À5, 1.4ꢂ10À4, and
4.8ꢂ10À4 molÀ1 LÀ1 sÀ1, respectively. From the Arrhenius plot
of lnkobs versus 1/T shown in Figure 2b, the activation
energy Ea is evaluated to be 96.1 kJmolÀ1 using the first-
order fit, which is smaller than the value of 106 kJmolÀ1 ob-
tained for the reaction on Nafion-silica composite.[51]
[a] Reaction conditions: substrate 2 mmol, solvent 1,4-dioxane 10 mL,
catalyst 0.3 g, Ar protection, heated to reflux. [b] Determined by
1H NMR spectroscopy.
rings. For instance, treatment of (2-benzylphenyl)methanol
in 1,4-dioxane heated to reflux for 4 h produced 9,10-dihy-
droanthracene (3b) as the single product in 92% yield. Sim-
ilarly, a compound of 9H-fluorene (3a, yield 88%) and
Material synthesis and general characterization: The catalyst
10,11-dihydro-5H-dibenzo
G
precursor precipitated from a solution mixture of acetone
were collected as mono prod-
ucts when employing biphenyl-
2-ylmethanol and (2-phenethyl-
phenyl)methanol as substrates,
respectively. The reaction rate
decreased in the sequence: 3b
> 3c > 3a, which could be ex-
plained by the increasing order
of strain energies (kcalmolÀ1):
Table 4. Reaction of p-xylene with different alcohols.[a]
Entry
Alcohol substrate
Product
T
[8C]
t
Conv.
[%]
Sel.
[%]
IY
[%]
[min]
1
2
4a
4b
135
110
10
15
>99
>99
>99[b]
>99[b]
93
95
cyclohexane (À0.6)
< cyclo-
heptane (5.6) < cylcopentane
3
4
4c
4d
110
110
20
45
>99
>99
>99[b]
>99[b]
95
96
(6.0).[50]
We expected that the MP cat-
alyst could also be applied to
substituted benzylic alcohol as
the alkylation agent. Catalytic
reactions were carried out using
p-xylene and various substitut-
ed benzyl alcohols (Table 4). As
a result, electron-rich alcohols
such as p- and o-methyl benzyl
alcohol (Table 4, entries 2 and
3) afforded the corresponding
benzylated p-xylene in excellent
yield (95%). Even with bulky
substituents at the benzylic
carbon such as 1-phenylethanol
5
6
4e
4 f
110
130
60
98[c]
>99[c]
>99[c]
88
90
180
>99[c]
7
8
4g
4h
130
130
180
180
>99[c]
>99[c]
93
–
0[c]
–
[a] Reaction conditions, see Table 1 footnote a. [b] Determined by GC. [c] Determined by 1H NMR spectros-
(Table 4, entry 4) and diphenyl- copy.
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745

F. Wang and W. Ueda
The ratio of (040) to (021) of
MP is 0.24, which is much
smaller than the 1.62 of MC-
MoO3, suggesting that the an-
AHCTUNGTERGiNNUN sotropic growth of MP is de-
creased and forms a thin pellet
structure.
Table 1 lists several proper-
ties of these oxides. The colors
of the samples are different.
The deep blue color of MP-NC
relative to the light blue color
of MP reflects a phase reduc-
tion during calcination under a
N2 atmosphere. Both MC-
MoO3 and MM have a small
surface area. After millling MM
Figure 2. a) The kinetic analysis of the benzylation of toluene over the MP catalyst. b) Arrhenius plot.
and water has the vibration frequencies at 917, 892, 841,
652, 577, and 478 cmÀ1 (see Figure S2 in the Supporting In-
formation). According to the literature,[52] these bands corre-
6À
spond to the Mo7O24 species. The isopolymolybdate struc-
ture of the heptamolybdate salt was not affected during the
precipitation process since the IR bands in the range of
1000–400 cmÀ1 are very sensitive to a small change in the
cluster structure.[52] Furthermore, there are no products from
acetone, as monitored by using mass spectrometry. Molyb-
dates in aqueous solution form a vast variety of structures
due to the flexibility in the Mo-O-Mo link. In this research,
acetone was used to decrease the outer polarity of the mac-
roanion molybdate clusters, which are highly hydrated and
interconnected by weak hydrogen bonds as proposed by
Mꢃller et al.[53] The interaction balance is disrupted by ace-
tone, and, therefore, the small clusters with acetone and
water periphery are driven to precipitation. As discussed
previously, the direct calcination of the molybdate salt
(NH4)6Mo7O24·4H2O (AHM) does not afford a highly active
catalyst. Thus, we reason that the small clusters are necessa-
ry to obtain the nanostructured and active catalyst. Acetone
is the best solvent for this purpose. We did not obtain any
precipitate by replacing acetone (dielectric constant e=21)
with glycerol (e=46), acetonitrile (e=37), methanol (e=
33), ethanol (e=24), benzyl 2-butanone (e=19), benzyl al-
cohol (e=12), and chloroform (e=10). This may be due to
the fact that the interaction of acetone with water is unique
and of medium strength. Solvents with larger e are miscible
with water and the ones with smaller e are immiscible with
water.
