제올라이트 베타상에서의 2-메톡시나프탈렌의 Friedel-Crafts 아실화반응

황규연, 이현구

서울대학교 응용화학부·화학공정 신기술 연구소

Friedel-Crafts acylation of 2-methoxynaphthalene over HBEA zeolites

Kyu-Yeoun Hwang and Hyun-Ku Rhee

School of Chemical Engineering and Institute of Chemical Processes,

Seoul National University, Seoul 151-742, Korea

INTRODUCTION

Friedel-Crafts acylation is the key step in the production of aromatic ketones which is of special importance in the synthesis of fragrances and pharmaceuticals [1]. In general, Lewis-acid metal chloride catalysts are employed for aromatic acylation. However, use of such conventional catalysts results in a number of problems. A major drawback of these catalysts is that they are non-regenerable and it is necessary to use the catalyst in excess. Besides, the work-up steps involving hydrolysis of the resultant intermediate complex produce hazardous corrosive waste products. Disposal of such waste products also leads to other environmental problems. Many of these drawbacks can be overcome by using recoverable and regenerable solid catalysts like zeolites. Indeed, acylation of anisole by Y and beta zeolites was already known to occur with good yields and selectivity [2], with beta giving the best results. Therefore, the use of zeolites and other solid acid catalysts in the manufacture of fine chemicals and chemical intermediates is gaining much more attention in recent years.

It would be special interest to develop a highly active catalyst for the selective acylation of 2-methoxynaphthalene (2-MON) with acetic anhydride (AA) or acid at the 6-position, in this way producing 6-acetyl-2-methoxynaphthalene (2,6-AMON), which is an important intermediate for the preparation of Naproxen. The reaction is actively catalysed by zeolites [3] or MCM-41 molecular sieves [4]. The results from all the previous investigations suggest that zeolite beta may be the most suitable catalyst for the production of 2,6-AMON since it was speculated that its formation is favored inside the pores of BEA.

Here, the acylation of 2-MON with AA has been studied over various  beta zeolites in a protonic form and with increasing Si/Al ratios. The aim of this paper is to elucidate the effect of the framework Si/Al ratio of beta zeolite on the activity and selectivity in catalyzing the 2-MON acylation for the formation of precursor to the nonsteroidal anti-inflammatory agent Naproxen.

EXPERIMENTAL

Materials

The zeolite beta (B20) was prepared according to the method described in ref. [5]. Three commercial zeolite beta samples were provided: two samples of zeolite beta (B11 and B37.5) of different Si/Al ratios by PQ (CP814 and CP811, respectively) and one zeolite beta (B25) by S□d-Chemie. The chemicals, 2-MON (Aldrich), 2,6-AMON (Aldrich), AA (Fluka) and chlorobenzene (Jassenca) were used in this study.

Reaction procedure

The reactions were carried out in a 50-ml three-neck round bottom flask, installed with a reflux condenser and with magnetic stirrer. A typical reaction was carried out as follows: A 0.2 g of the previously activated catalyst (4h at 773K) was added to a solution of 1 mmol of 2-MON and 2 mmol of AA dissolved in 20 ml of chlorobenzene under nitrogen atmosphere and the reaction mixture was heated to the reflux temperature of chlorobenzene (405K) and was stirred. Samples were taken out periodically during the course of reaction and analyzed by GC on HP-5 column with an FID detector using 1,3,5-triisopropylbenzene as internal standard. The products were confirmed by comparison with authentic samples and GC-MS analysis.

RESULTS AND DISCUSSION

The activity of HBEA catalysts

In the application of zeolite catalyst to acid-catalyzed reaction, the framework Si/Al ratio is one of the most important parameters which controls the activity and selectivity. This factor does not only define the number of potential protons, and therefore the number and strength of acid sites, but it also determines the adsorption property of the zeolite.

Table 1

Influence of the framework Si/Al ratio on the IRR and TOF in the acylation of 2-MON with AA over beta zeolites

Catalysts	Si/Al	Al/(Al+Si)	IRR (mmol min-1 g-1)	TOF (min-1)
B11	11	0.08333	0.3665	0.2639
B20	20	0.04762	0.4515	0.5689
B25	25	0.03846	0.6855	1.0694
B37.5	37.5	0.02597	1.475	3.406

Table 1 summarizes the activity of zeolite beta with different Si/Al ratios in the acylation of 2-MON. In order to establish an effective comparison between  different zeolites, the expression of result in terms of turnover frequency (TOF), obtained by dividing the initial reaction rate (IRR) by the framework aluminium molar fraction per gram, would be more persuasive. TOF is independent of total number of active sites by definition, so its value reflects both the acidic and hydrophobic property of one site. Firstly, IRR and TOF increases in a significant extent with increasing Si/Al ratios. This order of activities could be attributed, in a first approach, to the number of active sites (aluminium), consequently the strength of acid site. This is because it increases as the number of framework Al decreases.

