Francesca M Kerton is Assistant Professor (Green Chemistry) in the Department of Chemistry, Memorial University of Newfoundland, Canada. She gained her BSc in Chemistry with Environmental Science at the University of Kent and her PhD in Chemistry at the University of Sussex. For 2 years she was a Postdoctoral Fellow at the University of British Columbia in Canada followed by a Lecturer, then Royal Society University Research Fellow, at the University of York, UK. She has contributed to many books and journal articles and her research interests are green chemistry including solvent replacement, catalysis and renewable feedstocks.

Alternative Solvents for Green Chemistry

By Francesca M. Kerton

The Royal Society of Chemistry

Copyright © 2009 Francesca M. Kerton
All rights reserved.
ISBN: 978-0-85404-163-3

Contents

Chapter 1 Introduction,
Chapter 2 'Solvent free' Chemistry,
Chapter 3 Water,
Chapter 4 Supercritical Fluids,
Chapter 5 Renewable Solvents,
Chapter 6 Room Temperature Ionic Liquids and Eutectic Mixtures,
Chapter 7 Fluorous Solvents and Related Systems,
Chapter 8 Liquid Polymers,
Chapter 9 Tunable and Switchable Solvent Systems,
Chapter 10 Industrial Case Studies,
Subject Index, 218,

CHAPTER 1

Introduction

1.1 The Need for Alternative Solvents

One of the 12 principles of green chemistry asks us to 'use safer solvents and auxiliaries'. Solvent use also impacts some of the other principles and therefore, it is not surprising that over the last 10 years, chemistry research into the use of greener, alternative solvents has grown enormously. If possible, the use of solvents should be avoided, or if they cannot be eliminated, we should try to use innocuous substances instead. In some cases, particularly in the manufacture of bulk chemicals, it is possible to use no added solvent — so-called 'solvent free' conditions. Yet in most cases, including specialty and pharmaceutical products, a solvent is required to assist in processing and transporting of materials. Alternative solvents suitable for green chemistry are those that have low toxicity, are easy to recycle, are inert and do not contaminate the product. There is no perfect green solvent that can apply to all situations and therefore decisions have to be made. The choices available to an environmentally concerned chemist are outlined in the following chapters. However, we must first consider the uses, hazards and properties of solvents in general.

Solvents are used in chemical processes to aid in mass and heat transfer, and to facilitate separations and purifications. They are also an important and often the primary component in cleaning agents, adhesives and coatings (paints, varnishes and stains). Solvents are often volatile organic compounds (VOCs) and are therefore a major environmental concern as they are able to form low-level ozone and smog through free radical air oxidation processes. Also, they are often highly flammable and can cause a number of adverse health effects including eye irritation, headaches and allergic skin reactions to name just three. Additionally, some VOCs are also known or suspected carcinogens. For these and many other reasons, legislation and voluntary control measures have been introduced. For example, benzene is an excellent, unreactive solvent but it is genotoxic and a human carcinogen. In Europe, prior to 2000, gasoline (petrol) contained 5% benzene by volume but now the content is <1%. Dichloromethane or methylene chloride (CHCl) is a suspected human carcinogen but is widely used in research laboratories for syntheses and extractions. It was previously used to extract caffeine from coffee, but now decaffeination is performed using supercritical carbon dioxide (scCO2). Perchoroethylene (CCl2CCl2) is also a suspected human carcinogen and is the main solvent used in dry cleaning processes (85% of all solvents). It is also found in printing inks, white-out correction fluid and shoe polish. ScCO2 and liquid carbon dioxide technologies have been developed for dry cleaning; however, such solvents could not be used in printing inks. Less toxic, renewable and biodegradable solvents such as ethyl lactate are therefore being considered by ink manufacturers.

Despite a stagnant period for the solvent industry during 1997–2002, world demand for solvents, including hydrocarbon and chlorinated types, is currently growing at approximately 2.3% per year and approaching 20 million tonnes annually. However, when the less environmentally friendly hydrocarbon and chlorinated types are excluded, market growth is around 4% per year. Therefore, it is clear that demand for hydrocarbon and chlorinated solvents is on a downward trend as a result of environmental regulations, with oxygenated and green solvents replacing them to a large extent. It should be noted that these statistics exclude in-house recycled materials and these figures therefore just represent solvents new to the market; the real amount of solvent in use worldwide is far higher. It also means that annually a vast amount of solvent is released into the environment (atmosphere, water table or soil). Nevertheless the situation is moving in a positive direction, as in the USA and Western Europe environmental concerns have increased sales of water based paints and coatings to levels almost equal to the solvent based market. Therefore, it is clear that legislation and public interests are causing real changes in the world of solvents.

The introduction of legislation by the United States Food and Drug Administration (FDA) means that some solvents, e.g. benzene, are already banned in the pharmaceutical industry and others should only be used if unavoidable, e.g. toluene and hexane. FDA-preferred solvents include water, heptane, ethyl acetate, ethanol and tert-butyl methyl ether. Hexane, which is not preferred and is a hazardous air pollutant, is used in the extraction of a wide range of natural products and vegetable oils in the USA. According to the EPA Toxic Release Inventory, more than 20 million kg of hexane are released into the atmosphere each year through these processes. It may seem straightforward to substitute hexane by its higher homologue, heptane, when looking at physical and safety data for solvents (Table 1.1). However, heptane is more expensive and has a higher boiling point than hexane, so economically and in terms of energy consumption a switch is not that simple. Therefore, it is clear that much needs to be done to encourage the development and implementation of greener solvents.

1.2 Safety Considerations, Life Cycle Assessment and Green Metrics

In recent years, efforts have been made to quantify or qualify the 'greenness' of a wide range of solvents; both green and common organic media were considered. In deciding which solvent to use, a wide range of factors should be considered. Some are not directly related to a specific application, such as cost and safety, and these will generally rule out some options. For example, room temperature ionic liquids (RTILs) are much more expensive than water and they are therefore more likely to find applications in high value added areas such as pharmaceuticals or electronics than in the realm of bulk or commodity chemicals. However, a more detailed assessment of additional factors should be performed including a life cycle assessment, energy requirements and waste generation.

A computer-aided method of organic solvent selection for reactions has been developed. In this collaborative study between chemical engineers and process chemists in the pharmaceutical industry, the solvents are selected using a rules based procedure where the estimated reaction-solvent properties and the solvent-environmental properties are used to guide the decision making process for organic reactions occurring in the liquid phase. These rules (Table 1.2), whether computer-aided or not, could also be more widely used by all chemists in deciding whether to use a solvent and which solvents to try first.

The technique was used in four case studies; including the replacement of dichloromethane as a solvent in oxidation reactions of alcohols, which is an important area of green...