CHAPTER 5. ESTERIFICATION OF PHTHALIC ANHYDRIDE WITH n-butanol

Transcription

1 92 CHAPTER 5 ESTERIFICATION OF PHTHALIC ANHYDRIDE WITH n-butanol Esterification is a largely exploited reaction in pharmaceutical, perfumery and polymer industries. Despite several synthetic routes, the most acceptable method is the reaction between the corresponding acid and an alcohol (Carey 1990). The reaction is catalysed by a mineral acid and it is a reversible one. Phthalate esters such as dioctyl phthalate, diisoamyl phthalate and dibutyl phthalate are the important plasticisers for polymers. Phthalate esters constitute more than 70% of the plasticiser market in the world. They are mainly used in the polymerisation of olefins especially vinyl chloride, ethylene and propylene. Phthalate esters are prepared by reacting phthalic anhydride with appropriate alcohol in the liquid phase either with a monoester as intermediate or by direct route (Akubowwicz et al 1981 and Makoto et al 1977). A large number of liquid phase catalysts viz., sulphuric acid, p-toluenesulphonic acid, methanesulphonic acid, hydrochloric acid and phosphoric acid have been reported for the esterification of phthalic anhydride with various alcohols such as isoamyl alcohol, n-butanol and 2-ethylhexanol. However, these catalysts impart color to the product due to the formation of by-products, and the catalysts are also difficult to recover and reuse. Phthalate esters are also prepared by employing tetrabutyl titanate and tetrabutyl zirconate as catalysts but they are also not easily recoverable. Hence, there is a need for solid acid catalysts by which environmentally hazardous homogeneous catalysts can be replaced for the synthesis of a variety of phthalate esters. Solid acid catalysts are better than mineral acids

2 93 since they have the advantages of non-corrosiveness, high catalytic activity and ease of separation from the reaction mixture. Al-MCM-41, with its mild acidity has already been shown to exhibit catalytic activity in the synthesis of fine chemicals (Climent et al 1996). Hence, in the present study Al-MCM-41 (Si/Al=50, 100 and 150) have been attempted for the esterification of phthalic anhydride with n-butanol. Large pore H zeolite has also been used for comparison as this is also shown to be a good catalyst for fine chemical synthesis (Corma et al 1997). Heteropolyacids (HPA) are widely used in various acid - catalysed reactions such as esterification (Hu et al 1993), etherification, hydration of olefins, de-esterification (Okuhara et al 1990) and dehydration of alcohols (Okuhara et al 1995) in homogeneous and heterogeneous systems. Unlike conventional acids these catalysts do not impart colouration to the product and hence many of them have been used in the esterification of various alcohols and carboxylic acids (Thoart et al 1992, Koyano et al 1999, Baba and Ono 1986, Guttmann and Grassell 1983, Izumi et al 1992, 1995, 1997 and Izumi 1997). Supported HPA catalysts as well as insoluble HPA salts are advantages towards liquid-phase reactions in aqueous media because they are practically insoluble, thermally more stable than acidic resins and possess strong acidity. Carbon supported HPAs have been shown to catalyse liquidphase esterification in polar media (Izumi et al 1992). Schwegler et al (1992) applied carbon supported HPW for the esterification of phthalic anhydride with C 8 -C 10 alcohols in which they reported the formation of dialkyl phthalates. But the carbon support adsorbs polar organic molecules strongly which make the work-up procedure difficult. In the present study, Al-MCM-41, H zeolite and Al-MCM-41 (50) supported phosphotungstic acid (20 and 40 wt%) have been attempted in the esterification of phthalic anhydride. Unsymmetrical alcoholysis of phthalic anhydride has also been attempted for the first time and the results are discussed in this chapter.

