Acidity and Catalytic Activity of a Nafion-H/Silica Nanocomposite Catalyst Compared with a Silica-Supported Nafion Sample

Article abstract of DOI:10.1006/jcat.2000.2869

The characterization of acid sites in the recently developed Nafion-H/silica nanocomposite catalyst was carried out through pyridine adsorption using the surface FT-IR technique. For comparison, an impregnated Nafion-H/SiO2 catalyst was also studied. A large number of free surface SO3H groups was detected on the surface of the Nafion-H/silica nanocomposite, in contrast to the impregnated catalyst. Pyridine adsorption demonstrates that Broensted as well as Lewis acid sites are present on the surface of the samples. There was a large number of available Broensted sites on the surface of the Nafion-H/silica nanocomposite catalyst. As well as pyridine adsorption, the surface reaction of cyclopropane also revealed that the nanocomposite catalyst has a high number of acid sites. The impregnated sample exhibits much lower activity. The effect of different types of acidity and of acid strength were studied by means of the Friedel-Crafts alkylation (adamantylation of toluene) and various rearrangements. In agreement with the results of surface studies, the nanocomposite specimen exhibits activity which is several orders of magnitude higher than that observed for the other Nafion samples studied. This outstanding activity is attributed to the large surface area and to the increased accessibility of surface-active sites to reactants in catalytic transformations.

Full text of DOI:10.1006/jcat.2000.2869

Journal of Catalysis 193, 132–138 (2000)
doi:10.1006/jcat.2000.2869, available online at http://www.idealibrary.com on
Acidity and Catalytic Activity of a Nafion-H/Silica Nanocomposite Catalyst
Compared with a Silica-Supported Nafion Sample
,1
Be´la To¨ro¨k, Imre Kiricsi,† Arpa´d Molna´r, ,1 and George A. Olah‡
´
Department of Organic Chemistry, University of Szeged, Do´m te´r 8, H-6720 Szeged, Hungary; †Department of Applied and Environmental Chemistry,
University of Szeged, Rerrich te´r 1, H-6720 Szeged, Hungary; and ‡Department of Chemistry and Loker Hydrocarbon Research Institute,
University of Southern California, Los Angeles, California 90089-1661
Received December 23, 1999; revised February 11, 2000; accepted March 5, 2000
classes of these polymers are polystyrene-based materials
(Amberlyst, Dowex) (4) and perfluorinated resinsulfonic
acid based ion-exchanged resins (Nafion) (5). A new fam-
ily of siloxane-based polymers, Deloxane, has recently been
developed by Degussa (6).
The characterization of acid sites in the recently developed
Nafion-H/silica nanocomposite catalyst was carried out through
pyridine adsorption using the surface FT-IR technique. For com-
parison, an impregnated Nafion-H/SiO2 catalyst was also studied. A
large number of free surface SO3H groups was detected on the sur-
faceoftheNafion-H/silica nanocomposite, in contrast totheimpreg-
nated catalyst. Pyridine adsorption demonstrates that Brønsted as
well as Lewis acid sites are present on the surface of the sam-
ples. There was a large number of available Brønsted sites on the
surface of the Nafion-H/silica nanocomposite catalyst. As well as
pyridine adsorption, the surface reaction of cyclopropane also re-
vealed that the nanocomposite catalyst has a high number of acid
sites. The impregnated sample exhibits much lower activity. The
effect of different types of acidity and of acid strength were stud-
ied by means of the Friedel–Crafts alkylation (adamantylation of
toluene) and various rearrangements. In agreement with the results
of surface studies, the nanocomposite specimen exhibits activity
which is several orders of magnitude higher than that observed for
the other Nafion samples studied. This outstanding activity is at-
tributed to the large surface area and to the increased accessibility
of surface-active sites to reactants in catalytic transformations.
Due to their structural features, the perfluorinated ion-
exchanged polymers, commercially available under the
tradename Nafion, show high acidity and activity in acid-
catalyzed reactions (7). Nafion is a cross-linked perfluo-
rinated polymeric sulfonic acid (8–10). Since the perfluo-
roalkyl environment and especially the -CF2 moiety have
high electron-withdrawing character, the acidity of the sul-
fonic acid groups is very high. Nafion resins exhibit an acidic
character comparable to that of 100% sulfuric acid and tri-
flic acid in trifluoroacetic anhydride solution. The Hammett
acidity constant for Nafion-H was estimated to be
12,
i.e., it is a solid superacid (7, 11). Due to its superacidic
character, chemical resistance, thermal stability, and ease of
regeneration, Nafion-H is a frequently used solid acid cata-
lyst in organic transformations (7). Nafion-H can take the
place of common liquid acids in numerous transformations.
