PREDICTION OF PREFERRED PROTON LOCATIONS IN HMFI BENZENE COMPLEXES BYMOLECULAR MECHANICS CALCULATIONS - COMPARISON WITH NMR, STRUCTURAL AND CALORIMETRIC RESULTS

Sorbent/sorbate interactions in H.ZSM-5/benzene systems corresponding to both the monoclinic and the orthorhombic framework symmetries have been investigated by molecular mechanics calculations using the Buckin gham exp-6-1 model. By comparing the computer simulated energies with experimental calorimetric adsorption heat determinations, it is conclu ded that only some discrete protons attached to O1, O2, O5, O8, O11, O 18, O20, O21, O24 and/or O25 participate to the framework acidity. Mos t of these oxygen atoms are linked to the T2 or T1 2 sites, which have already been described as possible candidates for isomorphous Si/Al s ubstitution. The present work clearly establishes that the situation w here the protonic acid sites are spread over all the framework oxygen atoms should be rejected.