Effect of Calcination Temperature and Tmaoh on Catalytic Activity of Basic Clay
Montmorillonite K10 was intercalated with lithium hydroxide and used for the transesterification Methyl Laurate (ML) with glycerol. This modified clay was characterized to elucidate the influence of added Tetramethylammonium Hydroxide (TMAOH) on its catalytic activity. Effects of calcination temperature (350-650°C) on the catalytic properties of basic clay were investigated with the goal of determining the optimum pretreatment conditions. The basic clay calcined at 450°C exhibited the optimum activity when used together with TMAOH. The results showed that TMAOH played an important role in improving the efficiency of the basic clay. By using a glycerol/methyl laurate ratio of 8:1 without any addition of TMAOH at a temperature reaction of 130°C, 60.1% methyl laurate conversion and 56.5% selectivity to monolaurate were achieved over the LiK10 calcined at 450°C for 5 h. Meanwhile, the addition of TMAOH increased the ML conversion and selectivity to monolaurate to 82.5 and 88.2%, respectively.
Received: February 10, 2011;
Accepted: April 11, 2011;
Published: December 16, 2011
Monoglycerides are non ionic surfactants which have important applications
as emulsifier in food, pharmaceutical and cosmetics production due to their
physicochemical properties, basically the hydrophilic/hydrophobic balance (Lauridsen,
1976; Ahmed et al., 2000). These esters are
generally manufactured on industrial scale by continuous glycerolysis of fats
and oils at high temperatures (220-250°C) using alkaline catalysts under
a nitrogen gas atmosphere (Sonntag, 1982). Since the
required product is the highly pure monoglyceride, the mixture is molecular
distilled to obtain higher monoglycerides purity (~80%) (Fregolente,
A number of solid bases have been used as catalysts for the transesterification
of glycerol in order to avoid drawbacks associated with the use of homogeneous
catalysts such as low yield, dark color and burnt taste. Abro
et al. (1997) and Barrault et al. (2004)
showed that basic solid catalysts such as MgO, CeO2 and La2O3,
as well as alkali-doped MgO (Li/MgO and Na/MgO) were active catalysts for the
transesterification of methyl stearate with glycerol. The authors also noted
that the nature of the oxide had a small effect on monoester selectivity, and
that the distribution of the esters obtained was similar to that obtained with
homogeneous basic catalysts.
Corma et al. (2005) reported that the glycerolysis
of triolein with glycerol could be carried out over solid basic catalyst such
as hydrotalcites. Al-Li mixed oxide resulted in an active Lewis base catalyst
with better performance than MgO and hydrotalcite. From these heterogeneous
catalytic routes, glycerol esters were obtained at 90% yield with 75% selectivity
to onoglycerides. However, high temperature and a glycerol/triolein molar ratio
of 12 were necessary to reach such results.
In its origin or modified forms, montmorillonite K 10 is a type of acidic stratified
silicate mineral with a three-layer structure with an ideal chemical formula
of (Al2yMgy) Si4O10 (OH)2•nH2O
which widely used as adsorbent (Fan et al., 2006)
as acid catalyst, or as catalyst support (Iqbal et al.,
1988; Choudhary et al., 2004; Bahulayan
et al., 2003; Bokade and Yadav, 2009; Fatimah
et al., 2009). In its natural form, montmorillonite K 10 clay is
Brønsted acidic but it can be easily made basic by treating them with
a basic cation-containing solutions e.g., K2CO3 (Dintzner
et al., 2006), NaOH, KOH and LiOH (Beavers and
Culp, 2004). However, the use of a basic clay catalyst in the glycerolysis
often results in the adsorption of glycerol which can reduce the activity of
catalyst. Aserin et al. (1984) synthesized monoglyceride
by using a Phase Transfer Catalyst (PTC) to overcome the drawback of low solubility
of fatty acid salt in epichlorohydrin. Over 90% of pure non food monoglycerides
had been obtained in quantitative yields after short reaction time at relatively
low temperature. However, it should be noted that monoglycerides prepared by
this method are not food grade. The use of aqueous alkali and PTC in organic
reaction has been reported by some researchers. It was reported that the presence
of PTC was of great importance in the saponification of vegetable oil using
aqueous alkali catalyst (Bhatkhande and Samant, 1998)
and hydrolysis of small amount of lipid using organic basic solution (Woo
et al., 2002). Lopez and Pleixats (1998)
presented that treatment of sultam-derived N-(diphenyl-methylene) glycinate
equivalent to 1 with activated organic bromides and with Michael acceptors under
solid-liquid PTC conditions using potassium carbonate as base, produced monoalkylated
compounds with high diastereo selectivity (>97%). Zhang
et al. (2009) also demonstrated that base-catalyzed transesterification
rate for biodiesel synthesis was enhanced with PTCs such as cetyltrimethylammonium
bromide, tetrabutylammonium hydroxide and tetrabutylammonium acetate. The use
of basic clay with PTC provides a potential enhancement of the clay mineral
properties. The surfactant interacting with surface of the clay particles can
influence the stability and the flow behavior of clay-water system (Alemdar
et al., 2000). Clay mineral presents a proton rich environment when
mixed to organic molecules. On the other hand, the catalytic abilities can be
improved by providing organic cations in the interlamellar space, which enable
the access to/and interact effectively with organic molecules.
