Accepted Manuscript Ultrasound Assisted Synthesis of Methyl Butyrate using Heterogeneous Catalyst P.N. Dange, A.V. Kulkarni, V.K. Rathod PII: DOI: Reference:
S1350-4177(15)00046-2 http://dx.doi.org/10.1016/j.ultsonch.2015.02.014 ULTSON 2802
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Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
12 November 2014 11 February 2015 27 February 2015
Please cite this article as: P.N. Dange, A.V. Kulkarni, V.K. Rathod, Ultrasound Assisted Synthesis of Methyl Butyrate using Heterogeneous Catalyst, Ultrasonics Sonochemistry (2015), doi: http://dx.doi.org/10.1016/ j.ultsonch.2015.02.014
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Ultrasound Assisted Synthesis of Methyl Butyrate using Heterogeneous Catalyst P. N. Dange, A.V. Kulkarni, V. K. Rathod*
Department of Chemical Engineering, Institute of Chemical Technology, Matunga (E), Mumbai - 400019, India.
*Corresponding author Dr. V. K. Rathod Department of Chemical Engineering, Institute of Chemical Technology, Matunga (E), Mumbai400019 (India) E-mail: [email protected], Phone: +91-22-33612020, Fax: +91-22-33611020.
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Abstract Ultrasound assisted esterification of butyric acid with methanol was investigated in an ultrasound irradiated isothermal batch reactor using acid ion-exchange resin (amberlyst-15) as a catalyst. Effect of parameters, temperature (323-353 K), catalyst loading (0-8.5 % w/w), alcohol to acid ratio, M (2-6), ultrasound power (0-145 W), duty cycle (0-85 %) and amount of molecular sieves added (0-11 % w/w) on the rate of reaction was studied. At optimized parameters, a maximum conversion of 94% was obtained in 120 minutes in presence of ultrasound. Experimental kinetic data were correlated by using Eley-Rideal, and Langmuir-Hinshelwood-Hougen-Watson Models taking into account reverse reaction. Studies showed that single site LHHW with reactants and products both adsorbing on catalyst surface was most suited for the obtained experimental data. Activation energy determined based on heterogeneous kinetics was in the range 49.31-57.54 kJ/mol while it was 18.29 kJ/mol using homogeneous model.
Keywords: Ultrasound, kinetics, esterification, butyric acid, methanol, Amberlyst, kinetic model
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1. Introduction Organic esters are used widely in the manufacturing of flavors, adhesives, pharmaceuticals, pesticide, plasticizers, polymerization monomers, emulsifiers in the food and cosmetic industries. Derivatives of many esters form variety of useful chemical intermediates and monomers for high molecular weight polymers and resins. In general, esters can be defined as the products of reaction between carboxylic acids and organic alcohols. Chemically, an ester is formed as the condensation product when a carboxylic acid is reacted with an alcohol [1]. Esterification of carboxylic acids with alcohols represents a well-known class of liquid-phase reactions of considerable industrial interest owing to its wide industrial uses [2-3]. Methyl butyrate or methyl ester of butyric acid is an ester with a fruity odor of apple, pineapple, and strawberry. Present in trace amounts in several plant products, especially pineapple flavor is produced by distillation of oils from vegetable origin. This ester is also manufactured on a small scale for use in perfumeries or flavoring food [4, 5]. Organic esters can be synthesized by several routes, most of which have been briefly reviewed by Yadav and Mehta [5]. The traditional route for preparing esters is by the reaction of the carboxylic acid with an alcohol using homogeneous catalysts such as sulfuric acid or paratoluene-sulfonic acid [6, 7]. Although, homogeneous catalysts provide faster reaction rates, separation and recovery of the catalyst from the reaction mixture is major issue [6, 8]. In addition, the homogeneous acid catalysts are responsible for equipment corrosion, side reactions and additional neutralization cost of treatment of salt produced [9-10, 13]. These limitations of homogeneous catalysts can be circumvented by the use of heterogeneous catalysts such as sulphonic acid based ion-exchange resins. The solid type of material has good physical and chemical properties with enhanced catalytic activity in esterification [3, 7, 11]. Solid catalyst can
3
be mechanically separated from the reaction mixture and subjected to possible reuse [12]. Heterogeneous route offers promising option owing to its advantages as compared with the methods using homogeneous catalyst. Significant work is reported in the literature using variety of solid acid catalysts [7, 11, 14-15]. Despite advantages of heterogeneous method for the synthesis of organic esters, the reaction time is a concern which can be substantially reduced by using newer techniques. Use of ultrasound waves and microwaves in ester synthesis is still an evolving area and detailed studies are needed to shed some light on their applicability in heterogeneous catalysis. Therefore, an alternative esterification method is required to reduce processing time, to lower amount of catalyst and unreacted raw materials also to increase the mass transfer. The use of cavitation resulting from ultrasound waves appears to be an appealing option for heterogeneous synthesis of ester. Ultrasound (frequency ~20 kHz to l0 MHz) is cyclic sound pressure with a frequency greater than the upper limit of human hearing [16]. The main principle behind the application of ultrasonic waves to the reaction is a phenomenon known as cavitation. The ultrasonic waves technique has two cycles compression and rarefaction, during rarefaction a vacuum pressure creates a cavitation bubble, when the compression cycle occurs the bubble implodes in a very short period of time, producing localized heating, high pressures and liquid jet sprays with high velocities[17]. The ultrasound assisted techniques are playing an important role in chemical processes, especially in cases where conventional methods require extended reaction times. Applying Ultrasound to the chemical reaction process enhances mixing, shearing, transfer of materials and the rate of chemical reactions effectively reducing the reaction time [18-19, 20]. Use of ultrasound irradiation is a promising and untapped technology for chemical reactions, though it was in use for last few decades. It can improve the reaction conditions, accelerate the
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rate of reaction and produce a higher yield [21-22]. Recently, synthesis of methyl butyrate was carried out using conventional heterogeneous route with amberlyst-15 as catalyst [11]. The equilibrium conversion reported was greater than 90 % using amberlyst-15 catalyst under optimal reaction conditions. However, the time required to achieve this equilibrium conversion was longer (about 4 h) which needed to be reduced further. The objectives of this study were to test the suitability and efficacy of heterogeneous catalyst in presence of ultra sound irradiation for the synthesis of methyl butyrate and determine suitable kinetic model. Effects of time, temperature, catalyst loading, alcohol to acid ratio, addition of molecular sieves, ultra-sound power and duty cycle on esterification reaction were also undertaken.
2. Materials and Methods 2.1 Materials Butyric acid and methanol of 99.98% purity (w/w) were supplied by Merck. Both these chemicals were used as supplied. The acidic ion exchange resin (Amberlyst-15) was supplied by Alfa Aesar, USA. A cation exchange resin catalyst is insoluble polymer matrix which can exchange ions with the adjacent reacting mixture. It is a macro-porous type styrene-DVB (20%) resin. The resin is formed by the copolymerization reaction between styrene and divinyl-benzene which acts as cross-linking agent. For cation exchange resins, acid sites are deposited on the polymer matrix by the treatment of strong acids such as sulphuric acid which gives sulphonated cation exchange resins.
2.2 Batch Experiments
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Esterification reactions were performed in a 100 mL batch reactor made of borosilicate glass. The three necked reactor was equipped with sample port, provision for ultrasound probe and condenser to recycle back liquid after vapors condensed during reaction runs. Water bath fitted with automatic temperature control was used to ensure constant reaction temperature. Sound waves (by ultrasonic horn) generated at 22 kHz were used to ensure adequate mixing of reaction contents. Ultrasonic horn was supplied by Dakshin Ultrasonics, Mumbai, India with maximum power rating of 120 W, input supply voltage 230 V (50 Hz AC). Vertically mounted probe tip was made up of SS 304 with 11 mm diameter and 10 cm height. In a typical experiment, a measured quantity of methanol and catalyst (amberlyst-15) were charged into the reactor with no initial water present in the reaction mixture. The reactor was sealed and heating was started with condenser water on. To optimize parameters, ultrasound generator was used on varying power inputs and duty cycles. The type of reactor and ultrasonic probe distance from the surface of reaction mixture was used based on earlier reported work so as to ensure maximum cavitation (bubble generation) [23]. Thus, ultrasound probe was adjusted in such a way that the tip length of 0.5 cm dipped into the reaction mixture from the top liquid surface [23]. Once the desired temperature was reached, the required amount of butyric acid was injected into the reactor via syringe and sonication was initiated - the instant was marked as zero reaction time. Small amount of samples (0.2 mL) were withdrawn at specified time intervals over the first 2.5 h of reaction. Reaction mixture was filtered in the end to separate the sample solution from the used solid catalyst. All experiments were performed three times and average values have been reported with standard deviations.
2.3 Electrical Acoustic Intensity and Dissipation rate of Sound Energy
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Electrical acoustic intensity (I) can be defined as the ratio of electrical energy supplied to the cross sectional area of probe tip.
I (W/cm2)= π
(1)
Where Ei is the input electrical energy (W), r is the radius of the probe tip (cm). Calorimetric studies were performed to determine the dissipation rate of sound energy from the ultrasound probe tip. Fixed quantity of methanol (50 mL) was taken in a reaction vessel with tip of US horn dipped optimally into it. Measured quantity of input electrical energy was supplied to ultrasound generator. Strictly adiabatic conditions were ensured during the experiments. Dissipation rate of sound energy in the bulk of liquid was calculated by measuring the rate of change of temperature of liquid methanol as given by the following expression:
=
(2)
Where, m is the mass of methanol, Cp is average specific heat at constant pressure and dT/dt is the time rate of change o temperature.
