Alternative Solvents for Green Chemistryby Francesca M Kerton
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
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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.
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Alternative Solvents for Green Chemistry
By Francesca M Kerton, Ray Marriott
The Royal Society of ChemistryCopyright © 2013 FM Kerton and R Marriott
All rights reserved.
One of the twelve 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 chemistry research into the use of greener, alternative solvents has grown enormously. 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, biocatalysis, nanochemistry, and analytical chemistry. 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. 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 (CClCCl 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. 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. 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 benefits. For example, 2Me-THF is bio-derived and is a prefered alternative to THF in many respects. However, we must not be complacent and we need to take care when using it, as recently published toxicological data suggest that it has a similar toxicitiy to THF, and it is a VOC and flammable.
1.2 Safety Considerations, Life Cycle Assessment and Green Metrics
Efforts have been made to quantify or qualify the 'greeness' of a wide range of both green and common organic media. In deciding which solvent to use, a number of factors should be considered. Because of the cost and safety of particular alternatives, some options are often ruled out early in the decision-making process. For example, room temperature ionic liquids (RTILs) are much more expensive than water and, therefore, they are 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 occuring in the liquid phase. These rules (See Table 1.2) could also be more widely used by all chemists, whether computer-aided or not, 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 chemistry. 2-pentanone, other ketones and some esters were suggested as suitable replacement solvents. At this point, the programme was not able to assess the effects of non-organic solvents due to a lack of available data. However, this approach does hold promise for reactions where a VOC could be replaced with a far less hazardous or less toxic or a bio-sourced option. It should also be mentioned that computational modelling of solvation (aqueous and organic) and its effect on reactions has developed to a sophisticated level during the past ten years. Therefore, the use of solvent-models in understanding green chemistry will continue to grow in the future.
1.2.1 Environmental, Health and Safety (EHS)
EHS properties of a solvent include its ozone depletion potential, biodegradability, toxicity and flammability. Fischer and co-workers have developed a chemical (and therefore, solvent) assessment method based on EHS criteria. It is available at http://www.sustchem.ethz.ch/tools/ehs/. They have demonstrated its use on 26 organic solvents in common use within the chemical industry. The substances were assessed based on their performance in nine categories, Table 1.3.
Using this EHS method, high (environmentally poor) scores were obtained by formaldehyde, dioxane, formic acid, acetonitrile and acetic acid, Figure 1.1. Formaldehyde has acute and chronic toxicity, dioxane is persistent and the acids are irritants. Low scores, indicating a lower hazard rating, were obtained by methyl acetate, ethanol and methanol.
1.2.2 Life Cycle Assessment (LCA)
The function of life cycle assessment (LCA) is to evaluate environmental burdens of a product, process, or activity; quantify resource use and emissions; assess the environmental and human health impact; and evaluate and implement opportunities for improvements. It is important to realize that while this book focuses on solvents, VOC 'free' paints and other 'green' consumer items may not be entirely green or entirely VOC free when the whole life cycle is considered. For example, a VOC may be used in the preparation of a pigment or another paint component, which is then encorporated into the final non-VOC formulation (e.g. aqueous). The same can also be said for many synthetic procedures which are reported to be 'solvent free'. The reaction may be performed between neat reagents, however, a solvent is used in purifying, isolating and analyzing the product. Therefore, chemists should be aware of this and avoid over-interpreting what authors are describing.
Fischer and co-workers undertook a LCA of the 26 organic solvents which they had already assessed in terms of EHS criteria, see above. They used the Ecosolvent software tool, http://www.sustchem.ethz.ch/tools/ecosolvent/, which based on industrial data considers the 'birth' of the solvent (its petrochemical production) and its 'death' by either a distillation process or treatment in a hazardous waste incineration plant. For both types of end of life treatment, 'environmental credits' were granted where appropriate e.g. solvent recovery and re-use upon distillation. The results of this assessment are shown in Figure 1.2. THF, butyl acetate, cyclohexanone and 1-propanol are not good solvents from a LCA. This is primarily due to the environmental impact of their petrochemical production and, therefore, their LCA would improve if they came from a different source. For example, 1-propanol may one day become available through selective dehydration and hydrogenation of glycerol (a renewable feedstock). At the other end of this scale, diethyl ether, hexane and heptane are considered favourable solvents. However, it should already be apparent to the reader that diethyl ether is extremely hazardous in terms of flammability, low flash point and explosion risk through peroxide contamination. Therefore, the results from the EHS assessment and LCA were combined in an attempt to provide the whole picture, Figure 1.3.
