Wairakei geothermal silica; a low cost reagent for the synthesis of low, intermediate- and high-silica zeolites

A precipitated silica, produced as a waste product from the Wairakei geothe rmal field and power station in the central North Island of New Zealand, ha s been found to be active in the hydrothermal synthesis of a range of zeoli te molecular sieves of various structure types: Sodalite (SOD), Linde type A (LTA), Linde type Y (FAU), Mordenite (MOR), ZSM-5 (MFI) and Beta (*BEA). The natural silica reagent contains reactive aluminium (Al) and iron (Fe) i mpurities that can he either included or excluded from the zeolite framewor ks depending on the synthesis conditions. High alkalinity syntheses (SOD, L TA, FAU, MOR) tend to include Al, while Fe is excluded from the zeolite lat tices. The crystalline zeolite products that result are white to elf-white in colour. Lower alkalinity syntheses that require organic structure-direct ing agents (SDA), zeolites MFI and *BEA, include less Al but higher amounts of Fe, producing off-white to pale brown products. Both the crystallinitie s and compositions of zeolites MOR, MFI and *BEA are modified by rotation o f the autoclave during synthesis. (C) 2001 Elsevier Science B.V. All rights reserved.