Chemical industry and sustainable development

9.6 Chemical industry and sustainable development 9.6.1 Sustainability The problem of sustainability has been an issue of worldwide importance for o...
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9.6

Chemical industry and sustainable development

9.6.1 Sustainability The problem of sustainability has been an issue of worldwide importance for over 15 years and has been the object of two extremely important summit meetings (Earth Summit, Rio de Janeiro, 1992; World Summit on Sustainable Development, Johannesburg, 2002). It rose to the attention of world public opinion in 1987 with the presentation of the report Our common future (WCED, 1987) by the World Commission on Environment and Development. It is in this report that the most commonly cited definition of sustainability (often associated with the name of the Norwegian Prime minister, Gro Harlem Brundtland, who chaired the Commission) can be found: “sustainability is the ability to meet the needs of the present, without compromising the ability of future generations to meet their own needs”. The aspects which are most directly involved in problems of sustainability relate to population growth, food availability for nourishment, energy problems, the destruction of natural resources, global climate change, utilization of water sources, the generation and dissemination of toxic substances and waste products. These represent genuine challenges. Sustainable development and technological progress

Even if sustainability without development may seem to be a relatively simple solution at first sight, in reality, this option cannot be proposed; when examined more carefully, it cannot lead to the level of sustainability which should be guaranteed. For example, it is difficult not to take into consideration the above-mentioned correlation between a sustainable growth of the world population and an improvement in the quality of life in developing countries. The concept of sustainability therefore cannot but prove to be

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inextricably linked to the concept of development which, combined, form the well-known expression sustainable development. The utilization of scientific knowledge for practical purposes through the use of technology has grown enormously during the various industrial revolutions from the Eighteenth century onwards, and has led to a real explosion of technological development in our present times. Alongside the obvious benefits, a whole series of problems arose as a result of the uncontrolled exploitation of the Earth’s resources and the effects produced on the climate, environment and mankind. This led to an increasing sensitivity, which has developed in the last few decades, towards the question of protecting environmental resources. It has become evident that the environment is a critical resource that is rare and precious; it must be protected, defended, handled and preserved with care. Not only the problems, but also the solution to the problems associated with development should derive from scientific knowledge and its application in newly available technologies. The industrial system and sustainable development

To overcome the challenge of sustainability, the new technologies developed as a result of innovation must form part of a new model of industrial progress which is capable of combining economic growth with social and environmental responsibility. In reality, there is a growing awareness today that many of the concerns of a non-financial nature linked to the problem of sustainability have a direct impact on the long term value for shareholders. An increasing number of large international companies feels the need to focus their activities and base their final statements not only on economic profit, but also on the production (or less destruction) of an environmental

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and social value. This approach is commonly known as TBL (Triple Bottom Line): the three lines (which represent society, economy and environment) are in continuous movement as a result of economic, social, and environmental cycles and contrasts; they move independently of each other creating areas of friction from which conflicts emerge (Elkington, 1997). A significant example of this new awareness is represented by the World Business Council for Sustainable Development (WBCSD), founded in 1990, which brings together over 150 foremost worldwide companies, belonging to 34 countries and present in more than 20 industrial fields. In a series of studies and leaflets published during its activities, the WBCSD intends to act as a catalyst towards sustainable development and demonstrate that the business world, as a primary user of natural resources and new technologies, has much to gain from protecting the global ecosystem (Schmidheiny, 1992). Sustainabilityoriented actions can directly contribute towards both the tangible financial value of enterprises by favouring growth, lowering costs and reducing risks, and also towards improving the intangible assets, such as image, strategic relations, human capital and innovation. The system of Dow Jones Sustainability World Indexes, created in 1999, which comprises over 300 companies of 60 industrial groups in 34 different countries, with a market capitalization (February 2004) of approximately 6,500 billion dollars, represents the first example of global monitoring of the performances of the main world companies which operate paying careful attention to sustainability issues. The aspects of sustainability are evaluated according to a series of criteria which vary from generating long-term profit to management culture and company organization; from renewing production processes and products to the image and reputation that the company has acquired. More generally, what is assessed is the company’s ability to create value for the whole of its stakeholders, interest bearers, intended in a general sense and progressively in the widest sense: shareholders, employees, clients, as far as the local and worldwide community. A leading company in the field of sustainability must have a strategy which is capable of integrating an effective ability to compete and create profit with social, environmental and economic aspects over a long period of time. It must create innovative products and services, focused on technologies and systems which utilize natural, social and financial sources effectively and efficiently. It must pursue a policy of transparent communication towards shareholders and the public, as well as loyalty in relations with the clients. As sustainability is a property of the system in its entirety, re-planning the industrial system must be

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done within an extremely wide perspective, beyond the single process considered and even beyond the single company, just as an economic analysis must overcome classical cost and performance concepts to include environmental and socio-economic protection (Bakshi and Fiksel, 2004). A holistic vision of the whole system is essential for understanding and modelling the complex interactions between industry, society and ecosystem. It involves the intervention of various disciplines, from technical reference disciplines such as engineering, chemistry and biology, to medicine, economy, law, ethics and social sciences. Creating a sustainable industrial system must relate to the whole life cycle of the product or process; it must produce things eco-efficienly by reducing the environmental impact of the production activity to the minimum; a cyclic model, which is closer to the flows of natural ecosystems in which waste products have a value as bionutrients, must be favoured instead of a linear industrial system. All of this must naturally take place in small steps; the approach to industrial sustainability is in its early infancy and still a long way from an overall change in attitude to the commonly accepted way of ‘doing business’ which has been defined as downright ‘creative destruction’. Currently, the most common approach to sustainable development is limited to the setting up of single processes evaluated from a technological standpoint as more eco-compatible and therefore capable of obtaining greater consent. This is a relatively restricted approach, extremely distant from the planetary and multigenerational vision inherent in the definition of sustainability mentioned at the beginning, and also from the use of assessment techniques such as a detailed life cycle analysis, but it is important, however, for starting the whole process. Measuring sustainability

It can be seen, from what is described above, that sustainability can be considered as three-dimensional: ecological, economic and social. Sustainable development is therefore an optimization of these three dimensions. To put theory into practice, it is necessary to have instruments that quantify the reduction in the environmental impact, as well as the economic and social benefit generated by the transition to a new product, process or system. The social aspect of sustainability is the most difficult to quantify, and this issue is the least defined. The quantification of economic and environmental aspects has, on the contrary, been the object of considerable activities and is generally known as eco-efficiency analysis. During its activity, the WBCSD has publicized the concept of eco-efficiency (WBCSD, 2000) embodied in the 3 R

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principle: Reduce, Reuse, Recycle, which aims to integrate the environmental aspect with the economic aspect in the economically practical production of goods and services with a reduced ecological impact (creating more value with less impact). The objective of eco-efficiency is consequently to maximize the value, at the same time minimizing the negative impact on the environment. An eco-efficiency index is therefore defined, which takes into consideration the economic and ecological aspects: product or service value eco-efficiency index  1121111111244 environmental impact

The eco-efficiency can be quantified through an analysis of seven key indicators: a) reduction in the use of raw materials; b) reduction in the use of energy; c) reduction in the dispersion of toxic substances; d ) improved recyclability; e) maximization of the sustainable use of resources; f ) prolonged duration; g) growth in service performances. As already mentioned, the eco-efficiency index does not take into account all three pillars of sustainable development as it is based on economic and environmental efficiency, and does not consider the social aspects. It is, nevertheless, a key indicator for orienting decisions of the financial and political world. The most well-known instruments for quantifying eco-efficiency are Life Cycle Assessment (LCA) relating to the quantification of environmental impacts, and Total Cost Assessment (TCA) for value quantification. In life cycle assessments, all aspects concerning environmental impact are quantitatively taken into consideration by means of a ‘from cradle to grave’ analysis. This approach takes into account the whole life cycle of a product or system (and all parts of the relative production process), from raw materials to final disposal, including distribution, use, reuse and recycling. Energy, materials and water are used in all these phases, which can generate waste products. When the product becomes obsolete, it can be renewed, reproduced or recycled. It should be noted that this approach is very different from that adopted ten or twenty years ago, when the sole interest of manufacturers and legislators was directed at waste products and production scrap. In reality, the environmental impact of production is not necessarily the most important one. LCA must also be associated with total cost assessment (TCA), a financial analysis method which integrates conventional financial assessment and places a monetary value on environmental impacts during the single life cycle phases. In this way, a TCA highlights and takes into account a whole series of costs and savings which are not considered in a traditional approach, including future contingent costs

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associated with potential responsibility and internal or external intangible costs. For a correct TCA analysis, standardized methods developed and implemented in software are currently available.

9.6.2 Sustainability in the chemical industry The chemical industry has an extremely important role in the global industrial sector. On a world scale, it produces an annual turnover of over 1,700 billion euro, generating 9% of international commerce and offering work to over 10 million people. In the value generation chain, the chemical industry is positioned upstream of numerous sectors such as the building, transport, food, health, personal and household hygiene, clothes, electronics, etc., providing intermediates for downstream industries and also directly contributing to creating materials for the consumer market. Indeed, today’s society would not exist without the chemical industry. As a result of its extremely strategic nature connected to numerous sectors essential for modern-day society, and as a manufacturing industry which substantially transforms raw materials into products, the chemical industry is fully involved in the problems relating to industrial sustainability outlined above (Jenk et al., 2004). It is often considered by public opinion as being mainly responsible for problems relating to environmental degradation, and the negative consequences of production, distribution and the use of chemical products have now become of increasing social concern. Because it is situated upstream of numerous sectors producing both industrial intermediates and end-products for consumers, the chemical industry has quite a varied structure. It comprises fields ranging from petrochemicals (on the boundary between chemistry and refining) to bulk chemicals (large industrial intermediates), inorganic products, organic intermediates and polymers, through to fine and specialty chemicals, dyes, pharmaceuticals and agricultural products. This diversified reality obviously multiplies problems relating to sustainability, but at the same time, enhances opportunities for development that can derive from them. These problems can be summarized as follows: pollution caused by chemical processes and products; risks created by dangerous chemical products; exhaustion of sources of raw materials (Mestres, 2004). Chemical products are released into the environment more or less continuously during production processes as plant leakages, such as gaseous emissions, waste products sent for disposal, or

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waste water. A significant share of end-products of the chemical industry is dispersed into the environment, not only by the chemical industry which created them, but also by the industries that use them in the various applicative fields and by the final consumers. The difference between the negative effects deriving from polluting products and dangerous products should also be pointed out. Highly toxic, flammable, explosive products can be the cause of dramatic events, involving people and things to a considerable extent, but always on a local level. Polluting products are certainly harmful and dangerous for the environment; they can exert their effects over long periods of time (effects which cannot be so easily and unequivocally recognized) and on an global scale. Protection from accidents caused by the release of chemical products, and preventing the effects of pollution must therefore be kept quite distinct. The third problem identified is linked to the decline or exhaustion of raw materials (even for the local environment), whether they be of a mineral or fossil organic nature, leading to modifications in price and the economic balance of production and commercial activities. This will lead to the development and utilization of new or modified processes and products to allow a shift towards the use of renewable sources as raw materials. The objectives that the chemical industry must pursue to solve these problems can therefore be summarized as follows: a) reduction in the use and generation of polluting chemical products in chemical processes; b) reduction in the use of dangerous chemical products in chemical processes; c) reduction in the harmful effects of end-products; d ) reduction in the use of limited and non-renewable raw materials. The growing urgency of the inherent problems, together with the increasing awareness and sensitivity in this respect, has led in the last few decades to the introduction of increasingly stringent regulations aimed at regulating industrial processing procedures and the characteristics of the products used and sold, the quality and quantity of the emissions, as well as effluents and waste products generated (Gaertner et al., 2003). At the same time, the more responsive manufacturers (within the sphere of responsible care policies) have responded by voluntarily often adopting even more severe measures for the reduction of emissions, the treatment of waste products and, when possible, the recycling of end-products at their life-end. The limits of this approach, which can be defined as (self-)regulation, are evident. First of all, it only responds to the first two categories of problems outlined above, linked to pollution and safety, without paying any attention whatsoever to the issue of the exhaustion of raw material sources. Furthermore, based on the assumption of preserving the existing

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chemical status quo as far as possible, only limited improvements can be made, often at enormous and highly unproductive costs. Not by chance has the announced introduction in the European Union of the so-called REACH (Registration, Evaluation and Authorization of CHemicals) system been arousing controversy. On the one hand, it is considered by many as being too ambitious, complex and onerous, if seen as a further, more advanced attempt at regulation; on the other hand, too insufficient to contribute to the real leap forward in quality that many are awaiting in the construction of a sustainable industrial society. The REACH system has been defined (2001) in the White Book of the European Commission which outlines the new strategy for chemical substances, aiming at a sustainable development. The essential feature of the new system is that industry will have to take on an active role, and sustain the relevant costs, in providing essential information for evaluating the danger of existing and new products, assuming the responsibility and burden of the task. In other words, enterprises will have to take on the task of carrying out characterization tests (physical, chemical, toxicological, eco-toxicological properties) of the products, and providing a central data bank and users downstream (including consumers) with detailed information. The objective is to complete this operation within 2012, starting with the most widespread or most dangerous substances (the implementation times, however, are still under discussion). Green chemistry

A more radical solution to the problems specified above consists in the use of intrinsically cleaner processes and intrinsically safer products (Anastas and Williamson, 1998; Tundo et al., 2000). It is known that the risk can be defined as the product of the hazard of a certain agent, for example a chemical substance, times the exposure to the same agent: risk hazard exposure The regulative approach (command and control) imposed by laws and regulations is essentially directed towards minimizing the exposure factor, for example by controlling emissions or imposing the use of protective measures. The risk can be much more radically reduced, however, by diminishing or eliminating the other element of the expression, i.e. the hazard: if an agent is not dangerous, it obviously cannot give rise to a significant risk. This innovative approach is known by the name of green chemistry. Green chemistry arose from simple considerations, such as the fact that it is better to avoid the formation

