Alternative Solvents for Green Chemistry

Francesca Kerton, Ray Marriott

• Publisher: Royal Society of Chemistry
• Year: 2015
• ISBN:
  9781782626138
• Edition: 2
• Topic: Naturwissenschaften
• Subtopic: Organische Chemie

Everyone is becoming more environmentally conscious and therefore, chemical processes are being developed with their environmental burden in mind. This also means that more traditional chemical methods are being replaced with new innovations and this includes new solvents. Solvents are everywhere, but how necessary are they? They are used in most areas including synthetic chemistry, analytical chemistry, pharmaceutical production and processing, the food and flavour industry and the materials and coatings sectors. However, the principles of green chemistry guide us to use less of them, or to use safer, more environmentally friendly solvents if they are essential. Therefore, we should always ask ourselves, do we really need a solvent?

Green chemistry, as a relatively new sub-discipline, is a rapidly growing field of research. Alternative solvents - including supercritical fluids and room temperature ionic liquids - form a significant portion of research in green chemistry. This is in part due to the hazards of many conventional solvents (e.g. toxicity and flammability) and the significant contribution that solvents make to the waste generated in many chemical processes. Solvents are important in analytical chemistry, product purification, extraction and separation technologies, and also in the modification of materials. Therefore, in order to make chemistry more sustainable in these fields, a knowledge of alternative, greener solvents is important.

This book, which is part of a green chemistry series, uses examples that tie in with the 12 principles of green chemistry e.g. atom efficient reactions in benign solvents and processing of renewable chemicals/materials in green solvents. Readers get an overview of the many different kinds of solvents, written in such a way to make the book appropriate to newcomers to the field and prepare them for the 'green choices' available. The book also removes some of the mystique associated with 'alternative solvent' choices and includes information on solvents in different fields of chemistry such as analytical and materials chemistry in addition to catalysis and synthesis. The latest research developments, not covered elsewhere, are included such as switchable solvents and biosolvents. Also, some important areas that are often overlooked are described such as naturally sourced solvents (including ethanol and ethyl lactate) and liquid polymers (including poly(ethyleneglycol) and poly(dimethylsiloxane)). As well as these additional alternative solvents being included, the book takes a more general approach to solvents, not just focusing on the use of solvents in synthetic chemistry. Applications of solvents in areas such as analysis are overviewed in addition to the more widely recognised uses of alternative solvents in organic synthesis. Unfortunately, as the book shows, there is no universal green solvent and readers must ascertain their best options based on prior chemistry, cost, environmental benefits and other factors. It is important to try and minimize the number of solvent changes in a chemical process and therefore, the importance of solvents in product purification, extraction and separation technologies are highlighted.

The book is aimed at newcomers to the field whether research students beginning investigations towards their thesis or industrial researchers curious to find out if an alternative solvent would be suitable in their work.

CHAPTER 1

Introduction

1.1 Introduction

One of the twelve principles of green chemistry asks us to ‘use safer solvents and auxiliaries’.^1–3 Solvent use also impacts some of the other principles and therefore, it is not surprising that chemistry research into the use of greener, alternative solvents has grown enormously.^4–11 If possible, we should try to avoid using them and, if needed, we should try to use inocuous substances. In some cases, particularly in the manufacture of bulk chemicals, it is possible to use no added solvent, or so-called ‘solvent free’ conditions. Yet in most cases, including speciality 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. So-called ‘green’ solvents have been used in diverse areas, for example, polymer chemistry,¹² biocatalysis,¹³ nanochemistry,¹⁴ and analytical chemistry.¹⁵ There is no perfect green solvent that can be applied 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, in adhesives and in coatings (paints, varnishes and stains). Solvents are often VOCs (volatile organic compounds) and, therefore, are a major environmental concern as they are able to form low-level ozone and smog through free radical air oxidation processes.³ 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 (CH₂Cl₂) 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 coffee decaffeination is performed using supercritical carbon dioxide (scCO₂). Perchloroethylene (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 (e.g. Liquid Paper, Tipp-Ex) and shoe polish. ScCO₂ and liquid carbon dioxide technologies have been developed to perform dry cleaning, however, such a solvent could not be used in printing inks. Therefore, less toxic, renewable and biodegradable solvents such as ethyl lactate are being considered by ink manufacturers.

Despite a stagnant period for the solvent industry during 1997–2002, currently world demand for solvents, including hydrocarbon and chlorinated types, is growing at approximately 2.3% per year and approaching 20 million metric tons per annum. However, when the less environmentally friendly hydrocarbon and chlorinated types are excluded, market growth is around 4% per annum. 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.¹⁶ It should be noted that these statistics exclude in-house recycled materials and, therefore, these figures just represent solvent new to the market and 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 U.S. 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 U.S. and according to the EPA Toxic Release Inventory, more than 20 million kg of hexane are released into the atmosphere per year through these processes. For example, a hexane-based extraction process introduced in the 1930s is used to obtain soy oil from crushed soybeans. Hexane losses are of the order of 1 kg per ton of beans processed! Therefore, more environmentally friendly alternatives are in demand and a number of approaches have been studied.¹⁷ It may seem straight forward to substitute hexane with 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. Also, heptane does possess many of the same environmental health and safety hazards as hexane e.g. flammability. Therefore, it is clear that much needs to be done to encourage the development and implementation of greener solvents. Futhermore, it should be noted that even if one aspect of a solvent means it can be considered green, other properties of the solvent may detract from its potential benefit...