Use of short chain alkylamine as complexing-mobilizing agents: An alternative to the "fluoride route" for the synthesis of zeolite metallosilicate catalysts

A wide series of zeolite metallosilicates was successfully synthesized using a series of short alkyl chain amines as new potential mineralizing agents. In the particular case of ZSM-5 containing various framework heteroatoms, the mobilizing-complexing role of methylamine, as compared to that of fluoride ions, can be very different and basically depends on the nature of the metallic ion. The relative strength of the various fluoro or amino complexes could be exploited to monitor and control the selective (simultaneous or successive) incorporation of various metallic species in the MFI framework. The incorporation, occurring in variable amounts, can be arbitrary considered as total (i.e. with more than 2 ionic species incorporated per MFI unit cell) for Zn2+, Al3+, Ga3+, Fe3+, B3+ and Ti4+. Cd2+ and Cr3+ care only partly incorporated (M/u.c. < 2) while Cu2+, Co2+) Mn2+ and Mo5+ never incorporate the MFI lattice. Besides the fact that these ions do not readily achieve a tetrahedral coordination. the other possible reason could be that their polycondensation, if any possible, does not occur when these species are involved in too strong complexes with methylamine. In some cases (Cu2+, Fe2+, Ag+), methylamine behaves as a reducing agent in the synthesis medium and the corresponding metallic species are found admired with silicalite. At similar complexing power, the (alkali-free) "amine route" is preferred to the now more conventional "fluoride route", that involves lower pH and less supersaturated solutions, requiring very long crystallization times. These latter can also be drastically reduced by using fluoride-amine admixtures.