3 CHARACTERISATION The characterisation of Al-MCM-41 (Si/Al=50, 100 and 150) and Hβ has already been discussed in the previous chapter and hence the characterisation of Al-MCM-41(50) supported HPW has alone discussed below XRD of 20% and 40% HPW Al-MCM-41 (50) XRD pattern of calcined 20% and 40% HPW Al-MCM-41 (50) catalysts are shown in Figures 5.1 and 5.2 respectively. The 20% and 40% HPW Al-MCM-41 (50) catalyst exhibit (100) plane reflection at This illustrates the use of HPW to construct keggin phase within the pores. However, the intensity of peaks decreases upon the increasing HPW loading and lines appear above 20 (2 ) corresponding to the HPW crystalline phase. Comparison of the XRD pattern of Al-MCM-41(50) and 20% and 40% HPW Al-MCM-41(50) catalysts reveals that the mesoporous structure is rather intact even after the loading of HPW. The d 100 spacing and lattice parameter (a o ) calculated from 2d 100 / 3 are presented in Table 5.1.

4 95 Intensity (a.u) (degree) Figure 5.1 XRD pattern of 20% HPW Al-MCM-41 (50)

5 96 Intensity (a.u) (degree) Figure 5.2 XRD pattern of 40% HPW Al-MCM-41 (50)

6 97 Table 5.1 XRD d 100 spacing and lattice parameter (a 0 ) of 20% and 40% HPW Al-MCM-41 (50) Catalyst d 100 (Å) a 0 (Å) 20% HPW Al-MCM-41(50) % HPW Al-MCM-41(50) FT-IR spectra of 20% and 40% HPW Al-MCM-41(50) The supported HPW catalysts were analysed by FT-IR in order to confirm the presence of Keggin anion on Al-MCM-41 (50). The FT-IR spectra of 20% and 40% HPW Al-MCM-41 (50) catalyst are shown in Figures 5.3 and 5.4 respectively. The PW 12 O 3-40 Keggin ion structure consists of a PO 4 tetrahedron surrounded by four W 3 O 13 groups formed by edge sharing octahedral. These groups are connected to each other by cornersharing oxygen (Pope 1983). The spectra reveal the typical bands of keggin absorption at 1091, 968, 896 and 802 cm -1. This structure gives rise to four types of oxygen, which is responsible for the finger print bands of Keggin ion between 1200 and 700 cm -1. The bands at 1080 and 984 cm -1 are due to P-O and W=O vibrations respectively. The corner-shared and edge-shared vibrations of W-O-W bands occur at 892 and 800 cm -1 respectively (Rocchiccioli-Deltcheff et al 1983, Kozhevnikov et al 1995). These spectral features remain the same irrespective of HPW loading. A gradual increase in the absorbance of W-O-W corner shared vibrations at 892 cm -1 is observed for Al-MCM-41 supported HPW catalysts. Hence, it could be concluded that significant amount of crystallisation of Keggin phase starts only at and above 20 wt% loading of HPW.

7 98 Transmittance (%) Wavenumber (cm -1 ) Figure 5.3 FT-IR spectrum of 20% HPW Al-MCM-41 (50)

8 99 Transmittance (%) Wavenumber (cm -1 ) Figure 5.4 FT-IR spectrum of 40% HPW Al-MCM-41 (50)

9 P MAS-NMR spectra of 20% and 40% HPW Al-MCM-41 (50) 31 P MAS-NMR is the most revealing method to examine the state of phosphorus in heteropoly acids (Pope 1983). Figures 5.5 and 5.6 show the 31 P MAS NMR spectra of 20% and 40% HPW Al-MCM-41 (50) catalyst. The catalyst with HPW content exhibits a sharp resonance at 15.2 ppm, which is close to that of bulk HPW (Kozhevnikov et al 1995). This indicates unambiguously that Keggin structure is retained when HPW loaded on Al-MCM-41 (50). Intensity (a.u) ppm Figure P MAS-NMR spectrum of 20% HPW Al-MCM-41 (50)

10 101 Intensity (a.u) ppm Figure P MAS-NMR spectrum of 40% HPW Al-MCM-41 (50)

