102 CHAPTER 4: CATALYTIC PROPERTIES OF ZSM-5 ZEOLITES AND CUBIC MESOPOROUS MATERIALS Chapter summary The role of heterogeneous catalysts in organic reactions is included in this chapter. Two organic reactions, namely oxidation of styrene and esterification of benzyl alcohol were studied by using different synthesized catalysts. Oxidation of styrene by hydrogen peroxide and benzyl alcohol esterification by acetic acid with respect to time of reaction, nature of catalysts, amount of catalysts and temperature etc. are presented in this chapter. 4.1 Role of heterogeneous catalyst in organic reaction Generally organic reactions are carried out using homogeneous catalysts, such as pyridine in triethylamine [1], homogeneous Lewis acid catalysts (AlCl 3, BF 3 ) [2, 3], inorganic acids [2, 4] or traditional oxidizing agents such as dichromate, permanganate manganese dioxide etc [5]. But there are many drawbacks in the use of these homogeneous catalysts [2], such as generation of corrosive and toxic waste products, production of large aqueous effluents in the process multiplying the overall process cost, difficulty in reusing the catalysts, requirement of greater than stoichiometric amount etc. which make the process dirty and polluted. To overcome all these problems, it is necessary to use inexpensive, ecofriendly and reusable catalyst in organic reactions. Thus, to reduce the byproduct in synthetic route, the attention has been focused on the use of heterogeneous catalysts. In the recent years the solid acids catalysts like zeolites, clays and mesoporous materials have been found to play a key role in the chemical industry. The heterogeneous catalysts are governed by different steps [6] such as Transport of the reactant molecules to the surface of the catalyst followed by diffusion of the molecules to the active site. Adsorption of the reactant molecules to the active sites. Adsorbed reactant molecules undergo chemical reactions to form product(s) Desorption of the product molecule(s)
103 Diffusion of the product(s) from the active sites to the surface. Transfer of product(s) away from the porous catalyst surface. 4.2 Oxidation of organic compounds The selective catalytic oxidation reactions use cheap and ecofriendly oxidants such as molecular oxygen and hydrogen peroxide with heterogeneous catalysts those are more demanding in chemical industry [7-11]. The catalytic properties of zeolites in oxidation reactions have been studied by various workers [12-15]. Oxidation of styrene is one of the important reactions for the production of benzaldehyde and styrene oxide. Both the products are useful and value added products. 4.3 Oxidation of styrene by modified ZSM-5 and MCM-48 The oxidation of styrene at its side chain is of considerable academic and commercial interest for synthesis of few important products such as styrene oxide, benzaldehyde, benzoic acid and phenylacetaldehyde. In recent years, the use of hydrogen peroxide as oxidizing agent in the oxidation of organic substrates has received much attention due to its environmental implications, water being the only by-product in the oxidation. Depending on the reaction conditions and catalyst, the oxidation of styrene at the side chain produces different products. Thus, in the presence of metal complexes, benzaldehyde is the main product [16]. In presence of palladium salts, by Wacker process the styrene is mainly converted to acetophenone [17]. On the other hand, the oxidation can be stopped at the stage of epoxide by using CH 3 ReO 3 -H 2 O 2 system in anhydrous CD 3 CN, whereas 1, 2-diol is obtained in CH 3 CN/H 2 O solution [18]. In heterogeneous catalysis, over nanosize spinel-type Mg x Fe 3-x O 4 [19] or Nb(Co)-MCM-41 [20], the major product was benzaldehyde in styrene oxidation reaction with hydrogen peroxide. In Ti- containing molecular sieves such as TS-1 [21-24], TS-2 [22], BTS-2 [25] or TiMCM-41[26] for styrene oxidation with H 2 O 2 in organic solvent allow to increase the selectivity towards phenylacetaldehyde (PhAA). PhAA results by the in situ acid catalyzed
104 rearrangement of the styrene oxide, which is the primary product [27]. An increase in the productin of styrene oxide was obtained over TS-1 in presence of sodium hydroxide in the reaction medium [28] or using anhydrous urea-h 2 O 2 as an oxidizing agent [29]. The results of the epoxidation of styrene done by various workers are summarized in Table A. Table A Comparison of Styrene oxidation reactions under different conditions Ent ry Cataly st Condition Con vers ion (%) Selectivity (%) PhAA Epox ide Benzalde hyde Benz oic acid Gly col Ref 1 FeZS M-5 T=346 K Catalyst=0.2 g, Styrene: H 2 O 2 = 10: 9.8, Solvent- DMF, Time- 2h 5-36.9 63.1 0 0 [30( a)] 2 MCM- 48 3 FeMC M-41 Do 2.1-36.8 52.2 0 11 Do Do 3.5-45.7 45.9 0 8.4 Do 4 MnM CM-41 T= 60 0 C Catalyst= 0.2 g Time=24h, Solvent- DMF(1mL) +MeCN(9mL ), Styrene : TBHP= 10:10 17.9-81.9 18.1 0 0 [30( b)] 5 TiMC T= 343 K, Time= 5 h, 26.1 19.9 7.5 52.3 3.4 16.9 [30(
105 M-41 Solvent MeCN, Styrene: H 2 O 2 =2:1 c)] 6 TiMC M-41 T= 343 K, Time= 5 h, Solvent MeOH, Styrene: H 2 O 2 =2:1 15.4 9.7 11.7 14.1 - - Do 7 TiSBA -1 T= 343 K, Time- 8 h, catalyst= 0.3 g 46.7 4.0 (phenylac etate) 35.3 60.7 [31] Based on above results we consider the Scheme 4.1 for styrene oxidation [30(c)]. Scheme 4.1
