Free Phosphoric Acid of Diatomite-Phosphate Solid Acid and its Catalytic Performance for Propylene Oligomerization

Free phosphoric acid in the diatomite-phosphate solid acid catalysts was characterized by means of ion chromatography (IC), Hammett pH indicators and chemical titration. The results show that the free phosphoric acid is composed of 535 wt.% H₃PO₄, 17 wt.% H₄P₂O₇ and < 1 wt.% polyphosphoric acid, and its composition determines the acid strength distribution of the catalyst. When the total amount of free phosphoric acid containing >3 wt.% H₄P₂O₇ is within 9 26 wt.% (in P₂O₅), the catalyst shows higher catalytic activity for propylene oligomerization with propylene conversion >75wt.%. Besides, free phosphoric acid has an important effect on the catalytic selectivity. The relationship between them is discussed.