Glasses of Alkaline Earth Metaphosphates as Acid Catalysts: A Comparative Study with Crystalline Catalysts.

Abstract Surface properties of Mg-, Ca-, and Ba-metaphosphate glasses have been compared with those of their crystalline catalysts by investigating changes in their surface acidity and catalytic activity for 2-propanol dehydration resulting from their crystallization. All of the glassy samples contain two kinds of acid sites with different acid strengths, both of which are due to surface POH groups. The number of strong acid sites which are responsible for dehydration activity decreases significantly upon crystallization, while the number of weak acid sites remains almost constant. This reduction of surface acidity results from a solid-state condensation reaction of POH groups which always accompanies their crystallization. Thus, for the metaphosphates, glass formation is effective for increasing surface acidity. This increase is probably due to the characteristic structure of glasses.