106 The aim of the present study is to investigate the oxidation of styrene by H 2 O 2 over parent, modified zeolite ZSM-5 and mesoporous MCM-48 under different conditions. In this part, the detailed study of the liquid phase oxidation of styrene by H 2 O 2 using parent and Mo incorporated ZSM-5 zeolites and MCM-48 under different conditions is reported. The reactions were carried out in glass batch reactor under atmospheric pressure using acetonitrile as the solvent. The reason for using this solvent is that in the previous studies [30(c), 32], acetonitrile was found to be the best in term of activity for the styrene oxidation. The higher reactivity obtained in MeCN with respect to other protic solvents was explained in terms of electrophilicity of heteroatom sites in the catalysts structure. It was suggested that in the aprotic solvent (MeCN) a cyclic species was formed in which water, instead of alcohol, is a ligand of heteroatom as species II (Scheme 4.2). Then, due to lower donor properties of water with respect to alcohols, species II would have a higher electrophilic character than the species I and consequently would have a higher intrinsic reactivity for oxidation. The temperature of the reaction was controlled by a temperature controller and stirring was done by a magnetic stirrer. Before the use of catalyst in the reaction, it was activated at 373 K temperature for 4 h. The product(s) and the unreacted reactants of the reaction were collected at different time intervals and were filtered and diluted with acetone in order to obtain single homogeneous phase and then analysed by Gas Chromtography (PEKIN ELMER, Clarus 500, Elite 501 column) to calculate the percentage of conversion and selectivity of the products under different reaction conditions. Scheme 4.2
107 From the reaction scheme (Scheme 4.2) it is seen that when styrene is oxidized with hydrogen peroxide, first styrene oxide is formed which on hydrolysis produces benzaldehyde. Benzaldehyde on further oxidation produces benzoic acid. Again from styrene oxide phenylacetaldehyde may also be formed. The product identification was achieved from the retention time of the pure compounds. The conversion was measured as a function of reaction time. A blank GC analysis was carried out under the same conditions using a solution of styrene oxide in acetone to check reactivity of injector and column of GC. In this blank experiment no reaction was observed. 4.3.1 Analysis of oxidation of styrene by H 2 O 2 under different conditions 4.3.1:(i) Effect of reaction time Oxidation of styrene was carried out over ZN (parent) and MoZN1, MoZN2, MoZN3 and MoMCM-48 zeolites at 343 K temperature up to duration of 8 h in a batch reactor with magnetic stirrer, a reflux condenser and a thermometer under atmospheric pressure. The products and reactants, if any, were collected at different time intervals of 2 h, 4 h, 6 h, and 8 h. The catalytic results of the styrene oxidation reaction for the catalysts parent ZN, MoZN1, MoZN2, MoZN3, MoMCM-48 were reported in Tables 4.1, 4.2, 4.3, 4.4 and 4.5 and Figs 4.1 to 4.5 respectively. From the results, it is observed that the conversion of the reaction increased in all cases with increase of reaction time. The selectivity towards benzaldehyde increases while selectivity towards epoxide decreases with increase in reaction time. Interestingly, oxidation of styrene over the catalysts produced benzaldehyde and styrene oxide in the initial period of reaction almost under all conditions. If the reaction was continued beyond 4 h benzoic acid was appeared in the reaction mixture. This may be due to further oxidation of benzaldehyde. The percentage of conversion for catalyst ZN increased from 2.6 % to 3.0 % with increase in time up to 8 h. For catalysts MoZN1, MoZN2, MoZN3 and MoMCM- 48, the conversion of the reaction increased from 8.6 to 21.0%, 8.8-23.2%, 9.1-24.6 % and 9.8-29.2% respectively in the time period of 2 to 8 h.
108 The selectivity of epoxide for different ZSM-5 catalysts (ZN, MoZN1, MoZN2, MoZN3) remained more or less same with increase of Mo content in the structure. Table 4.1 Effect of reaction time on styrene oxidation over ZN catalyst [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3g] Styrene: H 2 O 2 : 4:1 (molar ratio) Substrate volume: 15 ml Temperature: 343 K Solvent: CH 3 CN Catalyst amount: 0.3 g Reaction Conversion Selectivity (%) Time(h) % (A) Epoxide ( B) Benzaldehyde ( C) Benzoic acid (D) 2 2.6 50.4 49.6 0 4 2.7 47.4 52.6 0 6 2.8 43.1 46.5 10.4 8 3.0 33.7 54.1 12.2 5 0 Conversion/ Selectivity (%) 4 0 3 0 2 0 1 0 A B C D 0 2 3 4 5 6 7 8 T im e ( h ) Fig 4.1: Effect of reaction time on styrene oxidation reaction over ZN catalyst. A-conversion %, B-% of selectivity of epoxide, C- % of selectivity of benzaldehyde, D- % of selectivity of benzoic acid. [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
109 Table 4.2 Effect of reaction time on styrene oxidation over MoZN1 [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3g] Styrene: H 2 O 2 : 4:1 (molar ratio) Substrate volume: 15 ml Temperature: 343 K Solvent: CH 3 CN Catalyst amount: 0.3 g Reaction Conversion Selectivity (%) Time(h) % (A) Epoxide ( B) Benzaldehyde ( C) Benzoic acid (D) 2 8.6 52.5 47.5 0 4 10.1 48.3 51.7 0 6 15.9 43.1 46.1 10.8 8 21.0 34.8 52.1 13.1 5 0 Conversion/ Selectivity (%) 4 0 3 0 2 0 1 0 A B C D 0 2 3 4 5 6 7 8 T i m e ( h ) Fig 4.2: Effect of reaction time on styrene oxidation reaction over MoZN1 catalyst. A- Conversion %, B-% of selectivity of epoxide, C- % of selectivity of benzaldehyde, D- % of selectivity of benzoic acid. [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
110 Table 4.3 Effect of reaction time on styrene oxidation over MoZN2 [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3g] Styrene: H 2 O 2 :4:1 (molar ratio) Substrate volume: 15 ml Temperature: 343 K Solvent: CH 3 CN Catalyst amount: 0.3 g Reaction Conversion Selectivity (%) Time(h) % (A) Epoxide ( B) Benzaldehyde ( C) Benzoic acid (D) 2 8.8 52.6 47.4 0 4 12.3 48.8 48.8 2.4 6 17.2 44.1 44.8 11.1 8 23.2 35.5 50.8 13.7 5 0 conversion/ Selectivity (%) 4 0 3 0 2 0 1 0 A B C D 0 2 3 4 5 6 7 8 T im e ( h ) Fig 4.3: Effect of reaction time on styrene oxidation reaction over MoZN2 catalyst. A- conversion %, B-% of selectivity of epoxide, C- % of selectivity of benzaldehyde, D- % of selectivity of benzoic acid. [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
111 Table 4.4 Effect of reaction time on styrene oxidation over MoZN3 [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3g] Styrene: H 2 O 2 : 4:1 (molar ratio) Substrate volume: 15 ml Temperature: 343 K Solvent: CH 3 CN Catalyst amount: 0.3 g Reaction Conversion Selectivity (%) Time(h) % (A) Epoxide ( B) Benzaldehyde ( C) Benzoic acid (D) 2 9.1 52.8 47.2 0 4 15.1 46.9 49.2 3.9 6 18.9 40.8 46.5 12.7 8 24.6 37.2 47.4 15.4 5 0 Conversion/ selectivity (%) 4 0 3 0 2 0 1 0 A B C D 0 2 3 4 5 6 7 8 T im e ( h ) Fig 4.4: Effect of reaction time on styrene oxidation reaction over MoZN3 catalyst. A- conversion %, B-% of selectivity of epoxide, C- % of selectivity of benzaldehyde, D- % of selectivity of benzoic acid. [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
