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Uncatalyzed esterification of biomass-derived carboxylic acids

ProQuest Dissertations and Theses, 2011
Dissertation
Author: Kehinde Seun Bankole
Abstract:
To shift from a petroleum-based to a biomass-based economy will require the development not only of biofuels, but also of biorenewable replacements for petroleum-derived chemicals. In this regard, environmentally friendly biomass-derived esters may serve as alternatives to fossil-derived chemicals such as toxic halogenated solvents and glycol ethers. Therefore, esterification of various carboxylic acids that find significant applications in the chemical, pharmaceutical, petrochemical, food, and cosmetic industries has been initiated by the chemical industry. At atmospheric condition, esterification is a reversible reaction limited by the low equilibrium conversion and slow reaction rate, and has recently been performed with excess alcohol to shift the equilibrium conversion. Heterogeneous or homogeneous acid catalysts are used to achieve acceptable reaction rates. The conventional acid-catalyzed process has been extensively developed; but it suffers from problems associated with the generation of side reactions, corrosion of equipment, expensive purification procedures, long reaction times and discharge of acidic wastes. Various attempts on esterification of carboxylic acids with ethanol have previously addressed important issues concerning product distribution, catalyst activity, and kinetics of acid-catalyzed esterification at lower reaction temperatures, but kinetics of uncatalyzed esterification at elevated reaction temperatures are still very limited. It is thus of great interest from a practical viewpoint that more information such as kinetic and thermodynamic parameters are required to develop a possible esterification process without using any catalyst. In this work, therefore, a fundamental study on the uncatalyzed esterification of different aliphatic carboxylic acids with stoichiometric amounts of ethanol was undertaken to examine the possibility of converting the biomass-derived carboxylic acids to ethyl esters and to determine the kinetic and thermodynamic parameters for the uncatalyzed esterification. Experiments were conducted with isothermal batch reactors at temperatures ranging from 298 K to 623 K. A 2 nd -order reversible kinetics rate expression was used to fit the experimental data. The thermodynamic and kinetic values estimated were found to vary for different esterification systems studied. The dependence of Keq on temperature for esterification of short-chain and long-chain carboxylic acids varied. Despite the nonlinearity of the Van't Hoff plot for esterification of linoleic acid, the Arrhenius and Eyring plots were linear. Two thermodynamic paths were developed for estimating the equilibrium conversions, and the theoretical values compared well with the experimental results reported in this study. Additional experiments performed to assess the corrosive and catalytic influences of metallic materials on esterification reaction indicated Inconel 625 alloy, nickel wire and stainless steel materials have potential corrosion problems on the uncatalyzed esterification reaction at elevated reaction. However, tantalum and grade 5 titanium materials showed acceptable level of compatibility for similar reaction conditions, and this can encourage the design of a flow reactor system.