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of waste products than to treat them and dispose of them after they have been produced; and that it is better to avoid the use of dangerous or potentially polluting products and processes than adopt expensive measures for reducing the risks and emissions. The green chemistry strategy is focused on concepts such as: a) eliminating the use of raw materials, intermediates and products which are toxic and dangerous; b) reducing as much as possible the quantity and hazard of emissions and waste products from production processes, also through the reuse of possible by-products; c) reducing energy consumption as much as possible; d) eliminating the use of solvents, especially those which are most harmful for the environment; e) increasing the use of renewable raw materials. Green chemistry is an essentially innovative approach, as it proposes to exploit the propensity towards innovation which has characterized the chemical industry right from the start. Green chemistry essentially means using chemistry to prevent risks and pollution. In addition to being innovative, the green chemistry approach is also non-regulatory and takes into account economic aspects. Using new processes which do not have to incur too many costs associated with increasing safety measures, risk protection, emission control and waste disposal (or that do not use raw materials close to exhaustion) constitute excellent business, if well planned. It should be remembered that economic advantages are considered as being a main boost for its success. The basic concepts of green chemistry have been encoded in 12 principles (Anastas and Warner, 1998), which identify the main guidelines in which the development and definition of new clean processes and products can be inserted. These include concepts already widely discussed before, starting with the first and fundamental concept which prescribes avoiding the formation of waste products rather than having to treat and dispose of them after they have been produced. At the basis of green chemistry is the

concept that the assessment parameters of a new chemical technology do not stop at the production yield, and properties and efficacy of the product or a particular reagent in the applications for which it has been designed. It must be taken into account that the chemical processes we are designing will have an impact on the people who come into contact with the materials produced and the production processes used, and on the environment in which they are present. In other words, the evaluation of a process or product cannot but completely fall within the extensive sustainability assessment methods described above, i.e. making use of total Life Cycle Assessment (LCA) and Total Cost Assessment (TCA) analysis concepts. From this point of view, green chemistry can be inserted within the context of industrial ecology, of which it uses instruments and guidelines (Graedel, 1999). The following objectives emerge: a) to adopt an assessment perspective for chemical products and processes extended to the whole life cycle; b) to take into account in the assessment the environmental impact of the activities of the suppliers and customers; c) in the case of long-lasting products, recyclability must also be taken into consideration; d ) in the case of dissipative products (those consumed in the process of carrying out their purpose), the distribution phases, circulation and use must be considered along with the production; e) to design both processes and products with a low environmental impact. Various specific indicators have been developed within the context of general sustainability measurement techniques and instruments (Curzons et al., 2001; Constable et al., 2002), illustrated in Table 1. These parameters are not directed towards major problems of overall sustainability and are not intended as tools for identifying great company strategies. They have been conceived as auxiliary means in defining research and development policies, aimed at developing new processes and improving those already existing within a short-medium term scenario.

Table 1. Measurement parameters of the sustainability of a chemical process Indicator Effective mass yield (%)

Definition Product mass 100/mass of non-benign materials used

E-factor Atom economy (%)

Waste-product mass/product mass Molecular weight useful product 100/molecular weights reagents sum

Mass intensity Carbon efficiency (%)

Total mass used in the process/product mass Quantity of carbon in the product 100/total carbon quantity in the reagents

Reaction mass efficiency (%)

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Product mass 100/reagent mass

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The effective mass yield measures in percentages the mass proportion of desired product obtained compared to the mass of all the non-benign materials involved in its synthesis. This indicator is extremely interesting as it emphasizes the hazard of the reagents, a consideration which is normally absent in yield evaluation. Benign materials refer to those reagents, solvents or by-products which involve no risks for health or the environment. The definition, however, suffers from an evident lack of clarity, especially for limited risk cases; moreover, information relating to risk is not always easily available, often making the use of this indicator difficult. The E-factor, on the other hand, has the advantage of simplicity, concentrating on the production of waste products accompanying a certain quantity of the desired product (Sheldon, 2000). The theoretical E-factor, linked to the stoichiometry of the reaction, obviously represents the minimum value; the real value, linked to the actual reaction yield, is normally much higher as the yield acts on both the numerator, increasing it, and the denominator, decreasing it. It is also evident that the evaluation criterion of the E-factor can vary surprisingly when passing from the evaluation of a petrochemical process to the synthesis of a pharmaceutical product, as shown in Table 2: for a petrochemical derivative produced in a quantity in the order of millions of tonnes, the E-factor is less, in fact much less than 0.1, whereas it can exceed the value of 100 for a pharmaceutical product. This is due to the fact that in the pharmaceutical sector the synthesis methods normally consist of multiple steps and use of reagents in stoichiometric quantities (those given by the molar ratio between the various reagents in the reaction formula) rather than catalytic (significantly less than the stoichiometric quantities due to the fact that the catalyst is reused in the reaction cycle after having interacted with the reactants to promote the reactive event). The E-factor is sometimes multiplied by a quotient of environmental risk (unfriendliness quotient) which aims to take into consideration the risk of the waste products produced; however, the definition of unfriendliness quotient is very

subjective. The result is called Environmental Quotient (EQ). Atom Economy (AE), otherwise defined as atom utilization or atom efficiency, is another indicator with an extremely simple and intuitive meaning. It indicates the amount of reagents that remains in the desired product on the basis of the reaction stoichiometry (Trost, 1991). The definition of atom economy is based on the stoichiometry, deliberately ignoring the yield and excess of one reagent compared to the others, as well as the presence of catalysts and solvents. In the context of the atom economy concept, a reagent is a substance which is at least partially incorporated in the product (or in a reaction intermediate). Unlike atom economy, the Mass Intensity (MI) not only takes into consideration the stoichiometry but also the yield, the solvents and auxiliaries used in the reaction and purification: in short, the total mass used in the process, except for that of water, and compares it with the product mass obtained. The following relation exists between the mass intensity and E-factor: E-factor MI 1 (to the extent in which the solvents and auxiliaries considered form a waste product). The mass intensity is sometimes transformed into mass productivity, expressed in a form homogeneous with the effective yield and atom economy: product mass 100 100 mass  111111331111244 12 productivity total mass used in the process  MI

The Carbon Efficiency (CE) is defined as the percentage of carbon in the reagents which remains in the product. It takes into account the yield and stoichiometric ratios of the reagents actually used in the reaction. Finally, the Reaction Mass Efficiency (RME) corresponds to the atom economy but, like the carbon efficiency, it also includes the yield and actual ratios between the reagents in the calculation. To a large extent, it represents the weight percentage of product obtained compared to the weight of the reagents used.

Table 2. E-factor in the various segments of the chemical industry (Sheldon, 2000) Product quantity (t/yr)

kgwaste/kgproduct

Petrochemicals and refining

106-108

0.1

Bulk chemicals

104-106

0.1-5

Fine chemicals

102-104

5-50

Pharmaceuticals

10-103

25-100

INDUSTRIAL SEGMENT

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There is a substantial difference between efficiency indicators and intensity or productivity indicators, in that the latter not only take into account the reagents but the whole mass used in the process, thus especially highlighting the use of solvents. In this sense (and to the extent in which the solvents and whatever else is used in the process form a waste product), there is an affinity between intensity indicators and the E-factor which, in turn, is essentially linked to the production of waste products. In addition to these indicators which are more widely known and commonly used, and can be defined as ‘related to the mass’, analogous quantitative evaluation parameters have been introduced. Some of these relate to the energy aspect, such as the total energy used in the process for the production of a weight unit of product or, more specifically, the energy used in the recovery of solvents, also in terms of greenhouse gas equivalent (CO2). Some others relate to aspects linked to environmental risks, such as the production of persistent materials, per unit of product, capable of causing bio-accumulation, or the relative contribution of the process for the photochemical formation of ozone, for example: solvent mass POCP vapour pressure 133114 1111111112111111124 4 product mass POCPtoluene vapour pressuretoluene wherein POCP is the photochemical ozone creation potential relating to the solvent used. The use of energy indicators highlights the importance of engineering and process aspects, together with the chemical aspects, for creating a truly sustainable chemistry. Instruments for sustainable chemistry

By identifying the objectives of the industrial system for sustainable chemistry, the description of an ideal process emerges which is designed for obtaining a safe and environmentally harmless product. This process must use safe, non-polluting, possibly renewable raw materials, and not form co-products or by-products; it must have a low energy intensity and not require the use of solvents either in the reaction phase or in the separation and purification phases. Great help is obtained from the mass of information currently available on the risks and toxicity of products, enabling a full evaluation of the positive or negative impact of the chemistry that is being developed. In past years, this knowledge was not so detailed and extensive, and this may have partly caused the environmental problems which arose then and the bad reputation that the chemical industry had acquired. Conversely, from the greater information which is currently available, the chemical community now has greater responsibility.

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Among the most powerful means available for achieving the objective of a sustainable chemistry, an absolutely predominant role is occupied by catalysis, which has been rightly indicated as being the foundational pillar of sustainable chemistry (Anastas et al., 2001). The design of new catalysts and catalytic systems fulfils the double objective of protecting the environment and economic benefit. Generally speaking, using a suitable catalyst can be a powerful sustainability tool in the chemical industry. It enables entirely new synthetic routes to be pursued and planned (which would otherwise be impossible or only practicable with a minimum or limited efficiency), aimed at the objectives and the process or product characteristics previously defined. Catalysis is able to provide numerous benefits, as illustrated in Fig. 1. It offers the key for access to alternative raw materials, such as those deriving from renewable sources. The reactions can be carried out under milder conditions, accompanied by less significant safety problems and a lower energy requirement. Less reactive reagents can be used, which are suitably activated through the catalyst only when necessary, for specific purposes and selectively, and they are consequently intrinsically less dangerous and often less toxic. Through catalysis, it is possible to eliminate the use of stoichiometric reagents, typically characterized by the formation of co-products, often in the form of inorganic waste products, thus improving the atom economy. Furthermore, a higher selectivity towards the desired product can be reached with improved results and with a reduction in by-products. The improved selectivity also leads to greater simplicity in separation and purification, with an energy saving on the one hand and a reduction in the use of solvents and auxiliary agents on the other. It also leads to a greater purity of the product, whose toxicological characteristics can vary significantly due to the absence of toxic impurities even in a minimum concentration. In industrial practice, many catalysts have an extremely high utilization duration, also for reasons linked to their cost, especially within the context of large petrochemical and bulk chemical processing, with the result that their use does not imply significant disposal problems. With the use of heterogeneous catalysts, moreover, many of which are particularly benign from a toxicological and environmental point of view, the separation of the catalyst from the reaction product is also particularly simple. Biotechnologies are, in turn, considered an extremely promising tool for pursuing industrial sustainability (Webster et al., 1996). They are also gaining ground in the production of bulk chemicals (so-called commodities), as new genetic manipulation techniques multiply their possibilities, overcoming the

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Fig. 1. Catalysis: one of the most powerful instruments for a sustainable chemistry.

more benign products

milder conditions

safer reagents controlled and targeted reactivity

CATALYSIS

high selectivity lower energy consumption

lower waste generation easier separation and purification

economic obstacles which have long limited their application in the pharmaceutical world and specialties. This application of biotechnologies appears to create fewer problems for public opinion than use in agriculture. Biotechnologies open up the field towards the use of biomasses (starch, cellulose) and sugars deriving therefrom as renewable raw materials also for chemical production and not only as energy vectors. In biorefinery (see below), biomasses are used for the combined production of chemical products with a more or less high added value together with energy vectors in a similar way to those of a traditional refinery which uses fuel/fossil raw material. The use of biomasses allows a mitigation in the emission of carbon dioxide into the atmosphere: as biomasses use CO2 for their own growth through photosynthesis, their use as raw material does not give rise to a net increase in the CO2 content in the atmosphere when the products reach their final destination in the environment. In addition to conventional productions, new uses of biomasses have become possible by introducing modified microorganisms and enzymes which act as biocatalysts and induce specific chemical transformations. The normal metabolism of a cell is modified through metabolic engineering techniques so that it contributes to the production of the desired chemical product. Furthermore, innovative biochemical engineering techniques can be used for a more effective separation of the products.

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Industrial biotechnology (white biotechnology) certainly cannot be considered immune from sustainability problems. On the one hand, it is associated with the generation of mud and waste water to be purified from potential pollutants (phosphates, heavy metals) and, on the other, with high energy consumptions due to the dilution of the process streams. However, the preferential use of renewable raw materials, the use and generation of highly biodegradable or reusable products and substrates with a low toxicity, the use of simple, highly selective processes carried out under mild conditions also in the production of complex molecules, and the strong innovation potential still unexploited, all lead to the assumption that white biotechnology will take on an increasingly important role in reducing the environmental impact induced by chemical processes. Process intensification is a term which, generally speaking, means ‘to do more with less’ within the context of optimizing the industrial production process. It was initially conceived as a strategy for significantly reducing plant dimensions and, consequently, the capital invested. It can be well adapted to the conception of sustainable production processes that are characterized by an optimization of the mass efficiency, reduction in energy consumption, greater control and safety, and better product quality (Jenk et al., 2004). At the basis of process intensification is the concept of adapting the process to the reaction and

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not vice versa, by suitably modulating the unitary operations for the physico-chemical characteristics of the single process steps. It also aims at combining, when possible, several operations in a single apparatus. It can therefore be easily understood how process intensification makes wide use of the development of innovative ideas such as membrane reactors, exchanger reactors, rotating bed or disk reactors, reactive and catalytic distillation, high-efficiency static mixers, microstructured equipment (reactors, exchangers, separators, extractors, etc.). The use of microstructured components which, today, can be manufactured at low cost in a variety of metallic, polymeric and ceramic materials, allows an extremely efficient mass and energy transfer. The industrial development of microstructured equipment is not considered in terms of conventional scale-up, but rather according to bio-mimetic criteria, through the replica of thousands of microstructures assembled and interconnected in a large-scale macroproductive unit, with the help of an integrated multiscale design. Other strong points linked to process intensification are alternative energy activation methods such as sonochemical and photochemical methods, methods based on the use of microwaves, etc.

