Efficient utilization of nanospace of layered transition metal oxide HNbMoO6 as a strong, water-tolerant solid acid catalyst

Article abstract of DOI:10.1021/ja802478d

Layered HNbMoO₆ was found to function as a strong solid acid catalyst, exceeding the activity of zeolites and ion-exchange resins for Friedel-Crafts alkylation. HNbMoO₆ also exhibited high catalytic activity for esterification of hydrocarboxylic acid and hydration. The catalytic performance of layered HNbMoO₆ is attributed to the intercalation of reactants into the interlayer and the development of strong acidity. Copyright

Full text of DOI:10.1021/ja802478d

Published on Web 05/13/2008
Efficient Utilization of Nanospace of Layered Transition Metal Oxide
HNbMoO6 as a Strong, Water-Tolerant Solid Acid Catalyst
Caio Tagusagawa,† Atsushi Takagaki,† Shigenobu Hayashi,‡ and Kazunari Domen*,†
Department of Chemical System Engineering, School of Engineering, The UniVersity of Tokyo, 7-3-1 Hongo,
Bunkyo-ku, Tokyo 113-8656, Japan, and Research Institute of Instrumentation Frontier, National Institute of
AdVanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
Received April 4, 2008; E-mail: domen@chemsys.u-tokyo.ac.jp
Table 1. Friedel-Crafts Alkylation over Some Solid Acid
There are inherent drawbacks to the use of liquid acids as
catalysts, such as the cost of separating the acid from the reaction
solution and the generation of large amounts of waste. Solid acid
catalysts, which are reusable and readily separable from the reaction
product, offer the opportunity to reduce the impact on the
environment and increase profits.1–4 For this reason, a number of
solid acids such as clay minerals,5 zeolites,6 Cs-heteropoly acids,7
sulfated zirconia,8 ion-exchange resins,9 and sulfonated-carbon
materials10 have been investigated. Among them, the use of a
layered material is one of the fascinating approaches to developing
a solid acid catalyst with enhanced activity. Cation-exchangeable
clay minerals such as montmorillonite exhibit catalytic activity for
acid-catalyzed reactions due to the good contact between interlayer
acid sites and the reactants.5
Like clay minerals, H+-exchanged layered transition-metal oxides
are promising materials for use as a solid acid catalyst due to the
generation of strong acidity within the interlayer space. However,
without modification, the reactants are generally inhibited from
penetrating into the interlayer region of layered transition-metal
oxides owing to the high charge density of the oxide sheets. One
approach to resolving this problem is to exfoliate the layered metal
oxide into metal oxide “nanosheets”.11 Most of the nanosheet
materials possess high surface area (ca. 100 m2 ·g-1) and exhibit
strong catalytic activity for acid-catalyzed reactions such as
esterification and hydrolysis, higher than that of zeolites and niobic
acid.11 Nevertheless, this method has a drawback that the acidity
of exfoliated nanosheets became weaker than that of the original
layered metal oxide.
In this study, the layered transition-metal oxide,
HNbMoO6 ·nH2O, is demonstrated to exhibit remarkable acid
catalysis without exfoliation. This is the first report of successful
acid catalysis using a layered transition-metal oxide. Layered
HNbMoO6 ·nH2O can be obtained easily from the Li form by a
proton-exchange reaction. The resultant material consists of layers
formed of randomly sited MO6 (M ) Nb and Mo) octahedral with
