Nitrogen and sulphur poisoning in alkene oligomerization over mesostructured aluminosilicates (Al-MTS, Al-MCM-41) and nanocrystalline n-HZM-5

Oligomerization of 1-hexene in the presence of model poisons such as thiophene (700–7000 ppm of sulphur) and n-butylamine (25–250 ppm of nitrogen), either alone or in combination, were tested at 200 °C, 50 bar and using n-octane as solvent, over three catalysts: two uniformly mesostructured silica–alumina (Al-MTS, Al-MCM-41) and a nanocrystalline HZSM-5 zeolite. A content of 700 ppm of sulphur (adding thiophene) or/and 25 ppm of nitrogen (adding n-butylamine) after a TOS = 240 min led towards roughly 10–20% decrease in conversion over nanocrystalline HZSM-5, without significant changes in selectivity. On the contrary, a feeding of 1-hexene with 7000 ppm of sulphur and 250 ppm of nitrogen showed a drastic drop of conversion (from 90 to 27%) over n-ZSM-5 zeolite with a significant increase in the selectivity towards lighter oligomers (dimers, C7–C8 isomers). This fact suggests that the strong acid sites of the zeolite are deactivated by poison adsorption and heavy oligomers/coke deposition both inside the micropores and over the external surface. In contrast, neither Al-MTS nor Al-MCM-41 catalysts were meaningfully affected by the poisons (especially Al-MTS), even for high concentration conditions, due to its high surface area, mesoporosity and medium acid strength distribution. TG analyses of the mesoporous catalysts indicate weight losses of ∼20–25%, with a contribution of 6–8% at 400–500 °C, assigned to the removal of deposited coke. Oligomerization of a FCC effluent under the same conditions over Al-MTS catalyst leads to a remarkable 58% conversion with a oligomer selectivity over 90% (32% of them C13–C18).

The oligomerization of 1-hexene in presence of model poisons such as thiophene and n-butylamine over mesostructured Al-MCM-41 and Al-MTS, and nanocrystalline n-HZSM-5 have been investigated. n-HZSM-5 was heavily deactivated by a concentrated poison mixture whereas Al-MTS and Al-MCM-41 retained almost completely their activity. Oligomerization of a true FCC effluent over Al-MTS led to 58% conversion with oligomer selectivity above 90% (32% C13–C18). Figure optionsDownload as PowerPoint slide