A trialkylphosphonium haloaluminate compound having a formula: where R1, R2, and R3 are the same or different and each is independently selected from C1 to C8 hydrocarbyl; and X is selected from F, Cl, Br, I, or combinations thereof is described. An ionic liquid catalyst composition incorporating the trialkylphosphonium haloaluminate compound, methods of making the trialkylphosphonium haloaluminate compound, and alkylation processes incorporating the trialkylphosphonium haloaluminate compound are also described.