2.4 Analysis: 2.4.1 Gas Chromatographic Analysis Samples from the reaction were taken in regular time intervals to ascertain the formation of butyric acid. Gas chromatograph (Chemito GC, Model 6890) was used to analyze liquid samples for a quantitative determination of methyl butyrate formed in the esterification reaction. GC was equipped with two detectors, a thermal conductivity detector and a flame ionization detector, connected in series. Use was made of a 30-m long HP-Innowax column (Polyethylene glycol 320 micrometers in diameter with 0.5 micrometer in thickness) with a temperature programmed analysis. Nitrogen was used as the carrier gas. 7
2.4.2 Titrimetric analysis Acid value is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance. The acid values (KOH mg/g) were determined by a standard titrimetry method. Samples from the reaction mixtures were titrated for the total acid content using 0.1 N alcoholic KOH with phenolphthalein indicator and methanol as a quenching agent. The Methyl butyrate obtained was expressed in terms of percent (%) conversion based on butyric acid consumption. The conversion obtained using both the analysis was similar with only 2% deviation. 3. Kinetic Modeling Kinetic modeling describes the rate of reaction as a function of reaction variables. Esterification reactions catalysed by solid acid exchange resins can be expressed by different kinetic models. Three models commonly subjected to correlate kinetic data are Pseudo-Homogeneous (PH) model [24], the Langmuir-Hinshelwood-Hougen-Watson (LHHW) model and the Elley-Rideal (ER) model [25]. Pseudo-Homogeneous model does not take into account the adsorption effects of the species in the reactant medium on the surface of catalyst. It has been successfully used in high polar reaction media [26]. A generalized esterification reaction is written as:
A + B E + W
(3)
Where, A and B are the reactants (carboxylic acid and alcohol), while E and W are the products (ester and water) of the reaction. The notation, kf is forward reaction rate constant whereas kr represents the rate constant for reverse reaction.
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3.1 Homogeneous Reaction Model: The pseudo-homogeneous model does not take into account the sorption effect on the catalyst sites in a reacting medium [6, 27]. If the reaction mixture is considered as a homogeneous liquid phase, outcome by the Pseudo-Homogeneous model could be considered as a satisfactory tool to correlate the esterification of butyric acid with methanol in the presence of amberlyst-15 as a catalyst. For the esterification reaction (3) where one of the reactants is in excess, the PH model can be represented by the following expression [28]. (– )
ln ( – ) = k C"# (M– 1)t
(4)
At equilibrium, '– r" ) = C"#
* *+
= k C" C, – k C C- = 0 and thus, the equilibrium constant
(Ke) can be calculated from:
K 0 = = ( 3
1 2 2 )(3 2 )
(5)
xAe is the conversion of butyric acid at equilibrium.
3.2 Heterogeneous Models In heterogeneous kinetic studies, the generally encountered problem is complicated rate expressions which may obey Langmuir-Hinshelwood-Hougen-Watson (LHHW) or Eley-Rideal (ER) model [29]. These reaction mechanisms can be of single site or dual site type. In single-site mechanism, the reactant is adsorbed on only one site whereas in dual site mechanism, reactant adsorbed on one site interacts with another site to form product. The LHHW and the ER models both include the adsorption effects of the species in the reactant medium. The basic assumption of LHHW model is that all reactants are adsorbed on the catalyst surface before chemical
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reactions occur. The ER model assumes the reaction taking place between adsorbed and nonadsorbed reactants in the bulk of liquid. The ER and LHHW models based on single site and dual site mechanisms have been reviewed by Ozdemir and Gultekin [30]. For the reversible reaction of butyric acid with methanol, considering adsorption effects of the reactants and products, we may have possible rate expression given by equation. (−r" ) = k 5
(6 7 68 78 369 79 6: 7: /6< ) ' =∑? @AB 6 7 )
C
--- (6)
Where, p=1 for single site mechanism; p=2 for dual site mechanism; i=1, 2, 3 and 4 for the esterification reaction; n=4 is the number of species involved in esterification reaction. For Eley-Rideal model, reactant A is adsorbed on catalytic sites and reacts with reactant B in the bulk liquid phase. In this case denominator becomes (1+ KACA) in equation (6). LangmuirHinshelwood- Hougen-Watson model can be based on reactants and products adsorbing on catalyst surface where reaction takes place. In first case, adsorption of product species is ignored and reaction takes place between A and B adsorbed on the surface of the catalyst. In these circumstances, denominator in equation (6), ignores product terms. In other case, LHHW model takes into account the sorption effects of product species along with reactants and equation (6) will be applicable in its entirety. Index p will be 1 or 2 depending upon the single site or dual site mechanism for all heterogeneous models.
4. Results and discussion 4.1 Effect of reaction temperature The effect of temperature on ultrasound assisted esterification between methanol and butyric acid was studied by conducting the reactions at 323 K, 333 K, 343 K and 353 K. The reaction was 10
performed at methanol to butyric acid ratio, M = 3, catalyst loading 6.5 % (w/w), added molecular sieves 8.5 % (w/w). The power input to the ultrasound horn was at 100 W with duty cycle of 75 %. The effect of temperature on conversion is shown in Fig.1. Conversion was found to be increasing markedly as reaction temperature varied from 323 K to 343 K. At low temperature diffusion phenomena is dominant owing to high cavitation intensity as at low temperature rate of bubble generation is low but they collapse with high intensity and hence more mass transport of the reactants on the catalytic sites. [31]. Temperature increase reduces the viscosity and surface tension of the reaction mixture which affects bubble formation and its subsequent collapse [32]. Higher temperature gives rise to more bubbles and frequent collisions with reduced intensity which causes limitation to transport of reactants from bulk to catalyst surface resulting into no significant rise in conversion [33]. Further at elevated temperatures, vapour pressure of the reactants increases and cavitation bubbles are filled by more vapor of the reactants causing low intensity cavitation. The maximum equilibrium conversion of 91.64 % was obtained at a temperature of 343 K. From the Fig. 1, it is seen that the equilibrium conversion decreased from 91.64 % to 70.65 % when temperature was raised from 343K to 353 K. This reverse trend is attributed to reduced cavitation intensity at temperature beyond 343 K which resulted in lower conversion. The other possible reason for lower conversion beyond 343 K is non-availability of methanol in the liquid reaction mixture due its evaporation [11, 34]. Therefore, the optimum reaction temperature was taken to be 343 K at which the conversion of butyric acid reached a stable value of 91.64 %.
4.2 Effect of Alcohol to Acid Ratio
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Theoretically, the esterification reaction requires one mole of methanol for each mole of butyric acid for the synthesis of methyl butyrate. However, in practice, the methanol should be in excess to drive the reaction towards completion as the esterification reaction is reversible. In order to study the effect of methanol/butyric acid molar ratio on esterification, different experiments were carried out using varied molar ratios of alcohol and acid. The mole ratio of methanol to butyric acid was varied from 2 to 6 and the results obtained are plotted in Fig.2. The maximum conversion of 91.6 % was obtained for an alcohol to acid molar ratio (M), 3 with a catalyst loading of 6.5 %, temperature of 343 K and added molecular sieves 8.5 %. The input power to the sonicator was 100 W with duty cycle of 75 %. Experiments show that with molar ratio of butyric acid to methanol of 1:3, maximum conversion was achieved in 100 minutes and remained constant thereafter over an extended reaction time. As seen in the Fig. 2, conversion increased from 84 % to 91.6 % as M is increased from 2 to 3 and decreased with further increase in alcohol to acid ratio, M. It can be reasoned that at the lower value of M, the effective viscosity of the reaction mixture is high and hence the effect of cavitation is minimal. This leads to scarce diffusional mass transport of reacting species on active catalyst sites hence limiting conversion. As the ratio M is increased up to 3, presence of more methanol lowers effective viscosity of the reaction mixture resulting into enhanced cavitation. This increase in the cavitation intensity gives rise to high turbulence in the reactor and diffusion of reacting species inside catalyst pores is dominant phenomena [35]. As the alcohol to acid ratio is increased beyond 3, conversion shows marginal decrease despite increased cavitation because of decreased viscosity owing to even higher amount of methanol present. At this stage, excessive bubbles are likely to coalesce and form larger and more stable bubbles. This creates a barrier to dissipative
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energy transfer hampering effective diffusion of reactant species and thus affecting the conversion marginally.
4.3 Effect of Catalyst Loading The effect of catalyst loading on the conversion of the butyric acid is shown in Fig.3. Each new run was performed using fresh resin catalyst. As expected, the conversion increases with an increase in the catalyst loading from0 to 6.5 % (w/w) due to more available active sites, resulting in higher reaction rate. However, there is decrease in the conversion of butyric acid when the catalyst loading is increased from 6.5 % to 8.5 %. The impact of cavitation is dictated by the properties of reaction mixture and ultrasound intensity. Viscosity of reaction mixture increases with its solid catalyst concentration hampering effective cavitation at higher catalyst loading [33]. It is also possible that increased catalyst content might hamper and reduce the ultrasound intensity in certain areas of reactor where effects of turbulence are not pronounced. Hence higher catalyst content affects sonication intensity, and cavitation bubbles may not develop in those areas in the reaction medium [37]. A higher loading of catalyst also results in reduction of the time required to reach the reaction equilibrium [34, 38]. It is also observed that the conversion curve is steep in the first 10 minutes indicating that rate of reaction is higher initially as the more catalyst sites are available for the unconsumed reactants and cavitation is effective owing to lower viscosity of the reaction medium. It can be reasoned further that the impact of cavitation bubbles is governed by the properties of reaction mixture and ultrasound. The propagation of ultrasound waves in a reaction medium is accompanied by unavoidable loss through attenuation, adsorption and dissipation. The behaviour of ultrasound dissipation would depend on the nature and extent of medium homogeneity. When the catalyst concentration increases from 0 % to 6.5 %, conversion 13
increased steadily up to more than 91 %. But as the catalyst % increased to 8.5 %, equilibrium conversion reduced to 89 % along with sluggish kinetics during initial period. This could be because of ultrasound energy loss due to attenuation and adsorption of ultrasound waves owing to higher solid catalyst content. This results in lower dissipation of ultrasound energy in the reaction and thus reduces the cavitational intensity. This led to insufficient turbulence to warrant uniform mixing of reactor contents resulting into reduced conversion. Addition of further catalyst mass beyond 6.5 % dampens the cavitational effects owing to hindrance to effective turbulence throughout reaction mixture resulting in decreased conversion. Hence the optimum catalyst loading was taken as 6.5 % and further esterification runs were carried out using this value.