However, the use of Nafion NR50 beads, which have an ex-
tremely low surface area ( 0.02 m2 g 1), generally requires
the use of relatively large amounts of catalyst and longer
reaction times. Furthermore, the acid sites of Nafion NR50
are not easily accessible and, consequently, the rate of the
gas phase reaction is rather low. Harmer and co-workers
recently reported a method for overcoming this problem
(12). They developed new solid superacidic catalysts called
Nafion-H silica nanocomposites. They are highly active
and have a large surface area (150–500 m2 g 1) with small
(<100 nm) Nafion particles entrapped in the porous silica
framework (12). Compared to Nafion-H beads, nafion par-
ticles have very high specific activity in 1-butene isomeriza-
tion (13), in Friedel–Crafts alkylation and acylation (12),
and in the nitration of benzene (12). These observations
were further confirmed in other transformations, namely,
in the dimerization of -methylstyrene (14), in the benzyla-
tion of aromatics (15), and in the Fries rearrangement (16).
c
2000 Academic Press
INTRODUCTION
The ever-growing demand for environmentally safe in-
dustrial processes requires the development and use of ef-
fective and safe catalysts. Many acid catalysts are hazardous
and corrosive, such as HF, sulfuric acid, and nitric acid (1).
A plethora of solid acids are, however, already available to
take the place of these aggressive and dangerous materials
in the laboratory practice and in industry. Molecular sieves,
such as HY and ZSM-5 zeolites (2) and heteropoly acids (3)
have already been introduced into the industrial produc-
tion of chemicals. Another class of solid acids is the acidic
form of ion-exchanged polymers, which exhibit a wide
range of acid strength, thermal stability, etc. Two important
1 Corresponding authors.
132
0021-9517/00 $35.00
c
Copyright
2000 by Academic Press
All rights of reproduction in any form reserved.

ACIDITY OF NAFION-H/SILICA NANOCOMPOSITE
133
We report here pyridine adsorption acidity measure- vals. After each heat treatment the IR cell was evacuated
ments, made by surface FT-IR spectroscopy, to determine and the spectrum recorded at room temperature.
the Brønsted and Lewis acidity of this new, very promising
Methods of Catalytic Studies
solid acid catalyst. Catalytic properties were also studied in
the ring opening of cyclopropane, in Friedel–Crafts alkyla-
tion (adamantylation of toluene), and in the ortho-Claisen
and Fries rearrangements. In addition, comparative studies
with Nafion beads and a silica-supported Nafion sample,
prepared by impregnation, were carried out to illustrate the
clear advantages of the nanostructured Nafion-H catalyst.
As well as pyridine adsorption, the ring-opening re-
action of cyclopropane was used to characterize surface
acidity. The surface reaction of cyclopropane was inves-
tigated in the same IR cell used for pyridine adsorp-
tion. Cyclopropane (1.33 kPa) reacted over the outgassed
self-supported wafer and the spectrum was recorded. The
change in the absorbance of the band at 3080 cm 1, due to
the adsorbed cyclopropane species, was considered to be a
measure of the transformation of the surface.
EXPERIMENTAL
Materials and Catalysts
The adamantylation of toluene was studied by refluxing
and stirring 0.212 g (0.001 mol) of 1-bromoadamantane and
0.2 g of catalyst in 5 ml of toluene.
Tetraethylorthosilicate [Si(OC2H5)4] and the silica cata-
lyst support (SBET = 500 m2/g) were purchased from
Aldrich; Nafion-H is a commercially available Dupont
product. The active H-form was generated from the potas-
sium salt after being treated with 25% nitric acid, washed
with water, and dried (105 C, 24 h) (7b). Nafion-H per-
fluorinated ion-exchanged solution (5% concentration),
1-bromoadamantane, pyridine, and the solvents were pur-
chased from Aldrich and cyclopropane from Merck. Lit-
erature procedures were applied to synthesize allyl phenyl
ether (17) and phenyl acetate (18). They proved to be 99%
pure by GC.
