Heterogenous catalysts are preferred for industrial applications because they are generally more cost effective, have higher stabilities, and are easier to separate from reaction mixtures than homogeneous catalysts.&lt;span style="font-size:10.8333px"&gt; &lt;/span&gt;Reactions of organometallics with sulfated metal oxides (SMO) form electrostatic ion pairs, and these well-defined sites are active in catalytic reactions. We recently showed that sulfated zirconium oxide (SZO) also supports the formation of [R3Si][SZO] sites that activate C-F bonds. These results indicate that these sites are very strong Lewis acids and that SZO acts as a weakly coordinating anion. In general, the degree of ion pairing correlates with the strength of the weakly coordinating anion’s conjugate acid; as the strength of the strong Brønsted acid increases the coordination strength of the weakly coordinating anion decreases. Using this concept, we sought to increase the acidity of an oxide material to access more weakly coordinating surface than SZO. In zeolites and mixed oxides Brønsted acidity originates from Si-O(H)-Al sites. This presentation will show reactions of partially dehydroxylated silica with strong Lewis acid aluminum alkoxides to form well-defined Si-O(H)-Al sites. These bridging silanol sites are strongly acidic and protonate aromatic Hammett indicators that do not react with zeolites, indicating that this material is more acidic than zeolites. Indeed, the Si-O(H)-Al sites in this well-defined Brønsted acid protonate 2,4-dinitrofluorobenzene, a Hammett indicator that is only protonated by acids ~2 orders of magnitude stronger than H2SO4. This material initiates oligomerization of propene at 50 °C, temperatures far lower than needed for zeolites. Functionalization of these surfaces will be presented.

Formation of well–defined strong acid sites on oxides

2019 Fall National Meeting

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