Esterification of palmitic acid over acid catalyst from modified bentonite.
Esterifications are among the most important organic reactions in industry, because of their ubiquitous applications as intermediate in the synthesis of fine chemicals, drugs, perfumes, food preservatives, and also as energy such as biofuel. The established process using inorganic acid, such as sulfuric acid, p-toluenesulfonic acid (PTSA) and dry hydrochloric acid  passed through a methanolic solution of the carboxylic acid leads to highly acidic waste streams posing environmental problem. Esterification of carboxylic acids by acid solution  has also been demonstrated. However, this process has disadvantage effects. These disadvantages have negative impact on ecoeconomic front, rendering these processes unviable both in terms of ecofriendliness and economy. Alternatives such as cationic exchangers like heteropolyacids supported on carbon , Si[O.sub.2]-[Al.sub.2][O.sub.3] and zeolites , sulphated zirconia and titania  have been developed. The use of clays as solid acids has received considerable attention in this reaction. The use of modified montmorillonite, such as [Zr.sup.4+]-montmorillonite as a catalyst for esterification of carboxylic acids with alcohols ; and [Al.sup.3+]-montmorillonite as a catalyst for etherification of [Me.sub.2]CHOH to [([Me.sub.2]CH).sub.2]O  have also been reported. The etherification of alkanes with different nucleophiles in the presence of metal-ion-exchanged montmorillonite clay catalysts were successfully demonstrated by Ballantine et al .
The modification of bentonite and its study for some uses has attracted much interest in recent year. Bentonite able to growth its pore size based on its surrounding. It cause the bentonite has low adsorption and catalytic selectivity. To increase its selectivity, some modifications of bentonite have been done. One of the modifications is clay intercalation using organic molecules and then the next step is pillarisation process using metal cationic [9-14]. This paper demonstrates the preparation, characterization and catalytic performance test of modified bentonite by intercalation and pillarisation method in esterification of palmitic acid.
The starting clay was a natural Pacitan bentonite, extracted from Pacitan region, East Java, Indonesia. The pillaring agent solution was prepared by mixing NaOH and Fe or Zn which has molar ratio of OH to Fe or Zn = 0.8. Pillarisation and intercalation process of the clay was carried out by mixing bentonite, pillaring agent solution and intercalating agent (hexadecyltrimethylammonium-bromide; HDTMA-Br) with ratio [gram bentonite/ volume of solution] = 1 gram/50 mL. The mixture was heated at 70-75 [degrees]C. After 5 hours, the mixture was cooled and washed with aquadest. The obtained solid was dried and calcined at 500[degrees]C for 4 hours with nitrogen and oxygen stream. All the materials were systematically characterized by Fourier Transform Infra Red (FTIR, Bruker Tensor 27, Germany) and powder X-ray diffraction (Shimadzu XRD 1000).
The catalytic esterification reaction of palmitic acid and methanol was carried out in a batch glass reactor equipped with thermometer and reflux condenser. It was open to the atmosphere and thoroughly stirring with magnetic stirrer. The reaction was conducted at 70[degrees]C for 6 hours. The molar ratio of palmitic acid to methanol was 1:2.5; the weight of catalyst was 0.25 g. The course of the palmitic acid conversion and methyl palmitic selectivity was followed by high performance liquid chromatography (HPLC) by means of Knauer. Conversion is defined as the ratio of consumed phenol over the fed phenol for the reaction. Yield is the ratio of the amount of ester palmitic to the amount of initial palmitic acid used.
Result and discussion
The materials obtained from modification of bentonite were encoded below (see table 1)
Figure 1 shows the comparison between fresh bentonite, intercalated- and pillared bentonite. It shows that there are difference pattern absorption peak in the wave number region of 500-1500 [cm.sup.-1].
The band at 500-600 [cm.sup.-1] corresponds to the pore character of the catalysts. The sharp peak in the fresh bentonite reflects that in natural there is the pore structure in the fresh bentonite, but it does not stabile because of its swelling properties. The similar band at around this wavenumber for PILB-HDTMA Zn and PILB-HDTMA ZnOH (Zn:OH=0,8) were wider and slightly shift. This indicates their pores structure relatively more stabile since the wider peak reflects that the tetrahedral silica structure is formed between two bentonite layers. In the case of PILB-Fe and PILB-HDTMA-Fe, the band at the region of 500-600 [cm.sup.-1] was sharper and had higher intensity than that of Zn-pillared bentonite. It indicates that the pore structure of Fe-pillared bentonite is better and more stabile than that of Zn-pillared bentonite. Phenomenon of pores structure formation is supported by the presence of band at the region of 900-1300 [cm.sup.-1] which is known to be assignable to Si-O-metal species [14,15]. The wider and the shifted band to the higher wavenumber in this region indicate that pillarisation process of the metal oxide is successful. In the case of PILB-HDTMA ZnOH band, it shows that there is new peak formed. It might be because of change of the crystal structure, since there is too much Zn metal penetrated in the crystal. It will cause its thermal stability decrease, so that the pores structure is broken during calcinations process.
[FIGURE 1 OMITTED]
Figure 2 shows that the main peak at 2[theta] of 5.82 reflects the characteristic of bentonite crystalline. This peak tends to be weak and even disappear for Zn-pillared bentonite. This indicates that the crystalline structures of the modified bentonite are not stable, so that the structures tend to be amorphous. This is because of Zn metal penetrated in high amount during pillarisation process. Since Zn has low thermal stability, it causes the crystalline structure broken during calcinations process. The main peak for PILB-HDTMA Fe is slightly shifted to the lower 2[theta]. It indicates that the crystalline structure of this material is stable. From this figure, it can be suggested that the role of HDTMA is to control the penetration Fe metal into bentonite interlayer and to arrange the Fe pillar at the vertical direction.
