Zeolite phenol acylation

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. 

Sfudies on phenol acylation wifh acefic acid (AAC) over ZSM-5 zeolife confirm thaf fhe reacfion is very complicafed. The reacfion is carried ouf in a flow reactor charged wifh the zeolite catalyst at 280°C. The main... 

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 ... 

Solid acid catalysts such as clays and zeolites are also utilized for phenol acylation however, these processes suffer from catalyst deactivation problems and lack C-selectivity. In the acylation of phenol with acetic anhydride, HZSM-5 zeolite shows a very high ort/io-selectivity (48% o-HAP yield, <1% p-HAP yield), although phenyl acetate is isolated in only approximately 20% yield [115]. The SAR value has a remarkable influence on the selectivity of the process when the reaction is carried out in the presence of HZSM-5(30), HZSM-5(150), and HZSM-5(280) zeolites, the o-HAP yields are 42,40, and 15%, respectively, whereas the O-acylation is noticeably increased. These results mean that C-acylation requires higher Brpnsted acidity and that lower acidity leads to phenyl acetate formation. It must be noted that the reaction performed with an amorphous aluminosilicate acid catalyst gives mostly phenyl acetate without isomer selectivity. These results suggest that the C-acylation of phenol occurs in the channels of zeolites and not on the external surface. 

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). 

As reported in the literature, the acylation of aromatic hydrocarbons can be carried out by using zeolites as catalysts and carboxylic acids or acyl chlorides as acylating agents. Thus toluene can be acylated by carboxylic acids in the liquid phase in the presence of cation exchanged Y-zeolites (ref. 1). The acylation of phenol or phenol derivatives is also reported. The acylation of anisole by carboxylic acids and acyl chlorides was obtained in the presence of various zeolites in the liquid phase (ref. 2). The acylation of phenol by acetic acid was also carried out with silicalite (ref. 3) or HZSM5 (ref. 4). The para isomer has been generally favoured except in the latter case in which ortho-hydroxyacetophenone was obtained preferentially. One possible explanation for the high ortho-selectivity in the case of the acylation of phenol by acetic acid is that phenylacetate could be an intermediate from which ortho-hydroxyacetophenone would be formed intramolecularly. 

The scheme proposed for the reaction over HFAU was that PA dissociates in phenol (P) and ketene and that o-HAP, which was highly favoured over the para isomer, results partly from an intramolecular rearrangement of PA, partly from acyl group transfer from PA to P whereas p-HAP results from this latter reaction only. In these experiments, the zeolite deactivation was very fast, as a result of coke deposition and zeolite dehydroxylation. Catalyst stability can be considerably improved by operating at lower temperatures and especially by substituting equimolar mixtures of PA and water or P and acetic acid for PA. Much higher HAP yields were obtained by using the P - acetic acid mixture as reactants.[68]... 

Figure 3.6 Yields, X (%) of phenyl acetate, PA ( ), o-hydroxyacetophenone, o-HAP (o) and p-hydroxyacetophenone, p-HAP ( ) as a function of the conversion of phenol, Xp (%), in an equimolar mixture with acetic acid over HMFI at 553 K. Reprinted from Journal of Molecular Catalysis, Vol. 93, Neves et al., Acylation of phenol with acetic acid over a HZSM-5 zeolite, reaction scheme, pp. 169-179, Copyright (1994), with permission from Elsevier...

Zeolites H-BEA and H-Y were found to be the most active catalysts, however all catalysts readily form the phenyl benzoate (Table 4.1). In the conditions of the reaction, the formation of phenyl benzoate (PB) occurs rapidly via O-acylation of phenol. Direct C-alkylation of phenol with benzoic anhydride (B) and Fries rearrangement of phenyl benzoate results in the formation of 2- and 4-hydroxy-benzophenones (2-HPB and 4-HPB) (Scheme 4.3). 

This paper is devoted to the acylation in liquid phase of phenol and anisole by aromatic acetates. Reaction mechanisms are presented and the effect of the solvent polarity which is known to play an important role on the rate and on the selectivity of zeolite catalysed reactions... 

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. 

The acetylation of alcohols and phenols with acetic anhydride and HSZ-360 zeolite at 60oC used no solvent. The acetate of 1-dodecanol was obtained in 98% yield, and the acetate from 1 naphthol in 100% yield.183 The acylation of anisole with acetic anhydride at 100oC without solvent using zeolite H-/8 gave 4-methoxyacetophenone (6.30) in 98%yield.184 The catalyst could be recovered, regenerated, and reused with no decrease in yield. These reactions show that not all zeolites are used at high temperatures in the vapor phase. 

Most studies in which acid solids were used concern the synthesis of hydroxyacetophenones either by Fries rearrangement of phenyl acetate [3,5-15] or by acylation of phenol with acetic acid or acetic anhydride [11,16-21]. These reactions were conducted in the gas or liquid phase, zeolites being generally chosen as catalysts (Section 5.3.1). These shape-selective catalysts can also be used... 