TABLE OF CONTENTS LIST OF TABLES ........................................................................................................... viii LIST OF FIGURES ........................................................................................................... xi CHAPTER 1: INTRODUCTION ....................................................................................... 1 1.1. Motivation ................................................................................................................ 1 1.2. Problem Identification ............................................................................................. 1 1.3. Research Justification .............................................................................................. 2 CHAPTER 2: BACKGROUND ......................................................................................... 4 2.1. Biomass-Derived products ....................................................................................... 4 2.2. Catalysis of Esterification Reaction ......................................................................... 4 2.2.1. Homogeneous Acid-Catalyzed Esterification ................................................... 7 2.2.2. Heterogeneous Acid-Catalyzed Esterification .................................................. 8 2.2.2.1. Ion exchange resins as catalysts ................................................................. 8 2.2.2.2. Enzymes as catalysts .................................................................................. 9 2.3. Development of Kinetic Model for Uncatalyzed Esterification Reaction ............... 9 2.4. Previous Kinetics Studies of Esterification Reactions ........................................... 13 2.5. Supercritical Fluids ................................................................................................ 19 2.5.1. Supercritical Alcohol ...................................................................................... 22 CHAPTER 3: RESEARCH OBJECTIVES ...................................................................... 23 CHAPTER 4: EXPERIMENTAL DESIGN AND DATA ANALYSIS .......................... 25 4.1. Design of Experiments ........................................................................................... 25 4.2. Experimental Data Analysis .................................................................................. 26 CHAPTER 5: KINETIC STUDIES OF UNCATALYZED ESTERIFICATION OF CARBOXYLIC ACIDS......................................................................... 28 5.1. Experimental Section ............................................................................................. 29 5.1.1. Materials ......................................................................................................... 29 5.1.2. Experimental Setup and Procedure ................................................................. 30 5.1.3. Catalysis and Corrosion Test .......................................................................... 31 5.1.4. Component Analysis Methods ........................................................................ 32 5.1.4.1. High Performance Liquid Chromatography ............................................ 32 5.1.4.2. Acid Base Titration of Esterification Reaction Sample ........................... 33 5.1.4.3. Raman Analysis for Linoleic Acid Esterification .................................... 35 5.2. Results and Discussions ......................................................................................... 35 5.2.1. Equilibrium Constant ...................................................................................... 35 5.2.2. Thermodynamic Parameters ........................................................................... 37 5.2.3. Theoretical Estimation of Thermodynamic Parameters ................................. 42 5.2.3.1. Ideal Gas Temperature Change Thermodynamic Path ............................ 43

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5.2.3.2. Liquid Temperature Change Thermodynamic Path ................................. 48 5.2.3.3. Thermodynamic Properties of Water ....................................................... 53 5.2.3.4. Comparison of Estimated Thermodynamic Properties ............................ 54 5.2.3.5. Theoretical Analysis of Equilibrium ........................................................ 58 Constant from Activity Coefficient .......................................................... 58 5.2.4. Reaction Rate Constant ................................................................................... 60 5.2.5. Kinetic Parameters .......................................................................................... 65 5.2.5.1. Activation Energy and Frequency Factor ................................................ 65 5.2.5.2. Activation Enthalpy and Activation Entropy ........................................... 68 5.2.6. Effect of Temperature ..................................................................................... 70 5.2.6.1. Uncatalyzed Lactic Acid Esterification System ...................................... 70 5.2.6.2. Uncatalyzed Levulinic Acid Esterification System ................................. 71 5.2.6.3. Uncatalyzed Acetic Acid Esterification System ...................................... 73 5.2.6.4. Uncatalyzed Formic Acid Esterification System ..................................... 74 5.2.6.5. Uncatalyzed Linoleic Acid Esterification System ................................... 75 5.2.7. Effect of Metallic Substance ........................................................................... 77 5.2.7.1. Conversion and Reaction Rate ................................................................. 77 5.2.7.2. Corrosion.................................................................................................. 79 5.3. Conclusions ............................................................................................................ 80 CHAPTER 6: IMPACT AND FUTURE WORK ............................................................. 82 REFERENCES ................................................................................................................. 84 APPENDIX ....................................................................................................................... 91 A.1. Preparation of Calibration Standards for .............................................................. 91 Esterification of Carboxylic Acids ........................................................................ 91 A.1.1. HPLC Standard Calibration Curves ............................................................... 91 A.1.2. Standard Calibration Curve for Formic Acid Titration .................................. 95 A.1.3. Raman Spectroscopy Standard Calibration Curves ....................................... 96 A.2. Derived Equation for Reaction Rate Constant in term of Carboxylic Acid Conversion................................................................................. 98 A.3. Experimental Results for Uncatalyzed Esterification Reaction .......................... 101 A.4. Estimation of Parameter for Thermodynamic properties .................................... 131

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LIST OF TABLES

Table 1. Selected Catalysts for Esterification Systems ....................................................... 6

Table 2. Published Kinetics Parameters for Esterification Reaction ................................ 18

Table 3. Comparison of fluid-phase characteristics .......................................................... 20

