CHAPTER 7. ACYLATION OF ANISOLE WITH ACETIC ANHYDRIDE OVER MnAPO-5 AND LEWIS ACID METAL ION-EXCHANGED MnAPO-5

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1 103 CHAPTER 7 ACYLATIN F ANISLE WITH ACETIC ANHYDRIDE VER MnAP-5 AND LEWIS ACID METAL IN-EXCHANGED MnAP INTRDUCTIN Friedel-Crafts acylation is one of the most important methods for the synthesis of aromatic ketones (lah 1973). Aromatic ketones are widely used in the synthesis of a large number of fine chemicals such as drugs, fragrances, dyes and pesticides (Kouwenhoven and van Bekkum 1997, March 1992). Acylation of aromatic compounds is of particular interest in the field of aromatic substitution and displays high regioselectivity towards substitution in the para position. Aromatic ketones are mainly prepared by acylation of aromatics with acid chlorides, carboxylic acids and their anhydrides in the presence of acid catalysts. Among the acylating reagents, acyl chlorides are more reactive but unfortunately they are too hazardous due to formation of corrosive hydrogen halides as by-products. Acid anhydrides and carboxylic acids are preferable as acylating agents because of their less problematic by-products (organic acid and water respectively). Acylation of aromatics have traditionally been carried out using homogeneous acid catalysts such as AlCl 3, FeCl 3, ZnCl 2, HF, etc. It has been observed that these homogeneous catalysts are toxic, corrosive and often difficult to be recovered from the reaction mixture. Hence, efforts have been made to develop eco-friendly process for

2 104 acylation reactions. The use of recoverable and regenerable solid acid catalysts can overcome many of the limitations associated with use of metal halides and Lewis acids. Acylation of anisole using acetic anhydride was performed over zeolites (Corma et al 1989, Gaare and Akporiaye 1996 and Smith et al 1998), clays (Bastock et al 1994 and Choudary et al 1998), Nafion-H on silica (Heidekum et al 1999) and heteropoly acids. Among the various heterogeneous catalysts zeolites are proved to be effective and selective to carry out the reaction with obvious advantages of maximise the selectivity, yield of single desired product and minimise the by-product. Acylation of anisole with acetic anhydride proceeds through the formation of acylium ion intermediates that are generated from the acylating agent by interaction with acid catalyst. At first acetic anhydride adsorbed over Bronsted acid sites eliminates acetic acid and form acylium ion which reacts with anisole in the liquid phase to yield ortho and para methoxyacetophenone. Among the two products, p-methoxyacetophenone is obtained with high selectivity and yield. This product is an industrially important fine chemical and it is used as pharmaceutical intermediate and as raw material for fragrance. 7.2 ACYLATIN F ANISLE WITH ACETIC ANHYDRIDE Acylation of anisole with acetic anhydride directly catalysed by Lewis acid ion-exchanged MeAP-5 has not been reported so far. In this investigation, we have established a dramatic improvement in the yield of p-methoxyacetophenone by using Lewis acid ion-exchanged MnAP-5 catalysts. Acylation reaction was carried over MnAP-5 and La 3, Ce 3, In 3 or Ga 3 ion-exchanged MnAP-5 catalysts in the liquid phase. The above reaction was carried out by changing the reaction parameters like temperature,

3 105 catalyst weight, feed ratio and reaction time for high yield at maximum conversion. The major product was found to be p-methoxyacetophenone Effect of Temperature Acylation of anisole with acetic anhydride was carried over MnAP-5 and La 3, Ce 3, In 3 and Ga 3 ion-exchanged MnAP-5 catalysts at 60, 80, 100 and 120 ºC. The feed ratio was maintained at 1:2 (anisole: acetic anhydride) and the reaction was carried for 12 h in all the cases. The major product was found to be p-methoxyacetophenone and a small amount of o-methoxyacetophenone was also obtained. The results of anisole conversion and yield of the products over all the catalysts are presented in Table 7.1. The formation of both products can be accounted by considering the reaction Schemes 7.1 and 7.2. Acetic anhydride chemisorbed on the Bronsted acid sites eliminates acetic acid and form the acyl cation which reacts with anisole electrophilically yielding o- and p-methoxyacetophenone (Scheme 7.1). Alternatively, acetic anhydride co-ordinates to M through its acetyl group and forms acyl cation and neutral M (CH 3 C) 2. The acyl cation further reacts with anisole to give o- and p-methoxyacetophenone. The anisole conversion increased with increase in temperature over MnAP-5 and ion-exchanged MnAP-5 catalysts. p-methoxyacetophenone showed higher yield than o-methoxyacetophenone due to steric hindrance for o-acylation.

