Alkylation catalysts Nafion

Nafion-H, a perfluorinated sulfonic acid resin, is another strongly acidic solid that has been explored as alkylation catalyst. Rprvik et al. (204) examined unsupported Nafion-H with a nominal surface area of 0.2 m2/g (surface area of a swellable polymer is difficult to define) in isobutane/2-butene alkylation at 353 K and compared it with a CeY zeolite. The zeolite gave a better alkylate and higher conversion than Nafion-H, which produced significant amounts of octenes and heavy-end products. The low surface area of the resin and questions about the accessibility of the sulfonic acid groups probably make the comparison inadequate. 

Figure 13.8 Catalyst decay during the isobutane alkylation on nafion/Si02, sulfated zirconia, beta-zeolite, and MCM-41-supported 12-tungstophosphoric acid.

Substantial progress has been made to carry out alkylation in the gas phase over solid superacid catalysts. Nafion-H, a perfluorinated resin-sulfonic acid, for example, catalyzes the methylation of benzene and methylbenzenes with methyl alcohol under relatively mild conditions. The reaction shows low substrate selectivity.203... 

The conventional resinsulfonic acids such as sulfonated polystyrenes (Dowex-50, Amberlite IR-112, and Permutit Q) are of moderate acidity with limited thermal stability. Therefore, they can be used only to catalyze alkylation of relatively reactive aromatic compounds (like phenol) with alkenes, alcohols, and alkyl halides. Nafion-H, however, has been found to be a suitable superacid catalyst in the 110-190°C temperature range to alkylate benzene with ethylene (vide infra) 16 Furthermore, various solid acid catalysts (ZSM-5, zeolite /3, MCM-22) are applied in industrial ethylbenzene technologies in the vapor phase.177... 

Alkyl mesylates undergo alkylation reaction with benzene rings in the presence of Sc(OTf)3. " Allylic acetates undergo alkylation with Mo(CO)g and allylic chlorides react in the presence of ZnCl2/Si02. " Montmorillonite clay (KIO) is an effective medium for alkylation reactions.Nafion-H, a super acidic perfluori-nated resin sulfonic acid, is a very good catalyst for gas phase alkylations with alkyl halides, alcohols,or alkenes. 

Another type of soHd supetacid is based on perfluorinated resin sulfonic acid such as the acid form of Du Font s Nafion resin, a copolymer of a perfluorinated epoxide and vinylsulfonic acid or soHd, high molecular weight petfluotoalkanesulfonic acids such as petfluotodecanesulfonic acid, CF2(CF2)qS02H. Such sohd catalysts have been found efficient in many alkylations of aromatic hydrocarbons (225) and other Friedel-Crafts reactions (226). 

As catalysts Lewis acids such as AICI3, TiCU, SbFs, BF3, ZnCh or FeCl3 are used. Protic acids such as FI2SO4 or FIF are also used, especially for reaction with alkenes or alcohols. Recent developments include the use of acidic polymer resins, e.g. Nafion-Fl, as catalysts for Friedel-Crafts alkylations and the use of asymmetric catalysts. ... 

Fewer examples are reported for organic electrode reactions some alkyl halides were catalytically reduced at electrodes coated with tetrakis-p-aminophenylporphy-rin carboxylate ions are oxidized at a triarylamine polymer and Os(bipy)3 in a Nafion film catalytically oxidizes ascorbic acid Frequently, modified electrodes fail to give catalytic currents for catalyst substrate combinations that do work in the homogeneous case even when good permeability of the film is proven... 

The availability of Nafion on silica has not only lowered the cost of the resin but also has made it versatile (Sun et al., 1997 Harmer et al., 1998). A number of industrially important reactions have been attempted, with considerable success, with these catalysts. Consider the Fries rearrangement of phenyl acetate to p-acetyl phenol (/t-hydroxy acetophenone). This has been accomplished by Hoelderich and co-workers (Heidekum, 1998). In the ca.se of alkylation of benzene with benzyl alcohol, Amberlyst-15 and p-toluene sulphonic acid are ineffective and Nafion on silica works well at 80 °C. 

The water-insoluble salts such as Cs2,5Ho., iPWi204o efficiently catalyse dehydration of 2-propanol in the gas phase and alkylation of m-xylene and trimethyl benzene with cyclohexene this catalyst is much more active than Nafion-H, HY-zeolite, H-ZSM-5, and sulphated zirconia (Okuhara et al., 1992). 

