25 years of energy and green chemistry: saving, storing, distributing and using energy responsibly

Under the heading “Design for energy efficiency”, a current formulation of the “energy principle” in the original twelve principle list (Fig. 1)¹ advocates the minimization of energy requirements in chemical processes.^1b Such a statement reflected that, in 1991, energy consumption was front and center; energy production somewhere off the monitor screen, due to the underpinning lucidity that most of the energy supply was fossil-based, hence a non-renewable source of costs and waste. Twenty-five years later, while energy remains a key (external) input parameter in green chemistry and engineering,² energy is also becoming more of a target for green chemistry itself: energy production and energy consumption, as well as their interconnecting links, energy storage and distribution, have become major challenges to be tackled in the field.

Fig. 1 The 12 Principles of Green Chemistry.¹

Green electrons

Within the past twenty-five years, a major event happened in physics and chemistry, altering how we produce, store and use energy more sustainably: some electrons turned green. They have the same reducing power as the old-fashioned ones, abide the same laws of physics and undergo the same chemistry and yet they possess one new asset: they can help us wane from our current addiction to fossil fuels as an energy source and help us transition to a more carbon-sober³ future.

Their existence was long known. As Giacomo Ciamician, the Italian chemist, said as early as 1912 “Inside [industrial colonies without smoke and without smokestacks] will take place the photochemical processes that hitherto have been the guarded secret of the plants, but that will have been mastered by human industry. (…) And if in a distant future the supply of coal becomes completely exhausted, civilization will not be checked by that, for life and civilization will continue as long as the sun shines!”.⁴

In this visionary statement green electrons were only the photogenerated ones; they are now also the electrons flowing in currents generated from renewable energies (RE): solar, wind, hydro- or geothermal power to mention the most often cited ones. The intermittence and/or the geographic uneven distribution of these renewable energies are among the biggest challenges connected with their integration in the current infrastructure. Chemistry connected to how to produce them (one by one in a valence band of a semiconductor or industrially through a wind turbine), distribute them (in an academic photoelectric cell or in an urban electric (smart)grid), store them and recover them back from the storing location (be it a battery or in a chemical bond) has definitely become green chemistry too, addressing directly the 6^th of the 12 principles. Furthermore, since the production, distribution, storage and utilisation of electrons embrace chemical reactions (such as the CO₂ reductions just to name one possible family of reactions),^3,5 materials (electrodes, membranes, conducting organic-based composites, and semiconductors, just to name a few) and reactors (as in photo-bioreactors, solid oxide electrolyzers or dye-sensitized solar cells to name just three), green chemistry is to be understood in its broad sense of relating to chemical sciences, advancements in molecular, heterogeneous and bio-chemistry, process engineering, material sciences… that is all the various facets which make chemistry a platform science in the energy challenge.⁶

Other forms of energy from renewables will complement RE-power generation such as heat from efficient biomass transformation, from solar thermal or from biogas, transport through biofuels.⁷ In these cases, as above, addressing the associated necessary scientific advancements connected with materials, process and, in the broad sense, chemistry, are undoubtedly targets of the field.

Efficiency (regardless of the color of energy) and disruption…

Even though electricity could eventually account for over a quarter of the final energy consumption by 2014, with renewables accounting for almost half of the new power generation – and even adding the contribution from renewable sources other than power generation – the overall anticipated increase in world energy consumption still leaves the major role to fossil sources.⁸

Just as it was twenty-five years ago, minimizing the energy consumption while keeping or improving the final chemical process output – i.e. maximizing energy intensity – remains a golden rule in green chemistry with repercussions far extending the direct perimeter of chemists: 95% of all manufactured goods involve at least one chemical process along their production route.⁹ Several indicators suggest that such dedication remains very high: for example, the European Chemical Industry Council (CEFIC) reports that between 1990 and 2010 the European chemical industry improved its energy intensity by an average of 3.7% p.a., achieving over a 50% drop in energy intensity in the 20 year span.¹⁰

Ammonia production is a good show-case example: the top chemical in terms of production volume (198 Mt in 2012), the top process in terms of energy consumption (its process using 17% of the overall global energy demand of the chemical industry, excluding feedstock preparation), the energetic efficiency of its industrial process has been constantly dropping, reaching in the best case practices a limit close to the thermodynamic one (ca. 8 GJ t⁻¹vs. the theoretical minimum of 5.8 GJ t⁻¹).⁹ This example shows that high temperatures and high pressures (the working conditions of industrial ammonia production) can be coupled with energy efficiency, and can corroborate that process engineering, rather than necessarily process depressurization or temperature reduction, can be a productive path.