4 106 Table 7.1 Effect of temperature on anisole conversion and product yield over different catalysts Catalyst Temperature ( C) Anisole conversion (%) Yield (%) p-map o-map MnAP LaMnAP CeMnAP InMnAP GaMnAP Reaction conditions: Catalyst amount: 0.1 g; Feed ratio: 1:2 (Anisole:Acetic anhydride); Time: 12 h

5 107 H 3 C CH 3 H 2- Mn P (CH 3 C) 2 2- Mn H P H 3 C CH 3 CH 3 H 3 C 2- Mn C P - CH 3 CH 2- Mn H P CH 3 CH 3 Scheme 7.1 Acylation of anisole with acetic anhydride over MnAP-5

6 108 CH 3 (CH 3 C) 2 M MnAP-5 M C C CH 3 CH 3 CH 3 CCH 3 CH 3 CCH 3 Scheme 7.2 Acylation of anisole with acetic anhydride over M Acetic anhydride is certainly protonated to yield acyl cation and the electrophilic reaction between acyl cation and anisole yields the product. Since one of the products is acetic acid, its influence was studied separately by conducting the reaction with acetic acid and anisole in the presence of the same catalyst. But this reaction did not form the product. The reaction between anisole and acetic anhydride at 120 C without the catalyst also did not form the product. The reaction was also studied with acetic anhydride with small amount of acetic acid and anisole. There was less than 4% conversion was noted with these reagents. This corroborates the assumption that acylium ions are hardly formed from acetic acid (Cardoso et al 2004 and Ma et al 1997). Hence this observation suggests that weak acid sites of MnAP-5 could also catalyse acylation. Since the catalyst carries strong acid sites, they might be the main active sites in catalysing the reaction. Comparison of the results over all the catalysts proved that MnAP-5 is the least active among the catalysts. This revealed that the reaction is largely controlled by Lewis acid sites. LaMnAP-5 showed higher

7 109 conversion of anisole and product yield than MnAP-5. Though the reduction of about 43% of strong acid sites in LaMnAP-5, the enhanced conversion clearly established the active role of La sites. Though the catalyst possessed less number of strong acid sites than MnAP-5, it showed higher activity. Co-ordination of both reactants to La could be possible as it possesses higher co-ordination capability than H. Another important observation is that ion-exchange of La 3 mainly occurred on strong acid sites leaving weak acid sites unaffected. Hence, the metal ion dependent route may be more pronounced than Bronsted acid sites dependent route. The results of CeMnAP-5 exhibited nearly similar trend in conversion and product yield as that of LaMnAP-5. However, there is further enhancement in conversion of anisole and yield of p-methoxyacetophenone over CeMnAP-5. This is attributed to slightly more Ce 3 content in CeMnAP-5 than La 3 in LaMnAP-5. Therefore Ce 3 may play similar role as that of La 3. The high conversion and yield are due to more ion-exchange of Ce 3 (70%) than La 3 (43%) in MnAP-5. The results of InMnAP-5 and GaMnAP-5 illustrate still higher conversion than the previous catalysts. This may due to still further ion-exchange of In 3 (89%) and Ga 3 (93%) in MnAP-5. Hence it could be concluded that M sites (where M = La 3, Ce 3, In 3 and Ga 3 ) are more active than Bronsted acid sites. The activity of the catalyst follows the order: GaMnAP-5>InMnAP-5>CeMnAP-5>LaMnAP-5. This study clearly established the effect of Lewis acid metal ions particularly M species in the acylation of anisole Effect of Feed Ratio The effect of feed ratio (anisole: acetic anhydride) viz., 1:1, 1:2 and 1:3 on anisole conversion and yield of the products was studied over

