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Applied Catalysis A: General 275 (2004) 247–255
Catalytic performance of Nafion/SiO2 nanocomposites
for the synthesis of a-tocopherol
Hai Wang, Bo-Qing Xu*
Innovative Catalysis Program, Key Lab of Organic Optoelectronics & Molecular Engineering,
Department of Chemistry, Tsinghua University, Beijing 100084, China
Received 12 April 2004; received in revised form 10 July 2004; accepted 23 July 2004
Available online 11 September 2004
Abstract
Synthesis of a-tocopherol starting from trimethylhydroquinone (TMHQ) and isophytol (IP) was performed over Nafion/SiO2 nano-
composite catalysts (Nafion content: 5–20 wt.%) and Nafion1 NR50 resin. The nanocomposites were made by in situ hydrolysis of
tetraethoxysilane in the presence of home-made Nafion solution. Nitrogen adsorption and catalytic dehydration of 2-propanol were
used, respectively, to characterize the texture and acid properties of the nanocomposites. It is found that acid strength of the Nafion-
based acidic sites was weakened in the nanocomposites and the weakening disappeared when the amount of Nafion exceeded 30 wt.%
of the nanocomposite. The pore structure and accessibility of the Nafion-based acidic sites in the nanocomposite catalysts showed
pronounced effects on the catalytic efficiency toward the desired a-tocopherol. In comparison with Nafion resin in the condensed
phase (Nafion1 NR50), 5 and 13 wt.% Nafion/SiO2 catalysts showed tenfold higher catalytic activities by turnover frequency for
a-tocopherol formation owing to increased Nafion dispersion and accessibility of the Nafion-based acid sites. Though the acid sites in
the 20 wt.% Nafion/SiO2 catalyst had similar accessibility to those in the 5 and 13 wt.% Nafion/SiO2 catalysts by the dehydration
of 2-propanol, smaller pore sizes of the 20 wt.% Nafion/SiO2 catalyst induced severe side reactions of the IP reactant, such as
dehydration to form phytadienes and furan derivatives, which resulted in much lower yield (or selectivity) and turnover frequency for
a-tocopherol.
# 2004 Elsevier B.V. All rights reserved.
Keywords: a-Tocopherol; Alkylation–condensation reaction; Nafion/SiO2 nanocomposite; Isophytol; Trimethylhydroquinone
1. Introduction
a-Tocopherol, an essential nourishment ingredient with
biological activity and antioxidant ability, is widely used as
an additive for foodstuffs, pharmaceuticals, cosmetics and
animal feeds [1–5]. The global productivity of a-tocopherol
is about 20 kilotons per year and the demand for this
compound is constantly increasing [1]. Hitherto, all
industrially syntheses of a-tocopherol have been based on
the acid-catalyzed Friedel-Crafts alkylation–condensation
reaction, Scheme 1, of trimethylhydroquinone (TMHQ) and
isophytol (IP) or phytol halides [1–9]. Various Bronsted
acids as well as Lewis acids, e.g. ZnCl2/HCl, AlCl3, BF3 and
* Corresponding author. Tel.: +86 10 62792122; fax: +86 10 62792122.
E-mail address: [email protected] (B.-Q. Xu).
0926-860X/$ – see front matter # 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2004.07.038
FeCl2/Fe/HCl, can serve as the catalyst for this reaction [2–
9]. But, all these catalysts suffer from disadvantages such as
high catalyst consumption, reactor corrosion, contamination
and/or waste disposal problems [2–9]. To overcome these
drawbacks, attempts have been made to use solid acid
catalysts, including metal triflates and their derivatives
(imides) [2–7], metal ion-exchanged montmorillonites [8]
and heteropoly acids [9], as greener alternative catalysts for
the synthesis of a-tocopherol. However, these attempts have
not been so successful on account of low product yield and
poor catalyst efficiency and reusability [2–9].