At the same time, even with remarkable efforts and achievements in terms of energy efficiency, the total energy consumption related to ammonia production has been steadily increasing over the years. The ammonia example therefore brings home at least two points of caution:

-The first one is that improvement in an intensive variable, the efficiency, is not per se an improvement in the connected extensive variable, a simple equation which reaches far beyond the ammonia case, impacting almost any energy-related challenge. Improving efficiency is laudable and necessary, but not enough, especially in a world growing in population and with a necessary per capita increase in energy consumption in some countries.¹¹

-The second one is that nowadays, even for the intensive variable, only incremental improvements are possible in some cases within this model of production.

Both these downsides carry the same silver lining: future major leaps will require, and hence favor, technology disruption.¹² All the ongoing efforts around process intensification or, to continue the ammonia example, in the photoproduction of NH₃,¹³ can be mentioned as possible tokens in this disruptive direction. The connected plant downsizing, or at least the end of a “one-fits-all” model might be considered the forbearers of possible future model changes – in the plural form since different site-specific solutions have to be devised in our diverse and complex world.¹⁴

…But “there is no such thing as free renewable energy”

Anticipating performing future (disruptive) technologies, several scenari based on renewable energies can sometimes convey the illusion of a form of environmentally ideal energy, for which an un-tapered consumption would not represent any major negative environmental or social impact. It is besetting to remind that appropriate metrics have become a useful complement to green chemistry principles such as established ones based on mass conservation¹⁵ such as the environmental e factor^15a or more complex multi-criteria systems analyses such as life cycle analyses and assessments.¹⁶ Similar handles dedicated to energy will undoubtedly also help us advance in the energy challenge through green chemistry.¹⁷

In conclusion, in twenty-five years, renewable energies, their production, their distribution and their storage have gained a major role in shaping how green chemistry addresses the “energy challenge”, without phasing out the original question of how to minimize (fossil) energy consumption. Facilitating the transition to a carbon-sober future has been mentioned above as the ultimate purpose of all these efforts. The necessity of a carbon-sober future becomes acute only against the menacing backdrop of the environmental climate context,¹⁸ compounded by, on one side, the societal costs linked to preserving access to current fossil fuel supplies for the current users and, on the other side, addressing the fact that still too few people have access to acceptable quantities of energy.¹¹

The exciting lines of investigations around principle 6 of the Green Chemistry Principles outlined above are only reinforced by the profound sense of responsibility that can emerge from the overarching context. The major event is that some electrons turned green in these past few decades: can we use them to address, among other challenges, the fact that 1.2 billion people do not have access to electricity,⁸ regardless its fossil or renewable origin, today?

References

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2. P. T. Anastas and J. B. Zimmerman, Design through the Twelve Principles of Green Engineering, Environ. Sci. Technol., 2003, 37(5), 94A–101A.
3. G. Centi, E. A. Quadrelli and S. Perthoner, Catalysis for CO₂ conversion to introduce renewable energy in the value chain of chemical industries, Energy Environ. Sci., 2013, 6, 1711–1731.
4. G. Ciamician, The Photochemistry of the Future, Science, 1912, 36, 385–394.
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6. Chemical Energy Storage, ed. R. Schlögl, Walter de Gruyter GmbH, Berlin/Boston, 2013.
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9. Dechema Gesellschaft für Chemische Technik und Biotechnologie e. V (DECHEMA), International Energy Agency (IEA) and International Council of Chemical Associations (ICCA), “Technology Roadmap” (2013) https://www.iea.org/publications/freepublications/publication/Chemical_Roadmap_2013_Final_WEB.pdf Accessed December 14^th, 2015.
10. CEFIC “The European chemical industry in worldwide perspective Facts and Figures 2012”.
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16. (a) D. Kralisch, D. Ott and D. Gericke, Towards a Holistic Approach to Metrics for the 21^st Century Pharmaceutical Industry, Green Chem., 2015, 17, 123; (b) W. Vigon, D. A. Tolle, B. W. Cornaby, H. C. Latham, C. L. Harrison, T. L. Boguski, R. G. Hunt and J. D. Sellers, Life-Cycle Assessment: Inventory Guidelines and Principles, EPA/600/R-92/036, U.S. Environmental Protection Agency, Cincinnati, 1992; (c) B. Sørensen, Life-Cycle Analysis of Energy Systems: From Methodology to Applications, RSC Publishing, 2011,  10.1039/9781849732864.
17. A. Sternberg and A. Bradow, Power-to-What? – Environmental assessment of energy storage systems, Energy Environ. Sci., 2015, 8, 389–400.
18. M. I. Hoffert, et al., Advanced Technology Paths to Global Climate Stability: Energy for a Greenhouse Planet, Science, 2002, 298(5595), 981–987.

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