CO₂ Capture with Silylated Ethanolamines and Piperazines

Abstract

Invited for this month's cover is the group of Marcus Herbig from the TU Bergakademie in Freiberg. The cover picture shows the reaction of CO₂ with a silyl derivative of the biogenic amine ethanolamine. The role of CO₂ as a contributor to climate change makes “carbon capture” a desirable goal. However, in addition to simply capture CO₂, aminosilanes form silylcarbamates, which represent starting materials for a variety of crucial chemicals. Thus, the entrapped CO₂ represents a useful C₁ building block. The ESF‐funded Junior Research Group CO₂‐Sil at the TU Bergakademie Freiberg (represented by their Logo and location) pursues that kind of goals. CO₂‐Sil studies these key reactions of CO₂ insertion in depth by syntheses, quantum chemical calculations and calorimetric experiments. CO₂ brought to the ground by our method shall be feedstock for various branches in chemistry. Read the full text of their Full Paper at 10.1002/open.201900269.

“…Charging a calorimeter with highly moisture sensitive compounds such as aminosilanes is just one side of the story… Find out more about the story behind the front cover research at 10.1002/open.201900269.

What was the biggest challenge (on the way to the results presented in this paper)?

The probably trickiest part of this work arose from calorimetry side. Charging a calorimeter with highly moisture sensitive compounds such as aminosilanes is just one side of the story. A more challenging part was the precise and rapid dosage of dry CO₂ into the aminosilane charged calorimeter. Last but not least, even with these hurdles overcome, the first attempts at decent calorimetric measurements failed because of the great amount of heat produced by the insertion reaction. It took many additional experiments to optimize the procedure to finally deliver reliable data.

Does the research open other avenues that you would like to investigate?

Of course, now we can investigate the influence of the Si‐ and N‐bound substituents on the thermodynamics of the reaction, which helps to distinguish between thermodynamic and kinetic effects in rather slow reactions. This is an important aspect when it comes to optimizing reaction conditions for a particular system of starting materials. Furthermore, methods for thermokinetic experiments could be developed with our calorimetric system. These would provide some insight into the reaction kinetics and hints at the reaction mechanism. Hence, this is one of the points on the agenda.

What other topics are you working on at the moment?

The insertion of CO₂ into Si‐N bonds is a very broad but nonetheless only little explored field so far. At the moment, we investigate the reaction kinetics by in‐situ IR and NMR measurements, and both syntheses and spectroscopic monitoring currently aim at developing a deeper understanding of the influence of the amine moiety on the reaction.

Who pays the bill for the research highlighted in the cover?

Funding was provided by the European Social Fund (ESF) and the Freistaat Sachsen. We have a “Nachwuchsforschergruppe”, i.e. a Junior Research Group named CO₂‐Sil, which is dealing with aminosilanes and the CO₂ insertion products.

In one word, how would you describe your research?

Breathtaking!

What prompted you to investigate this topic/problem?

Aminosilanes have been part of various investigations at TU Bergakademie Freiberg (Institut für Anorganische Chemie) over the past two decades, be it as protecting groups for silane syntheses or as precursors for Si/N/C containing materials (polymers, ceramics). Even though aminosilanes are moisture sensitive, they already proved useful for some potential applications. Thus, from the aminosilane side experimental know‐how (preparation under inert conditions) was abundant at our research institution. However, including the utilization of CO₂ into aminosilane chemistry was big leap forward. It was intriguing and tempting (because of the entire CO₂ and climate debate), and it was (and is) challenging at the same time.

What aspects of this project do you find most exciting?

Normally, syntheses of organic carbamates involve phosgene at some point (e.g., for the syntheses of isocyanates, which may be transformed into carbamates). Circumventing the use of phosgene is a desirable goal, of course. Considering CO₂ as an alternative starting point, carbamic acids formed by addition of CO₂ to amines are not stable, but can be stabilized as salts (i.e., by deprotonation). We used CO₂ as starting material to form this moiety without a catalyst at ambient temperature and pressure in good yields. Changing one hydrogen atom from an amine moiety to a silyl moiety does not inhibit the insertion reaction but changes the stability of the products in a favorable manner. Apart from the moisture sensitivity, the reaction products offer new access to the field of carbamate chemistry.

How would you describe to a layperson the most significant result of this study?

A great number of chemicals, which are important starting materials for, e.g., various everyday life applications, require an additional C₁‐building block in their synthesis. Phosgene, a highly toxic gas, is such a building block. CO₂, which is less dangerous, could become an alternative. The chemicals to be synthesized are located on their individual energy levels (height levels, one can see them sitting in their individual valleys within a range of mountains). Phosgene is something which can be regarded high in energy, and introducing C₁ via phosgene can be seen as providing C₁ from above, lowering it into all sorts of valleys via parachute – Easy! Basically there is no hindrance. CO₂, albeit the desirable (because less dangerous) starting point, is incredibly low in energy. Introducing C₁ via CO₂ is a journey from one valley into another, sometimes a convenient walk, but sometimes creeping across mountain passes into the desired valley. The energy required for this journey will be “paid” by the reactants in advance. CO₂ receives some credit, which may help to reach a valley higher than the starting point. If CO₂ fails to reach a new target valley, there might be two reasons: 1) The new valley is too high, the credit paid is too low. CO₂ refuses to walk this way. 2) The new valley is fine, but a very high mountain pass has to be overcome, the journey just takes too long (for us to observe any progress). In this case we may try and find a way to open tunnels for the CO₂ molecule to eventually enter the new valley. For further applications, for exploring the potential use of CO₂ as a starting material, it is important to tell the difference between scenarios 1) and 2), and our current study is making a contribution in this regard.

M. Herbig, L. Gevorgyan, M. Pflug, J. Wagler, S. Schwarzer, E. Kroke, ChemistryOpen 2020, 9, 893.