Figure 3. The X-ray diffraction patterns of various molybdenum oxides.
The intensities of MC-MoO3, MH, and MC-MoO2 were multiplied by the
number on the right to give a reasonable scale.
for 15 min, the surface area increased to 20.5 m2 gÀ1. The in-
crease in surface area could explain the higher activity of
MM-m15 and the lower selectivity of alkylated product
(Table 1).[54] The MP-NC has a larger surface area than MP
due to shear reduction.[55] The crystallite sizes are calculated
by Sherrerꢁs formula (Dhkl =0.9l/ACTHUNTGRNEUNG[bcosq]) on three diffrac-
tion peaks of (020), (110), and (021). The size of the (110)
and (020) facets of MP are 26% and 43% of that of MC-
MoO3, respectively. The milling process decreased the crys-
tallite size, as evidenced by comparing MM-m15 (MM after
milling for 15 min) and MM. The surface morphology was
observed by using electron microscopy (see Figure S3 in the
Supporting Information). MC-MoO3, MM, and MP have
pellet structures with an exposed (010) basal plane. Howev-
er, the thickness of MP is about 30 nm (Figure 4), which is
consistent with the X-ray diffraction data. The lateral length
of MP lies within a narrow distribution of 400–500 nm. The
MC-MoO3 is microscale with a broad distribution centering
at between 1.0 and 1.5 mm. (Figure 5).
The calcination of molybdate salt (MM), the commercial
MoO3 (MC-MoO3), and the MP are orthorhombic crystals,
exhibiting the typical (020), (110), (021), and (111) of a-
phase MoO3 (JCPDS 05–0508) (Figure 3). The calcination of
the catalyst precursor under a N2 atmosphere affords MP-
NC with
a reduced phase (monoclinic phase Mo4O11,
JCPDS 65–2473). The orthorhombic phase MoO3 is a lay-
ered structure stacked parallel to the (010) plane, in which
À
each layer is composed of bilayers of Mo O octahedrons.
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Nanostructured Molybdenum Trioxide
FULL PAPER
Figure 4. Representative SEM image of the MP catalyst. Inlet image
shows the thickness of the pellet.
Figure 7. Temperature-programmed decomposition in helium, and mass
spectrometry (TPD-MS) profile of the MP precursor.
is due to NH3, as monitored by m/z 15. The generation of
NH3 is accompanied by the release of water, as illustrated
by the similar onset temperature of water at about 1708C.
We note that the release temperature of NH3 is much lower
than the 3108C of calcining (NH4)6Mo7O24·4H2O (AHM),
suggesting that NH3 was bonded more weakly in the catalyst
precursor than in AHM. The peak at 3058C is attributed to
water. Notably, m/z 32, representative of oxygen, is evolved
at about 4008C. We did not observe the oxygen release
during calcining the AHM salt.
The composition of the precipitate precursor was estimat-
ed by assuming that the composition at 3308C (before re-
leasing oxygen) is equivalent to MoO3. A general formula
Figure 5. The histogram of the lateral length distribution. About 200 par-
ticles were counted along the x and y directions of the SEM images to
give the statistical result.
of ((NH4)2O)x·MoO3·ACHTNUTRGNE(NUG H2O)y proposed by Zeng et al. was
employed.[3] The weight loss at 3308C is 13.1%, which can
be ascribed to the combined loss of adsorbed water (4.3%)
and both NH3 and dehydrated water (8.8%). The calculated
values of x and y are 0.169 and 0.239, respectively. Thus, the
precursor formula can be expressed as ((NH4)2O)0.169·MoO3·-
Thermogravimetric analysis (TGA) of the precipitate and
the catalyst precursor were performed in air. The plot of the
weight loss against temperature is given in Figure 6, which
shows three distinctive peaks at 122, 305, and about 4008C,
and one shoulder at 2358C. On the basis of the tempera-
ture-programmed desorption mass spectrometric (TPD-MS)
profile (Figure 7), no acetone is detected. The peak at about
1228C corresponds to weakly adsorbed water, as represent-
ed by the mass–charge ratio (m/z) of 18. The peak at 2358C
ACHNUTRTGENN(GUN H2O)0.239. Compared to the hydrothermally prepared pre-
[3]
cursor of ((NH4)2O)0.0866·MoO ·
(H2O)0.231
,
the precipitate
precursor contains more interlinked NH3 and water, which
tends to create a larger interval space and generate edges
after releasing NH3 and water in calcination, as illustrated
by the card-house morphology of the MP catalyst (see
Figure 4).
