Toluene acylation with zeolites

Among the wide variety of organic reactions in which zeolites have been employed as catalysts, may be emphasized the transformations of aromatic hydrocarbons of importance in petrochemistry, and in the synthesis of intermediates for pharmaceutical or fragrance products.5 In particular, Friede 1-Crafts acylation and alkylation over zeolites have been widely used for the synthesis of fine chemicals.6 Insights into the mechanism of aromatic acylation over zeolites have been disclosed.7 The production of ethylbenzene from benzene and ethylene, catalyzed by HZSM-5 zeolite and developed by the Mobil-Badger Company, was the first commercialized industrial process for aromatic alkylation over zeolites.8 Other typical examples of zeolite-mediated Friedel-Crafts reactions are the regioselective formation of p-xylene by alkylation of toluene with methanol over HZSM-5,9 or the regioselective p-acylation of toluene with acetic anhydride over HBEA zeolites.10 In both transformations, the p-isomers are obtained in nearly quantitative yield. 

Using acyl nitrates as nitrating agents (compare with Section 4.5.4.2) and zeolite H-ZSM-11 treated with tributylamine, Nagy et al. (1991, 1994) were able to nitrate toluene with an even more impressive percentage of the isomers obtained—ortho 2-3%, meta 1-2%, and para 95-98%. 

Numerous acylations mediated by zeolite materials were described since 1986, when the first results were reported.76 The acylation of toluene and p-xylene with C2—C20 alkanoic acids over Ce-exchanged NaY at 150°C in a batch reactor was reported. Shape selectivity was observed maximum activity was found for Ci2 and C 4 acids. Other studies showed that Br0nsted acid centers are involved in acylation77 and an electrophilic mechanism is operative.78 A comprehensive review2 of the subject is available and, therefore, only the more recent results are discussed here. 

The same effect is found for toluene acylation over BEA zeolite with derivatives of isobutyric acid isobutyric anhydride presents a higher initial activity compared with isobutyric chloride.[6]... 

Norrish Type I cleavage of benzylketones occurs very readily to yield benzyl and acyl radicals. In competition with other processes, these can couple by attack of the acyl radical upon the aromatic nucleus of the benzyl radical to give as a product an acyl toluene derivative. Turro has examined the photochemical properties of the 2-phenylalkanones (326) in solution and complexed with cyclodextrins. In solution the major fate of the Type I biradicals formed is intr2unolecular recombination by attack of the acyl radical at the para position of the benzyl radical to generate (327). However, complexatlon of (326) within the cyclodextrin cavity inhibits formation of (327) and promotes disproportionation of the blradicals to give (328). Turro has also examined the photochemistry of dlbenzylketone Included in zeolites.Recombination of the Type I radicals in this case can produce a mixture of (2-methyl-phenyl) benzylketone and (4-methylphenyl)benzylketone decarbonylation of the acyl radical competes and leads to the isolation of 1,2-dlphenylethane. It is found that the relative yields of the products... 

Table 1. Acylation of toluene with different acids over Ce -exchanged Y zeolite (150 °C, 48 h).

If the acylation of toluene with acetic acid depicted above [8] failed, use of the more active acyl chlorides instead of acids and La-exchanged Y-zeolite leads to the para-acylated product [20],... 

Several BEA zeolites have been tested as catalysts for the liquid-phase acylation of toluene with AAN (Table 4.7). [S]-Named zeolites are hydro-thermally prepared using amorphous silica as the silicon source, whereas [T]-named zeolites are prepared by using tetraethylorthosilicate (TEOS) as the silicon source with different SAR. The acylation process represents a very selective route to 4-methylacetophenone 18 small amounts of ortho- and mete-methylacetophenone 19 and 20 are produced in less than 2% of the total yield of the acetylated products. For the [S]-type BEA zeolites, the acidity values are in good agreement with the catalytic activity, following the order [S]-BEA(32) > [S]-BEA(26) > [S]-BEA(54) [S]-BEA(76). In the [T]-BEA series, the catalytic activity decreases when the SAR increases, regardless of the increase of hydrophobicity. The conclusion is that low SAR lead to better yields in acetophenones (APs). However, the poisoning... 

The vapor-phase acylation of aromafic hydrocarbons wifh acyl chlorides over differenf zeolifes was invesfigafed. Comparing fhe acetylation of toluene with AC over ZSM-5, MOR, and REY zeolites, ZSM-5 was the best catalyst affording para-methylacefophenone with 60% conversion of... 

Botella, R, Corma, A., Lopez-Nieto, J. M., Valencia, S., and Jacquot, R. 2000. Acylation of toluene with acetic anhydride over Beta zeolites influence of reaction conditions and physicochemical properties of the catalyst. /. Catal. 195 161-168. 

