esterification under influence of external fields: a...

Esterification under Influence of External Fields: a Review

Dr. M.S.Patil*, Mr. V.D.Gurudasani**

** (Department of chemical engineering, Anuradha Engineering college Chikhli, India

Email: [email protected])

Abstract

Esters are very important chemical having wide application in perfumes, flavors,

Pharmaceuticals, plasticizers, solvents etc. An overview of esterification and transesterification

under non classical ways of energy input- microwave and ultrasound irradiation has been

presented here. General aspects of microwave and ultrasound irradiation are described.

Esterification reaction has been studied using variety of catalyst under microwave and

ultrasound irradiation. Rare literature is available on esterification using ionic liquid under

microwave and ultrasound irradiation. Many esterification reactions using several novel

heterogeneous catalysts are yet to be investigated under microwave and ultrasound irradiation.

Key words: Esterification,Microwaves,Ultrasound,Ionic liquids

Introduction

One of the ways to minimize the use of energy, is to make the energy input to the

process as efficient as possible. Microwave irradiation and ultrasonic waves are non

classical forms of energy input and are more efficient than the conventional ways

(classical forms) of energy input. These are non contact energy sources and are applied

externally. In order to minimize energy input in chemical processes, the "Reactions under

Influence of External field” is new hot topic.

Advantages of applying non contact energy sources, includes quick energy

absorption, selective heating, decreased thermal gradient. For chemical synthesis, such

more effective heating leads to faster start up, reduced equipment size and elimination of

process steps.

Journal of Interdisciplinary Cycle Research

Volume XI, Issue XI, November/2019

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*(Principal,Pratap Institute of Management and Technology, Washim, (M.S.) IndiaEmail: [email protected])

Microwaves

Microwaves have an extremely beneficial effect on reactions involving

compounds with high dielectric constant [1]. Microwaves are absorbed by these

compounds with a resultant increase in temperature and molecular activity. The rate of

the reaction, as a consequence, is found to be increased several fold [1].

Microwave heating of reactants is extremely advantageous since it does not

involve conductive heating. It has been reported that alcohols boil at much higher

temperatures under microwave heating as compared to their normal boiling points (for

example, methanol under microwave begins to boil at 98o as against the normal boiling

point of 65o). This enables higher reaction temperatures and consequently higher reaction

rates under microwave heating. Reaction times get reduced to minutes from hours.

The advantages of applying microwave power, a non contact energy source, into

the bulk of a material include: includes quick energy absorption, selective heating,

decreased thermal gradient. For chemical synthesis, such more effective heating leads to

faster start up, reduced equipment size and elimination of process steps.

With microwave knowledge and by combining several techniques in an intelligent

way it is possible to accelerate processes and to reduce the chemical uses. Microwave

technology can be very useful for chemical preparations , as products are heated directly

no convection and conduction is involved in heating. This result in a reduction of the

processing time, no overheating and degradation of the product . This preserve the

quality of product.



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Fig.1: Conventional heating and MW heating

Ultrasound

Ultrasound is the name given to sound waves having frequencies higher

than those to which human ears can respond, i.e. greater than 16 kHz and with

wavelength between 7.0 and 0.015 cm. It is transmitted through any substance – solid,

liquid or gas, which possesses elastic properties. The first commercial application of

ultrasonic appeared in 1917 with echo-sound technique for the estimation of depths of

water resulting in the system known as SONAR (sound navigation and ranging). Some of

the broader applications of ultrasound in various fields are for homogenization and cell

disruption in biology and biochemistry; to assist in drilling, grinding, cutting, welding of

hard materials and testing of materials in engineering; for cleaning and drilling teeth in

dentistry; for dispersal of pigments and solids in paints, inks, resins; for acoustic filtration

and ultrasound drying in industry; for ultrasound imaging in obstetrics and treatment of

muscle strains in medicine (frequency range 1 – 10 MHz); for welding of thermoplastics,

polymer degradation, curing of resins and initiation of polymerization in plastics and

polymers ; for breakdown of aromatic pollutants and cell disruption of bacteria in waste

treatment, to name a few.