The perfluorosulfonic acid Nafion resin, e.g. Nafion1
NR50, is a well-known strong solid acid (H0 � �12) with
high thermal stability (up to 280 8C) and chemical
resistance. The literature is rich in demonstrating that
applications of the Nafion resin as a strong solid acid catalyst
H. Wang, B.-Q. Xu / Applied Catalysis A: General 275 (2004) 247–255248
Scheme 1. Synthesis of a-tocopherol based on the alkylation–condensation
reaction of trimethylhydroquinone (TMHQ) and isophytol (IP).
can lead to efficient catalytic syntheses of a great variety of
organics [10,11]. It was shown by Schager and Bonrath [12]
that Nafion1 NR50 resin can also be a potentially efficient
catalyst for the synthesis of a-tocopherol, although the yield
of a-tocopherol was affected remarkably by the nature of the
reaction solvent. Owing to its extremely low surface area
(0.02 m2/g), the majority of the acid sites (sulfonic acid
groups) are buried in the bulk of the Nafion resin and are not
accessible to reactants, which greatly limits the applicability
of the Nafion resin catalyst, especially when the reaction
has to be conducted in nonpolar media or in gaseous phase
[10–12]. The dispersion and accessibility to the acid sites of
the Nafion resin were found to increase remarkably by
entrapping nanosized Nafion particles inside porous silica to
make Nafion/SiO2 nanocomposite catalysts [13,14]. The
most important feature of the Nafion/SiO2 nanocomposite
catalysts is the inclusion and exposure of the strong solid
acid sites of the Nafion resin in porous silica, which has
received considerable attention in acid catalysis [13–27].
Our preparation by using Si(OC2H5)4 for the silica source
also produced Nafion/SiO2 nanocomposite catalysts that
exhibited, as in [13,14], significantly enhanced catalytic
activity in the reactions of benzene-alkylation with olefins
[28,29] and of a-methylstyrene dimerization [30].
In the present study, we report the catalytic behavior of our
home-made Nafion/SiO2 nanocomposites for the synthesis
of a-tocopherol based on the alkylation–condensation reac-
tion of trimethylhydroquinone (TMHQ) and isophytol (IP)
(Scheme 1). Moreover, we show how the microstructures of
Table 1
The physicochemical properties of Nafion/SiO2 nanocomposites with different N
Nafion contenta (wt.%) Acid capacitya (mmol/g) Surface
5 0.045 415
13 0.12 396
20 0.18 352
100c 0.89 0.02
a The Nafion content and acid capacity were measured by thermogravimetric an
with the theoretical ones if one assumes no loss of Nafion during the preparatiob The specific surface area is obtained from the BET method, and the total pc Nafion1 NR50 resin (H+-form, 10–35 mesh).
the nanocomposites (acidity, accessibility of acid sites and
pore structure) affect their catalytic behavior.
2. Experimental
2.1. Reagents, sample preparation and characterizations
Trimethylhydroquinone (>90 wt.%) and isophytol
(>95 wt.%) were purchased from Tokyo Chemical Industry
Co. Ltd.; a-tocopherol (>95 wt.%) was purchased from
Aldrich. The other reagents (analytical grade) were obtained
from Beijing Chemical Reagent Plant. Nafion1 NR50 resin
(Lancaster Chemical, 10–35 mesh), whose physicochemical
properties are listed in Table 1 (last entry), was dissolved in a
mixture of deionized water and propanol under elevated
temperature and pressure to form a solution containing
5 wt.% Nafion according to the procedure described in [31].
The Nafion/SiO2 nanocomposites with different Nafion
contents (5–20 wt.%) were prepared by incorporation of the
dissolved Nafion in the 5 wt.% Nafion solution into the pore
system of silica by an in situ sol–gel method with
tetraethoxysilane for the silica source; details of the
preparation were reported in [32]. The nanocomposites
were dried overnight under vacuum at 150 8C and were
sieved into 40–80 mesh before they were used for any
physical characterizations and/or catalytic reaction tests.
Measurements of the nitrogen adsorption–desorption
isotherms on the nanocomposites were performed on a
Micromeritics ASAP 2010C instrument at �196 8C. TEM
measurements of some samples were performed on a Hitachi
H-800 transmission electron microscope.
2.2. Dehydration of 2-propanol
The catalytic dehydration of 2-propanol was carried out
with 25 mg catalyst (40–80 mesh) in a U-shaped tubular
quartz reactor under atmospheric pressure. The reactant was
introduced into the reactor by bubbling the carrier gas
(nitrogen, 50 ml/min) through 2-propanol maintained at ice
temperature (0 8C). The effluent from the reactor was
analyzed with an online GC. The reaction partial pressure
and weight hourly space velocity (WHSV) of 2-propanol
were 6.67 mmHg and 2.87 h�1, respectively.
afion contents
areab (m2/g) Pore volumeb (cm3/g) Pore sizeb (nm)
0.77 5.3
0.72 5.2
0.55 4.7
– –
alysis (TGA) and acid–base titration, respectively. The values are consistent
ns.
ore volume and the average pore size are derived from the BJH approach.