112 Table 4.5 Effect of reaction time on styrene oxidation over MoMCM-48 [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3g] Styrene: H 2 O 2 :4:1 (molar ratio) Substrate volume: 15 ml Temperature: 343 K Solvent: CH 3 CN Catalyst amount: 0.3 g Reaction Time(h) Conversion % (A) Selectivity (%) Epoxide ( B) Benzaldehyde ( C) Benzoic acid (D) 2 9.8 55.2 44.8 0 4 16.7 47.2 50.4 2.4 6 22.4 37.7 49.4 12.9 8 29.2 35 50.2 14.8 6 0 5 0 Conversion/Selectivity (%) 4 0 3 0 2 0 1 0 A B C D 0 2 3 4 5 6 7 8 T im e ( h ) Fig 4.5: Effect of reaction time on styrene oxidation reaction over MoMCM-48 catalyst. A- conversion %, B-% of selectivity of epoxide, C- % of selectivity of benzaldehyde, D- % of selectivity of benzoic acid. [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
113 4.3.1: (ii) Comparison of conversion (%) of styrene oxidation over different catalysts: The results of the styrene oxidation by H 2 O 2 over different catalysts at temperature 343K, Styrene: H 2 O 2 ; 4:1, solvent: CH 3 CN, Catalyst amount: 0.3 g are summarized in Table 4.6 and Fig 4.6. The oxidation of styrene in presence of ZN ( i.e, ZSM-5 with no Mo) occurred at very low rate. When Mo containing ZSM-5 zeolites (MoZN1, MoZN2, MoZN3), as well as MoMCM-48 were used as catalysts, the conversion of styrene was increased. This could be considered as the proof of role of molybdenum as active sites in the styrene oxidation reaction. Under similar reaction conditions styrene conversion was found to be sequenced in the order MoZN3> MoZN2> MoZN1 for the molybdenum containing ZSM-5. This means that with increase of metal (Mo) loading (Table 2.4, chapter 2), the activity of the catalysts increased showing role of molybdenum in the oxidation reaction. Under similar reaction conditions MoMCM-48 showed better activity in the reaction than MoZSM-5 catalysts. The high catalytic activity of MoMCM-48 can be explained by large pore MCM- 48 material (mesopores, pore size >2 nm) than those of ZSM-5 zeolites (0.54 x 0.56 nm). In this case diffusion coefficient and shape selectivity towards the transition state may be considered for explaining higher activity of MCM-48 compared to medium pore zeolite (ZSM-5). Similar results were obtained by Hulea et. al [30(c)] with TS-I, Tibeta and TiMCM-41. If we compare the conversion and product distribution of the styrene oxidation reaction for the different catalysts under a particular reaction conditions [styrene: H 2 O 2 = 4:1 (molar ratio), time of reaction = 8 h, catalyst amount 0.3 g, temperature= 343K], we observe that not only conversion increased marginally with increase of molybdenum content in the ZSM-5 structure (Table 4.12, Fig 4.12), but also selectivity towards epoxide and benzoic acid increased, while that towards benzaldehyde decreased with increase of molybdenum content in the structure [33, 34]. Apparently, presence of molybdenum in the structure effects not only conversion of styrene, but also helped to produce benzoic acid from benzaldehyde.
114 Table 4.6 Comparison of conversion (%) of Styrene oxidation over different catalysts [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3g] Time Conversion (%) (h) ZN MoZN1 MoZN2 MoZN3 MoMCM-48 (A) (B) (C) (D) (E) 2 2.6 8.6 8.8 9.1 9.8 4 2.7 10.1 12.3 15.1 16.7 6 2.8 15.9 17.2 18.9 22.4 8 3.0 21.0 23.2 24.6 29.2 30 25 Conversion (%) 20 15 10 A B C D E 5 0 1 2 3 4 5 6 7 8 9 Time (h) Fig 4.6 Comparison of conversion (%) of Styrene oxidation over different catalysts A- ZN, B- MoZN1, C- MoZN2, D- MoZN3, E- MoMCM-48, [Styrene: H 2 O 2 ; 4:1, T=343K, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
115 4.3.1:(iii) Effect of temperature: The effect of temperature on the styrene oxidation reaction by hydrogen peroxide was studied in the temperature range of 323 to 373 K using catalysts ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48. The reactions were done under atmospheric pressure taking 0.3 g catalyst with styrene to hydrogen peroxide molar ratio of 4:1 in acetonitrile solvent. Product distribution after 8 h reaction time is shown in the Tables 4.7 to 4.11 for the catalysts ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48 respectively and graphically presented in the Figs. 4.7, 4.8, 4.9, 4.10 and 4.11 respectively. It is observed that with the increase of reaction temperature, the overall conversion of styrene showed expected upward trend from 2.4 % to 5.5 %, 15.2 to 24.1%, 15.9 to 25.4%, 16.7 to 27.7% and 19.5 to 32.3 % for ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48 catalysts respectively when reaction temperature was increased from 323 to 373 K. However the selectivity towards epoxide decreased with increase in temperature from 39.2 to 22.5%, 41.3 to 24.3%, 42.4 to 34.2%, 45.8 to 36.5 % and 46.7 to 35% for ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48 respectively. At the same time selectivity towards benzaldehyde increased with the increase in temperature. This further proves that styrene oxidation follows the proposed Scheme 4.1 (route 3) under the present experimental conditions. The rate of the styrene oxidation increased with increase of temperature according to Arrhenius equation. The increase of temperature also helped to produce benzaldehyde and benzoic acid from the intermediate product styrene epoxide. The benzoic acid is the oxidation product of benzaldehyde. The amount of benzoic acid was found to be low for all catalysts under all the conditions of reaction. The change of amount of benzoic acid with the change of temperature was affected only to a small extent (after 8 h reaction, the concentration of benzoic acid increased from 11.6 to 12.4 %, from 12.5 to 13.9%, from 13.1 to 14.3 %, from 15.2 to 15.9 % and from 14.5 to 15.2 % for the catalysts ZN, MoZN1, MoZN2, MoZN3 and MoMCM- 48 respectively for an increases of temperature 323 K to 373 K).
