Friedel–Crafts acylation of anisole with octanoic acid over acid modified zeolites

Abstract

Friedel–Crafts acylation of anisole using octanoic acid as a green acylating agent was studied over zeolites free of solvent. It was found that a mixed organic acid, composed of tartaric acid and oxalic acid, modified Hβ (Mix-Hβ) zeolite showed the best catalytic performance among the catalysts studied. The conversion of octanoic acid and the selectivity for p-octanoyl anisole were 72.7% and 82.5%, respectively. Inductively coupled plasma analysis (ICP) and ²⁷Al MAS NMR indicated dealumination of the parent Hβ zeolite due to the treatment of the mixed organic acid, leading to more accessible active sites and accounting for the better catalytic activity of the Mix-Hβ zeolite. Furthermore, lower strength of Lewis acid sites and Brönsted acid sites of the Mix-Hβ zeolite, as demonstrated by Fourier Transform Infrared Spectrometry after adsorption of pyridine (Py-IR), are advantageous to suppress the demethylation of anisole and the subsequent esterification of thus formed phenol, accounting for its higher selectivity toward p-octanoyl anisole.

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1. Introduction

Friedel–Crafts acylation of aromatics is by far the most popular method for the production of aromatic ketones,^1,2 which are key intermediates of fine chemicals, pharmaceuticals, insecticides, plasticizers, dyes, perfumes, and agrochemicals.^3–5 Presently, Friedel–Crafts acylation is usually carried out using acid chlorides⁶ or acid anhydrides^7,8 as the acylating agents and catalyzed by Lewis acids,^9–14 such as AlCl₃, BF₃, or Brönsted acids, typically H₂SO₄ or HF. However, the procedures using acid chlorides and acid anhydrides suffer from severe corrosion to reactors as well as waste problems, mainly due to the formed HCl or carboxylic acids during the reaction. On the other hand, the above catalysts have limitations such as environmental pollution hazards arising from the disposal of potential toxic wastes, reactor corrosion, and difficulties in their handling. Furthermore, the catalyst amounts to be employed are sometimes much more than the stoichiometric quantity required, as the catalysts have a tendency for complex formation with either the reactants or the products. Thus, considering both the acylating agents and the catalysts are not eco-friendly for current methods, there has been great demand to develop a green method due to the environmental and economical concerns.

Carboxylic acids are regarded as potential green acylating agents in the Friedel–Crafts acylation because water is the only byproduct derived from carboxylic acid after the acylation.^15–18 On the other hand, solid acid catalysts, such as heteropoly acids and polyoxometalates^19–25 and zeolites,^26–29 are regarded as potential green catalysts. Thus, some researches have been carried out using carboxylic acids as acylating agents catalyzed by solid acid catalysts for the purpose of green synthesis. Particularly, zeolites have attracted special attention in this transformation due to their pore structures and unique acid properties. For example, Ma et al.¹⁶ have investigated the Friedel–Crafts acylation of anisole with substituted benzoic acids and alkanoic acids over Y zeolite and found Lewis acid sites were more effective and selective for the target reaction. Wagholikar et al.¹⁸ have studied the Friedel–Crafts acylation of anisole with different long-chain carboxylic acids over three large pore zeolites, and found that both the type of zeolites and the chain length of carboxylic acids can influence the reaction results. Although progresses have been made by such catalysts, problems still can not be totally solved because of the prolonged reaction time and/or high molar ratio of aromatics to carboxylic acids, as well as low conversion of carboxylic acids. Therefore, it is desirable to develop an efficient catalyst for the green Friedel–Crafts acylation.

In the continuation of our previous works,^2,30 we herein report our recent studies on the green Friedel–Crafts acylation with the acylation of anisole with octanoic acid to p-octanoyl anisole (p-OA) being chosen as a model system. A series of zeolites was tested and a mixed organic acid, composed of tartaric acid and oxalic acid, modified Hβ (Mix-Hβ) zeolite showed the best catalytic performance among the catalysts studied. The activities of the zeolites were correlated with their structures using appropriate characterizations and a possible mechanism was also proposed.

