Dealumination of Y zeolites.

摘要

In the dealumination of zeolites, aluminum atoms are extracted from the framework and, in some cases, replaced with silicon atoms. The extraction of aluminum can lead to significant losses in crystallinity and to the formation of amorphous silicon material. This dissertation focuses on the framework amorphization of zeolite Y during dealumination with ammonium hexafluorosilicate. NMR spectroscopy, as well as propane adsorption and diffusion, have been used to probe the structural changes that take place with dealumination.; A model for dealumination is proposed, with predictions in quantitative agreement with experimental data. This model incorporates two parameters: the probability p that extracted Al atoms are replaced with Si atoms and the fraction f of replacement silicon that originates from the sample. The model tracks Al and Si coordination, and allows for statistical probability of different Al atoms being removed, as well as bond strength differences depending on the number of Al atoms bonded to a particular Si atom. Best fits are obtained for a purely statistical likelihood of bond breaking.; In this model, for substitution probabilities less than unity, the resulting framework vacancies increase the likelihood that other atoms will not have the necessary connectivity and leave the lattice as well; this cascade leads to the formation of amorphous silicon material. By matching experimental data to model predictions, the factors influencing dealumination can be better understood and the quantities of framework defects and amorphous material in zeolites dealuminated under different conditions can be calculated.; Additionally, the effects of framework amorphization on the adsorption and diffusion behavior of propane in dealuminated Y zeolites are quantified. Pore blockage, mesopore formation, and surface pitting were found to alter the sorption capacities of dealuminated samples. These different factors, which are all necessary for a quantitative description of the effects of amorphization, depend solely on the structural changes undergone by the dealuminated materials. These changes, in turn, are determined by the specific dealumination conditions as described by the aforementioned parameters p and f.