116 Table 4.7 Effect of temperature on styrene oxidation over ZN (Parent ZSM-5) [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3g] Styrene: H 2 O 2 :4:1 (molar ratio) Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Catalyst amount: 0.3 g Temperature (K) Conversion % (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 323 2.4 39.2 49.2 11.6 343 3.0 33.7 54.1 12.2 373 5.5 22.5 65.1 12.4 Conversion/ Selectivity (%) 6 5 6 0 5 5 5 0 4 5 4 0 3 5 3 0 2 5 2 0 1 5 1 0 5 0 3 2 0 3 3 0 3 4 0 3 5 0 3 6 0 3 7 0 3 8 0 T e m p e ra tu re (K ) A B C D Fig 4.7: Effect of temperature on styrene oxidation reaction over ZN catalyst A- conversion %, B-% of selectivity of epoxide, C- % of selectivity of benzaldehyde, D- % of selectivity of benzoic acid [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
117 Table 4.8 Effect of temperature on styrene oxidation over MoZN1 [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3g] Styrene: H 2 O 2 :4:1 (molar ratio) Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Catalyst amount: 0.3 g Temperature(K) Conversion % (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 323 15.2 41.3 46.2 12.5 343 21.0 34.8 52.1 13.1 373 24.1 24.3 61.8 13.9 6 5 6 0 5 5 Conversion/ Selectivity (%) 5 0 4 5 4 0 3 5 3 0 2 5 2 0 1 5 1 0 5 0 3 2 0 3 3 0 3 4 0 3 5 0 3 6 0 3 7 0 3 8 0 T e m p e ra tu re (K ) A B C D Fig 4.8: Effect of temperature on styrene oxidation reaction over MoZN1 catalyst A- conversion %, B-% of selectivity of epoxide, C- % of selectivity of benzaldehyde, D- % of selectivity of benzoic acid, [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
118 Table 4.9 Effect of temperature on styrene oxidation over MoZN2 [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3g] Styrene: H 2 O 2 :4:1 (molar ratio) Substrate volume: 15 ml Solvent: CH 3 CN Time: 8 h Catalyst amount: 0.3 g Temperature(K) Conversion % (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 323 15.9 42.4 44.5 13.1 343 23.2 35.5 50.8 13.7 373 25.4 34.2 51.5 14.3 50 45 Conversion/ Selectivity (%) 40 35 30 25 20 15 10 A B C D 5 0 320 330 340 350 360 370 380 Temperature (K) Fig 4.9: Effect of temperature on styrene oxidation reaction over MoZN2 catalyst. A- conversion %, B-% of selectivity of epoxide, C- % of selectivity of benzaldehyde, D- % of selectivity of benzoic acid, [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
119 Table 4.10 Effect of temperature on styrene oxidation over MoZN3 [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3 g] Styrene: H 2 O 2 :4:1 (molar ratio) Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Catalyst amount: 0.3 g Temperature(K) Conversion % (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 323 16.7 45.8 39 15.2 343 24.6 37.2 47.4 15.4 373 27.7 36.5 47.6 15.9 50 45 40 Conversion/ Selectivity (%) 35 30 25 20 15 10 5 A B C D 0 320 330 340 350 360 370 380 Tem perature (K ) Fig 4.10: Effect of temperature on styrene oxidation reaction over MoZN3 catalyst. A- conversion %, B-% of selectivity of epoxide, C- % of selectivity of benzaldehyde, D- % of selectivity of benzoic acid, [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
120 Table 4.11 Effect of temperature on styrene oxidation over MoMCM-48 [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3 g] Styrene: H 2 O 2 :4:1 (molar ratio) Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Catalyst amount: 0.3 g Temperature(K) Conversion % (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 323 19.5 46.7 38.8 14.5 343 29.2 35 50.2 14.8 373 32.3 40.3 44.5 15.2 50 45 Conversion/ Selectivity (%) 40 35 30 25 20 15 10 A B C D 5 0 320 330 340 350 360 370 380 Temperature (K) Fig 4.11: Effect of temperature on styrene oxidation reaction over MoMCM-48 catalyst, A- conversion %, B-% of selectivity of epoxide, C- % of selectivity of benzaldehyde, D- % of selectivity of benzoic acid, [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3 g, Substrate volume: 15 ml]
121 Table 4.12 Comparison of conversion (%), selectivity of benzaldehyde(%) and styrene oxide of styrene oxidation over different catalysts [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, catalyst amount: 0.3g] Styrene: H 2 O 2 :4:1 (molar ratio) Substrate volume: 15 ml Time: 8 h Catalyst amount: 0.3 g Temperature: 343 K Samples name Conversion % (A) Selectivity (%) Epoxide ( B ) Benzaldehyde ( C) Benzoic acid (D) ZN 3.0 33.7 54.1 12.2 MoZN1 21.0 34.8 52.1 13.1 MoZN2 23.2 35.5 50.8 13.7 MoZN3 24.6 37.2 47.4 15.4 MoMCM-48 29.2 35 50.2 14.8
122 55 50 % of conversion % of styrene oxide % of benzaldehyde % of benzoic acid Conversion/ Selectivity (%) 45 40 35 30 25 20 15 10 5 0 ZN MoZN1 MoZN2 MoZN3 MoMCM-48 Samples Fig 4.12: Comparison of conversion (%), selectivity towards different products in the (%) styrene oxidation over different catalysts [Temperature = 343 K, Time of reaction 8 h, catalyst amount 0.3 g, styrene : H 2 O 2 = 4:1 (molar ratio), Substrate volume: 15 ml]
123 4.3.1: (iv) Effect of catalyst amount: The effect of catalyst amount on the styrene oxidation reaction by hydrogen peroxide was studied at 343 K over catalysts ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48 using styrene to hydrogen peroxide molar ratio of 4:1 in acetonitrile solvent. The reactions were done under atmospheric pressure taking 0.1, 0.3 and 0.5 g of different catalysts. Product distribution after 8 h reaction time is shown in the Tables 4.13, 4.14, 4.15, 4.16 and 4.17 for ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48 catalysts respectively and graphically presented in the Figs. 4.13, 4.14, 4.15, 4.16 and 4.17 respectively. It is observed that with the increase of catalyst amount, the overall conversion of styrene showed expected upward trend from 2.1 to 5.3%, 15.8 to 23.2%, 17.2 to 25.6%, 18.9 to 27.1% and 22.4 to 30.7% for catalysts ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48 respectively. Selectivity towards benzaldehyde decreased and that of styrene epoxide increased for increase in catalyst amount. This is because more amount of catalyst used in reaction always helps the reactants to convert over more number of available active sites. The selectivity towards styrene epoxide increased from 22 to 35.2 % for ZN, from 25 to 35.8 % for MoZN1, from 26.7 to 38.9 % for MoZN2, from 26.9 to 38.9 % for MoZN3 and from 32.2 to 47.2 % for MoMCM-48 when catalyst amount was increased from 0.1 g to 0.5 g. The selectivity towards benzaldehyde was decreased from 59.7 to 54.1%, 62 to 52.1%, 63.9 to 50.8%, 65.3 to 47.4% and 55.2 to 50.2 % for catalysts ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48 respectively with increase of catalyst amount from 0.1 g to 0.5 g. In case of ZSM-5 samples the selectivity towards benzoic acid decreased when catalyst amount was increased from 0.1 to 0.3 g. However, there was little change in selectivity for further increase of catalyst amount. Remaining all other conditions constant, increase of catalyst amount means increase of surface area of the catalyst, which helps to increase the rate of the reaction to produce more and more epoxide. However, it appears that it cannot help in the subsequent oxidation process.