11 Acidity Measurements of 20% and 40% HPW Al-MCM-41(50) FT-IR spectra of 20% and 40% HPW Al-MCM-41(50) were recorded after adsorption of pyridine followed by evacuation at elevated temperatures (Figures 5.7 and 5.8). The spectra show contribution of pyridine adducts in the region cm -1. Formation of pyridinium ion by adsorption at 1545 and 1490 cm -1 is characteristic of Brönsted acid sites and both Brönsted and Lewis acid sites respectively (Dias et al 1999, 2003). The band appeared at 1634 cm -1 is due to ring vibration of pyridine bound to Brönsted acid sites (Corma 1995). The bands at 1445 and 1613 cm -1 are assigned to hydrogen-bonded pyridine (Corma 1995). The acidity was calculated using the extinction co-efficient of the bands of Brönsted and Lewis acid sites adsorbed pyridine (Emeis et al 1993). The results are presented in the Table 5.2. Intensity (a.u) Wavenumber (cm -1 ) Figure 5.7 Brönsted and Lewis acidity of 20% HPW Al-MCM-41 (50)

12 103 Intensity (a.u) Wavenumber (cm -1 ) Figure 5.8 Brönsted and Lewis acidity of 40% HPW Al-MCM-41 (50)

13 104 Table 5.2 Brönsted and Lewis acidity values for 20% and 40% HPW Al-MCM-41 (50) Catalyst 20% HPW Al-MCM-41 40% HPW Al-MCM-41 Brönsted (B) acid site concentration (mmol/g) Lewis acid site concentration (mmol/g) B/L acid site ratio ESTERIFICATION OF PHTHALIC ANHYDRIDE Esterification of phthalic anhydride with n-butanol was carried out in the liquid phase over Al-MCM-41 (50,100 and 150) and H zeolite with the reactants ratio (phthalic anhydride:n-butanol) 1:3 at 80 C. The reaction pathway is shown in Scheme 5.1. The comparative activities of the catalysts towards esterification are depicted in Figure 5.9. The formation of monobutyl COOC 4 H 9 Si COOC 4 H 9 H + COOH O Al Si C + OH OH O Al n-c 4 H 9 OH Si H + O COOC 4 H 9 Al + COOC 4 H 9 Scheme 5.1 The possible pathway for the formation of symmetrical diester

14 Yield (%) MBP DBP Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H Catalyst Figure 5.9 Effect of temperature on the yield of products: Temperature 80 C; Phthalic anhydride: n-butanol molar ratio 1:3; Catalyst amount 0.1g. 105

15 106 phthalate (MBP) is instantaneous even in the absence of a catalyst as reported by Yadav et al (1999). Hence, the esterification of the second carboxyl group of MBP alone becomes a catalyst demanding and time dependent process. It is clearly evident that the amount of MBP, which is 100% in the beginning, decreases gradually with increase in time due to its subsequent esterification with n-butanol in the presence of Al-MCM-41 (50). The maximum yield (50%) of DBP is observed over Al-MCM-41 (50) compared to 20% over Al-MCM-41 (100 and 150). The low yield of DBP over Al-MCM-41 (100 and 150) is due to the less density of acid sites and stronger acid strength than Al-MCM-41 (50). It is once again confirmed by nearly the same yield of DBP over Al-MCM-41 (150) whose acid strength is more than that of Al-MCM-41 (100). Hence the low yield of DBP over these two catalysts compared to Al-MCM-41 (50) is attributed to their enhanced hydrophobicity with which the hydrophobic DBP once formed may be retained within the pores, thus preventing the diffusion of MBP into the pores for subsequent esterification with n-butanol. This demonstrates clearly the occurrence of reaction largely inside the pores of the catalyst rather than on the catalyst surface. The less hydrophobic and high hydrophilic property of Al-MCM-41 (50) is also important factor in driving out DBP from the pores, thus keeping the pore accessible for subsequent esterification of MBP. This leads to high yield of DBP over this catalyst. The enhanced hydrophobicity of Al-MCM-41 (100 and 150) is advantageous for esterification as water once formed in the esterification can be expelled immediately out of the pores. But the retainment of the product inside the pores prevents the reactants to diffuse into the pores, thus hinders further esterification. The results with H zeolite illustrate nearly the same yield of DBP as that of Al-MCM-41 (50). Since it is a microporous material there could be diffusional constrain for both MBP and DBP through the pores. Hence low yield is expected with H compared to Al-MCM-41 (50). However, the same

