Butene oligomerization by phosphoric acid catalysis: Separating the effects of temperature and catalyst hydration on product selectivity

Solid phosphoric acid (SPA) catalysts find commercial application in olefin oligomerization and in the alkylation of aromatics with olefins. The composition of strong phosphoric acids, the hydrolysis rate of different phosphoric acid species, and the effect of temperature and hydration (composition of phosphoric acid) on butene oligomerization, as well as two case studies to show these effects on commercial operation in a Fischer-Tropsch refinery, are discussed. It was found that the hydrolysis rate of higher phosphoric acid species is much faster than that of ortho-, pyro- and triphosphoric acids (P1-P3). This implies high hydration sensitivity for SPA catalysts when operated at low hydration levels. The initial product of hydrolysis is P3, and not P1 as previously suggested, indicating additional stability of the triphosphoric acid species, possibly due to intramolecular hydrogen bonding to create a P3 cage structure. Both temperature and the degree of catalyst hydration influence catalysis. The highest yield of the best quality hydrogenated motor gasoline (highest degree of branching) was obtained when operating in a "cold and wet" mode. Decreasing catalyst hydration, irrespective of temperature, increased distillate yield. No benefit was found for "hot and wet" operation. The operating region covered by the study is 150-250 degrees C and 100-115% H3PO4.