H. Wang, B.-Q. Xu / Applied Catalysis A: General 275 (2004) 247–255 249
Fig. 1. Adsorption–desorption isotherms of Nafion/SiO2 nanocomposites
with different Nafion contents.
2.3. Synthesis of a-tocopherol
The synthesis of a-tocopherol was done according to the
alkylation–condensation reaction using isophytol (IP) and
trimethylhydroquinone (TMHQ) for the reactants (Scheme
1). The reaction was carried out in a four-necked glass flask
equipped with a reflux condenser under flowing nitrogen at
atmospheric pressure. Isophytol (16 mmol) was added
dropwise for 1 h to a stirred mixture of trimethylhydroqui-
none (16 mmol) and catalyst in different solvents (20 ml) at
reflux. The solvents were thoroughly dried over 4 A
molecular sieves before being used.
When the addition of isophytol was completed, the
mixture was stirred for another 1 h and then the catalyst was
filtered off. The filtrate was concentrated under reduced
pressure to give a crude product. Yield and conversion were
determined by GLC analyses by comparison with authentic
external standards.
Fig. 2. Pore size distributions of Nafion/SiO2 nanocomposites with differ-
ent Nafion contents.
3. Results
3.1. Physicochemical properties of Nafion/SiO2
nanocomposites
The physicochemical properties of Nafion/SiO2 nano-
composites with different Nafion contents are shown in
Table 1. We found that the Nafion content and acid capacity
of the samples obtained from thermogravimetric analysis
(TGA) and acid–base titration, respectively, are consistent
with their theoretical values. All nanocomposite samples
featured high surface area, though the actual values
decreased with an increase in the Nation content from 5
to 20 wt.%. The surface areas of Nafion/SiO2 composites are
four orders of magnitude higher than that of the Nafion1
NR50 resin. Although the surface area of the composite is an
additive value for both silica and Nafion resin, it was verified
by a number of physicochemical characterizations including
TGA/TPD measurements of 2-propanol [13–15] and NH3
adsorptions [32] that the incorporated Nafion appears as
highly dispersed Nafion nanoparticles in porous silica during
the in situ sol–gel preparation of the composite sample, and
that the accessibility of Nafion-based acid sites to reactants
is greatly improved. We found that the specific surface area,
total pore volume and average pore size of the 20 wt.%
Nafion/SiO2 sample are remarkably lower than those of the
samples containing 5 and 13 wt.% Nafion; the differences in
the latter two samples are insignificant as judged by values
of the texture parameters.
The adsorption–desorption isotherms and pore size
distributions of Nafion/SiO2 composites with different
Nafion contents are presented in Figs. 1 and 2, respectively.
All samples give IV-type isotherms and H2-type hysteresis
loops. The H2-type hysteresis loop is usually attributed to a
combination of thermodynamic and pore connectivity
(network) effects and its relations to pore size distribution
and pore shape are not well defined [33]. The H2-type
hysteresis loop had been taken as an indication for the
presence of pores with narrow mouths (ink-bottle pores) but
was observed recently to appear on materials with relatively
uniform channel-like pores [33]. According to [33–35], the
inclination degree of the hysteresis loop can be used to
characterize pore size homogeneity and pore-connectivity of
solids. Clearly, the hysteresis loop of the 20 wt.% Nafion/
SiO2 composite shows a much higher inclination degree than
those of the two composites with lower Nafion content; this
higher inclination degree could imply a much higher
inhomogeneity in the pore size and poorer pore-connectivity
in the 20 wt.% Nafion/SiO2 composite. Accordingly, the
20 wt.% Nafion/SiO2 composite was narrower in pore-
size distribution and exhibited much smaller pore volume
than the composites with lower Nafion content (Fig. 2 and
Table 1).
H. Wang, B.-Q. Xu / Applied Catalysis A: General 275 (2004) 247–255250
Fig. 3. Pore size distributions of 13 wt.% Nafion/SiO2 nanocomposite
before and after calcination at 700 8C.
