Fries rearrangement of aryl esters catalysed by heteropoly acid

Fries rearrangement of aryl esters catalysed by heteropoly acid

Applied Catalysis A: General 245 (2003) 69–78 Fries rearrangement of aryl esters catalysed by heteropoly acid Elena F. Kozhevnikova, Juliette Quartar...

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Applied Catalysis A: General 245 (2003) 69–78

Fries rearrangement of aryl esters catalysed by heteropoly acid Elena F. Kozhevnikova, Juliette Quartararo, Ivan V. Kozhevnikov∗ Department of Chemistry, The Leverhulme Centre for Innovative Catalysis, University of Liverpool, Liverpool L69 7ZD, UK Received 16 October 2002; received in revised form 22 November 2002; accepted 22 November 2002

Abstract Heteropoly acids (HPAs) are active catalysts for the Fries rearrangement of aryl esters (phenyl acetate, phenyl benzoate, and p-tolyl acetate) to yield the acylated phenols and esters together with phenols in homogeneous or heterogeneous liquid-phase systems at 100–170 ◦ C. Amongst the HPA catalysts studied are bulk, silica-supported, and sol–gel silica-included H3 PW12 O40 (PW), as well as bulk Cs+ and Ce3+ salts, Cs2.5 H0.5 PW12 O40 and Ce0.87 H0.4 PW12 O40 . The reaction with bulk and silica-supported PW occurs homogeneously in aryl esters (without solvent) or in polar solvents, such as nitrobenzene and o-dichlorobenzene. In non-polar solvents such as dodecane, the reaction is heterogeneous. The Cs+ and Ce3+ salts and sol–gel PW catalysts perform heterogeneously in all these media. From heterogeneous systems, the catalysts can be separated and reused, albeit with reduced activity. The homogeneous and silica-supported PW are much more active than H2 SO4 and H-Beta zeolite, which is explained by the greater acid strength of HPA compared to the other acids. In contrast to silica-supported PW, the sol–gel PW catalysts show only a negligible activity in the Fries reaction of phenyl acetate, yielding mainly phenol with 92–100% selectivity. This may be explained by a weaker acid strength of the sol–gel catalysts and the presence of relatively high amount of water. 31 P MAS NMR shows that the state of the PW anion in the sol–gel and silica-impregnated catalysts is similar, but different from that in the bulk PW. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Fries rearrangement; Aryl ester; Heteropoly acid

1. Introduction The Friedel–Crafts aromatic acylation and related Fries rearrangement of aryl esters are the most important routes for the synthesis of aromatic ketones that are intermediates in manufacturing fine and speciality chemicals as well as pharmaceuticals [1–6]. The Fries rearrangement of aryl esters, e.g. phenyl acetate (Eq. (1); Ac = acetyl) to yield 2- and 4-hydroxyacetophenones (2HAP and 4HAP) and 4acetoxyacetophenone (4AAP) together with phenol ∗ Corresponding author. Tel.: +44-151-794-2938; fax: +44-151-794-3589. E-mail address: [email protected] (I.V. Kozhevnikov).

involves acylium ion intermediates that are generated from the ester by interaction with an acid catalyst. Mechanisms for the formation of products have been discussed ([4] and references therein). 2HAP, 4AAP and phenol are considered to be the primary products, 2HAP being formed by the intramolecular rearrangement of PhOAc and 4AAP and PhOH by the self-acylation: 2PhOAc → 4AAP + PhOH. In contrast, 4HAP appears to be the secondary product formed by the intermolecular acylation of phenol with PhOAc. Usually, the yield of phenol is greater than that of 4AAP, as part of PhOH results from the decomposition and/or hydrolysis of PhOAc that also produce ketene, acetic acid and acetic anhydride. Solvent plays a significant role in Fries reaction, polar

0926-860X/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0926-860X(02)00618-X

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solvents favouring the formation of the para-acylation products (4AAP and 4HAP). Present practice requires a stoichiometric amount of soluble Lewis acids (e.g. AlCl3 ) or mineral acids (e.g. H2 SO4 or HF) as catalysts, which results in substantial amount of waste and causes corrosion problems [2]. To overcome this, in the last couple of decades, considerable effort has been put into developing heterogeneously catalysed Friedel–Crafts chemistry using solid acid catalysts such as zeolites, clays, Nafion-H, heteropoly acids, etc. [2], zeolites being the most studied catalysts for both the acylation [2,3] and Fries rearrangement [2,4–6].