16 107 results over H should therefore be reasoned out. Derouane et al (1999) reported that low conversion and reaction inhibition in liquid phase reactions over zeolites are due to the action of zeolites as solid solvents by which the reactants and products are competitively adsorbed. This is also concurred by Rohan et al (1998) and Freese et al (1999) in separate studies. The deficit of acetic acid by-product in the acylation of anisole with acetic anhydride at long reaction times was attributed to partial dealumination of zeolite framework and/or reaction of acetic acid with silanol defects. Dealumination can reduce strong acid sites and silanol defect esterification can block the pores of zeolite, which are suggested to be the cause for less conversion (Smith et al 1998). But in the present study there is a gradual increase in conversion of MBP even after 24 h of reaction, and hence MBP may not undergo esterification with silanol defects to offer diffusional constrain as reported with acetic acid (Derouane et al 1999). In this context, the point to be noted in the work is that instead of the by product acetic acid, the reactant acetic anhydride might have been better considered for esterification of silanol defects of the parent zeolite or the dealuminated zeolite, as they are more reactive and do not require a catalyst. The recyclability of the spent catalyst in the present study after regeneration at 500 C in air exhibited nearly similar activity, thus illustrating absence of aluminium leaching and esterification of silanol defects. Again, if the anhydride or MBP enters esterification reaction with silanol defects, the free alcohol can easily cleave this as silanol defects are less nucleophilic than free alcohols due to delocalisation of oxygen electron pairs over the channel surface. All these catalysts catalyse esterification of MBP by protonation of its carboxyl function rather than ethanol by Eley-Ridel mechanism as

17 108 proposed by Koster et al (2001). n-butanol makes nucleophilic attack on the protonated carboxyl function of MBP to yield DBP Effect of Temperature The reaction was carried out at 100, 130 and 150 C to understand the influence of temperature on the esterification of MBP with the same reactants ratio and catalyst weight. The results are depicted in Figures 5.10, 5.11 and 5.12 respectively. Figure 5.10 illustrates decrease in the yield of DBP over all the catalysts compared to the yield at 80 C. The decrease is found to be 25% over Al-MCM-41 (50), 20% over H zeolite, 7% over Al-MCM-41 (100) and 10% over Al-MCM-41 (150). Since esterification of carboxyl function with alcohol is an equilibrium process, the yield of DBP should increase with increase in temperature but conversely it decreases at 100 C. Hence there could be some other controlling factor in addition to the reaction between protonated MBP and free butanol. Since water is one of the products, its influence should be taken into account for the decrease in the yield of DBP. Although the catalysts were dried at 100 C for 3 h prior to use, it cannot be expected to assume that the catalysts are completely free of water. This factor is especially important for Al-MCM-41 (50) and H due to their high hydrophilic property. Such entrapped water may play significant retarding effect in the esterification of MBP over Al-MCM-41 (50) and H zeolite. Hence the percentage decrease is high in the yield of DBP over these two catalysts. Since Al-MCM-41 (100 and 150) is hydrophobic, they cannot retain water inside the pores. Therefore, the decrease in the yield of DBP over Al-MCM-41 (100 and 150) is less and the yield of DBP is nearly the same as that at 80 C. The low yield of DBP can also be attributed to the hydrolysis of DBP to MBP due to some water retained in the pore, which could not expelled out even at 100 C. Thus water prevents

18 109

19 100 MBP DBP 80 Yield (%) Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H Catalyst Figure 5.10 Effect of temperature on the yield of products: Temperature 100 C; Phthalic anhydride: n-butanol molar ratio 1:3;Catalyst amount 0.1g. 109

20 Yield (%) MBP DBP Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H Catalyst Figure 5.11 Effect of temperature on the yield of products: Temperature 130 C; Phthalic anhydride: n-butanol molar ratio 1:3; Catalyst amount 0.1g 110

