First-principles molecular modeling of structure-property relationships and reactivity in the zeolite chabazite

Zeolites are crystalline, porous aluminosilicates; while a pure silicate structure is charge-neutral, the substitution of A1³⁺ for Si⁴⁺ creates in the framework a negative charge, which can be compensated by a proton that acts as a strong, acid-donating Bronsted site. Zeolites are widely used in industry, most commonly for catalysis and separations. Unfortunately, they have not yet been able to replace all homogeneous catalysts in industrial processes due to the difficulties in reactant and product diffusion to and from the zeolite surface in the absence of a solvent. However, it is believed that if we had a thorough understanding of how solid acids, especially zeolites, catalyze reactions, then we would be able to design heterogeneous catalysts to overcome these difficulties. The nature of the acid sites in zeolites and the factors contributing to enhanced catalytic activity have been the subject of much study in the literature. In particular, the issue of whether all of the acid sites in a particular zeolite are homogeneous or heterogeneous in acid strength requires the development of a systematic way to quantify acidity. To address this, a detailed density functional theory (DFT) investigation of the reactivity of the acid sites in the zeolite chabazite was performed. Energies of adsorption of bases, deprotonation energies, and vibrational frequencies were calculated on a periodic chabazite (SSZ-13) model with various loadings of acid sites per unit cell, and with various structural framework defects. The four acidic oxygens at the aluminum T-site were found to all have roughly the same proton affinity, and the deprotonation energy is not correlated to the O-H bond length or vibrational stretch frequency. Furthermore, the adsorption energy of various bases at

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Chemical Engineering.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

2005

Author dc:creator

• Lo, Cynthia

Advisor dc:contributor.advisor

• Bernhardt L. Trout.

• Chemical Engineering.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/28842
• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Identifier URI

http://dspace.mit.edu/handle/1721.1/28842

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/28842