Synthesis of boron-beta zeolite from near-neutral fluoride-containing gels

A borosilicate beta zeolite was prepared from almost neutral fluoride aqueous gels in the presence of 1,4-diazabicyclo[2.2.2]octane and methylamine as the templates. The influence of several physical and chemical parameters was studied. In comparison with the (Si,Al) system, this (Si,B) system appears less ‘reactive’ with longer crystallization times. Moreover, the products obtained display a narrower range of compositions. Only boron-rich (Si/B=2) and fluoride-poor media (F/Si=2) led to well-crystallized samples of beta zeolite characterized by a Si/B molar ratio close to 14. The differences observed between the two systems might be related to a stronger complexation of boron by the fluoride anions. The crystallization of the boron zeolite beta occurs probably through condensation reactions after hydrolysis of fluoroborate (BF₄⁻), hydroxyfluoroborate (BF₃OH⁻ and BF₂(OH)₂⁻) and fluorosilicate species. Zeolite beta was characterized by XRD, SEM, chemical analysis, thermal analysis and ¹¹B, ¹⁹F and ¹³C MAS NMR spectroscopy. The latter technique shows that the as-made beta samples contain, beside the dabconium cation, a polymeric compound identified as polyethylene piperazine.