Friedel-Crafts reactions using 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. 

At higher temperatures, C—H and C—C bonds may be similarly broken. Thus, zeolite catalysts may be used for (i) alkylation of aromatic hydrocarbons (cf. the Friedel-Crafts reactions with AICI3 as the Lewis acid catalyst), (ii) cracking of hydrocarbons (i.e., loss of H2), and (Hi) isomerization of alkenes, alkanes, and alkyl aromatics. 

In addition to large-scale industrial applications, solid acids, such as amorphous silica-alumina, zeolites, heteropoly acids, and sulfated zirconia, are also versatile catalysts in various hydrocarbon transformations. Zeolites are useful catalysts in fine-chemical production (Friedel-Crafts reactions, heterosubstitution).165-168 Heteropoly compounds have already found industrial application in Japan, for example, in the manufacture of butanols through the hydration of butenes.169 These are water tolerant, versatile solid-phase catalysts and may be used in both acidic and oxidation processes, and operate as bifunctional catalysts in combination with noble metals.158,170-174 Sulfated zirconia and its modified versions are promising candidates for industrial processes if the problem of deactivation/reactivation is solved.175-178... 

This chapter presents an alternative to classical Friedel-Crafts catalysis using heterogeneous catalysts such as zeolites, as we believe them to be the ultimate catalyst for the future production of aromatic ketones. After a short introduction to the literature, we focus on catalyst and process issues for different reactions. 

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. 

Friedel-Crafts reactions what possible technology might replace the old stoicheiometric Lewis acid route The use of acido/basic (particularly zeolites) materials to catalyse this reaction will be discussed as well as the industrial aspects such as recycling, regeneration, and process simplification. 

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. 

Alkylation by zeolites made a major entry in the field of industrial catalysis through the highly acclaimed Mobil-Badger process for ethylbenzene (Csicsery, 1984, Hoelderich et al., 1988 Hoelderich and Van Bekkum, 1991) by replacing the toxic, eco-unfriendly, corrosive Friedel-Crafts reaction. A modified version developed by the National Chemical Laboratory (the Albene process), using a similar class of catalysts known as Encilites, is particularly suited to alkylation by ethyl alcohol of any concentration down to 30% (Bhowmik and Ratnasamy, 1991). 

In the first stage, a Friedel-Crafts reaction is commonly carried out by treating benzene with ethylene in the liquid phase at 90-100°C at slightly above atmospheric pressure. The catalyst is aluminium chloride (with ethyl chloride as catalyst promoter). A molar excess of benzene is used to reduce the formation of polyethylbenzenes the molar ratio of reactants is generally about 1 0.6. The reactants are fed continuously into the bottom of a reactor whilst crude product is removed from near the top. The product is cooled and allowed to separate into two layers the lower layer, which consists of an aluminium chloride-hydrocarbon complex, is removed and returned to the reactor. The remaining ethylbenzene is then separated by distillation from polyethylbenzenes and benzene, which are recycled (since dealkylation also occurs under the reaction conditions). In newer plants, ethylbenzene is produced in a gas phase process. An excess of benzene is treated with ethylene at about 420°C and 1.2-2 MPa (12-20 atmospheres) in the presence of a zeolite catalyst. 

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. 

Hardacre et al. report the Friedel-Crafts benzoylation of anisole with benzoic anhydride to yield 4-methoxybenzophenone with various ILs and zeolite catalysts (USY, HZSM-5, H-beta, and H-mordenite). The rates of reaction were found to be significantly higher using ionic liquids compared with organic solvents.Continuous-flow studies of successful ionic liquid systems indicate that the bulk of the catalysis is due to the formation of an acid via the ion exchange of the cation with the protons of the zeolite as shown in the following reaction. Scheme 8. 

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). 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

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... 

---