Table 4. Critical Parameters of Common Substances ....................................................... 21

Table 5. Range of Reaction Conditions in Batch Reactors ............................................... 29

Table 6. HPLC Analysis Conditions for Esterification Systems ...................................... 33

Table 7. Values of Equilibrium Constants for Esterification of Carboxylic Acids .......... 37

Table 8. Values of Thermodynamic Parameters for Esterification of Short-chain Carboxylic Acids ............................................................................................... 39

Table 9. Values of Thermodynamic Parameters for Esterification of Linoleic Acid ....... 41

Table 10.Theoretical Thermodynamic Values using Ideal Gas Temperature Change Thermodynamic path ......................................................................................... 55

Table 11.Theoretical Thermodynamic Values using Liquid Temperature Change Thermodynamic path ......................................................................................... 56

Table 12. Experimental and Calculated Equilibrium Constants and Equilibrium Conversion for Esterification of Acetic Acids ................................................... 58

Table 13. Values of Forward Reaction Rate Constants (k f ) for Esterification of Carboxylic Acids ............................................................................................... 65

Table 14. Values of Kinetic Parameters for Uncatalyzed Esterification of Carboxylic Acids ............................................................................................... 67

Table 15. Values of Activation Enthalpy and Activation Entropy for Uncatalyzed Esterification of Carboxylic Acids ..................................................................... 69

Table A.1. Concentrations of Standard Compounds for Calibration Curves ................... 91

Table A.2. Concentrations of Standard Formic Acid for Calibration Curves .................. 95

Table A.3. Molar Ratio of Calibration Mixtures .............................................................. 97

Table A.4. Formic Acid Conversion at 373 K ................................................................ 102

Table A.5. Formic Acid Conversion at 423 K ................................................................ 103

Table A.6. Formic Acid Conversion at 473 K ................................................................ 104

Table A.7. Formic Acid Conversion at 523 K ................................................................ 105

ix

Table A.8. Acetic Acid Conversion at 373 K ................................................................. 106

Table A.9. Acetic Acid Conversion at 423 K ................................................................. 107

Table A.10. Acetic Acid Conversion at 473 K ............................................................... 108

Table A.11. Acetic Acid Conversion at 523 K ............................................................... 109

Table A.12. Lactic Acid Conversion at 298 K................................................................ 110

Table A.13. Lactic Acid Conversion at 313 K................................................................ 111

Table A.14. Lactic Acid Conversion at 333 K................................................................ 112

Table A.15. Lactic Acid Conversion at 353 K................................................................ 113

Table A.16. Lactic Acid Conversion at 373 K................................................................ 114

Table A.17. Lactic Acid Conversion at 423 K................................................................ 115

Table A.18. Lactic Acid Conversion at 473 K................................................................ 116

Table A.19. Lactic Acid Conversion at 523 K................................................................ 117

Table A.20. Lactic Acid Conversion at 523 K in the presence of Inconel 625 Material .................................................................................. 118

Table A.21. Levulinic Acid Conversion at 333 K .......................................................... 119

Table A.22. Levulinic Acid Conversion at 353 K .......................................................... 120

Table A.23. Levulinic Acid Conversion at 373 K .......................................................... 121

Table A.24. Levulinic Acid Conversion at 423 K .......................................................... 122

Table A.25. Levulinic Acid Conversion at 473 K .......................................................... 123

Table A.26. Levulinic Acid Conversion at 523 K .......................................................... 124

Table A.27. Linoleic Acid Conversion at 373 K ............................................................ 125

Table A.28. Linoleic Acid Conversion at 423 K ............................................................ 126

Table A.29. Linoleic Acid Conversion at 473 K ............................................................ 127

Table A.30. Linoleic Acid Conversion at 523 K ............................................................ 128

Table A.31. Linoleic Acid Conversion at 573 K ............................................................ 129

Table A.32. Linoleic Acid Conversion at 623 K ............................................................ 130

