Synthesis of MFI metallosilicate zeolites using metallic amino complexes as mineralizing agents: an overview

The present overview, essentially dealing with the authors' recent laboratory work, consists of a critical evaluation of a series of new synthesis pathways leading to efficient formation and stabilization of several industrially important framework metal substituted MFI zeolites. More precisely, we critically discuss the potentialities and limitations of short-chain alkylamines as mobilizing-complexing agents for use as mineralizers for a variety of metallic ions in zeolite syntheses. A wide range of MFI zeolites containing various framework heteroatoms could be prepared in the presence of short-chain alkylamines. In particular, the mobilizing-complexing role of methylamine was investigated in more depth. The mineralizing power of methylamine can be very different from that of fluoride ions and basically depends on the nature of the metallic ion. The relative strength of the various amino complexes could be exploited to monitor and control the selective (simultaneous or successive) incorporation of various metallic species in the MFI framework. More than two heteroatoms were found to be incorporated per MFI unit cell in the case of Zn, Al, Ga, Fe, B, Ge and Ti. Cd2+ and Cr3+ ions only partly incorporate the MFI lattice (Mn+ per unit cell<2). Copper, nickel, cobalt, manganese and molybdenum were found essentially admired within the silicalite crystals, but they can be also partly anchored to the A1-MFI (negative) lattice as (strong) amino complex cationic species. At a similar complexing power, the (alkali-free) 'amine route' is preferred to the more conventional 'fluoride route', which involves a lower pH and less-supersaturated solutions, requiring very long crystallization times. (C) 1999 Elsevier Science B.V. All rights reserved.