Modification of the adsorption and catalytic properties of molecular sieves by reaction with organometallic complexes

Molecular sieves can be modified by organometallic complexes using two diff erent methods (i) OMCVD (organometallic chemical vapor deposition), where a multilayered organometallic fragment is calcined and transformed into an o xide of rather indefinite composition, and (ii) SOMC (surface organometalli c chemistry), where an organometallic fragment is grafted with retention of its molecular integrity. Both methods may lead to a grafting, which can oc cur at the external surface and/or inside the channels of the zeolites. In the case of SOMC, by judicious choice of both the reaction conditions and t he organometallic complexes, it is possible to modify only the external sur face of the sieve, for example, of a mordenite. This can lead to a decrease of the surface acidity, allowing greater selectivity in catalytic reaction s (such as xylene isomerization). Another application is the modification o f the pore openings of the sieves using Molecular complexes channels by gra fting organometallic compounds near the pore entrances. This results in a m odification of the adsorption characteristics of the molecular sieve, allow ing a fine tuning of its properties. Depending on the size of the adsorbed molecule, two effects can be observed: (i) a total exclusion of the molecul e from the channels of the molecular sieve, due to steric effects ("thermod ynamic" control); (ii) a modification of the adsorption kinetics of the ads orbed molecule, depending greatly on its size and on the size of the organo metallic fragment ("kinetic" control). Finally, the grafting reaction can, in some cases, occur inside the channels, thereby leading to effects simila r to those described above and to a reduction of the adsorption capacity du e to the presence of the grafted organometallic moieties within the pores.