2. Experimental 2.1. Chemicals H3 PW12 O40 and H3 PMo12 O40 hydrates from Aldrich and Aerosil 300 silica from Degussa were used. Phenyl acetate, phenyl benzoate, p-tolyl acetate, nitrobenzene (PhNO2 ), o-dichlorobenzene and acetic anhydride, all ≥99% purity, were obtained from Aldrich and used as received. Other reagents and solvents were of analytical purity. 2.2. Catalysts

(1) Heteropoly acids (HPAs) are promising solid acid catalysts for aromatic acylation [7–15]. They are stronger than many conventional solid acids such as mixed oxides, zeolites, etc. The Keggin-type HPAs typically represented by the formula H8−x [XM12 O40 ], where X is the heteroatom (e.g. P5+ or Si4+ ), x its oxidation state, and M the addenda atom (usually Mo6+ or W6+ ), are the most important for catalysis [7–11]. They have been widely used as acid and oxidation catalysts for organic synthesis and found several industrial applications [11]. Silica-supported HPAs have been reported to catalyse the Fries rearrangement of phenyl acetate in the gas phase at 200 ◦ C [16]. Recently, we have communicated that H3 PW12 O40 is a very efficient catalyst for the rearrangement of phenyl acetate in liquid-phase systems [17]. Here we describe in detail the Fries rearrangement of aryl esters, such as phenyl acetate (PhOAc), phenyl benzoate (PhOBz), and p-tolyl acetate (p-TolOAc), catalysed by heteropoly acids in homogeneous and heterogeneous liquid-phase systems. Amongst the catalysts studied are bulk, silica-supported, and sol–gel silica-included H3 PW12 O40 (PW), the strongest acid in the HPA series, as well as its bulk acidic Cs+ and Ce3+ salts, Cs2.5 H0.5 PW12 O40 and Ce0.87 H0.4 PW12 O40 .

Silica-supported HPA catalysts, PW-SiO2 , were prepared by impregnating Aerosil 300 silica (SBET , 300 m2 g−1 ) with an aqueous solution of HPA. The mixture was stirred overnight at room temperature, followed by drying using a rotary evaporator, as described elsewhere [18]. The acidic salts Cs2.5 H0.5 PW12 O40 [19] and Ce0.87 H0.4 PW12 O40 [20], designated Cs2.5 H0.5 PW and Ce0.87 H0.4 PW, respectively, were prepared by literature methods. The bulk and silica-supported HPA catalysts were calcined at 150 ◦ C/0.1–0.5 Torr for 1.5 h, unless stated otherwise, and stored in a desiccator over P2 O5 . H-Beta zeolite (Si/Al = 12) was calcined at 550 ◦ C/2 h under N2 flow prior to use. Sol–gel catalysts, PW-SiO2 , comprising of H3 PW12 O40 included into silica matrix were prepared by hydrolysis of tetraethyl orthosilicate in aqueous ethanol in the presence of the heteropoly acid using the Japanese [19] or Hungarian [21] procedure, the latter being a modification of the former method [19]. The sol–gel catalysts thus prepared are designated as PW-SiO2 -J and PW-SiO2 -H, respectively. The hydrolysis of (EtO)4 Si was carried out at 40 ◦ C for 1 h and then at 80 ◦ C for 3 h. The amount of PW charged corresponded to 10–20 wt.% PW loading in anhydrous PW-SiO2 catalysts, H2 O/(EtO)4 Si = 5 (mol/mol), the EtOH/(EtO)4 Si molar ratio was 0.7 (Japanese method) or 2.7 (Hungarian method). The resulting hydrogel was dehydrated at 45 ◦ C/24 Torr, calcined at 130 ◦ C/0.1–0.5 Torr for 1.5 h, ground, extracted with hot water (80 ◦ C, 3 h) and finally, unless stated otherwise, heated at 130 ◦ C/0.1–0.5 Torr for 1.5 h. When calcined at higher temperatures, the sol–gel catalysts turned brownish. In our hands, the

E.F. Kozhevnikova et al. / Applied Catalysis A: General 245 (2003) 69–78

sol–gel catalysts prepared by the Hungarian method leached less PW upon water extraction than those made by the Japanese method. The leaching of PW was monitored by UV spectroscopy at 265 nm and the W and P content in the catalysts was determined by ICP. Catalyst characterisation is given in Table 1.