4.4 Effect of Addition of Molecular Sieves Water is one of the products formed during the esterification of butyric acid with methanol. Thus, the water content is another important parameter in esterification reactions. As the reaction progress, the water formed as product during reaction increases. Reaction may proceed in the reverse direction if the water formed during reaction is not separated forthwith. Addition of molecular sieves or silica gel adsorbes water formed during reaction and usually improves the equilibrium conversion [39-40]. In order to study the influence of water produced in the reaction, molecular sieves (3 Å type) were added in the range of 0-8.5 % (w/w). The experiments were carried out at ultrasound probe rating of 100 W with 75 % of duty cycle, catalyst loading of 6.5 % and a temperature of 343 K. Fig.4 shows that the conversion of methyl butyrate increased with increasing amount of molecular sieves. This could be because of the adsorption of produced water in the pores of
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molecular sieves and hence inhibition of the reverse reaction. The optimum value of molecular sieves added to reactor was 9.5 % beyond which there was no significant increase in conversion since the amount of water generated remains essentially same even though the molecular sieves added has more saturation capacity. When the reaction is carried out without adding molecular sieves the conversion obtained was at the minimal value indicating the presence of reverse reaction. This shows that water formed during the reaction needs to be separated in order to keep reaction going in forward direction.
4.5 Effect of Power Experiments were performed by varying input power from 50-145 W to identify the optimum power needed in order to achieve efficient cavitation for the esterification. Other parameters were kept at: alcohol to acid ratio of 3, catalyst loading of 6.5 % (w/w), reaction temperature of 343 K, added molecular sieves 8.5 % and duty cycle of 75%. As can be seen from the Fig. 5, conversion increases drastically by varying power from 50 W to 100 W. Calculated electrical acoustic intensities were 52.63, 78.95, 105.27, 131.59 and 152.65 W/cm2 for the input powers of 50, 75, 100, 125 and 145 W respectively. Electrical acoustic intensity increases with the increase in power consequently causing cavitation in more pronounced manner. This leads to more turbulence because of high rate of bubble generation at sub-microscopic scale giving rise to more interfacial area [41]. More interfacial area paves the way for increased diffusion for the reactants on the catalytic sites giving higher conversion. However, an increase in power beyond 100 W, resulted into mere liquid agitation (mixing) instead of cavitation. This attributed to poor propagation of ultrasound waves through the reaction mixture resulting in lower conversion [42]. When a large amount of ultrasonic power enters a system, a much larger quantity of ultrasonic
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cavitation bubbles are generated in the reaction mixture. Excessive bubbles are likely to merge and form larger and more stable bubbles and, thus, create a barrier to acoustic energy transfer [43].
4.6 Effect of Duty Cycle Duty cycle expressed in percentage indicates ratio of ultrasound probe ON time and that of ON and OFF time taken together. Duty cycle of 75 % pertains to ON time of 9 seconds and OFF time of 3 seconds. To investigate the effect on reaction conversion, duty cycle was varied from 25 % to 85 %. The other conditions were kept at the optimum values of temperature 243 K, alcohol to acid mole ratio of 3:1, molecular sieves 4 g (8.5 % w/w), catalyst loading 6.5 %, power 100 W. Effect of duty cycle on conversion of methyl butyrate are shown in Fig.6. As can be seen in the figure, when the ultrasound duty cycle varied from 25 % to 75 %, equilibrium conversion increased from 72 % to 91 %. This shows that the exposure time of ultrasound irradiation to the reacting mixture is increased subsequently increasing product formation. Although moderate to higher duty cycle of sonication employed improves the conversion, its use for extended period of time without pulse can damage the transducers which generate ultrasonic waves [44]. On further increase of duty cycle from 75 % to 85 %, the conversion is seen to be decreasing from 91.64 % to 81 %. This decrease in conversion can be attributed to excessive bubbles generated due to almost incessant operation of ultrasound generator. These bubbles are likely to coalesce and form larger and more stable bubbles. This creates a barrier to dissipative energy transfer inhibiting effective diffusion of reactant species [43]. Hence the optimum duty cycle was taken at 75 %.
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4.7 Estimation of Kinetic Parameters The kinetic data of esterification reaction was correlated with three kinetic models: PH model, the ER model and the LHHW model according to equations given in section 3. Firstly, the pseudo-homogeneous model was applied and plots were made using equation (4) to determine the slope of the line from which the values of kf were calculated (Table 1). These plots are as shown in the Fig.7. The dependence of the forward rate constant on the reaction temperature is described by the Arrhenius rate law, as given in Eq. (7): E9F
k = k # e GH
(7)
Where, k0 is the pre-exponential factor, Ea is the activation energy, R is the ideal gas constant and T is the reaction temperature. The parameters of the Arrhenius equation, activation energy and frequency factor were determined for the forward reaction. The data of ln (k) versus 1/T were fitted by linear regression and the result is plotted in the Fig.7. The Arrhenius plot of the rate constant for forward reaction in the temperature range of 323-343 K gives the regression coefficient of 0.99. The slope could be applied to calculate the activation energy. Calculated activation energy for the synthesis of methyl butyrate is 18.29 kJ/mol. This moderate value of Ea based on homogeneous kinetics shows that the conversion rate is competitively controlled by the reaction kinetics and the mass transport phenomena. Since the esterification reaction between butyric acid and methanol involves solid catalyst phase and the liquid phase reactants and products, the reaction is termed as heterogeneous type. The estimation of kinetic parameters for heterogeneous esterification is complicated owing to the complex surface-adsorbate interactions at catalyst surface. The values of surface reaction rate constant (ks), surface equilibrium constant, KS and adsorption equilibrium constants (KA, KB, KE,
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and KW) were determined by nonlinear regression using Microsoft EXCEL. The experimental data was subjected to models of type ER (Eley-Redeal), LHHW (Langmiur-HinshelwoodHougen-Watson) with reactants adsorbed and LHHW with reactants and products both adsorbed on the surface of the catalyst to find best fit. Surface reaction rate constant and adsorption equilibrium constants were calculated by minimizing sum of the squared value of the difference between experimental and predicted rates. The expressions for single site ER and LHHW type models are given in equation 8-10 [28-29]. (−rI0* ) = k 5 (−rI0* ) = k 5
J K J K 6 68 7 78 3 9 9 : : J<
( = 6 7 ) J K J K 6 68 7 78 3 9 9 : : J<
( = 6 7 = 68 78 ) J K J K 6 68 7 78 3 9 9 : :
(−rI0* ) = k 5 ( = 6
J<
7 = 68 78 = 69 79 = 6: 7: )
(8)
(9)
(10)
The values of kinetic and surface constants are summarized in table 2. Estimated rate parameters were further used to predict the rate of reaction. Experimental values of heterogeneous rates for the ester synthesis are compared with the values predicted by equation (8-10) and are shown in table 3. Predicted rates are in good agreement with the experimental values with correlation coefficient exceeding 0.9. All heterogeneous models predict rate increase with the increase in temperature from 323 K to 343 K and decrease thereafter. Activation Energies, Ea for the heterogeneously predicted rates was in the range of 49.31-57.54 kJ/mol which was higher than that earlier predicted by the homogeneous model. The maximum Ea (57.54 kJ/mol) was obtained from rate parameters determined by using equation (10). The high value of activation energy indicated that there is no external resistance for mass transport in the reaction mixture and the reaction is kinetically controlled. This could happen because of cavitation generated by 18
ultrasonic irradiation of the reactor contents paving the way for adequate turbulence and mixing. The comparative plots of predicted conversions with experimental values at various temperatures are given in Fig. 8 (a)-(d). As is evidenced from Fig. 8, heterogeneous models predicted conversions that are in good conformity with those obtained experimentally. The predicted conversion curves with respect to time are indistinct and resemble in nature for ER and LHHW kind of kinetics as seen in the Fig. 8. There is no significant difference in the minimized value of square of the summed error (which is of the order of 10-5) between experimental and predicted rates in all the tested models. However, the consistency in kinetic parameters determined by equation (10) could be seen in table 3. Therefore, it could be inferred that LHHW model with reactants and products adsorbing on the surface of catalyst was best representative of experimental data fit.
4.8 Comparison between conventional and ultrasound assisted reaction In order to compare the performance of ultrasound assisted esterification and conventional route, the heterogeneous reaction was also carried out in a stirred batch reactor in absence of ultrasound irradiation. Studies show that 71 % conversion was achieved in 15 minutes in presence of ultrasound and equilibrium conversion of 91.64 % reached in 120 minutes. The rate of reaction for methyl butyrate synthesis was lower in the absence of ultrasound. Conventional method showed fast reaction kinetics in early 20 minutes but rate of reaction slowed down thereafter. It took about 180 minutes to reach the conversion of 91.64 %. It can be concluded that ultrasound assisted reaction is faster and takes lesser time to reach equilibrium conversion. Possibility seems that cavitation generated in ultrasound system, eliminated mass transfer resistance favouring
19
higher reaction rates. It also helped in uniform mixing and turbulence creating bubble movements. These combined factors give synergetic effect resulting in improved conversion.
5. Conclusion The synthesis of methyl butyrate by the reaction between butyric acid and methanol with acidic ion exchange resin as a catalyst in presence of ultrasound irradiation was successfully carried out. The reaction rate increased with an increase in temperature over the range of 323-343 K and decreased on further rise in temperature. Optimum conversion was obtained at alcohol to acid ratio of 3 and catalyst loading of 6.5 % (w/w). Highest equilibrium conversion of 91.64 % was achieved at ultrasound power 100 W and duty cycle 75 % indicating good mixing effects and absence of film diffusion. This maximum conversion was achieved in 120 minutes at optimum reaction conditions. The ER and LHHW Single Site heterogeneous models could be successfully applied for representing ultrasound assisted esterification kinetics for the reaction between butyric acid and methanol. LHHW model with reactants and products both adsorbing on the surface of catalyst was best suited for fitting the experimental data. Thus combined use of ultrasound technique with heterogeneous catalyst appears to be a promising alternative for the synthesis of methyl butyrate owing to higher conversion and low reaction time.
