Formation of Specific Metal Complexes in Nanometric Zeolite Pores

Formation of Specific Metal Complexes in Nanometric Zeolite Pores 

Ag dimers and trimers have been observed in MFl zeohtes [26]. Thermal as well as photochemical activation leads to charge transfer from the zeolite to the 

Iron nanoparticles are formed in Fe-MFI during calcination in O2, with very short interatomic Fe—Fe distances (2.53 A) as determined by EXAFS [27]. Although the soUd is not completely reduced, this distance is close to the nearest-neighbour distance in metaUic iron, and the stracture of the iron cluster in the zeohte could be similar to that of the metallic center of ferrodoxin (II). A specific infrared band has been detected for NO interacting with this iron complex, which is linked to the catalytic activity (rather than the isolated iron). 

Using a zeolite support rather than a non-microporous soUd like alumina can be very fruitful, and the same silver nanoclusters on alumina and H-MFI have very different activity. On zeolite, they catalyse NOx SCR by methane, but they lead to combustion of methane at temperatures as low as 300 °C [28]. 

The Si/Al ratio in zeolites controls acidobasic properties, and influences electronic distribution, and thus metal ions in the stracture. Cu in faujasite-type zeolites is present as monomers and dimers at Si/Al = 5.6, but when Si/Al reaches high values (13.9 and 390), only monomers are observed. Photocatalytic activity is therefore higher on the sample with Si/Al = 13.9, although its Cu content is much lower [29]. 

---