It can be seen that formaldehyde, dioxane, organic acids, acetonitrile and THF are not desirable solvents. THF and formaldehyde are significant outliers on this last graph due to their particularly poor performance under one of the asessment methods. Methanol, ethanol and methyl acetate are preferred solvents based on their EHS assessment. Heptane, hexane and diethyl ether are preferred based on LCA. However, it must be noted that the LCA was performed based on petrochemical production of the solvents and if the first group of solvents was bio-sourced, perhaps methanol, ethanol and methyl acetate would be the outright winners! Unfortunately, assessment tools used in this study could not be applied to many currently favoured alternative solvent technologies, such as supercritical fluids and RTILs, as there is a lack of available data at this time to quantify them fully. A more qualitative LCA approach, however, has been used by Clark and Tavener to assess the neoteric solvents described in this book, Figure 1.4. The solvent must first be manufactured, usually from petroleum. This is relatively straightforward for simple and aromatic hydrocarbons that are obtained through cracking and distillation of crude oil. However, more complex synthetic routes are needed for others to introduce heteroatoms such as halogens. Others, such as acetone, are produced as by-products in the manufacture of some chemicals. In terms of the alternative solvents described in this book, fluorous solvents and RTILs typically require multistage syntheses. CO2 and water do not need preparing but do need purification prior to use. Other renewable solvents, such as ethanol and esters, would require separation/extraction and purification before use. A step often overlooked in LCA of chemicals is its distribution. CO2 and water are available globally and can therefore be sourced close to their location of use. Bioethanol would be a good solvent to use in Brazil but may not be readily available in other areas of the world. Therefore, the authors suggested a labelling system, similar to 'food miles' being introduced at supermarkets, where chemists can find out where their compounds or solvents were manufactured.
The third primary stage in the life cycle of a solvent is its use. Solvents are used in many areas and not just as media for reactions, Table 1.4. The choice of the right solvent can have significant effects on energy consumption and the Efactor of a process. Solvent effects can lead to different reaction pathways for a number of reasons, some of these effects will be briefly discussed later in this chapter. The E-factor is the mass ratio of waste to desired product. If the wrong solvent is chosen, it can significantly affect the yield of a process (99% in the 'right' solvent compared to 30% in the 'wrong' one). For this reason, it is not surprising to find tables within journal articles showing the conversions or yields for a range of solvents. Clearly, in process development laboratories worldwide a significant amount of time and effort is spent optimizing the reaction conditions and the solvent choice to optimize this part of the LCA. Often the physical properties of the solvent play a significant role here; the boiling/melting points, viscosity, volatility and density must all be considered alongside safety issues such as flash point, reactivity and corrosiveness that were discussed earlier. At this stage in the process and the life cycle, biphasic systems and processes can be considered as these usually lead to reduced energy and increased efficiency. Fluorous solvents can be advantageous for this reason. However, all alternative solvents have advantages and disadvantages. Unfortunately, in the chemical literature, most authors are biased and are trying to 'sell' their chosen reaction medium. For example, the pressures involved with supercritical fluids are a disadvantage, but its facile removal at the end of a process is an advantage. Therefore, Clark and Tavener used a scoring system to grade the solvents, Table 1.5, in an attempt to qualify the general level of 'greeness' of a range of alternative solvents. It becomes apparent that all the solvents have some drawbacks and therefore, solvent free approaches should deserve greater attention and that if a solvent is used, water should be considered first, followed by carbon dioxide. They also suggest that it is unrealistic to think that all VOCs can be replaced in every application, therefore, there is a growing role for VOCs derived from renewable resources in the alternative solvent field. In all areas, we need to balance the technical advantages of a particular solvent with any environmental, cost or other disadvantages. For example, in the coatings industry a reduction in the amount of VOC in a paint may lead to a range of problems, including the stability of the formulation, longer drying times, a lower gloss and a less hard-wearing finish. Although, there are significant EHS advantages to an aqueous emulsion paint – including reduced VOC emissions, reduced user exposure and less hazardous waste productions – manufacturers and consumers need to decide if the advantages outweigh the disadvantages.
Excerpted from Alternative Solvents for Green Chemistry by Francesca M Kerton, Ray Marriott. Copyright © 2013 FM Kerton and R Marriott. Excerpted by permission of The Royal Society of Chemistry.
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Meet the Author
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.
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