CHAPTER 5 ESTERIFICATION OF PHTHALIC ANHYDRIDE WITH...

31
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

Transcript of CHAPTER 5 ESTERIFICATION OF PHTHALIC ANHYDRIDE WITH...

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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

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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 liquid-

phase esterification in polar media (Izumi et al 1992). Schwegler et al (1992)

applied carbon supported HPW for the esterification of phthalic anhydride

with C8-C10 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.

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5.1 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.

5.1.1 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 2.32. 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 d100 spacing and lattice parameter

(ao) calculated from 2d100 3/ are presented in Table 5.1.

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Figure 5.1 XRD pattern of 20% HPW Al-MCM-41 (50)

2 (degree)

Inte

nsity

(a.u

)

0 10 20 30 40 50

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Figure 5.2 XRD pattern of 40% HPW Al-MCM-41 (50)

2 (degree)

Inte

nsity

(a.u

)

0 10 20 30 40 50

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Table 5.1 XRD d100 spacing and lattice parameter (a0) of 20% and

40% HPW Al-MCM-41 (50)

Catalyst d100 (Å) a0 (Å) 20% HPW Al-MCM-41(50) 44.02 38.12

40% HPW Al-MCM-41(50) 44.02 38.12

5.1.2 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 PW12O403- Keggin ion structure consists

of a PO4 tetrahedron surrounded by four W3O13 groups formed by edge

sharing octahedral. These groups are connected to each other by corner-

sharing 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.

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Figure 5.3 FT-IR spectrum of 20% HPW Al-MCM-41 (50)

Wavenumber (cm-1)

Tran

smitt

ance

(%)

4000 3000 2000 1000 400

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Figure 5.4 FT-IR spectrum of 40% HPW Al-MCM-41 (50)

Wavenumber (cm-1)

Tran

smitt

ance

(%)

4000 3000 2000 1000 400

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5.1.3 31P MAS-NMR spectra of 20% and 40% HPW Al-MCM-41 (50)

31P 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 31P 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).

Figure 5.5 31P MAS-NMR spectrum of 20% HPW Al-MCM-41 (50)

ppm

Inte

nsity

(a.u

)

-1

5.2

40 0 -40 -80

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Figure 5.6 31P MAS-NMR spectrum of 40% HPW Al-MCM-41 (50)

ppm

Inte

nsity

(a.u

)

-15.

2

40 0 -40 -80

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5.1.4 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 1650-1450 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.

Figure 5.7 Brönsted and Lewis acidity of 20% HPW Al-MCM-41 (50)

1600 1500 1400

Wavenumber (cm-1)

Inte

nsity

(a.u

)

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Figure 5.8 Brönsted and Lewis acidity of 40% HPW Al-MCM-41 (50)

Inte

nsity

(a.u

)

1600 1500 1400

Wavenumber (cm-1)

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SiO

Al

H+

COOC4H9

COOH

SiO

Al

COOC4H9

OH

OH

C+

n-C4H9OH

SiO

Al

H+

COOC4H9

COOC4H9

+

Table 5.2 Brönsted and Lewis acidity values for 20% and 40% HPW

Al-MCM-41 (50)

Catalyst Brönsted (B) acid site concentration

(mmol/g)

Lewis acid site concentration

(mmol/g)

B/L acid site ratio

20% HPW Al-MCM-41 0.25 0.32 0.78

40% HPW Al-MCM-41 0.20 0.27 0.74

5.2 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 80C. 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

Scheme 5.1 The possible pathway for the formation of symmetrical

diester

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0

20

40

60

80

100

Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H

Catalyst

Yie

ld (%

)

3h

3h 3h

3h

6h

6h 6h

6h

9h

9h 9h

9h

MBP DBP

Figure 5.9 Effect of temperature on the yield of products: Temperature 80C; Phthalic anhydride: n-Butanol molar

ratio 1:3; Catalyst amount 0.1g.

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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

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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 500C 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

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proposed by Koster et al (2001). n-Butanol makes nucleophilic attack on the

protonated carboxyl function of MBP to yield DBP.

