Computational studies oriented towards the development of a greener chemistry
- Autores: Jesús Antonio Luque Urrutia
- Directores de la Tesis: Albert Poater i Teixidor (dir. tes.), Miquel Solà Puig (codir. tes.)
- Lectura: En la Universitat de Girona ( España ) en 2021
- Idioma: español
- Tribunal Calificador de la Tesis: Josep M. Luis (presid.), Diego Marcelo Andrada (secret.), Laura Falivene (voc.)
- Materias:
- Enlaces
- Tesis en acceso abierto en: TDX
- Resumen
According to Dr. Paul Anastas quote:
“We are all in the same boat, and we only have one boat.” Minnesota Green Chemistry Forum, 2013 In this thesis, we emphasize this thought. For a long time, we have believed that everything is profitable, that resources are unlimited, and there are no consequences. However, reality is often disappointing. The use of non-renewable resources, the excessive waste production, and forfeiting the task of recycling has led us to walk over a fragile thread that, once broken, may never restore itself. Metaphors aside, we are talking about our planet, the Earth, and its unique characteristic of harboring life, including ourselves. Our world has its equilibrium; when the wind erodes a mountain, a beach appears, or when a fire devastates an area, eventually, new life rises from the ashes. However, humans have been distorting this balance for decades now. Our evolving way of living has increased the number of resources every person consumes, as either food, shelter, or energy; we overworked everything until exhaustion. We even have the Earth Overshoot Day, which has the “honor” of remembering the day we have consumed all the planet’s resources created over a year. Right now, this day is located on July 29, meaning that to supply every human being, we need the resources of 1.75 Earths at the standard recovery rate of the natural resources. However, this is just one problem lurking in the back of our minds like a bad omen: our planet is dying.
Scientists worldwide have already said actively and passively that we have in front of us one of the most significant problems we have ever faced: climate change. For many people, it just means that it will be a bit hotter in summer and a bit warmer in winter, but this is far from reality. Every living thing in this world has acclimated to its environment, like polar bears with their transparent fur to retain the most heat or elephants with their large ears to release it. However, this occurred through the evolution of several thousand years, and every being was able to adapt to it or die trying. Now, we are changing the climate at an alarming rate. The global surface temperature rose from +0.2 ºC approximately from 1970 until 2000, then +0.6 ºC until 2015, and close to +0.8 ºC in 2020. This temperature rise means that, in 100 years, we could see an increase in the global surface temperature between +2 ºC and +6 ºC. It is unsustainable, and we should try to revert it, or, if we are too late, to slow it down as much as possible. For that to happen, there are many possible ways to help. In this thesis, we research catalysts to use water as a source of energy and recycle gases like CO2 and N2O, but we also look towards non-catalytic ways of generating energy through solar cell production.
Starting from the catalysis of water to produce hydrogen, we look at the best catalyst involved in water oxidation catalysis (WOC) that we could find: Ru(bda)(pic)2. This catalyst performs as well as photosystem II (PSII), involved in the photosynthesis of a leaf, and the trick to its efficiency is the equatorial ligand bda. It includes carboxylates fixed around the Ru atom, leaving an opening angle large enough for a water molecule to react, converting an octahedral Ru into a reactive hepta-coordinated Ru. While this catalyst was experimentally tested, the full mechanism was still missing. Thus, we wanted to fill in this gap to understand it better. There were discrepancies among several researchers on how the mechanism works, making us want to study all the possibilities available. Knowing why and how it works makes it possible to introduce modifications and upgrade the catalyst into a better one. We demonstrate this catalysis’s entire mechanism, analyze the different proposals of other research papers, and study the system’s pH influence. We found that the mechanism varies slightly according to the pH since there are deprotonations involved, but it occurs only on intermediates with short life spans, and the overall reaction looks utterly unaffected by these changes. Following this work, we performed a second project modifying this catalyst; more precisely, we changed the carboxylate groups in the bda ligands for phosphonate groups, forming the new ligand bpaH2. Previous studies had shown that the bda outperforms the bpaH2 and its different protonation states, but the reason was still unknown.
