About the Author
Andrew J. Hunt is the Scientific Leader of the alternative solvent technology platform at the Green Chemistry Centre of Excellence, the University of York, UK. His research interests include elemental sustainability, solvents, supercritical fluids, waste utilisation and biorefineries.
James Sherwood is a Research Associate at the University of York, UK, where he works at the Green Chemistry Centre of Excellence. In addition to his work on solvent effects in organic synthesis, he is now conducting research on sustainability assessments for bio-based products.
Corrado Topi is a Senior Research Fellow at the Stockholm Environment Institute Research Centre at the University of York, UK, where he focuses on sustainability, resilience and change in a context of integrated social, economic and environmental systems.
Giulia Paggiola is a Scientist within the Green Chemistry team at GlaxoSmithKline, UK. Prior to this, she was investigating the applicability of green solvents in biocatalysed processes and studied the potential for their uptake in the pharmaceutical industry.
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Perspectives from Research, Business and International Policy
By James H. Clark
The Royal Society of ChemistryCopyright © 2017 James H. Clark, Andrew J. Hunt, Corrado Topi, Giulia Paggiola and James Sherwood
All rights reserved.
Introduction to Solvents and Sustainable Chemistry
1.1 Sustainable Solutions
As with all products, it is a matter of urgency that the necessary feedstocks, manufacturing processes and habits of end-users become sustainable. Solvents are an important type of chemical product, with a multi-million tonne annual market. The reputation of solvents is strongly linked to their association with volatile organic compound (VOC) emissions. This book offers an insight into the sustainable use of solvents, through discussion of relevant sustainability issues, and by providing data for many different solvents that the reader can take forward into their own sustainability assessments. Case studies illustrate sustainable solvent developments and applications, and methods of solvent selection are also described to help explain a way to introduce sustainable solvents and motivate users to embrace these products.
This book explains how sustainability is applied to solvents (Chapter 3), and how the philosophy of green chemistry can help manage solvent use in a sustainable manner (Chapter 5). Chapter 6 provides extensive data to satisfy sustainability criteria for bio-based solvents and neoteric solvents, which are described in Chapter 4. Trends in solvent use are explained in Chapter 2. A general introduction to solvents and sustainability is provided in this chapter for those readers less familiar with these topics.
The term solvent is applied to a vast number of different substances. They are unified by their role as inert fluids with the purpose of dissolving another substance (the solute). Solvents may be reactive under certain conditions, which must be avoided when applying the substance as a solvent. There are several alternative descriptions available, some of which are summarised below (Table 1.1). In this discussion it is also helpful to pinpoint the conditions that define a liquid as separate from other fluids (gases and supercritical fluids) and different from solids. Definitions of solvents usually specify that they are a liquid. Gases are not solvents because they do not have intermolecular bonds, but supercritical fluids are considered to be solvents. A supercritical fluid is a state of matter achieved when the temperature and pressure both equal or exceed a critical point, where the liquid and vapour phases reach the same density and become a single (supercritical) phase.
The substance then has properties intermediate of a liquid and a gas, with intermolecular forces, which is why it is capable of acting as a weak solvent. The United Nations (UN) Globally Harmonized System of Classification and Labelling of Chemicals (GHS), and the subsequent European Regulation 1272/2008 for the Classification, Labelling and Packaging (CLP) of substances, state that a liquid is a substance with a melting point (meaning the initial melting point if relevant) of no more than 20 °C at standard pressure. Also, a liquid is not completely gaseous at 20 °C, and at 50 °C a liquid has a vapour pressure of 300 kPa (3 bar) or less. Clearly this links to the definition of a gas, which is completely gaseous at 20 °C and at 50 °C has a vapour pressure above 300 kPa. For completeness, a solid is a substance that does not meet the criteria for either a liquid or a gas.
