The Effect of Outer‐Sphere Acidity on Chemical Reactivity in a Synthetic Heterogeneous Base Catalyst

Abstract

Rational outer‐sphere design can be used to optimize heterogeneous catalysts. Thus, imprinting of bulk silica was used to prepare tethered active sites with hydrophilic (1) and hydrophobic environments (2). In the Knoevenagel condensation of isophthalaldehyde with malonitrile, for example, 1 gave rate enhancements of about 50 and 30 relative to 2 and a commercial catalyst consisting of a monolayer of 3‐aminopropyl groups on silica, respectively.

Highly accessible catalytic sites on recyclable organosilane-functionalized magnetic nanoparticles: An alternative to functionalized porous silica catalysts

“…A significant drop in reaction rate was observed for the hydrophobic catalyst under identical reaction conditions, which is consistent with the results of Katz and co-workers for amino-functionalized silica catalysts [45]. It was proposed that hydrophilic supports could preactivate the reagents by interaction of the carbonyl groups with the acidic hydroxyl groups, affording reaction rate enhancement [45,46]. Indeed, rate enhancement by using acid-base bifunctional catalysts was also observed for other base-catalyzed reactions such as aldol or nitroaldol condensations, in which the carbonyl groups were also preactivated by the acidic sites prior to the base-catalyzed step [47].…”

“…5, there was a large difference in activity of the hydrophilic and the hydrophobic magnetic nanoparticle catalysts. A significant drop in reaction rate was observed for the hydrophobic catalyst under identical reaction conditions, which is consistent with the results of Katz and co-workers for amino-functionalized silica catalysts [45]. It was proposed that hydrophilic supports could preactivate the reagents by interaction of the carbonyl groups with the acidic hydroxyl groups, affording reaction rate enhancement [45,46].…”

“…Katz and co-workers [45] investigated the effect of outersphere acidity on chemical reactivity of amino-functionalized silica catalysts in the Knoevenagel reaction and indicated that differences between hydrophilic and hydrophobic silica supports resulted in significant effects on the reaction. A higher reaction rate was observed for hydrophilic silica supported catalyst with free acidic silanol groups surrounding the diamine species.…”

Catalytic Strategies for Changing the Energy Content and Achieving CC Coupling in Biomass‐Derived Oxygenated Hydrocarbons

“…Coupling at the molecular level has led to the development of inorganic-organic hybrid materials that may be used as heterogeneous catalysts in these reactions. [47][48][49][50] These new catalysts are intriguing be- Figure 6. Energies of combustion per unit volume (relative to the energy density of sugar) for various liquid fuels derived from glucose, along with the enthalpy changes and the amounts of CO 2 and H 2 O generated during the production of these fuels from glucose.…”

“…[47][48][49][50] These improvements in catalytic performance allow for the elimination of the homogeneous acid and base catalysts that are currently used in biomass-conversion reactions. [47][48][49][50] Thus, the multifunctional nature of this new class of catalysts presents opportunities for combining reduction and CÀC bond-formation reactions. Furthermore, the use of bifunctional catalysts (metals supported on basic metal oxides) to carry out condensation and hydrogenation cycles in a single reactor highlights the use of activesite coupling to combine CÀC bond formation with oxygen removal.…”