Microwave synthesis and their applicationsPresented by: Sheama Farheen Savanur

A green chemistry approach

Green chemistry is the utilization of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture and application of chemical products.

Out of the 12 principles of green chemistry, the following are taken care through MW synthesis

Prevention of waste Less hazardous chemical synthesis Design for energy efficiency Inherently safer chemistry for accident prevention

Introduction

Microwave chemical synthesis has become a method of choice for chemists throughout so many industries. The technology allows synthesis to be done faster and cleaner with reduced solvent consumption as a “greener” process.

Microwave chemistry is the science of applying microwave radiation to chemical reactions. Preparation of a desired compound from available starting materials via some (multi-step) procedure, involving microwave irradiation

The microwave region of the electromagnetic spectrum lies between IR and radio-frequency corresponding to wavelength 1 cm to 1 m (frequencies 30 GHz to 300GHz)

5 Principle ( Fundamental Theory)

Microwave radiation

Electric component

Dipolar polarization Ionic conduction

6Dipolar Polarization

• Loss Tangent (Energy Dissipation Factor) – a measure of the ability to absorb microwave energy and convert it into thermal energy (heat)• Derived from Maxwell’s equation tanδ = ε”/ε’• δ = dissipiation factor • ε” = loss factor• ε’ = dielectric constant• Reaction medium with high tanδ value

efficient absorption

rapid heating

7

According to Arrhenius equation:-Ea/RT

Rule of Thumb: for every 10°C increase in temperature the rate of reaction becomes twice

80 °C 90 °C 100 °C 110 °C 120 °C 130 °C 140 °C 150 °C 160 °C

8 hr 4 h 2 hr 1 hr 30 min 15 min 8 min 4 min 2 min

Increasing temperature

Decreasing reaction time

Ionic conduction• Due to translational motion of electric charges when an electric field is applied• Ions cause increased collision rate and convert kinetic energy to heat.

k =A*e

Microwave heating and Conventional heating

Microwave heating can be divided into two kinds:

Thermal effects Non-thermal effects

Caused by different temperature regime which can be created due to microwave dielectric heating

Caused by effects specifically inherent to the microwaves and are not caused by different temperature regime

In case of conventional heating the heat gradient is from the heating device to the medium and the heat transfer depends on thermal conductivity, on the temperature difference across the material and on convection currents, therefore temperature increase is often rather slow.

While in case of microwave heating the heat is dissipated inside the irradiated medium(mass heating) and heat transfers from the medium to outside. Due to mass heating effect much faster temperature increase can be obtained depending upon microwave power and loss factor of material being irradiated.

Microwave assisted organic reaction

Organic synthesis is an area which can benefit significantly from MW irradiation. Microwave assisted organic reaction is a fast developing area in synthetic organic chemistry. The basis being the observation that some organic reactions proceed much faster and with nhigher yields under microwave irradiation as compared with conventionl heating.

To demonstrate the versatility of more chemistry, a variety of organic reactions are discussed.

1. Catalytic Transfer Hydrogenation.

2. Preparation Of Oximes.

3. Condensation.

4. Nitrations.

1. Catalytic Transfer Hydrogenation.

Hydrogenation of benzaldehyde has been carried out using RuHCl(CO)PPh3)3. It has been found that with constant microwave irradiation the reaction was completed in 7 minutes as compared with 4 hours under standard reflux.

2. Preparation Of Oximes.

Puciova and Toma have shown that in the preparation of ferrocenyl oxime, in contrast to conventional heated reactions, the microwave assisted reactions gave only the thermodynamically stable isomer.

3. Condensation.

Quinoxalines are effectively synthesized in a few minutes by condensation reaction of o-phenylen diamine with α-dicarbonyl compounds in ethanol under MWI

4. Nitrations.

Nitrations of selected heterocyclic compound is carried out by using Cu(NO3)2, Pb(NO3)2, KNO3 and NaNO3 in glacial acetic acid under MWI. The comparative study shows that Cu(NO3)2 in glacial acetic acid is superior reagent than others. The products were obtained in fairly good yields enhanced rate of reaction using microwave irradiation.

Applications Of Microwave Heating

In Material Chemistry- SiC is a large volume ceramic used in industries for grinding wheels & in manufacturing of abrasion tools.

In Catalyst Preparations- Synthesis of a high permeance NaA zeolite from aluminate & silicate soln, Synthesis & study of vanadium dioxide & synthesis of 1D manganese dioxide catalyst(OMS-1).

In Nanotechnology- MW heating is used for the fabrication of nanoscale objects, so as to give definite structures in much shorter time.

In Polymer Synthesis- Faster heating rates with high quality/better yields can be attained in polymer synthesis by MW heating. Polyacrylamide(PAM) is used as a flocculating agent in waste water treatment was studied by MW heating.

In Waste Management- Handling of domestic & hazardous industrial waste & nuclear waste can be done by MW heating. MW technology can be applied for control of CFC, Methane, Greenhouse gases via microwave catalysis reactions.

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