For the MP catalyst, the release of oxygen at about 4008C
in air is a sign of weakly held oxygen species. We reason
that the surface molybdenum octahedron is severely distort-
ed on the nanostructured MoO3. As a result, the strength of
the Mo=O bond on the (010) facet is weakened (see below),
and the oxygen becomes more active compared with an
ideal MoO3 surface.[55] Both TGA and TPD-MS showed
three peaks at 4008C, suggesting terminal, bridging, and
three-coordinate oxygen atoms, which agrees well with the
literature.[15]
The catalytic activities and product selectivities are great-
ly dependent on the calcination temperature of the MP pre-
cursor (Figure 8). The catalyst exhibits low activity from
2508C, reaches a maximum of 19.2 mmolminÀ1 gÀ1 at 3308C,
Figure 6. The TGA and DTG profiles of the precursor of the MP cata-
lyst. Carrier gas was air. The temperature was kept at 4008C until no ob-
vious weight loss was observed.
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F. Wang and W. Ueda
Table 5. Control experiment of the MP catalysts and comparison with
MH catalyst.[a]
Cat
Facet ratio[b] Acidity[c]
S
A
Conv. Sel.
[%]
A
]
[m2 gÀ1
]
[%][d]
010 non-010
AP
BDE
MP
88
12
33
99
ca. 0
11
14.6
23.5
7.7
76
82
28
100
83
97
0
17
3
milled MP[e] 67
MH
1
4
[a] Reaction conditions: alcohol 0.24 mL, arene 15.0 mL, hexadecane (in-
ternal standard) 0.1 mL, reaction temperature 1108C, catalyst 0.3 g, sam-
pling time 10 min, Ar protection. [b] The facet ratio was estimated by
counting about 100 particles in SEM images. [c] The amount of ammonia
desorption. [d] AP: alkylated product; BDE: benzyl diether. [e] The MP
was manually milled for 10 min in a mortar.
Figure 8. The changes in specific activity, product selectivity, and catalyst
acidity as a function of calcination temperature.
of the MP catalyst by ball milling increases the ratio of non-
facet (010) from 12 to 33%. As a result, the milled MP has
an acidity of 11 mmolgÀ1, suggesting that acidic sites are cre-
ated on side facets by milling. The MH is a catalyst aniso-
tropically grown along the [010] direction to form a cuboid
structure 3 mm long and 100 nm thick. The ratio of the (010)
facet of MH is about 1%. Catalytic results show the milling
process increases the conversion from 76 to 82%, due to the
increased surface area. However, the milling process leads
to a decrease in the selectivity for the alkylated product.
About 17% of benzyl diether (BDE) is produced as a by-
product. These results are understandable since the milling
and decreases to 6.25 mmolminÀ1 gÀ1 at 4008C. Prior to
reaching the temperature of 3308C, the product is mono-
ACHTUNGTRENNUNGalkylated toluene. The byproduct of benzyl diether is pro-
duced at 3508C, and reached 33% at 4008C.
Ammonia molecules interact with Lewis (L) acid sites
À
through N metal bonding and with Brønsted (B) sites
through N-H-O bonding on the oxide surface.[56,57] Normally,
bulk MoO3 has an acidity of 20 mmolgÀ1.[46] However, as
shown in Figure 8, in addition to its high specific surface
area, the MP has no acidity (about 0 mmolgÀ1). Compared
to the acidity of MM of 12 mmolgÀ1, it is obvious that the
surface of nanostructured MoO3 is very different from the
surface of bulk MoO3. This also means that the nanostruc-
tured MoO3 contains fully oxygen-coordinated MoO6 octa-
hedrons on the surface, and the benzylation reaction on the
nanostructured MoO3 is not of an acid-catalyzed type.
Under the calcination temperatures of 350 and 4008C, ac-
cording to the results of the TPD-MS, the oxygen vacancies
are created, and correspondingly, the acidity is increased,
and reaches 25 mmolgÀ1 at 4008C. Simultaneously, the selec-
tivity to BDE is increased because the reaction is catalyzed
by acid sites.[49]
À À
process breaks the Mo O Mo bonds on the edge facets,
and creates acidic sites, which catalyze the formation of
BDE.[54] In comparison, MH is much less active than MP.
This also suggests that the benzylation takes place on a
(010) facet.
