We have compared the acidity and the catalytic properties of (i) zeoli
te H-Y, (ii) H-Y dealuminated by hydrothermal treatment (H-US-Y), and
(iii) H-US-Y realuminated by the KOH treatment (H-Real-US-Y). Quantita
tive IR measurements of pyridine sorption show that the concentration
of ''3460 cm(-1) hydroxyl groups'', the Bronsted acid sites active in
many catalytic reactions, is slightly lower in sample H-Real-US-Y than
in H-Y. This demonstrates that most of the 3460 cm(-1) hydroxyl group
s, present in the parent material and subsequently destroyed by dealum
ination, are restored by realumination and the K+/H+ exchange. The tur
nover frequency of the realuminated zeolite in m-xylene reactions is h
igher than in the parent sample, as a result of the higher strength of
the acid sites, caused by the higher population of the most strongly
acidic Si3Si-OH-AlSi3 groupings. Dealumination and realumination also
alter the selectivity in the transformation of m-xylene. Disproportion
ation of xylenes to toluene and trimethylbenzenes, which involves bulk
y transition step complexes, is hindered in the dealuminated zeolite,
where the channels are partly blocked by the extra-framework Al specie
s, but is restored in the realuminated material, in which the extra-fr
amework species are re-inserted into the framework.