The ion-exchange and catalytic properties of titanium phosphates and mixed titanium-zirconium phosphates

Abstract

[alpha]-Titanium phosphate, Ti(HPO4)2 * H2O, has layered structure isomorphous with [alpha]-zirconium phosphate, Zr(HPO4)2 * H2O. Thus, solid solutions of the two compounds were prepared and their properties correlated with reference to the pure analogues. These crystalline titanium-zirconium phosphates of composition 20, 33, and 80 mole 5 titanium were characterized from two standpoints; namely: 1.) ion-exchange behavior and 2.) catalytic properties. Their behavior towards Na+-H+ exchange was determined and interpreted on the basis of the known behavior of the pure phases. It was found that those high in zirconium behave as zirconium phosphates and exhibit relatively low hydrolysis of phosphate groups while those high in titanium behave like titanium phosphate in many respects. This analogy was also observed in their catalytic properties, i.e., surface area, acidity and activity in the dehydration of cyclohexanol. The dehydration reaction was found to be first order in alcohol concentration and to increase in rate with increase of pretreatment temperature. This was correlated to the finding that two types of acid sites are responsible for the catalytic activities of these compounds: 1.) a Brönsted acid site and 2.) a Lewis acid site. The former plays a predominant role at the lower temperatures while the latter is operative at the higher temperatures. Titanium phosphates of varying crystallinities were prepared and were likewise characterized for their catalytic properties. They were found to be more active catalysts in the dehydration of cyclohexanol than the zirconium phosphates.