Copper exchanged Silicalite-1: evidence of the location of copper oxide nanoclusters in the supermicropores of S-1

摘要

In the present contribution we report some results of a study concerning the texture features of the silicalite S-1 (which is a SiO2 polymorph having the zeolite MFI framework-type) as prepared and after the introduction of copper (1.38 % wt). Copper is loaded by an ion-exchange that is made possible by the internal silanol nests of the defective structure. The porosimetric analysis is able to reveal two important characteristics of these materials. It provides evidence about the presence (i) of supermicropores that are created by Si(IV) vacancies in the MFI framework, and (ii) copper oxide entrapped in these cavities as nanoclusters. The (CuO)_n nanoclusters are characterized by a low nuclearity (n). This is suggested by different techniques as X-ray powder diffraction (XRPD), UV-Vis diffuse reflectance spectroscopy (UV-Vis DRS) and magnetic susceptibility measurements. Indeed, if compared to the parent material, the copper-loaded silicalite shows a sharp reduction of the low-pressure hysteresis loop associated to a decreasing of both the total (BET) surface area and the micropore volume, all these findings supporting the picture that the copper oxide nanoclusters are confined in the supermicropores of the S-1 material.