Ten percent Nafion-H on a silica (10% Nafion-H/SiO2)
catalyst was prepared by impregnation. Ten grams of silica
support was suspended in a 2-propanol/1-propanol/water
mixture containing the calculated amount of Nafion-H. The
mixture was stirred at room temperature for 2 h, the sol-
vents were removed by a rotary evaporator, and the result-
ing free-flowing material was dried under vacuum (100 C,
5 Torr, 24 h). Nafion-H/silica nanoncomposite material
with 13% Nafion was prepared using tetraethylorthosili-
cate according to a procedure described in the literature
(12).
The Claisen rearrangement was carried out by refluxing
a continuously stirred mixture of 2.01 g (0.015 mol) of allyl
phenyl ether, 0.05 g of 1,3,5-tri-tert-butylbenzene (internal
standard), and the catalyst (10 mg of Nafion-H beads or an
appropriate amount of the supported catalysts containing
the same amount of Nafion resin) in benzene (10 ml).
The Fries rearrangement was performed by reacting a
stirred mixture of 0.544 g (0.004 mol) of phenyl acetate
and 0.2 g of catalyst in 5 ml of nitrobenzene at 220 C. In
one experiment a mixture of 2 ml of phenol and 3 ml of
nitrobenzene was used as the solvent.
In each of the above reactions, products that were with-
drawn at intervals were analyzed (GC) to determine con-
versions and compositions of the products (Carlo Erba
Fractovap 2350 equipment, thermal conductivity detector,
OV-17 column, or HP 5890 GC, FID detector, 60-m DB-1
column). Products were identified by GC-MS (HP 5890 GC
coupled with a HP 5970 mass selective detector, 60-m DB-1
column).
RESULTS AND DISCUSSION
Characterization of the Catalysts
Characterization of Surface Acidity
In situ Fourier transform infrared spectroscopy (FT-IR)
The results of a systematic study of pyridine adsorption
(Mattson Genesis 1 spectrometer) was used to character- and the skeletal isomerization of cyclopropane, used to de-
ize the acid centers of the catalysts. Self-supported wafers, termine the type, the relative amount, and the strength of
10 mg/cm2 thick, prepared from the powdered materials the acid centers of the two catalyst samples, are presented
were placed into an IR-cell connected to a vacuum sys- here. Both these techniques give important information
tem. The samples were outgassed at 200 C and their spectra with respect to the acidity of the samples and enable the de-
recorded.
termination of the correlation between acidity and activity.
Figure 1 shows the results of pyridine adsorption for the
In pyridine adsorption–desorption studies, 1.33 kPa of
pyridine was introduced into the optical cell at 25 C fol- 10% Nafion-H/SiO2 sample. In the spectrum registered af-
lowed by the registration of the spectrum related to the ter evacuation at room temperature, i.e., after the removal
adsorbed pyridine species. Adsorption equilibrium was of gaseous and physically adsorbed pyridine, three bands,
reached after 5 min as revealed by the spectra observed considered to be characteristic of acid sites developed
after the samples were treated with pyridine. The cell was (19–21) (Fig. 1b): a small, broad band at 1545 cm 1, a some-
evacuated, and the temperature was raised at regular inter- what sharper band at 1490 cm 1, and a sharp, more intense

¨ ¨
TOROK ET AL.
134
distinguish among different acid sites. Pyridine molecules
bonded to Lewis acid sites (complexing pyridine) absorb
1
at 1450 cm (ascribed to the 19b-ring mode of pyridine)
while those adsorbed on Brønsted acid sites (pyridinium
1
ion) show absorbance at 1540 cm as a result of the dif-
ferent types of surface bonding (19–21). The third band at
1
1490 cm is a combined band originating from pyridine
that is bonded to both Brønsted and Lewis acid sites. The
relative surface concentrations of Brønsted and Lewis acid
sites can be determined by comparing the corresponding
absorbance.
From the data presented in Figs. 1 and 2, it is obvious
that both Brønsted and Lewis acid sites are present on the
surface of the samples. The band at 1544–1545 cm 1 is char-
acteristic of the pyridinium ion resulting from the reaction
of pyridine with proton sites. The band appearing at 1445–
1
1450 cm represents the interaction of pyridine with the
silica matrix through donor–acceptor-type bonds. It is also
clear that this latter bonding is rather weak since complexed
pyridine can be almost completely removed from the sur-
face at 80 C. Pyridinium ions, in contrast, are more strongly
bound to the surface. In the case of the nanocomposite sam-
ple, the intensity of the band changed only slightly, even at
150 C. Moreover, the nanocomposite material exhibits ad-
ditional IR absorption at 1415 cm 1. This band appeared in
FIG. 1. FT-IR spectra illustrating pyridine adsorption on the 10%
Nafion-H/SiO2 sample. (B) Band characteristic of pyridine adsorbed on
Brønsted sites. (C) Band characteristic of pyridine interacting with the sil-
ica matrix. (a) Without pyridine; after pyridine adsorption and evacuation
at (b) room temperature, (c) 80 C, and (d) 150 C.
