Multiple adsorption process description of zeolite mordenite acidity

Sodium and protonic exchanged mordenite zeolites were characterized by XRPD, ICP-AES, TGA/DTA, Si-29 and Al-27 solid-state NMR, FTIR, DRIFTS, and Cal-Ad (calorimetric and adsorption studies of pyridine interaction with the solid in cyclohexane slurry). To describe the acidic site of \H\ [Al-Si-O]-MOR, a two site adsorption process showed the best results: 0.081 mmol g(-1) of Bronsted sites (DeltaH(1) = -82.9 kJ mol(-1)) and 0.422 mmol g(-1) of Lewis sites (DeltaH(2) = -36.7 kJ mol(-1)). These results were confirmed by FTIR experiments. In the protonic sample, only 18% of tetrahedral Al were assigned as Bronsted sites, while 56% of the sites (Bronsted and Lewis) are located in the main channels. Si-29 CP/MAS NMR spectra showed the presence of silanol groups and Al-27 MAS NMR spectra showed the presence of non-framework penta-coordinated Al species and/or non-framework amorphous silica-alumina. These species probably caused an overestimation of framework Si/Al calculation by Si-29 MAS NMR. The framework Si/Al ratio was calculated using ICP-AES and Al-27 MAS NMR, and the dealumination process was observed by both Si-29 CP/MAS and Al-27 MAS NMR. (C) 2004 Elsevier Inc. All rights reserved.