Table A.33. Constants for Heat Capacity ....................................................................... 131

x

Table A.34.Estimated C p dT and C p dT/T Values for Ideal Gas Temperature Change Thermodynamic Path ................................................. 132

Table A.35. Estimated C pliq dT and C pliq dT/T Values for Liquid Temperature Change Thermodynamic Path ................................... 132

Table A.36.Estimated Residual Properties for Ideal Gas Temperature Change Thermodynamic Path ................................................. 133

Table A.37. Estimated Residual Properties for Liquid Temperature Change Thermodynamic Path ................................... 133

Table A.38. Vapor Pressure Correlation Parameters for water ...................................... 135

Table A.39. Calculated Water Vapor Pressure ............................................................... 135

Table A.40. Estimated Pressure ...................................................................................... 136

Table A.41. Molar properties of formation at 298 K ...................................................... 136

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LIST OF FIGURES

Figure 1. Pressure-Temperature Phase Diagram for model Supercritical Fluid ............... 19

Figure 2. Pressure-Density Phase Diagram for Pure substance ........................................ 20

Figure 3. Experimental Setup of a Batch Reactor for Esterification Reaction ................. 31

Figure 4. Effect of Temperature on Equilibrium Constant for Uncatalyzed Esterification of Carboxylic Acids with Ethanol .......................... 38

Figure 5. Ideal Gas Temperature Change Thermodynamic Path Diagram ....................... 48

Figure 6. Liquid Temperature Change Thermodynamic Path Diagram ........................... 52

Figure 7. 2 nd -order rate plot for Uncatalyzed Esterification of Lactic Acid with Ethanol ................................................................................... 61

Figure 8. 2 nd -order rate plot for Uncatalyzed Esterification of Levulinic Acid with Ethanol .............................................................................. 62

Figure 9. 2 nd -order rate plot for Uncatalyzed Esterification of Acetic Acid with Ethanol ................................................................................... 62

Figure 10. 2 nd -order rate plot for Uncatalyzed Esterification of Formic Acid with Ethanol ............................................................................... 63

Figure 11. 2 nd -order rate plot for Uncatalyzed Esterification of Linoleic Acid with Ethanol ............................................................................. 63

Figure 12. Arrhenius Plot for Uncatalyzed Esterification of Carboxylic Acids with Ethanol ....................................................................... 66

Figure 13. Eyring Plot for Uncatalyzed Esterification of Carboxylic Acids with Ethanol ............................................................................................................ 68

Figure 14. Conversion-time profile at various temperatures for Uncatalyzed Esterification of Lactic Acid with Ethanol ..................................................... 70

Figure 15. Conversion-time profile at various temperatures for Uncatalyzed Esterification of Levulinic Acid with Ethanol ................................................ 72

Figure 16. Conversion-time profile at various temperatures for Uncatalyzed Esterification of Acetic Acid with Ethanol ..................................................... 73

Figure 17. Conversion-time profile at various temperatures for Uncatalyzed Esterification of Formic Acid with Ethanol .................................................... 75

Figure 18. Conversion-time profile at various temperatures for Uncatalyzed Esterification of Linoleic Acid with Ethanol .................................................. 76

Figure 19. Conversion vs. time for Uncatalyzed Esterification of Lactic Acid with and without Inconel 625 material at 523 K ..................................................... 77

xii

Figure 20. Reaction Rate Constant for Uncatalyzed Esterification of Lactic Acid with and without Inconel 625 material........................................ 78

Figure 21. Reactors with intensity of green color formation after Uncatalyzed Esterification of lactic Acid ....................................................... 79

Figure 22.Reactors with different metallic substances exposed to Uncatalyzed Esterification of lactic acid at 523 K. ......................................... 80

Figure A.1. Calibration Curve for Lactic Acid ................................................................ 92

Figure A.2. Calibration Curve for Ethyl Lactate ............................................................. 92

Figure A.3. Calibration Curve for Ethanol for lactic acid Esterification ......................... 93

