Reaction processes in ZSM-5 zeolites elucidated by quantum density functional theory and dynamic Monte Carlo.

摘要

This thesis presents a study of particular reaction processes occurring in CuZSM-5, and the effect of confinement on reaction processes in ZSM-5 zeolites. It begins with a presentation of quantum density functional theory, the major technique used. This presentation is geared towards chemical engineers and chemists, both experimentalists and theorists who have had minimal previous exposure to quantum computational methods.; Quantum density functional theory is used to evaluate the stability of Cu species thought to be involved in the autoreduction process, which occurs when aqueously prepared CuZSM-5 is dehydrated. In particular, {dollar}rm Cusp{lcub}2+{rcub}0sp-{dollar} associated with a single Al site is found to be stable, and is predicted to be one of the entities responsible for the loss in detectable electron spin during autoreduction. Cu{dollar}sp{lcub}2+{rcub}{dollar} associated with 2 Al sites is found to be unstable in a ring structure containing 6 T-sites, but is found to be stable in a ring structure containing 5 T-sites.; Quantum density functional theory is also used to calculate the thermodynamics of elementary steps thought to be involved in the process of NO{dollar}rmsb{lcub}x{rcub}{dollar} decomposition over CuZSM-5. Based on these calculations, a possible reaction pathway is presented. The pivotal species in this reaction pathway is {dollar}rm Cusp{lcub}2+{rcub}Osp-{dollar} associated with a single Al site.; Finally, dynamic Monte Carlo simulations are performed on a lattice model of ZSM-5, which realistically incorporates the effects of confinement. The various interaction sites in the zeolite are modeled as either weakly adsorbing inactive sites or strongly adsorbing reactive sites. It is found that because of the low connectivity in the zeolite, diffusivity as a function of occupancy and as a function of fraction of blocked sites, decreases more rapidly in the ZSM-5 lattice than in a square or cubic lattice. Moreover, because of confinement effects, diffusivity decreases as a function of concentration of reaction sites. Thus, optimal Si/Al ratios are found at which the rate of the reaction process is a maximum. These optimal Si/Al ratios depend on the rate of hopping from weakly adsorbing sites relative to the rate of desorption from strongly adsorbing sites, and may be larger than the minimum Si/Al ratio possible in ZSM-5.