Zeolites acylium ions

It is generally admitted that over zeolites, acetylation of aromatic substrates with acetic anhydride (AA) is catalyzed by protonic acid sites. The direct participation of Lewis sites was excluded by using two BEA samples with similar protonic acidities, but with very different Lewis acidities indeed, these samples were shown to have quasi-similar activities. The currently accepted mechanism is shown in Figure 12.6 for the anisole acetylation example. The limiting step of the process is the attack of anisole molecules by acylium ions. 

An interesting example of an indirect acylation of alkenes is the reaction of small olefins (ethylene, propylene, isobutylene) over H-ZSM-5 in the presence of CO at 23°C to give unsaturated ketones.122 In this case the acylium ion is formed in situ from the alkene and CO in the pores of the zeolite at ambient temperature ... 

Chemisorption of acetic anhydride on the zeolite protonic sites with formation of acylium ions and of acetic acid (Step 1). 

Advantage can be drawn from the positive effect of phenol on PA transformation into p-HAP to improve the yield and selectivity of p-HAP production.[82 84] Thus, with a HBEA zeolite the yield and selectivity for p-HAP passes from ca. 5 and 28 % respectively with cumene solvent to 24 and 60% with phenol as a solvent .[84] Again sulfolane was shown to have a very positive effect on the selectivity for p-HAP and limits the catalyst deactivation. To explain these observations as well as the effect of P and PA concentrations on the reaction rates, it was proposed that sulfolane plays two independent roles in phenol acylation solvation of acylium ion intermediates and competition with P and PA for adsorption on the acid sites.1831... 

A kinetic study of the acylation of phenol with phenyl acetate was carried out in liquid phase at 160°C over HBEA zeolite samples, sulfolane or dodecane being used as solvents. The initial rates of hydroxyacetophenone (HAP) production were similar in both solvents. However the catalyst deactivation was faster in dodecane, most likely because of the faster formation of heavy reaction products such as bisphenol A derivatives. Moreover, sulfolane had a very positive effect on p-HAP formation and a negative one on o-HAP formation. To explain these observations as well as the influence of phenol and phenyl acetate concentrations on the rates of 0- and p-HAP formation it is proposed that sulfolane plays two independent roles in phenol acylation solvation of acylium ions intermediates and competition with phenyl acetate and phenol for adsorption on the acid sites. Donor substituents of phenyl acetate have a positive effect on the rate of anisole acylation, provided however there are no diffusion limitations in the zeolite pores. 

Under mild conditions (liquid phase, 160°C) HBEA zeolites can catalyse the acylation of phenol with phenyl acetate. High selectivity to p-hydroxyacetophenone is obtained by using sulfolane as a solvent, which can be explained by a better dissociation of phenyl acetate into acylium ions due to a solvation effect. However a competition between sulfolane and phenyl acetate for adsorption on the active acid sites is also demonstrated. A preliminary investigation of the effect of the acylating agent shows that generally, donor groups in aromatic acetates have a positive effect on the rate of acylation provided they do not block the access of the acetate to the acid sites of the zeolite pores. 

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). °... 

The Fries rearrangement is specific in that published results are not always consistent, and that there is a dehate on intra-Zintermolecular reactivity. In the Fries rearrangement of phenyl acetate over zeolites (Eq. 16) it was first shown that the reaction is initiated by protonation of the aromatic ester, leading then to the formation of the acylium ion (Scheme 1) [33]. 

The acylation of benzofuran by acetic anhydride was carried out in the presence of Y zeolites in the liquid phase (60°C, atmospheric pressure). It is shown that the reaction procedure has a significant influence on the activity of the catalyst. Deactivation takes place but the zeolite can be completely regenerated by reactivation in air. A reaction mechanism is proposed in which the acylium ion adsorbed on the zeolite reacts with non activated benzofuran. 

Dealuminated zeolites can be fairly active in the acylation of compounds like benzofutan although their performance depends very much on the reaction procedure. The reaction kinetics agree with a deal-type mechanism in which the adsorbed acylium ion reacts with a non activated benzofuran molecule. However, benzofuran seems to be strongly physisorbed on the zeolite as indicated by a cinetic order less than unity and a very slow reaction when it is introduced first on the catalyst. 

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