MORDENITE ACIDITY - DEPENDENCE ON THE SI/AL RATIO AND THE FRAMEWORK ALUMINUM TOPOLOGY .2. ACIDITY INVESTIGATIONS

The acidity of a series of mordenites dealuminated by acid refluxing was studied by calorimetric measurements of the NH3 chemisorption, protein magic angle spinning nuclear magnetic resonance spectroscopy, and temperature-programmed desorption of NH3 and NH4+ ion exchange. The number of bridged OH groups determined by H-1 MAS NMR corresponds to the number of tetrahedrally coordinated Al atoms. Starting with a low Al content (a high Si/Al ratio), the number of the strong acidic sites increases with increasing Al number of the framework up to 4.6 Al atoms/u.c. (Si/Al ratio of 9.5) and then decreases. The decrease of the strong acidity above 4.6 Al atoms/u.c. may be explained by the appearance of an aluminum atom in the second coordination sphere of the Si-OH-Al group. The experimentally found value of the maximum (at m = 0.096) in the curve of the strong acidity versus the aluminum molar fraction coincides with a value calculated by Barthomeuf from the topological Al density of mordenite, thus confirming her theoretical concept of zeolite acidity.