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Nomenclature A
Butyric acid
B
Methanol
CA0
Initial concentration of butyric acid (mols/m3)
CA, CB, CE and CW
Concentration of butyric acid, methanol, methyl butyrate and water
respectively at any time, t (mols/m3) Cp
Specific heat of methanol at constant pressure (J/kg.K)
E
Methyl Butyrate
Ea
Activation energy( kJ*mol-1.)
Ed
Sound waves dissipated energy (J/see)
Ei
Input electrical energy, Ei= VI (J/sec)
i
Variable index indicative of species participating in the reaction (dimensionless)
I
Electrical acoustic intensity (W/cm2)
kf
Forward rate constant (l* mol-1 *min-1)
kr
Reverse rate constant (l* mol-1 *min-1)
ks
Surface reaction rate constant (l2/mol*gcat*Min)
KA, KB, KE and KW
Adsorption equilibrium constant for butyric acid, methanol, methyl
butyrate and water respectively (l/mol*gcat*sec) Ke
Equilibrium rate constant
KS
Surface Equilibrium rate constant
m
Mass of methanol (kg)
M
Methanol to butyric acid ratio (CB0/CA0)
n
Number of species involved in esterification (dimensioness)
p
Index in equation 5 (dimensionless) 21
(-rA ) Reaction rate obtained based on Butyric acid consumed (mol* l-1 min-1) (rexp)
Experimental reaction rate (mol*l-1*g-1*min-1)
(rpred) Reaction rate predicted by heterogeneous models (mol*l-1*g-1*min-1) R
Universal gas constant (8.314 KJ/mol.K)
T
Temperature (K)
W
Water
xA
Conversion of butyric acid [(CA0-CA)/CA0]
xAE
Equilibrium conversion of butyric acid at equilibrium
22
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25
List of Tables Table 1: Rate constants determined at various temperatures Table 2: Kinetic and equilibrium constants at various temperatures for heterogeneous models (Single Site Mechanism) Table 3: Experimental and calculated reaction rates at various temperatures for heterogeneous synthesis of methyl butyrate
26
Table 1: Rate constants determined at various temperatures Sr. No. 1
Temp. (K) 323
% Conv. (xA) 72.48
2
333
3 4
Keq
R2
kf (l*mol-1*min-1) 2.0 *10-3
0.83
0.97
75.54
3.0*10-3
1.03
0.96
343
91.64
5.0*10-3
4.82
0.93
353
75.08
4.0*10-3
0.69
0.86
27
Table 2: Kinetic and equilibrium constants at various temperatures for heterogeneous models (Single Site Mechanism) Temp. (K)
323
333
343
Kinetic and Adsorption Equilibrium Constants Heterogeneous Model
g*l *mol-1* s-1
Ks
KA
KB
KE
KW
ER
0.14
7.99
1.1*10-3
4.10
0.055
0.055
LHHW (Reactants)
0.17
7.69
0.30
3.14
1.52
1.52
LHHW (React + Prod)
0.12
7.70
1.57
3.17
1.43
1.43
7.16 6.39
0.108 2.00
0.108 2.00
1.74 4.41
2.79 1.02*10-5
2.79 8.61*10-6
0.05
4.40
0.23
0.23
1.46
3.38
0.91
0.91
6.10
0.18
0.18
4.34
2.65
2.65
1.64
2.86
2.86
ER
1.10
LHHW (Reactants)
1.21
LHHW (React + Prod)
1.40
ER
1.97
LHHW (Reactants)
2.09
8.66 8.26
6.8*10
7.95 7.90
2.17 1.1*10
2.09
ER
0.90
8.87
1.5*10
LHHW (Reactants)
0.79
8.03
0.81
7.94
0.33 2.09
LHHW (React + Prod)
-4
0.19
7.89 7.79
LHHW (React + Prod) 353
ks -1
-3
-3
28
Table 3: Experimental and calculated reaction rates at various temperatures for heterogeneous synthesis of methyl butyrate Sr. No.
Temperature (K)
rexp (*10-2) (mol*l-1*g-1 *min-1)
rpred (*10-2) (mol*l-1*g-1 *min-1) ER
1
323
3.37
3.22
LHHW (Reactants) 3.11
LHHW (React + Prod) 3.20
2
333
4.72
4.17
4.03
4.27
3
343
6.68
6.59
6.31
6.39
4
353
4.01
3.76
3.67
3.83
29
List of Figures Fig. 1 Effect of temperature on conversion of butyric acid at alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W Fig. 2 Effect of alcohol to acid ratio (M) on conversion of butyric acid at temperature 343 K, catalyst loading 6.5 %, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W Fig. 3 Effect of catalyst loading on the conversion of butyric acid at temperature 343 K, alcohol to acid ratio3, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W Fig. 4 Effect of addition of molecular sieves on the conversion of butyric acid at temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 %, duty cycle 75 %, power input 100 W Fig. 5 Effect of power on conversion of butyric acid at temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 % , molecular sieves 8.5 % duty cycle 75 %. Fig. 6 Effect of US duty cycle on conversion of butyric acid at temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 % , molecular sieves 8.5 %, power input 100 W Fig. 7 Effect of temperature on forward reaction rate constant (alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves added 8.5 %, duty cycle 75 %, power input 100 W) Fig. 8 Comparison of predicted versus experimental conversion at different temperatures (alcohol to acid ratio 3, catalyst loading 6.5 % , ultrasound power 100 Watts, duty cycle 75 %, molecular sieves 8.5 % )
30
1 0.9
Conversion (XA)
0.8 0.7 0.6 0.5
323 K
0.4
333 K
0.3
343 K
0.2
353 K
0.1 0 0
25
50
75 100 Time (minutes)
125
150
Fig. 1 Effect of temperature on conversion of butyric acid at alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W
31
1 0.9
Conversion ( XA)
0.8 0.7 0.6
M=2
0.5
M=3
0.4
M=4
0.3
M=5
0.2
M=6
0.1 0 0
25
50
75 100 Time (min)
125
150
Fig. 2 Effect of alcohol to acid ratio (M) on conversion of butyric acid at temperature 343 K, catalyst loading 6.5 %, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W
32
1 0.9
Conversion (XA)
0.8 0.7 0.6 0.5
0%
0.4
3.2 %
0.3
6.5 %
0.2
8.5 %
0.1 0 0
25
50
75 100 Time (min)
125
150
Fig. 3 Effect of catalyst loading on the conversion of butyric acid at temperature 343 K, alcohol to acid ratio3, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W
33
1 0.9
Conversion (XA)
0.8 0.7 0.6
0%
0.5
3.2 %
0.4
6.5 %
0.3
8.5 %
0.2
11 %
0.1 0 0
25
50
75 100 Time (min)
125
150
Fig. 4 Effect of addition of molecular sieves on the conversion of butyric acid at temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 %, duty cycle 75 %, power input 100 W
34
1 0.9
Conversion (XA)
0.8 0.7
0W
0.6
50 W
0.5
75 W
0.4
100 W
0.3
125 W
0.2
145 W
0.1 0 0
15
30
45
60 75 90 Time (min)
105 120 135 150
Fig. 5 Effect of power on conversion of butyric acid at temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves 8.5 % duty cycle 75 %.
35
1 0.9
Conversion (XA)
0.8 0.7
0%
0.6
25 %
0.5
40%
0.4
60 %
0.3
75%
0.2
85 %
0.1 0 0
15
30
45
60 75 90 Time (mins)
105
120
135
150
Fig. 6 Effect of US duty cycle on conversion of butyric acid temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves 8.5 %, power input 100 W
36
-2.25 0.0028 -2.3
1/T (K-1) 0.0029
0.003
0.0031
0.0032
-2.35 -2.4 ln(k)
-2.45 -2.5 -2.55 -2.6 -2.65 -2.7 -2.75
y = -2201.2x + 4.1065 R² = 0.993
Fig. 7 Effect of temperature on forward reaction rate constant (alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves added 8.5 %, duty cycle 75 %, power input 100 W)
37
0.8 0.7 Exp.
Conversion (XA)
0.6
ER
0.5
LHHW (R) LHHW (R+ P)
0.4 0.3 0.2 0.1 0 0
25
50
75 100 Time (min)
125
150
(a) 323 K
0.9 0.8
Conversion (XA)
0.7
Exp.
0.6
ER
0.5 LHHW (R) LHHW (R+ P)
0.4 0.3 0.2 0.1 0 0
25
50
75 100 Time (min)
125
150
(b) 333 K
38
1 0.9
Conversion (XA)
0.8
Exp.
0.7 0.6
ER
0.5
LHHW (R) LHHW (R+ P)
0.4 0.3 0.2 0.1 0 0
25
50
75 100 Time (min)
125
150
(c) 343 K
0.8
Conversion (XA)
0.7 0.6
Exp.
0.5
ER
0.4
LHHW (R) LHHW (R+ P)
0.3 0.2 0.1 0 0
25
50
75 100 Time (min)
125
150
(d) 353 K
39
Fig. 8 (a)-(d) Comparison of predicted versus experimental conversion at different temperatures (alcohol to acid ratio 3, catalyst loading 6.5 % , ultrasound power 100 Watts, duty cycle 75 %, molecular sieves 8.5 % )
40
Research Highlights •
Ultrasound assisted synthesis of Methyl butyrate using amberlyst-15catalyst
•
Optimization of various parameters was done to obtain maximum conversion
•
Ultrasound helps to reduce the reaction time compared with conventional process.
