Spring 2025 ChBE Seminar: Sievers

Mechanochemical processes use mechanical collisions in a ball mill or similar device to drive chemical reactions. The collisions can create transient surface sites with extraordinary catalytic activity and hot spots that are characterized by rapid local temperature rises followed by dissipation of heat to the environment. In addition, mechanical forces can create intimate contact between two solids, so that the conversion of a solid reactant over a solid catalyst becomes viable. 

 

The formation of hot spots is analyzed in a fundamental study of the conversion of CaCO3 to CaO [1]. Based on models for the impact of the milling ball and heat dissipation, each collision can be modeled as a transient batch reactor. The rates of CO2 formation in a flow-through milling vessels are determined at different milling frequencies to validate the model. The impact of a 20 mm steel ball with a net velocity of 4.5 m/s results in a hot spot temperature of above 800 °C.

 

These dynamic environments can be used for ammonia synthesis from the elements [2]. During milling in a mixture of N2 and H2, transition metals are converted into their nitrides. Additional collisions lead to the formation of more reactive excess nitride species. During the decay of the hot spot, the system passes through a regime in which hydrogenation of these reactive nitrides to ammonia is thermodynamically and kinetically feasible.

 

The ability to convert solid feedstock opens new possibilities for converting lignin [3] and waste plastics [4]. For example, the depolymerization of poly(ethylene terephthalate) (PET) occurs readily when the polymer is milled with NaOH [4]. After an initial period, in which monomers are produced at a constant rate, the reaction mixture is converted into a wax that coats the milling balls or is pressed into the sites of the milling vessel. After wax formation, the remaining polymers are converted much faster because the kinetic energy of the impinging milling ball is transferred to the reaction mixture more effectively. The reaction kinetics are explained with a modified shrinking core model. For depolymerization of poly(ethylene) we demonstrate a process, in which the polymer is partially oxidized in random positions of the backbone to facilitate cleavage of C-C bonds [5].

Carsten Sievers obtained his Diplom and Dr. rer nat. degrees in Technical Chemistry at the Technical University of Munich, Germany. Under the guidance of Prof. Johannes A. Lercher, he worked on heterogeneous catalysts for various petrochemical processes. In 2007, he moved to the Georgia Institute of Technology to work with Profs. Christopher W. Jones and Pradeep K. Agrawal as a postdoctoral fellow. His primary focus was the development of catalytic processes for biomass depolymerization and synthesis of biofuels. He joined the faculty at the Georgia Institute of Technology in 2009. His research group is developing catalytic processes for the sustainable production of fuels and chemicals. Specific interests are catalyst deactivation and regeneration, mechanocatalysis, plastics upcycling, catalytic processes in aqueous phase, surface chemistry of complex molecules, production of value-added chemicals from biomass, operando spectroscopy, and CO2 capture and conversion. He published over 120 peer-reviewed papers. He is one of the Chairs of the 29th North American Meeting of the North American Catalysis Society (NAM29) and Deputy Editor-in-Chief of Applied Catalysis A: General.