PREDICTION OF PREFERRED PROTON LOCATIONS IN HMFI BENZENE COMPLEXES BY MOLECULAR 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 Buckingham exp-6-1 model. By comparing the computer simulated energies with experimental calorimetric adsorption heat determinations, it is concluded that only some discrete protons attached to O1, O2, O5, O8, O11, O18, O20, O21, O24 and/or O25 participate to the framework acidity. Most 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 substitution. The present work clearly establishes that the situation where the protonic acid sites are spread over all the framework oxygen atoms should be rejected.