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or dodecane (1 wt.%) was added as a GC internal standard in the reactions of PhOAc and p-TolOAc and tetradecane in the reaction of PhOBz. To monitor the reaction, 0.1 ml samples of the reaction mixture were taken periodically, diluted to 1 ml with 1,2-dichloroethane and analysed by gas chromatography (Varian 3380 chromatograph with autosampler) using a 30 m × 0.25 mm BP1 capillary column.

2.3. Characterisation techniques 31 P

MAS NMR spectra were recorded at room temperature and 4 kHz spinning rate on a Bruker Avance DSX 400 NMR spectrometer using 85% H3 PO4 as a reference. The recycling time was 90 s and the 90◦ pulse was 5.5 ␮s. Surface area and porosity of HPA catalysts were measured by nitrogen physisorption on a Micromeritics ASAP 2000 instrument. Thermogravimetric analyses (TGA) were performed using a Perkin-Elmer TGA 7 instrument under nitrogen flow.

3. Results and discussion 3.1. Rearrangement of phenyl acetate The rearrangement of PhOAc (Eq. (1)) catalysed by PW occurs at 100–170 ◦ C, preferably at 130–150 ◦ C, to yield 2HAP, 4HAP, 4AAP, and phenol together with acetic acid and acetic anhydride. In contrast, H3 PMo12 O40 (PMo) is a very poor catalyst for this reaction (entry 11, Table 2) probably because of reduction of the HPA [10,11]. One important advantage of HPA, as compared to zeolites or mineral acids (e.g. H2 SO4 ), is that the reaction can be carried out both homogeneously and heterogeneously. The homogeneous process occurs in polar media, for example, in neat PhOAc or polar organic solvents like nitrobenzene or o-dichlorobenzene (Table 2) that are commonly used for Fries

2.4. Fries rearrangement The rearrangement of aryl esters was carried out in liquid-phase at 100–170 ◦ C under nitrogen atmosphere in a 25 ml glass reactor equipped with a condenser and a magnetic stirrer. The total weight of reaction mixture (ester + solvent) was 7.0 g. Decane Table 1 Catalyst characterisation Catalysta

SBET (m2 /g)

Pore sizeb (Å)

Pore volumec (cm3 /g)

H2 O contentd (wt.%)

W contente (wt.%)

P contente (wt.%)

PW 60% PW-SiO2 40% PW-SiO2 20% PW-SiO2 10% PW-SiO2 Cs2.5 H0.5 PW Ce0.87 H0.4 PW 11% PW-SiO2 -Hf 11% PW-SiO2 -Hf,g 15% PW-SiO2 -Jf 16% PW-SiO2 -Hf

8.1 86 143 200 266 119 0.8 196 196 394 344

74 146 110 107 97 29 7 18 18 18 17

0.015 0.32 0.39 0.53 0.65 0.086 0.001 0.089 0.089 0.178 0.146

4.30 3.23 2.73 2.58 2.03 1.79 1.87 6.50 2.60 7.01 6.41

8.33 8.33 11.33 12.46

0.09 0.09 0.15 0.15

Catalysts pretreated at 150 ◦ C/0.1–0.5 Torr for 1.5 h; PW content refers to anhydrous catalysts. From BET adsorption. c Single point adsorption total pore volume at a relative pressure p/p of 0.97–1.0. 0 d From TGA; weight loss from 30–300 ◦ C. e W and P content in anhydrous catalysts from ICP. f Sol–gel catalysts pretreated at 130 ◦ C/0.1–0.5 Torr for 1.5 h; PW-SiO -H prepared by the method [21], PW-SiO -J by the method [19]. 2 2 g The catalyst pretreated at 160 ◦ C/0.1–0.5 Torr for 2.5 h. a

b

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Table 2 Homogeneous Fries rearrangement of phenyl acetate (2 h) Catalyst (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14