Figure A.4. Calibration Curve for Levulinic Acid ........................................................... 93

Figure A.5. Calibration Curve for Ethyl Levulinate ........................................................ 93

Figure A.6. Calibration Curve of Ethanol for Levulinic Acid Esterification .................. 94

Figure A.7. Calibration Curve for Acetic Acid ................................................................ 94

Figure A.8. Calibration Curve for Ethyl Acetate ............................................................. 94

Figure A.9. Calibration Curve of Ethanol for Acetic Acid Esterification ....................... 95

Figure A.10. Titration of standard Formic Acid .............................................................. 96

Figure A.11. Spectra of standard mixtures of linoleic acid and ethyl linoleate in ethanol .............................................................................. 98

Figure A.12. Calibration curve for the conversion of linoleic acid to ethyl linoleate ..... 98

1

CHAPTER 1: INTRODUCTION 1.1. Motivation The increased demand for energy, concerns about climate change, national security, energy dependence, and the need to reduce the environmental impacts from traditional fossil based chemicals, solvents, and fuels have led to a growing interest in alternative and renewable resources. 1-4 Among the several renewable resource options available, biomass is an important resource that can be converted to energy, chemicals, and petroleum compatible products. For example, polysaccharides, which are major components of biomass, can be converted to ethanol and various organic acids (carboxylic acids) useful for esterification. For the conversion of biomass-derived resources into useful chemicals, esterification of biomass-derived carboxylic acids has been initiated by the chemical industry to synthesize corresponding esters, which have significant applications in various areas like pharmaceuticals, plasticizers, solvents, food flavors, coating, and fragrance. 5-8 Some esters are converted into their derivatives, which are useful as chemical intermediates and monomers for resins and high molecular weight polymers. 9 Therefore, the conversion of biomass-derived ethanol and carboxylic acids into value-added esters has motivated the focus of this study.

1.2. Problem Identification Esterification is a well-known process, from which important chemicals such as methyl, ethyl and butyl esters of carboxylic acids have been produced. However, at atmospheric condition, esterification is a reversible reaction limited by the low equilibrium conversion and slow reaction rate, and has recently been performed with

2

excess alcohol and/or by continuous removal of water by azeotropic distillation to shift the equilibrium conversion. This reaction is generally carried out in batch reactors in the presence of homogeneous or heterogeneous acid catalysts and also with supported heterogeneous acid catalysts in a fixed bed reactor with concurrent down-flow of the liquid phases to achieve acceptable reaction rates. 5-8, 10-37 Although the acid-catalyzed process has been extensively developed, it has at least the following inherently undesirable drawbacks: 11-18,29 the homogeneous acid catalyst can erode process equipment; miscibility of acid catalyst with the reaction medium requires expensive downstream separation operations; there are possible side reactions such as dehydration and etherification; and acid disposal can be an environmental issue. In addition, the heterogeneous acid-catalyzed reactions can be mass transfer limited, require complex catalyst pre-treatment, suffer from deactivation of solid catalyst, and require longer reaction time than the homogeneously acid-catalyzed esterification system. Hence, an uncatalyzed esterification reaction that provides cleaner routes for synthesizing a wide variety of industrial products due to ease of separation and purification of the products without contamination is essential. 1.3. Research Justification Supercritical fluid (SCF) technology has received considerable attention over the last few years in the chemical industry due to their favorable gas-like low mass transfer resistance and liquid-like high solvating capacity properties. Performing reactions under supercritical conditions rather than in the conventional gas or liquid phase could be an interesting option for improving the equilibrium conversion, enhancing the reaction rate and making the process more environmentally friendly. 38 It has also been reported that at

3

supercritical state, the dielectric constant of alcohol decreases, and this allows supercritical alcohol to become a better solvent for organic compounds, which consequently enables a homogeneous reaction system with more favorable kinetics. 39

