(114f) Opportunities of Mechanocatalytic Processes

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 short-lived hot spots. In addition, mechanical forces can create intimate contact between two solids, so that a solid reactant can be converted with a solid catalyst.

These dynamic environments can be used for ammonia synthesis [1]. During milling in N₂ and H₂, titanium metal is converted into TiN. Additional collisions lead to the formation of reactive nitride species. During the decay of the hot spot, the system passes through a regime in which hydrogenation of reactive nitrides to ammonia is thermodynamically and kinetically feasible.

The ability to convert solid feedstock opens new possibilities for converting waste plastics [2]. For example, the depolymerization of poly(ethylene terephthalate) (PET) occurs readily when the polymer is milled with NaOH [2]. 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. 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 [3].

1. A.W. Tricker, K.L. Hebisch, M. Buchmann, Y.-H. Liu, M. Rose, E. Stavitski, A.J. Medford, M.C. Hatzell, C. Sievers, ACS Energy Letters 5 (2020) 3362−3367.

2. A.W. Tricker, A.A. Osibo, Y. Chang, J.X. Kang, A. Ganesan, E. Anglou, F. Boukouvala, S. Nair, C.W. Jones, C. Sievers, ACS Sustainable Chem. Eng. 10 (2022) 11338.

3. V.S. Nguyen, Y. Chang, E.V. Phillips, J.A. DeWitt, C. Sievers, ACS Sustainable Chem. Eng. 11 (2023) 7617.