PW (0.60) PW (5.0) PW (3.0) PW (0.60) PW (0.60)b PW (0.60) PW (0.60) PW (0.60) PW (3.0) PW (0.60) PMo (0.60) H2 SO4 (1.4) PW (0.60) PW (0.60) a b

Solvent (PhOAc, wt.%)

T ( ◦ C)

PhOAc (100) PhOAc (100) PhOAc (100) PhNO2 (50) PhNO2 (50) PhNO2 (50) PhNO2 (50) PhNO2 (25) PhNO2 (25) PhNO2 (25) PhNO2 (25) PhNO2 (25) PhNO2 (10) o-Cl2 C6 H4 (25)

150 150 130 170 150 130 100 150 150 130 130 130 150 130

Conversion (%)

5.5 19.2 18.2 9.7 11.0 11.9 10.5 24.8 45.8 21.0 6.0 12.8 35.2 16.4

TOF (min−1 )a

12 11 11 14 15 13 5 14 6 10 1 0.08 9 9

Selectivity (%) PhOH

2HAP

4HAP

4AAP

49 52 53 57 44 45 55 49 52 46 82 67 53 48

5.2 5.7 7.3 7.5 7.5 6.5 5.1 9.7 12 7.8 2.6 9.4 9.4 9.9

5.6 15 14 6.0 8.0 11 10 16 24 18 0 7.6 22 14

40 28 26 30 40 38 29 25 12 27 15 16 16 28

Turnover frequency per proton corresponding to the initial reaction rate. 1.5 h.

reaction. All these media will easily dissolve PW at elevated temperatures (ca. 100 ◦ C). On the other hand, when using non-polar solvents, such as higher alkanes (e.g. decane or dodecane) that will not dissolve HPA, the reaction proceeds heterogeneously over solid HPA catalysts (Table 3). In the latter case, silica-supported PW is the catalyst of choice, as bulk PW possesses a low surface area (1–8 m2 g−1 ) [7–11] (Table 1). The heterogeneous catalysis by bulk and silicasupported PW in the PhOAc–dodecane media was clearly proved by filtering-off the catalyst from the reacting system which completely terminated the reaction. In contrast, filtration did not affect the reaction course in homogeneous systems, such as PhOAc– PhNO2 and PhOAc–o-Cl2 C6 H4 . In many cases, judgement could be made simply by appearance of the reaction system. When the reaction was homogeneous, the whole mixture quickly turned dark brown; when heterogeneous, the liquid-phase remained almost clear, with the solid catalyst gradually turning brown as the reaction progressed. The Cs+ and Ce3+ salts of PW act heterogeneously in both polar and non-polar media studied (Table 3). The HPA catalysts are easily separated from the heterogeneous system by filtration and could be reused (entries 4 and 18, Table 3), though with decreased activity. From 31 P MAS NMR, the structure of HPA in PW-SiO2 catalysts remained unchanged during

the reaction, as both fresh and used catalysts showed the same single line at ca. −15 ppm characteristic of the Keggin structure [10,11]. From the homogeneous systems, HPA can be separated without its neutralisation by extraction with water and reused or utilised otherwise. It should be noted that our attempts to precipitate HPA from the homogeneous systems by non-polar solvents such as toluene or hexane failed to recover the HPA. Catalyst thermal pretreatment controls the amount of water in the HPA catalysts and is essential for heterogeneous catalysis by HPA [7–11]. For bulk and silica-supported PW as well as for its Cs+ and Ce3+ salts, the heat treatment at 150 ◦ C/0.1–0.5 Torr for 1.5 h was found to be optimal for the Fries reaction, similar to that for anisole acylation [15]. With 40% PW-SiO2 pretreated at 250 ◦ C/0.1 Torr for 1.5 h, the reaction (1) went slower, making less phenol, as expected (cf. entries 3 and 5, Table 3). For the less hydrophilic Cs+ salt, such a pretreatment also slowed down the reaction, but without affecting the selectivity (cf. entries 17 and 19, Table 3). Strong inhibition of the HPA-catalysed process by reaction products was observed both in homogeneous and heterogeneous systems, which is not unexpected, as the same takes place with other catalysts (AlCl3 , zeolites, etc.) [2,6]. The inhibition is apparent from the reaction time course (Fig. 1), which is similar to that

