Investigation of Zeolite Nucleation and Growth Using NMR Spectroscopy

Abstract

Zeolite nucleation and growth is a complex problem that has been widely investigated but still not completely understood. However, a full understanding of this process is required in order to develop predictive models for the rational design and control of the zeolite properties. The primary objective of this dissertation is to determine the strength of organicinorganic interactions (i.e., the adsorption Gibbs energy) in transparent synthesis mixtures using PFG NMR spectroscopy, in order to provide more information for a better understanding of zeolite nucleation and growth.

Three main tasks were conducted in this work. The first was an investigation of the organocation role in precursor mixtures of silicalite-1, where the Gibbs energy of the organocation adsorption on the silica particles was determined at 25 degrees C. The findings showed that small changes in the adsorption Gibbs energy resulting from the differences in the molecular structure of the organocations lead to large changes in both the stability of the precursor particles and the rate of silicalite-1 formation. The second was an in situ PFG NMR investigation of silicalite-1 synthesis mixtures, where the adsorption Gibbs energy was determined at 25 degrees C and 70 degrees C, and the time evolution of silicalite-1 was monitored at synthesis conditions. The findings showed similar adsorption Gibbs energies at 25 degrees C and 70 degrees C. Also, a maximum in the organocation diffusion coefficients was observed during the time evolution of silicalite-1, which was associated with the exothermicendothermic transition occurring during the synthesis. The third was a systematic investigation of silicalite-1 precursor mixtures with varying degrees of dilution, where the effect of the composition of the mixtures on their conductivity, pH and particle size distribution (PSD) was studied. The results showed that conductivity, pH, and PSD are strongly affected by the mixture composition.

The main conclusion of this research is that the strength of the organic-inorganic interactions in transparent synthesis mixtures can be determined from experimental data of the organocation self-diffusion coefficients obtained with PFG NMR spectroscopy. The outcome information of this research should contribute to the development of a more detailed molecular-level description of the zeolite nucleation and growth, which is expected to allow the emergence of a new generation of materials by design.