•
Reaction was well predicated using Langmuir-Hinshelwood-Hougen-Watson Models
41
S1350-4177(15)00046-2 http://dx.doi.org/10.1016/j.ultsonch.2015.02.014 ULTSON 2802
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
12 November 2014 11 February 2015 27 February 2015
Please cite this article as: P.N. Dange, A.V. Kulkarni, V.K. Rathod, Ultrasound Assisted Synthesis of Methyl Butyrate using Heterogeneous Catalyst, Ultrasonics Sonochemistry (2015), doi: http://dx.doi.org/10.1016/ j.ultsonch.2015.02.014
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Ultrasound Assisted Synthesis of Methyl Butyrate using Heterogeneous Catalyst P. N. Dange, A.V. Kulkarni, V. K. Rathod*
Department of Chemical Engineering, Institute of Chemical Technology, Matunga (E), Mumbai - 400019, India.
*Corresponding author Dr. V. K. Rathod Department of Chemical Engineering, Institute of Chemical Technology, Matunga (E), Mumbai400019 (India) E-mail: [email protected], Phone: +91-22-33612020, Fax: +91-22-33611020.
1
Abstract Ultrasound assisted esterification of butyric acid with methanol was investigated in an ultrasound irradiated isothermal batch reactor using acid ion-exchange resin (amberlyst-15) as a catalyst. Effect of parameters, temperature (323-353 K), catalyst loading (0-8.5 % w/w), alcohol to acid ratio, M (2-6), ultrasound power (0-145 W), duty cycle (0-85 %) and amount of molecular sieves added (0-11 % w/w) on the rate of reaction was studied. At optimized parameters, a maximum conversion of 94% was obtained in 120 minutes in presence of ultrasound. Experimental kinetic data were correlated by using Eley-Rideal, and Langmuir-Hinshelwood-Hougen-Watson Models taking into account reverse reaction. Studies showed that single site LHHW with reactants and products both adsorbing on catalyst surface was most suited for the obtained experimental data. Activation energy determined based on heterogeneous kinetics was in the range 49.31-57.54 kJ/mol while it was 18.29 kJ/mol using homogeneous model.
Keywords: Ultrasound, kinetics, esterification, butyric acid, methanol, Amberlyst, kinetic model
2
1. Introduction Organic esters are used widely in the manufacturing of flavors, adhesives, pharmaceuticals, pesticide, plasticizers, polymerization monomers, emulsifiers in the food and cosmetic industries. Derivatives of many esters form variety of useful chemical intermediates and monomers for high molecular weight polymers and resins. In general, esters can be defined as the products of reaction between carboxylic acids and organic alcohols. Chemically, an ester is formed as the condensation product when a carboxylic acid is reacted with an alcohol [1]. Esterification of carboxylic acids with alcohols represents a well-known class of liquid-phase reactions of considerable industrial interest owing to its wide industrial uses [2-3]. Methyl butyrate or methyl ester of butyric acid is an ester with a fruity odor of apple, pineapple, and strawberry. Present in trace amounts in several plant products, especially pineapple flavor is produced by distillation of oils from vegetable origin. This ester is also manufactured on a small scale for use in perfumeries or flavoring food [4, 5]. Organic esters can be synthesized by several routes, most of which have been briefly reviewed by Yadav and Mehta [5]. The traditional route for preparing esters is by the reaction of the carboxylic acid with an alcohol using homogeneous catalysts such as sulfuric acid or paratoluene-sulfonic acid [6, 7]. Although, homogeneous catalysts provide faster reaction rates, separation and recovery of the catalyst from the reaction mixture is major issue [6, 8]. In addition, the homogeneous acid catalysts are responsible for equipment corrosion, side reactions and additional neutralization cost of treatment of salt produced [9-10, 13]. These limitations of homogeneous catalysts can be circumvented by the use of heterogeneous catalysts such as sulphonic acid based ion-exchange resins. The solid type of material has good physical and chemical properties with enhanced catalytic activity in esterification [3, 7, 11]. Solid catalyst can
3
be mechanically separated from the reaction mixture and subjected to possible reuse [12]. Heterogeneous route offers promising option owing to its advantages as compared with the methods using homogeneous catalyst. Significant work is reported in the literature using variety of solid acid catalysts [7, 11, 14-15]. Despite advantages of heterogeneous method for the synthesis of organic esters, the reaction time is a concern which can be substantially reduced by using newer techniques. Use of ultrasound waves and microwaves in ester synthesis is still an evolving area and detailed studies are needed to shed some light on their applicability in heterogeneous catalysis. Therefore, an alternative esterification method is required to reduce processing time, to lower amount of catalyst and unreacted raw materials also to increase the mass transfer. The use of cavitation resulting from ultrasound waves appears to be an appealing option for heterogeneous synthesis of ester. Ultrasound (frequency ~20 kHz to l0 MHz) is cyclic sound pressure with a frequency greater than the upper limit of human hearing [16]. The main principle behind the application of ultrasonic waves to the reaction is a phenomenon known as cavitation. The ultrasonic waves technique has two cycles compression and rarefaction, during rarefaction a vacuum pressure creates a cavitation bubble, when the compression cycle occurs the bubble implodes in a very short period of time, producing localized heating, high pressures and liquid jet sprays with high velocities[17]. The ultrasound assisted techniques are playing an important role in chemical processes, especially in cases where conventional methods require extended reaction times. Applying Ultrasound to the chemical reaction process enhances mixing, shearing, transfer of materials and the rate of chemical reactions effectively reducing the reaction time [18-19, 20]. Use of ultrasound irradiation is a promising and untapped technology for chemical reactions, though it was in use for last few decades. It can improve the reaction conditions, accelerate the
4
rate of reaction and produce a higher yield [21-22]. Recently, synthesis of methyl butyrate was carried out using conventional heterogeneous route with amberlyst-15 as catalyst [11]. The equilibrium conversion reported was greater than 90 % using amberlyst-15 catalyst under optimal reaction conditions. However, the time required to achieve this equilibrium conversion was longer (about 4 h) which needed to be reduced further. The objectives of this study were to test the suitability and efficacy of heterogeneous catalyst in presence of ultra sound irradiation for the synthesis of methyl butyrate and determine suitable kinetic model. Effects of time, temperature, catalyst loading, alcohol to acid ratio, addition of molecular sieves, ultra-sound power and duty cycle on esterification reaction were also undertaken.
2. Materials and Methods 2.1 Materials Butyric acid and methanol of 99.98% purity (w/w) were supplied by Merck. Both these chemicals were used as supplied. The acidic ion exchange resin (Amberlyst-15) was supplied by Alfa Aesar, USA. A cation exchange resin catalyst is insoluble polymer matrix which can exchange ions with the adjacent reacting mixture. It is a macro-porous type styrene-DVB (20%) resin. The resin is formed by the copolymerization reaction between styrene and divinyl-benzene which acts as cross-linking agent. For cation exchange resins, acid sites are deposited on the polymer matrix by the treatment of strong acids such as sulphuric acid which gives sulphonated cation exchange resins.
2.2 Batch Experiments
5
Esterification reactions were performed in a 100 mL batch reactor made of borosilicate glass. The three necked reactor was equipped with sample port, provision for ultrasound probe and condenser to recycle back liquid after vapors condensed during reaction runs. Water bath fitted with automatic temperature control was used to ensure constant reaction temperature. Sound waves (by ultrasonic horn) generated at 22 kHz were used to ensure adequate mixing of reaction contents. Ultrasonic horn was supplied by Dakshin Ultrasonics, Mumbai, India with maximum power rating of 120 W, input supply voltage 230 V (50 Hz AC). Vertically mounted probe tip was made up of SS 304 with 11 mm diameter and 10 cm height. In a typical experiment, a measured quantity of methanol and catalyst (amberlyst-15) were charged into the reactor with no initial water present in the reaction mixture. The reactor was sealed and heating was started with condenser water on. To optimize parameters, ultrasound generator was used on varying power inputs and duty cycles. The type of reactor and ultrasonic probe distance from the surface of reaction mixture was used based on earlier reported work so as to ensure maximum cavitation (bubble generation) [23]. Thus, ultrasound probe was adjusted in such a way that the tip length of 0.5 cm dipped into the reaction mixture from the top liquid surface [23]. Once the desired temperature was reached, the required amount of butyric acid was injected into the reactor via syringe and sonication was initiated - the instant was marked as zero reaction time. Small amount of samples (0.2 mL) were withdrawn at specified time intervals over the first 2.5 h of reaction. Reaction mixture was filtered in the end to separate the sample solution from the used solid catalyst. All experiments were performed three times and average values have been reported with standard deviations.
2.3 Electrical Acoustic Intensity and Dissipation rate of Sound Energy
6
Electrical acoustic intensity (I) can be defined as the ratio of electrical energy supplied to the cross sectional area of probe tip.
I (W/cm2)= π
(1)
Where Ei is the input electrical energy (W), r is the radius of the probe tip (cm). Calorimetric studies were performed to determine the dissipation rate of sound energy from the ultrasound probe tip. Fixed quantity of methanol (50 mL) was taken in a reaction vessel with tip of US horn dipped optimally into it. Measured quantity of input electrical energy was supplied to ultrasound generator. Strictly adiabatic conditions were ensured during the experiments. Dissipation rate of sound energy in the bulk of liquid was calculated by measuring the rate of change of temperature of liquid methanol as given by the following expression:
=
(2)
Where, m is the mass of methanol, Cp is average specific heat at constant pressure and dT/dt is the time rate of change o temperature.