2. Experimental

2.1. Materials

Most reagents were purchased from Baoding Huaxin Reagent and Apparatus Co., Ltd. and were used as received without further purification. Particularly, the HZSM-5, HY, MCM-41 and Hβ zeolites were commercial samples purchased from Tianjin Chemist Catalyst Co., Ltd.

2.2. Catalyst preparation

Oxalic acid modified Hβ zeolite was prepared by impregnating 1.0 g of the parent Hβ zeolite into 25 mL of an aqueous 1 M solution of oxalic acid at 80 °C for 1 h. Then, the sample was filtered, washed with deionized water for 5 times, dried at 130 °C and calcined at 500 °C for 4 h. The prepared catalyst was labeled as OA-Hβ. Moreover, lactic acid, citric acid and tartaric acid modified Hβ zeolites were prepared by the same method and labeled as LA-Hβ, CA-Hβ, TA-Hβ, respectively. 2 M-OA-Hβ, 2 M-TA-Hβ and Mix-Hβ zeolites were prepared by impregnating 1.0 g of the parent Hβ zeolite into 25 mL of an aqueous 2 M solution of oxalic acid, tartaric acid, or mixed organic acid (oxalic acid-to-tartaric acid molar ratio = 1 : 1) at 80 °C for 1 h. After that, the samples were filtered, washed with deionized water for 5 times, dried at 130 °C and calcined at 500 °C for 4 h.

2.3. Catalyst characterization

X-ray diffraction (XRD) measurements were carried out on a Bruker D8 advance diffractometer with Cu Kα radiation. Bulk compositions were identified by inductively coupled plasma analysis (ICP) on a Thermo VISTA-MPX spectrometer. BET surface area and total pore volume were calculated using a Micromeritics Tristar II 3020 surface area and pore analyzer.²⁷Al MAS NMR spectra were recorded on a Varian Infinityplus 300M Hz spectrometer. NH₃ temperature programmed desorption (NH₃-TPD) experiments were performed on a TP-5000 instrument equipped with a thermal conductivity detector. Fourier Transform Infrared Spectrometer after adsorption of pyridine (Py-IR) was measured on a Bruker EQUINOX55 spectrometer to determine the surface acidity of zeolites. Zeolite wafers were first mounted in a vacuum cell. The samples were then pretreated at 500 °C under vacuum for 0.5 h, after that pyridine vapor was adsorbed onto the sample, followed by infrared spectrometer characterization at 150 °C and 450 °C, respectively.

2.4. Catalyst activity test

Friedel–Crafts acylation of anisole with octanoic acid, as shown in Scheme 1, was carried out in a 25 mL round-bottom flask. In a typical run, anisole (2.70 g, 25.0 mmol), octanoic acid (1.80 g, 12.5 mmol), and catalyst (0.5 g) were charged into the flask and stirred at 155 °C for 6 h under nitrogen atmosphere. The reaction mixtures were analyzed by a gas chromatography (GC) using a 30 m SE-30 capillary column and the products were confirmed by GC-MS (Agilent 5975C). p-OA was also isolated by column chromatography on 200–300 mesh silica gel using petroleum ether–dichloromethane = 4 : 1 as eluent and further identified by ¹H-NMR (Bruker Avance III 600 MHz).

Scheme 1 Friedel–Crafts acylation of anisole with octanoic acid.

p-OA. Colorless crystals. ¹H NMR (600 MHz, CDCl₃) δ 7.98 (d, J = 8.4 Hz, 2H), 6.96 (d, J = 9.0 Hz, 2H), 3.89 (s, 3H), 2.94 (t, J = 7.2 Hz, 2H), 1.77–1.72 (m, 2H), 1.41–1.28 (m, 8H), 0.92 (t, J = 6.6 Hz, 3H).