9.6.3 Innovation areas for sustainable chemistry There are numerous areas in which it is possible to operate for sustainable chemistry and these can be divided into categories. Here we will adopt the most commonly used distinction which refers to the basic components of a chemical reaction (Anastas and Williamson, 1996): a) alternative raw materials; b) alternative synthesis methods or alternative reagents; c) alternative reaction conditions; d) alternative products. These four components are obviously strictly correlated with each other and the objectives in a varied, complex and, at times, inextricable manner. A new synthesis method primarily aimed at improving the selectivity or atom economy, with a reduction in the by-products through a smaller number of steps, can also envisage different raw materials and reaction conditions that are possibly more benign, such as the limited use or absence of solvents. It is therefore not objectively easy to insert a real case within a specific category, unless one of the components has such a predominant importance as to make the choice simple and obvious.

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Alternative raw materials The search for alternative raw materials is aimed at adopting more benign raw materials, which are renewable or reduce risks for human beings and the environment. In many cases, the selection of the raw material can represent an extremely important factor in determining the impact of the activity on human beings and on the environment. Most chemical productions currently derive directly or indirectly from petroleum. In the choice of alternative raw materials, several main trends can be highlighted: a) the use of natural gas as an alternative primary fossil source to oil; b) the use of raw materials from renewable sources, of an agricultural or vegetal origin (biomasses); c) the reuse of by-products or scraps from other processing or activities; d ) the reuse of carbon dioxide. Natural gas

The use of natural gas, whose main component is methane, is an alternative which is more sustainable than oil (Rostrup-Nielsen, 2004). Verified reserves of natural gas guarantee its use for a much longer period. With natural gas, effectively integrated systems can be constructed for the direct production of thermal and electric power, and – by means of direct or indirect conversion processes – other synthetic fuels and the main commodities of the chemical industry can be produced. Using natural gas in combined cycle gas turbines enables the generation of electric power with a high yield. Direct methane conversion processes are the object of intense studies, but until now, the key step in the use of natural gas remains the production of synthesis gas (syngas), a mixture of CO and H2 obtained by partial oxidation (with oxygen) or steam reforming (with water vapour): CH4 0.5O2 CO 2H2 CH4 H2O CO 3H2 䉴



Synthesis gas is a hydrogen source, also through the conversion of CO to CO2 by means of the so-called Water Gas Shift (WGS): CO H2O CO2 H2 䉴

or it can be converted to synthetic liquid fuels such as methanol, dimethylether or synthetic fuels (synfuel) for diesel and other hydrocarbon cuts obtained from the Fischer-Tropsch reaction. In addition to facilitating the exploitation of ‘remote gas’ (i.e. the large natural gas reservoirs located in areas far from developed regions) due to easier transportation, these synthetic fuels containing oxygen and, unlike many oil cuts, free of contaminants such as sulphur, are

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SUSTAINABILITY

refinery products alternative fuels

gas to hydrogen

hydrogen ammonia WGS

electric power

gas turbine

natural gas (methane)

partial oxidation

syngas

FischerTropsch

steam reforming

synfuels (FT diesel)

gas to liquids

petrochemicals olefins methanol

alternative fuels

MTO

gas to chemicals gasoline

dimethylether MTG Fig. 2. Use of natural gas as alternative raw material.

characterized by low polluting emissions and represent a valid answer to the new demands and regulations relating to fuels. Like synthesis gas, methanol is a fundamental and extremely versatile intermediate. It can be transformed on acid zeolites into olefins such as ethylene and propylene (MTO, Methanol To Olefins process), but also into gasoline (MTG, Methanol To Gasoline process), as well as being a progenitor of other important bulk chemicals such as formaldehyde and acetic acid, for example. A map of the numerous possible uses of natural gas as an alternative source is illustrated in Fig. 2. Biomasses

In general, biomasses of a vegetable origin can represent renewable raw materials for a whole series of oxygenated chemical products, in primis ethanol, but also for methane or synthesis gas through classical gasification, fermentation or high temperature pyrolysis processes, or through new developing catalytic process (for example, with the use of homogenous catalysts based on transition metals). In this context, the use of biomasses can be well integrated with the utilization potential of natural gas as alternative raw material. The use of raw materials of a vegetable origin understandably forms the hard core of green chemistry, a kind of green revolution which represents its most strongly developing sector (Stevens and Verhé, 2004). Typical examples of the use of renewable raw materials of an agricultural origin are represented by the substitution of raw materials

888

deriving from fossil sources with sugars (products obtained from starch or cellulose) or with oils and fats, often, but not always, through the use of biosynthetic processes. In some cases, the product obtained from raw materials of a vegetable origin is only functionally similar, but chemically different from the product currently obtained from a petrochemical source: an example is bio-diesel, consisting of a mixture of methyl esters of fatty acids produced from vegetable oils, used in place of or in a mixture with gas oil. In other cases, it is possible to obtain exactly the same products, currently obtained from a petrochemical source, from biomasses. In addition to well consolidated productions such as ethanol/ethylene and acetic acid, the wide range of products which can be obtained is continually expanding. Adipic acid, for example, which is the main intermediate for the production of nylon, is traditionally produced starting with benzene, a refinery derivative of petroleum: OH OH

(

O

)



O

O

HOC(CH2)4COH

An alternative raw material could be glucose, as demonstrated in a biosynthetic pathway which does

ENCYCLOPAEDIA OF HYDROCARBONS

CHEMICAL INDUSTRY AND SUSTAINABLE DEVELOPMENT

not exist in nature, through the use of a genetically modified microbial biocatalyst which transforms glucose into muconic acid; the latter is then hydrogenated to adipic acid (Drats and Frost, 1998):

O HOCH2

COOH

COOH

OH

OH OH

O

OPO3H2 OH

more competitive production cost (Nakamura and Whited, 2003):

HOCH2

OH HOCH2CHCH2OH

OH OH

 OH OH

O

HO O H2O3PO

OH

H

Use of by-products

OH

COOH

HO OH

OH OH

O

HOC(CH2)4COH

O

O

O

HOCCH

CHCH

CHCOH

Pyrocatechol, another industrial intermediate of great interest, can also be produced during this biosynthetic pathway. The widely quoted biosynthesis of adipic acid is a possibility that has only been demonstrated on a laboratory scale, and there are doubts as to the effective current economic compatibility of this form of synthetic pathway. A similar case, but which already seems to have effective applicative potentialities, is the synthesis of 1,3-propanediol. The latter is an intermediate for a new polyester whose development has so far been obstructed by the excessive cost of its production carried out with traditional methods, which involves among other things, the manipulation of highly toxic substances such as ethylene oxide and acrolein: O CH2 CO,H2

CH2 O

HOCH2CH2CH H2

HOCH2CH2CH2OH

OH

H2

HOCH2CH2CH2OH

In this case, the alternative biosynthetic method, which uses a genetically modified bacterium, allows the production of 1,3-propanediol from glucose at a

VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

The utilization of by-products and waste-products of other processings is a highly topical theme for sustainable development. In the oil and refining industry, there is, on the one hand, a shift in demand towards higher-quality fuels, subject to increasingly strict specifications, such as the limitation of the sulphur content, capable of guaranteeing higher performances and lower polluting emissions during use. On the other hand, there is a growth in the utilization of increasingly heavier crude oils and bituminous sands. This leads to an increased availability of heavy hydrocarbon fractions with reduced and economically disadvantageous possibilities of utilization as fuel oils, when it is not even necessary to proceed with their onerous disposal. Apart from traditional upgrading processes, new developing technologies are capable of offering a better answer to these problems. For example, the EST technology (Eni Slurry Technology; Panariti et al., 2000), based on hydrogenation in the presence of molybdenum catalysts in slurry phase and currently on a demonstrative plant scale, obtains the almost complete conversion of residues and non-conventional heavy crude oils into higher-quality hydrocarbon cuts (Fig. 3). New desulphurization technologies, in turn, allow the exhaustive desulphurization of medium distillates with a high sulphur content resistant to conventional HDS (Hydrodesulphurization) treatment. These would otherwise not be capable of satisfying the new specifications relating to the sulphur content aimed at reducing polluting emissions, enabling their use as high-quality fuels (Song and Ma, 2003). Desulphurization can be obtained oxidatively (ODS, Oxidative DeSulphurization) by oxidizing the sulphur containing components with peracids, with hydrogen peroxide in the presence of ultrasounds or with hydroperoxides, conveniently generated by treatment with air of suitable refinery cuts: sulphur containing molecules are thus obtained with a differentiated polarity (sulphones) which can be easily separated from gas oil (ULSD process, Ultra Low Sulphur Diesel; Fig. 4). Bio-desulphurization processes by means of bacterial strains, suitably enhanced for the purpose with genetic

889

SUSTAINABILITY

hydrogen

gas

catalyst make-up

gasoline gas oil slurry reactor

distillation

VGO

hydrocarbon residue solvent (propane)

deasphalting (SDA) DAO

recycle (asphaltenescatalyst)

purge

Fig. 3. EST process. DAO, DeAsphalted Oil; SDA, Solvent DeAsphalting; VGO, Vacuum Gas Oil.

engineering techniques, are also proving to be extremely promising (D’Addario et al., 2000). Another process of great interest which is currently being developed relates to the zeolitecatalyzed production of 2,6-dimethylnaphthalene, a key intermediate of PEN (polyethylene naphthalate), a high-performance polyester. Through this process, 2,6-dimethylnaphthalene is obtained directly from naphthalene cuts of heavy hydrocarbon fractions with a low value of petrochemical or refinery origin (Millini et al., 2003). Through a series of successive alkylation, dealkylation and isomerization reactions and a

selective crystallization, the process allows the practically quantitative transformation of all compounds with a naphthalene structure present in the starting material, into the desired product. It is also possible to obtain high-quality feedstocks for steam cracking, consisting of C2-C5 linear alkanes, for the production of light olefins (ethylene, propylene). This is done by processing heavy fractions (similar to those mentioned above) with hydrogen under cracking conditions, as well as the poorer-quality components of pyrolysis gasoline with a high content of aromatic compounds. These poorer-quality components will have increasing difficulty in finding a place on the

gas oil from HDS S300 ppm

refinery stream

desulphurized gas oil S10 ppm

peroxidation

air

oxidation to sulphones

separation (DeSOx)

(SOx)

Fig. 4. ULSD process.

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ENCYCLOPAEDIA OF HYDROCARBONS

CHEMICAL INDUSTRY AND SUSTAINABLE DEVELOPMENT

market due to the reduction in the content of aromatic products in gasoline (Ringelhan et al., 2004). Reuse of CO2

The reuse of carbon dioxide represents a problem of such current importance that it can be considered as being a genuine trend in itself as far as the use of by-products as alternative raw materials are concerned (Aresta, 2003). The first and main opportunity for the reuse of CO2 generally consists in the use of biomasses of a vegetable origin (see Chapter 6.5), as biomasses use CO2 for their own growth through photosynthesis, allowing a virtuous sustainable cycle to be established with zero emission of CO2. There are numerous important examples of the utilization of CO2 already in the context of large classical processes of traditional industrial chemistry. In reality, carbon dioxide has been adopted for many years in the production of urea (used as fertilizer) and acetylsalicylic acid (better known as aspirin), in addition to participating in the synthesis of methanol and other processes involving the equilibrium of carbon oxides. In addition to these is the more recent utilization of CO2 for the synthesis of organic carbonates, considered as building blocks of great interest for the creation of a sustainable chemistry due to their benign characteristics towards human beings and the environment (see below). Alkylene carbonates (ethylene, propylene) are industrially produced by the reaction of the corresponding oxides with CO2: R CH2

CHR O

 CO2

O

O O

R  H, CH3, C2H5

In the presence of a different catalytic system, the reaction evolves towards the formation of high molecular weight aliphatic polycarbonates, characterized by a high biodegradability. The amazing regularity of the alternation of the insertion of epoxide units and CO2 allows extremely high molecular weights to be reached: R n

CHR

CH2 O

 n CO2

H

O

OCH2CHOC n

The homologous reaction between CO2 and alcohols leads to the formation of alkyl carbonates. In this way, dimethylcarbonate (DMC) can be obtained from methanol and carbon dioxide: O 2CH3OH  CO2

CH3OCOCH3  H2O

VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

The feasibility of this reaction has been recently demonstrated, but industrial application has not yet been achieved, mainly due to difficulties linked to the unfavourable thermodynamic equilibrium. The synthesis of alkyl carbonates by the reaction of alcohols with urea, which also represents a means (indirect) for the utilization of CO2 as a raw material is, however, in a more advanced development phase. Extraordinary interest is being shown in reforming methane with CO2 (so-called dry reforming) as it offers a means for the direct conversion of large quantities of CO2 to synthesis gas, even if the benefits must be carefully assessed in the light of the overall energy balance, as a result of the reaction’s endothermicity: CH4 CO2 2CO 2H2 䉴