The technique of electron spin resonance (ESR) is very
sensitive to the electronic structure and the symmetry of the
surroundings of paramagnetic species.[58] Stoichiometric
MoO3 containing Mo6+ has no 4d electrons and is a diamag-
netic insulator. A disturbance in its stoichiometry by the for-
mation of oxygen vacancies causes the generation of 4d1
(Mo5+, S=1/2), which is expected to give an ESR signal.
The ESR spectrum of MM is found to be featureless
(Figure 9), indicating that the valence of molybdenum is 6+.
The MP-250 exhibits a detectable signal at g=1.956, show-
Combining the results of catalytic tests, TGA, TPD-MS,
and the TPD of ammonia (NH3-TPD), we came to the fol-
lowing conclusions: 1) the decomposition of the catalyst pre-
cursor to NH3 and H2O at 2358C does not generate active
sites; 2) the active sites are created at 3308C after elimina-
tion of NH3 and water; 3) the active sites are lost after re-
leasing oxygen, which leads to acid sites; 4) the selective
and active sites on the MP catalyst toward the alkylated
product are non-acidic, and the acidic sites produce
A
ACHTUNGTRENNUNGproducts.
morphology with the most exposed facet as (010).[15] There
have been many disputes concerning the active facet of
MoO3 in selective oxidation.[27,28] However, in benzylation,
we conclude that the active facet of the nanostructured
MoO3 is the basal (010) facet. The edge facets, such as (100)
and (001), are nonselective sites for benzylation. This con-
clusion is based on milling tests and comparison with the
MH catalyst. As shown in Table 5, the mechanical activation
Figure 9. The ESR spectra of MM, MP-250, and MP in a vacuum and
MP in air.
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Nanostructured Molybdenum Trioxide
FULL PAPER
ing the start of the formation of a Mo5+ center. For MP in a
vacuum, the signals appear at g values of 2.001, 1.956, 1.931,
and 1.872. The later g values, which are the most intense sig-
nals, are comparable to those reported for MoO3 single crys-
tals and are characterized as Mo5+ ions in an orthorhombic
distorted, sixfold coordination.[54] The distinctive asymmetry
of these signals indicates severely distorted Mo5+ centers.
The appearance of the signal at g=2.001 is indicative of un-
paired electrons, which are the reason for the blue color of
the MP catalyst. The unpaired electrons may be confined to
oxygen defects, which are located between two molybdenum
metal sites within the bulk phase, similar to F-type defect
centers.[59] Some other examples of blue materials with con-
fined electrons in defects or three-dimensional pores are
nanocrystalline MgO,[60] zeolites,[61] or inorganic electrides.[62]
The signal at g=2.001 did not disappear after exposing the
MP to air, which is evidence that the unpaired electrons are
located within the bulk phase.[63]
tected, which slightly decreases the corresponding binding
energies due to delocalized electrons.
The ultraviolet-visible (UV/Vis) spectra of MM, MP, and
MP-NC are given in Figure 11. The absorption edge energy
(AE) provides an accurate description of the electronic
The X-ray photoelection spectroscopy (XPS) spectra re-
corded on MM and MP are given in Figure 10. The doublets
in the binding energies reflect the Mo oxidation states pres-
Figure 11. Diffuse reflectance UV/Vis spectra of MM, MP, and MP-NC.
All spectra use the Kubelka–Munk formalism and assume indirect ligand
to metal charge transfer (LMCT) transitions. Edge energies (AE) are de-
fined by convention as the x intercept of a linearized near-edge region
for [F(R8)*hn]1/2 as a function of hn, where F(R8) is the Kubelka–Munk
function and hn is the energy of the incident photon.
properties of metal oxides because it corresponds to the
energy needed for electron transfer from the oxygen of an
oxide to the metal site.[65,66] The MM catalyst has an AE
value close to that of common bulk MoO3.[65,67] The blue
MP catalyst is darker than the light gray hue of MM. This is
quantitatively measured by a 0.11 eV smaller AE value than
that of MM. This suggests that the energy required to pro-
mote an electron from the highest occupied molecular orbi-
tal (HOMO) in an oxygen atom to the lowest unoccupied
molecular orbital (LUMO) in the Mo cation of MP decreas-
es. Correspondingly, for the MP catalyst, the backdonation
from an oxygen atom to a Mo atom becomes possible.
These results are also consistent with similar trends ob-
served for dispersed MoOx.[65,68] The absorption peak at
1.3 eV of MM can be ascribed to intervalence charge trans-
fer (IVCT) transitions of Mo6+-O-Mo5+. The blue shift to
1.4 eV and broadened band absorption of MP is a diagnostic
of diminution of particle size as well as the strain in the
oxide lattice induced by severely distorted MoO6 octahe-
drons.[13] This phenomenon was previously observed on a
nanocomposite material.[69,70] No absorption band for IVCT
was observed on MP-NC, suggesting the entire reduction of
the sample.
