Diesel-selective hydrocracking of an iron-based Fischer-Tropsch wax fraction (C15-C45) using a MoO3-modified noble metal catalyst

Iron-based low-temperature Fischer-Tropsch wax, in the carbon number range of C-15-C-45, was hydrocracked using an unsulfided Pt/SiO2-Al2O3 catalyst, modified with MoO3. The selective production of diesel having a low cloud point (-18 degrees C) combined with a high cetane number (71) was achieved. A bench-scale trickle-bed reactor was used to study the effect of operating conditions on product selectivities and yield. The effect of pressure and hydrocarbon chain length on reactivity and diesel properties was studied in particular. Low-pressure operation (3.5 MPa) improved the isoparaffin to n-paraffin ratio for the carbon number range C-15-C-22 but did not affect the degree of isomerization significantly after C-22. The higher pressure of 7.0 MPa suppressed the isomerization and the C23+ percent conversion and improved the diesel selectivity by inhibiting secondary cracking. Increasing the pressure resulted in an overall paraffin reactivity decrease. A maximum in paraffin reactivity at C-33 was observed at low-pressure operation, while high-pressure operation resulted in two maxima at C-21 and C-33. The observed unexpected paraffin reactivity pattern was the consequence of competing concentration and reactivity effects.