band at 1445 cm 1. After evacuation at 80 C, the band at
1
1445 cm disappeared (Fig. 1c), whereas the intensity of
the band at 1540 cm 1 decreased slightly. Further evacua-
tion at 150 C resulted in a further decrease in the intensity
of the latter band (Fig. 1d). The intensity of the band at
1490 cm 1 decreased gradually.
The IR spectrum of the nanocomposite sample after
1
evacuation showed a rather broad band at 1415 cm
(Fig. 2a). Upon adsorption of pyridine, two characteristic
changes were observed. As expected, all three bands asso-
ciated with acid centers observed in the above experiments
were detected (1544, 1490, and 1445 cm 1) (Fig. 2b). At
1
the same time, the band at 1415 cm disappeared. Upon
evacuation at 80 C, the band at 1450 cm 1 also disappeared
(Fig. 2c). In sharp contrast, the band at 1544 cm 1 remained
almost unchanged. Evacuation at 150 C resulted in a slight
decrease in the intensity of this band, too, with the concomi-
tant reappearance of the band at 1415 cm 1. The intensity
of the band at 1490 cm 1 decreased gradually.
Adsorption of pyridine as a base on the surface of solid
acids is one of the most frequently applied methods for
the characterization of surface acidity. This adsorption–
desorption technique enables the determination of the
strength of the interaction between pyridine and the cor-
responding acid sites present on the surface. The use of IR
FIG. 2. FT-IR spectra illustrating pyridine adsorption on the 13%
Nafion-H/silica nanocomposite sample. (B) Band characteristic of pyri-
dine adsorbed on Brønsted sites. (C) Band characteristic of pyridine inter-
acting with the silica matrix. (a) Without pyridine; after pyridine adsorp-
spectroscopy to detect the adsorbed pyridine enables us to tion and evacuation at (b) room temperature, (c) 80 C, and (d) 150 C.

ACIDITY OF NAFION-H/SILICA NANOCOMPOSITE
135
the spectrum of dry Nafion-H and was attributed to SO3H
groups (22). The nanocomposite sample, therefore, has a
high concentration of surface SO3H groups available for
catalytic transformation. The 10% Nafion-H/SiO2 speci-
men has the same weak Lewis acid sites that are thought to
be characteristic of the silica surface, but it has a number of
fairly strong Brønsted acid sites.
The intensities of the IR bands of the samples cannot be
used directly to compare the surface concentration of acid
sites of the samples. However, since the matrix (silica) is the
same for both materials, the relative intensities of the two
bands can be used to make at least a qualitative comparison.
The ratios of band intensities corresponding to pyridine
bonded to Brønsted and Lewis sites are 1 for 10% Nafion-
H/SiO2 and 21.1 for 13% Nafion-H/silica nanocomposite.
These normalized data show that the nanocomposite cata-
lyst has a much higher concentration of Brønsted sites.
Cyclopropane is an excellent probe molecule for testing
the acidity of solid catalysts (23). It has been shown that
FIG. 3. Consumption of the reactant as a function of contact time in
the isomerization of cyclopropane over Nafion-H-based catalysts followed
by FT-IR spectroscopy (᭿, 13% Nafion-H/silica nanocomposite, 80 C; ᭡,
ring-opening isomerization of cyclopropane on strongly 10% Nafion-H/SiO2, 150 C).
acidic samples leads to the formation of propene. The
product, however, readily undergoes secondary transfor-
The adamantylation of toluene with 1-bromoadaman-
mations: propene isprotonated and subsequentlytakespart
in oligomerization resulting in the formation of oligomers
with a carbocationic character on the surface (24–27). As a
==
double bond of propene cannot be detected spectroscopi-
cally (28).