Figure 10. The XPS spectra of Mo 3d3/2 and Mo 3d5/2 of the MM and MP
catalysts.
ent in these catalysts. The doublets in the binding energies
at 235.6 and 232.4 eV for MM are attributed, respectively, to
the binding energies of the 3d3/2 and 3d5/2 electrons of Mo6+
.
The integral areas between the two doublets are in a ratio
of 2:3, and the energy gap between them is 3.2 eV, in agree-
ment with standard data.[64] In comparison to the MM cata-
lyst, the two peaks of the MP catalyst are shifted slightly
lower in energy (235.4 and 232.1 eV for 3d3/2 and 3d5/2, re-
spectively), which is indicative of an extra electron around
the Mo6+ center, but not a complete reduction to Mo5+. It
is known that an extra electron can lower the inner electron
binding energy of the core metallic cations. Therefore, we
infer that for the MP catalyst, the electron from Mo5+ rela-
tive to Mo6+ is actually delocalized to a great degree, and
not bound tightly to an individual Mo6+ core cation. In this
regard, it is reasonable that only one set of doublets is de-
During our experiments, we observed that the light blue
MP catalyst changed to dark blue. Based on this, we infer
that some electrons may be delocalized to a Mo6+ site
during activation of the benzyl alcohol. As a result, the
nature of the Mo=O bond will be partially ionic if electrons
hop between metal centers and oxygen atoms on the MP
surface.[16] In an extreme state, in which one electron is en-
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749

F. Wang and W. Ueda
tirely back donated to a Mo6+ site, one unpaired electron
will be located around an oxygen atom (Scheme 1). As a
result, the terminal oxygen atom may have the ability to
close to 1ꢂ1016 unitcmÀ2. The two values are approximate,
suggesting that the adsorption site is terminal Mo=O. The
difference between the two values may be due to steric ef-
fects since some Mo=O sites,
such as those located in reverse
corners and steps, cannot be
reached by TCNE.
It would be of great interest
to know the effect of various
Scheme 1. The surface electron transfer from a terminal oxygen atom to a Mo atom or to an electron acceptor
(TCNE, tetracyanoethylene).
electron acceptors with differ-
ent electron affinity on the cat-
alytic performance of the MP in
donate the unpaired electron or to accept one extra electron
from an external source to form a stable bond. In our re-
search, we employed an electron acceptor to probe the un-
paired electron. Tetracyanoethylene (TCNE) is a common
electron acceptor with an electron affinity value of 2.3. The
benzylation. A series of electron donors are listed in Table
S1 in the Supporting Information. The dependence of specif-
ic activity on the electron affinity is shown in Figure 13. The
addition of nitrobenzene (electron affinity: 1.0 eV) has no
effect on the catalytic activity. The decrease of catalytic ac-
tivity starts with the addition of m-bromonitrobenzene
(1.3 eV) and becomes apparent by adding o-dinitrobenzene
(1.6 eV). With the large electron affinity of TCNE (2.3 eV),
the decrease reaches 85% of the original catalytic activity.
CÀ
appearance of a blue anion radical of TCNE , an electron-
transfer complex detectable by UV/Vis spectrometry at
about 330 nm,[71,72] is the evidence of such a reaction and is
given in Scheme 1.
The adsorption test of TCNE on the MP catalyst was con-
ducted at 808C in acetonitrile. The reaction reached equilib-
rium in 10 min as judged by UV/Vis spectroscopy. The color
of the catalyst changed to dark blue. The UV/Vis spectra of
a fresh TCNE acetonitrile solution and the reacted solution
are given in Figure 12. A distinctive adsorption peak shows
Although the electron affinity of tetracyano-p-quino
AHCTUNGTREGmNNUN ethane (TCNQ, 2.8 eV) is larger than that of TCNE, it has
ACHTUNGTRENNUNGdi-
almost no influence on the catalytic activity (Figure 13). In
this case, the steric effect has to be considered because ad-
sorption sites are located on flat surfaces or in corners. In
comparison, these electron acceptors have no effect on the
performances of the MM catalyst (Figure 13), suggesting
that the reaction on the bulk MoO3 is not catalyzed by ter-
minal oxygen sites.
We conclude the following points from these tests: 1) the
terminal oxygen of the nanostructured MoO3 catalyst, which
can be molecularly probed with electron acceptors, is very
different from that of bulk MoO3 surfaces; and 2) the termi-
nal oxygen of the nanostructured MoO3 is the active site for
electron transfer and also for benzyl alcohol activation.
