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 different methods: (i) OMCVD (organometallic chemical vapor deposition), where a multilayered organometallic fragment is calcined and transformed into an oxide of rather indefinite composition, and (ii) SOMC (surface organometallic chemistry), where an organometallic fragment is grafted with retention of its molecular integrity. Both methods may lead to a grafting, which can occur 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 the organometallic complexes, it is possible to modify only the external surface of the sieve, for example, of a mordenite. This can lead to a decrease of the surface acidity, allowing greater selectivity in catalytic reactions (such as xylene isomerization). Another application is the modification of the pore openings of the channels by grafting organometallic compounds near the pore entrances. This results in a modification of the adsorption characteristics of the molecular sieve, allowing 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 molecule from the channels of the molecular sieve, due to steric effects ('thermodynamic' control); (ii) a modification of the adsorption kinetics of the adsorbed molecule, depending greatly on its size and on the size of the organometallic fragment ('kinetic' control). Finally, the grafting reaction can, in some cases, occur inside the channels, thereby leading to effects similar to those described above and to a reduction of the adsorption capacity due to the presence of the grafted organometallic moieties within the pores.