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Table 3 Heterogeneous Fries rearrangement of phenyl acetate (2 h) Catalyst (wt.%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

PW (0.60) 60% PW-SiO2 (1.0) 40% PW-SiO2 (1.5) 40% PW-SiO2 (1.5)c 40% PW-SiO2 (1.5)d 40% PW-SiO2 (7.5) 40% PW-SiO2 (1.5) 40% PW-SiO2 (3.3) H-Beta (1.3)e H-Beta (1.3)f 20% PW-SiO2 (3.0) 10% PW-SiO2 (6.0) 10% PW-SiO2 (6.0)g 10% PW-SiO2 (6.0)h 40% PW-SiO2 (6.0)h 60% PW-SiO2 (6.0)h Cs2.5 H0.5 PW (0.67) Cs2.5 H0.5 PW (0.67)i Cs2.5 H0.5 PW (0.67)d Cs2.5 H0.5 PW (2.25) Cs2.5 H0.5 PW (4.0) Cs2.5 H0.5 PW (4.0) Ce0.87 H0.4 PW (0.62)

Solvent (PhOAc, wt.%) Dodecane (25) Dodecane (25) Dodecane (25) Dodecane (25) Dodecane (25) Dodecane (25) Dodecane (25) Dodecane (36) Dodecane (36) Dodecane (25) Dodecane (25) Dodecane (25) Dodecane (25) Dodecane (25) Dodecane (25) Dodecane (25) PhNO2 (25) PhNO2 (25) PhNO2 (25) PhNO2 (25) PhNO2 (25) Dodecane (25) PhNO2 (25)

T (◦ C)

130 130 130 130 130 130 100 160 160 130 130 130 130 130 130 130 130 130 130 130 130 130 130

Conversion (%)

TOFa (min−1 )

3.1 7.2 8.3 6.7 4.5 18.0 6.4 17.2 9.3 4.1 10.5 11.8 8.5 0.3 1.0 0.7 8.7 5.5 6.7 17.2 17.2 10.5 5.3

15b 3 4 2 2 2 1 5 0.1 0.1 6 8 2 0.3 0.1 1 15 5 17 6 5 3

Selectivity (%) PhOH

2HAP

4HAP

4AAP

69 61 62 51 52 77 64 62 38 64 63 66 41 0 0 18 49 56 47 50 55 74 51

8.0 8.2 10 10 9.8 12 5.6 11 32 15 8.6 8.0 14 0 0 0 6.1 6.1 6.6 7.6 7.4 6.0 9.3

0 5.5 6.0 5.0 8.5 5.0 7.3 9.9 6.4 0 8.1 9.6 4.4 0 14 0 4.4 1.8 3.7 13 10 5.8 0

23 25 22 32 29 6.0 23 17 24 22 20 16 41 100 86 82 41 37 43 30 28 14 39

a Turnover frequency per proton corresponding to the initial reaction rate, assuming that all H+ are accessible unless stated otherwise; the true TOF may be higher. b Assuming PW surface area of 8 m2 g−1 , PW anion cross section of 100 Å2 and three accessible H+ per anion. c Reuse of the run of entry 3. d The catalyst pretreated at 250 ◦ C/0.1 Torr for 1.5 h. e 5 h, Si/Al = 11 [4]. f 4.5 h, Si/Al = 12. g Acetic anhydride (1.5 wt.%) added. h Acetic anhydride (4 wt.%) added. i Reuse of the run of entry 17.

found for the acylation of anisole, where the inhibition of HPA catalysts by products has been demonstrated [15]. Addition of more HPA catalyst allowed reaching a higher PhOAc conversion (cf. entries 1, 2, 8 and 9 in Table 2 and entries 3, 6, 17 and 20 in Table 3). Some catalyst deactivation was also observed. For example, in a PhOAc–dodecane 25:75 (wt.%) system, the 40% PW-SiO2 catalyst in the second run showed ca. 80% of its initial activity (entry 4, Table 3). After the first run, the catalyst was significantly coked (C content, ca. 13%) which probably caused the deactivation. The total selectivity of reaction (1) to PhOH, 2HAP, 4HAP and 4AAP was found to be over 98%. The homogeneous reaction is more selective than

the heterogeneous one because it makes less phenol and more acetophenones, the selectivity to the more valuable para-acetophenones, 4AAP and 4HAP, being also higher. Addition of acetic anhydride (entries 13–16, Table 3) improved the selectivity towards the acylated products at the expense of phenol, as expected, although it decreased the catalytic activity. The turnover frequencies (TOF) corresponding to the initial reaction rate were calculated as the number of moles of PhOAc converted per mole of total protons in the catalyst. It should be pointed out that in solid HPA catalysts only a part of H+ are accessible, thus the true TOF may be higher. Being a much stronger acid [7–11], HPA is almost 100 times more