Fig. 4. TEM images of Nafion/SiO2 nanocomposites with differe
We also examined the textural properties (e.g. pore size
distribution) of the porous silica-matrix of the 13 wt.%
Nafion/SiO2 composite by calcination in air up to 700 8C to
completely remove the incorporated Nafion resin. Fig. 3
compares the pore size distributions of the sample before
(with Nafion resin incorporated) and after (with Nafion resin
removed) the calcination. Clearly, the pore size distribution
was broadened from 2–11 to 4–18 nm after the removal of
Nafion resin. Assuming that the pore-size broadening was
solely due to the removal of incorporated Nafion resin from
the pores of the silica-matrix, the particle size of Nafion1
NR50 resin in the 13 wt.% Nafion/SiO2 composite is thus in
the range of 2–15 nm, narrower than the 10–20 nm size
reported by Harmer et al. for a similar Nafion/SiO2 sample
prepared using tetramethoxylsilane for the silica source
[13,14,19]. These results suggest that the Nafion resin
entities in the Nafion/SiO2 composites are present as highly
dispersed nanoparticles within the pores of the silica-matrix,
confirming the nanocomposite nature of the samples [14,32].
nt Nafion contents: (A) 5 wt.%; (B) 13 wt.%; (C) 20 wt.%.
H. Wang, B.-Q. Xu / Applied Catalysis A: General 275 (2004) 247–255 251
Table 2
Dehydration of 2-propanol over Nafion/SiO2 nanocomposites with different Nafion contentsa
Nafion content (wt.%) Conversion (%) Turnover frequency (mmol/(mmol-H+�min)) Selectivity (%)
Propylene Diisopropyl ether
5 1.20 0.22 100 0
13 3.95 0.27 81.6 18.4
20 8.69 0.39 84.4 15.6
100b 1.80 0.02 100 0
a Reaction conditions: 80 8C; ambient pressure; the weight hourly space velocity (WHSV) and partial pressure of 2-propanol are equal to 2.87 h�1 and
6.67 mmHg, respectively; N2 used as balance gas.b Nafion1 NR50 resin (H+-form, 10–35 mesh).
Fig. 4 shows the TEM images of the Nafion/SiO2
composites. These images mainly reflect the particles of
silica in the samples since Nafion resin particles were
basically transparent to electrons. It can be seen that
particles of the silica-matrix became larger with increasing
the Nafion content. And, also, a much higher agglomeration
state is evident in the 20 wt.% Nafion/SiO2 sample.
However, SEM/EDX and other characterization results of
these nanocomposites reported earlier in [32] have shown
that the incorporated nanoparticles of the Nafion resin were
distributed quite evenly throughout the pore system of the
silca matrix.
3.2. Dehydration of 2-propanol
We made use of 2-propanol dehydration to characterize
the acidity and catalytic behavior of Nafion/SiO2 composites
and Nafion1 NR50 resin. The results are presented in Fig. 5
and Table 2. The lowest temperature for the dehydration of
2-propanol (also defined as the onset temperature of
dehydration reaction) in Fig. 5 was taken as the reaction
Fig. 5. The lowest temperature at which dehydration of 2-propanol occurs
(based on the conversion less than 1%) vs. the Nafion content in Nafion/SiO2
nanocomposites. Reaction conditions: ambient pressure; the weight hourly
space velocity (WHSV) and partial pressure of 2-propanol are 2.87 h�1 and
6.67 mmHg, respectively; N2 used as balance gas.
temperature which effected 0.5–1% conversion for the 2-
propanol reactant. The data were measured by extrapolating
the temperature–conversion curves of the reaction. It is seen
that the onset temperature of 2-propanol dehydration
decreases with increasing the Nafion content and finally
reaches the lowest value (60 8C) at the Nafion content of
30 wt.%. Apparently, the order of the acid strength of these
catalysts was the reverse of the onset temperature in Fig. 5.
When the Nafion content was increased from 5 wt.% to 13
and then further to 20 wt.%, the conversion of 2-propanol at
80 8C increases from 1.20 to 3.95% and then further to
8.69% (Table 2). The conversion of 2-propanol was
converted into the catalytic turnover frequency (TOF) based
on the number of acidic protons of the incorporated Nafion
resin in the composite catalyst. It is seen that the TOF
number, and hence the acid strength of the acidic protons,
increased with the increase in the Nafion content up to
20 wt.%. This conclusion is consistent with that of Palinko et
al. [36] who studied the acid strength with in situ FT-IR and
found that interactions between the sulfonic groups of
Nafion resin and the hydroxyl groups of SiO2-matrix led to a
decrease in acid strength of the acidic protons due to a
leveling effect of the hydrating environment in the
composites. The leveling effect becomes less effective
when the Nafion content is increased to more than 20 wt.%
[36].