2.4 Analysis: 2.4.1 Gas Chromatographic Analysis Samples from the reaction were taken in regular time intervals to ascertain the formation of butyric acid. Gas chromatograph (Chemito GC, Model 6890) was used to analyze liquid samples for a quantitative determination of methyl butyrate formed in the esterification reaction. GC was equipped with two detectors, a thermal conductivity detector and a flame ionization detector, connected in series. Use was made of a 30-m long HP-Innowax column (Polyethylene glycol 320 micrometers in diameter with 0.5 micrometer in thickness) with a temperature programmed analysis. Nitrogen was used as the carrier gas. 7
2.4.2 Titrimetric analysis Acid value is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralize one gram of chemical substance. The acid values (KOH mg/g) were determined by a standard titrimetry method. Samples from the reaction mixtures were titrated for the total acid content using 0.1 N alcoholic KOH with phenolphthalein indicator and methanol as a quenching agent. The Methyl butyrate obtained was expressed in terms of percent (%) conversion based on butyric acid consumption. The conversion obtained using both the analysis was similar with only 2% deviation. 3. Kinetic Modeling Kinetic modeling describes the rate of reaction as a function of reaction variables. Esterification reactions catalysed by solid acid exchange resins can be expressed by different kinetic models. Three models commonly subjected to correlate kinetic data are Pseudo-Homogeneous (PH) model [24], the Langmuir-Hinshelwood-Hougen-Watson (LHHW) model and the Elley-Rideal (ER) model [25]. Pseudo-Homogeneous model does not take into account the adsorption effects of the species in the reactant medium on the surface of catalyst. It has been successfully used in high polar reaction media [26]. A generalized esterification reaction is written as:
A + B E + W
(3)
Where, A and B are the reactants (carboxylic acid and alcohol), while E and W are the products (ester and water) of the reaction. The notation, kf is forward reaction rate constant whereas kr represents the rate constant for reverse reaction.
8
3.1 Homogeneous Reaction Model: The pseudo-homogeneous model does not take into account the sorption effect on the catalyst sites in a reacting medium [6, 27]. If the reaction mixture is considered as a homogeneous liquid phase, outcome by the Pseudo-Homogeneous model could be considered as a satisfactory tool to correlate the esterification of butyric acid with methanol in the presence of amberlyst-15 as a catalyst. For the esterification reaction (3) where one of the reactants is in excess, the PH model can be represented by the following expression [28]. (– )
ln ( – ) = k C"# (M– 1)t
(4)
At equilibrium, '– r" ) = C"#
* *+
= k C" C, – k C C- = 0 and thus, the equilibrium constant
(Ke) can be calculated from:
K 0 = = ( 3
1 2 2 )(3 2 )
(5)
xAe is the conversion of butyric acid at equilibrium.
3.2 Heterogeneous Models In heterogeneous kinetic studies, the generally encountered problem is complicated rate expressions which may obey Langmuir-Hinshelwood-Hougen-Watson (LHHW) or Eley-Rideal (ER) model [29]. These reaction mechanisms can be of single site or dual site type. In single-site mechanism, the reactant is adsorbed on only one site whereas in dual site mechanism, reactant adsorbed on one site interacts with another site to form product. The LHHW and the ER models both include the adsorption effects of the species in the reactant medium. The basic assumption of LHHW model is that all reactants are adsorbed on the catalyst surface before chemical
9
reactions occur. The ER model assumes the reaction taking place between adsorbed and nonadsorbed reactants in the bulk of liquid. The ER and LHHW models based on single site and dual site mechanisms have been reviewed by Ozdemir and Gultekin [30]. For the reversible reaction of butyric acid with methanol, considering adsorption effects of the reactants and products, we may have possible rate expression given by equation. (−r" ) = k 5
(6 7 68 78 369 79 6: 7: /6< ) ' =∑? @AB 6 7 )
C
--- (6)
Where, p=1 for single site mechanism; p=2 for dual site mechanism; i=1, 2, 3 and 4 for the esterification reaction; n=4 is the number of species involved in esterification reaction. For Eley-Rideal model, reactant A is adsorbed on catalytic sites and reacts with reactant B in the bulk liquid phase. In this case denominator becomes (1+ KACA) in equation (6). LangmuirHinshelwood- Hougen-Watson model can be based on reactants and products adsorbing on catalyst surface where reaction takes place. In first case, adsorption of product species is ignored and reaction takes place between A and B adsorbed on the surface of the catalyst. In these circumstances, denominator in equation (6), ignores product terms. In other case, LHHW model takes into account the sorption effects of product species along with reactants and equation (6) will be applicable in its entirety. Index p will be 1 or 2 depending upon the single site or dual site mechanism for all heterogeneous models.
4. Results and discussion 4.1 Effect of reaction temperature The effect of temperature on ultrasound assisted esterification between methanol and butyric acid was studied by conducting the reactions at 323 K, 333 K, 343 K and 353 K. The reaction was 10
performed at methanol to butyric acid ratio, M = 3, catalyst loading 6.5 % (w/w), added molecular sieves 8.5 % (w/w). The power input to the ultrasound horn was at 100 W with duty cycle of 75 %. The effect of temperature on conversion is shown in Fig.1. Conversion was found to be increasing markedly as reaction temperature varied from 323 K to 343 K. At low temperature diffusion phenomena is dominant owing to high cavitation intensity as at low temperature rate of bubble generation is low but they collapse with high intensity and hence more mass transport of the reactants on the catalytic sites. [31]. Temperature increase reduces the viscosity and surface tension of the reaction mixture which affects bubble formation and its subsequent collapse [32]. Higher temperature gives rise to more bubbles and frequent collisions with reduced intensity which causes limitation to transport of reactants from bulk to catalyst surface resulting into no significant rise in conversion [33]. Further at elevated temperatures, vapour pressure of the reactants increases and cavitation bubbles are filled by more vapor of the reactants causing low intensity cavitation. The maximum equilibrium conversion of 91.64 % was obtained at a temperature of 343 K. From the Fig. 1, it is seen that the equilibrium conversion decreased from 91.64 % to 70.65 % when temperature was raised from 343K to 353 K. This reverse trend is attributed to reduced cavitation intensity at temperature beyond 343 K which resulted in lower conversion. The other possible reason for lower conversion beyond 343 K is non-availability of methanol in the liquid reaction mixture due its evaporation [11, 34]. Therefore, the optimum reaction temperature was taken to be 343 K at which the conversion of butyric acid reached a stable value of 91.64 %.
4.2 Effect of Alcohol to Acid Ratio
11
Theoretically, the esterification reaction requires one mole of methanol for each mole of butyric acid for the synthesis of methyl butyrate. However, in practice, the methanol should be in excess to drive the reaction towards completion as the esterification reaction is reversible. In order to study the effect of methanol/butyric acid molar ratio on esterification, different experiments were carried out using varied molar ratios of alcohol and acid. The mole ratio of methanol to butyric acid was varied from 2 to 6 and the results obtained are plotted in Fig.2. The maximum conversion of 91.6 % was obtained for an alcohol to acid molar ratio (M), 3 with a catalyst loading of 6.5 %, temperature of 343 K and added molecular sieves 8.5 %. The input power to the sonicator was 100 W with duty cycle of 75 %. Experiments show that with molar ratio of butyric acid to methanol of 1:3, maximum conversion was achieved in 100 minutes and remained constant thereafter over an extended reaction time. As seen in the Fig. 2, conversion increased from 84 % to 91.6 % as M is increased from 2 to 3 and decreased with further increase in alcohol to acid ratio, M. It can be reasoned that at the lower value of M, the effective viscosity of the reaction mixture is high and hence the effect of cavitation is minimal. This leads to scarce diffusional mass transport of reacting species on active catalyst sites hence limiting conversion. As the ratio M is increased up to 3, presence of more methanol lowers effective viscosity of the reaction mixture resulting into enhanced cavitation. This increase in the cavitation intensity gives rise to high turbulence in the reactor and diffusion of reacting species inside catalyst pores is dominant phenomena [35]. As the alcohol to acid ratio is increased beyond 3, conversion shows marginal decrease despite increased cavitation because of decreased viscosity owing to even higher amount of methanol present. At this stage, excessive bubbles are likely to coalesce and form larger and more stable bubbles. This creates a barrier to dissipative
12
energy transfer hampering effective diffusion of reactant species and thus affecting the conversion marginally.
4.3 Effect of Catalyst Loading The effect of catalyst loading on the conversion of the butyric acid is shown in Fig.3. Each new run was performed using fresh resin catalyst. As expected, the conversion increases with an increase in the catalyst loading from0 to 6.5 % (w/w) due to more available active sites, resulting in higher reaction rate. However, there is decrease in the conversion of butyric acid when the catalyst loading is increased from 6.5 % to 8.5 %. The impact of cavitation is dictated by the properties of reaction mixture and ultrasound intensity. Viscosity of reaction mixture increases with its solid catalyst concentration hampering effective cavitation at higher catalyst loading [33]. It is also possible that increased catalyst content might hamper and reduce the ultrasound intensity in certain areas of reactor where effects of turbulence are not pronounced. Hence higher catalyst content affects sonication intensity, and cavitation bubbles may not develop in those areas in the reaction medium [37]. A higher loading of catalyst also results in reduction of the time required to reach the reaction equilibrium [34, 38]. It is also observed that the conversion curve is steep in the first 10 minutes indicating that rate of reaction is higher initially as the more catalyst sites are available for the unconsumed reactants and cavitation is effective owing to lower viscosity of the reaction medium. It can be reasoned further that the impact of cavitation bubbles is governed by the properties of reaction mixture and ultrasound. The propagation of ultrasound waves in a reaction medium is accompanied by unavoidable loss through attenuation, adsorption and dissipation. The behaviour of ultrasound dissipation would depend on the nature and extent of medium homogeneity. When the catalyst concentration increases from 0 % to 6.5 %, conversion 13
increased steadily up to more than 91 %. But as the catalyst % increased to 8.5 %, equilibrium conversion reduced to 89 % along with sluggish kinetics during initial period. This could be because of ultrasound energy loss due to attenuation and adsorption of ultrasound waves owing to higher solid catalyst content. This results in lower dissipation of ultrasound energy in the reaction and thus reduces the cavitational intensity. This led to insufficient turbulence to warrant uniform mixing of reactor contents resulting into reduced conversion. Addition of further catalyst mass beyond 6.5 % dampens the cavitational effects owing to hindrance to effective turbulence throughout reaction mixture resulting in decreased conversion. Hence the optimum catalyst loading was taken as 6.5 % and further esterification runs were carried out using this value.