Ultrasound has also been employed for specific chemical applications resulting in

a sub-discipline called ‘Sonochemistry’. Sonochemistry arises from acoustic cavitation:



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the formation, growth, and implosive collapse of bubbles in a liquid. Ultrasonic waves

have an extremely beneficial effect on reactions. Ultrasound has been used in a variety of

applications like polymerization, emulsification and esterification [2]. Cavitation refers to

the formation and the subsequent dynamic life of bubbles in liquids. Experimental results

have shown that these bubbles have temperatures around 5000 K, pressures of roughly

1000 atm, and heating and cooling rates above 1010 K/s[2]. In aqueous systems the

tearing of the bubbles can lead to the formation of hydrogen and hydroxyl radicals in the

gas zone. These can react further to produce hydrogen peroxide thus enhancing the rate

of an esterification reaction.

A number of common reactions used in synthetic organic chemistry can be carried

out more efficiently using ultrasound. There are several advantages. Generally, the yield

increases and the percentage of by-products decrease. Reactions occur faster, so that

lower temperatures can be used. Ultrasound provides alternative pathways for reactions,

due to the formation of high energy intermediates [1-3].

Esterification

Esterification is one of the most important unit processes in chemical

industry. Esters are strategically important because they are used in a wide range of

products such as armaments (moderants for propellants), cosmetics, biofuels,

pharmaceuticals and plasticizers. Various esterification reactions have been widely

investigated under conventional heating. There is very less investigation on the

esterification under non classical form of heating.

Catalyst for esterification

Catalyst is used in the esterification process to enhance or accelerates chemical

reaction process. Catalysts can be divided into two types, homogenous catalysts such as

the strong liquid mineral acids, such as sulphuric acids and hydrochloric acids,

heterogeneous catalyst such as solid acid catalysts with mainly Bronsted acid sites.



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The homogeneous catalysts suffer from several drawbacks, such as the existence

of side reaction with reactant, corrosive nature and the separation of catalyst from

products is difficult plus environmental threats. The alternative way to overcome these

drawbacks is using heterogeneous catalyst. Solid acid catalysts properties are not

corrosive, can be coated onto a support and easily reused. Ion–exchange

resins,zeolites,sulfated zirconia and niobium acid are the examples of catalysts used in

esterification reactions. In recent years research have been carried out on number of

esterification reaction in ionic liquids the ionic liquid acts as both solvent and as a

catalyst [4-8]

Esterification under microwaves

Most of published work on esterification has been performed using domestic

microwave oven (power up to 1000 W frequency 2.45 G Hz). Key reason for using

domestic microwave oven is that they are readily available and inexpensive. To avoid

explosion, domestic MW ovens are modified to provide reflux system. Continuous flow

microwave reactors for lab scale have also been developed. It consists of glass tube

placed in microwave cavity. Continuous flow heterogeneous catalyzed reaction can be

carried out by localizing the catalyst in the glass tube.

Commercial scale up of microwave heating is achieved in many industries

including food, ceramics and mineral processing [1].but it has not proved easy in organic

synthesis. One most important reason is microwave has limited penetration depth defined

by materials dielectric properties. Therefore retrofitting of microwave heating to large

stirred vessel is not possible option. Only outside layer gets heated. An in- pipe

configuration with continuous processing is best strategy to scale the microwave assisted

organic synthesis. The reaction time obtained in small scale laboratory study is translated

into residence time within the microwave heating system.



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Fig 2: Lab scale batch type microwave reactors

Ample of work is available on transesterification of triglycerides with alcohol to

produce biodiesel [9]. Best operating conditions have been worked out for several type of

vegetable oil under variety of homogeneous and heterogeneous catalyst. (viz NaOH,

Na2CO3. k2CO3 ,Ba(OH)2 ) .Application of microwave offer fast and easier rout to this

valuable biofuels production. Esterification of triglycerides was studied using variety of

heterogeneous catalysts [10-12] .Study shows that this methodology allows the use of

high FFA content feed stock including used cooking oil [11]. Selective heating is

significant in heterogeneously catalyzed reactions. Rate enhancement is achieved because

of microwave induced localized superheating [29-30]. In order to obtain milder reaction

condition ,great number of non ionic base have been developed and used as catalyst for

trans esterification .Among these bases amines triethylamins, piperidine,

pentamethylpiperidine, pyridine 2,6-di-tert-butylpyridine, 4-dimethyl-aminopyridine1,

nitroguanidines, are frequently used in organic synthesis [9]. Literature review shows that

most of catalysts are yet to be studied under microwave heating.

