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CHAPTER 3
ESTERFICATION OF MALEIC ANHYDRIDE
WITH ETHANOL
The catalyst plays an important role in many reactions such as
alkylation, acylation, cyclization, aldol condensation, Knoevenagel
condensation, oxidation, reduction, isomerisation, disproportionation,
polymerisation, esterification, protection, deprotection, acetalization, etc.
These reactions are catalysed by Brönsted and Lewis acid sites and the type
of requirement of the site is dependent on the type of reaction.
Esterification is an industrially important reaction as organic esters
are important intermediates in the synthesis of fine chemicals, drugs,
plasticisers, food preservatives, pharmaceuticals, cosmetics and auxiliaries
(Bushey et al 1972). These esters are produced by a batch process in
industries using mineral acid catalysts such as hydrofluoric acid, sulphuric
acid or Lewis acid catalysts like tin octoate (Kowalski et al 1998). Mineral
acid catalysts are corrosive and need to be neutralised after the reaction for
disposal. Lewis acid catalysts also require careful removal after the reaction
by adsorption on bleaching earth which also produces large amount of waste.
Hence, there is a need for ecofriendly heterogeneous catalysts for
esterification. Many heterogeneous catalysts viz., ion-exchange resin H-ZSM-
5, HY (Corma et al 1989), triolic acid, sulphated oxides (Lu 1995), hydrous
zirnoium oxide and supported heteropoly acids (Chu et al 1996) were reported
in the literature for esterification.
47
Al-MCM-41 materials with large pore diameter (20 to 100 Å) and
scattered distribution of acid sites were proved to be an important catalyst.
Recently Koster et al (2001) have reported esterification of acetic acid with
ethanol over MCM-41. Diaz et al (2000) have reported combined alkylation
and sulphonic acid functionalisation of MCM-41 type silica for esterification
of glycerol with fatty acids. These studies have instigated us to study
esterification of maleic anhydride with methanol over Al-MCM-41 molecular
sieves.
The ester, diethyl maleate, is used as an additive and intermediate
for plastics, pigments, pharmaceuticals and agriculture products. It is used as
a dienophile in Diels-Alder reaction. Diethyl maleate is manufactured by
reacting maleic anhydride with ethanol in the presence of catalytic quantities
of sulphuric acid (Makowka et al 1989). As esterification process involves
hazardous mineral acids, ecofriendly solid acid catalysts could be a
convenient alternative. Hence, in the present study esterification of maleic
anhydride with ethanol in the liquid phase over Al-MCM-41 molecular sieves
with different Si/Al ratios has been carried out.
3.1 CHARACTERISATION
3.1.1 Elemental Analysis
Elemental analysis of hydrothemally synthesised MCM-41 materials
was analysed by inductively coupled plasma atomic-emission spectroscopy
(ICP - AES). Table 3.1 contains the data of Si/M (M = Al) ratios of various
catalysts in the initial gel as well as in the calcined samples. Under the
synthesis conditions used, the crystallisation reaction was non-stoichimetric
and a high Si/Al ratio was noticed in the crystal (Biz and Occelli et al 1998).
The results indicate that almost all aluminium specimens employed during the
synthesis are incorporated in the mesoporous materials.
48
Table 3.1 ICP - AES analysis of Si / Al ratio for Al-MCM-41
molecular sieves
Catalyst (Si / Al) Ratio
Initial gel ICP Al-MCM-41 (50) 50 55
A1-MCM-41 (100) 100 104
A1-MCM-41 (150) 150 153
3.1.2 XRD of Al-MCM-41 Molecular Sieves
X-ray powder diffraction patterns of all as-synthesised and calcined
mesoporous Al-MCM-41 (Si/Al=50,100 and 150) are shown in Figures 3.1A
and 3.1B. The XRD patterns of MCM-41 exhibits typically three peaks with a
very strong peak at a low angle and two weak peaks at relatively higher
angles. These three peaks were indexed to hexagonal lattice corresponding to
(100), (110) and (200) diffraction lines. Since the materials were not
crystalline at atomic level, no higher order reflections were observed.