124 Table 4.13 Effect of amount of catalyst on styrene oxidation over ZN (parent ZSM-5) [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, T= 343 K] Styrene: H 2 O 2 :4:1 (molar ratio) Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Temperature: 343 K Catalyst amount (g) Conversion (%) (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 0.1 2.1 22 60.5 17.5 0.3 3.0 33.7 54.1 12.2 0.5 5.3 35.2 52.2 12.6 60 55 50 Conversion/ Selectivity (%) 45 40 35 30 25 20 15 10 5 0 0.1 0.2 0.3 0.4 0.5 Catalyst amount (g) A B C D Fig 4.13: Effect of amount of catalyst on styrene oxidation reaction over ZN catalyst A-conversion %, B- selectivity of epoxide, C- selectivity of benzaldehyde, D- selectivity of benzoic acid, [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
125 Table 4.14 Effect of amount of catalyst on styrene oxidation over MoZN1 [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, T= 343 K] Styrene: H 2 O 2 :4:1 (molar ratio) Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Temperature: 343K Catalyst amount (g) Conversion (%) (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 0.1 15.8 25 60.3 14.7 0.3 21.0 34.8 52.1 13.1 0.5 23.2 35.8 50.1 14.1 60 55 Conversion/ Selectivity (%) 50 45 40 35 30 25 20 15 10 5 0 0.1 0.2 0.3 0.4 0.5 Catalyst amount (g) A B C D Fig 4.14: Effect of amount of catalyst on styrene oxidation reaction over MoZN1 catalyst. A-conversion %, B- selectivity of epoxide, C- selectivity of benzaldehyde, D- selectivity of benzoic acid, [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
126 Table4.15 Effect of amount of catalyst on styrene oxidation over MoZN2 [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, T= 343 K] Styrene: H 2 O 2 :4:1 (molar ratio) Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Temperature: 343 K Catalyst amount (g) Conversion (%) (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 0.1 17.2 26.7 58.9 14.4 0.3 23.2 35.5 50.8 13.7 0.5 25.6 38.2 48.4 13.4 60 55 50 Conversion/ selectivity (%) 45 40 35 30 25 20 15 10 A B C D 5 0 0.1 0.2 0.3 0.4 0.5 C a ta lyst a m o u n t (g ) Fig 4.15: Effect of amount of catalyst on styrene oxidation reaction over MoZN2 catalyst, A-conversion %, B- selectivity of epoxide, C- selectivity of benzaldehyde, D- selectivity of benzoic acid, [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
127 Table 4.16 Effect of amount of catalyst on styrene oxidation over MoZN3 [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, T= 343 K] Styrene: H 2 O 2 :4:1 (molar ratio) Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Temperature: 343 K Catalyst amount (g) Conversion (%) (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 0.1 18.9 26.9 54.3 18.8 0.3 24.6 37.2 47.4 15.4 0.5 27.1 38.9 45.1 16 55 50 45 Conversion/ Selectivity (%) 40 35 30 25 20 15 10 5 A B C D 0 0.1 0.2 0.3 0.4 0.5 Catalyst amount (g) Fig 4.16: Effect of amount of catalyst on styrene oxidation reaction over MoZN3 catalyst, A-conversion %, B- selectivity of epoxide, C- selectivity of benzaldehyde, D- selectivity of benzoic acid, [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
128 Table 4.17 Effect of amount of catalyst on styrene oxidation over MoMCM-48 [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, T= 343 K] Styrene: H 2 O 2 :4:1 (molar ratio) Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Temperature: 343 K Catalyst amount (g) Conversion (%) (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 0.1 22.4 32.2 55.8 12 0.3 29.2 35 50.2 14.8 0.5 30.7 47.2 46.2 6.6 55 50 45 Conversion/ Selectivity (%) 40 35 30 25 20 15 10 5 A B C D 0 0.1 0.2 0.3 0.4 0.5 Catalyst amount (g) Fig 4.17: Effect of amount of catalyst on styrene oxidation reaction over MoMCM-48 catalyst, A-conversion %, B- selectivity of epoxide, C- selectivity of benzaldehyde, D- selectivity of benzoic acid, [Styrene: H 2 O 2 ; 4:1, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
129 4.3.1: (v) Effect of molar ratio of the reactant: The effect of change in molar ratios of the reactants i.e. styrene and hydrogen peroxide on the oxidation reaction is depicted in the Figs 4.18, 4.19, 4.20, 4.21 and 4.22 for the catalysts ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48 respectively. The reaction was carried out taking catalyst amount of 0.3 g in each case at 343 K. Conversion of styrene was found to increase with an decrease of molar ratio of styrene to hydrogen peroxide. The conversion decreased from 5.1 to 3.0 % for ZN, from 23.6 to 21 % for MoZN1, from 25.9 to 23.2 % for MoZN2, from 27.3 to 24.6 % for MoZN3 and from 32.4 to 29.2 % for MoMCM-48 samples when concentration of H 2 O 2 was decreased in the reaction mixture. Again the selectivity towards benzaldehyde decreased but to those of epoxide increased with increase of molar ratio of styrene to hydrogen peroxide. If we consider the efficiency of H 2 O 2 with respect to styrene oxide production according to Wang et. al [30(a)] the following equation H 2 O 2 efficiency = amount of styrene oxide formed (mol) / amount of H 2 O 2 consumed (mol), the efficiency was highest at lowest concentration. The increase in selectivity of epoxide is from 27.4 to 33.7 %, from 28.1 to 34.8 %, from 28.8 to 35.5 %, from 30.7 to 37.2 % and from 38.8 to 40% for ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48 respectively. The selectivity of benzaldehyde is from 59.7 to 54.1%, from 62 to 52.1%, from 63.9 to 50.8%, from 65.3 to 47.4% and from 55.2 to 50.2% for ZN, MoZN1, MoZN2, MoZN3 and MoMCM-48 respectively. In all cases benzoic acid is produced with low selectivities. The results for the different catalysts in two different molar ratios at 343 K, and with catalyst 0.3 g are summarized in the Table 4.23 and Fig 4.23.
