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 200ºC, 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 and medium acid strength distribution. TG analyses of the mesoporous catalysts indicate weight losses of ~ 20-25%, with a contribution of 6 to 8% at 400 - 500ºC, 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).
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