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Density functional theory has been used to determine the effects of local composition and structure on the proton affinity (PA) of H−ZSM-5. These calculations were performed with clusters ranging in size from 34 to 41 atoms. For isolated Al sites, the associated PA is 329 kcal/mol. Introduction of a defect in the form of a silanol group immediately adjacent to such a Bronsted acid site reduces the PA to 316 kcal/mol. For an isolated structure containing two Al atoms in next-nearest-neighbor T sites, the value of PA lies between 330 and 298 kcal/mol, depending on the location of the two charge-compensating cations, and is not affected by the presence of a silanol group on the Si atom situated between the two Al atoms. However, the presence of a trigonally coordinated Si atom immediately adjacent to the Bronsted acid site reduces the PA dramatically to 281 kcal/mol, but the presence of trigonally coordinated Al in the vicinity of a Bronsted acid site has no effect on the value of PA. However, the presence o...

Density Functional Theory Calculations of the Effects of Local Composition and Defect Structure on the Proton Affinity of H−ZSM-5