Gauthier et al. (1989) studied the activity of various cation-exchanged Y-type zeolites in the acylation of toluene with octanoic acid, obtaining selectivities to the para isomer of 94% at 75% yield of acylated product. The most efficient catalysts were rare-earth-exchanged zeolites (70% exchange), the following order of activity being observed Cr3+, Zr4+ < M2+, Cu2+, Co2+ <11 < Pr3+, La3+, Gd3+, Yb3+, Ce3+. 

In contrast to anisole, the acylation of toluene with HPA is far less efficient than that with H-Beta. These results can be explained by the well-known strong affinity of bulk HPA towards polar oxygenates, which would lead to the preferential adsorption of acetic anhydride on HPA, blocking access for toluene to the catalyst surface. It appears that the hydrophobic zeolites with high Si/Al ratios less strongly differentiate the adsorption than the hydrophilic HPA and, therefore, are more suitable catalysts for the acylation of nonpolar aromatics like toluene. 

Unfortunately, most heterogeneous catalysts investigated for this type of reaction have demonstrated only limited success, showing only high levels of activity for the acylation of activated rings with carboxylic acids. Rare earth exchanged zeolites have been shown to have some activity for acylation of toluene with carboxylic acids. ... 

The ion exchanged zeolite catalyst exhibits a maximum in activity for the acylation of toluene with decanoic acid. This trend has been previously observed by Chiche et al. when using this type of catalyst. The HPW30 catalyst appears to be much more active for reactions involving shorter chain acids however, there is a significant reduction in rate... 

Studies of the acylation of toluene with La-exchanged zeolite Y has shown the dependence of the activity on the rare-earth cation content and the high selectivity to the para isomer. Activities increasing in the order benzoyl < acetyl < propionyl chloride were found for all catalyst modifications. 

However, for liquid phase reactions, it is very difficult to correlate the results wiHi BrOnsted or Lewis acidity as the reaction conditions used are different from those used for characterization. When the Diels-Alder reaction is conducted in a solvent, it appears that the maximum for the activity of HY zeolites is obtained for a Si/Al ratio of 15. This maximum was also observed for esterification of carboiQ lic acids (14), methylthiolation of phenol with dimethyldisulfide (15), acylation of toluene with benzoic acids (15) or dehydration of fhictose (15), and in solvents such as alcohols, water or hydrocarbons. If we assume that Lewis species are transformed to Brdnsted ones in the presence of water as solvent, this would thus mean that the Diels-Alder reaction is preferentially catalyzed by BrSnsted species, the maximum observed at Si/Al=15 for HY zeolites being a good balance between the niunber 6ind the strength of the protonic species. 

Other liquid phase applications of zeolites on the laboratory scale have been reported, including a shape selective /7flr -acylation of toluene with carboxylic acids [15], and the production of 4,4 -diisopropylbiphenyl from biphenyl and propene is catalysed by the naturally occurring zeolite, mor-denite [16]. The latter reaction shows similar pore-imposed selectivity to the chlorination of biphenyl mentioned above. 

The hydrothermally prepared HBEA zeolites, using amorphous silica as the silicon source, give a very selective synthesis of 4-methylacetophenone 11 in the acylation of toluene with acetic anhydride. In particular, it is shown that HBEA(32) gives product 11 with 80% yield and 98% selectivity (Scheme 3.6) [31]. 

Cation-exchanged montmorillonites are utilized by Geneste et al. [67, 68] in the acylation of aromatics with carboxylic acids. Best results are achieved with AP -exchanged montmorillonite (60% yield of the ortho, meta, and para isomers). The influence of the chain length of different carboxylic acids on the yield of acylation appears to be similar on clays and on zeolites [35]. Indeed, acylation of toluene with CHj(CH2) COOH over Al -montmoriUonite gives 12% yield for n= 1,45% forn = 6, and 80% for = 14. 

Ordinary solid acids such as SiOz —AI2O3 and zeolites are almost or completely inactive for acylation reactions. However, a solid superacid, Zr02 — S04 (cf. Section 3.9) was recendy found to exhibit high activity for the acylation of chlorobenzene or toluene with benzoyl chloride or o-chlorobenzoyl chloride in liquid phase. This was ascertained by separation of the solid superacid from the reaction mixture during a reaction in which the solid superacid acted as the perfect heterogeneous catalyst. 

Reaction conditions ( )-1-Phenylethanol (0.122g, 1 mmol), acyl donors (1.5 mmol), zeolite beta with Si/AI = 150 (50mg), Novozym 435 (30 mg) and toluene (5 ml) at 60 °C. 