These materials are thus different from zeolites, which are highly
crystalline microporous materials. High loadings of heteroatom led to the
collapse of the structure. This is due to the fact that purely siliceous materials
are found to possess pore wall with highly flexible O-Si-O linkages. Thus,
substitution of heteroatom like Al results in less flexible O-Al-O linkages,
which might give rise to defective structure with less crystallographic order.
The d100 spacing and lattice parameter (ao) calculated from 2d100 3/ are
presented in Table 3.2, which revealed that incorporation of heteroatom in the
framework increases the cell parameter value slightly. Even though this
cannot be taken as evidence for the incorporation of heteroatoms in the
framework, it provides first hand information regarding the presence of
heteroatom.
49
Figure 3.1 XRD patterns of (A) as-synthesised (a) Al-MCM-41 (50),
(b) Al-MCM-41 (100), (c) Al-MCM-41 (150) and
(B) calcined (d) Al-MCM-41 (50), (e) Al-MCM-41 (100)
and (f) Al-MCM-41 (150)
A
B
Inte
nsity
(a.u
)
2 (degree)
Inte
nsity
(a.u
)
2 (degree)
(d)
(e)
(f)
(c)
(b)
(a)
50
Table 3.2 XRD d100 spacing and lattice parameter (a0) of Al-MCM-41
molecular sieves
Catalyst d100 (Å) a0 (Å)
Al-MCM-41 (50) 41.7 48.2
Al-MCM-41 (100) 40.6 46.9
Al-MCM-41 (150) 36.9 47.2
Corma et al (1994) and Luan et al (1995) mentioned that
introduction of aluminium decreases the order in these materials. It has also
been claimed by Janicke et al (1994) and Schmidt et al (1994) that when
sodium silicate was used as the precursor for silicon the ordering improved
but at the cost of framework substitution. The diffraction patterns, recorded
before and after calcination, confirmed the regular hexagonal array of uniform
channels characteristic of MCM-41 (Beck et al 1992). The as-synthesised
samples exhibit poorly ordered wall structure as shown by the lower peak
intensity than the calcined samples. The XRD patterns indicate that the long
range order structure was achieved (Kresge et al 1992) and the regular
mesoporous structure was retained after the introduction of metal atoms. The
removal of organic template is accompanied by contraction of the unit cell
owing to the release of strain imposed in the framework by the bulky organic
template and condensation of adjacent silanol groups. Further, the intensity of
(100) plane and the weak broadened peaks recorded from metal incorporated
catalysts suggests that the hexagonal array of mesopores in MCM-41 was
sustained even after the incorporation of metal ions in the framework. These
peaks are broadened and shifted slightly to higher angle with increasing metal
content. These results showed that the regularity of mesoporous structure
decreased and the pore size became slightly narrow with the introduction of
51
heteroatom. These XRD patterns coincide with the data already reported in
the literature for mesoporous aluminosilicate molecular sieves (Chen et al
1993, Corma 1997 and Biz and Occelli 1998).
3.1.3 XRD of H Zeolite
The XRD pattern of H is shown Figure 3.2. The most intense
peaks are positioned at 22.4, 25.7 and 27.8 (2). These peak positions and
the d spacing values are comparable to those reported in the literature.
Figure 3.2 XRD pattern of Hzeolite
Inte
nsity
(a.u
)
2 (degree)
52
3.1.4 FT-IR Spectra of Al-MCM-41 Molecular Sieves
Infrared spectroscopy has been used extensively to characterise
transition metal containing mesoporous molecular sieves. The infrared spectra
of as-synthesised and calcined Al-MCM-41 (Si/Al = 50, 100 and 150) are
shown in Figures 3.3A and 3.3B. The broad envelope around 3500 cm-1 is due
to O-H stretching of water, surface hydroxyl and bridged hydroxyl groups.
