(592a) Nuclear Quantum Effects on Adsorption in Nanoporous Structures

Nuclear quantum effects are typically not important for most adsorption and separation processes. Therefore, almost all molecular simulations of adsorption, diffusion, and separation in porous materials use a classical treatment of the nuclei. However, there are notable cases where differences in nuclear quantum effects provide the driving force for a separation process. These are rare, but important problems, primarily dealing with isotope separations. In this work, we have employed path-integral molecular dynamics simulations to rigorously account for quantum motion of the nuclei. We have used this method to study the selectivities of isotopologues of oxygen molecules in MFI zeolites and carbon nanotubes. We also use these methods to study structural changes in Calcium-based squarate frameworks brought about by adsorption of H₂O and D₂O. We use a combination of high-order splitting techniques and appropriate thermostats to achieve convergence of quantum mechanical properties at a much reduced computational cost. Our results indicate that a difference of nuclear quantum kinetic energy results in the preferential adsorption of heavier isotope of oxygen in MFI zeolites and a negative thermal expansion as a result of entrapping of heavier water in squarate-based frameworks.