CHAPTER 4 ISOPROPYLATION OF TOLUENE

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1 9 CHAPTER ISOPROPYLATION OF TOLUENE. INTRODUCTION Zeolites are largely exploited catalysts in industries. They catalyzed both the acid and base catalyzed reactions (Aiello et al 999, Costa et al 009, and Li et al 009). Alkylation, isomerization, cracking, disproportionation and trans alkylation are some of the petrochemical reactions catalyzed by H-form of zeolites (Corma et al 995, Li et al 009 and Gao et al 009). Dehydration of alcohols to olefins is the major class of reaction catalyzed by basic zeolites (Ramesh et al 009 and Ivanova et al 009). Among the zeolites, the most important is ZSM-5. It has medium pore size (5.5 ), strong acidity, high thermal stability and high resistance to coke formation. These are the characteristic features for its large application as catalyst in chemical industries (Mhamdi et al 008, Lee et al 009 and Holm et al 009). It has been commercially synthesized from template free route in alkaline medium, but the crystallinity was not good (Narlta 985 and Kang et al 009). Synthesis of ZSM-5 in fluoride medium in acidic ph yielded crystals of large size, and high crystallinity (Le et al 000). These characteristics are important for shape selective alkylation of aromatics (Chen et al 99). Pore size optimization by chemical vapour deposition in it was attempted to enhance product selectivity, but drastic decrease of conversion was a drawback. Since the shape selectivity is controlled by crystallinity and crystal size (Louis et al 00), in the present study synthesis of ZSM-5 (Si/

2 80 Al= 50, 5 and 00) in fluoride medium was attempted for shape selective isopropylation of toluene to p-. This attempt was based on the fact that the fluoride medium favours growth of large crystal size and high crystallinity (John et al 00). In the fluoride medium route, the formation of [SiO / F] - species could dissolve silica for high crystallinity, but it can also form AlF x complexes (Ioiseau et al 00). It retards incorporation of alumunium in the framework of ZSM-5. In our lab phosphoric acid was verified to act as a complexant of F - and enhance framework incorporation of Al. So, it was used in the synthesis of ZSM-5 in fluoride medium. The high crystallinity was expected to suppress isomerisation of p- on the external acid sites, and the large crystal size to enhance p-selective diffusion (Arichi et al 008). The reaction of isopropylation of toluene was attempted over zeolite (Parikh et al 99, Marvec et al 00 and Ghiaci et al 00), but not over ZSM-5, synthesized in fluoride medium to the best of our knowledge. Moreover, the ZSM-5 zeolite, synthesized in fluoride medium, possesses only weak and medium acid sites (Louis et al 00). It is an advantage, these sites might not much promote side reactions such as isomerization of p- and trans alkylation on the external surface. The success of this study carries commercial significance, as p- is an important precursor for the industrial manufacture of o-cresol used in the synthesis of dyes, pharmaceuticals, fragrances and heat transfer fluids (Ito et al 9 and Fraenkel et al 989). It is also a raw material for the production of isomers by trans-alkylation with toluene (Odedairo et al 0).. CATALYTIC STUDIES.. Effect of Temperature Isopropylation of toluene with isopropyl alcohol was carried out at 00, 5, 50, 5and 00 C over ZSM-5 catalysts with the feed ratio :

3 8 (Toluene: Isopropyl alcohol) and WHSV.5 h -. The results of toluene conversion and product selectivity are presented in Table.. The products were analyzed by GC-MS. (Gas chromatograph Coupled with Mass spectrometer).the major product was p-, and the minor products were o- and m-. The conversion increased from 00 to 50 C and then decreased over ZSM-5 (50). The decrease in conversion was due to coke formation. Presence of coke was physically verified at 5 C and 00 C after the reaction. The reduced selectivity of o- was due to hindrance to diffusion inside the pores (kinetic diameter of o-.9 ) and its steric hindrance to o-substitution. It can be formed only on the external acid sites of the catalyst (Morles- Pacheco et al 0), but they were expected to be minimum in the present catalyst, as it showed high crystallinity. A high crystallinity favours slow molecular diffusion and yields highly p-selective products (Arichi et al 008). Even if o- formed on the external acid sites, it is very much susceptible to isomerisation to p-. It was also suggested as one of the main reasons for low selectivity of o-. The major product was p-. As this product is steric free, it can very well be formed inside the pore where the active sites are present, and can also easily diffuse out of the pore as its kinetic diameter might be same as that of isopropyl benzene 5.. The selectivity of m- was low similar to o-. Its kinetic diameter is.9, and hence it cannot diffuse out of the pore. It can be formed on the external acid sites by isomerisation of p-, but the external acid sites were to be minimum as said above to isomerize the same. Hence isopropylation of toluene over ZSM-5 zeolite, synthesized in fluoride medium, could be good for selective formation of p-. It could be better than commercial ZSM-5 catalyst, as the later can favor more isomerisation than the former because of its strong acid sites and reduced crystallinity. During isopropylation, isoprophyl alcohol might be chemisorbed on the Brönsted acid sites to form isopropyl cation,