Specific catalytic activity of the composites obtained was at least several times higher than the same value for the random copolymer Nafion (even in an esterification reaction considered to be a diffusion-uncontrolled reaction). For the oligomerization reaction of decene-1 with strong diffusion control, the specific catalytic activity of the composites was 35 times higher than that for the random copolymer. Esterification of acrylic acid and alkylation of mesitilene by a substituted phenol were also performed using the composite catalyst. 

Alkyl nitrates in the presence of sulfuric acid" and Lewis acids/ like SnCU, AICI3, and BF3, have been used as nitrating agents. Nitrations in the presence of Nafion-H acidic resin have also been reported." Alkyl nitrates do not effect the nitration of aromatic substrates in the absence of an acid catalyst. 

Disproportionation (transalkylation) and positional isomerization usually take place simultaneously when either linear or branched alkylbenzenes are treated with conventional Friedel-Crafts catalysts or with Nafion-H. The reactivity of alkyl groups to participate in transalkylation increases in the order ethyl, propyl < isopropyl < tert-butyl.117 207 217... 

A comparative study of nanocomposites (16% Nafion-silica and commercial SAC-13) has been performed by Hoelderich and co-workers169 in the alkylation of isobutane and Raffinate II. Raffinate II, the remaining C4 cut of a stream cracker effluent after removal of dienes, isobutane, propane, and propene, contains butane, isobutylene, and butenes as main components. High conversion with a selectivity of 62% to isooctane was found for Nafion SAC-13 (batch reactor, 80°C). Both the quality of the product and the activity of the catalysts, however, decrease rapidly due to isomerization and oligomerization. Treating under reflux, the deactivated catalysts in acetone followed by a further treatment with aqueous hydrogen peroxide (80°C, 2 h), however, restores the activity. 

In the alkylation of ethylbenzene with ethylene, with conventional acid catalysts under usual conditions,. veobutyl benzene is a byproduct. rec-Butylbenzene was detected when the reaction was carried out over catalysts such as supported phosphoric acid,189 ferric phosphate,189 or AICI3—NiO—Si02-190 When Nafion-H or AICI3 are used, no such byproduct is detected, probably due to fast dealkylation of sec-butylbenzene under the more acidic conditions. 

The high acidity of the Nafion-H catalyst is further demonstrated by its ability to promote both polyalkylation and isomerization. In reaction between benzene and ethylene at 190°C, 20% of the alkylated products are diethylbenzenes.187 The isomer distribution of the diethylbenzenes is 1 % of the ortho, 75% of the meta, and 24% of the para isomers. This composition is very close to the equilibrium composition of diethylbenzenes determined in solution chemistry with AICI3 catalyst and indicates that the reaction is thermodynamically controlled. 

An alkene mixture of industrial source (equal amounts of C9-C13 alkenes and alkanes) was used in the alkylation of benzene on three Nafion-silica catalysts with 5%, 13%, and 20% loadings.195 20% Nafion-silica showed high and stable activity and its performance exceeded that of a Y-zeolite-based material. The selectivity to 2-phenylalkanes (25%) was higher than in the Detal process using fluorinated silica-alumina but decreased somewhat with increasing Nafion content. 

When olefins are used as alkylating agents, the catalytic activity of Nafion-H slowly decreases, most probably due to some polymerization on the surface, which deactivates the catalytic sites. The activity decreases faster when more reactive branched alkenes are used. The use of alcohols instead of olefins as the alkylating agents improves the lifetime of the catalyst. With alcohols, no ready polymerization takes place, since water formed as byproduct inhibits polymerization of any olefin formed (by dehydration) but does not affect the acidity of the catalyst at the reaction temperatures. 

The alkylation of aromatic hydrocarbons with methyl alcohol over Nafion-H catalysts, including the mechanistic aspects, has been studied in detail. The degree of conversion of methyl alcohol was much dependent on the nucleophilic reactivity of the aromatic hydrocarbon. For example, the reactivity of isomeric xylenes was higher than that of toluene or benzene. 

Table 5.11. Alkylation of Benzene with Alcohols over Nafion-H Catalyst...