Alternative synthesis methods and alternative reagents The development of new synthesis methods and the use of alternative reagents certainly represent the most interesting investigation areas with the aim of satisfying many of the 12 principles of green chemistry. An alternative synthesis which comprises a fewer number of steps is definitely a step in the right direction towards a higher overall yield, with a consequent reduced formation of by-products and an improvement in the E-factor. An alternative synthesis could exploit a reaction characterized by a better atom efficiency, especially a catalytic step for example, to substitute stoichiometric reagents which are the cause of waste-products and by-products. Finally, one of the key criteria in the selection of alternative methods relates to reducing the risk (for human beings and the environment) of reagents involved in the synthesis process. Replacement of hazardous reagents

Substances which are extremely toxic and dangerous for the environment are widely used in the chemical industry: a particularly significant example relates to the use of phosgene, a gas deriving from chlorine, in the extremely high-scale production of commodities such as isocyanates, intermediates for the production of polyurethanes, and polycarbonates, widely-used polymeric materials. The production of phosgene on a worldwide scale can be estimated as being in the order of 6-7 million t/yr. The use of carbonic esters, such as dimethylcarbonate (DMC) and its homologues, i.e. low-toxicity reagents, makes it possible to avoid the use of phosgene. Phosgene is highly toxic and dangerous, and has other problems deriving from its use: dependence on the chlorine cycle, the use of halogenated solvents such as methylene chloride and

891

SUSTAINABILITY

chlorobenzene, the formation of by-products and chlorinated saline waste-products. Carbonate esters, such as dimethylcarbonate and its homologous products, are reagents with a low toxicity and can be used in place of phosgene. Substituting the use of phosgene in industrial practice with organic carbonates undoubtedly represents one of the successes of green chemistry, and also of the research carried out by the Eni group, for many years at the forefront of promoting their industrial utilization (Rivetti, 2000a, 2000b). As the production of carbonates was traditionally linked to the use of phosgene, it was first of all absolutely necessary to develop alternative synthesis methods which did not envisage its use (Delledonne et al., 2001). Among those identified, apart from those starting from CO2 described above, the oxidative carbonylation of methanol to dimethylcarbonate, which uses carbon monoxide and oxygen, and gives water as a co-product, is of primary industrial importance: O 2CH3OCOCH3  2H2O

4CH3OH  2CO  O2

In the process industrialized by EniChem for the production of dimethylcarbonate, the oxidative

carbonylation is carried out on liquid methanol using copper halides as catalysts. Fig. 5 shows a simplified process scheme. The dimethylcarbonate can then be easily transformed into its higher homologous products by means of transesterification reactions. For example, diphenylcarbonate (DPC), which is more reactive as a carbonylation agent and therefore selected as preferred reagent in specific reactions, is obtained from dimethylcarbonate and phenol: OH

O CH3OCOCH3  2

O OCO

 2CH3OH

When a dialkyl or diarylcarbonate is used as carbonylation agent in substitution of phosgene, the carbonyl group (CO) is incorporated in the substrate, whereas the alcohol or phenol corresponding to the carbonate is released. It is important to appreciate how, in the whole sequence of reactions involved, the resulting alcohol or phenol can be recycled to the

CO recycle

purge CO, CO2, light ends

gas purification CO2

product recovery condenser

DMC, CH3OH, H2O

to CO generation unit

DMC/CH3OH azeo recovery

reactor H2O

DMC, CH3OH recycle

DMC, H2O O2

CO

CH3OH

DMC purification

DMC

Fig. 5. EniChem process for the production of dimethylcarbonate (Rivetti and Delledonne, 2003).

892

ENCYCLOPAEDIA OF HYDROCARBONS

CHEMICAL INDUSTRY AND SUSTAINABLE DEVELOPMENT

CH3

O CO, O2

CH3OH

OCO

HO

C

OH

CH3

OH

C H2O

CH3

CH3OCOCH3 O

C

O O

CH3

C n

Fig. 6. Production of aromatic polycarbonates without the use of phosgene:

molten mass polymerization of biphenol A with diphenylcarbonate.

synthesis of the carbonate used as reagent. For carbonylations carried out with organic carbonates, the result is a synthesis pathway which only uses CO and O2 (or CO2) and produces water alone as co-product. In the production of aromatic polycarbonates, for example, the diphenylcarbonate, produced from dimethylcarbonate, is reacted with bisphenol A in melt to produce the polycarbonate. The methanol co-produced in the synthesis of diphenylcarbonate can be recycled to the synthesis of DMC. Analogously, the phenol co-produced in the synthesis of polycarbonate is recycled to the synthesis of DPC (Fig. 6). It is also interesting to note that the whole cycle represents a brilliant example of a process carried out with the total absence of solvents. This synthesis method has become of considerable industrial importance: approximately a sixth of the world production of polycarbonate is obtained by it. Similarly, through the production of diallylcarbonate as intermediate, the synthesis has been carried out (without phosgene) of diethyleneglycol bis(allylcarbonate), the monomer (commercially known as CR-39 or RAV-7) traditionally most widely used in the production of high-quality optical and ophthalmic lenses (Romano et al., 1985). In addition to avoiding the use of phosgene in the above-mentioned reactions, dimethylcarbonate is also an excellent alternative to the use of other extremely toxic and polluting reagents, such as dimethylsulphate and methyl chloride, in alkylation reactions: in this way, it is possible to selectively methylate heteroatoms (O, N, S) to aliphatic and aromatic amines, phenols, sulphides, and also carbon atoms carrying activated hydrogen in position a to nitriles, ketones, esters, sulphones, etc.

VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

Replacement of stoichiometric reagents

The use of organic carbonates is an example of the success with which highly dangerous reagents can be substituted by more benign reagents which perform the same function. Equally striking is the elimination of stoichiometric reagents (often dangerous and the cause of waste products, polluting by-products and co-products) in favour of catalytic reactions characterized by an improved atom efficiency. There are numerous examples of this tendency, which demonstrates the considerable power of catalysis as a tool for a more sustainable chemistry; some of these, selected from the most representative and of greatest industrial interest, are cited hereunder. Typical stoichiometric reagents associated with toxicity for human beings and problems relating to environmental pollution are those adopted in oxidations with metals (such as chromium and manganese compounds) or, even if to a lesser extent, with organic peroxides and peracids. In this way, the oxidation of alcohols to aldehydes or to ketones are still frequently carried out with chromium trioxide, or sodium or potassium dichromate in the presence of sulphuric acid. This produces waste products containing a toxic metal, an acid, and organic residues combined together that are extremely difficult to dispose of. The example provided hereunder relates to the oxidation of 1-octanol to octanal: 3CH3(CH2)6CH2OH 2CrO3 3H2SO4 3CH3(CH2)6CHO Cr2(SO4)3 6H2O

893

SUSTAINABILITY

The same reactions can be carried out using air in the presence of a variety of catalysts, such as for example, 2,2,6,6-tetramethylpiperidine-N-oxide (TEMPO) combined with ruthenium complexes (Sheldon, 2000): 2CH3(CH2)6CH2OH O2

RuCl2(Ph3P)3 CH3 H3C

CH3 N

.

CH3

A historical method for the industrial preparation of hydroquinone (1,4-dihydroxybenzene) consists in the oxidation of aniline with manganese dioxide to give quinone, followed by the reduction of the latter with iron. The process gave as co-products ammonium sulphate and iron oxide in stoichiometric quantities compared to hydroquinone, and a quantity of manganese sulphate equal four times this stoichiometric value: NH2

O

O 2CH3(CH2)6CH 2H2O

The use of 4-hydroxy-TEMPO as catalyst for the production of bisnoraldehyde by means of oxidation with sodium hypochlorite of the corresponding alcohol represents the keystone of a new and more benign commercial synthesis method of progesterone (Hewitt, 1998). This method has replaced a prior process based on the oxidation of stigmasterol in two steps (oxidation according to Oppenhauer with aluminum ter-butoxide and cyclohexanone followed by ozonolysis and reduction with zinc/acetic acid), reducing aqueous refluents 5 times and organic waste 10 times. The new method also allows a more effective use of the raw material at the start of the process: soybean sterols (100% )

 4MnO2 5H2SO4

2

O 2

(NH4)2SO4 4MnSO4 4H2O

Fe, H2O

O OH  FeO

2 OH

The same product, together with the 1,2 isomer (pyrocatechol), can be obtained directly from phenol without the use of metals and without a significant generation of inorganic by-products, using hydrogen peroxide as an oxidant, which gives only water as co-product:

(15%) OH

OH OH

 H2O2

TS-1

OH 

OH

 H2O

OH HO cyclohexanone Al(OtBu)3

OH NaOCl

H3C H3C

N

CH3 CH3

O O 1. O3 2. Zn/CH3COOH

H O

894

This process represented the first industrial application of titanium silicalite (TS-1), a catalyst of the zeolite group with an MFI structure, synthesized for the first time and developed by research laboratories of the Eni group as a selective oxidation catalyst in numerous applications (Perego et al., 2001). Among other applications subsequently developed for titanium silicalite, mention should be made of the ammoximation of ketones with NH3 and H2O2, especially of cyclohexanone to give cyclohexanone oxime, an intermediate in the production of Nylon 6, also recently attaining industrial exploitation (Petrini et al., 1996). The conventional technology used in the production of cyclohexanone oxime (Raschig process) creates the problem of a high co-production of ammonium sulphate (approximately 2.8 kg/kg), which gives rise to onerous disposal problems:

ENCYCLOPAEDIA OF HYDROCARBONS

CHEMICAL INDUSTRY AND SUSTAINABLE DEVELOPMENT

O

CH3CH

.

CH2

 NH2OH H2SO4  2NH3

Cl2 H2O

Cl

NOH

OH NaOH

CH3CHCH2OH, CH3CHCH2Cl

 (NH4)2SO4  H2O NaOH

It is also jeopardized by the complexity of the operating cycle, linked to the synthesis of the intermediates involved, especially hydroxylamine, and to the inorganic raw materials used (hydroxylamine sulphate is produced from NH3, CO2 and SO2), and the consequent problems relating to polluting emissions (NOx, SOx). The ammoximation process represents a radical innovation in the sector. The term ammoximation indicates that the production of oxime is carried out directly starting with ammonia: the process is, in fact, based on a catalytic reaction between cyclohexanone, ammonia and hydrogen peroxide which, on the one hand, completely eliminates the critical aspects linked to the production and use of hydroxylamine and, on the other, the co-production of sulphates: O

O

The process via chlorohydrin is jeopardized by a series of problems, with evident implications of an environmental nature, such as the formation of inorganic and organic chlorinated by-products. A waste product is formed, equal to approximately 40 times the volume of the epoxide produced, consisting of a diluted solution of sodium or calcium chloride and organic chlorinated residues. The epoxidation process of propylene with H2O2 catalysed by titanium silicalite, on the other hand, gives water only as coproduct (Clerici et al., 1991):

 NH3  H2O2

It is an improvement even when compared to processes via hydroperoxides, which give styrene or MTBE (methyl-tert-butylether) as co-product, as it eliminates the necessity of recycling or placing the co-product on the market:

 2H2O

CH3 R-H

O2

C

O  NH3  H2O2

OH

C

OH

R

R– OOH

CH2

NOH

NHCCH3

C

MTBE

CH2  R – OH

CH3CH

O H

TS-1

R – OOH

CH3 CH3CH

CH3

CH2  H2O

CH3CH O

Another ammoximation process catalysed by titanium silicalite of great industrial importance is used in the production of paracetamol: CH3

TS-1

CH2  H2O2

CH3CH

NOH TS-1

CH2 NaCl

CH3CH

O CH

styrene

CH3 OH

New processes in the development phase for the production of propylene oxide and phenol are also extremely important. Propylene oxide is a basic intermediate of the chemical industry, used for producing a long series of other products and materials, in primis polyether polyols devoted to the production of polyurethanes. In the conventional process, propylene is reacted with chlorine in an aqueous solution, producing a mixture of chlorohydrins, from which propylene oxide is released, by treatment with lime or caustic soda:

VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

Similar problems, relating to marketing the acetone co-produced or recycling it as an alkylating agent of benzene, after reduction to isopropanol, are linked to the current production process of phenol, via cumene (Girotti et al., 2003): OOH CH(CH3)2 CH3CH

CH2

C(CH3)2 H

O2

OH

O  CH3CCH3

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SUSTAINABILITY

Also in this case, the direct oxidation of benzene to phenol with H2O2 catalysed by titanium silicalite, currently in the development phase, allows these problems to be solved (Balducci et al., 2003): OH  H2O2

TS-1

 H2 O

An alternative method for the direct synthesis of phenol from benzene uses nitrous oxide (N2O) as oxidant and a ZSM-5 zeolite containing iron as catalyst (Panov, 2000): OH  N2O

Fe-zeolite

 N2

Unfortunately, the cost of a production ad hoc of N2O is too high and the only plausible scenario for the application of this process envisages the reuse of N2O by-product from production plants of adipic acid (intermediate of nylon). The synthesis of phenol by the selective hydrodeoxygenation (HDO) of polyphenols such as catechol, appears to be potentially quite interesting, especially with the prospect of producing the latter from renewable raw materials such as lignocellulose biomasses: OH  H2