3. Results and discussion

3.1. Catalyst selection

It is well known that carboxylic acids are not active enough as acylating agents in the Friedel–Crafts acylations.¹⁶ Therefore, selection of a suitable catalyst is crucial to the efficient acylation of anisole with octanoic acid. Thus, some commercial zeolites, such as HZSM-5, HY, and Hβ zeolite, were first examined and the representative results are listed in Table 1. It is clear that the Hβ zeolite was the most effective one among them, which can be ascribed to the special structure of Hβ zeolite and the characteristics of this reaction. The conversion of octanoic acid was 43.3% and the selectivity for p-OA was 73.4% over Hβ zeolite, but still lower than our expectation. Considering the benefit of acid modification,^28–30 a series of organic acid modified Hβ zeolites was then investigated. As envisaged, these modified zeolites showed better catalytic performance than the parent Hβ zeolite, especially in the cases of the OA-Hβ and TA-Hβ zeolites. The conversion of octanoic acid was increased to 66.2% over the TA-Hβ zeolite and the selectivity for p-OA was increased to 83.7% over the OA-Hβ zeolite, both being the best results among the modified zeolites. However, the high conversion and selectivity can not be achieved simultaneously over one catalyst. Thus, to obtain a more efficient catalyst, we prepared a mixed organic acid, composed of tartaric acid and oxalic acid, modified Hβ zeolite and tested in this reaction. As expected, both good conversion and selectivity are obtained over this novel catalyst. The conversion of octanoic acid and the selectivity for p-OA were 72.7% and 82.5%, respectively. The catalytic performance of 2 M-OA-Hβ and 2 M-TA-Hβ zeolites were also tested for comparison. As can be seen, the conversions of octanoic acid were both improved at about 10.0% with the increase of the concentration of organic acids used for modification. However, the selectivities for p-OA were nearly unchanged, indicating that the selectivity is not affected by the concentration of organic acids used for modification. Thus, there should be certain synergistic effect between tartaric acid and oxalic acid during modification, accounting for the best catalytic performance of Mix-Hβ zeolite in this transformation. Therefore, Mix-Hβ zeolite was selected as the catalyst of choice for further investigation.

Table 1 Results of different zeolites for Friedel–Crafts acylation of anisole with octanoic acid^a

Zeolite	Conversion of octanoic acid (%)	Selectivity (%)
		p-OA	Methyl octanoate	Phenyl octanoate
a Reaction conditions: octanoic acid-to-anisole molar ratio: 1 : 2, reaction time: 6 h, zeolite amount: 0.5 g, reaction temperature: 155 °C.
HZSM-5	1.2	25.0	56.7	15.6
HY	2.5	84.3	4.9	6.4
MCM-41	0.2	80.0	0	0
Hβ	43.3	73.4	14.7	8.0
LA-Hβ	50.9	68.3	18.0	9.3
CA-Hβ	49.8	69.5	17.2	5.8
OA-Hβ	51.2	83.7	9.0	4.9
TA-Hβ	66.2	73.5	14.9	7.0
Mix-Hβ	72.7	82.5	7.4	4.8
2 M-OA-Hβ	62.5	84.9	7.1	6.0
2 M-TA-Hβ	75.8	76.0	12.6	5.7

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3.2. Catalyst characterization

XRD patterns of the parent Hβ, Mix-Hβ, TA-Hβ and OA-Hβ zeolites are shown in Fig. 1. As can be seen, the characteristic diffraction peaks of the modified zeolites are at about 7.5° and 22.5°, just like those of the parent Hβ zeolite, indicating that the modified zeolites still kept the Hβ type. However, the intensity of the characteristic diffraction peaks of the modified zeolites all decreased, especially for the Mix-Hβ zeolite, indicating that acid treatments led to small crystals of the modified zeolites.

Fig. 1 XRD patterns of the parent Hβ (a), Mix-Hβ (b), TA-Hβ (c) and OA-Hβ (d) zeolites.