Herein, we elucidate the role of TMAOH on a base modified clay-catalyzed transesterification of methyl ester with glycerol. The effects of parameters such as type of calcinations temperature, amount of TMAOH and other variables are thoroughly discussed.
MATERIALS AND METHODS
Reagents and material: Glycerol (99%), Methyl Laurate (ML) (98%) and montmorillonite K 10 used in this study were purchased as commercial products of Fluka. Lithium Hydroxide (LiOH) and Tetramethylammonium Hydroxide (TMAOH) were purchased from Merck. All the above materials were used without further purification. Deionized water was used throughout this work.
Preparation base modified clay: A 250 mL round bottom flask equipped with a reflux condenser was charged with 10 g of Montmorillonite K10 clay, 100 mL deionized water and an amount of LiOH solution. This mixture was stirred and heated at reflux for 6 h. The slurry was then allowed to cool to room temperature. Next, the solid were separated from liquid by centrifugation and the liquid portion was decanted and discarded. The solids were then washed by resuspending in 500 mL of deionized water followed by centrifugation. This sequence was repeated for two more times to ensure complete removal of all soluble species. The base modified clay (Li-K10) was calcined in an oven at 350, 450, 550, 650°C for 4 h to complete the work up.
Characterization of catalyst: Base strengths of the catalysts (H_) were determined by using Hammett indicators. About 25 mg of the catalyst sample was shaken with 5.0 mL of a solution of Hammett indicators diluted with methanol, and left to equilibrate for 2 h. After the equilibration, the color on the catalyst was noted. The following Hammett indicators were used: neutral red (H_ = 6.8), bromthymol blue (H_ = 7.2), phenolphthalein (H_ = 9.3), 2,4-dinitroaniline (H_ = 15.0) and 4-nitroaniline (H_ = 18.4).
Thermal Gravimetric Analysis (TGA) data of K 10 and base modified clay samples were taken using a thermal analyzer (TGA7 Perkin Elmer) instrument by heating the samples at 20°C min-1 heating rate from ambient temperature to 850°C under a high-purity nitrogen flow at 20 mL min-1.
Catalyst test: The transesterification reactions of glycerol with methyl
laurate were accomplished in a glass jacketed reactor equipped with a condenser
system. Glycerol, methyl laurate and catalyst (4% wt) were mixed under magnetic
stirring and heated in a silicone bath to the required temperature in absence
of any solvent. A Dean-Stark instrument was attached to the batch reactor to
remove methanol that was formed during the reaction. After the 5 h reaction,
samples of 100 μL were withdrawn from reaction vessels and transfered into
a sample vial containing 100 μL of water and 100 μL of methyl acetate
using a method adopted from Yang et al. (2003).
The samples were then vortexed, and the organic phase containing acylglycerols
and FA was separated by means centrifugation. To this, 0.1 mL of 0.2 M internal
standard and 0.5 mL acetone were added and then, a 1 μL sample was injected
into a gas chromatograph.
The product composition was analyzed using a Hewlett Packard instruments HP
5800 gas chromatograph equipped with an FID. The column used was Zebron ZB-5HT
Inferno (5%-Phenyl- dimethylpolysiloxane nonpolar, 15 mx0.32 mm i.d.x0, 1 μm
df). The temperatures of the injector and detector were set at 250 and 280°C,
respectively. The temperature of the column was held at 100°C for 0.5 min,
increased to 330°C at a rate of 10°C min-1 and then maintained
at 330°C for 3 min. The split ratio used was 1:5. The degree of transesterification
was expressed as the ratio of the amount of ML consumed in the reaction to the
initial amount of ML before the reaction. The MG and DG contents were expressed
as the sum of the wt. % of their regioisomers. The authentic standards TG, DG,
MG and FA were additionally used for identification.