As reported in the literature, the acylation of aromatic hydrocarbons can be carried out by using zeolites as catalysts and carboxylic acids or acyl chlorides as acylating agents. Thus toluene can be acylated by carboxylic acids in the liquid phase in the presence of cation exchanged Y-zeolites (ref. 1). The acylation of phenol or phenol derivatives is also reported. The acylation of anisole by carboxylic acids and acyl chlorides was obtained in the presence of various zeolites in the liquid phase (ref. 2). The acylation of phenol by acetic acid was also carried out with silicalite (ref. 3) or HZSM5 (ref. 4). The para isomer has been generally favoured except in the latter case in which ortho-hydroxyacetophenone was obtained preferentially. One possible explanation for the high ortho-selectivity in the case of the acylation of phenol by acetic acid is that phenylacetate could be an intermediate from which ortho-hydroxyacetophenone would be formed intramolecularly. 

Our pioneering work in 1986[1] has shown that acid zeolites are efficient catalysts in the Friedel-Crafts acylation of toluene and xylene with carboxylic acids and constitutes a breakthrough in environmentally friendly fine chemistry replacing the conventional AICI3 method by a heterogeneous catalysts. Since this initial study, a tremendous amount of work has been performed in this area[2] and particularly, in recent years, the acetylation reaction, which is a field of research with large potential for the production of fine chemicals, has been intensively investigated. 

Chiche, B., Finiels, A., Gauthier, C., Geneste, P., Graille, J. and Pioch, D. Friedel-Crafts acylation of toluene and /7-xylene with carboxylic acids catalyzed by zeolites. J. Org. Chem., 1986, 51, 2128-2130. 

Klisakova, J., Cerveny, L. and Cejka, J. On the role of zeolite structure and acidity in toluene acylation with isobutyric acid derivatives, Appl. Catal., A, 2004, 272, 79-86. 

A number of protic acids have been used to catalyze acylation reactions. It is assumed that the reactions involve the generation of acylium ions. Polymeric reagents such as Nafion-H have been used for example 2-fluorobenzoyl chloride and toluene give the benzophenone derivatives with an ortho.para ratio of 4 81. ° A zeolite-catalyzed acylation (equation 6) has been reported to afford 4-dodecenoyltol-uene in 96% yield but the yields are low with short chain carboxylic acids. Early examples of the use of trifluoroacetic and perchloric acids reported good yields of products. Some more recent examples are shown in equations (7) to (9). Phosphoric and polyphosphoric acids have been used together with carboxylic acids (equation 10),anhydrides and acylureas (equation 11). °... 

Pioneering and extensive work in this field was realized in 1985 [8] by use of exchanged Ce Y-zeolite as the catalyst for the Friedel-Crafts acylation of toluene (Eq. 3) and xylene with carboxylic acids. Different aspects of this initial work are of interest. Firstly, it shows that the mild acidity of zeolites is sufficient to effect the reaction, and, secondly, it shows that this reaction can be conducted with carboxylic acids and not the corresponding acid chlorides. Only the more lipophilic acids were found reactive whereas no acetylation occurs with acetic acid. The reaction with toluene (Table 1) is extremely para selective and more selective than a conventional aluminum chloride homogeneous process. The different reactivities reported in this paper are essentially because of differences between preferential adsorption of the substrates on the catalyst and not their intrinsic activity. 

In terms of reactivity benzoylation seems to be a much easier reaction. By use of zeolites, benzoylation of toluene [23], xylene [24], and benzene [25] have been described under mild conditions (80-140 °C). It is probable that benzoylation is favored because of the greater affinity of the acylating agent for the heterogeneous catalyst. Interesting results have been found for acylation of benzene with phthalic anhydride to form anthraquinone (Eq. 11) [26]. When the H/0 cata-... 

In a series of valuable studies, the activity of various cation-exchanged Y zeolites in the acylation of toluene and xylenes with aliphatic carboxylic acids was investigated In a model reaction between toluene and octanoic acid, the activity of rare earth-, transition metal-, and alkaline earth-cation-exchanged Y zeolites was considered. CeY zeolite exhibits the highest activity (3 yield = 75%) in the para-acylation (Scheme 4.3) in agreement with the results published in an early study qj., contrary, unmodified Y zeolite shows a lower activity (3 yield < 40%), and transition metal and alkaline-earth-exchanged Y are nearly inactive. 

The process can be carried out with aromatic substrates such as benzene, toluene, and isopropylbenzene and different acylating agents (56%-99% conversion). Zeolite ZSM-5 fails to give any producf with other substrates such as xylenes, mesitylene, and N,N-dimethylaniline. 

More interestingly, carboxylic acids can be employed in the acylation of aromatic compounds. The acylation of toluene and anisole with C2-C42 aliphatic carboxylic acids can be carried out with CsPW, affording the corresponding products in 41%-71% yield. These solid acids are superior in activity to the conventional acid catalysts such as sulfuric acid and zeolites, and can be reused after a simple workup, albeit with reduced activity. 

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