A completely isolated Al tetrahedron will have zero NNN and supports the strongest type of framework Brønsted acid site. Barthomeuf [?] extended this idea by using topological densities to include the effects of layers one through five surrounding the Al atom. Both the Al NNN and the topological density theory predict that by changing the framework Si/Al ratio, either by synthesis or by post chemical synthesis, it is possible to change not only the total number but also the electronic density on the bridging hydroxyl group, and therefore to change the acid strength of the Brønsted acid site. Thus, when reactions demanding low acidities are to be catalyzed, zeolites with lower framework Si/Al ratios will be preferred. In contrast, when strong acidities are required, zeolites with isolated framework Al (Si/Al ratios 9–10) will be chosen.