From figure 1 and figure 2, it can be concluded that when the adsorption peak (FTIR spectra) at wave number of 500-650 [cm.sup.-1] is sharp, then the crystalline structure is good and stable.
[FIGURE 2 OMITTED]
Catalytic activity test for esterification of palmitic acid
For all the reactions, %yield is defined as the ratio of the amount of ester (methyl palmitic) to the amount of initial palmitic acid used. Figure 3 describes the yields of methyl palmitic from esterification reaction over different catalysts.
[FIGURE 3 OMITTED]
As the esterification reaction proceeds via an acyl-oxygen cleavege bimolecular mechanism through a tetrahedral intermediate, it can be expected that a multivalent metal ion could affect the rate of esterification reaction due to transient complexation with the carbonyl group. This paper focus on the exploration of the increment of catalytic performance of the modified bentonite by Fe and Zn pillarisation. The esterification reaction of palmitic acid is shown below:
The palmitic acid yield shown in figure 3 is inline with the FTIR and XRD characterization, which is describe that pillarisation of bentonite using Fe metal causes increasing the pores size and stability of crystalline structure, whereas pillarisation of bentonite using Zn metal causes the destruction of crystalline structure during calcination process since Zn has low thermal stability. It affects the catalytic performance of the materials, i.e. the bigger pores size and the better crystalline structure, the better catalytic performance is. Figure 3 depicts this phenomenon very clearly, i.e. the yields of methyl palmitic from the esterification reactions of palmitic acid over Fe-pillared bentonite are higher than that of the esterification reactions of palmitic acid over Zn-pillared bentonite. In the case of Fe-pillared bentonite, it can be compared two materials based on the presence of intercalating agent (HDTMA). The figure illustrates that the presence of HDTMA causes increasing of methyl palmitic yield. This is also inline with the characterization result, which is describes that the presence of HDTMA might support the penetration of Fe into the bentonite layer to form more stable structure in term of the crystalline and the pores size. In the case of Zn-pillared bentonite, the presence of OH does not affect the catalytic performance of the material compared to the Zn-pillared bentonite without OH.
In conclusion, the work has given a brief phenomenological frame to understand the action of Fe and Zn metal in pillarisation of bentonite. It was found that Fe and Zn have a role to penetrate isnside the bentonite layer to form crystalline and increase the pore size. But Zn has low thermal stability that causes the destruction of crystalline structure during calcinations process. An interesting observation made during these experiments is that the Fe-pillared bentonite show esterification of palmitic acid is catalyzed by the Fe-pillared bentonite. Further characterization and investigation of the nanostructuring of this material play an important role in the chances of understanding and controlling the performance of esterification catalysts via the synthesis and activation of the final catalyst.
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Restu Kartiko Widi (1), *, Arief Budhyantoro (1) and Lieke Riadi (1)
(1) Department of Chemical Engineering, University of Surabaya, TG building 5th floor, Raya Kalirungkut, Surabaya 60293, Indonesia
* E-mail: email@example.com
Restu Kartiko Widi is born in Blitar, Indonesia in 1973. He joined the University of Surabaya at the Faculty of Engineering in 1999. His research topic is in the field of material engineering and catalysis.
Educational background. Restu Kartiko Widi graduated from the Gadjah Mada University, Yogyakarta, Indonesia in 1996 with a B.S. in chemistry. In 1998, he earned a M.Sc. in chemistry from the same University. In 2005, he earned a Ph.D. in Chemistry from the University of Malaya at Kuala Lumpur, Malaysia.
His research includes studies of the preparation and improvisation of natural clays and its performance in adsorption and catalytic activity.
Teaching. Restu Kartiko Widi's teaching at University of Surabaya reflects his multiple areas of expertise. He has taught courses on organic chemistry, physical chemistry, analytical chemistry. In addition, several scientific articles prepared by him and his team has been published, such as High-throughput experimentation for testing of catalytic activity of multi metal oxide catalyst in selective oxidation of propane, Combinatorial Technology as a New Emerging Technology for Chemical Process, Propane Oxidation over Nanostructured Molybdenum-Vanadium Mixed Oxides: Insitu Studies of the Geometric and Electronic Structure, Pillarisasion of Natural Bentonite with Mixed Metal Fe-Al And Its Application in Chromium Ion Adsorption, Effect of HDTMA on Pillarisasion of Bentonite with Metal Fe And Its Application in Copper Ion Adsorption, Modification of Bentonite by Pillarisation and Intercalation and Its Application in Phenol Hydroxylation, Synthesis of Organoclay from Natural Bentonite, The Effect of Diluent and Reaction Parameter on Selective Oxidation of Propane over MoVTeNb Catalyst by using nanoflow catalytic reactor, Hydroxylation of Phenol with Hydrogen Peroxide Catalyzed by Modified Bentonite, Kinetic Investigation of Propane Oxidation on Diluted [Mo.sub.1]-[V.sub.0.3]-[Te.sub.0.23]-[Nb.sub.0.125]-[O.sub.x] Mixed-Oxide Catalysts, etc.
Tabel 1: Catalyst materials code. Code Material Fresh bentonite Natural bentonite unmodified PILB HDTMA-Zn HDTMA intercalated and Zn pillared bentonite PILB HDTMA-ZnOH HDTMA intercalated and Zn-OH pillared bentonite PILB Fe Fe pillared bentonite PILB HDTMA-Fe HDTMA intercalated and Fe pillared bentonite
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|Author:||Widi, Restu Kartiko; Budhyantoro, Arief; Riadi, Lieke|
|Publication:||International Journal of Applied Chemistry|
|Date:||Jan 1, 2010|
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