BET surface areas and pore volumes of the parent Hβ, Mix-Hβ, TA-Hβ and OA-Hβ zeolites are listed in Table 2. We found that the BET surface areas and pore volumes of the three modified Hβ zeolites both increased after the acid treatments and Mix-Hβ zeolite exhibited the largest surface area and pore volume. Furthermore, the bulk Si/Al molar ratios of the parent Hβ, Mix-Hβ, TA-Hβ and OA-Hβ zeolites were calculated from the ICP results and are also listed in Table 2. The Si/Al molar ratios markedly increased after the organic acid treatments, especially for the oxalic acid modified zeolite, and thus dealumination of the zeolites should occur under the acid treatments, probably due to their strong coordination complexation with aluminum cation of the Hβ zeolite and in line with the previous reports.^30–32

Table 2 Structural properties of the zeolites

Zeolite	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Si/Al^a
a Calculated from the ICP results.
Hβ	432	0.28	12
Mix-Hβ	543	0.34	44
TA-Hβ	508	0.32	28
OA-Hβ	498	0.33	46

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In order to further investigate the dealumination effect of the mixed organic acid, ²⁷Al MAS NMR spectra of the parent Hβ and Mix-Hβ zeolites were tested and presented in Fig. 2. Both samples show two major peaks at about 0 and 53 ppm, corresponding to octahedrally coordinated nonframework Al sites and tetrahedrally coordinated framework Al sites, respectively.³³ It is obvious that the strength of both peaks markedly decreased by roughly 70% after the mixed organic acid treatment, ascribed to the dealumination of the parent Hβ zeolite and in accordance with the ICP results. Consequently, combined with the experimental data (Table 1), we suggested that dealumination of the parent Hβ zeolite resulted in the Mix-Hβ zeolite having more accessible active sites and then accounted for its higher activity.

Fig. 2 ²⁷Al MAS NMR patterns of the parent Hβ (a) and Mix-Hβ (b) zeolites.

NH₃-TPD analyses were carried out to determine the number and types of acidic sites on the surfaces of the parent Hβ, Mix-Hβ, TA-Hβ and OA-Hβ zeolites by the measurement of the amounts of NH₃ desorbed at different temperatures, as shown in Fig. 3. The strong peaks centered at about 250 °C are assigned to the weakly acidic sites and the peaks located approximately at 400 °C to the moderately-strong acidic sites according to the literatures.^34,35 It is obvious that the amounts of the weakly acidic sites of the modified Hβ zeolites markedly decreased compared with the parent Hβ zeolite. In contrast, the amounts of the moderately-strong acidic sites kept relatively stable. Thus, careful inspection of the reaction data (Table 1) together with the NH₃-TPD results suggests that a catalyst having an appropriate amount of weakly acidic sites is in favor of the desired transformation, in agreement with our previous report.³⁰

Fig. 3 NH₃-TPD profiles of the parent Hβ (a), Mix-Hβ (b), TA-Hβ (c) and OA-Hβ (d) zeolites.

Py-IR was then carried out to study the effect of acid treatment on the Lewis acid and Brönsted acid properties of the zeolites. Py-IR spectra of the parent Hβ and Mix-Hβ zeolites at 150 °C are shown in Fig. 4. It is clear that the amounts of both Lewis acid sites (at about 1450 cm⁻¹) and Brönsted acid sites (at about 1540 cm⁻¹) markedly decreased after the mixed organic acid treatment.^36,37 Furthermore, the relative amount of Brönsted acid sites decreased at about 68% (from 0.84 to 0.27), more than that of Lewis acid sites (Table 3), indicating that the acid modification causes different variation on different acid sites. When the desorption temperature further increased to 450 °C, the amounts of both Lewis acid sites and Brönsted acid sites of the two catalysts decreased in different extent, indicating the desorption of pyridine adsorbed on the weakly acid sites of zeolites (Table 3). It is obvious that the Mix-Hβ zeolite possesses a little more weakly Brönsted acid sites and much less both strongly Lewis acid sites and Brönsted acid sites, compared with the parent Hβ zeolite. Thus, we can conclude that the treatment by a mixed organic acid mainly reduced the amount of the strongly Lewis acid sites and Brönsted acid sites; whereas, the weakly acid sites still kept in a high level in the Mix-Hβ zeolite.