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active than H2 SO4 in homogeneous reaction, as well as more selective to acetophenones (cf. entries 10 and 12, Table 2). In heterogeneous systems, HPA is also two orders of magnitude more active than H-Beta zeolite, which is one of the best zeolite catalysts for this reaction (cf. entries 8, 9, 10 and 11, Table 3). However, H-Beta shows a higher total selectivity to acetophenones than HPA [4] (entry 9, Table 3). In our hands, though, H-Beta showed similar performance to that of the bulk PW (cf. entries 1 and 10, Table 3). It should be pointed out that HPA in homogeneous systems gives a higher selectivity to para-acetophenones 4AAP and 4HAP than H-Beta (Table 2). The efficiency of solid HPA (at constant loading) increases in the order PW < 40% PW-SiO2 < 10% PW-SiO2 in which the number of accessible proton sites increases [10,11] (cf. entries 1–3, 11 and 12, Table 3). Insoluble salt Cs2.5 H0.5 PW is an efficient solid catalyst for the reaction in polar media such as PhNO2 (entry 17, Table 3). Although less active per unit weight than the homogeneous PW or PW-SiO2 , it shows high TOF and is more selective to acetophenones than HPA, making less phenol. The salt Ce0.87 H0.4 PW performs similarly, although it is slightly less active than Cs2.5 H0.5 PW (entry 23, Table 3). Apparently, the less hydrophilic Cs+ and Ce3+ salts possess stronger proton sites than the solid PW or PW-SiO2 . The PW catalysts retain larger amounts of water than the salts after pretreatment at 150 ◦ C (Table 1). In a polar solvent PhNO2 , the Cs+ salt is more active and selective than in non-polar dodecane, producing more para-acylation products 4AAP and 4HAP at the expense of phenol and 2HAP (entries 21 and 22, Table 3). Similar results have been reported for zeolites [4]. 3.2. Rearrangement of phenyl benzoate

Fig. 1. Time course for the Fries reaction of phenyl acetate: (a) PW (5.0 wt.%), without solvent, 150 ◦ C (entry 2, Table 2); (b) 40% PW-SiO2 (1.5 wt.%), PhOAc–dodecane (25:75 (wt.%)), 130 ◦ C (entry 3, Table 3); (c) Cs2.5 H0.5 PW (0.67 wt.%), PhOAc–PhNO2 (25:75 (wt.%)), 130 ◦ C (entry 17, Table 3).

This reaction (Eq. (2); Bz = benzoyl) occurs similarly to that of PhOAc, yielding 2- and 4-hydroxybenzophenones (2HBP and 4HBP), 4-benzoxybenzophenone (4BBP) and phenol together with benzoic acid. Table 4 shows examples of homogeneous (with PW) and heterogeneous (with Cs+ salt) rearrangement of PhOBz in PhNO2 solution. The product selectivities and catalyst activities are quite similar to those observed for PhOAc. The difference is that the amount of phenol formed is nearly equal to that of 4BBP, indicating that the hydrolysis of PhOBz is less

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Table 4 Fries rearrangement of phenyl benzoate (130 ◦ C, 2 h) Catalyst (wt.%)

(0.60)a

PW Cs2.5 H0.5 PWO40 (0.67)b a b

Solvent (PhOBz, wt.%)

Conversion (%)

PhNO2 (25) PhNO2 (25)

18.6 4.3

TOF (min−1 )

3 4

Selectivity (%) PhOH

2HBP

4HBP

4BBP

40 48

5.2 2.3

19 2.5

36 47

Homogeneous reaction. Heterogeneous reaction.

significant in this case.