4 8 which attacked toluene electrophilically to form mainly p- (Scheme. and Scheme.). + CH HO CH CH + HO Si Al - CH + H C CH ZSM-5 CH Scheme. Formation of isopropyl cation on ZSM-5 CH CH CH CH CH + H C + CH C H H CH CH Scheme. Electrophilic substitution of isopropyl cation on toluene yields isomers

5 8 Table. Effect of temperature on toluene conversion and products selectivity Catalysts Temperature ( C) ZSM-5 (50) ZSM-5 (5) ZSM-5 (00) H-ZSM-5 (50) Conversion (Wt %) p Products selectivity (%) m- 8 8 O- Others Reaction conditions: Catalyst 0. g; WHSV:.5 h - ; Toluene: Isopropyl alcohol : The reaction was also carried out over ZSM-5 (5 and 00) and the results are presented in the same table and compared. ZSM-5 (5 and 00) showed lower conversion than ZSM-5 (50). It confirmed dependence of conversion on the density of acid sites. Both the catalysts showed high

6 8 selectivity to p- similar to ZSM-5 (50). The reaction was also studied over H-ZSM-5 (50) derived from Na-ZSM-5 (50) synthesized in alkaline medium. The conversion was higher than ZSM-5 (50) synthesized in fluoride medium but the p- selectivity was lower. Hence ZSM-5 (50) synthesized in fluoride medium verified better than the ZSM-5 synthesized in alkaline medium for isopropylation of toluene to p-. It was due to high crystallinality of ZSM- 5 synthesized in fluoride medium... Effect of Feed Ratio The effect of feed ratio on conversion, products selectivity was studied at 00 C and their results are presented in Table.. The results indicated : as the optimum one for the isopropylation of toluene. At : the conversion decreased which might be dilution of toluene by isopropyl alcohol. Such a dilution effect was also reported in the literature (Rajesh et al 00). At : the conversion was slightly less than :, since at : there might be reduced number of alkyl cations for alkylation. Table. Effect of feed ratio on toluene conversion and products selectivity Feed ratios (Toluene : Isopropyl alcohol) Conversion (Wt %) p- Product selectivity (%) m- o- Others : 9 : 8 9 : 9 Reaction conditions: Catalyst = ZSM-5(50); Catalyst 0. g; Temp = 50 C.

7 85.. Effect of WHSV The effect of Weight Hourly Space Velocity (WHSV) on conversion and products selectivity was studied and the results are present in Table.. The data showed.80 h - as the optimum one for high conversion to toluene and high selectivity to p-. The decrease in conversion at high WHSV was due to reduced adsorption of alcohol. The selectivity of o- and m- remained nearly the same, though a slight decrease was there. The selectivity of p- increased with the decrease in WHSV. It established formation of m- by isomerization of p-, and hence, there might be at least some acid sites on the external surface. Generally on the external surface isopropylation of toluene to steric free p- might be more favored than sterically hindered o-. But, isomerization of p- on the external surface might be minimum, as the acid sites might be few. Table. Effect of WHSV on toluene conversion and products selectivity WHSV (h - ) Conversion (Wt %) p- Product selectivity (%) m- o- Others Reaction conditions: Catalyst = ZSM-5(50); Catalyst Wt = 0. g; Temp =50 C; Feed ratio :.

8 8.. Effect of Time on Stream The effect of time on stream on conversion and products selectivity was studied and the results are presented in Table.. The toluene conversion was remained the same for h of time on stream. However the coke formation little significant to reduce conversion and affect product selectivity. The study also established occurrence of the same reaction in the entire time on stream. Again isopropyl alcohol might not be dehydrated to form propene at 50 C. The weight of the product mixture was also nearly equal to the initial weight of the feed. It is also in support of absence of coke formation and blocking of active sites. Table. Effect of time on stream on toluene conversion and products selectivity Time on stream (h) Conversion (Wt%) p- Product selectivity (%) m- o- Others Reaction conditions: Catalyst: ZSM-5(50); Catalyst Wt = 0. g; WHSV:.80 h - ; Temp = 50 C; Feed ratio :.

9 8 00 Toluene conversion and p-selectivity Time (h) % of toluene conversion % p- selectivity Figure. Effect of time on stream on toluene conversion and p- selectivity. CONCLUSION Synthesis of ZSM-5 in fluoride medium resulted in the formation of crystals of large size with high crystallinity. Only weak and medium acid sites were present in all of them. High crystallinity and large crystal size to confered high shape selectivity in the isopropylation of toluene. Both high crystallinity, high surface area were in support of reduced defects and number of external acid sites, hence these catalysts could be good to suppress isomerization of p- to m-isomers in alkylation of alkyl aromatics. Therefore, synthesis of ZSM-5 in fluoride medium was established as being more advantageous than in alkaline medium for obtaining catalysts for shape selective alkylation. The same catalyst could also be applied to shape selective alkylation of other alkyl aromatics.