CHAPTER 5 ESTERIFICATION OF PHTHALIC ANHYDRIDE WITH...
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
100
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
101
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
102
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
)
103
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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109
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.
110
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.