Hydroisomerization of N-decane in the presence of sulfur and nitrogen compounds Original Research Article

Isomerization of n-decane in the presence of 1000 ppm H2S and up to 770 ppm nitrogen (as tertiary butylamine (TBA)) was studied on bifunctional catalysts containing Pt supported on molecular sieves (MAPSO-31, SAPO-11, SAPO-34, SAPO-41, SAPO-5, MFI) or amorphous silica–alumina. Sulfur and nitrogen compounds selectively poison metal and acid catalyst active sites, affecting the acid–metal balance required for the high isomerization selectivity. It was demonstrated as to how to control n-paraffin isomerization selectivity in the presence of sulfur by the addition of TBA. The addition of up to 770 ppm TBA to feed containing 1000 ppm H2S suppressed catalyst activity by 100°C but improved isomerization selectivity. The isomerization selectivity of mildly acidic Pt catalysts (SAPO-11, SAPO-41, MAPSO-31) was close to that with no sulfur and TBA added. It was speculated that amine neutralizes strong acid sites, leading to low catalyst acidity and activity, while sulfur effectively poisoned the metal dehydrogenation function. To compensate for the activity loss, the reaction temperature has to be increased significantly. At this temperature, the weak and intermediate strength acid sites are active enough to provide high paraffin conversion. On the other hand, metal activity is significantly higher at elevated temperature and the perfect balance between the weak to intermediate acidity and strong metal function is established, resulting in high isomerization selectivity. The composition of the isomerized and cracked products supports the proposed mechanism.