By expanding the scope of sustainability to the entire lifecycle of chemical products, the concept of circular chemistry aims to replace today\u2019s linear \u2018take\u2013make\u2013dispose\u2019 approach with circular processes. This will optimize resource efficiency across chemical value chains and enable a closed-loop, waste-free chemical industry.<\/h5>\n
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Awareness of the finite nature of many resources \u2014 including the issue of element scarcity, shown in Fig.\u00a01<\/a>\u00a0\u2014 as well as the limited environmental tolerance towards our chemical industry has grown tremendously in the past few decades. It has become painfully obvious that the linear route of production, in which scarce resources are consumed and their value-added products are degraded to waste, is a route cause of several impending global crises such as climate change, diminished biodiversity, as well as food, water and energy shortages.<\/p>\n <\/p>\n \u00a9 EuChemS, reproduced from\u00a0https:\/\/www.euchems.eu\/euchems-periodic-table\/<\/a>\u00a0under a Creative Commons license\u00a0CC BY-ND 4.0<\/a><\/p>\n<\/div>\n In this representation of the periodic table prepared by the European Chemical Society (EuChemS) for the International Year of the Periodic Table, naturally occurring elements (except some of the rarest ones beyond uranium53<\/a><\/sup>) are depicted through tiles, the size of which gives an indication \u2014 on a logarithmic scale \u2014 of how much of the element is present in the Earth\u2019s crust and atmosphere. The areas are approximate for all elements and exaggerated in the case of the least abundant ones shown here (technetium, promethium, polonium, astatine, radon, francium, radium, actinium and protactinium) so that they are noticeable. Technetium and promethium, shown here in white and marked as synthetic elements, do also occur naturally on Earth, though only in very small amounts. This illustration highlights the speed with which elemental supplies are being used, and draws attention to elements that are at risk of being depleted completely unless recycling routes are developed, as well as those that come from countries in which wars are fought over the ownership of the relevant mineral rights. This table mentions 31 elements (though other sources list other numbers, up to around 70) that are used in smart phones, which are typically replaced more rapidly than necessary.<\/p>\n<\/div>\n<\/div>\n <\/p>\n <\/p>\n Advocates of the circular economy such as The Ellen MacArthur Foundation (https:\/\/www.ellenmacarthurfoundation.org<\/a>) cleared the path for the emergence of novel policy frameworks that aim to redesign current economic systems, exemplified by the European Union\u2019s 2013 \u2018manifesto for a resource-efficient Europe\u2019. A circular economy is defined as \u201crestorative and regenerative by design, and aims to keep products, components and materials at their highest utility and value at all times\u201d1<\/a><\/sup>. Chemistry is crucial for achieving this2<\/a>,3<\/a>,4<\/a>,5<\/a>,6<\/a>,7<\/a><\/sup>. Chemists understand their role in designing and developing indispensable materials and technologies, but also simultaneously recognize the potentially detrimental effects that this may have on their practice; they are therefore becoming increasingly aware that each step must be designed or reassessed with sustainability in mind.<\/p>\n <\/p>\n<\/div>\n Green chemistry for linear processes<\/strong><\/p>\n Since it was first introduced in the 1980s, green chemistry has provided a framework for teaching and performing sustainable chemistry, and has delivered an impetus for developing cleaner products and processes8<\/a>,9<\/a>,10<\/a><\/sup>\u00a0\u2014 which have enhanced chemical sustainability in industry and academia. Its twelve guiding principles (Box\u00a01<\/a>, GC 1\u201312) focus on the direct sustainability assessment of chemical reactions, and are perfectly suited for the optimization of linear production routes. The developments towards a circular economy, however, require a re-evaluation of what defines a sustainable chemical process, and needs to take into account the people, planet and profit level (referred to as the \u2018triple bottom line\u2019)11<\/a><\/sup>. Notably, innovative chemistry designed with sustainability in mind is only effective when translated into economically viable applications.<\/p>\n An illustrative example is the reported green synthesis of adipic acid \u2014 a key component for the manufacture of nylon-6,6 \u2014 by the direct oxidation of cyclohexene with hydrogen peroxide12<\/a><\/sup>. Solvent-free conditions are applied (GC 2), avoiding the use of the corrosive nitric acid (GC 3) and thus side-stepping the formation of the environmentally taxing gas nitrous oxide, N2<\/sub>O \u2014 a waste product of the current industrial synthesis (GC 4). The green method, however, requires hydrogen peroxide, H2<\/sub>O2<\/sub>, as starting material, which means that this process is currently not commercially viable, since H2<\/sub>O2<\/sub>\u00a0is more expensive than the adipic acid product. Although this route obeys green chemistry principles, it violates the value chain. As a result, this green adipic acid synthesis has not been applied industrially, and has therefore not led to an overall increase in sustainability. Thus, accounting for the profit level of the triple bottom line is an essential component in the design of sustainable chemistry.<\/p>\n Other chemical processes may satisfy the green chemistry principles while being economically viable, yet remain unsustainable. For example, the Haber\u2013Bosch process uses iron for the conversion of dinitrogen, N2<\/sub>, into ammonia, NH3<\/sub>, which in turn is used in the production of agricultural fertilizers. It is a key industry showcase for the use of catalysts (GC 9) in increasing energy efficiency (GC 6). The current process requires high temperatures and pressures, and further optimization has stagnated. After its use as fertilizer, large portions of the fixated nitrogen are lost to the environment, causing eutrophication, a global environmental concern, the importance of which should not be underestimated13<\/a><\/sup>. The cascade of environmental changes that results includes an increase in water and air pollution, both of which threaten to destabilize the Earth\u2019s system beyond the proposed \u2018planetary boundaries\u2019 or \u201csafe operating space\u201d for anthropogenic activities14<\/a><\/sup>. This highlights the importance of looking beyond the scientific discovery and analysing the global impact of chemistry using a systems approach15<\/a><\/sup>.<\/p>\n![\"Fig.](\"https:\/\/media.springernature.com\/m685\/springer-static\/image\/art%3A10.1038%2Fs41557-019-0226-9\/MediaObjects\/41557_2019_226_Fig1_HTML.png\")
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