Ionic liquids (ILs) and immobilized ionic liquids were used as acid catalysts for the liquid phase alkylation of raffinate II and isobutane. The influences of reactant concentration (molar ratio of iC(4/)C(4)(=)), time, temperature and acid strength of the ionic liquids were studied. Using a step-up design under batch conditions with a very dilute mixture of isobutane and alkene, the conversion for a variety of ionic liquids was found to follow a pattern based on acid strength of the catalyst. Imidazolium based ILs were found to be superior to phosphonium based ILs. Novel Lewis-Acid Catalysts II (NLAC II, immobilization by grafting on siliceous MCM 41 or on silica FK 700) are better than other solid acid catalysts tested, such as SAC 13, zeolite H-Beta (Si/Al = 14) and NLAC I (impregnation of ILs on silica FK 700). Possible leaching of the ionic liquid from the catalyst surface was followed by ICP measurements of the catalyst after reaction and of the reaction mixture. (c) 2006 Elsevier B.V. All rights reserved.