130 Table 4.18 Effect of reactant mole ratio on styrene oxidation over ZN (Parent ZSM-5) [catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K] Catalyst amount: 0.3 g Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Temperature: 343 K Reactant mole ratio (Styrene: H 2 O 2 ) Conversion (%) (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 1.1 5.1 27.4 59.7 12.9 4.1 3.0 33.7 54.1 12.2 Fig 4.18: Effect of reactants mole ratio on styrene oxidation reaction over ZN catalyst. A-conversion %, B- selectivity of epoxide, C- selectivity of benzaldehyde, D- selectivity of benzoic acid. [catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
131 Table 4.19 Effect of reactant mole ratio on styrene oxidation over MoZN1 [catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K] Catalyst amount: 0.3 g Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Temperature: 343 K Reactant mole ratio (Styrene: H 2 O 2 ) Conversion (%) (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 1.1 23.6 28.1 62 9.9 4.1 21.0 34.8 52.1 13.1 Fig 4.19: Effect of reactants mole ratio on styrene oxidation reaction over MoZN1 catalyst, A- conversion %, B- selectivity of epoxide, C- selectivity of benzaldehyde, D- selectivity of benzoic acid, [catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
132 Table 4.20 Effect of reactant mole ratio on styrene oxidation over MoZN2 [catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K] Catalyst amount: 0.3 g Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Temperature: 343 K Reactant mole ratio (Styrene: H 2 O 2 ) Conversion (%) (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 1.1 25.9 28.8 63.9 7.3 4.1 23.2 35.5 50.8 13.7 Fig 4.20: Effect of reactants mole ratio on styrene oxidation reaction over MoZN2 catalyst, A- conversion %, B- selectivity of epoxide, C- selectivity of benzaldehyde, D- selectivity of benzoic acid, [catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
133 Table 4.21 Effect of reactant mole ratio on styrene oxidation over MoZN3 [catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K] Catalyst amount: 0.3 g Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Temperature: 343 K Reactant mole ratio (Styrene: H 2 O 2 ) Conversion (%) (A) Selectivity (%) Epoxide (B) Benzaldehyde (C) Benzoic acid (D) 1.1 27.3 30.7 65.3 4 4.1 24.6 37.2 47.4 15.4 Fig 4.21: Effect of reactants mole ratio on styrene oxidation reaction over MoZN3 catalyst, A- conversion %, B- selectivity of epoxide, C- selectivity of benzaldehyde, D- selectivity of benzoic acid, [catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
134 Table 4.22 Effect of reactant mole ratio on styrene oxidation over MoMCM-48 [catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K] Catalyst amount: 0.3 g Solvent: CH 3 CN Substrate volume: 15 ml Time: 8 h Temperature: 343 K Reactant mole ratio (Styrene: H 2 O 2 ) Conversion (%) (A) Epoxide (B) Selectivity (%) Benzaldehyde (C) Benzoic acid (D) 1.1 32.4 38.8 55.2 6 4.1 29.2 40.1 50.1 9.8 Fig 4.22: Effect of reactants mole ratio on styrene oxidation reaction over MoMCM- 48 catalyst, A- conversion %, B- selectivity of epoxide, C- selectivity of benzaldehyde, D- selectivity of benzoic acid, [catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
135 Table 4.23 Comparison of conversion (%) of Styrene oxidation over different catalysts at different reactant molar ratios. [Catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K] Reactant molar ratio Conversion (%) ZN MoZN1 MoZN2 MoZN3 MoMCM-48 1:1 5.1 23.6 25.9 27.3 32.4 4:1 3.0 21.0 23.2 24.6 29.2 Fig 4.23: Comparison of conversion (%) of Styrene oxidation over different catalysts at different reactant molar ratio. [Catalyst amount: 0.3 g, Time: 8 h, solvent: CH 3 CN, T= 343 K, Substrate volume: 15 ml]
136 4.4 Mechanism of styrene oxidation reaction The oxidation of styrene under similar conditions (Time: 8 h, solvent: CH 3 CN, T= 343 K, styrene : H 2 O 2 = 4:1), in absence of catalyst was found to proceed in very very low rate (results not shown). However, when catalysts, particularly molybdenum loaded ZSM-5 and MCM-48 were used main product obtained was styrene epoxide, benzaldehyde and benzoic acid. It appears from these results that the oxidation of styrene proceeds through nucleophilic attack of H 2 O 2 on styrene oxide followed by a cleavage of the intermediate hydroxyl-hydroperoxistyrene (Scheme 4.3). Formation of benzaldehyde directly from styrene by oxidative cleavage of its side chain double bond through radical mechanism cannot also be ruled out. Scheme: 4.3 4. 5 Conclusion of styrene oxidation reaction Styrene oxidation reaction with hydrogen peroxide was carried out in presence of acetonitrile with different styrene to hydrogen peroxide ratios at different temperatures over different amounts of catalysts to study the course of the reaction. The reaction was carried
137 out by using parent and molybdenum incorporated ZSM-5 and MCM-48. The reaction was studied under different conditions such as reaction time, temperature, catalyst amount and reactants molar ratio. Under all the conditions the predominant products were benzaldehyde and epoxide. The styrene conversion increased with increase in reaction time, temperature, catalyst amount and decrease in styrene amount in the reaction mixture. The selectivity towards benzaldehyde increased with increase in time and temperature but that of epoxide increased with increase in catalyst amount and styrene : H 2 O 2 molar ratio. The decrease in epoxide selectivity with time corresponding to the increase in selectivity of benzaldehyde might be due to the secondary oxidation of the epoxide. With increase of temperature in styrene oxidation, the conversion increased and benzaldehyde selectivity also increased. It may be due to further oxidation of epoxide to benzaldehyde and then to benzoic acid as higher reaction temperature favors the oxidation of epoxide. All of the samples showed considerable activity, and the styrene conversion and epoxidation selectivity was found to depend strongly on the Mo content in the synthesized material. It is observed that styrene conversion as well as the selectivity for desired epoxide increased with increasing Mo content. It demonstrates that Mo in framework site is responsible for the oxidation of styrene. This is in accordance with the conclusion that the framework Mo is the active center for the epoxidation of olefins. Thus it is seen that the activity of the synthesized samples increases due to the incorporation of molybdenum.