The MoO3 (010) surface contains three types of oxygen
centers (Figure 14, inset geometric structure): one unshared
Figure 12. The UV/Vis spectra of fresh TCNE (tetracyanoethylene) ace-
CÀ
tonitrile solution (solid line) and charge-transfer complex of TCNE
(dashed line).
up at 328 nm, which is the evidence of the formation of the
CÀ
charge-transfer complex TCNE . We note that this is the
first time the electron donation ability of nanostructured
pure MoO3 was observed. Using the molar adsorption value
CÀ [73]
of 3800 molLÀ1 cmÀ1 for TCNE , the concentration was
calculated to be 6ꢂ10À8 molLÀ1. Given that one anion radi-
cal corresponds to one terminal Mo=O site in Scheme 1, the
density of Mo=O sites per surface area of the catalyst is esti-
mated to be 4ꢂ1015 unitcmÀ2. Assuming that the radius of
one octahedron is 1.8 ꢄ, the surface terminal Mo=O bond is
Figure 13. The effect of various electron acceptors with different electron
affinity on the catalytic activity of the MP and MM catalyst in the benzy-
lation of toluene with benzyl alcohol.
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Nanostructured Molybdenum Trioxide
FULL PAPER
Figure 14. IR spectra of MM, MP, and the benzyl alcohol adsorbed MP
catalyst. The scheme in the inset shows the geometric structure of oxygen
coordinated Mo octahedron.
Figure 15. The understanding of the nanostructured MoO3 by various
characterization techniques, and reaction mechanism in the benzylation
of toluene with benzyl alcohol.
terminal oxygen atom O1 located in the (010) plane, two
axial oxygen atoms O2 located asymmetrically between two
Mo centers, and three equatorial oxygen atoms O3’ located
symmetrically between two Mo centers and linking another
Mo center. The IR spectra of MM, MP, and pretreated MP
catalyst are given in Figure 14. Three main adsorption peaks
and their changes with samples are observed. The MM
shows characteristic IR patterns similar to those of bulk
MoO3.[54,74] A very sharp band at 990 cmÀ1 is due to the
nMo=O1 stretching vibration. The second intense band at
fully oxygen-coordinated Mo sites. The whole material can
be pictured as distorted octahedron layers supported on par-
tially reduced MoO3. The catalytic reactions take place on
the (010) facets. The oxygen atoms of terminal Mo=O sites,
which resemble oxygen anion radicals, adsorb and activate
benzyl alcohol by electrophilic attack. Due to the high den-
sity and uniform distribution of Mo=O sites on the (010)
facet, the nanostructured MoO3 shows excellent activity and
À
selectivity. The dissociation of the PhCH2 OH (Ph=
phenyl) bond forms a benzyl carbocation, which is further
reacts with toluene to form the product and water, the de-
sorption of which from the catalyst surface recovers the
active sites so that they are ready for the next catalytic
cycle. The absence of any oxidized product and the high re-
usability suggests that the surface Mo=O is not broken
during the reaction, otherwise the catalyst would soon lose
activity.
871 cmÀ1 is assigned to the Mo O2 Mo vibration. The third
À
À
broad peak at 594 cmÀ1 is ascribed to the vibration of O3
À
À
Mo O3’. For the MP spectrum, these three bands shift to
992, 877, and 618 cmÀ1, respectively. Such phenomena are
known for nanocomposite materials and are the evidence of
quantum confinement due to particle size.[75–77] The adsorp-
tion of benzyl alcohol on the MP catalyst leads to an appar-
ent shift of the peak at 992 cmÀ1 to 973 cmÀ1 and slightly af-
fects the other two peaks at 877 cmÀ1 and 620 cmÀ1. It was
reported that the introduction of an extra electron into
oxides, such as pure V2O5, could cause a redshift in the
stretching band of the V=O group[48] due to the relaxation
of surface energy.[78] Therefore, the most affected stretching
band of the terminal Mo=O strongly proves that the ad-
sorbed benzyl alcohol transfers an electron to a Mo=O site.
As a result, benzyl alcohol is activated on a Mo=O site and
forms a benzyl carbocation. One electron from an oxygen
atom is lent to a Mo6+ site, and, thus, the charge of Mo is
lower than 6+, close to 5+, which is the reason for the
color change of the catalyst to dark blue.
