Friedel-Crafts acylation reactions zeolites

Very interesting Friedel-Crafts acylation reactions using zeolites can also be found in the work of Bayer s team (ref. 3d) as well as Prins (ref. 3c). In both cases, work was realized on activated aromatics such as anisole. In the latter, Hb were found to exhibit particularly hight activity and selectivity independently of the Si-to-Al ratio of the zeolite. 

Figure 4.1 Structures of the most utilized zeolites in Friedel-Crafts acylation reactions. (From http //izasc.ethz.ch/ http //cpm.tnw.utwente.nl/ http //chemeducator.org/ http //www.zeolyst.com/ http //www.uni-giessen.de/cms/. With permission.)...

To hit these targets, a wide variety of solid cafalysfs have been utilized in Friedel-Crafts acylation reactions. Quite promising synthetic results have been achieved, and the reaction can be efficiently conducted with heterogeneous catalysts. These studies have led to the industrialization of at least one process, showing that all the problems of reactivity, catalyst deactivation, and engineering can be solved by using zeolites as catalysts. ... 

Friedel-Crafts acylation is widely used for the production of aromatic ketones applied as intermediates in both fine chemicals and pharmaceutical industries. The reaction is carried out by using conventional homogenous catalysts, which represents significant technical and environmental problems. The present work reports the results obtained in the Friedel-Crafts acylation of aromatic substrates (anisole and 2-methoxynaphthalene) catalyzed by Beta zeolite obtained by crystallization of silanized seeds. This material exhibits hierarchical porosity and enhanced textural properties. For the anisole acylation, the catalytic activity over the conventional Beta zeolite is slightly higher than with the modified Beta material, probably due to the relatively small size of this substrate and the weaker acidity of the last sample. However, the opposite occurred in the acylation of a bulky substrate (2-methoxynaphthalene), with the modified Beta showing a higher conversion. This result is interpreted due to the presence of a hierarchical porosity in this material, which favors the accessibility to the active sites. 

The Friedel-Crafts acylation of aromatic compounds is an important synthesis route to aromatic ketones in the production of fine and specialty chemicals. Industrially this is performed by reaction of an aromatic compound with a carboxylic acid or derivative e.g. acid anhydride in the presence of an acid catalyst. Commonly, either Lewis acids e.g. AICI3, strong mineral acids or solid acids e.g. zeolites, clays are used as catalysts however, in many cases this gives rise to substantial waste and corrosion difficulties. High reaction temperatures are often required which may lead to diminished product yields as a result of byproduct formation. Several studies detail the use of zeolites for this reaction (1). 

By in situ MAS NMR spectroscopy, the Koch reaction was also observed upon co-adsorption of butyl alcohols (tert-butyl, isobutyl, and -butyl) and carbon monoxide or of olefins (Ao-butylene and 1-octene), carbon monoxide, and water on HZSM-5 (Ksi/ Ai — 49) under mild conditions (87,88). Under the same conditions, but in the absence of water (89), it was shown that ethylene, isobutylene, and 1-octene undergo the Friedel-Crafts acylation (90) to form unsaturated ketones and stable cyclic five-membered ring carboxonium ions instead of carboxylic acids. Carbonylation of benzene by the direct reaction of benzene and carbon monoxide on solid catalysts was reported by Clingenpeel et al. (91,92). By C MAS NMR spectroscopy, the formation of benzoic acid (178 ppm) and benzaldehyde (206 ppm) was observed on zeolite HY (91), AlC -doped HY (91), and sulfated zirconia (SZA) (92). 

Friedel-Crafts acylation of 3-phenylpropanoyl chloride (0.59 mmol) in anisole (50 ml) at 408 K in the presence of acid zeolites (1,00 g) for 17 h of reaction time. 

The set of catalysts selected for the dehydration of 2-butanol was also tested for the Friedel-Crafts acylation of anisole [69, 70]. The catalytic test was performed in the liquid phase due to the high boiling points of the reactants and products of this reaction. Anisole was reacted with acetic anhydride at 120 °C in the absence of solvent. In principle, acylation can occur on both the ortho and para positions of anisole. The main product (>99%) over all catalysts in this study was para-methoxyacetophenone, indicating that the reaction predominantly takes place inside the zeolite micropores. The same trend in catalytic activity as in the 2-buta-nol dehydration reaction is observed the conversion of anisole into para-nicihoxy-acetophenone increases upon increasing Ge content of the catalyst (Fig. 9.17) [67]. The main cause of deactivation for this reaction is accumulation of the reaction products inside the micropores of the zeolite. The different behavior of Ge-ZSM-5, compared with ZSM-5, may therefore be due to improved diffusional properties of the former, as the presence of additional meso- and macropores allows for... 

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. 

As was emphasized in the introduction, the main cause of the significant waste production in the Fine Chemicals Industry (5 > 50 kg/kg product (1)) is the large use of homogeneous reactions carried out stoichiometrically or by using stoichiometric amounts of catalysts (e.g. A1C13 in Friedel Crafts acylation). The examples presented in this chapter show that cleaner, more simple and more economic processes using solid catalysts, especially zeolites, can be substituted for these... 

