Synthesis of alkali-free MFI borosilicates from methylamine-SiCl4 media Original Research Article

New synthesis routes leading to a rapid crystallization of alkali-free MFI type borosilicates from TPA-methylamine media have been thoroughly explored. When fumed silica was used as Si source, pure MFI borosilicate crystals were obtained rapidly. They stay hydrothermally stable for 20 days at 185°C. The evaluation of framework boron by 11B-NMR along the growth process suggests a liquid phase transportation type mechanism. Crystallization using SiCl4 as Si source is much slower, the hydrolysis of the chloride in methylamine being the rate limiting step for the nucleation to start. Appropriate amounts of boron species markedly increase the crystallization, probably by forming readily intermediate BOSi oligomeric associations in the reaction gel. This latter partly undergoes a solid state restructuring and, along with the remaining soluble SiB oligomers, yield crystalline MFI borosilicates.