138 4.6 Esterification reaction: In organic synthesis esterification reaction is one of the fundamental reactions and has great use [35]. Most of the esterification reactions were carried out in presence of sulphuric acid, hydrochloric acid and toxic materials which are environmentally hazardous and unacceptable. Considering the impact of these chemicals on environment, zeolites are used as acid catalyst in organic transformation [36-39]. The use of zeolite has great advantage because it can be reused. Further, zeolite is thermally and hydrothermally stable and more efficient catalyst for esterification reaction [40-42]. Microwave induced esterification was also carried out using ion-exchange resin which was compared with that of sulphuric acid [43]. In industries different esterification of organic acid by different alcohol is used extensively for different purposes [35, 44, 45]. Benzyl acetate is the product of the reaction of benzyl alcohol and acetic acid in presence of conc. sulphuric acid [46] and it is an important ester. In the present study the reaction of benzyl alcohol and acetic acid for esterification using modified ZSM-5, MCM-48 and SBA-1 catalysts. 4.7 Esterification of benzyl alcohol by acetic acid over modified ZSM-5 and MCM-48 In this part of the chapter, the detailed study of the esterification of benzyl alcohol by acetic acid using parent and modified ZSM-5 zeolites and mesoporous materials MCM-48 and SBA-1 under different conditions is reported. The reaction was carried out in glass batch reactor under atmospheric pressure. The temperature of the reaction was controlled by a temperature controller and stirring was done by a magnetic stirrer. Before the use of catalysts in the reaction it was activated at 373 K temperature for 4 h. The product and the unreacted reactants of the reaction were collected at different time intervals and analysed by Gas Chromtography (PEKIN ELMER, Clarus 500, Elite 501 column) to calculate the percentage of conversion under different reaction conditions. The reaction scheme is given below
139 OH O C CH 3 O CH 3 COOH prepared catalyst benzyl alcohol Scheme 4.4 Benzyl acetate 4.7.1 Analysis of esterification reaction of benzyl alcohol under different conditions 4.7.1:( i) Effect of Reaction Time Esterification of benzyl alcohol by acetic acid was carried out over parent ZSM-5 (ZP) and samples ZC1, ZC2, ZC3, SBA-1 and MCM-48 under 373 K temperature and up to duration of 10 h. The samples were collected at different time intervals such as 2 h, 4 h, 6 h, 8 h and 10 h. The catalytic results of the reaction for catalysts ZP, ZC1, ZC2, ZC3, SBA-1 and MCM-48 are reported in Tables 4.24, 4.25, 4.26, 4.27, 4.28 and 4.29 respectively and graphically presented in Figs 4.24 to 4.29 respectively. The results for benzyl alcohol esterification on different catalysts under similar conditions at 373 K are summarized in Table 4.30 and presented in Fig 4.30. When the progress of the reaction was studied in different time intervals, it was observed that the conversion of the reaction increased in all cases with increase of reaction time. It has been observed that the conversion increased from 20.5 to 33.9%, from 20.7 to 50.4%, from 21.3 to 54.3%, from 23.2 to 56.9%, from 25.1 to 58.1% and from 25.4 to 58.2% for the catalysts ZP, ZC1, ZC2, ZC3, SBA-1 and MCM-48 respectively. When time of reaction was increased from 2 to 4 h, there was a difference of pattern in increase of conversion in case of catalyst ZP but not for other catalysts for increase of time of reaction from 4 to 6 h under similar conditions. It appears from the plots that the reaction tends to equilibrate after 8 h of reaction.