Another well known example of successful application of Beta zeolite is the substitution of AICI3 for Friedel-Crafts acylation. This reaction is an important industrial process, used for the preparation of various pharmaceuticals, agrochemicals and other chemical products, since it allows us to form a new carbon-carbon bond onto an aromatic ring. Friedel-Crafts acylations generally require more than one equivalent of for example, AICI3 or BF3. This is due to the strong complexation of the Lewis acid by the ketone product. 

Alternative catalysts for this reaction are polymer-supported alkyl sulfonic acids [231], even if they show lower performances than the zeolite. Two of the problems in the reactions are the need to vaporize the reactant and the periodic regeneration of the rapidly deactivating zeolite catalysts. It was thus proposed recently that continuous catalytic Friedel-Crafts acylation can be performed in the biphasic medium of an ionic liquid and supercritical carbon dioxide [232]. 

An impressive number of papers and books has been published and numerous patents have been registered on the aq lation of aromatic compounds over solid catalysts. Recently Sartori and Maggi [1] have written an excellent review with 267 references on the use of solid catalysts in Friedel-Crafts acylation. In one section of this review, namely acylation of aromatic ethers or thioethers, the authors report work on acylation by solid catalysts such as zeolites, clays, metal oxides, acid-treated metal oxides, heteropolyacids or Nafion. When examining in details these results, it appeared very difficult for us to build upon these experimental results as the reaction conditions differ drastically from one paper to the next. This prompted us to reinvestigate the scope and limitations of the Friedel-Crafts acylation using heterogeneous solids as catalysts, trying as much as we could to rationalize the observed effects. 

To start our investigations, we examined the conversion of 2,3-dimethyl-2-butene (1) into 3,3,4-trimethyl-4-penten-2-one (2) as a model reaction (eq. 1). The choice of acetic anhydride as the acetylating agent was made in the light of related studies on the acylation of aryl ethers. Our work in this field had shown that acetic anhydride was the most effective reagent for the Friedel-Crafts acylation of anisole in the presence of Hp zeolite. A lower degree of conversion was achieved with acetyl chloride, while hardly any reaction occurred with ethyl acetate or acetic acid [6]. 

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. 

Maximum effort has been directed toward the use of solid acid catalysts. In fact, heterogeneous catalysts can be easily separated from the reaction mixture and reused they are generally not corrosive and do not produce problematic side products. Different classes of materials have been studied and utilized as heterogeneous catalysts for Friedel-Crafts acylations these include zeolites (acid treated), metal oxides, and heteropoly acids already utilized in hydrocarbon reactions. Moreover, the application of clays, perfluorinated resinsulfonic acids, and supported (fluoro) sulfonic acids, mainly exploited in the production of fine chemicals, are the subject of intensive studies in this area. 

Owing to the great interest in the argument, minireviews have been published on the use of solid catalysts in Friedel-Crafts acylation. Kouwen-hoven and van Bekkum, in a chapter of the Handbook of Heterogeneous Catalysis, faced the basic problem of the use of zeolites in the reaction. A further essential overview of the same argument was reported by Metivier in Fine Chemicals through Heterogeneous Catalysis Furthermore, Bezouhanova described the synthetic aspects of the zeolite-catalyzed preparation of aromatic ketones. ... 

The use of Y, BEA, and ZSM-5 zeolites in the Friedel-Crafts acylation of aromatics shows some limitations when large-sized molecules are utilized. In fact, because of the dimension of their pores, the access to the internal active sites of these zeolites is restricted to molecules with kinetic diameter up to 8 A. Trying to overcome this drawback, mesoporous molecular sieve MCM-41 can be utilized as interesting catalyst or support for catalysts for reactions involving larger molecules. 

Acylation reactions of arenes using zeolites may become industrially important because of their high selectivity. The Friedel-Crafts acylation of aromatic hydrocarbons by carboxylic, acids has been studied by Chiche et al. (1986) [see also Brunei et al. (1989)]. 

Geneste and coworkers in 1986 reported that zeolites catalyzed Friedel-Crafits acylations in presence of a Ce + exchanged zeolite Y catalyzed the acylation of toluene and xylenes with carboxylic acids (Table 11.2) [42]. They also mentioned that only mild acidity is sufficient for Friedel-Crafts acylations which can be induced by means of zeolites catalysts. The reaction exhibited a very high para-selectivity. 

Previously, Kantam et al. also carried out the Friedel-Crafts acylation of pyrrole with acetic anhydride over microcrystalline beta zeolites with less successful results [118]. The microcrystalline beta zeolite catalysts showed the highest activity in the acylation reaction due to their higher acidity as compared with ordinary beta zeolite. When a microcrystalline beta zeolite I (with a particle size of 1-10 gm obtained by mechanical disintegration of beta zeolite) was used as catalyst, 73% of conversion and 58% of selectivity to 2-acetylp5n role was reached after 2.5 h of reaction time. However, when a microcrystalline beta zeolite II (with a particle size of 10-50 gm obtained by decreasing the aging time to 48 h instead of 1 week) was used, 78% of conversion and 64% of selectivity to 2-acetylpyrrole was reached after 2h of reachon time. 

Friedel-Crafts acylation of indole with acetic anhydride on tungstophosphoric acid modified H-BEA zeolite under microwave and conventional heating [179] showed better catalytic performance under microwave radiation. The reaction proceeded selectively to 3-acetylindole compared with conventional heating. 

---