The unincorporated Nafion resin itself, i.e. Nafion1
NR50, which showed the lowest onset temperature for 2-
propanol dehydration and was the strongest in acidity, rather
exhibited the lowest activity by TOF for the dehydration
reaction. The TOF over the Nafion1 NR50 catalyst was 10–
20 times lower than that over the Nafion/SiO2 composites.
This unusually low activity of the Nafion1 NR50 catalyst is
an artifact of averaging the activity by all the acid sites
(sulfonic groups) since only a very small percentage of the
acid sites were accessible to the reactant molecules;
Nafion1 NR50 appears in a condensed state and has an
extremely low surface area (0.02 m2/g); the majority of acid
sites are buried in the bulk of the resin [13–15].
Only intramolecular dehydration product (propylene)
was detected over the Nafion1 NR50 and 5 wt.% Nafion/
SiO2 catalysts, while a significant amount of intermolecular
dehydration product (diisopropyl ether) was formed over the
13 and 20 wt.% Nafion/SiO2 composites. The formation of
H. Wang, B.-Q. Xu / Applied Catalysis A: General 275 (2004) 247–255252
Table 3
Synthesis of a-tocopherol in different solvents over 13 wt.% Nafion/SiO2 nanocompositea
Entry Catalyst loading (mg) Amount of Nafion (mg) Solvent Reaction temperatureb (8C) Yieldc (%)
1 400 52 Ethyl acetate 77 1.3
2 600 78 Ethanol 78.5 0.8
3 400 52 2-Butanone 80 0.4
4 600 78 Benzene 80 0.7
5 600 78 n-Heptane 98 90.4
6 600 78 Toluene 110 91.7
7 400 52 Toluene 80 1.5
8 600d 78 Toluene 110 68.8
a Reaction conditions: TMHQ/IP = 1:1 (16 mmol); 20 ml solvent; 2 h; flowing nitrogen gas as protective atmosphere.b Reaction temperature is also boiling point of solvent except for entry 7.c Yield based on isophytol.d Regenerated 13 wt.% Nafion/SiO2 nanocomposite after it was used in entry 6.
intermolecular dehydration product hints more or less
diffusion limitation for the reaction over the Nafion/SiO2
nanocomposite catalysts of higher Nafion content. Also, the
absence of diisopropyl ether in the products over Nafion1
NR50 further supports the conclusion that Nafion resin in the
Nafion/SiO2 composites is incorporated into the pores of the
SiO2-matrix.
3.3. Synthesis of a-tocopherol
Since pure isophytol (IP), a tertiary and allylic alcohol,
can easily dehydrate to form phytadienes in the presence of
acid catalysts, the synthesis of a-tocopherol was carried out
by adding dropwise the required IP reactant into the re-
fluxing solution containing TMHQ and the catalyst. Table 3
gives the synthetic results in different solvents over the
13 wt.% Nafion/SiO2 composite (entry 1–6). Note that the
reaction temperature was equal to the boiling point of the
solvent used. It appears that 13 wt.% Nafion/SiO2 composite
presents excellent catalytic performance (the yield of
Fig. 6. Influence of catalyst loading on the synthesis of a-tocopherol over
Nafion/SiO2 nanocomposites with different Nafion contents. Reaction
conditions: 110 8C; TMHQ/IP = 1:1 (16 mmol); 20 ml toluene; 2 h; flowing
nitrogen gas as protective atmosphere.
a-tocopherol being more than 90%) in nonpolar solvents
such as n-heptane (entry 5) and toluene (entry 6). On the
contrary, the yield of a-tocopherol is extremely low (ca. 1%)
in other solvents (entry 1–4). One possible reason for the
poor yields in the syntheses using ethyl acetate, ethanol, 2-
butanone and benzene for the solvents is that the boiling
points of these solvents are lower than 100 8C, which made it
impossible to carry out the reaction at temperatures enabling
evaporation of the water product from the reactor. Water
molecules formed during the reaction may interact with the
catalytic acid sites (sulfonic groups) and inhibit the desired
reaction. Evaporation of water during the reaction at
temperatures of 100 8C or higher can lower the concentra-
tion of water in the reaction mixture and promote desorption
of water molecules from the catalyst, thus greatly enhancing
formation of the desired a-tocopherol. This explanation is
verified by the fact that, with toluene for the solvent, a
lowering of the reaction temperature from 110 to 80 8Creduced the yield of a-tocopherol from 91.7% (entry 6) to
1.5% (entry 7).