H2O in the interlayer.12
The acid catalytic activity of layered HNbMoO6 is demonstrated
here through the Friedel-Crafts alkylation, acetalization, esterifi-
cation, hydrolysis, and hydration. Table 1 lists the catalytic
performance results for the Friedel-Crafts alkylation of anisole,
toluene, or benzene with benzyl alcohol in liquid phase over layered
HNbMoO6. For comparison, the results for niobic acid
(Nb2O5 ·nH2O), zeolites and ion-exchange resins are also shown.
As we have reported, other layered transition-metal oxides such as
HNb3O8 and HTiNbO5 did not exhibit the activity (Supporting
Information, Table S1). However, HNbMoO6 exhibits remarkable
activity for these reactions, much higher than that of other tested
Catalystsa
yield of alkylate (%)b
acid amount
(mmol g-1
R ) OCH3c
R ) CH3
R ) H
catalyst
)
d
g
HNbMoO6
HNbMoO6
1.9e
1.9e
0.3
0.9
4.8
0.2
1.0
99.0f (89)
94.0f (86)
1.2 (2)
42.3 (23)
42.1 (5)
8.6 (22)
30.6 (16)
74.1 (4.9)
22.2 (1.5)
N.D.
19.0 (2.8)
14.0 (0.4)
0.9 (0.6)
0.6 (0.1)
21.9 (1.4)
7.7 (0.5)
N.D.
9.7 (1.4)
6.7 (0.2)
N.D.
Nb2O5 ·nH2O
Nafion NR50
Amberlyst-15
H-ZSM5h
H-Betai
N.D.
a Reaction conditions: 1 (100 mmol), benzyl alcohol (10 mmol),
catalyst (0.2 g), 353 K, 4 h. b Values in parentheses are turnover rate
(/h-1). N.D. - not determined. c Reaction temperature, 373 K; reaction
time, 1 h. d Protonated with H3PO4. e Determined by 31P NMR using
trimethylphosphine oxide. f Reaction time, 30 min. g Protonated with
HNO3. h SiO2/Al2O3
JRC-Z-HB25.
)
90, JRC-Z-5-90H. i SiO2/Al2O3
)
25,
solid acids; the yield of benzyl anisole reached to 99% after 30
min, whereas those for ion-exchange resins reached to ca. 42%
even after 1 h. The turnover rate of HNbMoO6 for the alkylation
of anisole was more than three times higher that of Nafion NR50.
HNbMoO6 also catalyzes the alkylation of toluene or benzene,
achieving high yields.
During the alkylation, benzyl alcohol was rapidly consumed in
the initial stage of the reaction, while the product formed gradually.
This result implies that benzyl alcohol is rapidly intercalated into
the layered HNbMoO6, where it reacts with anisole, toluene, and
benzene to form the product. For confirmation of this process, the
adsorption of benzyl alcohol onto HNbMoO6 was investigated in
isolation by immersing HNbMoO6 ·nH2O (n ) 1.23) in benzyl
alcohol solution under constant shaking for 30 min. Figure 1c shows
the XRD pattern of the resultant material after drying at room
temperature. Immersion in benzyl alcohol shifts the (001) peak due
to HNbMoO6 to lower angles, corresponding to an increase in basal
spacing from 13.7 to 16.3 Å. This shift confirms the intercalation
of benzyl alcohol into the layered HNbMoO6. Given the interlayer
spacing of the fully dehydrated sample (10.8 Å),12 the total
expansion is estimated to be 5.5 Å. Although similar experiments
were conducted for anisole, toluene, and benzene, no intercalation
was indicated. The necessity of intercalated benzyl alcohol for this
reaction was further investigated by adding an intercalated sample
to anisole solution followed by heating at 373 K for 1 h. Alkylated
products were detected after the reaction, and the basal spacing of
HNbMoO6 was found to decrease to 14.1 Å (Figure 1d). Benzyl
† The University of Tokyo.
‡ National Institute of Advanced Industrial Science and Technology (AIST).
9
7230 J. AM. CHEM. SOC. 2008, 130, 7230–7231
10.1021/ja802478d CCC: $40.75
2008 American Chemical Society

