Ionic liquid-catalyzed alkylation of isobutane with 2-butene

Abstract

A detailed study of the alkylation of isobutane with 2-butene in ionic liquid media has been conducted using 1-alkyl-3-methylimidazolium halides-aluminum chloride encompassing various alkyl groups (butyl-, hexyl-, and octyl-) and halides (Cl, Br, and 1) on its cations and anions, respectively. The emphasis has been to delineate the role of both cations and anions in this reaction. The ionic liquids bearing a larger alkyl group on their cation ([C(8)mim]) displayed relatively higher activity than a smaller one ([C-6 or C(4)mim]) with the same anionic composition, due to the high solubility of reactants in the former. Among the ionic liquids with different halide groups, bromides ([C(8)mim]Br-AlCl3) showed outstanding activity, because of the higher inherent acidity relative to others. From the Al-27 NMR study, a major peak at similar to 99.5 ppm corresponding to [AlCl3Br](-) (similar to 99.5 ppm) was observed. Moreover, the anion showed a strong acidity based on FT-IR characterization; the largest peak related to acidity (1570 cm(-1)) was detected. Under various composition conditions, catalytic activity and amount of TMPs increased with concentration of anion. This is mainly attributed to a higher amount of strong acid ions [Al2Cl6Br](-) which can react with hydrogen atoms at the 2-position of an imidazolium ion to form Bronsted acid. However, the ionic liquid with strong acidity (X = 0.58) deactivated rapidly due to a higher sensitivity to moisture, causing decomposition. Under various reaction temperature conditions, optimum catalytic activity was observed at 80 degreesC. The result is also attributed to the effect of anion composition. The strong acidic anion increased with temperature. However, at higher reaction temperatures (120 degreesC), the ionic liquid showed a lower activity and TMP selectivity, since the solubility and Bronsted acid sites were reduced by decomposition of imidazolium ions. The selected ionic liquid sample ([C(8)mim]Br-AlCl3) was compared with one of the standard commercial catalysts, sulfuric acid. Under optimum experimental conditions, it was observed that both catalysts showed comparable catalytic behavior. However, ionic liquid showed higher activity, and lower TMP selectivity due to a more acidic nature and a lower amount of Bronsted acid sites, respectively. (C) 2004 Elsevier Inc. All rights reserved.