Aldol condensations of carbonyl-containing molecules form an intermolecular C-C bond and remove O-atoms as H2O. Such reactions occur on solid Bronsted acids, but with low selectivity and significant deactivation because of the formation of unsaturated species that undergo subsequent reactions. These acid catalysts, specifically MFI, BEA, and FAU, become selective and stable in condensation of C3- and C4-alkanones when Pt is present as physical mixtures of supported catalysts or ion-exchanges; in the presence of H2, metal sites selectively hydrogenate C=C bonds in a,�-unsaturated intermediates to a stable saturated alkanone. With a stable catalyst, a rigorous mechanistic study for aldol condensation was performed. Aldol condensation occurs on Br�nsted acid sites as determined by in-situ titrations with 2,6-di-tert-butyl-pyridine, which suppresses the rate irreversibly to zero. The reaction mechanism is similar to what is reported on metal oxides, where the acid site, here a proton, stabilizes the reactant oxygenate by H-bonding the oxygen and a basic site, the adjacent oxygen, abstracts the a-H to form a reactive enol species. The H-bonded oxygenate is the resting state of the catalyst as observed by infrared spectroscopy at reaction conditions (0.1 to 10 kPa oxygenate pressure, 473 K). The condensation rate is first order in oxygenate pressure, which is consistent with a transition state containing two oxygenate molecules forming a C-C bond when the surface is covered with the H-bonded oxygenate. Absence of a kinetic isotope effect (1.0, acetone-D6) and DFT cluster calculations support that C-C bond formation is the kinetically relevant step. Dependencies of the reaction on acid strength and confinement have been investigated by using isomorphously substituted MFI samples (Al, Fe, Ga, B, Si) and by using a range of zeolite frameworks of varying void size, respectively.

Selective C-C bond formation and oxygen removal from oxygenates on solid Br�nsted Acid catalysts

248th National Meeting (2014)

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