Acylation with zeolites

The influence of the acid pretreatment of BEA on ifs acfivity in the model reaction between 2-MN and AAN was analyzed in depth/ The contribution of the inner and outer surfaces of the catalyst was examined by considering the selectivity with respect to the bulky product 28 and the linear product 29, which are assumed to be formed on fhe outer and on both the inner and outer surfaces of fhe catalyst, respectively/ " It is shown that the production of EFAL species located in the micropores of BEA subjected to high heating rate calcination provokes the increase in the selectivity of the less hindered 29 because the formation of the bulky 28 is sterically hampered. Indeed, when the external surface of zeolite BEA is passivated by coating with amorphous silica, a significant increase in the selectivity of 29 is observed, and fhis resulf is a clear example of shape-selective acylation with zeolite catalyst. On the contrary, acid treatment increases the catalytic activity of fhe oufer surface due to the extraction of the catalytically active EFAL species out of the micropores, leading to the preferential formation of fhe bulky 28. 

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. 

Several metal oxides could be used as acid catalysts, although zeolites and zeo-types are mainly preferred as an alternative to liquid acids (Figure 13.1). This is a consequence of the possibility of tuning the acidity of microporous materials as well as the shape selectivity observed with zeolites that have favored their use in new catalytic processes. However, a solid with similar or higher acid strength than 100% sulfuric acid (the so-called superacid materials) could be preferred in some processes. From these solid catalysts, nation, heteropolyoxometalates, or sulfated metal oxides have been extensively studied in the last ten years (Figure 13.2). Their so-called superacid character has favored their use in a large number of acid reactions alkane isomerization, alkylation of isobutene, or aromatic hydrocarbons with olefins, acylation, nitrations, and so forth. 

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

In acylation with acidic ammonium and cesium salts of phosphotungstic acid, direct correlation was found between activity and the number of accessable Br0nsted acid sites.69,70 Moreover, cesium salts were found to be more active, than HY and H-Beta zeolites.70... 

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. 

Tridimensional Y and BEA zeolites are also utilized as efficient catalysts in the acylation of tetralin with acyl chlorides. By using octanoyl chloride and butyroyl chloride in the presence of zeolite Y(40), good yields in the acylation products are achieved (83% and 77% yield, respectively, after 8 h), whereas acetyl chloride (AC) leads only to less than 20% yield of the two isomeric ketones 1 and 2 (1/2 = 7.5) (Scheme 4.2). This trend is correlated to the influence of the chain length of the acylating agent on the efficiency of Friedel-Crafts acylation over zeolite catalysts previously discovered and rationalized. The lower activity of zeolite Y(40) with AC is related to the strong adsorption of organic species, either acetic acid (AAC) or acetyl tetralin. 

Y zeolites also show promising activity in the acylation of heterocyclic compounds. Acylation of benzofurans by AAN was extensively studied, and some good S mthetic results have been achieved. Benzofuran and 2-methylbenzofuran undergo acylation with AAN under mild conditions, giving 2-acetylbenzofuran 13 and 2-methyl-3-acetylbenzofuran 14 respectively, as the main products over Y(32) zeolite (Scheme 4.8). 

Table 4.9 Phenyl ether acylation with aliphatic anhydrides promoted by BEA zeolite...

BEA zeolite can be also applied in the acylation of different heteroaromatic compounds. Thus, a series of sulfur, nitrogen, and oxygen heterocycles can be acylated with AAN. Some synthetic results are reported in Table 4.10. The best conversion and selectivity are achieved with BEA zeolite showing a surface area of 350-620 m x g-p... 

Table 4.10 Heterocycle acylation with AAN in the presence of BEA zeolite...

Table 4.12 Anisole acylation with different carboxylic acids catalyzed by zeolite ZSM-5...

Anisole can be acylated with carboxylic acids over ZSM-5 catalyst. The reaction is carried out in a batch reactor under reflux for 2 d. Table 4.12 shows fhe resulfs obfained utilizing carboxylic acids with different chain length. Prom propanoic acid to stearic acid, the conversion decreases from 92% to 1%, showing a trend opposite to that observed with REY zeolites. This behavior can be ascribed to the small micropore size of ZSM-5, in which the formation of large molecules is difficulf (for penfanoic to octanoic acid) or impossible (for longer-chain carboxylic acids). The poor conversion of AAC can be affribufable to the low reflux temperature. 

Cerveny, L., Mikulcova, K., and Cejka, J. 2002. Shape-selective synthesis of 2-acetylnaphthalene via naphthalene acylation with acetic anhydride over large pore zeolites. Appl. Catal. A Gen. 223 65-72. 

Studies on catalyst deactivation during phenol acylation with PA in the presence of BEA zeolite were performed by recovering the organic material entrapped into the zeolite following two methodologies ... 

The aromatic acylation with carboxylic acids (Eq. 3) instead of acid anhydrides and acyl chlorides has attracted interest, because it is an environmentally benign reaction, resulting in the formation of water as the only by-product. It has been attempted with zeolites and clays as catalysts. Heteropoly acids have proved to be more active catalysts for this... 

O-methyl- and -octyl )S-D-glucopyranoside (7a and 7b, respectively) with ethyl butanoate without drying gave mainly the 6-O-acylated products 8, as would be expected, but with zeolite present complete conversion to the diesters took place. The 3,6-diesters 9 were mainly formed, together with the 2,6-isomers (10), depending on the size of the anomeric substituent. 

Acylation with acids is ideal from a waste perspeetive, but is rarely effective. Geneste et have shown that CeY zeolite (where the Na coun-... 

Another patent apphcation (28) describes the use of zeolite/TUD-1 with optionally a metal function for a variety of reactions. In an example, as-synthesized MCM-22 / TUD-1 was tested for acylation of 2-methoxynaphthalene with acetic anhydride to 2-acetyl-6-methoxynaphthalene at 240°C. After reaction for six hours, conversion of 2-methoxynaphthalene reached 56% with 100% selectivity to 2-acetyl-6-methoxynaphthalene. Other zeolite catalysts were similarly tested, but none were nearly as effective. 

Catalytic activity of the beta zeolite with enhanced textural properties in the Friedel-Crafts acylation of aromatic compounds... 

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 present work reports the results obtained in the Friedel-Crafts acylation of different aromatic substrates catalyzed by zeolite Beta obtained according to a novel method based on the crystallization of silanized seeds, as a way to perturb the subsequent crystal growth step and to modify the zeolite textural properties [5], The catalytic behavior of this material is compared with that of the conventional Beta zeolite. 

---