Nafion silica composites

Nafion is commercially available as Nafion NR50 (DuPont) in the form of relatively large (2-3 mm) beads. A disadvantage of these beads is the low surface area (typically 0.02 m g ) [8]. Water-selective Perma Pure membranes also consist of the polymer Nafion and were especially developed for removal of water from gas-phase streams [9]. Nafion is also available as a 5 % solution in a mixture of lower aliphatie alcohols and water [10]. 

What makes Nafion so useful for catalytic purposes is the superacidity of the acidic form of the polymers. Because the sulfonic acid group is attached to a highly electron-withdrawing perfluoroalkyl backbone, a relatively high polarization of the -OH bond results. Because methods for measuring solution acidity cannot be directly applied to heterogeneous solid materials, determination of the acidity of Nafion-H is difficult. Nevertheless, studies indicate that the acidic character of Nafion-H resin is comparable with that of 100% sulfuric acid, because the estimated Hammett Hq acidity function value of Nafion-H, approximately —12, is comparable with that of 100% sulfuric acid [1]. 

The properties of Nafion-H have led to an extensive development of different acid catalyzed reactions, especially by Olah and co-workers. Sharma et al. 

To overcome the disadvantage of the low surface area ( 0.02 m g ) of pure Nafion NR50 beads, researchers at DuPont developed Nafion-silica composites, in which small (20-60 nm) Nafion resin particles are embedded in a porous silica matrix [7]. The composites, available under the trade name SAC 13 (containing 13 % (w/w) Nafion) are prepared by a sol-gel technique. Because of the higher surface area and accessibility of the active sites the application of this material as a solid-acid catalyst has become attractive. 

In our study of the acylation of anisole with carboxylic acids [24], and the Nafion-silica composite as the catalyst, no leaching was detected during reaction conditions. This was also not observed in the study of Botella et al. [22] in the alkylation of 2-butene by isobutane. 

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Figure 20. Electro-osmotic drag coefficients of diverse membranes based on perfluorinated polymers (Dow - and Nafion/silica composites ) and polyarylenes (S—PEK/ PSU blends, ionically cross-linked S—PEK/PBP ), as a function of the solvent (water/methanol) volume fraction Xy (see text for references). Lines represent data for Nafion and S—PEK (given for comparison) for data points, see Figure 15. Dashed lines correspond to the maximum possible electro-osmotic drag coefficients for water and methanol, as indicated (see text).

The Pechmann reaction over Nafion/silica composites is an example that in some cases, zeolites show no advantage over other solid acids. Further progress, not only in the area of zeolites but also in the field of other solid acids like acidic ion-exchange resins, is unavoidable. 

The use of Nafion/silica composites and zeolite catalysts in this kind of reaction has been studied. These composites containing different amounts of Nafion catalyst are compared with the original material and with Amberlyst 15, a well known macroreticular ion-exchange resin, which achieves remarkable results in several acid catalyzed reactions (81). 

Figure 15.9 shows the esterification of different saturated carboxylic acids with dicyclopentadiene. In all depicted experiments the carboxylic acid/dicyclo— pentadiene molar ratio is about 4 and the amount of catalyst is 10% by weight of dicyclopentadiene at a reaction temperature of 80°C. In this reaction the Nafion/silica composite catalyst is more active compared to the Amberlyst resin, in particular with respect to the amount of the acid groups on the resin. 

Carrying out experiments with equal ratios of starting material 47 to sulfonic acid group of the specific catalysts reveals the same pattern as obtained in the experiments described above. Figure 5.2 shows the results obtained over some Nafion/silica composite materials with different amounts of Nafion compared to the Amberlyst ion-exchange resin. 

Fig. 15.10 Esterification of 47 with different carboxylic acids over Nafion/silica composites with different amount of Nafion and over Amberlyst 15...

All Nafion/silica composite catalysts show complete conversion independently of the amount of Nafion resin. In contrast, a conversion of only about 80% is obtained over the pure polymer. Again, the Amberlyst material achieves only poor conversion of about 12%. 

For instance, nafion composites give promising results. In the Pechmann reaction as well as in the esterification of dicyclopentadiene, the Nafion/silica composites are superior to pure Nafion, zeolites and ion exchange resins. We find high conversions at low temperatures, which can be attributed to accessible acidic sites with high acidic strength. 

