Catalytic performance of Nafion/SiO2 nanocomposites for the synthesis of α-tocopherol

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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).


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.%.


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


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


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.


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-


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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Catalytic performance of Nafion/SiO2 nanocomposites for the synthesis of α-tocopherol

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