4.4 Effect of Addition of Molecular Sieves Water is one of the products formed during the esterification of butyric acid with methanol. Thus, the water content is another important parameter in esterification reactions. As the reaction progress, the water formed as product during reaction increases. Reaction may proceed in the reverse direction if the water formed during reaction is not separated forthwith. Addition of molecular sieves or silica gel adsorbes water formed during reaction and usually improves the equilibrium conversion [39-40]. In order to study the influence of water produced in the reaction, molecular sieves (3 Å type) were added in the range of 0-8.5 % (w/w). The experiments were carried out at ultrasound probe rating of 100 W with 75 % of duty cycle, catalyst loading of 6.5 % and a temperature of 343 K. Fig.4 shows that the conversion of methyl butyrate increased with increasing amount of molecular sieves. This could be because of the adsorption of produced water in the pores of
14
molecular sieves and hence inhibition of the reverse reaction. The optimum value of molecular sieves added to reactor was 9.5 % beyond which there was no significant increase in conversion since the amount of water generated remains essentially same even though the molecular sieves added has more saturation capacity. When the reaction is carried out without adding molecular sieves the conversion obtained was at the minimal value indicating the presence of reverse reaction. This shows that water formed during the reaction needs to be separated in order to keep reaction going in forward direction.
4.5 Effect of Power Experiments were performed by varying input power from 50-145 W to identify the optimum power needed in order to achieve efficient cavitation for the esterification. Other parameters were kept at: alcohol to acid ratio of 3, catalyst loading of 6.5 % (w/w), reaction temperature of 343 K, added molecular sieves 8.5 % and duty cycle of 75%. As can be seen from the Fig. 5, conversion increases drastically by varying power from 50 W to 100 W. Calculated electrical acoustic intensities were 52.63, 78.95, 105.27, 131.59 and 152.65 W/cm2 for the input powers of 50, 75, 100, 125 and 145 W respectively. Electrical acoustic intensity increases with the increase in power consequently causing cavitation in more pronounced manner. This leads to more turbulence because of high rate of bubble generation at sub-microscopic scale giving rise to more interfacial area [41]. More interfacial area paves the way for increased diffusion for the reactants on the catalytic sites giving higher conversion. However, an increase in power beyond 100 W, resulted into mere liquid agitation (mixing) instead of cavitation. This attributed to poor propagation of ultrasound waves through the reaction mixture resulting in lower conversion [42]. When a large amount of ultrasonic power enters a system, a much larger quantity of ultrasonic
15
cavitation bubbles are generated in the reaction mixture. Excessive bubbles are likely to merge and form larger and more stable bubbles and, thus, create a barrier to acoustic energy transfer [43].
4.6 Effect of Duty Cycle Duty cycle expressed in percentage indicates ratio of ultrasound probe ON time and that of ON and OFF time taken together. Duty cycle of 75 % pertains to ON time of 9 seconds and OFF time of 3 seconds. To investigate the effect on reaction conversion, duty cycle was varied from 25 % to 85 %. The other conditions were kept at the optimum values of temperature 243 K, alcohol to acid mole ratio of 3:1, molecular sieves 4 g (8.5 % w/w), catalyst loading 6.5 %, power 100 W. Effect of duty cycle on conversion of methyl butyrate are shown in Fig.6. As can be seen in the figure, when the ultrasound duty cycle varied from 25 % to 75 %, equilibrium conversion increased from 72 % to 91 %. This shows that the exposure time of ultrasound irradiation to the reacting mixture is increased subsequently increasing product formation. Although moderate to higher duty cycle of sonication employed improves the conversion, its use for extended period of time without pulse can damage the transducers which generate ultrasonic waves [44]. On further increase of duty cycle from 75 % to 85 %, the conversion is seen to be decreasing from 91.64 % to 81 %. This decrease in conversion can be attributed to excessive bubbles generated due to almost incessant operation of ultrasound generator. These bubbles are likely to coalesce and form larger and more stable bubbles. This creates a barrier to dissipative energy transfer inhibiting effective diffusion of reactant species [43]. Hence the optimum duty cycle was taken at 75 %.
16
4.7 Estimation of Kinetic Parameters The kinetic data of esterification reaction was correlated with three kinetic models: PH model, the ER model and the LHHW model according to equations given in section 3. Firstly, the pseudo-homogeneous model was applied and plots were made using equation (4) to determine the slope of the line from which the values of kf were calculated (Table 1). These plots are as shown in the Fig.7. The dependence of the forward rate constant on the reaction temperature is described by the Arrhenius rate law, as given in Eq. (7): E9F
k = k # e GH
(7)
Where, k0 is the pre-exponential factor, Ea is the activation energy, R is the ideal gas constant and T is the reaction temperature. The parameters of the Arrhenius equation, activation energy and frequency factor were determined for the forward reaction. The data of ln (k) versus 1/T were fitted by linear regression and the result is plotted in the Fig.7. The Arrhenius plot of the rate constant for forward reaction in the temperature range of 323-343 K gives the regression coefficient of 0.99. The slope could be applied to calculate the activation energy. Calculated activation energy for the synthesis of methyl butyrate is 18.29 kJ/mol. This moderate value of Ea based on homogeneous kinetics shows that the conversion rate is competitively controlled by the reaction kinetics and the mass transport phenomena. Since the esterification reaction between butyric acid and methanol involves solid catalyst phase and the liquid phase reactants and products, the reaction is termed as heterogeneous type. The estimation of kinetic parameters for heterogeneous esterification is complicated owing to the complex surface-adsorbate interactions at catalyst surface. The values of surface reaction rate constant (ks), surface equilibrium constant, KS and adsorption equilibrium constants (KA, KB, KE,
17
and KW) were determined by nonlinear regression using Microsoft EXCEL. The experimental data was subjected to models of type ER (Eley-Redeal), LHHW (Langmiur-HinshelwoodHougen-Watson) with reactants adsorbed and LHHW with reactants and products both adsorbed on the surface of the catalyst to find best fit. Surface reaction rate constant and adsorption equilibrium constants were calculated by minimizing sum of the squared value of the difference between experimental and predicted rates. The expressions for single site ER and LHHW type models are given in equation 8-10 [28-29]. (−rI0* ) = k 5 (−rI0* ) = k 5
J K J K 6 68 7 78 3 9 9 : : J<
( = 6 7 ) J K J K 6 68 7 78 3 9 9 : : J<
( = 6 7 = 68 78 ) J K J K 6 68 7 78 3 9 9 : :
(−rI0* ) = k 5 ( = 6
J<
7 = 68 78 = 69 79 = 6: 7: )
(8)
(9)
(10)
The values of kinetic and surface constants are summarized in table 2. Estimated rate parameters were further used to predict the rate of reaction. Experimental values of heterogeneous rates for the ester synthesis are compared with the values predicted by equation (8-10) and are shown in table 3. Predicted rates are in good agreement with the experimental values with correlation coefficient exceeding 0.9. All heterogeneous models predict rate increase with the increase in temperature from 323 K to 343 K and decrease thereafter. Activation Energies, Ea for the heterogeneously predicted rates was in the range of 49.31-57.54 kJ/mol which was higher than that earlier predicted by the homogeneous model. The maximum Ea (57.54 kJ/mol) was obtained from rate parameters determined by using equation (10). The high value of activation energy indicated that there is no external resistance for mass transport in the reaction mixture and the reaction is kinetically controlled. This could happen because of cavitation generated by 18
ultrasonic irradiation of the reactor contents paving the way for adequate turbulence and mixing. The comparative plots of predicted conversions with experimental values at various temperatures are given in Fig. 8 (a)-(d). As is evidenced from Fig. 8, heterogeneous models predicted conversions that are in good conformity with those obtained experimentally. The predicted conversion curves with respect to time are indistinct and resemble in nature for ER and LHHW kind of kinetics as seen in the Fig. 8. There is no significant difference in the minimized value of square of the summed error (which is of the order of 10-5) between experimental and predicted rates in all the tested models. However, the consistency in kinetic parameters determined by equation (10) could be seen in table 3. Therefore, it could be inferred that LHHW model with reactants and products adsorbing on the surface of catalyst was best representative of experimental data fit.
4.8 Comparison between conventional and ultrasound assisted reaction In order to compare the performance of ultrasound assisted esterification and conventional route, the heterogeneous reaction was also carried out in a stirred batch reactor in absence of ultrasound irradiation. Studies show that 71 % conversion was achieved in 15 minutes in presence of ultrasound and equilibrium conversion of 91.64 % reached in 120 minutes. The rate of reaction for methyl butyrate synthesis was lower in the absence of ultrasound. Conventional method showed fast reaction kinetics in early 20 minutes but rate of reaction slowed down thereafter. It took about 180 minutes to reach the conversion of 91.64 %. It can be concluded that ultrasound assisted reaction is faster and takes lesser time to reach equilibrium conversion. Possibility seems that cavitation generated in ultrasound system, eliminated mass transfer resistance favouring
19
higher reaction rates. It also helped in uniform mixing and turbulence creating bubble movements. These combined factors give synergetic effect resulting in improved conversion.
5. Conclusion The synthesis of methyl butyrate by the reaction between butyric acid and methanol with acidic ion exchange resin as a catalyst in presence of ultrasound irradiation was successfully carried out. The reaction rate increased with an increase in temperature over the range of 323-343 K and decreased on further rise in temperature. Optimum conversion was obtained at alcohol to acid ratio of 3 and catalyst loading of 6.5 % (w/w). Highest equilibrium conversion of 91.64 % was achieved at ultrasound power 100 W and duty cycle 75 % indicating good mixing effects and absence of film diffusion. This maximum conversion was achieved in 120 minutes at optimum reaction conditions. The ER and LHHW Single Site heterogeneous models could be successfully applied for representing ultrasound assisted esterification kinetics for the reaction between butyric acid and methanol. LHHW model with reactants and products both adsorbing on the surface of catalyst was best suited for fitting the experimental data. Thus combined use of ultrasound technique with heterogeneous catalyst appears to be a promising alternative for the synthesis of methyl butyrate owing to higher conversion and low reaction time.