Nafion-silica composites were compared with zeolites such as H-Beta, H-USY and H-ZSM-5, in the Fries rearrangement of phenyl acetate (see earlier). The highest conversion was observed with H-Beta [88]. 

Bronsted acids can also be fixed to the surfaces of many common support materials. Surface attached perfluorosulfonic acids have been reported123 as well as the perfiuorinated sulfonic acid resin (Nafion)-silica composites reported a year earlier.124 The former are prepared by reacting... 

A wide variety of solid-acid catalysts is available [17] acidic clays, zeolites, silica-occluded heteropoly acids, sulfonated polysiloxanes, Nafion (a sulfonated perfluoroalkyl resin) and Nafion-silica composites, and a variety of hybrid sulfonated mesoporous systems (see Chapter 3). 

The Use of Nafion and Nafion-Silica Composites in Solid Acid Catalysis... 

Different alkylation reactions have been investigated with Nafion as a catalyst. An example is the propylation of benzene to cumene. Large rate enhancements were observed on use of Nafion-silica composites, compared with bulk polymeric catalysts such as pure Nafion NR50 and Amberlyst-15. The activity of the composite was ca 6-7 times higher than that of pure Nafion beads on the basis of the total amount of catalyst. If this correlation were made on the basis of the total number of acid sites on Nafion, the activity would be ca 50 times higher. Amberlyst-15 was about twice as active as the Nafion NR50 particles (on the basis of the total amount of catalyst) [7]. 

In the alkylation of 2-butene by isobutane the activity of Nafion-silica composite was very high compared with pure Nafion and similar to that of zeolite H-BEA [22]. 

The Fries rearrangement of phenyl acetate to o- and p-hydroxyacetophenone is also a very valuable reaction in the pharmaceutical industry. Nafion-silica composites containing 13 and 40% (w/w) Nafion were also studied in this reaction and compared with pure Nafion and with zeolites such as HBEA, HUSY, HZSM5, and HY. The Nafion-silica composites resulted in better conversion than the pure analog, but in this study the BEA zeolite resulted in the highest conversion and a relatively high selectivity. Unfortunately, no reactions rates were given so no conclusions can be drawn about the intrinsic activity of the different catalysts [17]. 

Another solid-aeid catalyzed reaction in which the Nafion-silica composites are much more active than the non-supported pure resin and Amberlyst-15 is the addition-esterification of carboxylic acids to cyclic olefins. For example, the addition of saturated carboxylic acids to dicyclopentadiene, leading to starting esters for the flavor and fragrance industry [27]. 

Nafion-silica composite has proved to be a promising catalyst in acid catalysis. Different studies with this catalyst revealed very high activities per weight of catalyst compared with the pure Nafion and with other solid-acid catalysts, e. g. zeo-... 

The Nafion-silica composite is rather expensive, thus the commercially available catalyst, SAC13 (containing 13% Nafion) costs approximately 1000 kg ... 

Interesting reactivity has recently been found in the use of Nafion/silica composite material [30]. Nafion resins are active in the acylation of anisole with acetyl chloride, but entrapping highly dispersed nanosized Nafion particles in a silica matrix leads to a much more active catalyst, as indicated in Table 5. 

Substrate Nafion catalyst yield (%) Nafion/silica composite yield (%)... 

Extension of the scope of the this reaction by use of Nafion/silica composite resin shows that these catalysts can be active in the acylation of xylenes yet have low activity toward toluene (Table 6). 

A variety of other heterogeneously catalyzed Diels-Alder reactions has been reported. Nafion-H, a perfluorinated resinsulfonic acid, catalyzed several Diels-Alder reactions and the isolated yield of the adducts was 80-95% [46]. We have found that a recently described Nafion-silica composite catalyst containing 13 % (w/w) Nafion [47] was approximately 30 times more active than the pure resin in the Diels-Alder reaction of 2,3-dimethylbutadiene with 1,4-naphthoquinone [48]. We also showed that another strong heterogeneous Brpnsted acid, tung-stophosphoric acid supported on silica gel, is a very active catalyst of Diels-Alder reactions of quinones [49] and other enones [50]. 

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