SSCU Ph. D. Thesis Colloquium
Name: Mr. S. A. M. Shamimul Ahsan
Research Supervisor: Prof. S. Yashonath and Prof. Govardhan Reddy
Title: Intermolecular Potential for 12-Ring Zeolites, Hydrogen Bond Dynamics and Diffusion of Uranyl Ion and Hydrocarbon Confined to Zeolite and Related Studies
Date &Time: Tuesday, 03rd March 2026 at 04:00 p.m.
Venue: Rajarshi Bhattacharya Memorial Lecture Hall, Chemical Sciences Building
Abstract:
Understanding the behavior of fluids and ions confined within nanoporous materials is critical for applications in catalysis, separations, extraction of metals and nuclear waste management. This thesis investigates the structure, hydrogen bond dynamics, and transport properties of water, hydrocarbons, and ions confined within zeolitic frameworks, with a particular focus on siliceous and aluminated faujasite (FAU) structures, using classical and ab initio molecular dynamics simulations.
Water confined in aluminated faujasite was found to exhibit shared hydrogen bonding with framework oxygen atoms (OAl), leading to prominent radial distribution (RDF) peaks at ∼ 2.9 Å (OW -OAl RDF) and ∼ 1.9 Å (HW -OAl RDF). These shared hydrogen bonds induce significant structural distortions in the zeolite framework, with ∠OAlO increasing by ∼ 7°, and demonstrate jump reorientation dynamics with very short lifetimes (∼ 50-100 fs) distinct from typical hydrogen bonding dynamics observed in bulk water.
Comparative studies between siliceous and aluminated FAU revealed that aluminium incorporation enhances framework water interactions, resulting in bimodal distributions in structural angles (e.g., ∠SiOAl), altered water orientational relaxation, and large amplitude angular jumps of confined water molecules. The presence of sodium cations in aluminated FAU further modulates these dynamics, with Na(1) sites exhibiting the strongest interactions with water.
For confined hydrocarbon mixtures, simulations of n-hexane and 2,2-dimethyl butane within zeolite NaY showed reduced mutual diffusivity compared to bulk mixtures, reflecting the restrictive effect of pore confinement on molecular mobility. Similarly, studies of aqueous uranyl nitrate solutions confined in zeolite NaY demonstrated drastic reductions in mutual diffusivity of UO22+ and NO3− relative to bulk solutions, with confinement affecting ion-ion and ion-solvent coupling.
To accurately capture zeolite structural behavior, an improved interatomic potential (aHSFF-G) was developed and validated against experimental window sizes, thermal expansion coefficients and ab initio DFT calculations enabling reliable simulations of 12-membered ring siliceous zeolites.
Overall, this thesis elucidates the unique hydrogen bond dynamics, structural perturbations, and transport properties of confined fluids and ions within zeolites, contributing to a deeper molecular-level understanding essential for the design of advanced materials for catalysis, separations, extraction and nuclear waste immobilization. Furthermore, these findings have direct implications for geoscience, particularly in modelling water mineral interactions, fluid transport in nanoporous geological materials and radionuclide behavior in subsurface repositories.