There are less intense peaks just below 3000 cm-1 in the spectra of the
as-synthesised samples which are assigned to symmetric and asymmetric
stretching modes of the -CH2 group of the locked-in template. Their
corresponding bending mode is observed at 1400 cm-1. The peaks between
500 and 120 cm-1 are assigned to framework vibrations. The asymmetric
stretching modes of T-O-T groups are observed around 1225 cm-1 and
1075 cm-1. The peak at 963 cm-1 is assigned to the presence of defective
Si-OH groups in the materials. Camblor et al (1992) reported similar
stretching vibrations of Si-OH groups at defect sites. The symmetric
stretching modes of T-O-T groups are observed around 800 and 544 cm-1. The
peak at 460 cm-1 is due to the bending mode of T-O-T. Upon introduction of
high metal content, most of the peaks are shifted to high wave numbers,
consistent with their incorporation in lattice position. Additionally, an
absorption band in the range 950-960 cm-1 is observed due to stretching
vibration of Si-O-M linkage. This is generally considered to be a proof for the
incorporation of heteroatoms into the framework. The symmetric and
asymmetric stretching modes of the -CH2 group of the template are absent in
the spectra of calcined samples (Figures 3.3b). These spectral features
resemble those reported by the previous workers (Chen et al 1993 and Biz and
Occelli et al 1998).
53
Figure 3.3 FT-IR spectra of (A) as-synthesised (a) Al-MCM-41 (50),
(b) Al-MCM-41 (100), (c) Al-MCM-41 (150) and
(B) calcined (d) Al-MCM-41 (50), (e) Al-MCM-41 (100) and
(f) Al-MCM-41 (150)
A
B
Tran
smitt
ance
(%)
Wavenumber (cm-1)
Tran
smitt
ance
(%)
Wavenumber (cm-1)
(f)
(e)
(d)
(a)
(b)
(c)
54
3.1.5 FT-IR Spectrum of H Zeolite
The mid infrared FT-IR spectrum of H zeolite is shown in
Figure 3.4. The broad band between 3000 and 3600 cm-1 is assigned to O-H
stretching of water and defective Si-OH groupings. The presence of water is
confirmed by its bending vibration at 1634 cm-1. The broad intense band
between 1000 and 1300 cm-1 is due to framework asymmetric (Si-O-Si and
Si-O-Al) vibrations. The corresponding symmetric vibration is assigned to
788 cm-1. The peaks below 600 cm-1 are due to bending modes of Si-O-Si
and Si-O-Al. The values are comparable to those reported by Flanigen et al
(1976).
Figure 3.4 FT-IR spectrum of Hzeolite
Tran
smitt
ance
(%)
Wavenumber (cm-1)
55
3.1.6 Thermogravimetric Analysis
The thermograms of as-synthesised Al-MCM-41 (Si/Al = 50, 100
and 150) catalysts are presented in Figures 3.5 - 3.7. Thermogravimetric
analysis of the catalysts shows distinct weight losses that depend on the
framework composition. Generally, when the metal content increases, there is
decrease in organic content and increase in water content. Three distinct
regions of weight losses are noticed in the temperature range 25-150°C,
150-400°C and above 400°C. The first weight loss corresponds to the
desorption and removal of water molecules physisorbed on the external
surface of the crystallites or occluded in the mesopores present between the
crystallite aggregates. A second weight loss between 150 and 400°C is
attributed to the removal of organic template. Finally, a third weight loss
above 400°C is related to loss of water from the condensation of adjacent
silanol groups to form siloxane bond (Chen et al 1993 and Occelli et al 1998).
However, the distribution of successive weight loss depends on the
framework or substituted Si/M ratio (Busio et al 1995).
