The role of pH in the performance of precipitated iron Fischer-Tropsch catalysts

Precipitated iron Fischer-Tropsch catalysts were prepared at various constant pH levels with a Kölbel in-line mixing unit using either sodium carbonate or ammonium hydroxide as the precipitant. In microreactor studies at 1.38 MPa and 548 K, the Na 2CO 3-precipitated catalysts prepared at low pH (3.7 and 4.7) showed unusually high olefin selectivity (C 2C 4) as well as activity maintenance superior to that of the catalysts prepared at higher pH (5.8, 7.6, and 9.8). While the NH 4OH-precipitated catalyst showed lower olefin selectivity, the activities of these catalysts, especially the copper-promoted ones, were quite stable for periods of up to 300 h. Thermogravimetric studies indicated that graphitic carbon deposition occurred to a greater degree on Na 2CO 3-precipitated catalysts prepared at high pH. The high soda content of these catalysts was identified as one factor accounting for this deposition and presumably also accounted for the higher rates of deactivation. Those factors responsible for the high olefin selectivity of the Na 2CO 3-precipitated catalysts prepared at low pH remain unidentified.