The Alkylation of 2-Butene with Isobutane over Large-Pore Zeolites

Abstract

The growing demand for higher quality gasoline coupled with stringent environmental regulations necessitates the reduction in aromatics, sulfur and nitrogen compounds. The alkylation process utilises low value olefinic gases produced in the refinery with isobutane to produce high octane gasoline that is free from aromatics, sulfur, and nitrogen compounds. Current alkylation technologies utilise hydrofluoric or sulfuric acids as catalysts for the reaction. Both acids however present health and environmental concerns along with high operational and mitigation costs. Zeolites are an ideal alternative to both acids due to their benign nature and acidic properties. Catalyst deactivation remains a major challenge for a zeolite-based alkylation process. Using a well-mixed reactor under ideal reaction conditions and relatively high flow rates, the alkylation of 2-butene with isobutane was studied over large pore zeolites. The effect of zeolite structure, of manipulating the acidity of the zeolite and of diluting the zeolite with additives on activity, stability and alkylate selectivity were examined. Examining the effect of the zeolite structure indicated that a three-dimensional channel system was required for the reaction. Zeolites Beta and Y exhibited higher stability and selectivity while MOR deactivated rapidly. The rapid deactivation in MOR was attributed to the lack of intersecting side-channels which prevented the bulky molecules from escaping the pores thus causing faster pore blockages. Varying the Si/Al ratio for zeolite Beta (12.5, 75, and 150) indicated that high aluminium content was required for the reaction. The siliceous Beta zeolites deactivated rapidly and mainly produced oligomerisation products. Lanthanum impregnation on the lowest Si/Al Beta zeolite reduced the Bronsted acidity and the impregnation procedure created Lewis acidity, both effects combined to reduce the activity and selectivity of the zeolite. The Si/Al ratio was varied for zeolite Y ranging from 2.6 to 30. The optimum ratio was the lowest, 2.6. Increasing the Si/Al ratio decreased the acidic sites available for the reaction and reduced the alkylate selectivity. Lanthanum exchange was shown to significantly improve the catalytic stability due to the reduction in the strong Lewis acidity. The optimum number of exchanges was three followed by calcination. This procedure created the highest ratio of strong Bronsted/strong Lewis acidity and enhanced the stability of the catalyst. Diluting the Y zeolite provided the most significant improvement on the catalytic stability. Silica additives, and particularly MCM-41, were shown to reduce the oligomerisation and enhance the alkylate production. The improvement was attributed to the separation of the zeolite clusters which reduced the olefin concentration and promoted alkylation. The optimum additive, MCM-41, had the highest surface area. Increasing the additive content did not provide additional improvements as the initial dilution seemed the most significant. Finally, the optimum catalysts from each section were compared for their stability and alkylate selectivity.

Date of Award	1 Aug 2018
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Arthur Garforth (Supervisor) & Stuart Holmes (Supervisor)