The effect of microwave on enzyme catalyzed esterification and transesterification

has been studied [12- 14] . In all cases, microwave heating was found to increase the



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reaction rate by 2.1-4.7 times. It is also found that stability of enzyme is superior under

microwave heating as compared to conventional heating. The enzyme catalyzed reactions

needs milder reacting conditions and selective heating. Microwave assisted enzymatic

esterification investigations reveals that under microwave apparent activation energy of

enzymatic reaction is reduced .It is one of the cause of increasing the reaction rate [27].

Fatty acid esters of sugar (sugar esters) can be obtained by esterification of fatty

acid or by transesterification of their corresponding alkyl esters with sugar. Sugar esters

are non ionic biodegradable surfactants. They are widely used in food, cosmetic,

pharmaceutical and detergent industries. Synthesis of sugar esters can be carried out

either chemically or enzymatically. Chemical process is carried out under high temp and

leads to coloration of final product. Enzymatic method of sugar ester synthesis is

performed under milder conditions. It needs long reaction time or gives low yields.

Advantages of microwave heating are evident in term of high reaction rate and good

purity of product. However application of microwave to enzymatic synthesis of sugar

ester is still little explored. More work is needed particularly on regioselective aspects in

enzymatic synthesis of sugar esters.

Large number of industrially important esters is synthesized by Fischer type

esterification between carboxylic acid and alcohols. Moderate work is available on

Fischer type esterification under microwave heating in presence of homogeneous as well

as heterogeneous catalyst [15-22].Rate enhancement is observed in all cases. Authors

point out that in case of heterogeneously catalyzed reaction, apart from increased

molecular vibration and superheating, the increased mass transfer rate between catalyst

and liquid contributes the major role in rate of reaction. Most of work is in lab scale batch

process. Study lacks in Lab scale continuous process. Also it needs to study the Fischer

esterification in pilot scale continuous process under microwave heating. Most of work is

done to find best operating conditions. Work is needed particularly in term of heat

modeling [30] along with kinetic modeling to predict of exact energy requirement.



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Now a day various ionic liquids are being invented as a reaction media for

organic synthesis. Use of ionic liquid as a solvent has many advantages. Ionic liquids

itself acts as a catalyst for esterification. Ionic liquids can be removed easily from

reaction mixture and can be recycled several times without significance loss of their

catalytic activity. Use of ionic liquids in transesterification and in esterification is

thoroughly documented in the literature [23-25]. Silica sulfuric acid (ionic liquid)

catalyzed Fischer esterifications have been studied under conventional heating. It takes

long time, from 2 hrs to 8 hrs to achieve 85-90% conversion [5]. Investigation on Fischer

esterification of alcohols with carboxylic acid using ionic liquid 1-butylpyridium chloride

alluminium (III) showed 65 -80% conversion in 2 hours [7]. Reaction time can be

reduced if same reaction is carried out under microwave. Since concept of using ionic

liquids as a catalyst for esterification is relatively new ,only few publications involving

esterification using ionic liquids under microwave can be found in literature .In Fischer

esterification of benzoic acid with variety of alcohol using Bronsted acid ionic liquids

under the microwaves yield was 77-98% [26]. Enzymatic esterification of lactic acid was

studied under microwave in phosphonium type ionic liquid. The combined advantages of

microwave and ionic liquid resulted in higher yield [28]. Most widely used ionic liquids

are probably N,N-dialkylimidozolium salts [8 ].Its water stable varieties can be used in

esterification. There is wide scope to study the esterification under microwave using the

water stable varieties of ionic liquids. Work should be done with aim to find scope and

limitations of microwave heating together with ionic liquids.