C O M M U N I C A T I O N S
recoverable by filtration and washing with water and acetone to
remove the residue within the interlayer, and the material was
confirmed to be reusable with no change in crystal structure or
activity after three reuse cycles (Table S1 and Table 2).
The acid property of the HNbMoO6 was evaluated by NH3 TPD.
The NH3-TPD profile for HNbMoO6 is composed of two peaks, at
601 and 682 K (Figure S5), the higher of which is much higher
than the temperature of the peaks of zeolites (582 K for H-ZSM5)
and niobic acid. The order of desorption peak of these solid acids
is consistent with that of the catalytic activity per acid sites for
Friedel-Crafts alkylation. 31P MAS NMR using trimethylphosphine
oxide (TMPO) was also carried out. The 31P chemical shifts of
protonated TMPO tend to move downfield, indicating that the
resonance peaks with high chemical shifts are due to strong protonic
acid. HNbMoO6 displays distinct peaks at 86.4 and 81 ppm,
attributable to strong acid sites (Figure S6). It was also confirmed
by XRD that TMPO intercalates in HNbMoO6 (Figure S7),
indicating that the strong acid sites of HNbMoO6 detectable in the
NMR spectrum are present in the interlayer.
HNbMoO6 intercalates alcohols, ketones, and alkene whereas
aromatics and acetic acid do not. The intercalation of protonated
layered materials is generally induced by acid-base interaction,
indicating that the basicity of substrate and its stability after
protonation are main factors to be intercalated or not. A more
detailed study is necessary to reveal the intercalation mechanism.
In summary, HNbMoO6 was found to function as a strong solid
acid catalyst, exceeding the activity of zeolites and ion-exchange
resins for the Friedel-Crafts alkylation. HNbMoO6 also exhibited
high catalytic activity for acetalization, esterification of hydrocar-
boxylic acid, and hydration. The catalytic performance of HNb-
MoO6 is attributed to the intercalation of reactants into the interlayer
and the development of strong acidity.
Figure 1. XRD patterns of HNbMoO6: (a) dehydrated, (b) hydrated (n )
1.23), (c) after immersion in benzyl alcohol for 30 min, (d) after reaction
of anisole, (e) during Friedel-Crafts alkylation.
Table 2. Esterifications of Acetic Acid and Lactic Acid and
Hydration of 2,3-Dimethyl-2-butene Catalyzed by HNbMoO6
esterification of acetic
acid,a reaction rate
esterification of lactic
acid, reaction rate
hydration,b
yield (%)
catalyst
(mmol h-1
)
(mmol h-1
)
HNbMoO6
Nafion NR50
Amberlyst-15
blank
2.2 (3)
5.7 (11)
6.0 (25)
4.5 (3)
1.4
3.4, 3.4c
2.2
3.7
16.0 (84)
13.7 (13)
0.9
0.4
a Reaction conditions: acetic acid or lactic acid (100 mmol), ethanol
(1 mol), catalyst (0.2 g), 343 K. Values in parentheses are turnover rate
(/h-1 b Reaction conditions: 2,3-dimethyl-2-butene (12.5 mmol), H2O
)
(0.42 mol), catalyst (0.2 g), 343 K, 5 h. c Third reuse.
alcohol was not detected in the solution. In the Friedel-Crafts
alkylation of toluene in the presence of benzyl alcohol, the interlayer
spacing of the layered HNbMoO6 was found by XRD of the
extracted catalyst (30 min after reaction) to increase to 16.6 Å
(Figure 1e).These results clearly indicate that intercalated benzyl
alcohol is consumed in the reaction and that the interlayer sites of
HNbMoO6 function as active sites.
Acknowledgment. This work was supported by Development
in a New Interdisciplinary Field Based on Nanotechnology and
Materials Science programs of the Ministry of Education, Science,
Sports and Culture (MEXT) of Japan and the Global COE Program
for Chemistry.
HNbMoO6 is also able to intercalate n-alkyl alcohols and ketones
as shown in Figure S1 and S2. For acetalization of cyclohexanone
with methanol HNbMoO6 exhibited a high activity (Table S2).
Table 2 shows esterifications of acetic acid (carboxylic acid) or
lactic acid (hydroxycarboxylic acid) with ethanol. Ion-exchange
resins catalyzed both esterifications and the production rate of ethyl
acetate was ca. 3 times higher than that of ethyl lactate. On the
other hand, HNbMoO6 did not catalyze esterification of acetic acid
despite the ability of intercalation of ethanol. However, for the
esterification of lactic acid HNbMoO6 exhibited a high catalytic
activity, comparable to ion-exchange resins. The XRD indicated
that lactic acid can intercalate into the oxide whereas acetic acid
does not (Figure S3). Considering that activation of carboxylic acid
by proton is necessary for the reaction, esterification of acetic acid
did not occur due to the difficulty of intercalation of acetic acid.
However, lactic acid which has an OH group adjacent to the
carboxyl group could intercalate into the oxide, resulting in a high
catalytic activity for the reaction. Similar results were obtained in
hydrolysis of ethyl acetate, ethyl lactate and cyclohexyl acetate
(Table S3). For three esters, HNbMoO6 only catalyzes hydrolysis
of ethyl lactate.
Supporting Information Available: Experimental section, Tables
S1-S3 and Figures S1-S7. This material is available free of charge
via the Internet at http://pubs.acs.org.
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Table 2 also lists the results for hydration of 2,3-dimethyl-2-
butene over HNbMoO6. Even in the presence of water, HNbMoO6
also functioned as an efficient catalyst. The yield of the corre-
sponding alcohol was higher than that of Nafion NR50 and
comparable to that of Amberlyst-15. This alkene was able to be
intercalated into the oxide which was determined by XRD (Figure
S4), resulting in the high catalytic activity. The HNbMoO6 was
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J. AM. CHEM. SOC. VOL. 130, NO. 23, 2008 7231