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SiO2 nanocomposites for the synthesis of α-tocopherol
Applied Catalysis A: General 275 (2004) 247–255 www.elsevier.com/locate/apcata
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 nanocomposite 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 Nafionbased 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 Bro¨nsted 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
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the nanocomposites (acidity, accessibility of acid sites and pore structure) affect their catalytic behavior.
2. Experimental 2.1. Reagents, sample preparation and characterizations
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 reaction of trimethylhydroquinone (TMHQ) and isophytol (IP) (Scheme 1). Moreover, we show how the microstructures of
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.
Table 1 The physicochemical properties of Nafion/SiO2 nanocomposites with different Nafion contents Nafion contenta (wt.%)
Acid capacitya (mmol/g)
Surface areab (m2/g)
Pore volumeb (cm3/g)
Pore sizeb (nm)
5 13 20 100c
0.045 0.12 0.18 0.89
415 396 352 0.02
0.77 0.72 0.55 –
5.3 5.2 4.7 –
a The Nafion content and acid capacity were measured by thermogravimetric analysis (TGA) and acid–base titration, respectively. The values are consistent with the theoretical ones if one assumes no loss of Nafion during the preparations. b The specific surface area is obtained from the BET method, and the total pore volume and the average pore size are derived from the BJH approach. c Nafion1 NR50 resin (H+-form, 10–35 mesh).
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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 trimethylhydroquinone (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.
3. Results 3.1. Physicochemical properties of Nafion/SiO2 nanocomposites The physicochemical properties of Nafion/SiO2 nanocomposites 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
Fig. 1. Adsorption–desorption isotherms of Nafion/SiO2 nanocomposites with different Nafion contents.
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 poresize distribution and exhibited much smaller pore volume than the composites with lower Nafion content (Fig. 2 and Table 1).
Fig. 2. Pore size distributions of Nafion/SiO2 nanocomposites with different Nafion contents.
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Fig. 3. Pore size distributions of 13 wt.% Nafion/SiO2 nanocomposite before and after calcination at 700 8C.
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].
Fig. 4. TEM images of Nafion/SiO2 nanocomposites with different Nafion contents: (A) 5 wt.%; (B) 13 wt.%; (C) 20 wt.%.
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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 13 20 100b
1.20 3.95 8.69 1.80
0.22 0.27 0.39 0.02
100 81.6 84.4 100
0 18.4 15.6 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 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.
1
and
temperature which effected 0.5–1% conversion for the 2propanol 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 Pa´ linko´ 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 2propanol 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
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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 2 3 4 5 6 7 8
400 600 400 600 600 600 400 600d
52 78 52 78 78 78 52 78
Ethyl acetate Ethanol 2-Butanone Benzene n-Heptane Toluene Toluene Toluene
77 78.5 80 80 98 110 80 110
1.3 0.8 0.4 0.7 90.4 91.7 1.5 68.8
a b c d
Reaction conditions: TMHQ/IP = 1:1 (16 mmol); 20 ml solvent; 2 h; flowing nitrogen gas as protective atmosphere. Reaction temperature is also boiling point of solvent except for entry 7. Yield based on isophytol. 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 refluxing 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, 2butanone 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 concentration 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 8C reduced 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
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
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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 atocopherol 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 Pa´ linko´ 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
Table 4 Synthesis of a-tocopherol over Nafion/SiO2 nanocomposites with different Nafion contentsa Catalyst loading (mg)
Nafion content (wt.%)
Weight of Nafion (mg)
Conversionb (%)
Yieldb (%)
Turnover frequencyc (mmol/(mmol-H+h))
1040 400 260 500
5 13 20 100d
52 52 52 500
98.4 100 80.4 100
98.4 82.9 11.8 85.7
170.1 143.3 20.4 15.4
a b c d
Reaction conditions: 110 8C; TMHQ/IP = 1:1 (16 mmol); 20 ml toluene; 2 h; flowing nitrogen gas as protective atmosphere. Conversion and yield based on isophytol. Turnover frequencies outside and inside the parentheses are based on a-tocopherol produced and isophytol reacted, respectively. Nafion1 NR50 resin (H+-form, 10–35 mesh).
(170.1) (172.9) (139.0) (18.0)
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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 of a-tocopherol because of the fact that, at complete conversion of IP, comparable yields of atocopherol 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 poreconnectivity 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 atocopherol, 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 byproducts 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-
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matrix of Nafion/SiO2 nanocomposites exhibit significantly enhanced catalytic activities for the synthesis of atocopherol 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.
