113 CHAPTER 7 FRIEDEL-CRAFTS ACYLATION OF TOLUENE WITH ACETIC ACID 7.1 INTRODUCTION Acylation of aromatic compounds are industrially prominent reaction as its products are intermediates in many organic synthesis especially in pharmaceuticals and fine chemicals. Lewis acids such as AlCl 3, BF 3, ZnCl 2, TiCl 4, FeCl 3 and Brönsted acids like polyphosphoric acid and hydrofluoric acid are the commercial catalysts employed in Friedel-Crafts acylation of aromatic compounds (Noller and Adams 1924 and Gardner 1954). Nevertheless, the use of these traditional catalysts produce problems such as environmental pollution arising from the disposal of hazardous by-products as waste, corrosion of reactor set-up, tendency to form complex with reactants or products due to use of more than stoichiometric amounts of the catalysts (Newman 1945 and Burton and Praill 1950). In view of the increasing stringent environmental legislation, application of heterogeneous catalysts has become attractive. Among the acylating reagents, acyl chlorides are more reactive but unfortunately they are also hazardous due to formation of corrosive hydrogen halides as by-products. Acid anhydrides and carboxylic acids are preferable acylating agents because of their less problematic by-products (organic acid and water). Efforts are now being made to overcome the above drawbacks by developing catalysts with high shape selectivity using ion-exchanged zeolites and zeo-type materials.
114 Botella et al (2000) reported toluene acylation over Hβ zeolite of different Si/Al ratios using acetic anhydride as acylating agent. They observed catalysts deactivation during long hours and hence the products were collected during the first hour of reaction period. Botella et al (2000) also observed hydroxylation of acetic anhydride to acetic acid. Though the reaction is highly selective to obtain p-methylacetophenone (p-map) the above said problems found to occur as drawback in this reaction. The catalytic activity of rare earth cations ion-exchanged zeolite β was studied for toluene acylation with acetic anhydride as an acylating agent (Sheemol et al 2004). La 3+ ion-exchanged catalyst was found to be more active than other catalysts with more than 95 % selectivity to p-isomer. Sheemol et al (2004) also compared the activity of rare earth cations ion-exchanged β zeolite catalysts with Naβ zeolite and reported the inactive nature of Naβ zeolite in toluene acylation. Kawamura et al (2006) reported Friedel-Crafts acylation of aromatic compounds using carboxylic acids as an acylating agent in the presence of Lewis and Brönsted metal triflates. They concluded that dehydrative Friedel-Crafts acylation of aromatic compounds with carboxylic acids was efficiently catalysed by Lewis acid catalysts. Brönsted acid catalysts were more efficient if the catalyst temperature is higher than the temperature required for Lewis acid catalysts. Vishnupriya et al (2008 and 2008a) and Mavrodinova et al (2004 and 2004a) proved the Lewis acidic nature of MO + (CeO + and InO + ) species for alkylation and disproportionation reactions. Hence the Lewis acidic nature of FeO +, LaO + and CeO + sites has been exploited for the acylation of toluene in the vapour phase using acetic acid as an acylating agent. 7.2 TOLUENE ACYLATION OVER MAPO-36 AND ION-EXCHANGED MAPO-36 Vapour phase acylation of toluene was carried out in a fixed-bed, vertical-flow type reactor as shown in Chapter 2. The reaction was carried out
115 over MAPO-36, Zn, Fe, La and CeMAPO-36. The effect of reaction temperature, catalysts, feed ratio, WHSV and time on stream was studied in order to get high toluene conversion and maximum p-methylacetophenone (p-map) selectivity. 7.2.1 Effect of Temperature on Toluene Acylation The vapour phase reaction of acetic acid and toluene was carried out over MAPO-36, Zn, Fe, La and CeMAPO-36. Figure 7.1 shows the results of toluene conversion and products selectivity over CeMAPO-36 with 1:1 feed ratio (acetic acid: toluene) in the temperature range 200-400 o C. 100 90 Conversion and Selectivity (%) 80 70 60 50 40 30 20 10 Toluene conversion Selectivity of p-map o-map Other products Figure 7.1 0 200 250 300 350 400 Temperature ( o C) Effect of temperature on toluene conversion and products selectivity The reaction involved the production of one molecule of water for every molecule of acetic acid consumed. Among the products obtained, the major product was found to be p-map with 85-89 % selectivity. Small
116 amount of o-map was also obtained in the product mixture. The toluene conversion increased from 200 to 300 o C and then decreased. The catalytic activity of all other catalysts was studied at 300 ºC. The decrease of toluene conversion above 300 o C could be literally attributed to coke formation on the surface of the catalyst. Formation of polyalkylated phenolics and polybutenes has been reported to be the main source of coke deposit (Mavrodinova et al 2001). It is suggested that coke deposits reduce the conversion by blocking the active sites of the catalysts by coke. Rare earth ion-exchanged MAPO-36 catalysts showed both Brönsted and Lewis acidity (Vishnupriya et al 2008 and 2008a) due to its high charge density. Acylation of toluene using acetic acid over ion-exchanged MAPO-36 proceeds through the mechanism that involves acylium ion intermediates, that are generated from the acylating agent by interaction with the catalyst as shown in Scheme 7.1. Friedel-Crafts acylation reaction mechanism as proposed by Olah (1973), involved acylation either by the catalyst adduct and the acylating agent or by free acylium ions depending upon the reaction conditions. 7.2.2 Effect of Catalysts The reaction was also carried out over calcined MAPO-36 and Zn, Fe and LaMAPO-36 under similar reaction conditions. The results shown in Table 7.1 reveal that MAPO-36 is the least active catalyst. This illustrates the fact that the reaction is largely controlled by Lewis acid sites. Among the ion-exchanged MAPO-36, CeMAPO-36 catalyst is found to be more active. Among the MO + ions formed CeO + is more active than other MO + ions, supporting the adsorption of toluene on catalyst surface in a better way. All the catalysts showed regular selectivity trend towards p-map. ZnMAPO-36 showed reduced p-map selectivity than other ion-exchanged catalysts. This is due to the presence of both strong and weak acid sites in ZnMAPO-36. Ce, La and FeMAPO-36 with more number of weak acid sites could be considered as better catalysts than ZnMAPO-36.
