Silicalite Characterization. Part II: IR Spectroscopy of the Interaction of CO with Internal and External Hydroxyl Groups

In paper 1 we have shown that the structural properties of silicalites depend upon the preparation procedure. In particular Na- and Al-free sample (S) prepared following a specifically designed method is characterized by the presence of internal atomic-dimension defects (nanodefects and microcavities). On the basis of spectroscopic data and computer graphic simulations, it is shown that these microcavities derive from one or more missing [SiO₄] units and can contain up to four OH groups per missing tetrahedron. These OH groups form chains of hydrogen-bonded species which, upon water elimination at high temperature, give Si-O-Si distorted bridges characterized by peculiar vibrational features. It is also shown that internal and external OH groups have distinguishable IR manifestations. Terminal and isolated hydroxyl groups located in internal positions give 1:1 hydrogen-bonded adducts with CO at 77 K, which are associated with red and blue shifts of the OH and CO stretching frequencies, respectively. External OH groups can be perturbed only by dosing CO at more elevated pressures.