[9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
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.
[23] [24] [25] [26] [27] [28] [29]
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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 nanocomposite 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 Nafionbased 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 Bro¨nsted 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
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the nanocomposites (acidity, accessibility of acid sites and pore structure) affect their catalytic behavior.
2. Experimental 2.1. Reagents, sample preparation and characterizations
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 reaction of trimethylhydroquinone (TMHQ) and isophytol (IP) (Scheme 1). Moreover, we show how the microstructures of
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.
Table 1 The physicochemical properties of Nafion/SiO2 nanocomposites with different Nafion contents Nafion contenta (wt.%)
Acid capacitya (mmol/g)
Surface areab (m2/g)
Pore volumeb (cm3/g)
Pore sizeb (nm)
5 13 20 100c
0.045 0.12 0.18 0.89
415 396 352 0.02
0.77 0.72 0.55 –
5.3 5.2 4.7 –
a The Nafion content and acid capacity were measured by thermogravimetric analysis (TGA) and acid–base titration, respectively. The values are consistent with the theoretical ones if one assumes no loss of Nafion during the preparations. b The specific surface area is obtained from the BET method, and the total pore volume and the average pore size are derived from the BJH approach. c Nafion1 NR50 resin (H+-form, 10–35 mesh).
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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 trimethylhydroquinone (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.
3. Results 3.1. Physicochemical properties of Nafion/SiO2 nanocomposites The physicochemical properties of Nafion/SiO2 nanocomposites 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
Fig. 1. Adsorption–desorption isotherms of Nafion/SiO2 nanocomposites with different Nafion contents.
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 poresize distribution and exhibited much smaller pore volume than the composites with lower Nafion content (Fig. 2 and Table 1).
Fig. 2. Pore size distributions of Nafion/SiO2 nanocomposites with different Nafion contents.
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Fig. 3. Pore size distributions of 13 wt.% Nafion/SiO2 nanocomposite before and after calcination at 700 8C.
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].
Fig. 4. TEM images of Nafion/SiO2 nanocomposites with different Nafion contents: (A) 5 wt.%; (B) 13 wt.%; (C) 20 wt.%.
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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 13 20 100b
1.20 3.95 8.69 1.80
0.22 0.27 0.39 0.02
100 81.6 84.4 100
0 18.4 15.6 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 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.
1
and
temperature which effected 0.5–1% conversion for the 2propanol 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 Pa´ linko´ 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 2propanol 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
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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 2 3 4 5 6 7 8
400 600 400 600 600 600 400 600d
52 78 52 78 78 78 52 78
Ethyl acetate Ethanol 2-Butanone Benzene n-Heptane Toluene Toluene Toluene
77 78.5 80 80 98 110 80 110
1.3 0.8 0.4 0.7 90.4 91.7 1.5 68.8
a b c d
Reaction conditions: TMHQ/IP = 1:1 (16 mmol); 20 ml solvent; 2 h; flowing nitrogen gas as protective atmosphere. Reaction temperature is also boiling point of solvent except for entry 7. Yield based on isophytol. 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 refluxing 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, 2butanone 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 concentration 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 8C reduced 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
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
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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 atocopherol 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 Pa´ linko´ 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
Table 4 Synthesis of a-tocopherol over Nafion/SiO2 nanocomposites with different Nafion contentsa Catalyst loading (mg)
Nafion content (wt.%)
Weight of Nafion (mg)
Conversionb (%)
Yieldb (%)
Turnover frequencyc (mmol/(mmol-H+h))
1040 400 260 500
5 13 20 100d
52 52 52 500
98.4 100 80.4 100
98.4 82.9 11.8 85.7
170.1 143.3 20.4 15.4
a b c d
Reaction conditions: 110 8C; TMHQ/IP = 1:1 (16 mmol); 20 ml toluene; 2 h; flowing nitrogen gas as protective atmosphere. Conversion and yield based on isophytol. Turnover frequencies outside and inside the parentheses are based on a-tocopherol produced and isophytol reacted, respectively. Nafion1 NR50 resin (H+-form, 10–35 mesh).
(170.1) (172.9) (139.0) (18.0)
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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 of a-tocopherol because of the fact that, at complete conversion of IP, comparable yields of atocopherol 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 poreconnectivity 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 atocopherol, 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 byproducts 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-
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matrix of Nafion/SiO2 nanocomposites exhibit significantly enhanced catalytic activities for the synthesis of atocopherol 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.
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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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