научная статья по теме KINETIC STUDY OF ESTERIFICATION OF ACETIC ACID WITH METHANOL OVER INDION 190 ACIDIC SOLID CATALYST Химия

Текст научной статьи на тему «KINETIC STUDY OF ESTERIFICATION OF ACETIC ACID WITH METHANOL OVER INDION 190 ACIDIC SOLID CATALYST»

КИНЕТИКА И КАТАЛИЗ, 2015, том 56, № 4, с. 421-429

УДК 541.124:542.951.3:547.292

KINETIC STUDY OF ESTERIFICATION OF ACETIC ACID WITH METHANOL OVER INDION 190 ACIDIC SOLID CATALYST

© 2015 Mekala Mallaiah*, Goli Venkat Reddy

Department of Chemical Engineering, National Institute of Technology, Warangal, 506004 India

* E-mail: mmyadav2001@gmail.com Received 27.08.2014

Esterification of acetic acid with methanol to synthesize methyl acetate in an isothermal well-mixed batch reactor was studied in the temperature range of 323.15—353.15 K. Indion 190 ion-exchange resin was used as a solid catalyst. Feed molar ratios were varied from 1 : 1 to 1 : 4. The influence of temperature, catalyst loading, stirring rate, catalyst particle size and initial molar ratio on the reaction rate was investigated. Experimental results showed that the reaction is kinetically controlled. The sorption experiments were carried out independently to find the adsorption constants. For the constituent components the values of adsorption constants decrease in the order of water > methanol > acetic acid > methyl acetate. The kinetic data were correlated with the pseudo-homogeneous (ideal and non-ideal), Eley—Rideal and Langmuir—Hinshelwood—Hougen— Watson (LHHW) models to determine the kinetic parameters. All the models were suitable to predict the experimental data, but with the LHHW model a more accurate match of the experimental data was achieved.

DOI: 10.7868/S0453881115040127

INTRODUCTION

Organic esters are very important chemicals. There is a wide range of applications of organic esters such as production of cosmetics, plasticizers, pharmaceutical substances, polymers, textiles, flavours and in food industry. Several synthetic processes are available to obtain organic esters. A comprehensive review of esters synthesis is available [1]. Methyl acetate manufactured commercially is in great demand. It is especially useful for the manufacturing nail polish removers, printing inks, perfumery, paints, dyes, industrial coatings and as a solvent in adhesives.

Methyl acetate is produced by the esterification reaction between the acetic acid and methanol. At room temperature, the reaction is very slow and reversible and several days are usually required to attain equilibrium in the absence of the catalyst. The addition of the catalyst increases the reaction rate and therefore decreases the time needed to reach an equilibrium state. One can discriminate between heterogeneous and homogeneous catalytic reactions. Homogenous catalysis occurs when the catalyst and the reactants are both in the same phase while in the case of heterogeneous catalysis the catalyst and the reactiants are in different phases. Homogeneous catalysts, such as HCl, HI, H2SO4, and HBr, provide an acid medium. Ion-exchange resins are frequently used as heterogeneous catalysts. Heterogeneous catalysts are preferable to the homogeneous catalysts due to several advantages like easy separation of catalyst from the post reaction mix-

ture, better selectivity towards desired product, high purity of the product due to suppression of side reactions and elimination of the corrosive environment [2].

One of the earliest works relating to kinetics of catalytic esterification of acetic acid with methanol was published by Rolfe and Hinshelwood [3]. Ronnback et al. [4] investigated the kinetics of esterification of acetic acid with methanol using a homogeneous hydrogen iodide as a catalyst. It was observed that hydrogen iodide also reacted with methanol and produced methyl iodide as a by-product. Agreda et al. [5] proposed a rate expression for the esterification reaction in which sulphuric acid was used as a homogeneous catalyst.

Many solid catalysts were used, such as solid acids and bases, ion-exchange resins, zeolites and acid clay catalysts. Ion-exchange resins are the most common heterogeneous catalysts used for esterification reaction [6—8]. These ion-exchange resins not only catalyse the reaction but also improve conversion because of selective adsorption of reactants and swelling nature [9, 10]. In the heterogeneous catalysis, the active solid surface can distort or even dissociate an absorbed reac-tant molecule and increase the rate of reaction [11].

Most of the esterification reactions were studied by using the solid catalyst Amberlyst 15 [12—18]. Liu et al. [19] investigated the similarities and differences between heterogeneous and homogeneous catalysed es-terifications. They studied the kinetics of acetic acid esterification with methanol using a commercial Nafion/silica nano composite catalyst (SAC-13) and

Table 1. Physico-chemical properties of Indion 190 catalyst

Physical property Indion 190

Manufacturer "Ion Exchange India Ltd."