21 100 MBP DBP 80 Yield (%) Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H Catalyst Figure 5.12 Effect of temperature on the yield of products: Temperature 150 C; Phthalic anhydride: n-butanol molar ratio 1:3; Catalyst amount 0.1g 111

22 112 ultimately esterification of MBP to DBP. Since the effect of water in the esterification is not well pronounced at 80 C, it may be presumed that water is not uniformly dispersed and blocked the active sites on the surface of the catalysts. Water blocks the active sites of the catalysts more at 100ºC due to high dispersion. As the decrease in the yield of DBP at 100ºC is ascribed to the activation of water present in the pores, the reaction was also carried out at 130 C in order to confirm this reason (Figure 5.11). But contrary to our expectation the yield of DBP increases over all the catalysts. The yield of DBP is about 50% over Al-MCM-41 (50) and H zeolite which is equal to the yield at 80 C. Al-MCM-41 (100) and Al-MCM-41(150) give higher yield of DBP at 130 C than at 80 C. These results suggest the absence of retarding effect of water in the esterification at 130 C. Water may be largely expelled out from the pores of the catalysts at 130 C, thus aiding esterification of MBP. Although the results prove evidently the decrease of retarding effect of water present in the pores, their presence in the pores is still evident from the results obtained over all the catalysts at 150 C (Figure 5.12). The yield of DBP is 50 to 60% over all the catalysts at the end of 9 h of the reaction. The equalisation of the yield of DBP over all the catalysts at the end of 9 h suggests the attainment of equilibrium. While comparing the activity of catalysts at the end of 3 h reaction, Al-MCM-41 (100 and 150) exhibits higher activity than Al-MCM-41 (50) and H zeolite. This result indicates that Al-MCM-41 (100 and 150) could expel adsorbed water even at the end of 3 h to give higher activity than Al-MCM-41 (50) and H zeolite. Moreover, the latter catalysts are more hydrophilic than the former, and hence the immediate removal of water is difficult.

23 Effect of Time As there is an increase in the yield of DBP over all the catalysts even up to 9 h without attaining steady state, the equilibrium is expected to attain sooner as the percentage increase in conversion is not high at 9 h compared to 6 h. Hence, the reaction was extended to 12 h duration at 130 C in order to verify the attainment of equilibrium and the results are depicted in Figure Although there is an increase in the yield of DBP over all the catalysts at the end of 12 h compared to 9 h, the total yield of DBP over Al-MCM-41 (50) and H zeolite is higher than over Al-MCM-41 (100 and 150) catalysts. In spite of long reaction time (12 h) the maximum yield of DBP over Al-MCM-41 (50) and H zeolite is only 60 to 70%. This observation suggests the unattainment of equilibrium even at the end of 12h and hence the reaction is assumed to be diffusion controlled. This reaction does not require diffusion of MBP into the pores. As mentioned already the second esterification step only requires a catalyst. But it is not necessary for the monoester to diffuse entirely into the bulk of the catalyst for protonation of the carboxyl group to facilitate nucleophilic attack of alcohol to produce diester. The protonation of the carboxyl group can occur even at the pore entry of zeolite or MCM-41 as there are acid sites at the pore entry. Once the acid function of the ester is protonated at the pore entry it will be prevented from diffusion into the bulk region of the particle due to retardation by electrostatic attraction between negative charge center of the zeolite or MCM-41 and the protonated acid function. In fact, there will be repulsion from other protons when it tries to diffuse deep into the pores. Hence the protonated monoester will be retained in the pore entry itself because of such charge based restriction to diffusion. Under these circumstances, the protonated monoester molecules are easily available for

24 100 MBP DBP Yield (%) h 12h 12h 12h Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H Catalyst Figure 5.13 Effect of time on the yield of products: Temperature 130 C; Phthalic anhydride: n-butanol molar ratio 1:3; Catalyst amount 0.1g. 114