Fig. 6 shows the effects of the catalyst loading in the
reactor and of the Nafion content in the catalyst on the yield
of a-tocopherol at 110 8C when toluene was used for the
solvent. When the 13 wt.% Nafion/SiO2 composite was used
for the catalyst, the yield of a-tocopherol increases steadily
with increasing the catalyst loading up to 400 mg, the
reaction was then slowed down to a completion with ca.
100% a-tocopherol yield on further increasing the catalyst
amount to 800 mg. The effects were quite similar when
either 5 or 20 wt.% Nafion/SiO2 composite was used for the
catalyst. When the yields of a-tocopherol were compared on
the basis of equal amounts of the catalyst loading, however,
the highest yield was obtained on 13 wt.% Nafion/SiO2
composite and the lowest yield on 20 wt.% Nafion/SiO2
catalyst. Since the acid sites in the Nafion resin should be
responsible for the catalysis, we tried to calibrate the
catalysis by plotting in Fig. 7 the a-tocopherol yield against
the ‘‘net’’ amount of Nafion inside the reactor when the
Nafion/SiO2 composites and also a ‘‘pure’’ Nafion1 NR50
were used to catalyze the reaction. Apparently, the yield of
a-tocopherol was in proportion to the ‘‘net’’ amount of
H. Wang, B.-Q. Xu / Applied Catalysis A: General 275 (2004) 247–255 253
Fig. 7. The dependence of yield for a-tocopherol on Nafion weight within
Nafion-based catalysts with different Nafion contents. Reaction conditions:
110 8C; TMHQ/IP = 1:1 (16 mmol); 20 ml toluene; 2 h; flowing nitrogen
gas as protective atmosphere.
Nafion in each case. However, the ascending rates for
different Nafion-based catalysts are different, viz., the
catalytic efficiency based on equal amounts of Nafion
decreases with increasing the Nafion content in the Nafion/
SiO2 composites.
Detailed information for the catalytic performances of
the Nafion-based catalysts is given in Table 4; the data were
obtained with a ‘‘net’’ amount of the Nafion resin (52 mg, or
an Nafion acidity of 0.046 mmol), except that the used
amount of Nafion1 NR50 was 500 mg and it had one order
of magnitude higher acidity (0.45 mmol) than composite
catalysts. The composite catalysts containing 5 and 13 wt.%
Nafion enabled similar IP conversion levels, but the former
catalyst gave a higher yield and a higher TOF than the latter
for the formation of the desired product, a-tocopherol. The
catalytic performance of the composite containing 20 wt.%
Nafion was the poorest for the synthesis of a-tocopherol.
The large amount (500 mg) of Nafion resin catalyst in its
condensed state, i.e. Nafion1 NR50, produced an IP
conversion that is similar to those of the 5 and 13 wt.%
Nafion/SiO2 catalysts, but the yield of a-tocopherol
appeared in between those over the 5 and 13 wt.%
Nafion/SiO2 catalysts. Moreover, it is important to note
Table 4
Synthesis of a-tocopherol over Nafion/SiO2 nanocomposites with different Nafio
Catalyst loading (mg) Nafion content (wt.%) Weight of Nafion (m
1040 5 52
400 13 52
260 20 52
500 100d 500
a Reaction conditions: 110 8C; TMHQ/IP = 1:1 (16 mmol); 20 ml toluene; 2b Conversion and yield based on isophytol.c Turnover frequencies outside and inside the parentheses are based on a-tocd Nafion1 NR50 resin (H+-form, 10–35 mesh).
that the number of TOF for producing a-tocopherol over the
Nafion1 NR50 catalyst was slightly lower than that over the
20 wt.% Nafion/SiO2 composite but it was one order of
magnitude lower than those over the 5 and 13 wt.% Nafion/
SiO2 catalysts. Also, it should be mentioned that the a-
tocopherol yield (86%) over the Nafion1 NR50 catalyst in
the present study was higher than that (75%) reported by
Schager and Bonrath [12] who used the same catalyst for the
reaction at 110 8C with toluene for the solvent.
