Ferrisilicate molecular sieve and use as a catalyst

Ferrisilicate molecular sieves of the ZSM-5 type, having SiO2/Fe2O3 mole ratios ranging from 20 to 400, are prepared by adding a silica source and a quaternary ammonium salt in that order to an acedified solution of an iron (III) compound, crystallizing the resulting gel to form a ferrisilicate molecular sieve, and thermally treating the molecular sieve with nitrogen, air and/or steam at 300 DEG to 700 DEG C. Preferred thermal treatment comprises treating with nitrogen first, then with air or steam. Thermally treated molecular sieves contain iron both in and outside the crystal framework; most of the non-framework iron is dispersed as very finely divided iron oxides on internal surfaces. Molecular sieves are useful as catalysts in Fischer-Tropsch and other iron oxide-catalyzied reactions.