MOLECULAR MECHANICS STUDIES ON MFI TYPE ZEOLITES .1. EFFECT OF PARA-XYLENE ADSORPTION ON THE ZEOLITE STRUCTURE

The effect of the adsorption of p-xylene on the all-silica MFI-structure is simulated with our molecular mechanics model. From experimental measurements it is known that the symmetry of the framework changes from monoclinic (space group P2(1)/n.1.1.) towards orthorhombic (space group P2(1)2(1)2(1)) upon adsorption of p-xylene up to eight molecules per unit cell. The experimentally observed change in the geometry of the zeolite upon p-xylene adsorption is compared with that calculated by molecular mechanics. For the calculations our force field for all-silica zeolites is used in combination with the MM3 force field for organic molecules. Parameters for the organic-zeolite interactions are obtained by combination of the two force fields.