RESULTS AND DISCUSSION
TGA result: One method for the characterization of basic clay is by
the use of thermal analysis technique. Such technique can be used to evaluate
the thermal stability of the basic clay and the characteristics of the thermal
decomposition process. Changes in the basic clay structure and thermal character
are related to three respective factors i.e., type of interlayer cation, interlayer
cation radius, and the movement of the interlayer cation (Emmerich
et al., 1999). TGA/DTG results of the five clays catalyst are shown
in Fig. 1 and 2. All the clay catalysts
showed an initial weight loss in the temperature range of 30-120°C with
K 10 exhibited a highest weight loss (ca. 11%). The initial weight loss was
first attributed to the volatilization of both adsorbed water from the external
surface and the water residing inside the interlayer space coordinated to the
exchangeable cations such as Na+, Ca2+ and Li+.
Sharp weight drop at lower temperature for K 10 suggested that more water being
physically adsorbed on the surface than that occurring in interlayer while Li
K 10 calcined at 650°C showed least water adsorbed on the surface. This
was due to heating of lithium saturated montmorillonite at a temperature higher
than 500°C that caused the collapse of the interlayers and decreased the
amount of water sorbed (Bujdak and Slosiarikova, 1994).
Furthermore, the acidic or basic character of montmorillonite depends largely
on the nature of both the water of hydration and the exchangeable cation (Azzouz
et al., 2006). The lithium exchanges have neutralized substantially
all of the acid sites (OH sites) within the clay and replaced them with their
conjugate basic sites (O-) and impregnated the clay with cations
from the base so that basicity of montmorillonite increased (Beavers
and Culp, 2004). Calcination of lithium saturated montmorillonite released
small and neutral molecules which are often anhydrides of inorganic acids. The
acid anhydrides then reacted with the hydroxyl groups in the clay affecting
the desired base-modification.
|| TGA thermogram of (a) K10, (b) LiK10-350, (c) LiK10-450,
(d) LiK10-550 and (e) LiK10-650
|| DTG curve of (a) K10, (b) LiK10-350, (c) LiK10-450, (d) LiK10-550
and (e) LiK10-650
Furthermore, the calcination of lithium saturated montmorillonite caused the
migration of the Li+ cations from the interlayer space to vacant
lattice octahedral sites and reduced the net negative layer charge due to fixation
of Li within the layers (Hrobarikova et al., 2001).
So the calcination of Li saturated montmorillonite affected water coordinated
to Li+ ions in the interlayer is bonded more strongly and that water
was liberated at higher temperature (>400°C) (Bujdak
and Slosiarikova, 1994). Calcination at higher than 400°C also caused
the removal of organic materials (Ahmed and Ramli, 2011).
As demonstrated by DTG curves in Fig. 2 that the Li K 10 calcined
at 350°C had higher peak for dehydroxylation of the silicate lattice at
The base strength and the activity of catalyst: Table 1 summarizes the basic strengths of a series of Li-K 10 catalysts calcined at different temperatures. The parent K 10 MMT was an acidic clay as it could not change the colour of bromthymol blue (H_ = 7.2). After the intercalation with LiOH followed by calcination at elevated temperature from 350 to 650°C, the intercalated clay samples exhibited significantly higher base strengths. The base strength of catalyst increased as the calcination temperature was increased, except for the Li-K10 calcined at 650°C (LiK10-650) that showed the weakest base strength (H_) in the range of 7.2-9.3. It demonstrated that the Li-K 10 calcined at 450°C (LiK10-450) had the highest base strength. When the calcination temperature was higher than 550°C, the base strength decreased.
Subsequently, we conducted the catalytic activity tests for the monoglyceride
synthesis at 130°C with an 8:1 molar ratio of the glycerol to methyl laurate.
The catalytic activities data in Table 1 indicate the conversion
of methyl laurate and the selectivity of monoglyceride after transesterification
for 5 h. The experimental result indicated that calcination at 450°C for
4 h was the optimal condition for pre-treatment, in which the highest conversion
of methyl laurate can be achieved. From this table, it can be observed that
the maximum conversion of methyl laurate of 60.1%, was obtained at a calcination
temperature of 450°C. Meanwhile, low level of conversion was observed at
a calcination temperature below 350°C and above 650°C. Obviously, the
basicity changes with calcination temperatures paralleled the change in the
catalytic activity for the glycerolysis of methyl laurate. The data obtained
by using Hammett indicator titration are in good agreement with the catalytic
finding. Thus, the Hammett titration method could give qualitative information
of the basic property of the solid catalysts.