5.2.1 Effect of Temperature

The reaction was carried out at 100, 130 and 150C 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 80C. 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

100C. 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 100C 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 80C. 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 100C. Thus water prevents

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0

20

40

60

80

100

Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H

Catalyst

Yie

ld (%

)

3h 3h 3h

3h

6h 6h 6h

6h

9h

9h 9h

9h

MBP DBP

Figure 5.10 Effect of temperature on the yield of products: Temperature 100C; Phthalic anhydride: n-Butanol molar

ratio 1:3;Catalyst amount 0.1g.

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0

20

40

60

80

100

Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H Catalyst

Yie

ld (%

)

3h 3h

3h

3h

6h 6h

6h 6h

9h

9h

9h 9h

MBP DBP

Figure 5.11 Effect of temperature on the yield of products: Temperature 130C; Phthalic anhydride: n-Butanol molar

ratio 1:3; Catalyst amount 0.1g

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0

20

40

60

80

100

Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H Catalyst

Yie

ld (%

)

3h 3h 3h

3h

6h 6h 6h

6h 9h 9h 9h 9h

MBP DBP

Figure 5.12 Effect of temperature on the yield of products: Temperature 150C; Phthalic anhydride: n-Butanol molar

ratio 1:3; Catalyst amount 0.1g

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ultimately esterification of MBP to DBP. Since the effect of water in the

esterification is not well pronounced at 80C, 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

130C 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 80C. Al-MCM-41 (100) and Al-MCM-41(150) give higher yield of

DBP at 130C than at 80C. These results suggest the absence of retarding

effect of water in the esterification at 130C. Water may be largely expelled

out from the pores of the catalysts at 130C, 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 150C (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.

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5.2.2 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 130C

in order to verify the attainment of equilibrium and the results are depicted in

Figure 5.13. 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

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0

20

40

60

80

100

Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H Catalyst

Yie

ld (%

)

3h 3h 3h

3h

6h 6h 6h

6h

9h 9h

9h

9h

12h 12h

12h

12h

MBP DBP

Figure 5.13 Effect of time on the yield of products: Temperature 130C; Phthalic anhydride: n-Butanol molar ratio 1:3;

Catalyst amount 0.1g.

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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 5.14. 20% 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.

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0

20

40

60

80

100

Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41 (150) H 20% HPW Al-MCM-41 (50)

40% HPW Al-MCM-41 (50)

Catalyst

Yie

ld (%

)

MBP DBP 3h

3h 3h

3h 3h 3h

6h 6h

6h 6h

6h 6h

9h 9h

9h

9h

9h

12h 12h

12h

12h 12h

12h 9h

Figure 5.14 Effect of HPW loading on the yield of products: Temperature 130C; Phthalic anhydride: n-Butanol molar

ratio 1:3; Catalyst amount 0.1g.

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5.2.3 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

20% HPW Al-MCM-41 (50)

3 80.1 19.9 70.9 29.0 72.9 27.0

6 69.6 30.4 47.9 52.0 50.9 49.1

9 44.9 55.1 21.7 78.3 26.9 73.2

12 24.9 75.1 3.0 96.9 4.8 95.1

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5.2.4 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 130C.

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%

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Table 5.4 Formation of unsymmetrical ester

Catalyst Time (h) MBP MEP DBP DEP Unsymmetrical

ester

20% HPW

Al-MCM-41(50)

3 65.27 6.5 7.85 7.69 19.19

6 55.58 2.9 8.75 11.67 24

9 44.84 0 11.11 13.89 30.16

Temperature: 130C; Catalyst amount: 0.1g.

Ethanol DEP MBP MEP Unsymmetrical ester n-Butanol

Scheme 5.2 Formation of unsymmetrical diester

DBP

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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.

5.2.5 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-

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butanol. Although esterification reactions are based on equilibrium and

influenced by increase of temperature, the reaction at 100C decreases the

yield of DBP compared to that at 80C. This is attributed to the activation of

water present in the pores at 100C. 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.