Significant Parameters in the Synthesis of Large Alkaline-Free MFI-Type Zeolites and AFI-Type Aluminophosphates

Abstract

Factorial experiments were successfully employed to determine significant synthesis variables allowing the crystallization of large AlPO4-5 and ZSM-5 crystals. Growth of AFI-type aluminophoshate up to 500 µm in length was achieved using a molar reaction composition of 0.5 Pr3N - Al2O3 - P2O5 - 300 H2O. Crystal size and yield were countercurrently affected by the water content during the synthesis . The alkaline-free crystallization of 300 µm uniform MFI-type zeolites at high conversion was possible in a batch of 6.5 (TPA)2O - 282 (NH4)2O - 96 SiO2 - 1400 H2O. Decreasing water content and increasing ammonia concentration improved both crystal size and yield. Length growth rates reached 3.0 ± 0.6 µm/h. Increasing the aluminium content caused a decrease in the size and yield of ZSM-5 after four days along with a reduction in the crystal aspect ratio compared to aluminium-free runs. Preliminary results with alkaline-free preparations of DOH-,DDR-, MTN-, and MEL-type zeolites indicate, that large crystals with sharp particle size distributions are frequently observed in ammonia-based zeolite synthesis processes.