Supercritical ethanol (SC EtOH ), whose critical temperature and pressure are 240 °C and 6.15 MPa, respectively, has received attention as an alternative reaction medium because of its positive effects on the reaction rate, selectivity, and yield. 40-41 Ethanol has been a good choice as a reaction medium because presently, ethanol can be easily produced from biomass. Thus, ethanol will be used both as reactant and reaction medium for the uncatalyzed esterification reaction system. In addition, research on production of esters from carboxylic acids and ethanol have addressed important issues concerning product distribution, catalyst activity, and kinetic studies of acid-catalyzed esterification at lower reaction temperatures, but kinetics of uncatalyzed esterification at elevated reaction temperatures is still very limited. It is thus of great interest from a practical point of view that more information such as kinetic and thermodynamic parameters are available to develop a possible esterification process without using any catalyst. In this work, therefore, a fundamental study on the uncatalyzed esterification of different aliphatic carboxylic acids with stoichiometric amount of ethanol was undertaken to investigate the possibility of converting the biomass-derived carboxylic acids to ethyl esters and to determine the kinetic and thermodynamic parameters for the uncatalyzed esterification using isothermal batch reactors.

4

CHAPTER 2: BACKGROUND 2.1. Biomass-Derived products Biomass, being a viable alternative to fossil material, is a biological material derived from living organisms such as plant, waste and other derived organic matter available on a renewable basis. Of the types of biomass, lignocellulosic biomass (e.g. wood and switchgrass), which is the main naturally available biomass resource, has been used in a variety of applications ranging from engineering materials to a source of energy. Lignocellulosic biomass is mainly a mixture of cellulose (38-50 wt%), hemicelluloses (23-32 wt%), and lignin (15-25 wt%) which are held together by covalent bonding, various intermolecular bridges, and van der Waals forces, forming a complex structure. 4 While lignocellulosic biomass material is not only useful as direct heating source, its degradation products are also potentially useful intermediates in industry. The significance of glucose from cellulose hydrolysis is evident in its fermentation product, cellulosic ethanol. Glucose, which can be produced from hydrolysis of cellulose, can be degraded into other small molecules such as 5-hydroxymethylfurfural (5-HMF), lactic, acetic, formic and levulinic acids. 42 These acids have important economic values. These products from the degradation of lignocellulosic biomass have recently been obtained with supercritical fluid technology. 4, 42

2.2. Catalysis of Esterification Reaction Esterification can be defined as the transformation of carboxylic acids or their derivatives into esters, and esterification is an important class of reactions in which the kinetics have been investigated, dating back to the innovative efforts of Berthelot and Gilles in 1862. 43 There are many available reaction routes, such as solvolytic,

5

condensations and free radical processes for the preparation of esters. 44 The most widely applied method is the direct esterification of carboxylic acid with alcohol in presence of homogeneous mineral acid or a heterogeneous catalyst. However, the conventional acid- catalyzed esterification system suffers from problems associated with the generation of side reactions (such as etherification, and dehydration), corrosion of equipments, expensive purification procedures, long reaction times and discharge of acidic wastes. 11- 18, 29, 44 Synthesis of ester compounds by the treatment of carboxylic acids with alcohol is a reversible reaction, wherein water is a byproduct (Reaction 1).

6

chain acids react more rapidly than branched acids; particularly, branching at the α- position lowers the rate of esterification. Esterification of aromatic acids, like benzoic acid, is generally slow. Esterification of benzoic acid with methanol containing isotopic oxygen ( 18 O) has shown that the oxygen in the water formed during acid-catalyzed esterification originates from the acid, not from the alcohol. 45 The mechanism of acid- catalyzed esterification reaction has been discussed in detail by Zimmermann and Rudolf. 46

Table 1. Selected Catalysts for Esterification Systems HOMOGENEOUS HETEROGENEOUS Sulfuric acid, Toluenesulfonic acid, Phosphoric acid, Hydrochloric acid, Ferric sulfate hydrate, Methyl thiosulphate, Aluminium trichloride, Niobic acid, Alkyl benzene sulfonic acid, Hydrogen Iodide. Ion exchange resins, Enzymes, Ion exchange membranes, Solid super acids, Polyolefin supported sulfonic acid, Solid super acid of Zr(OH) 4 exposing with H 2 SO 4 by calcinations, Solid acid having salts of H 3 PW 12 O 40 CS 2 .5PW 12 O 40 , Heteropolyacids supported on activated carbon, Solid acids of metal oxide promoted with SO 4 2- , H 3 PO 4 supported on silica gel.

