Green Nanomaterials. Siddharth Patwardhan

one. However, during LCA, one needs to consider how the new reactant is derived (reaction (1.11)). Solvent C (equation (1.12)) used to produce reactant 3 may be as hazardous as solvent A. It quickly becomes clear that the alternative (reaction (1.11)) is not as green as it appeared.

Reactant1⟶SolventCReactant3+waste.(1.12)

      Essentially, although the need for solvent A has been removed, in order to produce the same product, solvent C is required, thus simply shifting the problem to the synthesis of the new precursor. In addition, LCA can take into account the energy required to produce the desired products.

      Carbon footprint is another measure for assessing the environmental impact of a process or a product. It estimates the CO₂ equivalent emissions of greenhouse gases caused by a process or associated with a product. Carbon footprint analysis can help decide between processes where, for example, one option can produce a high quality product but require very high amounts of energy, while an alternative method could be lean on energy requirements but may produce a lower quality product.

1.2 Ways to improve sustainability

      Estimation of environmental impact using the metrics discussed above can help identify the problems with an existing process or product. This can then help device targeted plans to find alternatives. Environmental impact assessment can also highlight future challenges, such as the need for technical developments in order to improve product quality for greener processes such that they can be competitive.

      There are many ways to improve process sustainability, and most of them are based around finding alternatives to either the solvents used, the reactants used and/or the process conditions (e.g. temperature or pressure) [1]. Avoiding the use of solvents altogether is an excellent example of improving greenness of a process. Other alternatives include switching to solvents that are non-volatile organic compounds. The use of supercritical fluids (e.g. CO₂ and water) has also been reported as an alternative due to the ease of solvent separation and reuse. However, one should consider the energy required to operate processes under supercritical conditions (typically high temperatures and pressures; e.g. for water, the critical point is 374 °C and 220 bar). In the case of processes that require heating, alternative ways of providing energy, such as microwave or ultrasound, could be effective. These methods work on the principle of providing energy only to the desired location or chemicals without wasting energy by heating the entire medium or system.

      Various ways of estimating the environmental impact, as discussed above, are powerful tools, however they require involvement and inputs from chemists and process engineers [7]. It is critically important to analyse how the modifications made to a section of a process can affect the entire process and perhaps beyond to the entire business. For example, environmental impact estimates cannot predict how changes in feedstock can affect the product value, profits and market competition due to feedstock availability. Answers to such questions can be obtained by performing techno-economic evaluation of a process [8].

1.3 Green chemistry and nanomaterials

      When compared to the sectors listed in table 1.1, nanomaterials manufacturing is relatively young. Most attention has been focussed on the discovery and design of nanomaterials. Although the use of nanomaterials can be back-dated to the 9th century (see chapter 2, section 2.2), the large-scale commercial production of engineered nanomaterial developed and accelerated around the late 1990s and early 2000s [9]. This means that the manufacturing-related developments for nanomaterials are in their infancy, and for a vast majority of newly discovered nanomaterials, large-scale manufacturing does not exist. In other words, we are happy to have some process for the large-scale manufacturing of desired nanomaterials, while little or no attention is given to the sustainability of the process. However, this needs to change and the time is ripe to focus on the environmental impact of nanomaterials production, in order to apply the developments enjoyed by other sectors, listed in table 1.1, to nanomaterials production. These factors clearly stress the urgent need for developing fundamentally new production methods for nanomaterials that are green and sustainable. This change in thinking is very important, and previous experience suggests that when sustainability/green principles are included at the discovery stage, this provides the most benefits.

      This book focuses on stimulating a shift towards a sustainable approach for nanomaterials—from discovery to production. Chapter 2 will serve as an introduction to nanomaterials. Their properties will be discussed and benefits offered from their use in selected applications will be presented. In chapter 3, we will provide an overview of the analytical techniques used to probe various properties of nanomaterials. Chapter 4 will present, with examples, a range of current manufacturing processes used to produce nanomaterials at large scales. We will discuss these processes for sustainability by using the theory and concept of green chemistry covered in chapter 1. Chapter 5 will summarise the benefits of using nanomaterials, while highlighting the need for greener alternatives. In chapter 6, we will detail how biology produces a range of inorganic materials from macro to nanomaterials and point out the potential for learning from biology. This learning will be consolidated in chapter 7 where strategies for developing biologically inspired green routes to produce nanomaterials will be presented. The key advantages and opportunities from these alternative routes will be identified, and further explained in chapters 8 and 9 with two case studies.

References

[1] Lancaster M 2010 Green Chemistry: An Introductory Text 2nd edn (Cambridge: Royal Society of Chemistry)

[2] Gro Harlem Brundtland 1987 Development and international co-operation: environment, the United Nations, A/42/427 1987, http://www.un-documents.net/wced-ocf.htm

[3] Basics of Green Chemistry [cited 2016 23^rd March]; Available from: https://www.epa.gov/greenchemistry/basics-green-chemistry

[4] Anastas P T and Warner J C 1998 Green Chemistry: Theory and Practice (Oxford: Oxford University Press)

[5] Sheldon R A 1997 J. Chem. Technol. Biotechnol. 68 381

[6] Sheldon R A 2007 Green Chem. 9 1273

[7] Windsor R, Cinelli M and Coles S R 2018 Curr. Opin. Green Sustain. Chem. 12 69

[8] Drummond C, McCann R and Patwardhan S V 2014 Chem. Eng. J. 244 483 Lieberman M B 1989 Strat. Manage. J. 10 431

[9] Vance M E, Kuiken T, Vejerano E P, McGinnis S P, Hochella M F, Rejeski D and Hull M S 2015 Beilstein J. Nanotechnol. 6 1769

Section II