In gas-phase methylation reactions over Nafion-H using methyl alcohol as the alkylating agent, the consumption of methyl alcohol was higher than that calculated by product analysis.207,208 This is due to the formation of dimethyl ether as the byproduct [Eq. (5.82)]. Indeed, when neat methyl alcohol is passed over Nafion-H catalyst at temperatures over 150°C, dimethyl ether is the only product formed quantitatively with water as the byproduct.218... 

Friedel-Crafts alkylation of benzene,220 221 toluene,222para-xylene,220 and naphthalene223 with benzyl alcohols have been studied over Nafion-silica nano-composite catalysts, including the kinetics of alkylation.221,223 In most cases, 13% Nafion-silica showed the highest activity, testifying again to the much higher accessibility of the active sites. Complete conversion of para-xylene was found in the presence of triflic acid, and it was the only reaction when ether formation as side reaction did not occur. 

Satisfactory results were obtained in the Nafion-H-catalyzed gas-phase alkylation of aromatic hydrocarbons with alkyl halides235 [Eq. (5.88)]. Alkylhalides are reactive Friedel-Crafts alkylating agents and give high conversions when alkylating benzene in the gas phase over Nafion-H catalyst. For example, in the alkylation of benzene with isopropyl chloride, conversions as high as 87% were achieved (Table 5.17, run 11). Conversions, however, were temperature and contact time dependent (Table 5.17). 

The selectivity of the Nafion-H catalyst for monoalkylation has been found to be generally high. With a molar ratio of benzene isopropyl chloride being 5 1, about 94% of the alkylate is monoalkylbenzene. This result is comparable to the highly selective monoalkylation reaction reported by Langlois.236 They alkylated benzene with propylene (5.2 1 molar ratio) over H3P04-quartz catalyst at --200 C and obtained cumene in 95% yield. 

The results obtained in the gas-phase isopropylation of various aromatic hydrocarbons with isopropyl chloride over Nafion-H catalyst showed only a relatively small variation of reactivity in going from fluorobenzene to xylenes.235 Therefore, it has been assumed that the reaction rate is controlled by the formation of a reactive electrophilic intermediate (possibly, protonated alkyl halide 61, or some form of incipient alkyl cation) rather than by cr-complex formation between the electrophile and the aromatic nucleus [Eq. (5.89)]. 

Besides the advantage of their high reactivity toward alkylation reactions, primary and secondary alkyl halides show little tendency for dehydrohalogenation in Nafion-H-catalyzed gas-phase reactions.235 Although a minor amount of olefin is reported to be formed, no polymer formation was observed on the catalyst. As a result, the catalytic activity of Nafion-H stays constant over prolonged on-stream periods. 

As expected, the aptitude for disproportionation of the aromatic compound depends upon the nature of the alkyl group, and the order of reactivity is isopropyl > ethyl > methyl. Due to their higher nucleophilicity, polyalkylbenzenes react faster than monoalkylbenzenes. This effect is pronounced in the case of methylbenzenes. Toluene itself shows little reactivity over Nafion-H at 193°C. Diethylbenzenes react much faster than dimethylbenzenes. The rate of conversion of diethylbenzenes over Nafion-H at 193°C is 5 10-5 mol min 1 g-1 of catalyst.269 This is a low rate when compared with that using AICI3 HC1 in the liquid phase at room temperature (10-4 mol min-1 g-1 of catalyst).272 However, one should bear in mind that Nafion-H is a truly insoluble heterogeneous catalyst, whereas in the case of AICI3—HC1 a soluble complex is formed with the hydrocarbon and therefore the rates are not directly comparable. The equilibrium composition of the acid-catalyzed disproportionation of diethylbenzenes depends upon the nature of the catalyst. 

Table V summarizes several reactions that have been demonstrated on a laboratory scale 1 know of no industrialized chemical process using Nafion as a superacid catalyst. Although many of the reactions were carried out with stirring a mixture of reactants and Nafion-H, several alkylation, disproportionation, rearrangement, and esterification reactions were performed by means of the flow-reaction method in the liquid or gas phase. For instance, in the esterification of carboxylic acids with alcohols, when a mixture of the acid and alcohol was allowed to flow over a Nafion-H catalyst at 95-125°C with a contact time 5 s, high yields, usually S90%, of the corresponding ester were obtained (82). It had been found that no reactivation of the catalyst was needed because the catalytic activity of the Nafion remained unchanged for prolonged periods of operation.

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