EFFECT OF ACTIVATION PARAMETERS ON CONVERSION IN CLAY- CATALYSED ESTERIFICATION OF ACETIC ACID

Transcription

1 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 05 1 EFFECT OF ACTIVATION PARAMETERS ON CONVERSION IN CLAY- CATALYSED ESTERIFICATION OF ACETIC ACID Igbokwe, P.K a* Olebunne, F.L. b, ; Nwakaudu, M.S. b. a Department of Chemical Engineering, Nnamdi Azikiwe University, Awka, Nigeria. b Department of Chemical Engineering, Federal University of Technology, Owerrr, Nigeria. *Corresponding Author philoigbokwe@yahoo.com ABSTRACT The effects of clay activation method, strength of the activating acid, activation duration and temperature on conversion in esterification reaction were studied using Nigerian montmorillonite clay. The results indicated that acid activation is the best. Activating with 1M acid strength gave the highest conversion of 78.29% at 200 o C activation closely followed by 0.5M with its highest conversion of 74.29% also at 200 o C. This makes 0.5M the better option from economic point of view. Longer activation time gave higher conversion during the medium temperature activation but at high activation temperature, conversion decreased with longer activation duration. From the statistical analysis using ANOVA, the strength of the activating acid had the highest significance while the effects activation temperature and duration were less significant. KEYWORDS: Esterification, clay-activation, clay-catalysis. INTRODUCTION Esters are important class of chemicals which have their applications in a variety of areas such as solvent, plasticizers, pharmaceuticals and intermediates. They have characteristic pleasant, fruity odour that leads to their use in fragrance and flavour industries [1, 2]. Clays are assemblies of tetrahedral layers of silicate units and octahedral layers of aluminates units which result in planar sheets. Thus, adsorbents are constrained to diffuse in two-dimensional space in contrast to three-dimensional reaction volume. This gives rise to increased encounter frequencies between reactants thereby boosting the reaction rate [3]. Clay minerals are acidic in the nature and exhibit ion exchange ability. The acidity of clay can be enhanced by the activation process using acid or acidic metal solutions. Acid clay catalysts are widely used for reactions involving organic chemicals among which is esterification reaction. The use of naturally benign catalysts like clays for chemical reactions is an important aspect of the green chemistry crusade. Their obvious benefits include low cost, ease of separation, reduced waste generations and environmental friendliness. [4, 5]. Natural montmorillonite clays have almost no catalytic activity, but it is relatively easy to convert them into useful catalyst by activating them with acid or cationexchange using polyvalent ions Al 3 + and Cr + 3. The cations can polarize their coordinated water molecules to yield protons in the interlamellar zone [6]. Smectite clays are known for their activity in promoting acid-catalysed reactions like dimerization and polymerization of

2 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 05 2 unsaturated hydrocarbons. They are also used in synthesizing ethers from alkenes [7]. In their research on the activation saponite clay at room temperature and 90 o C using different acid/clay ratios, [8] found that the leaching of magnesium from the octahedral sheets is enhanced by increase in the acid/clay ratio and by increase in the temperature of activation. The type of reagents used in activation, the strength/molarity of the reagents, the temperature of the activation and the activation duration are all believed to have effects on the effectiveness of the clay catalysts. The aim of this research is to elucidate the effects of these activation parameters on a montmorillonite clay catalyst used in esterification reaction. MATERIALS AND METHOD CLAY ACTIVATION Three small samples of the clay obtained from Udi stream in Enugu state, Nigeria, were first activated using thermal, acid and alkaline methods. For the acid activation, the screened clay was treated with a predetermined amount of 1molar strength of sulphuric acid to form slurry which was heated in oven at 100 o C for four hours. The same procedure was used for the alkaline activation using 1molar strength sodium hydroxide. While for the thermal activation, screened clay was heated in oven at 100 o C for four hours without any chemical treatment. The dried activated clays were pulverized and stored in air-tight containers. THE REACTION Each of three activated clay samples was used in esterification reaction to assess their effectiveness. 2.5ml of acetic acid was pipetted into a 10ml stoppered bottle; 0.25g of the clay catalyst was added to it first before 2.5ml of ethanol was pipetted into it to ensure that the active sites of the catalyst were not blocked by the alcohol. The container was tightly closed, the contents shaken vigorously and immersed in a water bath maintained at 323K for 6hours after which the content was titrated with 1MNaOH. The summary of the reaction equation is CH 3 COOH + C 2 H 5 OH CH 3 COOC 2 H 5 + H 2 O The result is shown in Figure 1. SPECIFICATION OF ACTIVATION PARAMETERS Having confirmed from Figure 1 that acid activation gave much higher conversion than the thermal and alkaline methods, other activation parameters were checked which include: activation temperature, activation duration and the strength of the activating acid (molarity). The range of the parameters is:

