Theory of Brønsted Acidity in Zeolites

The nature of the chem. bond of protons in a zeolite is analyzed on the basis of theor. and spectroscopic results. Of interest is the dependence on zeolite structure as well as compn. The zeolitic OH bond is mainly covalent. Proton attachment to the zeolite lattice causes a weakening of neighboring Si-O and Al-O bonds. The effective increase in vol. of the bridging oxygen atom causes a local deformation, that changes the strength of the lattice-chem. bonds over a few bond distances. Proton concn. effects as well as lattice-compn. effects can be understood on the basis of the lattice-relaxation model. The energetics of proton transfer is controlled by the need to stabilize the resulting zwitter-ion. The pos. charge on the cation becomes stabilized by contact with basic lattice-oxygen atom