Shape Beads

Physical form Opaque, faint dark grey coloured

Size, 725

Apparent bulk density, g/cm3 0.55-0.60

Surface area, m2/g 28-32

Pore volume, ml/g 0.32-0.38

Operating temperature T °C J max' ^ 150

Hydrogen ion capacity, meq/g 4.7

Matrix type Styrene-DVB

pH range 0-7

Resin type Macroporous strong acid cation

Functional group SO-

Ionic form H+

H2SO4. The kinetic behaviour of esterification of acetic acid with methanol was investigated by Tsai et al. [20], who used Amberlyst 36 as a solid catalyst in a packed-bed reactor in the temperature range of 313.15—328.15 K with the molar ratio of methanol to acetic acid varied from 1 to 5.

In the present work, esterification of acetic acid with methanol was studied in the presence of the solid acid catalyst Indion 190. This catalyst has received little attention. For the first time adsorption of acetic acid, methanol, methyl acetate, and water on Indion 190 catalyst surface from binary mixtures was conducted. For the first time the swelling of Indion 190 catalyst in the presence of acetic acid, methanol, methyl acetate and water could be described. Moreover, in the present study ideal and non-ideal based kinetic models were used to correlate the experimental data. The effect of various parameters like stirring rate, size of catalyst particle, reaction temperature, reaction time, catalyst loading and initial reactant concentration on the esteri-fication was studied. Four types of kinetic models, pseudo-homogeneous (ideal and non-ideal), Eley—Rideal (ER) and Langmuir—Hinshelwood—Hougen—Watson (LHHW) models were evaluated and the best kinetic model was proposed for the esterification reaction.

EXPERIMENTAL

Chemicals

Methanol (purity of 99% w/w) and acetic acid (purity of 99.95% w/w), supplied by "SD Fine Chemicals Ltd." (India), were used in the present study.

Catalyst

The solid acid catalyst, Indion 190, used for the esterification reaction was supplied by "Ion-Exchange India Ltd.". Indion 190 has cross-linked three-dimensional structures of polymeric material, obtained by sulfonation of a copolymer of polystyrene and divinyl benzene (DVB). It is an opaque and faint dark grey coloured solid spherical bead. The ion-exchange resin was dried for 2 h in an air oven at temperature 363.15 K to remove the moisture. The physico-chemical properties of the solid ion-exchange resin catalyst are shown in Table 1.

Experimental setup

The esterification reaction was carried out in a 500 ml three neck round-bottom flask. The flask was placed in a heating rota mantle, which contained a heating knob, stirrer and a speed control knob. The rotational speed of magnetic stirrer was varied from 240 to 640 rpm, using the speed control knob. A spiral condenser was connected to the reaction flask vertically to condense the vapours and mix them back with the reacting mixture. A mercury thermometer was inserted into the flask to measure the temperature of the reaction mixture. The accuracy of the thermometer is within ±0.5 K.

Experimental procedure

The reactants were weighed using a digital electronic balance with an accuracy of ±0.0001 g. In the experiment, equimolar quantities of methanol (32 g) and acetic acid (60 g) were mixed and charged to the reactor. The reaction mixture was heated and when it reached the desired temperature, the catalyst was added to the mixture and the time was noted (t = 0). The temperature was measured by mercury thermometer within an error of ±0.5 K. Samples of the reaction mixture were withdrawn at regular intervals of time. The samples were placed in a refrigerator prior to analysis, to prevent further reaction. The reaction mixture was analyzed by Gas Chromatography (GC) for components of the mixture.

The following reaction takes place in the reacting mixture.

CH3COOH (A) + CH3OH (B) ^ ^ CH3COOCH3 (C) + H2O (D). Water is also formed along with methyl acetate.

Sorption equilibrium and swelling experiments

The sorption experiments were carried out for 4 nonreactive binary mixtures at a constant temperature of 298.15 K. The nonreactive mixtures studied were: water—methanol, water—acetic acid, methyl acetate— methanol, and methyl acetate—acetic acid. These experiments were carried out according to the procedure proposed by Popken et al. [12]. The binary sample of known quantity (a total of 10 g) was mixed with the known quantity of catalyst (1.0 g) in 20 ml glass vials. After reaching an equilibrium state in about 2—3 weeks, the samples were analysed using gas chromatography. The difference between the initial and final quantity of binary mixture in the vial was taken as the quantity of adsorbed material.

To find out the swelling nature of the catalyst in the presence of pure components like methanol, acetic acid, methyl acetate and water, experiments were conducted at a constant temperature of298.15 K in sealed graduated cylinder of volume of 20 ml. A known amount of dry catalyst (1.0 g) of known particle diameter was placed in the glass cylinder followed by adding a pure component. The experiments were carried out for each component separately. The solid catalyst and the pure component were allowed to remain in contact with each for about 2—3 weeks, till equilibrium

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