Preferential Siting of Bridging Hydroxyls and Their Different Acid Strengths in the Two-Channel System of MCM-22 Zeolite

The stretching frequency of the bridging hydroxyls in MCM-22 zeolite has been calculated by atomistic simulation techniques. In this way our model was able to reproduce the two infrared bands corresponding to bridging hydroxyls detected experimentally in MCM-22. It is shown that the OH stretching frequency correlates with the modulus of the electric field at the proton site, thus showing the influence of both short- and long-range factors on acidity. The relative substitution energy to form the different acid centers was calculated, and a span of 40 kJ/mol was found between the sites, which indicates that preferential siting is expected. The acidity distribution of the MCM-22 structure is presented showing the strength and location of the individual acid sites. This provides an understanding of the overall acidity of this zeolite as well as the acid features of each void system: the large pore supercage system and the medium pore sinusoidal system. A stronger acidity is predicted for the centers located in the supercages with respect to those in the sinusoidal chanels.