Surface area and pore structure of mixed hydroxides and oxides of magnesium and iron

Abstract

Two mixed hydroxides of magnesium and iron were prepared, one by coprecipitation (Mixed Hydroxide I), and the other by mixing suspensions of the hydroxide precipitates (Mixed Hydroxide II). X-ray analysis and differential thermal analysis of the mixed hydroxides reveal that the individuality of the component hydroxides is substantially altered, and that a slow process of interaction seems to take place between ferric and magnesium hydroxides in the suspension, leading probably to the formation of a hydrated ferrite. When the mixed hydroxides are progressively heated they give rise to products which at first become amorphous and then gradually pass into the crystalline magnesium ferrite. Adsorption isotherms of cyclohexane, benzene, and nitrogen were measured on the mixed hydroxides and their products formed by dehydration at various temperatures between 200°C. and 1000°C. Specific surface areas calculated by the application of the BET theory to the adsorption of the three adsorbates are in good agreement and are higher than the surface areas of a mixture of the two hydroxides having the same composition, but they sinter more readily. Pore volume distributions calculated from desorption isotherms of cyclohexane and nitrogen are in good agreement in the case of Mixed Hydroxide II and its dehydration products, but they differ slightly for Mixed Hydroxide I and its dehydration products.