3 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 05 3 Activation temperature, (x1) Activation duration, (x2) Molarity of acid, (x3) 100 o C o C 3hours - 6hours 0.1M - 1M The three-way Analysis of Variance (ANOVA) arrangement is as shown, X1 X2 3hrs 4.5hrs 6hrs 100 o C 1M 0.5M 0.1M 200 o C 0.5M 0.1M 1M 300 o C 0.1M 1M 0.5M RESULTS AND DISCUSSION A total of 18 clay samples were variously activated and used the esterification reaction. The results are shown in Figures (2) (7) and the ANOVA Table1. a. Effect of activation method From Figure 1, it can be seen that the acid-activated clay gave high conversion while the thermal and alkaline activation only improved the conversion a little above the uncatalysed reaction. This is an indication that the active ingredient of the catalysis is proton (H + ) from the acid. The thermally activated clay showed a better conversion than the alkaline and the uncatalysed. This is confirmation that heating of the clay to remove the interstitial water increases the surface area and its Bronsted acidity. The little improved conversion shown by alkaline activation indicates that while there was a small improvement in the catalytic property of the clay due to drying, the alkaline treatment reduced the acidity of the natural clay.

4 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 05 4 b. Effect of the strength of the activating acid Figures (2) (4) show the conversions recorded after six hours from the three clay samples activated with different molarity of sulphuric acid. In the graphs, the clay activated with very dilute acid (0.1M) has very low conversion about 30%. Conversion improved remarkably for the clay activated with medium strength acid (0.5M) up to 74%. However, doubling the acid strength to 1M only added about 5% to the conversion with 0.5M. These results also show that the proton is the active agent of the activation. The small difference in the conversion between 0.5M and 1M indicates that there is a limit above which increase in the acid strength does not produce corresponding increase in conversion. This may be the point where all the clay s possible active sites have been activated.

5 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 05 5 c. Effect of activation temperature Figures (2) (7) indicate that activating the clay between 100 o C 200 o C has favourable conversion while decline in conversion was noted above 200 o C. Clay catalysts are known to exhibit both Bronsted and Lewis acid sites depending on type of heating chemical treatment. The good conversion obtained at activation temperature 100 T 200 o C confirms that when clays are heated at moderate temperature to remove most of the interlamellar water, the Bronsted acidity is increased while heating at high temperatures, to 200 o C results in the collapse of the clay s interlayer structure which heightens Lewis aciditybut decreases the essential Bronsted acidity

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7 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 05 7 Effect of activation duration From Figures (5) (7),it can be seen that when the clay was activated in temperature range o C, longer heating time improves conversion but above 200 o C,longer heating time becomes unfavourable. This shows that as more interstitial water is driven out during the moderate temperature heating, the Bronsted acidity increases gradually while the resultant collapse of the clay s interlayer structure during high temperature heating brings corresponding decrease in the clay acidity as the heating lasts longer. Thus, if the clay should be activated at high temperature, it should be for a short time, just long enough to drive out the interlamellar water content of the clay. TABLE 1.Analysis of Variance Result using MATLAB Software Source Sum Sq. d.f. Mean Sq. F Prob. (F) X X X Error Total

8 International Journal of Basic & Applied Sciences IJBAS-IJENS Vol: 11 No: 05 8 CONCLUSION The various activation parameters that could affect the effectivness of a clay catalyst for esterification were examined. Acid activation was found to be the best compared with thermal and alkaline activation. The best activation temperature was found to be 200 o C. Longer activation time has good effect below 200 o C but at higher temperature, longer activation time has adverse effect. The strength of the activating acid was found to have the highest impact on the effectiveness of the activated clays. The little difference between the conversion obtained with 1M and 0.5M acid makes 0.5M more favourable from economic standpoint. REFERENCES 1. Kirbaslar, S.I; Baykal, Z.B. and Dramur, U. (2001) Esterification of Acetic Acid with Ethanol Catalysed by Ion-Exchange Resin Turkish Journal of Engineering and Environmental Sciences; Vol.25, Pp Wikipedia Esterification (Retrieved 2007) 3. Laszlo, P. (1990) Catalysis of Organic Reactions by Inorganic Solids, Journal of Pure and Applied Chemistry, Vol.62, No.10, pp Dintzner, M.R; Wucka, P; Lyons, T.W. (Retrieved 2008) Microwave-assisted Synthesis of Natural Insecticide on Basic Montmorillonite Clay Dept. of Chemistry, DePaul University Chicago. 5. Correa, K.S.; Bernin, R.B.; Mattos, M.C.; Aguiar, M.R.; and Guarino, A.W.; (2007) A New Environmental-Friendly Clay Catalyst for One-pot Coiodination and Epoxidation of Alkenes Brazilian Journal of Chemistry Society. Vol.18, No.8, Page www. designer-drug.com/pte /ded/chemistry/mw.clays.txt 7. Adams, J.M; Clement, D.E. and Graham, S.H. (1982) Synthesis of Methyl-t-Butyl Ether from Methanol and Isobutene usinga Clay Catalyst Clays and Clay Minerials, Vol.30, No.2, pp Kooli, F. and Jones,W.(1997) Characterization and Catalyst Properties of a Saponite Clay Minerals Journal of Fine Particle Sciences. Geoscience World, Vol.32, No.4, pp