tane is a Friedel–Crafts alkylation. The reaction yields
two isomeric compounds: p- and m-adamantyltoluene [1]
(Table 1). In each case the para isomer, expected on the
basis of the orientation effect of the methyl substituent, is
result of surface oligomerization and protonation, the C
C
The IR spectrum of cyclopropane adsorbed on 10%
Nafion-H/SiO2 showed an intense band at 3080 cm 1, which
is characteristic of cyclopropane species adsorbed on the
catalyst surface (23). The intensity of this band decreases
with increasing contact time and temperature. The transfor-
mation of cyclopropane was found to be slow over this cata-
TABLE 1
Conversions and Selectivities of the Adamantylation of Toluene
a
r0
Selectivity (% )
1
Catalyst
samples
Time Conversion (mmol min
1
(min)
(% )
gcat
)
p
m
1
lyst: the integrated absorbances of the band at 3080 cm
2
decreased significantly only at 150 C (Fig. 3).
Nafion-H beads
5
20
35
60
5.4
16
37.5
51.3
5.02 10
85
85
82
76
15
15
18
24
When the nanocomposite material was treated with cy-
clopropane, the band related to the adsorbed cyclopropane
species (3080 cm 1) was detected only at room tempera-
ture; the treatment at 80 C induced the transformation of
cyclopropane to propene oligomers at a high reaction rate
(Fig. 3) (24–27).
10% Nafion-H/SiO2
5
20
35
60
19
25
34
67
0.58
83
85
86
82
17b
15c
14
18d
The surface reaction of cyclopropane over the two cata-
lyst samples reveals that, once again, the nanocomposite
material displays much higher catalytic activity and, conse-
quently, possesses a large number of surface acid centers.
The much lower activity of the impregnated sample reflects
the considerably lower concentration of the surface-active
sites. The silica support was inactive in cyclopropane trans-
formation.
13% Nafion-H/silica
nanocomposite
5
20
35
60
47
100
100
100
3.38
80
79
55
61
20
21
45
39b
a Initial rates.
b 4% adamantane.
c 2.3% adamantane.
d 2% adamantane.
Catalytic Studies
The results of the catalytic test reactions with the above-
mentioned two catalyst samples and comparisons with
Nafion-H beads are shown in Tables 1–3.

¨ ¨
TOROK ET AL.
136
the main product. A large shift in selectivity with time was contrast, 2-methyldihydrobenzofuran (3), formed as a re-
observed, however, when the nanocomposite was used as sult of cyclization of 2, is the main product in the pres-
the catalyst. This sample exhibited the highest activity of ence of Nafion-H beads. Small amounts of phenol and some
the three catalysts studied. In some cases, small amounts oligomerization were also detected. The 10% Nafion/silica
of the adamantane by-product were detected.
and Nafion beads, however, exhibited very low and rapidly
It was already shown that, of the three possible isomeric declining activity. Again, the nanocomposite sample was
adamantyltoluenes, the ortho isomer isnot formed for steric more active. However, the rearrangement of the primary
reasons (29). It is also known that acidity exerts a great in- product 2, induced by the highly acidic nature of this cata-
fluence on regioselectivity (29, 30): the distribution of the lyst to form the 3 dihydrobenzofuran derivative, results in
kinetic product (high excess of the para isomer) can shift to decreasing selectivity with time. Similar conclusions were
the distribution of the thermodynamic product (increasing reached in a recent study by Sheldon et al. who applied
ratio of the meta isomer). Since isomerization by alkyl shift H-beta and H-mordenite zeolites (33). Oligomerization, a
is much slower than direct alkylation, this change in prod- major side reaction over the latter catalysts, is not very sig-
uct distribution occurs after complete conversion, provided nificant in the presence of the nanocomposite sample.
that the catalyst is sufficiently acidic. In the present case, the
Finally, our catalysts were tested in the Fries rearrange-
13% Nafion/silica nanocomposite sample which, according ment which is the acid-catalyzed transformation of phenol
to IR measurements, had the highest concentration of acid esters resulting in the formation of phenolic ketones (34).
sites, exhibits the highest activity. On the other hand, this The Lewis acids that were applied originally were later re-
catalyst induces a large shift in regioselectivity toward the placed by Brønsted acid catalysts; Nafion-H has been found
formation of the meta isomer, approaching a selectivity of to be just as active (35, 36).
1 : 1, in agreement with results of earlier studies.