We attempted to reuse the 13 wt.% Nafion/SiO2
composite for the synthesis of a-tocopherol; the results
are given in Fig. 8. It is seen that the catalytic efficiency was
reduced during the repeated reuse of the catalyst. A gradual
change in the color of the catalyst was observed and the
color became black after it was reused three times. The
deactivation and coloring of the catalyst may be caused by
adsorption of reactants and by-products (phytadienes and
furan derivatives) on the composite catalyst according to
[6,7,12]. We found that the deactivated catalyst can be
partially regenerated by washing several times with acetone
and nitric acid. The regenerated 13 wt.% Nafion/SiO2
catalyst gave a yield of ca. 69% for the desired a-tocopherol
(entry 8 in Table 3).
4. Discussion
The present data show that the Nafion nanoparticles
incorporated in the porous Nafion/SiO2 composites are
much more effective catalysts than the condensed Nafion1
NR50 resin for the synthesis of a-tocopherol from IP and
TMHQ (Scheme 1). For better understanding of the catalysis
leading to a high yield of the desired a-tocopherol product, it
is essential to correlate the catalyst performance with the
accessibility/dispersion and acid strength of the Nafion resin
in the Nafion/SiO2 composites, and with their pore structure.
The present measurement of the acid catalysis with the
dehydration reaction of 2-propanol agrees well with the
conclusion of Palinko et al. [36] that the acid strength of
Nafion-based acid sites is weakened due to interactions with
the silica matrix in the Nafion/SiO2 composites. However,
this weakening in acid strength of the acid sites becomes less
pronounced with increasing the Nafion content or with
Nafion particles of lower dispersion in the composite
n contentsa
g) Conversionb (%) Yieldb (%) Turnover frequencyc
(mmol/(mmol-H+�h))
98.4 98.4 170.1 (170.1)
100 82.9 143.3 (172.9)
80.4 11.8 20.4 (139.0)
100 85.7 15.4 (18.0)
h; flowing nitrogen gas as protective atmosphere.
opherol produced and isophytol reacted, respectively.
H. Wang, B.-Q. Xu / Applied Catalysis A: General 275 (2004) 247–255254
Fig. 8. The yield of a-tocopherol vs. the number of utilizations of 13 wt.%
Nafion/SiO2 nanocomposite. Reaction conditions: 0.40 g catalyst; 110 8C;
TMHQ/IP = 1:1 (16 mmol); 20 ml toluene; 2 h; flowing nitrogen gas as
protective atmosphere.
catalysts. No correlation exists between the catalyst acid
strength and the yield ofa-tocopherol because of the fact that,
at complete conversion of IP, comparable yields of a-
tocopherol were obtainable over Nafion1 NR50 and the
Nafion/SiO2 catalysts containing 5 and 13 wt.% Nafion (Table
4). This is in contrast with the alkylation reactions of isobutane
with 2-butene [23] and of benzene with linear C9–C13 mixed
alkenes [29], where stronger acid sites connected with less
dispersed Nafion particles in Nafion/SiO2 composites showed
higher activity, selectivity and stability in the catalysis.
The tenfold difference in TOF numbers for the
production of a-tocopherol (Table 4) between Nafion1
NR50 (15 h�1) and the 5 and 13 wt.% Nafion/SiO2 catalysts
(140–170 h�1) reveals that the dispersion or accessibility of
the Nafion-based acid sites is crucial for the required
catalysis. Compared with 5 wt.% Nafion/SiO2 catalyst, the
significantly lower yield for a-tocopherol based on the
‘‘net’’ amount of Nafion resin in the 13 wt.% Nafion/SiO2
catalyst (Fig. 7 and Table 4) would suggest an involvement
of diffusion limitation in the reaction. Indeed, the pore size
distribution of 13 wt.% Nafion/SiO2 catalyst was narrower
than that of 5 wt.% Nafion/SiO2 catalyst (Fig. 2). The
diffusion limitation can led to longer residence of reactant/
product molecules and has resulted in the formation of a
significant amount of diisopropyl ether in the dehydration of
2-propanol over the former catalyst.