25 115 nucleophilic attack by the alcohol to produce diester at the pore entry itself. This diester can enter the pore and get perfect shelter inside. At this stage it is important to realise that once the ester is perfectly sheltered in the bulk of the zeolite pore it may not diffuse out of the pore, as the zeolites are good solid solvents (Corma et al 1996). However, due to steric congession of products inside the pores, it is quite possible that some products may escape out of the pores. The force of attraction the product experiences inside the pores might be even more than outside. This fact may be the cause for increase in conversion in certain reported reactions (Rohan et al 1998) over zeolites due to reduction in particle size. Actually this is due to increase in the number of pore entries that provide less diffusional problems for the reactants to diffuse in and the products to diffuse out of the pores. Hence, it can be inferred that in all diffusion controlled reactions especially liquid phase reactions it may not be presumed that the reaction occur well within the pores as long as there is a possibility of protonation near the pore entry. As cyclodextrin was shown to catalyse hydrolysis of esters, the zeolite rings can also catalyse the esterification reaction at their pore entries (Saenger 1980). This cannot completely preclude a reaction well within the pores of a catalyst, but it is not necessary when there is a probability at the entry. The reaction was also carried out over Al-MCM-41 supported HPW catalysts. These catalysts possess Keggin structured HPW in the pores as well as on the surface. HPW is expected to restrict diffusion of reactants or products inside the pores. As the reaction was carried out using single time surface washed catalysts meets the expectation that conversion occur on pore entry as discussed above. The comparative results of all catalysts are depicted in Figure % and 40% HPW Al-MCM-41 (50) gave almost 100% conversion at the end of 12 h. The high activity of these catalysts is attributed to their high acidity.

26 Yield (%) h 12h 12h MBP DBP 12h 12h 12h 20 0 Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H 20% HPW Al-MCM-41 (50) Catalyst 40% HPW Al-MCM-41 (50) Figure 5.14 Effect of HPW loading on the yield of products: Temperature 130 C; Phthalic anhydride: n-butanol molar ratio 1:3; Catalyst amount 0.1g. 116

27 Effect of Feed Ratio The effect of feed ratio (1:2, 1:3 and 1:5) on the reaction was studied over 20% HPW Al-MCM-41 (50). The reaction was carried out for 12 h at each feed ratio and the results are given in Table 5.3. Maximum conversion of 75% is observed with a feed ratio 1:2. Although this catalyst can produces nearly 100% conversion by driving the reaction to the right by absorbing water, the less conversion is certainly due to gradual decrease in the concentration of n-butanol. As n-butanol molecules are well scattered over the catalyst surface, they may not be closer to the chemisorbed MBP for its subsequent esterification to DBP. When the feed ratio is changed to 1:3 the yield of DBP is increased to 97%, thus supporting the assertion of dilution of n-butanol in the feed ratio 1:2. Similar result is obtained with feed ratio 1:5. As the yield of DBP remains the same at the end of 12 h for feed ratios 1:3 and 1:5, chemisorption of n-butanol on the catalyst surface may not be involved in the rate-determining step. Hence the mechanism of this esterification follows Eelay-Ridel type involving the reaction of chemisorbed MBP through its carboxylic group on the active site and n-butanol in the free liquid phase. Table 5.3 Effect of feed ratio on the yield of products over 20% HPW Al-MCM-41(50) Catalyst Time (h) 1:2 1:3 1:5 MBP DBP MBP DBP MBP DBP % HPW Al-MCM-41 (50)