MO H + + C Mg 2+ O P 5+ CH3 COOH Mg 2+ O O O + OH OH P 5+ + CO H 2 O 2+ O Mg O P 5+ Others + CO + o- MAP CO p- MAP Scheme 7.1 Acylation of toluene over CeMAPO-36 117
118 Table 7.1 Effect of catalysts on toluene conversion and products selectivity Catalyst Toluene conversion Selectivity (%) (%) p-map o-map Others MAPO-36 21.0 65.9 22.8 11.3 ZnMAPO-36 26.0 68 24.2 7.8 FeMAPO-36 30.0 71.7 18.8 9.5 LaMAPO-36 33.0 76 15.3 8.7 CeMAPO-36 37.0 88.9 4.2 6.9 Reaction conditions: temperature: 300 ºC; feed ratio: 1:1; WHSV: 2.74 h -1 7.2.3 Effect of Feed Ratio The effect of different feed ratios viz., 1:1, 1:2 and 1:3 over CeMAPO-36 was also performed and the results are presented in Table 7.2. The conversion increased with increase of feed ratio from 1:1 to 1:2 and then decreased for 1:3. This may be due to preferential adsorption of the excess acid on the catalyst surface rather than toluene. The conversion was found to be high with 1:2 feed ratio than others. The decrease in conversion at 1:3 feed ratio is due to the presence of excess acetic acid which dilutes the concentration of toluene. In addition, selectivity was also decreased, suggesting that polyalkylated products could be avoided with feed containing less amount of acetic acid.
119 Table 7.2 Effect of feed ratio on toluene conversion and products selectivity Toluene Selectivity (%) Toluene : Acetic acid conversion (%) p-map o-map Others 1:1 37.0 88.9 4.2 6.9 1:2 49.0 90.5 6.5 3.0 1:3 32.7 81 12 7 Reaction conditions: catalyst: CeMAPO-36; temperature: 300 ºC 7.2.4 Effect of WHSV Different flow rates (WHSV) were studied over CeMAPO-36 with a feed ratio 1:2. From the results shown in Table 7.3 it is clear that conversion decreased with increase in WHSV which may be due to rapid diffusion of reactants. The selectivity to o-map decreased while that of p-map slightly increased with increase in WHSV. This supports the fact that selective formation of p-map required reaction between free toluene and acyl cation over the catalysts surface. Table 7.3 Effect of WHSV on toluene conversion and products selectivity WHSV (h -1 ) Toluene conversion ( %) Selectivity (%) p-map o-map Others 2.74 49.0 90.5 6.5 3.0 2.90 42.0 91.5 5.8 2.7 2.95 38.0 93.0 6.2 0.8 Reaction conditions: catalyst: CeMAPO-36; temperature:300 ºC; feed ratio: 1:2
120 7.2.5 Effect of Time on Stream The time on stream study was carried out for 6 h over CeMAPO-36 at 300 o C with 1:2 feed ratio and WHSV of 2.90 h -1 and the results are shown in Figure 7.2. The conversion decreased gradually due to coke formation. About 20% conversion was observed even at the end of 6 h time on stream. The selectivity increased up to 3 h and decreased from then on. The decrease in the selectivity after 3 h may be due to blocking of active sites by coke. It is quite interesting to note that the selectivity of o-isomer increased with increase in time on stream. 100 Conversion and Selectivity (%) 80 60 40 20 Toluene conversion Selectivity of p-map o-map Other products 0 1 2 3 4 5 6 Time (h) Figure 7.2 Effect of time on stream on toluene conversion and products selectivity
121 7.3 CONCLUSION The study revealed that Fe, La and CeMAPO-36 catalysts are active for vapour phase acylation of toluene with 84% selectivity to p-isomer. It was further observed that toluene conversion is catalyzed by Lewis acid sites rather than Brönsted acid sites. Acetic acid is an environmentally benign acylating agent because it produces water alone as side product irrespective of the percentage conversion of toluene. Hence it is concluded that Lewis acid ion-exchanged MAPO-36 could find significant use as catalyst in the selective acylation of toluene with acetic acid.