In general, the increased activity of zeolite with high Si/Al ratio can be explained by three different factors, zeolite acidity, increased hydrophobicity and the presence of extraframework aluminium (EFAL) [6]. Recently, M. Guisnet et al. [7] observed that EFAL species have no effect on the acylation of 2-MON and even they play a negative role by blocking the micropore or neutralization of the protonic sites. Even if some EFAL species are present, our analysis below would only be affected on a relative basis and our conclusion regarding TOF remains valid. Therefore, the effect of EFAL on the present reaction could be excluded. Moreover, TPD spectra of these catalysts indicate that the order of overall acidity strength follows the sequence B25 > B37.5 > B20 > B11. According to these results, the highest activity of B37.5 originates from the optimum balance between the strength of the acid sites and its hydrophobicity.

Influence of Si/Al ratio on the selectivity to 2,6-AMON

The previously published results showed that acylation occurs preferentially at position 1, which is the kinetically controlled position. However, the formation of the more linear 2,6-AMON was enhanced by using the steric constraints in the narrow pores of zeolite, especially zeolite beta. Difference in product selectivity between zeolites was interpreted by the shape selective effect. Besides, protiodeacylation of 1-acetyl-2-methoxynaphthalene (1,2-AMON) to 2-MON and isomerization of 1,2-AMON in the presence of MON to the thermodynamically stable 2,6-AMON were observed. Therefore, the synthesis of 2,6-AMON occurs through a combination of direct acylation of 2-MON and isomerization of 1,2-AMON. Although deacylation of 2,6-AMON to 2-MON was also detected, this secondary reaction is not taken into account in this study due to its low probability. The reactions occurring in acylation of 2-MON over zeolite catalysts could be organized as in Scheme 1.

Scheme 1

All the HBEA samples produce two major products, 1,2-AMON and 2,6-AMON. Also, traces of 1-acetyl-7-methoxynaphthalene and diacylated compounds are detected. However, the initial product distribution and its variation with the reaction time are somewhat different according to the value of framework Si/Al ratio. As the reaction time increases, the selectivity to 2,6-AMON increases owing to the secondary reactions which involve the protiodeacylation as well as isomerization of 1,2-AMON. As shown in Fig. 1(b), the reaction of 2-MON with AA using zeolite beta with various Si/Al ratios indicate that zeolite beta with low Si/Al ratio, relatively less acidic, may be suitable for the selective formation of 2,6-AMON. Considering the 1 min reaction time, B11 shows the highest selectivity to the 2,6-AMON and it decreases slightly from B20 to B37.5. After 4 hours reaction, the selectivity to 2,6-AMON follows the sequence B11 >> B37.5 > B20 ≒ B25. From these results, it may be seen that the direct acylation of 2-MON is the major reaction route to 2,6-AMON over B11, whereas in the case of B37.5, the isomerization of 1,2-AMON is more pronounced. A possible explanation for this difference is that under catalytic conditions using the catalyst having relatively low acid strength, the formation of kinetically favored product, 1,2-AMON, could be suppressed, so primary reaction would preferentially proceed at the 6-position. Even though the selectivity to 2,6-AMON enhancement on B37.5 is faster than on B11, it levels off rapidly due to  catalyst deactivation. As shown in Fig. 1(a), the fast deactivation occurs in all the cases. Such a loss of the catalytic activity may be due to the saturation of the active sites of the catalyst by the product formed, acetic acid or the polyaromatic ketones.

Fig. 1. Conversion (a) and selectivity to the 2,6-AMON (b) versus time on stream over beta zeolites with different Si/Al ratios

The UV-visible spectra of the deactivated catalyst shows the typical bands of acetophenones and polyaromatics, which were absent in the fresh samples. Such an

adsorption explains the fast deactivation of the catalyst as soon as the products are formed, which cannot be avoided in the batch conditions [8]. Therefore, the final selectivity to 2,6-AMON is largely influenced by the initial product distribution. It is also noticed that there is no apparent correlation between the framework Si/Al ratio and the selectivity to 2,6-AMON over the catalysts tested.

ACKNOWLEDGEMENT

The financial aid from the Brain Korea 21 Program supported by the Ministry of Education is greatly appreciated.

REFERENCE

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