SIEVING PROPERTIES OF ZEOLITES FOR C-6-C-8 HYDROCARBONS - THE EFFECTSOF A TRIMETHYLTIN COMPLEX GRAFTED ON THE PORE EDGE

To determine in which way an organometallic Sn(Me)(3) group, grafted o n the pore edge of a model dealuminated H-mordenite zeolite, changes t he diffusion process of two selected hydrocarbons (n-hexane and isooct ane) into the porous material, free-energy contour maps of the involve d potential of mean force have been determined on the basis of Monte C arlo simulations using statistical perturbation theory method. Their a nalysis lends us to conclude that the penetration of the hydrocarbons inside the zeolite is strongly modified by the grafting. The awaited e ffective pore-size reduction is confirmed, bur the most striking featu re is the appearance of an attractive potential well once the molecule s have penetrated into the zeolite pore (-80 +/- 15 kJ mol(-1) for rt- hexane. -180 +/- 30 kJ mol(-1) for isooctane). This may accelerate the initial diffusion of the hydrocarbons toward the solid but also may t rap the first incoming molecules and thus reduce the whale diffusion p rocess. In the case of n-hexane, no enhancement of the energy barrier involved in the penetration of the molecule within the solid is found (3 kJ mol(-1)), while for isooctane, a real change occurs (+60 kJ mol( -1)). A detailed analysis of the conformational properties of both the grafted group and the hydrocarbon molecules along the diffusion path is reported. As in the case of the plain H-mordenite, the flexibility of the penetrating molecules appears to play a decisive role.