3.1.7 Nitrogen Sorption Studies
Nitrogen adsorption-desorption isotherms provide information on
the textural properties of the prepared catalysts in addition to specific surface
area and pore size distribution (Thomas et al 1999, Rao 1999 and Storck et al
1988). The pore size distribution can be mapped directly from adsorption
isotherms of nitrogen along with surface area of the catalysts. The textural
properties of hydrothermally synthesised mesoporous materials were analysed
by nitrogen adsorption following BET procedure. The BET
surface area, pore volume and pore diameter (BJH method) for calcined
mesoporous Al-MCM-41 (Si/Al = 50, 100 and 150) are presented in
56
Figure 3.5 TGA-DTG curves of as-synthesised Al-MCM-41 (50)
Wei
ght l
oss (
%)
Temperature (oC)
W
/T
57
Figure 3.6 TGA-DTG curves of as-synthesised Al-MCM-41 (100)
Wei
ght l
oss (
%)
Temperature (oC)
W
/T
58
Figure 3.7 TGA-DTG curves of as-synthesised Al-MCM-41 (150)
Wei
ght l
oss (
%)
Temperature (oC)
W
/T
59
Table 3.3. The surface area of the samples varies from 910 to 1044 m2/g
(Biz and White 1999) and the pore diameter varies from 25.7 to 30.2 Å. The
pore volume ranges from 0.64 to 0.69 cm3/g. The typical isotherms of the
above calcined samples measured at liquid nitrogen temperature (77 K) and
the corresponding pore size distribution curve (inset) are shown in
Figures 3.8-3.10. All samples exhibit type IV adsorption-desorption
isotherms, featuring the shape characteristic of MCM-41 mesoporous
materials (Cesteros and Haller 2000, Lindlar et al 2000, Matsumoto et al
1999). The observed sharp inflection (P/P0=0.3-0.45) and the two
well- separated hysteresis loops in all the samples indicate the characteristics
of Al-MCM-41 materials and framework confined mesoporosity (Branton
et al 1994). The two inflections viz., P/P0 < 0.3 is accounted for the
monolayer adsorption of nitrogen on the walls of the mesopores and
P/P0 > 0.3 corresponds to capillary condensation within the uniformly sized
mesopores (Chen et al 1997). The steep increase and sharpness in inflection
position at P/P0 = 0.3-0.45 is indicative of relative uniformity of the pore size
distribution in the samples (Luan et al 1995). The sharpness and height of the
capillary condensation step are the indicative of pore size uniformity.
Deviation from sharp and well-defined pore filling step is the indication of
increases in pore size heterogeneity. Al-MCM-41 catalysts exhibit isotherms
with well-developed step in the relative pressure range P/P0 0.42, which is
characteristic of capillary condensation in uniform mesopores.
60
Table 3.3 Textural properties of calcined Al-MCM-41 catalysts
determined from N2 sorption studies
Catalyst BET surface
area (m2/g)
Pore diameter
(Å)
Pore volume (cm3/g)
Wall thickness
(Å)
Al-MCM-41 (50) 979 27.9 0.68 20.3
Al-MCM-41 (100) 1044 28.3 0.66 18.6
Al-MCM-41 (150) 1039 29.5 0.68 17.7
Figure 3.8 Adsorption isotherms of Al-MCM-41 (50)
Vol
ume
of N
2 ads
orbe
d (c
c/g)
Relative pressure (P/Po)
dV/d
D (c
c/g/
A)
10-3
Pore diameter (Å)
61
Figure 3.9 Adsorption isotherms of Al-MCM-41 (100)
Figure 3.10 Adsorption isotherms of Al-MCM-41 (150)
Vol
ume
of N
2 ads
orbe
d (c
c/g)
Relative pressure (P/Po)
Vol
ume
of N
2 ads
orbe
d (c
c/g)
Relative pressure (P/Po)
dV/d
D (c
c/g/
A)
10-3
Pore diameter (Å)
dV/d
D (c
c/g/
A)
10-3
Pore diameter (Å)
62
3.1.8 29Si MAS-NMR
The 29Si MAS-NMR spectra of all the calcined mesoporous
Al-MCM-41 (Si/Al = 50, 100 and 150) are shown in Figure 3.11. The spectra
show a broad signal at -110 ppm which can be assigned to Si(OSi) without
aluminum coordination. The net silica in Al-MCM-41 consists of rather
irregular structure and so a wide range of Si-O-Si angles are present. The
partly resolved signal at -92 ppm is assigned to Q2 species. The other signals
at -101 (Q3) and -106, -110 (Q4) appear similar to those reported in the
literature (Sun et al 1997).
