The speciation of M-Ox sites on high surface area metal oxides depends on the Brønsted acidity of surface -OH sites. For example, supporting an organometallic on SiO2 results in the formation of M-Ox sites however supporting an organometallic on sulfated zirconium oxide (SZO) results in the formation of [M][Ox] electrostatic ion-pairs, which can have obvious implications for catalytic activity of the well-defined sites. The acidity of -OH sites on SZO were suggested to show Brønsted superacid behavior because this material isomerizes n-butane at lower temperatures than neat H2SO4. Additionally, adsorption of colorimetric Hammett indicators suggested that -OH sites on SZO are approximately four orders of magnitude more acidic than neat H2SO4. In contrast, calorimetry studies and DFT calculations showed that the Brønsted sites on SZO are less acidic than zeolites. We studied the reaction of a series of di-tert-phenylphosphines with Brønsted sites on SZO to form [R3P][SZO]. These reactions cleanly form [R3PH][SZO], and do not form byproducts usually obtained when SZO is contacted with phosphines. Our results show that binding to SZO is at an equilibrium between [R3PH][SZO300] and free phosphine in MeCN slurry. The pKa of [R3PH] affects the equilibrium binding constant such that less acidic phosphoniums bind stronger to SZO300 than phosphines that form more acidic phosphoniums, which is typical of weak acid/base equilibria. Metalation of these [R3PH][SZO300] sites with Pd(0) organometallics results in catalysts resembling phosphine-sulfonate-palladium catalysts that are used for olefin polymerization reactions. The reactivity of this new heterogenous Palladium catalyst will be discussed.

Phosphines on sulfated zirconia: Probes for Brønsted acidity and precursors to catalytically active olefin polymerization sites

2019 Fall National Meeting

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.