Fig. 4 Py-IR spectra of the parent Hβ (a) and Mix-Hβ (b) zeolites at 150 °C.

Table 3 Brönsted acid sites and Lewis acid sites of the zeolites^a

Zeolite	Total acid sites 150 (°C)		Strongly acid sites 450 (°C)		Weakly acid sites
	Brönsted acid sites	Lewis acid sites	Brönsted acid sites	Lewis acid sites	Brönsted acid sites	Lewis acid sites
a Relative amounts calculated from the Py-IR results.
Hβ	0.84	3.54	0.77	2.11	0.07	1.43
Mix-Hβ	0.27	1.71	0.16	0.31	0.11	1.40

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3.3. Reaction mechanism

A possible mechanism is finally proposed based on the GC-MS results (Scheme 2). The main reaction is the C-acylation of anisole with octanoic acid over Mix-Hβ zeolite to generate p-OA 1, the target product.^38,39 This transformation is ascribed to the presence of Lewis acid sites on Hβ zeolites, which could chemisorb octanoic acid via coordination. Then, the octanoic acid coordinated on Lewis acid sites would preferentially attack at the aromatic ring in its para-position to generate product 1; in contrast, the attack at its ortho-position is markedly inhibited due to the steric effect of octyl group, making the ortho-position substituted byproduct not detectable by GC-MS.

Scheme 2 Possible mechanism for the acylation of anisole with octanoic acid.

In addition, octanoic acid will be protonated in the presence of Brönsted acid sites on Hβ zeolites. The protonated octanoic acid will attack at the oxygen atom of anisole (O-acylation) to produce phenyl octanoate 2, together with the formation of methanol.¹⁶ At the same time, Brönsted acid sites on Hβ zeolites will favor of the demethylation of anisole to produce phenol 3 as a major intermediate,^4,31 which will possibly react with octanoic acid to produce phenyl octanoate 2.⁴⁰ Moreover, the esterification of octanoic acid with methanol produces methyl octanoate 4.¹⁶ According to the reaction data (Table 1), compound 2 and 4 are the two major byproducts and the better selectivity for p-OA over Mix-Hβ zeolite is mainly attributed to the decrease of them. Thus, lower strength of Lewis acid sites and Brönsted acid sites of Mix-Hβ zeolite, as demonstrated by Py-IR, are advantageous to suppress the demethylation of anisole and the subsequent side reactions, accounting for its higher selectivity for p-OA in this reaction.

4. Conclusions

In conclusion, Friedel–Crafts acylation of anisole with octanoic acid over a novel Mix-Hβ zeolite is first established in this paper. Noticeably, all factors in this reaction system including the acylating agent, catalyst as well as the modifying agent are eco-friendly. Modification by a mixed organic acid markedly improved the catalytic performance of the parent Hβ zeolite. ICP and ²⁷Al MAS NMR results demonstrated the dealumination of the Hβ zeolite under the mixed organic acid treatment, promoting an increase in BET surface area and the accessibility of reactants to the active sites, accounting for the best catalytic performance of Mix-Hβ zeolite. Moreover, lower strength of Lewis acid sites and Brönsted acid sites of Mix-Hβ zeolite are advantageous to suppress the demethylation of anisole and in turn accounts for its higher selectivity toward p-octanoyl anisole.

Acknowledgements

Financial support by the National Natural Science Foundation of China (20806018 and 21376060) and the Natural Science Foundation of Hebei Province (B2014201024) are gratefully acknowledged.

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