2,6-Di-<i>tert-</i>butylpyridine
Sorption Approach to Quantify the External
Acidity in Hierarchical Zeolites
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Abstract
This work was aimed to evidence that
substituted pyridine, 2,6-di-<i>tert</i>-butylpyridine,
is a suitable probe for the quantitative
investigation of the external acidity in hierarchically structured
zeolites. The 2,6-di-<i>tert</i>-butylpyridine was too large
to enter the micropores, even in wide pore zeolites, and nearly no
sites in nonmesoporous zeolites were available. Accessibility studies
of acid sites in zeolites TNU-9 and BEA involving quantitative IR
measurements with hindered 2,6-di-<i>tert</i>-butylpyridine
as a probe were performed. The extinction coefficients of the 1615
cm<sup>–1</sup> diagnostic bands of 2,6-di-<i>tert</i>-butylpyridine interacting with Brønsted acid sites were determined.
Lewis acid sites were not detected with the probe. The accessibility
factor (AF) for the 2,6-di-<i>tert</i>-butylpyridine probe
molecule was defined as the number of sites detected by adsorption
of the dTBPy (external sites) divided by the total amount of acid
sites in the studied zeolites as quantified by pyridine sorption.
Upon desilication resulting in the fabrication of the secondary mesopores,
the enhanced accessibility of the protonic sites was observed. In
comparison to the mesoporous zeolites with the secondary system of
mesopores generated by alkaline leaching, considerably higher accessibility
of protonic sites was evidenced in both ultrathin ZSM-5 and delaminated
ITQ-2 zeolite