It has been found by means of quantum chemical calculations that by modifying the framework composition, i.e., changing the Si/Al ratio, and substitution of Al by other trivalent atoms such as Ga and B, in a given zeolite, the energy of the frontier orbitals of the zeolite changes and therefore the softness of the acid site changes. The acidity softness increases when increasing the Si/Al ratio and also when going from B to Ga and to Al in the framework. Since the acid softness can play a determinant role specially in orbital-controlled reactions, the catalytic implications of the changes in the acidity softness-hardness of zeolites have been shown by carrying out the alkylation of toluene by methanol and looking into the para-ortho selectivity changes obtained when using HY zeolites with different framework Si/Al ratios and H-beta zeolites with Al, Al + Ga, Ga, and B as trivalent framework elements.