Review of Isobutane Alkylation Technology Using Ionic Liquid-Based Catalysts-Where Do We Stand?

Conventional strong liquid acids such as H2SO4 and HF are used for the majority of current commercial isobutane alkylation processes to produce motor fuel alkylates, but these acids can have significant safety and sustainability concerns. Ionic liquid (IL) catalyst technologies offer potential advantages over current processes due to the negligible vapor pressure and molecularly tunable properties that can optimize both the chemistry and engineering for alkylate production. In this review, IL-based catalysts used in isobutane alkylation are reviewed. ILs are categorized and discussed by type: (1) metal-based Lewis acidic ILs, (2) metal-based Bronsted-Lewis acidic ILs, (3) nonmetal-based Bronsted acidic ILs, and (4) immobilized/supported ILs. A critical perspective on the use of these ILs in alkylation is presented, focusing on the effect of speciation and physicochemical properties on chemical reaction. Further, a summary of IL speciation is provided and examples of how the tunability of ILs can be used to overcome current limitations in alkylation chemistry. The reaction conditions and performance (e.g., conversion, C-8 selectivity, and trimethylpentane:dimethylhexane ratio) of literature reports are summarized. A comparison of IL-based catalysts with the incumbent H2SO4 process and the new ISOALKY Chevron process are also discussed. Gaps in the literature (e.g., mass transfer rates, material compatibilities, and phase equilibrium) associated with IL-based alkylation technology and our perspectives on solving the relevant issues in this field are summarized.