 H2O

OH

New catalysts

An extremely interesting trend which enjoys considerable success is the substitution of those catalysts which – as a result of their poor catalytic activity (or duration of their catalytic life) and/or due to the difficulty or impossibility of separation, work-up and recycling – are used in massive quantities and are also characterized by a high risk for human beings and the environment. Numerous acid catalysts widely used on an industrial scale such as sulphuric acid, hydrofluoric acid and aluminum trichloride belong to this category. Exceptional results in enabling their substitution have been obtained through the use of solid acid catalysts, especially synthetic zeolites. These silico-aluminates can be easily obtained in acid form and have orderly porous crystalline structures, which form ideal matrixes for directing and accelerating chemical reactions and also making them selective. As zeolites are heterogeneous catalysts, their use minimizes problems linked to the separation of the catalyst from the medium and reaction products. Zeolites are configured as inert solids from a toxicological and environmental point of view, they

896

have high catalytic activities, and normally have a long duration and are easy to regenerate, so that problems associated with their disposal are practically zero. Alkylation is a very important refinery process which allows isobutane and C3-C5 olefins to be converted to C7-C9 branched paraffins with a high octane number, ideal components for gasoline: CH3CH(CH3)2  CH3CH

CHCH3 CH3 (CH3)3CCHCH2CH3

This reaction is traditionally catalysed with concentrated sulphuric acid or with hydrofluoric acid, with an installed productive capacity which is more or less equally distributed. These are highly corrosive substances which create considerable environmental and safety problems if they spill, in addition to generating large quantities of inorganic by-products (HF forms stable aerosols at a ground level, whereas in the case of H2SO4, the production of spent acid to be disposed of reaches 70-100 kg per tonne of alkylate). Industrial alkylation processes are currently available that are competitive with traditional technologies, which use a variety of solid acid catalysts, from large-pore zeolites, with a Y-type structure, normally containing metals with a hydrogenating function, to Lewis or protic acids such as aluminum trichloride or trifluoro methanesulphonic acid supported on porous substrates such as alumina (Hommeltoft, 2001). The use of these processes is capable of eliminating most, if not all, of the disadvantages of processes based on liquid acids. Friedel-Crafts reactions (from the name of their discoverers), i.e. alkylation and acylation of aromatic compounds in the presence (prevalently) of aluminum trichloride (AlCl3), form a very important group of reactions that belong to a wide range of sectors of the chemical industry, from petrochemicals to fine chemicals and pharmaceuticals productions. The use of AlCl3, however, is not without problems. In alkylation reactions, it can be used in a catalytic quantity but it cannot be recovered at the end of the reaction, with the result that the quenching of the catalyst with water creates significant liquid and solid waste products containing aluminum and chlorine. It is mainly in acylation reactions, however, where AlCl3 must be used in a quantity which is even higher than the stoichiometric value (up to two times) that after the quenching of the catalyst, there is an enormous production of waste products. The use of zeolites in substitution of traditional catalysts for the alkylation of aromatic compounds is now a well-consolidated reality and is widespread in large petrochemical processes. Their use, for example, has

ENCYCLOPAEDIA OF HYDROCARBONS

CHEMICAL INDUSTRY AND SUSTAINABLE DEVELOPMENT

become prevalent in industrial synthesis reactions of ethylbenzene, an intermediate in the production of styrene, and cumene, an intermediate in the production of phenol. In these reactions, large-pore zeolites, such as beta zeolite, have substituted, as alkylation catalysts of benzene with ethylene and propylene, aluminum trichloride and phosphoric acid (H3PO4) supported on silica, respectively (Perego and Ingallina, 2002). In an ethylbenzene plant with a capacity of 500,000 t/yr, the production of an aqueous effluent of aluminum hydroxide in the order of several thousands of t/yr is thus avoided, whereas in the production of cumene, the generation of organic aromatic by-products is reduced by at least 20 times, and the necessity to periodically dispose of the catalyst in landfills is practically zero:  CH2

 CH3CH

CH2

CH2CH3

beta zeolite

(styrene) CH(CH3)2

beta zeolite

CH2

(phenol)

As far as acylation reactions are concerned, the use of beta zeolite allows the substitution of aluminum trichloride or hydrofluoric acid in the synthesis of 2-acetyl-6-methoxy naphthalene, an intermediate for the production of naproxen (the well-known antipyretic agent, a non-steroid analgesic and anti-inflammatory drug) and in the synthesis of isobutylbenzene of 4-isobutyl acetophenone, an intermediate of the equally well-known ibuprofen (Andy et al., 2000):  (CH3CO)2O

beta zeolite

H3CO COCH3  CH3COOH H3CO

which is extremely toxic and corrosive, in turn, requires the use of strict safety measures, and extremely costly equipment and materials. Beta zeolite is also adopted as catalyst in the acylation of other industrially important aromatic ethers such as anisole (to p-methoxy acetophenone) and veratrole. The acylation catalysed by the zeolite uses acetic anhydride as acylation agent in the absence of solvent and allows a higher yield (from 85-90% to over 95%). The classical process, on the contrary, uses acetyl chloride combined with more than one equivalent of AlCl3 and a chlorinated hydrocarbon as solvent. The new process thus avoids the generation of hydrochloric acid (HCl) deriving from both the use of aluminum trichloride and acetyl chloride (as well as the synthesis of the latter). The classical process generates 4.5 kg of aqueous effluent per kg of product, containing aluminum compounds, HCl or its salts and residues of chlorinated hydrocarbons. The catalytic alternative process generates only 0.035 kg of aqueous effluent per kg of product (i.e. more than 100 times less) containing less than 1% of organic compounds, mainly acetic acid. The product is isolated by the simple filtration of the catalyst (which can be recycled) and distillation, with the result that the number of unitary operations is reduced from 12 to 2 (Sheldon, 2000). Also in nitration reactions of aromatic compounds, the use of zeolites as catalysts, as also other solid acids (for example Nafion, a sulphonated perfluorinated polymer), represents an alternative to the use of the sulphonitric mixture (HNO3/H2SO4) traditionally adopted. In addition to eliminating the use of sulphuric acid with consequent problems of disposal, the formation of polynitrated products is significantly reduced, and in particular, with the use of beta zeolite, a high selectivity towards the para isomer is obtained (Bernasconi et al., 2004): CH3

CH2CH(CH3)2  (CH3CO)2O

CH2CH(CH3)2 beta zeolite

 CH3COOH COCH3

As already mentioned, this avoids the use of a higher quantity of aluminum trichloride than the stoichiometric value, necessary as a result of the strong complexing of AlCl3 with the carbonyl compound formed. Furthermore, the subsequent hydrolysis process necessary for decomposing this intermediate, which generates large quantities of inorganic by-product and is the cause of onerous separation processes, is also avoided. The use of HF,

VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

 HNO3  (CH3CO)2O

beta zeolite

CH3  2CH3COOH NO2

Finally, one of the most striking examples of the industrial use of a zeolite as solid acid catalyst relates to the substitution of oleum (a mixture of concentrated sulphuric acid and sulphur trioxide) used in a large quantity in the Beckmann rearrangement of cyclohexanone-oxime to caprolactam (Ichihashi and Sato, 2001):

897

SUSTAINABILITY

N

O

O

OH

NH

 H2SO4  2NH3

CCH3

 (NH4)2SO4 (CH3CO)2O

The new reaction completely avoids the co-production of 1.3-1.6 kg of ammonium sulphate per kg of caprolactam produced, deriving from the neutralization of oleum: N

silicalite MFI

1

(CH3)2CHCH2

NaOC2H5

(CH3)2CHCH2 COOC2H5

O

CH3

NH

CH

CHCH3

H, H2O

NH2OH

3

4

(CH3)2CHCH2

The coupling of the ammoximation process of cyclohexanone to cyclohexanone-oxime, indicated above, with the new catalytic rearrangement process has opened up the way to a production of caprolactam which is completely free of the co-production of salts.

2

CHO

O

OH

ClCH2COOC2H5

AlCl3

(CH3)2CHCH2 NOH

CHCH3

CN

COOH

CHCH3

CHCH3

H, H2O 5

(CH3)2CHCH2

6

(CH3)2CHCH2

(CH3)2CHCH2

Syntheses with a fewer number of steps

In many cases, the setting up of an alternate synthesis strategy aims not only at exploiting reactions characterized by a better atom efficiency and the use of less hazardous reagents, but also at reducing the number of steps necessary for obtaining the product. A fewer number of steps favours a higher overall yield and a reduction in the quantity of by-products. The search for alternative synthesis pathways has led to the development of logic protocols as a basis for computerized programs, based on the observation that the number of possible synthesis methods for objective molecules with even a modest complexity is normally huge (Hendrickson, 1996). It has been observed that within the range of these programs, the most reliable selection criteria of the synthesis strategy are those simply based on the number of steps, as predicting the yields is often so inaccurate as to be totally useless. The best synthesis methods are therefore normally the shortest. It is not surprising that the most significant examples of the reduction in the number of steps in complex synthesis procedures are mainly found in the pharmaceutical, or fine chemical and specialty sectors. Ibuprofen has already been mentioned as being one of the most widely used pharmaceutical aids among anti-inflammatory, analgesic and antipyretic drugs. The traditional synthesis of ibuprofen requires six steps with the use of large masses of solvent and corrosive reagents, used in a stoichiometric quantity. A calculation of the atom economy indicates that only 40% of the atoms present in the raw materials is incorporated in the product:

898

A new synthesis has been developed and used on an industrial scale (it currently covers approximately 25% of the whole world production of ibuprofen), which envisages only three steps, with a doubling of the atom efficiency to at least 80% and a consequent significant reduction in the quantity of by-products generated (Cann and Connelly, 2000): O CCH3 (CH3CO)2O

H2

HF

Ni-Raney

(CH3)2CHCH2

1

(CH3)2CHCH2

OH

2

COOH

CHCH3

CHCH3 CO Pd(II)

(CH3)2CHCH2

3

(CH3)2CHCH2

The new synthesis, which in current industrial practice still contemplates the use of HF as catalyst of the acylation step, will be able to obtain further benefit from the use of the zeolites described above. Another alternative synthesis which has proved to be valid up to a pilot scale for ibuprofen, as also for the homologous arylpropionic acids ketoprofen and naproxen, exploits the use of dimethylcarbonate as reagent, with yields exceeding 99%. Similarly, again through the use of dimethylcarbonate, a synthesis strategy has been elaborated which allows the production of the wide-range antibiotic cyprofloxacin using three

ENCYCLOPAEDIA OF HYDROCARBONS

CHEMICAL INDUSTRY AND SUSTAINABLE DEVELOPMENT

reaction steps less compared to the traditional method (Rivetti, 2000b). In the pharmaceutical industry, there are generally numerous and widely recognized examples of productions of active principles which have benefited from the identification and development of alternative synthesis strategies; they are characterized by a lesser complexity, greater efficiency, elimination of toxic reagents and auxiliaries, reduction in the use of solvents, and quantity of waste products generated (Dunn et al., 2004). An extremely important role in the definition of a synthesis characterized by a lesser complexity and greater efficiency is played, on the one hand, by the use of bio-catalysis and fermentative processes and, on the other, by the use of chiral catalysts for the stereo-selective synthesis of the desired isomer (asymmetrical catalyst). Well-known examples in this respect are L-dopa, l-menthol and (S)-metolachlor. These two aspects are often conveniently combined in the use of enzymatic catalysis. Alternative process conditions The conditions used in synthesis processes of chemical products can have important environmental and safety drawbacks: risks associated with severe reaction conditions, high energy consumption and harmful emissions. Many solvents (so-called VOCs, Volatile Organic Compounds), used in reactions and separation/purification operations, substantially contribute to polluting air and water. There are consequently well-founded reasons for widely directing the search for alternative process conditions towards eliminating the use of solvents or developing the use of alternative benign solvents. A study of reactions in aqueous, biphasic systems (which include, in addition to traditional aqueous-organic systems, ionic liquids and perfluorinated solvents) and also supercritical systems, especially supercritical CO2, forms part of this trend. A second tendency in the search for alternative process conditions relates to attempts at process intensification, already mentioned above. Process intensification aims to allow the use of milder reaction conditions and a more selective activation, and it includes alternative energy activation methods, such as the use of ultrasounds and microwaves; greater attention towards more classical energy sources, such as those used in photochemistry and electrochemistry. These unconventional activation techniques are often ideal for enabling reactions to be carried out without solvents. Alternative solvents

The radical solution to the problems created by the use of solvents obviously lies in their complete

VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

elimination (“the best solvent is no solvent”), with considerable benefits also from an economic point of view and for the simplicity of the processes. It is not by chance that many consolidated processes of the bulk chemical industry operate without solvents, generally in gaseous phase. This tendency is expanding as demonstrated by recent processes for the polymerization of ethylene and propylene in a fluid bed reactor in gas phase or the polymerization of ABS (Acrylonitrile Butadiene Styrene) in a fluid mass. There is ample space for the substitution of solvents with other less toxic, volatile and flammable products. For the washing of metal surfaces, chlorinated solvents, for example, can be substituted by: a) esters, such as dibasic esters (a mixture of methyl adipate, glutarate and succinate); b) methyl esters deriving from vegetable oils (also used as fuels for motor vehicles, the well-known biodiesel); c) dibutyl carbonate; d ) naturally occurring terpenes (limonene); e) or finally by water-based formulations containing surface-active agents. We will now examine the role of the benign solvents par excellence, i.e. water and carbon dioxide, either liquid or under supercritical conditions; and also other innovative solvents, such as ionic liquids. These are only a few examples of the imposing transformations which are taking place in the field of solvents; they involve not only the production and processing area but also, and above all, the formulation of products available to users, from products for personal and household hygiene to paints and varnishes (just consider, for example, water-based paints). The most benign and economic solvent is naturally water. Even if it is the main medium of biochemical reactions in living systems, water is traditionally considered as being a solvent which is difficult to use in chemical reactions. Many reagents and organic products, as also numerous homogeneous catalysts based on complexes of transition metals, are insoluble in water (even if there are significant and important exceptions), and the recovery of the products from aqueous solutions is generally quite onerous. Water, moreover, can be relatively reactive towards organic molecules and is capable of inhibiting numerous reactions. In spite of this, there are recent examples in which this traditional negative outlook has been completely reversed, as in the case of the use of an aqueous medium in Diels-Alder reactions (Breslow, 1998) and in Barbier-Grignard organometallic syntheses (Li, 1998; Paquette, 1998), a group of reactions of considerable synthetic utility, e.g. in the chemistry of carbohydrates. Under near-critical conditions (T250-350°C, P5-10 MPa), the solvency characteristics of water

