Studies on Mordenite by Monte Carlo Method Ⅲ. The Correlation between Al-Al Next-nearest Neighboring Distribution in Framework and its Acidity

Four types of lattice Al ions were defined for mordenite. Each type was distinguished by the present number （0,1,2,3） of other Al on its next-nearest-neighbors. We call the relative pop ulations of four types AlO₄^- the Al-Al nnn-distribution in the mordenite sample. The differences of the local environment of AlO₄^--, lead to the existence of four add strengthes: 0 ＞ 1 ＞ 2 ＞ 3.
In the present work, the definition of "acid site efficiency parameter" which scales the contribution of a type AlO₄^-- towards the acidity is introduced.
Based on this definition the efficienCy parameter α₀ introduced by Barthomeuf can be expressed quantitatively by the Al-Al nnn-distribution of the mordenite sample.
It has been emphasised before that the Al-Al nnn-distributions in parent and dealuminated mordenite samples are quite different. Both Al-Al nnn-distributions can be calculated by Month Carlo simulation. Since the strong-acid sites are preferentially removed, we predict theoretically that the acidity of the dealuminated mordenite is always weaker than the parent mordenite sample with the same Si/Al framework ratio. The Al content dependences of α₀ are quite different between parent and dealuminated mordenites. The theory explains the results of the ammonia TPD acidity measurement for the parent and dealuminated mordenite samples satisfactorily.