Activation of Solid Phosphoric Acid by Heat Treatment

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Abstract
Solid phosphoric acid is a calcined mixture of kieselguhr and phosphoric acid. The acid components on the finished catalyst surface are ortho-, pyro- and metaphosphoric acids.As is well known, pyrophosphoric acid is the most active catalyst, while orthophosphoric acid is fairly active and metaphosphoric acid almost inactive. Hence, it may be said that the aim of the heat treatment of the catalyst is to obtain the maximum conversion of orthophosphoric acid into pyrophosphoric acid on the catalyst surface.The authors observed the change in the acid composition on the catalyst surface in the course of heat treatment with special reference to its influence on the catalytic activity.The adhered phosphoric acid was successfully extracted from the carrier skeleton without changing its acid composition and analyzed according to the Wazer's pH titration method. The relationship between the catalytic activity and the acid composition of the solid phosphoric acids was fully discussed based on the analytical data of the extracted acid.The authors also investigated the properties of carrier kieselguhrs. Generally speaking, kieselguhrs having small bulk density and large micropore volume gave excellent catalysts by mild heat treatment. And the stability of their siliceous skeleton to heat and chemicals was found to have an appreciable effect on the activity and mechanical strength of the finished catalysts.

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