Dealumination of the ZSM-5 zeolite shows a great effect on ortho/ para selectivity in the acylation of phenol by AAC. Thus, for a phenol conversion of 20%, ortho/para selectivity is 7.0 when ZSM-5(41.8) is utilized, and becomes 13.0 in the presence of ZSM-5(42.4). This unexpected increase in the ortho/para ratio can only be explained by postulating that ortho-HAP and para-HAP result from different pathways. The ortho isomer is mainly produced in the pores, whereas para isomer production occurs only on the external acid sites. The ortho isomer can be formed by direct C-acylation of phenol with AAC this selective reaction can be related to the general mechanism reported in Scheme 5.1. ortho-HAP can also be obtained by the Fries rearrangement. On the contrary, the para-isomer is a secondary product and, therefore, it results from the acylation of phenol by PA according to Scheme 5.6. 

Unmodified BEA zeolite, on the other hand, shows the highest activity in the acylation of phenol wifh benzoic anhydride. Phenyl benzoate (PB) is the main product (61% yield), accompanied by C-acylated products (35%), with an interesting para-selectivity (ortho/para = 0.48). When the reaction time is increased from 4 to 20 h, an increase in para-hydroxy-benzophenone yield (from 11% to 23%) fogefher wifh a decrease in PB yield (from 79% to 64%) is observed however, a small increase in the orf/zo-hydroxybenzophenone yield (from 9% to 10%) cannof be avoided. The acfivify of the catalyst, together with its selectivity, does not distinctly decrease when the catalyst is used from fresh to firsf recycle. 

Phenol undergoes propanoylation with propanoyl chloride over BEA, Y, MOR, and ZSM-5 zeolites The main product is phenylpropanoate moreover, BEA (Scheme 5.7) is the most active in the C-acylation and selective for para-hydroxypropiophenone production (ortho/para = 0.53) compared to other zeolite catalysts. 

The CeY zeolite is utilized for the preparation of 4-methylcoumarin by the reaction of phenol with AAN. The formation of PA represents the first step the subsequent acylation at the ortho position, followed by an intramolecular aldol-like condensation, affords the final 4-methylcoumarin in 75% yield (Scheme 5.8). In the entire process, the cerium-catalyst shows a bifunctional character the active centers in the supercage of CeY zeolite, the Ce + ions, act as Lewis acid catalysts, whereas the acid centers H+, formed by the dissociation of water according to the equation Ce + + H2O [Ce(OH)]2+ + H+, act as Bronsted catalysts. 

Neves, L, Jayat, R, Magnoux, R, Perot, G., Riberito, R R., Guberlmarm, M., and Guisnet, M. 1994. Acylation of phenol with acetic acid over a HZSM5 zeolite, reaction scheme. /. Mol. Catal. 93 169-179. 

Subba Rao, Y. V., Kulkarni, S. J., Subrahmanyam, M., and Rama Rao, A. V. 1993. A novel acylative cyclization reaction of phenol over modified Y zeolites. /. Chem. Soc., Chem. Commun. 1456-1457. 

However, for liquid phase reactions, it is very difficult to correlate the results wiHi BrOnsted or Lewis acidity as the reaction conditions used are different from those used for characterization. When the Diels-Alder reaction is conducted in a solvent, it appears that the maximum for the activity of HY zeolites is obtained for a Si/Al ratio of 15. This maximum was also observed for esterification of carboiQ lic acids (14), methylthiolation of phenol with dimethyldisulfide (15), acylation of toluene with benzoic acids (15) or dehydration of fhictose (15), and in solvents such as alcohols, water or hydrocarbons. If we assume that Lewis species are transformed to Brdnsted ones in the presence of water as solvent, this would thus mean that the Diels-Alder reaction is preferentially catalyzed by BrSnsted species, the maximum observed at Si/Al=15 for HY zeolites being a good balance between the niunber 6ind the strength of the protonic species. 

The HBEA(20) deactivation in the reaction between phenol and phenyl acetate has also been studied [118]. In this case, the organic material trapped in the zeolite can be recovered following two methods (i) Soxhlet extraction of the zeolite [Ext] and (ii) extraction of coke by dissolution of the zeolite itself in a 40% solution of hydrofluoric acid [Coke]. The acylation reactions are carried out in two classical solvents, dodecane and sulfolane, and in both cases, a significant lowering of the rate of HAPs formation with time is observed. This deactivation is faster in dodecane ( 1 h) than in sulfolane ( 2h).Whatever the solvent, the two reactants are the main components of the material retained in the catalyst nevertheless, in the case of sulfolane, their contents in Ext and Coke are similar to that of the reaction mixture, whereas when the less polar solvent dodecane is employed, their contents in Ext and Coke are greater than that in the reaction mixture. In addition. 

Nevertheless, such reactions catalyzed by zeolites have been discussed in the review of 2001 (1) isomerization (double-bond shift, isomerization of tricyclic molecules, like synthesis of adamantane, isomerization of terpenes, diverse rearrangements, conversion of aldehydes into ketones), (2) electrophilic substitution in arenes (alkylation of aromatics, including the synthesis of linear alkylbenzenes, alkylation and acylation of phenols, heteroarenes and amines, aromatics nitration and halogenation), (3) cyclization, including the formation of heterocycles, Diels-Alder reaction, (4) nucleophilic substitution and addition,... 

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