899

SUSTAINABILITY

become similar to those of organic solvents such as ethyl alcohol or acetone. Its dielectric constant decreases from 80 to 20, and its density, from 1 to 0.7. This ensures that almost all organic compounds which are normally insoluble can be dissolved under these conditions and reacted in homogeneous phase in water, thus becoming an excellent reaction medium. The lowering of the temperature and pressure at the end of the reaction forms a simple separation method as a result of a reduction in the induced solubility, allowing easy recovery of the products. Furthermore, under these conditions, water becomes substantially dissociated, allowing acid- or base-catalysed reactions to be carried out without the actual addition of acids or bases, and without the consequent co-production of salts resulting from their neutralization. Examples of the use of water as solvent under near-critical conditions are classical base-catalysed reactions such as aldol and Claisen condensation, and the Cannizzaro reaction and Friedel-Crafts acylation, carried out without the use of Lewis acids as catalysts (Nolen et al., 2003). An example is the synthesis of anthraquinone: O

O H2O

O

275°C

O

OH

The use of water under supercritical conditions (T 374°C, P22 MPa) is at the basis of processes in which water acts as both reagent and solvent for destroying organic waste products by total oxidation; for the selective oxidation with oxygen of alkyl aromatic compounds (such as p-xylene to terephthalic acid) with MnBr2 as catalyst, which avoids the use of acetic acid as solvent; for the recycling of condensation polymers such as polyurethanes (PU) or polyethyleneterephthalate (PET) through conversion back into the starting monomers. This latter process is applied commercially for the recovery of toluene diamine from residues of the production of TDI (toluene diisocyanate): CH3

CH3 NCO

NCO

 2H2O

NH2

supercritical conditions

 2CO2

NH2

The identification of alternative solvents not only aims at using solvents with a low toxicity for human beings and the environment, but also at defining alternative process conditions which enable the easy recycling of the reagents, catalyst and solvent and the simple separation of the product (Keim, 2003). With

900

this prospect, an increasingly important role is being played by multiphase processes. Processes based on phase transfer catalysts in a heterogeneous aqueousorganic biphase environment were introduced over thirty years ago, and include consolidated examples of industrial application. In these processes, the aqueous phase acts as a reserve for the generation of reactive inorganic or organic anions, which migrate into the organic phase containing the reagents and catalyst, where the reaction takes place (Makosza, 2000). The catalyst (PCT, Phase Transfer Catalyst) is normally a quaternary ammonium or phosphonium salt, which is a source of lipophilic cations. As such, it is capable of transferring the anion into the organic phase through the formation of an ionic couple, thus allowing the reaction to proceed, which would otherwise not be able to take place. The use of this type of catalysis in an aqueous-organic biphase system avoids the necessity of having to resort to the use of aprotic polar solvents which are often toxic, such as dimethyl formamide (DMF) or dimethyl sulphoxide (DMSO), and allows better yields, selectivity and reaction rates. Selective oxidations with hydrogen peroxide in an aqueous-organic biphase medium, in which the activation of the H2O2 is catalysed by a mixture of phosphate (or arsenate) and tungstate anions, become possible through the use of phase transfer catalysis (Kozhevnikov, 1998). This selective oxidation method has been applied on a commercial scale for the production of 1,2-epoxydecane and isobutyl 3,4-epoxybutyrate, a key intermediate in the synthesis of the nootropic drug Oxiracetam: O CH2

2

CHCH2COCH2CH(CH3)2  H2O2 O

PCT, H2O

HO

CH2COCH2CH(CH3)2 O

3

WO4 /PO4

N

O

CH2CONH2

On the wave of successes of phase transfer catalysis, reactions catalysed by complexes of transition metals have been carried out in an aqueous-organic biphase system, in which the catalyst is situated in the aqueous phase where the reaction takes place. A simple phase separation is then sufficient for isolating the product from the catalyst which can be easily recycled. The most prominent example of this method is the hydroformylation of propylene to butyraldehyde (Kohlpaintner et al., 2001). The catalyst, a rhodium phosphine complex, is made hydrosoluble by the insertion of sulphonic groups in the phosphine ligand:

ENCYCLOPAEDIA OF HYDROCARBONS

CHEMICAL INDUSTRY AND SUSTAINABLE DEVELOPMENT

CATIONS

ANIONS

R1 R2

BF4



N

N R 4 R3

ammonium

H

N

N

R pyridinium

H pyrazolium





R2

R1

phosphonium

N

N

R2

imidazolium

N R2 R1 pyrrolidinium

R  C3-C16

SbF6

fluoborate hexafluorophosphate hexafluoroantimonate CF3SO 3

R1 P R 4 R3

PF 6

SO3

CH3

mesylate

tosylate

NO 3

CF3COO

nitrate

trifluoroacetate

AlCl 4

Al2Cl 7 chloroaluminates

Fig. 7. Ionic liquids.

O CH3CH

CH2  CO  H2

NaO3S L

RhLn H2O

CH3CH2CH2CH

SO3Na P

SO3Na

Systems based on perfluorinated or highly fluorinated solvents have been recently developed (FBS, Fluorous Biphase Systems), which overcome the limitations of aqueous-organic systems. Such limitations are linked to the fact that the reaction has to be carried out in an aqueous medium, whilst maintaining the characteristics which make them interesting from the sustainability perspective, in primis, the easy separation of the products (Horvath, 1998; Curran and Lee, 2001). Solvents of this type are generally non-toxic and biologically compatible. Their miscibility with products and organic solvents can be modulated in relation to the temperature, so that in many cases, homogeneous solutions can be obtained in the reaction phase, subsequently causing de-mixing by cooling into two phases to facilitate the separation of the product and recycling of the catalyst. Because ionic liquids have similar qualities, great interest has recently been shown in their use, among the neoteric solvents (Wilkes, 2002), even if their characteristics of toxicity and recycling stability, as well as problems relating to their production cost are still controversial. An ionic liquid is a compound,

VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

generally a salt of a quaternary ammonium or phosphonium cation with a variety of anions, which, although being of an ionic nature, is liquid or low-melting due to the size of the ions and the low molecular symmetry (an ionic liquid is conventionally distinguished from a molten salt when it has a melting point lower than 100°C). A material of this type is in liquid form within a temperature range which can extend to values in the order of 200 and up to 300°C (for water, this range is known to be 100°C). Ionic liquids are now commercially available (Fig. 7). Ionic liquids as such are characterized by zero vapour pressure, and their structure can be programmed so as to provide marked hydrophilic and hydrophobic properties. This characteristic can be exploited for the generation of biphase systems for both water and apolar organic phases which favour the separation of the reaction product. The use of ionic liquids has been studied in an extremely wide variety of reactions, which include among the most significant examples, Friedel-Crafts reactions, hydrogenation, oxidation, hydroformylation, dimerization and oligomerization of olefins. Ionic liquids generally dissolve most complexes of transition metals extremely well and can therefore be easily used in homogeneous catalysis. Other alternative solvent systems, such as carbon dioxide or perfluorinated solvents, often suffer from a low solubility of metal complexes and require the synthesis of specifically designed ligands. In the 1980s and early 1990s, a foremost process in the study of the use of ionic liquids was the conversion of synthesis gas into oxygenated organic compounds

901

SUSTAINABILITY

catalyst: Ni complex

n-octenes

ionic liquid:

light olefins (propylene, butenes, etc.)

catalyst in ionic liquid

H3C

N



N

AlCl 4

C4H9

1-butyl-3-methylimidazole tetrachloroaluminate

Fig. 8. Difasol process.

such as ethylene glycol, acetic acid, and alcohols on a molten catalytic system containing ruthenium in which ruthenium complexes are dispersed together with co-catalysts in low-melting quaternary ammonium and phosphonium salts. More recently (Olivier, 1999), a dimerization process of light olefins, such as propylene and butenes was developed, catalysed by nickel carried out in double phase, using an acid ionic liquid which acts both as solvent and co-catalyst (Difasol process). The cationic nickel catalyst is stabilized in the ionic phase without the necessity of additional ligands, whereas the reaction products (C6-C10 olefins) form a second less dense phase which can be easily separated. In this way, there is a higher catalytic activity, a reduced consumption of catalyst, and improved selectivity to the desired dimers compared to the traditional process in homogeneous phase (Fig. 8). Analogously, isobutene can be polymerized to high molecular weight polyisobutene in an acid ionic liquid, such as an alkylmethylimidazole or N-alkylpyridine chloroaluminate. Polymerization in ionic liquid can offer a series of advantages compared to the traditional process which uses aluminum trichloride: the polymer forms a separate layer and does not require subsequent aqueous washings for purification. The catalyst can be recycled, and undesired secondary reactions, such as isomerization, are kept under control without the necessity of an alkaline quenching of the reaction. The processes described make use of ionic liquids in the form of chloro-aluminates, with acid characteristics, the first to be used in Friedel-Crafts reactions and di/oligo/polymerization, alkylation reactions or the isomerization of olefins. The subsequent development of neutral ionic liquids in

902

 the form of anions BF 4 , PF6 , etc. enabled their interesting characteristics to be exploited in other reactions of olefins such as hydrogenations, hydroformylations and Diels-Alder reactions or as extraction solvents. An interesting example relates to the elimination of compounds containing S and N from gasoline, gas oils and other refinery products by extraction with ionic liquids, such as 1-butyl-3-methyl imidazole octylsulphate. These have good extraction properties also for sulphur containing compounds which are difficult to eliminate by means of hydrodesulphurization, such as dibenzothiophene and its alkyl derivatives (Esser et al., 2004). The use of carbon dioxide in liquid form or under supercritical conditions (Tc31.1°C, Pc7.38 MPa) also represents an alternative to the use of traditional solvents (Aresta, 2003). Carbon dioxide is available in large quantities, it is not toxic or flammable, and is inexpensive. Its solvent properties can be suitably modulated by varying the operating pressure, i.e. the density, in this way significantly influencing the solubility parameter. In spite of this, CO2 as a solvent always maintains an extremely low polar nature (its solvent properties can be compared to those of a hydrocarbon such as hexane), and it is therefore not able to dissolve salts or highly polar polymers and is not suitable for ionic reactions or reactions catalysed by ionic catalysts. In these cases, resort is made to the use of phase transfer agents or lipophilic ligands and counter-ions. Another limitation may be that carbon dioxide is an electrophilic reagent of a slightly acidic nature and is capable of interacting with basic substances. In practice, the most appropriate reactions are those involving gases with a poor solubility in

ENCYCLOPAEDIA OF HYDROCARBONS

CHEMICAL INDUSTRY AND SUSTAINABLE DEVELOPMENT

conventional solvents, such as hydrogen and oxygen. This exploits their complete miscibility with CO2 under supercritical conditions and, in the case of oxygen, also the fact that as the CO2 molecule is at its maximum oxidation state, it cannot be further oxidized (Morgenstern et al., 1996). A consolidated industrial application of carbon dioxide under supercritical conditions (or liquid) is the extraction of caffeine (and also other natural and pharmaceutical active principles); more recent applications or those in the pre-marketing phase are the washing of metallic parts, spray varnishing, the production of silica aerogels and fine powders, the deposition of thin films and dry cleaning of clothes, substituting trichloroethylene. The industrial use of carbon dioxide under supercritical conditions as reaction solvent in substitution of freon in the polymerization of tetrafluoroethylene has been announced, but there is no confirmation of the plant construction. More recently, its use has replaced acetone in a continuous multipurpose plant for the hydrogenation of isophorone to trimethylcyclohexanone with palladium catalysts, guaranteeing quantitative conversions and selectivity (Licence et al., 2003). Other reactions usefully studied in supercritical CO2 are hydroformylation, etherification and esterification, Friedel-Crafts reactions and various kinds of polymerizations. The latter ones (as for extractions) take advantage of the low viscosity, high diffusivity and ability to permeate the polymeric solid with plasticization of the precipitated polymer, as well as the ability not to leave residues of solvent in the polymer. This leads to an improvement in the yields and molecular weights in both radical polymerization and poly-condensation reactions. An important example is the use of supercritical CO2 in the production of aromatic polycarbonates carried out in melt or in the solid state. Carbon dioxide under supercritical conditions represents a potential substitute for organic solvents in biphase reactions with water, such as phase transfer reactions. In the same way, the combination of supercritical CO2 and (per)fluorinated solvents or ionic liquids can offer special advantages: neoteric solvents contain the catalyst, whereas the reaction products are extracted in supercritical CO2. Finally, carbon dioxide under supercritical conditions can be a particularly advantageous reaction medium because it also acts as reagent, as in the aforementioned syntheses of organic carbonates and polycarbonates obtained from CO2. Alternative activation methods

The use of alternative activation methods of chemical reactions (different from the normal