Catalytic esterification involves expensive procedure of separation of catalyst

from the product. It is found that under microwave heating, rate of esterification is

considerably high even in absence of catalyst. Non catalytic esterifications of fatty acid

have been studied. Results showed conversion up to 60% in one hour [31].Further

research in a non catalytic esterification under microwave is needed, because use of

catalyst in esterification is expensive.



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Esterification under ultrasound

Lab scale organic synthesis using ultrasound is generally carried out using

ultrasound horn or ultrasound bath (fig.3). These apparatus operates at frequency upto 40

Khz with rated power upto 240 W . Bath type reactors can be incorporated in the system

for pilot scale operation by increasing the number of transducers. Large scale ultrasound

reactors are either batch or continuous type. Batch types are appropriate where lower

power is required. Flow type reactor consists of an intense sonication zone made up of

transducers. The process liquid flows through sonication zone.



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Fig 3 : Schematic of the typical experimental setup used for ultrasound assisted reaction i) Horn dipped in reaction vessel ii)

Reaction vessel in ultrasonic bath

The production of alkyl esters using ultrasound has been studies by several

researchers [32-42]. In the study of the methanolysis of cotton, sunflower and sesame

oils, yield obtained was from 43 to 93% depending on the operating condition applied

[33]. Compared with traditional mechanical process, it is found that the yields of

biodiesel are always higher when ultrasound was applied [33]. In study of the

methanolysis of soybean oil,[34,35] yields was from 69 to 100%. In one of the

investigation in production of biodiesel using several types of alcohols, it was found that

increasing the length of the chain of the alcohol reduces the yields of biodiesel[36].

Transesterification using methanol resulted in yields from 68 to 98 %, while upon using

n-propanol the yield reduces to 92% under the best operating condition.

The biocatalytic rout of esterification is economical and environmentally benign.

Sugar esters are produced using enzymes as a catalyst. Enzymatic synthesis of sugar

esters in non aqueous media is very slow process. Enzymatic synthesis of sugar ester was

carried out by various researchers using variety of enzymes under ultrasound results were

compared with classical method[43,44] .In all cases yield under ultrasound method was

double than that under conventional method. In case of enzymatic esterification under

ultrasound in production of sugar esters, mechanical vibration from ultrasound increases

the mass diffusion between liquid media and immobilized enzymes. This increases the

frequency of collision between molecules of substrate and enzyme, resulting in increase

in rate of reaction. Under ultrasound enzyme structure become flexible, same enzyme

shows different stereo configuration depending upon intensity of ultrasound [43,44]. This

phenomenon is important in synthesis of regio, sterioselective sugar esters. Further

research is required in this direction.



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Combination of ultrasound and Microwave as a hybrid technology

Ultrasound provide large amount of concentrated energy and microwave provide

dielectric heating and selective heating of solid particle. Hybrid reactors combine these

features together. Simplest lab scale hybrid reactor consists of an ultrasonic horn inside of

a microwave zone (fig.4). Combined reactors are at lab scale only. Industrial scale hybrid

reactor has not been developed yet.

Fig.4: MW/US Hybrid Reactors: a) US horn inside MW field; b) US horn inside MW field; c) US horn outside MW field

Few papers are available on esterification under simultaneous radiation of MW

and ultrasound. Fori chemant etal [45] studied heterogeneously catalyzed esterification of

stearic acid with butanol under microwave and ultrasound separately then under

combination of these two fields. Further rate enhancement was observed under

combination of both types of fields.



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Conclusion

Microwave and ultrasound, the non classical ways of energy input are convenient

to use in organic synthesis. With these, heating is instantaneous and very specific. No

contact is required between energy source and reaction vessel.

Esters constitute an important class of synthetic organic compound. Conventional

homogeneous mineral acid catalysts for esterification are being substituted by novel

heterogeneous catalyst due to environmental reasons. Ionic liquid shows excellent

catalytic activities for esterification reaction under mild conditions. Esterification using

novel heterogeneous catalyst and the esterification using non classical solvent (ionic

liquid) under non classical heating may be new environmentally benign approach. It

is needed to develop the models predicting reaction kinetic and yield under microwave

and ultrasound for several industrially important esterification processes. These models

will be helpful in determination of optimum operating procedures for studied

esterification reaction.

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