(2)

3.3. Rearrangement of p-tolyl acetate

3.4. Sol–gel HPA catalysts

In the case of p-TolOAc, the acylation in the para-position is no longer possible. Hence, the major products are 2-hydroxy-5-methylacetophenone (2H5MAP) and p-cresol (Eq. (3)) together with acetic acid and acetic anhydride. A very small amount of the meta-acylation product 3-hydroxy-6-methylacetophenone (3H6MAP) may also be formed.

The sol–gel catalysts containing PW in the silica matrix prepared by hydrolysis of tetraethyl orthosilicate have been introduced by Izumi et al. [19]. These have been claimed to be efficient and non-leaching solid acid catalysts for reactions in polar media (e.g. ester hydrolysis in water) [19]. The sol–gel catalysts have also been reported to be advantageous over supported HPA for aromatic alkylations and other reactions [21]. On the other hand, it has been well established that HPAs (e.g. PW) supported on silica possess weaker acid sites than bulk HPA because of interaction between HPA protons and the silanol groups of silica ([10,11] and references therein). Hence, one might expect that HPA in sol–gel catalysts would be even weaker than that in silica-impregnated catalysts due to its stronger interaction with the silica matrix. This has been supported recently [22] by the observation that the gas-phase isomerisation of butene occurred much slower over the sol–gel PW than over silica-impregnated PW. We prepared several PW sol–gel catalysts containing 11–16% HPA (Table 1) and tested them in the rearrangement of PhOAc. Two preparation methods were used, the catalysts thus made were designated PW-SiO2 -J and PW-SiO2 -H. It should be noted that these catalysts are all mesoporous solids (17–18 Å pore size) in contrast to macroporous PW-SiO2 and

(3) The homogeneous reaction with PW in PhNO2 or o-dichlorobenzene gives almost equal amounts of 2H5MAP and p-cresol, no 3H6MAP being formed (Table 5). The heterogeneous reaction with the Cs+ salt gives 2H5MAP with a remarkably high selectivity of 82% and only 17% of p-cresol. A little of 3H6MAP (1.4%) is also formed. This indicates that the hydrolysis p-TolOAc is less significant with the Cs+ salt than with PW, which is in agreement with the water content in these catalysts (Table 1).

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Table 5 Fries rearrangement of p-tolyl acetate (130 ◦ C, 2 h) Catalyst (wt.%)

(0.60)a

PW PW (0.60)a Cs2.5 H0.5 PWO40 (1.4)b a b

Solvent (TolOAc, wt.%)

Conversion (%)

PhNO2 (25) o-Cl2 C6 H4 (25) o-Cl2 C6 H4 (25)

8.5 5.0 8.8

TOF (min−1 )

6 4 6

Selectivity (%) p-Cresol

2H5MAP

3H6MAP

46 51 17

54 49 82

0 0 1.4

Homogeneous reaction. Heterogeneous reaction.

contain relatively large amounts of water (Table 1). Regardless of the preparation method, the sol–gel solids were found to be very poor catalysts for the Fries reaction (Table 6). In this case, the hydrolysis of PhOAc to form phenol (92–100% selectivity) is by far predominant over the rearrangement. The hydrolysis is likely to be caused by water introduced with the catalysts that contained 5–7 wt.% H2 O after pretreatment at 130 ◦ C (Table 1). The catalyst pretreated at 160 ◦ C (2.6 wt.% H2 O) was even less active, giving 100% PhOH (entry 7). Therefore, the sol–gel PW catalysts show much poorer activity towards the Fries reaction compared to the bulk and silica-supported PW as well as to its Cs+ and Ce3+ salts. This may be explained by the weaker acid strength of the sol–gel catalysts and the presence of water in them. The lack of activity could hardly be explained by the small pore size of the sol–gel solids, as the Cs+ and Ce3+ salts, also having small pore sizes (Table 1), are very active catalysts for the Fries reaction.