Figure 3.11 29Si MAS-NMR spectra of calcined (a) Al-MCM-41 (50),
(b) Al-MCM-41 (100) and (c) Al-MCM-41 (150)
Inte
nsity
(a.u
)
ppm
(a)
(b)
(c)
63
3.1.9 27Al MAS-NMR
Solid state 27Al MAS-NMR study was performed by Luan et al
(1995) to understand the stability of aluminium in the tetrahedral
environment. The 27Al MAS-NMR spectra of the samples are shown in
Figure 3.12. The sharp resonance peak around δ = 54.3 ppm is attributed to
the presence of aluminium in tetrahedral coordination (Biz and Occelli 1998,
Wang et al 2002). The broad low intensity peak at δ = 0 ppm is due to the
presence of octahedral aluminium (Mokaya and Jones 1997, Englehardt and
Michel 1987, and Kolodziejski et al 1993). Generally materials with high
aluminium content are susceptible to framework leaching during calcination
(Matsumoto et al 1999). The small amount of aluminium at δ = 0 ppm is due
to the extra framework aluminium or due to water co-ordinated framework
aluminium (Chakraborty et al 1998). Busio et al (1995) reported that
incorporation of excess aluminium formed an impure crystal-phase tridimite.
The Lewis acid sites prevailed because of the octahedral non-framework
aluminium, accompanied with the collapse of the structure. When the catalyst
containing high Al was calcined, a substantial intensity loss of Si (2Si, 2Al)
and Si (3Si, 1Al) occurred while octahedral aluminium appeared. These
results could be explained by assuming that calcination leads to
dealumination. Reddy and Song (1996) reported that calcination of the
materials in nitrogen prior to air produce peak without any non-framework
aluminium.
64
Figure 3.12 27Al MAS-NMR spectra of calcined (a) Al-MCM-41 (50),
(b) Al-MCM-41 (100) and (c) Al-MCM-41 (150)
Inte
nsity
(a.u
)
ppm
(a)
(b)
(c)
65
3.1.10 Acidity Measurement of Al-MCM-41 molecular sieves
The acidity of mesoporous Al-MCM-41 has been estimated by
adsorption - desorption of bases such as pyridine, ammonia as probe
molecules (Corma et al 1994, Kosslick et al 1997, Mokaya and Jones 1996
and Renjo et al 1996). The acidity of calcined Al-MCM-41 (Si/Al= 50, 100
and 150) materials was measured by FT-IR spectroscopy using pyridine as a
probe. The FT-IR spectra of the above catalyst containing adsorbed pyridine
are shown in Figure 3.13. The samples give the expected bands due to Lewis
acid bound (1450, 1575, 1623 cm-1), Brönsted acid bound
(1545 and 1640 cm-1) and both Lewis and Brönsted acid bound pyridine
(1490 cm-1). 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 Table 3.4. The samples are found to contain both
Brönsted and Lewis acidity. The presence of both acidities in Al-MCM-41
was reported by Corma et al (1995a) and Climent et al (1999). It has been
reported that the acid strength of Al-MCM-41 is weaker than zeolites and
appears similar to that of amorphous silica-alumina (Corma et al 1996).
