2021 ushered in a new era for climate disasters. Four separate environmental calamities: Hurricane Ida, the summer flooding of China, the aggressive Mexico winter weather, and the July flooding of Europe; all prompted over 20 billion dollars of economic losses. The causality between these phenomena and the energy expenditure of humans is indisputable.
The healing of our climate via carbon emission reductions will aid the growing economy, mitigate the deaths caused by air pollution, and uplift the burden that is currently being placed on healthcare systems. There are varying ways of accomplishing this, two of which are Solar Thermal Fuels (STFs) and green chemistry applications for refining primary resources.
The Applications of Solar Thermal Fuels
The ubiquitous problem with many thermal energy schemes is that much of the retained solar energy is lost as the day turns to night. Solar power is an intermittent energy source, as such, this ephemeral reserve does not have the reliability we look for in truly sustainable energy. This poses the question, how can we convert photonic energy into thermal energy on demand, day or night?
Opposing traditional fuels, (STF) are materials that are renewable and would pose limited harm to the surrounding environment. These fuels gather the energy emitted by solar radiation and store it via the rearrangement/isomerization of its structural bonds. STFs possess the appealing quality of being able to function at lower temperatures (below 0 degrees Celsius). In addition, STFs possess extreme versatility. For example, the heat generated from the cycling of an engine could be stored within these photo-responsive materials matrixes, able to be reserved there until the time of expenditure.
Examples of these STFs are norbornadiene, quadricyclane, and azobenzene. Fortunately, these are also regarded as low-cost materials. Any, if not all these STFs cover the criteria listed in the “Principles of Green Chemistry.” Some of these include the choosing of selective catalytic reagents rather than superior stoichiometric reagents, maximizing atom economy, and the reduction of derivatization (the unnecessary addition of moieties throughout the physical/chemical reaction).
How Norbornadiene & Quadricyclane Can be Utilized as a Solar Thermal Fuel Source
Norbornadiene can be converted to quadricyclane via a +2-cycloaddition occurring in the forward reaction. This ultraviolet light coming into contact with norbornadiene will induce symmetry between the molecules HOMO and LUMO orbitals, and the resulting ring strain is where the excess 174kJ/Mol of energy can be stored. This can be performed with a green catalyst, with no excess energy expenditure, and is much more efficient than traditional fuel cells.
Even the most practical of fuel cells can only store 154kJ of energy, though this is per mole of hydrogen that is consumed. This material circumvents this necessity. However, there are drawbacks. For quadricyclane to revert to its inert state of norbornadiene, some UV light must still be present, as the material will have trouble isomerizing when once again under visible light. Other alternatives for fuel storage apparatus were thusly explored.
The Use of Azobenzene for Long-Term Solar Storage
Researchers have found an interest in azobenzene on account of its lower cost of production, and its ability to undergo various cycles of photoisomerization without degradation. As the trans molecule absorbs light and undergoes isomerization to the cis conformation, energy can be stored within the strained chemical bonds. This energy is accessed once the energy barrier, or the transition state, is overcome via absorption of visible light or UV light. The heat generated from this can be used later, released by azobenzene’s ability to reverse isomerize back to the trans conformation. This energy cycle is pivotal to all STFs, though this azobenzene proves superlative to the prior STF’s, on account of its ability to retain energy in the absence of UV light.
The Adoption of Waste Products in Generating Green Cement
It currently stands that the production of cement is non-renewable, which presents a great threat given the voluminous waste generated from stone cutting. Of all secondary resources, cement arguably has the greatest protentional for harm, standing as the third largest industry for worldwide pollution. Though cement production has reduced to less than half its peak volume in 2019, more than 500,000 tons of nitrogen oxide, carbon monoxide, and sulfur dioxide are released into the atmosphere per year.
A possible solution founded by Santiago Yagüe et al. proposes the use of recycled products within the “cement making” process. Rather than depositing these materials back into landfills, Dr. Yagüe proposes that 10% of granite sawmill waste be sued to sustain ecological cement, as well as other specified wastes. Using waste particles that are smaller than 15 µm can result in morphological matrices that can adopt the same function as cement. Compounds such as double-layered hydroxide compounds, and calcium silicate hydrate gels may fit. They demonstrate within the paper that these waste products can develop stable matrices over intervals longer than a year, though further research is still required.
- Calbo, J.; Thawani, A. R.; Gibson, R. S. L.; White, A. J. P.; Fuchter, M. J. Beilstein (2019), A combinatorial approach to improving the performance of azoarene photoswitches. J. Org. Chem. 15, 2753–2764
- Yuichiro Himeda, Nobuko Onozawa-Komatsuzaki, Hideki Sugihara, and Kazuyuki Kasuga (2005).
- Recyclable Catalyst for Conversion of Carbon Dioxide into Formate Attributable to an Oxyanion on the Catalyst Ligand. Journal of the American Chemical Society. 127 (38), 13118-13119
- Mihael A. Gerkman, Rosina S. L. Gibson, Joaquín Calbo, Yuran Shi, Matthew J. Fuchter, and Grace G. D. Han (2020). Arylazopyrazoles for Long-Term Thermal Energy Storage and Optically Triggered Heat Release below 0 °C Journal of the American Chemical Society 2020 142 (19), 8688-8695
- Yagüe, S., González Gaya, C., Rosales Prieto, V., & Sánchez Lite, A. (2020). Sustainable Ecocements: Chemical and Morphological Analysis of Granite Sawdust Waste as Pozzolan Material. Materials (Basel, Switzerland), 13(21), 4941.
- S. Chen, Y. Zhao, N. Ullah, Q. Wan and R. Zhang, Carbon (2019), 150, 439-445
- Dong L , Feng Y , Wang L , Feng W . (2018) Azobenzene-based solar thermal fuels: design, properties, and applications. Chem Soc Rev. Oct 1;47(19):7339-7368.