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 ch aracterized by means of ion chromatography (IC), Hammett pH indicators and chemical titration. The results show that the free phosphoric acid is compo sed of 5 similar to 35 wt.% H3PO4, 1 similar to7 wt.% H4P2O7 and < 1wt.% po lyphosphoric acid, and its composition determines the acid strength distrib ution of the catalyst. When the total amount of free phosphoric acid contai ning >3 wt.% H4P2O7 is within 9 similar to 26 wt.% (in P2O5), the catalyst shows higher catalytic activity for propylene oligomerization with propylen e conversion >75wt.%. Besides, free phosphoric acid has an important effect on the catalytic selectivity. The relationship between them is discussed.