66
Figure 3.13 Brönsted and Lewis acidity of (a) Al-MCM-41 (50),
(b) Al-MCM-41 (100) and (c) Al-MCM-41 (150)
Abs
orba
nce
Wavenumber (cm-1)
(a)
(b)
(c)
67
Table 3.4 Brönsted and Lewis acidity values for Al-MCM-41
molecular sieves
Catalyst Temperature (423 K)
B.Aa L.Aa
Al - MCM - 41 (50) 7.1 9.0
Al - MCM - 41 (100) 3.7 1.1
Al- MCM - 41 (150) 3.5 5.1
B.A - Brönsted acidity and L.A - Lewis acidity
3.2 ESTERIFICATION OF MALEIC ANHYDRIDE WITH
ETHANOL
3.2.1 Effect of Temperature
Esterification of maleic anhydride with ethanol over Al-MCM-41
(Si/Al = 50, 100 and 150) and Hβ zeolite was studied in the liquid phase at
80C with a feed ratio (maleic anhydride: ethanol) 1:3. The results are
illustrated in Figure 3.14. In the first step, maleic anhydride reacts with
ethanol yielding monoethyl maleate (MEM) with 100% conversion even at
room temperature in the absence of catalyst. The second step of esterification
only requires a catalyst as shown in the reaction Scheme 3.1.
The time dependence of the second step is clearly evident for the
catalyst Al-MCM-41 (50). The yield of diethyl maleate (DEM) increases
with increase in time and the amount of MEM decreases. Since in the second
step water is released, more hydrophobic catalysts could expel water from the
pore as and when it is formed, thus permitting the equilibrium to shift
towards the product side. Hence, more hydrophobic catalysts such as
68
Si
H+ CHCOOC2H5
CHCOOHAlO
CHCOOC2H5
CHCOOC2H5
CHCOOC2H5
CHOHOH
C2H5OH
Si AlO
C+
Si
H+
AlO +
Scheme 3.1 The possible pathway for the formation of symmetrical
diester
Al-MCM-41 (100) and Al-MCM-41 (150) were also tested for their activity
for comparison. As expected, Al-MCM-41 (100) shows slightly more activity
by converting more of MEM to DEM at the end of 9 h. About 10% of MEM
left unesterified. But Al-MCM-41 (150) shows less activity than Al-MCM-41
(100) contrary to our expectation. This result therefore illustrates that high
hydrophobicity beyond that of Al-MCM-41 (100) is not advantageous for this
reaction. The reduced activity of this catalyst is due to blocking of active
sites by the hydrophobic DEM as and when it is formed. In this context it
could be discussed that the reaction occurs mainly within the pores. The
percentage of unesterified MEM remains the same at the end of 6 and 9 h due
to establishment of equilibrium. To test the blocking of pores of the catalyst
by MEM and DEM, the FT-IR spectral analysis of the spent catalyst was
carried out. The spent catalyst was washed thrice with ethanol and air dried
for 30 min before the spectrum was taken. The spectrum of the spent
Al-MCM-41 (150) is shown in Figure 3.15. The characteristic peaks due to
69
esters are evident by the peaks at 1712, 1191 and 1041 cm-1. This confirms
complete blocking of the pores of Al-MCM-41 (150) with hydrophobic ester.
In addition, with increase in time more and more water can associate with
MEM. Such associated MEM may not diffuse into the pores of hydrophobic
Al-MCM-41 (150) for subsequent esterification.
The results over H are also shown in Figure 3.14. Maximum
conversion is reached, i.e. equilibrium is established, even at the end of 3 h of
the reaction suggesting the acid strength as well as the density of the acid sites
of the catalyst are important parameters for the esterification. H can also
retain more water inside the pores as it is more hydrophilic. But complete
loss of activity might require more amount of water. As the use of more
amount of catalyst may perturb the equilibrium towards ester, the reaction
was also studied with 0.1 and 0.15 g of the catalyst. The results are presented
in Table 3.5. More than 98% conversion was obtained at the end of 9 h in
line with our expectation.
70
0
10
20
30
40
50
60
70
80
90
100
Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41(150) H-BETA
Catalyst
Yie
ld (%
)
MEM DEM
3 h6 h
9 h
3 h
6 h
9 h
3 h
6 h9 h
3 h6 h 9 h
Figure 3.14 Effect of temperature on the yield of products: Temperature 80C; Maleic anhydride: Ethanol molar ratio
1:3; Catalyst amount 0.1g.