140 Table 4.24 Effect of reaction time on the esterification reaction over ZP (Parent) [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio) ] Catalyst: ZP Benzyl alcohol: acetic acid = 1:2 (molar ratio) Substrate volume: 16 ml Temperature: 373 K Catalyst amount: 0.2 g Time (h) Conversion (%) 2 20.5 4 28.4 6 31.2 8 33.7 10 33.9 3 4 C o n v e r s io n ( % ) 3 2 3 0 Conversion (%) 2 8 2 6 2 4 2 2 2 0 2 3 4 5 6 7 8 9 1 0 T im e ( h ) Fig 4.24 Effect of reaction time on the esterification reaction over ZP catalyst [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio), Substrate volume: 16 ml ]
141 Table 4.25 Effect of reaction time on the esterification reaction over ZC1 [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio) ] Catalyst: ZC1 Benzyl alcohol: acetic acid = 1:2 Substrate volume: 16 ml Temperature: 373 K Catalyst amount: 0.2 g Time (h) Conversion (%) 2 20.7 4 28.9 6 45.2 8 50.1 10 50.4 50 Conversion (%) 45 Conversion (%) 40 35 30 25 20 2 3 4 5 6 7 8 9 10 Time (h) Fig 4.25 Effect of reaction time on the esterification reaction over ZC1 catalyst [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio), Substrate volume: 16 ml ]
142 Table 4.26 Effect of reaction time on the esterification reaction over ZC2 [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio) ] Catalyst: ZC2 Benzyl alcohol: acetic acid = 1:2 Substrate volume: 16 ml Temperature: 373 K Catalyst amount: 0.2 g Time (h) Conversion (%) 2 21.3 4 28.9 6 47.1 8 54.1 10 54.3 55 Conversion(% ) 50 45 Conversion (%) 40 35 30 25 20 2 3 4 5 6 7 8 9 10 Time (h) Fig 4.26 Effect of reaction time on the esterification reaction over ZC2 catalyst [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio), Substrate volume: 16 ml ]
143 Table 4.27 Effect of reaction time on the esterification reaction over ZC3 [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio) ] Catalyst: ZC3 Benzyl alcohol: acetic acid = 1:2 Substrate volume: 16 ml Temperature: 373 K Catalyst amount: 0.2 g Time (h) Conversion (%) 2 23.2 4 30.5 6 50.7 8 56.5 10 56.9 60 C onversion (% ) 55 50 Conversion (%) 45 40 35 30 25 20 2 3 4 5 6 7 8 9 10 Time (h) Fig 4.27 Effect of reaction time on the esterification reaction over ZC3 catalyst [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio), Substrate volume: 16 ml ]
144 Table 4.28 Effect of reaction time on the esterification reaction over SBA-1 [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio) ] Catalyst: SBA-1 Benzyl alcohol: acetic acid = 1:2 Substrate volume: 16 ml Temperature: 373 K Catalyst amount: 0.2 g Time (h) Conversion (%) 2 25.1 4 33.4 6 51.1 8 57.8 10 58.1 6 0 C o n v e rs io n (% ) 5 5 5 0 Conversion (%) 4 5 4 0 3 5 3 0 2 5 2 0 2 3 4 5 6 7 8 9 1 0 T im e (h ) Fig 4.28 Effect of reaction time on the esterification reaction over SBA-1 catalyst [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio), Substrate volume: 16 ml ]
145 Table 4.29 Effect of reaction time on the esterification reaction over MCM-48 [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio) ] Catalyst: MCM-48 Benzyl alcohol: acetic acid = 1:2 Substrate volume: 16 ml Temperature: 373 K Catalyst amount: 0.2 g Time (h) Conversion (%) 2 25.4 4 32.3 6 54.2 8 58.1 10 58.2 60 Conversion (%) 55 50 Conversion (%) 45 40 35 30 25 2 3 4 5 6 7 8 9 10 Time (%) Fig 4.29 Effect of reaction time on the esterification reaction over MCM-48 catalyst [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio), Substrate volume: 16 ml ]
146 Table 4.30 Comparison of conversion (%) of esterification of benzyl alcohol with acetic acid over different catalysts at different time. [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio) ] Time (h) Conversion (%) ZP ZC1 ZC2 ZC3 SBA-1 MCM-48 2 20.5 20.7 21.3 23.2 25.1 25.4 4 28.4 28.9 28.9 30.5 33.4 32.3 6 31.2 45.2 47.1 50.7 51.1 54.2 8 33.7 50.1 54.1 56.5 57.8 58.1 10 33.9 50.4 54.3 56.9 58.1 58.2 Conversion (%) 60 55 50 45 40 35 30 25 20 ZP ZC1 ZC2 ZC3 SBA-1 MCM-48 15 10 5 0 2 4 6 8 10 Time (h) Fig 4.30: Comparison of conversion (%) of esterification of benzyl alcohol with acetic acid over different catalysts at different time. [Temperature = 373 K, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio), Substrate volume: 16 ml ]
147 From the above data it is seen that the conversion of benzyl alcohol remains almost same in lower time for blank and catalyzed reactions. The selectivity towards benzyl acetate in all the cases is 100%. 4.7.1: (ii) Effect of temperature The effect of temperature on the esterification reaction of benzyl alcohol was studied in the temperature range of 343 K to 383 K using the four ZSM-5 samples (ZP, ZC1, ZC2 and ZC3), and two mesoporous materials SBA-1 and MCM-48. The reaction was done under atmospheric pressure taking 0.2 g catalyst with reactant mixture in 1:2 molar ratio. The conversion of benzyl alcohol at different temperatures for reaction time of 8 h is shown in Tables 4.31, 4.32, 4.33, 4.34, 4.35 and 4.36 respectively for catalysts ZP, ZC1, ZC2, ZC3, SBA-1 and MCM-48 and also graphically presented in the Figs 4.31, 4.32, 4.33, 4.34, 4.35 and 4.36 respectively. The conversion of benzyl alcohol increases with increase in temperature. No side product was obtained up to 383 K temperature, which indicates the absence of dehydration of Friedel-Craft acylation. There was a sharp increase of conversion of benzyl alcohol from 20.8 to 35.2%, from 21.2 to 51.5 %, from 23.4 to 55.5%, from 34.4 to 58.5%, from 40.4 to 58.9% and from 45.6 to 59.1%, when temperature was increased from 343 K to 373 K. for the catalysts ZP, ZC1, ZC2, ZC3, SBA-1 and MCM-48 respectively. In this temperature range the increase was above 60 % for all the catalysts except for SBA-1 where it was 43.5 % When the temperature was increased further to 383 K, the reaction slowed down for all the catalysts. This increase was less than 5 % for all catalysts. This may be due to the blockage of adsorption sites of the catalysts at higher temperature.
148 Table 4.31 Effect of temperature on the esterification reaction over ZP [Time = 8 h, Catalyst amount = 0.2 g, Benzyl alcohol: acetic acid = 1:2 (molar ratio)] Catalyst: ZP Benzyl alcohol: acetic acid = 1:2 Substrate volume: 16 ml Time: 8 h Catalyst amount: 0.2 g Temperature (K) Conversion (%) 343 20.8 373 33.7 383 35.2 36 Conversion (%) 34 32 Conversion (%) 30 28 26 24 22 20 340 350 360 370 380 390 Temperature (K) Fig 4.31 Effect of temperature on the esterification reaction over ZP catalyst. [Time = 8 h, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio), Substrate volume: 16 ml ]
149 Table 4.32 Effect of temperature on the esterification reaction over ZC1 [Time = 8 h, Catalyst amount = 0.2 g, Benzyl alcohol: acetic acid = 1:2 (molar ratio)] Catalyst: ZC1 Benzyl alcohol: acetic acid = 1:2 Substrate volume: 16 ml Time: 8 h Catalyst amount: 0.2 g Temperature (K) Conversion (%) 343 21.2 373 50.1 383 51.5 55 Conversion (%) 50 45 Conversion (%) 40 35 30 25 20 340 350 360 370 380 390 Temperature (K) Fig 4.32 Effect of temperature on the esterification reaction over ZC1 catalyst. [Time = 8 h, Catalyst amount = 0.2 g, Benzyl alcohol : acetic acid = 1:2 (molar ratio), Substrate volume: 16 ml ]