Effect of the amount of TMAOH: The aforementioned results confirmed
that an increase in basicity of the clay catalyst could only promote faster
reaction rate while it did not favour the formation of monoglyceride.
||Catalytic activities and base strength of LiOH intercalated
clay at different calcination temperatures
|Reaction conditions: glycerol/methyl laurate molar ratio,
8:1; catalyst amount , 3%; reaction temperature 130°C; reaction time,
|| Effect of the amount of added TMAOH on the yield of GML and
ML respectively (Reaction condition: glycerol/methyl laurate molar ratio,
8:1; catalyst amount, 3 wt. %; reaction time 5 h; reaction temperature 130°C)
Thus, several experiments were performed in order to observing the influence
of the tetramethylammonium hydroxide (CH3)4 N+OH-)
on the activity of the catalysts. For the monoglyceride synthesis using heterogeneous
catalyst, the reaction mixture was a three-phase system: methyl laurate/glycerol/clay
The presence of TMAOH and the clay catalyst resulted in a synergistic effect in catalyzing the reaction of the methyl laurate with glycerol. The effect of the addition of TMAOH could enhance the effectiveness of LiK10 catalyst on the conversion of methyl laurate and the selectivity of monolaurate. As shown in Fig. 3, the yield of GML increased with an increase in the TMAOH amount from 0.5 to 2.2%.
The enhanced activity observed was attributed to a simple addition of TMAOH and clay activities. It is postulated that TMAOH functions both as a co-catalyst, maintaining the basic environment needed to enhance the activity of the heterogeneous catalyst and as a co-solvent, minimize the mass transfer problem normally encountered in the heterogeneous system.
TMAOH is a strong base solution which has a high activity in transesterification
reaction (Yang et al., 2003).
||Conversion and selectivity as a function of reaction time.
(Reaction conditions: glycerol/methyl laurate ratio, 8:1; LiK10 450 amount
3% (■) with and (▲) without TMAOH 2.2%; reaction temperature 130°C)
However, in the monoglyceride production which is a reverse reaction of the
biodiesel synthesis, the role of TMAOH appeared to suppress the di and triglyceride
formations so that high selectivity of monoglyceride can result. Moreover, the
different criteria such as the chain length, symmetry and lipophilicity could
also determine the activity of PTC. The increased of lipophilicity was due to
the presence of longer chain (Jayasree et al., 1998).
The lower lipophilicity of TMAOH was expected to result in high selectivity
to monolaurate which have a high hydrophobicity than the reactants. Thus, the
reaction and the selectivity of product were greatly enhanced by the addition
We have shown that the presence of TMAOH enhanced the catalytic activity of
LiK10-450. The swelling of montmorillonite immersed in glycerol suggested that
the material had an affinity for this compound so that it was absorbed from
the aqueous solution (Alemdar et al., 2000).
Even though the LiK10-450 was a hydrophobic clay, TMAOH was expected to reverse
the adsorption of glycerol on clay. To check this, we added the TMAOH in the
reaction and compared its activity for the glycerolysis of methyl laurate without
that of LiK 10-450 catalyst (Fig. 4).
In Figure 4, the conversion versus reaction time is presented. It can be seen that the activity catalyst of LiK10-450 with added TMAOH was higher than that of experimental run without TMAOH. For the reaction system with TMAOH, the conversion increased steadily with reaction time ranging between 0 and 1 h and thereafter remained nearly constant as a result of nearly equilibrium conversion. The maximum conversion of methyl laurate was achieved after 5 h. The highest conversion was found to be about 82.5% and the selectivity to monolaurate was about 88.2%.
Based on the modern theories of phase transfer catalysis, the presence of TMAOH
could assist in extracting the laurate anion in the organic phase via liquid
anion exchange mechanism similar with as reported by Aserin
et al. (1984). The initial rapid reaction was due to the initial
presence of tetramethylammonium cation (Me4N+) that would
ion pair in reaction involving hydroxide ion. TMAOH has two important roles
in this kind of reaction (Jwo, 2003). Firstly, it provides
a mechanism for transferring an anion into the organic phase that contains the
reactive substrate. Secondly, and more importantly, the anion is introduced
in a weak solvated resulting in highly reactive state of the reactants to promote
K10 intercalated with lithium hydroxide (LiK10) which was prepared by treating with a saturated aqueous basic solution of LiOH followed by calcination at high temperature in air showed high catalytic activities for the transesterification reaction. Based on the results of this work, the temperature of calcination could significantly affect the base strength and the catalytic activity of LiK10 catalyst. Higher calcination temperature of LiK10 could reduce the base strength and the activity of catalyst. The highest catalytic activity of LiK10 was obtained by calcining the catalyst at 450°C. The presence of TMAOH in the reaction could improve the activity of catalyst. The reaction rate and monoglyceride selectivity were significantly enhanced by the addition of a small amount of TMAOH. In the absence of a TMAOH, the conversion was low as a result of the limited contact of glycerol and methyl laurate.
The Sciencefund from the Ministry of Science, Technology and Innovation (MOSTI), Malaysia, a Research University grant and a short term grant from Universiti Sains Malaysia are gratefully acknowledged.
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