NH3 Adsorbed on All-Silica MFI Zeolites Silicalite

Defective Silicalite is an all-silica MFI zeolite, which is non-hydrophobic and weakly acidic as a consequence of the abundant polar defects (Si-OH nests) generated by the stmcture to compensate the Si atoms vacancies [7]. IR spectra in the vqh stretching frequency region give a clear evidence of the presence of Si-OH nests, which are characterized by a different geometrical arrangement according to the synthesis procedure and/or post-synthesis treatments, as illustrated in ref. [36]. 

The pair of Avqh and (-Aa//)o values measured for one of the investigated MFI-Silicalite samples (Sil-B, see Chap. 1 Fig. 1.15b) was found to deviate from the linear correlation illustrated in the plot. The zero-coverage adsorption enthalpy for Sil-B 

Also in this case, the apparently anomalous behavior of one sample was interpreted as an indication of the presence of phenomena others than a plain adsorption. In fact, the adsorption of NH3 on Sil-B was demonstrated to imply an endothermic step which lowered the measured heat value. 

Also in this case, the accurate interpretation at molecular detail of the volumetric-calorimetric results was achieved thanks to the joint use of microcalorimetry, IR spectroscopy and ab initio calculations. 

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