In the transformation of phenyl acetate (5), phenol
The aromatic ortho-Claisen rearrangement has been fre- (6) was formed with high selectivity over Nafion beads
quently studied: this transformation results in substituted and in the presence of the silica-supported catalyst [3];
phenols through the rearrangement of the corresponding i.e., deacetylation took place (Table 3). This was ob-
allyl ethers (31). Formally, the reaction appears to be a 1,3 served in earlier studies, and the highly reactive ketene
O → C migration of the allyl group. In fact, the thermal thus formed was assumed to bring about deactivation of
process is a 3,3 sigmatropic rearrangement and is regarded the catalyst (16, 35). The nanocomposite sample showed
as being concerted. It is known that acid catalysts, namely, some unique characteristics. Firstly, this catalyst again
zeolites, are also capable of promoting the Claisen rear- proved to be the most active of the three samples studied
rangement (32, 33).
(Table 3). Secondly, the Fries rearrangement, leading to
Our results with the Nafion samples show (Table 2) 2-hydroxyacetophenone (7), 4-hydroxyacetophenone (8),
that the rearrangement of allyl phenyl ether (1) to o-allyl and also 4-acetoxyacetophenone (9), was more significant
phenol (2), the product of the Claisen rearrangement, is over the nanocomposite sample: the combined yield of
the main direction of the transformation when the silica- these three acetylated products (7 + 8 + 9) exceeds 50% .
supported catalyst is used [2]. The nanocomposite mate- Moreover, the transformation becomes completely selec-
rial also shows high selectivity at short reaction time. In tive when carried out in the presence of phenol. This
TABLE 2
Conversions and Selectivities of the ortho-Claisen Rearrangement of Allyl Phenyl Ether
r0
Selectivity (% )
1
Catalyst
samples
Time
(h)
Conversion
(% )
(mmol min
1
gcat
)
2
3
4
oligomers
Nafion-H beads
1
5
1
8
0.25
25
35
75
59
—
6
—
—
10% Nafion-H/SiO2
1
5
0.5
9
0.025
1.04
100
56.5
—
5.5
—
—
33
13% Nafion-H/silica
nanocomposite
1
5
16
41
50
27
34.5
52
6
8
9
13

ACIDITY OF NAFION-H/SILICA NANOCOMPOSITE
137
TABLE 3
Conversions and Selectivities of the Fries Rearrangement of Phenyl Acetate
r0
Selectivity (% )
1
Catalyst
samples
Time
(h)
Conversion
(% )
(mmol min
1
gcat
)
6
7
8
9
2
3
Nafion-H beads
1
2
19
24.6
5.24 10
5.52 10
95.4
94.5
94.5
—
—
5.5
5.5
—
—
10% Nafion-H/SiO2
1
2
2
5
100
100
—
—
—
—
—
—
13% Nafion-H/silica
nanocomposite
1
45
54
40
45
46
—
5
5
—
24.5
26
100
25.5
23
—
2
1a
a In the presence of phenol.
dramatic change was assumed to be brought about by the posite material is about 1 order of magnitude higher than
replenishment of the starting material through the reac- that of the silica-supported sample. The difference in cata-
tion of ketene with phenol (16). A more likely possibility is lytic activity, however, is even more pronounced when the
that phenol acts as a scavenger of the acetyl cation formed initial rates of the catalytic reactions are compared. This in-
in the deacetylation step, thereby directly yielding prod- dicates that the concentration of surface-active sites is not
uct 8 through intermolecular acetylation. Such transacety- the sole factor determining catalytic activity. The excellent
lation has already been demonstrated with zeolite catalysts activity, i.e., the higher accessibility of protic sites on the
(37, 38).
surface in the Fries rearrangement, is probably due to the
Data for a comparison of the activity (initial rates) of the experimental conditions and the favorable steric demand
three catalysts in the above transformations are shown in of the reacting molecule.
Tables 1–3. It is clear that the 13% Nafion/silica nanocom-
posite catalyst is the most active of the catalyst samples
studied. This is especially true for the Fries rearrangement
ACKNOWLEDGMENTS
where the activity of the nanocomposite material is at least
We are most grateful for the financial support of the Loker Hydro-
3 orders of magnitude higher than that of the other Nafion
samples.
carbon Research Institute at USC and the Hungarian National Science
Foundation (Grants OTKA F023674 and T030156).
CONCLUSIONS
REFERENCES
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studies show clearly that the 13% Nafion-H/silica nanocom-
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groups compared to the silica-supported Nafion-H; this en-
sures excellent performance in catalytic transformations.
This outstanding activity is apparently due to the increased
accessibility of surface-active sites to reactants, ensured by
the large surface area and the porous structure of the silica
matrix.
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