Fig. 2 shows 31 P MAS NMR spectra for 11% PW-SiO2 -H, 10% PW-SiO2 , and bulk PW. These samples exhibit signals at −14.8, −14.9 and −15.5 ppm, respectively, which indicates that, the state of the PW anion in the sol–gel and silica-impregnated catalysts is quite similar, but different from that in the bulk PW. The signal for the sol–gel PW is more symmetrical than that for PW-SiO2 , reflecting a more isotropic environment in the sol–gel system, as expected. This may also be due to the higher water content in the sol–gel catalyst (Table 1). A small peak at −13.5 ppm may be attributed to lacunary or Dawson-type HPA [18]. The difference in chemical shifts for the bulk PW, on the one hand, and the silica-included and silica-impregnated catalysts, on the other, indicates a certain chemical interaction between the HPA and support [10,11]. Apparently, in the case of a sol–gel catalyst, all PW protons are involved in this interaction, whereas in the case of silica-impregnated PW only part of the protons. This may be thought to be

Table 6 Heterogeneous Fries rearrangement of phenyl acetate catalysed by sol–gel catalysts PW-SiO2 (130 ◦ C, 2 h)a Catalyst (wt.%)

1 2 3 4 5 6 7

11% 15% 15% 15% 15% 16% 11%

PW-SiO2 -H (6.0) PW-SiO2 -J (4.0) PW-SiO2 -J (4.0) PW-SiO2 -J (4.0)c PW-SiO2 -J (4.0)d PW-SiO2 -H (4.0) PW-SiO2 -H (6.0)e

Solvent (PhOAc, wt.%)

Conversion (%)

PhOAc PhOAc PhNO2 PhNO2 PhNO2 PhNO2 PhNO2

2.4 2.9 10.4 6.2 0.2 7.4 2.8

(100) (100) (25) (25) (25) (25) (25)

TOFb (min−1 )

4 6 5 5 0.1 4 0.5

Selectivity (%) PhOH

2HAP

4HAP

4AAP

96 92 94 97 67 99 100

1.3 1.8 1.1 0 0 0.2 0

0 0.7 0 0 0 0 0

3.0 5.1 4.5 2.9 33 1.2 0

Unless stated otherwise, the catalysts were pretreated at 130 ◦ C/0.1–0.5 Torr for 1.5 h. TOF per proton corresponding to the initial reaction rate assuming that all H+ are accessible. c Acetic anhydride (2.0 wt.%) added. d Acetic anhydride (4.0 wt.%) added. e The catalyst pretreated at 160 ◦ C/0.1–0.5 Torr for 2.5 h. a

b

E.F. Kozhevnikova et al. / Applied Catalysis A: General 245 (2003) 69–78

77

Fig. 2. 31 P MAS NMR spectra for: (a) bulk PW (−15.5 ppm); (b) 11% PW-SiO2 -H (−14.8 ppm); and (c) 10% PW-SiO2 (−14.9 ppm). The catalysts were pretreated at 130 ◦ C/0.1–0.5 Torr for 1.5 h.

the reason for the weaker acid strength of the former compared to the latter catalyst.

4. Conclusion Heteropoly acid H3 PW12 O40 (PW) and its acidic salts are active and environmentally friendly catalysts for the Fries rearrangement of aryl esters, such as phenyl acetate, phenyl benzoate, and p-tolyl acetate, in homogeneous or heterogeneous liquid-phase systems. The reaction occurs at 100–170 ◦ C (preferably

130–150 ◦ C) with 0.6–5 wt.% HPA charge. Amongst the most efficient catalysts are the PW itself in homogeneous systems and silica-supported PW as well as its bulk acidic Cs+ salt Cs2.5 H0.5 PW12 O40 in heterogeneous systems. Both in homogeneous and heterogeneous systems, the reaction is inhibited by products, which is typical of the Friedel–Crafts chemistry. Consequently, to achieve a higher ester conversion a larger amount of the catalyst is needed or a flow technique should be applied. In heterogeneous reaction, the silica-supported PW is more active than the bulk PW because of greater number of available proton sites in

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PW-SiO2 [10,11]. From heterogeneous systems, the catalysts can be separated by filtration and reused, albeit with reduced activity. In terms of turnover frequency, PW is almost two orders of magnitude more active than H2 SO4 or H-Beta zeolite, which is explained by the greater acid strength of PW compared to the other acids [10,11]. In contrast to silica-supported PW, the sol–gel PW catalysts show only a negligible activity in the Fries reaction. This may be explained by a weaker acid strength of the sol–gel catalysts due to a strong interaction of the HPA protons with the silica matrix and the presence of relatively high amounts of water.

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