Zeolites benzoyl chloride, activation

Jacob et al/12] found that o-xylene can be benzoylated selectively to 3,4-dimethyl-benzophenone using zeolites as catalysts and benzoyl chloride as benzoylating agent. As shown in Scheme 4.2, zeolite H-BEA exhibits higher activity and selectivity for 3,4-dimethylbenzophenone (3,4-DMBP) than that of the other zeolite catalysts (H-ZSM-5, H-Y and H-MOR), due on the one hand to its stronger acid sites compared with the other zeolite catalysts and on the other hand to smaller pore openings (7.6 x 6.4 A and 5.5 x 5.5 A) than H-Y (7.4 x 7.4 A) zeolite, leading to a shape selectivity. In another article on benzoylation of toluene and naphthalene/101... 

Benzoylation of benzene and other aromatic compounds by benzoyl chloride over H-BEA zeolite modified by indium oxides has been investigated.191 We report in Table 4.2 the time required for half reaction (L/2) f°r a series of aromatic substrates used in the above reaction. The benzoylation reaction rate (via L /2 value) depends strongly on the substituent group present in the aromatic substrate and increases due to the presence of the electron-donating group, depending upon its electron-donating ability. The activity order is as follows benzene toluene < p-xylene < anisole. 

Mesoporous aluminosilicates have attracted much recent attention because of their potential use as versatile catalysts and catalyst supports, especially for large molecules, but their acidity is always much weaker than that of zeolites. They must be modified to enhance their acidity. Mesoporous Si-MCM-41 supported metal chloride catalysts[29] have shown high activity in the acylation of benzene by benzoyl chloride even in the presence of moisture in the reaction mixture. The time required for 54 % conversion of benzoyl chloride at 80 °C was 2.2 h in the presence of InC13/Si-MCM-41, against 18 h with H-BEA. 

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. 

Halogenation179 and benzoylation of subtituted benzenes also favor the para-products. Nitration of toluene with mpropyl nitrate in the presence of H-ZSM-5 gives 95% para-selectivity.180 Benzene can be nitrated with 65% nitric acid in the vapor phase at 170°C over a mordenite catalyst.181 (Most zeolites are not stable to protonic mineral acids. Mordenite is an exception.) The nitration of aromatic compounds can also be carried out with a sulfuric acid on silica catalyst that is slurried in methylene chloride to give 97-98% yields.182 The catalyst could be recovered and reused with less than 3% loss in activity after three runs. These methods eliminate the problem of what to do with spent by-product sulfuric acid from conventional nitrations. 

More recently, Gaare and Akporiaye (1996) studied the acylation of anisole by acetyl chloride and acetic anhydride. They observed that the activity of La-exchanged H-Y zeolite depends on the lantanum content. In fact, as can be seen in fig. 11, the highest yield to acylated compounds was obtained on catalysts with a 93% exchange. However, the activity to acylation reactions depends not only on the catalyst compositions but also on the acylating agents. Thus, it has been shown that activities for the acylation of anisole increases in the order benzoyl < acetyl < propionyl chloride, but, in all cases, a high selectivity to the para isomer was observed (Akporiaye et al. 1993). 

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. 

The low activity of HY zeolite catalysts for the acylation of toluene using benzoyl and propionyl chloride is in contrast to the much enhanced activities found for the lanthanum modified equivalents. The importance of the lanthanum component is confirmed by vaiying the level of exchange with high selectivities for the para-product being found for all catalyst modifications. The La-modified/add chloride system is found to be a much more active system for the acylation reaction compared to the equivalent carboxylic add. Higher activity for the aliphatic acylation compared to the benzoylation appears to indicate additional factors restricting the activity in the benzoylation reacton. 

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