The acidity of a series of commercial dealuminated H−Y zeolites
was studied by means of MAS NMR and
XPS spectroscopy. 27Al MAS NMR has shown the
presence of three types of Al in the dealuminated
samples: framework tetrahedral Al, nonframework octahedral Al, and a
third type of Al. As inferred from
the framework Si/Al ratio obtained by 29Si MAS NMR
spectra and from the evolution of the unit cell parameter
determined by XRD, this third type of Al may be localized into the
zeolite framework. XPS characterization
of the samples in their acid form has allowed us to establish the
relative concentration of Al at the outer
surface of the particles. Upon adsorption of ammonia on the
dealuminated zeolites, three types of acid sites
have been evidenced by XPS. The analysis of the XPS data indicates
that the strength of the acid sites
associated with tetrahedral framework Al atoms is not influenced by the
other types of Al. The catalytic
results of heptane and decane hydroisomerization have evidenced that
the acid sites of certain dealuminated
H−Y zeolites may be very efficient. The information provided by
MAS NMR, combined with the XPS
results and literature data, has allowed us to partially elucidate the
origin of the high activity of some
dealuminated H−Y zeolites.
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