20
Nomenclature A
Butyric acid
B
Methanol
CA0
Initial concentration of butyric acid (mols/m3)
CA, CB, CE and CW
Concentration of butyric acid, methanol, methyl butyrate and water
respectively at any time, t (mols/m3) Cp
Specific heat of methanol at constant pressure (J/kg.K)
E
Methyl Butyrate
Ea
Activation energy( kJ*mol-1.)
Ed
Sound waves dissipated energy (J/see)
Ei
Input electrical energy, Ei= VI (J/sec)
i
Variable index indicative of species participating in the reaction (dimensionless)
I
Electrical acoustic intensity (W/cm2)
kf
Forward rate constant (l* mol-1 *min-1)
kr
Reverse rate constant (l* mol-1 *min-1)
ks
Surface reaction rate constant (l2/mol*gcat*Min)
KA, KB, KE and KW
Adsorption equilibrium constant for butyric acid, methanol, methyl
butyrate and water respectively (l/mol*gcat*sec) Ke
Equilibrium rate constant
KS
Surface Equilibrium rate constant
m
Mass of methanol (kg)
M
Methanol to butyric acid ratio (CB0/CA0)
n
Number of species involved in esterification (dimensioness)
p
Index in equation 5 (dimensionless) 21
(-rA ) Reaction rate obtained based on Butyric acid consumed (mol* l-1 min-1) (rexp)
Experimental reaction rate (mol*l-1*g-1*min-1)
(rpred) Reaction rate predicted by heterogeneous models (mol*l-1*g-1*min-1) R
Universal gas constant (8.314 KJ/mol.K)
T
Temperature (K)
W
Water
xA
Conversion of butyric acid [(CA0-CA)/CA0]
xAE
Equilibrium conversion of butyric acid at equilibrium
22
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25
List of Tables Table 1: Rate constants determined at various temperatures Table 2: Kinetic and equilibrium constants at various temperatures for heterogeneous models (Single Site Mechanism) Table 3: Experimental and calculated reaction rates at various temperatures for heterogeneous synthesis of methyl butyrate
26
Table 1: Rate constants determined at various temperatures Sr. No. 1
Temp. (K) 323
% Conv. (xA) 72.48
2
333
3 4
Keq
R2
kf (l*mol-1*min-1) 2.0 *10-3
0.83
0.97
75.54
3.0*10-3
1.03
0.96
343
91.64
5.0*10-3
4.82
0.93
353
75.08
4.0*10-3
0.69
0.86
27
Table 2: Kinetic and equilibrium constants at various temperatures for heterogeneous models (Single Site Mechanism) Temp. (K)
323
333
343
Kinetic and Adsorption Equilibrium Constants Heterogeneous Model
g*l *mol-1* s-1
Ks
KA
KB
KE
KW
ER
0.14
7.99
1.1*10-3
4.10
0.055
0.055
LHHW (Reactants)
0.17
7.69
0.30
3.14
1.52
1.52
LHHW (React + Prod)
0.12
7.70
1.57
3.17
1.43
1.43
7.16 6.39
0.108 2.00
0.108 2.00
1.74 4.41
2.79 1.02*10-5
2.79 8.61*10-6
0.05
4.40
0.23
0.23
1.46
3.38
0.91
0.91
6.10
0.18
0.18
4.34
2.65
2.65
1.64
2.86
2.86
ER
1.10
LHHW (Reactants)
1.21
LHHW (React + Prod)
1.40
ER
1.97
LHHW (Reactants)
2.09
8.66 8.26
6.8*10
7.95 7.90
2.17 1.1*10
2.09
ER
0.90
8.87
1.5*10
LHHW (Reactants)
0.79
8.03
0.81
7.94
0.33 2.09
LHHW (React + Prod)
-4
0.19
7.89 7.79
LHHW (React + Prod) 353
ks -1
-3
-3
28
Table 3: Experimental and calculated reaction rates at various temperatures for heterogeneous synthesis of methyl butyrate Sr. No.
Temperature (K)
rexp (*10-2) (mol*l-1*g-1 *min-1)
rpred (*10-2) (mol*l-1*g-1 *min-1) ER
1
323
3.37
3.22
LHHW (Reactants) 3.11
LHHW (React + Prod) 3.20
2
333
4.72
4.17
4.03
4.27
3
343
6.68
6.59
6.31
6.39
4
353
4.01
3.76
3.67
3.83
29
List of Figures Fig. 1 Effect of temperature on conversion of butyric acid at alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W Fig. 2 Effect of alcohol to acid ratio (M) on conversion of butyric acid at temperature 343 K, catalyst loading 6.5 %, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W Fig. 3 Effect of catalyst loading on the conversion of butyric acid at temperature 343 K, alcohol to acid ratio3, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W Fig. 4 Effect of addition of molecular sieves on the conversion of butyric acid at temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 %, duty cycle 75 %, power input 100 W Fig. 5 Effect of power on conversion of butyric acid at temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 % , molecular sieves 8.5 % duty cycle 75 %. Fig. 6 Effect of US duty cycle on conversion of butyric acid at temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 % , molecular sieves 8.5 %, power input 100 W Fig. 7 Effect of temperature on forward reaction rate constant (alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves added 8.5 %, duty cycle 75 %, power input 100 W) Fig. 8 Comparison of predicted versus experimental conversion at different temperatures (alcohol to acid ratio 3, catalyst loading 6.5 % , ultrasound power 100 Watts, duty cycle 75 %, molecular sieves 8.5 % )
30
1 0.9
Conversion (XA)
0.8 0.7 0.6 0.5
323 K
0.4
333 K
0.3
343 K
0.2
353 K
0.1 0 0
25
50
75 100 Time (minutes)
125
150
Fig. 1 Effect of temperature on conversion of butyric acid at alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W
31
1 0.9
Conversion ( XA)
0.8 0.7 0.6
M=2
0.5
M=3
0.4
M=4
0.3
M=5
0.2
M=6
0.1 0 0
25
50
75 100 Time (min)
125
150
Fig. 2 Effect of alcohol to acid ratio (M) on conversion of butyric acid at temperature 343 K, catalyst loading 6.5 %, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W
32
1 0.9
Conversion (XA)
0.8 0.7 0.6 0.5
0%
0.4
3.2 %
0.3
6.5 %
0.2
8.5 %
0.1 0 0
25
50
75 100 Time (min)
125
150
Fig. 3 Effect of catalyst loading on the conversion of butyric acid at temperature 343 K, alcohol to acid ratio3, molecular sieves 8.5 % , duty cycle 75 %, power input 100 W
33
1 0.9
Conversion (XA)
0.8 0.7 0.6
0%
0.5
3.2 %
0.4
6.5 %
0.3
8.5 %
0.2
11 %
0.1 0 0
25
50
75 100 Time (min)
125
150
Fig. 4 Effect of addition of molecular sieves on the conversion of butyric acid at temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 %, duty cycle 75 %, power input 100 W
34
1 0.9
Conversion (XA)
0.8 0.7
0W
0.6
50 W
0.5
75 W
0.4
100 W
0.3
125 W
0.2
145 W
0.1 0 0
15
30
45
60 75 90 Time (min)
105 120 135 150
Fig. 5 Effect of power on conversion of butyric acid at temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves 8.5 % duty cycle 75 %.
35
1 0.9
Conversion (XA)
0.8 0.7
0%
0.6
25 %
0.5
40%
0.4
60 %
0.3
75%
0.2
85 %
0.1 0 0
15
30
45
60 75 90 Time (mins)
105
120
135
150
Fig. 6 Effect of US duty cycle on conversion of butyric acid temperature 343 K, alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves 8.5 %, power input 100 W
36
-2.25 0.0028 -2.3
1/T (K-1) 0.0029
0.003
0.0031
0.0032
-2.35 -2.4 ln(k)
-2.45 -2.5 -2.55 -2.6 -2.65 -2.7 -2.75
y = -2201.2x + 4.1065 R² = 0.993
Fig. 7 Effect of temperature on forward reaction rate constant (alcohol to acid ratio 3, catalyst loading 6.5 %, molecular sieves added 8.5 %, duty cycle 75 %, power input 100 W)
37
0.8 0.7 Exp.
Conversion (XA)
0.6
ER
0.5
LHHW (R) LHHW (R+ P)
0.4 0.3 0.2 0.1 0 0
25
50
75 100 Time (min)
125
150
(a) 323 K
0.9 0.8
Conversion (XA)
0.7
Exp.
0.6
ER
0.5 LHHW (R) LHHW (R+ P)
0.4 0.3 0.2 0.1 0 0
25
50
75 100 Time (min)
125
150
(b) 333 K
38
1 0.9
Conversion (XA)
0.8
Exp.
0.7 0.6
ER
0.5
LHHW (R) LHHW (R+ P)
0.4 0.3 0.2 0.1 0 0
25
50
75 100 Time (min)
125
150
(c) 343 K
0.8
Conversion (XA)
0.7 0.6
Exp.
0.5
ER
0.4
LHHW (R) LHHW (R+ P)
0.3 0.2 0.1 0 0
25
50
75 100 Time (min)
125
150
(d) 353 K
39
Fig. 8 (a)-(d) Comparison of predicted versus experimental conversion at different temperatures (alcohol to acid ratio 3, catalyst loading 6.5 % , ultrasound power 100 Watts, duty cycle 75 %, molecular sieves 8.5 % )
40
Research Highlights •
Ultrasound assisted synthesis of Methyl butyrate using amberlyst-15catalyst
•
Optimization of various parameters was done to obtain maximum conversion
•
Ultrasound helps to reduce the reaction time compared with conventional process.
•
Reaction was well predicated using Langmuir-Hinshelwood-Hougen-Watson Models
41