28 Unsymmetrical Alcoholysis of Phthalic Anhydride: An Important Observation As the first step of esterification of phthalic anhydride with n-butanol is fast and catalyst independent and the second step of esterification is slow and catalyst dependent, it has been planned to produce unsymmetrical ester with different alcohol like ethanol. This type of study has not been reported previously for phthalic anhydride. The idea of preparing unsymmetrical ester for phthalic anhydride has been obtained as unsymmetrical esterification of maleic anhydride over solid acid catalysts is reported already (Bhagiyalakshmi et al 2004). Phthalic anhydride and n-butanol were mixed in 1:1 ratio and the reaction was conducted at 130 C. Ethanol was added to the reaction mixture after 1 h in such a way that the ratio was kept as 1:1:1 and the reaction was continued. The reaction results obtained with 20% HPW Al-MCM-41 (50) are presented in Table 5.4. The products are MBP, monoethyl phthalate (MEP), DBP, diethyl phthalate (DEP) and butylethyl phthalate. The amount of MBP decreases with increase in time while the yield of DBP and DEP increases with increase in time. The yield of unsymmetrical ester, which is more than either DEP or DBP, increases with increase in time. The yield of MEP, which is formed in low amount, decreases with increase in time and disappears at the end of 9 h. The reaction Scheme 5.2 represents the yield of all these products. Since phthalic anhydride and n-butanol were mixed in the ratio 1:1 there could be high amount of MBP and low amount of both unreacted phthalic anhydride and n-butanol in the reaction mixture. When ethanol is added to the reaction mixture, it reacts immediately with free phthalic anhydride to give MEP. MBP can react with n-butanol to give DBP but the reaction is slow as n-butanol amount is less. This is also evident from the low yield of 8%, 9%

29 119 Table 5.4 Formation of unsymmetrical ester Catalyst Time (h) MBP MEP DBP DEP Unsymmetrical ester 20% HPW Al-MCM-41(50) Temperature: 130 C; Catalyst amount: 0.1g. Ethanol DEP MBP MEP Unsymmetrical ester n-butanol DBP Scheme 5.2 Formation of unsymmetrical diester

30 120 and 11% at the end of 6, 9 and 12 h respectively. MBP can also react with ethanol to give unsymmetrical ester rapidly as free ethanol amount is high. It is clearly evident from the high yield of unsymmetrical ester at each time interval compared to either DEP or DBP. The formation of unsymmetrical ester can also be possible from the reaction of MEP and n-butanol. But this reaction cannot contribute so much to unsymmetrical ester because of the low concentration of MEP. Similarly the reaction of MEP with ethanol is also slow. Moreover, ethanol can easily react with MBP to yield unsymmetrical ester as the MBP concentration is high. Comparison of the yields of DEP and DBP reveals that they are formed at similar rate. This is due to low concentration of MEP and n-butanol for formation of DEP and DBP. It is also quite possible that there might be transesterification of DBP or unsymmetrical esterification to DEP. The same transesterification can also be applied to MBP. In order to confirm this, DBP was reacted with ethanol over 20% HPW Al-MCM-41 (50) under similar conditions. GC analysis of the product indicates the absence of DEP and MEP. This observation clearly confirms the absence of transesterification between DBP and ethanol. Hence transesterification is ruled out with any of the esters with ethanol Conclusion The study of esterification of phthalic anhydride with n-butanol over Al-MCM-41 (Si/Al=50, 100 and 150), H zeolite and (20% and 40%) HPW Al-MCM-41 (50) revealed that these catalysts are convenient and ecofriendly substitutes for the hazardous homogeneous mineral acids. The results conclude that 20% HPW Al-MCM-41 (50) supported catalyst is the most active one. The study of influence of feed ratio unequivocally establishes Eelay-Rideal mechanism prevailing between protonated MBP and free n-

31 121 butanol. Although esterification reactions are based on equilibrium and influenced by increase of temperature, the reaction at 100 C decreases the yield of DBP compared to that at 80 C. This is attributed to the activation of water present in the pores at 100 C. Since monoesterification of phthalic anhydride is observed to be fast and does not require the catalyst, the second esterification of MBP becomes catalyst dependent. This observation has provided a new approach to produce unsymmetrical ester using n-butanol and ethanol. The same route can be applied in the preparation of a wide variety of unsymmetrical esters using appropriate alcohols. Comparing the production of unsymmetrical ester by the reaction of sodium salt of MBP with benzyl halide which is actually employed in industries, the solid acid catalyzed esterification of monoester with alcohols in the present study is ecofriendly and cost effective.