The definition offered by Marcus, relies on our understanding of the liquid state as was previously discussed (Table 1.1). The more specific definition within the EU Industrial Emissions Directive 2010/75/EU, uses applications to establish what an organic solvent is. Although the inertness of the solvent is maintained, some of the applications listed do not require that the solvent is actually dissolving anything. We are also required to appreciate the meaning of VOC, which is also defined in the Industrial Emissions Directive as "any organic compound ... having at 20 °C a vapour pressure of 0.01 kPa or more, or having a corresponding volatility under the particular conditions of use". Wypych differentiates between solvents and plasticisers in his definition, while the Industrial Emissions Directive includes plasticisers as an application of solvents. Several of the solvents that will be discussed in this book have boiling points in excess of 250 °C (a plasticiser according to Wypych) and so it is more advantageous to follow the definitions of Marcus and the Industrial Emissions Directive. Also note that other formal definitions of a plasticiser exist without reference to boiling point, which only consider the context in which the substance is used (as a plasticiser). Wypych also suggests that certain solids and gases are solvents, and in doing so stretches the meaning of a solvent beyond what is useful.
The International Union of Pure and Applied Chemistry (IUPA C) decide upon the authoritative nomenclature and descriptions of chemicals and chemical phenomena. The definition of a solution offered by IUPA C alludes to the meaning of the term solvent, but it is not clearly stated. Instead, IUPA C defines different types of solvent separately. For instance, according to IUPA C a dipolar aprotic solvent is "a solvent with a comparatively high relative permittivity (or dielectric constant), greater than ca. 15, and a sizable permanent dipole moment, that cannot donate suitably labile hydrogen atoms to form strong hydrogen bonds", and an amphiprotic solvent is a "self-ionizing solvent possessing both characteristics of Brønsted acids and bases".
1.2.2 Types of Solvent and Their Origins
The history of solvents is nicely described by Estévez. Before the advent of the petrochemical industry, solvents were limited to water, naturally occurring oils and substances that could be easily fermented or distilled from biomass. By contrast the number of solvents available now is quite overwhelming. In order to provide satisfactory performance in the variety of processes, formulations, and cleaning applications that solvents are needed for, many different types of solvent are required. Large differences in boiling point, polarity, viscosity and several other physical properties can be found between solvents, and are documented in specialised texts. Fundamentally the solvent must dissolve the relevant substrates, sometimes selectively, and not react or decompose within the system. Hence many different chemical functionalities are found in both historically important solvents and contemporary examples. Protic solvents (in a hydrogen bonding sense) include water, alcohols, primary and secondary amines, acids, and multi-functional solvents containing any of these chemical groups. Glycol ethers would be one example. Aprotic solvents are more diverse, including aliphatic, olefinic, and aromatic hydrocarbons, halogenated hydrocarbons, ethers, esters, ketones, carbonates, nitriles, tertiary amines, nitrohydrocarbons, organophosphates, amides and sulphur containing compounds.
All the above solvents (with the exception of water) are routinely made from the major chemical building blocks of the petrochemical industry. These are syngas, ethylene, propylene, the butenes, butadiene, benzene, toluene, and xylenes, as produced from crude oil and natural gas (Figure 1.1). These base chemicals are transformed into functionalised solvents through the processes of oxidation, hydrogenation, dehydrogenation, hydration, dehydration, dimerisation, and esterification.
Before the essential base chemicals can be obtained, the crude oil must be processed accordingly. Firstly desalting occurs, and then the crude oil is subjected to fractional distillation. The naphtha cut is composed of short-chain aliphatic hydrocarbons. n-Hexane and n-heptane can be separated from processed naphtha with molecular sieves, for example. Naphtha is also suited to downstream chemical production, and through steam cracking is the main source of petrochemical olefins. Shale gas is another economically attractive source of olefins and syngas.
