Recent Survey of Catalysis by Solid Metal Sulfates

The relationships between the surface properties of ionic salts, in particular metal sulfates, and their catalytic activities have intrigued us for some time [l]. They offer a useful model for linking homogeneous to heterogeneous catalysis. A metal sulfate, for example NiSO4·7H2O, develops a significant degree of acidity-and hence catalytic activity-only after heating causes deaquation. In 1965, on the basis of the physical and chemical characterization of changes accompanying such a process, the mechanism of surface activation was rationalized in terms of an intermediate, mestastable phase formed during the transition from one crystal modification to another [29]. During this change the nickel system exhibits a maximum acidity at an acid strength corresponding to that of 50% H2SO4 about half way between the monohydrate and anhydrous form. These acidic sites, attributed to vacant coordination sites on the above intermmediate crystal lattice, are responsible for the catalytic activity of solid metal sulfates.