Zeolites and related microporous molecular sieves
consist of a three-dimensional network of metal–oxygen
tetrahedra (in a few cases also octahedra) which provide
the periodically sized microporous structure, in which
the active sites are part of the structure. Acid sites result
from the imbalance of the metal and the oxygen formal
charge in the primary building unit. This can easily be
recognised in the case of zeolites, which consist of a
three-dimensional network of Si–O tetrahedra. A lattice
comprising of only Si–O tetrahedra is neutral (the 4+
charge at the silicon is balanced by four oxygen atoms
with each 2 charge, however, belonging to two tetrahedra).
Replacing one Si4+ atomby Al3+ causes a formal
charge on the tetrahedron of 1. This negative charge
is then balanced by a proton or metal cation forming
an acid site. The bare, negatively charged tetrahedron
is then the corresponding base. Please keep in mind that
these acid and base properties are not just a function of
the chemical composition, since other factors, like the
framework density, the type of cation or the local strain
have an influence as well