Generation of Phosphonium Sites on Sulfated Zirconium Oxide: Relationship to Brønsted Acid Strength of Surface −OH Sites

The reaction of (tBu)2ArP (1a-h), where the para position of the Ar group contains electron-donating or electron-withdrawing groups, with sulfated zirconium oxide partially dehydroxylated at 300 °C (SZO300) forms [(tBu)2ArPH][SZO300] (2a-h). The equilibrium binding constants of 1a-h to SZO300 are related to the p Ka of [(tBu)2ArPH]; R3P that form less acidic phosphoniums (high p Ka values) bind stronger to SZO300 than R3P that form more acidic phosphoniums (low p Ka values). These studies show that Brønsted acid sites on the surface of SZO300 are not superacidic.