Esterification reaction is a slow equilibrium-limited reaction. The equilibrium must be shifted toward the product side by excess use of one of the reactants or continuous removal of one of the products, especially water, by azeotropic distillation. Ester synthesis through direct esterification between carboxylic acid and alcohol is conventionally conducted with the aid of external acid catalysts, but compatibility of the catalysts with other functional groups in the reaction medium is an important problem to address. Possible undesired side reactions of the conventional homogenously acid- catalyzed reaction are shown in Reactions 2 and 3.

7

C H OH H SO CH CH 2H O SO 2 5 2 4 2 2 2 3 Ethanol SulfuricAcid Ethylene Water Sulfite (2) + → = + + + → + +

C H OH H SO CH CH OCH CH 2H O SO 2 5 2 4 3 2 2 3 2 3 Ethanol SulfuricAcid Diethyl ether Water Sul fite (3) + → + + + → + +

Reactions 2 and 3. Possible Side Reactions in Catalytic Esterification Reaction

Thus, the catalyst-free (uncatalyzed) reaction which provides cleaner routes for ester synthesis due to ease of separation and purification of the products without contamination is more desirable. Uncatalyzed reaction is enabled when reaction is carried out at high temperatures. 2.2.1. Homogeneous Acid-Catalyzed Esterification Homogeneous catalysts such as sulfuric acid, hydrochloric acid, hydrogen iodide, phosphoric acid, p-toluenesulfonic acid, and mixtures of acids are efficient homogenous catalysts generally used for acid-catalyzed esterification. The homogenous acid-catalyzed esterification of carboxylic acids have been reported to give higher conversion than the heterogeneous acid-catalyzed esterification system, because the heterogeneous catalysts have been shown to exhibit limitations for catalyzing esterification due to low thermal stability, mass transfer resistance or loss of active acid sites (adsorption of reactants and swelling nature) in the presence of a polar medium. 14,36-37,76 But the disadvantage of homogenous mineral acids is their miscibility with the reaction medium leading to equipment corrosion and separation problems. The homogeneous method involves passing the alcohol through the mixture of carboxylic acid and homogenous acid catalyst already preheated to a temperature above the alcohol boiling temperature. 17, 31 Ronnback

8

et al. studied the kinetics of esterification of acetic acid with methanol in presence of hydrogen iodide. 2.2.2. Heterogeneous Acid-Catalyzed Esterification Heterogeneous acid-catalyzed esterification reactions have been investigated by various researchers. The process involves the reaction of carboxylic acid with alcohol in presence of insoluble solid acid catalyst at a temperature below the alcohol boiling temperature. Some of the solid-acid catalysts studied are ion-exchange resins, 5,11,15,18,22,32

acid-treated clays, 21 heteropolyacids, 20,29,30,34 Zeolite-T membrane, 33 and immobilized enzymes. 68 Nevertheless, of all the reported heterogeneous catalysts, ion-exchange resin and enzyme catalysts are reported to be most effective for heterogeneous phase esterification reactions. However, deactivation of solid catalysts does occur due to aging, aqueous adsorption, and swelling nature. 29, 66-68