Furthermore, the bpaH2 required Ceric Ammonium Nitrate (CAN) at pH=1 because otherwise, no reaction could occur. All these pieces were missing; thus, we developed the full mechanism of bpaH2 at pH=1 and pH=8. Thanks to that, we could observe the different protonation states of the bpaH2 in each pH and revealed the potentials needed to deprotonate water into an oxo ligand. Nevertheless, we also found out that the catalysis could not proceed through either known mechanisms, a dimerization that releases O2 (I2M), or a nucleophilic attack of water that deprotonates a water molecule again before removing the oxygen (WNA). The reason behind this is that in the case of the I2M, experimental results from other authors had shown that it is a first-order reaction according to the catalyst, meaning that the rate-determining step (r.d.s.) could not be a dimer between catalyst, but while the other choice, i.e. WNA, should have been confirmed, we found out it was not possible. As we looked into the second water deprotonation, the hydrogens’ pKa was too high for our given pH; therefore, the hydroperoxy ligand (-OOH2) could not deprotonate. Since neither WNA nor I2M could proceed, it left us with the last choice by elimination, i.e. an I2M between the catalyst and the CAN oxidant (Ru-CAN). The problem here lies in CAN, as the structure in the solvent phase is unknown. While we tried to find a suitable geometry, we decided that since we did not have a solid ground on this structure, we would stop this research as it could shift the study’s target towards the definition of this CAN structure.
The next projects present a dehydrogenative reaction with a new manganese catalyst that enabled it to become acceptorless since this type of reaction usually requires a sacrificial molecule, or acceptor, to accept the free hydrogens. By eliminating this acceptor, we thoroughly dispose of the waste. Thus, we found it as an excellent example of avoiding waste, and since it was still a new research, it lacked the full mechanism. Even more, this catalyst is one of the most researched types of catalyst nowadays, i.e. a PNP pincer catalyst which, as its name implies, has two phosphorous atoms and one nitrogen atom bound to the metal center, as a single ligand. The original theorized mechanism pales in comparison to the complete mechanism we discovered. Not only were we able to locate the r.d.s. of the process, but we also found out the relationship between two experimental isomers that were able to convert one into the other if left alone long enough at room temperature or by heating the mixture for 30 min.
Furthermore, we found that the initial mechanism had an ionic mixture as a possible intermediate, but actually, it is a bond between the Mn center and the substrate's CN ligand. Next, we wanted to delve further into the PNP pincer catalysis, as it shows promising and diversified reactivity; thus, we looked into a second catalyst. This time, we chose a similar catalyst, with an identical reaction temperature, but instead of forming imines like the previous one, it produced aldimines. We were expecting a similar mechanism; even though after several tries, we found out that the catalyst’s only job was to transform the reactant alcohol into an aldehyde, which in turn united with the amine to form the aldimines. This last step happens without the catalyst’s help, being this the r.d.s. of the reaction.
Continuing with green chemistry, we wanted to study processes related to recycling the waste already generated, such as N2O and CO2. We decided to follow up with a PNP pincer catalyst for the first gas, just like the previous projects, since the N2O converting reaction was tested experimentally with a very similar catalyst that we studied previously. We have determined the full mechanism of this reaction, including a suitable catalyst poisoning, which appears if the environment is dry, revealing that water is a great help as an assistant molecule. A volumetric study of the phosphonate ligands (isopropyl, phenyl, and tert-butyl) hinted into which one is optimal for this catalysis and, only through the help of a bond analysis we were able to determine that isopropyl is the best of the three. Additionally, after testing three different PNP pincer ligand catalysts, we propose to combine the three for future research based on the various researched mechanisms’ properties.
The last project is about waste recycling and it involves CO2, the gas most responsible for climate change. To recycle the atmospheric CO2, we looked into the cyclization of epoxides to obtain a cyclic carbonate. The experimental conditions are already promising since they require 100 ºC but 1 atm of CO2, meaning that we could reach optimal temperatures if tuned properly while not needing tremendous pressure. In this study, we tested the possibility of removing the commonly used halides as epoxide openers, since they create CO2 when produced, into N-labile ligands such as DMAP or pyridine. Even more, two possible mechanistic pathways were tested and determined that only one of them is viable. The results show that some ligands precipitate when in contact with CO2, while in other cases, the r.d.s. are too high in energy. Thankfully, there were two ligands, namely [4] and DMAP, which could perform the reaction and give hope to this research path, where [4] was found to be the best option for halide substitution.
Finally, yet importantly, we wanted to test a different perspective in green chemistry, such as solar cells’ production. Solar cells differ significantly in types, so we focused on a fullerene-based cell, which uses the fullerenes as a surface where the photosensitive dyes transfer the electrons through the surface, but we went deeper than that. Instead of using fullerenes, we tested fullerenes inside other fullerenes, also known as carbon nano-onions (CNOs). As solar cells rely on electron movement, we wanted to see if it would be possible to affect them by doping or trapping ions into de nano-onions, as they could emit an electronic field. For that to happen, the CNOs should be perfect Faraday cages. If we trap an ion inside, the outer fullerene should have the same charge as the ion, creating the field we wanted. Unfortunately, our study shows partial charge shielding, which means that these endohedral fullerenes are not perfect Faraday cages for the proposed role.
Overall, we tested several mechanisms and properties towards green chemistry, and we demonstrate that it is possible to improve their related industrial processes to save our planet.