Aromatics solvents (benzene, toluene, xylene(s), collectively known as BTX in oil refinery terms) are produced by the catalytic reforming of crude oil. The aromatics must be separated from their azeotropic mixtures with aliphatic hydrocarbons, and to do this an additional extraction solvent is required. Several processes exist, but the Shell sulpholane process is the most established. More than 90% of BTX separation processes use either this technology, the Udex diethylene glycol process (which the Shell sulpholane process is superseding), or the Lurgi Arosolvan process. The latter uses N-methyl pyrrolidinone (NMP) as the primary solvent, and is being introduced as new plants are built. All use a certain amount of water as an anti-solvent.
Biorefineries with the capacity to produce appreciable amounts of renewable solvents are now operational. Bio-ethanol is produced primarily for fuel, but is also a common solvent. Other fermentation products also have applications as solvents (e.g. acetone, 1-butanol, lactic acid, and their derivatives). Chemical processes and extractions of natural products can also be applied to produce solvents or their precursors from renewable feedstocks. Chapter 4 will discuss the production of bio-based solvents in greater detail.
Typical properties characteristic of the different classes of solvent have been helpfully tabulated by Wypych. Data for individual solvents can be found in Smallwood's 'Handbook of Organic Solvent Properties'. This book is concerned with the sustainability of solvents, not their physical properties as such, so please refer to the suggested texts for complete descriptions. Chapter 6 presents data relevant for determining the environmental impact and health and safety implications of both bio-based solvents and speciality petrochemical solvents.
1.2.3 Solvent Markets
Accessible data regarding the quantities of solvent used or purchased across large geographical regions is usually outdated. Statistics for western Europe were last published by the European Solvent Industry Group (ESIG) in 1993, even predating the European VOC Directive 1999/13/ EC. The United States Environmental Protection Agency (EPA) monitors chemical releases and wastes, and compiles the data annually. This data is categorised by substance and industry. Solvents must be identified while recognising that many solvents have other uses, so the data can significantly overestimate industrial solvent use. At the same time, reuse of solvents and the fact that solvents can be contained in final products also influences the accuracy of the dataset.
It is important to regard certain datasets as representative of past solvent use where appropriate, and also bear in mind modern trends, which have reacted to new environmental regulations and are steered by the implementation of greener, more effective technologies. Nevertheless, the information presented in Figure 1.2 is useful as a historical marker, indicating the large volumes of solvent that were used in paints and coatings especially, and a reminder of how widespread and diverse solvent use is. Modern data is reviewed subsequently.
Furthermore, US data for the year 1995 was collated by George Wypych in his impressively comprehensive, sector by sector analysis. American industry solvent releases in 1995 were dominated by methanol (Figure 1.3). The most transferred solvent (as waste) was the much less volatile ethylene glycol. Methanol, toluene and xylene, all possessing health hazards, together constitute over half of all solvent releases and transfers. Other solvents in the top ten combined releases and transfers are, in order of significance, methyl ethyl ketone (MEK), carbon disulphide, dichloromethane (DCM), n-hexane, methyl isobutyl ketone (MIBK) and t-butanol. Although non-solvent uses are included in the data, because solvents are regularly used as high volume auxiliaries in manufacturing, they are more prone to being released or otherwise eventually becoming waste. That said, the use of carbon disulphide as a solvent is minimal, but its continued use as a reactant in rayon manufacture is very significant in terms of volume, and its volatility is contributing to the release data shown (Figure 1.3).
Sector data from the same time shows that the manufacture of polymers, plastics, and organic chemicals contributes highly to solvent emissions (releases and disposal) and waste transfers (Figure 1.4). Together they account for 50% of the reported figures (Figure 1.5). Metal fabrication, the paper and pulp industry and the motor vehicle manufacturing sector are also large contributors, although the paper and pulp industry is overwhelmingly releasing and transferring methanol, which is not a solvent in this instance but a by-product of the Kraft pulping process. Some of the plastic and fiber industry emissions and waste will also be attributed to wastes, e.g. from condensation polymerisations, as well as volatile monomers and other reactants. Otherwise, cleaning solvents will make up a significant proportion of the data shown in Figure 1.4.
