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

Oligomerization of I-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 degrees C, 50 bar and using n-octane as solvent, over three catalysts: two uniformly rnesostructured 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. Oil 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, C-7 - C-8 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 similar to 20 - 25%, with a contribution of 6 - 8% at 400 - 500 degrees 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 C-13 - C-18). (c) 2007 Elsevier B.V. All rights reserved.