Conclusion
In summary, we have demonstrated that nanostructured
MoO3 is a highly active and selective catalyst for the alkyla-
tion reaction. The method was extended to synthesize diphe-
nylmethane derivatives from various substituted benzyl alco-
hols and substituted arenes. The ability to selectively acti-
À
vate the C X (X=OH, Cl) bond shows its potential appli-
cation to other catalytic reactions, such as selective oxida-
tion and dechlorination. The catalyst preparation is facile
and does not require hydrothermal processes and organic
templates. Catalyst characterization results suggest that the
nanostructured MoO3 platelet with a thickness about 30 nm
contains active Mo=O bonds on the (010) facet. The active
facet for the benzylation is the (010) facet. The terminal
oxygen atom has radical properties and is very active to-
wards the adsorption and activation of benzyl alcohol. The
reactions catalyzed by bulk MoO3 (MM and MC-MoO3)
with normal surface Mo=O bonds, or by a reduced nano-
structured MoO3 (MP-NC) are sluggish and less selective.
The catalyst structure and reaction mechanism is given in
Figure 15. The a-MoO3 contains a slab of two layers of
edge-shared Mo octahedrons. The slab, equal to one and
half octahedra, has a height of about 6 ꢄ. One pellet of
about 30 nm consists of roughly 50 slabs, which are connect-
ed along the [010] direction by van der Waals forces.
Oxygen vacancies are located within the bulk phase. Free
electrons are stabilized within the oxygen vacancies, similar
to electrides.[62] The surface layers of (010) facets contain
Chem. Eur. J. 2009, 15, 742 – 753
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751

F. Wang and W. Ueda
swept from 100 to 13000 G. The g values were calculated by uisng the
formula hn=gmBB, where h is the Planck constant (6.6261ꢂ10À34JS), n is
the microwave frequency (9.438 GHz), mB is the Bohr magneton (9.274ꢂ
10À24JTÀ1) and B is magnetic field strength measured in tesla (T). The
XP spectrum (XPS) was obtained by using a JPC-9010mC. A monochro-
mated MgKa (10 kV; 10 mA) line was used as X-ray source. The pressure
in the analyzing chamber was 1ꢂ10À6 Torr during measurement. Binding
energies of peaks were corrected by referencing the C 1s signal of adven-
titious contamination hydrocarbon to 284.2 eV.
The present work will be helpful in understanding heteroge-
neous catalysis on the surface of nanomaterials.
Experimental Section
All chemicals were purchased from Wako Pure Chemical Industries.
These chemicals were used without further purification. All reagents
were A.R. grade. Distilled water was prepared by using Yamato Autostill
WG25 (Tokyo, Japan).
The UV/Vis spectra were collected on a spectrophotometer (V-570,
JASCO). For solid samples, the reference sample was BaSO4. The proce-
dures employed for the tetracyanoethylene (TCNE) adsorption were the
following. Acetonitrile (HPLC grade) was dried over 4 ꢄ molecular
sieves overnight and degassed with Ar for 2 min prior to use. The tetra-
cyanoethylene (TCNE) solution (1 mgmLÀ1) was prepared in a 10 mL
volumetric flask. The fresh catalyst and magnetic stir bar were placed in
a 25 mL flask and pretreated under N2 flow at 808C for 10 min, followed
by addition of TCNE solution (8 mL). The reaction reached equilibrium
after 10 min. The spectra of the solutions were measured. IR spectra
were recorded on a Perkin Elmer FTIR spectrometer. The sample pellet
with a 1.5 mm diameter was prepared by pressing a mixture of sample
and KBr. The IR spectra were recorded by accumulating 32 scans at a
Preparation of MP: In a 2 L beaker, (NH4)6Mo7O24·4H2O (AHM; 15 g)
was added to water (500 mL). The solution was stirred for 5 min at room
temperature until a clear solution had formed. One liter of acetone was
poured into the solution to cause precipitation. The mixture was aged for
7 min, and then the solid was filtered off. The filtrate was washed with
acetone (2ꢂ100 mL) and pentane (2ꢂ20 mL), and then dried at 608C in
an oven for 14 h. The precipitate was calcined at 3308C for 2 h with a
ramp of 108CminÀ1 in air. The sample was designated as MP. The sam-
ples calcined at 250, 350, and 4008C were designated as MP-250, MP-350,
and MP-400, respectively. The sample calcined in
a N2 flow
(50 mLminÀ1) was designated as MP-NC.
spectra resolution of 2 cmÀ1
.
Preparation of MM: AHM (2 g) was spread as a thin layer in a crucible
and calcined in an oven at 5008C for 12 h. The sample was designated as
MM. The MM-m15 sample was prepared by milling MM for 15 min at
The measurements of thermogravimetric-differential thermal analysis
(TG-DTA) were performed on a TG-8120 (Rigaku) thermogravimetric
analyzer. Dry air or N2 provided by a pressure tank with a flow rate of
30 mLminÀ1 was used as the carrier gas. The catalyst sample and stan-
dard were loaded into two alumina pans and heated at 108C minÀ1 to the
desired temperature. The baseline was subtracted from a blank run with-
out loading the sample.