The Swedish Chemicals Agency (KEMI) has published more recent data for solvent use specifically in Sweden, up to the year 2010.25 The categorisation of industrial sectors is different to the ESIG data in Figure 1.2, but clear similarities and differences can be seen (Figure 1.6). Total industrial solvent use in Sweden was 228 thousand metric tonnes in 2010. We shall assume Sweden is representative of western Europe to enable comparison with the 1993 data shown previously in Figure 1.2, but obvious market demand differences will be highlighted in the following description. The printing industry is now the major user of solvents, with the paints and coatings sector shrinking relative to the ESIG 1993 data, as water-based paints have started to take a larger market share. Nevertheless, paints and coatings are still significant, and global estimates for the proportion of solvent use in this sector are higher still. The pharmaceutical industry was the second largest consumer of solvents in Europe in 1993, but is not represented specifically in the Sweden data. However, there are a number of pharmaceutical companies active in Sweden, and so this sector must be represented within the "manufacture of other chemicals" group (accounting for 6% of the total Swedish industrial solvent use).
Organic solvent use in Swedish consumer products equalled 50 thousand metric tonnes in 2010, about 80% of the quantity required by the paints and coatings industry (Figure 1.7). The data shown in Figure 1.7 considers consumer products for the general public, and also technical products for industry. Paints accounted for 9% of the consumer solvent use, behind antifreeze and windscreen washing fluids. The market share of antifreeze in Sweden is understandably higher than in warmer climates, and so the size of this contribution to total consumer solvent use is bound to vary considerably from nation to nation. Antifreeze is based on a glycol solvent, ethylene glycol or propylene glycol in applications where the toxicity of the former is an issue. Windscreen washing fluids are aqueous alcohol solutions, that may contain methanol, or again for toxicity reasons preferably ethanol or isopropanol. Most of the other consumer products are based on hydrocarbon solvents.
It follows that the composition of contemporary solvent use in Sweden is dominated by alcohols and hydrocarbons (Figure 1.8). Mixtures of aliphatic and aromatic solvents are categorised separately. The types of solvent used in industry are more diverse, but still a preference for alcohols and aliphatic hydrocarbons emerges. Esters are the third most common class of organic solvent, but are underrepresented in consumer products. Of course, water is the most popular solvent found in final products, and is used an order of magnitude more frequently than all the organic solvents combined. Note that fuel and chemical intermediates have been removed from this dataset.
It is notable from the types of solvent used (in any appreciable amount at least) that the most infamously toxic and environmentally damaging solvents are absent. The use of chlorinated solvents in Sweden was massively reduced in the 1990's through national legislation. Later, between 2004 and 2010, the demand for chloroform, dichloromethane (DCM), and perchloroethylene stabilised, as is true of total solvent use according to the KEMI dataset. 1,1,1-Trichloroethane was phased out by the Montreal Protocol. Of the remaining options, cumulatively about 500 tonnes of chlorinated solvent is used each year in Sweden (0.2% of total solvent use). Perchloroethylene is still used in dry cleaning, while DCM and chloroform can be used for cleaning if the operator has been issued with a permit. Specialised use in fine chemical manufacture continues. Data from GlaxoSmithKline (GSK), although not a Swedish company, suggests that although DCM use is decreasing, at least relatively, it is still one of their top ten solvents. Solvent use in the pharmaceutical industry is discussed in Chapter 2.
Excerpted from Sustainable Solvents by James H. Clark. Copyright © 2017 James H. Clark, Andrew J. Hunt, Corrado Topi, Giulia Paggiola and James Sherwood. Excerpted by permission of The Royal Society of Chemistry.
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Table of Contents
ContentsChapter 1 Introduction to Solvents and Sustainable Chemistry, 1,
Chapter 2 Modern Trends in Solvent Use, 35,
Chapter 3 Sustainability Applied to Solvents, 87,
Chapter 4 Alternative Solvents, 136,
Chapter 5 Green Chemistry Concepts and Metrics for Solvent Selection, 188,
Chapter 6 An Appendix of Solvent Data Sheets, 235,
Subject Index, 348,