H
71
Figure 3.15 FT-IR spectrum of the spent Al-MCM-41 (150)
Table 3.5 Effect of catalyst loading on the yield of products
Catalyst weight
Time (h)
0.10 g 0.15 g
MEM DEM MEM DEM
3 24.72 75.28 29.03 70.39
6 22.93 77.07 19.48 80.52
9 11.18 88.81 7.85 92.15
Catalyst: Al-MCM-41 (100); Temperature: 80C; Maleic anhydride: Ethanol
molar ratio 1:3.
Tran
smitt
ance
(%)
Wavenumber (cm-1)
72
As the reaction is an equilibrium process, the reaction was also
studied at 100C. Figure 3.16 presents the results obtained over all the
catalysts. Al-MCM-41 (50) shows less conversion of MEM at the end of 3 h
than at 80C but high conversion at the end of 6 and 9 h. High conversion is
also observed over Al-MCM-41 (100). Similar increase in conversion is also
observed over Al-MCM-41 (150) compared to 80C at the end of 3 h. But the
percentage of MEM is high at the end of 6 and 9 h. The establishment of
equilibrium is also evident at the end of 9 h as the conversion at the end of 6
and 9 h is nearly the same. The increase in the percentage of MEM at the end
of 6 h may be attributed to more adsorption of DEM in the pores of the
catalyst leaving most of the MEM outside the pores as it is less hydrophobic.
FT-IR spectral analysis of the spent catalyst showed similar results
as reported above thus confirming entrapment of DEM. H also shows slight
increase in conversion at the end of 9 h compared to 80C. Although increase
in conversion is observed over all the catalysts, the increase is not much
suggesting less influence of 20 rise in reaction temperature. Exactly similar
characteristics are observed at 120C as shown in Figure 3.17. There is a
slight increase in conversion of MEM is observed over H. But Al-MCM-41
(50) shows less conversion at the end of 3 h and more conversion at the end of
6 and 9 h similar to that at 100C. The conversion over Al-MCM-41 (100) at
120C is the same at 80C. Al-MCM-41 (150) exhibits similar characteristic
of less conversion at the end of 3 h and more conversion at the end of 6 and
9 h for MEM at 80C. The less conversion at 100 and 120C over
Al-MCM-41 (50) than at 80C is attributed to less entry of ethanol into the
pores of the catalysts, as it has less boiling point (b.p. of ethanol = 78C). The
hydrophobic pore can access more affinity for MEM than ethanol. Hence
increase in time only the conversion of MEM can also increase. Similar
results are also expected for Al-MCM-41 (100) but more conversion is
73
observed at the end of 3 h. As this catalyst is more hydrophobic, it can attract
and adsorb ethanol maintaining nearly similar amount of adsorption of MEM
thus facilitating more conversion. The high conversion over Al-MCM-41
(150) at 100 and 120C than 80C can also be accounted in a similar manner.
From the Figures 3.14, 3.16 and 3.17 it is evident for establishment
of equilibrium over Al-MCM-41 (150) and H catalysts at 80, 100 and
120C. But the equilibrium is not established even at the end of 9 h over
Al-MCM-41 (50) and Al-MCM-41 (100). Since 9 h was a long period, the
reaction was not carried out more than this time. From the study it is observed
that Al-MCM-41 (100) and H are better than other catalysts, as the yield of
DEM is not less than 90% over both the catalysts.
74
0
10
20
30
40
50
60
70
80
90
100
Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41(150) H-BETA
Catalyst
Yie
ld (%
)
MEM DEM
3 h
6 h9 h
3 h6 h
9 h
3 h6 h 9 h
3 h6 h 9 h
Figure 3.16 Effect of temperature on the yield of products: Temperature 100C; Maleic anhydride: Ethanol molar ratio
1:3; Catalyst amount 0.1g.