The 20 wt.% Nafion/SiO2 catalyst showed a much lower
efficiency for producing a-tocopherol (Table 4 and Figs. 6
and 7). Since Nafion resin in this 20 wt.% Nafion/SiO2
catalyst showed in Table 2 the highest catalytic TOF for the
dedydration of 2-propanol, the lower efficiency can not be
explained by a lower dispersion of the Nafion resin. It is clear
in Table 4 that the difference between the conversion of IP
and the yield of a-tocopherol is also the highest for this
20 wt.% Nafion/SiO2 catalyst. Using the IP conversion data,
we calculated another set of catalytic TOF for the reaction
and these values are put in the parentheses after the TOF
number for producing the desired a-tocopherol. The TOF
based on the converted IP molecules (139 h�1) over 20 wt.%
Nafion/SiO2 catalyst is only 20% lower than those (ca.
170 h�1) over 5 and 13 wt.% Nafion/SiO2 catalysts, but is 8
times higher than that (18 h�1) over Nafion1 NR50 resin. It
is therefore conclusive that diffusion limitation is the main
cause for the lower yield of a-tocopherol over the 20 wt.%
Nafion/SiO2 catalyst. The much higher inhomogeneity and
narrower distribution in the pore size, and poorer pore-
connectivity in the 20 wt.% Nafion/SiO2 catalyst, as
indicated by its textural parameters (Table 1 and Fig. 2),
give further support for the conclusion.
The difference between the two TOF numbers for the
Nafion/SiO2 catalysts could have connection with the reaction
kinetics. The huge difference over the 20 wt.% Nafion/SiO2
catalyst suggests that the majority of the reacted IP molecules
were converted to by-products other than the desired a-
tocopherol, even though the catalytic synthesis was designed
by dropwise adding IP to avoid undesired reactions of IP. As
were mentioned in the literatures [6,7], phytadienes and furan
derivatives were detected in this work as the major by-
products over 20 wt.% Nafion/SiO2 catalyst. Fortunately, the
undesired reactions of IP were effectively reduced over
13 wt.% Nafion/SiO2 catalyst and were successfully avoided
over 5 wt.% Nafion/SiO2 catalyst with more open textures.
Over Nafion1 NR50 resin, the consistency in the two TOF
numbers (15 h�1 versus 18 h�1) also reveals little diffusion
effect, but low accessibility of the acidic sites (sulfonic
groups) in this condensed state of the resin made it one order
of magnitude less active for the synthesis of a-tocopherol.
Thus, besides a confirmation of earlier observations that acid
sites connected with the incorporated Nafion nanoparticles in
Nafion/SiO2 composites are highly accessible for organic
reactions [13–30], the present catalytic data in the synthesis
of a-tocopherol further reveal the importance of pore size
distribution on the reaction selectivity. For a given synthesis,
optimization of the pore structure in the preparation of the
composite material could further reduce the required amount
of Nafion inside the pores of silica-matrix and could increase
selectivity of the desired product.
The exploratory data in Fig. 8 suggest that Nafion/SiO2
catalyst can be recyclable in the synthesis of a-tocopherol,
though our attempt to regenerate the reused catalyst did not
completely recover the catalytic efficiency (Table 3). With
systematic investigation on, and optimization of, the
recovery chemistry, it would be possible to further reduce
the activity loss in the recovered Nafion/SiO2 composites.
5. Conclusions
Compared with Nafion1 NR50 resin, highly dispersed
Nafion nanoparticles incorporated in the porous silica-
H. Wang, B.-Q. Xu / Applied Catalysis A: General 275 (2004) 247–255 255
matrix of Nafion/SiO2 nanocomposites exhibit significantly
enhanced catalytic activities for the synthesis of a-
tocopherol owing to the increased dispersion or accessibility
to reactants of the Nafion-based acid sites. In addition to the
dispersion of Nafion resin, the pore size distribution also
has pronounced effects on the catalytic efficiency for the
synthesis of a-tocopherol. Diffusion limitation in the
nanocomposite catalysts with higher Nafion content leads
to significantly lower yield for the desired a-tocopherol
because undesired reactions of the isophytol reactant
become more favorable in the narrower pores of the
composite catalysts. The acid strength of the nanocomposite
catalysts, which increases with increasing the Nafion content
or with decreasing the Nafion dispersion, shows little effects
on the catalytic efficiency.
Acknowledgments
The authors thank the National Natural Science
Foundation of China (grant: 20125310) and the National
Basic Research Program (grant: 2003CB615804) of China
for the financial support of this work.
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