VOLUME III / NEW DEVELOPMENTS: ENERGY, TRANSPORT, SUSTAINABILITY

administration of heat or the use of traditional reagents and catalysts) is generally considered an instrument of sustainable chemistry with an extremely high potential, even if it has still not been adequately studied and exploited. Among these activation methods, various relatively known and consolidated techniques can be mentioned on the one hand, such as the use of electricity (electrochemistry) and light (photochemistry); and emerging borderline techniques, on the other, such as the use of microwaves and ultrasounds (sonochemistry). All of these methods have various common characteristics which make them potentially interesting. They can activate molecules, for example, in an extremely specific manner, allowing reactions to take place which would otherwise be impossible, improving the selectivity, allowing the use of milder reaction conditions; they can avoid the use of reagents and catalysts, eliminating problems relating to separation and the generation of by-products; they can significantly reduce the reaction times. They are particularly suitable for solvent-free reactions or reactions using new or benign solvents such as ionic liquids or water. The economic aspects and difficulties of the industrial scale-up, however, which have different characteristics compared to the scale-up of conventional processes, have often been the cause of an under-utilization of even the most consolidated activation techniques. Electrochemical techniques, for example, are capable of providing many benefits from the sustainability point of view (Matthews, 2001). Electrodes can be seen as heterogeneous catalysts which can be easily separated from the reaction products. Electrons can be considered as a reagent which can be used with a 100% atom efficiency and under rather mild conditions of use. Oxidation and reduction reactions (characterized by the loss or gain of electrons by a molecule) are ideal because they are activated even by the sole passage of an electric current, without the necessity of additional reagents. Electricity can therefore be used for substituting toxic stoichiometric reagents, as in oxidations traditionally performed with halogens. A wide-scale use of electrochemistry can only happen with a significant and generalized reduction in the cost of electric energy. In a scenario of this kind, significant prospects could be opened up for the reuse of carbon dioxide through electrochemical techniques, as in the synthesis of methanol (Sánchez-Sánchez et al., 2001). Photochemistry also paves the way towards potential scenarios linked to the utilization of visible light as a source of energy and materials, following mimetic processes of photosynthesis. In more concrete

903

SUSTAINABILITY

terms, light is an activation system which does not leave residues to be removed after the reaction. It is capable of activating molecules in an extremely selective manner and promoting different reactivities, typically radical reactivity, and new reactions, otherwise extremely difficult or even impossible, such as certain isomerizations and cyclo-additions of olefins which involve thermally prohibited transition states (Albini et al., 2000). The activation of molecular oxygen also becomes possible under much milder conditions and providing better reaction selectivities: it has been demonstrated that the catalytic photo-oxidation with oxygen of cyclohexane to cyclohexanol/one, important intermediates of nylon, in the presence of catalysts such as titanium dioxide, polyoxotungstates or metallo-porphyrins, takes place at room temperature and atmospheric pressure. The use of visible light is an alternative to Friedel-Crafts acylation, and eliminates the use of Lewis acids such as AlCl3: O

OH O 

O

CHO

hn OH

In recent years, considerable interest has developed for alternative activation techniques such as those using microwaves (generally 2.45 GHz) and ultrasounds (20 kHz-1 MHz) – techniques which can still be considered as being cutting-edge, but whose actual industrial application is debatable. In many cases, the use of microwaves allows a marked reduction in the reaction times by 2-3 orders of magnitude, and is well suited to reactions carried out without solvents (for example, by mixing the reagents in the solid state, in water or in neoteric solvents such as ionic liquids which, due to their high dielectric constant, are very sensitive to microwaves). As a result of their characteristics, microwaves are proving to be extremely congenial for use in rapid screening, and for the definition of synthesis protocols in parallel and combinatorial syntheses for the construction of databases – in particular, through an impressive series of syntheses carried out in the solid state without solvents with reagents and catalysts supported on substrates, such as alumina, silica or polystyrene (PS; Varma, 2001). The use of ultrasounds gives rise, in relation to the power applied, to chemical and physical effects linked to acoustic cavitation phenomena, inside and around the bubbles which are formed. Under sonication conditions, homolithic fragmentations to radical species (sonolysis) can be observed, together with excited states and alteration of the solvation, as well as

904

improvements in the mass transfer and phase transfer effects. Beneficial results can be obtained such as: a) a reduction in the reaction times; b) increased yields; c) milder reaction conditions; d) a change in the reaction mechanisms with the establishment of modified or completely different reactivities; e) improved efficiency of the reactions carried out in water or under multiphase conditions. Alternative products One of the fundamental objectives of sustainable chemistry is to design and develop alternative products to those currently adopted. They must be safer and environmentally more benign, and produced with ‘cleaner’ processes which exploit the concepts examined above. This objective is as complex as it is obvious, and involves aspects which go far beyond strictly chemical concepts, enveloping the whole multi-faceted problem of product design: from aspects relating to environmental impact and safety of the material to energy consumption; to performance efficiency and the relationship between technical innovation and change in the production chain, and between technical innovation and change in user models. The ambitious aim of designing new products means the prospect of redesigning of an entire industrial system. Three main, partly correlated, lines can be distinguished to define new alternative products. The first relates to products characterized by a lower toxicity and risk, without sacrificing their functional efficacy. The second aspect consists of new products deriving from renewable sources, normally of a vegetable origin. The third line is most directly linked to applicative aspects, and relates to the development of products with improved performances and which can be used in a lower quantity, have a longer duration, require less maintenance, or can be totally or partly recycled, with obvious advantages from the environmental impact point of view. Less toxic products

A primary objective for sustainable chemistry is to direct production towards products which are less toxic, less dangerous and which have a lower impact on the environment, while guaranteeing the same functional performances of the products to be possibly substituted. The need to move towards less toxic and environmentally harmless products is a tendency which has been consolidated for several decades, with numerous examples also of considerable mass media importance, as in the case of prohibiting the use of chlorinated pesticides (above all DDT), PCBs (polychlorobiphenyls) and chlorofluorocarbons. All

ENCYCLOPAEDIA OF HYDROCARBONS

CHEMICAL INDUSTRY AND SUSTAINABLE DEVELOPMENT

these are examples in which widely-used substances have been banned only after recognition of the unpredicted disastrous environmental effects which they have been proved to cause. The objective is much more ambitious and concerns the ability to direct research towards the design and development of comparatively benign products. The challenge is to reduce the negative effects of a molecule without sacrificing its functional efficacy. In reality, in many cases, it has been observed that the part of the molecule that provides the desired activity is separated from the part responsible for toxicity or dangerous properties; it is therefore possible to obtain a reduction in risk maintaining the same performances by suitably modifying the molecular structure within the same group of compounds. More generally, based on an analysis of the function to be performed, a similar result could be obtained using completely different compounds, and without the risks which accompany traditional products. Different strategies can therefore be used to design safer products, and the choice essentially depends on the amount of information available on the action mechanism, activity-structure correlations, and so forth. When it is difficult to intrinsically reduce the molecular hazard, use of the product can be made less dangerous by reducing its bioavailability. In a biological context, for example, the hydrophilic and lipophilic characteristics or the molecular volume can be modified, which often control the capacity of a substance to pass through biological membranes. In the environmental field, a modification of the lifetime or physico-chemical properties of a product can prevent it from reaching or remaining for a sufficient time in areas where it can exert its harmful effects, e.g. the stratosphere for substances with a high ozone reduction potential (Anastas and Williamson, 1996). The fundamental objective of combining low risk and toxicity with adequate performances is also at the basis of ideas previously expounded concerning reagents, auxiliaries and solvents used in industrial processes. As far as end-products destined for consumer use are concerned, this objective has mainly been pursued in specific industrial sectors, such as the pharmaceutical field and in the production of pesticides. However, its validity is entirely general, as it is also followed for obtaining other large-consumer products, such as paints and varnishes. In the field of pesticides, for example, there is a gradual shift from: the use of inorganic products containing arsenic, copper, lead or sulphur; chlorinated organic products which are long-lasting in the environment; towards naturally occurring or biomimetic molecules, such as pyrethroids and pheromones. The latter act on target organisms (but

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are harmless to other organisms), and are capable of degrading to harmless end-products. Within the broad picture, there is the recent example of spinosad (spinosyn A/D). This is a natural insecticide with a chemical macro-cyclic lactone structure produced by fermentation, active against mosquito larvae with a high selectivity and general low toxicity, and a favourable environmental profile (Sparks et al., 2001). Surprising examples of opportunities provided by using alternative products can be found in the field of biocides. Even today, the most widely-used wood preservative consists of a mixture of copper, chromium and arsenic compounds (CCA, Chromated Copper Arsenate). Only recently have alternative products been introduced, such as a formulation based on a complex of bivalent copper and a quaternary ammonium compound dissolved in ammonia or ethanolamine, which eliminate problems induced by the presence of arsenic and hexavalent chromium (Award [...], 2002). In reality, the problem of eliminating toxic and polluting heavy metals from consumer goods is rather generalized and transversal to various product sectors, such as paints and varnishes. Pigments based on chromium, lead and cadmium, such as lead molybdate, lead chromate and cadmium sulphoselenide, are traditionally used for covering the range of colours red, orange and yellow. They can be validly substituted, also from an economic point of view, with low toxicity azoic dyes that contain harmless alkaline-earth metals such as calcium, strontium and barium, which can also be used safely in contact with food and drinks, and in household products (Ritter, 2004). Similarly, yttrium, a more common and less toxic element, can replace lead as primer in the cathodic electrodeposition technology (electro-coating) used for painting motor vehicles to protect metal sheets from corrosion (Cahn, 2002). Alternative products from renewable sources

The development of new alternative products that derive from renewable sources is probably the true core of the ‘green revolution’ in chemistry (Stevens and Verhé, 2004). The utilization of renewable sources is an aspect which has already been dealt with above, from another viewpoint, in relation to alternative raw materials. In addition to already consolidated products of the current industrial scenario, completely new products can also be ambitiously obtained from these raw materials which, apart from their derivation from renewable sources, have other benign characteristics. The production of these new products often, but not always, takes place through biotechnological and biocatalytic processes. A well-known and important example is biodiesel, an alternative fuel for diesel engines, consisting of a

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mixture of methyl esters of fatty acids produced by methanolysis of triglycerides such as soybean oil, rapeseed oil, palm oil or exhausted food oils (Knothe et al., 2004). Biodiesel improves the lubricating power of the fuel and, like most oxygenated fuels, is capable of reducing emissions deriving from combustion. Esters of fatty acids are also used as lubricants, cut oils and solvents. The fact is that nowadays, there are a large number of proposals for the upgrading of renewable raw materials to functional new generation materials. Among the foremost of these are polymers (Gross and Kalra, 2002): this marks a come-back given that, before the era of low-priced petroleum and the introduction of synthetic polymers, polymers deriving from renewable sources were already widely used. The production of bio-plastics based on starch, a product abundantly available in maize and other plants, is continuously increasing on the market. These bio-plastics are especially used in specific applications which require biodegradability as a functional characteristic: compostable bags for recycling food waste, disposable tableware, packaging materials, products for personal hygiene, agriculture and farming. However, applications with a greater technological content are also emerging, such as the use of bio-plastics in tyres for motor vehicles (Bastioli, 2002). As a result of their good biocompatibility, biodegradability and atoxicity properties, polysaccharides of a vegetable origin (cellulose, pectin, gum Arabic), of a marine (alginates, agar), microbial (dextran, xanthan, scleroglucane) or animal

BIO POLYMERS

PE/PP

PS

PET ABS PU PVC

Fig. 9. Polymers and sustainability.

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orgin (chitosan, glycogen) are predictably widely used in the medical-pharmaceutical and cosmetic field, but also in a wide variety of other industrial sectors (Lapasin and Pricl, 1995). Polylactic acid (PLA) and polyaspartate (TPA) are polymers produced from renewable raw materials that have been recently introduced. Lactic acid is obtained from the fermentation of sugars contained in maize flour and, after distillation, is transformed first into lactide and then into the high molecular weight polymer. Polylactic acid is biodegradable or can be recycled (lactic acid is re-obtained by hydrolysis), and is used in the field of textile fibres and packaging (Vink et al., 2003). Polyaspartate is a new hydrosoluble and biodegradable biopolymer synthesized from L-aspartic acid, a naturally occurring amino acid. Polyaspartate is used as an alternative to polyacrylates, which are not biodegradable, as a corrosion and scale inhibitor for water treatment, and as an anti-scale dispersing agent in detergents (Cann and Connelly, 2000). It is interesting to note that in an important product sector, i.e. surface-active agents for detergents, the hydrophobic fraction of the surface-active molecule prevalently derives from renewable sources (palm oil or coconut oil), also in traditional industrial lines. Surface-active agents have recently been developed in which the hydrophilic part also derives from renewable sources, substituting ethoxylated derivatives produced from ethylene oxide, with derivatives of sugars such as saccharose, glucose or sorbitol (von Rybinski and Hill, 1998). Both non ionic and anionic surface-active agents can therefore be obtained by suitably functionalizing the structure of the carbohydrate. Products with an improved performance

Among materials provided by the chemical industry, mention must be made of the predominant and growing importance of polymers in applications which, from case to case, can have a long life (as in constructions), a medium duration (such as motor vehicles), or short life (packagings). Excluding aspects relating to production processes, which are also extremely important, all improvements in the mechanical and applicative characteristics, duration, recyclability, easy disposal of products based on polymeric materials, have enormous repercussions on the ecosystem. The increasing capacity to control the molecular structure of polymers and new production technologies enable a continuous expansion of the types of polymers produced to satisfy market demands as specifically and precisely as possible, thus guaranteeing the production of better products (Romano and Garbassi, 2000): for example, new grades of high density polyethylene (HDPE), the most

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widely-used polymer, characterized by a bimodal molecular weight distribution, allow a decrease of 33% in the thickness of polyethylene tubes, with a corresponding saving of material. The greater versatility of the typologies available often favours the substitution of environmentally more problematic materials: this is the case, for example, of the substitution in primis of polyvinylchloride (PVC) and polyurethanes (PU; Fig. 9). Polyolefins (PE, polyethylene; PP, polypropylene) have the great advantage of consisting of carbon and hydrogen alone, and of being free of aromatic components. As a result of their physico-chemical and applicative characteristics, as well as their production technologies, they are considered as being among the most environmentally compatible plastic materials. Linear low density polyethylenes (LLDPE), for example, produced with metallocene catalysts, are eroding the polyurethane and PVC market in the field of motor vehicles and carpet underlay: in this latter application, unlike before, this allows an easy total recycling of both the lower plastic layer and also the upper layer made of nylon fibre, as the polyolefin is compatible with the de-polymerization of nylon to caprolactam monomer for subsequent reuse (Segars et al., 2003). Polypropylene reinforced with glass fibre is capable of competing, from the performance point of view, with PET and nylon. The field of reinforced polymers is fully evolving: the use of nanocomposites is reported to be revolutionary, and envisages an even fiercer competition between the various polymers and materials.