H
75
0
10
20
30
40
50
60
70
80
90
100
Al-MCM-41 (50) Al-MCM-41 (100) Al-MCM-41(150) H-BETA
Catalyst
Yie
ld (%
)
MEM DEM
3 h6 h
9 h
3 h
6 h
9 h9 h
6 h
3 h
3 h
9 h
6 h
Figure 3.17 Effect of temperature on the yield of products: Temperature 120C; Maleic anhydride: Ethanol molar ratio
1:3; Catalyst amount 0.1g.
H
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3.2.2 Effect of Feed Ratio
The effect of feed ratio was studied over Al-MCM-41 (100) and H
at 100C and the results are shown in Tables 3.6 and 3.7. With a feed ratio of
1:5, 95.3% yield of DEM was obtained over Al-MCM-41 (100). Nearly
similar conversion was also observed over H for the same feed ratio. The
requirement of feed ratio more than 1:3 illustrates strong influence of
equilibrium. When the feed ratio is changed from 1:3 to 1:5 only about 7%
increase in the yield of DEM is observed over Al-MCM-41(100) but over H
about 5% increase is observed.
Table 3.6 Effect of feed ratio on the yield of products over
Al-MCM-41(100)
Yield of DEM and amount of MEM (%)
Feed ratio
1:3 1:5 1:7 1:9
Time (h)
MEM DEM MEM DEM MEM DEM MEM DEM
3 24.72 75.28 21.79 78.9 24.9 75.1 29.8 70.2
6 22.93 77.07 15.16 84.84 26.5 73.5 13.5 86.5
9 11.18 88.81 4.69 95.3 3.9 96.1 2.8 97.2
Temperature: 80C; Catalyst weight: 0.1g.
77
Table 3.7 Effect of feed ratio on the yield of products over H zeolite
Yield of DEM and amount of MEM (%)
Feed ratio
1:3 1:5 1:7 1:9
Time (h)
MEM DEM MEM DEM MEM DEM MEM DEM
3 19.19 80.81 13.90 86.10 12.90 87.10 9.41 90.59
6 11.12 88.88 10.70 89.30 9.50 90.50 6.70 93.80
9 9.70 90.29 4.90 95.10 3.90 96.10 1.93 98.07
Temperature 80C; Catalyst weight: 0.1g.
3.2.3 Effect of Catalyst Weight
The effect of catalyst loading was studied with Al-MCM-41 (100).
The feed ratio was set at 1:3 and the temperature was maintained at 100C.
The results are shown in Table 3.5. The conversion increases from 88.81 % to
92.15% with 0.15 g the catalyst. Although the number of acid sites is
increased with the increase in the catalyst weight, the problem due to released
water on equilibrium is suggested to be more reduced as the total area of the
catalyst is increased.
3.2.4 Isomerisation of DEM to DEF
The important observation noted in this study is the absence
of isomerisation of diethyl maleate to diethyl fumurate over all
Al-MCM-41 molecular sieves. But the yield of diethyl fumurate is 1-2% over
Hβ and approximately 5% isomerisation (Makowoka et al 1989) over mineral
acids. The less acidic nature of Al-MCM-41 molecular sieves might be the
78
cause for it. This is important in the industrial production of diethyl maleate
as it avoids product purification.
3.2.5 Conclusion
Maleic anhydride can be esterified with ethanol over Al-MCM-41
and H zeolites. Monoesterification to monoethyl maleate is fast and catalyst
independent but the subsequent esterification to diethyl maleate is catalyst
dependent. Al-MCM-41 (50), Al-MCM-41 (100) and H are capable of
catalysing the reaction with nearly 95% yield. The advantage of these
catalysts is absence of isomerisation of diethyl maleate to diethyl fumarate.
This study also illustrates that these solid acid catalysts can conveniently
replace hazardous sulphuric acid in the industrial esterification of maleic
anhydride with ethanol. Although hydrophobic property of the catalysts is
important for esterification, the highly hydrophobic Al-MCM-41 (150) is not
desired, as it retained most of the esters in the catalyst pores leading to less
activity.
