SYNTHESIS OF ALKALI-FREE MFI BOROSILICATES FROM METHYLAMINE-SICL4 MEDIA

New synthesis routes leading to a rapid crystallization of alkali-free MFI type borosilicates from TPA-methylamine media have been thoroughl y explored. When fumed silica was used as Si source, pure MFI borosili cate crystals were obtained rapidly, They stay hydrothermally stable f or 20 days at 185 degrees C. The evaluation of framework boron by B-11 -NMR along the growth process suggests a liquid phase transportation t ype mechanism. Crystallization using SiCl4 as Si source is much slower , the hydrolysis of the chloride in methylamine being the rate limitin g step for the nucleation to start. Appropriate amounts of boron speci es 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 th e remaining soluble Si-B oligomers, yield crystalline MFI borosilicate s. (C) 1997 Elsevier Science B.V.