[80]
150 RPM. The procedures for preparing MH[79] and b-MoO3 are de-
tailed in the Supporting Information. MC-MoO3 and MC-MoO2 were
commercial products (Wako Chemicals).
Catalysis: A 50 mL round-bottom three-neck flask equipped with a
reflux condenser was used as a stirred bed reactor to test the catalytic ac-
tivities. Typically, 0.3 g of catalyst and a Teflon coated magnetic stir bar
were loaded into the reactor. The reactor was sealed and purged with N2
(10 mLminÀ1) under the desired temperature for 5 min. The oil bath was
removed and the reactor was cooled to ambient temperatures. A mixture
of benzyl alcohol (0.24 mL), arenes (15 mL), and an internal standard,
hexadecane (0.1 mL), were added to the reactor. The mixture was purged
with N2 (10 mLminÀ1) for 3 min. The reactor was then placed in the oil
bath. The mixture was stirred until the reaction reached completion, as
judged by TLC. Aliquots (0.1 mL) were collected at intervals. The con-
centrations of reactant and product were measured by gas chromatogra-
phy using a flame ionization detector (Shimazu Classic-5000, 60 m TC
WAX column), operated with a heating program: 1208C for 10 min,
ramp 108C minÀ1 to 2508C (kept for 20 min). The crude mixture was con-
centrated in vacuo to yield the crude product. Purification of the product
was carried out on silica gel using pure hexane to remove the remaining
solvent, followed by washing with hexane/ethyl acetate (2.5:1) to give the
desired diphenylmethane compound. 1H NMR and 13C NMR spectra
were recorded by using a JEOL ECX-600 or a JEOL ECX-400 at the
Temperature-programmed desorption (TPD) of ammonia, NH3-TPD, was
employed to measure oxide surface basicity. The experiment was carried
out on a BELSORP apparatus. The experimental procedure was as fol-
lows. The catalyst (ca. 80 mg) was set up between two layers of quartz
wool and pre-heated under helium (50 mLminÀ1) at 3008C for 1 h. Then,
ammonia was introduced at 1008C for 30 min. The desorption profile was
recorded with a mass spectrometer from 1008C to 6008C under helium
flow (50 mLminÀ1). Temperature-programmed decomposition mass spec-
trometry (TPD-MS) measurements were performed from 120 to 6008C
at a heating rate of 108C minÀ1 in helium flow (50 mLminÀ1). The de-
composed gas molecules were monitored by
a mass spectrometer
(ANELVA, Quadrupole Mass Spectrometer, M-100QA, BEL Japan), col-
lecting several mass fragments: acetone (m/z 58), NO2 (46), CO2 (44), O2
(32), NO (30), N2 (28), H2O (18, 17, 16), NH3 (17, 16, 15), H2 (2).
1
CRIS center of Hokkaido University. The H NMR and 13C NMR spectra
Acknowledgements
were conducted in CDCl3 under ambient conditions with TMS as the
standard (1 wt% in CDCl3) or with CHCl3 as reference (d=7.26 ppm).
The spectra of known compounds (Table 2 and Table 4) were identical to
those reported in the literature, and unavailable spectra and the spectra
of new compounds (Table 4, entry 2–7) are appended as Supporting
Information.
F. W. thanks Prof. M. Sadakane for his help with XPS measurements, and
Prof. Y. Hinatsu and Prof. M. Wakeshima from the faculty of science and
graduate school of science, Hokkaido University for their help with ESR
measurements. F. W. acknowledges the financial support of CREST-JST.
General characterization: Powder X-ray diffraction (XRD) measure-
ments were performed with a Rigaku, RINT Ultima+ diffractometer
with CuKa radiation (Ka 1.54056 ꢄ) and X-ray power of 40 kV/20 mA.
The crystallite size was calculated from the line broadening using the
Scherrer formula. Shirley background correction procedures and least-
squares fittings methods were adopted for data processing. Field-emission
scanning electron microscopy (FE-SEM) was performed on a JSM-7400F
(JEOL). Samples for SEM were dusted on an adhesive conductive
carbon paper attached on a brass mount. Specific surface areas were
measured by N2 adsorption at 77 K using the Brunauer–Emmett–Teller
method (BET)[81] over Autosorb 6 AG (Quantachrome Instruments).
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752
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Received: June 12, 2008
Revised: August 15, 2008
Published online: November 26, 2008
Chem. Eur. J. 2009, 15, 742 – 753
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