2.2.2.1. Ion exchange resins as catalysts Ion exchange resin catalysts have been used for several years in esterification reactions. Ion exchange material is broadly defined as an insoluble matrix containing labile ions capable of exchanging with ions in the surrounding medium without major physical change in its structure. 77 Typical resin catalysts are sulphonic acids fixed to a polymer carrier, such as polystyrene cross-linked with divinylbenzene (DVB). Several commercially available catalysts are Amberlyst resins such as Amberlyst–15 and Amberlite IR-120. Unlike catalysis by dissolved electrolytes, with resins diffusion, adsorption, and desorption processes are present, and the concentration of the reactants at the active site of the catalyst (where the reaction takes place) may be different from that

9

in the bulk solution. Also, the matrix with the fixed ionic group may have some influences that are not purely physical on the course of the reaction. 78

2.2.2.2. Enzymes as catalysts Enzymes have been widely used in esterification technology. In particular, lipase enzymes are used for the resolution of racemic alcohols and carboxylic acids through asymmetric hydrolysis of the corresponding esters. Yeast (Candida cylindracea) completely converts a carboxylic acid and an alcohol into the corresponding ester in the presence of organic solvents, in a highly stereoselective manner. 79 Enzymes are highly stable in organic solvents compared to water. To predict the behavior of batch enzymatic reactors, Mensah and co-workers developed a model incorporating reaction kinetics, water partitioning, and mass transfer effects. Also conducted were experimental and theoretical studies on immobilized enzyme catalyzed esterification of propionic acid with isoamyl alcohol in presence of hexane as solvent to understand the dynamic behavior of a continuous-flow packed-bed reactor. 80

Full document contains 155 pages
Abstract: To shift from a petroleum-based to a biomass-based economy will require the development not only of biofuels, but also of biorenewable replacements for petroleum-derived chemicals. In this regard, environmentally friendly biomass-derived esters may serve as alternatives to fossil-derived chemicals such as toxic halogenated solvents and glycol ethers. Therefore, esterification of various carboxylic acids that find significant applications in the chemical, pharmaceutical, petrochemical, food, and cosmetic industries has been initiated by the chemical industry. At atmospheric condition, esterification is a reversible reaction limited by the low equilibrium conversion and slow reaction rate, and has recently been performed with excess alcohol to shift the equilibrium conversion. Heterogeneous or homogeneous acid catalysts are used to achieve acceptable reaction rates. The conventional acid-catalyzed process has been extensively developed; but it suffers from problems associated with the generation of side reactions, corrosion of equipment, expensive purification procedures, long reaction times and discharge of acidic wastes. Various attempts on esterification of carboxylic acids with ethanol have previously addressed important issues concerning product distribution, catalyst activity, and kinetics of acid-catalyzed esterification at lower reaction temperatures, but kinetics of uncatalyzed esterification at elevated reaction temperatures are still very limited. It is thus of great interest from a practical viewpoint that more information such as kinetic and thermodynamic parameters are required to develop a possible esterification process without using any catalyst. In this work, therefore, a fundamental study on the uncatalyzed esterification of different aliphatic carboxylic acids with stoichiometric amounts of ethanol was undertaken to examine the possibility of converting the biomass-derived carboxylic acids to ethyl esters and to determine the kinetic and thermodynamic parameters for the uncatalyzed esterification. Experiments were conducted with isothermal batch reactors at temperatures ranging from 298 K to 623 K. A 2 nd -order reversible kinetics rate expression was used to fit the experimental data. The thermodynamic and kinetic values estimated were found to vary for different esterification systems studied. The dependence of Keq on temperature for esterification of short-chain and long-chain carboxylic acids varied. Despite the nonlinearity of the Van't Hoff plot for esterification of linoleic acid, the Arrhenius and Eyring plots were linear. Two thermodynamic paths were developed for estimating the equilibrium conversions, and the theoretical values compared well with the experimental results reported in this study. Additional experiments performed to assess the corrosive and catalytic influences of metallic materials on esterification reaction indicated Inconel 625 alloy, nickel wire and stainless steel materials have potential corrosion problems on the uncatalyzed esterification reaction at elevated reaction. However, tantalum and grade 5 titanium materials showed acceptable level of compatibility for similar reaction conditions, and this can encourage the design of a flow reactor system.