8 110 GaMnAP-5 at 100 ºC and the results are presented in Table 7.2. The conversion increased from 1:1 to1:2. The increase of conversion from 1:1 to 1:2 is due to enhanced adsorption and activation of anisole on the active sites of the catalyst. There is a slight decrease in conversion at 1:3 compared to 1:2. Since the reaction is mainly controlled by Ga ion, there may be slight reduction in the transport of anisole to the co-ordination sphere of Ga at 1:3 feed ratio where acetic anhydride is chemisorbed. There is no free acyl cation formation in this pathway of acylation as shown in the reaction Scheme 7.1. If there is formation of free acyl cations then decrease in conversion would not be observed. The yield of p-methoxyacetophenone is about 90% at 1:1, 1:2 and 1:3 as the catalyst do not have shape selective feature. The yield of o-methoxyacetophenone is very low due to steric hindrance. Table 7.2 Effect of feed ratio on anisole conversion and product yield over GaMnAP-5 Feed ratio Anisole conversion (%) p-map Yield (%) o-map 1: : : Reaction conditions: Temperature: 100 C; Catalyst amount: 0.1 g; Time: 12 h Effect of Catalyst Loading The effect of catalyst loading on anisole conversion and product yield was studied using 0.05, 0.1 and 0.15 g catalyst and the results are

9 111 presented in Table 7.3. The conversion increased with increase in the catalyst amount. Hence, once the product is formed it should be immediately desorbed from the catalyst surface without getting chemisorbed again. With increase in catalyst amount from 0.05 to 0.15 g there is tremendous increase in conversion but the conversion with 0.1 g is almost equal to that of 0.15 g. GaMnAP-5 with more active sites is better than other catalysts. This study clearly revealed that the reaction still depends on the number of active sites. So the optimum amount of catalyst is found to be 0.1 g. Table 7.3 Effect of catalyst amount on anisole conversion and product yield over GaMnAP-5 Catalyst amount (g) Anisole conversion (%) p-map Yield (%) o-map Reaction conditions: Temperature: 100 C; Time: 12 h; Feed ratio: 1:2 (Anisole: Acetic anhydride) Effect of Reaction Time The effect of reaction time on anisole conversion and product yield was studied over 0.1 g GaMnAP-5 with feed ratio 1:2 at 100 C and the results are presented in Table 7.4. The conversion and yield of p-methoxyacetophenone increased with increase in the reaction time and attained steady state illustrating the attainment of equilibrium at the end of 12 h. At the equilibrium condition the yield of p-methoxyacetophenone is very much higher than o-methoxyacetophenone. The major product was found to be p-methoxyacetophenone irrespective of reaction time.

10 112 Table 7.4 Effect of reaction time on anisole conversion and product yield over GaMnAP-5 Time (h) Anisole conversion (%) Yield (%) p-map o-map Reaction conditions: Temperature: 100 C; Catalyst amount: 0.1 g; Feed ratio: 1:2 (Anisole: Acetic anhydride) 7.3 CNCLUSIN It is concluded that ion-exchanged Lewis acid metal ions play the active role in acylation of anisole with acetic anhydride. All the metal ion-exchanged catalysts showed higher catalytic activity than MnAP-5. The reaction is found to be very selective as it yields mainly p-methoxyacetophenone. The reaction appeared to proceed without the formation of free acyl cation over metal ion-exchanged MnAP-5. Among the metal ion-exchanged catalysts, GaMnAP-5 is more active than others due to high Ga content and higher Lewis acidity.

CHAPTER 5 SYNTHESIS OF 7-HYDROXY-4-METHYLCOUMARIN OVER ZAPO-5 AND LEWIS ACID METAL ION-EXCHANGED ZAPO-5 MOLECULAR SIEVES

CHAPTER 5 SYNTHESIS OF 7-HYDROXY-4-METHYLCOUMARIN OVER ZAPO-5 AND LEWIS ACID METAL ION-EXCHANGED ZAPO-5 MOLECULAR SIEVES 79 CHAPTER 5 SYNTHESIS F 7-HYDRXY-4-METHYLCUMARIN VER ZAP-5 AND LEWIS ACID METAL IN-EXCHANGED ZAP-5 MLECULAR SIEVES 5.1 INTRDUCTIN Coumarins are an important group of naturally occurring compounds widely