Identifying Free Energy Landscapes of Proton-Transfer Processes between Brønsted Acid Sites and Water Clusters Inside the Zeolite Pores
Peng Liu, Donghai Mei
Abstract
In the aqueous-phase zeolite-catalyzed reactions, water molecules profoundly alter the reaction activity and reaction mechanisms. This can be ascribed to the changing nature of the Brønsted acid site (BAS) induced by surrounding water molecules. In this work, the effects of water clusters with different sizes on the BAS site were investigated using ab initio molecular dynamics simulations combined with an enhanced sampling methodology. The proton hopping processes between the BAS site and the water cluster or within the water cluster have been mapped out with free energy landscapes. When the H2O/BAS ratio equals to 1, the proton hops between two oxygen atoms of the BAS site. When the H2O/BAS ratio is 2, the proton prefers to shuttle between the zeolite framework and the water dimer. When the H2O/BAS ratio reaches 3, the proton moves away from the BAS site to the linear water trimer and shuttles between oxygen atoms within the water cluster chain. The increasing proton affinity with the expanding water cluster size can be attributed to the enhanced electron density in oxygen atoms of the water cluster.