9.6.4 Looking ahead It has been widely demonstrated how renewable sources represent an essential component of sustainable development. Producing chemical products from renewable sources is not in itself a new idea: up until the first part of the Twentieth century, in fact, most chemical products derived from these. Subsequently, due to the overwhelming development of the oil industry during the century, the situation changed and, today, only about 5% of chemical products derives from biological renewable sources. This situation seems destined to change again for various reasons (Wedin, 2004). The first relates to doubts as to the amount and availability of oil reserves still exploitable with consequent repercussions on the prices, also in view of the increases in consumption by the developing countries: various predictions estimate the peak of overall oil production as being between 2005 and 2010. The second reason concerns the production of carbon dioxide connected with the use

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of fossil fuels and controversial issues, following the accelerated alteration observed in the composition of the atmosphere. Finally, the recent impressive technical and scientific developments of biotechnology – from new enzymes to biocatalysis, from genomics to metabolic engineering – open up completely new prospects for the use of raw materials from renewable sources. Biorefinery

The concept of biorefinery emerges within the context of this expected future growth in the use of materials from renewable sources (Realff and Abbas, 2004). The development of the concept of biorefinery is associated with the idea that producing chemical products and materials from biological sources to substitute petrochemical ones must make use of similar concepts to those which led to the affirmation of traditional refineries producing from crude oil, but with inevitable essential differences. The possibility of completely replacing petroleum as a main energy source with biomasses is obviously not disputed: the consumption of crude oil for petrochemical production does not exceed 3% of the total consumption, and the replacement of fossil fuel as energy source cannot but envisage resort to other alternative forms of energy. An important aspect of the traditional chemical industry is that careful planning, and optimum management of materials and energy are vital for its success, and that the transformation of the greatest possible amount of raw material into high-quality products is at the basis of new process technologies. The essential characteristic of oil refineries is that they handle complex mixtures. In addition to useful molecules, these mixtures contain ones that seem useless and undesirable at first sight, but are upgraded (through suitable processes set up during technological development) to produce useful products for the market. Biorefinery follows the same model, accompanied by various specific aspects. Biological (agricultural) raw materials are also complex materials with components which cannot be used immediately. It will be up to biorefineries to find a way to make the best possible use of most of these renewable raw materials. Operations for the utilization of petroleum and biomasses have surprisingly similar aspects from a conceptual point of view. They comprise: stabilization and transportation, separation (for example, distillation), purification, decomposition (e.g. cracking), transformation (such as reforming). To carry out these operations, an oil refinery substantially relies on chemical or physico-chemical principles, whereas a biorefinery must also resort to agricultural and biological techniques.

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Alongside these similar aspects, there are also differences. One of the characteristics of fossil fuel is that it is highly localized and can be easily transported, because it has a high utilization density. This allows the formation of relatively few transformation knots, with consequent significant scale economies in the process phase. The inherent environmental problems are therefore concentrated around the area in question. Masses of a biological origin, on the other contrary, are normally much more widespread over the territory and long-distance transportation is often costly, also in relation to the low apparent densities and high water content. Furthermore, the differentiation already existing in the plant (leaves, seeds, ligneous part, etc.) allows an initial separation which exploits mechanical energy, rather than having to resort to a phase separation or separation at a molecular level which exploits thermo-chemical energy as in the case of petroleum. This opens up the prospect of a widespread relocation of transformation plants, at least as far as a first pretreatment phase is concerned, allowing cheaper and easier transportation. This seems reasonable also because of the fewer environmental problems involved: possible waste materials can be easily biodegraded and a simple disposal procedure is represented by their reuse as soil nutrients to help guarantee the renewal of sources in loco. Scale economies suggest an intermediate concentration of biomass in liquid form for conveyance to a central biorefinery. The widespread relocation of transformation plants has evident social implications. Once the biomass is conveniently concentrated for subsequent transformations, it must be restored to a limited number of intermediates (building blocks) from which consumer materials can be produced. The stronghold of an oil refinery is the distillation column which separates the hydrocarbons present in crude oil, substantially containing carbon and hydrogen, on the basis of the number of carbon atoms, from 3 to about 20-30. Other physical and chemical operations, such as extraction, cracking, etc., allow production to be focused on obtaining the intermediates most in demand for the production of the desired products and materials (fuels, solvents, ethylene, propylene, aromatic compounds, etc.). In biorefineries, the range of carbon atom numbers is very different, and varies from a significant fraction centred between 5 to 7 carbon atoms (simple sugars) to an intermediate fraction based on oligomeric sugars, up to molecules containing hundreds of carbon atoms in the fraction defined as lignocellulose (hemi-celluloses, cellulose and lignin). Biomasses, moreover, contain a large quantity of oxygen and alkaline metals in their molecules. These different characteristics modify and partially complicate the synthetic pathways towards

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the production of industrial end-products. The process operations can be of a biological nature or chemical fermentation, combined with extraction, thermo-chemical and hydrolysis processes. The development of biorefineries must have the cooperation of enterprises operating in the agricultural field and chemical sector with the participation of partners who possess the know-how relating to innovative biotechnologies. The development of more efficient and cheaper processes for the pretreatment and separation of the main components of biomasses is the first essential step for an effective development of biorefineries. These processes (on which a concerted effort is being made) must utilize a heterogeneity of fibrous lignocellulose materials and other waste by-products from herbaceous, arboreal or forest crops such as molasses, shells, stems, straw, brushwood, etc.; these derive, for example, from processing maize, sugar beet, soya, and are widely available at a very low cost. There are two main types of biomasses, those consisting of starches and those of lignocellulose products. Both contain polymers of sugars which must be reduced to their monomeric components before being subjected to chemical or fermentative transformation processes. Whereas the reduction of starches to glucose is relatively easy, the treatment of cellulose to give sugars is much more difficult due to its crystallinity, resistance to hydrolysis and its close association with hemi-cellulose and lignin through covalent bonds. There are likely to be extremely promising developments as a result of steam explosion treatment, for which one of the most important pilot plants exists in Italy (Zimbardi et al., 2002). This process subjects lignocellulose material to the action of saturated water vapour at a high pressure (15-30 bar) and temperature (180-230°C) for a short period of time (1-10 min), followed by rapid depressurization to atmospheric pressure which causes an explosive decompression with a fibre disruption of the biomass. The treatment allows an effective separation of the three different fractions that form the vegetable biomass (hemicellulose, cellulose and lignin). Other degradation strategies of lignocellulose materials are acid hydrolysis and supercritical extraction. The cellulose, thus separated and purified, can be more easily transformed into sugars through the action of suitable micro-organisms capable of producing enzymes (cellulase) which hydrolyse and break up the cellulose. Purified lignin can also, in turn, form the raw material for producing a variety of chemical products through more traditional chemical treatment. Once the sugar step has been reached, the next step involves converting the sugars to the desired products,

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ranging from pharmaceutical products to industrial intermediates, polymers, and solvents up to the production of fuels (Lichtenthaler and Peters, 2004). As in traditional refineries, biorefineries must be able not only to use all the raw material available, but also transform it into the multitude of products, with an extremely diversified and wide range of utilization and added value, which is now a prerogative in the oil industry. The oil industry begins with the refinery which produces fuels and extends as far as fine and specialty chemicals and pharmaceuticals. Similarly, biorefineries will also have to take on the responsibility of the most varied productions. Ethanol, for example, which is produced through the traditional fermentation of glucose or through more innovative fermentative processes using sugars that derive from lignocellulose, is an excellent alternative fuel already widely used for motor vehicles. Fundamental intermediates such as acetic acid, lactic acid and succinic acid can also be obtained by fermentation. Acetic acid is now already one of the most important industrial intermediates. Lactic acid is a basis for the production of new solvents (ethyl lactate) and biopolymers (the above-mentioned PLA, polylactic acid). Succinic acid is also a starting point for numerous useful products, such as solvents and intermediates for polymers. Three strategies can be basically identified for substituting products deriving from petroleum with products deriving from biomasses: direct substitution of the petroleum-derived product with the same product deriving from biomass (for example, ethanol, ethylene, acetic acid); indirect substitution with a product functionally similar but chemically different; substitution with an intrinsically new product, for example, substitution of a synthetic polymer with a degradable biopolymer.

These transformation pathways of biorefineries envisage a wide use of biocatalysis through the exploitation of recent progress in biotechnology (Webster et al., 1996). The term biocatalysis refers to a chemical reaction which is catalysed either by an entire living organism (such as a microbe) or by a specific enzyme deriving from an organism. Through metabolic engineering techniques (recombinant DNA technique), it is possible to modify part of the normal metabolism of a cell to gear it towards producing the desired product. There are numerous examples, at present mostly limited in practice to specialty chemistry, but with the prospect of being applied to commodities production, of products obtained using Escherichia coli or other suitably modified microorganisms. Some examples have already been provided above, relating to the utilization of raw materials from renewable sources. The future holds new exciting possibilities, such as the use of extremophilic enzymes, catalytic antibodies and ribozymes. Extremophilic enzymes are capable of acting under extreme conditions such as high temperature, high saline concentration, supercritical conditions, and organic solvents. Catalytic antibodies are obtained by injecting a hapten (i.e. a small molecule capable of generating an immunitary response when it is attached to a protein) into an animal, which is analogous to the transition state of the desired reaction. Catalytic antibodies have similar catalytic properties to enzymes, but they can be generated and isolated within a few weeks. An ambitious objective is the development of antibodies capable of catalysing new chemical reactions, unknown to their naturally occurring enzymatic counterparts. Finally, ribozymes are ribonucleic acids which, once suitably manipulated, have proved to be capable of exerting

Fig. 10. Biorefinery.

SUGARS biochemical platform

BIOMASS

HEAT POWER

FUELS CHEMICALS MATERIALS

SYNGAS thermochemical platform

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functions typical of protein enzymes in the specific catalysis of reactions. An effective scale-up of biochemical processes is a further obstacle to be overcome before biorefineries take hold. Many fermentative processes, for example, give rise to diluted aqueous solutions from which the desired products can be recovered. This requires separation and purification technologies which are different from the more consolidated technologies of the petrochemical industry, such as the use of membranes, liquid-liquid extraction techniques or electrodialysis techniques. The problem of the product inhibiting the reaction, which causes the low concentration of the final products, can be overcome by developing continuous removal techniques of the product itself, similar to what is done in reactions characterized by unfavourable thermodynamic equilibrium. The biorefinery concept is based on the use of sugars (but also phenol derivatives of lignin and triglycerides, for example biodiesel) deriving from biomasses, which uses largely biochemical conversion processes. Alongside this is a perhaps less innovative biorefinery concept based on thermo-chemical gasification processes of the biomasses or by-products of their conversion, followed by conventional operations and treatment for the production of energy or chemical products deriving from synthesis gas (Fig. 10). The integrated industrial approach (clustering) which characterizes biorefineries and associates them with traditional refineries is consequently not only at the basis of the production of a wide range of products, which includes fuels, intermediates and specialties, but also an integrated energy production for the generation of electricity and process heat for self-consumption or sale – again with the prospect of maximizing the value on the one hand, but also annulling emissions on the other (Gravitis et al., 2004). Zero emissions

Biorefineries represent one of the fundamental components of the concept of a zero emissions industrial system, which substitutes the traditional linear industrial model in which the production of waste products is considered the norm, with an integrated system wherein every component has its utility (Pauli, 1998). This new industrial system model evokes and emulates the sustainable cycles to be found in nature. According to the zero emissions concept, everything that enters an industrial transformation process is used in the final products or converted into added value raw material for other industries or processes. For this purpose, industries have to reorganize themselves in clusters so that the

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waste products and by-products of one industry are compensated by the raw materials required by another industry, and the sum of the integrated processes does not produce any waste. The exhaustive use of raw materials, combined with a shift towards raw materials deriving from renewable sources, aims to take the utilization of world resources back towards sustainable levels. In a biorefinery integrated cluster, thanks to photosynthesis, the production and utilization of carbon dioxide, also represent a closed cycle. This is one of the main advantages of biorefineries compared to the use of raw materials of a fossil origin. The concept of a zero emissions industrial system increases the ecoefficiency not only by completely eliminating waste- and by-products, but at the same time maximizing the productivity of resources. It intends to represent a third historical phase of the control and reduction of emissions, following the first phase which manages pollution issues once they have occurred, and the second phase which applies preventive strategies for the reduction of emissions and environmental risks.

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Ugo Romano EniTecnologie San Donato Milanese, Milano, Italy

Franco Rivetti Polimeri Europa Novara, Italy

ENCYCLOPAEDIA OF HYDROCARBONS