Abstract
Periodic DFT calculations have been performed to characterize the acidic properties of bridging and terminal OH groups and various defect sites present in zeolites. As a model system mordenite with Si/Al=23 has been used. Binding energies and vibrational frequencies of adsorption complexes of frequently used probe molecules such as CO and NH 3 have been calculated. Ammonia is protonated at bridging OH groups while it is H-bonded upon adsorption at terminal silanol groups. A defect with a two-membered silica ring is a weak Lewis site as indicated by very low interaction energies with probe molecules. The Brønsted acidity of a defect with the bridging OH group in the two-membered ring is mid-way between that of bridging OH groups and terminal silanol groups. A three-coordinated surface Al atom is a strong Lewis site.
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